Molecular design of organic hole transport materials for efficient and thermally stable perovskite solar cells

Organic hole transport materials (HTMs) must reconcile efficient charge collection with the stability of a multicomponent contact under illumination and heat. This Progress Review places three small-molecule HTM families we developed in the context of molecular design strategies reported by other groups. Conjugated-core, π-bridge, and methoxy/methylthio terminal-group modifications reveal that aggregation control, conductivity after doping, and interfacial selectivity must be optimized together. Mixed terminal functionality enabled an average power conversion efficiency of 23.6%, whereas sulfur-containing bridges substantially improved device durability at 85 °C. High initial efficiency and undoped mobility alone did not predict thermal stability or doped-device performance. The resulting design principles primarily concern chemically doped HTMs in n–i–p devices; their extension to dopant-free and inverted architectures requires validation of processing and interfacial compatibility. We distinguish observed structure–property relationships from proposed packing and passivation mechanisms and identify stable doping, interface control, and scalable film formation as priorities for practical HTMs.