Flexible dye-sensitized nanocrystalline TiO2 solar .Flexible Dye-Sensitized Nanocrystalline TiO2
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Flexible Dye-Sensitized NanocrystallineTiO2 Solar Cells
P.M. (Paul) Sommeling, Martin Spth, Jan Kroon, Ronald Kinderman, John van RoosmalenECN Solar Energy, P.O. Box 1, 1755 ZG Petten, The Netherlands,
tel. +31 224 564276, fax. +31 224 563214, email: email@example.com.
ABSTRACT: The Dye-sensitized nanocrystaline TiO2 solar cell (nc-DSC) developed by Grtzel  has the potentialto reach low costs in future outdoor power applications. In addition, due to its expected ease of production and possibilities toadjust its appearance, the potential for application is very broad. When plastic foil is used as a substrate for the nc-DSC theproduction and application possibilities are even bigger. Polymer foils are easier to handle in processing steps such as cutting oflarger entities into smaller individual modules. Moreover the processing of foil into complete flexible solar cells could be realizedby means of a continuous roll to roll proces. This implies that for the flexible nc-DSC a higher throughput in production could berealized. Also special applications requiring a certain flexibility of the solar cell, such as smart cards, come into play. In this paperthe differences in cell performance and -stability between glass based nc-DSC's and polymer foil based nc-DSC's are studied. TheITO coating on the polymer foil appears to be a critical factor with respect to the photoelectrode stability. TiO2 sintered at lowtemperatures (150 C) does not influence the cell stability in a negative way.Keywords:TiO2 - 1: Plastic - 2: Stability - 3.
A new type of solar cell based on dye-sensitized nanocrystalline titanium dioxide has been developed by M.Grtzel and coworkers [1,2]. Remarkably high quantumefficiencies have been reported for this type of solar cell (aso called nc-DSC), with overall conversion efficiencies upto 11 % . This fact , in combination with the expectedrelatively easy and low cost manufacturing makes this newtechnology an interesting alternative for existing solar celltechnologies.Realisation of stable efficiencies in the order of 10 % inproduction, however , requires a lot of effort on theresearch and development side. For this reason, the firstapplication of this type of solar cell will probably be one inwhich only a low power output is required, since this iseasier to achieve. Less stringent efficiency requirementsleave room for increased flexibility in the manufacturingprocess of these cells, i.e. the requirements that are put onthe materials used are less severe. This results in lowermanufacturing costs and more flexibility in materialschoice, opening the way for alternative productionprocesses.One alternative concept for the nc-DSC is based on theintroduction of polymer foils as a substrate instead of glass.This opens the way to attractive roll to roll productionprocesses and special applications in which flexibility ofthe solar cell is to some extent required. Moreoverprocessing and handling of polymer foil devices in generalis easier than for the glass version especially when it comesto cutting of glass or drilling of holes in glass substrates.The aim of this study is to investigate the criticalparameters regarding the stability of the nc-DSC's onpolymer foil in comparison to the glass version. Aprototype of a flexible DSC was already demonstrated in1998 and is shown in Fig.1. The aim of this paper is tostudy the stability and performance of nc-DSC's onpolymer foils in comprison to nc-DSC's on glass.
Figure 1: A 4 cell module of a flexible nc-DSC
2 WORKING PRINCIPLE
nc-DSCs are based on a wide bandgap semiconductor,usually TiO2, which is sensitized for visible light by amonolayer of adsorbed dye. The most frequently used dyeis, for reasons of best efficiencies, cis-(NCS)2bis(4,4-dicarboxy-2,2bipyridine)-ruthenium(II).The photoelectrode in such a device consists of ananoporous TiO2 film (approx. 10m thick) deposited on alayer of transparant conducting oxide (TCO, usuallySnO2:F) on glass (Fig.2). The counter electrode alsoconsists of TCO coated glass on which a small amount ofplatinum catalyst is deposited.
Figure 2: Working principle of a nc-DSC.
In a complete cell, photo- and counter electrode areclamped together and the space between the electrodes andthe voids between the TiO2 particles are filled with anelectrolyte. This electrolyte consists of an organic solventcontaining a redox couple, usually iodide/triiodide (I-/I3-).A dye monolayer on a flat surface absorbs less than 1 % ofthe incoming light (one sun conditions). To obtainreasonable efficiencies comparable to established solar celltechnologies, in the nc-DSC the surface area is enlarged bya factor of 1000, by using nanoparticles of TiO2 with adiameter of approximately 10-20 nm.The working principle of the nc-DSC is based on excitationof the dye followed by fast electron injection into theconduction band of the TiO2, leaving an oxidized dyemolecule on the TiO2 surface. Injected electrons percolatethrough the TiO2 and are fed into the external circuit. Atthe counter electrode, triiodide is reduced to iodide bymetallic platinum under uptake of electrons from theexternal circuit:
I3- + 2e- --> 3I-
Iodide is transported through the electrolyte towards thephotoelectrode, where it reduces the oxidized dye. The dyemolecule is then ready for the next excitation/oxidation/reduction cycle.
3. MANUFACTURING OF nc-DSC's
3.1 Manufacturing of nc-DSCs on glassIn this section the basic manufacturing technology for
nc-DSCs on glass substrates is described. The first processstep consists of the preparation of the photo electrode bydeposition of a layer of nanocrystalline titanium dioxideonto the SnO2:F coated glass substrate. On an industrialscale this can be realized by means of screen printing of anink, containing a TiO2 colloidal suspension. This layer isdried and sintered in a furnace at 450-550 C to eliminateorganic residues of the screen print medium and toestablish electrical contact between the TiO2 particles.After cooling down of the photoelectrode, it is stained byemerging the electrode in a solution of organic dye for a
certain period of time.After drying the stained photoelectrode is assembled withthe platinized counter electrode and a thermoplastic thinfilm in between (not on the active cell surface). The twoelectrodes are sealed together by heating up to 150C andapplying pressure.The cell is completed by filling with electrolyte throughsmall prefabricated holes in the counter electrode. Theelectrolyte is spread in the very small space between theelectrodes (approx. 10m) by capillary forces. Finally, theholes are sealed with a thermoplastic film and coverglasses.
3.2 nc-DSC's on polymer foilWhen 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 meansto achieve high throughput. Moreover, alternative sealingtechnology can be used. ECN has developed a method for thefilling of a nc-DSC with electrolyte without the use of afilling hole, which is required in the full glass version .This strongly facilitates the manufacturing of nc-DSC's.Some of the drawbacks of polymer foils are the fact thatpolymers do not resist high temperatures and that thesematerials are permeable for water and oxygen. This hasimmediate consequences for the solar cell processingparameters and for the performance and stability. The mainconsequences of using a polymer foil substrate instead ofglass are the following:
1. TCOSnO2:F cannot be applied to polymer foils because of highprocessing temperatures; room temperature sputtered indiumtin oxide (ITO) can be used instead.2. Sintering of TiO2.In the glass based manufacturing process TiO2 isscreenprinted using a paste which contains substantialamounts of organic additives that have to be burned out at450C. This procedure cannot be used for the manufacturingof the polymer foil solar cell, because organic residuescannot be removed effectively at temperatures as low as150C. Aqueous TiO2 paste without organic surfactantsshould be used. Sintering temperatures as low as 150C aresufficient to produce mechanically stable TiO2 films.3. Platinum.Platinum cannot be deposited using a thermal platinizationprocess with a platinum salt as precursor. Instead, Pt can bedeposited using a galvanic process or by room temperaturesputtering.4. Polymer foils are not gastight, oxygen and water vapor canpermeate through the polymer into the solar cell.
4. EXPERIMENTAL METHODS
4.1 Cell preparationPhoto electrodes have been prepared both on glass
substrates and on polymer foil, i.e. polyethyleneterephtalate(PET), by means of doctor blading . The titaniumdioxide paste used in this process has been developed forsintering at lower temperatures (150 C) but has also beenapplied to the electrodes on glass substrates which have
been sintered at 450 C. Titanium dioxide pastes werebased on in house synthesized colloid. The latter materialwas synthesized by hydrolysis of an organic titaniumprecursor according to the procedure described in literature.The colloid was processed to produce a titanium dioxidepaste that can be deposited onto glass or polymer foilsubstrates.Several sintering temperatures have been applied to thephotoelectrodes: 150 C for the polymer foil electrodes,150 C and 450 C for the glass electrodes.After sintering for 30 minutes the photoelectrodes havebeen immersed overnight in a solution of 3*10-4 M cis-(NCS)2bis(4,4-dicarboxy-2,2bipyridine)-ruthenium(II) inethanol.Counter electrodes have been prepared by deposition of athin layer of a 150 mM solution of H2PtCl6 in isopropanol