Fabrication And Characterization Of CuInS₂ And ZnS Thin Films Materials For Solar Cells

CuInS2(CIS) as aⅠ-Ⅲ-Ⅵ2 ternary semi-conductive inorganic compound is a direct band gap semiconductor, its absorptive coefficient is larger about 104-105 cm-1, its optical band gap is about 1.50 eV, very close to the best band gap (1.45 eV) of solar cell material. Comparing to CuInSe2, CuInGaSe2, CdTe and other thin film solar cell materials, CuInS2 contains no toxic ingredients, is a very promising solar cell materials.Currently, CIGS-based thin film solar cells generally use CdS films as buffer layer. However, the band gap of CdS films is only 2.4 eV, it restricts the response to short-wave of absorbing layer to a certain extent. Cd in CdS is toxic heavy metal, and the appropriate safety measures increase the cost of industrialization, and the use of CdS is not beneficial to environmental protection. Consequently, research and development of cadmium-free CIGS solar cells has become a global hot spot. ZnS is an n-type,Ⅱ-Ⅵ,wide and direct optical band gap semi-conductive compound. ZnS film becomes a thin film material alternative to CdS buffer layer because of its environmentally friendly, the wide optical band gap (3.5-3.8 eV), low production cost.Some relative researches on the materials of CuInS2 thin film solar cell were investigated in the thesis. First, Mo back electrode was prepared on glass substrate by magnetron sputtering. Second, copper indium alloy precursor, which had been fabricated by magnetron sputtering, was sulfured by heat treatment with solid-state sulfur in N2 to prepare CuInS2 absorbing layer thin film. Third, ZnS film, which replaces CdS buffer layer thin film, was prepared by chemical bath deposition. The influences of thin film process on the optical properties, phase structure, electrical properties, surface morphology and the related mechanisms were deeply researched. X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), UV-visible spectrum (UV-vis) and other test methods were used to test and characterize the performance of samples. The major experimental results were as follows:(1)Mo back electrode prepared by magnetron sputtering attached well to glass substrate with low resistance (3.2Ω／cm).(2) CuInS2 thin film prepared was smooth with dense structure. The results showed that the temperature of heat treatment had a great effect on formation of crystalline phase CuInS2, CuInS2 phase started to form at 500℃, high-quality crystalline CuInS2 phase formed at 550℃and CuInS2 film was preferred orientation growth on (103) plane. The CuInS2 film optical band gap by Tauc method increased with the decrease of annealing temperature. After sulfured at 550℃for 6 h, CuInS2 film diffraction peaks appeared in XRD pattern corresponded to the (103), (220), (312) plane of chalcopyrite structure, SEM picture indicated crystal particles was laminar and size is 1μm, the optical band gap of CuInS2 thin film was 1.5 eV. CuInS2 thin film is more suitable for absorbing layer of CuInS2 solar cell.(3) ZnS thin film prepared by chemical bath deposition was amorphous. The thickness of the deposited film was 60.7 nm after deposited 1h at 80℃, film transmittance was over 85%, the optical band gap varied between 3.8-3.9 eV. ZnS film prepared by chemical bath deposition was suit for replacing the CdS buffer layer in CIS/CIGS solar cell.