Fabricating The Absorber Layer Of CuInS₂ Thin Film Solar Cells By Spin-coating Method

Copper indium sulfide (CuInS₂) thin film solar cells have became photovoltaic research focus due to its advantages such as high conversion efficiency, long-term stability and nontoxic. In order to establish the foundation for large-scale industrialization of CuInS₂ solar cells, the non-vacuum fabrication technologies of the absorber layer of the cells have been researched in this thesis mainly from the view of high performance and low cost. Specific process route is that: for the first the mixed nano-powders of sulfides of copper and indium metals were prepared by co-precipitation, then the mixed powders were annealed in hydrogen gas, the mixture obtained were prepared slurry and spin coated, finally precursor films were annealed in sulfur containing atmosphere and CuInS₂ thin films with a pure chalcopyrite structure were obtained.In this paper, the influence of pH value on the product of the co-precipitation was studied first. PH value was selected 1, 3 and 5 as the precipitation reaction conditions respectively, and the obtained precipitate were then annealed in Ar atmosphere at 500oC for 30min. X-ray diffraction (XRD) results showed that when the pH value of the co-precipitation was 1, the obtained precipitation contained only CuInS₂ chalcopyrite phase after annealed, no other impurity phases such as In2O3 or CuO. So pH=1 was confirmed as a more suitable pH value of co-precipitation reaction.Second, the impact of H2 annealing on the thin films was studied. The mixed powders of sulfides were obtained by co-precipitation and they were then annealed in H2 gas to get another mixed powder, both of which were used as precursor to prepare CuInS₂ thin films. The results of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), XRD and Raman spectra showed that in both thin films there was a chalcopyrite phase of CuInS₂, but the performance of CuInS₂ polycrystalline thin films prepared from the precursor of the powders annealed in H2 gas improved very much compared with the precursor of mixed sulfides. Packing density was higher, impurity phases disappeared and the quality was better. Therefore, annealing the precipitate powders was a necessary process of the experiment. The energy gap of CuInS₂ thin film prepared was determined to be about 1.45 eV by absorption spectroscopy measurement. The reaction mechanism of sulfidation was also discussed in this thesis and it was found that the process of forming took place via decomposition of Cu-In alloys. It mean that atoms of Cu and In from Cu-In alloys diffused through the initially formed CuInS₂ to react with sulfur from the gas phase at the surface, it also mean that the sulfidation was diffusion-limited.Finally, process parameters of sulfidation were explored, such as temperature and time of the sulfidation. The experiment results showed that 520oC was a better curing temperature, and curing time of 15min was appropriate.