A feasible and effective solution-processed PCBM electron extraction layer enabling the high VOC and efficient Cu2ZnSn(S, Se)4 devices

Photo-generated carrier recombination loss at the CZTSSe/CdS front interface is a key issue to the open-circuit voltage (VOC) deficit of Cu2ZnSnSxSe4−x(CZTSSe) solar cells. Here, by the aid of an easy-handling spin-coating method, a thin PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) layer as an electron extraction layer has been introduced on the top of CdS buffer layer to modify CZTSSe/CdS/ZnO-ITO (In2O3:Sn) interfacial properties. Based on Sn4+/DMSO (dimethyl sulfoxide) solution system, a total-area efficiency of 12.87% with a VOC of 529 mV has been achieved. A comprehensive investigation on the influence of PCBM layer on carrier extraction, transportation and recombination processes has been carried out. It is found that the PCBM layer can smooth over the CdS film roughness, thus beneficial for a dense and flat window layer. Furthermore, this CZTSSe/CdS/PCBM heterostructure can accelerate carrier separation and extraction and block holes from the front interface as well, which is mainly ascribed to the downward band bending of the absorber and a widened space charge region. Our work provides a feasible way to improve the front interfacial property and the cell performance of CZTSSe solar cells by the aid of organic interfacial materials.