The dry and damp heat stability of chalcopyrite solar cells prepared with an indium sulfide buffer deposited by the spray-ILGAR technique

Cadmium free chalcopyrite solar cells based upon industrial Cu(In,Ga)(S,Se)2 absorber films with indium sulfide buffer layers deposited by the Spray-ILGAR technique have recently achieved certified efficiencies of 14.7%. Here we report for the first time on the stability of these cells. The cells were subjected to dry and damp heat conditions of 85 °C and 85% humidity for 100 h without encapsulation. The resulting cell parameters are measured and compared to cells prepared using a standard cadmium sulfide layer deposited by chemical bath deposition. Two different zinc oxide window processes were used for both buffers and the effect of changing the zinc oxide process is discussed. Before the damp heat tests, using an rf-sputtered zinc oxide process the indium sulfide buffers have an efficiency equal to the cadmium sulfide buffered cells and when using a second rf/dc-sputtered zinc oxide process a superior efficiency is obtained with the indium sulfide. The biggest loss in efficiency after damp heat testing is shown to arise from shunt paths at the scribe lines. The indium sulfide buffered cells degrade by only 11% under damp heat conditions when measured after rescribing. A difference between the cell efficiencies using two different zinc oxide windows highlights the interdependence of the process steps.