Numerical Simulation of Cadmium-Free Buffer Layers for Copper-Doped Antimony Selenide Solar Cells

Cu-doped Sb2Se3 is considered a promising absorber for photovoltaics. The main efficiency bottleneck is related to the n-type buffer layer: the commonly used CdS presents a strong parasitic absorption in the visible light and displays a non-optimal band alignment; moreover, CdS is toxic. This modelling work presents the simulated performances of alternative buffer layers to overcome these issues: ZnSe, SnS2 and In2(Ox,S1-x)3. First, the experimental Cu:Sb2Se3/CdS junction reported by some of the authors was simulated to obtain experimental values of Sb2Se3 simulation parameters. ZnSe was found as very promising to substitute CdS; the optimal value of the thickness is dependent on the type of defectivity at the interface. SnS2 also shows good potential, but strongly dependent on donor doping density, that should be as high as possible, considering the possibility of external doping too. For what concerns In2(Ox,S1-x)3, the outmatch on CdS is possible if the composition is optimized, sticking to the better ratio between oxygen and sulfur. The role of the defectivity at the surface is very important for all the buffer layers object of this work, therefore the selection of an adequate buffer layer and the control of the growth is mandatory to obtain a high-performing device.