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

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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 (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.
Funding Information
  • National Natural Science Foundation of China (12174152, 61974175)
  • Natural Science Foundation of Jilin Province (202513JC010277746, 20190201307JC)

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