Cu(In,Ga)Se2 superstrate solar cells: prospects and limitations

Superstrate solar cells were prepared by thermal evaporation of Cu(In,Ga)Se2 onto ZnO coated glass substrates. For the first time, photo-conversion efficiencies above 11% were reached without the necessity of additional light soaking or forward biasing of the solar cell. This was achieved by modifying the deposition process as well as the sodium doping. Limitations of the superstrate device configuration and possible ways to overcome these were investigated by analyzing the hetero-interface with electron microscopy and X-ray photoemission spectroscopy measurements, combined with capacitance spectroscopy and device simulations. A device model was derived that explains how on the one hand the GaOx, which forms at the CIGSe/ZnO interface, reduces the interface recombination. On the other hand how it limits the efficiency by acting as an electron barrier at the hetero-interface presumably because of a high density of negatively charged acceptor states like CuGa. The addition of sodium enhances the p-type doping of the absorber but also increases the net doping within the GaOx. Hence, a trade-off between these two effects is required. The conversion efficiency was found to decrease over time, which can be explained in our model by field-induced diffusion of sodium cations out from the GaOx layer. The proposed device model is able to explain various effects frequently observed upon light soaking and forward biasing of superstrate devices. Copyright © 2014 John Wiley & Sons, Ltd.