Magnesium‐Doped Nickel Oxide Hole Transport Layer Achieves High Efficiency and Stability Perovskite Solar Cells ^†

Comprehensive Summary Perovskite solar cells (PSCs) are promising thin‐film photovoltaic devices and achieve a high power conversion efficiency (PCE) of 27.3% (certified). Hole transport layer (HTL) composed of nickel oxide (NiO x ) and [4‐(3,6‐dimethyl‐9 H ‐carbazol‐9‐yl)butyl]phosphonic acid (Me‐4PACz) is extensively utilized in these devices. However, the dispersion and conductivity of NiO x are suboptimal, and it exhibits energy‐level mismatch. Meanwhile, the coverage of Me‐4PACz on NiO x is non‐uniform. Herein, we synthesized magnesium ion‐doped nickel oxide (Mg:NiO x ) with more surface hydroxyl groups to address these issues. More surface hydroxyl groups provided more binding sites for Me‐4PACz, resulting in a denser and more uniform coverage of Me‐4PACz. Consequently, fewer defects were present at the buried interface, and a better environment for the crystallization of perovskite (PVK) was established. Furthermore, Mg:NiO x /Me‐4PACz enabled better energy level alignment with PVK. The Mg:NiO x ‐based PSCs achieved a champion PCE of 25.86%, representing a notable improvement over the NiO x ‐based devices (24.51%). After 462 h of continuous illumination testing, the PSCs with Mg:NiO x retained 96.8% of their initial PCE, while those with NiO x only maintained 62.9% of their initial PCE. Thus, Mg:NiO x effectively enhanced both the PCE and stability of PSCs.