Mesoporous Titania Films Prepared by Flame Stabilized on aRotating Surface: Application in Dye Sensitized Solar CellsSaro Nikraz, Denis J. Phares, and Hai Wang*
Department of Aerospace and Mechanical Engineering, University of Southern California, Los Angeles, California 90089-1453,United States
ABSTRACT: We examine the properties and performance of mesoporous nanoparticle films of TiO2 prepared by flamestabilized on a rotating surface (FSRS) for application in dye sensitized solar cells (DSSCs). The fabrication of mesoporous TiO2layers, involving simultaneous flame synthesis of TiO2 nanoparticles and film deposition, followed by film densification andsintering, greatly simplifies photoanode preparation in comparison to the sol−gel method commonly used for TiO2 particlepreparation. To demonstrate the flexibility of the FSRS technique, three types of photoanodes were prepared and characterized,two of which are dominated by anatase but differ in particle size and the third has substantial rutile content. DSSCs made with anFSRS film with 12 μm thickness and 20 nm median particle size, containing predominantly anatase, with the use of abackscattering layer and sensitized by the N719 dye, produced ∼20 mA/cm2 short-circuit current density and a photon-to-electricity conversion efficiency of 8.2% at AM1.5 incident irradiance, which is comparable to highly efficient cells reported in theliterature without the use of an anti-reflective layer. The FSRS TiO2 film does not require pre- or post-TiCl4 treatment.Comparisons of the DSSCs prepared with the FSRS photoanodes show great reproducibility and high sensitivity of cellperformance with respect to particle size and crystal phase content. Films made with anatase particles 9 nm in median diameterproduced the lowest photoefficiency. Rutile content of ∼15% in weight percentage of the film deteriorates the photoefficiencysignificantly. Open circuit voltage decay measurements show the apparent correlation between photoefficiency and electronlifetime, and this lifetime appears to be associated with charge recombination through charge transfer from the TiO2 surface tothe oxidized species in the electrolyte.
1. INTRODUCTIONTitanium oxide (TiO2) has been widely recognized as amultifaceted compound since its early use as a pigment.1−4 Arecent application is in dye sensitized solar cells (DSSCs).5 Theanode of a typical DSSC is comprised of a porous film of lightlysintered nanoparticles of TiO2. Bulk TiO2 has a bandgap justoutside of the visible light spectrum (3.0 eV for rutile and 3.2eV for anatase).6 Photoexcitation in DSSCs occurs in anorganometallic dye bound to the TiO2 network. Excitedelectrons are transferred from the dye to the TiO2 conductionband.7 Electrons percolate through the TiO2 network and aretypically collected on a transparent conductive oxide (TCO)substrate. The counter electrode is usually comprised of a layerof functional nanophase platinum to catalyze the reduction ofan electrolyte that completes the circuit via a redox process.Typically, a liquid iodide/tri-iodide redox electrolyte fills thegap between the two electrodes to regenerate the photo-
sensitizer, although solid or quasi-solid electrolytes have alsobeen proposed.8
Solar energy-to-electricity conversion efficiencies between 11and 13% have been achieved for DSSCs under AM 1.5 solarirradiance.9−14 Since its discovery, the majority of the DSSCresearch has focused on the photoanode. In particular, themolecular structure of the sensitizing dye has been improveddrastically over the past decade. Improvements include anincreased light absorption especially into the red region and anenhanced efficiency of electron injection into the TiO2 from thesensitizer.7,15
Improvement of the mesoporous TiO2 film is another aspectcritical to DSSC performance. Recent studies suggested that
Received: October 4, 2011Revised: February 3, 2012
one of the main factors limiting the efficiency of DSSCs is theTiO2 layer, especially in its path to commercialization.13,16 It iswidely accepted that because of their small sizes, electric fieldsand, consequently, electron drift cannot exist in TiO2nanoparticles and a network comprised of them.17−19 Rather,the transport of the electrons through the TiO2 film is diffusivein nature. Experimental evidence suggests that the efficiency ofelectron diffusion is critical to cell performance, and thisefficiency is largely governed by the properties of the TiO2film.20,21 Characteristics of the TiO2 mesoporous film includeparticle properties such as median size, size distribution, andcrystal phase; and film properties including thickness, porosity,and interparticle necking, all of which can impact chargegeneration and collection, and charge losses due torecombination.22 A film of 12 μm thickness, composed ofphase-pure anatase nanoparticles of ∼20 nm in diameter, hasbeen shown to produce the highest conversion efficiency.12
Although our understanding of the porous TiO2 layer andhow it affects the efficiency of a DSSC has greatly improvedover the past decade, preparation of highly efficientmesoporous TiO2 films remains challenging at both laboratoryand commercial scales. Film preparation has typically relied onthe sol−gel method employing wet chemistry to produce apaste of TiO2 nanoparticles.
23 The paste is then transferred tothe TCO by screen printing. The sol−gel route requires alengthy and costly multistep procedure.22 In a recent paper,Gratzel and co-workers16 noted that “for the best performingTiO2 electrodes, the synthesis of TiO2 paste involves hydrolysisof Ti(OCH(CH3)2)4 in water to ethanol by three timescentrifugation. Finally, the ethanol is exchanged with α-terpineol by sonication and evaporation. Totally, it takes 3days. Such a long time procedure of TiO2 paste is economicallyunsuitable for industrial production and has to be reduced.”Direct mesoporous film fabrication bypassing the TiO2 paste
preparation with the sol−gel technique would be an attractiveroute to efficient anode preparation. For example, Gratzel,Kavan, and co-workers24 prepared thin mesoporous films ofTiO2 on a TCO by restrained hydrolysis of TiCl4 in thepresence of a block copolymer. Aerosol or spray pyrolysis isanother method capable of single-step deposition of meso-porous films.25 In this method, an aerosol or a fine spraycontaining an organometallic precursor of titanium is impingedonto a heated substrate, usually a TCO, leading to thedeposition of a mesoporous TiO2 film on that substrate.25−29
Spray pyrolysis also has been applied extensively for producingthe TiO2 blocking layer for the DSSC anode (see, for example,refs 30 and 31). In many cases, spray pyrolysis can lead tooriented growth of the crystal structure or highly intercon-nected films with their microstructures highly dependent on thesubstrate temperature and other film growth conditions.27,28
Typically, the photoefficiency of the DSSC prepared by spraypyrolysis is around 5%, which is substantially lower than themost efficient cells prepared by screening printing using pastesynthesized with the sol−gel method.Flame synthesis has shown promise as a fabrication method
for producing metal oxide nanoparticles of value to both basicresearch and manufacturing.32−34 Biswas and co-workers35 useda premixed flame aerosol reactor to prepare TiO2 films andshowed that in the columnar morphology and using theRuthenium 535-bisTBA (N719) dye, a 6.9 μm film generated20 mA/cm2 photocurrent and 6% solar energy to electricityefficiency at 124 mW/cm2 simulated solar irradiance.Unfortunately, the method involves the direct impingement
of a flame onto the substrate, thus excluding the possibility oflow-temperature fabrication of mesoporous films.Previously, we proposed and demonstrated a continuous
flame synthesis process for direct fabrication of TiO2
nanoparticles and deposition of these particles into a thinfilm in a single step.36−39 The method, called flame stabilizedon a rotating surface (FSRS), uses an aerodynamic nozzle togenerate a premixed flat flame in a laminar stagnating flowagainst a rotating surface. The flame was specially designed onthe basis of advanced combustion theories and can produce aporous thin film of ultrafine, phase pure metal oxidenanoparticles of narrow size distribution in a single step.Control over the flame synthesis of particles is achievedthrough uniformity of velocity and temperature profiles suchthat particles undergoing the growth process experiencingnearly identical temperature, concentration, and time history. Inthis technique, a flow of unburned gas mixture typicallycontaining ethylene, oxygen, and argon under the fuel leancondition is shaped into a plug flow by a converging nozzle.The specially shaped flow produces a quasi one-dimensionalflow field leading to a flat, disk-shaped flame and thus uniformparticle growth environment. It eliminates the conicalcharacteristic typically associated with a Bunsen flame inwhich the fluid flow experiences varied time histories. Theunburned gas is doped with an organometallic precursor. Uponheating by the flame, the precursor decomposes and undergoesoxidation uniformly across the flame disk, producing vapor-phase metal oxide precursors.40−42 Nucleation and particle sizegrowth follows. Because of the large temperature gradientbetween the flame (>2000 K) and the convectively cooledrotating surface (∼400 K), particles are quickly driven to thesurface by thermophoresis43 to form a film. The time fromparticle formation to deposition is typically a few milliseconds,controlled by the temperatures in the flame disk and therotating substrate.The FSRS technique differs from spray pyrolysis in many
ways. For example, in the FSRS technique, the particles aresynthesized from gas-phase nucleation and growth in a hotterflame followed by deposition of these particles onto a coolersubstrate to form a thin film. In aerosol or spray pyrolysis, acold spray impinges a hot surface, pyrolysis of the precursorcould occur in the gas phase adjacent to the surface or on thesurface directly. The FSRS technique is also scalable forcontinuous film deposition over a width of tens of centimeterssimply by enlarging the nozzle following well-known fluidmechanic principles or by the use of an array of nozzles. It hasbeen shown that FSRS is capable of producing narrowlydistributed, single-crystal TiO2 particles and easily controllablefilm thickness and crystal phase,38 all of which are essential toDSSC performance and fabrication. This film preparationmethod has already been tested for conductometric TiO2
chemical sensor applications. The results showed a notablyimproved sensitivity toward CO sensing.39
The purpose of the present article is to demonstrate that theflame technique outlined above is a useful and efficient methodfor preparing mesoporous TiO2 films for DSSC applications.Baseline relationships between flame condition, particle andfilm characteristics, and DSSC efficiency were established. Thephotoanodes are characterized with respect to a number of filmparameters. Commercial sol−gel anodes were also tested forcomparison.
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2. EXPERIMENTAL DETAILS2.1. Flame Synthesis. Details of the flame stabilized on a
rotating surface (FSRS) method are discussed elsewhere.36,38,41
Briefly, the burner, shown schematically in Figure 1, is an
aerodynamically shaped nozzle with a 1 cm exit diameter toform a laminar, flat jet impinging against a titanium disk 30.5cm in diameter. The disk, placed 3.0 cm from the nozzle exit,acts as the flow stagnation surface. A round flame about 2 cm indiameter is stabilized at about 0.3 cm above the stagnationsurface by flow divergence and stretch. An image of a typicalflame is shown on the lower-right corner of Figure 1. Thesynthesis is carried out inside a glovebox under atmosphericpressure.The unburned gas composition (3.77% C2H4/26.90% O2−
Ar; equivalence ratio, ϕ = 0.42) and the total flow rates of theunburned gas (14.3 L/min STP) were held fixed during thecourse of the study. The gas jet and the flame were isolated by ashroud of argon issued through a concentric tube at 11 L/min(STP). Flow rates of the unburned gas were controlledindividually with sonic nozzles (O’Keefe Controls) and GOpressure valves. The gases were mixed before flowing into theburner nozzle. Titanium tetraisopropoxide (TTIP, Aldrich,97%) was injected into the unburned mixture through ahypodermic needle by a syringe pump (Harvard Apparatus,PHD 2000 Infusion). All of the gas lines and the burner wereheated to 120 °C to prevent TTIP condensation. The liquidvolumetric flow rates of TTIP used were 25, 45, and 60 mL/h.Under these conditions, the partial pressure of TTIP issubstantially smaller than its saturation pressure (∼0.025 atm at100 °C). In addition, the gas exiting the nozzle has a linear flowvelocity of 400 cm/s at the operating temperature. The changein the flow rate due to TTIP doping was less than 0.5%.The titanium disk is mounted on a stepper motor (Aerotech
BM250_UF) with its speed controlled by a BAI Intellidrivecontroller. The center-to-center distance between the motoraxis and the gas jet is 12.1 cm. The disk spin rate used was 300rpm. Our previous study showed that the flame and particlecharacteristics are insensitive to the rotation speed for spin rate
≥100 rpm.36 A series of flat slots were machined on the disk formounting deposition substrates. As the disk rotates, thesubstrates were inserted below the flame repeatedly, and ateach pass, particles deposit onto the substrates, resulting in acontinuous film. The rotation of the disk keeps the stagnationsurface at ∼450 K, whereas the temperature inside the flamesheet just 0.3 cm away from the surface is well above 2000 K.36
For DSSC applications and using the current setup, filmpreparation requires 5−15 min deposition time typically,depending on the precursor loading.
2.2. DSSC Preparation. Transparent conductive oxide(TCO) glass (fluorine-doped tin oxide, Pilkington TEC15, 2.2mm thickness, sheet resistance of 15 Ω/□ and visible lighttransmittance of ∼80%) was used as the current collector. Highperformance DSSCs reported in the literature and most notablythe champion cells11,12 typically used an anti-reflective, anti-hazing layer to suppress light reflection.44,45 In addition, theanode TCOs were typically treated with a dilute solution ofTiCl4. In the present study, no anti-reflective or anti-hazingtreatment was performed on the TCO glasses. The TCOs weretypically stored in a sulfuric acid solution. Before filmdeposition, they were rinsed with deionized water, followedby treatment in an ultrasonic bath of acetone and in a UV-ozone cleaner, both for 15 min before film deposition. As forthe TiCl4 treatment, we have followed the procedure identicalto that of Ito et al.12 but found no systematic, discernibledifference in the performance of the cell prepared with thecurrent flame technique. The tests included pretreatment ofTCO along (the blocking layer), post-treatment along andboth. While further investigations are necessary to understandthe lack of sensitivity to TiCl4 treatment, we note that FSRScells do not require TiCl4 treatment to achieve reasonably highefficiencies, which is of considerable practical interest. Theresults reported herein were obtained without this treatment.Films as grown using the FSRS method have a porosity of
∼90% while the desired porosity has been reported to bebetween 50% and 60%.46,47 High porosity films typically exhibitweak adhesion with the substrate (e.g., peeling or crackingupon dipping in the dye solution), low mechanical integrity andhigh electric resistance. For this reason, as-synthesized filmswere densified by dipping them into a 15% wt solution of ethylcellulose (46080, Fluka) in ethanol. The films were thensintered on a hot plate in a stepwise manner, first at 200 °C for10 min, followed by 350 °C for 10 min, and finally 500 °C for30 min. Again, high-efficiency DSSCs typically treat the TiO2particle film with TiCl4 (see, for example, Ito et al.12), followedby washing and sintering before dye sensitization. We found nosystematic evidence that would support any benefits from suchtreatment for the FSRS films.The total time for film preparation, including flame
deposition, densification, and sintering, took approximately 2h. The films were sensitized by submerging them in a dyesolution composed of 0.5 mM ditetrabutylammonium cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′ dicarboxylato)ruthenium(II) (N719 dye, Solaronix) in n-butanol−acetonitrile(50:50 by volume) at room temperature for 24 h. The N719dye was used as received without further purification. Thesensitized films were dried in air and rinsed in acetonitrilebefore being used in the cell.Highly efficient DSSCs also use a double layer with the back
layer applied for photon-trapping.12,48−50 To compare theperformance of our cells against those highly efficient cells, weused the same approach in some of the cells studied here. The
Figure 1. Schematic of the experiment. The low-right corner shows animage of the flame.
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back layer was prepared by doctor blading using the DyesolWER2-O TiO2 paste.The cathodes were prepared using a Pt catalyst paste
(Solaronix). After screen printing, the cathodes were sinteredon a hot plate at 400 °C for 30 min. The electrolyte (EL-HPE,Dyesol) was used as received. Surlyn (25 μm, Solaronix) wasused both as the spacer between the electrodes and as thesealant for the electrolyte.2.3. Particle, Film, and DSSC Characterization. X-ray
diffraction measurements were made using a Rigaku diffrac-tometer on thin films of particles deposited on VWRmicroscope slides. For analysis by transmission electronmicroscopy (TEM, Akashi 002B), particles were dispersedand sonicated in ethanol and deposited on a copper-supported(200 mesh) holey grid. HRTEM was also performed using a300 kV Tecnai F30 field emission TEM. Scanning electronmicroscopy (SEM) analysis was carried out using a JEOL JSM-7001 SEM on both as-synthesized TiO2 films and densified/sintered films prepared on silicon wafers. UV−vis absorptionspectra were obtained using a Shimadzu 2401-PC spectrometer.Absorbance tests were made on sensitized films. To find theoptical bandgap of the particle material, UV−vis absorption wasalso carried out for a colloid of as synthesized particlesdispersed in ethanol. The solution was sonicated to ensurecolloidal uniformity before testing. The thickness of densified/sintered films was examined using a stylus profilometer(Ambios XP-2) with accuracy of ±1 μm.Measurements of photocurrent−voltage curves and open
circuit voltage decay51,52 (OCVD) were carried out on cellswith typical areas of ∼4 × 4 mm using an in-house testingfacility featuring a National Instrument DAQ card (PCI 6031-E) interfaced with LabVIEW 8.6 and a solar simulator at 100mW/cm2 using a 150 W xenon lamp (Newport 62553) with anAM1.5 filter. The test for anode 2a was carried out with tapemasking in a dark room to avoid the effect of stray light,
exposing only the active area to the light, while the other cellswere tested in a dark room but without tape masking. Theincident light intensity was calibrated carefully and periodicallyusing a precalibrated silicon photodiode (Hamamatsu S1787−04) to ensure the illumination intensity accuracy.
3. RESULTS AND DISCUSSION
3.1. Particle and Film Properties. The efficiency of aDSSC is highly dependent on the properties of the TiO2particles and the mesoporous film comprised of these particles,as mentioned before. These properties include the crystal phaseand particle size. In the present work, we demonstrate theflexibility of the FSRS technique for tailored synthesis ofmesoporous titania films, focusing on the aforementioned twoproperties as examples. Three types of mesoporous films wereprepared for this purpose, two of which are dominated byanatase but differ in particle size and the third has a substantialrutile content.Our previous study38 shows that while fuel-rich flames
produce mostly rutile particles, ultralean flames favor phase-pure anatase, which is decidedly favorable for DSSCapplications. For the present study, we kept the equivalenceratio (ϕ) of the base flame at 0.42. Flames doped with theTTIP precursor have a higher ϕ value. For example, at thehighest TTIP loading of 5640 PPM in unburned mixture, the ϕvalue increases to 0.80. It will be shown that using the baseflame composition, it is possible to tune the particle size andphase content simply by varying the TTIP dopant loading.Unburned synthesis flame mixtures were doped with 2350,
4230, and 5640 PPM of TTIP, corresponding to flames 1, 2and 3, respectively, as listed in Table 1. Figure 2 shows TEMimages of nanoparticles collected from the three flames. It isseen that the particles are nearly spherical and mostly singlecrystals with occasional sintered necks. Crystal features withdifferent lattice orientations are clearly visible in most of the
Table 1. Summary of Flame, Particle, and Cell Properties
aBase flame: 3.77%C2H4/26.90%O2−Ar (ϕ = 0.42); v0 = 400 cm/s (120 °C, 1 atm). bDetermined by TEM data (see Figure 3). cIncludes a 4 μmbackscattering layer on top of the 12 μm TiO2 film prepared by FSRS.
Figure 2. Selected TEM images of nanoparticles produced from flames 1 (left panel), 2 (center panel), and 3 (right panel).
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particles imaged. Particle size distributions may be extractedfrom TEM images, leading to diameter distribution datapresented in Figure 3. All of the distributions shown may bedescribed by the log-normal function.
=π σ
−− ⟨ ⟩
σ
⎡
⎣⎢⎢
⎤
⎦⎥⎥N
ND
D D1 dd log( )
12 log
exp(log log )
2(log )p g
p p2
g2
(1)
where N is the number density, Dp the particle diameter, ⟨Dp⟩is the median diameter, and σg is the geometric standarddeviation, which was found to range from 1.42 to 1.53 for theflames tested. These values are close to that in a self-preserveddistribution due to particle growth kinetics dominated bycoagulation.53,54
By increasing the TTIP loading, the particle median diameterexhibits a rise-then-fall behavior, with ⟨Dp⟩ = 8.9 ± 0.2, 21.1 ±0.4, and 17.1 ± 0.2 nm for flames 1, 2 and 3, respectively, asshown in Figure 3. By superimposing the data presented in themiddle and right panels of Figure 3, the size distributions fromflames 2 and 3 are not as distinctively different as the mediandiameter values would have suggested. Nonetheless, thevariation in the particle size may be explained by the differentTTIP dopant concentration and the resulting variation in theflame temperature. With an increased precursor loading, theparticle nucleation and coagulation rates increase, but theequivalence ratio of the flame also increases, leading to a higherflame temperature and a larger thermophoretic force.43 Hence,the increase of the median diameter from flame 1 to flame 2 iscaused largely by an increased rate of particle formation andgrowth, whereas compared to flame 2, the somewhat smallerparticle size from flame 3 is due to a shorter particle residencetime.The crystal phase content is sensitive to oxygen availability
and thus the flame equivalence ratio.38 Figure 4 shows the X-ray diffraction patterns of the three particle samples collectedfrom the flames. As seen, the powders as synthesized fromflames 1 and 2 show diffraction patterns of nearly phase pureanatase, whereas the powder from flame 3 is a mixture ofanatase and rutile. The weight percentages of the polymorphs,given in Table 1, were determined using the integratedintensities of the X-ray diffraction peaks. The anatase contentsin particles prepared with flames 1 and 2 are around 95% andthat flame 3 yielded particles containing a substantially larger
amount of rutile (15 wt %). It will be shown later that this rutilecontent has a direct impact on DSSC performance. Crystallinesizes were also determined and are found to be in agreementwith TEM results, as discussed by Memarzadeh et al.38
Absorption spectra of the three particle samples showfeatures typical of TiO2 nanoparticles, as shown in Figure 5.These spectra can be analyzed using the Tauc method.55 For anindirect band gap, a plot of (αhv)1/2 versus hv yields the opticalbandgap from the intercept. As shown in the inset of Figure 5,TiO2 samples produced from flames 1 and 2 give a bandgapvalue of 3.2 eV, as expected, since both particle samples aredominated by anatase. Flame 3 yields nanoparticles with asubstantially larger content of rutile. Consequently, the bandgap undergoes a red shift to slightly over 3.0 eV. The bandgapremains unchanged after densification and sintering.Upon densification with a solution of ethyl cellulose (15 wt
%) in ethanol and sintering, the film becomes notably denserand mechanically stronger. Figure 6 shows SEM images ofdensified and sintered TiO2 films prepared from flame 2 (leftpanel) and flame 3 (right panel). The morphology of the film
Figure 3. Size distribution functions of TiO2 nanoparticles synthesized in flames 1 through 3. Symbols are experimental data based on TEMmeasurements; lines are log-normal fits to data (eq 1).
Figure 4. Powder film X-ray diffraction patterns of unsintered TiO2films and sintered anode 2.
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prepared by flame 2 may be characterized by strands ofagglomerated primary particles and occasional, but regularly,occurring holes with feature size of several micrometers.Compared to undensified films (see, for example, Figure 9 ofTolmachoff et al.36), the densified film shown here has asignificantly reduced porosity. Comparisons of the diffractionpatterns show that with the exception of a growth of [004]facets in the sintered TiO2 film, the films remain nearlyidentical before and after sintering (see Figure 4). As we notedbefore, the FSRS titania films tend to be resistant towardsintering and phase change to form rutile at high temper-atures.39,40 The growth of [004] appears to be consistent with afurther growth of crystallinity upon sintering. Early observa-tions show that the most intense diffraction feature in highlycrystallized anatase TiO2 is [004] rather than [101]56 and thatthe strong [004] intensity in anatase TiO2 nanorods appears tobe correlated with preferential growth along the [001]direction.57 Hence, the diffraction patterns collected beforeand after sintering suggest that necking among particles may bepreferential along this same direction.The film prepared by flame 3 appears to have a significantly
fewer number of mesoscopic holes than that by flame 2, asshown in Figure 6. Prominent cracks are clearly seen. In fact,many of the films developed cracks during sintering, just likethose shown in the figure, while others show uniform, cracklessfeatures. The cracked films are usually characterized by the
formation of islands separated by narrow channels. Interest-ingly, no systematic trend was observed for DSSC performancewith respect to the presence of cracks and their dimensions,probably because the feature size is substantially smaller thanthe film thickness.
3.2. DSSC Characteristics. The purpose of this part of thestudy is to investigate the performance of anodes prepared withthe FSRS method in DSSCs. We first present in Figure 7 the
photocurrent density curve measured for a DSSC with a doublelayer anode, consisting of a 12 μm FSRS film with a 4 μmbackscattering layer (anode 2a of Table 1). At AM1.5irradiance, the photoconversion efficiency η was measured tobe 8.2%, with a short-circuit current density, jsc, of 20 mA/cm2
and an open-circuit voltage, Voc, of 0.80 V. The current densityjsc is, in fact, higher than those reported for typical championcells, e.g., 17.7 mA/cm2 reported by Gratzel10 and 18.2 mA/cm2 by Ito et al.12 at the same incident irradiance.The j-versus-V curve measured for cell 2a shows notable
internal electric resistances. These losses lead to a rather poorfill factor (FF = 0.52). High-performance cells reported in theliterature typically yield FF values close to 0.75. In all of thecells we prepared and tested, the fill factor is typically 0.6. Toexamine whether the poor fill factor is inherent to the FSRSmethod, we tested cells using the as-purchased TiO2 coated testcell glass plates from Dyesol. The size of the anode film wasreduced to 4 × 4 mm to provide results comparable to theFSRS cells. The resulting fill factor is roughly the same as thoseprepared with the FSRS method. On the basis of this evidence,the poor fill factor is probably the result of ohmic loss due tothe electric resistance in TCO or contact resistance betweenelectric leads and the TCO, especially because we did not use asilver coating on the TCO to reduce contact resistancesbetween the TCOs and the electric leads. The point-to-pointresistance in the TCO used is around 20 Ω. This resistancealone would produce 0.13 V of voltage loss at the ∼3 mA ofphotocurrent measured for cell 2a.Other causes may include poor interface contact between the
TCO and the mesoporous layer. Our atomic force microscopystudy of the TCO electron collector showed its surface to berather rough with distinctive edges, kinks, and steps of feature
Figure 5. UV−vis absorbance spectra of TiO2 particles dispersed inethanol and their band edges. The inset shows an analysis of theoptical bandgap using the Tauc method.55
Figure 6. SEM images of mesoporous TiO2 films after densificationand sintering. Left panel, flame 2; right panel, flame 3.
Figure 7. Photocurrent density−voltage characteristics of DSSCs usinganodes (2a) and (2b−e). The test for anode 2a was carried out withtape masking, exposing only the active area to the AM1.5 incidentlight.
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sizes that can vary from tens to a few hundred nanometers. It ispossible that because of diffusional deposition, the TiO2particles do not effectively fill the kinks or edges during theinitial phase of film deposition, leading to formation of spatialvoids, which could not be eliminated by film densification. Thisissue clearly requires further study. Nonetheless, the resultspresented here demonstrate the potential of FSRS as anefficient route to mesoporous TiO2 film preparation for DSSCapplications, especially if antireflection treatment is adopted.We examined the relationships among the synthesis
condition, film property, and cell performance, and especiallythe influences of particle sizes and phase purity on cellperformance. To minimize complications arising from electro-lyte transport and other factors, all of the photoanodespresented in this section are 6 μm in thickness afterdensification and sintering without the use of the backscatteringlayer, though we acknowledge that keeping the anode film atthis thickness does not necessarily provide normalization onother cell characteristics including dye loading, back reaction, orthe various factors governing electron transport. To verify thatthe results are reproducible, we first prepared four cells withidentical conditions of mesoporous film deposition andtreatments (anodes 2b−e) and characterized their j-versus-Vbehaviors, as shown in Figure 7. The variation of the areas ofthese cells (4 × 4 mm) was kept small (<∼20% difference). Itcan be seen that the photocurrent behavior of these cells ishighly reproducible. This is true for other TiO2 particle sizestested.It is worth noting that the photoefficiency of DSSCs
prepared with anodes 2b−e is around 6.7 ± 0.2%, despite amuch thinner anode (1/2 of the film thickness of highlyefficient DSSCs). Compared with anode 2a, it may beconcluded that much of the photon-to-electron conversionoccurs in the front layer adjacent to the TCO, as expected.As discussed earlier, DSSC performance is highly dependent
on the properties of the mesoporous TiO2 film. Thisdependency is evident in several functions within the cell.Charge injection into the TiO2 nanoparticles from the dye isinfluenced by relative Fermi levels of the TiO2 with respect tothe LUMO of the dye molecule.58−60 Charge diffusion throughthe mesoporous network depends on factors including particlesize, interparticle necking, film porosity, and the FTO/TiO2interface.61−64 Charge recombination is affected by the densityand type of surface and bulk trap states within thenanoparticles.65−70 Indeed, the current work shows thatDSSC performance is highly related to particle size and phasecontent of the TiO2 particles.We first compare the photocurrent density−voltage curves of
anodes 1 through 3. The DSSC characteristics of the threeanodes, including η, jsc, Voc, and FF, are presented in Table 1.Figure 8 shows that under comparable conditions, anode 2 hasthe highest photocurrent. The dependency of photocurrent onthe particle properties is not as straightforward to interpret aswe had hoped; the following discussion is speculative to anextent. Although the particles in anode 3 have sizes similar tothose in anode 2, its photocurrent is notably smaller than thatof anode 2. The lowered current production in anode 3 isclearly related to the presence of a significant amount of rutile.Light absorbance spectra shown in Figure 9 indicate that,compared to anode 2, anode 3 gives better light absorption inthe blue region but poorer absorption in the red. The overalldifference in the absorbance is perhaps not sufficient to explainthe change in jsc from anode 2 to 3. The mesoscopic film
features are the most likely cause for the different jsc observedfor anodes 2 and 3. The lack of micrometer sized holes inanode 3 probably limits the rate of electrolyte transport. Inaddition to this possibility, Park et al.71 observed that undercomparable conditions, dye sensitized rutile gives a jsc 30%lower than dye sensitized anatase. They attributed thisdifference to the smaller stacking efficiency of rutile particles,which results in a smaller surface area to volume ratio, andconsequently less dye adsorption. A second consequence of asmaller surface area to volume ratio is a smaller extent ofparticle neckings, which impedes electron diffusion.Anode 1 (8.9 nm particle diameter) produced the lowest
photocurrent. Light absorption tests on sensitized anode filmsshow that anode 1 has the poorest light absorbance across theentire range of the visible spectrum. Yet this absorbancedifference corresponds to only a 20% change in the lightabsorbed, which cannot explain the 40% reduction in jsc fromanode 2 to 1. The decreased light absorbance of anode 1 maybe explained by its smaller pore sizes in the mesoporous film,which limits the access of the dye molecules. The even larger
Figure 8. Photocurrent density−voltage characteristics of DSSCsprepared with 6 μm thick mesoporous TiO2 films (anodes 1, 2, and 3)prepared by the FSRS technique.
Figure 9. UV−vis absorbance spectra of sensitized anodes 1, 2, and 3.
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difference in jsc is probably the result of a smaller electrondiffusion rate associated with a TiO2 film having smaller grainsizes and a greater number of interparticle necks.Another interesting observation is the somewhat larger Voc
observed for anode 3 than anodes 1 and 2, whereas its bandgapis the lowest. This observation appears to be surprising since, tothe first order, the Voc value is determined by the energy level ofthe conduction band of TiO2 and the redox potential of theelectrolyte. Here, we point out that anode 3 contains about15% (wt) rutile and that its bandgap is predominantly impactedby the rutile contamination. The lower bandgap value is notindicative of the conduction band characteristics of theremaining 85% anatase phase in which most of the electroninjection into the TiO2 film takes place. As will be discussedlater, dye sensitization also appears to impact the energy level ofthe conduction band and consequently the Voc value.For all three cells, the light absorbance spectra show
significant decay in absorption toward long wavelengths. Thisresult may be contrasted by the persistently high incidentphoton to charge carrier efficiency (IPCE) into the red regionreported by Gratzel and co-workers.12 Measurements of IPCEon FSRS cells (not shown here) also show limited photon tocharge efficiency in the red region. The cause for this differenceis unclear at this time, but understanding this difference may becritical to an enhanced photoefficiency for FSRS anodes.The most interesting correlation between film properties and
DSSC performance lies in the electron lifetimes in the cellsusing the technique of open circuit voltage decay (OCVD).51,52
What can be learned from OCVD experiments remains asubject of controversy, but the common practice has been tointerpret them as the apparent electron lifetime or responsetime in the TiO2 layer under the quasi-equilibrium con-dition.72,73 It has been suggested that the electron lifetime is ameasure of kinetic rate processes involving charge transfer, andthe electronic states and transitions, all of which are expected toimpact the DSSC operation. The apparent correlation betweenthe longest electron lifetime and the highest photoefficiency asobserved for anode 2 suggests that charge recombination playsa critical role, as one would expect from a large body ofliterature studies.We carried out the OCVD experiment by illuminating the
cells at AM1.5 until they reach the steady state. The light wasthen turned off, and the open circuit voltage was recorded. Thetop panel of Figure 10 shows the OCVD curves (solid lines)measured for the three anodes. It is seen that the voltage decaysare notably different from cell to cell, not only in the initial rateof decay but also in the apparent plateau values for long decaytime, even though the voltage must decay to zero over longerperiods of time. The OCVD curves of anodes 1 and 3 arecharacterized by more complex, biexponential decay functions,which have been predicted theoretically.52 OCVD curves wereobtained also for DSSCs made with unsensitized anodes usingthe Xenon lamp without the AM1.5 filter, to provide insightinto the direct recombination of electrons from the TiO2 layerwith the electrolyte. Comparing the OCVD curves betweensensitized and unsensitized films for each anode, twoobservations can be made. First, dye sensitization greatlyincreases the electron lifetime under quasi-equilibrium, andsuch an effect is likely caused by the dye ligands insulating theTiO2 surface from the oxidized species in the electrolyte andpreventing charge recombination. Second, among the threeanodes, the order of voltage decay rates of the sensitized anodeis identical to that of the unsensitized anodes, suggesting that
charge recombination in the sensitized cells occurs in large partfrom the TiO2 surfaces to the oxidized species in theelectrolyte.The cell made with anode 2 gives the slowest voltage decay.
Coupled with its highest photoefficiency observed, the resultsobtained here are consistent with the notion that chargerecombination rates indeed play a critical role in thephotoefficiency. The fact that anode 1 has the lowestphotoefficiency observed is attributable to a combination oflowest light absorbance of the anode film (Figure 9), relativelyfast electron recombination (Figure 10), and possibly slowerelectron diffusion rates in the TiO2 layer.A widely accepted method for characterizing charge
recombination in DSSCs is to determine a characteristic timeconstant or electron lifetime τn. Although it has been arguedthat true electron lifetime is not accessible from transientvoltage measurements,66 this lifetime may be determined, atleast approximately, from the reciprocal of the time derivativeof Voc normalized by thermal voltage.51,52
τ = −−⎛
⎝⎜⎞⎠⎟
kTe
Vt
ddn
oc1
(2)
Figure 10. Open circuit voltage decay (top panel) and electronlifetime calculated from the OCVD curves (eq 2) (bottom panel).Solid lines, sensitized anodes; dashed lines, unsensitized anodes. Theinset in the top panel shows the OCVD curves at the early stage ofvoltage decay. The zero-time values are somewhat larger than thosereported in Table 1 because of a small temperature difference, as theOCVD experiments require voltage equilibration, leading to a warmercell at the onset of OCVD.
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where k is the Boltzmann constant, T is the temperature, and eis the elementary charge. The bottom panel of Figure 10 showsthe variation of the electron lifetime as a function of themeasured Voc. Of particular interest to us is the lifetime at thelarge Voc region (Voc > 0.6 V). Bisquert et al.52 attributed thelinear log(τn)−Voc response in this voltage domain totrapping−detrapping dominated kinetics, even though chargetransfer is still governed by conduction band states. Hence, theapparently longer electron lifetime of anode 2 is attributable toa greater density of surface traps than anode 1. These surfacetraps appear to be crucial to suppressing charge recombination.For anode 2, the region of exponential rise in the lifetimeextends to much lower Voc values than for anode 1. The latterexhibits a deep, inverted parabola of the origin alreadydiscussed in Bisquert et al., in contrast to a significantly mildercurvature of anode 2 at Voc ≈ 0.45 V. Another interestingobservation is the relatively short electron lifetime in anode 3across a large Voc region. The characteristic difference betweenanodes 2 and 3 indicates that anode 3 is in many waysfundamentally different from anode 2 in trapping−detrappingkinetics, and this observation is consistent with the notablydifferent crystal phase and the resulting optical bandgap.Another interesting feature as observed by the OCVD
measurements is exhibited in the inset in the top panel ofFigure 10. As seen, among unsensitized anodes tested, the Vocvalue of anode 3 is the lowest at the onset of OCVD, inagreement with the bandgap measurement. However, the Vocvalue of the stained anode 3 is the highest (see, also, Table 1)among the cells tested. This opposite trend is indicative ofalteration of the characteristics of the TiO2 conduction bandupon dye sensitization. In other words, Voc can be impacted bythe secondary factors in addition to the conduction bandenergy level and the redox potential of the electrolyte.Lastly, we note that the results presented herein are obtained
with anode films that required the post-treatment ofdensification and sintering. These aspects of the anodepreparation make the method proposed not as distant fromscreen printing as one would hope. Our past and ongoing worksuggests that by using the FSRS technique, it is possible toprepare DSSC anodes without having to rely on liquid-baseddensification and sintering, but the efficiencies of the resultingcells are not as high or as reproducible for the time being. Weare carrying out additional studies to explore the possibility ofachieving high photoefficiencies by flame deposition withoutthe need for densification, sintering, or other post-treatment.
4. CONCLUSIONSWe demonstrate that the flame stabilized on a rotating surface(FSRS) method may be tailored for direct synthesis of anataseTiO2 nanoparticles and deposition of these particles into a thinfilm in a single step. As a photoanode, the resulting film canachieve photoefficiencies in dye sensitized solar cells com-parable to those of highly efficient cells reported previouslywithout antiglaring, anti-reflective treatment. The simplicity andscalability of the FSRS method may be desirable to produceinexpensive anode films, especially because the film fabricationcan be completed rapidly in a single-step film depositionfollowed by densification.Variation of the particle size and crystal phase content can be
accomplished by tuning relevant synthesis flame conditions.The dependency of the DSSC performance to the properties ofthe TiO2 crystalline size and phase content is similar to thoseprepared by the sol−gel method. Pure anatase with crystal size
of around 20 nm appears to produce the highest photo-efficiency. The presence of rutile at around 15% (wt) alters theoptical band gap, reduces the electron lifetime, and lowers thecell photoefficiency. The mesoscale morphology can also be animportant factor in limiting cell performance. Smaller anatasecrystalline size leads to lowered light absorbance and fastercharge recombination.
NotesThe authors declare no competing financial interest.
■ ACKNOWLEDGMENTSWe wish to thank Ms. Erin Kampschorer, Ms. Wenbo Hou, andProfessor Stephen Cronin for their help in SEM filmcharacterization and Professor Andrea Hodge for profilometry.This work was supported by the Combustion Energy FrontierResearch Center (CEFRC), an Energy Frontier ResearchCenter funded by the U.S. Department of Energy, Office ofScience, Office of Basic Energy Sciences under Award NumberDE-SC0001198.
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