We have studied the density of states and the structural properties of a new class of material. called hydrogenated polymorphous silicon, prepared by decomposition of a mixture of silane and hydrogen in a radio frequency powered plasma enhanced chemical vapour deposition system. The evolution of the density of states and of the optoelectronic properties of these films upon light-soaking and annealing cycles is different from that observed on standard hydrogenated amorphous silicon. The presence of platelet-like microstructures. revealed by an infrared absorption band centred at 2030 cm(-1). may explain this evolution, (C) 2002 Elsevier Science B.V. All rights reserved.