Thin Solid Films 519 (2011) 8125–8134

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Comparison of dye solar cell counter electrodes based on differentcarbon nanostructures

Kerttu Aitola a,⁎, Janne Halme a, Niina Halonen b, Antti Kaskela a, Minna Toivola a, Albert G. Nasibulin a,Krisztián Kordás b, Geza Tóth b, Esko I. Kauppinen a,c, Peter D. Lund a

a Aalto University, Department of Applied Physics, P.O. Box 15100, 00076 Aalto, Finlandb Microelectronics and Materials Physics Laboratories, Department of Electrical and Information Engineering, University of Oulu, P.O. Box 4500, FI-90014 University of Oulu, Finlandc VTT Biotechnology, P.O. Box 1000, 02044 VTT, Finland

⁎ Corresponding author. Tel.: +358 50 316 0960.E-mail address: kerttu.aitola@aalto.fi (K. Aitola).

0040-6090/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.tsf.2011.05.078

a b s t r a c t

a r t i c l e i n f o

Article history:Received 13 August 2010Received in revised form 20 May 2011Accepted 30 May 2011Available online 6 June 2011

Keywords:Dye-sensitized solar cellCatalystConductive layerCarbon nanostructureCarbon nanotubeCarbon black

Three characteristically different carbon nanomaterials were compared and analyzed as platinum-freecounter electrodes for dye solar cells: 1) single-walled carbon nanotube (SWCNT) random network films onglass, 2) aligned multi-walled carbon nanotube (MWCNT) forest films on Inconel steel and quartz, and3) pressed carbon nanoparticle composite films on indium tin oxide-polyethylene terephtalate plastic. Resultsfrom electrochemical impedance spectroscopy and electron microscopy were discussed in terms of thecatalytic activity, conductivity, thickness, transparency and flexibility of the electrode films. The SWCNT filmsshowed reasonable catalytic performance at similar series resistance compared to platinized fluorine dopedtin oxide-coated glass. The MWCNTs had similar catalytic activity, but the electrochemical performance of thefilms was limited by their high porosity. Carbon nanoparticle films had the lowest charge transfer resistanceresulting from a combination of high catalytic activity and dense packing of the material.

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1. Introduction

For creating electricity from sunlight, several solar cell types havebeen developed, of which the dye solar cell (DSC) is one of the mostrecent ones [1]. The advantages of the DSC are its relatively cheapmaterials and the possibility for roll-to-roll production of flexible solarcells on plastic or metal foils. However, there are still challenges in theway of commercializing flexible DSCs, such as long term durability [2].Also some of the materials, such as platinum and indium, should bereplaced with cheaper alternatives.

A DSC is a thin layer electrochemical cell consisting of a dye-sensitized nanostructured TiO2 photoelectrode and a counter elec-trode (CE), laminated together to hold between them a layer of redoxelectrolyte, typically iodide-tri-iodide redox couple in an organicsolvent. While the primary steps of photovoltaic conversion, namelylight absorption and charge separation, take place at the photoelec-trode that releases electrons to the external circuit, the counterelectrode is responsible for returning these electrons to the cell byreducing the tri-iodide in the electrolyte in an electrochemical charge-transfer reaction. To minimize the overvoltage needed to drive thisreaction, the counter electrode has to display high electrocatalyticactivity towards the tri-iodide reduction. A standard choice for a high

performance CE is a fluorine-doped tin oxide (FTO) coated glass witha few nanometer thick layer of platinumnanoparticles deposited on it.However, the FTO glass is one of the most expensive components inDSCs [3] and requires slow batch processing in solar modulemanufacturing. For cheaper and faster roll-to-roll processing, thestandard alternative has been flexible indium tin oxide-coated (ITO)polyethylene terephtalahte (PET) plastic foils with sputtered Pt [4,5].However, both In and Pt are expensive and rare materials, so thatscientific efforts are directed towards finding alternative catalystsand transparent, conductive layers for DCSs, as well as for otheroptoelectronic devices.

One option for replacing the expensive, non-flexible materials inDSCs are carbon nanomaterials, such as nanoparticles (e.g. carbonblack) [6–10] and carbon nanotubes (CNT) [11–15]. Carbon is widelyavailable, and even though it is not as good a catalyst as platinum, itsperformance in the solar cell can be improved by employing it as aporous film with high specific surface area. Indeed, DSCs based onporous carbon CEs have demonstrated equal [6–8] or better [9]photovoltaic performance compared to standard cells with Pt as acatalyst. It is also possible to create conducting layers of carbonnanomaterials. This has been utilized in the so-called monolithic DSCstructure where a porous conducting carbon composite film acts asboth the catalyst and the lateral current collector in the cell, replacingthus one of the expensive FTO glass substrates and simplifyingmanufacturing [6,10]. In addition, transparent single wall carbonnanotube films have recently emerged as a promising option for

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ITO coated plastics in optoelectronic devices [16–19], adding thustransparency in the list of possible functional properties of carbonnanomaterials as active components in DSCs.

The rapid progress in the development of synthesis and depositionof carbon nanomaterials has led to a situation where there is a vastpool of options how to utilize them for creating new DSC counterelectrodes. However, integrating a promising material to a DSC is notstraightforward, since the requirements for conductivity, catalyticactivity, transparency, flexibility, durability, purity, and depositionconditions depend on the application and may be mutually conflict-ing. These properties originate from the micro-scale properties andstructure of the carbon nanomaterials but are strongly influenced bythe macro-scale electrode architecture in which they are introducedin the cell. Hence the performance of a given carbon nanomaterial as aDSC CE material is generally hard to predict, but needs to be tested inpractice.

In this paper, we used an experimental approach to address thisproblem by investigating three characteristically very different carbonnanomaterials and architectures as DSC CE shown in Fig. 1: a) randomnetwork type single-walled CNT films aimed to combine bothcatalytic and conducting functions to very thin semi-transparentfilm; b) thick, vertically alignedmulti-walled CNT “forest” films aimedto maximize the catalytic performance through high surface area, andc) carbon nanoparticle composite films aimed to realize flexible CEfilms on ITO plastic foils by simple manufacturing. The objective of thepaper is to illustrate the basic design and performance differences ofcarbon nanomaterial CEs for DSCs using the above mentioned threecases as practical examples.

2. General properties of carbon materials

The carbon nanomaterials, used here and reported previously asthe DSC counter electrode material, differ from each other in manyaspects (see Fig. 1). In this chapter we try to sketch some generalguidelines for understanding the carbon nanomaterials' propertiesand designing counter electrodes of them.

Individual carbon nanotubes can be electrically extremely con-ductive, and mechanically strong yet flexible toward bending[20,21,17]. These and other properties can be influenced by thechirality, length, number of walls, defectiveness, metal content and

Fig. 1. Schematic images of three different carbon nanomaterial counter electrodearchitectures. a) Random network carbon nanotube film, b) vertically aligned carbonnanotube “forest” film, and c) carbon nanoparticle-graphite composite film.

chemical functionalization of the tubes [22]. The interest for applyingCNTs to many optoelectrical devices is based on the expectation thatthe properties of individual tubes could be realized also inmacroscopic CNT films.

The structure of a CNT film can vary greatly. For example, a CNTfilm can be a random network of interconnected individual CNTs andCNT bundles, a vertically aligned CNT “forest”, or a horizontallyaligned CNT film [23]. The structure most likely has an effect on thebehavior of the film in a device. For instance, in broad terms, a randomnetwork SWCNT film conducts electricity quite well in the plane of thefilm, whereas a CNT forest conducts electricity best in the vertical,perpendicular-to-the-substrate-plane directionc [24,25].

The most studied CNT based counter electrode material in the DSCis the random network type film [11–15]. The sheet resistance of arandom network CNT film can be comparable to the values of ITO-PET(ca. 50 Ω/sq.) even when the films are very thin and almosttransparent [16–18]. Furthermore, the films seem to be catalyticallyactive towards the DSC redox reaction at the counter electrode[12,14]. The catalytic activity of CNTs has most often been ascribed todefects and end caps of the CNTs, and even the catalytic nanoparticlesfrom the CNT synthesis present in the films, but this view has beenrecently challenged, giving the CNT sidewalls a more prominent a rolein the electron transfer reaction [26]. Probably several tube and filmqualities affect the catalytic properties. It has also been proven thatCNT films are more durable toward bending than ITO-PET [17]. Oneinteresting feature of this type of films is that with them additionalconductive layers beneath the catalyst layer might be avoided.

In general, when a random network SWCNT or MWCNT film hasbeen used as DSC CEs, similar efficiencies compared to reference Ptsolar cells have been obtained [11,12]. Usually the CNT films havebeen prepared quite simply, by e.g. spray-coating [12,14] or drop-casting [11]. In the literature, the films have usually been on FTO glasssubstrates. As an exception, one study used stainless steel as thesubstrate and SWCNTs as the catalytic material, and a 3.9% efficiencywas obtained [27]. In another study a SWCNT film onmembrane filter,adhered on plain substrate glass, produced a 4.5% cell efficiency [11].

The aligned CNT forest has been tested in DSC a few times [28–31].With the CE type, good [29] and fair [28,30,31] DSC efficiencies havebeen reported, but another conductive layer along the substrate planehas always been used.

Carbon nanoparticle composite films have been used in the DSCboth on rigid and flexible substrates with conductive layers beneath[8,9,32,33], and good catalytic activities and DSC efficiencies havebeen obtained [8,9,33]. Even with the conductive layer, the nano-particles have to be blended with conductivity increasing compo-nents, such as graphiteflakes, and they have to be severalmicrometersthick. Obvious advantage of this structure is the cheapness of thematerials.

3. Performance characteristics of carbon nanomaterialcounter electrodes

3.1. Charge-transfer resistance

The key performance metric of the DSC counter electrode is thecharge-transfer resistance RCE (Ωm2) per geometric surface area of theelectrode film. RCE can be measured e.g. by impedance spectroscopy[34,35], and it should be as low as possible to minimize the charge-transfer overpotential that decreases the fill factor of the solar cell IVcurve. In the case of a porous carbon nanomaterial CE, RCE decreaseswith increasing film thickness d as

RCE = rCEd−1 ð1Þ

where rCE (Ωm3) is the charge-transfer resistance per unit volume ofthe film. This assumes that the whole film is active in the charge-

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transfer process, which is a basic criterion for a good porous CE.Understandably, rCE depends on the microscopic properties of theelectrode, such as its specific surface area per unit volume AV (m2/m3)and the charge-transfer resistance per unit area of this surface, rCE,A(Ωm2),

rCE = rCE;AA−1v : ð2Þ

Since charge-transfer is an interfacial process occurring at thecatalyst surface, rCE,A may be called the “catalytic activity” of theelectrode material, and it is the figure of merit for comparing differentcatalyst materials. In the case of composite electrodes where there areelectro-inactive binder particles or polymers in addition to thecatalyst material, it must be further considered that not all the(specific) surface area of the film is taking part in the charge-transferreactions. One measure of the electrically accessible surface area isgiven by the electrode capacitance CCE (Fm−2).

CCE = cCEd = cCE;AAvd ð3Þ

where cCE (Fm−3) is the capacitance per unit volume and cCE,A(Fm−2) the capacitance per unit microscopic surface area of theelectrode material. In the case of carbon nanomaterials of metallic-nature, the capacitance can be expected to be dominated by the(Helmholtz) electrochemical double layer capacitance. However, inthe case of the semiconducting CNTs, a voltage dependent chemicalcapacitance, also called quantum capacitance [22,36], can play a role.

In order to carry out a comparison of the catalytic activities of thedifferent carbon nanomaterials, the microscopic surface-area shouldbe known. This could bemeasured with somemethod, such as the BETmethod, but it is beyond the scope of this paper. Here, we compare thecatalytic activities by taking the product of the charge-transferresistance and the capacitance, since in the product their dependenceon the thickness and specific surface area of the film cancel out leavingthe product dependent only on microscopic properties of theelectrode material - electrolyte interface. From Eqs. 1, 2 and 3 we get

RCECCE = rCE;AcCE;A: ð4Þ

This product may be used as a figure of merit for catalytic activityof catalyst materials (here carbon), that have similar area-specificcapacitance, cCE,A, and used to compare samples even if their thicknessis not known. In this paper we use Eq. 4 for the comparison of differentcarbon nanomaterials, assuming that their area-specific capacitancecCE,A values are equal, meaning that the value depends only on thecapacitance of the Helmholtz double layer per microscopic surfacearea, which is similar in each “metallic” nanocarbon CE.

Based on this basic consideration it becomes clear that the charge-transfer performance of a carbon nanomaterial CE can be optimized(RCE minimized) by introducing the electrode material in the form ofnanosized particles or nanotubes, packed densely into a porouselectrode structure (high AV) and employed as a sufficiently thick film.The target for RCE is b10 Ωcm2, or preferably around 1 Ωcm2 that isobtained with optimized planar Pt counter electrodes with aqueousand organic electrolytes [37]. This is a value low enough to causereasonably low to negligible voltage loss at the 10–20 mA/cm2 currentdensities produced by a typical DSC at 1000 W/m2 standard solarillumination conditions. Similar values have been reached withsome CNT electrodes in aqueous electrolytes [14] and with severalmicrometers thick nanoparticle carbon electrode in organic electro-lytes [8,9,33], demonstrating the potential of the carbon nanomaterialCEs.

It should be noted that although poor catalytic activity of a givencarbon nanomaterial CE material can be compensated by applying itas a thick porous film, doing so at the expense of increased totalthickness of the electrolyte filled volume, can bring rise to adverse

secondary effects due to electrolyte mass transport limitations at thephotoelectrode. In general, the CE thickness should be kept below fewtens of microns, which is a typical optimum thickness of the freeelectrolyte layer in the case of a planar Pt CE. Also, durability tobending is more demanding the thicker the porous CE film and maylimit suitability to roll-to-roll processing and flexible devices. Thisgives motivation to maximize the catalytic performance of carbonnanomaterial CEs so that they can be applied as thin and flexible films.

3.2. Sheet resistance

In the case when a carbon nanomaterial CE film is designed to beused also as a laterally conducting current collector layer in a cell, itshould have low sheet resistance, Rsh (Ω, or “Ω/square”)

Rsh = ρ�d−1 = σdð Þ−1 ð5Þ

where ρ� (Ωm) and σ (S/m) are the electrical resistivity and con-ductivity of the film parallel to the substrate. This depends on thenature and quality of the carbon nanomaterial (e.g. nanotubes vs.nanoparticles) and the structure of the film, and can therefore varygreatly from one type of film to another. The electrical conductivity ofCNT films thicker than a few monolayers cannot be rigorouslydescribed by the percolation theory, but must comply with a bulkmaterial conductivity law. The electrical conductivity or specificconductance can be calculated from

σ = enM; ð6Þ

where e is the elementary charge, n is the charge carrier concentration(per unit volume of the film), and M is the electrical mobility.

An alternative way to describe the film thickness is its opticaltransmittance, T, measured at 550 nm light wavelength. Since thetransmittance is related to the amount of carbon (layers) in the film,and light scattering and reflection may be neglected, the transmit-tance can be related to the film thickness by the Beer–Lambert law

T = e−� λð Þ p−1ð Þd ð7Þ

where �(λ) is an extinction (or absorptivity) coefficient, whichdepends on the wavelength, λ, and p is the porosity of the film.Usually electrical and optical properties of films are presented as adependence of sheet resistance versus transmittance, T. Using Eqs. 5, 6and 7 the dependence can be described by the following expression

Rsh =� 1−pð Þ

enM ln Tð Þ : ð8Þ

This dependence is in good agreement with our experimentalresults with SWCNT networks. Obviously, for a certain type of CNTs(�=const) with a certain porosity (p=const), the quality of the filmwill be determined by the concentration and mobility of the chargecarriers. The first parameter can be altered by the doping level ofCNTs, whereas the length of the tube determines the mobility ofcharge-carriers in the CNT film.

To compete with the currently used transparent FTO-coated glass(such as Pilkington TEC-8 or TEC-15) and ITO-coated polyethylenenaphtalate plastic substrates, the Rsh of the carbon nanomaterial CEfilm should be ca. 10–15 Ω/sq. or lower with optical transmittance of80–90% in the visible region.

4. Experimental details

Three characteristically different carbon nanomaterial CEs werecompared: a) random network SWCNT film on glass substrate withoutadditional conductive or catalytic layers, b) aligned MWCNT forest filmon quartz and Inconel nickel–chromium–steel alloy substrates, and

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c) carbon nanoparticle composite film on ITO-PET plastic substrate[33]. With random network CNT films it is possible to achieve ratherlow sheet resistances already at transparent thicknesses, and alsocatalytic activity has been reported [12–14]. With CNT forest filmsperpendicular-to-the-substrate conductivity is feasible, and catalyticactivity has also been reported [31], but an additional conductive layermust be used under the film. Carbon nanoparticle composite filmmaterials are cheap and good catalytic activities can be achieved withrelatively thick films [8,9,33].

4.1. Random network SWCNT counter electrodes

4.1.1. SWCNT synthesisIn the process, the SWCNTs are first synthesized by an aerosol CVD

process, in which both the synthesis of iron catalyst particles andcatalytic growth of SWCNTs takes place in laminar flow of carbonmonoxide (CO) in a heated tube furnace in temperatures of about900 °C (for detailed description, see [38]). The synthesized SWCNTstend to form bundles in the aerosol flow, which are the conductiveelement in the SWCNT networks. The SWCNT network film can beformed by filtration of the gas through a nitrocellulose filter at theoutlet of the reactor. The formed SWCNT network can then betransferred from the nitrocellulose filter to the preferred substrate(plastic, glass, quartz) by simply pressing the membrane filter with aSWCNT network to the substrate with moderate pressure of about1 kPa. The transfer process eliminates liquid processing prior thedeposition making the process fast and non-defective on the SWCNTquality. The size and thickness of the SWCNT networks can be tunedsimply by varying the size of the filter and the duration of thefiltration, respectively.

4.1.2. Electrode preparationThree sample sets with different optical transmittance and

electrical sheet resistance were prepared for this study with thepreviously described aerosol CVD synthesis and filtration process on amembrane filter. The diameter of the SWCNT film collection area onthe circular membrane filter was 110 mm. The transmittances of thefilms were 82, 45, and 5%, corresponding to sheet resistances of 214,80, and 20–30 Ω/sq., respectively.

The SWCNT network was cut to a rectangular shape (13 mm×8 mm), while still supported by the membrane filter. The individualelectrodes were transferred by the press-transfer method to 1 mmthick borosilicate microscope glass slides (cut to the size of 20 mm×25 mm). The glass functions as a supporting structure for the SWCNTnetwork electrode. The SWCNT network was densified with ethanol(Etax Aa, Altia), by drop-casting ethanol on the SWCNT network onthe substrate and letting it evaporate in ambient conditions. Thesurface tension of the evaporating ethanol layer compresses thenetwork in out-of-plane direction compacting the film and improvesthe film adhesion [39].

4.2. Vertically aligned MWCNT forest counter electrodes

Catalytic chemical vapor deposition (CCVD) is a very versatilemethod for producing vertically aligned MWCNT forests in acontrolled manner [40]. A typical CNT growth temperature in themethod is 700–800 °C, and the CNTs grow on predefined growthtemplates, which are usually silicon, silicon dioxide, aluminum oxideor zeolite. The catalyst needed for the MWCNT growth is typically atransition metal such as iron, cobalt or nickel, and the catalyst isintroduced to the process together with the CNT precursor. The metalcatalyst nanoparticles, formed by decomposition of e.g. ferrocene, aredeposited all over the template surface, but depending on thetemplate material, the nanoparticles may interact with the templatesurface forming inactive compounds for MWCNT growth.Whenmetalis used under the catalyst nanoparticles, the selectivity is not so

obvious but still exists to some extent. However, if a thick (∼200 nm)titanium film is usedwith iron catalyst in the CCVD process, selectivityof CNT growth can be observed with quartz and Inconel templates, i.e.CNTs grow mainly on SiO2 and Inconel, but not so much on titanium.

4.2.1. Preparation of substratesThe substrates were i) nickel–chromium–steel alloy sheets

(tradename Inconel alloy 600, composition by weight-% Ni 72%, Cr16% and Fe 8%, Goodfellow Cambridge Limited) with thickness of0.2 mm and area 50 mm×50 mm and ii) quartz wafer (double sidepolished, Silicon Quest International, Inc.) with diameter of 100 mmand thickness of 0.5 mm.

On these substrates, 250 nm of titaniumwas deposited by physicalvapor deposition and patterned with a lift-off process (photoresistsLOR10B, Microchem Corp., and AZ1512HS, MicroChemicals GmbH),resulting in titanium covered substrates having openings onto nickelalloy and quartz surfaces to enable MWCNT growth. On the nickelalloy substrate, the MWCNT growth opening was uniform with size14 mm×6 mm. On the quartz substrate the MWCNT growth openingconsisted of an array of squares, square size was 100 μm×100 μmwith 50 μm spacing. The square array area was 14 mm×6 mm, andthe reason for such grid structure was to get sufficient electricalcontact to the MWCNT film on non-conducting quartz substrate.

After titanium deposition and patterning, the nickel alloy andquartz sheets were cut to the DSC counter electrode size, which was20 mm×16 mm. The nickel alloy substrates were cut by using Nd:YAG pulsed laser and the quartz substrates by using Nd:YVO4 pulsedlaser. The diced nickel alloy and quartz substrates were washed withacetone in an ultrasonic bath for 3 minutes in room temperature.

4.2.2. Catalytic chemical vapor deposition of MWCNT filmsA horizontal tube reactor was pumped to a base pressure below

70 Pa and purged with argon. The argon flow rate was then set to∼40 ml/min and the reactor was heated to 785 °C. ∼1 ml precursorsolution (20 g ferrocene in 1000 ml xylene) was injected into anevaporator column pre-heated to 185 °C, and then the vapor wasintroduced into the reactor with the pre-set flow rate of argon gas.After the pre-feeding step, the precursor flow rate was adjusted to0.1 ml/min and maintained until the end of the process, which were10 min for nickel alloy substrate and 4 min for quartz resulting inMWCNT films of thicknesses 10–50 μm.

4.2.3. Deposition of platinum catalyst on the MWCNT filmsSome of the MWCNT forest films were additionally deposited with

Pt catalyst particles. Platinum bis(acetylacetonate) dissolved intoluene (8.9 mg/ml) was drop casted on the MWCNT films until40 wt.-% Pt loading was reached. Platinum bis(acetylacetonate) wasreduced to Pt in H2/Ar (5/95%) stream at 105 °C for 30 min and at200 °C for 120 min.

4.3. Carbon nanoparticle composite electrodes

A previously published method for preparing carbon nanoparticlecomposite electrodes by spraying and room-temperature compres-sion was used [33]. In short, carbon nanoparticles (Printex XE2 fromDegussa) with average particle size of 30 nm, graphite and nanocrys-talline TiO2 were ground together in a mortar in a small amount ofethanol, after which more ethanol was added and the mixture wasstirred overnight to produce a suspension fluid enough for spray-deposition. The paste was deposited on 20 mm×20 mm sized ITO-PET plastic (Bekaert Specialty Films) substrates. Mechanical integrityof the CE films was achieved by pressing the dried powder layers in ahydraulic press at room temperature. Typical pressing forces variedbetween 40 and 60 kN (equaling 1000–1500 kg/cm2), yielding filmsof 10–20 μm thickness.

Fig. 2. The equivalent circuit used in the fitting of the EIS data of the CE–CE cells with aCNT film as the other CE (random network SWCNT film on glass or MWCNT forest filmon steel or quartz) and Pt on FTO glass as the other CE. R denotes a resistor and CPE aconstant phase element. The equivalent circuit was similar for the carbon nanoparticleon ITO-PET CE–CE cell and for the reference Pt on FTO CE–CE cell, but comprised of onlyone parallel connected resistor and constant phase element, since these CE–CE cell hadtwo identical CEs.

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4.4. Cell preparation

The counter electrode materials were tested in a thin layerelectrochemical cell configuration, where the two counter electrodesare faced with each other [34], called hereafter CE–CE cells. One of theelectrodeswas the studied carbon nanomaterial CE, whereas the otherone was conventional thermally deposited platinum on FTO glass, theproperties of which were determined independently in symmetricCE–CE cells. An exception to this is the pressed carbon nanoparticlecomposite CEs, which were measured in a symmetric CE-CE cell withtwo similar CEs. Also the reference Pt on FTO CE–CE cells comprised oftwo identical CEs.

The platinized FTO glass CEs were prepared by cutting an FTO glasssheet (TEC-15 from Pilkington, sheet resistance about 15 Ω/sq.) to16 mm×20 mm pieces, which were then washed with mild deter-gent, followed by 3 minutes in ethanol in ultrasonic bath, and finally3 min in acetone. Thermal platinization was carried out by applying5 mM PtCl4 (Sigma-Aldrich) in isopropanol (Merck) on the substratesand firing them in oven in 385 °C for 15 min.

The CE–CE cells were prepared by attaching the electrodes to eachother with a 25 μm Surlyn 1702 (DuPont) sealant by heating the cellsto 110 °C and applying pressure. In the case of the thick MWCNTforests, two layers of 45 μm Surlyn 1601 sealant was used, totalling in90 μm thickness, to exclude possible short-circuiting of the cells dueto thickest parts of the film. The electrolyte (0.5 M LiI, 0.03 M I2, and0.05 M 4-tert-butylpyridine in 3-methoxypropionitrile) was insertedto the cells through holes in the FTO glass electrode. The holes werethen closed with a 45 μm Surlyn 1601 and thin microscope glass byheating. Electrical contacts were made from copper tape on theelectrodes and applying conductive silver paint (Electrolube) on theinterface. Epoxy was applied on top of the silver to enhancemechanical durability, except in the case of random networkSWCNT film, since it was previously noticed that epoxy may havean adverse effect on the electrical contact at the silver-CNT interface.

4.5. Measurements

The MWCNT film thicknesses were determined with Veeco Dektak6M Stylus Profiler profilometer. The scanning electron microscopy(SEM) of the SWCNT films was carried out with a JEOL JSM 7500Fanalytical field-emission SEM (FESEM) at 2 kV acceleration voltageand transmission electron microscopy (TEM) with a Philips CM200FEG TEM at 200 kV. The MWCNT film was imaged with a Zeiss UltraPlus FESEM at 5 kV. The carbon nanoparticle composite films and itscomponents were characterized with a LEO DSN982 Gemini FESEM at2 kV.

The electrochemical impedance spectroscopy (EIS) measurementswere carried out in the symmetrical CE–CE cell configuration. Thespectra were obtained with a Zahner IM6 potentiostat at 0 V, in thefrequency range 1 MHz–1 Hz, starting from the high frequency end,going to low frequencies, and then back again to high frequencies. TheEIS spectra were fitted with ZView2 using equivalent circuit modelscomprising of resistors and constant phase elements, see Fig. 2. Thecharge-transfer resistances are obtained from the diameters of thesemi-circles ascribed to each electrode in the Nyquist plot of the EISspectra.

5. Results and discussion

5.1. Random network SWCNT film CEs

5.1.1. Structure and propertiesThe random network SWCNT film comprises of bundles of

individual CNTs on substrate. The bundles are somewhat alignedalong the substrate plane, but apart from that the structure is random,as can be seen in the SEM and TEM images of Fig. 3a–c. Individual

CNTs conduct electricity so well, that the bulk conductivity is dictatedby the high contact resistance between the tubes and bundles [19].The iron catalyst particles from the SWCNT synthesis are discernibleespecially in Fig. 3b–c.

Fig. 4 shows the dependence of the sheet resistance on the opticaltransparency in these kinds of SWCNT films. A model based on bulkconductivity and Beer–Lambert type light absorption (Eq. 8) describesthe data reasonably well, but fails to explain the saturation of the Tand RSH values. This may be an indication of a percolation limit to thefilm conductivity.

5.1.2. Electrochemical performanceFig. 5 displays EIS spectra (Nyquist plot) for typical CE–CE cells

with the carbon nanomaterial CEs and for the reference Pt on FTO CE–CE cells. The EIS spectra of the cells with a CNT CE (random networkSWCNT film on glass or MWCNT forest on steel or quartz) exhibit twoimpedance arcs, one ascribed to the CNT CE and the other one to the Pton FTO glass CE. The carbon nanoparticle composite CE–CE cell andthe Pt on FTO glass reference CE–CE cell spectra exhibit only one EISsemicircle for the CE, since the cells consisted of two totally identicalCEs. The electrochemical performance of the MWCNT forest CE andthe carbon nanoparticle CE is discussed in more detail in latersections.

Table 1 shows the charge-transfer resistance (RCE) of the SWCNTcounter electrode and the series resistance (RS) of the CE-CE cell fordifferent film thicknesses (in terms of optical transmittance T at550 nm) measured by EIS. The data shows quite a lot of variation, inparticular for the RCE, but with a general trend of lower RCE withthicker films, as expected from Eq. 1. The lowest values of ca. about50 Ωcm2 obtained with T=5% and 45%, can be considered reasonablefor low power DSC applications, but do not reach the target of10 Ωcm2 or less required in high performance DSCs at full sunlightconditions.

The RS of the cells, normalized to the active area of the films,decreases quite clearly with increasing SWCNT film thickness, i.e.decreasing transmittance (Table 1, Fig. 4), indicating that theresistance of the SWCNT film dominates the RS of the whole cell.Indeed, the sheet resistance of the two lowest transmittance SWCNTfilms were considerably higher than that of the FTO glass (15 Ω/sq.)used as the other substrate of the cell. However, at T=5%, the RS wassimilar (ca. 10 Ωcm2) as in a symmetrical CE-CE cell containing twoplatinized FTO glass CEs. In Fig. 4, an estimate for the RS contributionof the SWCNT CE (RS, TOTAL−RS, FTO) is plotted along the RSH of thesetype of SWCNT films. It can be seen from the figure, that the RS of thecells follows more or less linearly the RSH of the SWCNT films, inaccordance with the fact that they are related to each other by the cellgeometry only. There is quite a lot of variation in the RS contribution

Fig. 3. a) and b) SEM images of the random network SWCNT films at different magnifications. White spots are iron catalyst particles. c) TEM image of the SWCNT film with largemagnification. It can be seen that the CNT bundles comprise of individual CNTs. The black rounded shape is an iron catalyst particle.

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values, which is also seen in the variation of the RS values of the CE–CEcells with SWCNT films. The reason for this is not currently totallyclear, but might be due to for instance variation in the distance of thesilver contact to the active area of the cells or possible damage causedto the SWCNT film by the Surlyn sealant in the cell manufacturing.

Finally, the capacitance ascribed to the SWCNT CE grows withdecreasing transmittance (Table 1, Fig. 6). This implies that the wholefilm is active in the charge-transfer process, since the capacitance isexpected to increase with increasing film thickness (Eq. 3). Inquantitative terms, however, the observed linear relation betweenthe capacitance and transmittance (Fig. 6) is against the expectation

Fig. 4. Sheet resistance of the SWCNT films as a function of transmittance (lower x-axis). The fit (model) is based on Eq. 8. Also the series resistance contribution of theSWCNT CE to the total series resistance of the CE–CE cells (RtextrmS,TOTAL−RS,FTO) isplotted in the figure (upper x-axis). It can be seen that the shape of the latter followsthat of the former, since linear dependence between the resistances can be expected.

from Eqs. 3 and 7 that predict instead logarithmic dependence on T,but explaining the deviation is difficult based on this set of data.

5.2. MWCNT “forest” CEs

The MWCNT forest CEs consist of an array of individual 100 μm×100 μm sized MWCNT forest islands grown on Ti-free areas of thesubstrate as defined by the patterning of the Ti coating (Fig. 7a).According to profilometer measurements, the films were mostly 5–20 μm thick, but some films (or some parts of the films) wereconsiderably thicker, even 40–50 μm. Indeed, the SEM images revealthat the films have rather non-uniform surface (Fig. 7b). A closermagnification to the edge of the films (Fig. 7c) shows that theyconsist of a highly porous underlayer of MWCNT “trees” standing

Fig. 5. Typical EIS spectra (Nyquist plot) of the carbon nanomaterial CE–CE cells and thereference Pt on FTO CE–CE cells. The arrows point at the semicircles ascribed to thedifferent CE types, CNP refers to the carbon nanoparticle composite CE. For the CNP–CNP CE–CE cell, there are two impedance arcs, of which the one at the lowerfrequencies corresponds to the charge-transfer resistance, while the high frequency arcis an additional geometric effect described in detail in ref. [33]. The charge-transferresistances of the Pt–Pt CE–CE cells and the CNP–CNP CE–CE cells are obtained bydividing the semicircle diameters by two, since the cells contained two exactly identicalCEs.

Table 1Comparison of the charge-transfer resistance (RCE) and capacitance (CCE) of different carbon nanomaterial counter electrodes. The series resistance (RS) for the carbon nanoparticleCE cells can not be compared with the other cells, due to different cell geometry. Film thickness is given directly for the MWCNT (rough estimate) and nanoparticle films, but can beindicated only by the optical transmittance at 550 nm for the SWCNT films. Also RCECCE values are shown.

CE type No ofcells

RSH(Ωcm2)

T (%) RCE(Ωcm2)

RCE, ave(Ωcm2)

CCE(μFcm−2)

CCE, ave(μFcm−2)

RS(Ωcm2)

RS, ave(Ωcm2)

RCECCE(mΩF)

RCECCEave

Pt on FTO glass 4 15 ca. 80 10–13 11 5.0–5.8 5.5 9.1–10 9.9 0.083–0.095 0.088SWCNT network on glass 3 214 82 107–384 304 18–25 21 27–54 42 2.0–10 6.4

4 80 45 47–266 129 87–98 91 9.6–17 13 4.2–24 122 20–30 5 54–181 118 130–160 150 8–12 10 8.9–24 16

d(μm)MWCNT (pristine) on Inconel 3 – 5–20 52–57 54 270–420 340 6.5–9.0 7.8 14–24 19MWCNT+Pt on Inconel 6 – 1–10 6.5–11 7.5 61–740 250 5.4–7.0 6.0 0.54–2.9 1.3MWCNT+Pt on quartz 2 – 5–35 7.4–17 12 210–380 300 42–46 44 3.0–3.6 3.3Carbon nanoparticle composite 6 – 10–20 0.6–1.8 1.3 1200–3700 2400 – – 2.0–5.2 2.9

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perpendicular to the substrate connected to a denser top layer“foliage” of randomly oriented MWCNTs and possibly amorphouscarbon. Due to the structure of the films and curvature of the metalsubstrates, only rough estimates for the film thickness could bederived from profilometer measurements as listed in Table 1.

The RCE of pristine MWCNT films (i.e. without Pt) was ca. 55 Ωcm2

and did not thus reach the target b10 Ωcm2. To investigate whetherthe reason was due to low catalytic activity or low conductivity of theMWCNT forest, samples both on Inconel and quartz were depositedwith additional Pt. Quite expectedly, the RCE decreased significantly,below 10Ωcm2 in the best cases. However, the fact that 200 timesmore Pt was used per geometric area of the electrode compared to Pton FTO glass, demonstrates rather poor utilization of the Pt. This issurprising given that the Pt was present as nanoparticles of theMWCNT surface (Fig. 7), and hence had high active surface area. Thereason for the poor Pt utilization may be related to insufficientperpendicular-to-the-plane conductivity: electric current from thesubstrate does not penetrate the whole thickness of the film but istransferred to the electrolyte already close to the substrate due to highconcentration of Pt on tubes. This is in contradiction with previousstudies where it was noticed that CNT forest films conduct electricitybetter in the vertical direction than in the horizontal, in-planedirection [24,25]. Another reason for the bad vertical conductivitycould be a contact problem between the MWCNTs and the substrate.

Finally, the RS (Table 1) was relatively low (less than 10Ωcm2) forthe MWCNT films on Inconel substrates, as expected due to goodconductivity of these substrates, but for the CNT films on quartz it wasover 40 Ωcm2, indicating high sheet resistance of the titanium layer orits poor contact to the MWCNT forests.

Fig. 6. Capacitance of the CNT films decreases with decreasing film thickness, implyingthat the total microscopic surface area grows with growing film thickness. The fit isbased on Eqs. 3 and 7.

5.3. Pressed carbon nanoparticle composite CEs

The structure of the carbon nanoparticle composite films, pressedat room temperature on ITO-PET plastic substrate, is visible from theelectron micrographs in Fig. 8. The films have several micrometerssized graphite flakes (Fig. 8a) surrounded by much finer material(Fig.8b) consisting of a mixture of carbon black and TiO2 particleswith diameters of ca. 30 nm and 10 nm, respectively (Fig. 8c–d).From the structure, the role of the different materials can beunderstood: the graphite flakes provide good conductivity over largedistances in the film, whereas the small carbon black particlesincrease its microscopic surface area and catalytic activity.

Table 1 lists RCE of the carbon nanoparticle composite electrodesreported previously [33], and complemented here with capacitancedata. The low RCE values of 0.5–2 Ωcm2 [33] can be explained not onlywith the natural catalytic tendency of the carbon black nanoparticlesbut also with the high active surface area provided by the porous film.When compared to values typical for platinum CEs, e.g. 0.8–11 Ωcm2

for sputtered Pt [41,42] and 4–5 Ωcm2 for thermal Pt [33] this showsthat carbonmaterials have potential to replace Pt as a CE catalyst evenin flexible DSCs prepared at low temperatures—sufficient electricalcontact within the film and to the substrate can be reached by simplemechanical pressing instead of heat treatment. However, these filmsendured bending only at relatively low thicknesses (few microme-ters), implying that the physical contacts between individual particlesand of the film to the substrate should be improved. One option is tomix polymers as binders to the composite structure. Indeed, agelatinized carbon nanoparticle composite CE, modified from thepresent materials by addition of a gelling agent, was used previouslyto construct a photovoltaic fiber resulting in more durable films [43].

5.4. Comparison

Some qualitative conclusions on the differences of the materialscan be drawn. When comparing the catalytic and conductive perfor-mance of the different carbon nanomaterials, it must be borne in mindthat their properties depend greatly on the synthesis method. Forexample, CNTmaterials synthesized in large volumes display typicallya broad distribution of individual tube qualities (such as chirality,thickness, length, etc.), and effective large-scale techniques for sortingthem according to these properties do not presently exist, nor generalmethods for growing films with specific kind of tubes. This means thatin practice CNTs cannot be regarded as a simple raw material withpredictable behavior, and that the performance of CNT films in devicesdepends greatly on their properties.

In this study, the thickness of the random network CNT films wasin the few tens to few hundreds of nanometers range and the filmswere still more or less transparent, whereas the CNT forest and thecarbon nanoparticle film thicknesses seemed to be in the fewmicrometer range, making the films opaque. The random network

Fig. 7. A collage of SEM images of the MWCNT forest films. a)–d) MWCNT film at different magnifications. Images with larger magnification show individual MWCNT bundlesc) without and d) with Pt.

8132 K. Aitola et al. / Thin Solid Films 519 (2011) 8125–8134

CNT films were probably the most flexible of the materials [17]. Theyalso functioned both as the conductive and the catalyst layer. However,as with the pressed carbon nanoparticle film, substrate adherenceseemed to be problematic for the random network CNT film.

The catalytic activities were good for the carbon nanoparticle films,and fair for the random network SWCNT film and the MWCNT forestfilm without platinum. It is anyhow known from the literature that arandom network CNT film on a conductive substrate is catalyticenough for normal DSC operation. The literature on the exact catalyticactivity of the random network CNT is rather scarce, but the quitegood DSC efficiencies and fill factors obtained with such films onconductive substrates indicate that the RCE is not a limiting factor forthe DSC behavior [12,14].

Fig. 8. A collage of SEM images of the carbon nanoparticle composite. a) The whole structure ananoparticles which appear as larger and lighter particles in the structure compared to the Tat larger magnification.

Quantitative comparison of these materials is difficult. The mate-rials' microstructures differ greatly, making predicting the electro-chemical behavior challenging. Systematic literature on the propertiesof the different carbon nanomaterials as DSC CEs (or in general) doesnot presently exist. Here, reliable thickness dependent comparison ofthe materials was prohibited by the difficulty of measuring thethickness for the extremely thin SWCNT films and large uncertaintyin it for theMWCNT forest films. However, using the product RCECCE as afigure of merit for the catalytic activity (Eq. 4), a comparison can still bemade, assuming that the cCE,A values are equal for all the different CEtypes.

It is clear from the RCECCE values in Table 1 that Pt has superior(more than 100 times higher) catalytic activity than the carbon

t lowmagnification. G denotes graphite flake. b) Larger magnification. C denotes carboniO2 particles. c) Pure carbon black particles at larger magnification d) Pure TiO2 particles

8133K. Aitola et al. / Thin Solid Films 519 (2011) 8125–8134

nanomaterials. Note also, that the values are similar for the thickestSWCNT films and the pristine MWCNT films, revealing that they havesimilar catalytic properties, whereas the carbon nanocomposite filmhas comparably higher catalytic activity.

Finally, comparison of the active surface area of the different CEscan be made based on the measured average capacitance (Table 1).Using the Pt on FTO glass as a reference, the active area is 3.8–27 timeslarger in the SWCNT films, 45–62 times larger in the MWCNT forests,and more than 400 times larger in the carbon nanoparticle films. Thereason for the surprisingly low active surface area in theMWCNT filmscompared to the carbon nanoparticle films can be explained by muchhigher porosity of the former, as can be seen from Figs. 7c and 8. Thisdemonstrates the fact that for good catalytic performance, the carbonnanomaterials have to be packed in a dense porous film, as alreadymentioned in Section 3.1.

5.5. Future perspectives

The field of carbon nanomaterial counter electrode materials forDSCs is most likely to experience great changes in the future, if andwhen carbon nanomaterial synthesis and film formation methodsdevelop further. For instance, even though individual CNTs are strong,a film can be mechanically weak in the sense that bundles do notadhere to each other well, and it is likely that the CNTs should beinterlinked into each other by bonds [44–46] or be in some composite(matrix) to really utilize their strength. CNT composites have actuallyalready been used at the DSC CE, for instance SWCNT-Nafion mixture[47] and TiN-nanoparticle-functionalized MWCNTs [48]. Other optionmight be spinning the CNTs into yarn or rope [49] and then making a“fabric” out of this.

Utilizing carbon nanoparticle-CNT composites seems like naturalnext step in the field of carbon nanomaterial electrodes, but this kindof a material has not been tested in a DSC so far. CNTs have been usedas conductive additives in lithium ion batteries, both at the anode andcathode [50]. In DSCs, such structure might improve the conductivityand bendability of carbon nanoparticle composite electrodes. Yet,mixing with polymers might still be necessary to secure bonding tothe substrate. Graphene [51] may also be a future possibility forcarbon nanomaterial DSC CEs, and it might offer the best conductivityvs. transparency. There are indeed some very preliminary results withthis material [52,53], but the films have contained very smallgraphene flakes, and the intriguing properties of larger graphenepieces have not been fully utilized.

6. Conclusions

Different carbon nanostructures were tested and their perfor-mance as the DSC counter electrode was discussed in terms of theircatalytic activity, active surface area, conductivity, thickness andtransparency, relating these properties to electrical, electrochemicaland structural information obtained from impedance spectroscopyand electron microscopy.

The best catalytic performance was achieved with carbonnanoparticle composite electrodes, which could be explained bytheir high catalytic activity and large total active surface area. Thecatalytic activity of carbon nanotubes was comparable in the randomnetwork SWCNT films without conductive layer beneath and in theMWCNT forest films without additional activation, but lowercompared to the carbon nanoparticle films. The charge-transferresistance of the both CNT electrode types was further limited bylow total active surface area, as deduced from their lower capacitancecompared to the carbon nanoparticle films.

The results of the paper highlight the basic properties anddifferences of platinum free carbon nanomaterial CEs for DSCs.

Acknowledgments

This work was carried out in the project Carbon NanoBud basedEnergy systems for Mobile Devices, CNB-E, of the TKK/AaltoUniversity Multidisciplinary Institute of Digitalisation and Energy,MIDE. K. A. gratefully acknowledges the graduate school of NordicCenter of Excellence in Photovoltaics for the scholarship. N. H.acknowledges the National Graduate School in Nanoscience forgraduate school position. A. N. thanks Academy of Finland (ProjectNo 128445). We gratefully acknowledge Dr. Unto Tapperfor the SEMimaging of the carbon nanoparticle counter electrodes.

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