Although the beneficial effect of the buffer layer between the p- and i-layer of amorphous silicon solar cells has been known for many years, the role of this layer is controversial. This paper examines the effect of the buffer layer using a new equivalent circuit for these devices (Merten et al. IEEE Trans. Electron Dev. 45 (1988) 423-429 [1]). The parameters of this model can be easily assessed by variable illumination measurements (VIM) of the devices＇ I(V)-curve. With the model, collection of carriers in the bulk of the cell is easy and clearly separated from the diode behaviour of the device. The VIM-method allows for a complete analysis of the thin film cells, covering both technological and physical topics. It is shown that the dominant effect increasing the efficiency of the cells with buffer layer is the reduction of the hole injection from the p-layer which leads to a reduced diode term. The buffer layer only slightly reduces the recombination in the i-layer. This reduction mainly occurs in a region close to the p/i-interface and cannot be observed with red light (homogeneous carrier generation).