Balanced Ambipolar Organic Thin-Film Transistors Operated under

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Balanced Ambipolar Organic Thin-Film Transistors Operated under Ambient Conditions: Role of the Donor Moiety in BDOPV-Based Conjugated Copolymers Xu Zhou, Na Ai, Zihao Guo, Fang-Dong Zhuang, Yu-Sheng Jiang, Jieyu Wang, and Jian Pei Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b00018 • Publication Date (Web): 13 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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Balanced Ambipolar Organic Thin-Film Transistors Operated under Ambient Conditions: Role of the Donor Moiety in BDOPV-Based Conjugated Copolymers Xu Zhou, Na Ai, Zi-Hao Guo, Fang-Dong Zhuang, Yu-Sheng Jiang, Jie-Yu Wang,* and Jian Pei* Beijing National Laboratory for Molecular Sciences, Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, Center for Soft Matter Science and Engineering, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China ABSTRACT: Organic field-effect transistors (OFETs) are receiving increasing attention due to their intriguing advantages such as flexibility, low cost and solution processibility. Development of organic conjugated polymers with balanced ambipolar carrier transportation operated under ambient conditions, in particular, is considered to be one of the central issues in OFETs. In this work, 3,7-bis((E)-2-oxoindolin-3-ylidene)-3,7-dihydrobenzo[1,2-b:4,5-b']difuran-2,6-dione (BDOPV) unit as a good acceptor unit was copolymerized with three donor moieties, thieno[3,2-b]thiophene (TT), benzo[1,2-b:4,5-b']dithiophene (BDT) and benzo[1,2-b:4,5b']diselenophene (BDSe), to construct three donor-acceptor (D-A) conjugated polymers, BDOPV-TT, BDOPV-BDT and BDOPV-BDSe. Photophysical and electrochemical properties of all the polymers were characterized. The fabrication of OFETs using three polymers as the active layers demonstrated that all the three polymers showed balanced ambipolar transport property tested under ambient conditions, which is of great importance in complementary circuits. In particular, both electron and hole mobilities of BDOPV-TT were achieved above 1 cm2 V−1 s−1 under ambient conditions (1.37 and 1.70 cm2 V−1 s−1, respectively), showing great potential in balanced ambipolar OFETs.

INTRODUCTION Organic conjugated polymers with the advantages of low cost, large-area printing, mechanical flexibility, and tunable optoelectronic properties are of extensive scientific interest for application in organic field-effect transistors (OFETs).1−5 The performance of PFETs has been remarkably improved recently due to rational design of molecular structures and optimization of device fabrication.6,7 Especially, to develop ambipolar OFETs which operate under ambient conditions is very important for further fabrication of single-component organic logic circuits and light emitting transistors, as well as fundamental understanding of electron vs. hole transport in polymer films.8−13 Recently, the properties of ambipolar polymer semiconductors, in which both hole and electron can be transported, have been improved significantly.14,15 Nevertheless, most of ambipolar PFETs were fabricated and tested under nitrogen or in vacuum. Very few polymers are known to be operated under ambient conditions, and generally exhibit relatively low hole/electron mobilities (less than 1 cm2 V−1 s−1).16,17 One of the main challenges hampering the development of new high-performance ambipolar polymers has been the design and synthesis of new polymerizable π-electron deficient acceptors with good charge-transporting properties. Nearly all the reported ambipolar polymers with charge carrier mobilities larger than 1 cm2 V−1 s−1 (all tested in nitrogen) were based on diketopyrrolopyrrole (DPP) as an acceptor unit.18−21 To

achieve PFETs operated under ambient condition with high electron and hole mobilities, new types of polymers containing acceptors other than DPP are pursued. 3,7-Bis((E)-2oxoindolin-3-ylidene)-3,7-dihydrobenzo[1,2-b:4,5-b']difuran2,6-dione (BDOPV) as a new acceptor was developed in our group to construct polymers which show perfect OFET performance with electron mobility of up to 1 cm2 V−1 s−1.22−25 For instance, polymer BDOPV-2T exhibited n-type transport behavior with high electron mobility of up to 1.74 cm2 V−1 s−1 under ambient conditions; however, upon oxygen exposure, BDOPV-2T displayed ambipolar transport behaviors, which maintained high electron mobilities while hole mobilities significantly increased to 0.47 cm2 V−1 s−1.23 These results demonstrate that BDOPV unit is a promising acceptor to construct new polymers for high-performance ambipolar charge transport. In this work, we report the synthesis and characterization of three solution-processable ambipolar polymers, BDOPV-TT, BDOPV-BDT and BDOPV-BDSe. BDOPV acts as a strong electron-accepting moiety and efficiently reduces the LUMO energy levels of the desired polymers down to about −3.9 eV. Three units, thieno[3,2-b]thiophene (TT), benzo[1,2-b:4,5b']dithiophene (BDT) and benzo[1,2-b:4,5-b']diselenophene (BDSe), were applied as electron-donating moieties to finely adjust the energy levels to achieve the ambipolar transport behaviors. Moreover, OFETs based on these polymers as the active layers are all fabricated in nitrogen and tested

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Scheme 1. Synthetic Routes to Polymers BDOPV-TT, BDOPV-BDT, and BDOPV-BDSe

under ambient conditions. The device performance demonstates that donor moieties play a very improtant role on the ambipolar properties of the polymers. BDOPV-TT shows balanced high hole and electron mobilities over 1 cm2 V−1 s−1. However, although BDOPV-BDT and BDOPV-BDSe exhibit the balance ambipolar properties, both hole and electron mobilities are quite low relative to BDOPV-TT. Film morphology investigation by grazing incident X-ray diffraction (GIXD) and tapping-mode atomic force microscopy (AFM) indicates that these polymers with stronger crystallization tendency and more ordered interchain packing exhibit higher charge carrier mobility.

RESULTS AND DISCUSSION Synthetic Approach and Characterization. The synthetic routes of BDOPV-TT, BDOPV-BDT and BDOPV-BDSe are illustrated in Scheme 1. Donor moieties and BDOPV were synthesized following the literature precedures.22,23,26−28 Stillecoupling polymerizations between BDOPV and diffirent donor moieties were carried out to afford the corresponding polymers, BDOPV-TT, BDOPV-BDT and BDOPV-BDSe, in high yield.7 The purification of the crude polymers was performed by Soxhlet extraction with methanol and hexane and then with chloroform to extract polymer products, respectively. All the three polymers were dark black solids that gradually dissolved in chloroform. The molecular weights of BDOPV-TT, BDOPV-BDT and BDOPV-BDSe were

evaluated by gel permeation chromatography (GPC) at 150 oC in trichlorobenzene, and the results were summarized in Table S1. The thermal stability of these polymers was characterized by thermal gravity analysis (TGA). All polymers exhibited good thermal stability with high decomposition temperatures over 390 oC (Table S1 and Figure S1). Photophysical and Electrochemical Properties. The photophysical properties of all the polymers were investigated by UV-vis spectroscopy. As shown in Figure 1, a typical dual band absorption was observed in each polymer both in solution and in thin film, with π-π* transition band of the donor moiety in high energy range and intramolecular charge transfer band in low energy range. No obvious bathochromic shift in the film absorption spectra of these polymers was observed as compared with those in solution, indicating that there might be some pre-aggregation of these three polymers in solution (Figure S2).29 Moreover, all of these polymers displayed obvious 0-0 and 0-1 vibrational peaks both in dilute solution and in film due to their shape-persistent backbones. From BDOPVBDT to BDOPV-TT, the absorption maxima peak λmax of the π-π* transition bands varied from 475 nm to 520 nm while the 0-0 vibrational peaks of charge transfer bands red-shifted from 765 nm to 823 nm. The absorption features of BDOPV-TT largely red-shifted compared with those of BDOPV-BDT and BDOPV-BDSe due to the stronger electron-donating nature of TT moiety.

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Figure 1. UV-vis absorption spectra of polymers BDOPV-TT, BDOPV-BDT and BDOPV-BDSe (a) in chloroform (10−5 M) and (b) in thin film spin-coated on quartz glass substrates. (c) Cyclic voltammograms of BDOPV-TT, BDOPV-BDT, BDOPV-BDSe and ferrocene in n-Bu4NPF6/acetonitrile at a scan rate of 50 mV s−1.

Figure 2. DFT-calculated molecular orbitals of (a) BDOPV-TT, (b) BDOPV-BDT and (c) BDOPV-BDSe at the B3LYP/6-311G(d,p).

The cyclic voltammetry (CV) behaviors of these D-A conjugated polymers in film were measured in an acetonitrile solution containing 0.1 M n-Bu4NPF6 as supporting electrolyte. After calibrated using ferrocene as a reference, the HOMO and LUMO energy levels of these three polymers were estimated from the onset of oxidation and reduction features. As shown in Table S1, the LUMO levels of these three polymers are exactly the same due to their same electron-withdrawing unit BDOPV, whereas their HOMO levels are different, which is largely due to the different electron-donating parts. To further investigate the HOMO energy levels of these polymers, the

photoelectron spectroscopy (PES) was performed and the results are −5.70 eV for BDOPV-TT, −5.78 eV for BDOPVBDT and −5.74 eV for BDOPV-BDSe (Figure S3). Theoretical Calculations. To investigate the electronic properties of these polymers, theoretical calculation was performed by using density functional theory (B3LYP/6311G(d,p) level). The result of the calculation is shown in Figure 2. For computational simplicity, long alkyl chains were replaced by methyl groups. The results of calculation demonstrate that the HOMOs of these

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Figure 3. Transfer (a, b, e, f, i, j) and output characteristics (c, d, g, h, k, l) of (a-d) BDOPV-TT, (e-h) BDOPV-BDT, (i-l) BDOPVBDSe.

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Figure 4. (a-c) 2D-GIXD patterns and (d-f) tapping-mode AFM height images of (a, d) BDOPV-TT, (b, e) BDOPV-BDT and (c, f) BDOPV-BDSe.

polymers are well delocalized along the polymer backbones, whereas the LUMOs are almost localized on the BDOPV core. This is consistent with the result of electrochemical study and photophysical measurement, since the LUMO levels of the three polymers measured by CV are the same, while the HOMO levels of them, affected by both the donor and acceptor units, are different. OFET Performance. A top-gate/bottom-contact (TG/BC) device configuration was applied to fabricate FETs based on BDOPV-TT, BDOPV-BDT and BDOPV-BDSe. The polymer solutions (3 mg/mL in o-dichlorobenzene) were spincoated on patterned Au/SiO2/Si substrates as the active layers. After thermal annealing, a CYTOP solution as the dielectric layer was spin-coated onto the active layer, and then an aluminum layer as the gate electrode was thermally evaporated. Different from the other high-performance balanced ambipolar FETs, the evaluation of the three FETs was carried out under ambient conditions.14,15,18−21,30 After several annealing temperatures were tried, we found that the best device performance was achieved when the active layers were annealed at 180 °C. The transfer and output characteristics are shown in Figure 3 and the carrier mobilities and threshold voltages are summarized in Table 1. As shown in the transfer characteristics, all the three polymers displayed typical ambipolar behaviors. BDOPV-TT exhibited the highest electron mobility of up to 1.37 cm2 V−1 s−1 with an average mobility of 0.91 cm2 V−1 s−1 and the highest hole mobility of up to 1.70 cm2 V−1 s−1 with an average mobility of 0.80 cm2 V−1 s−1. Compared with BDOPV-TT, the electron mobilities (0.11 cm2 V−1 s−1 for BDOPV-BDT and 0.16 cm2 V−1 s−1 for BDOPV-BDSe) and hole mobilities (0.12 cm2 V−1 s−1 for BDOPV-BDT and 0.10 cm2 V−1 s−1 for BDOPV-BDSe) of the other two polymers are relatively low. The transfer characteristics of all the three polymers showed negligible hysteresis, indicating that there are only a few traps for electrons and holes. Table 1. OFET Device Performance of BDOPV-TT, BDOPV-BDT and BDOPV-BDSe Polymer

µe 2

−1 −1

µh 2

−1 −1

Vthe (V)

Vthh (V)

(cm V s )

(cm V s )

BDOPV-TT

1.37 (0.91/0.19)a

1.70 (0.80/0.33)

33

−65

BDOPV-BDT

0.11 (0.07/0.02)

0.12 (0.08/0.03)

38

−69

0.16 0.10 34 −66 (0.10/0.04) (0.08/0.02) a The data in the parentheses are the average values calculated from 25 devices/standard deviation for the measurements. BDOPV-BDSe

Thin-Film Microstructure Analysis. The film morphology of the three polymers were investigated to understand the intermolecular packing by using GIXD and AFM. Figure 4 displays GIXD images of all the three polymers. The film of BDOPV-TT shows a strong out-of-plane diffraction peak

indexed as (100) at 2θ of 2.65°, corresponding to a d-spacing of 26.8 Å (Figure S4). Another three orders of diffraction peaks were also observed, which were attributed to (h00) diffractions, suggesting a good lamellar edge-on packing mode in the film. Furthermore, an obvious in-plane (010) peak corresponding to a d-spacing of 3.45 Å was also observed, which was attributed to the π-π stacking distance. Compared to BDOPV-TT, the films of BDOPV-BDT and BDOPV-BDSe showed weaker out-of-plane diffraction peaks. Compared to the four orders of diffraction peaks observed in the BDOPVTT film, there are only three orders of diffraction peaks for BDOPV-BDSe and two orders of diffraction peaks for BDOPV-BDT. The diffraction peak (100) of BDOPV-BDSe at 2θ of 2.83o corresponds to a d-spacing of 25.1 Å, which is a little smaller than that of BDOPV-TT. As for BDOPV-BDT, the diffraction peak (100) at 2θ of 2.85o corresponds to a dspacing of 24.9 Å. Furthermore, compared to BDOPV-TT, a less obvious in-plane (010) peak can be observed in BDOPVBDSe, which corresponds to a π-π stacking distance of 3.50 Å. However, no signal could be observed in the in-plane direction of BDOPV-BDT film. These results are consistent with the obtained mobilities of field-effect transistors. As the packing order increases in the sequence of BDOPV-BDT, BDOPVBDSe and BDOPV-TT, the field-effect mobilities also increase. The same trend could also be observed in tapping-mode AFM. Figure 4 displays height images of the three polymer films, which were prepared by using the same procedures as those in the devices fabrication. All the three polymers show networklike morphology in films. The root-mean-square (RMS) deviations are 0.25 nm for BDOPV-BDT, 0.46 nm for BDOPV-BDSe and 0.90 nm for BDOPV-TT, which indicates that the one with the stronger crystallization tendency and more ordered interchain packing possesses the higher charge carrier mobilities.

CONCLUSION In summary, through donor moiety screening, we have synthesized three BDOPV-based donor-acceptor conjugated polymers, all of which are tested under ambient conditions and exhibit balanced ambipolar charge transport property. The highest electron mobility are optimized up to 1.37 cm2 V−1 s−1 and hole mobility up to 1.70 cm2 V−1 s−1, which are among the highest balanced ambipolar mobilities. BDOPV-TT is the first example that is tested under ambient conditions and possesses high balanced ambipolar mobilities larger than 1 cm2 V−1 s−1, which is significant in organic electronics. All the three polymers possess the same LUMO levels due to their same acceptor units, but show slightly different HOMO levels. BDOPV-TT with stronger electron-donating unit not only maintains the high electron mobility as those BDOPVbased polymers, but also shows comparable high hole mobility at the same time. Its higher electron and hole mobilities may be ascribed to the higher crystallization tendency and more ordered lamellar packing in film. Our results prove that BDOPV is an excellent electron-withdrawing moiety with

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which high-performance balanced ambipolar D-A polymers could be achieved. These ambipolar polymers largely broaden the promising application of BDOPV-based polymers in electronic devices.

EXPERIMENTAL SECTION Synthetic Details. All chemicals and reagents were used as received from commercial sources without further purification unless stated otherwise. The monomers BDOPV, TT, BDT and BDSe were prepared according to the literature procedures.22,23,26−28 BDOPV-BDT: BDOPV (60 mg, 0.035 mmol), 2,6bis(trimetylstannyl)benzo[1,2-b:4,5-b']dithiophene (18 mg, 0.035 mmol), Pd2(dba)3 (0.64 mg, 2 mol%), P(o-tol)3 (0.84 mg, 8 mol%) and 15 mL of toluene were added to a Schlenk tube. The tube was charged with nitrogen through a freeze-pump-thaw cycle for three times. The mixture was stirred for 48 h at 110 oC. Then, N,N'diethylphenylazothioformamide (20 mg) was added and the mixture was stirred for 3 h to remove all residual catalyst before being precipitated into methanol (100 mL). The precipitate was filtered through a PTFE filter and purified via Soxhlet extraction for 8 h with methanol, 12 h with hexane, and finally was collected with chloroform. The chloroform solution was then concentrated by evaporation and precipitated into methanol (200 mL) and filtered off as a dark solid (58 mg, yield: 95%). Elemental Anal. Calcd: for (C116H176N2O6S2)n: C, 79.22; H, 10.09; N, 1.59; Found: C, 78.02; H, 9.87; N, 1.49. BDOPV-BDSe: The synthetic procedure of BDOPV-BDSe is similar as described above (yield: 92%). Elemental Anal. Calcd: for (C116H176N2O6Se2)n: C, 75.21; H, 9.58; N, 1.51; Found: C, 73.25; H, 9.42; N, 1.41. BDOPV-TT: The synthetic procedure of BDOPV-TT is similar as described above (yield: 89%). Elemental Anal. Calcd: for (C112H174N2O6S2)n: C, 78.73; H, 10.26; N, 1.64; Found: C, 77.51; H, 10.06; N, 1.58.

ASSOCIATED CONTENT Supporting Information. Thermal properties, UV-vis, GIXD and PES spectra of polymers, and OFET performance. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTs This work was supported by the Major State Basic Research Development Program (2013CB933501) from the Ministry of Science and Technology, National Natural Science Foundation of China, Beijing Natural Science Foundation (2144049), and Doctoral Program Foundation from Ministry of Education of China (20130001120018). The authors thank beamline BL14B1 (Shanghai Synchrotron Radiation Facility) for providing the beam time. We thank Ms. Xinyue Zhang for the help in AFM measurements.

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