Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954
Synthesis, Structural and Optical Studies of Yb Doped CuGaS2 Thin Films Prepared By Facile Chemical Spray Pyrolysis Technique 1 S. Kalainathan1,2, N. Ahsan2, T. Hoshii2, Y. Okada2, T. Logu3, K. Sethuraman3 1 – Centre for Crystal Growth, School of Advanced Sciences, VIT University, Vellore, India 2 – Research Center for Advanced Science and Technology (RCAST), The University of Tokyo, Japan 3– School of Physics, Madurai Kamaraj University, Madurai, Tamil Nadu, India DOI 10.2412/mmse.66.48.915 provided by Seo4U.link
Keywords: thin films, intermediate band solar cells, spray pyrolysis, CuGaS2, optical properties.
ABSTRACT. Pristine and Ytterbium (Yb) doped (1-4%) chalcopyrite CuGaS2 (CGS) thin films were successfully prepared by facile homebuilt chemical spray pyrolysis technique and annealed in vacuum, nitrogen and argon atmospheres. X-ray diffraction characterization confirmed that all the prepared films are in tetragonal chalcopyrite structure with polycrystalline nature. The structural characterization of the thin films confirmed the formation of CGS without any presence of secondary phases in X-ray diffraction analysis. The optical band gaps of pristine and Yb doped CGS thin films were obtained from UV absorption spectra. The pristine CGS film shows a band gap of 2.40 eV. It is found that the band gap values decreases from 2.40 to 2.20 and 2.10 eV for 1 and 2 wt% Yb doping, and further widen from 2.4 to 2.2.47 and 2.61 eV for 3 and 4wt% of Yb. Fascinatingly, 1 and 2wt% Yb doped CGS thin films gives two band gaps 2.2- 1.1 eV and 2.1-1.0 eV, and this can be due to the formation of sub-band gap below the conduction band after doping. The presence of Yb in the host CGS thin film was confirmed by X-ray photoelectron spectroscopy studies. The photoelectric response of the sample has also been studied which shows significant photo current for the 1 wt% Yb doped CGS thin films.
Introduction. The incorporation of an impurity band within the semiconductor band gap can allow the absorption of low energy photons and thus can increase the efficiency of intermediate band (IB) solar cells [], [2], [3]. For a traditional photovoltaic semiconductor the electrons are excited directly from the valence band (VB) to the conduction band (CB) by absorbing photons, whereas in the case of IB semiconductors three photon transitions from VB to IB, IB to CB and VB to CB occurs due to the insertion of partially filled IB into the forbidden band gap which results in the enhancement of photocurrent without affecting the photo voltage. The percentage of upper limit efficiency was calculated to be 65.1% which was greater than the conventional Schokley-Queisser single junction solar cell whose efficiency was about 40.7%, and by increasing more the number of IBs will result in the increase of efficiency upto 80% [1], [2], [3]. Various IB materials such as thin films of highly mismatched alloys III-V dilute nitrides [4, 5], deep impurity doped hosts [6], and nanostructures using quantum dots [7], quantum rings [8], quantum wells [9], etc. makes the IB material to be easily fabricated and also its high density enhances absorption [6]. Ternary chalcopyrite semiconductor copper gallium sulphide (CuGaS2/CGS) attracts research interests for the optoelectronic and photovoltaic solar cell device applications due to the direct band gap of 2.49eV in the green region of the visible spectrum at room temperature [10]. Doping of transition metals in CGS has been found to be a potential candidate for IB solar cells [6, 11]. Earlier reports for doping of transition metals such as Fe [12], [13], [14], V [15], Mn [16], [17], [18], Cr [1921], Zn [22], [23], Ti [24] to the CGS hosts have been predicted for the creation of IB. Theoretical insights and experimental verifications have also been reported for transition metals doped CGS [21]. The valency match and the less distortion in lattice make the transition and rare earth elements to a 1
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