Investigation of coevaporated Cu(In,Ga)Se2 thin films in highly efficient solar cell devices

In order to further improve Cu(In,Ga)Se 2 based thin film solar cell devices, it is important to understand the electronic defects in the absorber layer material. For this study Cu(In,Ga)Se 2 thin film solar cells with a layer sequence Mo/Cu(In,Ga)Se2/CdS/ZnO/ZnO:Al have been produced on float glass substrates using a 3-stage coevaporation process together with a precise in-situ laser light scattering process control. The Cu/(In+Ga) ratio has been varied by means of altering the duration of the 3rd stage of the process. Although the spectral quantum efficiencies measured on the resulting solar cell devices have not been affected by this variation, the open circuit voltage V oc measured under AM1.5 illumination increases when the final Cu/(In+Ga) ratio of the absorber layer is closer to stoichiometry. Indium and gallium depth profiles, which are recorded as energy dispersive X-ray spectroscopy line scans along device cross-sections, together with the results from the previously performed defect spectroscopy on identical samples, are correlated with the increase in V oc .