Defect‐Competitive Equilibrium Driven 13.83% Efficiency Breakthrough in DMF‐Based CZTSSe Solar Cells

Abstract Open‐circuit voltage ( V OC ) and fill factor (FF) losses originating from harmful defects remain major challenges for achieving high‐efficiency Cu 2 ZnSn(S,Se) 4 (CZTSSe) solar cells. In this work, an ultra‐precise Zn/Sn chemical potential modulation‐induced defect competitive balance strategy is proposed to suppress high‐density detrimental bulk defects and tune the band alignment, thereby further reducing nonradiative carrier recombination losses. At a lower Zn/Sn ratio, defect compensation and Coulombic attraction reduce the formation energy of [2Cu Zn +Sn Zn ] clusters, which act as carrier recombination centers. An excessively high Zn/Sn ratio simultaneously suppresses Sn Zn defects and releases Cu Zn defects from [2Cu Zn +Sn Zn ] clusters, reigniting band‐tail states that degrade photovoltaic device performance. The defect competition achieves optimal equilibrium at Zn/Sn = 1.12, the reduced concentration of [2Cu Zn +Sn Zn ] defect clusters shifts upward the conduction band minimum, optimizing band alignment at the CdS/CZTSSe junction and minimizing band‐tail states and nonradiative recombination. Furthermore, the Zn/Sn chemical potential regulation can also drive Na enrichment in the fine‐grained layer, promoting grain growth and passivating grain boundary defects. Consequently, this strategy achieves the highest efficiency of 13.83% reported to date in N,N ‐dimethylformamide (DMF)‐based kesterite solar cells, with V OC increasing from 494 to 526 mV and FF improving from 64.34% to 70.36%.