We report on a direct measurement of the spatial distribution of electrical potential on cross sections of hydrogenated amorphous silicon (a-Si:H) n-i-p solar cells using scanning Kelvin probe microscopy. We found that most voltage has dropped near the p/i interface up to similar to500 nm into the i-layer, but that the voltage flattens out near the n/i region for a device with i-layer thickness of similar to800 nm. However, the potential distributes approximate-linearly on the entire i-layer for a device with i-layer of similar to250 nm. The potential measurements suggest that the i-layer is slightly n-type (6-13 x 10(15)/cm(3)), and the depletion width is similar to350-500 nm at the i/p interface. This depletion width provides a direct experimental evidence for the device design criteria that a-Si:H solar cells should be thinner than similar to500 nm, in the point view of electrical potential. (C) 2004 Elsevier B.V. All rights reserved.