Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2000-12-08
2003-11-18
Kelly, Cynthia H. (Department: 1774)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S001100, C428S001260, C428S473500, C428S917000, C313S502000, C313S506000, C313S509000
Reexamination Certificate
active
06649283
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to layers comprising polyimide and a functional material such as a hole transport material (hole conductor material), an electron transport material (electron conductor material) or an emitter material. Polyimide layers doped with said functional materials can be used for the production of novel electronic and optoelectronic devices.
BACKGROUND OF THE INVENTION
The use of polyimide layers as polyimide alignment layers to orient liquid crystalline materials is a standard technology in the liquid crystal (LC) display industry. A review of the state of the art in orienting layers for liquid crystals can be found in Ref. [1].
Polyimides for alignment layers are typically prepared from polyamic acid or polyamic ester precursor compounds. One example of a precursor polyimide is ZLI 2650 from Merck. Usually, the precursor polymer is first deposited and subsequently chemically or thermally converted into an insoluble polyimide. Alternatively, soluble preconverted polyimides can be used.
The orientation layer is prepared in order to give it aligning properties. Conventionally, surface preparation is made by rubbing with a cloth. Other methods for surface preparation known in the art make use of polarized ultraviolet light or flows of hot air. The liquid crystals are subsequently aligned by depositing them onto the alignment layer and letting them align at a temperature above the melting point and below the clearing point of the materials.
However, polyimide films are very poor hole or electron conductors. Thus, if layers are required which have a high hole or electron transport and injection ability, as in the case discussed in the following paragraph, undoped polyimide films would reduce the efficiency of the respective devices.
On the other hand, light-emitting devices (LEDs) based on organic and polymeric electroluminescent materials are known. Typically such devices require two electrodes of differing work function, at least one of which is transparent. A high work function anode (e.g. indium tin oxide (ITO), fluorine-doped tin oxide (FTO), or gold) serves for hole injection and a low work function cathode (e.g. Mg, Al, Li, Ba, Yb, Ca) for electron injection into the organic or polymeric material.
In order to make a high efficiency LED, it is desirable for current in the device to be space charge limited instead of injection limited. As will be described in detail below, a space charge limited current is indicated in J*d versus V/d plots by the independence of the plot from the interelectrode distance d.
For realizing space charge limited current, the barriers for charge injection have to be small. One way to achieve this is to fabricate devices comprising additional layers to facilitate injection and transport of electrons and holes. One type of layer comprises organic hole conductors, typically aromatic amines, which facilitate the injection and transport of holes in the device.
These materials are typically used in the undoped state and represent organic semiconductors with a low intrinsic conductivity. Such semiconducting hole conductor layers can increase the efficiency of the device by balancing injection and charge of positive and negative charge carriers and forcing the recombination of charge carriers to occur in a region well away from the electrodes, thereby preventing the emission efficiency from being reduced by semiconductor/metal interactions. The use of hole conducting layers in polymeric and organic LEDs is described e.g. in Refs. [2] and [3].
In most cases the multilayers are prepared by organic evaporation to avoid dissolving the previously applied layer, which can happen when solvent-based processing such as spin coating, doctor blading, dip coating, etc. are used.
Alternatively, metallically conductive doped conjugated polymers such as polythiophene or polyaniline may be used as anode modification layers to adjust the work function of the electrode and to provide a stable interface between semiconductor layer and anode. The use of such electrodes is described in Refs. [4] and [5].
The use of dendrimeric structures (Ref. [6]), sterically hindered structures using spirobifluorene or triptycene units (Refs. [3] and [7]) and crosslinkable structures (Ref. [8]) having a relatively high glass transition temperature (glass temperature, T
g
) and forming a stable glassy phase, in organic LEDs has been described. The glass transition temperature is phenomenologically characterized by the transition from a more or less hard, amorphous glass-like or partially crystalline state into a rubber-like to viscous melt-like state.
In recent years, the possibility of realizing polymeric LEDs emitting polarized light has been investigated. Thereby, polarized emission has been achieved to some extent by stretch orienting emitting, non liquid crystalline polymers (Ref. [9]). Also in this reference, the possible use of polarized polymer LEDs as backlights for liquid crystalline displays is addressed. In Ref. [10], polarized emission from rigid-rod molecules which were aligned by the Langmuir-Blodgett transfer method is described. In both cases, no orientation in a liquid crystalline phase was used to achieve polarized alignment.
A polymer LED in which a layer of polythiophene or polyaniline was rubbed and used to align a low molecular weight liquid crystal doped with a small amount of dichroic fluorescent dye is described in Ref. [11]. However, the aligning ability of polythiophenes is known to be not high and, indeed, alignment of polymeric liquid crystals on rubbed polythiophene was not shown.
Polarized emission was obtained on rubbed poly(p-phenylenevinylene) according to Ref. [12]. LEDs comprising polymeric liquid crystalline emitter materials aligned on rubbed polyimide are disclosed in Ref. [13]. However, polyimide cannot serve as a hole or electron conducting material. In Ref. [14], polarized electroluminescence by epitaxial growth of sexiphenylene onto a rubbed sexiphenylene film is described.
The use of a polyimide derivative in a photoconductive device is described in Ref. [15]. The polyimide derivative is a polyimide comprising, covalently linked, a moiety in the main chain that might serve as a hole conducting compound.
Finally, a hole conducting layer prepared by mixing the hole conductor N,N′-diphenyl-N,N′-di(m-tolyl)benzidine (TPD) with a polyamic acid precursor in the ratio 50/50 by weight, followed by thermal conversion, is described in Ref. [16]. TPD has a glass transition temperature (T
g
) of 65° C. and a molecular weight of below 500.
As described in Ref. [16], a two-layer LED can be made with the above TPD/polyimide layer by vacuum deposition of an aluminum complex as electron transport and emitting material. However, at concentrations high enough to produce good hole transport, the hole conductor TPD and the polyimide phase separate and the TPD crystallizes. This brings about the problem, firstly, that the phase separated TPD is susceptible to attack by solvents in subsequent coating processes and, secondly, that the film cannot be processed to be an orientation layer, especially by rubbing, as rubbing of the layer comprising crystallized TPD would result in severe damage.
As mentioned above in the context of characteristics of polyimide films, polyimide itself has only poor hole or electron transport and injection properties. Thus, the use of a polyimide layer in an undoped state reduces the efficiency of devices in which hole/electron transport is needed.
On the other hand, although some alignment can be achieved by rubbing electrically conducting or hole transporting polymers, these materials do not have such a high degree of aligning ability as polyimide. Thus, alignment layers based on such polymers are unsatisfactory.
In addition, one is restricted to the energy levels and hole mobilities provided intrinsically by the mate
Grell Martin
Lupo Donald
Meisel Andreas
Miteva Tzenka
Neher Dieter
Frommer William S.
Frommer & Lawrence & Haug LLP
Garrett Dawn
Kelly Cynthia H.
Megerditchia Samuel H.
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