Numerical Modelling of Cu2SnZn(Se,S)4 Monograin Layer Solar Cells

Abstract

Cu2SnZn(Se,S)4 monograin layer solar cells are based on a two-dimensional matrix of monograin crystals embedded into the epoxy resin. The rest of the solar cell resembles a thin film CIGS solar cell in the superstrate configuration. Laboratory AM1.5 conversion efficiency of these solar cells reaches up to 5.3% along with the open-circuit voltages up to 660 mV and fill-factors of up to 65%. One way to improve the efficiency is to increase the packing density of the monograins. An alternative is to enhance the properties of the solar cell structure. By using the numerical modelling approach we have investigated the latter possibility. The results indicate the presence of the surface defect layer between the buffer and the CZTSSe monograin absorber layers with the high concentration of the mid-gap defects. The open-circuit voltage seems to be limited by the non-thermodynamic effect. Besides the shallow acceptor doping in the CZTSSe layer, also wider distribution of deep compensating donor traps occurs, which might be responsible for the low fill-factor and for the specific temperature trend of the short-circuit current. A relatively narrow space charge region and a large difference between the absorbed flux and the collected current density points to good prospects for the short-circuit current improvement.