Polymer devices

Details Polymer devices Polymer devices

2001-06-06

2003-08-05

Jackson, Jerome (Department: 2815)

Active solid-state devices (e.g., transistors, solid-state diode

Organic semiconductor material

C257S290000, C438S082000

Reexamination Certificate

active

06603139

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to polymer devices, for instance transistors that comprise a semiconductive polymer material.
There has been extensive work on transistors made with organic materials. Insulated gate field effect transistors (FETs) have been made with polymer semiconductors deposited by solution processing of either the polymer itself or a precursor to form a layer of the final polymer.
FIG. 1
shows the general structure of such a device. Under the semiconductor polymer layer
1
are two spaced apart metallic electrodes, the drain electrode
2
and the source electrode
3
of the transistor. Below them are a layer
4
of Si/SiO
2
and a metallic gate electrode
5
. The device acts as a switch because current flow between the source and drain electrodes is greatly increased when a bias is applied to the gate electrode. One such device, in which the semiconductor polymer is regioregular poly-hexylthiophene (P3HT), is described in more detail in Z. Bao et al., Appl. Phys. Lett. 69, 4108 (1996).
Devices of this type have several problems (see A. R. Brown et al., Science 270, 972 (1995)). First, the through-current from the source to the drain is low because the electronic carrier mobility &mgr; is typically in the range from 10
−4
to 10
−6
cm
2
/Vs. (See J. H. Burroughes et al., Nature 335, 137 (1988) and A. R. Brown et al., Synthetic Metals 88, 37 (1997)). Most solution-processed polymers have a disordered structure, and it is believed that in these systems the carrier mobility is limited by variable-range hopping between polymer chains. This low mobility rules out such transistors for general current-supply applications. Second, the on-off ratio, i.e. the ratio between the through-current in the on and off states, is poor: less than 10
4
for example. Up to now a polymer transistor with a performance comparable to that of inorganic amorphous silicon transistors has not been demonstrated. As a consequence a preferred approach has been to use molecular (or oligomer) organic materials instead of polymers. Molecular devices tend to have improved electrical performance but have severe process shortcomings. First, the molecules are generally deposited by vacuum sublimation, typically at substrate temperatures around 100-200° C. This rules out the use of such molecular materials on heat-sensitive substrates. Second, the molecular materials are generally not robust; there are serious concerns about the effect of cracks and microcracks in highly crystalline sublimed molecular films, in particular if deposited on flexible plastic substrates. Third, molecular devices are highly sensitive to subsequent processing steps. Attempts to post-process sublimed molecular films, for example to deposit subsequent layers on top of the sublimed films for multilayer integrated devices, have generally resulted in greatly reduced performance of the buried FETs.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention there is provided an integrated circuit device comprising: a current drive switching element having an input electrode, an output electrode, a switchable region comprising a semiconductive polymer material electrically coupled between the input electrode and the output electrode, and a control electrode electrically coupled to the switchable region so as to allow the application of a bias to the control electrode to vary the flow of current through the switchable region between the input electrode and the output electrode; and a second circuit element, integrated with the switching element, and electrically coupled with the output electrode of the switching element for receiving a drive current from the switching element.
According to a second aspect of the present invention there is provided a method for forming an electronic device having a region comprising a semiconductive polymer material, the method comprising depositing the semiconductive polymer by a process which promotes ordering in the deposited polymer. The electronic device according to this aspect of the invention may suitably be a switching element, for example of the type as set out above in relation to the first aspect of the invention.
According to a third aspect of the present invention there is provided an integrated circuit device comprising: a switching element having an input electrode, an output electrode, a switchable region comprising a semiconductive polymer material electrically coupled between the input electrode and the output electrode, and a control electrode electrically coupled to the switchable region so as to allow the application of a bias to the control electrode to vary the flow of current through the switchable region between the input electrode and the output electrode; and an electro-optical circuit element, integrated with the switching element, and electrically coupled to one of the electrodes of the switching element.
The semiconductive polymer may, for instance, be a conjugated polymer (see, for example, PCT/WO90/13148, the contents of which are incorporated herein by reference) or an “intermolecular” semiconducting polymer like poly-vinylcarbazole (PVK) containing short conjugated segments connected by non-conjugated segments.
An insulating layer may be deposited directly or indirectly on top of the electronic device. Preferably this does not substantially degrade the performance of the device. A second circuit element (as in the first aspect of the invention) may also be formed, and is preferably integrated with the said electronic device.
The second circuit element (or the opto-electrical element of the third aspect of the invention) is preferably an element that stores or consumes (preferably significant) electrical energy, e.g. by converting current to an electrical or optoelectrical signal, or an element that converts an optical signal into an electrical signal, e.g. a voltage or a current. The second circuit element is preferably not a switching element. The second circuit element is suitably capable of emitting or detecting light and/or varying the transmission of light through itself. Examples include light-emissive devices, photovoltaic devices and devices such as liquid crystal devices. The device may suitably emit or detect an optical signal, it may be a display device and/or form part of a visual display. The second circuit element preferably requires a significant drive current for its operation.
Where the second circuit element is a light-emissive element it is preferred that it comprises one or more light-emissive organic materials. The or each light-emissive organic material may be a polymer material, preferably a conjugated or partially conjugated polymer material. Suitable materials include poly-phenylene-vinylene (PPV), poly(2-methoxy-5(2′-ethyl)hexyloxyphenylene-vinylene) (MEHPPV), PPV-derivatives (e.g. di-alkoxy or di-alkyl derivatives), polyfluorenes and/or co-polymers incorporating polyfluorene segments, PPVs and/or related copolymers (see, for example, PCT/WO90/13148). Alternative materials include organic molecular light-emitting materials, e.g. tris(8-hydroxyquinoline)aluminium (Alq3) (see, for example, U.S. Pat. No. 4,539,507, the contents of which are incorporated herein by reference), or any other small sublimed molecule or conjugated polymer electroluminescent material as known in the prior art (see, for example, N. C. Greenham and R. H. Friend, Solid State Physics (Academic Press, San Diego, 1995) Vol. 49, pp 1-149). The light emitted by the device may be inside or outside the visible spectral range (400-800 nm). In the latter case materials such as LDS-821 (A. Dodabalapur et al., IEEE J. Selected Topics in Quantum Electronics 4, 67 (1998)) may be used.
The light-emissive element preferably comprises a cathode for injecting negative charge carriers (electrons) and an anode for injecting positive charge carriers (holes). There is preferably a region (suitably in the form of a layer) of light emissive material (suitably with other layers to improve performance) between the electrodes. The ca

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