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Photosynthesis in a test tube- dye sensitized solar cells as a teaching tool
Renewable Energy 35 (2010) 1010–1013
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Photosynthesis in a test tube- dye sensitized solar cells as a teaching tool Atul Raturi*, Yoheni Fepuleai Division of Physics, School of Engineering and Physics, The University of the South Pacific, Suva, Fiji
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 January 2009 Accepted 26 October 2009 Available online 14 November 2009
Dye sensitized solar cells employing natural plant dyes as phosensitizers can be effectively used to train students in the science and technology of solar cells. This is especially relevant to developing countries where facilities for silicon cell fabrication are non-existent. The cross-disciplinary nature of this device makes it very attractive for student projects. The present work describes such a project where anthocyanin dye from hibiscus flowers has been used as the electron harvester. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Dye sensitized solar cells Natural dyes
1. Introduction The high dependence on imported fuel and lack of access to electricity make renewable energy (RE) the most suitable source of energy for a multitude of people living in the Pacific Island Countries (PICs). A large segment of the population lives on remote islands where national grid extension is not a possibility and RE based distributed generation systems could be the only solution to alleviate the energy poverty among these people. A number of RE projects are being developed in the region. Some of them are: Fiji: 100% RE based electrification by 2012 - a 12 MW windfarm was commissioned in 2007. Marshall Islands: 2 Mini-grid systems. Nauru: Wind and other RE to supply 10% of total demand. Tonga: Outer Islands Solar Electrification programme Papua New Guinea (PNG): Solar based rural electrification Tuvalu: wind and solar energy development PNG, Fiji, Solomon Islands etc.: Biofuel development (Bioethanol, Biodiesel) All PICs: Hybrid (wind-solar) systems for the communication sector.
training in renewable energy technologies. There are a number of courses being offered at undergraduate and graduate levels. In the case of photovoltaic technology, it has not been possible to set-up laboratory facilities to fabricate solar cells due to the high costs involved. However, dye sensitized solar cells (DSSC) or Gratzel cells offer an opportunity to fabricate solar cells without the need of expensive equipment [1]. Using synthetic ruthenium-based dyes efficiencies larger than 10% have been achieved [2] and a technology road map for DSSCs aims to develop cells with 20% efficiency in the next 10 years [3]. The high cost of these dyes remains an issue and efforts are underway to utilize cheaper, natural dyes. Anthocyanin dyes found in berries, flowers and other plant materials have emerged as possible alternatives [4–7]. It is possible to fabricate low-cost solar cells with these dyes acting as photon absorbers and electron generators. There is a lot of interest in this cross-disciplinary field, which combines studies in Physics (semiconductors), biology (plant dyes) and chemistry (electrochemistry), together with energy generation. Student projects based on this type of solar cells complement our RE teaching very well and expose the students to the science behind PV devices. This paper describes one such project where anthocyanin dye from Hibiscus flowers was used as the sensitizer. 2. Dye sensitized solar cells
One of the important requirements for the success of any renewable energy programme is the availability of trained manpower to develop, design, construct, and maintain RE systems. The University of South pacific, a regional university serving 12 pacific island countries is a main provider of education and
* Corresponding author. Tel.: þ679 3232430. E-mail address: [email protected] (A. Raturi). 0960-1481/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2009.10.035
A plant leaf can be thought to be made of millions of tiny photoelectrochemical cells, which utilize solar energy to convert CO2 and water into carbohydrates and oxygen. This involves a redox reaction of excited chlorophyll molecules. There have been many attempts to replicate this process artificially with very limited success. Seminal work done by Gratzel and co-workers in the late 80’s and early 90’s, resulted in a highly efficient photovoltaic device
A. Raturi, Y. Fepuleai / Renewable Energy 35 (2010) 1010–1013
(DSSC) that mimics the natural photosynthesis process. The most important step was to use an organic dye coated nanocrystalline titanium dioxide electrode as the photoanode. The nanocrystalline nature of the electrode increases the surface area of the photon harvester. One of the main differences between a DSSC and a conventional solar cell is that in DSSC, photon absorption and charge carrier transport processes are separated. Organic dyes are excited by the absorption of a photon and inject an electron into a porous (nanocrystalline) wide band -gap semiconductor (usually TiO2). This electron can be extracted by a transparent conducting oxide electrode and then flows through an external circuit to a second electrode called the counter electrode. The two electrodes are separated by a redox electrolyte (iodide–polyiodide). Oxidation and reduction reactions occur at the anode and the cathode respectively while charge transport takes place through the electrolyte. A DSSC is a majority carrier device as opposed to conventional p-n junction solar cell. The absorption spectrum of the dye and the chelation of the dye at the surface of the TiO2 electrode determine the efficiency of the solar cell [4]. The best results so far have been obtained with electrodes sensitized with ruthenium-based dyes [7]. Dye sensitized solar cells have emerged, as a serious alternative to conventional silicon solar cells. The manufacturing process is low-cost and less energy consuming. There is a growing global interest in DSSCs which are on the on the verge of being fully commercialized [8]. They are suitable for Building Integrated PV (BIPV) systems. In Queensland, Australia DSSCs are being used for powering environmental monitoring sensors. 3. Experimental techniques 3.1. TiO2 films Titanium dioxide films were grown onto fluorine-doped tin oxide (FTO) coated glass slides using the constant current electrophoresis method [9]. The FTO glass slides were TEC15 supplied by Hartford Glass Co. Inc., USA. The electrophoresis set-up is shown in Fig. 1. 0.8 g of TiO2 (Degussa P25, USA) particles were dispersed in 300 ml of deionized water to form a colloidal solution. When a constant current was applied between the aluminium anode and the FTO cathode, a nanocrystalline thin film of TiO2 was deposited onto the FTO electrode. Films were grown using 3 different constant currents (0.60 mA, 0.30 mA and 0.15 mA) .The growth duration was 10 min for all currents and after deposition, as-grown films were annealed in air at 350 C for 10–15 min.
Fig. 1. Electrophoresis set-up.
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The transmission/absorption spectrum of blank and TiO2 coated FTO films were studied using a Lambda 25 UV/VIS spectrophotometer.
3.2. Dye extraction Anthocyanin dye was obtained from Red hibiscus (Hibiscus rosasinensis) a tropical flower belonging to Malvaceae family (Fig. 2). A total of 12 flowers were immersed in 100 ml of methanol to extract the red dye. The dye was filtered and stored in a dark bottle. The optical properties of the dye were studied using the spectrophotometer described above.
3.3. DSSC fabrication Photoanodes were prepared by dipping the TiO2 coated FTO slides in the anthocyanin dye solution. After approximately 10–15 min, the dye was absorbed by the TiO2 film (as evident by the change in the colour of the film). The dye stained film was washed in water and ethanol. After drying, the film was ready to be used as a photoanode. A counter electrode was prepared by coating an FTO film with the carbon soot from the luminous flame of a Bunsen burner. DSSCs were fabricated by using the dye coated TiO2/FTO plate as photoanode and the carbon coated FTO plate as the counter electrode. Few drops of iodide/polyiodide (0.5 M KI/0.05 I in ethylene glycol) electrolyte were inserted between the two plates .The cell was completed by clamping the two electrodes together. The cells were tested for open-circuit voltage, short-circuit current and I-V characteristics using the standard solar cell test circuit. The cells were tested under natural sunlight.
4. Results and discussion 4.1. FTO film characterization The fluorine-doped tin oxide films used in this work were measured to have a sheet resistance of w15 U/,. The direct band gap was determined by plotting the square of the absorption coefficient against photon energy and was found to be w3.48 eV Fig. 3.
Fig. 2. Hibiscus rosa-sinensis.
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A. Raturi, Y. Fepuleai / Renewable Energy 35 (2010) 1010–1013
Fig. 3. Direct band gap determination.
4.2. TiO2 film growth The best films, which were visually smooth and crack –free were grown at the lowest current value (0.15 mA, sample 3) and were used in the fabrication of the DSSC. Fig. 4 shows the transmission spectra of FTO films coated with TiO2. It is seen that the absorption edge moves towards the lower energy side when TiO2 is deposited over FTO. Rutile TiO2 has a smaller band gap of w3.03 eV [10] which is responsible for this shift. 4.3. Optical properties of the dye solution Fig. 5 shows the transmission and absorption spectrum of the anthocynin dye. The absorption spectrum is slightly different from that reported by Fernando and Senadera [6] for Hibiscus rosa-sinensis dye extracted using acidified ethanol. Their dye exhibited an absorption peak at w540 nm. Absorption of photons in the visible region is strongly dependent on the pH and increases in acidic solutions. 4.4. DSSC characterization Fig. 6 shows the I-V characteristics of the DSSC fabricated. The cell had an open-circuit voltage of 250 mV and a short-circuit current
Fig. 4. Transmission spectra of coated FTO slides. (1) Blank FTO, (2) FTO þ sample1 (current ¼ 0.60 mA), (3) FTO þ sample 3 (current ¼ 0.15 mA).
Fig. 5. Transmission and absorption spectra of the dye (a) Transmission (b) Absorption.
density of 0.31 mA/cm2 under an insolation of 490 W/m2. The characteristics exhibit a fill factor of 0.4 and an efficiency of w0.07% can be deduced. Although the values obtained are quite small, they do demonstrate the applicability of plant dyes for photovoltaic energy conversion. Not all dyes are suitable for DSSC application. The dye should be able to inject an electron into the conduction band of TiO2. This is possible if the energy level of the dye is 0.2–0.3 eV above the
Fig. 6. I-V characteristics of the DSSC.
A. Raturi, Y. Fepuleai / Renewable Energy 35 (2010) 1010–1013
conduction band edge of TiO2 [11]. Fernando and Senadeera [6] have reported an efficiency of 1.02% in DSSCs with Hibiscus rosasinensis sensitizer. The photoanode consisted of TiO2 paste (thickness w4–5 mm) coated FTO and acidified ethanol was used to extract the dye. One of the reasons for small efficiencies in our case could be the small TiO2 film thickness together with presence of pinholes. A low dye concentration and its low absorption in the visible range could also be a contributing factor. The carbon counter electrode was very fragile and should be replaced by a platinum electrode. We are currently studying the effect of growth parameters of TiO2 films in conjunction with a variety of local plant dyes on the DSSC characteristics. 5. Conclusions Fabrication and characterization of dye sensitized solar cells made with local plant dyes as photosensitizer can play a major role in photovoltaics technology education in developing countries. Students appreciate the interdisciplinary nature of the project and the hands-on training. It is envisaged that in not too distant future, small Pacific Island Countries can have their own DSSC modules manufacturing facilities.
1013
References [1] Regan BO, Gratzel M. A low-cost high efficiency solar cell based on dyesensitized colloidal TiO2 films. Nature 1991;353:737–40. [2] Green MA, Emery K, Hisikawa Y, Wilhelm. Solar cell efficiency tables(version 30). Prog Photovoltaics Res Appl 2007;15:425–30. [3] Pagliaro M, Palmisano G, Ciriminna R. Flexible solar cells. WILEY-VCH verlag GmbH & Co, ISBN 978-3-527-32375-3; 2008. [4] Tennakone K, Kumarasinghe AR, Kuma GRRA, Wijayantha K, Sirimanne PM. Nanoporous TiO2 photoanode sensitized with the flower pigment Cyanidin. J Photochem Photobiol 1997;108:128. [5] Yamazaki E, Murayama M, Nishikawa N, Hashimoto N, Shoyama M, Kurita O. Utilization of natural caroteniods as photosensitizers for dye sensitized solar cells. Solar Energy 2007;81:512–6. [6] Fernando JMRC, Senadeera GKR. Natural anthocyanins as photosensitizers for dye- sensitized solar devices. Curr Sci 2008;95:663–6. [7] Wongcharee K, Meeyoo V, Chavade S. Dye-sensitized solar cell using natural dyes extracted from rosella and blue pea flowers. Solar Energy Mater Solar Cells 2007;91:566–71. [8] Available at: http://dyesol.com. [9] Fujimura K, Yoshikodo S. Preparation of TiO2 film for dye sensitized solar cell deposited by electrophoresis method. Key Eng Mater 2003;248:133–6. [10] Jin Y, Li G, Zhang Y, Zhang Y, Zhan L. Photoluminescence of anatase TiO2 thin films achieved by the addition of ZnFe2O4. J Phys Condens Matter 2001;13:L913–8. [11] Matthias J. Novel aqueous electrolyte films for hole conduction in dye. Sensitized solar cells and development of an electron transport model. PhD. Thesis. Free University of Berlin; 2007.
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Photosynthesis in a test tube- dye sensitized solar cells as a teaching tool Atul Raturi*, Yoheni Fepuleai Division of Physics, School of Engineering and Physics, The University of the South Pacific, Suva, Fiji
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 January 2009 Accepted 26 October 2009 Available online 14 November 2009
Dye sensitized solar cells employing natural plant dyes as phosensitizers can be effectively used to train students in the science and technology of solar cells. This is especially relevant to developing countries where facilities for silicon cell fabrication are non-existent. The cross-disciplinary nature of this device makes it very attractive for student projects. The present work describes such a project where anthocyanin dye from hibiscus flowers has been used as the electron harvester. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Dye sensitized solar cells Natural dyes
1. Introduction The high dependence on imported fuel and lack of access to electricity make renewable energy (RE) the most suitable source of energy for a multitude of people living in the Pacific Island Countries (PICs). A large segment of the population lives on remote islands where national grid extension is not a possibility and RE based distributed generation systems could be the only solution to alleviate the energy poverty among these people. A number of RE projects are being developed in the region. Some of them are: Fiji: 100% RE based electrification by 2012 - a 12 MW windfarm was commissioned in 2007. Marshall Islands: 2 Mini-grid systems. Nauru: Wind and other RE to supply 10% of total demand. Tonga: Outer Islands Solar Electrification programme Papua New Guinea (PNG): Solar based rural electrification Tuvalu: wind and solar energy development PNG, Fiji, Solomon Islands etc.: Biofuel development (Bioethanol, Biodiesel) All PICs: Hybrid (wind-solar) systems for the communication sector.
training in renewable energy technologies. There are a number of courses being offered at undergraduate and graduate levels. In the case of photovoltaic technology, it has not been possible to set-up laboratory facilities to fabricate solar cells due to the high costs involved. However, dye sensitized solar cells (DSSC) or Gratzel cells offer an opportunity to fabricate solar cells without the need of expensive equipment [1]. Using synthetic ruthenium-based dyes efficiencies larger than 10% have been achieved [2] and a technology road map for DSSCs aims to develop cells with 20% efficiency in the next 10 years [3]. The high cost of these dyes remains an issue and efforts are underway to utilize cheaper, natural dyes. Anthocyanin dyes found in berries, flowers and other plant materials have emerged as possible alternatives [4–7]. It is possible to fabricate low-cost solar cells with these dyes acting as photon absorbers and electron generators. There is a lot of interest in this cross-disciplinary field, which combines studies in Physics (semiconductors), biology (plant dyes) and chemistry (electrochemistry), together with energy generation. Student projects based on this type of solar cells complement our RE teaching very well and expose the students to the science behind PV devices. This paper describes one such project where anthocyanin dye from Hibiscus flowers was used as the sensitizer. 2. Dye sensitized solar cells
One of the important requirements for the success of any renewable energy programme is the availability of trained manpower to develop, design, construct, and maintain RE systems. The University of South pacific, a regional university serving 12 pacific island countries is a main provider of education and
* Corresponding author. Tel.: þ679 3232430. E-mail address: [email protected] (A. Raturi). 0960-1481/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2009.10.035
A plant leaf can be thought to be made of millions of tiny photoelectrochemical cells, which utilize solar energy to convert CO2 and water into carbohydrates and oxygen. This involves a redox reaction of excited chlorophyll molecules. There have been many attempts to replicate this process artificially with very limited success. Seminal work done by Gratzel and co-workers in the late 80’s and early 90’s, resulted in a highly efficient photovoltaic device
A. Raturi, Y. Fepuleai / Renewable Energy 35 (2010) 1010–1013
(DSSC) that mimics the natural photosynthesis process. The most important step was to use an organic dye coated nanocrystalline titanium dioxide electrode as the photoanode. The nanocrystalline nature of the electrode increases the surface area of the photon harvester. One of the main differences between a DSSC and a conventional solar cell is that in DSSC, photon absorption and charge carrier transport processes are separated. Organic dyes are excited by the absorption of a photon and inject an electron into a porous (nanocrystalline) wide band -gap semiconductor (usually TiO2). This electron can be extracted by a transparent conducting oxide electrode and then flows through an external circuit to a second electrode called the counter electrode. The two electrodes are separated by a redox electrolyte (iodide–polyiodide). Oxidation and reduction reactions occur at the anode and the cathode respectively while charge transport takes place through the electrolyte. A DSSC is a majority carrier device as opposed to conventional p-n junction solar cell. The absorption spectrum of the dye and the chelation of the dye at the surface of the TiO2 electrode determine the efficiency of the solar cell [4]. The best results so far have been obtained with electrodes sensitized with ruthenium-based dyes [7]. Dye sensitized solar cells have emerged, as a serious alternative to conventional silicon solar cells. The manufacturing process is low-cost and less energy consuming. There is a growing global interest in DSSCs which are on the on the verge of being fully commercialized [8]. They are suitable for Building Integrated PV (BIPV) systems. In Queensland, Australia DSSCs are being used for powering environmental monitoring sensors. 3. Experimental techniques 3.1. TiO2 films Titanium dioxide films were grown onto fluorine-doped tin oxide (FTO) coated glass slides using the constant current electrophoresis method [9]. The FTO glass slides were TEC15 supplied by Hartford Glass Co. Inc., USA. The electrophoresis set-up is shown in Fig. 1. 0.8 g of TiO2 (Degussa P25, USA) particles were dispersed in 300 ml of deionized water to form a colloidal solution. When a constant current was applied between the aluminium anode and the FTO cathode, a nanocrystalline thin film of TiO2 was deposited onto the FTO electrode. Films were grown using 3 different constant currents (0.60 mA, 0.30 mA and 0.15 mA) .The growth duration was 10 min for all currents and after deposition, as-grown films were annealed in air at 350 C for 10–15 min.
Fig. 1. Electrophoresis set-up.
1011
The transmission/absorption spectrum of blank and TiO2 coated FTO films were studied using a Lambda 25 UV/VIS spectrophotometer.
3.2. Dye extraction Anthocyanin dye was obtained from Red hibiscus (Hibiscus rosasinensis) a tropical flower belonging to Malvaceae family (Fig. 2). A total of 12 flowers were immersed in 100 ml of methanol to extract the red dye. The dye was filtered and stored in a dark bottle. The optical properties of the dye were studied using the spectrophotometer described above.
3.3. DSSC fabrication Photoanodes were prepared by dipping the TiO2 coated FTO slides in the anthocyanin dye solution. After approximately 10–15 min, the dye was absorbed by the TiO2 film (as evident by the change in the colour of the film). The dye stained film was washed in water and ethanol. After drying, the film was ready to be used as a photoanode. A counter electrode was prepared by coating an FTO film with the carbon soot from the luminous flame of a Bunsen burner. DSSCs were fabricated by using the dye coated TiO2/FTO plate as photoanode and the carbon coated FTO plate as the counter electrode. Few drops of iodide/polyiodide (0.5 M KI/0.05 I in ethylene glycol) electrolyte were inserted between the two plates .The cell was completed by clamping the two electrodes together. The cells were tested for open-circuit voltage, short-circuit current and I-V characteristics using the standard solar cell test circuit. The cells were tested under natural sunlight.
4. Results and discussion 4.1. FTO film characterization The fluorine-doped tin oxide films used in this work were measured to have a sheet resistance of w15 U/,. The direct band gap was determined by plotting the square of the absorption coefficient against photon energy and was found to be w3.48 eV Fig. 3.
Fig. 2. Hibiscus rosa-sinensis.
1012
A. Raturi, Y. Fepuleai / Renewable Energy 35 (2010) 1010–1013
Fig. 3. Direct band gap determination.
4.2. TiO2 film growth The best films, which were visually smooth and crack –free were grown at the lowest current value (0.15 mA, sample 3) and were used in the fabrication of the DSSC. Fig. 4 shows the transmission spectra of FTO films coated with TiO2. It is seen that the absorption edge moves towards the lower energy side when TiO2 is deposited over FTO. Rutile TiO2 has a smaller band gap of w3.03 eV [10] which is responsible for this shift. 4.3. Optical properties of the dye solution Fig. 5 shows the transmission and absorption spectrum of the anthocynin dye. The absorption spectrum is slightly different from that reported by Fernando and Senadera [6] for Hibiscus rosa-sinensis dye extracted using acidified ethanol. Their dye exhibited an absorption peak at w540 nm. Absorption of photons in the visible region is strongly dependent on the pH and increases in acidic solutions. 4.4. DSSC characterization Fig. 6 shows the I-V characteristics of the DSSC fabricated. The cell had an open-circuit voltage of 250 mV and a short-circuit current
Fig. 4. Transmission spectra of coated FTO slides. (1) Blank FTO, (2) FTO þ sample1 (current ¼ 0.60 mA), (3) FTO þ sample 3 (current ¼ 0.15 mA).
Fig. 5. Transmission and absorption spectra of the dye (a) Transmission (b) Absorption.
density of 0.31 mA/cm2 under an insolation of 490 W/m2. The characteristics exhibit a fill factor of 0.4 and an efficiency of w0.07% can be deduced. Although the values obtained are quite small, they do demonstrate the applicability of plant dyes for photovoltaic energy conversion. Not all dyes are suitable for DSSC application. The dye should be able to inject an electron into the conduction band of TiO2. This is possible if the energy level of the dye is 0.2–0.3 eV above the
Fig. 6. I-V characteristics of the DSSC.
A. Raturi, Y. Fepuleai / Renewable Energy 35 (2010) 1010–1013
conduction band edge of TiO2 [11]. Fernando and Senadeera [6] have reported an efficiency of 1.02% in DSSCs with Hibiscus rosasinensis sensitizer. The photoanode consisted of TiO2 paste (thickness w4–5 mm) coated FTO and acidified ethanol was used to extract the dye. One of the reasons for small efficiencies in our case could be the small TiO2 film thickness together with presence of pinholes. A low dye concentration and its low absorption in the visible range could also be a contributing factor. The carbon counter electrode was very fragile and should be replaced by a platinum electrode. We are currently studying the effect of growth parameters of TiO2 films in conjunction with a variety of local plant dyes on the DSSC characteristics. 5. Conclusions Fabrication and characterization of dye sensitized solar cells made with local plant dyes as photosensitizer can play a major role in photovoltaics technology education in developing countries. Students appreciate the interdisciplinary nature of the project and the hands-on training. It is envisaged that in not too distant future, small Pacific Island Countries can have their own DSSC modules manufacturing facilities.
1013
References [1] Regan BO, Gratzel M. A low-cost high efficiency solar cell based on dyesensitized colloidal TiO2 films. Nature 1991;353:737–40. [2] Green MA, Emery K, Hisikawa Y, Wilhelm. Solar cell efficiency tables(version 30). Prog Photovoltaics Res Appl 2007;15:425–30. [3] Pagliaro M, Palmisano G, Ciriminna R. Flexible solar cells. WILEY-VCH verlag GmbH & Co, ISBN 978-3-527-32375-3; 2008. [4] Tennakone K, Kumarasinghe AR, Kuma GRRA, Wijayantha K, Sirimanne PM. Nanoporous TiO2 photoanode sensitized with the flower pigment Cyanidin. J Photochem Photobiol 1997;108:128. [5] Yamazaki E, Murayama M, Nishikawa N, Hashimoto N, Shoyama M, Kurita O. Utilization of natural caroteniods as photosensitizers for dye sensitized solar cells. Solar Energy 2007;81:512–6. [6] Fernando JMRC, Senadeera GKR. Natural anthocyanins as photosensitizers for dye- sensitized solar devices. Curr Sci 2008;95:663–6. [7] Wongcharee K, Meeyoo V, Chavade S. Dye-sensitized solar cell using natural dyes extracted from rosella and blue pea flowers. Solar Energy Mater Solar Cells 2007;91:566–71. [8] Available at: http://dyesol.com. [9] Fujimura K, Yoshikodo S. Preparation of TiO2 film for dye sensitized solar cell deposited by electrophoresis method. Key Eng Mater 2003;248:133–6. [10] Jin Y, Li G, Zhang Y, Zhang Y, Zhan L. Photoluminescence of anatase TiO2 thin films achieved by the addition of ZnFe2O4. J Phys Condens Matter 2001;13:L913–8. [11] Matthias J. Novel aqueous electrolyte films for hole conduction in dye. Sensitized solar cells and development of an electron transport model. PhD. Thesis. Free University of Berlin; 2007.