Chemical characterisation of evaporated In2Sx buffer layers in Cu(In,Ga)Se2 thin‐film solar cells with SNMS and SIMS

Abstract

AbstractFor the series production of Cu(In,Ga)Se2 (CIGS)‐based thin‐film solar cells it is desirable to replace the thin CdS buffer layer between absorber and transparent front contact by non‐toxic, low‐absorbing semiconductors. In2S3, deposited by atomic layer deposition, has already been qualified as an alternative buffer material. In this work, results of indium sulphide buffer layers deposited by thermal evaporation are presented. Pressed powders with different compositions and morphology were used for evaporation at about 720 °C, resulting in different layer compositions and cell performances. The composition of the initial powder material and of the pellets after the deposition steps was determined by XRF. The deposited In2SxOy buffer layers and the buffer/absorber interface region were analysed by SIMS and sputtered neutral mass spectrometry (SNMS) depth profiling.Fine‐grained pressed In2S3 powder evaporates rather homogeneously during the entire deposition run, resulting in nearly stoichiometric In2S3 layers. S and Cl are evaporated preferentially from pellets of coarse‐grained, S‐poor In2S2.4 powder containing 2 at% Cl, leading to excess S and high Cl concentrations in the first deposited layer. The subsequent layers are S‐poor and the In/S ratios continue to increase. In all interface regions additional amounts of Se, In, and Ga are detected, which could be attributed to the Cu‐poor defect layer on top of the CIGS absorbers. The best solar cell performance with efficiences of about 13% was achieved with the S‐rich buffer layers. Stoichiometric In2S3 or S‐poor layers yield lower efficiences between 8 and 11%. Copyright © 2008 John Wiley & Sons, Ltd.