Organic n-Channel Transistors Based on [1]Benzothieno[3,2-b

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Organic Electronic Devices

Organic n-Channel Transistors Based on [1]Benzothieno[3,2b]benzothiophene–Rylene Diimide Donor–Acceptor Conjugated Polymers Suman Kalyan Samanta, Inho Song, Jong Heun Yoo, and Joon Hak Oh ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10831 • Publication Date (Web): 31 Aug 2018 Downloaded from http://pubs.acs.org on August 31, 2018

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ACS Applied Materials & Interfaces

Organic n-Channel Transistors Based on [1]Benzothieno[3,2-b]benzothiophene–Rylene Diimide Donor–Acceptor Conjugated Polymers Suman 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 Keywords: conjugated polymers, BTBT, OFETs, phototransistors, charge transport

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ABSTRACT

Improving the charge carrier mobility of conjugated polymers is important to developing highperformance,

solution-processed

optoelectronic

devices.

Although

[1]benzothieno[3,2-

b]benzothiophene (BTBT) has been frequently used as a high-performance p-type small molecular semiconductor and employed a few times as a building block for p-type conjugated polymers, it has never been explored as a donor moiety for high-performance n-type conjugated polymers. Here, BTBT has been conjugated with either n-type perylene diimide (PDI) or naphthalene diimide (NDI) units to generate donor-acceptor copolymer backbone, for the first time. Charge transport measurements of the organic field-effect transistors (OFETs) 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 of the highest among the PDI-containing polymers. The high π-π stacking propensity of BTBT significantly improves the charge carrier mobility in these polymers, as supported by atomic force microscopy and grazing incidence X-ray diffraction analyses. Phototransistor applications of these polymers in the n-type mode show highly sensitive photoresponses. Our findings demonstrate that incorporation of the BTBT donor unit within the rylene diimide acceptor-based conjugated polymers can improve the molecular ordering and electron mobility.


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1. Introduction Organic semiconductors consisting of donor-acceptor π-conjugated units offer many advantages including tunable band-gap properties, solution processability, and mechanical flexibility. New donor and acceptor components for the synthesis of conjugated copolymers are required to achieve high-performance semiconductors for organic field-effect transistors (OFETs) and organic photovoltaic devices.1-2 The charge carrier mobility of semiconducting polymers is currently comparable with that of amorphous silicon, as a result of the polymer chemistry and device engineering.3 However, obtaining new conjugated polymers with high charge transport properties still remains challenging. Among the high-performing conjugated polymers, there are relatively few n-type polymers compared with p-type counterparts. NDI4-10 and PDI11-16 are the most commonly used acceptor units for the synthesis of n-type polymers. Marder and coworkers described PDI-based copolymers with dithienothiophene17 and diethynylbenzene18 that show electron mobility (μe) up to 0.013 and 0.10 cm2 V1 s1, respectively. Facchetti and coworkers reported NDI-bithiophene copolymers showing μe as high as 0.85 cm2 V1 s1.19-20 Recently, Kim and coworkers reported NDI-based copolymers with thienylene-vinylene-thienylene21 and selenophene-vinylene-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, and it shows high charge transport properties, with hole mobility (μh) reaching up to 43 cm2 V1 s1 as a result of its layered herringbone packing by intermolecular ππ stacking and S···C interactions.23-24 While BTBT has been well-explored as a p-type small molecular semiconductor and a building block for p-type polymers, their performance in n-type


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copolymers has never been examined. In this regard, incorporation of BTBT within an n-type conjugated polymer backbone may be an effective strategy to improve the mobility of the resulting copolymer. However, a major drawback in such design strategies stems from the intrinsic poor solubility of BTBT due to strong intermolecular interactions. Takimiya et al. 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 highly intertwisted structures within the polymer backbone. Alternating donor-acceptor type copolymers containing diketopyrrolopyrrole and BTBT showed a poor μh (0.003 cm2 V1 s1).26 Meanwhile, random copolymerization with the incorporation of only a small amount of BTBT within the polymer main chain has been shown to increase μh significantly (up to 2.47 cm2 V1 s1).27-28 However, an alternating copolymer containing BTBT as a donor unit for high-performing n-type semiconductors is hitherto unknown. Organic phototransistors (OPTs), a type of photosensitive OFETs are gaining widespread attention recently, in which light detection by the semiconducting material and signal amplification can take place simultaneously in a single device.29-30 The photoresponsivity (R) of such devices for a few 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 these cases the semiconductor is either a p-type polymer or the OPT devices were operated in p-type mode. In general, the photoresponsivity values for a majority of polymers have been observed to remain below 5 A W1 when operated either with p-type mode3640

or ambipolar mode.41-42 Although small molecular n-type semiconductors are known for

phototransistor applications,43-44 n-type conjugated polymer-based OPTs have rarely been reported.45


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Here, we report BTBT-based alternating donor-acceptor copolymers NDI-BTBT and PDI-BTBT with NDI and PDI, respectively (Scheme 1). Our design strategy stems from the fact that while PDI and NDI represent high-performing n-type materials, BTBT represents a state-of-the art ptype material. Therefore, we conjugated these two individually best performing materials to generate alternating copolymers with superior properties contributed by both components. BTBTcontaining copolymers were synthesized by the Pd-catalyzed Stille-coupling reactions (Scheme 2). We performed in-depth studies on the structure-property relationship of the OFET devices based on NDI-BTBT and PDI-BTBT polymers, and also further applied them to OPTs. The OFET devices based on PDI-BTBT and NDI-BTBT showed n-type dominant behaviors with electron mobility reaching ~0.11 cm2 V1 s1 and ~0.050 cm2 V1 s1, respectively. In addition, highly sensitive photoresponses were observed in the OPT devices of these polymers in the n-type mode showing photoresponsivity (R) up to 5.3 A W1 for PDI-BTBT. Morphological and structural analyses revealed that the charge carrier mobility in these polymers was improved significantly due to the high crystallinity caused by high π-π stacking propensity of BTBT moiety, the unit which was not used to its full potential to generate high-performing conjugated polymers due to the lack of realization of a fully-working synthetic procedure.

2. Experimental Section 2.1. Materials and Methods. All reagents, starting materials, and silica gel for TLC and column chromatography were obtained from the commercial sources and were used without further purification. [1]Benzothieno[3,2-b]benzothiophene (BTBT), 2,6-dibromonaphthalene-1,4,5,8tetracarboxylic dianhydride and 1,7-dibromoperylene-3,4,9,10-tetracarboxylic dianhydride were


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purchased from well-known commercial sources. Solvents were distilled and dried prior to use. Reactions were carried out under argon atmosphere with the use of standard and Schlenk techniques. Solution phase 1H and 13C NMR of the monomers and polymers were recorded on a Bruker Ultrashield 400 Plus spectrometer. Chemical shifts were reported in ppm downfield from the internal standard, tetramethylsilane. Gel permeation chromatography (GPC) measurements were performed in a Shimadzu LC solution using polystyrene as internal standards at 40 oC. MALDI mass spectrometry was recorded in Autoflex Speed LRF (Bruker) instrument. Thermogravimetric analysis was performed on a TGA Q50 (TA Instruments) with a heating rate of 10 oC min-1. Differential scanning calorimetry was performed in a DSC 4000 (PerkinElmer) with a heating rate 10 oC min-1 in both exothermic and endothermic scans. 2.2. Absorption and PL Spectroscopy. The absorption spectra were measured on a Cary 5000 UV-Vis-NIR spectrophotometer and PL spectra were recorded on an FP-6500 spectrofluorometer (JASCO) both in solution (1 cm path length) and in thin films. A 20 μM concentration of each polymer (NDI-BTBT or PDI-BTBT) in chloroform was prepared by stirring the solution overnight to ensure complete dissolution of the polymers in the solvent and was used for the solution phase UV and PL measurements. Thin-film of the polymers were made in quartz plate by spin coating the polymer solutions (2 mg mL-1) in chloroform followed by optimal annealing process. Fluorescence quantum yields were measured in solution phase (50 μM in chloroform) at an excitation wavelength of 500 nm using a spectrofluorometer instrument, model FP-8500ST. 2.3. Cyclic Voltammetry Characterization. The electrochemical properties were characterized by Iviumstat Electrochemical Interface Potentiostat using a three-electrode cell with a polished 2 mm glassy carbon as working electrode, Pt as counter electrode, and Ag/AgCl as reference electrode.

The

electrolytic

solution

employed

were

0.1

M

tetra-n-butylammonium


6

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hexafluorophosphate (n-Bu4NPF6) in dry acetonitrile at a scan rate of 100 mV s-1 under Ar atmosphere. The reference electrode was calibrated using a ferrocene/ferrocenium redox couple as external standard, whose redox potential is set at –4.8 eV with respect to zero vacuum level. The lowest unoccupied molecular orbital (LUMO) levels were calculated following equation (1).46 Corresponding highest occupied molecular orbital (HOMO) levels were calculated using the optical HOMO-LUMO gap obtained from the onset of UV-vis spectra following equation (2) and (3). 𝐶𝑉 𝐸𝐿𝑈𝑀𝑂 = −((𝐸𝑜𝑛𝑠𝑒𝑡,𝑟𝑒𝑑 − 𝐸1/2(𝐹𝑒𝑟𝑟𝑜𝑐𝑒𝑛𝑒) ) + 4.8) [𝑒𝑉]

(1)

𝐸𝑔𝑂𝑃𝑇 = 1240/𝜆 [𝑒𝑉]

(2)

𝑂𝑃𝑇 𝐶𝑉 𝐸𝐻𝑂𝑀𝑂 = 𝐸𝐿𝑈𝑀𝑂 − 𝐸𝑔𝑂𝑃𝑇 [𝑒𝑉]

(3)

2.4. Theoretical Calculations. Density Functional Theory (DFT) calculations were performed using the Gaussian 09 package with the Becke, 3-parameter, Lee-Yang-Parr (B3LYP) function and the 6-31G(d) basis set. 2.5. Atomic Force Microscopy (AFM). An Agilent 5500 (Agilent, USA) scanning probe microscope (SPM) running with a Nanoscope V controller was used to obtain AFM images of the NDI-BTBT and PDI-BTBT polymer films. AFM images were obtained in the high-resolution tapping mode under ambient conditions. The polymer films were spin-coated from a 2 mg mL1 solution onto the OTS-treated substrate. Root-mean-square surface roughness (RRMS) was measured from AFM topographic images (2 μm × 2 μm). 2.6. Grazing Incidence X-ray Diffraction (GIXD). GIXD measurements were performed at PLS-II 9A U-SAXS beamline of Pohang Accelerator Laboratory in Korea.


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2.7. OFET Device Fabrication. Bottom-gate top-contact (BGTC) organic field-effect transistors (OFETs) were fabricated using heavily doped silicon wafers covered with a 300-nmthick SiO2 layer (Ci=11.5 nF cm2). Wafers were cleaned with a piranha solution for 30 min, followed by UV-ozone treatment. The wafer surface was treated with an octadecyltrichlorosilane (OTS) self-assembled monolayer. The OTS solution (3 mM in trichloroethylene) was spin-coated onto the wafers at 1500 rpm for 30 s, and the samples then kept overnight in a vacuum desiccator with a separate vial containing NH4OH. The wafers were then washed with toluene, acetone, and isopropyl alcohol and dried under nitrogen. Polymer NDI-BTBT and PDI-BTBT solutions in chloroform were spin-coated onto the OTS-treated wafers. Polymer films were thermally annealed on a hot plate. Gold electrodes (40 nm) were thermally evaporated on the polymer films and patterned using shadow masks. The LUMO levels of both polymers are relatively well-matched with the work function of Au electrodes. The source/drain patterns had a channel length (L) of 50 μm and a channel width (W) of 1000 μm (W/L = 20). The optimal post annealing treatment conditions were determined by confirming the electrical performance of OFETs. 2.8. Electrical Measurements. Current–voltage characteristics of OFETs were measured inside a N2 glove box, using the Keithley 4200-SCS semiconductor parametric analyzer. The optoelectronic properties were measured in the inert gas atmosphere due to their relatively low air stability. The monochromatic light was produced using the Oriel Cornerstone 130 1/8m monochromator. 2.9. Estimation of Optoelectrical Properties.44 In order to investigate photosensitivity for OPTs, photoresponsivity (R) and photocurrent/dark-current ratio (P) were calculated from transfer characteristics coupled with light irradiation. The R and P values are typically defined by the following equations:


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𝑅=

𝑃=

𝐼ph

=

𝑃inc

𝐼light −𝐼dark 𝑃inc

𝐼light −𝐼dark 𝐼dark

(1)

(2)

where Iph is the photocurrent, Pinc the incident illumination power on the channel of the device, Ilight the drain current under illumination, and Idark the drain current in the dark, respectively. In addition, the external quantum efficiency (EQE) (η) of OPTs was calculated which can be defined as the ratio of number of photogenerated carriers that practically enhances the drain current to the number of photons incident onto the OPT channel area, using the following equation (3):

𝜂=

(𝐼𝑙𝑖𝑔ℎ𝑡 −𝐼𝑑𝑎𝑟𝑘 )ℎ𝑐 𝑒𝑃𝑖𝑛𝑡 𝐴𝜆𝑝𝑒𝑎𝑘

(3)

where h is the plank constant, c the speed of light, e the fundamental unit of charge, A the area of the transistor channel, and λpeak the peak wavelength of the incident light, respectively. Detectivity usually describes the smallest detectable signal, which allows comparisons of phototransistor devices with different configurations and areas. D* was evaluated within this study using the following Equations (4) and (5): √𝐴

𝐷∗ = 𝑁𝐸𝑃

𝑁𝐸𝑃 =

√̅̅̅ 𝐼𝑛2 𝑅

(4)

(5)

In these equations, A is the phototransistor active area, NEP the noise equivalent power, and 𝐼̅𝑛2 the measured noise current. If the major limit to detectivity is shot noise from the drain current under dark conditions, D* can be simplified as:


9


𝐷∗ =

Page 10 of 31

𝑅

(6)

√(2𝑒∙𝐼𝑑𝑎𝑟𝑘 /𝐴)

3. Results and Discussion 3.1. Synthesis and Characterization of the BTBT-based Conjugated Polymers. The key to successful synthesis of soluble BTBT-containing copolymer is the length of the alkyl chains attached to the polymer backbone. Branched alkyl chains are often preferred over normal alkyl chains to improve the solubility and charge transport properties of conjugated polymers.47 Therefore, we synthesized dibrominated NDI (1) and PDI (2) monomers with 2-decyl-tetradecyl branched alkyl chains (cf. Experimental Section, Supporting Information (SI)). The 2,7bis(trimethylstannyl)BTBT (3) was synthesized from corresponding 2,7-dibromoBTBT (Figure S1 and S2 and Scheme S1). Finally, the polymers NDI-BTBT and PDI-BTBT were synthesized via the Pd-catalyzed Stille-coupling reaction (Scheme 2), producing soluble and high-molecularweight polymers (Figure S3 and Scheme S2 and S3). NDI-BTBT was more soluble in chlorinated or aromatic solvents compared with PDI-BTBT. The polymer characterization data revealed that the molecular weight of NDI-BTBT (Mn 90,500 g 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 the low solubility of PDI-BTBT in the reaction medium (toluene) due to high propensity of aggregation. However, both the polymers showed high thermal stability (Td > 400 C) in thermogravimetric analysis (Figure S4) and similar exothermic and endothermic transition temperatures in differential scanning calorimetry (Figure S5), as summarized in Table 1. 3.2. Optical

Properties and

HOMO-LUMO

gap

Estimations.

Absorption and

photoluminescence (PL) spectra were recorded for both polymers in thin films and in chloroform solutions (Table 1). In the spin-coated films, the NDI-BTBT polymer showed absorption bands at


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575/330 nm, while PDI-BTBT showed bands at 570/485/348 nm (Figure 1a). The optical HOMOLUMO gap (𝐸𝑔𝑂𝑃𝑇 ) of NDI-BTBT (1.77 eV) was slightly lower than that of PDI-BTBT (1.86 eV), as obtained from the onset of the absorption spectra for the thin films. Significant red-shifts in the maximum absorption wavelength (λmax,abs) and PL wavelength (λmax,PL) of the thin films were observed compared with those in solutions due to the aggregation-induced effects (Figure S6). In addition, NDI-BTBT and PDI-BTBT exhibited fluorescence quantum yields of 0.26% and 4.90% in solution at an excitation wavelength of 500 nm, respectively. Cyclic voltammetry (CV) was 𝐶𝑉 used to estimate the LUMO energies (𝐸𝐿𝑈𝑀𝑂 ) of the polymer thin films from their corresponding

reversible CV reduction waves (Figure 1b and c). The LUMO level of PDI-BTBT (–3.85 eV) was slightly low-lying compared with that of NDI-BTBT (–3.60 eV), indicating that the PDI-BTBT polymer is a better acceptor of electrons. However, no oxidation waves were detected in CV for these polymers, and therefore, corresponding HOMO levels were calculated from the optical 𝑂𝑃𝑇 𝐶𝑉 HOMO-LUMO gap values following the equation 𝐸𝐻𝑂𝑀𝑂 = 𝐸𝐿𝑈𝑀𝑂 − 𝐸𝑔𝑂𝑃𝑇 (Table 1). The

theoretical HOMO and LUMO values calculated using the density functional theory (DFT, B3LYP/6-31G(d) level) also support these experimental values (Table S1). Surprisingly, the theoretically optimized structures showed that the dihedral angle between BTBT and PDI (53.2o) was lower than that between BTBT and NDI (58.6o). This indicates a lower torsion and a higher backbone planarity in the PDI-BTBT polymer, which could improve charge transport (Figure S7). In the optimized structures, the LUMO is located mainly on the acceptor units (NDI/PDI); while the HOMO is located on the donor BTBT unit along with a certain extent of delocalized throughout the polymer backbone (Figure S8 and S9). 3.3. OFET Device Fabrication and Charge Transport Properties. OFETs were fabricated to investigate charge-transport properties using PDI/NDI-BTBT as the semiconducting layer on n-


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octadecyltrichlorosilane (OTS)-modified SiO2/Si substrates in a bottom-gate top-contact device architecture (cf. Experimental Section, SI). Both polymers showed n-type dominant behaviors in N2, as commonly observed in the NDI/PDI-based polymer OFETs,3-9 despite the presence of BTBT. The representative I-V and output curves of optimized OFETs (Figure 2 and S10) showed significantly enhanced μe of the as-cast polymer films upon gradual increase of the annealing temperature (Ta), reaching the maximum electron mobility (μe,max) for NDI-BTBT and PDI-BTBT at an optimal Ta of 350 and 300 C, respectively (Table S2). In particular, the as-cast films of NDIBTBT showed a μe,max of 0.00061 cm2 V1 s1, whereas the annealed films showed a μe,max of 0.050 cm2 V-1 s-1. Interestingly, the annealed films of NDI-BTBT showed ambipolar characteristics with a relatively low μh of 0.0054 cm2 V1 s1 due to their energetically high-lying LUMO and HOMO levels (Figure S11, Table S3). In contrast, PDI-BTBT only showed n-type unipolar behaviors, due to the energetically low-lying LUMO and HOMO levels. The as-cast and optimally annealed films of PDI-BTBT showed a much better μe,max of 0.0017 and 0.11 cm2 V1 s1, respectively, which is one of the highest among the optimized PDI-based polymer films.18 However, we observed some degree of non-linearity in |ID|1/2 in the transfer characteristics of OFETs, which leads to typical downward kink, invalidating the accurate mobility extraction, similarly to the reported polymer based OFETs.48-50 Thus, we estimated the charge carrier mobilities at the kinks48 and by calculating a reliability factor (rsat)51 to avoid mobility overestimation (Table S2 and S3), which confirmed that the re-estimated mobilities follow the same trend of Ta variation. For better validity in mobility estimation, we tested the devices in linear regime at VD of 10 V and VG of 100 V (Figure S12 and Table S4). Interestingly, μlinear exhibited transfer curves without the kink and corresponded to the mobility trends to μe,eff estimated using the reliability factor.


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3.4. Atomic Force Microscopy and Grazing Incidence X-ray Diffraction Analyses. To elucidate the observed FET performance, morphologies of the polymer films were investigated using a tapping-mode atomic force microscope (AFM) (Figure 3). The as-cast NDI-BTBT films showed small uniform granular structures (root-mean-square roughness RRMS = 0.58 nm), which upon thermal annealing at 350 C were transformed into highly developed nanofibrillar structures with interconnected domains (RRMS = 4.40 nm) due to the thermally induced strong intermolecular interactions, leading to a relatively better mobility in the annealed devices. On the other hand, PDIBTBT showed strongly self-assembled nanofibrillar networks with denser aggregates in both ascast and annealed films (RRMS = 0.88 and 1.21 nm, respectively), which is in agreement with the improved mobility for PDI-BTBT in their OFETs. To further explore the crystallinities and molecular orientations in the polymer films, twodimensional (2D) grazing incidence X-ray diffraction (GIXD) analyses were performed (Figure 4 and S13). The annealed films of both NDI-BTBT and PDI-BTBT exhibited strong ordered lamellar peaks (n00) in the out-of-plane direction, indicating orientations of edge-on domains.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 the possibility of 3D charge transport. Upon thermal annealing, the lamellar distance (d) and coherence length (Lc) of the (100) order peak of PDI-BTBT at 22.7/148.4 Å in the out-of-plane direction became slightly smaller than those of NDI-BTBT at 24.2/154.0 Å , indicative of closely packed organizations for the former.54 The numbers of layers, estimated based on the ratio of Lc(100)/d(100), were 6.3 and 6.5 for NDI-BTBT and PDI-BTBT, respectively, suggestive of a relatively well-ordered edge-on lamellae.55 The influence of thermal annealing was reflected in the d-spacing, Lc, number of layers and peak sharpness, suggesting enhanced crystallinities of the polymer films (Figure S13, Table S5). We observed peaks related to the chain backbone repeat,


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with orders (001) and (002).56 The chain backbone repeat distance and Lc of the (001) peaks were 16.8/93.8 Å and 15.84/167.3 Å , respectively, leading to numbers of layers of 5.6/10.6 for NDIBTBT/PDI-BTBT. While the numbers of layers for lamellar stacking were similar, the numbers of layers for the chain backbone repeat direction were quite different, which means PDI-BTBT films have much higher crystallinities for chain backbone repeats in the in-plane directions. This may result in improved charge carrier mobility in PDI-BTBT compared with NDI-BTBT. In addition, the higher crystallinity for PDI-BTBT could be correlated to a lower molecular weight compared to NDI-BTBT.57 Therefore, the higher electron mobility of PDI-BTBT can also be attributed to the larger crystallinity of PDI-BTBT thin films, in comparison with NDI-BTBT thin films.58 3.5. Phototransistor Applications. Optoelectronic properties of PDI-BTBT and NDI-BTBT were investigated by irradiating monochromatic light on OFET devices in vacuum condition (Figures 5a and S14). Upon illumination of light in the visible region ( = 500 nm), the sourcedrain current increased 350-fold (at VG = 16 V) with a negative threshold 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 carriers and the elimination of trap sites. 42-43 The photodetection ability of the polymer films was quantified by photoresponsivity (R), photocurrent/dark current ratio (P), external quantum efficiency (EQE), and detectivity (D*) parameters using the transfer characteristics coupled with light irradiation (300 μW cm-2, see details in the Experimental Section), showing ~six-fold higher R value and ~five-fold higher D* value for PDI-BTBT compared with NDI-BTBT OFETs (Figure 5b-c, Table 2). Especially, the R value, one of the most important factors in OPTs, reached as high as 5.3 A W-1 for the n-type PDIBTBT, which is better and/or comparable with most of other p-type or ambipolar polymer-based


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OPTs.36-42, 59-60 The enhanced performance of PDI-BTBT originated from the higher extinction coefficient, greater electrical performances and crystalline ordering, in which higher R and EQE values typically result from increased exciton diffusion length and charge carrier mobilities for separated charges.40 In addition, the EQE values larger than 100% indicate that the highly crystalline BTBT-containing polymer-based OPTs exhibit photomultiplication phenomenon, as observed in organic photodiodes and phototransistors in several reports.44,

61-63

In our OPTs,

photoexcitation may generate many accumulated holes at the semiconductor/electrode interface, resulting in photocurrent multiplication due to the tunneling electrons, similarly to the organic photodiodes and phototransistors.43 Real-time photoresponses of NDI-BTBT and PDI-BTBT OFETs were investigated under pulsed illuminated of light of different intensities at 30 s intervals with an external gate bias (VG = 20 V), as shown in Figure 5d and S15. PDI-BTBT OFETs showed highly sensitive, rapid, and reversible on/off switching of photocurrent upon the pulsed light illumination of different intensities as opposed to rather low photoresponses of NDI-BTBT, corresponding to better light detection properties of PDI-BTBT thin films due to better exciton diffusion length and charge carrier mobilities for charge separation. In addition, we tested photoresponse speed by checking real-time current change under monochromatic light irradiation (Figure S16). PDI-BTBT OFETs exhibited short rise (< 680 ms) and decay times (< 1.3 s) under 300 μW cm-2 of light irradiation. To further confirm potential of matching illumination power and electrical signals in real-time photodetection, we plotted current change depending on the light pulses of different intensities from 5 to 300 μW cm-2 (Figure S17). The photocurrent enhancement as a function of light intensities can be fitted by the power law (current ~ P0.54), leading to well-matched relationship between illumination power and electrical signals. Moreover, we found that the real-time photodetection critically depended


15


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on the field effects. PDI-BTBT OFETs exhibited highly sensitive photo-switching at VG > 0 (20 V), leading to enhanced electron transport, while at VG ≤ 0, no light was detected (Figure 5e). In other words, additional field effects enable different photodetecting modes such as photoconductor mode (without gate voltage) and phototransistor mode (with gate voltage), leading to modulation of the photodetection ability and optoelectronic performance in our OFETs. These rapid, reversible, and adjustable real-time photodetection demonstrated the potential within practical optoelectronic applications.

4. Conclusion In summary, we report the synthesis of two new alternating donor-acceptor copolymers composed of high-performing small molecular semiconductors: p-type BTBT and n-type PDI/NDI derivatives by the Pd-catalyzed Stille-coupling reaction. The presence of the BTBT moiety led to well-aggregated morphology in the polymer films due to its high π-π stacking propensity as evidenced from AFM and GIXD results. This in turn led to 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 behaviors because of the energetically high-lying HOMO and LUMO levels compared with 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 ~350, detectivity of 1.1 × 1011 Jones and fast response speed (tr = 0.68 s) in phototransistor applications for the OPT devices operating in n-type mode. Our results show that incorporation of the BTBT unit into an alternating donor-acceptor copolymer can improve the molecular ordering and charge carrier


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mobility. Therefore, we believe that BTBT would be a useful donor linker for new highperformance π-conjugated polymers.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Synthesis procedures, characterization data and images, including Figures S1−S17 and Tables S1−S5 (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Joon Hak Oh: 0000-0003-0481-6069 Inho Song: 0000-0002-9683-0804 Author Contributions ‡S.K.S. and I.S. contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF) grant (No. 2017R1E1A1A01074090) and Nano Material Technology Development Program (No. 2017M3A7B8063825) funded through the NRF by the Ministry of Science and ICT (MSIT), Korea.

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Scheme 1. Design strategy for the synthesis of alternating donor-acceptor conjugated polymers composed of BTBT and NDI/PDI derivatives.

Scheme 2. Molecular structures of the BTBT containing copolymers NDI-BTBT and PDI-BTBT.


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0.6 0.4 0.2 0.0

NDI-BTBT PDI-BTBT

30 0 -30 -60 -90

300

375

450

525

600

675

Wavelength (nm)

750

(c) -3.0 -3.60 -4.0

-5.0

-3.85

PDI-BTBT

0.8

60

NDI-BTBT

PDI-BTBT Film PDI-BTBT Solution NDI-BTBT Film NDI-BTBT Solution

Current (A)

(b)

1.0

Energy Levels (eV)

(a) Normalized Abs.

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 24 of 31

-5.37 -6.0

-5.71

-1.5 -1.0 -0.5 0.0

E / V vs Ag/AgCl

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) CV curves and (c) energy level diagram of NDI-BTBT and PDI-BTBT.


24

(a)

-5

10

-6

10

0.0090 cm2 V-1 s-1 rsat = 19%

6

30

-8

-9

10

-10

4 3

VG = 80 V

60 V

1 0

0 -20 0 20 40 60 80 100

10

0 20 40 60 80 100

VDS (V)

VGS (V) -4

10

-5 -6

10

30

-8

10

-11

10

-12

10

20 10

0 -20 0 20 40 60 80 100

1/2

-10

10

0.042 cm2 V-1 s-1 0.11 cm2 V-1s-1

-8

-9

10

(X10 A )

ID (A)

-7

40

14

1/2

10

50

(ID)

10

0.029 cm2 V-1 s-1 rsat = 26%

12

100 V

10

ID (A)

(b)

100 V

2

1/2

0.0072 cm2 V-1 s-1 10 0.050 cm2 V-1 s-1

10

-8

ID (A)

1/2

10

(X10 A )

20

-7

40 V 20 V 0V -20 V

5

(ID)

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


ID (A)

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90 V

8 6

80 V

4

70 V

2

VG = 60 V

0

VGS (V)

0 20 40 60 80 100

VDS (V)

Figure 2. Transfer (left) and output (right) curves of a) NDI-BTBT and b) PDI-BTBT OFETs after thermal annealing at 350 C and 300 C, respectively. Red, blue and grey fitting lines indicate the region for maximum mobility estimation, mobility at kinks and considering the reliability factor (rsat), respectively.


25


(a)

(c)

(b)

(d)

Page 26 of 31

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 C and 300 C, respectively (scale bar indicates 500 nm in each case).


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1.5

1.5

0.5

(200) (100)

0.0

0.0 -1.0

0.0

1.0

(b) 2.0

(200) (100)

1.5 1.0

-1.0

0.0

1.0

2.0

-2.0

-1.0

qxy (Å -1)

0.0

1.0

2.0

qxy (Å -1) (010)

(001) (002)

Annealed

(100) (001) (002)

(010) As-cast

0.3

0.6

0.9

1.2

-1

1.5

(h)

(100)

Annealed As-cast

0.3 0.6 0.9 1.2 1.5 1.8 2.1 -1

qz (A )

(001)

(010)

(002)

Annealed

(010) As-cast

0.3

1.8

qxy (A ) (200) (300) (400)

Intensity (a.u.)

(g)

Intensity (a.u.)

Intensity (a.u.)

2.0

0.0 -2.0

Intensity (a.u.)

1.0

0.5

0.0

(d)

0.0

2.0

qz (Å -1)

(100) (001) (002)

(010)

1.0

-1.0

qxy (Å -1)

(f)

1.5

0.5

-2.0

2.0

qxy (Å -1)

(010)

-2.0

(c)

(010)

1.0 (001)

(010)

0.5

(001)

(400) (300) (200) (100)

(002)

1.0

qz (Å -1)

(e) 2.0

qz (Å -1)

(a) 2.0

qz (Å -1)

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


(002)

Page 27 of 31

0.6

0.9

1.2 -1

1.5

1.8

qxy (A ) (100) (200) Annealed As-cast

0.3 0.6 0.9 1.2 1.5 1.8 2.1 -1

qz (A )

Figure 4. 2D GIXD images of the annealed (top) and as-cast (bottom) polymer films of (a, b) NDIBTBT and (e, f) PDI-BTBT. Corresponding diffractogram profiles of (c, d) NDI-BTBT and (g, h) PDI-BTBT before and after thermal annealing at 350 C and 300 C, respectively.


27


10

10

-1

1

10

-4

(X10 A )

-2

10

0

10

-3

10

-1

10

-4

10

1/2

-2

10

-5

-3

10 -20 0 20 40 60 80100

-6

10

VGS (V)

VG (V)

(d)

VG (V)

0.02

 = 500 nm

300 W cm 200

100 50 0.01

10 5

0.00 500

600

700

800

Drain Current (nA)

(e) -2

Drain Current (nA)

10 13 NDI-BTBT 4 10 10 PDI-BTBT 3 10 2 12 10 10 1 10 0 11 10 10 -1 10 -2 10 10 -3 10 10 -4 10 -5 9 10 10 -6 10 -20 0 20 40 60 80100

D* (Jones)

-1

10

10

5

(c)

3

10

2

0

10

1/2

ID (A)

NDI-BTBT PDI-BTBT

1

P

10 35 -6 10 30 -7 10 -8 25 10 -9 20 10 -10 10 15 -11 10 10 -12 10 -13 5 10 -14 0 10 -20 0 20 40 60 80100

2

10

(b)

EQE (%)

40

DrainI 300 W

R (A W )

(a) 10-4-5

(ID)

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 28 of 31

VGS 0.02

0.0012 0.0009

-20 V 0V 20 V

0.0006 0.0003

60

90

120

0.01 -2

 = 500 nm, 10 W cm

0.00

20

Time (s)

40

60

80

Time (s)

100 120 140

Figure 5. (a) Transfer curves of PDI-BTBT OFETs in the dark and under light irradiations ( = 500 nm, 300 μW cm-2); (b) R, P and (c) EQE, D* of NDI/PDI-BTBT polymer OFETs under light irradiations (300 μW cm-2); Photo-switching behaviors of PDI-BTBT OFETs depending on (d) light intensity (VG = 20 V and VD = 100 V) and (e) applied VG (VD = 100 V, inset showing magnified plot).


28

Page 29 of 31 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


Table 1. Physicochemical Properties of NDI-BTBT and PDI-BTBT Polymers.a) 𝑠𝑜𝑙/𝑓𝑖𝑙𝑚

𝑠𝑜𝑙/𝑓𝑖𝑙𝑚

Polymer

Mn/Đ

𝑇 𝑒𝑛𝑑𝑜,𝑒𝑥𝑜

Td

𝜆𝑚𝑎𝑥,𝑎𝑏𝑠

𝜆𝑚𝑎𝑥,𝑃𝐿

𝐸𝑔𝑂𝑃𝑇 film

NDI-BTBT PDI-BTBT

90.5/1.6 11.1/1.9

238.0, 227.5 239.5, 229.7

430 423

550, 575 558, 570

695, 712 670, 692

1.77 1.86

𝐶𝑉 𝑂𝑃𝑇 𝐸𝐿𝑈𝑀𝑂 𝐸𝐻𝑂𝑀𝑂

–3.60 –3.85

–5.37 –5.71

μh

μe

0.0054 -

0.050 0.11

Mn, T, λ, E, and μ in kDa, C, nm, eV, and cm2 V-1 s-1, respectively. Td was recorded at 5% weight

a)

𝑂𝑃𝑇 𝐶𝑉 loss. ELUMO was determined from the CV reduction potential onset; 𝐸𝐻𝑂𝑀𝑂 = 𝐸𝐿𝑈𝑀𝑂 − 𝐸𝑔𝑂𝑃𝑇 .

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

Table 2. Optoelectronic Properties of NDI-BTBT and PDI-BTBT Polymers. Polymer NDI-BTBT PDI-BTBT

R (A W–1) 0.86 5.3

P 13 350

EQE (%) 2.1×102 1.3×103

D* (Jones) 2.2×1010 1.1×1011


29


Page 30 of 31

(The English in this document has been checked by at least two professional editors, both native speakers of English. For a certificate, please see: http://www.textcheck.com/certificate/KxoVzs)


30

Page 31 of 31

ToC Figure TOC-Graphical Abstract_Adv Funct Mater

PDI-BTBT n-type OFET

Drain Current (nA)

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


0.02

300 W cm 200

-2

 = 500 nm

100 0.01

50 10

5

μe = 0.11 cm2 V1 s1 R = 5.3 A W1

0.00 High Photoresponsivity (R) 500 600 700 800

Time (s)


31

Organic n-Channel Transistors Based on [1]Benzothieno[3,2-b

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