Using Ar-induced promotion of microcrystallization to the Si:H network, P-doped μc-Si:H films were prepared at a deposition rate of 50 Å/min. However, a large fraction of an amorphous component was identified, and a definite structural inhomogeneity was evident in the Si:H matrix. Efficient defect elimination, structural reorientation, and grain growth were performed, and an improved microcrystalline network with enhanced dopability was attained by introducing H2 to the Ar-assisted SiH4 plasma and by using their individual advantages to facilitate the microcrystallization process. A high conductivity of approximately 2 × 101 S cm−1 and a homogeneous distribution of micrograins of average diameter approximately 80 Å, contributing to a crystalline volume fraction of 67% in the matrix, were obtained. A significant achievement was to maintain microcrystallinity at a very low thickness of the bulk layer. The n-type μc-Si:H film of dark conductivity approximately 100 S cm−1 was realized at a thickness of approximately 350 Å with a deposition rate of 24 Å/min. Using this material at the tunnel junction as well as at the bottom layer of a double-junction a-Si solar cell, a conversion efficiency of 11.7% was attained without compromising the other parameters.