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metal interfaces as a model interface of electroluminescent device studied by UV photoemission
ELSEVIER
Synthetic Metals 86
(1997) 2425-2426
Electronic Structure of Organic Carrier Transporting Material / Metal Interfaces as a Model Interface of Electroluminescent Device Studied by UV Photoemission K. Sugiyamaa, D. Yoshimuraa, E. Itoa, T. Miyazak?, Y. Hamatam ‘a, I. Kawamotoa, H. Ishiib, Y. Ouchia, K. Sekia a Department of Chemistry, Faculty of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan b Institute for Molecular Science, Myodaiji. Okazaki 444, Japan
Abstract Electronic structures of N,N’-diphenyl-N,N’-(3-methylphenyl)-l,l’-biphenyl-4,4’-diamine (TPD) / metal and N,N’-diphenyl1,4,5,8-naphtyltetracarboxylimide (DP-NTCI) / metal interfaces were directly investigated as a model interface of organic electroluminescent (EL) devices using UV photoemission spectroscopy (UPS). At the organic / metal interfaces, abrupt shift of vacuum level was observed, in contrast to the traditional assumption of common vacuum level at the interface. For understanding EL devices, we see the necessity of the direct observation of the interfacial electronic structure by UPS or other techniques for understanding EL devices. Keywords: 1.
Photoelectron
spectroscopy,
Metal/semiconductor
Introduction
Recently, electroluminescent (EL) devices based on organic thin layers have attracted much attention in relation to the application to a large-area light-emitting display, which operate at low drive voltages. For understanding its mechanism, it is indispensable to elucidate the electronic structure of interface between the organic layer and the metal electrode. However, only few reports have been published for the direct measurements of the interfacial electronic structure [ 11. In most studies, interfacial energy diagram have been estimated by combining separately observed electronic structures of the two components at the interface, with assumption of a common vacuum level at interface. However, recently we found a shift of vacuum level at the porphyrin / metal interface [2], by directly studying the interface by UV photoemission spectroscopy (UPS). This is in contrast to the concept of common vacuum level, indicating that direct examination of the interface is necessary for clarifying the interfacial electronic structure. In this work, we have extended our UPS study to hole or electron-injecting organic layer I metal interfaces as model interfaces of organic EL device. By comparing with the interfacial electronic structure deduced from UPS experiments, the validity of traditional assumption of common vacuum level is discussed. 2.
interfaces
Experiments
As hole and electron injecting materials, N,N’-diphenylN, N’-(3-methylphenyl)-1 ,I’-biphenyl-4,4’-diamine (TPD) and N,N’-diphenyl-1,4,5,8-naphtyltetracarboxylimide (DP-NTCI) were used, respectively. Their structures are shown in Fig. 1. The samples were supplied from Toshiba R&D Center, and were purified by sublimation. Photoelectron spectroscopic measurements were carried out using synchrotron radiation with an angle resolving UPS spectrometer at BL8B2 of UVSOR Facility at Institute for Molecular Science. TPD and DP-NTCI thin films were prepared by vacuum deposition onto evaporated films of Al or Au. UPS spectra were measured under 0379-6779/97/$17.00 8 1997 Elsevier Science S.A All rights reserved PII SO379-6779(%)04895-3
‘Cl-h
Had (b) DP-NTCI
(a) TPD
Figure. 1. Structures of (a) TPD and (b) DP-NTCI vacuum of 10m8Pa by a concentric hemispherical analyzer with a total resolution of about 0.2eV. In order to assign the observed spectra, the molecular orbital (MO) calculations were carried out with a MOPAC semi-empirical PM3 program. The simulated spectra were obtained by convoluting the delta functions located at each orbital energy with a Gaussian function with a half width of 0.8eV. 3.
Results
and
discussion
Fig. 2 (a) shows the UPS spectra of TPD (1Onm thick) on Au. Incident photon energy hv was 40eV. The abscissa is the binding energy Et, from the vacuum level. Ionization threshold energy Ith corresponds to the right-hand onset. Ith of TPD was 5.0 k O.leV. For hole-injection layer, materials of donor character, which have comparatively small I,t,, are suitable. The value of Ith of TPD is similar to those of other organic donors, (for example, tetrathiafulvalene with Itt, of 5.0eV [3]), showing adequacy of using TPD for hole-injecting layer. The simulated spectrum of T’PD is also shown in figure 2 (b), with each orbital energies marked by a vertical line. This simulated spectrum offers good agreement with the observed spectrum. The topmost feature at 6.0eV which dominates the electric properties of TPD is assigned to the n HOMO of a benzidin part and includes few contributions of phenyl side chains. In the case of DP-NTCI, the large It,, of 7.0 + O.leV is similar to those of other organic acceptors such as tetracyano-
K. Sugiyama et al. /Synthetic Metals 86 (1997) 2425-2426
(a) UPS spectrum of TPD
(b) Simulated spectrum ofTI’D
I\ /\
A AU
t
TPD IOnm
Al
EyE hd=0.87eV
TPD IOnm
A=-O.ZeV
0
Binding energy 1 eV Figure 2. (a) UPS spectrum and (b) Simulated spectrum of TPD quinodimethane with Ith of 7.4eV [3], showing that DP-NTCI is suitable for electron-injecting layer. Detailed assignments of the UPS spectrum of DP-NTCI will be discussed elsewhere ]41. Using the results of UPS measurements of the films with 1Onm thickness, interfacial energy diagrams of TPD and DPNTCI on Al and Au were determined as shown in Fig. 3. @m means the work function of the metal, while ,& and &gdenote the energies of the vacuum level and the HOMO of organic materials relative to the Fermi level of the metal substrate, respectively. E& is determined as the right-hand onset of the spectrum of organic films relative to the Fermi level of the metal substrate. The energy position of the lowest unoccupied molecular orbital (LUMO) was estimated by regarding optical excitation energy of 3.3eV (TPD) or 3&V (DP-NTCI) as the band gap. The actual band gap, however, may be slightly larger since optical excitation corresponds to the transition to an exciton state. At all the measured interfaces, there are abrupt vacuum level shifts A within the thickness of a few nms by organic layer deposition, in contrast to the traditional assumption of a common vacuum level at the interface. Similar abrupt shift of vacuum level was also observed at organic semiconductor / metal interfaces (Zinc tetraphenylporphyrin / metal [2]), inorganic semiconductor / metal interfaces [S], and organic I inorganic interfaces (merocyanine dyes / Ag halides [6]). A positive value of A observed at most interfaces indicates the formation of interfacial dipole with the metal side negatively charged. At DP-NTCI / Al interface, negative A was observed, suggesting the electron transfer from Al to DP-NTCI. At TPD / Au interfaces, the barrier height to hole injection corresponding to the energy difference between the HOMO of TPD and the Fermi level of Au is 1.25eV. Although this value is not small, the Fermi level of Au is nearer to the HOMO of TPD than the LUMO of TPD, indicating hole injecting nature Using a traditional assumption with a of this interface. common vacuum level at interface, the barrier height is estimated to be smaller, while direct observation offers a comparatively larger barrier height. At DP-NTCI / Au and DP-NTCI / Al interfaces, the barrier height to electron injection is very small, indicating good electron-injecting character of these interfaces. Usually, low work function metals are used for cathodes in order to reduce a barrier height to electron injection. However, at interfaces with strong electron acceptor such as DP-NTCI, electron transfer from metal to the organic material and negative A is realized, leading to a rather lager barrier height. Thus the
AU
DP-NTCI lomn
Al
DP-NTCI IOnm
Figure 3. Energy diagrams of TPD / Au (a), TPD / Al (b), DPNTCI / Au (c), and DP-NTCI / Al interfaces, determined directly by using the results of UPS measurements.
neglect of vacuum level shift at the interface A leads to an incorrect prediction of carrier-injecting nature of interfaces, since A plays an important role in determining the interfacial electrical structure. These findings demonstrate the necessity of the direct observation of the interfacial electronic structure by UPS or other techniques for understanding EL devices. Acknowledgment
The authors thank Dr. Syun Egusa and Dr. Takashi Sasaki of Advanced Research Laboratory, Toshiba R&D Center for the gift of the sample material. This work was performed as a Joint Studies Program of the UVSOR facility of the Institute for Molecular Science (No.6-H217). This work was supported in part by the Grant-in Aids for Scientific Research (Nos. 07CE2004, 08CE004, 07NP0303, 04403001) and by the Venture Business Laboratory Program, both from the Ministry of Education, Science, Sports, and Culture of Japan, References
111 R. Lazzaroni, J. L. Bredas, P. Dannetun, C. Fredriksson, S. Stafstrom, and W. R. Salaneck, Elecrrochim. Actu, 39 (1994) 235. PI S. Narioka, H. Ishii, D. Yoshimura, M. Sei, Y. Ouchi, K. Seki, S. Hasegawa, T. Miyazaki, Y. Harima, and K. Y amashita, Appl. Phys. L&t., 67 (1995) 25. 131 K. Seki, Mol. Cryst. Liq. Cryst., 171 (1989) 255. [41 K. Sugiyama, D. Yoshimura, E. Ito, T. Miyazaki, Y. Hamatani, I. Kawamoto, H. Ishii, Y. Ouchi, K. Seki, to be published.
[51 W. Month, Surf. Sci., 299/300 928 (1994) [61 K. Seki, H. Yanagi, Y. Kobayashi. T. Ohta, and T. Tani, Phys. Rev., B, 49 (1994) 2760.
Synthetic Metals 86
(1997) 2425-2426
Electronic Structure of Organic Carrier Transporting Material / Metal Interfaces as a Model Interface of Electroluminescent Device Studied by UV Photoemission K. Sugiyamaa, D. Yoshimuraa, E. Itoa, T. Miyazak?, Y. Hamatam ‘a, I. Kawamotoa, H. Ishiib, Y. Ouchia, K. Sekia a Department of Chemistry, Faculty of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan b Institute for Molecular Science, Myodaiji. Okazaki 444, Japan
Abstract Electronic structures of N,N’-diphenyl-N,N’-(3-methylphenyl)-l,l’-biphenyl-4,4’-diamine (TPD) / metal and N,N’-diphenyl1,4,5,8-naphtyltetracarboxylimide (DP-NTCI) / metal interfaces were directly investigated as a model interface of organic electroluminescent (EL) devices using UV photoemission spectroscopy (UPS). At the organic / metal interfaces, abrupt shift of vacuum level was observed, in contrast to the traditional assumption of common vacuum level at the interface. For understanding EL devices, we see the necessity of the direct observation of the interfacial electronic structure by UPS or other techniques for understanding EL devices. Keywords: 1.
Photoelectron
spectroscopy,
Metal/semiconductor
Introduction
Recently, electroluminescent (EL) devices based on organic thin layers have attracted much attention in relation to the application to a large-area light-emitting display, which operate at low drive voltages. For understanding its mechanism, it is indispensable to elucidate the electronic structure of interface between the organic layer and the metal electrode. However, only few reports have been published for the direct measurements of the interfacial electronic structure [ 11. In most studies, interfacial energy diagram have been estimated by combining separately observed electronic structures of the two components at the interface, with assumption of a common vacuum level at interface. However, recently we found a shift of vacuum level at the porphyrin / metal interface [2], by directly studying the interface by UV photoemission spectroscopy (UPS). This is in contrast to the concept of common vacuum level, indicating that direct examination of the interface is necessary for clarifying the interfacial electronic structure. In this work, we have extended our UPS study to hole or electron-injecting organic layer I metal interfaces as model interfaces of organic EL device. By comparing with the interfacial electronic structure deduced from UPS experiments, the validity of traditional assumption of common vacuum level is discussed. 2.
interfaces
Experiments
As hole and electron injecting materials, N,N’-diphenylN, N’-(3-methylphenyl)-1 ,I’-biphenyl-4,4’-diamine (TPD) and N,N’-diphenyl-1,4,5,8-naphtyltetracarboxylimide (DP-NTCI) were used, respectively. Their structures are shown in Fig. 1. The samples were supplied from Toshiba R&D Center, and were purified by sublimation. Photoelectron spectroscopic measurements were carried out using synchrotron radiation with an angle resolving UPS spectrometer at BL8B2 of UVSOR Facility at Institute for Molecular Science. TPD and DP-NTCI thin films were prepared by vacuum deposition onto evaporated films of Al or Au. UPS spectra were measured under 0379-6779/97/$17.00 8 1997 Elsevier Science S.A All rights reserved PII SO379-6779(%)04895-3
‘Cl-h
Had (b) DP-NTCI
(a) TPD
Figure. 1. Structures of (a) TPD and (b) DP-NTCI vacuum of 10m8Pa by a concentric hemispherical analyzer with a total resolution of about 0.2eV. In order to assign the observed spectra, the molecular orbital (MO) calculations were carried out with a MOPAC semi-empirical PM3 program. The simulated spectra were obtained by convoluting the delta functions located at each orbital energy with a Gaussian function with a half width of 0.8eV. 3.
Results
and
discussion
Fig. 2 (a) shows the UPS spectra of TPD (1Onm thick) on Au. Incident photon energy hv was 40eV. The abscissa is the binding energy Et, from the vacuum level. Ionization threshold energy Ith corresponds to the right-hand onset. Ith of TPD was 5.0 k O.leV. For hole-injection layer, materials of donor character, which have comparatively small I,t,, are suitable. The value of Ith of TPD is similar to those of other organic donors, (for example, tetrathiafulvalene with Itt, of 5.0eV [3]), showing adequacy of using TPD for hole-injecting layer. The simulated spectrum of T’PD is also shown in figure 2 (b), with each orbital energies marked by a vertical line. This simulated spectrum offers good agreement with the observed spectrum. The topmost feature at 6.0eV which dominates the electric properties of TPD is assigned to the n HOMO of a benzidin part and includes few contributions of phenyl side chains. In the case of DP-NTCI, the large It,, of 7.0 + O.leV is similar to those of other organic acceptors such as tetracyano-
K. Sugiyama et al. /Synthetic Metals 86 (1997) 2425-2426
(a) UPS spectrum of TPD
(b) Simulated spectrum ofTI’D
I\ /\
A AU
t
TPD IOnm
Al
EyE hd=0.87eV
TPD IOnm
A=-O.ZeV
0
Binding energy 1 eV Figure 2. (a) UPS spectrum and (b) Simulated spectrum of TPD quinodimethane with Ith of 7.4eV [3], showing that DP-NTCI is suitable for electron-injecting layer. Detailed assignments of the UPS spectrum of DP-NTCI will be discussed elsewhere ]41. Using the results of UPS measurements of the films with 1Onm thickness, interfacial energy diagrams of TPD and DPNTCI on Al and Au were determined as shown in Fig. 3. @m means the work function of the metal, while ,& and &gdenote the energies of the vacuum level and the HOMO of organic materials relative to the Fermi level of the metal substrate, respectively. E& is determined as the right-hand onset of the spectrum of organic films relative to the Fermi level of the metal substrate. The energy position of the lowest unoccupied molecular orbital (LUMO) was estimated by regarding optical excitation energy of 3.3eV (TPD) or 3&V (DP-NTCI) as the band gap. The actual band gap, however, may be slightly larger since optical excitation corresponds to the transition to an exciton state. At all the measured interfaces, there are abrupt vacuum level shifts A within the thickness of a few nms by organic layer deposition, in contrast to the traditional assumption of a common vacuum level at the interface. Similar abrupt shift of vacuum level was also observed at organic semiconductor / metal interfaces (Zinc tetraphenylporphyrin / metal [2]), inorganic semiconductor / metal interfaces [S], and organic I inorganic interfaces (merocyanine dyes / Ag halides [6]). A positive value of A observed at most interfaces indicates the formation of interfacial dipole with the metal side negatively charged. At DP-NTCI / Al interface, negative A was observed, suggesting the electron transfer from Al to DP-NTCI. At TPD / Au interfaces, the barrier height to hole injection corresponding to the energy difference between the HOMO of TPD and the Fermi level of Au is 1.25eV. Although this value is not small, the Fermi level of Au is nearer to the HOMO of TPD than the LUMO of TPD, indicating hole injecting nature Using a traditional assumption with a of this interface. common vacuum level at interface, the barrier height is estimated to be smaller, while direct observation offers a comparatively larger barrier height. At DP-NTCI / Au and DP-NTCI / Al interfaces, the barrier height to electron injection is very small, indicating good electron-injecting character of these interfaces. Usually, low work function metals are used for cathodes in order to reduce a barrier height to electron injection. However, at interfaces with strong electron acceptor such as DP-NTCI, electron transfer from metal to the organic material and negative A is realized, leading to a rather lager barrier height. Thus the
AU
DP-NTCI lomn
Al
DP-NTCI IOnm
Figure 3. Energy diagrams of TPD / Au (a), TPD / Al (b), DPNTCI / Au (c), and DP-NTCI / Al interfaces, determined directly by using the results of UPS measurements.
neglect of vacuum level shift at the interface A leads to an incorrect prediction of carrier-injecting nature of interfaces, since A plays an important role in determining the interfacial electrical structure. These findings demonstrate the necessity of the direct observation of the interfacial electronic structure by UPS or other techniques for understanding EL devices. Acknowledgment
The authors thank Dr. Syun Egusa and Dr. Takashi Sasaki of Advanced Research Laboratory, Toshiba R&D Center for the gift of the sample material. This work was performed as a Joint Studies Program of the UVSOR facility of the Institute for Molecular Science (No.6-H217). This work was supported in part by the Grant-in Aids for Scientific Research (Nos. 07CE2004, 08CE004, 07NP0303, 04403001) and by the Venture Business Laboratory Program, both from the Ministry of Education, Science, Sports, and Culture of Japan, References
111 R. Lazzaroni, J. L. Bredas, P. Dannetun, C. Fredriksson, S. Stafstrom, and W. R. Salaneck, Elecrrochim. Actu, 39 (1994) 235. PI S. Narioka, H. Ishii, D. Yoshimura, M. Sei, Y. Ouchi, K. Seki, S. Hasegawa, T. Miyazaki, Y. Harima, and K. Y amashita, Appl. Phys. L&t., 67 (1995) 25. 131 K. Seki, Mol. Cryst. Liq. Cryst., 171 (1989) 255. [41 K. Sugiyama, D. Yoshimura, E. Ito, T. Miyazaki, Y. Hamatani, I. Kawamoto, H. Ishii, Y. Ouchi, K. Seki, to be published.
[51 W. Month, Surf. Sci., 299/300 928 (1994) [61 K. Seki, H. Yanagi, Y. Kobayashi. T. Ohta, and T. Tani, Phys. Rev., B, 49 (1994) 2760.