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.