Metal-to-Acceptor Charge Transfer through a Molecular Spacer Layer

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Metal-to-Acceptor Charge Transfer through a Molecular Spacer Layer Patrick Amsalem,*,† Jens Niederhausen,† Johannes Frisch,† Andreas Wilke,† Benjamin Br€oker,† Antje Vollmer,‡ Ralph Rieger,§ Klaus M€ullen,§ J€urgen P. Rabe,† and Norbert Koch† †

Humboldt-Universit€at zu Berlin, Institut f€ur Physik, Newtonstr. 15, 12489 Berlin, Germany Helmholtz-Zentrum Berlin-BESSY II, Albert-Einstein-Str. 15, 12489 Berlin, Germany § Max Planck Institut f€ur Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany ‡

ABSTRACT: We investigate how the thermodynamic equilibrium is reached when a strong electron acceptor molecule (hexaaza-triphenylene-hexacarbonitrile, HATCN) is deposited on Ag(111) precovered with a physisorbed tris(8-hydroxyquinolinato)aluminum (Alq3) monolayer. Photoemission measurements reveal that equilibrium is achieved via a charge transfer from the metal to the HATCN layer through the Alq3 monolayer. The work function increase observed upon HATCN adsorption is explained by the formation of an interface dipole resulting from the metal-to-acceptor charge transfer and an electric field induced reorientation of the Alq3 molecule dipoles within the interlayer.

1. INTRODUCTION The understanding of physical and chemical processes at organic
organic (O
O) or metal
organic (M
O) heterojunctions is important for knowledge-based improvements of organic (opto)electronic device efficiencies. The energy level alignment at such interfaces critically determines the performance of devices.1
5 However, the fundamental mechanisms that determine the energy level alignment and the way thermodynamic equilibrium is established in these systems have not yet received a clear answer.6
10 For instance, predicting the position of the energy levels assuming vacuum level alignment fails in many cases.2,11,12 Alternative models involving integer charge transfer (ICT),7 induced density of interface states (IDIS),6 or polarization effects8 try to capture the complex phenomena at interfaces and have recently received growing interest. The ICT model is especially designed for weakly interacting systems, such as inert electrode
organic and O
O interfaces. It predicts integer charge transfer through tunneling between materials at interfaces and has the merit to provide a simple explanation for the formation of the observed interface dipoles. While the proposed theories can explain some of the observed behavior, the current level of understanding still does not allow explaining all experimental observations reported in the literature consistently. In this work, we study how thermodynamic equilibrium is reached in an O
O system that starts out in nonequilibrium. It consists of the strong electron acceptor hexaaza-triphenylenehexacarbonitrile (HATCN) deposited on top of a Ag(111) surface that is precovered with a physisorbed tris(8-hydroxyquinolinato)aluminum (Alq3) layer (chemical structure of both molecules shown in the inset Figure 1). In this case, the electron r 2011 American Chemical Society

acceptor has an electron affinity (EA) higher than the work function (WF) of the Alq3-covered metal substrate. Consequently, the lowest unoccupied molecular orbital (LUMO) of HATCN would be positioned below the occupied states of the substrate if vacuum level alignment was assumed. We find that equilibrium is established via electron transfer to HATCN and propose that these charges, (partly) filling the HATCN LUMO, originate from the metal substrate. Furthermore, the WF change of 0.75 eV observed upon HATCN adsorption is attributed to the combination of two effects. First, there are dipoles formed by the charge transfer between the electrode and HATCN which increases the WF. This, in turn, produces an electric field which induces a reorientation of the polar Alq3 molecules and counterbalances the WF increase. These findings contribute to the phenomenological understanding of the formation of O
O heterojunctions grown on a metal. Specifically, it helps to clarify the role of the electrode which is shown to provide electrons to molecular overlayers located above the first monolayer, to establish the thermodynamic equilibrium.

2. EXPERIMENTAL SECTION Ag(111) surfaces were prepared by repeated Ar-ion sputtering/annealing (450 °C) cycles. Alq3 (Aldrich) and HATCN (synthesized at the MPIP, Mainz) were evaporated from a resistively heated pinhole source (base pressure: 1  10
9 mbar). Film thickness was controlled with a quartz crystal microbalance. The secondary electron cutoff (SECO; for Received: June 7, 2011 Revised: July 26, 2011 Published: July 28, 2011 17503

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The Journal of Physical Chemistry C

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Figure 2. (a) Comparison between Alq3 core levels before (blue lines) and after (red lines) HATCN adsorption showing a rigid shift of Alq3 core levels. Note that the increase in intensity of the C1s peak at
2 eV relative to BE after HATCN deposition corresponds to the C1s component of the HATCN molecules as shown in (b). (b) C1s peak measured for 0.2 nm HATCN/1 ML Alq3 /Ag(111) (red circle). The C1s component of Alq3 (blue dotted line) was subtracted to obtain the C1s contribution due to HATCN (black circles).

Figure 1. (a) Secondary electron cutoff, (b) top valence band, and (c) large range valence spectra of, from bottom to top, Ag(111), 1.2 nm (1 ML) Alq3/Ag(111), and 0.2 nm HATCN/1 ML Alq3/Ag(111). The thin vertical lines in (b) show the Alq3 HOMO onset and in (c) denote shifts and changes of the spectrum induced upon HATCN deposition (see text). Inset: Chemical structure of Alq3 and HATCN.

determination of sample work function and ionization energy) was measured with the samples at
10 V bias to clear the analyzer WF. One set of the photoemission experiments reported here was done in an analysis chamber (base pressure: 3  10
10 mbar) connected to the preparation chamber (base pressure: 1  10
9 mbar). X-ray photoelectron spectroscopy (XPS) and UV photoelectron spectroscopy (UPS) measurements were performed using Mg KR and HeI radiation and a SPECS Phoibos 100 hemispherical energy analyzer (1.2 eV and 120 meV energy resolution in XPS and UPS, respectively). Contributions of HeIβ and HeIγ satellites to the UPS spectra were removed with standard procedures from the original spectra, except from that shown for pristine Ag(111). Their intensity (1.8% and 0.4% of the intensity of the HeIR line, respectively) was determined via the replica of the metal Fermi edge. The other set of measurements, where an excitation energy of 35 eV was used, were performed at the end station SurICat (beamline PM4) at the synchrotron light source BESSY II (Berlin, Germany).13 There, the ultrahigh vacuum system consists of interconnected sample preparation (base pressure: 1  10
8 mbar) and analysis (base pressure: 1  10
10 mbar) chambers. The spectra were collected with a hemispherical electron energy analyzer (Scienta SES 100) set to an energy resolution of 120 meV.

3. RESULTS AND DISCUSSION Figure 1 (a), (b), and (c) depicts the SECO and valence region spectra, recorded in normal emission, of Ag(111), 1 monolayer (ML, corresponding to 1.2 nm mass thickness) Alq3/Ag(111), and 0.2 nm HATCN/1 ML Alq3/Ag(111) (sequentially deposited). The deposition of 1 ML Alq3/Ag(111) was calibrated by incremental deposition of molecules until the Ag surface state completely vanished. The HATCN mass thickness corresponds to ca. one layer of flat-lying molecules.14 The sample WF, which is 4.5 eV for the pristine Ag(111), decreases by 0.9 eV upon deposition of 1 ML Alq3 and amounts to 3.6 eV. This decrease is mainly due to the push-back effect15
17 occurring at weakly interacting organic/metal interfaces and compares very well with the WF decrease observed for other physisorbed organic layers on Ag(111).2,10,18,19 The overall valence band (VB) spectrum of 1 ML Alq3 is similar to those previously reported and in addition, shows that the Ag 4d contribution, between 4 and 7 eV binding energy (BE), is largely attenuated.20 The low BE onset of emission from the HOMO (highest occupied molecular orbital) of Alq3 is at 1.80 eV BE, in good agreement with the literature.17 The deduced Alq3 monolayer ionization energy (IE) value of 5.4 eV is 0.3 eV smaller than the multilayer IE because the photoholes are more efficiently screened by the metal electrons.17 Note that the frontier peak of emission from Alq3, which is centered at 2.4 eV and has a full width of ca. 1 eV, is in fact constituted of the HOMO, HOMO-1, and HOMO-2 that are localized on each ligand of the molecule;20,21 for simplicity, we will refer to this emission as the HOMO peak of Alq3. It is also important to stress that no molecule-derived gap states are observed, which evidences that Alq3 interacts weakly with the silver surface. Upon deposition of 0.2 nm HATCN on top of 1 ML Alq3/ Ag(111), the WF increases by 0.75 eV. This WF change is accompanied by a shift of the Alq3 HOMO onset from 1.80 eV BE to 1.5 eV BE and a narrowing of the peak by 0.1 eV. This 0.3 eV shift toward lower BE is also observed for the core levels specific to Alq3 [C1s, O1s, and Al2p, see Figure 2(a); N1s core levels of Alq3 and HATCN cannot be resolved unequivocally]. In the large energy range (VB spectra presented in Figure 1(c)) a 17504

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Figure 3. From top to bottom: top valence band spectra of 0.3 nm HATCN/1 ML Alq3 /Ag(111), ∼0.2 nm HATCN/Ag(111), and 1.2 nm HATCN/Ag(111) (lying and standing HATCN on Ag(111), respectively; see text) taken with a photon energy of 35 eV using synchrotron radiation.

Figure 4. (a) Normalized Ag3d core levels measured on 1 ML Alq3/ Ag(111) (full blue line) and 0.2 nm HATCN/1 ML Alq3/Ag(111) (red circles). (b) Fit (full black line) of the O1s spectrum measured on 1 ML Alq3/Ag(111) (blue circles). (c) Fit (full black line) of the O1s spectrum measured on 0.2 nm HATCN/1 ML Alq3/Ag(111) (red circles). The measurements were performed on the same sample as that presented in Figure 1.

comparison of the spectra before and after HATCN deposition shows a rigid shift of the Alq3 spectrum similar to the one observed for the Alq3 HOMO and core levels. Let us note that, due to the small amount of deposited HATCN, the HATCN contributions are relatively weak and only lead to minor modifications of the shape of the spectrum (for instance, around 8 and 6 eV BE). This suggests a change in the electrostatic potential inside the Alq3 layer, as a strong chemical interaction can be ruled out on general grounds because the Alq3 IE is at least 1 eV higher than the HATCN EA. Turning back to the valence region near the Fermi level, an ca. 1 eV broad occupied density of states (DOS), with a peak maximum at ∼0.6 eV BE, clearly emerges upon HATCN deposition. Because of the high IE of HATCN (∼9.3 eV),14 this DOS cannot be attributed to the HATCN HOMO but indicates a (partial) filling of the LUMO of neutral HATCN whose EA is ca. 4
4.5 eV.22 Close inspection of this peak yields no evidence that this state is intersected by the Fermi energy (EF); i.e., HATCN adsorbed on 1 ML Alq3/Ag(111) is not metallic. In contrast, the corresponding gap states of HATCN deposited directly on a bare Ag(111) surface do intersect EF, indicating a metallic molecular adsorbate (see Figure 3). This was

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Figure 5. Valence band spectra of 4.8 nm HATCN/Ag(111) (a), 5 nm Alq3/Ag(111) (b), and 10 nm HATCN:Alq3 blend film (ratio 1:2) (c). The spectra of the prisitine materials in (a) and (b) are shifted to match with the features observed in the blend film. The spectra were taken with a photon energy of 35 eV using synchrotron radiation.

also reported earlier for both HATCN/Ag(111) monolayer phases (flat lying and vertically inclined) and explained by a strong hybridization of the HATCN LUMO and the continuum of metal states.14 Consequently, HATCN molecules interact differently with the Alq3/Ag(111) surface compared to a pristine Ag(111) surface. Because of the mismatch between the HATCN EA and Alq3 IE, as HATCN molecules are deposited on the (inert) preadsorbed Alq3 layer, it is thus reasonable to presume that electron transfer into the HATCN LUMO takes place from the silver substrate through the Alq3 spacer layer. However, interdiffusion of two sequentially deposited organic materials on metal substrates has recently been reported.23 Another issue to be considered is that the order of a metal-adsorbed molecular layer influences the electronic properties, as demonstrated for the case of PTCDA (perylene-3,4,9,10-tetracarboxylic dianhydride) submonolayers formed on Ag(111).24 For our present system, we must therefore evidence that HATCN is indeed adsorbed on the surface of Alq3 and that the particular line-shape and energy position of the LUMO-derived peak observed here are not just a consequence of HATCN diffused through Alq3 and chemisorbed in a disordered manner on the silver substrate. To establish that HATCN is indeed Alq3-surface adsorbed, we performed additional core-level photoemission measurements on our samples. We make use of the short photoelectron mean free path (∼2 nm) to determine the position of HATCN molecules. If they are indeed located at the very sample surface, HATCN adsorption should attenuate the intensity of Alq3 core levels in a manner similar to the intensity of the substrate core levels. Because both HATCN and Alq3 contain C and N, we focus here on the intensity variations of the O1s and Ag3d levels, as presented in Figure 4. As a first step, the Ag3d peak spectra with and without HATCN adsorbate, whose absolute intensity exhibited a decrease of about 13% after HATCN deposition, were normalized to the same intensity [Figure 4(a)]. The same normalization factors were then applied to the O1s spectra (of the Alq3), and the area under the peaks was determined using a fitting routine. The best fits were realized using a Voigt profile (65% Gaussian and 35% Lorentzian) as shown in Figure 4(b) and (c). This showed that the area of the peaks before and after HATCN deposition is the same within 1.5% (within an error of (3%). As the spectral intensity of O1s (Alq3 specific) varies like that of Ag3d (metal substrate specific), it is established that 17505

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The Journal of Physical Chemistry C HATCN indeed adsorbs on the surface of Alq3. Fitting also returned that the O1s peak full width at half-maximum (fwhm) is decreased by about 8% (0.13 eV) upon HATCN adsorption. The similar narrowing observed for the O1s core level and the Alq3 HOMO (see above) will be discussed further below. Having established that the HATCN layer is located at the very surface, we will now discuss the origin of the electron transfer to the LUMO of HATCN. The first possible mechanism is a charge transfer between the two organic materials. This assumption can be ruled out by observing the valence band spectrum of an Alq3:HATCN blend film, which is shown in Figure 5 (molar ratio 2 Alq3:1 HATCN). This blend spectrum can be described as a simple sum of spectra recorded for individual pristine HATCN and Alq3 films, shifted to higher and lower binding energy, respectively, to take account of the different sample WF values. Most importantly, no gap state close to EF is observed for the blend, revealing the absence of organic
organic charge transfer. This is actually to be expected due to the mismatch of the electron affinity, ≈4
4.5 eV, of HATCN22 and the Alq3 ionization energy of 5.7 eV. Consequently, charge transfer between the metal and the HATCN through the Alq3 spacer layer is concluded on. In the following, we turn toward estimating the amount of charge transferred per HATCN molecule, the behavior of the WF, and the shift of the Alq3 HOMO onset and core levels. Within the limit of not knowing the photoemission cross section as well as the molecular orientation-dependent photoelectron angular distribution,25,26 the number of extra charges per HATCN can be estimated by determining the intensity ratio of the Alq3 HOMO emission and that of the partially filled HATCN LUMO, taking into account the number of molecules probed in the experiment. The latter was done via the C1s spectrum of the Alq3/HATCN system. By subtracting the component of the pristine Alq3 monolayer (whose intensity is assumed to decrease like the Ag3d intensity; see above) from the overall C1s spectrum, the HATCN contribution can be determined as shown in Figure 2(b). By comparing the area of these two components a ratio of Alq3:HATCN of 5 ( 1 is obtained; this is consistent with an estimation using the reading of quartz crystal microbalance during sample fabrication. In the valence region, the ratio of the Alq3 HOMO and the HATCN LUMO is 35 ( 5.0. As noted at the beginning, the Alq3 HOMO peak contains in fact contributions of three distinct molecular orbitals, and it thus accommodates six electrons. Taking into account that there is 1 HATCN for 5 Alq3 molecules, we find that the HATCN LUMO is filled with 1.2 ( 0.3 electrons. Even though this is not a scrupulously justified estimation, the number is remarkably close to filling with an integer electron and strongly suggests the occurrence of a static charge transfer from the silver to the HATCN LUMO. As HATCN is adsorbed on top of the Alq3 layer, no hybridization of HATCN orbitals with metal states can occur, and the charge transfer, necessary to reach thermodynamic equilibrium, must be realized by tunneling through Alq3. The observed WF increase of 0.75 eV upon HATCN deposition may be described using the Helmholtz equation27 ΔWF ¼

qNp εε0

with q being the elementary charge, N the dipole density on the surface, p the dipole moment perpendicular to the surface, ε0 the vacuum permittivity, and ε the relative dielectric constant. The

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Figure 6. Schematic energy level diagram of the occupied DOS of HATCN on 1 ML Alq3/Ag(111). The upper part depicts the Alq3 reorientation induced by the electric field formed between HATCN and Ag(111) (see text).

footprint area of a flat-lying HATCN molecule is ca. 0.9 nm2. With a Alq3:HATCN ratio of 5:1 and assuming flat-lying HATCN, the surface density of HATCN (and thus the dipole density) is about 0.2 molecules/nm2. Further assuming a charge of one electron per HATCN and a relative dielectric constant ε of 2.5,28 a WF change of 0.7 eV is found when the distance HATCN
silver is about 0.5 nm. This distance is smaller than the expected HATCN adsorption distance from Ag, which can be deduced from the Alq3 approximate size of 1 nm. For a molecule
substrate distance of 1 nm, the Helmholtz equation shows that the WF increase should be 0.7 eV larger than the experimental observations. This discrepancy might be due to two different facts: First, a HATCN
Ag(111) distance of 1 nm is a rather rough estimation since, for instance, HATCN could partially penetrate the space between two ligands of Alq3, which would notably decrease the adsorption height. Second, Alq3 is a polar molecule with a permanent dipole moment μ of ca. 4 D.28 When the pristine Alq3 monolayer on Ag(111) is completed, the WF decrease is, as mentioned previously, similar to what is observed for other nonpolar organic molecular layers adsorbed on Ag(111) and is ascribed to the push-back effect. This suggests that for 1 ML Alq3/Ag(111) the vertical components of the intrinsic dipole moments of Alq3 contribute negligibly to the WF change. Consequently, the molecules likely adopt no preferential orientation or, alternatively, are arranged with their dipole moment aligned in an antiparallel fashion. However, the situation can be different for HATCN/Alq3/Ag(111). Indeed, this system is similar to a dielectric slab placed inside in a charged parallel plate capacitor (with the positive and negative charges located in the metal electrode and the HATCN layer, respectively). In this scenario, upon HATCN adsorption, the Alq3 molecules possibly undergo a reorientation to align their intrinsic dipole to the electric field created inside the plate capacitor, as depicted in Figure 6 (together with the corresponding energy level alignment). When going from a randomly toward a vertically oriented Alq3 dipole layer (with the positive pole pointing upward), the WF increase due to electron transfer to HATCN will be counterbalanced. Such an effect can decrease the WF by up to ca. 1 eV.29 Therefore, our experimental findings can be 17506

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The Journal of Physical Chemistry C conciliated with electrostatic considerations involving such a molecular reorientation. We finally remark that the 0.3 eV shift to lower BE of all the Alq3 energy levels upon HATCN adsorption likely results from the electric field of the positive and negative charges accumulated at the metal surface and in the HATCN layer. In addition, the proposed molecular reorientation is susceptible to account for the observed narrowing of the Alq3 energy levels upon HATCN adsorption.

4. CONCLUSION A strong electron acceptor molecule, HATCN, was deposited on top of a physisorbed Alq3 monolayer adsorbed on Ag(111). The location of HATCN at the very surface was deduced from core level photoemission measurements. Noteworthy, the valence band spectra reveal the emergence of an interface DOS in the vicinity of the Fermi level. This DOS is thought to stem from a charge transfer from the metal to the top HATCN molecular layer through the Alq3 layer, possibly involving the transfer of one electron per molecule. Because of the large distance between the metal surface and the HATCN layer, preventing hybridization of the HATCN and silver electron states, we propose that these charges tunnel through the Alq3 layer. Finally, the (relatively low) WF increase suggests a reorientation of Alq3 and its dipole moment upon HATCN adsorption. These conclusions entangle the effects of interface dipole formation and the establishment of thermodynamic equilibrium at organic
organic heterojunctions grown on metallic or, more generally, on conductive substrates. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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(14) Br€oker, B.; Hofmann, O. T.; Rangger, G. M.; Frank, P.; Blum, R.-P.; Rieger, R.; Venema, L.; Vollmer, A; M€ullen, K.; Rabe, J. P.; Winkler, A.; Rudolf, P.; Zojer, E.; Koch, N. Phys. Rev. Lett. 2010, 104, 246805. (15) Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K. Adv. Mater. 1999, 11, 605. (16) Kahn, A.; Koch, N.; Gao, W. Y. J. Polym. Sci., Part B: Polym. Phys. 2003, 41, 2529. (17) Hill, I. G.; M€akinen, A. J.; Kafafi, Z. H. Appl. Phys. Lett. 2000, 77, 1825. (18) Duhm, S.; Salzmann, I.; Koch, N.; Fukagawa, H.; Kataoka, T.; Hosoumi, S.; Nebashi, K.; Kera, S.; Ueno, N. J. Appl. Phys. 2008, 104, 033717. (19) Niederhausen, J. et al. , submitted. (20) Hill, I. G.; Kahn, A.; Cornil, J.; dos Santos, D. A.; Bredas, J. L. Chem. Phys. Lett. 2000, 317, 444. (21) Curioni, M.; Boero, W.; Andreoni Chem. Phys. Lett. 1998, 294, 253. (22) Calculations based on Density Functional Theory (DFT, B3LYP/ 6-31+G* level) give an EA of 4 eV. (23) Duhm, S.; Salzmann, I.; Br€oker, B.; Glowatzki, H.; Johnson, R. L.; Koch, N. Appl. Phys. Lett. 2009, 95, 093305. (24) Kilian, L; Hauschild, A.; Temirov, R.; Soubatch, S.; Sch€oll, A.; Bendounan, A.; Reinert, F.; Lee, T.-L.; Tautz, F. S.; Sokolowski, M.; Umbach, E. Phys. Rev. Lett. 2008, 100, 136103. (25) Okudaira, K. K.; Asegawa, S.; Ishii, H.; Seki, K.; Harada, Y.; Ueno, N. J. Appl. Phys. 1999, 85, 6453. (26) Kera, S.; Yamane, H.; Fukagawa, H; Hanatani, T.; Okudaira, K. K.; Seki, K.; Ueno, N. J. Electron Spectrosc. Relat. Phenom. 2007, 156, 135. (27) Samorjai, G. A. Introduction to surface chemistry and catalysis; Wiley: NewYork, 1994. (28) Nagata, Y. ChemPhysChem 2010, 11, 474. (29) Yanagisawa, S.; Lee, K.; Morikawa, Y. J. Chem. Phys. 2008, 128, 244704.

’ ACKNOWLEDGMENT The authors are grateful to Hendrik Glowatzki and Ingo Salzmann for technical support and thank Georg Heimel for valuable discussions. ’ REFERENCES (1) Blochwitz, J.; Pfeiffer, M.; Fritz, T.; Leo, K. Appl. Phys. Lett. 1998, 73, 729. (2) Koch, N. ChemPhysChem 2007, 8, 1438. (3) Zhou, X.; Pfeiffer, M.; Blochwitz, J.; Werner, A.; Nollau, A.; Fritz, T.; Leo, K. Appl. Phys. Lett. 2001, 78, 410. (4) Baldo, M. A.; Forrest, S. R. Phys. Rev. B 2001, 64, 085201. (5) Horowitz, G.; Hajlaoui, R.; Bouchriha, H.; Bourguiga, R.; Hajlaoui, M. Adv. Mater. 1998, 10, 923. (6) Vazquez, H; Oszaldowski, R.; Pou, P.; Ortega, J; Perez, R.; Flores, F.; Kahn, A. Europhys. Lett. 2004, 65, 802. (7) Braun, S.; Salaneck, W. R.; Fahlmann, M. Adv. Mater. 2009, 21, 1450. (8) Verlaak, S.; Beljonne, D.; Cheyns, D.; Rolin, C.; Linares, M.; Castet, F.; Cornil, J.; Heremans, P. Adv. Funct. Mater. 2009, 19, 1. (9) Wilke, A.; Amsalem, P.; Frisch, J.; Br€oker, B.; Vollmer, A.; Koch, N. App. Phys. Lett. 2011, 98, 123304. (10) Duhm, S.; Xin, Q.; Koch, N.; Ueno, N.; Kera, S. Org. Electron. 2011, 12, 903. (11) Hill, I. G.; Kahn, A. J. Appl. Phys. 1998, 84, 5583. (12) Gao, W.; Kahn, A. Appl. Phys. Lett. 2003, 82, 4815. (13) Vollmer, A.; Jurchescu, O. D.; Arfaoui, I.; Salzmann, I.; Palstra, T. T. M.; Rudolf, P.; Niemax, J.; Pflaum, J.; Rabe, J. P.; Koch, N. Eur. Phys. J. E 2005, 17, 339. 17507

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