Investigation of Photovoltaic Properties of In:ZnO/SiO 2 /p- Si Thin Film Devices

Universities Research Journal 2011, Vol. 4, No. 4 Investigation of Photovoltaic Properties of In:ZnO/SiO 2 /p- Si Thin Film Devices Kay Thi Soe 1, Moht Moht Than 2 and Win Win Thar 3 Abstract This study investigates photovoltaic properties of In: ZnO/ SiO 2 /p-si thin film devices indium (2mol% & 4mol%) doped zinc oxide [ZnO: In (2 mol % and 4 mol %)] thin films are deposited on SiO 2 / p-si substrates and heat treated at 700 C for 1h. Current density versus voltage (J-V) characteristic of these thin films is studied under dark and illumination conditions. The devices showed good diode characteristic in the dark condition. The diode parameters such as saturation current density (J o ), ideality factor (n) and barrier height (Φ bo ) are evaluated. For the photovoltaic application, open-circuit voltage (V oc ) and short-circuit current density (J sc ) are studied and important parametres such as fill factor (FF) and conversion efficiency (η) are observed. Key words: ZnO:In (2mol%& 4mol%), fill factor, conversion efficiency Introduction The most promising source of clean, safe, and abundant energy is the sun which made possible the life on earth itself. Photovoltaic devices (or simply solar cells) offer the direct conversion of sunlight into electricity. The discovery of the photovoltaic effect dates back to 1839 when Becquerel found a photovoltage between electrodes in electrolyte solutions [Schock H w et al. (2000) and Archer M L et al. (2001)]. From the electrical point of view, photovoltaic devices can be considered as p-n junctions; two semiconductors of different carrier type are in contact and the recombination of charge carriers across the interface leads to the formation of a space charge region (or depletion layer) 1. Student Researcher, Department of Physics, University of Yangon 2. Assistant Lecturer, Department of Physics, University of Yangon 3. Professor and Head, Dr, Department of Physics, University of Yangon

304 Universities Research Journal 2011, Vol. 4, No. 4 and a built-in potential. The junction shows rectifying behaviour and depletion region gives rise to a junction capacitance. Special materials are used for the construction of photovoltaic cells. These materials are called semiconductors. The most commonly used semiconductor material for the construction of photovoltaic cells is silicon. Several forms of silicon are used for the construction; they are single-crystalline, multicrystalline and amorphous. Other materials used for the construction of photovoltaic cells are polycrystalline thin films such as copper indium diselenide, cadmium telluride and zinc oxide [http://www.how Do a Photovoltaic Cell Work/html]. Transparent conduction oxide (TCO) films of tin, indium and zinc oxides (doped and undoped) have been extensively studied due to their high optical transmittance and electrical conductivity. These films are useful in photovoltaic and photothermal applications. Non-stiochiometric pure ZnO is an n-type semiconductor, but its optical and electrical properties are not very stable at high temperatures. However, doped films can be made which have very stable electrical and optical properties. Therefore, doped ZnO films are preferred for practical purposes [Paraguay F et al 2000]. Experimental Details Boron doped five-inch diameter single-crystalline p-si wafers with thickness of 625μm and oriented in (100) plane were used. They were cut into relatively small segments (1cm 1cm) to be used as base semiconductor for semiconductor-insulator-semiconductor (SIS) photovoltaic devices. The mixture of In (2 mol % and 4 mol %) doped ZnO thin films on SiO 2 /p-si substrates have been prepared using high purity (99.9% Laboratory reagents) ZnO and In 2 O 3 powders. These powders weigh on the basis of stiochiometric composition. The resultant stiochiometric composition of the Zn 1-x In x O (x = 0.02, 0.04) powders were ground by agate mortar to obtain the homogeneity. The mixed powders were heat-treated at 800 C for 2 h. 2-metoxyethanol (CH 3 OCH 2 CH 2 OH) is added to the mixture and then stirred and heated up to 100 C for 1hr. The homogeneous precursor solution or coating solution was obtained and these solutions were used to depose on SiO 2 /p-si (100) substrates using

Universities Research Journal 2011, Vol. 4, No. 4 305 spin coating technique. Before the spin coating deposition, the p-si (100) substrate was cleaned by standard semiconductor cleaning method. The cleaning sequences are: They were washed in boiling acetone, then in boiled propanol for 5 minutes to remove greasy films. They were immersed in nitric acid HNO 3 for 3 mins, in order to remove ionic contamination. The wafers were immersed in HCl: HNO 3 (3:1) for 3 mins, to remove metallic films. They were etched in buffered hydrofluoric acid (34.6% NH 4 F:6.8% HF: 58.6% H 2 O) for 2 min to remove oxide films. The silicon wafers were cleaned in distilled water and dried at room temperature. Then, the insulator SiO 2 layer deposited on p-si substrate by heating at 1200 C for 1 h by conventional furnace heating process. After deposition, they were heat-treated at 700 C for 1h. Finally ZnO: In (2mol% & 4mol%) /SiO 2 /p-si thin film devices were obtained. The process steps of the thin film devices are shown in figure 1(a). Results and Discussion The current voltage (I-V) measurement of ZnO: In (2mol% & 4mol%)/SiO 2 /p-si thin film devices in the dark condition were measured in the forward and reverse direction using a dc power supply as shown in figure 1(b). Figure 2(a & b) shows the current densityvoltage characteristics of ZnO:In(2mol% & 4mol%)/SiO 2 /p-si thin film devices at process temperatures 700 C for 1hr. From these figures, it can be seen that a positive applied voltage moves the current carriers so as to produce a conventional current density which flows to the right-hand side. This means electron vacancies (holes) move to the right as they are positively charged and electrons to the left as they carry a negative charge. The positive applied voltage yields an easy current flow, i.e. a bigger current density; while the same voltage applied in the opposite direction produces a smaller current density. The current flows through the devices exponentially increased with an increase in potential in

306 Universities Research Journal 2011, Vol. 4, No. 4 forward bias region, it shows an exponential growth of the current density that is main characteristics of a diode. ZnO In 2 O 3 Zn 1-x In x O(x= 0.02, 0.04) mol% Ground by agate mortar Mixed Crystalline powder Solution Precursor Solution Heat treated at 800 C for 2h Solvent (CH 3 OCH 2 CH 2 OH) Heated up 100 C for 1h Spin Coating ZnO:In(2mol% & 4mol%)/SiO 2 /p-si thin films Heat treated at 700 C for I-V characteristics Figure 1(a) The general processing for the ZnO:In(2mol%& 4mol%)/SiO 2 /p-si thin film devices

Universities Research Journal 2011, Vol. 4, No. 4 307 Figure 1(b) Current-Voltage (I-V) measurement in the dark condition 3.5e-5 3.0e-5 Current density [A/cm 2 ] 2.5e-5 2.0e-5 1.5e-5 1.0e-5 5.0e-6 0.0-6 -4-2 0 2 4 6 Voltage [V] Figure 2(a) Current density versus voltage characteristics of ZnO:In (2mol%)/SiO 2 /p-si thin films device Figure 2(a) Current density versus voltage characteristics of ZnO:In(2mol%)/SiO 2 /p-si thin films device

308 Universities Research Journal 2011, Vol. 4, No. 4 7e-4 Current density (A/cm 2 ) 6e-4 5e-4 4e-4 3e-4 2e-4 1e-4 0-6 -4-2 0 2 4 6 Voltage (V) Figure 2(b) Current density versus voltage characteristics of ZnO:In (4mol%)/SiO 2 /p-si thin films device Figure 2(b) Current density versus voltage characteristics of ZnO:In(4mol%)/SiO 2 /p-si thin films device Figure 3(a & b) shows the ln J versus V characteristics of ZnO:In (2mol% & 4mol%)/SiO 2 /p-si thin film devices. From the ln J-V graph, reverse saturation current density (J o ), diode ideality factor (n) and barrier height (Φ bo ) are carried out. A solar cell is in principle a diode, so that the I-V characteristics in the dark can be described by the diode equation, qv nkt ( e ) J (1) = J 0 1 where J is the current density, J o the saturation current density in the reverse direction, n the so called diode ideality factor, V the applied voltage. The reverse saturation current density (J o ) obtained from the intersection of the ln J versus V graph. The barrier height (φ bo ) can be obtained by using the Richardson constant R * (8.16 AK -2 cm -2 ), from the equation (2), = * 2 qφ bo Jo R T exp (2) kt where, T is the absolute temperature, q the electric charge and (kt = 0.02586 ev). The diode ideality factor n can be calculated as:

Universities Research Journal 2011, Vol. 4, No. 4 309 1 φ = 1 bo n (3) V The values of diode parameters of ZnO:In (2mol% and 4mol%)/SiO 2 /p-si thin film devices at process temperatures 700 C for 1h are listed in Table (1). Table (1) The values of saturation current density J o, barrier height Φ bo and ideality factor n of ZnO: In (2mol% and 4mol%)/SiO 2 /p-si thin film devices Sample J o (A/cm 2 ) Φ bo (ev) n ZnO:In(2mol%)/SiO 2 /p-si 0.29 10-6 0.739 1.161 ZnO:In(4mol%)/SiO 2 /p-si 6.27 10-6 0.659 1.167-10 -11-12 ln J [A/cm 2 ] -13-14 -15-16 0 2 4 6 Voltage [V] Figure 3(a) ln J versus V characteristics of ZnO:In(2mol%)/SiO 2 /p-si thin film device. Figure 3(a) ln J versus V characteristics of ZnO:In (2mol%)/SiO 2 /p-si thin film device

310 Universities Research Journal 2011, Vol. 4, No. 4-7 -8 ln J (A/cm 2 ) -9-10 -11-12 0 1 2 3 4 5 6 Voltage (V) Figure 3(b) ln J characterisitcs of ZnO:In(4mol%)/SiO 2 /p-si thin film device Figure 3(b) ln J versus V characteristics of ZnO:In (4mol%)/SiO 2 /p-si thin film device The basic characterization tool for photovoltaic behaviour of thin film device is the current-voltage (I-V) measurement under illumination. The voltage and current of ZnO:In(2 mol% and 4mol%)/SiO 2 /p-si thin film devices are measured by using Digital multimeter (DT 9205C) and (DT 9208A), as a voltmetre and an ammetre. For illumination, 160 WATT MERCURY BLENDED LAMP (220V, 50Hz) as a light source and EasyView Digital Light Metre Model EA30 was used to count the light intensity. Current-Voltage characteristics measurement under illumination is shown in figures 4 (a & b). Figure 5 (a & b) show the J-V characteristics of ZnO:In (2mol%)/SiO 2 /p-si thin film device under illumination 1000 lux and ZnO:In(4mol%)/SiO 2 /p-si thin film device under illumination 1.2 klux. In these figures, it is seen that the curve passes through the fourth quadrant and hence the device can deliver power. The characteristics in the fourth quadrant are represented in Figures 6 (a & b), obtained with a rotation of the current axis by 180 C around the voltage axis.

Universities Research Journal 2011, Vol. 4, No. 4 311 Figure 4(a) Current-voltage (I-V) measurement under illumination 1000 lux for ZnO:In(2mol%)/SiO 2 /p-si thin film device Figure 4(b) Current-voltage (I-V) measurement under illumination 1.2 klux for ZnO:In(4mol%)/SiO 2 /p-si thin film device

312 Universities Research Journal 2011, Vol. 4, No. 4 1.0 Current density (A/m 2 ) 0.8 0.6 0.4 0.2 0.0-8 -6-4 -2 0 2 4 6 8-0.2 Voltage (V) -0.4-0.6 J-V of ZnO:In(2mol%)/SiO 2 /p-si thin film device Figure 5(a) J-V characteristics of ZnO:In (2mol%)/SiO 2 /p-si ( undrer illumination 1000 lux) thin film device (under illumination 1000 lux) 0.6 Current density (A/m 2 ) 0.4 0.2 0.0-8 -6-4 -2 0 2 4 6 8-0.2 Voltage (V) -0.4 Figure 5(b) J-V characteristics of ZnO:In(4mol%)/SiO 2 /p-si thin film device ( undrer illumination 1.2klux). Figure 5(b) J-V characteristics of ZnO:In (4mol%)/SiO 2 /p-si thin film device (under illumination 1.2 klux)

Universities Research Journal 2011, Vol. 4, No. 4 313 0.25 J sc 0.20 Current density (A/m 2 ) 0.15 J mp 0.10 0.05 0.00 0.0 0.5 1.0 V mp 1.5 2.0 2.5 V oc 3.0 Voltage (V) Figure 6(a) J sc Figure versus 6(a) V oc J sc curve versus Vof oc curve ZnO:In of ZnO:In(2mol%)/SiO 2 /p-si 2 /p-si thin film thin device film device 0.14 J sc 0.12 Current density (A/m 2 ) 0.10 0.08 J mp 0.06 0.04 0.02 0.00 0.0 V 0.5 mp 1.0 1.5 2.0 V oc 2.5 Voltage (V) Figure 6(b) J sc versus V oc curve of ZnO:In 2 thin film device Figure 6(b) J sc versus V oc curve of ZnO:In (4mol%)/SiO 2 /p-si thin film device

314 Universities Research Journal 2011, Vol. 4, No. 4 The open-circuit voltage (V oc ) is the voltage measured on the thin film devices when no current flows through the devices (I = 0). Similarly, the short circuit current density (J sc ) is the current flowing through the devices when the voltage is zero (V = 0). Another defining term in the overall behaviour of photovoltaic thin film devices is fill factor (FF). This is the ratio of the maximum power point divided by the open-circuit voltage (V oc ) and the short-circuit current density (J sc ). The fill factor (FF) is evaluated by the equation, FF J V mp mp = (4) J sc Voc where, J mp and V mp are respectively the current density and the voltage obtained when the output power is maximum (maximum power point). The conversion efficiency (η) of a photovoltaic device is the ratio of the maximal electrical output power over the incident light power (P light ), J V J V FF mp mp sc oc η = = (5) Plight Plight where, P light = 1.464W/m 2 (1000 lux) and 1.757W/m 2 (1.2klux). Photovoltaic parameters of ZnO:In (2mol% & 4mol%)/SiO 2 /p-si thin film devices are listed in Table (2). Table (2) Photovoltaic parameters for ZnO: In(2mol% and 4mol%)/SiO 2 / p- Si thin film devices measured under illumination Sample V oc (V) J sc (A/m 2 ) FF η (%) ZnO:In(2mol%)/SiO 2 /p-si 2.576 0.225 0.301 11.927 ZnO:In(4mol%)/SiO 2 /p-si 2.109 0.122 0.267 3.914

Universities Research Journal 2011, Vol. 4, No. 4 315 Conclusion The electrical properties of ZnO:In 2(mol% & 4mol%)/SiO 2 /p-si thin film devices are determined. Processing parameter such as reverse saturation current density, barrier height and ideality factor are systematically investigated from the detail analysis of ln J-V characteristics at dark condition. Ideality factor for thin film devices are about 1.16. Current density - voltage characteristics of ZnO:In (2mol% & 4mol%)/SiO 2 /p-si thin film devices under illumination are investigated. Photovoltaic parameters such as open circuit voltage, shortcircuit current density, fill factor and conversion efficiency are evaluated. For the ZnO:In (2mol%)/SiO 2 /p-si thin film device, the following parameters were obtained: the value of FF = 0.301, V oc = 2.576 V, J sc = 0.225 A/m 2, η = 11.927 % under the illumination 1000 lux. For the ZnO:In (4mol%)/SiO 2 /p-si thin film device, the following parameters were obtained: the value of FF = 0.267, V oc = 2.109 V, J sc = 0.122 A/m 2, η = 3.914 % under the illumination 1.2 klux. The maximum value of conversion efficiency is 11.927 % for the ZnO:In (2mol%)/SiO 2 /p-si thin film device. According to the experimental results, ZnO:In (2mol%)/SiO 2 /p-si thin film device is more suitable for the fabrication of silicon based photovoltaic device application. Acknowledgements The authors wish to express their sincere thanks to Professor Dr Win Win Thar, Head of the Department of Physics, University of Yangon, for her kind permission to carry out this research. References Schock, H. W. et al. (2000). Thin film solar cells: The early years in Proceeding of the 16 th European Photovoltaic Solar Energy Conference p 270 Archer, M. L. et al. (2001). Clean Energy from Photovoltaics (Imperial College Press) Paraguay, F. et al. (2000). Thin Solid Films 373,137 http://www.how Do a Photovoltaic Cell Work/html (Retrieved on 18.8.09)