Article Cite This: Chem. Mater. 2019, 31, 4864−4872
pubs.acs.org/cm
Improving the Electrical Connection of n‑Type Conjugated Polymers through Fluorine-Induced Robust Aggregation Minjun Kim,†,∥ Won-Tae Park,§,∥ Seung Un Ryu,† Sung Y. Son,† Junwoo Lee,† Tae Joo Shin,*,‡ Yong-Young Noh,*,† and Taiho Park*,†
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†
Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), 77 Cheongam-Ro, Nam-Gu, Pohang, Gyeongbuk 37673, South Korea ‡ UNIST Central Research Facilities & School of Natural Science, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Eonyang-eup, Ulsan 44919, South Korea § Department of Electrical and Computer Engineering, University of Waterloo, 200 University Avenue West, Waterloo, ON N2L 3G1, Canada S Supporting Information *
ABSTRACT: Naphthalene diimide (NDI)-based conjugated polymers with bithiophene or dithienylethene (TVT) units can form large crystal domains through NDI-driven self-assembly and are widely used in organic electronic devices as n-type materials. However, improving electron transport in these semiconducting polymers has been a significant challenge mainly due to poor electrical connections between the crystal domains. Formation of an interconnected network of small domains with short-range ordering and mixed orientations could be an effective way of increasing the electron mobility (μe) of polymeric materials. The present study demonstrates the feasibility of this approach using an NDI-based polymer composed of fluorinated TVT (FTVT) units. The FTVT unit enhances intermolecular interactions between the polymer chains, leading to robust aggregation. This aggregation was found to suppress NDI-driven self-assembly, resulting in interconnected small crystal domains with short-range ordering and good thermal stability at elevated temperatures. This microstructure provided transistor devices with improved μe by lowering energetic disorder as well as consistent electrical connectivity at different annealing temperatures.
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INTRODUCTION Conjugated polymers based on fused aromatics have been widely employed in various electronic devices, such as organic photovoltaic solar cells (OPVs), organic field-effect transistors (OFETs), polymer light-emitting diodes, and perovskite solar cells.1−4 Highly developed molecular design approaches (e.g., donor−acceptor structures) have enabled the development of polymer-based devices with improved light absorption properties, enhanced light emission, and tunable energy levels.5−7 Tremendous efforts have also been made to understand the charge transport phenomena in conjugated polymers for furthering the development of high-efficiency devices. Despite these advances, investigations utilizing both experimental and theoretical approaches are still needed to thoroughly understand the origin of charge transport phenomena in conjugated polymers. In general, most conjugated polymers are composed of highly ordered (i.e., crystalline domains) and disordered (i.e., amorphous domains) regions. Many highly crystalline conjugated polymers have been reported to possess improved charge transport behaviors.8 However, it is difficult to obtain conjugated polymers with large-scale crystallinity due to the macromolecular morphologies of polymer chains, such as © 2019 American Chemical Society
chain-folding, loose loops, dangling segments, and tie molecules.9 These complex morphologies inevitably cause disordered regions, generating electronic traps that act as a significant impediment to charge transport.10 Moreover, largescale crystalline polymers are susceptible to mechanical failure due to their rigidity and brittleness, resulting in cracks in the polymer film when they undergo a morphological stress.11,12 Since the McCulloch group reported a large hole mobility (μh = 3.6 cm2 V−1 s−1) in the less crystalline, p-type conjugated polymer based on indacenodithiophene and benzothiadiazole (IDT-BT),13 various other less crystalline polymers have shown μh values that are comparable to those found in highly crystalline polymers.14−16 The Sirringhaus group provided one possible rationalization for why IDT-BT exhibited such a high μh compared to highly crystalline poly[2,5-bis(thiophen-2yl)thieno[3,2-b]thiophene]. According to their theoretical and experimental studies, a planar conformation and a torsion-free backbone in the amorphous region enhanced the structural Received: April 14, 2019 Revised: June 12, 2019 Published: June 13, 2019 4864
DOI: 10.1021/acs.chemmater.9b01469 Chem. Mater. 2019, 31, 4864−4872
Article
Chemistry of Materials
stronger than that of the NDI unit, the aggregation driven by the donor unit may be competitive with NDI-driven selfassembly. This aggregation could suppress the crystallinity of the polymer and form small, interconnected domains with short-range order (Figure 1b), which could reduce the energetic disorder that acts as a significant impediment to charge transport.17,18,30 Among the various synthetic strategies, fluorination has been shown to be effective in controlling intermolecular interactions in conjugated heterocyclic molecules.31−33 The high electronegativity of fluorine induces strong polarization of the C−F bond and enhances intermolecular interactions (i.e., C−H···F, F···F, and C−F···πF), enabling tight packing between the molecules.34,35 Although many reports on fluorinated polymers have demonstrated the improved electron transport of OFETs, the underlying effect of the fluorinated donor unit in NDIbased polymers remains unclear. Herein, we examine the crystallization, aggregation, and electron transport behavior of NDI-based polymers after annealing at various temperatures and demonstrate the feasibility of utilizing a fluorination design approach to improve electron transport in an NDI-based polymer. To this aim, we prepared two NDI-based polymers with 1,2-di(2-thienyl)ethene (TVT) (PNDI-TVT)36 or (E)-1,2-bis(3-fluorothiphen2-yl)ethene (FTVT) units (PNDI-FTVT)37 (Figure 1a). We observed significant differences in the solid-state properties of the two polymers. After increasing the annealing temperature (TA), the PNDI-TVT film exhibited large crystal domains, which resulted in poor electrical connectivity in the FET device, hindering further improvements in electron transport. On the other hand, the FTVT unit induced robust aggregation to produce interconnected domains with short-range order and a highly stable film morphology at elevated temperatures. The PNDI-FTVT-based FETs showed improved electron transport in comparison to the PNDI-TVT-based FETs at different TA’s due to consistent electrical connections.
resilience of the material, which prevented energetic disorder induced by the broadening of the density of states (DOS).17 Recently, we demonstrated that OFETs with a large μh based on the less crystalline p-type random copolymer of 3hexylthiophene and thiophene (RP33) could be realized by lowering the activation energy (EA) required for charge transportation.18 Randomly placed, highly planar thiophene units on the main backbone of poly(3-hexylthiophene) (P3HT) were capable of forming localized aggregates in the amorphous regions, acting as charge transport junctions due to strong π−π interactions. This feature led to an interconnected network in the amorphous region, resulting in a lowered EA for charge transport in the less crystalline RP33 polymer. RP33 exhibited a 1 order of magnitude greater μh (1.37 cm2 V−1 s−1) than P3HT (0.19 cm2 V−1 s−1). We further demonstrated that increasing the interconnectivity by installing a small portion (2.0−2.5 mol %) of a strong π−π interaction unit, such as 3,6carbazole or pyrene, in the random copolymers was key to improving the bulk charge transport and enabling efficient photovoltaic cells.19,20 Naphthalene diimide (NDI)-based donor−acceptor conjugated polymers, which are n-type semiconducting polymers, have attracted much attention as materials for OFETs and OPVs due to the high electron affinity of their NDI core and substantial electron-transporting properties.2,21 The NDI unit is very flat and large and thus capable of strong π−π stacking with another NDI unit, producing a semicrystalline structure through NDI-driven self-assembly.22,23 The crystallization driven by NDI self-assembly becomes more dominant after thermal treatment (Figure 1b). Sufficient thermal energy has
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EXPERIMENTAL SECTION
Materials and Characterization. All reagents and chemicals for this study were purchased from Sigma-Aldrich, TCI, and 1-Materials, unless otherwise specified. Synthesis of the TVT monomer and PNDI-TVT polymer followed a previously reported method.36 The FTVT monomer and PNDI-FTVT polymer were synthesized by methods we had recently developed.38 The detailed synthetic procedures and characterizations of the polymers are provided in the Supporting Information. 1H and 19F NMR spectra of the polymers were recorded on a Bruker BioSpin AG system operating at 500 MHz in Cl2CDCDCl2 solution at 100 °C. The elemental analyses of the polymers were carried out by the Korean Basic Science Institute at Daejeon using a Carlo Erba Instruments CHNSO EA 1108 analyzer. The number-average molecular weight (Mn) and dispersity (Đ, Mw/ Mn) of the polymers were determined by gel-permeation chromatography (GPC) from SHIMADZU with polystyrene as a control. UV− vis spectra of the polymer solution and film were obtained via a UV− vis spectrophotometer from Mecasys Optizen Pop. Cyclic voltammograms (CV) of the polymer thin films were measured using a PowerLab/AD instrument with three electrodes (quasi-reference electrode, platinum wire, and glassy carbon disk) in an acetonitrile solution of 0.1 M tetrabutylammoniumhexafluoro-phosphate (Bu4NPF6) electrolyte at 50 mV s−1 scan rate. To estimate the molecular energy levels of the polymer films, ferrocene was used as an internal standard. Differential scanning calorimetric (DSC) analyses were performed using TA Instruments discovery at a heating and cooling rate of 10 °C min−1 under a N2 atmosphere.
Figure 1. (a) Chemical structures of PNDI-TVT and PNDI-FTVT. (b) Schematic illustration of NDI-driven self-assembly (left) and FTVT-driven aggregation (right).
been shown to allow mobile polymer chains and conformational changes, propagating an ordered self-assembly to large lamellar structures.24−26 However, despite the significant increase in crystallinity after thermal treatment, the electron transport properties of NDI-based polymers have shown only marginal improvements.24,26−28 Although this behavior has been attributed to the poor electrical connectivity between the large crystal domains, the exact origin is still not fully understood. Therefore, it is important to explore how the thermally induced crystallization of NDI-based polymers impacts electron transport properties in thin films. In parallel, the strength of the intermolecular interactions of the donor units in the NDI-based polymer should also be considered, since controlling intermolecular interactions has been shown to greatly affect solid-state properties, such as morphology, crystallinity, aggregation, and orientation.8,29 If the intermolecular interaction of the donor unit is comparable to or 4865
DOI: 10.1021/acs.chemmater.9b01469 Chem. Mater. 2019, 31, 4864−4872
Article
Chemistry of Materials Table 1. Polymerization Results and Optical and Electrochemical Properties of the Polymers polymer
Mn [kg mol−1]
Đ [Mw/Mn]
Tm [°C]a
ΔHm [J g−1]a
Tc [°C]a
ΔHc [J g−1]a
Egopt [eV]b
LUMO [eV]c
HOMO [eV]c
PNDI-TVT PNDI-FTVT
69 79
2.09 2.58
248 277
5.21 6.39
216 250
7.48 9.27
1.41 1.46
−3.89 −3.96
−5.59 −5.82
Determined from DSC measurements. bOptical band gap estimated from the absorption edges in the film state. cLowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) levels were estimated from the onset of the first reduction and first oxidation potentials, respectively, with reference to ferrocene at −4.8 eV. a
Density Function Theory (DFT) Calculations. DFT calculations were conducted in the Gaussian 16 package at the B3LYP/631G (d,p) level for optimized backbone structures and potentialenergy variations. The long alkyl chains of model compounds were replaced with methyl groups to simplify and reduce calculations. Potential-energy profiles of the model compounds were calculated by varying the dihedral angle between the NDI and the electron-rich unit while fixing the other linkages. Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS) Analysis. GIWAXS measurements were performed in the PLS-II 6D UNIST-PAL beamline of the Pohang Accelerator Laboratory in Korea. X-rays emitted from the bending magnet were monochromated at 11.6 keV (wavelength: 1.0688 Å) using Si(111) double crystals and focused at the detector position by a combination of a sagittal-type monochromator crystal and a toroidal mirror system. The incidence angle of the X-rays was set to ca. 0.12°, which was close to the critical angle of the samples. GIXD patterns were extracted from a 2D CCD detector (MX225-HS, Rayonix L.L.C.) located approximately 245 mm from the sample center. The measured data were analyzed using the Igor-Pro software package. The polymer thin films used for GIWAXS analysis were prepared via spin-coating from a chloroform solution (10 mg mL−1) on a Si wafer at 4000 rpm for 20 s. The annealed films were prepared by thermal treatment at 100, 150, and 200 °C for 10 min using a hot plate. A Hosemann plot was used to calculate the true paracrystal size (L); δb is the value of the integral widths, calculated from the coherence length (LC) of diffraction peaks, and h is the diffraction orders. Through this plot, L can be obtained from the intercept of the straight line. Device Fabrication and Characterization. The organic fieldeffect transistors (OFETs) were fabricated with a top-gate/bottomcontact (TG/BC) structure. For source−drain electrode deposition, we used conventional lift-off photolithography with a positive-tone photoresist, and then Ni (3 nm) and Au (12 nm) were evaporated by thermal evaporation in a high-vacuum atmosphere (
