The spectral response of a typical thin-film a-Si:H p-i-n solar cell has been investigated using the simulation RAUPV2. The peak in the external quantum efficiency has been observed to shift towards the violet part of the spectrum on decreasing the cell thickness. Moreover, the height of the peak increases as cell thickness is decreased. This is correlated with an enhancement in cell performance for thinner cells, due to a general increase in the drift field within the cell. The external quantum efficiency of a cell with an optimal concentration of phosphorous in the intrinsic layer has also been investigated. The external quantum efficiency for this cell is similar to that of the thinner cell, and is associated with the enhancement of the drift field near the p/i interface that is brought about by the phosphorous doping of the intrinsic layer. However, the integrated recombination for the thinner cell and the phosphorous-profiled cell differ significantly at long wavelengths, despite their similarity at shorter wavelengths. This effect is due to the weakening of the drift field near the n/i interface in the phosphorous-profiled cell.