Effect of Annealing Temperature on the ... - ACS Publications

May 7, 2008 - recombination lifetime decreased with increasing the particle sizes.6 ..... the chance of entering the external circuit and contributing...
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8486

J. Phys. Chem. C 2008, 112, 8486–8494

Effect of Annealing Temperature on the Photoelectrochemical Properties of Dye-Sensitized Solar Cells Made with Mesoporous TiO2 Nanoparticles De Zhao,† Tianyou Peng,*,†,‡ Lanlan Lu,† Ping Cai,† Ping Jiang,† and Zuqiang Bian‡ College of Chemistry and Molecular Science, Wuhan UniVersity, Wuhan 430072, P. R. China, and State Key Laboratory of Rare Earth Materials Chemistry and Applications, Peking UniVersity, Beijing 100871, China ReceiVed: NoVember 8, 2007; ReVised Manuscript ReceiVed: March 12, 2008

Mesoporous TiO2 (m-TiO2) nanoparticles were used to prepare the porous film electrodes for the dye-sensitized solar cells (DSSCs). The obtained m-TiO2 porous electrodes were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), electrochemical impedance spectra (EIS), and open-circuit photovoltage decay curves (OCVD). Experimental results indicate that the effect of bulk traps and the surface states within the m-TiO2 porous films on the recombination processes of the photoinjected electrons in DSSCs depends on the annealing temperature. Moreover, the homemade m-TiO2 nanoparticles show much higher photoelectric conversion efficiency than the nonporous TiO2 nanoparticles (P25). The mesostructures within the m-TiO2 nanoparticles, which can be maintained even after annealing at 500 °C, are of great benefit to the dye adsorption, and then to the improvement of the photoelectrochemical properties of DSSCs. 1. Introduction In the past few decades, dye-sensitized solar cells (DSSCs) have been studied extensively because of their low costs and facile fabrication procedures in comparison with solid photovoltaic devices.1–4 Generally, DSSCs are mainly composed of three parts: the dye-sensitized semiconductor nanocrystalline film photoanode, the redox couple (usually I3-/I-) in organic solvent(s), and the platinized transparent conducting oxide (TCO) glass as the counter electrode.1–3 Among those, the photoanode usually consists of porous TiO2 nanocrystalline film adsorbed by dye molecules (such as a ruthenium complex), which can absorb the light energy especially the visible light of sunshine.1–5 The dye molecules move to the excited states when the DSSCs is exposed to light irradiation with suitable energy, and the excited-state electrons are injected quickly into the conduction band of TiO2; these injected electrons are collected through the conducting substrate of the TiO2 photoanode. The dye molecules can be regenerated by the iodide ions in the electrolyte, and then the resulting triiodide ions can accept electrons from the platinized TCO counter electrode to fulfill a complete current cycle in DSSCs.5 TiO2 is one of the most common materials that can be employed to make the porous film electrode of DSSCs. The morphology and particle size of TiO2 play critical roles in the photoelectric conversion efficiency of DSSCs.1,6–13 For example, TiO2 materials with different morphologies, such as nanoparticles,1,8 nanotubes,9–11 nanowires,12 and nanofibers,13 have been applied to fabricate the porous film electrodes. Nakade and his co-workers have reported that the diffusion coefficients of electrons in the TiO2 nanocrystalline film increased while the recombination lifetime decreased with increasing the particle sizes.6 Furthermore, some mesoporous TiO2 materials have also been applied to fabricate the porous electrode for DSSCs.14–17 * To whom correspondence should be addressed. Phone: 86-27-87218474; fax: 86-27-6875-4067; e-mail: [email protected]. † Wuhan University. ‡ Peking University.

The total photoelectric conversion efficiency has already enhanced to 11.1% from its original 7.9%.1,16 This improvement in efficiency can be attributed to the larger surface area of the porous film, more efficient light absorption of the dye molecules, and the electron transport and/or transfer between the interfaces of film material/electrolyte/counter electrode within DSSCs. Recently, TiO2 nanoparticles with intraparticle mesostructures (m-TiO2) have been fabricated and shown much better photoactivity than that of the commercial photocatalyst (P25, a nonporous TiO2 nanopowder) because of its high specific surface area and well-crystallized mesoporous wall within the nanoparticles in our previous publications.18,19 These m-TiO2 nanoparticles may also be used to prepare the photoelectrode because of their large surface area, which allows more efficient adsorption of dye molecules. However, there are no studies focusing on DSSCs fabricated by TiO2 nanoparticles with mesostructures to the best of our knowledge. Herein, these m-TiO2 nanoparticles were applied to prepare the porous film electrode for DSSCs. The electrical impedance spectroscopy (EIS) and the open-circuit voltage decay curve (OCVD) measurements were combined to examine the kinetic processes in DSSCs because these techniques have been developed as powerful tools to characterize the electrical and ionic processes involved in the operation of DSSCs.20–28 The effects of annealing temperature for the porous electrodes on the photoelectrochemical property of DSSCs were explored. The photoelectric conversion efficiencies of DSSCs fabricated from the homemade m-TiO2 nanoparticles and nonporous P25 were comparatively investigated. 2. Experimental Section 2.1. Materials Preparation. m-TiO2 Nanoparticle Preparation. The m-TiO2 nanoparticles were prepared via a CTAB surfactant templating process as described in our previous publication.18 The as-prepared samples were dried at 80 °C overnight and then calcined at 300 °C with a heating rate of 2 °C min-1 for 7 h. The microstructures and physical properties of the obtained m-TiO2 nanoparticles are listed in our previous publications.18,19

10.1021/jp800127x CCC: $40.75  2008 American Chemical Society Published on Web 05/07/2008

Effect of Annealing Temperature on Solar Cells Photoelectrode Preparation. The doctor-blade technique was adopted to prepare the porous film electrode on a conducting glass (FTO, 15-20 Ω/0), which has been rinsed with distilled water and immersed in isopropanol for 2 h to increase its hydrophilicity. A 1.0 g m-TiO2, 4.3 mL dry alcohol, 1.4 mL HCl solution (0.1 M), and 0.3 mL acetylacetone were mixed in a PTFE container and then dispersed through ball-milling. The resulting paste was dropped onto the conducting glass with adhesive tape that served as spacers, in which four pieces of FTO glasses were placed in parallel and the paste was spread at one time to obtain films with the same thickness. The films were dried under ambient conditions and then annealed at 350, 450, 500 and 600 °C for 1 h, respectively. The dye-sensitized electrode was prepared by immersing the obtained films in an ethanol solution of cis-bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II)-bistetra butylammonium (N719, 0.3 mM) overnight. For comparison, a porous electrode was also fabricated from a commercial photocatalyst (P25, Degussa, Germany), a nonporous TiO2 nanopowder. DSSCs Fabrication. The dye-sensitized electrode was assembled in a typical sandwich-type cell; namely, the identical Pt counter electrode was placed over the dye-sensitized photoanode, and then the photoelectrochemical property of DSSC was measured. The electrolyte solution, which consists of 0.5 M LiI, 0.05 M I2, and 0.1 M 4-tert-butylpyridine in 1:1 acetonitrile-propylene carbonate, was injected into the interspace between the photoanode and the counter electrode. 2.2. Characterization. Structure phase analyses with the X-ray diffraction (XRD) method were performed on a D8advance X-ray diffractometer (Bruker) with Cu KR radiation (λ ) 0.15418 nm). Transmission electron microscopy (TEM) studies were carried out on a LaB6 JEM-2010 (HT)-FEF electron microscope. For the film electrodes, TiO2 was scratched from the FTO substrate to carry out the TEM observations. A thin layer of platinum (∼2 nm) was sputtered on the surface of the TiO2 film for the scanning electron microscopy (SEM) observation on a JEOL-6700F electron microscope. The thicknesses of the films were measured with a TalyForm S4C-3D profilometer (U.K.). To estimate the adsorbed amount of dye on the TiO2 films, the sensitized photoelectrode was separately immersed into a 0.1 M NaOH solution in a mixed solvent (water/ ethanol ) 1:1), which resulted in the desorption of N719 from the porous electrode. The absorbance of the resulting solution was measured by a UV-240 UV-vis spectrophotometer. The adsorbed amount of dye was determined by the molar extinction coefficient of 1.41 × 104 dm3 mol-1 cm-1 at 515 nm as reported previously.29 2.3. Photoelectrochemical Measurements. Electrochemical Impedance Spectra and Open Circuit PhotoWoltage Decay CurWes. The electrochemical impedance spectra (EIS) measurements were carried out by applying bias of the open circuit voltage (Voc) and recorded over a frequency range of 0.005 to 100 kHz with ac amplitude of 10 mV. The cell was first illuminated to a steady voltage, and then the open circuit photovoltage decay curve (OCVD) was recorded once the illumination was turned off by a shutter. The above two measurements were carried out on a CHI-604C electrochemical analyzer (CH Instruments) combined with Xe lamp as the light source. DSSCs Properties Test. The DSSC was illuminated by light with energy of a 42 mW cm-2 from a 500 W Xe lamp. A computer-controlled Keithley 2400 source meter was employed to collect the I-V curves. The active area of DSSC was 5 × 5

J. Phys. Chem. C, Vol. 112, No. 22, 2008 8487

Figure 1. XRD patterns of the m-TiO2 nanoparticles (inset) and the film electrodes annealed at different temperatures.

TABLE 1: Properties of m-TiO2 Film Photoelectrodes Annealed at Different Temperatures electrodes and annealing temperature/°C m-TiO2-350 °C m-TiO2-450 °C m-TiO2-500 °C m-TiO2-600 °C P25-500 °C

mean particle size/nm

film thickness/ µm

adsorbed amount of dye/×10-8 mol cm-2

68.9

4.38 4.35 4.30 4.20 4.25

6.07 5.60 4.77 3.48 1.36

74.1

mm2. The photoelectric conversion efficiency was calculated according to eq 1:

η(%) )

VocIscFF × 100 Pin

(1)

For the photocurrent action spectra, a WDG-100 monochromator was used to obtain the monochromatic light from a 500 W Xe lamp. The active area was 4 × 4 mm2. The incident monochromatic photoelectric conversion efficiency (IPCE) was defined as (eq 2)

IPCE(%) )

12400 × Isc(µA cm-2) λ(nm) × Pin(µWcm-2)

(2)

In the above two formulas, η is the global efficiency, Voc, Isc, and FF are open circuit voltage, short circuit current density, and fill factor, respectively. Pin and λ are the light energy and wavelength of the incident monochromatic light, respectively. 3. Results and Discussion 3.1. X-ray Diffraction Analyses. Figure 1 depicts the XRD patterns of the film electrodes made with m-TiO2 nanoparticles after annealing at different temperatures, and the inset in Figure 1 shows that the crystal phase of m-TiO2 nanoparticles calcined at 300 °C is anatase (JCPDS, no. 21-1272). As can be seen, the FTO layer has an obvious influence on the diffraction pattern of anatase. One of the diffraction peaks of FTO overlaps with the (004) diffraction peak of anatase, and the strongest diffraction peak of FTO layer appears as a shoulder peak next to the (101) peak of anatase. This phenomenon may be attributed to the smaller thickness (∼4.3 µm) of the m-TiO2 film as shown in Table 1.

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Figure 2. TEM images of the m-TiO2 nanoparticles. (a and b) paste and (c) TiO2 film annealed at 500 °C.

Figure 3. FESEM images of m-TiO2 porous films annealed at different temperatures. (a and b) 350 °C; (c and d) 500 °C.

The (101) peak of anatase becomes sharper and stronger with enhancing the annealing temperature from 350 to 600 °C. It is reasonable to speculate that the mesostructures of m-TiO2 nanoparticles reconstructed along with the increase of crystallite size and crystallinity. Namely, the internal surface area of the m-TiO2 film decreased with enhancing the annealing temperature, which is consistent with our previous observation.18 This decrease in the internal surface area of m-TiO2 film can also be confirmed by the decreasing absorbed amount of N719 as shown in Table 1. The improvement in the crystallinity, decreases in the internal surface area, and the adsorbed amount of N719 will certainly affect the properties of DSSCs, which will be discussed further in the following section. 3.2. Microstructure Analyses. Figure 2 shows the TEM images of the paste containing the m-TiO2 nanoparticles after ball-milling and the m-TiO2 film after annealing at 500 °C. The m-TiO2 nanoparticles in the paste are in the range of 4.2 ∼ 17.5 nm with an average particle size of 9.1 nm. The mesoporous characteristics within nanoparticles can be observed clearly from Figure 2a, indicating that the m-TiO2 nanoparticles has comparatively good stability during the ball-milling process. Figure 2b shows the HRTEM image of m-TiO2 nanoparticles in the paste. The crystallized domains with mean size of 3.5 nm were surrounded by an amorphous structure, which is similar to that of m-TiO2 nanoparticles reported before.18 The average particle size of m-TiO2 nanoparticles in film after annealing at

500 °C increases to 10.4 nm with particle size distributions in the range of 6.2 ∼ 32.4 nm as shown in Figure 2c. The mesoporous characteristics within nanoparticles can still be observed clearly. The amorphous structures are completely crystallized after annealing at 500 °C. Two possible mechanisms are worthy of consideration during the crystallization of the surface amorphous structures. The first one is that the surface amorphous structures crystallized in situ and the small nanopaticles are merged and/or connected into larger ones. The second possible mechanism is that the surface amorphous structure of some particles diffused quickly to the surface of the other particles and then crystallized, which resulted in the increased particle size distribution although the merging and/ or connection of little particles were also taking place at the same time. The above two possible mechanisms may be the reasons that the average particle size of m-TiO2 in the porous film (Figure 2c) is larger than that in the paste (Figure 2a). Figure 3 shows the FESEM images of m-TiO2 films. The surface of the m-TiO2 film annealed at 350 °C is smooth, and the mean particle size is 68.9 nm with a particle size distribution in the range of 25.2 ∼ 151.4 nm (Figure 3b). This larger particle size in the porous film may be ascribed to the physical accumulation of the small nanoparticles as observation from TEM images, and large numbers of TiO2 nanoparticles are relatively discrete instead of sintering together because of the insufficient annealing temperature. After annealing at 500 °C,

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J. Phys. Chem. C, Vol. 112, No. 22, 2008 8489 TABLE 2: Kinetic Parameters of the Recombination Reactions within the m-TiO2 Porous Electrodes Annealed at Different Temperatures

Figure 4. EIS spectra for the DSSCs made with m-TiO2 electrodes annealed at different temperatures. Solid curves represent the fitted results. A: Bode phase plots; B: Nyquist plots.

the film surfaces become rough with the appearance of some pores (Figure 3c). The mean particle size increases to 74.1 nm with a particle size distribution in the range of 32.7 ∼ 178.3 nm as observed from Figure 3d. Many small nanoparticles are sintered together as illustrated by the enlarged area shown in Figure 3d (inset). The increase of mean particle size can be attributed to the crystallization of the surface amorphous structure and the merging and/or connection of small particles as discussed above. This merging and/or connection of those small nanoparticles at an elevated annealing temperature (i.e., 500 °C) are necessary for the electron transport within the m-TiO2 film. 3.3. Electrochemical Impedance Spectra Analyses. Generally, three characteristic frequency peaks can be obtained from EIS spectra (Bode phase plot) when DSSCs were controlled at the open-circuit conditions under light illumination.20 The lowfrequency peak (in the mHz range) is attributed mainly to the Nernst diffusion of I3- within the electrolyte. The middlefrequency peak (in the 10-100 Hz region) is related to the transport process of the injected electrons within TiO2 porous films and the charge transfer process of the injected electrons at the interfaces between TiO2 and the electrolyte/dye coating. The high-frequency peak (in the kHz range) is ascribed to the charge transfer process at the interfaces between the redox couple and the platinized counter electrode. Figure 4A shows the Bode phase plots of EIS spectra for the DSSCs made with m-TiO2 electrodes annealed at different temperatures. As can be seen, two main frequency peaks are observed for the charge transfer process at different interfaces. One frequency peak at the high-frequency region can be ascribed to the charge transfer at the interfaces of the electrolyte/Pt counter electrode, and the

electrodes

350 °C

450 °C

500 °C

600 °C

500 °C (P25)

Rct/Ω Rw/Ω τn/ms keff/s-1

67.37 17.78 2.282 438.25

25.89 21.35 91.57 10.92

20.91 20.12 51.51 19.42

27.90 18.61 42.53 23.51

30.65 13.19 5.05 197.92

other at the low-frequency region to the accumulation/transport of the injected electrons within TiO2 porous film and the charge transfer at the interfaces of the electrolyte/TiO2, respectively. The frequency peak related to the diffusion of I3- is not observed from EIS spectra because of the low viscosity of the present electrolyte. The low-frequency semicircles in Figure 4B can be fitted to a charge-transfer resistance (Rct), which is shunt-wound with a constant phase element (CPE) instead of the chemical capacitance (Cµ) as stated in the previous report,28 and both of them are connected in series with a transport resistance (Rw) of the injected electrons within the m-TiO2 film. According to the EIS model developed by R. Kern,20 the lifetime (τn) of injected electrons in m-TiO2 films can be drawn by the positions of the low-frequency peak in Figure 4A through the expression τn ) 1/ (πf), where f is the frequency of the superimposed ac voltage. 2 Furthermore, the effective rate constant (keff) for the recombination reaction can be also obtained according to the method proposed by Adachi.24 The fitted Rct, Rw, τn, and keff values for the m-TiO2 electrodes annealed at different temperatures are shown in Table 2. As can be seen, the electrode annealed at 500 °C shows the lowest Rct value, and the lowest Rw and τn values are obtained from the electrode annealed at 350 °C. Moreover, there is an increasing trend for keff values and a decreasing trend for Rw and τn values upon enhancing the annealing temperature except that the electrode annealed at 350 °C, which gives an exceptionally large keff value. Those experimental results can be attributed to the effect of the annealing temperature on the bulk traps (the localized electronic states within the band gap of TiO2) and the surface states (the electron states physically located either at the surface of nanoparticles or within its tunneling distance) in the m-TiO2 films, which further influence the recombination processes of the injected electrons in DSSCs. The detailed dependency of Rct, Rw, τn, and keff on the annealing temperature will be discussed separately in the following sections. 3.4. Overview of the Charge Transport/Transfer Processes in m-TiO2 Films. On the basis of the model proposed by Kern et al.,20 and the conceptual distinction between the bulk traps and the surface states,27 a modified model concerning the transfer and recombination of the injected electrons in m-TiO2 electrode is proposed in Figure 5. The electron transfer of bulk traps only occur between the bulk traps and the conduction band of TiO2,27 whereas the surface states can transfer electrons to the acceptor species in solution in addition to the electron exchanges with the conduction band of TiO2. In the present condition, the bulk traps can be regarded as the grain boundary and the amorphous domains between the crystallites within the m-TiO2 film; whereas the surface states are originate mainly from the surface amorphous structures or surface atoms on the m-TiO2 nanoparticles. Under light illumination and open circuit conditions, the charge recombination of DSSCs can take place not only between the oxidized species (such as I3- or dye cations) and the electrons in the conduction band but also between the oxidized species and the surface states of TiO2 at the interfaces of electrolyte/TiO2 or dye/TiO2. The recombina-

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Figure 5. Transport and recombination reactions of the photoinjected electrons in the TiO2 electrode of DSSCs during the EIS measurement.

tion process of electrons in the conduction band of TiO2 with the oxidized species is faster than that in the surface states.20 Therefore, it can be concluded that the Rct, which is contributed from the recombination process of the conduction band and/or surface state electrons with the oxidized species, should be small if the charge transfer processes is dominated by the recombination reaction between the conduction band electrons of TiO2 and the oxidized species; and the Rct should increase if the charge transfer process is related mainly to the surface state electrons. Namely, the Rct value can indicate whether the electrons in the conduction band or the surface state of TiO2 dominate the recombination reaction with the oxidized species to some extent. There are several surface states existing in the m-TiO2 electrode annealed at a lower temperature (i.e., 350 °C), and the electrons are trapped mainly by the surface states during the EIS measurement. These trapped electrons are difficult to transfer to the oxidized species in comparison with the electrons in the conduction band of TiO2. This is the reason that the m-TiO2 electrode annealed at 350 °C shows a much larger Rct as shown in Table 2. The crystallinity of the m-TiO2 film is improved along with a decease in the internal surface area within the m-TiO2 electrodes upon enhancing the annealing temperature, which can also be proven by the decreasing adsorbed level of N719 (Table 1). These changes result in a decrease in the density of the surface states, and then the charge transfer would be dominated mainly by the recombination process of the conduction band electrons of m-TiO2. Therefore, the Rct values become small in the case of 450 and 500 °C as shown in Table 2. After annealing at 600 °C, the internal surface area of the electrode decreased drastically and the porous structure of the electrode may collapse to some extent. Although those factors can lead to a decrease in the grain boundary, new surface states (contributed mainly from the surface atoms) appear on the surface of m-TiO2 nanoparticles and the density of surface states becomes larger again than that in the m-TiO2 electrodes annealed at 450 and 500 °C, which results in the increase of Rct again as illustrated in Table 2. The bulk trapping and/or detrapping of electrons only occur between the bulk traps in the film and the conduction band of TiO2 as described above.27 The injected electrons can be trapped rapidly and released slowly by the grain boundary, which serves as bulk traps in the porous electrode. Therefore, the transport of injected electrons within the TiO2 films becomes difficult. As shown in Table 2, the transport resistance (Rw) of injected electrons decreases along with enhancing the annealing temperature except for 350 °C. This decrease can be attributed to the gradual reduction of the grain boundary within the m-TiO2

Zhao et al. electrodes17 because the small nanoparticles must come into connection with each other and the grain boundary reduce gradually due to the formation of larger nanoparticles with further enhancing of the annealing temperature. On the contrary, it is thought that 350 °C is too low to sinter the nanoparticles together sufficiently. These discrete nanoparticles can be enwrapped by the electrolyte, namely, the lowest Rw for the electrode annealed at 350 °C is related mainly to the charge transport in the crystallized domain within the relatively isolated m-TiO2 nanoparticles with small particle sizes. The quick trapping and slow releasing of the electrons by the bulk traps also delay the sacrifice of the injected electrons. Namely, if the bulk trap density is high, then the lifetime (τn) of the electrons should be long. When the annealing temperature varied from 450 to 600 °C, the grain boundary (i.e., bulk traps) decreased gradually. Therefore, the τn decreases from 91.57 to 42.53 ms as illustrated in Table 2. In the case of annealing at 350 °C, the electrode possesses a large internal surface area and a high surface state density although there are many bulk traps within the m-TiO2 film. The injected electrons can mostly diffuse to the surfaces of m-TiO2 nanoparticles with a rapid rate because of the very small particle size and the lowest Rw in comparison with the other three electrodes. At the same time, the electrons are trapped by the surface states first and then transferred to the electrolyte. Its larger contact area between the m-TiO2 film and electrolyte admits a quick transfer of many electrons into the electrolyte and is responsible for the shortest electron lifetime. The sharp tendency of the losing electron for the electrode annealed at 350 °C can be also illustrated by the much larger effective rate constant (438.25 s-1) of the recombination reaction. Generally, the photoelectrons that possess long lifetimes have the chance of entering the external circuit and contributing to the photoelectric conversion efficiency. As can be seen from Table 2, the injected electrons have the longest lifetime (91.57 ms) for the electrode annealed at 450 °C. However, this long lifetime is originated mainly from the existence of lots of bulk traps within the m-TiO2 film instead of the suppressed surface recombination as reported.28 These bulk traps will not be beneficial for the electron transports within the m-TiO2 film. As for the electrode annealed at 600 °C, the high temperature is propitious to the elimination of bulk traps, but the surface states reappear and are contributed mainly by the surface atoms due to partial collapse of the mesostructures within the m-TiO2 film, which can also result in reduction of the adsorption of N719. Therefore, it is important to buck for the balance between the mesoporous characteristics of m-TiO2 films and the electrons lifetimes, which is related mainly to the transport and recombination processes of electrons. 3.5. Overview of the Charge Recombination Processes in m-TiO2 Films. In the model shown in Figure 5, the effects of bulk traps and surface states on the transport and/or recombination of electrons are considered separately in comparison with the proposal by Kern et al.20 The total effective rate constant (keff) for the charge recombination in the electrode, which is similar to the definition in ref 24, can be written as follows st keff ) Rkcb eff + βkeff

kcb eff cb keff

st keff

)

st st 2nckcb r , keff ) 2nskr

(3) (4)

where and represent the effective rate constant for the recombination reaction of conduction band and surface state electrons, respectively. krcb and krst represent the rate constant for the recombination reactions from the conduction band states and the surface states, respectively.

Effect of Annealing Temperature on Solar Cells

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nc and ns represent the steady-state electron density in the conduction band and the surface states, respectively. R and β cb st are the coefficients of the keff and keff , respectively. As reported before,27 the energy level of surface states overlaps with the main distribution of the fluctuating energy levels in solution. It is thought to be reasonable that both krcb and krst are independent of the annealing temperature by additionally neglecting the effect of annealing temperature on the bottom energy level of the TiO2 conduction band. Namely, cb st keff and keff for different electrodes are only determined by nc and ns, respectively. In the case of annealing at 350 °C, many of surface states exist within the electrode and the recombination originates mainly from this surface state because of its lower sintering. In other words, the surface state electron density (ns) of this electrode exceeds significantly that of the other electrodes annealed at 450, 500, and 600 °C. Therefore, the electrode annealed at 350 °C shows the largest keff value. Along with enhancing the annealing temperature from 450 to 600 °C, the surface state density decreased drastically due to the gradual crystallization of m-TiO2 films, which leads to reduction of the recombination and keff value. Moreover, the well-crystallized surfaces of the m-TiO2 film are thought to be beneficial for the injection of the excited-state electrons from the dye to the conduction band of TiO2, and therefore the nc increased with enhancing the temperatures. This, however, leads to the increase cb of keff and keff. On the basis of forementioned discussions, it is reasonable to think that the effect of the bulk traps decreases along with enhancing the annealing temperature and that of surface states decrease gradually from 350 to 500 °C and then reincrease at 600 °C according to the variations of Rw, Rct, τn, and keff. Therefore, an optimal annealing temperature for the m-TiO2 porous film electrodes is 500 °C, at which the mesostructures of m-TiO2 nanoparticles can be maintained and there is suitable low recombination probability. 3.6. Open-Circuit Photovoltage Decay Curves Measurement. The OCVD curves can show the main information of the recombination process between the injected electrons in TiO2 and the electrolyte under the dark state,26 whereas the recombination process between the injected electrons and the oxidized dye molecules cannot be measured by the OCVD.27 The lifetime (τn′, response time of recombination reaction) of the injected electrons in TiO2 is given by the reciprocal of the derivative of the decay curve normalized by the thermal voltage as shown in eq 5.26

τn ′ )

( )

kBT dVoc e dt

-1

(5)

According to Bisquert’s viewpoint,27 the τn′ ∼ Voc curves can be marked off the following three voltage-dependent regions: (1) a constant lifetime at the high photovoltage (Voc) region dominated mainly by the electron transfer process from the conduction band of TiO2 to the electrolyte; (2) an exponential increase region due to the internal trapping and/or detrapping; and (3) an inverted parabola at the low Voc region coming from the effect of surface states. Figure 6 shows the obtained τn′ ∼ Voc curves according to eq 5 through the OCVD measurement. Among the tested electrodes, the shape of τn′ ∼ Voc curve for the electrode annealed at 500 °C is in general consistent with the previous conclusion;27 its τn′ value is constant at the high Voc region. However, the τn′ values show a decrease tendency first at the high Voc region with the decay of Voc for the other electrodes. This decrease in τn′ values at the high Voc region is attributed mainly to the effect of surface-state electrons

Figure 6. τn′ ∼ Voc curves of the porous electrodes annealed at different temperatures. Solid curves represent the fitted results. (a) 350 °C, (b) 450 °C, (c) 500 °C, (d) 600 °C.

but not to the bulk traps although the bulk trapping can prolong the lifetime of electrons as discussed above. Moreover, if the deviations of τn′ from a constant are originated from the bulk traps in the m-TiO2 film at the high Voc region, then the τn′ values should be constant for the electrode annealed 600 °C due to its much higher crystallinity because the bulk traps become less and less in the film electrode with enhancing the annealing temperature. However, there is also a decreasing tendency in τn′ at the first high Voc region for this electrode, implying that the recombination reaction at the high Voc region is not affected by the bulk traps. On the contrary, once the electrons of surface states shift to the electrolyte at the high Voc region, the τn′ ∼ Voc curve would be controlled completely by the conduction band states, and then the τn′ value decreases with the decay of Voc. This assumed decreasing tendency is appropriately confirmed by the experimental facts in Figure 6 except the electrode annealed at 500 °C, which reveals that annealing at 500 °C can efficiently eliminate the effect of surface states. At the low Voc region, the shape of the τn′ ∼ Voc curve is affected by the bulk traps and surface traps differently. The linearly increasing of the τn′ value is controlled entirely by the bulk traps, whereas the inverted parabola at the lower Voc is dominated by the surface states. The typical shape of these two parts can be observed from the electrode annealed at 450 °C but can not be observed from the other electrodes in Figure 6. A possible explanation for the shapes of the τn′ ∼ Voc curves at the low Voc region can be drawn by analyzing the evolvement processes of all of the film electrodes. At low temperature (350 °C), many of bulk traps and surface states coexist within the m-TiO2 film. The injected electrons are trapped by the bulk traps and surface states first, and then some of the electrons are released to the conduction band by the bulk traps. Because the electron transfer of the surface states is much slower than the conduction band electrons, the recombination was therefore actually controlled by the bulk traps. This effect is proven by the very wide range of the linear part of the τn′ ∼ Voc curves until very low Voc as shown in Figure 6. When the annealing temperature is increased to 450 °C, the bulk trap density decreases so that the effect of surface states emerges on the recombination reaction. Therefore, the distinct typical linear part and inverted parabola of the τn′ ∼ Voc curves are all observed clearly for the electrode annealed at 450 °C. For the 500 °C treated electrode, however, the surfacestate density also decreases significantly while the bulk traps continue to diminish. The recombination comes back to be

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Zhao et al.

Figure 7. Photocurrent action spectra of the DSSCs sandwiched by using different photoelectrodes.

controlled by the bulk traps, which can be illustrated by the wide range of the linear part of the τn′ ∼ Voc curve in Figure 6. Upon annealing at 600 °C, the internal surface area reduced drastically, and new surface states appeared due to the surface atoms so that the surface state density cannot be neglected in comparison with the bulk traps. On the basis of the above discussion, the effect of surface states on the recombination reaction appears not only at the low Voc region but also at the high Voc region. The results of OCVD measurement reveal that the surface states showed an obvious impact on the recombination process at the high Voc region except for the electrode annealed at 500 °C, which coincides with the discussions in the EIS spectra section. Considering the maintained mesoporous characteristics of m-TiO2 nanoparticles as illustrated in Figure 2c, 500 °C is thought to be the optimal temperature at which high photoelectric conversion efficiency could possibly be obtained. 3.7. Photoelectrochemical Property Measurement of DSSCs. Figure 7 shows the IPCE curves of the prepared DSSCs, in which IPCE were normalized to a common value of 1 at their maximum (510 nm). As can be seen, the IPCE is almost proportional to the annealing temperature in the full range of 400-800 nm, and all of the porous m-TiO2 electrodes give much higher IPCE values than that of the porous electrode made with P25 except for the m-TiO2 electrode annealed at 350 °C. It has been reported that IPCE can be expressed in terms of the light harvesting efficiency (LHE),2 the quantum yield of the charge injection (φinj), and the efficiency (ηc) of collecting the injected charge at the back contact as shown in eq 6

IPCE ) LHE(λ)φinjηc

(6)

In the present experiment, the LHE (λ) is mainly proportional to the adsorbed dye molecules per square centimeter. φinj and ηc are related to the crystallinity of m-TiO2 nanoparticles. Additionally, ηc is more directly determined by the film resistance (Rw), electron lifetime (τn), and film thickness. It is also considered that the porosity of the thin-film electrode can affect the photocurrent that appears in the formula of determining the IPCE. However, this factor can be neglected because the monochromatic light intensity is very weak (∼µW cm-2) during the measurement, and thus there are no significant distinctions between the transport resistances of I3- for different electrodes. For the electrode annealed at 350 °C, the LHE (λ) is higher due to the larger adsorbed amount of N719 molecules (see Table

Figure 8. I-V curves of the DSSCs sandwiched by using different photoelectrodes.

TABLE 3: Operation Parameters of DSSCs Made with Different m-TiO2 Porous Electrodes electrodes 350 450 500 600 500

°C °C °C °C °C (P25)

Isc/mA cm-2

Voc/mV

FF

η

2.87 9.78 12.22 9.84 5.86

488 559 565 605 645

0.612 0.615 0.616 0.622 0.7046.33%

2.04% 8.00% 10.12% 8.81%

1), but the crystallinity of m-TiO2 nanoparticles is low and the excited-state electrons can not be effectively injected into the conduction band of TiO2, which means low φinj. In addition, the ηc is also very low because of the very short lifetime (2.282 ms, see Table 2) of the injected electrons, which means that the electrons can not be effectively collected by the conductive substrate before they recombine with the I3- (the main recombination process). The lowest IPCE was therefore observed for the electrode annealed at 350 °C among the explored electrodes as shown in Figure 7. The crystallinity of m-TiO2 nanopartciles in the film electrode increased gradually with enhancing the annealing temperature from 450 to 600 °C, which can lead to the decrease in the dye adsorption amount and LHE (λ), whereas the improved crystallinity is of great benefit to the increase of φinj and ηc because the transport resistance (Rw) decreased gradually as shown in Table 2. It is assumed that the effect of the electron lifetime and the film thickness (ref Table 1) can be neglected because the τn values are all enough for the electron injection and the equal quantity of paste has been spread on the FTO substrate to fabricate the electrodes. Under the cooperation of all of the factors, the sequence of photocurrent responses of different dye-sensitized m-TiO2 electrodes in Figure 7 can be understood qualitatively. As for the P25 electrode, the thickness of the TiO2 film is similar to those of the above three m-TiO2 films but with a much lower N719 adsorption amount than the electrodes annealed at above 450 °C as shown in Table 1, which can be ascribed to its much smaller specific surface area in comparison with the m-TiO2 film. The much lower adsorbed amount of dye molecules means a lower LHE (λ). Therefore, the electrode made with P25 gives a lower IPCE than the m-TiO2 films except for the electrode annealed at 350 °C as shown in Figure 7. The I-V curves of DSSCs made with different electrodes are plotted in Figure 8 and the operation parameters are summarized in Table 3. The results indicate that the Voc and FF are proportional to the annealing temperature for the DSSCs

Effect of Annealing Temperature on Solar Cells made with m-TiO2 materials, while the Isc has a maximum value at 500 °C. Hoshikawa et al.21 found that the FF was affected mainly by the internal resistances at FTO/TiO2, Pt/electrolyte interfaces and the conductive glass. In the present studies, the adopted FTO substrate has good thermal stability and the effect of Pt/electrolyte interface can be neglected because the same counter electrode was used for the different DSSCs. It is also reasonable to think that the electric contact between the m-TiO2 and FTO should become easier with enhancing the annealing temperature although the necessary information has not been detected. Therefore, the increase of FF with enhancing the annealing temperature as shown in Table 3 can be ascribed mainly to the contribution from the decrease in the internal resistances due to sintering of the m-TiO2 films. From 350 to 600 °C, the internal surface area of TiO2 films decreases while the crystallinity increases along with enhancing the annealing temperature. The adsorbed N719 molecules reduce gradually as illustrated in Table 1, and thus the light-harvesting efficiency will also decrease. However, the improved crystallinity of TiO2 films can lead to the more effective electron injection into the conduction band of TiO2. Moreover, it has been proposed that the Voc is related mainly to the electron density in the conduction band.26 Therefore, the increased Voc can be attributed to the enhanced electron injection efficiency. According to the previous report,21 a higher Rw means that the existence of the high bulk trap density in the TiO2 films can prevent the injected electrons from flowing smoothly. Moreover, the Isc can be also affected markedly by the porosity of porous m-TiO2 films. Under the light intensity of 42 mW cm-2, the effect of porosity cannot be neglected. In theory, the Isc should increase along with the gradual decrease in the Rw value for the electrodes annealed from 450 to 600 °C as can be seen from Table 2; however, the reduced porosity of m-TiO2 film limits this increase in Isc. Therefore, the maximum Isc was obtained for the m-TiO2 electrode after annealing at 500 °C under the cooperation of the above two factors. Although the above criterion is not fit for the electrode annealed at 350 °C, this electrode possesses the largest porosity and lowest Rw. As discussed above, the lowest Rw value originated in fact from the relatively separated and small nanoparticles that have not been sintered together sufficiently. Namely, there are several bulk traps and surface states within the electrode; those bulk traps can localize the electrons in the band gap while the surface states localize and transfer the electrons to the oxidized species. Therefore, the electron density in the conduction band of the m-TiO2 film annealed at 350 °C is very low in comparison with the other electrodes, and the smooth transport of electrons within the m-TiO2 film is very difficult, which gives a minimal Isc value. As shown in Table 1, the m-TiO2 film annealed at 500 °C can adsorb many more dye molecules than a P25 film with a similar thickness. This can be ascribed to the porous characteristics of m-TiO2 nanoparticles. Moreover, the movement of I3- in the electrolyte benefits from the mesostructure of the m-TiO2 nanoparticles, which can improve the Isc. The much higher Isc reveals that the m-TiO2 film annealed at 500 °C possesses a high porosity due to the mesostructures of the m-TiO2 nanoparticles in comparison with P25. On the contrary, the m-TiO2 based DSSC annealed at 500 °C shows a lower Voc than the P25-based DSSCs as can be seen from Table 3. It may be related to the much lower Rw for the P25-based film due to its smaller grain boundary within the film. However, the highest η (10.12%) for DSSCs made with the m-TiO2 electrode annealed at 500 °C is obtained among all of the fabricated DSSCs, and

J. Phys. Chem. C, Vol. 112, No. 22, 2008 8493 this efficiency is improved by 3.79% in comparison with that of the P25-based DSSC. 4. Conclusions The mesoporous TiO2 nanoparticles were applied to prepare the porous electrodes for the DSSCs. The influence of the annealing temperature on the properties of porous electrodes was studied by the electrochemical impedance spectra (EIS) and the open-circuit photovoltage decay (OCVD) techniques. The experimental results indicate that the bulk traps and the surface states within the porous electrodes have important impacts on the recombination reaction, which in turn influence the Isc and Voc of DSSCs. An optimal annealing temperature for the mesoporous TiO2 nanoparticle electrodes is 500 °C, at which the mesostructures of TiO2 nanoparticles can be maintained and therefore is suitable for a low recombination probability. The mesostructures within the m-TiO2 nanoparticles are also of great benefit to dye adsorption, and then to the improvement of the photoelectrochemical property of DSSCs. Moreover, the highest photoelectric conversion efficiency (10.12%) for the DSSCs made with the mesoporous TiO2 nanoparticles is obtained among all of the fabricated DSSCs, and this efficiency is improved by 3.79% in comparison with that of the P25-based DSSC after annealing at 500 °C, indicating the promising application in DSSCs of the present special homemade m-TiO2 nanoparticles. Acknowledgment. This work was supported by the National “863” Foundation of China (2006AA03Z344), Natural Science Foundation of China (20573078), Program for New Century Excellent Talents in University (NCET-07-0637), and Talented Young Scientist Foundation (2006ABB003) of Hubei Province, China. References and Notes (1) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (2) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Muller, E.; Liska, P.; Valchopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6382. (3) Hagfeldt, A.; Gra¨tzel, M. Chem. ReV. 1995, 95, 49. (4) Robel, I.; Subramanian, V.; Kuno, M.; Kamat, P. V. J. Am. Chem. Soc. 2006, 128, 2385. (5) Gra¨tzel, M. Photochem. Photobiol., C 2003, 4, 145. (6) Nakade, S.; Saito, Y.; Kubo, W.; Kitamura, T.; Wada, Y.; Yanagida, S. J. Phys. Chem. B 2003, 107, 8607. (7) Saito, Y.; Kambe, S.; Kitamura, T.; Wada, Y.; Yanagida, S. Sol. Energy Mater. Sol. Cells 2004, 83, 1. (8) Gra¨tzel, M. Photochem. Photobiol., A 2004, 168, 235. (9) Uchida, S.; Chiba, R.; Tomiha, M.; Masaki, N.; Shirai, M. Electrochemistry 2002, 70, 418. (10) Adachi, M.; Murata, Y.; Okada, I.; Yoshikawa, S. J. Electrochem. Soc. 2003, 150, 488. (11) Gopal, K.; Karthik Shankar, M.; Maggie, P.; Varghese, O. K.; Grimes, C. A. Nano Lett. 2006, 6, 215. (12) Adachi, M.; Murata, Y; Takao, J; Jiu, J. T; Sakamoto, M; Wang, F. M J. Am. Chem. Soc. 2004, 126, 14943. (13) Song, M. Y.; Kim, D. K.; Ihn, K. J.; Jo, S. M.; Kim, D. Y. Nanotechnology. 2004, 15, 1861. (14) Ngamsinlapasathian, S.; Pavasupree, S.; Suzuki, Y.; Yoshikawa, S. Sol. Energy Mater. Sol. Cells 2006, 90, 3187. (15) Wei, M. D.; Konishi, Y.; Zhou, H. S.; Yanagida, M.; Sugihara, H.; Arakawa, H. J. Mater. Chem. 2006, 16, 1287. (16) Chiba, Y.; Islam, A.; Watanabe, Y.; Komiya, R.; Koide, N.; Han, L. Y. Jpn. J. Appl. Phys., Part 2 2006, 45, L638. (17) Hou, K.; Tian, B. Z.; Li, F. Y.; Bian, Z. Q.; Zhao, D. Y.; Huang, C. H. J. Mater. Chem. 2005, 15, 2414. (18) Peng, T. Y.; Zhao, D.; Dai, K.; Shi, W.; Hirao, K. J. Phys. Chem. B 2005, 109, 4947. (19) Peng, T. Y.; Zhao, D.; Song, H. B.; Yan, C. H. J. Mol. Catal. A: Chem. 2005, 238, 119.

8494 J. Phys. Chem. C, Vol. 112, No. 22, 2008 (20) Kern, R.; Sastrawan, R.; Ferber, J.; Stangl, R.; Luther, J. Electrochim. Acta 2002, 47, 4213. (21) Hoshikawa, T.; Yamada, M.; Kikuchi, R.; Eguchi, K. J. Electrochem. Soc. 2005, 152, 68. (22) Fabregat-Santiago, F.; Garcia-Belmonte, G.; Bisquert, J.; Zaban, A.; Salvador, P. J. Phys. Chem. B 2002, 106, 334. (23) Wang, Q.; Moser, J. E.; Gra¨tzel, M. J. Phys. Chem. B. 2005, 109, 14945. (24) Adachi, M.; Sakamoto, M.; Jiu, J.; Ogata, Y. K.; Isoda, S. J. J. Phys. Chem. B 2006, 110, 13872. (25) Hauch, A.; Georg, A. Electrochim. Acta 2001, 46, 3457.

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