Stepwise Structural Evolution of a DTS-F2BT Oligomer and Influence

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Stepwise Structural Evolution of a DTS-FBT Oligomer and Influence of Structural Disorder on OFET and OPV Performance Chi-Feng Huang, Sin-Hong Huang, Chou-Ting Hsieh, Yi-Hsiang Chao, Chia-Hua Li, San-Lien Wu, Yi-Fan Huang, Chen-Yang Hong, Chain-Shu Hsu, Wei-Tsung Chuang, and Chien-Lung Wang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b03763 • Publication Date (Web): 28 Nov 2016 Downloaded from http://pubs.acs.org on November 30, 2016

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Chemistry of Materials

Stepwise Structural Evolution of a DTS-F2BT Oligomer and Influence of Structural Disorder on OFET and OPV Performance Chi-Feng Huang, a Sin-Hong Huang, a Chou-Ting Hsieh, a Yi-Hsiang Chao, a Chia-Hua Li, a San* Lien Wu,a Yi-Fan Huang,a Chen-Yang Hong, a Chain-Shu Hsu, a Wei-Tsung Chuang,b and Chien-Lung Wanga* a b

Department of Applied Chemistry, National Chiao Tung University, 1001 Ta Hsueh Road Hsinchu, 30010, Taiwan National Synchrotron Radiation Research Center, 101 Hsin-Ann Road, Hsinchu Science Park, Hsinchu, 300, Taiwan

*E-mail: [email protected], [email protected]

ABSTRACT: An A-D-A oligomer, DTS(F2BT)2, was synthesized; its structural evolution was studied with DSC, POM, 2DWAXD and in situ GI-XRD. The structural evolution of DTS(F2BT)2 is stepwise and kinetically slow. Both rapid drying and the presence of PC71BM trapped DTS(F2BT)2 in a less ordered nematic (N) phase. PDMS-assisted crystallization enabled a pristine DTS(F2BT)2 thin film to attain a more ordered equilibrium phase, and enhanced the OFET mobility of DTS(F2BT)2. In OPV devices, DIO additive drove the DTS(F2BT)2 domains in the DTS(F2BT)2:PC71BM blended film from the N phase toward the equilibrium phase, and resulted in enhanced OPV performances. These results reveal the slow ordering process of the A-D-A oligomer, and the importance of monitoring the degree of structural evolution of the active thin films in organic optoelectronics.

Introduction Organic optoelectronics attract academic and industrial attention because of their prospective mass production of flexible and cost-effective devices.1, 2 Progress in molecule design, processing and device architectures have led to improved charge mobility (μ) and efficiency of power conversion (PCE) of organic optoelectronics. On optimizing the molecular structures and processing techniques, hole mobilities (μh) exceeding 10 cm2 V−1 s−1 3-5and electron mobilities over 5 cm2 V−1 s−1 6-8 have been attained in organic field-effect transistors (OFET). Because of the excellent light-harvesting ability and μ, when blended with fullerene derivatives, donor-acceptor (D-A) conjugated copolymers and oligomers have also delivered PCE over 10 % in bulk-heterojunction (BHJ) organic photovoltaics (OPV).9, 10 Although high device performances were attained, D-A conjugated molecules remain complicated systems to investigate, because the active layers made with D-A molecules can easily incorporate both macroscopic heterogeneities (amorphous and mesophases) and microscopic heterogeneities (conformational isomers, side-chain enantiomerism etc.).11, 12 Conjugated molecules with an acceptor-donor-acceptor (A-D-A) framework have delivered large PCE. Combining the donor core, 4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5b’]dithiophene (DTS), with acceptor flanking units such as (6-fluorobenzo[c][1,2,5]thiadiazole) (F1BT),13-16 [1,2,5] thiadiazolo[3,4-c]pyridine,17, 18 and 3-ethyl-rhodanine19

have resulted in highly efficient A-D-A conjugated molecules. DTS-benzothiadiazole(BT)-based A-D-A conjugated oligomers have, in particular, demonstrated 9 % PCE in OPV devices.15 Linked with multiple inter-aryl single bonds, conformational changes of the A-D-A oligomers can significantly alter the molecular shapes and affect the assembly behavior of the molecules.11, 20, 21 Locking A-D-A molecules at a specific conformation require strong intramolecular interactions, such as CH···O and CH···N.12, 22 The energy barriers for the rotation of an inter-thienyl single bond23 or for a single bond between a thienyl unit and an acceptor unit are typically too small to hinder the conformational isomerization according to evidence from the results of calculations12, 22 and the single crystal structures of D-A oligomers.20, 24-27 The non-fused and extended D-A conjugated backbones thus increase conformational isomerism, and impede the crystallization of A-D-A conjugated molecules.28 Provided that conformational isomerization of A-D-A molecules occurs during the formation of a thin film, questions about the structural evolution in the solid-state, and whether a thermodynamic equilibrium phase can be attained during rapid drying (such as spin coating), or in an active layer containing fullerene derivatives, should be discussed. To address this issue, in this work, we synthesized an A-D-A conjugated molecule, 7,7'-(4,4-bis(2ethylhexyl)-4H-silolo[3,2-b:4,5-b'] dithio-phene-2,6diyl)bis(5,6-difluoro-4-(5'-hexyl-[2,2'-dithiophen]-5-

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Scheme 1. Synthetic route of DTS(F2BT)2 . (i) Pd(PPh3)2Cl2, toluene, 80 C, 24 h; (ii) Pd(PPh3)4, toluene, 110 C, 72 h

yl)benzo[c][1,2,5] thiadiazole) (DTS(F2BT)2 in Scheme 1). Its structural evolution in the solid state was investigated in experiments with a differential scanning calorimeter (DSC), polarized optical microscope (POM), and in situ grazing-incidence X-ray diffraction (GI-XRD) ; the lattice structure of the equilibrium crystalline phase was characterized with two-dimensional wide-angle X-ray diffraction (2D WAXD). A kinetically feasible metastable nematic (N) phase was invariably formed before a more ordered and more stable crystalline phase. Without appropriate conditions, the ordered phase at equilibrium was not attained in the active layers of the OFET and OPV devices because of lack of time, or the presence of a fullerene acceptor. Synthesis and thermal properties of DTS(F2BT)2 The synthetic route to prepare DTS(F2BT)2 is shown in Scheme 1. Reacting 5,6-difluoro-4,7-diiodobenzo[c] [1,2,5]thiadiazole (1) with stannylated hexylbithiophene (2) via Stille coupling resulted in the mono-functionalized F2BT arm unit (3) in 37 % yield. Figure S1 shows the 1HNMR spectra of 2 and 3, in which the Ha signal of 3 has a larger chemical shift than the Ha’ signal of 2. The downfield shift caused by the electron-withdrawing F2BT unit indicates that the F2BT and the bithiophene were successfully connected. In the mass spectrum (Figure S3a), the observed ratio m/z of 3 also matches its theoretical molecular mass. Reacting 3 (2 eq) with 4,4-bis(2-ethylhexyl)2,6-bis(trimethylstannyl)-4H-silolo[3,2-b:4,5b']dithiophene (4, 1 eq.) gave DTS(F2BT)2 in 34 % yield. Figures S2b and S3b show the mass spectra and 1H-NMR of DTS(F2BT)2. The appearance of the Hb signal of the DTS core in the 1H-NMR spectrum, and m/z ratio 1255.3069 confirmed the chemical identity of the final

product. DTS(F2BT)2 has satisfactory thermal stability; mass loss is 5 % at temperature 406 oC, determined from thermogravimetric analysis (TGA) (Figure S4). The phase behavior of DTS(F2BT)2 was investigated with a DSC. In Figure 1, when the DTS(F2BT)2 melt was cooled from 250 oC, two exothermic signals at 179 oC (ΔH = -0.413 kJ mol-1, ΔS = - 0.913 J K-1 mol-1) and at 148 oC (∆H = -26.5 kJ mol-1, ΔS = -62.96 J K-1 mol-1) were observed. A Schlieren texture (Figure 2a) was observed in a POM image of DTS(F2BT)2 at 160 oC, indicating that an exothermic transition at 179 oC is a transition from isotropic to nematic (N) phase. Below 148 oC, the POM image at 130 o C (Figure 2b) shows strong birefringence, indicating that the low-temperature crystalline phase possesses greater solid-state order than the N phase. In subsequent heating, a cold-crystallization signal at 135 oC (ΔH = -1.61 kJ mol-1, ΔS = -3.93 J K-1 mol-1) and a melting transition at 201 oC (ΔH = 37.96 kJ mol-1, ΔS = 80.1 J K-1 mol-1) were observed. The presence of cold crystallization indicates that formation of crystalline phase was kinetically slow and incomplete during cooling. The lack of the phase transition crystal to N during heating implies that the N phase might be metastable, and that the crystalline phase is an equilibrium phase. To confirm this deduction, we cooled the DTS(F2BT)2 melt from 250 oC and observed under a POM at 160 oC for 8 h. Given sufficiently protracted annealing, the Schlieren texture (Figure 2a) became a bright dendritic morphology (Figure 2c) at 160 oC. The results confirm the metastability of the N phase, and the monotropic phase behavior of DTS(F2BT)2.


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conformational order and molecular orientational order.29 Attaining a molecular orientational order before positional order and conformational order in the stepwise ordering of a DTS(F2BT)2 thin film indicates that aligning the long axes of DTS(F2BT)2 molecules is easier than positioning the molecules at lattice points and unifying the conformations of the molecules, which is reasonable because DTS(F2BT)2 has a curved backbone and many rotatable single bonds.30

Figure 1. DSC thermogram of DTS(F2BT)2 at scan rate 5 °C -1 min , showing a monotropic phase behavior.

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Figure 2. POM image of DTS(F2BT)2 at (a) 160 C, (b) 130 C, o o o and (c) cooling from 250 C to 160 C, and kept at 160 C 8 hrs.

Phase structure and phase evolution of DTS(F2BT)2 As the DSC result indicates that DTS(F2BT)2 forms the N phase before the crystalline phase, the progress towards the equilibrium phase in the DTS(F2BT)2 thin film was further monitored by the in-situ GI-XRD experiments. As shown in Figure 3a, when DTS(F2BT)2 was cooled from an isotropic melt to 160 oC, a broad diffraction arc immediately appeared at the meridional direction. The d-spacing of the arc is 1.5 nm, which is approximately the lateral dimension of DTS(F2BT)2. This result indicates that, as DTS(F2BT)2 enters the N phase, molecules align their long axes to the in-plane direction. After annealing at 160 oC for 5 min, three diffraction signals with positions 1: 2: 3 were assigned as (200), (400) and (600) along the meridional direction (Figure 3b), whereas the diffractions in the quadrant remained weak, indicating that a long-rangeordered lamellar structure developed after the formation of the N phase. The highly symmetric diffractions of the equilibrium phase formed were observed about 15 min (Figure 3c). The in-situ GI-XRD results thus confirmed that the equilibrium phase of DTS(F2BT)2 was developed through (1) setting up the in-plane orientation of the conjugated backbones on the substrate, (2) separating the flexible alkyl chains and rigid conjugated backbones to establish the long-range-ordered lamellar structure, and (3) adjusting the conformations and positions of DTS(F2BT)2 molecules to attain the equilibrium structure. According to Wulderlich’s classification of condensed phases, a crystalline phase is a phase with long-range positional order,

The single crystal of DTS(F2BT)2was difficult to obtain, probably due to the large number of conformational isomers and the slow crystallization kinetic of the molecule.11, 24, 28, 30 To know the crystalline phase of the equilibrium thin-film structure, we deduced lattice parameters (a, b, c, α, β, and γ) from a pole figure of the GIXRD pattern (Figure 3c), as shown in Figure 4a. For an orthorhombic lattice with a = 4.54 nm, b = 0.96 nm, c = 2.71 nm, we indexed (hkl) diffractions in Figure 4a. The (hoo) and (020) diffractions respectively appeared along meridional and equatorial directions, and out-of-plane diffractions show (hk0) and (hol) planes. These findings indicate that the a-axis of an orthorhombic lattice prefers to align perpendicularly to the substrate and the crystals of the thin film present a-axis rotation. The crystallographic data obtained from GI-XRD (Table S1) closely match those obtained from fiber diffraction method (Table S2). Figure 4b shows the 2D WAXD pattern of an extruded DTS(F2BT)2 fiber that was annealed at 160 oC before the measurements to ensure a complete formation of the equilibrium crystalline phase. The (h00) and (hk0) diffractions on the equator and the (00l) diffractions on the meridian indicate that the long axis of molecules DTS(F2BT)2 is aligned along the shear direction with the c-axis rotation. GI-XRD provides not only lattice parameters from Bragg signals but also information about the electron density within the crystallographic unit cell from the peak intensities. Conventional analysis through 2D Fourier transformation is a most commonly used approach for localization of missing atoms and the recognition of molecular conformation in the crystallographic lattice from X-ray diffraction data. Realizing the conformation and localization of designated molecules through the electron density map could enable a construction of a causal relation among the thin-film structure, fabrication and electrical performance to develop organic optoelectronics, in addition to optimum chemical structures of organic semiconductor and maximum orbital overlap continues. 2D electron density maps were generated with the general Fourier formula for 2D electron density as ρ, ∑ 2 or ∑ 2 , in which φ(hk or hl) are phases of structure factors (equal to 0 or π) and I(hk or hl) are intensities of Bragg signals



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Figure 3. In-situ GI-XRD pattern of a DTS(F2BT)2 thin film, (a) cooled from the isotropic melt to 160 C, (b) annealed at 160 C o for 5 min, and (c) annealed at 160 C for 15 min.

Figure 4. (a) Pole figure of 2D GI-XRD pattern of DTS(F2BT)2. (b) 2D WAXD pattern of the equilibrium phase, and 2D electrondensity maps in (c) plane ac and (d) plane ab .

obtained from the GI-XRD patterns after background subtraction and correlation of Lorentz and polarization factors, respectively. The correct phase can be determined on the basis of physical merits of the reconstructed electron-density map, aided by other information on the system from e.g. spectra or molecular simulation. The relative values to reconstruct electron-density maps are listed in Table S1. The 2D electron-density maps of planes ac and ab are shown in Figure 4c and 4d, respectively, indicating that DTS(F2BT)2 molecules adopt an edge-on orientation with tilt angle 5o to the substrate; dπ-π is ca. 0.36 nm obtained from Figure 4d. Absorption and Energy Levels of Frontier Orbitals of DTS(F2BT)2

gaps (Eg) of the compound. For the solution in chloroform, the two absorption bands of DTS(F2BT)2 at 380 and 570 nm are attributed to a localized π-π* transition and an intramolecular charge transfer (ICT) transition. The red-shifted absorption band (λmax = 590 nm) and the vibronic shoulder (640 nm) of the DTS(F2BT)2 thin film indicate a stronger molecular aggregation in the solid state. Eg of DTS(F2BT)2 is 1.62 eV, deduced from λonset at 742 nm. EHOMO and ELUMO of DTS(F2BT)2, determined from a cyclic voltammogram (Figure S5), are -5.46 and -3.67 eV. EHOMO and ELUMO of DTS(F2BT)2 lie lower than those of its analogue, DTS(F1BT)2 in the literature14 , and Eg of DTS(F2BT)2 is wider than that of DTS(F1BT)2. The additional two fluorine substituents thus lower EHOMO and ELUMO, and widen Eg of the conjugated system.

Figure 5 shows the normalized UV–visible absorption of a DTS(F2BT)2solution and thin film. Table 1 summarizes the optical data, including the wavelengths of absorption maxima (λmax), absorption edges (λonset), and optical band


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14

Table 1. Absorption and electrochemical data of DTS(F2BT)2, and DTS(F1BT)2 λmax /nm

Compd.

DTS(F2BT)2, 14

DTS(F1BT)2

a

λonset/nm

Eg/eV

Solution

Film

Solution

Film

570

590

642

742

1.79 (1.62 )

590

678

670

800

1.78 (1.55 )

EHOMO

ELUMO

/eV

/eV

a

b

-5.46

-3.67

a

b

-5.12

-3.34

b

based on redox potentials; based on the onset absorptions of thin films Table 2. OFET characteristics of DTS(F2BT)2. 2

-1 -1

Device treatment

SAM layer

μh,max./cm V s

PAC

ODTS

3.9×10

PAC

PTS

1.4×10

Spin-coated

ODTS

1.9×10

Spin-coated

PTS

3.6×10

*

*

2

-1 -1

Ion/Ioff

-3

-3

9.1×10

-2

-3

-3

-4

1.8×10

-3

-4

1.3×10

μh,ave. /cm V s

-3

2.4×10 ±1.5×10

-2

1.0×10 ±3.4×10

-3

1.6×10 ±5.9×10

-3

2.9×10 ±8.2×10

Vth/V

5

10.12

9.3×10

3

-23.87

4

-15.04

3

-5.42

Average value of 5 devices. Value behind “±” represents the standard deviation. Table 3. OPV characteristics of DTS(F2BT)2/PC71BM thin films

DTS(F2BT)2/PC71BM (6:4 w/w) DTS(F2BT)2/PC71BM (6:4 w/w) DTS(F1BT)2/PC71BM13,33 *

-2

Voc*/V

Jsc*/mA cm

FF*/%

PCE*/%

0.81±0.07

5.75±1.21

50.41±4.83

2.4±0.75

0.4 v% DIO

0.82±0.06

7.56±0.92

54.99±4.87

3.39±0.48

0.4 v% DIO

0.772

14.5

60

6.29

Additive -

Average value of 13 devices. Value behind “±” represents the standard deviation.

OFET Performance of DTS(F2BT)2

Figure 5. UV-visible absorption spectra of DTS(F2BT)2 in -6 CHCl3 solution (10 M) and as a thin film.

The charge-transport properties of DTS(F2BT)2 were investigated in OFET devices with bottom-gate and topcontact configuration. To investigate the degree of structural evolution in varied process conditions, we prepared the active layer by both spin coating and a PDMS-assisted crystallization (PAC).31 The output and transfer plots of the devices exhibit typical p-channel OFET characteristics (Figure 6). The hole mobility (μh) of DTS(F2BT)2 was obtained from the transfer characteristics of the devices in saturation regimes. As summarized in Table 2, the devices prepared via the PAC method gave greater μh than those prepared via spin coating. The GI-XRD pattern in Figure 7a shows that DTS(F2BT)2 molecules in the spin-cast film are in the N phase, whereas the (200), (400), (600) and the quadrant diffractions in Figure 7b indicate that



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the addition of 0.4 v% of DIO delivered more united and higher values. As a result, the active films prepared with DIO also gave PCE with higher averaged value and lower standard deviation as shown in Figure 8b. Considering a statistical significance α = 0.05 represents a 95% confidence to conclude that the active layers prepared with DIO have a higher average PCE than those prepared without DIO. To assess the statistical significance, the test's p-value is calculated.32 The p-value deduced from the PCEs of 13 devices prepared without DIO and 13 devices with DIO is 0.0022, which is less than α = 0.05. Thus, it can be concluded that the DTS(F2BT)2/PC71BM thin films prepared with DIO delivered better PCE than those prepared without DIO.

Figure 6. Output (left) and transfer (right) characteristics of DTS(F2BT)2 OFET devices prepared with (a) spin coating and (b) PAC technique. PTS served as the SAM layer.

Figure 7. GI-XRD patterns of DTS(F2BT)2 thin films peprepared with (a) spin coating and (b) PAC.

GI-XRD patterns of DTS(F2BT)2:PC71BM thin films prepared without and with DIO appear in Figure 9. The large scattering halo at q=17.5 nm-1 belongs to PC71BM. In direction qz, diffraction arcs belonging to the N phase and the lamellar packing of DTS(F2BT)2, i.e. (200), (400), (600) diffractions, are found, but quadrant diffractions were not observed. The results indicate that with the presence of PC71BM, DTS(F2BT)2 retained an edge-on orientation, but an equilibrium crystalline phase was not formed. The DIO additive decreased the fraction of the N phase, and increased the fraction of the phase with more ordered lamellar packing (Figure 9b). Correlating the OPV performances with the morphological results, it can be found that the better OPV performances, which were delivered by active films prepared with 0.4 v% of DIO, can be partly attributed to the improved solid-state order of the DTS(F2BT)2 domain in the thin film.

DTS(F2BT)2 molecules in the PAC film are in the equilibrium phase. The solid-state morphology of DTS(F2BT)2 is thus highly dependent on the process conditions. A protracted process and a controlled crystal growth in the PAC method allow DTS(F2BT)2 to attain greater solidstate order, and, consequently, improved μh from 3.6×10-3 cm2 V-1 s-1 to 1.4×10-2 cm2 V-1 s-1. OPV Characteristics of DTS(F2BT)2:PC71BM The OPV characteristics of DTS(F2BT)2:PC71BM active films were assessed in devices with architecture ITO/ZnO/DTS(F2BT)2:PC71BM (60:40 w/w)/MoO3/Ag. The DTS(F2BT)2:PC71BM active films were produced with or without diiodooctane (DIO). The thickness of the active films measured by atomic force microscope are 88.1 nm (without DIO) and 96.6 nm (with DIO). (Figure S6 ) Plots of current density vs voltage of the devices appear in Figure 8; device characteristics are summarized in Table 3. Comparing with the characteristics of its analogue, DTS(F1BT)2,13 DTS(F2BT)2 provides a greater open-current voltage (Voc), 0.86 V, because of its lower-lying HOMO. The statistical data in Figure S7 shows that the DTS(F2BT)2:PC71BM active films prepared without DIO gave scattered and lower values of Voc, short-circuit current (Jsc), and fill factor (FF), whereas those prepared with

Figure. 8 (a)Current density–voltage characteristics and (b)histograms of device PCE of DTS(F2BT)2–PC71BM-based inverted BHJ PSC processed with and without DIO (0.4 vol -2 %) under illumination of AM 1.5 G at 100 mW cm .


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reaction mixture was heated to 80 oC for 24 h. After reaction, the solution was concentrated under reduced pressure to remove the solvent and then loaded onto silica and purified with flash chromatography using toluene:hexane (3:7) as solvent. (The material dissolves with difficulty. To load onto the column, it can be dissolved in hot toluene.) After removal of the solvent, a fuchsia solid was obtained; yield 125 mg (37.4 %). 1

Figure 9. GI-XRD patterns of DTS(F2BT)2:PC71BM thin films prepared (a) without DIO, (b) with DIO (0.4 v %).

H NMR (400 MHz, CDCl3) δ/ppm 8.19 (d, J = 4.0 Hz, 1H), 7.23 (dd, J = 4.1, 1.4 Hz, 1H), 7.15 (d, J = 3.6 Hz, 1H), 6.74 (d, J = 3.6 Hz, 1H), 2.82 (t, J = 7.6 Hz, 2H), 1.70 (dt, J = 15.3, 7.5 Hz, 2H), 1.47 – 1.15 (m, 6H), 0.90 (t, J = 7.1 Hz, 3H). DTS(F2BT)2

Conclusion In this work, the structural evolution of an A-D-A conjugated oligomer, DTS(F2BT)2, were monitored via DSC and GI-XRD. The in-situ GI-XRD experiments show that the structural evolution of DTS(F2BT)2 is stepwise and kinetically slow. Rapid drying such as with spin casting and the presence of PC71BM thus prevent DTS(F2BT)2 from attaining its thermodynamically equilibrium phase, and trap the molecules in a less ordered N phase. The less ordered packing in the N phase resulted in the lower OFET and OPV performances. The PAC method and a DIO additive assisted DTS(F2BT)2 to approach the more ordered equilibrium phase in the pristine and blended thin films respectively, and led to the improved performances in the OFET and OPV devices. The results indicate that the A-D-A oligomer, DTS(F2BT)2, delivers different performances in different condensed phases. Like DTS(F2BT)2, many DTS based A-D-A oligomers gave only limited diffraction peaks from their thin films.34, 35 There is possibility that in the thin film, these A-D-A oligomers were also trapped in mesophases with different types and degrees of disorder. Monitoring the degree of structural evolution of these A-D-A oligomers via GI-XRD and electron-density maps, and correlate the device performances with the exact solid-state structure can lead to a more accurate structure-property relationship of A-D-A oligomers and provide useful information when process conditions need to be optimized.

5,5’-Bis(trimethylstannyl)-3,3’-di-2-ethylhexylsilylene2,2’-bithiophene (compound 4, 59 mg, 0.079 mmol), compound 3 (100 mg, 0.183 mmol), and toluene (8 mL) were added to a two-necked round-bottom flask (100 mL) and degased for 20 min. Catalyst Pd(PPh3)2Cl2 (21.45 mg) was added; the mixture was reacted near 295 K for 24 h. The reacted solution was poured into methanol, precipitated and filtered. The filtered solid was dissolved in trichloromethane. The solution was then loaded onto silica and washed with methanol:hexane (3:1) and propanone. The material was purified on a chromatography using tetrahydrofuran or trichloromethane, yielding a purple solution. An alumina plug using trichloromethane afforded the product. The solution was concentrated under reduced pressure to remove the solvent. The material was added to trichloromethane and heated until dissolution. Solvent methanol:hexane (3:1) was added to the solution for reprecipitation. The solution was stirred overnight, filtered and washed with purified hexane and propanone. After drying, a purple solid was obtained; yield 34 mg (33.86 %). 1

H NMR (400 MHz, CDCl3) δ/ppm 8.35 (t, J = 4.6 Hz, 1H), 8.17 (d, J = 4.0 Hz, 1H), 7.19 (d, J = 4.0 Hz, 1H), 7.12 (d, J = 3.5 Hz, 1H), 6.72 (d, J = 3.5 Hz, 1H), 2.81 (t, J = 7.6 Hz, 2H), 1.70 (dt, J = 15.3, 7.5 Hz, 2H), 1.48 – 0.99 (m, 18H), 0.91 (t, J = 6.7 Hz, 3H), 0.831-0.87 (m, 6H). [M+]: (1254.31) 1255.87; found: 1255.3069. Characterization of DTS(F2BT) 2 NMR spectra 1

EXPERIMENTS Materials All reagents and chemicals (Merck, Fischer, Aldrich, Lancaster, TCI and Acros) were used as received unless otherwise specified. The drying agents were dried from agents with sodium metal. The samples were maintained under standard conditions before characterization and analysis. Synthesis of DTS(F2BT)2 Compound 3. Compound 1 (253 mg, 0.597 mmol), compound 2 (259 mg, 0.627 mmol), and toluene (12 mL) were added to a two-necked round-bottom flask (100 mL) and degased for 20 min. Catalyst Pd(PPh3)2Cl2 (21.45mg) was added; the

H spectra were recorded (400 and 75 MHz). Chemical shifts are expressed in parts per million (ppm); splitting patterns are designated s (singlet), d (doublet), t (triplet) and m (multiplet). Coupling parameters J/Hz are reported. The reference for 1H NMR spectra was residual proton impurity in DCCl3 at δ 7.27 ppm. For 13C NMR spectra the reference was 13CDCl3 at δ 77.00 ppm. Mass Spectra Mass spectra were recorded (Micromass Trio 2000 mass spectrometer, Manchester, UK; high resolution, JEOL JMS- 700, Japan). Thermogravimetric Analysis (TGA)



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TGA were recorded (Perkin-Elmer Pyris, under a dry dinitrogen flow, heated from 23 oC to 750 °C at rate 10 °C min-1). Differential Scanning Calorimeter (DSC) For the differential scanning calorimeter (TA Instruments Unpacking Q series DSC), dinitrogen served as purge gas; samples were scanned in from 0 °C to 250 °C. The measurement operated two cycles at scan rates 5 °C min−1 and 1 °C min−1. Polarizing Optical Microscope (POM) Observations with a polarizing optical microscope (Zeiss Axiophot trinocular research polarizing optical microscope) yielded images recorded with a microscope camera (Axiocam ERC5s). The sample temperature was controlled with a hot stage (Mettler-Toledo FP82). UV-Visible absorption spectra UV-visible absorption spectra were recorded (Hitachi UV-400 UV-visible spectrophotometer). DTS(F2BT)2 in solution (10-6 M in trichloromethane) was recorded at 23 o C. For measurements of thin films, DTS(F2BT)2 was spincoated onto pre-cleaned glass slides (2.5 cm x 2.5 cm) from solution (10-6 M). Cyclic Voltammetry Cyclic-voltammetric curves were measured (CH Instruments model 611D) with a standard three-electrode configuration (working electrode glass-carbon, reference electrode Ag/AgNO3 (0.01 M in anhydrous acetonitrile), and counter electrode Pt wire). DTS(F2BT)2 was cast onto a glassy carbon working electrode from trichloromethane solution (10-6M) and dried under dinitrogen. The measurements were made in anhydrous acetonitrile with Bu4NPF6 (tetra-1-butylammonium hexafluorophosphate, 0.1 M) as supporting electrolyte under argon at scan rate 20 mV/s. CV curves were calibrated using ferrocene/ferrocenium redox couple (Fc/Fc+) as standard, of which the oxidation potential was set at −4.8 eV with respect to the zero vacuum level. The highest occupied molecular orbital (EHOMO ) and lowest unoccupied molecular orbital (ELUMO) energy levels of copolymers were calculated from onset oxidation potentials (Eonsetox) and onset reduction potentials (Eonsetred), respectively, according to equations: EHOMO = −e (Eonsetox − Eonset(ferrocene) + 4.8) eV, and ELUMO = −e (Eonsetred − Eonset(ferrocene) + 4.8) eV. 2D-wide angle X-ray diffraction (2D WAXD) To measure 2-D WAXS patterns we used a singlecrystal X-ray diffractometer (Bruker APEX DUO, microfocus air-cooled sealed Cu tube source, 50 W, (50 kV, 1 mA; Kα radiation 0.1542 nm) and an APEXII CCD camera. The DTS(F2BT)2 fiber sample was prepared on extruding the sample from a homemade stainless-steel extruder at ca. 210 °C. The diffraction pattern was recorded near 23 o C, exposure duration 40 s. Grazing Incidence X-ray Diffraction (GI-XRD) The GI-XRD experiments were performed at beamline BL13 of Taiwan Light Source at National Synchrotron

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Radiation Research Center (NSRRC). Incident X-rays (wavelength 1.03 Å) were delivered from a superconducting wavelength-shifting magnet and a Si (111) doublecrystal monochromator. The diffraction pattern was recorded with an imaging plate detector (Mar345). The angle between film surface and incident beam was fixed at 0.18°. GI-XRD patterns were recorded in situ of a DTS(F2BT)2 melt (2 mg) on a cleaned glass during cooling. Fabrication and Characterization of a Polymer Solar Cell The device structures for inverted PSC were ITO/ZnO/ DTS(F2BT)2 :PC71BM /MoO3 /Ag. The ITO glass substrates were cleaned with detergent, deionized water, propanone, and 2-methylethanol in an ultrasonic bath and then dried overnight in an oven at > 100 ° C. The ITO glass substrates were subjected to treatment with UV-ozone over 20 min. Zinc acetylacetonate hydrate dissolved in methanol (20 mg mL− 1) was spin-cast at 2000 rpm on pre-cleaned ITO substrates and baked at 170 °C for 15 min in glove boxes under dinitrogen atmosphere to form the ZnO layer of thickness 40 nm. Blends of DTS(F2BT)2and PC71BM were dissolved in chlorobenzene at 90 °C; the solutions were filtered through a poly(tetrafluoroethene) (PTFE, 1 μm) filter. In a glove box, the solution was kept at 120 ° C, and then spin-coated at 900 rpm onto the ZnO layer to form an active layer. The anode made of MoO3 (6 nm) and Ag (150 nm) was evaporated through a shadow mask under vacuum (< 10− 6 Torr). The devices were characterized in air under AM 1.5 G irradiation with intensity 100 mW/cm2 simulated light measurement (Yamashita Denso solar simulator). Current versus potential (J-V) curves were recorded with illumination (Keithley 2400 Solar) conforming to JIS Class AAA was provided with a solar simulator (SAN-EI 300 W) equipped with an AM 1.5G filter. The incident light intensity was calibrated with a silicon photodiode (Hamamatsu S1336-5BK). Atomic Force Microscope Measurements Atomic force microscopy (AFM) measurements were performed with a Veeco Diinnova atomic force microscope (AFM). DTS(F2BT)2:PC71BM active films with and without DIO are prepared as previous section. The active films was scratched by blade and were measured in tapping mode. OFET Fabrication and Characterization A heavily doped n-type Si wafer and a layer of dry oxidized SiO2 (300 nm, capacitance 11 nF cm-2) served as a gate electrode and gate dielectric layer, respectively. The substrates were placed in Piranha solution (H2SO4:H2O2= 7:3) for 2 h, cleaned using deionized water and dried under a dinitrogen stream. The surface was modified with octadecyltrichlorosilane (ODTS) or phenyltrimethoxysilane (PTS). The substrates were washed with EtOH and dried under dinitrogen stream. DTS(F2BT)2 was dissolved in chlorobenzene (20mg/mL). The active layers were prepared with spin-coating (2000 rpm) or PDMS-assist crystallization (PAC). The annealing was done in vacuum


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for 30 min at 80 °C. Gold source and drain contacts (thickness 40 nm) were deposited by vacuum evaporation on the organic layer through a shadow mask, affording a bottom-gate, top-contact OFET device. Field-effect characteristics of the devices were determined near 23 oC in air using a semiconductor parameter analyzer (Keithley 4156C, Agilent Technologies). The field-effect mobility was calculated in the saturation regime with equation Ids = (μWCi/2L)(Vg – Vt)2, in which Ids is the drain-source current, μ is the field-effect mobility, W is the channel width (1 mm), L is the channel length (0.1 mm), Ci is the capacitance per unit area of the gate dielectric layer, Vg is the gate voltage and Vt is threshold voltage.

SUPPORTING INFORMATION 1

H NMR spectra, mass spectra, TGA, index of XRD and GIXRD, reduction,oxidation cyclic-voltammetric curves, AFM images of OPV thin film and histograms of OPV characteristics of DTS(F2BT)2 are available in supporting information. The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXXX

AUTHOR INFORMATION Corresponding Author a

Chien-Lung Wang. E-mail: [email protected] b Wei-Tsung Chuang. E-mail: [email protected]

Present Addresses a

Department of Applied Chemistry, National Chiao Tung University, 1001 Ta Hsueh Road Hsinchu, 30010 (Taiwan) b National Synchrotron Radiation Research Center, 101 HsinAnn Road, Hsinchu Science Park, Hsinchu, 300, Taiwan

Funding Sources Ministry of Science and Technology, Taiwan (MOST 1032221-E-009-213-MY3, MOST 104-2628-E-009-007-MY3, MOST 104-2628-E-213 -001 -MY3) and “ATP” of the National Chiao Tung University and Ministry of Education, Taiwan..

ACKNOWLEDGMENT Ministry of Science and Technology, Taiwan (MOST 1032221-E-009-213-MY3, MOST 104-2628-E-009-007-MY3, MOST 104-2628-E-213 -001 -MY3) and “ATP” of the National Chiao Tung University and Ministry of Education, Taiwan supported this research. We thank National Synchrotron Radiation Research Center (NSRRC, Taiwan) for assistance with the GIWAXS measurements..

REFERENCES (1) Heeger, A. J., Semiconducting polymers: the third generation. Chem. Soc. Rev. 2010, 39, 2354-2371.

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Stepwise Structural Evolution of a DTS-F2BT Oligomer and Influence

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