Characterization of p-cu 2 O/n-ZnO Heterojunction Solar Cells

Characterization of p-cu 2 O/n-ZnO Heterojunction Solar Cells Verka Georgieva a, AtanasTanusevski b a Faculty of Electrical Engineering and Information Technology, The "St.Cyril & Methodius"Univrsity P.O.Box 574, 1000 Skopje, FYROM, vera@feit.ukim.edu.mk b Institute of Physics, Faculty of Natural Sciences and Mathematics, The "St. Cyril and Methodius"University, 1000 Skopje,FYROM Abstract. We report on the fabrication of heterojunction solar cells based on copper (I) oxide (p-cu 2 O) cathodically electrodeposited onto transparent conducting thin (TCO) films of zinc oxide (n-zno). The TCO films have been grown by low temperature electrodepositions in aques solution onto conductive SnO 2 -coated glass substrates. To complete the systems as solar cells, thin layer of graphite or nickel were deposited on the rear of the Cu 2 O. Front wall cells were formed. The performance of the solar cells thus prepared is discussed in terms of the structural and morphological properties of the films and the type of the back electrode (graphite or nickel). ). The structure of the films was studied by scanning electron microscopy (SEM). SEM micrographs indicate a polycrystalline structure, on which one could see small rounded grains with diameters between 100 and 200 nm, depending on electro deposition potential. An opencircuit voltage of 0.33 V, short circuit current density of 0.400 ma/cm 2, fill factor of 0.29 were obtained under 100 mw/cm 2 illumination. Also some differences with Schotky barrier solar cells based on cuprous oxide are discussed. Keyword: Heterojunction solar cells, electrodeposition, thin film, cuprous oxide, zinc oxide PACS: 73.40 Lq INTRODUCTION The interest for investigating in a technology of thin films as the effective transformers of solar insulation in electricity is increasing more and more during the last years. Since thin film solar cell materials require less material to generate a given amount of power, they make attractive alternatives to single crystalline devices. A photovoltaic device composed of a p-type semiconducting cuprous (I) oxide (Cu 2 O) and n-type zinc oxide (ZnO) has attracted increasing attention as a future thin film solar cell, due to a theoretical conversion efficiency of around 18% and an absorption coefficient higher than that of a Si singal crystal [1]. The interest in cuprous oxide, Cu 2 O, as a semiconductor began with the invention of the Cu 2 O rectifier by Grondhal in the 1920s. Photosensitive devices based on Cu 2 O were investigated in the 1930s. Considerable work was done on Cu 2 O characterization from 1930 to 1940. Interest in Cu 2 O revived during the mid seventies in the photovoltaic community [2]. Also, in the recent years the interest in Cu 2 O revived again. Cu 2 O with band gap energy in the range of 1.96 ev to 2.38 ev has the potential to form a solar cell with high open-circuit voltage by combination with a suitable n-type semiconductor [3]. The attractiveness of Cu 2 O as a photovoltaic material lies in the simple and inexpensive process for semiconductor layer formation. Among the other, electrochemical deposition technique is a simple, versatile and convenient method for producing large area devices. Low temperature growth and the possibility to control film thickness, morphology and composition by readily adjusting the electrical parameters, as well as the composition of the electrolytic solution, make it more attractive. At present, electrodeposition of binary semiconductors from aqueous solutions is employed in the preparation of solar cells. The success of the electrodeposition will open new doors for developing new functional electronics at a low cost [1]. In the literature, there are many reports on solar cells based on cuprous oxide (Cu 2 O) as active layer because this semiconductor shows many interesting characteristics useful for solar cells productiion such as CP1203, 7 th International Conference of the Balkan Physical Union, edited by A. Angelopoulos and T. Fildisis 2009 American Institute of Physics 978-0-7354-0740-4/09/$25.00 1074

nontoxic, good mobility, fairly high minority carrier diffusion length, energy band gap and theoretical efficiency of 20%. Despite that, the highest energy conversion efficiency of a Cu 2 O solar cell obtained up to now is much lower than the theoretical limit. The main restricted factor to the optimization of Cu 2 O solar cells is the difficulty in the doping process. Cu 2 O is a p-type semiconductor. All the efforts to dope in n- type and to form p-n homojunctions are unsuccessful up to now. The only exception is a very recent report for p-n homojunction using the same kind of solution, with different ph, in which no photovoltaic action is anyway claimed. In the absence of any way to make a p-n homojunction, the devices with the highest photovoltaic efficiencies were metal-cu 2 O Schottky-barrier solar cells. But nevertheless it is generally accepted that their efficiency and stability cannot be much improved. Better solar cells are made using a heterojunction between Cu 2 O and n-type TCO (Transparent Conducting Oxide). ZnO is a suitable partner since it has a fairly low work function. ZnO/Cu 2 O heterojunctions have therefore attracted the interest of several researches. The best performing cells with 2% efficiency were fabricated on Cu 2 O substrates obtained by oxidation of copper sheets at high temperature and ZnO deposited by Ion Beam Sputtering at room temperature [4]. Thin film ZnO/Cu 2 O heterojunctions have been obtained using the cheaper and low energy intensive electrodeposition technique also. This process involves the cathodic electrodeposition of ZnO on a TCO coated glass substrate followed by the cathodic electrodeposition of Cu 2 O to form the heterojunction. The high conversion efficiency obtained with this type of cells is 1,28% [1]. Even low efficiency it may be acceptable in countries where the other alternative energy sources are much more expensive. This paper reports preparing of ZnO and Cu 2 O thin films by electrochemical deposition technique on transparent conducting glass slides coated with SnO 2, their structural, morphological and optical properties and also some of the characteristics of the cells compared with Schotky barrier solar cells.. The structure of the films was studied by X-ray diffraction measurements (XRD) and scanning electron microscopy (SEM). RESULTS AND DISCUSSION Preparation of the Films ZnO/Cu 2 O heterojunction solar cells were made by consecutive cathodic electrodeposition of ZnO and Cu 2 O onto tin oxide covered glass substrates. A very simple apparatus was used for electrochemical deposition. It consisted of a thermostat, a glass cell with solution, two electrodes (cathode and anode) and a standard electrical circuit for electrolysis. A reference electrode, as saturated calomel electrode, (SCE) or any other, was not used. Zinc oxide (ZnO) was cathodically deposited on a conductive glass substrate covered with SnO 2 as cathode by a potentiostatic method [5,6,7]. Conducting glass slides coated with SnO 2 films are commercial samples. The electrolysis takes place in a simple aqueous 0,1M zinc nitrate [Zn (NO 3 ) 2 ] solution with ph about 6, maintained at 70 0 C temperature. The cathodic process possibly can be described by the following reaction equations [8]: Zn(NO 3 ) 2 Zn 2+ + 2NO 3 NO 3 + H 2 O +2e NO 2 +2OH Zn 2+ + 2OH Zn(OH) 2 ZnO +H 2 O ZnO films were electrochemically grown at constant potential of 0.8 V between the anode and cathode. For a fixed value of the potential, a current density decreased with increasing the film thickness. The deposition time was varying from 10 min to 30 min. Deposited films were rinsed thoroughly in distilled water and allowed to dry in air at room temperature. The anode was zinc of 99.99% purity. Thin films of Cu 2 O were electrodeposited by cathodic reduction of an alkaline cupric lactate solution at 60 0 C [9]. The deposition was carried out in the constant potential regime. The deposition parameters, as current density, voltage between the electrodes and deposition time were changed. The Cu 2 O films were obtained under following conditions: current density J = 1-3 ma/cm 2 - voltage between the electrodes V = 0.5 V and deposition time t = 55 min. The deposition potential is ph sensitive. It suggests, also and it has already been reported that the Cu 2 O layer was formed by the following reaction: 1075

Even this reaction does not explain the large ph dependence of deposition potential [1,10]. The present study was conducted, in a first instance, on undoped zinc oxide films and cuprous (I) oxide films. The structure of the films was studied by X-ray diffraction measurements using monochromatic Cu K radiation with a wavelength of 0,154 nm operated at 35 kv and 24 ma. Morphology and grain size was determined through micrographs on a JEOL JSM 6460 LV scanning electron microscope. Structural Properties The structure of the of ZnO films was studied by X ray diffraction. The X-ray diffraction patterns of ZnO film show crystalline structure. XRD peaks corresponding to ZnO and the substrate material SnO 2 were determined with JCPDS patterns. The XRD spectrum indicates a strong ZnO peak with a (0002) or (1011) preferential orientation. The structure of the Cu 2 O films was studied also by X ray diffraction. It was found that all films are polycrystalline and chemically pure Cu 2 O with no traces of CuO. XRD peaks corresponded to Cu 2 O and the substrate material. The XRD spectrums indicate a strong Cu 2 O peak with (200) preferential orientation. Morphological Structure Figure 1 shows a scanning electron micrograph of undoped electrodeposited ZnO film prepared at 0.8 V potential for 10 min. The photograph shows small rounded grains with different size (about 100 nm and less to 500 nm), that it is difficult to estimate the size of the grains. The influence of the electrodeposition potential and current on the morphology of the films was not examined so far. FIGURE 1. SEM micrograph of undoped electrodeposited ZnO film for 10 min FIGURE 2. SEM micrograph of potentiostatic electrodeposited Cu 2 O film for 50 min Figure 2 shows the scanning electron micrographs of Cu 2 O film deposited on glass coated by SnO 2. The photographs indicate a polycrystalline structure. The grains have a relatively uniform size distribution. Also they are very similar to each other in shape. The crystal grains appear as four sided pyramids. They are with an average size of 0.3-0.5 m for the film deposited on SnO 2 under the following conditions: current density J = 1-3 ma/cm 2 - voltage between the electrodes V = 0.5 V and deposition time t = 50 min. It is known that the grains size and the structure of the films can be influenced by various factors such as the substrate material, potential and the current between the electrodes. The experience shows that with increasing the current density, the grain size decrease. 1076

Some Characteristics of the Cells A Cu 2 O/ZnO/SnO 2 solar cell was prepared by a two-step electrochemical method. In the first step, Zno film was deposited on SnO 2 glass substrate. It was deposited potentiostatically at 0.8 V from an aqueous solution containing 0.1 M Zn(NO 3 ) 2. Cu 2 O film was deposited onto the ZnO/SnO 2 sample from alkaline lactate solution at a constant potential of 0.5V. Ohmic contact was achieved using carbon paste or carbon spray on the rear of the Cu 2 O or by vacuum evaporating a thin layer of nickel also on the rear. Front wall cells were formed. The total cell active area was 1 cm 2. Antireflectance coating or any special collection grids have not been deposited. The best values of the open circuit voltage V o c = 330 mv and the short circuit current density I sc = 400 μm/cm 2 were obtained by depositing carbon paste and illumination of about 100 mw/cm 2. A carbon back contact was chosen because of simplicity and economy of the cell preparation and because the cells with carbon give high values of the short circuit current density despite the evaporated layer of nickel. FIGURE 3. Dark and light volt-current characteristics of the Cu 2 O/ZnO/SnO 2 solar cell FIGURE 4.Temperature dependence of Voc and Jsc FIGURE 5. Volt-current characteristics of the Cu 2 O/SnO 2 solar cell Our investigation shows that the ZnO layer improves the stability of the cells. That results in a device with better performances despite of the Schhotky barrier solar cells (Cu 2 O/SnO 2 ). First, the cells show photovoltaic properties without annealing, because potential barrier was formed without annealing. The barrier fell for a few days which result in decreasing the open circuit voltage despite the values of V oc for just made cells. It decreases from 330 mv to 240 mv. But after that the values of V oc keep stabilized, because of stabilized barrier potential. It wasn t case with Schotkky barrier solar cells, because barrier potential height decreases with aging. 1077

Figure 3 shows the volt-current characteristics of the cell recorded upon the illumination 100mW/cm 2. That behavior is as near a real solar cell. Whit increasing the temperature the short current density increase up to 45 o C and after forward increasing the temperature we can note little decrease of the short current density, fig. 4. The open circuit voltage increase up to temperature of 40 o C and after that slightly decreases. It is quite different of Schottky barrier solar cells where the the short current density and open circuit voltage decrease continuously with increasing the tempearture fig.5. To improve the quality of the cells, we have to work on improving the quality of ZnO and Cu 2 O films, because they have very high resistivity, a factor which limits the cells performances. Doping of the ZnO films with In, Ga and Al will decrease the resistivity of the deposited films and increase their electroconductivity. Behind the ohmic contact, maybe one of the reason for low photocurrent is just recombination of the carriers and decreasing of the hole cocentracion with the time. Also, the transmittivity in a visible region have to increase. Also, it is necessary to improve the ohmic contact, consequently to increase the short circuit current density and to increase the fill-factor of the cells. CONCLUSION In this paper ZnO and Cu 2 O thin films have been used for preparing the heterojunction cuprous (I) oxide based solar cells. ZnO is a transparent oxide that is widely used in many different applications, including thin film solar cells. Our investigation shows that the ZnO layer improves the stability of the cells. That results in a device with better performances despite of the Schhotky barrier solar cells (Cu 2 O/SnO 2 ). First, the cells show photovoltaic properties without annealing, because potential barrier was formed without annealing. The Cu 2 O/SnO 2 cells without the ZnO layer show a lower V oc. The improvement in V oc could be due to the increase of the barrier height using ZnO layer as n-type semiconductor. For further improvement of the performances of the cells maybe inserting of a buffer layer at the heterojunction between Cu 2 O and ZnO films will improve the performance of the cells by eliminating the mismatch defects which act as recombination centers. Also it will be protection of reduction processes that maybe exists between ZnO and Cu 2 O. REFERENCES 1. Masanobu Izaki, Tsutomu Shinagawa, Ko-Taro Mizuno, Yuya Ida, Minoru Inaba and Akimas Tasaka, J.Phys.D:Appl.Phys.40 (2007) 3326-3329. 2. L.C.Olsen, F.W.Addis, W.Miller, Sol.Cells 7 (1982) 247. 3. J.Katayama, K.Ito, M.Matsuoka and J.Tamaki, 34: 687-692, 2004 4. Alberto Mittiga, Enrico Salza, Franceasca Sarto, Mario Tucci and Rajaraman Vasanthi, Applied Physics Letters 88, 163502 (2006) 5. E.A.Dalchiele, P.Giorgi, R.E.Marotti, at all. Solar Energy Materials &Solar Cells 70 (2001) 245-254. 6. F Ng-Cheng-Chin, M.Roslin, Z.H.Gu and T.Z.Fahidy, J.Phys.D.Appl.Phys.31 (1998) L71-L72. 7. M.Izaki, H.Ishizaki, A.Ashida, at all. J.Japan Inst.Metals, Vol. 62,No.11(1998),pp.1063-106. 8. M.Izaki,T.Omi,J.Electrochem.Soc.139(1992)2014. 9. A.E.Rakhshani, A.A.Al-Jassar and J.Varghese Thin Solid Films, 148,pp.191-201(1987) 10. Longcheng Wang and Meng Tao, Electrochemical and Solid-State Letters, 10 (9) H248-H250 (2007) 1078