Among various semiconductor technologies applied in photovoltaics, thin-film technologies offer several attractive features, both technically and economically. Recent developments of silicon thin-film devices predominantly based on amorphous silicon (a-Si), are described. Particular emphasis is placed on the importance of the substrates that consist of glass coated with a transparent conductive oxide (TCO) to serve as front contact layer of the photovoltaic film combination. The electrical conductivity and optical absorption of the TCO substantially affect the energy conversion efficiency of solar modules. An essential enhancement of the light absorption within the photoactive a-Si layer is derived from a surface texture of the TCO, that provides index-grading for lower front reflection, and light scattering to result in light trapping due to total internal reflections within the a-Si. The requirements for optimal TCO properties are reviewed in the context of their availability. Additionally, novel cell structures involving microcrystalline silicon, are expected to lead to higher efficiencies. Thin-film technologies, led by those based on a-Si, will take on an increased role in module production worldwide, which is consistent with favorable manufacturing cost expectations. © 1999 Elsevier Science S.A. All rights reserved.