Wide bandgap polymer donors for high efficiency non-fullerene acceptor based organic solar cells

Abstract

In the past few years, the power conversion efficiency (PCE) of organic solar cells (OSCs) has improved rapidly with the milestone value exceeding 18%, primarily owing to the development of novel non-fullerene acceptors (NFAs) as well as matching polymer donors. The molecular structure of a polymer donor fundamentally determines its molecular packing (crystal structure and morphology) and optoelectronic properties, which influence the photovoltaic processes and the ultimate PCE of the OSC device. The structure–property–cell performance relationships of polymer donors with respect to the specific acceptor are very complex, involving numerous parameters, but are extremely important towards the development of high-performance polymer donors to achieve high PCE. This review provides a timely analysis of the top-performing wide bandgap (WBG) polymer donors that have been developed to match the three most representative narrow bandgap NFAs, ITIC, IT-4F, and Y6, in terms of their structural design, fine-tuning of their optoelectronic properties, and control of the morphology and crystallinity of their blends with NFAs. We hope that this article provides deeper insight into the structure–property–cell performance relationships of polymer donors and a collection of useful guidelines and strategies for the design and processing of novel polymer donors for matching with NFAs for achieving ultrahigh performance OSCs.

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1 Introduction

Organic solar cells (OSCs) have attracted much attention as a promising technology to convert solar energy into electricity because of their advantages in fabricating flexible, lightweight, large-area, and low-cost solar cells.^1–4 Since the first report by Heeger et al.⁵ in 1995, OSCs with a bulk heterojunction (BHJ) structure composed of a blend active layer, comprising a p-type conjugated polymer as a donor and an n-type organic semiconductor as an acceptor, have attracted tremendous attention due to their excellent solution processability and mechanical properties that allow high throughput, roll-to-roll manufacturing. Significant improvements in the OSC performance (with the highest PCE exceeding 18%) have been achieved, largely by the judicious design and delicate synthesis of matching polymer donor and acceptor materials.^6–10

The first generation acceptor materials for BHJ OSCs comprise fullerene derivatives such as phenyl-C₆₁ (or C₇₁)-butyric acid methyl ester (PC₆₁BM or PC₇₁BM), which were first developed by Wudl et al.¹¹ They have good solubility in organic solvents, high electron mobility (μ_e), and high electron affinity.^12,13 Among various polymer donors developed to match these acceptors, a benzo[1,2-b:4,5-b′]dithiophene (BDT)-based polymer, PTB7, developed by Liang et al.⁷ in 2010, showed a high PCE of 7.4% and 9.2% using a conventional and inverted device structure, respectively, when blended with PC₇₁BM.^7,14 The highest PCE of 11.7% for single-junction binary-blend fullerene-based OSCs was achieved using PffBT4T-C₉C₁₃ as a donor by Zhao et al. in 2016.¹⁵ Since then, the development of fullerene-based OSCs has become subdued because of their weak absorption in the visible spectral region, limited energy level tunability, and inadequate long-term stability of the devices caused by the susceptibility to dimerization and gradual aggregation.

To overcome the drawbacks of fullerene-based acceptors, non-fullerene acceptors (NFAs) have been developed.^16,17 ITIC, developed by Lin et al. in 2015, is one of the most efficient NFAs. It has a rigid indacenodithienothiophene (IDTT) central unit and a narrow bandgap of 1.59 eV.⁸ OSCs using ITIC as an acceptor and PTB7-Th as a donor showed a moderate PCE of 6.8% due to their similar absorption range with poor absorption in the shorter wavelength region of the solar spectrum. The PCE of ITIC-based OSCs rapidly improved to ∼10% when WBG polymer donors with complementary absorption such as J51 (9.26%),¹⁸ J61 (9.53%),¹⁹ and PBDB-T (11.21%)¹⁶ were used. Recently, a new WBG polymer donor, PBTA-PSF, was developed by Li et al.²⁰ to match with ITIC to realize complementary absorption. OSCs based on PBTA-PSF:ITIC achieved a PCE of 13.91%, which is by far the highest among ITIC-based OSCs. At the same time, incorporation of electron-donating (e.g., methyl in IT-M and IT-DM)²¹ and electron-withdrawing (e.g., fluorine in IT-4F)²² groups, side chain engineering of the ITIC core structure (e.g. m-ITIC,²³ ITIC2,²⁴ ITIC-Th,²⁵ ITIC-Th1,²⁶ and IDIC²⁷) and optimization of the central core (AOIC,²⁸ INIC,²⁹ FNIC,³⁰ and FOIC³¹) further improved the PCEs when appropriate matching polymer donors were used. In 2019, Yuan et al.⁹ developed a new high performance NFA, Y6, which has a slightly electron-deficient dithienothiophen[3.2-b]-pyrrolobenzothiadiazole core that helps achieve a narrower bandgap of 1.33 eV compared to ITIC. Y6 achieved a very high PCE of 15.7% when blended with donor polymer PM6 owing to the largely improved photocurrent. The record PCE of 18.22% was obtained using Y6 as an acceptor and a novel WBG (1.98 eV) polymer D18 as a donor.¹⁰

Because BHJ OSCs utilize both donor and acceptor materials in the active layer blend, matching of the optoelectronic properties (frontier molecular orbital (FMO) energy levels, optical absorption, etc.) and morphological compatibility (miscibility, phase separation, crystallinity, etc.) between donor and acceptor materials are vital for achieving high photovoltaic performance. Compared to the previous review articles that focus on donors^4,32–35 and acceptors,^36–41 respectively, this review will provide a perspective from a different angle by placing emphasis on the matching between donors and acceptors to gain a better understanding of the relationships between the structures, properties, and device performances of representative donors with different prominent acceptor materials.

Firstly, we briefly discuss the working mechanism of OSCs and the general relationships between the molecular structure, nano-/microstructure, properties, and cell performance. Next, we select and classify some high-performance polymer donors that have been used to match the three most representative NFAs, ITIC, IT-4F, and Y6, to achieve PCEs above 10%. Then, we discuss the properties and photovoltaic performances of different donors and provide some guidelines for the design and processing of polymer donors to match with a certain acceptor to achieve high solar cell performance. Finally, a summary of the key findings in terms of the structure–property–cell performance relations of WBG high-performance polymer donors and an outlook for the future development of this type of materials and OSCs in general are provided.

2 Structure–property–cell performance relationships of polymer donors

A typical OSC device is made of an electron donor (commonly a polymer material) and an electron acceptor (commonly a small molecule material) that form a BHJ (photo)active layer with interpenetrating donor and acceptor phases at the nanometer scale.⁵ This active layer is sandwiched between an anode and a cathode, which collect the holes and electrons generated in the active layer, respectively. Usually, a hole transport layer (HTL) such as PEDOT:PSS or MoO₃ is inserted between the active layer and the anode to facilitate the collection of holes and/or blocking of the electrons. A counterpart electron transport layer (ETL) such as ZnO is placed between the active layer and the cathode to extract electrons and/or block holes. At least one electrode, the cathode or anode, is made of a transparent conductor (TC) such as indium-doped tin oxide (ITO) to allow transmission of light to reach the active layer. The two most commonly adopted OSC architectures are shown in Fig. 1a and b; the conventional one has a TC anode (Fig. 1a), while the inverted one has a TC cathode (Fig. 1b).

Fig. 1 (a) Conventional and (b) inverted OSC architectures; (c) solar cell figures-of-merit: short circuit current density (J_SC, mA cm⁻²), open circuit voltage (V_OC, V), fill factor (FF) and PCE;⁴² and (d) working mechanism of the BHJ layer in OSCs (red and blue areas represent donor and acceptor domains, respectively).⁴² Reproduced from ref. 42 with permission from John Wiley and Sons, copyright 2019.

The donor material in the active layer is a p-type semiconductor that has relatively high-lying highest occupied molecular orbital (E_HOMO) and lowest unoccupied molecular orbital (E_LUMO) energy levels, which can stabilize and transport the photogenerated holes. On the contrary, the acceptor material is an n-type semiconductor that has low E_HOMO and E_LUMO with respect to those of the donor material, which can stabilize and transport the photogenerated electrons. One or both of the donor and acceptor materials should strongly absorb sunlight.

The ultimate photovoltaic performance parameter of an OSC is the PCE or η, which is the percentage of electric energy generated by an OSC out of the total photo energy incident on the active layer conventionally under the standardized AM 1.5G solar spectrum with an intensity of 100 mW cm⁻². As shown in Fig. 1c, the PCE is contributed by three factors, the short circuit current density (J_SC, mA cm⁻²), open circuit voltage (V_OC, V), and fill factor (FF), obtained from the current density–voltage (J–V) curve of an OSC based on the relationship PCE = (J_SC × V_OC × FF)/P_in, where P_in is the input power density (100 mW cm⁻²) of the light source under AM 1.5G conditions. The FF represents the ratio of the product of J_mp × V_mp (at the maximum power point on the J–V curve) over the product of J_SC and V_OC. The FF values are typically 0.5–0.7 for high-performing OSCs. Broader light absorption in the solar spectrum by the active layer increases J_SC of the OSC; however, absorption of long wavelength (lower energy) photons results in a lower V_OC. Therefore, it is preferred that an active layer absorbs the maximum number of photons in the low wavelength region of ∼400–925 nm (or energies of ∼3–1.34 eV) based on the Shockley–Queisser (S–Q) limit.⁴³ The portion of sunlight with wavelengths longer than 925 nm can be effectively utilized by developing a tandem OSC with two or more OSC devices combined.^44–47 Research into tandem OSCs has also attracted much attention, but faces numerous issues since deposition of an increased number of well-defined thin active layers and interlayers is very challenging and the charge transport in these multilayered devices is too complicated to control. Although tandem OSCs can achieve higher PCE intrinsically, single junction OSCs still perform better at the moment.

A typical OSC device undergoes photovoltaic processes as follows (Fig. 1d):^48–50 (1) excitons (hole–electron pairs) are produced in the BHJ active layer (in the donor, the acceptor, or both phases) upon absorption of photons from sunlight; (2) the formed excitons diffuse towards the donor–acceptor (D–A) interfaces; (3) the excitons are dissociated into free holes and electrons at the D–A interfaces; and (4) the free holes and electrons travel through the donor and acceptor phases and are collected at the anode and cathode, respectively.

An OSC works through close collaboration of matching donor and acceptors. Fig. 2 illustrates the complex hierarchical relationships between the cell performance, properties, and the molecular and nano-/microstructures of polymer donor materials. Each of the cell performance parameters, J_SC, V_OC, or FF, is influenced by the photovoltaic processes and material properties, which are ultimately governed by the molecular structure of the polymer donor.

Fig. 2 Scheme of the structure–property–cell performance relationships of a polymer donor for OSCs, where α is the absorption coefficient, ε_r is the dielectric constant (or relative permittivity), μ is the mobility, and MW and Đ are the molecular weight and its distribution, respectively, of the polymer donor. Defects may include terminal groups, homo coupled units in a copolymer, random arrangements of comonomers in a copolymer, regio-irregular units, branching, lightly cross-linked units, oligomers, etc. Some or all of these relationships may apply to small molecule donors as well as polymer and small molecule acceptors.

Specifically, J_SC is influenced by all the photovoltaic processes, where (1) the number of excitons generated (or photons absorbed) depends largely on the absorption coefficient (α) and the bandgap (E_g) of the donor; (2) exciton diffusion is influenced by the electronic structure of the conjugated building block, the dielectric constant (ε_r), and the morphology (phase size) of the donor; (3) exciton dissociation is influenced by the electronic structure of the building block, the dielectric constant, and the HOMO and LUMO energy offsets, ΔE_HOMO and ΔE_LUMO, between the donor and the acceptor; and (4) transport and collection of charge carriers (holes) are determined by the hole mobility (μ_h) of the donor and its balance with the electron mobility (μ_e) of the acceptor. A higher dielectric constant can decrease the exciton binding energy, reducing exciton recombination events. If the donor phase is too large, excitons generated in a region with a distance to the donor and acceptor interface greater than the exciton diffusion length would recombine, leading to a reduction in J_SC.

V _OC is determined by the difference between E_LUMO of the acceptor and E_HOMO of the donor as well as the dielectric constant of the donor (and acceptor). A lower E_HOMO of the donor helps achieve a high V_OC, while a large dielectric constant can reduce the exciton binding energy, which would help reduce the required donor–acceptor energy offset ΔE_LUMO for exciton dissociation, achieving a higher V_OC. Additionally, the type of building block (electronic structure) often plays a critical role in determining the minimal energy offset required for exciton dissociation, which is directly related to V_OC.

The FF is critically influenced by μ_h of the donor and its balance with μ_e of the acceptor. High and balanced μ_h and μ_e of >10⁻⁴ cm² V⁻¹ s⁻¹ are usually required to achieve a high FF. The morphology of the donor and acceptor blend film also exerts some influence on the FF.^51–53 The carrier mobility is influenced by several factors including the building block, side chain, crystallinity (degree of crystallinity and crystal orientation), film morphology, and amount of structural defects. A face-on lamellar packing motif for the polymer donor in an OSC is desirable because the charge transport is facilitated through the vertically aligned π–π stacks in this type of crystal motif. Structural defects refer to the terminal groups, homo coupled units in a copolymer, random arrangements of comonomers in a copolymer, regio-irregular units, branching, lightly cross-linked units, oligomers, etc. Most structural defects are difficult to determine but are often largely responsible for the poor cell performance since they have adverse effects on the crystallinity, morphology, and charge transport.

Additionally, although not shown in Fig. 2, the properties of the interfaces between the active layer and the HTL and/or ETL determine the efficiency of charge collection, which influences J_SC, V_OC, and/or the FF.

As can be clearly seen in Fig. 2, the molecular structure fundamentally dictates the nano-/microstructure, properties, and OSC performance of a polymer donor. In particular, the π-conjugated building block used to construct a donor determines all the properties of the donor and the performance parameters of its OSC.

Characterization of the structures and properties of donors, acceptors, and their blends is nontrivial and often encounters great challenges. The widely used methodologies for characterization of their key structural features and properties are briefly described as follows:

(1) E_g: UV-vis-NIR spectroscopy is a simple and the most widely used method to obtain the bandgap of the donor or acceptor using the absorption onset wavelength. However, many materials, particularly polymers, are non-monodisperse and contain disordered structures, resulting in a tail in their absorption spectra with ill-defined onset edges. Measurements using the intersection of the normalized absorption and emission spectra can overcome this issue.^54,55 Alternatively, E_g can be calculated from the onset of the EQE spectrum of the OSC device to minimize the influence of the film morphology.⁵⁵

(2) E_HOMO/E_LUMO: due to its low-cost and easy operation, cyclic voltammetry (CV) is the most popular method used to estimate E_HOMO and E_LUMO of the donor or acceptor by using the onset oxidation and reduction potentials, respectively, with a reference having a known E_HOMO such as ferrocene (E_HOMO = −4.8 eV). However, most donors and acceptors only show the oxidation or the reduction process, respectively, in their CV diagrams. In such cases, the optical bandgap obtained by UV-vis-NIR and E_HOMO or E_LUMO obtained by CV are usually combined to calculate E_LUMO or E_HOMO of the donor or acceptor, respectively. The energy levels calculated in this way are often inaccurate because of the large exciton binding energy (up to ∼0.3–1 eV) associated with organic semiconductors. The energy levels determined by CV measurements also have large variations caused by different experimental conditions^56,57 and thus the values reported for the same materials in the literature often vary largely. Nonetheless, the E_HOMO and E_LUMO values obtained by CV under similar conditions are still very useful and quite reliable for comparing the energy levels of different materials. A combination of ultraviolet photoelectron spectroscopy (UPS) and (low-energy) inverse photoemission spectroscopy (IPES), which are much more expensive and complex to operate compared to the electrochemical setup used for the CV measurements, can also be used to more accurately measure the ionization potential (IP) and electron affinity (EA) values, which correspond to E_HOMO and E_LUMO, respectively.^56–58

(3) Crystallinity: wide-angle X-ray diffraction (WAXD) is the standard characterization method to study the crystal structure. When this method is used for thin films (∼100 nm), a grazing incidence geometry is used and the method is termed as GIWAXS or GIXD. GIWAXS can be used to obtain information about the structure, orientation, and ordering of crystallites.⁵⁹ Particularly, two-dimensional (2-D) GIWAXS measured with a powerful synchrotron X-ray source can reveal the in-plane and out-of-plane crystal structures of the finely mixed BHJ blend film at the nanometer scale.^60–62

(4) Morphology/phase separation: atomic force microscopy (AFM) is a surface analysis technique commonly used to probe morphological features (on the nanometer to micron-scale) such as film roughness, phase segregation, and domain size. Transmission electron microscopy (TEM) has also been used to extract information about phase segregation, grain size, and grain connectivity.⁵⁹ Resonant soft X-ray scattering (RSoXS) is another evolving method to investigate multi-component, multi-phase systems like the active layer of an OSC, where the contrast of the donor and acceptor components can be tuned by selection of the incident X-ray energy to quantitatively reveal the molecular orientation, domain purity, and domain spacing.^59,60,63,64

(5) Exciton dissociation, collection, and recombination: the trend of photocurrent density J_ph (J_ph = J_L − J_D, where J_L is the current density under illumination and J_D is the current density in the dark) versus effective voltage (V_eff = V − V₀) gives insights into the charge generation and exciton dissociation characteristics of the device. Here V₀ is called the compensation voltage or the voltage at which J_ph = 0 and V is the applied voltage.^65,66J_ph reaches a saturation value (J_sat) with increasing V_eff, which means that all the photogenerated excitons are dissociated into free carriers and collected by the electrodes with the assistance of large reverse bias. Thus, the exciton dissociation probability, defined as P_diss = J_SC/J_sat, reflects the efficiency of exciton dissociation, charge transport, and charge collection.⁶⁵ The photoinduced charge transfer efficiency (from the donor to the acceptor or vice versa) can also be probed via photoluminescence (PL) quenching and decay measurements of the blend films relative to the neat films.^67,68

The light intensity dependence of V_OC can directly indicate the role of trap-assisted recombination (or SRH recombination) versus bimolecular recombination under open circuit conditions. For this analysis the slope S₁ (or ideality factor) is defined as , where q is the elementary charge, k is the Boltzmann constant, T is the absolute temperature, and P_light is the light intensity.^69–71 A value of S₁ close to 1 indicates more ideal recombination, whereas values >1 indicate more trap-assisted/SRH recombination in the device.^65,71 Moreover, J_SC of an OSC follows a power law dependence with respect to the light intensity (P_light) (i.e.), where S₂ is the exponential factor. A value of S₂ close to unity indicates negligible bimolecular recombination during sweep-out under short circuit conditions.^72–74

(6) Mobility: the methods for measuring the carrier mobility along the direction vertical to the film (relevant to the charge transport in OSC devices) include space-charge-limited current (SCLC),^75–77 time of flight (TOF),^78,79 carrier extraction by linearly increasing voltage (CELIV),^80,81 photogenerated charges in CELIV (photoCELIV),^82–84 and impedance spectroscopy (IS).^85,86 The SCLC method is most widely used.

(7) Dielectric constant: the exciton binding energy (E_b) estimated by the coulomb interaction depends inversely on the dielectric constant, ε_r: E_b = q²/4πε₀ε_rr, where q is the elementary charge, ε₀ is the permittivity of a vacuum, and r is the electron and hole separation distance.^87–92 Therefore, to facilitate exciton diffusion and dissociation, semiconductors with high dielectric constant values are preferred. The capacitance–voltage (C–V) measurement at different frequencies (impedance spectroscopy) of a film sandwiched between two electrodes can be performed to obtain the dielectric constant value.^93–98

3 High performance WBG polymer donors for NFAs

On the basis of the optical bandgap, polymer donors can be divided into narrow-bandgap or low-bandgap (LBG, bandgap <1.6 eV), medium-bandgap (MBG, 1.6 eV < bandgap < 1.8 eV) and wide-bandgap (WBG, bandgap >1.8 eV).³⁵ To form complementary absorption with narrow-bandgap NFAs, polymer donors with medium- or wide-bandgaps are needed. Regioregular, head-to-tail poly(3-hexylthiophene-2,5-diyl) (P3HT), which can be synthesized at low cost and shows high μ_h, has been extensively studied as a donor for OSCs based on fullerene acceptors.^50,99,100 However, P3HT has a rather high E_HOMO (ca. −5.1 eV), which leads to a low V_OC, limiting its solar cell performance.^101–105 On the other hand, donor–acceptor (D–A) polymers containing alternating D and A building blocks on the backbone (Fig. 3) have been extensively developed as donor materials since their optical bandgaps and FMO (i.e. HOMO and LUMO) energy levels can be conveniently tuned through intramolecular charge transfer (ICT) by choosing different D and A units.^{32,35,49,106–108} D–A polymers also have enhanced μ_h due to intermolecular D–A interactions. D–A polymers have shown far better solar cell performances than P3HT in both fullerene and NFA-based OSCs.

Fig. 3 Chemical structures of representative D and A building blocks used for high performance D–A polymers, where R₁ or R₂ is a side chain chosen from alkyl, alkoxy, alkylthio, and aryl groups, etc., and x or y is an appropriate integer.

The D building blocks mostly used to construct high-performing D–A polymer donors for NFA-based OSCs are thiophene-containing moieties such as thiophene (T), thieno[3,2-b]thiophene (TT), and benzo[1,2-b:4,5-b′]dithiophene (BDT) (Fig. 3), owing to their appropriate electron-donating effect to tune E_HOMO, lower steric effect to maintain backbone coplanarity, rich chemistry, and good chemical stability. Two or more D blocks are often used to constitute a D unit in a D–A polymer. The frequently used A building blocks include 3-fluorothieno[3,4-b]thiophene-2-carboxylate (FC-TT),¹⁰⁹ thieno[3.4-c]pyrrole-4,6-dione (TPD),¹¹⁰ benzo[1,2-c:4,5-c′]dithiophene-4,8-dione (BDD),¹¹¹ difluorobenzotriazole (FBTA),¹¹² quinoxaline (Qx),¹¹³ and benzothiadiazole (BT),¹¹⁴ which have an electron-withdrawing ester, imide, fluorine, diketone, and N-containing heterocycle substituting or fused to the thiophene or benzene ring. One or more linear or branched alkyl side chains (R₁ and R₂) are anchored to D, A, or both units to render the polymer soluble and control the packing of polymer chains in the solid state.

By analysing the chemical structures of high-performing D–A polymer donors, one would notice that the majority of them contain the fused ring structure BDT as the D building block. Therefore, in this review, the polymer donors are classified as BDT-based polymers and non-BDT based polymers. The BDT-based polymers are further classified according to the A units: (1) ester or imide substituted building blocks (FC-TT and TPD), (2) diketone derivatives (BDD), (3) N-heterocycles containing sp² nitrogen (FBTA and Qx), and (4) other A units (such as BT) (Fig. 3).^4,49,106,114 The structures of the polymer donors discussed in this review are shown in Fig. 4 and 5 based on this classification. Their key optoelectronic properties are listed in Table 1.

Fig. 4 Chemical structures of BDT-based polymer donors discussed in this review. Their properties are listed in Table 1. Names in parentheses are used in the original publications.

Fig. 5 Chemical structures of non-BDT-based polymer donors discussed in this review. Their properties are listed in Table 2. Names in parentheses are used in the original publications.

Table 1 Key optoelectronic properties of polymer donors discussed in this review

Polymer	E HOMO (eV)	E LUMO (eV)	E ^gopt (eV)	λ max (nm)	λ onset (nm)	α f-max (×10^5 cm^−1)	α s-max (×10^5 M^−1 cm^−1)	μ h (×10^−4 cm^2 V^−1 s^−1)	Ref.
a Obtained from film absorption, αf-max represents the maximum extinction coefficient in a thin film, and αs-max represents the maximum extinction coefficient in solution. b The numbers of significant digits of the data are adopted from the original references and may be different.
PE1	−5.22	−3.64^b	1.58	625, 690	785	1.0^b		11.1^115	8
PE2	−5.37	−3.63	1.74	549	711				116
PE3	−5.47	−3.58	1.94		538	1^b			117
PE4	−5.51	−3.59	1.93	540	642	0.79		1^b	118
PE5	−5.50	−3.59	1.93	544	642	0.81		1.2^b	118
PE6	−5.54	−3.6^b	1.94	536	639	0.72		0.73	118
PE7	−5.49		1.97	538	629	0.726^b	0.607^b	1.11^b	119
PE8	−5.5		1.94	544	639	0.758	0.65^b	3.4	119
PE9	−5.48	−3.63	1.83		678				120
PE10	−5.51	−3.63	1.87		663				120
PE11	−5.55	−3.62	1.93		642				120
PE12	−5.59	−3.67	1.99	533	623	0.769			121
PE13	−5.50	−3.53	1.97	577	630	1.25			122
PE14	−5.46	−3.13	1.82		700				123
PE15	−5.05	−3.19	1.86	562, 613	667				110
PE16	−5.20	−3.45	1.78	586, 630	670				110
PE17	−5.15^7	−3.31^7	1.60	614, 674	775			5.8	124
PE18	−5.33	−3.52	1.51	655, 705	820			109^125	126
PE19	−5.63	−3.63	2.24	416	554		0.362	0.028	127
PE20	−5.39	−3.37	1.96	540	633		0.557	0.94	127
PD1	−5.23	−3.18	1.80	581, 622	689	0.86^128		1.11^128	111
PD2	−5.50	−3.56	1.80	570, 614	689			2.97	129
PD3	−5.52	−3.57	1.79	577, 606^128	692	0.99^128		6.67^128	130
PD4	−5.40	−3.60	1.80	626	688	1.08			22
PD5	−5.54	−3.27	2.11	479	588				131
PD6	−5.48	−3.47	1.78	622	697				131
PD7	−5.42	−3.36	1.90	556	652	0.697	0.331	7.56^132	133
PD8	−5.59	−3.53	1.85	627	671			13.4	134
PD9	−5.38	3.57	1.81	577	685	0.531		3.96	135
PD10	−5.5	−3.64	1.86	612	667	0.845		15.1	135
PD11	−5.48	−3.52	1.75	632, 625	710				136
PD12	−5.41	−3.58	1.83	621	678	1			137
PD13	−5.54	−3.59	1.95	551	636		2.63		138
PD14	−5.16	−3.45	1.71	657	726		5.54		138
PD15	−5.36	−3.63	1.73	645	718		7.37		138
PD16	−5.52	−3.72	1.80		689				139
PN1	−5.37	−2.91	1.99	538, 580	623	0.89			140
PN2	−5.40	−3.24	1.96	528, 573	633	0.96			141
PN3	−5.42	−3.29	1.98	533, 578	626	0.7			140
PN4	−5.46	−2.92	1.98	535, 576	626	0.69			140
PN5	−5.56	−3.06	1.99	526	623	0.68			140
PN6	−5.42	−2.93	1.93	553, 600	642				142
PN7	−5.50	−3.02	2.00	536	620	0.98			143
PN8	−5.57	−3.58	1.99		622	0.838			144
PN9	−5.56	−3.50	1.94	536	640	1.08		10.2	145
PN10	−5.23	−3.43	1.92	372, 550, 597	646		0.564		146
PN11	−5.29	−3.49	1.9	374, 550, 602	653		0.854		146
PN12	−5.33	−3.01	1.99	530, 575	623			7.28	147
PN13	−5.27	−3.06	1.96	539	632	0.523		7.23	148
PN14	−5.49	−3.22	1.99	533	623	0.651		7.86	148
PN15	−5.34	−3.4	1.94		639		0.73	5.59	20
PN16	−5.52	−3.54	1.98	537, 577	626		0.7	6.01	20
PN17	−5.32	−3.36	1.96	544, 589	633		0.62		149
PN18	−5.40	−3.40	2.00	542, 579	621		0.60		149
PN19	−5.44	−3.49	1.95		636				150
PN20	−5.34	−3.46	1.81	600	685	0.7			151
PN21	−5.42	−3.49	1.93	590	642		1.1		152
PN22	−5.18	−3.48	1.70	636	755	0.545	0.454		113
PN23	−5.34	−3.62	1.72	600	737	0.731	0.515		113
PN24	−5.49	−3.7	1.79	583	716	0.859	0.617		113
PN25	−5.31	−3.62	1.69	361, 435, 652	736	4.73			153
PN26	−5.38	−3.67	1.71	361, 443, 635	724	4.64			153
PN27	−5.33	−2.94	1.86		667			2.00	154
PN28	−5.46	−3.16	1.79		693			7.74	154
PN29	−5.24	−3.42	1.80	374, 450, 632	689		29		155
PN30	−5.31	−3.46	1.81	368, 452, 636	685		34.5		155
PO1	−5.35	−2.78	2.07		599		25		156
PO2	−5.39	−2.79	2.09		593		31		156
PO3	−5.41	−3.46	1.95	550	636	1.13			157
PO4	−5.47	−3.47	2.00	580	620		1.53	0.614	158
PO5	−5.47	−3.57	2.00	579	620		1.39	0.143	158
PO6	−5.64	−3.64	2.00	580	620		1.27	0.27	158
PO7	−5.62	−3.74	1.88	361, 565	660		0.896		159
PO8	−5.67	−3.77	1.90	364, 565	653		0.944		159
PO9	−5.45	−2.79	1.96		634			7.20^160	161
PO10	−5.52	−2.91	1.96		634				161
PO11	−5.48	−2.83	1.95	551, 588	636			11.9	160
PO12	−5.51	−2.77	1.98	584, 559	626			15.9	10
PO13	−5.36	−2.81	2.16	502, 535	574				162
PX1	−5.31	−3.00	1.90	551	653	1.05			163 and 164
PX2	−5.41	−3.61	1.80	576	689				165
PX3	−5.55	−3.80	1.75	583	708				166
PX4	−5.63	−3.86	1.77	580	700				166
PX5	−5.54	−2.98	1.92		646				167
PX6	−5.67	−3.74	1.93		642				168
PX7	−5.62	−3.71	1.92		646				168
PX8	−5.61	−3.69	1.92		646				168
PX9	−5.25	−3.63	2.02	583	627	0.818	1.2		169
PX10	−5.16	−3.10	1.94	541	639		0.804		170
PX11	5.24	−3.23	1.84	559, 617	674		1.28		170
PX12	−5.52	−2.76	1.95		637				171

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As mentioned in the previous section, good matching between donor and acceptor materials is vital for achieving high OSC performance. Therefore, developing a polymer donor material having its photophysical properties matching with those of a certain NFA material on purpose is the most adopted and effective strategy. From a large number of high performance NFAs,^36–38,40 we deliberately select the three most representative ones, ITIC, IT-4F, and Y6, which have been widely used for developing ultrahigh performance (PCE >10%) polymer donors. Their chemical structures are shown in Fig. 6, while their key optoelectronic properties are summarized in Table 2. It should be noted that the reported E_LUMO and E_HOMO values of the three NFAs, particularly ITIC, vary largely in the literature due to the different measurement conditions as mentioned earlier.

Fig. 6 Chemical structures (a), thin-film UV-vis-NIR spectra (b), and energy diagrams (c) of ITIC, ITIC-4F, and Y6.

Table 2 Key optoelectronic properties of NFAs discussed in this review

NFA	E HOMO (eV)	E LUMO (eV)	E ^gopt (eV)	λ max (nm)	λ onset (nm)	α f-max (×10^5 cm^−1)	α s-max (×10^5 M^−1 cm^−1)	μ e (×10^−4 cm^2 V^−1 s^−1)	Ref.
a Obtained from film absorption. b SCLC mobility obtained at the annealing temperature shown in brackets.
ITIC	−5.48	−3.83	1.59	702	780	1.10	1.72	3 (150 °C)	8
	−5.60	−3.85							172
	−5.64	−4.04							173
	−5.70	−4.00							174
IT-4F	−5.66	−4.14	1.52	726	803	1.16	2.10	5.05 (150 °C)	22
	−5.71	−4.15							175
	−5.69	−4.17							176
Y6	−5.65	−4.10	1.33	821	931	1.07	2.39	2.35 (100 °C), 14.8 (150 °C), 33.0 (200 °C)	9
	−5.70	−4.08							177
	−5.60	−4.10							130
	−5.65	−4.11							175

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3.1 High performance polymer donors matching with ITIC

As a milestone for NFAs, ITIC was developed in 2015 by Lin et al.,⁸ which has an electron-donating indacenodithienothiophene (IDTT) central unit and two electron-withdrawing 2-methylene-(3-(1,1-dicyanomethylene)-indanone) (IC) end groups. Hexyl phenyl side chains are introduced on IDTT to render the material soluble and to suppress the excessive aggregation of molecules in the solid state. ITIC has a good SCLC μ_e of 3.0 × 10⁻⁴ cm² V⁻¹ s⁻¹, which approaches that of fullerene-based acceptors (∼10⁻⁴–10⁻² cm² V⁻¹ s⁻¹).^179,180 Its rather high E_LUMO (−3.83 eV) helps achieve a high V_OC. However, ITIC has a relatively large bandgap (E_g = 1.59 eV), absorbing sunlight below 800 nm, which restricts J_SC when a WBG polymer donor is used. Nonetheless, ITIC has been widely used as a model acceptor for the study and development of various polymer donors to provide significant insights into their structure–property–cell performance relationships.

3.1.1 BDT-based polymer donors using ester or imide substituted building blocks as A units. PE1 (PTB7-Th) developed by Liao et al.,¹⁰⁹ which has 2-ethylhexyl-thienyl substituted BDT and FC-TT as D and A units, respectively, is one of the prominent D–A polymer donor materials for OSCs. PE1 has a rather narrow bandgap of 1.58 eV, which was designed as a donor material mainly for matching fullerene-based acceptors that absorb sunlight poorly. When ITIC is used as an acceptor and PE1 as a donor, their absorption profiles are mostly overlapped, which is undesirable for efficient light harvesting. Consequently, the devices based on the PE1:ITIC blend showed a moderate J_SC of 14.21 mA cm⁻² and PCE of 6.8%.⁸ In order to improve the light absorption, Kim et al.¹¹⁶ developed a series of polymer donors, modulating the energy levels and absorption spectra by controlling the BDT/FC-TT (D/A) ratio in the polymer backbone, and found that as the D/A ratio increases, E_HOMO downshifts and E_g widens. PE2 with a D/A ratio of 5 : 1 has the optimal E_HOMO of −5.37 eV and E_g of 1.74 eV to form complementary absorption with ITIC. As a result, a higher J_SC (17.24 mA cm⁻²) along with a higher V_OC (0.89 V) and FF (67%) and thus a higher PCE of 10.27% were achieved compared to polymers with other D/A ratios. An et al.¹¹⁷ developed a polymer PE3 (PBDT-S-2TC) having a weaker electron-withdrawing ester substituted thiophene as A units. A non-substituted thiophene spacer was inserted between the ester substituted thiophene and the BDT unit to maintain the planarity of the polymer backbone. In addition, alkylthio chains were introduced onto BDT to lower E_HOMO (−5.47 eV). A wide bandgap of 1.94 eV was obtained, which matches that of ITIC very well for complementary light absorption. The devices based on PE3:ITIC showed an improved J_SC and a very high V_OC of 0.96 V, resulting in a PCE of 10.12%.

3.1.2 BDT-based polymer donors using diketone derivatives as A units. Diketone substituted thiophene building block BDD is a relatively weak electron-accepting (A) unit. The D–A polymers based on BDT and BDD demonstrated excellent photovoltaic performance when paired with narrow bandgap NFAs. PD1 (PBDB-T or PBDTBDD) was the first polymer using BDD developed by Qian et al.,¹¹¹ which achieved a PCE of 6.67% when paired with PC₆₁BM. Later, Zhao et al.¹⁶ utilized PD1 as a donor to match ITIC since PD1 has a relatively wide bandgap of 1.80 eV. Compared with PE1:ITIC, PD1:ITIC exhibited much improved EQE values in the short wavelength range (300–500 nm) due to the strong absorption by PD1 (Fig. 7b). Therefore, a higher J_SC (16.81 mA cm⁻² for PD1:ITIC vs. 14.21 mA cm⁻² for PE1:ITIC) was obtained. The improved and more balanced μ_h and μ_e of PD1:ITIC (Table 3) led to the increased FF compared to PE1:ITIC (74.2% vs. 59.1%). In addition, although PD1 showed a similar E_HOMO (∼−5.2 eV) to PE1, a higher V_OC (0.899 vs. 0.810 V) was obtained owing to its smaller energy loss (E_loss). Combining all these improvements, the PD1:ITIC-based device displayed a much higher PCE of 11.21%.¹⁶

Fig. 7 (a) IPCE spectrum of the PE1:ITIC-based OSC, reproduced from ref. 8 with permission from John Wiley and Sons, copyright 2015; (b) EQE curve of the PD1:ITIC-based OSC, reproduced with permission,¹⁶ copyright 2016, John Wiley and Sons; (c) energy level schematic of PN22, PN23, PN24, and ITIC, reproduced from ref. 113 with permission from John Wiley and Sons, copyright 2016; and (d) chemical structures of J51, J52 and m-ITIC.

Table 3 Photophysical properties and OSC performance of polymer donor:ITIC blends

Blend	μ h, μe (× 10^−4 cm^2 V^−1 s^−1), μe(h)/μh(e)^a	EQE^b (%)	V OC (V)	J SC (mA cm^−2)	FF (%)	PCE (%)	S 1	S 2	P diss (%)	Ref.
a μ e/μh or μh/μe, whichever is >1. b Maximum value of EQE. c Obtained from VOC ∼ S1 log(Plight). d Obtained from Jph ∼ S2 log(Plight). e The numbers of significant digits of the data are adopted from the original references and may be different.
PE1:ITIC	0.43, 1.1, 2.63	72.6^e	0.81^e	14.21^e	59.1^e	6.8^e				8
PE2:ITIC	0.298, 0.346, 1.16		0.89	17.24	67^e	10.27^e	1.15	0.927		116
PE3:ITIC	2.11, 2.83, 1.34		0.96	16.4^e	64.3	10.12				117
PD1:ITIC	2.1, 3.13, 1.49		0.899^e	16.81	74.2	11.21				16
PD7:ITIC	2.59, 2.25, 1.15	79^e	1.01	18.1	59	10.8	1.01		86.8^e	133
PD12:ITIC	2.91, 2.67, 1.09	76	0.942	16.81	66.3	10.5				137
PN2:ITIC	3.78, 3.07, 1.23	76.5	0.94	17.32	69.77^e	11.41				141
PN1:m-ITIC	1.93, 8.4, 4.34	81.5	0.92	18.09	69.82	11.62				140
PN2:m-ITIC	1.09, 4.66, 4.28		0.944	18.09	70.59	12.05				140
PN3:m-ITIC	0.46, 3.77, 8.2		0.962	16.35	65.03	10.23				140
PN4:m-ITIC	0.38, 3.12, 8.2		0.974	16.45	66.87	10.71				140
PN5:m-ITIC	0.24, 4.0, 16.66		0.99	15.89	61.18	9.63				140
PN6:ITIC	3.66, 4.06, 1.11		0.95	15.27	73.08	10.6		0.94^e	96^e	142
PN6:m-ITIC	4.08, 4.67, 1.15		0.96	16.48	69.83	11.05		1.01	97	142
PN7:m-ITIC	1.016, 3.002, 2.95		0.984	18.03	65.54	11.63		1.028^e		143
PN10:ITIC	13.3, 8.7, 1.53	78	0.81	18.63	66.7	10.07				146
PN11:ITIC	24.7, 20.3, 1.22	80	0.84	19.51	75.1	12.31				146
PN12:ITIC	3.25, 2.62, 1.24	82	0.91	18.7	61.8	10.5	1.26	0.94		147
PN13:ITIC	4.36, 2.98. 1.46	72	0.74	15.7	49.8	5.8	1.8	0.93		148
PN14:ITIC	5.08, 3.96, 1.28	84	0.94	18.4	60.2	10.4	1.32	0.99		148
PN15:ITIC	4.94, 3.81, 1.3		0.94	18.23	69.19	11.85		0.93	93	20
PN16:ITIC	5.46, 5.25 1.04		1.01	18.51	74.4	13.91		0.97	95.4	20
PN17:ITIC	1.15, 3.93, 3.42		0.89	18.12	67.37	10.86		0.92	93	149
PN18:ITIC	4.16, 3.16, 1.32		0.95	18.76	73.85	13.16		0.96	95	149
PN19:ITIC	2.132, 3.397, 1.59	83	0.936	18.21	67.8	11.56				150
PN20:ITIC	0.86, 0.23, 3.74		0.87	18.29	64.34	10.24		0.971		178
PN22:ITIC	7.8, 8.2, 1.05		0.69	16.16	59.91	6.68	1.33	0.952	88	113
PN23:ITIC	8.6, 8.3, 1.04		0.83	17.16	62.49	8.9	1.23	0.981	95	113
PN24:ITIC	10, 10, 1		0.95	17.87	66.8	11.34				113
PN25:ITIC	1.13, 1.6, 1.42	75	0.9	16.88	69.24	10.52		0.96	97.2	153
PN26:ITIC	0.139, 0.68, 4.89	66.4	0.94	13.75	56.11	7.22		0.9	96.3	153
PN29:ITIC	1.8, 0.56, 3.2	71	0.87	16.21	64.6	9.11	1.37	0.92	89	155
PN30:ITIC	2.5, 1.3, 1.9	78	0.92	17.86	69.8	11.47	1.25	0.96	94	155
PO1:ITIC	8.5, 6.8, 1.25		1.01	17.15	67.7	11.72				156
PO2:ITIC	12, 7.2, 1.67		1.1	17.78	65.4	12.8				156
P3HT:ITIC	1.4, 1.68, 1.2		0.52	4.22	56.93	1.25	1.14	0.99	92	163
PX1:ITIC	2.4, 16.3. 6.79	75	0.94	16.5	65.67	10.16				163
PX2:ITIC	1.79, 1.20, 1.49	72.5	0.66	15.19	57	5.72				165

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Since then, PD1 was widely used as a star polymer donor and the PCE of devices based on PD1 was further improved to above 13% with a largely improved J_SC of up to ∼20 mA cm⁻² through rational design of new NFAs with narrower bandgaps.^181–185

Fan et al.¹³³ synthesized a novel polymer donor PD7 (PBPD-Th) with a low-lying E_HOMO of −5.42 eV by replacing the 5-alkylthienyl in PD1 with m-alkoxyphenyl on the BDT units. OSCs based on PD7:ITIC have a smaller E_loss and therefore achieved a high V_OC of up to 1.01 V. Moreover, the polymer also has a wide bandgap of 1.90 eV, resulting in complementary absorption with ITIC. High EQE values (up to ∼80%), especially in the short wavelength range (400–550 nm, ∼70% on average), were obtained, which led to a high J_SC (18.1 mA cm⁻²) and thus a high PCE of 10.8%.

By breaking the symmetry of the BDT unit, i.e., introducing different side chains on two sides of BDT, Li et al.¹³⁷ developed an asymmetric polymer donor PD12 (asy-PBDBTN). Alkoxyl side chains were introduced to modulate the absorption spectra and solubility of the polymers (achieving enhanced light-harvesting ability and compatibility with acceptor materials, simultaneously), while a β-position linked naphthalene unit with high ionization potential and low electron density was introduced to lower E_HOMO. PD12 showed a low-lying E_HOMO of −5.41 eV, a wide bandgap of 1.83 eV, and high light-harvesting ability with an α_f-max of up to 1 × 10⁵ cm⁻¹. OSCs based on the PD12:ITIC blend displayed a high PCE of 10.5%.¹³⁷

3.1.3 BDT-based polymer donors using N-heterocycles as A units. By copolymerizing BDT with FBTA, Li's group developed a J-series (J40 to J91) of polymer donors.^{19,23,142,143,186–188} Through optimization of the backbone conjugation dimension, variation of side chains, and fluorination of BDT units, the PCE was increased from 6.48% to 9.53% for polymers J40 to J61.^18,19,186 Furthermore, trialkylsilyl substituents were used to replace the alkyl chains on the BDT unit in J52 (Fig. 7d) to produce PN2 (J71), where the bond interaction of the low-lying σ* orbital of the Si atom with the π* orbital of the aromatic units would result in stabilization of E_LUMO and lowering of E_HOMO (−5.40 eV).¹⁴¹PN2 also showed a higher extinction coefficient of 0.96 × 10⁵ cm⁻¹ than that (0.73 × 10⁵ cm⁻¹) of its analogue J52 with alkyl chains. OSCs based on the PN2:ITIC blend showed a J_SC of 17.32 mA cm⁻², a V_OC of 0.94 V, and a FF of 69.77%, resulting in a high PCE of 11.41%. The annealed blend film showed high and balanced μ_h and μ_e of 3.78 × 10⁻⁴ cm² V⁻¹ s⁻¹ and 3.07 × 10⁻⁴ cm² V⁻¹ s⁻¹, respectively, which contributed to the high J_SC and high FF. It should be noted that ΔE_HOMO between PN2 and ITIC is only 0.11 eV, which suggests a high hole transfer efficiency from the acceptor to the donor.

The same research group systemically investigated the effects of the size and configuration of side chains (PN1–PN5 (J70–J74)) on the OSC performance of their blends with an ITIC isomer, m-ITIC (Fig. 7d), which has meta-alkyl-phenyl substitution and shows a higher degree of self-organization and crystallinity.¹⁴⁰ Among these five polymers, they found that the film absorption coefficients and hole mobilities decreased, while the E_HOMO values downshifted, with increasing length and complexity (linear to branched) of the alkyl substituents, which resulted in decreased J_SC and FF, but enhanced V_OC. Among this series of polymers, PN2 showed the highest PCE of 12.05%.

When the thiophene rings in the BDT units were replaced with furan rings to form benzo[1,2-b:4,5-b′]difuran (BDF), the resultant polymer PN6 (J81), which is analogous to PN1, showed a lower E_HOMO of −5.42 eV, achieving a high PCE of 11.05% when paired with m-ITIC.¹⁴² On the other hand, when the thiophene side chains on the BDT units in PN1 were di-substituted with electron-withdrawing fluorine atoms, the formed PN7 (J91) exhibited a further down-shifted E_HOMO (−5.50 eV).¹⁴³ OSCs based on PN7:m-ITIC showed an even higher PCE of 11.63%, resulting from the increased V_OC and J_SC.

Compared with thiophene, selenophene shows lower aromaticity and enhanced ground-state quinoid resonance character, which would result in improved planarity, increased effective conjugation length, and a lowered bandgap of the polymer. In addition, improved interchain interactions and charge carrier transport could also be observed for selenophene-containing polymers because of the stronger heteroaromatic interactions induced by selenium atoms.^189,190

When the thiophene side chains on BDT units of J51 (Fig. 7d)¹⁸ were replaced with selenophene side chains, the obtained PN10 (PBDT-Se-TAZ) exhibited similar E_HOMO (−5.23 vs. −5.26 eV) and E_g (1.92 vs. 1.91 eV) to those of J51.¹⁴⁶ However, the PN10:ITIC blend showed much higher μ_h and μ_e of 13.3 × 10⁻⁴ cm² V⁻¹ s⁻¹/8.7 × 10⁻⁴ cm² V⁻¹ s⁻¹ compared to J51 (4.32 × 10⁻⁴ cm² V⁻¹ s⁻¹/3.74 × 10⁻⁴ cm² V⁻¹ s⁻¹), indicating much improved charge transport in PN10 due to the presence of selenophene side chains. Consequently, the devices based on PN10:ITIC showed improved performance (J_SC = 18.63 mA cm⁻²; V_OC = 0.81 V; FF = 66.7%; and PCE = 10.07%) compared to the devices based on J51:ITIC (J_SC = 16.47 mA cm⁻²; V_OC = 0.82 V; FF = 0.69; and PCE = 9.26%). Once the alkyl selenophene side chains in PN10 were changed to alkylthio selenophene side chains, the resulting PN11 (PBDTS-Se-TAZ) showed a slightly lower E_HOMO (−5.29 vs. −5.23 eV), a slightly narrower E_g (1.90 vs. 1.92 eV), and a higher α_s-max (the maximum extinction coefficient in solution) (8.54 × 10⁴vs. 5.64 × 10⁴ M⁻¹ cm⁻¹) compared to PN10.¹⁴⁶ The PN11:ITIC blend showed greatly enhanced and more balanced μ_h and μ_e (2.47 × 10⁻³ cm² V⁻¹ s⁻¹/2.03 × 10⁻³ cm² V⁻¹ s⁻¹), and thus less recombination and higher exciton dissociation and charge collection efficiencies (Table 3). Therefore, PN11:ITIC displayed higher V_OC (0.84 V), J_SC (19.51 mA cm⁻²), FF (75.1%), and PCE (12.31%).¹⁴⁶

Replacing thiophene side chains on BDT units with benzene side chains is a strategy to lower E_HOMO as mentioned previously (in the case of PD7). Li et al.¹⁴⁷ synthesized PN12 (PBFZ-OP) based on meta-alkoxy-phenyl-substituted BDT (BDT-m-OP) and FBTA units. E_HOMO was lowered to −5.33 eV compared with J52 with thiophene side chains (E_HOMO = −5.21 eV).¹⁹PN12 also showed a wider bandgap of 1.99 eV than that of J52 (1.96 eV), which could form a more complementary absorption profile when paired with ITIC. Much improved cell performance (V_OC = 0.91 V; J_SC = 18.7 mA cm⁻²; FF = 0.618; and PCE = 10.50%) was obtained for the PN12:ITIC blend compared with the J52:ITIC blend (V_OC = 0.73 V; J_SC = 13.1 mA cm⁻²; FF = 0.578; and PCE = 5.50%).

Later, the same group changed the alkoxy substituent of the phenyl side chains on BDT from the meta to the para position (PN13 (PBZ1)) and introduced a CF₃ group on the meta position (PN14 (PBZ-m-CF₃)) to further lower E_HOMO.¹⁴⁸ Compared with PN12, both polymers showed a similar bandgap (1.96 eV for PN13 and 1.99 eV for PN14). PN13 showed a slightly higher E_HOMO of −5.27 eV, while PN14 showed a much lower E_HOMO of −5.49 eV. Between these two polymers, PN14 exhibited a higher α_f-max (6.51 × 10⁴vs. 5.23 × 10⁴ cm⁻¹), smaller π–π distance (3.60 vs. 3.69 Å), and slightly higher μ_h (7.86 × 10⁻⁴vs. 7.23 × 10⁻⁴ cm² V⁻¹ s⁻¹). The PN14:ITIC blend showed a much higher PCE of 10.4% than PN13:ITIC (5.8%).

Li et al.²⁰ synthesized another similar pair of polymers, using p-alkylthiophenyl without or with fluorine at the meta position as the side chains to BDT units, PN15 (PBTA-PS) and PN16 (PBTA-PSF), respectively. A wider bandgap of 1.98 eV and a much lower E_HOMO of −5.52 eV were obtained for PN16 compared to PN15 (E_g = 1.94 eV; and E_HOMO = −5.34 eV). When paired with ITIC, both showed a high V_OC of ∼1 V and high J_SC of ∼18 mA cm⁻². The device based on PN16:ITIC showed a much improved FF of 74.4% (vs. 69.19% for PN15:ITIC) due to its higher and more balanced μ_h and μ_e, less recombination and higher exciton dissociation and charge collection efficiencies (Table 3). Therefore, while PN15:ITIC showed a PCE of 11.85%, the PN16:ITIC blend exhibited a much higher PCE of 13.91%, which is the highest value achieved among ITIC-based OSCs by far. The results indicate that alkylthiophenyl substitution on BDT is very beneficial for the improvement of the OSC performance of the resulting polymer. On other hand, fluorination on the phenyl ring could further enhance the cell performance.

Chen et al.¹⁴⁹ conducted a further study to compare the effects of the position of alkylthio chains on phenyl (para or meta) on the OSC performance of this type of polymers. Compared with the polymer with alkylthio chains at the para position (PN17 (p-PBDTPS-FTAZ)), the one with the chains at the meta position, PN18 (m-PBDTPS-FTAZ), showed a lower E_HOMO and blue-shifted absorption. When blended with ITIC, higher and more balanced μ_h and μ_e, less recombination, and higher exciton dissociation and charge collection efficiencies were observed. As a result, a higher PCE of 13.16% was achieved for the PN18:ITIC blend compared to PN17:ITIC (10.86%).

Liu et al.¹⁵⁰ developed a series of polymer donors having the BDT unit asymmetrically substituted with an alkylthiolthienyl group on one side and a phenyl and naphthyl (or p-biphenyl) on the other. It was found that the long alkylthiolthienyl substituent can lower E_HOMO, broaden the absorption spectrum, and improve the solubility. The non-substituted rigid aryl substituent (benzene, naphthalene and p-biphenyl) can be regarded as a lever arm to disturb the ITIC phase and weaken the polymer chain entanglements, which helps realize a favorable morphology even without post-treatment. When the polymer with the naphthyl substituent, PN19 (PBDTTAZ-NaPh), was blended with ITIC, the obtained film exhibited a homogeneous morphology with finely separated domains. A high PCE of 11.56% was obtained with this blend film.

Fan et al.¹⁵¹ used alkylimide to replace the two fluorine atoms on benzotriazole in FBTA to form an acceptor unit, TZBI, which was used to make a copolymer PN20 (PTZBI) with BDT. PN20 possesses a more delocalized electron distribution across the conjugated backbone, which lowers E_HOMO and enhances the carrier mobility. A high PCE of 10.24% was obtained with the PN20:ITIC blend.¹⁷⁸

Qx is another moderate electron-accepting N-heterocylic building block that is widely used in constructing WBG conjugated D–A polymer donors for OSCs.^34,191 Zheng et al.¹¹³ conducted a systematic study on the correlations between the energy level alignment, performance, and device physics by finely tuning the FMO energy levels through introducing different numbers of F atoms. Three polymers with 0, 2 and 4 F atoms in one repeat unit, PN22 (PBQ-0F), PN23 (PBQ-QF) and PN24 (PBQ-4F), respectively, were synthesized. With an increasing number of F atoms, E_LUMO and E_HOMO downshifted gradually. PN24 showed the lowest E_HOMO of −5.49 eV, and so achieved the highest V_OC of 0.95 V when blended with ITIC. It should be noted that although the energy offsets between PN24 and ITIC are very small (ΔE_HOMO = 0.04 eV and ΔE_LUMO = 0.24 eV), the devices exhibited efficient charge generation, which is an outstanding advantage over fullerene-based OSCs, which require a large energy offset of >0.3 eV for efficient exciton dissociation.^{87,121,192–195} A high J_SC (17.87 mA cm⁻²) and FF (66.80%) were also obtained owing to the stronger inter-chain interaction, resulting in higher carrier mobility. Therefore, this blend afforded a high PCE of 11.34%. A mixture of non-halogenated solvents THF and IPA was used for processing the active layer, demonstrating the potential for environment-friendly fabrication of OSCs.

The thiophene donor building block possesses higher electron density and smaller steric hindrance, providing better absorption and charge transport to the resulting polymer than the benzene building block. Xu et al.¹⁵³ replaced the phenyl side chains of Qx units with alkyl substituted fluorothiophene and introduced alkyl chains with different lengths on BDT units to form polymers PN25 (TTFQx-T1) and PN26 (TTFQx-T2). When paired with ITIC, the blend based on PN25 with short alkyl chains on BDT showed higher and more balanced μ_h and μ_e, less recombination, and higher exciton dissociation and charge collection efficiencies (Table 3). Therefore, it exhibited a much improved J_SC (16.88 vs. 13.75 mA cm⁻²) and FF (69.24% vs. 56.11%) and thus a higher PCE (10.52% vs. 7.22%) compared with the PN26:ITIC blend. This result indicated that the alkyl chain length has notable influences on the photovoltaic properties.

Building blocks with an enlarged planar aromatic structure are favorable for ordered molecular packing and thus improved photovoltaic performance.^196,197 In addition, alkylthiothiophene side chains can further reduce E_HOMO. Based on these principles, Yu et al.¹⁵⁵ used quinoxalino[6,5-f]quinoxaline (NQx) as A units and introduced alkylthiophene and alkylthiothiophene as side chains of BDT units to form PN29 (PBDT-NQx) and PN30 (PBDTS-NQx), respectively. The latter showed a lower E_HOMO (−5.31 vs. −5.24 eV) and higher α_s-max (3.45 × 10⁶vs. 2.9 × 10⁶ M⁻¹ cm⁻¹). The devices based on PN30:ITIC showed a higher PCE of 11.47% than those based on PN29:ITIC (9.11%).

3.1.4 Other BDT-based polymers and non-BDT-based polymers. 1,3,4-Thiadiazole (TDZ) is highly polarizable with large electron affinity, which can be used as an excellent A building block for constructing high-performance D–A copolymers for OSCs.¹⁹⁸ Xu et al.¹⁵⁶ copolymerized TDZ with BDT units to form PO1 (PBDT-TDZ) and PO2 (PBDTS-TDZ), which employed alkylthiophene and alkylthiothiophene as side chains of BDT units, respectively. PO2 showed a slightly lower E_HOMO (−5.39 vs. −5.35 eV) and higher α_s-max (3.1 × 10⁶vs. 2.5 × 10⁶ M⁻¹ cm⁻¹) than PO1. The PO2:ITIC blend showed higher crystallinity and higher μ_h and μ_e (Table 3). As a result, a higher PCE of 12.8% for PO2:ITIC was obtained compared to PO1:ITIC (11.72%).

The primary driving force for OSC research is the potential of manufacturing OSC products at a much lower cost than Si solar cells. The synthetic complexity (SC), which considers the number of reaction steps, purification methods, reaction yields, use of hazardous materials, etc., can be used to estimate the synthetic cost of the materials.¹⁹⁹ Although BDT-based polymers showed high cell performance, the tedious synthesis of the BDT tin monomer alone would result in a high SC (>30%). P3HT can be synthesized with a very low SC (7.75%), but its OSC performance is rather low mainly due to its high E_HOMO. To overcome the disadvantageous high-lying E_HOMO of P3HT, Zhang et al.¹⁶³ introduced electron-withdrawing carboxylate substituents on half of the thiophene rings in the backbone of polythiophene to form PX1 (PDCBT). E_HOMO of PX1 down-shifted to −5.26 eV, much lower than that of P3HT (∼−5.0 eV). Therefore, a much higher V_OC for the PX1:ITIC blend was obtained (0.94 V for PX1:ITIC vs. 0.52 V for P3HT:ITIC). Furthermore, in contrast to P3HT, PX1 predominantly adopted a face-on crystal orientation in the blend film, which is beneficial for the charge transport in the vertical direction. Combined with lower recombination and higher exciton dissociation and charge collection efficiencies, the devices based on PX1:ITIC showed a largely improved PCE of up to 10.16% compared to the PCE of 1.25% for P3HT:ITIC, demonstrating the potential of polythiophenes for achieving high photovoltaic performance. Another strategy to improve the photovoltaic performance of polythiophenes is to design novel D–A polymer donors with conjugated D and A units located at the backbone and side chains, respectively, which was demonstrated by He et al.¹⁶⁵ with a polymer PX2 (PTIBT) that has an electron-rich polythiophene backbone and electron-accepting indolin-2-one side chains. The polymer with the so-called donor-backbone–acceptor-side-chain structure showed a very high dielectric constant of 7.70 due to the largely separated donor and acceptor units, which is beneficial for the exciton diffusion to the donor:acceptor interface and subsequent dissociation into free electrons and holes. Although the PCE was moderate (5.72%) for the PX2:ITIC blend, this novel polymer design showed potential for achieving high performance through increasing the dielectric constant of the material. It should be mentioned that this polymer could be synthesized with a very low SC of 23.5%.

3.2 High performance polymer donors matching with IT-4F

In the previous section it was pointed out that ITIC has a rather large bandgap (1.59 eV), which can only absorb sunlight below 800 nm. In 2017, Zhao et al.²² introduced F atoms on the end IC groups of ITIC to form IT-4F (Fig. 6). Compared with ITIC, IT-4F has a narrower bandgap (1.52 eV) with red-shifted absorption and higher α_s-max and α_f-max values along with lower E_LUMO (−4.14 eV) and E_HOMO (−5.66 eV). OSCs based on IT-4F have demonstrated improved photovoltaic performance compared to ITIC-based devices.

3.2.1 BDT-based polymer donors using ester or imide substituted building blocks as A units. We have mentioned earlier that in addition to F or Cl atoms, ester groups can also downshift E_HOMO and enhance the interchain interactions. Li et al.¹¹⁸ developed a novel polymer donor by using BDT and ester substituted thiophene (EST) as the D and A units, respectively, to form PDTB-EF-T polymers and investigated the influence of different alkyl chains of the ester groups on the cell performance. Three polymers with different alkyl chains, PE4, PE5 and PE6, showed similar FMO energy levels and bandgaps, while the polymer with linear decyl chains (PE5) exhibited the strongest absorption and highest μ_h due to the most planar backbone and strongest intermolecular interactions. When using IT-4F as an acceptor, similar V_OC values were obtained for the three blends, but the PE5:IT-4F blend showed the highest chain ordering, the highest and most balanced μ_h and μ_e, the most suppressed recombination, and the highest exciton dissociation and charge collection efficiencies (Table 4), resulting in the highest J_SC, FF, and PCE (14.2%).

Table 4 OSC device performances based on blends of polymer donors and IT-4F

Blend	μ h, μe (×10^−4 cm^2 V^−1 s^−1), μe(h)/μh(e)^a	EQE^b (%)	V OC (V)	J SC (mA cm^−2)	FF (%)	PCE (%)	S 1	S 2	P diss (%)	Ref.
a μ e/μh or μh/μe, whichever is >1. b Maximum value of EQE. c Obtained from VOC ∼ S1 log(Plight). d Obtained from Jph ∼ S2 log(Plight). e The numbers of significant digits of the data are adopted from the original references and may be different.
PE4:IT-4F	1.51, 4.12, 2.7		0.896^e	20.05^e	64	11.5		1^e	93^e	118
PE5:IT-4F	3.46, 3.95, 1.1	82^e	0.9^e	20.73	76	14.2		0.91^e	95	118
PE6:IT-4F	0.632, 2.35, 3.7		0.904	20.31	61	11.2			93	118
PE7:IT-4F	0.553, 0.918, 1.7		0.883	19.34	65	11.1	1.11	0.95	86	119
PE8:IT-4F	1.94, 2.21, 1.1	82	0.865	21.83	75	14.16			96	119
PE9:IT-4F (CB/DIO)			0.899	21.5^e	78	15.1				120
PE9:IT-4F (THF/NMP)			0.887	21.1	76	14.2	1.31	0.91		120
PE12:IT-4F	16, 6.4, 2.5	87	0.91^e	21.5	75	14.7				121
PE13:IT-4F	2.64, 2.21, 1.19	83	0.933	20.6	70	13.5				122
PD4:IT-4F	3.25, 4.32, 1.33	83	0.88	20.5	71.9	13	1.2	0.999^e	93	22
PD2:IT-4F	9.76, 7.15, 1.37		0.84	22.2	72.5	13.5			97.8^e	200
PD3:IT-4F			0.86	21.8	77	14.4				201
PD5:IT-4F			0.88	0.88	23.74	0.18				131
PD6:IT-4F	0.0574, 0.0516, 1.1		0.84	20.6	71.09	12.33		0.93		131
PD8:IT-4F	10.7, 5.4, 1.98		0.85	19.74	76	12.7			97	134
PD9:IT-4F	3.15, 1.86, 1.69		0.72	16.8	51.2	6.2	1.23	0.999	86.4	135
PD10:IT-4F	4.43, 3.46, 1.28		0.89	20.4	64.5	11.7	1.19	0.977	93.7	135
PD11:IT-4F	2.87, 1.02, 2.81	82.2^e	0.891	21.03	69.9	13.1		0.931	98	136
PN8:IT-4F	8.98, 4.53, 1.98	80	0.790	20.76	73.5	12.1	1.24	0.998	96	144
PN9:IT-4F	2.11, 2.57, 1.22	81	0.93	19.2	71.5	12.8	1.11	0.935	93.7	145
PO3:IT-4F	5.01, 2.71, 1.85		0.804	20.05	73.1	12.01		1.01	95.7	157
PO4:IT-4F	0.84, 4.1, 5		0.82	21.5	67.8	12.0	1.37	0.96	98	158
PO5:IT-4F	3.4, 2.9, 1.17		0.92	22.4	71.1	14.7	1.21	0.98	98	158
PO6:IT-4F	7.0, 5.7, 1.22		0.94	21.8	72.5	14.8	1.24	0.99	98	158
PO7:IT-4F	4.91, 5.60, 1.14	85	0.813	24.06	65.0	12.70				159
PO8:IT-4F	6.76, 6.11, 1.11	85	0.891	23.40	67.0	13.97				159
PX3:IT-4F	14.2, 5.49, 2.59		0.91	19.41	76	13.31				166
PX4:IT-4F	5.61, 2.21, 2.54		0.94	19.01	71	12.74		0.95		166
PX5:IT-4F			0.931	20.52	66.96	12.79		0.938	95.9	168
PX6:IT-4F	2.60, 2.24, 1.16		0.911	21.42	69.48	13.56		0.929	96.4	168
PX7:IT-4F	1.31. 1.81, 1.38		0.911	21.12	69.28	13.32		0.924	96.3	168
PX8:IT-4F	0.7, 1.64, 2.4		0.891	21.07	69.75	13.1		0.94	95.5	168

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Despite the high performance of EST-based polymer donors, the steric effects of the ester groups lead to twisted backbone structures. To improve the planarity of this kind of polymers, Li et al.¹¹⁹ inserted a TT unit and a difluorinated 2,2′-bithiophene (DFDT) unit as a π-spacer between two EST units to form PE7 (PBDE-TT) and PE8 (PBDE-DFDT). The latter had much better coplanarity due to the F⋯S non-bonding interaction, which enhanced the optical absorption, strengthened the interchain π–π interaction, and shortened the π–π distance, resulting in much improved μ_h (3.4 × 10⁻⁴ cm² V⁻¹ s⁻¹ for PE8vs. 1.11 × 10⁻⁴ cm² V⁻¹ s⁻¹ for PE7). When blended with IT-4F, more compact and ordered packing, higher and more balanced μ_h and μ_e, suppressed recombination, and higher exciton dissociation and charge collection efficiencies were obtained for the PE8:IT-4F blend. As a result, a higher PCE of 14.16% was obtained for PE8:IT-4F compared to PE7:IT-4F (11.1%).

PE12 (PTO2) having BDT and EST building blocks as D and A units, respectively, was developed by Yao et al.¹²¹PE12 showed a low-lying E_HOMO of −5.56 eV, so a high V_OC of 0.91 V was achieved when blended with IT-4F. ΔE_HOMO between PE12 and IT-4F is very small (0.07 eV), but highly efficient exciton dissociation and charge separation were still observed. Through DFT calculations, they found that PE12 and IT-4F have a very large difference in molecular electrostatic potential (ESP) and the induced intermolecular electric field (IEF) may assist the separation of excitons. A high J_SC and FF and thus a high PCE of up to 14.7% were achieved for PE12:IT-4F.

Although using ternary blends can enhance the optical absorption and thus the PCE, a third component would lead to a much more complicated blend morphology and mixing of incompatible materials would result in severe molecular disorder, hampering the device performance. As an alternative, incorporation of an additional building block to form a ternary polymer (terpolymer) is a feasible method to optimize the optical absorption and energy levels. Cui et al.¹²⁰ introduced different amounts of the EST building block into PD2 to tune the electron-withdrawing property of the A units. The introduction of EST led to larger steric effects, but the desired downshifted E_HOMO and broadened absorption. When PE9, which contained 20% EST in the A units, was used as a donor to blend with IT-4F, PCEs of 15.1% for the blend film processed from a chlorobenzene (CB)/1,8-diiodooctane (DIO) system and 14.2% for the blend film processed from a non-halogenated tetrahydrofuran (THF)/N-methyl pyrrolidone (NMP) system were obtained.

Compared with F atoms, CF₃ groups also have strong electron-withdrawing ability, and can be more easily introduced. Meng et al.¹²² introduced CF₃ groups at the terminal of the ester groups of PE12, but eliminating F atoms on the thiophene side chains, to form PE13 (F1). Compared with the polymer without the CF₃ groups (F0), PE13 showed a lower E_HOMO, increased absorption, and stronger intermolecular interactions. DFT results also suggested that the introduction of CF₃ groups did not lead to larger steric hindrance. OSC devices based on the PE13:IT-4F blend showed a much higher PCE of 13.5% than those based on the polymer without CF₃ groups (4.9%).

3.2.2 BDT-based polymer donors using diketone derivatives as A units. To match with IT-4F, a polymer donor PD4 (PBDB-T-SF) was developed by simultaneously introducing F and alkylthio chains on the thienyl substituents of BDT in PD1 (Fig. 8a).²²PD4 has a low-lying E_HOMO of −5.40 eV and a wide bandgap of 1.80 eV. The PD4:IT-4F blend showed a broader optical absorption band, a higher absorption coefficient, and more ordered intermolecular arrangements in comparison with the PD1:ITIC blend. As a result, a high PCE of 13% was obtained for PD4:IT-4F. In addition, the devices maintained a high PCE of 11.99% after storage for 1700 h in an N₂-filled glovebox, demonstrating the excellent device stability. Fan et al.²⁰⁰ used PD2 as a donor to blend with IT-4F. The strong crystallinity of PD2 is beneficial for forming a favorable blend morphology to suppress recombination, which led to a high PCE of 13.5%.

Fig. 8 (a) Molecular energy levels of PBDB-T, ITIC, PBDB-T-SF and IT-4F, reproduced from ref. 22 with permission from the American Chemical Society, copyright 2017; (b) chemical structures of ITIC-Th and IDIC; (c) the maximum PCE versus SC of some polymers; and (d) the AFOM versus ASC for the active layers showing high PCEs in single-junction OSCs. Reproduced from ref. 168 with permission from John Wiley and Sons, copyright 2017.

Although fluorination is an effective way to tune the FMO energy levels and molecular packing, the tedious synthesis and low yield would limit further industrial application. Zhang et al.²⁰¹ replaced the F atoms in PD2 with Cl atoms to form PD3 (PM7). The lower material cost, much shorter synthetic route, and simpler purification of the intermediates made the synthetic cost of PD3 potentially much lower than that for PD2. In addition, introduction of Cl atoms resulted in higher dipole moments and a more delocalized HOMO of the polymer. Except for a slightly lowered E_HOMO, the absorption profile, aggregation effect in solution, and film morphology of PD3 were similar to those of PD2. However, a very high PCE of 14.4% was obtained for the PD3:IT-4F blend. Furthermore, the same group introduced Cl atoms at different positions on the thiophene spacers of PD1 to form PD5 and PD6.¹³¹ These differences had dramatic effects on the molecular packing and thus the photovoltaic performance. PD6 showed a large redshift of 140 nm in its absorption spectrum with a narrower bandgap of 1.78 eV than PD5 (E_g = 2.11 eV). On the other hand, PD5 showed much weaker aggregation behaviour, poorer crystallinity, and almost no SCLC mobility due to the larger steric effects. Consequently, an extremely low PCE of 0.18% was obtained for PD5:IT-4F, while a much higher PCE of 12.33% was obtained for PD6:IT-4F.

Ye et al.¹³⁴ developed a similar polymer donor, PD8 (PBT1-C-2Cl), using p-alkylphenyl groups as the side chains of BDT units and introducing Cl atoms on the 4-position of the thiophene spacer. Compared with PD6, PD8 exhibited a lower-lying E_HOMO of −5.59 eV and a slightly wider bandgap of 1.85 eV. The devices based on PD8:IT-4F showed much enhanced μ_h and μ_e (Table 4), and therefore a higher FF of 76% than that of PD6:IT-4F (70.5%). Thus, a slightly higher PCE of 12.7% was obtained for PD8:IT-4F.

Li et al.¹³⁵ used fluorinated alkoxyphenyl groups as side chains of BDT to form PD10 (PFOPB). Compared with PD9 (POPB) without fluorination, PD10 possessed a deeper E_HOMO, higher extinction coefficient, and higher μ_h. Higher crystallinity with a smaller phase separation size was obtained for the PD10:IT-4F blend. As a result, a higher PCE of 11.7% was obtained. They also replaced the BDD unit with a naphtho[2,3-c]thiophene-4,9-dione (NTDO) unit as the latter possesses a planar structure and offers several loci in the phenyl ring for functionalization.²⁰² Alkylthio side chains were also introduced to lower E_HOMO. The resultant PD11 (PBN-S) showed a low-lying E_HOMO of −5.48 eV, high crystallinity, and ordered molecular packing. Benefitting from the efficient charge separation, high and balanced μ_h and μ_e, ordered molecular packing, appropriate aggregations, and complementary absorption in the active layer, the devices based on PD11:IT-4F demonstrated a high PCE of 13.10%. The devices showed good storage stability, retaining 88% of their initial PCE after 100 days storage in a nitrogen-filled glove-box.¹³⁶ A PCE of up to 10.21% with an active area of 100 mm² was obtained, demonstrating the area scalability of these OSCs.

3.2.3 BDT-based polymer donors using N-heterocycles as A units. Polymers based on BDT and FBTA units showed high cell performance when blended with ITIC. Liao et al.¹⁴⁴ optimized the side chains on FBTA units and introduced Cl atoms onto BDT units to synthesize PN8 in order to optimize the planarity, aggregation property, light-harvesting ability, energy levels, molecular face-on orientation, and packing distance. The devices based on PN8 (L810):IT-4F showed a high PCE of 12.1%. Based on the success of chlorination and alkylsilyl substitutions (PN1–PN5), Su et al.¹⁴⁵ employed these two strategies to obtain PN9 (PBZ-ClSi). This polymer showed a lower E_HOMO, an increased absorption coefficient, and improved charge mobility compared to its parent polymer PBZ.²⁰³ The PN9:IT-4F blend showed high and balanced μ_h and μ_e, less recombination, and higher charge separation and collection efficiencies, which led to a high PCE of 12.8% in OSCs.

3.2.4 Other BDT-based polymers and non-BDT-based polymers. Although alkylsilyl substituted BDT-based polymers showed a high PCE over 10%, the devices based on this kind of polymers always suffered from low FF. The thiazolo[5,4-d]thiazole (TZ) building block with excellent planarity and electron-withdrawing properties has been used to construct polymer donors with a high FF of ∼75% when blended with NFAs.²⁰⁴ Xu et al.¹⁵⁷ synthesized PO3 (PBDS-TZ) using alkylsilyl BDT and TZ as the D and A unit, respectively, to lower E_HOMO and improve the backbone planarity and charge transport. The PO3:IT-4F blend film showed high and balanced μ_h and μ_e, resulting in suppressed recombination and high exciton dissociation and charge collection efficiencies. A high FF of 73.1% was achieved, which contributed largely to a high PCE of 12.01% for the PO3:IT-4F-based OSCs.

Benzo[1,2-d:4,5-d′]bis(thiazole) (BBT) is a weak electron-withdrawing building block for structural modification to improve μ_h and the stability of polymer donors. Wen et al.¹⁵⁸ employed BDT and BBT both with alkylthiophene side chains as the D and A units, respectively, and introduced different halogen atoms (F and Cl) on the thiophene side chains of the BDT units to form polymers PO4 (PBB-H), PO5 (PBB-F), and PO6 (PBB-Cl). When blended with IT-4F, similarly high J_SC values were obtained for these three polymers due to their high extinction coefficient and favourable face-on orientation. Fluorination and chlorination lowered the energy levels, improved the charge transport, and suppressed recombination, which led to a higher V_OC and FF. Among them, the devices based on PO6:IT-4F showed the highest PCE of 14.8%.

Jeon et al.²⁰⁵ synthesized a simple, potentially low cost polymer donor P(Cl) by using BDT and 3-chlorothiophene as D and A units, respectively. P(Cl) showed a high PCE of 12.14% when blended with ITIC-Th (Fig. 8b). Although fluorination or chlorination on BDT units can improve the device performance, the molecular weight and solubility would decrease with the introduction of F or Cl atoms. To achieve a balance between molecular weight and solubility, they introduced a small amount of BDD as A units to form two ternary polymers, PO7 (P(F–Cl)(BDD = 0.2)) and PO8 (P(Cl–Cl)(BDD = 0.2)).¹⁵⁹ Both polymers had higher molecular weights and showed better solubility than those without BDD units (P(F–Cl) and P(Cl–Cl)). High PCE values of 12.7% and 13.97% were obtained for PO7:IT-4F and PO8:IT-4F-based OSCs, respectively, when processed from eco-friendly solvents. For the BDT-based polymers, the π-orbitals of BDT show a high degree of localization, which is not beneficial for intramolecular charge carrier delocalization, resulting in low mobilities.⁴⁹ Replacing the BDT units with imide-oligothiophene can greatly improve the carrier mobility.²⁰⁶ Yu et al.¹⁶⁶ developed two polymer donors with D–A₁–D–A₂ structures using thiophenes as D units and difluorobenzothiadiazole (ffBT) and phthalimide (PhI) or difluoro phthalimide (ffPhI) as the two A units to form PX3 (PhI-ffBT) and PX4 (ffPhI-ffBT), respectively. These polymers are highly crystalline and show a high field effect μ_h of 0.6–0.9 cm² V⁻¹ s⁻¹. PX4 has predominantly edge-on polymer backbone orientation, which is not beneficial for the charge transport in the vertical direction in an OSC. Therefore, the PX4:IT-4F blend showed a lower SCLC μ_h than that of PX3:IT-4F. The PX3:IT-4F blend exhibited tighter π–π stacking and less recombination and higher exciton dissociation and charge collection efficiencies. A higher PCE of 13.31% was obtained for PX3:IT-4F compared to PX4:IT-4F (12.74%).

Sun et al.¹⁶⁷ synthesized PX5 (PTQ10) using thiophene and difluorinated quinoxaline as D and A units, respectively, in only two reaction steps with a high overall yield of 87.4%. PX5 possessed a low E_HOMO of −5.54 eV and a wide bandgap of 1.92 eV and showed a high PCE of 12.70% when an ITIC derivative, IDIC (Fig. 8b), was used as an acceptor. Yuan et al.¹⁶⁸ adopted a combinatory side chain strategy to keep the low synthetic cost while achieving high efficiencies. Siloxane terminated alkoxy and alkoxy groups with different ratios were incorporated to modify PX5 to obtain PX6 (PQSi05), PX7 (PQSi10), and PX8 (PQSi25). Compared with PX5, these polymers showed similar bandgaps, but increased E_HOMO with an increasing amount of siloxane-containing side chains, which resulted in a decrease in V_OC of OSCs based on their blends with IT-4F. Among them, the PX6:IT-4F blend showed the highest and most balanced μ_h and μ_e, least recombination and highest exciton dissociation and charge collection efficiencies (Table 4). Consequently, the devices based on the PX6:IT-4F blend showed the highest PCE of 13.56%. In addition, all these polymers have a low SC and high figure-of-merit (FOM) (a parameter that is related to the manufacturability of OSCs using this material) as well as a low average SC (ASC) and average FOM (AFOM) when considering the cost of acceptors (Fig. 8c and d).¹⁶⁸

3.3 High performance polymer donors matching with Y6

In 2019, Yuan et al.⁹ developed a novel NFA, Y6, which incorporates a BT-based fused-unit as the central core and 2FIC as the end groups (Fig. 6). Compared with ITIC and IT-4F, Y6 has a narrower bandgap of 1.33 eV with a red-shifted optical absorption onset at 931 nm, and a higher α_s-max of 2.39 × 10⁵ M⁻¹ cm⁻¹. The bandgap of Y6 is almost ideal for achieving the maximum PCE based on the Shockley–Queisser limit for single junction solar cells (at an E_g of 1.34 eV).⁴³ The E_LUMO of −4.1 eV was between those of ITIC and IT-4F. Y6 is the acceptor that has achieved the record PCE of 18.22% so far.¹⁰

3.3.1 BDT-based polymer donors using imide derivatives as A units. Since benzotriazole functionalized with an imide moiety (TzBI) can lower E_HOMO, Ding et al.¹²³ developed a novel polymer donor, PE14 (P2F-EHp), using this building block, which showed a low-lying E_HOMO of −5.46 eV and a wide bandgap of 1.82 eV. They also synthesized a new NFA based on Y6 with a shorter central core, BTPT-4F, which has a wider bandgap of 1.45 eV than that of Y6 (1.33 eV). Therefore, the blend of PE14:Y6 showed much wider absorption (with an onset at about 900 nm) than that of PE14:BTPT-4F. In addition, the former blend exhibited more ordered packing with a face-on orientation and smaller domain size for efficient exciton dissociation. A much higher PCE of 16.02% was obtained for PE14:Y6 than that for PE14:BTPT-4F (1.09%). Another widely used electron-accepting building block containing the imide group is thieno[3.4-c]pyrrole-4,6-dione (TPD). Huang et al.¹¹⁰ synthesized two polymers, PE15 (PTPD) and PE16 (PbiTPD), using BDT and one or two TPD units in one repeat as the D and A units, respectively. The biTPD unit has a larger planar structure and stronger electron-withdrawing ability. PE16 showed a lower E_HOMO (−5.20 vs. −5.05 eV) and red-shifted absorption. The blend of PE16:Y6 showed favourable face-on backbone orientation and higher crystallinity. A significantly enhanced PCE of 14.2% was obtained for PE16:Y6 compared to PE15:Y6 (5.9%).

3.3.2 BDT-based polymer donors using diketone derivatives as A units. PD2 is the first donor used to match Y6.⁹ The blend showed a wide EQE curve with the onset at about 900 nm due to the small bandgap of Y6, resulting in a high J_SC of 25.3 mA cm⁻² (Fig. 9a). Together with a high V_OC of 0.83 V and a high FF of 74.8%, a high PCE of 15.7% was obtained. The PCE is relatively independent of the blend film thickness with a high PCE of 13.6% even at a thickness of 300 nm. Although PD2:Y6 achieved a high PCE over 15%, V_OC was relatively low due to the small difference between E_HOMO of PD2 and E_LUMO of Y6. Since PD3 with chlorine substitution had a lower E_HOMO than PD2 (−5.52 vs. −5.50 eV), the devices based on PD3:Y6 showed a higher V_OC of 0.897 V, leading to a much higher PCE of 17.037%.¹³⁰

Fig. 9 (a) EQE curves of PD2:Y6-based OSCs, reproduced from ref. 9 with permission from Elsevier, copyright 2019; and (b) J–V curve for a PO12:Y6-based OSC, reproduced from ref. 10 with permission from Elsevier, copyright 2020.

As PD8 had shown high performance when blended with IT-4F, Xie et al.²⁰⁷ employed Y6 as an acceptor to blend with PD8. Compared with the IT-4F blend, the Y6 blend showed much wider EQE curves due to the red-shifted absorption of Y6. A much improved J_SC of 26.1 mA cm⁻² was obtained for PD8:Y6 than that for PD8:IT-4F (19.74 mA cm⁻²), contributing to the higher PCE of 16.1% of the former.

As mentioned previously, the π-spacer between D and A units plays a key role in improving the coplanarity of the polymer backbone. As a π-spacer, selenophene with lower aromaticity than thiophene can endow a more planar backbone and longer conjugation. Chao et al.¹³⁸ synthesized three polymers, PD13 (PBB), PD14 (PBBSe-H), and PD15 (PBBSe-Cl), to investigate the influence of selenophene π-spacers as well as chlorination on the performance of OSCs. It was found that PD13 without π-spacers and chlorination showed the most twisted structure with blue-shifted absorption. On the other hand, PD15 with π-spacers and chlorination demonstrated the strongest sunlight harvesting capability with a medium bandgap of 1.73 eV, a deep E_HOMO, the largest dipole moment, the most planar polymer backbone, the strongest aggregation, and the highest charge mobility. Therefore, a high PCE of 14.44% was obtained when PD15 was blended with Y6.¹³⁸

As both F atoms and ester groups can downshift the FMO energy levels and enhance the planarity and hence the charge transport, Sun et al.¹³⁹ simultaneously introduced these two types of substituents onto the β positions of thiophene to form FE-T as A units and made several terpolymers with BDT and BDD units via random copolymerization. The resultant polymer PD16 (S1) had a low-lying E_HOMO of −5.52 eV and enhanced aggregation/crystallinity. Moreover, this ternary strategy enabled a favorable balance between crystallinity and miscibility. When this polymer was blended with Y6, a high PCE of 16.42% was obtained.

3.3.3 BDT-based polymer donors using N-heterocycles as A units. The molecular conformation is crucial for the molecular packing and crystallinity, and thus the photovoltaic performance of the polymer donor. Since many D–A polymer donors employ π-spacers between the D and A units, the type of π-spacer would influence the molecular conformation. Tang et al.¹⁵² utilized BDT with chlorothiophene side chains and FBTA as D and A units, respectively, and 3-hexylthieno[3,2-b]thiophene as π-spacers to develop a polymer PN21 (PE4). Compared with the polymer using thiophene as the π-spacers (J52-Cl), the molecular conformation of PN21 using this new spacer changed from a zigzag conformation to a linear conformation, which led to strong π–π stacking, ordered lamellar packing, small phase domains, and high mobility. OSC devices based on the PN21:Y6 blend showed a high PCE of up to 14.02%.

Phenazine (Pz) can be considered as a Qx derivative with extended conjugation. Ding et al.¹⁵⁴ employed dithieno[3,2-a:2′,3′-c]phenazine (DTPz) and 9,10-difluorodithieno[3,2-a:2′,3′-c]-phenazine (FDTPz) as A units, BDT as D units, and thiophene as π-spacers to form two novel polymer donors, PN27 and PN28, respectively. Both polymers formed complementary absorption with Y6, while the fluorinated polymer PN28 showed a lower E_HOMO of −5.46 eV than that of PN27 (−5.33 eV). The devices based on the PN28:Y6 blend showed a higher V_OC (0.867 vs. 0.777 V) and a similar J_SC compared with PN27:Y6. Moreover, due to the higher and more balanced μ_h and μ_e (Table 5), a higher FF (65.8% vs. 61.6%) was obtained for PN28:Y6. Thus, the PN28:Y6 blend showed a higher PCE of 15.14% than that of PN27:Y6 (12.3%).

Table 5 OSC device performances based on blends of polymer donors and Y6

Blend	μ h, μe (×10^−4 cm^2 V^−1 s^−1), μe(h)/μh(e)^a	EQE^b (%)	V OC (V)	J SC (mA cm^−2)	FF (%)	PCE (%)	S 1	S 2	P diss (%)	Ref.
a μ e/μh or μh/μe, whichever is >1. b Maximum value of EQE. c Obtained from VOC ∼ S1 log(Plight). d Obtained from Jph ∼ S2 log(Plight). e The numbers of significant digits of the data are adopted from the original references and may be different.
PE14:Y6	1.18, 0.83, 1.42	82	0.81	26.68	74.11	16.02	1.16	0.99		123
PE15:Y6	0.87, 1.32, 1.52	<70	0.66	19.5	46	5.9^e	1.27	0.98		110
PE16:Y6	5.61, 2.42, 2.32	83	0.83	25.6	66.7	14.2	1.59	0.9^e		110
PD2:Y6	5.90, 2, 2.95		0.83	25.3	74.8	15.7		0.975		9
PD3:Y6	7.46, 4.66, 1.60		0.897	25.644	74	17.037			96.7	130
PD8:Y6	7.1, 5.1, 1.39		0.84	26.1	72.5	16.1				207
PD13:Y6	0.84, 4.1, 5		0.84	4.25	42.36	1.51		0.9		138
PD14:Y6	3.4, 2.9, 1.17		0.65	22.23	56.4	8.17		0.95		138
PD15:Y6	7.0, 5.7, 1.22		0.82	24.07	73.16	14.44		0.96		138
PD16:Y6	7.01, 4.43, 1.67		0.877	25.402	73.7	16.421		0.978	95.8	139
PN21:Y6	8.04, 1.14, 7.05		0.84	22.21	75.43	14.02	1.27	1		152
PN27:Y6	1.33, 0.681, 1.95		0.777	25.7	61.6	12.3		0.933	87.9	154
PN28:Y6	3.42, 2.42, 1.41		0.867	26.53	65.8	15.14		0.958	94.9	154
PO9:Y6	5.62, 4.77, 1.18	76	0.8	23.93	74.7	14.63		0.954	96.7	161
PO10:Y6	1.60, 2.05, 1.28	68	0.87	20.52	70.2	12.57		0.951	94.7	161
PO11:Y6	2.82, 2.81, 1.01	83	0.85	25.41	74.9	16.22		0.981	97.8	160
PO12:Y6	1.49, 1.40, 1.06	87	0.859	27.70	76.6	18.22				10
PO13:Y6	3.23, 1.51, 2.14	85	0.88	25.87	70.7	16.16		0.959	94.6	162
PX5:Y6	32.6, 13.5, 2.41		0.826	26.65	75.1	16.53		0.986	99.4	208
PX9:Y6	2.82, 1.25, 2.26		0.74	22.7	65.73	11.05				169

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3.3.4 Other BDT-based polymers and non-BDT-based polymers. Ding et al.¹⁶¹ used a fused-ring aromatic lactone (FRAL) with strong electron-withdrawing capability and an extended molecular plane as a building block to develop D–A polymer donors. The resulting PO9 (L1) and PO10 (L2) showed complementary absorption spectra with Y6. The higher and more balanced μ_h and μ_e (Table 5) of PO9:Y6 contributed to a higher J_SC (23.93 vs. 20.52 mA cm⁻²) and FF (74.7% vs. 70.2%) and thus a higher PCE (14.36% vs. 12.57%) compared to PO10:Y6. They further replaced the lactone unit with the thiolactone unit to form PO11 (D16), which showed good planarity and thus stronger intermolecular π–π stacking and charge carrier transport, leading to a higher PCE of 16.22%.¹⁶⁰

Later, Ding et al.¹⁰ used dithieno[3′,2′:3,4;2′′,3′′:5,6]benzo[1,2-c][1,2,5]thiadiazole (DTBT) as the A units and BDT as the D units to synthesize a polymer donor, PO12 (D18), which showed a higher μ_e of 1.59 × 10⁻⁴ cm² V⁻¹ s⁻¹ than that of PO11 (1.19 × 10⁻⁴ cm² V⁻¹ s⁻¹) since DTBT has a larger molecular plane. While the high and balanced μ_h and μ_e of the PO12:Y6 blend were responsible for the high FF (76.6%), the complementary absorption contributed to the very high J_SC of 27.7 mA cm⁻² (Table 5). A PCE of 18.22% was achieved (Fig. 9b), which is the highest value reported for single junction OSCs. They also used fluorine- and alkoxyl-substituted benzene (FAB) as A units to synthesize a new polymer donor PO13 (W1) with a low E_HOMO of −5.36 eV and a wide bandgap of 2.16 eV.¹⁶² The PO13:Y6 blend achieved a PCE of 16.16%.

PX5 was synthesized with very low cost and showed high performance when blended with IDIC or IT-4F.^167,168 When PX5 was blended with Y6, much wider absorption and EQE curves were obtained owing to the red-shifted absorption of Y6.²⁰⁸ In addition, higher μ_h and μ_e were also obtained. A largely improved J_SC of 26.65 mA cm⁻² was achieved, leading to a high PCE of 16.53%.

When J52 was blended with Y6, a low V_OC (0.65 V) resulting from the high E_HOMO (−5.12 eV) of J52 and a low FF (53.82%) due to the imbalanced μ_h and μ_e were obtained, leading to a moderate PCE of 7.15%.¹⁶⁹ When IDTT was used to replace BDT in J52, the resulting PX9 (PIDTT-DTffBTA) had a lower E_HOMO of −5.25 eV.¹⁶⁹ When blended with Y6, more balanced μ_h and μ_e were obtained for the blend film. Consequently, a higher V_OC of 0.74 V, an improved FF of 65.73%, and an enhanced PCE of 11.05% were obtained.

Fig. 10 Schematic illustration of three regimes in the Flory–Huggins interaction parameter–polymer volume fraction (χ–ϕ) phase diagram. Regime I, II, and III represent high χ, medium χ (where the purity of the mixed domain is close to the small molecule acceptor percolation threshold), and low χ, respectively. The dashed rectangle marks the reported percolation threshold values where the polymer volume fractions are 0.8–0.9. The composition corresponding to the lower limit for electron transport is termed as the percolation threshold. Reproduced from ref. 228 with permission from John Wiley and Sons, copyright 2018.

3.4 General guidelines for the design of high-performance WBG polymer donors

Based on the above discussions, some useful guidelines for the design of high-performance WBG polymer donors for matching narrow bandgap NFAs are obtained, which are briefly summarized as follows:^42,209–212

(1) A combination of suitable D and A building blocks to form D–A polymers has been almost exclusively used as the strategy to develop WBG polymer donors for matching narrow bandgap NFAs since desired FMOs and bandgaps can be readily obtained by fine selection of donor and acceptor units with different abilities of electron withdrawing or donating properties.³⁵ Thiophene-containing building blocks have been used in the D units due to their appropriate electron-donating ability to obtain the desired E_HOMO and their lower steric effects on the neighbouring units to maintain backbone planarity to achieve high hole mobility. Among them, BDT has been most frequently used due to its large and planar fused-ring extended π-conjugated structure that can facilitate hole transport and more importantly induce face-on crystal orientation for efficient charge transport along the vertical direction in the active layer. Moderately electron-withdrawing building blocks that can maintain a relatively high E_LUMO have been utilized as the A units to construct WBG polymer donors. The electron-withdrawing groups among these building blocks are mainly esters, imides, ketones, N-heterocyles (containing sp² nitrogen), and their combinations. In addition, an electron donating, sterically less demanding π-spacer has often been inserted between the D and A units in a D–A type polymer donor to maintain the backbone planarity and the distance between the D and A units to increase the electronic polarization, which possibly helps stabilize the excitons.^213–216

(2) Several strategies can be used to lower E_HOMO of the polymer donors: (i) introducing electron-withdrawing groups such as fluorine, chlorine, ester, trifluoromethyl, and alkoxy groups;^{121,147,148,217,218} (ii) replacing thienyl side chains with alkoxy or alkylthio substituted phenyl side chains;^133,219 and (iii) replacing the alkyl side chains with alkylthio groups.²²⁰

(3) A coplanar and rigid backbone is needed for high charge carrier mobility, which can be achieved by: (i) extending the conjugation area of the backbone by using larger fused ring building blocks;¹⁹⁶ (ii) replacing thiophene with selenophene, which has lower aromaticity and enhanced ground-state quinoid resonance character, resulting in improved planarity and increased effective conjugation length;¹³⁸ (iii) fluorination or chlorination, which enhances the intermolecular interactions;^217,218 and (iv) strengthening intermolecular interactions through side chain engineering²²¹ and introduction of π-spacers.¹³⁸

3.5 Morphology control in WBG polymer donor:NFA systems

The morphology of the BHJ blend active layer directly impacts the exciton dissociation, charge separation, and transport processes. A bicontinuous network of the donor and acceptor with domain sizes between 10 and 20 nm could provide a large interfacial area between the two domains to facilitate exciton dissociation, while forming a continuous and relatively pure percolating pathway to transport free charges to the corresponding electrodes.^222–228 The extent of this phase separation is critical since domains that are too large (greater than the typical exciton diffusion length on the order of tens of nanometres) will favour geminate recombination, whereas too-mixed domains can increase the rate of non-geminate recombination.^4,229–231 A favourable morphology can be achieved by tuning the donor:acceptor miscibility and crystallinity.²²⁹

Herein, we discuss how changing the donor properties, choosing a certain acceptor in the BHJ blend, and modifying the processing conditions would impact the morphology of the donor:acceptor blend and hence the performance of the OSC device.

3.5.1 Donor and acceptor interaction and miscibility. In most high-performance WBG donor:NFA systems, the as-cast active layer is in the metastable state and the film formation is strongly dependent on the thermodynamics and kinetics of thin film formation from the blend solution.^232–234 The Flory–Huggins interaction parameter (χ) has been used to predict the morphology evolution in the active layer in a number of studies. Three distinct miscibility regions have been defined by Ye et al.²²⁸ as shown in Fig. 10 in the χ–ϕ phase diagram, where ϕ is the polymer volume fraction; an ideal morphology is neither too pure nor too mixed. Parameter χ can be calculated by differential scanning calorimetry (DSC) measurements (melting point depression method) or surface energy measurements of the donor and acceptor (details of the equations used for the calculation can be found elsewhere).^{222,228,232,233,235,236}

Fig. 11 Chemical structures of additional NFAs.

A computational approach to studying the charge separation efficiency between polymer donors and NFAs was proposed by Yao et al..¹²¹ They found that the larger electrostatic potential difference between a WBG polymer donor (PE12) and NFA (IT-4F) was responsible for a high PCE of 14.7%, even though the driving force for charge separation (energy level offsets) in the blend was quite small.¹²¹

3.5.2 Active layer morphology optimization methods. During film casting, the intermolecular forces among polymer chains play a critical role in determining the properties of the resulting condensed state. These properties include the aggregation, morphology, domain purity, molecular orientation, etc. of the polymer in the blend film. For example, rigid polymers (usually polymers with rigid BDT and BDD units) with strong π–π stacking lead to ordered BHJ films with increased domain purity. On the other hand, polymers with relatively weak π–π stacking and less pronounced molecular ordering in solution (such as PX1, PE17 (PTB7), PE1 and PE18 (PBDT-TS1)) tend to form much smaller and random polymer domains upon casting.^{163,236–238}

The morphology of the donor:acceptor blend can be controlled by pre- and post-deposition treatments of the as-cast film. These morphology optimization methods aim to achieve high average domain purity and highly ordered packing of both the donor and acceptor to minimize charge recombination in NFA-based OSCs.²²³

Processing solvent additives are well known for regulating the BHJ active layer morphology. Wang et al.²³⁹ studied the effect of different concentrations of a common solvent additive DIO (1,8-diiodooctane) in the PD1:ITIC system, using a vertical stratification study of the donor and acceptor. They found an optimal DIO content of 0.5%. It is noteworthy that while an increase in the DIO content (to 1%) leads to abruptly enhanced face-on packing of PD1 (beneficial in OSCs), excessive vertical component distribution and over crystallization of ITIC occur, which results in reduced vertical carrier transport paths for both PD1 and ITIC and a reduced PCE. The device with no additive demonstrated an average PCE of 8.74%, J_SC of 15.88 mA cm⁻², V_OC of 0.892 V, and FF of 61.7%. With 0.5% DIO an increased J_SC and FF of 17.23 mA cm⁻² and 69.3%, respectively, resulted in a high PCE of 10.75%. With 1% DIO, both J_SC and the FF dropped to 15.23 mA cm⁻² and 66.8%, respectively, thereby leading to a lower PCE of 9.27%.

Solvent vapor annealing (SVA) is an efficient post-processing technique to control the morphology of the active layer. It serves as a method for molecular level rearrangement by penetrating solvent vapor into the active layer.^240,241 Jiao et al.²⁴⁰ used four common annealing solvents, methylene chloride (CH₂Cl₂), chloroform (CHCl₃), tetrahydrofuran (THF), and carbon disulfide (CS₂), to study the effect of active layer SVA treatment on the performance of PD3:IT-4Cl-based OPVs, achieving significantly improved performance compared with that of the as-cast OPVs. SVA (especially with CS₂) enhanced the charge generation, transport, and collection efficiency. The PD3:IT-4Cl device with CS₂ solvent treatment showed the highest PCE of 13.76% with a J_SC of 20.53 mA cm⁻², a V_OC of 0.87 V and a FF of 77.05%, while the control device showed a lower J_SC (19.28 mA cm⁻²) and FF (70.36%), and a similar V_OC of 0.88 V with a PCE of 11.94%.

Thermal annealing is another widely used simple method for morphology optimization, mostly due to the temperature-dependent aggregation behaviour of donor–acceptor phases.^{223,233,242–244} For example, Bin et al.¹⁴¹ reported a more than 25% improvement in the PCE of the PN2:ITIC device upon annealing at 150 °C for 10 min due to enhanced crystalline domains with prominent face-on orientation leading to higher SCLC mobility (improved charge transport) as compared to unannealed devices. The as-prepared PN2:ITIC device showed a PCE of 9.03% with a J_SC of 14.81 mA cm⁻², a V_OC of 0.96 V and a FF of 63.63%, while, upon annealing, the device showed a significantly higher J_SC (17.32 mA cm⁻²) and FF (69.77%) and a similar V_OC (0.94 V), resulting in a PCE of 11.41%. Especially for the high performance WBG polymer PD1, with temperature dependent aggregation behavior, thermal annealing is necessary to achieve the optimal photovoltaic performance.¹⁴¹ The critical parameters that determine the effectiveness of thermal annealing for a polymer:NFA system are the glass transition temperature (T_g) and miscibility of the components (χ).²³³

4 Desirable properties of WBG polymer donors for high efficiency NFA-based OSCs

4.1 Molecular packing and charge transport

The choice of polymer donor will affect not only μ_h of the blend but also the molecular ordering of the NFAs significantly and thus μ_e and the domain purity (miscibility).²⁴⁵ A coplanar polymer with reduced steric hindrance in the backbone is favorable for ordered molecular packing and intermolecular charge transport and is correlated with higher μ_h of neat and blend films. For example, Li et al.¹¹⁹ employed covalent and noncovalent (F⋯S conformational lock) backbone rigidification strategies, respectively, to design two BDT and ester-modified oligothiophene-based copolymers (PE7 and PE8). Hydrogen-bonding interactions induced in the DFDT-based unit in polymer PE8 resulted in stronger interchain π–π interactions, a smaller π–π distance with a longer coherence length (L_c) and thus a significantly higher μ_h compared to its TT counterpart. A V_OC of 0.86 V, a high J_SC of 21.83 mA cm⁻² and a remarkable FF of 75%, and thus a high PCE of 14.16% were obtained for the PE8:IT-4F device, whereas the PE7:IT-4F device showed a significantly lower J_SC (19.34 mA cm⁻²) and FF (65%), and thus a low PCE of 11.10%. Liu et al.¹²⁷ designed two new WBG polymers (PE19 (PB2T) and PE20 (PB3T)) based on BDT and oligothiophene units. PE20 exhibits a more planar geometry, whereas PE19 has a highly twisted backbone conformation; therefore, a PCE of 11.9% was achieved for the PE20:IT-M (Fig. 11) device compared to the very poor performance (0.01%) of the PE19:IT-M device. Tang et al.¹⁷⁰ used a backbone fluorination strategy to improve the crystallinity (lamellar and π–π packing coherence length) and thus SCLC mobilities, making use of the introduced F⋯F, C–F⋯H, and F⋯S interactions in PX11 (PDTF-TZNT) compared to non-fluorinated counterpart PX11. Consequently, the PX10:IT-M device showed a relatively low PCE of 4.42% with a J_SC of 10.15 mA cm⁻², a V_OC of 0.75 V, and a FF of 58.1%, whereas the PX11:IT-M device obtained a higher PCE of 10.05% with an improved J_SC of 17.33 mA cm⁻², a V_OC of 0.80 V, and a significantly high FF of 72.5%.

Achieving high FF requires efficient charge extraction capability facilitated by high mobilities of both holes and electrons. As seen in Fig. 12a, for high performance systems, blend carrier mobilities in excess of 10⁻⁴ cm² V⁻¹ s⁻¹ can achieve a FF of >70%. To achieve such moderate to high mobilities in the device, the polymer donor is preferred to have a planar backbone with moderate aggregation capability, predominantly face-on orientation, and covalent and noncovalent backbone rigidification induced by strategies such as fluorination, conformation locking, and/or side chain modification. In addition, the mobility ratio, crystal sizes, and coherence length of the π–π stacking order of the polymer in the blend are also crucial (OSC performance dependencies on the mobility ratio and (010) coherence length are plotted in Fig. 12b and c, respectively). For perspective, most high-performance systems utilize polymers with neat film SCLC mobilities in excess of 10⁻⁴ cm² V⁻¹ s⁻¹ for achieving a high FF of >60% (Fig. 12d). Importantly, an overly crystalline polymer donor can also lead to decreased miscibility with the NFA in the blend, leading to large pure aggregated domains, which is detrimental to charge transport.²³⁶

Fig. 12 Plots of (a) SCLC electron and hole mobilities measured in blend films vs. FF, (b) SCLC mobility ratio (μ_h/μ_e) vs. the corresponding FF, (c) coherence length along the [010] direction of crystal packing in blend films vs. the respective PCE, and (d) neat film SCLC mobility vs. the corresponding FF in OSCs with WBG polymer donors and NFAs discussed in this review.

4.2 Matching energy levels for minimizing E_loss

For a long time, it has been considered that a sufficient energy level offset of ≥0.3 eV between organic donor and acceptor materials, ΔE_HOMO or ΔE_LUMO, is required for efficient charge separation mostly based on observations for fullerene-based OSCs.^{87,121,192–195} This is due to the nature of organic semiconductors, which typically have much larger exciton binding energies (∼0.3–1 eV) than inorganic semiconductors. However, due to the advent of efficient NFAs, several studies have demonstrated highly efficient OSCs with efficient and fast charge separation at a ΔE_HOMO (and/or ΔE_LUMO) much smaller than 0.3 eV. For example, Sun et al.¹⁷¹ used finely tuned WBG polymer donor:NFA acceptor system PX12 (PTQ11):TPT10 (Fig. 11) to achieve highly efficient exciton dissociation and charge transfer, resulting in a high PCE of 16.32% with a high V_OC of 0.88 V, a large J_SC of 24.79 mA cm⁻², and a high FF of 74.8%, despite the zero ΔE_HOMO value. Similar instances of a high J_SC (high EQE) despite an incredibly low or zero ΔE_HOMO or ΔE_LUMO have been reported elsewhere.^65,246–250

The voltage loss (V_loss), calculated from the difference between the optical bandgap of the solar cell active layer and V_OC of the device (V_loss = E_g/q − V_OC), is the key concern for obtaining the maximum performance from a given pair of a WBG polymer donor and NFA.^248,250 This voltage loss (also referred to as energy loss, E_loss = eV_loss) can be minimized by increasing the difference between E_LUMO of the acceptor and E_HOMO of the donor as V_OC of OSCs is proportional to this difference. Therefore, if fast and efficient charge separation can be achieved in blends with almost no energy offsets, the LUMO and HOMO of the donor polymer could align with the acceptor to minimize ΔE_HOMO and ΔE_LUMO, effectively decreasing V_loss.¹⁷¹ This was one of the two design criteria proposed by Qian et al.,²⁴⁸ where they used spectroscopic and quantum-chemistry approaches to identify key rules for minimizing voltage losses: (1) a low energy offset between the donor and acceptor molecular states and (2) high luminescence of the low-bandgap component.

Nevertheless, several studies have reported the need to fine tune the molecular energy alignment to provide a “certain minimum” driving force for balancing the exciton dissociation probability and V_OC loss. Yang et al.⁶⁵ used structurally similar donor and acceptor combinations (with similar blend morphologies and charge transport properties) to study the performance variations corresponding to their different energy-level offsets. They found that the energy offset has no impact on the charge transport process, but it does affect the exciton dissociation process in NFA-based OSCs. A larger ΔE_HOMO between the donor and acceptor can provide sufficient driving force, which contributes to a higher exciton dissociation probability (P_diss) and PCE. Although minimizing ΔE_HOMO can effectively reduce E_loss (or improve V_OC), the decreasing driving force will diminish the exciton dissociation process. Moreover, as ΔE_HOMO falls below a threshold, P_diss drops sharply, reducing the overall performance of the device.⁶⁵ In yet another study investigating the relationship between E_loss and the device performance of NFA-based OSCs, Ling et al.²⁴⁹ found that donor and acceptor blends with an E_loss below 0.6 eV demonstrated inefficient charge transfer between the polymer donor and NFA, resulting in a low EQE and PCE (an example of the trade-off between device performance and energy loss). The PD1:IE-4Cl (Fig. 11)-based device afforded a PCE of 11.1% (with a high J_SC of 21.49 mA cm⁻² and a moderate FF of 60%) and E_loss of 0.64 eV, whereas the PD1:IEIC based device obtained a low PCE of 3.8% (with a low J_SC of 7.65 mA cm⁻² and low FF of 45%) despite the lower E_loss of 0.52 eV.

The reasons as to why some polymer donor:acceptor blend systems require a very low ΔE_HOMO or ΔE_LUMO for efficient charge separation and show a very low E_loss are still not well understood, but seem to correlate with both the donor and acceptor materials. More examples are needed to probe the relationships of the structures (chemical and morphological) and optoelectronic properties of the polymer donors with E_loss of the OSC devices, which are extremely important for maximizing the potential of OSCs.

4.3 Characteristics of top performing polymer donors and OSCs

The OSC performance parameters, V_OC, J_SC, FF, and PCE, of the WBG polymer donors paired with ITIC, IT-4F, and Y6 discussed in this review are plotted in Fig. 13. It can be seen that V_OC decreases and J_SC increases from ITIC to IT-4F to Y6, while the FF is similar among the three NFAs even though the IT-4F based devices show a slightly higher FF. From ITIC to IT-4F to Y6, the FMO energy levels downshift and the absorption spectra red-shift, which leads to a narrowed optical bandgap, resulting in reduced V_OC, but increased J_SC. The overall PCE of the OSCs follows the order of ITIC < IT-4F < Y6, which is due to the relatively large contribution of J_SC that supersedes the reduction in V_OC. The PCE values of the top five high-performing polymer donors and the photovoltaic parameters of the best polymer donor for each NFA are shown in Fig. 14.

Fig. 13 V _OC (a), J_SC (b), FF (c), and PCE (d) of high-performance OSCs using WBG polymer donors and different NFAs, ITIC, IT-4F and Y6, discussed in this review.

Fig. 14 Device performances of the top performing polymer donors matching with ITIC, IT-4F and Y6, where the photovoltaic parameters shown are from the best performing PN16:ITIC, PE9:IT-4F and PO12:Y6 systems.

Some distinct characteristics of these high-performance polymer donors (in Fig. 14) can be observed. Firstly, the E_HOMO values of the donors matching with ITIC are −5.3 to −5.5 eV, while those of the donors matching with IT-4F and Y6 are −5.5 to −5.6 eV, which are decided by E_HOMO of the NFAs since a sufficient ΔE_HOMO for efficient dissociation of excitons generated in the NFA phase is required. Secondly, these WBG polymer donors have E_g values of 1.9–2.1 eV that cover the short wavelength region (where the NFAs absorb poorly) to form complementary absorption to achieve high J_SC. Lastly, these polymers have SCLC hole mobilities higher than 10⁻⁴ cm² V⁻¹ s⁻¹, which could achieve high and balanced μ_h and μ_e in their blends with NFAs, leading to high FF. All these blends have a μ_h/μ_e ratio very close to 1 except for PX5:Y6, which has a μ_h/μ_e ratio (2.41) greater than 2, where both μ_h and μ_e are higher than 10⁻³ cm² V⁻¹ s⁻¹, which would probably not impede charge transport even though μ_h and μ_e are not well balanced in this case.

5 Conclusions and outlook

In the past few years, tremendous progress has been made in the development of WBG polymer donors that match the emerging high-performance narrow bandgap NFAs, achieving remarkable PCEs of up to 18.22%, which has beaten the predicted PCE limit of ∼10% for OSCs based on fullerene acceptors.²⁵¹ In this review, we have examined the most representative WBG polymer donors that have been used to match three typical NFAs: ITIC, IT-4F, and Y6. The structure–property–cell performance relationships are probed by analysing the structural design strategies to control and finely tune the FMO energy levels, adjust the light absorption profile, and enhance the hole mobility to match those of different NFAs. The morphology control of the polymer blends to create desirable phase separation and crystallinity for achieving high device performance is also discussed.

Despite these recent advances in the development of polymer donors and OSCs in general, there is room for further improvement of the cell performance, while some challenges and issues need to be overcome in order to make OSCs a commercial reality.

(1) Further boosting the PCE of OSCs

Theoretical limit of the PCE for OSCs. According to Shockley and Queisser,⁴³ the maximum PCE of a single-junction solar cell is 33.7% (the S–Q limit) achievable with a semiconductor having a bandgap of 1.34 eV. Hou et al.¹⁷ predicted that a PCE of >20% for OSCs is possible if V_loss can be lowered to <0.45 V, a realistic value considering that a V_loss of 0.3 V has been reported recently for perovskite solar cells,²⁵² while the current state-of-the art OSCs show a V_loss of ca. 0.6 V.^253,254 Therefore, theoretically there is still much room for further enhancement of the PCE for OSCs.

Reducing the energy loss. Organic materials with large dielectric constants in the same range as silicon (∼11) could reduce the coulombic interaction in excitons, or the exciton binding energy, to enable highly efficient exciton dissociation.^87,90,95,255 Consequently, thicker layers could be used to maximize the absorption of light (and to ease large-scale fabrication). Moreover, the largely reduced exciton binding energy would only require very small ΔE_HOMO and ΔE_LUMO for exciton dissociation, which could greatly reduce the energy loss. Furthermore, the reduced exciton binding energy would enlarge the exciton diffusion distance, which may enable bilayer heterojunction OSCs without delicate and tedious morphological control of the blend layer to realize a nanoscale BHJ structure. This would be very desirable for single component-organic solar cell devices (SC-OSCs), where the polymer in the active layer having both donor and acceptor components could greatly simplify the device fabrication process and achieve high morphological stability.⁹⁸ Very recently, Jiang et al.²⁵⁶ achieved a record PCE of 8.40% in SC-OSCs using a double-cable conjugated polymer.

(2) Long-term stability

Despite the considerable efforts made over the past few decades, OSCs have not become a commercial reality yet. The stability of OSCs is one of the main obstacles in the path towards commercialization. Solar cells work under various operational stresses such as solar illumination, increased temperature, and exposure to air.^{102,257–262} The intrinsic degradation mechanisms of the active layer in OSCs include degradation in the dark caused by the morphological changes (rearrangement of donor and acceptor domains) that occur over time and photo-chemical reactions of the organic semiconductor materials in the presence of illumination.^263–267 In terms of morphological stability, all-polymer solar cells have attracted much attention since a polymer donor and a polymer acceptor can be entangled to form a much more metastable blend, leading to superior thermal and mechanical stabilities compared to small molecule-based OSCs.^268–274 Notable progress has been made in improving the lifetime of OSCs with extrapolated lifetimes (with 80% PCE retention) of over 20 years being demonstrated recently.^263,275

(3) Fabrication of large area devices

Another big obstacle towards the commercialization of OSCs is the poor scalability of OSC devices. Most, if not all, of the OSCs showing high PCEs greater than 10% are reported with an active area of less than 1 cm². Increasing the device active area usually results in a significant reduction in the PCE. Although promising large area modules have been reported,^276,277 the lab-to-manufacturing translation seems still very challenging and requires urgent attention.²⁷⁸

(4) Low cost

Commercialization of OSCs will require a competitive low manufacturing cost-to-PCE ratio along with energy payback and embodied energy metrics comparable to existing silicon solar cells.^279,280 It has been predicted that for OSCs having unique merits such as flexibility, lightness, and high throughput manufacturability, a module efficiency of 10% with an operational lifetime of 10 years and a production cost of <$0.5 per W_p would be enough to reach the threshold of commecialization.²⁸¹ For the donor:acceptor combination to be fully scalable, it should possess a high value of the industrial figure of merit (i-FOM).^282–284 This parameter takes account of the synthetic complexity (SC) of the donor and acceptor materials as well as the PCE and lifetime of the OSC device. Most of the high PCE polymer donors require tedious synthesis and use of toxic substances, resulting in very high SC and thus insufficient i-FOM. The importance of developing polymer donors with low SC has been increasingly realized. On the other hand, small molecule donor materials have found increased interest in the last couple of years for their industrial scalability owing to the advantages of well-defined chemical structures, synthetic ease, high purity of materials, and outstanding repeatability from batch to batch synthesis.^285,286

(5) Environmentally friendly manufacturing

Most of the high performance OSCs are fabricated using halogenated solvents such as o-dichlorobenzene, chlorobenzene, or chloroform, which are environmental hazards. These solvents might be suitable for proof-of-concept lab-based research, but for large scale production alternative eco-friendly solvents (green solvents) need to be used.^287,288 As discussed, some polymer donors that can be processed using green solvents and yet show high PCE have been reported.^120,289,290

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors are thankful for the support from the Discovery Grants (RGPIN-2016-04366) and Strategic Partnership Grants for Projects (STPGP 506317-17; STPGP 521458) of the Natural Sciences and Engineering Research Council of Canada (NSERC).

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