Structural, optical and electrical properties of undoped and al doped-ZnO films for use in photovoltaic devices

JOURNAL OF SCIENCE OF HNUE DOI: 10.18173/2354-1059.2016-0040 Mathematical and Physical Sci., 2016, Vol. 61, No. 7, pp. 122-127 This paper is available online at STRUCTURAL, OPTICAL AND ELECTRICAL PROPERTIES OF UNDOPED AND Al DOPED-ZnO FILMS FOR USE IN PHOTOVOLTAIC DEVICES Nguyen Dinh Lam, Tran Thi Mui, Le Thuy Trang, Pham Van Vinh, Vuong Van Cuong and Nguyen Van Hung Faculty of Physics, Hanoi National University of Education Abstract. ZnO films are deposited on glass substrates using a sol-gel spin coating method. To investigate the influence of Al doping concentration on structural, optical and electrical characteristics of films, Al doping concentrations are varied from 0% to 5%. The surface morphology, structure and optical transmittance characteristics of the films are studied using scanning electron microscopy, an X-ray diffractometer and a UV-Vis-NIR system, respectively. All the peaks shown in the XRD patterns are sharp and narrow, and they closely matched that of the hexagonal wurtzite ZnO structure. Average transmittance of the films in the visible region exceeds 95%, with the highest average transmittance obtained by 1% Al-doped film. Furthermore, the optical bandgaps of the films are determined by Tauc’s law and they show a tendency to increase with the incorporation of Al content. The I-V plots of 1%Al-doped ZnO film were carried out in the dark and under illumination. The film shows good photoconductivity response. The results suggest a potential application of Al-doped ZnO films for different optoelectronic and photovoltaic devices. Keywords: ZnO film, sol-gel spin coating, Al-doped, photovoltaic. 1. Introduction Zinc oxide (ZnO) films can be employed in thin film solar cells, transistors, sensors and other optoelectronic devices because of its wide band gap (3.37 eV) and large exciton binding energy (60 meV) [1-3]. The optical transmittance and electrical conductivity of ZnO films can be improved by adding dopants such as aluminum (Al), tin (Sn), and indium (In) [4-6]. ZnO films can be fabricated using growth techniques such as molecular beam epitaxy (MBE), metal-organic chemical vapor deposition (MOCVD) and high vacuum sputtering [7-9]. Using these techniques, high quality ZnO film can be obtained. However, the total cost to manufacture ZnO film will be high because of the need for sophisticated, high cost equipment. However, the sol-gel spin coating technique is simple and it’s not difficult to fabricate ZnO films. Furthermore, the sol-gel spin coating technique also has good composition controllability and it’s easy to coat a desired shape or area [10, 11]. For these reasons, the sol-gel spin coating technique is usually utilized to fabricate Received March 31, 2016. Accepted July 25, 2016. Contact Nguyen Dinh Lam, e-mail address: lam.nd@hnue.edu.vn 122 Structural, optical and electrical properties of undoped and Al doped-ZnO films for use... ZnO films. In this work, Al-doped ZnO films were deposited on glass substrates. The Al doping concentration varied from 0% to 5%. The influence of the Al content on the structural, optical, and electrical characteristics of ZnO films was investigated in detail. 2. Content 2.1. Experimental details Using zinc acetate dehydrate (Zn(CH3COO)2.2H2O) as a precursor, isopropanol (IPA) as a solvent and diethanolamine (DEA) as a stabilizer, ZnO films were prepared. Appropriate amounts of aluminum (Al) doping were achieved by adding aluminum nitrate to the precursor solutions with Al/Zn molar ratios of 0.0, 0.5, 1, 1.5, 2, 2.5, 3 and 5%. The solutions were stirred at room temperature for 2 hours to obtain homogeneous solutions. The obtained solutions were deposited on glass substrates using a spin coating system. Before film deposition, all contaminates were removed from the glass substrates by an NaOH solution, methanol and deionized water. The solutions were dropped onto glass substrates and spun at 3000 rpm for 30 s for all films. After deposition, the obtained films were dried at 150 ◦C for 20 minutes in an oven to evaporate the solvent and remove organic residuals. The procedures from coating to drying were repeated five times to get a desire thickness of ZnO films. After the coating procedure, the films were annealed at 500 ◦C for 1 h in air using an electric furnace. The structure, surface morphology, optical spectra and I-V curves were determined using a D5000 X-Ray Diffractometer with Cu.Kα radiation (λ = 1.5406 A˚), a Scanning Electron Microscope (SEM), an UV-VIS-NIR spectrophotometer in the wavelength range of 300 - 800 nm at room temperature, and a Keithley 2000 multimeter, respectively. For electrical measurements, silver paste was used to create ohmic contact. A mercury lamp was used to evaluate the photoconductivity response of the fabricated sample. 2.2. Results and discussion The structures of the ZnO films were characterized using a X-ray Diffractometer as shown in Figure 1. All of the peaks shown in the X-ray diffraction patterns (XRD) are sharp and narrow peaks and closely match that of the hexagonal wurtzite ZnO structure. Diffraction peaks related impurity phases were not observed in the XRD patterns. A small variation of interplaner spacing (dhkl) of Al doped-ZnO from that of ZnO is also observed which implies that aluminum incorporates into ZnO crystal lattice. This means that doping would induce a distorted crystal lattice manifested by the displacement of lattice indices. Moreover, the study of peak intensity also indicates that the ZnO films tend to orient along the c-axis when Al doping concentration increases. The crystallite size of the films is calculated using Scherrer’s formula d = 0.9λβcosθ [14], where d is the crystallite size, λ is the X-ray wavelength (1.54 A˚), β is the full width at half maximum (FWHM) and θ is the diffraction angle. The calculation indicates that average grain size is slightly reduced with the increase in Al-doping amount which might be due to the substitution of smaller Al atoms at the Zn sites in the lattice of ZnO [12, 13]. 123 N. D. Lam, T. T. Mui, L. T. Trang, P. V. Vinh, V. V. Cuong and N. V. Hung Figure 1. XRD patterns of fabricated ZnO films Figure 2 shows SEM images of fabricated ZnO thin films on glass substrates with different Al doping amount. The surface morphology of the films is strongly dependent on the Al doping concentration. The fabricated films are formed by many round nanoparticles. The particle size is bigger when the doping concentration of aluminum increases. Therefore, the surface of the films is rougher when the Al doping concentration increases. Figure 2. SEM images of ZnO films Figure 3 (a) shows the optical transmittance spectra of the films in the wavelength range of 300 - 800 nm at room temperature. As seen in Figure 3 (b), the average transmittance of the films in the visible region is higher than 95%. And, the highest average transmittance was obtained 124 Structural, optical and electrical properties of undoped and Al doped-ZnO films for use... when the Al doping concentration is 1%. Furthermore, the absorption edges of the doped thin films exhibit a blue shift. For estimating the band gap energy (Eg) of the fabricated films, the (αhν)2 = β(hν - Eg)m relationship is utilized [14] where, α is the absorption coefficient, β is a constant and hν is the photo energy. The ZnO is a direct band gap semiconductor so m is chosen as 12 [14]. The absorption coefficient α of the films is calculated from the transmittance spectra using the equation α = 1/t ln(1/T), where t is the thickness of film and T is the transmittance [14]. The band gap energy Eg is determined by extrapolating the linear region of (αhν)2 versus the photo energy in the Figure 3 (c) and depicted in Figure 3 (d). The optical band gap energy is enlarged from 3.21 to 3.31 eV when Al doping concentration is increased from 0% to 5%. The enlargement in the optical band gap energy can be attributed to the Burstein-Moss effect [15]. Figure 3. (a) Optical transmittance spectra of the ZnO films, (b) Average transmittance of the films vs. Al doping concentration, (c) The plots of (αhν)2 vs. photon energy of the ZnO films, (d) Optical band gap energy of the films vs. Al doping concentration The measured dark and illuminated I-V characteristics of the 1% Al doped-ZnO thin film are shown in Figure 4. The I-V curves show ohmic and quasi-liner behavior in the dark and under illumination. The current at a given voltage for the film in the dark is lower than that when under illumination. Therefore, the film shows good photoconductivity response. The influence of light on the electrical characteristics of the film indicates that fabricated ZnO films can be utilized as a photovoltaic material. 125 N. D. Lam, T. T. Mui, L. T. Trang, P. V. Vinh, V. V. Cuong and N. V. Hung Figure 4. I-V plots of 1% Al doped-ZnO thin film in the dark and under illumination 3. Conclusion Undoped and Al-doped ZnO thin films were successfully deposited on glass substrates using the sol–gel spin coating method. The structural, optical, and electrical characteristics of these films were investigated. The XRD results show that all films crystallize as a hexagonal wurtzite structure having a preferential orientation along the c-axis as the Al doping concentration increases. 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