Device and material characterization of Cu(InGa)Se2 solar cells with increasing band gap

Thin‐film solar cells have been fabricated from Cu(InGa)Se2 films which were deposited by four‐source elemental evaporation with [Ga]/([In]+[Ga]) from 0.27 to 0.69 corresponding to a band gap from 1.16 to 1.45 eV. The films were intentionally deposited with no grading of the Ga and In to avoid gradients in their electrical and optical properties. X‐ray diffraction, energy‐dispersive x‐ray spectroscopy, and Auger electron spectroscopy show that the films have uniform composition with no change in structure and morphology. Glass/Mo/Cu(InGa)Se2/CdS/ZnO devices have open‐circuit voltage increasing over the entire band gap range to 788 mV and 15% total area efficiency for band gap less than 1.3 eV, or [Ga]/([In]+[Ga]) less than 0.5. A decrease in device efficiency with higher Ga content is caused primarily by a lower fill factor. Analysis of current–voltage and quantum efficiency measurements show that this results from a voltage‐dependent current collection.