Surface State Recombination and Passivation in Nanocrystalline TiO2

Article pubs.acs.org/JPCC

Surface State Recombination and Passivation in Nanocrystalline TiO2 Dye-Sensitized Solar Cells Alexander R. Pascoe,† Laure Bourgeois,‡ Noel W. Duffy,§ Wanchun Xiang,† and Yi-Bing Cheng*,† †

Department of Materials Engineering, Monash University, Melbourne, Victoria 3800, Australia Monash Centre for Electron Microscopy, Monash University, Melbourne, Victoria 3800, Australia § CSIRO Energy Technology, Clayton, Victoria 3169, Australia ‡

S Supporting Information *

ABSTRACT: The relative role of surface state recombination in dye-sensitized solar cells is not fully understood, yet reductions in the recombination rate are frequently attributed to the passivation of surface states. We have investigated reports of trap state passivation using an Al2O3-coated TiO2 photoanode achieved through atomic layer deposition (ALD). Electrochemical characterization, performed through impedance measurements and intensity modulated photovoltage spectroscopy (IMVS), data showed that the Al2O3 deposition successfully blocked electron recombination and that the chemical capacitance of the film was unchanged after the ALD treatment. A theoretical model outlining the recombination kinetics was applied to the experimental data to obtain charge transfer rates from conduction band states, exponentially distributed traps, and monoenergetic traps. The determined electron transfer rates showed that the deposited Al2O3 coating did not selectively passivate trap states at the nanoparticle surface but reduced recombination rates equally from both conduction band states and surface states. These results imply that the reduction in the recombination rates reported in core−shell structured photoanodes cannot be attributed to a modification of surface traps, but rather to the weakened electronic coupling between electrons in the film and the electrolyte species.

1. INTRODUCTION It is widely acknowledged that the band gap of the nanocrystalline TiO2 used in dye-sensitized solar cells (DSSCs) is characterized by an exponential distribution of trap states.1 The nature and distribution of these trap states have been well described through the multiple-trapping model.2−5 Traps within the semiconductor bandgap delay the movement of electrons through the photoanode film.6 This delay results in a difference between the observed movement of a single charge and the effective movement of the electron population. Small perturbation measurements, transient measurements, and computational simulations present a disparity in electron transport data that stems from the transport limiting nature of these trap states.7 Strong evidence has been presented to show that these electron impeding traps are largely located at the surface of the TiO2 nanoparticles.8−13 Kopidakis et al. have indicated a direct correlation between the internal surface area of TiO2 nanoparticle films and the density of electron traps.8 As shown through photoluminescence imaging, Mercado et al. argue that these traps derive from the undercoordinated titanium atoms at the nanoparticle surface.9 Despite the evidence for surface localized electron traps, recent modeling of DSSC surface state recombination has assumed an even spatial distribution of trap states throughout the volume of the nanoparticle.14 Under this assumption, only the fraction of exponentially distributed traps that happen to reside at the © XXXX American Chemical Society

surface of the nanoparticle would participate in the recombination dynamics. The trapped electrons in the bulk material are not electronically coupled with the electrolyte molecules and therefore would not take part in recombination events. The inability to confirm the position and source of electron trap states further complicates the understanding of what role these states play in recombination dynamics. Previous studies have attributed the nonideal behavior of DSSC devices to the back-transfer of electrons from surface traps.15−17 Ideal statistics predict an increase of 59 mV for each decade increase in the illumination intensity as well as a linear relationship between the free electron concentration and the recombination rate. Yet the observed increase in the potential is typically greater than the predicted 59 mV increase. In addition, the recombination rate routinely exhibits a sublinear relationship with respect to the free electron concentration. It is understood that the back-transfer of excited electrons from trapped states may account for this deviation away from theoretical expectations. Whether surface states play a significant role in recombination is ultimately governed by the electron kinetics of the back-reaction. If we assume that the trapping and detrapping rates of electrons from surface states Received: August 12, 2013 Revised: November 5, 2013

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dx.doi.org/10.1021/jp408055g | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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important parameter to define.26 For the purposes of this study, the three components that contribute to the measured chemical capacitance are the conduction band states, the exponential distribution of traps, and the monoenergetic traps. The chemical capacitances of each of these constituents is shown in eqs 1, 2, and 3 respectively.27

are far greater than recombination rates, then it is expected that an electron is much more likely to be released from a trap state than to recombine with the oxidized electrolyte species. This assumption forms the foundation of the quasi-static approximation, which is widely used to model DSSC mechanisms.18 Although the quasi-static approximation considers surface state recombination negligible, multiple studies have alluded to surface states playing a significant role in the loss of conductive electrons in DSSCs.15,19,20 In addition to acknowledging the contribution of surface states to recombination dynamics, several studies have reported the passivation of these states.21−24 This passivation is essentially due to a treatment of the film surface which prevents the traps from playing part in the electron backreaction. Treatments include the deposition of a thin coating on the TiO2 nanoparticles either through a physical or chemical method. Two initial studies into the effects of trap state recombination after a surface treatment were performed by Hamann et al. and Fabregat-Santiago et al.21,22 Both studies reported the complete passivation of surface states, with an additional tunneling barrier suppressing conduction band recombination. DeVries, Pellin, and Hupp drew similar conclusions, arguing that approximately three cycles of atomic layer deposition (ALD) alumina were sufficient to passivate surface states and that additional cycles formed a tunneling barrier for conduction band electrons.23 Another report by Odersma and Hamann observed a similar increase in the electron lifetime as the previous studies after an ALD alumina treatment. This study, however, concluded that there was no evidence of alumina passivating surface states and further claimed that there was no evidence of surface state recombination in an untreated film.25 Their proof for the absence of surface state passivation was given by the unchanged chemical capacitance between an untreated and a treated film. Although, it should be noted that there is a difference between the removal of trap states and the passivation of trap states, which prevents electron transfer from these sites. A change in the chemical capacitance between cells is expected when there is either a change in the distribution of trap states or a shift in the conduction band edge. There have been mixed reports concerning the relative role of surface states in DSSC recombination as well as the apparent effects of surface treatments on electron back-transfer. If significant, recombination from surface states could severely compromise the device efficiency. It is necessary to note that previous works on surface state recombination differ to our own, in that we propose a model that moves beyond the analysis of electron lifetime plots to distinguish between conduction band and surface state recombination. Herein we explore the role of trap state recombination through a novel model describing not only experimental lifetime data but also the chemical capacitance and recombination resistance. By employing a model that separates conduction band and trap state components, it is possible to more accurately ascribe any changes in the total recombination dynamics to either a change in the conduction band or surface state recombination rates. Using this model, we aim test reports of trap state passivation achieved through the deposition of a surface coating on the TiO2 film.

Cμcb = q2

∂nc n = q2 c ∂E F kBT

Cμexp = q2

∂ ∂E F

∫E

Ec

(1)

gexp(E)fexp (E , E F) dE ≈ q2gexp(E F)

v

(2)

Cμme = q2 ≈ q2

∂ ∂E F

∫E

Ec v

gme(E)fme (E , E F) dE

Nme f (1 − fme ) kBT me

(3)

where g(E) is the density of states, f represents the Fermi− Dirac distribution, Nme is the total density of monoenergetic traps, kB is the Boltzmann constant, and EF is the quasi-Fermi level. In eq 3, the zero-temperature approximation has been employed to the Fermi−Dirac function in order to simplify the expression. The total chemical capacitance is calculated by the sum of the conduction band, exponentially distributed and monoenergetic contributions. In eq 1, nc represents the conduction band electron density which is proportional to the total effective density Nc of conduction band states, as shown in eq 4. nc = Nc exp[(E F − Ec)/kBT ]

(4)

The electron transfer rate k(i) et (i = cb, exp, me) for a given energy state has been defined in literature as being exponentially proportional to the reorganization energy λ of the electrolyte species.28−30 We may express the electron transfer rate as being directly proportional to a maximum value k(i) et,max, as shown in eq 5. 2⎤ ⎡ −(E(i) − E F,redox + λ) ⎥ (i) ⎢ ket(i) = ket, max exp ⎢⎣ ⎥⎦ 4λkBT

(5)

The recombination resistance provides us with an indication of the reluctance of charges to transfer from the semiconductor back to the electrolyte species. Combining eq 5 with eqs 1−3 allows us to define an expression for the recombination resistance Rr.

R r(i) =

1 Cμ(i)ket(i)

(6)

The total recombination resistance is calculated by summing the parallel resistance elements. Finally, we are able to obtain a complete expression for the electron lifetime as a function of the chemical capacitance and recombination resistance of each constituent energy state. τn =

2. THEORY The chemical capacitance of the TiO2 film provides us with an indication of the density of electron states, and it is an

Cμcb + Cμexp + Cμme [R rcb]−1 + [R rexp]−1 + [R rme]−1

= Cμ ,totalR r,total

(7)

The expressions for the chemical capacitance and the recombination resistance hold special significance, as they are the parameters that we obtain through impedance measureB



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ments. It is important to define these parameters theoretically, as they form the foundations of the model that has been used to fit the experimental data. The chemical capacitance and recombination resistance models have only been applied to the impedance data, while the effective electron lifetime model is relevant to both impedance and intensity modulated photovoltage spectroscopy (IMVS) techniques.

3. EXPERIMENTAL METHOD Fabrication of DSSCs. The fluorine-doped tin oxide (FTO) covered glass was initially cleaned in three stages using a Hellmanex solution, distilled water, and ethanol (96% purity). A dense TiO2 blocking layer was then formed on the FTO using spray pyrolysis at 450 °C of titanium diisopropoxide bis(acetylateacetonate) (Aldrich, 32525-2) in isopropanol with a volume ratio of 1:9. Films of approximately 1.7 μm in thickness were then printed using a 30 nm TiO2 paste (CCIC PST-30NR) and sintered at a maximum temperature of 500 °C for 30 min. ALD was performed using a Fiji F200 instrument at 250 °C. The printed TiO2 working electrodes were soaked in a 0.3 mM MK2 dye solution in toluene for a duration of 4 h. Counter electrodes were prepared using a 10 mM platonic acid (H2PtCl6) solution in isopropanol on FTO-covered glass, which were sintered at 400 °C for 15 min. Sandwich cells were made using a 25 μm Surlyn gasket and a 0.2 M [Co(bpy)3](TFSI)2, 0.06 M [Co(bpy)3](TFSI)3, 0.05 M LiTFSI, 1.00 M tert-butylpyridine electrolyte in acetonitrile. Cells were left to settle for approximately 17 h in the dark before characterization. Characterization Techniques. High-resolution transmission electron microscopy (HRTEM) and energy dispersive Xray (EDX) spectroscopy using a JEOL JEM-2100F instrument operated at 200 kV and equipped with a Si(Li) EDX detector were used to characterize the core−shell structure of the nanoparticles in the working electrode. The current voltage characteristics of each cell were measured using a xenon arc lamp with an AM1.5 spectral filter. These measurements were repeated until the performance of the device stabilized. Cells were then characterized by electrochemical impedance spectroscopy31,32 (EIS) and intensity modulated photovoltage spectroscopy2,11,33 (IMVS). Both EIS and IMVS techniques utilized a Zahner Zennium electrochemical workstation ECW IM6 as a frequency response analyzer. EIS measurements were performed in the dark using a 10 mV applied perturbation in the 30 mHz−500 kHz frequency range. The illumination source for the IMVS measurements was a 435 nm diode laser powered by a PP210 potentiostat. IMVS light perturbations were between 2 and 5% of the steady state illumination, and optical filters were used to achieve low dc illumination intensities. Impedance data and photovoltage curves were analyzed using Zview equivalent circuit modeling software (Scribner).

Figure 2. EDX maps of (a) aluminum, (b) titanium, and (c) oxygen as well as (d) overlay of the three elemental maps, for a sample region slightly larger than that shown in Figure 1. The thin layer of aluminum beyond the perimeter of the titanium oxide verifies the core−shell structure.

4. RESULTS AND DISCUSSION HRTEM and EDX. A HRTEM image of the alumina-coated TiO2 core−shell structure is shown in Figure 1. From this image it is possible to see the clear contrast between the lattice fringes of the TiO2 material at the core of the nanoparticles and a thin amorphous layer residing at the surface. The coating thickness appears to be in the order of 1 nm, which was achieved after five cycles of ALD. To determine which chemical element(s) are present in this coating layer, spatially resolved

the overlay of the aluminum, titanium, and oxygen atoms seen in Figure 2d, successfully illustrate the aluminum atoms located at the surface of the nanoparticle, extending past the edge of the titanium film. The EDX map gives clear evidence that the core−shell structure is achieved and that the coating material is indeed the intended alumina layer. Our conclusion may seem trivial; however, the significance of this result is that any differences in the electrochemical characterization can be properly ascribed to the formation of this alumina barrier and

Figure 1. HRTEM image of the TiO2−aluminum oxide core−shell structure. The crystalline anatase nanoparticles exhibit clear lattice fringes; the ≈1 nm thick shell is visible as a disordered region on the surface of the particles. Five cycles of ALD deposited alumina were applied to the printed TiO2 film.

EDX of the same sample region was performed. Only Ti, O, Al, and C (from the carbon support film of the TEM sample) were detected. The EDX maps shown in Figure 2, and in particular

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that changes in the electron transfer are not due to another film modification. From the HRTEM images and EDX maps, the deposited aluminum oxide layer appears to be continuous and relatively uniform across the surface of the TiO2 nanoparticles. Similar ALD processes have been shown to produce uniform coatings on a TiO2 substrate in previous experimentation.34−36 At the low ALD cycles, however, it is unclear whether the deposited material would provide complete coverage of the nanoparticle surface. It has been proposed that the initial deposition of material takes the form of an island growth.37 This nonuniform coating would influence the charge transfer at the nanoparticle surface, as there would still be contact between the TiO2 particles and the redox mediator. This may give rise to a higher degree of variation in electrochemical data for a film with a single ALD cycle of deposited material. Figure 4. Electron lifetimes as a function of the quasi-Fermi level for one to three cycles of ALD deposited alumina. Impedance measurements are shown in the circles, and IMVS measurements are shown in the triangles.

data and fits are presented in the Supporting Information in addition to information concerning the calculation of solar cell parameters. To determine the quasi-Fermi level of the TiO2 film, the potential drop across the series resistance, the Warburg impedance, and the counter electrode were subtracted from the applied voltage. The increase in the electron lifetime after successive ALD depositions of an alumina layer is in accordance with previous experimentation.22,23,25,40 This suppression of the recombination rate has also been shown in films with an alumina coating produced through sol−gel methods.36,41,42 The larger semiconductor band gap presented by the alumina coating produces a potential barrier that inhibits the transfer of electrons from the TiO2 film to the redox mediator. By stemming the leak of conductive electrons from the film, the electron concentration is increased and the quasiFermi level is pushed to more negative potentials. Figure 5 shows the (a) chemical capacitance and (b) recombination resistance of an untreated film determined through impedance measurements. The plot also describes the three types of electron states that were modeled to fit the experimental data. These states are the previously mentioned conduction band states, exponentially distributed traps, and monoenergetic traps. The modeling of the chemical capacitance and recombination resistance was performed in accordance with eqs 1, 2, 3, and 6. The chemical capacitance of the untreated film is characterized by two distinct regions. At the more negative potentials, the chemical capacitance is dominated by the trapped electron population. The higher concentrations of exponentially trapped electrons compared to conduction band electrons, at typical DSSC operating voltages, has been reported in previous experimentation.1,7,43 These reports are consistent with the larger contribution to the capacitance presented by trapped electrons compared to conduction band electrons, as seen in our work. In accordance with previous studies,8,9 we considered the trapped electron states to be located solely at the nanoparticle surface, and we did not consider the contributions of trap states within the bulk material. At the more positive potentials, the chemical capacitance is dominated by the capacitance of the monoenergetic traps. This dominant contribution of the monoenergetic traps results in the characteristic bump in the total chemical capacitance.

Figure 3. Current−voltage performance of the untreated and treated films. The table inset provides information about the open-circuit voltage, the short circuit current density, the fill factor, and the cell efficiency.

Current−Voltage Measurements. The photovoltaic performance of the untreated and Al2O3 treated films are shown in Figure 3. It is clear from the current−voltage data that each successive deposition of an alumina layer results in a small decrease in the device efficiency. Interestingly, this result differs from previous work using metal−oxide core−shell photoanodes.34−36,38 However, the decreasing efficiencies can be well qualified by similar effects outlined in previous literature.37,39 The drop in the device performance can be attributed to the decreasing short circuit current as a function of the higher coating thickness. While presenting a potential barrier for the back transfer of electrons from the TiO2, the alumina layer also presents a barrier to the injection of electrons from the excited dye species. The reduced injection rates lead to lower short circuit currents, which is reflected in our current−voltage data. The effect of the alumina barrier on the recombination dynamics is seen through the increasing open circuit voltage as a function of the coating thickness. The slower transfer rate of electrons from the TiO2 back to the electrolyte species bolsters the electron population within the film, which in turn raises the quasi-Fermi level and the VOC of the cell.35 However, this gain in the potential is not sufficient to offset the loss in the dye injection rate, and the total efficiency is compromised. Impedance and IMVS Lifetime Measurements. Electron lifetimes for the untreated film and the films with an Al2O3 ALD treatment are shown in Figure 4. Impedance and IMVS D



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Figure 6. (a) Chemical capacitance and (b) recombination resistance plots of impedance measurements (circles) and theoretical model (lines) for a film treated with three cycles of ALD deposited alumina.

Figure 5. (a) Chemical capacitance and (b) recombination resistance plots of impedance measurements (circles) and theoretical model (lines) for an untreated film. The orange, purple, and brown lines represent the contributions of the conduction band electrons, exponentially distributed trap states, and monoenergetic trap states, respectively. The green line shows the total chemical capacitance and recombination resistance due to the sum of the energy state contributions.

untreated and treated films does not provide a thorough indication of whether surface states have been passivated. It is true that if the trap states have been removed, and the density of traps has been reduced, then surface states would play a lesser role in the back transfer of electrons due to their diminished concentration. In this case there would be an observable difference between the chemical capacitance of a treated and untreated film. However, the fact that the untreated and treated films contain a similar concentration and distribution of traps does not tell us about how electrochemically active these trap states are. The main experimental difference between the treated and untreated films is evident in the plots of the recombination resistance. The higher recombination resistance for the treated film ultimately results in the higher observed lifetimes, as shown in Figure 4. We have already stated that the measured chemical capacitance for the treated and untreated films were nearly identical. This fundamentally means that when we compare the lifetimes of each film, we are comparing cells at equivalent electron concentrations and trap distributions. In this case the differences in the measured electron lifetimes can be solely attributed to the reduction in the recombination dynamics. The real motivation of this study, though, is to ascertain what the primary mechanisms are that cause this reduction in the electron transfer dynamics. As described in eq 7, the model for the total chemical capacitance and the total recombination resistance can be

Similarly, the plot of the recombination resistance in Figure 5b can be divided into distinct regions of operation. At the more negative potentials the recombination resistance is dominated by the recombination of conduction band electrons. Given the uncertainty in the impedance measurements at these potentials, the IMVS lifetime data were used in collaboration with the impedance results to produce a more accurate map of the lower potentials. The center region is controlled by the recombination resistance of the exponentially distributed traps. At the more positive potentials, the recombination resistance is determined by the monoenergetic states. Similarly to the chemical capacitance, the wedge shape contribution of the monoenergetic traps gives rise to the curved region seen in the total recombination resistance. Figure 6 shows the chemical capacitance and recombination resistance for a film with three ALD cycles of alumina. The regions that described the plots for the untreated film are seen again in the case of the alumina-treated film. Both treated and untreated films display a similar chemical capacitance, which was unsurprising, as the only modification to the films is the deposition of an insulating alumina layer. It is important to note that a comparison of the chemical capacitance between the E



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The application of a theoretical model describing charge transfer dynamics to fit our experimental data makes it possible to extract approximate values for the charge transfer rate from each energy state. These parameters are presented in Table 1.

combined to form a model for the electron lifetime. The plots of the electron lifetimes for the untreated and treated films, in conjunction with the theoretical models, are shown in Figure 7.

Table 1. Parameters Used for Chemical Capacitance, Recombination Resistance, and Lifetime Models parameter −3

Ncb (cm ) Nexp (cm−3) Nme (cm−3) T0 (K) Eme (eV) −1 kcb et,max (s ) −1 kexp et,max (s ) me ket,max (s−1)

untreated film 1.0 × 3.7 × 5.3 × 1200 0.61 1.6 × 1.2 × 4.2 ×

18

10 1018 1016

105 103 103

3 ALD cycles Al2O3 1.0 × 3.9 × 2.0 × 1150 0.56 5.5 × 4.0 × 1.4 ×

1018 1018 1016

104 102 103

The charge transfer models for each cell were calculated relative to a redox couple Fermi level of EF,redox = 0 eV. The conduction band energy level was defined as Ecb − EF,redox = 1 eV and the valence band energy level as Ev = −2.2 eV. Cobalt redox couples are reported to have relatively high internal reorganization energies, which is one of the defining features that makes them viable as outer-sphere DSSC electrolytes.48 For the purposes of the modeling, the total reorganization energy was derived from previous experimentation to be λ = 1.51 eV for a Co(bpy)33+/2+ redox couple.30 The more conventional iodide/triiodide system used in DSSCs has long displayed a successful balance between slow recombination kinetics at the photoanode and fast electron transfer at the counter electrode.49−51 The relatively slow recombination rates can be attributed to the multistep electron transfer process that is required for the reduction of the triiodide molecule. Yet, the precise reaction path of recombining electrons is not fully understood, and it is unknown what role each molecule plays in the back transfer from TiO2 trap states. For this reason, it is far simpler to understand the importance of trap state recombination through the employment of a single electron transfer redox mediator. The maximum electron transfer rates calculated for each grouping of energy states are shown in Table 2. By observing

Figure 7. Electron lifetimes for (a) an untreated film and (b) a film with three cycles of ALD deposited alumina. Impedance results are shown in the circles, and IMVS results are shown in the triangles. The modeled lifetime is presented in the green line.

Bisquert et al. have previously suggested that the parabolic region of the lifetime curve is caused by the recombination from surface states.44 Our model appears to account for this shape specifically through the influence of monoenergetic surface state recombination. It is true, however, that a reduction in the surface state recombination rate would result in the straightening of the lifetime curve, which has been reported by Bisquert and other groups.14,21,23,44 An important feature of the film fabrication that relates to the lifetime data is the use of the dense TiO2 blocking layer on the FTO substrate. Electron back-transfer from the substrate comprises a significant portion of the charge transfer rate, especially in the case of an outer-sphere redox couple.45−47 Existing work has suggested that the recombination directly from the FTO substrate can produce a similar characteristic shape in the electron lifetime as surface state recombination.25 If this is the case, a straightening of the lifetime curve may be incorrectly attributed to the passivation of surface states instead of a reduction in the substrate recombination rate. All films in used in our experiments contained a dense TiO2 blocking layer deposited through spray pyrolysis, which ensured the separation of the redox mediator and the FTO substrate.

Table 2. Maximum Electron Transfer Rates (ket,max) for Each Energy State ket,max (s−1) (rel change from untreated film) untreated film

1 ALD cycle Al2O3

conduction band

1.6 × 105

exponentially distrubuted traps monoenergetic traps

1.2 × 103

8.6 × 104 (−46%) 6.8 × 102 (−43%) 2.8 × 103 (−33%)

energy state

4.2 × 103

2 ALD cycles Al2O3 6.5 × 104 (−59%) 5.5 × 102 (−54%) 1.6 × 103 (−62%)

3 ALD cycles Al2O3 5.5 × 104 (−66%) 4.0 × 102 (−67%) 1.4 × 103 (−67%)

the relative change, compared to an untreated film, it is possible to see that the recombination rate is reduced evenly for all energy states due to the applied alumina barrier. This information implies that the alumina treatment does not preferentially passivate surface traps but affects recombination rates evenly across all energy states. These results contradict previous reports of selective surface state passivation.21−24 The F



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This result proved consistent with previous experimentation which illustrated the recombination blocking effects of the core−shell structure. By the application of a theoretical model used to describe the electron transfer process, it was found that the alumina coating suppressed recombination evenly from conduction band states, exponentially distributed traps, and monoenergetic traps. This conclusion does not match previous predictions that the reduced recombination rates were due to the selective passivation of electron traps located at the nanoparticle surface. It appears that the alumina coating purely serves as a tunneling barrier between conductive electrons and the redox mediator.

addition of the alumina coating acts as a tunneling barrier for electrons residing within the conduction band, at the exponentially distributed trap states, and in monoenergetic traps. It is evident from Table 2 that the surface traps play a considerably smaller role in recombination dynamics when compared to conduction band states. However, the relative comparison of electron transfer rates between energy states is highly dependent on the accuracy of the chemical capacitance fitting. This is not the case when comparing different films, as the chemical capacitance data are very similar between films, and differences in the recombination rates can be contrasted against a normalized chemical capacitance. When considering the absolute electron transfer rate of the conduction band electrons, it is necessary to accurately fit the chemical capacitance of the conduction band states. The capacitance of these states is only dominant at the very negative potentials, which is where there is considerable overlap between impedance elements. Therefore, it is difficult to produce an accurate fit of the conduction band capacitance, which affects the comparison of relative charge transfer rates within a single film. To gain greater confidence in the fitting of the conduction band electron transfer, our experimental results were compared to previous reports of charge transfer using outer-sphere redox couples. Previous literature has calculated the maximum electron transfer constant through current−voltage measurements in single crystal electrodes.30 To avoid confusion between the electron transfer constant ket (s−1) denoted in our work, the expression below shows the electron transfer rate e tr (cm 4 s −1 ) as determined through current density calculations. J(E) = −qetr[A]nc

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ASSOCIATED CONTENT

S Supporting Information *

Information concerning the calculation and interpretation of impedance and IMVS data as well the interpretation of previous electron charge transfer rates. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y.-B.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Doug Mair of the Melbourne Centre for Nanofabrication for assisting with the preparation of the working electrodes. We also acknowledge the Monash Centre for Electron Microscopy and the Victorian Organic Solar Cell Consortium for allowing us access to their facilities. Additionally, we thank the Solar Institute of Australia and the Australian Government Department of Industry, Innovation, Climate Change, Science, Research and Tertiary Education for their financial support.

(8)

In eq 8, J(E) refers to the current density and [A] is the acceptor concentration. Values for the maximum electron transfer rate etr,max calculated through the current voltage method are in the order of 10−17−10−16 cm4 s−1.30 Using the modification for a nanoporous film and the concentration Co3+ in our electrolyte (see Supporting Information), the previously determined etr,max is calculated to be approximately ket,max ≈ 7 × 104−7 × 105 (s−1). This value is in accordance with our experimental data, as shown in Table 2. The significance of these results is that film modifications should not be tailored to selectively passivate the surface of the nanoparticle, but rather DSSC materials should be chosen to maximize the separation between TiO2 electrons and the redox mediator. This point is exemplified through the incorporation of a recombination barrier into the dye structure to distance conductive electrons from oxidized electrolyte molecules.29,52−54 This design provides an elegant solution to limiting recombination dynamics without greatly compromising dye injection rates. It is this sort of initiative that has proven a catalyst for extending existing DSSC efficiencies beyond the 12% barrier.53

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REFERENCES

(1) Bailes, M.; Cameron, P. J.; Lobato, K.; Peter, L. M. Determination of the Density and Energetic Distribution of Electron Traps in Dye-Sensitized Nanocrystalline Solar Cells. J. Phys. Chem. B 2005, 109, 15429−15435. (2) Fisher, A. C.; Peter, L. M.; Ponomarev, E. A.; Walker, A. B.; Wijayantha, K. G. U. Intensity Dependence of the Back Reaction and Transport of Electrons in Dye-Sensitized Nanocrystalline TiO2 Solar Cells. J. Phys. Chem. B 2000, 104, 949−958. (3) Bisquert, J.; Zaban, A.; Salvador, P. Analysis of the Mechanisms of Electron Recombination in Nanoporous TiO2 Dye-Sensitized Solar Cells. Nonequilibrium Steady-State Statistics and Interfacial Electron Transfer via Surface States. J. Phys. Chem. B 2002, 106, 8774−8782. (4) Peter, L. M.; Ponomarev, E. A.; Franco, G.; Shaw, N. J. Aspects of the Photoelectrochemistry of Nanocrystalline Systems. Electrochim. Acta 1999, 45, 549−560. (5) Fabregat-Santiago, F.; Mora-Sero, I.; Garcia-Belmonte, G.; Bisquert, J. Cyclic Voltammetry Studies of Nanoporous Semiconductors. Capacitive and Reactive Properties of Nanocrystalline TiO2 Electrodes in Aqueous Electrolyte. J. Phys. Chem. B 2003, 107, 758−768. (6) Peter, L. “Sticky Electrons” Transport and Interfacial Transfer of Electrons in the Dye-Sensitized Solar Cell. Acc. Chem. Res. 2009, 42, 1839−1847. (7) Peter, L. M. Characterization and Modeling of Dye-Sensitized Solar Cells. J. Phys. Chem. C 2007, 111, 6601−6612. (8) Kopidakis, N.; Neale, N. R.; Zhu, K.; van de Lagemaat, J.; Frank, A. J. Spatial Location of Transport-Limiting Traps in TiO 2

5. CONCLUSION Core−shell structures were successfully created through the deposition of an alumina coating on a nanoporous TiO2 film by ALD. This coated layer was well characterized through electron microscopy and energy dispersive X-ray spectroscopy and was shown to be approximately 1 nm in thickness after five cycles of the ALD process. The deposited alumina coating on the TiO2 film resulted in an increase in the measured electron lifetimes. G



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Nanoparticle Films in Dye-Sensitized Solar Cells. Appl. Phys. Lett. 2005, 87, 202106(1)−202106(3). (9) Mercado, C. C.; Knorr, F. J.; McHale, J. L.; Usmani, S. M.; Ichimura, A. S.; Saraf, L. V. Location of Hole and Electron Traps on Nanocrystalline Anatase TiO2. J. Phys. Chem. C 2012, 116, 10796− 10804. (10) Zhu, K.; Kopidakis, N.; Neale, N. R.; van de Lagemaat, J.; Frank, A. J. Influence of Surface Area on Charge Transport and Recombination in Dye-Sensitized TiO2 Solar Cells. J. Phys. Chem. B 2006, 110, 25174−25180. (11) Franco, G.; Gehring, J.; Peter, L. M.; Ponomarev, E. A.; Uhlendorf, I. Frequency-Resolved Optical Detection of Photoinjected Electrons in Dye-Sensitized Nanocrystalline Photovoltaic Cells. J. Phys. Chem. B 1999, 103, 692−698. (12) Schlichthörl, G.; Park, N. G.; Frank, A. J. Evaluation of the Charge-Collection Efficiency of Dye-Sensitized Nanocrystalline TiO2 Solar Cells. J. Phys. Chem. B 1999, 103, 782−791. (13) Wang, H.; He, J.; Boschloo, G.; Lindström, H.; Hagfeldt, A.; Lindquist, S.-E. Electrochemical Investigation of Traps in a Nanostructured TiO2 Film. J. Phys. Chem. B 2001, 105, 2529−2533. (14) Ondersma, J. W.; Hamann, T. W. Measurements and Modeling of Recombination from Nanoparticle TiO2 Electrodes. J. Am. Chem. Soc. 2011, 133, 8264−8271. (15) Gonzalez-Vazquez, J. P.; Oskam, G.; Anta, J. A. Origin of Nonlinear Recombination in Dye-Sensitized Solar Cells: Interplay between Charge Transport and Charge Transfer. J. Phys. Chem. C 2012, 116, 22687−22697. (16) Villanueva-Cab, J.; Wang, H.; Oskam, G.; Peter, L. M. Electron Diffusion and Back Reaction in Dye-Sensitized Solar Cells: The Effect of Nonlinear Recombination Kinetics. J. Phys. Chem. Lett. 2010, 1, 748−751. (17) Villanueva-Cab, J.; Oskam, G.; Anta, J. A. A Simple Numerical Model for the Charge Transport and Recombination Properties of Dye-Sensitized Solar Cells: A Comparison of Transport-Limited and Transfer-Limited Recombination. Sol. Energy Mater. Sol. Cells 2010, 94, 45−50. (18) Bisquert, J.; Vikhrenko, V. S. Interpretation of the Time Constants Measured by Kinetic Techniques in Nanostructured Semiconductor Electrodes and Dye-Sensitized Solar Cells. J. Phys. Chem. B 2004, 108, 2313−2322. (19) Ansari-Rad, M.; Abdi, Y.; Arzi, E. Reaction Order and Ideality Factor in Dye-Sensitized Nanocrystalline Solar Cells: A Theoretical Investigation. J. Phys. Chem. C 2012, 116, 10867−10872. (20) Bisquert, J.; Fabregat-Santiago, F.; Mora-Sero, I.; GarciaBelmonte, G.; Gimenez, S. Electron Lifetime in Dye-Sensitized Solar Cells: Theory and Interpretation of Measurements. J. Phys. Chem. C 2009, 113, 17278−17290. (21) Fabregat-Santiago, F.; García-Cañ adas, J.; Palomares, E.; Clifford, J. N.; Haque, S. A.; Durrant, J. R.; Garcia-Belmonte, G.; Bisquert, J. The Origin of Slow Electron Recombination Processes in Dye-Sensitized Solar Cells with Alumina Barrier Coatings. J. Appl. Phys. 2004, 96, 6903−6907. (22) Hamann, T. W.; Farha, O. K.; Hupp, J. T. Outer-Sphere Redox Couples as Shuttles in Dye-Sensitized Solar Cells. Performance Enhancement Based on Photoelectrode Modification via Atomic Layer Deposition. J. Phys. Chem. C 2008, 112, 19756−19764. (23) DeVries, M. J.; Pellin, M. J.; Hupp, J. T. Dye-Sensitized Solar Cells: Driving-Force Effects on Electron Recombination Dynamics with Cobalt-Based Shuttles. Langmuir 2010, 26, 9082−9087. (24) Li, T. C.; Goes, M. S.; Fabregat-Santiago, F.; Bisquert, J.; Bueno, P. R.; Prasittichai, C.; Hupp, J. T.; Marks, T. J. Surface Passivation of Nanoporous TiO2 via Atomic Layer Deposition of ZrO2 for SolidState Dye-Sensitized Solar Cell Applications. J. Phys. Chem. C 2009, 113, 18385−18390. (25) Ondersma, J.; Hamann, T. Impedance Investigation of DyeSensitized Solar Cells Employing Outer-Sphere Redox Shuttles. J. Phys. Chem. C 2010, 114, 638−645.

(26) Bisquert, J. Chemical Capacitance of Nanostructured Semiconductors: Its Origin and Significance for Nanocomposite Solar Cells. Phys. Chem. Chem. Phys. 2003, 5, 5360−5364. (27) Bisquert, J. Theory of the Impedance of Charge Transfer via Surface States in Dye-Sensitized Solar Cells. J. Electroanal. Chem. 2010, 646, 43−51. (28) Marcus, R. On the Theory of Oxidation-Reduction Reactions Involving Electron Transfer. J. Chem. Phys. 1956, 24, 966−978. (29) Clifford, J. N.; Palomares, E.; Nazeeruddin, M. K.; Gratzel, M.; Nelson, J.; Li, X.; Long, N. J.; Durrant, J. R. Molecular Control of Recombination Dynamics in Dye-Sensitized Nanocrystalline TiO2 films: Free Energy vs Distance dependence. J. Am. Chem. Soc. 2004, 126, 5225−5233. (30) Hamann, T. W.; Gstrein, F.; Brunschwig, B. S.; Lewis, N. S. Measurement of the Dependence of Interfacial Charge-Transfer Rate Constants on the Reorganization Energy of Redox Species at n-ZnO/ H2O Interfaces. J. Am. Chem. Soc. 2005, 127, 13949−13954. (31) Wang, Q.; Moser, J. E.; Gratzel, M. Electrochemical Impedance Spectroscopic Analysis of Dye-Sensitized Solar Cells. J. Phys. Chem. B 2005, 109, 14945−14953. (32) Bisquert, J. Theory of the Impedance of Electron Diffusion and Recombination in a Thin Layer. J. Phys. Chem. B 2002, 106, 325−333. (33) van de Lagemaat, J.; Park, N. G.; Frank, A. J. Influence of Electrical Potential Distribution, Charge Transport, and Recombination on the Photopotential and Photocurrent Conversion Efficiency of Dye-Sensitized Nanocrystalline TiO2 Solar Cells: A Study by Electrical Impedance and Optical Modulation Techniques. J. Phys. Chem. B 2000, 104, 2044−2052. (34) Ganapathy, V.; Karunagaran, B.; Rhee, S.-W. Improved Performance of Dye-Sensitized Solar Cells with TiO2/Alumina Core−Shell Formation Using Atomic Layer Deposition. J. Power Sources 2010, 195, 5138−5143. (35) Lin, C.; Tsai, F.-Y.; Lee, M.-H.; Lee, C.-H.; Tien, T.-C.; Wang, L.-P.; Tsai, S.-Y. Enhanced Performance of Dye-Sensitized Solar Cells by an Al2O3 Charge-Recombination Barrier Formed by LowTemperature Atomic Layer Deposition. J. Mater. Chem. 2009, 19, 2999−3003. (36) Liberatore, M.; Burtone, L.; Brown, T. M.; Reale, A.; Di Carlo, A.; Decker, F.; Caramori, S.; Bignozzi, C. A. On the Effect of Al2O3 Blocking Layer on the Performance of Dye Solar Cells with Cobalt Based Electrolytes. Appl. Phys. Lett. 2009, 94, 73113−73113. (37) Antila, L. J.; et al. ALD Grown Aluminum Oxide Submonolayers in Dye-Sensitized Solar Cells: The Effect on Interfacial Electron Transfer and Performance. J. Phys. Chem. C 2011, 115, 16720−16729. (38) Chandiran, A. K.; Tetreault, N.; Humphry-Baker, R.; Kessler, F.; Baranoff, E.; Yi, C.; Nazeeruddin, M. K.; Gratzel, M. Subnanometer Ga2O3 Tunnelling Layer by Atomic Layer Deposition to Achieve 1.1 V Open-Circuit Potential in Dye-Sensitized Solar Cells. Nano Lett. 2012, 12, 3941−3947. (39) Antila, L. J.; Heikkilä, M. J.; Aumanen, V.; Kemell, M.; Myllyperkiö, P.; Leskelä, M.; Korppi-Tommola, J. E. I. Suppression of Forward Electron Injection from Ru(dcbpy)2(NCS)2 to Nanocrystalline TiO2 Film as a Result of an Interfacial Al2O3 Barrier Layer Prepared with Atomic Layer Deposition. J. Phys. Chem. Lett. 2010, 1, 536−539. (40) Brennan, T. P.; Bakke, J. R.; Ding, I. K.; Hardin, B. E.; Nguyen, W. H.; Mondal, R.; Bailie, C. D.; Margulis, G. Y.; Hoke, E. T.; Sellinger, A.; McGehee, M. D.; Bent, S. F. The Importance of Dye Chemistry and TiCl4 Surface Treatment in the Behavior of Al2O3 Recombination Barrier Layers Deposited by Atomic Layer Deposition in Solid-State Dye-Sensitized Solar Cells. Phys. Chem. Chem. Phys. 2012, 14, 12130−12140. (41) Palomares, E.; Clifford, J. N.; Haque, S. A.; Lutz, T.; Durrant, J. R. Control of Charge Recombination Dynamics in Dye Sensitized Solar Cells by the Use of Conformally Deposited Metal Oxide Blocking Layers. J. Am. Chem. Soc. 2003, 125, 475−482. (42) O’Regan, B. C.; Scully, S.; Mayer, A. C.; Palomares, E.; Durrant, J. The Effect of Al2O3 Barrier Layers in TiO2/Dye/CuSCN Photovoltaic Cells Explored by Recombination and DOS CharacterH



Article

ization Using Transient Photovoltage Measurements. J. Phys. Chem. B 2005, 109, 4616−4623. (43) Duffy, N. W.; Peter, L. M.; Rajapakse, R. M. G.; Wijayantha, K. G. U. A Novel Charge Extraction Method for the Study of Electron Transport and Interfacial Transfer in Dye Sensitised Nanocrystalline Solar Cells. Electrochem. Commun. 2000, 2, 658−662. (44) Bisquert, J.; Zaban, A.; Greenshtein, M.; Mora-Sero, I. Determination of Rate Constants for Charge Transfer and the Distribution of Semiconductor and Electrolyte Electronic Energy Levels in Dye-Sensitized Solar Cells by Open-Circuit Photovoltage Decay Method. J. Am. Chem. Soc. 2004, 126, 13550−13559. (45) Cameron, P. J.; Peter, L. M. How Does Back-Reaction at the Conducting Glass Substrate Influence the Dynamic Photovoltage Response of Nanocrystalline Dye-Sensitized Solar Cells? J. Phys. Chem. B 2005, 109, 7392−7398. (46) Cameron, P. J.; Peter, L. M.; Hore, S. How Important is the Back Reaction of Electrons via the Substrate in Dye-Sensitized Nanocrystalline Solar Cells? J. Phys. Chem. B 2005, 109, 930−936. (47) Cameron, P. J.; Peter, L. M. Characterization of Titanium Dioxide Blocking Layers in Dye-Sensitized Nanocrystalline Solar Cells. J. Phys. Chem. B 2003, 107, 14394−14400. (48) Hamann, T. W. The End of Iodide? Cobalt Complex Redox Shuttles in DSSCs. Dalton Trans. 2012, 41, 3111−3115. (49) Boschloo, G.; Hagfeldt, A. Characteristics of the Iodide/ Triiodide Redox Mediator in Dye-Sensitized Solar Cells. Acc. Chem. Res. 2009, 42, 1819−1826. (50) Montanari, I.; Nelson, J.; Durrant, J. R. Iodide Electron Transfer Kinetics in Dye-Sensitized Nanocrystalline TiO2 Films. J. Phys. Chem. B 2002, 106, 12203−12210. (51) J. Frank, A.; Kopidakis, N.; Lagemaat, J. v. d. Electrons in Nanostructured TiO2 Solar Cells: Transport, Recombination and Photovoltaic Properties. Coord. Chem. Rev. 2004, 248, 1165−1179. (52) Liu, Y. R.; Jennings, J. R.; Huang, Y.; Wang, Q.; Zakeeruddin, S. M.; Gratzel, M. Cobalt Redox Mediators for Ruthenium-Based DyeSensitized Solar Cells: A Combined Impedance Spectroscopy and Near-IR Transmittance Study. J. Phys. Chem. C 2011, 115, 18847− 18855. (53) Yella, A.; Lee, H. W.; Tsao, H. N.; Yi, C.; Chandiran, A. K.; Nazeeruddin, M. K.; Diau, E. W.; Yeh, C. Y.; Zakeeruddin, S. M.; Gratzel, M. Porphyrin-Sensitized Solar Cells with Cobalt (II/III)Based Redox Electrolyte Exceed 12% Efficiency. Science 2011, 334, 629−634. (54) Feldt, S. M.; Gibson, E. A.; Gabrielsson, E.; Sun, L.; Boschloo, G.; Hagfeldt, A. Design of Organic Dyes and Cobalt Polypyridine Redox Mediators for High-Efficiency Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2010, 132, 16714−16724.

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Surface State Recombination and Passivation in Nanocrystalline TiO2

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