Light emitting device and electric device using the same

Details Light emitting device and electric device using the same Light emitting device and electric device using the same

2003-02-26

2004-10-26

Vu, David (Department: 2821)

Electric lamp and discharge devices: systems

Plural power supplies

Plural cathode and/or anode load device

C315S169300, C345S076000, C345S077000

Reexamination Certificate

active

06809481

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Filed of the Invention
The present invention relates to an organic light emitting device having an anode, a cathode, and a layer containing organic compounds in which luminescence can be obtained by applying an electronic field (hereafter referred to as organic compound layer). The present invention particularly relates to an organic light emitting element having a longer drive life than that of conventional ones, and a light emitting device using the organic light emitting element.
2. Description of the Related Art
An organic light emitting element is an element which emits light when an electronic field is applied. A luminescence mechanism thereof has been said: applying a voltage to an organic compound layer interposed between electrodes, electrons injected from a cathode and positive holes injected from an anode recombine together at a center of luminescence in the organic compound layer to form excited molecules; energy is released and light is emitted when the molecule excitons returne to the base state.
In addition, kinds of molecule excitons formed by the organic compound can include a singlet excited state and a triplet excited state, while the specification of the present invention contains the case where either of the excited states contributes to luminescence.
In such organic light emitting device, an organic compound layer is normally formed in a thin film below 1 &mgr;m in thickness. Also, since the organic light emitting device is a self-luminescent type one, in which the organic compound layer itself emits light, a backlight used in a conventional liquid crystal display is not necessary. Accordingly, the organic light emitting device can be very advantageously formed to be thin and lightweight.
Also, with, for example, an organic compound layer of about 100 to 200 nm in thickness, a time period having elapsed from injecting of a carrier to recombination thereof is in the order of several tens of nanosecond taking account of the extent of movement of the carrier in the organic compound layer, and luminescence is achieved in the order of less than one micro second even when the procedure from the recombination of the carrier to luminescence is included. Accordingly, one of the features is that the speed of response is very quick.
Further, since the organic light emitting device is a carrier-injecting type light emitting device, it can be driven by DC voltage, and is hard to generate noise. With respect to drive voltage, an adequate luminance of 100 cd/m
2
is achieved at 5.5 V by first making the thickness of an organic compound layer a uniform, super-thin film of around 100 nm in thickness, selecting an electrode material, which reduces a carrier injection barrier with respect to the organic compound layer, and further introducing a single hetero structure (double structure) (Reference 1: C. W. Tang and S. A. VanSlyke, “Organic electroluminescent diodes”, Applied Physics Letters, vol. 51, No. 12, 913-915 (1987)).
Owing to such performances as thin and lightweight, high-speed response, DC low voltage drive, and the like, organic light emitting devices have been given attention as next-generation flat panel display devices. Also, since organic light emitting devices are of self-luminescent type and wide in angle of visibility, they are comparatively favorable in visibility and believed to be effective as devices used for displays in portable devices.
However, the reliability of the device can be a major problem of such a light emitting device. With respect to the reliability, the luminance being deteriorated over time is a particularly conspicuous problem, and a considerable improvement is needed. Further, being a super thin film device, defects such as short circuits of the device caused by the morphology, point defect or unevenness of the thin film must be prevented.
It can be considered that the luminance being deteriorated over time is a phenomenon basically caused by the materials, however, it is possible to prolong the half-life of luminance by driving methods of the device. For example, there is an example that the half-life of luminance being deteriorated over time has been improved dramatically by inserting copper phthalocyanine as a hole injecting layer, and employing a rectangular wave alternating current drive (constant currents in forward biases, constant voltages in reverse biases) instead of a direct current drive (Reference 2: S. A. Van Slyke, C. H. Chen, and C. W. Tang, “Organic Electroluminescent Devices with Improved Stability”, Appl. Phys. Lett., 69 (15), 2160-2162 (1996)).
According to reference 2, the luminance half-life at initial luminance 510 cd/m
2
can be prolonged to 4000 hours. As factors thereof, an excellent positive hole injecting characteristic of copper phthalocyanine which is a positive hole transportation layer, a superior heat-resistance of NPB which is a positive hole transportation layer, and a capability of excluding the accumulation of space charges by an alternating current drive can be given. In addition, normally, when the organic light emitting element is driven by a constant current, a drive voltage rises gradually along with the luminance deterioration, however, the rises of the drive voltage can be suppressed by the alternating current drive.
It has been reported that defects such as short circuits of the device caused by the morphology and point defects can be removed by such alternating current drive (Reference 3: Japanese Patent Laid-Open No. 08-180972). Reference 3 has pointed that the short circuits present frequently in a short period (200 to 300 hours) in the case of direct current drive as a result of comparing the lifetimes of the alternating current drive and the direct current drive.
Further, as techniques for preventing the short circuits of the element caused by unevenness of an electrode, a technique such as providing a buffer layer comprising a conductive polymer on the electrode is devised (Reference 4: Yoshiharu Sato, Organic Molecules and Bioelectronics (The Japan Society of Applied Physics, Vol. 11, No. 1 (2000), p. 86-99). Reference 4 has mentioned that by introducing an anode buffer layer of polymer, the surface unevenness of ITO which is an anode can be smoothed, thereby leads to the reduction of the defects of short circuits.
As described above, in order to improve the reliability of the organic light emitting element, techniques can be implemented not only from the viewpoint of improvements of materials, but from the viewpoint of the drive methods and the structure of the element.
The alternating current drive described in the document 3 is characterized in that the voltage applied to the device in reverse bias (hereafter, it is denoted by Vr) is equal to or greater than the voltage applied to the device in forward bias (hereafter, it is denoted by Vf) and the time for duration in reverse bias (hereafter, it is denoted by Tr) is shorter than the time for duration in forward bias (hereafter, it is denoted by Tf). More specifically, Vf≦Vr and Tf>Tr. Here, the voltages shown here are all handled as positive values.
Considering the effect that the reverse bias eliminates the accumulation of space charges, Vf≦Vr is more effective. However, in this case, it is necessary to take into account of the dielectric breakdown strength of the element. In short, Vr has to be set smaller than the dielectric breakdown voltage Vb. That is, Vf≦Vr<Vb.
However, when the luminance is being reduced over time, the current efficiency (the ratio of the luminance to the current density) itself is being reduced compared with the initial driving period. In other words, since the leakage current not contributing to light emission is increased, it is likely to destroy the element by dielectric breakdown with the voltage smaller than Vb in the long term.
That is, in the case of the alternating current drive method, the half-life of luminance is prolonged more than that by direct current, and short circuits generated in the initial driving period can be prevented. However, in the

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