Charge Recombination to Oxidized Iodide in Dye-Sensitized Solar

ARTICLE pubs.acs.org/JPCC

Charge Recombination to Oxidized Iodide in Dye-Sensitized Solar Cells John G. Rowley,† Shane Ardo,† Yali Sun,‡ Felix N. Castellano,‡ and Gerald J. Meyer*,† †

Departments of Chemistry and Materials Science and Engineering, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, United States ‡ Department of Chemistry and Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio 43403, United States ABSTRACT: The goal of this study was to determine whether electrons injected into TiO2 in dye-sensitized solar cells (DSSCs) react with di-iodide, I2•
, a known intermediate in sensitized iodide oxidation. The approach was to utilize time-resolved absorption spectroscopy to quantify the yield of I2•
disproportionation under conditions where I2•
reduction by electrons photoinjected into TiO2, TiO2(e
)s, could be competitive. The DSSC was based on [Ru(dtb)2(dcb)]2+, where dtb is 4,40 -(C(CH3)3)2-2,20 -bipyridine and dcb is 4,40 -(COOH)2-2,20 -bipyridine, sensitized mesoporous nanocrystalline TiO2 thin films sintered onto an optically transparent fluorine-doped tin oxide (FTO) conductive substrate. A transparent Pt counter-electrode and a 0.5 M LiI/0.05 M I2/acetonitrile electrolyte completed the DSSC. After pulsed 532 nm laser excitation, the first iodide oxidation product observed spectroscopically was I2•
. Under all conditions studied, there was no direct evidence for the reaction between TiO2(e
) and I2•
, and the kinetics for I2•
loss indicated quantitative disproportionation of I2•
to yield I3
and I
with a rate constant that was, within experimental error, the same as that measured in fluid acetonitrile solution, 2.2 + 1  109 M
1 s
1. This was true even when steady state illumination was utilized to increase the TiO2(e
) concentration. Data consistent with charge recombination to I3
, from TiO2(e
) or electrons at the Pt counter electrode, were quantified spectroscopically, with the Kohlrausch
Williams
Watts (KWW) function, at specific points on the current
potential curve. This reaction was found to be sensitive to steady state illumination incident on the DSSC. Transient absorption changes assigned to a Stark effect that were intimately coupled to the presence of transiently generated TiO2(e
) complicated charge recombination analysis.

’ INTRODUCTION The identification and characterization of charge recombination reactions at illuminated semiconductor interfaces has been and continues to be of considerable interest. A molecular level understanding of such reactions is emerging at dye-sensitized TiO2 interfaces.1,2 Electron transfer from TiO2 to oxidized ruthenium compounds is now reasonably well understood experimentally with theoretical models that take into account the driving force,3,4 transport,5,6 temperature,7 pH,8,9 and distance dependencies.10,11 In contrast, when iodide redox mediators are present in the external electrolyte, charge recombination reactions are not as well understood. This is unfortunate as electron transfer from TiO2 to oxidized iodide species is thought to be a key loss mechanism in efficient dye-sensitized solar cells, DSSCs, and molecular level descriptions may enable more efficient cells to be fabricated.3,4 In this manuscript, we have utilized nanosecond absorption spectroscopy to quantify charge recombination to iodide-derived acceptor(s) at open circuit, short circuit, and the point of maximum power. In previous work, a photoelectrochemical cell designed to characterize interfacial electron transfer at potentiostatically r 2011 American Chemical Society

controlled TiO2 thin film electrodes was employed to quantify the reactivity of TiO2 with tri-iodide and di-iodide, I3
and I2•
, in acetonitrile.12,13 Both ions were generated by direct excitation of iodide (266 nm), band gap excitation of TiO2 (355 nm), or both. Consistent with previous studies, after band gap excitation the first iodide oxidation product observed was I2•
. There was no direct kinetic evidence for a reaction between TiO2 and I2•
, even when the concentration of electrons, TiO2(e
)s, was increased with a forward bias to that expected under 100 suns of solar illumination. The I2•
formed was found instead to quantitatively disproportionate on the 10 μs time scale, eq 1. 2I2 •
f I3
þ I


ð1Þ

This was followed by data consistent with a slow millisecond reaction that concurrently depleted the TiO2(e
) and I3
concentrations. In the present work, we sought to understand how general these conclusions were and to specifically investigate Received: May 19, 2011 Revised: September 2, 2011 Published: September 06, 2011 20316

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The Journal of Physical Chemistry C the role molecular sensitizers might play in altering iodide oxidation and/or mediating charge recombination. A recent observation relevant to the analysis of spectroscopic data reported herein was recently described by two groups.14
17 Electric fields induced by ion adsorption or electron injection into TiO2 induce large spectral shifts in the absorption spectra of the sensitizers. The spectral changes were reminiscent of those generated in electro-absorption spectroscopy and have hence been termed “Stark effects”. The significance of this to the recombination processes reported herein is 2-fold. First, the spectroscopic signatures of the Stark effect can be larger than those generated from the injected electron and the oxidized iodide product(s), thereby complicating kinetic analysis. This issue is exacerbated by the fact that the magnitude of the Stark effect is environmental and time dependent. For this study, the sensitizer [Ru(dtb)2(dcb)]2+, where dtb is 4,40 -(C(CH3)3)22,20 -bipyridine and dcb is 4,40 -(COOH)2-2,20 -bipyridine, was selected since its Stark effect behavior has previously been characterized.15 Second, the influence interfacial electric fields might have on local motion of ions like iodide is poorly understood. The Stark effect was in fact shown to decrease under conditions where the TiO2(e
) concentration remained constant, behavior attributed to interfacial reorganization or “screening” of the electric field by lithium cations. It was thus of interest to see whether related chemistry might occur with iodide salts.

’ EXPERIMENTAL SECTION Materials. All chemicals were reagent grade or better unless otherwise specified and were used without further purification: acetonitrile (Burdick and Jackson, spectrophotometric grade); lithium perchlorate (Aldrich, 99.99%); n-tetrabutylammonium perchlorate (TBAP; Fluka, >99.9%); n-tetrabutylammonium iodide (TBAI; Aldrich, >99% or Fluka, >98%); lithium iodide (LiI; Aldrich, 99.999%); iodine (Aldrich, 99.999%); argon gas (Airgas, >99.998%); nitrogen gas (Airgas, >99.999%); oxygen gas (Airgas, industrial grade); and titanium(IV) isopropoxide (Sigma-Aldrich, 97%). The fluorine-doped SnO2-coated glass substrate was used as received (FTO; Hartford Glass Co., Inc., 2.3 mm thick, 15 Ohm/0). The synthesis of [Ru(dtb)2(dcb)](PF6)2 was described previously.14 Coordination Compound Sensitized TiO2 Electrodes. TiO2 mesoporous nanocrystalline (anatase) films were prepared by the hydrolysis of titanium(IV) isopropoxide using a sol
gel technique.15 The TiO2 paste was doctor bladed onto conductive optically transparent fluorine-doped tin oxide (FTO) glass electrodes. Scotch tape was employed as a spacer. The TiO2/FTO electrode was sintered at 420 °C for 20 min under O2 flow to form a mesoporous thin film. The TiO2/FTO electrode was TiCl4 treated by soaking in a 0.05 M TiCl4 solution at 25 °C for 12 h. After 12 h the slides were rinsed lightly with deionized H2O and sintered at 420 °C under O2 stream for 20 min. TiO2/FTO electrodes were sensitized out of a high-concentration [Ru(dtb)2(dcb)](PF6)2/CH3CN solution by soaking the electrode overnight. Preparing DSSC Analog Samples. A Ru(dtb)2(dcb)2+sensitized TiO2 nanocrystalline thin film sintered onto a fluorinedoped tin oxide electrode (Ru(dtb)2(dcb)/TiO2/FTO electrode) was sandwiched with a platinized FTO (Pt/FTO) electrode across the diagonal of an argon gas purged quartz cuvette. A platinum wire electrode (0.1 mm diam) was also included in the cuvette; the Pt wire extended all the way to the bottom of the cuvette. An argon-saturated solution of 0.5 M LiI/0.05 M

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Figure 1. Steady state absorption spectra of Ru(dtb)2(dcb)/TiO2 in neat acetonitrile (red) and 0.5 M LiClO4 (dashed black). Inset shows the absorption difference between the absorption spectrum of Ru(dtb)2(dcb)/TiO2 measured in neat acetonitrile from that measured in 0.5 M LiClO4/CH3CN.

I2/CH3CN was injected into the base of the cuvette. Capillary action drew some of the electrolyte up into the space between the Ru(dtb)2(dcb)/TiO2/FTO electrode and the Pt/FTO electrode. The remaining electrolyte formed a small pool at the base of the cuvette, in which the 0.1 mm diameter Pt wire was immersed. For all current
potential curves shown, the potential is referenced vs the Pt wire electrode immersed in the pool of 0.5 M LiI/0.05 M I2/CH3CN at the bottom of the cuvette, and the illuminated cell has an area of approximately 1 cm2. High optical transmittance was a requirement for the experiment; thus, the Pt/FTO electrode was optimized more for transmission than for power conversion efficiency, and this in fact necessitated the use of the separate Pt quasi reference electrode. Transient Absorption Spectroscopy. Transient absorbance in the visible region (350
800 nm) was performed using a Xe flash probe lamp and a frequency-doubled (532 nm) Q-switched Nd:YAG laser excitation with a pulse duration of ∼5 ns. The Xe flash lamp probe beam was optically coupled to a monochromator placed before the sample to minimize excitation and maximize shot-to-shot reproducibility. Unless otherwise indicated, the 532 nm pump laser fluence was 5 mJ/cm2. A “Z” configuration double monochromator and photomultiplier tube were used to select the probe wavelength and detect the transmitted light intensity. Signal information was digitized using an oscilloscope. The signal-to-noise ratio of the transient absorption data was increased with a moving average smooth. A continuous Ar+ laser (514.5 nm) was used to irradiate the sample during some transient absorption experiments.

’ RESULTS Figure 1a shows metal-to-ligand charge transfer (MLCT) absorption spectra of [Ru(dtb)2(dcb)](PF6)2 anchored to a mesoporous nanocrystalline (anatase) TiO2 thin film sintered onto a fluorine-doped tin oxide (FTO) substrate, abbreviated Ru(dtb)2(dcb)/TiO2, immersed in neat acetonitrile and 0.5 M LiClO4/CH3CN. Consistent with previous studies, a red shift 20317

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Figure 2. (a) Current
potential curve for a Ru(dtb)2(dcb)/TiO2-based DSSC measured in the dark (dashed line) and under 5 mW of 514.5 nm light illumination (blue). The power curve for the operational DSSC (product of current and voltage) is also shown in green. (b) Transient absorbance spectra recorded at the indicated delay times after pulsed 532 nm excitation of the DSSC. The potential of the operational DSSC was held at the power point during the collection of the transient absorption spectra.

of the MLCT absorption and increased absorptivity below ∼425 nm were observed with Li+ exposure.18 For comparisons with transient absorption data, these spectral shifts are conveniently represented as difference spectra where the absorption in 0.5 M LiClO4 was used as a reference. The inset shows the absorption spectrum measured in neat acetonitrile subtracted from the spectrum in 0.5 M LiClO4. Previous adsorption isotherm measurements revealed that the equilibrium binding constant for Li+ to a Ru(dtb)2(dcb)/TiO2 thin film was 15
80 M
1.14 The Ru(dtb)2(dcb)/TiO2 electrode was utilized in the regenerative DSSC described in the Experimental Section. The DSSC was illuminated with 5 mW/cm2 of 514.5 nm light, and a current
potential (i
P) curve was measured (Figure 2a). About 0.1 mW/cm2 of power was generated that corresponded to an overall power conversion efficiency of 2%. Transient absorption spectra recorded at the power point are shown at the indicated times after pulsed 532 nm excitation (Figure 2b). The transient spectra were accurately modeled with the known absorption spectra of I2•
, I3
, and TiO2(e
)s and the absorption difference spectrum shown in Figure 1b hereafter referred to as a Stark effect. The concentration of I2•
was highest immediately after laser excitation and subsequently decreased with a corresponding increase in I3
. The details of this disproportionation chemistry will be discussed further below. Full spectral data were recorded at the open and short-circuit conditions as well. The spectral data were qualitatively similar to that measured at the power point, but the spectral evolution differed significantly. Shown in Figure 3 is an i
P curve for another DSSC along with time-resolved absorption changes measured under the indicated conditions. The absorption changes were monitored at 433, 510, and 710 nm that are predominately due to I3
, Stark effect, and TiO2(e
), respectively. As discussed further below, the Stark effect arises from the change in the surface electric field that accompanies excited state injection. The data are shown on a 0.01 ms and longer time scale and thus are not complicated by the I2•
disproportionation chemistry that will be discussed further below. Under the opencircuit conditions shown in Figure 3b, absorption changes monitored at 433 and 710 nm returned cleanly to baseline on

a 0.1 s time scale consistent with the recombination of transiently generated TiO2(e
)s and I3
. On shorter time scales, the Stark effect decayed without change in the TiO2(e
) concentration, behavior that has previously been attributed to charge screening.14 On millisecond and longer time scales, loss of the absorption feature attributed to the Stark effect occurred coincident with TiO2(e
) loss. When the DSSC was held at the power point (Figure 3c), the absorption features attributed to TiO2(e
)s and oxidized iodide decreased on a shorter time scale than that observed under open-circuit conditions. The loss of the absorption feature at 510 nm also occurred much faster and changed to a positive absorption feature at longer observation times due to the weak underlying absorption of I3
. With the exception of the short-circuit condition, the kinetic data measured at 710 nm were well described by the Kohlrausch
Williams
Watts (KWW) kinetic model, eq 2.19,20 IðtÞ ¼ Io exp½
ðt=τo Þβ 

ð2Þ

Here β is inversely related to the width of the underlying L
evy distribution of rate constants, 0 < β < 1, and τo is a characteristic lifetime. A mean lifetime for the kinetic process was calculated from the first moment of the KWW function, eq 3

 τo 1 τKWW ¼ ð3Þ Γ β β where Γ(x) is the Gamma function.21,22 The best fit parameters are given in Table 1. The quality of the fits was poor at the shortcircuit condition, presumably because some of the injected electrons were removed from the optical path prior to reaction with I3
. Note that for this data the TiO2(e
) concentration decayed to zero well before the I3
concentration did. Pulsed laser excitation of Ru(dtb)2(dcb)/TiO2 in 0.5 M LiI/ CH3CN under open-circuit conditions was followed by monitoring at 433 nm, which is an isosbestic point for the Stark effect. At this wavelength, both I2•
and I3
absorb light, and disproportionation of I2•
and the subsequent reduction of I3
could be quantified without spectral complications due to transient electric fields. Figure 4b shows data for I2•
disproportionation, 20318

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Figure 3. (a) Current
potential curve for DSSCs measured with 514.5 nm illumination. Current
potential conditions relevant to operational DSSCs are indicated: open circuit (b), power point (c), and short circuit (d). Transient absorption changes for a [Ru(dtb)2(dcb)]2+-based DSSC monitored at 433 (black), 710 (green), and 510 nm (red) after pulsed 532 nm excitation measured at the (b) open-circuit, (c) power point, and (d) short-circuit condition. Overlaid on the data are best fits to the KWW kinetic model.

and Figure 4a reveals how ∼+10 nm changes in the observation wavelength led to significant kinetic contributions from the interfacial electric fields. Overlaid on the data in Figure 4b are fits to a second-order equal concentration kinetic model for the

ΔAbst ¼

reaction shown in eq 1. The kinetics were complicated somewhat as the appearance of the I3
product and the loss of the I2•
reactant both occur at this monitoring wavelength. Equation 4 was therefore used to fit the data

ΔAbsf ½ð2  εðI2 •
Þ  lsolution þ 2  εðTiO2 ðe
ÞÞ  lsolution þ 4k  t  ΔAbsf Þ ðεðI3
Þ  lsolution þ 2  εðTiO2 ðe
ÞÞ  lsolution þ 4k  t  ΔAbsf Þ

where ε(X) is the molar decadic absorption coefficient of species X; lsolution is the optical path length; k is the second-order rate constant; and ΔAbsf is the (final) change in absorbance after disproportionation (t = 10 μs). The other variables have their normal meanings. The derivation of eq 4 is given in the Appendix. With the previously measured path length of 10 μm,13 and the known extinction coefficients, a second-order rate constant of k = 2.2 ( 1  109 M
1 s
1 was abstracted from best fits to eq 4.

ð4Þ

Under the assumption that the disproportionation was complete by 10 μs, the initial absorption from I2•
necessary for a quantitative reaction was calculated and is depicted by the numbered box at time zero for six different irradiance conditions. For the lowest four irradiances, the kinetic fits accurately predicted the initial amplitudes consistent with quantitative disproportionation. The predicted amplitudes for the highest two irradiances were larger in magnitude than the experimental data. However, the kinetic fits again accurately predicted these initial amplitudes 20319

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with I2•
disproportionation reactions faster than the 10 ns instrument response function. Table 1. KWW Analysis of a Ru(dtb)2(dcb)/TiO2-Based DSSCa

a

current
potential condition

τo (s)

β

τKWWc (s)

open circuit

0.044

0.80

0.050

power point short circuitb

0.0098 0.0079

0.62 1

0.014 0.0079

Kinetic data abstracted from Figure 3. b The KWW function did not model this data significantly better than a first-order kinetic model. c The mean lifetime (τKWW) calculated with eq 3.

Transient absorption changes at the open-circuit condition of a DSSC with 1
10 mW/cm2 of 514.5 nm steady state irradiance are shown in Figure 5 along with the current
potential curves measured at these same irradiances. The short-circuit photocurrent was linear with the incident irradiance, and the opencircuit photovoltage was logarithmic with irradiance with an ideality factor of about two. The transient absorption data on time scales less than 10 μs were attributed to I2•
disproportionation and fit to a second-order model with a rate constant of 3  109 M
1 s
1. The yield of I2•
, the I2•
disproportionation rate constant, and the yield of I3
were all independent of the steady state irradiance. This is shown most clearly as the inset to Figure 5a.

Figure 4. (a) Transient absorbance changes monitored at 443 nm (black), 433 nm (red), and 422 nm (green) after pulsed 532 nm (1 mJ/cm2) excitation of Ru(dtb)2(dcb)/TiO2 immersed in 0.1 M LiClO4/0.5 M TBAI/CH3CN electrolyte under the open-circuit condition. (b) Transient absorbance changes monitored at 433 nm for Ru(dtb)2(dcb)/TiO2 as a function of an increased pulsed 532 nm laser fluence under the same experimental conditions as in panel a. Overlaid on the data are fits to a second-order equal concentration kinetic model. The number box at time zero corresponds to the I2•
absorption change necessary to account for quantitative conversion to I3
by 10 μs.

Figure 5. (a) Transient absorbance changes monitored at 433 nm for a DSSC at open-circuit condition after pulsed 532 nm laser excitation. The 514.5 nm steady state laser illumination was 10 (blue), 5 (green), 2 (red), and 1 (black) mW/cm2. The inset shows an expansion of the first 6 μs of absorption change with an overlaid fit to a second-order equal concentration kinetic model. (b) The corresponding current
potential curves for the DSSC at the indicated steady state 514.5 nm irradiances. 20320

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Table 2. Current
Potential and KWW Analysis of a Ru(dtb)2(dcb)/TiO2-Based DSSCa

Scheme 1

irradianceb (mW/cm2)

Voc (mV)

jsc (mA/cm2)

τo (s)

β

τKWWc (s)

1 2 5 10

270 300 360 400

0.16 0.32 0.73 1.5

0.076 0.054 0.045 0.021

0.65 0.57 0.53 0.47

0.10 0.087 0.081 0.047

a

Abstracted from the data in Figure 5. Experimental uncertainties are in the last significant figure. b The incident irradiance of 514.5 nm light. c The mean lifetime (τKWW) calculated with eq 3.

Absorption changes that occurred on time scales longer than 10 μs were attributed to the reaction of TiO2(e
)s with I3
(or I2 which was present in low equilibrium concentrations and was not detected spectroscopically). These transient data were nonexponential and well described by the KWW kinetic model, eq 2.18,19 The values of β ranged from 0.47 to 0.64 to model the data shown. The characteristic lifetimes and β decreased with increased steady state irradiance (Table 2). It should be noted that some of the absorption changes in Figures 2
5 occurred in the absence of the sensitizer as a result of direct excitation of I3
with the 532 nm light. Excited I3
is known to be dissociative and yields I2•
and the iodine atom, eq 5. The iodine atom rapidly reacts with iodide to form a second equivalent of I2•
in CH3CN, k = 2.5 + 0.4  1010 M
1 s
1, eq 6. I3
þ hv f I2 •
þ I• •




I þ I f I2

•


ð5Þ ð6Þ

The overall quantum yield (reactions 5 and 6) for I2•
photogeneration was measured to be 1.2.24 In the operational DSSCs, these reactions presented no issues and did not influence the kinetic data as they simply generated a little I2•
,