U.S. patent application number 13/257459 was filed with the patent office on 2012-03-29 for organic zener diode, electronic circuit, and method for operating an organic zener diode.
This patent application is currently assigned to NOVALED AG. Invention is credited to Kentaro Harada, Karl Leo, Frank Lindner, Bjoern Luessem.
Application Number | 20120075013 13/257459 |
Document ID | / |
Family ID | 42207950 |
Filed Date | 2012-03-29 |
United States Patent
Application |
20120075013 |
Kind Code |
A1 |
Leo; Karl ; et al. |
March 29, 2012 |
Organic Zener Diode, Electronic Circuit, and Method for Operating
an Organic Zener Diode
Abstract
This disclosure relates to an organic zener diode having one
electrode and one counter electrode, and an organic layer
arrangement formed between the electrode and the counter electrode,
wherein the organic layer arrangement includes the following
organic layers: an electrically n-doped charge carrier injection
layer on the electrode side, made from a mixture of an organic
matrix material and an n-dopant, an electrically p-doped charge
carrier injection layer on the counter electrode side, made from a
mixture of another organic matrix material and a p-dopant, and an
electrically undoped organic intermediate layer that is arranged
between the electrically n-doped charge carrier injection layer on
the electrode side and the electrically p-doped charge carrier
injection layer on the counter electrode side. An electronic
circuit arrangement with an organic zener diode and method for
operating an organic zener diode are also provided.
Inventors: |
Leo; Karl; (Dresden, DE)
; Harada; Kentaro; (Tokyo, JP) ; Lindner;
Frank; (Dresden, DE) ; Luessem; Bjoern;
(Dresden, DE) |
Assignee: |
NOVALED AG
Dresden
DE
|
Family ID: |
42207950 |
Appl. No.: |
13/257459 |
Filed: |
March 19, 2010 |
PCT Filed: |
March 19, 2010 |
PCT NO: |
PCT/DE2010/000332 |
371 Date: |
December 2, 2011 |
Current U.S.
Class: |
327/584 ; 257/40;
257/E51.008 |
Current CPC
Class: |
H01L 29/866 20130101;
H01L 51/4293 20130101; Y02E 10/549 20130101; H01L 51/0583 20130101;
H01L 51/002 20130101; H01L 51/0072 20130101; H01L 51/0081 20130101;
H01L 2251/308 20130101 |
Class at
Publication: |
327/584 ; 257/40;
257/E51.008 |
International
Class: |
H01L 29/866 20060101
H01L029/866; H01L 51/10 20060101 H01L051/10 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 20, 2009 |
DE |
102009013685.1 |
Claims
1. A zener diode comprising one electrode, one counter electrode,
and an organic layer arrangement, wherein the organic layer
arrangement is in electrical contact with the electrode and counter
electrode, and wherein the organic layer arrangement comprises: an
n-doped charge carrier injection layer, wherein the n-doped charge
carrier injection layer comprises a mixture of a first organic
matrix material and an n-dopant, a p-doped charge carrier injection
layer, wherein the p-doped charge carrier injection layer comprises
a mixture of a second organic matrix material, and a p-dopant,
wherein the p-doped charge carrier injection layer is arranged
closer to the counter electrode than the n-doped charge carrier
injection layer, and an electrically undoped organic intermediate
layer, wherein the electrically undoped organic intermediate layer
is arranged between the n-doped charge carrier injection layer and
the p-doped charge carrier injection layer.
2. The zener diode as recited in claim 1, wherein the n-dopant or
the p-dopant is a molecular dopant.
3. The zener diode as recited in claim 1, wherein the electrically
undoped organic intermediate layer has unipolar charge carrier
transport properties such that the mobility for charge carriers in
the form of electrons and the mobility for charge carriers in the
form of holes are different.
4. The zener diode as recited in claim 1, wherein the electrically
undoped organic intermediate layer has ambipolar charge carrier
transport properties such that the mobility for charge carriers in
the form of electrons and the mobility for charge carriers in the
form of holes are substantially the same.
5. The zener diode as recited in claim 4, wherein the electrically
undoped organic intermediate layer consists of one organic
material.
6. The zener diode as recited in claim 4, wherein the electrically
undoped organic intermediate layer comprises a mixture of multiple
organic materials.
7. The zener diode as recited in claim 1, wherein the electrically
n-doped charge carrier injection layer comprises the first organic
matrix material and the n-dopant in a ratio of at least 1 mol %
dopant to matrix material, and the electrically p-doped charge
carrier injection layer comprises the second organic matrix
material and the organic p dopant n a ratio of at least 1 mol %
dopant to matrix material.
8. The zener diode as recited in claim 1, wherein the n-doped
charge carrier injection layer and the p-doped charge carrier
injection layer are electrically doped via metal ions.
9. The zener diode as recited in claim 1, wherein the first organic
matrix material and the second organic matrix material are the
same, and the electrically undoped organic intermediate layer
comprises the first organic matrix material.
10. The zener diode as recited in claim 1, wherein the electrically
undoped organic intermediate layer has a layer thickness between
about 1 Angstrom and about 100 nm.
11. The zener diode as recited in claim 1, wherein at least one of
the following layers contains at least one inorganic material: the
electrically n-doped charge carrier injection layer, the
electrically p-doped charge carrier injection layer, and the
electrically undoped organic intermediate layer.
12. The zener diode as recited in claim 1, wherein at least one of
the electrically n-doped charge carrier injection layer, the
electrically p-doped charge carrier injection layer, and the
electrically undoped organic intermediate layer, comprises at least
one organic material selected from the following group of organic
materials: oligomer material and polymer material.
13. An electronic circuit arrangement comprising an organic zener
diode and a storage element, wherein the organic zener diode
comprises one electrode, one counter electrode, and an organic
layer arrangement, wherein the organic layer arrangement is in
electrical contact with the electrode and counter electrode, and
wherein the organic layer arrangement comprises: an n-doped charge
carrier injection layer, wherein the n-doped charge carrier
injection layer comprises a mixture of a first organic matrix
material and an n-dopant, a p-doped charge carrier injection layer,
wherein the p-doped charge carrier injection layer comprises a
mixture of a second organic matrix material and a p-dopant, wherein
the p-doped charge carrier infection layer is arranged closer to
the counter electrode than the n-doped charge carrier injection
layer, and an electrically undoped organic intermediate layer,
wherein the electrically undoped organic intermediate layer is
arranged between the n-doped charge carrier injection layer and the
p-doped charge carrier injection layer.
14. A method for operating an organic zener diode that is connected
in sequence with at least one other component comprising: applying
an electrical voltage to an electrode and a counter electrode of
the organic zener diode, wherein the electrical voltage is limited
to the value of the breakdown voltage, wherein a protective state
is created for components that are connected in sequence with the
organic zener diode, and draining the current flow created by the
applied voltage via the organic zener diode.
15. The zener diode as recited in claim 1, wherein the first
organic matrix material and the second organic matrix material are
the same.
16. The zener diode as recited in claim 10, wherein the
electrically undoped organic intermediate layer has a layer
thickness between about 1 nm and about 10 nm.
Description
FIELD OF THE INVENTION
[0001] The invention relates to an organic zener diode, an
electronic circuit and a method for operating an organic zener
diode.
BACKGROUND OF THE INVENTION
[0002] The steady advances in microelectronics are causing
structures to shrink continuously, even as more and more components
are being place on a given area. This trend is also evident in the
development of larger and larger data memories. The classic
silicon-based semiconductor technology is nearing its limits for
physical and financial reasons, and will soon not be able to keep
pace with the drive towards miniaturisation. Components that are
being manufactured today have structure sizes of several tens of
nanometres. New concepts and materials are needed that will enable
the structure sizes and thus entire components to be shrunk yet
further, to a few nanometres.
[0003] The demand for novel, inexpensive electronics, preferably
providing functionalities on flexible substrates continues to grow.
For example applications such as intelligent admission cards,
extremely inexpensive transponder labels or electronics integrated
in clothing are conceivable. Besides their other requirements, all
such applications also need memory components. Microelectronics
based on crystalline semiconductors can only offer a limited level
of functionality for these.
[0004] Passive storage concepts have the advantage of a relatively
simple construction and the ability to be integrated easily in 3D
concepts. Resistive storage concepts, that is to say memories that
can assume various electrical resistances and thus store
information content, are viewed as promising for purposes of mass
storage in the future because of their scalability to the molecular
magnitude. A simple construction in crossbar technology enables
these components to be produced cheaply and integrated in 3D
concepts. One disadvantage of this construction is that it is
susceptible to crosstalk with adjacent cells when programming or
deleting individual elements. In order to prevent this, and to
enable larger memory arrays to be produced, additional active and
passive components are necessary. One option consists in connecting
each memory cell individually to a zener diode. Thus, crosstalk is
prevented by the strongly non-linear characteristic curve. Zener
diodes are easy to implement and are used widely in classic silicon
technology to stabilise voltages and protect important modules from
destruction.
[0005] These diodes behave like normal diodes in the forward bias
direction, but in the reverse bias direction their resistance
suddenly falls dramatically above a certain voltage, the breakdown
voltage. The breakdown voltage can be adjusted from 3 to 100V by
selectively changing the doping of the electron-conducting layer
and/or the hole conducting layer and the modification this brings
about in the width of the depletion layer. Zener diodes are
currently also used in passive matrix memories. Since these
crossbar memories are theoretically scalable down to the molecular
level, silicon technology will shortly reach its limits in this
field as well.
[0006] Accordingly, the search for alternative methods and
materials to replace the classic silicon technology is being
conducted intensively all over the world.
[0007] Organic electronics has emerged as a promising alternative
to silicon-based electronics. Among its advantages is the fact that
it involves relatively simple processes such as printing or vapour
deposition at low temperatures, the ability to work on flexible
substrates, and the wide variety of molecular materials.
[0008] The filed of organic electronics is having its first
applications in organic light emitting diodes (OLEDs).
[0009] Following a relatively short development period, these can
already be found in many devices. Even now, in the research stage,
the efficiencies of these OLEDs are reaching record values that
most other light sources cannot rival. The development of OLEDs
provides an indication of the potential that is as yet untapped in
organic electronics. However, before organic electronics can be
treated as a fully developed system, it is necessary to produce not
only light emitting diodes but also organic transistors, organic
memories and other components in order to take full advantage of
the cost benefit in production and to avoid having to rely on a
combination of organic electronics and classic silicon technology.
Besides organic transistors, organic solar cells are the subject of
considerable research efforts all over the world. Although they do
not yet offer the same efficiency levels as classic solar cells,
they are easy to produce, and as such have the potential for an
enormous cost advantage over silicon solar cells. As the number of
components increases, components that protect the primary
electronics from external influences are needed in organic
electronics as well. Voltage stabilisation and overvoltage
protection are important considerations, among others.
[0010] A number of organic thin film zener diodes consisting of one
or more organic layers are known. Several different approaches for
such diodes are described in US 2004/0051096 A 1. Up to three
organic layers of various materials are applied between two
electrodes. The zener voltage can be adjusted through appropriate
selection of the organic material, electron-conducting
(n-conducting) or hole-conducting (p-conducting) for example. The
zener voltage can be changed by altering the sequence of layers of
organic materials. This document will also show that different
zener voltages also result from different electrodes. With the
appropriate selection of material, it is possible to achieve zener
voltages in the range from 0.1 V to 7V. If a specific zener voltage
is required, it is possible with a suitable combination of organic
material, electrodes, and layer structure. At the same time,
however, the current-voltage curve is also altered in the forward
direction, which represents a significant drawback. In the forward
direction, it is desirable for the diode behaviour to remain as
consistent as possible for different zener voltages. Another
disadvantage is that only certain electrode materials and
combinations can be used for a given zener voltage. This places
marked limitations on design freedom.
[0011] Another problem with this design is the poor electrical
contact properties between the electrodes and the organic material.
Injection of charge carriers is hindered by large barriers for
electrons as well as holes at the respective boundary surfaces
between the organic layers and the metal contacts.
[0012] Finally, electrical conductivity in undoped layers is highly
sensitive to the layer thickness (a cubic dependency is expected
under the precondition of ohmic injection: M. A. Lampert et. al,
Current injection in solids, Academic, New York, 1970). As a
result, the approaches in production described in US 2004/0051096
A1 are vulnerable to inconsistencies in the production process.
[0013] The object of the invention is to provide an improved zener
diode, of simple construction and offering improved performance in
conjunction with the breakdown voltage. The zener diode should
demonstrate stable, reproducible behaviour, and it should be
possible to adjust the breakdown voltage without altering the
forward bias characteristic curve.
SUMMARY OF THE INVENTION
[0014] This object is solved with an organic zener diode as
described in independent claim 1. In addition, electronic circuits
as described in independent claims 13 and 14 and a method for
operating an organic zener diode as described in independent claim
15 are also provided. Advantageous configurations of the invention
are the object of dependent claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] In the following, the invention will be explained in greater
detail using exemplary embodiments and with reference to the
figures of a drawing. In the drawing:
[0016] FIG. 1 is a schematic representation of a layer sequence for
an organic zener diode,
[0017] FIG. 2 is a current-voltage curve of an ideal zener
diode,
[0018] FIG. 3 is a schematic representation of a layer sequence for
an organic zener diode according to FIG. 2 with modifiable
transport layer,
[0019] FIG. 4 is a current-voltage curve for a first embodiment
with a 5 nm thick intermediate layer of TCTA:TPBI in a ratio of
1:1,
[0020] FIG. 5 is a current-voltage curve for a second embodiment
with a 10 nm thick intermediate layer of TCTA:TPBI in a ratio of
1:1,
[0021] FIG. 6 is a current-voltage curve for organic zener diodes
according to FIG. 1 with various intermediate layer thicknesses of
TCTA:TPBI in a ratio of 1:1,
[0022] FIG. 7 is a current-voltage curve for organic zener diodes
according to FIG. 1 with a 5 nm thick intermediate layer of
Balq:NPB in a ratio of 1:1,
[0023] FIG. 8 is a current-voltage curve for organic zener diodes
according to FIG. 1 with a 5 nm thick intrinsic intermediate layer
of the same material that is used as the matrix for the charge
carrier injection layers,
[0024] FIG. 9 is a current-voltage curve for organic zener diodes
with structure according to FIG. 1 and having a 7 nm thick
intrinsic intermediate layer of the same material that is used as
the matrix for the charge carrier injection layers, for different
doping concentrations of the hole conducting injection layer,
[0025] FIG. 10 is a current-voltage curve for organic zener diodes
with structure according to FIG. 1 and having a 7 nm thick
intrinsic intermediate layer of the same material that is used as
the matrix for the charge carrier injection layers, for different
doping concentrations of the electron conducting injection
layer.
[0026] FIG. 11 is a current-voltage curve for organic zener diodes
with structure according to FIG. 1 and having a 30 nm thick
intrinsic intermediate layer of a single molecule for the ambipolar
"low-gap" material pentacene, and
[0027] FIG. 12 is a current-voltage curve for organic zener diodes
with structure according to FIG. 1 and having an 8 nm thick
intrinsic intermediate layer of the unipolar materials Balq and
NPB.
DETAILED DESCRIPTION OF THE INVENTION
[0028] The reverse bias breakdown voltage of the organic zener
diode is adjustable simply by altering the thickness of the
intermediate layer. Alternatively or in addition thereto, the
reverse bias breakdown voltage may also be adjusted by changing the
doping concentration of the hole conducting charge carrier
transport layer and/or the doping concentration of the electron
conducting charge carrier transport layer. In this case, adjusting
the reverse bias breakdown voltage has no effect on the forward
bias behaviour of the diode. This results in an advantageous
capability to manufacture organic zener diodes having different
breakdown voltages simply and reproducibly.
[0029] Advantages over the prior art also consist particularly in
the fact that a semiconductor component of such kind may be
manufactured inexpensively using standard manufacturing methods.
Whereas the forward bias characteristic curve is difficult to
control in organic zener diodes that consist of only one organic
layer and two electrodes, with the invention the breakdown
behaviour is controllable, stable, and reproducible in both the
forward and reverse bias directions.
[0030] The charge carrier injection layer and the intermediate
layer may include inorganic materials.
[0031] A preferred refinement of the invention provides that the
n-dopant and/or the p-dopant is a molecular dopant. Because of the
relatively high current densities in the working range of organic
zener diodes, diffusion of the doping ions or doping molecules is
to be expected. Because of their size, the likelihood that
molecular dopants will be diffused is many times smaller than the
likelihood that ions will be diffused. Accordingly, it is possible
to operate the component at significantly higher current densities
and thus also at higher temperatures.
[0032] Doping with organic materials enables the use of "high gap"
materials. The use of these materials with a large energy gap
enables the manufacture of transparent components. The great
advantage of these is that visible light is neither absorbed nor
emitted. Consequently, these components may be used in direct
combination with OLED displays, for example.
[0033] Organic dopants as such are described for example in EP 1
988 587. The dopants described in examples 1 to 9 in that document
are preferred for the use under consideration. Other preferred
p-dopants are described in US 2005/0139810. Preferred n-dopants are
also disclosed in the documents US 2005/0061231, WO 2005/086251 as
well as EP 1 837 926 and EP 1 837 927. Preferred hole transport
materials (HTM semiconductors that are dopable by a p-dopant and
transport holes) are described for example in the document EP 1 988
587. Preferred electron transport materials (ETM semiconductors
that are dopable by an n-dopant and transport electrons) include
for example BPhen, BCP or other phenanthroline derivatives, Alq3,
C60, PTCBI, PTCDI, TCNQ, PBD, OXD, TAZ, TPOB, BAlq.
[0034] In one practical configuration of the invention, it may be
provided that the electrically undoped organic intermediate layer
has unipolar charge carrier transporting properties, so that the
mobility for charge carriers in the form of electrons differs from
the mobility for charge carriers in the form of holes. It is
preferred if |.mu.h/.mu.e| or |.mu.e/.mu.h| is greater than 10,
more preferably it is greater than 1000.
[0035] According to an advantageous embodiment of the invention,
the electrically undoped organic intermediate layer has ambipolar
charge carrier transporting properties, so that the mobility for
charge carriers in the form of electrons and the mobility for
charge carriers in the form of holes are essentially the same. In
order to enable a steep rise in the forward bias characteristic
curve and still keep voltages low, the intermediate layer should
preferably consist of an ambipolar material. This ensures that both
electrons and holes are involved in transporting the charge in the
forward direction, which in turn means that relatively high
currents are achieved even with low voltages.
[0036] A refinement the invention provides that the electrically
undoped organic intermediate layer preferably contains or consists
of exactly one organic material.
[0037] In an advantageous configuration of the invention, it may be
provided that the electrically undoped organic intermediate layer
contains or consists of a mixture of several organic materials.
[0038] A development of the invention may provide that the
electrode-side electrically n-doped charge carrier injection layer
contains the organic matrix material and the organic n-dopant in a
ratio of at least 1 mol % dopant to matrix material, and the
electrically p-doped charge carrier injection layer on the counter
electrode side contains the organic matrix material and the organic
p-dopant in a ratio of at least 1 mol % dopant to matrix material.
In another preferred configuration, the ratio is at least 2 mol %.
It is further preferred if the doping concentration of the doped
layers is at least 4 mol %.
[0039] A preferred refinement of the invention provides that the
charge carrier injection layers on the electrode and the counter
electrode sides are each electrically doped with metal ions.
[0040] According to a practical configuration of the invention, it
may be provided that the organic matrix material and the additional
organic matrix material are the same, and that the electrically
undoped organic intermediate layer contains the same organic matrix
material. In one configuration, the material for the injection
layers is used as the matrix material, and is n-doped or p-doped
respectively. In the intermediate layer, this material is used
undoped in its intrinsic form. A combination of this kind is
referred to as "homojunction".
[0041] An advantageous embodiment of the invention provides that an
electrically undoped organic intermediate layer having a layer
thickness between about 1 Angstrom and about 100 nm, preferably
between about 1 nm and about 10 nm, is formed.
[0042] A refinement of the invention preferably provides that at
least one of the following layers contains at least one inorganic
material: the electrically n-doped charge carrier injection layer
on the electrode side, the electrically p-doped charge carrier
injection layer on the counter electrode side, and the electrically
undoped organic intermediate layer.
[0043] In an advantageous configuration of the invention, it may be
provided that at least one of the organic layers, that is to say
the electrically n-doped charge carrier injection layer on the
electrode side, the electrically p-doped charge carrier injection
layer on the counter electrode side, and the electrically undoped
organic intermediate layer, contains at least one organic material
selected from the following group of organic materials: oligomer
material and polymer material.
[0044] A small energy barrier is preferably smaller than 0.5 eV,
more preferably 0 eV. The energy barrier is considered to be
barrier to the charge carrier injection of the charge carrier
injection layer into the intermediate layer when the component is
used in normal diode operation. The low barrier is preferred in
order to obtain the lowest possible threshold voltages and the
steepest possible characteristic curves.
[0045] In this context, the layers that are arranged between the
two electrodes are referred to as active layers. They may comprise
organic materials; in particular, the technical term used for these
molecules in the field of organic semiconductors is "small
molecules". The active layers may also comprise oligomers. The
active layers may also comprise polymers.
[0046] The layers, specifically the electrodes, the injection
layers, the semiconductor layers and/or the intermediate layers,
are preferably produced via one of the following methods: [0047]
Vacuum evaporation: This is the usual method for producing very
thin layers. The organic layers are evaporated mainly by thermal
evaporation or PVD ("Physical Vapour Deposition"). The inorganic
layers can be separated by thermal evaporation, sputtering, laser
ablation, spray pirolisys, CVD ("Chemical Vapour Deposition") and
other methods. These methods do not necessarily have to take place
in a vacuum, they may also be carried out in a shielding gas
atmosphere. [0048] Wet chemical procedures or deposition from
solution: This includes methods such as "spincoating", "blade-gap
coating", "stamping", printing (ink-jet) or similar. [0049]
"Organic vapour phase deposition": The production of mixed layers
by this method is explained in EP 1 780 816 A1 (see paragraphs
[0011] to [0013]). The production of doped layers by this method is
described in EP 1 780 816 A1 (see paragraphs [0017] to [0019]).
[0050] The deposition of the layers is always carried out onto a
substrate or onto previous layers that have already formed on the
substrate. Optionally, the substrate may also serve another
function besides just its carrier function. For example, the
substrate may be conductive and may also form the electrode of the
diode.
[0051] Other preferred aspects of the invention will be explained
in the following.
[0052] Operation of an organic diode in reverse bias with current
breakdown may be provided so that current essentially flows through
the diode, wherein the diode comprises the following layers between
two conductive electrical contacts: an electrically n-doped organic
semiconductor layer, and electrically undoped organic semiconductor
layer, and an electrically p-doped organic semiconductor layer.
[0053] Operation of an organic diode in reverse bias with current
breakdown may also be provided so that current essentially flows
through the diode, wherein the diode has layers between two
conductive electrical contacts (electrodes) in the following order:
an electrically n-doped organic semiconductor layer, and
electrically undoped organic semiconductor layer, and an
electrically p-doped organic semiconductor layer.
[0054] Further, a method may be provided for operating an organic
semiconductor element, particularly an organic zener diode, having
one electrode and one counter electrode as well as an organic layer
arrangement formed between the electrode and the counter electrode
and in electrical contact therewith, wherein the organic layer
arrangement comprises the following organic layers: a charge
carrier injection layer on the electrode side, a charge carrier
injection layer on the counter electrode side, and an intermediate
layer area arranged between the two, wherein a protective state for
subsequent components is achieved during the process by applying an
electrical voltage greater than the breakdown voltage so that the
electrical voltage is limited to the breakdown voltage value and
draining the current flow created by the applied voltage via the
organic zener diode.
[0055] The organic zener diode is preferably used in combination
with a storage element.
[0056] The invention further encompasses the idea of an organic
electronic semiconductor element with an electrode and a counter
electrode, and an organic layer arrangement formed between the
electrode and the counter electrode and in electrical contact
therewith. The organic layer arrangement comprises the following
organic layers: a charge carrier injection layer on the electrode
side and a large carrier injection layer on the counter electrode
side as well as a layer area with an intermediate layer located
between the two.
[0057] The electrode and the counter electrode are preferably made
from a highly conductive material, for example a metal.
Non-metallic electrode materials may also be used provided they
have a certain electrical conductivity. Non-metallic electrode
materials of such kind include for example highly conductive
oxides, SnO, In:SnO (ITO), F:SnO ZnO, heavily doped inorganic and
organic semiconductors such as a-Si, c-Si or similar, nitrides and
polymers.
[0058] Another configuration provides that the intermediate layer
consists of a hybrid layer of two different organic materials, one
material being particularly suitable for conducting electrons and
the other material being particularly suitable for conducting
holes.
[0059] The requirement for a high current with relatively low
voltages in the forward bias direction may also be satisfied by an
intermediate layer consisting of a material that has a very small
"energy gap" ("low gap"). In this case, the electrons and holes do
not have to overcome any energy barriers that would prevent the
charge from being transported. Larger currents are achieved with
lower voltages.
[0060] The charge carrier transport layers on the electrode and
counter electrode sides serve to effectively inject charge carriers
in the form of electrons or holes (defect electrons) into the
organic layer arrangement and there to transport them without
significant electrical losses.
[0061] Doping of organic materials is known in various forms
n-doping or p-doping of the organic material may be provided. The
n-dopant is usually selected from molecules or neutral radicals for
which the HOMO level (HOMO--"Highest Occupied Molecular Orbital")
is lower than 4.5 eV, preferably lower than about 2.8 eV, and more
preferably lower than about 2.6 eV. The HOMO level of the doping
material can be determined from cyclovoltammetric measurements of
the oxidation potential. Alternatively, the reduction potential of
the donor cation may be determined in a salt of the donor. The
donor should have an oxidation potential with reference to
Fc/Fc+(ferrocene/ferrocenium redox couple) less than or equal to
about -1.5V, preferably less than or equal to about -2.0V and more
preferably less than or equal to about -2.2V. The molar mass of the
n-doping material is preferably between about 100 and about 2000
g/mol, and more preferably between about 200 and 1000 g/mol. In a
preferred embodiment, a molar doping concentration for electric
n-doping is between 1:1000 (acceptor molecule: matrix molecule) and
1:2, preferably between 1:100 and 1:5, and more preferably between
1:100 and 1:10.
[0062] It may be provided that the donor is only formed from a
precursor as the organic layers are being manufactured or while the
subsequent layer manufacturing process is in progress, as is
described as such in DE 103 07 125. The values for the HOMO level
of the donor cited previously then refer to the species that is
created thereby. Alternatively, the doping of the organic material
may also be carried out by a different method. Such different
methods include for example co-evaporation of the organic material
with a metal that has a low work function. Lithium and caesium are
examples of substances that as suitable for n-doping.
[0063] The p-dopant is usually selected from molecules or neutral
radicals for which the LUMO level (LUMO--"Lowest Unoccupied
Molecular Orbital") is energetically below 4.5 eV, preferably below
4.8 eV, and more preferably below 5.04 eV. The LUMO level of the
acceptor for p-doping can be determined with the aid of
cyclovoltammetric measurements of the reduction potential. The
acceptor preferably has a reduction potential with reference to
Fc/Fc+ of at least -0.3V, more preferably at least -0.0V and most
preferably at least 0.24V. Acceptors having a molar mass of about
100 to 2000 g/mol, preferably a molar mass between about 200 and
1000 g/mol, and more preferably a molar mass between about 300
g/mol and 2000 g/mol are preferred. In a practical embodiment, the
molar doping concentration for p-doping is between 1:1000 (acceptor
molecule: matrix molecule) and 1:2, preferably between 1:100 and
1:5, and more preferably between 1:100 and 1:10. The acceptor does
not have to be formed from a precursor until the layers are being
manufactured or while the subsequent layer manufacturing process is
in progress. The LUMO level of the acceptor indicated above then
refers to the species created thereby.
[0064] Examples of such materials are given in documents DE 103 478
56 B8, EP 1 837 926 B1 or also U.S. Pat. No. 6,908,783 B1. Metals
such as caesium or lithium and others are also used in n-doping. In
addition, oxides such as vanadium pentoxide (V2O2) or even
molybdenum oxide (Mo2O3) may also be used as p-dopants.
[0065] One embodiment provides for the zener diode to be used in an
electronic circuit to generate a voltage reference.
[0066] Another embodiment provides for the zener diode to be used
in combination with other organic or inorganic components.
[0067] FIG. 1 is a schematic representation of a layer sequence for
an organic electronic zener diode. A charge carrier injection layer
3 on the electrode side, a charge carrier injection layer 4 on the
counter electrode side, and an intermediate layer 5 arranged
between the two are disposed between an electrode 1 and a counter
electrode 2.
[0068] FIG. 2 is a schematic representation of a current-voltage
characteristic curve of an ideal zener diode with the
characterizing voltages Ud as the forward bias voltage and Uz as
the breakdown voltage.
[0069] FIG. 3 is a schematic representation of a layer sequence for
an organic electronic zener diode.
[0070] A charge carrier injection layer 22 on the electrode side, a
charge carrier injection layer 24 on the counter electrode side,
and a transport layer 23 arranged between the two are disposed
between an electrode 21 and a counter electrode 25. The thickness
(x) of the intermediate layer in this case is variable.
[0071] In order for the component to function reliably, it is
beneficial to use highly pure forms of all organic materials, as
may be achieved for example by gradient sublimation in a vacuum.
This avoids leakage currents, which can occur as a result of "trap
states". Organic materials that have been purified by sublimation
are helpful for correct, reproducible breakdown behaviour.
EXAMPLES
Example 1
[0072] As a first embodiment, the following structure was selected:
[0073] (21.1) Anode: Indium tin oxide (ITO) [0074] (22.1) injection
layer for holes: 50 nm
2,2',7,7'-Tetrakis(N,N-di-p-methylphenylamino)-9,9'-spirobifluorene
doped with 4% by weight
2,2'-(Perfluoronaphthaline-2,6-diylidene)-dimalodinitrile [0075]
(23.1) Hybrid intermediate layer: 5 nm TCTA:TPBi [0076] (24.1)
Injection layer for electrons: 50 nm BPhen doped with caesium
[0077] (25.1) Cathode: 100 nm aluminium
[0078] All layers are produced in a vapour deposition process in a
vacuum. In principle, such layers may also be produced by other
methods, such as spin coating, blade gap coating, organic vapour
phase deposition, or self-assembly. The intermediate layer is
formed by a hybrid layer of an n-conductive and a p-conductive
organic material. The mixing ratio in the embodiment is 1:1.
[0079] FIG. 4 shows a current-voltage curve for an organic
component as shown in FIG. 3. Thickness x of the transport layer is
5 nm. It produces typical diode behaviour when a positive voltage
is applied to the anode (forward bias). When a negative voltage is
applied to the anode (reverse bias), the current increases sharply
after a voltage Uz. The breakdown voltage is usually measured with
a reference current of approximately 1 to 5% of the maximum
permitted reverse current.
[0080] An important parameter for zener diodes is their
differential resistance in the breakdown range. The smaller this
resistance is, the steeper is the characteristic curve in the zener
diode's breakdown range. One consequence of this is better voltage
stabilisation. This differential resistance in the reverse bias
direction may be lowered with a higher molecular ratio between the
dopant and the matrix. If higher doping is selected, more free
charge carriers are available for transporting the current. This
increases conductivity. This is particularly noticeable in the
reverse direction, since in the forward direction above a certain
doping the current is not limited by conductivity any more but by
the harriers at the boundary surfaces. The components shown, and
particularly the doping ratio of the injection layers, may thus be
further optimised and adapted to respective requirements.
[0081] In order to further improve the behaviour of the components
in the reverse direction, the surface area of the components may be
reduced, for example. The purpose of this is to reduce the
capacitive effects. Another option for lowering the differential
resistance and thus also improve the properties is to replace the
no with gold, for example, as the anode material. ITO has a
relatively high lateral resistance, which is also included in its
differential resistance, since the cross resistance is applied to
the layers that are actually active in series. If this resistance
is reduced, the differential resistance of the component as a whole
is lowered.
[0082] The hole transport layer 22 on the anode side is made from
2,2',7,7'-Tetrakis(N,N-di-p-methylphenylamino)-9,9'-spirobifluorene.
2,2'-(Perfluoronaphthaline-2,6-diyliden)-dimalodinitrile is used as
a molecular dopant. F4-TCNQ may also be used instead of the
material used in this embodiment,
2,2',7,7'-Tetrakis(N,N-di-p-methylphenylamino)-9,9'-spirobifluorene
and 2,2'-(Perfluoronaphthaline-2,6-diyliden)-dimalodinitrile.
Example 2
[0083] In a second embodiment of an organic zener diode according
to FIG. 1, the following structure is provided: [0084] (21.2)
Anode: Indium-tin oxide (ITO) [0085] (22.2) Injection layer for
holes: 50 nm
2,2',7,7-Tetrakis(N,N-di-p-methylphenylamino)-9,9'-spirobifluorene
doped with 4% by weight
2,2'-(Perfluoronaphthaline-2,6-diyliden)-dimalodinitrile [0086]
(23.2) Intermediate hybrid layer: 10 nm TCTA:TPBi [0087] (24.2)
Injection layer for electrons: 50 nm BPhen doped with caesium
[0088] (25.2) Cathode: 100 nm aluminium
[0089] FIG. 5 shows a current-voltage curve for an organic
electronic component according to FIGS. 1 and 3. In this case,
thickness x of the transport layer is 10 nm. In the forward bias
direction, the embodiment exhibits typical diode behaviour. Unlike
the embodiment with a 5 nm intermediate layer, the reverse bias
characteristic curve obtained is shifted significantly towards
larger negative voltages.
[0090] FIG. 6 shows several current-voltage curves for organic
zener diodes according to FIGS. and 3. Thickness x of the
intermediate layer is varied between 5 nm and 8 nm. The breakdown
voltage is shifted by 3V.
Example 3
[0091] In a third embodiment of an organic zener diode according to
FIG. 1, the following structure is provided: [0092] (21.3) Anode:
Indium-tin oxide (ITO) [0093] (22.3) Injection layer for holes: 50
nm Meo-TPD doped with 4% by weight
2,2'-(Perfluoronaphthaline-2,6-diyliden)-dimalodinitrile [0094]
(23.3) Hybrid intermediate layer: 5 nm Balq:NPB [0095] (24.3)
Injection layer for electrons: 50 nm BPhen doped with caesium
[0096] (25.3) Cathode: 100 nm aluminium
[0097] FIG. 7 shows a current-voltage curve for an organic
electronic component according to FIG. 1. In this case, thickness x
of the transport layer is 5 nm. In the forward direction, the
embodiment exhibits typical diode behaviour. In the reverse
direction, an exponential rise in current is observed for a given
Uz.
Example 4
[0098] In a fourth embodiment of an organic zener diode according
to FIG. 1, the following structure is provided: [0099] (21.4)
Anode: Indium-tin oxide (ITO) [0100] (22.4) Injection layer for
holes: 50 nm RE68 2% by weight
2,2-(Perfluoronaphthaline-2,6-diylidene)-dimalodinitrile [0101]
(23.4) Hybrid intermediate layer: 5 nm RE68 [0102] (24.4) Injection
layer for electrons: 50 nm RE68 2% by weight
Tetrakis(1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinato)ditun
gsten(II) [0103] (25.4) Cathode: 100 nm aluminium
[0104] This embodiment relates to an organic zener diode that
differs from the previous embodiments in that the cathode-side
injection layer is made from an n-doped material, the intermediate
layer is made from the same material in intrinsic form, and the
anode-side injection layer consists of the same material but with
p-doping. FIG. 8 shows a current-voltage characteristic curve for
this embodiment. In this example too, the reverse bias
characteristic curve may be shifted by varying the intrinsic
thickness of the intermediate layer.
[0105] FIG. 9 shows the current-voltage characteristic curve of a
component according to the fourth embodiment having an intrinsic
layer thickness of 7 nm. The characteristic curves for various
doping conditions of the hole conducting injection layer are
shown.
[0106] FIG. 10 shows the current-voltage characteristic curve of a
component according to the fourth embodiment having an intrinsic
layer thickness of 7 nm. The characteristic curves for various
doping conditions of the electron conducting injection layer are
shown.
Example 5
[0107] In a fifth embodiment of an organic zener diode according to
FIG. 1, the following structure is provided: [0108] (21.4) Anode:
Indium-tin oxide (ITO) [0109] (22.4) Injection layer for holes: 50
nm 4% by weight pentacene, doped with
2,2'-(Perfluoronaphthaline-2,6-diylidene)-dimalodinitrile [0110]
(23.4) Hybrid intermediate layer: 30 nm pentacene [0111] (24.4)
Injection layer for electrons: 50 nm BPhen doped with caesium
[0112] (25.4) Cathode: 100 nm aluminium
[0113] This embodiment relates to an organic zener diode that
differs from the previous embodiments in that the anode-side
injection layer is made from a p-doped organic low gap material.
The intermediate layer consists of the same material, but is
present intrinsically in the intermediate layer. The cathode-side
charge carrier injection layer consists of an organic high gap
material doped with metal ions. In this embodiment too, the reverse
bias characteristic curve may be shifted by varying the intrinsic
intermediate layer, also by varying the doping, of the injection
layers.
[0114] FIG. 11 shows the current-voltage characteristic curve of a
component according to the fourth embodiment having an intrinsic
layer thickness of 30 nm. The characteristic curves for a 30 nm
thick intrinsic pentacene layer from an intermediate layer are
shown.
Example 6
[0115] In a sixth embodiment of an organic zener diode according to
FIG. 1, the following structure is provided: [0116] (21.4) Anode:
Indium-tin oxide (ITO) [0117] (22.4) Injection layer for holes: 50
nm Meo-TPD 4 wt % by weight doped with
2,2'-(Perfluoronaphthaline-2,6-diylidene)-dimalodinitrile [0118]
(23.4) Hybrid intermediate layer: 8 nm BAlq and/or 8 nm NPB [0119]
(24.4) Injection layer for electrons: 50 nm BPhen doped with
caesium [0120] (25.4) Cathode: 100 nm aluminium
[0121] This embodiment relates to an organic zener diode that
differs from the previous embodiments in that the intrinsic organic
intermediate layer consists solely of a unipolar material. In this
example too, the reverse bias characteristic curve may be shifted
by varying the intrinsic intermediate layer, also by varying the
doping of the injection layers.
[0122] FIG. 12 shows the current-voltage characteristic curve of a
component according to the sixth embodiment having an intrinsic
layer thickness of 8 nm. The characteristic curves for an
intermediate layer thickness of 8 nm for the electron-conducting
material BAlq and the hole conducting material NPB are shown.
[0123] The optimisation approaches discussed with reference to the
first embodiment also apply for all the other embodiments
presented.
[0124] Charge carrier injection layer or only injection layer:
Layer that helps to transfer majority charge carriers from layer
disposed on one side to another layer disposed on the opposite
side.
[0125] The energy barrier refers to a barrier to charge carrier
injection from the charge injection layer into the intermediate
layer when the component is being used in normal diode operation
(forward bias).
[0126] An oligomer is a molecule that is constructed from a number
of identical or similar units. Oligomers include dimers, trimers
and larger molecules including up to 30 units. Molecules that are
composed of more than 30 identical or similar units are called
polymers.
[0127] Forward biasing and reverse biasing are the normal technical
terms as applied to the use of conventional diodes. In FIG. 4, the
diode is operated with forward biasing when it is operated with
positive voltage. The diode is operated with reverse biasing when
it is operated with negative voltage.
[0128] The current breakdown of a diode in reverse biasing is
defined by the negative voltage range after which current
essentially flows through the diode, which is represented in FIG. 4
by the range from about -2.5 V to more negative voltages. This is
also referred to as zener behaviour.
[0129] It should also be noted that when the zener diode is being
operated, the reverse bias current must be restricted if it is too
high, to prevent it from destroying the diode. The same applies for
normal diodes in the forward bias direction.
[0130] The technical terms used are explained in the following:
[0131] ITO Indium-tin oxide [0132] HTM Semiconductor material that
transports holes, also called a p-type conductor, can be p-doped,
[0133] ETM Semiconductor material that transports electrons, also
called an n-type conductor, can be n-doped, [0134] Bphen
4,7-Diphenyl-1,10-phenanthroline, [0135] BCP
2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline (usually used as
ETM), [0136] Alq3 Aluminium-tris(8-hydroxyquinoline) (usually used
as ETM), [0137] C60 Fullerene C60 (used as ETM), [0138] PTCBI
3,4,9,10-Perylenetetracarboxylic acid bisbenzimidazole, [0139]
PTCDI 3,4,9,10-Perylenetetracarboxylic acid diimide, [0140] TCNQ
Tetracyanoquinodimethane, [0141] F4-TCNQ
2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (strong
organic acceptor, usually used for doping HTM), [0142] PBD
2-(4-Biphenylyl)-5-(p-tort-butylphenyl)-1,3,4-oxadiazole, [0143]
OXD 1,3-Bis[(p-tert-butyl)phenyl-1,3,4-oxadiazoyl]benzene, [0144]
TAZ
3-(Biphenyl-4-yl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole,
[0145] TPOB
1,3,5-Tris(4-tert-butylphenyl-1,3,4-oxadiazolyl)-benzene, [0146]
TCTA 4,4',4''-Tris(N-carbazol-triphenylamine, [0147] TPBI
2',2''-(1,3,5-Phenylene)tris[1-phenyl-1H-benzimidazole], [0148] NPB
N,N'-Bis(naphthaline-1-yl)-N,N'-bis(phenyl)-benzidine, [0149]
MeO-TPD (N,N,N',N'-Tetrakis(4-methoxyphenyl)-benzidine), [0150]
RE68 Tris(1-phenylisoquinoline)iridium(III), [0151] Trap states
Deep states for electrons in the conduction band (LUMO), which
capture the electrons. For holes, the trap states are high states
in the valence hand (HOMO) which capture the holes, [0152] Donor
n-dopant, [0153] Acceptor p-dopant, [0154] Matrix molecule Matrix
material, matrix molecule that forms a layer in which the dopant
molecules are embedded. [0155] HOMO Highest Occupied Molecular
Orbital [0156] LUMO Lowest Unoccupied Molecular Orbital [0157]
Precursor A substance that is not converted into an active molecule
until it is modified. [0158] "High gap" material Material with an
optical band gap that is of such a size as to render the material
essentially transparent. The gap is typically larger than 2 eV.
[0159] "Low gap" material Material with an optical band gap that is
of such a size as to render the material essentially opaque for
layers of sufficient thickness. The band gap is typically smaller
than or equal to 2 eV. [0160] Homojunction Junction, typically a pn
junction, wherein both sides (p and n) are created essentially from
the same transport material. [0161] Zener diode Diode having a
relatively low reverse bias breakdown voltage and a steep
characteristic curve in the forward bias direction. In the passing
direction, they behave like normal diodes, but in the blocking
direction above a certain voltage, the blocking or breakdown
voltage, their resistance suddenly falls sharply. [0162] Injection
layer for holes Layer in an electronic device that has holes as
majority charge carriers under forward biased voltage and injects
them into another layer. [0163] Injection layer for electrons Layer
in an electronic device that has electrons as majority charge
carriers under forward biased voltage and injects them into another
layer. [0164] Organic vapour phase deposition Organic vapour phase
deposition
[0165] The features of the invention disclosed in the preceding
description, the claims and the drawing may be significant either
individually or in any combination for the realisation of the
invention in its various embodiments.
* * * * *