Electron-Transfer Processes in Zinc Phthalocyanine–Phosphonic Acid

Letter pubs.acs.org/JPCL

Electron-Transfer Processes in Zinc Phthalocyanine−Phosphonic Acid Monolayers on ITO: Characterization of Orientation and ChargeTransfer Kinetics by Waveguide Spectroelectrochemistry Hsiao-Chu Lin,† Nathan W. Polaske,† Luis E. Oquendo,† Matthew Gliboff,§ Kristina M. Knesting,‡ Dennis Nordlund,# David S. Ginger,‡ Erin L. Ratcliff,† Brooke M. Beam,† Neal R. Armstrong,† Dominic V. McGrath,† and S. Scott Saavedra*,† †

Department of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona 85721, United States Department of Physics and ‡Department of Chemistry, University of Washington, Seattle, Washington 98195, United States # Stanford Synchrotron Radiation Laboratory, 2575 Sand Hill Road MS69, Menlo Park, California 94025, United States §

S Supporting Information *

ABSTRACT: Using a monolayer of zinc phthalocyanine (ZnPcPA) tethered to indium tin oxide (ITO) as a model for the donor/transparent conducting oxide (TCO) interface in organic photovoltaics (OPVs), we demonstrate the relationship between molecular orientation and charge-transfer rates using spectroscopic, electrochemical, and spectroelectrochemical methods. Both monomeric and aggregated forms of the phthalocyanine (Pc) are observed in ZnPcPA monolayers. Potential-modulated attenuated total reflectance (PM-ATR) measurements show that the monomeric subpopulation undergoes oxidation/reduction with ks,app = 2 × 102 s−1, independent of Pc orientation. For the aggregated ZnPcPA, faster orientation-dependent chargetransfer rates are observed. For in-plane-oriented Pc aggregates, ks,app = 2 × 103 s−1, whereas for upright Pc aggregates, ks,app = 7 × 102 s−1. The rates for the aggregates are comparable to those required for redox-active interlayer films at the hole-collection contact in organic solar cells. SECTION: Energy Conversion and Storage; Energy and Charge Transport Herein, we present the first measurements of electrontransfer rates for surface-confined Pcs on transparent conducting oxide (TCO) electrodes as a function of molecular orientation and aggregation, made possible by using a combination of waveguide spectroscopy16,17 and potentialmodulated attenuated total reflectance (PM-ATR) spectroelectrochemistry.18−20 We explore these structure−function relationships using monolayers of an asymmetric zinc phthalocyanine phosphonic acid (ZnPcPA; Figure 1A) selfassembled on indium−tin oxide (ITO), which serves as a model for the donor layer/TCO interface in OPVs. The influence of molecular orientation on the charge-transfer kinetics of sensitizers tethered to semiconductor nanomaterials has been examined by several groups (see ref 21 for a review); however, in general, the orientation distribution of the linkerchromophore film was not measured, which makes it difficult to establish a definitive correlation such as that reported here for ZnPcPA films on ITO. In a recent paper, we showed that ZnPcPA binds strongly to ITO electrodes.22 On the basis of ultraviolet photoemission

C

harge-transfer efficiency at organic/electrode interfaces can critically affect the performance of organic electronic devices, including organic photovoltaics (OPVs).1−3 Rates of charge transfer across these interfaces are determined by offsets in frontier orbital energies, wave function overlap, reorganization energies, and charge mobilities, which in turn depend on structural parameters, for example, packing and orientation, of the interfacial molecular layers.4−9 Modification of this interface with a redox-active organic surface modifier may enhance charge injection across the interface by providing a facile electron-transfer pathway between the contact and the adjacent organic layer and/or by controlling chemical and physical interfacial compatibility and the effective work function of the contact.1,4,5,10,11 We ultimately envision formation of OPVs starting with a tethered donor monolayer at the hole-harvesting contact that is formed using an asymmetric phthalocyanine (Pc) bearing a carboxylic acid, phosphonic acid, or other anchoring group.12,13 These tethered chromophores may also be useful in emerging dye-sensitized solar cells (DSSCs).14,15 To implement this approach in an effort to improve the efficiency of both OPVs and DSSCs, it is essential that rates of heterogeneous electron transfer for the tethered monolayer be characterized as a function of structural parameters, including surface coverage, molecular orientation, and degree of aggregation of the chromophores. © 2012 American Chemical Society

Received: March 1, 2012 Accepted: April 11, 2012 Published: April 11, 2012 1154

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CV method formulated by Laviron,29 are 1.7 ± 0.2 and 2.4 ± 0.1 s−1 (n = 4 trials), respectively. Fitting the anodic portion of the CV to two peaks centered at the respective anodic peak potentials indicates that the adsorbed ZnPcPA monolayer is composed of ∼55 and 45% of monomeric and aggregated species, respectively. At lower surface coverages, a slightly greater percentage of the voltammetric response is assignable to the aggregated forms, indicating that their formation is thermodynamically favored relative to the monomer. Figure 2A shows the absorption spectra of a 100 μM ZnPcPA/acetonitrile/pyridine (7/3, v/v) solution and a

Figure 1. (A) Structure of ZnPcPA; (B) background-subtracted cyclic voltammogram showing the first oxidation of monomeric and aggregated forms of ZnPcPA tethered to ITO. The electrolyte was 0.3 M tetrabutylammoniumperchlorate (TBAP) in acetonitrile.

spectroscopy measurements, ZnPcPA monolayers do not pose an energetic barrier for hole collection on ITO from several Pc donor layers;22 however, electrochemical characterization (Figure 1B) showed that distinct monomeric and aggregatelike subpopulations are present in these monolayers. Aggregation of ZnPcPA in these films is presumably influenced by the density and heterogeneity of adsorption sites on ITO,5,23 the surface packing density, and the extent of intermolecular interactions. Aggregation in Pc molecular assemblies is known to affect frontier orbital energies and energy level alignments, as well as dark and photoelectrical properties at both organic/ organic and organic/electrode heterojunctions.12,13,24−27 However, for donor monolayers on TCO substrates, these structure−function relationships are poorly understood, particularly the effects of molecular aggregation and orientation on interfacial electronic properties and heterogeneous chargetransfer behavior. Chemisorption of ZnPcPA to ITO-coated waveguides was monitored at the Q-band absorbance maximum (685 nm) using ATR spectroscopy (see Supporting Information (SI)). A symmetric ZnPc without the phosphonic acid tethering group (Figure S1, SI) showed no detectable adsorption. The ZnPcPA surface coverage at saturation was estimated to be 2.6 × 10−10 mol/cm2, equivalent to ∼1.3 closed-packed monolayers in which ZnPcPA is adsorbed in edge-on orientation with a projected area of ∼85 Å2/molecule on a flat surface. Detailed calculations are shown in the SI. Figure 1B shows the background-subtracted cyclic voltammogram (CV) of the first oxidation of an adsorbed ZnPcPA monolayer on ITO (scan rate = 100 mV/s). The voltammetric response is resolved into two peaks, indicative of two electrochemically distinct subpopulations of molecules, with midpoint potentials (E0′) at 0.23 ± 0.01 and 0.42 ± 0.01 V versus Ag/Ag+ (n = 4 trials). These are assigned to aggregated and monomeric forms of ZnPcPA, respectively (see ref 22 and references therein for the basis of these assignments). Cofacial Pc aggregates are more easily oxidized than monomeric ZnPcPA due to “through-space” orbital overlap of the π systems,27,28 which stabilizes the product of the one-electron oxidation.27 Integrating over the entire voltammetric envelope, the electroactive surface coverage was calculated to be 2.2 ± 0.1 × 10−10 mol/cm2 (n = 4 trials) assuming one-electron oxidation for both monomeric and aggregated ZnPcPA. This value corresponds to ∼1.1 closed-packed monolayers of molecules adsorbed in edge-on orientation, in good agreement with the surface coverage determined using spectroscopy. The apparent heterogeneous electron-transfer rate constants (ks,app) for monomeric and aggregated ZnPcPA, determined using the

Figure 2. (A) Transmission spectrum of 100 μM ZnPcPA dissolved in acetonitrile/pyridine (7/3, v/v) (dashed line) and TE polarized ATR spectrum of a ZnPcPA film on ITO (solid line) in contact with the same solvent. Potential-dependent ATR spectra of a ZnPcPA film on ITO in (B) TE and (C) TM polarizations from VOC to 0.6 V versus Ag/Ag+. The electrolyte was 0.1 M TBAP in acetonitrile. (D) Plot of the absorbance of monomeric (■/□, at 680 nm) and aggregated (▲/△, at 630 nm) subpopulations of a ZnPcPA film on ITO in TE and TM polarizations, respectively, as a function of applied potential. Absorbance values are normalized to the value at VOC.

ZnPcPA film on ITO, measured in transmission and ATR geometries, respectively. The spectrum of dissolved ZnPcPA shows a sharp Q-band, indicative of monomeric Pc. For ZnPcPA films adsorbed on ITO, the Q-band absorbance is considerably broadened and accompanied by a blue-shifted band centered near 630 nm that is consistent with aggregate formation.30−34 The maximum of the monomeric species is red-shifted by ∼6 nm relative to the dissolved form (also see Figure S3 in the SI), consistent with studies of adsorption of sensitizing dyes on TiO2,32,35−39 which is known to alter their frontier orbital energies.40,41 Figures 2B and C show ATR spectra of adsorbed ZnPcPA films acquired in transverse electric (TE) and transverse magnetic (TM) polarizations, respectively, as a function of the potential applied to the ITO-coated waveguide. As the potential is stepped from ∼−0.13 to 0.6 V, a decrease in both monomer and aggregate absorption bands is observed. A plot of absorbance (normalized to that at VOC) versus applied potential (Figure 2D) shows that the aggregate species (630 nm) is oxidized at a less positive potential relative to the monomeric 1155

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species (680 nm), consistent with the spectral and electrochemical assignments in Figures 2A and 1B, respectively. Midpoint potentials for oxidative bleaching were ∼0.42 and 0.26 V for monomeric and aggregated species, respectively, consistent with the electrochemical data. The molecular orientations of the monomeric and aggregated forms in ZnPcPA films were assessed from measurements of the dichroic ratios at 680 and 630 nm using polarized ATR spectroscopy.42−44 The measurements and calculations are described in the SI. Mean tilt angles between the molecular plane normal and the normal to the ITO surface plane are 33 ± 1 and 57.8 ± 0.7° (n = 3 trials) for monomeric and aggregated ZnPcPA, respectively, showing that the orientation distributions of these two forms are different. The mean tilt angle of the entire ZnPcPA film (monomer plus aggregate) was also determined using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy.45 Measurements of the CC π* spectral intensity versus incident angle of polarized X-rays in Auger electron yield (AEY) and total electron yield (TEY) channels yielded tilt angles of 51.5 ± 0.5 and 53 ± 1° (n = 3 trials), respectively (see the SI). Although the ATR and NEXAFS results do not provide a measure of the width of the orientation distributions for the monomer, the aggregate, or the entire film, taken together, they clearly show that there is a broad distribution of tilt angles in ZnPcPA films. These results also point to the importance of measuring the orientation of a linker-chromophore on a semiconductor surface, as opposed to predicting orientation based on molecular structure and using that prediction as a basis for interpreting measurements of charge-transfer kinetics. PM-ATR was used to estimate ks,app for both the monomeric and aggregated forms of adsorbed ZnPcPA and for differently oriented subpopulations of these forms. PM-ATR is a form of electroreflectance spectroscopy46 that allows charge-transfer rates to be measured using TE or TM polarized light to probe molecules that are oriented predominately in-plane or out-ofplane, respectively. In a previous study, this approach was used to correlate heme group orientation with electron-transfer rates in an electroactive protein monolayer.18 Detailed descriptions of the theory for PM-ATR and its use in measuring ks,app for a variety of redox-active thin films is given elsewhere.18−20 Briefly, a sinusoidally modulated potential is applied to the waveguide electrode centered around a DC bias (Edc) near the E0′ for the redox couple, and the real (Re(Rac), in-phase) and imaginary (Im(Rac), out-of-phase) portions of electroreflectance responses are measured as a function of modulation frequency. Here, the Edc values were 0.375 and 0.25 V for the monomeric and aggregated forms of ZnPcPA, respectively, for which measurements were made at 680 and 630 nm, respectively, in both TE and TM polarizations (see the SI for details of other experimental variables). Complex plane plots were subjected to a polynomial fit to determine the optical switching frequency (ω) at which Re(Rac) = 0 (shown in Figure 3 for the aggregated forms and in the SI for the monomer). Rate constants were calculated from ks,app = 0.5ω2RsCdl, where Rs is the solution resistance and Cdl is the double-layer capacitance.18−20 Rs = 54 ± 6 Ω·cm2 and Cdl = 7 ± 1 μF/cm2 were determined by electrochemical impedance spectroscopy on independently prepared ZnPcPA films (n = 5 trials; see the SI). The four ks,app values determined for ZnPcPA films on ITO are listed in Table 1. All ks,app values determined by PM-ATR are significantly larger than those estimated from voltammetry, consistent with

Figure 3. Complex plane plots for the imaginary versus real components of electroreflectance for the aggregated forms in ZnPcPA films (λmax = 630 nm) measured in (A) TE polarization and (B) TM polarization. The frequency where the real component goes to zero is used to estimate ks,app.

Table 1. Electron-Transfer Rate Constants (n = 5 trials) for Subpopulations of ZnPcPA Tethered to ITO Determined by PM-ATR polarization

monomer, ks,app (sec−1)

aggregate, ks,app (sec−1)

TE TM

2.0 (±0.6) × 10 1.7 (±0.5) × 102

2.1 (±0.5) × 103 7 (±2) × 102

2

previous studies.18,47−49 This disparity is likely attributable to differences in the fraction of electroactive molecules probed by the two methods. Adsorption of ZnPcPA on a chemically and structurally heterogeneous surface such as ITO5,23 generates a monolayer that is structurally heterogeneous, which should be reflected in a distribution of E0′ values and apparent rate constants for electron transfer.50,51 In the voltammetric measurement, the entire ensemble of electroactive molecules is oxidized/reduced over a large potential range, whereas in the PM-ATR measurement, the potential is modulated in a narrow range around E0′, and only the molecules with redox potentials in this range are probed. This subpopulation should equilibrate more rapidly with the modulated electrode potential relative to the remainder of the film, and thus, the ks,app measured by PMATR is expected to be greater than that measured by CV. Focusing on the PM-ATR results, the ks,app values for the aggregated ZnPcPA are 3−10 times greater than that measured for the monomeric form, which is consistent with previous studies indicating that a Pc dimer has a smaller reorganization energy for electron transfer relative to the monomer.52,53 The ks,app values for the monomer exhibit no apparent dependence on molecular orientation, whereas for aggregated ZnPcPA, the rate constant obtained in TE polarization is 3-fold greater than the value obtained in TM polarization. This difference can be explained by considering the influence of a narrow versus a broad tilt angle distribution. In a redox-active film composed of molecules with D4h symmetry and a uniaxial orientation distribution in the electrode surface plane, molecules at all tilt angles (from 0 to 90°) contribute to the rate constants measured by PM-ATR in TE and TM polarizations, but their contributions are weighted by the extent to which their absorption transition dipoles project onto the electric fields of TE and TM polarized light.18 For a film with a broad distribution of tilt angles, the TE and TM measurements will probe molecular subpopulations that are overlapping but differentially weighted; molecules with large tilt angles contribute more to the TM ks,app value, whereas those with small tilt angles contribute more to the TE ks,app value. However, if the tilt angle distribution of the molecules is narrow, then the TE and TM measurements will not be differentially weighted, and the rate constants measured in both 1156

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polarizations will be equal. This hypothesis provides the most probable explanation for the observation that the ks,app values for the monomer are not dependent on the polarization of the probe beam. Due to the chemical heterogeneity and polycrystalline surface morphology of ITO,5,23 it is likely that cofacially aligned ZnPcPA aggregates will form in both upright and in-plane orientations, generating an orientation distribution broader than that of the monomer. The relative peak widths in the CV (Figure 1B) support this interpretation; the aggregate peak width is broader than that of the monomeric species, consistent with a broader distribution of electrochemical (and structural) environments. A broad orientation distribution for aggregated ZnPcPA will correspond to a broader distribution of Pc− electrode separation distances. The macrocycles in a Pc dimer with an in-plane orientation should be closer to the electrode surface than that in a dimer with an upright orientation, particularly when the Pc bears large solubilizing side chains as in ZnPcPA. The tunneling distance should therefore be shorter for the in-plane orientation, leading to more rapid electron transfer when measured in TE polarization, as observed here. In summary, we have demonstrated the capability to correlate molecular orientation and electron-transfer rates for subpopulations of molecules in a structurally heterogeneous Pc monolayer that is a model for the donor/TCO interface in an OPV. The experimental approach and results are also relevant to ongoing efforts by many groups to understand and optimize interfacial charge-transfer processes in DSSCs (see, e.g., ref 21). Interestingly, the highest ks,app measured, 2100 s−1 for the inplane Pc aggregate, exceeds the required value for an efficient hole-harvesting interlayer in an OPV.5 A photocurrent density of 15 mA·cm−2 corresponds roughly to 9.4 electrons s−1 Å−2, which corresponds to a charge-transfer rate of ∼800 s−1 per molecule for molecules having a projected area of 85 Å2 tethered to an electrode.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Procedures and data for film preparation, ZnPcPA adsorption kinetics, surface coverage calculation, determination of mean tilt angles by ATR and NEXAFS spectroscopies, and PM-ATR experiments. This material is available free of charge via the Internet at http://pubs.acs.org.

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Letter

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported as part of the Center for Interface Science: Solar-Electric Materials (CIS:SEM), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award No. DE-SC0001084. NEXAFS measurements were acquired at Stanford Synchrotron Radiation Laboratory, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences. 1157

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