In this paper, we review recent work on photoluminescence in the transition materials from hydrogenated amorphous silicon (a-Si:H) to microcrystalline silicon (pc-Si:H) and silicon-germanium alloy (a-SiGe:H). Also, the electroluminescence is reviewed as transport-controlled recombination. We focus on the new materials such as hydrogen-diluted a-Si:H, a-Si:H prepared by hot-wire chemical vapor deposition, high-growth rate films, and the new findings concerning the electronic structure in relation to the material microstructures and to the solar cell performance such as open voltage (V-oc). (C) 2003 Published by Elsevier Science B.V.