Design and synthesis of N-substituted perylene diimide based low band gap polymers for organic solar cell applications

Article abstract of DOI:10.1039/c8ra05232h

In this study, we report on the synthesis and device studies of a series of new copolymers containing N-substituted perylene dimide and dioctylfluorene units as part of the main backbone. A facile synthetic approach avoiding non-selective bromination was used to synthesize the monomer M1 by the reaction of perylene-3,4,9,10-tetracarboxylic dianhydride with 2-amino-7-bromo-9,9-dioctylfluorene. The copolymers P1 and P2 were synthesized by Suzuki polycondensation of M1 with 2,2′-(9,9-dioctyl-9H-fluoren-2,7-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) M2 and 9-(heptadecan-9-yl)-2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-carbazole M3, respectively. The copolymer P3 was synthesized by direct arylation polymerization of M1 with 4,7-bis(4-octylthiophen-2-yl)benzo[c]-1,2,5-thiadiazole M4. All the copolymers showed thermal stability greater than 380 °C as evidenced from thermogravimetric analysis. The copolymers exhibited a narrow optical band gap (1.80-2.08 eV) with their UV-visible absorption spectra extending up to the NIR region and they are found to be suitable for use in OSC applications. The molecular weights of the polymers P1-P3 were found to be in the range of 10.68 to 16.02 kg mol^-1 as measured from GPC analysis. The surface morphology of the active layers based on P1/P2/P3:P3HT blend films was investigated by AFM and the rms values from height images range from 0.65 to 2.90 nm. The polymers were blended with P3HT to fabricate BHJ solar cells in three different weight ratios i.e. 1 : 1, 1.5 : 1 and 2 : 1 and the best power conversion efficiency was observed for the binary film of P3:P3HT blend device in a 1 : 1 weight ratio which reached up to 1.96% with a V_oc of 0.55 V, J_sc of 10.12 mA cm^-2 and FF of 34.63% which is among the highest reported for BHJ solar cells with N-substituted PDI based acceptors.

Full text of DOI:10.1039/c8ra05232h

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PAPER
Design and synthesis of N-substituted perylene
diimide based low band gap polymers for organic
Cite this: RSC Adv., 2018, 8, 30468
solar cell applications†
a
Savita Meena,a Tauheed Mohammad,b Viresh Duttab and Josemon Jacob
*
In this study, we report on the synthesis and device studies of a series of new copolymers containing N-
substituted perylene dimide and dioctylfluorene units as part of the main backbone. A facile synthetic
approach avoiding non-selective bromination was used to synthesize the monomer M1 by the reaction
of perylene-3,4,9,10-tetracarboxylic dianhydride with 2-amino-7-bromo-9,9-dioctylfluorene. The
copolymers P1 and P2 were synthesized by Suzuki polycondensation of M1 with 2,20-(9,9-dioctyl-9H-
fluoren-2,7-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane)
M2
and
9-(heptadecan-9-yl)-2,7-
bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-carbazole M3, respectively. The copolymer P3 was
synthesized by direct arylation polymerization of M1 with 4,7-bis(4-octylthiophen-2-yl)benzo[c]-1,2,5-
thiadiazole M4. All the copolymers showed thermal stability greater than 380 ꢀC as evidenced from
thermogravimetric analysis. The copolymers exhibited a narrow optical band gap (1.80–2.08 eV) with
their UV-visible absorption spectra extending up to the NIR region and they are found to be suitable for
use in OSC applications. The molecular weights of the polymers P1–P3 were found to be in the range of
10.68 to 16.02 kg molꢁ1 as measured from GPC analysis. The surface morphology of the active layers
based on P1/P2/P3:P3HT blend films was investigated by AFM and the rms values from height images
range from 0.65 to 2.90 nm. The polymers were blended with P3HT to fabricate BHJ solar cells in three
different weight ratios i.e. 1 : 1, 1.5 : 1 and 2 : 1 and the best power conversion efficiency was observed
for the binary film of P3:P3HT blend device in a 1 : 1 weight ratio which reached up to 1.96% with a Voc
of 0.55 V, Jsc of 10.12 mA cmꢁ2 and FF of 34.63% which is among the highest reported for BHJ solar
cells with N-substituted PDI based acceptors.
Received 19th June 2018
Accepted 20th August 2018
DOI: 10.1039/c8ra05232h
rsc.li/rsc-advances
tunable energy levels, and ease of synthesis and purication.8,9
Solution processable, non-fullerene bulk heterojunction (BHJ)
Introduction
solar cells with power conversion efficiencies (PCEs) > 8% have
recently been reported which indicate the potential of non-
fullerene electron acceptors.10,11
Organic solar cells (OSCs) are a promising alternative to the
inorganic silicon based devices due to their low cost, ease of
fabrication, exibility and light weight.1,2 During the past
decade, fullerene and its derivatives have been extensively
investigated as electron acceptors for OSCs, PC61BM and
PC71BM in particular.3 However, these acceptors suffer from
some drawbacks such as limited light absorption in the visible
region, high cost, instability of surface morphology in the blend
lms, difficulty in synthesis and purication and limited energy
level tunability.4,5 To overcome these challenges, there has been
lot of reports in the recent literature on the design of non-
fullerene based acceptors6,7 with broader absorption, readily
Perylene diimides (PDIs) are the most widely studied class of
non-fullerene acceptors in OSCs12,13 because of their large
electron mobility with probable generation of a highly con-
ducting path along their p–p stacking axis due to their strong
tendency to form ordered aggregates and low-lying frontier
energy levels.14 PDIs are also advantageous due to their high
absorption ability, high photochemical and optical stability,
reversible redox properties, ease of synthetic modication and
excellent environmental/thermal stability.15–17 PDI based mole-
cules and polymers have progressively attracted signicant
attention in applications such as light-emitting diodes,18
sensors,19 organic eld-effect transistors,20,21 light-harvesting
arrays,22 molecular wires,23 photochromic materials24 and
photovoltaic cells.25,26 Further, in PDI unit, the optical, electro-
chemical and charge transport properties can be tailored by
introduction of appropriate substituents at the N-positions26,27
aDepartment of Materials Science and Engineering, Indian Institute of Technology
Delhi, Hauz Khas, New Delhi-110016, India. E-mail: jacob@polymers.iitd.ac.in; Tel:
+91-11-26591425
bPhotovoltaic Laboratory, Centre for Energy Studies, Indian Institute of Technology
Delhi, Hauz Khas, New Delhi-110016, India
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ra05232h
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or at the perylene bay positions,28,29 offering wider scope for synthesized by reported literature procedures.40–42 2,20-(9,9-
tuning the optical as well as processing properties.30
Dioctyl-9H-uoren-2,7-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaboro-
Janssen and co-workers were the rst to demonstrate the use lane) (M2) and 4,7-bis(4-octylthiophen-2-yl)benzo[c]-1,2,5-
of N-substituted PDI based polymers in OSCs.31 Mikroyannidis thiadiazole (M4) were synthesized by reported methods with
et al.32 synthesized alternating phenylenevinylene copolymer modied reaction conditions and improved yields43,44 (details were
with PDI moiety along the backbone by Heck coupling reaction given in ESI†).
for use as an acceptor in OSCs. This copolymer acts as an n-type
organic semiconductor with electron mobility of ꢂ0.85 ꢃ 10ꢁ2
General measurements and characterization
cmꢁ2 Vꢁ1 sꢁ1. A blend device consisting of the above acceptor
The monomers were analysed for C, H and N using a Perkin-
copolymer and a poly(3-phenyl hydrazonethiophene) donor
resulted in a PCE of 1.67% that further increased to 2.32% by
thermal treatment and reported one of the best efficiency for N-
substituted PDI polymers. In 2011, Z. Liang et al.33 developed
Elmer 2400 Series II elemental analyser. The H and 13C NMR
spectral data were recorded on a Bruker spectrospin DPX-400
1
spectrometer operating at 400.13 MHz and 100.61 MHz,
respectively. The chemical shis are referenced to CDCl3
a variety of photo stable polymers containing poly(ethylene
solvent (7.26 and 77.0 ppm for H and 13C NMR, respectively)
glycol) or poly(propylene glycol) segments in the N-substituted
relative to tetramethylsilane (TMS). Fourier transform infrared
PDI backbone and used as acceptor in a PSC with poly(3-
(FT-IR) spectra were obtained on KBr disks in the range of 4000–
hexylthiophene) (P3HT) as donor. The polymer/P3HT (1 : 1
450 cmꢁ1 using a Thermo Scientic Nicolet 6700 FT-IR spec-
ratio) blend device gave a PCE of 0.1% with low Jsc of only 0.6
trometer. Ultraviolet-visible spectra were measured in solution
mA cmꢁ2. The low performance was attributed to the improper
and in lm on a Perkin-Elmer Lambda 1050 UV-visible spec-
lm morphology and low charge transport in the polymer
trophotometer. Thermogravimetric analysis (TGA) was per-
blend. W. Fu et al.,34 Jin et al.35 and Koyuncu et al.36 have recently
formed on a Perkin Elmer Pyris 6 thermal analysis system under
demonstrated the use of PDI based polymers as acceptor
N2 atmosphere at a heating rate of 20 ꢀC minꢁ1. The molecular
material in BHJ solar cells with moderate power conversion
weight determination was carried out on a Waters series 1515
efficiency.
1
gel permeation chromatograph (GPC) equipped with WATERS
The N-substitution at imide position of PDI can affect the
1515 column. The measurement was performed using THF as
solubility and molecular packing of the polymer in the solid
eluent at a ow rate of 0.5 mL minꢁ1 at 30 ꢀC and samples were
state, without inuencing the conjugation effect of the PDI
prepared in THF (1 mg mLꢁ1). The number-average molecular
backbone and thus the opto-electrochemical properties. In our
weight (Mn), weight-average molecular weight (Mw) and dis-
efforts to develop PDI based acceptors for use in BHJ solar cells,
persities (Đ ¼ Mw/Mn) of the polymers were determined using
we herein describe the synthesis of a novel N-substituted PDI
calibration curves obtained from polystyrene standards. Cyclic
based monomer and its alternating donor and acceptor type
voltammograms (CV) for electrochemical properties were
copolymers containing ourene, carbazole and thiophene–
recorded by using a Zehnar Zehnnium potentiostat-galvanostat
benzothiadiazole–thiophene moieties with tunable electro-
instrument with a glassy carbon working electrode, a platinum
chemical and optical properties and examine their perfor-
counter electrode and a Ag/AgCl reference electrode. The
mances in OSCs. In this work, we report the synthesis of three
surface morphology of the blended lms was investigated by
new polymers P1, P2 and P3 obtained either by Suzuki or direct
atomic force microscopy (AFM) using a Nanoscope Bruker icon
arylation polymerization reactions.37,38 The optical, electro-
dimension with scan Asyst in the tapping mode. MALDI-TOF
chemical properties and OSCs application of these copolymers
MS spectrum of M1 was recorded on an ultraextreme Bruker
were carefully investigated and correlated with their chemical
Daltonics TOF system in HCCA matrix.
structures.
Synthetic procedures for synthesis of monomer and polymers
Experimental
Materials
Monomer synthesis
Synthesis of 2,9-bis(7-bromo-9,9-dioctyl-9H-uoren-2-yl)anthrax
The solvents dimethylsulfoxide (DMSO), dimethyl acetamide, [2,1,9-def:6,510-d,e,f0]diisoquinoline-1,3,8,10(2H,9H)tetraone
chloroform (CHCl3), methanol (CH3OH), acetone, 1,4-dioxane, (M1). Perylene-3,4,9,10-tetracarboxylic dianhydride (1.0 g, 2.54
tetrahydrofuran (THF) and toluene were dried and distilled by mmol), 4 (2.5 g, 5.16 mmol), anhydrous zinc acetate (0.42 g, 2.25
well known standard procedures39 and kept in a glovebox for mmol) and imidazole (5.0 g, 73.44 mmol) were stirred under N2
ꢀ
ꢀ
further use. Fluorene (1), 4,7-dibromobenzo[c]-1,2,5-thiadiazole atmosphere rst at 100 C for 12 h and then at 160 C for an
(7), 9-(heptadecan-9-yl)-2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxa- additional 12 h. The progress of the reaction was monitored by
borolan-2-yl)-9H-carbazole
(M3)
and
perylene-3,4,9,10- thin layer chromatography (TLC) using CHCl3. The reaction
ꢀ
tetracarboxylic dianhydride were purchased from Sigma- mixture was then allowed to cool to ca. 80 C and poured into
Aldrich (USA) and was used directly without purication. 2- 150 mL of 1 N aqueous hydrochloric acid solution. The red
Bromo-9,9-dioctyluorene
(2),
2-bromo-7-nitro-9,9- brown precipitate was ltered and washed sequentially with
dioctyluorene (3), 2-amino-7-bromo-9,9-dioctyluorene (4), 0.5 N HCl (2 ꢃ 20 mL), distilled water (2 ꢃ 20 mL) and CH3OH
2,7-dibromo-9,9-dioctyl-9H-uorene (5) and 4,4,5,5-tetramethyl- (3 ꢃ 20 mL). The solid was dissolved in CHCl3 (30 mL), washed
2-(4-octylthiophene-2-yl)-1,3,2-dioxaborolane
(6)
were with distilled water (30 mL) and dried over anhydrous Na2SO4.
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The solvent was evaporated on a rotary evaporator and the crude 1019, 865, 700. UV-vis spectrum lmax(lm)/nm: 354, 516 and
product was puried by column chromatography (silica gel, 954. GPC analysis (PS standard): Mw ¼ 10.68 kg molꢁ1. Dis-
CHCl3) to obtain M1 as a red brown solid. Yield: 3.0 g, 90%. persity (Đ): 1.28.
Anal. calc. for C82H88N2O4Br2: C, 74.31%; H, 6.69%; N, 2.11%.
Synthesis of poly[2,9-bis(9,9-dioctyl-9H-uoren-2-yl)anthra
1
Found: C, 74.61%; H, 6.47%; N, 2.21%. H NMR (400.13 MHz, [2,1,9-def:6,5,10-d0e0f0]diisoquinoline-1,3,8,10(2H,9H)-tetraone-
CDCl3, 25 ꢀC vs. TMS) d: 8.73 (d, 4H, J ¼ 8.0 Hz) 8.61 (d, 4H, J ¼ alt-4,7-bis(4-octylthiophen-2-yl)benzo[c][1,2,5]thiadiazole]
(P3).
8.0 Hz) 7.83 (d, 2H, J ¼ 8.0 Hz) 7.61 (d, 2H, J ¼ 8.0 Hz) 7.51–7.49 Pivalic acid (4.62 mg, 30 mol%) and caesium carbonate (0.12 g,
(m, 4H) 7.37–7.33 (m, 4H) 2.03–1.90 (m, 8H) 1.26–1.11 (m, 40H) 0.38 mmol) were added to a mixture of M1 (0.20 g, 0.15 mmol)
0.84 (t, 12H, J ¼ 7.2 Hz) 0.72 (m, 8H). 13C NMR (100.61 MHz, and M4 (0.08 g, 0.15 mmol) in anhydrous N,N-dimethylacetamide
CDCl3, 25 ꢀC vs. TMS) d: 162.1, 152.4, 150.3, 139.3, 138.3, 133.1, (1.5 mL, 0.1 M) with constant stirring at room temperature. The
133.0, 130.2, 129.1, 128.1, 126.7, 125.3, 124.9, 122.6, 122.4, reaction mixture was purged with N2 for 15 minutes, followed by
122.0, 120.4, 119.4, 54.6, 38.8, 30.8, 28.9, 28.6, 28.2, 28.1, 22.7, addition of palladium acetate (3.39 mg, 10 mol%). The reaction
21.6, 13.1. MALDI-TOF MS: calc. for C82H88N2O4Br2: 1325.40; mixture was heated at 100 ꢀC for 72 h under N2 atmosphere
found 1325.58 (M+). FT-IR (KBr disk, nmax/cmꢁ1): 2925, 2852, which was then cooled to room temperature and poured drop
1705, 1664, 1593, 1458, 1354, 1253, 1062, 796, 653. UV-vis wise into cold CH3OH (200 mL) to precipitate the polymer. The
spectrum lmax(THF)/nm: 455, 485 and 520.
isolated polymer was further puried by using Soxhlet extraction
steps similar to that of P1. The polymer collected in CHCl3 was
Polymers synthesis
Synthesis of poly[2,9-bis(9,9-dioctyl-9H-uoren-2-yl)anthra concentrated and precipitated from CH3OH (100 mL) again, to
1
[2,1,9-def:6,5,10-d0e0f0]diisoquinoline-1,3,8,10(2H,9H)-tetraone- get P3 as a dark red brown solid. Yield: 0.19 g, 75%. H NMR
alt-9,9-dioctyl-9H-uorene] (P1). M1 (0.20 g, 0.15 mmol), M2 (400.13 MHz, CDCl3, 25 ꢀC vs. TMS) d: 8.63 (br, 4H) 8.50 (br, 4H)
(0.10 g, 0.15 mmol), potassium carbonate (0.13 g dissolved in 8.02 (br, 2H) 7.76 (br, 4H) 7.53–7.43 (br, 8H) 7.18 (br, 2H) 2.62 (br,
0.45 mL of distilled water), Pd(PPh3)4 (3.48 mg, 2 mol%) and 4H) 1.89–1.64 (br, 8H) 1.18 (br, 72H) 0.76 (br, 18H). FT-IR (KBr
a phase transfer catalyst Aliquat 336 (3 drops) were added to N2 disk, nmax/cmꢁ1): 2920, 2852, 1705, 1663, 1580, 1456, 1353, 1261,
purged mixture of water : toluene (1 : 6) (1.75 mL). The mixture 1092, 1023, 866, 802, 695. UV-vis spectrum lmax(lm)/nm: 329,
was reuxed at 100 ꢀC with stirring for 48 h under N2 atmo- 520, 784 and 878. GPC analysis (PS standard): Mw ¼ 16.02 kg
sphere and then an end capping agent, bromobenzene (0.79 mL, molꢁ1. Dispersity (Đ): 1.29.
5 mol%) was added to the mixture and heated for an additional
6 h followed by addition of phenyl boronic acid (0.92 mg,
5 mol%) with further heating for an additional 6 h. The reaction
Fabrication of organic solar cell devices
mixture was cooled to room temperature and poured drop wise Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:
into CH3OH (200 mL) to precipitate out the polymer. The PSS), regioregular poly(3-hexylthiophene) (P3HT) and 1,2-dichlo-
polymer was isolated by ltration and puried by Soxhlet robenzene (DCB) were purchased from Sigma-Aldrich and used
extraction with CH3OH, acetone and CHCl3 successively. The without further purication. PEDOT:PSS was ltered through
polymer extract from CHCl3 was concentrated on a rotary 0.45 mm PVDF lter before use. The active layer was prepared by
evaporated and reprecipitated from CH3OH (100 mL). The dried mixing P1, P2 or P3 and P3HT in three different weight ratios of
polymer P1 was obtained as a red brown solid. Yield: 0.19 g, 1 : 1, 1.5 : 1 and 2 : 1 in DCB and all solutions were ltered
1
ꢀ
81%. H NMR (400.13 MHz, CDCl3, 25 C vs. TMS) d: 8.70 (br, through a 0.45 mm PTFE lter before use. PSCs were fabricated
4H) 8.54 (br, 4H), 7.84–7.69 (br, 12H) 7.49–7.38 (br, 6H) 2.05– with a structure of ITO/PEDOT:PSS/polymer/P3HT/Al using
1.85 (br, 12H) 1.15 (br, 72H) 0.83 (br, 18H). FT-IR (KBr disk, a standard solution processing method. Patterned indium tin
nmax/cmꢁ1): 2926, 2853, 1705, 1668, 1593, 1462, 1352, 1253, oxide (ITO)-coated glass substrates were cleaned using labolene
1120, 813, 745. UV-vis spectrum lmax(lm)/nm: 341, 502 and soap solution, deionized water, acetone and isopropyl alcohol
951. GPC analysis (PS standard): Mw ¼ 14.77 kg molꢁ1. Dis- with ultrasonic treatment each for 30 minutes at 50 ꢀC. The
persity (Đ): 1.53.
cleaned ITO substrates were nitrogen dried and preheated in an
Synthesis of poly[2,9-bis(9,9-dioctyl-9H-uoren-2-yl)anthra oven at 100 ꢀC for 60 minutes before oxygen plasma treatment
[2,1,9-def:6,5,10-d0e0f0]diisoquinoline-1,3,8,10(2H,9H)-tetraone- for 15 minutes. A layer of PEDOT doped with PSS was spin-
alt-9-(heptadecan-9-yl)-9H-carbazole] (P2). P2 was synthesized coated (3500 rpm for 45 seconds) on cleaned ITO surface, fol-
ꢀ
by a similar procedure as for P1 using M1 (0.20 g, 0.15 mmol), lowed by annealing at 150 C for 30 minutes. Then active layer
M3 (0.10 g, 0.15 mmol), potassium carbonate (0.13 g dis- of blend solution of P3HT:P1 or P2 or P3 (new accepters) in DCB
solved in 0.45 mL of distilled water), Pd(PPh3)4 (3.48 mg, was spin casted (1000 rpm for 30 seconds) on the top of
2 mol%), phase transfer catalyst Aliquat 336 (3 drops), PEDOT:PSS layer and then annealed at 150 ꢀC for 10 minutes. At
water : toluene (1 : 6) (1.75 mL), bromobenzene (0.79 mL, the end, aluminium electrode (100 nm) was deposited through
5 mol%) and phenyl boronic acid (0.92 mg, 5 mol%). P2 was a shadow mask using thermal evaporation on top of the pho-
obtained as a brown solid. Yield: 0.18 g, 77%. 1H NMR (400.13 toactive layer. All the devices were fabricated and tested in air
MHz, CDCl3, 25 ꢀC vs. TMS) d: 8.72 (br, 4H) 8.56 (br, 4H) 8.20– without any encapsulation. J–V characteristics of PSCs were
7.71 (br, 12H) 7.52–7.29 (br, 6H) 4.69 (s, 1H) 2.05–1.68 (br, measured using a Keithley 2440 source meter, under AM 1.5 G
12H) 1.18 (br, 72H) 0.83 (br, 18H). FT-IR (KBr disk, nmax
/
illumination of Oriel Sol 3A class AAA solar simulator. The
cmꢁ1): 2924, 2852, 1704, 1669, 1564, 1457, 1351, 1261, 1093, simulator was calibrated using a NREL certied Si solar cell.
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Results and discussion
Synthesis and characterization of monomers and polymers
The synthetic approach toward the monomer, M1 is shown in
Scheme 1, whereas for M2 and M4 are shown in Scheme S1.†
The synthesis of the target monomers 2,9-bis(7-bromo-9,9-
dioctyl-9H-uoren-2-yl)anthrax[2,1,9-def:6,510-d,e,f0]diisoqui-
noline-1,3,8,10(2H,9H)tetraone
(M1),
2,20-(9,9-dioctyl-9H-
uoren-2,7-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane)
(M2) and 4,7-bis(4-octylthiophen-2-yl)benzo[c]-1,2,5-thiadiazole
(M4) were carried out as follows: in the rst step bromination
followed by alkylation of uorene 1 in presence of N-
bromosuccinimide (NBS) and n-octyl bromide, respectively,
yielded 2-bromo-9,9-dioctyluorene 2 (90%) which on nitration
in the second step followed by reduction gave 2-amino-7-bromo-
9,9-dioctyluorene 4 in 74% overall yield.40 Reaction of 4 with
perylene-3,4,9,10-tetracarboxylic dianhydride in the presence of
imidazole and anhydrous zinc acetate on heating at 160 ꢀC
under N2 atmosphere resulted in M1 in 90% yield. Bromination
of 1 in presence of bromine gave dibromouorene which on
alkylation with n-octyl bromide using tetrabutylammonium
hydroxide at 80 ꢀC gave 2,7-dibromo-9,9-dioctyl-9H-uorene 5 in
90% yield.45 Reaction of 5 in presence of Pd(dppf)Cl2 and
potassium acetate with bis(pinacolato)diboron under N2
atmosphere at 105 ꢀC resulted in M2 (70% yield). Reaction of 7
with 6 in toluene/THF under N2 atmosphere in presence of
sodium carbonate, Pd(PPh3)4 and tetrabutylammonium
hydroxide gave M4 in 80% yield.
The structures of intermediates and monomers (M1, M2 and
M4) were conrmed by NMR, FT-IR spectroscopy and elemental
analysis. MALDI-TOF MS and UV-vis spectroscopy were also
carried out to characterise M1. In the aromatic region of the 1H
NMR spectrum of M1, there are two doublets of equal intensity
at 8.73 and 8.61 ppm, which correspond to protons of the PDI
unit. The presence of two doublet (7.83 and 7.61 ppm) and two
multiplet (7.51–7.49 and 7.37–7.33 ppm) signals of equal
intensity can be attributed to the two uorene moieties attached
to PDI unit. The disappearance of NH2 protons from interme-
diate 4 and the slight shi in the chemical shi of perylene
Fig. 1 Normalized absorbance spectrum of M1 in THF solution.
protons compared to perylene dianhydride, further conrm the
structure. The aliphatic region of the 1H NMR spectrum
constitute the signals of the alkyl substituents on the uorene
moieties and the ratio between integrated intensities of
aromatic and aliphatic regions of the spectrum is consistent
with the proposed structure of M1 (Fig. S1†). In 13C NMR
spectrum of M1, the C]O peaks of the PDI unit appear at
162.1 ppm. The presence of 17 signals in the down eld region
in the range of 152.4–119.4 ppm correspond to 44 different
aromatic carbon atoms and the signals in the range of 54.6–
13.1 ppm account for the different aliphatic carbon atoms of the
alkyl chains attached to the uorene moieties conrming the
structure of M1 (Fig. S2†).
The FT-IR spectrum of M1 clearly shows the presence of
characteristic aromatic C–H stretching appearing at 2925 cmꢁ1
,
aliphatic C–H stretching appeared at 2852 cmꢁ1, O]CN
stretching at 1705 and 1664 cmꢁ1, C]C stretching appeared at
1593 cmꢁ1 and C–N stretching appeared at 1354 cmꢁ1 and
found similar as reported for other PDI derivatives.46
Scheme 1 Synthetic routes to design intermediates and monomer M1.
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Scheme 2 Synthetic routes to design of polymers P1–P3.
Table 1 Molecular weights and thermal properties of polymers P1–P3
Polymers Mwa (kg molꢁ1
)
Mn (kg molꢁ1
)
PDIc (Mw/Mn) T5 (ꢀC)
b
d
P1
P2
P3
14.77
10.68
16.02
9.66
8.32
12.42
1.53
1.28
1.29
381
458
380
a
b
Weight-average molecular weight.
Number-average molecular
c
d
weight. Polydispersity index. Onset decomposition temperature
(5% weight loss).
The UV-vis spectrum of M1 (Fig. 1) in THF, having three
clearly distinguishable peaks in the region of 400–550 nm,
which showed their absorption maxima at lmax ¼ 455, 485 and
520 nm consistent with the reported absorption spectra of PDI
based monomers.47 The above peaks are found match with
different vibrational modes (2 ) 0, 1 ) 0 and 0 ) 0, respec-
tively) combined with the PDI main transition dipole moment,48
which is lined up along the long axis of the molecule. The more
direct and supporting evidence for M1 was obtained from the
MALDI-TOF MS spectrum shown in Fig. S3,† where it can be
seen that the experimental mass agrees well with the calculated
mass.
1
The H and 13C NMR spectra of M2 and M4 are shown in
Fig. S4–S7† and found to be in agreement with reported litera-
ture although in this work, M2 and M4 were synthesized by
modied procedures with improved yield.43,44
N-substituted PDI based polymers P1 and P2 were synthe-
sized by using Suzuki polymerization49 whereas P3 was
synthesized by direct arylation polymerization.50 Their synthetic
routes are outlined in Scheme 2. The Suzuki polycondensation
reaction of M1 with M2 or M3 in the presence of Pd(PPh3)4,
potassium carbonate and Aliquat 336 in mixture of toluene-
water at 100 ꢀC under N2 atmosphere resulted in P1 (81%
Fig. 2 Normalized absorbance spectra of P1, P2 and P3 (a) in THF
solution and (b) films.
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Table 2 Optical and electrochemical properties of polymers P1–P3
lmaxa (nm)
Energy levels (eV)
Egopt (eV)b (ledge/nm)c
EHOMO
ELUMO
d
e
Polymers
Solution
Film
P1
P2
P3
339, 519
352, 520
300, 510
341, 502, 951
354, 516, 954
329, 520, 784, 878
2.08 (597)
1.80 (688)
1.88 (660)
ꢁ5.66
ꢁ5.37
ꢁ5.50
ꢁ3.58
ꢁ3.57
ꢁ3.62
UV-vis absorption maxima in THF solution and lm. b Optical band gap Eogpt ¼ 1240/ledge
.
Onset of thin lms absorption spectra. d Calculated by
a
c
equation HOMO ¼ Eogpt + LUMO (eV). e Calculated from the onset reduction potential.
yield) and P2 (77% yield) respectively, whereas P3 was synthe- polymers P1–P3 (Fig. 2a) exhibited their absorption maxima in
sized by direct arylation polymerization of M1 with M4 in the range of 300 to 520 nm in THF solution. In these ranges, two
presence of pivalic acid, palladium acetate and caesium clearly dened absorption bands were noticed, the rst between
carbonate using N,N-dimethylacetamide at 100 ꢀC under N2 300 to 352 nm and the second in the range of 510 to 520 nm.
atmosphere in 80% yield. The purications of the synthesized These absorption bands located at lower wavelength can be
polymers were carried out by precipitation in cold CH3OH than assigned to p–p* transitions from the conjugated backbones
aer Soxhlet extractions and nally reprecipitation from localized on the donor or acceptor, whereas the higher wave-
CH3OH.
length absorption band can be assigned to intramolecular
1
The polymeric structures of P1–P3 were conrmed with H charge transfer (ICT) transitions inherent to the D–A system.51
NMR and FT-IR spectroscopy and their 1H NMR spectra are While comparing to lm spectra with solution, the absorption
shown in Fig. S8–S10.† In 1H NMR spectra of P1–P3, the peaks at maxima were red shied with broadened area owing to strong
8.63–7.29 ppm correspond to the aromatic protons whereas the inter-chain interaction and p–p stacking in the solid state
peaks at 2.62–1.15 ppm can be attributed to CH2 groups of alkyl which are benecial for charge transport.52 The solution and
chain. The signals in the range of 0.83–0.76 ppm arise from lm spectra of polymers have almost the same absorption
terminal CH3 of alkyl substituent. The 1H NMR spectrum of P2 maxima in the range of 502 to 520 nm and are very close to the
showed a singlet at 4.69 ppm conrming the presence of monomer M1 due to the presence of the same PDI unit in all the
carbazole moiety in the polymeric chain. In P3, the signal at polymers. Such close values in absorption maxima in monomer
7.18 ppm corresponds to aromatic protons of the benzothia- and polymers were also reported in other PDI based poly-
diazole moiety and at 2.62 ppm to the CH2 protons directly mers.53,54 The absorption spectra of the three polymers in lm
attached to thiophene units. The measured proton intensities (Fig. 2b) showed absorption maxima at near infrared (NIR)
agree well with the structures of P1–P3.
region, 951 nm for P1, 954 nm for P2 and 784 nm with
The molecular weights and dispersities of P1–P3 were a shoulder at 878 nm for P3. These absorptions at NIR region
determined by GPC against monodisperse polystyrene stan- are relatively broad and weak in the range of 750 to 1000 nm and
dards with THF as the eluent using the totalchrom soware and can be attributed to ICT between D–A units and simultaneously
their results are summarized in Table 1. The GPC measure- can increase the number of photons absorbed under solar
ments indicate that these polymers have weight-averaged radiation. The use of an OPV material that absorbs light in the
molecular weights (Mw) of 14.77, 10.68 and 16.02 kg molꢁ1 NIR region and even above 1000 nm could increase the effi-
with a narrow PDI of 1.53, 1.28 and 1.29 for P1, P2 and P3, ciency of devices in OSC applications.55 In the lm spectrum,
respectively. The thermogravimetric analysis (TGA) and differ- the absorption maximum at 520 nm of P3 was found to be
ential scanning colorimetry (DSC) studies were performed to slightly red shied by ca. 10 nm as compared to solution indi-
determine the thermal properties of the polymers P1–P3 (Table cating good intermolecular ordering in the solid state.56 The
1). DSC curves of polymers reveal that no transitions were optical band gaps (Eogpt) of all the polymers were estimated from
observed during the heating and cooling cycles whereas TGA ledge of the thin lm absorption spectra and found to be in the
analysis shows the onset decomposition temperature (T5) range of 1.80–2.08 eV. The absorption intensity and optical
ꢀ
higher than 380 C at 5% weight loss. This suggested that the band gap value of P3 are observed to be higher than those of P2
thermal stability of the polymers is sufficient for application in which may be due to the incorporation of carbazole unit into
OSCs. The TGA graphs of the polymers are shown in Fig. S11.† the polymer backbone of P2 leading to enhanced conjugation.57
The polymers P1–P3 are found to be soluble in chlorinated Further the presence of bulky octyl substituents in the thio-
solvents as well as in toluene and THF.
phene units probably twist atom induce the main chain of P3
leading to decrease coplanarity and intramolecular conjugation
in solid state of the polymer.58 All the polymers displayed
comparable optical band gap values with literature values as
reported for N-substituted PDI based alternating copolymers
containing thiophene (2.0 eV),36 oligothiophene (1.63–1.97 eV)59
and phenylenevinylene (1.66 eV) units.32 The narrower band
Optical properties
The UV-visible absorption spectroscopy was performed in THF
solution and thin lm to understand the optical properties of
polymers P1–P3. Fig. 2 shows the absorption spectra of all
polymers whereas their related data are given in Table 2. The
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using lms on a glassy carbon working electrode (made by drop
casting of their concentrated solution in CHCl3) in CH3CN
solution having [nBu4N]+[ClO4]ꢁ (0.1 mol Lꢁ1) as electrolyte at
a scan rate of 50 mV sꢁ1. The voltammograms are shown in
Fig. 3 and the results are summarized in Table 2. The highest
occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO) energy levels were calculated by
combining both absorption spectroscopy and CV. The LUMO
energy levels were calculated from the rst reduction onset
potential using as reported equation ELUMO ¼ ꢁe (Ered + 4.4) eV
and the HOMO energy levels were determined by Eogpt plus
LUMO energy levels.60
As displayed in Fig. 3, all the polymers show one or two
reversible or quasi-reversible reduction and oxidation waves. The
HOMO energy levels were calculated to be in the range of ꢁ5.37
to ꢁ5.66 eV for P1–P3, which are in remarkably good compliance
with the ideal range of HOMO energy levels for achieving high air
stability and high open-circuit voltage (Voc) in OSCs.61 In
comparison to the HOMO energy level of P3 (ꢁ5.50 eV), P1 and P2
(ꢁ5.66 and ꢁ5.37 eV, respectively) exhibit high lying and low
lying HOMO energy levels by ca. 0.16 eV and 0.13 eV, respectively.
The LUMO energy levels were calculated as ꢁ3.58 eV for P1,
ꢁ3.57 eV for P2 and ꢁ3.62 eV for P3. The LUMO energy levels of
P1 and P2 are almost comparable whereas P3 has slightly high
lying LUMO energy level which can be due to the combined effect
of two thiophene and one benzothiadiazole units. The HOMO
and LUMO energy levels of P1–P3 are found to be comparable
with that of alternating phenylenevinylene polymer with PDI unit
(HOMO and LUMO energy levels at ꢁ5.75 and ꢁ3.95 eV,
respectively), which has shown one of the highest efficiency (PCE
upto 2.32%) reported for N-substituted PDI based polymers.32
The comparable LUMO energy level values of P1–P3 illustrate
that they can be used as n-type materials in OSCs.
Organic solar cell applications
BHJ solar cell devices were fabricated by blending the polymers
P1–P3 and P3HT with the device conguration of ITO/
PEDOT:PSS/polymer:P3HT/Al and a pixel area of 0.04 cm2 (for
detail, see device fabrication) where PEDOT:PSS was used to
modify the ITO electrode and to act as a hole-transporting layer.
P3HT was used as the donor and N-substituted PDI based
polymers P1–P3 as the acceptor to design the photoactive layers.
DCB was used as a processing solvent since it was reported to
afford the perfect quality of blended lms.62 The OSC device
performance was optimized in three different binary composi-
tions of P1/P2/P3:P3HT weight ratios i.e. 1 : 1, 1.5 : 1 and 2 : 1 to
achieve best PCE. The current density versus voltage (J–V) curves
of the OSCs fabricated with the optimized D : A ratio are shown
in Fig. 4 and their corresponding open-circuit voltage (Voc),
short-circuit current (Jsc), ll factor (FF) and PCE are listed in
Table 3. The photovoltaic performances of PSCs are greatly
inuenced by lm morphology of P1–P3 and P3HT since the
data from photovoltaic studies reveal that the 1 : 1 weight ratio
of P1/P2/P3:P3HT blends exhibit higher PCEs (0.82%, 1.22%
Fig. 3 Cyclic voltammetry curves of polymers P1–P3.
gaps of P1–P3 can be assigned to well organised and planar
structure of the conjugated polymers.
Electrochemical properties
The cyclic voltammetry (CV) studied were carried out to inves- and 1.96%, respectively) in comparison to other weight ratios
tigate the electrochemical properties of the polymers P1–P3 i.e. 1.5 : 1 and 2 : 1 blends (PCEs ranges from 0.23 to 1.11%),
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Table 3 Photovoltaic properties of polymers P1–P3
Ratio
Polymer : Jsc
Polymers P3HT
(mA cmꢁ2
Voc
(V)
FF
(%)
PCE Roughness
)
(%)
(rms)
P1
P1
P1
P2
P2
P2
P3
P3
P3
1 : 1
1.5 : 1
2 : 1
1 : 1
1.5 : 1
2 : 1
1 : 1
1.5 : 1
2 : 1
6.43
5.82
5.21
6.70
3.94
1.62
10.12
6.32
5.98
0.49 25.83 0.82 1.79
0.51 24.79 0.74 2.34
0.45 24.03 0.57 2.85
0.59 30.81 1.22 1.67
0.60 21.26 0.51 2.43
0.66 21.41 0.23 2.90
0.55 34.63 1.96 0.65
0.56 31.22 1.11 1.44
0.55 24.21 0.80 2.66
lower rms values (1.79, 1.67 and 0.65 nm, respectively). The PCE
for the binary 1 : 1 lm of P3:P3HT blend device reached upto
1.96% (highest among all devices in all tested ratios) with Voc of
0.55 V, Jsc of 10.12 mA cmꢁ2 and a FF of 34.63%. In comparison,
P1:P3HT/P2:P3HT devices (for ratio 1 : 1) showed PCEs of 1.22/
0.82%, Voc of 0.59/0.49 V, Jsc of 6.70/6.43 mAcmꢁ2 and FF of
25.83/30.81%, respectively. The higher efficiency obtained with
P3 based device can be attributed to higher Jsc and FF owing to
better conjugation and broader absorption spectrum of poly-
mer. Jsc ideally correlates with the energetic difference between
HOMO–LUMO energy levels of D–A segments of blend lms and
consequently relates to the exciton dissociation. Comparing
OPV devices constructed with 1 : 1 weight ratio, P3 blend shows
a higher Jsc of 10.12 mA cmꢁ2 relative to P1 and P2 (Jsc of 6.70
and 6.43 mA cmꢁ2, respectively) analogues and hence higher
value of exciton dissociation which could be the reason for
higher PCE in P3 based OSC. However in the present study, the
Voc, Jsc, FF and PCE show a linear trend with all the tested weight
ratios of P1/P2/P3:P3HT blend lms, since these photovoltaic
device properties are inuenced by other features such as
molecular structure, charge transport and the surface
morphology of the blended lms as well. Even though the
device performances of the N-substituted PDI based polymers
were low, these are comparable to some of the previously re-
ported for the BHJ photovoltaic devices based on a blend of
tris(thienylenevinylene)-substituted polythiophene and poly
[perylene diimide-alt-bis(dithienothiophene)].63 The PCEs of
these devices are less than that reported for blend device of
alternating phenylenevinylene copolymer with PDI unit and
a poly(3-phenylhydrazonethiophene) donor, which resulted
a PCE of 2.32%. The improvement was attributed to thermal
annealing of the blend lm which initiate the ne mixing
morphology of two components and the higher hole mobility in
the lms.32 However, the lower PCE in our devices in compar-
ison to the above reported device efficiency is primarily
restricted by the low values of Voc and FF. The low values of FF of
our devices can be attributed to the electron traps present in
polymer phase which contributes to the recombination losses.
The high FF values of the solar cell devices can be achieved by
better fabrication or design with proper selection of device
engineering strategies,64,65 improving morphology of
Fig. 4 J–V curves of the OSCs based on (a) P1:P3HT and (b) P2:P3HT
and (c) P3:P3HT.
suggesting a further increase of polymer content and decrease
of P3HT content in the photoactive layer had an unfavourable
impact on both Jsc and FF resulting in an overall decrease in
PCE. The high polymer and low P3HT content in blend lms
probably attributed to the improper thickness of photo-active
layer leading to higher current leakage, which in turn account
for the poor photovoltaic performances observed in devices.
The relatively better performance of 1 : 1 P1/P2/P3:P3HT blends
can be due to more favourable lm morphology and proper
thickness of the active layers with smoothened surface having
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Fig. 5 AFM topographic (left) and phase (right) images of P1/P2/P3:P3HT blend films.
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photoactive layer with controlled thermal treatment66 and P2/P3:P3HT blend lms was less uniform for 1.5 : 1 and 2 : 1
proper selection of processing additives.67 It was reported that ratio as the polymer content is increased (weight% of polymer
additives are useful to enhance the miscibility between two are 60% and 66.67%, respectively) and in respect of polymer the
components of the photoactive layer and found necessary for P3HT content decreased (weight% of P3HT are 40% and
optimizing photovoltaic device performances by increasing 33.33%, respectively), the lms became more coarser owing to
FF.68 The PCEs obtained for N-substituted PDI based polymers poor solubility, high molecular stacking and electrostatic
are limited of their application in OSCs due to reduced exciton interactions resulting in self-aggregation of polymers with
dissociation efficiency and/or increased charge recombination. reduced compatibility with P3HT.72 The 1 : 1 blend ratio of
Therefore, further experiments are in progress to improve the P1:P3HT, P2:P3HT and P3:P3HT gave optimal morphology in
photocurrent for OSCs to achieve better PCE by modifying the lms for charge transport and correlates well with the observed
structure as well as improving the surface morphology of the device performances and their data are summarized in Table 3.
blend lms.
Conclusions
Morphological studies
In the present work, an efficient and facile route was developed
The surface morphology of the photoactive layer in the BHJ to synthesize the new N-substituted PDI based monomer M1.
solar cells was found to be an important aspect to control the Three novel N-substituted PDI based low band gap polymers P1,
charge carrier transport into blend lms and impact on the P2 and P3 were synthesized by using palladium catalysed
performance of OSCs.69 To further understand the different polycondensation reactions for organic solar cell applications.
photovoltaic performance of PSCs based on P1/P2/P3:P3HT The structures of M1 and the newly designed polymers were
blend lms, atomic force microscopy (AFM) studies were conrmed by NMR, FT-IR and UV-vis spectroscopy. Polymers
carried out to investigate the morphology of the active layers were further characterized by CV and GPC. The polymers exhibit
using tapping mode. Fig. 5 shows topographic images and good solubility in common organic solvents and thermal
phase images for P1:P3HT, P2:P3HT and P3:P3HT blend lms stability with T5 higher than 380 ꢀC at 5% weight loss. The
in DCB solution for 1 : 1 weight ratio whereas other topographic absorption spectra of P1–P3 were broad and extended up to
images and phase images for remaining 1.5 : 1 and 2 : 1 weight about 954 nm in to the NIR region with shorter wavelength
ratios are given in Fig. S12.† The root-mean-square (rms) maxima in the range of 502 to 520 nm and Eogpt in the range of
roughnesses of these lms measured from topographic images 1.80–2.08 eV. GPC studies revealed that all the polymers have
are in range of 0.65 to 2.90 nm. In all AFM images, we observed good molecular weight (Mw) ranging from 10.68 to 16.02 kg
a phase-separated interpenetrating network with sizable P3HT molꢁ1. From cyclic voltammetric studies, it was found that the
domains. Some of phase-separation are important and neces- HOMO energy levels ranged from ꢁ5.37 to ꢁ5.66 eV whereas the
sary for systematic formation of free carriers to generate LUMO energy levels ranged from ꢁ3.57 to ꢁ3.62 eV. P1/P2/
optimal photovoltaic properties of PSCs.70 The phase images of P3:P3HT based BHJs were fabricated in three different weight
P1:P3HT, P2:P3HT and P3:P3HT blend lms in the ratio of 1 : 1 ratios i.e. 1 : 1, 1.5 : 1 and 2 : 1 by solution processing and it was
are quite different (rms roughness 1.79, 1.67 and 0.65 nm, found that the weight ratio of 1 : 1 for binary P3:P3HT blend is
respectively) and revealed that the P3HT domains in the the most optimal system and showed a Jsc of 10.12 mA cmꢁ2
,
P3:P3HT blend lm were smaller than those in the P1:P3HT and a Voc of 0.55 V and FF of 34.63% with an PCE of 1.96% which is
P2:P3HT blend lms, indicating better compatibility between among the highest reported for PDI based BHJ solar cells.
the P3HT and polymer in case of P3. This can be attributed to Further, the high polymer and low P3HT content in blend lms
the high solubility and feasible solution processability of thio- adversely affect the lm morphology of the photo-active layer
phene–benzothiadiazole–thiophene71 containing polymer P3 resulting in charge recombination and current leakage owing to
which may allow easier incorporation of P3HT into polymer micro-phase separation and aggregation of polymer which in
matrix. Among all blend lms, more uniform morphology (rms turn lead to low photovoltaic performances of PSCs based on
0.65 nm) with lower defect density and larger aggregation 1.5 : 1 and 2 : 1 weight ratio.
domains was observed for P3:P3HT blend lm in 1 : 1 ratio,
which is favourable to electron transport leading to the highest
PCE 1.96% with highest FF value (34.63%). However, the P1/P2/
Conflicts of interest
P3:P3HT blend lms in the ratio of 1 : 1 each showed more There are no conicts of interest to declare.
intermixed, smooth and homogeneous morphology in
comparison to 1.5 : 1 and 2 : 1 blend lms. The blend lms with
weight ratios of 1.5 : 1 and 2 : 1 show higher rms values ranges
Acknowledgements
from 1.44 to 2.43 nm and ranges from 2.66 to 2.90 nm, The nancial support obtained from Department of Science &
respectively and therefore exhibit larger domain size resulting Technology, India (DST project no. SB/S1/OC-12/2013) is highly
in ineffective exciton dissociation and lower PCE. The phase appreciated by authors to complete this research. Authors are
images (Fig. S12†) of P1/P2/P3:P3HT blend lms having weight also thankful to DMSE, IIT Delhi for providing laboratory
ratios of 1.5 : 1 and 2 : 1 also support the above assertion. facilities, Central Research Facility (CRF) and Nanoscale
However, the distribution of P3HT units in the polymer for P1/ Research Facility (NRF), IIT Delhi for necessary spectral studies.
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S. M. acknowledges the University Grants Commission (UGC),
India for providing Teacher Research Fellowship under Faculty
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