Effect of substrate temperature and source grain size on the structural and electrical properties of CSVT grown Cu(In1−xGax)Se2 thin films

Abstract

Polycrystalline CIGS thin film layers, used as a solar cell absorber, have been prepared on glass substrates by a low cost Close Spaced Vapor Transport Technique (CSVT) at different growth conditions for different substrate temperatures (450, 500 and 550 °C) and source grain sizes. The structural quality and evolution of defects in the CIGS thin films have been studied in terms of the substrate temperature and the source grain size. Scanning electron microscopy and atomic force microscopy analyses show that for a substrate temperature of 500 °C and the large source grain size, the layers present larger columnar crystallites and better morphology. For a same substrate temperature of 500 °C and different source grain sizes, it was found that the crystallites structure disappears for source grain sizes less than 20 μm. These results are discussed in terms of supersaturation rate.Admittance spectroscopy measurements carried out on CIGS thin films obtained from large source grains size reveal the presence of two defects with an activation energy of 116 meV and 45 meV for layers grown at a substrate temperature of 550 °C while the defects detected in samples grown at 500 °C had much smaller activation energies around 35 meV.The observed deep trap levels are tentatively assigned to the well known N1 centres. The effect of growth conditions on the defect energy levels and the defect densities are discussed.