A low-cost, ligand exchange-free strategy to synthesize large-grained Cu2ZnSnS4 thin-films without a fine-grain underlayer from nanocrystals

Abstract

The first direct synthesis of CZTS nanocrystals in a formamide solvent system without using long hydrocarbon chain organic ligands is reported. The kesterite CZTS nanocrystals possess a mean size of 5.2 ± 1.2 nm. No secondary phases have been detected within the known limitations of XRD and Raman measurements. Experimental evidence suggests that excess S²⁻ is present on the surface of the nanocrystals, accounting for their dispersibility in polar solvents. The nanocrystals also exhibit a smaller weight loss of 8.7% at 500 °C compared to 24.4% for those capped by oleylamine. A description of the formation of CZTS FA nanocrystals and the role of formamide during synthesis is proposed. Annealing of spin-coated nanocrystal thin-films highlighted the difficulty of forming dense films from loose nanocrystal films. This work shows that this can be overcome using compaction with a combination of a reasonably soft metal and silicone. A means to compact the film uniformly on a centimeter scale with reduced delamination is thus demonstrated. Annealed compacted films possess crystal grains with a favorable size on the order of microns. More significantly, a large-grain layer is formed without an unwanted residual fine-grain underlayer. The absence of a fine-grain underlayer shows that this ligand exchange-free strategy is effective in resolving a key challenge associated with the nanocrystal approach of making CZTS thin-films while simultaneously being low-cost and having a smaller environmental footprint. The strategy presented here is equally applicable to other nanocrystal approaches requiring the synthesis of dense thin-films from nanocrystal films.