Adapting Ruthenium Sensitizers to Cobalt Electrolyte Systems - The

Letter pubs.acs.org/JPCL

Adapting Ruthenium Sensitizers to Cobalt Electrolyte Systems Sangeeta Amit Kumar,†,∥,⊥ Maxence Urbani,‡,§,⊥ María Medel,‡ Mine Ince,†,‡ David González-Rodríguez,‡ Aravind Kumar Chandiran,† Ashok N. Bhaskarwar,∥ Tomás Torres,*,‡,§ Md. K. Nazeeruddin,*,† and Michael Graẗ zel*,† †

Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, Swiss Federal Institute of Technology (EPFL), Station 6, CH 1015 − Lausanne, Switzerland ‡ Departamento de Química Orgánica, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain § Instituto Madrileño de Estudios Avanzados (IMDEA)-Nanociencia, c/Faraday, 9, Cantoblanco, 28049 Madrid, Spain ∥ Ion Adsorption Lab, Department of Chemical Engineering, Indian Institute of Technology at Delhi, Hauz Khas, New Delhi − 110016, India S Supporting Information *

ABSTRACT: In this work, we report the use of bulky substitutions in a new heteroleptic ruthenium(II) bipyridine complex, Ru(NCS)2LL′, coded TT-230 to obtain high open-circuit potential in a dye-sensitized solar cell (where L is a bipyridine ligand appended with two cyclopenta(2,1-b;3,4-bA)dithiophene moieties, and L′ = 4,4,′-dicarboxylic acid 2,2′-bipyridine). The electrolytes based on cobalt complexes have shown significant advantages in terms of attainable open-circuit potential compared to the standard iodide/tri-iodide redox mediators. These merits of the cobalt complexes were previously realized with a porphyrin sensitizer, achieving a VOC greater than 1 V in DSC. However, with conventional Ru(II)polypyridyl complexes such as the C101 dye, similar increase in the VOC could not be attained due to the enhanced recombination. In this work, we have shown that the use of bulky substituents can prevent the back reaction of photogenerated electron and subsequently increase the open-circuit potential of the device. The recombination processes were investigated by transient photovoltage decay measurements. SECTION: Energy Conversion and Storage; Energy and Charge Transport

D

cell.6 However, a major drawback of the conventional Ru(II)polypyridyl complexes is their lack of absorption in the visiblerange with low molecular absorption coefficients (in particular, the red-region). Therefore, the photocurrent is mainly limited by the absorption spectra of the dye. With the aim to improve their light harvesting efficiency (LHE) and hence JSC, IPCE, and power conversion efficiency (PCE) (see eq 1), many works have been devoted to enhance and extend the absorption of these dyes by incorporating conjugated systems in their molecular structure.7−9

ye-sensitized solar cells (DSSCs) act as a lucrative substitute to the conventional solar cells in terms of cost per kilowatt-hour of the power production. The conventional DSC contains a sandwich of mesoscopic sensitized titanium dioxide photoanode and a catalyst-coated counter electrode with a redox electrolyte in-between. Followed by the light absorption by the dye molecules, the excited electrons are injected into the conduction band of TiO2 and the oxidized dye is regenerated by the redox mediator adjacent to it. The circuit of electron flow is completed by the transport of electrons across the TiO2 nanoparticles reaching the counter electrode through the external circuit generating a photovoltage that is equivalent to the difference between the quasi Fermi level of electrons (nEF) in the TiO2 and the redox potential of electrolyte.1,2 The best performing Ru(II)-bipyridine sensitizers usually achieve high incident photons-to-photocurrent conversion efficiency (IPCE) at a given wavelength, due to their highly efficient electron injection/collection (ϕinj and ηc) with quantum yields close to unity and slow-charge recombination rates, which usually result in high photocurrent (JSC).3−5 In our previous work, we also evidenced that the bulky substitution in ruthenium heteroleptic analogues of C101 can prevent aggregation of the dye resulting in higher photocurrent of the © 2014 American Chemical Society

IPCE(λ) = LHE(λ) × φinj(λ) × ηc

(1)

At the current stage, another main factor that limits the efficiency of the conventional ruthenium dyes is their low photovoltage reached in DSSCs in comparison with other organic dyes, which usually lies far below half of the maximum theoretical attainable values.10 The main reasons of this drop of the VOC from the optimal values arise from interfacial charges recombination in the TiO2 film that determines the final Fermi Received: December 2, 2013 Accepted: January 16, 2014 Published: January 16, 2014 501

dx.doi.org/10.1021/jz402612h | J. Phys. Chem. Lett. 2014, 5, 501−505

The Journal of Physical Chemistry Letters

Letter

Chart 1. Molecular Structures of Ruthenium Dyes Studied in This Work: TT-230 and Benchmark C101

Information. The dye C101 was previously reported elsewhere26 and used as a benchmark in this study. Our results show that the VOC has considerably increased compared to our standard Ru(II) complex C101 by the virtue of reduced back reaction. The recombination dynamics of the photogenerated electrons to the Co3+ redox mediator were investigated by photovoltage transient decay technique.27 The UV−vis spectra in solution (CH2Cl2) of the heteroleptic complex TT-230 and its corresponding free ligand 5 are depicted in Figure 1. Bipyridine 5 displays a sharp absorption

level position (Ef), and from mismatches between the HOMO−LUMO levels of the dye, redox potentials of the electrolyte and the conduction band of TiO2.7 In the light of these limiting factors, different strategies have been attempted to increase the VOC of conventional Ru(II) dyes. One of them lies in the modification of their molecular structure and, for instance, Ru(II) heteroleptic complexes containing fluorine-substituents11 or modified N3 analogues incorporating rigid spacer between the carboxylic anchoring groups of the bipyridine12 have been reported with higher VOC values. Until recently, the conventional electrolytes based on iodide/ tri-iodide redox mediators13−15 were the best performing ones. However, the further possibility of improvement was hampered due to the need of high driving force (∼500 mV) for the regeneration of oxidized dye by I−/I3−.2,16,17 This loss represents a huge limitation in the overall attainable opencircuit potential (VOC) and hence the photovoltaic PCE of the device. On this scope, the development of other redox shuttles appears as an appealing alternative strategy. Recently, the cobalt-complexes cobalt bis(trithiacyclononane), 18 [Co(ttcn) 2 ] 3+/2+ , and cobalt tris(2,2′-bipyrine), 13,17 [Co(bpy)3]3+/2+, have been used in DSSCs as efficient redox shuttles to break this barrier. For instance, a record efficiency of 12.3% at full sun was achieved with a sensitizer based on porphyrin cosensitized with another organic dye.17 Unlike I−/ I3−, the redox potentials can be tuned in the cobalt complexes by modifying the ligands, and the regeneration overpotential needed is considerably lower, which lead to a record value of the VOC of 1.1 V.19,20 However, similar VOC improvements could not be achieved with conventional Ru(II) metal complexes and were attributed to the huge recombination process due to the closest possible approach of the cobalt redox mediators to the surface of the titanium dioxide and fast recombination kinetics.21 To overcome this issue, we designed and synthetized a new ruthenium dye TT-230 incorporating bulky ethylhexyl- and hexyl- chains in its molecular structure, tailored at the lateral and terminal positions of two cyclopentadithiophene22−25 (CDT) subunits (Chart 1). Detailed synthetic procedures and characterizations by HR-mass spectrometry and NMR spectroscopy of the dye TT-230 and intermediate products are provided in the Supporting

Figure 1. UV−visible spectra of 5 (black line) and TT-230 (blue line) in CH2Cl2 solution.

band centered at 410 nm ranging from 350 to 450 nm with relatively high molecular extinction absorption coefficient (λmax = 61 400 mol−1·dm3·cm−1) attributed to both CDT subunits. TT-230 displays three distinct bands at around 310, 440, and 580 nm. By comparison with the free ligand and similar heteroleptic complex bearing 2,2′-bipyridine-4,4′-dicarboxylic acid (dcapy) as the anchoring group, the first band at 310 nm is 502

dx.doi.org/10.1021/jz402612h | J. Phys. Chem. Lett. 2014, 5, 501−505

The Journal of Physical Chemistry Letters

Letter

assigned mainly to the π−π* dcapy intraligand transition (ILCT1); the second band at 440 nm is due to a contribution of both the Ru metal-to-ligand-charge transfer transitions (MLCT) and the π−π* CDT intraligand charge transfer transition (ILCT2). The low energy band at 580 nm was assigned to the second MLCT transition. The current−voltage characteristics TT-230 and C101 dyesensitized solar cells were measured at AM 1.5 G solar conditions (100 mW/cm2). The J−V curves are depicted in Figure 2, and photovoltaic data are summed up in Table 1.

Unfortunately, we concurrently observed a degradation of the current density for both TT-230 and C101 sensitized cells when using the cobalt-based electrolyte in comparison with the iodine-based one. We explain this drop in JSC by lower dye regeneration efficiencies (ηreg) caused by the diminishing of the overpotential for dye-regeneration. The current−voltage properties measured under dark shows a dark current onset at 550 mV for the reference device made with C101 sensitizer, whereas the onset shifts to higher voltages at around 625 mV for TT-230, which is 75 mV higher. The dark current represents the back flow of electrons from the titanium dioxide to the redox mediator.16,28−30 When the nEF of electrons in TiO2 reaches a potential of 550 mV with respect to the Nernst potential of the redox mediator, the electrons already start to flow from the TiO2 to the electrolyte. However, when the TT-230 dye is used, the onset shifts to 75 mV, which is responsible for higher open-circuit potential in the latter case. The current action spectra of the devices sensitized with dyes TT-230 (+Cheno) and C101 are presented in Figure 3. The

Figure 2. Current−voltage curves of the dye-sensitized solar cell under dark and under illumination (AM 1.5 G solar irradiance, 100 mW/cm2 photon flux).

Table 1. Photovoltaic Parameters Measured at AM1.5G Solar Irradiance (100 mW/cm2) of the Dye-Sensitized Solar Cellsa Employing the Cobalt-Based Electrolyteb dye

JSC (mA/cm2)

VOC (mV)

fill factor (%)

η (%)

TT-230 C101 TT-230+Cheno

3.3 6.5 3.0

774.0 735.1 804.4

72.5 74.7 70.0

1.8 3.6 1.6

a

The values reported in Table 1 were obtained for the best devices in each configuration. Two cells were made for each condition, and, in most cases, the power conversion efficiency errors are within 5% of the presented values. bThe cobalt complex used in this study is the Co(III/II) tris(2,2′-bipyridine) complex, [Co(bpy)3]3+/2+.

Figure 3. IPCE spectra for DSCs sensitized with TT-230+Cheno (black line and squares) and C101 (red line and dots).

absorption onset for both dyes are quite similar; however, the overall IPCE is less for the device containing the dye TT-230 compared to C101. The integrated area under the IPCE curve matches with the short-circuit current density measured from J−V characterization. The open-circuit potential of a standard DSC is affected by three different factors, namely, the distribution of the surface trap states in TiO2, the recombination rate (krr) of the photogenerated electrons from the conduction band of titanium dioxide to the oxidized redox species, and the redox potential of the electrolyte. In the discussion below, we analyzed each parameter separately to find the origin of the change in the open-circuit potential. As the electrolyte used in this study is based only on the [Co(bpy)3]3+/2+ redox mediator, the change in VOC due to the variation in the redox potential is negligible. Figure 4 shows the trap state distribution of electrons measured as a function of voltage by the transient photocurrent decay technique. The distribution shows an exponentially increasing trend when moving toward the conduction band of the titanium dioxide, consistent with the literature.15,31 However, there exists

With the [Co(bpy)3]3+/2+ redox mediator, the reference C101 exhibits an open-circuit potential of 735 mV with a current density of 6.5 mA/cm2. The lower short-circuit current observed for the TT-230 cell in comparison with C101 is explained most likely by lower dye coverage on the titania mesoporous film due to the bulkier substitution for the former. As a positive counterpart, however, the VOC of TT-230 dye, which contains bulky substitutions, strongly increased to ∼775 mV, which is 40 mV higher than our reference device C101. The addition of coadsorbent chenodeoxycholic acid (cheno) in the dye solution further enhanced the VOC to ∼805 mV for the device made with TT-230, whereas no changes in the photovoltaic performances were observed in the case of C101 (data not shown). It is worth noting that, when using a standard iodide/triiodide-based electrolyte, we obtained a VOC of only ∼650 mV with TT-230, whereas the VOC did not change significantly between the two electrolytes in the case of C101 dye (see Figure S17 in the Supporting Information). 503

dx.doi.org/10.1021/jz402612h | J. Phys. Chem. Lett. 2014, 5, 501−505

The Journal of Physical Chemistry Letters

Letter

electrons increasing the open-circuit potential of the device. The reduction in the recombination kinetics can be visualized based on the structure of the dye-sensitizers. Compared to the reference C101, the TT-230 dye has bulkier substituents that tend to keep the [Co(bpy)3]3+ complex away from the TiO2 surface, which in turn reduces the recombination loss of electrons to the oxidized redox mediator. Thus, in the case of Ru(II) bipyridyl complex sensitizers, a bulky attachment is needed to attain higher VOC similar to all-organic dyes. In this work we identified that bulky substituents are needed to block the back reaction in the Ru(II) complex based DSC when employing Co3+/Co2+ redox mediator. We have compared two different Ru(II) dye sensitizers, with different substituents (C101 and TT-230). The device made with the bulky TT-230 sensitizer exhibits higher open-circuit potential compared to our reference C101 dye. The reason for this variation is correlated to the higher dark current density for the latter. Our recombination studies show that devices made with TT-230 exhibit lower recombination rate and explain why the dark current onsets at higher voltages compared to the reference C101. In order to avoid decrease in the current densities when using cobalt-based electrolyte, further improvements will necessarily require adequate- and fine-tuning of the highest occupied molecular orbital (HOMO) of the Ru(II)-dye to keep enough overpotential needed for dye-regeneration by the redox mediator. Nevertheless, the dye TT-230 provides an example of a bulky Ru(II)-dye reaching high open circuit voltage in DSC in combination with the Co3+/Co2+ redox couple, and hence opens up an avenue to engineer a new generation of highly efficient sensitizers.

Figure 4. The distribution of surface electronic trap states in the titanium dioxide film sensitized with TT-230 and C101 dye molecules.

a difference when using C101 or TT-230 dyes. Due to the possible variation in the dipole moment of the dyes, C101 possesses deeper trap states distribution compared to TT-230, with and without cheno. Therefore, for a given charge density and recombination rate, the electron quasi-Fermi level of electrons in TiO2 is positioned more negatively for TT-230 than for C101. The second parameter, the recombination rate of the photogenerated electrons for two different dyes, was analyzed using transient photovoltage decay measurements (Figure 5).

■

ASSOCIATED CONTENT

S Supporting Information *

Detailed synthesis and characterizations of the Ru(II) dye TT230, mass and NMR spectra, and the information on device fabrication. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: mdkhaja.nazeeruddin@epfl.ch. *E-mail: michael.graetzel@epfl.ch. Author Contributions ⊥

S. A. Kumar and M. Urbani contributed equally to this work.

Notes

The authors declare no competing financial interest.

■

Figure 5. The recombination rate of the photogenerated electrons measured as a function of charge density for TT-230 and C101 dye sensitizers.

ACKNOWLEDGMENTS

Financial support is acknowledged from the European Union within the FP7-ENERGY-2012-1 framework, GLOBALSOL project, Proposal No 309194-2, the Spanish MEC and MICINN (CTQ2011-24187/BQU, CTQ2011-23659), CONSOLIDER INGENIO 2010, CSD2007-00010 on Molecular Nanoscience, PRI-PIBUS-2011-1128) and the Comunidad de Madrid (MADRISOLAR-2, S2009/PPQ/1533). S.N., A.K.C., M.K.N., and M.G. acknowledge the financial contribution from MOLESOL (the FP7-Energy-2010-FET Project Contract No. 256617).

It can be noticed that, at any given charge density (proportional to the capacitance), the krr is higher for the solar cells sensitized with the C101 compared to the TT-230 dye. Following the electron injection from the excited state of the dye into the TiO2 conduction band, the steady state electron density maintained in TiO2 is higher for the devices made with TT-230 (due to slower electron back reaction kinetics), which in turn shifts negatively the quasi-Fermi level of 504

dx.doi.org/10.1021/jz402612h | J. Phys. Chem. Lett. 2014, 5, 501−505

The Journal of Physical Chemistry Letters

■

Letter

Based Redox Electrolyte Exceed 12% Efficiency. Science 2011, 334, 629−634. (18) Xie, Y.; Hamann, T. W. Fast Low-Spin Cobalt Complex Redox Shuttles for Dye-Sensitized Solar Cells. J. Phys. Chem. Lett. 2013, 4, 328−332. (19) Feldt, S. M.; Wang, G.; Boschloo, G.; Hagfeldt, A. Effect of Driving Forces for Recombination and Regeneration on the Photovoltaic Performance of Dye Sensitized Solar Cells Using Cobalt Polypyridine Redox Couples. J. Phys. Chem. C 2011, 115, 21500− 21507. (20) Chandiran, A. K.; Tetreault, N.; Humphry-Baker, R.; Kessler, F.; Baranoff, E.; Yi, C.; Nazeeruddin, M. K.; Grätzel, M. Subnanometer Ga2O3 Tunneling Layer by Atomic Layer Deposition to Achieve 1.1 V Open-Circuit Potential in Dye-Sensitized Solar Cells. Nano Lett. 2012, 12, 3941−3947. (21) Nusbaumer, H.; Zakeeruddin, M.; Moser, J. E.; Grätzel, M. An Alternative Efficient Redox Couple for the Dye-Sensitized Solar Cell System. Chem.Eur. J. 2003, 9, 3756−3763. (22) Zhou, D.; Cai, N.; Long, H.; Zhang, M.; Wang, Y.; Wang, P. An Energetic and Kinetic View on Cyclopentadithiophene Dye-Sensitized Solar Cells: The Influence of Fluorine vs Ethyl Substituent. J. Phys. Chem. C 2011, 115, 3163−3171. (23) Tsao, H. N.; Yi, C.; Moehl, T.; Yum, J.-H.; Zakeeruddin, S. M.; Nazeeruddin, M. K.; Grätzel, M. Cyclopentadithiophene Bridged Donor−Acceptor Dyes Achieve High Power Conversion Efficiencies in Dye-Sensitized Solar Cells Based on the Tris-Cobalt Bipyridine Redox Couple. ChemSusChem 2011, 4, 591−594. (24) Zhu, Z.; Waller, D.; Gaudiana, R.; Morana, M.; Mühlbacher, D.; Scharber, M.; Brabec, C. Panchromatic Conjugated Polymers Containing Alternating Donor/Acceptor Units for Photovoltaic Applications. Macromolecules 2007, 40, 1981−1986. (25) Coppo, P.; Turner, M. L. Cyclopentadithiophene Based Electroactive Materials. J. Mater. Chem. 2005, 15, 1123−1133. (26) Gao, F.; Wang, Y.; Shi, D.; Zhang, J.; Wang, M.; Jing, X.; ̂̀ M. Humphry-Baker, R.; Wang, P.; Zakeeruddin, S. M.; GraItzel, Enhance the Optical Absorptivity of Nanocrystalline TiO2 Film with High Molar Extinction Coefficient Ruthenium Sensitizers for High Performance Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2008, 130, 10720−10728. (27) Barnes, P. R. F.; Miettunen, K.; Li, X.; Anderson, A. Y.; Bessho, T.; Grätzel, M.; O’Regan, B. C. Interpretation of Optoelectronic Transient and Charge Extraction Measurements in Dye-Sensitized Solar Cells. Adv. Mater. 2013, 25, 1881−1922. (28) Peter, L. M. Dye-Sensitized Nanocrystalline Solar Cells. Phys. Chem. Chem. Phys. 2007, 9, 2630−2642. (29) Listorti, A.; O’Regan, B.; Durrant, J. R. Electron Transfer Dynamics in Dye-Sensitized Solar Cells. Chem. Mater. 2011, 23, 3381−3399. (30) O’Regan, B.; Grätzel, M. A Low-Cost, High-Efficiency Solar Cell Based on Dye-Sensitized Colloidal TiO2 Films. Nature 1991, 353, 737−740. (31) Ito, S.; Murakami, T. N.; Comte, P.; Liska, P.; Grätzel, C.; Nazeeruddin, M. K.; Grätzel, M. Fabrication of Thin Film Dye Sensitized Solar Cells with Solar to Electric Power Conversion Efficiency over 10%. Thin Solid Films 2008, 516, 4613−4619.

REFERENCES

(1) Grätzel, M. Photoelectrochemical cells. Nature 2001, 414, 338− 344. (2) Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. DyeSensitized Solar Cells. Chem. Rev. 2010, 110, 6595−6663. (3) Zhu, K.; Jang, S.-R.; Frank, A. J. Impact of High ChargeCollection Efficiencies and Dark Energy-Loss Processes on Transport, Recombination, and Photovoltaic Properties of Dye-Sensitized Solar Cells. J. Phys. Chem. Lett. 2011, 2, 1070−1076. (4) Park, J.-K.; Kang, J.-C.; Kim, S. Y.; Son, B. H.; Park, J.-Y.; Lee, S.; Ahn, Y. H. Diffusion Length in Nanoporous Photoelectrodes of DyeSensitized Solar Cells under Operating Conditions Measured by Photocurrent Microscopy. J. Phys. Chem. Lett. 2010, 3, 3632−3638. (5) Villanueva-Cab, J.; Wang, H.; Oskam, G.; Peter, L. M. Electron Diffusion and Back Reaction in Dye-Sensitized Solar Cells: The Effect of Nonlinear Recombination Kinetics. J. Phys. Chem. Lett. 2010, 1, 748−751. (6) García-Iglesias, M.; Pelleja, L.; Yum, J.-H.; González-Rodríguez, D.; Nazeeruddin, M. K.; Grätzel, M.; Clifford, J. N.; Palomares, E.; Vazquez, P.; Torres, T. Effect of bulky groups in ruthenium heteroleptic sensitizers on dye sensitized solar cell performance. Chem. Sci. 2011, 3, 1177−1184. (7) Wu, K.-L.; Li, C.-H.; Chi, Y.; Clifford, J. N.; Cabau, L.; Palomares, E.; Cheng, Y.-M.; Pan, H.-A.; Chou, P.-T. Dye Molecular Structure Device Open-Circuit Voltage Correlation in Ru(II) Sensitizers with Heteroleptic Tridentate Chelates for Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2012, 134 (17), 7488−7496. (8) Gao, F.; Wang, Y.; Shi, D.; Zhang, J.; Wang, M.; Jing, X.; Humphry-Baker, R.; Wang, P.; Zakeeruddin, S. M.; Grätzel, M. Enhance the Optical Absorptivity of Nanocrystalline TiO2 Film with High Molar Extinction Coefficient Ruthenium Sensitizers for High Performance Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2008, 130, 10720−10728. (9) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Mueller, E.; Liska, P.; Vlachopoulos, N.; Grätzel, M. Conversion of light to electricity by cis-X2bis(2,2′-bipyridyl-4,4′-dicarboxylate)ruthenium(II) charge-transfer sensitizers (X = Cl-, Br-, I-, CN-, and SCN-) on nanocrystalline titanium dioxide electrodes. J. Am. Chem. Soc. 1993, 115, 6382−6390. (10) Raga, S. R.; Barea, E. M.; Fabregat-Santiago, F. Analysis of the Origin of Open Circuit Voltage in Dye Solar Cells. J. Phys. Chem. Lett. 2012, 3, 1629−1634. (11) Huang, W.-K.; Wu, H.-P.; Lin, P.-L.; Lee, Y.-P.; Diau, E. W.-G. Design and Characterization of Heteroleptic Ruthenium Complexes Containing Benzimidazole Ligands for Dye-Sensitized Solar Cells: The Effect of Fluorine Substituents on Photovoltaic Performance. J. Phys. Chem. Lett. 2012, 3, 1830−1835. (12) Abrahamsson, M.; Johansson, P. G.; Ardo, S.; Kopecky, A.; Galoppini, E.; Meyer, G. J. Decreased Interfacial Charge Recombination Rate Constants with N3-Type Sensitizers. J. Phys. Chem. Lett. 2010, 1, 1725−1728. (13) Jono, R.; Sumita, M.; Tateyama, Y.; Yamashita, K. Redox Reaction Mechanisms with Non-triiodide Mediators in Dye-Sensitized Solar Cells by Redox Potential Calculations. J. Phys. Chem. Lett. 2011, 3, 3581−3584. (14) Boschloo, G.; Gibson, E. A.; Hagfeldt, A. Photomodulated Voltammetry of Iodide/Triiodide Redox Electrolytes and Its Relevance to Dye-Sensitized Solar Cells. J. Phys. Chem. Lett. 2011, 2, 3016−3020. (15) Richards, C. E.; Anderson, A. Y.; Martiniani, S.; Law, C.; O’Regan, B. C. The Mechanism of Iodine Reduction by TiO2 Electrons and the Kinetics of Recombination in Dye-Sensitized Solar Cells. J. Phys. Chem. Lett. 2012, 3, 1980−1984. (16) O’Regan, B. C.; Durrant, J. R. Kinetic and Energetic Paradigms for Dye-Sensitized Solar Cells: Moving from the Ideal to the Real. Acc. Chem. Res. 2009, 42, 1799−1808. (17) 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)505

dx.doi.org/10.1021/jz402612h | J. Phys. Chem. Lett. 2014, 5, 501−505