Charge accumulation at the organic heterointerfaces in multilayer organic light-emitting diodes (OLEDs) is an important process for understanding their device operation, efficiency, and degradation properties. Charge accumulation behavior has typically been analyzed in terms of the energy barrier and difference of the charge carrier mobility across heterointerfaces. In this study, we demonstrate that permanent dipole moments and their orientational order also play a significant role in the charge behavior at organic semiconductor interfaces. The charge accumulation properties of bilayer devices composed of polar or nonpolar molecules deposited on a 4,4’-bis[N-(1-naphthyl)-N-phenylamino]-biphenyl layer between the anode and cathode were examined by displacement current measurement and impedance spectroscopy. In addition, Kelvin probe measurements for the corresponding bilayer structures excluding the cathode were performed to analyze the relationship between the potential profile and charge accumulation properties of the bilayer devices. We found that several polar molecules including tris-(8-hydroxyquinolate) aluminum, 1,3,5-tris(1-phenyl-1 H-benzimidazol-2-yl)benzene, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), and 1,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazo-5-yl]benzene (OXD-7) are spontaneously ordered in evaporated films, and orientation polarization remains in bilayer devices. Orientation polarization leads to interface charge which determines the least amount of accumulated charge in the device under operation. The estimated interface charge density for these molecules ranged from −2.3 (OXD-7) to −0.5 (BCP) mC/m2. Furthermore, impedance spectroscopy revealed that the presence of a permanent dipole moment can suppress the charge dispersion along the interface probably owing to the microscopic potential fluctuation formed in the vicinity of the interface. These results indicate that the permanent dipole moment and orientation polarization contribute to the efficient charge accumulation at organic heterointerfaces and are important parameters for understanding and controlling the charge carrier dynamics in multilayer OLEDs.
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1 June 2012
Research Article|
June 05 2012
Charge accumulation at organic semiconductor interfaces due to a permanent dipole moment and its orientational order in bilayer devices
Yutaka Noguchi;
Yutaka Noguchi
a)
1Center for Frontier Science,
Chiba University
, 1-33 Yayoi-cho, Inage, Chiba 263-8522, Japan
2Graduate School of Advanced Integration Science,
Chiba University
, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
3
PRESTO, Japan Science and Technology Agency
, 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan
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Yukimasa Miyazaki;
Yukimasa Miyazaki
2Graduate School of Advanced Integration Science,
Chiba University
, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
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Yuya Tanaka;
Yuya Tanaka
2Graduate School of Advanced Integration Science,
Chiba University
, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
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Naoki Sato;
Naoki Sato
2Graduate School of Advanced Integration Science,
Chiba University
, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
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Yasuo Nakayama;
Yasuo Nakayama
1Center for Frontier Science,
Chiba University
, 1-33 Yayoi-cho, Inage, Chiba 263-8522, Japan
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Tobias D. Schmidt;
Tobias D. Schmidt
4Institute of Physics,
University of Augsburg
, Augsburg 86159, Germany
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Wolfgang Brütting;
Wolfgang Brütting
4Institute of Physics,
University of Augsburg
, Augsburg 86159, Germany
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Hisao Ishii
Hisao Ishii
b)
1Center for Frontier Science,
Chiba University
, 1-33 Yayoi-cho, Inage, Chiba 263-8522, Japan
2Graduate School of Advanced Integration Science,
Chiba University
, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
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a)
Electronic mail: y-noguchi@faculty.chiba-u.jp.
b)
Electronic mail: ishii130@faculty.chiba-u.jp.
J. Appl. Phys. 111, 114508 (2012)
Article history
Received:
March 21 2012
Accepted:
April 30 2012
Citation
Yutaka Noguchi, Yukimasa Miyazaki, Yuya Tanaka, Naoki Sato, Yasuo Nakayama, Tobias D. Schmidt, Wolfgang Brütting, Hisao Ishii; Charge accumulation at organic semiconductor interfaces due to a permanent dipole moment and its orientational order in bilayer devices. J. Appl. Phys. 1 June 2012; 111 (11): 114508. https://doi.org/10.1063/1.4724349
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