Bandgap Engineering in Indium SulfideThin Films by Tin Mixing
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1012-Y12-14
Bandgap Engineering in Indium SulfideThin Films by Tin Mixing Meril Mathew1, C. Sudha Kartha1, Vijayakumar K.P.1, John Elgin2, and Parameswar Hari2 1 Department of Physics, Cochin University of Science and Technology, Cochin University of Science and Technology, Cochin, 682022, India 2 Physics and Engineering Physics, University of Tulsa, L-165, Keplinger Hall, University of Tulsa, Tulsa, OK, 74104 ABSTRACT In2S3 thin films are promising candidate for photovoltaic applications due to its stability, wide band gap and photosensitivity. The optical band gap of indium sulfide (In2S3) thin films reported in the literature varies from 2.0 eV to 2.4 eV depending on their synthesis process. These values are too small for application as buffer layer in solar cells. In the present work, we report that the incorporation of Sn using [SnCl4.5H2O] could increase the bandgap to wider ranges. In2S3/Sn thin films were deposited on soda lime glass substrate using the chemical spray pyrolysis (CSP) technique. Electrical, optical and structural characterization studies were done on films prepared using different Sn/In ratios. Depth profile of various constituent elements using x-ray photoelectron spectroscopy (XPS) showed that incorporation of Sn increased the concentration of oxygen in samples. Band gap of the In2S3 with Sn films showed a direct correlation with the increase in Sn/In ratios. Depending on the Sn/In ratio, bandgap of In2S3/Sn could be varied from 2.72 eV to 3.78 eV. At lower mixing levels of Sn wide band gap low resistive In2S3/Sn films could be obtained which are highly useful for buffer layer applications. Low resistive buffer layer will decrease the series resistance of the cell and wider band gap will improve light transmission in the blue wavelength, two important factors that will assist in increasing the short circuit current of the photovoltaic cell. Samples having higher Sn/In ratios showed wider band gap up to3.78 eV. Even though the In2S3/Sn samples had very high bandgap and high resistivity these samples were highly photosensitive. The results proved that tin incorporation modified the band gap and electrical properties of the In2S3/Sn films favorably over wider ranges making it highly suitable for optoelectronic applications. INTRODUCTION Indium sulfide is a promising candidate for optoelectronic and photovoltaic applications [1-4] and is an important material for many technological applications due to its stability, wide band gap and photoconductivity [5]. In2S3 can be used as an effective nontoxic substitute for cadmium sulfide (CdS) in copper indium gallium sulfide (CIGS) based solar cells. The motivation for using In2S3 as a buffer layer in CIGS solar cells is not only to eliminate toxic cadmium, but also to improve light transmission in the blue wavelength region by using a material having band gap wider than that of CdS. CIGS based solar cells, with In2S3 as the buffer layer could reach efficiencies (16.4 %) near to those obtained by devices made with standard CdS buffer layer [6]. β-In2S
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