J. Phys. Chem. C 2007, 111, 10741-10746
10741
Effect of the Particle Size on the Electron Injection Efficiency in Dye-Sensitized Nanocrystalline TiO2 Films Studied by Time-Resolved Microwave Conductivity (TRMC) Measurements Ryuzi Katoh,*,† Annemarie Huijser,‡ Kohjiro Hara,† Tom J. Savenije,‡ and Laurens D. A. Siebbeles‡ National Institute of AdVanced Industrial Science and Technology (AIST), AIST Tsukuba Central 5, Tsukuba, Ibaraki 305-8565, Japan, and Opto-Electronic Materials Section, DelftChemTech, Faculty of Applied Sciences, Delft UniVersity of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands ReceiVed: April 2, 2007; In Final Form: May 15, 2007
The efficiency of electron injection from photoexcited dye molecules into TiO2 nanoparticles with two different sizes (20 or 300 nm) was studied using the time-resolved microwave conductivity (TRMC) technique. For the ruthenium dye (N719) adsorbed on 20 nm TiO2 particles, we have found a near 100% injection efficiency, which is consistent with previous transient optical absorption studies. In contrast, the electron injection efficiency for N719 sensitized on 300 nm TiO2 particles was found to be less than 50%. A similar behavior was found for coumarine-sensitized TiO2 nanoparticle films. The difference is tentatively attributed to differences in the adsorption of dye molecules on the TiO2 surface. Interestingly, only for 20 nm TiO2 particles, a trap filling effect has been observed, that is, at low illumination intensities, the TRMC signal increases more than proportional with the illumination intensity. This is attributed to an increase in electron mobility due to occupation of deep traps. The absence of a significant trap filling effect for 300 nm particles points toward a considerably lower trap density.
1. Introduction Since highly efficient dye-sensitized solar cells were first reported,1 extensive research has been carried out to improve their performance. To this end, the primary processes in dyesensitized solar cells have been studied in great detail.2-6 To realize intimate contact between the dye and the electronaccepting wide band gap semiconductor, nanocrystalline semiconductor films are used in order to realize a large surface area. These porous semiconductor films are prepared by the calcination of nanometer-sized (typically 10-20 nm) particles on a conductive glass substrate. After sintering of the semiconductor film, organic dyes are chemically bound to the surface of the porous structure to sensitize the wide band gap semiconductor. The working principle of a dye-sensitized solar cell is based on the following photophysical processes.4-6 Upon excitation of the dye by visible light, electron injection from the excited dye into the semiconductor film occurs. The reduction of the oxidized sensitizer dye by the redox mediator molecules (I-/ I3-) is in competition with the unfavorable recombination of the injected electron with the oxidized dye. The electrons are transported by diffusion through the semiconductor nanostructured film to the electrode on top of the film. Obviously, electron injection is a very important primary process in a dye-sensitized solar cell. We7-16 and others17-26 extensively used transient absorption spectroscopy (TAS) to study the electron injection process. The electron injection dynamics were studied by femtosecond laser spectroscopy. Electron injection was found to occur within 10 ps.13-22 This * To whom correspondence should be addressed. E-mail: r-katoh@ aist.go.jp. † AIST. ‡ Delft University of Technology.
is much faster than the excited-state lifetime of typical sensitizer dyes, providing an explanation for the high electron injection efficiency. It is also feasible to evaluate the injection process on the basis of the electron injection efficiency using nanosecond time-resolved TAS.7-12,23-26 Even though it is complicated to determine the number of transient species using TAS, a few attempts to evaluate the electron injection efficiency using this technique were reported. Recently, we studied the efficiency for N3 dye (cis-bis-(4,4′-dicarboxy-2,2′-bipyridine)di(thiocyanato)ruthenium(II); Ru(dcbpy)2(NCS)2) adsorbed onto a nanocrystalline TiO2 film consisting of 20 nm particles (denoted as N3/20 nm). For this system, the estimated electron injection efficiency equaled 80 ( 15% under 532 nm excitation.10,11 In addition, the electron injection efficiency was investigated as a function of the excitation intensity,9 excitation wavelength,12 and under various conditions such as the concentration of Li+ ions.15 Although many efforts have been devoted to study the factors governing the efficiency of the primary processes in dyesensitized solar cells, it is still difficult to discuss the actual solar cell performance on the basis of the results obtained from the studies mentioned above. This is due to the fact that the nanostructured semiconductor films used for TAS studies are not the same as those typically applied in practical devices. For meaningful discussions, such differences should be eliminated. One discrepancy is the difference in particle size in the semiconductor films used. For many studies focusing on the electron injection process, transparent films prepared from 5-20 nm primary particles have been used, whereas the semiconductor films used in practical devices often include larger particles (100-400 nm in diameter) acting as effective light scattering centers.27,28 It has been pointed out that the particle size affects
10.1021/jp072585q CCC: $37.00 © 2007 American Chemical Society Published on Web 06/20/2007
10742 J. Phys. Chem. C, Vol. 111, No. 28, 2007
Katoh et al.
charge transport and recombination dynamics.29,30 To carry out quantitative optical measurements on such an opaque sample, conventional TAS is, however, not versatile. The semiconductor films scatter the incident light efficiently in the case of a primary particle size comparable with the wavelength of the incident light. Reduction of the scattering of the probe light can thus be achieved by using radiation with a longer wavelength, such as microwave (ca. 3 cm for 10 GHz) or terahertz radiation (ca. 0.3 mm for 1 THz). Two studies report on the use of the time-resolved microwave conductivity (TRMC) technique in order to study photophysical processes in dyesensitized nanocrystalline TiO2 films.31,32 In one of these works, the electron injection efficiency was evaluated quantitatively.32 Apart from this, the change in terahertz absorption by TiO2 films induced by the electron injection from optically excited dyes was investigated.33 Although all these studies have been carried out using transparent nanoparticle films, it is evident that transient absorption measurements using microwave and terahertz radiation could possibly be used to study actual solar cell semiconductor films including large particles. In the present study, we use the TRMC method to determine the electron injection efficiency in dye-sensitized films consisting of 20 or 300 nm particles. Apart from the well-known ruthenium dye (N719), also coumarine dyes have been examined. Dyesensitized solar cells based on these dyes investigated are known to exhibit a high performance.34 2. Experimental Section An organic-paste-containing TiO2 nanoparticle (20 nm in diameter) was prepared by the method reported by Gra¨tzel and co-workers.35 The crystallographic structure was anatase.27 To prepare large sized particle films (300 nm in diameter), a commercially available paste containing large size anatase TiO2 particles (Solaronix SA, Ti-Nanoxide 300) was used. The pastes were painted on a quartz plate with a screen printer (Mitani Electronics Co., MEC-2400). Nanocrystalline films were prepared by calcination of the painted substrate for 1 h at 420 °C. The films obtained had an area of 3 cm2 (1.18 cm × 2.55 cm). Layer thicknesses were measured using a Veeco Dektak 8 Stylus Profiler and equaled 1.2-1.5 µm and 3.8-4.0 µm for the 20 and 300 nm particle films, respectively. The sensitizer dyes were dissolved in dehydrated ethanol (Wako Chemicals) or a tert-butylalcohol-(Kanto, G grade) acetonitrile (Kanto, dehydrated) mixture solvent (50:50) at a concentration of 0.3 mM. These solvents were used without further purification. Figure 1 shows the molecular structures of the sensitizer dyes studied. The N719 dye (Solaronix SA) was used without purification. Detailed synthetic procedures of the coumarin dyes (NKX2677 and NKX2697) have been reported elsewhere.34 The semiconductor films were immersed in the dye solution and kept there at 25 °C for at least 10 h so that the dye could adsorb onto the semiconductor surface. The sample specimens were dried in air. Optical characterization of the films was carried out using a spectrophotometer (Perkin-Elmer, Lambda 900) equipped with an integrating sphere (Labsphere). Details of the procedure were described previously.32 Accordingly, the fraction of absorbed light FA can be expressed using the fraction of reflected light FR and transmitted light FT by
FA ) 1 - (FR + FT)
(1)
The FT and FR were determined from the transmission and the reflection spectra using the integrating sphere.
Figure 1. Molecular structure of the sensitizer dyes investigated.
Time-resolved microwave conductivity (TRMC) measurements were carried out using the same setup, as reported previously.32 The change of microwave power ∆P/P determined by TRMC measurements was proportional to the change in conductance ∆G of the irradiated film and expressed by
∆P ) -K∆G P
(2)
where K represents the sensitivity factor. In the case of a sample layer thickness much smaller than the microwave wavelength, ∆G is related to the conductivity at a depth z within the layer ∆σ(z) by
∆G ) β
∫0L ∆σ(z)δz
(3)
where L is the thickness of the excited area and β is the ratio between the broad and narrow inner dimensions of the waveguide and is equal to 2.08 for the X-band waveguide used. The conductivity ∆σ(z) can be expressed as
∑µ
∆σ(z) ) eN(z)
(4)
where e is the elementary charge, N is the charge carrier concentration at depth z, and ∑µ is the sum of the mobility of an electron and that of a hole. In the case that all photons are absorbed in the film and charge carriers are formed with an efficiency of electron injection per incident photon η, the integral in eq 3 is equal to I0η, and the conductance change ∆G follows from
∆G ) I0βeη
∑µ
(5)
where I0 is the incident light intensity. Finally, combining eqs 2 and 5 yields
η
1
1 1
∑µ ) I βe ∆G ) I βe K 0
0
( ) ∆P
-
P
(6)
TiO2 Films Studied by TRMC Measurements
J. Phys. Chem. C, Vol. 111, No. 28, 2007 10743
Figure 3. The traces of η∑µ for bare 300 nm and bare 20 nm TiO2 films excited with 300 nm pulses before and after annealing treatment. Excitation light intensities I0 (photons cm-2) are shown in the figure.
Figure 2. FR, FT, and FA as a function of wavelength for N719/20 nm films and N719/300 nm films.
The values of K and I0 could be determined separately,32 and therefore, the value of η∑µ could be evaluated quantitatively. The product of the initial quantum yield for electron injection (Φ0) and ∑µ was obtained from the maximum TRMC signal in the case that no decay occurred during the response time of the cavity using
Φ0
1
1
0 Aβe
K
∑µ ) I F
( ) ∆P
-
P
(7) MAX
where (-∆P/P)MAX is the maximum change of microwave power recorded. To deduce Φ0, the value of ∑µ was determined separately upon UV excitation of bare TiO2 and assuming that the quantum yield for charge carrier generation equals unity.36 3. Results and Discussion 3.1. Optical Characterization. Figure 2 shows the FR, FT, and FA as a function of wavelength for N719/20 nm and N719/ 300 nm films. The absorption band above 400 nm originates from the dye, whereas below 400 nm, the FA values are close to 0.9 due to strong absorption by TiO2. For the N719/20 nm film, the reflection is about 10%, whereas the N719/300 nm film exhibits a reflection >50% in the visible region. The FA of dye-sensitized 300 nm films in the visible wavelength range is limited due to this strong reflection. For a quantitative analysis of the TRMC data, the density of electrons in the film is an important parameter since the electron mobility and the recombination rate are known to be sensitive to the electron density.16,37 Thus, knowledge about the excitation volume () irradiated area × penetration depth) is required. Since the fraction of absorbed light at 500 nm equals only ∼0.4, the penetration depth of the incident light is assumed to equal the film thickness, and excitons are assumed to be formed homogeneously within the layer. On the contrary, for bare TiO2 films excited by 300 nm pulses, almost all photons are absorbed near the surface (Figure 2). The penetration depth, corresponding to
the reciprocal value of the absorption coefficient, is equal to 100 nm for excitation of TiO2 at λ ) 300 nm.38 3.2. TRMC Transients Obtained for Bare TiO2 Films. Figure 3 shows typical TRMC signals obtained upon direct band gap excitation at 300 nm of bare 20 nm and bare 300 nm films before and after annealing (450 °C, 20 min). As can be seen in Figure 3, the peak value of the η∑µ transient for 20 nm films equals 0.009 cm2 V-1 s-1, both for nonannealed and annealed films. Since the efficiency for charge generation upon direct band gap excitation of TiO2 is known to equal almost unity36 and the attenuation of the sample at λ ) 300 nm amounts to FA ) 0.9, the lower limit of the mobility of electrons in bare 20 nm films equals 0.01 cm2 V-1 s-1, which is close to values reported previously.32 In principle, both electrons and holes can contribute to the observed TRMC signal. Femtosecond transient absorption measurements on bare 20 nm TiO2 films revealed ultrafast trapping of the photogenerated charge carriers.39,40 In those works, it was found that the holes produced in TiO2 particles by direct band gap excitation were trapped rapidly at the surface, and only the electrons could move freely in the bulk of the particles. Accordingly, it is expected that only the electrons contribute to the observed TRMC signals, which is in agreement with conclusions made in previous work.32 The recombination dynamics between electrons and trapped holes were studied previously using nanosecond time-resolved TAS under direct band gap excitation.41 It turned out that, at higher intensities, the recombination rate strongly depended on the intensity of the excitation light. Using low excitation intensities (I0 < 0.1 mJ/cm2), the rate was not sensitive to I0, and recombination occurred in the microsecond time range, indicating that geminate recombination occurred. In fact, the number of hole-electron pairs per particle is estimated to be less than unity at the low excitation intensities. The TRMC signal of the nonannealed bare 20 nm films has a similar temporal profile as the TAS signal.41 This indicates that the decay of the TRMC signal reflects the population decay due to the recombination of electrons and holes in the particle rather than trapping of electrons. The minimum time required for recombination under the low excitation intensity condition could be estimated from the diffusion time in the particle. The diffusion time tdiff can be expressed as tdiff ) eL2/µκΤ, where L is the particle radius and µ is the mobility. From L ) 10 nm, µ ) 0.01 cm2 V-1 s-1, and T ) 293 K, we expect the recombination of electrons and holes in one particle to be completed within 4 ns. This is more than
10744 J. Phys. Chem. C, Vol. 111, No. 28, 2007 2 orders of magnitude faster than the observed recombination time of a microsecond for the nonannealed film. This remarkable difference might originate from a spatial separation of electrons and holes. Most likely, the electrons are located in the TiO2 nanoparticles, whereas holes are trapped at the surface of the particle, as discussed above and confirmed by the observation that holes in TiO2 react quickly with alcohols.41,42,43 For the nonannealed bare 300 nm particles, the half-life time (t1/2) of the TRMC signal decay equals about 100 µs, which is much longer than the half-life time of approximately 1 µs observed for bare 20 nm films. The longer lifetime of the mobile electrons in 300 nm particles suggests an efficient spatial separation of electrons and holes. The maximum TRMC signal observed for bare 300 nm films is 1 order of magnitude larger than that for bare 20 nm films and equals 0.1 cm2 V-1 s-1. Note that this value is 1 order of magnitude lower than the electron mobility for smooth polycrystalline layers of anatase, which has been reported to equal about 2 cm2 V-1 s-1.44 This tendency has been observed previously for AC conductivity measurements on nanoporous TiO2 using terahertz and microwave radiation. Microwave conductivity measurements reveal a decrease in mobility from 1.4 to 0.05 cm2 V-1 s-1 as the average particle size decreases from ca. 100 nm to ca. 10 nm.45 For smaller particles, mobility values on the order of 0.01 cm2 V-1 s-1 have been reported using microwave32 and terahertz radiation.46,47 The reason for the decrease in mobility with the particle size has not yet been established. One possible cause involves the scattering of electrons at the particle surface, resulting in a reduced effective mobility.32,46 Recently, another explanation was provided by Hendry et al.,47 in which they pointed out that the small mobility could be explained on the basis of the effective dielectric constant of an inhomogeneous mixture of vacuum and semiconductor. We have also investigated the effect of annealing of these films. The decay rate of the TRMC signal of bare 20 nm particle films significantly increases due to annealing, whereas almost no effect is observed for films with bare 300 nm particles. The annealing effect disappears after keeping the samples under ambient conditions. These observations imply that molecules adsorbed onto the surface of the TiO2 particles play an important factor in stabilizing the mobile charge carriers. The slower decay observed for the nonannealed bare 20 nm film might originate from an enhanced density of hole traps at the particle surface induced by the adsorption of surface molecules. This would result in a decrease of the charge recombination rate and hence yield a longer-lived signal. To confirm the cause of this phenomenon, a careful comparison of TRMC and TAS signals could be useful. 3.3. TRMC Transients Obtained for Dye-Sensitized TiO2 Films. Figure 4A and B presents the TRMC transients of NKX2677/20 nm and NKX2677/300 nm films, respectively, excited with 500 (closed circles) and 300 nm (triangles) laser pulses, together with the corresponding bare films excited at 300 nm (crosses). All traces are normalized at the maximum. Clearly, the decay profiles observed upon 500 nm excitation of the NKX2677-sensitized films are similar to those obtained upon 300 nm excitation of the NKX2677-sensitized and of the bare films. The weak absorption of the dye at 300 nm (Figure 4C) explains the similar transients obtained upon excitation at 300 nm of dye-sensitized and bare TiO2 films. In both cases, the incident photons are mainly absorbed by the TiO2 particles, resulting in the formation of mobile electrons and holes that are eventually trapped at surface states. Excitation at 500 nm
Katoh et al.
Figure 4. Normalized TRMC signal traces of dye-sensitized NKX2677/ 20 nm and NKX2677/300 nm films excited with laser pulses at a wavelength of 500 (closed circles) or 300 nm (triangles), together with data for the corresponding bare films excited at 300 nm (crosses). Excitation light intensities I0 (photons cm-2) are shown in the figure. Also included is the absorption spectrum of NKX2677 in ethanol.
of the dye-sensitized films, on the contrary, results in the production of mobile electrons by electron injection from the excited dye into the TiO2 particle. The similar decay profiles observed upon 300 and 500 nm excitation of bare and dyesensitized TiO2 films is consistent with the expectation that holes formed upon direct band gap excitation of bare TiO2 at 300 nm are trapped at the particle surface,41,42,43 and the observed TRMC signal mainly originates from mobile electrons. 3.4. Particle Size Dependency of the Electron Injection Yield. Charge transport in nanocrystalline films is known to be sensitive to the density of charge carriers due to a trap filling effect and bulk recombination.37 The TRMC signal at the end of the excitation pulse may also be reduced because of fast recombination of electrons and holes before trapping has occurred,39,40 which is expected to be most pronounced at high illumination intensities. To determine the absolute value of Φ0, we therefore recorded the TRMC transients at different excita-
TiO2 Films Studied by TRMC Measurements
J. Phys. Chem. C, Vol. 111, No. 28, 2007 10745 NKX2697-sensitized films, highly efficient electron injection can be expected since photovoltaic devices based on these materials possess high performances.34 This agrees with the 100 ( 10% efficient electron injection observed for the dyesensitized 20 nm films. The electron injection efficiencies obtained for dye-sensitized 300 nm TiO2 particle films are however significantly lower than those observed for the 20 nm based films. The discrepancy observed in Φ0 for the dye-sensitized 20 and 300 nm TiO2 particles might originate from differences during the dye adsorption onto the TiO2 surface. During the dye deposition, the following equilibrium reactions might occur, including the adsorption of a dye on a free adsorption site (NPfree) at the surface (1) and the formation of dye aggregates on the surface of the TiO2 (2)
Figure 5. Excitation density dependence of Φ0∑µ values of bare TiO2 films (open circles), N719 (closed circles), NKX2677 (triangles), and NKX2697 (crosses).
TABLE 1: The Electron Injection Yield in Various Dye-Sensitized 20 and 300 nm TiO2 Nanoparticle Films dye/particle size
electron injection yield
N719/20 nm NKX2697/20 nm NKX2677/20 nm N719/300 nm NKX2697/300 nm NKX2677/300 nm
1 ( 0.1 1 ( 0.1 1 ( 0.1 0.3 ( 0.03 0.4 ( 0.04 0.55 ( 0.05
tion intensities. Figure 5 shows the Φ0∑µ values of all samples studied as a function of the excitation density. We used the excitation volume () irradiated area × penetration depth) to evaluate the excitation density as mentioned section 3.1. The open and closed circles indicate the values for bare and dyesensitized films, respectively. The Φ0∑µ values appear to be sensitive to the excitation density. For 20 nm particle films, the dependence is similar to that observed earlier for similar films.32 Accordingly, at low excitation density (
