Bithiophene-based cost-effective hole transport materials for efficient n-i-p perovskite solar cells

Charge transport materials constitute a relatively large portion of the cost in the production of perovskite solar cells (PSCs). Therefore, developing cheap and efficient charge transport materials is of great significance for the commercialization of PSCs. Herein, three low-cost hole transport materials (HTMs), specifically TP-H, TP-OMe and TP-F, were designed and synthesized by employing bulky groups substituted 2,2'-bithiophene core and methoxy- or F-functionalized triphenylamine derivatives. Compared to HTMs without F atoms, F substitution confers upon TP-F enhanced intermolecular packing, a lower highest occupied molecular orbital (HOMO) energy level, and increased hole mobility and conductivity. PSCs incorporating doped TP-F as hole transport layer achieved highest power conversion efficiency (PCE) of over 24% among the three devices. The high performance of TP-F can be attributed to the passivation effect of S and F atoms on uncoordinated Pb²⁺ within the perovskite (PVSK) film, which significantly reduces the density of defect states and the incidence of trap-mediated recombination in PSCs. This work demonstrates the effectiveness of the 3,3'-bis(4-methoxy-2,6-dimethylphenyl)-2,2'-bithiophene building block for constructing cost-effective HTMs and highlights the impact of F-substitution on enhancing the photovoltaic performance of PSCs.