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 the device performance from exceeding 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 the doping and alloying of the material to suppress certain defects and improve crystallization, such as with 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, which are subsequently doped with Ge during selenisation to aid recrystallisation and reduce the effects of Sn species. Through Ge incorporation, we demonstrate structural changes with a negligible change in the energy bandgap but substantial increases in the crystallinity and grain morphology, which are associated with a Ge-Se growth mechanism, and gains in both the 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 the segregation of low local effective work function (LEWF) Sn(II) chalcogenide phases.

Correlating Molecular Precursor Interactions with Device Performance in Solution-Processed Cu2ZnSn(S,Se)4 Thin-Film Solar Cells.

Research efforts aimed at improving the crystal quality of solution-processed Cu2ZnSn(S,Se)4 (CZTSSe) absorbers have largely employed delicate pre- and postprocessing strategies, such as multistep selenization, heat treatment in mixed chalcogen atmospheres, and multinary extrinsic doping that are often complex and difficult to reproduce. On the other hand, understanding and tuning chemical interactions in precursor inks prior to the thin-film deposition have received significantly less attention. Herein, we show for the first time how the complexation of metallic and chalcogen precursors in solution have a stark influence on the crystallization and optoelectronic quality of CZTSSe absorbers. By varying thiourea to metal cation ratios (TU/M) in dimethylformamide (DMF) and isopropyl alcohol (IPA)-based inks, we observed the formation of nanoscale metal-organic complexes and submicron size aggregates which play a key role in the morphology of the precursor layers obtained by spin-coating and drying steps. We also identify the primary cations in the complexation and assembling processes in solution. The morphology of the precursor film, in turn, has an important effect on grain growth and film absorber structure after the reactive annealing in the presence of Se. Finally, we establish a link between metal complexes in precursor solutions and device performance, with power conversion efficiency increasing from approximately 2 to 8% depending on the TU/M and Cu/(Zn + Sn) ratios.