Effects of Driving Forces for Recombination and Regeneration on the

ARTICLE pubs.acs.org/JPCC

Effects of Driving Forces for Recombination and Regeneration on the Photovoltaic Performance of Dye-Sensitized Solar Cells using Cobalt Polypyridine Redox Couples Sandra M. Feldt, Gang Wang, Gerrit Boschloo,* and Anders Hagfeldt Department of Physical and Analytical Chemistry, Uppsala University, Box 259, 751 05 Uppsala, Sweden

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bS Supporting Information ABSTRACT: Dye-sensitized solar cells (DSCs) with open-circuit potentials above 1 V were obtained by employing the triphenylamine based organic dye D35 in combination with cobalt phenanthroline redox couples. A series of cobalt bipyridine and cobalt phenanthroline complexes with different redox potentials were investigated to examine the dependence of the driving force for recombination and dye regeneration on the photovoltaic performance. The photovoltage of the devices was found to increase and the photocurrent to decrease with increasing redox potential of the complexes. The halftime for regeneration of the oxidized dye by cobalt trisbipyrine was about 20 μs, similar to that found for the iodide/triiodide redox couple, whereas regeneration kinetics became slower for cobalt complexes with less driving force for regeneration. A driving force for dye regeneration of 390 mV for cobalt(II/III) tris(5-chloro-1,10-phenanthroline) was found sufficient to regenerate more than 80% of the D35 dye molecules, resulting in a conversion of incident photons to electric current of above 80%. The photocurrent of the D35 sensitized DSCs using cobalt phenanthroline complexes decreased, however, with increasing Nernst potential of the redox couples, due to the increased recombination and the decreased regeneration rate constants.

’ INTRODUCTION In a conventional dye-sensitized solar cell (DSC), solar energy is converted to electricity through light absorption by a dye molecule attached to mesoporous TiO2. Upon light absorption the dye is excited and injects an electron into the conduction band of the semiconductor and the oxidized dye is in turn regenerated by a redox couple in a surrounding electrolyte. The cycle is closed by the reduction of the redox mediator at a platinized counter electrode.13 The best certified efficiency reported to date for the standard DSCs using ruthenium based dyes in combination with the iodide/triiodide redox couple is 11.1%.4 The success of the I/I3 redox couple is mainly attributed to its slow interception of electrons at the TiO2 surface, which minimizes recombination losses in the DSCs. One of the drawbacks with the I/I3 redox couple is, however, the large driving force needed for the dye regeneration. For the standard ruthenium-based dye Ru(dcbpy)2(NCS)2 (known as N3 or N719) the driving force for regeneration in the DSC is about 0.75 V.5 The osmium-based analog Os(dcbpy)2(NCS)2 has, on the other hand, been found not to be efficiently reduced after electron injection by iodide/triiodide, despite a driving force for regeneration of about 0.45 V.6 Recently, however, Wenger et al.7 found that efficient regeneration of tetrathiafulvalene sensitizers by iodide/triiodide was obtained with a calculated driving force for regeneration of only 0.15 V. Despite this notable exception, it can be said that the large driving force needed for regeneration of the oxidized dye in DSCs using the I/I3 redox couple seriously limits the voltage r 2011 American Chemical Society

output and thus the efficiency of the solar cell. The reason for the large driving force needed is the complex regeneration kinetics that proceeds via formation of intermediates such as the I2 radical,8,9 effectively resulting in a potential loss of several hundred millivolts in the DSC.5 Other disadvantages of the I/I3 redox couple are competitive light absorption by the triiodide10 and its corrosiveness toward metals and sealing materials. One-electron redox couples with more favorable redox potentials can be employed to increase the photovoltage of the DSCs without diminishing the photocurrent, since it is expected that a lower driving force for dye regeneration is needed for these redox couples. Simply replacing the I/I3 couple in the DSC by a one-electron redox couples led, however, to poorly performing devices with low photovoltages and photocurrents, because of increased recombination of photoinjected electrons with oxidized redox species in the electrolyte.1114 Nevertheless, cobalt complexes are an interesting and feasible alternative to I/I3,1517 in particular when sensitizers with suitable steric properties are selected. Recently, we showed that cobalt polypyridine redox couples can give comparable efficiencies to iodide/triiodide-based DSCs using triphenylamine-based organic dyes with bulky substituents to prevent fast recombination processes.17 Electron transfer rates are known to be relatively Received: June 29, 2011 Revised: September 20, 2011 Published: September 23, 2011 21500

dx.doi.org/10.1021/jp2061392 | J. Phys. Chem. C 2011, 115, 21500–21507

The Journal of Physical Chemistry C

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Figure 1. (a) Schematic energy diagram for a nanostructured TiO2 electrode sensitized with D35 employing [Co(bpy)3]n+, [Co(dmb)3]n+, [Co(dtb)3]n+, [Co(phen)3]n+, [Co(5-chlorophen)3]n+, and [Co(5-nitrophen)3]n+ based electrolytes. (b) The chemical structure of the different cobalt polypyridine redox couples employed herein.

slow for cobalt complexes on account of the large internal reorganization energy when going from d7 (high spin) to d6 (low spin). This can be an advantage as it would slow down recombination kinetics of electrons in TiO2 with cobalt(III). It can, however, also be a disadvantage as regeneration kinetics of the oxidized dye could become too slow, and these aspects will be investigated in this paper. Here we have examined a series of different cobalt bipyridine and phenanthroline complexes with varying redox potentials to investigate the effect of the driving force for recombination and dye regeneration on the photovoltaic performance in cobaltbased DSCs. The efficient triphenylamine-based organic dye D3518 was used. An energy diagram of a DSC sensitized with D35 employing the different cobalt complexes employed herein and the chemical structure of the complexes are shown in Figure 1, panels a and b, respectively. The redox couples were cobalt(III/II) tris(4,40 -dimethyl-2,20 - bipyridine), [Co(dmb)3]n+, cobalt(III/II) tris(4,40 -ditert-butyl-2,20 -bipyridine), [Co(dtb)3]n+, cobalt(III/II) tris(2,20 - bipyridine), [Co(bpy)3]n+, cobalt(III/II) tris(1,10-phenanthroline), [Co(phen)3]n+, cobalt(III/II) tris(5chloro-1,10-phenanthroline), [Co(Cl-phen)3]n+, and cobalt(III/II) tris(5-nitro-1,10-phenanthroline), [Co(NO2-phen)3]n+. By using cobalt phenanthroline redox couples with more positive redox potentials than iodide/triiodide open circuit potentials of above 1 V can be obtained. Nanosecond laser spectroscopy measurements showed that a driving force for regeneration of 390 mV for [Co(Cl-phen)3]n+ was sufficient to regenerate more than 80% of the D35 dye molecules. The photocurrent of the devices decreased, however, with increasing redox potential of the complexes, because of slower regeneration of the oxidized dye and faster electronCo(III) recombination.

’ EXPERIMENTAL SECTION All chemicals were purchased from Sigma Aldrich unless otherwise noted. D35 was prepared according to the published procedure.18 Synthesis of Cobalt Complexes. The cobalt complexes Co(bpy)3(PF6)2, Co(dmb)3(PF6)2, Co(dtb)3(PF6)2, Co(phen)3(PF6)2, Co(Cl-phen)3(PF6)2, Co(NO2-phen)3(PF6)2, Co(phen)3(CF3SO3)2, and Co(Cl-phen)3(CF3SO3)2 were synthesized as described elsewhere.10,16,19 For cobalt complexes with the PF6 counterion, 1 equivalent of CoCl2 3 6H2O and 3.3 equivalent of the polypyridine ligand were dissolved in a minimal amount of

methanol (Merck), and for cobalt complexes with the CF3SO3 counterion, 1 equivalent of CoCl2 3 6H2O in water was added dropwise to 3.3 equivalent ethanolic solution of the polypyridine ligand to give a brown-yellow solution. The solution was left to stir at reflux for 2 h and an excess of the appropriate counterion, and if needed diethylether, was added to the solution to precipitate the compound. The product was then filtered; washed with methanol, ethanol, and diethylether; dried under vacuum; and used without further purification. Oxidation of the cobalt(II) complexes to cobalt(III) was performed by adding a slight excess of NOBF4 to an acetonitrile solution of the complex and then by removing the acetonitrile solution by rotary evaporation. The complex was then redissolved in acetonitrile and a large amount of the appropriate counterion was added to the solution. The final product was precipitated with diethylether, filtered, dried under vacuum, and used without further purification. Solar Cell Preparation. TiO2 electrodes were prepared on fluorine-doped tin oxide (FTO) glass substrates (Pilkington, TEC15). The glass substrates were cleaned in an ultrasonic bath overnight using (in order) ethanol, water and ethanol. The glass substrates were pretreated by immersion in 40 mM aqueous TiCl4 solution at 70 °C for 30 min, washed with water and dried. Mesoporous TiO2 films with a size of 0.5 0.5 cm2 were prepared by screen printing colloidal TiO2 paste (Dyesol DSL 30 NRD-T for the measurements using cobalt bipyridyl redox couples and Solaronix T37/SP for the measurements using cobalt phenanthroline redox couples). The thickness of the films was measured with a profilometer (Veeco Dektak 3) and was about 6 μm using either of the Dyesol or Solaronix TiO2 paste. For the optimization measurements, a light-scattering TiO2 layer (thickness 3 μm; PST400C, JGC Catalysts and Chemical LTD, gratefully received from JGC Catalysts and Chemical LTD) was deposited on top of 2 layers of mesoporous TiO2 film (12 μm thick, Dyesol DSL 30 NRD-T). The electrodes were sintered in an oven (Nabertherm Controller P320) in an air atmosphere using a temperature gradient program with four levels at 180 °C (10 min), 320 °C (10 min), 390 °C (10 min), and 500 °C (60 min). Prior to use, the electrodes were given an additional TiCl4 treatment as described above, followed by another heating step (500 °C (60 min)). When the temperature cooled to about 90 °C, the electrodes were immersed in a dye bath containing 0.2 mM D35 in ethanol and left overnight. The films were then rinsed in ethanol to remove excess dye. 21501

dx.doi.org/10.1021/jp2061392 |J. Phys. Chem. C 2011, 115, 21500–21507


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Table 1. CurrentVoltage Characteristics under AM1.5G Illumination for DSCs Sensitized with D35 Employing Cobalt Phenanthroline-Based Electrolytesa E00 (V vs NHE)

VOC (V)

EF,n, OC (V vs NHE)

JSC (mA cm‑2)

FF

η (%)

[Co(phen)3]

0.62

0.93

0.37

7.33

0.59

4.01

[Co(Cl-phen)3]n+ [Co(NO2-phen)3]n+

0.72 0.85

1.01 1.03

0.35 0.24

6.35 4.38

0.56 0.51

3.57 2.29

redox couple n+

The electrochemical potential in the TiO2 under OC conditions is calculated as follows: EF,n,OC = Eredox VOC. Eredox is calculated using the Nernst equation. a

Solar cells were assembled, using a 30 μm thick thermoplastic Surlyn frame, with a platinized counter electrode (TEC8), which was prepared by depositing 10 μL cm2, 4.8 mM H2PtCl6 solution in ethanol to the glass substrate followed by heating in air at 400 °C for 30 min. An electrolyte solution was then introduced through two holes predrilled in the counter electrode and the cell was sealed with thermoplastic Surlyn covers and a glass coverslip. Unless otherwise noted, the electrolyte consisted of 0.2 M Co(L)3(PF6)2, 0.03 M Co(L)3(PF6)3, 0.1 M LiClO4, and 0.2 M 4-tert butylpyridine (TBP) in acetonitrile, where L ascribes the different bipyridine ligands or 0.1 M Co(L0 )3(PF6)2, 0.01 M Co(L0 )3(PF6)3, 0.1 M LiClO4, and 0.2 M TBP in a 60:40 mixture of acetonitrile and ethylene carbonate, where L0 ascribes the different phenanthroline ligands. For comparison DSCs were prepared using 0.6 M tertbutylammonium iodide, 0.1 M LiI, 0.05 M I2, and 0.2 M TBP in acetonitrile. Solar Cell Characterization. Currentvoltage (IV) characteristics were measured using a Keithley 2400 source/meter and a Newport solar simulator (model 91160) giving light with AM 1.5 G spectral distribution, which was calibrated using a certified reference solar cell (Fraunhofer ISE) to an intensity of 1000 W m2. A black mask with an aperture slightly larger (0.6 0.6 cm2) than the active solar cell area was applied on top of the cell. Incident photon to current conversion efficiency (IPCE) spectra were recorded using a computer-controlled setup consisting of a xenon light source (Spectral Products ASB-XE-175), a monochromator (Spectral Products CM110) and a potentiostat (EG&G PAR 273), calibrated using a certified reference solar cell (Fraunhofer ISE). Electron lifetimes and photocurrent transients measurements were performed using a white LED (Luxeon Star 1W) as the light source.20 Voltage and current traces were recorded with a 16-bit resolution digital acquisition board (National Instruments) in combination with a current amplifier (Stanford Research Systems SR570) and a custom-made system using electromagnetic switches. Electron lifetimes were determined by monitoring photovoltage transients at different light intensities upon applying a small square wave modulation to the base light intensity. The photovoltaic responses were fitted using first-order kinetics to obtain time constants. Transient Absorption Spectroscopy (TAS) Measurements. TiO2 films for TAS measurements were prepared by doctor blading colloidal TiO2 paste (Dyesol DSL 18NR-T) diluted with terpineol (60 wt % paste + 40 wt % terpineol) on microscope glass slides. The film thickness was about 4 μm. The electrolyte was sandwiched between the sample and a glass microscope coverslip. TAS measurements were performed on an Edinburgh Instrument LP920 laser flash photolysis spectrometer using a near-infrared LED (Osram SHF 484, λmax 880 nm, fwhm 80 nm) as probe light and an amplified Si photodiode (Thorlabs PMA10A) as

Figure 2. Schematic energy diagram displaying the kinetic processes in the operation of a DSC described in text. kreg is the regeneration rate of the sensitizer by the redox mediator, krec1 is the recombination of photoinjected electrons with the oxidized dye and krec2 is the recombination of photoinjected electrons with the oxidized redox species.

detector. Laser pulses were generated using a frequency tripled Nd: YAG laser (Continuum Surelight II, 10 Hz repetition rate, pulse width 10 ns) in combination with an OPO (Continuum Surelight), tuned to 560 nm. The pulse intensity was attenuated to 0.1 mJ cm2 per pulse by a system using a movable λ/2 plate and a fixed polarizer in combination with a 10% transmittance filter (Thorlabs). The laser intensity was kept low so that only one or a few electron was produced per TiO2 particle upon pulse irradiation. Kinetic traces were obtained at a wavelength of 880 nm using 640 averages in order to monitor oxidized dye concentrations.

’ RESULTS AND DISCUSSION Effect of the Driving Force for Recombination on the Photovoltaic Performance. The cobalt phenanthroline com-

plexes have more positive redox potentials than the cobalt bipyridine complexes that we have reported on before. The redox potentials of [Co(phen)3]n+, [Co(Cl-phen)3]n+, and [Co(NO 2 -phen)3 ]n+ was measured by cyclic voltammetry (Figure S1 in Supporting Information) and were determined to be 0.62, 0.72, and 0.85 V versus NHE, respectively (Figure 1). The currentvoltage characteristics of DSC using these mediators were examined to investigate the dependence of the redox potential on the solar cell performance, see Figure 3a and Table 1. The VOC increased with increasing Nernst potential of the redox couples, and the highest VOC of 1.03 V was obtained using the [Co(NO2-phen)3]n+-based electrolyte, which is one of the highest values recorded to date for DSC. A similar trend in voltages was found by DeVries et al.22 for alumina-modified DSCs sensitized with the ruthenium dye N3 in combination with the cobalt phenanthroline mediators. The electrochemical potential in the TiO2 under open circuit conditions (equivalent to the quasi-Fermi level in the TiO2) is approximately the same using 21502



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Figure 3. (a) Current density versus applied potential curves under AM1.5G illumination and (b) electron lifetime as a function of the quasi-Fermi level of the TiO2 under open circuit conditions for DSCs sensitized with D35 employing cobalt phenanthroline based electrolytes.

[Co(phen)3]n and [Co(Cl-phen)3]n+ as redox mediator, but more than 100 mV more positive in case of [Co(NO2-phen)3]n+ (see Table 1), indicating significantly more recombination losses in the DSC when this mediator is used. As indicated in Figure 2, two different types of electron recombination losses can occur in the DSC: electrons in the TiO2 can recombine with oxidized dye molecules (krec1) and with the oxidized form of the redox mediator (krec2). Both options will be investigated here and discussed later. The short-circuit photocurrent in the DSCs decreased with increasing redox potential of the complexes, see Figure 3a and Table 1. A similar trend in currents and voltages was found using the ruthenium-based dye, Z907 (Figure S2, Table S1 in Supporting Information). The drop in photocurrent with increasing redox potential can be attributed to slow regeneration kinetics, fast recombination kinetics and/or diffusion limitations. These different aspects will be considered below. In contrast, DeVries et al.22 did not find a clear relation between photocurrent and redox potential in their study, but it should be noted that they obtained only low photocurrents of about 0.2 mA cm2. The electron recombination kinetics were further investigated studying the photovoltage response of the DSCs to a small amplitude light modulation. The measured electron lifetime is plotted as function of the internal potential in the TiO2 for the D35-sensitized DSCs employing cobalt phenanthroline complexes in Figure 3b. Electron lifetimes in DSCs usually only reflect the electron recombination with oxidized form of the redox species in the electrolyte (krec2), but in case of slow regeneration of the oxidized dye, also the effect of electron recombination to the oxidized dye (krec1) will be included in the measured lifetime. Under conditions of equal internal potential in the TiO2, implying also equal electron concentration in the TiO2 as no band edge shifts or changes in the trap distribution are expected, electron lifetime for [Co(NO2-phen)3]n+ was much shorter than for the other two cobalt phenanthroline mediators, while lifetime for [Co(Cl-phen)3]n+ was only slightly smaller than for [Co(phen)3]n+. As will be shown later, dye regeneration by [Co(NO2-phen)3]n+ is rather slow, and significant recombination of electrons in TiO2 to the oxidized dye may account for the short electron lifetime in this case. The observed trend of increase in recombination rate with driving force for recombination is in agreement with the results by DeVries et al.22 and can be explained on basis of the Marcus theory of electron transfer,

Figure 4. Spectra of incident photon to current efficiency (IPCE) for DSCs sensitized with D35 employing cobalt bipyridine-based electrolytes and cobalt phenanthroline-based electrolytes. Every fifth data point is marked with a symbol. Co (dmbpy) and Co(dtbbpy) yielded results indistinguishable to that of Co(bpy).

suggesting electron transfer in the normal region, where the driving force is smaller than the reorganization energy for the reaction. The electron transfer rate is also expected to depend on the structure of both the dye and the redox mediator, since the size of the donor and acceptor should affect the reorganization energy and electronic coupling. In our previous publication we showed that recombination was slowed down using a cobalt bipyridine complex with more bulky substituents, since the larger size of the complex decreased the electronic coupling by increasing the spatial separation of the donor and acceptor.17 The size of the substituents introduced on the cobalt phenanthroline complexes are, however, small and the differences in electron lifetimes are therefore suggested to be determined by the difference in the driving force for recombination rather than by the difference in size. The incident-photon-to-current conversion efficiency (IPCE) for DSCs sensitized with D35 employing cobalt bipyridine redox couples and cobalt phenanthroline redox couples is shown in Figure 4. The IPCE was about 90% for all cobalt bipyridine redox couples, but decreased for the cobalt phenanthroline complexes in order with increasing redox potential. The high IPCE values for D35 sensitized DSCs employing cobalt bipyridine complexes 21503



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Figure 5. Transient absorption kinetics of D35 sensitized TiO2 electrodes employing (a) cobalt bipyridine based electrolytes and (b) cobalt phenanthroline based electrolytes. The traces symbolizes D35 with (A) inert, (B) [Co(dmb)3]n+, (C) [Co(dtb)3]n+, (D) [Co(bpy)3]n+, (E) [Co(phen)3]n+, (F) [Co(Cl-phen)3]n+, and (G) [Co(NO2-phen)3]n+ electrolytes. Electrolyte A contains 0.1 M LiClO4 and 0.2 M TBP in acetonitrile; B-D contain additionally 0.2 M Co(II) and 0.03 M Co(III) complex; E-G contain 0.1 M LiClO4, 0.2 M TBP, 0.1 M Co(II), and 0.01 M Co(III) complex in a 60:40 mixture of acetonitrile and ethylene carbonate.

can be attributed to efficient regeneration of the oxidized dye (see next section) and to the efficient blocking of electron recombination to the electrolyte by the introduction of bulky substituents on the dye, as discussed in our previous publication.17 The same trend in IPCE was found for the Z907 dye (Figure S3 Supporting Information). Due to solubility problems of the cobalt phenanthroline redox couples in acetonitrile, a lower concentration of these complexes was used than for the cobalt bipyridine complexes reported in ref 17. The electrolyte was in addition changed to a 40:60 mixture of ethylene carbonate and acetonitrile. Mass transport limitations are therefore likely to have a larger impact on the currentvoltage characteristics of DSC with the cobalt phenanthroline complexes, but should not affect the IPCE measurements significantly, since low monochromatic light intensities (15 W m2) are used in these experiments giving only small photocurrents. The observed decrease in IPCE is therefore expected to be a result of the increased recombination with increasing Nernst potential of the complexes, as discussed in the previous section, and/or slow dye regeneration. The electron diffusion length (L), describing the average distance an electron can travel through the mesoporous TiO2 film before recombination, should be larger than the film thickness for efficient electron collection. L can be determined under steady-state conditions by analysis of the IPCE spectra, using the assumption that recombination is proportional to electron concentration.23 Although this was found not to be the case for in cobalt-based DSCs,17 it is possible to determine an effective electron diffusion length from the IPCE spectra, noting that this approximate value is only valid at low light intensities. L was estimated to 7.1 μm for [Co(phen)3]n+, 4.2 μm for [Co(Clphen)3]n+, and 2.3 μm for [Co(NO2-phen)3]n+ (see Figure S4 in Supporting Information), indicating that the charge collection efficiency is just sufficient for [Co(phen)3]n+-based DSCs with a film thickness of about 6 μm. The photovoltaic performance of the cobalt phenanthroline complexes with more positive redox potentials is, however, clearly limited by recombination losses. The short electron diffusion lengths obtained for the cobalt phenanthroline complexes with the most positive redox potentials can partially be a result of the slow regeneration rate, as will be discussed below.

Table 2. Regeneration Halftimes and Regeneration Efficiencies for D35-Sensitized DSCs Employing Cobalt Polypyridine Redox Couples and Iodide/Triiodide redox couple

ΔG0 (eV) a

D35 + inert electrolyte

a

t1/2 (μs)

Φreg

IPCE (%) b

323

iodide/triiodide

0.79

15

0.95

86

[Co(dmb)3]n+

0.68

11

0.98

88

[Co(dtb)3]n+

0.68

15

0.95

89

[Co(bpy)3]n+

0.55

19

0.94

88

[Co(phen)3]n+

0.49

61

0.81

85

[Co(Cl-phen)3]n+ [Co(NO2-phen)3]n+

0.39 0.26

79 143

0.76 0.56

75 62

Difference between E0(redox) and E0(dye). b At 450 nm.

Effect of the Driving Force for Regeneration on the Photovoltaic Performance. The effect of the driving force for

dye regeneration by the cobalt complexes was investigated by transient absorption spectroscopy (TAS) performed on a nanosecond laser setup. In addition, photoinduced absorption spectroscopy, performed under conditions similar to the operational conditions of DSCs, was used to obtain the spectra for the oxidized dye, electrons in TiO2 and the Stark effect due to the change in electric field across the dye molecules upon charge injection.17,24 This data is shown in Supporting Information. Transient absorption spectroscopy was performed to determine regeneration rates constants of the D35 dye. The transient absorption kinetics of D35 sensitized TiO2 electrodes employing cobalt bipyridine based electrolytes and cobalt phenanthroline based electrolytes are shown in Figure 5. The transient optical signal observed at 880 nm after the laser pulse excitation at 560 nm is dominated by the absorption of the oxidized state of the D35 dye, but contains also the weaker absorption of electrons in the TiO2. The decrease in absorbance signal in the presence of an inert electrolyte shows the recombination of conduction band electrons with the oxidized dye molecules (krec1), and a half time (t1/2) for the recombination of 323 μs was obtained. In the presence of redox electrolyte the decay of the signal is strongly accelerated for the cobalt bipyridine redox couples, suggesting rapid regeneration of the dye by the redox mediators. The 21504



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by HAB 2 ðΔG0 λÞ2 ket ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi exp 4λkB T 4πλkB T

Figure 6. Semilogarithmic plot of the inverse of the halftime for reaction between oxidized D35 and cobalt polypyridine redox mediators. The drawn curve is a fit according to the Marcus theory (eq 1) for of regeneration kinetics using 0.2 M Co(II) + 0.03 M Co(III) in acetonitrile (filled symbols). Cobalt phenanthroline data (triangles) was recorded for 0.1 M Co(II) + 0.01 M Co(II) in a 60:40 mixture of acetonitrile and ethylene carbonate. Co(Cl-phen)3 was measured in both systems, and this data was used to calculate expected values for the other Co(phen) mediators (symbols in brackets).

absorbance signal reached a plateau 3000 μs after the excitation pulse, which was assigned to the electrons in the TiO2 and this signal was subtracted from the transient absorption signal in order to calculate the regeneration time constants. A table with the half times for dye regeneration for the cobalt bipyridine and cobalt phenanthroline redox couples is shown in Table 2. For the cobalt bipyridine complexes, the regeneration half times were 19, 11, and 15 μs for [Co(bpy)3]n+, [Co(dmb)3]n+, and [Co(dtb)3]n+, respectively. The longer regeneration time found for [Co(dtb)3]n+ compared to [Co(dmb)3]n+, which has a similar redox potential, can be explained by that the increased steric bulk of [Co(dtb)3]n+ decreases the electronic coupling and slows down the regeneration rate. For comparison, the regeneration halftime using iodide/triiodide electrolyte under identical conditions was 15 μs, which is similar to that of the cobalt bipyridine complexes. Regeneration of the oxidized dye was significantly slower for the cobalt phenanthroline complexes and the regeneration halftimes decreased with more positive redox potential of the mediator. The halftime for regeneration of [Co(phen)3]n+ was about 3 times slower than that of [Co(bpy)3]n+, despite only small differences in driving force and size of the redox mediators. It should be noted that part of the slower kinetics may be attributed to the lower Co(II) concentration (0.1 M) and the different solvent (acetonitrile:ethylene carbonate 60:40) used for the cobalt phenanthroline complexes. In order to estimate the driving forces for dye regeneration in a real device, the difference between the redox potential of the redox couple in solution and the D35 dye absorbed onto a TiO2 film was calculated. The cyclic voltammogram of D35 absorbed onto a TiO2 film is shown in Figure S6 in Supporting Information. The calculated driving forces for regeneration of the D35 dye by the different cobalt mediators in listed in Table 2. Regeneration of the oxidized dye by cobalt mediators is expected to be reasonably well described by the Marcus theory for electron transfer in homogeneous medium. The rate constant for electron transfer between a donor and acceptor is given

! ð1Þ

where HAB is the electronic coupling between the donor and acceptor states, ΔG0 the reaction free energy, and λ the reorganization energy. Figure 6 shows a semilogarithmic plot of the inverse of the regeneration half time (proportional to ket) as function of ΔG0. The drawn line is a fit to the Marcus equation, assuming equal HAB and λ for all Co(II)-complex oxidized D35 combinations. A reasonable trend is found and a reorganization energy of 0.8 ( 0.1 eV is determined. A correction to account for difference in Co(II)concentration and solvent system in the cobalt phenanthroline complex electrolytes was made (see figure caption). It is useful to determine the regeneration efficiency, which gives the fraction of oxidized dye molecules that are regenerated by the redox mediator. It is given by eq 2, where kreg is the first order rate constant for the regeneration of the sensitizer by the redox mediator and krec1 the rate constant for the back electron transfer from the electrons in the TiO2 conduction band to the oxidized dye molecules, and can be approximated using the determined half times as follows: ϕreg ¼

t1=2redox kreg ¼ 1 kreg þ krec1 t1=2inert

ð2Þ

The calculated regeneration efficiency was about 95% for the different cobalt bipyridine redox couples investigated here, which indicates sufficiently fast kinetics for dye regeneration of these redox mediators. It should, however, be noted that regeneration efficiency values reported here are approximate, as conditions in the ns-laser experiments differ from that under full sun solar cell working conditions. The slower kinetics for regeneration of the oxidized dye found for the cobalt phenanthroline complexes lead to lower regeneration efficiencies, decreasing with increasing redox potential of the complexes. The regeneration half reaction time of the D35 dye by the cobalt phenanthroline complex with the most positive redox potential, [Co(NO2-phen)3]n+, was 143 μs and the regeneration efficiency only 56%. The transient absorption kinetics for the Z907 dye with the different cobalt redox couples have also been measured and are reported in the Supporting Information. These results can not directly be compared to the results with D35, since a high intensity white probe light was used in the measurements of the Z907 dye, significantly speeding up recombination reactions and giving therefore less reliable results (Figure S8 and Tables S1 and S2 in Supporting Information). Trends in regeneration halftime as function of driving force were qualitatively similar as for D35: the regeneration rate of the Z907 dye by cobalt phenanthroline complexes was found to become slower with the more positive redox potential of the complexes. This trend is in agreement with the trend of the lower IPCE values found with increasing redox potential of the cobalt phenanthroline complexes (Figure S3). The IPCE is determined by the light harvesting efficiency (ηLHE), the injection efficiency (ηinj), the regeneration efficiency (ηreg), and the charge collection efficiency (ηcoll), according to eq 3. IPCE ¼ ηLHE ηinj ηreg ηcoll

ð3Þ

The light harvesting efficiency and injection efficiency can be estimated to unity for D35 sensitized solar cells, since an IPCE of about 90% were obtained using cobalt bipyridine redox couples. 21505



Figure 7. Plots of current transients measured at an illumination intensity of 1000 W m2 for DSCs sensitized with D35 employing cobalt phenanthroline based electrolytes.

The regeneration efficiencies obtained for the cobalt polypyridine redox couples correlate well with the IPCE values obtained at a wavelength of 450 nm (see Table 2), indicating that the low IPCE values with increasing redox potential of the complexes are a result of slow regeneration kinetics. The good correlation suggests that the determination of the regeneration efficiency is quite accurate. Effect of Mass Transport Limitations on the Regeneration Efficiency. The lower regeneration efficiency obtained for cobalt phenanthroline complexes can also be attributed to slow diffusion of the species since a lower concentration of the redox couples and a more viscous electrolyte was used. The effect of slow mass transport in DSCs employing cobalt phenanthroline redox mediators was investigated by monitoring photocurrent transients using a large modulation (on/off) of the incident light. The photocurrent transients recorded for D35-sensitized DSCs with the different cobalt phenanthroline redox mediators, using a white LED with the output set to match the photocurrent obtained under 1000 W m2 AM1.5G illumination, is shown in Figure 7. A maximum in photocurrent was observed when the light is switched on, followed by a decrease in photocurrent, eventually reaching a constant value. Similar measurements were performed by Nelson et al.,25 who suggested that the observed decrease in photocurrent with time was a result of slow diffusion of Co(III) to the counter electrode. The ratio of the initial peak current to steady state current was herein about the same for the different cobalt phenanthroline complexes, indicating that the diffusion rate is about the same and not dependent on the substituents of the complexes. The diffusion coefficients for the cobalt phenanthroline redox mediators were determined from the diffusion limiting current measured by slow scan cyclic voltammetry (Figure S9 Supporting Information). The diffusion coefficients were similar for the cobalt complexes and diffusion coefficients of 8.2 106 cm2 s1 for [Co(phen)3]2+, 7.6 106 cm2 s1 for [Co(Cl-phen)3]2+, and 8.0 106 cm2 s1 for [Co(NO2-phen)3]2+ in acetonitrile were obtained. For comparison, a value of 18 106 cm2 s1 has been reported for I3 in acetonitrile.25 The lower IPCE values with the increasing redox potential of the complexes are therefore, as expected, not explained by a difference in diffusion rate between the complexes. The initial peak current decreased, however, with increasing redox potential of the cobalt phenanthroline complexes, and the difference in IPCE values are therefore most

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Figure 8. Transient absorption kinetics of D35 sensitized TiO2 electrodes employing different [Co(Cl-phen)3]n+ based electrolytes. Electrolyte (A) consisted of 0.1 M [Co(L0 )3(PF6)]2+, 0.01 M [Co(L0 )3(PF6)]3+, 0.1 M LiClO4, and 0.2 M TBP in a mixture of acetonitrile and ethylene carbonate (60:40). Electrolyte (B) consisted of 0.2 M [Co(L0 )3(CF3SO3)]2+, 0.03 M [Co(L 0 )3(CF3SO3)]3+, 0.1 M LiClO4 and 0.2 M TBP in acetonitrile. Electrolyte (C) consisted of 0.4 M [Co(L0 )3(CF3SO3)]2+, 0.08 M [Co(L0 )3(CF3SO3)]3+ , 0.1 M LiClO4, and 0.5 M TBP in acetonitrile.

Figure 9. Optimized current density versus applied potential curves under AM1.5G illumination for DSCs sensitized D35 employing cobalt phenanthroline based electrolytes.

probably a result of slow regeneration or recombination kinetics, as discussed above. Cobalt phenanthroline complexes with a triflate counterion were synthesized in order to increase the solubility of the complexes in acetonitrile to investigate the concentration dependence of the redox couples on the regeneration kinetics. The transient absorption kinetics of D35 sensitized TiO2 electrodes employing [Co(Cl-phen)3]n+ based electrolytes in different concentrations and electrolyte solutions is shown in Figure 8. The regeneration of the oxidized dye accelerated when the concentration of [Co(Cl-phen)3]2+ was increased from 0.1 to 0.2 M (t1/2 decreased from 79 to 58 μs) and the regeneration efficiency increased from 76% to 82%. This indicates that a driving force for dye regeneration of about 390 mV is enough to regenerate more than 80% of the D35 dye molecules. Further increase in concentration to 0.4 M did, however, not show a significant effect on the regeneration efficiency. Nusbaumer et al.15 observed a linear increase in the regeneration rate of the N719 dye with increasing concentration of Co(II) in the electrolyte after a 21506


The Journal of Physical Chemistry C threshold concentration of 0.01 M. The regeneration rate was also found to decrease with increasing concentration of Co(III) in the electrolyte. The concentration of Co(III) was not kept constant in our measurements, and the small change in regeneration efficiency found with increasing concentration of Co(II), going from electrolyte B to C in Figure 9, can be explained by the increase in concentration of Co(III). The regeneration efficiency could also be affected by the different counterions used; for instance, Pelet et al.9 observed faster dye regeneration for iodide/triiodide using cations in the electrolyte that adsorbed to the TiO2 surface. The solar cell performance of D35-sensitized DSCs using cobalt phenanthroline complexes was optimized and the resulting currentvoltage curves for [Co(phen)3]n+ and [Co(Cl-phen)3]n+ are shown in Figure 9. The electrolyte consisted of 0.4 M Co(L0 )3(CF3SO3)2, 0.08 M Co(L0 )3(CF3SO3)3, 0.1 M LiClO4 and 0.5 M TBP in acetonitrile. The solubility of [Co(NO2phen)3]n+ in acetonitrile using the triflate counterion was poor and this compound was therefore not included in the optimization study. The best efficiency of 6.25% was obtained using the [Co(phen)3]n+-based electrolyte, which can be compared to the 5.5% and 6.7% efficiencies that we reported previously for the D35 dye in combination with the iodide/triiodide or the [Co(bpy)3]n+ redox mediators, respectively (see Table S3).17 Despite the better VOC obtained using [Co(Cl-phen)3]n+, the conversion efficiency was about one percent unit less (5.1%) using this redox couple because of a decrease in photocurrent. The loss in photocurrent by increasing the redox potential by about 100 mV is attributed to the slower regeneration and faster recombination kinetics. Most importantly, we have shown that cobalt complexes with a low driving force for regeneration can be used to increase the voltage of DSCs.

’ CONCLUSION DSCs with open-circuit potentials of more than 1 V were obtained by using the D35 dye that efficiently suppresses recombination in combination with cobalt phenanthroline redox couples with more positive redox potentials than the iodide/ triiodide redox couple. The effect of the driving force for recombination and regeneration on the photovoltaic performance was investigated by tuning the coordination sphere of cobalt bipyridine and phenanthroline redox couples. For DSC using the cobalt phenanthroline redox couples the VOC was found to increase and the IPCE to decrease as the Nernst potential of the redox couples became more positive. The decrease in IPCE was attributed to an increase in recombination rate as the driving force for recombination increased and a decrease in regeneration rate as the driving force for regeneration decreased, in accordance with predictions from the Marcus theory. The regeneration rate constants obtained for the cobalt bipyridine complexes was comparable to that found for the iodide/triiodide redox couple, about 15 μs. A driving force for regeneration of 390 mV for [Co(Cl-phen)3]n+ was found sufficient to regenerate more than 80% of the D35 dye molecules. ’ ASSOCIATED CONTENT

bS

Supporting Information. Supplemental experimental details; cyclic voltammograms of cobalt phenanthroline complexes; currentvoltage characteristics for Z907 based DSCs; IPCE spectra for Z907 based DSCs; fits to the IPCE spectra for D35 sensitized DSCs; cyclic voltammogram of D35 sensitized

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TiO2 films; photoinduced absorption spectra of D35 and Z907 sensitized TiO2 electrodes; transient absorption kinetics of Z907 sensitized TiO2 electrodes; and diffusion limiting currents for cobalt phenanthroline complexes. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT We thank Peter Lohse for measuring the HOMO level of the D35 dye absorbed onto TiO2. We gratefully acknowledge financial support from the Swedish Energy Agency, the Swedish Research Council, Vinnova and the STandUP for Energy program. ’ REFERENCES (1) O’Regan, B.; Gr€atzel, M. Nature 1991, 353, 737–740. (2) Hagfeldt, A.; Gr€atzel, M. Chem. Rev. 1995, 95, 49–68. (3) Hagfeldt, A.; Gr€atzel, M. Acc. Chem. Res. 2000, 33, 269–277. (4) Chiba, Y.; Islam, A.; Watanabe, Y.; Komiya, R.; Koide, N.; Han, L. Jpn. J. Appl. Phys., Part 2 2006, 45, L638–L640. (5) Boschloo, G.; Hagfeldt, A. Acc. Chem. Res. 2009, 42, 1819–1826. (6) Kuciauskas, D.; Freund, M. S.; Gray, H. B.; Winkler, J. R.; Lewis, N. S. J. Phys. Chem. B 2000, 105, 392–403. (7) Wenger, S.; Bouit, P.-A.; Chen, Q.; Teuscher, J.; Di Censo, D.; Humphry-Baker, R.; Moser, J.-E.; Delgado, J. L.; Martin, N.; Zakeeruddin, S. M.; Gr€atzel, M. J. Am. Chem. Soc. 2010, 132, 5164–5169. (8) Clifford, J. N.; Palomares, E.; Nazeeruddin, M. K.; Gr€atzel, M.; Durrant, J. R. J. Phys. Chem. C 2007, 111, 6561–6567. (9) Pelet, S.; Moser, J.-E.; Gr€atzel, M. J. Phys. Chem. B 2000, 104, 1791–1795. (10) Nusbaumer, H.; Zakeeruddin, S. M.; Moser, J.-E.; Gr€atzel, M. Chem.—Eur. J. 2003, 9, 3756–3763. (11) Gregg, B. A.; Pichot, F.; Ferrere, S.; Fields, C. L. J. Phys. Chem. B 2001, 105, 1422–1429. (12) Feldt, S. M.; Cappel, U. B.; Johansson, E. M. J.; Boschloo, G.; Hagfeldt, A. J. Phys. Chem. C 2010, 114, 10551–10558. (13) Oskam, G.; Bergeron, B. V.; Meyer., G. J.; Searson, P. C. J. Phys. Chem. B 2001, 105, 6867–6873. (14) Zhang, Z.; Chen, P.; Murakami, T. N.; Zakeeruddin, S. M.; Gr€atzel, M. Adv. Funct. Mater. 2008, 18, 341–346. (15) Nusbaumer, H.; Moser, J.-E.; Zakeeruddin, S. M.; Nazeeruddin, M. K.; Gratzel, M. J. Phys. Chem. C 2001, 105, 10461–10464. (16) Sapp, S. A.; Elliott, C. M.; Contado, C.; Caramori, S.; Bignozzi, C. A. J. Am. Chem. Soc. 2002, 124, 11215–11222. (17) Feldt, S. M.; Gibson, E. A.; Gabrielsson, E.; Sun, L.; Boschloo, G.; Hagfeldt, A. J. Am. Chem. Soc. 2010, 132, 16714–16724. (18) Hagberg, D. P.; Jiang, X.; Gabrielsson, E.; Linder, M.; Marinado, T.; Brinck, T.; Hagfeldt, A.; Sun, L. J. Mater. Chem. 2009, 19, 7232–7238. (19) Klahr, B. M.; Hamann, T. W. J. Phys. Chem. C 2009, 113, 14040–14045. (20) Boschloo, G.; H€aggman, L.; Hagfeldt, A. J. Phys. Chem. B 2006, 110, 13144–13150. (21) Boschloo, G.; Hagfeldt, A. Inorg. Chim. Acta 2008, 361, 729–734. (22) DeVries, M. J.; Pellin, M. J.; Hupp, J. T. Langmuir 2010, 26, 9082–9087. (23) S€odergren, S.; Hagfeldt, A.; Olsson, J.; Lindquist, S.-E. J. Phys. Chem. 1994, 98, 5552–5556. (24) Cappel, U. B.; Feldt, S. M.; Sch€oneboom, J.; Hagfeldt, A.; Boschloo, G. J. Am. Chem. Soc. 2010, 132, 9096–9101. (25) Nelson, J. J.; Amick, T. J; Elliott, C. M. J. Phys. Chem. C 2008, 112, 18255–18263. 21507


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