The Preparation And Research Of Cu₂ZnSnS₄Thin Film Solar Cells

Kesterite materials Cu2ZnSnS4(CZTS), as a quaternary semiconductor, is an very promising solar cell materials due to its optical band gap of1.5eV and high optical absorption coefficient of larger than104cm-1. The structure of CZTS can be treat as a consequence of replacing half In atom in chalcopyrite CuInS2with Zn and Sn, respectively and the conduction type is p-type. Compared with CuInS2and CIGS, CZTS film is more suitable in mass production and application since the constitute elements in CZTS are nontoxic and inexpensive, moreover, they are earth abundant and environment benign.CZTS thin films is a kind of multiple compound. The atomic ratio and lattice mismatch may lead to structural imperfection, as a consequence, solar cell with CZTS photo-absorption layer may display a poor photoelectric conversion efficiency. In this work, therefore, we need to study the structure characteristics, photoelectric performance and mechanism of the film, after which some work like expanding and improving the preparation technology of high quality CZTS thin film should be done, then preparation process of CZTS thin film solar cell should be optimized.CZTS thin film solar cell works base on p-n junction, which consist of p-type CZTS and n-type ZnO. However, poor lattice matching between CZTS and ZnO can affect the output of the solar cell efficiency if they contact directly to form p-n junction. Consequently, it’s necessary to prepare a buffer layer between CZTS absorption layer and ZnO window layer. As a direct bandgap semiconductor with high light transmittance and band gap of2.42eV, CdS buffer layer can make the band gap steps and lattice mismatch between CZTS and ZnO descend and improve the quality of the p-n junction along with the performance of the solar cell. Some relative researches on the absorption layer, buffer layer and back electrode of CZTS solar cell were investigated in this thesis. First, Mo electrode was prepared on glass substrate through magnetron sputtering. Second, CZTS film was fabricated by sol-gel spin-coating method, after which the film was sulfered by heat treatment in a N2atmoshpere. Third, a new electrochemical deposition technique was used for fabricating CdS thin films from an alkaline solution. Finally, the window layer ZnO/AZO thin films was prepared by magnetron sputtering, and the top electrode Al prepared by a thermal evaporation method. The influences of thin film process on phase structure, surface morphology, atomic ratio and optical properties were deeply researched. Some test methods like X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrum (EDS), and UV visible spectrum (UV vis) were used to test and characterize the performance of thin film samples and electrode. The major experimental results were as follows:(1) Double-decker Mo back electrode was prepared by magnetron sputtering, the first layer attached well to glass substrate while the second layer crystallized well and packed closely.(2) CZTS thin film was prepared with a continuous uniform and smooth surface and1-2μm thickness after heat treatment. Through XRD spectrum we find a (112) plane preferred orientation growth of CZTS thin films. Under one hour heat treatment at500℃, four diffraction peaks appeared corresponded to the (103),(220),(312) plane of kesterite structure. Through UV-vis spectrum, we can find the optical band gap of CZTS thin film was1.5eV, which means CZTS thin film was a perfect solar cell absorption material. Moreover, the results also showed that the film-forming temperature and different Cu atomic ratio had great effect on formation of crystalline phase, morphology and optical properties of CZTS thin film.(3) A electrochemical deposition technique was described for fabricating CdS thin films from an alkaline solution. This technique was further applied to the preparation of CdS buffer layer. CdS thin films with smooth surface, uniform particle size, near stoichiometric atomic ratio, and a theoretical band gap of2.4eV were obtained under the condition as follows:pH value was9.36, Cd2+concentration was 0.025mol/L and deposition potential was1.7V. After being applied to the CZTS thin film solar cells, the as-synthesized buffer layer of CdS thin films matched well with the CZTS thin films, and improved quality of the CZTS/CdS p-n junction.(4) Combining the fabrication methods above with traditional process, we prepared CZTS thin film solar cell device. As a consequence, we got a low photoelectric conversion efficiency of0.12%. After a study about solar cell’s performance loss, we can find that the SC need to improve the interface contact between the films.