Quinacridone-Diketopyrrolopyrrole-Based Polymers for Organic Field-Effect Transistors

Incorporation of pigment or dye molecules as building units is of great interest in the development of semiconducting polymers, due to their strong intermolecular interactions arising from the strong local dipoles in the unit structure, which would facilitate the charge transport property. In this paper, semiconducting polymers based on well-known pigments, namely, quinacridone and diketopyrrolopyrrole, are synthesized and characterized. The π-stacking distances are found to be 3.5–3.8 Å, which is fairly narrow for semiconducting polymers, indicating that they possess strong intermolecular interactions. Interestingly, polymer orientation is influenced by the composition of alkyl side chains. While the edge-on orientation is observed when the linear alkyl groups are introduced for all the side chains, the face-on orientation is observed when the branched alkyl groups are introduced either in the quinacridone or diketopyrrolopyrrole unit. It is found that the electronic structure of the present polymers is mostly affected by that of the diketopyrrolopyrrole unit, as evidenced by the absorption spectra and computation. Although the field-effect mobility of the polymers is modest, i.e., in the order of 10−4–10−3 cm2/Vs, these findings could be important information for the development of semiconducting polymers.


Introduction
Owing to the excellent electrical and optoelectronic properties, semiconducting polymers have been attracting extensive attention in solution-processed organic electronics, such as field-effect transistors (OFETs) and photovoltaics (OPVs), which can offer large-area and flexible devices of next generation [1,2]. In particular, OFETs are an important fundamental component for such devices [3,4]. The main charge transport path in the semiconducting polymer films is interchain π-orbital overlaps of face-to-face π-stacked backbones, and thus enhancement of the intermolecular π−π interaction is crucial for the improvement of OFET performances [5].
Recently, we reported that the semiconducting polymers based on quinacridone (QA), a well-known red-violet pigment ( Figure 1) [22], have strong π-π interactions with an interchain distance of 3.6 Å and exhibit relatively high hole mobilities of 0.2 cm 2 /Vs in OFETs [23]. This reveals that QA is a good building unit for semiconducting polymers. As QA is a p-type molecule and thus is a donor unit, combining with an acceptor dye unit can offer stronger intermolecular interactions due to the enhanced donor-acceptor system throughout the main chain, which could be useful for organic devices. Here, we report the synthesis, properties, structures, and OFET performances of new semiconducting polymers having QA and DPP as the building units.

Results and Discussion
Scheme 1 shows the synthesis of QA-DPP polymers. 2,9-Dibromo-N,N-dialkylquino [2,3-b] acridine-7,14-dione (1) [24] was reacted with bis(pinacolato)diboron in the presence of Pd(PPh 3 ) 2 Cl 2 to afford 5,12-dialkyl-2,9-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)quinolino[2,3-b]acridine-7,14 (5H,12H)-dione (2) as the comonomer. 2 was then copolymerized with 3,6-bis-(5-bromo-thiophen-2-yl) -N,N′-bis(alkyl)-1,4-dioxopyrrolo [3,4-c]pyrrole (3) via the Suzuki-Miyaura coupling reaction, yielding the desired polymers. The alkyl groups introduced in 2 and 3 are n-hexadecyl (C16) and 2-decyltetradecyl (DT), and thus four polymers with different side chain combination were prepared (PQADPP-16, -16DT, -DT16, and DT). Molecular weight of the polymers is summarized in Table 1. While PQADPP-16DT, -DT16, and DT are soluble in chloroform, PQADPP-16 is only soluble in hot chlorobenzene or o-dichlorobenzene, which is likely as a result of the reduced solubility due to the introduction of linear alkyl groups in both the QA and DPP units. Whereas relatively high molecular weights of M n > 15 kDa are obtained for PQADPP-16DT, -DT16, and -DT, PQADPP-16 gives low molecular weight of M n = 7.4 kDa. This is probably due to the low solubility of the polymer as evidenced by the fact that the polymer had precipitated during the polymerization.   Photoelectron spectroscopy (PESA) revealed that the HOMO level (E HOMO ) of the polymers is ca. −5.2 eV. Figure 2 depicts the UV-Vis absorption spectra of the polymers in the chlorobenzene solution and in the film. Note that the spectrum for PQADPP-16 in the solution was measured at 100 °C, due to its low solubility. All polymers show similar spectra with the absorption maxima (λ max ) and absorption edge (λ edge ) of around 690 ~ 700 nm and 720 ~ 750 nm, respectively, in the solution. The λ max slightly red-shifts by ca. 10 nm in the thin film. These λ max in the present polymers are far red-shifted as compared to PQA2T, λ max ≈ 470 nm and λ edge ≈ 600 nm, but blue-shifted as compared to the common DPP-based polymers, e.g., for PDQT, λ max ≈ 790 nm and λ edge ≈ 900 nm.  E HOMO and E LUMO for QA and DPP2T were determined by electrochemistry. E HOMO and E LUMO used for QADPP are those of the polymer (PQADPP-16DT), in which E LUMO is determined by the addition of the optical bandgap calculated from the absorption onset (1.73 eV) to E HOMO (−5.2 eV). It is interesting to note that although HOMOs and LUMOs of both QA and DPP2T are distributed along the molecule long axes, those of QADPP localizes only on the DPP unit. It is likely that since E HOMO and E LUMO of DPP2T are higher and lower than those of QA, respectively, as depicted in Figure 3, the electronic structure of DPP2T is dominant in that of QADPP. This speculation may be supported by the fact that both E HOMO and E LUMO of the polymers are close to those of DPP2T.
OFETs are fabricated using a bottom-gate top-contact configuration, in which the polymer solution was spin-coated on the hexamethyldisilazane (HMDS)-treated Si/SiO 2 substrate and Au source and drain electrodes are vacuum-deposited after annealing the resulting polymer film at 150 °C for 30 min. Figure 4 shows the transfer curves and output curves of the polymer devices. While PQADPP-16, -16DT, -DT16 showed p-channel characteristics, PQADPP-DT was inactive. The maximum mobilities evaluated from the saturation regime are summarized in Table 1. Whereas modest mobilities, up to 6.8 × 10 −3 cm 2 /Vs, are obtained for PQADPP-16, lower mobilities with the order of 10 −4 cm 2 /Vs are obtained for PQADPP-16DT and -DT16. All mobilities obtained here are relatively low, and these results may relate to the localized HOMOs. Since the HOMO distribution play an important role in the charge (hole) transport both along the backbone and the intermolecular interaction, this localization might prevent the efficient hole transport. The variation of the mobility depending on the side chain composition can be well explained by the structural study using the two-dimensional grazing incidence X-ray diffraction (2D-GIXD) as described below. Figure 5 shows the 2D-GIXD patterns of the polymer films after annealing at 150 °C. Interestingly, the polymer orientation is strongly dependent on the side chain composition. In the film of PQADPP-16, the diffractions corresponding to the lamellar and π-π stacking structures appear along the q z and q xy axes, respectively, indicating that the polymer chains are oriented in an edge-on manner.
In the meantime, PQADPP-16DT and -DT16 exhibited the diffraction corresponding to the lamellar and π-π stacking structures along the q xy and q z axes, respectively, suggesting that the polymers form the face-on orientation. While the π-π stacking diffraction for PQADPP-16, -16DT, and -DT16 appears strong, that for PQADPP-DT is very weak. The π-π stacking distance (d π ) for PQADPP-16 is found to be 3.5 Å, which is relatively narrow for semiconducting polymers. The d π for PQADPP-16DT and -DT16, 3.6 Å, and -DT, 3.8 Å, are wider than that for PQADPP-16. These differences in the π-π stacking crystallinity can relate to the ratio of bulky branched group in the side chain. The long branched alkyl group may prevent the interchain interaction in the solid state, though it affords good solubility. Since the charge transport path in the semiconducting polymer films is primarily through the π-π interaction of face-to-face stacked polymer chains [5], such differences in the orientational motif and the π-π stacking is fairly consistent with the trend of the mobility.
Device fabrication and characterizations. OFET devices were fabricated in a "top-contact" configuration on heavily doped n + -Si (100) wafers with 200-nm-thick thermally grown SiO 2 (C i = 17.3 nF cm −2 ). The Si/SiO 2 substrates were carefully cleaned and then treated with hexamethyldisilazane (HMDS) to form a self-assembled monolayer (SAM), in which the silicon wafers were exposed to FDTS vapor at 100 °C in a closed desiccator for 3 h under nitrogen. Polymer layers were then spin-coated from warm (~80 °C) 3 g/L dichlorobenzene solution with 2500 rpm for 45 s, subsequently annealed at 150 °C for 30 min under nitrogen. On top of the polymer thin films, Au drain and source electrodes (thickness 80 nm) were deposited in vacuum through a shadow mask, where the drain-source channel length (L) and width (W) are 50 μm and ca. 1.5 mm, respectively. Current-voltage characteristics of the OFET devices were measured at room temperature in air with a Keithly 4200-SCS semiconductor characterization system. Field-effect mobilities were calculated in the saturation regime (V SD = −60 V) of the I SD using the following equation, where C i is the capacitance of the SiO 2 dielectric, I SD is the source-drain current, and V SD , V G and V th are the source-drain, gate and threshold voltages, respectively. Current on/off ratios (I on /I off ) were determined from the minimum current at around V G = 0-20 V (I off ) and the current at V G = −60 V (I on ). The mobility data were collected from more than 10 different devices.

Conclusions
We have synthesized and characterized semiconducting polymers based on QA and DPP pigment dyes. Although the transistor properties of these polymers were modest, some interesting information was found. The electronic structure of DPP is dominant in this system; E HOMO and E LUMO of the polymers are mostly affected by those of the DPP unit. Introduction of the two dye units in the polymer backbone leads to highly crystalline structure in the thin film with a close π-stacking distance of ~3.5 Å. The topology of the alkyl substituents significantly influence the orientation of the polymers, in which the polymer tends to orient in a edge-on manner when only the linear alkyl group is introduced, and in a face-on manner when, even partially, the long branched alkyl group is introduced. We believe that these results are of importance for the design of high-performance semiconducting polymers.