A microwave-based technique has been used to prepare microcrystalline Si (µc-Si) materials rates near 15 Å/s. The use of these materials as intrinsic layers (i-layers) for single and multi-junction devices has been assessed. Since the high deposition rates allow for fabrication of the required thicker µc-Si i-layers in a similar amount of time to that used for high quality a-SiGe i-layers (rates of 1-3 Å/s), the materials are attractive, low cost replacements for a-SiGe bottom cell i-layers in a-Si/a-SiGe and a-Si/a-SiGe/a-SiGe multi-junction cells. Single-junction nip, a-Si/µc-Si and a-Si/a-SiGe/µc-Si devices have been fabricated. For these devices, the doped and amorphous layers were deposited using conventional rf glow discharge processes and deposition equipment separate from that used to fabricate the µc-Si materials. 7.0% efficiencies have been achieved for single-junction devices while pre-light soaked 9.8 and 11.4% efficiencies have been achieved for the tandem and triple-junction devices, respectively. The single-junction devices exhibit a degradation of only 0-2% after long term (1000 hrs.) of light soaking demonstrating a high degree of stability. Based on the present status, the µc-Si material prepared at high rates qualifies as a reasonable candidate for the i-layer of a bottom cell of a triple-junction device. Improvements in the performance, particularly the FF, will be needed before use in single-junction and tandem devices can be considered.