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Non-Vacuum Single Step Synthesis of Large-Grain Size CZTS Photo Absorber for Thin Film Solar Cells by Flux Assisted Chemical Spray

One-step preparation of stoichiometric CZTS photo absorber thin films without the secondary or amorphous phases by a modified chemical spray pyrolysis (CSP) technique is described. By incorporating sodium (Na2S) as fluxing agent by intermittent spray deposition during CZTS film deposition or through post growth heat treatment with Na2S added by soaking, CZTS films with large grain size are realized. XRD patterns show characteristic diffraction lines with increased intensity and decreased FWHM in CZTS thin films both with Na flux during in-situ deposition and post deposition annealing step affirming the increase in crystallinity and grain size. This is supported by Raman studies. Whereas the average crystallite size of CSP-CZTS film is ~25 nm, using the Na2S layering within the CZTS, size increases to ~90 nm and by post growth soaking with Na2S aqueous solution a further increase in average grain size to ~0.25μm was realized as shown by the SEM studies. Optical data shows an increase in absorption coefficient with band gap energy in the range 1.49 eV-1.62 eV in Na2S in-situ or post annealed CSP-CZTS films which is ideal for heterojunction solar cells.