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

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 (Eg ). In this work, "upgraded" CsCl, Eu3+ -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 Eu3+ promotes the formation of high-quality Cs-FA films by adjusting the Pb-I cluster. CsCl:Eu3+ also offsets the local strain and lattice contraction induced by Cs+ , which maintains the inherent Eg of FAPbI3 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.