Exploring the application of metastable wurtzite nanocrystals in pure-sulfide Cu2ZnSnS4 solar cells by forming nearly micron-sized large grains

Abstract

An innovative approach to overcome the main challenge of solution-based pure-sulfide Cu₂ZnSnS₄ thin film solar cells by sulfurizing quaternary Cu₂ZnSnS₄ nanocrystals into nearly micron-sized large grains in a few minutes is presented. We developed an efficient phase-transition-driven grain growth strategy to explore the application of metastable wurtzite Cu₂ZnSnS₄ nano-materials in the photovoltaic field. The obtained Cu₂ZnSnS₄ thin film has a typical bilayer microstructure containing large grains on the top and fine grains at the bottom. Clear variations of phase, morphology, and component redistribution of the Cu₂ZnSnS₄ thin film were identified after the sulfurization process, which is critical to get a dense large-grained Cu₂ZnSnS₄ layer from quaternary nanocrystals. By tuning the composition of the wurtzite Cu₂ZnSnS₄ nanocrystals, annealing conditions, and sodium-containing compound, laboratory-scale photovoltaic cells with 4.83% efficiency were demonstrated without anti-reflection coatings. These results suggest the potential application of metastable wurtzite nanocrystals in pure-sulfide Cu₂ZnSnS₄ solar cells. This unique approach may also open up new opportunities to other optoelectronic devices, such as CuIn(S,Se)₂, Cu₂(In,Ga)Se₄, CdTe, and Cu₂ZnGe(S,Se)₄ solar cells.