Impact of annealing processes on the properties of CuIn 0.75Ga 0.25Se 2 thin films

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The efficiency of a solar cells utilizing polycrystalline Cu(In,Ga)Se 2 is to a large extent limited by crystalline defects of the semiconductor. Depending on the fabrication process the density of grain boundaries and dislocations, can vary considerably. However, the material properties can be improved significantly by the subsequent processing steps. In this paper results obtained using various post deposition methods to improve the structural and electro-optical properties of CuIn 0.75Ga 0.25Se 2 (CIGS) thin films have been presented and discussed. Films deposited by the evaporation of pre-reacted polycrystalline CIGS onto glass substrate were subsequently processed under several sets of conditions including vacuum, selenium, inert and forming gas ambients at different temperature and times. The structural and electro-optical properties of both as-deposited and annealed films were studied using a variety of analytical techniques. X-ray diffraction (XRD) and scanning electron microscopy (SEM) studies of the films showed a columnar structure with strong <112> orientation, which after heat treatments was relaxed to form a chalcopyrite structure. Raman analysis showed that full wave half maximum (FWHM) value reduced from 20 to 10 cm −1 with the annealing process indicating a change in both film composition and microstructure. In addition, investigations using energy dispersive X-ray analysis (EDAX), X-ray fluorescence (XRF) and Rutherford backscattering spectroscopy (RBS) revealed that the composition was approaching that of the starting polycrystalline material. Both n- and p-type conductivities were observed and gave resistivity values in the range 10 −1 to 10 6 Ω cm. Annealing in selenium changed the observed n-type conductivity of the as-deposited films to p-type. Photoacoustic spectroscopy (PAS) have also been applied to verify the improvement in the optical properties of annealed films.