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Thiophene-thiazole based Semiconducting Copolymers for High Performance Polymer Field-effect Transistors Jong Won Chung, Won-Tae Park, Jeong-Il Park, Youngjun Yun, Xiaodan Gu, Jiyoul Lee, and Yong-Young Noh ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12974 • Publication Date (Web): 19 Oct 2017 Downloaded from http://pubs.acs.org on October 20, 2017

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ACS Applied Materials & Interfaces

Thiophene-thiazole based Semiconducting Copolymers for High Performance Polymer Field-effect Transistors Jong Won Chung†, ‡, Won-Tae Park#, ‡, Jeong-Il Park†, Youngjun Yun†, Xiaodan Gu∥, Jiyoul Lee*,§, Yong-Young Noh*,#

† Organic Materials Lab, Samsung Advanced Institute of Technology, Samsung Electronics Co., Samsung-ro, Suwon, Gyeonggi 16678, Republic of Korea # Department of Energy and Materials Engineering, Dongguk University, 30 Pildong-ro 1-gil, Jung-gu, Seoul 04620, Republic of Korea ∥ School of Polymers and High Performance Materials, University of Southern Mississippi, Hattiesburg, MS 39406, USA § Department of Graphic Arts Information Engineering, Pukyong National University, 365 Sinseon-ro, Nam-gu, Busan 48547, Republic of Korea

KEYWORDS: conjugated polymer, field-effect transistor, thiophene-thiazole, charge-transport, film thickness, contact resistance

ACS Paragon Plus Environment

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ABSTRACT We report a newly-synthesized donor (D) – acceptor (A) type semiconducting copolymer, consisting of thiophene as an electron-donating and thiazole as an electron-accepting unit (PQTBTz-TT-C8) for the active layer of the organic field-effect transistors (OFETs). Specifically, this study investigates the structure and electrical property relationships of PQTBTz-TT-C8

with

comprehensive

analyses

on

the

charge-transporting

properties

corresponding to the spin rate of the spin-coater during the formation of the PQTBTz-TT-C8 film. The crystallinity of PQTBTz-TT-C8 films are examined with grazing incidence X-ray diffraction (GIXRD). Temperature-dependent transfer measurements of the OFETs are conducted to extract the density of states (DOS) and characterize the charge-transport properties. The comparative analyses on the charge-transports within the framework of the physical model, based on polaron hopping and Gaussian DOS, reveal that the pre-factors of both physical charge transport models are independent of the spin-coating condition for the films. For staggered structural transistors, however, the thickness of the PQTBTz-TT-C8 films, which strongly affect the series resistance along the charge transfer path in a vertical direction, is changed in accordance with the spin-coating rate. In other words, the spin-coating rate of the PQTBTz-TTC8 films influence the thickness of the polymer films, yet any significant changes in the crystallinity of the film or electronic coupling between the neighboring molecules upon the spincoating condition were barely noticeable. Since the PQTBTz-TT-C8 backbone chains inside the thin film are stacked up with the edge-on, the series resistances are changed according to the thickness of the film, and thus the performance of the device varies depending on the thickness.


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INTRODUCTION Solution-processable π-conjugated semiconducting polymers have been intensively studied and developed for the last three decades due to their great potential as the best candidate materials of ‘printed electronics’ that produce electronic circuitry on flexible substrates via cost-effective graphic arts printing processes.1-6 However, further developments in the performance of the πconjugated semiconducting polymers used in printed electronics applications are still required for the expansion of their applications to high-end electronics. Further development of the semiconducting polymers can be achieved with the rational design and synthesis of the materials based on the comprehensive understanding of the charge transport in the semiconductor films as well as the optimized device structures.7-8 The semiconducting polymer ink consists of a long molecular chain that is irregular and a twisted coil in the solvent. A certain chain in the polymer ink retains such a disordered part which converts into an amorphous phase and some of the chains are rearranged and then form a crystallized phase via self-organization during the coating process.9-13 Thus, the charge transports in the semiconducting polymer films mainly takes place through randomly-distributed crystalline and amorphous domains. According to the reports on the charge transports in the π-conjugated polymer films, the rate-limiting step for the transport is the amorphous phase, which is modeled as hopping transport between the localized subunits of the twisted and entangled polymer chains (by chemical or physical defects) assuming randomly-distributed energetic and positional functions. Meanwhile, conduction within the crystallized grain requires intermolecular hopping transport accompanied with the polaronic effect in the π-conjugated polymer chains.14-19


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In this work, we report a newly-synthesized donor (D)-acceptor (A) type semiconducting copolymer consisting of thiophene as an electron-donating and thiazole as an electron-accepting unit (PQTBTz-TT-C8, Figure 1) for an active layer of high performance organic field-effect transistors (OFETs).4, 20-24 We investigate the influence of the spin-coating rate of the PQTBTzTT-C8 solution on the device performance and charge transporting characteristics to optimize the fabrication of their applications. In addition, we examined the structure and electrical property relationships of the PQTBTz-TT-C8 film with comprehensive analyses on the chargetransporting properties corresponding to the spin-speed of the spin-coater during the formation of the semiconducting copolymer film. Grazing incidence X-ray diffraction (GIXRD) was performed to analyze the crystallinity of the copolymer films. The electrical characterization to investigate the charge-transporting properties in PQTBTz-TT-C8 films is performed with OFET platforms, and then the data obtained from the temperature-dependent transfer measurements on the PQTBTz-TT-C8 based OFETs were used to extract density of states (DOS) of the PQTBTzTT-C8 films.25-26 The charge transporting properties were also analyzed and compared by applying the obtained data to the polaronic effect model and the Gaussian DOS model, respectively.14-19 The comparative analyses performed within the framework of the physical charge transport modeling, based on polaron hopping and Gaussian DOS, show that the prefactors of the physical charge transport models are independent of the spin-coating condition of the PQTBTz-TT-C8 solution, while the thickness of the PQTBTz-TT-C8 films is dependent on the spin-coating rate. As the results, the lowest contact resistance (42.7 kΩ·cm) is observed at OFETs with the highest spin-coating rate (3,000 rpm) due to the thinnest thickness of the PQTBTz-TT-C8 layer because the copolymer backbone chains inside the thin film are stacked up along the edge-on direction and the series resistance induced by the charge transfer distance is


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changed according to the thickness of the film controlled by the spin-coating rate. In the end, a reasonably high field-effect mobility (µFET) of 0.8 cm2/Vs is achieved with the CYTOP gate insulator.

EXPERIMENTAL METHODS Materials: All chemicals were purchased from Aldrich and used without additional purification. The synthetic routes and characterizations of poly(5‐[2‐(3',4‐dioctyl‐5'‐{thieno[3,2‐b]thiophen‐2‐ yl}‐[2,2'‐bithiophen]‐5‐yl)‐4‐octyl‐1,3‐thiazol‐5‐yl]‐2‐{3',4‐dioctyl‐[2,2'‐bithiophen]‐5‐yl}‐4‐ octyl‐1,3‐thiazole) (PQTBTz-TT-C8) is described in the Supporting Information. 1H NMR spectra were recorded with Bruker Avance digital 300 (300 MHz) and Bruker Avance III (600 MHz) spectrometers. All NMR spectra were referenced to the solvent.

13

C NMR spectra were

also collected. Proton (0.1 ppm) chemical shifts were measured with respect to internal TMS in CDCl3.

13

C chemical shifts were reported in ppm relative to CDCl3. In addition, TLC analyses

were conducted on aluminum sheets coated with silica gel 60 (Merck 5554). Column chromatography was performed on Merck silicagel 60. Mass spectra were measured using a Bruker, Ultraflex III TOF/TOF 200 mass spectrometer. UV-Visible absorption spectra were recorded with a Varian cary 5000 UV-Vis-NIR spectrophotometer using thin-film samples. The melting temperature of the compounds was determined using differential scanning calorimetry (DSC). Sample preparation for characterization: To confirm the morphology and molecular orientation, PQTBTz-TT-C8 thin films were spin-coated using 1 wt% solutions in chlorobenzene on


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octadecyltrichlorosilane (ODTS) treated SiO2 substrates, and annealed at 180 °C, for 1 h in an N2 atmosphere.

Figure 1. (a) Molecular structure of PQTBTz-TT-C8 and (b) illustration of the staggered (topgate bottom contact (TG/BC)) configuration.

Device fabrication and electrical characterization: The electrical characterization of the PQTBTz-TT-C8 films were performed using the staggered (top-gated and bottom-contact (TG/BC)) structural OFETs. Figure 1 shows the chemical structure of the PQTBTz-TT-C8 copolymer solution and a schematic illustration of the TG/BC-structured OFET, which was fabricated mainly using spin-coating. First, before the spin-coating process of the PQTBTz-TTC8 copolymer solution, the thermally-evaporated 13-nm thick Au thin films on a glass substrate were photo-lithographically patterned to have source/drain (S/D) electrodes with a channel length and width of 20 µm and 1000 µm, respectively. Then, the polymer thin films were spun from a 1.0 wt% tetralin solution of the PQTBTz-TT-C8 copolymers with 1,000 rpm, 2,000 rpm, and 3,000 rpm for 60 s onto the Au S/D patterned substrate. The thickness of the semiconducting copolymer films were 51.8 nm, 28.3 nm, and 20.9 nm for the spin speeds of 1,000 rpm, 2,000 rpm, and 3,000 rpm, respectively. Next, CYTOP dielectrics were spin-coated to serve as the gateinsulating layers and then annealed at 200 °C for 30 min. The thickness of the CYTOP dielectric layer is ~380 nm, resulting in a capacitance (Ci) of 4.89 nF/cm2. Finally, the Al gate electrode


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was thermally evaporated through a shadow mask onto the CYTOP gate insulator. The thickness of the Al electrode was 50 nm. The fabricated PQTBTz-TT-C8 copolymer OFETs were characterized using a Keithley 4200-SCS semiconductor analyzer connected to a MS-Tech cryogenics vacuum probe station with a chamber pressure below 10−3 Torr. Liquid nitrogen was introduced into the cryogenic probe station to acquire current-voltage (I-V) characteristics at different substrate temperatures (200–300K). Film characterization for x-ray analysis: GIXRD images were collected in grazing incidence refection mode with a 2-dimensional (2D) area detector in a helium chamber at beamline 11-3 of the Stanford Synchrotron Radiation Lightsource. The sample-to-detector distance was 400 mm and the incidence angle was 0.12 °. The X-ray wavelength was 0.9758 Å, corresponding to a beam energy of 12.7 keV. The PQTBTz-TT-C8 copolymer films were prepared on bare Si wafers with a thin layer of native oxide.


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Figure 2. (a–c) Grazing incidence X-ray diffraction patterns and (d–e) diffractogram of the PQTBTz-TT-C8 thin film measured in the: (d) out-of-plane and (e) in-plane direction, fabricated by different spin-coated speeds of 1,000 rpm (51.9 nm), 2,000 rpm (28.3 nm), and 3,000 rpm (20.9 nm).


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RESULTS Characterization of the PQTBTz-TT-C8 thin films formed with different spin rates: According to recent reports, controlling the effective π-conjugation length and intermolecular side-chain interdigitation is essential to design semiconducting polymers with high mobility, reproducible device uniformity, environmental and electrical stability.1-4 However, tightlystacked and interdigitated polymers have high-melting temperatures and a strong tendency to crystallize, which necessitates a high-processing temperature to implement these materials into printed devices. Our novel strategy for lowering the processing temperature is to partially dishevel the side chains, while maintaining the backbone mismatches in such a way that does not disturb the π-π stacking. Detailed synthesis procedures of the PQTBTz-TT-C8 are shown in Scheme S1 in the Supporting Information. The crystallinity of the PQTBTz-TT-C8 thin films coated with spin rates of 1,000 rpm (t ~ 51.8 nm), 2,000 rpm (~ 28.3 nm), and 3,000 rpm (~ 20.9 nm) was characterized by GIXRD. From Figure 2, 2D-GIXRD patterns reveal that the spin-coated PQTBTz-TT-C8 films adopt mostly edge-on chain orientations to the glass substrate for all three spin-coating conditions. The scattering peak intensity tended to decrease with the reduced thickness of the films formed at higher spin-coating speed. In more detail, only the (010) peaks (i.e., scattering signals from the polymer backbone and π‒π (010) directions) of the thin film that formed with a spin rate of 1,000 rpm were obvious in the 2D-GIXRD patterns (Figure 2 (a)), but the (010) peaks of the films coated at spin rates of 2,000 rpm and 3,000 rpm were barely identifiable (Figures 2 (b) and 2 (c)). These results suggest that the thin semiconducting copolymer film coated at higher spin speeds have a relatively disordered phase compared to the relatively thick polymer film at the lower spin speed. Figure 2(d) shows the one dimensional profile of the in-plane direction (qxy) in


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the X-ray patterns. The (010) diffraction peak in the qxy direction corresponds to the π‒π stacking distance between the conjugated backbone, which was calculated to be 3.78 Å. This stacking distance is slightly larger than those of the emerging conjugated polymers, such as diketopyrrolopyrrole and isoindigo, based on the electron donor-acceptor polymers (3.6 Å ~ 3.8 Å).27-28 On the other hand, the (100) lamella peaks in the out-of-plane direction shifted with different coating conditions. The (100) diffraction peak reduced from qz = 0.365 Å−1 for 1,000 rpm coating speed to 0.352 Å−1 for 3,000 rpm coating speed, indicating that the inter-spacing between the side chain lamella packing direction increased from 17.2 nm to 17.8 nm. Figure 2 (e) shows the full width at half maximum (FWHM) of the (100) peaks of the films coated with three different spin-coating conditions, which was fitted to obtain the coherence length for the scattering peaks. The coherence length for the (100) calculated by the Scherrer equation was found to increase from 3.43 to 3.75 nm with increasing film thickness, indicating that the quantity of the ordered phase in the spin-coated film decreases with spin coating speed. Collectively, from the GIXRD results, it is speculated that the spin-coated PQTBTz-TT-C8 film initially formed with a less ordered-phase and then the film acquired a more ordered-phase as the thickness of the film gradually increased.


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(a)

(b)

Drain Current, IDS (uA)

1000 rpm

(c) 2000 rpm

10-4 L/W = 20/1000 µm

15 3000 rpm

VDS = -60 V

10-5

10

10-6 10-7

5

10-8 0 10-9 -60 -40 -20 0 -60 -40 -20 0 -60 -40 -20 0 Gate Voltage, VGS (V) Gate Voltage, VGS (V) Gate Voltage, VGS (V)

(d)

(e)

Drain Current, IDS (uA)

1000 rpm

Drain Current1/2, |ID|1/2 (mA1/2)

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


(f) 2000 rpm

3000 rpm

-80 -60 -8 -6 -4 -2 0 2 VGS = 0 ~ -60 V

-40

-8 -6 -4 -2 0 2

-8 -6 -4 -2 0 2

with -10V steps

-20 0

1000 rpm 20 -60

-40

-20

2000 rpm 0

Drain Voltage,VDS (V)

-60

-40

-20

3000 rpm 0


-60

-40

-20

0


Figure 3. (a–c) Transfer and (d–f) output characteristics of the PQTBTz-TT-C8 OFETs fabricated using different spin rates of 1,000 rpm (51.8 nm), 2,000 rpm (28.3 nm), and 3,000 rpm (20.9 nm).

I-V characteristics of the PQTBTz-TT-C8 based OFETs. Figures 3 (a)‒(c) show the transfer curves (upper panel) obtained in the saturation (VD = -60 V) regimes and the output curves (lower panel) [Figures 3 (d)‒(f)] were measured at VG = 0 V ~ -60 V with -10 V increments. The basic device parameters are summarized in Table I. From Figure 3, the transfer and output characteristics of all three OFETs containing the PQTBTz-TT-C8 films coated using different spin speeds of 1,000 rpm, 2,000 rpm, and 3,000 rpm exhibited typical p-channel transistor characteristics with the same turn-on voltages (VT) of ~ -2 V, meaning the same level of deep-


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trap concentrations at the interface between PQTBTz-TT-C8 and the CYTOP dielectrics. The PQTBTz-TT-C8-based OFETs also showed very small hysteresis between the forward and reverse bias sweeps, indicating stable operation. The field-effect mobility (µFET) of the PQTBTzTT-C8 OFETs strongly depends on the thickness of the semiconducting film. The µFET extracted from the |ID|1/2 versus VG curves were 0.16, 0.40, and 0.80 cm2/Vs for the OFETs with the PQTBTz-TT-C8 films coated using spin speeds of 1,000 rpm, 2,000 rpm, and 3,000 rpm, respectively. The inset figures of the lower panel (Figures 3(d–f)) which exhibit enlarged output characteristics of the PQTBTz-TT-C8-based OFETs near VD = 0 V. From the inset figures, it can be clearly found that the onset of ID shifted in the negative VD direction (i.e., the barrier voltage of VD is negatively increased) and the non-linear increase of the current in the linear regime is more dominant as the film thickness increases (i.e., the lower spin rate of the PQTBTz-TT-C8 films). These output characteristics in the linear regime correlate with the charge-injection property and contact resistance between the S/D electrodes and the semiconducting copolymer film channel.29 To confirm the correlation, we extracted the contact resistance, Rc of the PQTBTz-TT-C8-based OFETs with different film thickness using the Y-function method (see Supporting Information, Figure S3).30-32 The calculated average Rc’s were 112.93 kΩ∙cm, 75.01 kΩ∙cm, and 42.70 kΩ∙cm for the OFETs with the PQTBTz-TT-C8 films coated at spin rates of 1,000 rpm, 2,000 rpm, and 3,000 rpm, respectively. This result infers that the lower value of Rc in the thinner film can be the main reason for the higher apparent mobility in the transistors, which will be discussed in the followed section.


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Table 1. Parameters of the PQTBTz-TT-C8 OFETs depending on different spin rate and thickness of the semiconducting film Spin-rate

Mobility

Thickness

Ion/Ioff

2

S.S

RC× W

µeff

(V/dec.)

(KΩ∙cm)

(cm2/V·s)

(rpm)

(nm)

(cm /V·s)

1000

51.8

0.16

> 104

1.70

112.93

0.04

2000

28.3

0.40

> 104

1.05

75.01

0.13

3000

20.9

0.80

> 104

1.31

42.70

0.28

* Solution concentration 10mg/ml

Temperature (oC) 60

13

-23

-51

0.368

-73

-91

1000 rpm 2000 rpm 3000 rpm

0.135 0.050 0.018 0.007 0.002 0.001

(b) 1000 rpm 2000 rpm 3000 rpm

1020 DOS (eV-1 cm-3)

(a)

µh, lin (cm2/V⋅s)

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


1019

T = 27 oC 1018

0.000 3.0

3.5

4.0

4.5

5.0

5.5

1000/T (K-1)

140

160

180

200

Energy above EVB (meV)

Figure 4. (a) Linear mobility of the PQTBTz-TT-C8 OFETs as a function of temperature and (b) charge DOS below the valence band (VB) fabricated from different spin rates of 1,000 rpm (51.8 nm), 2,000 rpm (28.3 nm), and 3,000 rpm (20.9 nm). The DOS are fitted with the Gaussian distribution model.

Low-temperature I-V characteristics analysis. To investigate the charge transport properties in the PQTBTz-TT-C8 films with different spin-coating conditions, the temperature-dependent


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transfer-measurements on the PQTBTz-TT-C8 -based OFETs were conducted over a temperature (T) range of 200–300K. Figure 4(a) shows the temperature dependence of the linear mobility (µlin) of the PQTBTz-TT-C8-based OFETs with spinning speed. Generally, the charge transport in such a disordered polymer semiconductor system usually proceeds via thermallyactivated hopping between the localized states, which can be expressed as follows: 

E 

µ (T ) = µ ' exp  − A   k BT 

(1)

where µ’ is the charge carrier mobility in the trap-free states, EA is the activation energy for hopping between the localized states, kB is the Boltzmann constant, and T is the absolute temperature.18-19, 25-26, 29 As expected from the measured µFET in the previous section, among the OFETs with PQTBTz-TT-C8, the film of 20.9 nm showed the smallest activation energy of 115 meV and vice versa, indicating the most efficient hopping transfer between the localized sites within the thin polymer films. However, this result contradicts the general interpretation based on the framework of the multiple trapping and release (MTR) model, which claims that the freecharge carriers can easily hop only after filling the trap sites,18-19, 29 since the OFETs with the thinnest films showed a less-ordered crystalline phase, confirmed by the wide FWHM in the scattering peaks, resulting in enhanced drain currents without any loss from the trap-related defects. Moreover, the calculated Gaussian DOS of the PQTBTz-TT-C8 films from the temperature-dependent field-effect charge transport measurement25-26, 29 shown in Figure 4 (b) have an insignificant difference on the trap density (N) as well as the width (σ) of the Gaussian distribution of trap states. Table II. summarized these results also the parameters imply that the device performance of the PQTBTz-TT-C8 OFETs with the different spin-rate conditions does not follow the general structure and electrical property relationships.


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Table II. Summary of the key charge transport parameters: activation energy (Ea), charge trap density (N), and the width of the density of the localized states (σ)

σ

Spin-rate (rpm)

Thickness (nm)

Ea (meV)

N (#/cm3)

(meV)

1000 rpm

51.8

130

5.38 × 1021

65

2000 rpm

28.3

120

3.69 × 1021

67

3000 rpm

20.9

115

1.03 × 1021

70

Comparative analysis based on the physical charge transport models. For further comprehensive understanding of the charge transport in the inhomogeneously disordered PQTBTz-TT-C8 film where the disordered amorphous phases and the ordered crystalline domains coexist, we applied the results from the low-temperature I-V characteristics measurements to the physical charge transport model: the polaron hopping and the Gaussian DOS model. First, the polaron hopping model was used to focus on the charge transfer in the ordered part of the PQTBTz-TT-C8 films which contain both the disordered phases and the ordered crystalline domains. The polaron hopping model, that is mainly applied to the chargetransporting phenomena in an organic semiconductor single crystal, analyzes the charge transfer between adjacent donor-sites and accepter-sites by applying a charge-phonon coupling (called a ‘polaron’) effect between the molecule configuration, expressed as follows:

qa 2 ω s  − Eh  µ (T ) = exp  kB T  kT 

(2)


15


Polaron Hopping Model 1000 rpm

µ (cm2/V⋅s)

10-1 10-2 10-3

Gaussian Disorder Model -5 -10 -15 -20 -25 -30 -35 -40

(d) 100

10-4

10-2 10-3

-5 -10 -15 -20 -25 -30 -35 -40

10-4

10-5

10-5 3.5

4.0 1000/T

4.5

5.0

1.5 105/T2

(b) 100 2000 rpm

10-1

1.0

(K-1)

10-2 10-3

-5 -10 -15 -20 -25 -30 -35 -40

2.0

2.5

(K-2)

(e) 100 2000 rpm

10-1

µ (cm2/V⋅s)

3.0

µ (cm2/V⋅s)

1000 rpm

10-1

µ (cm2/V⋅s)

(a) 100

10-4

10-2 10-3

-5 -10 -15 -20 -25 -30 -35 -40

10-4

10-5

10-5 3.5

4.0 1000/T

4.5

5.0

1.5 105/T2

(c) 100 3000 rpm

10-1

1.0

(K-1)

10-2 10-3 10-4

-5 -10 -15 -20 -25 -30 -35 -40

(f)

2.0

2.5

(K-2)

100

3000 rpm

10-1

µ (cm2/V⋅s)

3.0

µ (cm2/V⋅s)

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

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10-2 10-3

-5 -10 -15 -20 -25 -30 -35 -40

10-4

10-5

10-5 3.0

3.5

4.0 1000/T

4.5 (K-1)

5.0

1.0

1.5 105/T2

2.0

2.5

(K-2)

Figure 5. (a–c) The fitted results with the polaron hopping model and (d–f) Gaussian DOS model for the data obtained from the low-temperature measurements of the PQTBTz-TT-C8 OFETs with three different spin rates.


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Here, a is the hopping length determined by the distance between neighboring molecules and Eh is defined as the polaron hopping energy. Note that the pre-exponential factor in Equation (2) is valid when the phonon frequency (ωs) is small compared with the bandwidth (i.e., in the adiabatic condition).16-18 Figures 5 (a–c), show the fitted results with the polaron hopping model to the data obtained from the low-temperature measurements on the OFETs with three different PQTBTz-TT-C8 film thicknesses. On the other hand, the analyses focusing on the charge transport in the amorphous parts of the semiconducting copolymer films were performed with the Gaussian DOS model, which considers the charge transport in the disordered organic semiconductor as sequential hopping processes via localized energy states distributed in a Gaussian function. Under small electric fields, the temperature-dependent mobility is approximated as follows:

  To  2  µ (T ) = µ o exp −      T  

with

To =

2σ 3k B

(3)

where µ0 is the mobility prefactor and σ is the disorder parameter corresponding to the width of the distribution of the localized states.14-15,

18

The measured mobility values with varying

temperatures were fitted with the Gaussian DOS model (Figures 5 (d–f)).


17


(a)

(b) 240 1000 rpm 2000 rpm 3000 rpm

40

1000 rpm 2000 rpm 3000 rpm

220

Eh (meV)

ωs (×103 cm-1)

50

30 20

200 180 160

10

140

0 0

-10

-20

-30

-40

Gate Voltage, VG (V)

(c)

0

-10

-20

-30

-40


(d)

1.5

75 1000 rpm 2000 rpm 3000 rpm

1000 rpm 2000 rpm 3000 rpm

70

σ (meV)

1.2

µ0 (cm2/V⋅s)

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

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0.9 0.6

65 60

0.3 55 0.0 0

-10

-20

-30

-40


0

-10

-20

-30

-40


Figure 6. (a) The extracted parameters: [(a) phonon frequency (ωs), (b) polaron hopping energy (Eh), (c) mobility pre-factor (µ0), and (d) disorder parameter (σ)] from the polaron hopping model and Gaussian DOS model with respect to the applied gate voltages on the OFETs.

The extracted parameters from both physical charge transport models with respect to the applied gate voltages on the OFETs are plotted in Figure 6. It was found that the ωs value, a main preexponential factor extracted from the polaron hopping model focusing on the crystalline phases in the PQTBTz-TT-C8 films, is larger with thinner film thickness. In particular, from Figure 6(a), these values initially increase to the maximum value and then saturate or decrease as the absolute value of the applied gate voltage increases. It should be noted that the extracted ωs values are about one order of magnitude larger than the previously reported values.33 However, the extracted ωs values are acceptable considering the phonon can be increased under the electric


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field induced by the gate bias and the pre-exponential factors are estimated between 5 ~ 50 cm2/Vs. On the other hand, the plot of the polaron hopping energy with respect to the applied gate voltage [Figure 6 (b)] shows that the value of the polaronic hopping energy is reduced to a similar level regardless of the coating conditions of the PQTBTz-TT-C8 active layer of the OFET. The pre-factor, µ0 and the disorder parameter, σ extracted from the Gaussian DOS model for analyzing the amorphous parts in the PQTBTz-TT-C8 films are also shown in Figures 6 (c) and (d), respectively. Like the ωs values of the polaron hopping model, the extracted µ0 value also gradually increases and then becomes saturated as the applied gate voltage increases, while

σ decreases to a similar value regardless of the film thickness as the gate bias increases. Overall, the polaron hopping model and the Gaussian DOS model showed consistent results, although they focused on different phases in the PQTBTz-TT-C8 copolymer films. More importantly, from the plots of the extracted parameters with respect to the applied gate bias shown in Figure 6, it is suggested that the pre-factors of both physical charge transport models depend on the semiconducting film thickness but the polaron hopping energy or the width of distribution of the localized site are relatively independent.


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Figure 7. A schematic illustration of the PQTBTz-TT-C8-based OFETs embedded with the channels containing the probable crystalline state within the copolymer films. The right panel shows the crystalline structure of the PQTBTz-TT-C8 semiconducting copolymer.

DISCUSSION The results presented in the previous section provided two important aspects for understanding the device performance and charge transport properties of the PQTBTz-TT-C8 films embedded in the OFETs. First, it was observed that a disordered amorphous phase in the PQTBTz-TT-C8 film is initially formed and then the alkyl-side chain becomes more effectively interdigitated, which resulted in the formation of an ordered crystalline phase as the thickness of the copolymer film increases. Second, it was found that the thinner PQTBTz-TT-C8 films show higher mobility in the transistors although the relatively thin films have lower crystallinity than the thicker films. However, there was only a slight difference in the charge trap density and the width of the Gaissian distribution of the trap sites. Based on the above results, a schematic illustration of the polymer transistors embedded with the channels containing the probable crystalline-phase state within the PQTBTz-TT-C8 copolymer films is displayed in Figure 7. From Figure 7, for the OFETs with the semiconducting copolymer channels having both the mainly disordered phase


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contact with the bottom S/D electrodes in the lower region and the relatively ordered phase interfacing with the CYTOP dielectrics surface in the upper region, the charge carriers injected into the PQTBTz-TT-C8 channel from the electrode have initially experienced the disordered amorphous regions. Subsequently, the charge carriers should pass through the insulating sheath of the alkyl side chain along the edge-on direction (i.e,. the [100] direction shown in the right of Figure 7) in which the PQTBTz-TT-C8 backbone chains are stacked onto the polymer backbone to move toward the charge-accumulated channel formed just beneath the gate dielectrics surface.34-36 These two steps in the charge transporting process are reflected in the Rc value. Therefore, the OFETs with a thicker PQTBTz-TT-C8 film, which have relatively longer paths passing through the insulating sheath in the [100] (edge-on) direction, will have a larger Rc value and vice versa. On the other hand, the charge carriers in the transporting path along with the channel, where the charge accumulated by the applied gate bias is present in the PQTBTz-TT-C8 film mainly consist of the ordered-crystalline dominant phase, can move without being hampered by the trap sites. Therefore, the passage of the charge carrier across this region cannot be a ratedeterminig step in the entire charge transport process. As a result, it is expected that the the channel resistance (Rch) value reflecting this region will not be greatly influenced by the PQTBTz-TT-C8 coating conditions. This explanation agrees well with the analysis on the charge transporting property obtained from the temperature-dependent transfer curve measurements in the previous section, showing that there is no significant difference in the distribution of trap density and the number of traps in the PQTBTz-TT-C8 film, irrespective of the crystalline state of the copolymer film. In summary, the OFETs with the PQTBTz-TT-C8 film coated at the higher spin-rate show better device performance because the higher spin-coating speed resulted


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in thinner PQTBTz-TT-C8 films, which have a lower Rc value obtained by reducing the path of transports in the region of the insulating alkyl side chains.

CONCLUSION In this study, we investigated the device performance of OFETs with the newly-synthesized PQTBTz-TT-C8 copolymer films coated under different spin-rate conditions and analyzed the charge transfer characteristics in the PQTBTz-TT-C8 thin film with the physical charge transport model — the polaron hopping model and the Gaussian DOS model. Unlike what is expected from the general structural and electrical characteristics relationship, the OFETs with the channel of PQTBTz-TT-C8 having a relatively low-crystalline (disordered) phase obtained at the highest spin-coating speed showed the best device performance in terms of apparent charge carrier mobility and activation energy for charge-hopping. The comparative analyses on the charge transports by applying the results from the low-temperature experiments on the OFETs with the different PQTBTz-TT-C8 copolymer films to the polaron hopping model and Gaussian DOS model revealed that the pre-factors of both physical charge transport models are dependent on the spin-coating condition for the semiconducting polymer films while the polaron hopping energy or the width of distribution of the localized site are more or less independent. In accordance with the crystalline structure study with GIXRD and the comparative analyses with both physical charge transport modeling, it is speculated that the copolymer backbone chains inside the thin film are stacked up along the edge-on direction and the higher spin-coating speed induced the thinner PQTBTz-TT-C8 films, showing a lower Rc value through the shortened charge-transporting passage in the region consisting of the alkyl side-chain insulating sheath in the edge-on direction.


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ASSOCIATED CONTENT Supporting Information (SI) UV−Vis absorption spectra, atomic force microscope (AFM) and contact resistance fitting (by the Y-function method) are included. This information is available free of charge via the inter at http://pubs.acs.org/

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Y.-Y.N.), [email protected] (J.L.)

ORCID Jong Won Chung: 0000-0002-9799-7438 Yong-Young Noh: 0000-0001-7222-2401 Jiyoul Lee: 0000-0002-0683-9443

Author Contributions ‡

These authors contributed equally.

Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT ‡

These authors contributed equally. This work was supported by the Global Leading Technology

Program of the Office of Strategic R&D Planning (OSP) funded by the Ministry of Trade, Industry and Energy, Korea (No. 10042537); the Development of R&D Professionals on LED Convergence Lighting for Shipbuilding/Marine Plant and Marine Environments funded by the Ministry of Trade, Industry and Energy, Korea (Project No: N0001363); the Basic Science Research Program through the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning of Korea (Code No. 2015R1C1A1A02037534); and the Center for Advanced Soft-Electronics funded by the Ministry of Science, ICT and Future Planning as a Global Frontier Project (2013M3A6A5073183), respectively.


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