Understanding the Key Factors of Enhancing Phase and Compositional Controllability for 6% Efficient Pure-Sulfide Cu₂ZnSnS₄Solar Cells Prepared from Quaternary Wurtzite Nanocrystals

Pure-sulfide Cu 2 ZnSnS 4 thin film solar cells were fabricated employing a facile and low-cost preparation procedure by depositing wurtzite CZTS nanocrystals, followed by high temperature sulfurization. The exploration of previously reported devices with >4.8% efficiency has demonstrated the potential application of wurtzite nanocrystals in optoelectronic devices. Moreover, TEM-EDS characterization revealed the presence of compositional fluctuations within the fine-grained sublayer that may limit the performance of the final devices. To reduce the fine-grained sublayer and further improve the crystalline quality of the active large-grained layer of our solar cells, the Na doping method and the sulfurization process were both systematically studied in this work. The crystal phase, morphology, elemental composition, and photovoltaic performance were characterized. These results indicate that, for the wurtzite material system, (1) tuning the Na amount is necessary, yet insufficient to ensure the good performance of solar cells and (2) the introduction of SnS powder in the sulfurization treatment provides leverage with which to improve the microstructure and compositional distribution of the final absorber. By employing this leverage to optimize our CZTS absorbers prepared from quaternary wurtzite nanocrystals, the performances of solar cells has been increased to 6.0% in the absence of an antireflection coating.