Tuning the Energy Levels of Aza-Heterocycle-Based Polymers for

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Tuning the Energy Levels of Aza-Heterocycle-Based Polymers for Long-Term n‑Channel Bottom-Gate/Top-Contact Polymer Transistors Suxiang Ma,† Guobing Zhang,*,†,‡ Feifei Wang,† Yanrong Dai,† Hongbo Lu,†,‡ Longzhen Qiu,†,‡ Yunsheng Ding,‡ and Kilwon Cho*,§

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National Engineering Laboratory of Special Display Technology, State Key Laboratory of Advanced Display Technology, Academy of Photoelectronic Technology, Hefei University of Technology, and ‡Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, Anhui Province Key Laboratory of Advance Functionl Materials and Devices, Hefei University of Technology, Hefei 230009, China § Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH) and Center for Advance Soft Electronics (CASE), Pohang 790-784, Korea S Supporting Information *

ABSTRACT: Conjugated polymer-based organic thin film transistors (OTFTs) have received tremendous attention due to their potential applications. In addition to their high performances, air stability is also essential for application and another main property that OTFTs have. In this paper, three aza-heterocycle (BABDF)-based polymers were designed and synthesized using strong donor thiophene−vinylene−thiophene (TVT), weak donor thiophene−cyanovinylene−thiophene (TCNT), and weak acceptor dithiazole (TZ) as counits. The lowest unoccupied molecular orbital (LUMO)/ highest occupied molecular orbital (HOMO) energy levels were effectively lowered by introducing TCNT and TZ units, especially for PBABDF-TZ, for which the too much deep LUMO/HOMO energy levels of −4.28/−6.06 eV were obtained. These levels are low enough for air-stable electron transport and large enough for the hole injection barriers in OTFTs. Consequently, the unencapsulated bottom-gate/top-contact (BG/ TC) devices exhibited unipolar electron transport under air conditions. Furthermore, these devices had high air stability and maintained unipolar electron transport with a mobility of up to 0.01 cm2 V−1 s−1 during the one-year characterization period. Very low LUMO and HOMO levels were necessary for electron transport and the hole barriers, respectively, and both were important for long-term, air-stable n-channel polymer transistors.

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INTRODUCTION Conjugated polymers are one of the most promising semiconductors for organic thin film transistors (OTFTs) due to their solution processability, various structures, and potential application in low-cost, lightweight flexible devices.1−4 In the past few years, extensive synthetic and device research has been conducted to improve OTFT performances, and charge mobilities of over 10 cm2 V−1 s−1 have been achieved for both p-type and n-type polymer semiconductors.5−7 Although the performances of conjugated polymerbased devices have increased dramatically, OTFT device air stability (or storage lifetime) has come under intense scrutiny.8 Many p-type polymers can be tested in air conditions even if the semiconductor layer is exposed to air directly and also maintain stable hole transport for a long time.1 Compared with the stable p-type polymers, the development of their n-type counterparts is obviously falling behind. For electron transport conjugated polymers, decreasing the lowest unoccupied molecular orbital (LUMO) energy level of conjugated © XXXX American Chemical Society

polymers to facilitate electron injection, stable transport, and air stability is necessary.9−11 The generally accepted suitable LUMO energy level as discussed by Facchetti et al. should fall below −4.0 eV.12 In recent years, a series of n-type conjugated polymers was created by incorporating a strong acceptor, such as NDI, PDI, or BDOPV, into the polymer backbone. These were synthesized and exhibited air-stable electron transport under air conditions.13−16 For instance, Facchetti et al. reported an NDI-based donor−acceptor polymer devices fabricated in the top-gate/bottom-contact (TG/BC) configuration that displayed unipolar electron transport with electron mobility as high as 0.85 cm2 V−1 s−1.13 However, most of the conjugated polymers that exhibited n-type transport were usually characterized in a vacuum and inner gas.17,18 Otherwise, the Received: April 20, 2018 Revised: July 5, 2018

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DOI: 10.1021/acs.macromol.8b00839 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthetic Procedures of PBABDF-TVT, PBABDF-TCNT, and PBABDF-TZ

electronegative nitrogen atoms, make the BABDF acceptor strongly electron deficient.25,26 Second, three co-units, namely thiophene−vinylene−thiophene (TVT), thiophene−cyanovinylene−thiophene (TCNT), and dithiazole (TZ), were incorporated into the polymer backbone. The presence of the cyano group in TCNT and electronegative nitrogen in TZ can strongly attract electrons and further lower the LUMO/ highest occupied molecular orbital (HOMO) energy levels in comparison to analogues that consist of electron-rich units such as TVT.27−29 The results indicated that the energy levels could be further lowered by introducing TCNT and TZ units, especially for PBABDF-TZ, since LUMO/HOMO energy levels of −4.28/−6.06 eV were obtained. Consequently, unencapsulated bottom-gate/top-contact (BG/TC) OTFTs based on PBABDF-TZ exhibited unipolar electron transport under air conditions and maintained high air stability with a mobility of up to 0.01 cm2 V−1 s−1 during a characterization period of one year. To the best of our knowledge, this is the first report of such a long-term, air-stable n-channel polymer transistor without any encapsulation of the semiconductor layer. These results indicated that the much deep LUMO and HOMO levels were essential for electron transport and hole barriers under air conditions, respectively, and both were important for long-term, air-stable n-channel polymer transistors. The findings also demonstrated that a long-term, airstable unipolar electron transport polymer can be obtained through rational macromolecular design guided by further lowering the LUMO/HOMO levels.

TG device structure should be used and the polymer semiconductor layer should be encapsulated by a dielectric layer, such as poly(methyl methacrylate) (PMMA) and CYTOP (Figure S1a,b).13,15,16 Air stability can be increased remarkably by encapsulation in TG configuration (Figure S1a,b), which can reduce the negative influences from water and oxygen under air conditions.19,20 The n-type polymers are vulnerable due to the charge carrier trapping water and oxygen under air conditions, and unencapsulated polymer semiconductors can hardly maintain n-type characteristics when espoused to air.21,22 As such, polymer transistors (Figure S1c,d) with semiconductor layers exposure directly to air conditions that still exhibit unipolar electron transport are very rare.23,24 Moreover, a long-term, air-stable n-channel polymer transistor without any encapsulation of the semiconductor layer (such as bottom-gate/top-contact configuration) has also rarely been reported. Considering that n-type polymer semiconductors play a significant role in transistors and complementary circuits, the development of air-stable polymer semiconductors is crucial to avoid costly vacuum or inert atmosphere-based fabrication steps and device encapsulation. Herein, we reported on the design and synthesis of three aza-heterocycle-based electron-transporting conjugated polymers (PBABDF-TVT, PBABDF-TCNT, and PBABDF-TZ, Scheme 1) that exhibited long-term, air-stable unipolar electron transport characteristics by effectively tuning the LUMO/HOMO energy levels of the conjugated polymer. First, the aza-heterocycles unit (3E,7E)-3,7-bis(6-bromo-1-(4decyltetradecyl)-2-oxo-7-azaindolin-3-ylidene)benzo[1,2-b:4,5b′]difuran-2,6(3H,7H)-dione (BABDF) was used as the acceptor since it has a symmetric and planar structure based on a large fused aromatic ring, and the lactone groups in the central core, in combination with both lactam groups and

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RESULTS AND DISCUSSION Synthesis and Characterization. The synthetic route to creating BABDF-based conjugated polymers is outlined in Scheme 1, and detailed procedures are presented in the B


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Macromolecules Table 1. Molecular Weights and Optical and Electrochemical Properties of Polymers λmax [nm] polymer

Mn/PDIa [kDa]

Tdb [°C]

solution

film

λonset [nm]

Egopt c [eV]

ELUMOd [eV]

EHOMOd [eV]

Egcv [eV]

PBABDF-TVT PBABDF-TCNT PBABDF-TZ

48.1/2.75 46.1/2.97 40.4/2.58

402 391 380

860 830 789

853 829 787

1022 960 880

1.21 1.29 1.41

−4.02 −4.15 −4.28

−5.63 −5.88 −6.06

1.61 1.73 1.78

a Mn and PDI of the polymers were determined using polystyrene standards in trichlorobenzene. bThe 5% weight loss temperatures. cEgopt = 1240/ red + + λonset (in film). dHOMO = −(4.75 + Eox onset), LUMO = −(4.75 + Eonset), the redox Fc/Fc was located at 0.05 V related Ag/Ag .

Experimental Section (Supporting Information). The M and three distannylated monomers were synthesized according to the previous literature.25,27,28 Polymers were created through Stille step-growth polymerization of the monomer M with distannylated TVT, TCNT, and TZ. These three polymers were purified via Soxhlet extraction and followed by precipitation from methanol as black solids. The molecular weights were determined by gel permeation chromatography (GPC) in trichlorobenzene. The corresponding data are listed in Table 1. The number-average molecular weights (Mn) and polydispersity indexes (PDI) values of three polymers are in the ranges 40.4−48.1 kDa and 2.58−2.97, respectively. The thermal properties of the polymers were characterized by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), as shown in Figure S2. The three polymers show good thermal stability with a 5 wt % weight loss for decomposition temperatures (Td) over 380 °C (Table 1). DSC curves clearly exhibited endothermic peaks at 15−21 °C while being heated, and corresponding exothermic peaks at 11−15 °C were also observed during cooling. The three polymers exhibited similar transition temperatures which may be attributed to the phase transition of the long alkyl chains.30 Optical and Electrochemical Properties. Figure 1 shows the normalized UV−vis−NIR absorption spectra of the three polymers in chlorobenzene solutions and solid thin films. All the three polymers showed typical dual-band signals; the intense and broad ones with clear shoulder peaks were located in the low-energy region, whereas the weak ones were located in the high-energy region. As shown in Table 1, the absorption maxima of PBABDF-TVT, PBABDF-TCNT, and PBABDFTZ in dilute solutions were observed at 860, 830, and 789 nm, respectively. During transition from a solution to a solid state, the absorption maxima of the three polymers were found to be slightly blue-shifts of about 1−7 nm compared to the corresponding ones characterized from the solution state. Previous studies also observed this similar blue-shift in isoindigo-, thieno-isoindigo-, and aza-isoindigo-based polymers, likely due to the formation of H-aggregates.31−33 Compared to PBABDF-TVT, the absorptions of PBIBDFTCNT and PBABDF-TZ were obviously blue-shifted in both solution and solid states. This can be attributed to the different electron characteristics of TVT, TCNT, and TZ unit. The cyano group is an electron-withdrawing group that makes the TCNT unit a weak donor compared to the TVT unit and results in a weak push−pull interaction.27 The TZ unit was considered a weak acceptor because of the presence of electronegative nitrogen atoms. Consequently, PBABDF-TZ should be considered an all-acceptor polymer since the absence of a push−pull interaction in the PBABDF-TZ backbone results in strong blue-shifts compared to the other two polymers. 34 The optical bandgaps of PBABDF-TVT, PBABDF-TCNT, and PBABDF-TZ were 1.21, 1.29, and 1.41 eV, respectively, which were determined from the thin

Figure 1. UV−vis−NIR absorption spectra of the three polymers in solution and in thin films.

film absorption onset. The different bandgaps may also be attributed to the different push−pull interactions within the polymer backbone. The cyclic voltammetry (CV) experiments were carried out to investigate the LUMO and HOMO energy levels of the three polymers, which are closed to the transporting characteristics of devices (Figure 2a). The corresponding data are summarized in Table 1, and LUMO/HOMO energy levels are schematically depicted in Figure 2b. Very deep LUMO/HOMO levels were obtained for the polymers due to the strong electron-withdrawing BABDF unit. The PBABDFTVT with a TVT co-unit showed LUMO/HOMO levels of −4.02/−5.63 eV, indicating the potential of facile electron injection from appropriate electrodes and n-type formation in OTFT devices. PBABDF-TCNT, which had the less electronrich TCNT co-unit, displayed deeper LUMO/HOMO levels C


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Thin Film Microstructural Characterization. The film microstructures were characterized by two-dimensional grazing incidence X-ray diffraction (2D-GIXD). Figure 3 shows 2DGIXD patterns of the thermally annealed films spin-coated on ocadecyltrichlorosilane (OTS)-treated SiO2/Si substrates. The corresponding 1D diffraction profiles and calculated crystallographic parameters are also presented in Figure 3 and Table S1, respectively. PBABDF-TVT exhibited well-defined (100)− (500) diffraction peaks along the out-of-plane qz-axis, indicating that PBABDF-TVT had long-range ordered lamellar structures, which are desired properties for achieving high mobilities in OTFTs devices. After introducing the TCNT and TZ unit into the BABDF-based polymer backbone, the polymer thin film showed relatively poor crystal structures, with defined (100)−(400) and (100)−(300) diffraction peaks for PBABDF-TCNT and PBABDF-TZ, respectively. This may result in relatively low performances for these two polymers. The lamellar distances of the three polymers derived from the (h00) diffraction peak were found to be 32.88 Å for PBABDFTVT, 29.90 Å for PBABDF-TCNT, and 30.63 Å for PBABDFTZ. Apart from the (h00), the (010) diffraction peaks were also clearly observed from the in-plane qxy-axis and attributed to the π−π stacking. These results clearly suggested that all polymer films tested favored edge-on orientation on the substrate and had close π−π stacking distances calculated to be 3.53, 3.42, and 3.51 Å for PBABDF-TVT, PBABDF-TCNT, and PBABDF-TZ, respectively. It should be noted that the PBABDF-TZ exhibited a fairly small π−π stacking distance even though the polymer backbone is an all-acceptor structure compared to those of the TVT- and TCNT-based polymers. The close π−π stacking may create kinetic barriers to prevent the diffusion of water and oxygen into the active channel region and would be beneficial for the air stability of OTFTs.36 Characteristics of OTFTs. To investigate the carrier transport performances of three conjugated polymers, OTFT

Figure 2. (a) Cyclic voltammograms of the three polymers. (b) Experimental LUMO/HOMO energy levels for the three polymers.

of −4.15/−5.88 eV, which is approximately 0.15 eV lower than those of the TVT analogue (PBABDF-TVT). PBABDF-TZ, which had the electron-deficient TZ co-unit, exhibited the deepest LUMO/HOMO levels at −4.28/−6.06 eV. Moreover, the reduction peak of PBABDF-TZ was reversible and had a much more intense current response than the irreversible oxidation peak, indicating the stability of reduced PBABDF-TZ as an electron carrier.35 PBABDF-TZ is among the most electron-withdrawing polymers ever reported and simultaneously had both deep LUMO and HOMO energy levels.34 The results indicated that PBABDF-TZ may be the best potential candidates for unipolar and stable electron transport semiconductor under air conditions because the too much deep LUMO levels would facilitate electron injection and stable transport, and the deep HOMO level would result in large hole injection barriers.

Figure 3. Crystalline nature and molecular orientation of the polymers after thermal annealing (PBABDF-TVT: 260 °C; PBABDF-TCNT and PBABDF-TZ: 290 °C). (a) 2D-GIXD patterns. (b) In-plane line cuts of GIXD. (c) Definition of edge-on structure relative to the substrate. (d) Out-of-plane line cuts of GIXD. D


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Figure 4. (a−c) Output and (d−f) typical transfer curves of OTFT devices at an optimized annealing temperature. All the BG/TC devices were characterized under vacuum conditions.

Figure 5. (a) BG/TC device configuration. (b−d) Output and (e−g) typical transfer curves of fresh devices at an optimized annealing temperature. All the BG/TC devices without any encapsulation of semiconductor layer were characterized under air conditions.

annealed at different temperatures ranging from 180 to 290 °C. As listed in Table S2, the unipolar electron mobilities were improved effectively by using thermal annealing technology for all conjugated polymers, likely due to the improvement of the ordering structure and the decreased effect of the residual solvent.37 The optimized temperatures were 260−290 °C for all three polymers, and the maximum electron mobilities were as high as 2.42 cm2 V−1 s−1 for PBABDF-TVT, 0.74 cm2 V−1 s−1 for PBABDF-TCNT, and 0.055 cm2 V−1 s−1 for PBABDFTZ. These results indicated that a LUMO level of about −4.0 eV was low enough for electron injection and stable transport

devices were fabricated by spin-coating the corresponding polymers on OTS-treated substrates with BG/TC configurations. These devices were initially characterized under vacuum conditions, and their mobilities were obtained from the saturation regimes. Typical I−V curves are shown in Figure 4, and the corresponding data are summarized in Table S2. Three polymer-based devices showed clear unipolar electron transport characteristics. The nonannealed (N/A) devices exhibited mobilities of 0.82, 0.45, and 0.011 cm2 V−1 s−1 with the Ion/Ioff ratios over 105 for PBABDF-TVT, PBABDFTCNT, and PBABDF-TZ, respectively. These devices were E


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Figure 6. Air stability of PBABDF-TVT and PBABDF-TCNT-based unencapsulated TG/BC OTFTs. All the devices were directly measured under air conditions.

Figure 7. Air stability of PBABDF-TZ-based unencapsulated BG/TC OTFTs. All the devices were directly measured under air conditions.

electrons and holes (Figure 6a), respectively. These results indicated that PBABDF-TVT could not exhibit unipolar electron transport and that electron mobility gradually decayed when exposed to air conditions directly. First, the LUMO energy level (−4.02 eV) of PBABDF-TVT was insufficiently low for the air-stable transport of electrons, and the negative (electron) carriers in PBABDF-TVT were easily trapped at interfaces generated by air oxidants, such as water and oxygen.10 Second, the occurring hole transport under air conditions was mainly attributed to the oxygen doping that resulted in the increase of the energy levels of the highest three occupied molecular orbitals, therefore reducing the hole injection barrier from the gold electrode. For PBABDFTCNT, the relatively lower LUMO/HOMO energy levels (−4.15/−5.88 eV) caused by introducing TCNT unit into the polymer backbone may lead to a positive effect on stable transport under air conditions. As shown in Figure 5 and Table S3, PBABDF-TCNT exhibited unipolar electron characteristics when just moving in air, and the electron mobility was as high as 0.16 cm2 V−1 s−1. The air stability was also monitored over the 90 days of storage under air conditions (Figure 6b). Unfortunately, PBABDF-TCNT maintained unipolar electron transport for only 2 days before ambipolar transport characteristics with gradually decreased electron mobility were observed. After 90 days of air storage, PBABDF-TCNTbased OTFTs exhibited electron-dominant ambipolar transport with the electron and hole mobilities of 0.017 and 4.26 × 10−4 cm2 V−1 s−1, respectively. Compared to the PBABDFTVT, PBABDF-TCNT has the ability to work as an n-channel

uner vacuum conditions. Compared with PBABDF-TVT and PBABDF-TCNT, PBABDF-TZ exhibited much lower device performances, probably because of the poorest crystal structures of PBABDF-TZ as shown in Figure 3. The atomic force microscopy (AFM) images of the annealing films are also shown in Figure S5. PBABDF-TVT exhibited highly connected polymer fibers, such uniform nanofibers have interconnected network, which probably form an efficient channel to obtain excellent carrier transport. PBABDF-TCNT showed relatively shorter fibrillar network compared with the PBABDF-TVT film. For the PBABDF-TZ film, the short, fine, and disordered nanofibrillar crystals were observed over the entire area; this might be responsible for the low device performance. Air Stability of OTFTs. To investigate device air stability, the BG/TC devices made using the three polymers were also directly characterized under air conditions (Figure 5a). The fresh device performances are summarized in Table S3, and the typical I−V curves are also shown in Figure 5. Once exposed to air conditions, PBABDF-TVT exhibited electron-dominant ambipolar transport characteristics with obviously degraded electron mobility and weak hole mobility as compared with devices measured under vacuum conditions. The highest electron mobility of 1.10 cm2 V−1 s−1 (Figure 5b,e) was obtained with a hole mobility of 0.09 cm2 V−1 s−1 (Figure S4). After an exposure time of 1 week, the PBABDF-TVT-based devices exhibited a gradual decay of electron mobilities and unchanged hole mobility (Figure 6a). After 90 days of air exposure, the performances continued to decrease, and the highest mobilities were 0.015 and 0.011 cm2 V−1 s−1 for the F


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small molecules since the resistance to charge carrier trapping under air conditions for the small molecule films occurred at a LUMO value of about −4.3 eV.9,11 Furthermore, in comparison to the above results, a deep HOMO level (below approximately −6.0 eV) was also essential for unipolar electron transport under air conditions for resistance to oxygen doping and maintaining a large hole-injection barrier. To the best of our knowledge, the HOMO level value for air-stable electron transport polymers had rarely caused concern in previous studies, likely due to the fact that air-stable unipolar transport polymers were very rare unless the TG configuration was used, in which the semiconductor layer was encapsulated by a dielectric layer.

semiconductor under ambient conditions but exhibited limited lifetime (2 days). Moreover, the electron mobility drop reduces with the introduction of an electron-withdrawing cyano group in the TCNT unit, presumably due to the lowering LUMO level. However, PBABDF-TCNT, which had deep LUMO/HOMO (−4.15/−5.88 eV) levels and close π−π stacking (3.42 Å), still had difficulty maintaining long-term, airstable unipolar electron transport characteristics. The fresh OTFT performances of PBABDF-TZ are listed in Table S3, and the I−V curves are shown in Figure 5. The highest mobility obtained was 0.038 cm2 V−1 s−1 with a high Ion/Ioff ratio of over 105. PBABDF-TZ showed far less degradation in carrier mobility measured in air vs vacuum conditions. Moreover, compared to the other two polymers, PBABDF-TZ mobility degraded least, indicating that PBABDF-TZ had the most air-stable characteristics. To further investigate their air stability, the devices annealed at 260 and 290 °C were both exposed to air conditions for a year, and their performance measures, such as mobility and Ion/Ioff, were monitored. As shown in Figure 7 and Figures S6−S9, PBABDF-TZ-based devices not only maintained unipolar electron transport after long-term exposure to air conditions but also exhibited excellent electron mobility of over 0.01 cm2 V−1 s−1 with an Ion/Ioff ratio over 104 one year later. The nchannel OTFTs showed no obvious degradation in performance during operation in air, mainly due to the deep LUMO energy level (−4.28 eV), which can stabilize the injected charge carrier against the reactions with air oxidants. This is the chemical block between the oxygen and the polymer. Furthermore, the close π−π stacking may be act as a kinetic barrier to slow the diffusion of oxygen and water in polymer film. It should be noted that the devices did not exhibit any hole transport characteristics during their one-year storage. This was because of the low HOMO level (−6.02 eV), which formed a large enough barrier. Consequently, the hole was difficult to inject into the semiconductor layer from the Au electrode even if the polymer was doped by oxygen. Detailed analysis of the relationship between polymer energy levels and transport characteristics indicated that a LUMO level of at least −4.3 eV is essential for stabilizing electrons during charge transport and resisting charge carrier trapping under air conditions. In comparison to the previously reported laddertype semiconductors,12 it is generally believed that the appropriate LUMO energy level of n-channel semiconductors should be below −4.0 eV for an effective charge carrier injection and air-stable transport.12,37−39 In this study, a LUMO level of about −4.0 eV is only sufficient for the unipolar electron transport under vacuum conditions, and the devices exhibited ambipolar transport and showed significant degradation in performance when moved to air. This may be because that the determinations of energy level by electrochemical method are not very strict, and thus there should be variation depending on the conditions as well as on research group. Moreover, the kinetic barriers can also slow down the diffusion of water/oxygen and endow remarkable air stability, although the conjugated polymer had relatively high LUMO energy level occasionally.40 In BG/TC OTFTs fabricated with PBABDF-TCNT thin film with a LUMO level slightly higher than PBABDF-TZ at ∼0.13 eV, the electron mobilities are also clearly vulnerable to charge trapping by air-atmosphere-based species. Interestingly, the needed LUMO level for BABDFbased air-stable polymers is very consistent with the reported naphthalene diimide (NDI) and perylene diimide (PDI) based

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CONCLUSION Three BABDF-based polymers with the TVT, TCNT, and TZ as co-units were designed and synthesized. The LUMO/ HOMO energy levels could be effectively lowered by introducing the electron-deficient group (CN) and unit (TZ). PBABDF-TZ had deep LUMO/HOMO energy levels of −4.28/−6.02 eV, which places it among the most electronwithdrawing polymers ever reported, and simultaneously had both deep LUMO and HOMO energy levels. PBABDF-TVT exhibited excellent unipolar electron transport (2.4 cm2 V−1 s−1) under vacuum conditions, and PBABDF-TCNT displayed short-time unipolar electron characteristics under air conditions due to the relatively lower LUMO/HOMO levels. Further low HOMO/LUMO levels were achieved by introducing the TZ unit; PBABDF-TZ exhibited long-term, air-stable n-channel characteristics during its one-year exposure. To the best of our knowledge, this is the first report of such a long-term, air-stable n-channel polymer transistor without any encapsulation of the semiconductor layer. These results in this study indicated that the long-term, air-stable polymer transistors need not only much deep LUMO levels for electron transport but also very low HOMO levels for the hole barrier. Furthermore, these findings demonstrated that a longterm, air-stable electron transport polymer can be obtained through rational macromolecular design guided by lowering the LUMO/HOMO levels.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00839. Experimental section and other data (such as TGA,

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DSC, and OTFT performances) (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G.Z.). *E-mail: [email protected] (K.W.). ORCID

Guobing Zhang: 0000-0001-6053-2015 Longzhen Qiu: 0000-0002-8356-6303 Kilwon Cho: 0000-0003-0321-3629 Notes

The authors declare no competing financial interest. G


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Macromolecules

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ACKNOWLEDGMENTS This study was supported by the National Natural Science Foundation of China (NSFC, Grants 51573036 and 51703047), the Fundamental Research Funds for the Central Universities (Grant No. JZ2018HGPB0276), the grant (Code No. 2011-0031628) from the Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Science, ICT, and Future Planning, Korea, the Distinguished Youth Foundation of Anhui Province (1808085J03), the Foundation of Anhui Provincial Education Department (kj2015jd15), and the China Postdoctoral Science Foundation (2018M632524). The authors thank 3C beamlines (the Pohang Accelerator Laboratory in Korea) for providing the beam time.

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