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

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.