Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2000-07-14
2003-02-11
Hess, Bruce H. (Department: 1774)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S917000, C252S301350, C257S088000, C313S503000, C313S504000, C313S506000
Reexamination Certificate
active
06517958
ABSTRACT:
TECHNICAL FIELD
The present invention relates to compositions used to fabricate light emitting devices, more specifically, the present invention is drawn to organic-inorganic hybrid light emitting devices (HLED) fabricating with light emitting compositions in which hole transport, electron transport, and emissive compounds are covalently linked to polyhedral silsesquioxanes.
BACKGROUND ART
Flat panel displays based on organic light emitting devices (OLEDs) have been studied extensively by hundreds of industrial and academic institutions worldwide for the last decade. OLEDs offer exceptional potential for thinner, more energy efficient displays with equal or better resolution and brightness compared to the current liquid crystal display (LCD) technology. OLEDs also offer high switching speeds, excellent viewing angles (>160°), red, green and blue (RGB) color selection possibilities, and because no backlighting is necessary, it may be possible to fabricate devices on flexible substrates. However, despite the enormous research and development effort on OLEDs, there is presently only one commercially available product using this technology. One of the apparent problems for this is the need for the development novel efficient materials to satisfy the electronic device requirements.
The scientific basis for OLEDs relies on an organic material's ability to emit light when subjected to electrical stimuli. In this process, electrons and holes are injected into organic materials from conducting electrodes and diffuse through a thin organic film to form electron-hole pairs or excitons within a highly conjugated organic molecule or polymer layer. The excitons then recombine creating an excited state within the organic molecule. The excited state can then undergo radiative decay emitting a photon. Depending on the organic polymer/molecule and its substituents, the wavelength of light emitted can be any color and even multicolored, e.g. red, green, blue or combinations thereof.
For optimal operation, it is important that the rate at which the holes and electrons diffuse into this emitting layer be similar, and preferably matched. Hence, numerous efforts have been made to optimize transport of both holes and electrons to the emitting layer and also to prevent trapping of holes or electrons that leads to destructive effects within the devices. Most recently, efforts have been made to incorporate organic molecules or polymers that promote movement of holes or electrons within the OLED device. Still more recently, efforts have been made to incorporate organic molecules or monomer units in polymeric systems such that one organic unit promotes hole or electron conduction and a second organic unit promotes emission. Such electronic tuning is designed to minimize the transport distances and maximize the hole/electron injection balance, thereby enhancing the potential for radiative decay rather than non-radiative decay. Considerable work in this area remains.
Early examples of organic electroluminescence were reported by Pope et al. in 1963 [Pope, M.; Kallmann, H.; Magnante, P.
J. Chem. Phys
. 1962, 38, 2042] who demonstrated blue light emission from single crystal anthracene using very high voltages, ≈400 V.
Advances on OLED processing over the next two decades were limited to forming thin, light emitting films of organic compounds by vacuum deposition techniques, [Vincett, P. S.; Barlow, W. A.; Hann, R. A.; Roberts, G. G.
Thin Solid Films
1982, 94, 476] and lowering driving voltages to <30 V, however these single-layer devices suffered from both poor lifetimes and luminescence efficiencies. In 1987, Tang and Van Slyke [Tang, C. W.; Van Slyke, S. A.
Appl. Phys. Lett
. 1987, 51, 913] at Eastman Kodak discovered how to make two-layer electroluminescent devices. As shown in
FIG. 1
, the OLED device
10
was prepared by sandwiching organic hole transport (HT) material
12
and emissive (EM) material
14
between an indium-tin-oxide (ITO) anode
16
and magnesium/silver alloy cathode
18
layers. A conventional electric potential source
20
was connected to the cathode
18
and anode
16
. A glass substrate
22
allowed light emission as shown by Arrow
24
. The hole transport (HT) and emissive (EM) materials used by Tang and Van Slyke are shown below.
Materials Used by Tang and Van Slyke
The key to device performance was the layered architecture sequence: cathode/emissive-electron transport/hole transport/anode. These devices demonstrated brightness, efficiencies, and lifetimes far exceeding anything reported at that time. The materials shown in
FIG. 1
were deposited onto indium tin oxide (ITO) coated glass by a vacuum sublimation process to a thickness of ≈25 nm.
In 1990 Burroughs et al. [Burroughs, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burn, P. L.
Nature
1990, 347, 539] developed polymeric OLED devices or PLEDs. In 1992, Braun et al. [Braun, D.; Gutafson, D.; McBranch, D.; Heeger, A., J.
J. Appl. Phys
., 1992, 72, 546] discovered that poly(p-phenylenevinylene) (PPV), and its derivatives will electroluminesce both green and red light when confined between ITO and aluminum electrodes.
This work was important because PPV polymers can be deposited by a spin coating process that is more cost effective than vacuum sublimation. Spin coating also facilitates coating larger areas. As a result of these pioneering examples, hundreds of OLED and PLED based papers have been reported by research groups around the world using the following two common materials deposition approaches:
1. Vacuum sublimation of molecular species; and
2. Dip, spin, and spray coating or printing of oligomeric or polymeric materials.
Each method has advantages and disadvantages as outlined below.
Vacuum sublimation works well only with relatively low molecular weight (MW) compounds (<300 g/mol). Such compounds must be purified by sublimation or column chromatography to purities >99.99% prior to deposition to obtain superior light emitting efficiencies and device lifetimes. Vacuum sublimation allows for multi-layer configurations and very precise control of film thickness, both of which are advantageous in OLED processing. Drawbacks to vacuum sublimation are that it requires very costly equipment and it is limited to deposition on surface areas that are much smaller than surfaces that can be coated using spin coating. Additionally, device performance is adversely affected by the tendency of some sublimed compounds to crystallize with time. To prevent premature crystallization, compounds are currently being designed with high glass transition temperatures (Tgs) and substituents that minimize or prevent crystallization.
Dip coating, spin coating, spray coating, and printing techniques are generally applicable to the deposition of oligomeric and polymeric materials. It permits precise film thickness control, large area coverage and is relatively inexpensive compared to vacuum sublimation. Multi-layer configurations are only possible if the deposited layers are designed with curable functional groups for subsequent cross-linking, or with differing solubilities to prevent re-dissolution during additional coatings. For example, current OLED polymer technology uses a water soluble prepolymer PPV (shown below), that is thermally cured after deposition rendering it insoluble. [Li, X. C.; Moratti, S. C.
Semiconducting Polymers as Light-Emitting Materials
; Wise, D. L., Wnek, G. E., Trantolo, D. J., Cooper, T. M. and Gresser, J. D., Ed., 1998].
Process for Converting Soluble PPV to Insoluble PPV
Initial luminescent properties for OLEDs based on polymers were often inferior to their molecular counterparts. This was partly due to the difficulty in obtaining high purity material (99.99%) (polydispersity, endgroups, residual solvents and byproducts such as HCl, catalysts, etc.) necessary for efficient devices. However recent studies have shown that carefully purified polymers have similar or even better resultant
Laine Richard M.
Sellinger Alan
Garrett Dawn
Hess Bruce H.
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