Influence of Molecular Orientation on Charge-Transfer Processes at

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Influence of Molecular Orientation on Charge-Transfer Processes at Phthalocyanine/Metal Oxide Interfaces and Relationship to Organic Photovoltaic Performance Hsiao-Chu Lin, Gordon A MacDonald, Yanrong Shi, Nathan W Polaske, Dominic V. McGrath, Seth R. Marder, Neal R. Armstrong, Erin L Ratcliff, and S. Scott Saavedra J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 27 Apr 2015 Downloaded from http://pubs.acs.org on April 27, 2015

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The Journal of Physical Chemistry

Influence of Molecular Orientation on Charge-Transfer Processes at Phthalocyanine/Metal-Oxide Interfaces and Relationship to Organic Photovoltaic Performance Hsiao-Chu Lin,† Gordon A. MacDonald,† Yanrong Shi,‡ Nathan W. Polaske,† Dominic V. McGrath,† Seth R. Marder,‡ Neal R. Armstrong,† Erin L. Ratcliff,*,§ S. Scott Saavedra*,† †

Department of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona 85721

‡

School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia

Institute of Technology, Atlanta, Georgia 30332 §

Department of Materials Science and Engineering, University of Arizona, Tucson, Arizona 85721

* Email: [email protected]; [email protected]

RECEIVED DATE

ACS Paragon Plus Environment

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ABSTRACT The effect of the molecular orientation distribution of the first monolayer of donor molecules at the hole-harvesting contact in an organic photovoltaic (OPV) on device efficiency was investigated. Two zinc phthalocyanine (ZnPc) phosphonic acids (PA) deposited on indium tin oxide (ITO) electrodes are compared: ZnPc(PA)4 contains PA linkers in all four quadrants and ZnPcPA contains a PA linker in one quadrant.

ZnPcPA monolayers exhibited a broad distribution of molecular orientations whereas

ZnPc(PA)4 adsorption produced a monolayer with a narrower orientation distribution with the molecular plane more parallel to the ITO surface. We used potential-modulated attenuated total reflectance spectroelectrochemistry (PM-ATR) to characterize the charge-transfer kinetics of these films, and show that the highest CT rate constants correspond to ZnPc subpopulations that are oriented more parallel to the ITO surface plane. For ZnPc(PA)4, rate constants exceeded 104 sec-1 and are among the highest ever reported for a surface-confined redox couple, which is attributable to both its orientation and the small ZnPc-electrode separation distance. The performance of OPVs with ITO hole-harvesting contacts modified with ZnPc(PA)4 was comparable to that achieved with highly activated bare ITO contacts, whereas for ZnPcPA-modified contacts, the OPV performance was similar to that observed with (holeblocking) alkyl-PA modifiers. These results demonstrate the synergism between of molecular structure, energetics, and dynamics at interfaces in OPVs.

INTRODUCTION Derivatization of metal-oxide electrodes with organic modifiers has been used widely to tune electrode properties for use in energy conversion and organic logic applications, including control of chemical composition, surface free energy, work function and the stability of the oxide surface. For example, various small-molecule dipolar modifiers, especially phosphonic acids (PAs), have been used to control the surface free energy of metal-oxide contacts and tune the work function over a range of 1.5 eV.1-11 Numerous redox-active surface modifiers have been developed for photoelectrochemical cells,


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such as those by Meyer and coworkers,12-20 Galoppini and coworkers,21-32 and Imahori, Sundström and coworkers,33,34 and have been used as sensitizers on TiO2, ZnO, SnO2 and hybrid metal-oxide surfaces. These studies show that the orientation of the redox-active moiety and the distance between it and the oxide surface play a critical role in dye sensitized solar cells (DSSCs) for: i) the efficiency of electron injection from the dye excited state into the conduction band of the oxide; ii) the rate of hole-hopping between adjacent chromophores; and iii) recombination efficiency, which can limit the photopotential.1,22,24-26,28 The orientation and distance effects elucidated for redox-active sensitizers in DSSCs are pertinent for numerous other thin-film energy conversion systems. For example, organic modifiers have been used to alter the contact work functions in organic photovoltaic devices (OPVs).3,8,10,11,35-37 Control of the contact work function can ensure proper energetic alignment at the electrode/organic interface and add a driving force that counters charge recombination through built-in voltage.11 In addition to simple work-function modification, preferred carrier extraction in an OPV might be enhanced via frontier orbital engineering, such as treating the metal-oxide contact with an organic surface modifier with a redox potential close to the transport energy levels relevant for charge (hole or electron) harvesting from the organic components of the OPV active layer. In a recent paper,38 we began to explore this concept using a zinc phthalocyanine (ZnPc) containing a alkyl phosphonic acid (PA) linker in one quadrant (Fig. 1A), which was used to tether ZnPcPA to indium tin oxide (ITO). ZnPcPA monolayers contained both monomeric and aggregated forms, contributing to a broad distribution of molecular orientations. We demonstrated a relationship between molecular orientation and heterogeneous charge-transfer rates using a suite of spectroscopic, electrochemical, and spectroelectrochemical methods. The fastest rate constants were observed for the subpopulation of ZnPcPA aggregates that was oriented largely parallel to the electrode surface plane. Based on the these results, we hypothesized that the use of at least three linkers would provide better control of the immobilization process, producing a monolayer with minimal aggregation and a narrower


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molecular orientation distribution that is parallel to the electrode surface plane. In the current study, we describe preparation and characterization of ITO electrodes modified with a ZnPc containing PA linkers in all four quadrants (ZnPc(PA)4, Fig. 1B). Here we compare the structural and functional properties of ZnPc(PA)4 and ZnPcPA monolayers, focusing on the relationships between molecular orientation and heterogeneous CT kinetics. We also compared the efficiency of OPVs fabricated on ITO modified with ZnPcPA and ZnPc(PA)4. The results show that a ZnPc(PA)4 monolayer is largely monomeric and planar to the ITO electrode, and exhibits both faster charge transfer and improved OPV performance in comparison to ZnPcPA-modified ITO.

Demonstrating a direct correlation between molecular

orientation, charge-transfer kinetics, and OPV performance is unprecedented.

(A)

(B)

Figure 1. Chemical structures of (A) ZnPcPA and (B) ZnPc(PA)4.

EXPERIMENTAL METHODS ZnPc(PA)4/ITO and ZnPcPA/ITO Sample Preparation.

ZnPcPA was synthesized as described

previously,39 and ZnPcPA films adsorbed to ITO (denoted ZnPcPA/ITO) were prepared as described previously.38 Procedures for ZnPc(PA)4 synthesis and purification are given in Supporting Information (SI). Indium tin oxide (ITO) on Eagle XG glass with a thickness of ca. 145 nm and a sheet resistance of ca. 20 Ω/

was purchased from Thin Film Devices Inc. ITO electrodes were cleaned by first scrubbing

with detergent (1% Triton X-100) for ca. 1 minute followed by successive sonication in diluted


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detergent, water (Barnstead Nanopure, 18.3 MΩ·cm) and ethanol for 15 minutes each. Cleaned ITO slides were stored in ethanol and dried with a stream of nitrogen prior to use. Immediately before use, ITO electrodes were activated by air plasma cleaning (Harrick PDC-3XG, Harrick Scientific) for 15 minutes at medium RF level (10.5 W). For characterization using electrochemistry, potential-modulated attenuated total reflectance (PMATR) spectroelectrochemistry, and ultraviolet photoelectron spectroscopy (UPS), adsorbed ZnPc(PA)4 films on ITO (denoted ZnPc(PA)4/ITO) were prepared by incubating plasma-treated ITO electrodes in a 10 μM ZnPc(PA)4 solution in dimethyl sulfoxide (DMSO) for 30 minutes. A water bath was used to heat the ZnPc(PA)4 solution to ca. 100 °C. Electrodes were allowed to cool to room temperature for 30 minutes prior to use.

ZnPc(PA)4/ITO samples for steady-state ATR spectroscopy and

spectroelectrochemistry were prepared by clamping two 1 in. × 3 in. ITO electrodes back-to-back, with the ITO sides facing out (to minimize adsorption on the glass sides of the substrates), and incubating them horizontally in a 10 μM ZnPc(PA)4 solution in DMSO with only the bottom halves of the electrodes immersed in the solution. A water bath was used to heat the ZnPc(PA)4 solution to ca. 100 °C. Upon removal, substrates were rinsed with copious amounts of DMSO followed by acetonitrile to remove weakly adsorbed molecules and then dried under a stream of nitrogen. The top (uncoated) half of each electrode was used as the blank for ATR measurements. Electrochemical Characterization. Cyclic voltammetry (CV) of ZnPc(PA)4/ITO was performed with a standard three-electrode configuration (ITO electrode area = 0.071 cm2) in acetonitrile containing 0.1 M tetrabutylammonium perchlorate (TBAP) using a CH420A potentiostat (CH Instruments) with an Ag/Ag+ (0.01 M AgNO3, 0.1 M TBAP in acetonitrile) non-aqueous reference electrode (BASi). CV of ZnPcPA/ITO was described previously.38 CV of dissolved ZnPcPA and ZnPc(PA)4 was performed in 0.3 M TBAP in dichloromethane/DMSO (1/1, v/v). Spectroscopic Measurements. Transmission UV-Vis spectra of ZnPc(PA)4 and ZnPcPA solutions were acquired with a Model 440 UV-Vis spectrophotometer (S.I. Photonics). Transmission UV-Vis


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spectra of copper phthalocyanine (CuPc; Alfa Aesar) layers deposited on ZnPc(PA)4/ITO and ZnPcPA/ITO were acquired with an Agilent Technologies Model 8453 UV-Vis spectrophotometer (see below for CuPc deposition procedure). The transmission spectra of ZnPcPA/ITO and ZnPc(PA)4/ITO were used as the blanks. Steady-state ATR spectroscopy and spectroelectrochemistry were performed using a custom built instrument described previously.38,40,41 Briefly, the collimated and polarized output of a 10 W halogen lamp (International Light Technologies) was coupled into/out of the ITO-coated glass slide, which functioned as a waveguide, using two BK-7 glass prisms positioned 42.5 mm apart. The internal reflection angle was 74°-‐75°, yielding six reflections at the ITO/solution interface. The outcoupled light was directed into a monochromator (Newport MS260i) and the light was detected with an Andor iDus 420A CCD. ATR spectra of ZnPc(PA)4/ITO and ZnPcPA/ITO were acquired in contact with acetonitrile, while potential-dependent ATR spectra were acquired with the samples in contact with acetonitrile containing 0.1 M TBAP. A flow cell was used to introduce and exchange solutions in contact with the electrode surface. The potential was controlled using a EG&G Princeton Applied Research Model 263A potentiostat/galvanostat with an Ag/Ag+ (0.01 M AgNO3, 0.1 M TBAP in acetonitrile) non-aqueous reference electrode (BASi). The mean tilt angle of ZnPc(PA)4 molecules in ZnPc(PA)4/ITO was determined using polarized ATR spectroscopy, as described previously for ZnPcPA/ITO.38 The method of Mendes et al.42 was used to correct for the unequal interfacial electric field intensities in transverse electric (TE) and transverse magnetic (TM) polarizations. The difference in intensities was normalized by measuring the dichroic ratio (ρfilm = ATE,film / ATM,film) for a dye solution (malachite green dissolved in acetonitrile) that did not exhibit measurable adsorption to the surface of ITO (i.e., when the dye was flushed from the ATR flow cell with solvent, the transmittance returned to 100%). Based on this result, it was assumed that dye was randomly oriented in the evanescent volume adjacent to the ITO waveguide. PM-ATR Measurements. Detailed descriptions of the theory and experimental setup for PM-ATR and its use in measuring the heterogeneous charge transfer rate constant (ks,opt) for a redox-active thin


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film on an ITO waveguide are available.38,43-48 The instrument and conditions for ZnPc(PA)4/ITO spectroscopic measurements described above were also used for PM-ATR measurements, with the following exceptions: The two BK-7 coupling prisms were positioned 20 mm apart, producing two total internal reflections at the ITO/solution interface at an internal reflection angle of 74° -‐ 75°. The ITO electrode active area was 0.8 cm2. The dispersed light was detected with a PMT (Hamamatsu R928; Newport 77360 housing and 70705 power supply). A 20 mVrms (0.028 V peak-to-peak) sinusoidally modulated voltage (Eac) was applied to the ITO electrode around a DC bias (Edc) = 0.21 V which was selected as described in SI. The Eac frequency was controlled with a function generator (DS335; Stanford Research). The real (Re(Rac), in-phase) and imaginary (Im(Rac), out-of-phase) portions of the electroreflectance signals at 690 nm were integrated for 60 seconds at each modulation frequency. Measurements were made over a modulation frequency range of 0.1 - 6300 Hz. Complex plane plots (see examples in ref 38) were subjected to a polynomial fit to determine the modulation frequency (ω) at which Re(Rac) = 0. Rate constants were calculated from ks,app = 0.5ω2RsCdl where Rs is the solution resistance and Cdl is the double layer capacitance.38 Rs and Cdl were measured by electrochemical impedance spectroscopy as described previously38 using the same spectroelectrochemical cell employed in the PM-ATR measurements. OPV Device Fabrication and Current-Voltage Measurements. CuPc, C60 fullerene (Materials and Electrochemical Research Corp.) and bathocuproine (BCP; Sigma Aldrich) were purified by triplesublimation and then deposited on ZnPcPA/ITO and ZnPc(PA)4/ITO by physical vapor deposition from Knudsen cell sources at a pressure of ca. 5 × 10-7 Torr and a rate of ca. 0.5 Å·s-1. Film thickness was monitored by a quartz crystal oscillator. Aluminum cathodes were deposited by physical vapor deposition from a boron nitride crucible (Kurt J. Lesker), resistively heated via a tungsten basket heater (Kurt J. Lesker) at a rate of 0.7-1.5 Å·s-1. The device structures were 20 nm CuPc/40 nm C60/10 nm BCP/100 nm Al. Current-voltage (J-V) measurements were performed in a N2-filled glovebox. A current controlled, 300 W xenon arc lamp (Newport) was used as the light source. Impinging light was


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filtered with an AM 1.5 filter (Melles Griot) to simulate the solar spectrum, diffused to improve uniformity (40 degree diffuser, Newport), and passed through a 0.1 OD neutral density filter. The power density at the device surface was calibrated with a silicon diode (Newport, Model 818-SL with OD3 attenuator) to achieve 100 mW/cm2. Ultraviolet Photoemission Spectroscopy (UPS). UPS measurements were performed with a Kratos Axis Ultra X-ray photoelectron spectrometer with a He(I) excitation source (21.2 eV) with a -10.0 V bias applied to the sample to enhance the yield of low kinetic energy electrons. The Fermi level reference was established on a clean Au surface and used in the analysis to determine both the work function and onset of observed photoemission (ionization energy).

Samples were ZnPcPA/ITO,

ZnPc(PA)4/ITO, and 1, 2, and 4 nm thick CuPc deposited on ZnPcPA/ITO and ZnPc(PA)4/ITO, prepared as described above.

RESULTS AND DISCUSSION The hypothesis underlying this work is that the structural properties of the first monolayer in a donor layer at the donor/TCO interface in an OPV determine the efficiency of charge collection at the donor/TCO interface, which will in turn affect the device performance. In a recent paper, we began exploring this hypothesis using ZnPcPA/ITO,38 a model that is relevant to vacuum deposited, small molecule planar heterojunction (PHJ) and bulk heterojunction (BHJ) OPVs in which Pcs are used as donors.49-56 Here we extend the earlier study to ZnPc(PA)4 which forms a more ordered monolayer on ITO. Characterization of the structural, energetic and charge-transfer properties of ZnPc(PA)4/ITO is described first, along with a comparison to the respective ZnPcPA/ITO properties, followed by an assessment of OPVs prepared on ZnPc(PA)4/ITO and ZnPcPA/ITO. Spectroscopic and electrochemical characterization of ZnPc(PA)4/ITO.

Protocols to prepare

adsorbed ZnPc(PA)4 on ITO were tested and optimized by measuring the dichroic ratio (ρ = ATE/ATM) of ZnPc(PA)4/ITO samples using polarized ATR spectroscopy. Applying the adsorption conditions that


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were used in our previous study to prepare ZnPcPA/ITO films (immersion of ITO in a 100 μM solution for 3 hours)38 produced ZnPc(PA)4/ITO films with low ρ values, implying a predominately out-of-plane molecular orientation. In addition, the ATR spectra contained a strong band at ca. 630 nm which is characteristic of cofacial aggregation. Higher ρ values and minimal cofacial aggregation were achieved by adsorption from dilute solution at 100 °C, as described in Experimental Methods. A comparison of the ATR spectra of ZnPc(PA)4/ITO, prepared using the optimized protocol, and ZnPcPA/ITO is shown in Figure 2. Also shown are the normalized absorbance spectra of dissolved ZnPc(PA)4 and ZnPcPA. Sharp Q bands at ca. 684 nm and associated vibronic band shoulders at 600– 650 nm are present in the dissolved spectra, features that are consistent with non-aggregated ZnPc molecules. The ATR spectra of the adsorbed films show small red shifts of the Q band maximum and broadening relative to the solution spectra, as has been observed for sensitizing dyes adsorbed on TiO2 surfaces.57-62 A significant difference in these spectra is the band at ca. 630 nm, which is characteristic of cofacial aggregation;38 this band is prominent in the ZnPcPA ATR spectrum whereas the much weaker shoulder present in the ZnPc(PA)4 ATR spectrum nm is indicative of much less aggregation. The mean tilt angle between the ZnPc(PA)4 molecular plane normal and the normal to the ITO surface plane in ZnPc(PA)4/ITO was 29° ± 2° (n = 4). This result indicates that a majority of the adsorbed molecules are oriented with the ZnPc molecular plane parallel to the ITO surface plane, making it likely that they are tethered via 3-4 PA groups. This finding is consistent with the ATR spectrum (Fig. 2B) which shows the film is largely composed of monomers. However, since the tilt angle is expected be closer to 0° for a completely in-plane monolayer, a minority of ZnPc(PA)4 molecules is likely bound to ITO via only 1-2 PA groups, and these molecules are oriented more out-ofplane than those bound via 3-4 attachment points (also see below and SI). In addition, the flexibility of the anchoring chains may allow for some degree of out-of-plane tilt even in cases where the molecule is anchored by via 3-4 PA groups.


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(C)

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(D)

Figure 2. Absorbance spectra (red dashed lines) of dissolved (A) ZnPcPA and (B) ZnPc(PA)4, and TE polarized ATR spectra of (A) ZnPcPA/ITO and (B) ZnPc(PA)4/ITO. Spectra were normalized to the absorbance value at λmax to enable comparsion of the absorbance band shape. Solutions: 100 µM ZnPc(PA)4 in DMSO and and 100 µM ZnPcPA in acetonitrile/pyridine (7/3, v/v). ATR spectra were acquired at open circuit with the ZnPc films in contact with acetonitrile. Cyclic voltammograms showing the first oxidation of (C) ZnPcPA/ITO and (D) ZnPc(PA)4/ITO, acquired at a 100 mV·s-1 scan rate in TBAP/acetonitrile electrolyte. The current density j is normalized by the electrode area (0.071 cm2).


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The largely in-plane orientation in ZnPc(PA)4/ITO is in marked contrast to the results reported previously for ZnPcPA/ITO.38 In that case, the film was composed of both monomeric and aggregated subpopulations, with respective mean tilt angles of 33° ± 1° and 57.8° ± 0.7°.38 The orientation distribution in ZnPcPA/ITO is clearly broader than that in ZnPc(PA)4/ITO and can be attributed to greater rotational degrees of freedom associated with attachment via a single PA group vs. multiple PA groups.59,63-66 A distribution of the aggregate sizes also may contribute to the greater disorder in ZnPcPA/ITO. In summary, the spectral and orientation data show that ZnPc(PA)4 films are more inplane and ordered relative to the more disordered, out-of-plane character of ZnPcPA films. Cyclic voltammetry (CV) was used to determine the midpoint potential (E0’), the electroactive surface coverage (Γ) and the apparent charge transfer rate constants (ks,app) for ZnPc(PA)4/ITO. The data are listed in Table 1, along with data for ZnPcPA/ITO to enable comparison. In the voltammogram of ZnPc(PA)4 (Fig. 2D), we observed a single, broad oxidation wave which is assigned to the monomer. In contrast, two peaks were observed for ZnPcPA/ITO (Fig. 2C), corresponding to electrochemically distinct aggregated and monomeric subpopulations.38 ZnPc(PA)4/ITO is oxidized at 0.2 V less positive than the monomer subpopulation of ZnPcPA/ITO, a difference that is consistent with the 0.27 V difference in E0’values for dissolved ZnPc(PA)4 and ZnPcPA (see SI). Using Laviron’s formalism to determine the rate constant from anodic-cathodic peak separation,67 we estimate ks,app = 3.1 ± 0.6 s-1 for ZnPc(PA)4/ITO (assuming a single population of adsorbed molecules), which is slightly higher than the values estimated for monomeric (1.7 ± 0.2 s-1) and aggregated ( 2.4 ± 0.1 s-1) molecules in ZnPcPA/ITO.38 The differences are qualitatively consistent with the ks,opt values determined using PMATR spectroscopy (vide infra).


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Table 1. The midpoint potential (E0’), electroactive surface coverage (Γ), and apparent CT rate constant (ks,app) determined by cyclic voltammetry for ZnPc(PA)4/ITO (n = 5 trials) and ZnPcPA/ITO (n = 4 trials). ZnPcPA/ITO a 0

ZnPc(PA)4/ITO

Monomeric

Aggregated

Monomeric

0.42 ± 0.01

0.23 ± 0.01

0.22 ± 0.01

+

E ’ads (V vs Ag/Ag ) -2 b

Γ (mol·cm ) ks,app (s-1) c a

2.2 ± 0.1 × 10-10 1.7 ± 0.2

2.4 ± 0.1

6.1 ± 0.2 × 10-11 3.1 ± 0.6

Data from reference 38.

b

The electroactive surface coverages were calculated from the mean of the integrated charges of the anodic and cathodic processes. For ZnPcPA/ITO, the entire voltammetric envelope was integrated assuming one-electron oxidation processes for both monomers and aggregates. c

The ks,app values were determined using Laviron’s formalism67 based on the anodic to cathodic peak separation at a scan rate of 0.1 V·s-1.

The Γ values were estimated by integrating the voltammetric responses and assuming one-electron oxidation for both monomers and aggregates. The Γ for ZnPcPA4/ITO is about 30% of the Γ for ZnPcPA/ITO which is consistent with molecular orientation results. The projected surface area of a flat-lying ZnPcPA4 molecule with its alkoxy side chains fully extended in an all-trans conformation is estimated to be 405 Å2, assuming the side chains are intercalated. At Γ = 6.1 ± 0.2 × 10-11 mol·cm-2, 405 Å2/molecule corresponds to ca. 1.5 closed-packed monolayers. The actual monolayer coverage is probably less because, as discussed above, it is likely that a minority of ZnPcPA4 molecules is oriented more upright relative to the predominantly planar orientation. For comparison, Γ for ZnPcPA/ITO corresponds to ca. 1.1 closed-packed monolayers assuming that the molecules are adsorbed in upright orientation with a projected surface area of 85 Å2/molecule.38 Spectroelectrochemistry and heterogeneous charge transfer kinetics. ATR spectroelectrochemistry was used to characterize the potential-dependent bleaching of ZnPcPA4/ITO absorbance. The spectra


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plotted in Figure 3 show a progressive decrease in the 690 nm band as the potential was stepped from 0.1 to 0.5 V versus Ag/Ag+. The midpoint potential for oxidative bleaching was 0.21 V in both TE and TM polarizations, consistent with E0’ value determined for ZnPcPA4/ITO by cyclic voltammetry. Relative to the TE polarized spectra, the TM polarized spectra show a significant enhancement of the 630 nm band which, as discussed above, is characteristic of cofacial aggregation.38 The presence of a subpopulation of vertically oriented aggregates is consistent with the ‘out-of-plane’ hypothesis for a minor fraction of the ZnPcPA4 film.

Figure 3. Potential-dependent ATR spectra of ZnPcPA4/ITO measured in TE and TM polarizations. The electrolyte was 0.1 M TBAP in acetonitrile. The applied potential was stepped progressively from -0.1 to 0.5 V vs. Ag/Ag+ and held while the spectrum was acquired. We used PM-ATR to measure optically detected charge-transfer rate constants, ks,opt, for ZnPcPA4/ITO. In this method, changes in the electroreflectance (ER) of a redox-active film of chromophores on an ITO-coated waveguide are produced by modulating the applied potential around a DC bias (Edc) near the E0’ for the redox couple.38,43-48 The real and imaginary components of the ER are measured as a function of modulation frequency, from which an estimate of ks,opt for the film is obtained. PM-ATR is better suited than electrochemistry for measuring charge-transfer events on short (µs) time scales due to the lack of interference from non-Faradaic processes.


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subpopulations of a redox-active film of chromophores can be probed by selection of Edc and the wavelength and polarization of light propagating in the waveguide. Here, TE and TM polarized light were used to probe absorbance dipole transitions that are predominately in-plane and out-of-plane, respectively. Apparent ks,opt values for ZnPcPA4/ITO were measured at 690 nm (λmax for the monomer) at an Edc of +0.21 V, which was the peak potential of the optically-detected ER voltammogram (see SI), using a potential modulation of ±20 mVrms. (Note: The absorbance of the aggregate shoulder at ca. 630 nm in the ZnPcPA4/ITO spectrum was too weak to permit PM-ATR measurements to be performed). Table 2 shows a comparison of ks, opt values measured in TE and TM polarizations for ZnPcPA4/ITO, as well as data for the monomer and aggregate subpopulations in ZnPcPA/ITO that were published previously.38 The ks,opt values for the ZnPc(PA)4 monomer measured in TE and TM polarizations were statistically equivalent, indicating no apparent dependence of charge transfer kinetics on molecular orientation. This equivalence, along with the data presented above, is consistent with a narrow distribution of ZnPc(PA)4 molecular tilt angles.

A narrow distribution implies that the difference in macrocycle-electrode

tunneling distances between the subpopulations of molecules probed with TE and TM polarized light is minimal; thus the rate constants measured in both polarizations are indistinguishable.

A similar

observation and interpretation was made for the monomeric subpopulation in ZnPcPA/ITO (Table 2).38 In contrast, we have observed orientation-dependent ks,opt values for the aggregate subpopulation in ZnPcPA/ITO:38 a) ZnPcPA aggregates with a predominately in-plane orientation exhibited higher ks,opt values vs. out-of-plane ZnPcPA aggregates, which was attributed to a difference in tunneling distance; and b) the ks,opt values for in-plane and out-of-plane aggregates were up to 12-fold greater than those for the monomers, which is consistent with a lower reorganization energy for the aggregate.


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Table 2. Apparent CT rate constants, ks,opt (s-1), of ZnPc(PA)4/ITO and ZnPcPA/ITO measured by PM-ATR.a ZnPc(PA)4/ITO

ZnPcPA/ITO

Polarization

Monomer (690 nm) b

Monomer (690 nm) c

Aggregate (630 nm) c

TE

1.9 ± 0.4 × 104

2.0 ± 0.6 × 102

2.1 ± 0.5 × 103

TM

1.8 ± 0.3 × 104

1.7 ± 0.5 × 102

7 ± 2 × 102

ks,opt values determined by PM-ATR are significantly greater than the ks,app values estimated from voltammetry (Table 1), which is consistent with observations and discussion in previous studies.38,41,68 a

b

ks,opt values obtained from n = 8 trials. ks,opt = 0.5ω2RsCdl, where ω is the optical switching frequency at which the real component of the ER signal = 0, Rs is the solution resistance (= 13.8 ± 0.9 Ω·cm2) and Cdl is the double-layer capacitance (= 6.6 ± 0.9 µm·cm-2). Rs and Cd were determined by electrochemical impedance spectroscopy on independently prepared ZnPc(PA)4/ITO samples (n = 9 trials). c

Values taken from reference 38. Different wavelengths and Edc values were selected were to target monomer and aggregate subpopulations. The ks,opt values for ZnPc(PA)4 are an order of magnitude greater than the highest values measured for ZnPcPA (Table 2) and are among the highest ever reported for a surface-confined redox couple. The magnitude of these values is attributable to the predominately planar orientation and concomitant close apposition to the electrode surface. Although the side chains on ZnPc(PA)4 and ZnPcPA are the same length (10 carbons), adsorption of ZnPc(PA)4 to ITO via 3-4 PA groups should produce adsorption geometries with shorter tunneling distances between the macrocycle and the electrode surface, resulting in rapid charge transfer.

A variety of conformations is possible for ZnPc(PA)4 adsorption in a

predominately planar geometry. For instance, introduction of one or more gauche defects at various positions in each side chain will produce conformations that allow more than two PA groups to form tridentate bonds to ITO.2,9 An example is shown in SI (Fig. S3); when one gauche defect occurs at the C9-C10 linkage of each side chain, the distance between the Pc chromophore and the electrode surface


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is estimated to be 2.8 Å. UPS studies of ZnPc/ITO and CuPc deposited on ZnPc/ITO.

Ultraviolet photoemission

spectroscopy (UPS) was used to evaluate the energetic alignment in CuPc/ZnPcPA4/ITO and CuPc/ZnPcPA/ITO multilayers. Figure 4A shows the UPS spectra for 0, 1, 2, and 4 nm of CuPc vacuum deposited on ZnPcPA/ITO (top to bottom, respectively). The corresponding spectra of 0, 1, 2, and 4 nm of CuPc on ZnPcPA4/ITO are in Figure 4B. The work functions (Φ) and ionization energies (IE) obtained from analysis of these spectra are listed in Table 3. The work function is defined as the energetic difference between the Fermi level (EFermi) and the surface vacuum level (EVAC), derived from fitting the onset of the secondary edge on the left side of the UPS spectra in Figures 4A and 4B. The IE is the energetic difference between the onset density of occupied states and EVAC. For unmodified (bare) ITO, Φ is 5.0 eV (spectrum not shown). Modification of ITO electrodes with ZnPcPA and ZnPcPA4 decreases Φ to 4.2 and 4.37 eV, respectively. Differences in Φ are expected due to a combination of differences in molecular orientation and Pc surface coverage. In the ZnPcPA/ITO spectrum, clear molecular orbital features can be observed (spectrum i, right panel, in Fig. 4A), with an onset of occupied states 0.2 eV below the Fermi level and an IE of 4.4 eV. However, molecular orbitallike features in the ZnPcPA4/ITO spectrum (i in Fig. 4B) could not be readily detected, which is likely due to the lower surface coverage relative to ZnPcPA/ITO. In lieu of a UPS-derived value, the IE listed in Table 3 for ZnPcPA4/ITO, 5.0 eV, was estimated from the voltammogram (Fig. 2D).

For

comparison, electrochemically-derived IE values for the monomeric and aggregated subpopulations in ZnPcPA/ITO (estimated from Fig. 2C) are also listed in Table 3. It is important to note that a voltammetric estimate of IE does not account for the formation of an interfacial dipole upon adsorption of a ZnPc film on ITO, and thus it may systematically differ from a UPS-derived IE; this is the probable source of the ca. 0.8 eV difference in the respective IE values for ZnPcPA/ITO. Returning to the energy alignment issue, the electrochemically-derived IE values for ZnPcPA4/ITO and ZnPcPA/ITO


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(C)

(D) ITO

ZnPcPA

CuPc

ITO

EVAC

ZnPc(PA)4

CuPc

EVAC EVAC

EVAC

EFERMI EFERMI

Figure 4. UPS spectra of (i) 0 nm; (ii) 1 nm; (iii) 2 nm; and (iv) 4 nm of CuPc on (A) ZnPcPA/ITO and (B) ZnPcPA4/ITO electrodes. Energy band diagrams for the (C) CuPc/ZnPcPA/ITO and (D) CuPc/ZnPcPA4/ITO interfaces. The green ellipsoids represent the ZnPc monolayer films. The gray shaded bands represent energy levels in the degenerate ITO electrode, including mid-gap states. The blue shaded bands represent distributions in the highest occupied and lowest unoccupied molecular orbitals of the CuPc film.


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Table 3. Work functions (φ) and ionization energies (IE) for ZnPcPA/ITO and ZnPcPA4/ITO before and after CuPc deposition.a Sample

φ (eV)

IE (eV) b

IE (eV) d

ZnPcPA/ITO

4.2 ± 0.1

4.4 ± 0.1

5.1, 5.3 e

1 nm CuPc/ZnPcPA/ITO

4.08 ± 0.03

4.66 ± 0.06


4.05 ± 0.04

4.65 ± 0.05


3.98 ± 0.04

4.65 ± 0.06

ZnPcPA4/ITO

4.37 ± 0.06

--- c

1 nm CuPc/ZnPcPA4/ITO

4.23 ± 0.04

4.69 ± 0.05


4.17 ± 0.02

4.67 ± 0.02


4.04 ± 0.04

4.69 ± 0.04

a

Standard deviations were calculated from n ≥ 4.

b

Determined from the UPS measurements.

5.0

c

An onset density of states is clearly observed at 4.37 eV, however, it is not assignable to ZnPcPA4 due to the absence of well defined molecular orbital features in the spectrum. d

Estimated from voltammetry data (Figs. 2C and 2D), using values of 4.48 eV vs vacuum for the potential of the normal hydrogen electrode (NHE)69 and 0.64 V vs NHE for the ferrocene/ferricenium (Fc/Fc+) redox couple70 that was used to calibrate the Ag/AgNO3 reference electrode. The potential of Fc/Fc+ vs Ag/AgNO3 was 0.09 V. IE was therefore estimated from IE = (Eox + 5.03) eV, where Eox is the onset potential for oxidation of the ZnPc film vs Ag/AgNO3.


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(aggregate) differ by only 0.1 eV, a difference that is not a large enough to account for the higher ks,opt values measured for ZnPc(PA)4. This comparison and the significant overlap in the voltammetric behavior (Figs. 2C-D) indicate that ZnPcPA/ITO and ZnPc(PA)4/ITO are very similar energetically. Likewise, it is important to note that ITO is a high carrier density, degenerately doped oxide with a significant density of mid-gap states. Thus, relative to the organic layer, ITO can be considered to be metallic-like, although deviations from the Schottky-Mott limit can be expected.11,71-73 Thus, the small difference in the IE values measured for ZnPcPA4/ITO and ZnPcPA/ITO is not expected to alter the driving force for hole collection from the donor layer into the ITO electrode. However, differences in energetic alignment between the CuPc donor and the ZnPc monolayers could create a barrier, and hence, the energetic alignment was assessed experimentally. Deposition of 1 nm of CuPc causes a decrease in Φ for both ZnPc-modified substrates. For ZnPcPA/ITO, the largest dipole shift occurs within the first nm, where the change in Φ is 0.12 eV and the IE occurs at -4.66 eV, 0.6 eV below the Fermi level. Subsequent increases in the CuPc layer thickness produce a 0.1 eV change in Φ while the IE is unchanged. For deposition of 1 nm of CuPc on ZnPcPA4/ITO, similar changes are observed: Φ shifts by 0.14 eV and the IE moves to -4.69 eV. The Φ shifts further, by 0.19 eV, as the CuPc layer thickness increases, which suggests a small amount of band bending at the interface, while the IE is unchanged. After deposition of 4 nm of CuPc, both interfaces show statistically equivalent work function and ionization energies, consistent with those of CuPc.74 The differences in Φ for ZnPcPA/ITO and ZnPcPA4/ITO coated with 1 and 2 nm thick CuPc layers suggest the possibility that the different molecular orientations of the ZnPc monolayers produce differences in the structure of the overlying CuPc layers. We examined this “templating” effect by measuring absorbance spectra as a function of CuPc layer thickness. The results, presented in SI, indicate that the structure of the first few monolayers of CuPc is different on ZnPcPA/ITO and ZnPcPA4/ITO. However, based on the similar absorbance spectra, structural differences in the CuPc layer are minimal when it is ≥ 4 nm thick which is consistent with the statistically equivalent Φ and IE


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for 4 nm CuPc/ZnPcPA/ITO and 4 nm CuPc/ZnPcPA4/ITO listed in Table 3. Energy level diagrams for the CuPc/ZnPcPA/ITO and CuPc/ZnPcPA4/ITO electrodes, derived the data in Table 3, are presented in Figures 4C and 4D, respectively. Both interfaces show energetic alignment that favors hole extraction from CuPc into the ZnPc monolayer and into the underlying ITO. However, it is important to note that the distance of the ZnPcPA core from the ITO is expected to affect the charge transfer rate across this interface, which is not described in the thermodynamic depiction in Figure 4C. The saturated C10 tether of ZnPcPA may create a kinetic tunneling barrier for hole extraction, which is consistent with the charge-transfer kinetics described above and the OPV device data described below. In contrast, the largely in-plane orientation of the ZnPcPA4 creates a short tunneling distance relative to ZnPcPA/ITO, as described above, and this should lower the kinetic barrier for hole extraction which also is consistent with the OPV data presented below. OPV Performance. We investigated the impact on device performance when ZnPc(PA)4/ITO and ZnPcPA/ITO were used as hole-harvesting electrode contacts in OPVs. The dark and photoresponse characteristics of CuPc/C60 planar heterojunction OPVs (a platform for which there is substantial prior understanding)36,37,75,76 prepared on both ZnPcPA- and ZnPc(PA)4-modified ITO contacts were compared (Figure 5). Device performance parameters are listed in Table 4. In the dark, both device types are rectifying, but those constructed on ZnPc(PA)4/ITO have a higher rectification ratio (comparing currents at +/- 0.5 V) and a significantly lower series resistance, RS, measured at far forward bias. Under illumination, devices with ZnPcPA/ITO contacts are significantly recombination limited near the open-circuit voltage (VOC), although at higher reverse bias, comparable short-circuit currents (JSC) are observed, indicating efficient extraction under high reverse bias. In comparison, devices on ZnPc(PA)4/ITO have slightly higher VOC and much higher fill factor (FF). These devices have an overall power conversion efficiency (η) of ca. 1.5%, which is comparable to that achieved for equivalent OPV devices built on ITO contacts pretreated by conventional protocols such as acid etching or oxygenor air-plasma cleaning, and/or the use of interlayers designed to enhance hole-harvesting


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efficiencies.37,77,78 The ZnPc(PA)4 modification also outperforms analogous CuPc/C60 OPVs in which the ITO contact was functionalized with alkyl-PA modifiers having chains as short as six carbons,78 which indicates that the use of the redox couple with proper orientation is an improved strategy in solid state electrode modification strategies over simple small molecule modifiers.

CONCLUSIONS ZnPc(PA)4 molecules chemisorbed on ITO are predominately monomeric, with an average orientation largely parallel to the ITO electrode surface that produces a short tunneling distance for charge transfer. The rate constant measured by PM-ATR for the one-electron oxidation of these films is greater than 104 sec-1, consistent with the hypothesized structure. In contrast, ZnPcPA/ITO is relatively disordered and a significant fraction of the molecules are aggregated. The rate constants measured by PM-ATR for the various subpopulations in a ZnPcPA monolayer are 10-100 fold lower than that measured for ZnPc(PA)4, which can be attributed to a more upright orientation distribution which produces a larger mean tunneling distance (and distribution of distances) between the electrode and the Pc macrocycle. The energy level alignment of both CuPc/ZnPc(PA)4/ITO and CuPc/ZnPcPA/ITO is favorable for hole extraction.

Both types of ZnPc-modified ITO electrodes were used as hole-

harvesting contacts in planar heterojunction OPVs (with CuPc/C60 active layers). Significant differences in OPV performance were observed, consistent with the structural differences between the ZnPc(PA)4 and ZnPcPA monolayers and their respective rate constants. These studies demonstrate how simultaneous characterization of the orientation- and distance-dependent charge-transfer kinetics for redox-active surface modifiers can be used to guide the development of new modifiers to enhance the performance of charge-harvesting electrical contacts in energy conversion platforms. A promising design strategy for next-generation surface modifiers is promotion of π-π interactions leading to enhanced (and possibly asymmetric) rates of charge transfer at OPV interfaces.


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Figure 5. (A) Linear and (B) semi-log J-V curves for ITO/CuPc/C60/BCP/Al OPVs with ZnPcPA/ITO (green) and ZnPc(PA)4/ITO (red) contacts.

Table 4. Summary of device results for ITO/ZnPc/CuPc/C60/BCP/Al OPVs (n = 12).a Voc (V)

Jsc (mA·cm-2)

FF

η (%)

Rs (Ω·cm2)

Rsh (kΩ·cm2)

ZnPcPA

0.43 ± 0.01

-4.9 ± 0.1

0.28 ± 0.01

0.60 ± 0.05

52 ± 8

45 ± 19

ZnPc(PA)4

0.46 ± 0.01

-5.5 ± 0.2

0.57 ± 0.01

1.47 ± 0.05

0.81 ± 0.08

42 ± 18

ZnPc layer

Open-circuit voltage (Voc), short-circuit current (Jsc), fill factor (FF), overall power conversion efficiency (η), series resistance (Rs), shunt resistance (Rsh). a


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ACKNOWLEDGMENT This research was supported as part of the Center for Interface Science: Solar Electric Materials (CISSEM), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award Number DE-SC0001084.

SUPPORTING INFORMATION DESCRIPTION Synthesis of ZnPc(PA)4, electrochemistry of dissolved ZnPcs, selection of Edc for PM-ATR measurements, side chain conformation of ZnPc(PA)4 tethered to ITO, and UV-Vis studies of ZnPc/CuPc films on ITO. This material is available free of charge via the Internet at http://pubs.acs.org.

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Imahori, H.; Kang, S.; Hayashi, H.; Haruta, M.; Kurata, H.; Isoda, S.; Canton, S. E.; 25 ACS Paragon Plus Environment


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He, J.; Benko, G.; Korodi, F.; Polivka, T.; Lomoth, R.; Akermark, B.; Sun, L.; Hagfeldt, 27 ACS Paragon Plus Environment


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TOC IMAGE

TCO modifier ZnPc

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PA

vs. ITO

ZnPc

PA

PA

ITO

PA

PA


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Influence of Molecular Orientation on Charge-Transfer Processes at

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