Revolutionary SAMs: transforming inverted perovskite solar cells

Abstract

A material has the potential to reshape the trajectory of a field. Currently, in the realm of inverted perovskite solar cells (PSCs), self-assembled monolayers (SAMs) with distinctive structures are at the forefront of this revolution. The team led by Steve Albrecht has developed a unique type of self-assembled monolayer (SAM) incorporating phosphonic acid and carbazole groups, applied as the p-type functional layer in inverted perovskite solar cells. Materials of this kind, along with their derivatives, have remarkably propelled the efficiency of inverted perovskite solar cells (PSCs) and perovskite/silicon tandems to the pinnacle. We are delighted to designate them as Revolutionary SAMs (R-SAMs). In this perspective, a brief overview of the developmental history, synthesis processes, and utilization methods of R-SAMs is provided. Most notably, we present our perspective on the advantages of R-SAMs in inverted PSCs, emphasizing molecular configurations and the interaction of electron clouds. Simultaneously, we address some opportunities and challenges for future R-SAMs design.

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n–i–p (regular) and p–i–n (inverted) configurations of perovskite solar cells have long been locked in a competitive stance. Prior to 2021, the regular structure drew significant attention from both laboratories and some enterprises owing to its remarkable photoelectric conversion efficiencies (PCEs >25.5%).¹ While the inverted perovskite structure offers advantages such as improved thermal stability and photoelectric stability by sidestepping the use of doped organic functional layers (e.g., Li-TFSI doped Spiro-OMeTAD), it has trailed behind in the efficiency race.

In 2019, the team led by Steve Albrecht developed a kind of self-assembled monolayer (SAM) incorporating phosphonic acid and carbazole groups, which was applied in inverted perovskite solar cells as a p-type functional layer.² The introduction of such SAMs, either replacing or complementing conventional poly(triarylamine) (PTAA) and NiO_x, swiftly sparked a leap in the performance of inverted perovskite solar cells, earning them the designation of Revolutionary SAMs (R-SAMs).^3,4 Researchers rapidly iterated on similar molecular configurations of R-SAMs, leading to a prompt enhancement in the power conversion efficiencies (PCEs) of inverted perovskite solar cells (Fig. 1a).^5–7 The efficiencies of these inverted photovoltaic devices have swiftly surpassed the 25% bottleneck and surpassed the world-record efficiency of the regular structure (26.0%, ISCAS), reaching new world-record efficiencies of 26.1% (USTC, NU/UT).⁸ Simultaneously, perovskite/silicon tandem solar cells have also reaped benefits from these R-SAMs, achieving remarkable progress with an efficiency exceeding 33.7%.^9,10 At the same time, different research teams worldwide consistently achieve high device performance through the utilization of R-SAMs, highlighting their universality and reliability.^11–13

Fig. 1 (a) Certified high-efficiency perovskite solar cells: semi-transparent blue balls correspond to perovskite/silicon devices without R-SAMs, blue represents perovskite/silicon devices with R-SAMs, and black denotes n–i–p structured perovskite solar cells (all without R-SAMs). Red illustrates p–i–n structured perovskite solar cells without R-SAMs, while orange stars indicate p–i–n structured perovskite solar cells with R-SAMs. (b) Some molecules of R-SAMs include 2PACz, (4-(2,7-dibromo-9,9-dimethylacridin-10(9H)-yl)butyl)phosphonic acid (DMAcPA) and poly[(4-(9H-carbazol-9-yl)butyl)phosphonic acid] (poly-4PACz). (c) Illustration of R-SAMs at the buried and grain boundaries/upper interfaces of inverted perovskite devices, and the potential local electric field.

The molecular configuration of R-SAMs is depicted in Fig. 1b, where the head groups consist of electron-donating moieties with strong coordinating abilities, especially phosphonic acid (PA) groups. A short-chain carbon linker serves as a spacer, while conjugated ring groups, such as carbazole, acridine, etc., function as π-delocalized systems. The quintessential structures in this category are (2-(9H-carbazol-9-yl)ethyl)phosphonic acid (2PACz) and (4-(3,6-dimethyl-9H-carbazol-9-yl)butyl)phosphonic acid (Me-4PACz). Additionally, polymers with similar molecular configurations, such as poly-4PACz, can also be classified within R-SAMs.¹⁴ The synthesis of R-SAMs is straightforward. Taking 2PACz as an example, it involves an ammonolysis reaction, a Michaelis–Arbuzov reaction, and a hydrolysis reaction, resulting in a total yield of over 40%.

The methods for modifying perovskite solar cells with R-SAMs are versatile and straightforward. Spin-coating and immersion are two simple preparation techniques. R-SAMs can be directly spin-coated or immersed on ITO or FTO substrates, as well as on p-type functional layers such as PTAA and NiO_x. In large-scale production, the potential use of blade-coating for R-SAMs is a viable technique. Additionally, introducing R-SAMs directly into the perovskite precursor solution allows for their spontaneous diffusion to the upper and buried interfaces, as well as grain boundaries, through antisolvent engineering. The advantage of this approach lies in streamlining the fabrication process by reducing the number of layers.¹⁵ The Co-SAM strategy, involving a mixture of two SAMs or a blend of R-SAM with a modified additive, and sequential interface engineering have paved the way for synergistic R-SAM action at the perovskite interface, thereby yielding novel interfacial properties.^4,11,16,17

The utilization of R-SAMs in fabricating perovskite solar cells can significantly enhance the fill factor, open-circuit voltage, and strengthen the quasi-Fermi level splitting (QLFS). R-SAMs create an energetically aligned interface with the perovskite absorber, improving carrier transport and minimizing non-radiative losses, achieved by fine-tuning the position of the work function and HOMO energy level. Additionally, R-SAMs have the capability to enhance the crystallinity of perovskite and mitigate defects. Some comprehensive reviews have discussed these beneficial influences mentioned above.^18,19 Here, we briefly outline the benefits from a molecular configuration perspective, using 2PACz as an example (Fig. 1C). 2PACz possesses a substantial dipole moment, with electron cloud density concentrated on the PA group. During the contact process between 2PACz and the metal oxide bottom interface (ITO/FTO/NiO_x), the PA group anchors to the oxide bottom interface. This process is self-limiting as the PA groups only attach to surface sites where there is still bare oxide.² In this anchoring process, the PA group tends to form strong P–O–M bonding and also provides coordinating electron pairs to metal ions, resulting in an exchange of electron clouds. The electron cloud density of the PA group shifts towards covering the metal oxide. At the other end, the π-delocalized system will utilize π–π conjugation to establish a certain order. Upon reaching equilibrium after the completion of molecular self-assembly, a local electric field is formed at the interface between 2PACz and the metal oxide. The direction of this local electric field, in synergy with the π-conjugated delocalized rings, is highly favorable for the additional transport of photogenerated holes to the metal oxide. Similarly, for the grain boundaries/upper interface of the perovskite, R-SAMs may also play a potential role. He et al. dissolved R-SAMs in the perovskite precursor solution and found that portion R-SAMs were present at the grain boundaries/upper interface of the perovskite, yielding positive effects.¹⁵

Prospects and challenges

The emergence of novel R-SAMs represents a revolution in the realm of inverted perovskite solar cells, accelerating the commercialization of both inverted perovskite solar cells and tandem solar cells. Here's a concise overview of the advantages of R-SAMs: (1) R-SAMs prevent direct contact between metal oxides and perovskite, thereby mitigating potential catalytic reactions, whether photoelectric, humidity or thermal, that metal oxides may induce on perovskite. (2) Designing the head groups, backbone, and terminal groups of R-SAMs allows for precise tuning of the work function and HOMO energy level. (3) R-SAMs effectively regulate the nucleation, crystallization, and growth processes of perovskite films, leading to a reduction in defects. (4) R-SAMs with acid groups, particularly phosphonic acid as anchoring groups, exhibit high binding energy with metal oxide substrates and possess exceptional thermal stability.²⁰ (5) Ultra-thin R-SAMs minimize resistance to charge transport, which is thickness-dependent.^21,22 And further research is warranted to design and iterate upon the molecular structures of R-SAMs to better match the perovskite interface. Key considerations may include:

Reducing charge carrier interface recombination

(1) Enhancing the compatibility of R-SAMs with the microstructure of the substrate (ITO/FTO or NiO_x, etc.), improving molecular coverage, and reducing local aggregation, particularly to ensure the reliability of R-SAMs in large-scale preparation of perovskite modules.²³

(2) Fine-tuning the work function and LUMO/HOMO level. (a) Considering the effective dipole moment originating from the molecule, while also assessing the intramolecular and intermolecular depolarization effects;^24–26 (b) considering the electron cloud densities overlapping from anchoring groups to substrates by the covalent interaction (e.g., P–O–M and P=O⋯M).

Reducing defects

(1) Improving perovskite precursor wetting behavior on R-SAMs and investigating the impact of R-SAMs on the growth of perovskite films. Further developing a Co-mix strategy and sequential interface engineering.

(2) Enhancing the interaction between R-SAMs' terminal groups and perovskite by substituent modification. Improving the van der Waals forces to achieve stronger interaction.

Relating to the stability

Enhancing the chemical stability of R-SAMs under the operating conditions of perovskite devices, including exposure to light, heat, bias and humidity.

Others

Modulating the arrangement angle of the π-delocalized conjugated system, favoring either an edge-on or a face-on orientation.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This perspective was supported by the National Natural Science Foundation of China (52102267).

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