Novel and economical approach of sulfurizationof Bi-facial CIGSe layers on flexible substrate

The surface sulfurization is significant method to enhance the VOC of Copper Indium Gallium Diselenide (CIGSe) solar cells. However, a large amount of sulfur (S) in the layer deteriorates the FF and JSC owing to the lower absorption of photons with low energy and increased deep-level recombination. Since sulfurization is temperature-dependent, it is difficult to control the diffusion depth of S in the CIGSe layer. Herein, we report the band gap engineering of CIGSe by the addition of sulfur using a simple and rapid thermal processing (RTP) technique without involving toxic H2S or H2Se gasses, and the bifacial growth of CIGSe was reported using a non-aqueous electrochemical technique. The films were heat treated in an RTP furnace in ambient S at 450 °C using a two-step program. Sulfurization was performed with 2 mg and 20 mg of S powder, and the effect of the amount of S on the various properties of the CIGSe layers was investigated. A significant improvement in the properties of CIGSe was observed upon sulfurization. X-ray Diffraction, Spectroscopy and Raman spectroscopy indicated that the sulfurization process resulted in the formation of a Copper Indium Gallium Sulfoselenide (CIGSSe) absorber layer, which was highly crystalline and did not exhibit any secondary phase formation. The development of the CIGSSe layer at the surface of CIGSe in the sulfurized sample was confirmed by the existence of distinguished S 2p doublet peaks in the CIGSe layer sulfurized with 20 mg S powder as compared to 2 mg S powder, whereas it was completely absent in the as-prepared layer in the X-ray photoelectron spectroscopy (XPS) core level spectra. Rietveld refinement confirms the formation of CIGSSe along with CIGSe in layer upon sulfurization.