Stable organic field-effect transistors by incorporating an electron-accepting molecule
10367144 ยท 2019-07-30
Assignee
Inventors
Cpc classification
H10K10/476
ELECTRICITY
H10K10/488
ELECTRICITY
H10K85/113
ELECTRICITY
International classification
Abstract
The present disclosure demonstrates that the introduction of electron deficient fullerene acceptors into thin films comprised of the high-mobility semiconducting polymers suppresses an undesirable double-slope in the current-voltage characteristics, improves operational stability, and changes ambipolar transport to unipolar transport. Examination of a variety of high polymers shows general applicability. The present disclosure also shows that instability is further reduced by tuning the relative electron affinity of the polymer and fullerene by creating blends containing different fullerene derivatives and semiconductor polymers. One can obtain hole values up to 5.6 cm.sup.2 V.sup.1 s.sup.1 that are remarkably stable over multiple bias-sweeping cycles. The results provide a simple, solution-processable route to dictate transport properties and improve semiconductor performance in systems that display similar non-idealities.
Claims
1. One or more organic field effect transistor (OFET)s, comprising: semiconducting polymers combined with electron acceptors so as to change an ambipolar transport of the semiconducting polymers to p-type transport; a source contact to a p-type channel including the semiconducting polymers; a drain contact to the p-type channel; and a dielectric between a gate contact and the p-type channel.
2. The OFETs of claim 1, wherein the semiconducting polymers comprise polymer chains each comprising donor-acceptor repeating units.
3. The OFETs of claim 1, wherein the semiconducting polymers comprise polymer chains each comprising regioregular donor-acceptor repeating units.
4. The OFETs of claim 1, wherein the electron acceptors are current stabilizing agents.
5. The OFETs of claim 1, wherein each of the electron acceptors are fullerenes.
6. The OFETs of claim 5, wherein: each of the OFETs further comprise a film, the film is cast from a solution comprising the fullerenes and the semiconducting polymers, a weight ratio of the fullerene in the film is in a range of 1%-95% wt. %, and the weight ratio is defined as (W.sub.F/(W.sub.F+W.sub.SP))100, wherein W.sub.F is the weight of the fullerene in the film and W.sub.SP is the weight of the semiconducting polymers in the film.
7. The OFETs of claim 5, wherein: each of the OFETs each comprise a film; the fullerenes are phase separated in the film; the electron acceptors are either of low mobility or are not interconnected so as to inhibit electron transport in the p-type channel; and the semiconducting polymers include a plurality of interconnected polymer chains.
8. The OFETs of claims 5, wherein each of the fullerenes comprise PC.sub.61BM.
9. The OFETs of claim 1, wherein each of the electron acceptors comprise at least one compound selected from PC.sub.61BM, PC.sub.71BM, C.sub.60, N2200, an N2200 derivative, naphthalene diimide (NDI), an NDI derivative, perylene diimide (PDI), and a PDI derivative.
10. The OFETs of claim 1, further comprising: a first amount and a first composition of the semiconducting polymers; a second amount and a second composition of the electron acceptors; wherein each of the OFETs are characterized by having a curve plotting (I.sub.d).sup.1/2 as a function of gate voltage (V.sub.g), the curve having a single slope for V.sub.g in a range of 10 V to -60 V and (I.sub.d).sup.1/2of 0.012 or less.
11. The OFETs of claim 1, further comprising: a first amount and a first composition of the semiconducting polymers; a second amount and a second composition of the electron acceptors; wherein each of the OFETs are characterized by their drain-source current (I.sub.ds) for a given gate voltage (V.sub.g) changing by 10% or less after 20 sweeps of the V.sub.g in a range of 10 V to -60 V.
12. The OFETs of claim 1, further comprising: a first amount and a first composition of the semiconducting polymers; a second amount and a second composition of the electron acceptors; and wherein each of the OFETs have a mobility in a saturation regime of at least 2 cm.sup.2 V.sup.1 s.sup.1.
13. The OFETs of claim 1, further comprising: a first amount and a first composition of the semiconducting polymers; a second amount and a second composition of the electron acceptors; wherein the OFETs each have the mobility in a saturation regime that is not reduced by more than 30% after 20 sweeps of the gate voltage in a range of 10 V to -60 V.
14. The OFETs of claim 1, further comprising: a first amount and a first composition of the semiconducting polymers; a second amount and a second composition of the electron acceptors; wherein the OFETs each have a threshold voltage that does not shift by more than 1 Volt after 20 sweeps of the gate voltage in a range of 10 V to -60 V.
15. The OFETs of claim 1, wherein the semiconducting polymers comprise a plurality of polymer chains each comprising a regioregular conjugated main chain section, said regioregular conjugated main chain section having a repeat unit that comprises a compound of the structure: ##STR00012## wherein: Ar is a substituted or non-substituted aromatic functional group or Ar is hydrogen; the pyridine or the fluorinated ring is regioregularly arranged along the conjugated main chain section; and the compound is bonded, in the repeat unit, to a dithiophene of the structure: ##STR00013## wherein each R is independently hydrogen or a substituted or non-substituted alkyl, aryl or alkoxy chain; and X is C, Si, Ge, N or P.
16. The OFETs of claim 1, wherein the semiconducting polymer is: poly[4-(4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b]dithiophen-2yl)-alt-[1,2,5]thiadiazolo[3,4-c]pyridine] (PCDTPT), or poly[5-fluoro-[2,1,3]benzothiadiazole-4,7-diyl(4,4-dihexadecyl-4H-cyclopenta[2,1-b:3,4-b]dithiophene-2,6-diyl)-5-fluoro-[2,1,3]benzothiadiazole-7,4-diyl(4,4-dihexadecyl-4H-cyclopenta[2,1-b:3,4-b]dithiophene-2,6-diyl)] (PCDTFBT).
17. A method of fabricating an organic field effect transistor (OFET), providing semiconducting polymers combined with electron acceptors so as to change an ambipolar transport of the semiconducting polymers to p-type transport; providing a source contact to a channel comprising the semiconducting polymers; providing a drain contact to the channel; and providing a dielectric between a gate contact and the channel.
18. The method of claim 17, further comprising: forming the OFET on a substrate; solution processing the semiconducting polymers and the electron acceptors from a solution onto the substrate, wherein: each of the electron acceptors comprise a fullerene and a concentration of the fullerenes in the solution is in a range of 1%-95% wt. %, and the semiconducting polymers comprise a regioregular donor-acceptor backbone.
19. The field effect transistor of claim 1, wherein: the semiconducting polymers comprise interconnected polymer chains and are phase separated from the electron acceptors, and the electron acceptors are either of low mobility or are not interconnected so as to inhibit electron transport in the p-type channel.
20. The field effect transistor of claim 1, wherein: the semiconducting polymers comprise interconnected polymer chains and are phase separated from the electron acceptors, and the electron acceptors are not interconnected so as to inhibit electron transport in the p-type channel.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) Referring now to the drawings in which like reference numbers represent corresponding parts throughout:
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DETAILED DESCRIPTION OF THE INVENTION
(26) In the following description of the preferred embodiment, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration a specific embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.
(27) Technical Description
(28) In several polymer and small molecule systems, field-effect charge carrier mobilities exceeding 1 cm.sup.2 V.sup.1 s.sup.1 have been reported..sup.[3-5,8-10] However, in many cases, how to calculate mobility may be a subject of debate. The reason for these uncertainties is illustrated in
I.sub.d=W/2LC.sub.i(V.sub.gV.sub.T).sup.2.
(29) In some reported polymer semiconductors, the dependence of I.sub.d.sup.1/2 vs. V.sub.g has been observed to increase with increasing magnitude of V.sub.g..sup.[11] In other cases, the opposite occurs; a larger slope is observed at low V.sub.g and decreases at high V.sub.g.sup.[12-14], see also Supporting Information.sup.[23].
(30) The cause of the double-slope remains under debate (and may not be universal for all high mobility organic semiconductors) but has been commonly observed in recent years (see Supporting Information.) Charge conduction in the bulk, rather than near the dielectric interface, contact resistance effects, and charge interaction into the dielectric layer have all been cited as possible causes of the double-slope..sup.[3] In addition to deviations from ideality, changes in device characteristics through normal device operation are undesirable for commercial application. For example, should be constant after multiple scans so that the measured current at a particular voltage does not change, V.sub.T should remain fairly constant during device operation (i.e., a device should not be ON at a particular voltage and then switch to OFF at the same voltage solely due to operational stresses), and the on/off ratio (I.sub.ON/I.sub.OFF) should remain high to give well-defined ON and OFF states. Discovering methods that circumvent complications due to unstable transport are needed to enable implementation of organic semiconductors in solution-processed electronics.
(31) PCDTPT, DT-PDPPTT-DT, PCDTFBT, or a blend comprising a fullerene additive and the PCDTPT, DT-PDPPTT-DT, PCDTFBT was blade-coated as the active layer in bottom-gate bottom-contact OFETs with DTS-treated SiO.sub.2 (300 nm) as the dielectric, and Ni/Au (5/50 nm) as the source/drain contacts. The structure obtained was annealed at 200 C.
(32) 1. First Example: PCDTPT
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(34) a. Pristine PCDTPT
(35)
(36) The first scan of the transfer characteristics displays qualities of ambipolarity. This can also be observed in the output characteristic where current saturation is not observed at high source-drain voltages (Supplementary Figure S1.sup.[23]).
(37) Devices exhibited an average =0.570.06 cm.sup.2 V.sup.1 s.sup.1 (extracted from V.sub.g=30 V to 50 V), V.sub.T ranging from 5.9 to 10.7 V, and I.sub.ON/I.sub.OFF less than 120 (the first scan of the devices never reaches a well-defined OFF state due to electron injection; therefore, calculations of I.sub.ON/I.sub.OFF used the maximum and minimum current from the scan).
(38) Device instability was subsequently examined through device cycling (
(39) In addition to changes in device performance, deviations from the ideal current-voltage characteristics were observed after 20 scans. The double-slope issue previously discussed was observed, and an average as high as 3.20.5 cm.sup.2 V.sup.1 s.sup.1 is extracted from a narrow, low V.sub.g regime between V.sub.g=10 V to 5 V.
(40) b. PCDTPT: PC.sub.61BM Blend
(41) Since ambipolar transport (in particular, electron injection) in PCDTPT has been implicated as a possible cause of the deviation from ideality, the inventors of the present invention hypothesized that a suitable electron acceptor in the channel may perturb device characteristics. Accordingly, PC.sub.61BM was added in solution at varying weight concentrations from 99.9:0.1 wt. % PCDTPT:PC.sub.61BM to 5:95 wt. % PCDTPT:PC.sub.61BM.
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(43) The 95:5 wt. % PCDTPT:PC.sub.61BM blend device was subjected to the device cycling outlined above. After 20 scans, the average =0.570.06 cm.sup.2 V.sup.1 s.sup.1, the maximum I.sub.ON/I.sub.OFF is 550000, and notably, the double-slope was not observed. Thus, remarkably, p-type characteristics and stability are improved by the addition of PC.sub.61BM.
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(46) Other device parameters, i.e., V.sub.T and I.sub.ON/I.sub.OFF, are also stabilized by the addition of PC.sub.61BM.
(47) In addition to these measurements, electrical instability was investigated by a stress-time test where V.sub.g and V.sub.d were held constant at 10 V and 80 V and V.sub.T was monitored over time. Similar experiments have been performed to extract a quantitative measurement of stability, namely a characteristic time constant extracted when fitting to a stretched exponential function..sup.[21] These measurements reveal a characteristic time constant that is improved by nearly an order of magnitude for 95:5% PCDTPT:PC.sub.61BM relative to pristine PCDTPT, consistent with the bias-sweeping stability test (Figure S10 in the Supplementary Information.sup.[23]).
(48) Device stability was also tested when scanning to high positive V.sub.g and after cycling for 15 minutes (Supplementary Figures 10-13.sup.[23]). The addition of PC.sub.61BM was again observed to improve p-type OFET characteristics and device stability. Based on the assumption that electron injection is the underlying cause of instability, the inventors hypothesized that an applied positive V.sub.g would cause a current-voltage drift, whereas an applied negative V.sub.g would not.
(49) c. PCDTFBT: C.sub.60
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(51) d. Choice of Dielectric
(52) SiO.sub.2 dielectrics are known to cause trapping effects.sup.[7]. Despite their relevance for scientific studies, solution processing, as well as their compatibility with standard photolithographic patterning techniques and dielectric modification (e.g., for polymer alignment), other dielectrics are under consideration for large area and flexible devices. Therefore, bottom-gate/top-contact OFETs on a divinyl-tetramethyl-siloxane-bis(benzocylcobutene) (BCB) polymer dielectric were also tested. BCB has been used to limit double-slope characteristics..sup.[20] The inventors hypothesized that use of the BCB dielectric would shift ambipolar to unipolar p-transport in the OFET (if the shift reflects a property of the semiconductor blend layer). Indeed, results show unipolar p-type characteristics for an OFET comprising BCB with as little as 5 wt. % PC.sub.61BM as an additive, as shown in Figure S8 of the Supporting information.sup.[23].
(53) e. Use of Nanogrooves
(54) Semiconductor layers were also cast on substrates containing 100 nm wide and 1-5 nm deep uniaxially oriented nanogroove channels, which can be used to align polymer fibers..sup.[21-24] Fullerene addition in the semiconductor atop this dielectric layer also leads to improved stability (Figure S16). Higher mobilities were obtained relative to planar dielectric substrates (
(55) Thus, the impact of the fullerene additive is thus applicable under circumstances where alignment of conjugated polymer networks (e.g., by the nanogrooved substrate) is used to improve carrier mobilities/charge transport properties.
(56) 2. Second Example: DT-PDPPTT-DT
(57) The general utility of fullerene addition was explored by applying the technique to another semiconducting polymer (DT-PDPPTT-DT) that displays ambipolar characteristics. DT-PDPPTT-DT (having the structure illustrated in
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(59) a. Pristine DT-PDPPTT-DT
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(61) b. DT-PDPPTT-DT: PC.sub.61BM
(62) Devices from a 50 wt. % DT-PDPPTT-DT:PC.sub.61BM blend solution (average hole =0.390.03 cm.sup.2 V.sup.1 s.sup.1 for 8 devices) were prepared.
(63) To provide more evidence of the improved p-type behavior and non-ideality suppression upon addition of PC.sub.61BM, the average percent change, V.sub.T shift, and I.sub.ON/I.sub.OFF range of 8 DT-PDPPTT-DT and DT-PDPPTT-DT:PC.sub.61BM devices was measured (results shown in the Supporting Information.sup.[23]). It is clear that the V.sub.T shift, I.sub.ON/I.sub.OFF range, and percent change in the lower absolute V.sub.g regime can be controlled after blending the polymer semiconductor with PC.sub.61BM. Differences in the extent to which these instabilities and non-idealities are controlled by PC.sub.61BM (as compared to PCDTPT) suggests that factors such as the electron of the ambipolar semiconductor, the electronic structure of the ambipolar semiconductor, and the morphology within the blend film may play a significant role in PC.sub.61BM modulating p-type OFET behavior.
(64) 3. Third Example: PCDTFBT
(65) In addition to DT-PDPPTT-DT and PCDTPT, another polymer, PCDTFBT was also blended with PC.sub.61BM.
(66) a. Pristine PCDTFBT
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(68) b. PCDTFBT: PC.sub.61BM
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(70) 4. Fourth Example: PCDTBT:PCBM Blends
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(72) PCDTBT:PC.sub.61BM devices were also cast on nanogrooved substrates to align the polymer and improve p. Aligned PCDTBT:PC.sub.61BM devices obtained as high as 5.6 cm.sup.2 V.sup.1 s.sup.1 (See Figure S21 of the Supporting Information.sup.[23]). This compares favorably to an OFET comprising the 50:50 wt. % PCDTPT:PC.sub.61BM blend with nanogrooves having =2.70.2 cm.sup.2 V.sup.1 s.sup.1.
(73) 5. Comparative Examples
(74) a. Choice of Fullerene
(75) Under the assumption that a significant fraction of injected electrons are ultimately located in the fullerene, the inventors of the present invention hypothesize that the driving force for the process increasing stability, suppressing double slope, and suppressing electron transport of the OFETs should be tunable by the electron affinity of the fullerene. The lowest unoccupied molecular orbital (LUMO) energy level for PCDTPT determined by cyclic voltammetry is 4.0 eV..sup.[19] The precise value of the PC.sub.61BM LUMO energy has been discussed in the literature, and is typically reported in the range of 3.7-4.3 eV..sup.[26] The effects of blending PCDTPT with different fullerene acceptors were examined, specifically, by blending PCDTPT with bis-PCBM, PC.sub.61BM, and PC.sub.85BM, which are listed in
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(77) b. Choice of Semiconducting Polymer
(78) The examples provided herein support that the LUMO offset between the conjugated polymer and the fullerene improve OFET stability for a variety of different additives and semiconducting polymers, including structurally-related polymers PCDTBT, PCDTFBT, and PCDTPT. The reported LUMO values for these polymers using cyclic voltammetry and their optical band gaps are 3.6, 3.8, and 4.0 eV for PCDTBT, PCDTFBT, and PCDTPT, respectively..sup.[15,25]
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(81) c. Stability Under Bias Stress as a Function of Blend Composition
(82) If a high enough V.sub.g is applied for a period of time to some fullerene blend devices, a drift in current-voltage characteristics can be observed. PCDTBT:PC.sub.61BM devices were therefore subjected to a variety of conditions to test variations in performance, with results illustrated in
(83) Biasing PCDTBT:PC.sub.61BM for five minutes at V.sub.g=30 V results in increasing by 283% and a V.sub.T of 42 V; these changes are similar to the PCDTPT:PC.sub.61BM devices when applying a lower V.sub.g=10 V (see Supporting Information.sup.[23]). Successively higher initial V.sub.g was applied to investigate the effects on the current-voltage characteristics (see
(84) For comparison, PCDTPT:PC.sub.61BM shows larger changes in current-voltage characteristics with increasing by 50% and V.sub.T=6 V.
(85) A PCDTBT:PC.sub.61BM device was also subjected to bias-sweeping from V.sub.g=15 to 60 V while holding V.sub.d=80 V for t=52200 seconds (s) (
(86) PCDTPT:PC.sub.61BM devices, on the other hand, are not nearly as robust with changes in current-voltage characteristics occurring even after only 900 s of continuous scanning (
(87) 5. Device Fabrication Example
(88) The OFETs comprising the PCDTPT, DT-PDPPTT-DT, PCDTFBT, PCDTBT and corresponding blends, as described in the first, second, third, fourth, and comparative examples (and characterized by the data in Figures 1-9) were fabricated according to the following method unless indicated otherwise.
(89) A 300 nm SiO.sub.2 dielectric/doped Si (University Wafer) substrate was cleaned with piranha. Where specified, diamond lapping films (from Allied High Tech Products, Inc.) with diamond particle size of 100 nm were used to introduce nanogrooves on the surface of the SiO.sub.2 gate dielectric layer. Source and drain contacts (5 nm Ni/50 nm Au) were deposited using two-step photolithography. Before polymer deposition, the substrates were cleaned by sonication in acetone and isopropanol for 3 minutes each, and were then dried in an oven under air, at 120 C. for 10 minutes. The substrates were treated with UV-03 for 15 minutes and underwent acid hydrolysis to etch the Ni adhesion layer. Subsequently, the substrates were oven dried, treated again with UV-03, and then passivated by using decyltrichlorosilane (from Gelest Chemicals) from a 0.2 vol % toluene solution at 80 C. for 25 minutes. The substrates were rinsed and sonicated in toluene and dried under air flow. Blend films were deposited by blade-coating immediately after. The blend concentration in solution (chlorobenzene) was 5 mg/mL with respect to the total fullerene+polymer semiconductor content. PCDTPT, PCDTBT, PCDTFBT, which were synthesized according to previous procedures.sup.[15,25] and various fullerenes were dissolved in chlorobenzene and mixed by volume to obtain the corresponding blend weight ratio.
(90) A blade-coat and stage was built using a LTA-HS actuator and integrated CONEX-CC controller. The blades used were glass microscope slides, which were cleaned by piranha solution. The substrate was coated by injecting 8 L of solution in between a 100 m channel formed between the blade and the substrate. The blade angle was set to 60 relative to the plane of the substrate. Blade-coating conditions were 100 C. and 1.2 mm/s. Bottom gate, bottom contact field-effect transistors were fabricated for all blend compositions. Devices were annealed for 200 C. for 8 minutes and tested immediately after annealing. The final device architecture (from bottom to top) for these bottom gate, bottom contact field-effect transistors was Si (500 m)/DTS-treated SiO.sub.2 (300 nm)/Ni (5 nm)/Au (50 nm)/blend layer.
(91) The mobility of blend devices were obtained by fitting the following equation to the saturation regime transfer characteristics: I.sub.DS=(W/2L)C.sub.i(V.sub.GSV.sub.th).sup.2, where W is the channel width (1 mm), L is the channel length (80 or 160 m), C.sub.i is the gate dielectric layer capacitance per unit area (10 and 11.5 nF/cm.sup.2 for normal and nanogrooved substrates.sup.1), V.sub.GS is the gate voltage, V.sub.th is the threshold voltage, and I.sub.DS is the source-drain voltage. Devices were measured under nitrogen in a glovebox using a Signatone 1160 probe station and Keithley 4200 semiconductor parametric analyser. Mobility values are calculated from a gate voltage range of 30 V to 50 V at a source-drain voltage of 80 V.
(92) Process Steps
(93)
(94) Block 1000 represents obtaining/providing and/or preparing a substrate. In one or more embodiments, the substrate comprises a flexible substrate. Examples of a flexible substrate include, but are not limited to, a plastic substrate, a polymer substrate, a metal substrate, or a glass substrate. In one or more embodiments, the flexible substrate is at least one film or foil selected from a polyimide film, a polyether ether ketone (PEEK) film, a polyethylene terephthalate (PET) film, a polyethylene naphthalate (PEN) film, a polytetrafluoroethylene (PTFE) film, a polyester film, a metal foil, a flexible glass film, and a hybrid glass film.
(95) Block 1002 represents optionally forming/depositing contacts or electrodes (e.g., p-type, n-type contacts, a gate, a source, and/or drain contacts) on or above (or as part of) the substrate.
(96) In an OFET embodiment comprising a top gate & bottom contact geometry, source and drain contacts are deposited on the substrate. Examples of materials for the source and drain contacts include, but are not limited to, gold, silver, silver oxide, nickel, nickel oxide (NiOx), molybdenum, and/or molybdenum oxide. In one or more embodiments, the source and drain contacts of the OFET further comprise a metal oxide electron blocking layer, wherein the metal in the metal oxide includes, but is not limited to, nickel, silver or molybdenum.
(97) In an OFET embodiment comprising a bottom gate geometry, a gate electrode is deposited on the substrate. In one or more embodiments, the gate contact (gate electrode) is a thin metal layer. Examples of the metal layer for the gate include, but are not limited to, an aluminum layer, a copper layer, a silver layer, a silver paste layer, a gold layer or a Ni/Au bilayer. Examples of the gate contact further include, but are not limited to, a thin Indium Tin Oxide (ITO) layer, a thin fluorine doped tin oxide (FTO) layer, a thin graphene layer, a thin graphite layer, or a thin PEDOT:PSS layer. In one or more embodiments, the thickness of the gate electrode is adjusted (e.g., made sufficiently thin) depending on the flexibility requirement.
(98) The gate, source, and drain contacts can be printed, thermally evaporated, or sputtered, for example.
(99) Block 1004 represents optionally depositing a dielectric on the gate electrode, e.g., when fabricating an OFET in a bottom gate configuration. In this example, the dielectric is deposited on the gate contact's surface to form a gate dielectric.
(100) Examples of depositing the dielectric include forming a coating, a dielectric coating, or one or more dielectric layers on the substrate (and selecting a thickness of the dielectric layers or coating), the layers or coating comprising one or more nanogrooves/nanostructures.
(101) Examples of dimensions for the nanogrooves include, but are not limited to, a nanogroove depth of 6 nanometers or less and/or a nanogroove width of 100 nm or less.
(102) Examples of dielectric layers include a single polymer (e.g., PVP) dielectric layer or multiple dielectric layers (e.g., bilayer dielectric). A single polymer dielectric layer may be preferred in some embodiments (easier processing, more flexible). In one embodiment, the dielectric layers form a polymer/SiO.sub.2 bilayer. In another embodiment, the dielectric layers form a polymer dielectric/SiO.sub.2/SAM multilayer with the Sift on the polymer and the alkylsilane or arylsilane Self Assembled Monolayer (SAM) layer on SiO.sub.2. In yet another embodiment, the dielectric layers form a SiO.sub.2/SAM bilayer with the alkylsilane or arylsilane SAM layer on the SiO.sub.2. Various functional groups may be attached to the end of the alkyl groups to modify the surface property of the SAM layer.
(103) The thickness of the SiO.sub.2 may be adjusted (e.g., made sufficiently thin) depending on the composition of the dielectric layers and the flexibility requirement. For example, in one embodiment, the dielectric layer might not include a polymer dielectric layer and still be flexible.
(104) In one or more embodiments, the nanogrooves/nanostructures are formed/patterned using nano imprint lithography. In one or more examples, fabricating the dielectric layers comprises nano-imprinting a first dielectric layer (e.g., PVP) deposited on a gate metal surface of the substrate; and depositing a second dielectric layer on the nanoimprinted first dielectric layer, wherein a thickness of the second dielectric layer (e.g., comprising SiO.sub.2) is adjusted.
(105) Block 1006 represents obtaining/fabricating a semiconducting polymer and a fullerene or electron accepting molecule/compound, and combining the semiconducting polymer with the fullerene or the electron accepting molecule/compound.
(106) In one or more examples, the combining comprises forming a solution comprising a fullerene and the semiconducting polymer, wherein a weight ratio of the fullerene/electron accepting compound/electron acceptors in the solution or in the combination is in a range of 1%-95% wt. % and weight ratio wt. % is defined as (W.sub.A/(W.sub.A+W.sub.SP))100 where W.sub.A is the weight or mass of the electron acceptor and W.sub.SP is the weight or mass of the semiconducting polymer. For example, the weight ratio of electron acceptor/fullerene to the polymer in the solution can be in a range of 1:99 wt. % (i.e., 0.05 mg of fullerene/electron acceptor and 4.95 mg polymer if the solution concentration is 5 mg/ml) to 95:5 wt. % fullerene:polymer. In one or more embodiments, a weight (W.sub.SP) of the semiconducting polymer added in the solution and a weight (W.sub.A) of the fullerene/electron accepting compound/electron acceptors added in the solution are such that W.sub.SP is in a range of 5%-99% or 1%-99% of the total weight of the solution including the semiconducting polymers, the electron acceptors, and the solvent.
(107) Examples of the fullerene include a pristine fullerene with the formula of C.sub.2n and fullerene derivatives. Examples of pristine fullerenes include, but are not limited to, C.sub.60, C.sub.70, C.sub.76, C.sub.78, C.sub.82 and C.sub.84. Examples of fullerene derivatives include, but are not limited to, a mono-, bis- or tris-functionalized fullerene, PC.sub.61BM, PC.sub.71BM, bis-PC.sub.61BM, bis-PC.sub.71BM, ICMA, and ICBA. In one example, the fullerene comprises an endohedral fullerene.
(108) In one or more embodiments, the fullerene comprises a mixture of various pristine fullerenes and fullerene derivatives. In one or more embodiments, the pristine fullerene or the fullerene core of the fullerene derivatives comprise heteroatoms (one or more of the carbon atoms in the pristine fullerene, or in the fullerene core of the fullerene derivatives, are substituted by heteroatoms).
(109) Examples of an electron acceptor or electron accepting molecule/compound include any chemical entity that accepts electrons transferred to it from another compound or injected to it from an electrode. The electron transfer process can be either reversible or irreversible. The electron acceptor or electron accepting molecule/compound can be an organic, inorganic or hybrid semiconductor. When the electron transfer is from an electron donor to an electron acceptor, the LUMO energy level of the electron acceptor shall be below the LUMO energy level of the electron donor. The electrons accepted by the electron acceptor or electron accepting molecule/compound can be in their ground state or excited state.
(110) Further examples of electron accepting molecules include, but are not limited to, P(NDI2OD-T2) (polymer acceptor, e.g., Activink N2200]), NDI (naphthalene diimide), PDI (perylene diimide), and derivatives of NDI, PDI and N2200.
(111) The mechanism of increasing stability of the thin film transistor using an electron-accepting molecule should be applicable to any type of polymer. Examples of polymers include, but are not limited to, a polymer comprising a DPP unit or a polymer selected from those illustrated in
(112) In one or more examples, the semiconductor polymer is an ambipolar polymer, which is capable of conducting both holes and electrons. Many low bandgap donor-acceptor copolymers show ambipolar charge conduction due to their low-lying LUMO and high-lying HOMO levels.
(113) In one or more examples, the semiconducting polymer comprises a copolymer with donor and acceptor repeating units.
(114) In one or more embodiments of the copolymer, the semiconducting polymer comprises a conjugated main chain section, said conjugated main chain section having a repeat unit that comprises a pyridine of the structure:
(115) ##STR00003##
(116) wherein Ar is a substituted or non-substituted aromatic functional group, or Ar is nothing and the valence of the pyridine ring is completed with hydrogen. In one or more embodiments, the pyridine is regioregularly arranged along the conjugated main chain section.
(117) Examples of the pyridine unit include, but are not limited to:
(118) ##STR00004##
(119) In one or more examples, the repeat unit further comprises a dithiophene of the structure
(120) ##STR00005##
(121) wherein each Ar is independently a substituted or non-substituted aromatic functional group, or each Ar is independently nothing and the valence of its respective thiophene ring is completed with hydrogen, each R is independently hydrogen or a substituted or non-substituted alkyl, aryl or alkoxy chain; and X is C, Si, Ge, N or P. In the dithiophene, the R comprising the substituted or non-substituted alkyl, aryl or alkoxy chain can be a C.sub.6-C.sub.30 substituted or non-substituted alkyl or alkoxy chain, (CH.sub.2CH.sub.2O)n (n=220), C.sub.6H.sub.5, C.sub.nF.sub.(2n+1) (n=220), (CH.sub.2).sub.nN(CH.sub.3).sub.3Br (n=220), 2-ethylhexyl, PhC.sub.mH.sub.2m+1 (m=1-20), (CH.sub.2).sub.nN(C.sub.2H.sub.5).sub.2 (n=220), (CH.sub.2).sub.nSi(C.sub.mH.sub.2m+1).sub.3 (m, n=1 to 20), or (CH.sub.2).sub.nSi(OSi(C.sub.mH.sub.2m+1).sub.3).sub.x(C.sub.pH.sub.2p+1).sub.y (m, n, p=1 to 20, x+y=3). In some embodiments, the R groups in the dithiophene are the same, in other embodiments, the R groups in the dithiophene are different.
(122) Examples of dithiophene units include those illustrated in
(123) In one or more examples, the dithiophene unit comprises:
(124) ##STR00006##
(125) where R is as described above.
(126) In one or more embodiments, the semiconducting polymer has the structure with repeating units D comprising the dithiophene and repeating units A comprising the pyridine, e.g., [D-A-D-A].sub.n where n is an integer representing the number of repeating units, D is a donor structure, and A is an acceptor structure. In one or more embodiments, the structure has a regioregular conjugated main chain section having 5-150, or more, contiguous repeat units. In some embodiments, the number of repeat units is in the range of 10-40 repeats. The regioregularity of the conjugated main chain section can be 95% or greater, for example. In one or more embodiments, combination of the pyridine and the dithiophene yields the polymer of the formula:
(127) ##STR00007##
(128) In other examples, the C.sub.16H.sub.33 are replaced with R groups as discussed above.
(129) In one or more further examples, the semiconducting polymer comprises polymer chains having a backbone including an aromatic ring, the aromatic ring comprising a side group (e.g., Fluorine) having reduced susceptibility to oxidization as compared to a pyridine ring.
(130) In one or more embodiments comprising fluorinated conjugated polymer chains, the semiconducting polymer has fluoro functionality such as an acceptor structure including a regioregular fluoro-phenyl unit.
(131) In one or more examples, the semiconducting polymer comprises a conjugated main chain section, the conjugated main chain section having a repeat unit that comprises a compound of the structure:
(132) ##STR00008##
(133) wherein Ar is a substituted or non-substituted aromatic functional group containing one, two, three or more aromatic rings, or Ar is nothing and the valence of the ring comprising fluorine (F) is completed with hydrogen. In one or more embodiments, the ring comprising F is regioregularly arranged along the conjugated main chain section.
(134) In one or more examples, the ring comprising the F has the structure:
(135) ##STR00009##
(136) Other examples include those illustrated in
(137) In one or more embodiments, the repeat unit further comprises a dithiophene as described previously.
(138) Thus, in one or more embodiments, the semiconducting polymer is a regioregular semiconducting polymer comprising a repeating unit of the structure:
(139) ##STR00010##
(140) where the ring comprising F is regioregularly arranged along the conjugated main chain section pointing toward the direction shown in the structures above, Ar is a substituted or non-substituted aromatic functional group containing one, two, three or more aromatic rings, or Ar is nothing and the valence of the ring comprising fluorine (F) or the valence of the dithiophene is completed with hydrogen, the R groups comprising the substituted or non-substituted alkyl, aryl or alkoxy chain can be a C.sub.6-C.sub.30 substituted or non-substituted alkyl or alkoxy chain, (CH.sub.2CH.sub.2O)n (n=220), C.sub.6H.sub.5, C.sub.nF.sub.(2n+1) (n=220), (CH.sub.2).sub.nN(CH.sub.3).sub.3Br (n=220), 2-ethylhexyl, PhC.sub.mH.sub.2m+1 (m=1-20), (CH.sub.2).sub.nN(C.sub.2H.sub.5).sub.2 (n=220), (CH.sub.2).sub.nSi(C.sub.mH.sub.2m+1).sub.3 (m, n=1 to 20), or (CH.sub.2).sub.nSi(OSi(C.sub.mH.sub.2m+1).sub.3).sub.x(C.sub.pH.sub.2p+1).sub.y (m, n, p=1 to 20, x+y=3).
(141) For example, the semiconducting polymer can be regioregular poly[5-fluoro-[2,1,3]benzothiadiazole-4,7-diyl(4,4-dihexadecyl-4H-cyclopenta[2,1-b:3,4-b]dithiophene-2, 6-diyl)-5-fluoro-[2,1,3]benzothiadiazole-7,4-diyl(4,4-dihexadecyl-4H-cyclopenta[2,1-b:3,4-b]dithiophene-2,6-diyl)] (P2F or PCDTFBT).
(142) In one or more embodiments, the semiconducting polymer comprises PCDTBT of the formula:
(143) ##STR00011##
(144) wherein the C.sub.16H.sub.33 can be replaced with R as defined above.
(145) Block 1008 represents solution casting/processing the solution comprising the blends of semiconducting polymer and fullerene/electron accepting molecule. In one or more embodiments, the solution is cast on or above the substrate or on the dielectric (e.g., patterned dielectric) to form a film on the substrate or dielectric.
(146) Solution casting methods include, but are not limited to, inkjet printing, bar coating, spin coating, blade coating, spray coating, roll coating, dip coating, free span coating, dye coating, screen printing, and drop casting.
(147) In one or more embodiments, nanogrooves provide nucleation sites for growth of polymer chains within the solution so that one or more of the polymer chains seed and stack within one or more of the nanogrooves.
(148) Block 1008 can represent one or more solution coating processes, or combination of solution coating, thermal evaporation and sputtering, to form a polymer/fullerene blend, bilayer, or multilayer film.
(149) The semiconducting polymer can comprise polymer chains stacked into one or more fibers. One or more of the nanogrooves can contact and align one or more of the fibers such that the fibers are continuously aligned with (and/or at least partially lie within) one or more of the nanogrooves. The width of an individual fiber can be about 2-3 nm. Fibers on the nanostructured/nanogrooved substrate can form fiber bundles having a width of 50100 nm (as compared to fiber bundles having a width between 3040 nm when fabricated on a non-structured substrate).
(150) The polymer chains can each have a main chain axis, each fiber can have a long axis and a short axis, and the main-chain axes can be aligned along the long-axis of the fiber while - stacking of the polymer chains can be in a direction along the short-axis of the fiber.
(151) The nanogrooves can provide nucleation sites for growth of the polymer chains within the solution so that one or more of the polymer chains seed and stack within one or more of the nanogrooves.
(152) Block 1010 represents further processing the polymer/fullerene film cast on the substrate (e.g., on the patterned dielectric layers). The step can comprise annealing/curing the film or allowing the film to dry. Annealing/curing the film may be used to adjust/optimize the blends' morphology and phase separation). The step can comprise depositing source and drain contacts, if necessary.
(153) Block 1012 represents the end result, an organic semiconductor device.
(154) In one or more embodiments, the device comprises one or more OFETs or an electronic circuit comprising the OFETs.
(155) Embodiments of the present invention are not limited to the particular sequence of depositing the source, drain, and gate contacts. For example, OFETs according to one or more embodiments of the present invention can be fabricated in a bottom gate & top contact geometry, bottom gate & bottom contact geometry, top gate & bottom contact geometry, and top gate & top contact geometry.sup.[24].
(156) In one or more embodiments, the OFET comprises means (e.g., grooves, nanogrooves or statutory equivalents thereof) for aligning the main chain axes 1208 of the polymer 1202 to the channel. In one or more embodiments, the semiconductor polymers in the OFET each comprise polymer chains, the polymer chains each having a backbone/main chain axis 1208, the semiconducting polymers stacked in one or more fibers (e.g., each having a width or diameter of 2 nm-3 nm), and the fibers bundled into fiber bundles. Each fiber bundle has a long axis and a short axis, and the main-chain axes are aligned along the long-axis of the fiber while - stacking of the polymer chains is in a direction along the short-axis of the fiber. In one or more embodiments, the nanogrooves contact and align one or more of the fibers such that the fibers are continuously aligned with (and/or at least partially lie within) one or more of the nanogrooves. Fibers on the nanostructured/nanogrooved substrate can form fiber bundles having a width of 50100 nm (as compared to fiber bundles having a width between 3040 nm when fabricated on a non-structured substrate).
(157) In one or more embodiments, the semiconducting polymers comprise aligned conjugated polymer chains stacked to form a crystalline structure, the polymer chains oriented with an orientational order parameter between 0.9 and 1.
(158) The nanogrooves align the semiconducting polymers such that conduction between the source contact and the drain contact is predominantly along the backbones/main chain axes substantially parallel to a longitudinal axis of at least one of the nanogrooves, although charge hopping between adjacent polymers in a fiber bundle is also possible. For example, the means can align the main chain axes to an imaginary line bounded by the source and the drain or the means can align the main chain axes 1208 to an alignment direction 1210 in the channel between Source S and Drain D. The source and drain can be positioned such that a minimum distance between the source contact and drain contact is substantially parallel to the longitudinal axis of the nanogrooves.
(159) In one or more embodiments, the dielectric layer is patterned with the nanogrooves that orient the semiconducting polymers.
(160) In other embodiments, means for aligning the semiconducting polymers comprises a fabrication method, including, but not limited to, blade coating, dip coating, and bar coating (or statutory equivalents thereof) of the semiconducting polymers on dielectric/substrate.
(161) In one or more embodiments, the source, the drain, the gate, and dielectric have one or more compositions, structures, or configurations, the semiconducting polymer has a structure (including regioregularity), composition and weight/amount, the fullerene/electron accepting molecule has a composition and weight/amount, the film has a quality (including crystallinity) and morphology, the solvent for the solution has a composition, and the OFETs, semiconducting polymer, and fullerene are fabricated/processed under conditions described herein, such that: each of the OFETs are characterized by having a curve plotting (I.sub.d).sup.1/2 as a function of gate voltage (V.sub.g), the curve having a single slope for V.sub.g in a range of 10 V to 60 V and for (I.sub.d).sup.1/2 of 0.012 or less (or for a source drain voltage of 80 V); each of the OFETs are characterized by their drain-source current (I.sub.ds) for a given gate voltage (V.sub.g) changing by 10% or less after 20 sweeps of the V.sub.g in a range of 10 V to 60 V (i.e., a current voltage curve having reduced variation/hysteresis after multiple sweeps of the gate voltage); the OFET has a (e.g., hole or electron) mobility in a saturation regime of at least 2 cm.sup.2 V.sup.1 s.sup.1; the OFET has a mobility in a saturation regime that is not reduced by more than 30% after the 20 sweeps of the gate voltage in a range of 10 V to 60 V; and/or the OFET has a threshold voltage that does not shift by more than 1 Volt after the 20 sweeps of the gate voltage in a range of 10 V to 60 V.
(162) Thus, it is unexpectedly found that an amount of the electron accepting molecule/compound (e.g., fullerene) can be controlled in order to control and obtain a desired threshold voltage, mobility, ideality, and/or stability (including a stability over time and cycling stability) of the OFET, to achieve the results described herein. An ordinarily skilled artisan may not predict the unexpected and surprising increase in threshold voltage stability and current-voltage curve stability represented by the results described herein and achieved by one or more embodiments of the present invention.
(163) While the exceptional properties of PC.sub.61BM or C.sub.60 as an electron acceptor clearly suppress instability and non-idealities, the exact mechanism is not entirely understood. Due to PC.sub.61BM's high electron affinity, it is reasonable that any injected electrons ultimately are located in the fullerene instead of the polymer. Moreover, without being bound to a particular scientific theory, examination of the film morphology leads the inventors of the present invention to propose a possible requirement for the observed operational enhancement: that the polymers are interconnected to maintain hole transport and that the additive phase (e.g., the fullerene or electron accepting compound) either be of low mobility or not interconnected to inhibit electron transport. Despite uncertainties in percolation on the length scale of the devices, that is unperturbed across all concentration of PC.sub.61BM indicates that an interconnected polymer phase favorable for hole transport exists under all conditions.
(164) Possible Modifications and Variations
(165) Although the polymer-fullerene composition is incorporated into OFETs, the high mobility composition comprising the semiconducting polymer and fullerene could be included other high mobility systems (e.g., light emitting diodes).
(166) In one or more embodiments, the organic semiconductor device of Block 1012 comprises a composition of matter useful as a current transport region in the organic semiconductor device, the composition of matter comprising one or more semiconducting polymers; and means for accepting electrons from the semiconducting polymers or accepting electrons injected into the current transport region (and statutory equivalents thereof). In one or more embodiments, the means comprise an electron accepting compound.
(167) Examples of the means include, but are not limited to, a fullerene, a fullerene derivative, P(NDI2OD-T2) (polymer acceptor, e.g., Activink N2200]), NDI (naphthalene diimide), PDI (perylene diimide), small molecule or polymer derivatives of NDI, PDI and N2200, a statutory equivalent thereof, or a compound having a similar electron accepting structure/property as a fullerene.
(168) In one or more embodiments, the means for accepting electrons are electron accepting compounds useful as a current and/or threshold voltage stabilizing agent in the organic semiconductor device.
(169) In one or more embodiments, the means accept electrons injected into the current transport region of the device, thereby impeding conduction of the electrons in the one or more semiconducting polymers. An effective amount of an electron accepting compound can be incorporated into the current transport region of the device in order to impede the conduction and/or suppress current and voltage instability to a desired/threshold level.
(170) In one or more embodiments, and without being bound by any particular scientific theory: the electron accepting compound scavenges electron charge and/or blocks electron charge from being injected into the semiconducting polymer (wherein the electron charge is injected from external circuitry applying voltages, e.g., source drain voltages applied across the source and drain, and/or gate voltage applied to the gate contact); the electron accepting compound can receive electrons from the semiconducting polymer and prevent the electrons from returning to the semiconducting polymer; the OFET/device comprises a SiO.sub.2 gate dielectric layer, and the electron accepting compound accepts electrons formed on the SiO.sub.2 dielectric layer due to exposure of the SiO.sub.2 dielectric layer to environmental conditions (such as moisture), thereby impeding the electron charge from reaching the semiconducting polymer, and/or maintaining a constant electric field imposed by the gate voltage; and the electron accepting compound in the film or active region comprises multiple domains electrically insulated from one another such that conduction of electrons between the domains is hindered, impeded, or prevented.
(171) Advantages and Improvements
(172) OFETs have garnered attention since the solution processability of the semiconductor raises the possibility of lower device fabrication costs, and considerable progress has been made on achieving high mobility systems. Much progress has been made on obtaining high mobilities, which suggests future commercial viability. However, device stability remains an issue. Some systems display instability in the current and mobility whereby normal operational stresses undesirably and temporarily impact device performance. Quality control would require device stability under normal operating conditions.
(173) Here, one or more embodiments of the invention outline a method of fabricating devices that include an electron-accepting molecule or additive (e.g., a fullerene such as PC.sub.61BM or C.sub.60) that inhibits the mechanism that causes device instability. The present disclosure shows how the additive improves device characteristics/stability (such as ideality cycling stability, and temporal stability) for OFET devices comprising PCDTPT, DT-PDPPTT-DT, PCDTFBT, or PCDTBT. More specifically, the present disclosure reports on the observation of suppression of the double-slope in the I.sub.d.sup.1/2 vs. V.sub.g plots, improved I.sub.ON/I.sub.OFF, and stable V.sub.T and by using a fullerene additive with ambipolar polymer semiconductors. Moreover, Fullerene addition converts ambipolar charge transport to p-type only, regardless of the dielectric layer.
(174) These results provide a convenient route to direct the choice of charge carrier and to stabilize device function. The fact that the present invention's method of adding an electron acceptor has been applied to a variety of polymeric semiconductors opens various avenues of research to explore the effects of morphology, investigate different combinations of donor and acceptor semiconductors, and to explore how the choice of components and the morphology in multicomponent p-/n-type semiconductor blends impact charge transport characteristics. Continuous bias-sweeping and bias-stressing of polymer the blends are consistent with injected electrons ultimately residing on the fullerenes. The inventors' current thinking is that under dilute fullerene conditions, electrons are impeded from transport because of their localization in fullerene sites that cannot form a percolated network between electrodes. When the concentration of fullerene is higher, electron transport may not be as high relative to the polymer-only situation because of a lower intrinsic electron in the fullerene phase due to electron trapping (consistent with previous results for other polymer semiconductor:fullerene blends used for organic photovoltaics.sup.[29]), but further work to confirm this scenario in the blends studied here should be considered. Despite that mechanistic uncertainties will require additional characterization tools, that the hole is unperturbed indicates a continuous polymer phase present under all conditions that enables hole transport in the channel. By varying the electron affinity and LUMO offset, device characteristics are stabilized under a variety of stressing conditions. With the most stable PCDTBT:PC.sub.61BM blends, devices can be subjected to V.sub.g as high as 100 V and cycled for up to 14.5 hours without significant changes in performance and achieve greater than 5 cm.sup.2 V.sup.1 s.sup.1.
(175) Not only do the present disclosure's results illustrate a convenient route for producing improved p-type OFETs, but the facile method of using the additive to improve stability of the high mobility transistors would be useful for a variety of commercial applications. Indeed, the inventors believe that the use of the additive is a significant step forward for high p-type semiconductors that are promising for implementation into consumer-ready electronic devices. For example, polymer-based OFETs according to one or more embodiments of the present invention have potential applications in flexible and low-cost electronics including applications with light emitting diodes and liquid crystal displays.
(176) Further information on one or more embodiments of the present invention can be found in Fullerene Additives Convert Ambipolar Transport to p-Type Transport while Improving the Operational Stability of Organic Thin Film Transistors, by Michael J. Ford, Ming Wang, Hung Phan, Thuc-Quyen Nguyen, and Guillermo Bazan, Advanced Functional Materials, Volume 26, Issue 25, Jul. 5, 2016, Page 4616.sup.[22] and the Supporting Information.sup.[23].
REFERENCES
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CONCLUSION
(178) This concludes the description of the preferred embodiment of the present invention. The foregoing description of one or more embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.