Effect of Gap States on the Orientation-Dependent Energy Level

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Effect of Gap States on the Orientation-Dependent Energy Level Alignment at the DIP/F16CuPc Donor
Acceptor Heterojunction Interfaces Jian Qiang Zhong,† Hong Ying Mao,‡ Rui Wang,† Dong Chen Qi,† Liang Cao,† Yu Zhan Wang,† and Wei Chen*,†,‡ † ‡

Department of Physics, National University of Singapore, 2 Science Drive 3, 117542, Singapore Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543, Singapore ABSTRACT: The interface properties of organic
organic heterojunctions (OOHs) between diindenoperylene (DIP) and copper hexadecafluorophthalocyanine (F16CuPc), including interface morphology, molecular orientation, and energy level alignment, have been investigated systematically by atomic force microscopy, in situ ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and synchrotron-based near-edge X-ray adsorption fine structure (NEXAFS) measurements. As revealed by NEXAFS measurements, DIP molecules adopt the standing orientation on the standing F16CuPc on the SiO2 substrate, while they have the lying-down configuration on the flat-lying F16CuPc on the highly oriented pyrolytic graphite substrate. From the UPS and XPS studies, there is a large interfacial charge transfer and a band-bending-like behavior in the standing DIP/F16CuPc OOHs, while the vacuum level is almost aligned at the lying-down DIP/F16CuPc OOH interface. We propose that the defects-induced gap states have a crucial effect on the observed orientation-dependent energy level alignment and band-bending behaviors at these OOHs interfaces.

1. INTRODUCTION Understanding the organic/inorganic and organic/organic interfaces is crucial for many technologically important organic electronic devices, such as organic photovoltaic cells (OPVs), organic light-emitting diodes (OLEDs), and organic field-effect transistors (OFETs).1
9 Intensive research efforts have been devoted to the optimization of these interface properties, including the interface morphology, molecular packing structures, and energy level alignment, to improve the device performance.10
14 For example, recently Helander, et al.7 demonstrated a surface chlorination process to significantly modify the work function of indium tin oxide (ITO) transparent electrodes. This can largely simplify the device fabrication processes and, at the same time, provide optimized matching of the energy levels between the active organic material and the ITO electrode to achieve high efficiency and brightness in OLED devices. For the energy level alignment at organic
organic heterojunction (OOH) interfaces, the offset between the highest occupied molecular orbital (HOMO) of the donor molecule and the lowest unoccupied molecular orbital (LUMO) of the acceptor molecule is essential to determine the open-circuit voltages in OPVs.15
17 On the other hand, suitable matching of the charge transport states in organic materials to the electrode Fermi level is also necessary to reduce the electron and hole injection barriers in OLEDs.18,19 However, the understanding of the energy level alignment mechanism is still controversial. Various models have been proposed to explain the vacuum level alignment (Schottky
Mott limit) and bandbending behaviors at different kinds of OOHs interfaces, such r 2011 American Chemical Society

as the integer charge-transfer model20
22 and the induced density of interface states model.23,24 It has been reported that the defects-induced gap states, such as the structure defects, including grain boundaries, imperfect molecular packing regions, or impurities, have a significant effect on the energy level alignment at the molecule/metal and OOH interfaces.25
27 As previously demonstrated, these gap states determine the energy level alignment at the organic donor
acceptor heterojunctions comprising copper hexadecafluorophthalocyanine (F16CuPc) and copper phthalocyanine (CuPc), hence resulting in an obvious interfacial band-bending-like behavior for the OOH with a standing configuration (i.e., molecular π plane oriented nearly perpendicular to the substrate surface).25 Therefore, it is necessary to carry out a systematic investigation of the OOHs with a well-defined molecular orientation and surface morphology, and hence to understand the energy level alignment mechanism at the OOH interfaces, thereby providing design rules for effective interface engineering approaches to improve device performance for practical applications of organic heterojunction-based electronic devices. In this paper, we study the interfacial energy level alignment mechanism using model donor
acceptor OOHs with a welldefined molecular orientation, comprising diindenoperylene (DIP) and F16CuPc molecules, which have been widely used Received: September 7, 2011 Revised: October 21, 2011 Published: November 15, 2011 23922

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31 The molecular structures of F16CuPc and DIP are displayed in Figure 1. The interface properties at these model donor
acceptor OOHs, including the interface morphology, molecular orientation, and energy level alignment, were systematically studied through the combination of atomic force microscopy (AFM), in situ ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and synchrotron-based near-edge X-ray adsorption fine structure (NEXAFS) measurements. It is revealed that the interface properties at these OOHs are strongly affected by the molecular orientation. It is also found that the gap states play a crucial role in determining the energy level alignment at these organic heterojunction itnerfaces.25,26

2. EXPERIMENTAL SECTION The morphology of organic thin films was measured ex-situ by AFM with tapping mode under ambient conditions. The collected data were analyzed using the Nanoscope VI program. Insitu UPS experiments were carried out in a customer-built ultrahigh-vacuum (UHV) system with He 1α (21.2 eV) as the excitation source.32 Vacuum level shifts were determined from the low kinetic energy part of UPS spectra with a
5 V sample

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bias. The sample work function j was obtained through the equation j = hv
W, where W is the spectrum width (the energy difference between the substrate Fermi level and low kinetic energy onset). In-situ XPS and synchrotron-based NEXAFS measurements were carried out at the Surface, Interface and Nanostructure Science (SINS) beamline of the Singapore Synchrotron Light Source.33 The NEXAFS measurements were performed in total-electron-yield (TEY) mode.33 The XPS studies were performed with an Al Kα source (1486.6 eV). Si(111) with native oxide, referred to as SiO2 in this paper, and freshly cleaved highly oriented pyrolytic graphite (HOPG) substrates were thoroughly degassed in the UHV chamber at around 700 K overnight before molecule deposition. Vacuumsublimation purified DIP and F16CuPc molecules were thermally evaporated onto the substrates at room temperature (RT) from separate commercial Knudsen cells (Creaphys, Germany) in the growth chamber. Deposition rates of ∼0.20 nm/min for DIP and ∼0.25 nm/min for F16CuPc were chosen in our experiments. The nominal thickness of the organic thin films was estimated from the attenuation of the Si 2p peak intensity from the SiO2 substrate before and after deposition. The binding energy of all UPS and XPS spectra were calibrated and referenced to the Fermi level of a sputtered clean gold. UPS spectra were measured at normal emission, and XPS spectra were taken at 50° with respect to the surface normal direction. All the UPS, XPS, and NEXAFS measurements were performed at RT.

3. RESULTS AND DISCUSSION

Figure 1. Schematic drawings of (a) F16CuPc and (b) DIP molecular structures.

3.1. Morphology of the DIP/F16CuPc Heterojunctions. The film morphology of the DIP/F16CuPc heterojunctions grown at RT was characterized by ex situ AFM for F16CuPc on DIP on SiO2 (Figure 2a
c) and the reversed OOH of DIP on F16CuPc on SiO2 (Figure 2d
f). DIP molecules grow in a layer-by-layer mode at the initial growth stage on SiO2 (Figure 2a). After completing one wetting layer, the molecules nucleate into

Figure 2. AFM morphology images (2 um  2 um) of (a) 2.5 nm DIP on SiO2, (b) 15.8 nm DIP on SiO2, (c) 16.0 nm F16CuPc on DIP-covered SiO2, (d) 1.8 nm F16CuPc on SiO2 (the inset shows a zoom-in image with a 3 nm height scale), (e) 16.0 nm F16CuPc on SiO2, and (f) 15.8 nm DIP on F16CuPc-covered SiO2. 23923

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Figure 3. DIP on SiO2: UPS spectra at (a) the low kinetic energy region (secondary electron cutoff) and (b) the low binding energy region near the Fermi level during the deposition of a DIP molecule on the SiO2 substrate. (c) XPS C 1s core-level evolution for DIP on SiO2. (d) Angledependent C K-edge NEXAFS spectra for DIP on SiO2. (e) The plot of the work function, HOMO edge, and C 1s core level of DIP on SiO2 as a function of film thickness.

dispersed single-layer islands with the average domain size exceeding 300 nm. It is worth noting that the third or even fourth layer of DIP starts nucleating on top of the second layer island, suggesting a Stranski
Krastanov growth mode of DIP on SiO2 at RT. From the line profile (not shown here), the height of the single-layer island is measured to be around 1.68 nm, close to the molecular length along the long axis. This indicates that DIP molecules adopt a typical standing-up configuration on the inert SiO2 substrate, consistent with previous reports.34,35 With further increasing the DIP thickness (Figure 2b), the island growth behavior is even pronounced.36 Interestingly, after the deposition of F16CuPc on DIP film on SiO2 (Figure 2c), these F16CuPc molecules aggregate into dispersed small textures comprising the assemblies of needle-like nanostructures.37 Such film morphology is very different from the cluster-like feature of F16CuPc grown on the bare SiO2, as shown in Figure 2d,e.38,39 This clearly demonstrates the template effect of the underlying crystalline DIP film, which can significantly modulate the top layer film growth behaviors and hence be able to alter the film morphology. Similarly, such an effect can also be reflected on the film morphology of DIP on F16CuPc on SiO2 (Figure 2f), where monodispersed small DIP islands nucleate on top of the F16CuPc film.40,41 3.2. Energy Level Alignment at the Standing DIP/F16CuPc OOH Interfaces. F16CuPc on DIP on SiO2. We fabricate model donor
acceptor OOHs to evaluate the effect of molecular orientation on the interfacial energy level alignment mechanism. The molecular orientation was measured by in situ angle-dependent

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Figure 4. F16CuPc on DIP/SiO2: UPS spectra at (a) the low kinetic energy region and (b) the low binding energy region near the Fermi level during the deposition of a F16CuPc molecule on DIP-covered SiO2. (c) XPS C 1s core-level evolution for F16CuPc on DIP/SiO2. (d) Angledependent C K-edge NEXAFS spectra for F16CuPc on DIP/SiO2. (e) The plot of the work function, HOMO edge, and C 1s core level of F16CuPc on DIP/SiO2 as a function of film thickness.

NEXAFS.13,42 The first example is the OOH of F16CuPc on DIP on SiO2. Figure 3d shows the angle-dependent C K-edge NEXAFS spectra of 8.5 nm DIP on the SiO2 substrate. The first three sharp absorption peaks (283
290 eV) at normal incidence light (θ = 90°) are due to the resonant transitions from the C 1s core level of the various carbon atoms into unoccupied molecular orbitals (π* orbitals). The broad absorption peaks (290
315 eV) at the higher photon energies are the transitions to the σ* states. In principle, for planar DIP molecules with a standingup configuration, the π* resonance peaks will be greatly enhanced at normal incidence since the electric field vector E of the incident linear polarized synchrotron light has a large projection along the direction of the π* orbitals. As shown in Figure 3d, the π* resonance intensity of planar DIP is greatly enhanced at normal incidence (θ = 90°) and depressed at grazing incidence (θ = 20°). The intensity I of the π2* resonance (at 284.9 eV) is related to the tilt angle α of the DIP molecular plane with respect to the substrate plane and the synchrotron light incidence angle θ by42 1 IðθÞ µ 1 þ ð3cos2 θ
1Þð3cos2 α
1Þ 2 Using the intensity ratio R(π2*) = I(90°)/I(20°), we can estimate the average tilt angle α for 8.5 nm DIP on SiO2 to be 79° ( 5°. This indicates that DIP stands up-right on SiO2, consistent with our AFM results (Figure 2a,b). Figure 3a,b shows the evolution of UPS spectra of the standing-up DIP on SiO2 with increasing thickness. Clearly, the vacuum level is almost aligned at the DIP/SiO2 interface. At the 23924

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Figure 5. F16CuPc on SiO2: UPS spectra at (a) the low kinetic energy region and (b) the low binding energy region near the Fermi level during the deposition of a F16CuPc molecule on the SiO2 substrate. (c) XPS C 1s core-level evolution for F16CuPc on SiO2. (d) Angle-dependent C K-edge NEXAFS spectra for F16CuPc on SiO2. (e) The plot of the work function, HOMO edge, and C 1s core level of F16CuPc on SiO2 as a function of film thickness.

Figure 6. DIP on F16CuPc/SiO2: UPS spectra at (a) the low kinetic energy region and (b) the low binding energy region near the Fermi level during the deposition of a DIP molecule on F16CuPc-covered SiO2. (c) XPS C 1s core-level evolution for DIP on F16CuPc/SiO2. (d) Angledependent C K-edge NEXAFS spectra for DIP on F16CuPc/SiO2. (e) The plot of the work function, HOMO edge, and C 1s core level of DIP on F16CuPc/SiO2 as a function of film thickness.

same time, we did not observe any apparent binding energy shift for the HOMO peak in Figure 3b, C 1s core level in Figure 3c, and their plot in Figure 3e, indicating that there is no apparent charge transfer at the DIP/SiO2 interface. From Figure 3a,b, the ionization potential (IP) for the standing DIP is measured to be 5.20 eV. By considering the large HOMO
LUMO gap (∼2.5 eV) of DIP43 and the SiO2 substrate work function of 3.98 eV, the Fermi level can be positioned 1.22 eV above the DIP HOMO and 1.28 eV below the LUMO by assuming the vacuum level alignment at the interface, making the interfacial charge transfer energetically unfavorable and leading to the vacuum level alignment at this interface. Figure 4d shows the angular-dependent C K-edge NEXAFS for 9.6 nm F16CuPc on DIP on SiO2, revealing that F16CuPc molecules adopt the standing-up configuration on the DIP thin film with an average tilt angle α = 77° ( 5°. As shown in Figure 4b, after the deposition of 3.0 nm of F16CuPc on the DIP film, the valence band spectrum is dominated by F16CuPc-related components. The HOMO peak is centered at the binding energy of 1.74 ( 0.02 eV and the HOMO leading edge is at 1.14 ( 0.02 eV when the F16CuPc film thickness increased to 13.6 nm. As shown in Figure 4a, with increasing F16CuPc coverage, an obvious upward vacuum level shift of 1.14 eV is observed, originating from the interfacial charge transfer via an electron transferring from the underlying DIP to the top F16CuPc layer. At the same time, both the HOMO peak (Figure 4b) and the C 1s core level (Figure 4c) of F16CuPc shift toward lower binging energy. Figure 4e displays the plot of the work function, HOMO

edge, and C 1s core level of F16CuPc as a function of F16CuPc thickness, clearly indicating an upward band-bending at this OOH interface. For these noninteractive F16CuPc/DIP OOHs, the vacuum level shift and the band-bending-like behaviors originate from the interfacial charge transfer and can be explained by the gap states model.25 Recent studies have shown that the defects-induced gap state can extend from the valence band edge into the band gap for small organic molecules as derived from the measurements of ultra-high-sensitivity UPS spectra and electrical characteristics of OFETs.26,44 The existence of these gap states determines the energy level alignment at the OOH interfaces. At this F16CuPc/ DIP interface, the work function of the standing DIP substrate (4.12 eV) is smaller than the electron affinity of the standing F16CuPc (4.60 eV), which is derived from the measured IP (∼6.40 eV) and the reported HOMO
LUMO gap of 1.80 eV for F16CuPc.45 By simply assuming that the vacuum level is aligned at the interface, this could lead to the placement of the F16CuPc LUMO below the Fermi level and result in a spontaneous electron transferring from the substrate Fermi level to the F16CuPc LUMO, thereby leading to the upward shift of the VL or increase of work function. According to the gap states model, as the integration of all the empty gap states in F16CuPc film is not sufficient to accommodate the transferred electrons, the Fermi level is pinned at the F16CuPc LUMO at the initial stage for the formation of the F16CuPc/DIP interface.25
27 When the thickness is further increased, the Fermi level gradually moves away from the F16CuPc LUMO, penetrates into the F16CuPc 23925

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Figure 7. F16CuPc on HOPG: UPS spectra at (a) the low kinetic energy region and (b) the low binding energy region near the Fermi level during the deposition of a F16CuPc molecule on the HOPG substrate. (c) XPS C 1s core-level evolution for F16CuPc on HOPG. (d) Angle-dependent C K-edge NEXAFS spectra for F16CuPc on HOPG. (e) The plot of the work function, HOMO edge, and C 1s core level of F16CuPc on HOPG as a function of film thickness.

HOMO
LUMO gap, and finally is pinned at the bottom of the empty gap states, of which the intensity exponentially decays away from the LUMO. This is accompanied by a noticeable upward band-bending behavior, that is, upward shift of the vacuum level (increase of work function) and the shift toward lower binding energy of the F16CuPc HOMO and C 1s core level, as shown in Figure 4a
c,e. DIP on F16CuPc on SiO2. For the reversed OOH structure comprising DIP on F16CuPc on SiO2, their standing-up configuration was also confirmed by the angle-dependent C K-edge NEXAFS measurements, as shown in Figures 5d and 6d. F16CuPc molecules adopt a standing-up configuration on SiO2 with a tilt angle α = 80° ( 5°. The upper DIP molecules also have a similar orientation with an average tilt angle α = 76° ( 5°. As shown by the UPS and XPS results in Figure 5a
c,e, there is a clear upward band-bending phenomenon at the F16CuPc/SiO2 interface. This situation is similar to the previous F16CuPc/DIP interface since the SiO2 substrate work function (3.98 eV) is smaller than the electron affinity of F16CuPc (4.64 eV). A spontaneous charge transfer from the substrate Fermi level to the F16CuPc (gap states + LUMO) appears until thermodynamic equilibrium is reached. We then focus on the energy level alignment at the standing DIP/F16CuPc OOHs interface after depositing DIP on F16CuPc on SiO2. It is found that the interfacial energy level alignment has a very similar trend, but in the opposite direction for the F16CuPc/DIP OOH, as shown in Figure 6a
c,e. Again, it can be explained by the defect-induced gap states model. The work

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Figure 8. DIP on F16CuPc/HOPG: UPS spectra at (a) the low kinetic energy region and (b) the low binding energy region near the Fermi level during the deposition of a DIP molecule on F16CuPc-covered HOPG. (c) XPS C 1s core-level evolution for DIP on F16CuPc/HOPG. (d) Angle-dependent C K-edge NEXAFS spectra for DIP on F16CuPc/ HOPG. (e) The plot of the work function, HOMO edge, and C 1s core level of DIP on F16CuPc/HOPG as a function of film thickness.

function of the standing F16CuPc substrate (5.34 eV) is large than the IP of the DIP (5.20 eV). If assuming the vacuum level alignment, this leads to the HOMO of DIP positioned above the substrate Fermi level. As a result, the HOMO electrons from the DIP layer spontaneously transfer to the underlying F16CuPc layer upon the formation of physical contact. When the DIP thickness is further increased, the degree of the interfacial charge transfer is reduced, and hence the integration of the gap states over a smaller energy range is enough to provide necessary electrons to reach the thermodynamic equilibrium at the interface. This results in the Fermi level pinning at a position even further away from the HOMO. Finally, the Fermi level moves into the band gap of the DIP film depending on the distance from the DIP/F16CuPc interface. As in the energy level alignment diagram shown in Figure 9a, there is a very strong interfacial charge transfer and a band-bending behavior for this standing DIP/ F16CuPc OOH. 3.3. Energy Level Alignment at the Lying-Down DIP/ F16CuPc OOH Interface. We also investigated the interfacial energy level alignment mechanism of the DIP/F16CuPc heterojunction with a lying-down configuration, as shown in Figures 7 (F16CuPc/HOPG interface) and 8 (DIP/F16CuPc interface). From the NEXAFS measurements in Figures 7d and 8d, F16CuPc molecules lie flat on HOPG with a tilt angle α = 12° ( 5°, and DIP molecules also adopt the lying-down configuration on top of F16CuPc films with a tilt angle α = 22° ( 5°. The lying-down configuration of F16CuPc or DIP molecules arises from the directional interfacial π
π interactions between 23926

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C
F polar bond in F16CuPc and the opposite C
H polar bond in DIP.46,48
51 Clearly, the molecular orientation has an apparent effect on the degree of charge transfer at the organic heterojunction interface, that is, a much stronger charge transfer appearing at the standing OOH interface than that at the lyingdown OOH interface. The appearance of the strong interfacial charge transfer at the standing DIP/F16CuPc OOHs is accompanied by the formation of an electron accumulation layer in the n-type F16CuPc film and a hole accumulation layer in the p-type DIP film at the OOH interface. Both the electron and the hole accumulation layers can serve as effective charge transport channels at the OOH interface and hence allow the efficient operation of ambipolar FET devices. Moreover, the stacking direction of the molecular π planes of both DIP and F16CuPc molecules in the standing OOH is parallel to the substrate plane, further facilitating the efficient charge transport in an FET device configuration. In contrast, for the lying-down OOHs, the molecular π planes stack along the direction perpendicular to substrate plane and hence ensure the efficient charge transport in p
n junction-based solar cell devices.

Figure 9. Schematic drawings showing the molecular orientation (top panel) and energy level alignment diagrams (bottom panel) for the DIP/ F16CuPc on (a) SiO2 and (b) HOPG.

HOPG and the lying-down F16CuPc, or between the lying-down DIP and F16CuPc molecules.46,47 The UPS spectra at the low kinetic energy region shows that the vacuum level is almost aligned at the F16CuPc/HOPG interface. This suggests that the interfacial charge transfer is negligible. The work function of the HOPG substrate (4.44 eV) is larger than the electron affinity of the lying-down F16CuPc (3.96 eV). By assuming the vacuum level alignment, the Fermi level is located in the middle of the HOMO
LUMO gap of the lying-down F16CuPc molecules, hence excluding the possibility of spontaneous interfacial charge transfer. The evolution of the HOMO (Figure 7b) and the C 1s core level (Figure 7c) further confirm the vacuum level alignment at the F16CuPc/HOPG interface (Figure 7e). We did not observe any apparent charge transfer at the DIP/F16CuPc interface, as shown in Figure 8a
c. Similarly, such a vacuum level alignment can be rationalized as follows: The work function of the lying-down F16CuPc substrate (4.56 eV) is smaller than the IP of the lying-down DIP (5.60 eV). In this case, the Fermi level can be positioned in the middle of the HOMO
LUMO gap of the top DIP film and hence the interfacial charge transfer is not energetically favorable, leading to the vacuum level alignment at the DIP/F16CuPc interface (Figure 8e). Figure 9 shows the summary of the molecular orientation and schematic energy level alignment diagrams for the DIP/F16CuPc OOHs on different substrates, clearly revealing an orientationdependent energy level alignment. It also reveals the orientationdependent IP of organic films; that is, the IP of the lying-down F16CuPc (IP = 5.76 eV) is 0.68 eV lower than that of the standing film (IP = 6.44 eV), whereas the IP for the lying-down DIP thin film (IP = 5.60 eV) is 0.40 eV higher than that of the standing film (IP = 5.20 eV). Such an orientation-dependent IP in F16CuPc and DIP and other ordered organic thin films has been previously reported, originating from the exposed surface polar intramolecular bonds at the surface of ordered organic thin films, such as the

4. CONCLUSIONS We have carried out systematic investigations on the energy level alignment mechanism at the standing and lying-down DIP/ F16CuPc OOH interfaces by using AFM, in situ UPS, XPS, and synchrotron-based NEXAFS measurements. A strong interfacial charge transfer and a band-bending-like behavior exist at the standing DIP/F16CuPc or F16CuPc/DIP OOH interfaces, while a nearly vacuum level alignment is observed at the lying-down DIP/F16CuPc OOH interface on HOPG. The energy level alignment at these OOH interfaces can be fully explained through the combination of the orientation-dependent ionization potential of ordered organic films and gap states model. Our detailed investigation by using model donor
acceptor OOHs of DIP/F16CuPc can help us better understand the interfacial energy level alignment, hence providing design rules to engineer the interface properties for their applications in organic electronic devices. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The authors acknowledge Jens Pflaum for providing purified DIP molecules, and the support from Singapore ARF grant R143-000-440-112 and NUS YIA grant R143-000-452-101. ’ REFERENCES (1) Peumans, P.; Uchida, S.; Forrest, S. R. Nature 2003, 425, 158. (2) Peumans, P.; Yakimov, A.; Forrest, S. R. J. Appl. Phys. 2003, 93, 3693. (3) Forrest, S. R. MRS Bull. 2005, 30, 28. (4) Yang, F.; Shtein, M.; Forrest, S. R. Nat. Mater. 2005, 4, 37. (5) Armstrong, N. R.; Wang, W.; Alloway, D. M.; Placencia, D.; Ratcliff, E.; Brumbach, M. Macromol. Rapid Commun. 2009, 30, 717. (6) McCarthy, M. A.; Liu, B.; Donoghue, E. P.; Kravchenko, I.; Kim, D. Y.; So, F.; Rinzler, A. G. Science 2011, 332, 570. (7) Helander, M. G.; Wang, Z. B.; Qiu, J.; Greiner, M. T.; Puzzo, D. P.; Liu, Z. W.; Lu, Z. H. Science 2011, 332, 944. (8) Wen, Y.; Liu, Y. Adv. Mater. 2010, 22, 1331. 23927

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The Journal of Physical Chemistry C (9) Koch, N. ChemPhysChem 2007, 8, 1438. (10) Smith, A. R. G.; Ruggles, J. L.; Cavaye, H.; Shaw, P. E.; Darwish, T. A.; James, M.; Gentle, I. R.; Burn, P. L. Adv. Funct. Mater. 2011, 21, 2225. (11) Dong, S.; Tian, H.; Huang, L.; Zhang, J.; Yan, D.; Geng, Y.; Wang, F. Adv. Mater. 2011, 23, 2850. (12) Halik, M.; Hirsch, A. Adv. Mater. 2011, 23, 2689. (13) Chen, W.; Qi, D.-C.; Huang, H.; Gao, X.; Wee, A. T. S. Adv. Funct. Mater. 2011, 21, 410. (14) Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K. Adv. Mater. 1999, 11, 605. (15) Brumbach, M.; Placencia, D.; Armstrong, N. R. J. Phys. Chem. C 2008, 112, 3142. (16) Blom, P. W. M.; Mihailetchi, V. D.; Koster, L. J. A.; Markov, D. E. Adv. Mater. 2007, 19, 1551. (17) Potscavage, W. J.; Yoo, S.; Kippelen, B. Appl. Phys. Lett. 2008, 93, 193308. (18) Gao, Y. L. Mater. Sci. Eng., R 2010, 68, 39. (19) Kulkarni, A. P.; Tonzola, C. J.; Babel, A.; Jenekhe, S. A. Chem. Mater. 2004, 16, 4556. (20) Braun, S.; Liu, X.; Salaneck, W. R.; Fahlman, M. Org. Electron. 2010, 11, 212. (21) Braun, S.; Salaneck, W. R.; Fahlman, M. Adv. Mater. 2009, 21, 1450. (22) Osikowicz, W.; de Jong, M. P.; Salaneck, W. R. Adv. Mater. 2007, 19, 4213. (23) Hwang, J.; Wan, A.; Kahn, A. Mater. Sci. Eng., R 2009, 64, 1. (24) Vazquez, H.; Flores, F.; Kahn, A. Org. Electron. 2007, 8, 241. (25) Mao, H. Y.; Bussolotti, F.; Qi, D.-C.; Wang, R.; Kera, S.; Ueno, N.; Wee, A. T. S.; Chen, W. Org. Electron. 2011, 12, 534. (26) Sueyoshi, T.; Fukagawa, H.; Ono, M.; Kera, S.; Ueno, N. Appl. Phys. Lett. 2009, 95, 183303. (27) Sueyoshi, T.; Kakuta, H.; Ono, M.; Sakamoto, K.; Kera, S.; Ueno, N. Appl. Phys. Lett. 2010, 96, 093303. (28) Kurrle, D.; Pflaum, J. Appl. Phys. Lett. 2008, 92, 133306. (29) Tripathi, A. K.; Pflaum, J. Appl. Phys. Lett. 2006, 89, 082103. (30) Wang, J.; Wang, H. B.; Yan, X. J.; Huang, H. C.; Yan, D. H. Appl. Phys. Lett. 2005, 87, 093507. (31) Wang, J.; Wang, H.; Yan, X.; Huang, H.; Jin, D.; Shi, J.; Tang, Y.; Yan, D. Adv. Funct. Mater. 2006, 16, 824. (32) Zhong, J. Q.; Huang, H.; Mao, H. Y.; Wang, R.; Zhong, S.; Chen, W. J. Chem. Phys. 2011, 134, 154706. (33) Yu, X. J.; Wilhelmi, O.; Moser, H. O.; Vidyaraj, S. V.; Gao, X. Y.; Wee, A. T. S.; Nyunt, T.; Qian, H. J.; Zheng, H. W. J. Electron Spectrosc. Relat. Phenom. 2005, 144, 1031. (34) Durr, A. C.; Schreiber, F.; Munch, M.; Karl, N.; Krause, B.; Kruppa, V.; Dosch, H. Appl. Phys. Lett. 2002, 81, 2276. (35) Huang, Y. L.; Chen, W.; Huang, H.; Qi, D. C.; Chen, S.; Gao, X. Y.; Pflaum, J.; Wee, A. T. S. J. Phys. Chem. C 2009, 113, 9251. (36) Zhang, X.; Barrena, E.; Goswami, D.; de Oteyza, D. G.; Weis, C.; Dosch, H. Phys. Rev. Lett. 2009, 103, 136101. (37) Zhang, Y.; Barrena, E.; Zhang, X.; Turak, A.; Maye, F.; Dosch, H. J. Phys. Chem. C 2010, 114, 13752. (38) de Oteyza, D. G.; Barrena, E.; Osso, J. O.; Sellner, S.; Dosch, H. J. Am. Chem. Soc. 2006, 128, 15052. (39) de Oteyza, D. G.; Barrena, E.; Sellner, S.; Osso, J. O.; Dosch, H. J. Phys. Chem. B 2006, 110, 16618. (40) Barrena, E.; de Oteyza, D. G.; Sellner, S.; Dosch, H.; Osso, J. O.; Struth, B. Phys. Rev. Lett. 2006, 97, 076102. (41) de Oteyza, D. G.; Krauss, N.; Barrena, E.; Sellner, S.; Dosch, H.; Osso, J. O. Appl. Phys. Lett. 2007, 90, 243104. (42) Stohr, J. NEXAFS Spectroscopy; Springer-Verlag: Berlin, 1992; Vol. 25. (43) Opitz, A.; Wagner, J.; Brutting, W.; Salzmann, I.; Koch, N.; Manara, J.; Pflaum, J.; Hinderhofer, A.; Schreiber, F. IEEE J. Sel. Top. Quantum Electron. 2010, 16, 1707. (44) Kalb, W. L.; Haas, S.; Krellner, C.; Mathis, T.; Batlogg, B. Phys. Rev. B 2010, 81, 155315.

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(45) Zahn, D. R. T.; Gavrila, G. N.; Gorgoi, M. Chem. Phys. 2006, 325, 99. (46) Chen, W.; Chen, S.; Huang, H.; Qi, D. C.; Gao, X. Y.; Wee, A. T. S. Appl. Phys. Lett. 2008, 92, 063308. (47) Chen, W.; Huang, H.; Chen, S.; Gao, X. Y.; Wee, A. T. S. J. Phys. Chem. C 2008, 112, 5036. (48) Duhm, S.; Heimel, G.; Salzmann, I.; Glowatzki, H.; Johnson, R. L.; Vollmer, A.; Rabe, J. P.; Koch, N. Nat. Mater. 2008, 7, 326. (49) Salzmann, I.; Duhm, S.; Heimel, G.; Oehzelt, M.; Kniprath, R.; Johnson, R. L.; Rabe, J. P.; Koch, N. J. Am. Chem. Soc. 2008, 130, 12870. (50) Duhm, S.; Salzmann, I.; Heimel, G.; Oehzelt, M.; Haase, A.; Johnson, R. L.; Rabe, J. P.; Koch, N. Appl. Phys. Lett. 2009, 94, 033304. (51) Chen, W.; Huang, H.; Chen, S.; Huang, Y. L.; Gao, X. Y.; Wee, A. T. S. Chem. Mater. 2008, 20, 7017.

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