Phenanthro[9,10-d]imidazole-Based Hole Transport Materials for Perovskite Solar Cells: Influence of π-Bridge Units

Hole transport materials (HTMs) play a critical role in achieving high efficiency and stability in perovskite solar cells (PSCs). In this study, we developed a series of easy-to-synthesize and cost-effective HTMs that exhibit excellent performance. These HTMs contain triphenylamine units substituted at the C6 and C9 positions of the phenanthro[9,10-d]imidazole (PTI-imidazole) core, which serves as the donor unit. Various π-conjugated units were introduced at the C2 position, including benzene (FDIMD-Ph and O-FDIMD-Ph), pyridine (O-FDIMD-Py), 2,2'-bithiophene (O-FDIMD-Th-Th), thieno[3,2-b]thiophene (O-FDIMD-TT), and dithieno[3,2-b:2',3'-d]thiophene (O-FDIMD-TTT). By modifying the π-linker, we tuned the optoelectronic properties of the materials. These materials have fused, planar and symmetrical structures have good solublity and promoted uniform film morphology. As a result, O-FDIMD-Ph achieved an average champion power conversion efficiency (PCE) of over 20.7%, outperforming spiro-OMeTAD (20.3%) based devices. Most HTMs retained ~95% of initial PCE after 500 hours of light exposure at 25 °C, except O-FDIMD-Py. Furthermore, thiophene-based π-linkers, including O-FDIMD-Th-Th, O-FDIMD-TT, and O-FDIMD-TTT, promote pronounced intermolecular interactions and strong π–π stacking, which contribute to the significantly enhanced thermal stability observed at 85 °C under continuous illumination. These results demonstrate that π-extension in PTI-imidazole-based HTMs is a promising strategy for developing efficient and stable PSCs.