Suppression of Deep‐Level Defects Recombination in Cu ₂ ZnSn(S, Se) ₄ Solar Cells Through Rear‐Interface Cesium Doping

Abstract Alkali metals (AMs) doping is commonly accepted as an indispensable strategy to enhance the efficiency of Cu ₂ SnZn(S, Se) ₄ (CZTSSe) thin‐film solar cells. However, while extensive research has been focused on light AMs, heavy AMs have lacked attention in studies. In this work, a novel solution‐based approach is employed to achieve cesium doping in CZTSSe‐based thin‐film solar cells compared with conventional post‐deposition methods. This strategy allows cesium to diffuse into the absorber from the rear side of the precursor film before selenization, enabling its participation in the crystallization process without affecting the phase structure of CZTSSe. Time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS) spectra demonstrate that cesium is detected in the middle and rear region of the absorber. Further studies reveal that cesium mainly accumulates at the grain boundaries of the absorber and effectively suppresses non‐radiative recombination in the bulk of the absorber, leading to enlarged grain size and improved electrical properties of the CZTSSe device. This ultimately resulted in enhanced open‐circuit voltage ( V _OC ) and fill factor ( FF ), thus improving the photovoltaic performance.