Anode modification for organic light emitting diodes

Details Anode modification for organic light emitting diodes Anode modification for organic light emitting diodes

2000-01-10

2002-12-31

Day, Michael H. (Department: 2879)

Electric lamp and discharge devices

With luminescent solid or liquid material

Solid-state type

C313S503000

Reexamination Certificate

active

06501217

ABSTRACT:

TECHNICAL FIELD
The present invention pertains to organic electroluminescent displays and methods for making the same.
BACKGROUND OF THE INVENTION
Organic electroluminescence (EL) has been studied extensively because of its possible applications in discrete light emitting diodes (LED), arrays and displays. Organic materials can potentially replace semiconductors in many LED applications and enable wholly new applications. The ease of organic LED (OLED) fabrication and the continuing development of improved organic materials promise novel and inexpensive OLED display possibilities.
Organic EL at low efficiency was reported many years ago in e.g. Helfrich et al., Physical Review Letters, Vol. 14, No. 7, 1965, pp. 229-231. Recent developments have been spurred by two reports of efficient organic EL: C. W. Tang et al., Applied Physics Letters, Vol. 51, No. 12, 1987, pp. 913-915, and Burroughes et al., Nature, Vol. 347, 1990, pp. 539. Tang used vacuum deposition of molecular compounds to form OLEDs with two organic layers. Burroughes spin coated a polymer, poly(p-phenylenevinylene), to form a single-organic-layer OLED. The advances described by Tang and in subsequent work by N. Greenham et al., Nature, Vol. 365, 1993, pp. 628-630, were achieved mainly through improvements in OLED design derived from the selection of appropriate organic multilayers and electrode metals.
The simplest possible OLED structure, depicted in
FIG. 1A
, consists of an organic emission layer
10
sandwiched between cathode
11
and anode
12
electrodes which inject electrons (e
−
) and holes (h
+
), respectively, which meet in the emission layer
10
and recombine producing light. It has been shown (D. D. C. Bradley, Synthetic Metals, Vol. 54, 1993, pp. 401-405, J. Peng et al., Japanese Journal of Applied Physics, Vol. 35, No. 3A, 1996, pp. L317-L319, and I. D. Parker, Journal of Applied Physics, Vol. 75, No. 3, 1994, pp. 1656-1666) that improved performance is achieved if the electrode work functions match the respective molecular orbitals (MO) of the organic layer
10
. Such an improved structure is shown in
FIG. 1B
where optimized electrode materials
13
and
14
reduce the energy barriers to carrier injection into the organic layer
10
. Still, single organic layer structures perform poorly because electrons can traverse the organic layer
10
reaching the anode
14
, or holes may reach the cathode
13
, in either case resulting in current without light, and lower OLED efficiency.
Balanced charge injection is also important. For example, an excellent anode is of limited use if the cathode has a large energy barrier to electron injection.
FIG. 2
illustrates a device with a large electron barrier
16
such that few electrons are injected, leaving the holes no option but to recombine non-radiatively in or near the cathode
15
. The anode and cathode materials should be evenly matched to their respective MOs to provide balanced charge injection and optimized OLED efficiency.
An improved structure in which the electron and hole transport functions are divided between separate organic layers, an electron transport layer
20
(ETL) and a hole transport layer (HTL)
21
, is shown in FIG.
3
. In C. W. Tang et al., Journal of Applied Physics, Vol. 65, No. 9, 1989; pp. 3610-3616, it is described that higher carrier mobility is achieved in a two organic layer OLED design, and this led to reduced OLED series resistance enabling equal light output at lower operating voltage. The electrodes
22
,
23
can be chosen individually to match to the ETL
20
and HTL
21
MOs, respectively, while recombination occurs at the interface
24
of organic layers
20
and
21
, far from either electrode. As electrodes, Tang used a MgAg alloy cathode and transparent Indium-Tin-Oxide (ITO) deposited on a glass substrate as the anode. Egusa et al. in Japanese Journal of Applied Physics, Vol. 33, No. 5A, 1994, pp. 2741-2745 have shown that proper selection of the organic multilayer materials leads to energy barriers blocking both electrons and holes at the organic interface. This is illustrated in
FIG. 3
in which electrons are blocked from entering the HTL
21
and holes from entering the ETL
20
by a clever choice of organic materials. This feature reduces quenching near the contacts (as illustrated in
FIG. 2
) and also promotes a high density of electrons and holes in the small interface volume providing enhanced radiative recombination.
With multilayer device architectures now well understood and widely used, a remaining performance limitation of OLEDs is the electrodes. The main figure of merit for electrode materials is the position of the electrode Fermi energy relative to the relevant organic MO. In some applications it is also desirable for an electrode to be either transparent or highly reflective to assist light extraction. Electrodes must also be chemically inert with respect to the adjacent organic material to provide long term OLED stability.
Much attention has been paid to the cathode, largely because good electron injectors are low work function metals which are. also chemically reactive and oxidize quickly in atmosphere, limiting the OLED reliability and lifetime. Much less attention has been paid to the optimization of the anode contact, since conventional ITO anodes generally outperform the cathode contact leading to an excess of holes. Due to this excess, and the convenience associated with the transparency of ITO, improved anodes have not been as actively sought as improved cathodes.
ITO is by no means an ideal anode, however. ITO is responsible for device degradation as a result of In diffusion into the OLED eventually causing short circuits as identified by G. Sauer et al., Fresenius J. Anal. Chem., pp. 642-646, Vol. 353 (1995). ITO is polycrystalline and its abundance of grain boundaries provides ample pathways for contaminant diffusion into the OLED. Finally, ITO is a reservoir of oxygen which is known to have a detrimental effect on many organic materials (see J. C. Scott, J. H. Kaufman, P. J. Brock, R. DiPietro, J. Salem, and J. A. Goita, J. Appl. Phys., Vol. 79, p. 2745, 1996). Despite all of these problems, ITO anodes are favored because no better transparent electrode material is known and ITO provides adequate stability for many applications.
While conventional OLEDs extract light through the ITO anode, architectures relying on light extraction through a highly transparent cathode (TC) are desirable for transparent OLEDs or OLEDs fabricated on an opaque substrate. Si is an especially desirable OLED substrate because circuits fabricated in the Si wafer can be cheaply integrated with drive circuitry providing display functions. Given the minaturization and outstanding performance of Si circuitry, a high information content OLED/Si display could be inexpensively fabricated on a Si integrated circuit (IC).
The simplest approach incorporating a TC is to deposit a thin, semi-transparent low work function metal layer, e.g. Ca or MgAg, followed by ITO or another transparent, conducting material or materials, e.g. as reported in Bulovic et al., Nature, Vol. 380, No. 10, 1996 p. 29, or in the co-pending PCT patent application PCT/IB96/00557, published on Dec. 11, 1997 (publication number W097/47050). To maximize the efficiency of such a TC OLED, a highly reflective anode which can direct more light out through the TC is desired. Consequently, the low reflectivity of ITO is a disadvantage in TC OLEDs.
Alternatively, for some applications it may be more important to increase the contrast ratio of the OLEDs or display based thereon. In this case, a TC OLED could benefit from a non-reflective, highly absorbing anode. Again the optical characteristics of ITO are a disadvantage.
High work function metals could form highly reflective anodes for TC OLEDs. Some of these metals, e.g. Au, have a larger work function than ITO (5.2 eV vs. 4.7 eV), but lifetime may be compromised because of high diffusivity in organic materials. Like In from ITO, only worse, Au diffuses easily through many organic materials

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