Significant Influence of Annealing Temperature and Thickness of Electrode on Energy Conversion Efficiency of Dye Sensitized Solar Cell: Effect of Catalyst

Dye sensitized solar cell (DSSC) shows great promise as an alternative to conventional p-n junction solar cells due to their low fabrication cost and reasonably high efficiency. DSSC was assembled by using natural dye extracted from red amaranth (Amaranthus Gangeticus ) as a sensitizer and different catalysts for counter electrode were applied for maximum energy conversion efficiency. Annealing temperature and thickness of electrode were also investigated and optimized. Catalyst, annealing temperature and thickness were optimized by the determination of cell performance considering photoelectrochemical output and measuring current and voltage; then calculating efficiency and other electrical parameters. The experimental results indicated that samples having 40 µm electrode thickness and prepared at 450 °C annealing temperature showed the best performance.


INTRODUCTION
Most of the power generated nowadays is produced using fossil fuels, which emit tons of carbon dioxide and other pollution every second. More importantly, fossil fuel will eventually run out. So people are looking for new source of substitute clean energy in order to make the development of our civilization sustainable and cause less harm to our environment. Although the photovoltaic devices promise to offer a clean solution to these problems, current silicon-based photovoltaic technologies are hindered by serious combustion pollution and high production cost. As a novel renewable and clean solar-to-electricity conversion system, dye-sensitized solar cells (DSSCs) have attracted the attention of researchers since they were introduced by O'Regan and Gratzel [1].
Since then, DSSCs have been widely explored as a matter of tremendous interest to academic and commercial researchers due to their low cost fabrication and relatively high conversion efficiencies (>10 %) [2][3][4]. These cells are extremely promising because they are made of low-cost materials and do not need elaborate apparatus to manufacture. Conventional DSSCs consist of mainly five components: (1) a conductive and transparent mechanical support, (2) a wide band gap semiconductor film, (3) a sensitizer, (4) redox couple (usually I 3ˉ/ Iˉ) in an organic solvents and (5) a counter electrode. The successful combination of the materials involved in cell fabrication, as well as the precise methods used, will determine the final cell performance. In order to further improve the performances of DSSCs, extensive efforts are on-going for each of its constituent components and conditions including dye for sensitizer, electrolyte, paste formulation, catalyst for counter electrode [5], thickness of electrode [6][7][8] and annealing conditions [9][10][11] etc. In this paper our concentration is on catalyst, thickness and annealing temperature. Among these parameters the counter electrode is one of the most important components in the dye sensitized solar cell (DSSC).
The task of the counter electrode is the reduction of the redox species used as a mediator in regenerating the sensitizer after electron injection or collection of the holes from the conducting material in a solid-state DSSC [12]. Recently, some carbon materials and conducting polymers were used as efficient catalysts on the counter electrode of DSSCs [13,14]. To make an inexpensive cell one should consider a low cost material such as a carbon catalyst for counter electrode. Here our objective is to find out a low cost suitable carbon catalyst for DSSC. On the other hand another parameter that influences the performance of DSSC significantly is thickness.
Therefore, it is necessary to analyze the influence of various TiO 2 film thickness values on electron transport to improve DSSC efficiency. A number of studies on the influence of TiO 2 film thickness have focused on the transition of open circuit voltage, current density, fill factor, and efficiency and it is found that the cell's overall energy conversion efficiency strongly depends on the surface and electronic properties of the electrode with thickness varying from several hundred nanometers up to 20 µm [15,16]. Recently, Shin et al. analyzed the effect of TiO 2 film thickness on DSSC characteristics using electrochemical impedance spectroscopy [17]. Kao et al. and Barglio et al. reported the effect of TiO 2 film thickness on DSSC performance and electrochemical behaviour [18,19].
K. Park et al. concluded that an excessively thick TiO 2 is not good for DSSC performance because of the electron recombination and the maximum photovoltaic conversion efficiency was achieved for a TiO 2 film thickness of 9 µm [20]. Again another most important parameter that affects Dye-Sensitized Solar Cell (DSSC) characteristics is porosity. There are many aspects that can influence the porosity such as particles size of TiO 2 , annealing temperature and processing parameter or fabrication itself. In this paper, one of important parameter will be discussed, i.e., effect of annealing on characterization of TiO 2 thin layer.
Several researchers investigated the effect of annealing conditions and concluded that it has an important role in the efficiency of DSSC [21,22]. It is assumed that annealing process will improve electrical properties of this cell due to porosity effect. Annealing process can remove the binding and solvent which can increase electrically-connected network of TiO 2 particles [23]. This condition will increase TiO 2 performance as the electron transport. According to Chang-Ryul et al. [24] porosity is related with annealing duration and temperature, which affecting photocurrent density and the amount of absorbed dye. Now our objective of this research is to optimize annealing temperature and thickness of electrode and find out available suitable catalyst for counter electrode and then fabricate a complete DSSC. For this purpose firstly we searched a suitable catalyst keeping thickness and annealing temperature remained unchanged. After choosing catalyst thickness is optimized keeping temperature remain unchanged. Finally annealing temperature is optimized. Optimal International Letters of Chemistry, Physics and Astronomy Vol. 39 parameters were determined by measuring current and voltage with the help of a precision multimeter and then calculating maximum power point, fill factor and cell efficiency.

Preparation of TiO 2 Paste
2 g of Degussa P25 powder was deposited in a beaker mixed with 0.1 M, 6 mL citric acid, 0.2 mL polyethylene glycol, 0.2 mL titanium (IV) isopropxide and 0.1 mL nonionic surfactant triton X-100. The mixture was well mixed with the help of a glass rod and then kept in an ultrasonic bath for half an hour for the production of suspension with a consistency of a thick paint produced. The prepared paste was coated on ITO glass with surface resistance of 10-30 ohms by using doctor blade technique.

3. Controlling the Thickness
The thickness of the coating was controlled using the simplest and most widely used doctor-blade method. With this technique, the thickness of the paste layer is determined by the thickness of a spacer placed on both sides. Using the Scotch tape which has a thickness of 20 µm, 2 Layer = 40 µm, 3 Layer = 60 µm, 4 Layer = 80 µm and 5 Layer = 100 µm, with the conductive side facing up, apply two parallel strips of tape on the edges of the glass plate. The area of the middle of the glass is uncovered and where the TiO 2 paste will be deposited. After depositing the paste remove the scotch tape. Two bare edges masked by the tape will give then room for future sealing and electrical contacts.

4. Annealing electrode
The electrodes were annealed at temperature ranging from 200 ºC -550 ºC (200 ºC, 250 ºC, 300 ºC, 350 ºC, 400 ºC, 450 ºC, 500 ºC, 550 ºC) for half an hour with the help of a furnace. After the annealing was completed, the TiO 2 coated conductive glass was allowed to cool slowly at room temperature and then the TiO 2 coated glass plate was prepared for dye soaking.

5. Extraction of natural dye and staining the electrode
Red amaranth leaves were collected from local market and then washed and kept them some times for dried. 100 g of leaves were weighted and crushed in a mortar and pastel adding 50 mL of solvent. The extracted dye was filtered three times with a cotton cloth and stored in a dark bottle covered with aluminium foil paper. The TiO 2 coated glass plate was soaked in dye and was kept at a dark place. The glass plate was washed by distilled water and then ethanol and dried in air for 30 minutes

6. Preparing counter electrode and Electrolyte
Carbon from various available sources has been collected to investigate its effect as a catalyst. Wood coal, char coal, soft pencil, hard pencil, carbon from recycled battery and tip of candle flame were used as catalyst. To prepare electrolyte 8.3 g of 0.5 M potassium iodide and 1.27 g of 0.05 M iodine was mixed in ethylene glycol until it is 100 mL. The solution was stored in a black bottle and was used when necessary.

7. Assembling the Cell
Electrode and counter electrode were combined together keeping TiO 2 paste coated surface and the carbon coated surface face to face. 2/3 drops of electrolyte solution was given in the contact of two glasses and by the capillary action the electrolyte was uniformly distributed throughout the stained TiO 2 film. Excess electrolyte from the exposed area of the glass was wiped off by using cotton or tissue. The complete cell was then taken to sunlight for harvesting energy.

8. Measuring electric properties
Electrical properties were measured by using two digital multimeter keeping the cell in sunlight of approximately 100 mW/cm 2 illumination. Current and voltage were measured by multimeters changing with resistance with the help of a variable resistor. Based on I-V curve, the fill factor (FF) was defined as P is the power of incident light.

1. Effects of Catalyst for Counter Electrode
Counter electrode (CE) of dye-sensitized solar cell (DSSC) can be prepared with different materials and methods. Here we prepared six samples of different catalyst and one for no catalyst. The photovoltaic performances of the DSSC based on different carbon catalysts counter electrode are shown in Fig. 1. Open circuit voltage (Voc), short circuit current density (J sc ) and power density obtained for each DSSC are summarized in Table 1. Amongst the different catalyst of counter electrode the DSSC with tip of candle flame has shown maximum open circuit voltage 0.480 V and short circuit current density 0.85 mA/cm 2 resulting maximum power density 0.408 mW/cm 2 . This indicates that tip of candle flame has strong catalytic activity toward the reduction of iodine electrolyte.

2. Effects of Thickness of TiO 2 Film on Electrode
TiO 2 layer is the carrier of electron after it is excited by the organic dye. So it is necessary to recognize the semiconductor layer thickness dependency. In this experiment the thickness of TiO 2 was controlled by the thickness of tape.
Thus, by layering several pieces of tape on both sides of the ITO glass and then following the same rolling procedure as used for a normal cell, we can control the thickness of TiO 2 in this simple way. However, it was only possible to measure several distinctive data point, i.e the integer times of thickness of the tape, because it is impossible to tape any decimal layer of tapes on the glass.
All of the measurements were taken at the same temperature, amount of illumination and pressure. Fig. 2. shows the photovoltaic performances of the DSSC based on different thickness (20-80 µm) of TiO 2 film. Open circuit voltage (Voc), short circuit current density (J sc ), fill factor (F.F.) and efficiency obtained for each DSSC are summarized in Table 2. The DSSC shows linear increase in efficiency, short circuit current density and fill factor with increase in thickness up to 40 µm and then decrease slightly as shown in Fig. 2.(A-C), respectively. Fig 2D shows that open circuit voltage decrease with increase in thickness. The efficiency, fill factor and short-circuit current density are maximum at the thickness of 40 µm and they are 0.20 %, 63 % and 0.70 mA/cm 2 respectively. Therefore, the thickness was optimized and it was 40 μm.        shows the photovoltaic performances of the DSSC based on different annealing temperature (250-550 °C) of the electrode. Open circuit voltage (Voc), short circuit current density (J sc ), fill factor (F.F.) and efficiency obtained for each DSSC are summarized in Table  3. Fig. 3 (A-C) shows that efficiency, short circuit current density and fill factor increase with temperature up to 450 °C and than decrease. Fig. 3D shows that open circuit voltage increase with increase in temperature up to 400 °C and after that V oc was almost constant in spite of increasing temperature. It was found that annealing temperature had significant effects on the cell performance. Maximum cell efficiency was 0.21 % and it was for the temperature of 450 °C.

CONCLUSION
In summary, counter electrodes for DSSC with different carbon materials were fabricated using the doctor blade method and was found that tip of candle flame is a suitable catalyst for energy conversion. Fabrication of photo-electrodes of different thicknesses (20 µm, 40 µm, 60 µm, 80 µm and 100 µm) revealed that an excessively thick TiO 2 is not good for DSSC performance because of the electron recombination. The maximum photovoltaic conversion efficiency was achieved for a TiO 2 film thickness of 40 µm. The influence of annealing temperature (250-550 °C) on the performance of DSSC with TiO 2 films was studied and it was optimized at 450 °C. So, this paper describes a low cost and effective technique to fabricate dye sensitized solar cell using a novel locally available natural dye having promising energy conversion efficiency.