Fabrication of band gap tuned Cu2Zn(Sn1-xGex)(S,Se)4 absorber thin film using nanocrystal-based ink in non-toxic solvent

We deposited Ge-alloyed Cu2ZnSn1-xGex(S,Se)4 (CZTG(S,Se)) films using nanocrystal ink, where x = 0, 0.3, 0.5, 0.7, 1 and butylamine was used as ink-solvent. By post-sulfurization and post-selenization, we improved crystal quality of as-coated films and then studied their electrical and optical properties. The band gap of sulfurized film was increased from 1.54 eV (x = 0) to 1.98 eV (x = 1) and that of selenized film from 1.07 eV (x = 0) to 1.48 eV (x = 1) as x was increased from 0 to 1. The change of band gap with Ge substitution agreed well with theoretical band bowing model. The Raman peaks of both sulfurized CZTGS(S) and selenized CZTGS(Se) thin films also showed the peak shift to higher wave number by substituting Sn with Ge, indicating successful fabrication of Ge alloyed films by using nanocrystal inks. In both sulfurized and selenized films, grain size was observed to be increased with increase of germanium content. Results of structural and optical properties indicate that butylamine based ink can be used for fabrication of CZTGS(S) and CZTGS(Se) thin film of absorber layer in solar cell.