Trivalent Europium‐Doped CsCl Quantum Dots for MA‐Free Perovskite Solar Cells with Inherent Bandgap through Lattice Strain Compensation

Abstract Cesium–formamidinium (Cs–FA) perovskites have garnered widespread interest owing to their excellent thermal‐ and photostability in achieving stable perovskite solar cells (PSCs). However, Cs–FA perovskite typically suffers from Cs + and FA + mismatches, affecting the Cs–FA morphology and lattice distortion, resulting in an enlarged bandgap ( E g ). In this work, “upgraded” CsCl, Eu 3+ ‐doped CsCl quantum dots, are developed to solve the key issues in Cs–FA PSCs and also exploit the advantage of Cs‐FA PSCs on stability. The introduction of Eu 3+ promotes the formation of high‐quality Cs–FA films by adjusting the Pb–I cluster. CsCl:Eu 3+ also offsets the local strain and lattice contraction induced by Cs + , which maintains the inherent E g of FAPbI 3 and decreases the trap density. Finally, a power conversion efficiency (PCE) of 24.13% is obtained with an excellent short‐circuit current density of 26.10 mA cm −2 . The unencapsulated devices show excellent humidity stability and storage stability, and an initial PCE of 92.2% within 500 h under continuous light illumination, and bias voltage conditions is achieved. This study provides a universal strategy to address the inherent issues of Cs–FA devices and maintain the stability of MA‐free PSCs to satisfy future commercial criteria.