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

Abstract

We report on a novel room temperature approach for the synthesis of environmentally-friendly copper zinc tin sulfide (Cu2ZnSnS4) nanoparticles. The method is shown to be compositionally robust and able to produce S2--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 CuxS-phases, which are detrimental for the solar cell performance when present in the final absorber layer. With nanoparticle sizes approaching the Bohr radius for Cu2ZnSnS4, the band gap can be broadened up to 1.7 eV. In addition, the conditions for forming stable, carbon-free aqueous inks of such Cu2ZnSnS4 nanoparticles are investigated and the stabilizing NH4+/S2--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.