Interface engineering and characterization at the atomic-scale of pure and mixed ion layer gas reaction buffer layers in chalcopyrite thin-film solar cells

In this work, we investigate the p-n junction region for two different buffer/Cu(In,Ga)(Se,S)(2) (CIGSSe) samples having different conversion efficiencies (the cell with pure In2S3 buffer shows a lower efficiency than the nano-ZnS/In2S3 buffered one). To explain the better efficiency of the sample with nano-ZnS/In2S3 buffer layer, combined transmission electron microscopy, atom probe tomography, and X-ray photoelectron spectroscopy studies were performed. In the pure In2S3 buffered sample, a CuIn3Se5 ordered-defect compound is observed at the CIGSSe surface, whereas in the nano-ZnS/In2S3 buffered sample no such compound is detected. The absence of an ordered-defect compound in the latter sample is explained either by the presence of the ZnS nanodots, which may act as a barrier layer against Cu diffusion in CIGSSe hindering the formation of CuIn3Se5, or by the presence of Zn at the CIGSSe surface, which may disturb the formation of this ordered-defect compound. In the nano-ZnS/In2S3 sample, Zn was found in the first monolayers of the absorber layer, which may lead to a downward band bending at the surface. This configuration is very stable (Fermi level pinning at the conduction band, as observed for Cd in Cu(In,Ga)Se-2) and reduces the recombination rate at the interface. This effect may explain why the sample with ZnS nanodots possesses a higher efficiency. This work demonstrates the capability of correlative transmission electron microscopy, atom probe tomography, and X-ray photoelectron spectroscopy studies in investigating buried interfaces. The study provides essential information for understanding and modeling the p-n junction at the nanoscale in CIGSSe solar cells. Copyright (c) 2014 John Wiley & Sons, Ltd.