Cascade Type‐II 2D/3D Perovskite Heterojunctions for Enhanced Stability and Photovoltaic Efficiency

Heterojunction engineering is essential to reduce energy loss and enhance the stability of perovskite solar cells (PSCs). Herein, 1‐naphthylmethylammonium bromide (NMABr) is introduced to in situ generate an ultrathin p‐type 2D perovskite with wide bandgap between the 3D perovskite film and the hole transport layer (HTL). Cascade 2D/3D perovskites in situ form a type‐II heterojunction, which largely contributes to the improvement of photovoltaic performance. The type‐II heterojunction not only blocks the electron transfer and reduces the charge recombination on the surface and grain boundaries of the 3D perovskite film, but also promotes the hole extraction. The microphotoluminescence indicates the reduction of nonradiative recombination on the surface, consistent with the reduced trap density in Mott–Schottky plots and the increased recombination resistance in impedance spectra. The champion power conversion efficiency (PCE) of the NMABr‐passivated 2D/3D PSC reaches 21.09% under the AM1.5 illumination. In addition, the NMABr‐passivated 2D/3D PSC remains 80% of the initial PCE for 105 h at 85 °C in nitrogen and retains 80% of initial PCE for 350 h in 70–80% relative humidity in the air. This work provides a crucial in situ fabrication of type‐II 2D/3D heterojunction to improve the stability and efficiency of PSCs.