Rylene Diimide Based Alternate and Random Copolymers for Flexible

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C: Energy Conversion and Storage; Energy and Charge Transport

Rylene Diimide Based Alternate and Random Copolymers for Flexible Supercapacitor Electrode Materials with Exceptional Stability and High Power Density Sandeep Sharma, Roby Soni, Sreekumar Kurungot, and Asha Syamakumari J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11229 • Publication Date (Web): 01 Jan 2019 Downloaded from http://pubs.acs.org on January 1, 2019

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The Journal of Physical Chemistry

Manuscript ID: jp-2018-112295

Rylene Diimide Based Alternate and Random Copolymers for Flexible Supercapacitor Electrode Materials with Exceptional Stability and High Power Density Sandeep Sharmaa, c+, Roby Sonia, b+, Sreekumar Kurungot*b, c and Asha Syamakumari*a, c

a) Polymer Science and Engineering Division, CSIR-National Chemical Laboratory, Pune, India- 411008, b) Physical and Materials Chemistry Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411008, India c) Academy of Scientific and Innovative Research, New Delhi, India-110025 +Authors

contributed equally to this work

Corresponding Authors: 1)Dr. Asha Syamakumari, E-mail: [email protected] 2) Dr. K. Sreekumar, E-mail: [email protected]

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Abstract Donor-acceptor pi conjugated polymers are emerging as interesting electrode materials for supercapacitor device applications. They offer the exciting possibility of charge storage in both positive and negative electrodes since they are both p and n dopable. The ambipolar charging enables higher operating voltage, which can afford higher specific energy and power densities. The donor-acceptor design can be either donor-alternate-acceptor or donor-randomacceptor. This architectural variation has the potential to modify the charge storage; yet surprisingly not much literature data is available exploiting this aspect. This paper explores the alternate and random geometries of donor-acceptor -conjugated polymers based on naphthalene diimide (NDI) or perylene diimide (PDI) as the acceptor component and benzodithiophene (BDT) as the donor component and their application as composite electrode materials in Type III supercapacitor device. Results show that the donor-acceptor alternate design involving P(PDI-alt-BDT) is an excellent supercapacitor electrode material with specific capacitance of 113 F g-1 with excellent stability up to 4000 cycles and almost 100% retention of the initial capacitance in single electrode setup in PC-LiClO4 organic electrolyte. Flexible supercapacitor device were also fabricated which shows areal capacitance of 35 mF cm-2 at a current density of 0.5 mA cm-2, which is promising for commercial application.


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Introduction Supercapacitors have become highly important energy storage devices possessing desirable properties like simple construction, high power density, high cycling stability, etc.1 However, low energy density has been a pertaining issue which has marred its widespread use as an energy storage device or in finding niche applications like emergency doors, camara flash lights and electric vehicles in conjunction with batteries. The most studied materials for supercapacitors include various forms of carbon (CNTs, Graphene, etc.), metal oxides (MnO2, RuO2) and conducting polymers (PEDOT, PANi, Ppy).2-5 Conducting polymers as a capacitive material have the advantage of easy synthesis, flexibility, high capacitance due to faradaic charge storage etc.4, 5 Based on the nature of polymers used and device fabrication, the supercapacitors can be classified as type I, II, III, and IV. Type I and type III supercapacitors have a symmetric configuration where both electrodes are fabricated with same materials. Type I supercapacitors include symmetric cell in which both electrodes are ptype polymers such as polythiophene,6-8 polypyrrole (PPY),9, 10 polyaniline(PANi).11 A type III supercapacitor comprises of an n-type or donor-acceptor polymers for electrode fabrication. Type II and Type IV supercapacitors are asymmetric type of devices where electrode with different polarization forms the anode and cathode. In Type-II supercapacitors, two different p-type polymer showing polarization in negative and positive regions are used;12-14 whereas, type IV are fabricated with n-type or donor-acceptor polymers.15,

16

Current research on

conducting polymers is focussed on the p-type polymers, where polymer chain is oxidized to induce charge delocalization and doping-dedoping pheneomena results in faradaic charge storage. However, p-type conducting polymers show limited charge storage capability. p-type polymers in symmetric configuration (type I) can only exploit half of its charge storage capability, thereby, exhibiting low energy density. Operating potential window is also narrow for type I device, usually 0.8-1.2 V. However, asymetric configuration (type II, type IV) invloving two different polymers with different potential polarization can provide higher voltage window upto 1.8-2.5 V depending upon the type of polymers and electrolyte. Although, the potential window is increased with type II configuration the output capacitance is reduced as the capcitance of the device is dictated by the polymer with low capacitance, given by the formula CT = C1C2 / C1+C2. On the other hand, high voltage discharges are obtained for type III and type IV designs, thus providing high energy density. A polymer with high potential window with significant capacitance in organic electrolytes (potential window 2.5-3.5 V) can be considered as a suitable material to attain high energy density. In this context, a polymer than can be p-doped and n-doped so that during charging cycle, it can store 3 ACS Paragon Plus Environment


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charge in both positive and negative electrode is very promising as a supercapacitor electrode material. Recently, donor-acceptor (D-A) organic polymers have been tested for their charge storage properties in supercapacitors and batteries.17-21 The donor-acceptor design strategy enables fine tuning of the electrochemical (redox) properties. Their ambipolar charging potential can significantly enhance the operating voltage when they are fabricated into a Type III or Type IV polymer-based supercapacitor. In 2014, DiCarmine et al. studied different types of donor-acceptor polymers for charge storage; they also studied the effect of position of acceptor molecules on the polymer chain on the polymer conjugation and charge storage.18 Gou fabricated asymmetric supercapacitor from poly[4,7-bis(3,6-dihexyloxy-thieno[3,2b]thiophen-2-yl)]benzo[c][1,2,5]thiadiazole (PBOTT-BTD) as a positive electrode and PEDOT as negative; the device showed electrochromic properies which changed color with the amount of charge stored.19 The donor-acceptor polymers demonstrated till now suffer from high equivalent series resistance, low capacitance and low energy denisty. A great deal of improvement in the performance of donor-acceptor polymers is needed to use them in commercial devices. Most of the reports available for pseudocapacitive conjugated polymers for supercapacitor applications are p-type polymers like the well-known polythiophenes, poly(3,4-ethylene dioxythiophene) (PEDOT), polypyrrole (PPy), and polyaniline (PANI).4,

5

Only very few

reports are available at present for the n-type or electron transporting -conjugated polymers or donor-acceptor type polymers. Zeigler et al. demonstrated capacitive properties of n-type TPA-3Th-NDI polymer; in this work, they fabricated an asymmetric supercapacitor with ntype polymer as a negative electrode. However, high charge-transfer resitance severely impacted its performance, delivering a capacitance of only 22 F g-1.20 Recently we have reported n-type donor-acceptor polymers based on naphthalenediimide as the acceptor and thiophene terminated phenylenevinylene as the donor as supercapacitor electrode materials employing 0.5 M H2SO4 as the aqueous electrolyte with excellent stability and very good capacitance.21 It has been generally accepted that the donor-acceptor repeating design strategy enables control over the optical band gap and energy levels; but, there are not many reports comparing the effect of random versus alternate arrangement of donor and acceptor across the polymer backbone on its application as energy storage materials. In fact, few reports that are available in this regard have studied their implication in the area of photovoltaics and are rather confusing with equal numbers supporting alternate connection and the random arrangement.22-26 For application as electrode materials for energy storage purpose, factors 4 ACS Paragon Plus Environment

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like the chain alignment, chain inhomogeniety assume more prominence since they influence the movement of counterions in redox reactions during charging and discharging. The random organization of D and A units across the polymer chain backbone could be expected to have a deleterious impact on electrolyte ion accessibility compared to a regular alterante arrangement of D and A units. In this report, we describe the synthesis and comparison of the capacitance characteristics of a series of alternate and random copolymers having naphthalene diimide (NDI) or perylene diimide (PDI) as acceptor moiety and benzodithiophene (BDT) as donor units. In both the PDI and NDI polymer series, the alternate copolymers exhibited higher specific capacitance compared to the corresponding random analogues. Solid-state flexible device were also succesfully fabricatied with P(PDI-alt-BDT) polymer. Experimental Section Materials 1,4,5,8-Naphthalenetetracarboxylicdianhydride

(NTCDA),

3,4,9,10-

perylene

tetracarboxylicdianhydride (PTCDA), 2-octyldodecanol, 3-thiophene carboxylic acid 97 %, 2ethylhexylamine, bis(triphenylphosphine) palladium (II) dichloride (Pd(Ph3)2Cl2) were purchased from Sigma Aldrich and used without further purification. 2-(tributylstannyl) thiophene-98 % was purchased from Lumtech and used without further purification. Measurements 1H

NMR spectra were recorded using 200 and 400 MHz Brucker NMR spectrophotometer in

CDCl3 containing a small quantity of TMS as an internal standard.The molecular weights of polymers were determined using gel permeation chromatography (GPC). GPC measurements were carried on a Thermo Quest (TQ) GPC at 25 oC using chloroform as the mobile phase. The analysis was carried out at a flow rate of 1 mL/min using a set of five µ-Styragel HT columns (HT-2 to HT-6) and a refractive index (RI) detector. Columns were calibrated with polystyrene standards, and the molecular weights are reported with respect to polystyrene. Absorption spectra were recorded using Perkin Elmer Lambda -35 UV-Vis spectrophotometer. Thermogravimetric analysis (TGA) was performed using a Perkin Elmer thermal analyzer STA 6000 model at a heating rate of 10 °C/min in a nitrogen atmosphere. Sample Preparation For the UV-Vis absorption studies, thin films were prepared by dissolving the polymer in chloroform (10 mg/mL) and spin coating (600 rpm/60 s) on quartz plates. Solution studies carried out in chloroform and solution was made 0.1 OD (optical density) at peak maximum. 5 ACS Paragon Plus Environment


Electrode Fabrication To fabricate electrode, slurry of polymers with carbon nanotubes in 16.6 wt % was prepared in N-methyl pyrrolidone (NMP) in a concentration of 1 mg cm-1. Carbon nanotubes were added as a conductive additive to improve charge collection and transfer in the polymers. The polymer slurry was then drop coated on the carbon current collector over an area of 1 cm-2. The electrodes were then dried under IR lamp. Characterization of Supercapacitor Devices Charge-storage characteristics of the polymers were analyzed through cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge-discharge. All the electrochemical test were conducted on BioLogic VMP-3 potentiostat-galvanostat. Synthesis of alternate and random copolymers a)

Poly{[2,6-Bis(2-thienyl)naphthalene-1,4,5,8-tetracarboxylic-N,N’-bis(2-octyldodecyl)

diimide(TNDIT]-alt-4,8-Bis(5-(2-ethylhexyl)thiophene-2-yl)benzo[1,2-b:4,5b’]dithiophene)}, P(NDI-alt-BDT) 2,6-Bis(2-bromothien-5-yl)naphthalene-1,4,5,8-tetracarboxylic-N,N’-bis(2-octyldodecyl) diimide Br-TNDIT-Br (3) (0.381 g, 0.33 mmol) and 2,6-bis(trimethyltin)-4,8-bis(5-(2ethylhexyl)thiophene-2-yl)benzo[1,2-b:4,5-b’]dithiophene (8) (0.300 g, 0.33 mmol) were taken in Schlenk tube under nitrogen atmosphere. Dry toluene (15 mL) was added to the tube and purged with nitrogen for half an hour. Bis(triphenylphosphine) palladium (II) dichloride (Pd(Ph3)2Cl2) (31 mg, 0.0211 mmol) was added to the tube quickly by opening rubber septa, and the whole mixture was degassed by nitrogen. The reaction mixture was stirred at 110 oC for 48 hours. 2-Bromothiophene (0.2 mL) was then added, and the reaction mixture was further stirred at 110

oC

for 2 hours followed by the addation of 2-trimethyl

stannanethiophene and stirred for 2 hours at 110 oC. Upon cooling to room temperature, a solution of potassium fluoride (1g) in 2 mL water was added and stirred for 2 hours. The reaction mixture was extracted with chloroform (150 mL x 3). The organic layer was washed with water, dried over anhydrous sodium sulfate and concentrated on a rotary evaporator. The obtained residue was dried in a vacuum oven and subjected to Soxhlet extraction with methanol (24 hours) then acetone (48 hours) followed by hexane (24 hours) and chloroform (12 hours). Half of the chloroform solution was evaporated on rotary evaporator, and the concentrated polymer solution was precipitated in 500 mL methanol, stirred for 2 hours, filtered on Buchner funnel, washed with methanol and dried in vacuum. The polymer was obtained as a solid. Yield = 465 mg (80 %). 1H NMR (400 MHz, CDCl3) δ ppm: 8.80 (br, 2H), 6 ACS Paragon Plus Environment

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7.76 (br, 2H), 7.32 (br, 4H), 6.94 (br, 4H), 4.09 (br, 4H), 2.90 (br, 4H), 1.96 (br, 2H), 1.311.21 (br, 124H), 0.82 (br, 24H). All the alternate and random copolymers were synthesized using the same procedure as that given for P(NDI-alt-BDT) but with different molar ratio of monomers. b)

Poly{[2,6-Bis(2-thienyl)perylene-3,4,9,10-

tetracarboxylicdiimide-N,N’-bis(2-

octyldodecyl) diimide(TPDIT]-alt-4,8-Bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5b’]dithiophene)}, P(PDI-alt-BDT) P(PDI-alt-BDT)

was

prepared

using

2,6-bis(2-bromothien-5-yl)perylene-3,4,9,10-

tetracarboxylicdiimide -N,N’-bis(2-octyldodecyl) diimide Br-TPDIT-Br (6) (422 mg, 0.33 mmol),

2,6-bis(trimethyltin)-4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-

b’]dithiophene (8) (0.300 g, 0.33 mmol) and bis(triphenylphosphine) palladium (II) dichloride (Pd(Ph3)2Cl2) (31 mg, 0.0211 mmol). Yield = 410 mg (73 %). 1H NMR (400 MHz, CDCl3) δ ppm: 8.88 (br, 2H), 8.34 (br, 4H), 7.70-6.93 (br, 10H), 4.10 (br, 4H), 2.86 (br, 4H), 1.98 (br, 4H), 1.55-1.20 (br, 124H), 0.82 (br, 24 H). c) Poly{([N N’ -bis(2-octyldodecyl)-naphthalene-1,4,5,8- bis-(dicarboximide)-2,6-diyl]]ran-2,5-thiophene-ran-4,8-Bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5b’]dithiophene) P(NDI-r-BDT) P(NDI-r-BDT) was prepared using N, N’-bis (2-octyldodecyl)-2,6-dibromo-1,4,5,8naphthalenediimide NDI-2OD-Br2 (1) (201 mg, 0.2 mmol), 2,5-bis (trimethylstannyl) thiophene

(168.1

mg,

0.40

mmol),

yl)benzo[1,2-b:4,5-b’]dithiophene

2,6-dibromo4,8-bis(5-(2-ethylhexyl)thiophene-2-

BDT-Br2

(9)

(150

mg,

0.2

mmol)

and

bis(triphenylphosphine) palladium (II) dichloride (Pd(Ph3)2Cl2) (31 mg, 0.0211 mmol). Yield = 310 mg (82 %). 1H NMR (400 MHz, CDCl3) δ ppm: 8.93-8.81 (br , 2H), 7.75 (br, 2H), 7.43-7.32 (br, 4H), 6.94 (br, 4H), 4.12 (br, 4H), 2.90 (br, 4H), 1.98 (br, 2H), 1.78(br, 2H), 1.31-1.21 (br, 124H), 0.82 (br, 24 H). d)

Poly{(N,N′-Bis(2-octyldodecyl)-3,4,9,10-perylene

tetracarboxylicdiimide-ran-2,5-

thiophene-ran-4,8-Bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b’]dithiophene) P(PDI-r-BDT) P(PDI-r-BDT) was prepared using N, N′-bis(2-octyldodecyl)-2,8-dibromo-3,4,9,10-perylene tetracarboxylicdiimide PDI-2OD-Br2 (4) (226 mg, 0.2 mmol), 2,5-bis(trimethylstannyl) thiophene

(168.1

mg,

0.40

mmol),

BDT-Br2

(9)

(150

mg,

0.2

mmol)

and

bis(triphenylphosphine) palladium (II) dichloride (Pd(Ph3)2Cl2) (31 mg, 0.0211 mmol). Yield = 280 mg (58 %). 1H NMR (400 MHz, CDCl3) δ ppm: 8.89 (br, 2H), 8.32 (br, 4H), 7.68-6.93 7 ACS Paragon Plus Environment


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(br, 10H), 4.10 (br, 4H), 2.86-2.58 (br, 4H), 1.98 (br, 4H), 1.68 (br, 2H) 1.55-1.20 (br, 124H), 0.82 (br ,24 H). Results and Discussion Synthesis and characterization Novel random and alternate copolymers were designed and synthesized where naphthalene and perylene diimide based moieties were used as acceptor and BDT and Thiophene units were used as donor unit. Naphthalene and perylene diimide are well-known acceptor units because of their electron deficient nature, while BDT exhibited a large planar conjugated structure and small steric hindrance between the adjacent repeating units. All the monomers were synthesized following previously reported literature procedures.27-30 Detailed synthetic procedure and characterization of monomers is given in the supporting information. All the alternate and random copolymers were polymerized via stille polycondensation in the presence of catalyst Pd(PPh3)2 Cl2 using toluene as solvent; 2-bromothiophene and 2trimethyl stannanethiophene were used as end-capping agents. Scheme 1 and Scheme 2 show the synthetic steps of polymerization. Scheme 1 shows the synthesis of alternate copolymers where thiophene terminated naphthalene (3) or perylene diimide (6), and BDT (8) monomers were used. O

R2 N

O

Br R1

S R2 N

O S Br

N R2

O

S

S

S

O

N R2

Pd(PPh3)2Cl2

S S

n S

O

R1

P(NDI-alt-BDT)

Toluene

Sn

S

S

S O 3

R1

S

O

Sn S S 8

O

R1

R2 N

R1 O

S

Pd(PPh3)2Cl2

O

R2 N

S O

S

Br

R1= 2-ethylhexyl R2= 2-octyldodecyl

S

Br

n S

S S

S

S

S

Toluene

O

N R2

O

R1

P(PDI-alt-BDT) O 6

N R2

O

Scheme 1: Synthesis of alternate copolymers P(NDI-alt-BDT) and P(PDI-alt-BDT). 8 ACS Paragon Plus Environment

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In Scheme 2 three different monomers were used for random copolymerization where 1 equivalent of thiophene monomer was used and 0.5 equivalent of naphthalene (1) or perylene diimide (4) were taken and 0.5 equivalents of BDT (9) monomers were used to maintain the same stoichiometry as an alternate copolymer. For a better understanding of the polymer properties model compounds representing the donor and acceptor fragments TNDIT (2), TPDIT (5) and BDT-T (7) were also prepared. The structure of the model compounds are shown in Scheme 3. R2 N

O

O

O

O

S S Br

S

S

O

O

Pd(PPh3)2Cl2

R2 N

O

R1

Toluene

S

Br

n O

O 4

N R2

S

S

S Br

R1= 2-ethylhexyl R2= 2-octyldodecyl

m

O

P(NDI-r-BDT) R1

R1

9

N R2

Sn

R2 N

O

S

S

n S O

S S

S

S

Toluene

O

N 1 R2

Sn

+

S

Pd(PPh3)2Cl2

Br

Br

R1 O

S

Br

R1

R2 N

O

N R2

S

S

m

S S

O

R1

P(PDI-r-BDT)

Scheme 2: Synthesis of random copolymers P(NDI-r-BDT) and P(PDI-r-BDT).

O

R2 N

R2 N

O

O

R1

O S

S

S

S

S

S

S O

N R2

O R2= 2-octyl dodecyl

TNDIT (2)

O

N R2

O

R1 = 2-ethyl hexyl

TPDIT (5)

S R1 BDT-T (7)

Scheme 3: Structure of model compounds. All polymers were purified via Soxhlet extraction with methanol, acetone, hexane, chloroform and precipitated in methanol. The structure of monomers and polymers were characterized by proton NMR spectroscopy. NMR spectra of the polymers are shown in Supporting Information figure S1 to S4. The polymers were sparingly soluble in common organic 9 ACS Paragon Plus Environment


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solvents like DCM, chloroform and chlorobenzene. The molecular weight and dispersity (Ð) was measured by gel permeation chromatography (GPC) at 25 0C with chloroform as solvent and polystyrene as standard; details are summarized in Table 1. All polymers exhibited Mw (Weight-average molecular weight) in the range of 21,100 to 41,600 g/mol , and dispersity (Ð) in the range of 2.3 to 3.2. Table 1. Molecular weight,and thermal characterization of the copolymers. Polymer

aWeight-average

Mwa (g/mol) Ðb Tdc (oC)

P(NDI-alt-BDT)

31,900

2.6

420

P(NDI-r-BDT)

41,600

2.7

432

P(PDI-alt-BDT)

31,200

3.2

431

P(PDI-r-BDT)

21,100

2.3

435

molecular weight (Mw). bdispersity (Đ). c The decomposition temperature (5

% weight loss) estimated using TGA under N2. Thermal characterization of the copolymers was carried out by thermogravimetric analysis (TGA) as well as differential scanning calorimetry (DSC) measurement under nitrogen atmosphere. TGA curves (in Supporting Information figure S5) showed good thermal stability for the polymers with onset decomposition temperature (Td) more than 400 C (values enlisted in Table 1). The DSC thermograms were recorded by heating the polymers from 0 C to 350 C at 10 oC /min under N2 atmosphere. Figure S6 (in Supporting Information) shows the DSC thermograms of all the copolymers. Except for the random copolymer of NDI (P(NDI-r-BDT)), none of the other polymers exhibited any transitions in their DSC thermogram, which indicated their amorphous nature. Optical Properties and Energy Level of Polymers Absorption spectra of the donor-acceptor alternate copolymers of both NDI and PDI were compared with that of a 1:1 physical mixture of the respective donor and acceptor fragments in order to understand the nature of the different absorption bands. Figure 1 compares the absorption spectra of the donor and acceptor model compounds (structure shown in Scheme 3) and their 1:1 mixtures recorded in chloroform.


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1.0

TNDIT BDT-T 1:1 P(NDI-alt-BDT)

a)

0.8 0.6 Normalized Absorbance (a. u.)

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


0.4 0.2 0.0 1.0

b)

TPDIT BDT-T 1:1 P(PDI-alt-BDT)

0.8 0.6 0.4 0.2 0.0 300

400

500

600

700

800

900

Wavelength (nm) Figure 1. Normalized absorption spectra of model compound and 1:1 mixtures and alternate copolymers. NDI model compound TNDIT (2) showed two absorption bands. The first band observed between 350 nm to 400 nm corresponded to the π-π* transition of NDI core and thiophene, while the second band observed between 415 and 565 nm was attributed to intramolecular charge transfer (ICT) band.31 BDT-T (7) (donor) model compound showed a single absorption band between 315 to 415 nm which corresponded to π-π* transition of the BDT core. The 1:1 mixture of the two model compounds (TNDIT and BDT-T) exhibited absorption bands, whose shape resembled the sum of their individual absorption bands. The perylene based model compound TPDIT (5) showed absorption band in the high energy region (370 nm) corresponding to the π-π* transition of PDI core and thiophene.32 TPDIT exhibited several peaks in the region from 400 to 650 nm of which those in the range 400 to 540 nm corresponded to typical perylene core transitions and broad shoulder beyond 550 nm was 11 ACS Paragon Plus Environment


assigned to the ICT band. Compared to the TNDIT model compound the peaks of TPDIT were red-shifted due to extended conjugation in the latter. Figure 1b also compares the absorption spectra of 1:1 mixture of BDT-T and TPDIT, which like its lower analogue had features similar to a sum of the corresponding absorption bands. Figure 1also compares the absorption spectra of alternate copolymers of donor and acceptor fragments namely P(NDIalt-BDT) and P(PDI-alt-BDT). A clear difference could be seen in the absorption spectra of the alternate copolymers compared to the respective 1:1 donor-acceptor model compound mixture. A clear redshift of the ICT band and overall broadening of all bands were observed, signifying increased conjugation in the polymer. Figure 2 compares the solution (chloroform) and thin film absorption spectra of the alternate copolymers P(NDI-alt-BDT) and P(PDI-altBDT). Absorption spectra of the film was more red shift compared to the solution which is a very common phenomenon indicative of planarization of the polymer backbone in the film relative to the solution. Figure 3 shows the thin film absorption spectra of the alternate as well as random copolymers. It could be seen from the figure that in P(NDI-r-BDT) copolymer the second absorption band was more red-shifted compared to its alternate analogue P(NDIalt-BDT). The band was shifted from 460 nm in the alternate copolymer to 500 nm in the random copolymer. This red shift in the second absorption band could reflect inhomogeniety in the incorporation of donor and acceptor units. In fact an indication of this was reflected in the proton NMR spectra (See in Supporting information Figure S3) of the P(NDI-r-BDT) copolymer. It showed two distinct peaks for the NDI aromatic core protons. In the alternate copolymer P(NDI-alt-BDT) there was regular alteration of NDI and BDT units and the two aromatic protons of NDI appeared at 8.81 ppm. On the other hand, in the random copolymer an additional peak was observed at 8.93 ppm which could correspond to NDI units bridged with thiophene moieties.33 Only the random copolymer design had the probability of continued arrangement of NDI and thiophene units along segments of the polymer chain without incorporation of BDT units. The chemical shifts of the NDI aromatic protons in such segment along the chain backbone would be slightly different from that of an NDI unit which had thiophene bridged BDT as neighbors.


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1.0

a)

P(NDI-alt-BDT) Solution P(NDI-alt-BDT) Thin Film

Normalized Absorbance

0.8 0.6 0.4 0.2 0.0 1.0

b)

P(PDI-alt-BDT) Solution P(PDI-alt-BDT) Thin Film

0.8 0.6 0.4 0.2 0.0 300

400

500

600

700

800

900

Wavelength (nm) Figure 2. Normalized absorption spectra of alternate copolymers in solution and thin films. 1.2

Normalized Absorbance (a. u.)

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


P(NDI-alt-BDT) P(NDI-r-BDT) P(PDI-alt-BDT) P(PDI-r-BDT)

Thin Film

1.0 0.8 0.6 0.4 0.2 0.0 300

400

500

600

700

800

Wavelength (nm)

Figure 3. Normalized absorption spectra of thin films of all copolymers.


900


0.03 0.02

Current (mA)

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

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0.01 0.00 -0.01

P(NDI-alt-BDT) P(NDI-r-BDT) P(PDI-alt-BDT) P(PDI-r-BDT) Ferocene

-0.02 -0.03 -0.04 -0.05 -2.0

-1.5

-1.0

-0.5

Potential (V) vs

0.0 Ag/Ag+

0.5

Figure 4: Cyclic voltammograms of Copolymers as thin film in 0.1 M (n-Bu)4NPF6 – acetonitrile solution The onset reduction (Ered) and oxidation (Eox) potential for the four polymers were determined using cyclic voltammetry experiments. Thin-films of polymer were drop casted on platinum working electrode for recording the CV. The measurement was done in acetonitrile solvent with ferrocene/ferrocenium as an internal standard and tetrabutylammonium hexafluorophosphate (n-Bu4NPF6 0.1M/ acetonitrile) as supporting electrolyte and scanned between -2.0 and 2.0 V. Cyclic voltammograms for copolymers highlighting the negative potential scanare shown in Figure 4. Generally, most of the NDI and PDI based copolymers show two reduction peaks.34-37 Two quasi reversible reduction peaks were observed for the copolymers. The onset reduction potentials for the polymers were determined as -0.90 V and 0.85 V for the P(NDI-alt-BDT) and P(NDI-r-BDT) respectively, while the corresponding values for the PDI analogues were -0.82 V and -0.84 V respectively. These values are tabulated in Table 2. The onset oxidation potential were also determined for these polymers from their positive potential scan, which were irreversible (Supporting information figure S7). These values were 0.78 V and 0.72 V for the P(NDI-alt-BDT) and P(NDI-r-BDT) respectively, while the corresponding value was 0.93 V for both the PDI analogues. The HOMO and LUMO were determined using the equation32 14 ACS Paragon Plus Environment

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HOMO = -(4.8 – E1/2. Fc/Fc+ + Eoxonset) LUMO = -(4.8 – E1/2. Fc/Fc+ + Eredonset) Table 2. Electrochemical and Optical band gaps of the copolymers. Polymer

Eredonset Eoxonset Egopt(eV) Egele(eV) LUMO(eV) HOMO(eV) -0.90

0.78

P(NDI-r-BDT) -0.85

0.72

1.41

P(PDI-alt-BDT) -0.82

0.93

-0.84

0.93

P(NDI-alt-BDT)

P(PDI-r-BDT)

1.45

1.67

-3.85

-5.53

1.56

-3.90

-5.47

1.51

1.75

-3.93

-5.68

1.53

1.77

-3.91

-5.68

These values are also tabulated in table 2. The electrochemical band gap (Egecl) was determined as 1.67 eV, 1.56 eV, 1.75 eV and 1.77 eV for the NDI and PDI based alternate and random copolymers respectively. The optical band gap (Egopt) of the copolymers calculated from lower energy absorption band edge of thin-film is also listed in Table 2. The cyclic voltammetry data correlated very well with the absorption measurements in thin films – both indicated similar electro-optical properties for the perylene based random and alternating polymers. The P(NDI-r-BDT) had shown some differences in the thin film absorption spectrum compared to its alternate analogue. This was reflected in the HOMO/LUMO energy levels determined from the CV data also. The random polymer exhibited a decrease in HOMO and increase in LUMO levels resulting in lower bandgap, which was reflected as a broader and red shifted absorption in the thin film. Among the NDI and PDI systems, NDI based polymers show a lower band gap compared to PDI based copolymers, in general.34,

35

The

redox behavior over a wide potential window (> 2V) indicated that these polymeric materials were potentially suitable for electochemical supercapactiors. Electrochemical Characterization of the Polymer Composite Supercapacitor. All liquid state electrochemical test of the polymer composite electrodes were performed in 1 M (PC-LiClO4) electrolyte with Ag/Ag+ as a reference electrode and graphite sheet as a counter electrode. Working electrodes of the respective polymers consisted of 1 mg cm-2 of active material (16.6 wt. % carbon nanotubes and 83.4 wt. % polymer) and area of the electrode was kept at 1 cm2. Figure 5a shows the cyclic voltammograms recorded at a scan rate of 5 mV s-1 for the two polymers with alternate arrangement of D-A molecules. High charging current was observed for both the polymers in the negative region of the potential 15 ACS Paragon Plus Environment


scanning which signifies reduction of polymers. Reduction charge measured in the potential region -1.5 to - 0.85 for P(PDI-alt-BDT) was 118 mC where as reduction charge measured for P(NDI-alt-BDT) was 105 mC. A higher reduction charge for P(PDI-alt-BDT) suggested greater degree of reduction of the polymer chain. The CV features observed here are similar to the features obtained for perylene dianhydride based alkali-ion batteries.38

b)

a)

1.5

1.0

1.0

0.5

0.5

0.0

0.0

-0.5 -1.0 -1.5 1.5

P(NDI-alt-BDT) P(NDI-r-BDT)

P(NDI-alt-BDT) P(PDI-alt-BDT)

c)

-0.5 -1.0 -1.5 1.5

d)

1.0

1.0

0.5

0.5

0.0

0.0

-0.5 -1.0 -1.5

-1.0

-0.5

Potential vs. Ag/Ag+

-1.5

P(NDI-alt-BDT) P(PDI-alt-BDT) P(NDI-r-BDT) P(PDI-r-BDT)

P(PDI-alt-BDT) P(PDI-r-BDT)

0.0

0.5

1.0

-1.5

-1.0

-0.5

0.0

Potential (V) vs. Ag/Ag+ P(NDI-alt-BDT) e) P(PDI-alt-BDT)

1.0

0.5

1.0

Current (mA)

1.5

Current (mA)

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

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-0.5 -1.0 -1.5

P(PDI-r-BDT) P(NDI-r-BDT)

0.5 0.0 -0.5 -1.0 0

200

400

600 Time (s)

800

1000

1200

Figure 5 Cyclic voltammetry of different polymers recorded at a scan rate of 5 mV s-1. a) CV depicting the electrochemical behavior of polymer with NDI and PDI as a acceptor molecules; b) and c) shows the change in the CV features for the alternate and random arrangement of NDI and PDI based polymers respectively; d) CV depicting the electrochemical behavior of all four polymer with different configurations, e) the GCD plots for all copolymers. The inception of reduction was observed at a more negative potential (-0.45 V) for P(PDI-altBDT) compared to P(NDI-alt-BDT), which was observed at -0.41 V. The NDI rings are more electron deficient than the PDI rings thus it is easier for the NDI to accept electron (reduction) compared to PDI, hence the reduction is more facile in NDI than PDI. The effect of alteration 16 ACS Paragon Plus Environment

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of the acceptor molecule along the polymer backbone (alternate vs random arrangement of D and A) on electrochemical behavior was investigated by cyclic voltammetry. Figure 5b compares the cyclic voltammograms recorded for the alternate and random polymers of NDI. A huge difference was observed in the area of CV for P(NDI-alt-BDT) and P(NDI-r-BDT) polymers. P(NDI-r-BDT) stored 58 mC of charge in the reduction region which was 55 % of the charge stored by its alternate counterpart. Similarly, Figure 5c compares the CVs of P(PDI-alt-BDT) and P(PDI-r-BDT). In this case also the D-A alternate copolymer exhibited better charge storage ability with the charge stored by the random polymer being 55 mC which was 47 % of the charge obtained for its alternate counterpart in the reduction region. The above results indicated that alternate arrangement of D-A molecules was better for improved charge storage. DiCarrmine et al. in their work on donor-acceptor molecules found improvement in the charge storage properties of acceptor polymer by installing an acceptor molecule next to it.18 They observed that delocalization of negative charges in the polymer chain was poor; however, incorporation of an acceptor molecule improved the negative charging stability and also its ambipolar nature. The alternate arrangements of donor-acceptor molecule permits better charge storage with significant reduction in leakage current. Similarly, in our work, the alternate arrangement of donor-acceptor molecules showed superior charge storage. Random counterparts showed nearly half charge-storage; this may be accounted for by the reduced homogeniety in the polymer chain caused by the irregular arrangement of donor and acceptor moieties. To get a picture of difference in the electrochemical behavior of all the four polymers, Figure 5d compares combined CV profiles of all polymers. It can be seen from the figure that the polymers with alternate arrangement of donor-acceptor units outperformed their random counterpart for charge storage. Capacitance of the respective polymers in a three electrode cell was measured by galvanostatic charge-discharge (GCD); the GCD plots for P(PDI-alt-BDT), P(NDI-alt-BDI), P(NDI-r-BDI) and P(PDI-r-BDT) recorded at a current density of 0.5 A g-1 are given in Figure 5e. A large discharge time was observed for polymer with alternate arrangement of acceptor and donor molecules whereas small discharge time was observed for the random counterparts. The CD results complemented the observation from CV indicating high charge storage for P(PDI-alt-BDT) and P(NDI-alt-BDI) compared totheir respective random analogues. The capacitance measured for P(PDI-alt-BDT) was the highest with a value of 113 F g-1 followed by P(NDI-alt-BDT) with capacitance of 80 F g-1; the capacitance obtained for random polymers P(PDI-r-BDT) and P(NDI-alt-BDT) were 48 and 44 F g-1, respectively. Full cell studies were carried out by fabricating type III device for P(PDI-alt-BDT) as it was the 17 ACS Paragon Plus Environment


best performing polymer in terms of capacitance. Electrodes were prepared in a similar manner to the single cell study. The loading of active material was kept at 1 mg cm-2 and 1 M PC-LiClO4 was used as the electrolyte. Cyclic voltammograms recorded at increasing scan for type III device is shown in Figure 6a; CV behavior was typical of the symmetric devices fabricated using two similar electrodes.

3 2 1

-1

5 mV s -1 10 mV s -1 20 mV s -1 50 mV s -1 100 mV s -1 200 mV s

50

b)

40 30

ZIm

4

a)

20

0 -1

10

-2 -3 0.0

2.5

0.5

1.0

1.5

Voltage (V)

2.0

c)

2.5 0 -1

0.5 A g -1 1Ag -1 2Ag -1 3Ag -1 4Ag

2.0 1.5

10

20

30

ZReal

40

d)

0 50

100 80 60

1.0

40

0.5

20

0.0

0

50

100

150

200

250 0

Time (s)

1000

2000

3000

4000

Capacitance (%)

Current (mA)

5

Voltage (V)

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

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0

Cycle number

Figure 6 Full cell characterization of P(PDI-alt-BDT) a) cyclic voltammograms recorded at different scan rates; b) Nyquist plot; c) galvanostatic charge-discharge measured at different current densities, d) durability test performed at a current density of 4 A g-1. The behavior became more resistive at higher scan rates. When the potential is scanned at high rate, the electrolyte ions are forced to move with greater speeds to and from the interphase. Thus, for better rate behavior the ions should be highly mobile and more conducting; also, the electrode must be easily accessible to the electrolyte ions. In this study, PC-LiClO4 electrolyte has low conductivity compared to aqueous electrolytes and solvated Li+ is large which experience large viscous drag. Therefore, at high scan rates, the behavior becomes resistive. Electrochemical impedance spectroscopy (EIS) can assess the internal resistance, ion diffusion, charge-transfer resistance and frequency behavior of the supercapacitor device. EIS 18 ACS Paragon Plus Environment

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analysis of the device was performed in the frequency range of 1 MHz-100 mHz in the open circuit voltage (OCV) condition with ac amplitude of 10 mV; the Nyquist plot obtained for the device is given in Figure 6b. Equivalent series resistance (ESR) measured for the device was 9.5 Ω, which is the total resistance mostly contributed by the resistance of organic electrolyte, electrode material and connecting wires. This ESR value was very low compared to other reports from literature for devices based on PBOTT-BDT, where resistance as high as 272 Ω was measured.20 Analysis of the Nyquist plot showed that the intercept at x-axis was followed by a semi-circle, which represented charge transfer resistance at the electrodeelectrolyte interface. A small semi-circle means low charge-transfer resistance (Rct) (indicates faster kinetics) and vice-versa. The Rct shown by P(PDI-alt-BDT) was lower than that exhibited by the other donor-acceptor polymers. Semi-circle in the Nyquist plot was followed by a small horizontal line which is called as Warburg diffusion line; this region represents the resistance towards the diffusion of ions in the electrode. The diffusion line was observed in the region of 10.5 to 11.2 Ω; this diffusion resistance could be attributed to the diffusion of solvated ions in the electrolyte. The Warburg diffusion line then changes to a vertical line, representative of the capacitive behavior. For ideal capacitive behavior, this line is parallel to the imaginary axis, however, the deviation observed for the device is caused by the diffusion resistance of the electrolyte. Capacitance measurement and its retention at increasing current densities were analyzed by galvanostatic charge-discharge; the corresponding plot is given in Figure 6c. Capacitance measured at a current density of 0.5 A g-1 for the type III device was 42 F g-1, the capacitance obtained here was twice the capacitance obtained for TPA-1Th-NDI based donoracceptor polymers in Type IV device reported by Zeigler et al.19 At a current density of 2 A g1

which is four times the initial current, the device retained 19.2 F g-1 i.e. 42.66 % of the

capacitance at 0.5 A g-1. This modest retention at a high current density is caused by the low mobility of the electrolyte ions and also due to inferior inter-chain charge-transfer in the polymer structure. Improving the inter-chain charge transfer in the polymer along with creation of porosity can ameliorate the rate capability. The energy density obtained for the device was 9.1 Wh Kg-1 which was higher than energy densities obtained for 6,6-BEDOT-iIBut2, PBOTT-BDT20 and PDDDBT18 based donor-acceptor polymers. The maximum power deliverable by the P(PDI-alt-BDT) device calculated by V2/4R, where V is the initial voltage R is the equivalent series resistance, was 82 kW kg-1. These results suggested its superior charge storage capabilities compared to other similar systems reported in literature. Figure 6d shows the durability test conducted at a current density of 2 A g-1 for 4000 cycles. The device 19 ACS Paragon Plus Environment


could retain almost 100 % of the initial capacitance. The cycling stability of P(PDI-alt-NDI) donor-acceptor polymer in organic electrolyte was superior than the durability results reported for many donor-acceptor polymers. Pulverization of the polymer due to expulsion-insertion of counter-ion during oxidation-reduction dissolution in electrolyte or detachment of the material from the current collector are some of the reasons for the capacitance decay of conducting polymers during cycling. The stable performance P(PDI-alt-BDT) showed that it did not suffer from aforementioned issues.

Figure 7. Solid flexible supercapacitor study, a) Photograph demonstrating the thin and flat nature of the flexible device; b) show the flexibility aspect of device where it can easily rolled on a pen; c) cyclic voltammogram recorded at different scan rates; d) Nyquist plot; e) galvanostatic charge-discharge curves and f) demonstration of device where it is to light a 2.5 V light emitting diode with a single device. Supercapacitor using liquid electrolytes has the problem of electrolyte leakage and packaging of device with metal casing makes the device quite expensive. To address this problem, gel electrolytes have been introduced recently and researchers are trying to replace liquid electrolytes with gel electrolytes.39 Also, flexibility of charge storage devices will be an 20 ACS Paragon Plus Environment

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added advantage as it will allow their application in flexible electronics, and they are expected to be more cost-effective when produced on a commercial scale. Considering all the above mentioned factors, we also fabricated a solid flexible supercapacitor with P(PDI-alt-NDI) polymer. A PMMA-based PC-LiClO4 gel electrolyte replaced the liquid electrolyte in the solid supercapacitor. To fabricate the solid device, active material was drop coated on a grafoil-scotch tape derived flexible conducting substrate with a loading of 3 mg cm-2 and the area of the device was 2 cm2. Digital photographs of the device demonstrating the flexible attributes are given in Figure 7a & 7b. The device was very thin with thickness of 0.418 mm; it could be bent, rolled and twisted also. Cyclic voltammogram of the flexible device given in Figure 7c showed features similar to the characteristics observed for the liquid counterpart. ESR measured for the solid device was 18.4 Ω (Figure 7d); the resistance measured here was higher than the resistance measured for the liquid counterpart. The electrolyte ion in gel electrolytes are less mobile due to high viscosity, thus they have low conductivity. The areal capacitance measured for the flexible device from GCD curve (Figure 7e) at a current density of 0.5 mA cm-2 was 35 mF cm-2, the capacitance at high current density of 2 mA cm-2 decreased to 3.2 mF cm-2. The reduction in capacitance at a high current density is caused by the slow diffusion of ions in the gel electrolyte. However, the areal capacitance obtained in this work is higher than the capacitance obtained for other donor-acceptor polymers.20 Finally, practical utility of the capacitor was demonstrated by lighting a 2.5 V light emitting diode which is shown in Figure 7f. The high output voltage allowed the device to power electronics that require high power and energy. Conclusions In summary, -conjugated alternate and random copolymers based on NDI and PDI as the acceptor and BDT as the donor were synthesized and successfully evaluated as composite electrode in supercapacitors in a type III device configuration. Despite similar optoelectronic properties for the perylene based random and alternate polymers, the alternate D-A copolymer exhibited better capactiance behavior. A specific capacitance of 113 F g-1 with excellent stability up to 4000 cycles with almost 100 % retention of the initial capacitance was observed for the P(PDI-alt-BDT) polymer in single electrode setup in PC-LiClO4 organic electrolyte. The energy and maximum power density was obtained as 9.1 Wh kg-1 and 82 kW kg-1 respectively. Flexible supercapacitor device was also fabricated to show the commercial application of the polymers. The areal capacitance of 35 mF cm-2 observed for the flexible



device at a current density of 0.5 mA cm-2 is much higher compared to similar reports based on other donor-acceptor polymers in the literature. The present work comparing the structure-property relationship in random and alternate donor - acceptor copolymers, is among the first of its kind of study in the area of conjugated polymeric materials for energy storage applications. It is expected that such studies will provide new physical insights into the significance of molecular structural design in influencing their device performance. SUPPORTING INFORMATION The Supporting Information is available free of charge on the ACS Publications website at DOI: The synthesis of the monomers and their structural characterization using 1H NMR. Characterization of the polymers using TGA, DSC and CV. ACKNOWLEDGMENT This work has been financially supported by CSIR network project (TAPSUN) NWP0054. S. K. Sharma and Roby Soni acknowledge UGC and CSIR respectively for research fellowship.


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TOC Graphic Rylene Diimide Based Alternate and Random Copolymers for Flexible Supercapacitor Electrode Materials with Exceptional Stability and High Power Density

C10H21

C10H21 O

N

C8H17 O

C8H17 O

N

O

S

S

S

S

S

S

S

n S

m S

O

N

S

S

S

O C8H17

O C8H17

N

S

S m

S

O

C10H21

C10H21

P(PDI-r-BDT)

P(PDI-alt-BDT)

Alternating

Random current density 0.5 A g-1

1.0 0.5

P(PDI-alt-BDT) P(PDI-r-BDT)

0.0 -0.5 -1.0

100

Capacitance (%)

S

Potential vs. Ag/Ag+

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


80 60 40 20

-1.5

0 0

200

400

600

Time (s)

800

1000

1200

0

1000


2000

3000

Cycle number

4000

Rylene Diimide Based Alternate and Random Copolymers for Flexible

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