Modeling Tin Sulfide Grain Growth during Post-Processing

Tin sulfide (SnS) is a semiconductor material with both an indirect and direct bandgap at 1.1 eV and 1.3 eV respectively. Due to the availability of tin and sulfur, SnS is seen as a feasible alternative to the thin film CIGS and CdTe solar cells. With a direct bandgap of 1.1 eV and the ability to be produced as a thin film, the SnS solar cell should achieve high levels of efficiency of approximately 32% according to the Shockley-Queisser limit (Shockley, Queisser, 1961). However, the efficiency of most SnS systems is around 4% in low sun conditions (Hartman, 2011). To understand how to improve this efficiency, further research is being done on the grain structure and how grain growth occurs under different annealing conditions. After thermal evaporation deposition, three different conditions were varied during annealing to affect grain growth: time, temperature, and annealing atmosphere. The samples were also deposited on two different substrates, glass and molybdenum. The samples were coated with Pt/Pd and characterized using SEM imaging. The SEM images were segmented to collect grain area information from each sample. The characterization revealed that longer annealing times and higher annealing temperatures lead to faster and greater grain growth. The annealing atmosphere of the samples affected surface diffusion in that the greater the partial pressure of S2 gas present in the environment the greater the facilitation of grain growth. The key conclusion based on the experimental data was that the annealing grain growth mechanism for SnS films is secondary or abnormal grain growth. This was evidenced by the initial columnar structure, the bimodal grain area distribution, and the non-uniform grains present in the SEM images. Further research on grain boundary diffusion with respect to time and texture of the thin films is needed although they suggest secondary grain growth as well.