U.S. patent application number 11/002465 was filed with the patent office on 2005-12-15 for nanocrystal electroluminescence device and fabrication method thereof.
This patent application is currently assigned to Samsung Electronics Co., Ltd.. Invention is credited to Ahn, Tae Kyung, Choi, Seong Jae, Jang, Eun Joo, Jun, Shin Ae, Lee, Sung Hun.
Application Number | 20050274944 11/002465 |
Document ID | / |
Family ID | 35459576 |
Filed Date | 2005-12-15 |
United States Patent
Application |
20050274944 |
Kind Code |
A1 |
Jang, Eun Joo ; et
al. |
December 15, 2005 |
Nanocrystal electroluminescence device and fabrication method
thereof
Abstract
A nanocrystal electroluminescence device comprising a polymer
hole transport layer, a nanocrystal light-emitting layer and an
organic electron transport layer wherein the nanocrystal
light-emitting layer is independently and separately formed between
the polymer hole transport layer and the organic electron transport
layer. According to the nanocrystal electroluminescence device,
since the hole transport layer, the nanocrystal light-emitting
layer and the electron transport layer are completely separated
from one another, the electroluminescence device provides a pure
nanocrystal luminescence spectrum having limited luminescence from
other organic layers and substantially no influence by operational
conditions, such as voltage. Further, a method for fabricating the
nanocrystal electroluminescence device.
Inventors: |
Jang, Eun Joo; (Daejeon-Si,
KR) ; Jun, Shin Ae; (Gyeonggi-Do, KR) ; Lee,
Sung Hun; (Gyeonggi-Do, KR) ; Ahn, Tae Kyung;
(Seoul, KR) ; Choi, Seong Jae; (Seoul,
KR) |
Correspondence
Address: |
BUCHANAN INGERSOLL PC
(INCLUDING BURNS, DOANE, SWECKER & MATHIS)
POST OFFICE BOX 1404
ALEXANDRIA
VA
22313-1404
US
|
Assignee: |
Samsung Electronics Co.,
Ltd.
Gyeonggi-Do
KR
|
Family ID: |
35459576 |
Appl. No.: |
11/002465 |
Filed: |
December 3, 2004 |
Current U.S.
Class: |
257/40 ; 257/79;
438/22; 438/99 |
Current CPC
Class: |
C09K 11/08 20130101;
H01L 33/08 20130101; H05B 33/14 20130101; B82Y 20/00 20130101; H01L
33/24 20130101; Y10S 977/779 20130101; H01L 51/5048 20130101; H01L
51/502 20130101 |
Class at
Publication: |
257/040 ;
257/079; 438/099; 438/022 |
International
Class: |
H01L 033/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 9, 2004 |
KR |
2004-42200 |
Claims
What is claimed is:
1. An electroluminescence device comprising a polymer hole
transport layer, a nanocrystal light-emitting layer and an organic
electron transport layer, wherein the nanocrystal light-emitting
layer in contact with the polymer hole transport layer is
independently formed between the polymer hole transport layer and
the organic electron transport layer.
2. The electroluminescence device according to claim 1, wherein the
electroluminescence device has a structure consisting of an anode,
a polymer hole transport layer, a nanocrystal light-emitting layer,
an organic electron transport layer, and a cathode layered in this
order on a transparent substrate.
3. The electroluminescence device according to claim 2, further
comprising a hole injection layer, an electron blocking layer, a
hole blocking layer or an electron/hole blocking layer interposed
between the polymer hole transport layer and the anode, or between
the organic electron transport layer and the nanocrystal
light-emitting layer.
4. The electroluminescence device according to claim 1, wherein the
nanocrystal light-emitting layer is made of at least one material
selected from the group consisting of metal nanocrystals, Group
II-VI compound semiconductor nanocrystals, and Group III-V compound
semiconductor nanocrystals and PbS, PbSe and PbTe, wherein the
metal nanocrystals include Au, Ag, Pt, Pd, Co, Cu and Mo, the Group
II-IV compound semiconductor nanocrystals include CdS, CdSe, CdTe,
ZnS, ZnSe, ZnTe, HgS, HgSe and HgTe, the Group III-V compound
semiconductor nanocrystals include GaN, GaP, GaAs, InP and InAs;
and when the nanocrystal light-emitting layer is made of a mixture
of two or more nanocrystals, the nanocrystals exist in the state of
a simple mixture, fused crystals in which the nanocrystals are
partially present in the same crystal structure, or an alloy.
5. The electroluminescence device according to claim 1, wherein the
polymer hole transport layer is made of a material selected from
the group consisting of poly(3,4-ethylenedioxythiophene)
(PEDOT)/polystyrene para-sulfonate (PSS), poly-N-vinylcarbazole
derivatives, polyphenylenevinylene derivatives, polyparaphenylene
derivatives, polymethacrylate derivatives, poly(9,9-octylfluorene)
derivatives, and poly(spiro-fluorene) derivatives.
6. The electroluminescence device according to claim 1, wherein the
nanocrystal light-emitting layer has a thickness of 3 nm.about.30
nm.
7. The electroluminescence device according to claim 1, wherein the
electron transport layer is made of a material selected from the
group consisting of oxazoles, isooxazoles, triazoles, isothiazoles,
oxydiazoles, thiadiazoles, perylenes,
tris(8-hydroxyquinoline)-aluminum (Alq3), Balq, Salq and Almq3; and
has a thickness of 10 nm.about.100 nm.
8. The electroluminescence device according to claim 3, wherein the
electron blocking layer, the hole blocking layer or the
electron/hole blocking layer is formed of a material selected from
the group consisting of
3-phenyl-4-(1-naphthyl)-5-phenyl-1,2,4-triazole (TAZ),
2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP),
phenanthrolines, imidazoles, triazoles, oxadiazoles, and aluminum
complexes; and has a thickness of 5 nm.about.50 nm.
9. A method for fabricating an electroluminescence device,
comprising the steps of: patterning a hole-injecting anode on a
substrate and forming a polymer hole transport layer thereon;
coating a nanocrystal dispersion on the polymer hole transport
layer to form a nanocrystal light-emitting layer; forming an
organic electron transport layer on the nanocrystal light-emitting
layer; and forming an electron-injecting cathode on the organic
electron transport layer.
10. The method according to claim 9, wherein the nanocrystal
light-emitting layer is formed by dispersing nanocrystals
surface-bound by a photosensitive compound in a solvent which does
not damage the polymer hole transport layer to obtain a nanocrystal
dispersion, and coating the nanocrystal dispersion on the polymer
hole transport layer; or dispersing nanocrystals surface-bound by a
material containing no photosensitive functional group and a
photosensitive compound in a solvent which does not damage the
polymer hole transport layer to obtain a nanocrystal dispersion,
and coating the nanocrystal dispersion on the polymer hole
transport layer.
11. The method according to claim 9, wherein the nanocrystal
light-emitting layer is made of at least one material selected from
the group consisting of metal nanocrystals, Group II-VI compound
semiconductor nanocrystals, Group III-V compound semiconductor
nanocrystals, PbS, PbSe and PbTe, the metal nanocrystals including
Au, Ag, Pt, Pd, Co, Cu and Mo, the Group II-IV compound
semiconductor nanocrystals including CdS, CdSe, CdTe, ZnS, ZnSe,
ZnTe, HgS, HgSe and HgTe, the Group III-V compound semiconductor
nanocrystals including GaN, GaP, GaAs, InP and InAs; and when the
nanocrystal light-emitting layer is made of a mixture of two or
more nanocrystals, the nanocrystals exist in the state of a simple
mixture, fused crystals in which the nanocrystals are partially
present in the same crystal structure, or an alloy.
12. The method according to claim 10, wherein the solvent which
does not damage the hole transport layer and disperses the
nanocrystals is selected from the group consisting of water,
pyridine, ethanol, propanol, butanol, pentanol, hexanol, toluene,
chloroform, chlorobenzene, THF, cyclohexane, cyclohexene, methylene
chloride, pentane, hexane, heptane, octane, nonane, decane,
undecane, dodecane, and mixtures thereof.
13. The method according to claim 9, wherein the electron transport
layer is formed by spin coating, dip coating, spray coating, or
blade coating.
14. The method according to claim 10, wherein the nanocrystal
dispersion has a concentration of 0.01 wt %.about.10 wt %.
15. The method according to claim 9, wherein the nanocrystal
light-emitting layer has a thickness of 3 nm.about.30 nm.
16. The method according to claim 9, wherein the polymer hole
transport layer is made of a material selected from the group
consisting of poly(3,4-ethylenedioxythiophene) (PEDOT)/polystyrene
para-sulfonate (PSS), poly-N-vinylcarbazole derivatives,
polyphenylenevinylene derivatives, polyparaphenylene derivatives,
polymethacrylate derivatives, poly(9,9-octylfluorene) derivatives,
and poly(spiro-fluorene) derivatives.
17. The method according to claim 9, wherein the electron transport
layer is formed on the nanocrystal light-emitting layer by thermal
deposition, molecular deposition or chemical deposition.
18. The method according to claim 9, further comprising the step of
exposing the nanocrystal light-emitting layer to UV light to
crosslink it, prior to coating the organic electron transporting
material on the nanocrystal light-emitting layer.
19. The method according to claim 10, wherein the photosensitive
compound surface-bound to the nanocrystals contains a double bond,
a carboxyl group, an amide group, a phenyl group, a biphenyl group,
a peroxide group, an amine group, or an acryl group.
20. The method according to claim 9, further comprising the step of
inserting a hole injection layer between the anode and the hole
transport layer; inserting an electron blocking layer between the
nanocrystal light-emitting layer and the hole transport layer; or
inserting a hole blocking layer between the nanocrystal
light-emitting layer and the electron transport layer.
Description
BACKGROUND OF THE INVENTION
[0001] This non-provisional application claims priority under 35
U.S.C. 119(a) on Korean Patent Application No. 42200 filed on Jun.
9, 2004 which is herein expressly incorporated by reference.
[0002] 1. Field of the Invention
[0003] The present invention relates to an electroluminescence
device, and a method for fabricating the electroluminescence
device. More particularly, the present invention relates to a
nanocrystal electroluminescence device comprising a polymer hole
transport layer, a nanocrystal light-emitting layer and an organic
electron transport layer wherein the nanocrystal light-emitting
layer is independently and separately formed between the polymer
hole transport layer and the organic electron transport layer, and
a method for fabricating the nanocrystal electroluminescence
device.
[0004] 2. Description of the Related Art
[0005] A nanocrystal is defined as a material having a crystal
structure at the nanometer-scale level, and consists of a few
hundred to a few thousand atoms. Since the small-sized nanocrystal
has a large surface area per unit volume, most of the atoms
constituting the nanocrystal are present at the surface of the
nanocrystal. Based on this structure, the nanocrystal exhibits
quantum confinement effects, and shows electrical, magnetic,
optical, chemical and mechanical properties different from those
inherent to the constituent atoms of the nanocrystal. That is, the
control over the physical size of the nanocrystal enables the
control of various properties.
[0006] Vapor deposition processes, such as metal organic chemical
deposition (MOCVD) and molecular beam epitaxy (MBE), have been
conventionally used to prepare nanocrystals. On the other hand, a
wet chemistry technique wherein a precursor material is added to an
organic solvent to grow nanocrystals to a desired size has made
remarkable progress in the past decade. According to the wet
chemistry technique, as the crystals are grown, the organic solvent
is naturally coordinated to the surface of the quantum dot crystals
and acts as a dispersant. Accordingly, the organic solvent allows
the crystals to grow to the nanometer-scale level. The wet
chemistry technique has an advantage in that nanocrystals can be
uniformly prepared in size and shape in a relatively simple manner
at low cost, compared to conventional vapor deposition processes,
e.g., MOCVD and MBE.
[0007] However, since nanocrystals prepared by the wet chemistry
technique are commonly separated and are then dispersed in an
organic solvent, techniques for forming a thin film of the
nanocrystals in a solid state are required in order to apply the
nanocrystals to electroluminescence devices.
[0008] In nanocrystal electroluminescence devices reported
hitherto, the nanocrystals are used as luminescent materials, or
have functions of light emission, in combination with charge
transport. The first electroluminescence device employing
nanocrystals was suggested in U.S. Pat. No. 5,537,000. The
electroluminescence device is formed using one or more layers of
nanocrystals as an electron transport layer, and preferably capable
of emitting light. Accordingly, the luminescence wavelengths of the
electroluminescence device are varied in response to the changes in
the voltages applied to the device.
[0009] PCT publication WO/03/084292 teaches a device wherein a
layer of an organic-inorganic hybrid matrix containing nanocrystals
is disposed between two electrodes. Specifically, the device is
fabricated by mixing nanocrystals and a low molecular weight hole
transporting material, such as
N,N-diphenyl-N,N-bis(3-methylphenyl)-(1,1-biphenyl)-4,4-diamine
(TPD), with a solvent, and spin coating the mixture on an
electrode. When the coating conditions and the mixing ratio between
the nanocrystals and the hole transporting material are
appropriately controlled, a nanocrystal layer is formed on top of a
hole transport layer due to the difference in the intermolecular
force or density between the nanocrystals and the hole transporting
material. However, although the nanocrystal layer is formed on top
of the hole transport layer, the hole transporting material is
mixed with the nanocrystals in the transport layer. Accordingly,
the overlying electron transport layer is in contact with the hole
transport layer, and thus the hole and electron transport layers as
well as the nanocrystal layer emit light. To solve this problem,
the PCT publication discloses a technique for arranging a hole
blocking layer on a thin film of the hole transport layer
containing the nanocrystals, followed by forming the electron
transport layer on the hole blocking layer. Meanwhile, the hole
transporting material mixed with the nanocrystals has a low
molecular weight. If a polymer is used as the hole transporting
material, its solubility is low and thus the polymer is limited to
material which can be dissolved in solvents which dissolve the
nanocrystal. Although the polymer which can be dissolved are used,
the solubility of the polymer is not sufficiently high, rendering
it difficult to control the thickness of the nanocrystal layer and
the hole transport layer.
[0010] U.S. Pat. No. 6,049,090 describes a device wherein a mixed
layer of nanocrystals and a matrix as a light-emitting layer is
disposed between two electrodes. According to the device, the
matrix is selected to have a wider bandgap energy, a higher
conduction band energy level and a lower valence band energy level
than the nanocrystals so as to allow the nanocrystals to emit light
well and trap electrons and holes in nanocrystals, thereby
enhancing the luminescence efficiency of the device.
[0011] As stated above, the conventional electroluminescence
devices employing nanocrystals as luminescent materials are devices
wherein the nanocrystals have functions of light emission in
combination with charge transport, are mixed with a hole
transporting material to form a mixed layer, or are mixed with a
hole transporting material and coated to form a nanocrystal layer
separately formed on a hole transport layer due to the density
difference depending on the processing conditions. However, since
none of these conventional electroluminescence devices provide a
pure nanocrystal luminescence spectrum, they have a problem of low
color purity.
OBJECTS AND SUMMARY
[0012] Therefore, the present invention has been made in view of
the above problems of the related art, and it is an object of the
present invention to provide an electroluminescence device
comprising a polymer hole transport layer, a nanocrystal
light-emitting layer and an organic electron transport layer
wherein the nanocrystal light-emitting layer is independently and
separately formed between the polymer hole transport layer and the
organic electron transport layer, thereby providing a pure
nanocrystal luminescence spectrum and increasing the color purity
of the electroluminescence device.
[0013] It is another object of the present invention to provide a
method for fabricating the electroluminescence device wherein
materials for a hole transport layer can be selected, regardless of
the solubility in a solvent which disperses nanocrystals.
[0014] In accordance with one aspect of the present invention, the
above objects can be accomplished by an electroluminescence device
comprising a polymer hole transport layer, a nanocrystal
light-emitting layer and an organic electron transport layer
wherein the nanocrystal light-emitting layer in contact with the
polymer hole transport layer is separately formed between the
polymer hole transport layer and the organic electron transport
layer.
[0015] In accordance with another aspect of the present invention,
there is provided a method for fabricating the electroluminescence
device, comprising the steps of: patterning a hole-injecting anode
on a substrate and forming a polymer hole transport layer thereon;
coating a nanocrystal dispersion on the polymer hole transport
layer to form a nanocrystal light-emitting layer; forming an
organic electron transport layer on the nanocrystal light-emitting
layer; and forming an electron-injecting cathode on the organic
electron transport layer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The above and other objects, features and other advantages
of the present invention will be more clearly understood from the
following detailed description taken in conjunction with the
accompanying drawings, in which:
[0017] FIG. 1 is a cross-sectional view schematically showing a
nanocrystal electroluminescence device according to one embodiment
of the present invention;
[0018] FIG. 2 shows cross-sectional views illustrating the steps of
a method for fabricating a nanocrystal electroluminescence device
according to one embodiment of the present invention;
[0019] FIG. 3a is a partial cross-sectional view schematically
showing a conventional electroluminescence device in which after a
mixture of nanocrystals and a hole transporting material is coated,
the resulting hole transport layer and nanocrystal layer are
incompletely separated from each other; FIG. 3b is a partial
cross-sectional view showing another conventional
electroluminescence device in which after a mixture of nanocrystals
and a hole transporting material is coated, the nanocrystals are
uniformly dispersed in the hole transporting material to form one
mixed layer; and FIG. 3c is a partial cross-sectional view
schematically showing a nanocrystal electroluminescence device of
the present invention in which after a hole transporting material
is coated to form a thin film, baked, and coated with nanocrystals,
a nanocrystal light-emitting layer is completely separated from a
hole transport layer;
[0020] FIG. 4 is a photoluminescence spectrum of silica
nanocrystals surface-bound by a photosensitive compound, which is
prepared in Preparative Example 1 of the present invention;
[0021] FIG. 5 is a photoluminescence spectrum of silica
nanocrystals surface-bound by a photosensitive compound, which is
prepared in Preparative Example 2 of the present invention;
[0022] FIGS. 6a and 6b are luminescence spectra of
electroluminescence devices fabricated in Examples 1 and 2 of the
present invention according to the changes in the voltages applied
to the devices, respectively;
[0023] FIG. 7 shows luminescence spectra of an electroluminescence
device fabricated in Example 3 of the present invention according
to the changes in the voltages applied to the device; and
[0024] FIG. 8 shows luminescence spectra of a conventional
electroluminescence device fabricated in Comparative Example 1
according to the changes in the voltages applied to the device.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0025] The present invention will now be described in more detail
with reference to the accompanying drawings.
[0026] An electroluminescence device according to the present
invention comprises a polymer hole transport layer, a nanocrystal
light-emifting layer and an organic electron transport layer
wherein the nanocrystal light-emitting layer in contact with the
polymer hole transport layer is separately and independently formed
between the polymer hole transport layer and the organic electron
transport layer.
[0027] FIG. 1 is a cross-sectional view schematically showing the
nanocrystal electroluminescence device according to one embodiment
of the present invention. Referring to FIG. 1, the
electroluminescence device of the present invention comprises an
anode 20, a polymer hole transport layer 30, a nanocrystal
light-emitting layer 40, an organic electron transport layer 50,
and a cathode 60 layered in this order on a transparent substrate
10. The polymer hole transport layer 30 is formed of a material
capable of transporting holes, and the electron transport layer 50
is made of a material capable of transporting electrons. When a
voltage is applied between the two electrodes, the anode 20 injects
holes into the hole transport layer 30, and the cathode 60 injects
electrons into the electron transport layer 50. The injected holes
are combined with the injected electrons at the same molecules to
form exciton pairs, and then the exciton pairs are recombined to
emit light.
[0028] Optionally, the electroluminescence device of the present
invention further comprises a hole injection layer interposed
between the anode 20 and the hole transport layer 30; an electron
blocking layer, a hole blocking layer or an electron/hole blocking
layer interposed between the hole transport layer 30 and the
nanocrystal light-emitting layer 40; or an electron blocking layer,
a hole blocking layer or an electron/hole blocking layer interposed
between the nanocrystal light-emitting layer 40 and the electron
transport layer 50.
[0029] The transparent substrate 10 used in the electroluminescence
device of the present invention may be a substrate used in common
organic electroluminescence devices. A glass or transparent plastic
substrate is preferred in terms of superior transparency, superior
surface smoothness, ease of handling, and excellent waterproofness.
Specific examples of the transparent substrate include
polyethyleneterephthalate, polycarbonate substrates, and the like.
The thickness of the transparent substrate 10 is preferably in the
range of 0.3.about.1.1 mm.
[0030] The anode 20 formed on the transparent substrate 10 may be
made of an electrically conductive metal or its oxide so that it
can easily injects holes. As specific examples, indium tin oxide
(ITO), indium zinc oxide (IZO), nickel (Ni), platinum (Pt), gold
(Au), silver (Ag), and iridium (Ir) may be mentioned.
[0031] Examples of materials for the hole transport layer 30
include, but are not limited to, poly(3,4-ethylenedioxythiophene)
(PEDOT)/polystyrene para-sulfonate (PSS), poly-N-vinylcarbazole
derivatives, polyphenylenevinylene derivatives, polyparaphenylene
derivatives, polymethacrylate derivatives, poly(9,9-octylfluorene)
derivatives, poly(spiro-fluorene) derivatives, and the like. The
thickness of the hole transport layer 30 is preferably in the range
of 10 nm to 100 nm.
[0032] Materials commonly used in the art can be used to form the
organic electron transport layer 50. Specific examples of materials
for the organic electron transport layer 50 include, but are not
limited to, oxazoles, isooxazoles, triazoles, isothiazoles,
oxydiazoles, thiadiazoles, perylenes, and aluminum complexes,
including tris(8-hydroxyquinoline)-aluminum (Alq3), Balq, Salq and
Almq3. The thickness of the organic electron transport layer 50 is
preferably between 10 nm and 100 nm.
[0033] Suitable materials for the electron blocking layer, the hole
blocking layer or the electron/hole blocking layer are those
commonly used in the art. Specific examples include, but are not
limited to, 3-phenyl-4-(1-naphthyl)-5-phenyl-1,2,4-triazole (TAZ),
2,9-dimethyl-4,7-diphenyl-1, 10-phenanthroline (BCP),
phenanthrolines, imidazoles, triazoles, oxadiazoles, and aluminum
complexes. The thickness of the electron blocking layer, hole
blocking layer and electron/hole blocking layer is preferably in
the range of 5 nm to 50 nm.
[0034] Examples of materials for the electron-injecting cathode 60
include, but are not limited to, metals having a sufficiently low
work function to easily inject electrons, such as [I], Ca, Ba,
Ca/Al, LiF/Ca, LiF/Al, BaF.sub.2/Al, BaF.sub.2/Ca/Al, Al, Mg, and
Ag:Mg alloys. The thickness of the cathode is preferably in the
range of 50 nm to 300 nm.
[0035] Nanocrystals that can be used in the present invention
include most of the nanocrystals prepared by a wet chemistry
technique, such as metal nanocrystals and semiconductor
nanocrystals. Specifically, the nanocrystal light-emitting layer 40
is made of at least one material selected from the group consisting
of metal nanocrystals, such as Au, Ag, Pt, Pd, Co, Cu and Mo, Group
II-VI compound semiconductor nanocrystals, such as CdS, CdSe, CdTe,
ZnS, ZnSe, ZnTe, HgS, HgSe and HgTe, and Group III-V compound
semiconductor nanocrystals, such as GaN, GaP, GaAs, InP and InAs,
and PnS, PbSe, PbTe. If the nanocrystal light-emitting layer is
made of a mixture of two or more nanocrystals, the nanocrystals may
exist in the state of a simple mixture, fused crystals in which the
nanocrystals are partially present in the same crystal structure,
or an alloy. The thickness of the nanocrystal light-emitting layer
is between 3 nm and 30 nm.
[0036] The present invention is directed to a method for
fabricating the nanocrystal electroluminescence device. According
to the method of the present invention, a polymer hole transporting
material is coated on a hole-injecting anode by various coating
processes, and baked to form a rigid thin film of a polymer hole
transport layer. A nanocrystal dispersion is coated on the polymer
hole transport layer by various coating processes to form a thin
film of a nanocrystal light-emitting layer. At this time, the
nanocrystal dispersion is prepared by dispersing nanocrystals in a
solvent which does not dissolve the polymer hole transport layer.
The nanocrystal light-emitting layer thus formed is separated from
the polymer hole transport layer. Thereafter, an organic electron
transport layer and an electron-injecting cathode are sequentially
formed on the nanocrystal light-emitting layer.
[0037] FIG. 2 shows cross-sectional views illustrating the steps of
the method for fabricating the electroluminescence device shown in
FIG. 1, in accordance with the present invention. Referring to FIG.
2, a hole-injecting anode 20 is patterned on a substrate 10, and
then a polymer hole transporting material is coated on the
substrate 10 by various coating processes, such as spin coating, to
form a polymer hole transport layer 30. The polymer hole transport
layer 30 is then baked into a rigid thin film so that the polymer
hole transport layer is not damaged in the subsequent formation
step of a nanocrystal light-emitting layer. Next, a nanocrystal
dispersion is coated on the polymer hole transport layer 30 by
various coating processes, such as spin coating, to form a
nanocrystal light-emitting layer 4. At this time, the nanocrystal
dispersion is prepared by dispersing nanocrystals in a solvent
which does not substantially dissolve the polymer hole transport
layer 30. Thereafter, an organic electron transport layer 50 is
formed on the nanocrystal light-emitting layer 40, and then a
cathode is formed thereon to form the final electroluminescence
device.
[0038] The substrate 10 on which the anode 20 is patterned is
commonly washed with solvents, such as a neutral detergent,
deionized water, acetone and isopropyl alcohol, and is then
subjected to UV-ozone and plasma treatment.
[0039] According to the method of the present invention, the
nanocrystal light-emitting layer is formed in accordance with the
following procedure. Nanocrystals surface-bound by a photosensitive
compound are dispersed in a solvent which does not damage the
polymer hole transport layer to obtain a nanocrystal dispersion.
The nanocrystal dispersion is coated on the polymer hole transport
layer to form a thin film of the nanocrystals. Altematively,
nanocrystals surface-bound by a material containing no
photosensitive functional group and a photosensitive compound are
dispersed in a solvent which does not damage the polymer hole
transport layer to obtain a nanocrystal dispersion. The nanocrystal
dispersion is coated on the polymer hole transport layer to form a
thin film of the nanocrystals.
[0040] The solvent which does not damage the hole transport layer
and can disperse the nanocrystals is selected from the group
consisting of water, pyridine, ethanol, propanol, butanol,
pentanol, hexanol, toluene, chloroform, chlorobenzene, THF,
cyclohexane, cyclohexene, methylene chloride, pentane, hexane,
heptane, octane, nonane, decane, undecane, dodecane, and mixtures
thereof.
[0041] Prior to coating the organic electron transporting material
on the nanocrystal light-emitting layer 40, the nanocrystal
light-emitting layer 40 can be exposed to UV light at a wavelength
of 200 nm to 450 nm to crosslink it. The luminescence wavelength of
the nanocrystal light-emitting layer 40 is in the range of 350 nm
to 1,300 nm.
[0042] The solvent which does not damage the nanocrystal
light-emitting layer, the polymer hole transporting material, and
the hole transport layer, and can disperse the nanocrystals is as
described above. The concentration of the nanocrystal dispersion is
preferably between 0.01 wt % and 10 wt %, more preferably between
0.1 wt % and 5 wt %, and most preferably between 0.2 wt % and 2 wt
%.
[0043] As the material for the organic electron transport layer 50,
a low- or high-molecular weight material can be used. Vacuum
deposition and wet coating can be employed as the coating
processes. The first process for forming the organic electron
transport layer 50 by wet coating is performed by the following
procedure. Nanocrystals surface-bound by a photosensitive compound
are formed into a thin film of the nanocrystals, and exposed to UV
light to crosslink the thin film, thereby making the thin film
insoluble in a solvent containing an electron transporting
material. Thereafter, the organic electron transporting material is
wet-coated on the nanocrystal layer to form the organic electron
transport layer 50. The second process for forming the organic
electron transport layer 50 by wet coating is performed by the
following procedure. Nanocrystals surface-bound by a material
containing no photosensitive functional group, and a photosensitive
compound are thoroughly mixed, formed into a thin film of the
nanocrystals, and exposed to UV light to crosslink the
photosensitive material, thereby forming a network structure. The
network structure traps the nanocrystals, which makes the thin film
insoluble in a solvent containing an electron transporting
material. Thereafter, the organic electron transporting material is
wet-coated on the nanocrystal layer to form the organic electron
transport layer 50.
[0044] The organic material surface-bound to the nanocrystals
contains at least one functional group selected from the group
consisting of acetyl, acetic acid, phosphine, phosphonic acid,
alcohol, vinyl, carboxyl, amide, phenyl, amine, acryl, silane,
cyano and thiol groups at one or both terminals of its alkyl chain
or aromatic moiety.
[0045] The photosensitive compound surface-bound to the
nanocrystals contains a double bond, a carboxyl group, an amide
group, a phenyl group, a biphenyl group, a peroxide group, an amine
group, an acryl group, or the like.
[0046] The method for fabricating the electroluminescence device
according to the present invention may further comprise the step of
inserting a hole injection layer between the anode and the hole
transport layer; inserting an electron blocking layer between the
light-emitting layer and the hole transport layer; or inserting a
hole blocking layer between the nanocrystal light-emitting layer
and the electron transport layer.
[0047] FIGS. 3a and 3b show states wherein a mixture of
nanocrystals and a organic hole transporting material is coated by
spin coating to form thin films. The organic hole transporting
material used herein may be a low- or high-molecular weight
material. FIG. 3a shows a state wherein after a mixture of
nanocrystals and a organic hole transporting material is
spin-coated, a nanocrystal layer is formed on top of a hole
transport layer due to the difference in the intermolecular force
or density between the nanocrystals and the hole transporting
material. However, although the nanocrystals 25 can form a layer
partially separated from the hole transporting material, most of
the hole transporting material is mixed with the nanocrystals in
the nanocrystal layer. FIG. 3b shows a state wherein nanocrystals
25 are uniformly dispersed in a hole transporting material to form
one mixed layer. FIG. 3c shows a state wherein a nanocrystal
light-emitting layer 40 is completely separated from a hole
transport layer 30 by the method of the present invention shown in
FIG. 2.
[0048] The fabrication of the electroluminescence device of the
present invention does not require particular fabrication
apparatuses and methods, in addition to the formation of the
independent and separate nanocrystal light-emitting layer. The
electroluminescence device of the present invention can be
fabricated in accordance with conventional fabrication methods
using common luminescent materials.
[0049] To form the hole transport layer 30 and the electron
transport layer 50 into thin films, spin coating, dip coating,
spray coating, blade coating, and other coating processes can be
used. The exposure of the thin films used in the method of the
present invention may be carried out by a contact exposure or
non-contact exposure process. The electron transport layer 50 can
be formed on the nanocrystal light-emitting layer 40 by thermal
deposition, molecular deposition or chemical deposition.
[0050] After formation of the thin films, drying can be carried out
at 20.degree. C..about.300.degree. C. and preferably 40.degree.
C..about.120.degree. C. In addition, the energy for
photosensitization treatment is dependent on the thickness of the
thin films, and is preferably between 50 mJ/cM.sup.2 and 850
mJ/cm.sup.2. When the exposure energy is out of this range,
sufficient crosslinking is not likely to take place, or there is a
risk of damage to the thin film. Light sources usable for the light
exposure preferably have an energy in the range of about 100 W to
about 800 W at an effective wavelength of 200.about.500 nm and
preferably 300.about.400 nm.
[0051] Hereinafter, the present invention will be explained in more
detail with reference to the following examples. However, these
examples are made only for illustrative purposes of preferred
embodiments and are not to be construed as limiting the scope of
the invention.
PREPARATIVE EXAMPLE 1.
Preparation of CdSeS Nanocrystals
[0052] 16 g of trioctyl amine (hereinafter, referred to as `TOA`),
0.5 g of oleic acid, and 0.4 mmol of cadmium oxide were charged
into a 125 ml flask equipped with a reflux condenser. The reaction
temperature of the mixture was adjusted to 300.degree. C. with
stirring. Separately, a selenium (Se) powder was dissolved in
trioctyl phosphine (hereinafter, referred to as TOP) to obtain an
Se-TOP complex solution (Se concentration: about 0.25 M), and a
sulfur (S) powder was dissolved in TOP to obtain an S-TOP complex
solution (S concentration: about 1.0 M). 0.9 ml of the S-TOP
complex solution and 0.1 ml of the Se-TOP complex solution were
rapidly fed to the previous mixture, and then reacted for 4 minutes
with stirring. After the reaction was completed, the reaction
mixture was cooled to room temperature as rapidly as possible.
Ethanol as a non-solvent was added to the reaction mixture, and the
resulting mixture was then centrifuged. After the obtained
precipitates were separated from the mixture by decanting the
supematant, 1 wt % of the precipitates were dispersed in toluene to
prepare a dispersion of CdSeS nanocrystals. The nanocrystals
emitted light green light under a UV lamp at 365 nm. FIG. 4 shows a
photoluminescence spectrum of the dispersion of CdSeS nanocrystals.
As shown in FIG. 4, a photoluminescence peak having a full-width at
half maximum (FWHM) of about 30 nm was observed around 552 nm.
PREPARATIVE EXAMPLE 2.
Preparation of CdSe/ZnS Nanocrystals
[0053] 16 g of TOA, 0.5 g of oleic acid, and 0.1 mmol of cadmium
oxide were simultaneously charged into a 125 ml flask equipped with
a reflux condenser. The reaction temperature of the mixture was
adjusted to 300.degree. C. with stirring. Separately, a Se powder
was dissolved in TOP to obtain an Se-TOP complex solution (Se
concentration: about 2 M). 1 ml of the S-TOP complex solution was
rapidly fed to the previous mixture, and then reacted for about 10
seconds with stirring. After the reaction was completed, the
reaction mixture was cooled to room temperature as rapidly as
possible. Ethanol as a non-solvent was added to the reaction
mixture, and the resulting mixture was then centrifuged. After the
obtained precipitates were separated from the mixture by decanting
the supematant, the precipitates were dispersed in toluene to
prepare a dispersion of CdSe nanocrystals.
[0054] On the other hand, 8 g of TOA and 0.4 mmol of zinc acetate
were simultaneously charged into a 125 ml flask equipped with a
reflux condenser. The reaction temperature of the mixture was
adjusted to 260.degree. C. with stirring. After the dispersion of
CdSe nanocrystals was added to the reaction mixture, the reaction
was allowed to proceed for about 1 hour while an S-TOP complex
solution was slowly added thereto. After completion of the
reaction, the reaction mixture was cooled to room temperature as
rapidly as possible. Ethanol as a non-solvent was added to the
reaction mixture, and the resulting mixture was then centrifuged.
After the obtained precipitates were separated from the mixture by
decanting the supernatant, the precipitates were dispersed in
toluene to prepare a dispersion of CdSe/ZnS nanocrystals. The
nanocrystals emitted light green light under a UV lamp at 365 nm.
FIG. 5 shows a photoluminescence spectrum of the dispersion of
CdSe/ZnS nanocrystals. As shown in FIG. 5, a photoluminescence peak
having a full-width at half maximum (FWHM) of about 30 nm was
observed around 527 nm.
EXAMPLE 1.
Fabrication of Electroluminescence Device Employing CdSeS
Nanocrystal Light-emitting Layer Dispersed in Octane
[0055] This example shows the fabrication of an electroluminescence
device wherein a nanocrystal light-emitting layer is independently
and separately formed. First, an ITO-patterned glass substrate was
sequentially washed with a neutral detergent, deionized water,
water and isopropyl alcohol, and was then subjected to UV-ozone
treatment. A solution of 1 wt % of
poly(9,9'-dioctylfluorene-co-N-(4-butylphenyl)diphe- nylamine (TFB)
in chlorobenzene was spin-coated on the ITO-patterned substrate to
a thickness of about 50 nm, and then baked at 180.degree. C. for 10
minutes to form a hole transport layer. A dispersion of the CdSeS
nanocrystals (1 wt %) prepared in Preparative Example 1 in octane
was spin-coated on the hole transport layer, and dried to form a
nanocrystal light-emitting layer having a thickness of about 5 nm.
At this time, the octane used herein is a solvent which does not
dissolve the hole transport layer.
[0056]
(3-4-Biphenyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (TAZ)
was deposited on the completely dried nanocrystal light-emitting
layer to form a hole blocking layer having a thickness of 10 nm,
and then tris(8-hydroxyquinoline)-aluminum (Alq3) was deposited
thereon to form an electron transport layer having a thickness of
about 30 nm. LiF and aluminum were sequentially deposited on the
electron transport layer to thicknesses of 1 nm and 200 nm,
respectively, to form a cathode, thereby fabricating the final
electroluminescence device.
[0057] FIG. 6a shows luminescence spectra of the
electroluminescence device according to the changes in the voltages
applied to the device. As shown in FIG. 6a, a luminescence peak
having a full-width at half maximum (FWHM) of about 40 nm was
observed around 556 nm.
EXAMPLE 2.
Fabrication of Electroluminescence Device Employing CdSeS
Nanocrystal Light-emitting Layer Dispersed in Chlorobenzene
[0058] First, an ITO-pattemed glass substrate was sequentially
washed with a neutral detergent, deionized water, water and
isopropyl alcohol, and was then subjected to UV-ozone treatment. A
solution of 1 wt % of
poly(9,9'-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine (TFB)
in chlorobenzene was spin-coated on the ITO-patterned substrate to
a thickness of about 50 nm, and then baked at 180.degree. C. for 10
minutes to form a hole transport layer. A dispersion of 1 wt % of
the CdSeS nanocrystals prepared in Preparative Example 1 in
chlorobenzene was spin-coated on the hole transport layer, and
dried to form a nanocrystal light-emitting layer having a thickness
of about 5 nm. At this time, the chlorobenzene used herein is a
solvent which does not dissolve the hole transport layer.
[0059]
(3-4-Biphenyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (TAZ)
was deposited on the completely dried nanocrystal light-emitting
layer to form a hole blocking layer having a thickness of 10 nm,
and then tris(8-hydroxyquinoline)-aluminum (Alq3) was deposited
thereon to form an electron transport layer having a thickness of
about 30 nm. LiF and aluminum were sequentially deposited on the
electron transport layer to thicknesses of 1 nm and 200 nm,
respectively, to form an electrode, thereby fabricating the final
electroluminescence device.
[0060] FIG. 6b shows luminescence spectra of the
electroluminescence device wherein the nanocrystal light-emitting
layer was independently and separately formed, according to the
changes in the voltages applied to the device. As shown in FIG. 6b,
a luminescence peak having a full-width at half maximum (FWHM) of
about 50 nm was observed around 556 nm.
EXAMPLE 3.
Fabrication of Electroluminescence Device Employing CdSe/ZnS
Nanocrystal Light-emitting Layer Dispersed in Octane and Including
No Hole Blocking Layer
[0061] First, an ITO-pattemed glass substrate was sequentially
washed with a neutral detergent, deionized water, water and
isopropyl alcohol, and was then subjected to UV-ozone treatment. A
solution of 1 wt % of
poly(9,9'-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine (TFB)
in chlorobenzene was spin-coated on the ITO-patterned substrate to
a thickness of about 50 nm, and then baked at 180.degree. C. for 10
minutes to form a hole transport layer. A dispersion of the
CdSe/ZnS nanocrystals (1 wt %) prepared in Preparative Example 2 in
octane was spin-coated on the hole transport layer, and dried to
form a nanocrystal light-emitting layer having a thickness of about
5 nm. At this time, the octane used herein is a solvent which does
not dissolve the hole transport layer.
[0062] Tris(8-hydroxyquinoline)-aluminum (Alq3) was deposited on
the completely dried nanocrystal light-emitting layer to form an
electron transport layer having a thickness of about 40 nm. LiF and
aluminum were sequentially deposited on the electron transport
layer to thicknesses of 1 nm and 200 nm, respectively, to form an
electrode, thereby fabricating the final electroluminescence
device.
[0063] FIG. 7 shows luminescence spectra of the electroluminescence
device wherein the nanocrystal light-emifting layer was
independently and separately formed, according to the changes in
the voltages applied to the device. As shown in FIG. 7, a
luminescence peak having a full-width at half maximum (FWHM) of
about 46 nm was observed around 530 nm.
COMPARATIVE EXAMPLE 1.
Fabrication of Conventional Electroluminescence Device from Mixed
Solution of Hole Transporting Material and CdSeS Nanocrystals
[0064] This comparative example realizes a method for fabricating a
conventional electroluminescence device wherein after a mixture of
nanocrystals and a hole transporting material is coated, the
resulting hole transport layer and nanocrystal layer are separated
from each other due to the difference in the density of the
nanocrystals, which results from phase separation arising during
the coating.
[0065] First, an ITO-pattemed glass substrate was sequentially
washed with a neutral detergent, deionized water, water and
isopropyl alcohol, and was then subjected to UV-ozone treatment. A
solution of 1 wt % of
poly(9,9'-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine (TFB)
and the CdSeS nanocrystals prepared in Preparative Example 1 in
chlorobenzene was spin-coated on the ITO-patterned substrate, and
then baked at 180.degree. C. for 10 minutes to form a hole
transport layer in which a nanocrystal light-emifting layer was
included. At this time, the weight ratio of the TFB to the CdSeS
nanocrystals was adjusted to 1:1.
[0066]
(3-4-Biphenyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (TAZ)
was deposited on the completely dried hole transport layer to form
a hole blocking layer having a thickness of 10 nm, and then
tris(8-hydroxyquinoline)-aluminum (Alq3) was deposited thereon to
form an electron transport layer having a thickness of about 30 nm.
LiF and aluminum were sequentially deposited on the electron
transport layer to thicknesses of 1 nm and 200 nm, respectively, to
form an electrode, thereby fabricating the final
electroluminescence device.
[0067] FIG. 8 shows luminescence spectra of the electroluminescence
device according to the changes in the voltages applied to the
device. It was confirmed from FIG. 8 that the hole transport layer
including the nanocrystal layer, as well as the electron transport
layers emitted light.
[0068] As apparent from the foregoing, the electroluminescence
device of the present invention has a direct transition-type
bandgap ranging from the visible to the infrared range, and
includes a nanocrystal light-emitting layer which is made of
nanocrystals with enhanced luminescence efficiency and is
independently and separately formed. Accordingly, the
electroluminescence device of the present invention provides a pure
nanocrystal luminescence spectrum having limited luminescence from
other organic layers and substantially no influence by operational
conditions, such as voltage, resulting in a high color purity. In
addition, according to the method of the present invention,
materials for a hole transport layer can be selected, regardless of
the solubility in a solvent which disperses nanocrystals.
Accordingly, the method of the present invention has an advantage
in terms of improved workability.
[0069] Although the preferred embodiments of the present invention
have been disclosed for illustrative purposes, those skilled in the
art will appreciate that various modifications, additions and
substitutions are possible, without departing from the scope and
spirit of the invention as disclosed in the accompanying
claims.
* * * * *