Metal Interfaces

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Site-Specific Charge-Transfer Screening at Organic/Metal Interfaces Heiko Peisert,*,† Daniel Kolacyak,†,‡ and Thomas Chasse´† Institute of Physical and Theoretical Chemistry, UniVersity of Tu¨bingen, Auf der Morgenstelle 8, 72076 Tu¨bingen, Germany, and Fraunhofer Institute for Manufacturing Technology and Applied Materials Research; Wiener Str. 12, 28359 Bremen, Germany ReceiVed: June 18, 2009; ReVised Manuscript ReceiVed: September 15, 2009

Site-specific core hole screening effects at organic/metal interfaces were studied using core-level X-ray photoemission spectroscopy (XPS), X-ray excited Auger electron spectroscopy (XAES), and valence band ultraviolet photoemission spectroscopy (UPS). As model molecules, we chose zinc phthalocyanine (ZnPc) and its perfluorinated derivative ZnPcF16 evaporated on Au(100). For the first molecular layer on the metal surface, we observe for both molecules a clear splitting of the Zn L3M4,5M4,5 signal due to strongly increased screening in the double-hole final states. In contrast, F KL2,3L2,3 Auger spectra do not show additional features at any stage of deposition. Site-dependent screening effects are discussed in terms of polarization and chargetransfer screening. 1. Introduction In previous years, numerous studies of thin films of organic molecular semiconductors on metals have been performed to investigate the electronic structure of organic/electrode interfaces. The observed interactions at organic/inorganic interfaces reach from physisorption to a reaction depending upon both the substrate and the organic molecule under investigation. Because of the unique nature of organic molecules, rules for inorganic semiconductor interfaces cannot be applied on organic semiconductors without limitations. Models for the electronic structure of organic semiconductor interfaces are discussed controversially and thus considerable research efforts are still required.1-12 In bulk organic materials, charges are screened by the environment due to the interaction of the excess charge with permanent and induced multipoles of the surrounding molecules. The energy of a localized charge on a molecule is modified by the electronic polarization P, and in addition coupling with phonons (charge-vibration coupling) takes place as a result of intermolecular geometric relaxation described by the reorganization energy. Thus, the polarization of the dielectric medium affects significantly the charge injection into a molecule and the charge carrier transport as well. As an example, the transport gap, that is the energy difference between the transport levels for holes and electrons, has a substantial polarization energy contribution.13-15 The situation becomes even more complex at surfaces and interfaces because the electronic polarization is different compared to the bulk organic material,14,15 having an impact on the interfacial charge-transfer dynamics as well as the energetic barriers at interfaces.16,17 Combined photoemission spectroscopy (PES) and X-ray excited Auger electron spectroscopy (XAES) can be used as a tool to study the screening mechanism of holes at organic interfaces.18-21 In photoemission spectroscopy, the screening of the photohole is described within the initial state-final state * To whom correspondence should be addressed. E-mail: heiko.peisert@ uni-tuebingen.de. Fax: + +49 7071 295490. † University of Tu¨bingen. ‡ Fraunhofer Institute for Manufacturing Technology and Applied Materials Research.

framework by the dynamical or one-hole relaxation energy RD. Because of the different final states in PES (one hole) and XAES (two holes), RD is about 3-fold higher in the case of Auger spectra and ∆RD may be semiquantitatively derived by the comparison of binding-energy (EB) shifts in XPS and XAES. ∆RD can be correlated with the change of the polarization energy induced by the redistribution of environmental charges presuming similar intramolecular screening and excluding extramolecular charge transfer within the time scale of the photoemission. Although RD is larger for core holes compared to valence holes, changes of these terms are comparable in a good approximation and correlate with frequently discussed changes of the electronic (valence) polarization energy (∆EP or ∆P+). We focus in this work on core hole screening effects at organic/metal interfaces using PES and XAES. As a model system, we chose zinc phthalocyanine (ZnPc) and perfluorinated zinc phthalocyanine (ZnPcF16) on Au(100). Screening contributions from different atomic sites are compared. Phthalocyanines have been chosen (i) as representatives for low molecular weight organic molecules with extended π-systems and (ii) as promising candidates for a variety of applications. Moreover, the interfaces between Pcs and gold are very well suited for model studies due to the low reactivity of the gold surface. By the fluorination, the type of conductivity can be changed - for example, copper phthalocyanine (CuPc) is known as a p-type material in air,22 whereas its perfluorinated relative copper hexadecafluorophthalocyanine (CuPcF16) is one of the few organic semiconductors that have demonstrated high performance and stability in air for n-channel operation.23 2. Experimental Section The measurements were performed using a multichamber UHV-system (base pressure 2 × 10-10 mbar), equipped with a Phoibos 100 cylindrical hemispherical analyzer (SPECS), a monochromatic Al KR source, and a He discharge lamp. The energetic resolution determined from the width of the Fermi edge for XPS and UPS was about 400 and 100 meV, respectively. Organic films were evaporated in a stepwise manner, the pressure during the evaporation was less than 5 × 10-8 mbar, and the evaporation rate was about 0.1 Å/s. The

10.1021/jp9057548 CCC: $40.75  2009 American Chemical Society Published on Web 10/09/2009

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Figure 1. N 1s photoemission spectra of incrementally deposited ZnPc (a) and ZnPcF16 on Au(100). Whereas the binding energy (BE) increases for ZnPc/Au with the layer thickness, partially opposite trends are visible for ZnPcF16/Au.

film thickness was controlled by a quartz microbalance calibrated using the attenuation of XPS intensities during the initial deposition steps. Because of careful comparison of single scans, possible radiation damages can be excluded, and several series of spectra with different radiation exposure times were taken. The Au(100) single crystal has been cleaned by cycles of sputtering and annealing. The cleanliness of the gold surface was checked using core-level X-ray photoemission spectroscopy (XPS) and low-energy electron-diffraction (LEED). No sign of contamination could be detected and a sharp LEED pattern (5 × 20 superstructure) was obtained. 3. Results and Discussion 3.1. Energy Level Alignment. First, we discuss the energy level alignment of both phthalocyanines at the interface to Au(100). It was shown for CuPc and CuPcF16, as well as for ZnPc and ZnPcF16, that the fluorination of the Pc molecules offers an ideal tool to modify mainly one parameter, the ionization potential (IP) of the organic, without significantly changing further electronic properties, such as the optical gap or the composition of the HOMO and the LUMO.24,25 The variation of the IP however has consequences for the electronic interface properties. As an example, we show N 1s core-level spectra as a function of the layer thickness in Figure 1. Clearly visible are changes of the energetic position, whereas the peak shape is independent of the thickness. Also, for the other corelevel spectra as well as for UPS valence band spectra no additional features were observed at the initial stages of deposition, pointing to a chemically inert interface. The energetic shifts in Figure 1 however behave differently in both cases: Whereas the binding energy (BE) increases for ZnPc/Au with the layer thickness, partial opposite trends are visible for ZnPcF16/Au. The relative BE shifts, related to the thickest film, of all core levels can be seen summarized in Figure 2. In general, chemically inert interfaces of clean materials are characterized by the alignment of the energetic levels (or more exactly of charge neutrality levels, cf. ref 9) at the organic/ metal interface, and the existence of an interfacial electric dipole layer ∆ has been reported for many cases.1,2,4-10,24 Alternatively, the mechanism is described by a pinning at polaron levels, in

particular if the organic material is a polymer.7,8 Possible factors that form or affect the interfacial dipole layer depending on both the substrate and the organic, important contributions are chemical interactions, ion formation, mirror charges, surface rearrangement, and the (re)arrangement of molecular dipole moments.1 In this context, a change of the polarization energy at surfaces or interfaces and the formation of molecular dipoles due to bending of molecules at the interface10,28 are crucial. The dipole formation is expressed in UPS by a shift of the vacuum level and in XPS by an energetic shift of all core levels. The strongest shifts are observed within the first 2 nm of the organic material, whereas at larger distances from the interface the observed shifts in photoemission are very small. From parts a and b of Figure 2, it is clear that energetic corelevel shifts for ZnPc and ZnPcF16 behave differently, the general trends remind to CuPc/Au and CuPcF16/Au.24 First, we observe shifts to higher BE for coverages up to 1-2 nm, which characterize the dipole formation. They might be partially explained by final state effects in photoemission (screening).24 Second, in contrast to ZnPc/Au (Figure 2) not only a kink in all energetic shifts at a coverage of 1-2 nm but also even a turnaround of all BEs is observed. Looking in detail at the position of the vacuum level relative to the Fermi energy of gold (i.e., the work function φ) in part c of Figure 2, we observe initially the same behavior for ZnPc and ZnPcF16 - a decrease of φ. This potential drop most likely arises from a redistribution of the electron cloud of the metal due to the adsorption causing a reduction of the metal work function, a certainty that has been discussed by several research groups.26 With increasing film thickness, the development of φ differs for ZnPc and for ZnPcF16: We observe a further decrease of φ for ZnPc, whereas φ increases in the case of ZnPcF16. As mentioned above, the fluorination changes mainly the electron affinity EA and the ionization potential IP. We determine average IPs of 5.3 and 6.3 eV for ZnPc and ZnPcF16, respectively. Therefore, ZnPc molecules act as an electron donor and ZnPcF16 as an electron acceptor in many cases. Consequently, the possible mechanism for the occurrence of energetic shifts in Figure 2 could be explained by a band-bending-like behavior as suggested for CuPc/CuPcF16 on Au24 (which imply charge-transfer processes

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Figure 3. F KL2,3L2.3 and Zn L3M4,5M4,5 Auger spectra of ZnPcF16 (F KLL) and ZnPc (Zn LMM) thin films on Au(100), the film thickness in both cases was about 11 nm.

Figure 2. Energetic core-level shifts for ZnPc/Au (a) and ZnPcF16/ Au (b) as a function of the film thickness. In contrast to ZnPc/Au we observe for ZnPcF16/Au not only a kink in all energetic shifts at a coverage of 1-2 nm but even a turnaround of all BEs. Note that the decrease of the BE at the earliest steps of deposition is more pronounced for Zn 2p.

across the interface), accompanied by final state screening effects. For an interfacial charge transfer an overlap of molecular orbitals of the organic with metal is necessary, which was recently presumed for several organic interfaces.9,10,20,21 We note in this context that a classical band-bending model is unlikely due to the low charge carrier concentration in organic materials,2,5 even if a band-bending-like mechanism could contribute to the observed potential changes at the interface, considering an exponential or Gaussian distribution of the transport states.27 3.2. Charge Transfer Versus Polarization Screening. Charge-transfer processes within the time scale of PES and XAES result in an effective screening of final states (chargetransfer screening), and additional features in XAES spectra of related compounds were recently ascribed to charge-transfer screening.20,21 In this context, we note that, for the more reactive interfaces, where a distinct charge transfer occurs in the initial state, additional lines in photoemission were observed, which can also be attributed to charge-transfer screening. This was

recently reported for cobalt porphyrins and for NTCDA (1,4,5,8naphthalene-tetracarboxylic acid dianhydride) on Ag(111).29,30 Because we do not observe clearly separated additional peaks in all photoemission features, charge-transfer screening as a reason for the energetic shifts in Figures 1 and 2 seems to be unlikely. Also visible from Figure 2 is that the core-level spectra of different atoms directly at the interface are differently shifted; the strongest shift is observed for the central metal atom for both ZnPc and ZnPcF16 on Au(100). The question arises whether or not such differences could be caused by different screening contributions, for example due to a variation of the distance to the metal substrate. To gain information on local screening effects, we look in detail at the F KLL and Zn LMM Auger spectra. We note that unfortunately C KVV and N KVV transitions are characterized by broad spectral features, arising for example from the large number of valence band states to be considered and from their delocalization. As a consequence, the detailed analysis of energy shifts and changes of the peak shape in particular for (sub)monolayer films is hindered. We focus therefore on F KLL and Zn LMM Auger spectra, which both include deeper valence levels. First, we discuss in Figure 3 the Auger peak shape of relatively thick films, that is a thickness where the interface region is not probed by XPS. The spectra were taken on thin films of ZnPcF16 (F KLL) and ZnPc (Zn LMM) on Au(100), and the film thickness in both cases was about 11 nm. The fluorine KLL Auger spectrum in part a of Figure 3 involves different double-ionized final states, which appear at different energetic positions in the spectrum: the 2s02p6 final state (KL1L1, the so-called inner-inner region), 2s12p5 (KL1L2,3, inner-outer region), and 2s22p4 (KL2,3L2.3, outer-outer region). A detailed description of the fluorine Auger spectrum can be found in refs 31 and 32. We discuss in part a of Figure 3 only the part of the spectrum that contains the most intense KL2,3L2.3 transitions. The multiplet structure arises from different coupling of holes

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Figure 4. Comparison of Zn 2p3/2 core-level photoemission spectra (a) to Zn L3M4,5M4,5 Auger spectra (b) of incrementally deposited ZnPc on Au(100). The appearance of a well-resolved additional feature at the earliest stages of deposition only in the Auger spectra points to a high screening at the interface to the metal.

in the final state, which result in the different spectroscopic terms. We distinguish between the 1D term with the highest intensity and the 1S term. Also visible is a small but distinct shoulder (A) at the high kinetic energy side of the spectrum, which was observed at all stages of deposition. Similar features were recently also observed for other C-F compounds.32,33 Such a split of the F KLL, 1D line can be caused by a delocalization of the holes in the final state.33,34 In part b of Figure 3, we briefly consider the more complex shape of the Zn L3M4,5M4,5 Auger signal. The multiplet splitting was comprehensively discussed in ref 35, and main contributions arise from 1G, 3F and 1S terms, which can be easily distinguished in part b of Figure 3. We note, that additional 3P and 1D terms with lower transition probability contribute to the Zn L3M4,5M4,5 Auger signal, they are superposed by the most intense 1G feature.35 Narrow atomic-like peaks dominate the Zn L3M4,5M4,5 spectrum because the interaction energy U for the two holes is larger than the valence bandwidth W for of the Zn 3d closedshell system.36 Auger spectra of strongly correlated systems as a function of the U/W ratio are discussed in detail in ref 37. In this context, we presume that the on-site Coulomb interaction U is not changed drastically as a function of the layer thickness, although a reduction of U due to the influence of image charges in the metal substrate was observed, for example for C60.38 Because of the low bandwidth in organic systems, however, we suppose that U does not become smaller than W. The possible description of all Auger spectra below with similar atomic-like shapes supports this assumption. As recently shown for MgPc on Au, Auger spectra are very sensitive to screening effects at the interface due to their doublehole final state.20,21 In Figure 4, we compare Zn 2p3/2 core-level photoemission spectra to Zn L3M4,5M4,5 Auger spectra of incrementally deposited ZnPc on Au(100). In both cases, we observe distinct energetic shifts of spectral features to higher EB with increasing film thickness as discussed for all photoemission core-level spectra in Figures 1 and 2. Most importantly in Figure 4, however, we observe a clear change of the peak shape only in the Auger spectra. The whole series can be described essentially by two components whose peak shape

corresponds to the Auger spectrum of the thickest film. Deviations from this model might be understood considering a possible change of the shape of the Auger spectrum, for example due to a (slight) change of the oxidation state of Zn at the interface as discussed below. Whereas the main feature (1G) at about 496 eV dominates the Zn L3M4,5M4,5 spectra in part b of Figure 4 up to 0.9 nm, the second peak at about 498-499 eV dominates at higher thicknesses. This behavior reminds to evidently MgKLL peaks for MgPc on Au,20,21 we distinguish between a lower and a higher BE peak arising from molecules directly at the interface and in the bulk of the organic film, respectively. The interface peak corresponds to molecules of the first monolayer. The fact that the clear splitting into two peaks is only obvious in the case of Zn LMM suggests an attribution predominantly to screening. Because the contribution of ∆RD to energetic shifts of Auger spectra is expected to be 3-fold higher compared to XP spectra (above), a possible related splitting of the Zn 2p photoemission signal may not be energetically resolved. However, the broadening in Zn 2p spectra in the earliest steps of deposition is very weak (about 0.2 eV), whereas the splitting of the Auger peak is about 2 eV. Therefore, analogously to MgPc on Au,20,21 polarization screening due to mirror charges is not sufficient to explain the observed large splitting in the Auger spectra, significant contributions by charge-transfer screening have to be considered. The extent of polarization screening at interfaces is discussed in detail below applying a dielectric continuum model. It has to be clarified whether or not the strong screening effects are limited to the central metal atom of the phthalocyanine. For this purpose, we compare the results for the ZnPc/ Au(100) interface to ZnPcF16/Au(100), where in addition a relatively narrow Auger signal of fluorine (F KLL) is available. In part a of Figure 5, Zn L3M4,5M4,5 spectra for ZnPcF16/Au are shown as a function of the layer thickness. The development of the peak shape is very similar to ZnPc/Au: We distinguish between a typical spectrum for monolayer coverages (marked by an arrow in part a of Figure 5) with the main feature (1G) at about 496 eV and a second spectrum at about 498-499 eV that dominates at thicknesses >0.8 nm. Therefore, analogous to

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Figure 5. Zn L3M4,5M4,5 (a) and F KL2,3L2,3 (b) Auger spectra of incrementally deposited ZnPcF16 on Au(100). An additional feature at the earliest stages of deposition appears only in the Zn Auger spectra and points to a local charge-transfer screening at the central metal atom of the Pc or to the LUMO, which is not localized at the fluorine.

Figure 6. Modified Auger parameter R′ for fluorine ZnPcF16 during the formation of the interface dipole to Au(100) within the first 2 nm of the film growth. Changes of R′ can be understood by polarization screening.

TABLE 1: Layer-Dependent Screening Contributions Estimated by Dielectric Continuum Modela

first layer second layer third layer 2 nm

d (nm)

∆RD (eV)

d (nm)

0.15 0.49 0.83

0.80 0.25 0.15

0.19 0.53 0.87 ∆RD )

∆RD (eV) 0.63 0.23 0.14 0.06 eV

d (nm)

∆RD (eV)

0.22 0.56 0.90

0.55 0.21 0.13

a For the distance of the first layer to the mirror plane of the metal, we apply the van der Waals radius of carbon in organic compounds corrected by a possible bending of the molecule (0.19 nm, middle). For comparison, we chose also a significant smaller (0.15 nm, left) and larger (0.22 nm, right) value.

ZnPc/Au contributions by charge-transfer screening have to be considered to explain the two different components in the Zn L3M4,5M4,5 spectra. However, the F KLL spectra behave differently in part b of Figure 5: During the first steps of deposition no additional

component occurs, pointing to another screening mechanism. The absence of a strongly shifted component indicates that the F KLL double-hole final state for molecules directly at the interface (i.e., the first monolayer) is poorly screened compared to Zn L3M4,5M4,5. We therefore propose that charge-transfer screening, which is the most effective screening mechanism, does not participate in the screening of the F KLL final state. The different screening mechanism for Zn and F might be explained by i) a local charge-transfer screening at the central metal atom due to an overlap particular of Zn wave functions with the wave functions of the Au, analogously to cobalt porphyrins on Ag29 or by ii) a charge transfer to the LUMO, which is not localized at the fluorine. Also visible from part b of Figure 5 however are the comparatively strong energetic Auger shifts of more than 1 eV during the formation of the interface dipole within the first 2 nm, whereas corresponding shifts of photoemission lines in Figure 2 are generally smaller. The stronger shift in Auger spectra points again to final state screening effects, the increased screening ability at metal interfaces would be in agreement with the observations discussed above (e.g. Figure 2). If charge-transfer screening is unlikely for the fluorine spectra, we are left with the scenario where polarization screening is responsible for the different energetic shifts in XPS and XAES. In this case, the polarization energy is described by the dynamic relaxation energy ∆RD (above), which can be estimated by the Auger parameter. Within the Auger parameter approach, the sum of the XPS binding energy and the XAES kinetic energy is referred to the modified Auger parameter R′. It can be shown, that ∆R′ correlates well with the change of the electronic polarization energy for the core hole (∆R′ = 2∆RD).39,40 In Figure 6, we show the modified fluorine Auger parameter R′ ) EB(F1s) + Ekin(F KLL) during the formation of the interface dipole within the first 2 nm of the film growth. The highest value is found directly at the interface, whereas R′ and thus the screening ability - decreases with the film thickness. To sum up, we observe within the first 2 nm a distinct variation of the modified Auger parameter by 0.9 eV, which corresponds to ∆RD ) 0.45 eV.

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Figure 7. Valence band photoemission spectra for ZnPc and ZnPcF16 on Au(100) (zoom into the HOMO region). The split of the HOMO at a coverage of about 2-3 monlayers (0.8 nm, arrows) point to polarization screening in photoemission.

Figure 8. Schematic illustration of possible effects that may cause different screening mechanism for Zn and F atoms in ZnPcF16: An overlap of Zn wave functions with the wave functions of the Au and molecular bending resulting in different distances to the metal mirror plane for F and Zn.

We will verify now whether a relaxation energy as high as 0.45 eV can be understood by polarization screening applying a dielectric continuum model. Layer-dependent screening contributions ∆EB can be estimated according to ∆EB(d) ∆EB(∞) ) -e2/(16πε0εd),18-21 where d is the distance from the mirror plane, ∆EB(d) and ∆EB(∞) are binding energies referred to the distance d and the infinitely thick film, respectively. The dielectric constant  is assumed to be about 3, and for the molecule-molecule distance we apply the bulk value of 0.34 nm.41 The distance of the first layer to the mirror plane of the metal d1 is not easy to determine, in a first approximation the van der Waals (vdW) radius of the adsorbate can be applied. For carbon in organic compounds, d1 is about 0.17 nm. We note however that d1 is different on a microscopic scale, for example the vdW radius of neutral Zn is about 0.14 nm. In addition, the molecules can be indeed distorted as found for CuPcF16 on other metal single crystals; in this case the distance metal-fluorine was about 0.02 nm lager than metal-carbon.28 Considering this possible bending, we calculate layer resolved ∆EB () ∆RD) for d1 ) 0.19 nm and for a distinct larger and smaller d1 value (0.22 and 0.15 nm, respectively); the results

are summarized in Table 1. The differences of ∆RD for the first layer and a 2 nm film are in good agreement with the results obtained from the Auger parameter above (∆RD ) 0.45 ( 0.1 eV); a reasonable agreement is obtained for d1 ) 0.19 and 0.22 nm (∆RD ) 0.57 and 0.49 eV). The relative small scattering of the data in Table 1 shows that, despite the uncertainties in the determination of d1, the order of magnitude of ∆RD originating from polarization screening can be estimated in a good approximation. Thus, we conclude that polarization screening is essentially sufficient to explain the observed shifts of the F KLL Auger spectra in part b of Figure 5. On the other hand, ∆RD directly affects the EB in photoemission according to ∆EB(XPS) ) ∆V - ∆RD, where V reflects the potential of the initial state charge contribution. Although in a multilayer system the different energetic positions may not be resolved, the energy separation arising from ∆RD between the first and the second layer may be possible considering the energy resolution of the spectrometer (about 400 meV for XPS and 100 meV for valence band photoemission spectroscopy, UPS). The better energetic resolution in UPS and the usually sharp valence band structures enable even the separation of photoemission features with small energetic distances. For this purpose, UPS spectra for ZnPc and ZnPcF16 on Au(100) as a function of the layer thickness are shown in Figure 7 (zoom into the highest occupied molecular orbital, HOMO). At the first deposition step at 0.5 nm (1-2 monolayers) a small, single HOMO peak with a comparably low EB appears. Clearly visible is a split of the HOMO into two peaks for both systems at a film thickness of about 0.8 nm (2-3 monolayers), which can be attributed to signals from the first and the second monolayer (arrows in Figure 7). The energy separation of both features is about 0.3 eV, in good agreement with the expectations for polarization screening effects between the first and the second layer in table 1. In this context, we emphasize that a similar splitting of the HOMO in the monolayer regime was recently observed for related systems and different underlying reasons were discussed.42 Furthermore, the question arises why are the shifts of F 1s relatively small (the overall shifts are less than 0.3 eV, Figure 2) and why do the energetic shifts vary for different atoms in

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Figure 2? This phenomenon can be only understood if several (partly opposite) effects contribute to the energetic shifts. We propose that the observed polarization screening is partially superposed by an weak initial state charge transfer: i) by an intramolecular charge transfer induced by the possible bending of the molecule at the interface and/or ii) by a (partial) charge transfer from the molecule to the metal as assumed for CuPcF16/ Au.23 In addition, different distances of atoms within the adsorbed molecule to the metal mirror plane causes a variation of polarization screening contributions (Table 1). IV. Conclusions In conclusion, we have shown that charge-transfer screening affects the Zn LMM Auger spectra of ZnPc and ZnPcF16 directly at the interface to Au(100) but not the corresponding F KLL spectra. The different screening mechanism for Zn and F atoms in ZnPcF16 might be explained by i) a local charge-transfer screening at the central metal atom due to an overlap particular of Zn wave functions with the wave functions of the Au or ii) by a charge transfer to the LUMO, which is not localized at the fluorine. The contribution of polarization screening can be estimated by the Auger parameter for fluorine. Binding-energy shifts at interfaces between organic molecules and metals can be explained to a large extent by polarization screening. For the understanding of different energy shifts for each atom, a bending of the molecule has to be considered. These effects are schematically illustrated in Figure 8. Acknowledgment. We are grateful to W. Neu and I. Biswas for valuable discussions and technical support. The work was supported by the German Research Council Ch 132/20-1. References and Notes (1) Ishii, H.; Sugiyama, K.; Ito, E.; Seki, E. K. AdV. Mater. 1999, 11, 605. (2) (a) Hill, I. G.; Ma¨kinen, A. J.; Kafafi, Z. H. Appl. Phys. Lett. 2000, 77, 1825. (b) Hill, I. G.; Ma¨kinen, A. J.; Kafafi, Z. H. J. Appl. Phys. 2000, 88, 889. (3) Crispin, X.; Geskin, V.; Crispin, A.; Cornil, J.; Lazzaroni, R.; Salaneck, W. R.; Bre´das, J. L. J. Am. Chem. Soc. 2002, 124, 8131. (4) Campbell Scott, J. J. Vac. Sci. Technol., A 2003, 21, 521. (5) Peisert, H.; Schwieger, T.; Auerhammer, J. M.; Knupfer, M.; Golden, M. S.; Fink, J. Appl. Phys. 2002, 91, 4872. (6) Knupfer, M.; Peisert, H. Phys. Status Solidi A 2004, 201, 1055. (7) Tengsted, C.; Osikowicz, W.; Salaneck, W. R.; Parker, I. D.; Hsu, C.-H.; Fahlman, M. Appl. Phys. Lett. 2006, 88, 053502. (8) Koch, N. ChemPhysChem 2007, 8, 1438. (9) Va´zquez, H.; Dappe, Y. J.; Ortega, J.; Flores, F. J. Chem. Phys. 2007, 126, 144703. (10) Romaner, L.; Heimel, G.; Bre´das, J.-L.; Gerlach, A.; Schreiber, F.; Johnson, R. L.; Zegenhagen, J.; Duhm, S.; Koch, N.; Zojer, E. Phys. ReV. Lett. 2007, 99, 256801.

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