Effect of ortho-fluorine substituted hole transport materials for perovskite solar cells: influence of rigid vs. flexible linkers†

The development of stable and efficient hole transporting materials (HTMs) is essential for the commercialization of perovskite solar cells (PSCs). In this study, we introduce four novel HTMs featuring a D–π–D molecular structure. These HTMs have 3-fluoro-N,N-bis(4-(methylthio)phenyl)aniline (TPASF) peripheral terminal groups, which are linked with various π-core moieties. Our investigation reveals that altering the π-linkers affects the film morphology of the HTMs, significantly influencing device performance. HTMs with planar backbones, CPDT-OFTPASMe2 [2-(2,6-bis(4-(bis(4-(methylthio)phenyl)amino)-2-fluorophenyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophen-4-ylidene)malononitrile] and TTT-OFTPASMe2 [4,4′-(dithieno[3,2-b:2′,3′-d]thiophene-2,6-diyl)bis(3-fluoro-N,N-bis(4-(methylthio)phenyl)aniline)], form films with more voids. In contrast, ThOEt-OFTPASMe2 [4,4′-(3,3′-diethoxy-[2,2′-bithiophene]-5,5′-diyl)bis(3-fluoro-N,N-bis(4-(methylthio)phenyl)aniline)] and DTP-OFTPASMe2 [4,4′-(4-(4-methoxyphenyl)-4H-dithieno[3,2-b:2′,3′-d]pyrrole-2,6-diyl)bis(3-fluoro-N,N-bis(4-(methylthio)phenyl)aniline)], with inhibitory effects caused by ethoxy and methoxy phenyl groups, respectively, prevent film aggregation and result in a pinhole-free morphology. Among the four tested HTMs, the device based on the newly developed DTP-OFTPASMe2 HTM demonstrates exceptional promisedelivering an average power conversion efficiency of 18.77% and exhibiting good thermal stability. Subsequently under amibient conditions to promote oxidation, the efficiency was boosted to 21.35% in unsealed devices. Furthermore, air-exposed DTP-OFTPASMe2-based devices maintain their initial efficiency under high-humidity conditions for approximately 83 days, underscoring their robust performance over time.