Post‐Assembled Alkylphosphonic Acids for Efficient and Stable Inverted Perovskite Solar Cells

The self‐assembled monolayer (SAM) is now widely applied at the hole‐selective interface of efficient inverted perovskite solar cells (PSCs). However, voids are unavoidable in SAMs due to the rough substrate and the deposition conditions, which lead to direct contact between the active layer and electrode, undermining the efficiency and stability of PSCs. Here, alkylphosphonic acids with different alkyl chain length are post‐assembled to fill the voids, thereby forming a compact po‐SAM layer. By further modifying the head group of alkylphosphonic acids, the efficiency and stability of PSCs are improved due to wetting and passivation. Finally, the po‐SAM layer formed by (2‐Bromoethyl) phosphonic acid and [4‐(3,6‐dimethyl‐9H‐carbazol‐9‐yl)butyl] phosphonic acid contributes to a champion device with the power conversion efficiency of 25.16% (certified 24.67%) and improved stability under operation or storage. This work demonstrates the advantages of po‐SAM layer in improving the performance of PSCs, offering ideas for SAM modification. Po‐SAM strategy is proposed to improve the coverage of SAM and to introduce functional groups, which reduces the current leakage of the devices, improves the wettability, facilitates the perovskite crystallization, and passivates the defects at the buried interface. As a result, the target device achieved a high efficiency of over 25% (certified 24.67%, 0.0817 cm2) with enhanced storage and operation stability.