Current-voltage characteristics of thin-filin semiconductor devices are greatly affected by surface and interface defects. Deep level transient spectroscopy has been used in this work to reveal information about defect levels in CuInGaSSe based solar cells. The defect spectra varied significantly from sample to sample. Majority and minority carrier traps were identified for each cell. An effort was made to correlate the traps with the chemical nature of defects. In particular, cells were investigated that had three different buffer layers namely CdS, ZnO and ZnS. In the case of ZnO and ZnS buffer layer devices cells with high and low efficiencies were studied. The lower efficiency cells were characterized by deeper trap levels. (c) 2006 Elsevier B.V. All rights reserved.