Grain growth of CZTSSe via nanocrystal selenization

Achieving micro-sized and closed packed grains is an essential requirement for high performance chalcogenide thin film solar cells. The grain growth mechanism of CZTSSe starting from nanocrystal films appears to be affected by the capping ligands composed of long hydrocarbon chains. The growth of grains relies heavily on the selenium vapor supplied in the thermal process. This study decouples the selenium evaporation temperature apart from the substrate temperature to allow the investigations of their individual effects. We found that the two temperatures have distinctive effects on grain morphology, and the combination of a lower substrate temperature and a higher selenium evaporation temperature is favorable to produce flat and closely-packed grains and also a thinner MoSe₂ layer. Cell efficiency of 8.0% has been achieved by the film with favorable morphology. Also we show that the removal of carbon from the as-deposit films effectively improves the grain size of the selenized films. The understanding of the roles of selenium vapor and carbon content may further improve the device performance.