Article pubs.acs.org/Macromolecules
Donor−Acceptor Conjugated Polymers Based on Indacenodithiophene Derivative Bridged Diketopyrrolopyrroles: Synthesis and Semiconducting Properties Hao Song,† Yunfeng Deng,*,†,‡ Yao Gao,§ Yu Jiang,§ Hongkun Tian,§ Donghang Yan,§ Yanhou Geng,*,†,‡ and Fosong Wang§ †
School of Materials Science and Engineering and Tianjin Key Laboratory of Molecular Optoelectronic Science, Tianjin University, Tianjin 300072, P. R. China ‡ Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, P. R. China § State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, P. R. China S Supporting Information *
ABSTRACT: Two indacenodithiophene derivative bridged diketopyrrolopyrroles (DPP), i.e., 2,7-bis(2,5-bis(2-decyltetradecyl)-3,6-dioxo-4-(thiophen-2-yl)-2,3,5,6-tetrahydropyrrolo[3,4-c]pyrrol-1-yl)-s-indaceno[1,2-b:5,6-b′]dithiophene4,9-dione (DDPP-PhCO) and 2,2′-(2,7-bis(2,5-bis(2-decyltetradecyl)-3,6-dioxo-4-(thiophen-2-yl)-2,3,5,6-tetrahydropyrrolo[3,4-c]pyrrol-1-yl)-s-indaceno[1,2-b:5,6-b′]dithiophene4,9-diylidene)dimalononitrile (DDPP-PhCN), were developed via intramolecular Friedel−Crafts acylation and Knoevenagel condensation. A series of donor−acceptor (D−A) conjugated polymers were synthesized by Stille or direct arylation polycondensation with these two novel units as acceptors and vinyl or thiophene derivatives as donors. The polymers with DDPP-PhCO as acceptor unit exhibited optical bandgaps (Eopt g ) of ca. 1.2 eV and the highest occupied molecular orbital (HOMO) energy levels of ∼−5.3 eV with the difference less than 0.1 eV, and their lowest unoccupied molecular orbital (LUMO) levels were in the range of −3.73 to −3.91 eV. The polymer based on DDPPPhCN showed similar HOMO level (−5.29 eV) but remarkably lower LUMO level (−4.21 eV). Top-gate/bottom-contact (TGBC) organic field-effect transistors (OFETs) of all the polymers exhibited ambipolar transport behavior with the highest hole mobility (μh) and electron mobility (μe) up to 1.09 and 0.44 cm2 V−1 s−1, respectively, in air. Owing to their favorable molecular orientation and frontier molecular orbital distribution, the polymers based on DDPP-PhCO displayed much higher hole and electron mobilities than that based on DDPP-PhCN.
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increase the π−π overlap or intermolecular interaction between polymer backbones, which would improve both intra- and intermolecular charge transport properties.14,21,30,34 Diketopyrrolopyrrole (DPP) and isoindigo (IID) are two very favored Aunits for high mobility CPs.35−42 Several IID derivatives such as BDOPV,11,21,22 INDF,26 NBDOPV,24,25 and DIID,27 which have more extended conjugated framework than IID itself, have been developed, allowing the synthesis of novel D−A CPs with great ambipolar or n-transport properties. However, the synthesis of DPP derivatives flanked or bridged with polycyclic aromatics is rather difficult for the lack of efficient synthetic methods. Recently, we found that DPP derivatives flanked with polycyclic aromatics were readily accessible via intramolecular Friedel−Crafts acylation.43 In the current paper, we successfully synthesized two DPP derivatives in which two DPP units were
INTRODUCTION Conjugated polymers (CPs) attract growing interest owing to their potential applications in various optoelectronic devices such as organic field-effect transistors (OFETs).1−3 Alternatively linking electron-rich and electron-deficient aromatic units has become the most popular design rule for such materials since the resultant donor−acceptor (D−A) CPs are characterized by strong intermolecular interaction, tunable electronic properties, and flexibility in material design.4−7 Thus, much effort has been devoted to study how the chemical structures of D- and A-units influence the photophysical properties as well as semiconducting properties of D−A CPs.8−14 In the past few years, the incorporation of polycyclic D- or (and) A-units into D−A CPs has been proved to be one of the most promising strategies to obtain high mobility polymeric semiconductors.8,9,11,14−33 The rigid and planar structures of these polycyclic aromatics not only help to reduce torsional disorder along the polymer backbone but also benefit to © XXXX American Chemical Society
Received: December 25, 2016 Revised: February 27, 2017
A
DOI: 10.1021/acs.macromol.6b02781 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Scheme 1. Synthetic Route to DDPP-PhCO-2Br and DDPP-PhCN-2Bra
Reaction conditions: (i) K2CO3, Pd(OAc)2, pivalic acid (PivOH), N,N′-dimethylacetamide (DMAc), 110 °C; (ii) trifluoroacetic acid, dichloromethane (CH2Cl2), rt; (iii) (1) oxalyl chloride, N,N′-dimethylformamide (DMF), CH2Cl2, rt; (2) AlCl3, CH2Cl2, 0 °C to rt; (iv) Nbromosuccinimide (NBS), chloroform (CHCl3), 0 °C to rt; (v) malononitrile, TiCl4, pyridine, CH2Cl2, 0 °C to rt. a
Scheme 2. Synthetic Route to Polymersa
a Reaction conditions: (i) Pd2(dba)3, P(o-tol)3, toluene, 110 °C, 48 h; (ii) Herrman’s catalyst, P(o-MeOPh)3, PivOH, Cs2CO3, toluene, 120 °C, 48 h. R = 2-decyltetradecyl.
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bridged with polycyclic aromatics, i.e., 2,7-bis(2,5-bis(2-decyltetradecyl)-3,6-dioxo-4-(thiophen-2-yl)-2,3,5,6-tetrahydropyrrolo[3,4-c]pyrrol-1-yl)-s-indaceno[1,2-b:5,6-b′]dithiophene4,9-dione (DDPP-PhCO) and 2,2′-(2,7-bis(2,5-bis(2-decyltetradecyl)-3,6-dioxo-4-(thiophen-2-yl)-2,3,5,6-tetrahydropyrrolo[3,4-c]pyrrol-1-yl)-s-indaceno[1,2-b:5,6-b′]dithiophene4,9-diylidene)dimalononitrile (DDPP-PhCN). Based on them, a series of D−A CPs using different D-units were synthesized, and their photophysical and semiconducting properties were studied in detail.
RESULTS AND DISCUSSION
Synthesis. The synthesis of DDPP-PhCO and DDPPPhCN was started from 3,6-bis(thiophen-2-yl)-N,N′-(2-decyltetradecyl)-1,4-diketopyrrol[3,4-c]pyrrole (1), as shown in Scheme 1. The use of long and branched 2-decyltetradecyl groups was intended to improve the solubility of the final polymers. First, Pd-catalyzed direct arylation between di-tertbutyl 2,5-dibromoterephthalate (2) and 1 in N,N′-dimethylacetamide (DMAc) gave compound 3 in 61% yield, which was converted to 4 quantitatively. After treatment with oxalyl chloride and AlCl3-catalyzed intramolecular Friedel−Crafts acylation, the compound 4 was converted to DDPP-PhCO as B
DOI: 10.1021/acs.macromol.6b02781 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Table 1. Molecular Weights and Thermal, Photophysical, and Electrochemical Properties of the Polymers λmax [nm] polymer
Mn [kDa]
Đ
Td,5% [°C]
PDOV PDOT PDOBT PDOTVT PDO4FTVT PDNTVT
122.0 97.6 102.8 94.1 48.2 59.0
4.15 4.58 2.46 2.42 1.97 2.79
389 398 397 395 397 397
solution 771, 733, 732, 735, 721, 785
831 832 832 833 796
film 769, 728, 727, 728, 725, 782
833 833 832 833 806
Eopt g [eV]
ELUMO/Ereonset [eV]/[V]a
a EHOMO/Eox onset [eV]/[V]
Ecv g [eV]
1.17 1.18 1.19 1.16 1.20 −b
−3.91/−0.55 −3.85/−0.61 −3.74/−0.72 −3.73/−0.73 −3.79/−0.67 −4.21/−0.25
−5.24/0.78 −5.29/0.83 −5.27/0.81 −5.24/0.78 −5.33/0.87 −5.29/0.83
1.33 1.44 1.53 1.51 1.54 1.08
re ox HOMO and LUMO energy levels were calculated from the formula EHOMO = −(4.46 + Eox onset) eV and ELUMO = −(4.46 + Eonset) eV, in which Eonset b re and Eonset are oxidation and reduction onset potentials, respectively, versus SCE. The bandgap of PDNTVT was difficult to be determined because of the broad absorption tail at the near-infrared region. a
Figure 1. Chemical structures (a), solution UV−vis−NIR absorption spectra (b, 1 × 10−5 mol L−1 in chloroform), solution cyclic voltammograms (CV, c), and frontier molecular orbital energy levels (d) of DDPP-Ph, DDPP-PhCN, and DDPP-PhCO. Solution CV measurements were conducted in anhydrous CH2Cl2 at a scan rate of 100 mV s−1 with Bu4NPF6 (0.1 mol L−1) as electrolyte and saturated calomel electrode (SCE) as reference electrode, and energy levels were calculated from the redox onset potentials. R = 2-decyltetradecyl.
permeation chromatography (GPC) with polystyrene as standard. All these polymers exhibited 5% mass loss temperatures ca. 390 °C, and no obvious phase transition was observed from room temperature (rt) to 280 °C, as revealed by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements (Figure S1 in the Supporting Information). Properties of DDPP-PhCO and DDPP-PhCN. To evaluate the impact of the polycyclic bridges on the properties of DPP derivatives, absorption spectra and cyclic voltammograms (CV) of DDPP-PhCO and DDPP-PhCN were measured. For comparison, DDPP-Ph, which has a dithienylphenyl bridge between two DPP units, was also synthesized and studied. As shown in Figure 1, compared to DDPP-Ph, the absorption maximum (λmax) of DDPP-PhCO was slightly blueshifted while that of DDPP-PhCN exhibited a small red-shift. Additionally, a weak absorption band was observed at long wavelength for DDPP-PhCO and DDPP-PhCN (please see the inset, Figure 1b), which can be assigned to the symmetryforbidden n−π* transition.43,45,46 In solution CV, all three compounds showed reversible oxidation process and HOMO
a dark-blue solid in a two-step yield of 78%. Subsequently, Knoevenagel condensation of DDPP-PhCO and malononitrile afforded DDPP-PhCN in high yield (93%). Finally, the bromination of DDPP-PhCO and DDPP-PhCN using Nbromosuccinimide (NBS) gave the monomers DDPP-PhCO2Br and DDPP-PhCN-2Br in yields of 80 and 84%, respectively. As shown in Scheme 2, six CPs based on DDPP-PhCO and DDPP-PhCN, i.e., PDOV, PDOT, PDOBT, PDOTVT, PDO4FTVT, and PDNTVT, were synthesized by using different D-units. PDO4FTVT was prepared by direct arylation polycondensation according to our previous report,42 and other polymers were synthesized by Stille polycondensation with Pd2(dba)3/P(o-tol)3 as the catalyst.44 All the polymers are readily soluble in chlorinated solvents such as chlorobenzene (CB) and o-dichlorobenzene (o-DCB) except for PDOV, which is only soluble in 1,2,4-trichlorobenzene (1,2,4-TCB) due to its highest rigidity caused by less C−C single bonds within the backbone as compared with other polymers. Number-average molecular weights (Mn) of the polymers are in the range of 48− 122 kDa (Table 1) as measured by high temperature gel C
DOI: 10.1021/acs.macromol.6b02781 Macromolecules XXXX, XXX, XXX−XXX
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Figure 2. Solution (a) and film (b) UV−vis−NIR absorption spectra of the polymers. Solution spectra were measured in o-DCB or 1,2,4-TCB (PDOV) with a concentration of 1 × 10−5 mol L−1 of the repeating unit. Films on quartz were prepared by spin-coating with o-DCB or 1,2,4-TCB (PDOV) as solvent.
energy values (EHOMO) are −5.16, −5.17, and −5.26 eV for DDPP-Ph, DDPP-PhCO, and DDPP-PhCN, respectively. DDPP-Ph displayed an irreversible reduction peak; therefore, its LUMO energy level (ELUMO), which is 3.30 eV, was calculated by using its EHOMO and optical bandgap (Eopt g = 1.86 eV, see Figure 1b). This value is very close to ELUMO (−3.36 eV) calculated from the reduction onset. In contrast, both DDPP-PhCO and DDPP-PhCN exhibited reversible reduction behavior, and their LUMO energy levels are −3.80 and −4.31 eV, respectively, which are remarkably lower than that of DDPP-Ph and are comparable with or lower than those of wellknown strong A-units such as naphthalenediimide (NDI) and perylenediimide (PDI).47 Obviously, the structures of the bridges have notable impact on LUMOs but small influence on HOMOs. The backbones of DDPP-PhCO and DDPP-PhCN are almost planar as revealed by density functional theory (DFT) calculations (Figure S2). The above results imply that these two new DPP derivatives should be promising building blocks for ambipolar CPs. Photophysical and Electrochemical Properties of the Polymers. Figure 2 shows UV−vis−NIR absorption spectra of the polymers, and the data are summarized in Table 1. In both solution and film states, all the polymers based on DDPPPhCO exhibited a broad absorption band in 600−1000 nm. In solution, PDOT, PDOBT, and PDOTVT displayed very similar absorption spectra with absorption maxima (λmax) of ca. 730 nm. Compared to these three polymers, the λmax of PDOV was significantly red-shifted, and the vibronic peak became more pronounced (Figure 1a). This phenomenon should be ascribed to its much higher molecular weight and more rigid polymer backbone as mentioned above.11,48−50 PDO4FTVT exhibited λmax at 721 nm, which was bathochromically shifted by 14 nm relative to PDOTVT. A relatively narrower absorption band maximized at 785 nm along with a long and weak absorption tail toward near-infrared region was observed for PDNTVT. Similar to its monomeric spectrum, this weak absorption tail should be assigned to the symmetry-forbidden n−π* transition. Dicyanovinylene functionalization can lead to a large red-shift of the n−π* absorption band due to the significant contribution of the strongly electron-withdrawing dicyanovinylene group to the frontier molecular orbital.46 Therefore, for the polymers based on DDPP-PhCO, the relative stronger n−π* absorption bands may overlap with intramolecular charge transfer (ICT) absorption band. These result in the absorption spectrum of PDNTVT very different from those of the polymers based on DDPP-PhCO. From solution to film state, the λmax of the polymers had a small blue-shift, implying H-aggregation type interchain packing in the solid state.26,27,51 Optical bandgaps
(Eopt g ) determined from the film absorption onsets are ca. 1.18 eV for the polymers based on DDPP-PhCO. The estimation of Eopt of PDNTVT is difficult due to the presence of long g absorption tail. Nevertheless, these polymers exhibited lower bandgaps than the similar DPP-based polymers reported in the literature. For example, the Eopt g of PDPP-TVT and PDPP3Talt-TPT (for their structures please see Figure S3) are 1.2852 and 1.43 eV,53 respectively. The low bandgaps of the polymers are in good agreement with the strong electron-deficient nature of DDPP-PhCO and DDPP-PhCN. Film CV curves of the polymers are shown in Figure 3, and their redox onsets as well as calculated frontier molecular
Figure 3. Film cyclic voltammograms (CV) of the polymers measured in anhydrous acetonitrile with Bu4NPF6 (0.1 mol L−1) as electrolyte and SEC as reference electrode. The films were prepared by spincasting o-DCB or 1,2,4-TCB (PDOV) solutions with a concentration of 4 mg mL−1 on the glassy carbon electrode.
orbital energy levels are outlined in Table 1. The HOMO/ LUMO levels are −5.24/−3.91 eV for PDOV, −5.29/−3.85 eV for PDOT, −5.27/−3.74 eV for PDOBT, −5.24/−3.73 eV for PDOTVT, −5.33/−3.79 eV for PDO4FTVT, and −5.29/− 4.21 eV for PDNTVT. These polymers exhibit similar HOMO values with the difference less than 0.1 eV, while LUMO levels show larger discrepancy, ranging from −3.74 to −4.21 eV. This phenomenon is related to the frontier molecular orbital distribution of the polymers as shown in Figure 4 and Figures S4−S9. The HOMOs tend to delocalize over the whole conjugated backbones while the distribution of LUMOs depends on the structures of the polymers. Compared to PDOBT and PDOTVT, PDOV and PDOT show more delocalized LUMOs and thereby have lower LUMO levels. The LUMOs of PDNTVT mainly localize in the very electron deficient 2,2′-(s-indaceno[1,2-b:5,6-b′]dithiophene-4,9diylidene)dimalononitrile unit, leading to the lowest LUMO level (−4.21 eV) among the six polymers. Electrochemical bandgaps (Ecv g ) of the polymers are in the range of 1.08−1.54 eV, as shown in Table 1. We also measured HOMO levels of D
DOI: 10.1021/acs.macromol.6b02781 Macromolecules XXXX, XXX, XXX−XXX
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Figure 4. DFT-calculated LUMO distribution and LUMO energy levels of the dimers of the polymers. The bulky alkyl chains were replaced by methyl groups for simplifying the calculation.
Table 2. TGBC OFET Device Performance of the Polymers p-channel polymer PDOV PDOT PDOBT PDOTVT PDO4FTVT PDNTVT PDPP3T-alt-TPT
μha (cm2 V−1 s−1) 0.074 0.48 1.09 0.76 0.86 0.021 0.18
(0.050) (0.36) (0.89) (0.65) (0.67) (0.014) (0.14)
n-channel
VTb (V)
Ion/Ioffc
μea (cm2 V−1 s−1)
VTb (V)
Ion/Ioffc
μe/μhd
−27 −32 −37 −29 −46 −62 −7
∼103 ∼104 ∼104 ∼105 ∼104 ∼105 ∼104
0.053 (0.046) 0.12 (0.088) 0.25 (0.16) 0.13 (0.10) 0.44 (0.31) 1.2 × 10−4 (0.91 × 10−4) N/A
18 28 36 35 28 16 N/A
∼102 ∼103 ∼102 ∼102 ∼104 ∼10 N/A
0.92 0.25 0.18 0.15 0.46 0.0064 N/A
a
Maximum mobilities measured under ambient conditions in saturation regime. The average values are in parentheses. bThreshold voltage. cCurrent on/off ratio. dμe/μh was calculated from average mobilities.
Figure 5. Typical p-channel (a, b, e, f) and n-channel (c, d, g, h) transfer (a, c, e, g) and output (b, d, f, h) curves of TGBC OFET devices based on PDOBT (a−d) and PDO4FTVT (e−h). The devices were annealed at 200 °C for 15 min prior to the measurements.
devices exhibited a hole-mobility as high as 5.64 cm2 V−1 s−1 at VGS, comparable to those of DPP-based high mobility conjugated polymers with similar alkyl chains and device structures.41 However, the mobility decayed to 0.12 cm2 V−1 s−1 at high VGS. This phenomenon is usually observed in bottom-gate OFETs with self-assembled monolayer (SAM)modified SiO2 dielectric27,54−56 and possibly originating from the interface traps or disorder.3,57 This device characteristic may lead to the overestimation of the mobility.3 To avoid the above problem, we fabricated top-gate/bottomcontact (TGBC) OFET devices with poly(methyl methacry-
the polymers by ultraviolet photoelectron spectroscopy (UPS). The ionization potentials (IPs) of PDOV, PDOT, PDOBT, PDOTVT, PDO4FTVT, and PDNTVT are −5.38, −5.52, −5.43, −5.46, −5.55, and −5.48 eV, respectively (Figure S10). Semiconducting Properties of the Polymers. To evaluate the semiconducting properties of the polymers, we first prepared bottom-gate/top-contact (BGTC) OFET devices. However, severe double slope behavior in the transfer curves was observed, and the mobility extracted from saturation regime exhibited strong dependence on the gate voltage (VGS). As shown in Figure S11 with PDOBT as an example, the E
DOI: 10.1021/acs.macromol.6b02781 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules late) (PMMA) as dielectric (please see fabrication details in the Supporting Information). All spin-coating processes were carried out in ambient, and polymer active layers were annealed at 200 °C for 15 min under nitrogen prior to measurements. OFET characteristics of the devices were measured under the ambient conditions, and mobility was extracted from the saturation regimes. All polymers displayed ambipolar properties; the device performance data are outlined in Table 2. Figure 5 shows the transfer and output characteristics of the devices based on PDOBT and PDO4FTVT. The output curves are characterized by the superposition of standard saturation behavior for one kind of carrier at high VGS and a superlinear current increased at low VGS and high VDS due to the injection of the opposite carrier, and V-shaped transfer curves were observed. These characteristics are typical for ambipolar OFETs. As shown in Table 2, PDOV showed the maximum μh = 0.074 cm2 V−1 s−1 and μe = 0.053 cm2 V −1 s−1. With extension of D-unit, both p- and n-channel performance was significantly improved. Maximum μh and μe up to 0.48 and 0.12 cm2 V−1 s−1 for PDOT, 1.09 and 0.25 cm2 V−1 s−1 for PDOBT, 0.76 and 0.13 cm2 V−1 s−1 for PDOTVT, and 0.86 and 0.44 cm2 V−1 s−1 for PDO4FTVT were achieved. The average electron to hole mobility ratios (μe/μh) for PDOV, PDOT, PDOBT, PDOTVT, and PDO4FTVT were 0.94, 0.25, 0.18, 0.15, and 0.46, respectively. Among these polymers, PDOV displayed the most balanced hole and electron mobilities. This phenomenon can be attributed to its favorable distribution of the frontier molecular orbitals. It is known that the frontier molecular orbitals overlap is crucial for charge transport in organic semiconductors. As shown in Figure 4 and Figure S4, both HOMO and LUMO of PDOV are delocalized along the polymer backbone. This would likely result in comparable HOMO−HOMO and LUMO−LUMO intermolecular interactions, leading to balanced hole and electron mobilities.58−60 In contrast, PDNTVT exhibited hole-dominated device performance with μh of 0.021 cm2 V−1 s−1 and μe of 1.2× 10−4 cm2 V−1 s−1, although it has the lowest LUMO level. The much low μe of PDNTVT could be attributed to its much localized LUMO (Figure 4) as well as poorer film morphology (see below). Charge carriers mobility of conjugated polymers relies on both intrachain and interchain transport. The strong localization of LUMO not only is detrimental to intrachain electron transport but also can result in poor LUMO−LUMO interinteraction, leading to poor interchain transport.58,61−63 For comparison, OFET devices based on PDPP3T-alt-TPT (Mn = 54.3 kDa, Đ = 2.24), which is similar to PDOT but without carbonyl bridges between thiophene and phenyl rings, were also fabricated at the same conditions (Figure S12). PDPP3T-alt-TPT solely displayed p-type transporting property with a maximum μh of 0.18 cm2 V−1 s−1, which is much lower than that of PDOT. This confirms that the introduction of polycyclic aromatic bridges has positive impact on semiconducting properties of D−A CPs. Film Microstructures and Morphology. Thin films of the polymers were characterized by out-of-plane X-ray diffraction (XRD) and taping mode atomic force microscopy (AFM). As shown in Figure 6, for pristine films, no distinct diffraction peaks were observed for PDOV. After annealing at 200 °C, a weak diffraction peak at 2θ = 4.11° appeared, indicating that this polymer prefers to adopt edge-on alignment respective to the substrate after annealing, but its selforganization ability is very poor. For the pristine films of PDOT, PDOBT, PDOTVT, and PDO4FTVT, the (010)
Figure 6. Out-of-plane film XRD profiles of pristine and annealed films of the polymers. The thermal annealing was done at 200 °C for 15 min.
diffraction peak related to π−π stacking was observed. After thermal annealing, this peak became stronger and sharper. The π−π stacking distances were 3.72, 3.62, 3.58, and 3.56 Å for PDOT, PDOBT, PDOTVT, and PDO4FTVT, respectively, which were decreased gradually with an extension of D-unit. Meanwhile, the (100) peak appeared for PDOT, PDOBT, and PDOTVT and became stronger for PDO4FTVT. This suggests that face-on and edge-on molecular orientations coexisted in their annealed films. PDOBT and PDO4FTVT displayed the highest packing order as indicated by the presence of the (200) peak. Only the (010) diffraction peak, corresponding to a π−π stacking distance of 3.60 Å, was observed for PDNTVT even after thermal annealing, implying its poor self-organization ability. These observations are consistent with the OFET performance of the polymers as discussed above. No signals were observed in the in-plane XRD profiles (Figure S17), indicating the lack of in-plane ordered microstructures for these polymers. It is worthwhile noting that PDOBT, PDOTVT, and PDO4FTVT exhibited rather high OFET mobilities although the packing order of these polymers is much lower than those of the reported polymers based on DPP unit. This may be related to their bimodal molecular orientation, i.e., face-on and edge-on alignments on the substrate, which can form 3D conduction channels, leading to efficient charge transport.35,64−66 On the other hand, the rigid and coplanar polymer backbones of these polymers may also contribute to the high mobility. It is believed that the polymers possessing highly coplanar backbones can achieve high mobility in the lack of long-range order. In this case, the polymer backbones play a leading role in constituting the charge transport pathway owing to their efficient intramolecular charge delocalization, and only occasional π−π stacking is required to relay the charge carriers for such polymers.31,67 Figure 7 shows film AFM height images of the polymers before and after annealing at 200 °C for 15 min. The films of PDOV, PDOT, PDOBT, PDOTVT, and PDNTVT displayed very smooth surfaces with low root-mean-square (RMS). For these films, domain sizes and RMS became slightly larger with an extension of D-unit, which is consistent with the enhanced mobility and self-organization capability as aforementioned. The PDO4FTVT film showed well-interconnected larger grains, which is regarded to be beneficial to charge transport. This explains why PDO4FTVT has rather high mobility despite its molecular weight is relatively low.
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CONCLUSIONS Two new A-units in which two DPP units were bridged with indacenodithiophene derivatives, i.e., DDPP-PhCO and DDPPPhCN, have been developed. Compared to DDPP-Ph, these F
DOI: 10.1021/acs.macromol.6b02781 Macromolecules XXXX, XXX, XXX−XXX
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Figure 7. AFM height images of pristine (a−f) and annealed (g−l) films of PDOV (a, g), PDOT (b, h), PDOBT (c, i), PDOTVT (d, j), PDO4FTVT (e, k), and PDNTVT (f, l). The thermal annealing was done at 200 °C for 15 min.
two units have lower LUMO levels and more planar πconjugation skeleton. The polymers based on these two A-units exhibited low bandgap (∼1.18 eV) and deep LUMO energy levels (
