Active solid-state devices (e.g. – transistors – solid-state diode – Organic semiconductor material
Reexamination Certificate
1999-02-08
2002-08-13
Lee, Eddie (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Organic semiconductor material
C257S103000
Reexamination Certificate
active
06433355
ABSTRACT:
TECHNICAL FIELD
The present invention pertains to organic electroluminescent devices, such as discrete light emitting devices, arrays, displays, and in particular to injection layers and contact electrodes suited for such devices. It furthermore relates to a method for making the same.
BACKGROUND OF THE INVENTION
Organic electroluminescence (EL) has been studied extensively because of its possible applications in discrete light emitting devices, arrays and displays. Organic materials investigated so far can potentially replace conventional inorganic materials in many applications and enable wholly new applications. The ease of fabrication and extremely high degrees of freedom in organic EL device synthesis promises even more efficient and durable materials in the near future which can capitalize on further improvements in device architecture.
Organic EL at low efficiency was observed many years ago in metal/organic/metal structures as, for example, reported in Pope et al., Journal Chem. Phys., Vol. 38, 1963, pp. 2024, and in “Recombination Radiation in Anthracene Crystals”, Helfrich et al., Physical Review Letters, Vol. 14, No. 7, 1965, pp. 229-231. Recent developments have been spurred largely by two reports of high efficiency organic EL. These are C. W. Tang et al, “Organic electroluminescent diodes”, Applied Physics Letters, Vol. 51, No. 12, 1987, pp. 913-915, and by a group from Cambridge University in Burroughs et al., Nature, Vol. 347, 1990, pp. 539. Tang et al. made two-layer organic light emitting devices using vacuum deposited molecular dye compounds, while Burroughs used spin coated poly(p-phenylenevinylene) (PPV), a polymer.
The advances described by Tang and in subsequent work by the Cambridge group, for example in “Efficient LEDs based on polymers with high electron affinities”, N. Greenham et al., Nature, Vol. 365, 1993, pp. 628-630, were achieved mainly through improvements in the design of EL devices derived from the selection of appropriate organic multilayers and contact metals.
Organic EL light emitting devices (OLEDs) function much like inorganic LEDs, except that light is commonly extracted through a transparent electrode deposited on a transparent glass substrate. The simplest possible structure, schematically illustrated in
FIG. 1A
, consists of an organic emission layer
10
sandwiched between two electrodes
11
and
12
which inject electrons (e
−
) and holes (h′), respectively. Such a structure has been described in the above mentioned paper of Burroughs et al., for example. The electrons and holes meet in the organic layer
10
and recombine producing light. It has been shown in many laboratories, see for example: “Conjugated polymer electroluminescence”, D. D. C. Bradley, Synthetic Metals, Vol. 54, 1993, pp. 401-405, “The effect of a metal electrode on the electroluminescence of Poly(p-phenylvinylene)”, J. Peng et al., Japanese Journal of Applied Physics, Vol. 35, No. 3 A, 1996, pp. 1317-1319, and “Carrier tunneling and device characteristics in polymer LEDs”, I. D. Parker, Journal of Applied Physics, Vol. 75, No. 3, 1994, pp. 1656-1666, that improved performance can be achieved when the electrode materials are chosen to match the respective molecular orbitals of the organic material forming the organic layer
10
. Such an improved structure is shown in FIG.
1
B. By choosing the optimized electrode materials
13
and
14
, the energy barriers to injection of carriers are reduced, as illustrated. Still, such simple structures perform poorly because little stops electrons from traversing the organic layer
10
and reaching the anode
14
, or the hole from reaching the cathode
13
.
FIG. 2A
illustrates a device with a large electron barrier
16
such that only few electrons are injected, leaving the holes no option but to recombine in the cathode
15
.
A second problem, illustrated in
FIG. 2B
, is that the mobilities of electrons and holes in most known organic materials, especially conductive ones, differ strongly.
FIG. 2B
illustrates an example where holes injected from the anode
18
quickly traverse the organic layer
19
, while the injected electrons move much slower, resulting in recombination near the cathode
17
. If the electron mobility in the organic layer
19
were larger than the holes′, recombination would occur near the anode
18
. Recombination near a metal contact is strongly quenched by the contact which limits the efficiency of such flawed devices.
Tang, as shown in
FIG. 3
, separated electron and hole transport functions between separate organic layers, an electron transport layer
20
(ETL) and a hole transport layer (HTL)
21
, mainly to overcome the problems described above. In “Electroluminescence of doped organic thin films”, C. W. Tang et al., Journal of Applied Physics, Vol. 65, No. 9, 1989, pp. 3610-3616, it is described that higher carrier mobility was achieved in the two-layer design, which led to reduced device series resistance enabling equal light output at lower operating voltage. The contact metals
22
,
23
could be chosen individually to match to the ETL
20
and HTL
21
molecular orbitals, respectively, while recombination occurred at the interface
24
between the organic layers
20
and
21
, far from either electrode
22
,
23
. As electrodes, Tang used a MgAg alloy cathode and transparent Indium-Tin-Oxide (ITO) as the anode. Egusa et al. in “Carrier injection characteristics of organic electroluminescent devices”, Japanese Journal of Applied Physics, Vol. 33, No. 5 A, 1994, pp. 2741-2745 have shown experimentally that the proper selection of the organic multilayer can lead to a blocking of both electrons and holes at an organic interface remote from either electrode. This effect is illustrated by the structure of
FIG. 3
which blocks electrons from entering the HTL
21
and vice versa by a clever choice of HTL and ETL materials. This feature eliminates non-radiative recombination at the metal contacts as illustrated in FIG.
1
A and also promotes a high density of electrons and holes in the same volume leading to enhanced radiative recombination.
The heterojunction molecular orbital energy alignment illustrated in
FIG. 3
actually reflects a trend in preferred OLED materials which is beneficial to device design (and to the present invention as will be discussed later). The trend is that materials which tend to transport electrons with high mobility
20
do so, in part, because their LUMOs lie at lower energy. Similarly, good hole conductive properties go hand-in-hand with HOMOs lying at higher energy. These facts make it more probable that a heterojunction formed between an electron and hole transporting organic layer will block the injection of one or both carriers at the interface due to the energy discontinuities of the respective molecular orbitals. The blocking effect localizes the carriers far from the quenching electrodes where they can recombine most efficiently.
Two technical terms are commonly used which describe the positioning of the two relevant organic molecular orbitals: the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of organic materials, or in the case of semiconductors, the positioning of their respective counterparts, the valence bands (VB) and conduction bands (CB). These terms in our treatment all have units of lower energies than a free electron, which reflects the fact that they are bound, and energy is required to remove electrons from nearly all known materials. For convenience, we arbitrarily define a free electron to have zero energy, and therefore speak in terms of the above quantities having negative energy values with respect to the free electron (or vacuum) state. The first of these terms is the work function, which describes how much energy is required to ‘pull’ the most weakly bound electron out of the material, i.e. to make it a free electron. In a metal or degenerate semiconductor (i.e. a semiconductor characterized by an extremely high free carrier concentration), the work function is identical t
Riess Walter
Strite Samuel C.
Baumeister Bradley William
International Business Machines - Corporation
Lee Eddie
McGinn & Gibb PLLC
Underweiser, Esq. Marian
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