Improving The Crystallinity Of The CZTSSe Absorber And It's Impact On The Solar Cell Efficiency

CZTSSe compounds have become the hotspot of the solar cells, since their nontoxic, low-cost, earth-abundant compositions, suitable direct band gap and high absorption coefficient properties. Compared with the traditional chalcogenide solar cells, CZTSSe solar cells could achieve a higher solar energy conversion efficiency using the solution method. Recently, the world record efficiency of CZTSSe solar cells(12.6%) using hydrazine-based solution method has been reported by Mitzi and his co-workers. However, hydrazine is a highly poisonous solvent, which is hard to apply for the large-scale production. Hydrazine-free molecular precursor solution approaches have been widely investigated, but it's hard to prepare absorber layers with excellent crystallinities. In this work, we systematically studied how to improve the crystallinities of CZTSSe absorbers by rationally designing experimental processes.Firstly, we found the crystallinities of CZTSSe absorbers had great relevance with systematically optimizing selenization time. When the selenization time was 10 min, the absorber had a triple-layer morphology consisting of two large-grain layers(top and bottom of the film) and a fine-grain middle layer, and achieved the conversion efficiency of 5.37%. When the selenization time was extended to 20 min and 30 min, the thickness of the fine-grain middle layer slightly decreased, and this will lead to a poor cell efficiency due to the presence of small holes in the surface and cracks in the fine-grain layer. As the selenization time increased to 45 min, the CZTSSe grains became dense and the fine-grain layer disappeared, the power conversion efficiency of the corresponding solar cell increased to 6.23%.Secondly, we presented detailed investigations to improve the crystallinities of CZTSSe absorbers by adding a small amount of Ge powder as assisted selenization reagent. Elemental Ge could be transformed to GeSe2 for the reaction between Ge and Se. GeSe2 has a higher vapor pressure, which will generate GeSe2 vapor. The selenium and GeSe2 vapor would promote the transformation from CZTS film to Ge-doped CZTSSe film. We found the insert of elemental Ge into CZTSSe crystal lattice could greatly improve the quality of crystal growth and eliminate the fine-grain layer. Furthermore, this strategy finally enhanced the open voltage and power conversion efficiency of CZTSSe solar cell.Based on systematically investigating the research status and the urgent problems of CZTSSe solar cells, we have reasoningly designed the experimental program, systematically analysed the experimental results, and achieved the satisfied results. This work provides the new solution for improving the crystallinities and the efficiency of CZTSSe solar cells.