Tailoring Self-Assembled Monolayers through Electron-Donating and -Withdrawing Substituents for Inverted Perovskite Solar Cells

As critical hole-transport layers (HTLs) in perovskite solar cells (PSCs), self-assembled monolayers (SAMs) functionalized with electron-donating or -withdrawing groups (EDGs/EWGs) offer a promising route to enhance device performance and stability. However, the mechanism by which these tailored functional groups modulate SAM properties and device performance remains unclear. Herein, we systematically engineered a series of triphenylamine-based SAMs through substituent modulation (−CH3 as EDG, −H as neutral, −CN as EWG) to establish clear structure–property–performance relationships. We demonstrated that SAM TP-Me with electron-donating −CH3 group exhibit increased molecular electron density, enhanced intermolecular π–π stacking, homogeneous indium tin oxide (ITO) surface coverage, further facilitating the hole transfer and the formation of high-quality perovskite films with suppressed nonradiative recombination. Conversely, the electron-withdrawing −CN group induces molecular electron deficiency, leading to degraded hole selectivity. Consequently, inverted PSCs based on the EDG-substituted SAM (TP-Me) achieves a power conversion efficiency (PCE) of 25.79% with superior operational stability, outperforming those with EWG-substituted (21.69%) and neutral group-substituted (25.29%) analogues. This work underscores the pivotal role of substituent engineering in SAM design and provides mechanistic insights for developing advanced interfacial materials for high-performance PSCs.