A new method to analyse the optical and thermal losses in amorphous silicon solar cells is presented. It is demonstrated that PDS measurements can provide important information on the losses that limit the performance of such cells. The major advantage of the method presented is the ability to determine parameters which are not accessible by other methods, i.e. the reflection, absorption without carrier separation, thermalization and losses in p- and n-layer, field losses, recombination and electrical gain of the solar cell under operating conditions. The method can be applied to thin film solar cells of all kinds complementary to DSR which can also be measured in the same set-up. Eliminating some experimental difficulties could lead to a new comprehensive and complementary characterisation method of solar cells of all kinds.