A “One‐Stone‐Three‐Birds” Strategy: Multifunctional Y‐Doping in CdS Buffer Layer Enables 15.19% Efficient Kesterite Solar Cells

ABSTRACT The substantial open‐circuit voltage ( V OC ) deficit in kesterite Cu 2 ZnSn(S,Se) 4 (CZTSSe) solar cells remains a major challenge, primarily attributed to interfacial recombination resulting from non‐ideal band alignment, low carrier concentration, and secondary phase formation within the CdS buffer layer. Conventional cationic doping strategies for CdS often improve carrier density but exacerbate the conduction band offset (CBO), limiting V OC enhancement. Herein, we propose a novel cyclic‐immersion technique for trifunctional yttrium (Y 3+ ) doping of the CdS layer. The incorporated Y 3 + ions substitute Cd sites and introduce donor defects (Y Cd and V S ), significantly increasing electron concentration. Simultaneously, Y doping induces a downward shift of the conduction band minimum, thereby reducing the CBO and facilitating better electron extraction. Moreover, Y incorporation effectively passivates interfacial defects (e.g., V Cd ) by suppressing the formation of impurity phases like CdO and Cd(OH) 2 , thus mitigating non‐radiative recombination. Consequently, the optimized device demonstrates a power conversion efficiency of 15.19% with a V OC of 572.1 mV, ranking among the most efficient CZTSSe solar cells. This work demonstrates a synergistic approach that integrates carrier concentration enhancement, band alignment engineering, and defect passivation through rare‐earth doping of CdS, providing a validated route to high‐performance kesterite photovoltaics.