ARTICLE pubs.acs.org/JPCC
Electron Transport Patterns in TiO2 Nanotube Arrays Based Dye-Sensitized Solar Cells under Frontside and Backside Illuminations Po-Tsung Hsiao,† Yong-Jin Liou,† and Hsisheng Teng*,†,‡ †
Department of Chemical Engineering and Research Center for Energy Technology and Strategy, and ‡Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 70101, Taiwan
bS Supporting Information ABSTRACT: TiO2 nanotube arrays (NTA), of 1737 μm in thickness, detached from anodic oxidized Ti foils were used as photoanodes for dye-sensitized solar cells (DSSCs). Photovoltaic measurements under frontside and backside illumination showed that frontside illumination geometry provided better cell performance than backside illumination did. A cell assembled with 30 μm thick NTA film produced the greatest photocurrent and light conversion efficiency. Despite an advantageous architecture for electron transport, electron trapping remained a limiting factor for both illumination geometries, due to the presence of crystal grains in the NTA walls. Intensity-modulated photocurrent spectroscopy (IMPS) analysis showed that electron transport in the front-illuminated cells comprises both trap-free and trap-limited diffusion modes, whereas electrons in the back-illuminated cells travel only by trap-limited diffusion. The trap-free diffusion mechanism determines front-illuminated cell performance. Electrochemical impedance spectroscopy analysis showed the front-illuminated NTA-based DSSCs have a charge collection efficiency of better than 90%, even at 30 μm NTA film thickness. Large crystal size results in low trap state density in the NTA film, and this effect may result in a more extensive trap-free diffusion zone in the films, which facilitates charge collection.
’ INTRODUCTION The architecture of highly ordered one-dimensional TiO2 nanotube arrays (NTA), obtained from the potentiostatic anodization of titanium foil, has attracted much attention because of its unique properties and potential applications.16 For applications in dye-sensitized solar cells (DSSCs) as a photoanode, a TiO2 NTA film is decorated with a monolayer of dye molecules and assembled with a redox electrolyte and a Pt counter electrode to form a cell. Upon illumination, optically excited dye molecules inject electrons into the TiO2 NTA film, and subsequently these oxidized dye molecules transfer the holes to the electrolyte in which holes transport to the counter electrode. The TiO2 NTA film serves as a recipient of injected electrons and provides a direct electron pathway perpendicular to the electron-collecting substrate. Compared with the typical nanoparticulate films containing a three-dimensional network of randomly packed nanoparticles, the TiO2 NTA film is thought to be more efficient for electron transport, since electron trapping and recombination probability are decreased by the absence of an extended random walk network.79 The use of TiO2 NTA as the photoelectrode of dye-sensitized solar cells needs to consider the illumination geometry (Scheme 1); in a frontside illumination arrangement, the light source is on the substrate side of the device, and for a backside illumination, the source is on the electrolyte side. Differences in electron transport behaviors for backside- and frontside-illuminated DSSCs are yet to be explored. TiO2 NTA is commonly grown on an opaque titanium foil, and this has hampered the development of TiO2 NTA DSSC applications. Direct utilization of TiO2 NTA grown on titanium r 2011 American Chemical Society
foil requires backside illumination. This reduces light-conversion efficiency, because light must first pass through the Pt-coated counter electrode and iodine electrolyte, resulting in attenuation of the incident light intensity.10,11 A DSSC consisting of nanoparticulate film allows for frontside illumination and generally provides superior light-conversion efficiency.1214 In order to overcome this limitation to TiO2 NTA use, researchers have fabricated a transparent electrode of TiO2 NTA, to allow for frontside illumination.1517 Additionally, a DSSC assembled with optimal thickness transparent TiO2 NTA electrodes has been reported to provide a nearly ideal configuration for harvesting incident light at improved efficiencies.18,19 A previous report on the electron dynamics of oriented TiO2 NTA described electron transit time and recombination time display power-law dependencies on the incident light intensity and proposed that the presence of trap states influences electron transport.20 TiO2 NTA is a polycrystalline material, with a broad distribution of trap states below the conduction band. The existence of these trap states allows for trapping and detrapping processes, which prolong electron transit time.8,21 The illumination geometry affects the location of electron injection into the TiO2 film, and previous studies have shown that the influence of trap states on traveling electrons varies with the distance from the electron-collecting substrate.20,22 Although previous studies have explored the electron transport of TiO2 NTA-based DSSCs Received: March 22, 2011 Revised: June 22, 2011 Published: June 24, 2011 15018
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Scheme 1. Illumination Geometries (Frontside and Backside) and the Corresponding Electron Transport Processes for TiO2 NTA-Based DSSCsa
a
The crystal defects in the grain boundaries induce trapping/detrapping process and result in a trap-limited diffusion.
illuminated from the electrolyte side, it is essential to understand the electron transport characteristics of NTA-based DSSCs with frontside illumination, not only because of the greater cell efficiency but also to shed light on how the illumination geometry affects electron transport in TiO2 films. We fabricated transparent TiO2 NTA electrodes of varying film thicknesses, for use in DSSC devices suitable for both backside and frontside illuminations. Electron transport and recombination dynamics in the TiO2 films were characterized using intensity-modulated photocurrent spectroscopy, intensitymodulated photovoltage spectroscopy, and electrochemical impedance spectroscopy. Our analysis revealed that electron transport occurs by different diffusion modes in backside- and frontside-illuminated DSSCs. We explain why the location of electron injection, which is dependent on the illumination geometry, is important to cell performance by using a distributed occupancy of trap states in the TiO2 films.
’ EXPERIMENTAL METHODS Titania NTA films with high aspect ratios of the constituting tubes were prepared from two-electrode anodic oxidation of 1 cm 2.5 cm 0.25 mm Ti foils (Sigma-Aldrich, 99.7% purity) using a dc power source (PPS-1206, Motech) and a Pt foil as the counter electrode. Prior to anodic oxidation, the Ti foils were sonicated in ethanol and then rinsed with deionized water. The Ti foil was patterned with Teflon tape to provide an active area of 0.5 0.5 cm2 and subjected to potentiostatic oxidation at 60 V in an ethyl glycol based electrolyte containing ammonium fluoride (0.25 wt %) and deionized water (2 vol %). TiO2 NTA films of 1434 μm thickness were prepared by varying the anodic oxidation time from 1 to 4 h. Following the anodic oxidation, mild sonication of the as-anodized NTA films was conducted in methanol, containing alumina submicroparticles, for several minutes to clean debris from the tops of the tubes. To separate TiO2 NTA from the Ti foils, the as-anodized NTA films were annealed in an air furnace at 450 °C for 30 min to induce crystallinity. The TiO2 film was then reanodized in the same electrolyte at 60 V for 1 h to form an amorphous TiO2 layer between the crystallized TiO2 NTA and the Ti foil. Finally, the resulting film was soaked in a H2O2 solution (35.5 wt %) to dissolve the underlying amorphous TiO2 layer and separate the crystallized TiO2 NTA from the Ti foil.17
Figure 1. SEM images of (a) the top view of the as-anodized TiO2 NTA film from oxidation in ethylene glycol at 60 V for 2 h, (b) the top view of the as-anodized film with higher magnification, (c) the bottom caps of the NTA film peeled from the Ti foil, (d) the side view of the peeled NTA film attached to the FTO substrate with a 3 μm TiO2 nanoparticle interlayer, and (e) the top view of the peeled NTA film after TiCl4 treatment. (f) Optical image of the TiO2 NTA-based electrode before and after dye loading.
The crystallized TiO2 NTA film was fixed onto a fluorinedoped SnO2 (FTO) conducting glass substrate (TEC 8, Hartford Glass Co.) with the bottom cap (closed-end) of the nanotubes attaching the FTO glass substrate. Prior to the NTA attachment, the FTO glass substrate had been coated with a thin layer of anatase TiO2 colloid dispersion, containing poly(ethylene glycol) (Fluka, avg. 20 000 g mol1). Synthesis of the TiO2 dispersion is reported elsewhere.23,24 The TiO2 NTAattached substrate was annealed at 450 °C for 30 min. The resulting TiO2 electrode was immersed into an aqueous TiCl4 solution (40 mM) at 70 °C for 30 min, rinsed with deionized water, and annealed at 450 °C for 30 min.25 For sensitization the TiO2 electrode was immersed in a N719 dye (Solaronix) solution (0.5 mM) in a mixture of acetonitrile and tert-butyl alcohol (1:1 volume ratio) for 24 h. To assemble a DSSC, the dyed electrode was assembled with a Pt-coated conducting glass as the counter electrode using a 60 μm thick thermoplastic frame (SX1170-60, Solaronix). The thicker frame avoided accidental contact between a thick TiO2 NTA film and the counter electrode. The electrolyte, composed of 0.1 M LiI (Strem Chemicals), 0.05 M I2 (Riedel-de Ha€en), 0.6 M 1,2-dimethyl-3-n-propylimidazolium iodide (Solaronix), and 0.5 M 4-tert-butylpyridine (Aldrich) in acetonitrile (J. T. Baker), was introduced into the interelectrode space through a predrilled hole on the counter electrode. We investigated the TiO2 NTA microstructure with a Jeol JSM-6700F scanning electron microscope (SEM) and the crystallite arrangement with a Jeol JEM-2100F high-resolution transmission electron microscope (TEM). The TiO2 NTA crystalline phases were determined by powder X-ray diffraction (XRD), using a Rigaku RINT2000 diffractometer with Cu KR radiation at 40 kV and 40 mA. DSSC photovoltaic measurements were made using an AM 1.5 solar simulator (sp91160A-4739, Newport) for both frontside and backside illumination orientations. The intensity of the simulated light was calibrated to 100 mW cm2, using a reference Si solar cell. Intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS) measurements were 15019
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Figure 3. Photocurrent densityvoltage characteristics of DSSCs assembled with TiO2 NTA films, of varying thicknesses, under frontside and backside illuminations at 100 mW cm2.
Figure 2. XRD patterns of (a) the as-anodized TiO2 NTA film from oxidation at 60 V for 2 h, (b) the 450 °C-annealed TiO2 NTA film, and (c) the free-standing TiO2 NTA detached from the Ti foil during the synthesis process.
carried out using a frequency response analyzer (XPOT, Zahner), which was used to drive the blue and red light emitting diodes (LEDs, λmax = 455 and 625 nm) for the frontside and backside illuminations on the DSSCs, respectively. The LEDs provided both the dc and ac components of illumination. A small sinusoidal intensity modulation ((5%) in a frequency range of 0.1 Hz1 kHz was superimposed onto a much greater dc illumination intensity, for both IMPS and IMVS. The dc illumination intensities were 15 mW cm2 for the blue LED and 13 mW cm2 for the red. The electrochemical impedance spectroscopy (EIS) measurements were conducted using a potentiostat equipped with a frequency response analyzer (IM6, Zahner) at frequencies ranging from 0.1 Hz to 100 kHz. Bias potential was set at 0.65 V, and ac potential amplitude to 10 mV, under AM 1.5 solar illumination of 100 mW cm2.
’ RESULTS AND DISCUSSION NTA morphological characteristics were studied by SEM at each step of the synthesis process. Figure 1a, b shows SEM images of a NTA film grown during 2 h of anodic oxidation. The nanotubes have a diameter of 110 nm and wall thickness of 15 nm. After the NTA film is detached from its underlying Ti foil using a two-step anodic oxidation process, the bottom caps shown in Figure 1c reflect that the nanotubes remain closely packed. In the fabrication of a transparent photoelectrode, the annealed TiO2 NTA film attaches to the FTO substrate via an intermediate layer of TiO2 nanoparticles of ca. 3 μm thickness (Figure 1d). After treatment with TiCl4, a small quantity of TiO2 crystals is present on the NTA film (Figure 1e), providing an improvement in dye absorption. NTA morphology was maintained during these multistep treatments. The opening size should be large enough for unobstructed electrolyte penetration. Figure 1f shows optical images of the TiO2 NTA-based electrodes before and after dye loading. The lateral dimension of the TiO2 NTA films was approximately 0.5 0.5 cm2. It is apparent from the figure that the square TiO2 NTA film on the transparent FTO substrate was intact and able to absorb dye. Figure 2 shows the XRD patterns of the NTA film at various steps in the synthesis. The XRD pattern in Figure 2a indicates that the as-anodized NTA film is amorphous after anodic oxidation,
Figure 4. Film thickness dependence of cell performances for DSSCs assembled with TiO2 NTA films under frontside and backside illuminations at 100 mW cm2.
since only diffraction peaks of the underlying Ti foil are present. Figure 2b shows that diffraction peaks for the anatase phase appear after annealing in air at 450 °C for 30 min, indicating the emergence of crystallinity, in agreement with previous studies.26,27 The XRD pattern of free-standing NTA, detached from the Ti foil (Figure 2c), shows only the well-defined anatase phase, without the presence of other phases. By applying the DebyeScherrer equation to the XRD pattern, we obtained a mean crystal size of ca. 35 ( 1 nm for anatase contained in the free-standing NTA with different anodic oxidation times. This crystal size is larger than that of ordinary nanocrystalline TiO2 films used in DSSCs. The anodic oxidation time had neglible influence on the crystal size of the free-standing NTA. This is probably due to the fact that the annealing temperature, which is the same for all the films, governs the crystal growth in the NTA. This study also explored the crystallite arrangement of annealed TiO2 nanotubes with TEM. The high-resolution TEM image of a TiO2 nanotube (see the Supporting Information) shows a random distribution of crystallites, in which the lattice fringes are visible. This indicates that the anodized TiO2 nanotubes are polycrystalline in nature and contain crystal grain boundaries that impede electron transport. The photovoltaic properties of the fabricated DSSCs were studied by the currentvoltage characterization under illumination using an AM 1.5 solar simulator. Figure 3 shows the photocurrentvoltage characteristics of DSSCs assembled with the free-standing NTA films, of varying thicknesses, under 100 mW cm2 illumination from both the frontside and backside. Figure 4 summarizes the performance indices, based on the 15020
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Figure 6. IMPS responses of front-illuminated DSSCs assembled with (a) a 3 μm TiO2 nanoparticle film and (b) a 23 μm reversely attached NTA film. The blue light emitting diodes was used as the modulation light source with a dc intensity at 15 mW cm2.
Figure 5. IMPS responses of DSSCs assembled with TiO2 NTA films of varying thicknesses under the frontside and backside illuminations. The blue and red light emitting diodes were used as the modulation light source with dc intensities at 15 and 13 mW cm2, respectively, for the frontside and backside illuminations.
data provided in Figure 3. For the frontside-illuminated cells, the short-circuit photocurrent (Jsc) increased with increasing NTA film thickness to a maximum at 30 μm and then decreased with further increases in thickness. The open-circuit photovoltage (Voc) and fill factor of the cells show a monotonic decrease with increasing thickness. As a consequence, the maximum efficiency occurs at a NTA thickness of 30 μm, at which point, Jsc is also at its maximum value. This suggests that Jsc governs cell efficiency, even though Voc and the fill factor vary with film thickness. Performance of the backside-illuminated cells is inferior to that of the frontside-illuminated DSSCs. The principal photonic losses during illumination from the rear are light absorption by the electrolyte and reflection by the Pt-electrode.10,11 Backside-illuminated cells of different NTA thicknesses experience similar photonic losses, because the same electrolyte and Pt-electrode are used. The value of Jsc is generally proportional to the incident photonic number and therefore reflects the extent of photonic loss. However, the data in Figure 4a show that the Jsc losses during backside illumination were 41, 40, 50, and 62% for the DSSCs with NTA films of 17, 23, 30, and 37 μm thickness, respectively. The observed trend in Jsc losses for increasing NTA thickness indicates that differences in electron transport are present between the front- and back-illuminated TiO2 NTA and significantly affect the cell performance. This is worthy of in-depth investigation, and an understanding of how the electron transport characteristics change with the distance of electron collection for the frontside and backside illuminations is necessary for further advancement in TiO2 NTA-based DSSC technology. Figure 5 shows complex-plane IMPS data for short-circuited DSSCs, assembled with varying thickness NTA films, under both frontside and backside illuminations. The spectra of frontilluminated DSSCs exhibit two distinct semicircles, while only
a single semicircle is present in the spectra. The electron transit time (τd) can be calculated by the relation τd = (2πfmin)1, where fmin is the characteristic frequency minimum of the imaginary component of the semicircle.28,29 Accordingly, spectra consisting of two semicircles represent a transport mechanism, where a fraction of electrons are collected within a smaller transit time constant, and residual electrons are further collected according to a larger time constant. The fmin values, indicated in the spectra of Figure 5, show electron transport processes with both short and long transit times for the front-illuminated DSSCs, while that with only a long transit time is present for the back-illuminated cells. The high performance of the front-illumined cells relative to that of the back-illuminated may result from the electron transport mode characterized by a short transit time. The low-frequency semicircle in the IMPS plot of the frontilluminated cells possibly arises from the nanoparticle interlayer between the NTA and FTO substrate. Figure 6a shows the IMPS plot of a cell assembly with the nanoparticle layer under frontside illumination. The plot shows a single-semicircular feature, with a fmin value about 1 order of magnitude larger than the lowfrequency fmin of the NTA-based cells. Although the nanoparticle film provides a random walk network for electron transport, this IMPS analysis rules out the possibility that the nanoparticle layer results in the appearance of the low-frequency semicircle in the front-illuminated spectra. As to the TiCl4 treatment, it improves the amount of dye absorption on the NTA films and has little influence on the electron transport pattern (see the Supporting Information). We also reversely attached a 23 μm NTA film onto the FTO substrate and carried out IMPS experiments while illuminating from the frontside. Figure 6b shows that the IMPS data, which are similar to those of the corresponding cell (23 μm NTA) shown in Figure 5. This indicates that the presence of morphological disorder, arising from bundling of nanotubes and microcracks at the NTA ends,19 is not responsible for the hindered electron transport, as detected by low-frequency IMPS. Electron transport in nanocrystalline TiO2 photoelectrodes is believed to occur by diffusion, since there is a negligible macroscopic electric field across the film.30 Several previous studies have characterized electron transport using two diffusion modes: trap-limited and trap-free modes.22,31,32 Since our NTA films were not single-crystalline, the semicircles of long and short transit times shown in the IMPS plots of the front-illuminated cells (Figure 5) should correspond to the trap-limited and trapfree diffusion modes, respectively. To provide a clear perspective 15021
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Figure 7. Variation of the trap-free and trap-limited electron transit times with the TiO2 film thickness for the NTA-based DSSCs under frontside illumination. The electron transit times were obtained by the fmin of IMPS plots in Figure 5. Figure 9. Variation of current densities collected by the trap-free and traplimited diffusion modes with the film thickness for the NTA-based DSSCs under frontside illumination. The trap-free and trap-limited current densities were determined by the Bode diagrams of the IMPS measurements (see the Supporting Information).
Figure 8. IMVS responses of DSSCs assembled with TiO2 NTA films of varying thicknesses under the frontside and backside illuminations. The blue and red light emitting diodes were used as the modulation light source with dc intensities at 15 and 13 mW cm2, respectively, for the frontside and backside illuminations.
of the electron transport mechanism for the TiO2 NTA films, Figure 7 summarizes the variation in transit time with film thickness for the front-illuminated DSSCs. The trap-limited transit time is about 2 orders of magnitude longer than the trap-free transit time. Crystal defects present at the grain boundaries prolong electron transit time by trapping and releasing electrons, resulting in a trap-limited diffusion mode (Scheme 1).33,34 Electrons generated near the electrolyte side must travel a long distance, so are more likely to undergo the trapping/detrapping process. Figure 7 illustrates the predicted increase in trap-limited transit time with NTA thickness. On the other hand, electrons induced from the FTO-neighboring zone (Scheme 1), where the trap states are filled with the electrons traveling from positions distant from the FTO, diffuse via a trap-free mode. These electrons exhibit a thickness-independent transit time, indicating that the increased diffusion length is counterbalanced by the increased trap occupancy. The region for trap-free diffusion depends on the distance from the electron-collecting substrate,
rather than on the trap-state density of the TiO2. The present and other previous studies have demonstrated such a situation.20,22 As an example, despite that the nanoparticle interlayer and the NTA end may have a higher trap-state density, they neighbor the FTO substrate and therefore would exhibit a trap-free diffusion pattern for electrons injected in this FTO-neighboring region. For the backside illumination scheme, most electrons are generated near the electrolyte side and move by trap-limited diffusion, resulting in the observed one-semicircle feature in the IMPS plot. Scheme 1 compares the mechanisms of electron transport for front- and back-illuminated NTA-based DSSCs. Figure 8 shows the IMVS responses of the front- and backilluminated DSSCs assembled with NTA of varying thicknesses at open circuit. Unlike the IMPS measurements, the illumination geometry does not affect the shape of the IMVS spectra. The electron lifetime (τn) describes the recombination rate between electrons and I3 ions. This parameter is determined from τn = (2πfmin)1.28,29 Figure 8 shows that the lifetime increases with increasing film thickness. We attribute this to the decrease in photon flux with distance from the incident position, thus reducing recombination rates and lengthening electron lifetimes. The back-illuminated cells show longer lifetimes than the frontilluminated cells do, and this is probably due to lower photon flux in the back-illuminated cells. However, the lifetime difference between the front- and back-illuminated cells was not as significant as the transit time difference. Thus, the diffusion mode for charge collection should be the governing factor for the cell performance. Figure 9 shows the film thickness dependence of trap-free and trap-limited photocurrent densities, as determined by Bode diagrams of the IMPS measurements (see the Supporting Information). Trap-free diffusion is the dominant mechanism for collection of photogenerated electrons. The trap-free photocurrent increases with increasing film thickness and reaches a maximum of 30 μm film thickness. Upon increasing the thickness further, the trap-free current density decreases slightly. The variation in trap-free current density shows a trend similar to those of Jsc and conversion efficiency of the front illuminated cells during one-sun illumination (Figure 4). This supports the argument that trap-free diffusion is the key factor in determining the performance of the front-illuminated NTA-based DSSCs. To further elucidate electron transport characteristics under one-sun illumination, we carried out EIS analysis of the 15022
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Cμ (=cμL), rt, and rct values, as determined from the spectra and equivalent circuit shown in Figure 10. The chemical capacitance provides the total density of free electrons in the conduction band and the localized electrons in the trap states.37,40 The chemical capacitance increases in proportion with film thickness, suggesting that trap states are uniformly distributed throughout the film. The 30 μm NTA film exhibits a minimum rt, possibly resulting from the predominant trap-free diffusion mode for electron transport (Figure 9). However, this diffusion mode could also promote interfacial charge recombination, since minimum rct occurred at a film thickness of 30 μm. Because the photogenerated electrons diffuse forward and recombine with I3, the electron collection resistance (Rcc) at the FTO substrate is given by 1 1 1 1 1 ¼ ¼ Rcc Rt Rct rt L rct =L
ð1Þ
Accordingly, the electron-collecting efficiency (ηcc) can be written41,42 1 Rcc
rt 2 ηcc ¼ 1 1 ¼ 1 rct L þ Rcc Rct
Figure 10. (a) Nyquist impedance plots of DSSCs assembled with the TiO2 NTA films of varying thicknesses measured at 0.65 V under frontside illumination at 100 mW cm2. The solid lines represent results simulated using the parameters in Table 1. (b) General equivalent circuit of a DSSC. rt, rct, and cμ correspond to the electron transport resistance, interfacial charge transfer resistance, and chemical capacitance in the TiO2 film. Rs is the sheet resistance of the conducting glass, RPt and CPt are the charge transfer resistance and capacitance at the Pt/electrolyte interface, respectively, and ZN is the Nernst impedance.
Table 1. Equivalent-Circuit Parameters for the TiO2 NTABased DSSCs of Varying Film Thicknesses Obtained at 0.65 V under Frontside Illumination at 100 mW cm2 film thickness (μm)
cμ (mF)
rt (Ω μm1)
rct (Ω μm)
ηcc (%)
17
1.4
0.18
860
94
23
1.7
0.12
820
93
30
2.0
0.07
750
91
37
2.5
0.08
850
86
front-illuminated NTA-based DSSCs. Figure 10a shows the Nyquist impedance spectra of DSSCs assembled with various thickness NTA films, and Figure 10b shows the corresponding equivalent circuit assembled to interpret the impedance spectra.35,36 The major semicircle of these spectra reflects electron transport dynamics in the TiO2 NTA films and is characterized by the chemical capacitance (cμ), electron transport resistance (rt), and interfacial charge recombination resistance (rct) in the equivalent circuit.3739 Table 1 shows the film thickness (L) dependence of
ð2Þ
Table 1 lists the ηcc values dependence on film thickness as determined by eq 2. The electron diffusion length (Ln), another parameter depicting the efficiency of charge collecting, can be calculated according to Ln = (rct/rt)1/2. The values of Ln and ηcc correlate as indicated by eq 2, and this study shows only the ηcc values. The electron collection efficiency of DSSCs assembled with nanocrystalline TiO2 films generally shows a significant decay with increasing thickness. However, Table 1 shows that cells assembled with NTA films exhibit only a small decay in ηcc values with increasing film thickness, and decay is insignificant for thicknesses below 30 μm. The larger crystal size of the NTA films results in lower trap state density and is responsible for the high ηcc values at large film thicknesses. A low trap state density can lead to a high degree of trap occupancy under illumination and thus extends the trap-free diffusion zone to facilitate charge collection. This important feature of the front-illuminated NTAbased DSSC devices is the reason for their superior performance over nanoparticle-based DSSC devices, and certainly the backilluminated NTA-based DSSC devices, in charge collection.
’ CONCLUSIONS Frontside and backside illumination schemes significantly influence electron transport and recombination mechanisms, which, in turn, determine the solar energy conversion performances of DSSC devices. The fabrication of transparent NTA electrodes is essential for cells operated under frontside illumination. From IMPS and IMVS studies, the pattern of electron transport in the NTA varied significantly with the illumination scheme, while the mechanism of electron recombination appeared relatively insensitive to orientation of the incident light source. Under frontside illumination, NTA film provides an additional electron diffusion mode, trap-free diffusion, which provides shorter transit times for electron collection when compared to the backside illumination scheme, where electrons diffuse by a trap-limited diffusion alone. Trap-limited diffusion exhibits longer average transit times and occurs for both illumination 15023
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The Journal of Physical Chemistry C schemes. The front-illuminated devices outperform the backilluminated cells, partially due to the availability of trap-free diffusion, which is the dominant contribution to the overall photocurrent collection process. Owing to large crystal size and presence of a trap-free diffusion zone, the front-illuminated NTAbased DSSC devices attain a charge collection efficiency greater than 90% at a NTA film thickness of 30 μm. The comparison of results from different illumination geometries elucidates factors governing the performance of the NTA-based DSSCs. This provides a useful insight for the development of effective TiO2 electron conductors constructed of nanotubes that align perpendicularly to the electron-collecting substrate, for numerous photovoltaic applications.
’ ASSOCIATED CONTENT
bS
Supporting Information. TEM image of a TiO2 nanotube, IMPS spectrum of a cell without TiCl4 treatment, and Bode diagrams of the IMPS measurements on the front-illuminated NTA-based DSSCs. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail
[email protected]; fax 886-6-2344496.
’ ACKNOWLEDGMENT This research is supported by the National Science Council of Taiwan (98-2221-E-006-110-MY3, 99-2622-E-006-010-CC2, 1003113-E-006-001, 100-3113-E-006-012, 100-3113-E-007-008, and 98-2221-E-006-112-MY2), the Bureau of Energy, Ministry of Economic Affairs, Taiwan (100-D0204-2). ’ REFERENCES (1) Kuang, D.; Brillet, J.; Chen, P.; Takata, M.; Uchida, S.; Miura, H.; Sumioka, K.; Zakeeruddin, S. M.; Gr€atzel, M. ACS Nano 2008, 2, 1113. (2) Mor., G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nano Lett. 2006, 6, 215. (3) Mor., G. K.; Varghese, O. K.; Paulose, M.; Shankar, K.; Grimes, C. A. Sol. Energy Mater. Sol. Cells 2006, 90, 2011. (4) Das, J.; Freitas, F. S.; Evans, I. R.; Nogueira, A. F.; Khushalani, D. J. Mater. Chem. 2010, 20, 4425. (5) Lin, C. J.; Yu, W. Y.; Lu, Y. T.; Chien, S. H. Chem. Commun. 2008, 6031. (6) Chen, Q.; Xu, D.; Wu, Z.; Liu, Z. Nanotechnology 2008, 19, 365708. (7) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. Nat. Mater. 2005, 4, 455. (8) Zhu, K.; Neale, N. R.; Miedaner, A.; Frank, A. J. Nano Lett. 2007, 7, 69. (9) Jiu, J.; Isoda, S.; Wang, F.; Adachi, M. J. Phys. Chem. B 2006, 110, 2087. (10) Ito, S.; Ha, N. C.; Rothenberger, G.; Comte, P.; Zakeeruddin, S. M.; Pechy, P.; Nazeeruddin, M. K.; Gr€atzel, M. Chem. Commun. 2006, 4004. (11) Kuang, D.; Brillet, J.; Chen, P.; Takata, M.; Uchida, S.; Miura, H.; Sumioka, K.; Zakeeruddin, S. M.; Gr€atzel, M. ACS Nano 2008, 2, 1113. (12) Shankar, K.; Mor, G. K.; Prakasam, H. E.; Yoriya, S.; Paulose, M.; Varghese, O. K.; Grime, C. A. Nanotechnology 2007, 18, 065707. (13) Xu, Y. M.; Fang, X. M.; Zhang, Z. G. Appl. Surf. Sci. 2009, 255, 8743.
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