The effects of pillar height and junction depth on solar cell characteristics are investigated to provide design rules for arrays of such pillars in solar energy applications. Radially doped silicon pillar arrays are fabricated by deep reactive ion etching of silicon substrates followed by the introduction of dopant atoms by diffusion from a phosphorus oxide layer conformally deposited by low-pressure chemical vapor deposition. Increasing the height of the pillars has led to doubling of the efficiency from 6% for flat substrates to 12% for 40 μm high pillars with a 900 nm junction depth because of an increase in the total junction area and lower optical reflection. For higher pillars, the current density and efficiency is decreased, which is attributed to the increasing presence of defect states at the surface introduced during the etching process. This effect can be counteracted by an Al2O3 passivation layer on the pillar surface. An optimum efficiency of 13% is found for a junction depth of 790 nm for 40 μm pillar height. At increased junction depths, the efficiency is decreased due to the ever thinner undoped core of the pillars, causing pillars with a large junction depth to become less efficient than flat silicon substrates.