Pore-filling of Spiro-OMeTAD determined by Rutherford backscattering spectrometry in templated TiO2 photoelectrodes

Organic Electronics 15 (2014) 9–15

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Organic Electronics

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Pore-filling of Spiro-OMeTAD determined by Rutherfordbackscattering spectrometry in templated TiO2 photoelectrodes

1566-1199/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.orgel.2013.10.016

⇑ Corresponding author. Address: LCIS-GrEEnMat, University of Liege,Allée de la Chimie 3, 4000 Liege, Belgium. Tel.: +32 43663438; fax: +3243663413.

E-mail address: [email protected] (J. Dewalque).

Jennifer Dewalque a,⇑, Pierre Colson a, Gopala Krishna V.V. Thalluri a, François Mathis b,c,Grégoire Chêne b,c, Rudi Cloots a,d, Catherine Henrist a,d

a LCIS-GrEEnMat, Chemistry Department B6, University of Liege, B-4000 Liege, Belgiumb European Center for Archeometry, Physics Department B15, University of Liege, B-4000 Liege, Belgiumc Institut de Physique Nucléaire Atomique et Spectroscopie (IPNAS), Physics Department B15, University of Liege, B-4000 Liege, Belgiumd Center for Applied Technology in Microscopy (CATl), Chemistry Department B6, University of Liege, B-4000 Liege, Belgium

a r t i c l e i n f o

Article history:Received 2 July 2013Received in revised form 15 October 2013Accepted 15 October 2013Available online 6 November 2013

Keywords:Dye-sensitized solar cellsPore fillingRutherford backscattering spectrometrySpiro-OMeTADTemplated mesoporous filmsTiO2

a b s t r a c t

Liquid-state dye-sensitized solar cells can suffer from electrolyte evaporation and leakage.Therefore solid-state hole transporting materials are investigated as alternative electrolytematerials. However, in solid-state dye-sensitized solar cells, optimal TiO2 films thickness islimited to a few microns allowing the adsorption of only a low quantity of photoactive dyeand thus leading to poor light harvesting and low conversion efficiency. In order to over-come this limitation, high surface area templated films are investigated as alternative tonanocrystalline films prepared by doctor-blade or screen-printing. Moreover, templatingis expected to improve the pore accessibility what would promote the solid electrolytepenetration inside the porous network, making possible efficient charge transfers. In thisstudy, films prepared from different structuring agents are discussed in terms of micro-structural properties (porosity, crystallinity) as well as impact on the dye loading andSpiro-OMeTAD (2,20 ,7,70-tetrakis-(N,N-di-p-methoxyphenylamine)9,90-spirobifluorene)solid electrolyte filling. We first report Rutherford backscattering spectrometry as an inno-vative non-destructive tool to characterize the hole transporting materials infiltration.Templated films show dye loading more than two times higher than nanocrystalline filmsprepared by doctor-blade or screen-printing and solid electrolyte infiltration up to 88%.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction In DSSCs, the function of light absorption and carrier

In the early nineties, dye-sensitized solar cells (DSSCs)have been reported by O’Regan and Grätzel as a very prom-ising alternative to conventional silicon devices [1]. Nowa-days, liquid electrolyte-based DSSCs show the bestefficiency with the present record value as high as 12.2%[2]. However, the use of liquid electrolyte induces somelimitations such as electrolyte evaporation and leakage.Therefore solid-state hole transporting materials (HTMs)are investigated in order to overcome these disadvantages.

transport are separated. Dye molecules absorb light andtransfer electrons to the conduction band of TiO2. Dye isfurther regenerated by hole injection into the electrolyte.Electrons and holes respectively diffuse through the TiO2

network and the HTM to be further collected by the elec-trodes and produce electricity [3–5].

Solid-state HTMs are characterized by shorter chargecarrier diffusion length than liquid electrolytes, leading tofaster recombination reactions [6,7]. Moreover, solid elec-trolyte does not penetrate easily deep inside the TiO2 filmusually leading to incomplete pore filling [7,8]. Therefore,TiO2 films thickness is limited to a few lm for solid-stateDSSC applications. Best efficiencies are reported for only2 lm-thick films [7,9]. However, due to the limited

10 J. Dewalque et al. / Organic Electronics 15 (2014) 9–15

amount of dye adsorbed, such thin films presents a poorlight harvesting leading to low conversion efficiencies.

In order to overcome this limitation, high surface areatemplated films are investigated as alternative to nano-crystalline films prepared by doctor-blade or screen-print-ing [10–13]. Indeed, templating allows controlling the filmmesostructure, leading to high values of accessible poros-ity, combined with a regular pores organization. This pro-motes very high dye loading and thus efficient lightharvesting and subsequent high photoelectron production.Templated films show dye loading more than two timeshigher than nanoparticles samples [14]. Moreover, thepore accessibility promotes the HTM penetration insidethe porous network, making possible an efficient regener-ation of oxidized dye molecules. However, pore size oftemplated films is limited to a few nanometers dependingon the structuring agent, which could hinder HTM infiltra-tion. Therefore, pore size has to be increased by usingappropriate surfactant. Unfortunately, the increase of poresize leads to a loss of surface area. A perfect balance be-tween high HTM infiltration and high dye loading has thusto be stroked.

Spiro-OMeTAD infiltration in nanoparticles samples hasalready been studied by several groups. Different charac-terization techniques have been used to quantify the porefilling.

Snaith et al. proposed a calculation model predictingthe Spiro-OMeTAD pore filling by knowing the initial con-centration of the Spiro-OMeTAD solution, the thickness ofthe TiO2 porous film and its porosity, the thickness of thewet Spiro-OMeTAD layer coated on the TiO2 porous filmbefore infiltration as well as the thickness of the drySpiro-OMeTAD overlayer after infiltration [15]. Thick-nesses of the different layers were determined bycross-sectional scanning electron microscopy. This modelassumes that the Spiro-OMeTAD which is not in the over-layer fills the pores. Filling fractions above 85% have beenreported for 2 lm-thick TiO2 films. Olson et al. furtheradded corrections to this model to take into account theviscosity of the Spiro-OMeTAD solution and the pore radiusin the TiO2 films [16].

Ding et al. characterized the Spiro-OMeTAD infiltrationby UV–visible spectroscopy from desorption solutions andby X-ray photoelectron spectroscopy (XPS) [7]. XPS cannotgive an absolute estimation of the pore filling due to thefaster degradation of organic compounds during the mea-surement compared to TiO2. However, results showed anhomogeneous infiltration of Spiro-OMeTAD in nanoparti-cles films up to 5 lm-thick. Besides, adsorption character-ization of the same samples brought to light an incompletepore filling. Values around 60–65% were evidenced for2.8 lm-thick nanoparticles layers, what is lower than the85% filling fraction reported by Snaith et al. [15]. Neverthe-less, it has to be mentioned that experimental settings,such as Spiro-OMeTAD solution concentration, soakingtime and spin-coating rate have been proved to highlyinfluence the pore filling [7].

Optical non-destructive characterizations were alsoinvestigated to determine the filling fraction. Moulé et al.used spectroscopic ellipsometry. However, this techniqueis limited to thin films (�500 nm) investigation due to

the scattering effects occurring in thick layers [17,18]. Doc-ampo et al. investigated optical reflectometry [18]. Fillingfraction above 80% was determined for a 2.6 lm-thicklayer.

In this study, films prepared from different structuringagents are discussed in terms of microstructural properties(pore size, crystallinity) as well as impact on the dye load-ing and Spiro-OMeTAD impregnation. Moreover, we firstimplement Rutherford backscattering spectrometry (RBS)as a quantitative non-destructive tool to characterize theSpiro-OMeTAD pore filling.

2. Experimental section

2.1. Templated thin films synthesis

P123 solution was prepared by mixing 22.4 mL of 1-butanol (Acros Organics), 2 g of P123 surfactant (Sigma–Al-drich), 7.9 g of titanium tetraisopropoxide (Acros Organics)and 4.1 mL of concentrated hydrochloric acid (Merck, 36wt%), as reported in references [19] and [20]. HCl wasslowly added to titanium tetraisopropoxide under vigorousstirring. Warm-up during the HCl addition was controlledusing an ice bath. P123 was separately dissolved in 1-buta-nol and further added to the HCl/Ti(i-PrO)4 solution. P123solution was aged for 3 h before use.

PSA solution was prepared by mixing 2.6 g of tetrahy-drofuran (Acros Organics), 3.3 g of ethanol (Sigma–Al-drich), 0.244 g of PSA surfactant (Polymer source), 1.024 gof titanium tetraisopropoxide (Acros Organics) and1.458 g of concentrated hydrochloric acid (Merck, 36wt%). PSA surfactant was dissolved in THF and ethanol un-der heating at 70 �C for a few minutes. Surfactant solutionwas cooled down before HCl addition. Ti(i-PrO)4 was final-ly added to the HCl/PSA solution.

Films were dip-coated on (001)-oriented silicon sub-strates, purchased from MEMC (Electronic Materials, It)and passivated in nitric acid (1 mol L�1) for 2 h.

Mesoporous films were obtained by dip-coating thesubstrate in the precursors solutions. The withdrawal ratewas set to 0.8 mm/s and the relative humidity in the dip-coating chamber was set at 25% during a few minutes at25 �C. After dip-coating, the fresh layer was directly treatedat 300 �C for 15 min on a pre-heated hot plate (stabiliza-tion step). This step helps in preventing the dissolution ofthis layer during a subsequent dip-coating step, allowingmultilayer deposition [21]. The number of layers requiredto produce 1 lm-thick films varies with the surfactantused: 3 layers for P123 and 4 layers for PSA. A calcinationstep was applied every micron. This calcination step is re-quired to degrade the surfactant and promote the filmcrystallization. Two calcination sequences have been sepa-rately studied. The first one consists in treating the multi-layer film at 400 �C under air, in a muffle furnace with aheating ramp of 1 �C/min. The temperature is set at400 �C for 2 h and the film is allowed to cool down in thefurnace. The alternative one is performed at 500 �C for30 min under oxygen flow (heating ramp 500 �C/h). Oxy-gen flow is expected to improve the surfactant elimination.

J. Dewalque et al. / Organic Electronics 15 (2014) 9–15 11

2 lm-thick films were prepared from these two thermalsequences for further characterizations.

2.2. Spiro-OMeTAD infiltration

Dye solution was prepared by mixing Z-907 dye(Solaronix) and 4-guanidinobutyric acid (Sigma–Aldrich)(1:1 mixture, 6 � 10�3 M) in 1-methoxypropan-2-ol. Elec-trodes were immersed in dye solution for 10 min at roomtemperature before being rinsed in acetonitrile and air-dried. Z-907 dye was used instead of N-719 dye becauseof its long aliphatic chain, which hinders the hole-electronsrecombinations occurring at the TiO2/Dye/electrolyteinterface during the cell operation [2,5,9].

Solid electrolyte was prepared by mixing 225 mg ofSpiro-OMeTAD (2,20,7,70-tetrakis-(N,N-di-p-methoxyphe-nylamine)9,90-spirobifluorene, purchased from Merck) in1 mL of chlorobenzene under stirring and heating at70 �C for 30 min. 22 lL tert-butylpyridine (TBP, purchasedfrom Sigma–Aldrich) were further added to the Spiro-OMeTAD solution. Separately, 170 mg of Lithium bis(tri-fluoromethylsulfonyl)imide (Li-TFSI, purchased fromSigma–Aldrich) were pre-dissolved in 1 mL of acetonitrile.47 lL of the Li-TFSI solution was added to the Spiro-OMe-TAD/TBP solution. 40 lL of electrolyte solution wasdeposited onto the TiO2 photoelectrode and left for 1 minbefore spin-coating at 2000 RPM for 45 s. After spin-coat-ing, samples were dried overnight.

2.3. Characterization techniques

Transmission electron microscopy (TEM) micrographswere taken at an acceleration voltage of 200 kV (TecnaiG2 Twin, FEI) on the films scratched off the substrate anddispersed in ethanol under ultrasound, then deposited oncarbon-coated copper grids.

Ellipsometry measurements were performed on a UV–visible (from 250 nm to 1000 nm) GES5E SpectroscopicEllipsometer from SOPRALAB, and the data analysis wasperformed with the WINSE software. The ellipsometerwas coupled with an ellipsometric porosimetry device foratmospheric poroellipsometric (AEP) measurements. Thisdevice consists in a chamber containing the film to analyzein which the environment is modulated by a pulsed airflow with controlled partial pressure of water. The recordof the optical index variation of the film as the functionof the water partial pressure allows the determination ofthe percentage of porosity of the films. A pore size distribu-tion (PSD) was calculated from the AEP data using a spher-ical pores model.

The thickness of the samples was checked by mechani-cal profilometry (Dektak Stylus Profiler Veeco).

The dye loading was determined from the desorptionsolutions by UV–vis spectroscopy with a Perkin ElmerUV–vis Spectrometer Lambda 14 P. Film area was preciselydetermined by image analysis (GIMP software). For N-719dye loading, the film was soaked in the dye solution (eth-anolic solution, 2.7 � 10�4 M) during one night at roomtemperature. After drying, N-719 dye was desorbed in aknown volume of KOH solution (10�3 M) for one hour. Acalibration curve was used to calculate the experimental

molar extinction coefficient of N-719 dye, which is12,900 (mol/L)�1 cm�1 at 500 nm. For Z-907 dye loading,the film was soaked in dye solution (Z-907/4-guanidinobu-tyric acid 1:1 in 1-methoxypropan-2-ol, 6 � 10�3 M) dur-ing 10 min at room temperature. After drying, Z-907 dyewas desorbed in a known volume of tetramethylammo-nium hydroxide solution in dimethylformamide (10�1 M)for one hour. A calibration curve was used to calculatethe experimental molar extinction coefficient of Z-907dye, which is 8500 (mol/L)�1 cm�1 at 516 nm.

X-ray diffraction (XRD) data were collected on a BrukerD8 grazing incidence diffractometer with CuKa radiation(incidence angle: 2�, step size: 0.04�, scan rate: 6 s/step).The presence of anatase was checked from the (101) peakat 25.36� 2h. The integrated intensity under the (101) ana-tase peak was used to compare the global crystallinity ofthe different samples. Crystallite size was evaluated fromthe Scherrer equation.

Rutherford backscattering spectrometry (RBS) was per-formed to characterize the Spiro-OMeTAD infiltration. Thisshortly consists in focusing a beam of high energy acceler-ated particles on a target sample. Backscattered particlesare further collected by a PIPS (Passivated Implanted Pla-nar Silicon) detector from Camberra�, 100 lm-thick, witha surface of 50 mm2 and energy resolution of 12 keV (Mod-el PD50-12-100AM) placed at 165� in respect of the beamdirection. The number of particles is then plotted as a func-tion of their energy. This energy depends on the mass ofthe targeted atom and its distance from the sample surface,while the height of the signal depends on the concentra-tion of elements at their corresponding depth, therefore,allowing depth profiling of elements in the target. In thisstudy, protons of 1 MeV in normal incidence have beenused as probe particles. Combining high depth penetrationand enhanced sensitivity to low mass atoms such as carbonor oxygen due to nuclear interactions, protons allow accu-rate characterization of such 2 lm-thick samples.

3. Results and discussion

3.1. Mesostructure characterization

Two structuring agents have been used to prepare TiO2

mesoporous thin films: Pluronic P123 triblock copolymer(EO20PO70EO20) and PS(16400)-PEO(36400) diblock copoly-mer, further reported to as P123 and PSA respectively.

Films obtained from the two surfactant solutions havebeen analyzed by TEM imaging.

P123 and PSA samples prepared under 25% relativehumidity (RH) conditions and only stabilized at 300 �Cpresent an isotropic order referred to as ‘‘wormlike’’ struc-ture (Fig. 1) [22–24]. From TEM data, pore sizes around7 nm and 20 nm and walls of approximately 4 nm and10 nm can be determined for P123 and PSA, respectively.

After thermal treatments at 400 �C for 2 h or 500 �C for30 min, the film mesostructure partially collapses due tothe excessive crystallites growth (Fig. 2). The crystallitesize exceeds the initial wall thickness. Films obtained fromboth thermal treatments present almost the samemicrostructure. Crystallite size of approximately 8 nmand 15 nm can be measured for P123 and PSA, respectively.

(a) (b)

100 nm100 nm 100 nm100 nm

Fig. 1. TEM micrographs of (a) P123 wormlike and (b) PSA wormlike films. Bigger pores are reached with PSA surfactant.

(a) (c)

(b) (d)

100 nm 100 nm

100 nm 100 nm

Fig. 2. TEM micrographs of the (a) P123 film calcined for 2 h at 400 �C, (b) P123 film calcined for 30 min at 500 �C, (c) PSA film calcined for 2 h at 400 �C, and(d) PSA film calcined for 30 min at 500 �C. Both thermal treatments lead to the same microstructure.

12 J. Dewalque et al. / Organic Electronics 15 (2014) 9–15

In parallel with TEM observations, the percentage ofporosity and the pore size have been studied byatmospheric poroellipsometry (AEP). Film thickness hasbeen determined by mechanical profilometry and crystal-lite size by XRD from Scherrer equation. Data are reportedin Table 1.

From XRD data, films calcined at 500 �C seem to beslightly less crystallized than layers treated at 400 �C (notshown here). This is in accordance with smaller crystallitesizes reported for the PSA samples calcined at 500 �C.Moreover, we can observe that the crystallite size, deter-mined by XRD, matches with TEM observations.

Table 1Film thickness (profilometry), percentage of porosity (AEP), pore size (AEP), crystallite size (XRD) and N-719 dye loading (UV–vis) of P123 and PSA templatedfilms.

Sample (�C) N� of layers Thick. (lm) % Of porosity Pore size (nm) Crystallite size (nm) Dye loading (mol/cm3)

P123 400 7 1.7 37 10 10 2.5 � 10�4

P123 500 7 1.6 32 8 10 2.4 � 10�4

PSA 400 8 1.7 39 17 15 1.9 � 10�4

PSA 500 8 1.6 34 15 13 2.0 � 10�4

Fig. 3. RBS spectra of the 2 lm-thick PSA sample calcined at 500 �C (a)only sensitized with Z-907 dye, (b) sensitized with Z-907 dye and filledwith Spiro-OMeTAD. Ti signal (in bold dashed curve) is flattened anddisplaced to lower energy after Spiro-OMeTAD addition. Two steps peaksobserved for O and C atoms are due to the different backscatteringbehaviors of the Spiro-OMeTAD overlayer (pure phase) and the Spiro-OMeTAD filled inside the TiO2 network (mixed phases).

J. Dewalque et al. / Organic Electronics 15 (2014) 9–15 13

If we compare porosity results of the two thermal se-quences, treatment at 500 �C leads to smaller percentageof porosity. This is attributed to a higher contraction duringthe films calcination, promoted by the fast heating ramp.Indeed, samples calcined at 500 �C are thinner than filmstreated at 400 �C and present smaller pores.

In DSSCs, the active material under visible light is thedye adsorbed onto the film surface. As a strictly monomo-lecular layer of dye is required to raise the best photovol-taic performances, the conversion efficiency is directlydependent on the accessible surface area of the anatasefilm [25,26]. N-719 dye loading of the different sampleshas been determined by UV–visible spectroscopy and isalso reported in Table 1.

P123 samples calcined at 400 �C or 500 �C present thesame dye loading. The same behavior is observed for PSAfilms. Besides, due to their higher pore size and larger crys-tallite size, PSA samples show lower dye loadings thanP123 films. However, the dye loadings of both templatedsamples are far above the dye loading of the nanoparticlesreference sample (1.1 � 10�4 mol/cm3) [14].

3.2. Spiro-OMeTAD filling

In this work, we have implemented Rutherford back-scattering spectrometry to quantify the Spiro-OMeTADinfiltration in Z-907-sensitized samples.

From RBS spectra, we can first observe the homogenousfilling of the HTM inside the whole porous network of bothP123 and PSA samples. For all the samples, the Ti charac-teristic peak widened due to the increase of the TiO2 layerdensity by the addition of the HTM. While Spiro-OMeTADreplaces air inside the pores, there are more targetedatoms in the sample and more energy is lost to probeatoms away from the surface. This peak is also flattenedbecause of the decrease of relative Ti atomic concentrationinto the HTM-filled TiO2 layer. The homogenous wideningand flattening of the Ti signal is typical of an homogenousmodification of the TiO2 film and thus of an homogenousinfiltration of the Spiro-OMeTAD. Moreover, Ti peak movedto lower energies due to the presence of the Spiro-OMeTADoverlayer. Accelerated particles lose energy to passthrough the overlayer before reaching the TiO2 layer andthus the Ti atoms. RBS spectra obtained with and withoutSpiro-OMeTAD for 2 lm-thick PSA sample are reported inFig. 3.

Fitting of the RBS spectra also allows quantifying theelemental composition of the samples. Non Rutherfordcontribution to the backscattered signal has to be takeninto account for accurate quantification of both carbonand oxygen. We used cross sections provided by Sigma-Calc, which calculates at any appropriate angle, differential

cross sections [27,28], derived from experimental data setscompiled in the IAEA-IBANDL database (http://www-nds.iaea.org/ibandl/). Ratio between Carbon and Titaniumatoms inside the porous layer, Ctot/Ti, can then be experi-mentally determined by RBS after simulating experimentalspectra using SIMNRA� program [29]. Ctot represents theoverall Carbon amount, which includes Carbon fromZ-907 dye, Cdye, and from Spiro-OMeTAD, CSpiro.

Regarding the molecular formula of Z-907 dye(C42H54O6N6S2Ru), the dye contribution to the RBS Carbonsignal, Cdye, can be calculated from the RBS Rutheniumpeak:

Cdye

Ti¼ 42:

RuTi

ð1Þ

Table 2Experimental and theoretical elemental compositions of the differentsamples. Pore filling fraction is estimated by comparing CSpiro/Ti andCSpiro 100%/Ti.

Sample(�C)

RBS Cdye/Ti

RBS Ctot/Ti

RBS CSpiro/Ti

Theor. CSpiro

100%/TiPFF(%)

P123 400 0.55 0.89 0.34 0.46 74P123 500 0.54 0.82 0.28 0.32 88PSA 400 0.46 0.82 0.36 0.58 62PSA 500 0.44 0.71 0.27 0.43 63

Table 3Thickness of the Spiro-OMeTAD coverage estimated from the PFF and poresize of the different samples.

Sample (�C) PFF (%) Pore size (nm) Spiro coverage (nm)

P123 400 74 10 1.8P123 500 88 8 2.0PSA 400 62 17 2.3PSA 500 63 15 2.1

Fig. 4. Spiro-OMeTAD coverage (in grey) and resulting pore-filling for2 lm-thick PSA sample with 17 nm pores (left) and 2 lm-thick P123sample with 10 nm pores (right) calcined at 400 �C. Smaller pores lead tohigher pore filling fraction considering the same Spiro-OMeTAD coverage.

14 J. Dewalque et al. / Organic Electronics 15 (2014) 9–15

where 42 is the stoichiometry ratio between Carbon andRuthenium for Z-907 dye and Ru/Ti is the RBS ratio be-tween Ruthenium and Titanium atoms inside the porouslayer.

RBS ratio between Carbon from Z-907 dye and Titaniumatoms inside the porous layer, Cdye/Ti, can be corroboratedby dye loading. Indeed, the dye contribution to the Carbonsignal, Cdye, can also be calculated from the followingequation:

nCdye ¼ 42:C’ ð2Þ

where nCdye is the number of moles of Carbon from the Z-907 dye (in mol/cm3), 42 is the number of Carbon per moleof Z-907 dye and C’ is the experimental dye loading nor-malized for 1 cm3.

As the TiO2 relative volume has been determined byAEP, the amount of Titanium per volume can be calculatedfrom the following equation:

nTi ¼VTiO2qTiO2

MmTiO2

ð3Þ

where nTi is the number of moles of Titanium from the TiO2

layer (in mol/cm3), VTiO2 is the volume of TiO2

(=100% � Vporosity), qTiO2 is the density of TiO2 anatase(=3.9 g/cm3) and MmTiO2 is the molar weight of TiO2.

Cdye/Ti ratios obtained by RBS and dye loading are in thesame range, which validates the RBS technique.

Ratio between Carbon from the Spiro-OMeTAD andTitanium atoms, CSpiro/Ti, can be determined from the fol-lowing equation:

Cspiro

Ti¼ Ctot

Ti¼ Cdye

Tið4Þ

Experimental CSpiro/Ti ratio can be compared to the the-oretical value, CSpiro 100%/Ti, calculated for a 100% filledsample in order to determine the pore filling fraction(PFF). CSpiro 100% represents the maximum amount of Car-bon from the Spiro-OMeTAD that can fill the pores.

To calculate the CSpiro 100%/Ti ratio, we have to know thevolume available for the Spiro-OMeTAD infiltration. Thepart of the porosity filled by the dye has thus to be takeninto account.

The volume of one molecule of Z-907 dye was approxi-mated to be 6.4 � 10�22 cm3, based on the Z-907 dye sur-face coverage, the height of the Ruthenium complexcentral part and the length of the aliphatic chains [30,31].The volume occupied by the dye, Vdye, can thus be esti-mated from the Z-907 dye molecular volume and the num-ber of Z-907 dye molecules per cm3. The volume remainingfor Spiro-OMeTAD infiltration, VSpiro100%, can finally be cal-culated from the following equation:

VSpiro100% ¼ Vporosity � Vdye ð5Þ

The maximum amount of Carbon from the Spiro-OMeTADthat can fill the pores, CSpiro100%, can thus be determinedfrom the following equation:

nCSpiro100% ¼81V spiro100%qspiro

Mmspiroð6Þ

where nCSpiro100% is the number of moles of Carbon reachedfor a 100% filled sample (in mol/cm3), 81 is the number of Cper mole of Spiro-OMeTAD, qSpiro is the density of Spiro-OMeTAD (=1 g/cm3) [15,16,18] and MmSpiro is the molarweight of Spiro-OMeTAD.

The pore filling fraction can be calculated by comparingthe RBS experimental CSpiro/Ti and the theoreticalCSpiro 100%/Ti ratios obtained for the different samples.

PFF ¼Cspiro

TiCspiro100%

Ti

� 100 ð7Þ

Elemental compositions of the different samples are re-ported in Table 2.

It seems that 8 nm pores of P123 samples are big en-ough to allow efficient Spiro-OMeTAD filling. PFF up to88% is determined by Rutherford backscatteringspectrometry.

Although they present bigger pores, PSA samples showlower PFF. Spiro-OMeTAD is usually assumed to form aquite conformal coverage onto the TiO2/dye interface[32], with a thickness limited by the solvent evaporation.

The thickness of the Spiro-OMeTAD coverage can beestimated from PFF and pore size. Spiro-OMeTAD coatingaround 2 nm is calculated for the different samples

J. Dewalque et al. / Organic Electronics 15 (2014) 9–15 15

(Table 3), which corresponds to a monolayer coverage ofSpiro-OMeTAD [7].

It has to be mentioned that, in real samples, perfectlyconformal coating of Spiro-OMeTAD and spherical poresused in this model are highly unlikely.

For same thickness of Spiro-OMeTAD coverage (�2 nm),films with large pores are thus characterized by lower PFFthan films with small pores (Fig. 4).

PFF obtained for P123 and PSA templated samples arevery promising compared to PFF reported in the literaturefor nanoparticles samples [7,15,16,18]. Templated layersthus prove their potentiality to be used as high efficiencyphotoelectrode in solid-state DSSCs. Moreover, we firstreport Rutherford backscattering spectrometry as aquantitative non-destructive tool to characterize the HTMinfiltration.

4. Conclusions

Due to their high surface area and the perfect control ofthe porous mesostructure, templated films have beeninvestigated as photoelectrode material in solid-stateDSSCs. However, depending on the structuring agent, poresize can be limited to a few nanometers, possibly leading toinefficient HTM filling. Therefore, two different surfactantswere studied, with pore sizes around 8 nm and 15 nmrespectively. These surfactants were chosen in order toavoid a dramatic drop of the surface area, caused by thepore size increase.

Templated films show dye loading more than two timeshigher than nanoparticles samples, which can overcomethe limitation of the film thickness and the resulting lightharvesting.

Besides, the high pore accessibility of templated filmsallows the efficient infiltration of the solid electrolyte in-side the porous network. Pore filling fractions up to 88%have been determined by Rutherford backscattering spec-trometry, even for pore size of only 8 nm, which is higherthan PFF reported for nanoparticles layers. This highlightsthe high potentiality of templated films to be used as pho-toelectrode materials in high efficiency solid-state DSSCs.Moreover, RBS technique has been proved to be a promis-ing non-destructive tool to characterize the Spiro-OMeTADinfiltration.

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