Application of indium zinc oxide window layers in Cu(In,Ga)Se2 solar cells

Abstract

Recently, indium-based transparent conductive oxides have become more interesting for application in Cu(In,Ga)Se2 (CIGS) solar cells. Although the fabrication costs may be higher than for standard ZnO:Al (AZO), amorphous indium zinc oxide (IZO) films are very attractive due to their high charge carrier mobilities leading to high conductivity. Additional advantages are that the substrate does not need to be heated during sputter coating and that the films demonstrate superior stability against humidity.We investigate the deposition of IZO films by RF and DC magnetron sputtering in dependence of sputtering power, total pressure, oxygen and hydrogen admixture, and substrate temperature. The IZO films on glass are analysed by means of conductivity, spectral transmittance, and Hall measurements as well as X-ray diffraction and scanning electron microscopy. Specific resistivity below 350μΩcm is obtained for 0.4μm thick films, half that of AZO. Best mobilities of about 50cm2/Vs are achieved for highly transparent films.The suitability of IZO as a window layer for CIGS solar cells is demonstrated with efficiencies as high as 19.5% comparable to the AZO reference. Results for stability tests of IZO-coated CIGS modules in damp heat will be discussed.