The influence of annealing processes on the structural, compositional and electro-optical properties of CuIn0.75Ga0.25Se2 thin films

Abstract

Crystalline defects, such as the density of voids, grain boundaries and dislocations, in Cu(In,Ga) Se2 absorber layers depend on the fabrication conditions and determine to a large extent the efficiency of photovoltaic devices. The material properties, however, can be improved significantly by using post-deposition processes. In this paper, the effects of post-deposition heat treatments on properties of CuIn0.75Ga0.25Se2 (CIGS) thin films are investigated. Selected flash evaporated samples were subsequently processed under several sets of conditions, including vacuum, selenium, inert (argon) and forming gas (a 9:1 mixture of N2:H2) 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) studies of the films showed a columnar grain structure with strong preferred orientation, which after heat treatments relaxed to give a chalcopyrite 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 addition, X-ray fluorescence (XRF) and Rutherford backscattering spectroscopy (RBS) revealed that the composition was approaching that of the polycrystalline starting material. Both n- and p-type conductivities were observed with resistivity values in the range 10−1 to 106 Ω 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.