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. In this study, three low-cost hole transport materials (HTMs), specifically 4,4'-(3,3'-bis(4-methoxy-2,6-dimethylphenyl)-[2,2'-bithiophene]-5,5'-diyl)bis(N,N-bis(4-methoxyphenyl)aniline) (TP-H), 4,4'-(3,3'-bis(4-methoxy-2,6-dimethylphenyl)-[2,2'-bithiophene]-5,5'-diyl)bis(3-methoxy-N,N-bis(4-methoxyphenyl)aniline) (TP-OMe), and 4,4'-(3,3'-bis(4-methoxy-2,6-dimethylphenyl)-[2,2'-bithiophene]-5,5'-diyl)bis(3-fluoro-N,N-bis(4-methoxyphenyl)aniline) (TP-F), were designed and synthesized using a bulky group-substituted 2,2'-bithiophene core and methoxy- or F-functionalized triphenylamine derivatives. Compared to the HTMs without F atoms, TP-F using F substitution exhibited enhanced intermolecular packing, a lower highest occupied molecular orbital energy level, and increased hole mobility and conductivity. The PSC incorporating the doped TP-F as the hole transport layer achieved the highest power conversion efficiency (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 study 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.