Materials for High Performance Single-layer OLED Devices

Materials for High Performance Single-layer OLED Devices Abstract / Dipolar compounds containing a dibenzothiophene S,S-dioxide core and two peripheral diarylamines have been synthesized. They readily form glass and emit from blue to orange. These materials exhibit good electron and hole mobilities of over 10-4 cm2/(V s), as measured by the timeof-flight (TOF) transient photocurrent technique. High performance single-layer green-emitting electroluminescent devices using these materials have been achieved due to the balanced bipolar carrier transport properties. One device appears to have the best performance (3.1%; 3.9 lm/W; 7.5 cd/A at 100 mA/cm2) among small-molecule, single-layered devices ever reported in the literature.

Tai-Hsiang Huang1, *Jiann T. Lin1, Li-Yin Chen2, Yu-Ting Lin,2 and *Chung-Chih Wu2

Introduction

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Organic light-emitting diodes (OLEDs) have attracted intensive interest because of their potential applications in full-color flat-panel displays and lighting sources. In order to assure color purity and good performance of OLEDs, the balance of electron and hole mobility is extremely important in confining excitons at the desired region. Towards this end, a common approach used for polymers is the blending of different components. In contrast, small molecules of different functions are generally vacuum-deposited successively to form a multi-layered device. For instance, a three-layered structure typical for OLEDs consists of hole-transporting (HTL), emitting (EL), and electron-transporting (ETL) layers. The device can be simplified to a two-layered structure if one of the carrier-transporting materials also possesses emission

characteristics. Further simplification to a singlelayer structure should be possible if the emitting material has balanced electron and hole mobilities. It is conceivable that the overall cost will be reduced with a more simplified device fabrication. So far, the single-layered small-molecule electroluminescent (EL) devices reported are far from satisfactory except for the one reported by us. The yellow green-emitting device fabricated from 3-cyano-9-diarylamino carbazoles, [1] exhibits current efficiency up to ~5.5 cd/A at 100 mA/cm2. In our continuing search for materials for single-layered devices, electron-deficient dibenzothiophene-S,S-dioxide moiety[2] was integrated with an electron-rich arylamine moiety. We have successfully synthesized dibenzothiophene-S,S-dioxide-based dipolar compounds and fabricated single-layered devices from these materials.

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Institute of Chemistry, Academia Sinica, Taipei, Taiwan Department of Electrical Engineering, Graduate Institute of Electro-optical Engineering and Graduate Institute of Electronics Engineering, National Taiwan University, Taipei, Taiwan

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【Fig 1】Structures of the compounds.

【Fig 2】Cyclic voltammograms of the compounds 1, and 3 measured in CH 2 Cl 2 and DMF, respectively. Ferrocene (Fc) was added as the internal standard. Significant Reserach Achievements of Academia Sinica

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Results and Discussion The new compounds synthesized in this study are illustrated in Figure 1. These compounds readily form glass with glass transition temperatures ranging from 102 to 138 ºC. They also have high thermal de-composition temperatures which range from 345 to 440 ºC. The large solvatochromism of the emission spectra indicates that these molecules possess a strong intramolecular charge transfer from the donor (amine) to the acceptor (dibenzothiophene-S,S-

【Fig 3】Luminance versus current density characteristics for the devices I-III for 2 and 3. Graphs in red and blue are for the single-layer devices.

dioxide). The λem value of the compound increases as the solvent polarity increases, and the emission color ranges from blue to orange. The quantum yields of 1-4 in toluene are 15%, 55%, 19%, and 11%, respectively. Each compound displays two quasi-reversible waves in the cyclic voltammetry (see Figure 2 for compounds 2 and 3). The two-electron process at positive potential and one-electron process at negative potential relative to ferrocene/ferrocenium are attributed to the oxidation of the two peripheral amine units, and the reduction of the dibenzoth-

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【Fig 4】The device structure for III.

iophene-S,S-dioxide core, respectively. The calculated HOMO (5.21-5.51 eV) and LUMO (2.562.74 eV) energy levels together with the quasireversible waves suggest that these compounds may be appropriate materials for single-layer OLEDs using an ITO anode (Fermi level = 4.7 eV) and an Al/LiF cathode (Fermi level = 4.2 eV).

to those of device I. In particular, device III (Figure 4) of 2 has the best performance (3.1%; 3.9 lm/W; 7.5 cd/A at 100 mA/cm 2) among small-molecule, single-layered devices ever reported in the literature. The amorphous 2 was found to have similar electron and hole mobilities of over 10 -4 cm2/(V s), as measured by the time-of-flight (TOF) transient photocurrent technique in vacuum at room temperature. These values are about one order lower than the hole mobility (~10-3 cm2/(V s)) of the typical HTL material NPB and two orders higher than the electron mobility (~10-6 cm2/(V s)) of the typical ETL material Alq3 (tris-(8-hydroxyquinoline) aluminum). The TOF transients indicate dispersive transport behaviors for both holes and electrons. The fielddependence of mobilities (Figure 3) follows the nearly universal Poole-Frenkel relationship:μ ∝ exp(βE 1/2), where βis the Poole-Frenkel factor.[3] The knowledge of bipolar transport properties of the present compounds provides better understanding of the characteristics of single-layered devices. Although the EL performances of the current single-layered devices (devices III) are still slightly lower than those of the best twolayered devices (devices I), one however should notice that the devices have not yet been subjected to thorough optimization, such as optimizing

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Compounds 2 and 3, having higher emission quantum yields, were subjected to EL studies using different device structures on the indium tin oxide (ITO) anode: (I) ITO/2 (or 3) (40 nm)/TPBI (40 nm)/LiF (1 nm)/Al (150 nm); (II) ITO/NPB (40 nm)/2 (or 3) (40 nm)/ LiF (1 nm)/Al (150 nm); (III) ITO/2 (or 3) (80 nm)/LiF (1 nm)/Al (150 nm). Compounds NPB (1,4bis[(1-naphthylphenyl)amino]biphenyl) and TPBI (1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene) were used as hole- and electron-transporting materials, respectively, while LiF and Al were used as the electron-injection layer and the cathode, respectively. Figure 1 shows the currentluminance (I-L) characteristics. All devices exhibit low turn-on voltages (2.0-3.5 V) and emissions occur in 2 (or 3). It is worth noting that although devices I gives the best device performances among the three types of devices, the unoptimized single-layered devices III of 2 and 3 show very promising performances comparable

【Fig 5】Electron and hole mobilities versus E1/2 for 2.

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the electrodes (e.g. cathode), carrier injection and the layer thicknesses, etc. These factors could have subtle influences on the balance of carrier injection and the position of the carrier recombination zone, etc.

Conclusion We have developed a convenient synthesis of dipolar compounds with a dibenzothiopheneS,S-dioxide core and two peripheral arylamines. These compounds exhibit intriguing ambipolar carrier transport properties and can be used to fabricate single-layered EL devices possessing excellent performances comparable to those of multi-layered devices. More recently we have also developed new ambipolar materials for high performance single-layer blue-emitting OLEDs via a similar approach.

The original paper was published in Advanced

Materials 18 (2006): 602-606.

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References 1. Justin Thomas, K. R.; Velusamy, M.; Lin, J. T.; Tao, Y.-T.; Chuen, C.-H. (2004) Adv. Funct. Mater. 14, 387-392. 2. Hughes, G.; Bryce, M. R. (2005) J. Mater. Chem. 15, 94-107. 3. Gill, W. D. (1972) J. Appl. Phys. 43, 5033-5040.