U.S. patent application number 11/781690 was filed with the patent office on 2007-11-15 for aligned polymers for an organic tft.
This patent application is currently assigned to Cambridge University Technical Services Limted. Invention is credited to Richard Henry Friend, Richard John Wilson, Henning SIRRINGHAUS.
Application Number | 20070264748 11/781690 |
Document ID | / |
Family ID | 26315688 |
Filed Date | 2007-11-15 |
United States Patent
Application |
20070264748 |
Kind Code |
A1 |
SIRRINGHAUS; Henning ; et
al. |
November 15, 2007 |
ALIGNED POLYMERS FOR AN ORGANIC TFT
Abstract
A method for forming an electronic device having a
semiconducting active layer comprising a polymer, the method
comprising aligning the chains of the polymer parallel to each
other by bringing the polymer into a liquid-crystalline phase.
Inventors: |
SIRRINGHAUS; Henning;
(Cambridge, GB) ; Henry Friend; Richard;
(Cambridge, GB) ; John Wilson; Richard; (Dorset,
GB) |
Correspondence
Address: |
SUGHRUE MION, PLLC
2100 PENNSYLVANIA AVENUE, N.W.
SUITE 800
WASHINGTON
DC
20037
US
|
Assignee: |
Cambridge University Technical
Services Limted
|
Family ID: |
26315688 |
Appl. No.: |
11/781690 |
Filed: |
July 23, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10758256 |
Jan 16, 2004 |
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11781690 |
Jul 23, 2007 |
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10018425 |
Apr 16, 2002 |
6723394 |
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10758256 |
Jan 16, 2004 |
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Current U.S.
Class: |
438/99 ;
257/E51.001 |
Current CPC
Class: |
Y10T 428/31504 20150401;
H01L 51/0015 20130101; Y10T 428/1086 20150115; H01L 51/0056
20130101; Y10T 428/10 20150115; H01L 51/0012 20130101; H01L 51/0036
20130101; H01L 51/0545 20130101; Y10T 428/1005 20150115; C09K
2323/02 20200801; C09K 2323/00 20200801; H01L 51/0076 20130101;
H01L 51/0043 20130101; C09K 2323/06 20200801; Y10T 428/3154
20150401; H01L 51/0541 20130101; H01L 51/0039 20130101; H01L
51/0052 20130101; Y10T 428/21 20150115; H01L 51/0516 20130101 |
Class at
Publication: |
438/099 ;
257/E51.001 |
International
Class: |
H01L 51/40 20060101
H01L051/40 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 21, 1999 |
GB |
9914489.1 |
Dec 1, 1999 |
GB |
9928435.8 |
Claims
1. A method for forming a transistor comprising the step of forming
an active interface of the transistor between a semiconductive
layer and a gate insulator layer by solution deposition of a second
polymer layer on top of a solution-processed polymer layer that has
not been converted into an insoluble form prior to the deposition
of the second polymer layer.
2. A method as claimed in claim 1 wherein the second layer forms
the gate insulator of the transistor.
3. A method as claimed in claim 1, wherein the second layer
comprises PVP.
4. A method as claimed in claim 1, wherein the said semiconductive
layer is the said solution-processed polymer layer.
5. A method as claimed in claim 1, wherein the said
solution-processed polymer layer is soluble in a non-polar organic
solvent, but is insoluble in a polar solvent.
6. A method as claimed in claim 57 wherein the second polymer layer
is deposited from a polar organic solvent, in which the said
solution-processed polymer layer is not soluble.
7. A method as claimed in claim 1, wherein the second polymer layer
is deposited from an alcohol, a protic polar solvent such as water
or an aprotic polar solvent such as DMF.
8. A method as claimed in claim 1, wherein a source-drain and/or
gate electrode of the transistor is also formed from solution.
9. A method according to claim 1, wherein the second layer is the
gate insulator layer, and the solution-processed polymer layer is a
uniaxially aligned liquid crystal polymer film.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This is a continuation of application Ser. No. 10/018,425
filed Dec. 20, 2001; the disclosure of which is incorporated herein
by reference.
[0002] This invention relates to aligned polymers, especially
aligned polymers suitable for use in devices such as polymer thin
film transistors, and methods of aligning polymers. The aligned
polymers are preferably substantially parallel aligned,
liquid-crystalline conjugated polymers.
[0003] Semiconducting conjugated polymer field-effect transistors
(FETs) have potential applications as key elements of integrated
logic circuits (C. Drury, et al., APL 73, 108 (1998)) and
optoelectronic devices (H. Sirringhaus, et al., Science 280, 1741
(1998)) based on solution processing on flexible plastic
substrates. One main criterion to obtain high charge carrier
mobilities has been found to be a high degree of structural order
in the active semiconducting polymer.
[0004] For some polymers it is known to be possible to induce
uniaxial alignment of the polymer chains in thin films by using
processing techniques such as Langmuir-Blodgett (LB) deposition (R.
Silerova, Chem. Mater. 10, 2284 (1998)), stretch alignment (D.
Bradley, J. Phys. D 20, 1389 (1987)), or rubbing of the conjugated
polymer film (M. Hamaguchi, et al., Appl. Phys. Lett. 67, 3381
(1995)). Polymer FET devices have been fabricated with uniaxially
aligned polymer films fabricated by stretch alignment ((P.
Dyreklev, et al., Solid State Communications 82, 317 (1992)) and LB
deposition (J. Paloheimo, et al., Thin Solid Films 210/211, 283
(1992)). However, the field-effect mobilities in these studies have
been low (<10.sup.-5 cm.sup.2/s).
[0005] Local order in thin polymer films can be achieved by making
use of the tendency of some polymers to self-organise. An example
is poly-3-hexylthiophene (P3HT) in which lamella-type ordered
structures can be formed by phase segregation of rigid main chains
and flexible side chains. By using suitable deposition techniques
and chemical modification of the substrate it is possible to induce
preferential orientations of the ordered domains of the polymer
with respect to the substrate surface. At present P3HT yields the
highest known field-effect mobilities of 0.05-0.1 cm.sup.2/Vs for
polymer FETs (H. Sirringhaus, et al., Science 280, 1741 (1998)). In
these known devices there is no preferential, uniaxial alignment of
the polymer chains in the plane of the film.
[0006] Some conjugated polymers and small molecules exhibit
liquid-crystalline (LC) phases. By definition, a liquid-crystalline
phase is a state of matter, in which the molecules have a
preferential orientation in space. This alignment is conventionally
regarded as being alignment with respect to a vector called the
director. Unlike in the solid, crystalline state the positions of
the molecules in the LC phase are randomly distributed in at least
one direction. Depending on the type of orientational and residual
positional order one distinguishes between nematic, cholesteric and
smectic LC phases. The nematic phase possesses long-range
orientational order but no positional order. Smectic phases are
characterized by a two-dimensional (2D) layered structure, in which
the molecules self-assemble into a stack of layers each with a
uniform orientation of the molecules with respect to the layer
normal, but either no positional order or a reduced degree of
positional order in the 2D layers, LC phases occur mainly in
polymers/molecules with a significant shape anisotropy. Examples of
conjugated LC polymers are main-chain polymers with a rigid-rod
conjugated backbone and short flexible side chains, so-called
hairy-rod or rigid-rod polymers, Examples are poly-alkyl-fluorenes
(M. Grell, et al., Adv, Mat. 9, 798 (1998)) or ladder-type
poly-paraphenylenes (U. Scherf, et al., Makromol. Chem., Rapid.
Commun. 12, 489 (1991)). Another type of LC polymers are side-chain
polymers with a flexible non-conjugated backbone and rigid
conjugated units in the side chains.
[0007] A special class of liquid-crystalline organic molecules are
disc-shaped molecules with a rigid 2D conjugated core and flexible
side chains such as hexabenzocoronenes (HBC) (P. Herwig, et al.,
Adv. Mater. 8, 510 (1996)) or triphenylenes (D. Adam, et al. Nature
371, 141 (1994)). They tend to form so-called discotic mesophases
in which 1-dimensional columns are formed by .pi.-.pi. stacking of
the disc-shaped conjugated cores (FIG. 8).
[0008] LC phases typically occur at elevated temperatures in the
undiluted organic material (thermotropic phases) or if the organic
material is dissolved in a solvent at a sufficiently high
concentration (lyotropic phases) (see, for example, A. M. Donald,
A. H. Windle, Liquid Crystalline Polymers, Cambridge Solid State
Science Series, ed. R. W. Cahn, E. A. Davis, I. M. Ward, Cambridge
University Press, Cambridge, UK (1992)).
[0009] LC polymers can be uniaxially aligned by suitable processing
techniques. In an aligned sample the orientation of the director,
that is, for example, the preferential orientation of the polymer
chains in a main-chain LC polymer, is uniform over a macroscopic
distance of >.mu.m-mm. This is the scale of practical channel
lengths in FET devices. Alignment can be induced by shear forces or
flow or by depositing the LC polymer onto a substrate with an
alignment layer exhibiting a uniaxial anisotropy in the plane of
the substrate. The alignment layer may be a mechanically rubbed
organic layer such as polyimide (M. Grell, et al., Adv, Mat. 9, 798
(1998)), a layer evaporated at an oblique angle onto the substrate,
or a layer with a grooved surface. For a review of the various
techniques which can be used to align LC molecules see for example,
J. Cognard, J. Molec. Cryst. Liq. Cryst. Suppl. Ser. 1, 1
(1982).
[0010] A particularly attractive technique is photoalignment which
is less prone to mechanical damage than rubbing. A photosensitive
polymer is polymerized by exposure with linearly polarized light.
The plane of polarization of the light defines a preferential
orientation of the chains of the photosensitive polymer. Such
layers can be used as alignment layers for a broad range of polymer
and small molecule liquid crystals (M. Schadt, et al., Nature 381,
212 (1996)).
[0011] Uniaxially aligned liquid-crystalline polymers have been
incorporated as active light-emissive layers into polymer light
emitting diodes to produce linearly polarized light (M. Grell, et
al., Adv. Mat. 9, 798 (1998); G. Lussem, et al., Liquid Crystals
211 903 (1996)).
[0012] EP 0786 820 A2 discloses the device structure of an organic
thin film transistor in which the organic semiconducting layer is
in contact with an orientation film, such as a rubbed polyimide
layer. The orientation film is intended to induce alignment of the
organic semiconducting layer when the latter is deposited on top of
the orientation film. However, for most organic semiconducting
materials, in particular for conjugated polymers processed from
solution, mere deposition onto an orientation film is not
sufficient to induce alignment in the organic semiconductor.
[0013] WO99/10929 and WO99/10939 disclose a method of forming a
polymer field-effect transistor involving building up a
cross-linked layer structure and a method of forming an
interconnect in such a structure. Each layer is converted into an
insoluble form prior to solution deposition of the next layer,
[0014] According to one aspect of the present invention there is
provided a method for forming an electronic device having a
semiconducting active layer comprising a polymer, the method
comprising aligning the chains of the polymer parallel to each
other by bringing the polymer into a liquid-crystalline phase. This
aspect of the invention also provides an electronic device formed
by such a method.
[0015] According to a second aspect of the present invention there
is provided an electronic device having a semiconducting active
layer in which the polymer chains have been aligned parallel to
each other by bringing the polymer into a liquid-crystalline phase.
Preferably the chains are aligned parallel to each other.
[0016] The alignment of the chains may suitably be referred to as
uniaxial alignment since at least within a localised domain of
orientation, and more preferably over a wider extent, the parallel
alignment of the polymer chains indicates a single axis of
alignment.
[0017] The electronic device is suitably a switching device. The
electronic device is preferably a transistor, most preferably a
thin-film transistor. The device may thus be a polymer
transistor.
[0018] The said liquid-crystalline phase may be a nematic phase or
a smectic phase.
[0019] The step of bringing the polymer into the liquid-crystalline
phase suitably comprises heating the polymer. There is preferably a
subsequent step of cooling the polymer to fix its structure. That
cooling is preferably sufficiently rapid that the polymer retains
the said alignment in a preferred uniaxial direction after the
cooling. The cooling may be sufficiently rapid that the polymer is
in an amorphous, glassy state after the cooling. The cooling may
involve quenching the polymer. The cooling is preferably from above
the glass transition temperature of the polymer. The cooling is
conveniently to ambient temperature, for example room temperature
(20.degree. C.).
[0020] The said method may comprise forming source and drain
electrodes of the transistor in locations relative to the active
layer such that the channel of the transistor is oriented parallel
to the alignment direction of the polymer chains. Accordingly, the
said device may have a channel that is oriented parallel to the
alignment direction of the polymer chains.
[0021] The method preferably comprises depositing the polymer on
top of an alignment layer capable of inducing the said alignment of
the polymer. The method preferably comprises the step of forming
the alignment layer, for example by mechanical rubbing of a
substrate.
[0022] Preferably the parallel alignment of the polymer chains
extends over a distance/domain size of at least 100 nm and more
preferably at least 1 .mu.m, most preferably at least 10 .mu.m.
[0023] It is preferred that the polymer chains have monodomain,
uniaxial alignment over the area of the electronic device. However,
performance improvements may already be obtained if the alignment
occurs only locally, that is, if the polymer is in a multidomain
configuration with several domains with randomly oriented directors
located within the active area of the device. In each domain the
polymer chains would be aligned uniaxially parallel to the
director, when brought into the LC phase. To produce films in a
multilayer configuration no alignment layer is needed.
[0024] The polymer may be a semiconducting polymer. The polymer may
be a rigid-rod liquid-crystalline polymer. The polymer may be a
conjugated polymer. The polymer may be a polyfluorene polymer, for
example a polyfluorene homo-polymer or a polyfluorene based block
copolymer. The polymer may, for example be F8 or F8T2.
[0025] The semiconducting polymer may suitably be deposited from
solution. It is preferred that it is soluble in a non-polar organic
solvent, but is insoluble in a polar solvent.
[0026] The method may also comprise the step of forming an active
interface of the transistor by solution deposition of a second
polymer layer. That second polymer layer may be deposited on top of
a solution-processed polymer layer that has not been converted into
an insoluble form prior to the deposition of the second polymer
layer. The solution-processed layer may be the aforesaid aligned
layer and/or semiconductor active layer,
[0027] The second layer may provide a gate insulator of the
transistor. The second layer may be deposited from a polar organic
solvent in which the said solution-processed polymer layer is not
soluble. The solution deposition of the second polymer layer is
preferably performed after the said alignment step. The second
layer may be soluble in an alcohol solvent, such as isopropanol or
butanol. It may comprise polyvinylphenol (PVP).
[0028] An aspect of the present invention also provides a logic
circuit comprising a transistor as set out above. Such a logic
circuit may also include at least one optical device An aspect of
the present invention also provides an active matrix display
comprising a transistor as set out above, for example as part of
voltage hold circuitry of a pixel of the display.
[0029] According to a further aspect of the present invention there
is provided a method for forming an electronic device (for example
a transistor) comprising the step of forming an active interface of
the device by solution deposition of a polymer layer directly on
top of a solution-processed polymer layer that has not been
converted into an insoluble form prior to the deposition of the
second polymer layer.
[0030] According to a further aspect of the invention there is
provided a method for forming an electronic device having a
semiconducting active layer comprising a polymer, the method
comprising inducing parallel alignment in the chains of the polymer
by bringing the polymer into a liquid-crystalline phase. p
According to a further aspect of the invention there is provided a
method for forming an electronic device having a semiconducting
active layer comprising a polymer, the method comprising the step
of bringing the polymer into a liquid-crystalline phase.
[0031] According to a further aspect of the invention there is
provided a method for forming an electronic device having a
semiconducting active layer comprising a polymer, the method
comprising aligning the chains of the polymer within domains by
bringing the polymer into a liquid-crystalline phase.
[0032] A method for forming an electronic device having a
semiconducting active layer comprising a polymer, the method
comprising aligning the chains of the polymer as a monodomain
oriented in a preferred uniaxial direction within the layer of the
electronic device by bringing it into a liquid-crystalline
phase.
[0033] Preferred aspects of the said further aspects of the
invention include analogously all those set out above in relation
to the other aspects of the invention.
[0034] The present invention will now be described by way of
example with reference to the accompanying drawings, in which:
[0035] FIG. 1 is a schematic diagram of the top-gate device
configuration for LC polymer TFTs and orientation of the devices on
the substrate with respect to the rubbing direction;
[0036] FIG. 2 shows the polarized optical absorption spectra of a
uniaxially aligned T2/PVP TFT measured in the metal-free regions
with polarization of the light parallel and normal to the rubbing
direction;
[0037] FIG. 3 shows an optical micrograph of an aligned T2/PVP TFT
viewed under crossed polarizers. The source and drain electrodes
were defined by photolithography (Channel length L=20.mu.m);
[0038] FIG. 4 shows a schematic diagram of bottom (a) and top-gate
(b) TFT configurations;
[0039] FIG. 5 shows output characteristics of a top-gate P3HT/PVP
TFT on a glass substrate;
[0040] FIG. 6 shows output characteristics (a) and transfer
characteristics (b) of an aligned top-gate T2/PVP TFT (L=210 .mu.m,
channel width W=1.5 mm);
[0041] FIG. 7 shows saturated (a) and linear (b) transfer
characteristics of T2/PVP TFTs with channels oriented parallel and
normal to the rubbing direction (L=210 .mu.m, W=1.5 mm);
[0042] FIG. 8 shows the structure of the discotic liquid crystal
molecule hexabenzocoronene (HBC) and the desired orientation of the
discotic columns on the alignment layer with respect to the
direction of in-plane FET charge transport; and
[0043] FIG. 9 shows alternative configurations for aligned polymer
TFTs.
[0044] The present method provides a means of forming uniaxially
aligned polymers suitable for use, for example, as an active
semiconductor layer in an electronic device such as a
transistor--especially a polymer thin film transistor (TFT). The
method involves alignment of the polymer by means of a liquid
crystal phase.
[0045] In a device such as a TFT formed with such polymer as the
active layer, current flow is suitably either preferentially along
the polymer chains or preferentially normal to the polymer chains
(FIG. 1). This can allow useful uniformity of charge transport
properties in the relevant direction,
[0046] The method is described below with specific reference to
polyfluorene-based homopolymers such as poly-9,9-dioctylfluorene
(F8) and block-copolymers such as
poly(9,9-dioctylfluorene-co-dithiophene (F8T2). These main-chain LC
polymers exhibit nematic LC phases above 160.degree. C. (F8), and
265.degree. C. (F8T2), respectively. However, the method is not
limited to these materials or materials of those types, and is
applicable to a wide range of rigid-rod and LC polymers.
[0047] In one important preferred step of the method a polymeric
gate insulator may be deposited by solution-deposition on top of
the aligned semiconductive polymer forming an abrupt active
interface between the semiconductor and the insulator.
[0048] The fabrication of an aligned polymer FET having a top-gate
thin film transistor (TFT) configuration will now be described.
(See FIG. 1). Other devices and devices of other formations may
alternatively be made. In a first step a thin (500 .ANG.) polyimide
precursor film (Merck ZLI 2630 Polyimide kit) is spin-coated onto a
glass substrate (7059 glass, Corning) and converted to polyimide by
heating at 65.degree. C. for 15 min and 300.degree. C. for 1 h.
This polyimide precursor was selected for its high thermal
stability and glass transition temperature providing good alignment
capability at elevated temperature. Other materials may be used.
Then the polyimide film is rubbed mechanically with a nylon cloth
mounted on a mechanical drum to allow it to act as an alignment
layer as will be described below. A suitable procedure for forming
such an alignment layer is described in M. Grell, et al., Adv. Mat.
9, 798 (1998). Care is preferably taken to minimize the particle
contamination during the rubbing process as particles are believed
to be primarily responsible for device failure and gate leakage in
finished devices
[0049] Other techniques to fabricate the alignment layer could be
used as well instead of the above technique. A particularly
attractive technique is photoalignment, as it does not involve any
mechanical treatments of the alignment layer that may cause
mechanical damage to the film and reduce the yield of devices (M.
Schadt, et al., Nature 381, 212 (1996).
[0050] Gold source-drain electrodes are then defined on the
polyimide/glass substrate either by evaporation through a shadow
mask or by conventional photolithography. For the photolithographic
patterning of the gold electrodes lift-off techniques are
preferred. Direct etching of the gold film has been found to
adversely affect the alignment capability of the underlying
polyimide presumably by modification of the polyimide surface when
exposed to the etching solution (aqua regia). With
lift-off-techniques in which the surface of the polyimide is only
exposed to the acetone solvent with which the photoresist and the
gold film is lifted-off, the alignment of the LC polymers between
the source and drain electrodes (FIG. 3) was as high as on plain
substrates without source-drain electrodes.
[0051] Other deposition and patterning of the electrodes such as
direct ink-jet printing of a conducting polymer such as
polyethylenedioxythiophene doped with polystyrene sulfonate
(PEDOT/PSS) may be used. On the same substrates devices are defined
in which the TFT channel is either parallel or perpendicular to the
rubbing direction.
[0052] As the next step the LC semiconducting polyfluorene polymer
is deposited by spin-coating from a 1 weight % solution in mixed
xylenes. The thickness of the polymer film is on the order of
150-1000 .ANG.. Thin polymer films are preferred to minimize
contact resistance effects caused by transport of charge carriers
through the bulk of the semiconducting film from the source-drain
contacts to the active interface. The polymer film is then heated
into its LC phase at 200.degree. C. for 24-48 h (F8) and
280.degree. C. for 1-15 minutes (F8T2), respectively. During the
annealing the polymer aligns on the surface of the rubbed polyimide
surface The films are then brought into a glassy state by rapid
quenching to room temperature. The quenching is performed by
quickly moving the substrates from the hot stage onto a metallic
surface at room temperature. The quenching step is thought to
preserve the uniaxial alignment of the LC phase and to suppress
crystallization and formation of grain boundaries that would form
if the films were cooled slowly through the phase transition
between the LC and the crystalline phase. Crystalline grain
boundaries may act as charge carrier traps and adversely affect the
transistor performance. During the annealing step care is taken not
to contaminate the surface of the polymer film, by performing the
annealing steps either in vacuum or under inert atmosphere.
[0053] To further enhance the structural order in the polymer film
and the degree of uniaxial alignment additional annealing steps at
lower temperatures may be performed. The sample may also be kept in
a saturated solvent atmosphere at elevated temperatures after the
alignment.
[0054] FIG. 2 shows optical absorption spectra measured in the
metal-free regions of the completed TFT substrate. The absorption
between 2.5-3 eV belongs to the T2 polymer. The absorption is
stronger for light polarized parallel to the rubbing direction than
for light polarized perpendicular to it. Since this optical
transition is polarized along the polymer chain we conclude that
the polymer chains are aligned parallel to the rubbing direction.
The dichroic ratio estimated from the absorption spectra is
.apprxeq.9.6, which is a measure of the high degree of uniaxial
alignment of the polymer film.
[0055] FIG. 3 shows an optical micrograph of the channel region of
a completed TFT device with a uniaxially aligned T2 layer. The
image is viewed in reflection mode through the glass substrate.
Under crossed polarizers the interdigitated gold source-drain
electrodes appear dark. If the polarizers make a 45.degree. angle
with the rubbing direction s (parallel to the channel length L) the
plane of polarization of the incident light is rotated when passing
through the uniaxially aligned polymer film and some of the
reflected light can pass through the second polarizer. As a
consequence the film appears bright in between the dark source
drain electrodes (FIG. 3a). However, if one of the crossed
polarizers is oriented along the rubbing direction no rotation of
the polarization of the incident light can occur and the channel
region appears dark as well (FIG. 3b). In FIG. 3b the intensity of
the incident light has been enhanced relative to FIG. 3a to make
the contrast between the Au fingers and the channel region visible.
These observations clearly show that the polymer chains are aligned
uniaxially in the channel of the TFTs.
[0056] After the processing and annealing steps of the alignment of
the active semiconducting polymer the TFT devices are completed by
solution-deposition of a gate insulating layer and a metallic gate
electrode on top. In order to allow the fabrication of the
transistor channel on the top surface of the aligned Le polymer
film some critical problems had to be solved. For the formation of
a device having optimal electrical properties it is greatly
preferred (a) that the underlying layers are not dissolved nor
swelled by the solvents used for the deposition of the gate
insulator and (b) that at the same time the wetting properties of
the solutions on the underlying layers allow the deposition of
smooth and continuous insulating films.
[0057] It should be emphasized that preferred feature (a) is
particularly critical since the current flow in a TFT is confined
to a typically 10 nm thick interfacial layer at the interface
between the active semiconductor and the gate insulator, where the
accumulation layer is formed. The performance of the TFT is
determined by the structural and electronic properties of this
interfacial layer, whereas the properties of the bulk of the
semiconductor layer are of secondary importance only, The
performance of the TFT is very sensitive to the structural
properties and abruptness of the active interface. Any intermixing
between the semiconductor and the insulator during the solution
deposition will result in high interfacial roughness and
degradation of the electronic properties of the accumulation
layer.
[0058] Preferred feature (b) implies that the gate-insulating layer
needs to be smooth and continuous without pinholes and have a high
dielectric strength in order to allow the application of a high
electric field to form the accumulation layer.
[0059] The above mentioned preferred features (a) and (b) are not
only relevant for fabricating liquid-crystalline polymer TFTs, as
demonstrated here, but apply in general to solution processed,
all-polymer TFTs with top-gate structure (FIG. 4b), and analogously
to devices with the more conventional bottom-gate structure (FIG.
4a).
[0060] Previous approaches to fabricate all-polymer TFTs have used
precursor routes. A soluble precursor material is
solution-deposited as a first layer, and then converted into the
final polymer that is not soluble in common organic solvents. The
conversion may involve the elimination of solubilizing side groups,
an intrachain chemical reaction to form a more rigid polymer
backbone or a cross-linking reaction, The conversion usually
involves a heat treatment and/or exposure to a chemical reagent. To
fabricate bottom-gate all-polymer TFTs precursor-route insulating
layers (FIG. 4a) such as polyimide (Z. Bao, et al., Chem. Mat. 9,
1299 (1997)) have been used. For top-gate devices precursor-route
poly-thienylene-vinylene (PTV) as the active semiconductor has been
used (C. Drury, et al., APL 73, 108 (1998)). However, the use of
precursor routes severely limits the choice of semiconducting and
insulating polymers. For many polymers such as high-mobility
poly-3-hexylthiophenes (P3HT) no suitable precursor routes are
known. To the best of our knowledge all-polymer top-gate FETs with
a soluble polymer as the active semiconductor have not been
fabricated. It is non-obvious and has not been demonstrated yet
that a sufficiently abrupt interface can be formed by
solution-deposition techniques.
[0061] Here we demonstrate a method by which high-mobility
all-polymer top-gate TFTs can be fabricated with
solution-processible polymers by making use of the different
solubility of the semiconducting and insulating polymers in polar
and non-polar solvents. We use non-polar semiconducting conjugated
polymers such as P3HT, F8, F8T2 or
poly(9,9'-dioctyl-fluorene-co-N-(4-butylphenyl) diphenylamine)
(TFB) that have a low solubility in polar solvents such as
dimethylformamide (DMF), Propylene-glycol-methyl-ether-acetate
(pygmea), water or alcohols. We use insulating polymers such as
poly-hydroxystyrene (also called poly-vinylphenol (PVP)),
poly-vinyl-alcohol (PVA) polymethylmethacrylate (PMMA) or
poly-isobutyl-methyl-methacrylate (PiBuMA) which can be dissolved
in polar solvents. PVP has been used previously to fabricate
all-polymer top-gate TFTs in combination with an insoluble
semiconducting polymer (PTV) fabricated by a precursor route (C.
Drury, et al., APL 731 108 (1998)).
[0062] The particular solvent/insulating polymer combination is
chosen according to the polarity of the solvent and the wetting
properties of the solution on the surface of the underlying
semiconducting polymer. Alcohols such as methanol, ethanol,
butanol, or isopropanol have been found to be particularly
suitable. Most non-polar semiconducting polymers have exceptionally
low solubility in alcohols. This is partly because of the highly
polar nature of the hydroxyl groups of the alcohol, partly because
of hydrogen bonding among the alcohol molecules in solution. This
further lowers the solubility of those hydrophobic polymers that
cannot take part in hydrogen bonding. Solutions of insulating
polymers such as PVP in alcohols also tend to exhibit a relatively
high viscosity. This is believed to be partly due to the hydrogen
bonding between alcohol solvent molecules and the hydroxyl groups
of the PVP polymer in solution. A moderately high viscosity
facilitates the deposition of continuous smooth films helping to
overcome the wetting problems of a polar solution on a non-polar
substrate. Alcohols with different boiling points can be used.
Films fabricated from high boiling point alcohols such as butanol
tend to have smoother film morphology than those prepared from low
boiling alcohols such as isopropanol or methanol. However, no
significant differences of TFT mobility were observed for devices
fabricated with the same gate insulating polymer deposited from
different alcohol solvents. This indicates that the integrity and
abruptness of the active interface is well preserved with alcohol
solvents.
[0063] Other polar solvents such as
Poly-propylene-glycol-ether-acetate (pygmea) have been used
successfully as well. Although the devices functioned reasonably
well, the TFT mobility was typically a factor of 2 lower than with
alcohol solvents. This is attributed to some small residual
solubility of the polyfluorene polymer in the polar pygmea solvent.
To test this hypothesis FBT2 films were immersed in pygmea for
several minutes. Small changes of the optical absorption spectrum
of the film were observed afterwards, indicating that the solvent
interacted with the surface of the F8T2 film (either by dissolution
or swelling). No such changes were observed in the case of alcohol
solvents.
[0064] With PVP gate insulators deposited from alcohol solutions
TFTs were fabricated with a performance comparable to that of
control devices fabricated on standard SiO.sub.2/Si FET substrates.
Before deposition of the PVP the substrate is coated by the same
alcohol solvent that is used for the PVP solution and spun-dried.
This step is intended to wash off any alcohol-soluble residual
components of the polymer film. The PVP gate insulator is then
spin-coated from a 5-10 weight % solution, preferably in an alcohol
such as isopropanol or butanol. The capacitance of 1.2 .mu.m thick
PVP films are on the order of 3-5 nF/cm.sup.2 as measured by
impedance spectroscopy on metal-insulator-semiconductor (MIS)
diodes, and plain capacitor structures. Thinner PVP gate insulators
with thicknesses of 200-500 nm are preferred and have been
fabricated as well. The devices are completed by deposition of a
gold top-gate electrode through a shadow mask.
[0065] The source-drain and gate electrodes of the TFT may also be
fabricated from a conducting polymer that is patterned by a
suitable technique such as ink-jet printing, soft-lithographic
patterning or screen printing.
[0066] FIG. 5 shows the output characteristics of a top-gate
P3HT/PVP TFT fabricated on a standard glass substrate (without the
polyimide alignment layer). The device characteristics are
comparable to that of devices fabricated in the same deposition run
on conventional TFT substrates with bottom-gate SiO.sub.2
insulators. The mobility extracted from the transfer
characteristics in the saturation regime is on the order of
0.01-0.02 cm.sup.2Vs. The OFF-conductivity of the device is
somewhat higher than that of the corresponding bottom-gate devices
which is believed to be due to doping of the surface of the P3HT
surface by residual oxygen. This is not related to the specific
device structure and could be solved by exposing P3HT to a
reductive dedoping treatment in hydrazine prior to the deposition
of PVP (H. Sirringhaus, et al., Science 280, 1741 (1998)).
[0067] Devices were also fabricated with FBT2 as the semiconducting
polymer. On a plain glass substrate, that is without an alignment
layer, both the mobility of 0.003-0.005 cm.sup.2/Vs and the ON-OFF
current ratio are comparable to those of bottom-gate transistors
fabricated on SiO.sub.2.
[0068] These results demonstrate clearly that it is possible to
fabricate abrupt interfaces between two solution-processed polymer
layers and maintain the high degree of interfacial order which is
required to obtain high charge carrier mobilities of
>10.sup.-3-10.sup.-2 cm/Vs in the accumulation layer of a
TFT.
[0069] The procedure to deposit a polymeric gate insulator from a
polar solvent has been applied to fabricate top-gate TFT devices on
top of uniaxially aligned LC polymer films. FIG. 6 shows the output
(a) and transfer characteristics (b) of an aligned T2/PVP TFT with
the channel oriented parallel to the rubbing direction. The device
characteristics show good current saturation and ON-OFF current
ratios >10.sup.4-10.sup.5. FIG. 7 compares the transfer
characteristics in the saturation (a) and linear (b) regime of
aligned T2/PVP TFT devices with channels parallel and perpendicular
to the rubbing direction s. The devices are fabricated on the same
substrate. The transistor current is higher by a factor of
typically 5-8 if the current flow is along the rubbing direction,
that is, along the preferential direction of the polymer backbones,
The mobilities extracted from the transfer characteristics are
0.009-0.02 cm.sup.2/Vs for transport parallel to the direction of
preferential alignment of the polymer chains and 0.001-0.002
cm.sup.2/Vs for the perpendicular direction.
[0070] These mobility values can be compared to those of isotropic
TFT devices, that underwent the same thermal treatment as the
uniaxially aligned devices, but exhibit no monodomain alignment.
These isotropic, multidomain devices (x) are prepared on areas of
the substrate that are not coated by polyimide, and typically
exhibit mobilities .mu..sub.x=0.003-0.005 cm.sup.2/Vs with
.mu..sub..perp.<.mu..sub.x<.mu.||.
[0071] This demonstrates that uniaxial alignment results in a
significant improvement of the mobility by typically a factor of 3
compared to isotropic devices. However, even the isotropic devices
exhibit significantly higher mobilities and better turn-on voltage
stability than devices in which the F8T2 film underwent no thermal
treatment (.mu..sub.as-spun<10.sup.-3 cm.sup.2/Vs). In the
multidomain, isotropic devices the domain sizes are on the order of
0.1-1 .mu.m, as estimated from optical micrographs, that is, the
TFT channel contains several LC domain boundaries. This indicates
that LC domain boundaries in the nematic glass do not act as
carrier traps to the same extent as microcrystalline grain
boundaries in as-spun films.
[0072] The observation of a significant mobility anisotropy shows
that with the device configuration demonstrated here it is possible
to exploit fast intrachain transport along the polymer chain using
conjugated polymers that form LC phases and can be aligned by using
alignment layers. Further optimisation of the alignment process,
which will include optimisation of the annealing and rubbing
procedure and treatment of the aligned films with solvent vapours
is likely to result in a further increase of the dichroic ratio in
the channel region and higher mobilities.
[0073] Other LC polymers may be used as well. Higher mobilities are
expected in materials that show stronger interchain interactions,
and facilitate interchain hopping of charge carriers between
adjacent polymer chains. The most desirable orientation to obtain
high mobilities would be a structure in which the direction of
.pi.-.pi. stacking was preferentially oriented in the plane of the
film perpendicular to the rubbing direction. This would require a
biaxial liquid crystalline polymer showing an anisotropy and
preferred orientation of the molecules in a plane normal to the
alignment direction of the polymer backbone (A. M. Donald, A. H,
Windle, Liquid Crystalline Polymers, Cambridge Solid State Science
Series, ed. R. W. Cahn, E. A. Davis, I. M. Ward, Cambridge
University Press, Cambridge, UK (1992)). LC polymers that yield a
higher degree of alignment and longer persistence length along the
polymer chain (M. Grell et al., Macromolecules 32, 5810 (1999)) are
also expected to yield higher mobilities.
[0074] In order to allow for efficient hole injection from the
source-drain electrodes the LC polymer should have a sufficiently
low ionisation potential, preferably below 5.5 eV, that is well
matched to the work function of common source-drain electrode
materials such as inorganic metal electrodes (gold, platinum,
aluminium, etc.) or conducting polymers such as PEDOT.
[0075] Other types of LC molecules to which this TFT fabrication
process is applicable are disc-shaped conjugated molecules with
flexible side chains forming discotic mesophases. Along the 1 D
columnar stacks high charge carrier mobilities can be obtained (D.
Adam, et al. Nature 371, 141 (1994)). In some discotic molecules
such as hexabenzocoronenes (HBC)) (P. Herwig, et al., Adv. Mater.
8, 510 (1996)) the columns tend to be oriented in the plane of the
film such that the high mobilities along the columns could be
exploited in in-plane transistor transport (FIG. 8). HBC has a low
solubility in polar solvents as well. Techniques to align discotic
molecules with the use of alignment layers have been developed
(Mori, Hiroyuki, European patent application 94116645.6; Kamada, et
al., European patent application 94114956.9).
[0076] Several modifications of the device configuration are
possible. The source/drain electrodes may be deposited after
deposition/and alignment of the LC polymer (FIG. 5a). This will
facilitate charge carrier injection at the source-drain electrodes,
and improve the alignment of the polymer in the vicinity of the
electrodes. The alignment layer may be used as the gate insulator
itself in a conventional bottom gate configuration either in a
single-layer (FIG. 5c) or double-layer configuration (FIG. 9b). In
the latter the second insulating layer provides additional
dielectric strength. The bottom gate structure may be less suitable
for alignment layers produced by mechanical rubbing due to defects
at the active interface, but it is believed to be suitable for
other alignment techniques such as photoalignment.
[0077] Applications of polymer TFTs according to this invention are
in polymer TFT logic circuits (C. Drury, et al., APL 73, 108
(1998)) or as pixel drive transistors in high-resolution, active
matrix displays (H. Sirringhaus, et al., Science 280, 1741 (1998)).
Examples of such displays are active matrix polymer LED displays,
liquid-crystal displays (LCD) or electrophoretic displays. The
enhanced charge carrier mobility along the direction of
preferential uniaxial alignment of the polymer chains compared to
the mobility of the isotropic polymer film can be used to increase
the operation speed and the drive current capability of the
TFTs.
[0078] Some preferred aspects of selection of semiconducting
polymers for use as described above will now be described.
[0079] The polymer should preferably be able to form monodomains on
the alignment layer at temperatures below 300.degree. C., most
preferably below 200/150.degree. C. To be able to inject charge
carriers and to obtain stable TFT characteristics the ionisation
potential of the material should preferably be below 5.8 eV,
preferably below 5.5 eV, most preferably below 5.1 eV.
[0080] The polymer needs preferably to have good stability against
chemical reaction with the atmosphere (oxygen, water) etc during
the high temperature annealing step. The polymer should have an
ionisation potential larger than 4.9 eV, preferably higher than 5.1
eV. The TFT should have a ON-OFF current switching ratio larger
than 10 3, most preferably larger than 10 4, and a turn-on gate
voltage V0 less than -30V, most preferably less than -10V.
[0081] A preferred class of materials to achieve good environmental
stability and high mobility are A-B rigid-rod block-copolymers
containing a regular ordered sequence of A and B blocks, Suitable A
blocks are structurally well defined, ladder type moieties with a
high band gap (e.g. greater than 2.5 eV), that have high ionisation
potentials larger than 5.5 eV as a homopolymer and good
environmental stability. These features are preferable
independently and in combination. Examples of suitable A blocks are
fluorene derivatives (for example those disclosed in U.S. Pat. No.
5,777,070), indenofluorene derivatives, or phenylene or ladder-type
phenylene derivatives (for example those disclosed in J. Grimme et
al., Adv. Mat. 7, 292 (1995)). Suitable B blocks are
hole-transporting moieties with lower bandgaps (for example less
than 2.5 eV) that contain heteroatoms such as sulphur or nitrogen,
and as a homopolymer have ionisation potentials less than 5.5 eV.
These features are preferable independently and in combination.
Examples of hole-transporting B blocks are thiophene derivatives,
or triarylamine derivatives. The effect of the B block is to lower
the ionisation potential of the block copolymer. The ionisation
potential of the block copolymer is preferably in the range of 4.9
eV.ltoreq.I.sub.p.ltoreq.5.5 eV. Examples of such copolymers are
F8T2 (ionisation potential 5.5 eV) or TFB (U.S. Pat. No.
5,777,070).
[0082] Instead of hole transporting semiconducting polymers soluble
electron transporting materials may be used as well. These require
a high electron affinity larger than 3 eV, preferably larger than
3.5 eV, to prevent residual atmospheric impurities such as oxygen
to act as carrier traps. AB-type block copolymers with a
structurally-well defined, ladder-type A block with a high
ionisation potential larger than 5.5 eV and an
electron-transporting B block that increases the electron affinity
of the copolymer to a value higher than 3 eV, preferably higher
than 3.5 eV are preferred. Examples of A blocks are fluorene
derivatives (for example those disclosed in U.S. Pat. No.
5,777,070), indenofluorene derivatives, phenylene or ladder-type
phenylene derivatives (for example those disclosed in J. Grimme et
al., Adv. Mat. 7, 292 (1995)). Examples of electron-transporting B
blocks are benzothiadiazole derivatives (for example those
disclosed in U.S. Pat. No. 5,777,070), perylene derivatives,
naphtalenetetracarboxylic diimide derivatives or fluorinated
thiophene derivatives.
[0083] Many of these block copolymers exhibit liquid-crystalline
phases at elevated temperatures (F8, FBT2), whereas others such as
TFB do not form LC phases. With TFB field-effect mobilities of
0.002 cm.sup.2/Vs have been obtained in the: top-gate device
configuration. In the case of TFB an annealing step at a
temperature of 200-290.degree. C., i.e., above the glass transition
temperature, followed by rapid quenching has been found to enhance
the mobility and threshold voltage stability. Although in the case
of TFB no LC transition could be detected,.this improvement of
top-gate TFB devices is attributed to a reduction of charge
trapping at structural defects such as crystalline domain
boundaries when the polymer is prepared in an amorphous glassy
state.
[0084] In general, polyfluorene-based block-copolymers are a
promising new class of conjugated polymers that have not hitherto
been used in polymer transistor devices. For the reasons indicated
above, polyfluorene-based block copolymers are one preferred class
of materials for aligned semiconductor layers of devices described
above.
[0085] The present invention may include any feature or combination
of features disclosed herein either implicitly or explicitly or any
generalisation thereof, irrespective of whether it relates to the
presently claimed invention. In view of the foregoing description
it will be evident to a person skilled in the art that various
modifications may be made within the scope of the invention.
* * * * *