High-brightness top-emissive polymer light-emitting diodes utilizing ...

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Ten-Chin Wen and Sung-Nien Hsieh ... Chia-Tin Chung and Ching-In Wu. Chi Mei ..... B. Pode, C. J. Lee, D. G. Moon, and J. I. Han, Appl. Phys. Lett. 84,.
APPLIED PHYSICS LETTERS 89, 051103 共2006兲

High-brightness top-emissive polymer light-emitting diodes utilizing organic oxide/Al/ Ag composite cathode Tzung-Fang Guo,a兲 Fuh-Shun Yang, Zen-Jay Tsai, and Guan-Weng Feng Institute of Electro-Optical Science and Engineering, Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan, Taiwan 701, Republic of China

Ten-Chin Wen and Sung-Nien Hsieh Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan 701, Republic of China

Chia-Tin Chung and Ching-In Wu Chi Mei Optoelectronics Corporation, Tainan Science-Based Industrial Park, Taiwan 741, Republic of China

共Received 27 March 2006; accepted 19 June 2006; published online 1 August 2006兲 This work presents the fabrication of high-brightness 共over 30 000 cd/ m2兲 top-emissive polymer light-emitting diodes 共PLEDs兲 using a hybrid semitransparent cathode capable of efficient injection of electrons. The composite cathode is comprised of the organic oxide/Al complex as the injection buffer layer covered by a thin Ag overlayer. The anode is made of Ag: Ag2O coated on the glass substrate. The electroluminescence 共EL兲 efficiency of 8.9 cd/ A for phenyl-substituted poly共para-phenylene vinylene兲 copolymer based top-emissive PLED markedly exceeds that of 4.3 cd/ A for the control device with the bottom-emissive configuration. The high performance is attributed to the balanced injection of charge carriers and the effective extraction of EL emission from the top cathode. The optical microcavity effect significantly promotes the EL emission in the direction along the surface normal. © 2006 American Institute of Physics. 关DOI: 10.1063/1.2234317兴 The top-emissive organic and polymer light-emitting diodes 共T-OLEDs and T-PLEDs兲 emit light from the top surface of devices, which have many favorable features than the bottom-emissive O/PLEDs in active-matrix 共AM兲 flat panel display applications.1–4 These diodes can be fabricated on opaque substrates, including rigid Si wafers and flexible steel foils,5,6 and can function at a lower bias voltage or current, while still exhibiting the same luminescence as bottomemissive devices. The AM panels made of top-emissive O/PLEDs are expected to present a much more vividly colored image and increased operating lifetime. The configurations of typical T-O/PLEDs comprise several organic layers with different functionalities sandwiched between the highly reflective bottom anode and the semitransparent top cathode. Ag is commonly adopted as a conductive bottom anode, because of its highest reflectivity of light in the visible region and its ability of efficient injection of holes after the appropriate surface treatment.7,8 In the semitransparent cathode, an ideal top electrode should be highly conductive of charges, support the efficient injection of electrons, be highly transparent to light, have a high emission out-coupling efficiency, and have a long-term operating stability. However, no single material satisfies all of the requirements of the top cathode simultaneously. Many research groups have applied an ultrathin layer of the low work function metals, LiF / Al, or copper phthalocynaine covered with a thin Ag or indium-tin-oxide 共ITO兲 overlayer to fabricate the top cathodes.3,9–11 The performance of T-O/PLEDs thus obtained was favorable.12–16 A composite electrode is required to develop the multifunctional cathode structure. In a兲

Author to whom correspondence should be addressed; electronic mail: [email protected]

this letter, an organic oxide/Al/ Ag composite cathode is reported for the fabrication of high-brightness and highperformance top-emissive PLEDs. The configurations of the top-emissive PLEDs here in this study are plotted in the inset of Fig. 3. The Ag: Ag2O electrode on the glass substrate, reported by Chen et al.,7 is used as a highly reflective anode and supports the efficient injection of holes in a top-emissive device. The thin layer of Ag2O is prepared by UV-ozone treatment of the Ag/glass substrate. The device configuration is comprised of poly共3,4ethylenedioxythiophene兲:polystyrenesulfonate 共PEDOT:PSS, Bayer Corp. 4083兲 as the hole transport layer, “high-yellow” phenyl-substituted poly共para-phenylene vinylene兲 copolymer 共HY-PPV; electroluminescence 共EL兲 emission centered at 560 nm兲 film as the light-emissive layer, and the hybrid organic oxide/Al/thin Ag as the semitransparent cathode. The organic-oxide film is deposited by thermally evaporating a thin polymer layer, 15 Å, of poly共ethylene glycol兲 dimethyl ether 共PEGDE兲 共Aldrich, Mn ca. 2000兲 on the surface of HY-PPV film inside a vacuum chamber 共10−6 torr兲. The semitransparent metal electrode, 15 Å thick Al layer and Ag of different thicknesses, is evaporated on the substrates sequentially without breaking the vacuum. No dielectric capping layer is utilized for the hybrid cathode in this study. The active pixel area of the device is 0.06 cm2. The details of the fabrication procedure and the current-brightness-voltage 共I-L-V兲 measurement can be found elsewhere.17,18 The usage of the organic oxide, PEGDE, as the cathode buffer layer enables the efficient injection of electrons through the Al cathode and blocks the metal-induced quenching sites in the EL layer.17,18 In addition, the enhanced performance is limited only to the use of Al metal.18–20 The device configurations of the bottom-emissive devices

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FIG. 1. I-L-V curves of 共쎲兲 the top- and 共䊊兲 the control bottom-emissive devices. The inset shows the plot of the luminous efficiency vs current density.

with organic oxide/Al cathodes with Al metal layers of different thicknesses are glass/ITO/ PEDOT: PSS/ HY-PPV/ PEGDE共15 Å兲 / Al共X Å兲 / Ag共1200 Å兲, with X = 5, 15, 50, and 100 Å. Ag is used as the covering layer on the organic oxide/Al composite cathode to retain the conductivity of the electrode. The maximum efficiency is 10.7 cd/ A at 6.70 V and 5142.0 cd/ m2 for the device with the PEGDE共15 Å兲 / Al共100 Å兲 / Ag共1200 Å兲 cathode 共which has a thick Al middle layer兲, but only 3.4 cd/ A at 6.20 V and 2831.9 cd/ m2 for the device with the PEGDE共15 Å兲 / Al共5 Å兲 / Ag共1200 Å兲 cathode structure 共which has a thin Al middle layer兲. The luminous efficiencies are 9.1 cd/A at 6.70 V, 4824.8 cd/ m2 and 4.3 cd/A at 6.80 V, 5186.2 cd/ m2 for the devices with PEGDE共15 Å兲 / Al共50 Å兲 / Ag共1200 Å兲 and PEGDE共15 Å兲 / Al共15 Å兲 / Ag共1200 Å兲 cathode structure, respectively. The higher performance of the devices with thicker Al middle layers follows from the balanced injection of charge carriers, according to our previous investigations.17,18 Optimizing the thickness of the organic oxide/Al complex layer is critical to the injection of electrons through the cathode as well as the improvement in the performance of the device. Figure 1 plots the I-L-V curves of the top- and the control bottom-emissive devices with the structures of glass/ Ag: Ag2O / PEDOT: PSS/ HY-PPV/ PEGDE共15 Å兲 / Al共15Å兲 / Ag共70 Å兲 and glass/ITO/ PEDOT: PSS/ HY-PPV / PEGDE共15 Å兲 / Al共15 Å兲 / Ag共1200 Å兲, respectively. The light intensity of the top-emissive device exceeds 30 000 cd/ m2 in the direction along the surface normal when biased at 8.80 V. The turn-on voltage of the light emission is under 3.0 V. As shown in the inset in Fig. 1, the maximum luminous efficiency of the top-emissive device is 8.9 cd/ A 共6677.3 cd/ m2, EL emission centered at ⬃560 nm兲 when the device is biased at 6.60 V 共74.98 mA/ cm2兲, in which the efficiency markedly exceeds that of 4.3 cd/ A 共5186.2 cd/ m2兲, biased at 6.80 V 共120.40 mA/ cm2兲, for the control device with the bottom-emissive configuration. Since the cathode parts of the top- and control bottom-emissive devices are comprised of the same composition of the organic oxide/Al complex 关PEGDE共15 Å兲 / Al共15 Å兲兴 as the injection buffer layer for electrons, the superior luminous efficiency for the top-emissive device is presumed to be re-

FIG. 2. Normalized EL spectra of 共䊊兲 the control bottom-emissive device and the top-emissive devices made of Ag overlayers of various thicknesses: glass/Ag:Ag: Ag2O / PEDOT: PSS/ HY-PPV/ PEGDE共15 Å兲 / Al 共15 Å兲 / Ag共Y Å兲, Y = 共쎲兲 70 Å, 共䊏兲 150 Å, and 共䉱兲 300 Å. The inset displays 共䉭兲 the simulated EL emission and the 共䉱兲 measured EL spectrum of the top-emissive device with the cathode structure, PEGDE 共15 Å兲 / Al共15 Å兲 / Ag共300 Å兲.

lated to the enhanced spontaneous emission and the modified emission distribution of the optical microcavity effect.12,21,22 The preferential direction of EL emission between the highly reflective Ag: Ag2O anode and the semitransparent organic oxide/Al/ Ag cathode is tuned from the internal reflection toward the out-coupling regime. Hence, the device with the top-emissive configuration exhibits a substantially enhanced EL intensity in the forward direction, with a higher luminous efficiency along the direction of surface normal than that of the control bottom-emissive device without the cavity effect. The internal reflections of the light that propagates between the highly reflective Ag: Ag2O anode and the semitransparent composite cathode are expected to have strong optical microcavity effects on both the spectral and the spatial distributions of the EL emission.21–25 Figure 2 presents the normalized EL spectra of the control bottom-emissive device, glass/ITO/ PEDOT: PSS/ HY- PPV/ PEGDE共15 Å兲 / Al共15 Å兲 / Ag共1200 Å兲, and the top-emissive devices made of Ag overlayers of various thicknesses, glass/ Ag: Ag2O / PEDOT: PSS/ HY- PPV/ PEGDE共15 Å兲 / Al共15Å兲 / Ag共Y Å兲, Y = 70, 150, and 300 Å. The full width at half maximum 共FWHM兲 of the control bottom-emissive device is ⬃92 nm, but that of the top-emissive device with the PEGDE共15 Å兲 / Al共15 Å兲 / Ag共70 Å兲 cathode structure is ⬃52 nm. The shift in the ␭max toward the shorter wavelength 共from ⬃560 to⬃ 548 nm兲 and the decline in the FWHM 共from ⬃52 to⬃ 24 nm兲 of the EL emissions were also observed for the top-emissive devices with the thick and uniformly covered Ag overlayer. The approximate calculation of the Fabry-Pérot cavity theory is as follows:12,21–23 Gcav共␭兲 =

兩Eout兩2 ␶cav ⫻ , 兩E0兩2 ␶non-cav

Scav共␭兲 = S0共␭兲 ⫻ Gcav共␭兲, 共1兲

where Gcav共␭兲 is the emission enhancement factor associated with the optical cavity at a single wavelength ␭. Eout and E0 are the out-coupled and the free-space electric-field intensities of the emissions, respecticely. ␶cav and ␶non-cav are the

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measuring the intensities of photocurrents through a photodiode inside an integrated sphere, which are approximately identical for both devices. Depositing an additional index matching layer onto the structure of the composite cathode is expected to increase the net EL out-coupling efficiency of the top-emissive devices. In summary, the organic oxide/Al/ Ag composite electrode was disclosed as an appropriate cathode structure in the fabrication of high-performance and high-brightness topemissive PLEDs. The optical microcavity effect is responsible for the redistribution of EL emissions, in which the top-emissive devices exhibit saturated color emission and enhanced luminous intensity in the direction of the surface normal. FIG. 3. EL spectra of the top-emissive device, with the cathode structure PEGDE共15 Å兲 / Al共15 Å兲 / Ag共70 Å兲, measured at viewing angles of 共쎲兲 0°, 共兲 30°, and 共⫻兲 60° from the surface normal. The inset shows the device configuration of the top-emissive PLEDs here in this study.

radiative lifetimes of the excited molecules in the cavity and in free space, respectively. S0共␭兲 is the intrinsic emission spectrum of an emitter and is assumed to be Gaussian. Scav共␭兲 represents the emission spectrum under the influence of the optical microcavity effect. The inset in Fig. 2 shows the simulated EL emission, Scav共␭兲, determined by the calculation based on Fabry-Pérot cavity theory and the EL spectrum obtained in the direction along the surface normal of the top-emissive device with the cathode structure PEGDE共15 Å兲 / Al共15 Å兲 / Ag共300 Å兲. The simulated EL spectrum overlaps substantially with the experimental result. The shift of ␭max from 560 nm 共for the top-emissive device with a 70 Å thick Ag covering layer兲 toward 548 nm 共with a 300 Å thick Ag covering layer兲, as observed in Fig. 2, is probably caused by the change in the reflectivity of the composite cathode, which forms a complete cavity structure, and thus, multiply reflects the EL emission and moves the ␭max toward the resonance wavelength of the optical cavity. The simulation of FWHM is determined from Eq. 共1兲 as follows:23

The authors would like to thank the National Science Council 共NSC兲 of Taiwan 共NSC94-2113-M-006-007 and NSC95-ET-7-006-001-ET兲 and the Asian Office of Aerospace Research and Development 共AOARD-06-4076兲 for financially supporting this research. Dr. Ruei-Tang Chen from Eternal Chemical Co., Ltd. is appreciated for providing the HY-PPV polymer. 1

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