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Currently, there are only very few dopant‐free polymer hole‐transporting materials (HTMs) that can enable perovskite solar cells (PVSCs) to demonstrate a high power conversion efficiency (PCE) of greater than 20%. To address this need, a simple and efficient way is developed to synthesize novel crossconjugated polymers as high performance dopant‐free HTMs to endow PVSCs with a high PCE of 21.3%, which is among the highest values reported for single‐junction inverted PVSCs. More importantly, rational understanding of the reasons why two isomeric polymer HTMs (PPE1 and PPE2) with almost identical photophysical properties, hole‐transporting ability, and surface wettability deliver so distinctly different device performance under similar device fabrication conditions is manifested. PPE2 is found to improve the quality of perovskite films cast on top with larger grain sizes and more oriented crystallization. These results help unveil the new HTM design rules to influence the perovskite growth/crystallization for improving the performance of inverted PVSCs. Two isomeric crossconjugated polymer hole‐transporting materials (HTMs) are developed to demonstrate significantly distinct device power conversion efficiencies (PCEs) under the same device fabrication conditions, 11.1% PPE1 and 19.3% for PPE2, which is found to be due to the improved quality of perovskite films made on top of PPE2. More excitingly, the PPE2‐based perovskite solar cells (PVSCs) can further achieve a more impressive PCE of 21.3% through suitable surface passivation.

Although quasi‐2D Ruddlesden‒Popper (RP) perovskite exhibits advantages in stability, their photovoltaic performance are still inferior to 3D counterparts. Optimizing the buried interface of RP perovskite and suppress energetic losses can be a promising approach for enhancing efficiency and stability of inverted quasi‐2D RP perovskite solar cells (PSCs). Among which, constructing polymer hole‐transporting materials (HTMs) with defect passivation functions is of great significance for buried‐interface engineering of inverted quasi‐2D RP PSCs. Herein, by employing side‐chain tailoring strategy to extend the π‐conjugation and regulate functionality of side‐chain groups, target polymer HTMs (PVCz‐ThSMeTPA and PVCz‐ThOMeTPA) with high mobility and multisite passivation functions are achieved. The presence of more sulfur atom‐containing groups in side‐chain endows PVCz‐ThSMeTPA with increased intra/intermolecular interaction, appropriate energy level, and enhanced buried interfacial interactions with quasi‐2D RP perovskite. The hole mobility of PVCz‐ThSMeTPA is up to 9.20 × 10−4 cm2 V−1 S−1. Furthermore, PVCz‐ThSMeTPA as multifunctional polymer HTM with multiple chemical anchor sites for buried‐interface engineering of quasi‐2D PSCs can enable effective charge extraction, defects passivation, and perovskite crystallization modulation. Eventually, the PVCz‐ThSMeTPA‐based inverted quasi‐2D PSC achieves a champion power conversion efficiency of 22.37%, which represents one of the highest power conversion efficiencies reported to date for quasi‐2D RP PSCs. By adopting side‐chain tailoring strategy, two polymer hole‐transporting materials with high mobility and multisite passivation functions are developed for the buried‐interface engineering of inverted quasi‐2D Ruddlesden‒Popper perovskite solar cells (PSCs). Among which, PVCz‐ThSMeTPA‐based inverted quasi‐2D PSCs achieve impressive power conversion efficiency of 22.37% along with excellent thermal and long‐term stability.

The structural modification of hole‐transporting materials (HTMs) is an effective strategy for enhancing photovoltaic performance in perovskite solar cells (PSCs). Herein, a series of dithienopyran (DTP)‐based HTMs (Me‐H, Ph‐H, CF3‐H, CF3‐mF, and CF3‐oF) is designed and synthesized by substituting different functional groups on the DTP unit and are used fabricating PSCs. In comparison with Me‐H having two methyl substituents on the dithienopyrano ring, the Ph‐H having two phenyl substituents on the ring exhibits higher PCEs. Notably, the incorporation of trifluoromethyl groups in CF3‐H endows the molecule with a larger dipole moment, deeper HOMO energy level, better film morphology, closer molecular stacking, more efficient defect‐passivation, enhanced hydrophobicity, and better photovoltaic performance when compared with the Ph‐H counterpart. Furthermore, the HTMs of CF3‐mF and CF3‐oF, which feature fluorine‐substituted triphenylamine, demonstrated excellent film‐forming properties, more suitable energy levels, enhanced charge mobility, and improved passivation of the buried interface between HTMs and perovskite. As a result, PSCs employing CF3‐mF and CF3‐oF gave impressive PCEs of 23.41 and 24.13%, respectively. In addition, the large‐area (1.00 cm2) PSCs based on CF3‐oF achieved a PCE of 22.31%. Moreover, the PSCs devices with CF3 series HTMs exhibited excellent long‐term stability under different conditions. A series of dithienopyran (DTP)‐based HTMs are reported by substituting different functional groups, which are applied for high‐performance perovskite solar cells (PSCs). The trifluoromethyl‐substituted CF3‐oF exhibits remarkable PCEs in both small‐area (0.09 cm2, 24.13%), and large‐area (1.00 cm2, 22.31%) devices, with good stability owing to its superior hole transport, film formation properties, and hydrophobicity.

Perovskite solar cells (PSCs) with efficiencies greater than 20% have been realized mostly with expensive spiro‐MeOTAD hole‐transporting material. PSCs are demonstrated that achieve stabilized efficiencies exceeding 20% with straightforward low‐cost molecularly engineered copolymer poly(1‐(4‐hexylphenyl)‐2,5‐di(thiophen‐2‐yl)‐1H‐pyrrole) (PHPT‐py) based on Rutin–silver nanoparticles (AgNPs) as the hole extraction layer. The Rutin–AgNPs additive enables the creation of compact, highly conformal PHPT‐py layers that facilitate rapid carrier extraction and collection. The spiro‐MeOTAD‐based PSCs show comparable efficiency, although their operational stability is poor. This instability originated from potential‐induced degradation of the spiro‐MeOTAD/Au contact. The addition of conductive Rutin–AgNPs into PHPT‐py layer allows PSCs to retain >97% of their initial efficiency up to 60 d without encapsulation under relative humidity. The PHPT‐py/ Rutin–AgNPs‐based devices surpass the stability of spiro‐MeOTAD‐based PSCs and potentially reduce the fabrication cost of PSCs. The low hysteresis of PSCs originates from better interconnectivity between perovskite and Rutin–Ag nanoparticle/poly(1‐(4‐hexylphenyl)‐2,5‐di(thiophen‐2‐yl)‐1H‐pyrrole) (PHPT‐py). A new‐type chelation‐like silver metal complex interaction in the structure of halide perovskite and the hole‐transporting material, created by mixing Rutin–AgNPs into PHPT‐py film, improves and restricts the halide ions from migration.

Dye-sensitized solar cells (DSSCs) have been intensely researched for more than two decades. Electrolyte formulations are one of the bottlenecks to their successful commercialization, since these result in trade-offs between the photovoltaic performance and long-term performance stability. The corrosive nature of the redox shuttles in the electrolytes is an additional limitation for industrial-scale production of DSSCs, especially with low cost metallic electrodes. Numerous electrolyte formulations have been developed and tested in various DSSC configurations to address the aforementioned challenges. Here, we comprehensively review the progress on the development and application of electrolytes for DSSCs. We particularly focus on the improvements that have been made in different types of electrolytes, which result in enhanced photovoltaic performance and long-term device stability of DSSCs. Several recently introduced electrolyte materials are reviewed, and the role of electrolytes in different DSSC device designs is critically assessed. To sum up, we provide an overview of recent trends in research on electrolytes for DSSCs and highlight the advantages and limitations of recently reported novel electrolyte compositions for producing low-cost and industrially scalable solar cell technology.

The hole transport material (HTM) of organic–inorganic perovskite solar cells (PVSCs) plays a very important role for achieving high power conversion efficiency and long‐term stability. 2,2’,7,7’‐tetrakis[N,N‐di(4‐methoxyphenyl)amino]‐9‐9’‐spirobifluorene (spiro‐OMeTAD) is the first solid‐state HTM used in PVSCs and has gained tremendous attention during the last decade. Herein, the concept of spirolinkage for synthesis of spiro‐based HTMs is discussed, followed by an overview of the desirable optical and electrical properties of spiro‐OMeTAD. Recent progress in efficiency improvements of spiro‐based PVSCs is analyzed systematically, and the impacts of interface engineering, dopant‐free spiro‐OMeTAD, and novel spiro‐based HTMs are reviewed in detail. The hole mobility of spiro‐OMeTAD depends on the types of dopants and doping concentration. Commonly used lithium bis(trifluoromethylsulfonyl)imide and 4‐tert‐butylpyridine additives reduce the PVSC stability due to hygroscopicity and corrosiveness, respectively. The effects of additives on device stability and the techniques to improve the long‐term stability of spiro‐based PVSCs are also discussed. The review and analysis of various methods and strategies presented is useful for the perovskite research community, providing guidance and directions toward the further development of spiro‐based HTMs for PVSCs with improved efficiency and stability. A critical analysis on the recent advances in spiro‐OMeTAD and analogs as hole transporting materials is summarized for perovskite solar cells (PVSCs). The dopant engineering, device interface engineering, new materials design, and synthesis are discussed for achieving highly efficient and stable PVSCs.

Perovskite solar cells (PSCs) represent undoubtedly the most significant breakthrough in photovoltaic technology since the 1970s, with an increase in their power conversion efficiency from less than 5% to over 22% in just a few years. Hole-transporting materials (HTMs) are an essential building block of PSC architectures. Currently, 2,2’,7,7’-tetrakis-(N,N’-di-p-methoxyphenylamine)-9,9’-spirobifluorene), better known as spiro-OMeTAD, is the most widely-used HTM to obtain high-efficiency devices. However, it is a tremendously expensive material with mediocre hole carrier mobility. To ensure wide-scale application of PSC-based technologies, alternative HTMs are being proposed. Solution-processable HTMs are crucial to develop inexpensive, high-throughput and printable large-area PSCs. In this review, we present the most recent advances in the design and development of different types of HTMs, with a particular focus on mesoscopic PSCs. Finally, we outline possible future research directions for further optimization of the HTMs to achieve low-cost, stable and large-area PSCs.

Developing dopant-free hole-transporting materials (HTMs) for high-performance perovskite solar cells (PVSCs) has been a very active research topic in recent years since HTMs play a critical role in optimizing interfacial charge carrier kinetics and in turn determining device performance. Here, a novel dendritic engineering strategy is first utilized to design HTMs with a D-A type molecular framework, and diphenylamine and/or carbazole is selected as the building block for constructing dendrons. All HTMs show good thermal stability and excellent film morphology, and the key optoelectronic properties could be fine-tuned by varying the dendron structure. Among them, MPA-Cz-BTI and MCz-Cz-BTI exhibit an improved interfacial contact with the perovskite active layer, and non-radiative recombination loss and charge transport loss can be effectively suppressed. Consequently, high power conversion efficiencies (PCEs) of 20.8% and 21.35% are achieved for MPA-Cz-BTI and MCz-Cz-BTI based devices, respectively, accompanied by excellent long-term storage stability. More encouragingly, ultrahigh fill factors of 85.2% and 83.5% are recorded for both devices, which are among the highest values reported to date. This work demonstrates the great potential of dendritic materials as a new type of dopant-free HTMs for high-performance PVSCs with excellent FF.

Perovskite solar cells have emerged as one of the most promising photovoltaic technologies for future clean energy sources to replace fossil fuels. Among the various components in a perovskite solar cell, the hole-transporting materials play significant roles in boosting device performance and stability. Recently, hole-transporting materials with helicene cores have received much attention due to their unique properties and ability to improve the performance and stability of the perovskite solar cells. The focus of this review is on the emerging special class of HTMs based on helicenes for perovskite solar cells. The optical, electrochemical, thermal and photovoltaic properties of helicene-based small molecules as HTMs or interfacial layer materials in n-i-p or p-i-n type perovskite solar cells are summarized. Finally, perspectives for the future development of helicene type hole-transporting materials are provided.