We demonstrate a relationship between the diffusion length lD, as measured by the surface photovoltage (SPV) method and the collection length lco, measured on the same thin films of a-Si:H. lco is the appropriate quantity to describe current collection in p-i-n cells of a-Si:H, which is normally electric-field dominated. We have shown previously that lco can be used to predict the fill factor of p-i-n cells. Therefore, this letter justifies the use of lD measurements by SPV for optimizing the quality of i layers for p-i-n cells. An expression for ambipolar diffusion is presented and the experimental results are used to place limits on the relative magnitudes of electron and hole mobilities and lifetimes.