Bar-Coated Ultrathin Semiconductors from Polymer Blend for One

Subscriber access provided by Kaohsiung Medical University

Organic Electronic Devices

Bar-Coated Ultrathin Semiconductor from Polymer Blend for One-Step Organic Field-Effect Transistors Feng Ge, Zhen Liu, Seon Baek Lee, Xiaohong Wang, Guobing Zhang, Hongbo Lu, Kilwon Cho, and Longzhen Qiu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07118 • Publication Date (Web): 06 Jun 2018 Downloaded from http://pubs.acs.org on June 6, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

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.

Page 1 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Bar-Coated Ultrathin Semiconductor from Polymer Blend for One-Step Organic Field-Effect Transistors Feng Ge†, Zhen Liu†, Seon Baek Lee§, Xiaohong Wang†, Guobing Zhang†, ‡, Hongbo Lu†, ‡, Kilwon Cho§, *, Longzhen Qiu†, ‡, * †

National Engineering Lab of Special Display Technology, State Key Lab of Advanced

Display Technology, Academy of Opto-Electronic Technology, Hefei University of Technology, Hefei 230009, China ‡

Key Laboratory of Advanced Functional Materials and Devices, Anhui Province School of

Chemistry and Chemical Engineering, Hefei University of Technology, Hefei 230009, China §

Department of Chemical Engineering, Pohang University of Science and Technology,

Pohang 790-784, South Korea Corresponding Authors E-mail: [email protected] and [email protected] ABSTRACT One-step deposition of bi-functional semiconductor-dielectric layers for organic field effect transistors (OFETs) is an effective way to simplify the device fabrication. However, the proposed method has rarely been reported in large-area flexible organic electronics. Herein, we demonstrate wafer-scale OFETs by bar-coating the semiconducting and insulating polymer blend solution in one-step. The semiconducting polymer poly(3-hexylthiophene) (P3HT) segregates on top of the blend film, whereas dielectric polymethyl methacrylate (PMMA) acts as the bottom layer, achieving by a vertical phase separation structure. The morphology of blend film can be controlled by varying the concentration of P3HT and PMMA solutions. The wafer-scale one-step OFETs, with a continuous ultrathin P3HT film of 2.7 nm, exhibite high electrical reproducibility and uniformity. The one-step OFETs extend to substrate-free arrays that can be attached everywhere on varying substrates. In addition, due to the well-ordered molecular arrangement, the moderate charge transport pathway is formed, which resulted in stable OFETs under various organic solvent vapors and lights of different wavelengths. The results demonstrate that the one-step OFETs have promising potential in the field of large-area organic wearable electronics. KEYWORDS: bar-coating; one-step organic field-effect transistors; ultrathin film; wafer-scale; substrate-free; stability 1. INTRODUCTION Organic field effect transistors (OFETs), based on solution-processed organic semiconductors have been rapidly developed for a wide range of applications, such as wearable devices, chemical biosensors, radio frequency identification (RFID) tags and ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 19

flexible electronic paper in the recent years.1-6 Due to the recent improvements in polymeric semiconductors with exceptional field-effect mobilities ( > 10 cm2 V−1 s−1), advanced applications in these fields are being considered.7-12 However, low-cost and high-volume production of high-performance transistors is the one of biggest challenges, hindering the successful commercialization of organic electronics.13 Therefore, the fabrication of large-area organic electronic devices should rely on simple approaches, i.e., the solution-based deposition of the organic semiconductors should be carried out in ambient conditions with high speed and throughput. The bar-coating method is an excellent candidate for active layer deposition over large areas due to its controllability, simplicity and compatibility to the roll-to-roll process for flexible and large-area electronics.14-17 The typical procedure of the OFETs fabrication consists of four steps: substrate preparation; dielectric deposition; semiconductors deposition; electrodes deposition, according to the IEEE standards.18 During the conventional fabrication of OFETs, dielectric and semiconductor deposition is carried out in two separate steps. Before semiconducting layer deposition, the substrates are usually pretreated with self-assembled monolayers (SAMs) to reduce the trap and obtain desirable molecular packing.19-20 However, the wettability of the organic semiconductor solution on SAMs modified substrates dramatically decreases due to the low surface energy. The pre-treatment adds processing steps and materials costs, enhances the processing difficulty and generates uncertainties, such as quality of SAMs in the large-area application. The successive deposition of semiconductor films may increase the risk of inducing impurities to the interface during the deposition of the second layer in air, which can significantly reduce the electrical properties. In addition, as the surface of polymer substrates is composed of randomly oriented polymer chains, there are no particular anchoring sites for specific binding with any organic functionality, which implies that traditional SAMs are not suitable for polymer substrates used in flexible electronics. In order to simplify the fabrication processes and overcome these issues, it is necessary to simultaneously form bi-functional semiconductor-dielectric composites for larger-area and flexible OFET arrays.21-26 Recent reports have shown that semiconducting-insulating polymer blend could bring several advantages in OFETs, such as improved electrical performance, enhanced solution processability and superior mechanical properties.27-31 The blend systems generate different kinds of phase separations, where vertical phase separation satisfies the one-step formation of semiconductor

and

dielectric

layers.32

Furthermore,

the

self-stratified

semiconductor/dielectric layers have desired mechanical properties that can satisfy ACS Paragon Plus Environment

Page 3 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


substrate-free devices. Hence, the substrate-free devices could be attached everywhere on varying substrates which is potential in electronic skin.37-40 Here, we report a one-step deposition of bi-functional semiconductor-dielectric layers in OFETs by bar-coating the polymer blend solution on large-area substrates. P3HT and PMMA were chosen as semiconducting and insulating polymer, respectively. The vertical phase separation of P3HT-top PMMA-bottom structure was fabricated in the bar-coating process of the blend solution. The morphological evolution of P3HT in blend films was systematically studied by varying the concentration of two polymers. The microstructural changes in the ultrathin P3HT films were observed by atomic force microscopy (AFM) and transmission electron microscopy (TEM). The electrical properties of ultrathin P3HT films on PMMA matrix were measured by fabricating one-step OFETs. Furthermore, the application of this technique was extended to fabricate ultrathin (total thickness about hundreds of nanometers) and ultralight OFETs. The transistors are demonstrated to be stable when exposed to the lights of different wavelengths and different organic solvent vapors. 2. EXPERIMENTAL SECTION One-step OFET devices fabrication: P3HT (4002-EE, Mw ≈ 50-70 kDa, regioregularity ≈ 91-94 %), was purchased from Rieke Metals, Inc., PMMA (Mw ≈ 996 kDa), polyvinyl alcohol (PVA, Mw ≈ 9-10 kDa) and chlorobenzene were purchased from Sigma Aldrich. The Si substrates were cleaned with piranha solution, washed in deionized water and dried with nitrogen. The glass substrates were cleaned in an ultrasonic bath with acetone, alcohol and deionized water for 10 min, followed by drying with nitrogen for substrate-free devices. The PVA aqueous solution (100mg/mL) was spin-coated on glass substrates. Aluminum electrodes were thermally evaporated as gate. Before coating with blend solution, the Si substrates (or glass-PVA-aluminum for substrate-free devices) were pretreated with UV/O3 to improve the wettability of the polymer solution. The P3HT of 0.5 mg/mL and PMMA of 40 mg/mL were dissolved in chlorobenzene and stirred overnight. After that, the wire-wound bar was lowered on top of the solution, which was dropped onto one edge of the patterned large size substrate. The bar was horizontally transported at a constant velocity of 10 mm/s over the substrate to achieve a uniform coating of the polymeric ink and dried in the air. The Au patterns used for the source and drain electrodes were thermally evaporated. The substrate-free devices were floated on water to strip the devices from glass substrates. Thin film characterization: The surface morphologies of the polymer thin films were investigated by using ACS Paragon Plus Environment


tapping-mode atomic force microscopy (Nanoscope, Veeco Instrument Inc.). The TEM observations were conducted on a JEM-2100F electron microscope, operating at an acceleration voltage of 200 kV. The samples for TEM observation were prepared by transferring the blend film onto copper mesh grids and etching PMMA in acetone. The reactive ion etching (Etch Lab 200, oxygen plasma pressure is 2.00 Pa and RF generator on power is 30.0 W) was used to etch the P3HT/PMMA blend films, followed by UV-vis-NIR absorption spectra characterization. The UV-vis-NIR absorption spectra were recorded on polymer films, cast onto a quartz glass, by using a Perkin-Elmer model λ 20 UV-vis-NIR spectrophotometers. The capacitance of PMMA film and P3HT/PMMA blend film was measured by using an Agilent 4284 precision LCR meter. The electrical characteristics of the OFETs were measured in accumulation mode by using Keithley 4200 semiconductor characterization system (SCS), under ambient conditions. The test of the one-step OFETs under different visible light and different kind of organic solvent vapor refers to our previously published methods.37-38 3. RESULTS AND DISCUSSION Figure 1a exhibits the home-built bar-coating machine and provides a schematic illustration of the coating process. The thickness and morphology of the films are mainly controlled by changing the coating speed, the solution concentration, the boiling point of the solvent and the solution viscosity. In previous work, we have found that vertically phase-separated P3HT/PMMA blend film served the P3HT channel at the top surface and the PMMA gate dielectric near the bottom substrate.26 To confirm the vertical composition profile of blend film, the ultraviolet-visible absorption spectroscopy, in combination with incremental oxygen plasma etching, was used (Figure 1c).27, 41 The P3HT/PMMA blend film (0.5 mg/mL: 40 mg/mL) on quartz shows a clear P3HT characteristic absorption peak at ca. 560 and 610 nm. The insert shows that the top surface is completely covered by P3HT, which corresponds to the water contact angle of 110°. After short oxygen plasma etching, the characteristic absorption peak of P3HT rapidly decreased, which demonstrates that the topmost layer was enriched with the P3HT.

ACS Paragon Plus Environment

Page 4 of 19

Page 5 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. (a) The photographic image of the bar-coating machine and schematic illustration of the vertical phase separation in bar-coating process, (b) the chemical structures of P3HT and PMMA and (c) UV absorption spectra of blend film after different times of etching by oxygen plasma, inset shows the water contact angle of original blend film. An important issue of using this self-stratified film in one-step OFET devices is that blend film should be fully phase-separated in a vertical direction. To form such a bilayer structure, P3HT is limited to a very low content because P3HT can easily migrates to the upper layer and form a P3HT ultrathin film on the top of PMMA layer. Before more accurate experiments for the large-area process, we have systematically investigated the morphological evolution of the P3HT/PMMA blend films by varying the concentration of two polymers. By using the chlorobenzene as a high boiling point solvent, the sufficient drying time (more than 10 seconds) was provided for phase separation. Figure 2 presents the phase-mode atomic force microscopy (AFM) images of blend films by changing P3HT and PMMA concentration from 0.1 mg/mL ~ 0.5 mg/mL and 20 mg/mL ~ 60 mg/mL, respectively. It has been observed that P3HT and PMMA content significantly altered the morphology of P3HT film surface. There were two main kinds of tendencies in these phase-mode AFM images. By increasing the P3HT concentration, the coverage of P3HT is significantly improved, which may be due to enhanced P3HT molecules migration towards



top surface in higher-concentration solutions. When the concentration of P3HT is more than 0.3 mg/mL, the top surfaces are completely covered by P3HT molecules. On the other hand, the increasing PMMA concentration leads to higher coverage and more dispersive distribution of P3HT nanowires. This phenomenon can be explained by given reasons. Firstly, higher PMMA concentration generates thicker film, which results in higher amount of P3HT gathered on top surface.40-42 Secondly, higher PMMA concentration leads to higher viscosity, which obstructs P3HT to form large aggregates. Therefore, P3HT nanowires are more slender and dispersed in high concentration PMMA solution. The contact angle of the blend films shows consistent results with AFM and the surfaces with higher P3HT content resulted in larger contact angle. These results indicate that the solution (P3HT 0.5 mg/mL, PMMA 40 mg/mL) can generate a continuous and ultrathin P3HT film on PMMA layer, which may provide a well-defined interface for charge transport in OFETs.

Figure 2. Tapping-mode AFM phase images of P3HT/PMMA films with different blend


Page 6 of 19

Page 7 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


concentrations. The obtained values of water contact angle are mentioned in respective image. The solutions with the concentration of P3HT 0.5mg/mL and PMMA 40 mg/mL are chosen for large-area bar-coating process. Figure 3a-b present the surface of large-scale bar-coated blend film, which show that the surface is completely covered by P3HT and smooth. After bar-coating of the P3HT/PMMA polymer blend on the hydrophilic substrates, the vertical phase separation of the blend film is spontaneously induced to form the bilayer of polymer dielectric (bottom) and organic semiconductor (top) on a silicon wafer. Notably, the self-stratified film does not require successive solvent vapor annealing or thermal annealing. To investigate the interfacial morphologies of the blend films, the P3HT/PMMA blend films are reversed by a transfer process, i.e. slowly immersed into deionized water, reverse-transferred to a SiO2/Si substrate, rinsed with acetone to etch PMMA and dried with nitrogen.26 The remaining P3HT ultrathin films are characterized by AFM and TEM, as shown in Figure. 3c-f. The continuous P3HT film has shown the ultrathin thickness of ~2.7 nm, with some globular P3HT aggregates on it. Except for the globular aggregates, the P3HT films have shown smooth morphology between PMMA layers, which indicates that phase separation has generated the desirable interface. The TEM image (Figure 3e) also shows that globular aggregates exist on the interface, which is consistent with AFM observations. The interchain π-π stacking distance can be extracted from the arcs representing (0 k 0) diffractions of electron diffraction pattern (Figure 3f), which corresponds to the dπ-π ≈ 3.76 Å.43



Figure 3. (a) The tapping-mode AFM height image, (b) phase image of P3HT/PMMA films, (c, d) AFM height image of ultrathin P3HT film, (e) TEM image of ultrathin P3HT film and (f) selected area electron diffraction (SAED) pattern of ultrathin P3HT film. The formation of globular P3HT aggregates on the ultra-thin P3HT film can be explained by the phase-separation mechanism reported by Heriot et al.44 According to their mechanism, when the blend solution was bar-coated onto hydrophilic substrates, P3HT with low surface energy tend to migrate towards the film/air interface, whereas the PMMA component with higher affinity for substrate preferentially migrated to the substrate interfaces. As a result the blend film first undergoes vertical stratification, leading to bilayer structure. Because the solvent at the film surface evaporates at a faster rate than the solvent in the bulk can diffuse through the film, the polymer concentration at the surface becomes significantly higher than that at the substrate. This results in a Maragoni-like instability at the P3HT/PMMA interface. Therefore, the interfaces formed by phase separation are generally rough. In our work, the content of P3HT in the P3HT/PMMA blend is very low. The P3HT layer in the bilayer structure is very thin, which restricts the development of the interface-destabilizing wavevector. As a result, a relatively uniform P3HT untra-thin film was obtained. But this interface instability cannot be completely avoided, leading to the formation of globular P3HT aggregates. We have revised Figure 4 as follow.


Page 8 of 19

Page 9 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. Schematic of vertical phase-separation during the drying process. The electrical characteristics of the OFETs were investigated based on the one-step bar-coated semiconductor/polymer dielectric bilayer formed by the phase-separation of P3HT/PMMA. The wafer-scale one-step OFET array is shown in Figure 5a, with top contact/bottom gate device structure. All devices show good field-effect characteristics. Figure 5b, c present the transfer and output characteristics of OFETs indicating that the interfaces between semiconductor and dielectric layers provide effective charge carrier transport pathways. The gate leakage currents are extremely low 10-12 ~ 10-9 A, as shown in Figure 5b. The output curves, passing through the origin, support the preparation of vertically stratified structures in these blend films. To examine the reproducibility of the devices, the electrical properties of 70 transistors on one silicon wafer are tested, as shown in Figure 5d. The capacitance of the underlying PMMA layer is approximately 5.9 nF/cm2. Figure 5e presents the statistical distribution of the field-effect mobilities and on/off current ratio of 70 transistors. The maximum, average value of field-effect mobility and on/off current ratio, calculated from the transfer characteristics in the saturation regime, were 0.02 cm2 V−1 s−1, 0.005 cm2 V−1 s−1 and ~106, ~104, respectively. Notably, since the charge transport mainly occurs in the continuously ultrathin film, the non-continuously globular P3HT aggregations on the interface increase the film roughness which have a certain adverse effect on charge transport, such as scattering. The electrical stability under continuous bias conditions is another important parameter for the practical application of OFETs.45-46 To further investigate the stability of one-step transistors, the constant bias of Vg = -60V, Vd = -3V was applied in the air. Figure 5f presents the normalized ID decay in the one-step transistors under constant bias, over a period of 10 min. The bias stress-induced drain current (ID) decay in OFETs can be described by a stretched-exponential time (t) -dependent formula, applicable to a wide variety of disordered systems:



I = 0exp − where ID (0) refers to the initial drain current at t = 0, β corresponds to the dispersion parameter of the barrier energy height for charge trapping and τ represents a characteristic time associated with the rate of charge trapping. In one-step OFETs, fitting parameters are τ = 1.78 ×104 and β = 0.33. These results show that bar-coated P3HT/PMMA bi-functional layers successfully provide a well-defined organic semiconductor channel and a dielectric gate layer. Moreover, the ultrathin P3HT film can serve as stable semiconducting layer in air ambient, even if their thickness is of a few molecular layers. The stability and hysteresis-free characteristics of one-step OFETs can be attributed to the simultaneous formation of bi-functional semiconductor-dielectric composites without introducing any impurities to the interfaces and well-defined self-assembly of P3HT molecular chains. (a)

(b)

P3HT Au

(c)

10-6

Au

VD=

10-7

PMMA Doped Si

10

-8

10-10 10-11 10

-0.2

ID (µA)

-ID (A)

0 -20 V -40 V -60 V GateI

10-9

-40

(e)

-5

-20

0

30

70 OFETs

10

-50

-40

-30

-20

-10

0

VD (V) 1.0 0.9

15

-9

10

-10

10

10-11

20 15

ID(t)/ID(0)

Device counts

-8

10

10

10 5

-13

-60

-40

-20 VG(V)

0

20

0 0.00

0.01 Mobility (cm2/Vs)

0 0.02 3

0.8 0.7 0.6

5

-12

10

10

-60

(f)

20

25

-7

0V -15 V -30 V -45 V -60 V

-1

20

VG(V)

10

10-6

VG=

-0.6 -0.8

10-13

(d)

-0.4

-12

10-14 -60

ID(A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 19

4

5

6

Log (Ion/Ioff)

0.5

0

200

400

600

Time(s)

Figure 5. Electrical characteristics of one-step OFETs. (a) The device structure and photographic images of one-step OFETs on silicon wafer, (b) transfer characteristics of one-step OFETs, (c) output characteristics of one-step OFETs, (d) transfer curves of 70 transistors, (e) statistical distribution of the field-effect mobilities and on/off current ratios and (f) the normalized current decay for devices as function of time under constant bias of VG = -60 V, VD = -3V. Removing of the substrate is necessary that lead to enhanced flexibility of the self-sustained device and can be attached on varying surface. Therefore, substrate-free self-sustained OFET devices have become an important issue for us. To demonstrate the bar-coated one-step freestanding OFETs, we have produced the P3HT/PMMA (0.5 mg/mL: 40 mg/mL) blend film on sacrificial layer (PVA) and fabricated substrate-free OFETs (4 × 4


Page 11 of 19

cm2), as shown in Figure 6a. The devices can be stripped from glass substrates on water and attached to everywhere such as the back of hands (Figure 6b, c). As observed in Figure 6d and 6e, the substrate-free OFETs exhibit desirable field-effect characteristics (device yield over 90 %) as well as on rigid silicon substrates. The device performance of the one-step transistors can be further improved by adjusting the concentration of the blend solution. The substrate-free devices have also demonstrated high transparency due to the ultrathin P3HT films. These results indicate that the one-step bar-coating approach can fabricate ultrathin (total thickness about hundreds of nanometers) and ultralight OFETs.

(a)

P3HT Au

(b)

(c)

Au

PMMA Al PVA Glass substrate Water-floatation P3HT Au

Au PMMA Al

(d)

(e) 10-6 10

-7

10

-8

10

-9

V D=

0 -40 V -60 V GateI

-0.05

ID (µA)

-ID (A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10-10 10-11 -60

-0.1

VG = 0V -15 V -30 V -45 V -60 V

-0.15 -0.2

-40

-20

0

20

-0.25

-60

-50

VG (V)

-40

-30

-20

-10

0

VD (V)

Figure 6. Electrical characteristics of one-step substrate-free OFETs. (a-c) The device structure and photographic images of one-step OFETs, (d) transfer characteristics, and (e) output characteristics of one-step OFETs. The organic electronics experience various harsh environments during fabrication and application, such as different organic vapor atmosphere and light with different wavelengths. Hence, the stability of these devices should be well controlled. Therefore, the response of the one-step OFETs to the different organic solvent vapors and the lights of different wavelengths have been investigated, as shown in Figure 7. The concentration of organic vapors are standardized and controlled at 500 parts per million (ppm) (Figure 7a). Surprisingly, the electrical properties are stable even in the good solvent atmosphere of PMMA, such as acetone, toluene and ethyl acetate. In reported ultrathin-film OFET devices,



the demonstrated high stability is rare.15, 47 The ultrathin semiconductor films generally have a higher sensitivity to chemical vapors, as compared to their thicker counterparts, which contributes to the direct contact between stimuli and charge transport layers. The insensitivity of one-step OFETs can be attributed to the well-packed P3HT molecular chains according to our previous work.48 Figure 7b presents the transfer curves when devices are exposed to the laser lights of different wavelengths. The intensities of different lights were ~7.6 mW cm-2 for white light, ~20.0 mW cm-2 for 532 nm, ~63.1 mW cm-2 for 650 nm and ~28.3 mW cm-2 for 808 nm. The transfer curves are nearly unchanged under these test conditions, which is different from other reports in blend-polymer OFETs.49 Almost organic semiconductor are in lack of light-stability due to their relatively small bandgap and interfacial trap located at semiconductor/dielectric interface in OFET structure.50 In this work, the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap of P3HT is 1.9 eV indicating that P3HT is intrinsically photo-instable. The light-stability of our ultrathin P3HT could contribute to the transparency and few traps of P3HT layer. The light absorption of the layer is very limited, harvesting very little photons. Other photons were reflected or transmitted, and thus did not contribute to the photo-current. In general, our ultrathin P3HT films are insensitive to organic vapors and lights, which results in excellent stability. It is mainly due to the well-ordered P3HT arrangements that provide a moderate charge transport pathway and external stimuli cannot make traps or photon-generated carrier separation.

(a)

0.0010

-6

Initial Acetone Cyclohexane Toluene Ethanol Aether Ethyl acetate

-ID (A)

10-7 10-8

0.0005 10

-9

(-ID)1/2 (A)1/2

10

10-10 10

-11

-60

-40

-20

0

20

0.0000

VG (V)

(b)

0.0015 10-6

Dark White

650 nm

0.0010

532 nm

10-8

0.0005

10-9 -60

-40

-20

0

20

(-ID)1/2 (A)1/2

808 nm

10-7

-ID (A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.0000

VG (V)


Page 12 of 19

Page 13 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. The response of the one-step OFETs to different stimuli. Transfer curves of devices when exposed to (a) different organic solvent vapors and (b) lights of different wavelengths. 4. CONCLUSIONS In summary, large-area flexible OFETs have been fabricated by bar-coating semiconductor and dielectric layers in one step, with a vertically phase-separated P3HT-top and PMMA-bottom structure. The morphologies of blend film have been controlled by varying the concentration of P3HT and PMMA. The uniform and continuous ultrathin P3HT film of 2.7 nm has been demonstrated. The one-step OFETs based on P3HT/PMMA blend film exhibit high reproducibility and uniformity, both on silicon wafer substrates and substrate-free configuration. Furthermore, due to the well-ordered molecular arrangement, the moderate charge transport pathway has formed, which resulted in stable OFETs in different organic solvent vapors and the lights of different wavelengths. The combination of simple one-step bar-coating process and device characteristics represent a significant advancement towards wearable electronics. ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] and [email protected] ACKNOWLEDGMENTS This study was supported by National Natural Science Foundation of China (NSFC, 51573036, 51703047), the Distinguished Youth Foundation of Anhui Province (1808085J03), and the Fundamental Research Funds for the Central Universities (JD2017JGPY0006, JZ2017HGBH0952, JZ2017HGBZ0919, and JZ2018HGPB0276). REFERENCES (1) Rogers, J. A.; Someya, T.; Huang, Y. Materials and Mechanics for Stretchable Electronics.

Science 2010, 327, 1603-1607. (2) Wang, Q. H.; Wu, S. H.; Ge, F.; Zhang, G. B.; Lu, H. B.; Qiu, L. Z. Solution-Processed Microporous Semiconductor Films for High-Performance Chemical Sensors. Adv. Mater.

Interfaces 2016, 3, 1600518. (3) Berggren, M.; Nilsson, D.; Robinson, N. D. Organic materials for printed electronics. Nat.

Mater. 2007, 6, 3-5. (4) Forrest, S. R. The path to ubiquitous and low-cost organic electronic appliances on plastic.

Nature 2004, 428, 911-918. (5) Xu, J.; Wang, S. H.; Wang, G. J. N.; Zhu, C. X.; Luo, S. C.; Jin, L. H.; Gu, X. D.; Chen, S. ACS Paragon Plus Environment


C.; Feig, V. R.; To, J. W. F.; Rondeau-Gagne, S.; Park, J.; Schroeder, B. C.; Lu, C.; Oh, J. Y.; Wang, Y. M.; Kim, Y. H.; Yan, H.; Sinclair, R.; Zhou, D. S.; Xue, G.; Murmann, B.; Linder, C.; Cai, W.; Tok, J. B. H.; Chung, J. W.; Bao, Z. N. Highly stretchable polymer semiconductor films through the nanoconfinement effect. Science 2017, 355, 59-64. (6) Di, C.-a.; Zhang, F.; Zhu, D. Multi-Functional Integration of Organic Field-Effect Transistors (OFETs): Advances and Perspectives. Adv. Mater. 2013, 25, 313-330. (7) Tseng, H.-R.; Phan, H.; Luo, C.; Wang, M.; Perez, L. A.; Patel, S. N.; Ying, L.; Kramer, E. J.; Thuc-Quyen, N.; Bazan, G. C.; Heeger, A. J. High-Mobility Field-Effect Transistors Fabricated with Macroscopic Aligned Semiconducting Polymers. Adv. Mater. 2014, 26, 2993-2998. (8) Lee, B. H.; Bazan, G. C.; Heeger, A. J. Doping-Induced Carrier Density Modulation in Polymer Field-Effect Transistors. Adv. Mater. 2016, 28, 57- 62. (9) Lei, Y. L.; Deng, P.; Lin, M.; Zheng, X. L.; Zhu, F. R.; Ong, B. S. Enhancing Crystalline Structural Orders of Polymer Semiconductors for Efficient Charge Transport via Polymer-Matrix-Mediated Molecular Self-Assembly. Adv. Mater. 2016, 28, 6687- 6694. (10) Zheng, Y. Q.; Lei, T.; Dou, J. H.; Xia, X.; Wang, J. Y.; Liu, C. J.; Pei, J. Strong Electron-Deficient Polymers Lead to High Electron Mobility in Air and Their Morphology-Dependent Transport Behaviors. Adv. Mater. 2016, 28, 7213- 7219. (11) Zhang, A. D.; Xiao, C. Y.; Wu, Y.; Li, C.; Ji, Y. J.; Li, L.; Hu, W. P.; Wang, Z. H.; Ma, W.; Li, W. W. Effect of Fluorination on Molecular Orientation of Conjugated Polymers in High Performance Field-Effect Transistors. Macromolecules 2016, 49, 6431-6438. (12) Luo, H. W.; Yu, C. M.; Liu, Z. T.; Zhang, G. X.; Geng, H.; Yi, Y. P.; Broch, K.; Hu, Y. Y.; Sadhanala, A.; Jiang, L.; Qi, P. L.; Cai, Z. X.; Sirringhaus, H.; Zhang, D. Q. Remarkable enhancement of charge carrier mobility of conjugated polymer field-effect transistors upon incorporating an ionic additive. Science Advances 2016, 2, e1600076. (13) Baeg, K.-J.; Caironi, M.; Noh, Y.-Y. Toward Printed Integrated Circuits based on Unipolar or Ambipolar Polymer Semiconductors. Adv. Mater. 2013, 25, 4210-4244. (14) Bucella, S. G.; Luzio, A.; Gann, E.; Thomsen, L.; McNeill, C. R.; Pace, G.; Perinot, A.; Chen, Z. H.; Facchetti, A.; Caironi, M. Macroscopic and high-throughput printing of aligned nanostructured polymer semiconductors for MHz large-area electronics. Nat. Commun. 2015,

6, 8394. (15) Khim, D.; Ryu, G. S.; Park, W. T.; Kim, H.; Lee, M.; Noh, Y. Y. Precisely Controlled Ultrathin Conjugated Polymer Films for Large Area Transparent Transistors and Highly Sensitive Chemical Sensors. Adv. Mater. 2016, 28, 2752-2759. ACS Paragon Plus Environment

Page 14 of 19

Page 15 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(16) Murphy, C. E.; Yang, L.; Ray, S.; Yu, L. Y.; Knox, S.; Stingelin, N. Wire-bar coating of semiconducting polythiophene/insulating polyethylene blend thin films for organic transistors.

J. Appl. Phys. 2011, 110, 093523. (17) Nketia-Yawson, B.; Lee, H. S.; Son, H. J.; Kim, B.; Noh, Y. Y. Bar-coated high-performance organic thin-film transistors based on ultrathin PDFDT polymer with molecular weight independence. Org. Electron. 2016, 29, 88-93. (18) IEEE Standard for Test Methods for the Characterization of Organic Transistors and Materials - Redline. 2011, 1, 27. (19) Ito, Y.; Virkar, A. A.; Mannsfeld, S.; Oh, J. H.; Toney, M.; Locklin, J.; Bao, Z. A. Crystalline Ultrasmooth Self-Assembled Monolayers of Alkylsilanes for Organic Field-Effect Transistors. J. Am. Chem. Soc. 2009, 131, 9396-9404. (20) Kang, B.; Park, N.; Lee, J.; Min, H.; Choi, H. H.; Lee, H. S.; Cho, K. Surface-Order Mediated Assembly of pi-Conjugated Molecules on Self-Assembled Monolayers with Controlled Grain Structures. Chem. Mater. 2015, 27, 4669-4676. (21) Kim, H. M.; Kang, H. W.; Hwang, D. K.; Lim, H. S.; Ju, B. K.; Lim, J. A. Metal-Insulator-Semiconductor Coaxial Microfibers Based on Self-Organization of Organic Semiconductor: Polymer Blend for Weavable, Fibriform Organic Field-Effect Transistors.

Adv. Funct. Mater. 2016, 26, 2706-2714. (22) Lee, S. J.; Kim, Y.-J.; Yeo, S. Y.; Lee, E.; Lim, H. S.; Kim, M.; Song, Y.-W.; Cho, J.; Lim, J. A. Centro-Apical Self-Organization of Organic Semiconductors in a Line-Printed Organic Semiconductor: Polymer Blend for One-Step Printing Fabrication of Organic Field-Effect Transistors. Scientific Reports 2015, 5, 14010. (23)

Liu,

C.;

Li,

Y.;

Minari,

T.;

Takimiya,

K.;

Tsukagoshi,

K.

Forming

semiconductor/dielectric double layers by one-step spin-coating for enhancing the performance of organic field-effect transistors. Org. Electron. 2012, 13, 1146-1151. (24) Marszalek, T.; Wiatrowski, M.; Dobruchowska, E.; Jung, J.; Ulanski, J. One-step technique for production of bi-functional low molecular semiconductor-polymer composites for flexible OFET applications. J. Mater. Chem. C 2013, 1, 3190-3193. (25) Qiu, L.; Lim, J. A.; Wang, X.; Lee, W. H.; Hwang, M.; Cho, K. Versatile use of vertical-phase-separation-induced bilayer structures in organic thin-film transistors. Adv.

Mater. 2008, 20, 1141-1145. (26) Wang, X. H.; Lee, W. H.; Zhang, G. B.; Wang, X. H.; Kang, B.; Lu, H. B.; Qiu, L. Z.; Cho, K. Self-stratified semiconductor/dielectric polymer blends: vertical phase separation for facile fabrication of organic transistors. J. Mater. Chem. C 2013, 1, 3989-3998. ACS Paragon Plus Environment


(27) Lu, G.; Blakesley, J.; Himmelberger, S.; Pingel, P.; Frisch, J.; Lieberwirth, I.; Salzmann, I.; Oehzelt, M.; Di Pietro, R.; Salleo, A.; Koch, N.; Neher, D. Moderate doping leads to high performance of semiconductor/insulator polymer blend transistors. Nat. Commun. 2013, 4, 1588. (28) Bu, L.; Hu, M.; Lu, W.; Wang, Z.; Lu, G. Printing Semiconductor–Insulator Polymer Bilayers for High-Performance Coplanar Field-Effect Transistors. Adv. Mater. 2018, 30, 1704695. (29) Wang, S.; Fabiano, S.; Himmelberger, S.; Puzinas, S.; Crispin, X.; Salleo, A.; Berggren, M. Experimental evidence that short-range intermolecular aggregation is sufficient for efficient charge transport in conjugated polymers. Proc. Nat. Acad. Sci. U.S.A. 2015, 112, 10599-10604. (30) Yu, K.; Park, B.; Kim, G.; Kim, C. H.; Park, S.; Kim, J.; Jung, S.; Jeong, S.; Kwon, S.; Kang, H.; Kim, J.; Yoon, M. H.; Lee, K. Optically transparent semiconducting polymer nanonetwork for flexible and transparent electronics. Proc. Nat. Acad. Sci. U.S.A. 2016, 113, 14261-14266. (31) Lu, G.; Di Pietro, R.; Koelln, L. S.; Nasrallah, I.; Zhou, L.; Mollinger, S.; Himmelberger, S.; Koch, N.; Salleo, A.; Neher, D. Dual-Characteristic Transistors Based on Semiconducting Polymer Blends. Adv. Electron. Mater. 2016, 2, 1600267. (32) Kang, B.; Ge, F.; Qiu, L. Z.; Cho, K. Effective Use of Electrically Insulating Units in Organic Semiconductor Thin Films for High-Performance Organic Transistors. Adv. Electron.

Mater. 2017, 3, 1600240. (33) Yi, H. T.; Payne, M. M.; Anthony, J. E.; Podzorov, V. Ultra-flexible solution-processed organic field-effect transistors. Nat. Commun. 2012, 3, 1259. (34) Caboni, A.; Orgiu, E.; Barbaro, M.; Bonfiglio, A. Flexible Organic Thin-Film Transistors for pH Monitoring. IEEE Sensors J. 2009, 9, 1963-1970. (35) Zhang, L.; Wang, H. L.; Zhao, Y.; Guo, Y. L.; Hu, W. P.; Yu, G.; Liu, Y. Q. Substrate-Free Ultra-Flexible Organic Field-Effect Transistors and Five-Stage Ring Oscillators. Adv. Mater. 2013, 25, 5455-5460. (36) Liu, Yang Hui; Zhu, Li Qiang; Feng, Ping; Shi, Yi; Wan, Qing Freestanding Artificial Synapses Based on Laterally Proton-Coupled Transistors on Chitosan Membranes. Adv.

Mater. 2015, 27, 5599-5604. (37) Wu, S. H.; Wang, G. H.; Xue, Z.; Ge, F.; Zhang, G. B.; Lu, H. B.; Qiu, L. Z. Organic Field-Effect Transistors with Macroporous Semiconductor Films as High-Performance Humidity Sensors. ACS Appl. Mat. Interfaces 2017, 9, 14974-14982. ACS Paragon Plus Environment

Page 16 of 19

Page 17 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(38) Wang, Q.; Zhu, M.; Wu, D.; Zhang, G.; Wang, X.; Lu, H.; Wang, X.; Qiu, L. Phototransistors based on a donor-acceptor conjugated polymer with a high response speed. J.

Mater. Chem. C 2015, 3, 10734-10741. (39) Bu, L.; Gao, S.; Wang, W.; Zhou, L.; Feng, S.; Chen, X.; Yu, D.; Li, S.; Lu, G. Film-Depth-Dependent Light Absorption and Charge Transport for Polymer Electronics: A Case Study on Semiconductor/Insulator Blends by Plasma Etching. Adv. Electron. Mater. 2016, 2, 1600359. (40) Lee, W. H.; Lim, J. A.; Kwak, D.; Cho, J. H.; Lee, H. S.; Choi, H. H.; Cho, K. Semiconductor-Dielectric Blends: A Facile All Solution Route to Flexible All-Organic Transistors. Adv. Mater. 2009, 21, 4243-4248. (41) Lee, W. H.; Kwak, D.; Anthony, J. E.; Lee, H. S.; Choi, H. H.; Kim, D. H.; Lee, S. G.; Cho, K. The Influence of the Solvent Evaporation Rate on the Phase Separation and Electrical Performances of Soluble Acene-Polymer Blend Semiconductors. Adv. Funct. Mater. 2012, 22, 267-281. (42) Yoo, H.; Choi, H. H.; Shin, T. J.; Rim, T.; Cho, K.; Jung, S.; Kim, J.-J. Self-Assembled, Millimeter-Sized TIPS-Pentacene Spherulites Grown on Partially Crosslinked Polymer Gate Dielectric. Adv. Funct. Mater. 2015, 25, 3658-3665. (43) Biniek, L.; Pouget, S.; Djurado, D.; Gonthier, E.; Tremel, K.; Kayunkid, N.; Zaborova, E.; Crespo-Monteiro, N.; Boyron, O.; Leclerc, N.; Ludwigs, S.; Brinkmann, M. High-Temperature Rubbing: A Versatile Method to Align pi-Conjugated Polymers without Alignment Substrate. Macromolecules 2014, 47, 3871-3879. (44) Heriot, S. Y.; Jones, R. A. L. An interfacial instability in a transient wetting layer leads to lateral phase separation in thin spin-cast polymer-blend films. Nat. Mater. 2005, 4, 782-786. (45) Choi, H. H.; Kang, M. S.; Kim, M.; Kim, H.; Cho, J. H.; Cho, K. Decoupling the Bias-Stress-Induced Charge Trapping in Semiconductors and Gate-Dielectrics of Organic Transistors Using a Double Stretched-Exponential Formula. Adv. Funct. Mater. 2013, 23, 690-696. (46) Lee, W. H.; Choi, H. H.; Kim, D. H.; Cho, K. 25th Anniversary Article: Microstructure Dependent Bias Stability of Organic Transistors. Adv. Mater. 2014, 26, 1660-1680. (47) Zhang, F.; Di, C.-a.; Berdunov, N.; Hu, Y.; Hu, Y.; Gao, X.; Meng, Q.; Sirringhaus, H.; Zhu, D. Ultrathin Film Organic Transistors: Precise Control of Semiconductor Thickness via Spin-Coating. Adv. Mater. 2013, 25, 1401-1407. (48) Ge, F.; Wei, S. Y.; Liu, Z.; Wang, G. H.; Wang, X. H.; Zhang, G. B.; Lu, H. B.; Cho, K.; Qiu, L. Z. Tailoring Structure and Field-Effect Characteristics of Ultrathin Conjugated ACS Paragon Plus Environment


Polymer Films via Phase Separation. ACS Appl. Mat. Interfaces 2018, 10, 9602-9611. (49) Lei, Y. L.; Li, N.; Chan, W. K. E.; Ong, B. S.; Zhu, F. R. Highly sensitive near infrared organic phototransistors based on conjugated polymer nanowire networks. Org. Electron. 2017, 48, 12-18. (50) Park, J. H.; Lee, H. S.; Park, S.; Min, S. W.; Yi, Y.; Cho, C. G.; Han, J.; Kim, T. W.; Im, S. Photo-Stable Organic Thin-Film Transistor Utilizing a New Indolocarbazole Derivative for Image Pixel and Logic Applications. Adv. Funct. Mater. 2014, 24, 1109-1116.


Page 18 of 19

Page 19 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC


Bar-Coated Ultrathin Semiconductors from Polymer Blend for One

Recommend Documents