Characterization of Cu(In1-xGax)Se-2 films prepared by three-stage coevaporation and their application to CIGS solar cells for a 14.48 % efficiency

An element coevaporation method was used to prepare Cu(In1-xGa)Se-2 (CIGS) films for absorbing layers of CIGS solar cells. Na diffusion from a soda-li me glass substrate into the CIGS film controlled the hole density to an ord er of 10(16) cm(-3) in the Cu-deficient composition and to an order of 10(1 5) cm(-3) in the Cu-rich composition. At the CIGS surface, Na was strongly accumulated and Se was deficient, indicating that the CIGS surface had a Na 2Se-rich composition. As the Ga content increased, the bandgap of the CIGS film increased, but the optical transmittance near the band edge decreased due to scattering by small grains. The short-circuit current of the CdS/CIG S solar cell decreased continuously with increased Ga content. The open-cir cuit voltage and the fill factor were highest when the Ga content was near 0.3. The highest efficiency, 14.48 %, was achieved in the 0.18-cm(2) CdS/CI GS cell with a composition of Cu-0.9(In0.7Ga0.3)Se-2.