THE INFLUENCE OF ANNEALING PROCESSES ON THE STRUCTURAL, COMPOSITIONALAND ELECTROOPTICAL PROPERTIES OF CUIN0.75GA0.25SE2 THIN-FILMS

Crystalline defects, such as the density of voids, grain boundaries an d dislocations, in Cu(In,Ga) Se-2 absorber layers depend on the fabric ation conditions and determine to a large extent the efficiency of pho tovoltaic devices. The material properties, however, can be improved s ignificantly by using post-deposition processes. In this paper, the ef fects of post-deposition heat treatments on properties of CuIn0.75Ga0. 25Se2 (CIGS) thin films are investigated. Selected flash evaporated sa mples were subsequently processed under several sets of conditions, in cluding vacuum, selenium, inert (argon) and forming gas (a 9:1 mixture of N-2:H-2) ambients, at different temperature and times. Structural, compositional 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) studie s of the films showed a columnar grain structure with strong < 112 > p referred orientation, which after heat treatments relaxed to give a ch alcopyrite structure. Raman analysis showed that the annealing process reduced the full-wave half-maximum (FWHM) value from 20 to 10 cm(-1) indicating a change in both film composition and microstructure. In ad dition, X-ray fluorescence (XRF) and Rutherford backscattering spectro scopy (RBS) revealed that the composition was approaching that of the polycrystalline starting material. Both n- and p-type conductivities w ere observed with resistivity values in the range 10(-1) to 10(6) Omeg a cm. Annealing in selenium atmosphere altered the n-type conductivity to p-type. To confirm improvements in optical properties of annealed films, photoacoustic spectroscopy (PAS) was employed.