Quantum efficiency in a polymer electroluminescence device is significantly improved by inserting a thin insulating layer with the thickness of tunneling range. Four times higher quantum efficiency was obtained without the increase of the threshold voltage. Poly͑methyl methacrylate͒ Langmuir-Blodgett films were used as the thin tunneling barrier. The enhancement may result from the lowering of the effective barrier height for electron injection while increasing the effective barrier for hole

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... ection. The effects improve the balanced injection of electrons and holes into the light-emitting devices. Electroluminescence ͑EL͒ from conjugated polymers is in active research recently since the first demonstration of it from poly͑p-phenylene vinylene͒ ͑PPV͒. 1 Subsequently, PPV derivatives and other conjugated polymers have been used to achieve increased quantum efficiencies and to provide a range of colors. 2-7 Various materials, such as indium tin oxide ͑ITO͒, semitransparent gold, or polyaniline, can be used as hole injecting electrodes, however ITO has the advantage of high conductivity and high transparency over the visible range combined with high work function for adequate hole injection. Metals such as aluminum, calcium, and magnesium-silver have been evaporated on the polymer films to provide electron injection electrodes. In most conjugated polymers, electron injection has proved more difficult than hole injection. To achieve good efficiency it has been necessary to use a low work function metal such as calcium to reduce the barrier to electron injection. 2,8 Unfortunately, low work function metals are reactive in air environment. Various methods have been tried to achieve balanced injection of holes and electrons without using the low work function metals. These include the formation of heterojunction or blending with electron transporting materials and synthesis of new emitting materials having high electron affinity. In this letter we will demonstrate another method to control hole and electron injection by inserting a thin insulating layer between an emitting layer and an electron injecting electrode. In the device, the effective barrier to electron injection can be lowered while the injection of holes to the emitting layer is reduced if the thickness of the insulating layer is within the tunneling range. As a result, more balanced injection of electrons and holes is expected and the quantum efficiency will increase significantly. The thin insulating layers in the tunneling range were formed by Langmuir-Blodgett ͑LB͒ technique. Poly͑methyl-methacrylate͒ ͑PMMA͒ was used as the insulating material. The material is uniformly transferred on various substrates by the LB technique with a very small number of pinholes. 9-11 The material has an energy band gap of about 6 eV and therefore shows good insulating properties. By using the LB technique, uniform insulating layers were able to be formed with precisely controlled thickness in nanometer range. The devices reported here were formed on glass substrates coated with ITO. A layer of poly͓2-methoxy-5-͑2-ethylhexyloxy͒-1,4-phenylene-vinylene͔, ͑MEHPPV͒, was deposited onto the ITO by spin coating from dichloroethane solution. The thickness of the layer was about 90 nm measured with an Alpha Step profilometer. After drying at 100°C for 1 h in an oven, PMMA LB films were transferred onto the emitting layer at the surface pressure of 10 mN/m. Atactic PMMA purchased from Polyscience Inc. was used. The thickness of the PMMA monolayer was measured to be 1 nm and the number of the monolayers was varied from 2 to 14. Details of the formation of the LB films were reported before. 9-11 Aluminum electrodes were vacuum evaporated onto the upper surface of the PMMA LB films after drying the films in an oven at 100°C for 1 h. Light output from the devices was measured by using an optical power meter ͑Newport 835͒ as a function of the applied field and current. Electroluminescent spectra were measured using a dual grating monochromator ͑Spex 270M͒ with the photomultiplier tube ͑Hamamatsu R955͒. All the experiments were performed in air and at room temperature. Current-voltage characteristics of the EL devices under forward bias are shown in Fig. 1 . In the device, the major component of the observed current is known to be the hole current because of the higher energy barrier to electron injection than to hole injection. 12 The current density at a certain electric field decreases as the number of PMMA LB monolayers increases. The behavior is expected because the blocking of the hole current by the PMMA LB films becomes more effective. On the contrary, electroluminescence at a certain electric field increases as the number of the LB layers increases up to six layers, as shown in Fig. 2 . If the number of the layers increases further, however, luminescence intensity at a constant electric field decreases. The electric field required for the electroluminescence to be observed ͑threshold electric fields͒ remains almost the same up to the six layers as that of the MEHPPV. If we assume that the electroluminance is proportional to electron current, the higher luminance in the device may indicate that higher electron current is injected to the devices. Further increase of the a͒ To whom the correspondence should be addressed. Electronic mail: jjk@ard.etri.re.kr 599