CHAPTER 6 FABRICATION OF DYE SENSITIZED SOLAR CELL

Transcription

1 129 CHAPTER 6 FABRICATION OF DYE SENSITIZED SOLAR CELL 6.1 INTRODUCTION The dye-sensitized solar cell technology has been recently developed for efficient and low-cost solar energy conversion Smestad et al (1994). It has attracted a lot of interest from the area of research and industries as it is a new generation of photovoltaics. There is a great potential for DSSC to be a major renewable energy converter in the future. DSSC, also called the Grätzel cell is recently reported to attain experimentally an efficiency of 10.4 ± 0.3 % with an effective area of cm 2 under global AM 1.5 spectrum (1000 W m -2 ) at 25 o C and exhibited a short-circuit current of 21.8 ma cm -2 and an open-circuit voltage of V with a fill factor of (M.A. Green et al 2008). In the last few years nano-crystalline TiO 2 thin films have received much attention for solar energy applications Ni et al (2006). In dye-sensitized solar cells TiO 2 nano-particles are used as the working electrode because of the higher efficiency exhibited by these cells when compared to other metal oxide semiconductor based solar cells. After the discovery of an efficient dye sensitized photovoltaic cell by Regan O & Gratzel (1991), the dye sensitized solar cells have received much attention as low-cost photovoltaic cells and have become a rapidly expanding field with potential applications. Nano porous TiO 2 electrode sensitized with ruthenium complexes have exhibited high power conversion efficiency up to (11%).

2 130 In this chapter, the preparation of working electrode, cathode, electrolyte, assembling of the cell, Photovoltaic Performance Measurement and efficiency of the DSSC investigated are discussed. 6.2 SURVEY OF LITERATURE The dye- sensitized solar cell using the carotenoids bixin and norbixin,which were obtained from the extract of achiote seeds and simple separation and purification processes. The dyes strongly absorb visible light between 400 and 550 nm, and the maximum absorption coefficient of bixin is about 13x times higher than that of the ruthenium-based dye N-719, while maximum absorption coefficient for norbixin is about the same for N-719. Despite strong light absorption, the cell efficiencies are low, with the best efficiency of 0.53% for bixin- sensitized TiO 2 cells has been studied by Gomez- z et al (2010). The prepared organic inorganic composite gel electrolyte based on TiO 2 gel, by the sol gel methods, gel electrolyte shows high ambient ionic conductivity of 7.63 ms cm -1, which is close to the data of liquid electrolyte with the same organic iodide salt and c-butyrolactone. Based on the gel electrolyte, a quasi-solid-state dye-sensitized solar cell was fabricated and the highest overall energy conversion efficiency of light-to-electricity of 3.06% was achieved under irradiation of 60 mw cm -2 has been reported by Zhang Lan et al ( 2006). The effects of solvent composition on the electron injection efficiency for nanocrystalline TiO 2 films sensitized with black dye and electron injection efficiency of the film dried in air was estimated as 0.4 using a TRMC technique using TA absorption measurements, the estimated electron injection efficiency in various solvents. AN, which showed the best performance, had the

3 131 highest efficiency, and other solvents (3MPN,GBL,andPC) had lower efficiencies, suggesting specific interaction between the black dye and AN Although these solvents are promising for long-term stability, since they are non- volatile, the relatively low electron injection efficiencies obtained using these solvents indicate that they are not ideal for constructing high-performance solar cells has been studied by RyuziKatoh et al(2009). The solar cell sensitized with infrared dyes, NK-4432 and NK- 6037, is proved that the two dyes have appropriate HOMO LUMO energy levels for dye sensitization of the TiO 2 photoelectrode. However, only the NK sensitized solar cell shows an efficient light-to-electric conversion because of the carboxyl groups available for binding the dye onto the TiO 2 surface for better electron injection. The TiO 2 photoelectrode was optimized by controlling the film thickness and light-confining electrolyte. The correlation between the electron lifetime and Voc was discussed by EIS measurements. An efficiency as high as 2.3% for the infrared-dye-sensitized solar cell was obtained by using the NK-6037 dye. The IPCE at 840nm was also as high as over 50%, and the IPCE on set was 970 nm in the infrared region has been studied by Takahikono et al (2009). The photovoltaic performance of two new fluoranthene dyes as sensitizers in mesoporous TiO 2 solar cells was investigated using liquid electrolytes containing ion liquid and quasi-solid-state electrolytes. The two sensitizers exhibited good stability during a long-term accelerated aging, in which the quasisolid- state DSSCs based on the dye they achieved the better photovoltaic performance of 1.33% (Jsc ¼ 3.53 ma cm_2, Voc ¼ 542 mv, ff ¼ 0.70) under AM 1.5 solar simulator (100mWcm -2 ). And also their finding demonstrates that introduction of a thiophene unit brought about improved

4 132 photovoltaic performance comparing with the phenyl unit counterpart in quasi-solid-state electrolytes has been studied by Wenjun Wu et al ( 2009). The screen-printed films can be efficiently sensitized by N 719 dye. Illumination of the cell under light power of 100 mwcm -2 produced an opencircuit voltage of 603 mv and a short-circuit photocurrent density as high as 8.24 cm -2. The energy conversion efficiency of the cell was 2.44% with fill factor of 0.49, indicating the promising application in DSSCs of the paste of changes including microstructures of film, fine tuning of the film thickness and better optimization of the screen printing conditions are necessary. From an industrial point of view, the screen-printing technique used here for preparing semiconductor films is effective and practical has been studied by Fan k e et al (2010). In this literature (M.C. Kao 2009), author prepared Nanocrystalline anatase TiO 2 thin films with different thicknesses (0.5 deposited on ITO coated glass substrates by a sol gel method and rapid thermal annealing for application as the work electrode for dye-sensitized solar cells (DSSC). From the results, the increases in thickness of TiO 2 films can increase adsorption of the N3 dye through TiO 2 layers to improve the short-circuit photocurrent (Jsc) and open-circuit voltage (Voc), respectively. However, the Jsc and Voc of DSSC with a TiO 2 film thickn ma/cm2 and 0.61 V) are smaller than those of DSSC with a TiO 2 film the increased thickness of TiO 2 thin films also resulted in a decrease in the transmittance of TiO 2 thin films thus reducing the incident light intensity on obtained in a DSSC with the TiO 2

5 133 The fabricated Dye-sensitized solar cell by using Rose Bengal dye (RB) for sensitization of nanocrystalline TiO 2 and that imparts extension in spectral response towards visible region by modifying the semiconductor surface. Further, the photo response of the cell was evaluated by analyzing its J V and impedance characteristics under illumination with metal halide light source of 400 W with an incident light of 73 mw/cm 2. Various photovoltaic parameters like Jsc, Voc, FF were evaluated and found to be 3.22mA, 890mV, 0.53, respectively, resulting conversion efficiency (Z) of 2.09 has been reported by Roy et al( 2007). The influence of compact layer of TiO 2 between FTO and nanoporous TiO 2 on the charge transport and photovoltaic properties of quasi-solid state dye sensitized solar cells with polymer gel electrolyte and perylene derivative dye as sensitizer. The PEDOT:PSS/graphite/FTO was used as counter electrodes for the investigation. The modification of photo-electrode significantly improve the power conversion efficiency (2.94%) of the solar cells. The compact layer provide a large TiO 2 /FTO contact area, reduce the electron recombination by blocking the direct contact with the redox couple in the electrolyte and efficient collection of electrons by FTO electrode. Finally, the incorporation of TiO 2 nano-particle in the polymer electrolyte further improves the power conversion efficiency (3.2%) of the device attributed to the improved ion transport has been studied by Balraju et al(2009). 6.3 FABRICATION OF TIO 2 WORKING ELECTRODE TiO 2 thin film solar cells have been fabricated using indium doped tin oxide glass substrates. The nanocrystalline TiO 2 layer was deposited on ITO substrate by sol-gel dip drive method. In a typical experimental

6 134 procedure, 0.5 g Titanium isopropoxide (Alfa Aaser 99.9%) was added into with absolute ethanol (Aldrich 99.9%) and acetic acid at room temperature. A small amount of polyethylene glycol (PEG) was added as a binder. Ethanol is used as a solvent and acetic acid acts as a catalyst controlling the ph of the hydrolysis/condensation reactions in sol gel solution. The final mixture solution was stirred for about 3 hrs. The ph of the solution was adjusted by changing the volume of acetic acid. The TiO 2 thin films are prepared at 3 different ph values such as 1, 3.5 and 9. By using the prepared sol TiO 2 thin film was deposited by dip drive method. The well cleaned ITO substrate was immersed (30 sec) into the solution and the substrate was withdrawn from the solution by dip drive unit. The active TiO 2 layer was preheated at 100 C for 10 min and then allowed to cool to room temperature. TiO 2 was again dip coated on the already coated TiO 2 film and heated at 100 C for 10 mins and then allowed to cool to room temperature. The dip coating, heating and cooling process was repeated several times in order to get thicker films. The films were then dried at room temperature for 2 hrs and then annealed in air atmosphere at 450 C for 1 hr using a heating rate of 2 C/min. The thickness of the film was about 5µm. 6.4 FABRICATION OF COUNTER ELECTRODE The counter electrode for the solar cell was prepared using platinum II chloride as follows: H 2 PtCl 6 solution in isopropanol (2mg/ml) was deposited onto the ITO glass by spin coating method. The film was dried at 80 C for 30 min in air and then sintered at 400 C for 30 min in air.

7 ASSEMBLING THE DYE SENSITIZED SOLAR CELLS In cell assembly section, the prepared TiO 2 thin film electrodes were immersed in a 3.0 x 10-4 M N719 dye solution at room temperature for 24 hrs, after that period the film was rinsed with anhydrous ethanol and dried. A Pt-coated ITO electrode was then placed over the dye-adsorbed TiO 2 thin film electrode, and the edges of the cell were sealed with a sealing sheet (PECHM-1, Mitsui-Dupont Polychemical) by heating with hot plate at 100 C for 2 minutes. A redox electrolyte was prepared by using 0.5 mol KI, 0.05 mol I 2, and 0.5 mol 4-tert-butylpyridine and a drop of electrolyte solution was injected into the drilled hole in the counter electrode and was driven into the cell. Finally, the hole was sealed using additional cello tapes and the size of the electrode used was 0.40 cm CHARACTERIZATIONS The photocurrent-voltage (I-V) curves were measured using white light from a xenon lamp (max. 150 W) using a sun 2000 solar simulator (ABE technologies). Light intensity was adjusted using a Si solar cell to ~AM-1.5. Incident light intensity and active cell area were 100 mwcm -2 (one sun illumination) and 0.4 cm 2 (0.1 x 0.4 cm), respectively. The recombination property was measured by intensity modulated photo voltage spectroscopy (IMVS) using Compact State electrochemical interface from IVIUM STAT technologies.

8 RESULTS AND DISCUSSION Figure 6.1 shows the photocurrent-voltage (I-V) characteristics of dye sensitized solar cells prepared at different ph values. It is observed that the samples prepared at ph value (ph=1) shows lesser efficiency of 0.95% with short-circuit current density (J sc ) = 5.2 ma/cm 2, open circuit voltage (V oc ) = 0.57 V and fill factor (FF) = The low short-circuit current density (J sc ) of the solar cells can be attributed to the ineffective gathering of the photoinjected electrons, and this is due to the presence of smaller nanoparticles on the surface. 12 =2.72% ph=1 ph=3.5 ph=9 9 =1.89% 6 =0.95% Voltage (V) Figure 6.1 I-V curves of the TiO 2 thin film solar cells prepared using different ph values

9 137 0 ph= V 1.0V 1.5V 2.0V 2.5V Real ( ) Figure 6.2 IMVS curves of the TiO 2 thin film solar cells prepared using ph=3.5 0 ph= V 1.0V 1.5V 2.0V 2.5V Real ( ) Figure 6.3 IMVS curves of the TiO 2 thin film solar cells prepared using ph=9

10 ph=3.5 ph= Current intensity (ma/cm 2 ) Figure 6.4 Relation between electron lifetime and current intensity The smaller nanoparticles may increase the series resistance between the ITO and TiO 2 nanoparticle structures. The reduction of dye adsorption area, increased diffusion length for photo excited electrons and easy charge recombination are also the reasons for the reduction of conversion efficiency. It is observed that solar cells prepared at higher ph =9 value shows short-circuit current density (J sc ) of 9.2 ma/cm 2 and conversion efficiency (CE) of 1.89% and fill factor( FF)= 0.68, which is less compared to the solar cells prepared using ph=3.5 The reason may be increased in particle size or the particle clump. The low conversion efficiency and low J sc of dye sensitized solar cell is attributed to in efficient light harvesting due to the low dye absorption on the particle clump. The particle clump also acts as a barrier for electron movement between the working electrode and the ITO substrate. These factors cause small collection of the photo-injected electrons at the TiO 2 photo-anode and increase the recombination of electron to holes of electrolyte.

11 139 Therefore, the low conversion efficiency and J sc may be attributed to small absorption of dye molecules by the particle clump. The TiO 2 working electrode prepared at ph=3.5 shows maximum power conversion efficiency of 2.72% with short-circuit current density (J sc ) of 12.6 ma/cm 2 and fill factor FF= The reason is the presence of dense network of well crystallized particles these particles gives us a large surface area in a small amount of space, since the thinner structure with larger surface area can hold more dye molecules. Usually the Photovoltaic conversion efficiency strongly depends on dye adsorption volume and surface area of TiO 2. High surface area is required to achieve high efficiency. This is due to the fact that dense network of nanoparticles adsorb more dye molecules than the other surface structures, resulting in an increase in the generation of electron-hole pairs and therefore the short-circuit current density is high. IMVS response is used as a powerful tool to determine the electron lifetime and electron hole recombination dynamics under open circuit voltage conditions. Only in that condition photo injected electrons are not extracted at - the substrate contact and back react instead with I 3 or with oxidized state of the dye. IMVS measurement was carried out over a range of illumination intensities of 5 orders of magnitude. Figure 6.2 and 6.3 shows the IMVS curve for the solar cells prepared at a ph value of 3.5 and 9 respectively. The response shows that the semicircle in both the dye sensitized solar cells and the circle of IMVS is shifted to the right side for the solar cell prepared at ph=3.5. The shift of semicircle towards fourth quadrant (positive real and negative imaginary) of the complex plane is the result of the time delay (slower recombination or lifetime) between the generation and collection of electrons, Franco et al (1999) or relaxation of electrons by back reaction with

12 140 cations in the hole conductor matrix. The reason for the increase of recombination in the solar cell prepared using ph=9 is the presence of particle clumps; this structure increases the recombination in so many ways. The recombination in dye sensitized solar cell occurs at various places among them the four areas are very important; (i) recombination between the excited electron and holes in the dye molecule, (ii) trapping of the excited electrons from the semiconductor or excited electrons from the dye molecule by the surface states present in the dye molecule, (iii) recombination of excited electrons from the semiconductor or excited electrons from the dye molecule with the hole acceptors present the electrolyte and (iv) back electron injection from the semiconductor to the dye molecule are important phenomena. In order to improve the dye sensitized solar cell performance fast transition time is needed with slow recombination. The lifetime can be calculated from n= 1/2 f imvs, where f imvs is the frequency of the minimum IMVS imaginary component, same as the expression used in dye sensitized solar cells (Senthil et al. 2013),(Hsiao et al. 2010). Figure 6.4 shows the variation of life time with the current density. The decreasing trend in this figure can be attributed to the higher recombination frequency under stronger illumination. Stronger illumination leads to the greater photo-voltage. The rate is slower in the dye sensitized solar cells prepared using ph=3.5. When the light intensity increases the frequency corresponding to the minimum in the complex plane increases (radius of semicircle decreases), this is due to the decrease of electron life time. This shows that the electron recombination is more pronounced at high illumination intensities. The same conclusion can be drawn from the plot (Figure 6.4) of the imaginary component versus the frequency.