Manipulation of Chain Conformation for Optimum Charge Transport

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Manipulation of Chain Conformation for Optimum Charge Transport Pathway in Conjugated Polymers Seolhee Jeon, Jung Hun Lee, Jai Il Park, Bonghyun Jo, Dong Ryeol Whang, Tae Kyu Ahn, Hui Joon Park, Sung Dong Kim, Wi Hyoung Lee, and Bong-Gi Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b03930 • Publication Date (Web): 20 Jun 2017 Downloaded from http://pubs.acs.org on June 25, 2017

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ACS Applied Materials & Interfaces

Manipulation of Chain Conformation for Optimum Charge

Transport

Pathway

in

Conjugated

Polymers Seolhee Jeon, a‡ Jung Hun Lee, a‡ Jai Il Park,b Bonghyun Jo,c Dong Ryeol Whang,d Tae Kyu Ahn,c Hui Joon Park,e Sung Dong Kim,a Wi Hyoung Lee,a and Bong-Gi Kima* a

Department of Organic and Nano System Engineering, Konkuk University, Seoul 05029,

Korea b

Nuclear Chemistry Research Division, Korea Atomic Energy Research Institute, Daejeon

34057, Korea c

Department of Energy Science, Sungkyunkwan University, Suwon 16419, Republic of

Korea d

Linz Institute for Organic Solar Cells (LIOS)/Institute of Physical Chemistry, Johannes

Kepler University Linz, 4040 Linz, Austria e

Department of Energy Systems Research, and Department of Electrical and Computer

Engineering, Ajou University, Suwon 16499, Republic of Korea

* To whom correspondence should be addressed. E-mail: [email protected]

Abstract. Two different diketopyrrolopyrrole-based conjugated polymers (CPs) were designed and synthesized to investigate the effect of chain conformation on their molecular assembly. Conformation management was achieved by the incorporation of different linkers during

polymerization.

Using

computational

calculations

and

UV-vis

absorption

measurements, the resulting CPs were found to exhibit partly modulated chain geometry. Grazing incident X-ray diffraction experiment with a 2-dimensional detector revealed that PDPP-T having a planar chain conformation exhibited an edge-on type molecular

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arrangement, which evolved to a face-on type chain assembly when the planar geometry was altered to a slightly twisted one as in PDPP-BT. In addition, it was verified that CPs’ directional electric carrier mobility was critically distinguished by the distinctive chain arrangement, in spite of their similar chemical structure. Concentration dependent absorption measurements could provide improved understanding of the assembly mechanism of CP chains: the planar conformation of PDPP-T facilitates formation of pre-assembled chains in a concentrated solution and further directs the edge-on stacking, while the twisted dihedral angle along the benzothiophene in PDPP-BT prevents chain-assembly, resulting in the faceon stacking. Since CP’s chain conformation is inevitably connected with the generation of pre-assembled chains, manipulating CP’s geometry could be an efficient tool to extract an optimum chain assembly which is connected with the principal charge transport pathway in CPs.

Keywords. Chain conformation, Molecular assembly, Aggregation, Charge transport, Conjugated polymer

Introduction Conjugated polymers (CPs) are promising materials with various optoelectronic applications, such as organic photovoltaic (OPV) cells, organic field effect transistors (OFETs) and organic light emitting diodes (OLEDs).1-9 Their electrical properties are highly anisotropic due to the 1-dimensional p-orbital overlap along their molecular backbone, and the performance of CP-based optoelectronic devices are strongly dependent on the direction of assembled chains as well as the degree of molecular arrangement originated from the interaction between individual chains.10-12 In the case of intramolecular charge transport, the migration pathway of electrical carriers, such as hole and electron, is affected by the degree

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of p-orbital hybridization in the conjugated framework, implying that a planar geometry in CPs would be beneficial for outperforming electrical properties, because of its better p-orbital hybridization.5-6 However, intermolecular carrier transport determining the long-range electrical property of CPs depends on the degree of chain assembly as well as the direction of assembled chains, because the assembled chains determine the shortest intermolecular distance corresponding to the effective intermolecular charge transport pathway.10,13 Several methods have been suggested to manipulate the direction of assembled polymer chains or to enhance the degree of intermolecular assembly. For example, designing CPs possessing a planar chain conformation is one of representative strategies for inducing intermolecular chain assembly because the planar chains facilitate face-to-face interaction between the aromatic units of the CP.14-18 Liquid crystalline polymers or directional film fabrication techniques have also been utilized to manage the direction of assembled polymer chains.10 Generally, lamellar-like chain assembly is predominantly formed during CPs’ aggregation, mostly due to their rigid rod-like molecular framework and the high rotational energy barrier along the conjugated backbone in a condensed solution.19-21 Here, two different lamellar-like chain assemblies (face-on and edge-on), against the surface of applied substrates, could be expected. Each case reflects parallel/transverse arrangements between the CP’s aromatic surface and the substrate, respectively. This implies that charge carrier mobility in the vertical or transverse direction, against the substrate, can be tailored by the chain-assembly tendency, because of the different direction of the shortest inter-chain distance governing the principal intermolecular charge transfer pathway in CPs. Various material design strategies have been also investigated to improve electrical properties of CPs, but studies on the modulation of CP assembly have been neglected, in spite of the profound effect of CP assembly on the performance of optoelectronic devices. For example, the vertical-direction carrier mobility provides significant effect to solar cells or

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light emitting diodes, but the function of field effect transistors (FET) is sensitively affected by the horizontal-direction mobility.22-23 Recently, it has been reported that the chain aggregation of CPs sharing a same conjugated skeleton can be altered, including its assembly tendency, with side chains of different chemical structures.24-26 However, this preliminary result is insufficient to elucidate a conclusive relationship between CP’s electrical properties and assembled chain tendency, because the different side chains affect the degree of molecular inter-digitation, which alters the shortest intermolecular distance corresponding to the optimum charge transport pathway. In addition, several research groups have been investigated the electrical properties of diketoprrolopyrrole-based or isoindigo-based conjugated polymers containing thiophene or phenyl derivatives as a linker unit.27-28 Although they focused on the effect of CP’s chain conformation on electrical properties, fundamental studies, regarding to the origin of inducing the distinctive chain assembly among similar structured CPs, has been neglected. In this contribution, in order to investigate the geometry dependent chain assembly of CPs, we designed two different CPs bearing same side chains in similar structured conjugated frameworks. The chain conformation of each was compared by means of computational calculation, and the resulting chain-assembled tendency was characterized with 2D grazing incident X-ray diffraction (GIXRD). In addition, the directional hole transport behavior was correlated with the assembled chains after fabrication of both OFET and hole dominant space charge limited current (SCLC) devices. Finally, the driving force resulting in different chain assembly in the similar structured CPs was discussed based on the generability of pre-assembled chains in a concentrated CP solution.

Experimental Section Materials. All starting materials were purchased from commercial suppliers (Aldrich

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and TCI). 9-(4-bromobutyl)nonadecane, 2,5-bis(trimethylstannyl)thiophene (3) and 1,3bis(trimethylstannyl)benzo[c]thiophene (4) were prepared as previously described manners.2930

Detailed synthetic procedures are summarized as follows, and the synthesized compounds

were fully characterized with 1H-NMR, 13C-NMR and mass spectroscopy (Fig S1 - S6). Compound 1. To a stirred solution of 3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole1,4(2H,5H)-dione (DPP, 0.278g, 0.925 mmol) in anhydrous DMF (10 mL), was added potassium carbonate (0.383 g, 2.776 mmol). The mixture was stirred at 120 °C for 1 hour. 9(4-bromobutyl)nonadecane (1.0 g, 2.776 mmol) was added and the mixture stirred at 130 °C for 24 hours. The reaction mixture was diluted with DI water and extracted with chloroform. The organic layer was washed with brine, dried over MgSO4, and concentrated in vacuo. Column purification by elution of 1:40 ratio of ethylacetate:hexane gave compound 1 as a purple solid (52% yield). 1H-NMR (400 MHz, CDCl3, ppm): 8.91 (d, 2H), 7.60 (d, 2H), 7.26 (t, 2H), 4.05 (m, 4H), 1.72 (m, 4H), 1.30 - 1.20 (m, 74 H), 0.88 (t, 12H); 13C NMR (CDCl3, 150 MHz, ppm) 161.34, 140.01, 135.23, 130.60, 129.79, 128.58, 107.70, 42.28, 37.31, 33.29, 31.92, 30.14, 29.71, 29.67, 29.66, 29.37, 29.36,26.67, 24.05, 22.68, 14.11; MALDI-TOF MS (m/z) 945.82 (M+H)+, calcd. 945.58; anal. calcd. for C60H100N2O2S2: C, 76.21; H, 10.66; N, 2.96; S, 6.78; found: C, 76.05; H, 10.81; N, 3.04; S, 6.70. Compound 2. N-bromosuccinamide (0.377 g, 2.116 mmol) was added portion-wise, with stirring to a solution of compound 1 (1.0 g, 1.058 mmol) in anhydrous chloroform (30 mL) at 0 °C. The reaction mixture was stirred at room temperature overnight in the absence of light. Then, the reaction mixture was poured into DI water and extracted with chloroform. The organic layer was separated, dried with MgSO4, and concentrated in vacuo. The crude was purified by flash column chromatography (1:4 ratio of ethylacetate:hexane) to give an solid (compound 2) with 82 % yield. 1H-NMR (400 MHz, CDCl3, ppm): 8.66 (d, 2H), 7.21 (d, 2H), 3.96 (m, 4H), 1.67 (m, 4H), 1.35-1.20 (m, 74 H), 0.86 (t, 12H); 13C NMR (CDCl3, 5



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150 MHz, ppm) 160.96, 138.94, 135.32, 131.60, 131.09, 119.10, 107.77, 61.60, 42.29, 37.24, 33.54, 33.23, 31.91, 30.14, 29.71, 29.66, 29.65, 29,36, 29.35, 26.64, 22.68, 14.95; MALDITOF MS (m/z) 1103.65 (M)+, calcd. 1102.54; anal. calcd. for C60H98Br2N2O2S2 : C, 65.31; H, 8.95; N, 2.54; S, 5.8; found: C, 65.10; H, 8.97; N, 2.57; S, 5.83. General procedure of Stille type polymerization. To a stirred solution of compound 2 in anhydrous toluene was added an equivalent amount of corresponding monomers containing two trimethyltin groups under argon condition. To the reaction vessel in the presence of inert atmosphere. Then, 5 mol% of Pd2dba3 and P(o-toyl)3 (3.5 eq to Pd(0)) were added to the reaction mixture and it was stirred at 110 ºC for 24 hours. Then, 2-tributylstannyl thiophene (39.5 µl) and 2-bromothiophene (12.5 µl) were added to the reaction mixture in 3 hours intervals, respectively and both for the termination of polymerization and the chain end modification. The final polymer was collected by precipitation technique using methanol as solvent and the solid was filtered through a 0.45 µm nylon membrane filter. The collected solid was purified by soxhlet method using methanol and hexane, then the insoluble material was dissolved in chloroform and reprecipitated with methanol to afford final polymer. PDPPT (Mn: 29369 and PDI: 2.57) and PDPP-BT (Mn: 51679 and PDI: 2.05). Computational details Ground states geometries of the model compounds were computed using the B3LYP level of density functional theory (DFT), in which the 6-31G** basis set was applied for all atoms. Vertical transition energies of the molecule was calculated at the time-dependent (TD)-DFT. All calculations were carried out using Gaussian 09. GI-XRD characterization. GIXD was performed on a 3C2 beamline at the Pohang Accelerator Laboratory (PAL) using monochromatized x-rays under vacuum. The samples were mounted on an X and Y axes goniometer. The scattered beam intensity was recorded with an SCX 4300-165/2 CCD detector (1242 × 1152 pixels, Princeton Instruments). Diffraction intensity plots of the 2D GIXD patterns were obtained using SAXS_FIT2D

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software provided by the European Synchrotron Radiation Facility. Thin film transistor fabrication. CPs were dissolved in chloroform (20 mg/ml). The solution was spin-casted on octadecyltrichlorosilane (ODTS)-modified silicon wafers with a spin speed of 3000 rpm. Silicon wafers with a 300 nm thick, thermally grown SiO2 layer were cleaned with acetone, isopropyl alcohol and UVO3 treatment prior to use. An ODTS layer was fabricated by immersing the silicon wafers in ODTS/toluene (40 µl/20 ml) solution. After 1 hour, the silicon wafers were annealed at 120 °C and unreacted ODTS was removed by sonication in toluene. Residual solvent was removed by storing the CP film in vacuum oven for 24 hours. Gold source/drain electrodes of 50 nm thickness were thermally deposited on the CP films through a shadow mask to complete the fabrication of OFETs. The channel length and width of the fabricated OFETs were 100 µm and 1500 µm, respectively. Hole dominant space charge limited current (SCLC) device. ITO-coated glass were sequentially cleaned with acetone, isopropyl alcohol (IPA), and deionized water (DI), via ultrasonication. A PEDOT:PSS layer was cast on the cleaned ITO substrate by spin-casting at 2000 rpm for 30 sec, and then baked at 150 oC for 10 min. The CP layer was prepared on the PEDOT:PSS layer by a spin-casting method, as a solution in chlorobenzene (20 mg/ml concentration) at 1500 rpm for 30 s, and then annealed at 175 oC for 20 min. The sample was transported into a thermal evaporator, then MoO3 (50 nm) and Al (80 nm) were sequentially deposited at a base pressure of 4 × 10−6 torr. J-V characteristics were recorded on a Keithley 2400 Semiconductor Characterization System, in the dark. Hole mobility values were calculated using the space-charge-limited current (SCLC) method at a low voltage: J = 9εoεrµV2/8L3, where ε0εr is the permittivity of the component, µ is the carrier mobility, and L is the thickness.

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Scheme 1. Chemical structures and detailed synthetic routes of the designed CPs. (i) K2CO3, anhydrous DMF, 120 °C, 24 hours; (ii) N-bromosuccinimide, CHCl3, 0 °C, overnight; (iii) Pd2dba3, P(o-toyl)3, toluene, 110 °C, 24 hours

Results and Discussion As shown in Scheme 1, two different CPs sharing an identical side chain in their conjugated skeletons were designed to investigate the effect of chain conformation on molecular assembly. To manipulate the molecular conformation of the designated CPs, thiophene and benzothiophene units were combined with a diketopyrrolopyrrole (DPP) moiety via a Stille type polymerization, respectively. The applied DPP derivative (compound 1 in scheme 1) is well established as a quasi-planar material because of the intramolecular SO interaction between the DPP moiety and adjacent thiophenes. Since the five-membered thiophene imparts dissimilar intra-chain steric hindrance according to whether side group is present at the 3 and/or 4 positions, thiophene and benzothiophene were adopted as linkers during polymerization to selectively alter the twisted angle between the linker and the

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thiophene unit of the applied DPP derivative in the designed CPs (PDPP-T and PDPP-BT). To validate their different molecular geometries, pre-optimization was carried out using the AM1 semi-empirical quantum chemistry model and the resulting molecular configuration was then further optimized with B3LYP as the exchange-correlation functional in the density functional theory (DFT) framework.31 As illustrated in Figure 1a, the torsional angles between DPP and thiophene (① and ④) are less than 1° for both CPs, indicating quasi-planar geometry. However, in the case of torsional angle between the applied linkers and thiophene (② and ③), PDPP-BT exhibited relatively a larger value than that of the corresponding bonds in PDPP-T, implying that PDPP-T holds a quasi-planar chain conformation along the overall conjugated skeleton, but PDPP-BT has a slightly twisted one. Interestingly, when the absorption behavior of the obtained CPs was investigated using UV-vis absorption spectroscopy, PDPP-T absorbed at a shorter wavelength than PDPP-BT having slightly twisted chain conformation. Generally, planar chains facilitate p-orbital overlap between conjugated moieties, resulting in a shift of absorption to longer wavelength region. However, the absorption tendency of conjugated materials is also governed by the degree of intramolecular charge transport (ICT) originating from the inter-connected chemical bonding between electron donating and accepting groups in a conjugated framework. Since the benzothiophene has a higher HOMO level than the thiophene,32 PDPP-BT is expected to have higher intramolecular charge transfer characteristics between the DPP and benzothiophene, resulting in a more red-shifted absorption compared with PDPP-T. To validate this interpretation of the absorption behavior of PDPP-BT, computational calculations (Gaussian 09 B3LYP/6-31G** in vacuo) with the dimeric units of each CPs were undertaken because the computational approach reflects the tendency of electron delocalization via the intramolecular charge transfer. As shown in Fig. S7, when the energy levels of each dimeric units of corresponding CPs was calculated, it was revealed that PDPP9



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BT provided a smaller optical band-gap than that of PDPP-T, indicating the capability of the more red-shifted absorption in PDPP-BT.

Figure 1. (a) Optimized molecular configuration of obtained CPs and (b) their mass extinction coefficients in diluted chloroform solution.

To investigate the effect of CP’s chain geometry on the assembly of individual chains in film state, CP films prepared under same condition were characterized by means of 2D grazing incident X-ray diffraction (GIXRD). As depicted in Figure 2, both CPs exhibited lamellar-like chain aggregations, but their assembled directions were contradictive. A vertical chain-arrangement (edge-on assembly, Fig. 2a) was dominant in PDPP-T, but PDPP-BT was arranged horizontally (face-on assembly, Fig. 2b). Although PDPP-BT also exhibited a weak scattering in the small angle region of the out-of-plane direction corresponding to the edge-on orientation, the predominant scattering in the out-of-plane direction could be found in the wide angle region. Thus, judging from both the in-plane scattering in the small angle region and the distinctive out-of-plane scattering in the wide angle region, PDPP-BT is concluded to organize principally in a face-on chain orientation. The different molecular assembly between PDPP-T and PDPP-BT was more obvious when the diffraction images were analyzed with individual diffractograms both in in-plane and out-of-plane directions (Fig. 2c). In addition,

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when the assembled chain dimension was calculated from the obtained diffractograms (Fig. 2d), both CPs provided a similar range of the shortest inter-chain distance (3.67 Å for PDPPT and 3.74 Å for PDPP-BT, respectively). The 2D GIXRD result indicates that the obtained CPs possess the comparable shortest inter-chain pathways having a contradictive direction.

Figure 2. 2D grazing incidence X-ray diffraction (GIXRD) patterns of (a) PDPP-T and (b) PDPP-BT. (c) and (d) indicate diffractograms extracted from the diffraction patterns and assembled chain configuration, respectively.

Since the CP’s principal charge transport pathway is critically affected by the shortest inter-molecular distance,10,13 we compared the directional hole mobility after fabrication of both organic FET and hole dominant SCLC devices, to verify the directional effect of the shortest inter-molecular distance on the electrical properties of the CPs. In the case of the hole mobility in horizontal direction, bottom-gate top-contact OFETs were fabricated with obtained CPs and their electric transport behaviors were characterized as illustrated in Figure 3a-b. The hole mobility was extracted from the saturation regimes of the obtained transport 11



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Figure 3. Transport characteristics of (a) PDPP-T and (b) PDPP-BT obtained from FET device and (c) J-V characteristics from hole dominant SCLC device curves according to the relationship ID = C·µhh·W·(VG − Vth)2/2·L, where W and L are the channel width and length, respectively; C is the specific capacitance of the gate dielectric (10.8 nF/cm2); Vth is the threshold voltage; and µhh indicates hole mobility in the horizontal direction. As summarized in Table 1, PDPP-T (3.6 cm2V-1s-1) had a much higher hole mobility than PDPP-BT (0.04 cm2V-1s-1), due to its same direction of the shortest inter-chain pathway with the hole transport direction in OFET. As demonstrated with the 2D GIXRD experiment, the horizontally arranged inter-facial direction of aromatic units in PDPP-T allow for effective intermolecular π-orbital overlap, facilitating charge carrier transport in the horizontal direction. In contrast, when the vertical hole transport (µhv) was characterized with the hole dominant SCLC devices by incorporation of the electron blocking MoO3 layer, PDPP-BT (3.42 × 10-3 cm2 V-1 s-1) exhibited a slightly higher value of the hole mobility than that of PDPP-T (1.29 × 10-3 cm2 V-1 s-1), as summarized in Figure 3c and Table 1. This result can be interpreted in a similar context to that obtained from the OFET experiment, because PDPP-BT produced a vertical arrangement of the shortest inter-chain distance via a face-on type chain assembly. This study of the directional hole mobility reflects the close correlation between the principal transport pathway for the electric charge carrier and CP chain assembly. To verify the origin of the dissimilar chain assembly between PDPP-T and PDPP-BT, we characterized their UV-vis absorption behavior under different solution concentrations

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and compared it with that of CP film in which the molecular arrangement has been fully developed. As illustrated in Figure 4a, the absorption spectrum of PDPP-T became slightly dilated in film state, including a newly developed absorption around 830 nm. When the diluted solution was concentrated beyond 160 mg/mL, the absorption behavior evolved to that of the film state, indicating the generation of partly assembled chains (immature lamellae). However, the PDPP-BT film exhibited almost identical absorption behavior with that of the diluted solution and the only difference was a slight shift in absorption maximum (Fig. 4b). In general, the generation of a new absorption band in CP film, as in the case of PDPP-T, suggests strong intermolecular aggregation between π-conjugated backbones.33 Thus, PDPP-BT can be considered as having inferior intermolecular assembly, likely due to the slightly twisted main-chain conformation. This interpretation is supported by the absorption behavior of a highly concentrated PDPP-BT solution. The absorption spectra of both diluted and concentrated (320 mg/mL) solutions were practically coincident in PDPPBT, which indicates no existence of immature lamellae in the condensed solution. Based on the concentration dependent absorption experiment, we believe that the formation of immature lamellae plays an important role in determining the assembly tendency of individual CP chains, because both CPs could have similar compatibility with substrates due to their comparable chemical structure. As depicted in Figure 5, both CPs are initially completely solvated by good solvents such as chlorobenzene (CB). During film casting with a diluted solution, abundant solvent should be evaporated and a well spread film allowed to form on the top of applied substrate, via highly concentrated solution state. Here, if there are no pre-assembled polymer chains, or if the dimension of pre-assembled lamellae is shorter than the transverse length of the fully extended individual polymer chain (Dy < Dx in Fig. 5) in the condensed state as for PDPP-BT, the CP plane surface originated from both planar aromatic units and partly extended side chains would be preferentially arranged with facing

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a

diluted solution 40 mg/mL 80 mg/mL 160 mg/mL 320 mg/mL film

400

500

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700

b

PDPP-T

diluted solution 320 mg/mL film

Absorption (a.u.)

Absorption (a.u.)

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800

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PDPP-BT

800 1000 1200 1400 1600 1800

Wavelength (nm)

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Figure 4. Concentration dependent UV-vis absorption behaviors of (a) PDPP-T and (b) PDPP-BT the surface of the applied substrate, forming a primitive layer to reduce the center of gravity. Then, the final face-on type chain assembly could be produced through the sequential accumulation of another polymer chains on the primitive layer. In contrast, if the lamellarlike chain assembly happens in the condensed state as for PDPP-BT as well as the dimension of pre-assembled lamellae is longer than the transverse length (Dy > Dx), the edge-on type molecular configuration would be preferred due to the better capability for acquiring a lowered gravity center of the pre-assembled lamellae. Since chain conformation is a critical factor in determining the dimension of pre-assembled lamellae in the condensed solution of CP, manipulation of the CP’s conformation would be an efficient strategy to direct the direction of assembled chains.

Table 1. Summary of the obtained electrical properties from OFET and SCLC devices. Hole mobility HOMO* LUMO* Polymers OFET SCLC (eV) (eV) µhh [cm2 V-1 s-1] µhv [× 10-3 cm2 V-1 s-1] -5.12 -3.81 3.60 ± 0.18 1.29 ± 0.21 PDPP-T -4.85 -3.96 0.042 ± 0.0019 3.42 ± 0.62 PDPP-BT * HOMO level of each polymer was directly measured by means of photoelectron spectroscopy (AC2, Fig. S8) and LUMO level was calculated based on the absorption edge of each polymer.

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Figure 5. Suggested chain assembly mechanism of CPs having different chain conformations

Conclusion Two different CPs having a similar chemical structure have been synthesized, and the effect of chain conformation on molecular assembly was investigated. Incorporation of modulated linkers (thiophene and benzothiophene) during polymerization effectively altered the chain conformation of the resulting CPs. The successful modulation of CP’s conformation was verified by both computational calculation and analysis of UV-vis absorption behaviors. 2D GIXRD analysis revealed that the planar PDPP-T and the slightly twisted PDPP-BT exhibited edge-on and face-on type molecular arrangements, respectively. The distinctive chain arrangement between PDPP-T and PDPP-BT, in spite of their similar chemical structure, could be traced from their different chain conformations. In addition, the systematic characterization of both FET and hole dominant SCLC devices showed that the assembled chain tendency critically affects the directional mobility of electric charge carriers, via the

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direction of the shortest inter-chain pathway. The concentration dependent absorption measurement showed that PDPP-T only produced pre-assembled chains when highly concentrated, which implies that its edge-on type chain assembly is likely connected with the generation of pre-assembled chains. After comparison of gravity center between individual and pre-assembled chains, it was concluded that the pre-assembled chains could tumble down to reduce their own gravity center, finally resulting in the edge-on type molecular assembly. Since CP chain conformation is inevitably connected with the generation of pre-assembled chains, manipulating molecular geometry could be an efficient strategy to manage the direction of the shortest inter-molecular distance connected with the pricipal charge transport pathway and hence for a superior optoelectronic device.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] ORCID: 0000-0003-1386-3705 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. Note The authors declare no competing financial interest

ACKNOWLEDGMENT This research was supported by Basic Science Research Program through the National

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Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2015R1C1A1A02036659). This research was also supported by the Industrial Fundamental Technology Development Program (10063268) funded by the Ministry of Trade, Industry & Energy (MOTIE) of Korea.

ASSOCIATED CONTENT Supporting Information Available: NMR and mass spectra of synthesized materials, computational results of energy levels of dimeric unit of CP’s repeating segment including their absorption spectra. This material is available free of charge via the internet at http://pubs.acs.org.

REFERENCES (1)

Günes, S.; Neugebauer, H.; Sariciftci, N. S. Conjugated Polymer-Based Organic Solar Cells. Chem. Rev. 2007, 107, 1324-1338.

(2)

Peet, j.; Kim, J. Y.; Coates, N. E.; Ma, W. L.; Moses, D.; Heeger, A. J. Bazan, G. C. Efficiency Enhancement in Low-Bandgap Polymer Solar Cells by Processing with Alkane Dithiols. Nat. Mater. 2007, 6, 497-500.

(3)

Chen, H. -Y.; Hou, J.; Zhang, S.; Liang, Y.; Yang, G.; Yang, Y.; Yu, L.; Wu, Y.; Li, G. Polymer Solar Cells with Enhanced Open-Circuit Voltage and Efficiency. Nat. Photon. 2009, 3, 649-653.

(4)

Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; LangeveldVoss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; de Leeuw, D. M. Two-Dimensional Charge Transport in Self-Organized, High-Mobility Conjugated Polymers. Nature 1999, 401, 685-688.

(5)

Venkateshvaran, D.; Nikolka, M.; Sadhanala, A.; Lemaur, V.; Zelazny, M.; Kepa, M.;

17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Hurhangee, M.; Kronemeijer, A. J.; Pecunia, V.; Nasrallah, I.; Romanov, I.; Broch, K.; McCulloch, I.; Emin, D.; Olivier, Y.; Cornil, J.; Beljonne, D.; Sirringhaus, H. Approaching Disorder-Free Transport in High-Mobility Conjugated Polymers. Nature 2014, 515, 384-388. (6)

Noriega, R.; Rivnay, J.; Vandewal, K.; Koch, F. P. V.; Stingelin, N.; Smith, P.; Toney, M. F.; Salleo, A. A General Relationship between Disorder, Aggregation and Charge Transport in Conjugated Polymers. Nat. Mater. 2013, 12, 1038-1044.

(7)

Zhu, Y.; Zhang, Y.; Yao, B.; Wang, Y.; Zhang, Z.; Zhan, H.; Zhang, B.; Xie, Z.; Wang, Y.; Cheng, Y. Synthesis and Electroluminescence of a Conjugated Polymer with Thermally Activated Delayed Fluorescence. Macromolecules 2016, 49, 4373-4377.

(8)

Schelkle, K. M.; Bender, M.; Jeltsch, K.; Buckup, T.; Müllen, K.; Hamburger, M.; Bunz, H. H. Light-Induced Solubility Modulation of Polyfluorene to Enhance the Performance of OLEDs. Angew. Chem. Int. Ed. 2015, 54, 14545-14548.

(9)

Lee, S. Y.; Yasuda, T.; Komiyama, H.; Lee, J.; Adachi, C. Thermally Activated Delayed Fluorescence Polymers for Efficient Solution-Processed Organic Light-Emitting Diodes. Adv. Mater. 2016, 28, 4019-4024.

(10) Kim, B. –G.; Jeong, E. J.; Chung, J. W.; Seo, S.; Koo, B.; Kim, J. A Molecular Design Principle of Lyotropic Liquid-Crystalline Conjugated Polymers with Directed Alignment Capability for Plastic Electronics. Nat. Mater. 2013, 12, 659-664. (11) Lee, B. H.; Hsu, B. B. Y.; Patel, S. N.; Labram, J.; Luo, C.; Bazan, G. C.; Heeger A. J. Flexible Organic Transistors with Controlled Nanomorphology. Nano Lett. 2016, 16, 314-319. (12) Diao, Y.; Tee, B. C-K.; Giri, G.; Xu, J.; Kim, D. H.; Becerril, H. A.; Stoltenberg, R. M.; Lee, T. H.; Xue, G.; Mannsfeld, S. C. B.; Bao, Z. Solution Coating of Large-Area Organic Semiconductor Thin Films with Aligned Single-Crystalline Domains. Nat.

18


Page 18 of 22

Page 19 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Mater. 2013, 12, 665-671. (13) Wanga, S.; Fabianoa, S.; Himmelbergerb, S.; Puzinasa, S.; Crispina, X.; Salleob, A.; Berggren, M. Experimental Evidence that Short-Range Intermolecular Aggregation is Sufficient for Efficient Charge Transport in Conjugated Polymers. Proc. Natl. Acad.

Sci. U.S.A. 2015, 112, 10599-10604. (14) Schott, S.; Gann, E.; Thomsen, L.; Jung, S. –H.; Lee, J. –K.; McNeill, C. R.; Sirringhasu,

H.

Charge-Transport

Anisotropy

in

a

Uniaxially

Aligned

Diketopyrrolopyrrole-Based Copolymer. Adv. Mater. 2015, 27, 7356-7364. (15) Wang, C.; Qin, Y.; Sun, Y.; Guan, Y. –S.; Xu, W.; Zhu, D. ThiopheneDiketopyrrolopyrrole-Based Quinoidal Small Molecules as Solution-Processable and Air-Stable Organic Semiconductors: Tuning of the Length and Branching Position of the Alkyl Side Chain toward a High-Performance n‑Channel Organic Field-Effect Transistor. ACS Appl. Mater. Interfaces 2015, 7, 15978-15987. (16) Lu, C.; Chen, H. –C.; Chuang, W. –T.; Hsu, Y. –H.; Chen, W. –C.; Chou, P. –T. Interplay of Molecular Orientation, Film Formation, and Optoelectronic Properties on Isoindigo- and Thienoisoindigo-Based Copolymers for Organic Field Effect Transistor and Organic Photovoltaic Applications. Chem. Mater. 2015, 27, 6837-6847. (17) Park, Y. J.; Seo, J. H.; Elsawy, W.; Walker, B.; Cho, S.; Lee, J. –S. Enhanced Performance in Isoindigo Based Organic Small Molecule Field-Effect Transistors through Solvent Additives. J. Mater. Chem. C 2015, 3, 5951-5957. (18) Kim, J.; Swager, T. M. Control of Conformational and Interpolymer Effects in Cconjugated Polymers. Nature 2001, 411, 1030-1034. (19) Schuettfort, T.; Thomsen, L.; McNeill, C. R. Observation of a Distinct Surface Molecular Orientation in Films of a High Mobility Conjugated Polymer. J. Am. Chem.

Soc. 2013, 135, 1092-1101. 19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(20) Zhao, K.; Yu, X.; Li, R.; Amassian, A.; Han, Y. Solvent-Dependent Self-Assembly and Ordering in Slow-Drying Drop-Cast Conjugated Polymer Films. J. Mater. Chem. C 2015, 3, 9842-9848. (21) Schulz, G.; Ludwigs, S. Controlled Crystallization of Conjugated Polymer Films from Solution and Solvent Vapor for Polymer Electronics. Adv. Funct. Mater. 2017, 27, 1603083. (22) Jeon, S.; Thakur, U. K.; Lee, D.; Wenping, Y.; Kim, D.; Lee, S.; Ahn, T. K.; Park, H. J.; Kim, B. –G. N-Phenylindole-Diketopyrrolopyrrole-Containing Narrow Band-Gap Materials for Dopant-Free Hole Transporting Layer of Perovskite Solar Cell. Org.

Electron. 2016, 37, 134-140. (23) Jeon, S.; Lee, J. H.; Lee, I. K.; Lee, W. H.; Kim, B. –G. Side Chain Assisted Different Types of Molecular Aggregation and Comparison of Aggregation Dependent Field Effect Mobility. Dyes Pigm. 2016, 133, 114-119. (24) Mei, J.; Bao, Z. Side Chain Engineering in Solution-Processable Conjugated Polymers.

Chem. Mater. 2014, 26, 604-615. (25) Cabanetos, C.; Labban, A. E.; Bartelt, J. A.; Douglas, J. D.; Mateker, W. R.; Fréchet, J. M. J.; McGehee, M. D.; Beaujuge, P. M. Linear Side Chains in Benzo[1,2-b:4,5b′]dithiophene–Thieno[3,4-c]pyrrole-4,6-Dione Polymers Direct Self-Assembly and Solar Cell Performance. J. Am. Chem. Soc. 2013, 135, 4656-4659. (26) Shimata, Y.; Ide, M.; Tashiro, M.; Katouda, M.; Imamura, Y.; Saeki, A. Charge Dynamics at Heterojunction between Face-on/Edge-on PCPDTBT and PCBM Bilayer: Interplay of Donor/Acceptor Distance and Local Charge Carrier Mobility. J. Phys.

Chem. C 2016, 120, 17887-17897. (27) Chen, M. S.; Niskala, J. R.; Unruh, D. A.; Chu, C. K.; Lee, O. P.; Fréchet, J. M. J. Control of Polymer-Packing Orientation in Thin Films through Synthetic Tailoring of

20


Page 20 of 22

Page 21 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Backbone Coplanarity. Chem. Mater. 2013, 25, 4088-4096. (28) Kim, H. G.; Kang, B.; Ko, H.; Lee, J.; Shin, J.; Cho, K. Synthetic Tailoring of SolidState

Order

in

Diketopyrrolopyrrole-Based

Copolymers

via

Intramolecular

Noncovalent Interactions. Chem. Mater. 2015, 27, 829-838. (29) Han, A. –R.; Dutta, G. K.; Lee, J.; Lee, H. R.; Lee, S. M.; Ahn, H.; Shin, T. J.; Oh, J. H.; Yang, C. ε-Branched Flexible Side Chain Substituted DiketopyrrolopyrroleContaining Polymers Designed for High Hole and Electron Mobilities. Adv. Funct.

Mater. 2015, 25, 247-254. (30) Qin, Y.; Kim, J. Y.; Frisbie, C. D.; Hillmyer, M. A. Distannylated Isothianaphthene: a Versatile Building Block for Low Bandgap Conjugated Polymers. Macromolecules 2008, 41, 5563-5570. (31) Stewart, J. J. P. Optimization of Parameters for Semiempirical Methods I. Method. J.

Comput. Chem. 1989, 10, 209-220. (32) Köse, M. E. Evaluation of Acceptor Strength in Thiophene Coupled Donor−Acceptor Chromophores for Optimal Design of Organic Photovoltaic Materials. J. Phys. Chem. A 2012, 116, 12503-12509. (33) Kim, Y.; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C.; Giles, M.; McCulloch, I.; Ha, C. –S.; Ree, M. A Strong Regioregularity Effect

in

Self-Organizing

Conjugated

Polymer

Films

and

Polythiophene:Fullerene Solar Cells. Nat. Mater. 2006, 5, 197-203.

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High-Efficiency


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TOC Figure

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Manipulation of Chain Conformation for Optimum Charge Transport

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