Flexible Dye-Sensitized NanocrystallineTiO2 Solar Cells

P.M. (Paul) Sommeling, Martin Späth, 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: sommeling@ecn.nl.

ABSTRACT: The Dye-sensitized nanocrystaline TiO2 solar cell (nc-DSC) developed by Grätzel [1] 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.

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 quantumefficiencies have been reported for this type of solar cell (aso called nc-DSC), with overall conversion efficiencies upto 11 % [3]. 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-DSC’s 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-2,2’bipyridine)-ruthenium(II).The photoelectrode in such a device consists of ananoporous TiO2 film (approx. 10µm 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-DSC’s on glassIn this section the basic manufacturing technology for

nc-DSC’s 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 150°C 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. 10μm) 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 [4].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 at450°C. This procedure cannot be used for the manufacturingof the polymer foil solar cell, because organic residuescannot be removed effectively at temperatures as low as150°C. Aqueous TiO2 paste without organic surfactantsshould be used. Sintering temperatures as low as 150°C 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 [2]. 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

TiO2

TCOglass

glassTCO

3I-

3I-

I3-

I3-

S S*S+

e-

Pt

e-

dyelight

counter electrode

photoelectrode

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[1].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-2,2’bipyridine)-ruthenium(II) inethanol.Counter electrodes have been prepared by deposition of athin layer of a 150 mM solution of H2PtCl6 in isopropanol and subsequent heating at 450°C for 30 minutes.Temperature sensitive polymer foil counter electrodes havebeen made by a galvanic platinisation process.Cell assembly has been carried out according to theprocedure described in section 3.1. The electrolytecomposition was as follows: 0.5 M LiI, 0.05 M I2 and 0.4M tertiair-butylpyridine in methoxypropionitrile.

4.2 Characterization and stability testingCells have been characterized using fluorescent tube

lamps with a light intensity of 50-5000 lux to mimic indoorartificial light conditions. IV characteristics have beendetermined with a Keithley 2400 source meter. Ageing hasbeen carried out using a sulfur lamp. The irradiationintensity was approximately 2 sun. Cells have been undercontinuous illumination and IV characteristics ofindividual cells have been monitored with time. Lightintensities have been measured using a lux meter (Licor188B integrating quantum radiometer).

5. RESULTS AND DISCUSSION

The items mentioned in section 3.2 will influencethe efficiency and the stablity of the solar cells.Cell efficiency:TiO2 sintered at 150 °C results in less performing nc-DSC'sthan TiO2 sintered at 450 °C, though comparable efficiencieshave been found for low temperature and high temperaturesintering of TiO2 in another study, with sintering times up to48 h [5]. The open circuit voltage and fill factor areunaffected but the short circuit current is decreased to abouthalf its value for TiO2 sintered at 450 °C. However, thepolymer foil nc-DSC's exhibit a performance which is stillenough 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.

0

2

4

6

8

10

12

14

16

0 0.1 0.2 0.3 0.4 0.5V (V)

I (uA

)

Figure 3: IV characteristic of a nc-DSC on PET at 250 lux

At an illumination intensity of 250 lux, typical for indoorconditions, the following cell parameters have beendetermined: Voc: 0.48 V, Isc: 15μA/cm2, FF: 67 %. Takinginto account that the average power demand of a calculator is10 μW, a 5 cm2 sized polymer foil nc-DSC is able to producethis power down to a light intensity of 100 lux. It is possibleto apply polymer foils which resist high temperaturetreatment. These materials however are temperature-stablebecause of the high content of polyaromatic systems whichare more or less intensely colored. This filters out animportant part of the light and results in lower power output.

Cell stability:In practice the power output of a nc-DSC on (ITO) coatedpolyethelenterephtalate (PET) foil decreases with time.Though cell stability is not yet established and subject ofstudy for all nc-DSC's, the decrease in performance of the nc-DSC on ITO-PET foil is substantially faster than on SnO2:F-glass. The following factors could play a role:

1. Instead of SnO2:F, sputtered ITO is used as a TCO. Thisspecific material has not been applied earlier in nc-DSC's.The suitability of the material for application as a substratefor nc-DSC's is uncertain.2. Effects of low temperature sintering on cell stability couldbe related to insufficient contact between TiO2 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 foilinto the electrolyte or undesirable components could leachout of the ITO. These kinds of compounds could interferewith the delicate cell chemistry. Second, oxygen and watervapor can permeate through the polymer influencing the cellstability in a negative way. the latter problem could beavoided by application of barrier coatings to the polymer foil.4. Galvanically deposited platinum on the counter electrodecould degrade during cell operation.

Efficiency and stability of nc-DSC's have been studied undercontinuous illumination by monitoring IV characteristicswith time. Both glass and PET substrates have been applied,also combined in one nc-DSC, and TiO2 films have beensintered at several temperatures.Table I gives an overview of the relevant manufacturingconditions 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 forthe various nc-DSC's, all measured under illumination of2000 lux. The cells have been aged under illumination ofapproximately 2 sun.

Figure 4: Short cicuit current vs time

Figure 5: Open circuit voltage vs time

From figures 4-6 it can be derived that nc-DSC's withpolymer photo-electrodes degraded within the time period ofthe measurements (3.5 months). As a reference nc-DSC's onSnO2:F coated glass have been used. The reference cells havebeen stable within the time period of testing. Interestingly thefull glass cells containing 150 °C sintered TiO2 have beenremarkably stable as well as hybrid cells, composed of a glassphoto-electrode and an ITO-PET counter electrode.

Figure 6: Fill factor vs time

From these results the following conclusions can be drawn:

1. low temperature sintering of TiO2 results in lower initialpeformance but this performance is stable. The degradationof the polymer foil cells cannot be attributed to the low sintertemperature of the TiO2.2. The fact that the ITO-PET is in contact with the electrolyteof the cell and serves as a sealing on one side does not affectthe cell stability. From this it can be derived that possibleleaching of disturbing compounds from the ITO-PET doesnot play a role. More importantly, also permeation of waterand oxygen does not seem to play a major role in degradationof the polymer nc-DSC's.3. The application of galvanically deposited platinum on ITOas a catalyst does not affect the cell stability in a negativeway.

At this stage it can be concluded that the instability of the nc-DSC's on polymer foil is related to the photoelectrodeexclusively. This could be due to the composition of the TiO2paste, which is strongly acidic. The sputtered ITO on PET isproven to be sensitive towards acidic media. However experi-ments using alkaline TiO2 paste resulted in the samedegradation behaviour.Polymer foil cells have been dissembled after degradation. Itappeared that the TiO2 film peeled off of the foil. Moreoverthe ITO coating of the photoelectrode had been subject todegradation; in several cases parts of the ITO coating haddisappeared completely.

This leads to the conclusion that the properties of the transpa-rant conducting oxide in combination with a polymer foil areof crucial importance when it comes to manufacturing nc-DSC's on polymer foil.

6 CONCLUDING REMARKS

nc-DSC cells on polymer foil substrates can meet thedemands required by calculators, watches etc. under typicalindoor conditions. From the point of view of productiontechnology and special applications, flexible solar cells arean attractive alternative to the existing rigid systems. Thecurrently used substrate PET foil is a good candidate fromthe point of view of light transmission, though thesputtered ITO coating causes stability problems in the solarcells. The sintering of TiO2 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 incell production processes. The permeation of water throughthe polymer foil does not seem to be a factor influencingcell 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 -containingphotovoltaic element and element manufactured accordingto 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 theElectrochem. Society, 146 (11) 4324-4326 (1999)

Acknowledgements

This work has been carried out with financial support fromthe ECN Cooperation Funding Programme.