Spiro[fluorene-9,9′-xanthene]-Based Hole-Transporting Materials for Photovoltaics: Molecular Design, Structure–Property Relationship, and Applications

ConspectusOrganic hole-transporting materials (HTMs) are of importance in the progress of new-generation photovoltaics, notably in perovskite solar cells (PSCs), solid-state dye-sensitized solar cells (sDSCs), and organic solar cells (OSCs). These materials play a vital role in hole collection and transportation, significantly impacting the power conversion efficiency (PCE) and overall stability of photovoltaic devices. The emergence of spiro(fluorene-9,9′-xanthene) (SFX) as a novel building block for organic HTMs has gained considerable attention in the field of photovoltaics. Its facile one-pot synthetic approach, straightforward purification, and physiochemical properties over the prototype HTM spiro-OMeTAD have positioned SFX as a highly attractive alternative.In this Account, we present a comprehensive and in-depth summary of our research work, focusing on the advancements in SFX-based organic HTMs in photovoltaic devices with a particular emphasis on PSCs and sDSCs. Several key objectives of our research have been focused on exploring strategies to improve the properties of SFX-based HTMs. (i) One of the critical aspects we have addressed is the improvement of film quality. By carefully designing the molecular structure and employing suitable synthetic approaches, we have achieved HTMs with excellent film-forming ability, resulting in uniform and smooth films over large areas. This achievement is pivotal in ensuring the reproducibility and efficiency of photovoltaic devices. Furthermore, (ii) our investigations have led to an improvement in hole mobility within the HTMs. Through molecular engineering, such as increasing the molecular conjugation and introducing multiple SFX units, we have demonstrated enhanced charge-carrier mobility. This advancement plays a crucial role in minimizing charge recombination losses and improving the overall device efficiency. Additionally, (iii) we have explored the concept of defect passivation in SFX-based HTMs. By incorporating Lewis base structures, such as pyridine groups, we have successfully coordinated to Pb2+ in the perovskite layer, resulting in a passivation of surface defects. This defect passivation contributes to better stability and enhanced device performance. Throughout our review, we highlighted the potential and opportunities achieved through these steps. The combination of enhanced film quality, improved hole mobility, and defect passivation resulted in remarkable photovoltaic performance. Our findings have demonstrated promising short-circuit current densities, open-circuit voltages, fill factors, and PCEs, with some HTMs even outperforming the widely used spiro-OMeTAD.We believe that this review will not only provide a better understanding of SFX-based HTMs but also open new avenues for enhancing the performance of organic HTMs in photovoltaic and other organic electronic devices. By providing unique perspectives and exploring different strategies, we aim to inspire ongoing advancements in photovoltaic technologies and organic electronics. Meanwhile, the success of SFX-based HTMs in improving photovoltaic device performance holds great promise for the continued development of efficient and stable photovoltaic devices in the years to come.