Surface Band Bending Engineered via Lithium‐Induced Reconstruction for Minimized Voltage Deficit in Kesterite Solar Cells

ABSTRACT The large open‐circuit voltage deficit ( V _OC,def ) is currently recognized as the primary obstacle hindering the development of kesterite solar cells. Surface electrostatic potential fluctuations and bandgap fluctuations resulting from Cu/Zn antisite defects hinder energy level alignment and lead to V _OC ‐limiting recombination at the CZTSSe/CdS interface. Herein, we develop a new strategy to reconstruct the absorber surface by spin‐coating a high‐concentration Li‐containing solution onto the CZTS precursor films. This treatment facilitates the formation of a more stable weak n ‐type phase and n ‐type Zn _Li defects on the surface of the absorber layer, thereby enhancing effective p‐n conversion at the heterojunction. Moreover, the emergence of Li _Zn shallow‐level defects contributes to suppressing Cu _Zn defects on the surface of the absorber layer, which in turn increases band bending and reduces non‐radiative recombination at the interface. As a consequence, after undergoing the Li‐containing interface treatment, CZTSSe solar cells with a champion efficiency of 14.92% (certified 14.52%) have been realized, mainly benefitting from remarkably high V _OC approaching 570 mV and greatly reduced V _OC,def of 0.266 V. This study presents a feasible strategy to modulate kesterite solar cell surfaces for minimizing interfacial losses and enhancing device efficiency.