Defect engineering enabling p-type Mo(S,Se)2:TM (TM = V, Nb, Ta) towards high-efficiency kesterite solar cells

One important issue limiting the development of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells is the severe recombination loss at CZTSSe/Mo(S,Se)2/Mo back contact, primarily arising from the non-matched potential barrier and inferior electric contact between absorber and weak n-type Mo(S,Se)2 interfacial layer. It is expected that a p-type Mo(S,Se)2 can effectively improve the hole extraction and enhance the device performance. In this work, we propose a plausible direction to build up an electrically benign back surface field by in-situ doping Mo(S,Se)2 with group VB elements (TM = V, Nb and Ta). It is turned out that the generated VMo, NbMo, or TaMo shallow acceptors could convert Mo(S,Se)2 from weak n-type to p-type conductivity and increase its work function (WF) and hole concentration. With the contact of p-type absorber (WF,absorber < WF,Mo(S,Se)2), a benign ohmic contact with favorable downward band bending is formed, thereby accelerating the holes extraction and minimizing interface recombination loss. With significant gains in open-circuit voltage (Voc) and fill factor (FF), the Mo(S, Se)2:Ta device finally increases the efficiency from 10.82 % to 12.72 %. This convenient in-situ Mo(S,Se)2 doping strategy is different from previous complicated or unstable cases and should serve as a basis for high-quality back contact engineering in kesterite photovoltaics.