Reduction of bandgap of ITO film and possibility of use as a window layer by incorporation of copper

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We report on reduction of the bandgap energy of ITO films by incorporation of copper along with thermal annealing for use as a window layer in optoelectronic applications such as solar cells. Using a conventional solid state reaction method, high purity In2O3 and SnO2 (99.99%) powders were mixed in a mass ratio of 75:25 in a ball milling for about one hour to fabricate ITO alloys. Electron beam evaporation was used to prepare an an ITO film with thickness of about 165 nm on a glass substrate. By immersing the films in a solution of Cu(NO3)2 (1g/1000ml) for 30 min, the prepared films were doped by ion exchange with Cu. Furthermore, the doped films were annealed in air at different temperatures for the diffusion of dopant Cu content (100, 150, 200, 250 and 300 oC) for 30 min. The (EDAX) was used to set the initial treatment composition. The XRD pattern was used to identify the cubic phase of the ITO films. Using spectroscopic ellipsomerty, the film thickness, refractive index as well as extinction coefficient of the film was determined in terms of a three layers model. We show that the refractive index and extinction coefficient increase with incorporation of Cu and the annealing temperature. The possible transition in the as deposited and annealed films were found to allow direct transitions with reduction of the band gap energy from 3.50 to 2.75 eV with the increase of the annealing temperature. This means that the lower layer of ITO behaves as a window layer and is a shallow layer available as a buffer layer of solar cell.