Cu(In1-xGax)Se-2 (CIGS)-based thin film solar cells fabricated using transparent conducting oxide (TCO) front and back contacts were investigated. The cell performance of substrate-type CIGS devices using TCO back contacts was almost the same as that of conventional CIGS solar cells with metallic Mo back contacts when the CIGS deposition temperatures were below 500degreesC for SnO2:F and 520 degreesC for ITO. CIGS thin film solar cells fabricated with ITO back contacts had an efficiency of 15.2% without anti-reflection coatings. However, the cell performance deteriorated at deposition temperatures above 520 degreesC. This is attributed to the increased resistivity of the TCO's due to the removal of fluorine from SnO2 or undesirable formation of a Ga2O3 thin layer at the CIGS/ITO interface. The formation of Ga2O3 was eliminated by inserting an intermediate layer such as Mo between ITO and CIGS. Furthermore, bifacial CIGS thin film solar cells were demonstrated as being one of the applications of semi-transparent CIGS devices. The cell performance of bifacial devices was improved by controlling the thickness of the CIGS absorber layer. Superstrate-type CIGS thin film solar cells with an efficiency of 12.8% were fabricated using a ZnO:Al front contact. Key techniques include the use of a graded band gap Cu(In,Ga)(3)Se-5 phase absorber layer and a ZnO buffer layer along with the inclusion of Na2S during CIGS deposition. (C) 2004 Elsevier Ltd. All rights reserved.