Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With particular semiconductor material
Reexamination Certificate
2001-05-21
2004-01-13
Ho, Hoai (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With particular semiconductor material
C257S040000, C257S047000, C257S291000
Reexamination Certificate
active
06677621
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light emitting device including a element (hereinafter referred to as an EL element) in which a thin film (hereinafter referred to as an EL film) made of a luminescent material capable of generating electroluminescence (hereinafter abbreviated as EL) is interposed between a pair of electrodes. Incidentally, in the present specification, an EL element using a luminescent material in which EL is obtained by singlet excitation is called a singlet EL element, and an EL element using a luminescent material in which EL is obtained by triplet excitation is called a triplet EL element.
2. Description of the Related Art
In recent years, an EL element using an organic EL film as a light emitting layer has been developed, and EL elements using various organic EL films have been proposed. An attempt to realize a flat panel display by employing a light emitting device, which uses an EL element of the kind as a light emitting element, has been made.
As a light emitting device using an EL element, a passive matrix type and an active matrix type are known. The passive matrix type is a light emitting device using an EL element made of a structure in which stripe-like anodes and cathodes are provided to intersect with each other at right angles and an EL film is interposed between them. The active matrix type is a system in which a semiconductor component is provided for each of pixels, and one of an anode and a cathode of an EL element is connected to the semiconductor component, so that a current flowing through the EL element is controlled by the semiconductor component.
However, in both the passive matrix type light emitting device and the active matrix type light emitting device, since the light emission performance of the EL element is greatly influenced by the physical properties of the EL film itself, the development of the EL element being bright and having high reliability has been just the development of a luminescent material.
Although various kinds of luminescent materials from a low molecular material to a high molecular material are developed, a theoretical upper limit of luminous efficiency has been always a problem. Especially, with respect to internal quantum efficiency, it has been thought that the ratio of the generation efficiency of singlet exciton to the generation efficiency of triplet exciton is 1:3, and only the singlet exciton contributes to light emission (fluorescent emission).
Thus, even if all carriers (electrons and holes) are recombined, 25% of the whole contributes to the light emission, and if extracting efficiency to the outside of the component is 20%, external quantum efficiency becomes 5% in total. That is, according to the calculation, only 5% of consumed energy can be extracted.
However, recently, a material in which light emission (phosphorescent emission) using triplet exciton is proposed, and its high luminous efficiency attracts attention. As examples which the triplet exciton is used and the external quantum efficiency is improved, there are following reports.
(1) T. Tsutsui, C Adachi, S. Saito, Photochemical Processes in Organized Molecular System, ed. K. Honda, (Elsevier Sci. Pub., Tokyo, 1991) p. 437.
(2) M. A. Baldo, D. F. O'Brien, Y. You, A. Shoustikov, S. Sibley, M. E. Thompson, S. R. Forrest, Nature 395 (1998) p. 151.
(3) M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, S. R. Forrest, Appl. Phys. Lett., 75 (1999) p.4.
(4) T. Tsutsui, M. -J. Yang, M. Yahiro, K. Nakamura, T. Watanabe, T. Tsuji, Y. Fukuda, T. Wakimoto, S. Mayaguchi, Jpn. Appl. Phys., 38 (12B) (1999) L1502.
The luminescent materials set forth in the above papers are examples in which the external quantum efficiency is improved by obtaining phosphorescence from the triplet exciton.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a light emitting device which is bright and has low electric power consumption, by using an EL element having high luminous efficiency.
Another object of the present invention is to provide an electrical appliance which has a bright display portion and low electric power consumption, by using the light emitting device.
Still another object of the present invention is to provide an electrical appliance having low electric power consumption by using the light emitting device of the present invention as a light source (typically, a backlight).
A light emitting device of the present invention is characterized in that a triplet EL element is electrically connected to a semiconductor component and is controlled. That is, the present invention is characterized by using the triplet EL element as a light emitting element in an active matrix type light emitting device. As the semiconductor component, a field effect transistor (FET), preferably a thin film transistor (TFT) can be used.
First, a process up to the present invention will be described. When a voltage is applied between an anode and a cathode of an EL element, a carrier (electron or hole) is injected into an EL film, and luminescence is generated by recombination. Thus, a proportionality relation is obtained between density of current flowing through the EL element and luminous brightness. Incidentally, in the present specification, a voltage applied to an EL element is called an operation voltage of the EL element.
FIG. 3
is a graph schematically showing this relation.
FIG. 3
shows the relation between the density of current flowing through an EL element and the luminous brightness. Reference numeral
301
indicates a characteristic of a conventional EL element using singlet excitation (singlet EL element); and
302
, a characteristic of an EL element using triplet excitation (triplet EL element) .
The characteristic of the singlet EL element shown as
301
is that although the proportionality relation (linear relation) is obtained when the current density is low, an inclination becomes small as the current density becomes high. That is, it is known that even if the current density increases, the luminous brightness becomes hard to increase over a certain point. This tendency is remarkable in the case of the characteristic of the triplet EL element shown as
302
. When the current density is low, it is in the proportionality relation having an inclination larger than the singlet EL element with respect to the luminous brightness. However, when the current density increases, the inclination becomes extremely small, and there occurs such a state where luminous brightness is hardly changed even if the current density increases.
From the graph of
FIG. 3
, it is understood that although the luminous brightness is several times higher than that of the singlet EL element when the triplet EL element emits light in the operation region of small current density, it becomes almost equal to the luminous brightness of the singlet EL element in the operation region of high current density.
FIG. 4
shows a relation between the operation voltage of an EL element and its luminous efficiency. Reference numeral
401
indicates a characteristic of a singlet EL element; and
402
, a characteristic of a triplet EL element. An operation voltage “a” indicates an operation voltage (8 to 12 V) where the luminous efficiency of the singlet EL element becomes highest, and an operation voltage “b” indicates an operation voltage (3 to 5 V) where the luminous efficiency of the triplet EL element becomes highest.
At this time, as shown in the graph of
FIG. 4
, the triplet EL element has a feature that its luminous efficiency becomes highest when the operation voltage is lower than the singlet EL element. That is, the triplet EL element shows higher luminous efficiency than the singlet EL element in the operation region of low current density. This is not contradictory to the graph of
FIG. 3
in which the triplet EL element shows higher luminous brightness than the singlet EL element in the operation region of low current density.
Here, from the characteristics of the triplet EL element shown in the graph
Inukai Kazutaka
Yamazaki Shunpei
Fish & Richardson P.C.
Ho Hoai
Nguyen Dao H.
Semiconductor Energy Laboratory Co,. Ltd.
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