IMPACT OF ANNEALING PROCESSES ON THE PROPERTIES OF CUIN0.75GA0.25SE2 THIN-FILMS

The efficiency of a solar cells utilizing polycrystalline Cu(In,Ga)Se- 2 is to a large extent limited by crystalline defects of the semicondu ctor. Depending on the fabrication process the density of grain bounda ries and dislocations, can vary considerably. However, the material pr operties can be improved significantly by the subsequent processing st eps. In this paper results obtained using various post deposition meth ods to improve the structural and electro-optical properties of CuIn0. 75Ga0.25Se2 (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 con ditions including vacuum, selenium, inert and forming gas ambients at different temperature and times. The structural and electro-optical pr operties 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 stru cture with strong <112> orientation, which after heat treatments was r elaxed to form a chalcopyrite structure. Raman analysis showed that fu ll wave half maximum (FWHM) value reduced from 20 to 10 cm(-1) with th e annealing process indicating a change in both film composition and m icrostructure. In addition, investigations using energy dispersive X-r ay analysis (EDAX), X-ray fluorescence (XRF) and Rutherford backscatte ring spectroscopy (RES) revealed that the composition was approaching that of the starting polycrystalline material. Both n- and p-type cond uctivities were observed and gave resistivity values in the range 10(- 1) to 10(6) Ohm cm. Annealing in selenium changed the observed n-type conductivity of the as-deposited films to p-type. Photoacoustic spectr oscopy (PAS) have also been applied to verify the improvement in the o ptical properties of annealed films.