Growth and characterisation of Cu(In,Ga)Se2 thin films for solar cell applications
The development of low cost, efficient photovoltaic devices is a major technological challenge which demands suitable materials and fabrication processes. Thin film polycrystalline heterojunction solar cells appear to be most appropriate with respect to cost and ease of manufacture, and it is anticipated that the next generation of photovoltaic devices will be based entirely on thin film technologies. Copper based ternary and multinary compounds are well established as exceptional semiconductors with potential applications in the fields of solar cells for both terrestrial and space applications, infra-red detectors, light emitting diodes etc. The chalcopyrite forms of these compounds have large absorption coefficients and exhibit superior radiation resistance. Among these compounds, CuInSe2 (CIS) and CuIn1-xGaSe2 (CIGS) have raised the most interest and recent thin film heterojunction photovoltaic devices based on these materials have achieved efficiencies of the order 15.5% and 16.9% respectively. The higher efficiencies realised in CIGS based devices is due to the fact that the band gap of the material can be adjusted to wards the optimum value (1.45eV) by the partial substitution of gallium for indium. In this work, thin films of both CIS and CIGS were deposited onto glass substrates by flash evaporation of the respective pre-reacted source materials. The substrate temperature was varied between room temperature and 200 degrees C. Two types of evaporation sources, a flat tungsten strip and a molybdenum twin chimney were used. The effect of the growth conditions on the film properties was observed. The structural, compositional and electro-optical properties were studied using a variety of analytical techniques including x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive analysis with x-ray (EDAX), x-ray fluorescence (XRF), Rutherford backscattering spectroscopy (RBS), four point and thennal probe techniques, photoconductivity (PC) and photoacoustic spectroscopy (PAS). The as-grown films were found to have a columnar structure and a strong preferred orientation with the <112> plane parallel to the substrate. Results from EDAX, XRF and RBS indicated that the as-grown films were slightly deficient in selenium, otherwise the composition was comparable with that of the starting polycrystalline material. Electrical measurements revealed both n- and p-type conductivities with resistivity values in the range 10-2 to 106 gcm. The as-grown films were subsequently processed under several sets of conditions including vacuum, selenium, inert and forming gas ambients at different temperature and times. A two stage post-deposition heat treatment of the films was developed to improve the composition and crystal structure and to optimise the electro-optical properties. It was observed that the first annealing stage (in a selenium ambient) produced an excellent improvement in the composition of the film. An increase in the film grain size (to > 2pm) was observed when the films were subsequently annealed in a forming gas ambient. Significant improvements were also observed in the optical properties. The as-grown and annealed films were analysed using the PAS technique which revealed the existence of several donor and acceptor states originating from intrinsic defect levels. The results were compared with those obtained from single crystals. Photoconductivity measurements were also performed on the as-grown thin films.