Liquid crystal device and organic electroluminescence device...

Details Liquid crystal device and organic electroluminescence device... Liquid crystal device and organic electroluminescence device...

2002-05-23

2004-05-18

Wu, Shean C. (Department: 1756)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

With charge transferring layer of specified composition

C428S001300, C428S917000, C313S506000, C313S509000, C252S299300

Reexamination Certificate

active

06737127

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an organic electroluminescence device (to be referred to as “organic EL device” hereinafter) to be used for a display or the like as light-emitting device and also to a liquid crystal device having a high electric current carrying ability and an excellent carrier injection property and adapted to be used for such an organic EL device.
2. Related Background Art
Intensive research efforts are currently being paid for developing applications of organic EL devices that can be used as light-emitting devices showing a quick responsiveness and a high light-emitting efficiency.
FIGS. 1 and 2
of the accompanying drawings schematically illustrate the basic configuration of the organic EL device (see “Macromol. Symp.” 125, 1-48 (1997)). Referring to
FIGS. 1 and 2
, there are shown an organic compound layers
10
, a metal electrode
11
, a light-emitting layer
12
, a hole transport layer
13
, a transparent electrode
14
, a transparent substrate
15
and an electron transport layer
16
. As shown in FIGS.
1
and
2
, organic EL devices generally have an organic compound layers
10
having a multilayer structure and arranged between a transparent electrode
14
formed on a transparent electrode
15
and a metal electrode
11
.
In the instance of
FIG. 1
, the multilayer organic compound layer
10
include a light-emitting layer
12
and a hole transport layer
13
. The transparent electrode
14
is typically made of ITO that has a large work function so that the device may show an excellent hole injection characteristic for injecting holes from the transparent electrode
14
to the hole transport layer
13
. The metal electrode
11
is typically made of Al, Mg or an alloy thereof, which have a small work function so that the device may show an excellent electron injection characteristic for injecting electrons into the organic compound layer
10
. The electrodes typically have a thickness of 50 to 200 nm.
The light-emitting layer
12
is typically made of an aluminum-quinolinol complex derivative that shows an electron transport property and also a light-emitting property. The chemical structure of Alq3 is shown below as a typical example of such derivatives. The hole transport layer
13
is typically made of a phenyldiamine derivative having an electron providing ability such as &agr;-NPD as shown below.
An organic EL device having a configuration as described above shows an rectifying property when a voltage is applied thereto and injects electrons from the metal electrode
11
into the light-emitting layer
12
and holes from the transparent electrode
14
when an electric field is applied thereto in such a way that the metal electrode
11
and the transparent electrode
14
operate respectively as a cathode and anode. The injected holes and electrons are recombined to produce excitons in the light-emitting layer
12
and emit light. At this time, the hole transport layer
13
takes a role of blocking electrons to improve the recombination efficiency along the interface of the light-emitting layer and the hole transport layer, which hence increases the light-emitting efficiency.
With the arrangement of
FIG. 2
, an electron transport layer
16
is provided between the metal electrode
11
and the light-emitting layer
12
. With this arrangement, the light-emitting operation and the electron/hole transporting operation are separated to further improve the carrier blocking effect and the light-emitting efficiency. The electron transport layer
16
may typically be made of a oxadiazole derivative. The above described organic compound layer,
10
is typically made to have a two-layered or three-layered structure with a total film thickness of about 50 to 500 nm.
In any of the above illustrated organic EL devices, the degree of luminance of emitted light of the device depends on the performance of injecting electrons and holes from the respective electrodes of the device. When amorphous materials such as Alq3 and &agr;-NPD are used in a manner as described above, it is believed that the device may not necessarily show a satisfactory carrier injecting performance because of the problem of interfaces of the electrodes/organic-compound layer.
On the other hand, attempts have been made to utilize the structural regularity of liquid crystal as will be discussed hereinafter for the purpose of improving the carrier injection characteristic and the carrier transport characteristic of the device.
Liquid crystal materials having a high carrier transporting ability includes discotic liquid crystal compounds and smectic liquid crystal compounds that have a well-ordered structure. These liquid crystal materials normally show a degree of mobility that is as high as 10
−6
to 10
−3
cm
2
/Vsec. It is expected to realize a high productivity and an excellent performance on the part of organic electroluminescence devices by using such liquid crystal compounds. Applications of such compounds to solid electrolytes are also being studied.
Some of the characteristic aspects of the carrier transport effect that can be achieved by using liquid crystal materials include the following.
(1) A high carrier transporting ability can be achieved by the regular spatial structure obtained by the orientation of liquid crystal itself.
(2) A high electron injecting property can be achieved as a result of orientation of the &pgr; electron conjugate planes of liquid crystal molecules toward the electrode interface.
Reports on attempts for doping a material having a carrier transporting ability with a compound having an electron receiving property or an electron providing property relative to the organic compound layer are also known. They include the following.
(1) Yamamoto et al., Applied Physics Ltter Vol. 72, No. 17, p. 2147 (1998)
(2) Kido et al., Applied Physics Letter Vol. 73, No. 20, p. 2866 (1998)
The above reference (1) reports that the authors have succeeded in raising the luminance of emitted light by forming a hole transport layer, using a hole transporting polymer material prepared by mixing a salt containing SbCl
6
— with a polymer material by 20 mol %, producing holes in the hole transport layer and thereby raising the carrier density.
The above reference (2) reports that the electron injecting performance is improved by doping the electron transport layer with metal Li.
Reports on doping liquid crystal materials include the following.
(3) Boden et al, J. Am. Chem. Soc. Vol. 116, No. 23, p. 10808 (1994)
(4) J. Material Science: Materials in Electronics 5, p. 83 (1994)
The above reference (3) reports that an n-type semiconductor whose main carriers are electrons is formed by doping a discotic liquid crystal compound having a tricycloquinazoline skeleton with potassium by 6 mol %.
The above reference (4) reports that a p-type semiconductor whose main carriers are holes is formed by doping a discotic liquid crystal compound having a triphenylene skeleton with AlCl3.
However, when devices, which may not necessarily be light-emitting devices, are formed by using a liquid crystal composition prepared by doping a liquid crystal material with an inorganic compound in a manner as describe above, there arises a problem that not only carriers (holes or electrons) that operate as electrons but also ionized (cationized or anionized) dopants move in the liquid crystal material to cause an ionic electric current to flow in the device when an external electric field is applied thereto.
An ionic electric current is generated when the dopant itself moves. It is poorly reversible in terms of current characteristics and therefore not only the initial performance but also the durability of the device becomes problematic. Particularly, since liquid crystal materials have properties that are intermediary between crystal and liquid, the generation of an ionic electric current is more serious if compared with amorphous materials and polymer materials.
Additionally, if a discotic liquid crystal compound is used as a liquid crystal material,

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