Characterization of Cu(InGa)Se2 solar cells with high Ga content

Cu(InGa)Se₂ thin films were deposited by elemental evaporation with x≡[Ga]/([In]+[Ga]) from 0.27 to 0.81 and bandgap (E_g) from 1.16 to 1.54 eV. The Cu(InGa)Se₂ films were deposited with no gradients in the Ga and In and have no change in the structure or morphology as the Ga content increased. Glass/Mo/Cu(InGa)Se₂/CdS/ZnO solar cells have ~15% efficiency with x<0.5 or E_g<1.3 eV. V_oc increases over the entire bandgap range up to 820 mV but the cell efficiency falls off as E_g increases above 1.3 eV. Analysis of current-voltage and quantum efficiency measurements as a function of voltage show that this is attributable primarily to a voltage dependent current collection which results in a fall-off of fill factor, J_sc and, to a lesser extent, V_oc. Finally, Cu(InGa)Se₂ films with a Ga gradient are shown to improve fill factor in the high bandgap solar cells, apparently by improving the current collection