Exciplex formation at the organic solid-state interface: Yellow emission in organic light-emitting diodes using green-fluorescent tris(8-quinolinolato)aluminum and hole-transporting molecular materials with low ionization potentials

Itano, Koji; Ogawa, Hiromitsu; Shirota, Yasuhiko

doi:10.1063/1.120826

Cited by 163 publications

(91 citation statements)

References 13 publications

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“…This device also exhibits a redshift in the EL spectrum relative to the Alq emission, which has been attributed to emission from the MTDATA-Alq exciplex. 17 With increasing current density, the EL spectrum shows a gradual blueshift along with an increase in the current efficiency. This appears to indicate a decrease in exciplex emissions at the MTDATA/Alq interface in favor of Alq emissions due to increased hole injection into the bulk of Alq.…”

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Current efficiency in organic light-emitting diodes with a hole-injection layer

Wang

Klubek

Tang

2008

Applied Physics Letters

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We have systematically investigated the effect of layer structures on the current efficiency of prototypical hole-injection layer ͑HIL͒/hole-transport layer ͑HTL͒/electron-transport layer ͑ETL͒ organic light-emitting diodes based on 4 , 4Ј ,4Љ-tris͓N-͑3-methylphenyl͒-N-phenylamino͔-triphenylamine ͑MTDATA͒ as the HIL, 4 , 4Ј-bis͓N-͑1-naphthyl͒-N-phenylamino͔biphenyl ͑NPB͒ as the HTL, and tris͑8-quinolinolato͒aluminum ͑Alq͒ as the ETL. With bilayer devices, the current efficiency is limited by exciplex emissions in the case of MTDATA/Alq and quenching of Alq emissions by NPB + radical cations in NPB/Alq. The improved current efficiency in trilayer MTDATA/NPB/Alq devices can be attributed to a reduction in NPB + radical cations at the NPB/Alq interface and a strong electric field in the NPB layer. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2978349͔Since they first appeared in the 1980s, 1 organic lightemitting diodes ͑OLEDs͒ have received much attention due to their potential use as a full-color display technology. Although the efficiency and the operational life of OLEDs have vastly improved through intensive research effort, the mechanism underlying these key performance parameters are not well understood. The basic OLED device has a bilayer organic thin-film structure such as indium tin oxide ͑ITO͒/4,4Ј-bis͓N-͑1-naphthyl͒-N-phenylamino͔biphenyl ͑NPB͒/Alq/LiF/Al, where ITO is the anode and LiF/Al is the cathode, and NPB and Alq are the hole-transport layer ͑HTL͒ and the electron-transport layer ͑ETL͒, respectively. During operation, the injected holes and electrons recombine at or near the HTL/ETL interface, producing electroluminescence ͑EL͒. It has been shown that much improved OLED performance can be realized using a HIL/HTL structure, where HIL is the "hole-injection" layer inserted between the anode and the HTL. For example, with CuPc 2 as the HIL as in CuPc/NPB/Alq where Alq also functions as the emissive layer, long-lived OLEDs have been obtained. Another common HIL material is 4 , 4Ј ,4Љ-tris͓N-͑3-methylphenyl͒-N-phenylamino͔triphenylamine ͑MTDATA͒, 3 with which enhanced current efficiency and operational stability have been demonstrated. High-efficiency OLEDs have also been reported in various HIL/HTL configurations. 4-8 Furthermore, low-voltage and high-efficiency OLEDs can be realized with a p-doped HIL 9-11 in which the layer thickness can be readily adjusted for optimal light extraction. It has been suggested that the enhanced performance in HIL/HTL devices is due to a sequence of cascaded hole-injection barriers present in the HIL/HTL/ETL structure, which produces a "balanced" electron-hole recombination at the HTL/ETL interface. 12,13 In this paper, we systematically investigate the effect of the HIL on OLED performance characteristics, specifically the cause for current efficiency enhancement. Figure 1 shows the multilayer OLED structure and the molecular structures for the HIL ͑MTDATA͒, HTL ͑NPB͒, and ETL ͑Alq͒. The Fabrication of OLEDs by physical vapor deposition is described elsewhe...

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confidence: 99%

“…The low current efficiency for device B has been attributed to the formation of exciplex, 13,17 at the MTDATA/Alq interface, where the EL emission quantum yield is controlled by the exciplex emission with a lower quantum yield than Alq. Being a strong donor, MTDATA has a strong propensity to form exciplex with the acceptor Alq.…”

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confidence: 99%

Current efficiency in organic light-emitting diodes with a hole-injection layer

Wang

Klubek

Tang

2008

Applied Physics Letters

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“…Exciplex formation at the solid interface between Alq3 and the electron-rich multiple triarylamine hole-transporting materials m-MTDATA and t-Bu-TBATA was observed in a study by Itano et al, [11]. Exciplexes can also be observed in the PL spectra of donoracceptor blends [10,[12][13][14][15].…”

Section: Introductionmentioning

confidence: 63%

Exciplex Electroluminescence of the New Organic Materials for Light-Emitting Diodes

Kaplunov¹,

Nikitenko²,

Krasnikova³

2012

Organic Light Emitting Devices

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“…5,12 In this experiment, exciplex emission was mostly accompanied by exciton emission, resulting in the multiple component spectra, 5,6,10,11 an effect which has been exploited to produce almost white emitting OLEDs. 5,6 We will show that in our devices the exciplex emission is voltage dependent, a phenomenon also found in other works 11,12 wherein the related mechanism of color tuning is discussed. Because the fraction of radiative exciplex recombination 7 is small, the appearance of exciplexes generally lowers the efficiency of OLEDs.…”

Section: Introductionmentioning

confidence: 99%

“…[5][6][7][8][9][10][11][12] In these, exciplex emission is often reported to be redshifted with respect to the bulk EL emission. 10,12 This is related to the energy difference between the highest occupied molecular orbital ͑HOMO͒ level ͑ionization potential͒ of the hole transporting material and the lowest-unoccupied molecular orbital ͑LUMO͒ level ͑electron affinity͒ of the electron transporting material, which favors generation of bound states of lower energy rather than excitons.…”

Section: Introductionmentioning

confidence: 99%

Competition between excitons and exciplexes: Experiments on multilayered organic light emitting diodes

Castellani

Berner²

2007

Journal of Applied Physics

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The electronic processes responsible for charge transport and electroluminescence in multilayered organic light emitting diodes ͑OLEDs͒ are very sensitive to the properties of the organic heterojunction. In particular, the height of the energy barrier affects the way in which electrons and holes meet at the heterojunction, the way in which the barrier is crossed, and the probability for photon creation. We investigate these aspects experimentally using a family of OLED devices in which different hole transporting materials are used in otherwise identical device architectures to vary the interfacial hole barrier over a wide energy range. We find that the quantum efficiency of the device is maximum for low-energy barriers and drops for high barrier values where a redshifted electroluminescence spectrum is observed. This shift is attributed to exciplex generation at the heterojunction. The contributions of exciton and exciplex annihilation in radiative and nonradiative channels to the charge flow within the heterojunction region are separated and quantified.

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Exciplex formation at the organic solid-state interface: Yellow emission in organic light-emitting diodes using green-fluorescent tris(8-quinolinolato)aluminum and hole-transporting molecular materials with low ionization potentials

Abstract: Electron mobility in tris(8-hydroxy-quinoline)aluminum thin films determined via transient electroluminescence from single-and multilayer organic light-emitting diodes

Cited by 163 publications

References 13 publications

Current efficiency in organic light-emitting diodes with a hole-injection layer

Current efficiency in organic light-emitting diodes with a hole-injection layer

Exciplex Electroluminescence of the New Organic Materials for Light-Emitting Diodes

Competition between excitons and exciplexes: Experiments on multilayered organic light emitting diodes

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