Bright single-active layer small-molecular organic light-emitting diodes with apolytetrafluoroethylene barrierYudi Gao, Liduo Wang, Deqiang Zhang, Lian Duan, Guifang Dong, and Yong Qiu Citation: Applied Physics Letters 82, 155 (2003); doi: 10.1063/1.1536031 View online: http://dx.doi.org/10.1063/1.1536031 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/82/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Organic light-emitting diodes with direct contact-printed red, green, blue, and white light-emitting layers Appl. Phys. Lett. 101, 153304 (2012); 10.1063/1.4757279 High-brightness, high-color-purity, white organic light-emitting diodes featuring multiple emission layers J. Appl. Phys. 105, 064318 (2009); 10.1063/1.3097285 Multilayer white polymer light-emitting diodes with deoxyribonucleic acid-cetyltrimetylammonium complex as ahole-transporting/electron-blocking layer Appl. Phys. Lett. 92, 251108 (2008); 10.1063/1.2948864 Triarylamine siloxane anode functionalization/hole injection layers in high efficiency/high luminance small-molecule green- and blue-emitting organic light-emitting diodes J. Appl. Phys. 101, 093101 (2007); 10.1063/1.2719276 High-performance polymer light-emitting diodes fabricated with a polymer hole injection layer Appl. Phys. Lett. 83, 183 (2003); 10.1063/1.1589185

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Bright single-active layer small-molecular organic light-emitting diodeswith a polytetrafluoroethylene barrier

Yudi Gao, Liduo Wang, Deqiang Zhang, Lian Duan, Guifang Dong, and Yong Qiua)

Organic Optoelectronics Lab, Department of Chemistry, Tsinghua University, Beijing 100084, China

~Received 6 September 2002; accepted 18 November 2002!

Single-layer organic light-emitting diodes~OLEDs! with a small molecule, tris~8-hydroxyquinoline!aluminum (Alq3) as the only active material, have been prepared. In order to achieve an efficienthole injection, a thin layer of polytetrafluoroethylene~Teflon! was inserted between the anode andthe Alq3 layer. The effect of the Teflon layer thickness upon the device performance has also beeninvestigated. A brightness of 16 000 cd/m2 with 6 nm thick Teflon layer was achieved, whereas theconventional double-layer OLEDs with N,N8-bis-~1-naphthyl!-N,N8-diphenyl-1,18 biphenyl4,48-diamine and Alq3 showed only 9500 cd/m2 in our experiments. The single-layer structuretechnology is of great importance to the OLED’s commercialization due to its possible lower costand higher production efficiency. And it is reasonable to infer that, based on this work, higher deviceperformance could be realized by screening both the active material and the barrier layer material.© 2003 American Institute of Physics.@DOI: 10.1063/1.1536031#

Since Tang and Van Slyke1 have developed multilayerorganic light-emitting diodes~OLEDs!, many efforts havebeen made to improve the device performance.2–4 It wasrecognized that the external quantum efficiency of OLEDsdepends on the carrier injection efficiency and the carrierrecombination efficiency. However, enhanced carrier injec-tion and transport to the emission region do not ensure theimproved device luminous efficiency, and the balance ofholes and electrons is required for obtaining high-performance devices.5

In order to further improve the device performance, adouble-hole-transporting layer structure has been introducedand was found to perform well.6 It has also been demon-strated that inserting an organic conducting layer, such aspoly-3,4-ethylenedioxythiophene7–9 or an insulating layersuch as Al2O3

2 and poly~methylmethacrylate!,10,11 at the in-terface between the electrode and the organic layer helps toincrease the device efficiency. It seems that for OLEDs, themore layers there are in the device structure, the better thedevice performance. It is known, however, that the morecomplicated the device structure, the more difficult the de-vice fabrication process. There is no doubt that a single-layerOLED, which possesses only a small-molecule active layer,will greatly simplify the device fabrication process, thus con-ducive to lowering the cost and improving the productionefficiency. Hu and Matsumura12 reported that the single-layerOLEDs with CuOx-coated anode achieved a luminance ashigh as 2500 cd/m2, and a luminance–current efficiency of1.2 cd/A. However, no such improvement has been reported,to our knowledge, although high efficient single-layer poly-mer light-emitting diodes~PLEDs! have been successfullyprepared.13

In this work, an approach to fabricate single-layerOLEDs has been proposed. A common emissive small mol-ecule, tris~8-hydroxyquinoline! aluminum (Alq3), is used as

the only active material. The Alq3 layer functions as theelectron-transporting and the light-emitting layer. In order toachieve a balance between the electrons and holes in thedevice, an insulating plastic material, polytetrafluoroethylene~Teflon!, is introduced. The effect of the Teflon layer thick-ness upon the device efficiency has been investigated.

Figure 1 shows the schematic structure of the single-layer OLED. The device consists of an indium-tin-oxide~ITO!-coated glass substrate, Teflon layer, Alq3 , and metalcathode. The thickness of Alq3 is about 60 nm, while thethickness of Teflon layer varies from 4 to 12 nm. Both Alq3

and Teflon are deposited at a pressure of 231023 Pa. Theelectroluminescent spectra are measured with a spectropho-tometer~Photo Research PR-650!. All the measurements arecarried out in ambient atmosphere and at room temperature.

Five kinds of diodes are prepared with a Teflon layer of4, 6, 8, 10, 12 nm. Based on our observation, we find that thegreen-light emission is originated from Alq3 in all devices.

X-ray photoelectron spectroscopy~XPS! was used tocharacterize the chemical bond state of the Teflon films. TheC 1s spectra~Fig. 2 left-hand side! and F 1s spectra~Fig. 2right-hand side! show 298.1 eV and 689.1 eV peaks corre-sponding well with CF2 moiety of the Teflon material.

In Fig. 3, the current density–voltage curves are shownas a function of the OLEDs with different thickness of theTeflon buffer layer. Since Teflon is an insulating materialwith an extremely high resistivity of 1018 V cm, it is easy to

a!Electronic mail: qiuy@mail.tsinghua.edu.cn FIG. 1. Schematic structure of the single-layer OLED.

APPLIED PHYSICS LETTERS VOLUME 82, NUMBER 2 13 JANUARY 2003

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find that the current densities of OLEDs decrease with theincreasing thickness of the Teflon buffer layer.

Figure 4 shows the luminance–voltage characteristics ofthe single-Alq3 layer devices with various thicknesses of theTeflon layer. The brightness strongly depends on the thick-ness of the Teflon layer. When the Teflon thickness is 6 nm,an impressive brightness of more than 16 000 cd/m2 wasmeasured, whereas the conventional double-layer OLEDswith N,N8-bis-~1-naphthyl!-N,N8-diphenyl-1,18 biphenyl4,48-diamine and Alq3 showed only 9500 cd/m2 in our ex-periments. Therefore, it demonstrates, for the small-moleculeOLEDs with a single-active layer, that with the help of cer-tain buffer layer, a high bright emission could be achieved.

Similarly, the luminous efficiency of the single-Alq3

OLEDs was greatly increased due to the introduction of thethin Teflon layer~Fig. 5!. For the device with the Teflonthickness of 6 nm, an external luminous efficiency of 0.85lm/W was achieved. Under our experimental conditions, theTeflon layer with a thickness of 6 nm seems to be optimumfor obtaining high-performance device.

It is known in literature that electrons are the majorcharge carriers in the single-Alq3 layer OLEDs, and the ef-ficiency strongly depends on charge balance. Therefore, theefficient hole injection is expected to enhance the electrolu-minescent efficiency in the OLEDs. Ultraviolet photoelec-

tron spectroscopy was employed to measure the ionizationpotential of the deposited Teflon film, and a large value ofabout 9.8 eV was determined. Shown from the energy dia-grams of the single-layer OLEDs~Fig. 6!, a big energy gap,which seems to be disadvantageous for hole injection, existsat the interface between the ITO anode and Teflon layer. Onone side, the thickness of Teflon buffer is so small that thehole carriers can be injected into the Alq3 layer through tun-neling injection. On the other side, the redundant holes canbe blocked off by the Teflon barrier, and a balance combina-tion can be obtained in the devices.

In conventional OLEDs, the holes and electrons gener-ally combine at the interface between hole-transporting layerand electron transporting layer. Azizet al.14 revealed thatinjection of the redundant carriers would reduce the effi-ciency of OLEDs greatly. Schlatmannet al.15 indicated thatalthough the ITO electrode is known to form an excellentelectrode on a glass substrate, it heavily contaminates theorganic layer covered on top. And, Leeet al.16 reported in-dium diffusion from the ITO electrode into an organicmultilayer structure after prolonged operation and a substan-tial reduction in quantum efficiency due to the presence ofindium in organic emissive layers. These experiences can beused to explain our results of enhanced performance insingle-layer OLEDs with a Teflon buffer layer. On one side,

FIG. 3. Current density–voltage characteristics of the single-Alq3 layer de-vices with various thickness of Teflon layer.

FIG. 4. Luminance–voltage characteristics of the single-Alq3 layer deviceswith various thicknesses of Teflon.

FIG. 2. XPS measurements on a Teflon film grown by vacuum evaporation~Left-hand side: C 1s spectra; right-hand side F 1s spectra!.

156 Appl. Phys. Lett., Vol. 82, No. 2, 13 January 2003 Gao et al.

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the Teflon layer deposited on ITO in our work may act as abuffer to impede indium diffusion from the ITO and, thus,enhance device efficiency. On the other side, it also forms anew interface contacting with the Alq3 emitting layer, andthe carriers can efficiently combinate at the new interface. Itis reasonable to infer that, based on this work, higher deviceperformance could be realized by screening both the activematerial and the buffer layer material.

In summary, when introduced into single-layer OLEDs

as a barrier layer, a Teflon layer can efficiently enhance thehole injection in order to achieve a hole–electron balance inthe OLEDs. Characteristics of OLEDs employing Teflonwith different thicknesses have been investigated, and theresults indicate that the OLED with the 6 nm thick Teflonbuffer layer exhibits the most efficient light emission. A lu-minance of 16 000 cd/m2, which is much higher than that ofthe conventional double-layer OLED, and an efficiency of0.85 lm/W, can be obtained. The single-layer structure tech-nology is of great importance to the commercialization ofOLEDs due to its possible lower cost and higher productionefficiency.

This work is supported by the National Natural ScienceFoundation of China.~No. 90101029!.

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FIG. 5. Luminous efficiency–voltage characteristics of the single-Alq3 layerdevices with various thicknesses of Teflon.

FIG. 6. Schematic energy diagrams of the single-Alq3 layer OLEDs.

157Appl. Phys. Lett., Vol. 82, No. 2, 13 January 2003 Gao et al.

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