Role of One-Dimensional Ribbonlike Nanostructures in Dye

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Role of One-Dimensional Ribbonlike Nanostructures in Dye-Sensitized TiO2-Based Solar Cells Jiazang Chen,†,‡ Bo Li,†,§ Jianfeng Zheng,† Suping Jia,† Jianghong Zhao,† Huanwang Jing,†,§ and Zhenping Zhu*,† †

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China Graduate University of Chinese Academy of Sciences, Beijing 100039, China § College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China ‡

bS Supporting Information ABSTRACT: In dye-sensitized solar cells, there is a competition between transport of electrons through the porous semiconductor electrode toward the conducting substrate and backreaction of electrons to recombination with I3 ions on the semiconductorelectrolyte interface, which determines the charge collection efficiency and is strongly influenced by the electronic site distribution in intraband and geometrical structure of the semiconductors. Herein, we systematically analyze the electrochemical parameters of TiO2 nanoribbon- and nanoparticle-based electrodes by electrochemical impedance spectroscopy. The results show that the intrinsic one-dimensional crystalline structure of TiO2 nanoribbons can promote formation of a space charge layer on the surface of the semiconductor, which effectively blocks the recombination of electrons with I3 ions in the semiconductorelectrolyte interface, resulting in an increase of electron lifetime and a higher cell voltage. Furthermore, the boundaryless structure of the TiO2 nanoribbons provides efficient channels for electron transport and therefore increases electron diffusion length. The combination of TiO2 nanoparticle-based electrode with TiO2 nanoribbons can significantly improve energy conversion efficiency of ∼60%. These data provide a basic understanding of the role of TiO2 geometrical structure in solar energy conversion.

1. INTRODUCTION Since the breakthrough work done by Oregan and Gr€atzel in 1991,1 regenerative dye-sensitized solar cells (DSSCs) have attracted intensive attention because of their inexpensive costs and excellent energy conversion efficiency.24 Typically, a DSSC is constructed with a dye-anchored metal oxide semiconductor supported on a transparent conducting oxide (TCO) substrate, an electrolyte containing redox couples (usually I/I3 couple), and a platinized counterelectrode. During the operation of the DSSC, electrons are injected from the photoexcited dye into the conduction band of the semiconductor, followed by diffusion toward the TCO substrate. The transport of holes from the oxidized dye to the counterelectrode is carried out by the I/I3 species in the electrolyte, which is in contact with the semiconductor electrode. The photoelectrode is the key component of the DSSC, since a majority of the photoinduced steps take place in the bulk phase of the semiconductor network or on the surface. In these steps, the competition between transport of electron through the porous semiconductor and recombination of electrons with I3 ions in electrolyte determines the electron collection efficiency in DSSCs.59 Therefore, separation of r 2011 American Chemical Society

the electronhole pairs, transport of electrons through the semiconductor films toward the conducting substrate, and recombination of electrons with hole carriers on the semiconductorelectrolyte interface (SEI) are key factors to be considered for improving the photovoltaic performance of the DSSCs. For a DSSC, a large surface area of semiconductor materials is necessary to enhance dye absorptions for efficient light harvesting. Large surface area always requires a high porosity of the semiconductor network and small-sized particles.10 Highly porous structures are also beneficial to a fast diffusion of I/I3 species, which can improve regeneration of the oxide dye with I ions and suppress the recombination of electrons with I3 ions.11 However, high porosity of semiconductor film and small-sized particles generally result undesirably in poor electron-transport properties,1214 because numerous interparticle boundaries are involved, which greatly increases electron-transport resistance13 and trapping/detrapping events by surface state.15 Furthermore, Received: January 15, 2011 Revised: February 24, 2011 Published: March 16, 2011 7104

dx.doi.org/10.1021/jp2004369 | J. Phys. Chem. C 2011, 115, 7104–7113

The Journal of Physical Chemistry C small size means dense surface state sites, which will act as intermediates to trap electrons and increase the probability of electron recombination with I3 ions.11,1518 Theoretically, one-dimensional (1D) semiconductor nanostructures have less grain boundaries and will facilitate electron transport.1928 However, experimental results for 1D nanostructured TiO2 and ZnO photoelectrodes exhibit low energy conversion efficiency, because of low surface area of the 1D materials.24,29 In order to use the merits of both 1D TiO2 nanostructures and zerodimensional (0D) nanoparticles, they have been combined as composite photoelectrodes, which can greatly improve the energy conversion.3036 About the significant effect of the 0D1D combination, details on the intrinsic roles of 1D TiO2 nanostructures still need to be systemically understood. In this paper, we focus on semiconductor electrochemical properties of 1D ribbonlike TiO2 nanostructures and employ electrochemical impedance spectroscopy (EIS) to evaluate the specific parameters, including charge-transport resistance (RT), interfacial charge-transfer resistance (RCT), chemical capacitance (Cμ), electron diffusion coefficient (Dn), electron lifetime (τn), and electron diffusion length (Ln), with a comparison to those of 0D TiO2 nanoparticles. The obtained data give a good explanation on the intrinsic role of 1D TO2 nanostructures in the 1D/0D composite photoelectrode.

2. EXPERIMENTAL SECTION 2.1. Preparation of TiO2 Nanoparticles and Nanoribbons. TiO2 nanoparticles were prepared according to a modified literature procedure by hydrothermal synthesis starting from titanium(IV) n-butoxide.37 Briefly, 40 mL of titanium(IV) butoxide was added dropwise into 240 mL of 0.1 M HNO3 under vigorous stirring. Then the obtained slurry was rapidly heated to 80 °C and kept for 8 h. After peptization, the colloidal solution was moved to a self-sealed Teflon-lined autoclave that was heated for 12 h at 230 °C to grow TiO2 nanoparticles. TiO2 nanoribbons were prepared from a commercial P25 TiO2 powder (Degussa). One gram of TiO2 powder was mixed with 35 mL of NaOH (10 M) aqueous solution under stirring at room temperature for 30 min. The mixture was then heated at 160 °C for 12 h in a 50 mL selfsealed Teflon-lined autoclave. The collected precipitates were washed with water and dilute HCl several times. The final product was obtained by calcining the washed precipitates at 600 °C for 2 h. 2.2. Fabrication of DSSCs. The TiO2 pastes with various ratios of nanoribbon/nanoparticle were prepared by ultrasonically mixing nanoparticle (∼5 wt % from the hydrothermal process) and TiO2 nanoribbon (10 wt % in 0.1 M HNO3) dispersions for 2 h. The mixed dispersion was then concentrated in a rotary evaporator and evaporated at 40 °C to a final TiO2 concentration of ∼15 wt %. After that, poly(ethylene glycol) (PEG, molecular weight 20 000) was added in a proportion of 50% of the TiO2 weight. The as-prepared pastes were deposited by doctor-blade technique on fluorine-doped tin oxide conducting glass (FTO, 14 Ω/square, Nippon Glass Sheet) by preparing an active area of 0.50 cm2. Five pastes, with TiO2 nanoribbon concentrations of 0, 5%, 10%, 20%, and 100%, were used to the prepare TiO2 photoelectrodes, which were respectively labeled as TNP, TPR05, TPR10, TPR20, and TNR. Thickness of the films was controlled by adjusting the thickness of adhesive tapes (here the thickness of the TiO2 films after calcination was about 12 μm). The film was then heated to 500 °C at a rate of 15 °C/min

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and kept at 500 °C for 30 min. After cooling to 80 °C, the TiO2 electrode was immersed overnight in a solution of N719 dye [cisdiisothiocyanatobis(2,20 -bipyridyl-4,40 -dicarboxylato)ruthenium(II) bis(tetrabutylammonium) that was dissolved in acetonitrile at a concentration of 0.5 mM]. The electrode was then rinsed with acetonitrile and dried. One drop of iodine-containing electrolyte was deposited onto the surface of the electrode and penetrated inside the TiO2 film via capillary action. The electrolyte was composed of 0.1 M lithium iodide (LiI), 0.6 M tetrabutylammonium iodide (TBAI), 0.05 M iodine (I2), and 0.5 M 4-tertbutylpyridine that was dissolved in acetonitrile. A platinized FTO counterelectrode was then clipped on top of the TiO2 photoelectrode to form a photovoltaic device. 2.3. Photoelectrochemistry and Electrochemistry. The photovoltaic properties of the DSSCs were characterized by recording the photocurrentvoltage (IV) curves under illumination of A.M. 1.5 G (100 mW/cm2). The illumination was provided by a San-Ei solar simulator. Recording of incident photon conversion efficiency (IPCE) was carried out with a Crown instrument. Electrochemical impedance measurements of the cells were performed with a computer-controlled Zahner Im6ex potentiostat with a frequency of 0.005500K Hz; the amplitude of the AC signal is 10 mV. The obtained spectra were fitted with Z-view software. To determine the flat-band potential of the semiconductor electrodes, MottSchottky plots were obtained by performing potentiodynamic electrochemical impedance spectroscopy. Photocurrent transient measurements were carried out by using a light-emitting diode (LED) providing a white light bias that was incident on the FTO front contact of the cells. 2.4. Characterization of Materials. TiO2 nanoparticles and nanoribbons were characterized by X-ray diffraction (XRD) on a Bruker D8 X-ray powder diffractometer. Transmission electron microscopy (TEM) characterization was taken by a JEM-2010 electron microscope operated at 200 kV. Scanning electron microscopy (SEM) images were recorded on a JEOL 5610 electron microscope. Ultravioletvisible (UVvis) absorption spectra of the TiO2 films were recorded on a Shimadzu UV 3600 spectrometer. Specific area of the TiO2 was determined by use of a nitrogen adsorption apparatus (TriStar 3000).

3. RESULTS AND DISCUSSION 3.1. General Characterizations of TiO2 Materials. The morphologies and structures of the TiO2 materials, including nanoparticles, and nanoribbons were characterized by SEM, TEM, and XRD. Figure 1a shows a TEM image of TiO2 nanoparticles prepared via hydrothermal process. As it shows, the nanocrystals exhibit a particle size of about 15 nm. Figure 1b is a TEM image of TiO2 nanoribbons prepared from hydrothermal treatment of P25 TiO2 powder; it can be seen from the image that the length of the nanoribbons ranges from hundreds of nanometers to several micrometers. Also, the nanoribbons exhibit a width distribution of 200350 nm, and thickness estimated from the image is ∼40 nm. At high magnification, the high-resolution TEM image (Figure 1c) taken from the edge of a nanoribbon indicates a well-crystallized structure with lattice spacing of about 0.35 nm, which corresponding to the (101) interplanar spacing of anatase structure (ICDD-JCPDF 21-1272). XRD patterns (Figure 1d) show that both TiO2 nanoribbons and nanoparticles are anatasestructured. The BrunauerEmmettTeller (BET) surface area is 7105

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Figure 1. Characterization of TiO2 materials. (a, b) TEM images of hydrothermal synthesized TiO2 (a) nanoparticles and (b) nanoribbons. (c) High-resolution TEM image taken at the edge of the nanoribbon. (d) XRD patterns of TiO2 nanoparticles and nanoribbons. (eg) SEM images for (e) TNP, (f) TNR, and (g) TPR10 electrodes. (h) This SEM image indicates that the nanoparticles are combined with the nanoribbons.

Table 1. Parameters of Photovoltaic Devices with Various Photoelectrodes photoelectrode

Figure 2. Photocurrentvoltage curves for DSSCs with various ratios of nanoparticle to nanoribbon in TiO2 photoelectrode.

65.3 m2/g for the TiO2 nanoparticles, while it is only 28.9 m2/g for the TiO2 nanoribbons. SEM images of the electrodes made from TiO2 nanoparticles, nanoribbons, and their composite are shown in Figure 1eh. It can be seen that the TiO2 nanostructures remain their intrinsic morphologies in the TNP and TNR electrodes (Figure 1e,f). The SEM image (Figure 1g) of the TPR10 electrode shows that the TiO2 nanoribbons are embedded into the nanoparticle network. It is conformed from Figure 1h that the nanoparticles are combined with the nanoribbons in the composite electrode. 3.2. Photovoltaic Performances. The photocurrentvoltage (IV) characteristics of the DSSCs with various concentrations of TiO2 nanoribbons in the photoelectrodes are shown in Figure 2, and the detailed photovoltaic parameters are listed in Table 1. The values of open-circuit voltage (VOC), short-circuit photocurrent density (JSC), fill factor (FF), and overall energy conversion efficiency (η) of the DSSCs varied with the concentration of TiO2 nanoribbons. A cell with TNR electrode exhibits

VOC (mV)

JSC (mA/cm2)

FF

η (%)

TNP

698

9.67

0.643

4.34

TNR

794

6.14

0.652

3.18

TPR05

730

12.25

0.627

5.61

TPR10

748

13.27

0.697

6.91

TPR20

769

11.92

0.677

6.2

the lowest value of JSC and η, due to the very low surface area, which implies that only a very small amount of dye molecules could be adsorbed and result in poor light harvesting. Photovoltaic parameters for the TNP cell are VOC = 698 mV, JSC = 9.67 mA/cm2, FF = 0.643, and η = 4.34%. By incorporation of nanoribbons into the TiO2 films, the VOC value changes dramatically from 698 to 794 mV as the concentration of nanoribons increases. Meanwhile, the JSC and FF value of the cells also increased when the nanoribbons were employed. As a result of the good photovoltaic parameters for the cell consisting of nanoribbon/nanoparticle composites, the highest η of 6.91% is obtained at an optimized condition where the nanoribbon percentage is 10%. Compared with the cell based on pure nanoparticles, an increase of ∼60% is achieved. The effects of nanoribbons on photovoltaic performance in our cells are very similar to those of 1D nanostructures, such as nanotubes, nanowires, and nanorods, introduced into the porous semiconductor films.30,32,34,35 The incident photon to collected electron conversion efficiency (IPCE) of a DSSC can be defined as IPCEðλÞ

jðλÞ ¼ ηlh ðλÞηinj ðλÞηcol ðλÞ eI0 ðλÞ

ð1Þ

where j(λ) is the photocurrent density, e is the charge of an electron, I0(λ) is the incident photon flux, ηlh(λ) is the light 7106



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Figure 5. General equivalent circuit model of a DSSC.3942 rct is the charge-transfer resistance of the charge-recombination process between electrons and I3 in the electrolyte; cμ is the chemical capacitance of TiO2 film; ZD is the Warburg element showing the Nernst diffusion of I3 in the electrolyte; RPt and CPt are the charge-transfer resistance and double-layer capacitance at the counterelectrode (platinized TCO glass); RSU and CSU are the charge-transfer resistance and the corresponding double-layer capacitance at the exposed TCO/electrolyte interface; RCO and CCO are the resistance and the capacitance at the TCO/TiO2 contact; and RS is the series resistance, including the sheet resistance of TCO glass and the contact resistance of the cell.

Figure 3. (a) IPCE curves for DSSCs with TNP and TNR electrodes. (b) UVvis absorption spectra of dye-sensitized TNP and TNR photoelectrodes.

Figure 4. Nyquist diagrams of the DSSCs with TNP, TNR, and TPR10 under forward bias of 0.6, 0.64, and 0.72 V. (Inset) Enlarged spectra in the high-frequency region.

harvesting efficiency of the sensitized TiO2 layer, ηinj(λ) is the efficiency of electron injection from the excited sensitizer to the TiO2, and ηcol(λ) is the electron collection efficiency. Clearly the IPCE curve of a DSSC contains information about light absorption, electron injection, and electron collection efficiency. Figure 3a shows the IPCE curves for the DSSCs constructed with TNP and TNR electrodes. The IPCE curves of both TNP and TNR cell exhibit maxima at 530 nm, which is very close to the position where the maximum absorbance of sensitized electrode occurred (Figure 3b). While the IPCE values are quite different, the TNP cell shows a maximum value of 58.6%, and the photovoltaic device with TNR only exhibits a value of 33.2%. Although the TNR device would probably possesses good chargetransport properties, as reported in some publications,29,31,34,38 the unexpected value is probably due to the relatively low specific

surface area of the nanoribbons, resulting in poor dye absorption and lower light harvesting (Figure 3b). 3.3. Charge Transport and Recombination. 3.3.1. General Description of Impedance Spectroscopy. In order to investigate the effect of TiO2 nanoribbons on the charge-transport properties and recombination of electron with I3 ions in the electrolyte, the cells made of pure TiO2 nanoparticles, nanoribbons, and their composite (TNP, TNR, and TPR10) were characterized by impedance spectroscopy. For cells with good carrier collection efficiencies, electron transport in the TiO2 films appears as Warburg-like diffusion behavior in the high-frequency range, and the interfacial charge-recombination process grows to a large semicircle in the low-frequency region.39 Figure 4 presents Nyquist diagrams of the cells at the potentials in which all three photoelectrodes present similar transport resistance for electrons in TiO2. In these conditions we observe that recombination resistance is much larger for the TNR cell than the other devices, indicating that the nanoribbons can effectively prevent the electrons from recombination with the oxidized-state species, like I3 in the electrolyte. The electronic processes in the DSSC are well described by a transmission line model developed by Bisquert and coworkers,3942 as shown in Figure 5. Upon forward bias, electrons are injected from the FTO substrate into the TiO2 and the film is charged by electron propagation through the TiO2 network. Meanwhile, a fraction of the injected conduction band electrons are lost by reaction with the I3 ions in the electrolyte.43 By fitting impedance data with the transmission line, we can obtain characteristic elements that describe the effect of electrode architecture on the electronic process occurring in the cell. The main elements that have been analyzed are electron-transport resistance (RT), charge-recombination resistance (RCT) on the SEI between the electron and the I3 in the electrolyte, and chemical capacitance (Cμ). RT appears to have a straight line in the high-frequency region in the Nyquist diagram (Figure 4), which can be described as series connection of the rt (in Figure 5) originating from the intergrain boundaries: RT = rt n (in which n is the number of intergrain boundaries as the electrons diffused along the electrode). Both rct and cμ in Figure 5 are connected in parallel; thus RCT and Cμ can be characterized as RCT = rct/n and Cμ = cμ n, respectively. It has been established that the conductivity of the electrons in the TiO2 network is exclusively dependent on the number of free electrons in extended state:44 σ = eμn (where e is the elementary 7107



Figure 6. Comparison for potentials of TNP, TNR, and TPR10 cells as a function of conductivity obtained from impedance spectroscopy.

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Figure 7. Charge-recombination resistance of TNP, TNR, and TPR10 electrodes in the SEI under various electronic states.

charge and μ is the mobility). And thus it provides a reference of the position of the electronic Fermi level (EF) with respect to the conduction band edge (Ecb):44,45 EF Ecb σ ¼ σ0 exp ð2aÞ kB T where σ0 is a constant, kB is the Boltzmann constant, and T is the temperature. In a multiple trapping scheme, the electron conductivity is independent of the number of traps, because σ0 relates only to the steady-state transport and reflects the rate of displacement in the transport band. In contrast, the electron diffusion coefficient of a semiconductor network is strongly dependent on the occupation of traps, as a recent publication described.19 Thus it is reasonable to take the electron conductivity as a parameter to compare the electrochemical properties of different semiconductor electrodes at the same electron density. Conductivity of the TiO2 network can be calculated from RT by use of the geometrical parameters. 3.3.2. Effect of TiO2 Nanostructures on Voltage of the Cells. In a certain cell, the redox potential (Eredox) of I/I3 in electrolyte is stationary. The bias potential of the cell is related to the difference between the electronic Fermi level in TiO2 and Eredox of I/I3. Therefore, the relationship of potentialconductivity can also provide us with information on the difference between Ecb and Eredox of I/I3: e Eredox Ecb Vþ σ ¼ σ0 exp ð2bÞ kB T e Figure 6 presents a comparison of curves for potential versus conductivity of the cells with TNP, TNR, and TPR10. The TNR device exhibits the highest potential in the measured conductivity range; meanwhile the TPR10 cell also shows relatively high potential compared with that of TNP, indicating that introduction of TiO2 nanoribbons can increase the voltage of the solar cells. The potentialσ plots in Figure 6 are in quite good agreement with the various open-circuit voltages for the cells determined above (see Figure 2 and Table 1). Since the electrolyte employed in these cells is the same, the Eredox of I/I3 is fixed, and thus the high voltage of the cells could be attributed to band-edge movement in the TiO2 nanoribbon-based electrode rather than the downshift of the Eredox. Further confirmation for band-edge upshift will be discussed later in section 3.4.

Figure 8. Chemical capacitance of TNP, TNR, and TPR10 electrodes as a function of conductivity.

3.3.3. Charge Recombination on SemiconductorElectrolyte Interface. Due to the relatively slow transport through the TiO2 electrode, recombination of electrons with I3 ions in the electrolyte cannot be ignored. The recombination process always competes with the collection of electrons. Charge recombination on the SEI can be described by the RCT value, which can be obtained from impedance results. Figure 7 presents RCT of TNP, TNR, and TPR10 cells. It is clear that the RCT value of TNR cells is much larger than that of TNP, indicating that nanoribbons in the TiO2 electrode can significantly prevent the interfacial reaction of the electron with the I3 ions in the electrolyte. Similar information obtained from electrochemical techniques such as open-circuit voltage decay (OCVD),24,29,33 and intensity-modulated photovoltage spectroscopy (IMVS)34 in literature reports and our results (OCVD and IMVS patterns; see Figures S1 and S2 in Supporting Information) also reveal that the employment of 1D nanostructures in the electrode can retard the recombination of electrons with the I3 ions in electrolyte. 3.3.4. Chemical Capacitance. Figure 8 shows the chemical capacitance of cells with various electrodes. It is clearly shown that TiO2 nanostructures in the electrodes significantly influence the value of capacitance. Across the measured range, the capacitance of TNP is larger than that of TNR or TPR10, indicating the density of trap sites below the conduction band edge is much higher. By careful investigation of the variation of the plots, it can be seen that capacitance increases only slightly as conductivity increases for the TNP cell, while for the TNR cell, exponential behaviors with larger slope can be found over the measurement range. 7108



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Figure 9. Electron lifetime in TNP, TNR, and TPR10 electrodes as a function of conductivity.

There are many factors for lowering the slope of Cμσ plot, such as proton uptake in nanocrystalline TiO2, exponential distribution of traps below the conduction band edge, and small contribution by the Helmholtz layer and the surface-adsorbed ionic sepcies (CH||Cad).4648 Uptake of proton into nanocrystalline TiO2 has been demonstrated by Hupp and co-workers,49,50 using a photoelectrochemical quartz crystal microbalance technique. They found intercalation of cations into the TiO2 lattice can result in high density of intraband site, and therefore a great increase in the capacitance. Here in our case, the slope for the TNP electrode is very small over the measurement range; this may be due mainly to the intercalation of Liþ ions into the lattice and formed intraband sites, providing very high density of electronic sites in the band gap, and consequently a relatively large capacitance. In the case of TNR, the capacitance increases significantly as the density of electrons in the extended state increases, which is very similar to the standard behavior of crystalline TiO2.39 3.3.5. Electron Lifetime in Semiconductor Electrode. According to the quasi-static treatment developed by Bisquert and Vikhrenko,51 the apparent electron lifetime, τn, is related to the conduction band electron lifetime, τ0, by the expression Dnt τn ¼ ð3Þ τ0 Dnc where nt is the trapped electron density, nc is the conduction band electron density, and τ0 is the inverse of the pseudo-firstorder rate for back-transfer of electrons from the conduction band. The relationship set up between the occupancy of these traps and the electron density in the conduction band is perturbed when variables such as the illumination intensity or the voltage are changed. Clearly, the electron lifetime is a consequence of trapping and detrapping of electrons at states located in the band gap of the oxide. Therefore, the electron lifetime, τn, is strongly influenced by the voltage of the cells. Figure 9 presents the electron lifetime calculated from the charge-recombination resistance, RCT, and chemical capacitance, Cμ: τn = RCTCμ. As it shows, TNR electrode with a very large RCT value exhibits a relatively long lifetime, as compared with thoses in TNP and TPR10 electrodes. 3.3.6. Charge Transport in Semiconductor Electrode. Electron transport in DSSC is driven mainly by diffusion, due to the effective electrolyte shielding of space charge.19 It was found that electron transport in DSSC depends strongly on light intensity because of the broad distribution of band gap traps.52 The

Figure 10. (a) Diffusion coefficient and (b) diffusion length for electrons in TNP, TNR, and TPR10 electrodes.

simplest approach to take trapping into account is the classical multiple trapping (MT) framework,53,54 which has been applied to DSSCs and other types of photoelectrochemical cells.15,5557 In this model, transport through extended states is slowed down by trapping/detrapping events, while direct hopping between localized states is neglected.51,58 The waiting time for the thermal release of electrons to an extended state strongly influences the time constant of charge transport. It is therefore easily to be elicited that the effective diffusion coefficient, Dn, of electrons is decided by density of the localized state and depth of the trap sites. The effective diffusion coefficient for a cell is given by Dnc ð4Þ D0 Dn ¼ Dnt where D0 is the diffusion coefficient of electrons at the low edge of the conduction band. Under quasi-steady-state conditions, an effective diffusion coefficient of the electron in TiO2 electrode can be obtained by using small perturbation techniques such as intensity-modulated photocurrent spectroscopy (IMPS) and impedance spectroscopy. Here we calculate Dn from the transport resistance RT and chemical capacitance Cμ: Dn = (RTCμ)1. The evaluated diffusion parameter for each electrode plotted versus conductivity is shown in Figure 10a. In the low-conductivity range, the slope of the curve for TNR electrode is low, indicating that the influence of trappingdetrapping events during the charge transport is weak and/or the trap sites are located in the superficial level, rather than in deep levels. In the case of TNP and TPR10, the diffusion coefficients grow exponentially over the conductivity ranges, and the slopes of the plots are relatively large, indicating exponential distribution of localized state in the deep level of the band gap, especially for TNP. In the high-conductivity region, in 7109



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which the density of electrons in TiO2 is high, the relatively high slope (compared with the low-conductivity range) of the plot for TNR electrode indicates that the exponential distribution of localized state in the shallow level below the conduction band plays a crucial role in charge transport. By comparison with these electrodes in the measured range, the diffusion coefficient of TNR is only slightly higher than that for the other two films, as evaluated from the impedance data. The unexpected results of chemical diffusion coefficient are probably because many of the TiO2 nanoribbon building blocks are randomly oriented, rather than perpendicular to the FTO substrate. Thus, some of the electrons in the TNR electrodes still have to cross the interribbon boundaries. Charge collection is a key factor influencing the performance of DSSC and is strongly dependent on charge transport and recombination. Competition between electron transport to the conducting substrate and loss by back-transfer to the I3 ions can be expressed in terms of electron diffusion length. Under steadystate conditions, the charge collection in a certain photoelectrode can be expressed59 D0

D2 nc nc þG ¼ 0 Dx2 τ0

ð5Þ

where G is the charge-generation rate, which has been discussed in detail elsewhere.60 Since there is no information on trapping events in the continuity equation, the electron diffusion length is given by a constant, Ln = (D0τ0)1/2. On the other hand, since the effective electron diffusion coefficient and lifetime are dependent on the quasi Fermi level and hence the trap occupancy, the factor (∂nt/∂nc) in Dn and τn can be neutralized when forming the product Dnτn:51,55,61,62 pffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffi Ln ¼ D0 τ0 ¼ Dn τn ð6Þ By use of the aforementioned parameters obtained from impedance spectroscopy, the diffusion lengths for electrons in TNP, TNR, and TPR10 electrodes at various conductivities are plotted in Figure 10b. Due to a longer electron lifetime, larger electron diffusion coefficient, and larger charge-recombination resistance, the TNR cell exhibits a relatively large effective diffusion length. Therefore, TiO2 nanoribbons can significantly promote electron-transport properties and reduce the charge lose during the transport, suggesting better charge collection efficiency. It should be noted that the electron diffusion length is only weakly dependent on the conductivity (varying by less than a factor of 2 over the measured conductivity range), which is very similar to the behavior obtained by Fisher et al.,55 who determined Ln by IMPS and IMVS. The Lnconductivity behaviors obtained in our experiment for each cell are different from the constant value.61,63 Although several similar behaviors have been reported by impedance study of DSSCs,25,45,55,64 the reason is still not clear yet. 3.4. Band-Edge Movement and Band Bending. When a semiconductor contacts an electrolyte containing a redox system, the Fermi level of the semiconductor and the redox system must be equal on both sides of the interface. Then the energy bands are bent upward or downward by an energy of eΔφSC depending on the density of doping.60,65 For the n-type semiconductor of TiO2, when it contacts the electrolyte, electrons will deplete from the semiconductor into the electrolyte, and a space charge layer on the semiconductor side will be formed, reflected in an upward bending of the band edge.66 However, in DSSCs consisting of

Figure 11. (a) MottSchottky curves of TNP and TNR electrodes. (b) Schematic diagram for band bending.

nanocrystalline semiconductors, the individual particle size is too small to support a space charge, because the particle size is smaller than the Debye length. When the semiconductor contacts the electrolyte, all of the nanoparticles are depleted of electrons.17,67 In the case of 1D nanostructures, such as nanorods and nanowires immersed into solution, the outer surface of the material is depleted of carriers, forming surface band-bending in the radial direction, while retaining conduction in the tubular region in the central region of the rods.19,68 Thus the charge carriers in the central region are separated by the formed depletion layer and can effectively avoid interaction with the solution at the interface of the rod and wire.68 Here in our study, the nanoribbons would exhibit similar photoelectrochemical properties as nanorods and nanowires. Information on band-edge displacement can be sought through differential capacitance measurements on the SEI. In the simplest case, as more fully discussed elsewhere,60,66,69 one obtains the MottSchottky (MS) relationship of the semiconductor space charge layer capacitance (CSC) by invoking the Poisson equation: 1 2 kT ¼ j j ð7Þ fb CSC 2 εε0 e0 ND e0 where jfb is the so-called flat-band potential, that is, the applied potential (j) at which the semiconductor energy band are “flat”, which facilitates location of the energetic position of the conduction band edge of a given semiconductor material. ND is the doping density, k is the Boltzmann constant, T is the temperature, ε is the relative dielectric constant of the semiconductor, and ε0 is the permittivity of free space. In order to confirm the difference of the band edge and obtain the flat-band potentials for each electrode material, potentiodynamic electrochemical impedance spectroscopy with a frequency of 1 kHz was performed. Figure 11 presents MS plots of the TNP and TNR electrodes. The slope is inversely proportional to the effective donor concentration in the space charge layer of semiconductor, and the flat-band potential can be determined by extrapolation to CSC = 0. The flat-band potentials of both TiO2 electrodes are 7110


The Journal of Physical Chemistry C obtained from the plots. The calculated jfb values for nanoparticle and nanoribbon electrodes are 0.71 and 1.0 V, respectively. The donor concentration of the TNP and TNR electrodes was calculated to be 1.90 1019 and 3.85 1019 cm3, respectively, via a linear fitting method. Many factors, such as composition of the electrolyte, density distribution of surface state, light soaking, intercalation/deintercalation of cations into the semiconductor surface, surface coating, and the preparing and manufacturing process of the electrode, can result in band-edge movement.8,7073 Many publications indicate that LixTiO2 could be formed at the surface of the electrode by intercalation of cations like Liþ into the lattice of the semiconductor, resulting in a positive shift of the bandedge position and lower open-circuit voltage.46,47,71 Similar results were also found in a lithium-free aqueous photoelectrochemical system.7476 Associated with the capacitance conductivity behavior shown in Figure 8, the difference of bandedge position between the two kinds of TiO2 would probably originate from the different extent of proton intercalation.77 The space charge layer between the bulk semiconductor and liquid electrolyte plays an important role in photogenerated charge separation. The local electrostatic field present in the space charge layer aids in separation of light-induced electronhole pairs and suppresses the electrons from recombination with the hole carriers.11 For large particles, the potential drop across the space charge layer is equivalent to that of planar electrodes: kB T W 2 ð8aÞ ΔφSC ¼ 2e LD where W is the width of the space charge layer and LD is the Debye length, given by LD = (ε0εkBT/2e2ND)1/2. The key point, however, is that nanosized semiconductors, like TiO2 nanoparticles in our work, cannot support a large electric field or sustain a space charge layer, since the particle size is rather smaller than the Debye length, and the total band bending within the semiconductor, eΔφSC, is limited by radius r: kB T r 2 ð8bÞ ΔφSC ¼ 6e LD As calculated from eqs 7, 8a, and 8b with the ND value and the equivalent diameters of 15 and 60 nm, the eΔφsc values for nanoparticles and nanoribbons are 0.002 and 0.056 eV, respectively. The large electrostatic field at the surface of the nanoribbons can promote separation of the electronhole pairs and therefore enhance the collection efficiency of the photogenerated electrons.68 Moreover, the formed space charge layer in the surface of the TiO2 nanoribbons can prevent the electrons in the conduction band from recombining with the hole carriers (I3 ions), resulting in a relatively large RCT and electron lifetime, which is consistent with the impedance results (Figure 7 and 9). 3.5. Surface Effects. It is well-known that the surface state of the semiconductor electrode significantly influences the performance of the photoelectrochemical system.11 Many publications revealed that electron transfer from the solid to the redox electrolyte takes place predominantly via surface state.14,70 The surface state sites below the conduction band act as intermediates for recombination of electrons with the acceptor in the electrolyte. This may lead to a lowering of the photocurrent as well as a decrease of voltage.16 Therefore, it is important to investigate

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Figure 12. Photocurrent transient plots for DSSCs consisting of (a) TNP and (b) TNR electrodes. The photocurrent was normalized to unity.

the surface state sites induced recombination on the SEI of the DSSCs. Here we use transient photocurrent response to observe the surface state induced recombination. Figure 12 presents the photocurrent response curves recorded in the short circuit condition of the cells. Upon illumination, for the TNP cell, the photocurrent reached its maximum value and subsequently decayed due to the trapping of photogenerated charge carriers. Then the photocurrent approaches a steady-state value that depends on the relative rates of recombination and charge transfer to redox species.79 When the light is interrupted, the carriers trapped on surface states still continue to recombine, and thus the current changes sign since it is now only due to the photogenerated charge carriers flowing to surface states.79,80 In contrast to the TNP cell, the TNR cell exhibits a different behavior. During the period of illumination, the photocurrent remains its initial value. When the light is suddenly interrupted, the photocurrent decreasg rapidly, while the “overshoot” here was absent. The different behaviors of the two cells suggest the TNR cell possesses a relatively low density of surface states.79,80 Further confirmation of the quality of the semiconductor was carried out by evaluating the transfer constant for charge recombination. Transfer constant for interfacial charge recombination corresponds to the reciprocal value of the diode quality factor. By plotting RCT (Figure 7) versus potential (see Figure S3 in Supporting Information), we obtain the transfer constant following the exponential law expression: eβ V ð9Þ RCT ¼ RCT, 0 exp kB T where β is the transfer constant. The direct transfer of electron from the conduction band edge to the hole carriers in the electrolyte should give β = 1. Typically the values found experimentally are lower and in the range between 0.5 and 1, due to electron trapping by the surface state prior to interfacial charge-transfer. The transfer constant evaluated from eq 9 is 0.55 and 0.66 for the TNP and TNR electrodes, respectively, corresponding to diode quality factors of 1.8 and 1.5. The high quality of the semiconductor can effectively alleviate the interfacial charge recombination between the electrons and the I3 ions. 7111


The Journal of Physical Chemistry C As discussed above, the improvement in the overall energy conversion efficiency in the cells based on TiO2 nanoribbon/ nanoparticle composite electrodes is due to the following reasons. The increase in VOC is caused by band-edge movement of the nanoribbons, resulting in a larger difference between Eredox and electronic Fermi level. The formed space charge layer and the low density of the surface state can retard and reduce charge recombination on the SEI. Introduction of nanoribbons into the mesoporous electrode provides a long uninterrupted pathway for electron diffusion toward the conducting substrate and thus increases the charge collection efficiency. Although the surface area of TiO2 nanoribbons is low, the introduction of this material into composite electrode would not severely decrease the light absorption when the ratio of nanoribbons to nanoparticles is not high.

4. CONCLUSIONS TiO2 nanoparticles and nanoribbons exhibit different semiconductor electrochemical behaviors when they are employed in dye-sensitized solar cells. In this photoelectrochemical system, charge separation, transport, and recombination strongly depend on the nanostructures in the photoelectrode. For the cell containing TiO2 nanoribbons, formation of a space charge layer at the surface of the electrode effectively promotes the separation of photogenerated charge carriers and prevents recombination of the electron with the hole carriers. The uninterrupted long pathway with less boundaries is beneficial for transport of electron toward the conducting substrate. Simultaneously, the decrease of trap sites in the band gap and the surface state can effective enhance the charge-diffusion coefficient and suppression of the interfacial charge transfer. Due to the good charge-transport properties and separation, large resistance for the back-charge transfer, and less intraband site, cells consisting of the nanoribbons exhibit higher voltage, longer electron lifetime, and larger electron diffusion length. With an optimized ratio of nanoribbon/nanoparticle in the photoelectrode, an increment of energy conversion of ∼60% was achieved, as compared with that consisting of pure TiO2 nanoparticles. ’ ASSOCIATED CONTENT

bS

Supporting Information. Three figures showing intensity-modulated photovoltage spectroscopy (IMVS) patterns, open-circuit voltage decay (OCVD) patterns, and charge-transfer resistance versus potential. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT J.C. thanks Dr. Yinghua Xu (Zhejiang University of Technology) and Dr. Tiancheng Xu (HzCell Electrochem. Corp.) for the fruitful discussion on impedance results. ’ REFERENCES (1) Oregan, B.; Gr€atzel, M. Nature 1991, 353, 737–740.

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Role of One-Dimensional Ribbonlike Nanostructures in Dye

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