FLUORINE SUBSTITUTION INFLUENCE ON BENZO[2,1,3]THIODIAZOLE BASED POLYMERS FOR FIELD-EFFECT TRANSISTOR APPLICATIONS

Abstract

Four conjugated copolymers with a donor/acceptor architecture comprising 4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b′]dithiophene as the donor structural unit and benzo[2,1,3]thiodiazole fragments with varying degrees of fluorination have been synthesized and characterized. It has been shown that the HOMO levels were decreased after the fluorine substitution. The field-effect charge carrier mobility was similar for all polymers with less than an order of magnitude difference between different acceptor units.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to high mobility organic field effect transistors (OFETs).

2. Description of the Related Art

(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers in parentheses, e.g., (x). A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)

Instability of semiconducting polymers developed to date limit practical applications. One or more embodiments of the present invention overcome these problems.

SUMMARY OF THE INVENTION

One or more embodiments of the present invention describe various semiconductor polymers comprising fluorinated conjugated polymer chains, one or more methods for fabricating the semiconducting polymers, and one or more devices incorporating the semiconducting polymers.

The devices and methods are embodied in many ways, including, but not limited to, the following embodiments 1-11 listed below.

1. One or more organic field effect transistor (OFET)s each comprising a channel including semiconducting polymers, wherein each of the semiconducting polymers have a main chain section and a repeating unit of the structure:

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 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. The OFETs each further comprise a source contact to the semiconducting polymers; a drain contact to the semiconducting polymers; and a gate contact on or above the channel.

2. The OFETs of embodiment 1, wherein each of the semiconducting polymers have the repeating unit of the structure:

and wherein the fluorine (F) is regioregularly arranged along each of the main chain sections.

3. The OFETs of any of the preceding embodiments 1-2, wherein the semiconducting polymers are exposed to air for at least one day (24 hours).

4. The OFETs of any of the preceding embodiments 1-3, wherein carrier mobility of the OFET is at least 0.03 cm²V⁻¹s⁻¹ after exposure to the air for 5 days.

5. A package comprising the OFETs of any of the preceding embodiments 1-4, wherein the package exposes the semiconducting polymers to the air for at least 5 days.

6. The OFETs of any of the preceding embodiments 1-5, wherein the OFET is encapsulated in an air permeable material such as plastic.

7. The OFETs of any of the preceding embodiments 1-6, wherein the semiconducting polymers are non-aligned (or have no clear alignment as measured in a top surface of an AFM image) and/or the semiconducting polymers are disposed on a planar, non-grooved surface, and the OFETs each have a hole mobility of at least 1.2 cm²V⁻¹s⁻¹ in a saturation regime (e.g., in a range of 1.2-10 cm²V⁻¹s⁻¹ in the saturation regime).

8. The OFETs of any of the preceding embodiments 1-7. wherein the semiconducting polymers are stacked into a crystalline structure, and the crystalline structure is characterized by observation of a diffraction peak measured by grazing incidence wide-angle X-ray scattering (GIWAXS) of the film.

9. The OFETs of any of the preceding embodiments 1-8, wherein a π-π distance between adjacent semiconducting polymers is no more than 0.35 nm.

10. A method of fabricating the semiconducting polymers of any of the preceding embodiments 1-9, comprising reacting a first compound and second compound in a solution to form a product, wherein the first compound comprises a dithiophene and the second compound has the structure:

11. The method further comprises purifying the product in a solvent mixture using chromatography to obtain a third compound having the structure:

and polymerizing the third compound to obtain the semiconducting polymer.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers represent corresponding parts throughout:

FIG. 1 illustrates a scheme for polymer synthesis according to one or more embodiments of the present invention.

FIG. 2(a) illustrates UV-vis absorption of polymers in chlorobenzene solution and FIG. 2(b) illustrates UV-vis absorption of polymers in thin films, according to one or more embodiments of the present invention.

FIG. 2(c) illustrates GIWAXS measurement out of plane line-cut profiles and FIG. 2(d) illustrates GIWAXS measurement in plane line-cut profiles, of non-aligned semiconducting polymers (or semiconducting polymers with no clear alignment as measured in a top surface of an AFM image of the semiconducting polymers) according to one or more embodiments of the present invention.

FIGS. 3(a)-3(d) illustrate output curves of four polymer devices comprising non-aligned semiconducting polymers (or semiconducting polymers with no clear alignment as measured in a top surface of an AFM image of the semiconducting polymers) according to one or more embodiments of the present invention.

FIG. 4 plots polymer OFET mobilities tested in ambient condition, wherein the semiconducting polymers are non-aligned (or have no clear alignment as measured in a top surface of an AFM image of the semiconducting polymers) according to one or more embodiments of the present invention.

FIG. 5 is a flowchart illustrating a method of fabricating an OFET according to one or more embodiments of the invention.

FIG. 6a illustrates examples for the ring unit comprising Fluorine in the semiconducting polymer, according to one or more embodiments of the present invention. FIGS. 6b-6d illustrate examples for the dithiophene unit in the semiconducting polymers according to one or more embodiments of the present invention.

FIG. 7 illustrates a method of fabricating semiconducting polymers according to one or more embodiments of the present invention.

FIG. 8 illustrates a device comprising one or more OFETs according to one or more embodiments of the invention.

FIG. 9 illustrates a device comprising storage for a plurality of OFETs, including source and drain contacts/connections for the OFETs, according to one or more embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following description of the preferred embodiment, reference is made to the accompanying drawings which 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.

Technical Description

Solution-processed OFETs are under intense investigation because of the potential they offer in terms of production costs and fabrication of flexible, light-weight substrates.¹ Saturation regime mobilities of over 10 cm²v⁻¹s⁻¹ have been achieved through coupled developments in molecular design strategies and improvements in film processing methods.² Conjugated polymers containing a backbone with alternating donor-acceptor units comprise one of the most important approaches for achieving high mobility organic semiconductors used as the OFET transport material.³ Such structures consist of an electron rich moiety (donor, D) and an electron deficient moiety (acceptor, A) in each repeat unit. For example, Mullen et al. developed a polymer containing 4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b′]dithiophene (CDT) as the donor unit and benzo[2,1,3]thiadiazole (BT) as the acceptor unit,⁴ namely PBT in Scheme 1 (CDTBTZ in the original publication), which was shown to be suitable for thin film polymer alignment thereby able to attain mobility of 6.5 cm2V⁻¹s⁻¹.⁵ The molecular structure has been further refined by substituting the BT unit with a [1,2,5]thiadiazolo[3,4-c]pyridine (PT) unit through synthetic protocols that yield regioregular backbone structures.⁶ This regioregular polymer has yielded some of the highest mobilities in the literature (over 20 cm²V⁻¹s⁻¹) through molecular weight control and novel film processing techniques that improve chain alignment.^2d

Despite successes in achieving high charge carrier mobilities, copolymers containing CDT and BT structural units exhibit relatively high-lying highest occupied molecular orbital

(HOMO) levels (-5.0±0.2 eV);⁷ not an ideal situation for long-term air stability (below −5.27 eV).⁸ For structures containing the more electron withdrawing PT fragments, the basic nature of the PT heterocycle can destabilize relevant electronic properties when adjacent to acidic surfaces or exposed to air.⁹ Based on the above concerns, one or more embodiments of the present invention sought to design D-A copolymers containing CDT in conjunction fluorine-substituted BT derivatives as the acceptor. This approach was anticipated to lower the orbital levels of the resulting polymers and improve stability both in OFET and optoelectronic devices.¹⁰

First Embodiment: Polymers on Non-Grooved Substrates

a. Fabrication

The present disclosure describes synthesis of four polymers with increasing levels of fluorination and structural precision, and then studies their chemical properties, thin films feature, and application in OFETs to investigate the effects of fluorinated BT units. As shown in FIG. 1, the previously reported non-fluorinated reference polymer PBT was prepared via Stille polymerization using (4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b′]dithiophene-2,6-diyl)bis(trimethylstannane) (M1 in Scheme 1) and 4,7-dibromobenzo[2,1,3]thiadiazole in the presence of catalytic Pd(PPh3)4 in anhydrous o-xylene under microwave heating (200° C.). These reaction conditions were used for all other polymer preparations. Single fluorine substitution on BT creates an asymmetric structure and raises the issue of needing to achieve a regioregular backbone for maximizing order within the chain, and ultimately between chains in the solid state.¹¹ Accordingly, equimolar quantities of M1 and 4,7-dibromo-5-fluorobenzo[2,1,3]thiadizaole were reacted directly, ultimately producing a regiorandom polymer referred to in FIG. 1 as PRF. To synthesize the regioregular polymer (P2F), in which the fluorine atoms in alternating structural units point toward the same CDT unit, the symmetric monomer, M2 in FIG. 1, was first prepared by fine control of the Stille reaction temperature and the ratio of M1 and dibromide,⁹a then carried out the polymerization with M1 and M2. Polymer PDF, with difluorinated BT fragments, was obtained by the polymerization of M1 and 4,7-dibromo-5,6-difluorobenzo[2,1,3]thiadiazole. The molecular weight influence was also considered, particularly in view of subsequent comparisons of physical properties.^2d,5,12 The monomer ratio was varied and the Soxhlet extraction purification procedure was optimized by using different solvents. Synthetic details are provided in the Supporting Information (SI)²². These efforts provided molecular weights in the range of 50-70 kDa for each polymer.

b. Polymer Characterization

With these four polymers in hand, the UV-vis absorption, energy levels, OFET device performance, and film organization and morphologies were investigated with the goal of obtaining insight into the effect of fluorine substitution.

UV-vis absorption was used to examine optical transitions. Solution absorption measurements were carried out in chlorobenzene solvent. As shown in FIG. 2(a), the four polymers show peaks near 430 nm assigned to π-π* transitions. Peaks in the near-IR and IR regions are attributed to intramolecular charge transfer (ICT) excitations.⁷ By examination of the absorption maxima and onsets, one observes that PBT is the most red-shifted chromophore, while the di-fluorinated polymer PDF is the most blue-shifted, see Table 1 for a summary of relevant data. The excitation profiles of the two mono-substituted polymers PRF and P2F lie between those of PBT and PDF and are almost identical to each other, with their maximum peaks at 780 nm and 782 nm, respectively.

TABLE 1
	λsol	λfilm	Egopt	EHOMO	ELUMO
Polymer	(nm)	(nm)	(eV)	(eV)	(eV)
PBT	436,	436,	1.25	−4.80	−3.55
	803	793
PRF	431,	431,	1.27	−5.05	−3.78
	780	778
P2F	432,	431,	1.25	−5.03	−3.78
	782	781
PDF	427,	427,	1.30	−5.19	−3.89
	756	755
Summary of absorption characteristics and orbital energy levels. λsol and λfilm refer to absorption peak values in chlorobenzene and in the thin film, respectively; Egopt is the optical band-gap determined by film absorption onset; EHOMO and ELUMO are the energy levels from CV measurements and optical band-gaps.

Thin films were prepared by spin-casting from the corresponding chlorobenzene solution and the absorption spectra were measured. As shown in FIG. 2(b) and Table 1, the maxima of the ICT peaks remain largely unchanged, relative to solution. However, one observes a larger contribution from the low energy shoulder, leading to substantial differences in the absorption onsets. The optical band gaps are determined by corresponding thin film absorption onsets, which provide band-gaps of 1.25 eV, 1.27 eV, 1.25 eV and 1.30 eV for PBT, PRF, P2F and PDF respectively. It is interesting to note that the regiorandom (PRF) and regioregular (P2F) versions of the conjugated polymer containing F-BT are nearly identical. However, as demonstrated below, these two materials have very different charge transport characteristics.

Cyclic voltammetry was used to estimate HOMO energy levels by measuring the oxidation potential onsets.¹³ As shown in Figure S3 in the SI²², the HOMO level of PBT is −4.80 eV. After mono fluorine substitution, the HOMO levels decrease, with PRF and P2F displaying HOMO levels about −5.05 eV and −5.03 eV respectively. When two fluorine atoms are introduced to the BT unit, the HOMO level decreases further to −5.19 eV. These results highlight the influence of fluorine substitution, which reduces the HOMO levels by as much as 0.39 eV. From a practical perspective the lower HOMO value suggests that, among the polymers studied here, PDF should have the best air-stability in OFET applications.⁸ Lowest unoccupied molecular orbital (LUMO) energy levels were calculated by adding the optical band-gaps to the HOMO levels. As shown in Table 1, the LUMO levels also decrease with increasing fluorine substitution, in agreement with the stronger electron deficient nature of FBT and DFBT units relative to BT. It is interesting to note that the stabilization of HOMO and LUMO levels by F substitution is similar, which may be surprising in view of the A units greater participation in determining the LUMO levels.¹⁴

Grazing-incidence wide-angle X-ray scattering (GIWAXS) was used to investigate thin film organization.²¹ The 2D diffraction images are provided in the SI Figure S4-S7²², and the line-cut profiles are shown in FIG. 2(c) and FIG. 2(d). In the out of plane direction, there is a strong peak around 0.25 Å⁻¹. A peak around 0.5 Å⁻¹ corresponding to the 2^nd order for all polymers, is assigned to alkyl chain stacking features. The alkyl chain stacking distances of all polymers are almost identical at about 2.4 nm, consistent in that they all contain similar hexyldecyl solubilizing unit and suggesting that all polymers share similar lattice organizations. There is no observable peak in the region of 1.0-2.0 Å⁻¹. In the in-plane direction, there is a strong peak around 1.8 Å⁻¹ for all polymers, which is assigned to π-π stacking. Again, the π-π distances are nearly identical (0.35 nm). These features indicate that these polymers present edge-on orientation relative to the substrate plane and also indicate that fluorine substitution in this general backbone framework does not change the crystallite organizations under the film processing conditions disclosed here.

c. OFET Device Characterization

Bottom contact, bottom gate OFETs were fabricated using devices with the following architecture: polymer/Au/SAM/SiO₂/Si (doped). Decyltrichlorosilane (DTS) was used as the self-assembled monolayer on the silicon oxide substrate. Thin films were prepared via doctor-blading¹⁵ and the complete details are provided in the SI²². Table 2 provides average and maximum mobilites, as determined under nitrogen inside a glovebox by examination of the saturation current regime, together with the corresponding on/off ratios. Average mobilites were calculated from 8 devices.

Typical output curves obtained with OFET devices are provided in FIGS. 3(a)-3(d). At high V_g (−30−−60V), I_drain is saturated when V_drain in is greater than −70 V. At lower V_g, one observes I_drain increases with V_drain in the range of −40 to −100 V. This feature suggests injection of electrons and therefore an ambipolar nature of the devices.¹⁶ Comparison of FIG. 2(a) vs. 2(d) shows that when the hydrogen atoms are substituted with fluorine atoms, the ambipolar characteristics are more pronounced. One possible reason is that the lower LUMO levels are decreased as more fluorine substitutions have been introduced and that more electron-withdrawing groups can further stable the negative charges that electron transport would improve.¹⁷ The transfer curves are shown in SI Figure S10²². The I_drain^1/2 vs. V_g curves show nearly-ideal single slope characteristics.¹⁸

TABLE 2
Polymer OFET highest and average mobilities.
	Mn		Mobility		Vth
Polymer	(kDa)	PDI	cm2V−1s−1	On/off	(V)
PBT	68	3.6	1.4 (1.1 ± 0.1)	6.3 × 102	10.1
PRF	53	3.3	0.4 (0.3)	5.5 × 10	16.4
P2F	62	3.4	1.2 (0.9 ± 0.2)	5.5 × 10	14.6
PDF	67	4.1	0.3 (0.3)	2.8 × 10	9.6

Hole mobilities determined through measurements of OFETs containing different semiconductor polymers are summarized in Table 2. These data indicate that the polymer PBT, at least under these experimental conditions, shows a maximum mobility of 1.4 cm²V⁻¹s⁻¹. The regiorandom polymer PRF obtains a maximum mobility of 0.4 cm²V⁻¹s⁻¹, while its regioregular countpart P2F obtains a maximum mobility of 1.2 cm²V⁻¹s⁻¹. The molecule weight (53 kDa) for the PRF is slightly lower than the other three polymers (60-70 kDa). To verify the molecule weight influence on the mobility, a higher molecular weight batch of PRF was synthesized having a M_n of 128 kDa and a PDI of 4.4. However, the higher molecule weight batch exhibits a maximum mobility of 0.3 cm²V⁻¹s⁻¹ (See SI Table-S3²²). Therefore, there seems to be no strong molecular weight influence on the performance for PRF at least within the range of the molecular weights investigated in these studies. When comparing PRF and P2F, which have similar chemical structures and absorption profiles, but differ with respect to the structural precision of the backbone, the regioregular polymer (avg. 0.9±0.2 cm²V⁻¹s⁻¹) exhibits substantially improved performance than the regiorandom counterpart (avg. 0.3 cm²V⁻¹s⁻¹). The di-fluorinated PDF obtains the lowest mobility, of about 0.3 cm²V⁻¹s⁻¹ for both the maximum and average mobility. It is interesting since the fluorine substitution could enhance the packing and crystallinity, and in the past, DFBT has been widely reported as a better acceptor unit relative to the BT both in OFET and OPV devices,¹⁹ but here seems not the case.²⁰

Atomic force microscopy (AFM) was used to investigate the surface topographic features of devices, as a function of fluorine substitution. Height images are shown in the SI Figure S3²². There is no clear alignment in any of them in agreement with the result that their mobilities are not dependent on the blading direction. All films form continuous polymer domains. It seems there is no obvious difference in the film topography that could support the mobility difference in the device.

The air stability of PBT, P2F and PDF OFET devices were investigated to verify the benefit of lowering the HOMO levels as a function of fluorine substitutions. Their OFET device performance was measured in ambient conditions after exposure in air for one day and five days, respectively. Their transfer curves are provided in the SI Figure S11²², and their mobilities as a function of time are shown in FIG. 4. It is very obvious that the device with PBT is very instable in the air as the mobility decreased to 0.002 cm²V⁻¹s⁻¹ after 5 days, which is in agreement with the previous report.^5a After 5 days, mobility of the mono-fluorinated polymer P2F decreased to 0.03 cm²V⁻¹s⁻¹ and mobility of the di-fluorinated polymer PDF reduced to 0.07 cm²V⁻¹s⁻¹. The higher mobility of PDF in air addressed the importance of low-lying HOMO level for OFET applications.

The above results show the present disclosure has developed three novel low band-gap polymers for OFET applications. Fluorine substitution was shown to significantly lower the energy levels. Regioregularity was once again demonstrated as an important design strategy since the mobility of P2F is improved when compared to PRF. However, there appears to be no influence of the molecular structure on the alkyl chain or π-π stacking distance. This is really a subtle feature. It is interesting that the regioregular mono-fluorinated P2F exhibits a similar mobility but the di-fluorinated PDF displayed a lower mobility relative to the non-fluorinated PBT polymer. Nevertheless, PDF exhibited superior advantages in the air stable OFET device applications relative to PBT and P2F as it displayed remarkably deeper HOMO levels.

Example Synthesis

(4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b′]dithiophene-2,6-diyl)bis(trimethylstannane) (M1) was synthesized according to the reported procedure. 4,7-Dibromo-benzo[c][1,2,5]thiadiazole (BT) was purchased from Sigma-Aldrich Co. and purified via flash column using silica gel and chloroform and hexane mixture as the eluent. 4,7-Dibromo-5-fluoro-benzo[c][1,2,5]thiadiazole (FBT) and 4,7-Dibromo-5,6-difluoro-benzo[c][1,2,5]thiadiazole (DFBT)was purchased from Lumtec Co. and used as received. Pd(PPh3)4 was purchased from Strem Co. Anhydrous o-xylene was purchased from Acros Co.

5,5′-bis{(4-(6-fluoro-7-bromo-[1,2,5]thiadiazolobenzene)}-{4,4-bis(hexadecyl)cyclopenta-[2,1-b: 3,4-b′]-dithiophene} (M2):

A 25 mL microwave reaction vial was charged with M1 (952 mg, 1 mmol, 1.0 eq), FBT (780 mg, 2.5 mmol, 2.5 eq), Pd(PPh₃)₄ (60 mg, 0.05 mmol, 0.05 eq), and dry toluene (15 mL) inside the glovebox box. The reaction vial was then sealed using a Teflon®cap and moved out of the dry box. The reaction mixture was stirred at 90° C. in a conventional oil bath for 72 hours. Then the product was purified via silica-gel (200 mesh) column chromatography (20-33% chloroform in hexanes gradient, usually ¼, ⅓, ½, chloroform/hexane, v/v). The yield is about 650 mg˜60%, dark solid. ¹H NMR (CDCl₃, 500 MHz, δ) 8.06 (s, 2H), 7.73 (d, 2H), 1.98 (m, 4H), 1.00-1.30 (m, 56H), 0.87 (t, 6H). C¹³ NMR (CDCl₃, 125 MHz, δ) 161.81, 160.22, 159.81, 154.37, 154.31, 148.76, 139.99, 138.97, 128.13, 128.05, 123.36, 114.58, 114.44, 95.43, 95.23, 54.52, 37.78, 31.90, 29.96, 29.66, 29.65, 29.63, 29.62, 29.58, 29.57, 29.33, 24.65, 22.67, 14.10

PBT: M1 (200 mg, 0.21 mmol), BT (59 mg, 0.20 mmol), Pd(PPh3)4 (11 mg, 0.01 mmol), anhydrous o-xylene (2 mL) and DMF (0.4 mL) were added to a 2-5 mL microwave tube in the nitrogen atmosphere glovebox. The tube was and subjected to the following reaction conditions in the microwave reactor: 80° C. for 2 min, 130° C. for 2 min, 160° C. for 2 min, and 200° C. for 40 min. The reaction was allowed to cool to room temperature, and then the vail was transferred to the glovebox. The vail was opened to add Pd(PPh₃)₄ (3 mg), 2-bromothiophene (0.1 mL) and xylene (2 mL) for end-capping reaction. Then the vail was sealed again and subjected to the microwave reactor under the conditions of 80° C. for 2 min, 130° C. for 2 min, 160° C. for 20 min. The reaction was allowed to cool to room temperature and the polymer was precipitated in methanol. The precipitates were collected by filter paper and extracted with methanol, dichloromethane and chloroform respectively via a Soxhlet extractor. The chloroform solution was concentrated under vacuum. Then concentrated polymer solution was passed through a short silica-gel (60-100 mesh) column. Then it was concentrated again and was dropwise to the methanol under stirring. The polymer was precipitated and collected via filter paper, dried over in the vacuum to provide dark solid 130 mg, yield 85%.

PRF (M_n=53 kDa): M1 (209 mg, 0.22 mmol), FBT (62 mg, 0.20 mmol), Pd(PPh₃)₄ (11 mg, 0.01 mmol), anhydrous o-xylene (3 mL) were added to a 2-5 mL microwave tube in the nitrogen atmosphere glovebox. Further synthesis procedure is similar to the synthesis of PBT to provide dark solid 137 mg, yield 88%.

PRF (M_n=128 kDa): M1 (200 mg, 0.21 mmol), FBT (62 mg, 0.20 mmol), Pd(PPh₃)₄ (11 mg, 0.01 mmol), anhydrous o-xylene (3 mL) were added to a 2-5 mL microwave tube in the nitrogen atmosphere glovebox. The microwave-assist polymerization and end-capping procedures are similar to the synthesis of PBT. The reaction was allowed to cool to room temperature and the polymer was precipitated in methanol. The precipitates were collected by filter paper and extracted with methanol, dichloromethane, chloroform and toluene respectively via a Soxhlet extractor. The toluene solution was concentrated under vacuum. Then concentrated polymer solution was passed through a short silica-gel (60-100 mesh) column. Then it was concentrated again and was dropwise to the methanol under stirring. The polymer was precipitated and collected via filter paper, to provide dark solid 147 mg, yield 94%.

P2F: M1 (110 mg, 0.115 mmol), M2 (109 mg, 0.10 mmol), Pd(PPh₃)₄ (6 mg, 0.005 mmol), anhydrous o-xylene (3 mL) were added to a 2-5 mL microwave tube in the nitrogen atmosphere glovebox. Further synthesis procedure is similar to the synthesis of PBT to provide dark solid 140 mg, yield 90%.

PDF: M1 (209 mg, 0.115 mmol), DFBT (66 mg, 0.20 mmol), Pd(PPh₃)₄ (11 mg, 0.01 mmol), anhydrous o-xylene (3 mL) were added to a 2-5 mL microwave tube in the nitrogen atmosphere glovebox. Further synthesis procedure is similar to the synthesis of PBT to provide dark solid 140 mg, yield 88%.

Process Steps

FIG. 5 is a flowchart illustrating a method for fabricating an OFET. The method can comprise the following steps.

Block 500 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.

Block 502 represents optionally forming/depositing contacts or electrodes (e.g., p-type, n-type contacts, or a gate, source, and/or drain contacts) on or above (or as part of) the substrate.

In an OFET embodiment comprising a top gate & bottom contact geometry, source and drain contacts are deposited on the substrate. Examples of 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 includes, but is not limited to nickel, silver or molybdenum.

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.

The gate, source, and drain contacts can be printed, thermal evaporated, or sputtered. Block 504 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.

The step can comprise forming a coating (e.g., a dielectric coating) or one or more dielectric layers, on the substrate. The dielectric layers can comprise silicon dioxide, a polymer (e.g., PVP) dielectric layer, a polymerized ionic liquid (PIL), or multiple dielectric layers (e.g., a bilayer dielectric). The dielectric layers can be solution coated on the substrate. A single polymer dielectric layer may be preferred in some embodiments (for easier processing, more flexibility). In one embodiment, the dielectric layers can form a polymer/SiO₂ bilayer. In another embodiment, the dielectric layers form a polymer dielectric/SiO_2/SAM multilayer with the SiO₂ on the polymer and the alkylsilane or arylsilane Self Assembled Monolayer (SAM) layer on the SiO₂. In another embodiment, the dielectric layers form a SiO₂/SAM bilayer with the alkylsilane or arylsilane SAM layer on the SiO₂. Various functional groups may be attached to the end of the alkyl groups to modify the surface property of the SAM layer.

The thickness of the coating/dielectric (e.g., SiO₂) may be adjusted/selected. For example, the thickness 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.

Block 506 represents fabricating or obtaining one or more semiconducting polymers. In one or more embodiments, each of the semiconducting polymers comprise polymer chains having a backbone comprising an aromatic ring, the aromatic ring comprising an element (e.g., fluoro functionality) having reduced susceptibility to oxidization as compared to nitrogen.

In one or more embodiments, the semiconducting polymers have fluoro functionality comprising an acceptor structure including a regioregular fluoro-phenyl unit.

In one or more embodiments, the semiconducting polymer comprises a (e.g., regioregular) conjugated main chain section, the conjugated main chain section having a repeat unit that comprises a compound of the structure:

wherein Ar is a substituted or non-substituted aromatic functional group, 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.

In one or more examples, the ring comprising the F has the structure:

Other examples include those illustrated in FIG. 6a, where each R is independently a substituted or non-substituted alkyl chain, which can be a C₆-C₃₀ substituted or non-substituted alkyl chain, —(CH₂CH₂O)n (n=2˜20), C₆H₅, —C_nF_(2n+1) (n=2˜20), —(CH₂)_nN(CH₃)₃Br (n=2˜20), —(CH₂)_nN(C₂H₅)₂ (n=2˜20), 2-ethylhexyl, PhC_mH_2m+1(m=1-20), —(CH₂)_nSi(C_mH_2m+1)₃ (m, n=1 to 20), or —(CH₂)_nSi(OSi(C_mH_2m+1)₃)_x(C_pH_2p+1)_y (m, n, p=1 to 20, x+y=3), for example; in some embodiments, the R groups can be the same.

In one or more embodiments, the repeat unit further comprises a dithiophene of the structure:

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 some embodiments, the

R groups can be the same. In the dithiophene, the R comprising the substituted or non-substituted alkyl, aryl or alkoxy chain can be a C₆-C₃₀ substituted or non-substituted alkyl or alkoxy chain, —(CH₂CH₂O)_n (n=2˜20), C₆H₅, —CnF_(2n+1) (n=2˜20), —(CH₂)_nN(CH₃)₃Br (n=2˜20), 2-ethylhexyl, PhC_mH_2m+1(m=1-20), —(CH₂)_nN(C₂H₅)₂ (n=2˜20), —(CH₂)_nSi(CmH_2m+1)₃ (m, n=1 to 20), or —(CH₂)_nSi(OSi(C_mH_2m+1)₃)_x(C_pH_2p+1)_y (m, n, p=1 to 20, x+y=3).

Examples of dithiophene units include those illustrated in FIGS. 6b-6d of the present disclosure and Table B (FIG. 30B) in U.S. Utility patent application Ser. No. 14/426,467, filed on Mar. 6, 2015, by Hsing-Rong Tseng, Lei Ying, Ben B. Y. Hsu, Christopher J. Takacs, and Guillermo C. Bazan, entitled “FIELD-EFFECT TRANSISTORS BASED ON MACROSCOPICALLY ORIENTED POLYMERS,” Attorney's Docket No. 30794.0514-US-WO (UC REF 2013-030), wherein the definition of R, R1 and R2 in FIGS. 6b-6d is the same as the definition for the R groups given above for FIG. 6a.

In one or more examples, the dithiophene unit comprises:

In one or more embodiments, the semiconducting polymer has the structure with repeating units D comprising the dithiophene and repeating units A comprising the ring comprising the fluorine, e.g., [D—A—D—A]_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. Thus, in one or more embodiments, the semiconducting polymer is a regioregular semiconducting polymer comprising a repeating unit of the structure:

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) is completed with hydrogen, the R groups comprising the substituted or non-substituted alkyl, aryl or alkoxy chain can be a C₆-C₃₀ substituted or non-substituted alkyl or alkoxy chain, —(CH₂CH₂O)_n (n=2˜20), C₆H_5, —CnF_(2n+1) (n=2˜20), —(CH₂)_nN(CH₃)₃Br (n=2˜20), 2-ethylhexyl, PhC_mH_2m+1(m=1-20), —(CH₂)_nN(C₂H₅)₂ (n=2˜20), —(CH₂)_nSi(CmH_2m+1)₃ (m, n=1 to 20), or —(CH₂)_nSi(OSi(CmH_2m+1)₃)_x(C_pH_2p+1)_y (m, n, p=1 to 20, x+y=3).

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, also called PCDTFBT).

Further additives or compositions may be added to the solution, e.g., to form a blend.

FIG. 7 is a flowchart illustrating a method of preparing the semiconducting polymers.

Block 700 represents reacting a first compound and second compound in a solution. In one or more embodiments, the first compound has the structure:

However, in one or more embodiments, the C₁₆H₃₃ are replaced with R as indicated above to achieve the desired R in the semiconducting polymers. In one or more embodiments, the Sn is not required and the first compound is terminated with a proton on either side.

In one embodiment, the second compound has the structure:

In one or more embodiments, the reacting comprises mixing a solution comprising the first compound and the second compound to form a mixture and heating the mixture (e.g., to 90° C.) in an oil bath, obtaining a product.

Block 702 represents purifying the product in a solvent mixture using chromatography. In one or more embodiments, the solvent mixture comprises chloroform:hexane (with more hexane than chloroform, e.g., 20%-33% chloroform). In one or more embodiments, the chloroform/hexane v/v ratio is ¼, ⅓, or, ½, or between ¼ and ½. Surprisingly and unexpectedly, it was discovered that selection of the proper solvent mixture (more hexane than chloroform) obtained the regioregularity of the third compound (this is unexpected/surprising because a solvent comprising less hexane than chloroform (e.g., hexane:chloroform ratio of 1:3) is used during purification to obtain regioregularity of PCDTPT polymers.

Block 704 represents the end result of steps 700-702, a third compound. In one or more embodiments, the third compound has the structure:

Where each Y is bromide, iodide, pseudo-halides or triflate; Z is hydrogen, alkenyl, borate, Sn(Me)₃ or Sn(Bu)_3; each R is independently hydrogen or a substituted or non-substituted alkyl, aryl, or alkoxy chain.

In one or more embodiments, the third compound has the structure:

wherein, in one or more embodiments, C₁₆H₃₃ is replaced by R as described herein.

Block 706 represents polymerizing the third compound to obtain the regioregular semiconducting polymers described above.

Block 508 represents solution casting/processing the solution comprising the semiconducting polymers, such that the semiconducting polymers are deposited in a film on or above the substrate or on the dielectric.

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.

Block 510 represents further processing the polymer film cast on the dielectric/substrate. The step can comprise annealing/curing/drying the polymer (or allowing the polymer to dry). The step can comprise depositing source and drain contacts, if necessary.

Block 512 represents the end result, a device. In one or more embodiments, the device 800 includes one or more OFETs, e.g., as illustrated in FIG. 8. Each OFET comprises a source contact S and a drain contact D to a film comprising the one or more semiconducting polymers 802; and a gate connection G on a dielectric 804, wherein the gate connection G applies a field to the one or more semiconducting polymers 802 across the dielectric 804 to modulate conduction along one or more backbones of the one or more semiconducting polymers in a channel/active region between the source contact S and the drain contact D.

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 [23].

In one or more embodiments, the semiconducting polymers are stacked into a crystalline structure, wherein the crystalline structure is characterized by observation of a diffraction peak measured by grazing incidence wide-angle X-ray scattering (GIWAXS) of the film.

In one or more embodiments, the fabrication of the OFET, including selection of one or more compositions, one or more structures, and one or more configurations of the source, the drain, the gate, the dielectric, and/or the SAM; selection of a composition, structure (including regioregularity), crystallinity, and/or stability of the semiconducting polymers; selection of the solution casting conditions (e.g., solvent composition, casting speed) and annealing conditions (e.g., annealing time and/or temperature) for fabrication of the film comprising the semiconducting polymers; and selection of the quality and/or morphology of the film, are such that:

• • the semiconducting polymers are disposed in a film on a planar, non-grooved surface, wherein the film is characterized by the OFETs each having a hole and/or electron mobility of at least 1.2 cm²V⁻¹s⁻¹ in a saturation regime (e.g., in a range of 1.2 -10 cm²V⁻¹s⁻¹); and/or
  • mobility of the OFETs is no less than 0.03 cm²V⁻¹s⁻¹ after exposure to the air for between 1 and 5 days; and/or
  • the semiconducting polymers comprise polymer chains stacked into film having a crystalline structure, and the crystalline structure is characterized by observation of a diffraction peak measured by grazing incidence wide-angle X-ray scattering (GIWAXS) of the film; and/or
  • a π-π distance between adjacent polymer chains is no more than 0.35 nm.

Thus, one or more embodiments of the present invention have discovered that semiconducting polymers with fluoro functionality have unique properties. Specifically, the inventors have unexpectedly and surprisingly discovered that some fluorinated semiconducting polymers with specific fluorine moieties (e.g., P2F or PCDTFBT) can be used to fabricate OFETs having increased stability in air as evidenced by carrier mobility that is less degraded by exposure to air as compared to OFETs fabricated using PCDTPT or PBT . In addition, the inventors have unexpectedly and surprisingly discovered that some fluorinated semiconducting polymers with specific fluorine moieties (e.g., P2F or PCDTFBT) have increased mobility (e.g., 1.2 cm²V⁻¹s⁻¹) as compared to OFETs fabricated using their non-fluorinated counterpart PCDTPT (having a hole mobility of 0.6 cm²V⁻¹s⁻¹ as measured in reference⁶). Thus, the OFET with a P2F (PCDTFBT) channel according to one or more embodiments of the present invention exhibited a 200% higher hole mobility as compared to the OFET with the PCDTPT channel reported in other work⁶. These discoveries are unexpected and surprising at least because OFETs fabricated using some fluorinated polymers (e.g., PDF) had reduced mobility. Moreover, the molecular structure of the fluorinated semiconducting polymers did not appear to have a negative impact on the alkyl chain or π-π stacking distance of the semiconducting polymers. The relative stability of the fluorinated polymers enables fabrication of devices that could not previously have been envisaged. Specifically, devices where the OFETs are exposed to air can be fabricated. Moreover, much cheaper and simpler packaging that exposes the OFETs to air can be used.

Device Embodiments

FIG. 8 illustrates a device comprising storage (e.g., a substrate 806 and packaging 808) for a plurality of the OFETs each comprising a source S, drain D, and gate (e.g., at least 8 of the OFETs). The storage/packaging exposes the semiconducting polymers to air. In one or more embodiments, the device 800 comprises an optoelectronic or electronic device storing the OFETs in an electrical/electronic circuit. In one or more embodiments, the OFETs do not comprise encapsulation layers or the semiconducting polymers are covered by layers/encapsulation/packaging permeable to air. In one or more embodiments, the device 800 further comprises electrical connections and/or voltage sources to bias the OFETs with the appropriate threshold voltages and source-drain voltages to achieve the above described desired mobility and/or performance stability, and outputs to receive the outputs from each of the OFETs. In one or more embodiments, the OFETs are connected in a circuit to form logic gates. In one or more embodiments, the device comprises a display or array of sensors comprising the logic gates.

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The following references are incorporated by reference herein.

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Conclusion

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.

Claims

1. A composition of matter, comprising: a semiconducting polymer having a main chain section and a repeating unit of the structure:

2. The composition of matter of claim 1, wherein the semiconducting polymer comprises the main chain section having the repeating unit of the structure:

3. The composition of matter of claim 1, wherein the semiconducting polymer comprises the main chain section having the repeating unit of the structure:

4. The composition of matter of claim 1, wherein each of the semiconducting polymers have the repeating unit of the structure:

5. The composition of matter of claim 1, wherein carrier mobility of an OFET comprising the semiconducting polymer is at least 0.03 cm2V−1s−1 after exposure of the semiconducting polymer to the air for at least 5 days.

6. A package comprising the composition of matter of claim 1, wherein the package exposes the semiconducting polymers to the air for at least 5 days.

7. The composition of matter of claim 1, further comprising an air permeable material encapsulating the composition of matter.

8. The composition of matter of claim 1, further comprising: the semiconducting polymers are stacked into a crystalline structure, wherein the crystalline structure is characterized by observation of a diffraction peak measured by grazing incidence wide-angle X-ray scattering (GIWAXS) of the film.

9. The composition of matter of claim 8, wherein a π-π distance between adjacent semiconducting polymers is no more than 0.35 nm.

10. One or more organic field effect transistors (OFETs) comprising the composition of matter of claim 1, each of the OFETs comprising: a channel including the semiconducting polymers;a source contact to the channel;a drain contact to the channel; anda gate contact on or above the channel.

11. The OFETs of claim 10, wherein the semiconducting polymers are disposed on a planar, non-grooved surface, and the OFET has a hole mobility of at least 1.2 cm2V−1s−1 in a saturation regime.

12. The OFETs of claim 10, wherein the hole mobility is in a range of 1.2 -10 cm2V−1s−1 in the saturation regime.

13. An electronic circuit comprising the OFETs of claim 10.

14. A method of preparing a donor-acceptor copolymer, comprising: preparing a monomer, wherein the monomer comprises a donor structure having an electron rich moiety in combination with an acceptor structure including fluorine and having electron deficient moiety;reacting the monomer to produce a donor-acceptor copolymer that comprises a regioregular conjugated main chain section, and performing an end-capping reaction on the donor-acceptor copolymer; wherein:the charge carrier mobility of the regioregular donor-acceptor copolymer is greater than the charge carrier mobility of a regiorandom donor-acceptor copolymer of similar composition.

15. The method of claim 14, further comprising: forming a source contact and a drain contact to a channel comprising semiconducting polymers including the regioregular donor-acceptor copolymer;forming a gate contact on or above the channel;providing a dielectric between the semiconducting polymers and a gate; andwherein:each of the semiconducting polymers have a main chain section and a repeating unit of the structure:

16. The method of claim 15, further comprising packaging the OFET in package that exposes the semiconducting polymers to the air for at least 5 days.

17. The method of claim 15, further comprising encapsulating the OFET in an air permeable material.

18. The method of claim 15, further comprising encapsulating the OFET in plastic.

19. The method of claim 15, wherein carrier mobility of the OFET is at least 0.03 cm2V−1s−1 after exposure to the air for 5 days.

20. The method of claim 15, further comprising: depositing the semiconducting polymers from a solution onto a substrate, wherein the semiconducting polymers stack into a crystalline structure, wherein the crystalline structure is characterized by observation of a diffraction peak measured by grazing incidence wide-angle X-ray scattering (GIWAXS) of the film.

21. The method of claim 20, wherein a π-π distance between adjacent semiconducting polymers is no more than 0.35 nm.

22. The method of claim 15, further comprising assembling the OFETs in an electronic circuit.

23. The method of claim 14, wherein each of the donor-acceptor copolymers have the repeating unit of the structure:

24. The method of claim 14, wherein the donor-acceptor copolymers comprising semiconducting polymers are fabricated using a process comprising: reacting a first compound and second compound in a solution to form a product, wherein the first compound comprises a dithiophene and the second compound has the structure:

25. The method of claim 14, wherein the preparing is such that a regioregularity of the conjugated main chain section is 95% or greater.

26. The method of claim 14, wherein the method comprises Stille polymerization under microwave heating.