Phytol-derived Alkyl Side-chains for Ï-Conjugated Semiconducting

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Phytol-derived Alkyl Side-chains for #Conjugated Semiconducting Polymers Fanji Wang, Kyohei Nakano, Hiroshi Segawa, and Keisuke Tajima Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b05240 • Publication Date (Web): 22 Feb 2019 Downloaded from http://pubs.acs.org on February 22, 2019

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Chemistry of Materials

Phytol-derived Alkyl Side-chains for π-Conjugated Semiconducting Polymers Fanji Wang1,2, Kyohei Nakano1, Hiroshi Segawa3,4, and Keisuke Tajima1* 1

RIKEN Center for Emergent Matter Science (CEMS), 2-1 Hirosawa, Wako, Saitama

351-0198, Japan, 2Department of Applied Chemistry, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, 3Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan, and 4Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 1538902, Japan

*[email protected]

Abstract Diketopyrrolopyrrole (DPP)-based semiconducting polymers were synthesized with the alkyl side-chains of 3,7,11,15-tetramethylhexadec-2-en-1-yl (TMHDe) and 3,7,11,15-tetramethylhexadecyl (TMHD) that are derived from phytol, a naturallyoccurring diterpene alcohol. The properties of these polymers were compared to those of DPP-2-octyldodecyl (DPP-OD), a polymer comprising a conventional branched alkyl side-chain having the same number of carbon atoms as TMHDe and TMHD. DPPTMHDe and DPP-TMHD showed good solubility in organic solvents and had longer effective conjugation lengths than DPP-OD in diluted solutions. In thin films, DPP-

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TMHDe and DPP-TMHD showed higher crystallinity and orientational order than DPP-OD, resulting in 3–5-fold higher hole mobility in organic field effect transistors (OFETs). The device performance of OFETs based on DPP-TMHD was characterized by a particularly high thermal stability, up to a temperature of 250 °C, resulting from the robust packing structure of the polymer. Use of these phytol-based solubilizing groups in extended π-conjugated molecules can be a useful design strategy to strike a good balance between molecule solubility and relevant thin-film crystallinity.

INTRODUCTION Development of π-conjugated semiconducting polymers has been the subject of intense research, due to the potential applications of these compounds as low-cost, light-weight, and large-area printed flexible electronic devices that can be employed in organic field-effect transistors (OFETs)1-5, organic light-emitting diodes6, and organic photovoltaics (OPVs)7-10. Although the solution-processability of these polymers is regarded as a characteristic that distinguishes them from inorganic semiconductors, the solubility of the π-conjugated cores in common organic solvents is generally low, because of their planar structure and the strong interchain interactions. In fact, the solubility of π-conjugated semiconducting polymers largely relies on alkyl side-chains, and a large proportion of soluble semiconducting polymers with high performance comprise large amounts of insulating alkyl chains (typically 40–60 wt%). Alkyl sidechains also have a significant impact on interchain packing, molecular orientation, and polymer morphology in films, which are features that are known to affect device 2 ACS Paragon Plus Environment

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performance.11-19 The commonly used side-chains consist of linear and branched alkyl chains. Linear alkyl chains, such as n-hexyl, n-dodecyl, and n-octadecyl groups, have their own crystalline nature, which can influence intermolecular interactions of the πconjugated cores and the film crystallinity (known as a fastener effect).20-21 This often facilitates the charge transport in organic semiconducting materials. However, linear alkyl chains have small contribution to the solubility of the extended π-conjugated backbones with high rigidity and planarity. On the other hand, branched alkyl chains, such as 2-ethylhexyl, 2-hexyldecyl, and 2-octyldodecyl (OD) groups, could increase the solubility of semiconducting polymers to a more substantial extent as a consequence of their lower tendency to form crystalline structures. However, due to their bulkiness, these side-chains disturb interchain packing of the π-conjugated backbones, which negatively affect the electronic properties of the polymers. Several groups recently reported the new design of alkyl side-chains to strike a good balance between polymer solubility and main-chain packing. For example, it was shown that moving the branching point of the branched alkyl chains farther away from the π-conjugated polymers’ main chains can reduce the steric hindrance of the side-chains and facilitate the orderly packing of the main chains.22-24 Use of oligosiloxanes attached at the end of linear alkyl chains can enhance the polymer solubility due to their high flexibility.25-27 These side-chain modifications have a substantial influence on the electronic performance of π-conjugated polymers, indicating the importance that side-chain design has in these molecular devices. We also want to point out that the engineering on the side chains is very important as well for improving the performances of emerging 3 ACS Paragon Plus Environment


n-type π-conjugated semiconducting polymers28 and non-fullerene acceptors for OPVs29-30.

Figure 1. Chemical structures of DPP-TMHDe and DPP-TMHD with phytolderived alkyl side-chains. DPP-OD is used as the reference polymer in this study. In this study, we propose the use of phytol-derived alkyl side-chains in π-conjugated polymers. Namely, we propose using 3,7,11,15-tetramethylhexadec-2-en-1-yl (TMHDe) and 3,7,11,15-tetramethylhexadecyl (TMHD) as alkyl side-chains (Figure 1). Phytol is a naturally-occurring constituent of chlorophyll and commonly used as the precursor for the synthesis of vitamin E31 and vitamin K132. This diterpene alcohol derived from the natural source is commercially available at relatively low cost. TMHDe and TMHD contain 20 carbon centers in total (just like OD chain) and four methyl groups connected onto the linear backbone spaced at regular intervals. Derivatives of TMHDe and TMHD have been used to synthesize colloidal graphene quantum dots33-35 employed as light absorbers in photovoltaics36 and nanographenebased complexes with metal ions utilized for electrocatalytic and photocatalytic CO2 4 ACS Paragon Plus Environment

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reduction37. However, to the best of our knowledge, phytol-derived alkyl chains have never been used in the synthesis of the π-conjugated semiconducting polymers. We expected that the structure of phytol-derived alkyl side-chains could provide the properties in between linear and branched alkyl chains to tune the balance between the crystalline order of π-conjugated polymers and their solubility. A diketopyrrolopyrrole (DPP)-based polymer was chosen as the π-conjugated backbone because of its simplicity and its intrinsically low solubility resulting from its planar and linear structure (Figure 1).38-40 DPP-based polymers with n-dodecyl, n-hexadecyl, n-octadecyl, and 2-butyldodecyl alkyl side-chains have been reported to be insoluble in any organic solvent.40 DPP-based polymers with bulky branched chains (2-hexadecyl and OD chains) have been reported to be soluble but their crystallinity in the films was compromised and their hole mobility was limited to 10−2 cm2/Vs.40 To probe the potential of the DPP-based polymer backbone, TMHD and TMHDe were introduced as side-chains, and their effects on polymer properties like solubility, molecular structure in films, and electronic activity were investigated and compared with the polymer with OD chain.



Scheme 1. Syntheses of DPP-OD, DPP-TMHDe, and DPP-TMHD by Yamamoto homocoupling reaction.

RESULTS AND DISCUSSION The synthesis of the polymers with different alkyl chains was conducted by Yamamoto homocoupling reaction using bis(cyclooctadiene)nickel(0) (Ni(COD)2) and 2,2’bipyridine in tetrahydrofuran (THF), as detailed in Scheme 1. The polymerization of DPP-OD was conducted under microwave irradiation at an elevated temperature following the protocol described in previous reports.40-41 The syntheses of DPPTMHDe and DPP-TMHD were conducted at room temperature for 15 min, and followed with a purification process by preparative gel permeation chromatography (GPC) to remove a small amount of oligomer portion. This optimized condition allows to produce the soluble polymers with controlled relative molecular mass and minimize the yield loss from the insoluble fraction. The number-average relative molecular masses (Mn) of DPP-OD, DPP-TMHDe, and DPP-TMHD were 6.9×104, 5.1×104 and 5.0×104, respectively, and the relevant polydispersity indexes (PDIs) were 2.32, 1.98, 6 ACS Paragon Plus Environment

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and 2.06, respectively (Table 1). At room temperature, in common organic solvents like chloroform (CF), chlorobenzene (CB), and o-dichlorobenzene (DCB), all three polymers were characterized by a solubility value over 5 g L−1, which would be high enough to ensure film deposition. The Supporting Information (SI) describes the experimental details of the syntheses of the monomers and polymers utilized in the present study. Note that the TMHDe chain is a mixture of cis and trans isomers in a 33:67 ratio, as confirmed by 1H NMR spectroscopy.



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Table 1. The number-average relative molecular mass, polydispersity index and photophysical properties of DPP-OD, DPP-TMHDe, and DPP-TMHD in diluted solutions.

a

a

λmaxmono

FWHMmono

λmaxpoly

FWHMpoly

λmaxpoly

FWHMpoly

Polymers

Mn

DPP-OD

6.9 × 104

2.32

567

81

917

152

898

226

5.1 × 104

1.98

565

79

936

228

916

254

5.0 × 104

2.06

568

77

938

222

916

253

DPPTMHDe DPPTMHD

PDI

(nm)

b

(nm)

b

(nm)

b

(nm)

b

(nm)

c

(nm)

a

c

Determined by gel permeation chromatography using chloroform as the eluent at 40 °C. b Measured at room temperature in diluted chloroform solution. c Measured at 100 °C in diluted chlorobenzene solution. Figure 2a reports normalized optical absorption spectra of the monomers and polymers in CF solutions. Table 1 summarizes the wavelengths of absorption maxima (λmax) and full-width-half-maxima (FWHM). The absorption spectra of monomers decorated with different alkyl side-chains were very similar to each other, suggesting

Figure 2. Normalized optical absorption spectra for (a) monomers (dotted lines) and polymers (solid lines) of DPP-OD, DPP-TMHDe, and DPP-TMHD measured at room temperature in diluted chloroform solutions and (b) polymers measured at

100 °C in diluted chlorobenzene solutions. 8 ACS Paragon Plus Environment

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that the side-chains have a negligible influence on the photophysical properties of the monomers. The absorption spectra of the polymers were red-shifted regarding the corresponding monomers, as a consequence of an increase in effective conjugation length along the polymer backbone. In contrast to the monomer case, the alkyl sidechains had a significant influence on the absorption spectra of the polymers in solution. The absorption bands of DPP-TMHDe and DPP-TMHD (λmax: 936–938 nm; FWHM: 222–228 nm) were broader and more red-shifted than those of DPP-OD (λmax = 917 nm and FWHM = 152 nm). DPP-TMHDe and DPP-TMHD solutions had similar maximum extinction coefficients (6 × 104 L mol−1 cm−1), which were lower in value than that of DPP-OD (8 × 104 L mol−1 cm−1), see the non-normalized spectra in Figure S1. However, the integrated absorption intensities of the three polymers were comparable to each other, due to the fact that the absorption bands of DPP-TMHDe and DPP-TMHD extended into the longer wavelength region. Therefore, the light absorption abilities of the three polymers in solution are similar to each other, reflecting the fact that the relative molecular masses of the alkyl side-chains are the same for DPP-OD and DPPTMHD, and only two-hydrogen-atoms lighter for DPP-TMHDe. To elucidate the reasons for the described differences in the absorption spectra of the three polymers, the spectra of the polymers were recorded in CB and DCB solutions to investigate whether any solvent effect could be identified (Figure S2). In these experiments both the shifts in the positions of the peaks and the broadening of the absorption bands were still observed in the spectra of the phytol-based polymers versus the OD-based one. When the CF solution of each polymer was diluted from 0.02 g L−1 9 ACS Paragon Plus Environment


to 0.0025 g L−1, the normalized absorption spectra recorded were identical for each polymer, indicating that the features of the absorption spectra do not depend on polymer concentrations in the concentration range evaluated (Figure S3). Based on this evidence, the possibility of intermolecular interactions taking place in solution can be excluded. Furthermore, the absorption spectra of the polymers in diluted CB solution (0.0025 g L−1) were recorded at temperatures as high as 100 °C (Figure S4). In this case, a pronounced decrease in intensity of the lowest energy absorption bands and a blue shift of the λmax were observed for all three polymers at elevated temperatures, which suggests a decrease in effective π-conjugation length due to thermally-induced twisting of the main chain conformations.42 Figure 2b reports the normalized absorption spectra of the three polymers at 100 °C. Despite the negligibility of interchain interactions at low concentration and at high temperature, the absorption spectra of DPP-TMHDe and DPP-TMHD were still characterized by a red-shifted λmax and an extension of the absorption features into the longer wavelength regions, when compared with the absorption spectrum of DPP-OD (Table 1). These results indicate that the single-chain conformations of DPP-TMHDe and DPP-TMHD in solution are more extended than their DPP-OD counterpart, resulting in larger effective conjugation lengths.43 This effect may be attributed to the less bulky nature of TMHDe and TMHD; the methyl substitution with small steric hindrance is three carbon atoms away from the nitrogen atom of DPP in these polymers, while the octyl substitution with large steric hindrance is two carbon atoms away in DPP-OD. The solvation of the side chains and the


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Table 2. Optical, electronic, and thermal properties of DPP-OD, DPP-TMHDe, and DPP-TMHD polymers.

λmaxfilm

FWHMfilm

(nm)

(nm)

DPP-OD

940

275

1.15

−5.31

−4.16

398

DPP-TMHDe

945

265

1.17

−5.16

−3.99

348

DPP-TMHD

946

273

1.16

−5.20

−4.04

410

Polymers

Eg (eV)

a

IE (eV)

b

EA (eV)

c

Td (°C)

d

a

Estimated from the onset of the absorption spectra. b Ionization energy determined by ultraviolet photoelectron spectroscopy. c Electron affinity calculated by the formula of EA = IE + Eg. d Determined at the temperature corresponding to 5% weight loss.

intramolecular interactions between the polymers’ main-chains and side-chains may also affect the chain conformation differently in DPP-TMHD (or DPP-TMHDe) regarding DPP-OD. Figure 3a presents the absorption spectra of the polymer films. In contrast to the solution spectra, the three polymer films showed similar absorption bands with λmax in the 940–946 nm range and FWHM in the 265–275 nm range. The absorption peaks of

Figure 3. (a) Normalized optical absorption spectra and (b) UPS spectra in cut-off and Fermi-edge regions for the thin films of DPP-OD (black), DPP-TMHDe (blue), and DPP-TMHD (red). The films were spin-coated on quartz and ITO substrates from 5 g L−1 chloroform solutions. 11 ACS Paragon Plus Environment


the film spectra are more red-shifted and broader than those recorded for the solutions; additionally, the differences between polymers observed in solution were diminished in the case of the polymer films. This evidence suggests that polymer solidification forces DPP backbones to elongate and induces the intermolecular interactions in similar degree, resulting in the observed similar absorption profiles of the three polymer films. The values of the optical band gap (Eg) of DPP-OD, DPP-TMHDe, and DPP-TMHD were estimated from the absorption onset as 1.15, 1.17, and 1.16 eV, respectively (Table 2). Ionization energy (IE) of the three polymers were determined by ultraviolet photoelectron spectroscopy (UPS) (Figure 3b). Based on the cut-off energy of secondary electrons and the onsets of the band derived from the highest occupied molecular orbital (HOMO), IE values for DPP-OD, DPP-TMHDe, and DPP-TMHD were calculated to be −5.31, −5.16, and −5.20 eV, respectively. Electron affinity (EA), including exciton binding energy, was estimated from the equation EA = IE + Eg to be −4.16, −3.99, and −4.04 eV for DPP-OD, DPP-TMHDe, and DPP-TMHD, respectively (Table 2).


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Thermal analyses of the three polymers were conducted by performing differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). In DSC, none of the polymers displayed clear glass transition, crystallization and melting temperatures (Figure S5a). In TGA, DPP-OD, DPP-TMHDe, and DPP-TMHD exhibited a 5% weight loss at Td = 398, 348, and 410 °C, respectively (Figure S5b). DPP-TMHD was slightly more thermally stable than DPP-OD, whereas DPP-TMHDe was characterized by a significantly lower Td value than the other two polymers. This observation could be attributed to the existence of the double bond in TMHDe.

Figure 4. (a) The transfer characteristics in p-type operation without thermal annealing and (b) the summary of the effective mobilities with various thermal annealing conditions for DPP-OD (black), DPP-TMHDe (blue), and DPP-TMHD DPP is known to show ambipolar transport40, but due to the ease of charge injection, the hole mobilities of the three polymers were evaluated using the bottom-gate topcontact-type OFETs in the present study. The polymer films were prepared on the 13 ACS Paragon Plus Environment


water-soluble sacrificial layer of sodium polystyrene sulfonate (PSS) and transferred upside down onto the CYTOP passivated Si/SiO2 (300 nm) substrate following the reported procedures.44-50 In these OFETs, the charge transport channels next to the insulating layer are originally the surface of the spin-coated polymer films where the chain order is higher than that in the bulk, which could lead to higher carrier mobility than the spin-coated interfaces.44 Figure 4a shows the typical transfer characteristics of OFETs in p-type operation based on three polymers without thermal annealing. The channel width and length were 1000 μm and 200 μm, respectively. The typical output characteristics are reported in Figure S6. The on-currents of DPP-TMHDe and DPP-TMHD were larger than that of DPP-OD, indicating that a higher hole mobility could be obtained when the phytolderived alkyl side-chains are introduced. To avoid a possible overestimation of hole mobility,51-53 we calculated the effective hole mobility following the reported approach, in which deviation of the transfer characteristics from the ideal Shockley equation is corrected.51 The calculated average effective hole mobility of DPP-OD, DPP-TMHDe, and DPP-TMHD were 0.07, 0.20, and 0.32 cm2/Vs, respectively. Note that for these DPP-based polymers, the deviation from the ideal Shockley equation was small, with reliability factors above 85%, indicating that operation of the device did not suffer from charge traps or insufficient charge injection from the electrodes. The hole mobility value measured for DPP-OD in the present study was higher than that reported previously (0.006 cm2/Vs)40. This discrepancy may be due to the structural differences between the devices utilized in the present study versus the cited study and the 14 ACS Paragon Plus Environment

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interfaces of the films used as OFET channels (i.e. original film surfaces in the present study vs. film/substrate interfaces formed by spin-coating in the cited study). Our results show that DPP-TMHDe and DPP-TMHD outperform DPP-OD by factors roughly between 3 and 5, an observation that can be ascribed to the difference in the alkyl side-chains. The thermal stress stability of the films produced during the patterning processing is a crucial feature for the commercialization of OFETs.54-56 After transfer, the DPP-based polymer films were thermally annealed in the dry, N2-filled glovebox for 30 min at a given temperature, and the performance of OFETs was subsequently measured. The SI (Table S1) summarizes the detailed OFET properties of the three polymers. Figure 4b reports the effective hole mobility plotted against the thermal annealing temperature. The annealing temperature at 25 °C means without thermal annealing. The hole mobility of DPP-OD dropped drastically when the film was annealed at temperatures above 100 °C, indicating that the chain order and/or the film morphology at the charge transport channel are characterized by poor thermal stability. In contrast, DPP-TMHDe and DPP-TMHD exhibited better stability in term of the hole mobilities compared to DPP-OD. In particular, the hole mobility of DPP-TMHD remained almost unchanged after thermal annealing was performed at 250 °C. The hole mobility of DPP-TMHDe displayed lower stability than that of DPP-TMHD at temperatures higher than 150 °C. Note that 1H NMR measurements for the monomer of DPP-TMHDe before and after annealing in solid state at 250 °C for 30 min confirmed that there was no detectable change in the spectrum and the ratio of cis and trans isomers remained unchanged. 15 ACS Paragon Plus Environment


Therefore, the difference in the thermal stability of the hole mobility is not due to the chemical change of the double bond but may be attributed to structural disorder of DPPTMHDe polymer chains thermally induced at the semiconductor/dielectric interfaces, which will be discussed in the latter section. Although the value of the threshold voltage (Vth) did not display a clear correlation with the annealing temperature, values for the on-off current ratios (Ion/Ioff) increased with the annealing temperature for all three polymers; furthermore, such values where characterized by the following trend: DPPTMHD > DPP-TMHDe > DPP-OD (see Figure 4a and Table S1). We also examined thermal stability of the films at a relatively low temperature (100 °C) and for longer time (up to 5 h) to simulate a more realistic thermal stress during the device processing. The experiments were performed with the similar procedure above and the results lead to basically the same conclusion; DPP-TMHDe and DPPTMHD films show much higher stability toward the thermal stress than DPP-OD film (Figure S7).

Figure 5. AFM height images of (a) DPP-OD (Ra = 1.08 nm), (b) DPP-TMHDe (Ra = 1.00 nm), and (c) DPP-TMHD (Ra = 1.25 nm) films obtained by performing thermal annealing at 250 °C. 16 ACS Paragon Plus Environment

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To understand the influence that the different alkyl side-chains and thermal annealing temperatures had on OFET device performance, we studied the surface morphology of the films by atomic force microscopy (AFM). Figure 5 reports the AFM height images of the three polymer films thermally annealed at 250 °C on Si substrates. The surface roughness was similar for all three polymers, and the arithmetic mean roughness (Ra) in all three cases was in the 1.00–1.25 nm range. These results indicate that the different alkyl side-chains have little influence on surface roughness. AFM images of all the film samples with different annealing temperatures are reported in the SI (Figure S8). The negligible variation of Ra ranged from 0.69 to 1.25 nm indicates that the different



thermal stress stability of OFETs observed for the three polymer films has little correlation with the film surface morphology.

Figure 6. 2D GIWAXS patterns of (a) DPP-OD, (b) DPP-TMHDe, and (c) DPPTMHD films thermally annealed at 250 °C. The corresponding GIWAXS line-cut profiles of DPP-OD (black), DPP-TMHDe (blue), and DPP-TMHD (red) films along the (d) out-of-plane and (e) in-plane directions. Figure 6a–c reports the 2D grazing-incidence wide-angle X-ray scattering (GIWAXS) patterns of DPP-OD, DPP-TMHDe, and DPP-TMHD films obtained after performing thermal annealing at 250 °C. Line-cut profiles of the patterns in the out-of-plane and in-plane directions are presented in Figure 6d and e, respectively. The DPP-OD film shows 100, 200, and 010 diffractions along the out-of-plane direction, and 100 diffraction along the in-plane direction. This observation indicates that the crystalline domains in the DPP-OD film have a bimodal orientation of edge-on and face-on. The 18 ACS Paragon Plus Environment

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values for the d spacings of 100 (i.e. lamellar distance) and 010 (i.e. π-π stacking distance) in the DPP-OD film are 20.05 and 3.74 Å, respectively. The DPP-TMHDe film was characterized by more intense peaks of higher order diffractions from 100 to 400, and a very weak 010 diffraction in the out-of-plane direction. In the in-plane direction, there are a very weak 100 and a strong 010 diffractions. This evidence indicates that the DPP-TMHDe film has a strong edge-on orientation in the film. The lamellar distance in the DPP-TMHDe film (25.55 Å) is larger than that in the DPP-OD film (20.05 Å), reflecting the longer alkyl chain length in DPP-TMHDe. The π-π stacking distance in the DPP-TMHDe film (3.54 Å) is shorter than that in the DPP-OD film (3.74 Å). The DPP-TMHD film was characterized by an edge-on rich orientation similar to that of the DPP-TMHDe film, but slightly more face-on orientation fraction coexists, judging from the larger 010 peak in the out-of-plane and the larger 100 peak in the inplane directions. The lamellar distance is larger in the DPP-TMHD film (27.62 Å) than in the DPP-TMHDe film (25.55 Å). This observation can be ascribed to the fact that TMHD has a more elongated conformation than TMHDe, with the latter characterized by the presence of a mixture of cis and trans isomers made possible by the presence of the double bonds. The π-π stacking distance in the DPP-TMHD film (3.52 Å) is comparable to that in the DPP-TMHDe film (3.54 Å). The measured π-π stacking distances of DPP-TMHDe and DPP-TMHD are also shorter than those of the DPPbased semiconducting polymers previously reported with 2-hexyldecyl (3.75 Å)40 and with triethylene glycol (3.60 Å)57 side chains. 19 ACS Paragon Plus Environment


The results of GIWAXS measurements suggest that the introduction of the TMHDe and TMHD side-chains into the DPP-based semiconducting polymer leads to the higher crystallinity with highly ordered edge-on oriented polymer backbones and shorter π-π stacking distance than the introduction of the branched OD chain. These differences could be largely responsible for the observation that OFET devices based on DPPTMHDe and DPP-TMHD display higher hole mobility than their counterpart based on DPP-OD. However, we should note that the crystallinity differences observed in GIWAXS experiments are averaged over the bulk of the films, and they may not be directly correlated to OFET hole mobility, since such mobility is very sensitive to the structural features at the semiconductor/dielectric interfaces, where charge transport takes place. We also conducted 2D GIWAXS analyses on polymer films generated at different annealing temperatures to observe how crystallinity and molecular orientations were affected by the annealing temperature (data and detailed discussions are presented in SI). From these analyses, we conclude that DPP-OD and DPP-TMHDe have a certain degree of freedom that allows the polymer chains to move during thermal annealing, thus leading to changes in the molecular orientations and the interlamellar distances in the bulk of the films (Figures S9 and S10). This additional degree of freedom probably originates from the fact that the OD chain has large bulkiness and the TMHDe chain contains the double bonds with a mixture of cis and trans isomers. At the semiconductor/dielectric interfaces, however, the highly mobile chains may disturb the as-prepared high structural orders (which is formed at the film surface during the spin20 ACS Paragon Plus Environment

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coating process) by facilitating the interaction with the dielectric surface, resulting in low thermal stress stability in OFET device performance of DPP-OD and DPP-TMHDe. In contrast, as a consequence of thermal annealing, DPP-TMHD films displayed an enhancement in the level of crystallinity but no change in molecular orientation, and the smallest variation in interlayer spacing observed among the three polymer films (Figure S11). These observations may result from the fact that the TMHD side-chain is less bulky and more linear than the other two side-chains (TMHDe and OD), which could in turn lead to strong interchain stacking in the as-prepared film and inhibit mainchain movements at the temperatures tested. These effects may contribute to the higher thermal stress stability observed for DPP-TMHD than for DPP-OD and DPP-TMHDe in OFET device performances through more stable interfacial structures.

CONCLUSIONS We have demonstrated that phytol-derived alkyl groups can be useful as solubilizing side-chains of DPP-based polymers. With their presence, these side-chains facilitate the packing of the main polymer chain and improve the performance and thermal stress stability of OFET devices when compared with the case of branched alkyl chains. These findings indicate that phytol-derived side-chains are a good and easily accessible additional option for controlling the solubility and improving the solid film crystallinity of semiconducting polymers and molecular materials with highly extended πconjugated systems.



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EXPERIMENTAL SECTION Materials and General Characterizations 3,6-bis(5-bromothiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione

was

purchased from SunaTech, Inc. All other chemicals and solvents were purchased from Wako, TCI Chemical and Sigma-Aldrich Co., Ltd. and used without further purification. All air- or moisture-sensitive reactions were carried out under a dry N2 atmosphere implementing standard Schlenk techniques. Column chromatography was performed on silica gel with particle diameter of 40–50 μm. 1H and 13C solution NMR spectra were recorded on a JNM-AL300 spectrometer (JEOL) at 300 MHz in CDCl3 with tetramethylsilane as internal standard. Data are reported as chemical shift in ppm (δ), multiplicity (s = singlet, d = double, t = triplet, and m = multiplet), coupling constant (Hz), and integration. High-resolution mass spectrometry was carried out on a JEOL JMS-T100GCV. Preparative GPC was performed on a preparative recycling highperformance liquid chromatography system (LC-92XX II, Japan Analytical Industry) equipped with a preparative column (JAIGEL-2H-40, Japan Analytical Industry). The Mn values of the polymers were determined by analytical GPC with a liquid chromatography system (LC, Shimadzu) using CF as eluent at 40 °C and calibrated with a polystyrene standard. The optical absorption spectra were measured with a UVvis-NIR spectrophotometer (V-670, JASCO) equipped in the case of the temperaturedependent absorption measurements with a mini circulation bath (MCB-100, JASCO). UPS was performed on a surface analysis instrument (PHI5000 VersaProbe II, ULVAC-PHI Inc.) with He(I) excitation (photon energy 21.2 eV). For all UPS 22 ACS Paragon Plus Environment

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measurements, film samples were spin-coated on ITO substrates at 1000 rpm for 60 s using a CF solution with a 1 mg mL−1 concentration and applying a −5 V bias. DSC was performed on a DSC system (DSC 8230, Rigaku) implementing a heating rate of 10 °C min−1 in a 30–300 °C temperature range under an N2 atmosphere. TGA was performed on a TGA system (TG 8120, Rigaku). TGA experiments were conducted under an N2 atmosphere implementing a heating rate of 10 °C min−1 between room temperature and 500 °C. The 2D GIWAXS analysis was performed at the BL46XU beamline of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute. The X-ray energy was 12.39 keV and the incident angle of the measurements was fixed at 0.12° using a Huber diffractometer. The 2D GIWAXS pattern was obtained using a 2D image detector (Pilatus 300 K, Dectris). For UV-vis-NIR, 2D GIWAXS, and AFM measurements, film samples were spin-coated on quartz (for UV-vis-NIR) and Si (for 2D GIWAXS and AFM) substrates at 1000 rpm for 60 s using a CF solution with a 5 mg mL−1 concentration. As-deposited and thermally-annealed samples at 100, 150, 200, and 250 °C were prepared. OFET Device Fabrication and Evaluation The surface of the pre-cleaned n+ Si substrates (0.2–0.6 Ω cm) with 300 nm SiO2 insulating layer (E&M) was covered by a CYTOP insulating layer to eliminate the charge traps in SiO2. A 0.15 wt% CYTOP solution was spin-coated on the SiO2 at 3000 rpm, resulting in the formation of an approximately 3-nm-thick passivation layer. The total capacitance of SiO2 and the CYTOP layer was 11.7 nF/cm2. DPP polymer films approximately 50-nm-thick were prepared onto glass/PSS substrates to conduct the 23 ACS Paragon Plus Environment


contact film transfer. The details of the film transfer process can be found elsewhere.44 After the film transfer, thermal annealing was conducted for 30 min at a designated temperature inside a glovebox filled with dry N2 gas. 15 nm Au and 30 nm Ag were sequentially thermally evaporated for the source and drain electrodes through a metal shadow mask under the pressure of 10−4 Pa. The channel width and length of FETs were 1000 μm and 200 μm, respectively. The source-drain current and gate leakage current were measured with Keithley source measurement unit 6430 and 2400, respectively. FET measurement was conducted in vacuum (~10−4 Pa).

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Synthesis details, UV-vis, DSC, TGA, OFET performances, AFM images, additional discussion on annealing temperature dependence, GIWAXS and 1H and 13C NMR.

ACKNOWLEDGMENTS GIWAXS experiments were performed at beamline BL46XU of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI; Proposals 2018A1568 and 2018B1595). F. W. thanks the Junior Research Associate (JRA) program of RIKEN for financial support. Mass spectral data were acquired at the mass spectrometry facility run by Molecular Structure Characterization Unit (RIKEN CSRS,


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Wako). We thank Dr. Tomoyuki Koganezawa (JASRI) for support with the GIWAXS measurements and Prof. Liang‐shi Li (Indiana University) for fruitful discussions.

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Phytol-derived Alkyl Side-chains for Ï-Conjugated Semiconducting

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