Experimental Section - Modulation of Intermolecular Distance Incorporating Alkoxy Thiophene and

Chapter Ⅴ. Modulation of Intermolecular Distance Incorporating Alkoxy Thiophene and Alkyl

4.4 Experimental Section

Materials measurement: All starting materials were purchased from Sigma-Aldrich and Acros and used without further purification. All solvents were ACS grade unless otherwise noted. ¹H NMR spectra were recorded on VNMRS 600 (Agilent, USA) spectrophotometer using C2H2Cl4 as solvent and tetramethylsilane (TMS) as the internal standard. UV-Vis-NIR spectra were recorded on a UV-1800 (SHIMADZU) spectrophotometer. DFT calculations were performed using the Gaussian 09 package with the nonlocal hybrid Becke three-parameter Lee-Yang-Parr (B3LYP) function and the 6-31G* basis set to elucidate the highest occupied molecular orbital (HOMO) and LUMO levels after optimizing the geometry of polymers using the same method. The number-average (Mn) and weight average (Mw) molecular weights, and the polydispersity index (PDI) of the polymer products were determined by HT- GPC with Agilent 1200 HPLC and miniDAWN TREOS using polystyrene as standards in 1,2,4- trichlorobenzene at 120°C (HPLC grade). Differential scanning calorimetry (DSC) were measured by differential scanning calorimeter (TA instruments, USA) with a scan rate 10°C per a minute. Cyclic voltammetry (CV) measurements were performed on Solartron electrochemical station (METEK, Versa STAT3) with a three-electrode cell in a 0.1 M tetra-n-butylammonium hexafluorophosphate (n-Bu4NPF6) solution in acetonitrile at a scan rate of 100 mV/s at room temperature. Ag/Ag⁺ (0.01M AgNO3 in acetonitrile) electrode, a platinum wire, and a polymer-coated platinum electrode were used as the reference electrode, counter electrode, and working electrode, respectively. The Ag/Ag⁺ reference electrode was calibrated using a ferrocene/ferrocenium redox couple as an internal standard, whose oxidation potential is set at -4.8 eV with respect to zero vacuum level. The HOMO energy levels were obtained from the equation HOMO (eV) = LUMO - Egopt. The LUMO levels of the polymers were

obtained from the equation LUMO (eV) = -4.8 – (E1/2redfirst – E1/2oxFe/Fc+).

Synthesis of PNDI-Fu10: The monomer NDI-Br2 (100 mg, 0.10 mmol), T2-Sn2 (44 mg, 0.09 mmol), Fu-Sn2 (3.9 mg, 0.01mmol), Pd2(dba3) (2.0 mg, 3.0 mol) and P(o-tolyl3) (3.0 mg, 12 mol) were dissolved in toluene (10 mL) in argon condition. The sealed reaction flask was then stirred at 100 ^oC for 48 h. Then, excess 2-(trimethylstanny) thiophene was added and the reaction mixture was stirred at 100 ^oC for another 24 h. Finally, excess 2-bromothiophene was added and the reaction mixture was stirred for another 24 h. The mixture was cooled to room temperature and precipitated in methanol (100 mL). The precipitate was filtered and washed with methanol, acetone, hexane successively in a Soxhlet apparatus to remove oligomers and catalyst. Finally, the polymer was extracted with chloroform. The chloroform fraction was concentrated and precipitated in methanol. The precipitate was filtered and dried in a vacuum. The dark blue product was isolated in 81% yield. ¹H NMR (600 MHz, 1,1,2,2- tetrachloroethane-d2): δ(ppm) 9.23 (br, 0.2H), 8.88 (br, 2H), 7.65 (br, 0.2H), 7.41 (br, 4H), 4.18 (br, 4H), 2.08 (br, 2H), 1.41-1.23 (br, 64H), 0.90 (br, 12H). Elemental analysis: Anal. Calcd. C, 75.52; H, 9.03;

N, 2.86; S, 5.89. Found: C, 75.38; H, 9.19; N, 2.75; S, 5.62. Mn = 55.4 kDa, PDI = 2.19.

Synthesis of PNDI-Th10: Using the same procedure described above, except for using Th-Sn2 (4.1 mg, 0.01mmol). The dark blue product was isolated in 90% yield. ¹H NMR (600 MHz, 1,1,2,2- tetrachloroethane-d2): δ(ppm) 8.98 (br, 0.2H), 8.88 (br, 2H), 7.52 (br, 0.2H), 7.41 (br, 4H), 4.18 (br, 4H), 2.08(br, 2H), 1.42-1.28 (br, 64H), 0.91 (br, 12H). Elemental analysis: Anal. Calcd. C, 75.40; H, 9.02;

N, 2.85; S, 6.21. Found: C, 75.26; H, 9.18; N, 2.74; S, 5.94. Mn = 85.9 kDa, PDI = 2.12.

Synthesis of PNDI-Se10: Using the same procedure described above, except for using Se-Sn2 (4.6 mg, 0.01 mmol). The dark blue product was isolated in 90% yield. ¹H NMR (600 MHz, 1,1,2,2- tetrachloroethane-d2): δ(ppm) 9.02 (br, 0.2H), 8.89 (br, 2H), 7.72 (br, 0.2H), 7.42 (br, 4H), 4.19 (br, 4H), 2.10 (br, 2H), 1.42-1.33 (br, 64H), 0.92 (br, 12H). Elemental analysis: Anal. Calcd. C, 75.04; H, 8.98;

N, 2.84; S, 5.85. Found: C, 75.30; H, 8.86; N, 3.02; S, 5.64. Mn = 61.7 kDa, PDI = 2.29.

Fabrication and Characterization of Solar Cells: A conventional device (ITO/PEDOT:PSS/active layer/PDINO/Al ) was fabricated. PEDOT:PSS (Bayer Baytron 4083) was spin-coated at 4000rpm onto ITO substrate by annealing at 140°C for 20 minutes in air. The active layer was spin-coated from chloroform solution of donor and acceptor polymers (12mg ml^-1) onto the PEDOT:PSS layer and subsequent thermal annealing at 120°C for 10 minutes. After then PDINO dissolved in methanol was deposited onto the active layer with a spin rate of 3000 rpm for 30s and 100nm aluminum (Al) was thermal evaporated under vacuum (<5.0 X 10^-5 Pa). In all device conditions, 1vol% of DIO was added.

The J-V characteristics were performed with Keithley 2400 source under illumination of an AM 1.5G solar simulator with an intensity of 100Mw cm^-2. The EQE measurement were investigated in ambient air using an EQE system (Model QEX7) by PV measurements lnc. (Boulder, Colorade). The hole and

electron mobilities were measured by SCLC method with the structure of ITO/PEDOT:PSS/active layer/Au for hole-only and ITO/ZnO/active layer/PDINO/Al for electron-only.

4.5 Additional Experiment

The random terpolymer with 50% chalcogenophene (-Fu, -Th, -Se) additionally were synthesized, and they were measured optical and electrochemical properties by UV-Vis spectroscopy and cyclic voltammetry (CV). From data, we can confirm their electronic energy levels. These polymers were also measured performance of all polymer solar cells (All-PSCs).

3.5.1 Characterization of random terpolymers

The random terpolymers were investigated by ¹H-NMR. (Figure 22, 23, 24) There were two separated peaks in aromatic regions;  = 8.88–9.23 and  = 7.72–7.42. In these regions, we can confirm to compare the number of protons through integral.

Figure 22. Investigation of ¹H NMR related to random terpolymers, P(NDI-Fu50).

Figure 23. Investigation of ¹H NMR related to random terpolymers, P(NDI-Th50).

Figure 24. Investigation of ¹H NMR related to random terpolymers, P(NDI-Se50).

3.5.2 Optical and electrochemical properties

The optical properties were measured by UV-Vis spectroscopy. (Figure 25) There are two broad peaks;

low-energy band region, corresponding to intra-charge transition (ICT) and high-energy band region, corresponding to π-π* transition. P(NDI-Fu50) showed highest intra-charge transition (ICT) and more red-shifted wave regions, indicating most aggregated state. All polymers were red-shifted as thin film state.

Figure 25. The absorption spectra of random terpolymers in dilute chloroform solution (a) and as thin films (b).

The Electrochemical properties were measured by cyclic voltammetry (CV). (Figure 26) All polymers showed two reversible redox waves, however their electronic energy levels, lowest unoccupied molecular orbitals (LUMOs) were not changed. The summarized values are shown in Table 8.

47 Figure 26. Electrochemical properties.

Table 8. Photovoltaic parameters of all-PSCs based on the random terpolymers.

Polymer λmax sol (nm) λmax film (nm) HOMO (eV)^a

LUMO (eV)^b

Egopt

(eV)^c

P(NDI-Fu50) 621 661 -5.33 -3.79 1.54

P(NDI-Th50) 576 641 -5.41 -3.81 1.60

P(NDI-Se50) 579 644 -5.39 -3.78 1.61

aHOMO = LUMO – Egopt;^bLUMO = -4.8 – (E1/2redfirst –E1/2oxFc/Fc+); ^cCalculated from the absorption onset in CF-cast film.

48 4.5.3 Photovoltaics Properties

The structure of OPVs were same in previous work. The current density-voltage (J-V) curves of the performance based on FTQ:P(NDI-Fu50), FTQ:P(NDI-Th50), and FTQ:P(NDI-Se50) are shown in Figure X, and the related photovoltaic values, including short-circuit current density (Jsc), open-circuit voltage (Voc), and fill factor (FF) are shown in Table 9. All values of Voc is similar, however Jsc is changed dramatically followed by P(NDI-Fu50) < P(NDI-Th50) < P(NDI-Se50). FTQ:P(NDI-Se50) blending film was shown highest PCE of 3.57%, including Jsc of 7.94 mAcm^-2, a Voc of 0.837 V and a FF of 53.79%, while P(NDI-Th50) and P(NDI-Fu50) showed low PCE of 2.03% and 1.37%. External quantum efficiency (EQE) curves are shown in Figure 27.

Figure 27. Current density-voltage (J-V) curves (a) and EQE spectra (b) of the random terpolymers.

Table 9. Photovoltaic parameters of all-PSCs based on the random terpolymers.

Devices V_oc (V)^a

J_sc FF PCE EQE

(mA cm^-2)â (%)â (%)â (mA cm^-2)

FTQ:P(NDI- Fu50)

0.791 (0.787±0.006)

3.37 (3.27±0.13)

51.31 (49.48±4.30)

1.37

(1.27±0.10) 3.24

FTQ:P(NDI- Th50)

0.847 (0.837±0.011)

4.31 (3.41±0.55)

55.45 (47.23±4.63)

2.03

(1.96±0.05) 4.11

FTQ:P(NDI- Se50)

0.837 (0.832±0.006)

7.94 (7.91±0.43)

53.79 (53.19±2.52)

3.57

(3.52±0.07) 7.57

aThe values in the parentheses are obtained average values from over 10 devices with standard deviation.

3.5.4 Morphological Properties

The performance of OPVs are related to morphology of blending film systems. Therefore, we measured atomic force microscopy (AFM), however the results showed similar root-mean-square roughness in range of 0.67-0.78 nm. (Figure 28), indicating it is ambiguous to investigate

morphologies of three random terpolymers. To exactly understand we investigated grazing incident wide-angle X-ray scattering (GIWAXS) (Figure 28, 29). Although all films showed (010) diffraction peaks in out-of-plane (qz), FTQ:P(NDI-Se50) film showed stronger intensity among the others, meaning preferential π-face-on structures.

Figure 28. AFM height images (a) and 2D GIWAXS images (b) of the blend films.

Figure 29. GIWAXS line-cut profile of blend film (a) in-plane and (b) out-of-plane.

Chapter Ⅴ. Modulation of Intermolecular Distance Incorporating Alkoxy Thiophene and Alkyl Spacer Chains in n-type polymers

5.1 Introduction

Organic field effect transistors (OFETs) has been developed over the past decade, and the performance has achieved up to 10cm²V^-1s^-1, designing molecular structures and device constructions.^128-131 Through many researches, importance of intramolecular and intermolecular charge transfer was understood.^132-

135 Specifically, studies of π-conjugated backbones and side chains with specific functional groups were done based on importance of charge transfer for high planarity and molecular packing. Typically, polymers based on diketopyrrolopyrrole (DPP) showed high hole mobilities dependent on alkyl spacer chains, tuning branched points up to 4, 5, and 6 positioned spacers.⁷⁸ Unfortunately, studies related to the fact has lagged in p-type polymers. Herein, we studied the effect of intermolecular charge transfer based on napthalenediimide (NDI), which is well known as n-type organic materials.

The well-known polymer, N2200, which is composed of 2-octyldodecyl naphthalenediimide (NDI) and bithiophene (T2) monomers showed excellent electron mobilities of 0.85 cm²V^-1s^-1. Based on these two components, mono-dodecoxy bithiophene (TO) was synthesized for enhanced planarity by S-O interactions. In addition, NDI monomers of 5-positioned octyl-dodecyl spacer chains (C5NDIOD) were used for modulating lamellar distance between polymers.

As a result, 2 donors and 2 acceptor units were taken such as TO, T2, NDI2OD, and C5NDIOD in this work. Stille polymerization was done for forming alternative and random copolymers to understand the effect of alkoxy counterparts and alkyl spacer chains in n-type materials.

53 5.2 Results and Discussion

5.2.1 Synthesis and Characterization

The synthetic route of compound 2, 3-dodecyloxy-5,5’-bis(trimethylstannyl)-2,2’-bithiophene is shown in Scheme 3. The compound was synthesized with Stille-coupling, and compound 1 and 3 were synthesized following previous work. All polymers were synthesized by Stille-coupling for 2days at 100^oC. The detail procedure was shown in Experimental Section. After than the mixtures were precipitated in methanol and Soxhlet purification was done, following methanol, acetone, hexane and chloroform to remove catalyst or unreacted monomers. The final polymers were dark blue solids. In addition, the products were well soluble in chloroform (CF). Molecular weight of polymers was determined by high temperature gel permeation chromatography (HT-GPC) at 120^oC, using polystyrene standards as calibrants with 1,1,2,2-tetrachloroethane (TCE).

54 Scheme 3. The synthetic route of monomer and polymers^a

aReagents and conditions: (i) n-BuLi, (CH3)3SnCl, THF (ii) 1-dodecanol, P-toluenesulfonic acid monohydrate, Toluene, 120^oC, 20h (iii) NBS, DMF (iv) Pd(PPh3)4, Toluene, 90^oC (v) n-BuLi, (CH3)3SnCl, THF

5.2.2 Optical and Electrochemical Properties

The optical properties were measured by UV-Vis spectroscopy. (Figure 30) There are two broad peaks;

low energy band (600-800nm), corresponding to intra charge transition (ICT) and high energy band (340-440nm), corresponding to π-π* transition. The alternative polymers, P(NDI2OD-TO) and P(C5NDIOD-TO) showed higher ICT intensity than random copolymers, P(DNI2OD-T2-TO) and P(C5NDIOD-T2-TO) in CF solution state. Instead, the random copolymers have more red-shifted wavelength than alternative polymers. All polymers showed increased ICT in CF film state. In detail,

random copolymers have higher ratio between π-π* transition and ICT transition than alternative polymers, indicating strong aggregation between molecules.

Figure 30. The absorption spectra of polymers in dilute chloroform solution (a) and as thin films (b).

The electrochemical properties were investigated by cyclic voltammetry (CV). (Figure 31) There are two reversible redox waves and all polymers have similar lowest unoccupied molecular orbital (LUMO) energy levels. The summarized values are shown in Table 10.

Figure 31. Electrochemical properties of the polymers.

57 Table 10. Photophysical and electrochemical properties of polymers

(KDa) PDI λ

max

(nm)

λonset

(nm)

HOMO (eV)

LUMO (eV)

(eV)

P(NDI2OD-TO) 51.5 2.51 402.13,

734.42 949.73 -5.08 -3.78 1.30

P(NDI2OD-T2-TO) 121.1 1.73 725.54,

395.02 959.16 -5.04 -3.75 1.29

P(C5NDIOD-TO) 61.6 2.85 721.10,

395.46 942.40 -5.06 -3.75 1.31

P(C5NDIOD-T2-TO) 36.6 3.15 741.28,

399.48 997.55 -5.04 -3.80 1.24

5.3 Conclusion

We synthesized thiophene with alkoxy functional group for enhanced planarity of backbones due to S- O interaction. In addition, the donor counterpart was used for random copolymers based on P(NDI2OD- T2), which is widely known as n-type polymers. The synthesized polymers were investigated optical and electrochemical properties. Furthermore, study related to film morphology and performance of OFETs is demanded.

5.4 Experimental Section

GPC with Agilent 1200 HPLC and miniDAWN TREOS using polystyrene as standards in 1,2,4- trichlorobenzene at 120°C (HPLC grade). Differential scanning calorimetry (DSC) were measured by differential scanning calorimeter (TA instruments, USA) with a scan rate 10°C per a minute. Cyclic voltammetry (CV) measurements were performed on Solartron electrochemical station (METEK, Versa STAT3) with a three-electrode cell in a 0.1 M tetra-n-butylammonium hexafluorophosphate (n-Bu4NPF6) solution in acetonitrile at a scan rate of 100 mV/s at room temperature. Ag/Ag⁺ (0.01M AgNO3 in acetonitrile) electrode, a platinum wire, and a polymer-coated platinum electrode were used as the reference electrode, counter electrode, and working electrode, respectively. The Ag/Ag⁺ reference electrode was calibrated using a ferrocene/ferrocenium redox couple as an internal standard, whose oxidation potential is set at -4.8 eV with respect to zero vacuum level. The HOMO energy levels were obtained from the equation HOMO (eV) = LUMO - Egopt. The LUMO levels of the polymers were obtained from the equation LUMO (eV) = -4.8 – (E1/2redfirst – E1/2oxFe/Fc+).

Synthesis of P(NDI2OD-TO): The monomer NDI-Br2 (100 mg, 0.10 mmol), TO-Sn2 (68.6mg, 0.1 mmol), Pd2(dba3) (2.0 mg, 3.0 mol) and P(o-tolyl3) (3.0 mg, 12 mol) were dissolved in toluene (10 mL) in argon condition. The sealed reaction flask was then stirred at 100 ^oC for 48 h. Then, excess 2- (trimethylstanny) thiophene was added and the reaction mixture was stirred at 100 ^oC for another 24 h.

Finally, excess 2-bromothiophene was added and the reaction mixture was stirred for another 24 h. The mixture was cooled to room temperature and precipitated in methanol (100 mL). The precipitate was filtered and washed with methanol, acetone, hexane successively in a Soxhlet apparatus to remove oligomers and catalyst. Finally, the polymer was extracted with chloroform. The chloroform fraction was concentrated and precipitated in methanol. The precipitate was filtered and dried in a vacuum. The dark blue product was isolated in 95% yield.

Synthesis of P(C5NDIOD-TO): The procedure is similar to P(NDI2OD-TO), excepting the amount of toluene (20 mL). The dark blue product was isolated in 85% yield.

Synthesis of P(NDI2OD-T20.5-TO0.5): The monomer NDI-Br2 (100 mg, 0.10 mmol), TO-Sn2 (34.32mg, 0.05 mmol), T2-Sn2 (24.96mg, 0.05mmol), Pd2(dba3) (2.0 mg, 3.0 mol) and P(o-tolyl3) (3.0 mg, 12

mol) were dissolved in toluene (10 mL) in argon condition. The sealed reaction flask was then stirred at 100 ^oC for 48 h. Then, excess 2-(trimethylstanny) thiophene was added and the reaction mixture was stirred at 100 ^oC for another 24 h. Finally, excess 2-bromothiophene was added and the reaction mixture was stirred for another 24 h. The mixture was cooled to room temperature and precipitated in methanol (100 mL). The precipitate was filtered and washed with methanol, acetone, hexane successively in a Soxhlet apparatus to remove oligomers and catalyst. Finally, the polymer was extracted with chloroform.

The chloroform fraction was concentrated and precipitated in methanol. The precipitate was filtered and dried in a vacuum. The dark blue product was isolated in 90% yield.

Synthesis of P(C5NDIOD-T20.5-TO0.5): The procedure is similar to P(NDI2OD-T20.5-TO0.5) except for the amount of toluene (20mL). The dark blue product was isolated in 80% yield.

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Dalam dokumen Synthesis and Characterization of Semiconducting Polymers Based on Naphthalenediimide for Optoelectronics (Halaman 56-88)