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J. Phys. Chem. C 2007, 111, 1378-1384
Photoelectron Spectroscopic Study of the Electronic Band Structure of Polyfluorene and Fluorene-Arylamine Copolymers at Interfaces Jaehyung Hwang,† Eung-Gun Kim,‡ Jie Liu,§ Jean-Luc Bre´ das,‡ Anil Duggal,§ and Antoine Kahn*,† Department of Electrical Engineering, Princeton UniVersity, Princeton, New Jersey 08544; School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332; and General Electric Global Research Center, Niskayuna, New York 12309 ReceiVed: October 25, 2006
The occupied and unoccupied states of poly(9,9′-dioctylfluorene) (F8) and poly(9,9′-dioctylfluorene-co-bisN,N′-(4-butylphenyl)diphenylamine) (TFB) are investigated using ultraviolet photoelectron and inverse photoemission spectroscopies, cyclic voltammetry, and density functional theory calculations. Hole injection barriers are determined for interfaces between substrates with work function ranging from 4.3 to 5.1 eV and these two polymers as well as poly(9,9′-dioctylfluorene-co-bis-N,N′-(4-butylphenyl)-bis-N,N′-phenyl-1,4phenylenediamine) (PFB). Vacuum level alignment with flat bands away from the interface is found when the interface hole barrier is 0.6 eV or larger. Band bending away from the Fermi level occurs when the hole barrier is smaller than 0.4 eV. This is due to the accumulation of excess interface charges on the polymer when the barrier is small. The resulting field shifts the polymer levels to limit charge penetration in the bulk of the film.
1. Introduction The performance of polymers in light-emitting devices has considerably improved since the first demonstration of practical devices based on poly(p-phenylene vinylene) (PPV).1 The ease of processing and the scalability for large-area applications are key advantages of the polymeric materials. Yet, some fundamental properties of these materials and their interfaces in devices are difficult to determine and are still under intensive investigation.2 Of particular importance for charge injection and transport through the thin films in devices are the energy and density of electron and hole transport states and their relative positions across electrode-organic interfaces.3,4 The determination of the interface energetics is more difficult for polymer films than for small organic molecular films because of the very nature of the materials, which consist of long chains along which carriers can be delocalized and present varying degrees of interchain interactions. The electronic structure of polymer films along the polymer chains is usually more bandlike than that of small molecule films. This is due to the relatively long conjugation along the chain backbone, as compared to the highly localized nature of states in small π-conjugated molecules. Yet, charge carriers are not as delocalized as in inorganic materials. The film morphology, which is difficult to control, greatly affects the electronic and optical properties of these materials.5,6 The complex morphology of the polymer films is therefore a source of increased difficulties in the interpretation of information from spectroscopic techniques used in the investigation of interfaces, although direct and inverse photoemission spectroscopies provide the most direct experimental path to the observation of occupied and * Corresponding author. E-mail:
[email protected]; phone +1-609258-4642. † Princeton University. ‡ Georgia Institute of Technology. § General Electric.
Figure 1. Chemical structures of (a) F8, (b) TFB, and (c) PFB.
unoccupied electronic states in the bulk and at interfaces. Considerable insight into issues such as the position of molecular levels across interfaces and the resulting energy barriers for injection and transport is therefore obtained from comparison with the theoretically calculated density of states. In this report, three polymeric systems, poly(9,9′-dioctylfluorene) (F8, also known as PFO) and two fluorene-arylamine copolymers, poly(9,9′-dioctylfluorene-co-bis-N,N′-(4-butylphenyl)diphenylamine) (TFB) and poly(9,9′-dioctylfluorene-co-bisN,N′-(4-butylphenyl)-bis-N,N′-phenyl-1,4-phenylenediamine) (PFB), are chosen for investigation (Figure 1). F8 has been extensively studied2,5,7 and is a candidate for commercial applications. TFB has recently been shown to improve the efficiency of polymer light emitting devices.8 We investigate these materials with ultraviolet photoelectron spectroscopy
10.1021/jp067004w CCC: $37.00 © 2007 American Chemical Society Published on Web 12/16/2006
Polyfluorene and Fluorene-Arylamine Copolymers (UPS) and inverse photoemission spectroscopy (IPES). Reports comparing UPS to calculated spectra have been published for F8,2 but unoccupied levels have not been investigated, in spite of their importance for optical and transport properties. Our UPS and IPES data for F8 and TFB are compared with theoretical calculations performed at the density functional theory (DFT) level. Very good agreement is obtained between measured and calculated densities of states, which provides a solid basis for further studies of the materials. Interfaces of the three polymers with various substrates are investigated. Vacuum level alignment holds at most of these interfaces, and band bending is observed only with high work function substrates (i.e., low interface hole barriers). The excess hole accumulated at the interface due to the low hole barrier accounts for the band bending. 2. Experimental Section F8 and TFB were obtained from Dow Chemical and PFB was obtained from American Dye Source. The materials were stored under nitrogen atmosphere without further treatment. The polymers were weighed in air and introduced into a nitrogenpurged glove box where they were dissolved in anhydrous p-xylene (Sigma-Aldrich) at 70 °C for 1 h. Following complete dissolution, the solution was dispensed on the substrates through a 0.45 µm PTFE filter and spun at 3000 RPM for 40 s. The polymer film thickness vs solution concentration was calibrated in independent experiments with atomic force microscopy (AFM). Each film was then annealed in the glove box at 130 °C for 10 min to drive out any residual solvent and transferred to the ultrahigh vacuum (UHV) chamber for measurements. For UPS measurements, the films were transferred in a nitrogen vessel compatible with the load-lock of the analysis system, allowing transfer without ambient exposure. For IPES measurements, the samples were briefly exposed to air (less than 3 min). The comparison between UPS spectra from air-exposed and unexposed films indicated no significant difference in their energy levels. The substrates were (polycrystalline) Au, indium tin oxide (ITO) (Applied Films), and poly(3,4-ethylenedioxythiophene)/ poly(styrene sulfonate) (PEDOT‚PSS) (Baytron P VP CH 8000, OLED grade, H.C. Starck). Au was cleaned in ultrasonic baths of acetone and 2-propanol. ITO was first scrubbed using a soft brush with Alconox and treated with the same sequence of solvents as Au. UV-ozone cleaning was used when a higher work function was needed for both substrates. Finally, PEDOT‚ PSS was spun on UV-treated ITO and heated to 180 °C for 1 h in nitrogen. The thickness of the PEDOT‚PSS film was 40 nm. Measurements of occupied and unoccupied electronic states of the polymer films were performed with UPS and IPES, respectively. UPS was carried out with the He I (21.22 eV) and He II (40.8 eV) photon lines from a He-discharge lamp and a double-pass cylindrical mirror analyzer for electron collection. The ionization energy (IE) of each film was determined as the energy difference between the vacuum level (Evac) and the leading edge of the highest occupied molecular orbital (HOMO), following a procedure described elsewhere.9 The low-energy photoemission cutoff was measured for each substrate and film to determine Evac and the work function. IPES was carried out in the isochromat mode with an electron current density of 0.5-1 µA/cm2 impinging on the surface, and photons were collected with a KCl-coated channeltron through a SrF2 window.10 The electron affinity (EA) for each film was determined as the energy difference between Evac and the leading edge of the lowest unoccupied molecular orbital (LUMO). The
J. Phys. Chem. C, Vol. 111, No. 3, 2007 1379 IPES data were analyzed after removing a quadratic polynomial background.11 No background correction was performed for the UPS data. The spectral resolution was 150 meV for UPS and 450 meV for IPES, as determined from the Fermi step measured on a clean polycrystalline Au substrate. Polymer films with thickness ranging from 30 to 160 Å were measured with UPS. IPES was done on 160 Å films. Care was taken to ensure that no charging occurred in either experiment. Cyclic-voltammetry (CV) measurements were also made using the CH Instruments CHI660 EC Workstation. Thin films of each polymer were spun on a Pt-disk (0.03 cm2) working electrode (CH Instruments CHI102). A three-electrode electrochemical cell was employed with a Pt auxiliary electrode and a 0.01 M Ag/AgNO3 nonaqueous reference electrode. The electrolyte solution used for electrochemical measurements was prepared by dissolving 0.1 M dry tetrabutylammonium tetrafluoborate (Assay 98% min., GFS Chemicals) in acetonitrile (HPLC grade, J.T. Baker). The reference potential was calibrated with a 1.3 mM ferrocene standard with a redox potential of 4.8 eV with respect to vacuum. All voltammograms were recorded at a scan rate of 1 V/s. HOMO and LUMO energy levels were determined from the onset of the first oxidation and reduction peak of the polymer, respectively. In particular, the HOMO and LUMO energy levels were defined as the intersection of the linear extrapolation of the onset slope of the respective peak and the baseline. 3. Theoretical Methodology F8 and TFB were modeled as an infinite chain consisting of four monomeric units in the supercell under periodic boundary conditions. The periodic boundary conditions are effectively onedimensional along the chain axis because the box edge lengths along the other two directions (22 × 23 Å for F8 and 30 × 18 Å for TFB) are large enough that interactions among image chains are minimal. The use of multiple monomeric units in the unit cell was necessary in order to accommodate the nonplanar structure of the polymer. At the number of four monomeric units, the geometric parameters, especially the torsion angles, near the connecting rotatable bonds were found to remain almost unchanged for both polymers with respect to those of the respective oligomers of five units. The torsion angles were 139.7/141.4/-137.8/-135.5° for F8 and 145.8/147.5/ 138.1/132.7° for TFB. The end-to-end distance (the supercell edge length along the chain backbone) was 33.56 Å for F8 and 30.97 Å for TFB. All alkyl side groups, a dioctyl group on the fluorene unit and a butyl group on the triphenylamine unit, were taken into account in the calculations, which allowed quantitative comparison with experimental photoelectron spectra over a large binding energy window covering both the π (π*) and deeplying σ (σ*) states. The conformations of the alkyl groups, which would hardly affect the electronic structure, were not explored but instead were allowed to relax, starting from all-trans conformations (two CsC bonds flanking the methylene bridge of the fluorene unit were initially set to gauche+/gauche+ to avoid excessive steric hindrance to start with). DFT calculations were carried out within the generalized gradient approximation by using the BLYP exchange-correlation functionals. Norm-conserving numerical pseudopotentials generated with the procedure of Troullier and Martins12 were used for C and N, and a local analytic pseudopotential for H, with a plane-wave energy cutoff of 70 Ry. An efficient linear-scaling Newton-Raphson method (L-BFGS), in combination with an adaptive wavefunction convergence scheme, was used for
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Figure 2. Combined UPS/IPES spectra of 160 Å-thick films of (a) F8 and (b) TFB spun on PEDOT‚PSS. The onset of photoemission, which yields the vacuum level (Evac), is shown on the left of each panel. The Fermi level (EF) is the reference energy.
geometry optimization.13 The wavefunction optimization was carried out using a direct inversion in the iterative subspace (DIIS) method.14 The use of efficient geometry and wavefunction optimization schemes was critical because of the very large system sizes (276 and 226 atoms for F8 and TFB, respectively). The canonical molecular orbitals were calculated subsequently by using an iterative Lanczos diagonalization method.15 All the calculations reported here were carried out using the CPMD code unless specified otherwise.16 In order to simulate UPS and IPES spectra, we used a standard procedure:17,18 the calculated density of states was first convolved with a Gaussian function of fwhm (full width at halfmaximum) ) 0.55 eV and the energy-level scale was then expanded by a factor of 1.2 before a shift along the energy axis was applied to the whole spectrum to match against experimental peaks. Studies on various systems have shown that this simulation procedure effectively allows solid-state polarization to be taken into account.17-20 An expansion of the energy levels was found necessary. Separate calculations on a fluorene oligomer with the BLYP and B3LYP functionals using an atomic orbital 6-31G(d, p) basis set in the Gaussian program21 indicated that the need for expansion, which does not affect the overall spectral shape, becomes less significant when the exact Hartree-Fock exchange is included.22 4. Results and Discussion The combined UPS and IPES spectra from 160 Å thick F8 and TFB films on PEDOT‚PSS are shown in Figure 2. The work
Hwang et al.
Figure 3. Cyclic voltammograms of F8 (b) and TFB (O) on the Pt plate. Both oxidation (HOMO) (a) and reduction (LUMO) (b) cycles are shown.
function of the PEDOT‚PSS substrates, measured on a control sample prepared at the same time and under the same conditions as the substrates used for the polymer films, is 5.15 ( 0.05 eV. The IEs of F8 and TFB are 5.75 ( 0.05 eV and 5.50 ( 0.05 eV, and the corresponding EAs are 2.50 ( 0.2 eV and 2.45 ( 0.2 eV, respectively. Evac is located at 5.15 and 4.8 eV above EF for F8 and TFB, respectively. The edge-to-edge gap, measured between the onsets of occupied and unoccupied states () IE - EA), is 3.3 ( 0.25 eV for F8 and 3.05 ( 0.25 eV for TFB. The CV measurements on F8 yield HOMO and LUMO positions at 5.7 and 2.1 eV (Figure 3) below Evac, resulting in a 3.6 eV gap. Similar measurements on TFB give HOMO and LUMO at 5.3 and 1.9 eV, respectively, with a gap of 3.4 eV. The spectroscopic and electrochemical measurements are therefore in very good agreement, although this accord might appear fortuitous, considering the drastically different environments in which the measurements are made.23 The calculated densities of states and simulated spectra of F8 and TFB are compared to measured UPS and IPES spectra in Figures 4 and 5, respectively; the agreement is excellent in both cases. The expanded views of the gap area ((b) panels in the figures) indicate the positions of the calculated occupied and unoccupied molecular levels with respect to EF, and the energy gaps (peak-to-peak and edge-to-edge). The convolution of the calculated density of states with a Gaussian function of fwhm ) 0.55 eV is validated by the excellent agreement between theory and experiment on the overall spectra, in particular, near the edges of the occupied and unoccupied states.
Polyfluorene and Fluorene-Arylamine Copolymers
Figure 4. Comparison between measured DOS (black line) and calculated DOS (blue line) of F8: (a) broad spectra of occupied and unoccupied states and (b) an enlarged view of the energy region close to EF (HOMO and LUMO).
The good agreement between simulated and experimental spectra, presented in Figures 4 and 5, allows a precise determination of the relative positions of the HOMO and LUMO levels, and of the energy gap between them. This energy gap is known as the transport gap Et, or single-particle gap, of the material (although some corrections need to be applied;10,11 see below). This transport gap represents the energy difference between a single “free” electron (or negative polaron) and a single “free” hole (or positive polaron), separated in space and uncorrelated, moving through the polymer.6,11 It should also be equal to the sum of the electron and hole injection barriers from a given electrode. Note that, with some corrections related to surface vs bulk polarization and molecular (or chain) relaxation energy, the separation between the LUMO and HOMO determined by IPES and UPS is precisely this transport gap. This gap is fundamentally different from the optical absorption gap, Eopt, which represents the energy necessary to excite an electron on a polymer chain and create a bound electron-hole pair, i.e., an exciton. The energy difference between Et and Eopt represents the energy necessary to break the exciton to create a free electron (or negative polaron) and a free hole (or positive polaron), i.e., the exciton binding energy EB. Using the procedure developed for molecular films,10,11 we find here that Et, as adjusted for
J. Phys. Chem. C, Vol. 111, No. 3, 2007 1381
Figure 5. Same as Figure 4, for TFB.
polarization effects, is equal to 3.5 eV for F8 and 3.2 eV for TFB. Given Eopt of ∼3.1 eV and ∼3.0 eV24,25 for these materials, the estimated EB is 0.4 and 0.2 eV for F8 and TFB, respectively. These numbers should be considered as estimates with fairly large error bars of 0.2-0.3 eV, but give the sense that exciton binding energies in these polymer films, while significantly smaller than in small molecular systems,11 are nonetheless significant. Although F8 and TFB have different IEs and slightly different bandgaps, UPS measurements at the surface of 160 Å films show that the energy difference between EF and HOMO is the same for both materials (∼0.65 eV). If one assumes that the energy levels are flat across the film from the free surface to the interface with the substrate, this energy difference represents the hole injection barrier at the electrode/polymer interface. If, on the other hand, band bending and/or interface dipole formation occurs at the interface with the electrode, the energy difference measured at the free surface of the film must be corrected to obtain the hole injection barrier. For F8, the assumption of flat bands holds, as the hole injection barrier (measured at the surface of the film) is nearly identical to the difference between the IE of the polymer (5.75 eV) and the work function of the PEDOT‚PSS substrate (5.15 eV), which indicates an interface electronic structure defined by vacuum level alignment between the polymer film and substrate. On
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Figure 6. AFM micrograph of (a) 80 Å TFB on PEDOT‚PSS and (b) 30 Å TFB on PEDOT‚PSS. In part b, the PEDOT‚PSS substrate can be seen through pin holes in the TFB layer, with a step size of ∼30 Å.
the other hand, the energetics of the TFB/PEDOT‚PSS interface appears to be somewhat different. If vacuum level alignment did occur at the interface, the hole injection barrier would be the difference between the IE of TFB (5.5 eV) and the work function of the substrate (5.15 eV), equal to 0.35 eV. Yet, an apparent hole barrier of 0.65 eV is measured at the free surface of the film, which leads to a value of the work function of the polymer film (i.e., Evac - EF) ∼0.3 eV smaller (4.8 eV) than that of the PEDOT‚PSS substrate. The difference must therefore be accounted for by the formation of an interface dipole barrier or by band bending in the polymer near the interface, or both. Unlike small organic molecules vacuum-deposited on clean metal surfaces, which can form interfaces with a dipole barrier of up to 1 eV,26 polymer films on metals are known to form interfaces that closely follow vacuum level alignment.27 This can be rationalized by considering that polymer films are usually spun from solution in a nitrogen or ambient environment on a substrate surface that is generally contaminated with hydrocarbons and/or oxides. When organic molecules are vacuumdeposited on an ambient-exposed metal substrate, the interface dipole barrier decreases significantly compared to that obtained on the same but atomically clean metal, and the interface energetics approach the Schottky-Mott limit for this semiconductor/metal interface.28 Similarly, the unavoidable contamination layer between the polymer film and the metal surface reduces the polymer-substrate interaction. Therefore, it is likely that the discrepancy between the work function of the PEDOT‚ PSS substrate and that of the polymer film is due to band bending near the TFB/PEDOT‚PSS interface, rather than to an interface dipole barrier. To further investigate this point, UPS measurements were performed on ultrathin films prepared by spinning diluted polymer solutions. The thickness of these films, estimated from the concentration and spinning rates, was confirmed by AFM. The thinnest films (∼30 Å) did not exhibit sharp steps and showed poor coverage of the substrate (Figure 6). The height of the patches in AFM images was taken as the film thickness. Thicker films (40 Å, 80 Å and 160 Å) were more homogeneous and AFM provided a more reliable estimation of the thickness. No gap states or additional interface-related features are detected via UPS on films with decreasing thickness, suggesting
Figure 7. UPS spectra of films with varying thickness spun on PEDOT‚PSS: (a) F8 and (b) TFB. Enlarged views of the HOMOs are shown in the inset.
minimal chemical interactions between the polymer and the substrate (Figure 7). A negligible shift of the HOMO level toward the Fermi level is observed for F8 with decreasing thickness, but the shift is consistently larger than 0.1 eV in the case of TFB. The extrapolation of an exponential fit to the data (Figure 8) intersects the zero-thickness axis, i.e., the interface with the PEDOT‚PSS substrate, around 0.35 eV on the EF HOMO scale. This is close to the expected 0.3 eV for vacuum level alignment between the polymer and substrate (given IE(TFB) ) 5.5 eV and the work function of PEDOT‚PSS ) 5.15 eV). Similar experiments with ultrathin TFB films on the lower work function substrates, i.e., Au (4.70 eV) and ITO (4.40 eV), are summarized in Figure 9. Like F8 on PEDOT‚PSS, TFB on Au and on ITO exhibits negligible band bending. Interfaces between PFB and the same substrates were also investigated. IE(PFB) equals 5.35 eV, which is the smallest of the three polymers. On relatively high work function substrates (i.e., PEDOT‚PSS and ozone-treated Au), EF - HOMO is
Polyfluorene and Fluorene-Arylamine Copolymers
Figure 8. Energy position of the polymer HOMO level below EF as a function of film thickness spun on PEDOT‚PSS for F8 (b) and TFB (O). Dashed (TFB) and dot-dashed (F8) curves are exponential fits to the data.
Figure 9. Energy diagrams of (a) F8/PEDOT‚PSS, (b) TFB/PEDOT‚ PSS, (c) TFB/Au, and (d) TFB/ITO interfaces.
around ∼0.45 eV close to the interface and increases to 0.600.65 eV as the thickness of the film increases (Figures 10a and 10b), in a way that is very similar to the case of TFB on PEDOT‚PSS. On lower work function substrates, the PFB films exhibit near-flat band conditions (Figures 10c and 10d). The interface dipole barrier is negligible (
