FABRICATION AND EVALUATION OF CuO/ZnO HETEROSTRUCTURES FOR PHOTOELECTRIC CONVERSION

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1 IJRRAS 13 (1) October FABRICATION AND EVALUATION OF / HETEROSTRUCTURES FOR PHOTOELECTRIC CONVERSION Hiroki Kidowaki, Takeo Oku & Tsuyoshi Akiyama Department of Materials Science, The University of Shiga Prefecture, 2500 Hassaka, Hikone, Shiga Japan * oku@mat.usp.ac.jp ABSTRACT Cuprous oxide ()-based solar cells with zinc oxide () were fabricated on indium tin oxide by an electrodeposition. Microstructure and cell performance of the solar cells with ITO// or ITO// structures were investigated. Photovoltaic devices based on the / or / heterojunction structures provided photovoltaic properties under illumination. Structural, morphological and absorption measurement of the / heterojunction were studied by using X-ray diffraction, transmission electron microscopy and light current-voltage characteristics. The energy levels of the present solar cells were also discussed. Keywords: copper oxide, thin film, solar cell, photovoltaic property, optical property 1. INTRODUCTION Solar cell technology for future energy resources has been progressed recently. Silicon is used as the semiconductor material for conventional solar cells, and the cost reduction of the solar cells is one of the most important issues. Cu oxides such as and Cu 2 O are one of the candidate materials. The features of copper oxide semiconductors are high optical absorption coeffcient and non toxicity and low cost fabrication [1,2]. and Cu 2 O are p-type semiconductors with band gaps of ~1.5 ev and ~2.0 ev, respectively, which are close to the ideal energy gap for solar cells and allows for good solar spectral absorption due to these direct band gap. The highest efficiency of ~2% for Cu 2 O solar cells has been obtained by using the high-temperature annealing method and an expensive vacuum evaporation technique [3]. Efficient heterojunction solar cells with p-cu 2 O and n- fabricated by electrodeposition and photochemical deposition methods have been investigated and reported [4,5]. However, the solar cells with a / structure have not been reported. Since the ideal energy band gap for solar cells is ~1.4 ev, could be an ideal material for solar cells. The purpose of the present work is to fabricate and characterize solar cells with / structures. The heterojunction solar cells were denoted as /. is an n-type semiconductor with a wide band gap of ~3.37 ev, which can be applied to solar cells [6]. 2. EXPERIMENTAL layers were prepared on pre-cleaned indium tin oxide (ITO) glass plate by electrodeposition using platinum as counter electrode. Copper(II) sulfate (CuSO 4, 0.4 mol/l, Wako 97.5%) and l-lactic acid (3 mol/l, Wako) were dissolved into distilled water. ph of the electrolyte solution was adjusted to 12.5 by adding NaOH. - - Al + Al + ITO ITO Glass Glass Figure 1: Structure of ITO///Al and ITO///Al solar cells. 67

2 Absorbance IJRRAS 13 (1) October 2012 The temperature of electrolyte solution was kept at 65 C during electro deposition. Preparation of layers were carried out at voltages of V and quantity of electric charge of 2.2 C cm -2. After the deposition, the sample was rinsed with water and transferred into the electro deposition bath. layers were electro-doposited from M aqueous solution of Zn(NO 3 ) on the ITO/ substrate. The sample thickness ranged between 1 and 2 μm depending on the deposition time and current. Finally, the substrates were rinsed with water, dried with air and quickly transferred into a thermal evaporator for the vacuum deposition of the aluminum (Al) back contact. Structure of heterojunction solar cells were denoted as ITO///Al, with a schematic illustration as shown in Fig. 1 Current density-voltage (J-V) characteristics (Hokuto Denko Corp., HSV-100) of the solar cells were measured using potentiostat (Hokuto Denko, HSV-100) in the dark and under illumination at 100 mw/cm 2 by using an AM 1.5 solar simulator (San-ei Electric, Electric, XES-301S). The solar cells were illuminated through the side of the ITO substrate, and the illuminated area was 0.16 cm 2. Optical absorption of the solar cells was investigated by means of UV visible spectroscopy (Hitachi, Ltd., U-4100). Microstructures of the copper oxides were investigated by X-ray diffractometer (XRD, PHILIPS X Pert-MPD System) with CuKα radiation operating at 40 kv and 40 ma. Transmission electron microscopy (TEM, Hitachi H-8100, 200 kv operating voltage) was also carried out for nanostructure analysis. 3. RESULTS AND DISCUSSION The / structure showed characteristic curves with short-circuit current and open-circuit voltage. Table 1 shows measured parameters of -based solar cells. A solar cell with a / structure provided a power conversion efficiency (η) of %, fill factor (FF) of 0.25, short-circuit current density (J sc ) of 1.9 macm -2 and open-circuit voltage (V oc ) of V. Figure 2 shows measured optical absorption of thin films. The thin film shows high optical absorption in the range of 400 nm and 700 nm. Table 1: Measured parameters of -based solar cells V oc (V) J sc (ma cm -2 ) FF η (%) (5 min)/ (10 min)/ / (5 min) Wavelength (nm) Figure 2: UV-visible absorption spectra of thin films prepared by electrodeposition. Transmittance spectrum of 100 nm thick film, deposited on ITO, is presented in Fig. 3. From this spectrum, the optical absorption coefficients (α) of this film was determined from the spectral transmittance using the next equation, α = 1/d ln(1/t). Where d is the film thickness and T is the transmittance [7]. For determination of the optical band gap energy (E g ), the method based on the relation of αhν = A(hν-E g ) n/2 was used, where n is a number that depends on the nature of the transition. In this case, its value was found to be 1, which corresponds to direct 68

3 Transmittance (%) (αhν) 2 (a.u.) Transparence (%) (αhν) 2 (a.u.) IJRRAS 13 (1) October 2012 band to band transition. Figure 3 is a Tauc plot, which shows (αhν) 2 versus hν for the film. The intersection of the straight line with the hν-axis determines the optical band gap energy E g [8]. It was found to be ~2.1 ev which is wider than the ideal band gap of the crystal Wavelength (nm) hν (nm) Figure 3: Optical transmission spectrum of 100 nm thick film, and Tauc plot for the thin film Wavelength (nm) hν (ev) Figure 4: Optical transmission spectrum of 100 nm thick film, and Tauc plot for the thin film. All the crystalline components in the and thin films were investigated by XRD, as shown in Fig. 5. Diffraction peaks corresponding to and are observed in thin films, which consisted of cupric phase with monoclinic system (space group of C2/c). The particle size was estimated using Scherrer s equation: D= 0.9λ/Bcosθ, where λ, B, and θ represent the wavelength of the X-ray source, the full width at half maximum (FWHM), and the Bragg angle, respectively [9]. The crystallite sizes of and were determined to be 49.0 nm and 82.0 nm, respectively. Lattice constants of and were summarized as listed in Table 2 and Table 3, comparing with reported values [10,11]. From the lattice constants, the crystal structures of and have some crystal distortions. This would result in charge carriers transport in a reverse direction through the device [12], resulting in recombination. To increase the efficiency of the / solar cells, small grain size of and higher crystallinity of would be necessary. 69

4 Intensity (a.u.) Intensity (a.u.) IJRRAS 13 (1) October 2012 ITO Cu θ (degree). Figure 5: XRD patterns of and thin films. 2θ (degree) Table 2: Lattice parameters of. a (nm) b (nm) c (nm) Present data Previous data [10] Table 3: Lattice parameters of. a (nm) c (nm) Present data Previous data [11] A TEM image and an electron diffraction pattern of prepared by electrodeposition are shown in Fig. 6. Figure 6 indicated nanowires with the diameters of 50~60 nm, which agrees with XRD results. Debye-Scherrer rings in Fig. 6 indicate microcrystalline structures of. Figures 7 are a TEM image and an electron diffraction pattern of a layer prepared by electrodeposition. Debye-Scherrer rings in Fig. 7 indicate microcrystalline structures of. The conversion efficiency of the solar cells depends on morphology of interfaces in solar cells. A schematic microstructure of the present solar cells fabricated by electrodeposition is shown in Fig. 8. The layer has a nanowire structure, and interface area of the p-n junction would be increased, which would result in charge separation at the interface. Energy level diagram of the / solar cell is summarized as shown in Fig. 9. Previously reported values were used for the energy levels [9]. It has been reported that V oc is nearly proportional to the band gap of the semiconductors, and control of the energy level is important to increase efficiency [13]. Compared to silicon with an indirect transition band structure, with a direct transition band structure is more suitable for the optical absorption property. In addition, the ultrathin film of the layers could provide efficient charge injection because of the high optical absorption. 70

5 IJRRAS 13 (1) October 2012 a b nm Figure 6: TEM image and electron diffraction pattern of prepared by electrodeposition. a b nm Figure 7: TEM image and electron diffraction pattern of prepared by electrodeposition. layer layer Figure 8: Schematic microstructure of the present solar cells fabricated by electrodeposition. 71

6 IJRRAS 13 (1) October ev 2.1 ev -4.3 ev -4.7 ev 2.1 ev -4.8 ev 4.2 ev 4.2 ev ITO Al ITO Au Figure 9: Energy level diagram of ITO///Al and ITO///Au inverted solar cells. 4. CONCLUSIONS ITO///Al solar cells were produced and characterized, which provided η of %, FF of 0.25, J sc of 1.6 macm -2 and V oc of V. The / structure showed high optical absorption in the range of 400 nm and 700 nm, and the E g of was found to be ~2.1 ev from the Tauc plot, which is wider than that of the ideal band gap of the crystal. A crystallite size of was determined to be 49.0 nm, and had a nanowire structure, which would be expected for increase of the efficiency of / solar cells. Energy level diagram for the / structure was proposed, and separated holes could transfer from the valence band of the to the ITO, and separated electrons could transfer from the conduction band of the to the Al electrode, respectively. Formation of the / active layer with homogeneously distributed nanowires would improve the efficiencies of the solar cells. To increase power conversion efficiency, structures of the solar cells should be optimized. 5. REFERENCES [1] L.C.Olsen, R.C.Bohara, M.W.Urie, Explanation for low-efficiency Cu 2 O Schottky-barrier solar cells, App. Phys. Lett (1979). [2] J.Herion, E.A.Niekisch, G.Schari, Investigation of metal oxide/cuprous oxide heterojunction solar cells. Sol. Energy Mater. Sol. cells (1980). [3] A.Mittiga, E.Salsa, F.Sarto, M.Tucci, and R.Vasanthi, Heterojunction solar cell with 2% efficiency based on a Cu 2 O substrate, App. Phys. Lett (2006). [4] M.Izaki, K.Mizuno, T.Shinagawa, M.Inaba and A.Tasaka, Photochemical construction of photovoltaic device composed of p-copper(i) and n-zinc oxide, J. Electrochem. Soc. 153 C668-C672 (2006). [5] M.Izaki, T.Shinagawa, K.Mizuno, Y.Ida, M.Inaba and A.Tasaka, Photochemically constructed p-cu 2 O/n- heterojunction diode device, J. Phys. D : App. Phys (2007). [6] R.Vinodkumar, K.J.Letthy, D.Beena, A.P.Detty, I.Nava, U.V.Nayar, V.P.Mahadevan, V.Ganesan, V.R.Rdddy, Effect of ITO buffer layers on the structural, optical and electrical properties of multilayer thin films prepared by pulsed laser deposition technique, Sol. Energy. Mater. Sol. Cells (2010). [7] J.W.C.Chen, D.C.Perng, J.F.Fang, Nano-structured Cu 2 O solar cells fabricated on sparse nanorods, Sol. Energy. Mater. Sol. Cells (2011) [8] V.Georgieva, M.Ristov, Electrodeposited cuprous oxide on indium tin oxide for solar applications, Sol. Energy Mater. Sol. Cells (2002). [9] T.Oku, T.Noma, A.Suzuki, K.Kikuchi, S.Kikuchi, Fabrication and characterization of fullerene/porphyrin bulk heterojunction solar cells, J. Phys. Chem. Solids (2009). [10] M.I.Domnina, S.K.Filatov, I.Zyuzyukia and L.P.Vergasova, Neorgan. Materialy (1986). [11] Albertsson, S.C.Abrahams and Kvick, Acta Cryst. B45 34 (1989). [12] T.Gershon, Musselman, P.Kevin, A.Marin, Friend, H.Richard, L.MacManus-Driscoll Judith, Thin-film /Cu 2 O solar cells incorporating an organic buffer layer, Sol. Energy Mater. Sol. Cells (2012). [13] T.Oku, R.Motoyoshi, K.Fujimoto, T.Akiyama, B.Jeyadevan, J.Cuya, Structures and photovoltaic properties of copper oxides/fullerene solar cells, J. Phys. Chem. Solids (2011). 72