Sara Lena Josefin Engberg

Synthesis of large CZTSe nanoparticles through a two-step hot-injection method

Grain boundaries in Cu2ZnSn(SxSe1−x)4 (CZTSSe) thin films act as a defect that reduces the mobility of the charges. Hence one way to improve the performance of these thin film solar cells is to increase the grain size in the films. Most of the synthesis methods published so far for CZTSSe colloidal nanoparticles can achieve a general size distribution range from 5–20 nm. This is where the particle size will saturate for most recipes used today. The assumption is that uniform size distribution is good for grain growth in a thin film but based on packing considerations, an optimal mixture of large and small nanoparticles that can easily be dispersed in non-polar solvents could be better. Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) nanoparticles are synthesized using the hot-injection method with oleylamine, trioctylphosphine, and hexadecane as the solvents. Selenium (Se) is introduced in the liquid phase to encourage grain growth – liquid selenization. This eliminates the need to anneal the film in a Se-containing atmosphere and allows for a more environmentally friendly process with lower temperatures and shorter annealing times. We show that a good dispersion can be achieved by choosing suitable surfactant molecules, solvents and precursors, and by controlling the initial monomer concentration. Additionally, we show how our new synthesis route can be utilized to achieve targeted ratios of CZTS and CZTSe nanoparticles to be used for mixed-phase CZTSSe thin films.

Liquid phase assisted grain growth in Cu2ZnSnS4 nanoparticle thin films by alkali element incorporation

The effect of adding LiCl, NaCl, and KCl to Cu2ZnSnS4 (CZTS) nanoparticle thin-film samples annealed in a nitrogen and sulfur atmosphere is reported. We demonstrate that the organic ligand-free nanoparticles previously developed can be used to produce an absorber layer of high quality. The films were Zn-rich and Cu-poor, and no secondary phases except ZnS could be detected within the detection limit of the characterization tools used. Potassium was the most effective alkali metal to enhance grain growth, and resulted in films with a high photoluminescence signal and an optical band gap of 1.43 eV. The alkali metals were introduced in the form of chloride salts, and a significant amount of Cl was detected in the final films, but could be removed in a quick water rinse.