APPLIED PHYSICS LETTERS 91, 043504 共2007兲
Hole-transporting-layer-free high-efficiency fluorescent blue organic light-emitting diodes Jwo-Huei Jou,a兲 Po-Hsuan Chiang, Yu-Pu Lin, and Chin-Yeh Chang Department of Materials Science and Engineering, National Tsing Hua University, Hsin-Chu, Taiwan 30013, Republic of China
Chun-Liang Lai Labeltek Inc., Taipei, Taiwan 100, Republic of China
共Received 16 March 2007; accepted 24 June 2007; published online 24 July 2007兲 High-efficiency fluorescent blue organic light-emitting diodes were fabricated using hole-injection and carrier-confining effective device architecture without a hole-transporting layer. The diodes consist of 1250 Å indium tin oxide/300 Å blue-emissive layer/400 Å electron-transporting layer/ 10 Å lithium fluoride/1500 Å aluminum. A high efficiency of 6.0% 共12.5 lm/ W兲 at 100 cd/ m2 with a pan-blue emission of 共0.19, 0.31兲 when trans-1,2-bis-共6-共N , N-di-p-tolylamino兲naphthalene2-yl兲ethene was used in the emissive layer, or 3.9% 共6.7 lm/ W兲 with a blue emission of 共0.15, 0.20兲 when 2-共N , N-diphenylamino兲-6-关4-共N , N-diphenylamino兲-styryl兴naphthalene was used. Besides having a thinner device structure, the marked improvement in efficiency is attributable mainly to the relatively low hole-injection barrier. © 2007 American Institute of Physics. 关DOI: 10.1063/1.2759984兴 Organic light-emitting diodes 共OLEDs兲 have attracted considerable attention owing to their highly promising uses in full-color flat-panel displays and area illumination.1–4 In these applications, a high-efficiency OLED is very desirable. Numerous efficient phosphorescent OLEDs have been reported.5–7 For fluorescent OLEDs, the best reported power efficiency at 100 cd/ m2 was 7.9 lm/ W 共Ref. 8兲 for pan-blue emission or 2.5 lm/ W 共Ref. 9兲 for blue emission. Besides the electroluminescent characteristics of the materials, many structural factors, such as layer thickness, carrier-injection barriers,10 and carrier/exciton confinement,11–13 affect the device efficiency. A multiple-layered device architecture, which generally consists of a hole-transporting layer 共HTL兲, a holeinjection layer, an electron-transporting layer 共ETL兲, and an emissive layer 共EML兲, is frequently employed.14–17 However, a multiple-layered structure complicates the manufacturing process, so OLEDs of higher efficiency and simplified device structure should be developed. This letter presents high-efficiency fluorescent blue OLEDs using hole-transporting-layer-free device architecture. The device consisted of a 1250 Å indium tin oxide 共ITO兲 layer, a 300 Å blue emissive layer, a 400 Å 1,3,5-tris 共N-phenylbenzimidazol-2-yl兲benzene 共TPBi兲 layer, a 10 Å lithium fluoride layer, and a 1500 Å aluminum layer. Three blue-emissive materials were investigated: greenish-bluelight-emitting trans-1,2-bis共6-共N , N-di-p-tolylamino兲-naphthalene-2-yl兲ethene 共BNE兲,18 blue-light-emitting 2 - 共N , Ndiphenylamino兲- 6 -关4-共N , N-diphenylamino兲styryl兴naphand bluish-green-light-emitting thalene 共DPASN兲,19 di共triphenyl-amine兲-1,4-divinylnaphthalene 共DPVP兲. The device with BNE exhibited a high efficiency of 6.0% 共12.5 lm/ W兲 at 100 cd/ m2 with a pan-blue emission of 共0.19, 0.31兲, while the DPASN one exhibited 3.9% 共6.7 lm/ W兲 with a blue emission of 共0.15, 0.20兲. Figure 1 shows the device architecture with or without a HTL, the a兲
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chemical structures, and photoluminescence spectra of the three blue-emissive materials. Figure 2 compares the power efficiencies of the three blue OLEDs with and without a HTL. Since the HTL material may greatly affect the resultant power efficiency, N , N , N⬘ , N⬘-tetra共naphthalene-2-yl兲phenylenediamine 共t-NPD兲, which was the best HTL studied previously,19 was selected for comparison. The efficiency of the BNE-composing device with a HTL was 5.7% 共10.2 lm/ W兲 at 100 cd/ m2 and 3.8 V. Its HTL-free counterpart exhibited an efficiency of 6.0% 共12.5 lm/ W兲 at 100 cd/ m2 and 3 V. The efficiency of the DPASN-composing device with a HTL was 3.4% 共6.0 lm/ W兲 at 100 cd/ m2 and 4 V. That of the HTL-free counterpart was 3.9% 共6.7 lm/ W兲 at 100 cd/ m2 and 3.5 V. The above two devices demonstrated that the use of a HTL-free architecture markedly improved the efficiency. However, the absence of a HTL does not necessarily improve the efficiency of the other device. For ex-
FIG. 1. 共Color online兲 Schematic illustrations of the device structure of the blue OLEDs with or without a hole-transporting layer, the molecular structures, and the photoluminescence spectra of BNE, DPASN, and DPVP.
0003-6951/2007/91共4兲/043504/3/$23.00 91, 043504-1 © 2007 American Institute of Physics Downloaded 19 Dec 2010 to 140.114.66.106. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions
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FIG. 2. 共Color online兲 Power efficiency results of the three different blue OLEDs, with and without HTL, on the Commission Internationale de L’Eclairage 共CIE兲 1931 chromaticity diagram. The inset shows the power efficiency vs PL peak wavelength of the three blue OLEDs studied here and those by others 关BD1: mono共styryl兲amine-based deep-blue emissive dopant 共Ref. 9兲 and DSA-ph: p-bis共p-N,N-diphenyl-aminostyryl兲-benzene兲 共Ref. 8兲兴.
ample, when DPVP was used as the emissive layer, the efficiency decreased from 3.7% to 3.0% 共6.6 to 5.7 lm/ W兲 at 100 cd/ m2 when the HTL was eliminated. The absence of a HTL results in a thinner device structure and sometimes increases the efficiency. However, besides the device thickness, other crucial efficiency-affecting factors, such as carrier-injection barriers and carrier/exciton confinement, may change greatly when the HTL is removed. The resulting effect may be positive or negative, depending on whether the barriers are increased or decreased and the confinement is improved or degraded. Taking the BNE-composing device as an example, the hole-injection barrier decreased from 0.02 to 0 eV, while the electron-injection barrier remained unchanged when the HTL 共t-NPD兲 was removed, as revealed by devices I and IV in Fig. 3. The hole-confining function of the interface between EML and ETL, TPBi, remained unchanged when the HTL was removed. The electron-confining function provided by the interface between the HTL and EML significantly changed, and the electron-blocking barrier decreased from 0.77 to 0 eV, potentially negatively affecting the efficiency. However, since electrons could be effectively constrained at the EML/ETL interface due to the high barrier of 0.54 eV, holes and electrons could recombine effectively. Recombination indeed occurred mainly near the EML side, as indicated by the strong blue emission. All these factors can explain why the HTL-free BNE-composing device had a higher efficiency than the device with a HTL. The same reasons explain why the HTL-free DPASN-composing device also had a higher efficiency, because the energy levels of the highest
FIG. 4. 共Color online兲 共a兲 Power efficiency-voltage characteristics, 共b兲 current density-voltage characteristics, and 共c兲 luminance-voltage characteristics of the studied devices using the three different blue emissive materials, with and without a hole-transporting layer.
occupied molecular orbital 共HOMO兲 and the lowest unoccupied molecular orbital 共LUMO兲 of DPASN are similar to those of BNE, as also shown in devices II and V in Fig. 3. The methods used to estimate the HOMO and LUMO energies for these compounds are described elsewhere.14 The HTL-free DPVP-composing device had a holeinjection barrier of 0.1 eV, which was much higher than that of its counterpart with a HTL, as in devices III and VI 共Fig. 3兲. Meanwhile, all of the other functions remained unchanged. The unfavorably higher hole-injection barrier may explain why removing the HTL from the DPVP-composing device reduced the efficiency. Figure 4共a兲 shows the effect of the presence or absence of HTL on the efficiency of the three different blue OLEDs at varying voltages. The removal of HTL in the best performed BNE-composing device did not affect the dependency of efficiency on voltage at voltages under 6 V, at which the corresponding brightness was 10 000 cd/ m2. Above 6 V, the efficiency of the HTL-free device dropped more markedly than that of the HTL-containing counterpart, due to the lack of an efficient electron blocking barrier between EML and ITO. Figure 4共b兲 shows the J-V curves of the three devices with and without a HTL. Removing the HTL made all of the devices thinner, increasing the current density in all cases.
FIG. 3. Energy-level diagrams of the three blue OLEDs studied 共a兲 with hole-transporting layer and 共b兲 without hole-transporting layer; where devices I and IV comprise BNE, devices II and V comprise DPASN, and devices III and VI comprise DPVP. Downloaded 19 Dec 2010 to 140.114.66.106. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions
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TABLE I. Electroluminescent characteristics of BNE-, DPASN-, and DPVP-composing devices, with and without a hole-transporting layer.
Emissive layer
Driving voltagea 共V兲
Max. luminance 共cd/ m2兲
Power efficiencyb 共Im/W兲
Ext. quantum efficiencyb 共%兲
CIE 1931 共x , y兲 coordinatesb
With HTL
BNE DPASN DPVP
3.2 3.3 2.9
8,700 6,700 17,200
10.2 6.0 6.6
5.7 3.4 3.7
共0.18, 0.33兲 共0.15, 0.17兲 共0.21, 0.39兲
HTL-free
BNE DPASN DPVP
2.7 3.0 3.4
20,200 6,900 7,900
12.5 6.7 5.7
6.0 3.9 3.0
共0.18, 0.33兲 共0.15, 0.20兲 共0.22, 0.43兲
Driving voltage is define as the resultant value obtained at luminance 艌10 cd/ m2. Power efficiency, external quantum efficiency, and CIE coordinates as the resultant value obtained at luminance at 100 cd/ m2.
a
b
The zero hole-injection barrier of a HTL-free BNE—or DPASN-composing device may explain why its current density, even at very low voltages, was much higher than that of its HTL-containing counterpart. However, the current density of the HTL-free DPVP-composing device was slightly higher at low voltages while having the same thinner structure and was significantly higher only at voltages of over 7 V. The difference may be attributed to the variation in the hole-injection barrier when the HTL is removed: the holeinjection barrier increased in the former case and decreased in the latter. The effect of removing the HTL on the hole-injection barrier can be well understood with reference to the luminance results of the three devices shown in Fig. 4共c兲. The driving voltage required to emit 10 cd/ m2, for example, decreased from 3.2 to 2.7 V or from 3.3 to 3.0 V, when the HTL was removed from the BNE—or DPASN-composing device, respectively. The driving voltage was increased from 2.9 to 3.4 V when the HTL was removed from the DPVP counterpart, as summarized in Table I. In conclusion, this work has demonstrated the high efficiency of fluorescent blue OLEDs using HTL-free device architecture. The diodes consist of ITO/blue-emissive layer/ electron-transporting layer/lithium fluoride/aluminum. A high efficiency of 6.0% 共12.5 lm/ W兲 at 100 cd/ m2 and 3 V with a pan-blue emission of 共0.19, 0.31兲 can be obtained using greenish-blue-light-emitting BNE. Using blue-lightemitting DPASN, an efficiency of 3.9% 共6.7 lm/ W兲 at 100 cd/ m2 and 3.5 V is obtained with a blue emission of 共0.15, 0.20兲. As well as the thinner device structure, the high efficiency is attributable mainly to the relatively low holeinjection barrier between the ITO and blue-emissive layers. The authors would like to thank C. H. Cheng of Department of Chemistry, National Tsing Hua University, for help with the External Quantum efficiency measurements. This work was financially supported under Grant Nos.
NSTN92J0162J4, NSC93-2216-E-007-018, and AFOSRAOARD-05-0488. 1
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