Stable, carbon-free inks of Cu2ZnSnS4 nanoparticles synthesized at room temperature designed for roll-to-roll fabrication of solar cell absorber layers

We report on a novel room temperature approach for the synthesis of environmentally-friendly copper zinc tin sulfide ($Cu_2ZnSnS_4$) nanoparticles. The method is shown to be compositionally robust and able to produce $S^{2-}$-stabilized carbon-free nanoparticle inks that are suitable for an absorber layer in solar cells. No organic residues from the process were detected. The metal-composition and the occurrence of secondary phases is here correlated with synthesis conditions: By utilizing a reactant concentration of Cu/Sn < 1.8 and Sn(II) as tin-source it is possible to avoid the formation of $Cu_xS$-phases, which are detrimental for the solar cell performance when present in the final absorber layer. With nanoparticle sizes approaching the Bohr radius for $Cu_2ZnSnS_4$, the band gap can be broadened up to 1.7 eV. In addition, the conditions for forming stable, carbon-free aqueous inks of such $Cu_2ZnSnS_4$ nanoparticles are investigated and the stabilizing $NH_4^+/S^{2-}$-ion concentration affects the quality of the deposited absorber layer. The use of room temperature synthesis and stable aqueous ink formulations make the method suitable for roll-to-roll fabrication and upscaling.