Pyrazine-based sensitizers for dye-sensitized solar cells

Abstract

Dye-sensitized solar cells (DSSCs) have emerged as a major technology in solar energy conversion. Ruthenium dyes are commonly used in DSSCs due to their high stability and power conversion efficiency (PCE), but the complex and costly synthesis of ruthenium complexes hinders their commercialization. Metal-free sensitizers attract significant attention in DSSC technologies. They are eco-friendlier with more facile synthesis and offer diverse structural designs for tuning electronic, optical and morphological properties. Metal-free dyes have been reported with PCEs surpassing Ru-based N3 and N719 benchmark sensitizers in DSSCs. Pyrazine-based sensitizers demonstrate favorable photophysical and electrochemical properties due to their unique structural features and versatile synthetic approaches enabling functionalization at different positions. Several pyrazine sensitizers have been reported with strong absorption extending to the near-infrared region, high molar extinction coefficients, and balanced hole and electron transport. The donor–π–acceptor (D–π–A) design with pyrazine as the π-bridge is conventional to favor intramolecular charge transfer in DSSCs. Other pyrazine architectures, e.g., D–A–π–A′, demonstrated high PCEs, reaching up to 12.5%. This review highlights the advances in pyrazine-based sensitizers focusing on the pyrazine core as a principal electron acceptor, π-auxiliary acceptor, and even as a unit for functionalization as an electron-donating moiety. The reported sensitizers for DSSCs since 2008 are summarized, including metal-free dyes and pyrazines conjugated to Ru and porphyrin dyes. The dyes are classified into quinoxaline, thienopyrazine, pyridopyrazine, and pyrrolopyrazine cores in different sections. The DSSC parameters are summarized, discussing the electronic structure–property and structure–function relationships and offering insights into future architectures that accelerate their commercialization.

---

Ravulakollu Srinivasa Rao

Ravulakollu Srinivasa Rao completed his PhD at the CSIR-Indian Institute of Chemical Technology (IICT), Hyderabad, India in the group of Dr Surya Prakash Singh. After working as an Assistant Manager in the industry of UQUIFA in India from 2020 to 2021, he joined the group of Prof. Sung Yeon Jang at the Ulsan National Institute of Science and Technology (UNIST) in South Korea as a postdoctoral fellow in 2022. His research there focused on the synthesis of non-fullerene acceptors for organic photovoltaics. Currently, he is working as a postdoctoral fellow with Prof. Janah Shaya at the Khalifa University of Science and Technology (KUST) in Abu Dhabi. His research is focused on the synthesis of near-IR materials for organic photovoltaics and the synthesis of iron-based materials for various applications, utilizing different synthetic approaches via mechanochemistry.

Jonnadula Venkata Suman Krishna

Jonnadula Venkata Suman Krishna received his PhD in Chemistry from the CSIR-Indian Institute of Chemical technology (IICT), Hyderabad, India in 2019 under the guidance of Dr L. Giri Babu. Later, he worked with Prof. Tan Swee Ching (National University of Singapore, 2020–2021) and Prof. Han Young Woo (Korea University, Seoul, 2022–2023) as a Postdoctoral Fellow. After that, he worked as a Senior Researcher at Dxome Co. Ltd. (South Korea, 2023). Currently, he is an Assistant Professor in the group of Prof. Igor F. Perepichka at the Silesian University of Technology, Poland. His main research interests include synthesis of novel organic π-conjugated materials and tetra pyrrolic dyes for various optoelectronic and electronic applications.

Upendar Reddy Gandra

Upendar Reddy Gandra received his PhD in Chemical Sciences under the supervision of Dr Amitava Das from the CSIR-National Chemical Laboratory in 2016. After his first postdoctoral research with Prof. Alexander Schiller at Friedrich Schiller University, Jena, Germany (2015–2018), he joined Texas A&M University at Qatar (TAMUQ) to work with Prof. Hassan S. Bazzi on olefin metathesis derived functional polyolefins for sensing and drug release. In December 2021, he moved to the Ulsan National Institute of Science and Technology (UNIST) in South Korea, where his research focused on organelle-targeted self-assembly and supramolecular polymerization for drug-free cancer research and photodynamic therapy applications. He is currently working as a Research Scientist with Prof. Mohamed Infas Haja Mohideen at the Khalifa University of Science and Technology (KUST), Abudabi, UAE on the design and synthesis of functional solid-state materials, particularly metal organic frameworks.

Igor F. Perepichka

Igor F. Perepichka is a Professor and ERA Chair (European Research Area) at the Silesian University of Technology, Poland. He received his PhD in Chemistry from the National Academy of Sciences of Ukraine (1987) where he started his career in 1981. He was an Alexander von Humboldt Fellow at Würzburg University (1995–1997) and a visiting scientist/visiting professor at CNRS, Angers University (2000–2002), Durham University (2003–2007) and McGill University (2022–2023). In 2007, he was appointed as a Reader in Chemistry at the University of Central Lancashire and then moved to Bangor University as a Professor of Chemistry and LCRI Chair (Low Carbon Research Institute) in 2010. In 2019, he accepted professorship at Northwestern Polytechnical University in China and in 2023 he moved to SUT, Poland. His main research interests are in the field of organic π-functional materials for electronic and optoelectronic applications, from design and synthesis to studies in devices.

Janah Shaya

Janah Shaya received his MSc in Synthesis, Catalysis and Sustainable chemistry from the University of Lyon, France and his PhD degree in Chemistry from the University of Nice, France with honor distinction in 2016. He pursued postdoctoral fellowships at Ecole Normale Supérieure de Lyon and the University of Strasbourg, France in collaboration with Kyushu University, Japan up to 2019. He was appointed as a Senior Lecturer of chemistry at Khalifa University in 2019 and completed a postdoctoral degree in Artificial intelligence and Machine learning at the University of Texas at Austin, USA. Dr Shaya joined the Chemistry Department at KU in 2023 as an Assistant Professor. He has published in high impact factor journals including Angewandte Chemie, Nucleic Acid Research (Oxford Press), Journal of Photochemistry and Photobiology, and ACS Chemical Biology. His research interests involve organometallic chemistry and catalytic methods and their applications in sensing and two-photon absorption materials.

---

1. Introduction

Energy is the primary driving force for human progress in our modern world, with fossil fuels being the predominant energy source. However, depletion of fossil fuels is occurring much faster than their regeneration, urging the search for alternative energy sources to overcome the energy crisis. Extensive research is focused on challenges in solar energy conversion and storage due to its abundance, sustainability and ease of harnessing.^1–5 The conversion of light energy into electrical energy has been achieved by creating photovoltaic devices through several generations of technology. These include different categories in terms of the timeline of their development as well as the materials they use:⁶

– 1st generation solar cells, which include monocrystalline and polycrystalline silicon-based solar cells;^7,8

– 2nd generation solar cells based on thin-film technology. CdTe, copper indium gallium selenide (CIGS), and amorphous silicon (α-Si) are the three most widely commercialized thin-film solar cells.^9–11 Other thin-film semiconductor-based cells have also been developed (e.g., GaAs);¹²

– 3rd generation solar cells¹³ include semiconductor quantum dot (QD) solar cells (e.g., PbS, PbSe, CdS, CdSe, and CdTe QDs),^14–17 dye-sensitized solar cells (DSSCs),^18–22 organic solar cells based on small π-functional molecules or π-conjugated polymers (OSCs),^23–28 and perovskite solar cells (PSCs);^29–34

– the concept of 4th generation solar cells comprises hybrid materials, including flexible inexpensive polymer films and novel inorganic nanostructures (metal nanoparticles and metal oxides, carbon nanotubes etc.), combined with advances in device architecture and technology, with emphasis on improving efficiency and reducing costs.^6,35

In third-generation solar cells, OSCs occupy their own niche, offering a cost-effective and lightweight alternative to silicon-based solar cells, benefiting from high optical absorption, the ability to be processed onto flexible substrates and an almost infinite range of structural variations for tuning spectral, electrical and morphological properties to optimize device performance, long-range stability and commercialization perspectives. They have passed a long (over three decades) journey of research and development. In the “infant age” of OSC research, intense studies were focused on conjugated polymers as electron donors and fullerene derivatives as electron acceptors. These were used as blends to form the bulk heterojunction (BHJ) structure of the active layer in OSCs. During the last decade of the 20th century/early 2000s, their performance was improved from ∼0.1% to ∼3–5%, with the most studied system being P3HT:PC₆₁BM (poly-3-hexylthiophene:[6,6]-phenyl-C₆₁-butyric acid methyl ester), with hundreds of publications on it. Next steps in OSC development included (i) turning attention toward low band gap conjugated polymer donors for more efficient sunlight irradiation,^25–27 (ii) development of non-fullerene acceptors with intramolecular charge transfer,^27,28 (iii) OSCs based on small molecule electron donors and acceptors for solution processing,^23,27 and more recently (iv) search of efficient single component OSC materials.^36–38 With these enormous efforts, the best current OSCs have achieved PCE exceeding 20% under AM 1.5 G,^39–41 with the very recently reported record value of 20.82% for a single-junction OSC.^42,43

PSCs are a younger, third-generation solar cell technology, which made a splash and “game changes” in the fields of photovoltaics, energy and optoelectronic materials since their discovery, with immediate booming of research on perovskite materials and devices.^29–33 Metal halide perovskite materials are rapidly gaining attention as a promising photovoltaic technology due to their tunable bandgap, long carrier diffusion length (∼1 μm), and low exciton binding energy, together with cheap materials for PSC fabrication. PSCs stand out for their high efficiency, cost-effective fabrication, and adaptability to flexible substrates, making them strong contenders against traditional silicon-based solar cells. However, large-scale commercialization remains an ongoing challenge. Evolving from dye-sensitized solar cells, PSCs have seen remarkable progress, with efficiencies rising from 3.8% in 2009⁴⁴ to over 26% today.^45–47 Key innovations, such as replacing liquid electrolytes with solid-state hole-transporting layers, have significantly improved their stability and performance. Recent breakthroughs, including triple-cation perovskite compositions, have further enhanced efficiency and durability, reinforcing PSCs as a leading next-generation solar technology for commercialization.⁴⁸

DSSCs have emerged as one of the potential photovoltaic technologies due to their low production cost, efficient energy payback, flexibility, thermal stability, and ease of fabrication.^21,49 Currently, the efficient DSSCs demonstrate a PCE of over 12%. For example, PCE = 12.89% was demonstrated for tandem DSSCs,⁵⁰ and the record value of PCE = 15.2%.⁵¹ The device structure of a DSSC (Fig. 1) comprises a dye-sensitized TiO₂-coated conductive glass (FTO: fluorine-doped tin oxide), a redox electrolyte (I⁻/I₃⁻, Cu^+/2+, and Co^2+/3+) and platinum as the counter electrode. In DSSCs, charge separation and recombination mechanisms are essential for device efficiency. The cell consists of several key components: a photoanode (typically TiO₂), a dye sensitizer, an electrolyte (iodide/triiodide redox couple), and a counter electrode (Pt).⁵² The sensitizer on TiO₂ gets excited upon absorbing sunlight, and the excited electrons are injected into the TiO₂ semiconductor, followed by conduction through the conductive glass. The electrons then flow through the external circuit. Regeneration of the sensitizer is achieved through the redox electrolyte (I⁻/I₃⁻), which receives the electrons from the external circuit through the counter electrode (Pt).⁵³ Maximizing charge separation, through efficient electron injection from the dye into the TiO₂ conduction band, along with effective dye regeneration by the electrolyte, and minimizing recombination processes are critical to enhancing the overall efficiency of DSSCs.

Fig. 1 Schematic diagram of DSSC operation.

The performance of the DSSC device is evaluated using several parameters, including short-circuit current density (J_SC), open-circuit voltage (V_OC), fill factor (FF), and incident photon-to-electron conversion efficiency (IPCE), also known as external quantum efficiency (EQE) and power conversion efficiency (PCE).⁵² Short-circuit current density is influenced by EQE and reflects changes in EQE based on the emitted wavelength. V_OC is the electrical potential difference between two terminals of an electronic device when disconnected from any circuit. The fill factor (FF) measures the efficiency of a photovoltaic cell and represents the ratio of the maximum output power (P_max) of the solar cell per unit area to the product of open-circuit voltage (V_OC) and short-circuit current (J_SC) (eqn (1)):

FF = P_max/(J_SC × V_OC) (1)

IPCE or EQE measures how efficiently the solar cells convert the incident light into electricity (at a given wavelength), i.e. the efficiency of photon to electron energy conversion. It shows the percentage of photons that generate charge carriers (electrons or holes) collected by the cell (eqn (2)):

PCE is a metric that quantifies how effectively a photovoltaic device, such as a solar cell, converts sunlight energy into usable electrical power. It is defined as the ratio of the electrical power output (P_out) created by the device to the incident light power (P_in) from the sun and can be estimated from experimentally defined parameters V_OC, J_SC, and FF (eqn (3)):

J _SC is another important parameter that quantifies the conversion ratio of the incident photon flux to photocurrent density. Here, e is the elementary charge, I₀(λ) is the incident photon flux, and IPCE(λ) is the incident photon-to-current conversion efficiency (eqn (4)):^22,54

Charge injection efficiency plays a key role in achieving high efficiency in DSSCs. It refers to how efficiently charge carriers (electrons or holes) are transferred between phases, such as from a dye to a semiconductor in DSSCs. This process is essential for converting light into electricity. Efficient electron injection leads to increased J_SC and V_OC, improving the overall power conversion efficiency. The electron injection efficiency (Φ_inj) is defined as follows (eqn (5)):⁵⁵

Here, k_inj and k_r are the rate constants for electron injection from the excited dye into the TiO₂ layer and the rate of the excited state relaxation to the ground state, respectively. For efficient electron injection, the electron injection kinetics should be sufficient to compete effectively with excited state decay to ground (k_inj ≫ k_r). Under these conditions near unity injection yield is achieved while minimizing recombination losses.⁵⁶ Charge injection efficiency is a critical parameter in DSSCs, determining how efficiently photoexcited electrons are transferred from the dye to the TiO₂ conduction band. Improving this efficiency enhances both J_SC and V_OC, leading to higher PCE. Imperfect electron injection leads to recombination losses at the dye/TiO₂ interface, reducing J_SC. The charge injection efficiency of DSSCs is influenced by several factors:

• Energy alignment: the free energy change for electron injection (–G_inj). It is determined by the energy difference between the excited dye's LUMO and the conduction band (CB) of TiO₂.

• External conditions: solvent polarity, pH, excitation wavelength, and inclusion of cationic species influence injection dynamics.^57,58

• Dye–semiconductor interface: the adsorption mode of dye molecules on the TiO₂ surface (e.g., unidentate, bidentate chelating, and bidentate bridging) impacts charge transfer efficiency.⁵⁹

• Additives and surface modifications: common DSSC additives such as 4-tert-butylpyridine (tBP), which shifts the quasi-Fermi level of the TiO₂ photoanode to a higher potential and raises the energy of the conducting band (E_CB), reducing injection efficiency (while improving V_OC), and Li⁺ ions, which lower E_CB, enhance injection, alter charge injection dynamics.^60,61

• Dye aggregation: excessive dye loading on TiO₂ can lead to molecular aggregation, reducing charge injection efficiency.⁶²

To overcome current limitations, strategies such as molecular engineering of organic dyes, surface modifications, and optimizing dye loading are being explored. Understanding and optimizing charge injection efficiency are essential for advancing DSSC technology and achieving higher photovoltaic performance. Two types of recombination processes, labeled as (R1) and (R2) in Fig. 1, hinder the performance of DSSCs. The first (R1) occurs when an electron injected into TiO₂ recombines with the oxidized dye, resulting in a loss of charge carriers. The second (R2) takes place when electrons in the TiO₂ conduction band recombine with the oxidized species of the redox electrolyte (e.g., triiodide I₃⁻ anions in the I⁻/I₃⁻ redox pair). These recombination mechanisms lead to a decrease in both V_OC and J_SC values, negatively impacting overall PCE. The charge separation and recombination mechanisms in DSSCs are governed by the interaction between the dye, TiO₂, and the redox electrolyte.⁶³ Efficient charge separation through fast electron injection and sensitizer regeneration, coupled with the minimization of recombination at various interfaces, are critical for achieving high PCEs.

Redox electrolytes play a crucial role in DSSCs, being involved in an electrochemical process as redox shuttles (oxidation at the interface with a dye and reduction at the cathode interface), thus assisting charge carriage, maintaining electrochemical balance, and improving stability.⁶⁴ They allow efficient electron drive, support redox reactions by replenishing oxidized dye molecules, and mitigate dye degradation and cell corrosion. DSSCs utilize liquid, solid-state, and quasi-solid-state electrolytes, each of which has its own pros and cons. Liquid electrolytes (e.g., iodide/triiodide in acetonitrile) offer high efficiency but suffer from leakage, volatility and corrosion, which can typically reduce the device stability.⁶⁵ Ionic liquids provide better stability but have high viscosity. Redox mediator-based electrolytes (e.g., ferrocene and cobalt complexes) improve stability and open-circuit voltage.⁶⁶ Solid-state DSSCs may include solid-state or quasi-solid-state redox-electrolytes, including gel electrolytes (polymer-based, thermoplastic, and thermosetting), hole-transporting materials and/or quasi-solid-state ion conductive polymers, which improve the flexibility and durability of the devices.^67,68 Iodide/triiodide (I⁻/I₃⁻), cobalt (Co^2+/3+) and copper salt complexes (Cu^+/2+) as redox couples, which are well-soluble in organic solvents, are extensively utilized in liquid electrolytes for DSSCs due to their suitable redox potentials and excellent ionic conductivity.⁶⁹ These Co, Cu or Fe salts as redox electrolytes are normally used in the form of complexes with various N-ligands, e.g., 2,2′-bipyridyl (bpy), 4,4′,6,6′-tetramethyl-2,2′-bipyridyl (tmby), 1,10-phenanthroline (phen), 2,9-dimethyl-1,10-phenanthroline (dmb), 6-(1H-pyrazol-1-yl)-2,2′-bipyridine (bpy-pz), 2,2′-(ethane-1,1-diyl)dipyridine (bpye) and others.^69,70

Recently Cu-based electrolytes have gained prominence in DSSCs due to their advantageous redox properties, stability, and cost-effectiveness. Their importance can be emphasized through several key aspects of adjusting the redox potential of Cu complexes via ligand modifications. The incorporation of NIR-absorbing dyes provides an opportunity for cosensitization with Cu complex redox shuttles, enhancing light absorption and boosting overall device performance.^69,71 Additionally, Cu-based electrolytes exhibit superior long-term stability due to their lower volatility and reduced corrosiveness compared to other counterparts. Cu complexes are less corrosive toward metal electrodes, enabling the use of alternative counter electrodes like copper sulfide (Cu₂S) and carbon-based materials, which enhances both the durability and cost-effectiveness of DSSCs.^51,72

Sensitizers are currently an intensive area of research due to their significant light-harvesting abilities, effective charge separation, low cost, and ease of synthesis. Various inorganic and organic sensitizers have been engineered, including metal complexes such as Ru(II) polypyridyl complexes^73–77 and porphyrins^78–84 as well as metal-free sensitizers.^85,86 Ruthenium-based sensitizers are widely used in DSSCs due to their promising photophysical features and their ability to achieve high efficiency in converting light into electricity. These complexes typically consist of a central ruthenium ion surrounded by ligands that absorb light in the visible region of the spectrum. Ruthenium complexes used in DSSCs often feature polypyridyl ligands such as bpy or phen attached to the Ru(II) center. These ligands can be modified to tune their absorption spectra and improve electron transfer kinetics. They exhibit strong absorption bands in the 400–600 nm visible region, aligning with the solar spectrum and ensuring efficient light absorption. The excited state of the ruthenium complex facilitates rapid injection of electrons into the oxide semiconductor and subsequent electron transport. Their stability under prolonged light exposure and in various electrolyte systems is also advantageous for practical sensitization applications. Polypyridyl ruthenium complexes are renowned for their high efficiency in converting light to electric power, achieving high PCE up to 11% with the commonly used standard N719 dye in DSSCs.⁸⁷ However, the limited resources of ruthenium and the associated high costs, along with the environmental implications of its production, hinder the broader adoption of Ru complexes. Ru is an expensive rare metal, and the synthesis of its complexes still needs to find greener alternative routes.⁸⁸ Organic sensitizers have become strong competitors to metal-based ones in several applications due to their desirable properties such as ease of synthesis, high molar extinction coefficients, and affordable costs.^89–92

The donor–π–acceptor (D–π–A) architecture in metal-free sensitizers has demonstrated its impressive efficiencies. Various donor and acceptor moieties, incorporated through supporting or internal acceptors known as π-auxiliary acceptors, have been reported in metal-free organic dyes. These π-auxiliary acceptors are considered admirable building blocks for sensitizers due to their high polarizability and tunable spectroscopic and electrochemical properties. These acceptors improve the performance of DSSCs by enhancing light absorption, charge dissociation, and electron incorporation. As a result, the introduction of π-auxiliary acceptors in metal-free dyes has become a standard practice in dye conception and development. Introducing an auxiliary acceptor in the D–π–A type structure can transform it into a D–A–π–A′/D–π–A–A′ type architecture, which can lead to higher absorption and charge transfer, thereby enhancing the power conversion efficiency. In this aspect, a variety of π auxiliary acceptors such as heterocyclic compounds like benzothiadiazole, benzotriazole, diketopyrrolopyrrole, phthalimide, and pyrazine derivatives have been designed and tethered to dye molecules. The association of these moieties in sensitizers causes a substantial increase in π-conjugation and leads to red shifts in absorption spectra.^93–97

2. Pyrazine

Pyrazine is a heterocyclic aromatic compound, categorized by a six-membered ring structure having two nitrogen atoms at positions 1 and 4 in the ring (Fig. 2). Pyrazine plays a momentous part in a broad range of applications, covering from solar energy conversion to pharmaceutical development. Its diverse electronic properties, which arise from the presence of nitrogen atoms and the conjugated π-electron system, enable pyrazine to participate in various chemical processes,⁹⁸ while its tunability through the introduction of different chemical substituents further enhances its versatility, making it a valuable building block in multiple industries, including electronics, materials science, and drug synthesis. In the specific context of DSSCs, pyrazine-based dyes present an attractive balance of chemical stability, structural tunability, and excellent power conversion efficiency. These features have attracted researchers to tune several pyrazine-based designs aiming to achieve higher performance levels than metal-based dyes such as ruthenium complexes.

Fig. 2 Two general synthetic pathways to pyrazines.

Pyrazine is a strong electron-deficient heterocycle due to the presence of two more electronegative (compared to carbon) nitrogen atoms at sp² hybridization, which is widely used as a building block in the design of materials for optoelectronics and other various applications.^98,99 By modifying the pyrazine core with different substituents, the photophysical properties, such as light absorption and electron injection efficiency, can be tuned, which is valuable for device performance in solar cells. In comparison with several π-auxiliary acceptors, pyrazine derivatives are considered the most efficient heterocyclic building blocks for sensitizers due to their ability to diminish aggregation and charge recombination processes because of their nonplanar structures. Furthermore, the electron-accepting property of pyrazine derivatives enhances light absorption, charge separation efficiency, and stability, leading to higher solar cell efficiencies. Their versatile structure allows for tailored design and optimization, facilitating electron transport to the TiO₂ conduction band, making them valuable for advanced DSSC development.

In 2020, Gong et al. reviewed pyrazine derivatives for optoelectronic applications,¹⁰⁰ and Saha et al. explored quinoxaline-based sensitizers for DSSC applications in 2022.¹⁰¹ However, these reviews did not explore ruthenium complexes and porphyrins with quinoxaline derivatives and the key properties and applications of such designs incorporating metal complexes in DSSCs. Additionally, sensitizers based on other pyrazine derivatives such as thieno[3,4-b]pyrazine, pyrido[2,3-b]pyrazine, and pyrrolo[2,3-b]pyrazine and their applications in DSSCs were not previously discussed, to the best of our knowledge. This review summarizes all classes of pyrazine-based derivative dyes as sensitizers in DSSC applications, discussing advances from 2008 to 2024 and offering insights into future effective designs of these sensitizers.

2.1. Synthesis of pyrazine derivatives

Pyrazine scaffolds have been synthesized using various reaction methods for decades, but most of these methods are limited by their low yields and long reaction times. Recent advances include the work of Ghosh and Mandal in 2012, who reported an efficient, greener one-pot chemistry approach for pyrazine synthesis via the condensation of 1,2-diamine and 1,2-dicarbonyl in the presence of a strong base (t-BuOK) at room temperature (Fig. 2a).¹⁰² In 2018, Turner et al. reported the use of amino transaminase (ATA-113) enzyme as the catalyst in the condensation of 1,2-diketones and relevant amine donors to form aminoketones that undergo oxidative polymerization yielding the final symmetric pyrazine adduct (Fig. 2b).¹⁰³

In addition, pyrazine and its derivatives can easily be involved in different nucleophilic substitution and cross-coupling reactions.^{101,104–106} A wide diversity of coupling reactions have been reported using halogenated pyrazine scaffolds, such as Sonogashira,¹⁰⁷ Suzuki,¹⁰⁸ Negishi and Heck reactions, including a recent review on metal-catalyzed functionalization of pyrazines.¹⁰⁹ Based on the presented structural moieties annulated to the pyrazine ring, derivatives can be categorized into four major types: quinoxalines (benzene), thienopyrazines (thiophene), pyridopyrazines (pyridine), and pyrrolopyrazines (pyrrole) (Fig. 3). These structural motifs of pyrazine-derived electron-accepting moieties incorporated into dyes (as well as further side-chain functionalization in them) allow efficient tuning of the intramolecular charge transfer in the dyes and consequently their optical and electronic properties.

Fig. 3 Pyrazine-containing cores used as electron-deficient moieties in the design of sensitizers for DSSCs.

Quinoxaline (benzo[3,4-b]pyrazine) is a planar, electron-deficient aromatic heterocycle with two nitrogen atoms that enhance electron delocalization and LUMO stabilization. However, its moderate π-conjugation and visible absorption limit efficiency, often requiring modifications with electron-donating groups or extended conjugation.^110–112 While quinoxaline itself has limited absorption in the visible region, derivatives with donor groups (D–π–A systems) show significantly enhanced visible and NIR absorption.

Thieno[3,4-b]pyrazine (TPz) incorporates a thiophene ring, extending π-conjugation and improving electron delocalization. The presence of sulfur modifies the electron density distribution, leading to a lower band gap and red-shifted absorption, enhancing visible to NIR absorption and charge transport, making TPz-based sensitizers ideal for broad-spectrum DSSCs.¹¹³ Also, linking through the 2,5-position of the thiophene moiety has several advantages such as: (i) the 5-membered thiophene ring brings less steric hindrance in the interaction with the neighboring moieties in the backbone and (ii) the thiophene ring is less aromatic than the benzene ring shifting aromatic-to-quinoid resonance structure, thus facilitating ICT and longer wavelength absorption of a dye.

Pyrido[3,4-b]pyrazine possesses the topology similar to quinoxaline, but the electron-deficient pyridine moiety in it leads to further stabilization of the LUMO and improves electron-withdrawing ability. This further enhances the intramolecular donor–acceptor interaction in dyes, extending the absorption of dyes to long wavelength and NIR spectral regions and facilitating charge injection from them (in the excited state) into the TiO₂ photoanode.¹¹⁴

Data on the pyrrole-fused pyrazine core as a moiety for DSSC dyes are limited by a few dipyrrolo[2,3-b:2′,3′-e]pyrazine structures.¹¹⁵ In the given structure(s), two electron-rich pyrrole moieties annulated to the pyrazine core significantly destabilize the LUMO. While the device performance with this unit was low (0.06–1.44%), it is difficult to make predictions at this time due to limited data on this class of DSSC dyes.

Dye sensitizers for DSSCs based on the above highlighted four classes of fused-ring pyrazine derivatives (Fig. 3) are discussed in detail in the next section.

3. Quinoxaline-based sensitizers

The pyrazines fused with benzene moieties are commonly termed as quinoxalines. Quinoxaline-based sensitizers have proven to be very valuable in DSSCs owing to their strong light absorption and high stability. This is the most popular and widely studied electron-deficient unit exploited in the design of pyrazine-based dye sensitizers for DSSCs, particularly due to easy synthetic accessibility and functionalization of quinoxaline derivatives and good electron-accepting characteristics.

3.1. Quinoxaline derivatives as metal-free sensitizers

Table 1 summarizes the photovoltaic parameters of the reported quinoxaline-based sensitizers discussed in this section. In 2011, Beak et al. reported two quinoxaline sensitizers, named RC21 and RC22 (Fig. 4), for DSSC applications.¹¹⁶ The triphenylamine-bearing electron donor connected to the 1,2-positions of the quinoxaline in the vertical direction is referred to as RC21 and the one bonded in the horizontal direction into the fused benzene ring is named RC22. Both molecules exhibited good absorption properties (Fig. 5a). The absorption of RC22 was broad shifting to the NIR region due to an extensive conjugation and effective formation of an intramolecular charge-transfer (ICT) state. RC22 exhibited better a PCE of 5.56%, compared to RC21 (3.30%). The IPCE shown in Fig. 5b and c was observed to be good with respect to RC21 and RC22. The RC22 coated TiO₂ film exhibited better light-harvesting capabilities with higher IPCE and overall efficiency. The oxidation potential values of RC21 and RC22 were found to be 1.03 V and 1.01 V, respectively, vs. the normal hydrogen electrode (NHE). These values are more positive than the redox couple (I⁻/I₃⁻, ∼0.4 V), indicating a sufficient driving force for dye regeneration, which allows for the effective recovery of injected electrons by the dye cation. The excited-state oxidation potentials of these dyes were found to be more negative than the conduction band of TiO₂ (ca. −0.5 V vs. NHE), indicating efficient electron injection. Therefore, dyes RC21 and RC22 possess suitable electronic energy levels, making them promising sensitizers.

Table 1 DSSC performance parameters of quinoxaline-based metal-free sensitizers

Sensitizer^a	Architecture^a	Electrolyte	J SC [mA cm^−2]	V OC [V]	FF	PCE^b [%]	Ref.
a In the architectures of sensitizers, acceptor moieties containing the pyrazine ring are denoted as A, the other acceptor moieties and donor moieties are denoted as A1, D, and D1, and acceptor moieties attached to or part of the anchoring group are denoted as A′; A|D or D|A abbreviations denote the pyrazine-containing acceptor moiety fused to electron-donor heteroaromatic ring(s); π-bridges in some cases (e.g. thiophene, furan, bithiophene, thienothiophene, and dithienothiophene) can also act as electron-donor units; anchoring groups (CO2H or others) are, as a rule, at the right side of these abbreviated architectures. b PCEs above 7.5% are highlighted in a bold italic font. c The sensitizer was used in DSSCs together with the chenodeoxycholic acid co-adsorbent (CDCA).
RC21	(D)2–π–A	I^−/I3^−	6.4	0.72	0.71	3.30	116
RC22	D–A–π–A′	I^−/I3^−	11.4	0.66	0.74	5.56	116
LI37	D–π–A–π–A′	I^−/I3^−	11.50	0.69	0.67	5.38	117
LI38	D–π–A–π–A′	I^−/I3^−	13.56	0.72	0.65	6.51	117
LI39	D–π–A–π–A′	I^−/I3^−	14.34	0.74	0.67	7.20	117
LI40	D–π–A–π–A′	I^−/I3^−	14.59	0.73	0.64	6.75	118
LI41	D–π–A–π–A′	I^−/I3^−	6.43	0.60	0.67	2.63	118
FNE44	D–π–A–π–A′	I^−/I3^−	5.70	0.73	0.78	3.27	119
FNE45	D–π–A–π–A′	I^−/I3^−	14.60	0.67	0.75	7.40	119
FNE46	D–π–A–π–A′	I^−/I3^−	16.16	0.68	0.75	8.27	119
FNE47	D–π–A–π–A′	I^−/I3^−	6.18	0.74	0.75	3.45	119
FNE48	(D–π)2–A–π–A′	I^−/I3^−	13.2	0.72	0.65	6.1	120
FNE49	(D–π)2–A–(π–A′)2	I^−/I3^−	11.6	0.72	0.63	5.2	120
FNE53	(D–π)2–A–π–A′	I^−/I3^−	13.45	0.60	0.65	5.19	121
FNE54	D–π–A–π–A′	I^−/I3^−	13.88	0.62	0.69	6.0	122
FNE55	D–π–A–π–A′	I^−/I3^−	15.57	0.63	0.69	6.8	122
FNE56	D–π–A–π–A′	I^−/I3^−	17.76	0.67	0.68	8.2	122
FNE60	D–A–π–A′	I^−/I3^−	13.97	0.65	0.67	6.1	123
FNE61	D–A–π–A′	I^−/I3^−	9.23	0.62	0.68	3.9	123
FNE62	D–π–A–π–A′	I^−/I3^−	17.59	0.68	0.68	8.2	123
FNE63	D–π–A′–π–A′	I^−/I3^−	10.81	0.64	0.70	4.9	123
TQ1	D–A–π–A′	I^−/I3^−	10.83	0.67	0.71	5.19	124
TQ2	D–A–π–A′	I^−/I3^−	13.51	0.74	0.70	7.08	124
IQ1	D–A–π–A′	I^−/I3^−	13.60	0.68	0.67	6.24	124
IQ2	D–A–π–A′	I^−/I3^−	15.65	0.77	0.70	8.50	124
IQ4	D–A–π–A′	I^−/I3^−	17.55	0.74	0.71	9.24	125
IQ6	D–π–A–π–A′	I^−/I3^−	15.91	0.71	0.68	7.77	125
IQ7	D–π–A–π–A′	I^−/I3^−	14.70	0.71	0.61	6.39	125
IQ8	D–A–π–A′	I^−/I3^−	14.92	0.71	0.67	7.09	125
IQ9	D–A–π–A′	I^−/I3^−	5.70	0.69	0.73	2.91	126
IQ10	D–A–π–A′	I^−/I3^−	16.72	0.64	0.72	7.75	126
IQ11	D–A–π–A′	I^−/I3^−	12.59	0.72	0.72	6.56	126
IQ12	D–A–π–A′	I^−/I3^−	17.97	0.71	0.68	8.76	126
SC1	D–A–D–A′	I^−/I3^−	18.33	0.63	0.68	8.05	111
SC2	D–A–D–A′	I^−/I3^−	15.06	0.60	0.72	6.64	111
SC3	D–A–D–A′	I^−/I3^−	15.12	0.62	0.72	6.81	111
Q1	D|A–π	I^−/I3^−	5.84	0.77	0.72	6.5	127
DBP-L	(D)2–A	I^−/I3^−	2.24	0.52	0.62	0.73	128
DTP-L8	D2–D|A	I^−/I3^−	2.12	0.44	0.58	1.60	129
Y123	π–D–π–A′	Cu^+/2+	13.33	1.02	0.75	10.3	130
HY63	π–D–A1–π–A′	Cu^+/2+	13.71	0.99	0.76	10.3	130
HY64	π–D–A–π–A′	Cu^+/2+	15.76	1.02	0.77	12.55	130
CR204	D–A–π–A′	I^−/I3^−	14.87	0.66	0.66	6.49	131
DJ104	D–A–π–A′	I^−/I3^−	17.7	0.69	0.66	8.06	132
DJ112	D–A–π–A′	I^−/I3^−	16.9	0.67	0.66	7.47	132
DJ115	D–A–π–A′	I^−/I3^−	11.7	0.73	0.65	5.65	132
DJ125	D–π–A–π–A′	I^−/I3^−	15.8	0.62	0.64	6.27	132
DJ142	D–A–π–A′	I^−/I3^−	12.3	0.69	0.63	5.44	132
DJ152	D–A–π–A′	I^−/I3^−	11.5	0.72	0.62	5.24	132
CRCW1	D–A–π–A′	I^−/I3^−	15.6	0.65	0.65	6.59	132
PQ1	D–A–π–A′	I^−/I3^−	13.13	0.76	0.68	6.86	133
PQ2	D–π–A–π–A′	I^−/I3^−	1.86	0.43	0.43	0.34	133
PPL1	(D)2–A	I^−/I3^−	7.83	0.68	0.70	3.74	134
PPL2	(D)2–A	I^−/I3^−	8.08	0.69	0.72	4.04	134
WQ1	D–A	I^−/I3^−	4.10	0.662	0.69	1.89	135
WQ2	D–π–A	I^−/I3^−	5.51	0.612	0.66	2.25	135
ZW002	D–π–A–π	Co^2+/3+	12.43	0.96	0.69	8.23	136
ZW003	D–π–A–π	Co^2+/3+	11.33	0.95	0.69	7.43	136
AQ201	D–π–A–π–A′	Co^2+/3+	12.52	0.83	0.67	7.10	137
AQ202	D–π–A–π–A′	Co^2+/3+	14.61	0.83	0.69	8.37	137
AQ203	D–π–A–π–A′	Co^2+/3+	14.39	0.79	0.70	8.06	137
AQ308	D–π–A–π–A′	Co^2+/3+	16.75	0.83	0.70	9.81	138
DQ1	D–D|A–A′	I^−/I3^−	13.11	0.74	0.65	6.37	139
DQ2	D–D|A–A′	I^−/I3^−	9.31	0.66	0.65	4.03	139
DQ3	D–π–D|A–A′	I^−/I3^−	9.01	0.65	0.68	4.01	139
DQ4	D–A–D|A–A′	I^−/I3^−	14.51	0.72	0.65	6.78	139
DQ5	D–A–D|A–A′	I^−/I3^−	17.61	0.68	0.59	7.12	139
TQ-01	D–π–A–π–A′	I^−/I3^−	17.09	0.69	0.66	7.78	140
TQ-02	D–π–A–π–A′	I^−/I3^−	16.24	0.68	0.66	7.29	140
TQ-03	D–π–A–π–A′	I^−/I3^−	8.16	0.61	0.81	4.03	140
IT	D–π–A	I^−/I3^−	14.40	0.70	0.61	6.22	141
IQT	D–A–D|A–A′	I^−/I3^−	16.87	0.71	0.67	7̲.9̲8̲	141
IQ-C	D–A–D|A–π–A′	I^−/I3^−	13.79	0.65	0.66	5.96	142
IQ-D	D–A–D|A–π–A′	I^−/I3^−	15.13	0.68	0.61	6.39	142
IQ-F	D–A–D|A–π–A′	I^−/I3^−	15.21	0.67	0.64	6.60	142
QBT-1	D–D|A–A′	I^−/I3^−	13.76	0.72	0.73	7.36	143
QBT-3	D–D|A–A′	I^−/I3^−	13.29	0.746	0.75	7.73	143
QBT-4	D–D|A–π–A′	I^−/I3^−	15.48	0.754	0.72	8.41	143
QBT-5	D–D|A–π–A′	I^−/I3^−	14.93	0.75	0.71	8.08	143
QX22	(D)2–A–π–A′	I^−/I3^−	11.9	0.79	0.63	6.05	144
QX23	(D)2–A–π–A′	I^−/I3^−	12.9	0.81	0.67	7.09	144
QX24	(D)2–A–π–A′	I^−/I3^−	9.03	0.70	0.68	4.35	144
QX25	(D)2–A–π–A′	I^−/I3^−	9.71	0.72	0.68	4.81	144
TPDC2	(D)2–A–π–A′	I^−/I3^−	7.31	0.62	0.57	2.58	145
TPDC4	(D)2–A–π–A′	I^−/I3^−	10.41	0.68	0.60	4.27	145
TPDC6	(D)2–A–π–A′	I^−/I3^−	8.91	0.66	0.55	3.17	145
TPDC12	(D)2–A–π–A′	I^−/I3^−	5.84	0.60	0.62	2.17	145
CS9	(D)2–A–(π–A′)2	I^−/I3^−	10.58	0.65	0.645	4.43	146
CS10	(D)2–A–(π–A′)2	I^−/I3^−	4.89	0.66	0.665	2.16	146
LC4	D–A–π–A′	I^−/I3^−	15.17	0.69	0.70	7.36	147
QC5-1	D–D|A–A′	I^−/I3^−	12.92	0.66	0.75	6.48	148
QC5-2	D–π–D|A–A′	I^−/I3^−	12.47	0.69	0.73	6.33	148
PC5-1	D–D|A–A′	I^−/I3^−	15.63	0.69	0.71	7.77	148
PC5-2	D–π–D|A–A′	I^−/I3^−	13.76	0.66	0.74	6.81	148
PC5-3	D–D|A–A′	I^−/I3^−	11.08	0.64	0.73	5.23	148
ZHG5	π–D–A–π–A′	I^−/I3^−	12.63	0.73	0.61	5.64	149
ZHG6	π–D–A–π–A′	I^−/I3^−	12.06	0.73	0.60	5.32	149
ZHG7	π–D–A–π–A′	I^−/I3^−	7.12	0.70	0.54	2.74	149
NQX1	(D–π)2–A	I^−/I3^−	8.72	0.65	0.73	4.10	150
NQX2	(D)2–A	I^−/I3^−	7.60	0.70	0.75	3.98	150
NQX3	(D)2–A	I^−/I3^−	9.77	0.64	0.67	4.18	150
NQX4	(D)2–A	I^−/I3^−	9.99	0.61	0.71	4.36	150
NQX5	(D)2–A	I^−/I3^−	10.64	0.61	0.69	4.46	151
NQX6	(D)2–A	I^−/I3^−	8.01	0.58	0.70	3.21	151
NQX7	(D)2–A	I^−/I3^−	8.19	0.62	0.78	3.94	151
NQX8	(D–D)2–A	I^−/I3^−	12.53	0.64	0.67	5.41	152
NQX9	(D–D–D)2–A	I^−/I3^−	9.16	0.60	0.70	3.83	152
NQX10	(D–π)2–A	I^−/I3^−	10.00	0.70	0.75	5.22	152
NQX11	(D)2–A–(π)2	I^−/I3^−	10.08	0.65	0.75	4.94	152
NQX12	(D)2–A–(π–A′)2	I^−/I3^−	7.58	0.56	0.77	3.30	152
FHD4	D–A–π–A′	I^−/I3^−	9.74	0.68	70.29	4.69	153
FHD5	D–A–π–A′	I^−/I3^−	8.16	0.65	69.85	3.73	153
FHD6	D–A–π–A′	I^−/I3^−	7.47	0.70	70.91	3.69	153
FHD4-1	D–A–π–A′	I^−/I3^−	10.44	0.72	60.86	4.54	154
FHD4-2	D–A–π–A′	I^−/I3^−	12.94	0.68	59.06	5.16	154
FHD4-3	D–A–π–A′	I^−/I3^−	11.00	0.71	0.62	4.87	154
MM1	(D)2–A–π–A′	I^−/I3^−	7.75	0.58	0.66	2.77	155
MM2	(D)2–A–π–A′	I^−/I3^−	8.05	0.58	0.67	3.11	155
MM3	(D)2–A–π–A′	I^−/I3^−	7.93	0.66	0.67	3.53	155
MM4	(D)2–A–π–A′	I^−/I3^−	9.08	0.66	0.64	3.88	155
MM5	(D)3–A–π–A′	I^−/I3^−	14.0	0.66	0.63	5.86	155
MM6	D–A–π–A′	I^−/I3^−	17.1	0.66	0.59	6.70	155
MM3 + MM6	—	I^−/I3^−	17.7	0.71	0.62	7.92	155
LY01	D–A–π–A′	I^−/I3^−	12.76	0.61	0.64	5.06	156
LY02	D–A–π–A′	I^−/I3^−	14.11	0.66	0.65	6.08	156
LY03	D–A–π–A′	I^−/I3^−	14.32	0.91	0.53	7.04	156
FS08	D–A–π–A′	I^−/I3^−	6.02	0.61	0.72	2.69	157
FS10	D–A–π–A′	I^−/I3^−	11.10	0.67	0.70	5.27	158
FS11	D–A–π–A′	I^−/I3^−	11.29	0.65	0.68	5.10	158
FS12	D–A–π–A′	I^−/I3^−	11.84	0.63	0.65	4.92	158
FS13	D–A–(π–A′)2	I^−/I3^−	4.73	0.63	0.73	2.21	157
FS14	D–A–(π–A′)2	I^−/I3^−	5.08	0.67	0.74	2.56	157
TPP	(D–π)^2–A	I^−/I3^−	3.84	0.72	0.65	1.80	159
TPPS	(D–π)^2–A–π–A′	I^−/I3^−	4.01	0.59	0.55	1.31	159
TPPF	(D–π)^2–A–π–A′	I^−/I3^−	5.69	0.60	0.67	2.64	159

---

Fig. 4 Molecular structures of the quinoxaline-based sensitizers reported by Baek et al.¹¹⁶ and Li et al.^117,118

Fig. 5 (a) Absorption spectra of dyes RC21 and RC22 recorded in chloroform and tetrahydrofuran (THF) solutions, respectively. (b) J–V characteristics and (c) IPCE spectra of RC21 and RC22 dyes. Reprinted with permission from ref. 116 Copyright 2011 American Chemical Society.

Li et al. reported three organic sensitizers, LI37–LI39 (Fig. 4), bearing the dibenzo[a,c]phenazine (DBP) moiety. Increasing the electron-donating effect of the side groups from LI37 to LI39 using methyl/methoxy substituents led to enhanced absorption and bathochromic shifts, thus improving the device performance. The bulkiness and extended π-conjugation of the DBP core by aromatic fused-ring accommodation in the sensitizers efficiently decreased the aggregation of the dyes and charge recombination. Absorption spectral studies revealed a red shift for the film, which contributed to the increased photocurrent response of the dyes. This improvement was achieved with LI-39, containing methoxy substituents (PCE = 7.18%), compared to CH₃ or H in the other dyes.¹¹⁷ Later, the same group reported dyes LI40 and LI41 with the 6,7-bis(hexyloxy)-2,3-dimethylquinoxaline acceptor and thiophene or furan conjugated bridges (Fig. 4). The solar cell based on LI40 demonstrated a broad IPCE spectrum and a high PCE of 6.75%.¹¹⁸

Zhou and colleagues also developed a series of quinoxaline sensitizers, namely FNE44–FNE47, by replacing the thiophene moiety fused onto pyrazine in FNE32 with a benzene moiety (Fig. 6 and Table 1).¹¹⁹ The thienopyrazine derivative FNE32 based DSSCs (see Table 4) exhibited an extended IPCE, covering the NIR region up to ∼850–900 nm. However, IPCE intensity was lower compared to quinoxaline-based analogue dyes FNE44 and FNE47 (Fig. 6), likely due to inefficient electron injection from dyes into the TiO₂ film, and no PCE improvement was observed (3.66% vs. 3.27–3.45%, Tables 1 and 4). The structural modification of sensitizers FNE44–FNE47 was aimed at tuning the energy levels of the sensitizers to improve the device performance. In particular, the unsubstituted thiophenes attached to the quinoxaline moiety in FNE45 and FNE46 resulted in a more planar molecular structure compared to analogues FNE44 and FNE47 with hexyl-substituted thiophenes. This planarity enhanced the conjugation along the backbone and led to effective ICT interactions between the triphenylamine donor and the cyanoacrylic acceptor groups. A bathochromic shift up to 525 nm for the long-wavelength band in FNE45–FNE46 was observed, which is ∼163 nm red-shifted compared to FNE44 (Fig. 7). This resulted in improved light-harvesting efficiency and the device performance (Fig. 7), reaching a PCE in quasi-solid-state DSSCs of 6.44% and 7.14% for FNE45 and FNE46, respectively. The highest efficiency of 8.27% was demonstrated for FNE46 in a liquid-electrolyte DSSC (Table 1). These results illustrate the significance of the efficient conjugation along the backbone by controlling the planarity between the units that decreases the HOMO–LUMO gap extending the dye absorbance to the long-wavelength region and thus increasing the power conversion efficiency. Subsequently, the same group synthesized the X-shaped dyes FNE48 and FNE49 using quinoxaline as a bridge, differing in the number of their anchoring groups (Fig. 6).¹²⁰FNE48 with one anchoring group exhibited a superior charge transfer compared to FNE49 with two anchoring groups, which dispersed the electron push–pull effect and weakened the ICT interactions. DSSCs with FNE48 achieved 6.2% efficiency when utilized with a liquid electrolyte and 5.2% in the case of quasi solid-state DSSCs.

Fig. 6 Molecular structures of quinoxaline-based sensitizers reported by Zhou et al.^119–123

Fig. 7 Absorption spectra of FNE32 and FNE44–FNE47: (a) in chloroform and (b) on TiO₂ films. (c) IPCE spectra and (d) J–V curves of DSSCs based on FNE32 and FNE44–FNE47 with a liquid electrolyte. Reprinted with permission from ref. 119 Copyright 2012 American Chemical Society.

Later, the FNE53 sensitizer containing the dithieno[2,3-a:3′,2′-c]phenazine moiety (Fig. 6) was reported.¹²¹ The co-sensitization approach of using two (or more) dyes with different and complementary absorption characteristics is widely used to expand the spectral absorption range, to decrease the dye aggregation and thus to increase DSSC performance. Using this approach (combined with hydroxamic acid pre-adsorption to improve the dye molecular packing), an impressive PCE of 15.2% was recently demonstrated.⁵¹ The “cocktail approach” of using FNE53 and FNE46 as co-sensitizers in solid-state DSSCs showed complementary absorption properties (Fig. 8) and suppression of aggregation between dye molecules, paving the way for achieving the best device performance. To optimize the DSSC performance, various molar ratios between the co-sensitizers were used, and the highest performance of the co-sensitized quasi-solid-state DSSCs with PCE = 8.04% was demonstrated for the molar ratio FNE46 : FNE53 = 8 : 2 (Fig. 8).

Fig. 8 UV-vis absorption spectra of sensitizers FNE46, FNE48, and FNE53: (a) in toluene solutions and (b) on TiO₂ films. (c) J–V curves and (d) IPCE spectra of FNE46, FNE53, and their co-sensitized quasi-solid-state DSSCs. Reprinted with permission from ref. 121 Copyright 2014 American Chemical Society.

Furthermore, FNE54 (without fluorine) and the fluorinated sensitizers FNE55 and FNE56 (Fig. 6) were developed by incorporating the 6,7-difluoroquinoxaline group.¹²² The incorporation of fluorine atoms enhanced the electron-acceptor nature of the quinoxaline unit, resulting in increased push–pull interactions along the main chain. This, in turn, led to a bathochromic shift in the absorption wavelength from FNE54 to FNE56, while decreasing the band gap. Despite the lower-lying LUMO (lowest unoccupied molecular orbital) level in the fluorinated quinoxaline dyes, the driving force was sufficient for injecting the electrons from the excited states of the dyes into the titania semiconductor. Consequently, the quasi-solid-state DSSC with FNE56 exhibited the highest PCE in the series of 8.2%.

Another series of organic dyes FNE60–FNE63 (Fig. 6) were synthesized using 3,4-ethylenedioxythiophene (EDOT) as a bridge and cyanoacrylic acid or rhodanine-3-acetic acid as the anchoring group.¹²³ The sensitizers with two EDOT units attached on both sides of the quinoxaline moiety (FNE62–FNE63) revealed broader absorption spectra than sensitizers with a single EDOT (between the quinoxaline and the anchoring groups, FNE60–FNE61), thereby favoring the photon absorption ability and photocurrent generation ability. The rhodanine-3-acetic acid in FNE61 and FNE63 strengthened the intramolecular charge transfer interactions, resulting in a significant red shift in the absorption spectra. Despite this favorable red shift in absorption, the LUMO localization on the anchoring group (which was substituted with the methylene group of rhodanine-3-acetic acid) led to poor electron infusion from the LUMO of the dye (after its excitation) into TiO₂. Accordingly, the photovoltaic performance of quasi-solid-state DSSCs with FNE60 and FNE62 was higher (6.1% and 8.2%, respectively) than the cells with FNE61 and FNE63 (3.9% and 4.9%, respectively).

Zhu et al. reported four quinoxaline-containing sensitizers from two related series with D–A′–π–A architecture, i.e.TQ1, TQ2 and IQ1, IQ2 (Fig. 9).¹²⁴ These sensitizers were decorated with donor triphenylamine (TPA) groups or indoline-based units (IQ) and acceptor/anchoring cyanoacrylic acid groups. By substituting the octyloxy groups (IQ2 or TQ2) instead of methoxy groups (IQ1 or TQ¹), the electron injection lifetime increased. The resultant enhancement in photovoltaic performance was attributed to the increased prevention of aggregation by octyloxy groups. The IQ2 and TQ2 dyes performed well in reducing the charge recombination in the system, and the DSSC constructed using IQ² demonstrated the highest PCE efficiency of 8.50%. Later, the same group studied the impact of various π-linkers composed of 2,3-diphenylquinoxaline and thiophene units, complementing the IQ series bearing indoline-based donors with IQ4 and IQ6–IQ8 dyes (Fig. 9).¹²⁵ Reduced absorption intensities on TiO₂ were observed in IQ6 and IQ7 due to the presence of the thienyl unit/alkyl side group near the donor. IQ6, IQ7, and IQ8 exhibited low charge collection efficiency (Φ_col) compared to electron injection efficiency (Φ_inj), resulting in inadequate IPCE. DSSCs based on IQ6–IQ8 displayed shorter electron collection lengths with the increase in thiophene units, leading to decreased photocurrent and unfavorable photovoltaic performance. Consequently, the DSSC based on IQ₄ was identified as the most promising architecture in the series, achieving a notable efficiency of 9.24%.

Fig. 9 Molecular structures of quinoxaline-based sensitizers reported by Zhu et al.^{111,124–126,160}

Following this, IQ9–IQ12 (Fig. 19) were developed with benzene (IQ9 and IQ11) or thiophene (IQ10 and IQ12) π-linkers.¹²⁶ The enhanced parameters in IQ10 were primarily attributed to the thiophene bridge, which induces a smaller twist in the molecular planarity, thereby enhancing light-capturing (favorable for J_SC) but promoting more charge recombination (unfavorable for V_OC), termed as the trade-off effect. The trade-off amid J_SC and V_OC was mitigated by incorporating the 2,3-dithienylquinoxaline unit (IQ11 and IQ12) as an auxiliary component, which favors improving molar absorption coefficients and prevents the dye self-aggregation, subsequently reducing charge recombination. Notably, IQ12 exhibited a favorable balance between J_SC (17.97 mA cm⁻²) and V_OC (715 mV), resulting in a PCE of 8.76%, surpassing the PCE of the related dyes IQ9 (2.91%), IQ10 (7.75%), and IQ11 (6.56%).

Subsequently, a series of SC1–SC3 dyes without terminal triarylamine donor groups (Fig. 9) was developed based on quinoxaline and cyclopentadithiophene (CPDT) units, with different numbers and locations of long alkyl chains in their structures. When long alkyl chains were incorporated, the dye absorption showed a blue shift due to a decrease in planarity (and consequently π-conjugation), attributed to the distribution of many alkyl chains in a limited space. The electron accepting nature of the quinoxaline unit in these dyes improved the intramolecular charge transfer compared to the reference dye CPDT-3. The SC-1 dye exhibited a PCE of 8.05% in an I⁻/I₃⁻ electrolyte-based DSSC (Table 1).¹¹¹ This efficiency was 20% higher than that of CPDT-3 (PCE = 6.7%).¹⁶⁰

Liu et al. synthesized an interesting quinoxaline-containing sensitizer Q1 incorporated tetrathiafulvalene donor fused to the benzene ring of the quinoxaline moiety (Fig. 10).¹²⁷ The D–π–A system of Q1 exhibited strong absorption up to the deep red region (610 nm), attributed to strong ICT, while the HOMO is significantly stabilized. As a result, the DSSC performance with Q1 demonstrated a wider IPCE spectrum with greater than 70% absorption at 500–600 nm and increased photocurrent density, achieving PCE as high as 6.5%, which is the highest PCE to date for tetrathiafulvalene-based dyes for DSSCs. To further explore the structure–property relationships of sensitizers in DSSCs, Reynolds and coworkers synthesized a series of four quadrupole organic sensitizers.¹²⁸ These sensitizers were constructed using two acceptor hubs, namely dibenzo[a,c]phenazine (DBP) and dithieno[3,2-a:2′,3′-c]phenazine (DTP), with attached 4-hexylthien-2-yl donor units on both sides (Fig. 10). Regioisomers were generated based on their positional relation to the core of the sensitizers. The absorption properties revealed that the DTP series exhibited longer absorption wavelengths than the DBP congeners due to the favorable effect of thiophene donors. The electronic properties demonstrated encouraging electron injection for the DTP systems onto TiO₂. Despite variations in absorption and electronic properties among the regioisomers, the reported device performances for DSSCs were similar and in all cases quite low. Thus, the highest PCE of 0.73% was demonstrated for DBP-L. In a similar study, another series of six sensitizers were developed with dithieno[2,3-a:3′,2′-c]phenazine and dithieno[3,2-a:2′,3′-c]phenazine attached to thiophene derivatives. Photophysical properties revealed a rigid structure designed for linear isomers (conjugation extended with a donor moiety), while non-radiative decay was observed in branched isomers (conjugation extended with an acceptor). The IPCE of these molecules reached 38% in the visible region. In this series, DTP-L8 and DTP-B8 (Fig. 10) showed slightly better (albeit low) device performance due to the additional thiophene donor units in the chain, which improved the absorption and charge transfer properties, leading to an increase in PCE to 1.60% and 1.78%, respectively.^128,129

Fig. 10 Molecular structures of quinoxaline-based sensitizers reported by Liu et al.¹²⁷ and Reynolds et al.^128,129

In 2020, Grätzel and coworkers introduced the HY64 organic sensitizer (Fig. 11a) for copper-electrolyte-based DSSCs. In that regard, the Y123 dye (Fig. 11a) is a well-known sensitizer for this type of DSSC, but it exhibits a large band gap with a narrow IPCE region. To overcome these shortcomings, a new HY64 dye was developed by introducing the auxiliary π-extended acceptor consisting of the phenanthrene unit fused to quinoxaline, aiming to increase the potential of copper redox shuttles with an extended wavelength. HY64 with the phenanthreno-quinoxaline moiety compared to HY63 (Fig. 11a) with the benzothiadiazole moiety as the auxiliary acceptor demonstrated a much higher J_SC (15.76 and 13.71 mA cm⁻², respectively) (Table 1). The absorption of Y123, HY63, and HY64 in films is shown in Fig. 11b, and the molar extinction coefficients of HY63 (ε = 8.46 × 10⁴ M⁻¹ cm⁻¹) and HY64 (ε = 6.56 × 10⁴ M⁻¹ cm⁻¹) were found to be higher than that of Y123 (ε = 5.24 × 10⁴ M⁻¹ cm⁻¹) due to the extension of the π-system. These properties were utilized to improve light absorption in the long-wavelength visible/NIR region, addressing a prerequisite for enhancing the J_SC of DSSCs. The photovoltaic performance of HY64 achieved a PCE of 12.5%, which was higher than that of the other two dyes (∼10%). The device data of these dyes are depicted in Fig. 12.¹³⁰

Fig. 11 (a) Chemical structures of dyes HY63 and HY64 and the reference dye Y123. (b) Absorption spectra of dyes adsorbed on 2 μm thick transparent TiO₂ films. (c) LUMO and HOMO energy levels of sensitizers. Conducting band level of TiO₂ and redox potential of the used electrolyte salt are also shown. Reprinted with permission from ref. 130 Copyright 2020 Wiley.

Fig. 12 (a) J–V curves of optimized DSSCs based on dyes Y123, HY63, and HY64, with [Cu(tmby)₂]^+/2+ as a redox shuttle (tmby is the 4,4′,6,6′-tetramethyl-2,2′-bipyridyl ligand), measured under AM 1.5G illumination. (b) A summary of J_SC and PCE of reported high performing copper-based DSSCs with a single sensitizer, showing improvement in the performance of the device with HY64. (c) Histogram plots of solar cell efficiencies from 16 individual devices. (d) IPCE spectrum and current density integration. Reprinted with permission from ref. 130 Copyright 2020 Wiley.

Sun et al. developed a series of electron-deficient diphenylquinoxaline sensitizers and demonstrated that bulky and hydrophobic diphenylquinoxaline and peripheral phenyl moieties can suppress the infusion of I₃⁻ ions to the TiO₂ surface and inhibit the unfavorable charge recombination. Among these sensitizers, the fabricated solar cell with CR204 (Fig. 13) demonstrated the best PCE of 6.49%.¹³¹ Furthermore, the molecular structures of seven 2,3-diphenylquinoxaline-based sensitizers DJ104, DJ112, DJ115, DJ142, DJ125, DJ154, and CRCW1 (Fig. 13) were designed with TPA as an electron donor and cyanoacrylic acid as the anchoring group. The architectures aimed at enhancing charge separation by including electron-rich π-spacers such as thiophene, which reduced the charge-trapping effect, and diphenyl groups at the quinoxaline moiety as additional hydrophobic side substituents that shield the TiO₂ face to delay charge recombination kinetics. The IPCE spectral coverage for DJ104, DJ152, and DJ125-based DSSCs extended into the NIR region, reaching 800 nm for DJ125. However, the lack of the tetraphenyl barrier in DJ125 led to faster charge recombination and low V_OC = 0.62 V. DSSCs benefited from the dense adsorption of the dyes on TiO₂, avoiding dye aggregation due to the bulky donor group and diphenylquinoxaline moiety, thus improving the performance of solar cells. Among the studied sensitizers, DJ104 achieved the best PCE of 8.06%.¹³²

Fig. 13 Molecular structures of quinoxaline-based sensitizers reported by Sun et al.^131,132 and Yang et al.^132,134

Lian-Ming Yang et al. developed the D–A–π–A′ sensitizers PQ1 and PQ2 (Fig. 13) by incorporation of pyrazino[2,3-g]quinoxaline as a π-linker with an unusual broad and intense visible to NIR absorption.¹³³ The resultant power conversion efficiencies of PQ1 and PQ2 were 6.86% and 0.34% respectively. Subsequently, they reported PPL1 and PPL2 dyes (Fig. 13) by introducing pyrazino-[2,3-f][1,10]phenanthroline (PPL) as an electron withdrawing/anchoring group. PPL1 and PPL2-sensitized DSSCs displayed 3.74% and 4.04% efficiencies, respectively.¹³⁴

Xichuan Yang and coworkers reported WQ1 and WQ2 dyes (Fig. 14) with quinoxaline-2,3-diol as the electron-deficient and anchoring group. The DSSC with WQ2 showed a PCE of 2.25% and the IPCE value reaching up to 73%.¹³⁵ In 2020, the same group reported ZW002 and ZW003 dyes (Fig. 14) with 2,3-dihexylquinoxaline and 2,3-diphenylquinoxaline acceptor moieties, respectively, aiming to study the effect of side substituents in the π-bridge.¹³⁶ The absorption spectra in dichloromethane (DCM) solution of ZW003 exhibited a 25 nm red shift compared to that of ZW002 due to the extended π-conjugation by phenyl groups (Fig. 15a). Upon coating these dyes onto TiO₂ films, the absorption properties of the ZW dyes were enhanced compared to the solution (Fig. 15b). This enhancement facilitated light harvesting, allowing higher photocurrents to be achieved. Upon adding the chenodeoxycholic acid (CDCA) co-adsorbent, some shifts were observed in absorption peaks, indicating strong π–π interaction on the TiO₂ film (Fig. 15c and d). The larger shift in ZW003 (5 nm) compared to that of ZW002 (1 nm) suggests that ZW003 experiences more intermolecular forces and stronger self-aggregation due to the phenyl moieties. The alkyl chains in the auxiliary acceptor create steric hindrance, which helps in reducing dye aggregation. DSSCs with ZW series dyes were fabricated using Co²⁺/Co³⁺ complexes as the liquid electrolyte and their performance was evaluated through J–V and IPCE characterization (Fig. 15e and f). ZW002, featuring long alkyl chains with enhanced absorption and suppressed aggregation, demonstrated a higher PCE (7.64%) compared to ZW003 (PCE = 7.06%). The cells tested with a 1 mM CDCA co-adsorbent showed improved efficiencies by enhancing both V_OC and J_SC (Table 1). Under identical conditions, the photovoltaic performance of DSSCs with CDCA was found to be PCE  =  8.23% for ZW002 and 7.43% for ZW003, because CDCA additionally contributed to higher IPCE values for both dyes by decreasing the dye aggregation.

Fig. 14 Molecular structures of quinoxaline-based sensitizers reported by Yang et al.^135,136 and Hua et al.^137,138

Fig. 15 UV-vis absorption spectra of ZW002 and ZW003: (a) in CH₂Cl₂ solution and (b) on TiO₂ films. (c) and (d) The same spectra (as in (b)) with or without CDCA on TiO₂ films: (c) as measured and (d) on the normalized scale. (e) J–V curves and (f) IPCE spectra of DSSCs based on ZW002 and ZW003 with and without 1 mM CDCA. Reprinted with permission from ref. 136 Copyright 2020 Wiley.

Hua et al. developed three quinoxaline dyes (AQ201–AQ203; (Fig. 14)) by incorporating thiophene, 3,4-ethylenedioxythiophene (EDOT), and cyclopentadithiophene (CPDT) moieties as electron-rich π-systems between the TPA donor and the quinoxaline acceptor moieties.¹³⁷ The incorporation of the different electron-rich π-bridges gradually decreased the HOMO energy levels from AQ201 to AQ203 without affecting the LUMO energy, leading to reduced band gaps. A broadened absorption in the visible region accompanied by a red shift of the absorption maxima was observed from AQ201 to AQ203 in DCM solutions. The sensitizers were tested with [Co(bpy)₃]^2+/3+ and I⁻/I₃⁻ based electrolytes, showing superior performance. Thus, the DSSCs with AQ202 exhibited the highest performance in this series (PCE = 8.37%).

In the same year, Hua et al. adopted a unique approach to improve the efficiency of DSSCs by employing the AQ308 quinoxaline-based sensitizer (Fig. 14). They scrutinized the role of stacked graphene platelet nanofibers (SGNF) mixed in the liquid electrolyte, which contained the Co²⁺/Co³⁺ redox couple. The dispersion of a specific amount of SGNF in the electrolyte was assumed to have a dual role, i.e. as a solid-state electron mediator and as a movable counter electrode, which enhances the charge transport and catalytic reactions of the Co²⁺/Co³⁺ redox couple, thereby increasing J_SC. For implementing this strategy, they designed an organic diphenylquinoxaline-based dye, AQ308, with a triarylamine donor. Due to the presence of the bulky alkoxy-substituted TPA donor, this sensitizer efficiently inhibited charge recombination on the TiO₂ surface and reduced the mediator species in the electrolyte. The quinoxaline acted as an acceptor, enhancing ICT and broadening the absorption range. Optimization was achieved by adding different concentrations of SGNF to the electrolyte. The J_SC of DSSCs exhibited improvement with SGNF concentrations ranging from 0 to 0.2 mg mL⁻¹ (Fig. 16). However, when SGNF concentration was further increased above 0.2 mg mL⁻¹, the J_SC started to decrease, while V_OC fluctuations were smaller. Remarkably, the presence of 0.2 mg mL⁻¹ SGNF in the electrolyte demonstrated the highest power conversion efficiency of 9.81%.¹³⁸

Fig. 16 (a) IPCE spectra and (b) J–V curves of AQ308-based DSSCs with various amounts of SGNF dispersed in the electrolyte under AM 1.5 G simulated sunlight irradiation (100 mW cm⁻²). Reprinted with permission from ref. 138 Copyright 2015 Royal Society of Chemistry.

Cao and coworkers reported DQ1–DQ5 dyes comprising dipentyldithieno[3,2-f:2′,3′-h]quinoxaline and different end donor groups and the π-bridges (Fig. 17).¹³⁹ The incorporation of donor EDOT (DQ3) or acceptor benzothiadiazole units (DQ4–DQ5) was found to expand the absorption spectra up to 800 nm, thereby narrowing the band gap and noticeably enhancing the photovoltaic performance. Among these dyes, the DSSC utilizing DQ5 demonstrated the highest efficiency of 7.12%. The same group studied the impact of different quinoxaline moieties in TQ-01–TQ-03 (Fig. 17) on DSSC performance.¹⁴⁰ The solar cell constructed with TQ-01 featuring 2,3-dioctylquinoxaline as the spacer exhibited the highest efficiency of 7.78%. A significantly lower efficiency was found for TQ-03 (4.03%), which was attributed to severe intermolecular aggregation through the dithieno[2,3-a:3′,2′-c]phenazine core. IT and IQT dyes (Fig. 17) with the dipentyldithieno[3,2-f:2′,3′-h]quinoxaline (DPDTQ) unit as a π-bridge were subsequently reported.¹⁴¹IQT, featuring 2,3-dipentylquinoxaline as the auxiliary acceptor, demonstrated a photovoltaic efficiency of 7.98%. The introduction of an auxiliary acceptor with reasonable electron deficiency was found to red-shift the absorption, tune molecular orbital energy levels, and enhance the device performance. Another series of dithieno[3,2-f:2′,3′-h]quinoxaline-based organic dyes IQ-C–IQ-F with three π-spacers were investigated (Fig. 17).¹⁴² In this series, dithieno[3,2-f:2′,3′-h]quinoxaline served as electron-accepting π-spacer, and 2,3-diphenylquinoxaline or dibenzo[a,c]phenazine acted as the auxiliary π-spacer. In this series of sensitizers, the PCE was somewhat increased in the order IQ-C < IQ-D < IQ-F, and IQ-F exhibited superior absorption in solution and in films and a good device performance of 6.60% due to the extended flat π-spacer dibenzo[a,c]phenazine.

Fig. 17 Molecular structures of quinoxaline-based sensitizers reported by Cao et al.^139–142

Lin and coworkers investigated a series of five (D–A–A′) type sensitizers QBT-1–QBT-5 (Fig. 18) with triarylamine as the end electron donor, the 2,3-bis(4-n-hexyloxyphenyl)-dithieno[3,2-f:2′,3′-h]quinoxaline moiety as the internal acceptor, and 2-cyanoacrylic acid as the anchoring group.¹⁴³ Sensitizers QBT-4 and QBT-5 showed an intense absorption that shifted towards the red region compared to their D–π–A congeners. The large dihedral angles (34.1–42.7° degrees) between the DTQ moiety and the phenyl rings helped to reduce intermolecular H-aggregation. In addition, the presence of n-hexyloxy chains on the phenyl rings enhanced the solubility of the dyes and decreased further aggregation. The fabricated DSSCs using these dyes displayed PCE in the range of 6.11 to 7.59%. The efficiencies were further increased to 8.41% by adding the CDCA co-adsorbent.

Fig. 18 Molecular structures of quinoxaline-based sensitizers reported by Lin et al.,¹⁴³ Hou et al.¹⁴⁴ and Koyuncu et al.¹⁴⁵

Hou et al. developed T-shaped sensitizers containing two triphenylamine donor end groups, π-bridges (thiophene or furan), acceptor anchoring groups (cyanoacrylic acid or 2-(1,1-dicyanomethylene)rhodanine), and indolo[2,3-b]quinoxaline as the supporting acceptor.¹⁴⁴ These sensitizers showed broader absorption profiles in films compared to dichloromethane solutions, which may arise from dye aggregation and contacts between the molecules and TiO₂. Among studied dyes QX22–QX25 (Table 1 and Fig. 18), QX23 with furan as the π-bridge and cyanoacrylic acid as the anchoring group achieved the highest PCE of 7.09%. The promising design (D)₂–A–π–A′ of QX23 with indolo[2,3-b]quinoxaline as the acceptor and triphenylamine as the donor contributed to the best photophysical characteristics and the highest device performance of 7.09%.

Koyuncu et al. reported the TPDC quinoxaline series with terminal carbazole donor groups bearing linear alkyl substituents of different lengths (2C, 4C, 6C, and 12C, (Fig. 18)) to study the effect of the side chain length on the dye performance.¹⁴⁵ The steric hindrance between the carbazole units and quinoxaline moieties resulted in non-planarity of the system, so the alkyl groups were out of the quinoxaline-anchoring group plane, thus reducing dye aggregation and enhancing IPCE. TPDC₁₂ showed low charge recombination due to its thick dielectric layer (from the long alkyl chains) blocking dye regeneration. Density functional theory (DFT) computational results indicated that more planar geometries were attained with alkyl chains containing 6C and 12C due to van der Waals interactions, which facilitate intermolecular electron transfer. Impedance analysis showed that TPDC₄ has greater charge transfer from the dye to TiO₂. Among all the dyes, TPDC₄ with a 4C chain length (n-butyl) exhibited the optimal molecular interactions and maximal charge transfer to TiO₂, showing the highest PCE in the series (4.27%).

Liu et al. developed two quinoxaline sensitizers of the (D)₂–A–(π–A′)₂ architecture (CS9 and CS10; (Fig. 19)), containing carbazole as a donor and cyanoacrylic acid as an anchoring group, but differing in the π spacers, i.e. thiophene (CS9) or benzene (CS10).¹⁴⁶ Changing the π-spacer affected the photophysical properties of the dye, particularly its frontier orbital energy levels, molecular planarity and light-harvesting ability. The dye CS9 with the thiophene bridge was found to exhibit better molecular planarity, thereby improved ICT characteristics and reduced energy losses. The stronger absorption and smaller band gap of CS9 gave a better PCE of 4.4% for its device (Table 1).

Fig. 19 Molecular structures of quinoxaline-based sensitizers reported by Liu et al.,¹⁴⁶ Feng et al.,¹⁴⁷ Lin et al.,¹⁴⁸ and Zheng et al.¹⁴⁹

In 2018, Feng and coworkers reported an LC4 sensitizer containing fused ring pyrazino-benzotriazole π-spacer (Fig. 19).¹⁴⁷ The extended fused ring system and electron deficiency of this condensed heteroaromatic moiety, combined with the TPA donor end group and cyanoacrylic acid anchoring moiety, gave a dye with broadened spectral response and notable device performance (PCE = 7.36%). In the same year, Lin and Wong reported a series of dyes with two electron-deficient π-conjugated fused-ring bridges bis(thienothiophene)quinoxaline (QC5-1 and QC5-2) and bis(thienothiophene)phenazine (PC5-1–PC5-3, Fig. 19).¹⁴⁸PC5-based dyes showed broader absorption, which was attributed to the extended lateral π-electron delocalization. The overall efficiency of the devices followed the trend: PC5-1 (7.77%) > PC5-2 (6.81%) > QC5-1 (6.48%) > QC5-2 (6.33%) > PC5-3 (5.23%). These studies demonstrated that lateral expansion of π-conjugation is an effective tool for the design of dyes for DSSCs, in addition to the common and widely exploited strategy of an extension of π-conjugation.

The impact of the bulkiness of the end donor groups in sensitizers was investigated by Zheng and coworkers using ZHG5–ZHG7 dyes (Fig. 19).¹⁴⁹ The tilt angles of ZHG5 and ZHG6 on TiO₂ were similar, while ZHG7 was found to be near orthogonal (due to its bulkiness). This perpendicular orientation of the end groups led to intermolecular π–π aggregation, thereby causing unfavorable charge recombination, and ZHG7 showed the lowest molar extinction coefficient and PCE (2.74%). In contrast, DSSCs based on ZHG5–ZHG6 exhibited higher PCE values of 5.64% and 5.32%, respectively.

Jaung and coworkers developed the NQX1–NQX4 series of dyes using phenothiazine or triphenylamine with alkoxy substituents as donor end groups and quinoxaline with two carboxy groups as the electron acceptor/anchoring moiety (Fig. 20).¹⁵⁰ Optical studies revealed broad absorption up to 600 nm for all sensitizers. In photovoltaic studies, both triarylamino-containing (NQX1 and NQX2) and phenothiazene-containing (NQX3 and NQX4) sensitizers displayed comparable efficiency in the range of 3.98–4.36% (Table 1). To understand the effect of substitution patterns on device performance, they reported the regioisomeric quinoxaline-based dyes NQX5–NQX7 with anchoring carboxylic groups at either pyrazine or benzene rings of the quinoxaline moiety (Fig. 20). The IPCE of these sensitizers indicated that introducing phenothiazine groups at the 2- and 3-positions of quinoxaline (i.e. at the pyrazine ring, NQX5–NQX6) gives a more efficient molecular architecture for DSSC applications due to the smaller torsion angle between the donor and acceptor. The current density–voltage curves of devices with these sensitizers revealed the highest J_SC = 10.64 mA cm⁻² (for NQX5), resulting in PCE = 4.46%.¹⁵¹ Subsequently, the same group investigated the elongation of π-conjugation for synthesizing NQX8–NQX12 dyes. The sensitizer with a pair of dimeric phenothiazine donor units (NQX8 (Fig. 20)) showed the best PCE of 5.41% in the series, owing to its strong light absorption. The efficiency of DSSCs with this dye was further improved to 6.48% by using the CDCA co-adsorbent.¹⁵²

Fig. 20 Quinoxaline-based sensitizers reported by Jaung et al.^150–152

Fang and coworkers developed novel sensitizers with the spiro[dibenzo[3,4:6,7]cyclohepta[1,2-b]quinoxaline-10,9′-fluorene moiety and different π-spacers between it and the cyanoacrylic acid anchoring group (FHD4–FHD6, (Fig. 21).¹⁵³ The orthogonal arrangement of the fluorene moiety in the spiro-system to the skeleton of quinoxaline was responsible for reducing H-aggregation, facilitating the device efficiency. According to the molecular exciton theory, dye molecules with H-aggregation (plane-to-plane stacking) show a blue shift in their absorption spectra (vs. non-aggregated state) due to a higher energy splitting of the excitonic states, whereas dyes with J-aggregation (head-to-tail arrangement) show a bathochromic shift. The absorption maxima of FHD4–FHD6 exhibited a clear red shift of their absorption spectra on TiO₂ films compared to their solution spectra, assuming J-aggregation in these films, which is beneficial for solar cell applications. Among studied dyes, FHD4 exhibited a wider spectral absorption and high molar extinction, resulting in the best PCE of 4.61%, which was attributed to a smaller dihedral angle leading to coplanarity. Also, the PCE was further slightly improved to 4.69% upon adding the co-adsorbent CDCA. In order to further suppress aggregation completely, the same group developed a series of related dyes based on the best FHD4 structural motif using three different donor units: TPA (FHD4-1), dialkylamine (FHD4-2), and carbazole (FHD4-3) (Fig. 21).¹⁵⁴ Comparison of the optical properties of these compounds in solution and in films indicated that the dyes do not aggregate. This aggregation-free tendency was explained by the large dihedral angles of 39.6° (FHD4-1), 38.5° (FHD4-2), and 41.2° (FHD4-3) between the donor and quinoxaline skeletons. The dyes showed PCEs of 4.54%, 5.16%, and 4.87%, respectively. Interestingly, co-adsorption of these dyes with CDCA, which typically improves PCE by obstructing aggregation on TiO₂, led not to increase but to decrease in the efficiencies (4.17%, 5.08%, and 4.34%, respectively). This study demonstrates that aggregation-free dyes perform better without auxiliary additives (like CDCA), which can only reduce the adsorption of the dye and hence lower the J_SC and the device performance, without any positive effect.

Fig. 21 Molecular structures of quinoxaline-based sensitizers reported by Fang et al.^153,154 and Chow et al.¹⁵⁵

Chow and Chang reported a series of six Y-shaped sensitizers MM-1–MM-6 containing quinoxaline or related (pyridopyrazine and fluorenopyrazine) units (Fig. 21).¹⁵⁵ The reasonably good device performance of MM-5 and MM-6 (PCE = 5.86% and 6.70%, respectively, Table 1) underlines the favorable intermolecular charge transfer when the electron donors are at the 5,8-positions of the quinoxaline moiety in comparison to the 2,3-positions in MM-3 and MM-4 dyes. MM-6, co-deposited with deoxycholic acid (DCA) to control intermolecular aggregation and ensure proper orientation on TiO₂, demonstrated improved photovoltaic performance. Co-deposition of MM3 and MM6 optimized the dye coverage on TiO₂, achieving J_SC = 17.7 mA cm⁻², V_OC = 717 mV, FF = 0.63, and PCE = 7.92%. Notably, these dyes performed well in indoor photovoltaics, achieving 30.45% PCE under TL84 fluorescent lamp illumination.

El-Shafei et al. developed a series of novel rigid quinoxaline-based dyes LY01–LY03 (Fig. 22).¹⁵⁶ The DSSC based on LY03 showed the highest PCE of 7.04%. These results indicate that introducing long alkyl chains into both the acceptor and donor, along with increasing the rigidity of the sensitizer, represents a way to avoid the “trade-off” effect and to enhance the V_OC of the devices. In 2020, the same group reported FS10–FS12 dyes using indolo[2,3-b]quinoxaline as the central unit with three different donors (Fig. 22).¹⁵⁸ Under AM 1.5G solar light conditions, FS10 showed the best device performance (PCE = 5.27%). Co-sensitization effects were further assessed using FS10 with a ruthenium-based sensitizer, improving the efficiency to 8.32%. The optimized anchoring mode of FS10 and the ruthenium sensitizer on the TiO₂ surface led to prevention of the dye aggregation and charge recombination. Another study of dyes with the indolo[2,3-b]quinoxaline core was aimed to investigate the effectiveness of dual-channel anchorable sensitizers using FS13–FS14 dyes and their comparison with the FS08 dye, which has one anchoring group (Fig. 22). However, lower device performance was observed with these di-anchoring dyes, which was attributed to high spatial hindrance of the closely parallel branches, thus leading to lower photocurrent (Table 1). Nevertheless, co-sensitization of these dyes with Ru complexes allowed achieving better photovoltaic performances of 8.16% and 8.67% for FS13 and FS14 dyes, respectively, compared to 7.94% for FS08. This was rationalized as reduced aggregation and charge recombination of the di-anchoring dye on the TiO₂ surface.¹⁵⁷

Fig. 22 Quinoxaline-based sensitizers reported by El-Shafei et al.^156–158 and Lee et al.¹⁵⁹

In 2023, Songyi Lee and coworkers reported three dyes, TPP, TPPS, and TPPF (Fig. 22), and tested them in DSSCs using three different types of TiO₂ photoelectrodes: a double-layered nanoporous TiO₂ photoelectrode (SPD) and single-layered nanoporous TiO₂ photoelectrode (D type and SP type).¹⁵⁹ The sensitizers tested with the double-layered SPD photoelectrode achieved efficiencies ranging from 1.31% to 2.64%, which were similar to those of SP-type electrodes (1.31–2.50%) but exceeded D-type photoelectrodes (0.90–1.54%). This study found that increased interfacial resistance of the D-type TiO₂ photoelectrode can negatively impact the J_SC and FF, leading to reduced photovoltaic performance.

Summarizing the above, this section covered the different designs of metal-free sensitizers based on quinoxaline derivatives, detailing their photophysical properties and impact on DSSC applications. Among them, the D–A–π–A′ architecture sensitizers showed excellent photophysical and charge transfer properties, with the dye HY64 achieving a PCE of up to 13%.

3.2. Quinoxaline/quinoxalone moieties in the electron-donor terminal groups

Table 2 summarizes the photovoltaic characteristics of the dyes with quinoxaline or quinoxalone moieties as a part of the terminal electron-donating groups or anchoring groups.

Table 2 DSSC performance parameters of dyes with quinoxaline moieties in the terminal donor or anchoring groups

Sensitizer	Architecture	Electrolyte	J SC [mA cm^−2]	V OC [V]	FF	PCE^a [%]	Ref.
a PCEs above 7.5% are highlighted in a bold italic font.
3	A|D–A′	I^−/I3^−	2.66	0.50	0.64	0.86	161
5	A|D–π–A′	I^−/I3^−	9.29	0.57	0.64	3.45	161
8a	A|D–π–A′	I^−/I3^−	4.43	0.54	0.68	1.65	161
8b	A|D–π–A′	I^−/I3^−	2.73	0.59	0.67	1.08	161
8c	A|D–π–A′	I^−/I3^−	7.38	0.56	0.65	2.72	161
8d	A|D–π–A′	I^−/I3^−	7.77	0.56	0.62	2.68	161
JY01	A|D–π–A′	I^−/I3^−	16.04	0.70	0.67	7.62	162
JY02	A|D–D–π–A′	I^−/I3^−	14.84	0.70	0.63	6.48	162
JY03	A|D–D–π–A′	I^−/I3^−	14.13	0.74	0.67	7.03	162
QX05	A|D–π–A′	I^−/I3^−	8.9	0.67	0.68	4.10	163
QX06	D–π–A′	I^−/I3^−	14.0	0.70	0.69	6.82	163
QX07	A|D–D–π–A′	I^−/I3^−	15.3	0.75	0.71	8.28	163
QX08	A|D–D1–π–A′	I^−/I3^−	14.2	0.74	0.71	7.56	163
MQ1	D–A	I^−/I3^−	0.29	0.42	0.61	0.08	164
MQ2	D–A	I^−/I3^−	0.003	0.025	0.24	0.001	164
MQ3	D–D–A	I^−/I3^−	0.98	0.47	0.65	0.31	164
MQ4	A–D–A	I^−/I3^−	1.22	0.48	0.67	0.40	164
MQ5	D–A	I^−/I3^−	0.35	0.40	0.60	0.09	164

---

Several reports demonstrated the use of an electron-deficient quinoxaline unit as a part of the π-extended moiety of the total electron-donor character in the design of DSSC sensitizers. Thomas and coworkers reported a series of sensitizers (3, 5, and 8a–8d in Fig. 23) containing the indolo[2,3-b]quinoxaline moiety as the terminal electron-donating group, showcasing a rare class of sensitizers with heterocyclic donors.¹⁶¹ DSSCs were tested in two different bath solutions, DCM and an acetonitrile:tert-butanol:dimethylsulfoxide mixture (MeCN:t-BuOH:DMSO). The dyes in the mixed solution showed good device performance. Among them, dye 5 demonstrated the highest PCE of 3.45%. In 2015, Zhen et al. reported other dyes with indolo[2,3-b]quinoxaline terminal groups linked to the anchoring group via π-conjugated terthiophene, carbazolylthiophene or carbazolylfuran bridges (JY01, JY02 and JY03, respectively; Fig. 23).¹⁶²JY01 showed broad absorption up to 770 nm, covering almost the entire visible spectrum, and the DSSC with this dye achieved the highest efficiency of 7.62%.

Fig. 23 Molecular structures of sensitizers containing indolo[2,3-b]quinoxaline terminal moieties^161–163 and quinoxalone moieties as anchoring groups.¹⁶⁴

A year later, Hou et al. reported organic dyes QX05–QX08 based on indolo[2,3-b]quinoxaline or phenothiazine units (Fig. 23).¹⁶³ The absorption spectra of these dyes in solution and in films are shown in (Fig. 24a and b). The four dyes exhibited two absorption bands, in the 350–400 nm and 400–600 nm regions, corresponding to the local π–π* excitation and the ICT transitions, respectively. The peak at 484 nm for QX07 was red-shifted compared to the other dyes due to a stronger ICT progression. The high electron-richness of the thiophene unit increased the donor–acceptor interaction between the groups, inducing a stronger ICT (compared to the furan unit). When coated onto TiO₂ films, all four dyes showed slightly blue-shifted absorption peaks, which may be attributed to the deprotonation of the carboxylic acid acceptor affecting the interactions between the dye and TiO₂. The J–V and IPCE curves of the dyes are depicted in Fig. 24c and d. The highest PCE of 8.28% was achieved with QX07 due to the thiophene π-bridge, which led to a well-adjusted structure, high ICT, broader absorption, and the largest J_SC in this series. The QX08 dye with the furan bridge resulted in a PCE of 7.56% with a smaller J_SC.

Fig. 24 UV-vis absorption spectra of QX05–QX08: (a) in DCM solutions and (b) on TiO₂ films. (c) J–V dependence and (d) IPCE spectra of the DSSCs based on QX05–QX08. Reprinted with permission from ref. 163 Copyright 2016 Elsevier.

3.3. Quinoxaline derivatives as anchoring groups

Koiti et al. tested quinoxaline moieties as anchoring groups in the dyes on the TiO₂ conduction band. Based on this design approach, a series of sensitizers MQ1–MQ5 (Fig. 23) were developed with different terminal donors: triphenylamine (MQ1), ferrocene (MQ2), (E)-4,4′-(ethene-1,2-diyl)-bis(N,N-diphenylaniline) (MQ3), 4,4′-(bisquinoxalin-2(1H)-one)-triphenylamine (MQ4), and N,N-dimethylaminobenzene (MQ5) through a Knoevenagel condensation reaction. However, due to the poor electron-accepting nature of quinoxaline-2(1H)-one, the electron transfer process on TiO₂ was found to be inefficient, resulting in drastic decrease in the PCE to 0.08%, 0.001%, 0.31%, 0.40%, and 0.09%, respectively.¹⁶⁴

3.4. Quinoxalines with porphyrins

Table 3 summarizes the photovoltaic parameters of quinoxaline-porphyrin sensitizers. In essence, the extension of quinoxaline moieties to porphyrin macrocycles facilitates increased conjugation, shifting the absorption towards the NIR region and thereby enhancing the photon-to-electron conversion efficiency of DSSCs. Imahori and coworkers reported ZnQMA and ZnQDA sensitizers (Fig. 25) to investigate the impact of β,β-carboxyquinoxaline on the photovoltaic properties of porphyrins.¹⁶⁵ZnQMA and ZnQDA with one and two carboxylic acid units as binding groups, respectively, were evaluated as photosensitizers for DSSCs. Both compounds exhibited red-shifted and improved light absorption through π-extensions, resulting in narrower band gaps. It was found that ZnQMA with one carboxylic acid employs a bidentate attachment to the TiO₂ surface, whereas ZnQDA utilizes one bidentate and one monodentate binding mode. Device studies revealed that the PCE of ZnQMA- and ZnQDA-coated with P25 (TiO₂ nanoparticles) was 5.2% and 4.0%, respectively.

Table 3 DSSC performance parameters of porphyrin and ruthenium-based quinoxaline sensitizers

Sensitizer	Architecture	Electrolyte	J SC [mA cm^−2]	V OC [V]	FF	PCE [%]	Ref.
Porphyrins
ZnQMA	Por|A	I^−/I3^−	11.2	0.72	0.68	5.2	165
ZnQDA	Por|A	I^−/I3^−	9.3	0.67	0.64	4.0	165
ZnQCA	Por|A	I^−/I3^−	2.3	0.51	0.68	0.80	166
ZnBQA	Por|A	I^−/I3^−	11.1	0.68	0.67	5.1	166
ZnPQ	Por|A	I^−/I3^−	13.2	0.71	0.67	6.3	167
ZnPBQ	A|Por|A	I^−/I3^−	10.6	0.66	0.67	4.7	167
ZnPQI	D–Por|A	I^−/I3^−	13.9	0.68	0.71	6.8	168
FNE58	D–A–π–Por–π–A′	I^−/I3^−	9.08	0.62	0.70	3.99	169
FNE59	D–Por–π–A–π–A′	I^−/I3^−	12.06	0.70	0.71	6.02	169
LP11	D–π–Por–π–A–π–A′	I^−/I3^−	13.42	0.68	0.70	6.46	170
P2-PZ	(Por)2–A′	I^−/I3^−	6.24	0.53	0.60	2.00	171
PoZ	A–π–Por–π	I^−/I3^−	2.58	0.57	0.63	0.94	172
PmZ	A–π–Por–π	I^−/I3^−	5.47	0.61	0.68	2.29	172
Ru quinoxalines
M101	—	I^−/I3^−	0.54	0.45	0.39	0.12	173
M102	—	I^−/I3^−	1.91	0.43	0.56	0.58	173
M103	—	I^−/I3^−	1.02	0.47	0.65	0.39	173
M104	—	I^−/I3^−	0.74	0.52	0.66	0.34	173

---

Fig. 25 Molecular structures of quinoxaline-based porphyrin sensitizers.^165–172

ZnBQA and ZnQCA quinoxaline-porphyrin sensitizers were subsequently reported and compared to ZnQMA (Fig. 25).¹⁶⁶ The solar cell efficiencies of these sensitizers were 5.1%, 0.80%, and 6.3% respectively with the CDCA adsorbent. The very low efficiency of ZnQCA was due to weaker electronic interaction between the dye and TiO₂, leading to poor electron injection and IPCE. The same group developed ZnPBQ and ZnPQ dyes (Fig. 25), which showed PCEs of 4.7% and 6.3%, respectively. The weak interaction between ZnPBQ dye and TiO₂ resulted in low electron injection, charge collection efficiency, and IPCE.¹⁶⁷ Later, ZnPQI was reported featuring a porphyrin with a triarylamine donor group at the β,β′-edge through a fused imidazole group and carboxyquinoxaline anchoring group at the opposite β,β′-edge (Fig. 25). The UV-vis spectra showed red-shifted Soret and Q bands, indicating the improved light capturing ability of the dye. The ZnPQI-based solar cell showed an efficiency of 6.8%, surpassing that of the reference ZnPQ-sensitized solar cell (PCE = 6.3%) with the co-adsorbent CDCA under optimized conditions.¹⁶⁸

Wang et al. developed a series of zinc porphyrin sensitizers, including FNE58–FNE59, with an electron-deficient 2,3-diphenylquinoxaline (DPQ) as a π auxiliary acceptor at different positions on the porphyrin moiety (Fig. 25).¹⁶⁹ Placing the DPQ between the porphyrin and acceptor in FNE59 had a greater influence on absorption properties compared to its placement between the donor and porphyrin in FNE58. Theoretical studies revealed that FNE59 is more advantageous for delocalizing the LUMOs and enhancing electronic asymmetry, facilitating intramolecular charge transfer. The FNE59-based quasi-solid-state DSSCs exhibited efficient photovoltaic performance (PCE = 6.02%), representing a 51% improvement over FNE58-based DSSCs. Based on FNE59 architecture, Feng and coworkers developed the DPQ-containing porphyrin sensitizer LP11 (Fig. 25), achieving an improved performance with a PCE of 6.46% in the optimized DSSC.¹⁷⁰

Gust et al. developed the Y-shaped P₂-PZ sensitizer (Fig. 25) containing a quinoxaline ring linked to two porphyrin macrocycles and two carboxylic acid anchoring groups.¹⁷¹ The dodecyloxy groups on its molecular structure enhanced the dye's solubility and led to reduced aggregation, attributed to the minimized rate of charge recombination on the TiO₂ metal surface. Yet, the DSSC with P₂-PZ yielded an efficiency of 2.0% only. After co-sensitization with the porphyrin sensitizer P (Fig. 25), the efficiency increased to 3.5%. Batista et al. introduced the ethynyl-linked panchromatic dyes composed of dibenzophenazines coupled from ortho- (PoZ) and meta- (PmZ) positions to the meso position of porphyrin, followed by a carboxylic anchoring group grafted onto the TiO₂ surface (Fig. 25).¹⁷² Optical and computational measurements revealed a broad absorption from 300 to 636 nm and differences in the planarity of the two sensitizers associated with different points of attachment of dibenzophenazines to the porphyrin core. The respective photovoltaic performance of PoZ and PmZ was found to be PCE = 0.68% and 2.29%, respectively.

In conclusion, the introduction of quinoxaline moieties was shown to enhance photophysical properties when linked to porphyrins, resulting in strong absorption and efficient charge transfer, thus leading to high PCE. In this regard, the best performance of dyes with quinoxaline-porphyrin design reached up to 7% PCE with NIR incident photon-to-current efficiencies, showing potential for advancing DSSC technology.

3.5. Quinoxaline in ruthenium complexes

Table 3 also summarizes the photovoltaic parameters of ruthenium-based quinoxaline-containing dyes. To date, there is limited research on this class of sensitizers for DSSC applications. Extending quinoxaline derivatives with different moieties, especially with ruthenium complexes, may be an interesting approach as it establishes a strong interaction between the molecular structure and the electron infusion process, thereby regulating the IPCE.

Shahroosvand et al. synthesized four Ru(II) polypyridyl complexes, named M101–M104, which incorporated various ancillary ligands as illustrated in Fig. 26.¹⁷³ These ligands included 1,10-phenanthroline (phen) and 2,2′-bipyridyl (bpy), both demonstrating promising outcomes. Specifically, the amphiphilic ligand E101, featuring an aliphatic alkyl chain, helped to reduce aggregation. Ancillary ligands containing bpy, phen, and isothiocyanate (–N=C=S) enhanced light absorption abilities. Moreover, an anchoring 6,7-dicyanodipyrido[2,2-d:2′,3′-f]quinoxaline (dicnq) ligand with CN groups facilitated the coordination with the semiconductor surface. Unfortunately, for all four dyes, the device performance was extremely low (PCE = 0.12–0.58%) with the highest PCE observed for M102. The DSSC with the M102 dye without the E101 ligand did not show the photovoltaic effect, indicating the significance of the E101 ligand.

Fig. 26 Molecular structures of quinoxaline-based ruthenium sensitizers.¹⁷³

Ancillary ligands can improve light absorption, which is essential for enhancing PCE. By including aliphatic and aromatic units, these ligands effectively minimize charge recombination compared to other sensitizers. This may offer additional benefits for ruthenium-based sensitizers through coordination linkage in DSSCs.¹⁷⁴

4. Thieno[3,4-b]pyrazine-based sensitizers

Table 4 summarizes the photovoltaic parameters of thienopyrazine-based sensitizers. Several studies reported the fusion of the thiophene unit onto the pyrazine moiety as an extension to quinoxaline sensitizers (thieno[3,4-b]pyrazine). Based on this structure, Zhou et al. developed three NIR sensitizers, FNE32, FNE34, and FNE36 (Fig. 27).¹⁷⁵ These sensitizers exhibited long-wavelength absorption with a maximum around 625 nm due to intramolecular electron transfer from the donor to the acceptor unit. The presence of an extended acenaphtho[1,2-b]thieno[3,4-e]pyrazine unit in FNE34 and FNE36 resulted in decreasing the LUMO levels compared to FNE32 and the quasi-solid-state DSSCs with FNE34 and FNE36 showed high PCEs of 5.30% and 4.93%, respectively. To further optimize the LUMO levels, a subsequent study reported similar dyes with different anchoring groups: cyanoacrylic acid (FNE64 and FNE66) or carboxylic acid (FNE65 and FNE67) (Fig. 27).¹⁷⁶ The LUMO levels FNE66 and FNE67 bearing an additional phenylene bridge between the terminal donor triarylamine group and thieno[3,4-b]pyrazine moiety became more negative compared to FNE64 and FNE65, improving the driving force for electron injection from the excited dyes to the semiconductor's conduction band. A hypsochromic shift was observed in the absorption spectra of FNE66 and FNE67. The PCE of 7.2% was achieved with FNE66, which was higher than that for the FNE32 reference dye (3.22%).

Table 4 DSSC performance parameters of thienopyrazine-based sensitizers

Sensitizer	Architecture	Electrolyte	J SC [mA cm^−2]	V OC [V]	FF	PCE^a [%]	Ref.
a PCEs above 7.5% are highlighted in a bold italic font.
FNE32	D–π–A–π–A′	I^−/I3^−	11.29	0.47	0.69	3.66	175
FNE34	D–π–A–π–A′	I^−/I3^−	16.24	0.48	0.68	5.30	175
FNE36	D–π–A–π–A′	I^−/I3^−	14.66	0.48	0.70	4.93	175
FNE64	D–A–π–A′	I^−/I3^−	13.38	0.59	0.69	5.5	176
FNE65	D–A–π	I^−/I3^−	12.55	0.62	0.73	5.7	176
FNE66	D–π–A–π–A′	I^−/I3^−	17.25	0.61	0.68	7.2	176
FNE67	D–π–A–π	I^−/I3^−	14.44	0.64	0.67	6.2	176
TP1	D–A–π–A′	I^−/I3^−	10.4	0.63	0.68	4.40	177
TP1 + QX2	—	I^−/I3^−	13.1	0.67	0.71	6.20	177
MD4	D–π–A–π–A′	I^−/I3^−	10.11	0.68	0.66	4.58	178
MD5	D–π–A–π–A′	I^−/I3^−	17.16	0.72	0.67	8.39	178
MD6	D–π–A–π–A′	I^−/I3^−	11.58	0.70	0.65	5.26	178
MD7	D–π–A–π–A′	I^−/I3^−	17.65	0.75	0.68	9.03	178
AP2	(D)2–A–(π)2	I^−/I3^−	5.1	0.47	0.74	1.8	179
AP3	(D)2–A–(π)2	I^−/I3^−	12.4	0.56	0.68	5.0	179
AP4	(D)2–A–(π)2	I^−/I3^−	2.8	0.46	0.69	0.9	179
AP5	(D)2–A	I^−/I3^−	0.6	0.39	0.66	0.2	179
AP7	(D)2–A–(π)2	I^−/I3^−	4.5	0.47	0.64	1.4	179
AP10	(D)2–A–(π)2	I^−/I3^−	12.7	0.57	0.68	5.1	179
JD41	(D)2–A	I^−/I3^−	2.6	0.48	0.75	1.0	179
TPI	D–A–π–A′	I^−/I3^−	10.38	0.68	0.73	5.16	180
TP2	(D)2–A–(π)2	I^−/I3^−	8.67	0.68	0.68	4.03	180

---

Fig. 27 Molecular structures of thieno[3,4-b]pyrazine-based sensitizers.^175–180

Kono, Yamashita et al. developed a new NIR-absorbing thieno[3,4-b]pyrazine dye TP1 and quinoxaline-based dye QX2 (Fig. 27). The DSSCs with TP1 showed an efficiency of 4.4%, while the device with QX2 demonstrated a lower performance of PCE = 3.22% only. The efficiency of the devices was increased up to 6.2% by mixing TP1 with QX2, as a result of more efficient light capturing and steric hindrance in QX2.¹⁷⁷ Later, MD4–MD7 dyes, featured by bis(alkoxy)phenyl substituents connected to the pyrazine ring of thieno[3,4-b]pyrazine (TPz) or quinoxaline as auxiliary acceptor bridges, were developed (Fig. 27).¹⁷⁸ The absorption studies of MD sensitizers in THF solution and in films are shown in Fig. 28a and b. The sensitizers exhibited broad, close to panchromatic absorption over the visible region of the spectrum due to π–π* electron transition and efficient ICT. The dyes MD4 and MD6 with the thieno[3,4-b]pyrazine moiety exhibited absorption up to the NIR region. In contrast, the quinoxaline-based dyes MD5 and MD7 showed absorption at shorter wavelengths due to their relatively lower conjugation through the quinoxaline moiety compared to thieno[3,4-b]pyrazine (as a result of increased dihedral angles and lower bond length alternation due to higher aromaticity of benzene compared to thiophene). The device characterization of MD series dyes is shown in Fig. 28c–f. MD5 and MD7 demonstrated high performance, achieving PCEs of 7.88% and 8.22%, respectively. MD5 and MD7 occupied 80% of the IPCE at the 400–620 nm region. On the other hand, MD4 and MD6 exhibited smaller PCEs of 3.92% and 5.08%, respectively, occupying up to 60% of the IPCE. The superior performance of MD7 was attributed to (i) its higher absorption property extending into the longer wavelength spectral region and (ii) higher dye loading in the cells, which improved its light harvesting properties. Consequently, with the help of a co-adsorbent (CDCA), reducing the dye aggregation, the efficiency of MD7 was improved, reaching 9.03%. Under dim light conditions, the efficiencies of DSSCs based on MD7 reached 18.95% and 27.17% at the 300 lux and 6000 lux irradiances, respectively.

Fig. 28 Absorption spectra of MD4–MD7: (a) in THF solution and (b) on the TiO₂ thin film. (c) J–V curves and (d) IPCE spectra of DSSCs with MD4–MD7, without CDCA. (e) J–V curves and (f) IPCE spectra of the same DSSCs with added CDCA measured under simulated AM 1.5G illumination. Reprinted with permission from ref. 178 Copyright 2018 Royal Society of Chemistry.

Delcamp and coworkers synthesized seven organic sensitizers, namely AP2–AP5, AP-7, AP10, and JD41 (Fig. 27), introducing thienopyrazine as a π-bridge.¹⁷⁹ Notably, devices with AP3 and JD41 sensitizers drastically differed in efficiencies (PCE = 5.0% and 1.0%, respectively), due to the presence of the phenyl spacer between the π-bridge and the anchor group for AP3, whereas JD41 lacked the phenyl spacer. Structure–performance relationships were analyzed, indicating that AP3 was suitable for co-sensitization with the D35 dye (Fig. 27), thus displaying the highest device performance with a PCE of 7.5%. Furthermore, it exhibited a panchromatic IPCE with an onset at 800 nm.

Very recently, Daniele Franchi and coworkers reported TPI (D–A–π–A′ type) and TP2 ((D)₂–A–(π)₂ type) dyes (Fig. 27) of different symmetries. DSSCs constructed with these sensitizers and iodide-based electrolytes achieved the efficiency of 5.16% and 4.03%, respectively.¹⁸⁰

5. Pyrido[3,4-b]pyrazine-based sensitizers

The photovoltaic parameters of pyrido[3,4-b]pyrazine- and pyrrolopyrazine-based sensitizers are summarized in Table 5. Several sensitizers for DSSC applications were reported by exploiting the pyrido[3,4-b]pyrazine core. In 2014, Grätzel and coworkers synthesized a series of sensitizers, namely PP-I and APP-I–APP-IV (Fig. 29), by introducing alkoxy chains onto the terminal triphenylamine donor group and aryl groups of the quinoxaline moiety and varying the structure of additional π-spacers. Specifically, the incorporation of electron-donating octyloxy groups in APP-IV resulted in an elevation of the HOMO energy level. Among all these sensitizers, the lowest band gap was observed for APP-IV, which showed a red shift in its absorption spectra compared to other dyes. Furthermore, APP-IV revealed improved J_SC and V_OC when combined with a liquid electrolyte, leading to the highest PCE of 7.12% in DSSCs. Notably, when the same APP-IV sensitizer was utilized with solvent-free ionic-liquid electrolytes, the PCE was lower (6.20%).¹⁸¹

Table 5 DSSC performance parameters of pyridopyrazine- and pyrrolopyrazine-based sensitizers

Sensitizer	Architecture	Electrolyte	J SC [mA cm^−2]	V OC [V]	FF	PCE^a [%]	Ref.
a PCEs above 7.5% are highlighted in a bold italic font.
Pyrido[3,4-b]pyrazine-based sensitizers
PP-I	D–A–π–A′	I^−/I3^−	7.10	0.57	0.76	3.11	181
APP-I	D–A–π–A′	I^−/I3^−	12.11	0.67	0.76	6.14	181
APP-II	D–A–π–A′	I^−/I3^−	8.37	0.63	0.73	3.93	181
APP-III	D–A–π–A′	I^−/I3^−	8.20	0.69	0.76	4.35	181
APP-IV	D–A–π–A′	I^−/I3^−	13.56	0.69	0.76	7.12	181
IQ13	D–A–π–A′	I^−/I3^−	14.2	0.74	0.74	8.0	182
1Q17	D–A–π–A′	I^−/I3^−	14.2	0.79	0.77	9.0	182
Q-85	D–A–π–A′	I^−/I3^−	19.55	0.72	0.66	9.41	183
Q-93	D–A–π–A′	I^−/I3^−	19.53	0.67	0.61	8.17	183
DT1	π–D–A–π–A′	I^−/I3^−	16.08	0.80	0.66	8.57	184
DT2	π–D–A–π–A′	I^−/I3^−	14.99	0.69	0.69	7.21	184
DTN-1	π–D–A–π–A′	I^−/I3^−	10.67	0.78	0.73	6.10	185
SH3 (liquid)	π–D–A–π–A′	I^−/I3^−	5.41	0.69	0.77	5.74	114
SH3 (solid)	π–D–A–π–A′	I^−/I3^−	4.79	0.85	0.69	5.56	114
SH4 (liquid)	π–D–A–π–A′	I^−/I3^−	4.12	0.56	0.76	3.52	114
SH4 (solid)	π–D–A–π–A′	I^−/I3^−	3.73	0.71	0.62	1.69	114
IPT	D–A–A|D–A′	I^−/I3^−	12.89	0.65	0.64	5.32	141
Dipyrrolo[2,3-b:2′,3′-e]pyrazine-based sensitizers
3a	D–D|A–A′	I^−/I3^−	4.54	0.54	0.58	1.44	115
3b	D–D|A–A′	I^−/I3^−	1.38	0.54	0.50	0.38	115
3c	D–D|A–A′	I^−/I3^−	2.96	0.57	0.65	1.10	115
3d	D–D|A–A′	I^−/I3^−	0.24	0.44	0.53	0.056	115

---

Fig. 29 Molecular structures of pyrido[3,4-b]pyrazine-based sensitizers.^{114,141,181–185}

In 2015, Grätzel et al. reported two novel pyrido[3,4-b]pyrazine-based sensitizers, IQ13 and IQ17 (Fig. 29), which were employed in cobalt- and iodine-based redox electrolyte DSSCs.¹⁸² Optoelectronic transient measurements indicated that IQ17 with the pyrido[3,4-b]pyrazine moiety bearing long-chain alkoxyphenyl substituents effectively protects the TiO₂ surface from the oxidized species of the cobalt electrolyte. This resulted in reduced aggregation, lowered charge recombination, and consequently, led to a higher PCE of 9.0%. The same sensitizers with iodine electrolyte showed somewhat lower performance with a PCE of 8.65%. Consequently, pyridopyrazine-containing dyes incorporating long alkoxyphenyl chains are deemed to be suitable sensitizers for cobalt electrolyte systems.

Hua et al. synthesized two dyes with indoline (DT-1) and triphenylamine (DT-2) donors (Fig. 29).¹⁸⁴ Theoretical studies (DFT B3LYP/6-31G(d)) showed that both DT-1 and DT-2, which differ in the auxiliary donor moiety, have similar frontier orbital energies (HOMO = −4.82 / −4.77 eV and LUMO = −2.78 / −2.78 eV, respectively). Apparently, the DT-1 sensitizer due to the more bulky nature of the indoline group reduces dye aggregation and charge recombination. Thus, DSSCs fabricated with DT-1 showed a higher PCE of 8.57% with cobalt redox electrolytes than DT-2. Considering the high efficiency of the DT-1 dye, its regioisomer DTN-1 with the pyridine nitrogen atom adjacent to the anchoring group (rather than to the indoline donor, as in DT-1) was synthesized and compared with the DT-1 dye (Fig. 29).¹⁸⁵ Optical studies indicated that DTN-1 exhibited a greater red shift in its absorption spectrum compared to dye DT-1. However, the device made with DT-1 still demonstrated superior performance with PCE = 8.57%, which was higher than that of devices made with DTN-1 (6.10%). This highlights the negative impact of such a structural change on photovoltaic performance. Electrochemical impedance spectroscopy (EIS) studies revealed that DTN-1 exhibited increased charge recombination compared to DT-1, leading to a lower open-circuit voltage.

Additionally, the π-bridge in DT-1 was replaced with CPDT (dye SH₃) and EDOT (dye SH₄) (Fig. 29) to see how the nature of the bridge in this class of sensitizers affects their photophysical and electrochemical properties.¹¹⁴SH4 with the EDOT bridge exhibited poor absorption and lower photovoltaic performance. Both liquid-state and solid-state DSSCs fabricated with SH4 exhibited poor efficiencies of 3.52% and 1.69%, respectively, as compared to SH3 (5.74% and 5.56%, respectively). Theoretical studies revealed that in SH4, interactions and repulsions within the EDOT hindered the ICT process, leading to a reduced molar extinction coefficient and increased charge recombination.

Cao and colleagues developed the IPT dye (Fig. 29) using DPDTQ as a π-spacer and a 2,3-dipentylpyridopyrazine unit as a strong auxiliary acceptor.¹⁴¹ Although the strong electron-accepting nature induced a red shift and reduced band gap, it also hindered electron transfer, leading to a lower device performance (PCE = 5.32%). These results indicate that dyes with moderate electron-accepting properties could be promising for DSSC applications.

Marder et al. reported a D–A–π–A′ architecture of the dyes with pyrido[3,4-b]pyrazines as the auxiliary acceptor moieties (Q-85 and Q-93), in addition to the B-87 sensitizer made with benzothiadiazole as the auxiliary acceptor (Fig. 30).¹⁸³

Fig. 30 Schematic representation of the D–A–π–A′ featured structures of B-87, Q-85 and Q-93 with different auxiliary acceptors. Reprinted with permission from ref. 183 Copyright 2016 Royal Society of Chemistry.

The dithienosilole (DTS) unit was incorporated between the auxiliary acceptor and cyanoacrylic acid anchoring group that facilitated charge transfer and increased light harvesting properties due to the coplanarity of the system and improved conjugation. Additionally, incorporation of 2-ethylhexyl substituents onto DTS helped in reducing charge recombination and dye aggregation and contributed to higher performance in devices. The absorption spectra of these three sensitizers were studied in both solution and on a TiO₂ film (Fig. 31a and b). The dyes exhibited electron transitions in the UV (300–350 nm) and visible (550–590 nm) regions. High coplanarity and accepting properties of Q-93 resulted in a broader absorption than that for other dyes. All three sensitizers showed high performance in DSSCs achieving a PCE of 10.0%, 9.4%, and 8.2% for B-87, Q-85 and Q-93, respectively, in a standard I⁻/I₃⁻ liquid electrolyte system. After co-adsorption with CDCA, the PCE of DSSCs with these dyes was somewhat increased to 10.20%, 10.01%, and 8.43%, respectively. The J–V and IPCE characteristics of the sensitizers are shown in Fig. 31c and d.

Fig. 31 Absorption spectra of B-87, Q-85 and Q-93: (a) in DCM solution and (b) adsorbed on TiO₂ transparent films. (c) J–V curves of DSSCs based on dyes B-87, Q-85 and Q-93 under illumination of AM 1.5G simulated sunlight (100 mW cm⁻²). (d) IPCE spectra of the same DSSCs. Reprinted with permission from ref. 183 Copyright 2016 Royal Society of Chemistry.

6. Pyrrolo[2,3-b]pyrazine-based sensitizers

Structurally, dipyrrolo[2,3-b:2′,3′-e]pyrazine seems to be an interesting building block for design of D–A systems and possibly for design of DSSC sensitizers (although its symmetrical structure with two donor pyrrole moieties fused to the central pyrazine ring may rise a question of its potential). Based on this core, Gong and coworkers synthesized a series of novel dipyrrolopyrazine-based sensitizers, 3a–3d (Fig. 32), featuring different donors (N,N-dimethylamine and thiophene) and acceptors (pyridine, –COOH, and –CN) attached to the central core.¹¹⁵ The data are limited by four synthesized sensitizers of this class and among them, dye 3a showed the highest PCE of 1.44%. Photovoltaic parameters of dipyrollopyrazines dyes 3a–3d are depicted in Table 5. Potentially, future development of dipyrrolopyrazine-based dyes can provide a pathway for design of more efficient DSSCs. Some dyes containing indolo[2,3-b]quinoxaline moieties (as fused-ring extension of the pyrrolo[2,3-b]pyrazine core) have already been described in Sections 3.1 and 3.2 (see Fig. 18, 22 and 23), and higher performance of DSSCs with these acceptor fragments has been achieved, e.g., PCE = 7.09% (QX23, Table 1) and 4.10–8.28% (QX01–QX08, Table 2).

Fig. 32 Dipyrrolo[2,3-b:2′,3′-e]pyrazine-based sensitizers.¹¹⁵

7. The effect of donor–acceptor architectures of dyes on the device performance

With pyrazine-based dyes as sensitizers for DSSCs, different donor–acceptor architectures have been synthesized and studied (Tables 1–5). To comprehend the influence of different architectures on photovoltaic performance, it is essential to examine their structural modifications and effects on key factors such as light absorption, charge injection/transport and recombination. Investigation of variations in donor and acceptor moieties and π-bridges between them allow understanding major principles of their influence on PCE. Additionally, computational modeling, detailed spectroscopic studies, and device analysis provide deeper insights into the molecular orientation, dipole moments, and energy level alignment, contributing to the advancement of highly efficient and stable DSSCs.^18,186 Throughout the history of DSSC research, various architectures have been developed to enhance efficiency. Among them, the D–π–A architecture has been widely studied since its inception and remains a prominent choice due to its excellent photophysical properties and efficient intermolecular charge transfer.^137,187 To further optimize performance, researchers introduced the D–A–π–A′ system, an extension of D–π–A predecessor, which incorporates an auxiliary electron acceptor (A′) to fine-tune energy levels and modify photophysical properties. This advancement enhances stability and photostability, resulting in more durable photosensitizers.^188,189

A wide range of architectures of pyrazine-based sensitizers reported in the literature have been highlighted, including D–π–A, D–A–π–A′, D–π–A–π–A′, D–D–π–A, (D)₂–π–A and others (as outlined in the tables). Of all the structures described, in particular, the D–π–A and D–A–π–A′ architectures play an important role in the efficiency of photovoltaic systems, providing a fast electron transfer from the excited state of the dye to the semiconductor, followed by regeneration through a redox process. This efficient charge transfer mechanism contributes to the overall performance of DSSCs.¹⁹⁰ Among all the architectures explained here, the most efficient designs of D–π–A and D–A–π–A′ dyes have demonstrated promising performance. Meanwhile, research on D–D–π–A dyes and other architectures is ongoing, and they may have the potential to exhibit better photophysical properties than D–π–A type dyes. However, currently their efficiency remains limited.^191,192

The key structural features of the D–A–π–A′ system, which explain how it impacts all the properties to achieve good photophysical characteristics and photovoltaic performance, are as follows: the presence of an additional auxiliary acceptor (A′) enhances the electron-withdrawing capability, thereby strengthening the overall system.¹³⁰ This modification lowers the LUMO energy level, improves electron injection efficiency into the semiconductor, and facilitates charge separation and reduces recombination losses. As a result, it leads to a higher open-circuit voltage (V_OC) and ultimately enhances PCE. Meanwhile, the donor (D) group in the system donates electrons upon photoexcitation and significantly influences dye absorption. This enhances light-harvesting capability, thereby increasing the short-circuit current (J_SC). A stronger electron donor strengthens ICT interactions, shifting the absorption spectrum toward the NIR region, thus enabling better sunlight capture.^193,194 The π-bridge, functioning as a conjugated linker between donor and acceptor units, plays a crucial role in facilitating delocalized electron movement and improving charge transport. It enhances effective charge transfer from the donor to the acceptor while minimizing energy loss. Additionally, the π-bridge contributes to increased molecular rigidity, reducing non-radiative decay and improving dye stability. Furthermore, it affects the molecular orientation on the semiconductor surface, which can influence electron injection efficiency.^123,126,139 In DSSCs, the anchor acceptor group (A′) plays an important role by increasing the electron-withdrawing nature, thereby enhancing charge separation, tuning the energy level alignment, and binding the dyes to the TiO₂ surface. Additionally, it can affect the electron injection rate and the packing of adsorbed dyes.¹⁹⁵ Carboxylic acid is considered the best anchor group due to its stability and ease synthesis of dyes with a terminal –COOH group. The impact of different anchoring groups on photovoltaic performance has been well established.¹⁹⁶ Among various structural architectures, the D–π–A and D–A–π–A′ systems have demonstrated the best performance in different pyrazine derivatives. Future research can focus on optimizing these systems to develop highly efficient sensitizers and achieve the highest possible PCE in DSSCs.

8. Summary and outlook

This review explores the current design and development of dyes based on donor–acceptor pyrazine derivatives, as both metal-free and metal-containing sensitizers for dye-sensitized solar cells. Pyrazine-based sensitizers are noted for their superior efficiency, attributed to their broad absorption spectrum, efficient tuning of their suitable energy levels, strong electron-withdrawing properties, and effective electron injection into TiO₂ electrode. Ongoing research on various pyrazine structures, such as quinoxalines, thienopyrazines, pyridopyrazines, and pyrrolopyrazines showed promising results. Various design strategies of pyrazine derivatives for DSSC applications were reported, including D–π–A and D–A–π–A′ systems, demonstrating high efficiencies due to their favorable electronic arrangement, resulting in enhanced absorption, effective charge transport, and pronounced IPCE. The best efficiency for the D–π–A architectures was achieved with AQ202 and AQ203 dyes, reaching 8.37% and 8.06%, respectively, whereas for the D–A–π–A′ architectures a PCE of 12.5% has been achieved (for the HY64 dye). In DSSC development, expanding the π-conjugation by incorporating additional conjugated rings or systems, rather than extending linearly, is a promising strategy. This approach offers advantages of a wider range of light absorption, effective charge transport, and higher stability. Various quinoxaline sensitizers with different π-conjugated expansion moieties, such as triphenylamine, benzodithiazole, porphyrin, and carbazole, were reported in this context. In contrast, π-conjugated extensions, represented by polyenes and linear oligothiophenes, were found to improve absorption but often encountering problems with aggregation and less efficient charge transport properties.

Aggregation significantly impacts device performance, with J-aggregation being more favorable for red-shifted absorption, charge transport, and photostability. The torsion angle in dye molecules plays key roles in the electronic properties of the dyes and their tendency to aggregate and hence significantly affects the DSSC performance. In the NQX series, bulky groups influenced the torsion angle affecting the electronic coupling, light absorption, and molecular packing, and leading to variations in PCE. Co-adsorbents like CDCA and deoxycholic acid play crucial roles in enhancing power conversion efficiency by reducing dye aggregation and charge recombination. CDCA improves the device efficiencies, e.g. without and with the CDCA adsorbent, the values for ZW002 are 7.64% and 8.23%, for MD7 they are 8.22 and 9.03%, and for Q-85 they are 9.41% and 10.01%, respectively. Co-sensitization further enhances efficiency by producing complementary absorption and reducing aggregation. The “cocktail approach” of using FNE-53 and FNE-46 in solid-state DSSCs achieved 8.04%, while co-sensitization of FS14 with a Ru complex resulted in 8.67% efficiency. Some reports have shown that aggregation-free molecular dyes can perform better without auxiliary additives, which could reduce the adsorption of these aggregate-free dyes and lower the J_SC and device performance.¹⁵⁴

In DSSCs, the vertical orientation of the molecules plays a pivotal role, leading to strong intermolecular π–π aggregation, e.g., the study of the ZHG5–ZHG7 series reported enhanced photon capturing, charge transport properties, and DSSC performance. Structural modifications of pyrazine moieties in electron-donating fragments have shown to improve photophysical properties compared to their classical congeners with intrinsic electron-withdrawing nature. QX05–QX08 sensitizers represent examples of this approach, with the indolo[2,3-b]quinoxaline group acting as the donor, demonstrating the highest efficiency of 8.28% in this series.

Counter electrodes play a crucial role in enhancing the performance of DSSCs. Although the most commonly used platinum electrode is highly effective, its cost is a drawback. Researchers have found that structural modifications of counter electrodes using polymeric materials or TiO₂ can replace platinum electrodes and achieve high efficiency.¹⁹⁷ TiO₂ is a leading semiconducting material in DSSCs. However, its wide band gap of 3.2 eV limits power conversion efficiency. The band gap of TiO₂ can be optimized by incorporating different metals, such as Zn, Ni, Fe, and Cu, and bimetals like Pd/Pt and Fe/Nb, and it reached the highest PCE in DSSCs.¹⁹⁸ More research should be carried out on optimizing counter electrode materials.

Metal-free sensitizers are cost-effective and promising for future industrial use compared to ruthenium-based sensitizers. Further research should focus on increasing DSSC efficiency and better understanding the structure–property and structure–function relationships for expanding the commercialization of this technology. This includes the design of new sensitizers with pyrazine derivatives and different electronic architectures, investigation of suitable co-adsorbents and co-sensitizers, optimization of the structures by controlling the dihedral angles in the dye backbone and reducing their aggregation and studying the morphology of counter electrodes and their interactions with sensitizers.

Abbreviations

Ac	Acetyl
ATA-113	Amino transaminase
BHJ	Bulk heterojunction
bpy	2,2′-Bipyridyl
bpye	2,2′-(Ethane-1,1-diyl)dipyridine
bpy-pz	6-(1H-Pyrazol-1-yl)-2,2′-bipyridine
CDCA	Chenodeoxycholic acid
CPDT	Cyclopentadithiophene
DBP	Dibenzo[a,c]phenazine
DCA	Deoxycholic acid
DCM	Dichloromethane
DFT	Density functional theory
dicnq	6,7-Dicyanodipyrido[2,2-d:2′,3′-f]quinoxaline
dmb	2,9-Dimethyl-1,10-phenanthroline
DMSO	Dimethyl sulfoxide
DPQ	2,3-Diphenylquinoxaline
DPDTQ	2,3-Dipentyldithieno[3,2-f:2′,3′-h]quinoxaline
D–π–A	Donor–π–acceptor
DSSCs	Dye-sensitized solar cells
DTP	Dithieno[3,2-a:2′,3′-c]phenazine
DTS	Dithienosilole
EDOT	3,4-Ethylenedioxythiophene
EIS	Electrochemical impedance spectroscopy
EQE	External quantum efficiency
eV	Electron volt
Φ col	Charge collection efficiency
Φ inj	Electron injection efficiency
FF	Fill factor
FTO	Fluorine-doped tin oxide
HOMO	Highest occupied molecular orbital
ICT	Intramolecular charge transfer
IPCE	Incident photon-to-current efficiency
J SC	Short-circuit current
kcal	Kilocalorie
LUMO	Lowest unoccupied molecular orbital
MeCN	Acetonitrile
NHE	Normal hydrogen electrode
NIR	Near-infrared
OPV	Organic photovoltaics
OSCs	Organic solar cells
P3HT	Poly-3-hexylthiophene
PC61BM	[6,6]-Phenyl-C61-butyric acid methyl ester
PCE	Power conversion efficiency
PEDOT:PSS	Poly[2,5-(3,4-ethylenedioxy)thiophene] polystyrene sulfonate
phen	1,10-Phenanthroline
PPL	Pyrazino[2,3-f][1,10]phenanthroline
PSC	Perovskite solar cells
QDs	Quantum dots
SGNF	Stacked graphene platelet nanofibers
tBP	4-tert-Butylpyridine
t-BuOH	tert-Butanol
t-BuOK	Potassium tert-butoxide
THF	Tetrahydrofuran
tmby	4,4′,6,6′-Tetramethyl-2,2′-bipyridyl
TPz	Thieno[3,4-b]pyrazine
TPA	Triphenylamine
UV	Ultraviolet light
Vis	Visible light
V OC	Open circuit voltage

Data availability

The authors confirm that the data supporting the findings of this study are available within the article and cited original literature.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was supported by the FSU-2023-021 grant 8474000527 from the Khalifa University of Science and Technology. J. V. S. K. and I. F. P. further acknowledge support from the EU's Horizon 2020 ERA-Chair project ExCEED (grant agreement no. 952008).

References

1. S. C. Ameta and R. Ameta, Solar Energy Conversion and Storage: Photochemical Modes, CRC Press, 2016.
2. H. Tian, G. Boschloo and A. Hagfeldt, Molecular Devices for Solar Energy Conversion and Storage, Spinger, 2018.
3. Solar Energy Harvesting, Conversion, and Storage: Materials, Technologies, and Applications, ed. M. Khalid, R. Walvekar, H. Panchal and M. Vaka, Elsevier, 2021.
4. C. Shao, Y. Zhao and L. Qu, Recent advances in highly integrated energy conversion and storage system, SusMat, 2022, 2, 142–160.
5. Q. He, J. Ning, H. Chen, Z. Jiang, J. Wang, D. Chen, C. Zhao, Z. Liu, I. F. Perepichka, H. Meng and W. Huang, Achievements, challenges, and perspectives in the design of polymer binders for advanced lithium-ion batteries, Chem. Soc. Rev., 2024, 53, 7091–7157.
6. J. Pastuszak and P. Węgierek, Photovoltaic cell generations and current research directions for their development, Materials, 2022, 15, 5542.
7. C. Battaglia, A. Cuevas and S. De Wolf, High-efficiency crystalline silicon solar cells: status and perspectives, Energy Environ. Sci., 2016, 9, 1552–1576.
8. M. Di Sabatino, R. Hendawi and A. S. Garcia, Silicon solar cells: Trends, manufacturing challenges, and AI perspectives, Crystals, 2024, 14, 167.
9. M. A. Scarpulla, B. McCandless, A. B. Phillips, Y. Yan, M. J. Heben, C. Wolden, G. Xiong, W. K. Metzger, D. Mao, D. Krasikov, I. Sankin, S. Grover, A. Munshi, W. Sampath, J. R. Sites, A. Bothwell, D. Albin, M. O. Reese, A. Romeo, M. Nardone, R. Klie, J. M. Walls, T. Fiducia, A. Abbas and S. M. Hayes, CdTe-based thin film photovoltaics: Recent advances, current challenges and future prospects, Sol. Energy Mater. Sol. Cells, 2023, 255, 112289.
10. S. Sivaraj, R. Rathanasamy, G. V. Kaliyannan, H. Panchal, A. J. Alrubaie, M. M. Jaber, Z. Said and S. Memon, A comprehensive review on current performance, challenges and progress in thin-film solar cells, Energies, 2022, 15, 8688.
11. T. D. Lee and A. U. Ebong, A review of thin film solar cell technologies and challenges, Renew. Sustain. Energy Rev., 2017, 70, 1286.
12. N. Papež, R. Dallaev, Ş. Ţălu and J. Kaštyl, Overview of the current state of gallium arsenide-based solar cells, Materials, 2021, 14, 3075.
13. N. Shah, A. A. Shah, P. K. Leung, S. Khan, K. Sun, X. Zhu and Q. Liao, A Review of third generation solar cells, Processes, 2023, 11, 1852.
14. X. Lan, S. Masala and E. H. Sargent, Charge-extraction strategies for colloidal quantum dot photovoltaics, Nat. Mater., 2014, 13, 233–240.
15. O. E. Semonin, J. M. Luther and M. C. Beard, Quantum dots for nextgeneration photovoltaics, Mater. Today, 2012, 15, 508–515.
16. Quantum Dot Solar Cells, ed J. Wu and Z. M. Wang, Springer, 2014.
17. V. T. Chebrolu and H.-J. Kim, Recent progress in quantum dot sensitized solar cells: an inclusive review of photoanode, sensitizer, electrolyte, and the counter electrode, J. Mater. Chem. C, 2019, 7, 4911–4933.
18. A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo and H. Pettersson, Dye-Sensitized Solar Cells, Chem. Rev., 2010, 110, 6595–6663.
19. A. B. Muñoz-García, I. Benesperi, G. Boschloo, J. J. Concepcion, J. H. Delcamp, E. A. Gibson, G. J. Meyer, M. Pavone, H. Pettersson, A. Hagfeldt and M. Freitag, Dye-sensitized solar cells strike back, Chem. Soc. Rev., 2021, 50, 12450–12550.
20. M. Kokkonen, P. Talebi, J. Zhou, S. Asgari, S. A. Soomro, F. Elsehrawy, J. Halme, S. Ahmad, A. Hagfeldt and S. G. Hashmi, Advanced research trends in dye-sensitized solar cells, J. Mater. Chem. A, 2021, 9, 10527–10545.
21. M. Freitag, J. Teuscher, Y. Saygili, X. Zhang, F. Giordano, P. Liska, J. Hua, S. M. Zakeeruddin, J.-E. Moser, M. Grätzel and A. Hagfeldt, Dye-sensitized solar cells for efficient power generation under ambient lighting, Nat. Photonics, 2017, 11, 372–378.
22. K. Zeng, Z. Tong, L. Ma, W.-H. Zhu, W. Wu and Y. Xie, Molecular engineering strategies for fabricating efficient porphyrin-based dye-sensitized solar cells, Energy Environ. Sci., 2020, 13, 1617–1657.
23. A. Mishra and P. Bäuerle, Small molecule organic semiconductors on the move: Promises for future solar energy technology, Angew. Chem., Int. Ed., 2012, 51, 2020–2067.
24. Y. Che, M. R. Niazi, T. Yu, T. Maris, C.-H. Liu, D. Ma, R. Izquierdo, I. F. Perepichka and D. F. Perepichka, Nitrofluorene-based A–D–A electron acceptors for organic photovoltaics, J. Mater. Chem. C, 2023, 11, 8186–8195.
25. J.-S. Wu, S.-W. Cheng, Y.-J. Cheng and C.-S. Hsu, Donor–acceptor conjugated polymers based on multifused ladder-type arenes for organic solar cells, Chem. Soc. Rev., 2015, 44, 1113–1154.
26. Z.-P. Yu, K. Yan, W. Ullah, H. Chen and C.-Z. Li, Conjugated polymers for photon-to-electron and photon-to-fuel conversions, ACS Appl. Polym. Mater., 2021, 3, 60–92.
27. G. Zhang, F. R. Lin, F. Qi, T. Heumüller, A. Distler, H.-J. Egelhaaf, N. Li, P. C. Y. Chow, C. J. Brabec, A. K.-Y. Jen and H.-L. Yip, Renewed prospects for organic photovoltaics, Chem. Rev., 2022, 122, 14180–14274.
28. A. Wadsworth, M. Moser, A. Marks, M. S. Little, N. Gasparini, C. J. Brabec, D. Baran and I. McCulloch, Critical review of the molecular design progress in non-fullerene electron acceptors towards commercially viable organic solar cells, Chem. Soc. Rev., 2019, 48, 1596–1625.
29. J. Han, K. Park, S. Tan, Y. Vaynzof, J. Xue, E. W.-G. Diau, M. G. Bawendi, J.-W. Lee and I. Jeon, Perovskite solar cells, Nat. Rev. Methods Primers, 2025, 5, 3.
30. P. Chen, Y. Xiao, S. Li, X. Jia, D. Luo, W. Zhang, H. J. Snaith, Q. Gong and R. Zhu, The promise and challenges of inverted perovskite solar cells, Chem. Rev., 2024, 124, 10623–10700.
31. C. Yang, W. Hu, J. Liu, C. Han, Q. Gao, A. Mei, Y. Zhou, F. Guo and H. Han, Achievements, challenges, and future prospects for industrialization of perovskite solar cells, Light: Sci. Appl., 2024, 13, 227.
32. J. Y. Kim, J.-W. Lee, H. S. Jung, H. Shin and N.-G. Park, High-efficiency perovskite solar cells, Chem. Rev., 2020, 120, 7867–7918.
33. S. He1, L. Qiu, L. K. Ono and Y. Qi, How far are we from attaining 10-year lifetime for metal halide perovskite solar cells?, Mater. Sci. Eng., R, 2020, 140, 100545.
34. J. Xie, Q. Chen, Q. Xue, I. F. Perepichka and G. Xie, H₂O₂–modified NiO_x for perovskite photovoltaic modules, Innovation, 2024, 5, 100650.
35. F. Rehman, I. H. Syed, S. Khanam, S. Ijaz, H. Mehmood, M. Zubair, Y. Massoud and M. Q. Mehmood, Fourth-generation solar cells: a review, Energy Adv., 2023, 2, 1239–1262.
36. J. Roncali, Single-material organic solar cells based on small molecule homojunctions: An outdated concept or a new challenge for the chemistry and physics of organic photovoltaics?, Adv. Energy Mater., 2021, 11, 2102987.
37. B. Li, X. Yang, S. Liac and J. Yuan, Stable block copolymer single-material organic solar cells: progress and perspective, Energy Environ. Sci., 2023, 16, 723–744.
38. Y. He, N. Li, T. Heumüller, J. Wortmann, B. Hanisch, A. Aubele, S. Lucas, G. Feng, X. Jiang, W. Li, P. Bäuerle and C. J. Brabec, Industrial viability of single-component organic solar cells, Joule, 2022, 6, 1160–1171.
39. Z. Chen, J. Ge, W. Song, X. Tong, H. Liu, X. Yu, J. Li, J. Shi, L. Xie, C. Han, Q. Liu and Z. Ge, 20.2% Efficiency organic photovoltaics employing a π-extension quinoxaline-based acceptor with ordered arrangement, Adv. Mater., 2024, 36, 2406690.
40. Y. Jiang, S. Sun, R. Xu, F. Liu, X. Miao, G. Ran, K. Liu, Y. Yi, W. Zhang and X. Zhu, Non-fullerene acceptor with asymmetric structure and phenyl-substituted alkyl side chain for 20.2% efficiency organic solar cells, Nat. Energy, 2024, 9, 975–986.
41. R. Zhang, H. Chen, T. Wang, L. Kobera, L. He, Y. Huang, J. Ding, B. Zhang, A. Khasbaatar, S. Nanayakkara, J. Zheng, W. Chen, Y. Diao, S. Abbrent, J. Brus, A. H. Coffey, C. Zhu, H. Liu, X. Lu, Q. Jiang, V. Coropceanu, J.-L. Brédas, Y. Li, Y. Li and F. Gao, Equally high efficiencies of organic solar cells processed from different solvents reveal key factors for morphology control, Nat. Energy, 2025, 10, 124–134.
42. H. Chen, Y. Huang, R. Zhang, H. Mou, J. Ding, J. Zhou, Z. Wang, H. Li, W. Chen, J. Zhu, Q. Cheng, H. Gu, X. Wu, T. Zhang, Y. Wang, H. Zhu, Z. Xie, F. Gao, Y. Li and Y. Li, Organic solar cells with 20.82% efficiency and high tolerance of active layer thickness through crystallization sequence manipulation, Nat. Mater., 2025, 24, 444–453.
43. L. Zhu, M. Zhang, G. Zhou, Z. Wang, W. Zhong, J. Zhuang, Z. Zhou, X. Gao, L. Kan, B. Hao, F. Han, R. Zeng, X. Xue, S. Xu, H. Jing, B. Xiao, H. Zhu, Y. Zhang and F. Liu, Achieving 20.8% organic solar cells via additive-assisted layer-by-layer fabrication with bulk p-i-n structure and improved optical management, Joule, 2024, 8, 3153–3168.
44. A. Kojima, K. Teshima, Y. Shirai and T. Miyasaka, Organometal halide perovskites as visible-light sensitizers for photovoltaic cells, J. Am. Chem. Soc., 2009, 131, 6050–6051.
45. S. Liu, J. Li, W. Xiao, R. Chen, Z. Sun, Y. Zhang, X. Lei, S. Hu, M. Kober-Czerny, J. Wang, F. Ren, Q. Zhou, H. Raza, Y. Gao, Y. Ji, S. Li, H. Li, L. Qiu, W. Huang, Y. Zhao, B. Xu, Z. Liu, H. J. Snaith, N.-G. Park and W. Chen, Buried interface molecular hybrid for inverted perovskite solar cells, Nature, 2024, 632, 536–542.
46. M. A. Green, E. D. Dunlop, M. Yoshita, N. Kopidakis, K. Bothe, G. Siefer, D. Hinken, M. Rauer, J. Hohl-Ebinger and X. Hao, Solar cell efficiency tables (Version 64), Prog. Photovoltaics, 2024, 32, 425–441.
47. https://www2.nrel.gov/pv/interactive-cell-efficiency (accessed May 29, 2025).
48. S. Moradi, S. Kundu, M. Awais, Y. Haruta, H.-D. Nguyen, D. Zhang, F. Tan and M. I. Saidaminov, High-Throughput Exploration of Triple-Cation Perovskites via All-in-One Compositionally-Graded Films, Small, 2023, 19, 2301037.
49. B. O'Regan and M. Grätzel, A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO₂ films, Nature, 1991, 353, 737–740.
50. S. A. Badawy, E. Abdel-Latif and M. R. Elmorsy, Tandem dye-sensitized solar cells achieve 12.89% efficiency using novel organic sensitizers, Sci. Rep., 2024, 14, 26072.
51. Y. Ren, D. Zhang, J. Suo, Y. Cao, F. T. Eickemeyer, N. Vlachopoulos, S. M. Zakeeruddin, A. Hagfeldt and M. Grätzel, Hydroxamic acid pre-adsorption raises the efficiency of cosensitized solar cells, Nature, 2023, 613, 60–65.
52. S. Rahman, A. Haleem, M. Siddiq, M. K. Hussain, S. Qamar, S. Hameed and M. Waris, Research on dye sensitized solar cells: recent advancement toward the various constituents of dye sensitized solar cells for efficiency enhancement and future prospects, RSC Adv., 2023, 13, 19508–19529.
53. G. Boschloo and A. Hagfeldt, Characteristics of the iodide/triiodide redox mediator in dye-sensitized solar cells, Acc. Chem. Res., 2009, 42, 1819–1826.
54. Z. Li, Q. Li, C. Li and Y. Xie, Panchromatic porphyrin-based dye-sensitized solar cells: from cosensitization to concerted companion dye approaches, Mater. Chem. Front., 2024, 8, 652–680.
55. R. Katoh and A. Furube, Electron injection efficiency in dye-sensitized solar cells, J. Photochem. Photobiol., C, 2014, 20, 1–16.
56. S. A. Haque, E. Palomares, B. M. Cho, A. N. Green, N. Hirata, D. R. Klug and J. R. Durrant, Charge separation versus recombination in dye-sensitized nanocrystalline solar cells: the minimization of kinetic redundancy, J. Am. Chem. Soc., 2005, 127, 3456–3462.
57. S. M. Mousavi, H. Daghigh Shirazi, R. Ranta, M. I. Asghar, S. Kasurinen, J. Halme and J. Vapaavuori, Addressing the efficiency loss and degradation of triple cation perovskite solar cells via integrated light managing encapsulation, Mater. Today Energy, 2024, 46, 101707.
58. L. Tian, E. Bi, I. Yavuz, C. Deger, Y. Tian, J. Zhou, S. Zhang, Q. Liu, J. Shen, L. Yao, K. Zhao, J. Xu, Z. Chen, L. Xiao, Z. Yang, P. Shi, X. Zhang, S. Wang, S. Chu, M. Haider, J. Xue and R. Wang, Divalent cation replacement strategy stabilizes wide-bandgap perovskite for Cu(In,Ga)Se₂ tandem solar cells, Nat. Photonics, 2025, 19, 479–485.
59. L. V. Hublikar, S. V. Ganachari, F. A. Shilar and N. Raghavendra, Recent advances in transition metal oxide nanomaterials for solar cell applications: A status review and technology perspectives, Mater. Res. Bull., 2025, 187, 113351.
60. N. M. Saidi, N. K. Farhana, S. Ramesh and K. Ramesh, Influence of different concentrations of 4-tert-butyl-pyridine in a gel polymer electrolyte towards improved performance of dye-sensitized solar cells (DSSC), Sol. Energy, 2021, 216, 111–119.
61. K. D. Benkstein, N. Kopidakis, J. V. D. Lagemaat and A. J. Frank, Influence of the percolation network geometry on electron transport in dye-sensitized titanium dioxide solar cells, J. Phys. Chem. B, 2003, 107, 7759–7767.
62. L. Zhang and J. M. Cole, Dye aggregation in dye-sensitized solar cells, J. Mater. Chem. A, 2017, 5, 19541–19559.
63. N. V. Krishna, J. V. S. Krishna, M. Mrinalini, S. Prasanthkumar and L. Giribabu, Role of co-sensitizers in dye-sensitized solar cells, ChemSusChem, 2017, 10, 4668–4689.
64. H. Iftikhar, G. G. Sonai, S. G. Hashmi, A. F. Nogueira and P. D. Lund, Progress on electrolytes development in dye-sensitized solar cells, Materials, 2019, 12, 1998.
65. S. Yanagida, Y. Yu and K. Manseki, Iodine/iodide-free dye-sensitized solar cells, Acc. Chem. Res., 2009, 42, 1827–1838.
66. M. Ye, X. Wen, M. Wang, J. Iocozzia, N. Zhang, C. Lin and Z. Lin, Recent advances in dye-sensitized solar cells: from photoanodes, sensitizers and electrolytes to counter electrodes, Mater. Today, 2015, 18, 155–162.
67. Masud and H. K. Kim, Advanced polymeric matrices for gel electrolytes in quasi-solid-state dye-sensitized solar cells: Recent progress and future perspective, Mater. Today Energy, 2023, 38, 101440.
68. N. Wang, J. Hu, L. Gao and T. Ma, Current progress in solid-state electrolytes for dye-sensitized solar cells: A mini-review, J. Electron. Mater., 2020, 49, 7085–7097.
69. Masud and H. K. Kim, Redox shuttle-based electrolytes for dye-sensitized solar cells: comprehensive guidance, recent progress, and future perspective, ACS Omega, 2023, 8, 6139–6163.
70. M. Dhonde, K. Sahu, M. Das, A. Yadav, P. Ghosh and V. V. S. Murty, Review – Recent advancements in dye-sensitized solar cells; from photoelectrode to counter electrode, J. Electrochem. Soc., 2022, 169, 066507.
71. T. Higashino and H. Imahori, Emergence of copper(I/II) complexes as third-generation redox shuttles for dye-sensitized solar cells, ACS Energy Lett., 2022, 7, 1926–1938.
72. K. Samdhyan, P. Chand, H. Anand and S. Saini, Development of carbon-based copper sulfide nanocomposites for high energy supercapacitor applications: A comprehensive review, J. Energy Storage, 2022, 46, 103886.
73. I. M. Abdellah and A. El-Shafei, Efficiency enhancement of ruthenium-based DSSCs employing A–π–D–π–A organic co-sensitizers, RSC Adv., 2020, 10, 27940–27953.
74. S. Dayan, N. Kayaci and N. Kalaycioğlu Özpozan, Improved performance with molecular design of Ruthenium(II) complexes bearing diamine-based bidentate ligands as sensitizer for dye-sensitized solar cells (DSSC), J. Mol. Struct., 2020, 1209, 127920.
75. T. Jella, M. Srikanth, R. Bolligarla, Y. Soujanya, S. P. Singh and L. Giribabu, Benzimidazole-functionalized ancillary ligands for heteroleptic Ru(II) complexes: synthesis, characterization and dye-sensitized solar cell applications, Dalton Trans., 2015, 44, 14697–14706.
76. A. Mary, N. Jain, R. Sakla, D. A. Jose, B. S. Yadav and A. R. Naziruddin, Ruthenium (II) complexes bearing N-heterocyclic carbene-based C^N donor sets in dye-sensitized solar cells, Appl. Organomet. Chem., 2022, 37, e6873.
77. T. Banerjee, A. K. Biswas, U. Reddy G, T. S. Sahu, A. Das, B. Ganguly and H. N. Ghosh, Superior grafting and state-of-the-art interfacial electron transfer rates for newly designed geminal dicarboxylate bound ruthenium(II)– and osmium(II)–polypyridyl dyes on TiO₂ nanosurface, J. Phys. Chem. C, 2014, 118, 3864–3877.
78. Y. Zhang, T. Higashino and H. Imahori, Molecular designs, synthetic strategies, and properties for porphyrins as sensitizers in dye-sensitized solar cells, J. Mater. Chem. A, 2023, 11, 12659–12680.
79. N. Vamsi Krishna, J. Venkata Suman Krishna, S. P. Singh, L. Giribabu, A. Islam and I. Bedja, Bulky nature phenanthroimidazole-based porphyrin sensitizers for dye-sensitized solar cell applications, J. Phys. Chem. C, 2017, 121, 25691–25704.
80. N. V. Krishna, J. V. S. Krishna, S. P. Singh, L. Giribabu, L. Han, I. Bedja, R. K. Gupta and A. Islam, Donor-π–acceptor based stable porphyrin sensitizers for dye-sensitized solar cells: Effect of π-conjugated spacers, J. Phys. Chem. C, 2017, 121, 6464–6477.
81. S. Huang, Q. Li, S. Li, C. Li, H. Tan and Y. Xie, Recent advances in the approaches for improving the photovoltaic performance of porphyrin-based DSSCs, Chem. Commun., 2024, 60, 4521–4536.
82. J. Luo, Y. Wang, S. Shi, Y. Wu, T. Ma, L. Wang, G. Baryshnikov, X. Wu, C. Li and Y. Xie, Solar cells sensitized by donor-linked concerted companion dyes, J. Mater. Chem. C, 2023, 11, 5450–5460.
83. K. Zeng, Y. Chen, W.-H. Zhu, H. Tian and Y. Xie, Efficient solar cells based on concerted companion dyes containing two complementary components: an alternative approach for cosensitization, J. Am. Chem. Soc., 2020, 142, 5154–5161.
84. J. Luo, Q. Lu, Q. Li, Z. Li, Y. Wang, X. Wu, C. Li and Y. Xie, Efficient solar cells based on porphyrin and concerted companion dyes featuring benzo 12-crown-4 for suppressing charge recombination and enhancing dye loading, ACS Appl. Mater. Interfaces, 2023, 15, 41569–41579.
85. L. Giribabu, N. Duvva, S. P. Singh, L. Han, I. M. Bedja, R. K. Gupta and A. Islam, Stable and charge recombination minimized π-extended thioalkyl substituted tetrathiafulvalene dye-sensitized solar cells, Mater. Chem. Front., 2017, 1, 460–467.
86. D. Devadiga, M. Selvakumar, P. Shetty, M. Santosh, R. S. Chandrabose and S. Karazhanov, Recent developments in metal-free organic sensitizers derived from carbazole, triphenylamine, and phenothiazine for dye-sensitized solar cells, Int. J. Energy Res., 2021, 45, 6584–6643.
87. J. Oh, W. Ghann, H. Kang and F. Nesbitt, S. Providence and J. Uddin, Comparison of the performance of dye sensitized solar cells fabricated with ruthenium based dye sensitizers: Di-tetrabutylammonium cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II) (N719) and tris(bipyridine)ruthenium(II) chloride (Ru-BPY), Inorg. Chim. Acta, 2018, 482, 943–950.
88. Ö. Birel, S. Nadeem and H. Duman, Porphyrin-based dye-sensitized solar cells (DSSCs): A review, J. Fluorescence, 2017, 27, 1075–1085.
89. J.-M. Ji, H. Zhou and H. K. Kim, Rational design criteria for D–π–A structured organic and porphyrin sensitizers for highly efficient dye-sensitized solar cells, J. Mater. Chem. A, 2018, 6, 14518–14545.
90. G. S. Selopal, H.-P. Wu, J. Lu, Y.-C. Chang, M. Wang, A. Vomiero, I. Concina and E. W.-G. Diau, Metal-free organic dyes for TiO₂ and ZnO dye-sensitized solar cells, Sci. Rep., 2016, 6, 18756.
91. J. Shaya, F. Fontaine-Vive, B. Y. Michel and A. Burger, Rational design of push–pull fluorene dyes: synthesis and structure–photophysics relationship, Chem. – Eur. J., 2016, 22, 10627–10637.
92. J. Shaya, P. R. Corridon, B. Al-Omari, A. Aoudi, A. Shunnar, M. I. H. Mohideen, A. Qurashi, B. Y. Michel and A. Burger, Design, photophysical properties, and applications of fluorene-based fluorophores in two-photon fluorescence bioimaging: A review, J. Photochem. Photobiol., C, 2022, 52, 100529.
93. R. S. Rao, Suman and S. P. Singh, Near-infrared (>1000 nm) light-harvesters: Design, synthesis and applications, Chem. – Eur. J., 2020, 26, 16582–16593.
94. H. Takahashi, H. Watanabe, S. Ito, K. Tanaka and Y. Chujo, Design and synthesis of far-red to near-infrared chromophores with pyrazine-based boron complexes, Chem. – Asian J., 2023, 18, e202300489.
95. A. Tang, W. Song, B. Xiao, J. Guo, J. Min, Z. Ge, J. Zhang, Z. Wei and E. Zhou, Benzotriazole-based acceptor and donors, coupled with chlorination, achieve a high VOC of 1.24 V and an efficiency of 10.5% in fullerene-free organic solar cells, Chem. Mater., 2019, 31, 3941–3947.
96. N. Luo, G. Zhang and Z. Liu, Keep glowing and going: Recent progress in diketopyrrolopyrrole synthesis towards organic optoelectronic materials, Org. Chem. Front., 2021, 8, 4560–4581.
97. Y. Wu and W. Zhu, Organic sensitizers from D–π–A to D–A–π–A: effect of the internal electron-withdrawing units on molecular absorption, energy levels and photovoltaic performances, Chem. Soc. Rev., 2013, 42, 2039–2058.
98. F. B. Mortzfeld, C. Hashem, K. Vranková, M. Winkler and F. Rudroff, Pyrazines: Synthesis and industrial application of these valuable flavor and fragrance compounds, Biotechnol. J., 2020, 15, 2000064.
99. J. Zhang, Y. Jiang, X. Cheng, Y. Xie, J. Zhao and J. Weng, Recent advances in versatile pyridazine-cored materials: Principles, applications, and challenges, J. Mater. Chem. C, 2023, 11, 5563–5584.
100. P. Meti, H.-H. Park and Y.-D. Gong, Recent developments in pyrazine functionalized π-conjugated materials for optoelectronic applications, J. Mater. Chem. C, 2020, 8, 352–379.
101. G. Yashwantrao and S. Saha, Perspective on the rational design strategies of quinoxaline derived organic sensitizers for dye-sensitized solar cells (DSSC), Dyes Pigm., 2022, 199, 110093.
102. P. Ghosh and A. Mandal, Greener approach toward one pot route to pyrazine synthesis, Green Chem. Lett. Rev., 2012, 5, 127–134.
103. J. Xu, A. P. Green and N. J. Turner, Chemo-enzymatic synthesis of pyrazines and pyrroles, Angew. Chem., Int. Ed., 2018, 57, 16760–16763.
104. E. A. Sokolova and A. A. Festa, Synthesis of pyrazino[1,2-a]indoles and indolo[1,2-a]quinoxalines (microreview), Chem. Heterocycl. Compd., 2016, 52, 219–221.
105. C. Shalini, N. Dharmaraj, N. S. P. Bhuvanesh and M. V. Kaveri, Suzuki Miyaura cross-coupling of 2-chloropyrazine with arylboronic acids catalyzed by novel palladium(II) ONO pincer complexes, Inorg. Chim. Acta, 2022, 540, 121028.
106. Y. Li, M. Lai, Z. Wu, M. Zhao and M. Zhang, Synthesis of 2-acyl-substituted pyrazine derivatives through silver-catalyzed decarboxylative coupling reactions, ChemistrySelect, 2018, 3, 5588–5592.
107. V. P. Mehta, A. Sharma and E. Van der Eycken, The first palladium-catalyzed desulfitative Sonogashira-type cross-coupling of (hetero)aryl thioethers with terminal alkynes, Org. Lett., 2008, 10, 1147–1150.
108. S. Sharma, M. Kumar and V. Bhalla, Pyrazine derivative as supramolecular host for immobilization of palladium nanoparticles for efficient Suzuki coupling, Eur. J. Org. Chem., 2023, e202300594.
109. N. I. Nikishkin, J. Huskens and W. Verboom, Transition metal-catalyzed functionalization of pyrazines, Org. Biomol. Chem., 2013, 11, 3583–3602.
110. G. Yashwantrao and S. Saha, Recent advances in the synthesis and reactivity of quinoxaline, Org. Chem. Front., 2021, 8, 2820–2862.
111. C. Shen, Y. Wu, W. Zhang, H. Jiang, H. Zhang, E. Li, B. Chen, X. Duan and W.-H. Zhu, Incorporating quinoxaline unit as additional acceptor for constructing efficient donor-free solar cell sensitizers, Dyes Pigm., 2018, 149, 65–72.
112. M. Xie, Z. Wei and K. Lu, Quinoxaline-based Y-type acceptors for organic solar cells, Chem. Sci., 2024, 15, 8265–8279.
113. W. E. Meador, N. P. Liyanage, J. Watson, K. Groenhout and J. H. Delcamp, Panchromatic NIR-absorbing sensitizers with a thienopyrazine auxiliary acceptor for dye-sensitized solar cells, ACS Appl. Energy Mater., 2023, 6, 5416–5428.
114. Z. Shen, X. Zhang, F. Giordano, Y. Hu, J. Hua, S. M. Zakeeruddin, H. Tian and M. Grätzel, Significance of π-bridge contribution in pyrido[3,4-b]pyrazine featured D–A–π–A organic dyes for dye-sensitized solar cells, Mater. Chem. Front., 2017, 1, 181–189.
115. P. Meti, G. Nagaraju, J.-W. Yang, S. H. Jung and Y.-D. Gong, Synthesis of dipyrrolopyrazine-based sensitizers with a new π-bridge end-capped donor–acceptor framework for DSSCs: a combined experimental and theoretical investigation, New J. Chem., 2019, 43, 3017–3025.
116. D. W. Chang, H. J. Lee, J. H. Kim, S. Y. Park, S.-M. Park, L. Dai and J.-B. Baek, Novel quinoxaline-based organic sensitizers for dye-sensitized solar cells, Org. Lett., 2011, 13, 3880–3883.
117. J. Shi, J. Chen, Z. Chai, H. Wang, R. Tang, K. Fan, M. Wu, H. Han, J. Qin, T. Peng, Q. Li and Z. Li, High performance organic sensitizers based on 11,12-bis(hexyloxy) dibenzo[a,c]phenazine for dye-sensitized solar cells, J. Mater. Chem., 2012, 22, 18830–18838.
118. J. Shi, Z. Chai, J. Su, J. Chen, R. Tang, K. Fan, L. Zhang, H. Han, J. Qin, T. Peng, Q. Li and Z. Li, New sensitizers bearing quinoxaline moieties as an auxiliary acceptor for dye-sensitized solar cells, Dyes Pigm., 2013, 98, 405–413.
119. X. Lu, Q. Feng, T. Lan, G. Zhou and Z.-S. Wang, Molecular engineering of quinoxaline-based organic sensitizers for highly efficient and stable dye-sensitized solar cells, Chem. Mater., 2012, 24, 3179–3187.
120. X. Lu, X. Jia, Z.-S. Wang and G. Zhou, X-shaped organic dyes with a quinoxaline bridge for use in dye-sensitized solar cells, J. Mater. Chem. A, 2013, 1, 9697–9706.
121. X. Lu, T. Lan, Z. Qin, Z.-S. Wang and G. Zhou, A near-infrared dithieno[2,3-a:3′,2′-c]phenazine-based organic co-sensitizer for highly efficient and stable quasi-solid-state dye-sensitized solar cells, ACS Appl. Mater. Interfaces, 2014, 6, 19308–19317.
122. X. Jia, W. Zhang, X. Lu, Z.-S. Wang and G. Zhou, Efficient quasi-solid-state dye-sensitized solar cells based on organic sensitizers containing fluorinated quinoxaline moiety, J. Mater. Chem. A, 2014, 2, 19515–19525.
123. T. Lan, X. Lu, L. Zhang, Y. Chen, G. Zhou and Z.-S. Wang, Enhanced performance of quasi-solid-state dye-sensitized solar cells by tuning the building blocks in D–(π)–A′–π–A featured organic dyes, J. Mater. Chem. A, 2015, 3, 9869–9881.
124. K. Pei, Y. Wu, W. Wu, Q. Zhang, B. Chen, H. Tian and W. Zhu, Constructing organic D–A–π–A–featured sensitizers with a quinoxaline unit for high-efficiency solar cells: The Effect of an auxiliary acceptor on the absorption and the energy level alignment, Chem. – Eur. J., 2012, 18, 8190–8200.
125. K. Pei, Y. Wu, A. Islam, S. Zhu, L. Han, Z. Geng and W. Zhu, Dye-sensitized solar cells based on quinoxaline dyes: Effect of π-linker on absorption, energy levels, and photovoltaic performances, J. Phys. Chem. C, 2014, 118, 16552–16561.
126. H. Zhu, B. Liu, J. Liu, W. Zhang and W.-H. Zhu, D–A–π–A featured sensitizers by modification of auxiliary acceptor for preventing “trade-off” effect, J. Mater. Chem. C, 2015, 3, 6882–6890.
127. A. Amacher, C. Yi, J. Yang, M. P. Bircher, Y. Fu, M. Cascella, M. Grätzel, S. Decurtins and S.-X. Liu, A quinoxaline-fused tetrathiafulvalene-based sensitizer for efficient dye-sensitized solar cells, Chem. Commun., 2014, 50, 6540–6542.
128. C. A. Richard, Z. Pan, A. Parthasarathy, F. A. Arroyave, L. A. Estrada, K. S. Schanze and J. R. Reynolds, Quadrupolar (donor)₂acceptor-acid chromophores for dye-sensitized solar cells: influence of the core acceptor, J. Mater. Chem. A, 2014, 2, 9866–9874.
129. C. A. Richard, Z. Pan, H.-Y. Hsu, S. Cekli, K. S. Schanze and J. R. Reynolds, Effect of isomerism and chain length on electronic structure, photophysics, and sensitizer efficiency in quadrupolar (donor)₂–acceptor systems for application in dye-sensitized solar cells, ACS Appl. Mater. Interfaces, 2014, 6, 5221–5227.
130. H. Jiang, Y. Ren, W. Zhang, Y. Wu, E. C. Socie, B. I. Carlsen, J.-E. Moser, H. Tian, S. M. Zakeeruddin, W.-H. Zhu and M. Grätzel, Phenanthrene-fused-quinoxaline as a key building block for highly efficient and stable sensitizers in copper-electrolyte-based dye-sensitized solar cells, Angew. Chem., Int. Ed., 2020, 59, 9324–9329.
131. S.-R. Li, C.-P. Lee, C.-W. Liao, W.-L. Su, C.-T. Li, K.-C. Ho and S.-S. Sun, Structural engineering of dipolar organic dyes with an electron-deficient diphenylquinoxaline moiety for efficient dye-sensitized solar cells, Tetrahedron, 2014, 70, 6276–6284.
132. S.-R. Li, C.-P. Lee, P.-F. Yang, C.-W. Liao, M. M. Lee, W.-L. Su, C.-T. Li, H.-W. Lin, K.-C. Ho and S.-S. Sun, Structure–performance correlations of organic dyes with an electron-deficient diphenylquinoxaline moiety for dye-sensitized solar cells, Chem. – Eur. J., 2014, 20, 10052–10064.
133. L.-P. Zhang, K.-J. Jiang, G. Li, Q.-Q. Zhang and L.-M. Yang, Pyrazino[2,3-g]quinoxaline dyes for solar cell applications, J. Mater. Chem. A, 2014, 2, 14852–14857.
134. L.-P. Zhang, K.-J. Jiang, Q. Chen, G. Li and L.-M. Yang, Pyrazino-[2,3-f][1,10]phenanthroline as a new anchoring group of organic dyes for dye-sensitized solar cells, RSC Adv., 2015, 5, 46206–46209.
135. L. Wang, X. Yang, X. Wang and L. Sun, Novel organic dyes with anchoring group of quinoxaline-2,3-diol and the application in dye-sensitized solar cells, Dyes Pigm., 2015, 113, 581–587.
136. W. Zhang, X. Yang, J. An, L. Zhang, B. Cai and K. Yang, Effect of Side Substituents incorporated into π-bridges of quinoxaline-based sensitizers for dye-sensitized solar cells, Energy Technol., 2020, 8, 2000032.
137. X. Li, Y. Hu, I. Sanchez-Molina, Y. Zhou, F. Yu, S. A. Haque, W. Wu, J. Hua, H. Tian and N. Robertson, Insight into quinoxaline containing D–π–A dyes for dye-sensitized solar cells with cobalt and iodine based electrolytes: The effect of π-bridge on the HOMO energy level and photovoltaic performance, J. Mater. Chem. A, 2015, 3, 21733–21743.
138. X. Li, Y. Zhou, J. Chen, J. Yang, Z. Zheng, W. Wu, J. Hua and H. Tian, Stacked graphene platelet nanofibers dispersed in the liquid electrolyte of highly efficient cobalt-mediator-based dye-sensitized solar cells, Chem. Commun., 2015, 51, 10349–10352.
139. Z.-S. Huang, X.-F. Zang, T. Hua, L. Wang, H. Meier and D. Cao, 2,3-Dipentyldithieno[3,2-f:2′,3′-h]quinoxaline-based organic dyes for efficient dye-sensitized solar cells: Effect of π-bridges and electron donors on solar cell performance, ACS Appl. Mater. Interfaces, 2015, 7, 20418–20429.
140. L.-W. Ma, Z.-S. Huang, S. Wang, H. Meier and D. Cao, Impact of π-conjugation configurations on the photovoltaic performance of the quinoxaline-based organic dyes, Dyes Pigm., 2017, 145, 126–135.
141. H.-X. Ji, Z.-S. Huang, L. Wang and D. Cao, Quinoxaline-based organic dyes for efficient dye-sensitized solar cells: Effect of different electron-withdrawing auxiliary acceptors on the solar cell performance, Dyes Pigm., 2018, 159, 8–17.
142. T. Xiao, Z.-S. Huang, L. Wang and D. Cao, Impact of π-spacers of dithieno[3,2-f:2′,3′-h]quinoxaline-based organic dyes with three π-spacers on the solar cell performance, J. Mater. Sci.: Mater. Electron., 2019, 30, 647–657.
143. J.-S. Ni, W.-S. Kao, H.-J. Chou and J. T. Lin, Organic dyes incorporating the dithieno[3,2-f:2′,3′-h]quinoxaline moiety for dye-sensitized solar cells, ChemSusChem, 2015, 8, 2932–2939.
144. X. Qian, X. Lan, R. Yan, Y. He, J. Huang and L. Hou, T-shaped (D)₂–A–π–A type sensitizers incorporating indoloquinoxaline and triphenylamine for organic dye-sensitized solar cells, Electrochim. Acta, 2017, 232, 377–386.
145. D. Kiymaz, M. Sezgin, E. Sefer, C. Zafer and S. Koyuncu, Carbazole based D–A–π–A chromophores for dye sensitized solar cells: Effect of the side alkyl chain length on device performance, Int. J. Hydrogen Energy, 2017, 42, 8569–8575.
146. R. Wang, Q. Chen, H. Feng and B. Liu, Simple adjustments to the molecular planarity of organic sensitizers: Towards highly selective optimization of energy levels, New J. Chem., 2017, 41, 11853–11859.
147. F. Lu, G. Yang, Q. Xu, J. Zhang, B. Zhang and Y. Feng, Tailoring the benzotriazole (BTZ) auxiliary acceptor in a D–A′–π–A type sensitizer for high performance dye-sensitized solar cells (DSSCs), Dyes Pigm., 2018, 158, 195–203.
148. L. Huang, P. Ma, G. Deng, K. Zhang, T. Ou, Y. Lin and M. S. Wong, Novel electron-deficient quinoxalinedithienothiophene- and phenazinedithienothiophene-based photosensitizers: The effect of conjugation expansion on DSSC performance, Dyes Pigm., 2018, 159, 107–114.
149. S.-G. Chen, H.-L. Jia, X.-H. Ju and H.-G. Zheng, The impact of adjusting auxiliary donors on the performance of dye-sensitized solar cells based on phenothiazine D–D–π–A sensitizers, Dyes Pigm., 2017, 146, 127–135.
150. C. Y. Jung, C. J. Song, W. Yao, J. M. Park, I. H. Hyun, D. H. Seong and J. Y. Jaung, Synthesis and performance of new quinoxaline-based dyes for dye sensitized solar cell, Dyes Pigm., 2015, 121, 204–210.
151. J. M. Park, C. Y. Jung, Y. Wang, H. D. Choi, S. J. Park, P. Ou, W.-D. Jang and J. Y. Jaung, Effect of regioisomeric substitution patterns on the performance of quinoxaline-based dye-sensitized solar cells, Electrochim. Acta, 2019, 298, 650–662.
152. J. M. Park, C. Y. Jung, Y. Wang, H. D. Choi, S. J. Park, P. Ou, W.-D. Jang and J. Y. Jaung, Effect of additional phenothiazine donor and thiophene π-bridge on photovoltaic performance of quinoxaline cored photosensitizers, Dyes Pigm., 2019, 170, 107568.
153. M. Xu, X. Hu, Y. Zhang, X. Bao, A. Pang and J.-K. Fang, Novel organic dyes featuring spiro[dibenzo[3,4:6,7]cyclohepta[1,2-b]quinoxaline-10,9′-fluorene] (SDBQX) as a rigid moiety for dye-sensitized solar cells, ACS Appl. Energy Mater., 2018, 1, 2200–2207.
154. J.-K. Fang, M. Xu, X. Hu, C. Wu, S. Lu, H.-J. Yu, X. Bao, Y. Wang, G. Shao and W. Liu, Aggregation-free organic dyes featuring spiro[dibenzo[3,4:6,7]cyclohepta[1,2-b]quinoxaline-10,9′-fluorene] (SDBQX) for dye-sensitized solar cells, Global Challenges, 2019, 3, 1900034.
155. M. L. Jiang, J.-J. Wen, Z.-M. Chen, W.-H. Tsai, T.-C. Lin, T. J. Chow and Y. J. Chang, High-performance organic dyes with electron-deficient quinoxalinoid heterocycles for dye-sensitized solar cells under one sun and indoor light, ChemSusChem, 2019, 12, 3654–3665.
156. L. Lyu, R. Su, S. Y. Al-Qaradawi, K. A. Al-Saad and A. El-Shafei, Three-component one-pot reaction for molecular engineering of novel cost-effective highly rigid quinoxaline-based photosensitizers for highly efficient DSSCs application: Remarkable photovoltage, Dyes Pigm., 2019, 171, 107683.
157. R. Su, S. Ashraf, L. Lyu and A. El-Shafei, Tailoring dual-channel anchorable organic sensitizers with indolo[2,3-b]quinoxaline moieties: Correlation between structure and DSSC performance, Sol. Energy, 2020, 206, 443–454.
158. R. Su, L. Lyu, M. R. Elmorsy and A. El-Shafei, Structural studies and photovoltaic investigation of indolo[2,3-b]quinoxaline-based sensitizers/co-sensitizers achieving highly efficient DSSCs, New J. Chem., 2020, 44, 2797–2812.
159. M.-R. Kim, T. C. Pham, H.-S. Yang, S. H. Park, S. Yang, M. Park, S. G. Lee and S. Lee, Photovoltaic effects of dye-sensitized solar cells using double-layered TiO₂ photoelectrodes and pyrazine-based photosensitizers, ACS Omega, 2023, 8, 14699–14709.
160. Y. Hu, A. Abate, Y. Cao, A. Ivaturi, S. M. Zakeeruddin, M. Grätzel and N. Robertson, High absorption coefficient cyclopentadithiophene donor-free dyes for liquid and solid-state dye-sensitized solar cells, J. Phys. Chem. C, 2016, 120, 15027–15034.
161. A. Venkateswararao, P. Tyagi, K. R. Justin Thomas, P.-W. Chen and K.-C. Ho, Organic dyes containing indolo[2,3-b]quinoxaline as a donor: synthesis, optical and photovoltaic properties, Tetrahedron, 2014, 70, 6318–6327.
162. X. Qian, H.-H. Gao, Y.-Z. Zhu, L. Lu and J.-Y. Zheng, 6H-Indolo[2,3-b]quinoxaline-based organic dyes containing different electron-rich conjugated linkers for highly efficient dye-sensitized solar cells, J. Power Sources, 2015, 280, 573–580.
163. X. Qian, X. Wang, L. Shao, H. Li, R. Yan and L. Hou, Molecular engineering of D–D–π–A type organic dyes incorporating indoloquinoxaline and phenothiazine for highly efficient dye-sensitized solar cells, J. Power Sources, 2016, 326, 129–136.
164. M. Caicedo, C. A. Echeverry, R. R. Guimarães, A. Ortiz, K. Araki and B. Insuasty, Microwave assisted synthesis of a series of charge-transfer photosensitizers having quinoxaline-2(1H)-one as anchoring group onto TiO₂ surface, J. Mol. Struct., 2017, 1133, 384–391.
165. S. Eu, S. Hayashi, T. Umeyama, Y. Matano, Y. Araki and H. Imahori, Quinoxaline-fused porphyrins for dye-sensitized solar cells, J. Phys. Chem. C, 2008, 112, 4396–4405.
166. A. Kira, Y. Matsubara, H. Iijima, T. Umeyama, Y. Matano, S. Ito, M. Niemi, N. V. Tkachenko, H. Lemmetyinen and H. Imahori, Effects of π-elongation and the fused position of quinoxaline-fused porphyrins as sensitizers in dye-sensitized solar cells on optical, electrochemical, and photovoltaic properties, J. Phys. Chem. C, 2010, 114, 11293–11304.
167. H. Imahori, H. Iijima, H. Hayashi, Y. Toude, T. Umeyama, Y. Matano and S. Ito, Bisquinoxaline-fused porphyrins for dye-sensitized solar cells, ChemSusChem, 2011, 4, 797–805.
168. H. Hayashi, A. S. Touchy, Y. Kinjo, K. Kurotobi, Y. Toude, S. Ito, H. Saarenpää, N. V. Tkachenko, H. Lemmetyinen and H. Imahori, Triarylamine-substituted imidazole- and quinoxaline-fused push–pull porphyrins for dye-sensitized solar cells, ChemSusChem, 2013, 6, 508–517.
169. S. Fan, K. Lv, H. Sun, G. Zhou and Z.-S. Wang, The position effect of electron-deficient quinoxaline moiety in porphyrin based sensitizers, J. Power Sources, 2015, 279, 36–47.
170. F. Lu, X. Wang, Y. Zhao, G. Yang, J. Zhang, B. Zhang and Y. Feng, Studies on D–A–π–A structured porphyrin sensitizers with different additional electron-withdrawing unit, J. Power Sources, 2016, 333, 1–9.
171. B. L. Watson, B. D. Sherman, A. L. Moore, T. A. Moore and D. Gust, Enhanced dye-sensitized solar cell photocurrent and efficiency using a Y-shaped, pyrazine-containing heteroaromatic sensitizer linkage, Phys. Chem. Chem. Phys., 2015, 17, 15788–15796.
172. S. H. Lee, A. J. Matula, G. Hu, J. L. Troiano, C. J. Karpovich, R. H. Crabtree, V. S. Batista and G. W. Brudvig, Strongly coupled phenazine–porphyrin dyads: Light-harvesting molecular assemblies with broad absorption coverage, ACS Appl. Mater. Interfaces, 2019, 11, 8000–8008.
173. H. Shahroosvand and M. Eskandari, Ultrafast interfacial charge transfer from the LUMO+1 in ruthenium(II) polypyridyl quinoxaline-sensitized solar cells, Dalton Trans., 2018, 47, 561–576.
174. B. Pashaei, H. Shahroosvand, M. Graetzel and M. K. Nazeeruddin, Influence of ancillary ligands in dye-sensitized solar cells, Chem. Rev., 2016, 116, 9485–9564.
175. X. Lu, G. Zhou, H. Wang, Q. Feng and Z.-S. Wang, Near infrared thieno[3,4-b]pyrazine sensitizers for efficient quasi-solid-state dye-sensitized solar cells, Phys. Chem. Chem. Phys., 2012, 14, 4802–4809.
176. J. Wu, G. Li, L. Zhang, G. Zhou and Z.-S. Wang, Energy level engineering of thieno[3,4-b]pyrazine based organic sensitizers for quasi-solid-state dye-sensitized solar cells, J. Mater. Chem. A, 2016, 4, 3342–3355.
177. T. Kono, T. N. Murakami, J.-I. Nishida, Y. Yoshida, K. Hara and Y. Yamashita, Synthesis and photo-electrochemical properties of novel thienopyrazine and quinoxaline derivatives, and their dye-sensitized solar cell performance, Org. Electron., 2012, 13, 3097–3101.
178. M. B. Desta, N. S. Vinh, C. H. Pavan Kumar, S. Chaurasia, W.-T. Wu, J. T. Lin, T.-C. Wei and E. Wei-Guang Diau, Pyrazine-incorporating panchromatic sensitizers for dye sensitized solar cells under one sun and dim light, J. Mater. Chem. A, 2018, 6, 13778–13789.
179. H. Cheema, A. Peddapuram, R. E. Adams, L. McNamara, L. A. Hunt, N. Le, D. L. Watkins, N. I. Hammer, R. H. Schmehl and J. H. Delcamp, Molecular engineering of near infrared absorbing thienopyrazine double donor double acceptor organic dyes for dye-sensitized solar cells, J. Org. Chem., 2017, 82, 12038–12049.
180. D. Franchi, M. Bartolini, F. D’Amico, M. Calamante, L. Zani, G. Reginato, A. Mordini and A. Dessì, Exploring different designs in thieno[3,4-b]pyrazine-based dyes to enhance divergent optical properties in dye-sensitized solar cells, Processes, 2023, 11, 1542.
181. W. Ying, J. Yang, M. Wielopolski, T. Moehl, J.-E. Moser, P. Comte, J. Hua, S. M. Zakeeruddin, H. Tian and M. Grätzel, New pyrido[3,4-b]pyrazine-based sensitizers for efficient and stable dye-sensitized solar cells, Chem. Sci., 2014, 5, 206–214.
182. B. Liu, F. Giordano, K. Pei, J.-D. Decoppet, W.-H. Zhu, S. M. Zakeeruddin and M. Grätzel, Molecular engineering of pyrido[3,4-b]pyrazine-based donor–acceptor–π–acceptor organic sensitizers: Effect of auxiliary acceptor in cobalt- and iodine-based electrolytes, Chem. – Eur. J., 2015, 21, 18654–18661.
183. Y. Gao, X. Li, Y. Hu, Y. Fan, J. Yuan, N. Robertson, J. Hua and S. R. Marder, Effect of an auxiliary acceptor on D–A–π–A sensitizers for highly efficient and stable dye-sensitized solar cells, J. Mater. Chem. A, 2016, 4, 12865–12877.
184. X. Zhang, J. Mao, D. Wang, X. Li, J. Yang, Z. Shen, W. Wu, J. Li, H. Ågren and J. Hua, Comparative study on pyrido[3,4-b]pyrazine-based sensitizers by tuning bulky donors for dye-sensitized solar cells, ACS Appl. Mater. Interfaces, 2015, 7, 2760–2771.
185. Y. Zhou, X. Li, X. Li, J. Chen, F. Yu and J. Hua, The effect of pyridyl nitrogen atom position in pyrido[3,4-b]pyrazines in donor–acceptor–π–acceptor dyes on absorption, energy levels, and photovoltaic performances of dye-sensitized solar cells, Asian J. Org. Chem., 2016, 5, 293–300.
186. P. S. Saud, A. Bist, A. A. Kim, A. Yousef, A. Abutaleb, M. Park, S.-J. Park and B. Pant, Dye-sensitized solar cells: Fundamentals, recent progress, and optoelectrical properties improvement strategies, Opt. Mater., 2024, 150, 115242.
187. A. Sen and A. Groβ, Effect of electron-withdrawing/-donating groups on the sensitizing action of the novel organic dye “3-(5-(4-(diphenylamino)styryl)thiophen-2-yl)-2-cyanoacrylic acid” for N-type dye-sensitized solar cells: A theoretical study, J. Phys. Chem. C, 2020, 124, 8526–8540.
188. T. Hua, K. Zhang, Z.-S. Huang, L. Wang, H. Tang, H. Meier and D. Cao, Effect of structural engineering of π-spacers on anti-aggregation of D–A–π–A dyes, J. Mater. Chem. C, 2019, 7, 10379–10388.
189. X. Qian, Y.-Z. Zhu, W.-Y. Chang, J. Song, B. Pan, L. Lu, H.-H. Gao and J.-Y. Zheng, Benzo[a]carbazole-based donor–π–acceptor type organic dyes for highly efficient dye-sensitized solar cells, ACS Appl. Mater. Interfaces, 2015, 7, 9015–9022.
190. P. Naik, M. R. Elmorsy, R. Su, D. D. Babu, A. El-Shafei and A. V. Adhikari, New carbazole based metal-free organic dyes with D–π–A–π–A architecture for DSSCs: Synthesis, theoretical and cell performance studies, Sol. Energy, 2017, 153, 600–610.
191. F. Liu, Y. Yang, S. Cong, H. Wang, M. Zhang, S. Bo, J. Liu, Z. Zhen, X. Liu and L. Qiu, Comparison of second-order nonlinear optical chromophores with D–π–A, D–A–π–A and D–D–π–A architectures: diverse NLO effects and interesting optical behavior, RSC Adv., 2014, 4, 52991–52999.
192. C. Lin, Y. Liu, G. Wang, K. Li, H. Xu, W. Zhang, C. Shao and Z. Yang, Novel dyes design based on first principles and the prediction of energy conversion efficiencies of dye-sensitized solar cells, ACS Omega, 2021, 6, 715–722.
193. X. Ran, C. Zhao, M. Yin, K. Li, X. Yu and L. Jin, Design and screening on D–A–π–A type sensitizers for dye-sensitized solar cells: Effects of various donors on photovoltaic performances, Comput. Theor. Chem., 2025, 1245, 115101.
194. A. R. Al-Marhabi, R. M. El-Shishtawy, K. O. Al-Footy, E. Atilgan, A. Atilgan and A. Yildiz, The impact of aryl amine donors on the performance of dye-sensitized solar cells based on quinoxaline D–π–A–π–A sensitizers, J. Mol. Struct., 2025, 1333, 141746.
195. N. Martsinovich and A. Troisi, Theoretical studies of dye-sensitised solar cells: from electronic structure to elementary processes, Energy Environ. Sci., 2011, 4, 4473–4495.
196. L. Zhang and J. M. Cole, Anchoring groups for dye-sensitized solar cells, ACS Appl. Mater. Interfaces, 2015, 7, 3427–3455.
197. G. Richhariya, A. Kumar, A. K. Shukla, K. N. Shukla and B. C. Meikap, Effect of different counter electrodes on power conversion efficiency of DSSCs, J. Electron. Mater., 2023, 52, 60–71.
198. C.-Y. Hsu, H. N. K. Al-Salman, Z. H. Mahmoud, R. M. Ahmed and A. F. Dawood, Improvement of the photoelectric dye sensitized solar cell performance using Fe/S–TiO2 nanoparticles as photoanode electrode, Sci. Rep., 2024, 14, 4931.

---