Impact of Local Electric Fields on Charge-Transfer Processes at the

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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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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:

14 15 16


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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

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∆ = − ∙

∙

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.

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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-

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]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

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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

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region of the fittings. The electron density is calculated based on the laser intensity, film thickness and film

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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),

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respectively. In the inert electrolyte, the decay of the oxidize dyes is related to the charge

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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

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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

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observed kinetics. (The original spectra together with the spectra for other cations are shown in

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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:

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+,.- = +, ∗ /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

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in the case of the inert and redox electrolyte for four cations (Figure 3c and 3d, respectively),

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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

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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

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that the decay processes are now dominated by the regeneration processes by the redox mediator,

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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

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such a case. Nevertheless, the fitted results of t0 can still serve as a quantitative indicator of the

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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.


Na

+

,

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Figure 4. Relationship between the strength of the relative electric field with (a) the charge recombination time

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constants (b) regeneration time constants. The dash line represents a linear fitting. The error in the y-axis is the

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standard derivation taken from the fitting, while the error in the x-axis is estimated as 10 % of the normalized

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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

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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

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field and the recombination time constant t0 (Figure 4a); the strongest electric field in the case of

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Ca2+ also results in the shortest time constant, namely the fastest recombination kinetics. On the

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contrast, the t0 of the regeneration processes demonstrates, however, little dependence on the

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electric field strength. In another word, these results imply that the spatial confinement of the

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electric field across the dye layer has a strong effect on the charge transfer processes in-between,

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i.e. the charge recombination processes. Its impact on the dye regeneration process is, however,

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still required more investigation. The later consideration has also been implied by recent studies,

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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.

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Figure 5. Calculated dye regeneration efficiency of LEG4/TiO2 films versus the concentration of TiO2(e-) for 0.1M

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Li+/Na+/Mg2+/Ca2+ perchlorate salts in the case of the cobalt electrolyte (0.22 M Co(bpy)3(PF6)2, 0.05 M

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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

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by the state-of-art cobalt bipyridyl redox mediator at different concentration of TiO2(e-) by the

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equation 328 (Figure 5). The regeneration efficiency is found to decrease significantly when [e-]

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increases, which conclusion is in agreement to the previous studies where the [e-] is controlled by

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use of a electrochemical potentiostat.29,30 Significantly, the nature of cations is found to have

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strong impact on the regeneration efficiency of the oxidized dye, in the order of 4 > 4 >

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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

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facilitating dye injection21,32,33.

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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

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theory.24,34 Meanwhile, the vibrational coupling of sensitizer-TiO2 has been recently reported to

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change upon application of an electric field on the basis of Raman spectroscopy measurements.35

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This change of coupling could also impact on the interaction between TiO2 and dye cations, and

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therefore affect the interfacial electron transfer kinetics. By contrast, the electron transfer

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processes occurring farther away from the interface, such as the dye regeneration process

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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

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environment.15 Recently, there is an emerging awareness of the role of the local electric field at

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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

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transfer processes.

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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

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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

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Corresponding Author

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[email protected]; [email protected]

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ACKNOWLEDGMENT

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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

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Impact of Local Electric Fields on Charge-Transfer Processes at the

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