Molecular Orientation Dependent Energy Level Alignment at Organic

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Molecular Orientation Dependent Energy Level Alignment at Organic-Organic Heterojunction Interfaces Wei Chen,*,†,‡ Dong Chen Qi,† Yu Li Huang,† Han Huang,† Yu Zhan Wang,† Shi Chen,† Xing Yu Gao,† and Andrew Thye Shen Wee*,† Department of Physics, National UniVersity of Singapore, 2 Science DriVe 3, 117542, Singapore, and Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3, 117543 Singapore ReceiVed: April 6, 2009; ReVised Manuscript ReceiVed: May 18, 2009

Molecular orientation dependent energy level alignments at organic-organic heterojunction (OOH) interfaces have been investigated with synchrotron based high-resolution photoemission spectroscopy and near-edge X-ray absorption fine structure (NEXAFS) measurements. Model systems of the lying-down 3,4,9,10-perylenetetracarboxylic-dianhydride (PTCDA) films on both standing-up and lying-down copper hexadecafluorophthalocyanine (F16CuPc) and copper(II) phthalocyanine (CuPc) thin films have been used to illustrate the molecular orientation dependent interface properties. The formation of different interface dipoles at the heterojunction interfaces is strongly influenced by the orientation dependent ionization potentials of the underlying F16CuPc or CuPc thin films. This is attributed to the intrinsic surface dipoles induced in the standing-up F16CuPc (CuPc) film due to the polar intermolecular C-F (C-H) bonds formed at the interface. In situ NEXAFS measurements reveal that the room-temperature deposition of PTCDA layers does not alter the molecular orientation of the underlying lying-down or standing-up F16CuPc thin films. We also demonstrate that the binding energies of both the C 1s core level and the highest-occupied-molecular-orbital (HOMO) of PTCDA on the lying-down F16CuPc thin film is 0.3 eV higher than those on the standing-up F16CuPc thin film. This shows that it is possible to manipulate the energy level alignment at OOH interfaces by choosing the appropriate molecular orientation. In contrast, the HOMO positions of PTCDA on both lying-down and standing-up CuPc films are almost identical. This suggests that an orientation independent Fermi-level pinning occurs at the PTCDA/CuPc interfaces involving interfacial charge transfer. 1. Introduction In the past decade, much attention has been devoted to the fabrication of organic-organic heterojunctions (OOH) with tailored film properties for applications in low-cost, large-scale, and flexible organic electronic devices, in particular organic photovoltaic cells.1-4 It is known that the energy level alignment at OOH interfaces has a crucial impact on the device performance.5-12 For example, at OOH interfaces, the offset of the highest occupied molecular orbital (HOMO) or lowest unoccupied molecular orbital (LUMO) between the molecular acceptor (n-type) and donor (p-type) films is important for efficient charge separation or exciton dissociation. Furthermore, the energy difference between the LUMO of the acceptor film and the HOMO of the donor film largely determine the open circuit voltage of the organic photovoltaic cells.13-17 Another important interface property is molecular orientation, which can significantly affect light absorption and charge transport in the films.18-22 A few studies have been dedicated to the understanding and control of molecular orientation at OOH interfaces.23-30 At noninteractive OOH interfaces (without covalent bond formation), the molecular orientation is largely controlled by noncovalent intermolecular interactions such as hydrogen bonding and the interfacial interactions such as dispersion forces and the interlayer π-π interaction.28-33 * Corresponding authors. E-mail: [email protected] (W.C.); phyweets@ nus.edu.sg (A.T.S.W.). † Department of Physics. ‡ Department of Chemistry.

Recently, orientation-dependent ionization potentials (IPs) of organic thin films have been reported,34-43 where IP refers to the energy difference between the HOMO and the vacuum level (Evac). However, the orientation dependent energy level alignment at the interfaces of OOH with tailored molecular orientation is less understood. The typical organic-organic p-n heterojunction (p-n OOH) involves interfacial charge transfer. The widely used π-conjugated molecules such as the perylene derivatives and various phthalocyanines are highly anisotropic. This can lead to orientation dependent interfacial charge transfer.44,45 Therefore, both the charge transfer and IPs at p-n OOH interfaces can possess the strong orientation dependence. This complicates the understanding of the orientation dependent energy level alignment at OOH interfaces with well-defined molecular orientation. To simplify this problem, we use the n-n OOH model system of two molecular acceptor films, comprising 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) films on copper hexadecafluorophthalocyanine (F16CuPc) to minimize the interfacial charge transfer. We also compare the orientation dependent interfacial energy level alignment with that at the p-n OOH model system of PTCDA films on copper(II) phthalocyanine (CuPc), which involves interfacial charge transfer. The molecular structures of CuPc, F16CuPc, and PTCDA are shown by schematic drawings in Figure 1a-c, respectively. It is known that disk-like planar phthalocyanine molecules such as F16CuPc and CuPc lie flat on graphite arising from the directional interfacial π-π interaction;28-30 they also adopt the lying-down configuration on metal substrates such as Ag(111) and Au(111) due to the effective coupling between the metal

10.1021/jp903139q CCC: $40.75  2009 American Chemical Society Published on Web 06/24/2009

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Figure 1. Schematic drawings for (a) CuPc, (b) F16CuPc, and (c) PTCDA.

d-bands and conjugated π system (d-π interaction).46,47 On the other hand, by terminating the Au(111) surface with various self-assembled monolayers (SAMs) of aromatic thiols, the d-π interface interaction is minimized, thereby leading to a standingup configuration of F16CuPc or CuPc on SAMs/Au(111).28-30,48,49 In this paper, using in situ synchrotron based high resolution photoemission spectroscopy (PES) and near-edge X-ray absorption fine structure (NEXAFS) measurements, we investigate PTCDA films on both standing-up and lying-down F16CuPc and CuPc thin films to evaluate the effect of molecular orientation on the energy level alignment at OOH interfaces. 2. Experimental Section PES and NEXAFS measurements were carried out at the Surface, Interface and Nanostructure Science (SINS) beamline of the Singapore Synchrotron Light Source.50,51 All of the PES and NEXAFS measurements were performed at room temperature (RT). The NEXAFS measurements were performed in total-electron yield (TEY) mode with a photon energy resolution of 0.1 eV. The linear polarization factor of synchrotron light at the SINS beamline was measured to be about 0.95. The sample vacuum level shift was determined from PES spectra at the lowkinetic energy onset (secondary electron cutoff) using a photon energy of 60 eV with negative 5 V sample bias. The sample work function φ was obtained through the equation φ ) hν W, where W is the spectrum width (the energy difference between the substrate Fermi level and low kinetic energy onset).7-12 The φ of the electron analyzer was measured to be 4.30 ( 0.05 eV. Monolayers of octane-1-thiol (C8-SAM) (Sigma-Aldrich) were formed by spontaneous adsorption on Au(111)/mica substrates (SPI, USA). The Au(111)/mica samples were immersed in 3 mL of 1 mM solutions in an N2 environment for 48 h, using distilled ethanol as the solvent.52 After deposition, the samples were thoroughly rinsed using ethanol and immediately transferred into the UHV end station of the SINS beamline. Freshly cleaved highly ordered pyrolytic graphite (HOPG) substrates were thoroughly degassed in the UHV chamber at around 500 °C overnight before deposition. For NEXAFS and PES experiments, F16CuPc (Sigma-Aldrich, 85%), CuPc (Sigma-Aldrich, sublimated grade), and PTCDA (SigmaAldrich, 98%) were deposited in situ from K-cells onto HOPG, C8-SAM/Au(111) or SiO2 [Si(111) with native oxide] substrates at RT in the main chamber of the SINS beamline (base pressure better than 6 × 10-11 mbar). Prior to deposition, F16CuPc and PTCDA were purified twice by gradient vacuum sublimation (Creaphys, Germany). Deposition rates of 0.1 nm/min for PTCDA and 0.2 nm/min for F16CuPc and CuPc were used in our NEXAFS and PES experiments. The deposition rates for all molecules were precalibrated by a quartz crystal microbalance (QCM) under similar growth conditions. The actual deposition rates were further calibrated by monitoring the attenuation in intensity of the Au 4f7/2 peak before and after deposition on a sputter-cleaned poly Au sample.53

Figure 2. Angle-dependent N K-edge NEXAFS spectra for the 4 nm F16CuPc on (a) HOPG and (c) C8-SAM/Au(111) substrates. Panels b and d show the corresponding C K-edge NEXAFS spectra after the deposition of 8 nm PTCDA, respectively.

3. Results and Discussion 3.1. Molecular Orientation at the OOH Interfaces. The model systems of the OOHs studied comprise PTCDA thin films on both standing-up and lying-down F16CuPc thin films. The molecular orientations of the OOHs are probed by in situ angular dependent NEXAFS measurements. NEXAFS was used to monitor the resonance from the core level of a specific atomic species of a molecule (for example, the 1s core level of nitrogen atoms in a molecule) to its unoccupied molecular orbitals (π* and σ* orbitals).54 In principle, the resonance to the unoccupied π* or σ* orbital is strong when the electric field vector E of the incident linearly polarized synchrotron light has a large projection along the direction of the π* or σ* orbital, and vanishes when E is perpendicular to the π* or σ* orbital.54 For the planar PTCDA and F16CuPc molecules, the σ* and π* orbitals are directed essentially in-plane and out-of-plane, respectively. Therefore, angle-dependent NEXAFS can be used to probe the molecular orientations in PTCDA and F16CuPc thin films. Figure 2, panels a and c, shows the angle-dependent NEXAFS spectra (N K-edge) of 4 nm F16CuPc on HOPG and C8-SAM terminated Au(111), respectively. In Figure 2a, the first three sharp absorption peaks (397-404 eV) are assigned to excitations from the N 1s core level to individual π* states and the broad absorption peaks (404-415 eV) at higher photon energies to

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Figure 3. Angle-dependent N K-edge NEXAFS spectra for the 4 nm F16CuPc on HOPG (panels a and b) and C8-SAM terminated Au(111) (panels c and d) after the deposition of 0.5 nm (panels a and c) and 1.0 nm (panels b, and d) PTCDA.

transitions to the σ* states.28-30 The π* resonances of F16CuPc are greatly enhanced at grazing incidence and depressed at normal incidence. As such, the angular-dependence of NEXAFS spectra in Figure 2a confirms that F16CuPc molecules lie flat on HOPG with the molecular plane slightly tilted away from the substrate surface,55 in consistent with our previous lowtemperature scanning tunneling microscopy study.56 Because of the 3-fold symmetry of the graphite and Au(111) substrates, the intensity I of the π1* resonance is related to the tilt angle R of the molecular plane with respect to the substrate surface plane and the synchrotron light incidence angle θ by54

1 I(θ) ∝ 1 + (3 cos2 θ - 1)(3 cos2 R - 1) 2 Using the intensity ratio R(π1*) ) I(90°)/I(20°), we calculate the average tilt angle R for F16CuPc on HOPG to be 15° ( 5°. In Figure 2c, the angle-dependence of NEXAFS spectra for 4 nm F16CuPc on C8-SAM terminated Au(111) is reversed, i.e., the π* resonances are enhanced at normal incidence and depressed at grazing incidence. This suggests that F16CuPc molecules stand upright with the tilt angle R of 78° ( 5° on C8-SAM terminated Au(111), consistent with our previous results.28-30,48,49 Figure 2(b) shows the angle-dependent C K-edge NEXAFS spectra of 8 nm PTCDA on 4 nm F16CuPc (lying-down) on HOPG. The four sharp absorption peaks appearing at the absorption edge from 284 to 289 eV with grazing incident light (θ ) 20°) are due to resonant transitions from the C 1s core

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Figure 4. Synchrotron wide-range (panels a and b) and C 1s core level (panels c and d) PES spectra for the sequential deposition of F16CuPc and PTCDA on HOPG (panel a and c) and C8-SAM/Au(111) (panels b and d). The wide-range PES spectra are measured with photon energy of 740 eV, and the C 1s PES spectra with photon energy of 350 eV. All binding energy are relative to the Fermi level position of the electron analyzer. All spectra are measured under the normal emission condition.

levels of the various carbon atoms into unoccupied molecular orbitals of PTCDA. The first two peaks (peaks A and B: 284-286 eV) are attributed to the transitions from carbon atoms in the aromatic perylene core to the LUMOs; peaks C1 and C2 (287-289 eV) are transitions within the anhydride functional groups.57,58 The angle-dependent NEXAFS spectra in Figure 2b with the intensity of π* resonant peaks greatly enhanced at θ ) 20° and suppressed at θ ) 90° reveal that PTCDA molecules lie flat with the tilt angle R of 18° ( 5° on the lying down F16CuPc. Figure 2d shows the angle-dependent C K-edge NEXAFS spectra of 8 nm PTCDA on 4 nm F16CuPc (standingup) on C8-SAM terminated Au(111). The spectra possess the same angular dependence as that for PTCDA on the lying-down F16CuPc on HOPG, revealing that PTCDA molecules also lie flat with a slight larger tilt angle R of 25° ( 5° on the standingup F16CuPc thin film. In our previous study, we found that PTCDA molecule always adopt the lying-down configuration on standing-up and lying-down CuPc thin films and on other inert substrates such as SAM terminated Au(111).28 The formation of multiple in-plane hydrogen bonding within the PTCDA thin film is responsible for the preferential lying-down configuration of PTCDA films on various substrates, thereby stabilizing the flat-lying PTCDA molecules on both standingup and lying-down F16CuPc thin films. In order to confirm whether the deposition of PTCDA layers can change the molecular orientation of the underlying F16CuPc


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Figure 5. Evolution of synchrotron PES spectra for the sequential deposition of F16CuPc and PTCDA on HOPG (panels a-c) and C8-SAM/ Au(111) (panels d-f): (a and d) valence band spectra at the low-binding energy part and (b and e) corresponding near the EF region spectra from panels a and d, and (c and f) PES spectra at the low-kinetic energy part (secondary electron cutoff). All spectra are measured with photon energy of 60 eV. All binding energy are relative to the Fermi level position of the electron analyzer. All spectra are measured under the normal emission condition.

thin films, we monitor the evolution of N K-edge NEXAFS spectra of the F16CuPc films after the first few depositions of PTCDA. Figure 3, panels a and b, shows the angle-dependent N K-edge NEXAFS spectra of the lying-down F16CuPc thin film on HOPG after the deposition of 0.5 and 1.0 nm PTCDA, respectively. We did not observe any apparent changes in the NEXAFS spectra and their angular dependence as compared to the pristine F16CuPc film [Figure 2a], suggesting that the underlying F16CuPc film retains the lying-down configuration. The standing-up F16CuPc film on C8-SAM terminated Au(111) also maintains its original orientation configuration after the deposition of the PTCDA top layers [Figure 3c, 0.5 nm, and Figure 3d, 1.0 nm]. This suggests that the deposition of PTCDA layers does not alter the molecular orientation of underlying lying-down or standing-up F16CuPc thin films. As such, we have fabricated idea model systems of the flat-lying PTCDA films on both standing-up and lying-down F16CuPc thin films. 3.2. Energy Level Alignments at the OOH Interfaces. Figure 4, panels a and b, shows the evolution of wide-scan PES spectra (incident photon energy of 740 eV) after the sequential deposition of PTCDA on 4 nm F16CuPc films on HOPG and C8-SAM terminated Au(111), respectively. The intensity of the characteristic F 1s and N 1s core level peaks of the pristine F16CuPc films greatly attenuate after deposition of 4 nm PTCDA and almost vanishes after 8 nm PTCDA deposition. This suggests that the RT deposition of the PTCDA top layer does not induce significant interlayer molecular diffusion (e.g., F16CuPc diffusion from bottom to top layer). As shown in Figure 4c, the C 1s peak for 4 nm F16CuPc on HOPG is dominated by

three peaks: C(C) at the binding energy of 285.3 ( 0.05 eV, C(N) at 286.4 ( 0.05 eV and C(F) at 287.4 ( 0.05 eV. These F16CuPc-related C 1s components almost disappear after the deposition of 8 nm PTCDA and the spectrum is dominated by the PTCDA related-components: C(H,C) at the binding energy of 285.5 ( 0.05 eV and C(O) at 289.1 ( 0.05 eV. A similar trend is observed in Figure 4d for PTCDA on the standing-up F16CuPc on C8-SAM terminated Au(111). Our results suggest that there is no significant interlayer molecular diffusion during the RT deposition of PTCDA on both lying-down and standingup F16CuPc films. The orientation dependent energy level alignments of the flat-lying PTCDA films on both standing-up and lying-down F16CuPc thin films have been investigated by in situ high resolution PES. Figure 5 shows the evolution of PES valence band (VB) spectra at the low-binding energy region (Figure 5, panels b, c, e, and f) and the secondary electron cutoff at the low-kinetic energy region (Figure 5, panels a and d) during the sequential deposition of F16CuPc and PTCDA molecules. The hole injection barriers (∆h) can be measured from the energy difference between the substrate Fermi level and the HOMO leading edge (linear extrapolation of the lowbinding energy onset).7-12 The Evac or φ values were measured with -5 V sample bias by linear extrapolation of the lowkinetic energy onset (secondary electron cutoff) in the PES spectra. The IP equals to the sum of φ and ∆h. These values are summarized in Table 1 for the different films. As shown in Figure 5, panels b and e, the PES spectra show the apparent orientation dependence of the valence band features for both

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TABLE 1: Summary of Sample Parameters: Hole Injection Barriers (∆h), Work Functions (O), Ionization Potential (IP: IP ) ∆h + O), and the Tilt Angle r of the Molecular Plane with Respect to the Substrate Surface (Obtained from NEXAFS Results)a F16CuPc on HOPG PTCDA on F16CuPc on HOPG F16CuPc on C8-SAM PTCDA on F16CuPc on C8-SAM CuPc on HOPG PTCDA on CuPc on HOPG CuPc on SiO2 PTCDA on CuPc on SiO2 a

∆h (eV)

φ (eV)

IP (eV)

1.2 (F16CuPc) 1.75 (PTCDA) 1.3 (F16CuPc) 1.45 (PTCDA) 0.85 (CuPc) 1.90 (PTCDA) 0.85 (CuPc) 1.95 (PTCDA)



R (F16CuPc or CuPc)

R (PTCDA)

15° (lying) 18° (lying) 78° (standing) 25° (lying) 18° (lying) 23° (lying) 80° (standing) 25° (lying)

The error bar for PES results (∆h, φ, and IP) and R are (0.05 eV and (5°, respectively.

Figure 6. Schematics for the energy level diagrams of flat lying 8 nm PTCDA thin films on (a) the lying-down F16CuPc on HOPG and (b) the standing-up F16CuPc on C8-SAM.

standing-up and lying-down F16CuPc films.29 In particular, the Evac or φ of the standing up F16CuPc is 0.6 ( 0.05 eV higher than that of the lying-down film. ∆h does not show strong angle dependence for both F16CuPc films. Therefore, the IP of the standing-up F16CuPc is 0.7 ( 0.05 eV higher than that of the lying-down film. Such orientation dependent IP of ordered organic thin films has been reported previously.34-36,59 It can be attributed to the intrinsic surface dipoles induced in the ordered molecular layers due to the polar intermolecular bonds exposed at the surfaces.34-36,59 A downward pointing surface dipole originating from the C-F bond polarity forms at the surface of the standing-up F16CuPc film, leading to a larger IP than that of the lying-down F16CuPc film. It is worthy noting that the polarization from the substrate60,61 and surrounding molecules34 can also contribute to the observed orientation dependent IP. However, because of the finite number of free electrons on HOPG surface or the attenuation by the C8-SAM, the polarization or the photohole screening of the F16CuPc films from the underlying HOPG or C8-SAM terminated Au(111) is negligible in our experiments. As recently reported by Duhm, S., et al., the variation of polarization energy of different oriented molecular films is much smaller than the observed orientation dependent IP of sexithiophenes.34 Therefore, the different polarization or photohole screening by the neighboring standing-up or lying-down molecules is not the major source to cause the observed large variation of IP in differently oriented F16CuPc organic films.

In Figure 5, panels a and d, after the deposition of 8 nm PTCDA, the surface work function increases by 0.1 ( 0.05 eV and the vacuum level is 4.8 ( 0.05 eV above the substrate Fermi level (φ ) 4.8 ( 0.05 eV) for PTCDA on the lying-down F16CuPc film. In contrast, the surface work function decreases by 0.35 ( 0.05 eV and the vacuum level lies 4.95 ( 0.05 eV above the substrate Fermi level (φ ) 4.95 ( 0.05 eV) for PTCDA on the standing-up F16CuPc film. Both PTCDA and F16CuPc are typical molecular acceptors or n-type organic semiconductors. Although there is a slight difference in their electron affinities,8,62,63 the interfacial charge transfer at both OOH interfaces is expected to be negligible for the lying PTCDA on the lying F16CuPc under dark condition. As shown in Figure 5d, a downward shift in vacuum level (0.35 ( 0.05 eV) has been observed for PTCDA on the standing-up F16CuPc film. As a nonchemically interactive OOH interface, such downward shift in vacuum level can be attributed to the interfacial charge transfer involving electron transferring from PTCDA to the underlying standing-up F16CuPc. This suggests that it is possible to manipulate the interface charge transfer by controlling the molecular orientation. The orientation dependent IP due to the polar intramolecular bonds exposed at the surface can induce a vacuum level misalignment or the formation of interface dipoles at the OOH interfaces. More experiments are needed to confirm this hypothesis. In Figure 5(c), the ∆h for 8 nm PTCDA on the lying-down F16CuPc is measured to be 1.75 ( 0.05 eV with the HOMO peak centered at 2.3 ( 0.05 eV. For PTCDA on the standingup F16CuPc, the ∆h is 1.45 ( 0.05 eV with the HOMO peak centered at 2.0 ( 0.05 eV, as shown in Figure 5f. Figure 6, panels a and b, displays the energy level diagrams for 8 nm PTCDA on the lying-down and the standing-up F16CuPc thin films, respectively, clearly revealing the orientation dependent energy level alignments at both OOH interfaces. Although both PTCDA thin films are generally lying flat on both the standingup and the lying-down F16CuPc thin films, there is some difference in molecular tilt angles, as revealed by the small changes in the angular dependence of the C K-edge NEXAFS spectra for both PTCDA films in Figure 2, panels b and d. This can lead to the small variations of IPs for both PTCDA films, i.e., IP ) 6.55 ( 0.05 eV for PTCDA (R ) 18° ( 5°) on the lying-down F16CuPc and IP ) 6.40 ( 0.05 eV for PTCDA (R ) 25° ( 5°) on the standing-up F16CuPc. In particular, Figure 7 shows that the binding energies of both the C 1s core level [Figure 7a] and HOMO [Figure 7b] of the top PTCDA layers on the lying-down F16CuPc thin film is 0.3 eV higher than those on the standing-up F16CuPc thin film, demonstrating the possibility of engineering the energy level alignments at OOH interfaces by manipulating the molecular orientation of the underlying organic thin films.


Figure 7. (a) C 1s core level and (b) valence band PES spectra for the 8 nm PTCDA films on the lying-down (on HOPG) and the standingup (on SAM) F16CuPc thin films. The C 1s PES spectra are measured with photon energy of 350 eV, and the valence band PES spectra with photon energy of 60 eV. All binding energy are relative to the Fermi level position of the electron analyzer. All spectra are measured under the normal emission condition.

J. Phys. Chem. C, Vol. 113, No. 29, 2009 12837 We also carried out the experiment to study the energy level alignment at the typical p-n OOH of PTCDA on both standingup (on SiO2) and lying-down (on HOPG) CuPc thin films, which involves interfacial charge transfer. As previously reported, PTCDA always lies flat on CuPc thin films, irrespective of the molecular orientation of the underlying CuPc films.28 The molecular tilt angles of both PTCDA and CuPc films are summarized in Table 1. The evolution of the secondary electron cutoff at the low-kinetic energy region (Figure 8, panels a and d) and PES valence band (VB) spectra at the low-binding energy region (Figure 8, panels b, c, e, and f) and during the sequential deposition of PTCDA on CuPc films is shown in Figure 8. The obtained values of ∆h, φ, and IP for PTCDA on CuPc films from PES measurement are summarized in Table 1. The schematic drawings of the energy level diagrams for 10 nm PTCDA on the lying-down (on HOPG) and the standing-up (on SiO2) CuPc thin films are displayed in Figure 9, panels a and b, respectively. After the deposition of 10 nm PTCDA, an upward shift of the vacuum level by 0.15 ( 0.05 eV was

Figure 8. Evolution of synchrotron PES spectra for the sequential deposition of CuPc and PTCDA on HOPG (panels a-c) and SiO2 (panels d-f): (a and d) valence band spectra at the low-binding energy part and (b and e) corresponding near the EF region spectra from panels a and d, and (c and f) PES spectra at the low-kinetic energy part (secondary electron cutoff). All spectra are measured with photon energy of 60 eV. All binding energy are relative to the Fermi level position of the electron analyzer. All spectra are measured under the normal emission condition.

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Chen et al. choosing the appropriate molecular orientation. In situ NEXAFS measurements also reveal that RT deposition of top PTCDA layers does not alter the molecular orientation of the underlying F16CuPc thin films. However, for the model systems of the p-n OOH comprising flat lying PTCDA on the lying-down (on HOPG) and the standing-up (on SiO2) CuPc thin films, the presence of the interfacial charge transfer leads to a Fermi-level pinning at such p-n OOH interface and the HOMO position of the PTCDA top layers is independent of the molecular orientation of the underlying CuPc films. This detailed understanding of molecular orientation dependent energy level alignments at the interface of organic heterojunctions has important implications to organic electronic devices comprising multiple tailored organic layers, in particular for improving the power conversion efficiency of OOH-based organic photovoltaic cells through the interface engineering.

Figure 9. Schematics for the energy level diagrams of flat lying 10 nm PTCDA thin films on (a) the lying-down CuPc on HOPG and (b) the standing-up CuPc on SiO2.

Acknowledgment. Authors acknowledge the support from the A*STAR Grant R-398-000-036-305 and ARF Grants R-143000-392-133 and R-144-000-192-116. References and Notes

observed on the lying-down CuPc film; while a larger upward shift of 0.55 ( 0.05 eV was found on the standing-up CuPc film, originated from the combination of the interfacial charge transfer and orientation dependent IP. In contrast to the PTCDA on F16CuPc (n-n OOH), the HOMO positions of PTCDA on both CuPc films are almost identical, i.e., 1.90 ( 0.05 eV for PTCDA on the lying-down CuPc on HOPG and 1.95 ( 0.05 eV for PTCDA on the standing-up CuPc on SiO2. This suggests that a Fermi-level pinning occurs at the PTCDA/CuPc interfaces originated from the interfacial charge transfer.64 Such interfacial charge transfer induced Fermi-level pining has been previously observed for pentacene65,66 or C6067 on various conducting polymer substrates with different work functions. By comparing the energy level alignments at the n-n OOH of PTCDA on F16CuPc and p-n OOH of PTCDA on CuPc, the HOMO position of PTCDA top layers is largely affected by the molecular orientation of the underlying F16CuPc films in the absence of an interfacial charge transfer; while in the presence of an interfacial charge transfer, a Fermi-level pinning occurs at the p-n OOH interface and the HOMO position of the PTCDA top layers is independent of the molecular orientation of the underlying CuPc films. 4. Conclusion In summary, we have fabricated model systems of n-n OOHs comprising flat lying PTCDA thin films on lying-down F16CuPc film on HOPG and standing-up F16CuPc film on C8-SAM terminated Au(111). We observe orientation dependent energy level alignments at the interfaces of both OOHs by synchrotron based high-resolution PES and NEXAFS measurements. The formation of the orientation dependent interface dipoles is attributed to the orientation dependent IPs of the underlying F16CuPc thin films. This is explained by the intrinsic surface dipoles induced in the standing-up F16CuPc film due to the polar interamolecular bond (C-F) exposed at the surfaces. We observed that the energy level alignment at the OOH interfaces can be tuned by the molecular orientation of the underlying F16CuPc thin films, i.e., the binding energies of both the C 1s core level and the HOMO of the top PTCDA layers on the lyingdown F16CuPc thin film is 0.3 eV higher than those on the standing-up F16CuPc thin film, demonstrating the possibility of manipulating the energy level alignment at OOH interfaces by

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JP903139Q

Molecular Orientation Dependent Energy Level Alignment at Organic

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