Article pubs.acs.org/cm
Quinacridone-Based Semiconducting Polymers: Implication of Electronic Structure and Orientational Order for Charge Transport Property Itaru Osaka,*,† Masahiro Akita,† Tomoyuki Koganezawa,‡ and Kazuo Takimiya*,†,§ †
Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8527, Japan ‡ Japan Synchrotron Radiation Research Institute, 1-1-1, Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan § Institute for Advanced Materials Research, Hiroshima University, Higashi-Hiroshima 739-8530, Japan S Supporting Information *
ABSTRACT: We report the synthesis, characterization, and field-effect transistor properties of novel semiconducting polymers, PQA2T and PQA3T, incorporating a quinacridone unit, and discuss the structure−property relationships. Comparison of the optical and electrochemical properties between the monomer, repeat unit, and polymer suggests that the effective π-conjugation and the delocalization of HOMO along the backbone are relatively limited. X-ray diffraction studies revealed that the polymers form a π−π stacking with a short distance of 3.6 Å and that the orientational order was enhanced by an increase of molecular weight. The hole mobilities are found to be around 0.2 cm2 V−1 s−1 and are, interestingly, insensitive to the molecular weight and to the orientational order; the randomly oriented low molecular weight polymer showed similar mobilities to the edge-on oriented high molecular weight polymer. We speculate that the relatively localized HOMO might hinder the charge transport along the backbone, and thus the longer polymer chain is not necessary to facilitate the charge transport. The locally but strongly π−π interacted polymer crystallites seem to be sufficient for the effective charge transport in the QA-based polymer system. These features in the present polymers offer great interest of using QA moieties as the building block for semiconducting polymers and give new insight into the design of a new class of semiconducting polymers. KEYWORDS: semiconducting polymers, pigment, quinacridone, organic field-effect transistors, intermolecular interaction
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INTRODUCTION With their excellent electrical and optoelectronic properties as well as the great film forming property, semiconducting polymers/π-conjugated polymers have been attracting extensive attention in printed electronics, such as organic light-emitting diodes, field-effect transistors (OFETs), and solar cells, which can differentiate from the conventional silicon technologies.1 In particular, OFETs are an important fundamental component for the next generation of applications, for example, flexible large-area displays, integrated circuits, and RFID tags.2 Recent advances in the development of new semiconducting polymers have achieved charge carrier mobilities of reaching 1 cm2 V−1 s−1 or even higher, showing the bright prospect of polymer-based OFETs.3 Since the charge transport in semiconducting polymers is governed by charge hopping through the π-orbital overlaps of face-to-face π-stacked backbones,4 enhancement of the interchain π−π interaction is a crucial point to create high performance semiconducting polymers for OFETs. Incorporation of a π-extented fused ring into the polymer backbone5 and construction of a donor−acceptor (D−A) backbone3c−g,6−8 have been successful approaches to develop such © 2012 American Chemical Society
polymers. On the other hand, incorporation of a dye/pigment molecule into the backbone can also be beneficial to enhance the π−π interaction. Diketopyrrolopyrole,3e,f,8 rylenes such as perylenedicarboximide9 and naphthalenedicarboximide,10,11 and isoindigo12 (Figure 1) are successful dye/pigment molecules as the building unit for semiconducting polymers. Although much attention is focused on their electron deficient property and hence on the ambipolar or n-channel characteristic or their use as the acceptor unit for D−A polymers, an important nature of high electrical performances in such semiconducting polymers should originate in their strong intermolecular interactions, owing to the large local dipoles across the electron donating nitrogen to the electron withdrawing carbonyl group in the unit structure. Quinacridones (QAs, Figure 1) are well-known red−violet pigments that also possess such a dipole within the molecule13 and have been used as emitters in organic light emitting diodes,14 photoconductive materials for organic photovoltaics,15 and active layers in OFETs,16 which showed hole Received: February 15, 2012 Published: February 21, 2012 1235
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that the electronic structure of the present polymers is quite characteristic. We then study the OFET properties of the polymers, in which the hole mobilities are estimated to be around 0.2 cm2 V−1 s−1, and the X-ray diffraction measurements of the polymer thin films to clarify their ordering structures. We finally discuss the structure−property relationship of the polymers with the OFET properties, electronic structure, and ordering structure, which reveals that the mobility of the polymers is independent of the molecular weight and orientation in the thin film, somewhat resembling the other pigment-based polymers, and that the high mobility mostly originates in the strong π−π stacking of the polymers.
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RESULTS AND DISCUSSIONS Synthesis. Scheme 1 shows the chemical structures and synthetic route to the polymers. The very long branched side chain was chosen as the solubilizing group to ensure sufficient solubility and thus processability. As mentioned above, simple unsubstituted bithiophene and terthiophene were employed as the linker unit to clearly understand the nature of QA when incorporated into the polymer backbone. The bithiophene and terthiophene linkers are also beneficial to better reduce the possible steric hindrance between the outermost benzene rings of the QA unit and the linker. QA was first alkylated at the N-positions to afford 1, and the following bromination at the 2 and 9 positions gave monomer 2.17 2 was copolymerized with distannylated bithiophene (3) and terthiophene (4) via the Stille coupling reactions using Pd2(dba)3/P(o-Tol)3 as the catalyst system to afford poly{[N,N-di(2-decyltetradecyl)quino[2,3-b]acridine-7,14-dione2,9-diyl]-alt-5,5′-(2,2′-bithiophene)}, PQA2T, and poly{[N,Ndi(2-decyltetradecyl)quino[2,3-b]acridine-7,14-dione-2,9-diyl]alt-5,5″-(2,2′:5′,2″-terthiophene)}, PQA3T, respectively. The chemical properties of the polymers are summarized in Table 1.
Figure 1. Chemical structure of the representative dyes used in semiconducting materials.
mobilities of ∼0.1 cm2 V−1 s−1. All these results in the small molecular and oligomeric QAs and their devices well prove the great potential of QAs as the building block for p-type organic semiconductors. However, to our surprise, very limited studies have been reported so far for QA-based polymers as semiconducting materials.17 In this paper, we show the synthesis of QA-based semiconducting polymers, where simple bithiophene or terthiophene is used as the linker unit to clearly understand the benefit and/or the nature of QA moieties when incorporated in the polymer backbone (Scheme 1). We first discuss the electronic structure of the polymers by comparing the optical and electrochemical properties of the polymers with the corresponding monomer unit and repeat unit, which reveals
Scheme 1. Synthetic Routes to the Quinacridone-Based Semiconducting Polymersa
a
Polymerization conditions: (i) (a) Pd2(dba)3, P(o-Tol)3, chlorobenzene. (b) Pd(PPh3)2Cl2, toluene. (c) Pd(PPh3)2Cl2, toluene, where monomers 2 and 3 were recrystallized twice before polymerization. 1236
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545 nm and the high-energy band appears at 344 nm, which may correspond to a part of the transition of the high-energy band in 1 and be assigned to the transition contributed mainly from the chromophore related to the outer benzene ring along with the thiophene ring introduced. However, it is found that while the red-shift (Δλ) of the high-energy band is 46 nm, that of the HOMO−LUMO band (ca. 20 nm) is much less. In the polymer, PQA2T-12, the shoulder at around 530−570 nm, can be assigned to the HOMO−LUMO band, whereas the absorption with λmax at 453 nm likely corresponds to the highenergy band as observed in 1 and QA2T. These bands are red-shifted from those of QA2T, but, again, Δλ of the HOMO−LUMO band, approximately 30 nm, is much less than that of the high-energy band, approximately 110 nm. These relatively small Δλ of the HOMO−LUMO band compared to that of the high-energy band, as it goes from 1 to PQA2T via QA2T, implies that the effective conjugation along the PQA2T main chain is limited, probably because the HOMO is rather localized or stabilized on the QA moiety and thus is not much affected by linking the QA moiety with bithiophene. This speculation may be supported by the fact that the HOMO is not well distributed to the end of the thiophene rings, as predicted by the DFT calculation (Figure 3), and that the difference of the oxidation potential (HOMO energy level) between QA2T and PQA2T is small (vide infra). Figure 2b shows the absorption specra of PQA2T-12 and PQA3T, with the similar molecular weight, both in the solution and in the film. PQA3T gives λmax at 470 nm in the solution, which is red-shifted from PQA2T-12, likely owing to the additional thiophene ring in the backbone unit. However, the shoulder that appears at around 572 nm, corresponding to the HOMO−LUMO transition, is mostly located in the similar region to that of PQA2T-12. This implies, again, that the effective conjugation is limited and HOMO is not delocalized over the backbone in this system. Both polymers exhibit slightly broadened sepectra to the longer wavelength region in the thin film as compared to those in the solution. PQA2T-24 and -46 exhibited similar spectra to PQA2T-12 in both the solution and the film (Supporting Information Figure S2), suggesting that the molecular weight does not affect the electronic structure. Figure 4 shows the cyclic voltammograms of 1 and QA2T in dichloromethane and PQA2T-12 and PQA3T thin films. 1 gave oxidation (Eoxonset) and reduction (Eredonset) onset potential at +0.99 and −1.36 V (vs Ag/AgCl), which corresponds to the HOMO and LUMO energy level (EHOMO and ELUMO) of −5.4 and −3.0 eV, respectively. In QA2T, Eoxonset and Eredonset were estimated to be +0.94 and −1.29 V, which were slightly lower and higher than that of 1, giving EHOMO of −5.3 eV and ELUMO
The number averaged molecular weights (Mn) of PQA2T and PQA3T were determined to be 12 kDa with PD = 2.5 (PQA2T12) and 11 kDa with PD = 1.9, respectively, by high-temperature GPC. In addition, PQA2T was also synthesized using Pd(PPh3)2Cl2 as the catalyst, Mn = 24 kDa with PD = 2.1 (PQA2T24), and the polymerization with further purified 2 and 3 by recrystallization from acetone and ethanol, respectively, using Pd(PPh3)2Cl2 led to even higher Mn of 46 kDa (PQA2T-46) with PD = 3.4. All polymers were soluble in chlorinated solvents; PQA2T-12 and -24 and PQA3T were soluble in chloroform, chlorobenzene (CB), and o-dichlorobenzene (DCB) even at room temperature, and PQA2T-46 was soluble in warm CB and DCB. 2 was also cross-coupled with 2-trimethylstannylthiophene to yield 5,12-bis(2-decyltetradecyl)-2,9-di(thiophen-2-yl)quinolino[2,3-b]acridine-7,14(5H,12H)-dione (QA2T), which is regarded as the repeat unit of the present polymers and is used for UV−vis absorption spectroscopy and cyclic voltammetry. Table 1. Chemical Properties of the Polymers polymer
Mna (kDa)
Mwa (kDa)
PDb
DPnc
PQA2T-12 PQA2T-24 PQA2T-46 PQA3T
12 24 46 11
28 39 155 21
2.5 2.1 3.4 1.9
10.0 20.9 39.7 9.1
a
Number average (Mn) and weight average (Mw) molecular weight evaluated using high-temperature GPC (DCB as the eluent, at 140 °C) calibrated with polystryrene standard. bPolydispersity indexes. c Determined from Mn.
Electronic Structure of the Polymers. Optical and electrochemical properties of the polymers are characterized by UV−vis absorption spectroscopy, cyclic voltammetry, and photoelectron spectroscopy, and these data are summarized in Table 2. We first compare the UV−vis absorption spectra of the monomer unit (1), repeat unit (QA2T), and polymer (PQA2T-12) in chloroform, as shown in Figure 2a, and discuss the electronic structure of the present polymer system. 1 provides two characteristic absorption bands with the maximum peak (λmax) at 298, and 489 and 523 nm (black line). The latter band with λmax at 489 and 523 nm is assignable to the HOMO−LUMO transition, typical for QA compounds.18 The high-energy band at 298 nm may be contributed from the chromophore including outer benzene rings and/or inner benzene rings of QA. In QA2T, these bands are red-shifted from those in 1, reflecting the π-extension by the addition of two thiophenes, where the HOMO−LUMO transition band appears at 508 and
Table 2. Optical and Electrochemical Properties of 1, QA2T, and the Polymers λmaxa (nm) compound 1 QA2T PQA2T-12 PQA2T-24 PQA2T-46 PQA3T
solution 298, 344, 453, 456, 470, 470,
489, 523 508, 545 535sh, 574sh 535sh, 574shb 535sh, 576shb 572sh
film
453, 472, 470, 484,
Eredonsetc (V)
ELUMOd (eV)
Eoxonsetc (V)
EHOMOd (eV)
−1.36 −1.29
−3.0 −3.1
0.99 0.94 0.77
−5.4 −5.3 −5.2
0.73
−5.2
474, 533sh, 575sh 534sh, 574shb 535sh, 573shb 572sh
IPe (eV)
5.1 5.1 5.1 5.1
a
Absorption maxima in the DCB solution and the spin-coated thin film. bSee Supporting Information for the absorption spectra. cOnset potentials (V vs Ag/AgCl) from reduction (Eredonset) and oxidation (Eoxonset). All the potentials were calibrated with the Fc/Fc+ (E1/2 = +0.43 V measured under identical conditions). dEstimated with the following equation: ELUMO = −4.80 − Eredonset; EHOMO = −4.80 − Eoxonset. eDetermined by photoelectron spectroscopy in air. 1237
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Figure 2. UV−vis absorption spectra of monomer 1, QA2T, and PQA2T-12 in the solution (a) and PQA2T-12 and PQA3T in the solution and in the thin film (b).
and +0.73 V, respectively, with which EHOMO for both polymers were determined to be −5.2 eV. The small diffence of EHOMO between 1, QA2T, and the polymers implies that HOMO delocalization along the polymer main chain is rather limited and is in good agreement with small Δλ of the HOMO−LUMO transition band in the absorption spectra. The ionization potentials (IP) of PQA2T-12 and PQA3T were also evaluated using photoelectron spectroscopy in air and were both determined to be 5.1 eV (see Supporting Information), which is consistent with EHOMO estimated from the electrochemistry. OFET Properties. Bottom-gate top-contact OFETs with the channel length of 50 μm and the width of 1.5 mm were used to evaluate the charge transport properties of the polymers. Polymer thin films were spin coated onto 1H,1H,2H,2Hperfluorodecyltriethoxysilane (FDTS)-modified Si/SiO2 substrates,3b,19 which were subsequently annealed at 150 °C for 30 min, and then the Au source and drain were vacuum deposited to afford OFET devices. Figure 5 depicts typical transfer and output characteristics of OFET devices based on PQA2Ts (Figure 5a,b) and PQA3T (Figure 5c,d), respectively. Small hysteresis and negligible contact resistance are observed in the transfer and output curves, respectively, in all the polymer devices. The hole mobility of PQA2T-12 in the saturation regime is as high as 0.27 cm2 V−1 s−1, with the average of 0.18 cm2 V−1 s−1 and on/off ratios (Ion/Ioff) of 10−6 (Table 3). Interestingly, the mobilities of PQA2T-24 and PQA2T-46 are calculated to be ∼0.17 cm2 V−1 s−1 and ∼0.21 cm2 V−1 s−1, respectively, which are similar to those of PQA2T-12. This result contrasts to the fact that usually the mobility increases as the molecular weight increases in semiconducting polymers,3g,20 which will be discussed in the next section. The insensitivity of the mobility on the molecular weight could be advantageous in terms of batch-to-batch synthetic reproducibility. PQA3T also exhibits similar mobilities to PQA2T, ∼0.24 cm2 V−1 s−1. The air stability of the devices using PQA2T-12 and PQA3T was also examined in the humid air (relative humidity of >50%). Both polymers show some degradation with time (see Supporting Information), probably because IP of around 5.1 eV is not sufficiently large to ensure high stability, particularly in highly humid air. Ordering Structures in the Thin Film. The ordering structures of the polymers in the thin film on the FDTSmodified Si/SiO2 substrate were studied by X-ray diffraction
Figure 3. Calculated HOMOs and LUMOs of 1 and QA2T using the DFT method at the B3LYP-6-31 g(d) level. The substituents are replaced with the methyl group to simplify the calculation.
Figure 4. Cyclic voltammograms of 1 and QA2T in dichloromethane and PQA2T-12 and PQA3T in the thin film.
of −3.1 eV. Both PQA2T-12 and PQA3T provided only an oxidation voltammogram with similar Eoxonset of +0.77 1238
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Figure 5. OFET characteristics of the polymer-based devices. (a) Transfer curves of PQA2Ts with three different molecular weights (at VSD = −60 V), (b) output curves of PQA2T-12, (c) transfer (at VSD = −60 V), and (d) output curves of PQA3T.
π−π stacking distances (dπ) are estimated to be approximately 3.6 Å, which are comparable to or even closer than common highperformance polymers based on thiophene and/or thiophenefused aromatic systems.5,21 It should be noted that dπ of 3.6 Å for PQA2T and PQA3T can be surprisingly short considering that the polymer backbones have linkages of benzene (at the end of the QA unit)−thiophene. The thiophene−thiophene linkage favors the anti arrangement, and is coplanar, and thus leads to the well-defined backbone structure, resulting in the high π−π stacking crystallinity with dπ of
