Influence of the hole transport layer on the performance of organic light-emitting diodes

Giebeler, C.; Antoniadis, Homer; Bradley, Donal D. C.; Shirota, Yasuhiko

doi:10.1063/1.369413

Cited by 137 publications

(81 citation statements)

References 24 publications

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“…However, it has been shown that the OLED quantum efficiency is independent of the HTL mobility. 8 Finally, it has been shown that exciplex formation between the HTL and the emission layer reduces the device efficiency, 6 however we find no evidence of exciplex emission in these devices.…”

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confidence: 45%

“…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.…”

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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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confidence: 99%

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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“…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.…”

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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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“…Because the fraction of radiative exciplex recombination 7 is small, the appearance of exciplexes generally lowers the efficiency of OLEDs. Exciplex related investigations in OLEDs have also been conducted in relation to aging: Giebeler et al 7 considered the effects of heat or electrical stress on exciplex emitting OLEDs and found a reduction of exciplex emission as a function of time. In another paper, 8 exciplex emission was used to estimate the mobility of one of the constituent organic materials of the OLED.…”

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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Influence of the hole transport layer on the performance of organic light-emitting diodes

Cited by 137 publications

References 24 publications

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

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

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

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

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