Surface Modification of IndiumâTin Oxide with Functionalized

Article pubs.acs.org/JPCC

Surface Modification of Indium−Tin Oxide with Functionalized Perylene Diimides: Characterization of Orientation, Electron-Transfer Kinetics and Electronic Structure Yilong Zheng,† Anthony J. Giordano,‡ R. Clayton Shallcross,† Sean R. Fleming,† Stephen Barlow,‡ Neal R. Armstrong,† Seth R. Marder,*,‡ and S. Scott Saavedra*,† †

Department of Chemistry & Biochemistry, University of Arizona, Tucson, Arizona 85721, United States School of Chemistry & Biochemistry and the Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States

‡

S Supporting Information *

ABSTRACT: Charge-transfer efficiency at the active layer/transparent conducting oxide (TCO) interface is thought to be a key parameter contributing to the overall efficiency of organic electronic devices such as organic photovoltaics (OPVs). Modification of the TCO surface with a redox-active surface modifier is a possible approach toward enhancing OPV efficiency by providing an efficient charge-transfer pathway between either hole- or electron-harvesting contacts and the organic active layer. Here we report on the modification of indium−tin oxide (ITO) electrodes with two perylene diimides (PDIs), coupled to phosphonic acid (PA) binding groups through a p-phenylene bridge or a biphenyl-4,4′-diyl bridge (PDI− phenyl−PA and PDI−diphenyl−PA, respectively). We used two different deposition techniques: adsorption from solution (SA) and spin coating (SC), to create three types of monolayer films on ITO: SA PDI−phenyl−PA, SA PDI−diphenyl−PA, and SC PDI−phenyl−PA. These thin films, designed to act as “chargetransfer mediators”, were used to study relationships between molecular structure, electron-transfer (ET) kinetics, and electronic structure. Molecular orientation was assessed using polarized attenuated total reflectance (ATR) spectroscopy; the average tilt angle between the PDI molecular axis and the ITO surface normal for both SA films was about 30°, while films deposited using spin-coating were more in-plane, with an average tilt angle of 45°. To our knowledge, these are the first reported measurements of orientation in PDI monolayers on ITO electrodes. Electrochemical and ultraviolet photoemission spectroscopy studies showed that all three PDI−PA films have similar reduction potentials, electron affinities, and ionization energies, indicating that differences in bridge length and molecular orientation did not measurably affect the interfacial electronic structure. ET rate constants ranging from 5 to 50 × 103 s−1 were measured using potential-modulated ATR spectroscopy. The kinetic and thermodynamic data, along with a photoelectrochemical comparison of electron injection efficiency, show that PDI−PA films are capable of serving as a charge-transfer mediator between an ITO electrode and an organic active layer, and thus have potential for use as electron-collection contacts in inverted OPV devices.

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INTRODUCTION The efficiency of charge transfer at organic (active layer)/ transparent conducting oxide (TCO) interfaces can significantly affect the performance of organic electronic devices, including organic light emitting diodes and organic photovoltaics (OPVs).1,2 Charge transfer across these interfaces is influenced by a number of parameters, including the energy offsets of the frontier orbitals of the organic layer and the contact material, wave function overlap, and surface energy, which in turn depend on molecular variables such packing and orientation in the first monolayer adjacent to the contact.3−6 Indium−tin−oxide (ITO) substrates have high transparency and low resistivity and thus are widely used as the transparent TCO contact in OPVs.7 However, the hydrophilic surface of ITO is poorly wetted by hydrophobic organic semiconductors. © XXXX American Chemical Society

Moreover, the native work function of ITO, which varies from ca. 4.2 to 5.0 eV, depending on the commercial source and surface cleaning method,8,9 is either too low or too high for it to function as an efficient hole collection or electron collection electrode, respectively.9−11 Modification with an organic monolayer can be used to alter the surface properties of ITO, including surface energy and wettability, which improves its compatibility with organic active layer materials.12 In addition, the native work function can be altered through generation of an interface dipole.4,5,7−9,13 For example, Khodabakhsh et al.13 modified ITO electrodes using chlorobenzene-based selfReceived: July 7, 2016 Revised: August 21, 2016

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DOI: 10.1021/acs.jpcc.6b06812 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

sheet resistance of 20−30 Ω/cm2 was purchased from Thin Film Devices. ITO electrodes were cut into either 1 in. × 3 in. pieces for attenuated total reflectance (ATR) measurements or 0.5 in. × 0.5 in. pieces for electrochemical measurements. ITO electrodes were cleaned by lightly scrubbing with detergent (1% Triton X-100) for about 1 min, followed by successive sonication in detergent, water (Barnstead Nanopure, measured resistivity of 18.3 MΩ·cm), and ethanol for 15 min each. Cleaned ITO electrodes were stored in ethanol for later use. Immediately prior to use, ITO electrodes were dried under flowing N2 and then activated in an air plasma (Harrick PDC3XG, Harrick Scientific) for 15 min at medium RF level. The synthesis of PDI−phenyl−PA and PDI−diphenyl−PA is described in the Supporting Information. To prepare SA PDI films, activated ITO electrodes were immersed into: (a) a 20 μM PDI−phenyl−PA solution in 90% dimethylfomamide (DMF) + 10% chloroform (CHCl3) containing 20 μM tetrabutylammonium hydroxide (TBA−OH) at room temperature for 1 h or (b) a 20 μM PDI−diphenyl−PA solution in 50% DMF + 50% CHCl3 containing 20 μM TBA−OH at room temperature for 2 h. The addition of TBA−OH was found to hinder self-aggregation of dissolved PDI−PAs (see the Supporting Information). The ITO electrodes were then rinsed with a copious amount of CHCl3 and dried with either flowing N2 (for electrochemical measurements) or in air (for ATR measurements) before characterization. SC films were prepared by spinning a 500 μM PDI−phenyl− PA solution in CHCl3 onto ITO electrodes at 2000 rpm for 1 min at room temperature. Immediately after, electrodes were rinsed with a copious amount of CHCl3 and annealed at 120 °C for 2 min. For ATR experiments, only half of the electrode area (1/2 in. × 3 in.) was coated; the other half was masked with tape. The uncoated half was used to measure the blank ATR spectrum. Cyclic Voltammetry of PDI Films. Cyclic voltammetry (CV) was performed with a standard three-electrode configuration consisting of an ITO working electrode (electrode area = 0.754 cm2), a platinum counter electrode and a Ag/AgNO3 (0.01 M) nonaqueous pseudoreference electrode, controlled by a CH420A potentiostat (CH Instruments, Inc.). Tetrabutylammonium perchlorate (TBAP) dissolved in acetonitrile (0.1 M) was used as the electrolyte; it was degassed with a gentle flow of Ar for 1 h inside the fully assembled electrochemical cell in a glovebag, which was also filled with Ar. Both PDI−phenyl−PA and PDI−diphenyl−PA are insoluble in acetonitrile, which eliminated the possibility of desorption from ITO during the electrochemical measurements. All CV measurements were performed in a glovebag filled with Ar to minimize the effect of dissolved O2. The system was cycled at 0.5 V/s between −0.6 and −1.5 V with a sample interval of 1 mV and a sensitivity of 100 μA/V. ATR Spectroscopy. Steady-state ATR spectroscopy and spectroelectrochemistry were performed using a custom-built, broadband instrument described in previous publications.3,21 ITO electrodes were used as planar waveguides. Light was coupled into and out of waveguides using two BK7 prisms (Thorlabs) positioned 43 mm apart. The internal reflection angle was 73−74°, which produced eight total internal reflections at the ITO/solution interface. Adsorption isotherms of PDI−PA molecules were measured using transverse magnetic (TM) polarized light under open circuit conditions. TM polarized light also was used to measure ATR spectra under potential control. A platinum wire was used as the

assembled monolayers which changed the wettability and increased the work function by 0.4−0.6 eV. Hotchkiss et al.9 used a series of benzyl phosphonic acid (PA) modifiers to demonstrate that the effective ITO work function is tunable over 1 eV by varying the modifier structure, which controls interface dipole magnitude and orientation. Redox activity is another modifier property that may influence interfacial charge transfer. Specifically, a modifier with a oxidation or reduction potential close to the transport energy levels for charge (hole or electron) harvesting from the organic active layer is hypothesized to function as a ‘chargetransfer mediator,’3,4,6,12,14 in addition to providing for adjustment of the aforementioned surface properties. Perylene diimide (PDI) molecules15,16 are potential candidates for redox-active modification of ITO/acceptor interfaces in inverted configuration OPVs (where the ITO electrode is the electron collection contact) because their reduction potentials are close to those of commonly used acceptor molecules, such as fullerenes.17,18 Furthermore, the reduction potential can be tuned through substitution at the bay positions.19,20 Herein we examine two PA-functionalized PDIs with different bridge lengths, phenyl and diphenyl, between the PA moiety and the PDI core (PDI−phenyl−PA and PDI−diphenyl−PA, respectively; see Figure 1) as redox-active surface modifiers of ITO.

Figure 1. Structures of (A) PDI−phenyl−PA and (B) PDI−diphenyl− PA.

PDI monolayers on ITO were prepared by adsorption from solution (SA) and spin-coating (SC), and were characterized using a variety of techniques to assess relationships between film structure, electrochemical, spectroelectrochemical and photoelectrochemical properties, electron affinities, ionization energies, and electron-transfer (ET) kinetics. On the basis of this evaluation, PDI films are capable of serving as a chargetransfer mediator between an ITO electrode and an organic active layer, and thus have potential for use as electron collection contacts in inverted OPV devices. These studies also provide guidance for development of new TCO surface modifiers for organic electronics, which continues to be a very active research area.12

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EXPERIMENTAL SECTION Preparation of PDI Films on ITO. ITO on Schott BoroFloat glass with a layer thickness of ∼150 nm and a B


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quantitative measurements were made, the photocurrent was measured over a potential sweep from −0.2 to +0.7 V at a scan rate of 10 mV/s while chopping the incident beam at ca. 0.5 Hz. An applied potential of 0.6 V was selected to maximize the photocurrent and minimize the background current.

counter electrode, and a Ag/AgNO3 (0.01 M) electrode was used as the nonaqueous pseudoreference electrode. The supporting electrolyte was 0.1 M TBAP in acetonitrile, as described above. The potential was stepped from −0.6 to −1.6 V with a step size of 0.1 V, and spectra were acquired at each potential after a ca. 10 s hold time. The mean tilt angles of the absorbance dipoles in PDI−PA films were determined using polarized ATR spectroscopy,3,6 and the method of Mendes et al.22 was used to correct for the unequal interfacial electric field intensities in TM and transverse electric (TE) polarizations. The intensity difference was normalized by measuring the dichroic ratio, ρfilm (=ATE,film/ ATM,film, where A is the integrated absorbance of the film in the respective polarization), of a dextran−fluorescein film adsorbed on the ITO waveguide (see ATR spectra in Figure S6). It was assumed that fluorescein moieties were randomly oriented with respect to the ITO waveguide plane. PM-ATR Spectroscopy. Detailed descriptions of the PMATR instrument and methodology have been provided previously.3,21 Briefly, a sinusoidally modulated potential was applied to the PDI-modified ITO electrode centered around midpoint potential for the redox couple (Edc). The in-phase (Re(Rac)) and out-of-phase (Im(Rac)) portions of electroreflectance (ER) signals were measured as a function of modulation frequency. The ET rate constant was calculated from ks,opt = 0.5ω2RsCdl, where ω is the optical switching frequency at which Re(Rac) = 0, Rs is the solution resistance, and Cdl is the double-layer capacitance. Detailed descriptions of the theory underlying PM-ATR and its use for determining ks,opt for redox-active thin films have been published.23−25 The ITO electrode active area was 0.8 cm2 defined by a silicone gasket. Collimated, TM polarized light was coupled into/out of the ITO-coated glass slide by BK7 prisms positioned 20 mm apart. The internal reflection angle was 73−74°, which gave two total internal reflections at the ITO/ solution interface. The sinusoidally modulated voltage (Eac) was 20 mVrms (0.028 V peak-to-peak). The Edc was 0.91 V versus the Ag/AgNO3 (0.01 M) nonaqueous pseudoreference electrode. The ER signal was recorded at 460 nm. Rs and Cdl were measured using electrochemical impedance spectroscopy with an EG&G Model 263A potentiostat/galvanostat coupled with a Model 1025 frequency response detector operated with PowerSuite software (Princeton Applied Research), and using the same ATR spectroelectrochemical flow cell described above. Ultraviolet Photoemission Spectroscopy. UPS studies were performed with a Kratos Axis Ultra X-ray photoelectron spectrometer with a He(I) excitation source (21.2 eV), as described in previous papers.3,6,26 A −9.00 V bias was applied to the sample under measurement to enhance the collection of low kinetic energy electrons. The Fermi level reference was established on a freshly sputtered Au surface. Photoelectrochemistry. Photoelectrochemical measurements were performed using the same three-electrode cell used for the CV measurements, with ITO as the working electrode, a Pt wire as the counter electrode, and a AgCl-coated silver wire (Ag/AgCl) as the pseudoreference electrode, controlled by an EG&G Princeton Applied Research Model 263A potentiostat/galvanostat The electrolyte solution was 0.1 M TBAP in acetonitrile containing 500 μM of aluminum phthalocyanine hydroxide (AlOHPc) as the electron donor. The cell was illuminated with a HeNe laser (632.8 nm) to selectively excite AlOHPc. To select the potential at which

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RESULTS AND DISCUSSION This work is motivated by the hypothesis that the structural properties of organic modifiers tethered to a charge-collection contact in an OPV device may significantly influence the efficiency of charge transfer across that interface. In recent papers, we began exploring the structural and functional properties of zinc phthalocyanine molecules3,6,26 to modify the donor/ITO interface in an OPV. Here we extend those studies to the acceptor/ITO interface, using PDIs tethered to ITO as modifiers of the electron collection contact in an inverted OPV. We examined the influence of modifier bridge length by comparing the properties of SA PDI−phenyl−PA and SA PDI−diphenyl−PA films, and we prepared SA and SC films composed of PDI−phenyl−PA to compare the two deposition methods. Self-Assembly and Electrochemistry of PDI Films. The self-assembly of PDI−PA molecules on ITO electrodes was monitored using ATR spectroscopy. As described in the Supporting Information, saturated adsorption was observed after 1−2 h of static incubation, and these conditions were used to produce SA films for further analysis. Spin-coating parameters were optimized to generate SC films having an electroactive surface coverage similar to that of the SA films (see below for surface coverage data). The CVs of each type of PDI−PA film on ITO, plotted in Figure 2, exhibited the two step, two electron reduction that is

Figure 2. Representative cyclic voltammograms of the three types of PDI−PA films on ITO electrodes. The electrolyte was 0.1 M TBAP in acetonitrile and the scan rate was 0.5 V/s.

characteristic of PDI electrochemistry.18,27,28 The midpoint reduction potentials (E0′), listed in Table 1, were equivalent, showing that the different bridge lengths and deposition methods did not affect the redox thermodynamics of PDI−PA films. In addition, these reduction potentials are equivalent to those measured for the dissolved PDI−PAs (see Figure S4 and Table S1) and consistent with literature data,19,20 showing that adsorption to ITO also did not significantly affect the electrochemistry. However, the reduction and reoxidation peaks of the PDI−PA films were broader (fwhm of 150−200 mV) than those of the dissolved PDI−PAs (fwhm of 50−80 mV).18,28,29 The broadening of these voltammetric peaks is consistent with PDI−PA aggregation, as described below. C


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Table 1. Molecular Tilt Angles (θ), Electroactive Surface Coverages (Γ), Reduction Midpoint Potentials (E0′) and ET Rate Constants (ks,opt) for the Three Types of PDI−PA Films on ITOa long axis tilt angle (θ) Γ (10‑11 mol/cm2) E0′ (1st, V vs Ag/AgNO3) E0′ (2nd, V vs Ag/AgNO3) ks,opt (s‑1) a

SA PDI−diphenyl−PA

SA PDI−phenyl−PA

SC PDI−phenyl−PA

31° ± 4° 7±1 −0.88 ± 0.03 −1.15 ± 0.04 (0.5 ± 0.2) × 104

33° ± 4° 11 ± 2 −0.91 ± 0.04 −1.15 ± 0.05 (1.4 ± 0.3) × 104

45° ± 2° 10 ± 2 −0.91 ± 0.04 −1.14 ± 0.07 (5 ± 2) × 104

n ≥ 3 for all measurements.

Figure 3. (A) Representative ATR spectra of PDI−PA films on ITO in TE (dashed lines) and TM polarizations (solid lines): SA PDI−diphenyl−PA (blue), SA PDI−phenyl−PA (green), and SC PDI−phenyl−PA (red). (B) Schematic of the orientation parameters of an adsorbed PDI−phenyl−PA molecule.

ATR Spectra, Molecular Orientation, and Surface Coverage of PDI Films. Figure 3A shows TE- and TMpolarized ATR spectra of the three types of PDI−PA films. The broadened absorbance bands and small 0−0/0−1 absorbance peak ratios indicate that the molecules are highly aggregated in all three films.30−33 The spectra of the SC film exhibited greater broadening than the spectra obtained on SA films, indicative of a greater degree of aggregation. The molecular orientation in these films was assessed from the dichroic ratio determined from integration of the TE- and TM-polarized absorbance bands (450−550 nm), 22 and assuming that the PDI absorption transition dipole is aligned with the long molecular axis.34,35 The long axis tilt angles listed in Table 1, defined relative to the surface normal to the ITO substrate plane (θ, Figure 3B), were 33° ± 4° for SA PDI− phenyl−PA films and 31° ± 4° for SA PDI−diphenyl−PA films, indicating a predominately out-of-plane orientation in both cases. To our knowledge, these are the first reported measurements of orientation in a monolayer of a PDI derivative on ITO, although a few studies have been performed on perylene derivatives deposited as monolayers on TiO2.36,37 Molecular orientation has been assessed in numerous other types of monolayers on ITO, including PA-bound films which were surveyed in a recent review.12 Gliboff et al.38 reported tilt angles between the alkyl chain axis and the surface normal in the range of 30° - 41° for fluorinated and nonfluorinated octyl PAs adsorbed to ITO. Much smaller angles have been calculated and measured for simple aryl PAs on ITO and indium−zinc oxide.38,39 In contrast, the orientation of extended “rigid rod” aryl PAs similar to those used in the present work has not been studied extensively; the only known example is a tilt angle of 66° reported by Gundlach et al.36 for a perylene attached to TiO2 via a three-ring bridge with a PA anchor. Their result, along with a forthcoming paper from our

laboratory, suggests that as the size of the surface modifier increases, the tilt angle increases as well, possibly due to differences in the degree of cofacial π−π interactions between PDI cores in the monolayer. The spin-coated PDI−phenyl−PA films showed a more inplane orientation with a mean tilt angle of 45° ± 2°. Studies of self-assembled films of alkyl and aromatic phosphonic acids on TCOs have shown that binding occurs mainly through bidentate and tridentate modes.38,40,41 In spin-coated films, however, a kinetic barrier arising from rapid solvent evaporation may give rise to a different distribution of binding modes, possibly including monodentate binding as well as adsorptive interactions that do not involve the PA moiety, and these may be further altered by thermal annealing. The net result may be a different molecular orientation distribution than that obtained when PDI−phenyl−PA is adsorbed from dilute solution. The electroactive surface coverages (Γ) for PDI−PA films, listed in Table 1, were determined by voltammetric integration over both reduction peaks and assuming a two-electron reaction. For SA PDI−phenyl−PA films, Γ was 11 × 10−11 mol/cm2. This result can be expressed in monolayer units (ΓML) by considering the projected area of an adsorbed molecule, which was estimated to be 180 Å2 based on MM2 energy minimization of a PDI−phenyl−PA molecule and assuming a tilt angle when adsorbed on ITO of 33° (see Figure S5); this estimate does not consider possible effects of PDI aggregation and alkane chain interactions. The result is that for SA PDI−phenyl−PA films, ΓML was about 1.2 monolayers. ΓML values for SA PDI−diphenyl−PA films and SC PDI−phenyl− PA films were 0.95 and 1.4 monolayers, respectively, using projected areas of 230 Å2/molecule and 240 Å2/molecule, respectively, which were estimated using the respective tilt angles listed in Table 1 and subject to the same assumptions D


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Figure 4. (A) Representative potential-dependent ATR spectra of a SA PDI−phenyl−PA film on ITO acquired in TM polarization from −0.6 to −1.6 V versus a Ag/AgNO3 pseudoreference electrode. (B) Plots of absorbance of different PDI−phenyl−PA charge states (neutral state (PDI) at 460 nm, dianion (PDI2−) at 569 nm, and anion radical (PDI−•) at 690 nm) as a function of applied potential. Absorbance values are normalized to the most intense absorbance values of each charge state. The solid lines are sigmoidal fits to the data at 460 nm (blue) and 569 nm (green).

for r = 0 and βr is the exponential decay coefficient.47 Fitting this equation to the ks,opt data for the two SA films affords an estimate for βr of 0.28 Å−1, which is within the range of calculated and measured values reported in the literature for electron transfer through unsaturated bridges.45,48−51 For example, Sachs et al.48 measured a βr value of 0.57 Å−1 for oligo(phenylethynyl)-linked ferrocene monolayers on gold using an indirect laser-induced-temperature-jump (ILIT) method. Creager et al.49 measured βr = 0.36 Å−1 for similar films using AC voltammetry. The βr estimate of 0.28 Å−1 is smaller than those reported for alkyl-linked redox molecules (typically βr ≈ 1 Å−1)47,52,53 or for direct through-space tunneling (for which βr is even larger),54 which suggests that electron transfer in the SA PDI films is mediated by electronic coupling through the bridges. The ks,opt for SC PDI−phenyl−PA films was about four times greater than that measured for SA PDI−phenyl−PA films. This difference is less easily accounted for in terms of the throughbond tunneling distance, however, as discussed above, the molecular orientation is different in SC films. The more inplane tilt angle distribution may support higher ET rates due to shorter ITO-PDI core tunneling distances; it also is indicative of a different distribution of binding modes, suggesting that the coupling between the ITO and the p-phenylene bridge may be different in the SC films. The greater degree of aggregation in SC films relative to the both types of SA films, as noted above, also may play a role. In a more highly aggregated film, the PDI cores will be more closely spaced and this is anticipated to enhance the rate of intermolecular electron self-exchange between surface-bound PDI−• and PDI species.55,56 Several studies have shown that the rate of electron self-exchange in redox-active films on TCOs is enhanced as the surface coverage of the modifier is increased.50,57,58 The dependence of ks,opt on the degree of aggregation in PDI−PA monolayers will be the subject of a forthcoming paper. Silva et al.59 used impedance spectroscopy to measure the ET rate constant of another PDI derivative, N,N′-di(2phosphonoethyl)perylene-3,4:9,10-bis(dicarboximide) (denoted PPDI) adsorbed onto ITO. They reported a rate constant of 41 s−1, much lower than the rate constants measured here. However, their experiments were performed in aqueous solution using KCl as the supporting electrolyte, while here acetonitrile/TBAP was used. It is well-known that different solvents and different supporting electrolytes can shift the reorganization energy of a redox reaction, which can alter the ET rate constant.60−62 To assess this possibility and to obtain data more directly comparable to Silva’s, the ET rate

described above. In summary, ΓML for all three types of PDI− PA films was about one monolayer. Spectroelectrochemistry of PDI−PA Films. Figure 4A shows TM-polarized ATR spectra of a SA PDI−phenyl−PA film acquired as a function of potential applied to the ITO electrode. In the potential range from −0.6 to −1.1 V versus Ag/AgNO3, the major PDI absorbance bands in the region 600 nm which is assigned to the radical anion (PDI−•).42−44 At potentials more negative than −1.1 V, the PDI−• absorbance decreased, while new bands assigned to the formation of the dianion (PDI2−) arose in the 500−600 nm region.42−44 A near-complete bleach of the neutral-state absorbance was observed, which shows that nearly the entire film was electroactive. E0′ values for the first and second reductions were obtained by plotting the absorbance versus applied potential at wavelengths assigned to PDI (460 nm) and PDI2− (569 nm) in Figure 4B. The values obtained, −0.92 and −1.18 V, respectively, are consistent with the CV data. The 690 nm curve tracks the appearance and subsequent disappearance of PDI−• over the −0.6 to −1.6 V potential range. The spectroelectrochemical responses of SA PDI− diphenyl−PA and SC PDI−phenyl−PA films were similar (see spectra in Figure S7), and were also consistent with the electrochemical data. Some differences in band intensity were observed which may be due to differences in the molar absorptivities of the PDI−• and PDI2− species and the degree of aggregation in these films.43 PM-ATR Measurements of ET Rate Constants. The ET rate constants (ks,opt) of the first reduction for the three types of PDI−PA films were measured by PM-ATR in TM polarization. Figure S8 shows an example of a complex plane plot for a SA PDI−phenyl−PA film. Rs and Cdl values for all PDI−PA films were determined by electrochemical impedance spectroscopy and are summarized in Table S2. Table 1 summarizes the ks,opt data. The ks,opt value for SA PDI−phenyl−PA films was about three times greater than that for SA PDI−diphenyl−PA films; this is consistent with the shorter bridge of PDI−phenyl−PA, which should produce a shorter tunneling distance between the PDI core and the electrode. The tunneling distance (r) for electron transfer in the SA PDI−PA films, assuming a through-bond pathway dominates,45 was estimated from the geometric straight-line distance between the phosphorus atom in the PA anchoring group and the nearest diimide nitrogen atom.46 The dependence of the standard ET rate constant (kΓ,s) on r is described by kΓ,s = k0e−βrr, where k0 is the extrapolated value of the rate constant E


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Figure 5. Energy level diagrams of the three types of PDI−PA films on ITO electrodes determined by cyclic voltammetry (A) and UPS (B). The standard deviation of the UPS measurements is about 0.1 eV whereas that of the voltammetric measurements is about 0.05 eV. Thus, the frontier orbital energies of the three types of surfaces are equivalent within experimental error.

constant for SA PDI−phenyl−PA films on ITO was measured by PM-ATR in aqueous solution with KCl as the supporting electrolyte, yielding ks,opt = 93 ± 43 s−1. The similarity of this value with Silva’s result is perhaps surprising given that their PPDI monolayers were linked to ITO via a saturated bridging group; on the other hand, an ethyl bridge is somewhat shorter than a phenylene group, its flexibility can potentially allow direct PDI-to-ITO electron transfer, and there may be differences in the PA binding modes. The 150-fold slower rate constant for SA PDI−phenyl−PA in aqueous KCl relative to that measured in acetonitrile/TBAP may reflect greater structural reorganization required for the penetration of hydrated potassium ions vs tetrabutylammonium ions into the PDI monolayer, along with a greater solvent contribution to the reorganization energy. This comparison demonstrates the extent to which the solvent and/or supporting electrolyte influence ET kinetics in PDI−PA films. Lastly, we note that the ET rate constants determined in nonaqueous solution for all three types of PDI−PA films exceed the estimated charge collection rate in an OPV operating at a photocurrent density of 15 mA/cm2 (AM 1.5), which is ∼1700 s−1 per molecule assuming a projected molecular area of 180 Å2.3,63 This suggests that interfacial ET through the ITO/PDI−PA interface would not be an efficiency bottleneck. We also note that the ks,opt measurements were performed under negative potential bias and using small driving forces, which are not conditions at which an OPV would be operated for maximum performance. The driving forces for PDI•− to ITO electron transfer in an operating OPV likely would be higher. Characterization of Frontier Orbital Energies of PDIModified ITO Electrodes. The electron affinity (EA) and the ionization energy (IE) of the PDI−PA films relative to vacuum were estimated from the electrochemical data by calibration with the ferrocene/ferrocenium (Fc/Fc+) redox couple for which the midpoint potential was assumed to be −5.1 eV vs vacuum.64 The midpoint potential of Fc/Fc+ was measured to be 0.11 V vs the Ag/AgNO3 pseudoreference electrode. The onset reduction potential (Ered), −0.79 V vs Ag/AgNO3, was similar for all three types of PDI−PA films. Therefore, EA = |−(Ered + 4.99)| = 4.2 eV (Figure 5A). The onset of the 0−0 absorbance transition occurred at approximately 580 nm for all three types of PDI−PA films (Figure 4); this corresponds to an optical band gap of 2.1 eV which, neglecting the effects of exciton binding, yields IE = 6.3 eV (Figure 5A). These voltammetric estimates do not account for the formation of a

surface dipole, which is reflected in local vacuum level shifts, nor do they address the issue that a solvated monolayer may not reflect the local environment in a solid-state OPV. Ultraviolet photoemission spectroscopy (UPS) therefore was used to assess the energetics of PDI−PA-modified ITO electrodes. UPS spectra of the three types of modified surfaces as well as AP-ITO are shown in Figure 6. The left and right

Figure 6. UPS spectra of air plasma cleaned (AP) ITO and the three types of PDI−PA-modified ITO electrodes with respect to the Fermi energy (EFermi): low kinetic energy (LKE) portion (left panel); high kinetic energy (HKE) portion (right panel). The HKE panel is magnified 5× for display purposes. The values of the energetic edges were determined by linear extrapolation of the intensity to the baseline, as illustrated by the lightweight solid lines in the right panel.

panels show the low kinetic energy (LKE) and the high kinetic energy (HKE) portions of the spectra, respectively, which are plotted with respect to the Fermi energy (EFermi). The LKE and HKE edges are determined by linear extrapolation of the intensity to the baseline. The work function (Φ), defined as the energetic difference between EFermi and the surface vacuum level (EVAC), is determined by subtracting the He I photon energy (21.22 eV) from the LKE edge. The IE is determined by adding the HKE to Φ and represents the energetic difference between the onset density of occupied states and EVAC. The EA values of the PDI−PA-modified electrodes were estimated by subtracting the optical band gap (2.1 eV) from the respective IE.26,65 Figure 5B shows the resulting energy level diagrams for APand PDI−PA-modified ITO. For AP-ITO, the measurement yielded Φ = 4.9 eV, consistent with previous reports.6,9,13,26 Modification of ITO with SA PDI−diphenyl−PA, SA PDI− phenyl−PA, and SC PDI−phenyl−PA films shifted the local F


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films with a more in-plane molecular orientation compared to SA phenyl−PA films. All three PDI−PA films have similar reduction potentials, estimated electron affinities and ionization energies, showing that differences in bridge length and molecular orientation do not measurably affect the interfacial electronic structure. ET rate constants ranging from 5 to 50 × 103 s−1 were measured, and for SA films were correlated with the estimates of the through-bond tunneling distance between the PDI core and ITO. The presence of a PDI−PA monolayer on ITO enhances photoinitiated electron injection from a solution-phase donor into ITO. In summary, the thermodynamic and kinetic data indicate that PDI−PA films are capable of serving as the charge-transfer mediator between an ITO electrode and an organic active layer and thus have potential for use as electron-collection contacts in inverted OPV devices. From a broader perspective, the results of these studies also provide guidance for ongoing development of TCO surface modifiers.

vacuum down by 0.6−0.8 eV, decreasing the Φ to 4.1−4.3 eV. This decrease is consistent with the data of Bardecker et al.14 who modified ITO with several different aryl PAs and reported Φ decreases of 0.58−0.69. Similar decreases in Φ, about 0.7 eV, also were observed when ITO was modified with alkyl PAfunctionalized zinc phthalocyanines.6,26 The EA values of PDI− PA-modified ITO show that it can function as an electron acceptor from some commonly used C60 acceptors (e.g., indene-C60 bisadduct (ICBA) which has ELUMO = 3.5 eV66,67), and may serve as a charge-transfer mediator. For use in an inverted OPV, however, ideally the Φ would be lower than 4.1−4.3 eV; e.g., if the electron acceptor layer is ICBA with an EA of 3.5 eV, the loss in open circuit voltage will be ca. 0.7 eV. Comparing Figures 5A and 5B, the ca. 0.7 eV difference between IE and EA values determined by electrochemistry and UPS is likely due to (a) the significantly different dielectric environments in which the measurements were made68 and (b) the fact that the electrochemical measurements are not corrected for local vacuum shifts; it is assumed that surface dipoles are compensated in high ionic strength electrolyte environments.69 However, both the electrochemical and UPS data sets show that IE and EA do not vary with PDI−PA bridge length and orientation, which indicates that these variables do not affect the interfacial electronic structure. Photoelectrochemistry. To assess the function of a PDI− PA film as a charge-transfer mediator, photoelectrochemical measurements were performed using AlOHPc dissolved in acetonitrile as an electron donor to a SA PDI−phenyl−PA film on ITO. An energy level diagram of the photoelectrochemical cell is shown in Figure S9. Figure S10 shows the CV and the UV−visible absorbance spectrum of AlOHPc from which HOMO and LUMO energy levels were estimated relative to vacuum by calibration with Fc/Fc+. The Fc/Fc+ midpoint potential was measured to be 0.49 V vs the Ag/AgCl pseudoreference electrode. The onset oxidation potential (Eox) for AlOHPc was 1.04 V, from which the AlOHPc HOMO level was estimated from EHOMO = −(Eox + 4.61 eV) = −5.65 eV. The low energy onset of the AlOHPc 0−0 absorbance transition occurred at approximately 715 nm (Figure S9B); this corresponds to an optical band gap of 1.73 eV which yielded ELUMO = −3.92 eV. The EHOMO and ELUMO values for the SA PDI−phenyl−PA film shown in Figure S9 are the voltammetric estimates described above. Photocurrent was measured at applied potential of 0.6 V vs Ag/AgCl (−5.2 eV on the vacuum scale) using 632.8 nm light to excite dissolved AlOHPc and photoinitiate electron transfer to PDI−phenyl−PA. The photocurrent measured for air plasma-cleaned (AP) ITO was 8 (±1) nA while that measured for PDI−phenyl−PA-modified ITO was 60 (±8) nA (n = 5 for both values). Control experiments showed that AlOHPc adsorption on both surfaces was minimal. Thus, the presence of the adsorbed PDI−PA film enhanced the electron collection efficiency across the AlOHPc/ITO interface by a factor of about seven, demonstrating that the film functions as a mediator for electron transfer between AlOHPc and ITO.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b06812. Detailed synthesis procedures, spectral and electrochemical characterization of dissolved PDIs, PDI film preparation and adsorption kinetics, additional data from electrochemical, spectroelectrochemical, impedance, and PM-ATR on PDI−PA films, a photoelectrochemical energy level diagram, and CV and UV−vis absorbance data for AlOHPc (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(S.S.S.) E-mail: [email protected]. *(S.R.M.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was partially 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. This material is based upon work that was partially supported by the National Science Foundation under Grant No. DMR1506504. A.J.G. thanks the National Science Foundation and the Department of Defense for a Graduate Research Fellowship (DGE-0644493) and a National Defense Science and Engineering Graduate Fellowship, respectively.

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REFERENCES

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CONCLUSION Using two PA-functionalized PDI molecules and two different deposition methods, three types of PDI−PA monolayer films on ITO electrodes were prepared and characterized. Although PDI−diphenyl−PA and PDI−phenyl−PA have different bridge lengths, their out-of-plane molecular tilt angles are equivalent in SA films. Spin coating PDI−phenyl−PA onto ITO produces G


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