Influence of OLA and FA ligands on the optical and electronic properties of Cu2ZnSn(S,Se)4 thin films and solar cells prepared from nanoparticle inks

The use of formamide (FA) as an alternative capping ligand to oleylamine (OLA) during the preparation of Cu⁠2ZnSnS⁠4 (CZTS) nanoparticle inks was investigated. Photoluminescence properties of the selenised CZTS absorber layers revealed quasi-donor acceptor pairs (QDAP) as the main transition for carrier recombination with similar electrostatic potential fluctuations in both type of ligands. In OLA layers the transition shifts from QDAP to band-impurity as temperature rises. Solar cell devices fabricated from FA Cu⁠2ZnSn(Se,S)⁠4 were shunted due to the high porosity of the film while in contrast, the combination of FA+OLA absorber yielded reduced back contact barrier height, lower series resistance and higher fill factor and efficiency compared to OLA devices. This is a direct result of the removal of the fine grain carbon rich layer from the back contact interface usually present in films prepared with OLA only.