Highly efficient and bright organic electroluminescent devices with an aluminum cathode

Jabbour, Ghassan E.; Kawabe, Yutaka; Shaheen, Sean E.; Wang, J. F.; Morrell, M. M.; Kippelen, Bernard; Peyghambarian, N.

doi:10.1063/1.119392

Cited by 347 publications

(189 citation statements)

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“…The insertion of thin insulating layers of LiF in between the organic semiconductor and the metal contacts has shown to improve the performance of organic LEDs. [6][7][8] The effect was explained by the modification of the carrier injection barriers, information inferred from photoemission experiments. [9][10][11][12][13] In this article we report on the interfacial energy level alignment between Co and pentacene in the case of insertion of a thin aluminum oxide ͑AlOx͒ tunnel barrier in between.…”

Section: Introductionmentioning

confidence: 99%

Energy level alignment at Co/AlOx/pentacene interfaces

Popinciuc

Jonkman

Wees

2007

Journal of Applied Physics

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X-ray and ultraviolet photoemission spectroscopy ͑XPS and UPS͒ experiments were performed in order to study the energy level alignment and electronic structure at Co/AlOx/pentacene interfaces as a function of the aluminum oxide ͑AlOx͒ tunnel barrier thickness and the oxidation state of Co. XPS was used to determine the oxygen exposure for the optimum oxidation of 6, 8, and 10 Å thin layers of Al deposited on Co. The Fermi level ͑FL͒ position in the band gap of AlOx depends on the oxidation state of the underlying Co and on the thickness of the tunnel barrier. The energy level alignment at Co/AlOx interfaces is consistent with an interfacial dipole, its magnitude being sensitive to the oxidation of Co, and band bending phenomena in the thin AlOx tunnel barrier. UPS experiments revealed no chemical interaction at Co/AlOx/pentacene interface in contrast with hybridization effects found at Co/pentacene interface. The vacuum level of pentacene aligns with that of AlOx, following the position of AlOx energy levels with respect to FL. The hole injection barrier was found to increase with the thickness of the tunnel barrier and to decrease with the oxidation of Co at a fixed thickness of the AlOx layer.

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Section: Introductionmentioning

confidence: 99%

Energy level alignment at Co/AlOx/pentacene interfaces

Popinciuc

Jonkman

Wees

2007

Journal of Applied Physics

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“…They display excellent photovoltaic and photoconductive properties [8][9][10]. The performance of organic light emitting devices based on QA have been widely investigated [11][12][13][14]. The QA molecular structure can be varied by the substitution of C atoms by N and O. QA can also be modified by attaching lateral alkyl chains to N heteroatoms and by the formation of QAnC, as shown in Fig.…”

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Role of Lateral Alkyl Chains in Modulation of Molecular Structures on Metal Surfaces

et al. 2006

Phys. Rev. Lett.

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We use low energy electron diffraction, scanning tunneling microscopy, first-principles densityfunctional theory, and molecular mechanics calculations to analyze the adsorption and growth of quinacridone derivatives (QA) with alkyl chains of 4 and 16 carbon atoms on a Ag(110) substrate. Surprisingly, we find that the alkyl chains determine the orientation of the molecular overlayers. While the interaction of QA and the Ag substrate is primarily due to chemical bonding of oxygen to the silver substrate, determining the molecular orientation and preferred adsorption site, the intermolecular arrangement can be adjusted via the length of alkyl chains. We are thus able to fabricate uniform QA films with very well controlled physical properties. DOI: 10.1103/PhysRevLett.96.226101 PACS numbers: 81.07.Nb, 68.43.ÿh, 81.16.Dn In recent years, the structure and growth of functional molecular thin films have been widely investigated due to their potential application for molecular devices [1][2][3][4]. However, understanding the interactions between organic molecules and noble metal substrates is not straightforward and has proven to be rather challenging [1][2][3][4][5]. In this respect, the ability to control the structure of molecular thin films provides a method of tuning the functional properties in a discrete manner. Previous work demonstrated that linear aromatic hydrocarbons attach to a silver substrate via interactions of the lowest unoccupied molecular orbital (LUMO) and silver 4d orbitals [6]. The match of the LUMO and the silver 4d orbitals also dominates the structural properties of the ensuing molecular film. Subsequent work concerned the lateral functional groups interacting with silver substrates, such as CN and CO groups in DMe-DCNQI and PTCDA [7]. The work suggested that the adsorbate-substrate geometry of large aromatic molecules on noble metal surfaces can be precisely controlled by functional groups of the molecule. However, the intermolecular geometry is less well researched due to lack of experimental data. Correspondingly, the understanding of intermolecular interaction is still somewhat shallow.For example, quinacridone and its derivatives (QA) are well-known chemically stable pigments. They display excellent photovoltaic and photoconductive properties [8][9][10]. The performance of organic light emitting devices based on QA have been widely investigated [11][12][13][14]. The QA molecular structure can be varied by the substitution of C atoms by N and O. QA can also be modified by attaching lateral alkyl chains to N heteroatoms and by the formation of QAnC, as shown in Fig. 1(a), where n denotes the number of carbon atoms in each alkyl chain. Accordingly, the photoelectric or electrical property can be adjusted by structural alterations of the molecule. It is quite possible that the lateral alkyl chains act as spacers and modulate the intermolecular distance. In this case the energy transfer between QAnC functional units is modified by noncovalent interactions between molecules, which are tuned ...

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“…Multilayer devices consisting of thermally deposited hole transport layer ͑HTL͒ and emission layers have been shown to have high performance and good operational stability. 3,4 The HTL typically consists of a triphenyldiamine ͑TPD͒ or similar compound which is known to have high hole mobility. TPD also has an ionization potential ͑IP͒ which is well positioned between the work function of indium-tin-oxide ͑ITO͒ (ϳ4.7 eV) and the IP of many emission materials.…”

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“…Details of the device fabrication and characterization are given elsewhere. 3 Single-layer devices with a thickness of 90 nm were fabricated by either thermal deposition of small-molecule TPD or spin coating of polymer P2 onto ITO substrates followed by deposition of an Al cathode. All thermal depositions were done in a small bell jar (diameterϭ25 cm) with a source-to-sample distance of 17 cm and a fixed sample holder.…”

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confidence: 99%

“…These same trends were also seen in optimized devices which included doping the Alq 3 emission layer with quinacridone and replacing the Mg cathode with a bilayer LiF/Al cathode. 3 Figure 3 shows the luminance and luminous efficiency versus applied voltage for an optimized device using polymer P3 as the HTL. At an applied voltage of 3.0 V, the luminance is 15 cd/m 2 , and the luminous efficiency is 20 lm/W ͑corresponding to approximately 4.5% external quantum efficiency͒.…”

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confidence: 99%

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Organic light-emitting diode with 20 lm/W efficiency using a triphenyldiamine side-group polymer as the hole transport layer

Shaheen

Jabbour

Kippelen

et al. 1999

Applied Physics Letters

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We have used triphenyldiamine side-group polymers as hole transport layers in multilayer organic light-emitting diodes using 8-hydroxyquinoline aluminum (Alq 3 ) as an emission layer. The device efficiency systematically increases as the ionization potential of the hole transport layer is shifted further from the work function of the indium-tin-oxide anode. We attribute this trend to better balance of hole and electron charges in the device. An optimized device consisting of a fluorinated version of the polymer as the hole transport layer, quinacridone doped Al as the emission layer, and a LiF/Al cathode results in a peak external luminous efficiency of 20 lm/W. © 1999 American Institute of Physics. ͓S0003-6951͑99͒02421-3͔Research interest in organic light-emitting diodes ͑OLEDs͒ 1,2 continues to grow as their performance approaches a commercially viable level for applications such as low-cost, flat-panel displays. In order to be useful, these devices must have high brightness and efficiency, while requiring a low operating voltage. Multilayer devices consisting of thermally deposited hole transport layer ͑HTL͒ and emission layers have been shown to have high performance and good operational stability. 3,4 The HTL typically consists of a triphenyldiamine ͑TPD͒ or similar compound which is known to have high hole mobility. TPD also has an ionization potential ͑IP͒ which is well positioned between the work function of indium-tin-oxide ͑ITO͒ (ϳ4.7 eV) and the IP of many emission materials. Initial studies addressing the effects of varying the IP of the HTL on the device performance have led to differing results. 5,6 However, more recent studies have shown that the device quantum efficiency increases as the difference between the IP of the HTL and the emission layer is decreased. 7,8 These studies have generally been done using thermally deposited small-molecule hole transport materials. One disadvantage to this approach is that the morphological properties of the HTL film are affected by the particular molecular design. Possible crystallization of the hole transport material and poor interfacial contact with the ITO anode result in decreased device performance. In this study, we use a series of functionalized polymers with TPD derivative side groups as the HTL. The IP of these polymers can be controlled to provide a systematic way to investigate the importance of the IP of the HTL to the device performance while maintaining a consistent film morphology.The IP of TPD has been measured to be 5.38 eV using ultraviolet photoelectron spectroscopy. 9 This value can be systematically decreased ͑shifted toward the vacuum level͒ by adding an electron-donating moiety, such as p-OCH 3 , or increased ͑shifted further from the vacuum level͒ by adding an electron-withdrawing moiety, such as m-F. This principle is demonstrated by the three polymer TPD derivatives shown in Fig. 1, P1-P3, that have an IP that ranges from 5.06 to 5.56 eV. In this study, we used polymers P1-P3 as the HTL in double-layer OLEDs with a thermally evapora...

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Highly efficient and bright organic electroluminescent devices with an aluminum cathode

Cited by 347 publications

References 10 publications

Energy level alignment at Co/AlOx/pentacene interfaces

Energy level alignment at Co/AlOx/pentacene interfaces

Role of Lateral Alkyl Chains in Modulation of Molecular Structures on Metal Surfaces

Organic light-emitting diode with 20 lm/W efficiency using a triphenyldiamine side-group polymer as the hole transport layer

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