Efficient Cu2ZnSnSe4 thin film solar cells prepared from an ethanolamine-dimethyl formamide based sulfur/selenium-free solution

Abstract

Most reported solution processed Cu2ZnSn(S,Se)4 thin film solar cells are fabricated from precursor solution with sulfur source. However, the use of sulfur-containing solution is undesirable from the viewpoint of environment and security due to the release of sulfur-containing harmful gases when drying precursors. Here, a novel sulfur/selenium-free solution method using ethanolamine-dimethyl formamide as solvent was developed for the preparation of Cu2ZnSnSe4 thin films. It was found that Cu-Sn alloy phases were formed in precursor during drying process. The formation and phase transition mechanisms of the Cu-Sn phases were illustrated. The aggregation level of the alloy phases and their distribution in the precursor depend on the drying time. Effects of different drying time on structural, morphological and photoelectric properties of the resulted Cu2ZnSnSe4 films were systematically investigated. The Cu2ZnSnSe4 absorbers exhibit a sandwiched structure or a four-layer structure, which is determined by the drying time of precursor, implying two different grain growth mechanisms. The devices with drying time of 6 min show larger grain size, longer carrier life time, higher external quantum efficiency and better average photovoltaic performance.