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Photoemission study of interfaces in organic light-emitting diodes
ELSEVIER
Synthetic Metals 102 (1999) 1014-1015
Photoemission
study of interfaces
in organic
light-emitting
diodes
Quoc Toan Le,” Li Yan,” V.-E. Choong,“* E. W. Forsythe,” M. G. Ma~on,~ C. W. Tang.” and Yongli Gao ” a Department of Physics and Astronomy, University of Rochester, Rochester. NY 14627, USA b Imaging Research and Advanced Development, Eastman Kodak Co., Rochester, NY 14650. USA * Present uddress: Phoenix Corporute Reseurch Laborutory, Motorola 11x., Tempe, AZ 8.5284. USA Abstract The importance of the interfacial properties in organic light-emitting devices (OLEDs) is well recognized. We have investigated the interface formation between a metal, namely Al or Ca, and tris-(%hydroxyquinoline) aluminum (Alq,) using X-ray and ultraviolet photoemission spectroscopy (XPS and UPS). In the case of AI/AIq,, the metal was found to react preferentially with the quinolate oxygen as soon as it was deposited onto Alq,. UPS spectra show a quick disappearance of the Alq, features as early as 1 A of Al deposition, and also suggest the formation of a rather poorly defined gap state induced by Al. On the other hand, in the case of Ca/ Alq,, the interface is characterized by a staged interface reaction: for low Ca coverages (< 4 A of 0). electron transfer from the Ca to the nitrogen of the pyridine side of the quinolate ligand occurs. At higher coverages. the Ca reacts with the phenoxide oxygen resulting in the decomposition of the Alq, molecule. Keywords: quinoline) 1.
Light-emitting diodes; X-ray photoemission aluminum; MetaVorganic interfaces
spectroscopy;
Introduction
Tris-(8-hydroxyquinoline) aluminum (Alq,) is one of the most widely used as materials for organic light-emitting devices (OLEDs) due to its excellent stability and luminescent properties [l-2]. A typical OLED consists of, in sequence, a high work function anode such as indium tin oxide (ITO), one or multiple layers of organic films, and a low work function metal as the cathode. In such devices, the injection behavior of the contacts strongly depends on the nature of the metal/organic interface [3-41. Hence, an understanding of the interface formation between the metal electode and the underlying organic film is important. Using Alq, as a substrate, we show in this paper different interfaces for low work function metals such as Al and Ca deposited onto Alq,. Although the detailed behavior of the two interfaces is very different, in both cases the results indicate that the metal reacts with Alqg. 2.
Experimental
studies were performed at room The photoemission temperature with a VG ESCA Lab Mark II equipped with a monochromatized Al KCY source (1486.6 eV) for XPS analysis The and unfiltered He I excitation (21.2 eV) for UPS analysis. base pressures in the preparation chamber were I x10-’ and 6x10~“’ for Al and Ca evaporaion, respectively, and 4x10.” mbar in the spectrometer. UPS spectra were recorded with a bias
Ultraviolet
photoemission
spectroscopy;
tris-(X-hydroxy
of - 4.0 V to enable observation of the low energy secondary cutoff. Metal films were grown by sequential thermal evaporation onto a IS0 A-Alq, film deposited on a Au coated Si substrate. 3.
Results
and
discussion
The evolution of the XPS Al 2p. N Is. and 0 Is spectra recorded at different Al coverage is shown in Fig. I, At O,, = 0 A, the 0 Is core level signal appears at 53 1.2 eV and is comprised of only one component. Upon Al deposition, even at a coverage as low as I A, the emergence of a new component on the high binding energy (BE) side is observed. This new component at high BE can be associated with an interfacial species resulting from the reaction with Al atoms. The Al 2p signal also reflects the interaction between Al and Alq,. For clean Alq,. the Al 2p peak appears at - 74.2 eV, which is typical for the Al in Alq,. After deposition of a small amount of Al (l-2 A), the AI 2p begins to broaden on its high binding energy side. resulting from the reaction between Al atoms and the phenoxide oxygen. With increasing Al coverages, the oxidized Al is accompanied by the progressive formation of metallic Al that appears at lower BE. The emergence of metallic Al begins for O,, = 4 A and increases in intensity as a function of Al coverage. The evolution of the N Is and C Is signal (C Is not shown) suggests that the reaction has not occurred at the quinolatc ring.
0379.6779/99/$ see front matter 0 1999 Elsevier Science S.A. All rights reserved. PII: SO379-6779(98)01263-6
Y. Gao
et al. I Synthetic
Metals
102 (1999)
1014-1015
1015
position of the reacted N Is component remains almost constant; the 0 Is peak splits and a new reacted species formed at low binding energy which increases in intensity and shifts back to higher binding energy with increasing coverage. It was found using UPS that the deposition of Ca induces new state in the energy gap.
78
76
74
72
7CdO4
402
400
396
396
534
530
526
B,nd,ng Energy (eV)
Fig. 1. Evolution of the XPS Al 2p, N 1s, and 0 Is spectra as a function of the Al coverage on Alq,. In Fig. 2, the UPS spectra are shown for clean Alq, and upon Al deposition. The spectrum for clean Alq, exhibits features consistent with those found in the literature [5,6]. Deposition of an Al layer as thin as 1 A represents a significant change in the valence spectrum of Alq, and induces the formation of new states in the energy gap. The appearance of the Al Fermi level for I6 A coverage indicates that the reaction is limited within a thin layer at the Alq, surface.
Fig.3. The evolution of C Is, N Is, and 0 Is core level spectra as a function of Ca coverage. 4.
We have used photoemission techniques to characterize the AI/Alq, and Ca/Alq, interfaces. In the case of Al, XPS results suggest that Al reacts preferentially with the quinolate oxygen, In the case of Ca, a strong interaction with Alq, is observed using XPS. At low coverages, Ca interacts with nitrogen before oxygen. At higher coverages, Ca reacts with the phenoxide oxygen resulting in the decomposition of the Alq, molecule. Both metals induce the formation of new states in the energy gap.
Al coverage (A)
5.
-6
-5
-4
-3
-2
-1
0
Conclusion
1
Acknowledgement
This work was supported in part by DARPA 0086 and NSF DMR-9612370.
DAALOI-96-K-
Binding Energy (eV) Fig.2. The UPS spectra of Alq, before and after Al deposition. The formation of new states in the energy gap is already observed after 1 A Al coverage on Alq,. In the case of Ca deposited onto Alq,, the reaction is fundamentally different. In Fig. 3, the evolution of the XPS C Is, N Is, and 0 1s signals is plotted as a function of Ca coverage. The 0 1s peak remains single component until Oc, = 4 A. Conversely, as early as O,, = I A, the emergence of a new component in the N 1s spectrum is observed, indicating that This result the nitrogen is the first site for Ca interaction. show that Ca interacts with N before 0, and suggests that charge transfer has occurred from Ca to Alq, [6,7]. Recent molecular orbital calculations have found that an electron transferred into Alq, is localized on the pyridyl side of the quinolate ligand and results in a shorter AI-N bond on this particular quinolate ligand [7]. For Oc, > 4 A, the intensity and
6.
References
III
C. W. Tang and S. A. Vanslyke, Appl. Phys. Lett., 51 (1987) 913. S. A. Vanslyke, C. H. Chen. and C. W. Tang, Appl. Phys. Lett. 69 (1996), 2160. Y. Gao, K. T. Park, and B. R. Hsieh, J. Appl. Phys. 73 (1993), 7894. H. Razafitrimo, K. T. Park, E. Ettedgui, Y. Gao, and B. R. Hsieh, Polymer international 36 (1995), 147. M. Probst and R. Haight, Appl. Phys. Lett. 70 (1997), 1420. V.-E. Choong, M. G. Mason. C. W. Tang, and Y. Gao, Appl. Phys. Lett.. 72 (1998) 2689. P. E. Burrows, Z. Shen, V. Bulovic, D. M. McCarty, S. R. Forrest, J. A. Cronin. and M. E. Thompson, J. Appl. Phys. 79 (1996), 7991.
VI [31 ]41 151
[61 t71
Synthetic Metals 102 (1999) 1014-1015
Photoemission
study of interfaces
in organic
light-emitting
diodes
Quoc Toan Le,” Li Yan,” V.-E. Choong,“* E. W. Forsythe,” M. G. Ma~on,~ C. W. Tang.” and Yongli Gao ” a Department of Physics and Astronomy, University of Rochester, Rochester. NY 14627, USA b Imaging Research and Advanced Development, Eastman Kodak Co., Rochester, NY 14650. USA * Present uddress: Phoenix Corporute Reseurch Laborutory, Motorola 11x., Tempe, AZ 8.5284. USA Abstract The importance of the interfacial properties in organic light-emitting devices (OLEDs) is well recognized. We have investigated the interface formation between a metal, namely Al or Ca, and tris-(%hydroxyquinoline) aluminum (Alq,) using X-ray and ultraviolet photoemission spectroscopy (XPS and UPS). In the case of AI/AIq,, the metal was found to react preferentially with the quinolate oxygen as soon as it was deposited onto Alq,. UPS spectra show a quick disappearance of the Alq, features as early as 1 A of Al deposition, and also suggest the formation of a rather poorly defined gap state induced by Al. On the other hand, in the case of Ca/ Alq,, the interface is characterized by a staged interface reaction: for low Ca coverages (< 4 A of 0). electron transfer from the Ca to the nitrogen of the pyridine side of the quinolate ligand occurs. At higher coverages. the Ca reacts with the phenoxide oxygen resulting in the decomposition of the Alq, molecule. Keywords: quinoline) 1.
Light-emitting diodes; X-ray photoemission aluminum; MetaVorganic interfaces
spectroscopy;
Introduction
Tris-(8-hydroxyquinoline) aluminum (Alq,) is one of the most widely used as materials for organic light-emitting devices (OLEDs) due to its excellent stability and luminescent properties [l-2]. A typical OLED consists of, in sequence, a high work function anode such as indium tin oxide (ITO), one or multiple layers of organic films, and a low work function metal as the cathode. In such devices, the injection behavior of the contacts strongly depends on the nature of the metal/organic interface [3-41. Hence, an understanding of the interface formation between the metal electode and the underlying organic film is important. Using Alq, as a substrate, we show in this paper different interfaces for low work function metals such as Al and Ca deposited onto Alq,. Although the detailed behavior of the two interfaces is very different, in both cases the results indicate that the metal reacts with Alqg. 2.
Experimental
studies were performed at room The photoemission temperature with a VG ESCA Lab Mark II equipped with a monochromatized Al KCY source (1486.6 eV) for XPS analysis The and unfiltered He I excitation (21.2 eV) for UPS analysis. base pressures in the preparation chamber were I x10-’ and 6x10~“’ for Al and Ca evaporaion, respectively, and 4x10.” mbar in the spectrometer. UPS spectra were recorded with a bias
Ultraviolet
photoemission
spectroscopy;
tris-(X-hydroxy
of - 4.0 V to enable observation of the low energy secondary cutoff. Metal films were grown by sequential thermal evaporation onto a IS0 A-Alq, film deposited on a Au coated Si substrate. 3.
Results
and
discussion
The evolution of the XPS Al 2p. N Is. and 0 Is spectra recorded at different Al coverage is shown in Fig. I, At O,, = 0 A, the 0 Is core level signal appears at 53 1.2 eV and is comprised of only one component. Upon Al deposition, even at a coverage as low as I A, the emergence of a new component on the high binding energy (BE) side is observed. This new component at high BE can be associated with an interfacial species resulting from the reaction with Al atoms. The Al 2p signal also reflects the interaction between Al and Alq,. For clean Alq,. the Al 2p peak appears at - 74.2 eV, which is typical for the Al in Alq,. After deposition of a small amount of Al (l-2 A), the AI 2p begins to broaden on its high binding energy side. resulting from the reaction between Al atoms and the phenoxide oxygen. With increasing Al coverages, the oxidized Al is accompanied by the progressive formation of metallic Al that appears at lower BE. The emergence of metallic Al begins for O,, = 4 A and increases in intensity as a function of Al coverage. The evolution of the N Is and C Is signal (C Is not shown) suggests that the reaction has not occurred at the quinolatc ring.
0379.6779/99/$ see front matter 0 1999 Elsevier Science S.A. All rights reserved. PII: SO379-6779(98)01263-6
Y. Gao
et al. I Synthetic
Metals
102 (1999)
1014-1015
1015
position of the reacted N Is component remains almost constant; the 0 Is peak splits and a new reacted species formed at low binding energy which increases in intensity and shifts back to higher binding energy with increasing coverage. It was found using UPS that the deposition of Ca induces new state in the energy gap.
78
76
74
72
7CdO4
402
400
396
396
534
530
526
B,nd,ng Energy (eV)
Fig. 1. Evolution of the XPS Al 2p, N 1s, and 0 Is spectra as a function of the Al coverage on Alq,. In Fig. 2, the UPS spectra are shown for clean Alq, and upon Al deposition. The spectrum for clean Alq, exhibits features consistent with those found in the literature [5,6]. Deposition of an Al layer as thin as 1 A represents a significant change in the valence spectrum of Alq, and induces the formation of new states in the energy gap. The appearance of the Al Fermi level for I6 A coverage indicates that the reaction is limited within a thin layer at the Alq, surface.
Fig.3. The evolution of C Is, N Is, and 0 Is core level spectra as a function of Ca coverage. 4.
We have used photoemission techniques to characterize the AI/Alq, and Ca/Alq, interfaces. In the case of Al, XPS results suggest that Al reacts preferentially with the quinolate oxygen, In the case of Ca, a strong interaction with Alq, is observed using XPS. At low coverages, Ca interacts with nitrogen before oxygen. At higher coverages, Ca reacts with the phenoxide oxygen resulting in the decomposition of the Alq, molecule. Both metals induce the formation of new states in the energy gap.
Al coverage (A)
5.
-6
-5
-4
-3
-2
-1
0
Conclusion
1
Acknowledgement
This work was supported in part by DARPA 0086 and NSF DMR-9612370.
DAALOI-96-K-
Binding Energy (eV) Fig.2. The UPS spectra of Alq, before and after Al deposition. The formation of new states in the energy gap is already observed after 1 A Al coverage on Alq,. In the case of Ca deposited onto Alq,, the reaction is fundamentally different. In Fig. 3, the evolution of the XPS C Is, N Is, and 0 1s signals is plotted as a function of Ca coverage. The 0 1s peak remains single component until Oc, = 4 A. Conversely, as early as O,, = I A, the emergence of a new component in the N 1s spectrum is observed, indicating that This result the nitrogen is the first site for Ca interaction. show that Ca interacts with N before 0, and suggests that charge transfer has occurred from Ca to Alq, [6,7]. Recent molecular orbital calculations have found that an electron transferred into Alq, is localized on the pyridyl side of the quinolate ligand and results in a shorter AI-N bond on this particular quinolate ligand [7]. For Oc, > 4 A, the intensity and
6.
References
III
C. W. Tang and S. A. Vanslyke, Appl. Phys. Lett., 51 (1987) 913. S. A. Vanslyke, C. H. Chen. and C. W. Tang, Appl. Phys. Lett. 69 (1996), 2160. Y. Gao, K. T. Park, and B. R. Hsieh, J. Appl. Phys. 73 (1993), 7894. H. Razafitrimo, K. T. Park, E. Ettedgui, Y. Gao, and B. R. Hsieh, Polymer international 36 (1995), 147. M. Probst and R. Haight, Appl. Phys. Lett. 70 (1997), 1420. V.-E. Choong, M. G. Mason. C. W. Tang, and Y. Gao, Appl. Phys. Lett.. 72 (1998) 2689. P. E. Burrows, Z. Shen, V. Bulovic, D. M. McCarty, S. R. Forrest, J. A. Cronin. and M. E. Thompson, J. Appl. Phys. 79 (1996), 7991.
VI [31 ]41 151
[61 t71