Flexible Dye-Sensitized Nanocrystalline TiO2 Solar Cells P.M. (Paul) Sommeling, Martin Späth, Jan Kroon, Ronald Kinderman, John van Roosmalen ECN Solar Energy, P.O. Box 1, 1755 ZG Petten, The Netherlands, tel. +31 224 564276, fax. +31 224 563214, email: sommeling@ecn.nl. ABSTRACT: The Dye-sensitized nanocrystaline TiO 2 solar cell (nc-dsc) developed by Grätzel [1] has the potential to reach low costs in future outdoor power applications. In addition, due to its expected ease of production and possibilities to adjust its appearance, the potential for application is very broad. When plastic foil is used as a substrate for the nc-dsc the production and application possibilities are even bigger. Polymer foils are easier to handle in processing steps such as cutting of larger entities into smaller individual modules. Moreover the processing of foil into complete flexible solar cells could be realized by means of a continuous roll to roll proces. This implies that for the flexible nc-dsc a higher throughput in production could be realized. Also special applications requiring a certain flexibility of the solar cell, such as smart cards, come into play. In this paper the differences in cell performance and -stability between glass based nc-dsc's and polymer foil based nc-dsc's are studied. The ITO coating on the polymer foil appears to be a critical factor with respect to the photoelectrode stability. TiO 2 sintered at low temperatures (150 C) does not influence the cell stability in a negative way. Keywords:TiO 2-1: Plastic - 2: Stability - 3. 1 INTRODUCTION A new type of solar cell based on dye-sensitized nanocrystalline titanium dioxide has been developed by M. Grätzel and coworkers [1,2]. Remarkably high quantum efficiencies have been reported for this type of solar cell (a so called nc-dsc), with overall conversion efficiencies up to 11 % [3]. This fact, in combination with the expected relatively easy and low cost manufacturing makes this new technology an interesting alternative for existing solar cell technologies. Realisation of stable efficiencies in the order of 10 % in production, however, requires a lot of effort on the research and development side. For this reason, the first application of this type of solar cell will probably be one in which only a low power output is required, since this is easier to achieve. Less stringent efficiency requirements leave room for increased flexibility in the manufacturing process of these cells, i.e. the requirements that are put on the materials used are less severe. This results in lower manufacturing costs and more flexibility in materials choice, opening the way for alternative production processes. One alternative concept for the nc-dsc is based on the introduction of polymer foils as a substrate instead of glass. This opens the way to attractive roll to roll production processes and special applications in which flexibility of the solar cell is to some extent required. Moreover processing and handling of polymer foil devices in general is easier than for the glass version especially when it comes to cutting of glass or drilling of holes in glass substrates. The aim of this study is to investigate the critical parameters regarding the stability of the nc-dsc's on polymer foil in comparison to the glass version. A prototype of a flexible DSC was already demonstrated in 1998 and is shown in Fig.1. The aim of this paper is to study the stability and performance of nc-dsc's on polymer foils in comprison to nc-dsc's on glass. Figure 1: A 4 cell module of a flexible nc-dsc 2 WORKING PRINCIPLE nc-dsc s are based on a wide bandgap semiconductor, usually TiO 2, which is sensitized for visible light by a monolayer of adsorbed dye. The most frequently used dye is, for reasons of best efficiencies, cis-(ncs) 2 bis(4,4 - dicarboxy-2,2 bipyridine)-ruthenium(ii). The photoelectrode in such a device consists of a nanoporous TiO 2 film (approx. 10µm thick) deposited on a layer of transparant conducting oxide (TCO, usually SnO 2 :F) on glass (Fig.2). The counter electrode also consists of TCO coated glass on which a small amount of platinum catalyst is deposited.
Figure 2: Working principle of a nc-dsc. In a complete cell, photo- and counter electrode are clamped together and the space between the electrodes and the voids between the TiO 2 particles are filled with an electrolyte. This electrolyte consists of an organic solvent containing a redox couple, usually iodide/triiodide (I - /I 3 - ). A dye monolayer on a flat surface absorbs less than 1 % of the incoming light (one sun conditions). To obtain reasonable efficiencies comparable to established solar cell technologies, in the nc-dsc the surface area is enlarged by a factor of 1000, by using nanoparticles of TiO 2 with a diameter of approximately 10-20 nm. The working principle of the nc-dsc is based on excitation of the dye followed by fast electron injection into the conduction band of the TiO 2, leaving an oxidized dye molecule on the TiO 2 surface. Injected electrons percolate through the TiO 2 and are fed into the external circuit. At the counter electrode, triiodide is reduced to iodide by metallic platinum under uptake of electrons from the external circuit: I 3 - + 2e - --> 3I - counter electrode TiO 2 Iodide is transported through the electrolyte towards the photoelectrode, where it reduces the oxidized dye. The dye molecule is then ready for the next excitation/oxidation/ reduction cycle. 3. MANUFACTURING OF nc-dsc's e - S photoelectrode I 3 - e - I 3 - S* glass TCO TCO glass 3.1 Manufacturing of nc-dsc s on glass In this section the basic manufacturing technology for nc-dsc s on glass substrates is described. The first process step consists of the preparation of the photo electrode by deposition of a layer of nanocrystalline titanium dioxide onto the SnO 2 :F coated glass substrate. On an industrial scale this can be realized by means of screen printing of an ink, containing a TiO 2 colloidal suspension. This layer is dried and sintered in a furnace at 450-550 C to eliminate organic residues of the screen print medium and to establish electrical contact between the TiO 2 particles. After cooling down of the photoelectrode, it is stained by emerging the electrode in a solution of organic dye for a S + light 3I - 3I - Pt dye certain period of time. After drying the stained photoelectrode is assembled with the platinized counter electrode and a thermoplastic thin film in between (not on the active cell surface). The two electrodes are sealed together by heating up to 150 C and applying pressure. The cell is completed by filling with electrolyte through small prefabricated holes in the counter electrode. The electrolyte is spread in the very small space between the electrodes (approx. 10μm) by capillary forces. Finally, the holes are sealed with a thermoplastic film and cover glasses. 3.2 nc-dsc's on polymer foil When a polymer foil is used as a substrate for nc- DCS's instead of glass, the production process is different. Polymer foils allow roll to roll production which is a means to achieve high throughput. Moreover, alternative sealing technology can be used. ECN has developed a method for the filling of a nc-dsc with electrolyte without the use of a filling hole, which is required in the full glass version [4]. This strongly facilitates the manufacturing of nc-dsc's. Some of the drawbacks of polymer foils are the fact that polymers do not resist high temperatures and that these materials are permeable for water and oxygen. This has immediate consequences for the solar cell processing parameters and for the performance and stability. The main consequences of using a polymer foil substrate instead of glass are the following: 1. TCO SnO 2 :F cannot be applied to polymer foils because of high processing temperatures; room temperature sputtered indium tin oxide (ITO) can be used instead. 2. Sintering of TiO 2. In the glass based manufacturing process TiO 2 is screenprinted using a paste which contains substantial amounts of organic additives that have to be burned out at 450 C. This procedure cannot be used for the manufacturing of the polymer foil solar cell, because organic residues cannot be removed effectively at temperatures as low as 150 C. Aqueous TiO 2 paste without organic surfactants should be used. Sintering temperatures as low as 150 C are sufficient to produce mechanically stable TiO 2 films. 3. Platinum. Platinum cannot be deposited using a thermal platinization process with a platinum salt as precursor. Instead, Pt can be deposited using a galvanic process or by room temperature sputtering. 4. Polymer foils are not gastight, oxygen and water vapor can permeate through the polymer into the solar cell. 4. EXPERIMENTAL METHODS 4.1 Cell preparation Photo electrodes have been prepared both on glass substrates and on polymer foil, i.e. polyethyleneterephtalate (PET), by means of doctor blading [2]. The titanium dioxide paste used in this process has been developed for sintering at lower temperatures (150 C) but has also been applied to the electrodes on glass substrates which have
been sintered at 450 C. Titanium dioxide pastes were based on in house synthesized colloid. The latter material was synthesized by hydrolysis of an organic titanium precursor according to the procedure described in literature [1]. The colloid was processed to produce a titanium dioxide paste that can be deposited onto glass or polymer foil substrates. Several sintering temperatures have been applied to the photoelectrodes: 150 C for the polymer foil electrodes, 150 C and 450 C for the glass electrodes. After sintering for 30 minutes the photoelectrodes have been immersed overnight in a solution of 3*10-4 M cis- (NCS) 2 bis(4,4 -dicarboxy-2,2 bipyridine)-ruthenium(ii) in ethanol. Counter electrodes have been prepared by deposition of a thin layer of a 150 mm solution of H 2 PtCl 6 in isopropanol and subsequent heating at 450 C for 30 minutes. Temperature sensitive polymer foil counter electrodes have been made by a galvanic platinisation process. Cell assembly has been carried out according to the procedure described in section 3.1. The electrolyte composition was as follows: 0.5 M LiI, 0.05 M I 2 and 0.4 M tertiair-butylpyridine in methoxypropionitrile. 4.2 Characterization and stability testing Cells have been characterized using fluorescent tube lamps with a light intensity of 50-5000 lux to mimic indoor artificial light conditions. IV characteristics have been determined with a Keithley 2400 source meter. Ageing has been carried out using a sulfur lamp. The irradiation intensity was approximately 2 sun. Cells have been under continuous illumination and IV characteristics of individual cells have been monitored with time. Light intensities have been measured using a lux meter (Licor 188B integrating quantum radiometer). 5. RESULTS AND DISCUSSION The items mentioned in section 3.2 will influence the efficiency and the stablity of the solar cells. Cell efficiency: TiO 2 sintered at 150 C results in less performing nc-dsc's than TiO 2 sintered at 450 C, though comparable efficiencies have been found for low temperature and high temperature sintering of TiO 2 in another study, with sintering times up to 48 h [5]. The open circuit voltage and fill factor are unaffected but the short circuit current is decreased to about half its value for TiO 2 sintered at 450 C. However, the polymer foil nc-dsc's exhibit a performance which is still enough to power indoor applications such as watches, calculators or smart cards. A typical IV characteristic of a nc- DSC on ITO-PET is given in figure 3. I (ua) 16 14 12 10 8 6 4 2 0 0 0.1 0.2 0.3 0.4 0.5 V (V) Figure 3: IV characteristic of a nc-dsc on PET at 250 lux At an illumination intensity of 250 lux, typical for indoor conditions, the following cell parameters have been determined: Voc: 0.48 V, Isc: 15μA/cm 2, FF: 67 %. Taking into account that the average power demand of a calculator is 10 μw, a 5 cm 2 sized polymer foil nc-dsc is able to produce this power down to a light intensity of 100 lux. It is possible to apply polymer foils which resist high temperature treatment. These materials however are temperature-stable because of the high content of polyaromatic systems which are more or less intensely colored. This filters out an important part of the light and results in lower power output. Cell stability: In practice the power output of a nc-dsc on (ITO) coated polyethelenterephtalate (PET) foil decreases with time. Though cell stability is not yet established and subject of study for all nc-dsc's, the decrease in performance of the nc- DSC on ITO-PET foil is substantially faster than on SnO 2 :Fglass. The following factors could play a role: 1. Instead of SnO 2 :F, sputtered ITO is used as a TCO. This specific material has not been applied earlier in nc-dsc's. The suitability of the material for application as a substrate for nc-dsc's is uncertain. 2. Effects of low temperature sintering on cell stability could be related to insufficient contact between TiO 2 particles, causing the nanoporous film to fall apart. 3. The ITO/polymer foil itself could influence the solar cell. Plasticizers or stabilizers could leach out of the polymer foil into the electrolyte or undesirable components could leach out of the ITO. These kinds of compounds could interfere with the delicate cell chemistry. Second, oxygen and water vapor can permeate through the polymer influencing the cell stability in a negative way. the latter problem could be avoided by application of barrier coatings to the polymer foil. 4. Galvanically deposited platinum on the counter electrode could degrade during cell operation. Efficiency and stability of nc-dsc's have been studied under continuous illumination by monitoring IV characteristics with time. Both glass and PET substrates have been applied, also combined in one nc-dsc, and TiO 2 films have been sintered at several temperatures. Table I gives an overview of the relevant manufacturing conditions for the nc-dsc's that have been tested.
Table I: Cell no. Photoelectr. Counter electr. sinter T ( C) 1 glass polymer 450 2 glass polymer 150 3 glass glass 150 4 glass glass 450 5 polymer polymer 150 Figures 4,5 and 6 represent the Jsc, Voc and FF with time for the various nc-dsc's, all measured under illumination of 2000 lux. The cells have been aged under illumination of approximately 2 sun. Figure 4: Short cicuit current vs time Figure 6: Fill factor vs time From these results the following conclusions can be drawn: 1. low temperature sintering of TiO 2 results in lower initial peformance but this performance is stable. The degradation of the polymer foil cells cannot be attributed to the low sinter temperature of the TiO 2. 2. The fact that the ITO-PET is in contact with the electrolyte of the cell and serves as a sealing on one side does not affect the cell stability. From this it can be derived that possible leaching of disturbing compounds from the ITO-PET does not play a role. More importantly, also permeation of water and oxygen does not seem to play a major role in degradation of the polymer nc-dsc's. 3. The application of galvanically deposited platinum on ITO as a catalyst does not affect the cell stability in a negative way. At this stage it can be concluded that the instability of the nc- DSC's on polymer foil is related to the photoelectrode exclusively. This could be due to the composition of the TiO 2 paste, which is strongly acidic. The sputtered ITO on PET is proven to be sensitive towards acidic media. However experiments using alkaline TiO 2 paste resulted in the same degradation behaviour. Polymer foil cells have been dissembled after degradation. It appeared that the TiO 2 film peeled off of the foil. Moreover the ITO coating of the photoelectrode had been subject to degradation; in several cases parts of the ITO coating had disappeared completely. This leads to the conclusion that the properties of the transparant conducting oxide in combination with a polymer foil are of crucial importance when it comes to manufacturing nc- DSC's on polymer foil. 6 CONCLUDING REMARKS Figure 5: Open circuit voltage vs time From figures 4-6 it can be derived that nc-dsc's with polymer photo-electrodes degraded within the time period of the measurements (3.5 months). As a reference nc-dsc's on SnO 2 :F coated glass have been used. The reference cells have been stable within the time period of testing. Interestingly the full glass cells containing 150 C sintered TiO 2 have been remarkably stable as well as hybrid cells, composed of a glass photo-electrode and an ITO-PET counter electrode. nc-dsc cells on polymer foil substrates can meet the demands required by calculators, watches etc. under typical indoor conditions. From the point of view of production technology and special applications, flexible solar cells are an attractive alternative to the existing rigid systems. The currently used substrate PET foil is a good candidate from the point of view of light transmission, though the sputtered ITO coating causes stability problems in the solar cells. The sintering of TiO 2 at temperatures as low as 150 C does not seem to have a negative influence on the
stability of the nc-dsc. This is an important advantage in cell production processes. The permeation of water through the polymer foil does not seem to be a factor influencing cell stability. REFERENCES [1] B.O Regan and M. Grätzel, Nature 353 (1991) 737 [2] M.K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry- Baker, E. Müller, P. Liska, N. Vlachopoulos and M. Grätzel, J. Am. Chem. Soc. 115 (1993) 6382 [3] M.A. Green, K. Emery, K. Bücher, D.L. King and S. Igari, Prog. Photovoltaics: Res. Appl. 6 (1998) 35-42 [4] Int. Pat. Method for manufacturing a liquid -containing photovoltaic element and element manufactured according to this method, A.C. Tip, M. Späth and P.M. Sommeling, appl. No PCT/NL99/00370 [5] F. Pichot, S. Ferrere, R.J. Pitts, B.A. Gregg, J. of the Electrochem. Society, 146 (11) 4324-4326 (1999) Acknowledgements This work has been carried out with financial support from the ECN Cooperation Funding Programme.