Stereo‐Hindrance Induced Conformal Self‐Assembled Monolayer for High Efficiency Inverted Perovskite Solar Cells

Abstract Self‐assembled monolayers (SAMs) are employed as hole‐selective contacts in inverted perovskite solar cells (PSCs) and have achieved record power conversion efficiency (PCE) over 26%. However, the tendency of extensively employed SAM [2‐(3,6‐Dimethoxy‐9H‐carbazol‐9‐yl)ethyl]phosphonic acid to aggregate leads to its uneven coverage to the transparent conducting oxide substrate, which subsequently compromises the photovoltaic performance. Herein, a novel tert‐butyl functionalized phosphonic acid carbazole SAM is developed, i.e., (4‐(3,6‐di‐tert‐butyl‐9H‐carbazol‐9‐yl)butyl)phosphonic acid (tBu‐4PACz), and introduced to a mixed SAM system as the hole‐extraction layer in inverted PSCs. The stereo‐hindrance of the bulky tert ‐butyl group prevents undesired aggregation and leads to better conformality, which facilitates more efficient hole‐extraction and suppresses interfacial recombination losses. The tBu‐4PACz SAM‐based inverted PSC has achieved record level PCE of 26.25% (26.21%, certificated) with outstanding fill factors over 86%. Moreover, the mixed SAM based inverted PSC devices maintained over 94.7% of their initial efficiency after 500 h continuous maximum power‐point tracking under simulation 1‐sun irradiation.