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Journal of Electrostatics 68 (2010) 205e211

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Journal of Electrostatics

journal homepage: www.elsevier .com/locate/elstat

Review

A nano-porous TiO2 thin film coating method for dye sensitized solar cells (DSSCs)using electrostatic spraying with dye solution

Ji-Tae Hong a, Hyunwoong Seo a, Dong-Gil Lee a, Jin-Ju Jang a, Tae-Pung An b, Hee-Je Kim a,*

aDepartment of Electrical Engineering, Pusan National University, Jangjeon, Geumjeong, Busan 609-735, Republic of Koreab ENTEC E&E Co., LTD 78-2 Buncheon-Ri, Bongdam-Eup, Hwaseong-City, Gyungki-Do 445-894, Republic of Korea

a r t i c l e i n f o

Article history:Received 6 October 2009Received in revised form31 December 2009Accepted 16 February 2010Available online 10 March 2010

Keywords:Electrostatic sprayingDye sensitized solar cellsDye coloring processDyeing capacity

* Corresponding author. Tel.: þ82 51 510 2770; faxE-mail address: heeje@pusan.ac.kr (H.-J. Kim).

0304-3886/$ e see front matter Crown Copyright � 2doi:10.1016/j.elstat.2010.02.002

a b s t r a c t

The dye coloring process on a nano-porous TiO2 thin film for DSSCs was studied using an electrostaticspraying (ESS) method. In this study, dye coating experiments were performed using homemade ESSequipment. The coating patterns on the TiO2 thin film are changed by adjusting the applied voltages ofthe ESS system. The geometry of the coating patterns is observed by a charge-coupled device (CCD)camera. The TiO2 thin film is fabricated by controlling the electric field which allows a uniform colordistribution to be obtained with a high dyeing capacity. The colored TiO2 thin films are compared withthose obtained using a conventional immersed dye solution by means of an UVevis spectrometer. Theperformance of this novel coloring process was confirmed by measuring IeV curve characteristics.

Crown Copyright � 2010 Published by Elsevier B.V. All rights reserved.

1. Introduction

DSSCs are expected to be one of the next generation of solar cellsbecause of their low fabrication cost, simple structure and use ofraw materials [1], as well as the fact that they operate on the sameprinciple as photosynthesis in plants [2]. In DSSCs, an electron ismoved from the excited dye acting as a sensitizer to the conductionband of a metal oxide. Then, the negative charge diffuses outside ofthe transparent conductive oxide (TCO) glasses because of theexcess concentration of electrons in the conduction band ofthe metal oxide [1e3]. A maximum efficiency of 11% was achievedfor DSSCs by Grätzel et al. [4].

A DSSC consists of a dye colored metal oxide, electrolyte andPt catalyst between two TCO plates forming a sandwich structure.The fabrication process of DSSCs is divided into three processes,viz. the photo-electrode fabrication process, counter electrodefabrication process, and the joining of the electrodes and elec-trolyte injection [1e5]. The photo-electrode fabrication processtakes the longest time, because of the dye coloring time (morethan 20 h) on the metal oxide thin films. Photo-electrodes withcolloidal TiO2 paste are widely used as the nanostructure ofthe metal oxide and N719 dye solution is used as a sensitizer. Thebeginning of the photo-electrode fabrication process involves the

: þ82 51 513 0212.

010 Published by Elsevier B.V. All

printing of the TiO2 paste by the screen-printing or doctor-blademethod. Then, the TiO2 paste is sintered at 450 �C for 1 h to makea nanostructure which has the anatase phase. The sintered TiO2film is slowly cooled down to 100 �C and then immersed in thedye solution at room temperature for about 24 h [6]. This processrequires about 30 h. Therefore, a novel process of photo-electrodefabrication is needed. In this study, the ESS method is adopted asa dye coloring process for the purpose of reducing the fabricationtime.

The electro-hydro dynamics of the ESS method, which is relatedto the applied power at the droplets, the movement of the trajec-tories and the coating patterns of the droplets in the electric field,are described in [7e11]. Electro-hydro dynamics is widely used inmany industries, including micro- and nano- sized industrialapplications. ESS systems can make micro- and nano- sized drop-lets or particles and they have a very high transfer efficiency(capillary to target) of 75e85%, whereas conventional spraysystems have an efficiency as low as 20e30% [12]. Due to theirperformance, ESS systems are widely used in coating industriesinvolving thin film fiber fabrication, automobile coloring, anti-T.B.and so on. In general, the cone-jet mode is used as the coatingmethod of the ESS system. At this time, the liquid type dye orparticles are attached to the substrate surface, but do not penetrateinside it. In the dye coloring of the photo-electrode of DSSCs, it isnecessary for the liquid dye to be absorbed in the porous TiO2 thinfilm layer in order to achieve the best light harvesting. Therefore,

rights reserved.

Fig. 1. The controlled electric field at various elapsed times versus the applied voltage for the coloring process of DSSCs.

J.-T. Hong et al. / Journal of Electrostatics 68 (2010) 205e211206

micro-dripping and cone-jet mode controls are needed in an ESSsystem to improve the dyeing capacity.

2. Experiments

In the ESS method, the voltage of the circuit is adjusted toachieve micro-dripping and to activate the cone-jet mode of theESS system. The details of the ESS condition are described inchapter 2.1. Each coated cell is immersed in ethanol (99.9%) for10 min to remove the extra dye that accumulates. UVevis (V-570,JASCO) spectroscopy is used to measure the performance of dyeabsorption.

In this study, the two modes (micro-dripping and cone-jet) arecontrolled by adjusting the operating voltage of the ESS system andcompared with the single micro-dripping and cone-jet modes.Fig. 1 show the controlling electric field obtained at various elapsedtimes versus the applied voltage. In these experiments, the cone-jetmode appears in the high voltage region of approximately 6 kV. Theconcept of these experiments is used the phenomenon of themicro-dripping and cone-jet modes. In the micro-dripping mode,the dye solution is supplied to the TiO2 thin film surface (region I).

Fig. 2. Schematic diagram of th

While the dye solution is supplied to the TiO2 surface, the evapo-ration of ethanol continuously occurs due to the heat of the TiO2surface. The dye concentration of the droplet is raised by theevaporation process and, then, other droplets are ejected from thenozzle. If the micro-dripping mode is switched to the cone-jetmode (region II), the particle size becomes very small. The ethanolin the droplet evaporates and, then, the droplet consists almostentirely of dye particles when it reaches the porous TiO2 thin filmsurface. The electric field becomes higher and this helps theabsorption of the droplet on the TiO2 surface. During this process,the dye solution turns to solid states which are supplying dropletson the TiO2 surface continuously and the dye concentration isincreased. The droplet affects the electric field and is absorbed inthe TiO2 thin film. During the absorption process, the droplet issituated between the porous TiO2 layers with near solid state by theheating. The high electric field causes the ethanol to evaporate, thusleaving the colored TiO2 film (region III) in the dry state. The electricfield is lowered for the next step (region IV). These processes arerepeated for several minutes to achieve a higher dyeing capacity.

ESS colored DSSCs (Active area: 0.25㎠) were fabricated tomeasure the dyeing capacity performance. The DSSC fabrication

e ESS dye coloring system.

Fig. 3. Schematic diagram of the ESS circuit.

J.-T. Hong et al. / Journal of Electrostatics 68 (2010) 205e211 207

process uses the conventional method [1] and compares with theESS dye coloring method. The TiO2 paste used for the screen-printing step is prepared bymixing TiO2 nano powder (Degussa P25)with a-terpinol. Nano crystalline TiO2 is coated on the FTO glasssubstrate having an electric conductivity of 9.3 U/square and anoptical transmittance of 80% in the visible region. The nano-porousTiO2 film is fabricated using the doctor-blademethod. The TiO2 pasteused in this process should be sintered at 450 �C for 2 h in air (filmthickness: 10e12 mm). The electrolyte composition is iodolyte AN-50(iodide based low viscosity electrolyte with tri-iodide). Hot meltsealing sheets (SX 1170-60, 60 micron thick foil) are used as theprimary sealing materials.

The prepared, porous TiO2 thin film is colored by the conven-tional soaking and ESS methods. The dye solution is prepared ata concentration of 0.5 mM in ethyl alcohol. In the conventional dyecoloringmethod, the TiO2 film is soaked in the dye solution at a filmtemperature of 110 �C. The soaked film is kept at room temperatureduring 24 h.

2.1. Preparation of the ESS system

A diagram of our dye coloring system with the homemade ESScircuit is shown in Fig. 2. The ESS system consists of a homemade

Fig. 4. Schematic diagram of the ESS phenomenon (a) the micro-drip

high voltage DC power supply (ESS circuit), a syringe pump(KDS100, KD scientific) to supply a fixed quantity of dye solution,a circular iron plate as a negative electrode, a nozzle as a positiveelectrode (diameter: 0.1 mm) and a hot plate (KDS100, KRAnalytical) to heat the glass in order to evaporate the dye solution.The TiO2 coated FTO glass is on the negative electrode of the ESSsystem. The dye solution is prepared using ethyl alcohol (99.9%,Aldrich) with 0.5 mM N719 dye (Ruthenium 535 bis TBA, Solar-onix). To evaporate the ethanol, the hot plate temperature is fixedat 45 �C. The gap size between the two-electrodes (h) is fixed at6 cm. The syringe pump injection rate is 5 mL/h.

2.2. The ESS circuit design

Fig. 3 shows the circuit of the homemade ESS system. The ESScircuit can supply amaximumvoltage of 20 kV, a maximum currentof 200 mA and a variable frequency (90e200 kHz). The circuitconsists of a high power resonant inverter with a half-bridgeconfiguration, a high frequency step-up transformer and a highvoltage rectifier using a CockcrofteWalton voltage multiplier. Thehigh voltage DC supply includes a two-stage voltage multiplierwhich is driven by a regulated half-bridge inverter operating belowthe resonance frequency. The inverter drives the step-up

ping mode (b) the cone-jet mode (c) the shape of the droplets.

Fig. 5. The dye coloring mechanism of the ESS system in the micro-dripping mode (a) and the cone-jet mode (b).

J.-T. Hong et al. / Journal of Electrostatics 68 (2010) 205e211208

transformer and the secondary side of the transformer is applied bythe above voltage multiplier. This combination guarantees a smallparasitic capacitance in the transformer stage and a fast dynamicresponse.

The purpose of this circuit is to control the ESS jet mode whichhas a variable high voltage in the range of 5e10 kV. The outputpower of the ESS circuit (ZCS LC series resonant circuit) is

Pout ¼ 2fCV2in (1)

Where (f) is the operating frequency, (C) is the capacitance of thecharging capacitor (5 kV, 1000 pF), and (Vin) is the input voltage.According to Eq. (1), there are two ways to control the power ofthe ESS circuit. One is to vary the input peak voltage (Vin) and theother is to adjust the drop voltage by controlling operatingfrequency (f) at a constant pulse width (duty: 50%). For thecontrol of the voltage, the input voltage (Vin) is fixed at 330VDC(regulated at 220 V AC), and the operating frequency (f) isadjusted. A microprocessor is adopted to control the voltage andoperating frequency of the ESS circuit. The voltage is changedfrom 5 to 10 kV by adjusting the operation frequency from 90 to110 kHz. The power of the ESS circuit is adjusted by changing theoperating frequency (f).

Table 1Constants proposed for Eq. (2) to determine droplet size[13].

Authors aQ a3 ar as ag

Fernandez de la Mora and Losccrtales [14] 0.3 0.3 0 0 0.3Gañan-Calvo [15] and Gañan-Calvo et al. [16] 0.5 0.15 0.15 0.15 0.15Hartman et al. [17] 0.5 0.15 0.15 0.15 0.15

2.3. Atomization of the dye solution & the dye coloring mechanism

In the ESSmethod, the power of the electric field is related to thevelocity of the droplets and the repulsive power of the samepolarity ions. Also, the volume of the droplets decreases withincreasing strength of the applied electric field [13e15]. The dyecoloring mechanism can be considered as the effectiveness thata side of the positive electrode and the other side of negativeelectrode. Fig. 4 shows the dye coloring mechanism using the ESSsystem at the positive electrode. When the nozzle is connected tothe positive electrode, the strength of the electric field and the flow

rate of the dye solution control the size of the droplets. The dropletssprayed from the nozzle are accelerated by the positive charge ofthe latter. The dye concentration of the sprayed droplets isincreased due to the evaporation of ethanol from the dye solution. Ifthe size of the sprayed droplets is big enough compared with themicro-dripping mode, they drop onto the TiO2 porous film in theliquid state, as shown in Fig. 4(b). However, if the particle size is toosmall (the cone-jet mode), droplets turn to almost solid state, asshown in Fig. 4(c).

On the circular plate used as the negative electrode, the posi-tively charged droplets are accelerated by the attractive force of thenegative charges. At the negative electrode, there are manyvariables affecting the dye coloring mechanism. The strength of theapplied electric field (the repulsive and attractive forces), the size ofdroplets dropped on the TiO2 surface and the temperature of theTiO2 film are themain factors affecting the dyeing capacity. Fig. 5(a)shows the dye coloring mechanism of the ESS system at thenegative electrode in the micro-dripping mode. An attractive forceexists between the grounded FTO glass plate and the dropletsdropped on the porous TiO2 thin film. This attractive force elevatesthe absorption strength on the TiO2 thin film. Besides, the dropletson the porous TiO2 film have much more absorption force, due tothe repulsive force from the droplets sprayed from the nozzle (thepositive electrode).

Fig. 5(b) shows the dye coloring mechanism using the ESSsystem at the negative electrode in the cone-jet mode. The dropletssprayed on the porous TiO2 film are almost solid state. Therefore,

Fig. 6. Schematic diagram of the evaporation of ethyl alcohol from the dye solution.

J.-T. Hong et al. / Journal of Electrostatics 68 (2010) 205e211 209

the droplets on the porous TiO2 thin film cannot be absorbed intothe TiO2 thin film, but are percolated on the TiO2 thin film surface.The droplet size in the micro-dripping mode can be measured bya high speed CCD camera. However, the droplet size in the cone-jetmode is usually given by the following equation [13e17]:

d ¼ aQaQ 3a30 rar1sas1 gag1

(2)

The constant a in Eq. (2) depends on the liquid permittivity.The dye solution consists almost entirely of ethyl alcohol. So, thepermittivity of the liquid is approximately 22.3 e j 6.0 F/m(25 �C, 300 MHz). The variables reported for the sprayingparameter vary depending on the author (cf. Table 1). Thisscaling law has been confirmed by trial and error for a single,

Fig. 7. The dye coloring pattern on the porous TiO2 film at applied v

coaxial jet and a jet generated within an insulating liquid.Nowadays, this equation is commonly accepted and widely usedin the literature.

The temperature of the TiO2 surface is one of the conditionsdetermining the dye coating mechanism. The temperature on theTiO2 surface can cause the ethanol in the dye solution to quicklyevaporate. Fig. 6 shows a schematic diagram of the evaporation ofethanol within the absorbed dye solution in the porous TiO2 thinfilm. The accelerated droplets have a high velocity due to theattractive and repulsive force. These droplets are absorbed in theTiO2 layer by the strength of the electric field. The ethyl alcohol inthe droplets is evaporated due to the temperature of the TiO2 thinfilm. Then, the droplet size dwindles down to the solid state andthe dye concentration is increased by the evaporation of ethylalcohol.

oltages of 4 kV(a), 5 kV(b), 6 kV(c), 7 kV(d), 8 kV(e) and 9 kV(f).

Fig. 8. The N719 dye coated on the TiO2 films: the absorbance (a) and transmittance (b) at different applied voltages as a function of the wavelength.

J.-T. Hong et al. / Journal of Electrostatics 68 (2010) 205e211210

3. Results and discussion

3.1. The geometry of the dye coloring region and performance

The N719 dye absorbs visible light in the wavelength rangefrom 450 to 600 nm. So the dye absolution can be measured by aCCD camera. A CCD camera (CASIO EX-F1) and light box (LP-400,MEDAL-LIGHT) are used to measure the dye coloring pattern onthe TiO2 thin film in the ESS system. Then, the images in RGBvalues of the thin films are accumulated and shown in graphicalgeometry. Fig. 7 shows the geometry of the dye coloring regionin the variable voltage range of 4e9 kV in the case where thefilm temperature is maintained at 45 �C for 5 min and thenwashed with ethyl alcohol to remove the extra dye molecules onthe film. The size of the TiO2 film is 5 � 5 cm2. The maximumred color corresponds to the light intensity through the TiO2 filmwithout dye. The minimum value of blue corresponds to thelight intensity of the absorbed dye. Fig. 7(a) shows an image ofthe TiO2 film at an applied voltage of 4 kV in the ESS system. Atthis moment, the dye molecules are accumulated on the TiO2surface because the droplet size is greater than the ethanolevaporation rate of the droplets. Fig. 7(b) shows the best dyecoloring performance. The TiO2 film has a monodisperse layer ofdye molecules on the TiO2 particle surface. At a voltage of morethan 7 kV, the cone-jet mode appears rather than the micro-dripping mode, where the dye molecules are located on the TiO2

film surface because of the evaporation of ethanol from theemitted droplets.

Fig. 9. The absorbance of the N719 dye coated TiO2 films (a) and

3.2. The dye absorption performance using the ESS system

Fig. 8 shows the absorbance (a) and transmittance (b) of theN719 dye on the TiO2 thin films colored by the conventional 24 hsoaking method at single voltages in the range from 5 to 8 kV(5 min). In the absorbance graph (Fig. 8(a)), the characteristics oflight absorption differ. However, the 5 kV ESS treatment showsthe most similar curve to that obtained with the conventionalmethod.

Fig. 9 shows the absorbance (a) and transmittance (b) of theN719 dye colored TiO2 thin films which were prepared by theconventional 24 h soaking method and mode controlled ESSmethod (voltage varied in the range of 5e10 kV, period 5 s) forvarious treatment times. The colored TiO2 film is measured in thewavelength range of 400e800 nm for identification of difference. Inthis graph, the light absorption of the colored TiO2 film varies withthe ESS treatment time. After a treatment time of 6 min, the lightabsorption rate increases exponentially. In this result, the timerequired for the coloring process of the TiO2 film is reduced from24 h in the soaking method to 4e10 min in the ESS method.Moreover, it is possible to make a DSSC that has a variable lightabsorption rate.

3.3. IeV curve measurements of the fabricated DSSC for eachcondition

Fig. 10 shows the IeV curve graphs of the DSSCs (Active area:0.25 cm2) which were fabricated using the conventional and

the transmittance (b) for various treatment times of the ESS.

Fig. 10. The IeV curve (Active area: 0.25 cm2) of the DSSCs fabricated using theconventional method and the ESS method adapted for the coloring process withvarious treatment times.

J.-T. Hong et al. / Journal of Electrostatics 68 (2010) 205e211 211

colored TiO2 films obtained by the ESS method (with varioustreatment times). The conventional DSSC fabrication time is morethan 30 h, due to the coloring processes (soaking: over 24 h) of thephoto-electrode. However, by using the ESS system, it is possible toreduce the treatment time to less than 6 h, due to the rapid coloringprocess of the photo-electrode. If the ESS treatment is conductedfor more than 4 min, the IeV curve characteristic is similar to thatobtained by the conventional method. Moreover, the cells obtainedusing an ESS treatment time of 6 min which have an efficiency of3.9% show higher performance than the conventional cells thathave an efficiency of 3.5%. A previous paper about the dye solutionreported that 1 wt% (at the maximum concentration) dye in theTiO2 film [6]. However, if the concentration is increased, the currentdensity will be increased.

4. Conclusion

A homemade ESS system was fabricated and used to coatfilms at various working voltages and compared to the conven-tional coating method. At voltages below 5 kV, the coatingpattern of the dye in the porous film exhibits dye accumulation.Therefore, the ESS system should be operated at workingvoltages in the range of 5e10 kV to achieve a uniform dyeabsorption layer in the porous TiO2 thin film with the variouselectric fields. The UVevis measurement shows that there wasgood similarity between the films obtained by the conventional

dye soaking (24 h) method and ESS (5 min) method at variousworking voltages in the range of 5e10 kV. The ESS method canreduce the fabrication time of DSSCs in the photo-electrodefabrication process. In addition, when using the ESS system forthe fabrication of photo-electrodes used in DSSCs, it is notnecessary to maintain dye concentration in the dye solution.Moreover, DSSCs with various designs can be made by themicro-dripping and cone-jet control modes. The geometry of thedye absorption in the porous TiO2 thin film was observed bya simple light source and CCD camera. Both the geometry of thedye absorbed in the porous TiO2 tin film and the uniformity ofthe TiO2 film were able to be measured. Furthermore, the DSSCperformance was investigated for each of the fabricated DSSCsby IeV curve measurements.

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