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Feb 20, 2017 - A standard dye-sensitized solar cell (DSSC) architecture relies on the sensitization of ... however, tempered by the low solubility of ...
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High-Voltage Dye-Sensitized Solar Cells Mediated by [Co(2,2′bipyrimidine)3]z Kitty Y. Chen,† Chuan Du,† Brian O. Patrick,† and Curtis P. Berlinguette*,†,‡ †

Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia V6T 1Z1, Canada Department of Chemical and Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver, British Columbia V6T 1Z1, Canada

‡

S Supporting Information *

to the value of +0.57 V measured for [Co(2,2′-bipyridine)3]z (Co-bpy). The compact size of the complex and high transparency in the visible region are also compelling features for a DSSC mediator. Our hypothesis was validated by the finding that DSSC devices mediated by Co-bpm did yield a strikingly high photovoltage of +1.02 V in devices sensitized with 5-[4-[bis(4-iododiphenyl)amino]phenyl]thiophene-2-cyanoacrylic acid (Dye; Figure 1a).28,29 This promising result is, however, tempered by the low solubility of the complex precluding meaningful photocurrents.

ABSTRACT: The cobalt complex [Co(2,2′bipyrimidine)3](PF6)2 (Co-bpm) was tested as a redox mediator in the dye-sensitized solar cell. The measured photovoltages for the cells were in excess of 1 V, which is approximately 3-fold greater than that measured for devices where [Co(2,2′-bipyridine)3](PF6)2 (Co-bpy) was used as the redox mediator under the same experimental conditions. The root cause of this voltage enhancement is the CoIII/CoII redox potential for Co-bpm being positively shifted by 0.50 V relative to the Co-bpy mediator. This result highlights how the number and position of the N atoms in aromatic ligands can have a profound effect on the measured photovoltage.

A standard dye-sensitized solar cell (DSSC) architecture relies on the sensitization of TiO2 by a molecular dye to convert light into electrical energy.1−3 A redox mediator plays the critical role of transporting charge from the counterelectrode to the photooxidized dyes attached to the TiO2 film. The redox mediator also defines the open-circuit voltage (Voc), which corresponds approximately to the energy difference between the redox couple of the mediator and the quasi Fermi level of TiO2 (EF). It is therefore desirable to develop redox mediators with lowerenergy redox couples to drive up the photovoltage of the DSSC, while remaining sufficiently high in energy to regenerate the dye.4−8 The majority of DSSCs used iodide as the redox mediator until the recent discovery that tris(diimine)cobalt complexes could also produce high power conversion efficiencies (PCEs).9−13 Indeed, the most efficient DSSCs reported to date contain cobalt mediators.14−16 A distinct advantage of cobalt-based electrolytes over those with iodide is the ability to modulate the CoIII/CoII redox couple through ligand modification.9,10,17−21 Not only are higher photovoltages observed for cobalt complexes bearing electron-withdrawing substituents, but higher ligand denticities are also thought to engender higher stability.22,23 Given that Voc increases with anodically shifted CoIII reduction potentials, we set out to test [Co(2,2′-bipyrimidine)3]z (Cobpm) in the DSSC. This study was prompted by the 0.23−0.42 V anodic shift in metal-based potentials when the 2,2′-bipyridine (bpy) ligands of octahedral homoleptic nickel,24 iron,24,25 and ruthenium26,27 coordination complexes are substituted with 2,2′bipyrimidine (bpm) ligands. The CoIII/CoII redox couple for Cobpm is +1.07 V vs NHE, which is very positively shifted relative © XXXX American Chemical Society

Figure 1. (a) Summary of key DSSC energy levels in this work, including EF of the TiO2 semiconductor, ground- and excited-state reduction potentials for Dye, and the CoIII/CoII redox couples for Cobpm and Co-bpy. The molecular structures of Dye and Co-bpm are indicated. (b) UV−vis absorption spectra and (c) CV for CoIII-bpm (orange) and CoIII-bpy (gray) recorded in MeCN at a scan rate of 100 mV with a Pt working electrode, a Ag/AgCl reference electrode, a Pt counter electrode, and a 0.1 M NBu4PF6 supporting electrolyte.

The divalent Co-bpm complex was isolated by following a modified synthetic method reported by Ruminski and Petersen, who first reported the title complex 3 decades ago.30 The oxidation of Co-bpm is nontrivial because of solubility issues, but we found that NOPF6 served as an effective oxidizing agent. The structure of Co-bpm determined by single-crystal X-ray diffraction techniques (Figure S1 and Tables S1 and S2) Received: January 11, 2017

A

DOI: 10.1021/acs.inorgchem.7b00082 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry Table 1. Summary of DSSC Experimental and Performance Parameters mediator a

solvent

Co-bpy Co-bpma

MeCN MeCN

Co-bpmb

1:1 DMSO/MeCN

additive

PCE (%)

Voc (V)

Jsc (mA cm−2)

FF

none none 0.2 M TBP none 0.2 M TBP

0.26 (±0.02) 0.079 (±0.009) 0.41 (±0.05) 1.04 (±0.1) 0.57 (±0.02)

0.36 (±0.02) 0.90 (±0.07) 0.99 (±0.02) 1.007 (±0.007) 0.980 (±0.008)

1.74 (±0.09) 0.22(±0.02) 1.1 (±0.2) 2.3(±0.4) 2.2 (±0.2)

0.41 (±0.03) 0.390 (±0.009) 0.39 (±0.03) 0.44 (±0.02) 0.26 (±0.01)

a 0.02 M CoII, 0.002 M CoIII, and four replicates measured. b0.15 M CoII and 0.015 M CoIII. Values shown indicate mean values (and standard deviations in parentheses) for at least three devices.

DSSC devices were constructed with Co-bpm (counterion = PF6−) and sensitized by the organic sensitizer Dye (Figure 1a). We previously demonstrated that this particular sensitizer yields high PCEs, and that the energy of the highest occupied molecular orbital at 1.27 V can accommodate regeneration by Co-bpm.28,29 The first iteration of cells, prepared with Co-bpm electrolyte solutions with 0.022 M CoII and 0.002 M CoIII in MeCN devoid of other additives, produced a 2-fold increase in Voc but a considerably lower Jsc than those benchmarked against Co-bpy under the same cell conditions (Table 1). A mean Voc value of 0.90 V was measured across four devices, but the cell efficiencies were held below 0.1% with the Co-bpm mediator because of the low photocurrents. Regeneration by Co-bpm is only thermodynamically favorable by 200 meV, which is lower than the threshold value of 400 meV for efficient regeneration by cobalt mediators.21 We then explored the use of additives, namely, lithium perchlorate and 4-tert-butylpyridine (TBP), to improve the DSSC photocurrents. These widely used additives are known to affect the semiconductor band edge and can also passivate the surface to inhibit recombination between the electron injected into the TiO2 conduction band and the CoIII species.38−41 Lithium perchlorate was deemed unsuitable because anion exchange with Co-bpm yielded an insoluble salt in MeCN. We also did not add supporting electrolytes such as LiPF6 or TBAPF6 to improve the ionic conductivity because excess PF6− could shift the equilibrium toward a solid cobalt complex and further lower the solubility. The TBP additive did have a positive effect and raised the Voc to 1.02 V (Figure S3) and the mean PCE to 0.41% (Table 1). This improvement in Voc was attributed to TBP negatively shifting EF of TiO2, with protection of the surface from reaction with CoIII in solution increasing Jsc.42 We further examined a total of 16 electrolyte solvents to overcome low Co-bpm solubility (Figure S4), and the five most soluble systems were tested in fully assembled cells. The concentration of CoII was varied from 0.05 to 0.2 M and did not contain CoIII and additives. There were clear improvements in the PCEs in all five solvent systems relative to those of devices prepared with MeCN, but the increase did not track with the mediator concentration (Figure S5). The highest PCE of 0.3% was achieved for 0.15 M CoII-bpm in a 1:1 dimethyl sulfoxide (DMSO)/MeCN solvent mixture (Table S4). The solvent diffusion and donor ability are speculated to affect Jsc.43,44 Our final stage of experiments for this study involved the addition of 0.015 M CoIII-bpm and 0.2 M TBP to the 1:1 DMSO/MeCN solvent system with 0.15 M CoII-bpm. The devices produced PCEs of ca. 1% (Table 1 and Figure S3). The I−V curve profile is indicative of high series resistance in the device contributing to low fill factor (FF). A significant increase in Jsc and PCE was found for cells using 1:1 DMSO/MeCN as the solvent system instead of MeCN when all other experimental parameters were held at parity. These collective results pointed

confirmed a distorted octahedral geometry with Co−N bond distances of ca. 2.1 Å, which is similar to those measured for Cobpy.31 The 1H NMR spectrum for the divalent form of the complex CoII-bpm in CD3CN contained three signals found over the 20−100 ppm range (Figure S2b) consistent with a paramagnetic metal center. The proton closer to the cobalt experienced greater paramagnetic relaxation enhancement and hyperfine shift and therefore was assigned to the broader, more downfield signal.32 The chemical shifts were comparable to the relevant protons for related tris(diimine)cobalt(II) complexes.33 The NMR spectrum of diamagnetic CoIII-bpm yielded signals that were no further downfield than 10 ppm and did not overlap with the spectrum of the free ligand (Figure S2). UV−vis spectra for Co-bpm recorded in acetonitrile (MeCN) revealed one resolved absorption band centered at 935 nm and two overlapping bands at 243 and 465 nm. In accordance with other high-spin d7 octahedral cobalt complexes,34,35 intraligand π−π* and 4T1(F) → 4T2(F) d−d transitions were assigned to bands with maxima at 243 and 935 nm, respectively, and the 4 T1(F) → 4T1(P) transition was assigned to the shoulder at 465 nm (Figure 1b and Table S3). Nonetheless, Co-bpm demonstrated exceptionally low absorbance in the visible range, even below that of Co-bpy. Cyclic voltammetry recorded in MeCN confirmed a CoIII/CoII redox couple for Co-bpm at 1.07 V vs NHE, which is in close agreement with the previously reported value of 1.03 V.30 This value is positively shifted by ca. +0.5 V relative to that measured for Co-bpy (Figure 1c) and [Co(1,10-phenanthroline)3](PF6)2,22 despite their very similar molecular sizes and structures. The electron-withdrawing N atoms decrease the basicity and electron-donating ability of the bpm ligand, rendering the destabilized metal center harder to oxidize. The number and position of the N atoms in aromatic diimine ligands causing a drastic change in the metal-based redox potential is well established for ruthenium complexes (e.g., homoleptic complexes ligated by bpy and bpm yield RuIII/RuII couples of 1.51 to 1.93 V, respectively26,27,36). These N atoms are clearly operative in affecting the electron density at the Co center given the similar Co−N bond metrics between Co-bpm and Co-bpy. A challenge in using Co-bpm for DSSCs is the low solubility in MeCN being incongruous with the higher mediator concentrations that are normally required (ca. 0.2 M). Four different counteranions were tested for Co-bpm, and the highest measured solubility was merely 27 mM with the PF6− anion (Table S4). The differences in the solubilities were attributed, in part, to the lipophilicity of the counterion, which enabled inorganic salts to dissolve better in organic solvents and followed the trend ClO4− < BF4− < B(C6H5)4− < PF6−.37 PF6− was therefore selected as the counteranion of choice for DSSC studies because it enabled the highest mediator concentration in MeCN. B

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(9) Feldt, S. M.; Gibson, E. a.; Gabrielsson, E.; Sun, L.; Boschloo, G.; Hagfeldt, A. Design of Organic Dyes and Cobalt Polypyridine Redox Mediators for High-Efficiency Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2010, 132, 16714−16724. (10) Nusbaumer, H.; Moser, J.-E.; Zakeeruddin, S. M.; Nazeeruddin, M. K.; Grätzel, M. CoII(dbbip)22+ Complex Rivals Tri-iodide/Iodide Redox Mediator in Dye-Sensitized Photovoltaic Cells. J. Phys. Chem. B 2001, 105, 10461−10464. (11) Bomben, P. G.; Gordon, T. J.; Schott, E.; Berlinguette, C. P. A Trisheteroleptic Cyclometalated RuII Sensitizer That Enables High Power Output in a Dye-Sensitized Solar Cell. Angew. Chem., Int. Ed. 2011, 50, 10682−10685. (12) Zhou, D.; Yu, Q.; Cai, N.; Bai, Y.; Wang, Y.; Wang, P. Efficient Organic Dye-Sensitized Thin-Film Solar Cells Based on the tris(1,10phenanthroline)cobalt(II/III) Redox Shuttle. Energy Environ. Sci. 2011, 4, 2030−2034. (13) 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. (14) 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 (Washington, DC, U. S.) 2011, 334, 629−634. (15) Mathew, S.; Yella, A.; Gao, P.; Humphry-Baker, R.; Curchod, B. F. E.; Ashari-Astani, N.; Tavernelli, I.; Rothlisberger, U.; Nazeeruddin, M. K.; Grätzel, M. Dye-Sensitized Solar Cells with 13% Efficiency Achieved through the Molecular Engineering of Porphyrin Sensitizers. Nat. Chem. 2014, 6, 242−247. (16) Kakiage, K.; Aoyama, Y.; Yano, T.; Oya, K.; Fujisawa, J.; Hanaya, M. Highly-Efficient Dye-Sensitized Solar Cells with Collaborative Sensitization by Silyl-Anchor and Carboxy-Anchor Dyes. Chem. Commun. 2015, 51, 15894−15897. (17) Sapp, S. A.; Elliott, C. M.; Contado, C.; Caramori, S.; Bignozzi, C. A. Substituted Polypyridine Complexes of Cobalt(II/III) as Efficient Electron-Transfer Mediators in Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2002, 124, 11215−11222. (18) Nusbaumer, H.; Zakeeruddin, S. 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. (19) Yum, J.-H.; Baranoff, E.; Kessler, F.; Moehl, T.; Ahmad, S.; Bessho, T.; Marchioro, A.; Ghadiri, E.; Moser, J.-E.; Yi, C.; et al. A Cobalt Complex Redox Shuttle for Dye-Sensitized Solar Cells with High OpenCircuit Potentials. Nat. Commun. 2012, 3, 631. (20) Aribia, K. B.; Moehl, T.; Zakeeruddin, S. M.; Grätzel, M. Tridentate Cobalt Complexes as Alternative Redox Couples for HighEfficiency Dye-Sensitized Solar Cells. Chem. Sci. 2013, 4, 454−459. (21) Feldt, S. M.; Lohse, P. W.; Kessler, F.; Nazeeruddin, M. K.; Grätzel, M.; Boschloo, G.; Hagfeldt, A. Regeneration and Recombination Kinetics in Cobalt Polypyridine Based Dye-Sensitized Solar Cells, Explained Using Marcus Theory. Phys. Chem. Chem. Phys. 2013, 15, 7087−7097. (22) Feldt, S. M.; Wang, G.; Boschloo, G.; Hagfeldt, A. Effects of Driving Forces for Recombination and Regeneration on the Photovoltaic Performance of Dye-Sensitized Solar Cells Using Cobalt Polypyridine Redox Couples. J. Phys. Chem. C 2011, 115, 21500−21507. (23) Kashif, M. K.; Nippe, M.; Duffy, N. W.; Forsyth, C. M.; Chang, C. J.; Long, J. R.; Spiccia, L.; Bach, U. Stable Dye-Sensitized Solar Cell Electrolytes Based on Cobalt(II)/(III) Complexes of a Hexadentate Pyridyl Ligand. Angew. Chem., Int. Ed. 2013, 52, 5527−5531. (24) Ruminski, R. R.; Petersen, J. D. Tris(2,2′-bipyrimidine)M (M = Fe(II), Co(II), Ni(II)) Perchlorate Complexes. Spectroscopic Properties for Precursor Complexes in the Preparation of Polymetallic Systems. Inorg. Chim. Acta 1985, 97, 129−134. (25) Braterman, P. S.; Song, J.-I.; Peacock, R. D. Electronic Absorption Spectra of the Iron(II) Complexes of 2,2′-Bipyridine, 2,2′-Bipyrimidine,

to the low solubility of Co-bpm being the key reason that the PCEs were held to low values. However, the devices using 1:1 DMSO/MeCN were not robust because both DMSO and TBP increased the rate of dye desorption. This study shows that the very positive CoIII/CoII redox potential of Co-bpm can yield strikingly high photovoltages in excess of 1 V in the DSSC. This rare achievement for conventional DSSC architectures was made possible because Co-bpm has the most positively shifted CoIII/CoII redox potential of any tris(diimine)cobalt complex reported in the DSSC to date. A fundamental challenge to using Co-bpm in the DSSC is that its low solubility precludes sufficiently high concentrations to yield high current densities. Studies are underway to increase the solubility of the complex so that the positively shifted redox potential can be better utilized.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00082. Experimental details, NMR spectra, current−voltage curves and characteristics, and electrochemical, optical, and solubility data (PDF) Crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Curtis P. Berlinguette: 0000-0001-6875-849X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the Canadian Natural Science and Engineering Research Council, Canadian Institute for Advanced Research, Canadian Foundation for Innovation, and Canada Research Chairs.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b00082 Inorg. Chem. XXXX, XXX, XXX−XXX