Patent Application
20090181509
Fabrication of printed organic electronics (POE) is of profound interest, as such devices are ultra-low cost, are solution processable, and possess mechanical durability and structural flexibility. One type of POE, a printed thin film transistors (TFT), has received much attention in recent years as it is a promising, low cost alternatives to silicon technology for application in, for example, active-matrix liquid crystal displays (LCDs), organic light emitting diodes, e-paper, radio frequency identification tags (RFIDs).
The above characteristics are desirable for fabricating flexible TFTs on a number of substrates, including plastic substrates. The use of a plastic substrate with a polymer semiconductor can transform a rigid silicon TFT into a mechanically more durable and structurally flexible TFT. This transformation can be of particular value to large area devices such as large-area image sensors, electronic paper and other display media.
Polymer semiconductors, such as poly[5,5′-bis(3-dodecyl-2-thienyl)-2,2′-bithiophene] (PQT), have a mobility of 0.1-0.2 cm2/V·s. However, if two substituted or unsubstituted thiophene rings in the repeating unit of PQT are fused together, the mobilities of the PQT derivatives such as poly(2,5-bis-(3-alkylthiophen-2-yl)-thieno[3,2-b]thiophene) and poly(2,5-bis(2-thienyl)-3,6-dialkylthieno[3,2-b]thiophene), increase by at least a factor of 2. This result illustrates that a larger fused ring induces better intermolecular pi conjugation and better intramolecular pi-pi stacking, thus better mobility. However, if the two thiophene rings are fused linearly or parallel to the polymer's backbone, low solubility often results.
Further, heating the substrate or “annealing” is an important step in manufacturing current POE devices based on the PQT and it's derivative polymers. This time and energy consuming annealing process is not compatible with a roll-to-roll massive manufacturing process utilized to fabricate a low cost POE.
The above and other issues are addressed by the present application, wherein in embodiments, the application relates to a class of polymer semiconductors with pi conjugation extended perpendicular to the backbone direction to yield a weak side chain interaction. A polymer semiconductor with the pi conjugation extended perpendicular to the backbone promotes solubility and crystallinity. Also, a polymer semiconductor with a weak side chain interaction allows for the preparation of high performance polymer semiconductors without the need for high temperature annealing.
The present application thus achieves advances over prior polymer semiconductors and discloses a polymer semiconductor where various side chains are arranged in a manner to provide no or weak side chain interactions, and thus achieves high field-effect mobility without thermal annealing. The present application also achieves advances over prior polymer semiconductors and discloses a polymer semiconductor wherein the fusing ring structure is extended perpendicular to the conjugated backbone, thus achieving both good solubility and high crystallinity for high mobility.
In embodiments, a polymer semiconductor compound of Formula 1,
wherein R1, R2, R3 and R4 are side chains independently selected from the group consisting of a hydrogen atom, a hydrocarbon group, a heteroatom, and combinations thereof, and wherein at least one of the R1, R2, R3 and R4 side chains are not a hydrogen atom, wherein k represents the number of repeating units of from 1 to about 200; A is a divalent linkage; and B and C are independently selected from the group consisting of a bond, a divalent linkage, and combinations thereof, wherein m, n, p and q each individually represent from 0 to about 20, wherein if the both B and C are thieno[3,2-b]thiophene, m and n cannot be 1 and p and q cannot be 1, and wherein R1, R2, R3 and R4 are arranged in a manner promoting high mobility and providing a weak side chain interaction.
In further embodiments, a thin-film transistor comprised of a polymer semiconductor layer having a polymer semiconductor compound of Formula 1,
wherein R1, R2, R3 and R4 are side chains independently selected from the group consisting of a hydrogen atom, a hydrocarbon group, a heteroatom, and combinations thereof, and wherein at least one of the R1, R2, R3 and R4 side chains are not a hydrogen atom, wherein k represents the number of repeating units of from 1 to about 200; A is a divalent linkage; and B and C are independently selected from the group consisting of a bond, a divalent linkage, and combinations thereof, wherein m, n, p and q each individually represent from 0 to about 20, wherein if the both B and C are thieno[3,2-b]thiophene, m and n cannot be 1 and p and q cannot be 1, and wherein R1, R2, R3 and R4 are arranged in a manner promoting high mobility and providing a weak side chain interaction.
A method of manufacturing a thin-film transistor, which comprises a substrate, a gate electrode, a gate dielectric layer, an insulating layer, a source electrode and a drain electrode and in contact with the source/drain electrodes and the gate dielectric layer, a semiconductor layer comprised of a polymer semiconductor compound of formula 1,
wherein R1, R2, R3 and R4 are side chains independently selected from the group consisting of a hydrogen atom, a hydrocarbon group, a heteroatom, and combinations thereof, and wherein at least one of the R1, R2, R3 and R4 side chains are not a hydrogen atom, wherein k represents the number of repeating units of from 1 to about 200; A is a divalent linkage; and B and C are independently selected from the group consisting of a bond, a divalent linkage, and combinations thereof, wherein m, n, p and q each individually represent from 0 to about 20, wherein if the both B and C are thieno[3,2-b]thiophene, m and n cannot be 1 and p and q cannot be 1, and wherein R1, R2, R3 and R4 are arranged in a manner promoting high mobility and providing a weak side chain interaction, the method comprising: forming the gate electrode and gate dielectric layer, depositing the semiconductor compound of formula 1 on the substrate to form an active layer of the semiconductor compound on the surface of the substrate, wherein no high-temperature annealing is conducted following the formation of the active layer of the semiconductor compound; and depositing the source electrode and the drain electrode.
Illustrated in
Illustrated in
Illustrated in
Illustrated in
In embodiments, the polymer semiconductor can be represented by formulas (1) through (6).
wherein each R, R1, R2, R3 and R4 can be a hydrogen atom, a hydrocarbon group, a heteroatom, m, n, p and q can represent the number of thienylene units of from about 1 to about 5000, of from about 2 to about 3000 or from about 5 to about 1000, from about 5 to about 800, from about 5 to about 200 or from about 5 to about 75; k can represent the number of repeating units of from about 1 to about 5000, of from about 2 to about 3000 or from about 5 to about 1000, from about 5 to about 800, from about 5 to about 200 or from about 5 to about 75. Formula 1 represents the generic structure of the polymer semiconductor compounds.
Unless otherwise indicated, in identifying the substituents for R, R′, R1, R2, R3 and R4, the phrase “hydrocarbon group” encompasses both unsubstituted hydrocarbon groups and substituted hydrocarbon groups. The unsubstituted hydrocarbon group may contain, for example, from about 1 to about 50 carbon atoms, from about 5 to about 40 carbon atoms or from about 10 to about 35 carbon atoms. However, at least one substituent, R1, R2, R3 and R4, in the polymer semiconductor compound must not be a hydrogen atom. Examples of the unsubstituted hydrocarbon groups may include, for example, a straight chain alkyl group, a branched alkyl group, a cycloalkyl group, an aryl group, an alkylaryl group, and an arylalkyl group. Example alkyl groups may include, for example, methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, cyclopentyl, cyclohexyl, cycloheptyl, and isomeric forms thereof.
The substituted hydrocarbon group may contain, for example, from about 1 to about 70 carbon atoms, from about 10 to about 60 carbon atoms and from about 20 to about 50 carbon atoms substituted with, for example, fluorine, bromine, chlorine, iodine, sulfur, amino, nitro, cyano, methoxyl, ethoxyl, propoxy, or combinations thereof. Substituted hydrocarbon groups may be, for example, a straight chain alkyl group, a branched alkyl group, a cycloalkyl group, an aryl group, an alkylaryl group, and an arylalkyl group with a heteroatom. Example alkyl groups may include, for example, methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, cyclopentyl, cyclohexyl, cycloheptyl, and isomeric forms thereof In embodiments, the hydrocarbon group may be optionally substituted alkyl and optionally substituted aryl.
Unless otherwise indicated, in identifying the substituents for R, R′, R1, R2, R3 and R4, the term “heteroatom” includes fluorine, bromine, chlorine, iodine, sulfur, nitrogen, oxygen, or combinations thereof. Further the heteroatom can be a single atom such as, for example, chlorine or fluorine or the single atom contained in a compound such as, for example, the nitrogen atom (N) in an amino compound (NH2) and the sulfur atom (S) in a SO2 compound.
In embodiments, the polymer semiconductor may have a symmetrical repeating unit. As used herein, the term “symmetrical repeating unit” refers to a polymer semiconductor with the same substituent or hydrocarbon group repeated in a symmetrical fashion down the backbone of the polymer semiconductor.
The number average molecular weight (Mn) of the polymers can be, for example, from about 500 to about 400,000, including from about 1,000 to about 150,000, and the weight average molecular weight (Mw) can be from about 600 to about 500,000, including from about 1,500 to about 200,000, both as measured by gel permeation chromatography using polystyrene standards.
In embodiments, A is a conjugated fused-ring divalent linkage. The fused ring divalent linkage has at least 8 electrons to form a pi-conjugation. For example from 8 to 30 electrons, or from 10 to 20 electrons to form a pi-conjugation. In embodiments, the pi-conjugation of the fused ring is extended perpendicular to the backbone.
In further embodiments, A in the polymer semiconductor compound can be selected from any divalent linkage having at least 4 carbon atoms, such as, for example, from about 4 to about 40 carbon atoms, from about 6 to about 30 carbon atoms, from about 8 to about 24 carbon atoms and from about 8 to about 16 carbon atoms. Examples of divalent linkage may include, for example,
or combinations thereof. Examples of R′ may include a hydrogen atom, hydrocarbon group, heteroatom or combinations thereof.
In even further embodiments, B and C in the polymer semiconductor compound can be selected from a bond and any divalent linkage having at least 4 carbon atoms, such as, for example, from about 4 to about 40 carbon atoms, from about 6 to about 30 carbon atoms, from about 8 to about 24 carbon atoms and from about 8 to about 16 carbon atoms. Examples of divalent linkage may include, for example,
or combinations thereof. Examples of R′ may include a hydrogen atom, hydrocarbon group, heteroatom or combinations thereof.
As used herein, the phrase “bond” between the substituents of the polymer semiconductor is a chemical bond. The chemical bond can take the form of, for example, covalent bonding, hydrogen bonding, coordination complex bonding, or ionic bonding, or a mixture of different chemical bondings. In embodiments, the bond is a covalent bond.
In embodiments, if both B and C are thieno[3,2-b]thiophene,
the number of repeating units characterized as m and n of Formula 1 and Formula 2 both cannot equal 1. Further, if both B and C are thieno[3,2-b]thiophene,
the number of repeating units characterized as p and q of Formula 1 and Formula 2 both cannot equal 1.
The polymer semiconductor can be arranged in any combination of copolymer or unsymmetrical repeating units, except where two substituents or hydrocarbon groups are present at the 3 and 3′ positions for any two adjacent thiophene units within the structure of the polymer semiconductor, as further illustrated by the figure below.
In other words, simultaneous substitutions at the third ring positions (3 and 3′) of two adjacent thiophene units are not allowed. The simultaneous presence of substituents at the third ring positions may cause torsional deviation of the two adjacent thiophene units from coplanarity, and thus significantly shorten the pi-conjugation of the polymer semiconductor. Shortening the pi-conjugation may lead to short pi-delocalization and thus resulting in both poor charge carrier transport capability and low mobility.
In further embodiments, the structures of formulas (1) and (2) of the polymer semiconductor can be arranged in any manner that promotes high mobility due to a weak side chain interaction and does not require high-temperature thermal annealing. As used herein, the phrase “high mobility” is any mobility of the polymer semiconductor compound greater of 0.1 cm2/V·s, which, as stated above, is the upper limit of the mobility for the PQT compound.
The weak side chain interaction within the polymer semiconductor may be achieved by 1) narrowing the repeating distance of the side chains resulting in no side chain interdigitation, 2) widening the repeating distance between the side chains to form a very loose side chain interdigitation, or 3) substituting a bulky hydrocarbon group perpendicular to the backbone of the polymer semiconductor.
In embodiments, the polymer semiconductor does not achieve this undesired strong side chain interaction so long as the side chains are arranged in a manner results in a weak side chain interaction. In terms of thermal properties of the polymer semiconductor, there is no side chain phase transition in its DSC thermal diagram such as, for example, side chain melting.
In embodiments, the weak side chain interaction of the polymer semiconductor may be achieved by narrowing the repeating distance between the side chains so that there is no side-chain interdigitation.
In embodiments, the side chains on the polymer semiconductor may be narrowed to a distance of from about 10 angstroms to about 4.5 angstroms and from about 8 angstroms to about 5 angstroms. It is understood that this distance may change depending on the side chain distribution manner. For example, if a space is needed to adopt two side chains at the same time as shown in the example above, the required distance to avoid side chain interdigitation may be different from above.
The weak side chain interaction of the polymer semiconductor may also be achieved by widening the repeating distance between the side chains so as to promote loose interdigitation. As seen from
In embodiments, the loose interdigitation may be the widening of the distance between the side chains to a distance of from about 14 angstroms to about 50 angstroms and from about 16 angstroms to about 25 angstroms.
The weak side chain interaction may also be achieved by substituting a bulky hydrocarbon group perpendicular to the backbone of a polymer semiconductor. As used herein, the word “bulky” in the phrase bulky hydrocarbon group is any hydrocarbon group containing at least 10 carbon atoms such as, for example, from about 10 to about 500 carbon atoms, from about 20 to about 200 carbon atoms, from about 30 to about 100 carbon atoms and from about 40 to about 75 carbon atoms.
In embodiments, a polymer semiconductor with a “fused aromatic” or increased pi conjugation extended perpendicular to the backbone increases the solubility and promotes crystallinity. The solubility of the polymer semiconductor may increase from a factor of about 1.5 to about 3, from about 1.75 to about 2.75 and from about 2.0 to about 2.25. The crystallinity of the polymer semiconductor may increase by a factor of 1.5 to about 3, from about 1.75 to about 2.75 and from about 2.0 to about 2.25. As used herein, the phrase “fused aromatic” may be defined as any two aromatic compounds linked by a shared C—C bond. Examples may include the following aromatic compounds or mixtures thereof:
A polymer semiconductor with a weak side chain interaction promotes high mobility without the need for a high temperature annealing temperature to reorder the polymer backbones. However, if annealing is required, the annealing temperature for the polymer semiconductor may be below about 180 ° C., such as, for example, below about 150° C. below about 100° C., below about 75° C. or below about 50° C.
The polymer semiconductors of Formula (1) are soluble or substantially soluble in common coating solvents, for example, in embodiments they possess a solubility of at least about 0.1 percent by weight, and more specifically, from about 0.5 percent to about 10 percent, or to about 50 percent by weight in such solvents as methylene chloride, 1,2-dichloroethane, tetrahydrofuran, toluene, xylene, mesitylene, chlorobenzene, dichlorobenzene, and the like. Moreover, the polymer semiconductors of the formulas as illustrated herein provide a stable conductivity of, for example, from about 10−9 S/cm to about 10−4 S/cm, and more specifically, from about 10−8 S/cm to about 10−5 S/cm as determined by conventional four-probe conductivity measurements.
In yet other embodiments, there is provided a thin-film transistor comprising:
In embodiments and with further reference to the present disclosure, the substrate layer may generally be a silicon material inclusive of various appropriate forms of silicon, a glass plate, a plastic film or a sheet, and the like depending on the intended applications. For structurally flexible devices, a plastic substrate, such as for example polyester, polycarbonate, polyimide sheets, and the like, may be selected. The thickness of the substrate may be, for example, from about 10 micrometers to about 100 millimeters with a specific thickness being from about 50 to about 100 micrometers, especially for a flexible plastic substrate, and from about 1 to about 10 millimeters for a rigid substrate such as glass or silicon.
The gate dielectric layer, which can separate the gate electrode from the source and drain electrodes, and in contact with the semiconductor layer, can generally be an inorganic material film, an organic polymer film, or an organic-inorganic composite film. The thickness of the gate dielectric layer can be, for example, from about 10 nanometers to about 1 micrometer with a more specific thickness being about 100 nanometers to about 500 nanometers. Examples of inorganic materials suitable as the dielectric layer may include silicon oxide, silicon nitride, aluminum oxide, barium titanate, barium zirconate titanate and the like. Examples of organic polymers for the dielectric layer may include polyesters, polycarbonates, poly(vinyl phenol), polyimides, polystyrene, poly(methacrylate)s, poly(acrylate)s, epoxy resin and the like. Examples of inorganic-organic composite materials may include nanosized metal oxide particles dispersed in polymers, such as polyester, polyimide, epoxy resin and the like. The gate dielectric layer is generally of a thickness of from about 50 nanometers to about 500 nanometers depending on the dielectric constant of the dielectric material used. More specifically, the dielectric material has a dielectric constant of, for example, at least about 3, thus a suitable dielectric thickness of about 300 nanometers can provide a desirable capacitance, for example, of about 10−9 to about 10−7 F/cm2.
Situated, for example, between and in contact with the dielectric layer and the source/drain electrodes is the active semiconductor layer comprised of polymer semiconductors of the formulas as illustrated herein, and wherein the thickness of this layer is generally, for example, about 10 nanometers to about 1 micrometer, or about 40 to about 100 nanometers. This layer can generally be fabricated by solution processes such as spin coating, casting, screen, stamp, or jet printing of a solution of polymer semiconductors of the present disclosure.
The gate electrode can be a thin metal film, a conducting polymer film, a conducting film generated from a conducting ink or paste, or the substrate itself (for example heavily doped silicon). Examples of the gate electrode materials may include gold, chromium, indium tin oxide, conducting polymers, such as polystyrene sulfonate-doped poly(3,4-ethylenedioxythiophene) (PSS/PEDOT), a conducting ink/paste comprised of carbon black/graphite or colloidal silver dispersion contained in a polymer binder, such as Electrodag available from Acheson Colloids Company, and silver filled electrically conductive thermoplastic ink available from Noelle Industries, and the like. The gate layer may be prepared by vacuum evaporation, sputtering of metals or conductive metal oxides, coating from conducting polymer solutions or conducting inks, or dispersions by spin coating, casting or printing. The thickness of the gate electrode layer may be, for example, from about 10 nanometers to about 10 micrometers, and a specific thickness may be, for example, from about 10 to about 200 nanometers for metal films, and about 1 to about 10 micrometers for polymer conductors.
The source and drain electrode layer can be fabricated from materials which provide a low resistance ohmic contact to the semiconductor layer. Typical materials suitable for use as source and drain electrodes may include those of the gate electrode materials such as gold, nickel, aluminum, platinum, conducting polymers, and conducting inks. Typical thickness of this layer may be, for example, from about 40 nanometers to about 1 micrometer with the more specific thickness being about 100 to about 400 nanometers. The TFT devices contain a semiconductor channel with a width W and length L. The semiconductor channel width may be, for example, from about 10 micrometers to about 5 millimeters with a specific channel width being about 100 micrometers to about 1 millimeter. The semiconductor channel length may be, for example, from about 1 micrometer to about 1 millimeter with a more specific channel length being from about 5 micrometers to about 100 micrometers.
The source electrode is grounded and a bias voltage of generally, for example, about 0 volts to about −80 volts is applied to the drain electrode to collect the charge carriers transported across the semiconductor channel when a voltage of generally, for example, about +10 volts to about −80 volts is applied to the gate electrode.
In embodiments, the annealing temperature for a thin-film transistor comprised of a polymer semiconductor layer having the polymer semiconductor compounds according to the Formulas herein is about or below about 150° C., about or below about 125° C. or about or below about 100° C.
Other known suitable materials not recited herein for the various components of the TFT devices of the present disclosure can also be selected in embodiments.
The synthesis reaction for poly(4,8 didodecyl-2,6-bis-(3-methylthiophen-2-yl)-benzo[1,2-b:4,5-b′]dithiophene), a polymer semiconductor according to Formula 1, can be represented the following diagram. This polymer has a weak side chain interaction due to wide side chain spacing.
A 100 mL three-necked round-bottomed flask was flushed with argon gas and then evacuated under reduced pressure with heating. This procedure was repeated twice before magnesium turnings (1.65 g, 67.8 mmol) were added. After the flask was again flushed with argon for 10 minutes, 10 mL of anhydrous ether was added with stirring and followed by the slow addition of 5 mL solution of 2-bromo-3-methylthiophene (10.0 g, 56.5 mmol) in 20 mL anhydrous ether. The reaction mixture was slightly heated with a heat gun to initiate reaction that occurred about 3 minutes later with gas evolution. Subsequently, all the remaining 2-bromo-3-methylthiophene solution was added, and the reaction was allowed to proceed at room temperature for another 2 hours.
The resulting reaction mixture was transferred to another 100 mL flask and cooled to −78° C., after which 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (17.3 mL, 169.5 mmol) was added by a syringe. After stirring the resulting solution at room temperature for 18 hours, 15 mL of water was added to the reaction mixture and the organic and aqueous layers were separated. The aqueous portion was extracted with CH2Cl2 (3×50 mL), and the extract combined with the organic portion, dried with anhydrous MgSO4 and evaporated to dryness. Purification of the residue via column chromatography on silica gel (fluent: hexane/dichloromethane, 4/1, v/v) yielding 10.1 grams (Yield: 80%) of pure product as white powder after drying in vacuum.
To a 100 mL 3-necked reaction flask 1.0 g (1.46 mmol) of 2,6-dibromo-4,8-didodecylbenzo[1,2-b:4,5;b′]dithiophene, 0.82 g (3.65 mmol) of 3-methylthiophene-2-boronic acid pinacol ester, and 25 mL of toluene were added. The resulting mixture was stirred and purged with argon before a mixture of 0.034 g of tetrakis(triphenylphosphine palladium(0)) (Pd(Ph3P)4), 0.36 g of Aliquat in 5 mL toluene, and 4.2 mL of 2 M aqueous Na2CO3 was added. Subsequently, the reaction mixture was heated at 105° C. for 26 hours with stirring and cooled to room temperature.
Next, 100 mL of toluene was added and the organic layer was separated and washed 3 times with water, dried over anhydrous MgSO4, and filtered. After evaporation, the filtrate gave a solid residue, which was purified by column chromatography on silica gel (eluent: hexane/dichloromethane, 7/1, v/v) and then further recrystallized from 2-propanol to yield 0.57 grams of pure product (Yield: 57%) as a yellow needle-like crystal.
A solution of 0.41 g 4,8-didodecyl-2,6-bis-(3-methylthiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene in 10 mL of chlorobenzene was added dropwise to a well-stirred mixture of 0.46 g FeCl3 in 10 mL of chlorobenzene in a 50 mL round-bottom flask under an argon atmosphere over a period of 1 minute. The resulting mixture was heated to 65° C. and maintained at this temperature for 48 hours under a blanket of argon.
After cooling down to room temperature, 15 mL of chlorobenzene was added. The resulting mixture was poured into 200 mL of methanol, and ultrasonicated for 20 minutes before being stirred at room temperature for 1 hour to form the precipitate. The precipitated product was filtered and added to a well-stirred mixture of 200 mL methanol and 50 mL 30% aqueous ammonia solution. Next, the precipitated product was ultrasonicated again for 5 minutes and stirred at room temperature for 3 days to form a sold. The solid was filtered and subjected to Soxhlet extraction with methanol and then with heptane to remove low molecular weight materials. The remaining solid residue was extracted with 200 mL chlorobenzene for 16 hours to give a brown chlorobenzene extract yielding 0.12 g (yield 46%) of pure brown metallic-like polymer semiconductor upon solvent removal and drying in a vacuo.
Device Fabrication and Evaluation
Bottom-gate, top-contact TFT test devices with poly(4,8-didodecyl-2,6-bis-(3-methylthiophen-2-yl)-benzo[1,2-b:4,5-b′]dithiophene) as the semiconductor layer were prepared on silicon wafer in ambient conditions without taking any special precautions to exclude air, moisture and ambient light. A heavily n-doped silicon wafer with a 200 nm top layer of thermal silicon dioxide (SiO2) was used as the substrate/gate electrode with the SiO2 top layer serving as the gate dielectric. After cleaning the SiO2 surface of the wafer with oxygen plasma, the cleaned wafer was immersed in a 0.1 M solution of octyltrichlorosilane (OTS-8) in toluene at 60° C. for 30 minutes. The wafer was then rinsed with toluene and 2-propanol, and then air-dried.
The polymer semiconductor layer was then deposited on top of silane-treated SiO2 surface by spin coating a warm solution (˜50° C.) of poly(4,8-didodecyl-2,6-bis-(3-methylthiophen-2-yl)-benzo[1,2-b:4,5-b′]dithiophene) in 1,2-dichlorobenzene (0.5 weight %) at 1000 rpm for 30 seconds and vacuum drying the substrate at 70° C. for 10 minutes to form a 40-100 nm-thick polymer semiconductor layer. No thermal annealing was applied. Subsequently, a series of gold source/drain electrode pairs were deposited on the surface of the polymer semiconductor layer by vacuum evaporating through a shadow mask. This creates a series of TFTs with various channel length (L) and width (W) dimensions. The resulting TFT devices with channel lengths of 40, 90 or 190 μm and channel widths of 1 or 5 mm TFTs were patterned and characterized on a Keithley SCS-4200 in a black metal box in air.
The FET mobility was extracted using the following equation in the saturation regime from the gate sweep:
I
D
=μC
i(VG−VT)2(W/2L)
where ID is the drain current, μ is the field-effect mobility, Ci is the capacitance per unit area of the gate dielectric layer (SiO2, 200 mm, Ci=15 nF cm−2), and VG and VT are respectively gate voltage and threshold voltage. VT was derived from the relationship between the square root of ID at the saturated regime and VG by extrapolating the measured data to ID=0. W and L are respectively channel width and length. The TFT device showed a mobility 0.15 cm2/V·s and a current on/off ratio 106.
It will be appreciated that various of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also, various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art, and are also intended to be encompassed by the following claims.
