Organic electroluminescent device

Abstract

An organic electroluminescence device comprising a cathode, an anode and at least one layer comprising a phosphorescent light emitting material and a host material which is sandwiched between the cathode and the anode and further comprising an electron injecting layer which is adhered to the light emitting layer and is capable of transporting electrons, wherein an ionization potential of the host material is 5.9 eV or smaller, and wherein an energy gap of the electron transporting material in the electron injecting layer is smaller than that of the host material in the light emitting layer or wherein a triplet energy of the electron transporting material in the electron injecting layer is smaller than that of the host material in the light emitting layer. It emits phosphorescent light with enhanced efficiency because it comprises a light emitting layer and an electron injecting layer both satisfying specified condition and employs a light emitting layer capable of electron transporting.

Description

TECHNICAL FIELD [0001] The present invention relates to an organic electroluminescent device (an electroluminescent device will be referred to as an electroluminescence device, hereinafter) and, more particularly, to an electroluminescence device which utilizes phosphorescent light emission, exhibits a great efficiency of light emission under the application of lower driving voltage. BACKGROUND ART [0002] An organic electroluminescence (“electroluminescence” will be occasionally referred to as “EL”, hereinafter) device is a spontaneous light emitting device which utilizes the principle that a fluorescent substance emits light by energy of recombination of holes injected from an anode and electrons injected from a cathode when an electric field is applied. Since an organic EL device of the laminate type driven under a low electric voltage was reported by C. W. Tang of Eastman Kodak Company (C. W. Tang and S. A. Vanslyke, Applied Physics Letters, Volume 51, Pages 913, 1987), many stud...

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Benefits of technology

[0013] The present invention has an object of overcoming the above problems and providing an organic EL device utilizing phosphorescent light emission which exhibits a great efficiency of light emission even with relatively lower driving voltage.

[0014] As the result of intensive researches and studies to achieve the above object by the present inventors, the first aspect of the present invention provides an organic EL device, which comprises an light emitting layer consisting of at least phosphorescent light emitting material and a host material, a cathode, and an electron injecting layer adhered to the light emitting layer and at the same time sandwiched between the light emitting layer and the cathode, without employing any hole blocking layer. Further, the first aspect of the invention made the energy gap of electron transporting material in electron injecting layer smaller than that of the host material in the light emitting layer. According to the conventional comprehension by the persons skilled in the art, the above settlement will let the electron injecting layer deactivates the excitation state generated in the light emitting layer and only the EL devices with extremely low efficiency will be provided. In the present invention, however, enabling the light emitting layer electron transporting, an electron-hole recombination zone will separate from the interface between the electron injecting layer and the light emitting layer resulting in avoidance of deactivation. Besides, an EL device with high efficiency without the use of the hole blocking layer in the conventional organic EL device is provided by making the ionization potential of the host material 5.9 eV or lower in order to enable holes easily inject into the host material in the light emitting layer. Further, the driving voltage can be decreased because the holes are injected into the host material in the light emitting layer thereby allowing the transportation, and an organic EL device with long lifetime can be obtained because any hole blocking layer that easily deteriorates is not employed. Furthermore, the organic EL device is easily producible because its constitution became simple. Moreover, it was ensured that the energy gap of electron transporting material in electron injecting layer smaller than that of the host material in the light emitting layer has also an effect of promoting injection of electrons from cathode as well as decreasing the driving voltage.

[0015] The second aspect of the invention provides an organic EL device, which comprises an light emitting layer consisting of at least phosphorescent light emitting material and a host material, a cathode, and an electron injecting layer adhered to the light emitting layer and at the same time sandwiched between the light emitting material and the cathode, wherein a triplet energy of an electron transporting material in the electron injecting layer is smaller than that of the host material in the light emitting layer. This reduces the energy gap of electron transporting material and improves the injection of electrons from cathode remarkably. According to the conventional comprehension by the persons skilled in the art, the above settlement will let the electron injecting layer deactivates the excitation state generated in the light emitting layer and only the EL devices with extremely low efficiency will be provided. In the present invention, however, enabling the light emitting layer electron transporting, an electron-hole recombination zone will separate from the interface between the electron injecting layer and the light emitting layer resulting in avoidance of deactivation. Besides, an EL device with high efficiency without the use of the electron injecting layer with large energy gap in the conventional organic EL device is provided by making the ionization potential of the host material 5.9 eV or lower in order to enable holes easily inject into the host material in the light emitting layer. The organic EL device with improved long lifetime was obtained as the effect of making energy barrier for electron injection smaller. Still further, the driving voltage can be decreased because holes can be injected into the host material in the light emitting layer, thereby allowing transportation. Furthermore, the organic EL device is easily producible because its constitution became simple.

Problems solved by technology

(iii) It was necessary to dispose an electron injecting layer having smaller energy gap than the hole blocking layer between the hole blocking layer and the cathode because a direct connection between the hole blocking layer and the cathode metal furiously degrade the property such as lifetime or efficiency.

However, it was found that the conventional constructions for electron injection have problems.

Further, although many compounds used for the hole blocking layer held favorable hole barrier capability, they tended to deteriorate, and failed to provide an organic EL device with long lifetime.

In this case, there are problems that the energy barrier against the electron injection from cathode becomes so great that the driving voltage elevates, and in the case where the energy barrier is great, a continuation of an electric current injection will induce degradation of the EL device and will make the lifetime of the EL device short as a result.

However, there are problems that, because the host material employed for the EL device had an ionization potential of 5.9 eV or more, it fails to inject holes into the host material, prohibiting the host material from transporting holes thereby causing an elevation of the driving voltage.

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