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 formamidinium lead iodide (FAPbI 3 ) 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). To address these issues, we developed "upgraded" CsCl-Eu 3+ -doped CsCl quantum dots (QDs) for FAPbI 3 PSCs 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+ promoted the formation of high-quality Cs-FA films by adjusting the Pb-I cluster structure. CsCl:Eu 3+ also offsets the local strain and lattice contraction induced by Cs + , which maintains the inherent Eg of FAPbI 3 and decreases the trap density. Finally, we obtained a power conversion efficiency (PCE) of 24.13% with an excellent short-circuit current density (J sc ) of 26.10 mA/cm 2 , which are among the highest in methylammonium (MA)-free PSCs. The unencapsulated devices showed excellent humidity and storage stability owing to the relieved lattice strain and ligand protection. We achieved an initial PCE of 92.2% within 500 h under continuous light illumination of 1.5 G and bias voltage conditions, demonstrating excellent operational stability. 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. This article is protected by copyright. All rights reserved.