Iodide Chemistry in Dye-Sensitized Solar Cells: Making and

PERSPECTIVE pubs.acs.org/JPCL

Iodide Chemistry in Dye-Sensitized Solar Cells: Making and Breaking I-I Bonds for Solar Energy Conversion John G. Rowley, Byron H. Farnum, Shane Ardo, and Gerald J. Meyer* Departments of Chemistry and Materials Science & Engineering, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, United States

ABSTRACT The photo-oxidation of iodide (I-) results in the formation of I-I bonds relevant to solar energy conversion. The making (and breaking) of I-I bonds is specifically important to the operation of high-efficiency dye-sensitized solar cells. In this Perspective, the redox chemistry of iodide in aqueous solution is briefly reviewed, followed by recent photoinduced studies in nonaqueous solution. Analogous to thermal electron-transfer studies, two mechanisms have been identified for photodriven I-I bond formation in solution. With regard to breaking I-I bonds, the photodriven cleavage of I-I bonds has been quantified by the reduction of diiodide (I2•-) and triiodide (I3-). Studies at the solution-semiconductor interface present in dye-sensitized solar cells have also revealed that I-I bonds are formed, and I2•- is a product of iodide oxidation. Rapid disproportionation of I2•- to yield I3- and I- products that are not easily reduced by electrons injected into TiO2 is proposed to be key to the success of the I-/I3- redox mediator in dyesensitized solar cells.

T

he oxidation of iodide ions in fluid solution results in the formation of covalent I-I bonds.1,2 Therefore, when iodide oxidation is driven by photons, it provides an opportunity to study the conversion and storage of light energy in the form of chemical bonds. This process is of fundamental interest to scientists that hope to generate solar fuels.3 Molecular details of light-initiated electron-transfer reactions that result in chemical bond formation are rare, particularly when compared to photodissociative excited states where bonds are broken. Iodide redox chemistry is also of central importance to the operation of dye-sensitized (Gr€atzel) solar cells.4 To date, organic solutions comprised of iodide and triiodide, I-/I3-, represent the only mediator that provides >10% light-to-electrical power conversion efficiencies under one sun AM 1.5 illumination.5 The unique feature of this redox mediator lies not in its ability to reduce the oxidized sensitizer as many electron donors accomplish this yet still yield negligibly small power conversion efficiencies. Rather, it is the capability of the oxidized iodide products to avoid recombination with injected electrons as they diffuse through the mesoporous nanocrystalline TiO2 thin film. It has long been speculated that this reaction is inhibited by a large activation barrier associated with breaking I-I bonds; however, direct proof for this is lacking.5,6 Therefore, there are ample reasons to study the making and breaking of I-I bonds with light. This Perspective is focused on recent mechanistic advances in our understanding of light-driven iodide redox chemistry in nonaqueous solution. The general approach was to use the intensively characterized metal-to-ligand charge-transfer (MLCT) excited state of Ru(bpy)32þ and related diimine compounds7 to initiate the I-I bond making and/or breaking

r 2010 American Chemical Society

When iodide oxidation is driven by photons, it provides an opportunity to study the conversion and storage of light energy in the form of chemical bonds. reactions (Figure 1). The use of light to trigger such reactions has enabled mechanistic studies on short time scales. The MLCT excited state, Ru(bpy)32þ*, is both a stronger oxidant and reductant than is the ground state and was often employed as a primary reactant. Alternatively, the reduced RuII(bpy-)(bpy)2þ or oxidized RuIII(bpy)(bpy)23þ forms of the compound can be utilized as secondary reactants following reductive or oxidative quenching of the excited state, respectively. This affords a wide range of formal reduction potentials that can be further tuned by substitution of bipyridine with alternative ligands like those shown in Figure 1. Ruthenium polypyridyl compounds are also the most successful sensitizers in dye-sensitized solar cells; therefore their relevance toward fundamental iodide redox chemistry is duly noted.

Received Date: September 20, 2010 Accepted Date: October 4, 2010 Published on Web Date: October 13, 2010

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DOI: 10.1021/jz101311d |J. Phys. Chem. Lett. 2010, 1, 3132–3140


positive, E°(I•/I-) = þ1.33 V, and hence, a potent photo-oxidant must be generated in order to make iodine atoms with sunlight. However, a less energetically demanding pathway exists in which two iodides are involved in a concerted bond formation/ oxidation to yield diiodide, E°(I2•-/2I-) = þ1.03 V. Two parallel mechanisms were identified for iodide oxidation by transition-metal compounds as oxidants, Mox, with stopped-flow techniques, reactions 1 and 2.1 Mox þ I - f Mred þ I•

ð1Þ

Mox þ 2I - f Mred þ I2 • -

ð2Þ

In these studies, a nonlinear dependence on the observed rate constants with respect to the oxidant allowed for extraction of two individual rate constants, one being overall second-order and the other third-order in nature. Proposed mechanisms for reaction 2 included I- reacting with a [Mox,I-] ion pair or Mox reacting with an [I-,I-] ion pair. Using these techniques, dozens of transition-metal compounds have been characterized, and linear-free-energy relationships for both reactions 1 and 2 were established.1 Also of note from Figure 2 is the one-electron reduction of I3- that has been proposed to occur at E°(I3-/(I2•-,I-)) = þ0.04 Vand yield diiodide and iodide as dissociated products. Many studies have been reported for the simultaneous reduction of I3- and I2 present in equilibrium, reaction 3.10-12 However, the resulting analyses typically focused on iodine.

Figure 1. Latimer diagram for RuII(bpy)32þ in H2O. Visible light absorption by RuII(bpy)32þ results in a relatively long-lived chargetransfer excited state with an electron localized on a bpy ligand and an oxidized metal center. The excited state may undergo reductive quenching with electron donors to yield the reduced compound, RuII(bpy-)(bpy)2þ, or oxidative quenching with electron acceptors to yield the oxidized compound, RuIII(bpy)33þ.7 Also shown are other commonly used diimine ligands mentioned throughout this Perspective.

I - þ I2 h I3 -

Experimentally determined rate constants for the reduction of I2 and I2•- were shown to agree well with the Marcus cross relation.11 Similar analyses of I3- reduction are absent. An [I32-] intermediate may be transiently formed, although no evidence has been brought forth.12,13 The existence of such a species would indicate that electron transfer occurred prior to dissociation of the I-I bond.14-16 In summary, iodide redox chemistry is relatively well understood in aqueous solution. Iodide oxidation results in the formation of chemical bonds, bonds that may be broken by chemical reduction. It is therefore of great interest to understand these reactions in the context of photodriven processes, whereby solar energy may be converted and stored. Making I-I Bonds with Light. Early attempts to sensitize iodide oxidation to visible light with the MLCT excited state of Ru(bpy)32þ in aqueous solution revealed inefficient excited-state electron transfer.17 This presumably resulted from the very positive E°(I•/I-); Ru(bpy)32þ* was simply not a strong enough oxidant. Indeed, Ru(II) compounds based on 2,20 -bipyrazine, which are potent photo-oxidants, were efficiently quenched by iodide.18 Evidence that this quenching results in iodide oxidation to the iodine atom has recently been reported utilizing Ru(bpz)2(deeb)2þ* as the photooxidant, reaction 4.19,20

Figure 2. Latimer diagram indicating relevant standard thermodynamic redox potentials and equilibrium constants for various iodide redox species in aqueous solution.1,2,5,9 All concentrations are at 1 M and 25 °C.

Before describing this fundamental research, a discussion of the relevant aqueous pulse radiolysis and stopped-flow kinetic literature is provided as background. For consistency, all reduction potentials cited in this text are reported versus NHE. Care was taken to cite the original work as well as reviews in the area so that interested readers can locate additional details. The focus of the Perspective follows with specific sections on the making and breaking of I-I bonds in homogeneous organic solutions. A final summary describes the possible relevance of these studies to dye-sensitized solar cells and a discussion of unresolved issues that provide directions for future research. Early pulse radiolysis studies provided the spectroscopic and kinetic data necessary to establish equilibria and one-electron reduction potentials.8 Additional studies were later described and are now available in the outstanding reviews by Stanbury and Nord.1,2 Shown in Figure 2 is a Latimer diagram for the aqueous redox chemistry of iodide. A few values are of particular note. The formal reduction potential of the iodine atom is very


ð3Þ

RuIII ðbpz - ÞðbpzÞðdeebÞ2þ þ I f RuII ðbpz - ÞðbpzÞðdeebÞþ þ I•

ð4Þ

In these studies, the MLCT excited state of Ru(bpz)2(deeb)2þ (E°(Ru2þ*/þ) = 1.6 V) was quenched by iodide in acetonitrile

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in a purely dynamic process with a rate constant very close to that expected for a diffusion-limited reaction, 6.6 1010 M-1 s-1. The reduced ruthenium compound, Ru(bpz-)(bpz)(deeb)þ, appeared with an equal rate constant, indicating that it was a primary photoproduct. Diiodide appeared with a rate constant roughly three times slower, 2.4 1010 M-1 s-1, indicating that it was not a primary photoproduct. This observation was in accord with the initial formation of an iodine atom that subsequently reacted with iodide to form the I-I bond of I2•-, reaction 5.9 The observed rate constant also agreed well with numerous reports in the pulse radiolysis field.2,9 It is of note that excited-state electron transfer was near-quantitative, but the yield of Ru(bpz-)(bpz)(deeb)þ and I2•- was low, behavior attributed to a low cage escape yield, φCE = 0.042. Nonetheless, the sensitized photo-oxidation of iodide to iodine atoms with visible light yields I-I bonds in the form of I2•-. I• þ I - hI2 • -

Figure 3. X-ray crystal structure of [Ru(bpy)2(deeb)](I-)2 with hydrogens removed for clarity.22 Iodide anions are shown in purple. (a) Shown along the plane of the deeb ligand. (b) Shown from above the plane of the deeb ligand.

Interestingly, when iodide-Ru(bpy)2(deeb)2þ adducts were allowed to crystallize, X-ray diffraction studies revealed that one iodide anion was associated with each of the ester groups on the single deeb ligand, Figure 3. This observation was surprising on electrostatic grounds. The closest associated atoms for both iodides were the carbonyl carbons of the ester groups that presumably possess some partial positive charge, a fact that could explain the relief of Coulombic repulsion between the two iodides. The measured I-I distance in the crystal structure was determined to be 6.2 Å, about twice that of the I-I bond distance in I2•-.23 This enhances the intriguing possibility that I-I bond formation could occur in one concerted step with electron transfer to the MLCTexcited state. We note that Walter and Elliott also provide evidence for iodide association with a polypyridyl chromium compound where the bipyridine rings appeared to be specifically involved.24 Breaking I-I Bonds with Light. Perhaps the simplest examples of photoinduced I-I bond breaking are direct excitation of I2 and I3-. Direct excitation of iodine results in homolytic dissociation of the I-I bond, yielding two iodine atoms, reaction 7. An analogous mechanism exists for direct excitation of triiodide, where, again, excited-state dissociation results in diiodide and an iodine atom, reaction 8.19,25,26

ð5Þ

A more intriguing route to making I-I bonds with light is to do so through ion pairs in low dielectric solvents. For example, the addition of iodide to dichloromethane solutions of Ru(bpy)32þ or Ru(bpy)2(deeb)2þ gave rise to significant changes in the visible absorption spectrum that were consistent with ground-state ion pairing.21,22 Further evidence for ion pair formation was found in the high degree of static quenching of the MLCT excited state upon addition of iodide. With some assumptions, Stern-Volmer and Benesi-Hildebrand analysis of the spectral data provided a self-consistent estimate of the 1:1 adduct equilibrium constant for iodide-Ru(bpy)2(deeb)2þ ion pairing in dichloromethane, K = 59 700 M-1. Nanosecond absorption studies clearly demonstrated an electron-transfer quenching reaction with transient formation of I2•-. At high iodide concentrations, the rate constant for generation of diiodide was rapid and could not be timeresolved, k > 108 s-1, reaction 6. It was proposed that under these conditions, further ion pairing yielded a 1:2 adduct suited for I-I bond formation by a mechanism in which the bond formation was concerted with electron transfer. hν, k>108 s-1 ½RuII ðbpyÞ2 ðdeebÞ2þ , ðI - Þ2 s f RuII ðbpyÞ2 ðdeeb - Þþ þ I2 • -

ð7Þ

I3 - þ hν f I2 • - þ I•

ð8Þ

The I3- photochemistry has been known for some time with ultraviolet excitation;26 however, it can also be initiated with visible light.27 In fact, visible light absorption by I3- in regenerative dye-sensitized solar cells is significant. This can complicate spectroscopic analysis of complete solar cells unless long-wavelength excitation is utilized. Since the iodine atom generated from the photodissociation of triiodide reacts rapidly with I- in the redox electrolyte to yield a second equivalent of I2•- via reaction 5, the quantum yield for reaction 8, measured in acetonitrile to be 0.6, effectively doubles when this is taken into account.19 The use of ultraviolet and high-energy visible photons to photodissociate I-I bonds highlights the large energy requirements to perform such a reaction. Attempts to drive I3reduction with a MLCT excited state have been pursued. Ru(bpy)2(deeb)2þ* was quenched by triiodide in both dichloromethane and acetonitrile solution.27 In dichloromethane,

ð6Þ Excited-state quenching was quantitative, yet the measured yields of I2•- were φ = 0.25 for Ru(bpy)2(deeb)2þ and φ = 0.50 for Ru(bpy)32þ. The larger yield for Ru(bpy)32þ could be explained by specific interaction between iodide and the Ru(bpy)2(deeb)2þ compound, as described below.

A more intriguing route to making I-I bonds with light is to do so through ion pairs in low dielectric solvents.


I2 þ hν f 2I•

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Figure 4. Mechanistic description of I-I bond cleavage during reduction of I2•-. An analogous scheme can be drawn for I3reduction.

both static and dynamic mechanisms were observed. Stern-Volmer analysis of the photoluminescence data produced self-consistent estimates for a ground-state equilibrium constant to that for iodide, K = 51 000 M-1, and a bimolecular quenching rate constant, 4.0 1010 M-1 s-1. In acetonitrile, ion pairing was absent under the conditions studied, and a similar dynamic quenching rate constant of 4.7 1010 M-1 s-1 was extracted from the experimental data. The most likely mechanism for excited-state quenching was proposed to be reduction of triiodide with quantitative back electron transfer within the solvent cage. This data indicated that triiodide may be an efficient quencher of surface-bound MLCT excited states that do not rapidly inject electrons into TiO2 in dye-sensitized solar cells. Convincing evidence for reduction of I2•- and I3- that resulted in I-I bond dissociation has been realized with reactions of reduced Ru(II) compounds. As was previously discussed, iodide oxidation by Ru(bpz)2(deeb)2þ* yielded the reduced compound, Ru(bpz-)(bpz)(deeb)þ. The back reaction of Ru(bpz-)(bpz)(deeb)þ with I2•- was well described by a second-order equal concentration kinetic model, kcr = 2.1 1010 M-1 s-1, reaction 9.

Figure 5. Description of the flash-quench technique employed to study the reductive cleavage of I2•- and I3-.

also recently been described, reactions 10 and 11.28 In the flash-quench experiment, iodide oxidation of Ru(deeb)32þ* generated Ru(deeb-)(deeb)2þ, where reaction with I2•- or I3could be monitored, Figure 5. Titration of I3- into solution allowed reaction 11 to be the predominate pathway. RuII ðdeeb - ÞðdeebÞ2 þ þ I2 • - f RuII ðdeebÞ3 2þ þ 2I - ð10Þ RuII ðdeeb - ÞðdeebÞ2 þ þ I3 • - f RuII ðdeebÞ3 2þ þ I2 • - þ I ð11Þ Transient absorption data confirmed that I2•- was a product of triiodide reduction. Kinetic analysis for the loss of Ru(deeb-)(deeb)2þ and I3-, as well as the growth of I2•-, revealed a linear dependence of the observed rate constant with triiodide concentration that yielded a self-consistent second-order rate constant of 5.1 109 M-1 s-1 for reaction 11. Similar to I2•- reduction, no evidence for an [I32-] intermediate was found. Despite the self-consistency of the rate constant for triiodide reduction between loss of I3- and growth of I2•-, no conclusions could be made as to the correct pathway for reductive cleavage. Sutin's description of Marcus theory in the context of diffusional bimolecular reactions allowed the reduction potential to be calculated, E°(I3-/(I2•-,I-)) = -0.34 V.15 In this calculation, a reorganization energy (λ = 1.0 eV) and preexponential factor (νnκel = 1011 s-1) were assumed. This study provided the first experimental estimate for the oneelectron triiodide reduction potential in acetonitrile.28 Fluid Solution. There exists extensive literature of iodide redox chemistry in solution that reveals a tendency for iodide oxidation to be associated with the formation of I-I bonds.1,2 Initiating these reactions with light has already provided mechanistic insights into bond-forming reactions of possible relevance to the generation of solar fuels.19-22 While the rate laws for the photoinitiated reactions have not been firmly established, perhaps unsurprisingly, they appear to follow pathways similar to those identified for thermal electron transfer.1,2 The formation of I-I bonds has been shown to occur through light excitation of ion pairs to generate I-I bonds on subnanosecond time scales,22 as well as by diffusional electron transfer from a single iodine atom that subsequently reacts with a second iodide to yield I2•-.19,20 Evidence for the generation of iodine atoms with visible light excitation of Ru(bpz)2(deeb)2þ was reported for the first time. In this reaction, iodide transferred an electron to the t2g

RuII ðbpz - ÞðbpzÞðdeebÞþ þ I2 • - f RuII ðbpzÞ2 ðdeebÞ2þ þ 2I -

ð9Þ This diffusion-limited value was consistent with the expected large driving force for the reaction, ΔG° = -1.64 eV. The reaction cleanly produced the initial Ru(II) ground state with concomitant loss of diiodide. The mechanism for reductive cleavage of the I-I bond has been proposed to proceed through an [I22-] intermediate followed by bond dissociation or to occur in one concerted step, Figure 4.12,13 No evidence for an [I22-] intermediate was observed in the visible region, and competitive light absorption prevented the quantification of the iodide product.

Convincing evidence for reduction of I2•- and I3- that resulted in I-I bond dissociation has been realized with reduced Ru(II) ground-state compounds. Reactivity of both I2•- and I3- with Ru(deeb-)(deeb)2þ, generated by the conventional flash-quench technique, has


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Figure 6. (a) Extinction coefficient spectra of I3-, I2•-, and TiO2(e-) in CH3CN at 25 °C, in the presence of tetrabutylammonium cation. Absorption changes due to the cation-induced Stark effect for Ru(dtb)2(dcb)/TiO2 immersed in a 0.5 M LiI/CH3CN solution is also shown. (b) Absorption change measured 10 μs after pulsed 532 nm light excitation of Ru(dtb)2(dcb)/TiO2 immersed in a 0.5 M LiI/CH3CN solution. The absorption at 600 nm is due to TiO2(e-). The large derivativelooking feature centered near 475 nm has been attributed to the absorption of sensitizers influenced by TiO2(e-), that is, a Stark effect.

orbitals of the formal RuIII metal center in the excited state. The reaction can thereby be referred to as hole transfer from the MLCTexcited state to iodide. While a potent photo-oxidant was used to initiate the reaction, stopped-flow kinetic measurements by Stanbury and co-workers indicate that iodide oxidation is slightly more facile in acetonitrile, E°(I•/I-) = þ1.23 V (converted from þ0.60 V versus Fe(Cp)þ/0 using E°(Fe(Cp)þ/0) = þ0.63 V versus NHE29), than it is in water.30 Solid-state studies of [Ru(bpy)2(deeb)](I)2 ion pairs revealed that the Coulombic barrier for [I-,I-] association could be offset through specific Lewis acid-base interactions. The use of organic solvents with low dielectric constants also greatly facilitates ion pairing. Collectively, the observations of more favorable energetics and enhanced ion pairing suggest that a wide variety of excited-state compounds can be utilized to initiate I-I bondforming reactions in organic solvents. Studies of this type will certainly provide valuable information on chemical bond formation mechanisms that will help elucidate the low photochemical yields of oxidized iodide products observed thus far. Taking this a step further, light excitation of compounds with metal iodide bonds may allow these reactions to be driven by an inner-sphere mechanism. Coordination of iodide to metalloporphyrins, cobalt macrocycles, and compounds of the type cis-Ru(dcb)2I2 have been known for some time.31-33 In these examples, iodide is ligated to a transition metal that is also part of a chromophore. In some cases, photogeneration of iodine atoms was demonstrated, leading to subsequent I-I bond formation, reaction 5.31 The breaking of I-I bonds with light has been quantified by direct excitation of I3- and I2, which are known to possess dissociative excited states.25,26 Mechanistic studies have shown that at high thermodynamic driving force, diiodide reduction by a transition-metal compound can proceed near the diffusion limit in acetonitrile.19 The reduction of I3- clearly yielded I2•- as a product through dissociative electron transfer.28 Application of Marcus theory resulted in a surprisingly negative formal reduction potential, E°(I3-/(I2•-,I-)) = -0.34 V.


This value is certainly subject to considerable uncertainty as it is based on a single rate constant and several assumptions. Studies with a larger body of chemical reductants will help constrain the absolute reduction potential. Nevertheless, this value is in remarkable agreement with literature expectations and reveals a dramatic solvent dependence. The contrasting behavior of iodide equilibria in acetonitrile and aqueous solution is expanded upon below. It would be tremendously useful if a Latimer diagram like that shown in Figure 2 existed for organic solvents such as acetonitrile. We and many others have found that electrochemical measurements with metal electrodes report only on two-electron transfer reactions that are less relevant to photoinduced redox reactions that generally occur one electron at a time. Alternatively, the aqueous reduction potentials could be utilized to calculate values in organic solvents through thermochemical cycles. In practice, this method has not yielded consistent results as reliable estimates of the solvation energies are absent.30 Therefore, the reduction potentials must be determined through application of theory to kinetic data. There was in fact early evidence that solvent could play an important role; the equilibrium constant for I-,I2 with I3was reported to be >107 M-1 in acetonitrile and only 700 800 M-1 in water, reaction 3.5 Therefore, perhaps it is not surprising that formal reduction potentials differ considerably from the established values in water. For example, the I3reduction potential abstracted from flash-quench data described above was roughly 400 mV more negative in acetonitrile than that established for water (see Figure 2). The experimental value in acetonitrile was in reasonable agreement with a recently published lower limit for E°(I3-/(I2•-,I-)) g -0.35 V6 apparently estimated from Stanbury's upper limit for E°(I2•-/2I-) e þ0.93 V30 and the I3-/I- two-electron reduction potential. The strong reductant required to break the I-I bond of I3- in acetonitrile and the possible implications to dye-sensitized solar cells are discussed below. TiO2 Interfaces. The accepted electron-transfer sequence for an operational Gr€ atzel-type dye-sensitized solar cell is

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interfacial ionic reorganization of small cations such as Li þ. The screening kinetics were nonexponential but were well-described by the Kohlrausch-Williams-Watts model, from which a characteristic rate constant of 1.5 105 s-1 was abstracted. It was therefore necessary to understand the influence of the Stark effect on the spectral data before mechanistic studies of I-I bond formation could be made.

The origin of a previously unassigned transient absorption feature was determined to be from an underlying electroabsorption Stark effect, attributed to the electric field emanating from the TiO2 surface after injection of an electron.

Figure 7. Energetic scheme depicting estimated formal one-electron (green line) and two-electron (red line) reduction potentials for iodide species pertinent to dye-sensitized solar cells along with the free-energy density of states (DOS) of a TiO2 thin film measured in 0.5 M LiI/CH3CN.

widely recognized to be excited-state injection into TiO2 followed by iodide oxidation. Iodide is said to “regenerate” the sensitizer. Therefore, it is the oxidized compound that initiates I-I bond formation, not the excited state, as was the case for the previously described experiments in fluid solution. There are in fact photogalvanic cells where donor oxidation occurs before electron transfer to the semiconductor.34 In either case, the mechanism(s) for I-I bond formation at TiO2 interfaces remains speculative. There is no doubt that I2•- is formed as an intermediate after sensitizer or band gap excitation,35-38 but the mechanism for I-I bond formation remains tentative since diiodide could be formed in one concerted step, reaction 2, or through iodine atom intermediates, reaction 1, as was previously discussed. On the basis of the potentials alone, one might conclude that the concerted reaction is operative in dye-sensitized solar cells since iodine atom formation would be thermodynamically unfavorable by ∼120 mV for champion sensitizers like N3, E°(Ru(dcb)2(NCS)2þ/0) = 1.1 V.33 However, it must be considered that iodide may be in an activated state prior to oxidation due to ion pairing and/or electronic interactions with the sensitizer or TiO2 surface, thus possibly yielding iodine atom formation thermodynamically accessible. While some evidence for iodide ion pairing with sensitized TiO2 has been reported,39 it has not been observed in our laboratories, despite many attempts. In this regard, a new experimental observation provides an alternative interpretation of this data as well as some absorption transients previously assigned to iodide redox reactions.40-42 Photophysical studies of Ru(II) sensitizers anchored to mesoporous nanocrystalline TiO2 thin films immersed in LiI/CH3CN solutions revealed a previously unassigned transient absorption feature upon excitation with visible light. The origin of this feature was determined to be from an underlying electroabsorption Stark effect, attributed to the electric field emanating from the TiO2 surface after injection of an electron. Given the spectral range of the phenomenon, it could easily have been ascribed erroneously to diiodide redox chemistry, Figure 6a.43 The amplitude of the Stark effect decreased over time periods where charge recombination was absent, behavior attributed to “screening” of the electric field through


The newly discovered Stark effect gives rise to transient absorption changes that could be incorrectly assigned to ion pairs or other intermediates. Moreover, it complicates spectroscopic distinction of I2•- from I3- by visible absorption spectroscopy. Figure 6a shows the visible absorption spectra of I2•-, I3-, and TiO2(e-) in acetonitrile. Both iodide ions absorb blue light, but only I2•- absorbs in the red region. Hence, the ratio of the absorption at 400 nm relative to that at 750 nm is often utilized to quantify the concentrations of the two ions. While this approach is complicated by the presence of injected electrons, Figure 6b shows that the Stark effect for the sensitizer Ru(dtb)2(dcb)2þ represents a more significant issue as it can dominate the spectra. The mechanism(s) by which I-I bonds are broken at sensitized TiO2 interfaces is also poorly established. These reactions are important as recombination of injected electrons with oxidized iodide species lowers the TiO2 Fermi level and thus the open-circuit voltage. Time-resolved and steadystate photovoltage spectroscopies are widely used as indirect, yet in situ, probes of these reactions. Data from such studies indicate that sensitizers with binding site(s) for I2 or I3- can facilitate charge recombination.44 In addition, studies with sensitizers that have phenyleneethynyl linkers between surface binding groups and the metal center indicate that there may be an advantage to making I-I bonds distant from the TiO2 surface.45,46 There is also evidence that Ru(II) diimine compounds with low-lying π* orbitals can facilitate back electron transfer to I3-.47,48 It is worthwhile to collect the previously described oneelectron reduction potentials, abstracted from kinetic data in acetonitrile, and overlay them with the free-energy density of states of a mesoporous TiO2 thin film measured in

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0.5 M LiI/CH3CN, Figure 7. In creating this figure, we emphasize that the reduction potentials are sometimes based on a single measurement and are therefore subject to revision in the future. Even though the E°(I2•-/2I-) and E°(I3-/(I2•-/I-)) values are limits, they are given as equalities. In addition, the solar cell does not operate under standard conditions, and Nernstian shifts are significant. Likewise, the TiO2 density of states depends on a number of parameters and may vary significantly with materials fabrication procedures, the sensitizer, and/or additives present in the acetonitrile electrolyte. Finally, we note that when under constant sunlight illumination, the solar cell is no longer in equilibrium but at a steady-state condition. Despite all of these caveats, we believe it is still worthwhile to consider the possible consequences of the interfacial energetics for charge recombination. We note also that an excellent review of iodide redox chemistry in dye-sensitized solar cells was recently reported by Boschloo and Hagfeldt.6 It is generally agreed that regeneration of the oxidized sensitizer by iodide is quantitative, and ignoring impurities, recombination of the injected electron only occurs to the oxidized iodide species I3-, I2•-, I2, and I•.49 The electrons responsible for recombination are widely believed to be oneelectron transfer reductants trapped at TiIV sites (i.e., as TiIII) within the TiO2 nanocrystallites.5,50 A striking observation from even brief inspection of Figure 7 is that the reaction of I3- with an electron in a deep trap state is endergonic by several hundred millivolts. Therefore, the reaction is expected to be slow and the trapped electrons can only react rapidly with one of the other acceptors. Diiodide reduction is favored for all injected electrons, and there is indeed some evidence that I2•- is a relevant acceptor at dye-sensitized TiO2 interfaces.51 Recent spectroscopic data have also indicated that I2•- disproportionation within the TiO2 mesopores occurs with the same rate constant as that measured in fluid solution, 3 109 M-1 s-1.38 Reduction of I2•- by TiO2(e-) would therefore have to compete with this fast disproportionation. However, since disproportionation is a bimolecular redox reaction, a low local concentration of I2•- in the mesopores may have a significant lifetime. Molecular iodine reduction is also favored yet may be limited by low concentration. The previously defined equilibrium constant for reaction 3 indicates that with the 0.5 M LiI and 0.05 M I2 concentrations typically used in dye-sensitized solar cell preparation, the equilibrium concentration of I2 is vanishingly small. Iodine atoms are also potential acceptors with the largest driving force for reduction. However, the rapid regeneration rates measured spectroscopically by many groups indicate that the local iodide concentration is high, such that iodine atoms, if they are indeed generated, should rapidly react with iodide. A reasonable but speculative hypothesis for the unparalleled success of the I /I3 redox mediator in dye-sensitized solar cells may be stated as follows: sensitizer regeneration •forms I2 , which rapidly disproportionates to products whose one-electron reduction potentials are unfavorable for reaction with electrons injected into TiO2. If this hypothesis is correct, it would imply that oxidized donors with large driving forces for


disproportionation would also be effective mediators in dyesensitized solar cells.52

A reasonable but speculative hypothesis for the unparalleled success of the I-/I3- redox mediator in dye-sensitized solar cells may be stated as follows: sensitizer regeneration forms I2•-, which rapidly disproportionates to products whose one-electron reduction potentials are unfavorable for reaction with electrons injected into TiO2.

AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed.

Biographies John G. Rowley is currently a graduate student at Johns Hopkins University. He received his A.S. from Flathead Valley Community College in 2003 and his B.S. in Chemistry from the University of Alaska, Fairbanks in 2005. His research interests lie in understanding the mechanisms of interfacial and interparticle electron-transfer within high-surface-area metal-oxide thin films pertinent to dye-sensitized solar cells Byron H. Farnum is currently a graduate student at Johns Hopkins University. He received his B.S. in Chemistry from the University of South Carolina in 2008. His research interests lie in understanding the photochemically driven redox reactions of iodide species within dyesensitized solar cells. Shane Ardo received his Ph.D. from Johns Hopkins University in 2010. His dissertation was entitled “Photoinduced Charge, Ion, & Energy Transfer Processes at Transition-Metal Coordination Compounds Anchored to Mesoporous, Nanocrystalline Metal-Oxide Thin Films”. He is currently a postdoctoral scholar in Prof. Nathan Lewis' laboratories at the California Institute of Technology. Gerald J. Meyer is the Bernard N. Baker Professor of Chemistry and Materials Science & Engineering at Johns Hopkins University. His research interests include excited states, photoelectrochemistry, redox reactions, and solar energy conversion.

ACKNOWLEDGMENT This work was supported by a grant from the Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, U.S. Department of Energy (DE-FG0296ER14662). The authors thank Dr. James Gardner, Dr. Andras Marton, Dr. Chris Clark, and Dr. Amanda Morris, whose thesis work was described in part herein and has enabled our current studies.

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Iodide Chemistry in Dye-Sensitized Solar Cells: Making and

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