Band bending near grain boundaries of Cu2ZnSn(S,Se)(4) thin films and its effect on photovoltaic performance

Although the family of polycrystalline Cu2ZnSn(S,Se)(4) (CZTSSe) thin films are well-known light absorber materials for photovoltaic solar cells and have been studied extensively in the past, the behaviors of their grain boundary (GB) still remain elusive. By using a combination of experimental techniques, we have systematically investigated the compositions and electronic structures of the grain interior (GI) and GB of the polycrystalline CZTS, CZTSe and CZTSSe films at nanometer scales. In particular, we have for the first time independently determined the band edge positions for both the conduction band and the valence band using scanning tunneling spectroscopy. While the composition of GB was nearly the same as that of GI for both CZTS and CZTSe films, opposite band bending behaviors near GBs were discovered for them. For CZTS, both the conduction band and valence band were found to bend towards the forbidden gap near GBs, resulting in enhanced carrier recombination and relatively poor device performance. For CZTSe, in contrast, both the conduction band and valence band were found to bend away from the forbidden gap near GBs, which could be responsible for the relative better device performance due to the potentially impeded carrier recombination. In the case of CZTSSe thin film, by actively substituting Se with S near GBs, both conduction band and valence band at GBs were demonstrated to bend away from the forbidden gap, leading in principle to a lower carrier recombination at GBs and improved device performance. Our findings could provide a direction for manipulating the band bending between GB and GI to improve the performance of CZTSSe family solar cells.