Quaternary chalcopyrite semiconductors, Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe), have attracted increasing attention for photovoltaics (PV) application in recent years. However, due to the cell architecture borrowed from CuInxGa(1-x)Se2 (CIGS) devices, the open-circuit voltage is the limiting factor preventing further increases in solar cell efficiency. In the present study, band edge energies of Cu2ZnSnS4 and Cu2ZnSnSe4 were analysed electrochemically in order to show band energy alignments of CZTS and CZTSe. The electrochemical steady-state potential windows were also investigated; this provides vital information for various applications of both materials. The valence band energy offset between CZTS and CZTSe was found to be 0.5 eV. Compared with the flat band potential of CdS (-0.8 V vs Ag/AgCl), the open circuit potential in the dark between CZTS and CdS is therefore 0.4 V and 0.9 V for CZTSe. Moreover, from chronoamperometric measurements using an electrochemical field-effect transistor, the conductivity of CZTSe is found to be three orders higher than CZTS, which proves that CZTSe is significantly better for charge transfer.
Keywords chalcopyrite, Cu2ZnSnS4, Cu2ZnSnSe4, solar cells, electrochemical field-effect transistor