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Letter
Impact of Local Electric Fields on Charge Transfer Processes at the TiO2/Dye/Electrolyte Interface Wenxing Yang, Nick Vlachopoulos, and Gerrit Boschloo ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00568 • Publication Date (Web): 14 Dec 2016 Downloaded from http://pubs.acs.org on December 16, 2016
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ACS Energy Letters
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Impact of Local Electric Fields on Charge Transfer
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Processes at the TiO2/Dye/Electrolyte Interface Wenxing Yang*†, Nick Vlachopoulos‡, Gerrit Boschloo*†
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†Department of Chemistry, Ångström Laboratory, Uppsala University, Box 523, SE 75120
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Uppsala, Sweden;
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‡Laboratory of Photomolecular Science, Institute of Chemical Science and Engineering, Ecole
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Polytechnique Fédérale de Lausanne, EPFL-FSB-ISIC-LSPM, Chemin des Alambics, Station 6,
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CH-1015 Lausanne, Switzerland.
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Email:
[email protected]; Phone: +46 76 16 47221.
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[email protected]; Phone: +46 18 4713303.
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ABSTRACT
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Photoinduced electron transfer processes at the TiO2/dye/electrolyte interface are vital for
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various emerging technologies. Here, the impact of the local electric field at this interface on the
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charge transfer processes was investigated in two aspects: a) charge recombination between the
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electrons accumulated within TiO2 and the photoxidized dye; b) regeneration of the dyes by the
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cobalt bipyridyl redox mediators. The amplitude of the local electric field was changed by use of
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different cations in the electrolytic environment, in the order of ECa2+> EMg2+> ENa+> ELi+
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characterized by the transient absorption spectroscopy. For the charge recombination process, the
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kinetic time constant showed a remarkable linear correlation with the relative electric field
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strength, while for the regeneration process, no evident dependence was observed. These results
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collectively suggest the spatial confinement of the effects of the local electric field on the
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interfacial electron transfer processes.
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Graphical abstract:
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In the past decades, sensitization of nanocrystalline semiconductors by chromophores has
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attracted enormous research interest in the solar energy related research, i.e. dye-sensitized solar
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cells (DSSCs) and water-splitting devices.1–4 Such dye-sensitized mesoporous oxide
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semiconductor assemblies take advantages of the efficient light harvesting by the anchored
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sensitizers, as well as the desirable electronic properties of the base mesoporous semiconductor.
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Further development of the solar energy conversion devices based on these concepts require a
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more throughout mechanistic understanding of parameters affecting the interfacial electron
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transfer processes.5 Recently, the presence of the local electric field at the TiO2/dye/electrolyte
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interface was characterized and elucidated (Scheme 1a), which is generated by the photoinjected
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electrons within TiO2 (TiO2(e-)) and the surrounding cations at the interface.6,7 This local electric
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field was characterized by the shift of the absorption spectrum of the dye through a so-called
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Stark effect (Scheme 1b)8,9, which predicts that the change of the absorption spectrum, ∆A1st, due
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to the electric field can be expressed into equation 16,10
14
∆ = − ∙
∙
µ∙
(1)
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where dA/dλ is 1st derivative of the absorption spectrum of dye in its ground state with respect to
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the wavelength in the absence of the electric field, ∆µ is the difference of the dipole moment
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between the ground and excited state of the dye, ∆E is the vector of the change of local electric
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field, h is Plank’s constant and c is the speed of light.
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Therefore, this spectral change can serve as a unique optical probe to determine the strength of
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the local electric field after electron accumulation within TiO2. The strength of this local electric
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field was previously established to be in the order of MV cm-1,10–13 and has shown strong
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dependence on the nature of the cations in the surrounding electrolyte, reported in the order of
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ENa+< ELi+< EMg2+< ECa2+ according to the previous studies on ruthenium dye-sensitized TiO2.12
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Scheme 1. (a) Pictorial description of the local electric field (generated by the electrons
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accumulated within TiO2) and the related electron transfer reaction (process 1 and 2) investigated
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in the present study (b) Effects of the local electric field on the absorption spectrum of the dye,
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i.e. Stark effect. The red line schematically shows the difference in the absorption spectrum,
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∆A1st, caused by the electric field, as predicted by the equation 2.
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Considering the strong strength of the local electric field and its confinement of dye
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molecules within it, a vital question arises about how the charge transfer processes across the
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TiO2/dye/electrolyte interface will be affected by the presence of this field. The answer to this
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question is imperative in unrevealing mechanisms in control of related interfacial electron
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transfer processes. Previous studies have been conducted to investigate the effects of the local
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electric field on the charge recombination kinetics between the TiO2(e-) and the mobile electron
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acceptor in the electrolyte.14,15 However, it is still unknown how the charge recombination
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between TiO2(e-) and the oxidized dye after charge separation would be affected by the presence
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of the local electric field with respect to their stronger coupling and proximity. Moreover, the
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efficient regeneration process of the oxidized dye molecules is important to drive the reaction at
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the electrolyte side; its dependence on the cations is not clear either, in spite of its extreme
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importance for the successful operation of devices.
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Herein, we aim to investigate the role of the local electric field in the determination of: a) the
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charge recombination between the TiO2(e-) and the oxidized dyes, referred as the charge
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recombination process (Scheme 1a); b) the regeneration processes of the redox couple by a state-
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of-art cobalt bipyridyl redox mediator16, referred as the regeneration process, which resulted to a
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remarkable 13.6% power conversion efficiency in DSSCs.17Due to the large dependence of
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kinetic constants on the TiO2 (e-) on these sensitized-TiO2 assemblies18, studies are therefore
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conducted spanning a large range of TiO2 (e-) densities. A correlation between electron transfer
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kinetics and the relative amplitude of the local electric field was made and provides new insights.
14 15
Figure 1. (a) Molecular structure of the dye, LEG4, used in the present study (b) Steady-state UV-Vis absorption
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spectra of LEG4-sensitized TiO2 films on a fluorine-doped tin oxide (FTO) glasses in pure ACN and in 0.1 M
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Li+/Na+/Mg2+/Ca2+ perchlorate salts in ACN. The spectral modelling of the individual spectrum was shown in the
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Figure S1 to estimate the maxima. The baselines were taken from the bare TiO2 film on FTO glass in ACN.
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The dye used in the present study is 3-{6-{4-[bis(2’,4’-dibutyloxybiphenyl-4-yl)amino-
4
]phenyl}-4,4-dihexyl-cyclopenta-[2,1-b:3,4-b’]dithiophene-2-yl}-2-cyanoacrylic
acid
(named
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LEG4 and shown in Figure 1a), with a typical Donor--Acceptor structure. The broad absorption
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of the dye and blocking effects of the alkyl group on the triphenylamine donor part have led to its
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development of efficient DSSCs.19 It has been previously characterized20 and have spectroscopic
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features suitable for the present study (vide infra). Shown in Figure 1b are the UV-Vis absorption
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spectra of LEG4-sensitized TiO2 films, abbreviated LEG4/TiO2, in pure acetonitrile (ACN) and
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in 0.1 M Li+/Na+/Mg2+/Ca2+ perchlorate salts in ACN. The absorption peak of LEG4/TiO2 was
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found to be gradually red-shifted in the presence of a cationic environment compared to the
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spectrum measured in neat ACN: ACN(479 nm) < Li+(484 nm) < Na+(489 nm) < Mg2+(494
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nm)< Ca2+(498 nm). This spectrum shift is associated with the change of the local electric field
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upon cations adsorption onto the meso-TiO2 film.10,12 The larger red-shift of the absorption
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spectrum in the presence of divalent cations compared with monovalent cations is in consistency
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with the previous study on a ruthenium dye-sensitized TiO2 films.12
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Figure 2. Transient absorption spectra of LEG4/TiO2 films in 0.1 M Li+/Na+/Mg2+/Ca2+ perchlorate salts in ACN
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after ~ 10 ns laser excitation. (a) The TAS in the case of 0.1 M Ca2+ was shown as an example of the analysis. The
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measured TAS (Blue solid line) consists of two spectral contributions: (i) the change of the absorption spectrum of
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LEG4/TiO2 in the case of electrochemical oxidation (∆Asp, Black dash line, oxidized at 0.8 V vs NHE, ELEG4 =1.1 V
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vs NHE) and (ii) the Stark effect contribution (∆A1st, Red dash line, i.e. the first derivative of the UV-Vis spectrum
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of LEG4/TiO2 in ACN). The ∆Asp and ∆A1st spectra were taken from a previous publication20 and red-shifted ~ 15-
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20 nm in order to achieve the best simulation. Solid cyan line: the linear summation of the black and red line with
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appropriate coefficients. (b) Comparison of the normalized TAS for different cations after normalizing at ∆A = 800
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nm. The solid lines represent the data smoothed by averaging 5 adjacent data points, with the original data plotted
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with 50 % transparency. The noise represents around ~ 10 % error in the determination of ∆A at around 580 nm.
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Laser intensity: ~5 mJ/pulse. TAS delay time: ~10 ns.
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The transient absorption spectroscopy (TAS) was used here to demonstrate the presence of the
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Stark effect at the TiO2/dye/electrolyte interface after electron accumulation in TiO2, and to
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characterize the relative strength of the electric field tuned by the presence of different cations in
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the electrolyte (Figure 2). The laser excitation of the LEG4/TiO2 films results in rapid electron
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injection into TiO2 within ps to fs21 and therefore creates charge separated states with
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distinguishable spectral features. Herein, we use the transient spectrum of LEG4/TiO2 films
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immersed in 0.1 M Ca2+ solution in ACN (after ~ 10 ns) as an example to assign these spectral
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features (Figure 2a, other cations have similar results and not included for clarity). The transient
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absorption spectra of LEG4/TiO2 films, were demonstrated to consist of three contributions: the
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ground state dye bleach due to laser excitation (∆As, ~ 520 nm), the oxidized LEG4+ absorption
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due to electron injection into TiO2 (∆As+, ~ 670 nm and 770 nm20), and the Stark effect due to
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accumulation of injected electrons within TiO2 (∆A1st, ~ 580 nm), with the relationship of: ∆A =
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∆As+ - ∆As + ∆A1st. The former two can be measured by electrochemically scanning the
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LEG4/TiO2 film in the anodic direction, with the dye molecules oxidizing through a hole-
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hopping mechanism22, resulting in a difference spectrum (∆Asp ) contributed from both ∆As and
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∆As+: ∆Asp = ∆As+ - ∆As. The latter one can be taken from 1st-order derivative of the UV-Vis
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absorption spectrum of LEG4/TiO2, as dictated by the Equation 1. Figure 2a shows that the
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spectral simulation based on ∆Asp and ∆A1st indeed qualitatively reproduce the spectral features of
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the measured transient absorption spectrum. The mismatch at below 500 nm was caused by the
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high absorption of the samples in that range, which results in low light transmittance and limits
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the detection. The relative shift of the spectrum, necessary for better simulation of the spectrum,
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is not clear at the moment, and could be due to the sample difference in the present and previous
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studies. Nevertheless, these spectral analyses are sufficient to conclude that the two peaks at 670
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nm and 770 nm in TAS are attributed to the absorption of the oxidized LEG4, while the peak at
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around 580 nm is contributed from both the ground-state dye bleach and the Stark bleach. These
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arguments were further confirmed by the fact that in the presence of the redox mediator, a
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residual Stark feature was observed in the TAS after the regeneration of the dye by the redox
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mediator (Figure S2). A recent global analysis of the TAS spectrum in the decay-associated
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spectra has also led to a similar conclusion.23
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In order to characterize the electric field at TiO2/dye/electrolyte interface tuned by the cations in
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the electrolyte after electron accumulation, the transient absorption spectra of LEG4/TiO2 were
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measured in different cationic environments and normalized at 800 nm (Figure 2b). This
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normalization ensures the equal spectral contribution from the oxidized dyes to the bleach at 580
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nm. The remaining difference of the bleach at 580 nm can therefore indicate the relatively
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strength of the local electric field after the charge separation (equation 1, ∆A ∝ ∆E), which is
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found in the order of > > > . This result is consistent with the previous
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study12, where the local electric field, corresponding to approximate 20 electrons per TiO2
2
particle, conforms to a similar order of
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1.3 "#$%& ' > 1.1 "#$%& '. The slight difference in the order of Na+ and Li+
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suggests that the electric field may be also influenced by the dye structures, as previously
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suggested by A. Zaban et.al.24 Considering the spectral overlay between the ground state bleach
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and the Stark bleach here, it is not straightforward to determine the absolute strength of the local
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electric field according to the equation 1. Therefore, in the following discussion, only the relative
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electric field strength is used by normalizing each of the residual signals at 580 nm to that of the
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case Ca2+. However, this simplification shall be sufficient for the scope of the present discussion.
2.2 "#$%& ' > 1.8 "#$%& ' >
10
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Figure 3. Normalized kinetic decays at 750 nm, characteristic absorption from the oxidized LEG4, for LEG4/TiO2
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at different light intensities in contact with (a) the inert electrolyte consisting of 0.1 M LiClO4 in ACN. (b) the redox
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electrolyte consisting of 0.1 M LiClO4, 0.22 M Co(bpy)3(PF6)2, 0.05 M Co(bpy)3(PF6)3 in ACN. The solid line in (a)
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and (b) was fitted with the KWW function but only used to get the half-time of the decay (The results for other
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cations are shown in the supporting information.). (c) and (d) represent the plot of the acquired half-times from
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above decays as a function of the electron density in TiO2 for different cations environments in the inert electrolytes
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and redox electrolytes, respectively. The error in the half time reading at different light intensity was estimated to be
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~ 10%. The inset graphs show the corresponding electron transfer process investigated. In the case of the redox
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electrolyte, the kinetics is dominated by the charge regeneration process (solid line), due to its faster kinetics
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compared to the recombination process (dash line). The fitting curves between TiO2 (e-) and t0.5 in (c) and (d) are
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based on Equation 2, with fitting results summarized in Table 1. The shaded areas represent the 95% confidence
13
region of the fittings. The electron density is calculated based on the laser intensity, film thickness and film
14
absorbance at the excitation wavelength (shown in the Supporting Information).
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To investigate the effect of the local electric field on the charge transfer reactions, we measured
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the kinetic decay of ∆A at 750 nm, corresponding to the characteristic absorption from the
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oxidized LEG4, at different concentration of TiO2(e-) in the presence of the inert electrolyte (0.1
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M Li+/Na+/Mg2+/Ca2+ perchlorate salts in ACN) and the redox electrolyte (0.1 M
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Li+/Na+/Mg2+/Ca2+ perchlorate salts, 0.22 M Co(bpy)3(PF6)2, 0.05 M Co(bpy)3(PF6)3 in ACN),
20
respectively. In the inert electrolyte, the decay of the oxidize dyes is related to the charge
21
recombination process between the TiO2(e-) and oxidized dye. In the redox electrolyte, the decay
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of the oxidize dyes represents both the charge recombination process as well as the dye
23
regeneration by the redox mediator. Due to the fact that the dye regeneration is much faster than
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the recombination processes, the kinetics can be approximated to be contributed only from the
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dye regeneration (vide infra). Figure 3a and 3b show the normalized decay of the inert and redox
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Li+ electrolyte as an example to demonstrate the effects of the electron concentration on the
2
observed kinetics. (The original spectra together with the spectra for other cations are shown in
3
Figure S4 and Figure S5.) Quantification of these decays were achieved by fitting the original
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decay traces with a previous widely-used KWW function, however, for the sole purpose to
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obtain the decay half-time, t0.5, defined as the time required for a decay to half of its initial value.
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The extracted t0.5 from the individual decay curve is then plotted as a function of the
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concentration of TiO2 (e-) ([e-]) shown in Figure 3c and 3d for the inert and redox electrolyte,
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respectively. In the case of the inert electrolyte, t0.5 decreased dramatically with the increase of
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electron densities in TiO2 for all the cations. This dependence between [e-] and t0.5 has been
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previously found to follow equation 225,26, and explained by a transport-limited recombination
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model. According to this model, the electrons are suggested to trap and detrap several times
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before reaching the recombination center26,27:
13
+,.- = +, ∗ /0 & 1&2
(2)
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where β could be interpreted as a characteristic parameter of the multiple-trapping model in
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nanocrystalline TiO226,27 and +, is a characteristic constant of the recombination kinetics.
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The curve fitting in the Figure 3c confirms the above power-law relationship between the t0.5
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measured individually and the corresponding [e-], with the fitting results summarized in Table 1.
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Because of the small difference between β, t0 can therefore serve as a characteristic parameter to
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describe the decay kinetics for four cations regardless of [e-]; t0 is found to have strong
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dependence on the cations, in the order of t0,Ca2+< t0,Mg2+< t0,Na+< t0,
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cationic dependence of the recombination processes.
Li
+
, indicating the strong
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Table 1. Fitting results of the decay half-time of ∆A at 750 nm at different light intensities both
2
in the case of the inert and redox electrolyte for four cations (Figure 3c and 3d, respectively),
3
according to the equation 2. Li+
Na+
Mg2+
Ca2+
t0,µs
28.0±2.4∗
23.6±2.2
18.9±0.8
7.3±2.0
β
0.66±0.03
0.66±0.03
0.63±0.02
0.55±0.08
t0,µs
1.1±0.1
0.4±0.0
1.4±0.3
0.9±0.0
β
0.20±0.02
0.10±0.02
0.15±0.06
0.17±0.02
Inert electrolyte:
Redox electrolyte:
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∗Errors are the standard derivation of the fittings.
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Similarly, Figure 3d shows the relationship between the t0.5 of the oxidized dye at 750 nm in the
6
presence of a redox electrolyte and [e-]. By comparison with the inert electrolytes, the t0.5 decay
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of the regeneration process show little dependence on the light intensity, which is due to the fact
8
that the decay processes are now dominated by the regeneration processes by the redox mediator,
9
which is independent of the light intensity. Only a slight reduction of the time constant was
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observed at high concentration of TiO2 (e-), contributed from the charge recombination process.
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Similar data fittings with the equation 2 were also performed to data in Figure 4b as above.
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However, it should be noted that there is no real physical model corresponding to this fitting in
13
such a case. Nevertheless, the fitted results of t0 can still serve as a quantitative indicator of the
14
decay kinetics (Table 1) at the different TiO2(e-), in the order of t0,Mg2+ ≥ t0,Li+≥ t0,Ca2+ ≥ t0,
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which implies that the regeneration processes have no distinguishable dependence on the cationic
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environments.
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+
,
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Figure 4. Relationship between the strength of the relative electric field with (a) the charge recombination time
3
constants (b) regeneration time constants. The dash line represents a linear fitting. The error in the y-axis is the
4
standard derivation taken from the fitting, while the error in the x-axis is estimated as 10 % of the normalized
5
electric field.
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To understand the cation-dependent charge-transfer processes mentioned above, we further
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correlate the relative electric field (by normalizing the bleach at 580 nm of different cations to
8
that of Ca2+ in Figure 2, with ~ 10% estimated errors) and the time constant t0 in the table 1.
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Remarkably, a linear relationship emerges between the relatively strength of the local electric
10
field and the recombination time constant t0 (Figure 4a); the strongest electric field in the case of
11
Ca2+ also results in the shortest time constant, namely the fastest recombination kinetics. On the
12
contrast, the t0 of the regeneration processes demonstrates, however, little dependence on the
13
electric field strength. In another word, these results imply that the spatial confinement of the
14
electric field across the dye layer has a strong effect on the charge transfer processes in-between,
15
i.e. the charge recombination processes. Its impact on the dye regeneration process is, however,
16
still required more investigation. The later consideration has also been implied by recent studies,
17
where the recombination between TiO2(e-) and mobile electron acceptor in the electrolytic
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environment has been described either by the electric field effects14 or by thermodynamic
2
effects15.
3 4
Figure 5. Calculated dye regeneration efficiency of LEG4/TiO2 films versus the concentration of TiO2(e-) for 0.1M
5
Li+/Na+/Mg2+/Ca2+ perchlorate salts in the case of the cobalt electrolyte (0.22 M Co(bpy)3(PF6)2, 0.05 M
6
Co(bpy)3(PF6)3 in ACN)
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Based on these results, we further calculated the regeneration efficiency (4) of the oxidized dyes
8
by the state-of-art cobalt bipyridyl redox mediator at different concentration of TiO2(e-) by the
9
equation 328 (Figure 5). The regeneration efficiency is found to decrease significantly when [e-]
10
increases, which conclusion is in agreement to the previous studies where the [e-] is controlled by
11
use of a electrochemical potentiostat.29,30 Significantly, the nature of cations is found to have
12
strong impact on the regeneration efficiency of the oxidized dye, in the order of 4 > 4 >
13
4 > 4 ; this cationic dependent regeneration efficiencies could explain the difference of
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the DSSCs performance reported previously31 based on different cations besides the effects of
15
facilitating dye injection21,32,33.
16
4=5
5678 678 9567:
;. Δ? > Δ? in the present study, which would therefore result
7
in faster kinetics if the electron transfer occurs within the normal region of the Marcus
8
theory.24,34 Meanwhile, the vibrational coupling of sensitizer-TiO2 has been recently reported to
9
change upon application of an electric field on the basis of Raman spectroscopy measurements.35
10
This change of coupling could also impact on the interaction between TiO2 and dye cations, and
11
therefore affect the interfacial electron transfer kinetics. By contrast, the electron transfer
12
processes occurring farther away from the interface, such as the dye regeneration process
13
discussed here, show little dependence on the local electric field. The rapid screening of the
14
electric field outside the dye layer would allow dye regeneration to occur in a field-free
15
environment, or alternatively, in a more complicated electric field and thermodynamic
16
environment.15 Recently, there is an emerging awareness of the role of the local electric field at
17
these widely used TiO2/dye/electrolyte interfaces, e.g. in DSSCs6,7,36–38 and dye sensitized
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photoelectrosynthesis cells for water splitting.39 These findings are therefore of importance to be
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taken into consideration for the future mechanistic study of the related interfacial electron
20
transfer processes.
21 22
ASSOCIATED CONTENT
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1
Supporting Information. Experimental sections and data treatment details; Comparison of the
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kinetic traces measured at 750 nm by the oscilloscope with 10 µs, 100 µs and 1000 µs; Spectral
3
modelling of the UV-Vis absorption spectra in Figure 1b; Transient absorption spectra of LEG4-
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senitized TiO2 films on FTO in contact with 0.1 M LiClO4 Redox electrolyte after different time
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delay; Original and normalized kinetic decays at 750 nm in the case of inert and redox electrolyte
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at different light intensities. Calculation of the electron density within the TiO2 (e-);
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AUTHOR INFORMATION
8
Corresponding Author
9
[email protected];
[email protected] 10
ACKNOWLEDGMENT
11
Luca D'Amario, ShiHuai Wang and Dr. Leif Häggman are thanked for helpful discussions and
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kind help. Brian N. DiMarco (University of North Carolina at Chapel Hill) is acknowledged for
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sharing his expertise for the laser measurements. We gratefully acknowledge the Swedish
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Energy Agency, the Swedish Research Council (VR) and the STandUP for Energy program for
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financial support. Wenxing Yang sincerely acknowledges the Chinese Scholarship Council
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(CSC) for a PhD study fellowship.
17
REFERENCES
18
(1)
Youngblood, W. J.; Lee, S.-H. A.; Maeda, K.; Mallouk, T. E. Visible Light Water
19
Splitting Using Dye-Sensitized Oxide Semiconductors. Acc. Chem. Res. 2009, 42, 1966–
20
1973.
21
(2)
Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. Dye-Sensitized Solar Cells.
ACS Paragon Plus Environment
16
Page 17 of 22
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Energy Letters
1 2
Chem. Rev. 2010, 110, 6595–6663. (3)
Hu, K.; Blair, A. D.; Piechota, E. J.; Schauer, P. A.; Sampaio, R. N.; Parlane, F. G. L.;
3
Meyer, G. J.; Berlinguette, C. P. Kinetic Pathway for Interfacial Electron Transfer from a
4
Semiconductor to a Molecule. Nat. Chem. 2016, 8, 853–859.
5
(4)
6 7
Farnum, B. H.; Wee, K.-R.; Meyer, T. J. Self-Assembled Molecular p/n Junctions for Applications in Dye-Sensitized Solar Energy Conversion. Nat. Chem. 2016, 8, 845–852.
(5)
Kamat, P. V. Manipulation of Charge Transfer Across Semiconductor Interface. A
8
Criterion That Cannot Be Ignored in Photocatalyst Design. J. Phys. Chem. Lett. 2012, 3,
9
663–672.
10
(6)
Cappel, U. B.; Feldt, S. M.; Schöneboom, J.; Hagfeldt, A.; Boschloo, G. The Influence of
11
Local Electric Fields on Photoinduced Absorption in Dye-Sensitized Solar Cells. J. Am.
12
Chem. Soc. 2010, 132, 9096–9101.
13
(7)
Ardo, S.; Sun, Y.; Staniszewski, A.; Castellano, F. N.; Meyer, G. J. Stark Effects after
14
Excited-State Interfacial Electron Transfer at Sensitized TiO2 Nanocrystallites. J. Am.
15
Chem. Soc. 2010, 132, 6696–6709.
16
(8)
17
Liptay, W. Electrochromism and Solvatochromism. Angew. Chemie Int. Ed. English 1969, 8, 177–188.
18
(9)
Boxer, S. G. Stark Realities. J. Phys. Chem. B 2009, 113, 2972–2983.
19
(10)
Yang, W.; Hao, Y.; Vlachopoulos, N.; Eriksson, A. I. K.; Boschloo, G. Studies on the
20
Interfacial Electric Field and Stark Effect at the TiO2 /Dye/Electrolyte Interface. J. Phys.
ACS Paragon Plus Environment
17
ACS Energy Letters
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1 2
Chem. C 2016, 120, 22215–22224. (11)
3 4
Page 18 of 22
O’Donnell, R. M.; Ardo, S.; Meyer, G. J. Charge-Screening Kinetics at Sensitized TiO2 Interfaces. J. Phys. Chem. Lett. 2013, 4, 2817–2821.
(12)
O’Donnell, R. M.; Sampaio, R. N.; Barr, T. J.; Meyer, G. J. Electric Fields and Charge
5
Screening in Dye Sensitized Mesoporous Nanocrystalline TiO2 Thin Films. J. Phys.
6
Chem. C 2014, 118, 16976–16986.
7
(13)
Sampaio, R. N.; Li, G.; Meyer, G. J. Continuous Surface Electric Field Contraction
8
Accompanying Electron Transfer from TiO2 to Oxidized Sensitizers. ACS Energy Lett.
9
2016, 1, 846–851.
10
(14)
Sampaio, R. N.; O’Donnell, R. M.; Barr, T. J.; Meyer, G. J. Electric Fields Control TiO2
11
(e– ) + I3– → Charge Recombination in Dye-Sensitized Solar Cells. J. Phys. Chem. Lett.
12
2014, 5, 3265–3268.
13
(15)
DiMarco, B. N.; O’Donnell, R. M.; Meyer, G. J. Cation-Dependent Charge
14
Recombination to Organic Mediators in Dye-Sensitized Solar Cells. J. Phys. Chem. C
15
2015, 119, 21599–21604.
16
(16)
Feldt, S. M.; Gibson, E. a; Gabrielsson, E.; Sun, L.; Boschloo, G.; Hagfeldt, A. Design of
17
Organic Dyes and Cobalt Polypyridine Redox Mediators for High-Efficiency Dye-
18
Sensitized Solar Cells. J. Am. Chem. Soc. 2010, 132, 16714–16724.
19 20
(17)
Mathew, S.; Yella, A.; Gao, P.; Humphry-Baker, R.; Curchod, B. F. E.; Ashari-Astani, N.; Tavernelli, I.; Rothlisberger, U.; Nazeeruddin, M. K.; Grätzel, M. Dye-Sensitized Solar
ACS Paragon Plus Environment
18
Page 19 of 22
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Energy Letters
1
Cells with 13% Efficiency Achieved through the Molecular Engineering of Porphyrin
2
Sensitizers. Nat. Chem. 2014, 6, 242–247.
3
(18)
Palomares, E.; Clifford, J. N.; Haque, S. a; Lutz, T.; Durrant, J. R. Control of Charge
4
Recombination Dynamics in Dye Sensitized Solar Cells by the Use of Conformally
5
Deposited Metal Oxide Blocking Layers. J. Am. Chem. Soc. 2003, 125, 475–482.
6
(19)
Gabrielsson, E.; Ellis, H.; Feldt, S.; Tian, H.; Boschloo, G.; Hagfeldt, A.; Sun, L.
7
Convergent/Divergent Synthesis of a Linker-Varied Series of Dyes for Dye-Sensitized
8
Solar Cells Based on the D35 Donor. Adv. Energy Mater. 2013, 3, 1647–1656.
9
(20)
Yang, W.; Pazoki, M.; Eriksson, A. I. K.; Hao, Y.; Boschloo, G. A Key Discovery at the
10
TiO2/dye/electrolyte Interface: Slow Local Charge Compensation and a Reversible
11
Electric Field. Phys. Chem. Chem. Phys. 2015, 17, 16744–16751.
12
(21)
Koops, S. E.; O’Regan, B. C.; Barnes, P. R. F.; Durrant, J. R. Parameters Influencing the
13
Efficiency of Electron Injection in Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2009,
14
131, 4808–4818.
15
(22)
Bonhôte, P.; Gogniat, E.; Tingry, S.; Barbé, C.; Vlachopoulos, N.; Lenzmann, F.; Comte,
16
P.; Grätzel, M. Efficient Lateral Electron Transport inside a Monolayer of Aromatic
17
Amines Anchored on Nanocrystalline Metal Oxide Films. J. Phys. Chem. B 1998, 102,
18
1498–1507.
19
(23)
Ward, C. L.; O’Donnell, R. M.; DiMarco, B. N.; Meyer, G. J. Kinetic Resolution of
20
Charge Recombination and Electric Fields at the Sensitized TiO2 Interface. J. Phys. Chem.
21
C 2015, 119, 25273–25281.
ACS Paragon Plus Environment
19
ACS Energy Letters
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1
(24)
2 3
(25)
(26)
Nelson, J.; Chandler, R. E. Random Walk Models of Charge Transfer and Transport in Dye Sensitized Systems. Coord. Chem. Rev. 2004, 248, 1181–1194.
(27)
8 9
Durrant, J. R.; Haque, S. A.; Palomares, E. Towards Optimisation of Electron Transfer Processes in Dye Sensitised Solar Cells. Coord. Chem. Rev. 2004, 248, 1247–1257.
6 7
Zaban, A.; Ferrere, S.; Gregg, B. a. Relative Energetics at the Semiconductor/Sensitizing Dye/Electrolyte Interface. J. Phys. Chem. B 1998, 102, 452–460.
4 5
Page 20 of 22
Nelson,
J.
Continuous-Time
Random-Walk
Model
of
Electron
Transport
in
Nanocrystalline TiO2 Electrodes. Physical Review B, 1999, 59, 15374–15380. (28)
Feldt, S. M.; Wang, G.; Boschloo, G.; Hagfeldt, A. Effects of Driving Forces for
10
Recombination and Regeneration on the Photovoltaic Performance of Dye-Sensitized
11
Solar Cells Using Cobalt Polypyridine Redox Couples. J. Phys. Chem. C 2011, 115,
12
21500–21507.
13
(29)
Robson, K. C. D.; Hu, K.; Meyer, G. J.; Berlinguette, C. P. Atomic Level Resolution of
14
Dye Regeneration in the Dye-Sensitized Solar Cell. J. Am. Chem. Soc. 2013, 135, 1961–
15
1971.
16
(30)
Haque, S. A.; Tachibana, Y.; Klug, D. R.; Durrant, J. R. Charge Recombination Kinetics
17
in Dye-Sensitized Nanocrystalline Titanium Dioxide Films under Externally Applied Bias.
18
J. Phys. Chem. B 1998, 102, 1745–1749.
19 20
(31)
Wang, H.; Peter, L. M. Influence of Electrolyte Cations on Electron Transport and Electron Transfer in Dye-Sensitized Solar Cells. J. Phys. Chem. C 2012, 116, 10468–
ACS Paragon Plus Environment
20
Page 21 of 22
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Energy Letters
1 2
10475. (32)
Jennings, J. R.; Wang, Q. Influence of Lithium Ion Concentration on Electron Injection,
3
Transport, and Recombination in Dye-Sensitized Solar Cells. J. Phys. Chem. C 2010, 114,
4
1715–1724.
5
(33)
Li, R.; Liu, D.; Zhou, D.; Shi, Y.; Wang, Y.; Wang, P. Influence of the Electrolyte Cation
6
in Organic Dye-Sensitized Solar Cells: Lithium versus Dimethylimidazolium. Energy
7
Environ. Sci. 2010, 3, 1765.
8
(34)
9 10
Anderson, N. A.; Lian, T. Ultrafast Electron Transfer at the Molecules-Semiconductor Nanoparticle Interface. Annu. Rev. Phys. Chem. 2005, 56, 491–519.
(35)
Sevinc, P. C.; Dhital, B.; Govind Rao, V.; Wang, Y.; Lu, H. P. Probing Electric Field
11
Effect on Covalent Interactions at a Molecule-Semiconductor Interface. J. Am. Chem. Soc.
12
2016, 138, 1536–1542.
13
(36)
Saavedra Becerril, V.; Franchi, D.; Abrahamsson, M. Ionic Liquid-Induced Local Charge
14
Compensation: Effects on Back Electron-Transfer Rates in Dye-Sensitized TiO 2 Thin
15
Films. J. Phys. Chem. C 2016, 120, 20016–20023.
16
(37)
Oum, K.; Lohse, P. W.; Flender, O.; Klein, J. R.; Scholz, M.; Lenzer, T.; Du, J.;
17
Oekermann, T. Ultrafast Dynamics of the Indoline Dye D149 on Electrodeposited ZnO
18
and Sintered ZrO2 and TiO2 Thin Films. Phys. Chem. Chem. Phys. 2012, 14, 15429–
19
15437.
20
(38)
Pastore, M.; Angelis, F. D. F. Computational Modeling of Stark Effects in Organic Dye-
ACS Paragon Plus Environment
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1 2
Page 22 of 22
Sensitized TiO2 Heterointerfaces. J. Phys. Chem. Lett. 2011, 2, 1261–1267. (39)
Song, W.; Luo, H.; Hanson, K.; Concepcion, J. J.; Brennaman, M. K.; Meyer, T. J.
3
Visualization of Cation Diffusion at the TiO2 Interface in Dye Sensitized
4
Photoelectrosynthesis Cells (DSPEC). Energy Environ. Sci. 2013, 6, 1240.
5
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