Influences of Sn doping concentration on characteristics of ZnO films for solar cell applications

JOURNAL OF SCIENCE OF HNUE DOI: 10.18173/2354-1059.2015-0030 Mathematical and Physical Sci., 2015, Vol. 60, No. 7, pp. 41-46 This paper is available online at INFLUENCES OF Sn DOPING CONCENTRATION ON CHARACTERISTICS OF ZnO FILMS FOR SOLAR CELL APPLICATIONS Nguyen Dinh Lam, Le Thuy Trang, Nguyen Thi Mui, Pham Van Vinh, Vuong Van Cuong and Nguyen Van Hung Faculty of Physics, Hanoi National University of Education Abstract. Undoped and Sn-doped ZnO films are deposited on glass substrates using a sol-gel spin coating technique with a doping concentration varying from 0% to 3 mol%. The effects of Sn doping concentration on surface morphology, structural and optical properties of the ZnO films are investigated using scanning electron microscopy, an X-ray diffractometer and a UV-Vis-NIR system. As the Sn doping concentration increases, the ZnO films have a preferential orientation along the [002] direction and the crystalline particle size becomes bigger. Average optical transmittances of all fabricated films are higher than 95% in the visible range. The optical bandgap energy of the films is enlarged by introducing Sn dopant. Keywords: ZnO, sol-gel spin coating, Sn-doped, solar cell applications. 1. Introduction ZnO films is one of promising candidate for sensitized solar cell application because of its unique properties such as non-toxicity, high optical transparency in the visible region, high surface activities, chemical inertia, and it’s also an economical material [1-9]. In order to fulfill requirement of this application, ZnO thin films must be modified with regards to its electrical, structural and optical properties. Many techniques are used to modify characteristics of ZnO films and introducing another element in the ZnO structure by means of a doping process is a very useful one [10-14]. ZnO films can be deposited onto substrates using techniques such as pulsed laser deposition, RF sputtering, chemical vapor deposition, spray pyrolysis and sol-gel spin coating [15-19]. Of these, the sol-gel spin coating technique has some merits such as easy control of the chemical components, there is good uniformity of the thin films, low temperature synthesis is possible, and production approaches economical. The sol-gel spin coating technique is also a promising technique for the preparation of nano structure materials. In this work, undoped and Sn-doped ZnO films were deposited on glass substrates using the sol-gel spin coating technique. The influence of Sn doping concentration on the structural, surface morphology, and optical properties of the films was investigated in detail to find optimal conditions for sensitized solar cell fabrication. Received October 15, 2015. Accepted November 16, 2015. Contact Nguyen Dinh Lam, e-mail address: lam.nd@hnue.edu.vn 41 N. D. Lam, L. T. Trang, N. T. Mui, P. V. Vinh, V. V. Cuong, N. V. Hung 2. Content 2.1. Experimental details ZnO films were prepared using 0.45 M zinc acetate dehydrate (Zn(CH3COO)2.2H2O) as a precursor, isopropanol (IPA) as a solvent and diethanolamine (DEA) as a stabilizer. Appropriate amounts of tin (Sn) doping were achieved by adding tin (IV) chloride pentahydrate (SnCl4.5H2O) to the precursor solutions with Sn/Zn molar ratios of 0.0, 0.5, 1, 1.5, 2, 2.5 and 3%. The solutions of undoped and Sn-doped ZnO were stirred at room temperature for 2 h to obtain homogeneous solutions. The obtained solutions were spread on glass substrates using a spin coating system at a spin speed of 3000 rpm and spin time of 30 s. Before the coating process, the glass substrates were cleaned using a NaOH solution, methanol, and deionized water in sequence. The films were then dried at 150 ◦C for 20 minutes in an oven to evaporate the solvent and remove organic residuals. The spin-coating and drying processes were repeated five times to get a desired thickness and the precursor films were then annealed at 500 ◦C for 1 h in air. X-ray diffraction patterns of the films were measured using an X-Ray Diffractometer (XRD) D5000 with CuKalpha radiation (λ = 1.5406 A˚) at room temperature. The surface morphologies of ZnO films were investigated using a Scanning Electron Microscope (SEM). The optical spectra of the films were studied using an UV-VIS-NIR spectrophotometer with wavelengths of 300 - 800 nm at room temperature. 2.2. Results and discussion Figure 1. XRD patterns of fabricated ZnO films Figure 1 shows the typical X-ray diffraction patterns of the films with different Sn doping concentrations. The XRD patterns indicate that the structure of the films is polycrystalline. The presence of the (100), (002), (101), (102), (110), 103, and (112) peaks in the XRD patterns also indicates that ZnO has a hexagonal wurtzite structure. Diffraction peaks related to other impurity phases are not observed in the XRD patterns. A comparison of the peak intensity of the (002) peak with the others showed that the crystal of ZnO films has a preferential orientation along the [002] direction when the Sn doping concentration increases. Furthermore, the interplaner 42 Influences of Sn doping concentration on characteristics of ZnO films for solar cell applications spacing (d002) is slightly changed as the variation of the Sn doping concentration. This may attribute to the substitution of Sn4+ for Zn2+ [14]. The crystallite size calculated using Scherrer’s formula increases with the incorporation of Sn dopant, reaching a maximum value for the 2% Sn-doped films. Figure 2. SEM images of Sn-doped ZnO films at (a) 0%, (b) 1%, (c) 2%, and (d) 3% SEM images of the ZnO films with various Sn doping concentrations are shown in Figure 2. The images indicated that the films are made up of tens-nanometer particles. In addition, particle size is bigger and the surface of the films becomes rougher as Sn content increases. Therefore, the surface roughness of films can be adjusted by Sn doping. Figure 3. (a) Optical transmittance spectra of the fabricated ZnO films (b) Plot of average optical transmittance of fabricated samples vs. Sn doping concentration High optical transparency is one of the most important characteristics of ZnO films. The optical transmittance spectra of ZnO films are shown in Figure 3 (a). All the optical transmittance spectra show sharp absorption edges in the wavelength region around 378 nm. The results also 43 N. D. Lam, L. T. Trang, N. T. Mui, P. V. Vinh, V. V. Cuong, N. V. Hung indicate that the optical transmittance of the films depends strongly on Sn doping concentration. Average optical transmittances of the doped films are over 99% in the visible range while that of undoped films is about 93%. This indicates that Sn incorporated in the ZnO would significantly improve optical transmittance. Furthermore, as seen from the SEM images in Figure 2, the higher transmittance in the films may also be attributed to the increase in optical scattering caused by the mixing of small and large particles as well as its rough surface morphology. However, when Sn doping concentration increases, the optical absorption of the samples decreases sharply at short wavelengths which have energies higher than the bandgap energy of the ZnO films. Therefore, films that exhibits a high optical transmission in the visible region wavelengths and high optical absorption at short wavelengths might be useful for solar cell application. Figure 4 (a) First derivative (dT/dλ) plot of the optical transmittance spectra of the ZnO films (b) Optical bandgap energy vs. Sn doping concentration The first derivative of the optical transmittance spectra are presented in Figure 4 (a). The bandgap energies that correspond to the peaks for all of the films are extracted and plotted in Figure 4 (b). The result indicates that the bandgap can be enlarged slightly with an increase in Sn doping concentration. The blue shift of the absorption edge might be attributed to an increase of carrier doping concentration. The doping increases the carrier concentration, when the Zn ions are replaced by Sn ions, which may shift the Fermi level leading to a widening of the bandgap and an increase in transmission [20]. 3. Conclusions Highly transparent Sn-doped ZnO films have been successfully fabricated on glass substrates using the sol-gel spin coating technique. Transmittance of the doped ZnO films is higher than that of the undoped-ZnO films and this can be attributed to an enhancement of the surface roughness. The Sn-doped ZnO films exhibits a preferred orientation along the [002] direction. The optical transmittances of all the films are over 95% in the visible range. 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