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Tailoring Structure and Field-Effect Characteristic of Ultrathin Conjugated Polymer Films via Phase-Separation Feng Ge, Shiyu Wei, Zhen Liu, Guiheng Wang, Xiaohong Wang, Guobing Zhang, Hongbo Lu, Kilwon Cho, and Longzhen Qiu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19171 • Publication Date (Web): 28 Feb 2018 Downloaded from http://pubs.acs.org on March 1, 2018

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ACS Applied Materials & Interfaces

Tailoring Structure and Field-Effect Characteristic of Ultrathin Conjugated Polymer Films via Phase-Separation Feng Ge,† Shiyu Wei,† Zhen Liu,† Guiheng Wang,† Xiaohong Wang,† Guobing Zhang,†‡ Hongbo Lu,†‡ Kilwon Cho,⊥ and Longzhen Qiu*†‡ †Key Lab of Special Display Technology, Ministry of Education, 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] KEYWORDS: semiconductor/insulator blend; ultrathin film; conjugated polymer; OFET; 2D charge transport ABSTRACT A phase-separation method has been developed to control the semiconductor thickness and molecular arrangement via semiconducting/insulating polymer blend system. The thickness of poly(3-hexylthiophene) (P3HT) film have been regulated from 10.5 ± 1.4 nm down to 1.9 ± 0.8 nm with favorable self-assembly degree and the mobility ranging from 0.21 to 0.03 cm2 V-1 s-1, respectively. The ultrathin-films show the high bias stability and weak decay after 24 days with bottom gate configuration. Benefited from well molecular order, the

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films have low activation energy and 2D charge transport profile in semiconductor layers. Moreover, this blending process can be used as general strategy of thickness control in flexible low-voltage devices and donor-acceptor conjugated polymers. 1. INTRODUCTION The rapid development of polymeric semiconducting materials and devices promote organic field-effect transistors (OFETs) a promising technology for wearable devices, chemical biosensors, radio frequency identification (RFID) tags, and flexible electronic paper.1-6 The precise control of the semiconducting film thickness and molecular arrangement in OTFTs is of vital importance because the unclear charge transport mechanism of organic semiconductor can be easier to investigate in ultrathin films by surface sensitive detection method,7-8 and high-quality ultrathin semiconducting films can be used in the manufacture of sensors and optical transparent devices.9 Furthermore, ultrathin semiconducting film can dramatically reduce the materials consumption for next-generation large-area electronic devices.10 Therefore, for the high-performance devices and novel applications, it is necessary to research the correlation between film microstructure and electrical properties of the conjugated polymers with a thickness of less than 10 nm. Until now, several technologies have been developed to deposit ultrathin organic semiconducting films by solution method, such as spin coating,11-12 bar coating,10,

13

Langmuir−Schäfer (LS),14-17 dip coating,8, 18 and the electrical performance are within the comparable range to the thicker counterparts. P3HT is a “classic” semi-crystalline conjugated polymer which is an effective model for understanding the process−structure−property relationships19-24. Although P3HT has been demonstrated to ultrathin film, their mobility are


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still at very low level (less than 0.01 cm2 V-1 s-1) by above-mentioned solution processes. 9, 25 It is mainly contributed to that these common solution processes generally produce the P3HT ultrathin film with low self-assembly degree and poor compactness, resulting in very low field-effect performance. More importantly, the low performance limits the further study of charge carrier transport in ultrathin film, such as stability, activation energy, and dimensionality of charge transport. Our previous studies have found that well self-assembly P3HT film could be obtained through the vertical phase separation in the blends, which might provide a good solution for preparing P3HT ultrathin films.26-28 When a blend of P3HT and poly(methyl methacrylate) (PMMA) was spin-coated onto a hydrophilic SiO2/Si substrate, P3HT with low surface energy was shown to tend to migrate to the film/air interface, while the PMMA component with higher affinity for substrate preferentially migrated to the substrate interfaces. As a result, a P3HT-top/PMMA-bottom bilayer structure forms.29-37 Here, the vertical phase separation has been used to control the P3HT ultrathin film thickness and microstructure via changing its concentration in the insulating polymer matrix in blend system. Though carefully investigating the films by atomic force microcopy (AFM), grazing incidence X-ray diffraction (GIXD) and UV-vis measurements, the micro/nano-structure of ultrathin P3HT films have been researched. The accordingly charge transport behaviors and devices stability also have been systematically investigated to discuss the relationship between microstructure and electric properties. Notably these ultrathin film process also applied to flexible low-voltage devices and used as general strategy to another donor-acceptor (D-A) conjugated polymer. 2. EXPERIMENTAL SECTION



2.1

Materials

P3HT (P100, Mw~41.9 kDa, Rieke Metals, Inc.), PMMA (Mw∼996 kDa, Sigma Aldrich), isoindigo (IID)- bithiophene (BT) based acceptor–donor conjugated polymers (PIIDBT) is home-made, polystyrene (PS, Mw∼230 kDa, Sigma Aldrich), polyvinyl akohol (PVA, Sigma Aldrich), Cytop (purchased from AGC Asahi Glass), n-octadecyl phosphonic acid (purchased from TCI (Shanghai) Development Co., Ltd.). 2.2

Ultrathin film OTFTs fabrication

Pure P3HT and PMMA were separately dissolved in chlorobenzene, and mixed at ultimate concentration of P3HT (0.38, 0.5, 0.75, 1.00, 1.50, 3.0 mg/ml) and PMMA (3 wt. %). PIIDBT and PS were dissolved at same condition. The films were spin coated on silicon substrate (dealt with piranha solution, washed in deionized water, and dried with nitrogen flow) at 2000 rpm for 60 s in glovebox, and dry in vacuum oven overnight to remove solvent without heating (for PIIDBT/PS blend films were spin coated on pre-deposited PVA-silicon substrate). The films were immersed in KOH solution (5 wt. %) to strip itself from silicon substrate (for PIIDBT/PS blend films were immersed in DI water). The floated films were rinsed in deionized water for 3 times and transferred with Cytop-treated SiO2/Si substrates (300 nm-thick SiO2 or AlOx-SAMs hybrid dielectrics for low-voltage devices) followed by a washing step with acetone. Thermally evaporated gold source/drain electrodes (30 nm) were prepared on P3HT films with different thickness using shadow masks (W: L = 780 µm : 130 µm). The fabrication of AlOx-SAMs hybrid dielectrics:38 Aluminum film was thermally evaporated onto a PS sacrificial layer through a shadow mask. Then, UV Adhesive coated PET were covered on aluminum, and solidified. The composite films were immersed in


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cyclohexane to dissolve PS to obtain aluminum-PET. The high-k aluminum oxide gate dielectric layer was formed by potentiostatic anodization under ambient conditions (CHI660D electrochemical workstation, Shanghai Chen Hua Instrument Co., Ltd). Phosphonic acid self-assembled monolayers were prepared into solutions of 3 mM n-octadecyl phosphonic acid in isopropanol at room-temperature. AlOx-SAM hybrid dielectrics on flexible aluminum with good dielectric characteristics have been fabricated. The ultrathin P3HT films with different thickness were transferred to the AlOx-SAM hybrid dielectrics with the procedure as shown in Figure 1a and source and drain electrodes were evaporated as shown in Figure 6a. 2.3

Thin film characterization:

The surface morphologies and the thickness of the polymer thin film were investigated using tapping-mode atomic force microscopy (Nanoscope, Veeco Instrument Inc.). The UV-vis transmission were measured by CARY 5000 spectrophotometer (Agilent). 2D GIXD were measured at Pohang Accelerator Laboratory (PAL), 11.07 keV photons (1.12 Å) with a grazing angle of 0.12° were directed onto the sample. 2.4

Electrical measurement:

The FET electrical characteristics were measured using a Keithley 4200-SCS instrument in air ambient. The temperature-dependent electrical measurement was performed in a Lake Shore cryogenic vacuum probe-station. 3. RESULTS AND DISCUSSION 3.1. Film Thickness Control and Microstructure Analysis. A semiconductor-top and insulator-bottom bilayer structure can be obtained by surface-induced

vertical

phase

separation

during

spin

coating

organic

semiconducting/insulating polymer blends solution on a hydrophilic substrate.26, 28 To form



such a bilayer structure, the ratio of semiconducting polymer should control to be a small amount in a major part of the insulating polymer. Otherwise, the lateral or dual phase separation would emerge due to the compositional fluctuations parallel to the substrate surface which is dominated by mass transport limitations.39 The thickness of P3HT layer can be controlled by changing the concentrations of semiconducting and insulating polymer, spin-coating speed and solvents. To realize an accurate experiment, the spin-coating speed (2000 rpm), solvent (chlorobenzene), and PMMA concentration (3 wt. %) have been fixed. In the spin coating process, a long drying time (about 10 seconds) was generated by high boiling point of chlorobenzene and high viscosity of solution. P3HT assembled to the top layer sufficiently, and PMMA gather to the bottom on the hydrophilic silicon substrate within the extended drying time as shown in Figure 1a (left). The vertical phase separation of P3HT/PMMA layer was verified by contact angle and UV-visible absorption spectra in combination with incremental O2 plasma etching which show the most P3HT was gathered on the top (see Figure S1). Figure 1a (right) depicts the film transfer method developed in present research. First, the wafer with bilayer film was slowly immersed into the potassium hydroxide (KOH) aqueous solution, the film was stripped and floated on the liquid surface. Then the film was transferred to the deionized water for several times to rinse KOH solution, and caught out from water by the Cytop-treated hydrophobic dielectric substrate, and the P3HT-gathered top surface was contact to the target substrate. At last, PMMA was etched by acetone to form the P3HT ultrathin films, and the films were put into vacuum oven to remove acetone without any thermal annealing process. The transmittance of different thickness film have been demonstrated in Figure S2. When thickness reduced to 10 nm, transparency can


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increase to 90%, which could be applied in transparent transistors potentially.

Figure 1. (a) Schematic illustration of vertical phase separation and the transfer and etching process for P3HT ultrathin film. (b) AFM images of P3HT films with different thickness. (c) Trend of P3HT film thickness versus solution concentration. To investigate the microstructure and thickness of the ultrathin P3HT films, atomic force microscope (AFM) measurement was used. Figure 1b and Figure S9-S15 shows height-mode AFM images of films surface morphology after the removal of the PMMA. The thickness of the film is measured by scratching the step of films on silicon substrates. All P3HT films exhibit the continuous morphology with full coverage, which proves that a uniform layer of conjugated polymer assembled on the top surface of blend film. There are some high points can be observed on the films which could relate to P3HT aggregates at the interface between



P3HT and PMMA. The thickness of continuous films are range from 10.5 ± 1.4 to 1.9 ± 0.8 nm, which could be precisely controlled by varying the P3HT concentration of 3.00, 1.50, 1.00, 0.75, 0.50, and 0.38 mg/ml with linear-like relationship as Figure 1c shown. We have further increased P3HT concentration to 5.00 mg/ml and reduced it to 0.30 mg/ml, finding that non-uniform morphology and discontinuous films were arisen after transfer-etching process, respectively (see Figure S3, S4). The optimized concentration of 0.38 mg/ml P3HT could generate the 1.9 ± 0.8 nm-thick ultrathin film containing 1-2 molecular layers. The reported ultrathin P3HT films thickness were 1.6 nm and 2.5 nm by spin-coating and LS method, respectively.9, 40 The different thickness of P3HT monolayer film would attribute to the change of molecular stacking and hexyl chain folding. Figure 2 (a-e) and Table S1 show the GIXD results of different P3HT films. The films with thickness of 4.5 ± 1.9 nm, 2.8 ± 1.0 nm and 1.9 ± 0.8 nm have highly crystalline structure with edge-on alignment to the substrate according to GIXD analysis (Figure 2a-c).41-43 The (k00) peaks in the out of plane (qz) direction corresponding to the d-spacing of these films. Notably, for ultrathin film ~1.9 ± 0.8 nm, we observed several weak Bragg diffraction rods at qz direction which should not be observed in monolayer; this indicates that ultrathin P3HT film has redundant molecular layers such as the short nanofibrils and salient points as Figure 1b shows. For different thickness P3HT films (4.5 ± 1.9, 2.8 ± 1.0 and 1.9 ± 0.8 nm), the (k00) peak position get slightly decreased (qz = 0.388, 0.378, 0.356 Å-1), when the thickness reduced, related to the increased d-spacing distance of 16.19, 16.62 and 17.65 Å, respectively. Meanwhile, the neat P3HT film (spin coated with chlorobenzene) with the thickness of 95.1 nm have the closer d-spacing of 16.00 Å (see Figure S5a-c in supporting


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information). The (010) peaks, at the qxy = 1.65, 1.65, 1.66 Å-1, indicate π-π distance of 3.81, 3.81, and 3.78 Å which means the ultrathin films have the close π-π stacking.44

Figure 2. (a-c) 2D-GIXD scattering patterns of the different P3HT films with thickness of 4.5 ± 1.9, 2.8 ± 1.0 and 1.9 ± 0.8 nm respectively, and corresponding 1D-GIXD profiles of (d, e) out-of-plane and in-plane direction, (f) normalized UV-visible absorption spectra of ultrathin P3HT films via different concentration, (g) evolution of exciton bandwidth W of ordered aggregates in the films as a function of various concentration, (h and i) the multilayer and monolayer P3HT molecular stacking. UV-visible absorption spectra was measured and the results of different thickness P3HT films were depicted in Figure 2f (normalized absorbance at 560 nm). The absorbance shows all P3HT films have low energy features at ca. 560 and 605 nm associated with the vibronic bands of (0-1) and (0-0) transitions, respectively. Compared to the thick film, six films via blending method show slight red shift, and obviously enhanced (0-0) transitions. According to Spano's model,45-46 interchain coupling leads to vibronic bands in the absorption spectrum. And the free exciton bandwidth (W) could be extracted from the values of vibronic bands,



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which inversely related to intrachain ordering or exciton conjugation length along the backbone and its planarity.47 An increased W is associated with decreased molecular order. Intensities of (0-0) and (0-1) transitions are used to calculate the W values by equation 1.

I0-0 1-0.24W/EP 2 ={ } I0-1 1+0.073W/EP

(1)

I0-0 and I0-1 represent the intensities of the 0-0 and 0-1 transitions, respectively. EP is the vibrational energy of the symmetric vinyl stretch taken as 0.18 eV. As shown in Figure 2g, the value of W for different P3HT films decreases from 69 to 58 meV related to film thickness down from 10.5 ± 1.4 nm to 1.9 ± 0.8 nm, while the thick P3HT film has a large W with 130 meV, indicating that the thinner films have longer extent of conjugation and more ordered molecular packing.48 Figure 2h and i exhibit the molecular stacking of thicker film with multilayer and monolayer, respectively. The monolayer has the narrow π-stacking of 3.78 Å and large lamellar distance of 17.65 Å, while the multilayer has a normal π-stacking of 3.83 Å compact lamellar distance of 16.19 Å. The reason of the high order of ultrathin P3HT film could contribute to the so-called effect “2D nano-confinement”. This effect nanoconfinement of polymers into nanometerscale dimensions is known to result in peculiar thermodynamic and kinetic properties due to the finite-size effect and the interface effect. The ultrathin P3HT have enhanced polymer chain dynamics to align the backbone and extend the hexyl side chain. So, large d-spacing, narrow π-π stacking and long conjugation length are formed in phase-separation process. This conclusion is validated in the both results of GIXD and UV-visible absorption spectra.5 3.2. Electrical Characteristics and Stability of Ultrathin P3HT OFETs. To investigate the electrical properties of these ultrathin film, bottom-gate/top-contact OTFTs have been fabricated. The experimental details are included in Experimental Section


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and Support Information. Figure 3 shows the transfer, output curves, mobilities and threshold voltage (Vth) of P3HT ultrathin film with different thickness. Table 1 exhibits the detailed electrical data of all the transistors based on P3HT films by the blending method and the thick film (directly spin-coated film). The thinnest film with 1.9 ± 0.8 nm thickness shows the stable mobility of 0.03 cm2 V-1 s-1 indicating the effective charge transport in such an ultrathin film with 1-2 molecular layers. This mobility is even three times larger than the mobility of thick film by direct spin coating with the thickness of 95.1 nm. Increasing the film thickness to 10.5 ± 1.4 nm results in a remarkable improvement of hole mobility with 0.21 cm2 V-1 s-1 almost approaching to the highest mobility value for P3HT at the absence of chemical doping, which indicates the polymer chains are well ordered and provide favorable charge carriers transport pathway. A higher concentration 5.00 mg/ml of P3HT in blend solution was employed to investigated whether the thickness-dependent mobility could be further improved when film get thicker. The high concentration results in non-uniform morphology about 17.0 nm (see Figure S4) due to the dual phase-separation and the mobility has slight reduced. Notably, the devices exhibit good field effect characteristics with high mobility, well-defined linear and saturation regimes, straight line of the square root of drain current, and no hysteresis, which all of these features indicate good carrier injection and effective charge transfer pathway.49 In addition, the high on/off ratio 106 of P3HT OFETs which is rarely observed in other reports is due to low bulk conductance of ultrathin film that can dramatically reduce the off current.50 The electrical performance is much higher than the reported monolayer film via spin coating and Langmuir-Blodgett method.9, 25, 51-52



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Figure 3. Electrical characteristics of ultrathin polymer FETs. (a) Transfer characteristics of P3HT OFETs via different concentration, (b) Output characteristics of 3 mg/ml P3HT OFETs, (c) The evolution of the field-effect mobilities and (d) the threshold voltages by varying P3HT film thickness. Table 1. Electrical characteristics of ultrathin P3HT OFETs. P3HT

Mobility Thickness

concentration

2

-1

-1

Mobility 2

-1

-1

(cm V s )

(cm V s )

(max)

(average)

0.039

0.028 ± 0.008

(nm) (mg/ml) 0.38

1.9 ± 0.8

Vth (V)

Ion/Ioff

SS (V/dec)

-6.35

2×105

5.33

5

4.31

0.50

2.8 ± 1.0

0.071

0.045 ± 0.018

-5.09

2×10

0.75

3.0 ± 1.6

0.090

0.079 ± 0.019

-4.98

3×105

4.28

1.00

4.5 ± 1.9

0.131

0.099 ± 0.012

-4.98

1×106

3.73

6

3.42

1.50

5.6 ± 1.8

0.169

0.156 ± 0.006

-2.64

2×10

3.00

10.5 ± 1.4

0.211

0.192 ± 0.014

-2.35

9×105

2.80

5.00

17.0 ± 5.1

0.204

0.173 ± 0.033

-2.48

9×105

4.16

95.1 ± 4.1

0.011

0.009 ± 0.001

-2.48

6×103

7.85

Thick film (5 mg/ml)

The electrical properties of bottom gated P3HT OFETs would have obvious decline due to direct exposure in air, which contributed to the poorly chemical structure stability, grain


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boundary, and traps from water and oxygen.53 This effect may magnify in an ultrathin film devices. In order to test the electrical stability of P3HT ultrathin films based OFETs, constant gate voltage stress was applied with VG = -30 V and VD = -30 V in air ambient with relative humidity (R.H.) ~30% (Figure 4a).54 The measured channel current as a function of time was detected for a total duration of 1000 s. The devices with thicker films (>4.5 nm) exhibit high bias stability which maintained higher than 85 % from their initial values. With decreasing the film thickness to 2.8 ± 1.0 and 1.9 ± 0.8 nm, the current decay to 50% and 30% respectively, which were relatively more sensitive than thicker films. The devices were preserved in low vacuum environment for a certain time and tested again to investigate the instability of ultrathin films. Figure 4b shows the transfer curves of 10.5 ± 1.4 nm-thick film FET at initial condition and after 24 days. Figure 4c exhibits the mobility variation of these film after 24 days. All the FETs based on different films degenerate after certain time. The degree of degeneration seriously increased for thinner film with the thickness of 2.8 ± 1.0 nm and 1.9 ± 0.8 nm. Taken together, ultrathin films below a certain threshold value tend to have more serious instability than the thicker ones, due to thinner P3HT layers thickness and more defects. Ultrathin P3HT film without protection of upper molecules and the existence of some pinhole (shown as figure 4d) or grain boundary cause more traps which can be attacked by water and oxygen in ambient condition.



Figure 4. (a) The normalized current decay for P3HT devices as function of time under constant bias of VG = −30 V, VD = −30V, (b) Transfer characteristics of 10.5 ± 1.4 nm-thick P3HT OFET for 0 day and after 24 days, (c) The average mobilities decay of different P3HT films after 24 days, (d) the pin hole distribution on 1.9 ± 0.8 nm-thick P3HT film. 3.3. Charge Transport of Ultrathin P3HT. To get deep insight into the relationship between ultrathin films structure and charge transport, we investigated temperature-dependent FET characteristics of P3HT OTFTs. The measured transfer curves of FETs with different P3HT thickness at variable temperature (T), and Arrhenius plots of the T-dependent mobilities of different thickness films were shown in Figure S6. Figure 5a shows the activation energies (EA) of different P3HT FETs which can be extracted using the Arrhenius relation, µ ∝ exp (−EA / kT), where k is the Boltzmann constant. The higher EA can be observed in 1.9 ± 0.8 nm-thick film to overcome the local hole barrier between ordered regions.55-57 When the thickness improved, charge transport


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pathway increase, EA decrease and the lowest value is down to 46 meV. Taken together, charge transport in the thinnest film with 1.9 ± 0.8 nm need more thermal EA due to pinholes or grain boundary. More inter-layer charge jumping from first monolayer to upper layers decreased EA in thicker films. A fitting method developed by Brondijk et al. and Sirringhaus et al.58-61 is used to investigate the carrier distribution profiles in P3HT ultrathin films with two-dimensionality (2D) or 3D nature of charge transport. Temperature and VG dependences of the transfer plots can be described by a power-law formula, ID ∝ (VG-Vth) -γ. Here, the power γ as a function of temperature could distinguish dimensionality of the charge transport with γ 2D = (T0 /T) + 1, γ 3D = 2T0 /T. For 2D transport, the γ value would be extrapolated to 1 at infinite temperature. For 3D transport, this value is extrapolated to 0. T0 is a characteristic temperature associated the width of the density of states which can be extracted from the slope of γ versus 1/ T. Figure 5b shows the hole charge transport of all P3HT films with the thickness from 1.9 ± 0.8 nm to 10.5 nm take place in 2D profile. This results means the charge transport occurs in the 1~2 molecular layers near the dielectric layer. Notably, in the other reports, most bottom gate configuration exhibit the 3D charge transport, while the top gate configuration exhibit the 2D charge transport.62-64 Combining these works and our result, we can conclude that the top surface of polymeric semiconductor films provide good stacking of molecules and desired charge carriers transport pathway, especially in the semiconducting/insulating polymer blend systems. Proposed relationship between ultrathin P3HT films microstructure and properties was shown in Figure 5c. The vertical phase separation occurs in the spin coating process along



with solvent evaporation and polymer dissolving out. With low P3HT concentration, there is enough free space for the conjugated polymer self-assembly on the top surface and the hexyl side chains could sufficiently extend, thus the close π-π distance, large d-spacing, and long conjugate length can be formed in 1-2 molecular layers. Although the order of 1.9 ± 0.8 nm P3HT is high, monolayer leads to the non-perfect coverage with some pinhole. This structure generate much charge traps and inevitably decreasing of charge pathway resulting in the lower mobility, high EA and poor stability. For higher P3HT concentration, there is no sufficient space for P3HT chain self-assembly than lowest concentration, so the degree of crystallinity slightly decrease, conjugate length is short. Notably this conclusion is within the scope of blending-method ultrathin film, but still better than pristine P3HT film. The enough P3HT molecular layers allow the better charge transport and trap-tolerant capability. Therefore the thicker films have higher mobility, lower EA, desired stability.

Figure 5. (a) Extracted EA from Arrhenius relations for different P3HT thin films, (b) values of γ extracted from transfer characteristics, (c) mechanism of charge transport in monolayer (left) and multilayer (right) P3HT films. 3.4. Low Voltage and Flexible OFETs based on Ultrathin P3HT Films. One of the key challenges of OFETs for flexible display and wearable electronics comes from the high operating voltages, which results in overmuch energy consumption. To


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this end, the ultrathin, highly transparent P3HT films were applied to fabricate to flexible and low-voltage devices which method had been developed in our previous work.38 AlOx-SAM hybrid dielectrics were grown onto flexible aluminum under ambient conditions with good dielectric characteristics (negligible leakage, 114.3 nF cm-2 capacitance) via potentiostatic anodization and n-octadecyl phosphonic acid treatment. The ultrathin P3HT films with different thickness were transferred to the AlOx-SAM hybrid dielectrics and source and drain electrodes were evaporated as Figure 6a shows. The 1.9 ± 0.8 nm-thick film exhibit high transparency. The field effect characteristic curves are shown in Figure 6 (b and c), as our expectations, the films exhibit good performance and can be operated at low voltage. As the thickness increases, the film properties are improved. On/off current of ultrathin flexible low-voltage devices have no significant change after bending 200 times with 0.5 cm radius, demonstrating that the devices have a stable mechanical flexibility.

Figure 6. (a) The schematic device structure and an image of the low voltage, flexible



ultrathin P3HT OFETs, (b) transfer characteristics of OFETs with different P3HT thickness, (c) output characteristics of 10.5 ± 1.4 nm-thick (black) and 1.9 ± 0.8 nm-thick (violet) P3HT OFET, (d) on and off current of 2.8 ± 1.0 nm-thick P3HT low voltage a function of bending cycles. 3.5. Morphology and Electrical Characteristics of Ultrathin PIIDBT Films. In order to investigate the universality of this method, we also used another home-made donor-acceptor conjugated polymer based on isoindigo called PIIDBT which structural formula is shown in Figure 7c insert. By changing the proportion of the conjugated polymer, films with different thickness were obtained (see Figure 7 and Figure S7). Figure 7a, b show the thickness and morphology of 16.8 and 2.9 nm-thick PIIDBT films. Completely different from P3HT ultrathin films, the PIIDBT films exhibit highly ordered supramolecular morphology which is similar to the ultrathin film morphology reported in the literature.10 No matter the thickness of the conjugated polymer film, the relatively roughness is high due to the pin holes. The GIXD analysis of the ultrathin film have been measured (see Figure S8), and the film is crystalline with an edge-on alignment relative to the substrate. The bottom gate/top contact OTFTs have been fabricated to study the electrical properties of these films. Figures 7d, e show the transfer and output curves with good field-effect performance. The extracted mobility is shown in Figure 7f, with a significant thickness dependence. These properties of P3HT and PIIDBT films demonstrate that the proper film thickness about 10 nm provides favorable carrier transport pathway.


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Figure 7. (a, b) height-mode AFM images of thin and ultrathin PIIDBT film via blending method, (c) the concentration-dependent thickness of PIIDBT film thickness, (d) transfer curves of ultrathin PIIDBT films with different thickness, (e) output curves of 16.8 and 2.9 nm-thick PIIDBT films, (f) mobilities of PIIDBT ultrathin films with different thickness. 4. CONCLUSION Precisely control the conjugated polymer film thickness have been realized in blending system and with a brief transferring-etching method. The P3HT film thickness can be precisely controlled from 10.5 ± 1.4 nm down to 1.9 ± 0.8 nm. The low free exciton bandwidth, close π-π distance, can be observed in ultrathin film which indicates the favorable molecular stacking of P3HT chains. The film with 10.5 ± 1.4 nm thickness has the high mobility of 0.21 cm2 V-1 s-1 which almost approach to the highest electric property of reported P3HT OTFTs at several nanometers, while the thinnest film with 1.9 ± 0.8 nm also has the high mobility of 0.03 cm2 V-1 s-1. These films show the desired 2D charge transport in the active layers. Thickness-dependent EA and stability have been demonstrated relating to the microstructure of ultrathin films. Our films can also be applied in flexible low-voltage



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devices which have potential in next-generation electronics and another D-A conjugated polymer was tested to prove the universality of this method. ASSOCIATED CONTENT Supporting Information Contact angle of P3HT/PMMA blend film and UV-visible absorption spectra in combination with incremental O2 plasma etching results, AFM images of other P3HT films, GIXD results of

thick

P3HT

film,

molecular

stacking

distance

of

different

P3HT

films,

temperature-dependent transfer curves and Arrhenius plot of the T-dependent µ-values of different-thick P3HT films, AFM image of PIIDBT ultrathin films, GIXD results of PIIDBT film, AFM and Spectra Thick Series results of different P3HT films. The Supporting Information is available free of charge on the ACS Publications website at DOI: AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (Grant No. 51573036, 51703047), the Program for New Century Excellent Talents in University (Grant no. NCET-12-0839) and the Fundamental Research Funds for the Central Universities (JZ2017HGBH0952). REFERENCES (1) Rogers, J. A.; Someya, T.; Huang, Y. Materials and Mechanics for Stretchable Electronics. Science 2010, 327, 1603-1607.


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