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Influence of Dye Architecture of Triphenylamine Based Organic Dyes on the Kinetics in Dye-Sensitized Solar Cells Hanna Ellis, Ina Schmidt, Anders Hagfeldt, Gunther Wittstock, and Gerrit Boschloo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b04436 • Publication Date (Web): 02 Sep 2015 Downloaded from http://pubs.acs.org on September 8, 2015

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Influence of Dye Architecture of Triphenylamine Based Organic Dyes on the Kinetics in DyeSensitized Solar Cells Hanna Ellis,a⊥ Ina Schmidt,b⊥ Anders Hagfeldt,a, c, d Gunther Wittstock,b* Gerrit Boschlooa*

a) Physical Chemistry, Centre of Molecular Devices, Department of Chemistry – Ångström Laboratory, Uppsala University, SE-75120 Uppsala, Sweden

b) Carl von Ossietzky University of Oldenburg, Faculty for Mathematics and Natural Science, Center of Interface Science, Institute of Chemistry, D-26111 Oldenburg, Germany

c) École Polytechnique Fédérale de Lausanne, Laboratory of Photomolecular Science, Institute of Chemical Sciences and Engineering, EPFL-FSB-ISIC-LSPM, Station 6, CH-1015 Lausanne, Switzerland

d) Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah 21589, Saudi Arabia ⊥ Authors share first authorship

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ABSTRACT The impact of the dye architecture on the kinetics in the dye-sensitized solar cell (DSC) was investigated for two structurally similar organic dyes, adsorbed to a mesoporous TiO2 film. Differences in the HOMO and LUMO levels of the triphenylamine dyes D35 and D45 were negligible, indicating that the changes in kinetics of the electron transfer processes in the solar cells can be attributed to structural differences of the organic dyes. The electron transfer kinetics of various processes was investigated by scanning electrochemical microscopy (SECM), transient absorption spectroscopy (TAS) and impedance spectroscopy (IS). SECM was used for the first time to determine the rate constants of the regeneration (reduction) of a photooxidized organic dye by a one-electron cobalt mediator. Both TAS and IS measurements showed differences in recombination of electrons in TiO2 with oxidized D35 and D45. D45 with its shorter di-methoxyphenyl units yielded faster recombination and regeneration than D35, as measured by SECM and TAS. The results of this study show that small details in the dye structure significantly affect the kinetics of organic triphenylamine dye based dyesensitized solar cells.

Key words: dye-sensitized solar cells, all organic dye, cobalt mediator, scanning electrochemical microscopy, transient absorption spectroscopy, electrochemical impedance spectroscopy


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INTRODUCTION Dye-sensitized solar cells (DSCs) have attained a lot of academic and industrial attention, since they can serve as a low-cost alternative to existing photovoltaic technologies for converting solar energy to electricity.1-2 The standard DSC consists of a photoanode, usually a mesoporous TiO2 electrode sensitized with a dye, an electrolyte containing a redox couple, and a counter electrode with a catalyst for accelerating electron transfer reactions. All components can be made from abundant elements using low cost production processes. Generation of voltage by photoelectrochemical systems has been known since 1839 when Becquerel reported his pioneer experiments with metal halide salts.3 A breakthrough for the DSC came in 1991 when Grätzel and O’Regan reported a cell with 7.1% efficiency,4 using sensitized TiO2 nanoparticles and an iodide/triiodide electrolyte (I-/I3-). This electrolyte came to dominate the DSC field. However, the I-/I3- system is far from ideal. The cell voltage is limited due to the relatively large difference between its redox potential and the redox potential of the dye. Redox electrolytes based on cobalt complexes have been considered as alternatives because of their lower absorption coefficient compared to I-/I3-. Additionally, the redox potential can be varied by the design of the ligands allowing to fine-tune the driving force to find the best compromise with respect to minimizing internal loss potential and sufficient driving force for the reduction of the photooxidized dye.5 The iodide system is also more corrosive than cobalt electrolytes.6-9 Besides redox reactions (which is required for any redox mediator), iodide can promote the breakdown of passive oxide layers and the oxidative dissolution of noble metals (e.g. from the counter electrode) due to its ability to complex noble metal ions. Furthermore, iodine and iodide may enter into organic reactions with electrolyte components, sealing and so forth. A breakthrough in the use of alternative redox electrolytes was achieved only after a tailored combination of a cobalt bipyridine redox couple and an organic dye with bulky donor units had been found.10-11 This illustrates the ACS Paragon Plus 3Environment


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importance of dye design on the kinetics in the DSC and thus on DSC performance paving way for further record efficiencies.12 The current world record for a liquid-based DSC is 13% using a porphyrin dye with a cobalt based redox electrolyte.13 As the dye design is of great importance for the performance of DSCs,14-16 many different kinds of dyes are explored2 and new dyes continue to be developed.17 For optimum performance, the dye should absorb as much light as possible, thus the spectral absorption range needs to be extended towards long wavelengths. Besides this requirement, dye design should also favour all forward reaction steps (blue arrows in Figure 1) and inhibit loss processes (grey arrows in Figure 1). After absorption (abs), electrons are injected (inj) from the excited dye molecule D* into the conduction band (CB) of the semiconductor (TiO2), in which they are transported to the back contact of the photoelectrode, and cause a current in the external circuit. During transport in the mesoporous TiO2 electrons undergo multiple trapping-detrapping events in trap states that appear to have an exponential distribution below the conduction band edge (Figure 1), as has been extensively studied by Bisquert and others.18 The current loop is closed at the counter electrode where the oxidized form of the redox electrolyte is reduced (not shown in Figure 1). The reduced redox mediator is transported by diffusion and migration to the photoanode where the dye is regenerated by electron transfer from the reduced form of the redox electrolyte to the photooxidized dye D+ (reg). There are a number of loss processes that decrease the efficiency of the DSC: D* can be deactivated to D without an electron injection into the CB (rec3 in Figure 1), electrons in TiO2 may be transferred back to the photooxidized dye (rec2) or to the oxidized form of the redox electrolyte (rec1). The kinetics of the latter two processes are complex. A second order rate law would be expected for rec2 if only CB electrons would be involved (Figure 1). However, it may deviate from this value due to effects of electron trapping and detrapping.19-21 The recombination kinetics will then depend not only on the energetic difference between acceptor and donor, but also on the trap state distribution. ACS Paragon Plus 4Environment

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Figure 1. Energy levels of the D35 and D45 dyes and the recombination and regeneration processes present in the DSC. The forward reactions are indicated by blue arrows while the recombination reactions (or backward reactions) are shown by grey arrows. The energy levels and redox potentials as determined below are given on the NHE scale. In this study two structurally very similar organic dyes, D35 and D45, are studied as sensitizers in DSC with a cobalt bipyridine redox electrolyte. The two dyes differ in the length of the di-alkoxyphenyl group on the triphenylamine unit (Figure 2). Specifically, the kinetics of processes reg, rec1, and rec2 are investigated by complementary techniques, namely transient absorption spectroscopy (TAS), impedance spectroscopy (IS) and scanning electrochemical microscopy (SECM). It is revealed how a slight change in molecular structure of the sensitizer has a marked effect on electron transfer kinetics and performance in dyesensitized solar cells.

Figure 2. The organic triphenylamine dyes D35 (left) and D45 (right). ACS Paragon Plus 5Environment


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EXPERIMENTAL SECTION Materials. Detergent solution (RBS 25 from Fluka analytical), ethanol (VWR DBH Prolabo 99.9% purity), acetonitrile (anhydrous 99.8%, Sigma-Aldrich), 4-tert-butylpyridine (TBP, 96% Aldrich), LiClO4 (battery grade, dry, 99.99%, Aldrich), TiO2 pastes (DSL 30NR-D, Dyesol), scattering paste (WER2-O reflector, Dyesol), 1 M solution of tetrabutylammonium hydroxide (TBAOH) in methanol (Sigma-Aldrich, for desorption measurements) were used as received. Electrolytes for SECM measurements were prepared immediately before use from tetrabutylammonium hexafluorophosphate (TBAPF6, ≥99.0 %, Sigma-Aldrich, Steinheim, used without further purification stored in an oven at 80°C for at least 24 h prior to prepare electrolyte solutions) and acetonitrile (HPLC grade, VWR International, Darmstadt, Germany) stored above freshly regenerated 3 Å molecular sieve. The redox mediators cobalt(II)-trisbipyridine hexafluorophosphate [Co(bpy)3](PF6)2 and cobalt(III)-trisbipyridine hexafluorophosphate [Co(bpy)3](PF6)3 as well as organic triphenylamine dyes D35 and D45 were synthesized according to published procedures.22-24 Identification of the redox electrolyte by NMR is documented in the Supporting Information (SI-1), Figure SI-1. Fabrication of DSCs. Working electrodes were prepared according to procedure published by our group22 with the difference being the pastes used. For the absorption layer Dyesol DSL 30NR-D was used and for the scattering layer Dyesol WER2-O reflector. The pastes were used as received. The total thickness of the mesoporous TiO2 working electrodes was 7 µm for TAS and IS measurements and 19 µm for SECM measurements. For details of fabrication of DSCs see SI-2. Surface coverage measurements. After sensitization, one electrode from each batch were immersed in 0.1 M TBAOH in ethanol for 1 h, after which the absorption spectra of ACS Paragon Plus 6Environment

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dissolved dye were measured with a UV-VIS NIR spectrometer (DH-2000-BAL from Mikropack and HR2000 spectrometer from Ocean Optics). The dye loadings were determined from Lambert-Beer’s law with molar extinction coefficients for D35 and D45 obtained from solution of known concentration in the desorption solution (SI-3, Table SI-1). DSC characterization. Current-voltage (I-V) measurements were carried out with a source meter (Keithley 2400) and a solar simulator (model 91160, Newport). The light intensity was calibrated with a certified reference solar cell (Fraunhofer ISE, Freiberg, Germany) to an intensity of 1000 W m-2. For the I-V-measurements a mask of 0.4×0.4 cm2 was used in order to avoid significant additional contribution from light falling on the device outside the active area (0.5×0.5 cm2). Determination of energy levels. Absorption spectra were recorded by a UV-VIS-NIR Spectrometer (Varian Cary 5000). Fluorescence spectra were measured with a Fluorolog 1650 0.2 m double spectrometer (SPEX industries, Edison, NJ) in the right angle mode. Both absorption and fluorescence spectra were normalized and the intercepts were taken as E0-0 gaps (SI-3, Figure SI-3). The position of the E°'(D/D+) level on an electrochemical scale was determined as the formal potential of the dye sensitized on TiO2 from cyclic voltammograms (CVs) in 0.1 M LiClO4 in acetonitrile in the dark measured with a potentiostat (IviumStat.XR, Ivium Technologies) in three electrode-arrangement of the sensitized TiO2 film electrodes as working electrode, a Ag|10 mM AgNO3 reference electrode and a graphite rod auxiliary electrode (SI-3, Figure SI-2). Transient absorption measurements. An ns-laser (Quanta Ray, Spectra Physics) in combination with an Edinburgh Instruments LP920 spectrometer was used for TAS experiments, see SI-4 for details. The excitation pump intensity (532 nm) was 0.28 mJ pulse-1 cm-2 (results from other pulse intensities 0.18, 0.14 and 0.12 mJ pulse-1 cm-2 are reported in SI-5, Figure SI-5 and Figure SI-6). The pump intensity was adjusted so the numbers of photo-



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injected electrons and oxidized dyes were close to the same for both dyes, D35 and D45, in all measurements (see SI-4 and Table SI-3). The probe wavelength (880 nm) was chosen according to the oxidized dyes’, D35+ and D45+ (collectively referred to as D+), absorption spectra (SI-5, Figure SI-4). 2000 averages were taken for every signal. When measuring the recombination (rec2 and rec3 in Figure 1) the electrolyte was 0.2 M TBP and 0.1 M LiClO4 in acetonitrile, referred to as stock solution. For the regeneration (reg) and recombination (rec1, rec2 and rec3 in Figure 1), solutions of 0.2 M, 0.1 M or 0.05 M [Co(bpy)3](PF6)2 with 0.2 M TBP and 0.1 M LiClO4 in acetonitrile were used. The time constants obtained from the double exponential decay fitting were used for calculating logarithmic weighted averages τlog (SI-4 and Equation SI-2). Separated recombination and regeneration rate constants were also calculated (SI-4 and Equation SI-3). Impedance measurements. A computer controlled electrochemical instrument (AutoLab PGSTAT100) with a frequency response analyzer was used to record impedance spectra on complete DSCs in the dark and at 1 sun (1000 W m-2). In order to have the same Fermi-levels giving the same driving force for recombination in the D35 and D45 systems, the same potential was applied to both systems during measurements at 1 sun irradiation and in dark. The selected applied potential was 95% of the average VOC-values of both systems. The recombination resistance Rrec was determined from the diameter of the second semicircle in the Nyquist plots (SI-9, Figure SI-13). The electron lifetime τe is an average time an injected electron remains in the TiO2 before recombining to oxidized dye molecules, or to the oxidized form of the redox electrolyte. τe was calculated from the frequency, f, at the peak value of the second semicircle by Equation 1.

=

(1)

Scanning electrochemical microscopy. The SECM measurements were performed on a home build instrument25-27 similar to procedures published by our group for I-/I3ACS Paragon Plus 8Environment

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mediator.26,

28

For handling the cobalt mediator, an Au microelectrode (ME) was used. Its

preparation and further modifications of previous procedures are detailed in the SI-6.

RESULTS AND DISCUSSION DSC characteristics. There is a significant difference in the power conversion efficiency (PCE) for solar cells with D35-sensitized or D45-sensitized photoanodes and a cobalt-based redox electrolyte (Table 1 and Figure 3 for the best solar cells with poly(3,4ethylenedioxythiophene) (PEDOT) counter electrodes). The most prominent reason for the difference in PCE is the VOC (0.86 V for D35 vs. 0.80 V for D45). The difference for the platinized (Pt) counter electrodes is similar (0.86 V for D35 vs. 0.81 V for D45). The shortcircuit current density values do not differ significantly for DSCs with the PEDOT counter electrode. For the Pt counter electrodes slightly larger currents are observed for DSCs with D35-sensitized photoanodes. The larger difference in PCE between the DSCs with the two dyes for Pt counter electrodes (15%) compared to DSCs with PEDOT counter electrodes (9%) is attributed to the difference in charge transfer resistance for the reduction reaction of the [Co(bpy)3]3+/2+ redox couple at the counter electrode resulting in different fill factors (FF).29 The change of dye does not affect the fill factor (FF) in the DSC, and the difference in FF is not significant (Table 1, D35-PEDOT FF = 0.72 and D45-PEDOT FF = 0.71).



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Figure 3. I-V-curves at 1 sun (solid line) and 0.32 sun (dashed line) of the master solar cells assembled with the D35 (black) or D45 (red) dye and PEDOT counter electrodes. Table 1. I-V-data of solar cells assembled with the D35 and D45 dyes at 1 sun intensity (1000 W m-2 AM1.5G) a VOC / V JSC / mA cm-2 FF PCE / % Dye CEb D35 PEDOT 0.86 ± 0.01 9.47 ± 0.26 0.72 ± 0.01 5.9 ± 0.2 D35 Pt 0.86 ± 0.01 9.29 ± 0.11 0.68 ± 0.01 5.4 ± 0.1 D45 PEDOT 0.80 ± 0.01 9.50 ± 0.09 0.71 ± 0.01 5.4 ± 0.1 D45 Pt 0.81 ± 0.01 8.62 ± 0.19 0.66 ± 0.02 4.6 ± 0.2 a) Arithmetic mean of three solar cells with standard deviation b) Catalyst material of the counter electrode

Energy levels. The HOMO and LUMO energy levels of the D35 and D45 systems are very similar (SI-3, Table SI-2 and Figure 1). The zero-zero electronic transition energy, E0-0, was obtained as the photon energy of the intercept between normalized absorption and fluorescence spectra (SI-3, Figure SI-3, Table SI-2). The energetic position of the E°'D/D+ on the electrochemical scale as given in Figure 1 was obtained from cyclic voltammetry of the dyes adsorbed on TiO2 (SI-3, Figure SI-2). There is only a slight difference in E°'D/D+ between D35 and D45, which is smaller than the measurement uncertainty. E°'D*/D+ was calculated as the sum of E°'D/D+ and E0-0 of the dyes. Since the energy levels of HOMO and LUMO are similar, the driving force for the reduction of D+ by the cobalt mediator, whose redox potential11 is also shown in Figure 1, will not be significantly different for the two dyes. Instead the chemical structure of the dyes should have a dominant influence. The different lengths of the di-alkoxyphenyl groups on the triphenylamine units may influence how close [Co(bpy)3]3+/2+ may approach the dye thus affecting the regeneration kinetics. The positive charge of D+ is located on the triphenylamine nitrogen.30-31 Bulkier substituents at the periphery of the dye, i.e. longer di-alkoxyphenyl groups, may slow down the regeneration by hindering the approach of the Co complex to the positive charge of D+. However, the influence on the DSC performance is likely to be more complicated, because the substituents are also expected to influence the kinetics of the recombination. This prompted us to undertake a detailed investigation of the kinetics in the DSC by TAS, SECM and IS. By TAS

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the regeneration kinetics (reg) and recombination kinetics (rec2) were obtained, while SECM will be sensitive to the combined effect of dye regeneration (reg), all recombination processes (rec1, rec2 and rec3) and internal transport processes in the mesoporous electrode. IS will provide recombination kinetics from TiO2 electrons to D+ (rec2) and the oxidized form of the redox electrolyte (rec1, Figure 1). Transient absorption measurements. Time constants for the decay of the oxidized dye absorption in transient absorption measurements have been used to derive rate constants for the regeneration process both for the cobalt redox couple32 and the iodide redox couple.33 In TAS the dye attached to the TiO2 is pumped to an excited state from which electrons are injected into the CB of TiO2. The probe light measures the absorption of D+ that is formed after injection. In the absence of redox mediator the concentration of D+ is decaying due to process rec2, recombination of electrons from TiO2 to the oxidized dye. Special care was taken in our experiments to ensure that the average number of electrons and oxidized dyes generated per TiO2-particle were very similar for the D35 and D45 systems (SI-5, Table SI-3). The electron–oxidized dye recombination was found to be clearly faster for D45 than for D35 (Figure 4a). In the presence of a redox couple the decay of the D+ concentration is much faster due to additional reduction of the oxidized dye by [Co(bpy)3]2+ (reg, Figure 4b). The regeneration process of D35 and D45 for the different concentrations of [Co(bpy)3](PF6)2 is shown in SI-5, Figure SI-7. To evaluate the kinetic processes by TAS in more detail, time constants are derived from a fitting procedure by taking half-lifetime, τ1/2, or lifetime values, τlife (at which the signal is reduced to 1/e of its initial value). In some studies a stretched exponential function was used, where the origin of stretched exponential behavior was attributed to an electron trapping/detrapping process in the TiO2 nanoparticles.21 Here we use a biexponential function, which yields equally good fit results, but is easier to apply.16 The fundamental reason for the ACS Paragon Plus11 Environment


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successful biexponential fit has not been fully clarified. It might, however, represent recombination of oxidized dye by CB electrons (fast) and by trapped electrons (slower process).34 The relative amplitudes and the time constants of the biexponential fit of the transient absorption traces are presented in Table 2. In order to qualitatively compare the kinetics, it is convenient to extract an effective lifetime constant of first order. This can be done by calculating a weighted average.16 However, weighted averages can sometimes be misleading, especially in cases when there is major difference between the time constants, and when the amplitude of the longer time constant is small. The slower process will contribute significantly even if the amplitude for this process is small. The logarithmic weighted average values, τlog (SI-4, Equation SI-2), can be more suitable.


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a)

b)

Figure 4. Transient absorption traces for D35 (black) and D45 (red) absorbed on mesoporous TiO2 in 0.2 M TBP and 0.1 M LiClO4 in acetonitrile in the absence (a) and presence (b) of 0.2 M [Co(bpy)3](PF6)2. Dotted lines show the measured signal and solid lines are fits using biexponential decay functions. Table 2. Biexponential fit parameters of the normalized transient absorption traces of D35 and D45-sensitized TiO2-films. Dye+[ Co2+] M A1 A2 τ1 /µs τ2 /µs D35+stock 0.474 69.6 0.526 764 D35+0.05 0.642 7.02 0.359 41.7 D35+0.1 0.543 2.77 0.457 75.2 D35+0.2 0.700 3.60 0.300 80.1 D45+stock 0.617 15.2 0.384 142 D45+0.05 0.796 5.15 0.204 52.9 D45+0.1 0.873 1.62 0.126 65.5 D45+0.2 0.823 1.61 0.176 69.6 The recombination of electrons in TiO2 to D35+ molecules is well-fitted by a biexponential function with time constants of 69.6 µs (amplitude 0.474) and 764 µs (amplitude 0.526) ACS Paragon Plus13 Environment


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(Figure 4a and Table 2). For D45, τ1 = 15.2 µs has an amplitude of 0.617 and can therefore be considered as the dominating recombination process for D45. From a comparison of the relative amplitudes and the time constants (Table 2), D45 has a 7-fold faster recombination than D35. This is surprising considering the similarity of the dye structure and the nearidentical energy levels. A good explanation for this observation is lacking at present, but it may be related to differences in the reorganization energy, which could be affected by the length of di-alkoxyphenyl groups, as well as by the dye packing (SI-3, Table SI-1). This will cause different possibilities for the cation in the stock solution (Li+) to interact with the TiO2 surface, which may affect electron transfer kinetics. The regeneration reaction occurs by electron transfer from [Co(bpy)3]2+ to D+ if the reduced form of the redox electrolyte is present (reg in Figure 1). D45 has higher amplitude and shorter time constants than D35 for this reaction at all investigated concentrations (Table 2). In addition, τlog values were calculated using Equation 2 (Table 3). They show a clear trend, i.e. recombination (rec2) and regeneration (reg) are faster for D45 with its shorter dimethoxyphenyl units. An analogous result has been reported for organic carbazole and coumarin dyes having only hydrogen or hexyl-chains on the π-linker unit in an iodide-based electrolyte.35 The τlog values from TAS were used for calculating the effective rate constants κobs (κobs = 1/τlog). When no redox species was added, the effective rate constant κobs equals the recombination rate constant κrec. The regeneration rate constant κreg, can thus be obtained by subtracting κrec from the effective rate constant κobs measured in the presence of redox species (SI-4, Equation SI-3). The τlog values confirm that D45 shows faster regeneration and faster recombination compared to D35. Please note that this approach required adjustment of the laser fluence in such a way that the average number of electrons and oxidized dyes generated per TiO2-particle (SI-4, Equation SI-1) are very similar for the D35 and D45 systems (SI-5, ACS Paragon Plus14 Environment

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Table SI-3). The difference in regeneration and recombination is caused by the less-bulky dimethoxyphenyl groups on D45 compared to the di-butoxyphenyl groups on the D35 dye. Table 3. Kinetic parameters obtained from TAS measurements. Dye+[Co2+] M τlog / µs κobs / s-1 κrec / s-1 κreg / cm3 s-1 mol-1 D35+stock D35+0.05 D35+0.1 D35+0.2 D45+stock D45+0.05 D45+0.1 D45+0.2

246 13.3 12.5 9.12 36 8.30 3.01 3.10

4.07×103 7.52×104 8.00×104 11.00×104 2.79×104 12.05×104 33.22×104 32.26×104

4.07×103 4.07×103 4.07×103 4.07×103 2.79×104 2.79×104 2.79×104 2.79×104

-

1.42×103 7.59×102 5.30×102 -

1.85×102 3.04×103 1.47×103

A plot of the effective rate constants κobs vs. the concentration of [Co(bpy)3]2+ yields an average bimolecular rate constant k as the slopes of 4.65×102 cm3 s-1 mol-1 for D35 and 15.3×102 cm3 s-1 mol-1 for D45 (SI-5 Figure SI-8). SECM feedback mode investigation of D35/D45 sensitized TiO2 films. SECM has been applied for determining rate constants in the DSC with I-/I3- redox electrolyte systems before.25-26, 28, 36-37 The essential idea is the use of the redox electrolyte as mediator for SECM feedback studies. For this purpose the working solution must contain the oxidized form of the redox mediator only from which the reduced form is generated locally at the ME (Figure 5). The ME-generated reduced redox mediator diffuses from the ME to the TiO2/dye interface, reduces the oxidized dye D+ while being oxidized itself (Figure 5). While the ME is approaching the illuminated photoanode, the current at the ME depends more and more on the regeneration of the [Co(bpy)3]3+ by the dye regeneration reaction (and thus on the kinetic limitations of the reduction of D+) at the photoanode and less on the [Co(bpy)3]3+ diffusion from the surrounding solution to the ME. The SECM measurements are sensitive to the overall effective rate constant keff of the sequence of light absorption, electron injection, dye regeneration and various recombination processes. With the knowledge of dye loading, photon flux etc., keff values can be related to individual steps of the reaction sequence. In ACS Paragon Plus15 Environment


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particular an oxidation rate constant kox can be determined as a fitting parameter. Previous studies have already used [Co(bpy)3](PF6)3 as one electron redox SECM mediator, however, the evaluation was based on a formula for the stoichiometry of the I-/I3- system.38-39 For the present study, the data treatment is analogous to previous studies for the I-/I3- system28, 36 but taking into account the stoichiometry of the [Co(bpy)3]2+/3+ redox electrolyte (SI-7). Furthermore, we used Au MEs because attempts to use Pt as ME material as in most SECM studies had to be abandoned due to persistent adsorption phenomena preventing quantitative determination of diffusion controlled currents at Pt MEs. The CV of [Co(bpy)3] 3+ at the Au ME in the is shown in SI-8, Figure SI-11, from which the ME potential of ET = –0.28 V was selected for diffusion controlled reduction. During the approach curves to D35- and D45-sensitized TiO2 films in solution with different concentrations of [Co(bpy)3]3+, the samples were illuminated with a fixed wavelength of 455 nm and a photon flux density Jhν = 6.57×10-8 mol cm-2 s-1 (at 4.85 mW cm2

) from the backside. Both dyes show a strong adsorption in acetonitrile (for absorption on

TiO2 see SI-8, Figure SI-10). The photon energy of 2.72 eV is too low to excite the semiconductor

TiO2

(Eg = 3.2 eV,

λ = 387 nm).

Under

these

conditions,

the

photoelectrochemical reactions (2 and 3) occur at the sample, the electrochemical reaction at the ME is given in Equation 4 under quasi-stationary conditions.

at the dye/TiO2:

+ ∗ + $

+ [( )" ] + [( )" ]" at the ME:

[( )" ]" + → [( )" ]


(2) (3) (4)

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Figure 5. Schematic of the SECM feedback mode investigation of the dye regeneration with the [Co(bpy)3]2+/3+ redox mediator.

The redox mediator concentration of [Co(bpy)3]3+ was varied from 0.153 to 2.56 mM (i.e. 0.153×10-6 - 2.56×10-6 mol cm-3) starting with the lowest concentration. The approach curves (Figure 6) are given in dimensionless normalized coordinates where the normalized current IT is the ratio between the distance-dependent current at the ME iT(d) and the current in the bulk solution iT,∞. The normalized distance L is obtained as the ratio of the ME-tosample-distance d and the ME radius rT. The experimental approach curves were fitted by a least-square method to an analytical approximation by Cornut and Lefrou40 describing the finite irreversible kinetics of first order with respect to the local mediator concentration. The experimental curves fit very well to these predictions. The obtained normalized heterogeneous rate constants κ for the dye regeneration are given in SI-8, Table SI-4. Effective rate constants keff are obtained by Equation 5 from κ and the diffusion coefficient of the mediator D = 6.95×10-6 cm2 s-1 (determined from linear sweep voltammograms at a rotating disc electrode, SI-8, Figure SI-12).

&'' =

(∙* +

(5)

The diffusion coefficient of [Co(bpy)3]2+ is assumed as equal to that of [Co(bpy)3]3+.41 For each electrode two approach curves were recorded on two different, but equivalent, regions ACS Paragon Plus17 Environment


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with a lateral distance of 1000 µm between them in order to ensure reproducibility of the obtained values. The resulting keff values of both spots differ by 0.5% to 7.9% above D35 and by 4.8% to 20.0% above D45. In both cases the approach curves above the second region give higher keff values. This observation could be explained by the longer time the sensitized TiO2 film had been illuminated before the recording of the approach curve. Although the electron conduction through the TiO2 should be fast, an equilibrium of the system is not reached instantaneously and this may contribute to this small effect.

Figure 6. Normalized SECM feedback approach curves for the approach of a Au disk ME towards a D35-sensitized (grey shades) and a D45-sensitized (red tones) TiO2 film illuminated with a blue (455 nm) LED with a photon flux density 6.57×10-8 mol cm-2 s-1 for the following [Co(bpy)3]3+ in (10-6 mol cm-3) above D35 (1) 1.78, (2) 1.14, (3) 0.707, (4) 0.333, (5) 0.153 and above D45 (6) 2.56, (7) 1.26, (8) 0.875, (9) 0.421 and (10) 0.182. Top and bottom dash-dotted lines correspond to diffusion-controlled feedback and hindered diffusion, respectively. Symbols indicate normalized experimental approach curves whereas lines fittings to theory.40

Determination of the regeneration rate constant from SECM data. The keff values (SI-8, Table SI-4) summarize the influence of absorption, electron injection, recombination and regeneration. The effective rate constant keff can be calculated from a formal kinetic treatment (SI-7) providing the expression (6).

&'' =

, - ∙ . ∙/. ∙$ ["0 ]∗ ∙$ . ∙/.


(6)

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Here [Co3+]* stands for the bulk concentration of [Co(bpy)3]3+, Jhν denotes the photon flux density and ϕhν the excitation cross section, kox is the regeneration rate constant. Figure 7 shows the experimentally determined keff values (symbols) and the fit to Equation 6 (lines) for different mediator concentrations using kox and ϕhν as fitting parameters. ΓD° was determined independently (SI-3, Table SI-1) and the photon flux density Jhν was obtained from the wavelength of 455 nm and the light intensity.

Figure 7. Effective heterogeneous first-order rate constants keff as a function of the bulk redox mediator concentration c*. Symbols correspond to experimental values, lines are least-square fits to Equation 9 using kox = 2.40×103 cm3 mol-1 s-1 and ϕhν = 4.95×104 cm2mol-1 (ϕhν = 8.22×10-20 cm2 molecule-1) for D35 (triangle) and kox = 3.50×103 cm3 mol-1 s-1 and ϕhν = 1.39×105 cm2 mol-1 (ϕhν = 2.30×10-19 cm2 molecule-1) for D45 (reversed triangle).

The best fit of the experimental data to Equation 6 yields kox = 2.40×103 cm3 mol-1 s-1 for D35 and 3.50×103 cm3 mol-1 s-1 for D45 with comparable ΓD° (SI-8, Figure SI-10). Since kox can also depend on internal mass transport conditions,37 it is important that the photoanodes are of comparable thickness, porosity and dye loading to provide equivalent conditions for internal mass transport of the mediator. This condition is satisfied here due to the complete equivalence in the preparations of the D35-sensitized and D45-sensitized photoanodes. Furthermore, the sequence of kox values is in agreement with the results from TAS in Table 3.



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Impedance spectroscopy. IS was measured in the dark and under 1 sun (1000 W m-2) at the same applied potential. They are given as Nyquist plots in SI-9, Figure SI-13. The application of the external potential results in the same Fermi level in the D35 and D45sensitized TiO2 electrodes. The potential was selected to correspond approximately to the situation at VOC at 1 sun, from which the difference in recombination resistance between the two dyes and the effect of illumination could be derived. Under illumination, injected electrons in the TiO2 can recombine with the oxidized form of the redox electrolyte but also with D+. In dark, the recombination only occurs to the electrolyte. Two DSCs denoted as sample 1 and sample 2 were prepared for each dye and measured at 1 sun (Table 4) and in the dark (Table 5) with the application of 0.78 V (for samples 1 with D35 and D45) or 0.76 V (samples 2 with D35 and D45). D35 shows a larger recombination resistance Rrec both at 1 sun and in the dark. This is in line with D35 having higher VOC-values and longer electron lifetimes τe than D45 (Table 1, Table 4 and Table 5). The slower recombination of D35 compared to D45 can be related to its higher VOC (Table 1). The VOC is determined by the difference between the Fermi-level of the TiO2 and the redox potential of the electrolyte. Since [Co(bpy)3]2+/3+ was used with both dyes, the Fermi-level governs VOC. As shown before, D35 has relatively long τ and high VOC values with cobalt electrolyte.5, 11, 22 The bulky di-butoxyphenyl groups in D35 have already shown to increase the VOC by protecting the surface from the oxidized redox species and preventing recombination.11 Here, the difference in VOC for DSC from the two dyes amounts to about 60 mV (Table 1). In principle, VOC can be related to Rrec by using the diode equation (SI-10) showing that an increased Rrec increases the VOC. The difference between the Rrec values at 1 sun and in dark for the same dye (from IS) could be taken as a measure of recombination resistance to D+. The average difference amounts to 77 Ω for D35 and 66 Ω for D45. The higher Rrec for D35 is in line with its bulkier substituents and contributes to the higher VOC ACS Paragon Plus20 Environment

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values for the corresponding solar cells compared to the ones assembled with D45. The magnitude of the observed VOC change is, however, larger than that expected from the diode equation for the determined changes in Rrec from IS. Table 4. Characteristics of solar cells measured at the same applied potential at 1 sun, 1000 W m-2. τe / ms Dye Vapplied / V Rrec / Ω C / µF D35_sample 1 0.780 30 151 199 D45_sample 1 0.780 10 50 200 D35_sample 2 0.760 15 87 172 D45_sample 2 0.760 10 58 172 Table 5. Characteristics of solar cells measured at the same applied potential in dark. Dye Vapplied / V τe / ms Rrec / Ω C / µF D35_sample 1 0.780 38 240 158 D45_sample 1 0.780 15 91 165 D35_sample 2 0.760 27 152 178 D45_sample 2 0.760 19 148 128 The results from the IS, the τe and the Rrec (Table 4 and Table 5) are in line with the results of the TAS measurements also showing slower recombination for D35 compared to D45 (Table 2, Table 3 and Figure 4a). The capacity measurements reveal no significant difference between the electrodes with the two dyes. Since the IS is performed on assembled DSCs, the results should resemble the kinetics of the complete device.

CONCLUSION The kinetics of the dye regeneration and recombination were investigated for two structurally similar triphenylamine dyes, D35 with di-butoxyphenyl chains and D45 with dimethoxyphenyl chains, by a combination of SECM, TAS and IS. The kinetic information of the photoanodes from those complementing techniques was related to performance characteristics of completely assembled DSCs with both dyes. D35 and D45 possess similar energy levels and HOMO-LUMO differences. Therefore, the differences in the kinetics and device performance should not be caused by differences in the driving force for dye ACS Paragon Plus21 Environment


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regeneration by [Co(bpy)3]2+, but rather stem from the different distance to which the redox species can approach the positive charge on the triphenylamine unit.30-31 In order to achieve improved light harvesting, the dye must fulfill competing demands such as high absorption cross section, fast regeneration kinetics and slow recombination kinetics. IS shows that the longer di-butoxyphenyl groups on D35 seem to prevent recombination both to D+ and to the oxidized form of the redox electrolyte. This shows that the longer di-butoxyphenyl groups do not only prevent the oxidized redox couple from reaching the surface but also prevent recombination to the positive charge on the oxidized dye. The regeneration rate constants kox obtained from a formal treatment of SECM approach curve data under consideration of the proper stoichiometry of the cobalt mediator yield kox = 2.40×103 cm3 mol-1 s-1 for D35 and kox = 3.50×103 cm3 mol-1 s-1 for D45. These results are in the same order as the regeneration rate constants obtained from TAS. It can be concluded that the di-butoxyphenyl groups create a barrier around the positive charge on the triphenylamine unit30-31 affecting both the regeneration and recombination kinetics.

AUTHOR INFORMATION Corresponding authors * Gerrit Boschloo, [email protected], tel +4618-471 3303 * Gunther Wittstock, [email protected], fax +49-441-798 3979

ACKNOWLEDGMENTS The authors would like to gratefully acknowledge the Swedish Energy Agency, Knut and Alice Wallenberg Foundation, the Swedish Research Council and the STandUP for Energy


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program for financial support. I.S. thanks the Carl von Ossietzky University of Oldenburg for support within the programme “Focus on Research Oriented Teaching”. The authors would also like to thank Mohammad Mirmohades for technical support with the laser measurements as well as Prof. Leif Hammarström and Dr. Liisa Antila (Uppsala University) for discussions about the logarithmic weighted average value.

ASSOCIATED CONTENT Supporting Information. NMR-spectrum of [Co(bpy)3](PF6)3, fabrication of DSCs, formal potentials of D35 and D45 on TiO2 films, molar extinction coefficients, surface coverage of D35 and D45 on TiO2 films, specifications for TAS measurements, photo-induced absorptions spectra of D35 and D45, average number of electrons in the TiO2 particles and oxidized dyes during laser measurements, TAS measurements at different irradiation intensities and different concentration of [Co(bpy)3]2+, procedure of the SECM measurements, derivation of the SECM model for dye regeneration for a one electron redox mediator, absorption spectra of D35 and D45 on TiO2 films, electrochemistry of [Co(bpy)3]3+ in acetonitrile at an Au ME, linear sweep voltammograms, determination of the diffusion coefficient of [Co(bpy)3]3+, Nyquist plots of solar cells assembled with D35 and D45, correlation of VOC to recombination resistance. This information is available free of charge via the Internet at http://pubs.acs.org.



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Grätzel, M. Photoelectrochemical Cells. Nature 2001, 414, 338-344.

4. O'Regan, B.; Grätzel, M. A Low-Cost, High-Efficiency Solar Cell Based on DyeSensitized Colloidal TiO2 Films. Nature 1991, 353, 737-740. 5. Feldt, S. M.; Wang, G.; Boschloo, G.; Hagfeldt, A. Effects of Driving Forces for Recombination and Regeneration on the Photovoltaic Performance of Dye-Sensitized Solar Cells Using Cobalt Polypyridine Redox Couples. J. Phys. Chem. C 2011, 115, 21500-21507. 6. Martinson, A. B. F.; Hamann, T. W.; Pellin, M. J.; Hupp, J. T. New Architectures for Dye-Sensitized Solar Cells. Chemistry – A European Journal 2008, 14, 4458-4467. 7. Yanagida, S.; Yu, Y.; Manseki, K. Iodine/Iodide-Free Dye-Sensitized Solar Cells. Acc. Chem. Res. 2009, 42, 1827-1838. 8. Cazzanti, S.; Caramori, S.; Argazzi, R.; Elliot, C. M.; Bignozzi, C. A. Efficient NonCorrosive Electron-Transfer Mediator Mixtures for Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2006, 128, 9996-9997. 9.

Peter, L. M. The Grätzel Cell: Where Next? J. Phys. Chem. Lett 2011, 2, 1861-1867.

10. Nusbaumer, H.; Moser, J.-E.; Zakeeruddin, S. M.; Nazeeruddin, M. K.; Grätzel, M. CoII(Dbbip)22+ Complex Rivals Tri-Iodide/Iodide Redox Mediator in Dye-Sensitized Photovoltaic Cells. J. Phys. Chem. B 2001, 105, 10461-10464. 11. 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 DyeSensitized Solar Cells. J. Am. Chem. Soc. 2010, 132, 16714-16724. 12. Yella, A.; Lee, H.-W.; Tsao, H. N.; Yi, C.; Chandiran, A. K.; Nazeeruddin, M. K.; Diau, E. W.-G.; Yeh, C.-Y.; Zakeeruddin, S. M.; Grätzel, M. Porphyrin-Sensitized Solar Cells with Cobalt (II/III) Based Redox Electrolyte Exceed 12 Percent Efficiency. Science 2011, 334, 629-634. 13. Mathew, S.; Yella, A.; Gao, P.; Humphry-Baker, R.; CurchodBasile, F. E.; AshariAstani, N.; Tavernelli, I.; Rothlisberger, U.; Nazeeruddin, M. K.; Grätzel, M. Dye-Sensitized Solar Cells with 13% Efficiency Achieved through the Molecular Engineering of Porphyrin Sensitizers. Nat. Chem. 2014, 6, 242-247. 14. Ardo, S.; Meyer, G. J. Photodriven Heterogeneous Charge Transfer with TransitionMetal Compounds Anchored to TiO2 Semiconductor Surfaces. Chem. Soc. Rev. 2009, 38, 115-164. 15. Murakami, T. N.; Koumura, N.; Kimura, M.; Mori, S. Structural Effect of Donor in Organic Dye on Recombination in Dye-Sensitized Solar Cells with Cobalt Complex Electrolyte. Langmuir 2014, 30, 2274-2279. 16. Robson, K. C. D.; Hu, K.; Meyer, G. J.; Berlinguette, C. P. Atomic Level Resolution of Dye Regeneration in the Dye-Sensitized Solar Cell. J. Am. Chem. Soc. 2013, 135, 19611971. 17. Misra, R.; Maragani, R.; Patel, K. R.; Sharma, G. D. Synthesis, Optical and Electrochemical Properties of New Ferrocenyl Substituted Triphenylamine Based DonorAcceptor Dyes for Dye Sensitized Solar Cells. RSC Adv. 2014, 4, 34904-34911. 18. Bisquert, J.; Fabregat-Santiago, F.; Mora-Seró, I.; Garcia-Belmonte, G.; Giménez, S. Electron Lifetime in Dye-Sensitized Solar Cells: Theory and Interpretation of Measurements. J. Phys. Chem. C 2009, 113, 17278-17290.


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19. Haque, S. A.; Tachibana, Y.; Willis, R. L.; Moser, J. E.; Graetzel, M.; Klug, D. R.; Durrant, J. R. Parameters Influencing Charge Recombination Kinetics in Dye-Sensitized Nanocrystalline Titanium Dioxide Films. J. Phys. Chem. B 2000, 104, 538-547. 20. Hasselmann, G. M.; Meyer, G. J. Diffusion-Limited Interfacial Electron Transfer with Large Apparent Driving Forces. J. Phys. Chem. B 1999, 103, 7671-7675. 21. Nelson, J.; Chandler, R. E. Random Walk Models of Charge Transfer and Transport in Dye Sensitized Systems. Coord. Chem. Rev. 2004, 248, 1181-1194. 22. Ellis, H.; Eriksson, S. K.; Feldt, S. M.; Gabrielsson, E.; Lohse, P. W.; Lindblad, R.; Sun, L.; Rensmo, H.; Boschloo, G.; Hagfeldt, A. Linker Unit Modification of Triphenylamine-Based Organic Dyes for Efficient Cobalt Mediated Dye-Sensitized Solar Cells. J. Phys. Chem. C 2013, 117, 21029-21036. 23. Hagberg, D. P.; Jiang, X.; Gabrielsson, E.; Linder, M.; Marinado, T.; Brinck, T.; Hagfeldt, A.; Sun, L. Symmetric and Unsymmetric Donor Functionalization. Comparing Structural and Spectral Benefits of Chromophores for Dye-Sensitized Solar Cells. J. Mater. Chem. 2009, 19, 7232-7238. 24. Tian, H.; Gabrielsson, E.; Lohse, P. W.; Vlachopoulos, N.; Kloo, L.; Hagfeldt, A.; Sun, L. Development of an Organic Redox Couple and Organic Dyes for Aqueous DyeSensitized Solar Cells. Energ. Environ. Sci. 2012, 5, 9752-9755. 25. Shen, Y.; Tefashe, U. M.; Nonomura, K.; Loewenstein, T.; Schlettwein, D.; Wittstock, G. Photoelectrochemical Kinetics of Eosin Y-Sensitized Zinc Oxide Films Investigated by Scanning Electrochemical Microscopy under Illumination with Different Led. Electrochim. Acta 2009, 55, 458-464. 26. Tefashe, U. M.; Loewenstein, T.; Miura, H.; Schlettwein, D.; Wittstock, G. Scanning Electrochemical Microscope Studies of Dye Regeneration in Indoline (D149)-Sensitized ZnO Photoelectrochemical Cells. J. Electroanal. Chem. 2010, 650, 24-30. 27. Kirchner, C. N.; Hallmeier, K. H.; Szargan, R.; Raschke, T.; Radehaus, C.; Wittstock, G., Evaluation of Thin Film Titanium Nitride Electrodes for Electroanalytical Applications. Electroanal. 2007, 19, 1023-1031. 28. Tefashe, U. M.; Nonomura, K.; Vlachopoulos, N.; Hagfeldt, A.; Wittstock, G., Effect of Cation on Dye Regeneration Kinetics of N719-Sensitized TiO2 Films in Acetonitrile-Based and Ionic-Liquid-Based Electrolytes Investigated by Scanning Electrochemical Microscopy. J. Phys. Chem. C 2012, 116, 4316-4323. 29. Ellis, H.; Vlachopoulos, N.; Häggman, L.; Perruchot, C.; Jouini, M.; Boschloo, G.; Hagfeldt, A. PEDOT Counter Electrodes for Dye-Sensitized Solar Cells Prepared by Aqueous Micellar Electrodeposition. Electrochim. Acta 2013, 107, 45-51. 30. Westermark, K.; Tingry, S.; Persson, P.; Rensmo, H.; Lunell, S.; Hagfeldt, A.; Siegbahn, H. Triarylamine on Nanocrystalline Tio2 Studied in Its Reduced and Oxidized State by Photoelectron Spectroscopy. J. Phys. Chem. B 2001, 105, 7182-7187. 31. Nyhlen, J.; Boschloo, G.; Hagfeldt, A.; Kloo, L.; Privalov, T. Regeneration of Oxidized Organic Photo-Sensitizers in Grätzel Solar Cells: Quantum-Chemical Portrait of a General Mechanism. ChemPhysChem 2010, 11, 1858-1862. 32. Feldt, S. M.; Lohse, P. W.; Kessler, F.; Nazeeruddin, M. K.; Grätzel, M.; Boschloo, G.; Hagfeldt, A. Regeneration and Recombination Kinetics in Cobalt Polypyridine Based Dye-Sensitized Solar Cells, Explained Using Marcus Theory. Phys. Chem. Chem. Phys. 2013, 15, 7087-7097. 33. Anderson, A. Y.; Barnes, P. R. F.; Durrant, J. R.; O’Regan, B. C. Quantifying Regeneration in Dye-Sensitized Solar Cells. J. Phys. Chem. C 2011, 115, 2439-2447. 34. 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.



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35. Ogawa, J.; Koumura, N.; Hara, H.; Mori, S. Deceleration of Dye Cation Reduction Kinetics by Adding Alkyl Chains to the Pi-Conjugated Linker of the Dye Molecule. JSAP 2014, 53, 127301-1 - 127301-5. 36. Shen, Y.; Nonomura, K.; Schlettwein, D.; Zhao, C.; Wittstock, G. Photoelectrochemical Kinetics of Eosin Y-Sensitized Zinc Oxide Films Investigated by Scanning Electrochemical Microscopy. Chemistry – A European Journal 2006, 12, 58325839. 37. Tefashe, U. M.; Rudolph, M.; Miura, H.; Schlettwein, D.; Wittstock, G. Photovoltaic Characteristics and Dye Regeneration Kinetics in D149-Sensitized ZnO with Varied Dye Loading and Film Thickness. Phys. Chem. Chem. Phys. 2012, 14, 7533-7542. 38. Zhang, B., et al., Investigation of Dye Regeneration Kinetics in Sensitized Solar Cells by Scanning Electrochemical Microscopy. ChemPhysChem 2014, 15, 1182-1189. 39. Zhang, B.; Yuan, H.; Zhang, X.; Huang, D.; Li, S.; Wang, M.; Shen, Y. Investigation of Regeneration Kinetics in Quantum-Dots-Sensitized Solar Cells with Scanning Electrochemical Microscopy. ACS Appl. Mater. Interfaces 2014, 6, 20913-20918. 40. Cornut, R.; Lefrou, C. New Analytical Approximation of Feedback Approach Curves with a Microdisk Secm Tip and Irreversible Kinetic Reaction at the Substrate. J. Electroanal. Chem. 2008, 621, 178-184. 41. Bard, A. J.; Fan, F. R. F.; Kwak, J.; Lev, O. Scanning Electrochemical Microscopy. Introduction and Principles. Anal. Chem. 1989, 61, 132-138.


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TOC figure



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Energy levels of the D35 and D45 dyes and the recombination and regeneration processes present in the DSC. The forward reactions are indicated by blue arrows while the recombination reactions (or backward reactions) are shown by grey arrows. The energy level positions and redox potentials as determined below are given on the NHE scale.

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The organic triphenylamine based dyes D35 (left) and D45 (right). 53x35mm (600 x 600 DPI)



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I-V-curves at 1 sun (solid line) and 0.32 sun (dashed line) of the master solar cells assembled with the D35 (black) or D45 (red) dye and PEDOT counter electrodes. 82x61mm (300 x 300 DPI)


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Transient absorption traces for D35 (black) and D45 (red) absorbed on mesoporous TiO2 in 0.2 M TBP and 0.1 M LiClO4 in acetonitrile in the absence (a) and presence (b) of 0.2 M [Co(bpy)3](PF6)2. Laser fluence was 0.28 mJ pulse-1 cm-2. Dotted lines show the measured signal and solid lines are fits using biexponential decay functions. 61x46mm (300 x 300 DPI)



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Transient absorption traces for D35 (black) and D45 (red) absorbed on mesoporous TiO2 in 0.2 M TBP and 0.1 M LiClO4 in acetonitrile in the absence (a) and presence (b) of 0.2 M [Co(bpy)3](PF6)2. Laser fluence was 0.28 mJ pulse-1 cm-2. Dotted lines show the measured signal and solid lines are fits using biexponential decay functions. 82x61mm (300 x 300 DPI)


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Schematic of the SECM feedback mode investigation of the dye regeneration with the [Co(bpy)3]2+/3+ redox mediator. 54x35mm (300 x 300 DPI)



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Normalized SECM feedback approach curves for the approach of a Au disk ME towards a D35-sensitized (grey shades) and a D45-sensitized (red tones) TiO2 film illuminated with a blue (455 nm) LED with a photon flux density 6.57×10-8 mol cm-2 s-1 for the following [Co(bpy)3]3+ in (10-6 mol cm-3) above D35 (1) 1.78, (2) 1.14, (3) 0.707, (4) 0.333, (5) 0.153 and above D45 (6) 2.56, (7) 1.26, (8) 0.875, (9) 0.421 and (10) 0.182. Top and bottom dash-dotted lines correspond to diffusion-controlled feedback and hindered diffusion, respectively. Symbols indicate normalized experimental approach curves whereas lines fittings to theory. 64x49mm (300 x 300 DPI)


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Effective heterogeneous first-order rate constants keff as a function of the bulk redox mediator concentration c*. Symbols correspond to experimental values, lines are least-square fits to Equation 9 using kox = 2.40×103 cm3 mol 1 s 1 and ϕhν = 4.95×104 cm2mol-1 (ϕhν = 8.22×10-20 cm2 molecule-1) for D35 (triangle) and kox = 3.50×103 cm3 mol-1 s-1 and ϕhν = 1.39×105 cm2 mol-1 (ϕhν = 2.30×10-19 cm2 molecule-1) for D45 (reversed triangle). 63x48mm (300 x 300 DPI)


Influence of Dye Architecture of Triphenylamine ... - ACS Publications

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