Eliminating secondary phases: Understanding kesterite phase evolution of Cu_2ZnSnS_4 thin films grown from ethanol based solutions with high photosensitivity

摘要

Non-toxic solution based approaches need to suppress the formation of secondary phases in the growth of Cu2ZnSnS4 (CZTS) thin films, which otherwise detrimentally affect the device performance especially by limiting the open circuit voltage. While growing CZTS films from ethanol based solutions containing common metal salts, we have conclusively established the reaction pathway leading to the formation of the single phase kesterite CZTS from the precursor solution via intermediate solid state binary and ternary compounds during deposition and the subsequent sulfurization treatment. Differently with the previous reports, we found that the presence of monoethanolamine (MEA) - a routinely used complexing agent - in the initial stage of solution mixing, inhibits complete reduction (oxidation) of Cu2+ (Sn2+), which subsequently affected the reaction pathway resulting in undesirable secondary phases. The activation energy for the decomposition of the precursor complex was found to be smaller, surmounted in the early formation of the single phase CZTS in the absence of MEA. Having a bandgap of 1.44 eV, these phase pure CZTS films exhibited very high photoresponse behavior (about 275% change in current upon white light illumination at a bias of 6 V).