Fabrication of Cu\u3csub\u3e2\u3c/sub\u3eZnSnSe\u3csub\u3e4\u3c/sub\u3e Thin-film Solar Cells by a Two-stage Process

Copper zinc tin selenide (Cu2ZnSnSe4 or CZTSe) is a quaternary compound semiconductor material that has attained more and more attention for thin film photovoltaic applications. CZTSe is only comprised of abundant and non-toxic elements. People have concerns about availability and cost of indium from CIGS and tellurium from CdTe, also about cadmium’s toxicity. These concerns have promoted CZTSe as an alternative thin film solar cell material. The major issues about CZTSe absorber fabrication are: tin loss during selenization process and existence of secondary phases. Recent improvements of CZTSe absorber have increased the efficiency of CZTSe thin film solar cell to 9.7% in laboratory, and this was accomplished by using H2Se as selenium source in a “two-stage” process. [1] However “one-stage” vacuum co-evaporation technique is still the most popular technique for CZTSe thin-film solar cells fabrication. In this research, Cu2ZnSnSe4 thin-film solar cells have been fabricated by using a two-step rapid thermal selenization process. The first step selenization is operated at 375℃, a relatively low annealing temperature, which helps avoiding the most common issue of tin loss. The second step selenization is carried out at a higher annealing temperature, 400℃ to 500℃, at where the formation of CZTSe quaternary compound can be completed, and fewer secondary phases remain in the CZTSe absorber bulk. A specially designed metallic precursor stacks deposition order has been developed to inhibit tin loss and zinc loss during selenization. Vacuum co-evaporation technique is not feasible to mass production, due to facility difficulty and bad uniformity. And H2Se is toxic and dangerous. We have developed these metallic precursor stacks vacuum deposition process and two-step selenium vapor selenization process. We believe this technique is more suitable for potential mass production in future. The properties of CZTSe thin-films and the performance of CZTSe thin-film solar cells have been characterized using techniques, including J-V, Raman spectroscopy, spectral response, and SEM/EDS. The best performance CZTSe thin-film solar cell that have been accomplished, has an open circuit voltage of 0.42 volt, shirt circuit current densities of 14.5 mA/cm2, fill factor of 47%, and efficiency of 2.86%