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Performance Enhancement and Limitations of Cobalt Bipyridyl Redox Shuttles in Dye-Sensitized Solar Cells Benjamin M. Klahr and Thomas W. Hamann* Department of Chemistry, Michigan State UniVersity, East Lansing, Michigan 48824-1322 ReceiVed: April 14, 2009; ReVised Manuscript ReceiVed: May 27, 2009
A series of one-electron outersphere cobalt bipyridyl redox couples were used as redox shuttles in dyesensitized solar cells (DSSCs). Atomic layer deposition was used to deposit an ultrathin coating of alumina on nanoparticle-based TiO2 DSSC photoanodes, which results in significantly improved quantum yields for all of the DSSCs containing outersphere redox systems. However, a significant discrepancy in performance remains between DSSCs containing the different cobalt redox shuttles. Variation of the driving force for regeneration by ∼500 mV, by employing [Ru(bpy)2(4,4′-dicarboxy-bpy)](PF6)2 as a dye, combined with concentration dependence studies indicates that the cobalt redox couples are not limited by dye regeneration; however, in certain cases the iodide electrolyte was, one of the very few systems where alternate redox couples perform significantly better than triiodide/iodide. Electron lifetimes were measured with the open circuit voltage decay technique. The differences in the lifetimes (recombination kinetics) of DSSCs employing cobalt redox couples correlate well with the differences in the incident photon-to-current efficiencies (IPCEs), providing strong evidence that the external quantum efficiencies of DSSCs with cobalt polypyridyl redox couples are limited by recombination. We further found that, contrary to previous reports, the cobalt(III/II) tris(4,4′-dimethyl-2,2′-bipyridine) couple can produce comparable external quantum yields to cobalt(III/II) tris(4,4′-di-tert-butyl-2,2′-bipyridine) when employed as redox shuttles in DSSCs. However, the photovoltaic performances of both are constrained by mass transport of the oxidized species through the nanoparticle photoelectrode. Introduction The seminal report by Gratzel in 1993 of a 10% efficient dye-sensitized solar cell, DSSC, proffered the exciting possibility of very inexpensive yet efficient solar-to-electrical energy conversion.1,2 In this device [Ru(4,4′-dicarboxy-2,2′-bipyridine)2(NCS)2], N3, was used to sensitize high surface area TiO2 nanoparticle electrodes immersed in an I3-/I- electrolyte.1 Since that time, there has been continued interest and research to further improve and understand DSSCs. However, the maximum efficiency has remained essentially constant, with a current record of 11%.3-5 Although there have been a plethora of dyes prepared and incorporated in DSSCs,6-12 the most efficient (>10%) DSSCs all continue to use either N3, the partially deprotonated N719, (Bu4N)2[Ru(4-carboxy,4′-carboxylato-2,2′-bipyridine)2(NCS)2], or the closely related black dye, (Bu4N)[Ru(4,4′,4”-tricarboxy-2,2′: 6′,2”-terpyridine)(NCS)3].13,14 The lack of success in identifying a dye significantly superior to N3 can be attributed, at least in part, to the almost exclusive use of I3-/I- as the redox shuttle. Although I3-/I- appears to be an ideal redox shuttle with N3, exhibiting essentially unity dye regeneration efficiency, the regeneration of alternate dyes by iodide is often inefficient resulting in diminished performance.7,15 The mechanism of dye regeneration by iodide also remains unclear; however, the overall regeneration yield appears to be dependent upon interaction of a second iodide ion with a [dye+/I-] complex, making the intelligent design of new dyes very challenging.15 Furthermore, it has recently been demonstrated that certain dyes form dye/I3- complexes resulting in a large local concentration of triiodide at the semiconductor-dye interface, * To whom correspondence should be addressed. E-mail: hamann@ chemistry.msu.edu.
which enhances recombination.16 Formation of such complexes can also lead to the deleterious side reaction of direct quenching of the dye excited state by I3-.17 Another drawback of relying on I3-/I- is the complicated, potentially multielectron, kinetics of I3-/ I- redox processes, which hinders the understanding of several key steps in the operation of DSSCs.18,19 Additionally, strict reliance on I3-/Iprevents having molecular control of the redox shuttle thwarting systematic studies on the electrolyte. In general, however, oneelectron, outersphere redox reagents such as ferrocenes have not proved to be useful mediators in DSSCs.20-22 In a particularly interesting report, DSSCs using the easy-to-prepare cobalt(III/II) tris(4,4′-di-tert-butyl-2,2′-bipyridyl), [Co(tBu2bpy)3]3+/2+, as a redox shuttle exhibited excellent efficiencies, with incident photon-to-current efficiencies (IPCEs) ∼80% of a comparable cell employing I3-/I-.23 Surprisingly, similar redox couples such as cobalt(III/II) tris(4,4′-dimethyl-2,2′-bipyridyl), [Co(Me2bpy)3]3+/2+, exhibited relatively poor performance.23 In this work we return to re-examine these cobalt-based electrolytes in order to determine the cause of the discrepancy in photovoltaic performance. This represents our initial attempt to overcome the limitations associated with outersphere redox couples in DSSCs. Experimental Section 4,4′-Di-tert-butyl 2,2′-bipyridine (t-Bu2bpy), 4,4′-dimethyl 2,2′-bipyridine (Me2bpy), 2,2′-bipyridine (bpy), cobalt chloride (CoCl2 · 6H2O), lithium perchlorate (LiClO4), ammonium hexafluorophosphate (NH4PF6), and NOBF4 were used as received from Aldrich. 4-tert-Butylpyridine was purified by distillation. Solvents were the highest grade available and were used as received.
10.1021/jp903431s CCC: $40.75 2009 American Chemical Society Published on Web 07/01/2009
TRH: Cobalt Bipyridyl Redox Shuttles in Solar Cells The compounds [Co(t-Bu2bpy)3](PF6)2, [Co(Me2bpy)3](PF6)2, and [Co(bpy)3](PF6)2 were prepared by a modified literature method.23 Briefly, 1 equiv of CoCl2 · 6H2O dissolved in a minimal amount of methanol was added to a methanolic solution containing 3 equiv of the bipyridyl ligand, and the solution was stirred for 2 h. An excess of ammonium hexafluorophosphate was used to precipitate a yellow compound that was filtered, washed with ethanol, methanol, and ether, dried under vacuum, and used without further purification. Photoelectrodes were prepared on 12 Ω cm-2 FTO-coated glass (Hartford Glass) cleaned by sonicating in a soap water solution, sonicating in acetone, and then heating to 500 °C. Blocking layers of TiO2 were deposited using 500 ALD cycles of titanium isopropoxide (TIPS, Aldrich) and water as precursors with a Savannah 100 instrument (Cambridge Nanotech, Inc.). TiO2 was grown at 225 °C using reactant exposure times of 0.3 and 0.015 s for TIPS and H2O, respectively, and nitrogen purge times of 5 s between exposures. The thickness of the TiO2 blocking layer was determined to be ∼7 nm by ellipsometry performed on Si samples coated concurrently in the ALD reactor in accord with literature reports.24 A transparent TiO2 nanoparticle layer was prepared by doctor blading a paste of TiO2 nanoparticles (DSL 18NR-T, Dyesol) on the FTO. A scattering layer of TiO2 nanoparticles (DSL 18NR-AO, Dyesol) was subsequently deposited on top of the transparent layer. The resulting electrodes were annealed at 500 °C in air for 30 min. Alumina was deposited immediately following removal from the oven by ALD using trimethylaluminum (TMA, Aldrich) and water as precursors. Al2O3 was grown at 250 °C using reactant exposure times of 10 s for both precursors and nitrogen purge times of 10 s between exposures. The TiO2 electrodes were heated to 500 °C for 30 min, cooled to 100 °C, and immersed in a 0.5 mM solution of [Ru(4,4′dicarboxy-2,2′-bipyridine)2(NCS)2] (N3, Dyesol, B4 dye) in ethanol. Alternatively, electrodes were soaked in a ∼0.6 mM solution of [Ru(bpy)2(4,4′-dicarboxy-bpy)](PF6)2 (generously donated by the McCusker Group) in acetonitrile. After 20-24 h, they were rinsed with ethanol and acetonitrile. A ∼25 µm thick Surlyn frame (Solaronix) was sandwiched between the TiO2 nanoparticle electrode and a platinized FTO electrode, and light pressure was applied at 140 °C to seal the cell. Unless specifically mentioned in the text below, each electrolyte consisted of acetonitrile solutions containing 0.22 M Co(II) species, 0.20 M LiClO4, 0.20 M 4-tert-butyl pyridine, and 0.02 M NOBF4. Because there is some difficulty with accurately and reproducibly weighing and adding NOBF4 to electrolytes, thus introducing uncertainty in the concentration of oxidized Co(III) species, we prepared a stock solution of NOBF4 dissolved in acetonitrile that was stored in the freezer. A known constant volume of the NOBF4 solution was added to each electrolyte, thus keeping the concentration of oxidized cobalt species as constant as possible across the series. In addition, every electrolyte was prepared fresh on the same day, introduced into TiO2 cells, and examined immediately. For comparison, an iodide electrolyte was prepared consisting of 0.6 M butyl-methyl imidazolium iodide, 0.04 M I2, 0.20 M LiClO4, and 0.20 M 4-tert-butyl pyridine in acetonitrile. Each series reported was repeated several times; while the absolute values varied somewhat between batches of DSSCs prepared, the trends (relative performance) were very consistent and reproducible. Cyclic voltammetry was performed with an CHI600 potentiostat with a Pt or Au disk electrode, a high surface area Pt counter electrode, and Ag+/Ag as a reference; ferrocene was
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Figure 1. IPCE plots for DSSCs employing [Co(t-Bu2bpy)3]3+/2+ (red b), [Co(Me2bpy)3]3+/2+ (orange 2), [Co(bpy)3]3+/2+ (yellow 9), and I3-/I- (green () with (a) bare TiO2 electrodes and (b) 1 ALD layer of Al2O3. All electrolytes contain 0.2 M LiClO4 and 0.2 M tert-butyl pyridine.
used as an internal reference and determined to be 0.35 V versus Ag+/Ag. Photoelectrochemical measurements were performed with an Autolab PGSTAT 126N interfaced with a Xe arc lamp. An AM 1.5 solar filter and neutral density filters were used to simulate sunlight at 100 mW/cm2. A Horiba Jobin Yvon MicroHR was used for monochromatic light. Results and Discussion Incident photon-to-current efficiency (IPCE) measurements were performed for dye-sensitized solar cells (DSSCs) containing [Co(t-Bu2bpy)3]3+/2+, [Co(Me2bpy)3]3+/2+, [Co(bpy)3]3+/2+, and I3-/I- redox shuttles, and the resulting IPCE plots are displayed in Figure 1. The maximum IPCE for DSSCs employing [Co(t-Bu2bpy)3]3+/2+ is 63% (∼80% as large as DSSCs employing I3-/I-), which is comparable to previous reports.23 Contrary to previous reports, DSSCs employing [Co(Me2bpy)3]3+/2+ and [Co(bpy)3]3+/2+ electrolytes exhibited maximum IPCE values of 43% and 25%, respectively, as opposed to [Co(Me2bpy)3]3+/2+ > [Co(bpy)3]3+/2+, which indicates that the recombination rate constants decrease in the reverse order. A previous study employing single-crystal ZnO electrodes in contact with [Os(Me2bpy)3]3+/2+ and [Os(t-Bu2bpy)3]3+/2+ demonstrated that tert-butyl groups act as insulating spacers, which decrease the electronic coupling by a factor of ∼20 relative to the methyl-substituted compound.38 Although the lifetimes measured by the OCVD technique are less precise compared to those of single-crystal electrodes, the approximately 5-fold longer electron lifetime for [Co(t-Bu2bpy)3]3+/2+ compared to that of [Co(Me2bpy)3]3+/2+ can be attributed to decreased coupling due to the steric hindrance imposed by the tert-butyl groups. The larger size of the tert-butyl substituted cobalt shuttles should also produce a lower outersphere reorganization energy, leading to an increased rate of recombination, which can partially offset the steric effect. In other words, the 5-fold longer lifetime can be accounted for by a 20-fold decreased electronic coupling offset by a 4-fold increase in the recombination rate constant, resulting from the smaller reorganization energy. Another study using single-crystal electrodes in contact with [Co(bpy)3]3+/2+ to probe the dependence of interfacial electron transfer reactions via changes in solution pH indicated that a ∼150 mV increase in driving force could produce a ∼10fold increase in the rate constant.39 Therefore, the ∼10-fold decrease in lifetime observed for [Co(bpy)3]3+/2+ compared to that of [Co(Me2bpy)3]3+/2+ can be accounted for by the larger driving force (∼130 mV) of the electron transfer reaction (recombination). The trend of the electron lifetime decreasing with increasing driving force is also consistent with the lithium ion concentration dependence in the electrolyte previously reported.36 The electron diffusion distance, L, is proportional to the square root of the lifetime according to L ) (Dτn)1/2, where D is the electron diffusion coefficient.5 Decreasing lifetimes should therefore result in decreased diffusion distances.19,33,40 Although the diffusion distances were not measured directly herein, the differences in the lifetimes for the cobalt redox shuttles can readily account for the discrepancy in IPCEs observed for DSSCs employing [Co(t-Bu2bpy)3]3+/2+, [Co(Me2bpy)3]3+/2+, and [Co(bpy)3]3+/2+ (Figures 1 and 2). We further compared the electron lifetimes for each of the redox species with and without the alumina layer. In all cases, the lifetimes increased by ∼6-fold with one ALD cycle of Al2O3 compared to those of the bare TiO2 electrode (Figure S2 of the Supporting Information). The increase in lifetime can account for the improvement in the IPCEs observed for [Co(tBu2bpy)3]3+/2+, [Co(Me2bpy)3]3+/2+, and [Co(bpy)3]3+/2+ with the Al2O3 layer (Figure 1). This result supports the case that the cobalt redox shuttles are limited by recombination, which limits the electron diffusion distance and hence IPCEs. No improvement is observed for the IPCEs of DSSCs employing I3-/I- with the addition of an Al2O3 layer, however. It has been estimated that the electron diffusion distance of DSSCs employing I3-/I- is ∼200 µm, which is already greater than the electrode thickness.19 If the diffusion distance is greater than the electrode thickness, the IPCE is limited strictly by light harvesting; therefore, increasing the
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Klahr and Hamann for the redox species examined herein. Similar to a recent report on [Co(t-Bu2bpy)3]3+/2+, we observe that DSSCs with cobalt complexes have a relatively large initial current density immediately following illumination, which then quickly decays over ∼0.5 s to a lower steady-state current density.27 This behavior has been attributed to mass transport limitations of the electrolyte.27,28 Integration of IPCE curves with the simulated solar spectrum produces the predicted Jsc, in the absence of mass transport effects.11 Integration of the IPCE curves shown in Figure 1b with the solar spectrum yield predicted Jsc values of 9.1, 9.0, 7.9, and 4.0 mA cm-2 for I3-/I-, [Co(tBu2bpy)3]3+/2+, [Co(Me2bpy)3]3+/2+, and [Co(bpy)3]3+/2+, respectively, which are in reasonable agreement with the instantaneous current density in the current transient measurements (Figure 4b). The steady-state current densities for [Co(t-Bu2bpy)3]3+/2+ and [Co(Me2bpy)3]3+/2+ are significantly lower than the instantaneous current densities, however, attributed to mass transport limited current density. The associated mass transport resistance likely causes the very poor fill factors for DSSCs with these electrolytes. Conclusions
Figure 4. (a) Plots of current density versus applied potential, J versus E, and (b) plots of current transients of DSSCs employing [Co(tBu2bpy)3]3+/2+ (red), [Co(Me2bpy)3]3+/2+ (orange), [Co(bpy)3]3+/2+ (yellow), and I3-/I- (green) under AM 1.5 illumination.
TABLE 1: Relevant Photovoltaic Parameters of DSSCs Employing Different Redox Shuttlesa Jsc Voc predicted Jscb (mA cm-2) (mA cm-2) (V) - -
I3 /I [Co(t-Bu2bpy)3]3+/2+ [Co(Me2bpy)3]3+/2+ [Co(bpy)3]3+/2+
9.1 9.0 7.9 4.0
10.1 7.4 6.9 4.1
0.56 0.50 0.47 0.53
ff
η (%)
0.66 0.32 0.41 0.60
3.7 1.2 1.3 1.3
a Parameters are in response to AM 1.5 simulated light (100 mW cm-2) from plots shown in Figure 4a. b Predicted from integration of the IPCE curves with the AM 1.5 solar spectrum.
diffusion distance further by increasing the electron lifetime should not improve the IPCE.5,11,19 All else being equal, however, each factor of 10 increase in electron lifetime should improve the Voc by ∼59 γ mV (where γ is the diode quality factor) in accordance with the diode equation. We observed a ∼50 mV increase in Voc upon the addition of an Al2O3 layer for DSSCs employing I3-/I-, consistent with the increased lifetime. Turning to photovoltaic behavior, Figure 4a shows plots of the current density, J, versus applied potential curves for DSSCs containing [Co(t-Bu2bpy)3]3+/2+, [Co(Me2bpy)3]3+/2+, [Co(bpy)3]3+/2+, and I3-/I- electrolytes under 100 mW cm-2 AM 1.5 simulated white light illumination. The overall photovoltaic performances (efficiencies) of the cobalt-based redox shuttles are fairly comparable, and all significantly worse than the iodide electrolyte (Table 1). The [Co(tBu2bpy)3]3+/2+ and [Co(Me2bpy)3]3+/2+ electrolytes especially exhibited poor fill factors and lower-than-expected shortcircuit current densities, Jsc based on the IPCE data. Figure 4b shows plots of the short circuit current density versus time
We found that all of the cobalt polypyridyl redox shuttles investigated were able to effectively regenerate N3 and [Ru(bpy)2(4,4′-dicarboxy-bpy)](PF6)2 dyes. The generally used iodide electrolyte, however, performed much worse with [Ru(bpy)2(4,4′dicarboxy-bpy)](PF6)2, which we attribute to inefficient regeneration. We therefore believe that [Co(t-Bu2bpy)3]3+/2+ should be used as a redox shuttle in future investigations of new dyes because it should overcome the mechanistic complications of iodide, which may exhibit misleading poor performance. Minor variations of the ligand substituents on the cobalt redox shuttles investigated, [Co(t-Bu2bpy)3]3+/2+, [Co(Me2bpy)3]3+/2+, and [Co(bpy)3]3+/2+, result in fairly different rates of recombination. It is recombination of electrons in the TiO2 to the Co(III) complex that limits the IPCE of these electrolytes. Coating the photoanode with an ultrathin layer of Al2O3 decreases the rate of recombination for all electrolytes investigated, which improved the IPCE for all cobalt shuttles (because they were limited by recombination) but not iodide (which is not limited by recombination). The ability to further decrease the rate of recombination, for example, if thicker tunneling barriers could be used without loss of injection efficiency, should thus make redox couples with more positive potentials such as [Co(bpy)3]3+/2+ competitive with I3-/I-. We think it is important to reiterate here, however, that even if this could be achieved the overall photovoltaic performance would still likely be limited by mass transport.27 The development of new electrode architectures and higher extinction “super chromophores” used in conjunction with such redox couples, however, could result in DSSCs that can ultimately achieve higher efficiencies. Acknowledgment. T.W.H. thanks Michigan State University for providing a generous start-up package in support of this work. Supporting Information Available: IPCE plots for DSSCs with TiO2 electrodes with 1 ALD layer of Al2O3 and electron lifetimes of DSSCs. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphrybaker, R.; Muller, E.; Liska, P.; Vlachopoulos, N.; Gratzel, M. J. Am. Chem. Soc. 1993, 115 (14), 6382–6390.
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JP903431S
