Quantifying the Extent of Contact Doping at the ... - ACS Publications

Letter pubs.acs.org/JPCL

Quantifying the Extent of Contact Doping at the Interface between High Work Function Electrical Contacts and Poly(3-hexylthiophene) (P3HT) R. Clayton Shallcross,*,† Tobias Stubhan,‡ Erin L. Ratcliff,∥ Antoine Kahn,§ Christoph J. Brabec,‡ and Neal R. Armstrong*,† †

Department of Chemistry and Biochemistry, University of Arizona, 1306 East University Boulevard, Tucson, Arizona 85721, United States ‡ Institute of Materials for Electronics and Energy Technology (I-MEET), Friedrich-Alexander-University Erlangen-Nuremberg, Martensstrasse 7, Erlangen 91058, Germany ∥ Department of Materials Science and Engineering, University of Arizona, 1235 East James E. Rogers Way, Tucson, Arizona 85721, United States § Department of Electrical Engineering, Princeton University, Olden Street, Princeton, New Jersey 08544, United States S Supporting Information *

ABSTRACT: We demonstrate new approaches to the characterization of oxidized regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) that results from electronic equilibration with device-relevant high work function electrical contacts using highresolution X-ray (XPS) and ultraviolet (UPS) photoelectron spectroscopy (PES). Careful interpretation of photoemission signals from thiophene sulfur atoms in thin (ca. 20 nm or less) P3HT films provides the ability to uniquely elucidate the products of charge transfer between the polymer and the electrical contact, which is a result of Fermi-level equilibration between the two materials. By comparing high-resolution S 2p core-level spectra to electrochemically oxidized P3HT standards, the extent of the contact doping reaction is quantified, where one in every six thiophene units (ca. 20%) in the first monolayer is oxidized. Finally, angle-resolved XPS of both pure P3HT and its blends with phenyl-C61-butyric acid methyl ester (PCBM) confirms that oxidized P3HT species exist near contacts with work functions greater than ca. 4 eV, providing a means to characterize the interface and “bulk” region of the organic semiconductor in a single film.

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low work function contacts (relative to the oxidation and reduction potential of the organic semiconductor, respectively) and vacuum level alignment when the contact work function is between the organic semiconductor HOMO and LUMO energy.5 In the pinning regimes, interfacial charge transfer or “contact doping” has been posited; however, the extent of doping has rarely been quantified.6 Doping of the organic semiconductor may be significantly enhanced by the addition of strongly electron-donating (n-type) or -accepting (p-type) dopants,7 and while both forms of doping influence electrical properties, the molecular components (i.e., radical cations/ anions) resulting from the charge-transfer process can be difficult to both identify and quantify spectroscopically, especially for ultrathin films. These charge-transfer processes with organic semiconductors are not only dependent on the work function of the neighboring contact and the presence of molecular dopants but also are thought to depend strongly on

he interface region between electrical contacts and organic semiconductors is critical to the overall performance of organic electronic devices, especially as these regions control charge injection in organic light-emitting diodes (OLEDs) and organic field effect transistors (OFETs), charge extraction/ surface recombination in organic photovoltaics (OPVs), and device lifetime/degradation in all of these technologies.1,2 Interlayer materials are often used to tune the contact work function (Φ), ultimately controlling the energetic barrier for charge injection/extraction and turn-on/open-circuit voltage of OLEDs and OPVs, respectively.3 Understanding the energetics and the relationship between energetics and interfacial structure and the possible chemical reactions between electrical contacts and the active semiconducting materials are important issues when designing and optimizing optoelectronic devices. Energy level alignment at the contact/organic semiconductor interface is dependent on the work function of the contact relative to the frontier orbital energies (i.e., HOMO and LUMO) of the semiconductor.4 Different scenarios are possible for weak interactions (e.g., no chemical bonding) between an organic semiconductor and a contact: Fermi-level pinning near the organic semiconductor frontier orbitals for both high and © XXXX American Chemical Society

Received: March 3, 2015 Accepted: March 24, 2015

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DOI: 10.1021/acs.jpclett.5b00444 J. Phys. Chem. Lett. 2015, 6, 1303−1309

Letter

The Journal of Physical Chemistry Letters

that oxidative doping of P3HT films can result in either polaronic (singly charged) or bipolarionic (doubly charged) forms of the thiophene polymer, we simplify the argument by referring to positively charged P3HT as “oxidized.” For detailed discussions on electrochemically oxidized poly(thiophene) polymers and the nature of the cationic species, we refer the reader to a review by Heinze and coworkers20 as well as seminal work by Heeger et al.21,22 and Brédas et al.22,23 Our PES results confirm that oxidized P3HT is formed when P3HT and blends of P3HT/PC60BM come into contact with substrates that have a work function greater than ca. 4 eV, which is consistent with previous reports.5,24 For the first time, our results provide a detailed and quantitative picture of the contact doping reaction for P3HT with a variety of contacts. Electronic and energetic equilibration between high work function electrical contacts and P3HT is illustrated in Scheme 1. Charge transfer upon interface formation at the contact/

“mid-gap states” of the organic semiconductor8,9 and the electrical contact,10,11 both of which are thought to originate from structural disorder or impurities. Charge-transfer processes resulting from Fermi-level pinning between electrical contacts and organic semiconductors are typically inferred from work function measurements resulting from either UPS or Kelvin probe techniques on relatively thick films (e.g., > 20 nm).12,13 For electrode/organic semiconductor combinations that fall within the pinning regimes, equilibration of the contact and semiconductor Fermi energies results in a constant work function of the organic semiconductor despite changing the contact work function; hence, the work function of the organic semiconductor is “pinned” at a defined energy above or below the HOMO or LUMO energy for high and low work function electrodes, respectively. The integer chargetransfer (ICT) model, which has been invoked by the organic electronics community, postulates that the pinned organic semiconductor states are related to fully relaxed (in regards to both geometric and electronic structure) “positive integer charge-transfer states” (i.e., radical cations) and “negative integer charge-transfer states” (i.e., radical anions) of the organic semiconductor.4 Koch et al. have elucidated the molecular components of the contact doping reaction at the interface between C60 and Ag(111), where XPS of the C 1s core level affords two clear peaks, which correspond to neutral (C600) and anionic (C60−•) fullerene molecules (i.e., Fermi level equilibration at this interface results in electron transfer from the Ag electrode into the C60 overlayer).14 Similarly, Brédas et al. have shown that deposition of 4,4′-N,N′-dicarbazolebiphenyl (CBP) onto stoichiometric MoO3 induces the formation of reduced Mo5+ species due to oxidation of the CBP overlayer, as evidenced from the evolution of a new low binding energy (BE) Mo5+ peak in the XPS spectra of the Mo 3d core level; however, they did not elaborate on the relative doping concentration or report on any spectroscopic features associated with the corresponding CBP+• radical cation.15 Lowenergy optical absorption features of oxidized molecular species have been observed at the interface between organic semiconducting polymers and a variety of contacts due to contact doping; however, these measurements have not yet provided quantitative information as to the extent of doping.6,16,17 Herein, we utilize high-resolution PES to provide a quantitative molecular-level description of the contact doping process that occurs when P3HT, a prototypical electron donor material, comes into contact with a range of electrical contacts with varying work function. The work function of ITO substrates is varied over a 1 eV range by modification with thin metal oxide and polymeric interlayers. Conformal P3HT films are deposited by spin coating, and the film thickness is systematically controlled from submonolayer to multilayer by modifying the P3HT solution concentration. The ability to realize conformal P3HT films, even at very low coverages, is a key aspect of this study; solution processing and characterization of ultrathin polymer and small molecule films is a challenge because dewetting and aggregation phenomena may dominate film growth.18,19 UPS spectra provide information on the frontier orbital energetics of the contacts and P3HT films. The oxidation state of P3HT and, in one case, the contact is determined by analyzing XPS spectra. The extent of P3HT oxidation is quantified by analyzing S 2p core level spectra using a rigorous fitting procedure, which is made possible by characterization of electrochemically doped P3HT standards that are quantitatively oxidized. While it has been suggested

Scheme 1. Illustration of the Energetic and Chemical Equilibration between a High Work Function Contact and P3HT

semiconductor interface has been presumed to leave an oxidatively p-doped near-surface layer, equilibrating the Fermi energies of the two materials (Scheme 1a). In this schematic, we show a Gaussian distribution of states (DOS) for the filled HOMO level, where the transferred electrons originate from the upper tail of the DOS (red area above the dashed line representing the Fermi energy, EF). We note that recent work by Brédas and coworkers has shown that the distribution may in fact be exponential or Gaussian in shape, depending on the carrier concentration in the organic semiconductor material.27 The oxidized species concentration is shown to decrease systematically away from the interface until only neutral P3HT is found in the bulk of the film. The width of this concentration gradient defines the depletion layer thickness, which is described by the Schottky−Mott model typically invoked to describe semiconductor/metal interfaces.28 High-resolution core-level (XPS) and valence band (UPS) spectra, along with the resulting energy level diagram, for a series of P3HT films of various thicknesses on acid-etched29 indium tin oxide (AE-ITO, Φ = 4.5 eV) are shown in Figure 1. 1304


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work function (Figure 1b, left panel). Similarly, the high kinetic energy edge (HKE), which defines the ionization potential (IP) with respect to the Fermi energy, shifts ca. 0.7 eV to lower kinetic energy with increasing P3HT thickness (Figure 1b, right panel). The shifts associated with the C 1s core level, LKE, and HKE are displayed in Figure 1c. These shifts to lower kinetic energy are consistent with band bending via oxidation of the P3HT layer.9,30,32,33 These energetic shifts all saturate around 20 nm, defining the width of the depletion layer. An energy level diagram is compiled from these results and displayed in Figure 1d. The ionization potential is pinned to the Fermi energy at the interface, implying that there is a negligible injection/extraction barrier for holes at the AE−ITO/P3HT interface. The shifts in the vacuum level and ionization potential are illustrated as curved lines that asymptote after ca. 20 nm. Similar to our results, a recent study reports that band bending on the order of 40 nm is observed for P3HT films on oxygen plasma-treated ITO.34 The extent of band bending observed for organic semiconductors is strongly dependent on the electronic structure of the as-cast polymer film and the work function of the substrate.9 The contact doping reaction of P3HT with high work function contacts, which produces oxidized P3HT species, is quantified by analyzing high-resolution S 2p spectra. Electrochemical methods are used to prepare a range of quantitatively doped P3HT films,18,35,36 which are used as standards that afford controllable concentrations of oxidized P3HT (Figure 2). A representative cyclic voltammogram (CV) of a ca. 15 nm

Figure 1. Thickness-dependent energetics of P3HT on acid-etched ITO (AE-ITO) via photoelectron spectroscopy. (a) XPS spectra of the C 1s core level. (b) UPS valence band spectra plotted with respect to the Fermi level. Magnified view of the low kinetic energy edge (LKE, left panel). Magnified view of the valence band states near the Fermi level (right panel). The thickness values are the same as seen in panel a. (c) Energetic position of the work function (top), ionization potential (middle), and C 1s core level (bottom) as a function of P3HT thickness. The lines are a guide to the eye. (d) Energy level diagram constructed from the UPS data, showing band bending at the AE-ITO/P3HT interface.

These ITO substrates are used for these initial thickness studies because of their convenience and the fact that the work function is significantly above the 4 eV threshold necessary to induce charge redistribution with P3HT.25 P3HT films with readily controlled thicknesses (nominally 1−19 nm) are spin-cast from P3HT−chlorobenzene solutions (0.3−5 mg mL−1). The incremental increase in the C 1s intensity with film thickness is reminiscent of layer-by-layer growth typically observed for pinhole-free molecular films deposited via vacuum evaporation (Figure 1a).15,30,31 At the same time, the In 3d5/2 line of the underlying ITO substrate is systematically attenuated with an increase in P3HT thickness, providing a means to estimate the film thickness. (See the Supporting Information (SI), Figure S1.) These initial XPS results imply that we have unprecedented control of P3HT film thickness. Atomic force microscopy (AFM) images of each of the P3HT films clearly show the underlying ITO surface morphology and do not display any discernible signs of aggregation or dewetting (SI Figure S2), indicating that all of the P3HT films studied here are conformal with the ITO surface. For comparison, PCBM films on ITO deposited by spin coating under similar conditions display clear aggregates and discontinuities, which are not observed for our conformal P3HT layers (SI Figure S2). In addition to the aforementioned intensity increase, the C 1s peak shifts ca. 0.7 eV to higher BE with increasing P3HT thickness (Figure 1a). Normalized valence band spectra for the same series of P3HT films show thickness-dependent shifts (ca. 0.7 eV) to lower kinetic energy for the low kinetic energy edge (LKE), demonstrating changes to the local vacuum level and

Figure 2. Quantitatively doped P3HT films as standards for XPS S 2p core level neutral and oxidized fitting protocol. (a) Representative CV (n = 50 mV s−1) of a P3HT film (ca. 15 nm) on ITO. (b) XPS spectra (black spheres) of the S 2p core level as a function of elelctrochemical doping potential. A neutral (blue) and oxidized (red) doublet is used to provide the fitting function (green). (c) The calculated doping percentage from integrated CV measurements (black squares) is compared with the relative oxidized fraction from the S 2p fitting protocol. (d) Direct comparison of the S 2p core level spectra for a ca. 0.6 nm P3HT film on MoOx (black circles) and of a ca. 15 nm film electrochemically doped at +0.60 V (red squares). 1305


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5.5 eV) and a P3HT film electrochemically doped at +0.6 V (Figure 2d). The spectral shapes are almost identical, indicating that the doping/oxidation percentage is nearly equal at ca. 20%. (See Figure 2c.) The confirmation of charge transfer between P3HT and a high work function electrical contact is most clear for ITO contacts modified with ca. 10 nm of a substoichiometric molybdenum oxide (MoOx) interlayer.38 The MoOx contact is unique in comparison with ITO in that the relative fraction of reduced and oxidized species can be determined by analyzing the Mo 3d spectral region (see later).15 S 2p spectra for a range of P3HT film thicknesses (ca. 0.6 to 20 nm) show quantifiable spectral features associated with oxidized P3HT (Figure 3a). S

thick P3HT film on ITO is shown in Figure 2a. The film is neutral at −0.4 V; by scanning the potential positively, the polymer film is systematically oxidized, where more positive potentials correspond to higher percentages of oxidized thiophene species. The extent of electrochemical oxidation can be determined by integrating the oxidative scan of the CV (SI Figure S3). After initial voltammetric characterization, P3HT films are doped to desired levels using potential steps (choronoamperometry); the film is stepped to the desired potential for 60 s and then rapidly removed from solution under potential control. (See the SI for full experimental details.) High-resolution S 2p spectra for a range of electrochemically doped P3HT thin films (ca. 15 nm), along with that of a neutral film, are shown in Figure 2b (black spheres). Qualitatively, the S 2p spectra broaden and shift to lower binding energy with an increase in oxidation potential. The shift to lower binding energy with increasing oxidation potential originates from increased doping density within the P3HT film; this shift is analogous to that seen for the thickness-dependent data in Figure 1, where thinner films contain a relatively higher fraction of oxidized P3HT near the contact interface. We note that similar shifts are observed for the C 1s core level for these electrochemically oxidized P3HT films. (See SI Figure S4.) Electrochemically oxidized35,36 and photo-oxidized37 P3HT films have provided similar spectral broadening of the S 2p line, which is indicative of the addition of a higher binding energy oxidized doublet to the already present neutral doublet. To quantify the extent of oxidative doping using the S 2p spectra of P3HT thin films, we have developed a rigorous fitting procedure that is based on neutral and quantitatively oxidized P3HT standards (Figure 2b). The S 2p spectrum for a nondoped, neutral film is fit with a single doublet (i.e., two peaks) with spin−orbit splitting of 1.17 eV, full width at halfmaximum (fwhm) of 0.735 eV, and a 3/2:1/2 peak area ratio of 2:1. S 2p spectra for the electrochemically oxidized P3HT films require the addition of an oxidized component (red, high BE) to the existing neutral doublet (blue, low BE). As determined from peak fitting of the neutral P3HT film, the spin−orbit peak splitting and fwhm are held constant for the oxidized sample. The additional oxidized double is shifted +0.635 eV relative to the neutral species and increases in magnitude with increasing doping potential. The percentage of oxidized thiophene species in P3HT films is determined by calculating the relative area of the oxidized doublet to the total area of the S 2p spectrum. The curve-fitting procedure for electrochemically doped P3HT films and thus all P3HT S 2p spectra, is validated by comparing the relative percentage of oxidized thiophene from the S 2p spectra to the doping percentage quantified by integrating the oxidative scan from CV measurements (Figure 2c). SI Figure S3 provides the complete details pertaining to the determination of the doping percentage from electrochemical measurements. The percentage of oxidized P3HT from integrated CV data (black squares) and S 2p fitting (red circles) is plotted with respect to the electrochemical doping potential. The strong correlation between the doping percentage obtained from XPS and electrochemical results confirms that the S 2p peak fitting procedure provides a means for quantifying the extent of doping in P3HT films using XPS measurements. The surprisingly high extent of P3HT contact doping can be elucidated when directly comparing the normalized S 2p spectra for a thin P3HT film (nominally 0.6 nm) on a high work function molybdenum oxide contact (Φ =

Figure 3. (a) XPS spectra of the S 2p core level (black spheres) as a function P3HT thickness on MoOx. The doublet is fit with a neutral (blue) and oxidized (red) component, providing the overall fit (green). While it is typical to anneal (140 °C, 5 min) the P3HT films, the bottom spectrum is for a 0.6 nm film that is not annealed (NA). (b) XPS spectra of the Mo 3d core level (black spheres) of a MoOx contact at 0° takeoff angle (bottom) and below a nonannealed (NA) 0.6 nm P3HT film at 0 (middle) and 60° (top). The doublet is fit with a Mo6+ (green) and reduced Mo5+ (blue) component.

2p spectra for the thinnest (0.6 nm) annealed and nonannealed P3HT films are identical, suggesting that formation of oxidized P3HT is not thermally activated, in contrast with characterization of P3HT films on the conducting polymer interlayer PEDOT:PSS, where thermal activation appeared to be necessary.17 Qualitatively, the spectra become narrower and shift to higher binding energy with increasing P3HT thickness. The shift in the peak energy of ca. 0.8 V is similar in magnitude and direction, as seen for P3HT films on AE-ITO, confirming, as expected, that similar band bending takes place on this high work function MoOx contact. Quantitatively, the P3HT film, assuming that the nominal 0.6 nm layer represents submonolayer coverage, is 19 ± 3% oxidized (i.e., on average, approximately one charge per every five thiophene units) at the interface. The next thickest P3HT layer of ca. 1.5 nm, which is equivalent to a monolayer thickness for edge-on orientations (ca. 2 nm),18 contains 17 ± 3% oxidized P3HT species (ca. one charge for every six thiophene monomers). The percentage of oxidized P3HT decreases exponentially with increasing P3HT thickness and is no longer observable for the ca. 20 nm thick P3HT layer. Similar S 2p spectra and oxidized percentages are observed for submonolayer P3HT layers on AE-ITO contacts. 1306


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the top ca. 10 nm of the organic semiconductor thin film, with the remaining ca. 5% (corresponding to 5λ) of the PES signal originating from a 10−15 nm window below the organic semiconductor/vacuum interface. By increasing the takeoff angle to 60°, we decrease the probe depth by half (i.e., > 95% of the collected PES signal comes from ca. 5 nm below the organic semiconductor/vacuum interface). With these angular-dependent probe depths in mind, we examine ca. 15 nm thick films of both P3HT and blends, where a takeoff angle of 0 and 60° allows probing of the contact/organic semiconductor interfacial region and the bulk of the film, respectively. Angle-resolved XPS measurements for ca. 15 nm thick P3HT and blend films on a variety of device-relevant contacts are presented in Figure 4. The ability of XPS to probe the buried interface and bulk in a single thin film can be readily seen when

(See SI Figure S5.) While doping of P3HT by a number of different substrates has been qualitatively observed via the evolution of oxidized optical absorption features,39 inferred from UPS,5,34 optical modeling,40 and electrical measurements,6 these results are the first to quantify the degree of contact doping using a set of known standards. Furthermore, the measurements herein suggest that the degree of interface doping for the first monolayer (i.e., on the order of one charge for every six thiophene units) is significantly more than has been previously reported (e.g., Gomez et al. suggest that polyelectrolyte interface modifiers dope P3HT at comparable interfaces to ca. one charge per every 2 × 104 thiophene units).6 XPS spectra of the Mo 3d core level for the substoichiometric MoOx substrate are analyzed to further elucidate the contact doping reaction (Figure 3b). The background-corrected Mo 3d spectra are deconvoluted by fitting with a pair of doublets corresponding to fully oxidized Mo6+ (green) and reduced Mo5+ (blue) species, which are shifted −1.20 eV to lower binding energy. The peak fitting parameters correspond to a 5/2:3/2 spin−orbit splitting of 3.10 eV, a fwhm of 1.20 eV, and a peak area ratio of 3:2. At a photoelectron takeoff angle of both 0 (i.e., normal to the substrate plane) and 60°, the bare MoOx substrate is composed of 18 ± 1% of reduced Mo5+ species. After spin coating an ultrathin P3HT layer, there is a small but noticeable increase in the fraction of reduced Mo5+ (22 ± 2%) at normal takeoff. We see a further enhancement of the reduced component when using a more interface sensitive takeoff angle of 60°, corresponding to a Mo5+ content of 26 ± 2%. The increase in the relative fraction of reduced Mo species under the thin P3HT film can be attributed to electron transfer (i.e., reduction) from neutral P3HT to form the oxidized interfacial layer, which can be written as a redox reaction P3HT + Mo6 + → P3HT+ + Mo5 +

(1)

These results pertaining to reduction of a molybdenum oxide contact beneath a donor semiconductor layer due to Fermilevel equilibration are consistent with previous reports.15,26 The presented thickness-dependent PES measurements demonstrate that a large fraction of oxidized P3HT is created upon interface formation with two high work function contacts (i.e., AE-ITO and MoOx). We extend these contact doping studies to P3HT:PC60BM blends (1:1 by wt %, subsequently referred to as “blends”) and a range of device-relevant contacts and interlayers, which include a low work function surface modifier, poly(ethylenimine ethoxylated) (PEIE, Φ = 3.3 eV);41 solvent-cleaned ITO (SC-ITO, Φ = 4.0 eV);29 solutionprocessable substoichiometric WOx nanocrystals (WOx NCs, Φ = 4.8 eV);42 the ubiquitous conductive polymer interlayer, PEDOT:PSS (Φ = 5.2 eV); and an additional solutionprocessable substoichiometric metal oxide interlayer, NiOx (Φ = 5.4 eV).43,44 The preparation of these substrates and active layers is described in the SI. For these device-relevant materials sets, we present angleresolved XPS measurements as a means to probe, in a single film, the buried contact/organic semiconductor interface region and the bulk region (i.e., away from the contact interface) to confirm the presence and confinement of oxidized P3HT near the interface. We take advantage of the angular dependence of the sampling depth of photoemission in solids to control the probe depth within the active layer thin films.45,46 Approximately 95% (corresponding to three times the inelastic mean free path, 3λ, which is defined as the escape depth) of the photoelectrons collected at a takeoff angle of 0° originate from

Figure 4. Angle-resolved XPS of ca. 15 nm P3HT and blend (1:1 P3HT/PC60BM) films on substrates with different Φ. (a) In 3d region under a P3HT layer where the substrate is just barely visible at 0° (red) and no longer visible at 60° (black). (b) XPS spectra of the S 2p core level at 60 (black) and 0° (red) and the difference spectrum (green). The arrows indicate the loss of neutral (blue) and gain of oxidized (red) species when switching from 60 to 0°. (c) Relative percentage of oxidized component from S 2p spectral fitting as a function of substrate work function. All substrates with a work function of 4.0 eV and higher show oxidized species for the pure P3HT and blend films at the electrode interface (0°), indicating charge transfer due to Fermi level pinning. Oxidized species are not observed within the bulk of the film (60°). 1307



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looking at AR-XPS spectra for a representative P3HT layer on AE-ITO (Figure 4a). Spectra of the In 3d core level from the underlying substrate are just barely visible at 0° and vanish at 60°. Similarly, area-normalized spectra of the S 2p core level for the same P3HT film on AE-ITO display subtle spectral differences that are elucidated by analyzing the difference spectrum (Figure 3b). When subtracting the 0° (interface sensitive) spectrum from the 60° (bulk sensitive) spectrum, we observe a decrease in the neutral peaks and an increase in the oxidized fraction, adding further confidence in the ability of angle-resolved XPS to discern depth-dependent chemical differences in these P3HT thin films. Compiled angle-resolved XPS results for both blends and pure P3HT films on the aforementioned array of device-relevant contacts are shown in Figure 4c, where we use our S 2p core-level fitting procedure to analyze the fraction of oxidized P3HT at 0 and 60°. We observe oxidized P3HT in both neat P3HT and blend films near the contact/organic semiconductor interface (0°) of all substrates with a work function greater than ca. 4 eV. It is especially notable that low work function, PEIE-modified contacts do not show evidence of oxidative doping. All bulk sensitive (60°) spectra are devoid of oxidized species, implying that the majority of the doping, when it occurs, is confined near the contact interface. We note that angle-resolved XPS data is a qualitative indicator of oxidized P3HT species near the interface because the measured spectra are a weighted average of all thiophene photoelectrons emitted from the entire P3HT sampling depth. Provided the active layer film thickness is on the order of the photoelectron escape depth, angle-resolved XPS provides the means to evaluate the interfacial and bulk regions of a single film to ascertain if more rigorous thicknessdependent measurements are necessary for a given system. In conclusion, we have shown that Fermi-level equilibration in the pinning regime is responsible for the formation of an oxidized P3HT gradient (i.e., band bending) in P3HT thin films on device-relevant high work function contacts. Highresolution photoelectron spectroscopy results indicate that band bending on AE-ITO (Φ = 4.5 eV) and MoOx (Φ = 5.5 eV) contacts extends up to ca. 20 nm away from the contact/ P3HT interface. Electrochemically doped polymers provide a unique methodology for quantifying the extent of contact doping using XPS measurements, where as many as 20% of the polymer subunits can be doped/oxidized at the near-surface region. The charge exchange picture is completed by showing the complementary fraction of reduced species in contact materials that have well-defined spectral features associated with a change in oxidation state (e.g., molybdenum oxide). Finally, differences in chemical composition between the buried contact/P3HT interface and “bulk” of a thin film can be investigated using angle-resolved XPS, providing a facile way to determine if more stringent thickness-dependent measurements are necessary to describe a specific substrate/semiconductor interaction. These results ultimately provide a means to quantify the extent of interface charge transfer or contact doping and show that, contrary to some calculated results, the near-surface region can be highly doped, which affects efficiencies of charge injection/extraction and surface recombination. Current and future studies are focused on determining the extent of charge transfer at the interface between device-relevant contacts and high-performance donor polymer materials with higher ionization potentials (e.g., PCDTBT and PTB7) and the influence of contact doping on interfacial molecular structure.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental procedures, In 3d attenuation with P3HT thickness, AFM images, electrochemical analysis of P3HT films, and a detailed protocol for S 2p core level fitting. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*R.C.S.: E-mail: [email protected]. *N.R.A.: E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Research 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. T.S. is supported by the German Science Foundation (DFG) through SPP1355, grant number: BR4031/ 1-2.

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REFERENCES

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