Ex-situ Ge-doping of CZTS Nanocrystals and CZTSSe Solar Absorber Films

Cu2ZnSn(S,Se)4 (CZTSSe) is a promising material for thin-film photovoltaics however the open-circuit voltage (VOC) deficit of CZTSSe prevents device performance to exceed 13% conversion efficiency. CZTSSe is a heavily compensated material that is rich in point defects and prone to the formation of secondary phases. The landscape of these defects is complex and some mitigation is possible by employing non-stoichiometric conditions. Another route used to reduce the effects of undesirable defects is doping and alloying of the material to suppress certain defects and improve crystallization such as germanium. The majority of works deposit Ge adjacent to a stacked metallic precursor deposited by physical vapour deposition before annealing in a selenium rich atmosphere. Here we use an established hot-injection process to synthesise Cu2ZnSnS4 nanocrystals of a pre-determined composition, subsequently doped with Ge during selenisation to aid recrystallisation and reduce the effects of Sn species. Through Ge incorporation we demonstrate structural changes with negligible change in energy bandgap but substantial increase in crystallinity and grain morphology which is associated to a Ge-Se growth mechanism and gains in both VOC and conversion efficiency. We use surface energy-filtered photoelectron emission microscopy (EF-PEEM) to map the surface work function terrains and show an improved electronic landscape which we attribute to a reduction in segregation of low local effective work function (LEWF) Sn(II) chalcogenide phases.