A Competitive Electron Transport Mechanism in Hierarchical

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A Competitive Electron Transport Mechanism in Hierarchical Homogeneous Hybrid Structures Composed of TiO2 Nanoparticles and Nanotubes Jong Min Choi, Gyeongho Kang, and Taiho Park Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 02 Feb 2015 Downloaded from http://pubs.acs.org on February 5, 2015

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Chemistry of Materials

A Competitive Electron Transport Mechanism in Hierarchical Homogeneous Hybrid Structures Composed of TiO2 Nanoparticles and Nanotubes Jongmin Choi, Gyeongho Kang, and Taiho Park* Department of Chemical Engineering, Pohang University of Science and Technology, San31, Nam-gu, Pohang, Kyoungbuk, 790-780, Korea ABSTRACT: We prepared well-defined hierarchical structures comprising doubly open-ended TiO2 nanotube (NT) arrays covered with various layers of few-nm sized TiO2 nanoparticles (NPs) to investigate the electron collection mechanisms in homogeneous hybrid structures. We found that competitive electron transport pathways (direct transport through the NT and randomized transport through the NPs) are present in the homogeneous hybrid structures. Photoinduced electrons generated at the few-nm sized TiO2 NPs directly connected with TiO2 NTs (e.g., isolated and single layered NPs on the surface of NTs) dominantly traveled to the NTs. With increasing the number of TiO2 NP layers, photoinduced electrons are randomly transported through the TiO2 NP layers. Enhanced light harvesting and efficient charge collection (~ 95%) caused by the increased amounts of dye loading and the direct transport through the NT, respectively, are achieved in a structure with ca. 1.4 layer of few-nm sized TiO2 NPs, resulting in a power conversion efficiency of 11.3% with 22.9 mA/cm2 of JSC value close to the theoretical value (~26 mA/cm2) of a N719-based DSC.

■ Introduction Dye-sensitized solar cells (DSCs) have been considered a promising alternative to conventional silicon-based solar cells due to their low cost and high efficiency.1-4 Electric power in DSCs is generated through charge collection from photoexcited sensitizers, which transfer electrons to an n-type semiconductor.5-8 A deep understanding of the electron transfer mechanism at the semiconductor interface with the sensitizer is critical for designing new device structures and improving the photovoltaic performances of DSCs (e.g., neutralization of oxidized sensitizers,9,10 hole transporting in electrolytes.11,12) Meanwhile, the efficiency of electron transfer could be primarily influenced by the identity and structure of the receiving semiconductor.13-15 Efficient electron transfer requires that the injected electrons should quickly pass through the semiconductor to the external circuit, thereby preventing interfacial charge recombination with the oxidized dye or other redox species present in the electrolyte.16 Titanium dioxide (TiO2) provides excellent electrical properties. The energy band position is appropriate for receiving an electron from a sensitizer excited by visible light and the transferred electrons display a high mobility within the TiO2.17-19 Electron transfer in a TiO2 semiconductor is primarily governed by its structure. The most widely used semiconductor is 3D randomly packed TiO2 nanoparticles (NPs). This TiO2 NP film has a large surface area and, thus, permits large levels of dye loading, resulting in good light harvesting.20-22 However, a large number of grain boundaries and random pathway in NPs enhance electron trapping events and interfacial charge recombination reaction,23-26 thereby diminishing the overall electron charge collection (Figure 1a).27

Figure 1. Illustration of charge transport pathways in 3D randomly packed TiO2 nanoparticles (a) and aligned 1D TiO2 NT decorated with small sized TiO2 nanoparticles layer.

In order to improve this charge collecting behaviour, researches have been prompted towards the use of onedimensional (1D) TiO2 nanostructures (e.g., nanotubes,28,29 nanorods,30 nanowires31 etc). These 1D nanostructures are expected to significantly enhance the electron transport properties due to their decreased grain boundaries and direct electron pathway, reducing an interfacial charge recombination. Frank et al. demonstrated32 that the recombination time constant associated with TiO2 NTs on a Ti foil is 10 times larger than the value measured in TiO2 NPs because the TiO2 NT structure forms direct electron pathways. We reported similar results that were obtained using noncurling, freestanding, large-area aligned doubly open-ended TiO2 NTs.33 Recently, TiO2 NP–NT hierarchical structures where the NT covered with small sized NPs have been investigated in an effort to combine the large surface areas of NPs34,35 and the formation of direct electron pathways in NT.36-38

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Figure 2. (a) Schematic illustration of the structure of hierarchical TiO2 photoanodes. (b) TEM image of NT hierarchical electrodes (80 min). (c) HR-SEM images of top and cross-section parts of the bare NTs and NT hierarchical electrodes prepared using doubly open-ended NTs at the immersion time (in minutes) in the Ti(OH)4 solution. Red arrows indicate regions of bare TiO2 NTs at 40 min. Red circles shows incomplete bilayer regions. (d) Schematic illustration of NT hierarchical electrodes with isolated TiO2 NPs, more 1.4 layered NPs, and 2.6 layered NPs. Most investigations of these materials have focused on how to fabricate such hierarchical structures.35-41 Decoration of NPs on NTs may induce changes of the interfacial charge recombination at the interface between an electrolyte and a photoanode. Lamberti et al.42 and Jeon et al.43 independently reported a high resistance to recombination at the interface. In contrast, we44,45 and other research groups46-48 independently observed an opposite behavior at the interface. The TiO2 NP–NT hierarchical structures could potentially form two electron transport pathways, depending on the position at which photoinduced electrons were generated (Figure 1b). When the photoanode is a heterogeneous hybrid system (the electrodes are comprised of different materials), the electron movements are predictable due to their different electrical properties such as an energy band position, conductivity and so on, as reported by Wang et al. in ZnO nanowires with TiO2 NPs49-50 and Snaith et al. in Al2O3 nanoparticle with a perovskite layer.51 However, in a homogeneous hybrid system (the electrodes are composed of same material) such as TiO2 NP– NT hierarchical structures, it is difficult to predict electron movements due to their similar electrical properties. In particular, the TiO2 hierarchical homogeneous structures exhibited more chemically stable characteristics than those of other metal oxides including ZnO.52 Systematical investigation of the charge transport properties in the TiO2 homogeneous system is highly required to design advanced photoanode. Here, we describe a competitive electron transport mechanism in a hierarchical homogeneous hybrid structure comprising doubly open-ended TiO2 NT arrays covered with various layers of TiO2 NPs. This structure offers the advantage of a high surface area that increases dye loading, resulting in a better light harvesting. Therefore, extraction of the photoinduced electrons to the NTs, followed by transport through the NT, prevailed over transport pathways through the NPs might lead a remarkably improved electron collection as well as the better light harvesting.

■ Results and Discussion Preparation of hierarchical TiO2 NP–NT electrodes. Figure 2a shows a schematic illustration of the noncurling, aligned doubly open-ended TiO2 NTs standing (11 µm) on a FTO substrate with a thin (2 µm) TiO2 NP binding layer.33 The assembled NTs were annealed at 550°C for 30 min and immersed in a 0.2 M Ti(OH)4 aqueous solution at 70°C for different immersion times (40, 60, or 80 min) to control the quantities of the few-nm sized TiO2 NPs that were loaded. The electrodes were then re-annealed to create anatase TiO2 NPs (see Figure S1 for details). Transmittance electron microscopy (TEM) revealed that few nm-sized TiO2 NPs covered on the surface of the NTs (Figure 2b). High-resolution scanning electron microscopy (HR-SEM) images more clearly showed the formation of layers with few-nm sized TiO2 NPs (ca. 3-5 nm) on the surface of the NTs (Figure 2c). The average inner diameters of the doubly open-ended TiO2 NTs were 120 nm at the top of the layer (Figure S2a) and 66 nm at the bottom of the layer (Figure 2c). The inner NT diameter at the bottom surface decreased with the immersion time (Figure 2c). The few-nm sized TiO2 NPs were evenly distributed across the outer NT walls. The NTs retained a sufficiently large inner diameter. The inside walls of the NTs were evenly covered with few nm-sized TiO2 NPs (Figure S2). For the NTs immersed for 40 min, isolated NPs were clearly visible on the NT walls (Figure 2c) and the inner diameter (ca. 60 nm) was slightly smaller than that (ca. 66 nm) of the bare NTs. With immersion, the diameters of the NTs were decreased (ca. 55 and 45 nm for 60 and 80 min, respectively). Figure 2d shows schematic illustration of an average number of NP layers present on the NTs, estimated from eq. 1. Number of NP layers = (DNT - DNT#)/(2DNP)


(eq. 1)

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Figure 3. (a) XRD and XPS of the (b) Ti 2p and (c) O 1s patterns of the bare NT-, NT0.8-, NT1.4-, and NT2.6-based photoanodes. All NT electrodes contain thin (~2 µm) NP binding layer on FTO glass. The numbers of NT represent immersing time of Ti(OH)4 solution. The A in the spectra represent anatase peak of TiO2, and inset arrow represent position of peaks. Notice that two peaks corresponding to Ti 2p 3/2 at 459.5 ± 0.05 eV and Ti 2p 1/2 465.2 ± 0.2 eV in the Ti 2p binding energy region and two peaks corresponding to lattice oxygen (OL) in TiO2 (530.7 ± 0.05 eV) and the surface OH species (532.2 ± 0.1 eV) in the O 1s binding energy region.

where, DNT and DNT# (#: 40, 60, and 80 min) are the inner diameters of the NTs before and after immersion in a Ti(OH)4 solution, respectively. DNP is an average diameter (~ 4 nm) of the NPs. The number of a NP layer on the NTs immersed for 40 min (denoted to NT0.8) is ca. 0.8, which is less than 1.0. This result indicated that the NPs were not completely connected each other and, thus, we could observe the isolated TiO2 NPs as marked as arrows in Figure 2c. Meanwhile, the numbers of NP layers on the NTs immersed for 60 min (denoted to NT1.4) are ca. 1.4, indicating that one layer of the NPs fully covered the NTs and the extra NPs formed connection with the first layer. For the TiO2 NTs immersed for 80 min (denoted to NT2.6), ca. 2.6 were estimated, suggesting that more than two layers of NPs covered the NT surfaces. Characterization of hierarchical TiO2 NP–NT electrodes. The properties of the hierarchical homogeneous hybrid photoanodes were characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS) methods. The XRD patterns obtained from the various photoanodes were identical, indicating that all electrodes had an anatase crystal structure (Figure 3a). In addition, the XPS spectra exhibited no differences in the Ti 2p and O 1s binding energy regions (Figure 3b), respectively, excerpted from the survey spectra (Figure S3). These results suggested that the binding energies, Ti3+ formation, and oxygen vacancies were indistinguishable in the bare NTs and the hybrid photoanodes. The valence band maxima of each photoanode appeared at 3.3 eV below the Fermi level (EB = 0 eV), as determined by the edge of UPS spectra (Figure S4), consistent with previous reports.53 These meant that all electrodes were electronically identical after the Ti(OH)4 treatment. The amounts of dyes (N719) adsorbed to individual electrode, which is proportional to the active surface areas of the electrodes in this study, were measured using UV-Vis spectrophotometry after detaching the dyes from the TiO2 electrodes in a 0.1 M KOH solution (Figure 4).54-55 The quantity of N719 adsorbed onto NT0.8 was slightly larger than that adsorbed onto NT due to the presence of few-nm sized TiO2 NPs (the inset in Figure 4). The amounts of N719 increased exponentially, indicating the rapid generation of TiO2 NPs on the surfaces of the TiO2 NTs, consistent with the SEM images (Fig-

ure 2c). The quantity of N719 adsorbed onto NT2.6 approached the levels reported for conventional NP electrodes. It should be noted that the quantity of N719 molecules adsorbed onto the NTs was 0.104 µmol/cm2, 27% lower than the quantity adsorbed to conventional TiO2 NP electrodes.

Figure 4. Representative UV-Vis spectra of the dye (N719) detached from the electrodes. 0.1 M KOH aqueous solution was used for the detachment of N719. An insert: the quantity of adsorbed N719 on the electrodes as a function of immersion time in 0.2M Ti(OH)4 aqueous solution.

Photovoltaic performances. Figure 5a shows the photocurrent–photovoltage (J–V) properties of DSCs prepared from the various electrodes. The photovoltaic parameters are summarized in Table 1 (See Figure 5c and Figure S5 for the deviation of photovoltaic parameters). The front-illuminated NT-based DSC exhibited a JSC = 13.6 mA/cm2, a VOC = 0.759 V, and a FF = 65.5, providing a power conversion efficiency (PCE) of 6.8% at a tube length of 11 µm. The conventional NP-based DSC with a thickness of 13 µm displayed JSC = 14.3 mA/cm2, a VOC = 0.674 mV, FF = 67.6, and provided PCE = 6.5%. The dye loading value was 27% lower in the NT-based DSC than in the NP-based DSC, but the JSC for the NP-based DSC was similar to that obtained for the NT-based DSC. The PCE val-



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ues of the NT-based DSCs with NPs were 7.3–11.3%, better than the value (6.8%) obtained without NPs.

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The IPCE spectra revealed excellent photocurrent responses over the absorption range (Figure 5b). The trend in the IPCE values obtained at 525 nm agreed well with the trend in the JSC values (Figure 5c), indicating that the IPCE value was also not proportional to the quantity of dye adsorbed onto the electrodes. These results suggested that the remarkable increase in the IPCE and JSC values in the NT0.8- or NT1.4-based DSC resulted from effects other than simply an increase in light harvesting by the dyes. Analysis of photovoltaic and charge collection parameters. In general, JSC values can be estimated using eq. 2:32 JSC = qⅹηlhⅹηinjⅹηccⅹIo,

Figure 5. (a) Representative photocurrent–photovoltage (J–V) properties of the NT electrodes based DSCs, measured under AM 1.5 solar illumination, and (b) corresponding incident photon-tocurrent efficiency (IPCE). (c) Deviation of photovoltaic PCE of 6 devices. (d) The JSC and IPCE (at 525 nm) values as a function of the quantity of dye adsorbed onto the electrodes. Inset: %Transmittance for various TiO2 film thicknesses.

Table 1. The photovoltaic parameters of the NT-, NT0.8-, NT1.4-, NT2.6-, and NP-based DSCs, measured under AM 1.5 solar illumination.a Devices

Relative [dye]b

IPCEc (%)

VOC [V]

JSC [mA/cm2]

FF [%]

η [%]

NT

0.104

56.5

0.759

13.6

65.5

6.8

NT0.8

0.106

82.0

0.771

17.6

65.8

8.9

NT1.4

0.111

95.3

0.747

22.9

65.8

11.3

NT2.6

0.122

61.4

0.729

15.2

66.2

7.3

NP

0.132

54.8

0.674

14.3

67.6

6.5

(eq. 2)

where q is the fundamental charge of an electron, Io is the intensity of the incident light, ηlh is the light harvesting efficiency, ηinj is the electron injection efficiency from excited dye molecules to the TiO2 conduction band, and ηcc is the charge collection efficiency of the injected electrons to the transparent conductive oxides (TCO) layer. Among these parameters, Io was constant for all electrodes used in this study and Io on devices with dye loading levels exceeding 0.08 µmol/cm2 (7.8 µm thickness) was completely absorbed by the dyes (see the transmittance spectra in the inset of Figure 5c). Assuming that ηinj and ηcc were constant for all devices, the JSC value (or IPCE) should be proportional to the quantity of dyes adsorbed onto the electrodes for a given dye absorption coefficient, which is related to ηlh; however, the measured increase in JSC and the IPCE value were not proportional to the quantity of dye adsorbed onto the electrodes (Figure 5c). Assuming that the ηinj values were similar in the DSCs, this result suggested that the ηcc values of the DSCs varied and depended strongly on the type of electrodes.

a

Values obtained using the average over 4 devices, for 5 individual experiments (Figure S4). NTs were placed on 2 µm thick NPs (20 nm size). Cell size: 0.16 cm2 with black mask. b The values were determined by measuring UV-Vis absorption spectra (see insert in Figure 4) c Values at 525 nm.

These improved PCE values could be firstly attributed to an increase in JSC due to the increased dye loading; however, the increase in JSC was inconsistent with the increase in the quantity of dye molecules adsorbed onto the electrodes. For instance, the best photovoltaic performance (PCE = 11.3 %) was obtained from the NT1.4-based DSC, which adsorbed 0.111 µmol/cm2 dye molecules, and not NT2.6 which adsorbed more dye molecules (0.122 µmol/cm2), and produced a JSC = 22.9 mA/cm2, FF = 65.8, a VOC = 0.747 V. It is noticed that the JSC value was much greater than those (14.84 ~ 17.47 mA/cm2) employing singly open-ended NTs.36,40 The PCE (7.3%) of the NT2.6-based DSC was even lower than that of the NT1.4-based DSC, which adsorbed 0.106 µmol/cm2 dye molecules.


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Figure 6. (a) Illustration of the competitive reactions that occur at the dye/TiO2/electrolyte interface. (b) Nyquist plots for the NT-, NT0.8-, NT1.4-, NT2.6-, and NP-based DSCs measured at –0.69 V in the dark. The recombination resistances (R2) were calculated by fitting to a simplified circuit (the model given in the figure annotation). (c) The VOC and R2 values as a function of the quantity of dye adsorbed onto the electrodes.

photometers to determine τtrans (an electron transport time across the photoanode films) and τrec (an electron recombination time with I3-), respectively (Figure 7). 32 The charge collection efficiency (ηcc) was calculated according to the following eq. 4:32

ηcc ∼ 1 – (τtrans/τrec), Within a few picoseconds of electron injection (ηinj) from a photo-excited dye molecule into the quasi conduction band (CB), the electron relaxes to the lowest energy level of the CB and is subsequently trapped in the intra-band states of the surface. The photoinduced electrons [e–(TiO2)] in the trap states could then recombine with dye cations (Dye+, at a rate of kDye+)57 and the oxidized species (I3–, at a rate of k2)58 as described in Figure 6a. These recombination reactions increased (the characteristic τrec was short) in the presence of a greater number of trap states in the lower-lying quasi-Fermi level of the TiO2 electrode. The JSC measured from such DSCs was, correspondingly, reduced. The JSC drop due to recombination reactions (k2) decreased VOC relative to a theoretical VOC, determined by the energy difference between the quasi-Fermi level of a nanocrystalline TiO2 electrode under illumination and the redox potential of the I3–/I– pair in the electrolyte (Figure 6a), as described by eq. 3:59 VOC ∼ (nkT/q)ⅹln(JSC/JS),

(eq. 4)

The τtrans values for the NT- and NP-based DSCs were not significantly different (Figure 7a), due to the identical contact with FTC substrate using 20 nm-sized TiO2 NPs, consistent with previous reports.32,33 The τtrans values obtained from the NT1.4–NT2.6 were slightly smaller than the value obtained from NT, thereby yielding faster transport to the working electrode. This was due to the enhanced electrical connections at the interfaces between the TiO2 NP interlayer and the NTs by the few nm-sized NPs. In addition, we observed that there were many defects at the bare NT surfaces (Figure S6), which disappeared after treatment with the Ti(OH)4 solution for a relatively short period of time (40 min, NT0.8).

(eq. 3)

where, n is the device ideality factor, k is the Boltzmann constant, T is the temperature in Kelvin, q is the fundamental charge, and JS is the saturation current density. A higher number of surface trap states decreased k2. The trap states were thought to increase in number in the presence of a greater number of few-nm sized NPs on the NT surface or in the presence of NT surface defects. Figure 6b shows Nyquist plots of DSCs employing hierarchical hybrid electrodes, NTs, and NPs, measured at –0.69 V in the dark using electrochemical impedance spectroscopy (EIS). The measured impedance spectra were fit to the simplified equivalent circuit (the inset of Figure 6b). The small semicircle at high frequencies and the larger semicircle at low frequencies (in the dark, under a forward bias) were attributed to charge transfer at the electrolyte/counter electrode (R1) and electron recombination (R2) at the TiO2/electrolyte interface, respectively. Rs was related to the series resistance.60 The R2 value of the NP was 80 Ω, much lower than the value obtained from the NT (310 Ω) due to an increased number of surface trap states, which increased recombination of the photoinduced electrons with I3– in the NT-based DSCs. As the thickness of the few-nm sized NPs on the NTs increased (thereby increasing the surface trap states), the R2 values decreased (e.g., 164 Ω for the NT2.6), except for NT0.8. This result indicated the presence of a dominant photoinduced electron loss mechanism (eq. 3). The trend in the R2 values agreed well with the trend in the VOC values (Figure 6c). The strong dependence of the photovoltaic performances on electrode type might be ascribed to the effects of ηcc in eq. 1. Therefore, we further investigated photoinduced electron loss mechanism using the intensity-modulated photocurrent (IMPS, short-circuit) and photovoltage (IMVS, open-circuit) spectro-

Figure 7. Comparison of electron transport times (τtrans) (a), electron recombination times (or lifetime) (τe) (b), charge collection efficiencies (ηcc) (c), and effective electron diffusion lengths (DL) (d) of the NT-, NT0.8-, NT1.4-, NT2.6-, and NP-DSCs as a function of the incident photon flux.

Table 2. Summary of the charge transport parameters of the NT-, NT0.8-, NT1.4-, NT2.6- and NP-DSCs, estimated using electrochemical impedance spectroscopy (EIS), intensity-modulated photocurrent spectroscopy (IMPS), and intensity-modulated photovoltage spectroscopy (IMVS). TiO2

R2 (Ω)

τtrans (ms)

τrec (ms)

ηcc (%)

DL (µm)

NT

310

16

214

93

24

NT0.8

405

12

264

96

31

NT1.4

263

10

173

95

28

NT2.6

164

11

95

88

19

NP

80

13

43

69

12

Meanwhile, with increasing the number of few nm-sized NPs, the photoinduced electron loss mechanism via the re-



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combination reaction (k2) on few nm-sized NPs become dominant over the enhanced the electrical connections. The τrec value of NT0.8 exceeded that of NT; however, the τrec value of NT1.4 was smaller than that of NT, and this value was again further reduced in NT2.6. Notice that the trend of the ηcc values was consistent with the trend in the τrec values, which was also consistent with the trend in the R2 values (Table 2). According to the above result, a best performance with a highest JSC value should have been obtained from NT0.8-based DSC. However, NT1.4-based DSC shows the best PCE (11.3%) with an outstanding JSC (22.9 mA/cm2),61 indicating that the increased ηlh value still dominant over the recombination reaction. Notice that the VOC value of NT1.4-based DSC was slightly decreased compared to the value in NT0.8-based DSC due to the recombination reaction on the few nm-sized NP layer. Competitive electron transport mechanisms. Figure 8 shows the available charge transport pathways in the in hierarchical homogeneous hybrid structures composed of TiO2 NPs and NTs.

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generated on the isolated TiO2 NPs, were easily extracted to the NT layer, as evidenced by the high values of R2 and ηcc (Figure 8a). In the NT1.4-based DSC, extraction of the photoinduced electrons to the NTs, followed by transport through the NT, prevailed over transport pathways through the first layered NPs. However, some of the photoinduced electrons generated over the first layered TiO2 NPs could not be transferred to the NT as evidenced by the small decrease in R2 and ηcc (Figure 8b) This trend become dominant in the multilayered NPs (NT2.6), in which the dyes were mainly adsorbed onto the outer surface of the TiO2 NPs (with a relatively large distance between the NTs and the photoinduced electrons). The photoinduced electrons were transported through the fewnm sized TiO2 NPs, leading to a shorter effective electron diffusion length as a result from the great recombination reaction (Figure 8c). Verification of the mechanism. Finally, the dominant electron transport pathway through the NPs in NT2.6 was confirmed by preparing an even thicker layered of TiO2 NPs on the NTs. The immersion time for this electrode was extended to 100 min (denoted NT100-based DSC). Figure 9a shows the surface morphologies of a conventional NP layer and the NT100 electrodes. Numerous few-nm sized TiO2 particles were aggregated on the surfaces of the NTs; thus, the inner diameter of the NTs in NT100 was much smaller than that in NT2.6. The dye loading was correspondingly higher (0.127 µmol/cm2) (Figure S7), comparable to the levels reached by NP layers (0.132 µmol/cm2).

Figure 8. Illustration of two competitive electron pathways on NTs with isolated or thin-layered (a), and thick-layered few nmsized TiO2 NPs (b).

Unlike direct transport of the photoinduced electron through the NTs, photoinduced electrons generated from dyes adsorbed onto the surfaces of the NPs could travel from particle to particle (random diffusion). Given the NT coverage by fewnm sized TiO2 NPs, more dye molecules were adsorbed onto the surfaces of the few-nm sized TiO2 NPs rather than onto the NTs in the presence of a greater number of few-nm sized TiO2 NPs grown on the NTs surface. The two transportation pathways may be hybridized in NTs having few-nm sized TiO2 NPs and may depend on the number of the NP layer. The effective electron diffusion length (DL) was estimated according to the eq. 6 proposed by Bisquert:62 DL = (d2τrec /4τtrans)1/2,

(eq. 6)

where d is the thickness of the TiO2 electrode. The DL value (28 µm) of NT1.4 is larger than that (24 µm) of NT, but was smaller than that (31 µm) of NT0.8, and this value was again further reduced in NT2.6 (19 µm), indicating that comparable electron transfer mechanisms were functional in these electrodes. Therefore, most photoinduced electrons in the NT0.8,

Figure 9. (a) SEM images of the conventional TiO2 NP surface and the NT100 surface. An insert: the NT2.6 surface for comparison with the NT100. (b) Comparison of photocurrent– photovoltage (J–V) properties of the NT1.4-, NT100-, and NPbased DSCs, measured under AM 1.5 solar illumination. An insert: Nyquist plots for the NT1.4-, NT100-, and NP-based DSCs, measured at –0.69 V in the dark.

The J–V properties of the NT100-based DSC were similar to those observed in NP-based DSCs, providing JSC = 15.0


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mA/cm2, FF = 63.9, VOC = 0.705 V, and PCE = 6.8%. The VOC value was the lowest yet measured among the NT-based DSCs examined here and was slightly larger than the value measured for the NP-based DSC (0.674 V), as expected from the R2 value, as seen in the inset of Figure 9b. The R2 for NT100based DSC was similar to the value obtained from the NPbased DSC. In addition, the ηcc and DL values were 85.9% and 32.0 µm, respectively, values that are intermediate between those obtained from the NT2.6- and NP-based DSCs (Figure S8). This result supported the conclusion that a randomized charge transport pathway through the few-nm sized TiO2 NPs was dominant in the multilayered NPs.

■ Conclusion In conclusion, competitive electron transport pathways were found in the hierarchical homogeneous hybrid systems composed of TiO2 NPs and NTs: direct pathways through the TiO2 NTs or randomized pathways through the TiO2 NPs. The features and prevalence of the pathways depended on the number of the TiO2 NPs layer on the surface of the aligned doubly open-ended TiO2 NTs. Photoinduced electrons moved through the aligned TiO2 NTs upon introduction of many isolated 4 nm-sized TiO2 NPs or a less than bilayer of TiO2 NPs onto the surface of the aligned TiO2 NTs. This resulted in a high ηcc and a large R2, along with an increase in dye loading. These effects provided an extremely efficient DSC. This study suggests guidelines for balancing the light harvesting efficiency and charge collection efficiency in hierarchical homogeneous hybrid photoanode.

■ ASSOCIATED CONTENT Supporting Information. Experimental details, extra HR-SEM images, XPS, UPS, and UV-Vis. spectra, and charge collection parameters for NT100-based DSC

■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENT This work was supported by the Nano·Material Technology Development Program (2012M3A7B4049989), the Center for Next Generation Dye-sensitized Solar Cells (No. 2008– 0061903) and the Basic Science Research Program (No. 2012R1A1A2044697) through a NRF funded by MSIP (Korea).

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■ TABLE OF CONTENTS


9

A Competitive Electron Transport Mechanism in Hierarchical

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