(39c) Band Gap Tuning with CZTS-Based Nanocrystal-Ink Solar Cells

Cu₂ZnSn(S,Se)₄ is of particular interest due to the achievement of 9.6% efficient laboratory scale devices using a non-vacuum, hydrazine-based deposition process.  While hydrazine poses safety challenges from a manufacturing perspective, an alternative non-vacuum deposition approach has achieved 7.2% efficiency via the selenization of Cu₂ZnSnS₄ (CZTS) nanocrystals deposited by roll coating. The resulting Cu₂ZnSn(S,Se)₄ (CZTSSe) film has a band gap of approximately 1.1 eV, similar to CuInSe₂.  CuInSe₂ device performance is improved by incorporation of Ga, which widens the band gap.  We hypothesized that similar bandgap tuning and improvements to CZTS devices may be possible by substituting a lower atomic number group IV element for some of the Sn.  Substitution with germanium is an attractive route, as germanium’s crustal abudance of 1.5 mg/kg compares favorably with the 2.3 mg/kg crustal abundance of Sn, the rarest element in CZTS.  Here we report a synthesis of Cu₂Zn(Sn_1-xGe_x)S₄ (CZTGS) nanocrystals and the performance of Cu₂Zn(Sn_1-xGe_x)(S_ySe_1-y)₄ (CZTGSSe) solar cells fabricated by sintering the nanocrystal films with elemental selenium vapor.  We find that the band gap of the CZTGS nanocrystals and the CZTGSSe solar cell can be rationally controlled by adjusting the Ge/(Sn+Ge) ratio. Solar cells fabricated from Cu₂ZnGeS₄ nanocrystal films yielded a power conversion efficiency of 0.51%. However,  Cu₂Zn(Sn_xGe_1-x)S₄ nanocrystals with a Ge/(Ge+Sn) ratio 0.7 yielded devices with an efficiency of 6.8% when synthesized to be Cu-poor and Zn-rich.  This result opens the possibility of forming Ge gradients to direct minority carriers away from high recombination interfaces and significantly improve the device efficiency of CZTSSe-based solar cells.