Micro/nano pillar arrays are a promising architecture for high-efficiency solar cells that employ inexpensive photovoltaic materials with short minority carrier diffusion lengths (L n, p). To investigate design tradeoffs of the radial junction array solar cells, we fabricated 25 μm tall c-Si pillar array devices having different diameters and pillar filling ratios. The high-aspect-ratio radial n +-p + junctions were formed by gas phase diffusion of an n-type dopant into etched p-type Si pillars. The c-Si pillar arrays showed clear rectifying properties. The spectral reflectance decreased as the pillar filling ratio increased from 0.2 to 0.5, and no subsequent decrease was observed above a filling ratio of 0.5. Approximately two times higher cell efficiency was obtained with an 8 μm diameter (<1 L n) pillar array than with a 32 μm diameter (>3 L n) pillar array having the same pillar filling ratio.