Controlled Grain Growth for High Performance Nanoparticle-Based Kesterite Solar Cells

Large-grain absorber formation through selenization techniques is a promising route for high performance chalcogenide solar cells. Understanding and subsequently controlling such grain growth is essential in improving absorber quality and developing absorbers with unique optoelectronic and morphological properties. We explain the essential role of liquid selenium in the grain growth of Cu2ZnSnSe4 (CZTSe) absorbers from Cu2ZnSnS4 nano particles by proposing a liquid-assisted grain growth mechanism. Through the use of a multizone rapid-thermal processing furnace, control of liquid Se delivery to the film and the Se-(g) atmosphere during processing is shown to result in novel absorbers with tunable properties. Additionally, the processing parameters necessary for high quality CZTSe absorbers, the role of nanoparticle properties, and the role of alkali metal dopants in the liquid-assisted growth mechanism are shown. Ultimately, record nanoparticle-based device performance of 9.3% is achieved for selenized CZTSe absorbers.