Elimination of the carbon-rich layer in Cu2ZnSn(S, Se)4 absorbers prepared from nanoparticle inks

Kesterite Cu2ZnSn(S, Se)4 (CZTSSe) is a promising photovoltaic material attracting significant research interests in recent years. Among the variety of techniques employed for preparation of the absorber thin films, the best results are observed for a hydrazine-based method with efficiency up to 12.6 %. On the other hand, Cu2ZnSnS4 (CZTS) nanoparticle inks annealed in the presence of Se have shown efficiency as high as 9.3 %. Importantly, CZTS nanoparticle inks have the power to be compatible with high volume, high value manufacturing with a variety of substrates including flexible foils, plastics and ultra-thin glass. However, one of the current limitations of the nanoparticle ink technology is the presence of a fine-grain (FG) layer between the CZTSSe large grain (LG) layer and the back contact. The presence of this FG layer is likely to reduce device performance via carrier recombination through traps, interface states and increased grain boundary density. CZTS nanoparticles were synthesized by injection of cold sulphur $({25}\ \hat{\mathrm{A}}{\ }^{\circ}\mathrm{C})$ into hot metallic precursors ( $({225}\ \hat{\mathrm{A}}{\ }^{\circ}\mathrm{C})$ ). The long carbon chain molecule, oleylamine used in the nanoparticle synthesis step is believed to be the direct reason of the FG layer. Herein, a higher soft-baking temperature of ${400}\ \hat{\mathrm{A}}{\ }^{\circ}\mathrm{C}$ is studied to evaporate the carbon rich solvent efficiently from the nanoparticle precursor thin films before the selenization process. As a result, the absorber is found to be composed of a single LG CZTSSe layer where the carbon-rich FG layer is eliminated.