Flexible Organic Thin‐Film Transistors with Liquid‐Crystalline Organic Semiconductor, Ph‐BTBT‐10

Flexible organic thin‐film transistors with liquid‐crystalline organic semiconductor, 2‐decyl‐7‐phenyl‐benzothienobenzothiophene (Ph‐BTBT‐10), which is suitable for flexible transistor material because the polycrystalline thin films can be easily fabricated at about 100 °C or lower by solution process, are fabricated. 25 μm‐thick polyethylene terephthalate (PET) film and photosensitive resin poly(vinyl cinnamate) (PVCi) are used as flexible substrate and gate insulator, respectively. The PVCi gate insulator layer is crosslinked by UV light and the crosslinking reaction increases solvent resistance of PVCi gate insulator. Uniform Ph‐BTBT‐10 polycrystalline thin films are fabricated on the PVCi gate insulator and the crystallinity and electrical characteristics of Ph‐BTBT‐10 are not damaged by PVCi gate insulator. The entire process is performed below 130 °C considering the heat resistance temperature of the PET film. The fabricated transistors show the mobility of 0.24±0.084 cm2 V−1s−1$0.24 \pm 0.084 \left(\text{cm}\right)^{2} \left(\text{V}\right)^{- 1} \left(\text{s}\right)^{- 1}$ and threshold voltage of 0.57 ± 0.38 V.


Introduction
Organic thin-film transistors (OTFTs) have many attractive features including light weight, low-temperature processing, and mechanical flexibility.In addition, it has an advantage of being easier to fabricate than inorganic thin-film semiconductors which require high temperature over 300 °C and vacuum process.[6] The flexible transistors make it possible for many kinds of applications to operate on curved surfaces and also can realize devices that harmonize with the living body.These applications cannot be possible with rigid semiconductors and substrates such as silicon and glass substrates, respectively.As flexible substrates, polyimide (PI), polyethylene naphthalate (PEN), and polyethylene terephthalate (PET) are commonly used.Each substrate material has its advantages and disadvantages.In particular, the heat resistance temperature determines the maximum process temperature, which may limit the process for fabricating gate insulators and organic semiconductor layers.The high heat resistance temperature(about 500 °C) of PI allows films on PI substrate to anneal at high temperatures and even low-temperature polysilicon thin films can be fabricated on PI. [7,8] However, its low transmittance for visible light is not suitable for imaging applications such as displays and image sensors.PEN also has higher heat resistance than PET and can be heated to about 200 °C. [9,10]However, PEN is more expensive than PET.PET has high transmittance and low cost, but its heat resistance temperature is lower than other two materials.Thus, the fabrication temperature is limited to %100 °C or lower on PET substrate. [11,12]n the fabrication of the thin-film semiconductor layer, the low-temperature fabrication process is required in order to realize flexible transistors on PET substrate.Conjugated polymer materials having high mobility require high thermal anneal temperature. [13,14]The polycrystalline thin films of liquid-crystalline organic semiconductors are fabricated by a conventional and easy process such as spin coating and dip coating at the temperature of liquid-crystal phase. [15,16]In fact, 2-decyl-7-phenylbenzothienobenzothiophene (Ph-BTBT-10) (Figure 1a) can easily fabricate high-mobility transistors at approximately 100 °C or lower.It exhibits a highly ordered liquid-crystal phase of smectic E (SmE) and it provides uniform and molecularly flat polycrystalline thin films reproducibly when liquid-crystalline thin films as the precursor of flat and uniform thin films are crystallized. [17]pin coating a Ph-BTBT-10 solution at the temperature of SmE phase forms highly orderd polycrystaline thin films.The crystallization temperature of Ph-BTBT-10 during cooling is below 100 °C.Therefore, it can be fabricated on flexible substrates having low heat resistance temperature such as PET.
Additionally, the selection of an appropriate gate dielectric material is necessary to fabricate flexible transistors.To cope with both fine insulation properties and favorable Flexible organic thin-film transistors with liquid-crystalline organic semiconductor, 2-decyl-7-phenyl-benzothienobenzothiophene (Ph-BTBT-10), which is suitable for flexible transistor material because the polycrystalline thin films can be easily fabricated at about 100 °C or lower by solution process, are fabricated.25 μm-thick polyethylene terephthalate (PET) film and photosensitive resin poly(vinyl cinnamate) (PVCi) are used as flexible substrate and gate insulator, respectively.The PVCi gate insulator layer is crosslinked by UV light and the crosslinking reaction increases solvent resistance of PVCi gate insulator.Uniform Ph-BTBT-10 polycrystalline thin films are fabricated on the PVCi gate insulator and the crystallinity and electrical characteristics of Ph-BTBT-10 are not damaged by PVCi gate insulator.The entire process is performed below 130 °C considering the heat resistance temperature of the PET film.The fabricated transistors show the mobility of 0.24 AE 0.084 cm 2 V À1 s À1 and threshold voltage of 0.57 AE 0.38 V.
[20] High-k inorganic oxide materials are employed to reduce gate leakage current and operating voltage.The advantages of polymer dielectric materials are high mechanical flexibility, low surface polarity, and also, they can be easily formed by solution process.[23] PS can be easily dissolved in the p-xylene solution with Ph-BTBT-10.So, if organic semiconductors are also formed by the solution process, additional treatment is required for polymer gate insulator because high solvent resistance is required in bottom gate OTFTs.However, since CL-PVP and PDMS require thermal annealing of 150 °C, they are not suitable for PET substrate.Photocrosslinking is more suitable than thermal crosslinking for PET substrates which have low heat resistance temperatures.Poly(vinyl cinnamate) (PVCi) is crosslinked by UV light and improving solvent resistance. [24,25]Thus, it enables us to fabricate organic semiconductor layers on PVCi gate insulator by solution process below the heat resistance temperature of PET substrate.As shown in Figure 1b, UV light cleaves the double bond of PVCi and forms a cyclobutane ring.
In this study, we demonstrate the utilities of PVCi as a gate dielectric to ralaize flexible OTFTs with Ph-BTBT-10.We describe the crosslinking reaction of PVCi, interface of PVCi and Ph-BTBT-10, crystallinity of Ph-BTBT-10 on PVCi, and fabrication of flexible OTFTs.

Experimental Section
We used 25 μm-thick PET films (Lumirror T-60 TORAY Industries Inc) as flexible substrate.The PET film is recommended to treat at 130 °C or less to use for long time.Thus, we conducted the entire fabrication process below 130 °C or lower.The PET film is resistant to organic solvents such as acetone and xylene, which were used as solvents for cleaning and spin-coating processes, respectively.We fabricated bottom gate-bottom contact OTFTs as shown in Figure 2a.At first, PET film was temporary attached to supporting glass.Dimethylpolysiloxane (PDMS) (Shin-Etsu SIM-260) was spin coated on glass substrate and cured at 150 °C for 30 min.Then, PET substrate was placed on PDMS.Also, 500 nm poly(methyl-methacrylate) (PMMA) buffer layer was deposited by spin coating a PMMA solution with p-xylene (10 wt%) as solvent to improve the surface morphology of PET substrate.50 nm aluminum gate electrode was thermal evaporated and 20 nm alumina was deposited by radio frequency (RF) sputtering.PVCi (Sigma-Aldrich) and 4-methoxychalcone (Tokyo Chemical Industry Co., Ltd. Figure 2b) were dissolved in cyclopentanone solvent at a weight ratio of 10:1 and passed through a 0.45 μm PTFE membrane filter.4-methoxychalcone acts as a photosensitizer for UV light of 365 nm.Then, the PVCi solution was spin coated on substrate on alumina layer and the films were baked at 80 °C for 10 min to remove cyclopentanone solvent.Crosslinking reaction was produced by UV light of 365 nm wavelength 7mW cm À2 for 3 min and DUV of 185 and 254 nm wavelength 8mW cm À2 for 2 min.Au source and drain electrodes were  thermal evaporated.The channel length and width were 100 and 500 μm, respectively.Self-assembled monolayer, pentafluorobenzenethiol (PFBT: Figure 2c) was deposited on Au electrodes by vapor treatment at 105 °C for 15 min and the Au electrodes were cleaned by ethanol.PFBT monolyaer was formed on the Au electrodes and the contact between metal and organic semiconductor was improved. [26]We used 0.5 wt% p-xylene solution of Ph-BTBT-10 and heated at 130 °C to exhibit SmE phase in cooling process.Substrates were also heated at 130 °C for 1 min before spin coating Ph-BTBT-10 solution.After spin-coating, Ph-BTBT-10 thin film was thermally annealed at 120 °C for 5 min.After thermal annealing, changed its crystal structure from monolayer crystal structure to bilayer crystal structure which phenyl-BTBT groups facing each other, leading to improve the mobility.Finally, Ph-BTBT-10 was patterned by photolithography and oxygen plasma ashing.CYTOP CTL-107MK (AGC Inc.) was spin coated on Ph-BTBT-10 layer to protect from damage by the solvent of photoresist.2 nm aluminum was thermal evaporated to improve wettability of CYTOP layer.Then, photoresist OFPR-800 (Tokyo Ohka Kogyo Co., Ltd.) was spin coated and baked at 90 °C for 90 s.After exposure and development for photo resist, all the areas except channel area were etched by oxygen plasma ashing.

UV Spectra of PVCi
We investigated the effect of 4-methoxychalcone on UV crosslinking of PVCi. Figure 3 shows UV absorption coefficient spectra of 500 nm-thick PVCi with/without 4-methoxychalcone on quartz glass substrate.Absorbance coefficient peak at wavelength from 270 to 280 nm decreases after UV exposure.The decline indicates cleavage of the double bond of PVCi and crosslinking reaction was produced.Without 4-methoxychalcone, the peak was not changed after UV light of 365 nm wavelength exposure because PVCi doesn't absorb 365 nm UV light and crosslinking reaction hardly occurred.On the other hand, with 4-methoxychalcone, the peak at wavelength of 270-280 nm decreased after UV light of 365 nm wavelength exposure.4-methoxychalcone has an absorption near the wavelength of 365 nm and it acts as a photosensitizer to promote crosslinking of PVCi at this wavelength.After exposure, there is almost no absorption at this wavelength, and 4-methoxychalcone is assumed to be decomposed and incoporated into the polymer, and PVCi is not further crosslinked by UV light of 365 nm.However the degree of crosslinking is not enough.The desired solvent resistance was not achieved because the PVCi film thickness became smaller after dipping into the cyclopentanone solvent.Additionally, due to the inherently absorptive wavelengths of PVCi, DUV of 185 nm and 254 nm wavelength exposure encouraged more crosslinking reactions.After additional crosslinking reaction with DUV, the PVCi film thicknesses were not changed after dipping into the cyclopentanone solvent.However, since the absorbance of PVCi in DUV was very strong, DUV light did not penetrate in whole film and the entire film may not be crosslinked, especially for the thick film.The penetration depth at which the light intensity bacomes 1/e was calculated to be 69 nm for PVCi without 4-methoxychalcone before crosslinking at the wavelength of 254 nm.Therefore, it is effective to crosslink the entire PVCi film with UV light of 365 nm wavelength and further crosslink the PVCi surface with DUV light of 185 and 254 nm wavelength at the interface with the organic semiconductor with DUV.

Contact Angle of Water-PVCi
DUV exposure produces ozone which is regarded to modify the surface of PVCi and affect electrical characteristics of OTFTs.DUV exposure to atmosphere produces ozone from oxygen and hydrogen peroxide from moisture. [27]Ozone decomposes double bonds and produce ozonide. [28]The reaction of ozonide and hydrogen peroxide creates a OH group containing product. Figure 4 shows the contact angle of water on the PVCi surface.Contact angle was not changed after exposure of UV light of 365 nm wavelength, but it became smaller after exposing DUV light of 185 and 254 nm wavelength under air.The hydrophilic nature of the PVCi surface suggests the formation of OH groups.On the other hand, contact angle is not changed if DUV is exposed under nitrogen atmosphere.It can effectively suppress ozone formation by DUV.wavelength under nitrogen atmosphere.It has a peak at 2θ ¼ 1.71°, corresponding to two molecular lengths of Ph-BTBT-10, 52 Å.It indicates bilayer crystal structure of Ph-BTBT-10. Figure 5b shows the in-plane XRD of Ph-BTBT-10 polycrystalline thin films fabricated on PVCi layer crosslinked UV and DUV lights.The peaks corresponding to (110), (020), and (120) are observed.The lattice constants were calculated to be a ¼ 6.1Å and b ¼ 7.8Å.These values are approximately consistent with those previous reported. [29]These results indicated that the PVCi layer did not dissolve in the spin coating of Ph-BTBT-10 and maintained flatness because of the same crystal structure, as previously observed on rigid substrates.Furthermore, the out-of-plane and in-plane XRD peaks of Ph-BTBT-10 polycrystalline thin films on PVCi layer-fabricated exposure of DUV under air and nitrogen atmosphere are hardly changed, which indicate that the crystallinity of Ph-BTBT-10 is also hardly changed.In other words, ozone modification of the PVCi gate insulator surface and differences in surface energy did not affect the crystallinity of polycrystalline thin films of Ph-BTBT-10.

Electrical Characteristics of TFTs
The mobility, threshold voltage, and gate leakage current were evaluated from the transfer characteristics in saturation region.Figure 6 shows typical transfer characteristics of OTFT with PVCi gate insulator DUV crosslinked under air and OTFT with PVCi gate insulator DUV crosslinked under nitrogen atmosphere.The mobility and threshold voltage in the OTFT devices with PVCi gate insulator exposed by DUV under nitrogen atmosphere were 0.24 AE 0.084 cm 2 V À1 s À1 and 0.57 AE 0.38 V, respectively, in ten samples.On the other hand, the mobility and threshold voltage in OTFT devices with PVCi gate insulator exposed by DUV under air devices were 0.10 AE 0.086 cm 2 V À1 s À1 and 3.7 AE 0.83 V, respectively, in ten samples.Measuerd mobility was smaller than previously reported Ph-BTBT-10 OTFTs.This seems to be due to the fact that the active layer was damaged by oxygen plasma ashing when patterning Ph-BTBT-10 layer, and the mobility was generally above 1 cm 2 V À1 s À1 before oxygen plasma ashing.The threshold voltage was positively shifted in the  OTFT devices treated by DUV under air.This is thought to be due to the modification of the PVCi surface due to the difference in crosslinking conditions.As described in the result of contact angle of PVCi layer, it is suggested that there are OH groups on the surface of PVCi layer if it is crosslinked by DUV under air.Interfacial charges such as OH groups are known to the cause of threshold voltage shift. [30,31]Because OH groups have large dipole moments, the surface of gate insulator induces free holes in the organic semiconductor layer like chemical doping.It is thought to be the cause of the positive shift in threshold voltage.The exposure of DUV under nitrogen atmosphere is effective for crosslinking PVCi layer, and suppressing surface modification of OH groups by ozone can prevent threshold voltage positive shifts.

Conclusion
We fabricated flexible OTFTs with liquid-crystalline organic semiconductor, Ph-BTBT-10.We used PET film as flexible substrate and PVCi as gate insulator.The entire fabrication process was performed below 130 °C.The crosslinking of PVCi gate insulator was enhanced by the photosensitizer of 4-methoxychalcone.Highly crystalline thin films of Ph-BTBT-10 could be formed by spin coating on PVCi gate insulator having high solvent resistance after crosslinking by UV and DUV lights.The fabricated transistors showed mobility of 0.24 AE 0.084 cm 2 V À1 s À1 and threshold voltage of 0.57 AE 0.38 V. Crosslinking PVCi under a nitrogen atmosphere prevented modification of the PVCi surface and suppressed the threshold voltage shift.From the result of water contact angle on PVCi layer, it was considered that OH groups are formed on the PVCi surface by ozone generation by DUV exposure under atmospheric conditions.These results suggest that liquid-crystalline organic semiconductor Ph-BTBT-10 is a suitable material to realize flexible OTFTs.PVCi is also suitable gate dielectric material to fabricate transistors below about 100 °C.

Figure
Figure 5a shows the out-of-plane XRD of Ph-BTBT-10 polycrystalline thin films fabricated on PVCi layer crosslinked by UV light of 365 nm wavelength and DUV light of 185 and 254 nm

Figure 3 .
Figure 3. Absorption coefficient spectra of 500 nm-thick PVCi layer before and after UV and DUV exposure a) with and b) without 4-methoxychalcone.

Figure 4 .
Figure 4. Contact angles of water on PVCi gate insulator.a) Without crosslinking.b) After crosslinking by UV light exposure.After crosslinking by DUV light under c) air and d) nitrogen atmosphere in addition to crosslinking by UV light.

Figure 5 .
Figure 5. XRD of Ph-BTBT-10 polycrystalline fabricated on PVCi crosslinked by UV light and DUV light under air and nitrogen atmosphere.a) Out-of-plane and b) In-plane XRDs.

Figure 6 .
Figure 6.Drain current I d (solid and dot lines) and gate current I g (dash line) of fabricated Ph-BTBT-10 transistor on PET film in transfer characteristics in saturation region (V ds ¼ À5V).