The admittance analysis method has been used to calculate the collection efficiency and the short circuit current density in a-Si:H p-i-n Solar cell, as a function of the thickness of i-layer. It is evident that the results of the short circuit current can be used to determine the optimal thickness of the i-layer of a cell, and it will be more accurate in comparison with previous studies using a constant generation rate or an empirical exponential function for the generation of charge carriers throughout the i-layer. (C) 2004 Published by Elsevier Ltd.