SEMICONDUCTING POLYMERS WITH MOBILITY APPROACHING ONE HUNDRED SQUARE CENTIMETERS PER VOLT PER SECOND
20170133597 ยท 2017-05-11
Assignee
Inventors
- Byoung Hoon Lee (Goleta, CA, US)
- Ben B. Y. Hsu (Santa Barbara, CA)
- Chan Luo (Santa Barbara, CA, US)
- Ming Wang (Goleta, CA, US)
- Guillermo C. Bazan (Santa Barbara, CA)
- Alan J. Heeger (Santa Barbara, CA)
Cpc classification
C08G61/126
CHEMISTRY; METALLURGY
C08G2261/3243
CHEMISTRY; METALLURGY
C08G2261/3246
CHEMISTRY; METALLURGY
H10K85/113
ELECTRICITY
International classification
Abstract
One or more embodiments of the present invention report here a comparative study of field effect transistors (FETs) fabricated with semiconducting polymer PBT, regioregular semiconducting polymers, PCDTPT and their fluorinated analogue (P2F, PCDTFBT), in the transistor channel. The present invention shows that simple fluorination of PBT and PCDTPT to PCDTFBT leads to air-stability and reliable transistor characteristics. The FETs fabricated from aligned PCDTFBT yielded stable threshold voltages (at zero volt) and a narrow distribution of saturation hole mobilities of 65 cm.sup.2 V.sup.1 s.sup.1 (average over 50 independent FET devices). At higher source-drain voltage (higher electric field in the channel) the mobility approaches 100 cm.sup.2 V.sup.1 s.sup.1, the highest value for semiconducting polymers reported to date. High mobility is retained over 150 hours in ambient air without any encapsulation layers. The results obtained in one or more embodiments of the invention represent important progress for solution-processed plastic transistors, and provide molecular design guidelines for high-mobility and air-stable conjugated polymers.
Claims
1. One or more organic field effect transistors (OFETs), wherein each of the OFETs comprises: a channel comprising semiconducting polymers, wherein: each of the semiconducting polymers have a conjugated main chain section having a repeating unit of the structure: ##STR00011## 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, 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; a source contact to the channel; a drain contact to the channel; and a gate contact on or above the channel.
2. The OFETs of claim 1, wherein each of the semiconducting polymers have a repeating unit of the structure: ##STR00012## and wherein the fluorine (F) is regioregularly arranged along the semiconducting polymer's conjugated main chain section.
3. The OFETs of claim 1, wherein the acceptor structure is selected from
4. The OFETs of claim 1, wherein: the semiconducting polymers are disposed in a film on a planar, non-grooved surface, and the film has a crystallinity characterized by the OFETs each having a hole mobility of at least 1.2 cm.sup.2V.sup.1 s.sup.1 in a saturation regime.
5. The OFETs of claim 4, the hole mobility is in a range of 1.2-10 cm.sup.2V.sup.1 s.sup.1 in the saturation regime.
6. The OFETs of claim 1, wherein the semiconducting polymers are on a grooved surface and the semiconducting polymers have an alignment with respect to each other characterized by the OFETs each having a hole mobility between 1 cm.sup.2 V.sup.1 s.sup.1 and 97 cm.sup.2 V.sup.1 s.sup.1 in a saturation regime.
7. The OFETs of claim 6, wherein the OFETs each have a threshold voltage within +/1 volt of zero volts.
8. The OFETs of claim 7, wherein the OFETs each have a threshold voltage of zero volts.
9. The OFETs of claim 6, wherein 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.
10. The OFETs of claim 6, wherein a - stacking of the semiconducting polymer chains in the film is characterized by a peak having a full width at half maximum of 2 nm.sup.1 or less, as measured by a grazing incidence wide-angle X-ray scattering (GIWAXS) measurement of the film.
11. The OFETs of claim 6, wherein a - distance between adjacent polymer chains is no more than 0.35 nm.
12. A device comprising storage for twenty of the OFETs of claim 1, wherein each of the OFETs are characterized by: a threshold voltage such that an average threshold voltage for the twenty OFETs is within +/1 volt of zero volts, and/or the threshold voltage that is within +/1 V of zero volts) after multiple sweeps of the gate source voltage, and/or the threshold voltage within +/1 V of zero volts when the gate source voltage varies between 120 V to 0.5 V.
13. A device comprising storage for fifty of the OFETs of claim 1, wherein each of the OFETs are characterized by: a threshold voltage such that an average threshold voltage for the twenty OFETs is within +/1 volt of zero volts, and/or the threshold voltage that is within +/1 V of zero volts) after multiple sweeps of the gate source voltage, and/or the threshold voltage within +/1 V of zero volts when the gate source voltage varies between 120 V to 0.5 V.
14. The device of claim 13, wherein a stability of each of the OFETs is further characterized by the fifty OFETs having an average carrier mobility of at least 65 cm.sup.2 V.sup.1 s.sup.1 in a saturation regime.
15. A device comprising storage for the OFETs of claim 1, wherein the storage exposes the semiconducting polymers to air.
16. The device of claim 15, wherein: the device comprises an optoelectronic or electronic device storing the OFETs in a circuit, and the OFETs do not comprise encapsulation layers or the semiconducting polymers are covered by layers permeable to air.
17. The device of claim 16, wherein: a carrier mobility of each of the OFETs is reduced by less than 20% as exposure of the OFETs is increased from 4 hours to 100 hours in ambient air at a temperature of 22 C.-30 C., the ambient air having a relative humidity of 45%-70%, and the exposure including 50 hours in a nitrogen ambient.
18. The device of claim 15, wherein carrier (hole and/or electron) mobility of each of the OFETs is at least 0.03 cm.sup.2V.sup.1 s.sup.1 after exposure to the air for 5 days.
19. The OFETs of claim 1, wherein the source and drain contacts of the OFETs further comprise a metal oxide electron blocking layer.
20. The OFETs of claim 19, wherein the metal comprises nickel, silver, or Molybdenum.
21. A method of fabricating a plurality of organic field effect transistors, comprising: providing a source contact and a drain contact to a channel comprising semiconducting polymers; and providing a dielectric between the semiconducting polymers and a gate; wherein each of the semiconducting polymers have a repeating unit of the structure: ##STR00013## wherein the fluorine (F) is regioregularly arranged along the semiconducting polymer's conjugated main chain section; and wherein the R are each independently an alkyl, aryl, or an alkoxy chain.
22. The method of claim 21, further comprising solution processing at least fifty of the OFETs onto a substrate from a same batch of solution comprising the semiconducting polymers.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0050] Referring now to the drawings in which like reference numbers represent corresponding parts throughout:
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DETAILED DESCRIPTION OF THE INVENTION
[0067] 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
[0068] 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 (28). Saturation regime mobilities of over 10 cm.sup.2 v.sup.1 s.sup.1 have been achieved through coupled developments in molecular design strategies and improvements in film processing methods (29). 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 (30). Such structures consist of an electron rich moiety (donor, D) and an electron deficient moiety (acceptor, A) in each repeat unit. For example, Millen 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 (31), namely PBT in Scheme 1 (CDTBTZ in the original publication), which was shown to be suitable for thin film alignment thereby able to attain mobility of 6.5 cm.sup.2V.sup.1 s.sup.1 (32). 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 (33). This regioregular polymer PCDTPT has yielded some of the highest mobilities in the literature (over 20 cm.sup.2V.sup.1 s.sup.1) through molecular weight control and novel film processing techniques that improve chain alignment (29(d)).
[0069] Indeed, the realization of -electron wavefunctions delocalized over many repeat units along the backbone of highly aligned semiconducting polymer chains with minimal structural disorder is a primary requirement for achieving high mobility in semiconducting polymers (1, 2). To achieve this goal, one or more of the inventors of the present invention developed a series of crystalline semiconducting polymers including asymmetric regioregular PCDTPT (see
[0070] Despite successes in achieving high charge carrier mobilities, one or more of the inventors of the present invention have found, however, that copolymers containing CDT and BT structural units exhibit relatively high-lying highest occupied molecular orbital (HOMO) levels (5.00.2 eV) (34); not an ideal situation for long-term air stability (below 5.27 eV) (35). For structures containing the more electron withdrawing PT fragments, the basic nature of the PT heterocycle can de-stabilize relevant electronic properties when exposed to air or acidic surfaces (7, 36). As a result, FETs fabricated from PCDTPT produced unreliable device properties (causing variable threshold voltage and mobility) (7). The development of stable semiconducting polymers with high mobility is essential for the growth of plastic electronics. 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 (37).
[0071] One or more embodiments of the present invention report here a derivative of PCDTBT: 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)] (abbreviation as P2F or PCDTFBT; see
First Embodiment: Polymers on Smooth, Planar Substrates
[0072] a. Polymer Fabrication
[0073] 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
[0074] b. Polymer Characterization
[0075] 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.
[0076] UV-vis absorption was used to examine optical transitions. Solution absorption measurements were carried out in chlorobenzene solvent. As shown in
TABLE-US-00001 TABLE 1 Summary of absorption characteristics and orbital energy levels. .sub.sol and .sub.film refer to absorption peak values in chlorobenzene and in the thin film, respectively; E.sub.g.sup.opt is the optical band-gap determined by film absorption onset; E.sub.HOMO and E.sub.LUMO are the energy levels from CV measurements and optical band-gaps. .sub.sol .sub.film E.sub.g.sup.opt E.sub.HOMO E.sub.LUMO 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
[0077] Thin films were prepared by spin-casting from the corresponding chlorobenzene solution and the absorption spectra were measured. As shown in
[0078] Cyclic voltammetry was used to estimate HOMO energy levels by measuring the oxidation potential onsets (40). As shown in Figure S3 in the SI (49), 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 (35(a)). 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 (41).
[0079] Grazing-incidence wide-angle X-ray scattering (GIWAXS) was used to investigate thin film organization (48). The 2D diffraction images are provided in the SI
[0080] c. OFET Device Characterization
[0081] Bottom contact, bottom gate OFETs were fabricated using devices with the following architecture: polymer/Au/SAM/SiO.sub.2/Si (doped). Decyltrichlorosilane (DTS) was used as the self-assembled monolayer on the silicon oxide substrate. Thin films were prepared via doctor-blading (42) and the complete details are provided in the SI (49). 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.
[0082] Typical output curves obtained with OFET devices are provided in
TABLE-US-00002 TABLE 2 Polymer OFET highest and average mobilities. M.sub.n Mobility V.sub.th Polymer (kDa) PDI cm.sup.2V.sup.1s.sup.1 On/off (V) PBT 68 3.6 1.4 (1.1 0.1) .sup.6.3 10.sup.2 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
[0083] 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 cm.sup.2V.sup.1 s.sup.1. The regiorandom polymer PRF obtains a maximum mobility of 0.4 cm.sup.2V.sup.1 s.sup.1, while its regioregular countpart P2F obtains a maximum mobility of 1.2 cm.sup.2V.sup.1 s.sup.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 of 128 kDa and a PDI of 4.4. However, the higher molecule weight batch exhibits a maximum mobility of 0.3 cm.sup.2V.sup.1 s.sup.1 (See SI Table-S3 (49)). 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. 0.90.2 cm.sup.2V.sup.1 s.sup.1) exhibits substantially improved performance than the regiorandom counterpart (avg. 0.3 cm.sup.2V.sup.1 s.sup.1). The di-fluorinated PDF obtains the lowest mobility, of about 0.3 cm.sup.2V.sup.1 s.sup.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 (46), but here seems not the case (47).
[0084] 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 S3 (49). 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.
[0085] 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
[0086] 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 - 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.
Second Embodiment: Polymers on Grooved Substrates with Alignment
[0087] a. Polymer Fabrication
[0088] Preparation of PCDTFBT (P2F) in this embodiment uses the same technique invented by the inventors and described in the first embodiment described above in
[0089] b. Film Characterization
[0090] (i) Absorption
[0091] The UV-Vis-NIR absorption spectra of PCDTPT and PCDTFBT thin films were obtained by using an OLIS 14 UV/VIS/NIR spectrophotometer (On-Line Instrument Systems, Inc.). The samples were prepared by spin casting PCDTPT and PCDTFBT from chlorobenzene solutions (5 mg mL.sup.1) at 600 rpm for 60 s on pre-cleaned quartz substrates.
[0092] The measured thicknesses of both PCDTPT and PCDTFBT thin films, 70 nm, were used for estimation of absorption coefficients.
[0093] The PCDTFBT thin films exhibit a relatively small optical bandgap (E.sub.g1.15 eV), though slightly higher than that of a PCDTPT film (E.sub.g1.02 eV), as shown in
[0094] (ii) X-Ray Photoelectron Spectroscopy (XPS)
[0095] XPS measurement: The PCDTPT and PCDTFBT thin films were prepared by sandwich casting on pre-cleaned Au substrates in a nitrogen-filled glove box. While inside the glovebox, the samples were mounted and sealed into a capped stainless steel sample holder. The capped holder was then transferred into the XPS antechamber and the cap was removed from the holder after a preliminary vacuum of 110.sup.5 Torr had been achieved. The XPS results were obtained using a Kratos Axis Ultra XPS system (Kratos Analytical Ltd.) at a base pressure of 110.sup.8 Torr using monochromatized Al K X-ray photons (hv=1486.6 eV). High-resolution XPS spectra (N is) were obtained at constant pass energy of 40 eV and a step size of 0.1 eV, while survey XPS spectra were taken at pass energy of 160 eV with a step size of 0.5 eV. Data analysis was carried out with CasaXPS software.
[0096] The environmentally stable molecular structure of PCDTFBT was confirmed by X-ray photoelectron spectroscopy (XPS) measurements. While .sup.1H nuclear magnetic resonance (NMR) data prove the fluorinated molecular structure of PCDTFBT (
[0097] (iii) GIWAXS
[0098] The samples were prepared onto the Si/n-PVP and Si/n-PVP/SiO.sub.2 (2 nm) substrates using the sandwich casting system. GIWAXS measurements were performed at beamline 11-3 at the Stanford Synchrotron Radiation Lightsource (SSRL) with an X-ray wavelength of 0.9752 , at a 400 mm sample to detector distance. Samples were scanned for 300 s in a He environment at an incident angle of 0.10. The measurements were calibrated using a LaB6 standard. The CCL of - stacking for aligned PCDTPT and PCDTFBT were estimated by using fitted GIWAXS line profiles (-) and Scherrer equation, CCL=2/FWHM, where FWHM is the full width at half maximum (1).
[0099] The high crystallinity and associated anisotropy essential for achieving the high mobility of aligned PCDTFBT thin films parallel to the nano-grooves in the substrate were confirmed by GIWAXS experiments.
[0100] (iv) Ultraviolet Photoelectron Spectroscopy (UPS)
[0101] UPS measurements were carried out using a Kratos Axis Ultra UPS system (Kratos Analytical Ltd.) at a base pressure of 110.sup.8 Torr using the He I source (hv=21.2 eV). The samples were prepared by electron beam deposition of Au (50 nm) and Au/Ni (50/10 nm) onto glass substrates, and were ultraviolet/ozone-treated for 10 min. While inside the glovebox, the samples were mounted and sealed into a capped stainless steel sample holder. The capped holder was then transferred into the UPS antechamber and the cap was removed from the holder after a preliminary vacuum of 110.sup.5 Torr had been achieved. During UPS measurements, a sample bias of 9 V was used. The UPS results were obtained at constant pass energy of 20 eV with a step size of 0.025 eV. Data analysis was carried out with CasaXPS software.
[0102] (v) GPC Measurement
[0103] GPC was performed in chloroform (CHCl.sub.3) on a Waters Breeze2 Separation Module equipped with a Waters 2414 Refractive Index Detector. Polystyrene standards were used to calibrate the system. The molecular weight (M.sub.n) was estimated to be 46 KDa and 50 KDa for PCDTBT and PCDTFBT, respectively. The polydispersity index (PDI) was estimated to be 3 for both polymers.
[0104] (vi) CV Measurements
[0105] CV measurement was conducted using a standard three electrode configuration under an argon atmosphere. A three electrode cell equipped with a glassy carbon as a working electrode, an Ag wire as a reference electrode, and a Pt wire as a counter electrode. The measurements were performed in absolute acetonitrile with tetrabutylammonium hexafluorophosphate (0.1 M) as the supporting electrolyte at a scan rate of 50-100 mV s.sup.1 (Figure S3 (50)).
[0106] c. OFET Characterization
[0107] The OFETs were fabricated onto n-SiO.sub.2 substrates prepared by rubbing a n++Si (500 m)/SiO.sub.2 (300 nm) substrate (International Wafer Services Co.) with a diamond lapping disc with particle sizes of 100 nm (Allied High Tech Products Inc.) as described in detail in the previous reports (1, 2). The Ni (5 nm)/Au (50 nm)/Ni (10 nm) source and drain electrodes (W/L=1,000/200 m) were patterned on the dielectrics through a conventional photolithography process. All metal electrodes were deposited by electron beam evaporation at 710.sup.7 Torr. After ultraviolet/ozone treatment of pre-cleaned n-SiO.sub.2 substrates for 10 min, the substrates were passivated with the n-decyltrichlorosilane (Gelest Inc.) in toluene solution (1% by volume) at room temperature for 17 h in a nitrogen-filled glove box. The PCDTPT and PCDTFBT (both materials were purchased from 1-Material Inc., but wherein both polymers were synthesized according to the procedures invented by one or more of the inventors and described in
[0108] The devices were then cured at 200 C. for 2 min prior to measurements, and were tested using a probe station (Signatone Co.) in a nitrogen-filled glove box. For mobility measurement at various annealing temperatures, devices were mounted on a hot stage (STC 200; Instec Inc.) for 1 min at each temperature. Data were collected by a Keithley 4200 system.
[0109] To investigate the charge transport and reliability of PCDTFBT, one or more embodiments of the present invention fabricated 50 FETs with aligned PCDTFBT thin films as the electronically active semiconductor in the channel. The device properties were analyzed and compared with those of 50 reference devices fabricated with aligned PCDTPT thin films (
I.sub.DS=(W/2L)C(V.sub.GSV.sub.T).sup.2 (1)
[0110] where, I.sub.DS is the source-drain current, W is the channel width, L is the channel length, C is the gate dielectric capacitance per unit area, is the saturation carrier mobility, V.sub.GS is the gate-source voltage, and V.sub.T is the threshold voltage. The inventors of the present invention note that measured saturation mobility is V.sub.GS dependent with higher mobility obtained in the low V.sub.GS regime as shown in
[0111] The representative transfer curves of 5 FETs fabricated from PCDTPT or PCDTFBT as the FET channel are displayed in
[0112] The enhanced reliability of PCDTFBT can be more clearly seen by comparing V.sub.T and mobility values extracted from 50 independent FETs with PCDTPT or PCDTFBT as displayed in
[0113] Stable transistor characteristics are essential requirements for practical applications of plastic circuits. The inventors note that the high performance FETs described in the present disclosure are stable under continuous operating condition. To minimize possible electron injection into traps that causes electrical instability, one or more embodiments of the present invention introduced oxidized nickel electrodes as electron-blocking layers on top of gold source and drain electrodes in the FET configuration. As a result, devices fabricated with oxidized nickel electrodes exhibit stabilized transistor characteristics with less electron injection as displayed in
[0114] The transistor characteristics of devices with aligned PCDTFBT thin films are also stable at various operating voltages. To assess operational stability of PCDTFBT FETs, electric field dependent transconductance measurements for FETs fabricated from PCDTFBT were performed. Transfer curves were taken at varying V.sub.DS from 120 V to 0.5 V (
[0115] Strikingly high saturation mobilities approaching 100 cm.sup.2/V-s, the highest value for semiconducting polymers reported to date (6, 14-18), were obtained at high V.sub.DS (V.sub.DS120 V). The implied electric field dependence is a real effect; the extracted average hole mobilities taken at V.sub.DS=20 to 40 V (t40-53 cm.sup.2/V-s) are still high (
[0116] The thermal stability of aligned PCDTFBT thin films was investigated by measuring FET mobilities at various annealing temperatures.
[0117] More importantly, enhanced air-stability for aligned PCDTFBT thin films was observed.
[0118] Through facile fluorination of PCDTPT and PBT, the stable and high-mobility semiconducting polymer, PCDTFBT, has been demonstrated and thoroughly characterized. By demonstrating that aligned PCDTFBT leads to a record high mobility approaching 100 cm.sup.2/V-s at higher source-drain operating voltage and that the associated FET characteristics were electrically, thermally, and air-stable, the results presented in the present disclosure provide molecular design guidelines for stable and high-mobility semiconducting polymers for high-performance plastic electronics, and demonstrate that semiconducting polymers can compete with their inorganic counterparts in a variety of low-cost electronics applications.
Process Steps
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[0120] 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.
[0121] Block 1002 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.
[0122] 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.
[0123] 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.
[0124] The gate, source, and drain contacts can be printed, thermal evaporated, or sputtered.
[0125] 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.
[0126] 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 dielectric/SiO.sub.2 bilayer. In another embodiment, the dielectric layers form a polymer dielectric/SiO.sub.2/SAM multilayer with the SiO.sub.2 on the polymer and the alkylsilane or arylsilane Self Assembled Monolayer (SAM) layer on the SiO.sub.2. In 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.
[0127] The thickness of the coating/dielectric (e.g., SiO.sub.2) 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.
[0128] The dielectric or coating can be structured or patterned to form one or more grooves or structures (such as nanogrooves/nanostructures, e.g., having a depth of 6 nanometers or less and/or a width of 100 nm or less) in the dielectric. 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 (e.g., a minimum distance between the source contact and drain contact can be substantially parallel to the longitudinal axis of the nanogrooves).
[0129] In one or more embodiments, the nanogrooves are formed by nano-imprinting (i.e., the nanogrooves are nanoimprinted into the dielectric or substrate). For example, the step of fabricating the dielectric layers can comprise nano-imprinting a first dielectric layer (e.g., PVP) deposited on the substrate; and depositing a second dielectric layer on the nanoimprinted first dielectric layer, wherein a thickness of the second dielectric layer comprising SiO.sub.2 is adjusted.
[0130] Block 1006 represents fabricating or obtaining one or more semiconducting polymers in solution.
[0131] In one or more embodiments, each of the semiconducting polymers comprise polymer chains having a backbone including an aromatic ring, the aromatic ring comprising an element (e.g., fluoro functionality) having reduced susceptibility to oxidization as compared to pyridine nitrogen.
[0132] In one or more embodiments, the semiconducting polymers have fluoro functionality comprising an acceptor structure including a regioregular fluoro-phenyl unit.
[0133] 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:
##STR00002##
[0134] 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.
[0135] In one or more examples, the ring comprising the F has the structure:
##STR00003##
[0136] Other examples include those illustrated in
[0137] In one or more embodiments, the repeat unit further comprises a dithiophene of the structure:
##STR00004##
[0138] 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 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).
[0139] Examples of dithiophene units include those illustrated in
[0140] In one or more examples, the dithiophene unit comprises:
##STR00005##
[0141] 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].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.
[0142] Thus, in one or more embodiments, the semiconducting polymer is a regioregular semiconducting polymer comprising a repeating unit of the structure:
##STR00006##
[0143] 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 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=2 20), 2-ethylhexyl, PhC.sub.mH.sub.2m+1 (m=1-20), (CH.sub.2).sub.nN(C.sub.2H.sub.5).sub.2 (n=2 20), (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).
[0144] 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).
[0145] Further additives or compositions may be added to the solution, e.g., to form a blend.
[0146]
[0147] Block 1200 represents reacting a first compound and second compound in a solution. In one or more embodiments, the first compound has the structure:
##STR00007##
[0148] However, in one or more embodiments, the C.sub.16H.sub.33 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.
[0149] In one embodiment, the second compound has the structure:
##STR00008##
[0150] 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.
[0151] Block 1202 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 , , or, , or between and . Surpisingly 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.
[0152] Block 1204 represents the end result of steps 1200-1202, a third compound.
[0153] In one or more embodiments, the third compound has the structure:
##STR00009##
[0154] where each Y is bromide, iodide, pseudo-halides or triflate; Z is hydrogen, alkenyl, borate, Sn(Me).sub.3 or Sn(Bu).sub.3; each R is independently hydrogen or a substituted or non-substituted alkyl, aryl, or alkoxy chain.
[0155] In one or more embodiments, the third compound has the structure:
##STR00010##
[0156] wherein, in one or more embodiments, C.sub.16H.sub.33 is replaced by R as described herein.
[0157] Block 1206 represents polymerizing the third compound to obtain the regioregular semiconducting polymers described above.
[0158] Block 1008 represents solution casting/processing the solution comprising the semiconducting polymers, such that the semiconducting polymers are deposited (from the solution) in a film on or above the substrate or on the dielectric. 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.
[0159] 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.
[0160] Block 1010 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.
[0161] Block 1012 represents the end result, a device 1300. In one or more embodiments, the device 1300 includes one or more OFETs, e.g., as illustrated in
[0162] 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 (51).
[0163] In one or more embodiments, a plurality of the OFETs are solution processed onto a substrate from a same batch of solution comprising the semiconducting polymers. In one or more embodiments, the semiconducting polymers are deposited from solution onto a substrate to form a film comprising the semiconducting polymers, wherein a plurality of source contacts, a plurality of drain contacts, and a plurality of gate contacts are patterned/formed onto the film to form a plurality of the OFETs each comprising one of the sources, one of the drains, and one of the gates. In one or more embodiments, a plurality of such OFETs (e.g., 50 OFETs) have a mean mobility between 1 and 100 cm.sup.2V.sup.1 s.sup.1 with a standard deviation of 13 cm.sup.2V.sup.1 s.sup.1 or less.
[0164] In one or more embodiments, the OFET comprises means (e.g., grooves, nanogrooves or statutory equivalents thereof) for aligning the main chain axes 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, 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 50-100 nm (as compared to fiber bundles having a width between 30-40 nm when fabricated on a non-structured substrate).
[0165] 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.
[0166] 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 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 to an alignment direction 1310 in the channel between Source S and Drain D.
[0167] In one or more embodiments, the dielectric layer is patterned with the nanogrooves that orient the semiconducting polymers.
[0168] 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.
[0169] In one or more embodiments, the semiconducting polymers are fabricated as active regions in devices other than OFETs, such as in an organic light emitting device or in a photovoltaic cell.
[0170] 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: [0171] one or more threshold voltages of the one or more OFETs are within +/1 volt (V) of 0 V; [0172] one or more of the threshold voltages are equal to zero or shifted back towards zero; [0173] variability of the one or more threshold voltages is reduced (e.g., twenty or fifty of the OFETs have an average threshold voltage within +/1 V of 0 V); [0174] each of the OFETs are characterized by having a threshold voltage that is stable (e.g., within +/1 V of zero volts) after multiple sweeps of the gate source voltage; [0175] each of the OFETs are characterized by having a stable threshold voltage at various operating voltages (e.g., the threshold voltage of each of the OFETs is within +/1 V of zero volts when the gate source voltage varies between 120 V to 0.5 V). [0176] one or more carrier mobilities (e.g., hole and/or electron mobilities) of the one or more OFETs are between 28 cm.sup.2 V.sup.1 s.sup.1 and at least 97 cm.sup.2 V.sup.1 s.sup.1 (e.g., at least 40 cm.sup.2 V.sup.1 s.sup.1 or at least 97 cm.sup.2 V.sup.1 s.sup.1) in a saturation regime; [0177] hole and/or electron mobility is in a range of 28.1 cm.sup.2 V.sup.1 s.sup.1-200 cm.sup.2 V.sup.1 s.sup.1, or between 28.1 cm.sup.2 V.sup.1 s.sup.1-100 cm.sup.2 V.sup.1 s.sup.1, e.g., for a source drain voltage in a range of 120 V to 1 V and a gate-source voltage in a range of +20 V to 80 V, or for a gate-source voltage in a range of 20 V to 40 V at a source-drain voltage of 120V); [0178] the OFETs' mobility is not dependent on blade coating direction and/or the semiconducting polymers are not aligned (or there is no clear alignment of the semiconducting polymers as measured in top surface of an AFM image of the semiconducting polymers) and/or the OFETs each have a hole and/or electron mobility of at least 1.2 cm.sup.2 V.sup.1 s.sup.1 (or at least 0.03 cm.sup.2 V.sup.1 s.sup.1 after 5 days exposure to air). [0179] the carrier mobilities of fifty or less of the OFETs have an average (e.g., average field effect hole or field electron mobility in the saturation regime) of at least 65 cm.sup.2 V.sup.1 s.sup.1; [0180] one or more of the OFETs have their carrier mobility reduced by less than 12% or less than 20% as exposure of the OFETs is increased from 4 hours to 100 hours in ambient air at a temperature of 22 C.-30 C., the ambient air having a relative humidity of 45%-70%, and the exposure including 50 hours in a nitrogen ambient; and/or [0181] the semiconducting polymers have higher average mobility and higher stability in ambient air as compared to PCDTPT or PBT.
[0182] Moreover, the fabrication can be under conditions wherein the semiconducting polymer comprises aligned conjugated polymer chains stacked to form a crystalline structure. In one or more embodiments, a - stacking of the semiconducting polymer in the film is characterized by a peak having a full width at half maximum of 2 nm.sup.1 or less, as measured by a grazing incidence wide-angle X-ray scattering (GIWAXS) measurement of the film. In one or more embodiments, a - distance between adjacent polymer chains is no more than 0.35 nm.
[0183] The above described crystallinity and OFET performance can be achieved using the semiconducting polymers fabricated using the synthesis procedure described in
[0184] Thus, one or more embodiments of the present invention have discovered that semiconducting polymers with fluoro functionality have unique combinations of properties. Specifically, the present disclosure has 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, threshold voltage closer to zero, high on/off ratio, and less variation in performance (e.g., mobility and threshold voltage) between devices, while the OFETs have mobility that is at least comparable to OFETs fabricated using PCDTPT or PBT. The discovery is unexpected and surprising because some fluorinated polymers (e.g., PDF) had reduced mobility. Furthermore, it was unexpectedly and surprisingly discovered that molecular structure of the fluorinated semiconducting polymers appeared to have no influence on the alkyl chain or - stacking distance of the semiconducting polymers.
[0185] The relative stability of the fluorinated polymers enables fabrication of devices that could not previously have been envisaged. For example, batch processing or flow processing can be used to fabricate the OFETs. Moreover, a plurality (e.g., at least 50) of OFETs having the above described desirable, unique, and discovered properties can be mass produced (e.g., each from the same batch or the same solution comprising the semiconducting polymers). An optoelectronic (e.g., display) or electronic device comprising a plurality of the OFETs can be fabricated. Other devices or systems requiring a plurality of stable OFETs (e.g., stable threshold voltage 0 V), such as sensor systems, can also be fabricated.
[0186] Device Embodiments
[0187]
ADVANTAGES AND IMPROVEMENTS
[0188] As we described above, 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 (5.00.2 eV) (34); not an ideal situation for long-term air stability (below 5.27 eV) (35). For structures containing the more electron withdrawing PT fragments, the basic nature of the PT heterocycle can de-stabilize relevant electronic properties when exposed to air or acidic surfaces (7, 36). 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 (37).
[0189] Indeed, one or more embodiments of the present invention achieve a hole mobility of a highly aligned semiconducting polymer, P2F or PCDTFBT (see
[0190] Since the FET is the most important circuit element in modern electronics, the high values of the mobility and stability reported here will enable plastic transistors to be processed from solution at low cost. Moreover, the results obtained in this disclosure are reproducible. The dimensions of the solution processed FETs can be reduced to sizes that are consistent with industrial product needs.
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CONCLUSION
[0243] 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.