On the role of non-doped ZnO in CIGSe solar cells

We investigate the role of the non-doped i-ZnO layer in thin-film Cu(In,Ga)Se2 solar cells. A large number of laboratory scale module cells with different non-doped ZnO thicknesses are inspected by JV analysis, lock-in thermography and optical spectroscopy. We find that the role of the i-ZnO layer in our samples is to reduce the impact of randomly occurring electrical shunt paths in the devices. Accordingly, the i-ZnO layer leads to a somewhat smaller spread of device parameters. In the case of absent shunt paths, the device parameters of cells and modules without i-ZnO are similar to those with a thin i-ZnO layer. An effect of i-ZnO on the Voc is not systematically observed. However, for larger i-ZnO thickness, Jsc is systematically reduced, which is identified as due to optical absorption in the i-ZnO layer. We further find by high-resolution lock-in thermography that the junction homogeneity is not affected by the i-ZnO layer. Only the non-linear reverse bias current reduces with increasing i-ZnO thicknessan effect that appears to be independent from the saturation current density. Copyright (c) 2011 John Wiley & Sons, Ltd.