The Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) semiconductor is a compelling emerging light harvesting materials for low-cost,environment-benign,and high-efficiency thin-film photovoltaics.The highest power conversion efficiency (PCE) so far for kesterites are 11% and 12.6 % for pure sulphide kesterite and se-containing kesterite,respectively.The current state-of-the-art kesterite devices suffer from the long-standing challenge of controlling defects (e.g.cation—disordering defects and defect clusters),generally resulting in severe potential fluctuation,low minority carrier lifetime and thereby unsatisfactory device performance.Understanding the defects formation mechanism,and more importantly,identifying ways of controlling these defects is of vital importance for achieving further efficiency breakthroughs.In our recent work,we found that some of these defects can be modulated via both in-situ control of the local chemical environment (i.e.the oxidation states of cations and modifying the local chemical composition) during the synthesis of kesterite phase and lithium post deposition treatment.Notable that with the former in-situ defect control strategy,we demonstrated a confirmed 12.5% efficiency for CZTSe solar cells with high VOC of 491 mV,showing the promise of these defect control strategies.In this talk,the correlation of suppressed formation of detrimental intrinsic defects and associated improved kesterite device performance upon these defect-controlling strategies will be discussed.