Chalcopyrite thin-film solar cells with an In2S3 buffer showed high efficiencies above 15%, but only after annealing at 200C. One explanation was Cu interdiffusion at the absorber/buffer interface. It was possible to measure Cu interdiffusion directly using high kinetic energy X-ray photo-electron spectroscopy. Due to the increased information on depths of 10 to 20nm, the high kinetic energy spectroscopy was able to provide chemical data even through thin closed overlayers. The spectroscopy end-station also allowed in situ heating. A layer stack of In2S3/ Cu(In,Ga)Se2 was prepared and heated in situ up to 300C while continuously measuring the X-ray photo-electron spectra. A marked increase in the Cu signal was found above about 200C. With this set-up, it was possible to monitor directly Cu diffusion from the absorber and into the buffer layer.

Cu in In2S3 - Interdiffusion Phenomena Analysed by High Kinetic Energy X-ray Photoelectron Spectroscopy. P.Pistor, N.Allsop, W.Braun, R.Caballero, C.Camus, C.H.Fischer, M.Gorgoi, A.Grimm, B.Johnson, T.Kropp, I.Lauermann , S.Lehmann, H.Mönig, S.Schorr, A.Weber, R.Klenk: Physica Status Solidi A, 2009, 206[5], 1059-62