Article pubs.acs.org/Langmuir
Role of the Triiodide/Iodide Redox Couple in Dye Regeneration in p‑Type Dye-Sensitized Solar Cells Elizabeth A. Gibson,*,†,§ Loïc Le Pleux,‡ Jérôme Fortage,‡ Yann Pellegrin,‡ Errol Blart,‡ Fabrice Odobel,‡ Anders Hagfeldt,†,∥ and Gerrit Boschloo† †
Department of Physical and Analytical Chemistry, Uppsala University, Box 259, SE-751 05 Uppsala, Sweden CEISAM, Chimie et Interdisciplinarité, Synthèse, Analyse, Modélisation, CNRS, UMR 6230, Faculté des Sciences et des Techniques Université de Nantes, 2 rue de la Houssinière, BP 92208, 44322 Nantes Cedex 3, France
‡
S Supporting Information *
ABSTRACT: A series of perylene dyes with different optical and electronic properties have been used as photosensitizers in NiO-based p-type dyesensitized solar cells. A key target is to develop dyes that absorb light in the red to near-infrared region of the solar spectrum in order to match photoanodes optically in tandem devices; however, the photocurrent produced was found to decrease dramatically as the absorption maxima of the dye used was varied from 517 to 565 nm and varied strongly with the electrolyte solvent (acetonitrile, propionitrile, or propylene carbonate). To determine the limitations of the energy properties of the dye molecules and to provide guidelines for future sensitizer design, we have determined the redox potentials of the diiodide radical intermediate involved in the charge-transfer reactions in different solvents using photomodulated voltammetry. E°(I3−/I2•−) (V vs Fe(Cp)2+/0) = −0.64 for propylene carbonate, −0.82 for acetonitrile, and −0.87 for propionitrile. Inefficient regeneration of the sensitizer appears to be the efficiency-limiting step in the device, and the values presented here will be used to design more efficient dyes, with more cathodic reduction potentials, for photocathodes in tandem dye-sensitized solar cells.
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INTRODUCTION Despite the enormous attention focused on n-type dyesensitized solar cells (DSCs) over the last two decades,1,2 comparatively little work has been reported on the sensitization of p-type semiconductors (p-DSCs).3,4 Dye sensitization of ptype semiconductors gives rise to a photoactive electrode whereby cathodic photocurrents are generated upon visible light excitation. Functional, mesoporous, NiO-based p-type devices have been reported in which the iodide−triiodide redox couple5 and dyes including coumarin 343, perylene derivatives,6 and triphenylamine derivatives7 have been incorporated. IPCE values of up to 64% have so far been reported for such devices.8 The incorporation of a photocathode in tandem with a TiO2based n-type photoanode produces a device whereby the VOC is the difference between the Fermi level close to the conduction band in the n-type semiconductor and that close to the valence band in the p-type semiconductor, potentially giving a substantial increase over the VOC of the individual n- and pDSCs.9,10 By choosing sensitizers that absorb high-energy photons on one electrode and low-energy photons on the other, the full solar spectrum can be utilized. For the best ntype dye-sensitized solar cells, the spectral response is optimal at wavelengths shorter than 700 nm (1.77 eV).11,12 This is significantly lower than the ideal frontier orbital separation for a single junction solar cell of 925 nm (1.34 eV), which would have a maximum theoretical efficiency of 33.68%. However, a tandem cell assembled from two electrodes connected in series, © 2012 American Chemical Society
using sensitizers with frontier orbital separations of 0.94 eV (1320 nm) and 1.60 eV (775 nm), could operate with a maximum theoretical efficiency of 45.7%.13,14 Therefore, there is a real incentive to produce a photocathode that operates in the red to near-infrared region of the solar spectrum. Whereas gains in conversion efficiency brought about by this alternative configuration are likely to be substantial, the field of p-type DSCs is still in its infancy. The highest reported efficiency for an NiO-based p-DSC is 0.41%. The tandem cell incorporating this photocathode operated with an efficiency of 2.42%, which was lower than the corresponding n-DSC (5.9%) because there was significant spectral overlap.10 Evidently, a number of improvements to the devices are needed before competitive efficiencies are reached. Previously, we made substantial improvements to NiO-based p-DSCs by designing dyes that promote charge separation and limit recombination between the sensitizer and the semiconductor.15−17 Alternative redox electrolytes have also been used; when used in conjunction with a suitable sensitizer, these give a 3-fold increase in photovoltage over that produced by triiodide/iodide p-type DSCs.18 Despite our successes in improving the efficiency of the p-type system, we have encountered a number of hurdles that limit the performance Received: January 15, 2012 Revised: March 14, 2012 Published: March 20, 2012 6485
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Figure 1. Structural formulas of the PI sensitizers used in this study. electrodes were assembled face-to-face with a platinized TEC8 (Pilkington, sheet resistance 8 Ω/square) counter electrode using a 30-μm-thick thermoplastic frame (Surlyn-30, Dyesol). The electrolyte, containing 1.0 M LiI and 0.1 M I2 in propylene carbonate, propionitrile, or acetonitrile, was introduced through the predrilled hole in the counter electrode, which was then sealed. Incident photon to current conversion efficiency (IPCE) spectra were recorded using a computer-controlled setup consisting of a xenon light source (Spectral Products ASBXE-175), a monochromator (Spectral Products CM110), and a potentiostat (EG&G PAR 273), calibrated using a certified reference solar cell (Fraunhofer ISE). Absorption and emission data of the compounds were recorded in dichloromethane using a 2501PC Shimadzu spectrophotometer and a SPEX Fluoromax fluorimeter, respectively. The zero−zero singlet excited-state level (E0−0) was calculated with the wavelength at the intersection (λinter) of the emission and absorption spectra with the equation E0−0 (eV) = 1240/λinter (nm). Redox potentials of the dyes in solution were recorded in 0.15 M (Bu4N)PF6 in dichloromethane as the supporting electrolyte with an Autolab PGSTAT 302N potentiostat with a saturated calomel electrode as the reference. Values were calibrated against an external standard Fe(Cp)2+/0. Reduction potentials of the sensitizers adsorbed on NiO were measured using differential pulse voltammetry using 0.1 M LiClO4 in acetonitrile as a supporting electrolyte and a Ag/AgNO3 reference electrode. Values were calibrated against an external standard, Fe(Cp)2+/0. We have performed these experiments for a range of dyes and have found that the redox potentials are affected little by the solvent once adsorbed on the NiO surface in the presence of a lithium-containing supporting electrolyte. The rest potentials of the electrolytes in each solvent were measured in a two-electrode setup using a Ag/AgNO3 reference electrode in the corresponding solvent and a platinum working electrode. Photomodulated voltammetry was carried out on 6 × 6 mm2 sandwich cells, assembled as above. The working electrode was TEC15 glass dipped in a 10 mM aluminum isopropoxide solution in isopropanol for 20 min at 70 °C and sintered at 450 °C for 30 min to limit the dark current. The counter electrode (and reference) was platinized TEC8 glass. Excitation of the sample was provided by light from a blue LED (470 nm), which was modulated (on/off) at a frequency of 40 Hz by electronic means using an LED driver system.
of the device. In particular, following light absorption by the dye, the rate of the charge-transfer reaction from the photoreduced dye molecules to the redox mediator (dye regeneration) must be competitive with the extremely fast (10− 100 ps) charge recombination process between the reduced dye molecules and photoinjected holes in the NiO electrode. The dye regeneration reaction must be sufficiently thermodynamically favorable to compete with recombination, and energy losses in this step are one of the main causes of the low efficiencies measured with photocathodic DSCs. To improve this efficiency and achieve photocurrents comparable to those obtained with the state-of-the-art n-type DSCs for tandem cell applications, the electronic properties of the dyes must be optimized so that there is sufficient driving force for electron transfer from NiO to the photoexcited dye and from the reduced dye to the electrolyte; at the same time, the frontier orbital separation in the dye should be minimal so that photons can be collected over as wide an energy range as possible. Here we explore the effects of tuning the electronic properties of a series of perylene dyes to maximize the performance of NiObased p-DSCs. These results represent guidelines for the better design of the next generation of NiO photosensitizers.
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MATERIALS AND METHODS
All chemicals were purchased from Sigma-Aldrich unless otherwise stated. The synthesis of perylene imide (PI) sensitizers 1−6 was described previously.19,20 A triblock copolymer F108-templated precursor solution of NiO was prepared according to the published procedure by Sumikura et al.21 Layers of precursor solution were applied to conducting glass substrates (Pilkington TEC15, sheet resistance 15 Ω/square) by doctor blading using Scotch tape as a spacer (0.25 cm2 active area) until the film thicknesses (measured with a Sloan Dektak profilometer) were found to be approximately 2 μm. The films were held in a furnace at 450 °C for 30 min after each deposition step. The NiO electrodes were soaked in a methanol solution of 1, an acetone solution of 2, or a toluene solution of 3−6 (0.1 mM) for 16 h at room temperature. The sensitized NiO 6486
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Table 1. Optical and Electrochemical Properties of the Dyes dye
λabs (ε/M−1 cm1)/nma
λem/nma
E°′(D+/D)/V vs Fe(Cp)2+/0
E0−0/eVc
1 2 3 4 5 6
413(7 200); 487(21 200); 517 (31 900) 419 (1 700); 522 (7 500) 528 (shoulder); 566 (10 000) 364(8 500); 419(5000); 560(37 400) 374(10 300); 390(10 300); 453(17 400); 565 (39 600); 607 (62 700) 375(11 400); 389(11 500); 451(18 300); 563(45 600); 602(66 000)
587 588 624 641 647 645
0.65 0.74 0.74 0.60 0.55 0.88
2.24 2.20 2.09 2.06 1.97 1.99
E°′(D/D−)/V vs Fe(Cp)2+/0b − − − − − −
1.48 1.35 1.35 1.43 1.12 1.12
ΔGhinj/eVd − − − − − −
0.88 0.90 0.86 0.75 0.97 0.99
a The absorption and emission spectra were measured in dichloromethane solution. bThe ground-state oxidation potential (E°′(D+/D)) and reduction potential (E°′(D/D−)) of the dyes were determined in dichloromethane with 0.15 M Bu4NPF6 as a supporting electrolyte with a Pt working electrode and using SCE as a reference. cThe 0−0 transition energy (E0−0) was estimated from the intersection of the normalized absorption and emission curves with the equation E0−0(eV) = 1240/λinter(nm). dCalculated from the simplified Rehm−Weller equation according to ΔGhinj = EVB(NiO) − (E00 + Ered) using EVB(NiO) = −0.12 V vs Fe(Cp)2+/0 (Figure S1).
Table 2. Photovoltaic Performance of the Dye-Sensitized NiO Solar Cells Containing the Different PIs dye
solvent
1 2 3 4 5 6 1 2 3 4 5 6 1 2 3 4 5 6
propionitrile propionitrile propionitrile propionitrile propionitrile propionitrile propylene carbonate propylene carbonate propylene carbonate propylene carbonate propylene carbonate propylene carbonate acetonitrile acetonitrile acetonitrile acetonitrile acetonitrile acetonitrile
E°′(D/D−)/V vs FeCp2+/0a − − − − − − − − − − − − − − − − − −
1.3 1.2 1.2 1.1 1.0 1.0 1.3 1.2 1.2 1.1 1.0 1.0 1.3 1.2 1.2 1.1 1.0 1.0
η/%b
VOC/mVb
JSC/mA cm−2b
FFb
IPCE/%c
0.04 0.02 0.03 0.02 0.02 0.02 0.05 0.04 0.04 0.04 0.03 0.02 0.04 0.03 0.04 0.03 0.02 0.01
105 100 100 100 95 95 90 100 100 95 105 85 85 70 85 80 70 75
1.14 0.70 0.97 0.48 0.39 0.41 1.71 1.42 1.33 1.04 0.83 0.75 1.42 1.41 1.41 1.07 0.94 0.54
0.32 0.32 0.32 0.35 0.30 0.41 0.30 0.30 0.33 0.33 0.34 0.33 0.34 0.33 0.30 0.33 0.33 0.33
20 18 14 8 2 3 36 25 17 16 18 9 22 23 14 5 5 3
a For dye adsorbed on NiO with 0.1 M LiClO4 in acetonitrile as the supporting electrolyte. bElectrolyte: 1.0 M LiI, 0.1 M I2 in the solvent stated. cAt λmax for the dye.
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carbonate).15,24 Nonetheless, during recent studies we have found that some dyes perform better when propylene carbonate is used compared to acetonitrile. Therefore, in this work we have chosen to compare the devices using three different electrolyte solvents while keeping the ratio of I3− to I− constant. This comparison is also included in Table 2. A number of trends were observed. First, the devices that used propylene carbonate as a solvent had higher efficiencies than those using acetonitrile or propionitrile. Second, the IPCEs decreased with increasing λmax for the dyes. The IPCE of a device can be separated into a number of key components as given in eq 1.
RESULTS AND DISCUSSION
A series of perylene imide (PI) compounds (Figure 1) were used as sensitizers for NiO-based DSCs using the standard triiodide−iodide electrolyte. PIs are attractive sensitizers in ntype and p-type DSCs owing to their high extinction coefficients, photostability, and the ease with which they can be functionalized with different anchoring groups and electron donating/withdrawing substituents.22,23 Because our target is to prepare optically matched tandem DSCs, we have prepared a series of perylene dyes with different electronic properties and absorption maxima that can be matched with complementary sensitizers on n-type photoanodes. The optical and electrochemical properties of the sensitizers are given in Table 1. The photovoltaic properties of the devices using the different sensitizers are listed in Table 2, and the photocurrent action (IPCE) spectra are shown in Figure 2. In our early studies, we typically used propylene carbonate as a solvent for the electrolyte because of its electrochemical stability and low volatility.7 Later, the best-performing devices were obtained with acetonitrile as a solvent, possibly because of its lower viscosity, which leads to a greater diffusion coefficient for triiodide over propylene carbonate (1.18 × 10−5 cm2 s−1 for acetonitrile and 1.56 × 10 −6 cm 2 s −1 for propylene
IPCE(λ) = LHE(λ) × ηCC × ηinj × ηreg
(1)
where LHE is the light-harvesting efficiency according to eq 2 LHE(λ) = 1 − 10−A(λ)
(2)
where A(λ) is the absorbance of the film at wavelength λ. Because the LHE(λ) for each of the sensitized films was greater than 90%, we assume that this is not the limiting factor to the photocurrent. ηinj is the hole injection efficiency according to eq 3 6487
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Figure 2. Photocurrent action spectra for DSCs sensitized with different PIs using 0.1 M I2 and 1.0 M LiI in (a) propylene carbonate, (b) acetonitrile, and (c) propionitrile.
ηinj =
k inj τ−1 + k inj
ηCC = 1 − (3)
τtr τr = τh τr + τCC
(4)
where τr is the time constant for recombination, τCC is the time constant for charge collection, τtr is the hole transport time constant, and τh is the hole lifetime. ηCC is largely a property of the electrolyte and semiconductor unless the dye is able to inhibit recombination between the redox mediator and the semiconductor or promote charge transfer through the dye layer. This can be assumed to be constant for this dye series because they are similar in size and nature. Perylene dyes are known to suffer from aggregation problems in solution and can stack on semiconductor surfaces, but in this series, the dyes have been prepared with bulky t-butyl groups to prevent this (Figure 1). Accordingly, we will assume that ηCC is not the efficiency-limiting parameter here. In p-type DSCs, two fundamental parameters affecting the performance are the rate of charge recombination between the reduced dye and the oxidized NiO and the efficiency of charge interception by the redox mediator, which both contribute to the regeneration efficiency, ηreg in eq 1. ηreg is usually approximately unity in the TiO2-based system because charge recombination between the semiconductor and dye molecules is relatively slow compared to the rate of dye regeneration by the redox couple; therefore, for discussions of n-type devices, it is usually neglected.25 In the NiO p-type system, the rate of charge recombination is very fast (sub-nanosecond scale).16−18,20,26,27 Such a short charge-separated-state lifetime means that the regeneration reaction must be very fast
where τ−1 is the excited-state lifetime of the dye in the absence of hole injection and kinj is the rate of charge transfer to the semiconductor. In our previous study, we demonstrated that the time constant for electron transfer from NiO to the excited PI sensitizers is approximately 0.5 ps and the excited-state lifetime is on the order of a few nanoseconds.17−19 Furthermore, using the reduction potential of the dye in dichloromethane solution, the E0−0 value of the dyes in dichloromethane solution, and the valence band potential of NiO (EVB(NiO) = −0.12 V vs Fe(Cp)2+/0 in 100 mM LiClO4 in MeCN, estimated using spectroelectrochemistry; see Figure S1 in the Supporting Information), the Gibbs free energy of the hole injection reaction (ΔGhinj) was estimated (Table 1). Clearly, the latter process is strongly exergonic for all of the dyes. A recent communication by Smeigh et al.20 reports the hole injection and recombination dynamics measured using transient absorption spectroscopy. For each of the perylene dyes used in this study, the hole injection reaction is rapid (with time constants of less than 5 ps) compared to the excited-state lifetime (4 to 5 ns), indicating that charge injection is theoretically close to unity and at the very least unlikely to be the efficiency-limiting step, and ηinj can be ignored and will not be further discussed in this article. ηCC is the charge collection efficiency according to eq 4 6488
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(although we have yet to find evidence of the reaction proceeding faster than a microsecond timescale12) and efficient in order to intercept the charge before the dye returns to its neutral ground state. In early publications on n-type (and ptype) DSCs, simple mechanistic illustrations have simplified the regeneration process to be between E(D*/D+) (or E(D/D−) and E°(I3−/I−)) to avoid the ambiguity that exists with regard to the exact reaction processes. However, it is now generally understood that the regeneration processes are more complex, involving radical iodine-based intermediates.28,29 In the n-type system, E°(I2•−/I−) and E°(I•/I−) are the important redox potentials to consider for the regeneration of the dye, whereas in the case of the p-type system it is E°(I3−/I2•−) that is important.17,30,31 The complex redox chemistry of iodide/ triiodide may partially explain its success as a redox mediator in the Grätzel cell, but it adds constraints to the choices of energy levels and optical properties of the chosen dyes. In addition, it limits the maximum obtainable photovoltage as the potential of the counter electrode (and the built-in voltage of the device) is imposed by the E°(I3−/I−) couple, which is more positively shifted than the E°(I3−/I2•−) couple involved in the reoxidation of the reduced sensitizer in p-DSCs. The consequences partially explain the lower performance of p-DSCs compared to that of n-DSCs. Figure 2a shows the trend in IPCE for the cells with different PI sensitizers and with propylene carbonate as the electrolyte solvent. The IPCE is highest for the dye with the absorption maxima at the highest energy, despite the trend in increasing extinction coefficient and dye polarity (charge-transfer character) going in the opposite direction. The IPCE is lowest for 6, which has an absorption maximum at the lowest energy apart from 5, which is similar. For the dyes in this series, as the HOMO−LUMO energy gap is decreased and the absorption maximum is shifted to longer wavelength, the reduction potential of the dyes adsorbed on NiO (Table 2) is less negative. For 5 and 6, the reduction potential and absorption maxima are similar. Although several factors influence the photocurrent in the device, such as the polarity and permittivity of the solvent and the location of the electrons on the reduced dye, the general trend is attributed to the lower driving force for regeneration despite the lower driving force for recombination, which is approximately the difference in energy between the reduced dye and the NiO valence band edge. The time constants for recombination, reported previously, are on the order of 100 ps for dyes 1−4 and slightly longer for dyes 5 (470 ps) and 6 (260 ps).16,20 The shorter charge-separated-state lifetime for 6 compared to that for 5 may explain the differences in IPCE between the two dyes. As mentioned above, we have ignored the change in the reduction potential for the excited state energy because the driving force for hole injection is large for all of the dyes in this series. From Table 1, the driving force for hole injection follows the series 6 > 5 > 2 > 1 > 3 > 4, which does not match the trend in IPCE (Table 2), hence our assumption is validated. Figure 2b shows the IPCE for the equivalent cells using acetonitrile as the electrolyte solvent. The IPCE for these cells follows the same trend as in Figure 2a, but the IPCE values are systematically lower. For these cells, a significant peak in the IPCE spectra arises at lower wavelength (ca. 380 nm) than the absorption maxima of the dyes absorbed on the film. In Figure 2c, the IPCE spectra for the equivalent cells with propionitrile as the electrolyte solvent are shown. For these cells, the IPCE at the wavelength corresponding to the maximum absorption of
the dyes in the visible region is even lower than in Figure 2a,b, but there is also a contribution to the photocurrent at shorter wavelength. The photocurrent at 380 nm could be generated from a higher-energy electronic transition of the dye, the direct excitation of NiO, or the photoreaction of I3− in eqs 5 and 7. I3− + hν → I 2•− + I• photodissociation
(5)
2I 2•− → I3− + I− disproportionation
(6)
I 2•− + NiO → NiO(+ ) + 2I− hole injection to NiO (7)
We are confident that the excitation of I3− is responsible rather than the excitation of the NiO or the dyes because the IPCE curve best matches the absorption spectrum of triiodide.20 Furthermore, the maximum IPCE at 380 nm varies significantly when the triiodide concentration is varied.5,32 One explanation for the solvent dependence could be the shift in the iodine−triiodide equilibrium in the different solutions. Both I3− and I2•− are favored in solvents with a high dielectric constant (65 for PC, 36 for MeCN, and 27.2 for PrCN) and a high donor number (DN(MeCN) = 14.1, DN(PC) = 15.1, DN(PrCN) = 16.1).33−35 An increase in the quantity of triiodide should simultaneously increase the contribution to photocurrent from the I2•− photoproduct in addition to that from the sensitizer. The rate constant for the disproportionation of I2•− is reasonably solvent-independent; however, an increase in the concentration of I2•− would strongly favor disproportionation reaction 6, decreasing the radical lifetime.36,37 However, with such large equilibrium constants for process 8, (K = 6.3 × 106 L mol−1 in acetonitrile, K = 6.3 × 107 L mol−1 in propylene carbonate), the difference in triiodide in propylene carbonate available for regeneration of the dye is only 16 nM, so solvent should have a negligible effect.38,39 I3− ⇋ I 2 + I− triiodide dissociation
(8)
We also do not expect the aprotic solvents chosen for this study to differ significantly in their interaction with the electrolyte or dye on account of their similar donor strengths and strong dielectric constants.40 Indeed, NMR spectroscopy and conductance measurements have indicated that in propylene carbonate and acetonitrile, iodide salts such as LiI form contact ion pairs, where the anion is present in the inner solvation shell. In contrast, in most polar nonaqueous solvents I3− salts are completely ionized, forming solvent-separated ion pairs.41,42 Because it is likely to be fully solvated in all three solvents chosen here, we believe the interactions between the triiodide anion, responsible for the regeneration reaction, and the dye to be similar in all cases. It is likely that Li+ may interact with the dye, especially the dye radical anion, and it is known to stabilize the excited state of perylenes adsorbed on nanostructured TiO2.43 However, at such high ionic strength and considering the similarity between the behavior of lithium salts in acetonitrile and propylene carbonate, we do not propose that this will be a dominating factor. Therefore, we propose that the general decrease in IPCE in the visible region as the solvent is changed from propylene carbonate to acetonitrile to propionitrile is due to the shift in redox potentials of iodine species in different solvents. A plot of this trend is provided in the Supporting Information (Figure S4) The standard potentials of the I3−/I− redox couple in different solvents have been 6489
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Table 3. Electrochemical Potentials for Electrolytes in Different Solventsa
a
solvent
Erest(I3−/I−)b
E°′(I3−/I−)b
E°′(I3−/I2•−)b
E°′(I2•−/I−)b
ΔG°reg (5)c
ΔG°inj(I2•−)d
propionitrile propylene carbonate acetonitrile
−0.41 −0.27 −0.30
−0.38 −0.24 −0.28
−0.87 −0.64 −0.82
0.12 0.16 0.26
−0.13 −0.36 −0.18
−0.24 −0.28 −0.38
1.0 M LiI and 0.1 M I2 in propylene carbonate, propionitrile, and acetonitrile. bValues in V versus FeCp20/+. cValues in eV, described in eq 13. Values in eV, described in eq 14.
d
Figure 3. Potential energy diagram of the solvent-dependent charge-transfer processes involved in p-DSCs. The arrows indicate the direction of electron flow. (1) Hole injection from NiO to the electrolyte. (2) Dye regeneration reaction. The NiO valence band edge was estimated from spectroelectrochemical methods (Figure S1) as described by Boschloo and Hagfeldt.51
The redox processes were found to be solvent-dependent. With nitrile solvents, E°′(I3−/ I2•−) was found to lie close to E°′(D/D−) for dyes 5 and 6, which may give rise to poor ηreg accounting for the low IPCE for the PI sensitizers that absorb at longer wavelength. The driving force for regeneration (eq 13, listed for 5 in Table 3) for each of the dyes follows the order propionitrile < acetonitrile < propylene carbonate and agrees with the overall differences in photocurrent arising from dye sensitization. The driving force for hole injection from I2•− (eq 14, Table 3) is largest for acetonitrile accounting for the contribution to the photocurrent at ca. 380 nm when this solvent is used. The driving force was smaller for propylene carbonate but sufficiently large for charge transfer to occur as illustrated as a potential energy diagram in Figure 3. The driving force was lowest for propionitrile but, again, sufficient for charge transfer, suggesting that there may also be a kinetic limitation to this process. The magnitude of the signal in the PMV experiment was highest for acetonitrile and lowest for propylene carbonate, indicating that the I2•− diffusion length was longer for acetonitrile than for propylene carbonate, which could explain the similarity in photocurrent at 380 nm between the nitrile solvents, despite the difference in driving force for the reaction.46 The lower diffusion length of I2•− in propylene carbonate is likely to be the reason for the lack of hole injection from I2•− to NiO. We attribute the small differences in IPCE between dyes at 380 nm in the nitrile solvents to differences in the local concentration of triiodide at the electrode surface brought about by interactions with the adsorbed PI molecules. Supporting evidence for this can be observed in the publication by Zhu et al. where the spectral response at 380 nm almost doubles in magnitude in a NiO p-DSC sensitized with coumarin 343 compared to that for an unsensitized NiO film in the presence of LiI/LiI3 in methoxypropionitrile.5 The increase in the local concentration of redox species could be different for different dyes. For example, Myashita et al. have postulated that dye molecules may associate with I3− and Li+
thoroughly investigated in the literature; however, the solvent dependence of the redox chemistry of the intermediates is not very well documented.38−41 Because we observe such a large differences in IPCE with different solvents, E°(I3−/ I2•−) must lie close to E°(D/D−). Small changes in the redox potential of the electrolyte due to different solvents appear to have a significant impact on the regeneration of the dye, prompting us to investigate these processes further. To estimate the potentials of the radical intermediates of importance in the DSC, we have recently applied a technique known as photomodulated voltammetry. For a detailed description of the experiment, we direct the reader to the article by Boschloo et al.41 In brief, an on/off modulated blue LED is used to excite the electrolyte that is sandwiched between two conducting glass electrodes. At the same time, the voltage across the cell is swept and the current is recorded. Examples of photomodulated voltammograms along with a schematic illustration of the experimental setup are given in Figures S2 and S3 in the Supporting Information. The magnitude of the PMV signal depends greatly on the viscosity of the electrolyte, which affects the diffusion coefficient and diffusion length of I2•−.45Two waves can clearly be observed, corresponding to reactions 9 and 10: I3− + e− → I 2•− + I− E°(I3−/I 2•−)
(9)
I 2•− + e− → 2I− E°(I 2•−/I−)
(10)
I3− + 2e− → 3I− E°(I3−/I−)
(11)
For each of these reactions, the potentials in V versus E°′(I3−/ I−) were estimated from the point of inflection of the corresponding wave. The rest potential for each redox couple was measured versus an Ag+/0 reference electrode in the same solvent, and these were calibrated against FeCp2+/0. A summary of each of the values is given in Table 3. 6490
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was estimated by Farnum et al. using a flash-quench technique and is in reasonable agreement with our results.56 There are few dyes that have been applied in NiO p-DSCs by comparison to n-DSCs, so a comparison with similar systems with optimal frontier orbital energies is difficult. One dye that has sufficient driving force to be regenerated by all of the electrolyte solutions used in this study is coumarin C343. From an analysis of the literature, it appears that the choice of solvent does not affect the photocurrent density or IPCE at wavelengths corresponding to the dye.5,7,57 Unfortunately, the performance of p-DSCs with this dye is weak because the charge-separated-state lifetime is extremely short, which limits the IPCE to approximately 20%.32,57 In contrast, triphenyl amine dyes P4 and P1 reported by Qin et al. have an order of magnitude longer charge-separated-state lifetime, but in these cases, the IPCE is higher when acetonitrile is used rather than propylene carbonate on account of the greater diffusion coefficient of triiodide in acetonitrile compared to that in propylene carbonate (in MeCN, D = 1.18 × 10−8 m2 s−1; in propylene carbonate, D = 1.56 × 10−9 m2 s−1).15,27 However, a similar dye that had a reduction potential of −0.75 V versus FeCp2+/0 produced a negligible photocurrent (6% IPCE with P3 compared to 63% IPCE with P1, which has a reduction potential of −1.31 V vs FeCp2+/0) with triiodide/iodide in acetonitrile despite a similar structure and charge-separatedstate lifetime. The development of new sensitizers with sufficient driving force for regeneration as well as long charge-separated-state lifetimes is the focus of our current work. The short charge-separated-state lifetimes (10−100s ps) of organic dyes bound to NiO call into question how regeneration can compete with recombination at all. The optimum triiodide concentrations used in p-DSCs are, counterintuitively, relatively small (ca. 0.1 M) compared to those for iodide (ca. 0.9 M). Because the regeneration reaction is expected to occur on the nanosecond to microsecond timescale, either a small amount of reduced dye that is very efficiently oxidized remains on a longer timescale or triiodide is preassociated, lying close in space to the dye. Solid-state adducts of triiodide with bipyridine ligands of transition-metal complexes have been observed,58,59 and investigations are ongoing to determine if and where these interactions may take place for organic dye−NiO systems. The strength of these dye−electrolyte interactions and the rate of charge transfer between strongly coupled species should also be solvent-dependent. Whereas the substitution of the triiodide/ iodide electrolyte for a one-electron, outer sphere redox couple such as cobalt polypyridyl complexes would relieve the constraint that the complexities of the iodine-based system impose on the energetics of the dye, the regeneration reaction can be too slow to compete with fast charge recombination.16,18 Therefore, the next generation of p-DSCs will need to be energetically better matched to NiO and the chosen electrolyte and will enable long-lived charge-separated states.
ions, and O’Regan et al. have proposed that dye molecules may bind I2, each of which could promote recombination in n-type DSCs or regeneration in p-type DSCs.47,48 DeAngelis has investigated the ion interaction with coumarin dyes assembled on TiO2 in vacuo and in MeCN solution using computational methods and has demonstrated that the association of triiodide with the aromatic region is energetically favorable, as is iodine with oxygen and sulfur, and in agreement with Mori’s hypothesis, lithium is likely to associate with a carbonyl oxygen, but this does not affect triiodide binding.49 Li+ (1 M) could help screen the charge from the radical anion, as could a high dielectric solvent, increasing the current. Indeed, high concentrations of LiI are needed in the cell for it to work, and it is possible that this is not entirely because of the increased voltage from the altered valence band edge. The I2•− radicals have a lifetime of 0.2 ms under the conditions used in our photomodulated voltammetry experiments and solar cells, indicating a concentration of approximately 1 μM. It is likely that an increase in concentration at the electrode surface could have a sizable effect on the photocurrent density. We are carrying out experiments to support this hypothesis, but in a related article, we have shown that the dark current from a TiO2 electrode can be catalyzed by dyes that are efficient NiO photosensitizers.27 This peak in the IPCE spectra for the cells using nitrile solvents differs in magnitude slightly. It is known that perylene dyes have a tendency to associate with iodine, which may assist the regeneration of these dyes by increasing the local concentration of triiodide despite their short charge-separated-state lifetime.50 For example, PI 3 contains a nitrile group that is thought to interact favorably with triiodide.49 PI 5 contains a second carboxylic acid group that may alter the arrangement of the dye molecules on the surface through hydrogen bonding, preventing the approach of the redox couple, or the electropositive carbon could bind iodide as proposed by Meyer or the electronegative oxygen atoms could associate with Li+.29,47 We note that the absorptivity of the films for all dyes was similar (above 1 absorbance unit), despite the differences in the extinction coefficient; however, we are unable to correlate the dye loading to the differences in IPCE at both 380 nm and at the maximum absorbance of the dye from these data. We tentatively propose that the differences in IPCE where the dye does not contribute to the photocurrent are due to differences in the film thickness (which deviate by less than 10%) or the local concentration of triiodide as a result of interactions of different strengths with the different perylene dyes. We conclude that where the driving force for charge transfer is small, the distance between I2•− and NiO+ is a dominant factor. Because of the complexity of this system, we avoid emphasizing small differences in current and voltage and instead focus on the global trend. ΔG°reg = e(E°(D/D−) − E°(I3−/I 2•−)) •−
•−
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(13) −
ΔG°inj(I 2 ) = e(E° VB(NiO) − E°(I 2 /I ))
CONCLUSIONS We have shown that the IPCE for a series of perylene sensitizers is dependent on the driving force for regeneration and that this regeneration step is one of the major efficiencylimiting processes in the p-type system. In addition, differences in dye−electrolyte and dye−solvent interactions play a role in assisting the charge-transfer reactions, and these may vary with the dielectric constant of the solvent. We have measured the electrochemical potential for the key intermediate in the regeneration process, E°(I3−/ I2•−), and have shown that this
(14)
The values listed in Table 3 are in good agreement with reports on electron-transfer reactions with iodine-based redox couples. The value for E°(I2•−/I−), which is more relevant for n-type DSC systems, lies within the limits of iodide oxidation given by, for example, Wang and Stanbury, Kuciauskas et al., Alebbi et al., and Sauvé et al.52−55 The value E°′(I3−/ I2•−) = −0.58 V versus SCE (ca. −0.96 V vs FeCp0/+) in acetonitrile 6491
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(7) Qin, P.; Zhu, H.; Edvinsson, T.; Boschloo, G.; Hagfeldt, A.; Sun, L. Design of an organic chromophore for p-type dye-sensitized solar cells. J. Am. Chem. Soc. 2008, 130, 8570−8571. (8) Li, L.; Gibson, E. A.; Qin, P.; Boschloo, G.; Gorlov, M; Hagfeldt, A.; Sun, L. Double-layered NiO photocathodes for p-type DSSCs with record IPCE. Adv. Mater. 2010, 15, 1759−1762. (9) He, J.; Lindström, H.; Hagfeldt, A.; Lindquist, S.-E. Dyesensitized nanostructured tandem cell-first demonstrated cell with a dye-sensitized photocathode. Sol. Energy Mater. Sol. Cells 2000, 62, 265−273. (10) Nattestad, A.; Mozer, A. J.; Fischer, M. K. R.; Cheng, Y.-B.; Mishra, A.; Bäuerle, P.; Bach, U. Highly efficient photocathodes for dye-sensitized tandem solar cells. Nat. Mater. 2010, 9, 31−35. (11) 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% efficiency. Science 2011, 334, 629−634. (12) Han, L.; Islam, A.; Chen, H.; Malapaka, C.; Chiranjeevi, B.; Zhang, S.; Yang, X.; Yanagida, M. Porphyrin-sensitized solar cells with cobalt (II/III)-based redox electrolyte exceed 12% efficiency. Energy Environ. Sci. 2012, 5, 6057−6060. (13) Yum, J.-H.; Baranoff, E.; Wenger, S.; Nazeeruddin, M. K.; Grätzel, M. Panchromatic engineering for dye-sensitized solar cells. Energy Environ. Sci. 2011, 4, 842−857. (14) Bremner, S. P.; Levy, M. Y.; Honsberg, C. B. Analysis of tandem solar cell efficiencies under AM1 . 5G spectrum. Prog. Photovoltaics 2008, 16, 225−233. (15) Qin, P.; Linder, M.; Brinck, T.; Boschloo, G.; Hagfeldt, A.; Sun, L. High incident photon-to-current conversion efficiency of p-type dye-sensitized solar cells based on NiO and organic chromophores. Adv. Mater. 2009, 21, 2993−2996. (16) Le Pleux, L.; Smeigh, A. L.; Gibson, E. A.; Pellegrin, Y.; Blart, E.; Boschloo, G.; Hagfeldt, A.; Hammarström, L.; Odobel, F. Synthesis, photophysical and photovoltaic investigations of acceptor-functionalized perylene monoimide dyes for nickel oxide p-type dye-sensitized solar cells. Energy Environ. Sci. 2011, 4, 2075−2084. (17) Morandeira, A.; Boschloo, G.; Hagfeldt, A.; Hammarström, L. Photoinduced ultrafast dynamics of coumarin 343 sensitized p-typenanostructured NiO films. J. Phys. Chem. B 2005, 109, 19403−19410. (18) Gibson, E. A.; Smeigh, A. L.; Le Pleux, L.; Fortage, J.; Boschloo, G.; Blart, E.; Pellegrin, Y.; Odobel, F.; Hagfeldt, A.; Hammarström, L. A p-type NiO-based dye-sensitized solar cell with an open-circuit voltage of 0.35 V. Angew. Chem., Int. Ed. 2009, 48, 4402 −4405. (19) Fortage, J.; Severac, M.; Houarner-Rassin, C.; Pellegrin, Y.; Blart, E.; Odobel, F. Synthesis of new perylene imide dyes and their photovoltaic performances in nanocrystalline TiO2 dye-sensitized solar cells. J. Photochem. Photobiol., A 2008, 197, 156−169. (20) Smeigh, A. L.; Pleux, L. L.; Fortage, J.; Pellegrin, Y.; Blart, E.; Odobel, F.; Hammarström, L. Ultrafast recombination for NiO sensitized with a series of perylene imide sensitizers exhibiting Marcus normal behaviour. Chem. Commun. 2012, 48, 678−680. (21) Sumikura, S.; Mori, S.; Shimizu, S.; Usami, H.; Suzuki, E. Syntheses of NiO nanoporous films using nonionic triblock copolymer templates and their application to photo-cathodes of p-type dye-sensitized solar cells. J. Photochem. Photobiol., A 2008, 199, 1−7. (22) Edvinsson, T.; Li, C.; Pschirer, N.; Schöneboom, J.; Eickemeyer, F.; Sens, R.; Boschloo, G.; Herrmann, A.; Müllen, K.; Hagfeldt, A. Intramolecular charge-transfer tuning of perylenes: spectroscopic features and performance in dye-sensitized solar cells. J. Phys. Chem. C 2007, 111, 15137−15140. (23) Würthner, F. Perylene bisimide dyes as versatile building blocks for functional supramolecular architectures. Chem. Commun. 2004, 1564−1579. (24) Hanson, K. J.; Tobias, C. W. Electrochemistry of iodide in propylene carbonate I . Cyclic voltammetry monitored by optical spectroscopy. J. Electrochem. Soc. 1987, 134, 2204−2210. (25) Hagfeldt, A.; Grätzel, M. Molecular photovoltaics. Acc. Chem. Res. 2000, 33, 269−277.
value is strongly solvent-dependent. This solvent dependency should be taken into account in the testing of new dyes for solar cell applications. For significant gains in efficiency for p-type DSCs to be made, the regeneration step must be optimized and a sufficient driving force is essential if this process is to compete with the fast recombination reaction observed in p-type systems such as these. However, for applications in tandem cells, flexibility in the choice of the absorption wavelength of the dye is necessary for optimal optical matching of the electrodes. This problem may be overcome either by better engineering of the electrochemical properties of the dye or by substituting the electrolyte for one-electron outer sphere alternatives such as an organometallic redox system.
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ASSOCIATED CONTENT
S Supporting Information *
Illustration of the experimental setup for photomodulated voltammetry of the redox electrolytes and plots of modulated photocurrent as a function of voltage for the sandwich cells containing 1.0 M LiI and 0.1 M I2 in different solvents. Illustrations of the electron-transfer reactions involved in pDSCs. Plot of IPCE versus the driving force. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Addresses §
Now at the School of Chemistry, The University of Nottingham, University Park, Nottingham NG7 2RD, U.K. ∥ Also at the Center for Molecular Devices, Department of Chemistry, School of Chemical Science and Engineering, Royal Institute of Technology (KTH), 100 44 Stockholm, Sweden. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the Knut and Alice Wallenberg Foundation and the Swedish Energy Agency for funding. We thank Prof. Leif Hammarström, Dr. Amanda Smeigh, and Dr. James Gardner for their assistance with the project.
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REFERENCES
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In our previous paper, we determined the lifetime of I2•− to be 0.2 ms in acetonitrile by an analysis of the frequency dependence of the PMV signal.41 Assuming that disproportionation occurs at the same rate inside a porous film as outside and that the diffusion coefficient is the same as that for triiodide, the maximum is L = 3 μm for MeCN and 1 μm for PC.29 (46) The diffusion coefficient (and diffusion length) depends on the size and shape of the molecule, interaction with the solvent, and viscosity of the solvent, and the mass-transport-limited current density at an electrode surface is directly related to the diffusion coefficient according to
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− J(0, t ) =
⎡ ∂C(x , t ) ⎤ i = D⎢ ⎥ ⎣ ∂x ⎦x = 0 nFA
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