Effect of substrate temperature and post-annealing on the properties of CIGS thin films deposited using e-beam evaporation

Cu(InGa)Se2 (CIGS) thin films were prepared using e-beam evaporation on a soda-lime glass substrate. The effect of substrate temperature and the difference between substrate temperature and post-annealing on the properties of the CIGS thin films and solar cells were studied. X-ray diffraction (XRD) and Raman spectroscopy, energy-dispersive spectroscopy, scanning electron microscopy, UV–Vis–NIR and the Hall effect were used to characterize the structural properties, composition, morphology, optical properties and electrical properties of the as-prepared CIGS thin films, respectively. The results demonstrated that the photoelectric properties of CIGS thin films prepared at a substrate temperature of 300 °C were optimal, with an efficiency of 7.1%. As the substrate temperature increased to over 300 °C, element gallium tended to evaporate from the substrate, which resulted in the variation of the Cu/(In + Ga) and Ga/(In + Ga) ratios of the films. The post-annealing process with in situ annealing temperatures of 300 °C and 400 °C was also studied. The results indicated that the post-annealing process, unlike the process of direct deposition at certain substrate temperatures, was able to avoid the element loss. At 300 °C in situ post-annealing temperature, there formed a 'polygon grains' Cu2−x Se phase, which disappeared when the in situ post-annealing temperature rose to 400 °C. The XRD patterns revealed that the post-annealing process made the element diffusion in films more uniform. The post-treated sample with an in situ post-annealing temperature at 400 °C, as a result, showed the highest efficiency of 9.0%, accompanied by the highest open-circuit voltage, short circuit current and fill factor.