Enhanced exciton separation through negative energy band bending at grain boundaries of Cu2ZnSnSe4 thin-films

Local surface potential of Cu2ZnSnSe4 thin-films was investigated by Kelvin probe force microscopy. The surface potential profile across grain boundaries (GBs) shows a rise of 200-600 meV at GBs in a Cu-poor and Zn-poor film with 3.8% efficiency, which means positively charged GBs. In contrast, the GBs in a Cu-poor and Zn-rich film with 2% efficiency exhibit lowering of surface potential by 40 meV. The results indicate that GBs of Cu2ZnSnSe4 films play a role for exciton separation and governing defects for high efficiency could be not only Cu-Zn but also V-Cu as explained theoretical predictions. (C) 2011 American Institute of Physics. [doi:10.1063/1.3626848]