Research On Preparation Of CuInS₂ And ZnS Thin Films By Solvothermal Method

With the consumption of conventional energy, the problems of energy crisis and environmental pollution bring challenge to sustainable development of our nation economy. Solar energy possesses the advantages of renewable energy, widely distributed and friendly to the environment, which is considered to be the most potential replacement for energy. Thin film solar cells with less materials, low technological temperature and convenient operation, which makes it as a main and promising development direction of future solar cell. CuInS2 has a band gap of 1.53 eV with large optical absorption coefficients, It can act as p-type or n-type semiconductors depending on the types of point defects in CuInS2 semiconductor. Thus, the CuInS2 nanomaterial is one of the most useful absorption materials for thin film solar cells. At present, multi-source evaporation and sulfurization of metallic precursors are the more successful CuInS2 preparation methods. But the two methods require expensive devices and fastidious conditions, which seriously hinder the development of CuInS2 thin film cell.ZnS belongs to green n-type semiconductor with direct band gap of 3.6-3.8 eV, which can enhance the solar spectral response in short wave. It is a good buffer layer as a substitute of virulent CdS for thin films solar cells. Now ZnS thin film mainly prepared by chemical bath deposition method, but this method with fabrication process not mature, repeatability not good and complex chemical components. Therefore, how to search for a low-cost, mature and environment-friendly method for synthesis of the thin film will be a key point.In this paper, CuInS2 and ZnS thin films were deposited by solvothermal synthsis included the following 3 parts:In the first part, we have successfully prepared CuInS2 thin film on FTO glass with copper chloride, indium chloride and thiourea (TU) as starting materials under the open-air condition. The thin films were affected by dynamic conditions such as reaction time, TU concentration and oxalic acid concentration. By controlling the concentration of the reaction time, we can adjust the thickness and the crystallinity of the CuInS2 thin films. The typieal Uv-vis optical spectra show t hat all these CulnS2 thin films present a strong and borad adsorption band ranging between 400 and 950 nm, with the time increasing, the trend of the transmittance was decreased, the band-gap energy of the CuInS2 thin films ranges from 2.01 to 1.49 eV. Variation of the atomic compositions was as a function of TU concentration for CuInS2 thin film. XPS analysis that the Cu:In:S atom ratio was near-stoichiometry 1:12 with 0.3 mol/L TU, and with a band gap of 1.52 e V which is well matched to the visible light emission of sunlight (1.45 eV). The addition of H2C2O4 can improve the dispersion of the nanocrystals and the film formation. It was found that the solvothermal reactant oxalic acid played a very important role in controlling the growth of CuInS2 thin films. The oxalic cid not only acted as the reducing reagent but also helped to form the worm-like structure. The photoelectrochemical properties of the CuInS2 films were determined by Zahner CIMPS PEC cells using CuInS2 thin film as active photoelectrode (1 cm2), Na2S and Na2SO3 mixed solution as electrolyte, under the simulated sunlight (100 mW·cm-2), an open-circuit photo voltage (Voc) of 0.669 V, a short-circuit photocurrent (Jsc) of 18.38 mA/cm2 and fill factor (FF) of 29.6%, indicating an efficiency of 3.64%.In the second part, an in-situ growth of CuInS2 thin films on the copper substrate was developed by a facile solvothermal synthsis, where the ehanol was used as a solvent, InCl3.4H2O and TU were used as reactants, respectively. Study found that in-situ growth not only simplify the preparation process and greatly reduce the reaction time. Over time, homogeneous deposition was in a fully developed condition, some microspheres were covered on the dispersion of nanoplates, which are also composed of nanoplates. Reaetant coneentration plays a significant role in controlling the morphology of CulnS2 thin films. If InCl3.4H2O concentration was relatively high, nanoplates spread over the substrate and become dense to form net-like film. The addition of PEG is unfavorable for the growth of CulnS2 thin films with lower crystallinity, and, it prefers to form a doubled film, which the lower layer similar CulnS2 ordered nanosheets while the upper layer composed of flower or sphere shaped superstruetures. The photoelectrochemical properties of the CuInS2 films were studied by PEC, which an open-circuit photovoltage (Voc) of 0.662 V, a short-circuit photocurrent (Jsc) of 101.8 mA/cm2 and fill factor (FF) of 27.5%, indicating an efficiency of 15.4%.In the third part, we have successfully prepared ZnS thin film on glass with zinc acetate, and TU as starting materials under the solvothermal condition, where ehanol and water was used as a mixed solvent. All substrates were composed of microspheres wtih different density. The ZnS thin film obtained at 150 ℃ for 7 h containing Zn(CH3COO)2.2H2O(0.01 mol/L) and TU(0.02 mol/L) in the autoclave composed of a great many of spheres with a high and evenly density. ZnS thin films present a steep absorption edge at a wavelength of about 320-340 nm, the films with band gap of 3.7 eV is very suitable for the buffer layer of CuInS2 thin film solar cell.