Decamethylcobaltocene as an efficient n-dopant in organic electronic materials and devices

Organic Electronics 9 (2008) 575–581

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Decamethylcobaltocene as an efficient n-dopant in organic electronic materials and devices Calvin K. Chan a,*, Wei Zhao a, Stephen Barlow b, Seth Marder b, Antoine Kahn a a b

Department of Electrical Engineering, Princeton University, E-Quad, Olden Street, Princeton, NJ 08544, USA Department of Chemistry and Biochemistry, The Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, GA 30332, USA

a r t i c l e

i n f o

Article history: Received 4 January 2008 Received in revised form 28 February 2008 Accepted 5 March 2008 Available online 21 March 2008

PACS: 72.80.Le 68.55.Ln 79.60.Fr 73.61.Ph Keywords: Organic semiconductor n-Type doping Metallocene Photoemission spectroscopy Phthalocyanine

a b s t r a c t n-Doping of copper phthalocyanine (CuPc), which has an electron affinity (EA) of 3.52 eV, by decamethylcobaltocene (DMC) is demonstrated. DMC has a remarkably low solid-state ionization energy (IE) of 3.3 eV, as measured by ultra-violet photoemission spectroscopy (UPS). Further UPS measurements show a large 1.4 eV upward shift of the Fermi-level within the single particle gap of CuPc between the p- and n-doped films. n-Doping is also confirmed by current–voltage (I–V) measurements, which show a 106-fold increase in current density due to improved electron injection and enhanced conductivity of the bulk film. An organic p–i–n CuPc homojunction is also fabricated using F4-TCNQ and DMC as p- and n-dopants, respectively. Current–voltage characteristics demonstrate excellent rectification with a turn on voltage of approximately 1.3 eV, which is consistent with the built-in voltage measured by UPS and capacitance–voltage (C–V) measurements. Ó 2008 Elsevier B.V. All rights reserved.

1. Introduction Electronic doping of organic electronic materials with electron-donating or accepting compounds is an important, and increasingly used, technique for improving charge injection and transport in organic electronic devices, and for creating semiconductor device junctions. Doping also shows promise for enhancing the environmental and lifetime stability of organic devices. p-Doping, particularly with the electronegative molecule tetrafluorotetracyano-quinodimethane (F4-TCNQ), has been amply demonstrated with a number of hole-transport materials [1–9]. On the other hand, n-type doping is often hindered by the energetic requirements for electron transfer from dopant to host. The paucity of compounds with adequately * Corresponding author. Tel.: +1 609 258 2152; fax: +1 609 258 6279. E-mail address: [email protected] (C.K. Chan). 1566-1199/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2008.03.003

low ionization energy (IE) required for charge transfer to the unoccupied states of most host molecules of interest is exacerbated by the facile oxidation of these donor molecules by air. Until recently, practical n-doping of organic semiconductors employed alkali metals (i.e., Li or Cs), which exhibit exceptionally low IEs [10–17]. However, the small size of the alkali counter-ion presents two problems: the ion diffuses readily in the organic matrix, causing difficulties in device engineering and stability; and the metal ion often remains close to the doped organic molecule, forming a Coulomb trap for the donated electron. Bulkier molecular donors could assist in circumventing both of these limitations. Several groups have reported on molecular n-type doping using donor/host pairs with a specific match between the donor IE and the host electron affinity (EA) [18–21]. However, most of these stable molecular n-dopants have

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relatively high IEs, which limit the range of possible host materials to those with high EAs. One approach for achieving lower IE molecules uses the thermal decomposition of precursor molecular salts to sublimate molecular dopants, but this method introduces additional uncontrolled impurities into the organic film [21–24]. n-Doping with bis (terpyridine) ruthenium has also been demonstrated [19], but such compounds are not readily available and are difficult to synthesize. We recently showed that cobaltocene (CoCp2), with an IE of 4 eV as measured by ultraviolet photoemission spectroscopy (UPS), efficiently and controllably n-dopes a tris(thieno)-hexaazatriphenylene (THAP) derivative, which has an EA of 4.5 eV as measured by inverse photoemission spectroscopy (IPES). n-Doping was measured through a shift of the Fermi-level (EF) towards the unoccupied states of the host, and through a 103-fold increase in the current density in CoCp2-doped THAP devices [25]. Chemical composition and depth profiling experiments showed that the doping concentration is controllably varied and that dopant diffusion is negligible [26]. Gas-phase decamethylmetallocenes have previously been shown to possess considerably lower IEs as compared

Fig. 1. UPS and IPES spectra of DMC (top) and CuPc (bottom) showing the favorable energy level alignment of the dopant donor level with the host acceptor level. The DMC and CuPc molecular structures are shown in insets.

to their unsubstituted metallocene derivatives [27,28]. Given that the ionization energy of solid-phase cobaltocene (IEs) is reduced by 1.5 eV with respect to the gas-phase value (IEg), assuming a comparable polarization effect (P = IEg  IEs) for decamethylcobaltocene (DMC or CoCp2 ; Fig. 1 top) would place its IEs at only 3.2 eV. This hypothesis is verified by the measurement of the energetic levels of DMC by UPS and IPES. n-Doping of copper phthalocyanine (CuPc; Fig. 1 bottom) by DMC is demonstrated by measured shifts of the valence states towards high binding energy with respect to EF, and by the construction of n-doped CuPc diodes and homojuction devices. CuPc has an EA of 3.52 eV, which is substantially lower than the IE of most molecular donors investigated to date. DMC is therefore introduced here as a very promising candidate for efficient n-type doping of a number of organic molecular semiconductors. 2. Experimental Experiments were conducted in an interconnected three-chamber ultra-high vacuum (UHV) system equipped for organic film growth, UPS and IPES analysis, and in situ current–voltage (I–V) and capacitance–voltage (C–V) measurements. DMC was loaded, as received from Sigma–Aldrich, under inert N2 atmosphere into a sealed UHV gashandling apparatus that allows the release of a precisely regulated amount of dopant into the vacuum chamber [25,26]. When the DMC source is heated to 100 °C, it provides a background pressure (pd) of 106 Torr in the vacuum chamber when the gas release aperture is fully opened. The DMC source was mounted on the sample preparation chamber equipped with a thermal evaporation cell containing CuPc (Sigma–Aldrich), which was previously purified by three cycles of gradient sublimation. This chamber, in which all doped films were grown, is connected to the growth chamber and the analysis chamber, allowing for preparation of doped films, growth of undoped and metal films, and sample characterization without breaking vacuum. Experiments were conducted at base pressures of 2  1010, 5  1011, and 1  109 Torr in the preparation, analysis, and growth chambers, respectively. For analysis of the electronic structure of DMC, pristine films of the dopant were condensed onto indium-tin-oxide (ITO) substrates cooled to 80 K by a closed-cycle helium refrigerator. The ITO surfaces were prepared by mechanical, detergent, and solvent cleaning, followed by 1 h of UV-ozone exposure, and immediately transferred into vacuum. Doped CuPc films were obtained by evaporating the host onto a polycrystalline Au/Ti/Si(100) substrate in a controlled background pressure of DMC varying between pd = 108 to 106 Torr to obtain different doping concentrations. The deposition rate for CuPc was monitored with a quartz crystal microbalance, assuming a bulk density of 1.5 g cm3, and the background pressure of gas-phase DMC was monitored with a standard pressure ionization gauge located near the dopant source. Undoped CuPc was grown in the separate growth chamber. Occupied electronic states of the organic films were measured with UPS using the He I (hm = 21.22 eV) and He

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II (hm = 40.8 eV) photon lines of a helium discharge lamp, while unoccupied electronic states were measured using IPES in the isochromat mode [29]. All spectra presented here are referenced to, and aligned at, the Fermi-level (EF) of the experimental system determined by UPS and IPES measurements on a clean Au sample. The position of the vacuum level (Evac) of each surface was determined using the low-energy secondary electron cut-off [30]. Experimental resolutions for UPS and IPES are 0.15 and 0.5 eV, respectively. I–V measurements were conducted in vacuum on 100– 200 nm undoped, interfacially doped, or uniformly doped CuPc films grown on solvent-cleaned Au. These Au surfaces, used as-loaded, are covered with physisorbed contaminants, particularly water and hydrocarbons, and will be hereafter referred to as Au*. Top contacts consisting of 25 nm thick circular Au pads of varying diameter were then evaporated through a shadow mask. Measurements were done in situ at room temperature using a HP4155A semiconductor parameter analyzer. CuPc homojunction devices, structured with a 150 nm organic layer between an ITO bottom contact and a 25 nm thick Au top contact, were constructed using F4-TCNQ as the acceptor and DMC as the donor. The devices included: (i) an intrinsic CuPc layer, and (p–i–n) a 10 nm p-region, 120 nm i-region, and a 20 nm n-region. Each device was characterized using I–V measurements, while p–i–n devices were additionally characterized using C–V measurements with a HP4274A multi-frequency LRC meter. The built-in potential in the (p–i–n) device was also determined directly via UPS measurement on the n-, i- and p-layers at various stages of the fabrication.

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Fig. 2. (a) UPS spectra of the vacuum level onset (left) and filled valence states (right) of: (i) solvent-cleaned, as-loaded Au, ‘‘Au*”; (ii) undoped CuPc; (iii) 3% DMC-doped CuPc; (iv) 15% DMC-doped CuPc; and (v) 30% DMC-doped CuPc. (b) Corresponding energy-level diagrams showing frontier filled (thick lines) and empty (thick dashes) states, EF and Evac.

3. Results and discussion 3.2. Decamethylcobaltocene as a donor in CuPc 3.1. Energetic levels of DMC The occupied and unoccupied states of a 15 monolayer films of DMC (assuming a sticking coefficient equal to unity at low temperature) measured by UPS and IPES are plotted in Fig. 1, along with the corresponding states of a 6.5 nm CuPc film on Au*. Conventionally, adiabatic IE and EA of the condensed phase materials are determined as the energy difference between the vacuum level (Evac) and the onset of the highest and lowest energy features in UPS and IPES, respectively [30–32]. DMC exhibits an EA of 2.06 eV and a remarkably low IE of only 3.3 eV. This solid-state IE is in precise agreement with the hypothesized value derived from subtracting the 1.5 eV polarization energy observed for CoCp2 from the 4.7 eV gas-phase IE for DMC [25,27]. CuPc shows an IE of 5.35 eV, an EA of 3.52, and therefore a transport gap Egap of 1.83 eV, which is in good agreement with previously published solid-state spectroscopy data on phthalocyanines [4,5,14,15,33]. The most relevant energy level for n-type doping, i.e., the dopant IE is highlighted by a bold line spanning the two plots, and leads to the prediction of an energetically favorable electron transfer from DMC to CuPc in a simplified charge transfer model.

Doping of CuPc by DMC was investigated with UPS on CuPc layers undoped or doped with varying DMC concentrations deposited on Au*. The occupied state spectrum of Au* is plotted in Fig. 2a(i). The metal work function is 4.72 eV, which is typical of a contaminated Au surface. Such a surface has been shown to produce lower hole injection barriers (/Bh) to organic molecular films when compared to a pristine, atomically clean Au surface [34]. In agreement with these findings, undoped CuPc deposited on the Au* substrate is found in Fig. 2a(ii) to form a 0.67 eV hole injection barrier, with the Fermi level in the lower part of the gap. A series of 6.5 nm thick CuPc films were also deposited onto Au* substrates at 2 Å s1 under DMC partial pressures of 1  107, 5  107, and 1  106 Torr. The interface between the doped CuPc and Au* was mediated by a 2.5 nm layer of pristine CuPc to prevent direct interaction of DMC-molecules with the substrate that could result in modification of the Au* work function. The Co2p/C1s core level peak intensity ratio measured in X-ray photoemission spectroscopy (XPS) reveals that the CuPc sample with pd = 106 Torr is doped at a concentration of 30%. Assuming a linear extrapolation of the concentration as a function

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of DMC partial pressure during growth, the varying pd result in doping concentrations of 3%, 15%, and 30%, respectively. Previous Rutherford backscattering (RBS) experiments on THAP doped with CoCp2 suggest the validity of these estimated doping concentrations [26], but the exact doping concentrations for DMC-doped CuPc remains to be confirmed. The UPS spectra of the doped films are plotted in Fig. 2a(iii)–(v) and summarized in the energy-level diagram shown in Fig. 2b. They show the progressive shift of the CuPc molecular levels towards higher binding energy with increasing doping. This corresponds to a shift of EF towards the unfilled CuPc states, and is indicative of significant and controlled n-doping. In the highest doping scenario, EF shifts 0.95 eV to the upper part of the gap, with a final position only 0.31 eV below the edge of the CuPc empty states. The 1.1 eV shift of the local vacuum level, Evac, is slightly larger than the molecular level shift, and is likely the result of some DMC molecules adsorbed on the CuPc surface. For the highest doping concentrations, pd = 5  107 and pd = 1  106 Torr, a 0.1 eV broadening of all molecular peaks is observed, which indicates a high level of dopant incorporation. Additionally, an observable density of filled states within the single-particle gap, labeled ‘‘gap state” in Fig. 2a, may be the result of the formation of charge-transfer complexes at high doping concentrations [35]. Nevertheless, the spectra of the highly doped samples retain all the features of the CuPc film. 3.3. DMC-doped CuPc devices Undoped CuPc devices were grown on Au* and capped with freshly evaporated Au top contacts. The I–V measurements, normalized to current density and electric field, for the 100 nm undoped CuPc device are plotted in Fig. 3a, and show a distinctly asymmetric current in forward bias (open circles) and reverse bias (closed circles). Forward (or reverse) bias corresponds to positive bias on, and thus hole injection from, the bottom (or top) electrode. The hole

Fig. 3. Current–voltage measurements of contaminated-Au/100 nm organic/clean-Au diodes, where the organic layer consists of: (a) undoped CuPc; (b) 3% interface doped CuPc (see inset); (c) 3% homogenously doped CuPc; (d) the uniformly doped CuPc exposed to air for 15 min.; and (e) the air-exposed sample reintroduced into vacuum and pumped on for 1 h.

Table 1 Electron and hole barriers (/Be, /Bh) corresponding to undoped and doped CuPc films deposited on Au and Au* Electrode material

/Be (eV)

/Bh (eV)

Undoped CuPc on Au* (WF = 4.72 eV) Undoped CuPc on Au (WF = 5.20 eV) Doped CuPc on Au* and Au

1.16 0.92 0.31

0.67 0.91 1.52

/Bh is the energy difference measured by UPS at the interface between the electrode Fermi level and edge of the filled states. /Be is obtained by subtracting /Bh from the single particle edge-to-edge gap of CuPc (1.83 eV in Fig. 1).

injection barriers for undoped and doped CuPc on Au and Au* listed in Table 1 are determined from the UPS data in Section 3.2. Electron injection barriers are extrapolated from the EAs assuming that CuPc has an Egap = 1.83 eV as found from UPS and IPES in Section 3.1. As the CuPc molecular level positions at the contact interfaces indicate, the substantially lower hole injection barrier at the Au* interface (/Bh = 0.67 eV) as compared to the evaporated Au interface (/Bh = 0.91 eV) results in significantly improved carrier injection from the bottom interface, and therefore the observed 104 times increase in current density for the device under bottom hole injection. It must be emphasized that the dominant carriers in Au*/undoped CuPc/Au devices are likely to be holes, since /Bh is nearly 0.25 eV less than the electron injection barrier (/Be). Interface-doped CuPc devices consisting of a Au* bottom contact, 10 nm of 3% DMC-doped CuPc, 80 nm of undoped CuPc, an additional 10 nm of n-doped CuPc, and a freshly evaporated Au top contact (Fig. 3, inset) was fabricated. The I–V measurements, shown in Fig. 3b, indicate a dramatic increase in the current density ranging from a factor of 103 at moderate electric fields to 107 at low electric fields. However, in contrast to undoped devices, this current is likely dominated by efficient electron injection and transport in CuPc, since as observed in Section 3.2, the electron injection barrier (/Be = 0.31 eV) is now considerably smaller than the hole injection barrier (/Bh = 1.52 eV). This is an important finding because it dispels the common misconception that CuPc is exclusively a hole transport materials. Indeed, the current density for interface-doped devices is comparable with the hole-only devices observed by Gao et al. for zinc phthalocyanine (ZnPc) [4,5], an isoelectronic analog to CuPc, suggesting that when proper electron injection contacts are formed on CuPc, the electron transport characteristics of the material are comparable to its hole transport properties. The symmetric forward and reverse bias currents indicate that the interface energetics are identical on both sides, as would be expected given that equal doping was applied to both contacts, and is dominated by the dopant-determined alignment of EF with the empty states of CuPc. More importantly, n-doping of CuPc near the Au contacts demonstrates that use of reactive low work function anodes for electron injection can be circumvented by locally ndoping the host material at the electron injecting contact and using stable high work function metals. Finally, devices consisting of 100 and 200 nm CuPc films doped with pd = 5  107 Torr of DMC are investigated using I–V measurements (Fig. 3c). The superposition of

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the I–V curves normalized to electric field for two different thickness devices indicates that the current is limited by bulk transport through the film. The homogenously doped films demonstrate an additional 103 times increase in current density that results from increased carrier density in the bulk of the film. The fact that curve (c) parallels curve (b) provides further indication that even in the interfacedoped device, the metal–organic interface is no longer the limiting factor in charge transport. When the fully doped CuPc devices are exposed to air for 30 min, ex situ I–V experiments show a 100 times decrease in current density (Fig. 3d). This effect is attributed to oxygen diffusion into the CuPc film, which traps the DMC-induced excess electrons in the bulk film [36,37]. The reduction in bulk conductivity is partially reversible when the sample is reintroduced into vacuum and measured after 1 h of pumping in situ (Fig. 3e). However, despite the negative impact of air exposure on the overall current density of the doped film, good agreement between the I–V curves shown in Fig. 3c, i.e. the interfacedoped device, and Fig. 3d suggests that the improvement in current injection due to doping at the interfaces is not compromised. Given that improvement in carrier injection has been explained by carrier tunneling through the dopant-induced space-charge layer at the metal/organic interface [7,24], it should not be surprising that oxidizing species would negligibly affect the already electron depleted layer near the contacts. 3.4. CuPc p–i–n homojunction The application of DMC doping of CuPc to p–i–n homojunction diodes was investigated by fabricating 150 nm thick devices with an ITO bottom contact and an Au top contact. A fully intrinsic CuPc device was fabricated as a control sample for comparison with a p–i–n device consisting of a 10 nm CuPc p-region doped at 1% with F4-TCNQ, a 120 nm i-region, and a 20 nm n-region doped with DMC gradually from 3% at the intrinsic interface to 30% at the

Fig. 4. Current–voltage measurements of: (a) ITO/150 nm CuPc/Au and (b) ITO/10 nm 1% F4-TCNQ:CuPc/120 nm CuPc/20 nm 3–30% graded DMC:CuPc/Au p–i–n devices. The inset shows the device structure for the p–i–n homojunction.

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metal interface (Fig. 4 inset). The gradual doping was accomplished by increasing the dopant partial pressure over the course of the deposition of the n-CuPc layer. The large intrinsic region is used as a buffer layer to prevent carrier tunneling that results from the otherwise small space-charge region resulting from a direct organic p–n junction. Current density measurements of intrinsic and p–i–n devices shown in Fig. 4a and b, respectively, are plotted with respect to conventional forward and reverse diode conditions, and are in good qualitative and quantitative agreement with previous results obtained by Harada et al. on CuPc homojunctions made using a Ru compound as the n-dopant [19]. The results shown here exhibits a clear 103-fold increase in current density as a result of pand n-doping, but does not demonstrate an improvement in the rectification ratio of the p–i–n device, as would have been expected. This is likely the result of an unidentified source of leakage current at reverse bias, which can also be seen at low positive biases in the horizontal reflection of the reverse bias current onto the positive bias abscissa (dotted lines). If the leakage component of the current density can be minimized, the improvements observed for the p–i–n device would directly translate into a similar increase in the rectification ratio. For the p–i–n structure, UPS measurements were taken after deposition of the p- and n-layers to establish independently the magnitude of the built-in potential. The total shift between the two spectra is the built-in potential /bi, and is equal to 1.45 eV as shown in Fig. 5. Given that /bi in the p–i–n junction is simply given by: /bi ¼ Egap  ðEA  EFn Þ  ðEFp  IEÞ

ð1Þ

where EFn and EFp are the positions of the Fermi-level in the n- and p-doped regions, respectively, the 0.38 eV difference between Egap and /bi deviates from the sum of EA  EFn = 0.31 eV (Section 3.2) and EFp  IE = 0.2 eV [4]. Although the 130 meV discrepancy is within our experimental error, it could also be the result of UPS-induced charging in the 120 nm-thick i-layer of the device, which would result in an exaggerated energy level shift of the n-doped layer towards lower kinetic energy.

Fig. 5. UPS spectrum of the p-CuPc and n-CuPc films in the p–i–n device showing a large built-in potential.

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Fig. 6. Capacitance–voltage measurements of the CuPc p–i–n homojunction diode. The 1/C2 plot is shown in the inset.

To further investigate the value of the built-in potential of the CuPc homojunction, quasi-static C–V measurements were conducted on the p–i–n structure using a 2 kHz sinusoidal signal with an amplitude modulation of ±0.05 V. The C–V results plotted in Fig. 6 show a gradual decline of the specific capacitance of the device with increasing negative bias, and likely corresponds to an enlargement of the depleted space-charge regions at the reverse-biased p–i, i– n, and metal/organic interfaces [38]. At low positive bias, the observed exponential increase of the capacitance is correlated with the rise in diffusion capacitance, which results from the exponentially increasing current flow under forward bias found in Fig. 4b [38,39]. The drop in the capacitance at higher forward bias (>0.75 V), which is inductive in nature, has been attributed to a variety of mechanisms, including short-base diode effects [40,41], charge trapping and transport through localized trap states [42,43], injection through interfacial states [44], or carrier recombination within the film [45]. At the present time, however, it is uncertain whether any of these models apply to doped organic systems, or whether new models must be developed to explain the negative differential capacitance phenomena observed here. Nevertheless, using the convention that the extrapolated x-intercept of a 1/C2 vs. voltage plot corresponds roughly to the built-in potential of a p–i–n diode [38], the inset of Fig. 5 suggests a 1.29 eV offset between the levels in the p- and n-regions of the CuPc homojunction. This is in excellent agreement with /bi = 1.32 eV expected from Eq. (1). 4. Summary The solid-state ionization energy of decamethylcobaltocene (DMC) was found to be 3.3 eV, which makes this compound a powerful donor for many molecular semiconductors used in organic electronics. Efficient n-type doping of CuPc by DMC was demonstrated by evaporation of the phthalocyanine films in a controlled partial pressure of the dopant. The fabrication of n-doped CuPc films was confirmed by the shift of the Fermi-level towards the unoccupied states, with a final position of only 0.3 eV below the

frontier empty states of CuPc. I–V measurements were conducted on homogenously and interface-doped DMC/CuPc films sandwiched between Au contacts. These devices exhibited an increase of several orders of magnitude in current density due to enhanced injection and conductivity. Exposing the uniformly doped film to air decreases only the bulk conductivity due to the introduction of electron-trapping oxygen molecules, but this effect is found to be partially reversible. These findings have important consequences for device applications, by eliminating the need for low work function cathodes for electron injection. Finally, CuPc p–i–n homojunctions with F4-TCNQ as the pdopant and DMC as the n-dopant were constructed and examined with UPS, I–V, and C–V measurements. Although the exact mechanisms and theory behind the results are currently not well understood, the experiment unambiguously demonstrates the potential of molecular doping of organic films for stable, high current OLEDs and for OPVs with large open circuit voltages. Acknowledgements Support of this work by the National Science Foundation (DMR-0705920) and the Princeton MRSEC of the National Science Foundation (DMR-0213706) is gratefully acknowledged. Work at the Georgia Institute of Technology was supported in part by the STC Program of the National Science Foundation under Agreement Number DMR-0120967), and by the Office of Naval Research (N00014-04-1-0120). References [1] J. Blochwitz, M. Pfeiffer, T. Fritz, K. Leo, Appl. Phys. Lett. 73 (1998) 729. [2] M. Pfeiffer, A. Beyer, T. Fritz, K. Leo, Appl. Phys. Lett. 73 (1998) 3202. [3] J. Blochwitz, T. Fritz, M. Pfeiffer, K. Leo, D.M. Alloway, P.A. Lee, N.R. Armstrong, Org. Electron. 2 (2001) 97. [4] W. Gao, A. Kahn, Appl. Phys. Lett. 79 (2001) 4040. [5] W. Gao, A. Kahn, Org. Electron. 3 (2002) 53. [6] W. Gao, A. Kahn, J. Appl. Phys. 94 (2003) 359. [7] W. Gao, A. Kahn, J. Phys.: Condens. Matter 15 (2003) S2757. [8] A. Kahn, N. Koch, W. Gao, J. Polym. Sci. Part B: Polym. Phys. 41 (2003) 2529. [9] C. Chan, W. Gao, A. Kahn, J. Vac. Sci. Technol. A 22 (2004) 1488. [10] J. Kido, T. Matsumoto, Appl. Phys. Lett. 73 (1998) 2866. [11] Q.T. Le, L. Yan, Y. Gao, M.G. Mason, D.J. Giesenand, C.W. Tang, J. Appl. Phys. 87 (2000) 375. [12] M.G. Mason, C.W. Tang, L.-S. Hung, P. Raychaudhuri, J. Madathil, D.J. Giesen, L. Yan, Q.T. Le, Y. Gao, S.-T. Lee, L.S. Liao, L.F. Cheung, W.R. Salaneck, D.A.d. Santos, J.L. Bredas, J. Appl. Phys. 89 (2001) 2756. [13] G. Parthasarathy, C. Shen, A. Kahn, S.R. Forrest, Journal of Applied Physics 89 (2001) 4986. [14] L. Yan, N.J. Watkins, S. Zorba, Y. Gao, C.W. Tang, Appl. Phys. Lett. 79 (2001) 4148. [15] Y. Gao, L. Yan, Chem. Phys. Lett. 380 (2003) 451. [16] K. Ihm, T.-H. Kang, C.-C. Hwang, Y.-J. Park, K.-B. Lee, B. Kim, C.-H. Jeon, C.-Y. Park, K. Kim, Y.-H. Tak, Appl. Phys. Lett. 83 (2003) 2949. [17] J. Liu, A.R. Duggal, J.J. Shiang, C.M. Heller, Appl. Phys. Lett. 85 (2004) 837. [18] A. Nollau, M. Pfeiffer, T. Fritz, K. Leo, J. Appl. Phys. 87 (2000) 4340. [19] K. Harada, A.G. Werner, M. Pfeiffer, C.J. Bloom, C.M. Elliott, K. Leo, Phys. Rev. Lett. 94 (2005) 036601. [20] S. Tanaka, K. Kanai, E. Kawabe, T. Iwahashi, T. Nishi, Y. Ouchi, K. Seki, Jap. J. Appl. Phys. 44 (2005) 3760. [21] F. Li, A. Werner, M. Pfeiffer, K. Leo, X. Liu, J. Phys. Chem. B 108 (2004) 17076. [22] A.G. Werner, F. Li, K. Harada, M. Pfeiffer, T. Fritz, K. Leo, Appl. Phys. Lett. 82 (2003) 4495.

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