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Organic Electronic Devices

Low-Voltage, High Performance Flexible Organic Field Effect Transistors Based on Ultrathin Single Crystal Microribbons Hongming Chen, Xing Xing, Miao Zhu, Jupeng Cao, Muhammad Umair Ali, Aiyuan Li, Yaowu He, and Hong Meng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b13871 • Publication Date (Web): 28 Aug 2019 Downloaded from pubs.acs.org on August 28, 2019

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Materials & Interfaces

Low-Voltage, High Performance Flexible Organic Field Effect Transistors Based on Ultrathin Single Crystal Microribbons Hongming Chena, Xing Xingb, Miao Zhuc, Jupeng Caoa, Muhammad Umair Alid, Aiyuan Lia, Yaowu Hea, Hong Menga*

aSchool

of Advanced Materials, Peking University Shenzhen Graduate School, Peking

University, Shenzhen 518055, P.R. China

bResearch

& Development Institute of Northwest Polytechnical University (Shenzhen),

Northwestern Polytechnical University, Shenzhen 518057, P. R. China

cCollege

of Physics Science and Technology, Lingnan Normal University, Zhanjiang

524048, P. R. China

1 ACS Paragon Plus Environment

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dDepartment

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of Materials Science and Engineering, College of Engineering, Peking

University,

Beijing, 100871, P.R. China

ABSTRACT: Organic field effect transistors (OFETs) have acquired increasing attention

owing of their wide range of potential applications in electronics, nevertheless, high

operating voltage and low carrier mobility are considered as major bottlenecks in their

commercialization. In this work, we demonstrate low-voltage, flexible OFETs based on

ultrathin

single

crystal

microribbons.

Flexible

OFETs

fabricated

with

2,7-

dioctylbenzothieno[3,2-b]benzothiophene (C8-BTBT) based solution processed ultrathin single crystal microribbon as the semiconductor layer and high-k polymer, polysiloxane-

poly(vinyl alcohol) composite (HPCPS), as an insulator layer manifest a significantly low operating voltage of -4 V and several devices showed a high mobility of > 30 cm2 V-1 s-1.

Besides, the carrier mobility of the fabricated devices exhibits a slight degradation in static

bending condition, which can be retained by 83.3% compared with its original value under

a bending radius of 9 mm. As compared to the bulk C8-BTBT single crystal based OFET


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which showed a large crack only after 50 times dynamic bending, our ultrathin single

crystal based counterpart demonstrates much better dynamic force stability. Moreover,

under a 20 mm bending radius, the mobility of the device decreased by only 11.7% even

after 500 bending cycles and no further decrease was observed until 1,000 times bending.

Our findings reveal that ultrathin C8-BTBT single crystal based flexible OFETs are

promising candidates for various high performance flexible electronic devices.

KEYWORDS: Single Crystal, Microribbons, OFETs, High-k, Polymer Insulator, Flexible

1. INTRODUCTION

Organic semiconductors, owing to their versatile chemical structures, high flexibility and

low-cost processing, are considered as promising materials for flexible electronics. In

recent years, flexible organic field effect transistors (OFETs) have attracted great

attention due to their extensive applications in various flexible electronic devices, such as flexible organic light-emitting diode (OLED) displays1, wearable electronics2-4 and printable radiofrequency identification tags (RFID) etc.5 However, the reported flexible OFETs usually suffer from high operating voltage (|Vd| >20 V)6-7 or low carrier mobility83 ACS Paragon Plus Environment


10,

Page 4 of 40

which limits their real applications. Therefore, it is necessary to realize flexible OFTEs

with low operating voltage and high carrier mobility for their potential applications in

flexible electronics.

The operating voltage is mainly related to the dielectric constant of the gate insulator and high dielectric constant (high-k) materials are preferred to achieve satisfactory operating

voltage in flexible OFETs. In previous reports, various approaches have been adopted to realize low-voltage flexible OFETs by employing high-k gate insulators. The operating

voltage of flexible OFETs is reported to be decreased to below 5 V via employing AlOx modified with ultrathin self-assembled monolayer (SAM) as the gate insulator.2,

9, 11-12

Incorporation of various high-k polymer or inorganic films, such as PVDF-TrFE7,

13,

BST,14-15 and sol-gel silica16 etc. as the gate insulators is also observed to yield flexible OFETs with < 5 V operating voltage. We recently reported a novel high-k polymer,

polysiloxane-poly(vinyl alcohol) composite (HPCPS), and demonstrated HPCPS/Si based OFETs with a low driving voltage (|Vd| 30 cm2 V-1 s-1 under a low operating voltage of -4 V only, which is the best performance

among low-voltage, flexible OFETs reported to date. Moreover, the carrier mobility of our

flexible SCFETs demonstrated a good stability in static and dynamic bending conditions.

Interestingly, no noticeable damage on the ultrathin single crystal microribbons was

observed even after 1,000 times of dynamic bending tests, while bulk C8-BTBT single

crystal (with a thickness of about 135 nm) showed a large crack after only 50 times of

dynamic bending. Our results demonstrate that OFETs with a rational combination of ultrathin single crystal microribbon as the active layer and HPCPS as the high-k gate

insulator could be promising candidates for applications in high performance flexible

electronic devices.

2. EXPERIMENTAL SECTION

C8-BTBT

single

crystal

microribbons

growth.

The

2,7-dioctylbenzothieno[3,2-

b]benzothiophene (C8-BTBT) powder was synthesized by a previously reported method24

and purified three times before use. The C8-BTBT based ultrathin single crystals were 7 ACS Paragon Plus Environment


Page 8 of 40

grown by previously reported ‘space-confirmed strategy’.23 C8-BTBT powder was

dissolved in chlorobenzene and dichloromethane mixed solvent in a concentration of 2

mg/mL (the solvent ratio of chlorobenzene and dichloromethane was set to be 4:1 in

volume). Then, 15 µL of the solution was dropped onto the surface of sodium dodecyl sulfate (SDS) aqueous solution (1×10-3 mg/mL). After the complete evaporation of the

solvent, C8-BTBT single crystal microribbons were obtained on the water surface.

Device fabrication. We fabricated flexible organic SCFETs in a bottom-gate, top-contact (BG-TC) configuration. The ITO/PET (230 nm/160 U 3 substrates were pre-treated by

oxygen plasma (80 W) for 120 seconds. Then, the HPCPS sol-gel was spin-coated onto

the substrates at 4000 rpm for 50 seconds with a typical thickness of ~130 nm. The synthesis method for HPCPS sol-gel has been reported elsewhere.17 In next step, the HPCPS film was thermal annealed at 80 oC for 30 min in air. After that, the C8-BTBT single crystal microribbon was transferred onto the HPCPS layer.22 Finally, gold

electrodes were evaporated onto the single crystal microribbon through a copper grid (45 nm) mask.25


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Characterization. The polarized optical microscopy (POM) was used to capture the POM images of single crystal microribbons and flexible SCFETs. X-ray diffraction (XRD)

patterns were obtained using a Bruker D8 with an X-ray wavelength of 1.541 Å 2

?HW

source). Transmission electron microscopy (TEM) and selected-area electron diffraction

(SAED) characterizations were conducted with a FEI Tecnai G2F30. Atomic force

microscopy (AFM) on SPA400HV instrument with SPI 3800 controller (Seiko Instruments)

was used to investigate the surface morphology of the microribbons and substrates.

All the electrical characterizations were performed by an Agilent B1500A semiconductor

parameter analyzer at room temperature (25 oC) in ambient atmosphere. The hole mobility (U) of flexible SCFETs at the saturation region is estimated from the transfer curve

based on the following equation (1):

I ds

WCi (Vg Vth ) 2 2L

(1)

where Ids, W, L, Vg and Vth refer to the drain-source current, channel width, channel length, gate voltage and threshold voltage, respectively. Ci represents the capacitance per unit



Page 10 of 40

area of the gate dielectric layer, which is obtained by capacitance-frequency

measurement with Agilent E4980A in MIM (metal-insulator-metal) structure. The value of

Ci measured at a frequency of 20 Hz is 88.6 nF/cm2, as shown in Figure S1 (a) (Supporting Information). The dielectric constant of HPCPS can be calculated using

equation (2), as given below:

ci d

k

(2)

0

where d is the thickness of the film and

0

is the vacuum permittivity. The dielectric

constant for HPCPS is found to be ~13.0, which is much higher than our previous report where the Ci was measured at 10 kHz.17 It is to be noticed that the Ci value at a frequency of 20 Hz is widely used to calculate the carrier mobility in several previous reports [26-28].

Also, 20 Hz is the lowest possible frequency of our capacitance measurement equipment. Therefore, we calculated the carrier mobility based on the Ci value of 88.6 nF/cm2. We further performed a theoretical calculation for capacitance vs frequency curve in the

frequency range of 0.1 - 20 Hz (Figure S1(c)-(d)) and predicted the carrier mobility at a

frequency of 0.1 Hz. The detailed procedure adopted herein for the projection of carrier


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mobility can be found in Supporting Information. The current on/off ratio is defined as the

Ids, max/ Ids, min.

3. RESULTS AND DISCUSSION

Characterization of C8-BTBT single crystal microribbons. After complete evaporation of the solvent, the C8-BTBT microribbon was transferred on a pre-treated n-doped SiO2 (300 nm)/Si wafer. Figure 1(a) shows C8-BTBT microribbon film image obtained by a

bright-field optical microscope which revealed no obvious cracks or grain boundaries,

indicating the excellent quality of our single crystal. The length of the prepared

microribbons was more than one millimeter (Figure S2). The color of the sample changed

from brown to complete dark when it was rotated from 0 degree to 45 degrees under a

cross polarized optical microscope (Figure 1(b)-(c)). This birefringence phenomenon in

C8-BTBT microribbon film under the cross polarized mode verifies that the prepared

microribbon films possess single crystalline nature. As demonstrated in Figure 1 (d), a

typical TEM image revealed uniform morphology of the fabricated film while SAED pattern

(Figure 1 (e)) also confirmed the single crystalline nature of C8-BTBT microribbon film.



Page 12 of 40

Besides, the morphology of the single crystal microribbon is also investigated using AFM

(Figure S3 (a)), which showed the surface roughness of an ultrathin C8-BTBT single

crystal to be 0.255 nm in the frame selected area, indicating that the single crystal formed

a very flat surface on SiO2 /Si wafer.

As compared to the XRD pattern of C8-BTBT powder in Figure 1(f), the XRD pattern of

the C8-BTBT single crystal microribbons only exhibited (001), (002) and (003) peaks, indicating that the single crystal microribbons are arranged along the c-axis.23, 29-30 The

thickness of C8-BTBT microribbon is 21.69 nm, i.e. about 7-8 molecular layers (Figure

S3 (c)). The thickness difference of 2.96 nm in the crystal (red axis in Figure S3 (a)) fits-

well with the thickness of a single molecular layer of C8-BTBT (2.918 nm), which confirms

the layer by layer crystal growth.


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(a)

(b)

(c)

150 m

150 m (d)

(f)

(e) b

150 m (001) S

(020)

C8H17 C8H17 S

a (200) (002)(003)

single crystal powder

1 m

5 1/nm

Figure 1. (a)-(c) Bright-field and cross POM images of C8-BTBT single crystal microribbon; (d)-(e)

TEM image and the corresponding SAED pattern of C8-BTBT microribbon; (f) Powder and single

crystal XRD patterns of C8-BTBT on SiO2/Si substrate, the inset shows the chemical structure of C8BTBT.

Electrical properties of C8-BTBT SCFETs. To evaluate the performance of C8-BTBT based SCFETs, C8-BTBT single crystal microribbon was transferred onto the

HPCPS/ITO/PET substrate followed by thermal evaporation of Au electrodes. Figure 2(a)

shows the device structure (in BG-TC configuration). The optical microscope image of the



Page 14 of 40

device with a single crystal microribbon is shown in Figure 2(b). Furthermore, all the

devices fabricated in this work contain only one single crystal microribbon aligned along

its long axis. The operating voltage, threshold voltage and on/off ratio for this device were measured to be -4 V, -1.1 V and 9.18×104, respectively as presented in Figure 2 (c). This fabricated SCFETs exhibited a high saturated mobility of 33.4 cm2 V-1 s-1 with a leakage current of < 10 nA. Furthermore, the trap density (Ntrap) of the device is 2.48×1012 cm-2 (calculated by Equation S2, Supporting Information), which is much smaller than a C8-

BTBT SCFET device based on an OTS modified SiO2/Si substrate (Ntrap =1.21×1014 cm-2, Figure S4(d)). It indicates low trap states at the interface between C8-BTBT single crystal

and HPCPS dielectric, which means HPCPS dielectric film can effectively suppress the

interfacial traps as compared to the OTS modified SiO2/Si substrate. It is well-proved by previous reports that high quality single crystals can obtain a very high carrier mobility

because of their single crystalline nature and low trapping effect between the singlecrystal surface and the dielectric layer.31-32 Therefore, such a high mobility in our SCFETs

could be attributed to the low charge traps in our high quality C8-BTBT single crystals


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and further reduction in the interfacial traps by the polar chloropropyl chain in HPCPS dielectric layer. 17, 32

To critically evaluate the extracted mobility in our devices, the reliability factor “r” and the effective mobility “Ueff” of SCFETs were calculated by the equation (3) and (4) 26:

| I ds |max | I ds |0 2 WCi r ( ) /( ) | Vg |max 2L

eff

r

(3)

(4)

where |Ids|max, |Ids|0, Vg, W, L, Ci, and U refer to the experimental maximum source-drain current at the maximum gate voltage |Vg|max, source-drain current at Vg = 0, gate voltage, channel width, channel length, capacitance per unit area of the gate dielectric layer and

the calculated carrier mobility based on equation (1), respectively. By substituting the equation (1) into equation (3), the reliability factor, r could be expressed as:

| I ds |max | I ds |0 2 r ( ) (Vg Vth ) 2 / I ds max | Vg | (5)



Page 16 of 40

The reliability factor, r of our SCFET was measured to be 39.6%, which indicates the effective mobility, Ueff of 13.3 cm2 V-1 s-1. According to the equation (5), we found that the reliability factor is strongly related with the threshold voltage Vth: the lower the threshold voltage in an OFET device, the larger would be the reliability factor. Therefore, the high effective mobility in our devices is mainly due to the low threshold voltage Vth and high output current Ids, indicating the high intrinsic carrier transport in C8-BTBT single crystal microribbons.

A nonlinear behavior at low Vd is observed (Figure 2(d)), which is usually considered as an effect of the contact resistance between the electrodes and semiconductors.33-34 The

contact resistance effect can also be inferred from transfer curve (Figure 2(c)), i.e. the transfer curve demonstrates downward curvature at the highest Vg, indicating that the Ids may become more contact-limited as the interface of accumulation layer becomes more conducting at high Vg.

35-36

The mobility and threshold voltage against counts of 30

devices (fabricated in this work) can be seen in Figure 2(e) and (f), respectively, which


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Page 18 of 40

Figure 3(a), the AFM image of the channel in SCFET shows a roughness difference

between the single crystal and HPCPS surfaces. The roughness of HPCPS film surface

is 1.71 nm (Figure 3(a)), while that of single crystal surface is 1.48 nm (Figure 3(b)). Considering the surface of the microribbon to be very flat (Figure S3 (a), Ra=0.255 nm) on SiO2 substrate, the flexible substrate should not change the molecular packing and surface morphology during the transfer process as it would result in roughness mismatch

between the flat C8-BTBT single crystal and relatively rough flexible substrate, as

illustrated by the schematic diagram in Figure 3(d). Based on the calculation in Support

information, it is evident that the roughness mismatch between the two materials has little

influence on the capacitance. The trap density of C8-BTBT SCFET in Figure 2(c) is 2.48×1012 cm-2, while it is smaller for the device fabricated on HPCPS/Si substrate17. In fact, there are fewer defects in C8-BTBT single crystal due to high quality single crystalline nature and no grainboundaries effect as compared to the bulk C8-BTBT based OFET. Therefore, the roughness mismatch between the two materials introduces more interfacial traps in the device, which hinder efficient carrier transport thereby deteriorating the device performance.[37-38] The

roughness mismatch between the microribbon and insulator surface appears randomly,

i.e. some flexible organic SCFETs could have higher mismatch density while the others


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may have relatively less. For the devices with smaller mismatch, the charge carriers can

transport efficiently with a mobility much closer to the intrinsic mobility of the single crystal.

While, for the devices with higher mismatch between the semiconductor and insulator

interface, the effect of mismatch defects will decrease the carrier mobility. Therefore, the carrier mobility for our 30 devices ranges from 9 cm2V-1s-1 to 33.4 cm2V-1s-1. This suggests

that the distribution of carrier mobility can be further reduced by decreasing the roughness mismatch. The highest mobility among recent reports on low operating voltage (|Vd| < 10 V) flexible OFETs based on various semiconductors and insulators is 6.3 cm2V-1s-1 (with a carrier mobility of >2 cm2V-1s-1) as evident in Table 1,16 whereas, we attained about 5.5 times higher mobility (33.4 cm2 V-1 s-1) as compared to the best reported low-voltage

flexible OFET devices so far.




Page 20 of 40

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Semiconducto

Dielectric

Umax

r

(nF/cm2)

(cm2V-1s-1)

Vd

YearRef

200739 200840 200941 200942 200916 200943 201144 201212 20132 201345 201828

This

Pentacene

Pentacene

Pentacene

Pentacene

Pentacene

C60

Polyvinyl alcohol(400) Sol-gel silica (260) BaTiO2 nanoparticles (106) BaSrTiO2 nanoparticles (216) Sol-gel silica (160) AlOx/polystyrene (300)

Substrate

(V)

2.6

-5

2×103

PET

3.5

-3

105

PET

3.5

-3

2 ×104

PET

3.4

-5

103

PET

6.3

-3

2 ×105

PET

2.2 (8e)

9.5

105

PEN

DNTT

AlOx/SAM (100)

2.1

-1.5

108

PEN

C10-DNTT

AlOx/SAM (68)

4.3

-2

108

PEN

DNTT

SAM+Al2O3 (280)

3

-5

107

PEN

PDVT-8

AlOx/SAM (-)

2.41

-1

106

PI

5.6

-3

33.4

-4

Pentacene

PAA+PI copolymer (20)

1.4 × 106

PET

C8-BTBT HPCPS (88.6)

Work

Ion/Ioff

105

PET

single crystal

Flexibility of C8-BTBT SCFETs. A C8-BTBT based SCFET with tensile strain loading and the bending test diagram can be seen in Figure 4(a). The device was attached on the



Page 22 of 40

surface of a glass bottle in ambient atmosphere for 5 minutes, and then its electrical

properties in the bending state were tested. The carrier mobility for this device before bending was measured to be 18.9 cm2 V-1 s-1 (Figure S5). This device was tested at

bending radii (R) of 20 mm, 15 mm and 9 mm with tensile strains of 0.40%, 0.53% and

0.88%, respectively. Figure 4(b) shows the transfer curves for the device during bending

test, which indicate that the carrier mobility of the device decreased as a function of an

increase in tensile strain. A noticeable phenomenon in these transfer curves is the downward curvature at the highest Vg caused by the contact resistance effect, as discussed above. With a bending radius of 20 mm and 15 mm, the carrier mobility of the

device decreased very slightly, (1.1% and 3.8%, respectively). The mobility of the device

can still be retained by 83.3% compared with its original value under a bending radius of

9 mm. The variation in saturation mobility and the threshold voltage of the organic SCFET

at different tensile strains is shown in Figure 4(d). AFM image of the device channel

(Figure 4(c)) revealed no noticeable damage on the microribbon after bending tests. The

bending test induces an interfacial strain on the single crystal and the dielectric layer,

which may change the intermolecular spacing inside the single crystal leading to the 22 ACS Paragon Plus Environment

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degradation of the carrier mobility.46-47 It is worth noticing that the negative shifts in

threshold voltage of the device after 0.88% tensile strain loading were reduced by 13.3%

(-1.3 V to -1.5 V) compared with the initial state (Figure 4(d)). It may have caused by the

cracking or buckling of the gold electrodes with increased resistance of the electrodes, resulting in the need of a higher bias for transistor operation.48

(b)

(a)

Device L R Bottle

(d) (e)

(c)

Crystal

Substrate Substrate 4 m

Figure 4. (a) A C8-BTBT based flexible SCFET attached on the surface of a glass bottle and the

bending test schematic diagram. The radii of the bottles were 20 mm, 15 mm and 9 mm. Bending test



Page 24 of 40

with bottles of different radii for 5 min: (b) Transfer curves for an organic SCFET and (c) the AFM image after bending test. (d) The change in the relative mobility and the threshold voltage of the device under different tensile strains.

Furthermore, a flexible organic SCFET was tested after various bending cycles with a

radius of 20 mm, as shown in Fig. 5(a)-(b). The carrier mobility for this device before bending was measured to be 18.3 cm2 V-1 s-1 (Figure S6). The device was bent every one

second for one time and the electrical performance was measured on a flat plate after bending, the schematic diagram for this bending test can be seen in a previous report13.

The saturated mobility of the device bent on the bottle surface can be retained by 88.3%

compared with its original value after 500 bending cycles, which was maintained until

1,000 bending cycles, as evident in Figure 5(b). For this device, the threshold voltage

also showed a negative shift from -1.2 V to -1.4 V. The AFM image of the device channel

shows no noticeable crack after bending test (Figure 5(c)). However, a bulk single crystal

bent under the same radius shows a large crack, as can be observed in Figure 5(d) and

5(e). The thickness of this bulk C8-BTBT single crystal microribbon was about 135 nm

(Figure 5(f)), which indicates that the bulk C8-BTBT single crystal cannot stand dynamic


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bending test even for 50 times. These results suggest that ultrathin C8-BTBT single

crystal is a more suitable option for flexible OFETs as compared to the bulk counterpart.

(a)

(b)

(c) 21.2 nm

2 m

(d)

(e)

(f) 135 nm

25 m

25 m

2 m

Figure 5. (a) Transfer curves, (b) the relative mobility and the threshold voltage with different bending times

and (c) the AFM image of a flexible SCFET bent on a 20 mm radius bottle for different bending times.

(d) The POM image in original state; (e) the POM image after 50 times bending and (f) the

corresponding AFM image for the frame selected area in (e) of a bulk C8-BTBT single crystal bent on

a 20 mm bottle with different bending times.

4. CONCLUSION 25 ACS Paragon Plus Environment


Page 26 of 40

In conclusion, we fabricated low-voltage, ultrathin single crystal microribbons based

flexible OFETs with ultra-high carrier mobility and decent flexible stability. The solution

processed C8-BTBT single crystal microribbons used in our OFETs are as thin as ~21 nm. Our flexible devices exhibit an average saturated mobility of 18.5 cm2V-1s-1 and several devices showed a high mobility of > 30 cm2 V-1 s-1 under a low operating voltage

of -4 V only, which is the best performance for low-voltage, flexible OFETs reported to

date. We ascribe this high mobility observed in our flexible OFETs to the low charge traps

in C8-BTBT single crystal, and possibility of further reduction in the interfacial traps by the

polar chloropropyl chain in novel HPCPS dielectric layer. Besides, the carrier mobility of

the fabricated devices exhibited a small degradation without noticeable damage on the

single crystal microribbon after static bending test. With the bending radius of 20 mm and

15 mm, the carrier mobility of our devices showed only a slight decrease (1.1% and 3.8%,

respectively). Furthermore, it can still be retained by 83.3% compared with its original

value under a bending radius of 9 mm. As compared to the bulk C8-BTBT single crystal

based OFET which exhibited a large crack after 50 times dynamic bending test, our

ultrathin single crystal based counterpart demonstrates a much better dynamic force 26 ACS Paragon Plus Environment

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stability. Under a 20 mm bending radius, the mobility of the device decreased by only

11.7% after 500 bending times and without further decrease until 1,000 times bending

cycles. Our results reveal that OFETs with a rational combination of ultrathin single crystal microribbon as the active layer and HPCPS as the high-k gate insulator could be potential

candidates for applications in high performance flexible electronic devices.

ASSOCIATED CONTENT

Supporting Information.

The Supporting Information is available free of charge on the ACS Publications website

(http://pubs.acs.org).

Capacitance vs. voltage plots; optical image of C8-BTBT single crystal microribbon; POM

image of a SCFET and corresponding transfer curve with a reverse sweep; AFM image

of C8-BTBT single crystal on SiO2 substrate.

AUTHOR INFORMATION



Page 28 of 40

Corresponding Author

*E-mail: [email protected] (Hong Meng)

ORCID

Muhammad Umair Ali: 0000-0003-4115-0702

Yaowu He: 0000-0003-2887-735X

Hong Meng: 0000-0001-5877-359X

Notes

All authors have given approval to the final version of the manuscript and declare no

competing financial interest. ACKNOWLEDGMENTS

This work was financially supported by the China (Shenzhen)-Japan Technology

Collaboration Project (GJHZ20170313145614463). H. M. thanks the support from the

Guangdong Key Research Project (No. 2019B010924003), Shenzhen Engineering

Laboratory SFG (2016)1592. H.C. is supported by the Shenzhen Science and


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Technology Research Grant (JCYJ20180302153514509). M.Z. is supported by the

Shenzhen Science and Technology Research Grant (JCYJ20170412150946440). A.L.

and Y.H. acknowledge funding from the Shenzhen Science and Technology Research

Grant (JCYJ20160510144254604 and JCYJ20170412151139619).

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