Solvent-free synthesis of Cu2ZnSnS4 nanocrystals: a facile, green, up-scalable route for low cost photovoltaic cellsElectronic supplementary information (ESI) available: XPS spectra of the as-synthesized CZTS NCs, XRD, Raman, and XPS spectra of the CZTSe NCs right after the synthesis and after being stored for one year, the detailed procedure used to evaluate the device parameters, and TEM cross-sections of the CZTSe solar cell. See DOI: 10.1039/c4nr02564d

Efficient Cu 2 ZnSnSe 4 (CZTSe) solar cells were fabricated with a simple, environmentally friendly, and scalable synthetic method for Cu 2 ZnSnS 4 (CZTS) nanocrystals. CZTS nanoparticles were mechanochemically synthesized from elemental precursors on a relatively large scale (∼20 g), during which no solvents or additives were used, thus alleviating the complex process of particle synthesis. An analysis of the time evolution of the crystalline phase and morphology of precursor powders revealed that the formation of the CZTS compound was completed in 0.5 h once initiated, suggesting that the mechanochemically induced self-propagating reaction prevails. CZTS ink was prepared by dispersing the as-synthesized nanoparticles in an environmentally benign solvent (160 mg mL −1 in ethanol) without using any additives, after which it was cast onto Mo-coated glass substrates by a doctor-blade method. Subsequent reactive annealing at 560 °C under a Se-containing atmosphere resulted in substantial grain growth along with the nearly complete substitution of Se. The CZTSe solar cells therefrom exhibited power conversion efficiency levels as high as 6.1% (based on the active area, 0.44 cm 2 ) with a relatively high open-circuit voltage (0.42 V) in comparison with the bandgap energy of 1.0 eV. Cu 2 ZnSnS 4 synthesized by a solvent-free mechanochemical process were successfully applied to fabricate Cu 2 ZnSnSe 4 solar cells with efficiencies over 6%.