U.S. patent application number 12/533891 was filed with the patent office on 2010-02-04 for transparent, thermally stable light-emitting component having organic layers.
Invention is credited to Jan Blochwitz-Nomith, Karl Leo, Martin Pfeiffer, Xiang Zhou.
Application Number | 20100026176 12/533891 |
Document ID | / |
Family ID | 43525310 |
Filed Date | 2010-02-04 |
United States Patent
Application |
20100026176 |
Kind Code |
A1 |
Blochwitz-Nomith; Jan ; et
al. |
February 4, 2010 |
Transparent, Thermally Stable Light-Emitting Component Having
Organic Layers
Abstract
The presently described subject matter relates to transparent
and thermally stable light-emitting components having organic
layers, and in particular to a transparent organic light-emitting
diode having a charge carrier transport layer which is electrically
doped with an organic dopant.
Inventors: |
Blochwitz-Nomith; Jan;
(Dresden, DE) ; Leo; Karl; (Dresden, DE) ;
Pfeiffer; Martin; (Dresden, DE) ; Zhou; Xiang;
(Guangzhou, CN) |
Correspondence
Address: |
BAKER BOTTS L.L.P.
30 ROCKEFELLER PLAZA, 44TH FLOOR
NEW YORK
NY
10112-4498
US
|
Family ID: |
43525310 |
Appl. No.: |
12/533891 |
Filed: |
July 31, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10496414 |
Sep 19, 2005 |
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PCT/DE03/01021 |
Mar 27, 2003 |
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12533891 |
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Current U.S.
Class: |
313/504 |
Current CPC
Class: |
H01L 2251/5323 20130101;
H01L 51/0059 20130101; H01L 51/005 20130101; H05B 33/14 20130101;
H01L 51/0077 20130101; H01L 51/506 20130101; H01L 51/5076 20130101;
Y02E 10/549 20130101; Y10T 428/31855 20150401; C09K 2211/1007
20130101; H01L 51/002 20130101; H01L 51/5096 20130101; H01L 51/0032
20130101; C09K 11/06 20130101; H01L 51/006 20130101; H01L 51/5092
20130101; H01L 51/0072 20130101; H01L 51/0051 20130101; B32B 27/18
20130101; H01L 51/5048 20130101; H01L 51/5088 20130101; C09K
2211/1014 20130101; H01L 51/5234 20130101; H01L 51/5012
20130101 |
Class at
Publication: |
313/504 |
International
Class: |
H01J 1/62 20060101
H01J001/62 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 28, 2002 |
DE |
102 15 210.1 |
Claims
1. A transparent, thermally stable light-emitting component having
organic layers, comprising: a transparent substrate; a transparent
anode; a hole transport layer adjacent to the anode; at least one
light-emitting layer; a charge-carrier transport layer for
electrons; and a transparent cathode, wherein the transparency in
the visible spectral region is at least 75%, wherein the hole
transport layer is p-doped with an acceptor organic material and
the electron transport layer is n-doped with a donor organic
material, and the molecular masses of the dopants are each greater
than 200 g/mole, and wherein the transparent, thermally stable
light-emitting component having organic layers is an organic
light-emitting diode.
2. A light-emitting component according to claim 1, further
comprising: at least one of a hole-side blocking layer located
between the doped hole transport layer and the light-emitting layer
or an electron-side blocking layer located between the doped
electron transport layer and the light-emitting layer.
3. A light-emitting component according to claim 1, further
comprising: a electrode layer located between the anode and the
hole transport layer and a electrode layer located between the
charge-carrier transport layer and the cathode.
4. A light-emitting component according to claim 1, wherein the
doping concentration of the organic dopants is such that an ohmic
injection takes place from the anode into the charge-carrier
transport layer or from the cathode into the hole transport
layer.
5. A light-emitting component according to claim 3, wherein the
electrode layers comprise indium tin oxide (ITO) or a degenerate
oxide other than ITO.
6. A light-emitting component according to claim 1, wherein the
cathode includes a metallic intermediate layer adjacent to the
subjacent doped, charge-carrier transport layer when the cathode is
located on top or the anode includes a metallic intermediate layer
adjacent to the subjacent doped, hole transport layer when the
anode is located on top and wherein the metallic layer has a
nominal thickness between 0.1 nm and 3 nm.
7. A light-emitting component according to claim 1, wherein no
metal layer is located between the doped hole transport layer and
the anode when the anode is on top or between the doped electron
transport layer and the cathode when the cathode is on top.
8. A light-emitting component according to claim 1, where the anode
and cathode are located between the substrate and encapsulation
cover and the transparency is at least 70% for each wavelength
between at least 400 nm and 800 nm.
9. A light-emitting component according to claim 1, wherein the
molar concentration of admixture in the hole transport layer or in
the electron transport layer or in both the hole transport layer
and the electron transport layer is in the range of 1:100,000 to
1:10, calculated on the ratio of doping molecules to main-substance
molecules.
10. A light-emitting component according to claim 1, wherein the
molar concentration of admixture in the hole transport layer or in
the electron transport layer, or in both the hole transport layer
and the electron transport layer, is at least 1 wt %, calculated on
the ratio of doping molecules to main-substance molecules.
11. A light-emitting component according to claim 2, wherein the
thickness of each of the hole transport layer or the electron
transport layer, of the light-emitting layer and of the at least
one of a hole-side blocking layer or an electron-side blocking
layer lies in the range of 0.1 nm to 50 .mu.m.
12. A light-emitting component according to claim 1, wherein the
cathode is in direct contact with a doped transport layer and is
facing away from the substrate when the cathode is on top or the
anode is in direct contact with a doped transport layer and is
facing away from the substrate when the anode is on top and wherein
the doped transport layer is a hole transport layer or an electron
transport layer.
13. A light-emitting component according to claim 1, wherein the
organic n-dopant material is selected from the group consisting of
heterocyclic radicals, diradicals, dimers, an oligomer, a polymer,
a dispiro compound, and a polycycle thereof, having the structure
according to one of the following formulae: ##STR00027## wherein
structures 3 and 4 have one or more cyclic linkages A and/or A1
and/or A2, wherein A, A1 and A2 are selected from the group
consisting of carbocyclic, heterocyclic, polycyclic ring systems,
and any combination thereof, which may be substituted or
unsubstituted, ##STR00028## wherein A1 and A2 are present
individually or together and A1 and A2 are selected as in
structures 3 and 4 and T=CR22, CR22R23, N, NR21, O or S, and
##STR00029## wherein structure 7 has one or more bridge bonds Z and
Z1, Z or Z1, Z1 and Z2, or Z1 or Z2, and Z, Z1 and Z2 are
independently selected from the group consisting of alkyl, alkenyl,
alkynyl, cycloalkyl, sililyl, alkylsililyl, diazo, disulphide,
heterocycloalkyl, heterocyclyl, piperazinyl, dialkyl ether,
polyether, primary alkylamine, arylamine, polyamine, aryl, and
heteroaryl.
14. A light-emitting component according to claim 1, wherein the
organic acceptor organic material is a quiniode derivative or a
triylidene derivative, with a reduction potential in the range of
0V vs. Fc/Fc+ to 0.4V vs. Fc/Fc.sup.+.
15. A light-emitting component according to claim 12 or 13, wherein
the n-doped, donor organic material is an asymmetrically
substituted phenanthroline with the following structure
##STR00030## wherein: R1 and R2 are selected from the group
consisting of substituted or unsubstituted Aryl, Heteroaryl, and
Alkyl; and R3 is selected from the group consisting of H, CN,
substituted or unsubstituted Aryl, Heteroaryl, and Alkyl; R4 is
selected from the group consisting of H, CN, COOR with R=Alkyl,
Heteroalkyl, Aryl or Heteroaryl, substituted or unsubstituted Aryl,
Heteroaryl, Alkyl mit C1-C20, and Cycloalkyl mit C3-C20.
16. A light-emitting component according to claim 12 or 13, wherein
the n-doped, donor organic material has the structure: ##STR00031##
wherein M is selected from the group consisting of Ti, Zr, Hf. Nb,
Re, Sn and Ge, each R is independently selected from the group
consisting of hydrogen, C.sub.1-C.sub.20-Alkyl,
C.sub.1-C.sub.20-Alkenyl, C.sub.1-C.sub.20-Alkinyl, Aryl,
Heteroaryl, Oligoaryl, Oligoheteroaryl, Oligoarylheteroaryl,
--OR.sub.x, --NR.sub.xR.sub.y, --SR.sub.x, --NO.sub.2, --CHO,
--COOR.sub.x, --F, --Cl, --Br, --I, --CN, --NC, --SCN, --OCN,
--SOR.sub.x, SO.sub.2R.sub.x, and where R.sub.x and R.sub.y are
selected from the group consisting of C.sub.1-C.sub.20-Alkyl,
C.sub.1-C.sub.20-Alkenyl, and C.sub.1-C.sub.20-Alkinyl.
17. A light-emitting component according to claim 12 or 13, wherein
the n-doped, donor organic material has the structure: ##STR00032##
wherein R.sub.1, R.sub.2, R.sub.3, and R.sub.4 are independently
selected from the group consisting of H, halogen, CN, substituted
or unsubstituted aryl, heteroaryl, alkyl, heteroalkyl, alkoxy, and
aryloxy.
18. A light-emitting component according to claim 1, wherein the
anode is between the substrate and the at least one light-emitting
layer.
19. A light-emitting component according to claim 1, wherein the
cathode is between the substrate and the at least one
light-emitting layer.
20. A light-emitting component according to claim 3, wherein the
electrode layers include different transparent contact
materials.
21. A light-emitting component according to claim 1, further
comprising a contact-improving layer located between the electron
transport layer and cathode and a contact-improving layer located
between the anode and the hole transport layer, wherein the
contact-improving layers are configured not to prevent charge from
passing through.
22. A light-emitting component according to claim 1, further
comprising a contact-improving layer located between the electron
transport layer and cathode or a contact-improving layer located
between the anode and the hole transport layer, wherein the
contact-improving layers are configured not to prevent charge from
passing through.
23. A light-emitting component according to claim 1, wherein the
light-emitting layer includes a mixed layer of several
materials.
24. A light-emitting component according to claim 1, wherein the
p-doped hole transport layer includes a mixture of an organic main
substance and an acceptor doping substance and an acceptor doping
substance and the molecular mass of the dopants is greater than 200
g/mole.
25. A light-emitting component according to claim 1, wherein the
electron transport layer includes a mixture of an organic main
substance and a donor doping substance and an acceptor doping
substance and the molecular mass of the dopants is greater than 200
g/mole.
26. A light-emitting component according to claim 1, wherein when
the transparent cathode is on top, the transparent cathode includes
a transparent protective layer or when the transparent anode is on
top, the transparent anode includes a transparent protective
layer.
27. A light-emitting component according to claim 1, wherein when
the transparent cathode is on top, the transparent cathode includes
a metallic intermediate layer adjacent to the subjacent doped
charge-carrier transport layer or when the transparent anode is on
top, the transparent anode includes a metallic intermediate layer
adjacent to the subjacent doped hole transport layer, wherein the
transparency of the metal intermediate layer in the visible
spectral region is at least 75% and the thickness of the metal
intermediate layer is between 0.3 nm and 3 nm.
28. A light-emitting component according to claim 1, wherein the
sequence of p-doped hole transport layer and transparent anode is
repeated.
29. A light-emitting component according to claim 1, wherein the
sequence of n-doped electron transport layer and transparent
cathode is repeated.
30. A light-emitting component according to claim 2, further
comprising a metallic electron-injection-promoting layer located
between the doped electron transport layer and either the
electron-side blocking layer or the light-emitting layer, wherein
the transparency of the metallic electron-injection-promoting layer
in the visible spectral region is at least 75%.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S. patent
application Ser. No. 10/496,414, filed Sep. 19, 2005, which is a
national phase of and claims priority to International Application
PCT/DE03/01021, filed Mar. 27, 2003, which claims priority to
German Patent Application DE 102 15 210.1, filed Mar. 28, 2002, all
of which are incorporated by reference in their entireties.
FIELD OF THE SUBJECT MATTER
[0002] The presently described subject matter relates to the
organic semiconductor technology concerning transparent organic
light-emitting diodes with doped charge carrier transport
layers.
BACKGROUND
[0003] Ever since the demonstration, by Tang et al., 1987 [C. W.
Tang et al., Appl. Phys. Lett. 51 (12), 913 (1987)], of low
operating voltages, organic light-emitting diodes (OLED) have been
promising candidates for the realization of large-area displays.
They include a sequence of thin (typically 1 nm to 1 mu m) layers
of organic materials, which can be vacuum-deposited or deposited
from the solution, e.g., by a spin-on operation. For this reason,
these layers are often more than 80% transparent in the visible
spectral region. Otherwise, the OLED would have a low external
light efficiency due to reabsorption. Contacting of the organic
layers with an anode and a cathode is typically effected by means
of at least one transparent electrode having, in many cases, a
transparent oxide (e.g., indium tin oxide) and a metallic contact.
This transparent contact (e.g., the ITO) can be located directly on
the substrate. In the case of at least one metallic contact, the
OLED as a whole is not transparent, but reflective or scattering
(due to appropriate modifying layers, which do not belong to the
actual OLED structure). In case of the typical structure with the
transparent electrode on the substrate, the OLED emits through the
substrate situated on its lower side.
[0004] In the case of organic light-emitting diodes, light is
produced and emitted by the light-emitting diode by the injection
of charge carriers (electrons from one side, holes from the other
side) from the contacts into the organic layers situated
there-between, as a result of an externally applied voltage, the
subsequent formation of excitons (electron-hole pairs) in an active
zone, and the radiant recombination of these excitons.
[0005] One feature of such organic components as compared with
conventional inorganic components (semiconductors such as silicon,
gallium arsenide) is that it is possible to produce very large-area
display elements (visual displays, screens). Compared with
inorganic materials, organic starting materials are relatively
inexpensive (e.g., less expenditure of material and energy).
Furthermore, these materials, because of their low processing
temperature as compared with inorganic materials, can be deposited
on flexible substrates, which opens up a wide variety of novel uses
in display and illuminating technology.
[0006] The usual arrangement of such components having at least one
non-transparent electrode includes a sequence of one or more of the
following layers: [0007] 1. Carrier, substrate; [0008] 2. Base
electrode, hole-injecting (positive pole), typically transparent;
[0009] 3. Hole-injecting layer; [0010] 4. Hole-transporting layer
(HTL); [0011] 5. Light-emitting layer (EL); [0012] 6.
Electron-transporting layer (ETL); [0013] 7. Electron-injecting
layer; [0014] 8. Cover electrode, in most cases a metal having a
low work function, electron-injecting (negative pole); [0015] 9.
Encapsulation, to shut out environmental influences.
[0016] The above structure represents one general case; in some
cases some layers are omitted (except 2, 5 and 8), or else one
layer combines several properties.
[0017] In the case of the above-described layer sequence, the light
emission takes place through the transparent base electrode and the
substrate, whereas the cover electrode includes non-transparent
metal layers. Some materials for the transparent base electrode
include indium tin oxide (e.g., ITO) and related oxide
semiconductors as injection contacts for holes (e.g., a transparent
degenerate semiconductor). Used for electron injection are base
metals such as aluminum (Al), magnesium (Mg), calcium (Ca) or a
mixed layer of Mg and silver (Ag), or such metals in combination
with a thin layer of a salt such as lithium fluoride (LiF).
[0018] These OLEDs are usually non-transparent. However, there are
applications for which the transparency is of decisive importance.
Thus, a display element may be produced which in the switched-off
state appears transparent, i.e., the surroundings behind it can be
perceived, but will, in the turned-on condition, provide the viewer
with information. In this connection, one could think of car
windshields or displays for persons who must not be limited in
their freedom of movement by the display (e.g., head-on displays
for surveillance personnel). Such transparent OLEDs, which
represent the basis for transparent displays, are known, e.g., from
[0019] 1. G. Gu, V. Bulovic, P. E. Burrows, S. R. Forrest, Appl.
Phys. Lett. 68, 2606 (1996); [0020] 2. G. Gu, V. Khalfin, S. R.
Forrest, Appl. Phys. Lett. 73, 2399 (1998); [0021] 3. G.
Parthasarathy et al., Appl. Phys. Lett. 72, 2138 (1997); [0022] 4.
G. Parthasarathy et al., Adv. Mater. 11, 907 (1997); [0023] 5. G.
Gu, G. Parthasarathy, S. R. Forrest, Appl. Phys. Lett. 74, 305
(1999).
[0024] In reference (1) above, the transparency is achieved by
using the traditional transparent ITO anode as a base electrode
(that is, directly on the substrate). Here, it should be mentioned
that it is favorable for the operating voltage of the OLED if the
ITO anode is pretreated in a special way (e.g., ozone sputter,
plasma incineration) in order to increase the work function of the
anode (e.g., C. C. Wu et al., Appl. Phys. Lett. 70, 1348 (1997); G.
Gu et al., Appl. Phys. Lett. 73, 2399 (1998)). The work function of
ITO can be varied, e.g., by ozonization, ozone or oxygen plasma
treatment, and/or oxygen-plasma incineration from about 4.2 eV to
about 4.9 eV. In that case, it is possible to inject holes from the
ITO anode into the hole transport layer in a more efficient manner.
However, this pretreatment of the ITO anode is mostly possible if
the anode is situated directly on the substrate. This structure of
the OLED is denoted as non-inverted, and the structure of the OLED
with the cathode on the substrate as inverted. In (1), a
combination of a thin, semitransparent layer, a base metal
(magnesium, stabilized through the admixture of silver) and a
conductive transparent layer of the known ITO is used as a cover
electrode. The reason why this combination is necessary is that the
work function of the ITO is too high for electrons to be
efficiently injected directly into the electron transport layer and
thereby make it possible to produce OLEDs having low operating
voltages. This is avoided by means of the very thin magnesium
intermediate layer. Because of the thin metallic intermediate
layer, the resulting component is semitransparent (transparency of
the cover electrode is about 50-80%), whereas the transparency of
the ITO anode considered as fully transparent is over 90%. In
reference (1), an additional ITO contact is deposited on the
metallic intermediate layer by the sputter process, in order to
ensure the lateral conductivity to the connection contacts of the
OLED surroundings. The consequence of the ITO sputter process is
that the metallic intermediate layer, in some embodiments, may not
be thinner than 7.5 nm (1), as otherwise the sputter damage to the
subjacent organic layers can be unacceptable. Structures of this
type are also described in the following patents: U.S. Pat. No.
5,703,436 (S. R. Forrest et al.), applied for on Mar. 6, 1996; U.S.
Pat. No. 5,757,026 (S. R. Forrest et al.), applied for on Apr. 15,
1996; U.S. Pat. No. 5,969,474 (M. Arai), applied for on Oct. 24,
1997. Two OLEDs, one on top of the other, with the cathodes
described in reference (1), are described in reference (2). Here, a
green and a red OLED arranged one upon the other ("stacked OLED")
are prepared. Since both OLEDs are semitransparent, it is possible,
through suitable voltages at the now 3 electrodes, to choose the
emission color in a targeted manner.
[0025] It is also known that an organic intermediate layer can be
used to improve the electron injection (references 3-5). In this
case, an organic intermediate layer is arranged between the
light-emitting layer (e.g., aluminum tris-quinolate, Alq3) and the
transparent electrode (e.g., ITO) used as a cathode. In some cases,
this intermediate layer is copper phthalocyanine (CuPc). This
material is a hole-transport material (higher hole mobility than
electron mobility). It exhibits high thermal stability. Thus, the
sputtered-on cover electrode cannot do as much damage to the
subjacent organic layers. An additional feature of this CuPc
intermediate layer is the small band gap (distance between
HOMO--highest occupied molecular orbital--and LUMO--lowest
unoccupied molecular orbital). Because of the low LUMO position,
electrons can be injected from ITO relatively easily. However,
because of the small band gap, the absorption in the visible region
is high. For this reason, the thickness of the CuPc layer is
limited to below 10 nm. Moreover, the injection of electrons from
CuPc into Alq3 or another emission material is difficult, since
their LUMOs lie generally higher. A further realization of the
transparent cathode at the top of the OLED was proposed by Pioneer
[U.S. Pat. No. 5,457,565 (T. Namiki), applied for on Nov. 18,
1993]. In this case, a thin layer of an alkaline earth metal oxide
(e.g., LiO2) is used instead of the CuPc layer. This improves the
otherwise poor electron injection from the transparent cathode into
the light-emitting layer.
[0026] A further realization of the transparent OLED (G.
Parthasarathy et al., Appl. Phys. Lett. 76, 2128 (2000), WO Patent
01/67825 A1 (G. Parthasarathy), applied for on Mar. 7, 2001,
provides for an additional electron transport layer (e.g.,
BCP=bathocuproine having a high electron mobility) in contact with
the transparent cathode (e.g., ITO). There is an approximately 1 nm
thick pure layer of the alkali metal lithium (Li) either between
the light-emitting layer and the thin (e.g., 10 nm) electron
transport layer or between the electron transport layer and the ITO
cathode. This Li intermediate layer drastically increases the
electron injection from the transparent electrode. This effect is
explained by a diffusion of the Li atoms into the organic layer and
subsequent "doping," with the formation of a highly conductive
intermediate layer (e.g., degenerate semiconductor). Then, a
transparent contact layer (e.g., mostly ITO) is placed on the
latter.
[0027] The above studies make the following points clear: [0028] 1.
The choice of transparent electrodes includes ITO and similar
degenerate inorganic semiconductors. [0029] 2. The work functions
of the transparent electrodes mainly favor hole injection, but for
this, too, a special treatment of the anode is required, in order
to further reduce its work function. [0030] 3. Previous worked was
aimed at finding a suitable intermediate layer which improved the
injection of electrons into the organic layers.
SUMMARY
[0031] The presently described subject matter relates to
transparent and thermally stable light-emitting components having
organic layers, and in particular to a transparent organic
light-emitting diode having a charge carrier transport layer which
is electrically doped with an organic dopant.
[0032] It was determined that employing dopants which can dope
electron transport materials (ETM) with a LUMO (of the ETM) less
negative than Alq3 are useful for a low voltage, high efficient and
long lifetime transparent OLED. Materials with a LUMO less negative
than ETM-11 can be useful.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] FIGS. 1a-b show energy diagrams of the transparent OLED in
one example embodiment.
[0034] FIG. 2a shows OLED structures according to some embodiments
of the described subject matter.
[0035] FIG. 2b is an energy diagram of a transparent OLED according
to another example embodiment.
[0036] FIG. 3 shows a luminance vs. voltage curve of Example 1.
[0037] FIG. 4 shows the normalized luminance over time in an
accelerated aging test of one device of the presently described
subject matter. The figure compares the bottom emitting device with
the transparent device. The measured points almost completely
overlap each other.
[0038] FIG. 5 shows the comparison of the optical transmittance of
one device of the presently described subject matter (102) compared
with other devices (103). The transmittance of the glass substrate
with the ITO layer is also shown for comparison purposes (101).
[0039] FIG. 6 shows the luminance vs. voltage curve of a
transparent OLED according to an embodiment with a non-inverted
structure.
DETAILED DESCRIPTION
[0040] It is known, that for light-emitting diodes from inorganic
semiconductors, it is possible, through highly doped peripheral
layers, to obtain thin space charge zones which, even in the
presence of energy barriers, lead to efficient injection of charge
carriers by tunneling. Here, the term "doping" includes the
targeted influencing of the conductivity of the semiconductor layer
through admixture of foreign atoms/molecules (as is possible for
inorganic semiconductors). For organic semiconductors, the term
"doping" includes the admixture, to the organic layer, of specific
emitter molecules; here, a distinction should be made. The doping
of organic materials was described in U.S. Pat. No. 5,093,698,
applied for on Feb. 12, 1991. However, in the case of practical
applications of the described doping, this leads to problems with
the energy adaptation of the different layers and to reduction of
the efficiency of the LEDs having doped layers.
[0041] In addition, electrical doping includes the phenomenon where
a charge transfer occurs from the HOMO (LUMO) of the n-dopant
(p-dopant) to the LUMO (HOMO) of the n-type (p-type) semiconductor
which transports the charge carriers (also called matrix material).
The charge density in equilibrium and the Fermi Level can be thus
modified.
[0042] One object of the presently described subject matter is to
provide a fully transparent (e.g., 70% transmission) organic
light-emitting diode that can be operated at a low operating
voltage, the organic light-emitting diode having a high
light-emission efficiency. At the same time, the described subject
matter includes the protection of organic layers, in particular of
the light-emitting layers, against damage during preparation of the
transparent cover contact. The described subject matter includes
stable components (e.g., operating temperature range up to 80
degrees C., long-term stability).
[0043] According to the presently described subject matter, some
objects are achieved in combination with the following features: a
transparent, thermally stable light-emitting component having the
following sequence of organic layers: a transparent substrate; a
transparent anode; a hole transport layer adjacent to the anode; at
least one light-emitting layer; a charge-carrier transport layer
for electrons; and a transparent cathode; in such a way that the
hole transport layer is p-doped with an acceptor-type organic
material and the electron transport layer is n-doped with a
donor-type organic material, and the molecular masses of the
dopants are greater than 200 g/mole.
[0044] The presently described subject matter further includes a
transparent, thermally stable light-emitting component, having the
following organic layers: a transparent substrate; a transparent
cathode; a charge transport layer for electrons adjacent to the
cathode; at least one light-emitting layer; a charge-carrier
transport layer for holes; and a transparent anode; in such a way
that the electron transport layer is n-doped with a donor-type
organic material and the hole transport layer is p-doped with an
acceptor-type organic material, and the molecular masses of the
dopants are greater than 200 g/mole.
[0045] As described in Patent Application DE 101 35 513.0 (Leo et
al., submitted on Jul. 20, 2001), the layer sequence of the OLED
can be reversed, thus the hole-injecting (transparent) contact
(anode) can be a cover electrode. As a result, in the case of
inverted organic light-emitting diodes the operating voltages can
be considerably higher than with comparable non-inverted
structures. One reason for this phenomenon is that the injection
from the contacts into the organic layers is less efficient,
because optimization of the work function of the contacts in a
targeted manner can be more difficult.
[0046] In the solution according to the described subject matter,
the injection of charge carriers from the electrodes into the
organic layers (whether hole- or electron-transporting layers) does
not depend so strongly on the work function of the electrodes
itself. As a result it is also possible to use, on both sides of
the OLED component, the same electrode type, thus, e.g., two equal
transparent electrodes, e.g., ITO.
[0047] The term side includes extending along a plane parallel to
the substrate. The term bottom includes a position of a layer that
is closer to the substrate than another layer. The bottom electrode
includes an electrode located somewhere between the substrate and
at least one organic light-emitting layer. The term top includes a
position of a layer that is further from the substrate than another
layer. The top electrode includes an electrode located somewhere
not between the substrate and at least one organic light-emitting
layer.
[0048] Some embodiments include a transparent, thermally stable
light-emitting component having organic layers, including a
transparent substrate, a transparent anode, a hole transport layer
adjacent to the anode, at least one light-emitting layer, a
charge-carrier transport layer for electrons, and a transparent
cathode, wherein the transparency in the visible spectral region is
at least 75%, wherein the hole transport layer is p-doped with an
acceptor organic material and the electron transport layer is
n-doped with a donor organic material, and the molecular masses of
the dopants are each greater than 200 g/mole, and wherein the
transparent, thermally stable light-emitting component having
organic layers is an organic light-emitting diode.
[0049] Some embodiments further include at least one of a hole-side
blocking layer located between the doped hole transport layer and
the light-emitting layer or an electron-side blocking layer located
between the doped electron transport layer and the light-emitting
layer. Some embodiments further include a electrode layer located
between the anode and the hole transport layer and a electrode
layer located between the charge-carrier transport layer and the
cathode.
[0050] In some embodiments, the doping concentration of the organic
dopants is such that an ohmic injection takes place from the anode
into the charge-carrier transport layer or from the cathode into
the hole transport layer. In some embodiments, the electrode layers
comprise indium tin oxide (ITO) or a degenerate oxide other than
ITO. In some embodiments, the cathode includes a metallic
intermediate layer adjacent to the subjacent doped, charge-carrier
transport layer when the cathode is located on top or the anode
includes a metallic intermediate layer adjacent to the subjacent
doped, hole transport layer when the anode is located on top and
wherein the metallic layer has a nominal thickness between 0.1 nm
and 3 nm.
[0051] In some embodiments, no metal layer is located between the
doped hole transport layer and the anode when the anode is on top
or between the doped electron transport layer and the cathode when
the cathode is on top. The anode and cathode can be located between
the substrate and encapsulation cover and the transparency can be
at least 70% for each wavelength between at least 400 nm and 800
nm. The molar concentration of admixture in the hole transport
layer or in the electron transport layer or in both the hole
transport layer and the electron transport layer can be in the
range of 1:100,000 to 1:10, calculated on the ratio of doping
molecules to main-substance molecules. The molar concentration of
admixture in the hole transport layer or in the electron transport
layer, or in both the hole transport layer and the electron
transport layer, can be at least 1 wt %, calculated on the ratio of
doping molecules to main-substance molecules.
[0052] In some embodiments, the thickness of each of the hole
transport layer or the electron transport layer, of the
light-emitting layer and of the at least one of a hole-side
blocking layer or an electron-side blocking layer lies in the range
of 0.1 nm to 50 .mu.m. In some embodiments, the cathode is in
direct contact with a doped transport layer and is facing away from
the substrate when the cathode is on top or the anode is in direct
contact with a doped transport layer and is facing away from the
substrate when the anode is on top and wherein the doped transport
layer is a hole transport layer or an electron transport layer. In
some embodiments, the organic n-dopant material is selected from
the group consisting of heterocyclic radicals, diradicals, dimers,
an oligomer, a polymer, a dispiro compound, and a polycycle
thereof, having the structure according to one of the following
formulae:
##STR00001##
wherein structures 3 and 4 have one or more cyclic linkages A
and/or A1 and/or A2,
[0053] wherein A, A1 and A2 are selected from the group consisting
of carbocyclic, heterocyclic, polycyclic ring systems, and any
combination thereof, which may be substituted or unsubstituted,
##STR00002##
wherein A1 and A2 are present individually or together and A1 and
A2 are selected as in structures 3 and 4 and T=CR22, CR22R23, N,
NR21, O or S, and
##STR00003##
wherein structure 7 has one or more bridge bonds Z and Z1, Z or Z1,
Z1 and Z2, or Z1 or Z2, and Z, Z1 and Z2 are independently selected
from the group consisting of alkyl, alkenyl, alkynyl, cycloalkyl,
sililyl, alkylsililyl, diazo, disulphide, heterocycloalkyl,
heterocyclyl, piperazinyl, dialkyl ether, polyether, primary
alkylamine, arylamine, polyamine, aryl, and heteroaryl.
[0054] The organic acceptor organic material can be a quiniode
derivative or a triylidene derivative, with a reduction potential
in the range of 0V vs. Fc/Fc+ to 0.4V vs. Fc/Fc+. In some
embodiments, the n-doped, donor organic material is an
asymmetrically substituted phenanthroline with the following
structure
##STR00004##
wherein: R1 and R2 are selected from the group consisting of
substituted or unsubstituted Aryl, Heteroaryl, and Alkyl; and R3 is
selected from the group consisting of H, CN, substituted or
unsubstituted Aryl, Heteroaryl, and Alkyl; R4 is selected from the
group consisting of H, CN, COOR with R=Alkyl, Heteroalkyl, Aryl or
Heteroaryl, substituted or unsubstituted Aryl, Heteroaryl, Alkyl
mit C1-C20, and Cycloalkyl mit C3-C20.
[0055] In some embodiments, the n-doped, donor organic material has
the structure:
##STR00005##
wherein M is selected from the group consisting of Ti, Zr, Hf, Nb,
Re, Sn and Ge, each R is independently selected from the group
consisting of hydrogen, C.sub.1-C.sub.20-Alkyl,
C.sub.1-C.sub.20-Alkenyl, C.sub.1-C.sub.20-Alkinyl, Aryl,
Heteroaryl, Oligoaryl, Oligoheteroaryl, Oligoarylheteroaryl,
--OR.sub.x, --NR.sub.xR.sub.y, --SR.sub.x, --NO.sub.2, --CHO,
--COOR.sub.x, --F, --Cl, --Br, --I, --CN, --NC, --SCN, --OCN,
--SOR.sub.x, SO.sub.2R.sub.x, and where R.sub.x and R.sub.y are
selected from the group consisting of C.sub.1-C.sub.20-Alkyl,
C.sub.1-C.sub.20-Alkenyl, and C.sub.1-C.sub.20-Alkinyl.
[0056] The n-doped, donor organic material can have the
structure:
##STR00006##
wherein R.sub.1, R.sub.2, R.sub.3, and R.sub.4 are independently
selected from the group consisting of H, halogen, CN, substituted
or unsubstituted aryl, heteroaryl, alkyl, heteroalkyl, alkoxy, and
aryloxy.
[0057] In some embodiments, the anode is between the substrate and
the at least one light-emitting layer. In some embodiments, the
cathode is between the substrate and the at least one
light-emitting layer. The electrode layers can include different
transparent contact materials.
[0058] Some embodiments further include a contact-improving layer
located between the electron transport layer and cathode and a
contact-improving layer located between the anode and the hole
transport layer, wherein the contact-improving layers are
configured not to prevent charge from passing through. Some
embodiments further include a contact-improving layer located
between the electron transport layer and cathode or a
contact-improving layer located between the anode and the hole
transport layer, wherein the contact-improving layers are
configured not to prevent charge from passing through. The
light-emitting layer can include a mixed layer of several
materials. The p-doped hole transport layer can include a mixture
of an organic main substance and an acceptor doping substance and
an acceptor doping substance and the molecular mass of the dopants
can be greater than 200 g/mole. The electron transport layer can
include a mixture of an organic main substance and a donor doping
substance and an acceptor doping substance and the molecular mass
of the dopants can be greater than 200 g/mole. In some embodiments,
when the transparent cathode is on top, the transparent cathode
includes a transparent protective layer or when the transparent
anode is on top, the transparent anode includes a transparent
protective layer. In some embodiments, when the transparent cathode
is on top, the transparent cathode includes a metallic intermediate
layer adjacent to the subjacent doped charge-carrier transport
layer or when the transparent anode is on top, the transparent
anode includes a metallic intermediate layer adjacent to the
subjacent doped hole transport layer, [0059] wherein the
transparency of the metal intermediate layer in the visible
spectral region is at least 75% and the thickness of the metal
intermediate layer is between 0.3 nm and 3 nm. The sequence of
p-doped hole transport layer and transparent anode can be repeated.
The sequence of n-doped electron transport layer and transparent
cathode can be repeated. Some embodiments further include a
metallic electron-injection-promoting layer located between the
doped electron transport layer and either the electron-side
blocking layer or the light-emitting layer, wherein the
transparency of the metallic electron-injection-promoting layer in
the visible spectral region is at least 75%.
[0060] In some embodiments, the top transparent contact layer
(which is facing away from the substrate) is in direct contact with
the doped transport layer, which doped transport layer is a hole
transport layer or an electron transport layer.
[0061] In some embodiments, the transparent organic light-emitting
diode includes a thin (e.g., 1 to 10 nm thick) doped charge
transport layer at the interface with the top electrode (this layer
being localized between the light-emitting region and the
electrode); the dopant concentration being greater than 40 wt %, in
some embodiments greater or at least 50 wt %. In some embodiments,
the transparent organic light-emitting diode includes a thin (e.g.,
0.5 nm to 3 nm) pure dopant layer as a buffer layer at the
interface with the top electrode (between the charge transport
layer and the top electrode).
[0062] In some embodiments, ohmic injection occurs when the
dependence of the current with the applied voltage is linear (e.g.,
can be measured in single carrier type devices (e.g., hole only
devices)). In some embodiments, if a line fit (I=F(V)) to the I-V
curve fits to at least 95% in a range of at least 1 V (layer
thickness of at least 50 nm) then the injection is ohmic. For a
layer thinner than 10 nm, a dopant concentration greater than or
equal to 5%, perhaps greater than or equal to 10% may be required.
For layers thicker than 10 nm, the concentration may be higher than
0.2%, perhaps higher than 1%, and if the layer is under the top
electrode, then the doping concentration may be higher than 5%.
[0063] The cause of the increase of conductivity can be an
increased density of equilibrium charge carriers in a layer. Here,
the transport layer can have higher layer thicknesses than is
possible with undoped layers (e.g., 20-40 nm), without drastically
increasing the operating voltage. Similarly, the electron-injecting
layer adjacent to the cathode can be n-doped with a donor-type
molecule (e.g., an organic molecule or fragments thereof, see
Patent Application DE 102 07 859.9). This n-doping leads to an
increase in the electron conductivity due to higher intrinsic
charge-carrier density. The transport layer can also be made
thicker in the component than would be possible with undoped
layers, since that would lead to an increase in the operating
voltage. Thus, both layers are thick enough to protect the
subjacent layers against damage during the production process
(e.g., sputter process) of the transparent electrode (e.g., formed
from ITO).
[0064] In the doped charge-carrier transport layers (holes or
electrons) on the electrodes (anode or cathode), a thin space
charge zone may be created through which the charge carriers can be
injected in an efficient manner. Because of the tunnel injection,
the injection is not hindered by the very thin space charge zone,
even in case of an energetically high barrier. The charge-carrier
transport layer can be doped by an admixture of an organic or
inorganic substance (e.g., dopant). These large molecules are
incorporated in a stable manner into the matrix molecule skeleton
of the charge-carrier transport layers. As a result, a high degree
of stability is obtained during operation of the OLED (e.g., no
diffusion) as well as under thermal load.
[0065] In Patent Application DE 100 58 578.7, filed on Nov. 25,
2000 (see also X. Zhou et al., Appl. Phys. Lett. 78, 410 (2001)),
it is described that organic light-emitting diodes having doped
transport layers show an efficient light emission when the doped
transport layers are combined with blocking layers in an
appropriate manner. Hence, in an embodiment, the transparent
light-emitting diodes are also provided with blocking layers. The
blocking layer can be located between the charge-carrier transport
layer and a light-emitting layer of the component, in which the
conversion of the electric energy into light takes place. The
electric energy of the charge carriers can be injected by current
flow through the component. According to the described subject
matter, the substances of the blocking layers can be selected so
that when voltage is applied in the direction of the operating
voltage, because of their energy levels, the majority charge
carriers (HTL side: holes, ETL side: electrons) are not too
strongly hindered at the doped charge-carrier transport
layer/blocking layer interface (e.g., low barrier), but the
minority charge carriers are efficiently arrested at the
light-emitting layer/blocking layer interface (e.g., high barrier).
Moreover, the barrier height for the injection of charge carriers
from the blocking layer into the emitting layer can be small enough
that the conversion of a charge-carrier pair at the interface into
an exciton in the emitting layer is energetically advantageous.
This prevents exciplex formation at the interfaces of the
light-emitting layer, which reduces the efficiency of the light
emission. Since the charge-carrier transport layers can have a high
band gap, the blocking layers can be chosen to be very thin. In
spite of this, no tunneling of charge carriers from the
light-emitting layer in energy conditions of the charge-carrier
transport layers is possible. This permits obtaining a low
operating voltage despite blocking layers.
[0066] One embodiment of a transparent OLED according to the
described subject matter includes the following layers
(non-inverted structure) (FIG. 2a): [0067] 1 Carrier, substrate;
[0068] 2 Transparent electrode, e.g., ITO, hole-injecting
(anode=positive pole); [0069] 3 p-Doped, hole-injecting and
transporting layer; [0070] 4 Thin hole-side blocking layer made of
a material whose band positions match the band positions of the
layers enclosing it; [0071] 5 Light-emitting layer (possibly doped
with emitter dye); [0072] 6 Thin electron-side blocking layer of a
material whose band positions match the band positions of the
layers enclosing it; [0073] 7 n-Doped electron-injecting and
transporting layer; [0074] 8 Transparent electrode,
electron-injecting (cathode=negative pole); [0075] 9 Encapsulation,
to shut out environmental influences.
[0076] Another embodiment of a transparent OLED according to the
described subject matter includes the following layers (inverted
structure) (FIG. 2a): [0077] 1 Carrier, substrate; [0078] 2a
Transparent electrode, e.g., ITO, electron-injecting
(cathode=negative pole); [0079] 3 n-Doped, electron-injecting and
transporting layer; [0080] 4a Thin electron-side blocking layer of
a material whose band positions match the band positions of the
layers surrounding it; [0081] 5a Light-emitting layer (possibly
doped with emitter dye); [0082] 6a Thin hole-side blocking layer of
a material whose band positions match the band positions of the
layers surrounding it; [0083] 7a p-Doped hole-injecting and
transporting layer; [0084] 8a Transparent electrode, hole-injecting
(anode=positive pole), e.g., ITO; [0085] 9 Encapsulation, to keep
out environmental influences.
[0086] The described subject matter includes structures with one
blocking layer, because the band positions of the injecting and
transporting layer and of the light-emitting layer can match one
another on one side. Furthermore, the functions of charge-carrier
injection and of charge-carrier transport into layers 3 and 7 may
be divided among several layers, of which at least one (namely that
adjacent to the electrodes) is doped. When the doped layer is not
directly located on the respective electrode, then layers between
the doped layer and the respective electrode may be thin enough
that they can efficiently be tunneled through by charge carriers
(e.g., 10 nm). These layers can be thicker when they have a higher
conductivity (the bulk resistance of these layers may be smaller
than that of the neighboring doped layer). The intermediate layers
can then be considered to be a part of the electrode. The molar
doping concentrations can lie in the range of 1:10 to 1:10000. The
dopants can include organic molecules having molecular masses above
200 g/mole.
[0087] The n-dopant, or dopant donor, can include a molecule or a
neutral radical or combination thereof with a HOMO energy level
(e.g., ionization potential in solid state) more positive than -3.3
eV, or more positive than -2.8 eV, or more positive than -2.6 eV
and its respective gas phase ionization potential is more positive
than -4.3 eV, or more positive than -3.8 eV, or more positive than
-3.6 eV. The HOMO of the donor can be estimated by
cyclo-voltammetric measurements. An alternative way to measure the
reduction potential is to measure the cation of the donor salt. The
donor can exhibit an oxidation potential that is smaller than or
equal to -1.5 V vs Fc/Fc+ (Ferrum/Ferrocenium redox-pair), or
smaller than -1.5 V, or smaller than or equal to approximately -2.0
V, or smaller than or equal to -2.2 V. The molar mass of the donor
can be in a range between 200 and 2000 g/mole, or in a range from
300 and 1000 g/mole. The molar doping concentration is in the range
of 1:10000 (dopant molecule:matrix molecule) and 1:2, or between
1:100 and 1:5, or between 1:100 and 1:10. Sometimes doping
concentrations larger than 1:2 can be applied, e.g., if large
conductivities are required. The donor can be created by a
precursor during the layer forming (e.g., deposition) process or
during a subsequent process of layer formation. The above given
value of the HOMO level of the donor refers to the resulting
molecule or molecule radical.
[0088] A p-dopant, or dopant acceptor, can include a molecule or a
neutral radical or combination thereof with a LUMO level more
negative than -4.5 eV, or more negative than -4.8 eV, or more
negative than -5.04 eV. The LUMO of the acceptor can be estimated
by cyclo-voltammetric measurements. The acceptor can exhibit a
reduction potential that is larger than or equal to approximately
-0.3 V vs Fc/Fc+ (Ferrum/Ferrocenium redox-pair), or larger than or
equal to 0.0 V, or larger than or equal to 0.24 V. The molar mass
of the acceptor can be in the range of 200 to 2000 g/mole, or
between 250 and 1000 g/mole, or between 300 g/mole and 1000 g/mole.
The molar doping concentration can be in the range of 1:10000
(dopant molecule:matrix molecule) and 1:2, or between 1:100 and
1:5, or between 1:100 and 1:10. Sometimes, doping concentrations
larger than 1:2 can be applied, e.g., if large conductivities are
required. The acceptor can be created by a precursor during the
layer forming (e.g., deposition) process or during a subsequent
process of layer formation. The above given value of the LUMO level
of the acceptor refers to the resulting molecule or molecule
radical.
[0089] An n-dopant of the following structure can be employed in
the transparent p-i-n OLED:
##STR00007##
where M is a transition metal, e.g., Mo or W; and where [0090] the
structural elements a-f can include: a=CR.sub.9R.sub.10,
b=CR.sub.11R.sub.12, c=CR.sub.13R.sub.14, d=R.sub.15R.sub.16,
e=CR.sub.17R.sub.18 and f=CR.sub.19R.sub.20, where R.sub.9R.sub.20
independently of one another are hydrogen, C.sub.1C.sub.20 alkyl,
C.sub.1C.sub.20 cycloalkyl, C.sub.1C.sub.20 alkenyl,
C.sub.1C.sub.20 alkynyl, aryl, heteroaryl, NRR or OR, where
R=C.sub.1C.sub.20 alkyl, C.sub.1C.sub.20 cycloalkyl,
C.sub.1C.sub.20 alkenyl, C.sub.1C.sub.20 alkynyl, aryl or
heteroaryl, where R.sub.9, R.sub.11, R.sub.13, R.sub.15, R.sub.17,
R.sub.19=H and R.sub.10, R.sub.12, R.sub.14, R.sub.16, R.sub.18,
R.sub.20=C.sub.1C.sub.20 alkyl, C.sub.1C.sub.20 cycloalkyl,
C.sub.1C.sub.20 alkenyl, C.sub.1C.sub.20 alkynyl, aryl, heteroaryl,
NRR or OR, or [0091] in the case of structural elements c and/or d,
C can be replaced by Si, or [0092] optionally a or b or e or f is
NR, with R=C.sub.1C.sub.20 alkyl, C.sub.1C.sub.20 cycloalkyl,
C.sub.1C.sub.20 alkenyl, C.sub.1C.sub.20 alkynyl, aryl, heteroaryl,
or [0093] optionally a and f or b and e are NR, with
R=C.sub.1C.sub.20 alkyl, C.sub.1C.sub.20 cycloalkyl,
C.sub.1C.sub.20 alkenyl, C.sub.1C.sub.20 alkynyl, aryl, heteroaryl,
[0094] where the bonds a c, b d, c e and d f, but not
simultaneously a-c and c-e and not simultaneously b-d and d-f, may
be unsaturated, [0095] where the bonds a-c, b-d, c-e and d-f may be
part of a saturated or unsaturated ring system, which may also
contain the heteroelements O, S, Se, N, P, Se, Ge, Sn, or [0096]
the bonds a-c, b-d, c-e and d-f are part of an aromatic or
condensed aromatic ring system, which may also contain the
heteroelements O, S, Si, N, [0097] where the atom E is a main group
element, selected from the group C, N, P, As, Sb, [0098] where the
structural element a E-b is optionally part of a saturated or
unsaturated ring system, which may also contain the heteroelements
O, S, Se, N, P, Si, Ge, Sn, or [0099] the structural element a E-b
is optionally part of an aromatic ring system, which may also
contain the heteroelements O, S, Se, N.
[0100] The dopant can have the following structure II:
##STR00008##
[0101] Suitable n-dopant precursors include the heterocyclic
radicals, diradical, a dimers, an oligomer, a polymer, a dispiro
compound or a polycycle thereof, having the structure according to
the following formulae:
##STR00009##
where structures 3 and 4 have one or more cyclic linkages A and/or
A1 and/or A2, where A, A1 and A2 may be carbocyclic, heterocyclic
and/or polycyclic ring systems, which may be substituted or
unsubstituted;
##STR00010##
where A1 and A2 may be present individually or together and A1 and
A2 are as defined for structures 3 and 4 and T=CR22, CR22R23, N,
NR21, O or S;
##STR00011##
where structure 7 has one or more bridge bonds Z and/or Z1 and/or
Z2, and Z, Z1 and Z2 may independently be selected from alkyl,
alkenyl, alkynyl, cycloalkyl, sililyl, alkylsililyl, diazo,
disulphide, heterocycloalkyl, heterocyclyl, piperazinyl, dialkyl
ether, polyether, primary alkylamine, arylamine and polyamine, aryl
and heteroaryl; Organic n-dopant compounds include the heterocyclic
radicals or diradicals, the dimers, oligomers, polymers, dispiro
compounds and polycycles of:
##STR00012##
where the bridges Z, Z1 and Z2 can be independently selected from
alkyl, alkenyl, alkinyl, cycloalkyl, silyl; alkylsilyl, diazo,
disulfide, heterocycloalkyl, heterocyclyl, piperazinyl,
dialkylether, polyether, alkylamine, arylamine, polyamine, Aryl and
heteroaryl; X and Y can be O, S, N, NR.sub.21, P, or PR.sub.21;
R.sub.0-19, R.sub.21, R.sub.22 and R.sub.23 are independently
chosen from substituted or unsubstituted: aryl, heteroaryl,
heterocyclyl, diarylamine, diheteroarylamine, dialkylamine,
heteroarylalkylamine, arylalkylamine, H, F, cycloalkyl,
halocycloalkyl, heterocycloalkyl, alkyl, alkenyl, alkinyl,
trialkylsilyl, triarylsilyl, halogen, styryl, alkoxy, aryloxy,
thioalkyl, thioaryl, silyl and trialkylsilylalkanyl, or R.sub.0-19,
R.sub.21, R.sub.22 and R.sub.23, are part of a (hetero)aliphatic or
(hetero)aromatic ring system alone or in combination. Preferred
n-dopants are those with the structure:
##STR00013##
where R1 is methyl or isopropyl and R2 is phenyl or cyclohexyl.
[0102] Illustrative examples of suitable organic n-dopants include
the following dimer structures, their diradical state and their
monomer:
##STR00014## ##STR00015##
[0103] Other examples include (ED-9)
2,2'-diisopropyl-4,5-bis(2-methoxyphenyl)-4',5'-bis(3-methoxyphenyl)-1,1'-
,3,3'-tetramethyl-2,2',3,3'-tetrahydro-1H,1'H-2,2'-biimidazole;
(ED-10)
2,2'-Diisopropyl-4,5-bis(2-methoxyphenyl)-4',5'-bis(4-methoxyphenyl)-1,1'-
,3,3'-tetramethyl-2,2',3,3'-tetrahydro-1H,1'H-2,2'-biimidazole;
(ED-11)
2,2'-Diisopropyl-1,1',3,3'-tetramethyl-2,2',3,3',4,4',5,5',6,6',7,7'-dode-
cahydro-2,2'-bibenzo[d]imidazole; (ED-8)
2,2'-Diisopropyl-4,4',5,5'-tetrakis(4-methoxyphenyl)-1,1',3,3'-tetramethy-
l-2,2',3,3'-tetrahydro-2,2'-biimidazole; (ED-12)
2-Isopropyl-1,3-dimethyl-2,3,6,7-tetrahydro-5,8-dioxa-1,3-diaza-cyclopent-
a[b]naphthene; (ED-13)
Bis-[1,3-dimethyl-2-isopropyl-1,2-dihydro-benzimidazolyl-(2)];
(ED-14)
1,1',2,2',3,3'-hexamethyl-4,4',5,5'-tetraphenyl-2,2',3,3'-tetrahydro-1H,1-
'H-2,2'-biimidazole;
[0104] Electron transport materials (ETM) which can be used as host
for the n-dopants include phenanthrolines, metal quinolinates,
metal quinoxalinates, diazapyrenes and others.
[0105] Asymmetrically substituted phenanthrolines are described in
the European patent application EP07400033.2. Asymmetrically
substituted phenanthrolines which can be used as ETM can have the
following structure
##STR00016##
where: R1 and R2 are chosen from substituted or unsubstituted Aryl,
Heteroaryl, Alkyl; R3 is chosen from H, CN, substituted or
unsubstituted Aryl, Heteroaryl or Alkyl; R4 is chosen from H, CN,
COOR with R=Alkyl, Heteroalkyl, Aryl or Heteroaryl; substituted or
unsubstituted Aryl, Heteroaryl, Alkyl mit C.sub.1-C.sub.20,
Cycloalkyl mit C.sub.3-C.sub.20.
[0106] Examples of phenanthrolines to be used as n-doped ETM
include:
##STR00017## ##STR00018## ##STR00019##
[0107] Other ETM include metal complexes, such as metal chelates. A
form of the metal chelates are metal quinolates and quinoxalines.
Some materials are those with the structure:
##STR00020##
where M is chosen from Ti, Zr, Hf, Nb, Re, Sn and Ge, each R is
independently chosen from hydrogen, C.sub.1-C.sub.20-Alkyl,
C.sub.1-C.sub.20-Alkenyl, C.sub.1-C.sub.20-Alkinyl, Aryl,
Heteroaryl, Oligoaryl, Oligoheteroaryl, Oligoarylheteroaryl,
--OR.sub.x, --NR.sub.xR.sub.y, --SR.sub.x, --NO.sub.2, --CHO,
--COOR.sub.x, --F, --Cl, --Br, --I, --CN, --NC, --SCN, --OCN,
--SOR.sub.x, SO.sub.2R.sub.x, where R.sub.x, and R.sub.y are chosen
from C.sub.1-C.sub.20-Alkyl, C.sub.1-C.sub.20-Alkenyl and
C.sub.1-C.sub.20-Alkinyl.
[0108] Examples of quinoxalines include:
##STR00021##
[0109] Other ETM include compounds according to the following
formulae:
##STR00022##
where R.sub.1, R.sub.2, R.sub.3, and R.sub.4 are in each occurrence
independently selected from H, halogen, CN, substituted or
unsubstituted aryl, heteroaryl, alkyl, heteroalkyl, alkoxy and
aryloxy.
[0110] Examples of such ETM are:
##STR00023## ##STR00024##
[0111] Hole transport materials (HTM) that are used as host for the
p-dopants include phenylamines, triphenyl-amines, fluorenes,
benzidines.
[0112] Examples of such HTM include:
4,4',4''-tris(N-3-methylphenyl-N-phenyl-amino) triphenylamine
(m-MTDATA),
4,4',4''-tris(N-(2-naphthyl)-N-phenyl-amino)triphenylamine
(2-TNATA), MeO-TPD (N,N,N',N'-tetrakis(4-methoxy-phenyl)benzidine),
(2,2',7,7'-tetrakis-(N,N-diphenylamino)-9,9'-spirobifluoren
(spiro-TTB), N,N'-bis(naphthalen-1-yl)-N,N'-bis(phenyl)-benzidine,
N,N'-bis(naphthalen-1-yl)-N,N'-bis(phenyl)-9,9-spiro-bifluorene,
9,9-bis[4-(N,N-bis-biphenyl-4-yl-amino)phenyl]-9H-fluorine,
N,N'-bis(phenanthren-9-yl)-N,N'-bis(phenyl)-benzidine,
2,2'-bis[N,N-bis(biphenyl-4-yl)amino]9,9-spiro-bifluorene,
1,3,5-tris{4-[bis(9,9-dimethyl-fluorene-2-yl)amino]phenyl}benzene,
and tri(terphenyl-4-yl)amine;
N,N'-Bis(naphthalen-1-yl)-N,N'-bis(phenyl)-benzidine (NPD).
[0113] The p-dopant can have a reduction potential in the range of
0V vs. Fc/Fc+ to 0.4V vs. Fc/Fc+. Fc/Fc+, as usual the
Ferrocene/Ferrocenium redox couple. Reduction potentials can be
considered as measures for the LUMO of a molecule.
[0114] Examples of p-dopants include:
TABLE-US-00001 Name Chemical name MW OA-1
2,2'-(perfluorocyclohexa-2,5-diene-1,4- 276 (F4TCNQ)
diylidene)dimalononitrile OA-2
(perfluoronaphthalene-2,6-diylidene)dicyanamide 314 OA-3
N,N'-bicyano-2,5-dichloro-1,4-chinodiimine(2,5-dichloro- 261
3,6-difluorocyclohexa-2,5-diene-1,4-diylidene)dicyanamide OA-4
N,N'-bicyano-2,5-dichloro-3,6-difluoro-1,4- 225
chinodiimine(2,5-dichlorocyclohexa-2,5-diene-1,4-
diylidene)dicyanamide OA-5
N-(2,3,5,6-tetrafluoro-4-iminocyclohexa-2,5- 203
dienylidene)cyanamide OA-6
1,4,5,8-Tetrahydro-1,4,5,8-tetrathia-2,3,6,7- 384
tetracyanoanthrachinone OA-7 1,3,4,5,7,8-Hexafluoronaphtho-2,6- 362
chinontetracyanomethane OA-8 3,6-bis(cyano(4-cyano-2,3,5,6- 586
tetrafluorophenyl)methylene)-2,5-difluorocyclohexa-1,4-
diene-1,4-dicarbonitrile OA-9
2,2',2''-(cyclopropane-1,2,3-triylidene)tris(2- 651
(perfluorophenyl)acetonitrile) OA-10
2,2',2''-(cyclopropane-1,2,3-triylidene)tris(2- 1095
(perfluorobiphenyl-4-yl)acetonitrile); OA-11
2,2',2''-(Cyclopropane-1,2,3-triylidene) tris (2-(2,6- 672
dichloro-3,5-difluoro-4-(trifluoromethyl) phenyl)acetonitrile);
OA-12 2,2',2''-(cyclopropane-1,2,3-triylidene)tris(2-(2,6- 902
dichloro-3,5-difluoro-4-(trifluoromethyl)phenyl)-acetonitrile) Some
dopants have MW > 300. Some compounds have a MW > than
500.
[0115] Asymmetric phenanthrolines can be used as an electron
transport layer in the devices of the described subject matter.
Asymmetric phenanthrolines can also be used when they are n-doped
with dopants that are, or that form, neutral radicals (or, e.g.,
diradicales, their dimers, oligomers).
[0116] The dopants that are, or that form, neutral radicals (or,
e.g., diradicales, their dimers, oligomers) can form stable layers
when used as dopants in a matrix having asymmetric
phenanthrolines.
[0117] Metal quinoxalines can be used as electron transport
materials doped with dopants that are, or that form, neutral
radicals (or, e.g., diradicales, their dimers, oligomers).
Precursor dopants can form stable layers when used as dopants in a
matrix having metal quinoxalines.
[0118] Diazapyrenes can be used as electron transport materials
doped with dopants that are, or that form, neutral radicals (or,
e.g., diradicales, their dimers, oligomers). Precursor dopants can
form stable layers when used as dopants in a matrix having metal
quinoxalines.
[0119] Stability, low voltage, and high efficiency can be achieved
in devices where organic mesomeric compounds are used as organic
p-doping agents for the doping of an organic semiconductive hole
transport matrix material. The organic mesomeric compound can be a
radialene compound with the following formula:
##STR00025##
in which each X is
##STR00026##
where each R.sub.1 is independently selected from aryl and
heteroaryl and aryl and heteroaryl are at least partially or
completely substituted with electron acceptor groups.
[0120] Examples of emitter materials include Fluorescent emitters
such as
4-(Dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-
-pyran (DCJTB); CBP, antracene, Metal chelates such as 3 quinoline
Aluminum (Alq3); Phosphorescent emitters such as Ir-chelates;
Ir(ppy)3 Fir-pic.
[0121] Emitter materials can be mixed with an emitter host. The
host can also contribute to the emission. Examples of emitter hosts
include:
3,9-di(naphthalen-2-yl)perylene+3,10-di(naphthalen-yl)perylene
mixture (DNP); and NPD.
[0122] FIGS. 1a and 1b are energy diagrams of a transparent OLED in
one embodiment of the described subject matter without doping. The
position of the energy levels are shown in the upper part (HOMO and
LUMO) without external voltage and in the lower part with applied
external voltage. In this embodiment, both electrodes have the same
work function. Here, for the sake of simplicity, the blocking
layers 4 and 6 are also shown.
[0123] FIG. 2b is an energy diagram of a transparent OLED with
doped charge-carrier transport layers and matching blocking layers
according to an embodiment of the described subject matter. Note
the band bending adjacent to the contact layers, here of ITO in
both cases.
[0124] FIG. 3 shows the luminance vs. voltage curve of the
embodiment presented in example 1; the monitor luminance of 100
cd/m 2 is attained already at 4 V. The efficiency is 2 cd/A.
However, here, no transparent contact (e.g., ITO) is used as anode
material. The transparent contact is simulated by a semitransparent
(e.g., 50%) gold contact. Thus, this is a semitransparent OLED.
[0125] FIG. 4 shows the normalized luminance over time in an
accelerated aging test (current density=60 mA/cm.sup.2). The figure
compares the bottom emitting device with the transparent device. It
can be seen that the devices have the same behavior; the measured
points almost completely overlap each other. The extrapolated
lifetime is in excess of 10,000 h.
[0126] FIG. 5 shows the comparison of the optical transmittance of
an exemplary device of the described subject matter (102) compared
with an existing device (103). The device 102 exhibits superior
transmittance in comparison with the device 103. Note that the
transmittance was measured through the glass substrate and through
the encapsulation substrate and is greater than 70% in the visible
range and greater than 75% between 460 nm and 800 nm. In contrast,
the device 103 exhibits a transmittance less than 62% and largely
less than 50% of a larger range of the visible spectrum. The
transmission spectra of the device 103 is also more wavelength
dependent (i.e., the spectra is less flat and has stronger color).
The transmittance of the glass substrate with the ITO (101) is also
shown for comparison purposes.
[0127] In the embodiment shown in FIGS. 1a-b, no space charge zone
occurs at the contacts. This embodiment has a high energy barrier
for the charge-carrier injection. This high energy barrier, under
certain circumstances, cannot be overcome or overcome with
difficulty when using available materials. Hence, the injection of
charge carriers from the contacts is less effective. The OLED shows
an increased operating voltage.
[0128] According to the described subject matter, increased
performance is achieved, in some embodiments, by transparent OLEDs
with doped injection and transport layers, optionally in
combination with blocking layers. FIG. 2a shows one exemplary
arrangement. In this embodiment, the charge-carrier-injecting and
conducting layers 3 and 7 are doped, so that space charge zones are
formed at the interfaces to contacts 2 and 8. The doping is
sufficient to allow for the space charge zones to be easily
tunneled through. Such doping has been shown to be possible for the
p-doping of the hole transport layer for non-transparent
light-emitting diodes (e.g., X. Q. Zhou et al., Appl. Phys. Lett.
78, 410 (2001); J. Blochwitz et al., Organic Electronics 2, 97
(2001)).
[0129] The foregoing arrangements exhibit various characteristics:
(1) increased injection of charge carriers from the electrodes into
the doped charge-carrier transport layers; (2) independence from
the detailed preparation of the charge-carrier-injecting materials
2 and 8 (e.g., (I) injection layers may not be required if doping
is used; (II) the layers which contact the electrodes may not need
"special" treatment to improve injection (such as annealing,
surface modification of ITO, etc); (III) arrangements such as
inverted structures with the ETL on the bottom electrode, i.e.,
cathode on the substrate or non-inverted structures can be created
without great constraints); (3) gives the option of choosing, for
the electrodes 2 and 8, materials having comparatively high
barriers for the charge-carrier injection (e.g., the same material
in both cases such as ITO).
EXAMPLES
Example 1
[0130] In the following example, the electron transport layer is
not yet n-doped with stable large organic dopants. An embodiment
with the nonstable n-doping of a known electron transport material
(Bphen=bathophenanthroline) with Li demonstrates the effectiveness
of the transparent OLED with doped organic transport layers (U.S.
Pat. No. 6,013,384 (J. Kido et al.), applied for on Jan. 22, 1998;
J. Kido et al., Appl. Phys. Lett. 73, 2866 (1998)). This
approximately 1:1 mixture of Li and Bphen demonstrates the
effectiveness of the doping. This layer is not stable thermally and
operationally. It is assumed that the mechanism of doping is
different because of the high doping concentration. On doping with
organic molecules and doping ratios of between 1:10 and 1:10000, it
can be assumed that the dopant does not significantly affect the
structure of the charge-carrier transport layer. Where the
concentration is 1:1 of doping metals, e.g., Li, the same cannot be
assumed.
[0131] The OLED in Example 1 has the following layer structure
(inverted structure): [0132] 1 a Substrate, e.g., glass; [0133] 2 a
Cathode: ITO as purchased, untreated; [0134] 3 a n-Doped
electron-transporting layer: 20 nm Bphen:Li, 1:1 molecular mixing
ratio; [0135] 4 a Electron-side blocking layer: 10 nm Bphen; [0136]
5 a Electroluminescent layer: 20 nm Alq 3, may be mixed with
emitter dopants in order to in-create the internal quantum yield of
the light production; [0137] 6 a Hole-side blocking layer: 5 nm
triphenyldiamine (TPD); [0138] 7 a p-Doped hole-transporting layer:
100 nm Starburst m-MTDATA 50:1 doped with F 4-TCNQ dopant
(thermally stable to about 80 degrees C.); [0139] 8 a Transparent
electrode (anode) indium tin oxide (ITO).
[0140] The mixed layers 3 and 7 are prepared by a vapor deposition
process in vacuo by mixed evaporation. In principle, such layers
can also be prepared by other processes as well, such as, e.g.,
vapor deposition of the substances one upon the other, followed by
a possibly temperature-controlled diffusion of the substances into
one another; or by another type of deposition (e.g., spin-on
deposition) of the already mixed substances in or outside of
vacuum. The blocking layers 3 and 6 are likewise vapor-deposited in
vacuo, but can also be prepared by another process, e.g., by
spin-on deposition in or outside of vacuum.
[0141] FIG. 3 shows the luminance vs. voltage curve of a
semitransparent OLED. For test purposes, a semitransparent gold
contact (e.g., 50% transmission) was used. For a luminance of 100
cd/M 2 an operating voltage of 4 V is used. This value represents a
low operating voltage for transparent OLEDs, especially those with
an inverted layer structure. This OLED demonstrates the feasibility
of the described subject matter. Because of the semitransparent
cover electrode, the external current efficiency is limited to a
value of about 2 cd/A, short of 5 cd/A as expected for OLEDs with
pure Alq3 as the emitter layer.
[0142] Devices of the described subject matter demonstrate
increased efficiency, lifetime, and transparency and decreased
voltage.
[0143] The described devices can be fabricated more easily and
reliably than existing OLEDS. The use of doping layers allows for
directly depositing transparent conductive oxides over a charge
carrier transport layer without the necessity of buffer light
absorbing buffer layers such as CuPc or metal layers. Also, a
multi-step deposition procedure for the ITO is not necessary.
[0144] The following examples demonstrate the features of the OLEDs
of the described subject matter. For reference, the OLED
performance is compared with bottom emitting OLEDs that are made in
the same batch. The bottom emitting OLEDs are produced in parallel
with the transparent OLEDs. The difference is that, on the bottom
emitting devices, Al is deposited as a cathode instead of ITO.
[0145] Using a non-optimized structure, a variation of the ETM
doped with ED-8 demonstrates some favorable ETMs. The structure was
made on Glass/ITO substrate with the following layer sequence: 50
nm of NPD p-doped with OA-11; 10 nm of NPD as EBL; emitter host
doped with 0.5 wt % of rubrene; 10 nm of ETM-6 as HBL; the ETM in
the following table doped with ED-8 followed by 100 nm of ITO.
TABLE-US-00002 Voltage increase at 10 mA/cm{circumflex over ( )}2,
Doping compared to the reference (e.g., ETM concentration the
reference cathode is Ag) Alq3 10 2 V ETM-4 8 1.8 V ETM-6 8 0.8 V
ETM-11 8 0.8 V ETM-6 10% + 1 nm 0.4 V pure dopant ETM-6 10% + 1 nm
0.4 V pure dopant
[0146] The following layer sequence is a non-optimized OLED
structure which was used for the experiments (the thickness is
given in parenthesis):
[0147] ITO (90 nm)
[0148] NPD (50 nm) doped with 3 wt % of OA-11
[0149] NPD (10 nm)
[0150] DNP:Alq3:DCJTB in the ratio 70:29:1 (20 nm)
[0151] ETM-6 (10 nm)
[0152] ETL=n-doped ETM (70 nm)
[0153] top ITO (100 nm)
[0154] The comparative bottom emitting devices have a 100 nm
Aluminum layer in place of the top ITO layer.
[0155] A variation of the n-doping concentration of the ETL
demonstrates a favorable doping concentration. The structure was
made on Glass/ITO substrate with the following layer sequence: 50
nm of NPD p-doped with OA-11; 10 nm of NPD as EBL; emitter host
doped with 0.5 wt % of rubrene; 10 nm of ETM-6 as HBL; the ETM:
dopant system in the following table; followed by 100 nm of
ITO.
TABLE-US-00003 Voltage increase at 10 mA/cm{circumflex over ( )}2,
ETM: dopant Doping compared to the reference (e.g., system
concentration the reference cathode is Ag) ETM-4: ED-8 8 1.53 V
ETM-6: ED-8 2 2.7 V ETM-6: ED-8 4 1.5 V ETM-6: ED-8 8 0.9 V ETM-11:
ED-14 8 0.8 V ETM-11: ED-8 8 1.1 V ETM-11: ED-14 10 0.76 V
[0156] It can be seen in the table above that the optimum doping
concentration to achieve a low voltage, with a comparative voltage
increase of less than 1 V, compared to the bottom emitting device,
is higher than or equal to 8%. A dopant concentration greater than
25% is less desirable in the ETL. However, a highly doped buffer
can be used in addition to the doped ETL.
[0157] Another embodiment includes a thin (e.g., 1 to 15 nm thick)
highly doped charge transport layer at the interface of the top
electrode (this layer being localized between the light-emitting
region and the electrode). Another embodiment includes a thin
(e.g., 0.5 nm to 3 nm) pure dopant layer as a buffer layer at the
interface of the top electrode (between the charge transport layer
and the top electrode).
ED-8:doped ETM with 1 nm ED-8 interlayer
TABLE-US-00004 Voltage increase at Voltage increase at 10
mA/cm.sup.2, compared to the 100 cd/m.sup.2, compared to the
reference (e.g., the reference reference (e.g., the reference ETM
cathode is Al) cathode is Al) ETM-4 1 0.75 ETM-6 0.43 0.30 ETM-9
0.39 0.31 ETM-11 0.19 0.26 ETM-4 0.84 0.59 ETM-6 0.47 0.27 ETM-9
0.67 0.44 Voltage at 10 Voltage at 10 Voltage at 10 mA/cm.sup.2 for
ED-8 mA/cm.sup.2 for ED-14 mA/cm.sup.2 for ED-8 ETM (100
cd/m.sup.2) (100 cd/m.sup.2) (100 cd/m.sup.2) ETM-4 (2.72) ETM-6
2.77 (2.4) 2.58 (2.27) ETM-9 3.09 (2.63) 2.76 (2.42) 2.6 (2.28)
ETM-11 3.9 (3.5) 2.55 (2.79) ETM-40 2.9 (2.69)
[0158] Similar results as those with ED-14 were obtained with ED-3
and ED-4.
[0159] The devices of the described subject matter exhibit a good
life-time behavior. The time before the device exhibits half of the
initial brightness can be more than 10,000 h, under accelerated
aging (See FIG. 4).
Comparative Examples
[0160] Comparative devices were constructed according to known
techniques, without using doped layers. The anode (e.g, ITO) was
treated with oxygen plasma before the deposition of the organic
layers, to enhance the hole injection. A thin layer of Mg:Ag with
an atomic ratio of 40:1 was deposited, as part of the cathode
(e.g., electron injection layer), on top of the organic layers. A
sputtered ITO layer followed the thin metal layer.
[0161] The performance of the comparative device is poor, even if
the same organic stack is used. The comparative devices exhibit a
voltage (at a current density of 10 mA/cm2) more than 1 V higher.
The (cd/A) efficiency is reduced due to the additional absorption
of the thin metal layer. The overall power efficiency is further
reduced because of the additional effects of the absorption of the
metal layer and the increased operating voltage.
[0162] The samples with doped layers exhibit a higher yield,
especially the samples using the described diazapyrenes,
asymmetrical phenanthrolines, and metal quinoxalines as doped
ETM.
[0163] The comparative devices exhibited a low yield. Many included
short circuits immediately after being produced. The cause is
believed to be due to metal diffusion and sputter damage during
metal and ITO deposition. the doped layers can improve the
robustness of the device, not only against the sputtering process.
By using doped layers (with organic doping), the yield and device
efficiency can be higher, e.g., because these layers offer
protection against sputtering. The doping effect can be stable and
strong such that even after sputtering, the device performs
well.
Example Embodiment
[0164] An example OLED has the following layer structure
(non-inverted structure):
[0165] Substrate, glass
[0166] Anode, ITO (90 nm)
[0167] doped hole transport layer, NPD (50 nm) doped with 3 wt % of
OA-11
[0168] non doped interlayer NPD (10 nm) (optionally an electron
blocking layer)
[0169] Emitter layer DNP:Alq3:DCJTB in the ratio 70:29:1 (20
nm)
[0170] non doped interlayer E.TM.-6 (10 nm) (optionally a hole
blocking layer)
[0171] electron transport layer, ETM-6 n-doped with 10 wt % ED-14
(70 nm)
[0172] Cathode, ITO (100 nm)
[0173] Another example OLED with an inverted structure has:
[0174] Substrate, glass
[0175] Cathode, ITO (e.g., 100 nm)
[0176] electron transport layer, ETM-6 n-doped with 4 wt % ED-14
(40 nm)
[0177] non doped interlayer E.TM.-6 (10 nm) (optionally a hole
blocking layer)
[0178] Emitter layer DNP:Alq3:DCJTB in the ratio 70:29:1 (20
nm)
[0179] non doped interlayer NPD (10 nm) (optionally an electron
blocking layer)
[0180] doped hole transport layer, NPD (80 nm) doped with 8 wt % of
OA-11
[0181] Anode, ITO (90 nm)
[0182] FIG. 6 shows the luminance vs. voltage curve of a
transparent OLED according to an embodiment with a non-inverted
structure. For a luminance of 100 cd/m.sup.2 an operating voltage
of 2.14 V is used. This operating voltage is one of the lowest
voltages for transparent OLEDs.
[0183] The high transparency and the flatness of the optical
transmittance of the inventive OLEDs are especially useful for
white OLEDs. White OLEDs were constructed by different methods,
such as mixing multiple emitters in one light-emitting region, or
stacking OLEDs through so-called connecting units.
[0184] The use of doped layers according to the described subject
matter makes it possible to attain nearly the same low operating
voltages and high efficiencies in a transparent structure as occur
in a traditional structure with one-sided emission through the
substrate. This is due, as described, to the efficient
charge-carrier injection, which, thanks to the doping, is
relatively independent of the exact work function of the
transparent contact materials. In this way the same electrode
materials (or, e.g., transparent electrode materials of only
slightly different work functions) can be used as
electron-injecting contacts and hole-injecting contacts.
[0185] From the examples and the knowledge of one ordinarily
skilled in the art, it is obvious to a person skilled in the art
that many modifications and variations of the described subject
matter are possible which fall within the scope of the described
subject matter. For example, transparent contacts other than ITO
can be used as anode materials (e.g., as in H. Kim et al., Appl.
Phys. Lett. 76, 259 (2000); H. Kim et al., Appl. Phys. Lett. 78,
1050 (2001)). Furthermore, some embodiments include transparent
electrodes made by combining a sufficiently thin intermediate layer
of a nontransparent metal (e.g., silver or gold) and a thick layer
of the transparent conductive material. In that case, the thickness
of the intermediate layer is thin enough so that the device is
still transparent (e.g., 75% transparent in the entire visible
spectral region). Because of the thick doped charge-carrier
transport layers, no damage to the light-emitting layers is to be
expected during sputter. A further embodiment uses, for the doped
electron transport layer, a material whose LUMO level is too deep
(in the sense of FIGS. 1a-b and 2a-b layers 7 or 3a) to be able to
efficiently inject electrons into the blocking layer and
light-emitting layer (6 or 4a, and 5 or 5a, respectively) (thus,
greater barriers than those shown in FIG. 2a). In that case, it is
possible to use between the n-doped electron transport layer (7 or
3a) and blocking layer (6 or 4a) or the light-emitting layer (5 or
5a) a thin (2.5 nm) layer of a metal having a lower work function
than the LUMO level of the doped transport layer. The metal layer
is thin enough so that the overall transparency of the component is
mostly maintained (see L. S. Hung, M. G. Mason, Appl. Phys. Lett.
78, 3732 (2001)).
* * * * *