Functionalizing benzothiadiazole with non-conjugating ester groups as side chains in a donor-acceptor polymer improves solar cell performance

Article abstract of DOI:10.1039/C8NJ05850D

Herein, the effect of non-conjugated ester functionalization at the 5,6-position of 2,1,3-benzothiadiazole (BT) in a donor (D)-acceptor (A) conjugated polymer (CP) used for photovoltaic devices has been investigated. Positions 5 and 6 of BT were functionalized with methyl acetate groups and the structure property relationship was compared to BT with methyl groups at the 5 and 6 positions in four types of D-A CPs. Alternate co-polymers of newly synthesized methyl and methyl acetate derivatives of BT and Th-BT-Th with commonly used donors such as dithiophene (DTh) and benzodithiophene (BDT) were synthesized using Stille coupling reactions: namely, P(1,2,3,4)-Me and P(1,2,3,4)-Ac. All CPs were extensively characterized using GPC, UV-visible, ¹H-NMR, ¹³C-NMR, TGA and CV. The optimized geometry, along with changes in the dihedral angle upon substitution with acetate groups, was analyzed by density functional theory (DFT) using B3LYP/6-31G(d,p). The side chain ester groups lower the dihedral angle, improve the optical and electrochemical properties of CPs in polymer solar cells (PSCs), improve phase separation of the active layer and performance of the fabricated PSCs compared to methyl counterparts, as investigated for P(1,2,3,4)-Me and P(1,2,3,4)-Ac. Upon fabrication of a BHJ solar cell with ITO/PEDOT:PSS/P-PC₇₁BM/LiF/Al device geometry, CPs with methyl acetate functionalization (P2-Ac, P3-Ac and P4-Ac) resulted in higher PCEs of 1.36%, 1.17% and 0.35%, compared to their methyl counterpart CPs, P2-Me, P3-Me and P4-Me, which exhibited PCEs of 0.9%, 0.54% and 0.31%, respectively. With 1,8-diiodooctane (DIO) as an additive, a higher PCE of 1.96% was achieved for P3-Ac. Atomic force microscopy (AFM) and thin film X-ray diffraction (XRD) were used to determine the impact of side chain ester groups on π-π stacking distance among CP main chains in the film state and the morphology of the active layer of the fabricated PSCs, respectively.

Full text of DOI:10.1039/C8NJ05850D

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Substituting non-conjugating ester group on benzothiadiazole side
chain in donor-acceptor polymer improves the solar cell
performance
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
a
DOI: 10.1039/x0xx00000x
Radhakrishna Ratha, Mohammad Adil Afroz, Ritesh Kant Gupta and Parameswar Krishnan
Iyer*
a,b
www.rsc.org/
Herein, the effect of non-conjugated ester functionalization at 5,6-position of 2,1,3-benzothiadiazole (BT) in donor (D)-
acceptor (A) conjugated polymer (CP) used for photovoltaic devices have been investigated. Position 5 and 6 of BT have
been functionalized with methyl acetate group and its structure property relationship have been compared with BT having
methyl group at 5,6-positions in four types of D-A CPs. Alternate co-polymers of newly synthesized methyl and methyl
acetate derivatives of BT and Th-BT-Th with commonly used donors such as dithiophene (DTh) and benzodithiophene
(
BDT) have been synthesized using Stille coupling reaction namely P(1,2,3,4)-Me and P(1,2,3,4)-Ac. All CPs have been
extensively characterized by GPC, UV-visible, H-NMR, 13C-NMR, TGA and CV. Optimized geometry along with change of
1
dihedral angle on substitution of acetate group have been analysed by density functional theory (DFT) by B3LYP/6-31G
(d,p). The side chain ester group lowers the dihedral angle, improves optical and electrochemical properties of CPs used
for PSC, improves phase separation of active layer and performance of the fabricated PSC compared to its methyl
counterpart, as investigated for P(1,2,3,4)-Me and P(1,2,3,4)-Ac. On fabrication of BHJ solar cell with geometry of device as
ITO/PEDOT:PSS/P-PC71BM/LiF/Al, CP having methyl acetate functionalization such as P2-Ac, P3-Ac and P4-Ac resulted in
higher PCE of 1.36%, 1.17% and 0.35% compared to their methyl counterpart CP P2-Me, P3-Me and P4-Me with PCE of
0
.9%, 0.54% and 0.31% respectively. With 1,8-diiodooctane (DIO) as additive, higher PCE of 1.96% was achieved from P3-
Ac as well. Atomic force microscopy (AFM) and thin film X-ray diffraction (XRD) have been used to determine the impact of
side chain ester group on π-π stacking distance among CP main chain in film state and morphology of active layer of
fabricated PSCs respectively.
suitably positioned HOMO and LUMO corresponding to n-type
acceptor used for blending during fabrication. To further
increase the PCE and establish in-depth structure property
Introduction
Harvesting solar energy using a blend of donor(D)-acceptor(A)
containing p-type conjugated polymer (CP) donors and n-type
relationship, design of new backbones and modification of old
ones are necessary. Though the major focus among the
fullerene
heterojunction inverted / ternary
polymer solar cells (PSCs), is approaching a power conversion
/
non-fullerene-based acceptor in
a
bulk-
10,11
modifications have been on donor CP main chain,
various
1
2
3,4,5
6
/ tandem geometry
research groups have also functionalized the side chain of
these CPs such as: phosphite functionalized side chain CP
1
,7
efficiency (PCE) close to 11-13% and durability of the order
12
(
used as electron transporting layer
and additional
8
of 7 years. Besides designing several acceptors (fullerene/
13,14
interlayer),
side chain cyanide substituted CP (reported to
have improve dielectric constant), azide substituted CP (to
9
non-fullerene) and device engineering, the journey to a PCE of
11% was viable during last 10-15 years due to the large
number of CPs having desired properties such as, (i)
absorption maximum in 700-800 nm region of solar spectrum
15
>
16
improve crosslinking property)
functional group to enhance the solubility.
and incorporation of
17
Moreover, significant efforts have also been given to improve
the active layer morphology to enhance the charge separation
and to reduce the recombination of generated exciton. An
3
-1
with molar absorption coefficient of the order of 5*10 L mol
-1
2
2
-1 -1
cm , (ii) hole mobility of the order 1*10 cm V s , (iii) having
optimized
active
layer
(polymer-PCBM/non-fullerene)
18,19
morphology of a PSC device should: be less aggregated,
orderly arranged/crystalline film with bi-continuous phase
and having least RMS roughness with small grain size. These
preferred morphological variations have been achieved
a.
Centre for Nanotechnology, Indian Institute of Technology Guwahati, Guwahati-
81039, Assam, INDIA, Email: pki@iitg.ac.in
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati-
81039, Assam, INDIA.
Electronic Supplementary Information (ESI) available: [materials, methods, solar
20-22
7
b.
23
7
†
1
13
24,25
13
cell fabrication procedure, H-NMR, C-NMR, GPC and TXRD figures]. See majorly by; (i) use of additives
/mixture of solvents
/
DOI: 10.1039/x0xx00000x
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solvents to enhance solubility of the active layer,26 as and CH-π interactions both within the CP chain and/or inter-
preferential solubility of PCBM in one solvent over other molecularly in film state of CP and/or CP-PCBM blend.
allows stepwise deposition of blend layer sequentially
providing appropriate phase separation in the active layer
allowing distinct path for transport of holes and electrons; and
View Article Online
DOI: 10.1039/C8NJ05850D
Experimental
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7
(ii) high temperature annealing during device fabrication,
Detailed stepwise synthesis scheme of monomers and
polymers have been given in Fig. 1 and Fig. 2 respectively.
Synthetic procedures
providing sufficient energy for the film to be well organized or
upside down solvent vapour annealing.
incorporation of few functional group viz. fluorine / oxygen /
nitrogen into CP backbone has also been shown as a proficient
approach to improve morphology as well.
In line with the discussion of designing CPs as solar harvester
with good photo-physical property, mobility and having
functional group which improves morphology of active layer, it
is worth mentioning that CPs with main/side chain substituted
functional groups such as fluorine, oxygen, sulphur, ester,
2
8
In addition,
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,6-Dimethyl-benzo[1,2,5]thiadiazole (2). Monomer 2 was
prepared as per the literature procedure.45
2
9-31
4
,7-Dibromo-5,6-dimethylbenzo[c][1,2,5]thiadiazole
(3).
Monomer 2 (0.5g, 1.55 mmol) and concentrated HBr (48 wt%
ca. 30 mL) was added to a double neck reaction flask,
connected with a condenser at one neck. The flask was fitted
with an addition funnel at other neck to which Br
2
(0.470 mL,
1
0,16,32-35
9.134 mmol) solution in concentrated HBr (ca. 15 mL) was
ketone, cyanide and azide
are often being synthesized
to improve solar cell performances. Such functional groups added slowly (2-3 drops per second) while stirring at room
lower dihedral angle within CPs and bring planarity and temperature. After few drops of addition the solution turns
linearity into it,11 leading to improved π-π stacking in film state orange and a precipitate appears. After completion of addition
with desired morphology. The reason for improvement of the suspension was refluxed for 6 hours at 130 ⁰C. To the
PSC performance in case of main chain/side chain suspension, excess amount of H O was added to quench
functionalized CPs are enhanced intra-molecular/inter-
molecular oxygen-sulphur (O-S)/fluorine-hydrogen (F-H) /
nitrogen(N)-hydrogen(H)/oxygen-hydrogen (O-H) interaction
3
6
2
excess of bromine present. It was then filtered, washed with
H O repeatedly to give crude product, which upon further
2
purification by chromatographic column (SiO
2
, 50% CHCl
3
in
3
7,16,38,39
of CP chains in film state.
property in film state and segregation between CP and PCBM
acceptor), for the reason of enhanced interaction within
This improves optical
1
hexane) yielded 75% cream coloured solid 3. H-NMR (400
, δ in ppm): 2.672 (s, 6H). 13C-NMR (150 MHz,
MHz, CDCl
3
(
CDCl
3
, δ in ppm): 152.12, 140.61, 114.68, 22.03. HR-MS: m/z
S 322.868, found 322.868.
,7-Dibromo-5,6-bis(bromomethyl)benzo[c][1,2,5]thiadiazole
polymer-polymer chain, which repels the PCBM and gives the
most anticipated morphology called segregation rather than
undesired morphology called aggregation.
+
[
M+H] calcd for C
8 6
H
Br
2
N
2
4
(
4). Monomer 4 was synthesized following free radical
Ester group is one such important functional group among all,
being used both in the side chain and main chain of CPs for
solar cell. It has been reported that ester group improves
thermal stability of fabricated devices due to cross linking
bromination procedure as per literature procedure45 using 4,7-
Dibromo-5,6-dimethylbenzo[c][1,2,5]thiadiazole 3 (0.5 g, 1.553
mmol), NBS (0.683 g, 3.883 mmol), benzoyl peroxide (0.037 g,
property,4
0,41
4
0.155 mmol) and 30 mL CCl . After chromatographic
purification (SiO , 50% CHCl in hexane) of crude product, gave
2 3
compound 4 (cream coloured solid) with a yield of 92%.
4,7-Dibromobenzo[c][1,2,5]thiadiazole-5,6-diyl)bis(methylene)
thermo cleavable nature of ester group helps the
active layer of PSC resistant to solvent during multilayer device
fabrication,42 impart promising solubility, along with mobility
and improves morphology of active layer in PSC as well.
In this approach BT has been modified to improve the design diacetate (5). Monomer 4 0.9 g (1.87 mmol) and anhydrous
and synthesize acceptor units with methyl acetate group NaOAc 2.55 g (5.6 mmol) was mixed inside a round bottom
substituted as side chain at 5,6-position of BT (Fig. 1). flask containing 25 mL of glacial ethanoic acid and refluxed
Synthesis of D-A kind of CPs have been carried out (namely P1-
Ac, P2-Ac, P3-Ac, P4-Ac) from corresponding ester
functionalized BT-based acceptors with commonly used
donors (D1, D2, D3 and D4, Fig. 2) using Stille coupling.
Corresponding methyl counterpart CPs (namely P1-Me, P2-Me,
P3-Me and P4-Me) have also been synthesized to compare
influence of ester group at 5,6-position of BT. Herein, it has
been established that side chain ester group at 5,6-position of
BT, (i) can lower the dihedral angle along the conjugation
main chain resulting in increase in the molar absorption
coefficient (ii) red-shift in the absorption spectra in film state
43
4
4
2
overnight at 120 ⁰C. The suspension was transferred into H O
and neutralized with sodium bicarbonate solution in water. It
was followed by work up with CHCl and water. The organic
portion was dried and desired product was separated with
column chromatography using eluent (10% ethyl acetate in
hexane) to get 5 (0.75 g, yield = 91.7%) as white colored solid.
, δ in ppm): 5.58 (s, 4H), 2.1 (s, 6H).
, δ in ppm): 170.44, 152.88, 137.28,
18.01, 63.83, 20.84. HRMS (ESI): m/z [M+H]+ calcd for
3
1H-NMR (600 MHz, CDCl
13C-NMR (150 MHz, CDCl
3
3
1
C
12 2 2 4
H10Br N O S 438.878, found 439.215.
4
(
0
,7-Di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole-5,6-diyl)bis
methylene) diacetate (6). To a 50 ml flask monomer 5 (0.3 g
.687 mmol), tributyl(thiophen-2-yl)stannane 0.77 g (2.06
(iii) lowers optical LUMO level due to electron withdrawing
nature and (iv) enhances the solar cell performance of CP
compared to its methyl counterpart. These enhanced property
of side chain ester groups are due to oxygen-sulphur (O-S)/π-π
mmol) and 25 mL of dry toluene was transferred. The flask was
2
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Fig. 1 Reaction scheme for synthesis of monomers.
purged for 5 minutes with argon before adding Pd(PPh
.024 g (0.034 mmol). It was then refluxed at 100 ⁰C for 40 coloured solid (yield = 81%). H-NMR (600 MHz, CDCl
hours under argon flow. After cooling to RT, solvent was ppm): 7.56-7.55 (d, 2H), 7.24-7.23 (dd, 2H), 7.129-7.123 (d,
evaporated in rotary evaporator, and purified with column 2H), 2.48 (s, 6H). 13C-NMR (75 MHz, CDCl
, δ in ppm): 153.79,
chromatography (5% ethyl ethanoate in hexane), to get 0.25 g 139.78, 137.41, 128.74, 127.04, 126.79, 125.50, 18.99. HRMS
3
)
2
Cl
2
hexane) to give 0.33 g of compound 8 as greenish yellow
1
0
3
, δ in
3
1
+
12 2 3
of greenish yellow colored product (6) with 81% yield. H-NMR (ESI): m/z [M+H] calcd for C16H N S 329.028, found 329.024.
(
2
600 MHz, CDCl
3
, δ in ppm): 7.61-7.60 (d, 2H), 7.31-7.30 (d,
H), 7.26-7.23 (dd, 2H), 5.30 (s, 4H), 2.13 (s, 6H). 13C-NMR (75 dimethylbenzo[c][1,2,5]thiadiazole (9). Following the procedure
, δ in ppm): 170.15, 154.39, 135.03, 134.80, 130.08, used to synthesize monomer 7, Monomer 9 was synthesized
29.77, 128.16, 127.16, 61.81, 20.91. HRMS (ESI): m/z [M+H]+ by taking monomer 8 (0.25 g, 0.75 mmol) and NBS (0.4 g, 2.28
calcd for C20 445.038, found 445.0336. mmol) with room temperature stirring in dark. 10% CHCl in
,7-Bis(5-bromothiophen-2-yl)benzo[c][1,2,5]thiadiazole-5,6- hexane as eluent was used in column chromatography to yield
diyl)bis(methylene) diacetate (7). Inside a round bottom flask, pure monomer 9 as yellow coloured solid (0.3 g, 81% yield).
monomer 6 (0.1 g, 0.22 mmol) was dissolved in CHCl , δ in ppm): 7.182-7.76 (d, 2H), 6.954-
: glacial 1H-NMR (600 MHz, CDCl
acetic acid (1:1) and NBS (0.12 g, 0.675 mmol) was poured to it 6.948 (d, 2H), 2.50 (s, 6H). 13C-NMR (75 MHz, CDCl
, δ in ppm):
4,7-Bis(5-bromothiophen-2-yl)-5,6-
MHz, CDCl
1
3
H
16
N
2
O
S
4 3
3
4
3
3
3
in one portion. It was refluxed overnight in dark and then 153.34, 140.02, 138.79, 129.90, 129.26, 124.92, 113.71, 19.01.
evaporated to get crude product. Further column HRMS (ESI): m/z [M+H] calcd for C16
chromatographic (SiO
purification of the crude gave 0.09 g of monomer 7 (yellow
+
H
2
10Br N
2
S
3
486.848, found
2
,
eluent 5% EtOAc in hexane) 486.846.
General synthetic methodology for polymerization using Stille
, δ in ppm): coupling. Donor monomer (D1/D2/D3/D4) and acceptor
1
colored solid, 66% yield). H-NMR (600 MHz, CDCl
3
7
.19-7.18 (d, 2H), 7.06-7.05 (d, 2H), 5.30 (s, 4H), 2.13 (s, 6H). monomer (3/5/7/9) in equivalents of 1:1 were taken into a 50
13C-NMR (75 MHz, CDCl
, δ in ppm): 170.10, 154.02, 136.31, mL reaction flask and 25 mL of dry toluene was added to it.
35.28, 130.25, 130.12, 129.40, 115.54, 61.65, 20.94. HRMS Before addition of 0.2 equivalent of Pd(PPh , it was purged
3
1
3 4
)
+
(ESI): m/z [M+H] calcd for C20
H14Br N O S
2 2 4 3
602.858, found with argon for 5 minutes. It was then degassed and purged
6
02.858.
with argon thrice. It was then refluxed at 100 ⁰C for 72 hours in
5
,6-Dimethyl-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (8). presence of Ar. After reducing the temperature to 25 ⁰C, the
Synthesis of monomer 8 was performed as like monomer 6, by crude was concentrated and precipitated into 400 ml of
using monomer 3 (0.4 g, 1.24 mmol), tributyl(thiophen-2-yl) methyl alcohol. The precipitates were collected washed with
stannane (1.39 g 3.7 mmol) and Pd(PPh
mmol). The product was separated from the mixture by doing was filtered twice to remove insoluble part, concentrated and
3
silica gel column chromatography (with eluent 15% CHCl in again precipitated with 300 mL of methyl alcohol.
3 2 2
) Cl (0.043 g, 0.0621 MeOH, dried and then dissolved in chloroform. The solution
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Fig. 2 Reaction scheme for synthesis of CPs.
The precipitates were carefully collected to get desired CP.
2H), 7.26-7.20 (br, 2H), 4.31-4.23 (br, 4H), 2.67-2.58 (br, 6H),
P1-Me. It was synthesized using monomer D1 (0.1 g, 0.1207 1.95-1.83 (br, 2H), 1.62-1.54 (br, 4H), 1.53-1.39 (br, 12H), 1.16-
mmol), monomer 3 (0.038 g, 0.1207 mmol) and Pd(PPh (14 0.94 (br, 12H). GPC: M = 11580 g/mol.; M = 15300 g/mol.;
mg, 0.02414 mmol). P1-Me was formed as yellow colored PDI = 1.29; degree of polymerization, n = 15.
sticky solid (45 mg, yield 56%). H-NMR (600 MHz, CDCl
ppm): 7.14-7.12 (br, 1H), 7.09-7.07 (br, 1H), 2.71-2.69 (br, 2H), mmol), monomer 9 (0.0544 g, 0.09042 mmol) and Pd(PPh )
3
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w
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3
, δ in
P2-Ac. It was synthesized using monomer D2 (0.07 g, 0.09042
3 4
2
1
=
.62-2.55 (br, 6H), 1.78-1.68 (br, 4H), 1.43-1.38 (br, 2H), 1.26- (20.8 mg, 0.018 mmol). P2-Ac was formed as deep red colored
1
.25 (br. 36H), 0.88-0.86 (br, 6H). GPC: M
15883 g/mol.; PDI = 1.26; degree of polymerization, n = 16.
P1-Ac. It was synthesized using monomer D1 (0.1 g, 0.1207 7.28 (br, 2H), 5.48-5.39 (br, 4H), 4.32-4.21 (br, 4H), 2.24-2.15
mmol), monomer 5 (0.0526 g, 0.1207 mmol) and Pd(PPh (br, 6H), 1.95-1.85 (br, 2H), 1.64-1.53 (br, 4H), 1.53-1.40 (br,
27.8 mg, 0.02414 mmol). P1-Ac was formed as reddish yellow 12H), 1.16-1.09 (br, 12H). 13C-NMR (150 MHz, CDCl
, δ in ppm):
colored powder (60 mg, yield 67.3%). H-NMR (600 MHz, 170.18, 154.16, 144.5, 144.33, 140.55, 136.07, 135.31, 135.21,
CDCl , δ in ppm): 7.26-7.25 (br, 1H), 7.20-7.17 (br, 1H), 5.39 132.54, 130.99, 130.17, 129.73, 129.46, 125.28, 124.54,
br, 4H), 2.78-2.68 (br, 2H), 2.23-1.98 (br, 6H), 1.74-1.62 (br, 118.25, 116.79, 114.09, 76.24, 62.0, 40.64, 31.94, 30.46, 29.23,
n
= 12600 g/mol.; M
w
3
solid (74 mg, yield 92%). H-NMR (600 MHz, CDCl , δ in ppm):
7.71-7.65 (br, 1H), 7.60-7.51 (br, 1H), 7.45-7.36 (br, 2H), 7.30-
3 4
)
(
3
1
3
(
4
H), 1.44-1.37 (br, 2H), 1.37-1.12 (br, 36H), 0.91-0.80 (br, 6H). 23.99, 20.99, 14.21, 11.37. GPC: M
= 4986 g/mol.; M = 5396 g/mol.; PDI = 1.08; degree 11959 g/mol.; PDI = 1.03; degree of polymerization, n = 13.
of polymerization, n = 7. P3-Me. It was synthesized using monomer D3 (0.07 g, 0.077
P2-Me. It was synthesized using monomer D2 (0.07 g, mmol), monomer 7 (0.037 g, 0.077 mmol) and Pd(PPh (17.8
n w
= 11581, g/mol.; M =
GPC: M
n
w
3 4
)
0.09042 mmol), monomer 7 (0.04385 g, 0.09042 mmol) and mg, 0.015 mmol). P3-Me was formed as red colored solid (55
1
Pd(PPh
3
)
4
(20.8 mg, 0.018 mmol). P2-Me was formed as red mg, yield 79%). H-NMR (600 MHz, CDCl
3
, δ in ppm): 7.69-7.59
1
colored powder (62 mg, yield 89%). H-NMR (600 MHz, CDCl
δ in ppm): 7.69-7.66 (br, 1H), 7.63-7.59 (br, 1H), 7.51-7.46 (br,
3
,
4
| J. Name., 2012, 00, 1-3
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7
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9
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1
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1
1
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2
2
2
2
2
2
2
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3
3
3
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6
View Article Online
DOI: 10.1039/C8NJ05850D
0
1
2
3
4
5
6
7
8
9
0
1
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3
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5
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7
8
9
0
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1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Fig. 3 DFT calculation showing lowering of LUMO on functionalization of methyl acetate at 5,6-positon of BT-based CPs (model is a monomer).
(br, 2H), 7.36-7.29 (br, 2H), 7.22-7.12 (br, 2H), 7.12-7.02 (br, (19.5 mg, 0.0169 mmol). P4-Me was formed as deep red
2H), 6.91-6.79b (br, 2H), 2.87-2.75 (br, 4H), 2.55-2.39 (br, 6H), colored solid (53 mg, yield 76%). H-NMR (600 MHz, CDCl
1
3
, δ in
1
0
1
1
1
3
1
.70-1.59 (br, 2H), 1.59-1.45 (br, 4H), 1.33-1.21 (br, 12H), 0.92- ppm): 7.20-7.16 (br, 2H), 7.16-7.08 (br, 2H), 7.01-6.93 (br, 2H),
.78 (br, 12H). 13C-NMR (150 MHz, CDCl
, δ in ppm): 153.45, 2.78-2.70 (br, 2H), 2.55-2.40 (br, 6H), 1.69-1.59 (br, 4H), 1.41-
46.14, 139.91, 139.18, 138.77, 137.69, 137.37, 137.19, 1.31 (br, 2H), 1.25-1.06 (br, 36H), 0.84-0.74 (br, 6H). 13C-NMR
36.83, 130.13, 129.32, 128.89, 127.87, 125.54, 125.29, (150 MHz, CDCl , δ in ppm): 153.60, 143.48, 140.45, 139.79,
25.18, 124.04, 123.52, 121.19, 119.49, 114.11, 41.46, 34.33, 138.62, 137.79, 137.01, 136.74, 134.97, 129.94, 129.52,
2.51, 28.95, 25.80, 23.07, 19.29, 14.23, 10.95. GPC: M 126.62, 125.39, 123.93, 114.02, 31.96, 31.94, 30.58, 29.70-
1142 g/mol.; M = 13924 g/mol.; PDI = 1.24; degree of 29.38 (many peaks), 22.72, 19.24, 14.17. GPC: M = 15325.
= 17370 g/mol.; PDI = 1.33; degree of
polymerization, n = 18.
P4-Ac. It was synthesized using monomer D4 (0.06 g, 0.0724
17.8 mg, 0.015 mmol). P3-Ac was formed as deep red colored mmol), monomer 9 (0.0436 g, 0.0724 mmol) and Pd(PPh
3
3
n
=
w
n
polymerization n = 12.
g/mol.; M
w
P3-Ac. It was synthesized using monomer D3 (0.07 g, 0.077
3 4
mmol), monomer 9 (0.0466 g, 0.077 mmol) and Pd(PPh )
(
3 4
)
1
solid (60 mg, yield 76%). H-NMR (600 MHz, CDCl
3
, δ in ppm): (16.7 mg, 0.0144 mmol). P4-Ac was formed as deep red
1
7
7
.72-7.63 (br, 2H), 7.37-7.30 (br, 2H), 7.30-7.24 (br, 2H), 7.24- colored solid (59 mg, yield 87%). H-NMR (600 MHz, CDCl
.15 (br, 2H), 6.91-6.81 (br, 2H), 5.36-5.22 (br, 4H), 2.86-2.76 ppm):7.27-7.15 (br, 4H), 7.04-6.93 (br, 2H), 5.37-5.27 (br, 4H),
3
, δ in
(br, 4H), 2.12-2.03 (br, 6H), 1.68-1.61 (br, 2H), 1.61-1.47 (br, 2.80-2.69 (br, 2H), 2.14-2.04 (br, 6H), 1.69-1.61 (br, 4H), 1.40-
4H), 1.32-1.23 (br, 12H), 0.94-0.78 (br, 12H). 13C-NMR (150 1.32 (br, 2H) 1.25-1.06 (br, 36H), 0.84-0.74 (br, 6H). 13C-NMR
MHz, CDCl , δ in ppm): 170.14, 154.17, 146.14, 140.58, 139.28, (150 MHz, CDCl , δ in ppm): 170.24, 154.28, 140.98, 139.31,
3 3
138.95, 138.12, 137.41, 136.71, 135.29, 135.17, 130.85, 135.24, 135.0, 134.55, 130.88, 130.50, 129.72, 129.20, 129.0,
130.15, 129.66, 127.94, 125.55, 125.34, 124.51, 123.68, 126.77, 125.62, 124.51, 114.08, 62.1, 31.94, 31.92, 30.58, 29.7-
1
2
19.97, 115.54, 114.08, 61.98, 34.33, 32.53, 28.94, 25.77, 29.39 (many peaks), 22.65, 21.04, 14.20. GPC: M
3.10, 20.98, 14.20, 10.95. GPC: M = 9937 g/mol.; M =10605 g/mol.; M = 18199 g/mol.; PDI = 1.29; degree of
polymerization, n = 14.
n
= 14008
n
w
w
g/mol.; PDI = 1.06; degree of polymerization, n = 9.
P4-Me. It was synthesized using monomer D4 (0.07 g, 0.0844
3 4
mmol), monomer 7 (0.0409 g, 0.0844 mmol) and Pd(PPh )
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ARTICLE
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View Article Online
DOI: 10.1039/C8NJ05850D
0
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
Fig. 4 Absorption spectra of synthesized CPs, showing red-shift and broadening of spectra for methyl acetate substituted CPs compared to their methyl counterpart.
comparison of absorption property for both solution and film
shows bathochromic shift of both absorption maximum of ICT
Results and discussion
Theoretical calculations
transition and absorption onset with broadening of UV-visible
spectra for ester functionalized CPs compared to their methyl
counterpart (Table 1, Fig. 4a to 4d). All CPs exhibit tri-humped
peak (Fig. 4a to 4d) in general both in solution and film state,
which refers to high energy intrinsic band for donor only,
moderate energy π-π* transition for CP backbone and low
energy ICT band. In film state, intrinsic band for P1-Me, P1-
Ac and P2-Me is not clearly visible due to broadening of
spectra (Fig. 4a and 4b). In solution state, in addition to small
variation in intrinsic band and π-π* band, a red-shift in ICT
transition has been observed for methyl acetate substituted
CPs compared to their methyl counterpart. In film state a
significant bathochromic shift of absorption spectra has been
observed along with peak broadening for λmax (ICT band)
and/or UV-visible (onset), signifying a substantial π-π stacking
Influence of ester group on HOMO-LUMO levels and dihedral
angle between conjugated units of the polymers have been
studied using DFT calculation at B3LYP/6-31G (d,p). For
simplifying calculations, methyl chain have been used for any
kind of alkyl chain and investigation on all CPs have been
carried out taking 1 unit of each CP. Ester functionalised CPs
displayed required distribution of charge density for intra-
molecular charge transfer (ICT) from donor to acceptor
4
7
(distribution of charge density all over HOMO and localization
4
6
of charge density on acceptor only in LUMO) in all four CPs
P(1,2,3,4)-Ac] with 4 different donors (D1 to D4). Ester
[
functionalized CP has low dihedral angle in case of D2 and D4
compared to their methyl counterpart (Fig. 3c,d and 3g,h), due
to possible oxygen (negatively charged in ester group)-sulphur
4
8
of CP main chain (Fig. 4a to 4d), when side chain ester was
introduced into it, due to enhanced CH-π, π-π and oxygen-
sulphur interaction (intramolecular and intermolecular). Molar
(positively charges in thiophene ring) interactions. But in case
of D1 and D3 dihedral angle increases in ester functionalized
CPs (Fig. 3a,b and 3e,f) compared to its methyl counterpart
because of steric hindrance caused by D1 with two dodecyl
chains in close position (at 3,3ˊ) and bulkiness of D3 (due to
two thiophene substituted 2-ethylhexyl chain on BDT), which
predominates over any possible oxygen sulphur interactions.
In case of D2 and D4 dihedral angle reduced by 10⁰ in ester
substituted CP.
absorption coefficient (ε) of π-π* band and ICT band in CHCl
was higher for ester substituted CPs P2-Ac (27373, 26197 L
3
-1
-1
-1
-1
mol cm ) and P4-Ac (29827, 20113 L mol cm ) compared to
-1
-1
methyl substituted CP P2-Me (13820, 15372 L mol cm ) and
-1
-1
P4-Me (18124, 19819 L mol cm ) respectively (Table 1), due
to lowering of dihedral angle. However for similar instance,
ester substituted CP P1-Ac and P3-Ac molar absorption
coefficient gets lowered compared to their methyl counterpart
P1-Me and P3-Me due to higher dihedral angle in ester
Optical properties
UV-visible spectra for both methyl acetate substituted CPs substituted CPs caused due to steric hindered donor D1 (Fig. 2)
P(1,2,3,4)-Ac] and methyl substituted CPs [P(1,2,3,4)-Me] at with closely positioned dodecyl chain
in case of P1-Ac and
,6-position of BT have been recorded from dilute CHCl
solution and in film state (drop casted from chlorobenzene). A
[
5
1
3
6
| J. Name., 2012, 00, 1-3
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Page 7 of 12
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Journal Name
ARTICLE
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Table 1 Optical properties and voltammetry for synthesized CPs with degradation temperature corresponding to 5% weight loss.
View Article Online
DOI: 10.1039/C8NJ05850D
P
λ
max
λ
onset
λ
max film
[nm]
λ
onset film
[nm]
E
ox
HOMOel LUMOopt
ε (π-π*, ICT)
T
5D
-1
-1
[
nm]
[nm]
470
496
511
543
526
555
510
554
[Volt]
+0.801
+0.92
+1.03
+1.04
+0.77
+0.74
+0.66
+0.64
[eV]
-5.49
-5.609
-5.39
-5.40
-5.49
-5.46
-5.38
-5.36
[eV]
-2.94
-3.32
-3.33
-3.47
-3.38
-3.55
-3.16
-3.51
[L mo cm ]
16757, 10282
16942, 8247
13820, 15372
27373, 26197
41536, 40772
23934, 20532
18124, 19819
29827, 20113
[⁰C]
368 (T10 D
272
P1-Me
P1-Ac
P2-Me
P2-Ac
P3-Me
P3-Ac
P4-Me
P4-Ac
316, 326, 369
312, 324, 421
328, 379, 445
326, 382, 454
329, 404, 440
339, 404, ~470
327, 401,
327, 374
326, 433
388, 480
400, 515
490
558
602
644
586
648
557
670
)
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
275
244
333, 416, 462
352, 420, 523
330, 404, 460
327, 401, 512
275
265
348
325, 390, ~457
279
bulky 2-ethylhexylthiophene substituted BDT-Th donor D3 (Fig. than 100 ⁰C (Fig. 6a) corresponding to 5% weight loss and
2) in case of P3-Ac, which predominantly controls the might have occurred due to the presence of moisture (which
structure, eliminating possible oxygen-sulphur interactions by might have been responsible for its sticky nature). In Table 1
ester group with near Th-units which could stabilize the for P3, T10D (degradation corresponding to 10% weight loss)
structure.
Electrochemical properties
was at 368 ⁰C. All ester functionalized CPs have low T5D and
show bi-humped degradation pattern compared to their
methyl counterpart (Fig. 6a and 6b), which might have
occurred due to (fragile/thermocleavable nature of side chain
Cyclic voltammetry was carried out for all synthesized CPs and
they exhibited only an oxidation potential within -0.6 Volt to -
4
2
ester group (here methyl acetate).
Solar cell performance
1
.04 Volts (Fig. 5a and 5b). Calculated HOMO of all CPs is in the
range from -5.36 to -5.6 eV. HOMO of P1-Ac is 0.11 eV deeper
than P1-Me (Table 1), which might have caused due to high Both methyl ester and methyl functionalized on BT-based CPs
sterically hindered donor D1, leading to weak donating have been used with PC71BM to fabricate PSC and similar
tendency to the acceptor BT having methyl acetate group device architecture (ITO/PEDOT:PSS/CP:PC71BM (1:1)/LiF/Al)
(higher the π-electron donating tendency of donor in D-A CP,
higher the HOMO). Minor fluctuation (maximum observed
4
9
change 0.03 eV) of HOMO levels has been observed in P2-Ac,
P3-Ac and P4-Ac compared to their methyl counterpart,
indicating less control of ester group on HOMO of synthesized
CPs and influence of methyl acetate group on CP photo
physical property purely due to contribution to LUMO only.
None of the CPs exhibited a reduction potential, hence their
optical
LUMO
have been calculated from absorption onset using
opt
formula (band gap = λonset/1240 and LUMO = HOMO + band
gap). As all ester substituted CPs show a high absorption onset,
hence all ester functionalized CPs showed a low LUMO
compared to its methyl counterpart. Low lying LUMO in
ester functionalized CPs will help improve charge separation in
active layer of fabricated PSC due to small energy barrier
between LUMO levels of CP and PC71BM.
opt
opt
5
0
Thermal stability of polymers
Thermal stability for all side chain ester and methyl
functionalized CPs have been investigated using TGA analysis
under nitrogen atmosphere. All CPs are stable up to 250 ⁰C,
corresponding to 5% weight loss of CP (Table 1, Fig. 6a and 6b)
on a constant heating rate of 10 ⁰C per minute, which is
Fig. 5 CV for CPs, showing oxidation potential (a) P1-Me, P1-Ac, P2-Me, P2-Ac (b) P3-
Me, P3-Ac, P4-AMe, P4-Ac.
5
1,52
adequate temperature for device Fabrication.
exception, sticky natured CP P1-Me starts degrading at less
With
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1
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5
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7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
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4
4
4
4
4
4
4
4
4
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5
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5
5
5
5
5
5
5
5
6
been observed subsequently with PCE in case of ester
View Article Online
DOI: 10.1039/C8NJ05850D
functionalized CPs compared to their methyl counterpart. This
establishes side chain ester group at 5,6-position of BT
improves solar cell performance, as studied in three different
set of CPs with varying three donors (D2, D3, D4). In addition
2
P1-Ac:PC71BM (2:1) showed a PCE of 0.02% (Jsc = 0.50 mA/cm ,
V
oc = 0.17 V and FF = 26.2%), which is too low to study any
comparison with its methyl counterpart P1-Me. Both P1-Me
and P1-Ac have very poor film forming property, due to high
0
1
2
3
4
5
6
7
8
9
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2
3
4
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9
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2
3
4
5
6
7
8
9
0
5
7
dihedral angle along the conjugation main chain. The cause
of improvement of solar cell performances in case of ester
substituted CPs is due to higher absorption range (red-region),
better phase separation in active layer due to cross linking
nature of ester group within CP and/or PC71BM, improved ICT
Fig. 6 TGA spectra for CPs showing T5D (corresponding to 5% weight loss) (a) methyl
substituted at 5,6-position of BT-based CPs (b) methyl acetate counterpart CP.
was chosen to establish the influence of side chain ester group
on solar cell performance. It has been established that ester
substituted CPs [P(2,3,4)-Ac] show high solar cell performance
(PCE, Voc and Jsc) than their methyl counterpart CP [P(2,3,4)-
Me]]. With P2-Me:PC71BM (1:1) as active layer a PCE of 0.9%
2
(J
sc = 5.09 mA/cm , Voc = 0.68 V and FF = 26.2%) has been
achieved, whereas ester functionalized CP P2-Ac in similar
device configuration results in a PCE of 1.36% with Jsc of 7.4
2
mA/cm , Voc of 0.69 V and FF of 26.5% (Table 2, Fig. 7a).
Similarly with ester functionalized CP P3-Ac a PCE of 1.17% (Jsc
2
=
7.65 mA/cm , Voc = 0.63 V, FF = 29.3%) was achieved
2
compared to a PCE of 0.54% (Jsc = 3.5 mA/cm , Voc = 0.57 V, FF
=
27.1%) for its methyl counterpart (P3-Me). Unexpected
variation of Voc with respect to HOMO for P3 set of polymers,
might have resulted due to difference of side chain, from
methyl (P3-Me) to acetate (P3-Ac). This dissimilarity of side
chain changes the intermolecular interaction of polymer with Fig. 7 Solar cell performance for side chain methyl acetate and methyl containing BT-
PC71BM5
3,54
and affects the Voc as well. PSC performance for based CPs (a) J-V curve (b) external quantum efficiency.
P3-Ac has further improved to a PCE of 1.96% by the use of 3%
,8-diiodooctane (DIO) as additive into active layer (Table 2,
Table 2 Solar cell parameters for fabricated bulk-heterojunction devices.
1
2
4
Fig. 7a). The reason for improvement of PSC performance
with DIO is mainly due to film reorganization and better phase
separation (Fig. 8c).55
Active layera
V
Volt]
oc
J
sc
FF
Max PCE
[%]b
2
[
[mA/cm ] [%]
P4-Me and P4-Ac follow similar pattern as like P2-Me and P2-
Ac in fabricated devices with PCE of 0.35% (P4-Ac) and 0.31%
P2-Me
P2-Ac
0.68
0.69
0.57
0.63
0.70
0.47
0.51
5.09
7.40
3.50
7.65
8.62
2.54
2.62
26.2
26.5
27.1
29.3
32.3
26.3
26.2
0.91 [0.86±0.09]
1.36 [1.29±0.06]
0.55 [0.54±0.01]
1.17 [1.13±0.05]
1.96 [1.95±0.01]
0.31 [0.28±0.03]
0.36 [0.32±0.05]
(P4-Me). Solar cell performance with P4-Me and P4-Ac (CP
P3-Me
with DTh as donor) was very poor (PCE in the range of 0.3%)
compared to BDT based CPs. This is due to poor film forming
ability of P4-Me and P4-Ac with high dihedral angle (~18⁰)
between two thiophene units (Fig. 3d) of the donor
dithiophene (DTh), this leads to low rigidity of CP backbone
and hence poor film forming property. Moreover maximum
external quantum efficiency of 37% (Fig. 7b) have been
achieved with P3-Ac:PC71BM (1:1)-3% (DIO) as active layer for
fabricated solar cells and improvement of EQE values have
P3-Ac
P3-Ac-3% DIO
P4-Me
5
6
P4-Ac
a
active layer for each CP was CP:PC71BM (1:1), b Average PCE of three cell given in
parenthesis.
8
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due to lowering in dihedral angle along conjugation main chain substituted CPs in order to investigate influeVniecweArtoiclne Onπli-nπe
and lowering of LUMO which facilitates ease of separation of stacking distance in thin-film state wDiOthI: 1s0i.d10e39/cCh8aNinJ05e8s5t0eDr
exciton from LUMO of CP to LUMO of PC71BM.
Morphology of active layer of fabricated PSCs
substitution at 5,6-position of BT. For P4-Me and P4-Ac the
diffraction peaks were observed at 2θ = 24.21⁰ and 25.2⁰
which corresponds to 0.3673 nm and 0.3531 nm π-π stacking
distance respectively (Fig. S1c, †ESI). Close π-π stacking
distance in case of ester substituted CP P4-Ac compared to P4-
Me further confirms improved π-π stacking within CP chain in
Tapping mode AFM images have been recorded to investigate
nano-morphology of active layer for fabricated devices by spin
coating polymer:PC71BM blend on a pre-cleaned ITO coated
glass slide. As compared for P3-Me:PC71BM (1:1), P3-Ac:
PC71BM (1:1) and P3-Ac: PC71BM (1:1)with DIO, it was
observed that both the polymer-PC71BM blend have proper
miscibility in the active layer with RMS (root mean square)
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film state and hence better solar cell performance.
In
addition among P3-Me and P3-Ac, ester substituted at 5,6-
position of BT CP P3-Ac exhibited a similar/small increase in π-
π stacking distance (2θ = 23.16⁰, π-π stacking distance =
.3837 nm) as obvious due to small increase in dihedral angle
Fig. S1b, †ESI) (determined by DFT) compared to its methyl
counterpart P3-Me (2θ = 23.25⁰, π-π stacking distance =
.3822 nm). Similarly in case of P2-Me and P2-Ac no change in
π-π distance was observed (Fig. S1a, †ESI). DTh and BDT-Th)
resulting in new CPs having methyl acetate group at 5,6-
location of BT. All newly synthesized methyl and methyl
acetate substituted 5,6-BT-based CPs were well soluble in
commonly used solvent for device fabrication (CB, DCB
47
roughness less than 3 nm. Ester functionalized polymer P3-
0
(
Ac results in better phase separation between P3-Ac and
PC71BM in the blend film compared to its methyl counterpart
P3-Me (Fig. 8a and 8b). When 3% DIO was added into the P3-
0
Ac:PC71BM (1:1)
a significant improvement of phase
has been observed (Fig. 8c) allowing
55,58
separation
distinguished path for electron and hole to travel and reducing
recombination of generated excitons (as can be seen in the
height and 3D view of height images in Fig. 8). As a result PCE
improved to 1.96% with P3-Ac:PC71BM (1:1)-3% DIO as active
layer. This further confirms the positive influence of side chain
ester group in gaining crtystallinity in the active layer of
fabricated PSC with tunability to external effects like additives
and various solvent processing.
3
CHCl , THF) and additionally the ester substituted CPs are
59
stable up to 250 ⁰C. With this study it has been established
that functionalizing ester group at 5,6-position of BT, reduces
the dihedral angle in the CP backbone due to oxygen-sulphur
interaction, improves π-π stacking in film leading to red-shift
Thin film X-ray diffraction pattern
optical
and broadening of absorption spectra, lowers LUMO
CP, improves phase separation of active layer of fabricated
for
Thin film X-ray diffraction has been carried out by spin coating
equal concentration of methyl and corresponding ester
Fig. 8 Morphology of active layer (CP:PC71BM blend) of fabricated PSCs show improvement of phase separation with ester substituted CPs and the phase separation
further improved to large extent when 3% DIO was added to P3-Ac:PC71BM (1:1). (a) P3-Me:PC71BM (1:1) (b) P3-Ac:PC71BM (1:1) (c) P3-Ac:PC71BM (1:1)-3% DIO.
This journal is © The Royal Society of Chemistry 20xx
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View Article Online
DOI: 10.1039/C8NJ05850D
1
5 N. Cho, C. W. Schlenker, K. M. Knesting, P. Koelsch, H. L. Yip,
solar cell and hence improves solar cell performance
compared to methyl counterpart. This phase separation can be
further enhanced using additives, as in case of P3-Ac:PC71BM,
PCE of 1.17% was improved to 1.96% by addition of 3% DIO as
D. S. Ginger, A. K. Y. Jen, Adv. Energy Mater., 2014, 1301857.
16 H. J. Kim, A. R. Han, C. H. Cho, H. Kang, H. H. Cho, M. Y. Lee, J.
M. J. Fréchet, J. H. Oh, B. J. Kim, Chem. Mater., 2012, 24,
2
15-221.
additive. Such side chain ester group with good photo-physical 17 Y. L, J. Zou, H. L. Yip, C. Z. Li, Y. Zhang, C. C. Chueh, J.
Intemann, Y. Xu, P. W. Liang, Y. Chen, A. K. Y. Jen,
Macromolecules, 2013, 46, 5497-5503.
18 R. N. Brookins, E. Berda, J. R. Reynolds, J. Mater. Chem.,
and morphological influence on CP film state can be
introduced into side chain of synthetically feasible conjugated
units and used to synthesize CP with superior properties for
solar cells and other optoelectronic devices.
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009, 19, 4197-4204.
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9 F. S. U. Fischer, K. Tremel, A. K. Saur, S. Link, N. Kayunkid,
M. Brinkmann, D. H. Carvajal, J. T. L. Navarrete, M. C. R.
Delgado, S. Ludwigs, Macromolecules, 2013, 46, 4924-4931.
Acknowledgements
Authors acknowledge the Department of Science and
20 D. D. Nuzzo, A. Aguirre, M. Shahid, V. S. Gevaerts, S. C. J.
Meskers, R. A. J. Janssen, Adv. Mater., 2010, 22, 4321-4324.
2
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1 L. M. Chen, Z. Hong, G. Li, Y. Yang, Adv. Mater., 2009, 21,
1434-1449.
Technology
INDIA
(No.
SR/S1/PC-02/2009,
2 F. Liu, Y. Gu, J. W. Jung, W. H. Jo, T. P. Russell, J. Polym. Sci.,
Part A: Polym. Chem., 2012, 50, 1018-1044.
3 E. Wang, L. Hou, Z. Wang, S. Hellström, F. Z. O. Inganäs, M. R.
Andersson, Adv. Mater., 2010, 22, 5240-5244.
4 Y. Liang, Z. Xu, J. Xia, S. T. Tsai, Y. Wu, G. Li, C. Ray, L. Yu, Adv.
Mater., 2010, 22, E135-E138.
5 C. V. Hoven, X. D. Dang, R. C. Coffin, J. Peet, T. Q. Nguyen, G.
C. Bazan, Adv. Mater., 2010, 22, E63-E66.
6 W. Cai, P. Liu, Y. Jin, Q. Xue, F. Liu, T. P. Russell, F. Huang, H.
L. Yip, Y. Cao, Adv. Sci., 2015, 2, 1500095.
7 J. Hou, H. Y. Chen, S. Zhang, Y. Yang, J. Phys. Chem. C, 2009,
113, 21202-21207.
8 M. Zhang, F. Zhang, Q. An, Q. Sun, W. Wang, X. Ma, J. Zhang,
W. Tang, J. Mater. Chem. A, 2017, 5, 3589-3598.
9 J. W. Jo, S. Bae, F. Liu, T. P. Russell, W. H. Jo, Adv. Funct.
Mater., 2015, 25, 120-125.
DST/TSG/PT/2009/11 and DST/TSG/PT/2009/23), DST–Max
Planck Society, Germany (IGSTC/MPG/PG(PKI)/2011A/48),
Department of Electronics & Information Technology, (Deity)
No. 5(9)/2012-NANO (Vol. II) for financial assistance. RR thanks
UGC-INDIA for financial support. The Central Instruments
Facility and Department of Chemistry, IIT Guwahati, are
acknowledged for instrument facility. Sophisticated Analytical
Instrumentation Centre, Tezpur University is acknowledged for
GPC facility. Department of Chemistry, Guwahati University is
acknowledged for NMR facility as well.
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View Article Online
DOI: 10.1039/C8NJ05850D
Graphical Abstract
Substituting non-conjugating ester group on benzothiadiazole side chain in donor-
acceptor polymer improves the solar cell performance
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Radhakrishna Ratha , Mohammad Adil Afroz , Ritesh Kant Gupta and Parameswar Krishnan
Iyer *1, 2
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Centre for Nanotechnology and Department of Chemistry, Indian Institute of Technology
Guwahati, Guwahati-781039, Assam, INDIA
Text for graphical abstract:
Side chain ester substitution on donor-acceptor based conjugated polymers
used as solar harvester in bulk-heterojunction (BHJ) polymer solar cell (PSC)
can further improve harvesting property, phase separation in active layer and
PSC performance.
Graphical abstract figure:
Figure: Graphical abstract.