Organic n‑Channel Transistors Based on[1]Benzothieno[3,2‑b]benzothiophene−Rylene Diimide Donor−Acceptor Conjugated PolymersSuman Kalyan Samanta,†,‡ Inho Song,† Jong Heun Yoo, and Joon Hak Oh*

Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Korea

*S Supporting Information

ABSTRACT: Improving the charge-carrier mobility of conjugated polymers isimportant for developing high-performance, solution-processed optoelectronicdevices. Although [1]benzothieno[3,2-b]benzothiophene (BTBT) has beenfrequently used as a high-performance p-type small molecular semiconductorand employed a few times as a building block for p-type conjugated polymers, ithas never been explored as a donor moiety for high-performance n-typeconjugated polymers. Here, BTBT has been conjugated with either n-typeperylene diimide (PDI) or naphthalene diimide (NDI) units to generate adonor−acceptor copolymer backbone, for the first time. Charge-transportmeasurements of organic field-effect transistors show n-type dominant behaviors,with the electron mobility reaching ∼0.11 cm2 V−1 s−1 for PDI−BTBT and∼0.050 cm2 V−1 s−1 for NDI−BTBT. The PDI−BTBT mobility value is one ofthe highest among the PDI-containing polymers. The high π−π stackingpropensity of BTBT significantly improves the charge-carrier mobility in thesepolymers, as supported by atomic force microscopy and grazing incidence X-ray diffraction analyses. Phototransistorapplications of these polymers in the n-type mode show highly sensitive photoresponses. Our findings demonstrate thatincorporation of the BTBT donor unit within the rylene diimide acceptor-based conjugated polymers can improve themolecular ordering and electron mobility.

KEYWORDS: conjugated polymers, BTBT, OFETs, phototransistors, charge transport

1. INTRODUCTION

Organic semiconductors consisting of donor−acceptor π-conjugated units offer many advantages, including tunableband gap properties, solution processability, and mechanicalflexibility. New donor and acceptor components for thesynthesis of conjugated copolymers are required to achievehigh-performance semiconductors for organic field-effecttransistors (OFETs) and organic photovoltaic devices.1,2 Thecharge-carrier mobility of semiconducting polymers iscurrently comparable to that of amorphous silicon, as a resultof polymer chemistry and device engineering.3 However,obtaining new conjugated polymers with high charge-transportproperties still remains challenging.Among the high-performance conjugated polymers, there

are relatively few n-type polymers compared to p-typecounterparts. Naphthalene diimide (NDI)4−10 and perylenediimide (PDI)11−16 are the most commonly used acceptorunits for the synthesis of n-type polymers. Marder and co-workers described PDI-based copolymers with dithienothio-phene17 and diethynylbenzene18 that show electron mobility(μe) up to 0.013 and 0.10 cm2 V−1 s−1, respectively. Facchettiand co-workers reported NDI−bithiophene copolymersshowing μe as high as 0.85 cm2 V−1 s−1.19,20 Recently, Kimand co-workers have reported NDI-based copolymers with

thienylene−vinylene−thienylene21 and selenophene−vinyl-ene−selenophene22 units showing μe up to 1.8 and 2.4 cm2

V−1 s−1, respectively.[1]Benzothieno[3,2-b]benzothiophene (BTBT) is one of

the best performing p-type small molecules in OFET devices,which shows high charge-transport properties, with holemobility (μh) reaching up to 43 cm2 V−1 s−1 as a result of itslayered herringbone packing by intermolecular π−π stackingand S···C interactions.23,24 While BTBT has been well exploredas a p-type small molecular semiconductor and a buildingblock for p-type polymers, their performance in n-typecopolymers has never been examined. In this regard,incorporation of BTBT within an n-type conjugated polymerbackbone may be an effective strategy to improve the mobilityof the resulting copolymer. However, a major drawback in suchdesign strategies stems from the intrinsic poor solubility ofBTBT due to strong intermolecular interactions. Takimiya etal. reported p-type copolymers of BTBT and thiophene.25

However, only low-molecular-weight polymers were generated,and the polymers did not function in OFET devices due to the

Received: June 29, 2018Accepted: August 31, 2018Published: August 31, 2018

Research Article

www.acsami.orgCite This: ACS Appl. Mater. Interfaces 2018, 10, 32444−32453

© 2018 American Chemical Society 32444 DOI: 10.1021/acsami.8b10831ACS Appl. Mater. Interfaces 2018, 10, 32444−32453

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highly intertwisted structures within the polymer backbone.Alternating donor−acceptor-type copolymers containing dike-topyrrolopyrrole and BTBT showed a poor μh (0.003 cm2 V−1

s−1).26 Meanwhile, random copolymerization with theincorporation of only a small amount of BTBT within thepolymer main chain has been shown to increase μh significantly(up to 2.47 cm2 V−1 s−1).27,28 However, an alternatingcopolymer containing BTBT as a donor unit for high-performance n-type semiconductors is hitherto unknown.Organic phototransistors (OPTs), a type of photosensitive

OFETs, have gained widespread attention recently, in whichlight detection by the semiconducting material and signalamplification can take place simultaneously in a singledevice.29,30 The photoresponsivity (R) of such devices for afew polymer-based OPTs has reached very high values,31−34

and in a special case up to ∼106 A W−1.35 However, in all ofthese cases, the semiconductor is either a p-type polymer or

the OPT devices were operated in the p-type mode. In general,the photoresponsivity values for a majority of polymers havebeen observed to remain below 5 A W−1 when operated ineither p-type mode36−40 or ambipolar mode.41,42 Althoughsmall molecular n-type semiconductors are known forphototransistor applications,43,44 n-type conjugated polymer-based OPTs have rarely been reported.45

Here, we report BTBT-based alternating donor−acceptorcopolymers NDI−BTBT and PDI−BTBT with NDI and PDI,respectively (Scheme 1). Our design strategy stems from thefact that while PDI and NDI represent high-performance n-type materials, BTBT represents a state-of-the art p-typematerial. Therefore, we conjugated these two individually bestperforming materials to generate alternating copolymers withsuperior properties contributed by both components. BTBT-containing copolymers were synthesized by Pd-catalyzed Stillecoupling reactions (Scheme 2). We performed in-depth studies

Scheme 1. Design Strategy for the Synthesis of Alternating Donor−Acceptor Conjugated Polymers Composed of BTBT andNDI/PDI Derivatives

Scheme 2. Molecular Structures of the BTBT-Containing Copolymers NDI−BTBT and PDI−BTBT

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on the structure−property relationship of the OFET devicesbased on NDI−BTBT and PDI−BTBT polymers and alsofurther applied them to OPTs. The OFET devices based onPDI−BTBT and NDI−BTBT showed n-type dominantbehaviors with electron mobility reaching ∼0.11 and ∼0.050cm2 V−1 s−1, respectively. In addition, highly sensitivephotoresponses were observed in the OPT devices of thesepolymers in the n-type mode showing photoresponsivity (R)up to 5.3 A W−1 for PDI−BTBT. Morphological and structuralanalyses revealed that the charge-carrier mobility in thesepolymers was improved significantly due to the highcrystallinity caused by the high π−π stacking propensity ofthe BTBT moiety, the unit which was not used to its fullpotential to generate high-performance conjugated polymersdue to the lack of realization of a fully working syntheticprocedure.

2. EXPERIMENTAL SECTION2.1. Materials and Methods. All reagents, starting materials, and

silica gel for thin-layer chromatography and column chromatographywere obtained from commercial sources and were used withoutfurther purification. [1]Benzothieno[3,2-b]benzothiophene (BTBT),2,6-dibromonaphthalene-1,4,5,8-tetracarboxylic dianhydride, and 1,7-dibromoperylene-3,4,9,10-tetracarboxylic dianhydride were purchasedfrom well-known commercial sources. Solvents were distilled anddried prior to use. Reactions were carried out under argon atmospherewith the use of standard and Schlenk techniques. Solution-phase 1Hand 13C NMR spectra of the monomers and polymers were recordedon a Bruker Ultrashield 400 Plus spectrometer. Chemical shifts werereported in ppm downfield from the internal standard, tetramethylsi-lane. Gel permeation chromatography measurements were performedin a Shimadzu LC solution using polystyrene as internal standard at40 °C. Matrix-assisted laser desorption/ionization mass spectrometrywas recorded in an Autoflex Speed LRF (Bruker) instrument.Thermogravimetric analysis was performed on a TGA Q50 (TAInstruments) at a heating rate of 10 °C min−1. Differential scanningcalorimetry was performed in a DSC 4000 (PerkinElmer) at a heatingrate of 10 °C min−1 in both exothermic and endothermic scans.2.2. Absorption and Photoluminescence (PL) Spectroscopy.

The absorption spectra were recorded on a Cary 5000 UV−vis−NIRspectrophotometer, and the PL spectra were recorded on an FP-6500spectrofluorometer (JASCO) in both solution (1 cm path length) andthin films. And 20 μM concentration of each polymer (NDI−BTBTor PDI−BTBT) in chloroform was prepared by stirring the solutionovernight to ensure complete dissolution of the polymers in thesolvent and was used for the solution-phase UV and PL measure-ments. Thin films of the polymers were made in a quartz plate by spincoating the polymer solutions (2 mg mL−1) in chloroform, followedby an optimal annealing process. Fluorescence quantum yields weremeasured in solution phase (50 μM in chloroform) at an excitationwavelength of 500 nm using a spectrofluorometer instrument, modelFP-8500ST (JASCO).2.3. Cyclic Voltammetry (CV) Characterization. The electro-

chemical properties were characterized by Iviumstat ElectrochemicalInterface Potentiostat using a three-electrode cell with a polished 2mm glassy carbon as the working electrode, Pt as the counterelectrode, and Ag/AgCl as the reference electrode. The electrolyticsolution employed was 0.1 M tetra-n-butylammonium hexafluor-ophosphate (n-Bu4NPF6) in dry acetonitrile at a scan rate of 100 mVs−1 under Ar atmosphere. The reference electrode was calibratedusing a ferrocene/ferrocenium redox couple as an external standard,whose redox potential is set at −4.8 eV with respect to the zerovacuum level. The lowest unoccupied molecular orbital (LUMO)levels were calculated following eq 1.46 The corresponding highestoccupied molecular orbital (HOMO) levels were calculated using theoptical HOMO−LUMO gap obtained from the onset of UV−visspectra following eqs 2 and 3.

= − − + [ ]E E E(( ) 4.8) eVLUMOCV

onset,red 1/2(ferrocene) (1)

λ= [ ]E 1240/ eVgOPT

(2)

= − [ ]E E E eVHOMOOPT

LUMOCV

gOPT

(3)

2.4. Theoretical Calculations. Density functional theory (DFT)calculations were performed using the Gaussian 09 package with theBecke, three-parameter, Lee−Yang−Parr (B3LYP) function and the6-31G(d) basis set.

2.5. Atomic Force Microscopy (AFM). An Agilent 5500(Agilent) scanning probe microscope running with a Nanoscope Vcontroller was used to obtain AFM images of the NDI−BTBT andPDI−BTBT polymer films. AFM images were obtained in the high-resolution tapping mode under ambient conditions. The polymerfilms were spin-coated from a 2 mg mL−1 solution onto theoctadecyltrichlorosilane (OTS)-treated substrate. The root-mean-square surface roughness (RRMS) was measured from AFM topo-graphic images (2 μm × 2 μm).

2.6. Grazing Incidence X-ray Diffraction (GIXD). GIXDmeasurements were performed at PLS-II 9A U-SAXS beamline ofPohang Accelerator Laboratory in Korea.

2.7. OFET Device Fabrication. Bottom-gate top-contact organicfield-effect transistors (OFETs) were fabricated using heavily dopedsilicon wafers covered with a 300 nm thick SiO2 layer (Ci = 11.5 nFcm−2). Wafers were cleaned with piranha solution for 30 min,followed by UV−ozone treatment. The wafer surface was treated withan octadecyltrichlorosilane (OTS) self-assembled monolayer. TheOTS solution (3 mM in trichloroethylene) was spin-coated onto thewafers at 1500 rpm for 30 s, and the samples were then kept overnightin a vacuum desiccator with a separate vial containing NH4OH. Thewafers were then washed with toluene, acetone, and isopropyl alcoholand dried under nitrogen. NDI−BTBT and PDI−BTBT polymersolutions in chloroform were spin-coated onto the OTS-treatedwafers. Polymer films were thermally annealed on a hot plate. Goldelectrodes (40 nm) were thermally evaporated on the polymer filmsand patterned using shadow masks. The LUMO levels of bothpolymers are relatively well matched with the work function of Auelectrodes. The source/drain patterns had a channel length (L) of 50μm and a channel width (W) of 1000 μm (W/L = 20). The optimalpostannealing treatment conditions were determined by confirmingthe electrical performance of OFETs.

2.8. Electrical Measurements. Current−voltage characteristicsof OFETs were measured inside a N2 glovebox, using a Keithley 4200-SCS semiconductor parametric analyzer. The optoelectronic proper-ties were measured in the inert gas atmosphere due to their relativelylow air stability. Monochromatic light was produced using the OrielCornerstone 130 1/8 m monochromator.

2.9. Estimation of Optoelectrical Properties.44 To investigatethe photosensitivity of OPTs, photoresponsivity (R) and photo-current/dark-current ratio (P) were calculated from transfercharacteristics coupled with light irradiation. The R and P valuesare typically defined by the following equations

= =−

RI

P

I I

Pph

inc

light dark

inc (4)

=−

PI I

Ilight dark

dark (5)

where Iph is the photocurrent, Pinc is the incident illumination poweron the channel of the device, Ilight is the drain current underillumination, and Idark is the drain current in the dark. In addition, theexternal quantum efficiency (EQE) (η) of OPTs was calculated,which can be defined as the ratio of the number of photogeneratedcarriers that practically enhances the drain current to the number ofphotons incident on the OPT channel area, using eq 6

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ηλ

=−I I hc

eP A

( )light dark

int peak (6)

where h is Plank’s constant, c is the speed of light, e is the fundamentalunit of charge, A is the area of the transistor channel, and λpeak is thepeak wavelength of the incident light.Detectivity usually describes the smallest detectable signal, which

allows comparisons of phototransistor devices with differentconfigurations and areas. D* was evaluated within this study usingeqs 7 and 8

* =DA

NEP (7)

=I

RNEP n

2

(8)

In these equations, A is the phototransistor active area, NEP is the

noise equivalent power, and In2 is the measured noise current. If the

major limit to detectivity is shot noise from the drain current underdark conditions, then D* can be simplified as

* =·

DR

e I A(2 / )dark (9)

3. RESULTS AND DISCUSSION3.1. Synthesis and Characterization of BTBT-Based

Conjugated Polymers. The key to the successful synthesis ofsoluble BTBT-containing copolymers is the length of the alkylchains attached to the polymer backbone. Branched alkylchains are often preferred over normal alkyl chains to improvethe solubility and charge-transport properties of conjugatedpolymers.47 Therefore, we synthesized dibrominated NDI (1)and PDI (2) monomers with 2-decyl-tetradecyl branched alkylchains (cf. Experimental Section, Supporting Information(SI)). 2,7-Bis(trimethylstannyl)BTBT (3) was synthesizedfrom the corresponding 2,7-dibromoBTBT (Figures S1, S2and Scheme S1). Finally, the polymers NDI−BTBT and PDI−BTBT were synthesized via a Pd-catalyzed Stille couplingreaction (Scheme 2), producing soluble and high-molecular-

weight polymers (Figure S3 and Schemes S2, S3). NDI−BTBT was more soluble in chlorinated or aromatic solventscompared to PDI−BTBT. The polymer characterization datarevealed that the molecular weight of NDI−BTBT (Mn, 90 500g mol−1; Đ, 1.6) was significantly higher than that of PDI−BTBT (Mn, 11 100 g mol−1; Đ, 1.9), probably because of thelow solubility of PDI−BTBT in the reaction medium (toluene)due to the high propensity of aggregation. However, both thepolymers showed high thermal stability (Td > 400 °C) inthermogravimetric analysis (Figure S4) and similar exothermicand endothermic transition temperatures in differentialscanning calorimetry (Figure S5), as summarized in Table 1.

3.2. Optical Properties and HOMO−LUMO GapEstimations. Absorption and photoluminescence (PL)spectra were recorded for both polymers in thin films and inchloroform solutions (Table 1). In the spin-coated films, theNDI−BTBT polymer showed absorption bands at 575/330nm, while PDI−BTBT showed bands at 570/485/348 nm(Figure 1a). The optical HOMO−LUMO gap (Eg

OPT) ofNDI−BTBT (1.77 eV) was slightly lower than that of PDI−BTBT (1.86 eV), as obtained from the onset of the absorptionspectra for the thin films. Significant red shifts in the maximumabsorption wavelength (λmax,abs) and PL wavelength (λmax,PL) ofthe thin films were observed compared to those in solutionsdue to the aggregation-induced effects (Figure S6). In addition,NDI−BTBT and PDI−BTBT exhibited fluorescence quantumyields of 0.26 and 4.90% in solution, respectively, at anexcitation wavelength of 500 nm. Cyclic voltammetry (CV)was used to estimate the LUMO energies (ELUMO

CV ) of thepolymer thin films from their corresponding reversible CVreduction waves (Figure 1b,c). The LUMO level of PDI−BTBT (−3.85 eV) was slightly low-lying compared to that ofNDI−BTBT (−3.60 eV), indicating that the PDI−BTBTpolymer is a better acceptor of electrons. However, nooxidation waves were detected in CV for these polymers,and therefore, the corresponding HOMO levels werecalculated from the optical HOMO−LUMO gap valuesfollowing the equation EHOMO

OPT = ELUMOCV − Eg

OPT (Table 1).The theoretical HOMO and LUMO values calculated using

Table 1. Physicochemical Properties of NDI−BTBT and PDI−BTBT Polymersa

polymer Mn/Đ Tendo,exo Td λmax,abssol/film λmax,PL

sol/film EgOPT

film ELUMOCV EHOMO

OPT μh μe

NDI−BTBT 90.5/1.6 238.0, 227.5 430 550, 575 695, 712 1.77 −3.60 −5.37 0.0054 0.050PDI−BTBT 11.1/1.9 239.5, 229.7 423 558, 570 670, 692 1.86 −3.85 −5.71 0.11

aMn, T, λ, E, and μ in kDa, °C, nm, eV, and cm2 V−1 s−1, respectively. Td was recorded at 5% weight loss. ELUMO was determined from the CVreduction potential onset; EHOMO

OPT = ELUMOCV − Eg

OPT. Highest μ values are listed (bottom-gate top-contact devices, saturation regime).

Figure 1. (a) UV−vis spectra in chloroform (20 μM) and spin-coated film (from a solution of 2 mg mL−1 of the polymers in chloroform); (b) CVcurves; and (c) energy level diagram of NDI−BTBT and PDI−BTBT.

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density functional theory (DFT, B3LYP/6-31G(d) level) alsosupport these experimental values (Table S1). Surprisingly, thetheoretically optimized structures showed that the dihedralangle between BTBT and PDI (53.2°) was lower than thatbetween BTBT and NDI (58.6°). This indicates a lowertorsion and a higher backbone planarity in the PDI−BTBTpolymer, which could improve charge transport (Figure S7). Inthe optimized structures, the LUMO is located mainly on theacceptor units (NDI/PDI), while the HOMO is located on thedonor BTBT unit along with a certain amount of delocalizedelectrons throughout the polymer backbone (Figures S8 andS9).3.3. OFET Device Fabrication and Charge-Transport

Properties. OFETs were fabricated to investigate the charge-transport properties using PDI−NDI−BTBT as the semi-conducting layer on n-octadecyltrichlorosilane (OTS)-modi-fied SiO2/Si substrates in a bottom-gate top-contact devicearchitecture (cf. Experimental Section, SI). Both polymersshowed n-type dominant behaviors in N2, as commonlyobserved in the rylene-based polymer OFETs,3−9 despite thepresence of BTBT. The representative I−V and output curvesof optimized OFETs (Figures 2 and S10) showed significantly

enhanced μe of the as-cast polymer films upon a gradualincrease of the annealing temperature (Ta), reaching themaximum electron mobility (μe,max) for NDI−BTBT andPDI−BTBT at optimal Ta’s of 350 and 300 °C, respectively(Table S2). In particular, the as-cast films of NDI−BTBTshowed a μe,max of 0.00061 cm2 V−1 s−1, whereas the annealedfilms showed a μe,max of 0.050 cm2 V−1 s−1. Interestingly, theannealed films of NDI−BTBT showed ambipolar character-

istics with a relatively low μh of 0.0054 cm2 V−1 s−1 due to their

energetically high-lying LUMO and HOMO levels (Figure S11and Table S3). In contrast, PDI−BTBT only showed n-typeunipolar behaviors, due to the energetically low-lying LUMOand HOMO levels. The as-cast and optimally annealed films ofPDI−BTBT showed much better μe,max’s of 0.0017 and 0.11cm2 V−1 s−1, respectively, which is one of the highest amongthe optimized PDI-based polymer films.18 However, weobserved some degree of nonlinearity in |ID|

1/2 in the transfercharacteristics of OFETs, which leads to typical downwardkink, invalidating the accurate mobility extraction, similar tothe reported polymer-based OFETs.48−50 Thus, we estimatedthe charge-carrier mobilities at the kinks48 and by calculating areliability factor (rsat)

51 to avoid mobility overestimation(Tables S2 and S3), which confirmed that the reestimatedmobilities follow the same trend of Ta variation. For bettervalidity in mobility estimation, we tested the devices in thelinear regime at a VD of 10 V and a VG of 100 V (Figure S12and Table S4). Interestingly, μlinear exhibited transfer curveswithout the kink and corresponded to the mobility trends toμe,eff estimated using the reliability factor.

3.4. Atomic Force Microscopy and Grazing IncidenceX-ray Diffraction Analyses. To elucidate the observed FETperformance, morphologies of the polymer films wereinvestigated using a tapping mode atomic force microscope(AFM) (Figure 3). The as-cast NDI−BTBT films showedsmall uniform granular structures (root-mean-square rough-ness RRMS = 0.58 nm), which upon thermal annealing at 350°C were transformed into highly developed nanofibrillarstructures with interconnected domains (RRMS = 4.40 nm)due to the thermally induced strong intermolecular inter-actions, leading to relatively better mobility in the annealeddevices. On the other hand, PDI−BTBT showed strongly self-assembled nanofibrillar networks with denser aggregates inboth as-cast and annealed films (RRMS = 0.88 and 1.21 nm,respectively), which is in agreement with the improvedmobility for PDI−BTBT in their OFETs.To further explore the crystallinities and molecular

orientations in the polymer films, two-dimensional grazingincidence X-ray diffraction (GIXD) analyses were performed(Figures 4 and S13). The annealed films of both NDI−BTBTand PDI−BTBT exhibited strong ordered lamellar peaks (n00)in the out-of-plane direction, indicating orientations of edge-ondomains.52,53 On the other hand, both films showed the (010)peak at qxy ≈ 1.5 Å−1 due to π-stacks (∼4.2 Å), indicating thepossibility of three-dimensional charge transport. Uponthermal annealing, the lamellar distance (d) and coherencelength (Lc) of the (100) order peak of PDI−BTBT at 22.7/148.4 Å in the out-of-plane direction became slightly smallerthan those of NDI−BTBT at 24.2/154.0 Å, indicative ofclosely packed organizations for the former.54 The numbers oflayers, estimated based on the ratio of Lc(100)/d(100), were6.3 and 6.5 for NDI−BTBT and PDI−BTBT, respectively,suggestive of a relatively well-ordered edge-on lamella.55 Theinfluence of thermal annealing was reflected in the d-spacing,Lc, the number of layers, and peak sharpness, suggestingenhanced crystallinities of the polymer films (Figure S13 andTable S5). We observed peaks related to the chain backbonerepeat, with orders (001) and (002).56 The chain backbonerepeat distance and Lc of the (001) peaks were 16.8/93.8 and15.84/167.3 Å, respectively, leading to the numbers of layers of5.6/10.6 for NDI−BTBT/PDI−BTBT. While the numbers oflayers for lamellar stacking were similar, the numbers of layers

Figure 2. Transfer (left) and output (right) curves of (a) NDI−BTBT and (b) PDI−BTBT OFETs after thermal annealing at 350and 300 °C, respectively. The red, blue, and gray fitting lines indicatethe regions for maximum mobility estimation, mobility at kinks, andconsidering the reliability factor (rsat), respectively.

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for the chain backbone repeat direction were quite different,which means that PDI−BTBT films have much highercrystallinities for chain backbone repeats in the in-plane

directions. This may result in improved charge-carrier mobilityin PDI−BTBT compared to NDI−BTBT. In addition, thehigher crystallinity for PDI−BTBT could be correlated to a

Figure 3. AFM height (left) and phase (right) images of the polymer films of (a, b) NDI−BTBT and (c, d) PDI−BTBT before (top) and after(bottom) thermal annealing at 350 and 300 °C, respectively (the scale bar indicates 500 nm in each case).

Figure 4. Two-dimensional GIXD images of the annealed (top) and as-cast (bottom) polymer films of (a, b) NDI−BTBT and (e, f) PDI−BTBT.The corresponding diffractogram profiles of (c, d) NDI−BTBT and (g, h) PDI−BTBT before and after thermal annealing at 350 and 300 °C,respectively.

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lower molecular weight compared to NDI−BTBT.57 There-fore, the higher electron mobility of PDI−BTBT can also beattributed to the larger crystallinity of PDI−BTBT thin films,in comparison to NDI−BTBT thin films.58

3.5. Phototransistor Applications. The optoelectronicproperties of PDI−BTBT and NDI−BTBT were investigatedby irradiating monochromatic light on OFET devices invacuum conditions (Figures 5a and S14). Upon illumination oflight in the visible region (λ = 500 nm), the source/draincurrent increased 350-fold (at VG = 16 V) with a negativethreshold voltage shift (∼4.7 V) in the transfer curves of PDI−BTBT devices, which is indicative of a photodoping effect.This was due to the generation of photoexcited charge carriersand the elimination of trap sites.42,43 The photodetectionability of the polymer films was quantified by photo-responsivity (R), photocurrent/dark-current ratio (P), externalquantum efficiency (EQE), and detectivity (D*) parametersusing the transfer characteristics coupled with light irradiation(300 μW cm−2, see details in the Experimental Section),showing an approximately 6-fold higher R value and anapproximately 5-fold higher D* value for PDI−BTBTcompared to NDI−BTBT OFETs (Figure 5b,c and Table2). Especially, the R value, one of the most important factors inOPTs, reached as high as 5.3 A W−1 for the n-type PDI−BTBT, which is better and/or comparable to most of the otherp-type or ambipolar polymer-based OPTs.36−42,59,60 Theenhanced performance of PDI−BTBT originated from thehigher extinction coefficient, greater electrical performances,

and crystalline ordering, in which higher R and EQE valuestypically result from increased exciton diffusion length andcharge-carrier mobilities for separated charges.40 In addition,the EQE values larger than 100% indicate that the highlycrystalline BTBT-containing polymer-based OPTs exhibit aphotomultiplication phenomenon, as observed in organicphotodiodes and phototransistors in several reports.44,61−63

In our OPTs, photoexcitation may generate many accumulatedholes in the semiconductor/electrode interface, resulting inphotocurrent multiplication due to the tunneling electrons,similarly to the organic photodiodes and phototransistors.43

Real-time photoresponses of NDI−BTBT and PDI−BTBTOFETs were investigated under pulsed light illumination ofdifferent intensities at 30 s intervals with an external gate bias(VG = 20 V), as shown in Figures 5d and S15. PDI−BTBTOFETs showed highly sensitive, rapid, and reversible on/offswitching of photocurrent upon the pulsed light illumination ofdifferent intensities as opposed to rather low photoresponses ofNDI−BTBT, corresponding to better light detection proper-ties of PDI−BTBT thin films due to better exciton diffusionlength and charge-carrier mobilities for charge separation. Inaddition, we tested the photoresponse speed by checking real-time current change under monochromatic light irradiation(Figure S16). PDI−BTBT OFETs exhibited short rise (<680ms) and decay times (<1.3 s) under 300 μW cm−2 of lightirradiation. To further confirm the potential of matchingillumination power and electrical signals in real-time photo-detection, we plotted current change depending on the lightpulses of different intensities from 5 to 300 μW cm−2 (FigureS17). The photocurrent enhancement as a function of lightintensity can be fitted by the power law (current ∼ P0.54),leading to a well-matched relationship between illuminationpower and electrical signals. Moreover, we found that the real-time photodetection critically depended on the field effects.PDI−BTBT OFETs exhibited highly sensitive photoswitchingat VG > 0 (20 V), leading to enhanced electron transport, while

Figure 5. (a) Transfer curves of PDI−BTBT OFETs in the dark and under light irradiation (λ = 500 nm, 300 μW cm−2). (b) R, P and (c) EQE,D* of NDI−BTBT and PDI−BTBT polymer OFETs under light irradiation (300 μW cm−2). Photoswitching behaviors of PDI−BTBT OFETsdepending on (d) light intensity (VG = 20 V and VD = 100 V) and (e) applied VG (VD = 100 V, inset showing magnified plot).

Table 2. Optoelectronic Properties of NDI−BTBT andPDI−BTBT Polymers

polymer R (A W−1) P EQE (%) D* (Jones)

NDI−BTBT 0.86 13 2.1 × 102 2.2 × 1010

PDI−BTBT 5.3 350 1.3 × 103 1.1 × 1011

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at VG ≤ 0, no light was detected (Figure 5e). In other words,additional field effects enable different photodetecting modessuch as the photoconductor mode (without gate voltage) andphototransistor mode (with gate voltage), leading tomodulation of the photodetection ability and optoelectronicperformance of our OFETs. This rapid, reversible, andadjustable real-time photodetection demonstrated the potentialfor practical optoelectronic applications.

4. CONCLUSIONSIn summary, we report the synthesis of two new alternatingdonor−acceptor copolymers composed of high-performancesmall molecular semiconductors: p-type BTBT and n-typePDI−NDI derivatives by a Pd-catalyzed Stille couplingreaction. The presence of the BTBT moiety led to a well-aggregated morphology in the polymer films due to its highπ−π stacking propensity, as evidenced from AFM and GIXDresults. This in turn led to a high μe for PDI−BTBT (0.11 cm2

V−1 s−1), which is one of the highest for PDI-based polymers.NDI−BTBT exhibited n-type dominant ambipolar behaviorsbecause of the energetically high-lying HOMO and LUMOlevels compared to those of n-type unipolar PDI−BTBT.These polymers showed highly sensitive photoresponses (R =5.3 A W−1 for PDI−BTBT) along with the photocurrent/dark-current ratio of ∼350, detectivity of 1.1 × 1011 Jones, and highresponse speed (tr = 0.68 s) in phototransistor applications forthe OPT devices operating in n-type mode. Our results showthat incorporation of a BTBT unit into an alternating donor−acceptor copolymer can improve the molecular ordering andcharge-carrier mobility. Therefore, we believe that BTBTwould be a useful donor linker for new high-performance π-conjugated polymers.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.8b10831.

Synthesis procedures, characterization data, and images(Figures S1−S17 and Tables S1−S5) (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: joonhoh@postech.ac.kr.ORCIDJoon Hak Oh: 0000-0003-0481-6069Present Address‡S.K.S.: Department of Chemistry, Indian Institute ofTechnology Kharagpur, Kharagpur 721302, IndiaAuthor Contributions†S.K.S. and I.S. contributed equally to this work. Themanuscript was written through contributions of all authors.All authors have given approval to the final version of themanuscript.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by the National ResearchF o u n d a t i o n o f K o r e a ( N R F ) g r a n t ( N o .2017R1E1A1A01074090), Nano Material Technology Devel-opment Program (No. 2017M3A7B8063825), and the Center

for Advanced Soft Electronics under the Global FrontierResearch Program (No. 2013M3A6A5073175) fundedthrough the NRF by the Ministry of Science and ICT(MSIT), Korea.

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