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(Semi)ladder-Type Bithiophene Imide-Based AllAcceptor Semiconductors: Synthesis, Structure-Property Correlations, and Unipolar n-Type Transistor Performance Yingfeng Wang, Han Guo, Alexandra Harbuzaru, Mohammad Afsar Uddin, Iratxe Arrechea-Marcos, Shaohua Ling, Jianwei Yu, Yumin Tang, Huiliang Sun, Juan Teodomiro López Navarrete, Rocio Ponce Ortiz, Han Young Woo, and Xugang Guo J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b02144 • Publication Date (Web): 14 Apr 2018 Downloaded from http://pubs.acs.org on April 14, 2018
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Journal of the American Chemical Society
(Semi)ladder-Type Bithiophene Imide-Based All-Acceptor Semiconductors: Synthesis, Structure-Property Correlations, and Unipolar n-Type Transistor Performance Yingfeng Wang,†, ‖ Han Guo,†, ‖ Alexandra Harbuzaru,‡ Mohammad Afsar Uddin, § Iratxe ArrecheaMarcos,‡ Shaohua Ling,† Jianwei Yu,† Yumin Tang,† Huiliang Sun,† Juan Teodomiro López Navarrete,‡ § Rocio Ponce Ortiz,*,‡ Han Young Woo,*, Xugang Guo*,† †
Department of Materials Science and Engineering and The Shenzhen Key Laboratory for Printed Organic Electronics, South University of Science and Technology of China, No. 1088, Xueyuan Road, Shenzhen, Guangdong 518055, China
‡
Department of Physical Chemistry, University of Málaga, Campus de Teatinos s/n, Málaga 29071, Spain
§
Research Institute for Natural Sciences, Department of Chemistry, Korea University, Seoul 02841, South Korea
ABSTRACT: Development of high-performance unipolar n-type organic semiconductors still remains as a great challenge. In this work, all-acceptor bithiophene imide-based ladder-type small molecules BTIn and semiladder-type homopolymers PBTIn (n =1-5) were synthesized and their structure-property correlations were studied in depth. It was found that Pd-catalyzed Stille coupling is superior to Ni-mediated Yamamoto coupling to produce polymers with higher molecular weight and improved polymer quality, thus leading to greatly increased electron mobility (µe). Due to their all-acceptor backbone, these polymers all exhibit unipolar ntype transport in organic thin-film transistors, accompanied by low off-currents (10−10-10−9A), large on/off current ratios (106), and small threshold voltages (~15-25 V). The highest µe up to 3.71 cm2 V−1 s−1 is attained from PBTI1 with the shortest monomer unit. As the monomer size is extended, the µe drops by two orders to 0.014 cm2 V−1 s−1 for PBTI5. This monotonic decrease of µe was also observed in their homologous BTIn small molecules. This trend of mobility decrease is in good agreement with the evolvement of disordered phases within the film, as revealed by Raman spectroscopy and X-ray diffraction measurements. The extension of the ladder-type building blocks appears to have a large impact on the motion freedom of the building blocks and the polymer chains during film formation, thus negatively affecting film morphology and charge carrier mobility. The result indicates that synthesizing building blocks with more extended ladder-type backbone does not necessarily lead to improved mobilities. This study marks a significant advance in the performance of all-acceptor type polymers as unipolar electron transporting materials and provides useful guidelines for further development of (semi)ladder-type molecular and polymeric semiconductors for applications in organic electronics.
Introduction Organic thin-film transistors (OTFTs) show great potentials in realizing low-cost, large-area, and flexible devices such as displays, sensors, and radio-frequency identification tags, etc.14 The rise of donor-acceptor (D-A) type organic semiconductors has greatly advanced the OTFT technology, most of them being p-type,5-10 while the performance of n-type polymers greatly lags behind.11-19 In addition, D-A copolymers often exhibit ambipolar transport characteristics, resulting in high off-currents (Ioffs), small on/off current ratios (Ion/Ioffs), and large threshold voltages (VTs) in OTFTs.20,21 The injection of the minority charge carrier is difficult to eliminate due to their low-lying lowest unoccupied molecular orbitals (LUMOs) and high-lying highest occupied molecular orbitals (HOMOs), which originate from the intramolecular charge transfer (ICT) characteristics between neighboring electron-donating and accepting units in a polymeric backbone.22 The donor-weak
acceptor strategy has been demonstrated to be effective to fine-tune the frontier molecular orbitals (FMOs) of organic semiconductors, leading to high-lying LUMOs, which greatly suppress electron injection and the semiconductors show unipolar p-type characteristics with nearly ideal transistor performance characteristics.23-25 On the other hand, constructing donor-donor (D-D) or acceptor-acceptor (A-A) type polymers could minimize the ICT characteristics and offers an effective approach to obtain wide band gaps (Egs) with appropriately positioned FMOs (both LUMOs and HOMOs) and achieve unipolar transport in OTFTs. Comparable mobilities to those of D-A polymers have been widely demonstrated in p-type D-D polymers25-28 but very limited success has been reported in n-type A-A polymers.29-40 On the basis of the energetic consideration, the acceptoracceptor (or all-acceptor) type polymers are highly desired, which should lead to the semiconductors with both low-lying LUMOs and HOMOs. The deep LUMOs facilitate electron
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injection and low-lying HOMOs suppress hole accumulation, therefore resulting in improved n-type performance with suppressed p-type characteristics. Such semiconductors are beneficial to achieve unipolar n-type performance with more ideal transistor characteristics, which are critical for real applications. However, the development of A-A type polymers is largely hindered by the limited choices of acceptor units, constraining the available combinations when designing and synthesizing new polymer semiconductors.41 Furthermore, reducing the backbone torsion is highly challenging for the majority of A-A type polymers, since the strong electronwithdrawing substituents, such as (C=O, C≡N, etc.), which can enable n-type performance, are usually bulky and exert significant steric hindrance on the neighboring units.30,42,43 Twisted polymer chains are detrimental not only to intra-chain delocalization of charge carriers, but also to inter-chain transport as a result of poor molecular packing and low film crystallity.36 Among various A-A type polymer semiconductors reported in literature, their electron mobility (µe) is generally below 0.1 cm2 V−1 s−1 (Figure 1). The previous record was set by the homopolymer P(BTImR) based on the bithiophene imide (BTI) unit with a µe of 0.14 cm2 V−1 s−1,31 benefiting from the minimized steric hindrance between consecutive BTI units and the resulting planar backbone geometry. Recently, the newly developed thiazolothienyl imide (TzTI) homopolymer PDTzTI greatly improves the highest µe of A-A type polymers up to 1.61 cm2 V−1 s−1, which also enables suppressed Ioffs of 10−10– 10−11 A and remarkable Ion/Ioffs of 107–108 for unipolar n-type OTFTs.40 In both BTI and TzTI moieties, the strong electron withdrawing imide group is located on the center of bithiophene or thiazolothiophene, such unique structure leads to minimized steric hindrance and remarkable n-type performance of their homopolymers. Inspired by the excellent device performance of BTI-based polymers, we recently reported a series of ladder-type BTI small molecules BTIn up to 15 fused rings and 5 imide groups with well-defined chemical structures and tunable opto-electrical properties.44 The unique geometries and electrical properties of BTIn offer a remarkable platform for materials development and elucidation of the materials structure-property correlations. Ladder-type oligomers and polymers possess interesting features including rigid, planar backbone, and high intra-chain charge carrier mobility.29,45-51 Planar backbones can effectively increase π-conjugation length, decrease reorganization energies, and are less susceptible to disorder, thus greatly facilitating charge transport.10,12,52,53 In fact, µe > 0.1 cm2 V−1 s−1 in polymer semiconductors was first demonstrated by a ladder-type polymer poly(benzobisimidazobenzophenanthroline) (BBL).54 Incorporation of ladder-type building blocks into polymer semiconductors is widely used in the community, to take advantage of those benefits, mainly in D-A copolymer.55-57 The longest ladder-type monomer applied into polymer semiconductors to date is up to 11 rings.58,59 Based on the fused BTIn, a straightforward idea is to synthesize A-A type homopolymers through direct linkage of these ladder-type acceptors, the simplest being homopolymers with identical monomers towards unipolar n-type semiconductors. Herein, we report a series of ladder-type small molecules (BTI1 to BTI5) and semiladder-type homopolymers (PBTI1 to
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PBTI5) as shown in Figure 2, containing 3 to 15 fused rings per building block, respectively. It was found that the polymerization methods (Stille and Yamamoto coupling) show great impact on the molecular weight and polymer quality, and the polymer synthesized via Stille coupling yielded greatly improved OTFT performance versus polymer synthesized via Yamamoto coupling. These A-A type polymers all exhibit unipolar n-type transport characteristics in OTFTs, with low Ioffs (10−9–10−10 A), high Ion/Ioffs (106), and small VTs (~15-25 V). In addition, the OTFTs show kink-free transfer curves, thus avoiding overestimation of mobilities in many D-A type copolymers.60-62 For instance, the benzodifurandione-based oligo(p-phenylene vinylene) (BDOPV) D-A copolymer F4BDOPV-2T (Figure 1) exhibits a remarkable µe of ~ 15 cm2 V−1 s−1 in the low gate-voltage (VG) regime, however the µe is greatly reduced to ~0.6 cm2 V−1 s−1 in the high VG regime. Among these A-A polymers, the highest µe up to 3.71 cm2 V−1 s−1 was obtained from PBTI1 having the smallest monomer size. These remarkable results reveal the high intrinsic mobility of BTI-based A-A type homopolymers, approaching that of the best D-A type electron-transporting polymers.63 An unexpected result is the monotonic µe decrease upon increasing the fused monomer size in this series of polymers. The highest µe is gradually reduced to 1.34 (PBTI2), 0.53 (PBTI3), 0.21 (PBTI4), and 0.014 (PBTI5) cm2 V−1 s−1. In addition, the building blocks BTI1-BTI5 show a similar trend with the µe decreasing from 0.12 to 0.0088 cm2 V−1 s−1, which is attributed to the reduced molecular mobility as the repeating unit is extended, leading to poorer self-assembly and lower degree of film crystallinity. The results indicate that developing more extended ladder-type building blocks may not necessarily improve charge carrier mobility. This systematic study demonstrates the excellent potentials of A-A type polymers in realizing unipolar n-type transport with large mobility in OTFTs.
Results and Discussion Materials Synthesis The ladder-type BTI building blocks were synthesized following our published procedure,44 which were then subjected to bromiation64 using Br2 to afford the dibrominated monomers in excellent yields of 70-90% (SI). In the previous study, Ni-mediated Yamamoto coupling was utilized to synthesize the homopolymer PBTI1 from the dibrominated monomers,31 resulting in a low number-averaged molecular weight (Mn) of 7.2 kDa. In order to investigate the effect of polymerization methods on molecular weight, polymer structure, and device performance, two different polymerization methods were employed here, the Ni-mediated Yamamoto coupling and Pd-catalyzed Stille coupling (Figure 3a). The final product polymer PBTI1 synthesized via the Stille coupling shows a higher number-averaged molecular weight (Mn) of 12.7 kDa than that (5.4 kDa) of PBTI1* via the Yamamoto coupling, as determined by high-temperature gel permeation chromatography (GPC) using 1,2,4trichlorobenzene as the eluent at 150 °C. In addition to molecular weight, polymer chain structures were characterized by matrix assisted laser desorption ionization-time of flight (MALDI-TOF) measurements (Figure 3b). The MALDI-TOF spectra of both PBTI1* and PBTI1 all show oligomers (n = 4−9) separated by 514 amu, consistent
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with the calculated mass of the BTI repeating unit. We observed that the molecular weights obtained from MALDITOF are far smaller than those from GPC measurements, which is likely due to the difficulty of ionization of the high molecular weight portion. In the mass spectra of PBTI1* synthesized via Yamamoto coupling, the strong peak at m/z = 2728 can be assigned to the pentamer having Br atom at both termini and the weak peak at m/z = 2649 is attributed to the pentamer with one Br atom and one H atom at each end. In the mass spectra of PBTI1 synthesized via Stille coupling, the strong peak at m/z = 2570 can be assigned to the pentamer having H atom at both ends and the weak peak at m/z = 2649 is attributed to the pentamer with one Br and one H end group. No oligomers with two Br atom end groups have been detected for PBTI1. Therefore, the polymer PBTI1* prepared via Yamamoto coupling should contain more Br end groups than the polymer analogue PBTI1 prepared via Stille coupling. It has been reported that Br atom can act as a charge carrier trap,65-67 which likely leads to reduced charge carrier mobility for the polymer prepared via Yamamoto coupling. Moreover, the UV-vis absorption spectra show a distinctive difference between PBTI1* and PBTI1. PBTI1 exhibits a stronger 0–0 transition than PBTI1* both in solution and in film state, suggesting stronger aggregation and more ordered structure in PBTI1. Compared to PBTI1, PBTI1* shows a ~15 nm blueshift in the low energy band maximum, which can be attributed to the shorter conjugation length and/or structural defects of the polymer prepared via Yamamoto coupling. Due to the higher Mn, improved polymer quality, and greatly enhanced OTFT performance (vide infra) of polymer PBTI1 prepared via Stille coupling, therefore this polymerization method was then used for the preparation of all other homopolymers PBTI2-PBTI5. The detailed synthetic procedures are provided in Supporting Information. As summarized in Table 1, all these homopolymers show similar Mns in the range of 9–13 kDa with a narrow polydispersity index (PDI) of ~2. To be strict, on the basis of the numbers of repeating units, these PBTIn should be defined as oligomers, especially for PBTI4 and PBTI5. All PBTIn (n = 1-5) are readily soluble in common organic solvents due to the long and branched 2-octyldodecyl side chains, which enable facile materials processing and device fabrication at room temperature. However, based on the absorption spectra in solution and film state (vide infra), these polymers exist in the solvent as an aggregated form instead of as a molecularly dissolved state. Thermal, Optical and Electrochemical Properties Thermal properties of the BTIn small molecules and their corresponding homopolymers PBTIn were first studied by thermogravimetric analysis (TGA). As shown in Figure S1a, the BTIn small molecules exhibit high thermal stability, showing a decomposition temperature (Td) at ~450 °C with 5% weight loss except BTI1 (Td = 365 °C), and the PBTIn homopolymers also exhibit good thermal stability with Tds in the range of 370-415 °C (Figure S1b). Differential scanning calorimetry (DSC) was then used to characterize the materials crystallinity. The thermal transitions of BTIn small molecules appear to be greatly affected by the molecular size (Figure S2a). The smallest BTI1 goes into a liquid state slightly above room temperature, hence no peaks are observed within the test
range. The BTI2 shows the strongest melting/crystallization peaks located at 186/171 °C, which indicate its highest crystallinity among the series. The endothermic/exothermic peaks (melting/crystallization) shift to higher temperatures of 220/205 °C for BTI3 and 242/222 °C for BTI4. This could be understood on the basis of the more extended backbone. For the longest BTI5, the thermal transition peaks completely disappear in the measured temperature range, indicative of its reduced crystallinity. Therefore, as the backbone is extended, the crystallinity is gradually decreased from BTI2 to BTI5, which is likely attributed to the reduced molecular mobility and poorer self-assembling properties. Similar trend has also been observed in the ladder-type benzo[k]tetraphene oligomers.68 For all PBTIn homopolymers, DSC curves show no observable thermal transitions when measured up to 350 °C (Figure S2b), which is attributed to the greatly reduced molecular mobility when the BTIns are linked together. The optical properties of these (semi)ladder-type small molecules and polymer semiconductors were investigated by measuring their ultraviolet-visible (UV-vis) absorption spectra, which provide direct evidence of the molecular size dependent aggregation. On the basis of the absorption of BTI1-BTI5 in chlorobenzene at room temperature and 100 °C, and in the film state, BTI1-BTI4 are interpreted to exist as a disaggregated form in solution at room temperature. However, BTI5 exhibits pronounced aggregation at this temperature, showing a similar absorption spectrum (Figure 4a) with that (Figure 4c) in film. As the temperature is elevated to 100 °C, the aggregation of BTI5 is greatly suppressed. From BTI1 to BTI5, the absorption onset (λabsonset) in thin films gradually redshifts from 392 to 643 nm, corresponding to an optical band gap (Egopt) reduction from 3.16 to 1.93 eV, as a result of extended conjugation in the elongated molecules. The optical band gap change (∆Egopt) from BTI1 to BTI2 is large (0.68 eV), then ∆Egopt becomes smaller between BTI2/BTI3 (0.29 eV), BTI3/BTI4 (0.19 eV), and finally approaches zero for BTI4/BTI5 (0.07 eV). This suggests that the effective conjugation is close to saturation for BTI5, hence further increasing the BTIn length would not be necessary from this point of view. In comparison to the small molecules, all homopolymers PBTIn are strongly aggregated even in heated chlorobenzene solutions at 100 °C (Figure S3a-e), showing strong 0–0/0–1 vibronic transitions. From solution to film state, the absorption spectra display minimal bathochromic shifts (~10 nm) (Figure 4d and 4f), further validating the strong inter-chain aggregation of these homopolymers in solution. From PBTI1 to PBTI5, the λabsonset red-shifts from 626 to 700 nm, corresponding to a small Egopt reduction from 1.98 to 1.77 eV, suggesting similar level of electron delocalization in these homopolymers. The ∆Egopt from the small molecule to the corresponding homopolymer is 1.18 eV for BTI1/PBTI1, which is decreased to only 0.15 eV for BTI5/PBTI5, signifying that electron delocalization occurs over multiple building blocks in PBTI1 while it is primarily localized onto the long building block BTI5 in the polymer PBTI5, which further corroborates that the effective conjugation of BTI5 is close to saturation. Cyclic voltammetry (CV) measurements were performed using thin films in anhydrous acetonitrile under nitrogen
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atmosphere. On the basis of the cyclic voltammograms in Figure S4, all PBTIn homopolymers show distinctive reduction peaks with greatly suppressed oxidation peaks, indicative of their pronounced n-type character due to their allacceptor backbones. As the backbone of building blocks is extended, the CV shows multiple reduction peaks, which are attributed to the increased number of imide groups. The ELUMO levels were determined from the reduction onsets, and the EHOMO levels were calculated from ELUMOs and the corresponding Egopts using the equation: ELUMO = EHOMO + Egopt. The results are compiled in Table 1. For the PBTIn homopolymers, ELUMOs show a similar trend with that in the corresponding small molecules.44 From PBTI1 to PBTI5, ELUMO is gradually lowered from −3.48 to −3.72 eV. The derived EHOMOs are relatively deep in the range of −5.39 to −5.49 eV. These results show a direct correlation between the ELUMO and the fused polymer building blocks, indicating that the ELUMO can be effectively tuned by increasing the number of strong electron-withdrawing imide groups in the fused BTIn, providing useful guidelines for designing new n-type polymers based on ladder-type BTIn units, which should have profound implications for all-polymer solar cell application.69 Molecular Geometry and Frontier Molecular Orbital Computation In order to gain information on molecular geometry and FMO energetics, density functional theory (DFT)-based calculations were performed on dimeric models (two repetitive building blocks) of the polymers. The results obtained for an isolated dimer indicate basically a planar skeleton with dihedral angles
