LIQUID JUNCTION PHOTOCELLS SYNTHESIZED WITH DYE COATED ZINC OXIDE FILMS

Zinc oxide (ZnO) films fabricated using low cost methods were employed to synthesize a liquid junction photocell explained in this report. Methyl violet dye has been coated on ZnO films to enhance the light absorption. The thickness of the ZnO film and the separation between the Platinum electrode and ZnO film were varied in order to obtain the maximum efficiency of the photocell. The best photocurrent and photo-voltage could be measured for ZnO films coated with methyl violet dye for 12 hours. The optimum photocurrent obtained was 0.24 mA/cm 2 , which was a higher photocurrent for Methyl violet coated ZnO films. But the photo-voltage measured was comparable to the photovoltage of ZnO films measured by some other researchers. Hence, a considerably higher optimum efficiency such as 2.4% could be obtained for these ZnO films consist of ZnO nanoparticles. The higher effective surface area provided by ZnO nanoparticles is the possible reason for this higher photocurrent.


INTRODUCTION
Because zinc oxide (ZnO) is electrically conductive and visually transparent, ZnO thin films are extensively used in photo-voltaic applications, electric transducers, nanowires, integrated optics including optical wave guides, displays and heaters. Materials with wide band gaps such as ZnO can be used in solar cells to absorb the UV part of the solar spectrum. ZnO thin films have been previously synthesized by rf magnetron sputtering on glass substrates (Jeong et al., 2004) and by pulsed laser ablation on sapphire substrates (Cao et al., 1998). ZnO:Al thin films have been grown by off-axis rf magnetron sputtering on amorphous silica substrates (Jayaraj et al., 2002). ZnO nanowires have been deposited by the thermal evaporation /condensation method (Jo et al., 2003). For solar energy applications, chemical vapor deposition (Maruyama, 1993), the quasi-closed space vacuum sublimation technique (Bobrenko et al., 1994) and the screen printing technique (Knodler et al., 1993) have been also employed to grow thin films of ZnO.
Earlier thin films of ZnO have been sputtered by us (Samarasekara et al., 2002). The photo-voltage of these ZnO thin films sputtered using DC reactive sputtering varied with the sputtering conditions such as sputtering pressure, sputtering time period and substrate temperature. Nevertheless, the photo-current of these sputtered ZnO thin films were found to be really small because of the high band gap of ZnO. In this paper, the photocurrent variation of methyl violet dye coated ZnO films prepared using a low cost method are discussed.

EXPERIMENTAL
High purity ZnO nanoparticles produced by American Elements Company were ground well to make a uniform sample of fine nanoparticles. The original size of these nanoparticles was about 50 nm. These nanoparticles were mixed with ethyl alcohol to prepare a paste. A layer of this paste was uniformly applied on the conductive surface of conductive glass plates which were cleaned in an ultrasonic cleaner. The alcohol of this sample evaporated within one hour by leaving a layer of ZnO on conductive glass. Then the ZnO film sample was annealed in air in an oven at 450 0 C for 30 minutes to crystallize the ZnO phase and to make the film adhesive to conductive glass substrate. After the sample cooled down in oven, it was taken out for dye treatment and measurements.
Samples with area of 1 cm x1 cm were prepared for the measurements. But the thicknesses of the samples were varied from 0.2 mm to 2 mm in order to investigate the effect of thickness on photocurrent. The samples were then immersed in methyl violet dye solutions and left in dye for 4-24 hours.
The photocell was prepared using this dye coated ZnO film sample and a platinum electrode. A platinum plate with same area as ZnO film sample was used as the counter electrode. The separation between platinum plate and ZnO film of this photocell was varied in the range of 0.4 -1.4 mm in order to obtain optimum efficiency. Thereafter, the well sealed photocell was filled with KI/I 2 solution. KI/I 2 solution with 0.01M/0.0001M concentration dissolved in distilled water and redox couple I 3 -/Iwas used as the electrolyte in this liquid junction photocell, as the maximum efficiency could be obtained at this concentration. The variation of photocurrent with applied voltage was measured by means of computerized Keithly 236 source measurement unit, and a lamp calibrated similar to Liquid junction photocells 27 solar spectrum was employed to illuminate the photocell from the side of conducting glass.
X-ray diffraction (XRD) was performed to investigate the structure of ZnO samples.

RESULTS AND DISCUSSION
The I-V characteristic curves of ZnO film sample coated with methyl violet dye for 12 hours are shown in Figure 1. The area of all the samples described in this paper is 1 cm 2 . The photocurrent (or photo-voltage) has been defined as the difference between light and dark currents (or voltages). The minus signs of photocurrent and photo-voltage were disregarded. The photocurrent at V= 0 is 0.24 mA/cm 2 , and the photo-voltage at I= 0 is V= 0.22 V. Here dashed and solid lines indicate the dark and light I-V curves, respectively.
The efficiency corresponding to this photocurrent and photo-voltage is 2.4%. The photocurrent and photo-voltage vary with dye coating time, and they reach maximum values at dye coating time of 12 hours. Also the maximum photo-voltage and photocurrent could be obtained for film thickness of 0.6 mm. Therefore, all the ZnO film samples explained in this paper are 0.6 mm thick. The separation between the ZnO film sample and the platinum electrode of photocell is 0.8 mm in this case. The photocurrent and the photovoltage were optimum, when the separation between the ZnO film sample and the platinum plate of the photocell is 0.8 mm.   The XRD pattern of a film sample is shown in Figure 5. According to this XRD pattern, a single phase of pure ZnO nanoparticles has been crystallized in the film sample.  conditions is less than that at optimum conditions. Although it is possible to obtain a higher photo-voltage for ZnO films because of the higher band gap of ZnO, it is difficult to obtain a higher photocurrent due to the same reason. Especially, the optimum photocurrent obtained by this low cost method is higher than that obtained by some other researchers using expensive methods.