Impact of sulfur and gallium gradients on the performance of thin film Cu(In,Ga)(Se,S)2 solar cells

A graded bandgap structure proved to be an important factor for increasing an overall efficiency of the chalcopyrite-based thin film solar cells. This contribution is focused on the effects of sulfur incorporation into the surface region of industrial sequentially grown Cu(In,Ga)(Se,S)2 absorbers. A front grading due to such a sulfurization step enhances the bandgap in the space charge region, whereas the bulk of the absorber exhibits a lower bandgap which determines absorption and photocurrent. It will be demonstrated that such graded bandgap structures allow separating the absorption and recombination processes, therefore resulting in highly efficient solar cells with improved open circuit voltages without compromising short circuit currents. Moreover, a segregation of a gallium rich layer at the back contact as a result of sequential deposition reactions is discussed in terms of a back contact passivation that prevents injection of electrons to the back contact and suppresses phototransistor effects often observed at low temperatures. Furthermore, an influence of longer diffusion times on gallium distribution throughout the absorber layer has been investigated. High temperature deposition processes for prolonged time enhance gallium diffusion towards the absorber/buffer interface therefore leading to an overall increase of the absorber bandgap energy when both recombination and absorption processes are being affected.