Surface engineering in Cu(In,Ga)Se2 solar cells

Surface modifications of three-stage co-evaporated Cu(In,Ga)Se2 (CIGS) thin films are investigated by finishing the evaporation with gallium-free (CuInSe2, CIS) stages of various lengths. Secondary-ion mass spectrometry shows substantial interdiffusion of indium and gallium, smearing out the Ga/(Ga+In) profile so that the addition of a CIS layer merely lowers the gallium content at the surface. For the thinnest top layer, equivalent to 20nm of pure CIS, X-ray photoelectron spectroscopy does not detect any compositional difference compared with the reference device. The modifications are evaluated electrically both by temperature-dependent characterisation of actual solar-cell devices and by modelling, using the latest version of scaps-1d (Electronics and Information Systems, Ghent University, Belgium). The best solar-cell device from this series is obtained for the 20nm top layer, with an efficiency of 16.6% after antireflective coating. However, we observe a trend of decreasing open-circuit voltage for increasingly thick top layers, and we do not find direct evidence that the lowering of the gallium concentration at the CIGS surface should generally be expected to improve the device performance. A simulated device with reduced bulk and interface defect levels achieves nearly 20% efficiency, but the trends concerning the CIS top layer remain the same. Copyright (c) 2011 John Wiley & Sons, Ltd.