Effect of mono alkoxy-carboxylate-functionalized benzothiadiazole-based donor polymers for non-fullerene solar cells

Article abstract of DOI:10.1016/j.dyepig.2018.12.058

Structural modification of benzo[c]-1,2,5-thiadiazole (BT) has been proved to be the prominent way to fine-tune the frontier energy levels and the intermolecular and intramolecular interactions in organic conjugated materials. In this study, a new acceptor unit, alkyl benzo[c][1,2,5]thiadiazole-5-carboxylate (BT-Est), was designed and synthesized by drafting mono alkoxy-carboxylate substituent on 5-position of BT core. Its compatibility in the conjugated system was investigated by co-polymerizing BT-Est with well-known benzo[1,2-b:4,5-b’]dithiophene monomers containing either 2-(2-ethylhexyl)thienyl or 2-((2-ethylhexyl)thio)thienyl side chains to form two new polymers, P1 and P2, respectively. The BT-Est yielded polymers with good solubility, medium bandgap (～1.71 eV), and deep highest occupied molecular orbital energy levels (?5.48 to ?5.54 eV). Among the polymers, P1 exhibited broader absorption, compact molecular packing, high charge carrier mobility, and effective exciton dissociation, despite of the torsion angle caused by the free rotation of the carboxylate group in the polymer backbone. Consequently, the best power-conversion efficiency of 6.9%, with a J_SC of 14.6 mA cm^?2, V_OC of 0.9 V, and FF of 52.5% were obtained for P1-based devices with the well-known non-fullerene acceptor ITIC. We systematically expounded the structure-property relationship of the BT-Est polymers using diverse characterization methods. Our results demonstrated that the mono carboxylate-substitution on the BT core can be used as the alternate strategy to modulate the optoelectronic properties and control the aggregation in the conjugated polymers. Thus, BT-Est has the potential to produce new donor–acceptor conjugated polymers and small molecules for application in organic electronics.

Full text of DOI:10.1016/j.dyepig.2018.12.058

Accepted Manuscript
Effect of mono alkoxy-carboxylate-functionalized benzothiadiazole-based donor
polymers for non-fullerene solar cells
Gururaj P. Kini, Jun Young Choi, Sung Jae Jeon, Il Soon Suh, Doo Kyung Moon
PII:
S0143-7208(18)32467-7
https://doi.org/10.1016/j.dyepig.2018.12.058
DYPI 7263
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 9 November 2018
Revised Date: 5 December 2018
Accepted Date: 26 December 2018
Please cite this article as: Kini GP, Choi JY, Jeon SJ, Suh IS, Moon DK, Effect of mono alkoxy-
carboxylate-functionalized benzothiadiazole-based donor polymers for non-fullerene solar cells, Dyes
and Pigments (2019), doi: https://doi.org/10.1016/j.dyepig.2018.12.058.
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ACCEPTED MANUSCRIPT
Effect of mono alkoxy-carboxylate-functionalized benzothiadiazole-based
donor polymers for non-fullerene solar cells
Gururaj P. Kini, Jun Young Choi, Sung Jae Jeon, Il Soon Suh and Doo Kyung Moon*
Nano and Information Materials (NIMs) Laboratory,
Department of Chemical Engineering,
Konkuk University, 120, Neungdong-ro, Gwangjin-gu, Seoul 05029, Korea.
Corresponding Author
*
E-mail: dkmoon@konkuk.ac.kr; Fax: +82-2-444-0765; Tel: +82-2-450-3498.
(Additional Supplementary Information may be found in the online version of this article)
Keywords: polymer solar cells, benzothiadiazole, carboxylate substituted benzothiadiazole,
carboxylate effect, non-fullerene solar cells
1

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Abstract
Structural modification of benzo[c]-1,2,5-thiadiazole (BT) has been proved to be the
prominent way to fine-tune the frontier energy levels and the intermolecular and
intramolecular interactions in organic conjugated materials. In this study, a new acceptor
unit, alkyl benzo[c][1,2,5]thiadiazole-5-carboxylate (BT-Est), was designed and
synthesized by drafting mono alkoxy-carboxylate substituent on 5-position of BT core. Its
compatibility in the conjugated system was investigated by co-polymerizing BT-Est with
well-known benzo[1,2-
ethylhexyl)thienyl or 2-((2-ethylhexyl)thio)thienyl side chains to form two new polymers,
P1 and P2, respectively The BT-Est yielded polymers with good solubility, medium
bandgap (~1.71 eV), and deep highest occupied molecular orbital energy levels (−5.48 to
5.54 eV). Among the polymers, P1 exhibited broader absorption, compact molecular
b:4,5-b']dithiophene monomers containing either 2-(2-
.
−
packing, high charge carrier mobility, and effective exciton dissociation, despite of the
torsion angle caused by the free rotation of the carboxylate group in the polymer
backbone. Consequently, the best power-conversion efficiency of 6.9%, with a
JSC of 14.6
−
2
mA cm ,
VOC of 0.9 V, and FF of 52.5% were obtained for P1-based devices with the
well-known non-fullerene acceptor ITIC. We systematically expounded the structure-
property relationship of the BT-Est polymers using diverse characterization methods. Our
results demonstrated that the mono carboxylate-substitution on the BT core can be used as
the alternate strategy to modulate the optoelectronic properties and control the
aggregation in the conjugated polymers. Thus, BT-Est has the potential to produce new
donor–acceptor conjugated polymers and small molecules for application in organic
electronics.
2

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Introduction
Benzo[c]-1,2,5-thiadiazole (BT) [1-4] and its heteroannulated modifications like
benzobisthiadiazole (BBT), [5-7] naphthobisthiadiazole (NTz), [8-11]
dithieno[3',2':3,4;2'',3'':5,6]benzo[1,2-c][1,2,5]thiadiazole (fDTBT) [12-14] have emerged as
prominent electron-deficient acceptor cores for synthesizing donor–acceptor (D–A) polymers
and small molecules for application in organic electronics. The chemical structure of BT has a
strong electron-withdrawing 1,2,5-thiadiazole core attached to a benzene ring, allowing easy
electron delocalization via o-benzoquinoidal nature, and thus lowering the energy bandgap and
favoring ordered intermolecular packing. [15] The optoelectronic properties (energy levels,
bandgap, etc.) of BT-based polymers can be easily tuned via functionalization of a 5,6-positions
of the BT with either electron-deficient [1-2,16-25] or -donating [26-29] functional groups.
Usually, introduction of an electron-deficient heteroatom on BT lowers the frontier energy
levels and influences the intramolecular and intermolecular interactions in the D–A polymers,
thereby promoting effective charge transport. [1-2,4,18-22] Particularly, the incorporation of an
fluorine or chlorine atoms on BT unit has attracted significant attention in recent years since it
yields deep highest occupied molecular orbitals (HOMO), large dipole moments, good
photostability, and impressive performances of the resultant polymers in polymer solar cells
(PSCs).[4,16,18-20,23-25] Despite the high power-conversion efficiencies (PCEs) obtained
using these halogenated BT derivatives, their high aggregation tendencies, limited solubility and
the use of hazardous halogenating reagents hinder their use as the active layer material in large-
area roll-to-roll applications, where large quantities of raw materials required. [4,20,30]
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Therefore, it has become necessary to look for alternative techniques to overcome these
drawbacks.
Nowadays, conjugated polymers having electron withdrawing carboxylate or ester (–
COOR) substituents have demonstrated an impressive photovoltaic result over 10 %, indicating
their promising potential for application in PSCs. [31-36] These reports emphasize that the
insertion of ester groups onto the conjugated polymer backbone have various advantages such as
(1) easy tuning of solubility through alkyl chain modifications; (2) lower HOMO energy levels
due to the higher electron-withdrawing ability; (3) enhanced planarity and crystallinity due to
non-covalent interchain interactions; (4) good stability under continuous light illumination
conditions. [31-36]. However, there are only limited studies which emphasizes the detailed opto-
electronic properties of the carboxylate substituted BT polymers have been reported. This is due
to the free rotation of the carboxylate group, which can negatively affect absorption and
molecular packing in the conjugated backbone. [37] Recently, controlling number of substituents
on the acceptor units is considered as one of the effective methods to overcome the planarity
issues arising from insertion of the substituents with the larger size. [17-20,38] For instance, Hu
et al. substituted different number of chlorine (Cl) atom on BT core with an aim of fine tuning
energy levels and aggregation in the resulted polymers. [18] Interestingly, the authors observed
superior PCE of 8.21% for mono-Cl substituted polymer i.e. PCBT4T-2OD, which is
approximately 34% higher than two-Cl counterpart (PCE of 6.12%). This enhancement of PCE
is stemmed in from increase in short-circuit current density (J ) and fill factor (FF), which was
SC
ascribed mainly due to the favorable molecular orientation, larger crystallite sizes and superior
morphology showed by PCBT4T-2OD:PC BM blends. In another case, He and co-workers
7
1
achieved impressive PCEs up to 9.11% with same acceptor, despite of large torsion angle in the
4

ACCEPTED MANUSCRIPT
backbone induced from the inserting Cl atom. [19] Likewise, Zhang et al. synthesized mono-
chlorinated quinoxaline-based copolymer (Cl-PBTQT) based on the similar principle, which
showed PCE of 8.16%. [20] These results clearly suggest that the higher performance can be
achieved even with polymers having non-planar conjugated backbone, unless insertion of
substituents with the larger size is not negatively affecting molecular packing in blend films.
In this report, we synthesized the new acceptor unit namely
benzo[c][1,2,5]thiadiazole-5-carboxylate (BT-Est) by incorporating mono carboxylate
group at the 5-position of dithienyl-flanked BT core, with a goal of achieving lower
HOMO energy levels and easy tuning of solubility in the resulted polymers (Scheme 1).
To check the compatibility of BT-Est, theoretical calculations were first performed using
density functional theory (DFT) at the B3LYP/6-31G (d) level from the program
Gaussian 09. The detailed comparison of the optimized molecular geometries, direction
and magnitude of the dipole moment and HOMO and lowest unoccupied molecular
orbital (LUMO) levels of newly synthesized BT-Est and previously reported BT
derivatives
such
as
5-fluorobenzo[
c][1,2,5]thiadiazole
(BT-F),
and
5-
chlorobenzo[
c][1,2,5]thiadiazole (BT-Cl) are shown in Figure 1 and Table S1.
Interestingly, BT-Est showed not only deeper HOMO and LUMO levels, but also higher
dipole moment than BT, BT-F, and BT-Cl (Figure 1b and c). This indicates that
carboxylate substitution yields a stronger acceptor unit than either F or Cl substitution.
Moreover, higher dipole moments can further aid self-assembly and charge separation in
PSCs. [39-41] Notably, all BT derivatives exhibited similar electronic distributions
(
Figure 1d), signifying that carboxylate functionalization has a minor effect on molecular
5

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orbital distributions. Therefore, BT-Est acceptor unit can be utilized for synthesizing D–A
copolymers having low-lying frontier energy levels and good solubility.
Figure 1. Examples of chemical modification of the 2,1,3-benzothiadiazole (BT) system at the 5-
position: (a) chemical structures; (b) direction and magnitude of the dipole moment; (c)
6

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theoretical HOMO and LUMO energy levels; and (d) spatial LUMO distributions (all data
obtained using Gaussian 09 at the B3LYP/6–31G* level).
Herein, we report the synthesis of BT-Est via mono-carboxylation of BT. To
explore the compatibility of this acceptor in conjugated systems, we polymerized BT-Est
with
two
well-known
benzodithiophene
donor
monomers,
2-
ethylhexylthienylbenzodithiophene and 2-ethylhexylthiothienylbenzodithiophene to
obtain two novel medium bandgap polymers, poly-{2-hexyldecyl 7-(5-(4,8-bis(5-(2-
ethylhexyl)thiophen-2-yl)benzo[1,2-
-yl)benzo[ ][1,2,5]thiadiazole-5-carboxylate} (P1) and poly-{2-decyltetradecyl 7-(5-
4,8-bis(5-((2-ethylhexyl)thio)thiophen-2-yl)benzo[1,2- :4,5- ']dithiophen-2-yl)thiophen-
-yl)-4-(thiophen-2-yl)benzo[ ][1,2,5]thiadiazole-5-carboxylate (P2). We systematically
b:4,5-b']dithiophen-2-yl)thiophen-2-yl)-4-(thiophen-
2
c
(
b
b
2
c
evaluated the structure-property relationship between new acceptor in conjugated system
in detail by analysing and comparing DFT calculations, optical- electrochemical analysis,
molecular ordering, and nanoscale film morphologies in the resultant D–A polymers as
below.
2
. Results and Discussion
The syntheses of the BT-Est monomers and the polymers are shown in detail in
Scheme 1. The key electron-deficient intermediate (
2) [42] and the benzodithiophene
(
BDT) monomers (M3 and M4) [43-44] were synthesized according to previously
reported procedures. On the other hand, compound
3 was prepared via cyclo-addition
between
2
and methyl propiolate in toluene at 110 °C for 36 h. Then, the bromination of
7

ACCEPTED MANUSCRIPT
compound
3
using
N-bromosuccinimide (NBS) afforded compound
4, which was directly
converted to its acidic intermediate (
with different alkyl
5) via hydrolysis. Next, intermediate 5 was reacted
8

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Scheme 1. Synthesis routes for BT-Est acceptor and polymers: (i) thionyl aniline,
chlorotrimethyl silane, pyridine, RT, 3 h (90%); (ii) methyl propiolate, toluene, reflux, 36 h
(70%); (iii) NBS, THF, RT, 16 h (64%); (iv) LiOH, MeOH–THF (1:1), reflux, 16 h (74%); (v)
alcohol, DCC, DMAP, RT, 24 h (80-85%); (vi) Pd (dba) , tris(o-tolyl)phosphine, chlorobenzene,
2
3
microwave (74-77%).
alcohols, using
DMAP) to produce the respective alkylated target monomers M1 and M2 with yields
N,N'-dicyclohexylcarbodiimide (DCC) and 4-dimethylaminopyridine
(
over 80%. Thus, BT-Est minimized the use of hazardous halogenated reagents and
allowed direct tuning of the solubility by modifying alkyl side chains than halogenated
BT derivatives. Finally, the polymers P1 and P2 were obtained by a typical Stille
coupling reaction between corresponding the monomers M1
+ M3 and M2 + M4,
respectively, under microwave conditions. The detailed synthesis procedures are given in
ESI†. The new copolymers showed good solubility in common organic solvents used for
PSCs device fabrication, such as chloroform, toluene, and chlorobenzene (CB). Initially,
P2 having 2-hexyldecyl side chains on the BT-Est was synthesized, since it is well known
that the variation of alkyl side chain length and position affect the optoelectronic
properties, crystallinity, and PSC performances in conjugated polymers. [27,45]
However, because of its poor solubility, a longer alkyl chain (2-decyltetradecyl) was
introduced. The number-averaged molecular weights (Mn) of P1 and P2 were 61.9 and
4
5.6 kDa, respectively, and their polydispersity indices (PDIs) were 1.79 and 2.13,
respectively; estimated using gel-permeation chromatography (GPC) against polystyrene
standards. Both P1 and P2 displayed good thermal stability, with decomposition
temperatures ( d
T
) above 330 °C (at 5% weight loss) at a heating rate of 10 °C/min; the
9

ACCEPTED MANUSCRIPT
values of
T
d
were evaluated by thermogravimetric analysis (TGA) in a nitrogen
atmosphere (Table 1 and Figure S1, ESI†).
The UV–vis spectra of the polymers are shown in Figure 2, and the related optical data
are summarized in Table 1. The BT-Est polymers displayed typical dual absorption
bands, with the main absorption region ranging from 300 to 700 nm, complementary to
non-fullerene
acceptor
,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-
′]dithiophene (ITIC); which would allow them to harvest more photons in PSCs. The
3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-
5
d
:2 ,3 ]- -indaceno[1,2- :5,6-
′
′
-
d
′
s
b
b
first band around 300–400 nm in Figure 2a and b is ascribed to the localized π–π*
transitions in the conjugate backbone, while the second band corresponds to the
intramolecular charge transfer from a donor BDT unit to the BT-Est acceptor. [46]
Compared to solutions, the absorption spectra of P1 and P2 in the thin-film state
displayed ~20–40 nm red shifts, attributed to the
1
0

ACCEPTED MANUSCRIPT
Figure 2. Absorption spectra in (a) a chloroform (CF) solution and (b) the thin-film state,
(
c) cyclic voltammograms, and (d) energy levels of the BT-Est polymers.
1
1

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Table 1 Thermal, optical, and electrochemical properties of the BT–Est polymers.
Thermal
Electrochemical
properties
Optical properties
property
Polymer Mn [kDa]/
a
PDI
λmax
λmax
[nm],
thin film thin film
λonset
[nm],
4
opt
d
ε (10 )
Eg
[eV]
HOMO LUMO
b
Td [°C]
[nm],
solution
−1
−1
c
e
[
M cm ]
[eV]
[eV]
P1
P2
61.9/1.79
45.6/2.13
383
334
592
581
4.24
3.40
614
596
744
724
1.67
1.71
-5.48
-5.54
-3.81
-3.83
a
Number-averaged molecular weight determined by GPC.
b
d
Decomposition temperature (T ) was determined by TGA (with 5% weight loss).
c
The molar extinction coefficient of polymers at 600 nm in CF solution.
d
opt
Estimated values from the UV-vis absorption edge of the thin film (
E
g
= 1240/
λonset, eV).
enhanced planarity and interchain
π-π stacking of polymers. [22,29] Moreover, P1
exhibited an absorption spectrum that is ~20 nm wider than that of P2 due to the stronger
interchain interaction introduced by the shorter alkyl side chains in the polymer backbone.
[
47] However, there was no prominent vibrionic peak was observed in either of the
polymers in the thin-film state. This indicating that the carboxylate group on BT has
negative effect on the polymer packing in the solid state. Using the Beer–Lambert law (
εbc), the molar absorptivity coefficients ( ) of P1 and P2 were calculated to be 42485
A
=
ε
1
2

ACCEPTED MANUSCRIPT
−
1
−1
and 34058 M cm , respectively (Figure 2a). The higher
ε values of P1 would assist in
light harvesting, thus boosting the JSC in PSCs. The optical bandgaps of P1 and P2 were
determined from their absorption onsets were found to be 1.67 and 1.71 eV, respectively.
The frontier energy levels of the polymers were estimated by cyclic voltammetry (CV)
(
Figure 2c and d). The HOMO energy levels of P1 and P2 were determined to be −5.48
and −5.54 eV, respectively, from their onset oxidation potential (Eox) by using the
equation HOMO = −(
ox + 4.8) (eV). [48] The corresponding LUMO levels were found
to be −3.81 and −3.83 eV by subtracting HOMO from the optical bandgap of polymers in
E
opt
g
opt
g
the thin-film state (
E
), i.e., LUMO =
E
− HOMO. Thus, both HOMO and LUMO
energy levels in the conjugated backbone were effectively lowered due to the increased
electron affinity of the BT-Est. At the same time, the frontier energy levels of P2 were
further lowered compared to P1. This change is ascribed to the combined effects of (a)
the steric factor, ascribed to the higher-branched chain hindering planarization and
compact
from the thioalkyl side chains acting as
donating ability of the BDT unit. [49-50] As the open-circuit voltage (
π
-stacking in the polymer backbone [27, 47] and (b) the electronic factor, arising
-electron acceptors and reducing the electron-
OC) in PSC is
π
V
directly proportional to the energy offset between the polymer donor’s HOMO and the
polymer acceptor’s LUMO, both polymers could yield higher VOC in the corresponding
PSCs owing to their lower HOMO energy levels.
Theoretical calculations for the structures of P1 and P2 were performed using DFT to
further understand the impact of the new BT-Est acceptor on the molecular orientation
and energy levels of the polymers (Figure 3). To simplify the computation, alkyl chains
1
3

ACCEPTED MANUSCRIPT
were minimized to methyl chains, and calculations based on two repeating monomers
were
Figure. 3. Optimized geometries, isodensity surface plots, energy levels and map of electrostatic
potential (ESP) surfaces of dimer model molecules of (a) P1 and (b) P2 polymers, calculated
using the Gaussian 09 package at the B3LYP/6–31G* level.
performed. Notably, in Figure 3, no steric hindrance was observed between the BDT units
and the adjacent thiophene for both polymers (the dihedral angle,
and 0.7°, respectively, for P1 and P2). In contrast, a prominent dihedral angle,
was observed between the BT-Est unit and the adjacent thiophene (next to the carboxylate
1
θ , was found to be 2.2°
2
θ , of ~48°
1
4

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substitution), which resulted from the steric effect of the carboxylate group on the BT
unit. The dihedral angles between the P1 and P(BT2Est-BDT) structures were also
estimated to understand the effect of the number of carboxylate substituents on the BT
units (Figure S2, ESI†). Thus, limiting one carboxylate substituent on BT significantly
reduced the steric repulsion between BDT and the adjacent thiophene unit when
compared to their BT-2Est counterparts (θ1 = 2.2° and 48.2°, respectively, for P1 and
P(BT2Est-BDT)). In both P1 and P2, the HOMO energy levels were delocalized over
BDT and the adjacent thiophene, while the LUMO energy levels were concentrated on
the BT-Est acceptor. These HOMO–LUMO energy level trends are in good agreement
with experimental CV results, as shown in Table 1. The electrostatic potential (ESP)
surface images of the both the polymers demonstrated identical potential along the
backbone, expect strong electron withdrawing carboxylate group on the BT-Est core
which has a positive potential (highlighted in dotted line). This suggesting BT-Est has a
strong accepting nature. Consequently, the DFT results indicate that well-defined D–A
type structures were formed in the conjugated backbone, which could facilitate effective
π
-electron delocalization for intramolecular charge transfer.
To evaluate the effect of the new BT-Est acceptors on the photovoltaic performances
of the polymers, PSCs with an inverted configuration (indium–tin oxide (ITO)/zinc oxide
ZnO)/active layer/MoO /Ag) were fabricated. The non-fullerene acceptor ITIC was used
(
3
as the acceptor unit to test the efficiency of the polymers for the following reasons. First,
the energy levels of both polymers are better matched with ITIC than with PC BM,
7
1
which can yield efficient charge separation in PSCs. Second, ITIC exhibits absorption
that is complementary to that of P1 and P2, which would boost SC by utilizing the broad
J
1
5

ACCEPTED MANUSCRIPT
absorption spectrum. Therefore, BT-Est based polymers are expected to show good
compatibility with the ITIC acceptor. At the beginning of the evaluation, the blend ratio
of the polymer–ITIC system, spin-coating speed, solvent additive, and other parameters
were optimized to maximize the efficiency. Figure 4 show the current-density versus
voltage (J–V) and external quantum efficiency (EQE) plots of the optimized devices, and
the corresponding photovoltaic results are summarized in Table 2. The polymers P1 and
P2 exhibited PCEs of 5.8% and 5.3%, respectively with optimized active layer blend ratio
of 1:1 in CB and 0.5 vol% 1,8-diiodooctane (DIO). It is evident that the strong electron-
Table 2 Photovoltaic performance of polymer–ITIC (1:1) devices processed in CB with 0.5
−
2
vol% DIO under AM 1.5G illumination at 100 mW cm .
Annealing
temperature
a
2
PCE
%]
µ [cm
h
a
−2 a
a
Polymer
VOC [V]
J
SC [mA cm ]
FF [%]
−
1 −1 b
[
V s ]
[°C]
–
0.92 (0.90 ± 0.02) 13.3 (13.1 ± 0.2) 47.2 (47.8 ± 0.7) 5.8 (5.6)
0.90 (0.88 ± 0.02) 14.5 (14.2 ± 0.3) 48.9 (48.1 ± 0.8) 6.4 (6.2)
9
0
−5
P1
110
0.90 (0.90)
.90 (0.89 ± 0.01)
14.6 (14.5 ± 0.1) 52.5 (51.9 ± 0.6) 6.9 (6.7) 5.99 × 10
14.4 (14.3 ± 0.1) 51.9 (52.4 ± 0.3) 6.7 (6.6)
0
1
1
30
50
–
0.90 (0.88 ± 0.02) 14.3 (14.5 ± 0.3) 50.6 (51.1 ± 0.4) 6.5 (6.4)
0.98 (0.98) 10.5 (10.1 ± 0.4) 51.4 (50.8 ± 0.6) 5.3 (5.0)
9
0
0.96 (0.95 ± 0.01) 11.2 (10.9 ± 0.3) 52.9 (52.2 ± 0.7) 5.7 (5.4)
0.96 (0.95 ± 0.01) 12.1 (11.4 ± 0.7) 53.1 (53.0 ± 0.1) 6.2 (5.9)
0.80 (0.80 ± 0.02) 10.5 (10.2 ± 0.3) 49.6 (49.4 ± 0.2) 4.2 (4.1)
−5
P2
3.74 × 10
1
1
10
30
Device architecture: ITO/ZnO/active layer/MoO /Ag.
3
a
The average PCE in parentheses was obtained from 6–10 independent devices.
b
3
The hole-only device had the configuration ITO/PEDOT:PSS/active layer/MoO /Au.
1
6

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Figure 4. J–V curves of devices with inverted configurations based on (a) P1–ITIC (1:1), (b)
P2–ITIC (1:1) prepared with different annealing temperatures and processed in a solution of CB
and 0.5 vol% DIO. and, (c) corresponding EQE profiles of the best devices.
withdrawing ability of the BT-Est acceptor enabled both polymers to exhibit high
VOC in
the range of 0.92–0.98 V. Meanwhile, P1 showed a high SC because of broad UV
J
absorption and higher absorptivity. In terms of PSC engineering, method like thermal
annealing have proved to enhance PSC efficiency by improving the nanoscale bulk
heterojunction (BHJ) morphology. [51-53] Thus, we employed the same technique to
1
7

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further improve the PCEs of our polymers. Notably, the PCEs of the polymers were
substantially enhanced as the thermal annealing temperature was increased to 110 °C,
while temperatures above 110 °C had a negative effect (Figure 4a and b; Table 1). The P1
devices thermally annealed at 110 °C for 10 min exhibited the best PCE of 6.9%, with a
−
2
significantly higher JSC of 14.6 mA cm , fill factor (FF) of 52.5%, and a slightly lower
VOC of 0.9 V (average PCE was 6.7% for over 10 devices). The Likewise, the PCE of the
−
2
P2 devices was also improved from 5.3% to 6.2% (
JSC = 12.44 mA cm , FF = 53.1%,
and OC = 0.96 V) after thermal annealing. Details of the optimization of the polymers are
V
provided in Figure S3, S4 (ESI†) and Table S2, S3 (ESI†). The improvement in the PCEs
was primarily due to improved JSC and FF, which indicate thermal annealing benefits the
optimal phase-separated BHJ morphology and effective charge transport between the
active layer and the electrode. [51] Furthermore, the VOC of the P2 devices decreased
dramatically to 0.80V after thermal annealing at 130 °C. These results were attributed to
the poor thermal stability and intramolecular π-π stacking of P2 introduced by 2-decyl
tetradecyl alkyl chains in polymer backbone (can be seen clearly in TGA and XRD
images), which also negatively affected morphological stability P2:ITIC blend films in
PSCs.
The EQE profiles of the best PSCs were compared to understand the JSC variations in
P1 and P2 (Figure 4c). The calculated current from the EQE curve was in good
agreement (within 5% error) with the value of JSC obtained from J–V measurements. The
P1–ITIC device showed a broad spectral response, with absorption in the 400–750 nm
range and maximum EQE reaching 65% at 675 nm, signifying that the complementary
absorption of both P1 and ITIC facilitated effective light harvesting. In contrast, the P2
–
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ITIC device displayed a much poorer EQE response over the entire spectrum, indicating
poor photocurrent generation. [15] These results are well-matched with the absorbance
spectra and the absorption coefficients of the polymers.
As high and balanced charge-carrier mobility will determine the PCE of the PSCs, we
estimated the hole mobility (µ ) and electron mobility (µ ) of the polymer–ITIC blend
h
e
films using the space-charge-limited current (SCLC) method. Hole-only and electron-
only devices were fabricated under optimal device fabrication conditions to estimate the
charge-carrier mobilities; the corresponding device structures and their results are listed in
−
5
Figure S5 (ESI†). The hole and electron mobilities values were found to be 5.99 × 10
−
5
2
−1 −1
−5
−5
2
and 7.73 × 10 cm V s (µ / µ = 0.77) for P1 and 3.74 × 10 and 7.42 × 10 cm
h
e
−
1
−1
V s (µ / µ = 0.50) for P2. The P1 showed a slightly higher hole mobility than P2
,
h
e
possibly because of its higher molecular weight and shorter alkyl chains, which favoured
effective intermolecular interaction in the polymer backbone. [54] As reported in previous
works, high charge mobility can minimize charge-recombination, thereby boosting the
current densities and performances of the photovoltaic devices. [55] Thus, owing to its
broader EQE, higher and more balance balanced charge mobility, PSCs employing P1
exhibited higher JSC and PCE.
To gain a clear understanding of the molecular ordering in the BT-Est based polymers,
X-ray diffraction (XRD) of the pristine polymers were measured. Figure 5a and b
represent the related out-of-plane (OOP) and in-plane (IP) profiles, respectively of the
BT-Est polymers. The XRD patterns of the neat polymer films exhibit prominent lamellar
(
100) peaks in both OOP and IP directions, with corresponding d-spacings of 21.32 and
2.18 Å, respectively, for P1 and 23.11 and 26.59 Å, respectively, for P2. They also
2
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exhibit (010) peaks along the OOP direction at 2
θ
= 23.81 and 25.57°, which correspond
to π–π stacking distances of 3.73 and 3.77 Å, respectively, for P1 and P2. These results
suggest the coexistence of edge-on and face-on crystallites in both pristine P1 and P2
films. Moreover, when compared to P2, P1 displayed shorter distances in both the OOP
and IP directions because of its shorter alkyl chains. Several studies already emphasized
that the shorter π–π stacking distance between polymers will facilitates charge transport
by decreasing the energy barrier for charge hopping. [56] Thus, P1 exhibited higher
crystallinity, which could facilitate effective π-electron transport and charge-carrier
mobility. [16,29] These results are in good agreement with the high charge-carrier
mobility and JSC values observed for the P1–ITIC blend films.
Figure 5. (a) Out-of-plane and (b) in-plane profiles of the XRD patterns obtained from pristine
polymers P1 and P2
.
Finally, the BHJ morphology, charge transfer, and exciton-dissociation behavior of the
polymer–ITIC blends were studied using atomic force microscopy (AFM) and
photoluminescence (PL) spectra to examine their effects on the variation of the photocurrent in
the polymers (Figure 6). It is well known that a smooth BHJ morphology combined with a
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homogeneous intermixed blend will lead to a larger D–A interface area, thus facilitating efficient
exciton dissociation and transport. [16,55] In the AFM height images, the blend films of both
polymers showed nearly flat and smooth morphology, but there was aggregation of distinct
polymer and ITIC phases -
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Figure 6. AFM height (top) and three-dimensional topography images (bottom) of the (a, c) P1
–
ITIC and (b, d) P2–ITIC and (e–h) photoluminescence spectra (PL) of polymers P1 and P2
(excited at 600 nm) and ITIC (excited at 700 nm) films, as well as polymer–ITIC blend films
(excited at 600 and 700 nm), processed under optimized device-fabrication conditions
(polymer/ITIC mixing ratio of 1:1, 110 °C, 10 min).
-
(Figure 6a and b). These macro-phase separations adversely limit the efficient exciton
dissociation by lowering donor/acceptor interfacial area and thereby leading to lower Jsc
,
FF and PCEs. However, P1 blend film exhibits higher root-mean-square (RMS)
roughness (0.98 nm) than P2 blend (RMS roughness = 0.78 nm), indicating the higher
crystallinity of P1 which well-matched with the crystallinity of the polymer as discussed
in XRD studies (Figure 5). The higher roughness will further reduce the internal resistance
and facilitate light absorption via higher light scattering, thereby boosting JSC and PCE of
the PSC as reported previously. [57] To further substantiate these observations, PL
quenching data of the polymer blends were measured as shown in Figure 6e to h. The
pristine P1 and P2 films showed PL peaks around 620–900 nm, whereas ITIC showed
peaks around 720–900 nm. However, after mixing with ITIC, this PL emission was
quenched by about 84.5 and 70%, respectively, for polymers P1 and P2 under optimized
device-fabrication conditions (Figure 6e and f). This emphasizes the better electron
transfer between the P1 and ITIC. [16,29] Similarly, both the blend films were also
excited at 700 nm to quantify the effective hole transfer to the polymer (Figure 6g and h).
Noticeably, the emission was quenched by 90 and 86% for the P1–ITIC and P2–ITIC
blend films, respectively. Consequently, the P1–ITIC blend film has efficient exciton-
dissociation and charge-transport with lower recombination, which benefitting overall JSC
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2

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and PCE in the PSCs. [58] These results agree with morphology and photovoltaic results.
Thus, the major factors that boosted the JSC values of the P1–ITIC blend were determined
to be broad absorption, high absorption coefficient, high charge-carrier mobility, and
effective exciton dissociation.
3
. Conclusion
We synthesized a new electron-deficient acceptor core, BT-Est, by incorporating a
mono alkoxy-carboxylate group at the 5-position of the thienyl-flanked benzothiadiazole
unit via cyclo-addition and its donor–acceptor polymers for application in organic
photovoltaics. The BT-Est polymers demonstrated broad absorption and lower frontier
energy levels simultaneously owing to the introduction of the electron withdrawing
carboxylate substituent to the BT unit. Moreover, the carboxylate group provided an
additional alkylation site, which facilitated easy control over the solubility and
intermolecular interactions of the final polymers. Among the PSCs, the device employing
P1 exhibited high absorption coefficients and hole mobility, shorter intermolecular π–π
stacking distance, and effective exciton dissociation and charge transport rate, resulting in
the best PCE of 6.9%, with high JSC. Although, the PCEs of the BT-Est based PSCs were
moderate compared to current high-PCE non-fullerene solar cells, there remains
substantial scope for further improvement of PCEs by improving the FF and JSC. Further
tests such as optimization of polymer design by employing more appropriate donor units,
tuning alkyl chains to enhance JSC and device engineering like modifying hole and/or
electron layers, optimizing BHJ morphology by methods such as using more appropriate
solvent additives and solvent vapor annealing to improve the FF of these PSCs are
underway. Our results demonstrate that BT-Est acceptor can be used for synthesizing
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versatile D–A conjugated polymers and small molecules for organic electronic
applications.
Acknowledgements
This research was supported by the New & Renewable Energy Core Technology
Program (No. 20153010140030) and Human Resources program in Energy Technology
(
No. 20174010201540) of the Korea Institute of Energy Technology Evaluation and
Planning (KETEP) grant funded by the Ministry of Trade, Industry & Energy, Republic
of Korea. This research was supported by the 2018 KU Brain Pool of Konkuk University.
Appendix A. Supplementary data
1
13
Experimental section, detailed synthesis of intermediates ( H and C NMR), polymers
and additional figures and tables as mentioned in the main text. Supplementary data
related to this article can be found at http://dx.doi.org/~
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