Stable Quasi‐2D Perovskite Solar Cells with Efficiency over 18% Enabled by Heat–Light Co‐Treatment

2D perovskite solar cells (2D PVSCs) show good stability for commercialization. However, their power conversion efficiency (PCE) is relatively low. In this work, a post‐treatment strategy by simultaneously applying light and heat to quasi‐2D PVSCs, obtaining a record PCE of 18.24% is developed. It is found that heat‐treating PVSCs in the dark slightly increases the device performance over time at temperatures below 75 °C, whereas the performance deteriorates rapidly at temperatures above 100 °C. Upon illumination, the device efficiency is significantly improved, particularly when the thermal‐treatment temperature is increased to 100 °C. A comprehensive carrier dynamic study reveals that the enhanced performance can be attributed to the reduced quasi‐2D perovskite defect states and improved charge collection. In addition, this strategy enables better stability, and an unencapsulated device can retain 90% of its original PCE after 1340 h of direct exposure to air with a humidity of 50 ± 5%. Thus, the strategy paves the way for the commercialization of quasi‐2D PVSCs. High‐performance quasi‐2D perovskite solar cells (PVSCs) are demonstrated via heat–light co‐treatment. The optimized quasi‐2D PVSC presents a maximum power conversion efficiency of 18.24% with excellent stability. The underlying mechanism of the light and heat co‐treatment in improving the device performance lies in its synergistic effect in reducing the trap states and improving the charge transport.