Benzodithiophene and benzotriazole bearing conjugated polymers for electrochromic and organic solar cell applications

Article abstract of DOI:10.1149/2.0201707jes

Herein, alternating donor-acceptor type benzodithiophene and benzotriazole bearing copolymers were synthesized and thieno[3,2-b]thiophene and furan units were incorporated as π-bridges. The application of these polymers for electrochromic and photovoltaic

Full text of DOI:10.1149/2.0201707jes

Journal of The Electrochemical Society, 164 (6) G71-G76 (2017)
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0013-4651/2017/164(6)/G71/6/$37.00 © The Electrochemical Society
Benzodithiophene and Benzotriazole Bearing Conjugated
Polymers for Electrochromic and Organic Solar Cell Applications
Naime Akbasoglu Unlu,a Serife O. Hacioglu,a Gonul Hizalan,a Dilber Esra Yildiz,b
Levent Toppare,a,c,d,e and Ali Cirpana,c,e,f,z
aDepartment of Chemistry, Middle East Technical University, 06800 Ankara, Turkey
bDepartment of Physics, Hitit University, 19030 Corum, Turkey
cDepartment of Polymer Science and Technology, Middle East Technical University, 06800 Ankara, Turkey
dDepartment of Biotechnology, Middle East Technical University, 06800 Ankara, Turkey
e
¨
The Center for Solar Energy Research and Application (GUNAM), Middle East Technical University,
06800 Ankara, Turkey
fDepartment of Micro and Nanotechnology, Middle East Technical University, 06800 Ankara, Turkey
Herein, alternating donor-acceptor type benzodithiophene and benzotriazole bearing copolymers were synthesized and
thieno[3,2-b]thiophene and furan units were incorporated as π-bridges. The application of these polymers for electrochromic
and photovoltaic studies were performed. Spectroelectrochemical studies illustrate that both polymers showed multichromic behav-
ior due to tailoring of polaron bands in visible region. Photovoltaic properties of P2 were performed by conventional device structure.
The best performance device based on P2: PC71BM (1:3, w/w) exhibited the power conversion efficiency of 2.98%, with Voc of
0.74 V, Jsc of 6.31 mA/cm2, and FF of 64%. The carrier mobility of the P2: PC71BM was calculated as 1.37 × 10−3 cm2/V.s via
space-charge-limited current method.
© 2017 The Electrochemical Society. [DOI: 10.1149/2.0201707jes] All rights reserved.
Manuscript submitted March 23, 2017; revised manuscript received April 13, 2017. Published April 27, 2017.
Conjugated polymers have been utilized as active components
for electrochromic devices,1–7 organic solar cells,8–10 light-emitting
diodes,11,12 and field effect transistors.13 Large numbers of conjugated
polymers have been synthesized by combination of a wide range of
electron donor and acceptor groups leading to fine-tuning of optical
and electronic properties.14 Optimization of optoelectronic properties
can be achieved not only by combination of different donor and ac-
ceptor segments but also by the modification of π-bridges in polymer
backbone.
Synthetic Pathway
Synthetic pathways for P1 and P2 are outlined in Figure 1.
By keeping in mind that modification of the alkyl side chains on
a polymer backbone not only enhences the physical properties
such as solubility but also affects intramolecular interactions and
optoelectronic characteristics, such as light absorptivity and charge
transport behavior. We functionalized benzotriazole with branched
alkyl chain to ensure the solubility of polymers.24–26 Synthesis
of 4,7-dibromo-2-(2-octyldodecyl)-2H-benzo[d][1,2,3]triazole (1)
was performed according to previously published procedure.27
4,7-Bis(5-bromothieno[3,2-b]thiophen-2-yl)-2-(2-octyldodecyl)-2H
benzo[d][1,2,3]triazole (TTBTBT-Br) and 4,7-bis(5-bromofuran-2-
yl)-2-(2-octyldodecyl)-2H-benzo[d][1,2,3]triazole (FBT-Br) were
prepared in two steps. In the first step, Stille cross-coupling between 1
and tributyl(thieno[3,2-b]thiophen-2-yl)stannane (2) or between 1 and
tributyl(furan-2-yl)stannane (3) was performed with a Pd(PPh3)2Cl2
catalyst. In the second step, bromination was done with NBS to
synthesize TTBTBT-Br and FBT-Br in moderate yields. Poly-
merization reactions of TTBTBT-Br and FBT-Br were performed
with 2,6-bis(trimethylstannyl)-4,8-bis(2-ethylhexyloxy)benzo[1,2-b:
4,5-bꢀ]dithiophene in the presence of catalytic Pd2dba3 and a ligand
P(o-tolyl) to synthesize P1 and P2, respectively. After purification
of polymers, their solubility was tested in common organic solvents
such as THF, chloroform, chlorobenzene and 1,2-dichlorobenzene.
P2 is readily soluble in these solvents. Although, branched alkyl
chain is anchored on BTz moiety, P1 suffered from solubility and
it showed limited solubility in hot chloroform, chlorobenzene and
dichlorobenzene. Thermogravimetry analysis (TGA) and differential
scanning calorimetry (DSC) were used to assess the thermal stability
and thermal transitions of polymers. Both polymers showed no
significant degradation up to 300◦C. DSC scan indicated that
polymers did not show any thermal transition.
Chalcolophene derivatives are commonly utilized π-bridges in
donor-acceptor (DA) architecture. Among them, thiophene derivatives
are predominantly used compared to other chalcolophene derivates,
i.e furan, selenophene and tellurophene.15 The chalcogen atom has
an impact on bandgap, HOMO-LUMO energy levels, solubility, and
reactivity. Electronegativity of chalcogens is different, thus donation
of electrons into five membered rings and their dipole moment change
depending upon the chalcogen atom. Furan has the highest dipole mo-
ment resulting higher solubility in polar solvents compared to thio-
phene. Therefore, molecular weight of the resulting polymer is high
that affects the film quality of active layer. This is significant for effi-
cient charge generation and collection for organic solar cells (OSCs).
Furthermore, furan also stabilizes HOMO energy level and furan-
containing polymers have higher bandgap compared to thiophene
analogs due to low aromaticity. However, low-lying HOMO energy
is also another important parameter for air stability and achievement
of higher open circuit voltage in solar cell applications. Another in-
teresting property of furan is its production from renewable sources.
Hence, it is counted as “green building block” that makes this block
suitable for large-scale production and low cost applications.16–21
In addition to chalcolophene derivatives, fused aromatic structures
have gained much attention as π-bridges in recent years. Among them,
thieno[3,2-b]thiophene showed interesting optical and electrochem-
ical properties due to its centrosymetric, coplanar and rigid struc-
ture. First of all, it provides red shifted absorption, low bandgap
and high charge mobility compared to thiophene containing poly-
mers due to high delocalization of π-electrons and better intermolec-
ular π-stacking interaction.22,23 Therefore, in this study benzotri-
azole and benzodithiophene containing conjugated polymers with
thieno[3,2-b]thiophene and furan units as π-bridges have been syn-
thesized and their potential use for electrochromic and photovoltaic
application were investigated.
Electronic Properties
Evaluation of frontier orbitals is essential in order to evaluate poly-
mers’ potential for organic solar cell applications. Therefore, HOMO
and LUMO energy level as well as their oxidation and reduction po-
tential were investigated with cyclic voltammetry (CV). Cyclic volta-
mograms of polymers are outlined in Figure 2a and relevant data are
presented in Table I. P1 showed two reversible oxidation and reduc-
tion peaks. Oxidation potentials of P1 were observed at 0.69/0.96 V
with reversible dedoping peaks at 0.60 and 0.89 V. Reduction
zE-mail: acirpan@metu.edu.tr
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Journal of The Electrochemical Society, 164 (6) G71-G76 (2017)
Figure 1. Synthetic pathways of P1 and P2.
potential of P1 was observed at –1.84 V with a reversible dedoping
peak at –1.67 V. HOMO and LUMO energy level P1 were calcu-
lated from HOMO = −(4.75 − Fc + Eoxonset), LUMO = −(4.75 −
Fc + Eredonset) equations and found as −4.96 and −3.11 eV, respec-
tively. P2 showed only reversible oxidation. Oxidation potential of
P2 is observed at 1.03 V with a reversible dedoping peak at 0.84
V. HOMO energy level of P2 was calculated as −5.09 eV. It showed
lower HOMO energy level compared to P1. LUMO energy level of P2
was calculated from HOMO energy level and optical bandgap (2.05
eV) as −3.04 eV. Both polymers have suitable HOMO energy levels
to favor higher Voc values in polymer:PCBM solar cells and a suitable
LUMO energy level for photoinduced charge transfer.
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Journal of The Electrochemical Society, 164 (6) G71-G76 (2017)
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Figure 2. a) Cyclic voltammograms of the P1 and P2 on ITO electrode in 0.1 M TBAPF6, CH3CN solution with a scan rate of 100 mV/s Fc/Fc+ redox couple
was used as internal standard) b) Absorption spectra of P2 and P1 in chlorobenzene solution and in thin film.
to thin film can be ascribed to the enhanced conjugation length and
Table I. Summary of optical and electrochemical properties of P2
and P1.
intermolecular interaction between polymer chains in the solid state
as a result of molecular organization in the thin film to form more
opt
opt
ec
ordered structures. Eg of polymers were calculated from onset of
Eg
HOMO
(eV)
LUMO
(eV)
Eg
(eV)
opt
π-π∗ transitions in thin film and P1 has smaller Eg of 1.93 eV
sol
film
Polymers
λmax
λmax
(eV)
compared to P2 (2.05 eV) due to strong electron donating ability of
thieno[3,2-b]thiophene.
P2
P1
508
547/588
520
544/590
2.05
1.93
−5.09
−4.96
−3.04
−3.11
-
1.85
Spectroelectrochemistry
Optical Properties
Figure 3 shows spectroelectrochemisty of P1 and P2. π–π∗ tran-
sitions of P1, and P2 were observed at 544/590 and 520 nm, respec-
tively. P1 revealed red-shifted absorption compared to that of P2 due
to stronger electron donating nature of thieno[3,2-b]thiophene than
furan. P1 showed purple color in its neutral state and oxidation of
polymer resulted new absorptions at around 815 and 1000 nm that
were intensified due to formation of free charge carriers such as po-
larons and bipolarons. P1 switched between purple and blue at neutral
and bleaching states. Partial oxidation resulted in gray color. Besides,
polymer showed light green in its reduced state. P2 exhibited red color
in its neutral state and upon stepwise oxidation the visible absorption
gradually decreased and a gray color was formed as an intermediate
state due to creation of polaron and bipolaron.30 P2 showed transmis-
sive sky blue in its oxidized state (Figure 3). Both polymers showed
multichromism due to tailoring of polaron bands in visible region.
The absorption profiles and light harvesting potential of polymers
were investigated by UV-Vis spectroscopy in chlorobenzene solution
and in thin films. The UV-Vis absorption spectra of polymer in solution
and film are depicted in Figure 2b and the results are summarized
in Table I. Both polymers exhibited different behaviors in solution
and thin film. P2 showed absorption maximum at 508 nm which
is assigned to π-π∗ transition. It showed 12 nm bathochromic shift
from solution to thin film that is indicative of chain planarization and
strong aggregation of polymer in thin film.28 In addition to this, it
showed a vibronic shoulder in thin film as a consequence of stronger
interchain π–π stacking of the polymer chains in thin film. P1 has
two absorption peaks in solution at around 547/588 nm and it did not
show any redshift although intensity of peak at longer wavelength
is changed. The absence of redshift can be attributed to substantial
aggregation in solution.29 However, the intensity change from solution
Figure 3. UV–Vis–NIR spectra of (a) P2 (b) P1.
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Journal of The Electrochemical Society, 164 (6) G71-G76 (2017)
Figure 4. Percent transmittance changes of (a) P2, (b) P1 film in monomer free 0.1 M TBAPF6/ACN solution at its maximum absorption wavelengths.
Kinetic Studies
Optical contrast and electrochromic switching times of the poly-
mers were determined by sweeping potential between neutral and
oxidized states with a switching time interval of 5 s in 0.1 M
TBAPF6/ACN at two different wavelengths (Figure 4). While the
optical contrast could be defined as the change in percent transmit-
tance between neutral and oxidized states, switching time is the time
required for one full switch. Synopsis of optical contrast and switching
times are summarized in Table II. Both polymers showed moderate
optical contrast and fast switching times at their dominated wave-
lengths. P1 has 18% and 28% optical contrast with switching times
of 0.2 s at 545 and 1520 nm, respectively. Optical contrast of P2 was
15% and 29% at 520 and 1420 nm. Switching time of P2 was 0.4 and
0.2 s at 520 nm and 1420 nm.
Photovoltaic Properties
Although, both polymers posses suitable absorption profiles and
energy levels for OSC fabrication, P1 forms bad quality film, yield-
ing poor photovoltaic performance. Therefore, photovoltaic prop-
erties of P2 were studied. Fabrication of organic solar cell of P2
Figure 5. J-V characteristics of conventional devices based on different ratio
of P2: PC71BM.
was performed with a conventional configuration of ITO/PEDOT:
polymer chain organization to form suitable thin film morphology.31
Weight ratio of P2 to PC71BM was varied between 1:1 and 1:4. The
current density-voltage (J-V) characteristics of OSCs are depicted in
Figure 5 and photovoltaic parameters are presented in Table III. The
optimized weight ratio of P2 to PC71BM is 1:3. Devices fabricated
from the P2: PC71BM ratio of 1:1, 1:2, 1:3 and 1:4 gave PCEs of
1.66%, 1.50%, 2.98%, and 2.18%, respectively. VOC theoretically is
equal to the difference in HOMO energy level of donor and LUMO
energy level of the acceptor. Some loss in VOC was observed due
to interfacial electron–hole pairs located on the DA heterointerface
(Charge transfer state complexes), vacuum level misalignment, poor
dielectric properties, and disorder-induced changes in the hole and
PSS/polymer: PC71BM/Ca/Al to evaluate photovoltaic performance
of P2. PC71BM was selected as the acceptor due to its stronger ab-
sorption in visible region compared to PC61BM. PEDOT:PSS was
inserted as the hole transport layer in this device configuration in
order to smooth the ITO electrode and increase the work function
of ITO to collect holes effectively. Photovoltaic cells were prepared
and tested under 100 mW/cm2 AM1.5G illumination. Optimization
studies of OSC were performed by investigating effect of polymer:
PC71BM ratios, film thicknesses, additives and methanol treatment.
Device fabrication of P2 was performed with low-vapor-pressure
solvent; o-dichlorobenzene (DCB). It was selected due to its good
solvation properties and low evaporation rate which allows time for
Table III. Photovoltaic performance of P2 with PC71BM in
different weight composition.
Table II. Summary of optical contrast and switching times of P2
and P1.
Polymers
Optical Contrast (ꢀT%)
Switching times (s)
BHJ P2: PC71BM
VOC (V)
JSC (mA/cm2)
FF
PCE (%)
P2
15% (520 nm)
29% (1420 nm)
18% (545 nm)
28% (1520 nm)
0.4
0.2
0.2
0.2
1:1
1:2
1:3
1:4
0.73
0.72
0.74
0.73
4.36
4.75
6.31
5.07
52
44
64
59
1.66
1.50
2.98
2.18
P1
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Journal of The Electrochemical Society, 164 (6) G71-G76 (2017)
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Figure 8. SCLC mobilities of P2: PC71BM.
Figure 6. Incident to photon conversion efficiency (IPCE) spectra of
P2:PC71BM (1:3).
more effective scattering compared to polymer due to its high electron
density. It is important to note taking that both blend films showed
larger domains, which limits device performance.
electron quasi-Fermi levels.32–35 The VOC’s of the solar cells are in
the range of 0.72–0.74 V. This means that HOMO energy level of
donor and LUMO energy level of the acceptor are the main factor
for determination of VOC. JSC and FF were significantly affected from
active layer composition. The best device gave a PCE of 2.98%, with
Voc of 0.74 V, Jsc of 6.31 mA/cm2, and FF of 64%. Neither addition
of additive (1,8-diiodooctane) nor methanol treatment improved the
power conversion efficiency of devices.
Incident photon to current efficiency (IPCE) is defined as the ratio
of photocurrent to the incident photons, which are converted to carriers
collected at the electrodes under short circuit conditions. Therefore,
the integration of an IPCE spectrum is proportional to the short circuit
current.36 Hence, IPCE spectra of the P2: PC71BM based OSCs were
investigated to verify the changes in JSC (Figure 6). Photocurrent
responses of both systems started from 340 nm and ended at 700 nm
that is consistent with the absorption spectra of blends. Both P2 and
PC71BM made contribution to photon to electron conversion. It has a
photoresponse with highest IPCE of 16%.
SCLC Mobilities
The space charge limited current (SCLC) model were examined
from current density-voltage characteristics (J–V curve) under dark.
Mobilities were calculated with Child’s law under the trap free SCLCs
situation37–39
9εrθε0μV2
JSCLC
=
8L3
Where J is the current, μ is the charge carrier mobility, ε0 is the
permittivity of free space, εr is the relative permittivity of the mate-
rial, V is the effective voltage, and L is the thickness of the active
layer
SCLC graph is shown in Figure 8 and carrier mobilities are pre-
sented in Table IV. The average carrier mobilities for the blend are
correlated with device performance and highest carrier mobility was
found as 1.37 × 10−3 cm2/V.s.
Morphology
Conclusions
We studied atomic force microscopy (AFM) and transmission elec-
tron microscopy (TEM) to gain deeper insight for film morphologies
of P2: PC71BM (1:3) blends. AFM image is depicted in Figure 7. The
blend film exhibited relatively course phase separation with a root-
mean-square (RMS) of 7.38 nm. TEM images of P2: PC71BM blend
film elucidate PCBM and polymer rich domains. The dark areas in
the TEM images are attributed to PCBM rich domains since it causes
For the design of novel copolymers, benzodithiophene was cou-
pled with benzotriazole and electrochromic and optoelectronic ap-
plications were performed. Furthermore, during polymer design
thieno[3,2-b]thiophene and furan was inserted as π-bridging units. P1
had red-shifted absorption profile and P2 has low-lying HOMO en-
ergy level. Besides both polymers showed multichromism, fast switch-
ing time and moderate optical contrast. OSC fabrication of P2 was
performed with conventional device configuration and optimization
studies like usage of different polymer: PCBM blend ratio, addition
of additives and solvent treatment were performed. Best performance
device based on P2: PC71BM (1:3, w/w) showed the PCE of 2.98%,
with Voc of 0.74 V, Jsc of 6.31 mA/cm2, and FF of 64%. It was also
found that high RMS roughness and larger domains could be the
reason for limiting the performance of solar cell.
Table IV. Summary of SCLC mobilities of P2: PC71BM blends.
BHJ P2: PC71BM
M (cm2/V.s)
1:1
1:2
1:3
1:4
2.48 × 10−4
1.29 × 10−4
1.37 × 10−3
1.21 × 10−3
Figure 7. AFM and TEM images of P2: PC71BM (1:3). Scale bar is 2 μm
(AFM), scale bar is 1 μm (TEM).
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Journal of The Electrochemical Society, 164 (6) G71-G76 (2017)
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