Amidinium‐Based 2D Spacer Cations Enhance Efficiency and High‐Temperature Photostability of Perovskite Solar Cells

2D/3D perovskite heterojunctions represent a promising approach to enhance the efficiency and stability of perovskite solar cells (PSCs). However, the photostability at elevated temperatures of conventional 2D/3D heterostructures, employing ammonium-based spacer cations, is severely limited by deprotonation reactions, hindering their practical application. In this study, amidinium-based 2D spacer cations as an alternative, leveraging their higher acid dissociation constants, to mitigate deprotonation-induced instability while providing excellent defect passivation effect is introduced. Amidinium passivation not only facilitates formation of thermally stable 2D/3D heterostructures but also suppresses non-radiative recombination and enhances carrier transport dynamics. PSCs with amidinium-based bulk and surface passivation achieve a state-of-the-art power conversion efficiency of 26.52% for 2D/3D PSCs and exhibit outstanding high-temperature photostability, retaining 90.6% of initial efficiency after 1000 h of continuous illumination at maximum power point at 85 °C. This work offers valuable insights into designing high-performance, durable PSCs under challenging conditions.