Mesoporous Titania Films Prepared by Flame Stabilized on a Rotating

Article pubs.acs.org/JPCC

Mesoporous Titania Films Prepared by Flame Stabilized on a Rotating Surface: Application in Dye Sensitized Solar Cells Saro 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 flame stabilized on a rotating surface (FSRS) for application in dye sensitized solar cells (DSSCs). The fabrication of mesoporous TiO2 layers, involving simultaneous flame synthesis of TiO2 nanoparticles and film deposition, followed by film densification and sintering, greatly simplifies photoanode preparation in comparison to the sol−gel method commonly used for TiO2 particle preparation. 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 an FSRS film with 12 μm thickness and 20 nm median particle size, containing predominantly anatase, with the use of a backscattering layer and sensitized by the N719 dye, produced ∼20 mA/cm2 short-circuit current density and a photon-toelectricity conversion efficiency of 8.2% at AM1.5 incident irradiance, which is comparable to highly efficient cells reported in the literature 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 cell performance with respect to particle size and crystal phase content. Films made with anatase particles 9 nm in median diameter produced the lowest photoefficiency. Rutile content of ∼15% in weight percentage of the film deteriorates the photoefficiency significantly. Open circuit voltage decay measurements show the apparent correlation between photoefficiency and electron lifetime, and this lifetime appears to be associated with charge recombination through charge transfer from the TiO2 surface to the oxidized species in the electrolyte.

1. INTRODUCTION Titanium oxide (TiO2) has been widely recognized as a multifaceted compound since its early use as a pigment.1−4 A recent application is in dye sensitized solar cells (DSSCs).5 The anode of a typical DSSC is comprised of a porous film of lightly sintered nanoparticles of TiO2. Bulk TiO2 has a bandgap just outside of the visible light spectrum (3.0 eV for rutile and 3.2 eV for anatase).6 Photoexcitation in DSSCs occurs in an organometallic dye bound to the TiO2 network. Excited electrons are transferred from the dye to the TiO2 conduction band.7 Electrons percolate through the TiO2 network and are typically collected on a transparent conductive oxide (TCO) substrate. The counter electrode is usually comprised of a layer of functional nanophase platinum to catalyze the reduction of an electrolyte that completes the circuit via a redox process. Typically, a liquid iodide/tri-iodide redox electrolyte fills the gap between the two electrodes to regenerate the photo© 2012 American Chemical Society

sensitizer, although solid or quasi-solid electrolytes have also been proposed.8 Solar energy-to-electricity conversion efficiencies between 11 and 13% have been achieved for DSSCs under AM 1.5 solar irradiance.9−14 Since its discovery, the majority of the DSSC research has focused on the photoanode. In particular, the molecular structure of the sensitizing dye has been improved drastically over the past decade. Improvements include an increased light absorption especially into the red region and an enhanced efficiency of electron injection into the TiO2 from the sensitizer.7,15 Improvement of the mesoporous TiO2 film is another aspect critical to DSSC performance. Recent studies suggested that Received: October 4, 2011 Revised: February 3, 2012 Published: February 7, 2012 5342

dx.doi.org/10.1021/jp2095533 | J. Phys. Chem. C 2012, 116, 5342−5351

The Journal of Physical Chemistry C

Article

of a flame onto the substrate, thus excluding the possibility of low-temperature fabrication of mesoporous films. Previously, we proposed and demonstrated a continuous flame synthesis process for direct fabrication of TiO 2 nanoparticles and deposition of these particles into a thin film in a single step.36−39 The method, called flame stabilized on a rotating surface (FSRS), uses an aerodynamic nozzle to generate a premixed flat flame in a laminar stagnating flow against a rotating surface. The flame was specially designed on the basis of advanced combustion theories and can produce a porous thin film of ultrafine, phase pure metal oxide nanoparticles of narrow size distribution in a single step. Control over the flame synthesis of particles is achieved through uniformity of velocity and temperature profiles such that particles undergoing the growth process experiencing nearly identical temperature, concentration, and time history. In this technique, a flow of unburned gas mixture typically containing ethylene, oxygen, and argon under the fuel lean condition is shaped into a plug flow by a converging nozzle. The specially shaped flow produces a quasi one-dimensional flow field leading to a flat, disk-shaped flame and thus uniform particle growth environment. It eliminates the conical characteristic typically associated with a Bunsen flame in which the fluid flow experiences varied time histories. The unburned gas is doped with an organometallic precursor. Upon heating by the flame, the precursor decomposes and undergoes oxidation uniformly across the flame disk, producing vaporphase metal oxide precursors.40−42 Nucleation and particle size growth follows. Because of the large temperature gradient between the flame (>2000 K) and the convectively cooled rotating surface (∼400 K), particles are quickly driven to the surface by thermophoresis43 to form a film. The time from particle formation to deposition is typically a few milliseconds, controlled by the temperatures in the flame disk and the rotating substrate. The FSRS technique differs from spray pyrolysis in many ways. For example, in the FSRS technique, the particles are synthesized from gas-phase nucleation and growth in a hotter flame followed by deposition of these particles onto a cooler substrate to form a thin film. In aerosol or spray pyrolysis, a cold spray impinges a hot surface, pyrolysis of the precursor could occur in the gas phase adjacent to the surface or on the surface directly. The FSRS technique is also scalable for continuous film deposition over a width of tens of centimeters simply by enlarging the nozzle following well-known fluid mechanic principles or by the use of an array of nozzles. It has been shown that FSRS is capable of producing narrowly distributed, single-crystal TiO2 particles and easily controllable film thickness and crystal phase,38 all of which are essential to DSSC performance and fabrication. This film preparation method has already been tested for conductometric TiO2 chemical sensor applications. The results showed a notably improved sensitivity toward CO sensing.39 The purpose of the present article is to demonstrate that the flame technique outlined above is a useful and efficient method for preparing mesoporous TiO2 films for DSSC applications. Baseline relationships between flame condition, particle and film characteristics, and DSSC efficiency were established. The photoanodes are characterized with respect to a number of film parameters. Commercial sol−gel anodes were also tested for comparison.

one of the main factors limiting the efficiency of DSSCs is the TiO2 layer, especially in its path to commercialization.13,16 It is widely accepted that because of their small sizes, electric fields and, consequently, electron drift cannot exist in TiO 2 nanoparticles and a network comprised of them.17−19 Rather, the transport of the electrons through the TiO2 film is diffusive in nature. Experimental evidence suggests that the efficiency of electron diffusion is critical to cell performance, and this efficiency is largely governed by the properties of the TiO2 film.20,21 Characteristics of the TiO2 mesoporous film include particle properties such as median size, size distribution, and crystal phase; and film properties including thickness, porosity, and interparticle necking, all of which can impact charge generation and collection, and charge losses due to recombination.22 A film of 12 μm thickness, composed of phase-pure anatase nanoparticles of ∼20 nm in diameter, has been shown to produce the highest conversion efficiency.12 Although our understanding of the porous TiO2 layer and how it affects the efficiency of a DSSC has greatly improved over the past decade, preparation of highly efficient mesoporous TiO2 films remains challenging at both laboratory and commercial scales. Film preparation has typically relied on the sol−gel method employing wet chemistry to produce a paste of TiO2 nanoparticles.23 The paste is then transferred to the TCO by screen printing. The sol−gel route requires a lengthy and costly multistep procedure.22 In a recent paper, Grätzel and co-workers16 noted that “for the best performing TiO2 electrodes, the synthesis of TiO2 paste involves hydrolysis of Ti(OCH(CH3)2)4 in water to ethanol by three times centrifugation. Finally, the ethanol is exchanged with αterpineol by sonication and evaporation. Totally, it takes 3 days. Such a long time procedure of TiO2 paste is economically unsuitable 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 attractive route to efficient anode preparation. For example, Grätzel, Kavan, and co-workers24 prepared thin mesoporous films of TiO2 on a TCO by restrained hydrolysis of TiCl4 in the presence of a block copolymer. Aerosol or spray pyrolysis is another method capable of single-step deposition of mesoporous films.25 In this method, an aerosol or a fine spray containing an organometallic precursor of titanium is impinged onto a heated substrate, usually a TCO, leading to the deposition of a mesoporous TiO2 film on that substrate.25−29 Spray pyrolysis also has been applied extensively for producing the TiO2 blocking layer for the DSSC anode (see, for example, refs 30 and 31). In many cases, spray pyrolysis can lead to oriented growth of the crystal structure or highly interconnected films with their microstructures highly dependent on the substrate temperature and other film growth conditions.27,28 Typically, the photoefficiency of the DSSC prepared by spray pyrolysis is around 5%, which is substantially lower than the most efficient cells prepared by screening printing using paste synthesized with the sol−gel method. Flame synthesis has shown promise as a fabrication method for producing metal oxide nanoparticles of value to both basic research and manufacturing.32−34 Biswas and co-workers35 used a premixed flame aerosol reactor to prepare TiO2 films and showed that in the columnar morphology and using the Ruthenium 535-bisTBA (N719) dye, a 6.9 μm film generated 20 mA/cm2 photocurrent and 6% solar energy to electricity efficiency at 124 mW/cm 2 simulated solar irradiance. Unfortunately, the method involves the direct impingement 5343



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≥100 rpm.36 A series of flat slots were machined on the disk for mounting deposition substrates. As the disk rotates, the substrates were inserted below the flame repeatedly, and at each pass, particles deposit onto the substrates, resulting in a continuous film. The rotation of the disk keeps the stagnation surface at ∼450 K, whereas the temperature inside the flame sheet just 0.3 cm away from the surface is well above 2000 K.36 For DSSC applications and using the current setup, film preparation 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, 2.2 mm thickness, sheet resistance of 15 Ω/□ and visible light transmittance of ∼80%) was used as the current collector. High performance DSSCs reported in the literature and most notably the champion cells11,12 typically used an anti-reflective, antihazing layer to suppress light reflection.44,45 In addition, the anode TCOs were typically treated with a dilute solution of TiCl4. In the present study, no anti-reflective or anti-hazing treatment was performed on the TCO glasses. The TCOs were typically stored in a sulfuric acid solution. Before film deposition, they were rinsed with deionized water, followed by treatment in an ultrasonic bath of acetone and in a UVozone cleaner, both for 15 min before film deposition. As for the TiCl4 treatment, we have followed the procedure identical to that of Ito et al.12 but found no systematic, discernible difference in the performance of the cell prepared with the current flame technique. The tests included pretreatment of TCO along (the blocking layer), post-treatment along and both. While further investigations are necessary to understand the lack of sensitivity to TiCl4 treatment, we note that FSRS cells do not require TiCl4 treatment to achieve reasonably high efficiencies, which is of considerable practical interest. The results 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 be between 50% and 60%.46,47 High porosity films typically exhibit weak adhesion with the substrate (e.g., peeling or cracking upon dipping in the dye solution), low mechanical integrity and high electric resistance. For this reason, as-synthesized films were densified by dipping them into a 15% wt solution of ethyl cellulose (46080, Fluka) in ethanol. The films were then sintered on a hot plate in a stepwise manner, first at 200 °C for 10 min, followed by 350 °C for 10 min, and finally 500 °C for 30 min. Again, high-efficiency DSSCs typically treat the TiO2 particle film with TiCl4 (see, for example, Ito et al.12), followed by washing and sintering before dye sensitization. We found no systematic evidence that would support any benefits from such treatment for the FSRS films. The total time for film preparation, including flame deposition, densification, and sintering, took approximately 2 h. The films were sensitized by submerging them in a dye solution composed of 0.5 mM ditetrabutylammonium cisbis(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 N719 dye was used as received without further purification. The sensitized films were dried in air and rinsed in acetonitrile before 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 the performance of our cells against those highly efficient cells, we used the same approach in some of the cells studied here. The

2. EXPERIMENTAL DETAILS 2.1. 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

Figure 1. Schematic of the experiment. The low-right corner shows an image of the flame.

aerodynamically shaped nozzle with a 1 cm exit diameter to form a laminar, flat jet impinging against a titanium disk 30.5 cm in diameter. The disk, placed 3.0 cm from the nozzle exit, acts as the flow stagnation surface. A round flame about 2 cm in diameter is stabilized at about 0.3 cm above the stagnation surface by flow divergence and stretch. An image of a typical flame is shown on the lower-right corner of Figure 1. The synthesis is carried out inside a glovebox under atmospheric pressure. The unburned gas composition (3.77% C2H4/26.90% O2− Ar; equivalence ratio, ϕ = 0.42) and the total flow rates of the unburned gas (14.3 L/min STP) were held fixed during the course of the study. The gas jet and the flame were isolated by a shroud of argon issued through a concentric tube at 11 L/min (STP). Flow rates of the unburned gas were controlled individually with sonic nozzles (O’Keefe Controls) and GO pressure valves. The gases were mixed before flowing into the burner nozzle. Titanium tetraisopropoxide (TTIP, Aldrich, 97%) was injected into the unburned mixture through a hypodermic needle by a syringe pump (Harvard Apparatus, PHD 2000 Infusion). All of the gas lines and the burner were heated to 120 °C to prevent TTIP condensation. The liquid volumetric flow rates of TTIP used were 25, 45, and 60 mL/h. Under these conditions, the partial pressure of TTIP is substantially smaller than its saturation pressure (∼0.025 atm at 100 °C). In addition, the gas exiting the nozzle has a linear flow velocity of 400 cm/s at the operating temperature. The change in 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 Intellidrive controller. The center-to-center distance between the motor axis and the gas jet is 12.1 cm. The disk spin rate used was 300 rpm. Our previous study showed that the flame and particle characteristics are insensitive to the rotation speed for spin rate 5344



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Table 1. Summary of Flame, Particle, and Cell Properties TTIP flame/ anodea 1 2 2ac 2b−e 3

PSDF

phase (wt%)

DSSC performance

ϕ

rate (ml/ h)

conc (ppm)

⟨Dp⟩ (nm)b

σg

anatase

rutile

0.58 0.70

25 45

2350 4230

8.9 21.1

1.53 1.42

96 94

4 6

0.80

60

5640

17.1

1.44

85

15

thickness δ (μm)

η (%)

FF (%)

Voc (V)

jsc (mA/cm2)

6 6 12 6.0 ± 0.5 6

4.4 6.7 8.2 6.7 ± 0.2 5.6

55 60 52 60 ± 2 54

0.73 0.74 0.80 0.711 ± 0.004 0.78

11.2 16.0 19.7 15.7 ± 0.2 13.5

Base 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 μm backscattering layer on top of the 12 μm TiO2 film prepared by FSRS. a

Figure 2. Selected TEM images of nanoparticles produced from flames 1 (left panel), 2 (center panel), and 3 (right panel).

exposing only the active area to the light, while the other cells were tested in a dark room but without tape masking. The incident light intensity was calibrated carefully and periodically using a precalibrated silicon photodiode (Hamamatsu S1787− 04) to ensure the illumination intensity accuracy.

back layer was prepared by doctor blading using the Dyesol WER2-O TiO2 paste. The cathodes were prepared using a Pt catalyst paste (Solaronix). After screen printing, the cathodes were sintered on a hot plate at 400 °C for 30 min. The electrolyte (EL-HPE, Dyesol) was used as received. Surlyn (25 μm, Solaronix) was used both as the spacer between the electrodes and as the sealant for the electrolyte. 2.3. Particle, Film, and DSSC Characterization. X-ray diffraction measurements were made using a Rigaku diffractometer on thin films of particles deposited on VWR microscope slides. For analysis by transmission electron microscopy (TEM, Akashi 002B), particles were dispersed and sonicated in ethanol and deposited on a copper-supported (200 mesh) holey grid. HRTEM was also performed using a 300 kV Tecnai F30 field emission TEM. Scanning electron microscopy (SEM) analysis was carried out using a JEOL JSM7001 SEM on both as-synthesized TiO2 films and densified/ sintered films prepared on silicon wafers. UV−vis absorption spectra were obtained using a Shimadzu 2401-PC spectrometer. Absorbance tests were made on sensitized films. To find the optical bandgap of the particle material, UV−vis absorption was also carried out for a colloid of as synthesized particles dispersed in ethanol. The solution was sonicated to ensure colloidal 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 cells with typical areas of ∼4 × 4 mm using an in-house testing facility featuring a National Instrument DAQ card (PCI 6031E) interfaced with LabVIEW 8.6 and a solar simulator at 100 mW/cm2 using a 150 W xenon lamp (Newport 62553) with an AM1.5 filter. The test for anode 2a was carried out with tape masking in a dark room to avoid the effect of stray light,

3. RESULTS AND DISCUSSION 3.1. Particle and Film Properties. The efficiency of a DSSC is highly dependent on the properties of the TiO2 particles and the mesoporous film comprised of these particles, as mentioned before. These properties include the crystal phase and particle size. In the present work, we demonstrate the flexibility of the FSRS technique for tailored synthesis of mesoporous titania films, focusing on the aforementioned two properties as examples. Three types of mesoporous films were prepared for this purpose, two of which are dominated by anatase but differ in particle size and the third has a substantial rutile content. Our previous study38 shows that while fuel-rich flames produce mostly rutile particles, ultralean flames favor phasepure anatase, which is decidedly favorable for DSSC applications. For the present study, we kept the equivalence ratio (ϕ) of the base flame at 0.42. Flames doped with the TTIP precursor have a higher ϕ value. For example, at the highest TTIP loading of 5640 PPM in unburned mixture, the ϕ value increases to 0.80. It will be shown that using the base flame composition, it is possible to tune the particle size and phase 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, 2 and 3, respectively, as listed in Table 1. Figure 2 shows TEM images of nanoparticles collected from the three flames. It is seen that the particles are nearly spherical and mostly single crystals with occasional sintered necks. Crystal features with different lattice orientations are clearly visible in most of the 5345



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Figure 3. Size distribution functions of TiO2 nanoparticles synthesized in flames 1 through 3. Symbols are experimental data based on TEM measurements; lines are log-normal fits to data (eq 1).

particles imaged. Particle size distributions may be extracted from TEM images, leading to diameter distribution data presented in Figure 3. All of the distributions shown may be described by the log-normal function. 1 dN = N d log(Dp)

⎡ (log D − log⟨D ⟩)2 ⎤ p p 1 ⎥ exp⎢ − 2 ⎢ ⎥ 2π log σg 2(log σg ) ⎣ ⎦ (1)

where N is the number density, Dp the particle diameter, ⟨Dp⟩ is the median diameter, and σg is the geometric standard deviation, which was found to range from 1.42 to 1.53 for the flames tested. These values are close to that in a self-preserved distribution due to particle growth kinetics dominated by coagulation.53,54 By increasing the TTIP loading, the particle median diameter exhibits 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, as shown in Figure 3. By superimposing the data presented in the middle and right panels of Figure 3, the size distributions from flames 2 and 3 are not as distinctively different as the median diameter values would have suggested. Nonetheless, the variation in the particle size may be explained by the different TTIP dopant concentration and the resulting variation in the flame temperature. With an increased precursor loading, the particle nucleation and coagulation rates increase, but the equivalence ratio of the flame also increases, leading to a higher flame temperature and a larger thermophoretic force.43 Hence, the increase of the median diameter from flame 1 to flame 2 is caused largely by an increased rate of particle formation and growth, whereas compared to flame 2, the somewhat smaller particle size from flame 3 is due to a shorter particle residence time. The crystal phase content is sensitive to oxygen availability and thus the flame equivalence ratio.38 Figure 4 shows the Xray diffraction patterns of the three particle samples collected from the flames. As seen, the powders as synthesized from flames 1 and 2 show diffraction patterns of nearly phase pure anatase, whereas the powder from flame 3 is a mixture of anatase and rutile. The weight percentages of the polymorphs, given in Table 1, were determined using the integrated intensities of the X-ray diffraction peaks. The anatase contents in particles prepared with flames 1 and 2 are around 95% and that flame 3 yielded particles containing a substantially larger

Figure 4. Powder film X-ray diffraction patterns of unsintered TiO2 films and sintered anode 2.

amount of rutile (15 wt %). It will be shown later that this rutile content has a direct impact on DSSC performance. Crystalline sizes were also determined and are found to be in agreement with TEM results, as discussed by Memarzadeh et al.38 Absorption spectra of the three particle samples show features typical of TiO2 nanoparticles, as shown in Figure 5. These spectra can be analyzed using the Tauc method.55 For an indirect band gap, a plot of (αhv)1/2 versus hv yields the optical bandgap from the intercept. As shown in the inset of Figure 5, TiO2 samples produced from flames 1 and 2 give a bandgap value of 3.2 eV, as expected, since both particle samples are dominated by anatase. Flame 3 yields nanoparticles with a substantially larger content of rutile. Consequently, the band gap undergoes a red shift to slightly over 3.0 eV. The bandgap remains unchanged after densification and sintering. Upon densification with a solution of ethyl cellulose (15 wt %) in ethanol and sintering, the film becomes notably denser and mechanically stronger. Figure 6 shows SEM images of densified and sintered TiO2 films prepared from flame 2 (left panel) and flame 3 (right panel). The morphology of the film 5346



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formation of islands separated by narrow channels. Interestingly, no systematic trend was observed for DSSC performance with respect to the presence of cracks and their dimensions, probably because the feature size is substantially smaller than the film thickness. 3.2. DSSC Characteristics. The purpose of this part of the study is to investigate the performance of anodes prepared with the FSRS method in DSSCs. We first present in Figure 7 the

Figure 5. UV−vis absorbance spectra of TiO2 particles dispersed in ethanol and their band edges. The inset shows an analysis of the optical bandgap using the Tauc method.55

Figure 7. Photocurrent density−voltage characteristics of DSSCs using anodes (2a) and (2b−e). The test for anode 2a was carried out with tape masking, exposing only the active area to the AM1.5 incident light.

photocurrent density curve measured for a DSSC with a double layer anode, consisting of a 12 μm FSRS film with a 4 μm backscattering layer (anode 2a of Table 1). At AM1.5 irradiance, the photoconversion efficiency η was measured to be 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 density jsc is, in fact, higher than those reported for typical champion cells, e.g., 17.7 mA/cm2 reported by Grätzel10 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 poor fill factor (FF = 0.52). High-performance cells reported in the literature typically yield FF values close to 0.75. In all of the cells we prepared and tested, the fill factor is typically 0.6. To examine whether the poor fill factor is inherent to the FSRS method, we tested cells using the as-purchased TiO2 coated test cell glass plates from Dyesol. The size of the anode film was reduced to 4 × 4 mm to provide results comparable to the FSRS cells. The resulting fill factor is roughly the same as those prepared with the FSRS method. On the basis of this evidence, the poor fill factor is probably the result of ohmic loss due to the electric resistance in TCO or contact resistance between electric leads and the TCO, especially because we did not use a silver coating on the TCO to reduce contact resistances between the TCOs and the electric leads. The point-to-point resistance in the TCO used is around 20 Ω. This resistance alone would produce 0.13 V of voltage loss at the ∼3 mA of photocurrent measured for cell 2a. Other causes may include poor interface contact between the TCO and the mesoporous layer. Our atomic force microscopy study of the TCO electron collector showed its surface to be rather rough with distinctive edges, kinks, and steps of feature

Figure 6. SEM images of mesoporous TiO2 films after densification and sintering. Left panel, flame 2; right panel, flame 3.

prepared by flame 2 may be characterized by strands of agglomerated primary particles and occasional, but regularly, occurring holes with feature size of several micrometers. Compared to undensified films (see, for example, Figure 9 of Tolmachoff et al.36), the densified film shown here has a significantly reduced porosity. Comparisons of the diffraction patterns show that with the exception of a growth of [004] facets in the sintered TiO2 film, the films remain nearly identical before and after sintering (see Figure 4). As we noted before, the FSRS titania films tend to be resistant toward sintering and phase change to form rutile at high temperatures.39,40 The growth of [004] appears to be consistent with a further growth of crystallinity upon sintering. Early observations show that the most intense diffraction feature in highly crystallized anatase TiO2 is [004] rather than [101]56 and that the strong [004] intensity in anatase TiO2 nanorods appears to be correlated with preferential growth along the [001] direction.57 Hence, the diffraction patterns collected before and after sintering suggest that necking among particles may be preferential 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, as shown in Figure 6. Prominent cracks are clearly seen. In fact, many of the films developed cracks during sintering, just like those shown in the figure, while others show uniform, crackless features. The cracked films are usually characterized by the 5347



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sizes that can vary from tens to a few hundred nanometers. It is possible that because of diffusional deposition, the TiO2 particles do not effectively fill the kinks or edges during the initial phase of film deposition, leading to formation of spatial voids, which could not be eliminated by film densification. This issue clearly requires further study. Nonetheless, the results presented here demonstrate the potential of FSRS as an efficient route to mesoporous TiO2 film preparation for DSSC applications, especially if antireflection treatment is adopted. We examined the relationships among the synthesis condition, film property, and cell performance, and especially the influences of particle sizes and phase purity on cell performance. To minimize complications arising from electrolyte transport and other factors, all of the photoanodes presented in this section are 6 μm in thickness after densification and sintering without the use of the backscattering layer, though we acknowledge that keeping the anode film at this thickness does not necessarily provide normalization on other cell characteristics including dye loading, back reaction, or the various factors governing electron transport. To verify that the results are reproducible, we first prepared four cells with identical conditions of mesoporous film deposition and treatments (anodes 2b−e) and characterized their j-versus-V behaviors, as shown in Figure 7. The variation of the areas of these cells (4 × 4 mm) was kept small ( 0.6 V). Bisquert et al.52 attributed the linear log(τn)−Voc response in this voltage domain to trapping−detrapping dominated kinetics, even though charge transfer is still governed by conduction band states. Hence, the apparently longer electron lifetime of anode 2 is attributable to a greater density of surface traps than anode 1. These surface traps appear to be crucial to suppressing charge recombination. For anode 2, the region of exponential rise in the lifetime extends to much lower Voc values than for anode 1. The latter exhibits a deep, inverted parabola of the origin already discussed in Bisquert et al., in contrast to a significantly milder curvature of anode 2 at Voc ≈ 0.45 V. Another interesting observation is the relatively short electron lifetime in anode 3 across a large Voc region. The characteristic difference between anodes 2 and 3 indicates that anode 3 is in many ways fundamentally different from anode 2 in trapping−detrapping kinetics, and this observation is consistent with the notably different 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 of Figure 10. As seen, among unsensitized anodes tested, the Voc value of anode 3 is the lowest at the onset of OCVD, in agreement with the bandgap measurement. However, the Voc value of the stained anode 3 is the highest (see, also, Table 1) among the cells tested. This opposite trend is indicative of alteration of the characteristics of the TiO2 conduction band upon dye sensitization. In other words, Voc can be impacted by the secondary factors in addition to the conduction band energy 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 of densification and sintering. These aspects of the anode preparation make the method proposed not as distant from screen printing as one would hope. Our past and ongoing work suggests that by using the FSRS technique, it is possible to prepare DSSC anodes without having to rely on liquid-based densification and sintering, but the efficiencies of the resulting cells are not as high or as reproducible for the time being. We are carrying out additional studies to explore the possibility of achieving high photoefficiencies by flame deposition without the need for densification, sintering, or other post-treatment.

of around 20 nm appears to produce the highest photoefficiency. The presence of rutile at around 15% (wt) alters the optical band gap, reduces the electron lifetime, and lowers the cell photoefficiency. The mesoscale morphology can also be an important factor in limiting cell performance. Smaller anatase crystalline size leads to lowered light absorbance and faster charge recombination.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (213) 740-0499. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We wish to thank Ms. Erin Kampschorer, Ms. Wenbo Hou, and Professor Stephen Cronin for their help in SEM film characterization and Professor Andrea Hodge for profilometry. This work was supported by the Combustion Energy Frontier Research Center (CEFRC), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001198.

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REFERENCES

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4. CONCLUSIONS We demonstrate that the flame stabilized on a rotating surface (FSRS) method may be tailored for direct synthesis of anatase TiO2 nanoparticles and deposition of these particles into a thin film in a single step. As a photoanode, the resulting film can achieve photoefficiencies in dye sensitized solar cells comparable to those of highly efficient cells reported previously without antiglaring, anti-reflective treatment. The simplicity and scalability of the FSRS method may be desirable to produce inexpensive anode films, especially because the film fabrication can be completed rapidly in a single-step film deposition followed 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 of the TiO2 crystalline size and phase content is similar to those prepared by the sol−gel method. Pure anatase with crystal size 5350



Article

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Mesoporous Titania Films Prepared by Flame Stabilized on a Rotating

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