Heterocyclic and heteropolycyclic moieties in organic hole transport materials for perovskite solar cells: Design, synthesis, and performance

This review focuses on the pivotal role of heterocyclic and heteropolycyclic moieties in the development of organic hole-transporting materials (HTMs) for application in perovskite solar cells (PSCs). The escalating demand for sustainable energy sources has heightened interest in PSCs due to their exceptional efficiency and low-cost manufacturing potential. In this review, we highlight the importance of organic-HTMs in PSCs, detailing their innovative molecular design, synthesis, thermal and electrochemical properties, and influence on photovoltaic parameters. The synthesis of heteroatom substituted organic-HTMs from readily available precursors is systematically presented, highlighting key C-N/C-C bond-forming reactions including Buchwald-Hartwig amination, Suzuki, Stille, and Glaser-Hay couplings, as well as Knoevenagel and Horner−Wittig condensations, and Arbuzov reaction. We emphasize the significance of organic synthetic chemistry in designing HTMs with desirable characteristics, such as enhanced solubility, ease of synthesis, simple purification, and characterization by modern analytical techniques. Additionally, the review explores how organic chemistry can enhance critical parameters of HTMs, including energy level alignment, thermal stability, hydrophobicity, hole mobility, and defect passivation ability. Overall, this review describes various organic-HTMs, such as highly π-extended organic molecules, organic passivators, and organic self-assembled monolayers, as emerging HTMs for lead- and tin-based PSCs. To gain insights into the molecular structure, single crystal structures of potential organic-HTMs for PSCs are discussed. In addition, hole-transporting organic passivators playing a role in suppressing the formation of metallic lead (Pb⁰) on the perovskite surface are highlighted. Future directions of PSC research are discussed, highlighting the potential for further advancements in organic-HTM design and synthesis to drive improvements in PSC performance. Overall, this review is a valuable resource for researchers in the field, offering insights into recent advances and guiding future research endeavors.