Microcrystalline silicon (muc-Si:H) prepared by hot-wire chemical vapour deposition (HWCVD) at low substrate temperature T-S and low deposition pressure exhibits excellent material quality and performance in solar cells. Prepared at T-S below 250 degreesC, muc-Si:H has very low spin densities, low optical absorption below the band gap, high photo sensitivities, high hydrogen content and a compact structure, as evidenced by the low oxygen content and the weak 2100 cm(-1) IR absorption mode. Similar to PECVD material, solar cells prepared with HWCVD i-layers show increasing open circuit voltages V-oc with increasing silane concentration. The best performance is achieved near the transition to amorphous growth, and such solar cells exhibit very high V-oc up to 600 mV. The structural analysis by Raman spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM) shows considerable amorphous volume fractions in the cells with high V-oc. Raman spectra show a continuously increasing amorphous peak with increasing V-oc. Crystalline fractions X-C ranging from 50% for the highest V-oc to 95% for the lowest V-oc were obtained by XRD. XRD-mcasurements with different incident beam angles, TEM images and electron diffraction patterns indicate a homogeneous distribution of the amorphous material across the i-layer. Nearly no light induced degradation was observed in the cell with the highest X-C, but solar cells with high amorphous volume fractions exhibit up to 10% degradation of the cell efficiency. (C) 2003 Elsevier Science B.V. All rights reserved.