Photoresponsive Transistors Based on a Dual Acceptor-Containing

Subscriber access provided by Binghamton University | Libraries

Article

Photo-Responsive Transistors Based on a Dual Acceptor-Containing Low Bandgap Polymer Min Je Kim, Shinyoung Choi, Myeongjae Lee, Hyojung Heo, Youngu Lee, Jeong Ho Cho, and BongSoo Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 19 May 2017 Downloaded from http://pubs.acs.org on May 30, 2017

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 free 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 accessible to all readers and 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.

ACS Applied Materials & Interfaces 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 22

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

Photo-Responsive Transistors Based Containing Low Bandgap Polymer

on

a

Dual

Acceptor-

Min Je Kim,1,† Shinyoung Choi,2,† Myeongjae Lee,3 Hyojung Heo,4 Youngu Lee,4 Jeong Ho Cho1,* and BongSoo Kim,2,* 1

SKKU Advanced Institute of Nanotechnology, School of Chemical Engineering, Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon-si, Gyeonggi-do 16419, Republic of Korea 2 Department of Science Education, Ewha Womans University, 52 Ewhayeodae-gil, Seodaemun-gu, Seoul 03760, Republic of Korea 3 Department of Chemistry, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul, 02841, Republic of Korea 4 Department of Energy Systems Engineering, Daegu Gyeongbuk Institute of Science and Technology (DGIST), 333 Techno Jungang-daero, Hyeonpung-myeon, Dalseong-gun, Daegu 42988, Republic of Korea †

Min Je Kim and Shinyoung Choi contribute equivalently to this work.

*Corresponding authors: Professor Jeong Ho Cho: E-mail: [email protected] Professor BongSoo Kim: E-mail: [email protected]

1 ACS Paragon Plus Environment


Page 2 of 22

ABSTRACT In this manuscript, low bandgap pTTDPP–BT polymers based on electron−accepting 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

pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione

(DPP)

and

benzothiadiazole

(BT)

and

electron−donating

thienothiophene (TT) moieties were synthesized. Phototransistors have been fabricated using ambipolarbehaving pTTDPP–BT polymers as active channel materials. The electrical and photo−responsive properties of the pTTDPP–BT phototransistors were strongly dependent on the film annealing temperature. As−spun pTTDPP–BT phototransistors exhibited a low hole mobility of 0.007 cm2/(Vs) and a low electron mobility of 0.005 cm2/(Vs), which resulted in low photocurrent detection due to the limited transport of the charge carriers. Thermal treatment of the polymer thin films lead to a significant enhancement in the carrier mobilities (hole and electron mobilities of 0.066 and 0.115 cm2/(Vs), respectively, for 200 °C annealing) and thus significantly improved photo−responsive properties. The 200 °C-annealed phototransistors showed a wide-range wavelength (405‒850 nm) of photo−response, and a high photocurrent/dark-current ratio of 150 with a fast photoswitching speed of less than 100 milliseconds. This work demonstrates that a dual acceptorcontaining low band gap polymer can be an important class of materials in broadband photo-responsive transistors and the crystallinity of the semiconducting polymer layer has a significant effect on the photo−response characteristics.

Keywords: Phototransistor, Low bandgap polymer, Carrier mobility, Photo-response, Photo-switching


Page 3 of 22


1. INTRODUCTION 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

Organic pi-conjugated polymers have drawn a great deal of attention in the last few decades due to the merits of economical cost in fabrication, large−area printing compatibility, mechanical robustness, and manageable optoelectronic properties for application in polymer field-effect transistors (PFETs).1-7 Electrical performance of PFETs has been enhanced significantly through the logical design of polymer structures (i.e., both polymer backbones and substituted alkyl side chains) as well as painstaking development of the device fabrication process.8-11 Recently, a variety of donor−acceptor (D−A) lowbandgap polymers, typically consisting of electron−donating and electron−accepting groups alternating along the polymer backbone, have displayed impressive carrier mobilities.10,

12-17

In particular, carrier

mobilities over 1 cm2/(Vs) have been achieved from pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPP)−based conjugated copolymers mainly because of the large amount of interchain interactions via planar geometries of DPP moieties.18-21 It is worth noting that these D−A polymers can harvest a wide range of wavelength photons because their bandgaps are low.22-23 Phototransistors, a type of optical transducer, are considered to be a feasible candidate for use in conjugated polymers because they can detect light and amplify photo-response signals in a single device with a smaller amount of noise.24-25 Upon light-illumination on phototransistors, charge carriers, i.e. holes and electrons, were generated; holes are formed in the highest occupied molecular orbital (HOMO) and electrons are moved to the lowest unoccupied molecular orbital (LUMO) levels. Applying gate voltages facilitates carrier migration at the channel, which also contributes to photocurrent enhancement in the devices. Although phototransistors employ the principle of photodiodes, the amplifying characteristics of the transistor devices can provide higher photosensitivity compared to the photodiode; this is because both gate voltage and light illumination can tune the photocurrent.25-27 Therefore, several studies about phototransistors based on conjugated polymers have been reported.25,

28-30

However, the detectable

wavelengths of organic phototransistors have been limited to ultraviolet and short-wavelength visible light because of the relatively large bandgap of typical organic semiconductors. In this regard, wide-range light absorbing DPP-based polymers can be an important class of materials in generating photo-induced intramolecular charge separation in organic phototransistor devices. However, the photo-responsive behavior of DPP-based polymers has been rarely investigated.31-33 Moreover, the relationship between crystalline microstructures of the active channel layer and the photo−response characteristics needs to be established. In this manuscript, we report the synthesis of poly(3-(5-(benzo[c][1,2,5]thiadiazol-4-yl)thieno[3,2b]thiophen-2-yl)-2,5-bis(2-octyldodecyl)-6-(thieno[3,2-b]thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)dione) (pTTDPP–BT) containing electron–donating thienothiophenes (TTs),11,

14, 34-35

electron–accepting

DPP and benzo[c][1,2,5]thiadiazole (BT) groups,36-39 as well as its electrical and photo-responsive properties in the transistor format. TT moieties can stabilize holes, while DPP and BT units can facilitate the accommodation of electrons in the polymer backbone. The introduction of BT moieties can lower the 3 ACS Paragon Plus Environment


Page 4 of 22

LUMO level of typical oligothiphene-DPP based polymers and facilitate charge separation and electron 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

transport, which was guided by density functional theory (DFT)-calculations. As–spun pTTDPP–BT PFETs revealed ambipolar charge transport with a hole mobility of 0.007 cm2/(Vs) and an electron mobility of 0.005 cm2/(Vs). Thermal annealing increased carrier mobilities of the PFETs considerably; 200 °C‒thermal annealing increased the hole mobility to 0.066 cm2/(Vs) and the electron mobility to 0.115 cm2/(Vs). Photo– induced charge transport characteristics of the pTTDPP–BT films annealed at various temperatures was systematically investigated under illumination of various wavelength light sources. The pTTDPP–BT PFETs with low carrier mobilities exhibited a negligible photocurrent, whereas the drain current increased considerably for the thermally–annealed devices thanks to the high carrier mobilities and highly crystalline microstructure development. Optimized phototransistors exhibited a broad spectral light–response up to 850 nm and a high photo-current/dark-current ratio, as well as a fast switching response.

EXPERIMENTAL SECTION Chemicals. 4,7-Dibromo-2,1,3-benzothiadiazole, bis(pinacolato)diboron, anhydrous 1,4-dioxane, triphenyl phosphine, 2-bromothiophene, and diethylammonium diethyldithiocarbamate were bought from Alfa Aesar. 3,6-bis(thieno[3,2-b]thiophen-2-yl)DPP was bought from SunaTech Inc. (China). Anhydrous tripotassium phosphate was bought from Acros Organics, toluene from Sigma-Aldrich, catalysts such as [1,1’bis(diphenylphosphino)ferrocene]

dichloropalladium(II)

(Pd(dppf)2Cl2)

and

tris(dibenzylideneacetone)dipalladium(0) (Pd2dba3) from Strem Chemicals Inc., and Aliquat 336 from TCI C., Ltd (Japan). Potassium acetate and other organic solvents were acquired from Daejung. Tetrabutylammonium hexafluorophosphate (TBAPF6) was used as an electrolyte for cyclic voltammetry measurements. Acetonitrile was bought from Alfa Aesar. CDCl3 used for 1H-NMR spectroscopy was acquired from Cambridge Isotope Laboratories. Aside from solvents used during polymerization (anhydrous toluene and demineralized water that were deoxygenated by three freeze-pump-thaw degassing cycles in a Schlenk line), other solvents were used without further purification.

Synthesis

of

monomer

1.

4,7-dibromo-2,1,3-benzothiadiazole

(655

mg,

2.229

mmol),

bis(pinacolato)diboron (1.160 g, 4.570 mmol), Pd(dppf)2Cl2 (32.6 mg, 0.0446 mmol), and potassium acetate (1.203 g, 12.261 mmol) were added to a 50 ml flame-dried one-neck round bottom flask containing a magnetic stirring bar, and dissolved in anhydrous 1,4-dioxane (9.15 ml). The reaction was performed under an argon atmosphere. The reaction mixture was gradually heated to 80 °C and stirred vigorously for 1 d. The resulting brown mixture was cooled down to 22 °C, then quenched with demineralized H2O (10 ml). The organic layer was obtained by extraction using chloroform and demineralized H2O. The organic layer was dried over anhydrous Na2SO4, and the organic solvent was reduced through rotary evaporation. The remaining concentrate was recrystallized by boiling in hexanes (80 ml) followed by cooling back to 22 °C. The resulting brown solid was obtained by gravity filtration and rinsed with methanol, and then dried under 4 ACS Paragon Plus Environment

Page 5 of 22


high vacuum to yield 257 mg (30.0 %). 1H-NMR (300 MHz, chloroform-d, δ ppm) 8.14 (s, 2H), 1.44 (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

24H). 13C-NMR (75 MHz, chloroform-d, δ ppm) 157.14, 137.91, 84.58, 25.05. MALDI-MS: m/z: 388.14 (1H-NMR, 13C-NMR, and MALDI-MS spectra are shown in the supporting information (Figure S1 through S3)).

Synthesis of monomer 2. 3,6-bis(thieno[3,2-b]thiophen-2-yl)DPP (900 mg, 2.181 mmol), 2-octyldodecyl iodide (2,967 mg, 7.269 mmol), potassium carbonate (1,002 mg, 7.263 mmol), and 18-crown-6 (5.1 mg, 0.03 mmol) were added to a 50 ml flame-dried one-neck round bottom flask containing a magnetic stirring bar and then anhydrous DMF (30 ml) was added. The reaction was performed under an argon atmosphere. The reaction mixture was heated to 125 °C and stirred vigorously for 1 d. The resulting purple mixture was cooled down to 22 °C, then quenched with demineralized H2O (30 ml). The organic layer was obtained by extraction using dichloromethane and demineralized H2O. The organic layer was dried over anhydrous sodium sulfate (Na2SO4), and then using a rotary evaporator the organic solvent was evaporated. The crude product was refined by silica gel column chromatography (eluent: dichloromethane:hexane = 1:3) and recrystallized in dichloromethane/methanol solution to yield 2,5-bis(2-octyldodecyl)-3,6-bis(thieno[3,2b]thiophen-2-yl)DPP 170 mg (24 %). 1H-NMR (chloroform-d, δ ppm) 9.29 (s, 2H), 7.60 (d, 2H), 7.31 (d, 2H), 4.07 (d, 4H), 1.99 (m, 2H), 1.33-1.20 (64H), 0.88-0.81 (t, 12H). 13C-NMR (chloroform-d, δ ppm) 162.04, 143.57, 140.90, 140.59, 132.35, 131.54, 127.85, 119.64, 108.70, 46.89, 38.20, 32.24, 31.54, 30.3829.64, 36.55, 23.02, 14.46. MALDI-MS: m/z: 972.95 (1H-NMR spectrum,

13

C-NMR spectrum, and

MALDI-MS spectrum are shown in the supporting information (Figure S4 through S6). To a 25 ml flamedried one-neck round bottom flask containing a magnetic stirring bar, 3,6-bis(5-bromothieno[3,2b]thiophen-2-yl)-2,5-bis(2-octyldodecyl)DPP (150 mg, 0.154 mmol)was dissolved in chloroform (10 ml). The Br2 solution (55 mg, 0.309 mmol) diluted in chloroform (10 ml) was inserted to the reaction flask dropwise and waited for two hours at 22 °C. The organic solvent was reduced using a rotary evaporator and methanol was subsequently poured into the crude product, which was again recrystallized using CHCl3/methanol solution. The resulting monomer 2 was dried under high vacuum to yield 164 mg (94 %). 1

H-NMR (chloroform-d, δ ppm) 9.21 (s, 2H), 7.32 (s, 2H), 4.04 (d, 4H), 1.95 (m, 2H), 1.31-1.20 (64H),

0.88-0.82 (t, 12H). 13C-NMR (chloroform-d, δ ppm) 14.14. 22.71, 26.23, 30.05-29.33, 31.20, 31.93, 37.92, 46.62, 108.47, 119.00, 122.08, 126.88, 130.44, 140.27, 140.42, 142.00, 161.58, MALDI-MS: m/z: 1128.64. (1H-NMR spectrum,

13

C-NMR spectrum, and MALDI-MS spectrum are shown in the supporting

information (Figure S7 through S9)).

Synthesis of pTTDPP-BT. Monomer 1 (51.460 mg, 0.133 mmol), monomer 2 (150 mg, 0.133 mmol), Pd2dba3 (4.860 mg, 0.00530 mmol), PPh3 (3.480 mg, 0.0133 mmol), tripotassium phosphate (141.84 mg, 0.668 mmol), and 2 drops of Aliquat 336 were inserted to a flame-dried 10 ml one-neck round bottom flask containing a magnetic stirring bar. Anhydrous toluene (2.25 ml) and demineralized water (1.25 ml), which 5 ACS Paragon Plus Environment


Page 6 of 22

were degassed separately via four freezing-pumping-thawing cycles, were added. The reaction solution was 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

kept under Ar atmosphere. The reaction mixture was gradually heated from 60 °C to 90 °C by increasing the temperature by 5 °C every 10 min while stirring. Then, the solution was vigorously stirred at 90 °C for 40 min. 2-Bromothiophene (0.1 ml) was added to the solution, which was stirred further for 30 min. After cooling down to 22 °C, the reaction mixture was transferred to a 250 ml round bottomed flask using CHCl3 (4 ml). Diethylammonium diethyldithiocarbamate (10.0 mg) diluted with demineralized water (2 ml) was added to the solution, which was stirred vigorously and heated at 50 °C for 1 h. The desired polymer was extracted using CHCl3 (15 ml) and demineralized water (15 ml). The solvent from the collected organic layer was removed through rotary evaporation. The resulting polymer was rediluted in a small amount of CHCl3 and precipitated in methanol. The precipitated polymer was purified via Soxhlet extraction process by using solvents of methanol, acetone, hexane, and chloroform sequentially. The polymer fraction in chloroform was re-precipitated in CH3OH solution (300 ml) and was then collected by gravity filtration. The pure pTTDPP–BT polymer was dried under high vacuum to yield 107.1 mg (73.0 %). 1H-NMR (chloroform-d, δ ppm) 9.05 (s, 1H), 8.92 (s, 1H), 8.26 (s, 1H), 7.42 (s, 1H), 7.26 (s, 1H), 7.07 (s, 1H), 4.05 (m, 4H), 2.00-1.00 (q, 66H), 1.00-0.83 (t, 12H) (see Figure S10 as well). Elemental Analysis: Calculated percent C, 69.52; H, 8.02; N, 5.07; O, 2.89. Found percent C, 66.44; H, 8.08; N, 4.54; O, 2.95. Material characterization. 300 MHz Proton (1H-NMR) and 75 MHz carbon (13C-NMR) nuclear magnetic resonance spectra were measured using a Advance-300 spectrometer (Bruker). The molecular weight and polymer dispersity index (PDI) of the pTTDPP-BT polymer were determined by gel permeation chromatography (GPC) analysis, which was calibrated with poly(styrene) standards before use. A 2414 refractive index (RI) detector and a 1515 isocratic pump was appended to the GPC system, and odichlorobenzene was used as the eluent in the GPC running at 80 °C. A TGA 2100 thermogravimetric analyzer (TA) was used for thermal analyses in a N2 atmosphere at a temperature scan rate of 10 oC/minute. UV-visible (UV-vis.) absorbance was measured on a Cary 300 UV-vis spectrophotometer (Agilent technologies). Microscope slides (Paul Marienfeld GmbH&Co.KG) were exposed to UV/ozone for 20 minutes

in

advance

to

spin-casting

of

poly[3,4-ethylenedioxythiophene]-poly[styrenesulfonate]

(PEDOT:PSS) which was bought from Clevious (AI4083). PEDOT:PSS solution was spin-cast at 4,000 rpm, then dried at 110 °C for 15 minutes. On top of PEDOT:PSS substrates, pTTDPP-BT solution in CHCl3 with a concentration of 5.0 mg/ml was spin-casted at 1,000 rpm and heated on top of a hot plate at four temperatures: RT, 100 °C, 150 °C, and 200 °C. UV-vis. absorption of all the polymer films were obtained using PEDOT:PSS coated microscope slides as a reference. An Electrochemical Workstation CS120 (CorrTest instruments) was used for cyclic voltammetry (CV) under a nitrogen atmosphere at room temperature. CV was measured in a solution of TBABF6 (0.10 M) in anhydrous CH3CN and the voltage was swept at a rate of 50 mV/second. The solution was degassed by N2-purging. A Pt wire was employed as an auxiliary electrode, a polymer-coated Pt wire as a working electrode, and an Ag wire was employed as a 6 ACS Paragon Plus Environment

Page 7 of 22


pseudo-reference electrode. The potential of the Ag reference electrode was calibrated with the ferrocene 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

(Fc) oxidation potential (EFc/Fc+), which was supposed to be -4.8 eV with respect to 0.0 eV, i.e. the vacuum level. To obtain surface images of polymer films, AFM5100N (Hitachi) atomic force microscopy (AFM) with micro cantilever PRC-DF40P was run in non-contact mode. Grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements were carried out at the Pohang Accelerator Laboratory (9A beamline). The source beam (0.1115 nm) was incident on the polymer films with an angle of 0.13o. A CCD detector (model: Rayonix 2D SX165, PI-SCX: 4300) was used.

PFET fabrication and measurements. Top-contact bottom-gate (TCBG) PFETs were made for this study. A highly-doped silicon wafer having a 300 nm–thick silicon dioxide (SiO2) layer was used to fabricate PFETs based on pTTDPP–BT polymers. The SiO2 layer and the doped silicon acted as the gate dielectric layer and the electrode to apply gate biases, respectively. The organic contaminants on the SiO2 gate dielectric surface were removed with piranha (H2SO4/H2O2) solution treatment for 30 minutes at 100 degree Celsius and washed with a copious amount of deionized water. An octadecyltrichlorosilane (ODTS, Gelest, Inc.) monolayer was then treated onto the SiO2 layer to remove electron trapping sites. The pTTDPP–BT solution in chloroform at a concentration of 5 mg/ml was spin-cast onto the ODTS-treated substrates. The resulting pTTDPP-BT films were stored under high vacuum (~2×10-6 Torr) condition for 12 h. The resulting film thickness was ca. 50 nm. The pTTDPP-BT films were then annealed for a half hour in a vacuum (~2×10-6 Torr) condition at temperatures of RT, 100, 150 and 200 degree Celsius. Next, the fifty nanometer thick gold (Au) was deposited thermally through a patterned mask onto the annealed pTTDPP-BT polymer film at a pressure of ~2×10-6 Torr to form the source/drain electrodes. The channel width was 100 micrometers and the channel length were 1000 micrometers. The electrical characteristics of the pTTDPP– BT PFETs were measured at RT under dark and illuminated conditions using a Keithley 4200 parameter analyzer. The light sources were set up using monochromatic lasers (Susemicon) with various wavelengths (optical power = 4 mW, beam radius = 3 µm). The optical power was controlled using an optical attenuator (ThorlabsNDC–50C–4M) and was measured using an optical power meter (Thorlabs PM 100D).

RESULS AND DISCUSSION The synthesis of pTTDPP-BT polymer is depicted in Scheme 1. The detailed procedures are described in the Experimental section. Monomer 1 was synthesized following previous literature.40 Monomer 1 and monomer 2 were polymerized with Suzuki coupling to obtain crude pTTDPP-BT polymers. Purification of pTTDPP-BT polymer was conducted by a sequential extraction using Soxhlet procedure. The polymer in CHCl3 fraction was collected and the residual palladium catalyst in the solution was removed using diethyldithiocarbamic acid diethylammonium salt. The weight average molecular weight (Mw) of purified pTTDPP-BT polymer was 95,000 Da with a PDI of 5.6 (Figure S11). The high PDI value is a result of limited polymer solubility in hot hexane, which can be expected from the planar polymer structure based 7 ACS Paragon Plus Environment


Page 8 of 22

on multiple fused rings. pTTDPP-BT polymer can be dissolved in organic solvents like chloroform, toluene, 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

chlorobenzene, and o-dichlorobenzene. Thermogravimetric analysis revealed a 95%-decomposition temperature of 342 °C (Figure S12). Normalized UV-vis. absorption characteristics of the pTTDPP-BT polymer films placed at four temperatures (RT, 100, 150, and 200 °C) for 30 min are displayed in Figure 1a. A broad range of light absorption was observed and the absorption edge was found around 950 nm. With increasing annealing temperatures, the light absorption red-shifted gradually. Absorption peaks appeared at 711, 725, 732, and 735 nm for RT, 100, 150, and 200 °C, respectively. The red-shift upon thermal annealing reflected that the interchain interactions of polymer chains were enhanced by thermal treatment. In addition, CV measurements were carried out to evaluate the energy levels of pTTDPP-BT films. The CV curve is displayed in Figure 1b. The polymer oxidation was clearly observed and the onset of oxidation (Eoxonset) was found to be -0.49 V. The HOMO energy level of pTTDPP-BT was calculated to be -5.29 eV from Eoxonset and the assumption that the oxidation potential of the ferrocene molecule is -4.8 eV. However, the polymer’s reduction was not observed. Accordingly, the polymer’s LUMO level was acquired by considering the optical bandgap determined from the UV-vis. absorption onset. Since CV was conducted at RT, the onset wavelength of RT pTTDPP-BT film was used to calculate the optical bandgap, corresponding to 1.31 eV. Therefore, the LUMO level was determined to be -3.97 eV. Ambipolar PFET operation can be feasible with this low-lying LUMO level. Compared to the pDPPT3-OD that we have synthesized in the same method,4142

the HOMO and LUMO levels were similar. The similar characteristics in the UV-vis. absorption and the

energy levels between pTTDPP-BT and pDPPT3-OD may reflect that the thienyl-DPP-thienyl moieties appear to play a major role in determining these energy levels. DFT calculations were run to obtain the electronic structure of the pTTDPP-BT polymer using the Gaussian 09 package. For simplicity, 2-octyldodecyl groups were replaced with methyl groups, and the trimeric structure of the (TTDPP-BT)3 molecule was calculated. The geometry optimization of all model molecules in the ground state has been carried out without any symmetry constraints at the DFT level of theory, with the B3LYP (Becke, three-parameter, Lee-Yang-Parr) exchange-correlation functional and a standard 6-311G(d,p) basis set for all atoms. The DFT-calculated molecular orbitals and energy levels are displayed in Figure 2. The electronic density in the HOMO at -4.94 eV is well spread along the molecular backbone with a major contribution of thienyl-DPP-thienyl units. The electronic distribution in the LUMO at -3.47 eV is formed over the two monomeric regions with much contribution of benzothiadiazole moieties. For comparison, the pDPPT3-OD polymer was separately calculated using the same method and is displayed in Figure S13. The HOMO and LUMO levels of the pDPPT3-OD are respectively -4.93 and -3.13 eV. Note that though the HOMO levels of pTTDPP-BT and pDPPT3-OD are nearly the same, the LUMO level of pTTDPP-BT lies lower than that of pDPPT3-OD. It should be noted that though the absolute HOMO and LUMO level values are not identical with the experimental values, in particular for the LUMO levels due to the difficulty in determining directly in CV measurements, these slightly different properties 8 ACS Paragon Plus Environment

Page 9 of 22


partly explained the different electrical behavior between pTTDPP-BT and the pDPPT3-OD; that is, electron 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

transport is faster in pTTDPP-BT (see below), whereas hole transport is faster in pDPPT3-OD.41 GIWAXS patterns of pTTDPP-BT polymer thin films were acquired to examine the evolution of the crystallinity with an aid of thermal treatment. The 2D GIWAXS images of the pTTDPP-BT films annealed at 25, 100, 150, and 200 °C are shown in Figure 3. Upon thermal treatment, the crystallinity of polymer films increased; the peaks in the qz direction associated with the polymer lamellar structure became stronger and the peak in the qxy direction associated with the interchain π‒ π stacking became sharper. As-spun polymer films displayed only (100) and (200) peaks in the out-of-plane direction, whereas 200 °C-annealed films exhibited higher order peaks of (300) and (400) in the out-of-plane direction and a strong peak of (010) in the in-plane direction. Moreover, the (010) peak was radially distributed at the lower temperature, whereas it was more condensed in between qz = 0.0‒0.7 Å-1 at the higher annealing temperature. The 1D line-cut profiles in Figures 3e and 3f compare the difference between the annealed films more clearly. As the annealing temperature increased, the (100) peak was significantly intensified and sharpened in the outof-plane profile. The lamellar spacing, which was estimated from the spacing between (100) and (200) of 0.295 Å-1, was 21.3 Å. This spacing suggested that the 2-octyldodecyl chains would be interdigitated in the film state or tilted out to the aromatic backbone plane of polymer. For the in-plane profile, the (100) peak was weakened, but the (010) peak increased with annealing temperature. These observations indicated that thermal annealing enhanced the crystallinity, the long-range ordering in films, and the edge-on alignments of pTTDPP-BT polymers. The surface topographies of pTTDPP-BT films were examined by non-contact mode AFM and the surface images are presented in Figure 4. Each film was annealed at four temperatures: RT, 100, 150, and 200 degree Celsius. 25 and 100 °C-annealed films displayed simple polymer aggregates. As the annealing temperature increased to the higher temperatures of 150 and 200 °C, the polymer aggregates became connected to form networked thicker pTTDPP-BT polymer fibrils in the films, which is in accordance with the development of the long range ordering upon thermal annealing found in the above GIWAXS study. Further, the films annealed at higher temperatures exhibited rougher surface morphologies; the root meansquare (rms) values of 0.66, 0.67, 0.92, and 1.53 nm for each annealed film were found. Both the wider fibril structure and larger roughness at higher temperature can be ascribed to the increased crystalline characteristic and lengthened crystalline order, as shown in the GIWAXS results. BGTC PFETs were fabricated atop an Si substrate having an self-assembled monolayer of ODTS to investigate the electrical properties of the pTTDPP–BT semiconductor. More than 10 devices were fabricated and the electrical performances of the devices were probed. At applied drain voltages (VD) were – 60 or 60 V transfer characteristics were obtained for the PFETs based on pTTDPP-BT films annealed at 25, 100, 150, and 200 degree Celsius. The transfer characteristics exhibited V-shaped indicating the device are ambipolar (Figure 5a); hole transport was dominant at VD = −60 V and electron transport was dominant at VD = 60 V mode operations. The ION/IOFF ratios ranged from 104 to 105. Charge mobilities of the PFETs 9 ACS Paragon Plus Environment


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 22

were calculated from the respective saturation regimes of the transfer curves using ID = CsµW(VG – Vth)2/2L,43-44 where Cs is the specific capacitance of the SiO2 gate dielectric (11 nF/cm2), µ the carrier mobility, Vth the threshold voltage, W is the channel width, and L the channel length. The as–spun pTTDPPBT PFETs exhibited a hole mobility of 0.007 cm2/(Vs) and an electron mobility of 0.005 cm2/(Vs). To improve the charge transport, the pTTDPP–BT films were thermally annealed. When the film was annealed at 100 °C, the hole and electron mobilities improved slightly to 0.008 and 0.013 cm2/(Vs), respectively. However, thermal annealing above 150 °C yielded dramatic enhancement of the carrier mobilities. For example, 200 °C–annealed pTTDPP–BT PFETs exhibited a hole mobility of 0.066 cm2/(Vs) and an electron of 0.115 cm2/(Vs). The evolution of hole and electron mobilities as a function of annealing temperature was summarized in Figure 5b as well as in Table 1. In addition, the drain current level at the same VG increased dramatically, and the ION/IOFF ratio also increased with thermal treatment temperature. These enhanced electrical characteristics of the PFETs originated from the highly–developed crystalline microstructure and film morphologies of the polymers as found in the UV-vis. absorption and GIWAXS studies, which is consistent with previous reports.15-17 Figure 5c shows the output characteristics (ID vs. VD) at different values of VG related with electron or hole accumulation for the 200 °C–annealed pTTDPP-BT PFETs, which exhibited typical ambipolar characteristics, including diodic curve at low VG and saturation feature at high VG. Table 1. Hole/electron mobilities of OFETs based on pTTDPP-BT films annealed at indicated temperatures (unit: cm2/(Vs)) Temperature (°C)

RT

100

150

200

hole

0.007 (±0.001)

0.008 (±0.003)

0.035 (±0.011)

0.066 (±0.015)

electron

0.005 (±0.001)

0.013 (±0.002)

0.072 (±0.019)

0.115 (±0.049)

Carrier type

The photo−responsive properties of the PFETs based on pTTDPP-BT films were investigated. Figure 6a shows representative photo–induced transfer characteristics (total ID (dark current plus photocurrent) versus VG) at VD = 60 V under 405‒980 nm light illumination. The incident illumination power was fixed to be 1 mW. The black curve indicated the dark drain current of the device. As–spun pTTDPP–BT PFETs did not respond to light in the 405‒850 nm wavelength range because diffusion of the photo–generated charge carriers in the semiconducting layer was limited by the low hole/electron mobilities (0.007/0.005 cm2/(Vs), respectively). In contrast, 150 °C– and 200 °C–annealed pTTDPP–BT PFETs exhibited an obvious increase in the photocurrent as the wavelength decreased below 950 nm, corresponding to the optical bandgap of pTTDPP–BT. Specifically, the hole currents remained constant in the experimental error range, while the current of electrons increased significantly upon illumination. In addition, the gate voltage at the minimum conductance point shifted toward a negative direction. 10 ACS Paragon Plus Environment

Page 11 of 22


When light with a photon energy higher than the optical bandgap of pTTDPP–BT films was 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

absorbed in the film, excitons were formed and subsequently dissociated into free holes and electrons by the electric field.45-47 These charge carriers migrated to the channel region under the applied gate electrodes. Under positive gate bias, electrons were accumulated in the channel, while holes accumulated upon applying a negative gate bias. In our pTTDPP-BT phototransistors, the photo-generated electrons accumulated and facilitated migration to the channel under a positive gate bias, which resulted both in a dramatic enhancement in the electron current and in a negative shift of the gate voltage at the conductivity minimum point. In the same manner, photo-generated holes contributed to the hole current under an applied negative gate bias. However, variations of the photo–induced hole currents were negligible, as shown in the transfer curves of Figure 6a. This low photo-response in the hole current mode can be ascribed to the fact that the photo–generated holes could not contribute much to the total hole current because the high dark hole current was dominant compared to the photo–generated hole current. It should be noted that pTTDPP-BT PFETs exhibited a broad bandwidth from ultraviolet, visible, and near-infrared light due to the low bandgap of pTTDPP-BT (1.31 eV). Figure 6b shows the photocurrent changes (i.e., the differences between ID under illumination (Iillumination) and ID under dark (Idark)) versus wavelength at VG = 30 V and VD = 60 V for the thermally-annealed pTTDPP–BT PFETs. The photocurrent response increased significantly with increasing annealing temperature, which may be associated with the increased carrier mobilities of the high temperature-annealed films. The wavelength–dependent photocurrent behavior for thermally–annealed pTTDPP–BT PFETs (150 and 200 °C) agreed with the UV–vis absorption spectrum of pTTDPP–BT film (Figure 1a). The photocurrent/dark-current ratio (Iillumination/Idark) in Figure 6c indicated that the thermally– annealed pTTDPP-BT films exhibited a significantly enhanced Iillumination/Idark ratio, which is consistent with the trends of the photocurrents. For 200 °C–annealed pTTDPP-BT PFETs, the current ratio was found to be ~150 under light illumination at 405 nm and 1 mW. The high current ratio at 405 nm might be ascribed to the fact that electrons photo-excited to the higher-lying excited states have a long lifetime and have a good chance to make a transition to the lower-lying excited states,48 so that free carrier generation would be efficient. Nevertheless, the current ratio needs to be improved further for the real application, which can be done by chemical structure tuning of polymers, for instance, for improving light absorptivity and forming good morphology for better carrier transport. Figure 7a shows the illumination power–dependent photo–response of the 200 °C–annealed pTTDPP–BT PFETs under light illumination with optical powers from 10 µW to 1 mW. The photocurrents of the devices were measured at a fixed VD of 60 V and a 405 nm light illumination. The electron currents increased and the gate voltage at the minimum conductance point shifted gradually in the negative voltage direction with increasing illumination optical power. The photocurrent and Iillumination/Idark at VG = +30 V as a function of the incident optical power are summarized in Figures 7b and 7c, respectively. Both photocurrent and Iillumination/Idark became larger with the increased light power because the photogeneration of the excitons in the transistor channel increased as the illumination optical power increased.45,49 11 ACS Paragon Plus Environment


Page 12 of 22

Finally, the time–resolved photo–response of the thermally-annealed pTTDPP–BT PFETs was 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

investigated over multiple illumination cycles (405 nm and 1 mW), as shown in Figure 8. As expected, as– spun and 100 °C–annealed pTTDPP-BT PFETs exhibited negligible photoresponse. By contrast, the higher temperature–annealed devices with higher mobilities exhibited an enhanced ON–OFF switching ratio, which agrees well with the features displayed in Figure 6. For 150 °C– and 200 °C–annealed pTTDPP–BT PFETs, the drain current increased to a high value under light-illumination, called ON state, and then turned back to a low drain current value when the light disappeared, called OFF state. The rising and falling times were less than 100 milliseconds, which is the lowest measurement time that can be attained from our experimental setup. The ON–OFF switching was well maintained during perennial ON-OFF cycles, demonstrating the high stability and reproducibility of the photodetector characteristics.

CONCLUSIONS In conclusion, we synthesized a low–bandgap pTTDPP–BT semiconducting polymer and demonstrated its application to photo-transistors. Characterizations using UV-vis. absorption and GIWAXS measurements and AFM imaging indicated that the interchain interactions between polymer chains in the film state were improved by thermal annealing process. Accordingly, while as–spun pTTDPP–BT PFETs exhibited low carrier mobilities, thermal annealing was found to greatly improve not only carrier mobilities but also the photo–responsive behavior. The optimized pTTDPP–BT phototransistors exhibited a broad spectral photo-response (infrared, visible, and ultraviolet), high Iillumination/Idark ratio (150), and fast ON–OFF switching response (< 100 milliseconds).

SUPPORTING INFORMATION 1

H-NMR spectra,

13

C-NMR spectra, and MALDI-MS spectra of materials, GPC and TGA thermogram of

pTTDPP-BT. This material is available free of charge via the Internet at http://pubs.asc.org.

ACKNOWLEDGEMENTS Financial support by a grant (NRF–2015R1D1A1A01058493 and NRF-2017R1A2B2005790) from the Basic Science Program through the National Research Foundation (NRF) funded by the Ministry of Science, ICT and Future Planning, a grant (NRF–2015M1A2A2056218) from the Technology Development Program to Solve Climate Changes of the NRF, and the New & Renewable Energy of the Korea Institute of

Energy Technology Evaluation and Planning grant funded by the Korea Government Ministry of Knowledge Economy (KETEP 20163030013900).

REFERENCES (1) Li, Y.; Sonar, P.; Singh, S. P.; Soh, M. S.; van Meurs, M.; Tan, J. Annealing-Free High-Mobility Diketopyrrolopyrrole-Quaterthiophene Copolymer for Solution-Processed Organic Thin Film Transistors. J. Am. Chem. Soc. 2011, 133, 2198-2204. 12 ACS Paragon Plus Environment

Page 13 of 22

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


(2) Crone, B.; Dodabalapur, A.; Lin, Y.-Y.; Filas, R.; Bao, Z.; LaDuca, A.; Sarpeshkar, R.; Katz, H.; Li, W. Large-Scale Complementary Integrated Circuits Based on Organic Transistors. Nature 2000, 403, 521-523. (3) Facchetti, A. π-Conjugated Polymers for Organic Electronics and Photovoltaic Cell Applications. Chem. Mater. 2011, 23, 733-758. (4) Zaumseil, J.; Sirringhaus, H. Electron and Ambipolar Transport in Organic Field-Effect Transistors. Chem. Rev. 2007, 107, 1296-1323. (5) Lee, J.; Han, A.-R.; Kim, J.; Kim, Y.; Oh, J. H.; Yang, C. Solution-Processable Ambipolar Diketopyrrolopyrrole–Selenophene Polymer with Unprecedentedly High Hole and Electron Mobilities. J. Am. Chem. Soc. 2012, 134, 20713-20721. (6) Katz, H. E. Recent Advances in Semiconductor Performance and Printing Processes for Organic Transistor-Based Electronics. Chem. Mater. 2004, 16, 4748-4756. (7) Ho, P. K.; Thomas, D. S.; Friend, R. H.; Tessler, N. All-Polymer Optoelectronic Devices. Science 1999, 285, 233-236. (8) Mei, J.; Bao, Z. Side Chain Engineering in Solution-Processable Conjugated Polymers. Chem. Mater. 2014, 26, 604-615. (9) Mei, J.; Kim, D. H.; Ayzner, A. L.; Toney, M. F.; Bao, Z. Siloxane-Terminated Solubilizing Side Chains: Bringing Conjugated Polymer Backbones Closer and Boosting Hole Mobilities in Thin-Film Transistors. J. Am. Chem. Soc. 2011, 133, 20130-20133. (10) Meager, I.; Ashraf, R. S.; Rossbauer, S.; Bronstein, H.; Donaghey, J. E.; Marshall, J.; Schroeder, B. C.; Heeney, M.; Anthopoulos, T. D.; McCulloch, I. Alkyl Chain Extension as a Route to Novel Thieno [3,2-b] Thiophene Flanked Diketopyrrolopyrrole Polymers for Use in Organic Solar Cells and Field Effect Transistors. Macromolecules 2013, 46, 5961-5967. (11) He, M.; Li, J.; Sorensen, M. L.; Zhang, F.; Hancock, R. R.; Fong, H. H.; Pozdin, V. A.; Smilgies, D.M.; Malliaras, G. G. Alkylsubstituted Thienothiophene Semiconducting Materials: Structure− Property Relationships. J. Am. Chem. Soc. 2009, 131, 11930-11938. (12) Chen, M. S.; Niskala, J. R.; Unruh, D. A.; Chu, C. K.; Lee, O. P.; Fréchet, J. M. Control of PolymerPacking Orientation in Thin Films through Synthetic Tailoring of Backbone Coplanarity. Chem. Mater. 2013, 25, 4088-4096. (13) Guo, X.; Puniredd, S. R.; He, B.; Marszalek, T.; Baumgarten, M.; Pisula, W.; Müllen, K. Combination of Two Diketopyrrolopyrrole Isomers in One Polymer for Ambipolar Transport. Chem. Mater. 2014, 26, 3595-3598. (14) McCulloch, I.; Heeney, M.; Chabinyc, M. L.; DeLongchamp, D.; Kline, R. J.; Cölle, M.; Duffy, W.; Fischer, D.; Gundlach, D.; Hamadani, B.; et al. Semiconducting Thienothiophene Copolymers: Design, Synthesis, Morphology, and Performance in Thin‐Film Organic Transistors. Adv. Mater. 2009, 21, 10911109. (15) Kim, M. J.; Park, J. H.; Kang, B.; Kim, D.; Jung, A.-R.; Yang, J.; Kang, M. S.; Lee, D. Y.; Cho, K.; Kim, H.; et al. Low-Band-Gap Polymer-Based Ambipolar Transistors and Inverters Fabricated Using a Flow-Coating Method. J. Phys. Chem. C 2016, 120, 13865-13872. (16) Kim, M. J.; Jung, M.; Kang, W.; An, G.; Kim, H.; Son, H. J.; Kim, B.; Cho, J. H. Well-Balanced Carrier Mobilities in Ambipolar Transistors Based on Solution-Processable Low Band Gap Small Molecules. J. Phys. Chem. C 2015, 119, 16414-16423. (17) Kim, M. J.; Choi, J. Y.; An, G.; Kim, H.; Kang, Y.; Kim, J. K.; Son, H. J.; Lee, J. H.; Cho, J. H.; Kim, B. A New Rigid Planar Low Band Gap PTTDPP-DT-DTT Polymer for Organic Transistors and Performance Improvement through the Use of a Binary Solvent System. Dyes Pigments 2016, 126, 138-146. (18) Venkateshvaran, D.; Nikolka, M.; Sadhanala, A.; Lemaur, V.; Zelazny, M.; Kepa, M.; Hurhangee, M.; Kronemeijer, A. J.; Pecunia, V.; Nasrallah, I.; et al. Approaching Disorder-Free Transport in High-Mobility Conjugated Polymers. Nature 2014, 515, 384-388. (19) Lee, H.-S.; Lee, J. S.; Cho, S.; Kim, H.; Kwak, K.-W.; Yoon, Y.; Son, S. K.; Kim, H.; Ko, M. J.; Lee, D.-K.; et al. Crystallinity-Controlled Naphthalene-alt-Diketopyrrolopyrrole Copolymers for HighPerformance Ambipolar Field Effect Transistors. J. Phys. Chem. C 2012, 116, 26204-26213. (20) Li, W.; Hendriks, K. H.; Wienk, M. M.; Janssen, R. A. Diketopyrrolopyrrole Polymers for Organic Solar Cells. Acc. Chem. Res. 2016, 49, 78-85. 13 ACS Paragon Plus Environment


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 14 of 22

(21) Ashraf, R. S.; Meager, I.; Nikolka, M.; Kirkus, M.; Planells, M.; Schroeder, B. C.; Holliday, S.; Hurhangee, M.; Nielsen, C. B.; Sirringhaus, H.; et al. Chalcogenophene Comonomer Comparison in Small Band Gap Diketopyrrolopyrrole-Based Conjugated Polymers for High-Performing Field-Effect Transistors and Organic Solar Cells. J. Am. Chem. Soc. 2015, 137, 1314-1321. (22) Hendriks, K. H.; Li, W.; Wienk, M. M.; Janssen, R. A. Small-Bandgap Semiconducting Polymers with High near-Infrared Photoresponse. J. Am. Chem. Soc. 2014, 136, 12130-12136. (23) Li, W.; Hendriks, K. H.; Furlan, A.; Roelofs, W. C.; Wienk, M. M.; Janssen, R. A. Universal Correlation between Fibril Width and Quantum Efficiency in Diketopyrrolopyrrole-Based Polymer Solar Cells. J. Am. Chem. Soc. 2013, 135, 18942-18948. (24) Kim, F. S.; Ren, G.; Jenekhe, S. A. One-Dimensional Nanostructures of π-Conjugated Molecular Systems: Assembly, Properties, and Applications from Photovoltaics, Sensors, and Nanophotonics to Nanoelectronics. Chem. Mater. 2010, 23, 682-732. (25) Baeg, K. J.; Binda, M.; Natali, D.; Caironi, M.; Noh, Y. Y. Organic Light Detectors: Photodiodes and Phototransistors. Adv. Mater. 2013, 25, 4267-4295. (26) Liu, Y.; Wang, H.; Dong, H.; Tan, J.; Hu, W.; Zhan, X. Synthesis of a Conjugated Polymer with Broad Absorption and Its Application in High-Performance Phototransistors. Macromolecules 2012, 45, 1296-1302. (27) Mukherjee, B.; Sim, K.; Shin, T. J.; Lee, J.; Mukherjee, M.; Ree, M.; Pyo, S. Organic Phototransistors Based on Solution Grown, Ordered Single Crystalline Arrays of a π-Conjugated Molecule. J. Mater. Chem. 2012, 22, 3192-3200. (28) Zhang, G.; Guo, J.; Zhang, J.; Li, W.; Wang, X.; Lu, H.; Qiu, L. Benzodithiophenedione and Diketopyrrolopyrrole Based Conjugated Copolymers for Organic Thin-Film Transistors by Structure Modulation. Dyes Pigments 2016, 126, 20-28. (29) Nam, S.; Han, H.; Seo, J.; Song, M.; Kim, H.; Anthopoulos, T. D.; McCulloch, I.; Bradley, D. D.; Kim, Y. Ambipolar Organic Phototransistors with p-Type/n-Type Conjugated Polymer Bulk Heterojunction Light-Sensing Layers. Adv. Electron. Mater. 2016, 2, 1600264. (30) Li, F.; Chen, Y.; Ma, C.; Buttner, U.; Leo, K.; Wu, T. High-Performance Near-Infrared Phototransistor Based on N-Type Small-Molecular Organic Semiconductor. Adv. Electron. Mater. 2017, 3, 1600430. (31) Yang, D. S.; Kim, K. H.; Cho, M. J.; Jin, J. I.; Choi, D. H. Donor–Acceptor Alternating π-Conjugated Polymers Containing Di(thiophen-2-yl)pyrene and 2,5-Bis(2-octyldodecyl)pyrrolo[3,4-c]pyrrole1,4(2H,5H)dione for Organic Thin-Film Transistors. J. Polym. Sci. A Polym. Chem. 2013, 51, 1457-1467. (32) Palai, A. K.; Lee, J.; Das, S.; Lee, J.; Cho, H.; Park, S.-U.; Pyo, S. A Diketopyrrolopyrrole Containing Molecular Semiconductor: Synthesis, Characterization and Solution-Processed 1D-Microwire Based Electronic Devices. Org. Electron. 2012, 13, 2553-2560. (33) Xu, H.; Li, J.; Leung, B. H.; Poon, C. C.; Ong, B. S.; Zhang, Y.; Zhao, N. A High-Sensitivity NearInfrared Phototransistor Based on an Organic Bulk Heterojunction. Nanoscale 2013, 5, 11850-11855. (34) Wang, S.; Tang, J.-C.; Zhao, L.-H.; Png, R.-Q.; Wong, L.-Y.; Chia, P.-J.; Chan, H. S.; Ho, P. K.-H.; Chua, L.-L. Solvent Effects and Multiple Aggregate States in High-Mobility Organic Field-Effect Transistors Based on Poly (Bithiophene-Alt-Thienothiophene). Appl. Phys. Lett. 2008, 93, 162103. (35) Beaujuge, P. M.; Fréchet, J. M. Molecular Design and Ordering Effects in Π-Functional Materials for Transistor and Solar Cell Applications. J. Am. Chem. Soc. 2011, 133, 20009-20029. (36) Beaujuge, P. M.; Pisula, W.; Tsao, H. N.; Ellinger, S.; Müllen, K.; Reynolds, J. R. Tailoring Structure− Property Relationships in Dithienosilole−Benzothiadiazole Donor−Acceptor Copolymers. J. Am. Chem. Soc. 2009, 131, 7514-7515. (37) Hou, J.; Chen, H.-Y.; Zhang, S.; Li, G.; Yang, Y. Synthesis, Characterization, and Photovoltaic Properties of a Low Band Gap Polymer Based on Silole-Containing Polythiophenes and 2,1,3Benzothiadiazole. J. Am. Chem. Soc. 2008, 130, 16144-16145. (38) Zhang, M.; Tsao, H. N.; Pisula, W.; Yang, C.; Mishra, A. K.; Müllen, K. Field-Effect Transistors Based on a Benzothiadiazole−Cyclopentadithiophene Copolymer. J. Am. Chem. Soc. 2007, 129, 3472-3473. (39) Zhang, Y.; Kim, C.; Lin, J.; Nguyen, T. Q. Solution-Processed Ambipolar Field‐Effect Transistor Based on Diketopyrrolopyrrole Functionalized with Benzothiadiazole. Adv. Funct. Mater. 2012, 22, 97-105. (40) Amb, C. M.; Beaujuge, P. M.; Reynolds, J. R. Spray-Processable Blue-to-Highly Transmissive Switching Polymer Electrochromes Via the Donor–Acceptor Approach. Adv. Mater. 2010, 22, 724-728. 14 ACS Paragon Plus Environment

Page 15 of 22

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


(41) Lee, J. S.; Son, S. K.; Song, S.; Kim, H.; Lee, D. R.; Kim, K.; Ko, M. J.; Choi, D. H.; Kim, B.; Cho, J. H. Importance of Solubilizing Group and Backbone Planarity in Low Band Gap Polymers for High Performance Ambipolar Field-Effect Transistors. Chem. Mater. 2012, 24, 1316-1323. (42) Li, Y.; Sonar, P.; Murphy, L.; Hong, W. High Mobility Diketopyrrolopyrrole (DPP)-Based Organic Semiconductor Materials for Organic Thin Film Transistors and Photovoltaics. Energy. Environ. Sci. 2013, 6, 1684-1710. (43) Horowitz, G. Organic Field-Effect Transistors. Adv. Mater. 1998, 10, 365-377. (44) Kang, M. S.; Frisbie, C. D. A Pedagogical Perspective on Ambipolar Fets. ChemPhysChem 2013, 14, 1547-1552. (45) Zhu, M.; Lv, S.; Wang, Q.; Zhang, G.; Lu, H.; Qiu, L. Enhanced near-Infrared Photoresponse of Organic Phototransistors Based on Single-Component Donor–Acceptor Conjugated Polymer Nanowires. Nanoscale 2016, 8, 7738-7748. (46) Yu, H.; Bao, Z.; Oh, J. H. High-Performance Phototransistors Based on Single-Crystalline N‐Channel Organic Nanowires and Photogenerated Charge-Carrier Behaviors. Adv. Funct. Mater. 2013, 23, 629-639. (47) Hamilton, M. C.; Martin, S.; Kanicki, J. Thin-Film Organic Polymer Phototransistors. IEEE Trans. Electron Dev. 2004, 51, 877-885. (48) Skoog, D. A.; Holler, F. J.; Crouch, S. R. Principles of Instrumental Analysis, 6th ed; Brooks/Cole, Cengage Learning, 2007. (49) Lim, B. T.; Cho, J.; Cheon, K. H.; Shin, K.; Chung, D. S. Photoconductive Behaviors of Difluorinated 5, 11-Bis (Triethylsilylethynyl) Anthradithiophene. Org. Electron. 2015, 18, 113-117.



Scheme 1. Synthetic Route of pTTDPP-BT 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 16 of 22

Page 17 of 22

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) UV-vis. absorption spectra of pTTDPP-BT films annealed at RT, 100, 150, and 200 °C. (b) Cyclic voltammogram of the pTTDPP-BT film.

Figure 2. DFT-calculated molecular orbitals and energy levels of (TTDPP-BT)3.



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 18 of 22

Figure 3. 2D GIWAXS images of pTTDPP-BT polymer films annealed at (a) RT, (b) 100, (c) 150, and (d) 200 °C. 1D line-cut profiles in (e) in-plane and (f) out-of-plane directions are collected.


Page 19 of 22

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. AFM images of pTTDPP-BT polymer films annealed at (a) RT, (b) 100 °C, (c) 150 °C, and (d) 200 °C.

Figure 5. (a) Transfer characteristics (ID–VG plot) at VD values of –60 V and +60 V for the PFETs based on the pTTDPP–BT layers annealed at various temperatures (25, 100, 150, and 200 °C). (b) Carrier mobilities of the pTTDPP–BT PFETs as a function of the annealing temperature. (c) Output characteristics (ID–VD plot) of the PFETs based on the 200 °C–annealed pTTDPP–BT film.



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 20 of 22

Figure 6. (a) Representative transfer characteristics (VD = +60 V) of the PFETs based on as–spun and 200 °C–annealed pTTDPP–BT films under different illumination wavelengths at a fixed incident illumination power of 1 mW. (b) Photocurrents at VG = +30 V of the PFETs as a function of the illumination wavelength. pTTDPP–BT films were thermally annealed at the indicated temperatures. (c) Iillumination/Idark vs. wavelength of the illumination light.


Page 21 of 22

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. (a) Representative transfer characteristics (VD = +60 V) of the PFETs based on 200 °C–annealed pTTDPP–BT films under different illumination powers at a fixed wavelength of 405 nm. (b) Photocurrents at VG = 30 V of the pTTDPP–BT PFETs as a function of the illumination power. (c) Iillumination/Idark vs. illumination power.

Figure 8. Photo-switching characteristics of the PFETs based on the pTTDPP–BT films annealed at various temperatures (25 °C (black), 100 °C (red), 150 °C (green), and 200 °C (blue)) under alternating dark and light illumination (1 mW and 405 nm).



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 Figure


Page 22 of 22

Photoresponsive Transistors Based on a Dual Acceptor-Containing

Recommend Documents