The Photoelectric Performance Of Cu₂ZnSnS（Se）₄Films Prepared By A Two-stage Process

Recently, the research of copper indium gallium selenide （CIGS） thin film solar cells has made asignificated progresss, the power conversion efficiencies of the CIGS cell has been beyond20%. However,the cost is expensive owing to the Ga and In are rare element, which limit the development of large-scaleCIGS cells. Cu₂ZnSnS₄（CZTS） material was used to be one of the ideal substitute duo to that the materialis composed naturally abundant and nontoxic elements, and the band gap is very close to the optimumvalue for being used as an absorbed layer in solar cells. At the same time, CZTS material was applied indye-sensitized solar cells （DSSCs） as counter electrode （CE）.Currently, various methods have been introduced to the fabrication of CZTS films. Among them,the solution based approach was the most efficient. Nevertheless, long chain hydrocarbon molecules ortoxic solvent were inevitably used in the fabrication process, oleylamine and hydrazine, for example. Thesesolvent would be harmful to the environment. And a amorphism carbon layer always appeared whenannealed the films in sulfurization, which would block the electronic transferring. Finally, the conversionefficiency will be decreased. Whereas the advantge of less pollution, higher compactness of the films, andso on, sputtering is one of the ideal approach of preparig CZTS films. Meanwhile, CZTS is one ofquaternary compound, which increase the difficulty of the technology. Then, a new process of preparingCZTS films is necessary. In the paper, CZTS films were produced by a simple sputtering process and a hightempreture annealing process. Furtherly, these obtained CZTS films were used as absorbed layer forassembling CZTS film solar cells, and DSSCs were fabricated with CZTS CEs. The main work completedas follows:1. A Cu-Zn-Sn （CZT） precursor film was deposited by magnetron sputtering of a single ternary alloy target. The kesterite Cu₂ZnSnS₄films were fabricated after annealing process with plentiful S vapor,which can prevent the oxidation of the metal phase efficiently. A high quality film with high density andlarge grain was obtained after the annealing process at550oC. The composition of the CZTS films wasidentified to be pure with XRD and Raman tests. Cu₂ZnSnS₄films were produced by the sameexperimental process. These results proved that the two-stage process was available to prepare CZTS films,and the technology was comparatively facile and feasible. The ratio of S/Se was regulated by controling thesulfurization and selenization time, furtherly, the quality and property of the films was adjusted. TheUV-vis spectra indicated that the CZTS films possessed the absorb ability in the visible region.2. CZTS thin film solar cells were attempted to be assembled. Different methods were applied toprepare Mo back contact, CdS buffer layer, i-ZnO/Al:ZnO window layer, Al contact. Then the thin filmsolar cells were assembled and the I-V tests were performed. The result of the tests was not desired, owingto difficult the technology of assembly.3. CZTS counter electrodes with different ratio of S/Se were prepared by the two-stage process.Then DSSCs were fabricated with these counter electrodes, and the photovoltaic performance of the cellswas optimized by adjusting thickness and selenization time of the CZTS films. When the selenization timeachieved80min, the highest PCE8.42%was measured with a thickness of the film about600nm. It hasexceeded the efficiency of Pt CE8.16%, which was measured under the same experimental condition. TheCV and EIS tests demonstrate that the CEs produced by the two-stage process possess excellentelectrocatalytic activity and electrical conductivity.