Modification of mesoporous TiO 2 electrodes by surface treatment with titanium(IV), indium(III) and zirconium(IV) oxide precursors: preparation, characterization and photovoltaic performance in dye-sensitized nanocrystalline solar cells This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2007 Nanotechnology 18 125608 (http://iopscience.iop.org/0957-4484/18/12/125608) Download details: IP Address: 129.8.242.67 The article was downloaded on 16/09/2013 at 09:32 Please note that terms and conditions apply. View the table of contents for this issue, or go to the journal homepage for more Home Search Collections Journals About Contact us My IOPscience
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Modification of mesoporous TiO2 electrodes by surface treatment with titanium(IV), indium(III)
and zirconium(IV) oxide precursors: preparation, characterization and photovoltaic
performance in dye-sensitized nanocrystalline solar cells
This article has been downloaded from IOPscience. Please scroll down to see the full text article.
Modification of mesoporous TiO2electrodes by surface treatment withtitanium(IV), indium(III) andzirconium(IV) oxide precursors:preparation, characterization andphotovoltaic performance in dye-sensitizednanocrystalline solar cellsDavid B Menzies1,2, Qing Dai1,3, Laure Bourgeois2,Rachel A Caruso4, Yi-Bing Cheng1,2, George P Simon2 andLeone Spiccia1,3,5
1 ARC Australian Centre for Electromaterials Science, Monash University, Victoria 3800,Australia2 Department of Materials Engineering, Monash University, Victoria 3800, Australia3 School of Chemistry, Monash University, Victoria 3800, Australia4 Particulate and Fluids Processing Centre, School of Chemistry, Melbourne University,Victoria 3010, Australia
Received 24 October 2006, in final form 5 January 2007Published 21 February 2007Online at stacks.iop.org/Nano/18/125608
AbstractPost-treatment of titanium dioxide (TiO2) films for use in dye-sensitized solar cells has been carried outwith titanium(IV), indium(III) and zirconium(IV) oxide precursor solutions. The nanostructured electrodeswere characterized using nitrogen gas sorption (NGS), x-ray diffraction (XRD), x-ray photoelectronspectroscopy (XPS), energy dispersive x-ray spectroscopy (EDX), field emission scanning electronmicroscopy (FEGSEM) and high resolution transmission electron microscopy (HRTEM). The change in thenanostructure was quantified and the thicknesses of the core–shell coatings determined. An evaluation ofthe dependence of thickness by HRTEM concluded that one coating step of either the indium or zirconiumprecursor gave thicknesses of 0.5 nm, with EDX and XPS confirming the presence of either In or Zr at theTiO2 electrode surface, respectively. These working electrodes were then used to fabricate dye-sensitizednanocrystalline solar cells (DSSCs) whose performance was tested under AM1.5G 100 mW cm−2
illumination. TiCl4 post-treatment was found to improve the photovoltaic efficiencies from 3.6% to 5.3%.Single coatings of either In2O3 or ZrO2 on the TiO2 working electrode resulted in an increased efficiencyfrom 3.6% up to 5.0%. Thinner coatings gave the highest solar cell efficiency. The drop in performance wasmainly due to a decrease in short circuit current density (Jsc) with the greater shell thicknesses. ZrO2-coatedTiO2 electrodes subjected to microwave heat treatment using a 2.45 GHz microwave produced the highestefficiencies (5.6%) largely due to an increase in short circuit current from 11.4 to 13.3 mA cm−2.S Supplementary data are available from stacks.iop.org/Nano/18/125608
(Some figures in this article are in colour only in the electronic version)
5 Author to whom any correspondence should be addressed.
The dye-sensitized solar cell (DSSC, figure 1) has beenresearched extensively since it was reported in 1991 [1].The most attractive features of this technology are reducedproduction costs and ease of manufacture when compared totraditional solid-state silicon solar cells [2, 3]. Technologiesthat enable the mass production of DSSCs, including screenprinting and low-temperature pressureless sintering, are beingused to produce cells with reasonably good performance [4].Microwave processing, at 28 [5, 6] and 2.45 GHz [7],has been explored as a method for reducing the cost ofmanufacture of the TiO2 working electrodes as it can leadto the use of lower temperatures and production times.Recently, a lift-off technique has been used to construct DSSCswith a high efficiency on flexible substrates [8]. Theserecent investigations were aimed at making the commercialproduction of DSSCs more cost-effective, thereby improvingthe economic viability of the technology.
One source of energy loss in DSSCs, and other energyproducing devices, is charge recombination processes. Suchprocesses predominantly occur at the semiconductor–dye–electrolyte interface of the DSSC, when the electron is eitherdonated from the TiO2 conduction band back to the oxidizeddye molecules or to the I−3 in the electrolyte [2, 9, 10].However, charge recombination can also occur at the TCOglass support where oxidized species in the electrolyte canbe reduced by electrons flowing out of the cell [11, 12], theoverall effect being a reduction in the device current. Inprevious studies, dense TiO2 thin films have been depositedby spray pyrolysis in order to reduce the recombination at theconductive support [12–14]. Recently, Ito and co-workers [13]have compared the effect of TiCl4 pre-treatment and spraypyrolysis with the combination of TiCl4 post-treatment andfound that the latter is superior. However, there has beenrelatively little attention paid to full structural characterizationof the post-treated working electrodes, and to the use ofalternate titanium precursors [15]. This study focuses onelectrode post-treatment as well as the passivation of theTCO glass substrate to reduce charge recombination with theoxidized electrolyte species.
One further promising approach to minimizing chargerecombination involves the deposition of a thin oxide layeraround the semiconductor core, the so-called ‘core–shell’
TiO2
Platinumcatalyst
Glass Glass
TCO film
red
electrolyte
I3-
Ruthenium dye monolayer
TCO film
Glass Glass
TCO film
I- I3-
ox
I- I3-
semiconductor
Figure 1. Schematic diagram of the dye-sensitized solar cell.
approach (figure 2). This shell is generally formed bycoating the semiconducting working electrode with a metaloxide that has a higher conduction band edge than thesemiconductor itself [10, 16–25]. The coating allowsthe electrons generated upon photoexcitation of the dyeto tunnel through the shell to the semiconductor coreand, subsequently, into the conducting oxide layer, whilstsimultaneously creating an energy barrier for recombination.Recombination at the semiconductor/dye/electrolyte interfaceis reduced if the conduction band of the coating lies abovethe lowest unoccupied molecular orbital (LUMO) energy levelof the sensitizer and/or the conduction band of the coresemiconductor [20]. Previous studies utilizing anatase TiO2
have included shells of Nb2O5 [23], ZrO2 [18–20], ZnO [26],Al2O3 [20–22], SiO2 [20, 21], MgO [21, 25], SrTiO3 [17] andY2O3 [21]. As the shell thickness increases, the efficiencyshould decrease due to the increased electron tunnelling paththrough the oxide layer into the core semiconductor [23]. Inthis work, indium oxide or zirconium oxide were coated ontitanium dioxide, with bulk conduction band energies for thethree materials being −3.88, −3.41 and −4.21 eV versusthe absolute vacuum scale (AVS), respectively, [27] to forma core–shell nanostructure. It should be noted, however, thatwhen nanometre thick coatings are utilized, due to the quantumsize effect the conduction band energy may not be the same asfor single-crystal materials. The respective bandgaps for bulkindium oxide, zirconium oxide and titanium dioxide are 2.8,5.0 and 3.2 eV and surface pH (pHpzc) for the three materialsare 8.64, 5.80 and 6.70, respectively [27].
Although many methods are available for depositing a thinoxide coating on the surface of the porous semiconductor inDSSCs, conventional sol–gel post-treatment [16] is generallypreferred as it is more amenable to mass productionand results in a lower production cost in comparison toother methods, such as vapour deposition techniques. Inthis process, a conformal layer can be deposited on themesoporous semiconductor, as the solution can penetratethe pores in the electrode [23], resulting in a true core–shell nanostructured electrode. Kay and Gratzel [21] foundthat better device performance could be achieved when theoxide films were deposited directly on the semiconductorfilm rather than on the individual nanocrystals. In this
Ru dye monolayer
Shell
TiO2 core
Figure 2. Schematic diagram of the core–shell TiO2 workingelectrode.
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work, thin films of three oxide materials (TiO2, In2O3 andZrO2) were deposited onto the TiO2 electrodes from dilutemetal alkoxide precursor solutions. In the case of In2O3,multiple coatings were applied to evaluate the effect ofcoating thicknesses on DSSC efficiency. For the ZrO2-coated working electrodes sintering was carried out usingboth conventional and microwave heating. The films wereanalysed via powder x-ray diffraction, nitrogen gas sorption,field emission scanning electron microscopy, energy dispersivex-ray spectroscopy, x-ray photoelectron spectroscopy and highresolution transmission electron microscopy to allow accuratedetermination of the resulting morphology. They were thenused to assemble DSSCs so that the effect of the oxide materialcoatings, film thickness and sintering media on photovoltaicperformance could be determined.
2. Experimental methods
2.1. Working electrode preparation
A slurry of TiO2 was produced by combining TiO2
nanoparticles (Degussa P25) with acetylacetone (BDH), TritonX-100 (Lancaster) and distilled water. The slurry was groundin a mortar and pestle until a smooth consistency was achieved.Films were doctor-bladed into a well created by placingScotch TapeTM (3M) on F-doped SnO2 (14 �/�) glass (TCO,Hartford). The TiO2 film was then cut to 0.25 cm2 electrodesand sintered at 450 ◦C for 30 min in air and then stored ina desiccator under vacuum until required for use in DSSCtesting.
Three dilute TiO2 precursor solutions were used to post-treat the working electrodes. Solutions of titanium(IV)isopropoxide (Aldrich) and titanium(IV) butoxide (Aldrich)were prepared by diluting the commercial reagents (Aldrich)to 0.05 M in the parent alcohol (Merck) while the titanium(IV)chloride coating solution was produced by diluting theprecursor (Aldrich) down to 2 M with ethanol at 0 ◦C under N2.This stock solution was further diluted with ethanol to 0.05 Mprior to use. Precursor solutions consisting of indium(III)acetate (Aldrich) dissolved in a 1:1 acetic acid and distilledwater mixture, and of zirconium(IV) isopropoxide (Aldrich)dissolved in isopropanol were used to deposit In2O3 andZrO2 coatings, respectively (both with metal concentrations of0.05 M). These coating solutions were applied to the substratesby spin-coating (Headway Research Inc.) at 2000 rpm for10 s. The working electrodes were then hydrolyzed in a water-saturated atmosphere at 70 ◦C for 30 min. Some electrodeswere subjected to multiple coatings of In2O3 by repeating thespin coating and hydrolysis procedure for up to five times.
After hydrolysis, most of the working electrodes werecalcined at 450 ◦C for 15 min in air to remove residualorganics and to promote interparticle connection. Some of theZrO2-coated working electrodes were calcined in a 2.45 GHzmicrowave oven (MPC, Model 941) utilizing the variablepower (0–1 kW) and a three-stub tuner to control the heatingrate using a technique described previously [7, 28]. A forwardpower of 500 W was used until 450 ◦C was achieved, then thepower was adjusted to ensure that 450 ± 5 ◦C was held constantfor 15 min. For comparison, electrodes that had not beentreated with the titanium(IV), indium(III) and zirconium(IV)
precursor solutions were sintered alongside the post-treatedelectrodes.
After heat treatment, the working electrodes were cooledto 80 ◦C and placed into a 0.3 mM ethanolic solution ofN719 ((n-Bu4N)2[Ru(Hdcbpy)2(NCS)2], where dcbpy is 4,4′-dicarboxyl-2,2′-bipyridine) for >24 h at room temperature.They were carefully rinsed in acetonitrile after removal fromthe dye solution.
2.2. Working electrode characterization
Profilometry (AltiSurf 500, Cotec) was used to determine thefilm thickness, surface roughness and topography of the TiO2
films. Gas sorption (Tristar 3000, Micromeritics) with a N2
degas at 150 ◦C overnight was used to obtain the Brunauer,Emmett and Teller (BET) surface area. Powder x-raydiffraction (XRD, Philips 1130) enabled the characterization ofcrystalline phases present in the core of the working electrodes,with a penetration depth of a few microns. Cu Kα radiation wasused at 40 kV and 25 mA, with a step size of 0.05◦ 2θ and ascan rate of 1◦ 2θ min−1.
Field emission scanning electron microscopy (FEGSEM,Phillips XL30) enabled the characterization of the morphologyintrinsic to each sample. FEGSEM measurements were takenat a working distance of 11 mm at 5 keV. In addition, energydispersive x-ray spectroscopy (EDX, Oxford Instruments LinkISIS) was used to determine the chemical species at the surfaceof the working electrodes. Measurements were made at 20 keVon an area of 1.45 μm × 2 μm.
High resolution transmission electron microscopy(HRTEM) was used to characterize the core–shell morphologyand determine the thickness and surface coverage of the In2O3
coating. Samples were prepared by scratching the films di-rectly onto a carbon-coated copper grid. The instrument (JEOLJEM2011) has a thermionic electron source and is operated at200 kV, at which voltage its point resolution is 0.23 nm. Energydispersive x-ray spectroscopy (EDX, Oxford Instruments Inc.)was conducted using a stationary electron probe of diameterranging from 10 to 25 nm, in order to establish the interparticlenecks and core–shell morphologies.
X-ray photoelectron spectroscopy (XPS) experimentswere performed on an axis ultra spectrometer (KratosAnalytical Ltd, UK) with monochromated Al Kα x-rays(hν = 1486.6 eV) operating at 150 W. Survey and highresolution region spectra were acquired using 160 and 20 eVpass energies, respectively. The spectrometer energy scalewas calibrated using the Au 4f7/2 peak at binding energy(Eb = 83.98 eV). Spectra were charge-corrected assumingthe hydrocarbon component occurred at Eb = 285.0 eV.The analysis area was 700 μm × 300 μm. Spectra werequantified using the elemental sensitivity factors provided bythe manufacturer after background subtraction and fitting ofGaussian (70%)/Lorentzian (30%) component peaks.
2.3. Dye-sensitized solar cells
Counter-electrodes were produced by coating TCO glass using5 mM hexachloroplatinic acid hydrate (Fluka). The counter-electrodes were then heated at a rate of 2 ◦C min−1 to 385 ◦Cfor 15 min and cooled to room temperature, resulting in atransparent platinum catalyst coating [29]. A 25 μm thick
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Nanotechnology 18 (2007) 125608 D B Menzies et al
SurlynTM (Dupont) gasket was then cut to fit around the0.25 cm2 electrodes and placed on the counter-electrodes at100 ◦C. Working electrodes were then sandwiched to thecounter-electrodes to seal the cell. The electrolyte (0.6 Mtetrabutylammonium iodide (Aldrich), 0.1 M LiI (Lancaster),0.1 M I2 (Lancaster) and 0.5 M 4-tert-butylpyridine (Aldrich)in acetonitrile) was then administered by a syringe througha hole in the counter-electrode. This was filled into thecavity between the two electrodes by placing the DSSC withelectrolyte over the hole in the counter-electrode into vacuumand then releasing the vacuum after all the air had escapedfrom between the two electrodes. The hole at the rear of thecounter-electrode was then cleaned and sealed with a Surlyn-coated glass slide at 80 ◦C.
The photocurrent–voltage (J –V ) curves were determinedunder illumination (100 mW cm−2, AM1.5) with a SolarSimulator (1000 W Xe, Oriel). Labview 6.1 software (NationalInstruments) was used to drive a Keithley 2400 source meterand to record the current across a linear voltage sweep from0.9 to 0 V. Efficiencies were calculated by equation (1), whereη is the efficiency, (J × V )max is the maximum power andPlight is the power of the incident light. Plight was achievedby measuring the output of a KG filtered photodiode (S1787-04, Hamamatsu) that was calibrated externally under the samemodel solar simulator to 100 mW cm−2
η (%) = (J × V )max
Plight× 100. (1)
3. Results and discussion
This study examines the effect of three types of coatings onthe structure, properties and photovoltaic performance of TiO2
nanostructured electrodes:
(i) titanium dioxide (TiO2) deposited using dilute alcoholsolutions of TiCl4 and titanium isopropoxide (Ti(Oi Pr)4,denoted TiP) and titanium butoxide (Ti(Oi Bu)4, denotedTiB);
(ii) indium oxide (In2O3) using a dilute solution of indiumacetate in a 1:1 mixture of water and acetic acid;
(iii) zirconium dioxide (ZrO2) using a dilute solution ofzirconium isopropoxide (Zr(Oi Pr)4) in isopropanol.
3.1. Working electrode preparation
Half of the TCO glass substrates were coated with a densethin film of TiO2 (<20 nm), by applying 0.05 M TiCl4
in ethanol, hydrolyzing and finally calcining at 450 ◦C for15 min, to analyse the effect of this treatment on DSSCperformance. TiO2 films made from the Degussa P25powder were then deposited on the TiCl4 pre-treated anduntreated TCO glass substrates and were sintered at 450 ◦Cfor 30 min in a conventional furnace in air. Some of theseworking electrodes were then further coated with the titanium-,indium-or zirconium-containing precursor solutions. Thecoating solutions were stable and clear, indicating that theprecursor had not undergone condensation reactions leading toprecipitation during storage. The solutions were applied to theworking electrodes via spin-coating and were then hydrolyzed
in a water-soaked atmosphere. For some of the In2O3-coated working electrodes this process was repeated to achievemultiple coatings. No visible changes in the morphologyof the working electrodes were noticed from this depositiontechnique. With the exception of some ZrO2-coated workingelectrodes, which were sintered in a 2.45 GHz microwavefurnace, the coated electrodes were calcined at 450 ◦C for15 min in a conventional muffle furnace. No problemswith sparking or interference with the conductive oxideor semiconductor coatings was observed during microwaveheating using a published methodology [7].
3.2. Working electrode characterization
The detailed characterization of the nanostructured TiO2
films, via profilometry, BET, powder XRD, FEGSEM, EDX,XPS and HRTEM, have been carried out to determine theeffect of the various types of oxide coatings on the surfacemicrostructure and morphology of the electrodes.
3.2.1. General characterization methods. Profilometry foundthat the nanostructured TiO2 working electrode films to be10(±1) μm in thickness. As expected, the thickness wasunaffected by the coating solution used and by the applicationof multiple coatings of In2O3. The surface topographies ofthe In2O3 coatings have been highlighted previously [30].The consistent thickness of the films used facilitated theinterpretation of the DSSC performance (vide infra).
BET analysis found that surface areas for the uncoatedTiO2 electrodes either conventionally or microwave heatedwere 50 m2 g−1. This was reduced to 46–47 m2 g−1 forTiCl4, TiP and TiB-coated electrodes. Application of 1 to 3of coatings of In2O3 to produce In2O3 core–shell electrodesgave the same surface area (46–47 m2 g−1), but a drop to43 m2 g−1 was found for five coatings. The surface area ofthe microwave sintered ZrO2-coated TiO2 working electrodeswas 50–52 m2 g−1 and was the same for conventionally andmicrowave heated TiO2 electrodes. The slight decreases inthe surface areas of the coated working electrode films mayreduce the photocurrents of the DSSC devices by lowering dyeadsorption.
The XRD traces (figure 3) indicate that the three types ofcoatings have no effect on the TiO2 phase composition as thereis no apparent shift in the existing peaks of the dual phase TiO2
layer (rutile and anatase) which is estimated to consist of 20%rutile and 80% anatase, consistent with the crystal data for thecommercial powder (Degussa P25).
3.2.2. FEGSEM and EDX analysis. The FEGSEM images(figure 4) show that the morphology of the TiO2 workingelectrode is a disordered network of nanoparticles (25 nmaverage particle size). The expected pore-size distribution ofthis nanoporous film would therefore be quite broad. Themorphology was unchanged after sol–gel deposition of thethree types of TiO2 precursor solutions (figures 4(b)–(d)).
The morphology of the TiO2 electrodes remainedunchanged even after multiple coats with In2O3 (figure 4).Since the In2O3 coating on the TiO2 substrate is �1 nm,and the electrons have a penetration depth of approximately2.5 μm, the presence of indium (Lα) peak at ∼3.3 eV) in
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Table 1. XPS data of the conventionally heat-treated uncoated, titania-and indium-oxide-coated TiO2 electrodes showing binding energies(BE, eV) and the relative atomic concentrations (C , %).
Figure 3. XRD spectra of (a) bare TiO2, (b) TiCl4-coated TiO2,(c) In2O3-coated TiO2, (d) 2.45 GHz microwave calcined bare TiO2
and (e) 2.45 GHz microwave calcined ZrO2-coated TiO2 workingelectrodes; where A is anatase TiO2 and R is rutile TiO2.
the spectra was relatively low (figure 5(B)) but, as expected,samples subjected to multiple coatings of In2O3 displayed anincreased In Lα peak (supplementary figure S1 available atstacks.iop.org/Nano/18/125608). The bare and ZrO2-coatedTiO2 working electrodes that were calcined in the 2.45 GHzmicrowave had the same morphology as the conventionallysintered working electrodes (figure 4). EDX (figure 5(C))showed the presence of the zirconia Lα peak (at ∼2.05 eV)in the coated sample and that the Ti Kα and Kβ compositionswere unaltered.
3.2.3. XPS analysis. The x-ray photoelectron spectroscopy(XPS) results summarized in table 1 show that the content ofTi, O and C at the surface of the nanoparticles is relativelyunchanged after the three titanium precursor post-treatments.However, changes in the surface composition were evident forthe In2O3-coated electrodes (table 1), the most obvious beingthe presence of the indium peaks. XPS is more sensitive thanEDX to surface chemistry changes due to the lower penetrationdepth of 10 nm by the incident x-ray beam. The position of theIn 3d peak is consistent with the presence of In2O3 and displaysa steady increase with In2O3 coating thickness. In addition, asteady decrease in the titanium content is observed, which isindicative of the formation of a shell coating on the TiO2 core.
Table 2. XPS data of the 2.45 GHz microwave calcined uncoatedand zirconia-coated TiO2 electrodes showing binding energies (BE,eV) and the relative atomic concentrations (C , %).
In support of the formation of the core–shell nanostructure isthe gradual decrease in the oxygen content due to the loweroxygen to cation ratio for In2O3, compared with TiO2. Further,there is an increase in carbon content with the increase in In2O3
coatings, which is possibly due to incomplete combustion ofresidual acetic acid in the film [31].
The microwave calcined ZrO2-coated films (table 2) showno visible difference in the surface composition comparedto the conventionally heated electrodes. The presence ofzirconium at the surface of the TiO2 nanostructure can benoted. The oxygen and carbon contents are similar for the bareand ZrO2-coated TiO2 samples and show that both microwavecalcinations and conventional heat treatment at 450 ◦C wereeffective at removing the organics from the films (table 1).This is the same as seen with conventional heat treatments withother coating solutions [20, 30].
3.2.4. HRTEM analysis. HRTEM images (figure 6) arein agreement with the FEGSEM data (figure 4), confirmingan average particle size of about 25 nm, consistent withthe starting material. Due to the irregular connection ofthe nanoparticles, the pore structure in the TiO2 film is nothomogeneous. The particle size was unaffected after TiCl4post-treatment and clear lattice fringes reflected the anataseand rutile crystalline nature of the P25 powder. However,after this TiCl4 post-treatment a rougher surface was observed(figure 6), indicating the formation of an amorphous coating,and interparticle connections (arrowed in figure 6) have been
Figure 4. FEGSEM of (a) bare, (b) TiCl4-coated, (c) single indium-oxide-coated and (d) 2.45 GHz microwave calcined zirconia-coated TiO2
working electrodes.
A
Ti Kϒ
O KϒC
1000
2000
3000
4000
5000
000
a
b
B
c
d
a
C b
A Ti K°
Ti K°
O K°C K°
a
b
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c
d
a
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0
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Cou
nts
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nts
Cou
nts
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3.25 3.3 3.353.2 3.4 1.95 2 2.05 2.1 2.15100
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keV
1.9 2.2
keV0 6
Figure 5. EDX of the (A) TiO2 working electrode; (B) expanded region for the (a) bare TiO2, and films coated with (b) one, (c) three and(d) five layers of In2O3, and (C) 2.45 GHz microwave calcined (a) bare and (b) ZrO2-coated TiO2 working electrodes.
enhanced. These interparticle connections are clearly broaderthan in the uncoated TiO2 due to the accumulation of titaniaoligomers during the coating procedure via capillary forces.Subsequent calcination results in formation of TiO2 in these
critical regions. The interparticle connections provide thephoto-injected electrons in the TiO2 conduction band with aless tortuous pathway from particle to particle through thenanostructure to the conductive glass. This is advantageous as
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10nm10nm 10nm10nm
a b
10nm10nm 10nm10nm
c d
10nm10nm 10nm10nm10nm10nm 10nm10nm
a b
10nm10nm 10nm10nm
c d
Figure 6. HRTEM pictures of (a) bare and (b)–(d) TiCl4-coated TiO2 with broadened interparticle neck regions with the broadened necksindicated by arrows.
a
10 nm
10 nm 10 nmc d
10 nm
b
Figure 7. HRTEM pictures of (a) uncoated TiO2, (b) 1 In2O3 coat on TiO2, (c) 3 In2O3 coats on TiO2 and (d) 5 In2O3 coats on TiO2 with theshell coatings indicated by arrows.
the internal resistance for electron migration through the TiO2
film to the TCO-coated substrate would be reduced, facilitatingcharge collection [4, 13].
Amorphous coatings are observed in the HRTEM forelectrodes with thin films of In2O3 deposited on the surfaceof the crystalline TiO2 (figure 7), supporting the XPS data(table 1). The lattice fringes are again evident in the TiO2
nanocrystals. In addition, an amorphous surface is presentwhich translates to the core–shell nanostructure. In2O3 isclearly deposited on the oxide surface as a shell and hasa thickness of 0.5 nm for the first coating. Although thebare TiO2 electrodes appear to have an amorphous surface,the irregularity of the surface is increased upon depositionof the In2O3. The XPS data indicated that the In2O3 shell
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Nanotechnology 18 (2007) 125608 D B Menzies et al
10 nm
ba
10 nm
Figure 8. HRTEM of the 2.45 GHz microwave calcined (a) TiO2 and (b) ZrO2-coated TiO2 working electrodes.
Table 3. Photovoltaic data for DSSCs (this data is the average of tests recorded on three different devices) comprising the Ti(IV) pre- andpost-treated working electrodes tested with an active area of 0.25 cm2 under AM1.5G 100 mW cm−2.
a Jsc is the short circuit current density.b Voc is the open circuit voltage.c FF is the fill factor.d η is the overall conversion efficiency.
thickness increases to 0.7 and 1 nm for 3 and 5 coatings,respectively (arrowed in figure 7) [30]. The presence of theindium at the surface of the TiO2 nanostructure was establishedusing EDX on the HRTEM (supplementary figure S2 availableat stacks.iop.org/Nano/18/125608). Thus, the In2O3 shellthickness clearly increases with the number of successivecoatings of the precursor solution that are applied. In addition,as the number of In2O3 coatings increases broadening of theinterparticle necks increases (figure 7(d)). This supports theconclusion, reached here for TiCl4 post-treatments (figure 6)and in previous reports [4], that the precursor solution isdrawn into the neck regions by capillary forces upon drying.This study confirms that the precursors are deposited onto thesurfaces of the TiO2 nanostructure, where the hydrolysis andpolycondensation reactions, followed by calcination, broadenthese regions resulting in In2O3 containing necks.
The morphology and crystallinity of bare and ZrO2-coatedTiO2 after microwave heating (figure 8) are the same asthose for conventionally heated films coated with a nanolayer(0.5 nm) of In2O3. The presence of the zirconium atthe surface of the TiO2 nanostructure was established usingEDX on the HRTEM (supplementary figure S3 availableat stacks.iop.org/Nano/18/125608). The ZrO2 film is about0.5 nm thick, and our core–shell morphologies shown havesimilar thicknesses to those shown by Palomares et al [20].
3.3. Performance of dye-sensitized solar cells
3.3.1. Titania treatments. The effect of two types oftreatments on the performance of dye-sensitized solar cellswas examined: (i) post-treatment of the photoanodes followingtitania film deposition; (ii) pre-treatment of the conductive
oxide glass followed by titania film deposition and then post-treatment of the photoanode. The photovoltaic data (table 3)shows that working electrodes that had been undercoated witha thin dense layer of TiO2 using TiCl4 prior to the depositionof the TiO2 film results in devices with better performance. At100 mW cm−2, the cell efficiency increases from 3.6% (bareTiO2) to 4.0% (TiCl4 underlayer), due to a slightly higherfill factor (FF) indicating a reduction in charge recombinationfrom electrons in the TCO thin film to the electrolyte. Thisimprovement in photovoltaic performance compares well withTiCl4 post-treatment results where coating of the TiO2 workingelectrode with a thin film of TiO2 overlayer, applied usinga TiCl4 solution, has resulted in a 20% improvement inefficiency [4, 32–34].
Dark current measurements of the DSSCs indicate that therate of recombination between the electrons in the TCO filmand I−3 in the electrolyte is lower for the TiCl4 pre-treatment,cf. no pre-treatment (supplementary figure S4 available atstacks.iop.org/Nano/18/125608). This data is not quantitative,but can be used as a comparison between solar cells. Areduction in the recombination rate is indicated by a positiveshift in the breakdown voltage for the solar cells and in thedark current onset [16, 17, 23].
Post-treatment of the TiO2 working electrodes was foundto further improve DSSC performance (table 3). The highestcurrent of 12.2 mA cm−2 was achieved with TiB and TiCl4
post-treated TiO2 working electrodes. Moreover, all threetitanium dioxide precursor post-treatments resulted in anincrease in Voc. The best cell efficiencies were achieved withTiCl4 (5.3%, figure 9) and TiP (5.2%) post-treatment whereasTiB post-treatment films gave efficiencies of 4.5%. The poorerperformance of the latter was due to a combination of a lower
Figure 9. J–V curves for the DSSC comprising (a) bare TiO2 and(b) TiCl4 post-treated TiO2 working electrodes with correspondinginset of dark currents.
short circuit current, fill factor (ff) and open circuit voltage (seetable 3).
The variation in DSSC performance resulting from theapplication of the titania coatings via three different precursorsolutions was unexpected. We envisage that, since TiCl4 ismore reactive than the TiP and TiB solutions, it may be easierto form linkages at the interparticle neck regions of the workingelectrodes, as shown with the HRTEM (figure 6). Furthermore,the hydrolysis of TiCl4 generates hydrochloric acid, which cancatalyze hydrolytic processes (dissolution/precipitation) thatlead to improved connectivity between particles (necking) orcause subtle variations in surface chemistry. Thus, hydrolysisand polycondensation reactions will occur that cross-linktitanium centres, via titanium–oxygen linkages, or bridges,filling the interparticle neck regions. This is consistent withthe BET analysis, as the overall surface area of the workingelectrodes was decreased but resulted in increased deviceperformance. The broadened neck regions result in loweringthe internal resistance for electron conduction through theTiO2 film, and improved charge collection is expected. Thisexplanation is also consistent with the dark current data(figure 9 inset).
3.3.2. Indium oxide post-treatment. DSSCs constructedwith In2O3-coated TiO2 core–shell nanostructured workingelectrodes were found to show significant variation in deviceperformance with the applied treatment (table 4). In particular,the photovoltaic results for both the bare and TiCl4-coatedTCO substrates showed a consistent variation in performanceas a function of the number of In2O3 coatings deposited overthe TiO2 working electrode. One In2O3 coating enhancedthe DSSC performance for both the bare and TiCl4-coatedTCO substrate working electrodes (forming a thin TiO2 layeron the substrate). The best performance was achieved withTiO2 films coated with a 0.5 nm thick In2O3 layer (1 In2O3
coating) and no underlayer (4.9% efficiency under AM1.5100 mW cm−2 (figure 10)). This improvement of performancecan be attributed to an increase in short circuit current (Jsc)from 10.0 to 11.9 mA cm−2 and open circuit voltage (Voc) from690 to 730 mV. Additional In2O3 coatings resulted in majordecreases in the efficiencies, from 4.9% down to 2.8 and 2.7%
a b
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Figure 10. J–V curves for DSSCs constructed with (a) bare and(b) 1 In2O3 coating on TiO2 working electrodes with correspondinginset of dark currents.
Table 4. Photovoltaic data for DSSCs (this data is the average oftests recorded on three different devices) for In(III) post-treatedworking electrodes with and without a TiCl4 pre-treatment testedwith an active area of 0.25 cm2 under AM1.5G 100 mW cm−2.
for devices constructed from TiO2 films with 3 and 5 In2O3
coats and no underlayer (table 4).For electrodes that had been pre-treated with TiO2, the
application of one coating of In2O3 resulted in a slightly poorerperformance (4.3%) than found for the untreated films (4.9%).However, this cell efficiency was still better than for electrodesthat had not been subjected to any pre-treatment and/or post-treatment (4.0%). In contrast, pre-treated electrodes that werefurther coated (post-treated) with In2O3 gave similar results tothe untreated films. The overall cell efficiency observed forthe electrodes post-treated with 3 and 5 In2O3 coats was 2.9%,cf 2.8% and 2.7%, respectively, for devices constructed fromfilms with no underlayer.
These results, obtained with multiple coatings of In2O3,demonstrate the effect of the shell thickness in core–shell
9
Nanotechnology 18 (2007) 125608 D B Menzies et al
Table 5. Photovoltaic data for DSSCs (this data is the average oftests recorded on three different devices) comprising the Zr(IV)post-treated working electrodes calcined conventionally or with2.45 GHz microwaves (m/w) tested with an active area of 0.25 cm2
nanostructures on the photovoltaic performance of DSSCs.Thinner shells (∼0.5 nm) clearly lead to better DSSCperformance than thicker films (0.75–1 nm). Due to a higherconduction band energy, thicker shells more effectively reducecharge recombination but they also reduce the rate of electroninjection into the semiconductor. With the increased distancefor electrons to tunnel through the In2O3 shell to the TiO2 corethe current would decrease due to a reduced interfacial chargetransfer rate [23]. Thus, in these core–shell nanostructures, theshell thickness should be minimized whilst at the same timeensuring that monolayer coverage of the nanostructured TiO2
layer is achieved.The increased Voc for the core–shell electrodes with 1
In2O3 coating indicates that the introduction of the surfaceIn2O3 shell on the nanostructured TiO2 working electrode hasaltered the recombination rate to the electrolyte by changingthe conduction band edge. The dark current curves (figure 10inset) have a positive shift in the breakdown voltage whencore–shell electrodes are used in comparison to the bare TiO2,similar to that shown by the TiCl4 treatments. Thus, theelectrons in the TiO2 conduction band have a lower rate ofrecombination to the oxidized electrolyte species (I−3 ) andoxidized dye molecules at the surface of the nanostructuredworking electrode. This is clearly evident in the performanceof the devices constructed with one In2O3 coat on the TiO2
working electrodes, although In2O3 has a slightly smallerbandgap in comparison to TiO2; 2.8 and 3.2 eV for singlecrystals, respectively. However, since the thickness of thecoating will be too small and internal strain too high toallow crystallization, the bandgap will not be the same as thatmeasured for the single-crystal material.
3.3.3. Zirconia post-treatment. Conventionally andmicrowave sintered ZrO2-coated TiO2 working electrodeswere used in DSSCs to compare the effect of thin shells ofzirconia on DSSC performance with other oxide (vide infra)and to examine the effect of two different sinterings on thisperformance. DSSCs constructed from conventionally sinteredZrO2-coated TiO2 working electrodes showed improvedperformance from 3.6% (no ZrO2 treatment) to 4.7% (table 5).Microwave heating at 450 ◦C for 15 min of the ZrO2-coatedTiO2 working electrodes gave an even better performance of5.6% (figure 11). This was better than that observed formicrowave heated bare TiO2 electrodes (5.0% for microwaveheated, cf. 3.6% for conventionally heated electrodes).The better performance of the bare TiO2 films (no ZrO2
coating) may be attributed to the TCO coating of the glasssubstrate being the microwave absorbing material [7] and
ba
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Figure 11. J–V curves of DSSCs comprising (a) TiO2 and(b) 2.45 GHz microwave calcined ZrO2-coated TiO2 workingelectrodes with corresponding inset of dark currents.
localized heating at this interface will result in improved ohmiccontact between the TiO2 nanostructure and the TCO substrateitself. Dark currents (figure 11 inset) analysed between 400and 800 mV show that the microwave heating suppressedthe charge recombination. In the case of the ZrO2-coatedTiO2 working electrodes, a similar suppression of the chargerecombination between the semiconductor and the electrolytewas observed. Microwave calcination in the presence of aninorganic precursor solution of Zr(Oi Pr)4 can also increaselocal microwave absorption. When the precursor is coated onthe TiO2 nanostructure, the absorption of microwave radiationby surface organics and hydroxyl groups is stimulated at thesurface. Thus, the coating will be able to densify to form ananostructure with superior quality with reduction in surfacetraps. Therefore the microwave calcined surfaces may providea more efficient DSSC due to reduction in the trap sitesfor electron recombination [28]. This is borne out by theimproved photovoltaic performances for DSSCs constructedfrom both the uncoated-TiO2 and the ZrO2-coated TiO2
working electrodes after 2.45 GHz microwave calcination.
4. Conclusion
We have investigated the post-treatment of titania photoanodeswith a series of titania precursor solutions, and with indiumoxide and zirconia precursor solutions. For the indium oxideand zirconia coatings, core–shell nanostructures are formedin which these oxides form a thin layer around the titaniacore. Detailed characterization has been carried out on thesematerials to show the formation of core–shell nanostructuresand that these coatings enhance the interparticle connections(between titania particles). Moreover, HRTEM, supported byother characterization techniques, has been used to determinethe coating thickness for the In2O3 and the ZrO2-coated TiO2
films. For both In2O3 and ZrO2, one coating with a precursorsolution was found to deposit a 0.5 nm thick layer of theoxide. As the number of In2O3 coatings was increased, theshell thickness increased to 0.7 nm (3 coats) and 1 nm (5coats). Microwave calcination had resulted in no change inthe surface structures of both the bare and ZrO2-coated TiO2
nanostructures.
10
Nanotechnology 18 (2007) 125608 D B Menzies et al
The photovoltaic data clearly indicates that improvementsin the performance of dye-sensitized solar cells can be achievedby surface modification of the TiO2 nanostructure and byapplying blocking layers on the transparent conducting glass(pre-treatment). Undercoating of TCO glass with TiCl4
prior to titania film deposition increased the efficiency ofDSSCs from 3.6 to 4.0%, primarily due to a reduction inthe recombination rate. Post-treatment of these photoanodeswith three titania precursor solutions (two metal alkoxides andTiCl4) further improved the performance of the DSSCs; thebest efficiency (5.3%) again obtained with films that had beenpost-treated with TiCl4. DSSCs were also constructed usingcore–shell nanostructured electrodes with good efficienciesachieved using TiO2 working electrodes coated with 0.5 nmthick shell of In2O3 (4.9%) and ZrO2 (4.7%). As thethickness of the In2O3 shell was increased (up to 1 nm), asubstantial decrease in the efficiency of the DSSC deviceswas observed. These results indicate that thinner films areeffective at minimizing charge recombination at the interfacesbetween the electrode and electrolyte, whilst at the same timemaintaining high rates of electron injection into the titaniasemiconductor. A comparison of microwave sintering of theTiO2 working electrodes with conventional heat treatmentrevealed that microwave sintering resulted in an increase inefficiency from 3.6% to 5.0% for normal titania workingelectrodes, which increased further to 5.6% on introduction ofa ZrO2 coating. To our knowledge, this is the first known reportindicating that heat treatments with 2.45 GHz microwavescan significantly enhance photovoltaic performances in thepresence of metal–organic precursors (e.g. Zr(Oi Pr)4).
Acknowledgments
The authors would like to thank Karen Hands, Mark Greavesand John Ward of the Division of Forestry, CSIRO for the useof their profilometer and FEGSEM facilities and Dr Udo Bach(Materials Engineering and Chemistry, Monash University) forvaluable discussions. XPS data was collected by Dr NarelleBrack, Department of Physics, La Trobe University. Thisproject was supported by the Australian Research Council(ARC), the ARC Centre of Excellence for ElectromaterialsScience (ACES), a Monash Engineering Research Grant,a Monash Graduate Scholarship (DM) and Monash LoganFellowship (LB).
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