Electric lamp and discharge devices – With luminescent solid or liquid material – Solid-state type
Reexamination Certificate
2001-12-18
2003-09-30
Gushi, Ross (Department: 2833)
Electric lamp and discharge devices
With luminescent solid or liquid material
Solid-state type
C313S506000, C313S112000, C349S069000
Reexamination Certificate
active
06628068
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a luminescent device. In particular, it relates to a luminescent device which has two or more light-emitting regions. It also relates to a liquid crystal display device incorporating such a luminescent device.
BACKGROUND ART
One well-known luminescent device is an electroluminescent device. An electroluminescent (“EL devices”) generates light as a result of electron—hole recombination. An EL device typically has a multilayer structure, in which a light-emitting layer is confined between an anode layer and a cathode layer. The emitter layer may be either an organic material or an inorganic material. Charge carrier recombination occurs in the emitter layer, and photons are generated. It is possible to vary the wavelength of light emitted by an EL device by using different materials for the emitter layer or by applying different drive conditions to the emitter layer, and it is also possible to manufacture an EL device that emits white light.
Many organic emitting materials have relatively broad emission and absorption spectra. If single colour emission, or a narrow colour range, is required, it is possible to use colour filters as disclosed in J. Kido et al, “Science” Vol 267, page 1332 (1995). An alternative approach to obtaining a narrow wavelength range is to use cavity effects to narrow the emission spectrum, as described A. Dodabalapur et al, “Journal of Applied Physics” Vol 80, page 6954 (1996). Although the output wavelength can be narrowed using cavity effects, this approach has the disadvantage that the output becomes very directional and this is undesirable in a display intended to be viewed from a wide range of angles.
In many applications it is desirable to provide a full colour display. One possible way to achieve a full colour EL device involves dividing each pixel into three sub-pixels, with the three sub-pixels lying side by side. One of the sub-pixels emits red light, one green light and one blue light. A device of this type is disclosed in U.S. Pat. No. 5,294,869. One disadvantage of this known device is that each colour is emitted from only one third of the total active area of the device, so that the intensity of the device is low.
An alternative approach to providing a full colour EL device consists of stacking two or more EL devices above one another. Devices of this type are disclosed in P. E. Burrows et al, “Applied Physics Letters” Vol 69, No. 20, Nov. 11, 1996, pages 2959-2961, and in S. R. Forrest et al, “Synthetic Metals” Vol. 91, pages 9-13 (1997).
A stacked, three-layer EL device is shown schematically in FIG.
1
. This consists of a red EL element
1
disposed over a green EL element
2
which in turn is disposed over a blue EL element
3
. Each EL element comprises a cathode layer
4
B,
4
G,
4
R, an emitter layer
5
B,
5
G,
5
R, and an anode layer
6
B,
6
G,
6
R. Although
FIG. 1
shows the EL elements as being separate from one another, in practice they would be stacked with an insulating layer separating each anode-cathode interface.
In the stacked EL device shown in
FIG. 1
, emission of light of each colour occurs over the entire active cross-sectional area of the device, so that the intensity of the device is improved compared to the device described above which uses laterally divided sub-pixels. However, the EL device shown in
FIG. 1
has the disadvantage that light emitted by the red EL element must pass through the other two elements before it is emitted from the device, and that light emitted from the green EL device must pass through the blue EL device. This is a particular problem if organic materials are used to farm the emitter layers in the EL elements, since organic emitting materials generally have relatively broad emission and absorption spectra.
Forrest et al have attempted to address the problem of light emitted in one EL element being absorbed in a subsequent EL element. They have made use of the Stokes effect which provides a shift between the peak emission wavelength and the peak absorption wavelength.
The Stokes shift is illustrated in
FIG. 2
, which shows the emission and absorption spectra for the three EL elements of the EL device of FIG.
1
. The letter “a” indicates the absorption spectra, and the letter “e” indicates the emission spectra. The Stokes shift appears as a shift between the absorption spectrum and the emission spectrum for an EL emitter layer. Forrest et al have chosen materials which have large Stokes shifts so as to minimise the absorption of radiation emitted by one EL element in other EL elements.
The devices proposed by Forrest at al have the following disadvantages. Firstly, the choice of materials for the emitter layers of the EL elements is restricted, owing to the need to use only materials with a large Stokes shift. Moreover, Forrest et al are constrained to use the particular order of the red, green and blue EL elements shown in
FIG. 1
, so that the red light (with a low energy) subsequently passes through emitter layers having a higher band gap. However, even if the red light is not absorbed across the band gap of the emitter layers in the green and blue EL elements, sole absorption of the red light will inevitably occur as it passes through the blue and green EL elements. The red EL element currently has the lowest intensity of the red, green and blue EL elements. It would thus be preferable to put the red EL element to the front so that the red light did not have to pass through the green and blue EL elements, rather than place it at the back as required by Forrest et al.
A further disadvantage with the prior art is that the EL devices will emit light in both the forward direction (as shown in
FIG. 1
) and in the backward direction. It would be desirable to utilise the light emitted in the backward direction, as well as the light emitted in the forwards direction, so as to increase the intensity of the device. It is possible to provide a mirror (not shown) above the red EL element of
FIG. 1
to reflect the light emitted in the backward direction back towards the blue EL element
3
. However, light emitted in the backwards direction by the green or blue EL elements will have to pass through the red EL element twice, once before it reaches the mirror and once after it has been reflected, so that significant absorption will occur. Thus, even if a mirror is provided much of the light emitted in the backward direction will be lost.
In an EL device having an organic emitting layer, the emitting layer is usually evaporated, or spun-down. This will produce an amorphous emitting layer, which emits light having no polarisation. In many applications, it would be desirable to produce an organic EL device that emits polarised light.
One known approach to providing an organic EL device that emits light having some degree of polarisation is to deposit the organic emitting layer with some degree of orientation. This can be done by techniques such as Langmuir-Blodgett deposition, mechanically deforming an organic emitting layer, or rubbing a pure conjugated—polymer emitter layer. An alternative technique is to deposit a polymer layer on a highly aligned orientation layer such as polytetrafluoroethylene or polyimide, or by stretching a polymer layer. A further known technique is disclosed by Weder et al in “Advanced Materials” Vol. 9, page 1035 (1997), in which they disclose the tensile deformation of a guest-host system, so that the guest molecules adopt the orientation of the host.
An alternative approach to providing an organic emitting layer that emits light having some degree of polarisation is the cross-linking of polymeric materials using polarised UV light. This method eliminates the mechanical rubbing step, which is desirable since rubbing may introduce charge, inhomogeneities and dirt into the organic layer. M. Hasegawa et al, “J Photopolym Sci Technol” Vol. 8, page 241 (1995), M. Schadt et al, “Japanese Journal of Applied Physics” Vol. 31, page 2155 (1992) and M. Schadt et al, “Nature” Vol. 381, Page 212 (1996) disclose studies on cross-link
Bock Harald Reinhart
Hudson Andrew James
Rorison Judy Megan
Towler Michael John
Weaver Michael Stuart
Gushi Ross
Renner Otto Boisselle & Sklar
Sharp Kabushiki Kaisha
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