The Preparation Of Copper Indium Sulfide Semiconductor Thin Films And Its Application In Solar Cells

CuInS2(CuInS2, CIS) thin film is Ⅰ-Ⅲ-Ⅵ ternary compound semiconductor material, its band gap is1.53eV which is very close to the optimal band gap of solar cells, within the range of light, its light absorption coefficient can be up to105cm-1, and can absorb90%sunlight only within1～2μm thick. Therefore, using it as the absorbing layer of thin film solar cells can substantially reduce the cost of production of solar cells. In this paper, we used solvothermal technology, used copper (Ⅱ) chloride dehydrate as a copper source, indium (Ⅲ) nitrate as an indium source, thiourea as a sulfur source and complexing agent, hexadecyl trimethyl ammonium bromide (CTAB) as a cationic surfactant, oxalic acid as a reducing agent, and ethanol as a solvent to prepare precursors, and to grow CIS thin films on SnO2:F (FTO) substrate directly, and solar cells with a glass/FTO/CIS/CdS/i-ZnO/Al-ZnO/Ag structure were prepared basing on the preparation of CIS thin films. The effects of thiourea concentrations, precursor concentrations, oxalic acid concentrations, CTAB concentrations, and reaction time on the performance of CIS thin films were studied systematically, and the photovoltaic performance of CIS thin film solar cells was tested and analysise, the main results are as follows:1. Using solvothermal technology, with copper (Ⅱ) chloride, indium (Ⅲ) nitrate, thiourea, oxalic acid, and ethanol as reaction precursor solution, to form CIS thin films on FTO substrate directly. By controlling the concentration of precursor solution, we can realize the controlled-growth of microstructure and morphology of the thin films, when the concentration is relatively low,"CIS nanosheet array thin films" are formed with nanosheet arrays are nearly perpendicular to the FTO substrate surface; when the concentration is relatively high,"CIS nanosheet microsphere thin films" are formed with nanosheet gathering together.2. Through the study of XRD, HRTEM and Raman spectrum, the results indicated that: no matter the nanosheets were composed to be array thin films or the microshphere thin films, they were all with single chalcopyrite, and single crystal which was preferred growing along the (112) plane, and the thick of nanosheets are about20-30nm; The Cu/In/S atomic ratios of CIS thin films were very close to the stoichiometric ratio of CuInS2of1/1/2, but with different preparation process, thin film could be In-rich n-type semiconductor thin film with Cu/In<1, and also could be Cu-rich p-type semiconductor thin film with Cu/In>1; CIS thin films had stronge light absortion property in the wavelength range of400-850nm, and the band-gap range was1.48-1.54eV.3. The effects of kinds and concentrations of surfactant on the morphology and growth mechanism of CIS thin films were studied. It was found that we could control the morphology of CIS thin films by changing the concentration of CTAB. When the concentration of CTAB was relatively low, nanosheet array thin films were formed; When the concentration of CTAB was relatively high, nanosheets tended to clump together to form nanosheet microshperes which was beneficial to reduce the total energy. Cationic surfactant CTAB could reduce the interfacial tension, CTAB was adsorbted at the surface of the newly formed crystal nucleus, interacting with anion, limiting the growth of CIS in the two-dimensional plane, and forming into nanosheet arrays. Because CTAB has the effect of reducing interfacial intension, when the concentration of CTAB was relatively high, nanosheet clumped together to form into CIS naosheet microspheres. At the same time, using polyvinylpyrrolidone (PVP) and sodium dodecyl benzene sulfonate (SDBS) insteaded of CTAB to carry out experiments, the results showed that SDBS could replace CTAB as the role of surfactants, but using PVP instead of CTAB could not form CIS nanosheet array thin films on FTO substrate.4. The effects of substrates on the growth of CIS thin films were discussed, in the same reaction precursor, used silicon plate, quartz plate, titanium plate and FTO conductive glass as substrate to carry out solvothermal reaction. The results showed that:no CIS thin films were formed on surface of silicon plate, quartz plate and the back side of FTO conductive glass, the same morphology were formed on the surface of titanium plate and the conductive side of FTO, this indated that the conductivity of substrate played an important role on the nucleation and growth of CIS thin films. In addition, the lattice structure of FTO is similar to CIS and the conductive characteristics and roughness of FTO surface may be the main reasons which lead to the nucleation and growth of CIS on FTO substrate directly.5. By studing the morphology, structure and composition of CIS thin films prepared at different reaction times, we explored the growth mechanism of CIS thin films. In the experiments, metallic ion complexed with thiourea (Tu) to form complexes [Cu(Tu)n(H2O)x]2+and [In(Tu)n(H2O)x]3+, and [Cu(Tu)n(H2O)x]2+was deoxidized by oxalic acid and formed into [Cu(Tu)n(H2O)x]+. When the temperature of the precursor and the pressure of Teflon liner reached a certain extent, Tu would released S2", S2" reacted with [Cu(Tu)n(H2O)x]+and [In(Tu)n(H2O)x]3+to form the final product CIS, nanosheet nucleus which grew along every direction were formed on FTO substrate. With different concentrations of reaction solution, different morphology of CIS thin films may form during the growth of nucleus. When the concentration of reaction solution was relative low, nucleus may form nanosheet array thin films through heteroepitaxial growth; when the concentration was relative high, flower-like nanosheet microspheres were formed fistly through heterogeneous growth, and then adsorbted on the FTO substrate to form nanosheet microsphere thin films.6. We have taken a preliminary stab at the fabrication of CIS thin film solar cells with a glass/FTO/CIS/CdS/i-ZnO/Al-ZnO/Ag structure. For the best solar cell, the open-circuit voltage is341mV, the short-circuit current desity is1.09mA·cm-2, the fill factor is0.29, and the efficiency is0.21%. To improve the photoelectric conversion efficiency of solar cells, further study needs to be pursued on the preparation process of solar cells.