Sodium Assisted Sintering of Chalcogenides and Its Application to Solution Processed Cu₂ZnSn(S,Se)₄ Thin Film Solar Cells

Abstract

Cu<SUB>2</SUB>ZnSn(S,Se)<SUB>4</SUB> thin layers processed from solution-deposited earth-abundant precursors emerge as absorber materials for low-cost thin film solar cells. A frequently observed drawback of the chemical solution processing—poor crystallinity of the chalcogenide absorber—can be overcome by employing a sodium-containing reactive agent. We demonstrate a massive improvement in grain growth in the presence of sodium. It enhances the surface chemisorption of selenium molecules and can promote the formation of liquid Na<SUB>2</SUB>Se<I><SUB>x</SUB></I> phases during reactive annealing of the precursor. The sodium is also incorporated into the semiconductor absorber and significantly modifies its electronic properties. By adjusting the sodium precursor quantity, it is possible to tune doping levels and gradients to maximize the collection of photogenerated carriers in thin film Cu<SUB>2</SUB>ZnSn(S,Se)<SUB>4</SUB> solar cells. The presented approach can be extended to other solution-processed metal chalcogenides to enhance their structural and electronic properties, which are critical for applications such as thin film solar cells and transistors.