Enhancing the power conversion efficiency of polymer solar cells: Via selection of quinoxaline substituents

Article abstract of DOI:10.1039/c7nj03312e

In this study, three random copolymers including quinoxaline derivatives with different substituent groups, 2,6-bis(trimethylstannyl)-4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene and 4,7-dibromo-2-(2-octyldodecyl)-2H-benzo[d][1,2,3]triazole, wer

Full text of DOI:10.1039/c7nj03312e

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Enhancing Power Conversion Efficiency of Polymer Solar Cells via
Selection of Quinoxaline Substituents
Özge Karagaçtıa, Sevki Can Cevhera, Gonul Hizalana, Serife O. Hacioglua, Levent Topparea,b,c,d, Ali
Cirpana,b,c,e
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
In this study, three random copolymers including quinoxaline derivatives with different substituent groups; 2,6-
bis(trimethylstannyl)-4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene and 4,7-dibromo-2-(2-octyldodecyl)-2H-
benzo[d][1,2,3]triazole were designed and synthesized according to donor-acceptor (D-A) approach. 1H NMR spectroscopy,
UV-Vis absorption spectroscopy, cyclic voltammetry, and gel permeation chromatography were utilized for the
characterization of the polymers. Optical studies were performed in order to investigate absorption behaviors of polymers.
P1, P2 and P3 demonstrate broad visible light absorption behavior with maximum absorption peaks around 374/539,
493/535 and 488/532 nm and their optical band gap values were calculated as 1.74, 1.87 and 1.70 eV respectively. Bulk
heterojunction photovoltaic devices were fabricated using the synthesized polymers as electron donors with fullerene
derivatives where the best power conversion efficiencies were found as 2.03 % for P1: PC71BM (1:3, w:w), 2.22 % for
www.rsc.org/
P2:PC71BM (1:4, w:w) and 1.52
%
for P3: PC71BM (1:2, w:w) with methanol treatment.
an acceptor material in photoactive layer.6,7 Working principle
of an organic solar cell can be divided into five main steps
which include light absorption, generation of bounded
electron-hole pair (exciton), exciton diffusion from donor to
donor-acceptor interface, charge separation and charge
extraction towards the respective electrodes.8,9 BHJ provides
effective exciton dissociation without recombination by
extending the interfacial area between donor and acceptor
material.10 By this means, photovoltaic performance of PSCs
can be enhanced by bicontinuous interpenetrating network.
Undoubtedly that, donor-acceptor approach is another
considerable way to achieve high PCE by lowering the band
gap. Based on this approach, electron rich donor and electron
deficient acceptor units are combined in polymer backbone
and create a new narrow band gap energy that leads to
absorption of longer wavelengths in solar spectrum,
enhancement of optical and electrochemical properties and
also improvement of exciton charge separation and charge
mobility.11,12 With its strong electronegative imine groups,
relatively stable quinoid form and high solubility property
quinoxaline unit is one of the most popular acceptor moiety to
achieve low band gap conjugated polymers.6,13,14 Furthermore,
quinoxaline units offer the possibility of tuning electron
withdrawing character by introducing substituent groups
either on the benzene or on the pyrazine rings.15,16 Moreover,
they can be combined with different electron rich donor
groups such as benzodithiophene (BDT), carbazole and
fluorene to obtain solar cells with a high PCE.17–19 Among these
donor units, BDT demonstrates excellent electron donor
property due to its planar conjugated structure and ability to
generate π–π stacking easily.20 Its benzene coherent thiophene
Introduction
In today’s world, based upon depletion of resources and
increase in demand for energy due to the industrialization,
development of technology and population growth, the
seeking of new energy sources gathered fast. Renewable
energy sources are the solution for both addressing the energy
need and elimination of environmental concerns. Solar power
is the cleanest and the most abundant type of renewable
energy. Due to this, over the past decades, polymer solar cells
(PSCs) have gained great popularity as renewable energy
sources due to their low cost, flexibility, facile processing and
portability1–3 Still, PSCs are not suitable for commercialization
because of the low power conversion efficiency (PCE)
compared to their inorganic counterparts.4 Obtaining high PCE
necessitates low band gap, suitable HOMO and LUMO levels in
order to increase short-circuit current (JSC), open-circuit
voltage (VOC) and high carrier mobility to make hole
transportation easier.5 Bulk Heterojunction (BHJ) is one of the
remedy to increase PCE of PSC’s that includes a composition of
p-type polymer as a donor and n- type fullerene derivatives as
a.Department of Chemistry, Middle East Technical University, 06800 Ankara,
Turkey.
b.Department of Polymer Science and Technology, Middle East Technical University,
06800 Ankara, Turkey
c. Department of Biotechnology, Middle East Technical University, 06800 Ankara,
Turkey
d.The Center for Solar Energy Research and Application (GUNAM), Middle East
Technical University, 06800 Ankara, Turkey
e. Department of Micro and Nanotechnology, Middle East Technical University,
06800 Ankara, Turkey
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out under air. Column chromatography was used for
purification of materials by using Merck Silica Gel 60.
structure provides low-lying highest occupied molecular orbital
(HOMO) level for relevant polymer. Besides, alkyl or alkoxy
substituents assigned to BDT derivatives increase the solubility
of the polymer in different organic solvents.21 When combined
with BDT derivatives, quinoxaline based polymers achieve high
Measurements
17,22,23
Nuclear Magnetic Resonance Spectroscopy technique was
used to confirm chemical structures of both monomers and
polymers. 1H and 13C NMR shifts were taken using Bruker
Spectrospin Avance DPX-400 Spectrometer according to
trimethylsilane (TMS) internal reference in deuterated
chloroform (CDCl3). For polymers, gel permeation
chromatography (GPC) was used to estimate average
molecular weights by taking polystyrene standard as the
PCE as a result of high VOC.
Another widely used acceptor moiety is the benzotriazole (BTz)
that plays an important role in photovoltaic studies due to its
ascendant electrochromic and photovoltaic properties.24 Many
BTz derivatives are proper for single layer electrochemical
devices due to their switching ability between multicolored
and transmissive state.25,26 In addition to its usage in
electrochemical devices, BTz units have high demand in
internal
standard
in
chloroform
(CHCl3).
photovoltaic applications since placing a benzotriazole in
7,27,28
Spectroelectrochemical studies of synthesized polymers were
done by Varian Cary 5000 UV-Vis Spectrometer. For cyclic
voltammetry studies, polymers dissolved in CHCl3 were
sprayed onto indium tinoxide (ITO) coated glass and studies
were done by a Gamry 600 potentiostat in a three electrode
cell consisting ITO-coated glass slide as the working electrode,
Pt wire as the counter electrode, and Ag wire as the reference
electrode. Measurements were collected in a solution of 0.1 M
polymer chain results with a low-lying LUMO level.
They
demonstrate strong electron transporting ability thanks to
electron withdrawing nature of imine bond. In addition,
solubility of final polymer product can be enhanced by long
alkyl chain substitution on nitrogen position of BTz.29 Solubility
of a polymer is a crucial parameter for not only to ease of
purification steps but also to produce high quality thin films for
device applications.30 Finally, easy modification of N-H bond of
BTz unit provides the improvement of structural and electronic
properties for good processing ability BTz containing
polymers.31
of
tetrabutylammonium
hexafluorophosphate
(TBAPF6)/acetonitrile (ACN) solution at a scan rate of 100
mV/s.
In this study, three quinoxaline moieties with different
substitutents were chosen to combine with BTz and BDT units
to achieve broad visible light absorption. Broad light
absorption can be achieved by covering a considerable part of
visible spectrum. In literature, benzodithiophene and
quinoxaline based copolymers demonstrate maximum
absorption values around 600-730 nm.32–35 Also, copolymers
including benzotriazole and benzodithiophene derivatives
reveal a maximum absorption at around 500 nm.36,37 Addition
of benzotriazole unit to quinoxaline- benzodithiophene
containing polymer backbone atones the lack of absorption at
green part of visible region. Herein, 4,7-dibromo-2-(2-
Syntheses
9-(Bromomethyl)nonadecane,
4,7-dibromo-1H-
benzo[d][1,2,3]triazole,
benzo[d][1,2,3]triazole,
4,7-dibromo-2-(2-octyldodecyl)-2H-
5,8-dibromo-2,3-di(thiophen-2-
yl)quinoxaline, 5,8-dibromo-2,3-diphenylquinoxaline, 10,13-
dibromodibenzo[a,c]phenazine with some modifications were
synthesized according to literature reports.38,39
Synthesis of P1. For the synthesis of P1 Stille coupling reaction
procedure was followed by putting together 4,7-dibromo-2-(2-
octyldodecyl)-2H-benzo[d][1,2,3]triazole (4) (0.246 g, 0.442
mmol) and 5,8-dibromo-2,3-di(thiophen-2-yl)quinoxaline (5)
(0.200 g, 0.442 mmol) and 2,6-bis(trimethyltin)-4,8-bis(2-
ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene (0.653g, 0.884
mmol). After exposing monomer blend to argon gas, reactants
were dissolved in 10 mL dry tetrahydrofuran (THF) under
argon atmosphere. Reaction solution was bubbled with argon
for 40 minutes to remove air. After, catalytic amount of
tris(dibenzylideneacetone)dipalladium(0) (0.040 g, 0.044
mmol) (5% of benzodithiophene unit) and tris(o-
tolyl)phosphine (0.027g, 0.088 mmol) as the co-catalyst (10%
of benzodithiophene unit) were added to the medium and
reaction was refluxed at 70°C for 39 hours. Before addition of
bromothiophene (0.216 g, 1.327 mmol), catalyst
tris(dibenzylideneacetone)dipalladium(0) (0.020g, 0.022 mmol)
and co-catalyst tris(o-tolyl)phosphine (0.013 g, 0.044 mmol)
were put into the solution and then reaction was stirred for 6
hours. Later, trimethyl(thiophen-2-yl)stannane (0.990 g, 2.654
mmol) was added with same amount of extra catalyst
mentioned previously. Another 6 hours were spent before
reaction was stopped and cooled to room temperature.
octyldodecyl)-2H-benzo[d][1,2,3]triazole
bis(trimethyltin)-4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-
b′]dithiophene were randomly polymerized with 5,8-dibromo-
2,3-di(thiophen-2-yl)quinoxaline, 5,8-dibromo-2,3-
and
2,6-
diphenylquinoxaline and 10,13-dibromodibenzo[a,c]phenazine
to provide broad visible light absorption and enhance power
conversion efficiency of PSCs.
Experimental
Materials
All chemicals and reagents used in the syntheses of monomers
and polymerization processes were purchased from Sigma
Aldrich Chemical Co. Ltd. Metallic sodium and benzophenone
were used for drying tetrahydrofuran (THF) and toluene.
Besides these, solvents were used without any purification or
drying steps. PC71BM was purchased from Solenne for solar
cell application. Moisture and air sensitive procedures were
done under nitrogen atmosphere and the others were carried
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Solvent was evaporated under reduced pressure and reaction catalyst tris(o-tolyl)phosphine were put into the solution.
was precipitated in cold methanol. After, solid residue was Then, reaction was stirred for hours. After,
6
filtered by vacuum and purification was performed by Soxhlet trimethyl(thiophen-2-yl)stannane (0.508 g, 1.362 mmol) was
apparatus. Respectively, acetone, hexane and chloroform were added with extra catalyst. Another 6 hours were consumed
used to discard oligomers, small molecules, catalyst and other before reaction was stopped and cooled to room temperature.
reactants. Polymer was collected by chloroform. After Solvent was evaporated under reduced pressure and reaction
evaporation of the solvent, polymer was precipitated in cold was precipitated in cold methanol. After, solid residue was
methanol again and vacuum filtration was used to obtain pure filtered by vacuum and purification was performed by Soxhlet
product. (0.520 g, 74 % yield)
apparatus. Polymer was collected by chloroform. After
evaporation of solvent, residue was precipitated in cold
methanol again and pure polymer was achieved by filtration.
(GPC: Mn: 17.7 kDa, Mw: 35.0 kDa, PDI: 1.9)
Synthesis of P2. Same procedure with P1 was pursued for the (0.311 g, 86 % yield)
synthetic pathway of P2. Thus, 4,7-dibromo-2-(2- (GPC: Mn: 14.0 kDa, Mw: 23.2 kDa, PDI: 1.7)
octyldodecyl)-2H-benzo[d][1,2,3]triazole (5) (0.126 g, 0.227
mmol), 5,8-dibromo-2,3-diphenylquinoxaline (6) (0.100 g, Fabrication and characterization of polymer solar cells.
0.227
mmol)
and
2,6-bis(trimethyltin)-4,8-bis(2- Basically a BHJ polymer solar cell includes the layers of ITO,
ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene (0.351 g, 0.454 PEDOT:PSS, Polymer:PC71BM, LiF and Al. ITO glass substrates
mmol) were mixed in reaction medium and kept under argon that purchased from Visiontek Systems were etched by acid
before addition of 8 mL of anhydrous tetrahydrofuran (THF). solution and cleaned with toluene, Hellmanex as the
Reaction solution was bubbled with argon for 40 minutes to detergent, water and isopropyl alcohol in ultrasonic bath for
remove
air.
After,
catalytic
amount
of 15 min. After drying with N2, oxygen plasma cleaning step was
tris(dibenzylideneacetone)dipalladium(0) (0.021 g, 0.022 done with Harrick Plasma Cleaner to remove residual organic
mmol) and tris(o-tolyl)phosphine (0.014 g, 0.045 mmol) as the impurities on ITO. Then, poly(3,4-ethylenedioxythiophene)-
co-catalyst were added to the medium and reaction was poly(styrenesulfonate) (PEDOT:PSS) (Heraus V P Al 4083) was
refluxed at 70°C for 34 hours under argon atmosphere. Before coated on clean ITO surface via spin coating technique.
addition of bromothiophene (0.111g, 0.682 mmol), catalytic Substrates were heated to 135˚C for 15 min to remove residual
amount of tris(dibenzylideneacetone)dipalladium(0) and co- water. Polymer-PC71BM blends with different weight ratios
catalyst tris(o-tolyl)phosphine were put into the solution. were prepared and coated with using spin coating under N2
Then, reaction was stirred for
6
hours. Later, atmosphere. Then, LiF and Al were thermally evaporated on
trimethyl(thiophen-2-yl)stannane (0.508 g, 1.362 mmol) was active layer as the electron transport layer and the cathode
added with extra catalyst. Another 6 hours were consumed and respectively. Keithley 2400 source meter was used in
before reaction was stopped and cooled to the room order to record J-V curves under AM 1.5 G illumination.
temperature. Solvent was evaporated under reduced pressure
and reaction was precipitated in cold methanol. After, solid
Results and Discussion
residue was filtered by vacuum and purification was
performed by Soxhlet apparatus. Polymer was collected by Optical Studies
chloroform. After evaporation of solvent, residue was
Absorption spectra of all three polymers in both thin film and
precipitated in cold methanol again and pure polymer was
chloroform solution are illustrated in Figure 1 and results are
achieved by filtration. (0.296 g, 85 % yield)
summarized in Table 1. P1, P2 and P3 demonstrated broad
(GPC: Mn: 49.6 kDa, Mw: 149.5 kDa, PDI: 3.0)
visible light absorption as expected. Copolymers P1, P2 and P3
Synthesis of P3. Same procedure for P1 and P2 was applied for
showed two absorption maxima that includes one in a short
wavelength zone due to π-π* transition of conjugated polymer
the synthetic pathway of P3. Thus, 4,7-dibromo-2-(2-
octyldodecyl)-2H-benzo[d][1,2,3]triazole (4) (0.127 g, 0.228
backbone and the other in long wavelength zone due to
intramolecular charge transfer from donor to acceptor units.40
mmol), 10,13-dibromodibenzo[a,c]phenazine (7) (0.100 g,
0.228
mmol)
and
2,6-bis(trimethyltin)-4,8-bis(2-
Maximum absorption values of P1, P2 and P3 thin films were
recorded as 374/539, 493/535 and 488/532 nm respectively.
All polymers have similar donor and acceptor groups but they
were synthesized via random copolymerization. Thus, all have
different molecular weights and also the number and order of
the repeating units are different. Actually P2 also has a peak at
around 400 nm but the peak height is smaller compared to P3.
ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene (0.352 g, 0.456
mmol) were mixed in reaction medium in 8 mL of anhydrous
tetrahydrofuran (THF). Reaction solution was bubbled with
argon for 40 minutes to remove air. After, catalytic amount of
tris(dibenzylideneacetone)dipalladium(0) (0.021 g, 0.022
mmol) and tris(o-tolyl)phosphine (0.014 g, 0.045 mmol) as the
co-catalyst were added to the medium and reaction was
refluxed at 70°C for 44 hours under argon atmosphere. Before
addition of bromothiophene (0.111 g, 0.682 mmol), catalytic
amount of tris(dibenzylideneacetone)dipalladium(0) and co-
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Scheme 1. Synthetic procedure of monomers and polymer
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Although all polymers have absorption peaks at 400, 500 and energy levels are crucial parameters for solar cell application.
550 nm the intensity of those are different. The colors of To achieve a successful hole transfer in solar cell application,
polymers also support the explanation. Also, λmax values of P1, HOMO level of the donor unit should be higher than that of
P2 and P3 in solution were observed as 369/530, 491/534, acceptor. All polymers show higher HOMO levels when
486/529 nm correspondingly. Maximum absorption peaks of compared with that of PC71BM which has a -6.10 eV HOMO
P1, P2 and P3 in thin film were nearly identical to the peaks in energy level.43 As seen, P1 has the highest lying HOMO level
solution state that were red shifted by only 9, 1 and 3 nm that may lead lower power conversion efficiency via lowering
respectively. Intermolecular stacking and conformational VOC value. With the replacement of thiophene substituent by
alterations cause such shifts in the spectra .41 Red shift in thin benzene, lower HOMO level than P1 was obtained with P2. P2
film form can be explained by decrease in conformational has the lowest lying HOMO level which can provide high VOC.
freedom, aggregation or due to interaction between solvent Also, LUMO level of the polymer is another crucial criterion for
and polymer chains.42 Such low differences between thin film polymers in order to perform a good charge separation. For an
and solution state attest the shortage of aggregation and weak effective charge separation, energy difference between LUMO
solvent-polymer interaction. Apparently, polymers made level of donor unit and LUMO level of acceptor should be
reunion in thin film state and showed pre-aggregates in around 0.3 eV.44 According to CV studies, LUMO levels of
solution because of the strong inter chain interactions.
polymers were found as -3.45, -3.47 and -3.63 eV for P1, P2
and P3. All of the polymers will show effective charge transfer
to PCBM.
Table 1. Summary of the optical studies in chloroform solution and thin film form
Solution λmax
(nm)
Thin Film λmax
(nm)
P1
P2
P3
530
534
529
539
535
532
Electrochemical studies
Cyclic voltammetry (CV) studies were performed in order to
examine redox potentials and HOMO-LUMO energy levels of
three random polymers. For measurements, copolymers were
dissolved in CHCl3 (5mg/mL) and spray coated on ITO coated
glass substrates. CV studies were carried out in a three
electrode system. Recorded electrochemical data of all
polymers were obtained from cyclic voltammograms as
indicated in Figure 2 and summarized in Table 2. All polymers
have ambipolar character as shown in the Figure. Oxidation
potentials of the polymers (p-doping) were recorded at 1.10 V,
1.08V and 1.00V for P1, P2 and P3 respectively. n-type doping,
potentials of P1, P2, and P3 were recorded at -1.50V/-1.95V, -
1.43V/-1.93V and -1.48V/-1.91V in return. When comparing all
three polymers in terms of their oxidation-reduction
potentials, as expected P3 demonstrated lower oxidation
potential due to high electron density of its fused phenyl
structure compared to P2. Although thiophene unit has more
electron donating ability and P1 is expected to perform
oxidation and reduction processes easier than benzene
substituted P2, oxidation potential of P1 is nearly the same
with P2. This result can be attributed to the low conjugation
length of P1 when compare to its counterparts.
Figure 1. Absorption spectra of P1, P2 and P3 in solution and thin film
Electronic band gap of random copolymers were calculated by
the differences of HOMO and LUMO levels. From the HOMO-
LUMO equation, electronic band gaps of polymers were
calculated as 1.70, 2.05, and 1.72 eV, respectively. It is a
common result that Egel of a conjugated polymer is higher than
op
Eg since charge species such as polarons can locate on
HOMO and LUMO energy levels were estimated by taking
normal hydrogen electrode value as -4.75 eV. HOMO-LUMO
energy levels were calculated using equations of HOMO= -
(4.75 + E onset) and LUMO= - (4.75 + E onset). HOMO energy
polymer chain and some energy barriers may occur at
electrodes during charge transfer. Although P2 and P3 follow
this usual trend, P1 demonstrates higher optical band gap
value compared with its electronic band gap. Such an unusual
result can be explained by uncompleted oxidation of the
ox
red
levels of random copolymers were calculated as -5.15 eV, -5.52
eV and -5.35 eV for P1, P2 and P3 correspondingly. HOMO
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Table 2. Electrochemical Properties of P1, P2 and P3
el
Ep-doping
(V)
Ep-dedoping
(V)
En-doping
(V)
En-dedoping
(V)
HOMO
(eV)
LUMO
Eg
(eV)
-3.45
-3.47
-3.63
(eV)
P1
P2
P3
1.10
1.08
1.00
0.32
0.73
0.50
-1.50/-1.95
-1.43/-1.93
-1.48/-1.91
-1.28/-1.50
-1.36/-1.53
-1.15/-1.52
-5.15
-5.52
-5.35
1.70
2.05
1.72
polymer during doping procedure. P3 shows lower electronic A conductivity value of 0.01 S/cm is measured for a free
band gap than P2 due to its fused phenyl ring structure that standing film of 30 microns thickness. However this value does
enhances planarity and makes easier the electron not refer to that of the component used in the device
transportation with its two connected benzene ring. P1 with construction. The differences between HOMO, LUMO and
thiophene bearing quinoxaline unit has lower band gap than electronic band gap values of polymers are mainly because of
P2.
the change in electron density and donating abilities of side
Band gap of a polymer is decreased with the quinoid groups. It can be concluded that conjugated substituent
resonance stabilization.3 Thiophene unit can generate quinoid groups certainly affects the electronic properties of the
form easier than the benzene ring due to the low aromatic polymers.
resonance stabilization which resulted with low band gap of Spectroelectrochemical Studies
P1.45
For spectroelectrochemical studies, external potential was
applied to the polymer film stepwise and spectra are obtained
from UV–Vis–NIR spectroscopy as illustrated in Figure 3.
Potential was applied between 0.0 V and 1.2 V for P1 and P2,
0.0 V and 1.1 V for P3 steadily. In order to obtain accurate
neutral state absorption, polymer thin films were exposed to a
constant potential to remove any trapped charge or ion before
the measurements. Dual maximum absorption wavelengths
were recorded for all polymers at 374/539, 493/535 and
488/532 for P1, P2 and P3 respectively. During the p type
doping process, while the absorption in visible zone started to
reduce due to the formation of polarons and bipolarons which
are the charge carriers on polymer chain, new absorption
bands at high wavelengths arouse. The peaks around 1600 nm
in Figure 3c appeared due to instrumental noise, unfortunately
common for those wavelengths. Optical band gap (Egop) values
of P1, P2 and P3 were calculated from the onsets of lowest
op
π-π* transitions according to Eg =1241/λonset and found as
1.74 eV, 1.87 eV and 1.70 eV correspondingly as illustrated in
Table 3. Relatively low band gap of thiophene based polymer
provides opportunity to make high absorption at high
wavelength region of solar spectrum.32 Aim of having a broad
visible absorption was successfully achieved by covering red,
blue and green region of spectrum.
Table 3. Spectroelectrochemical results of P1, P2 and P3
op
λmax (nm)
E
(eV)
g
P1
P2
P3
374/539
493/535
488/532
1.74
1.87
1.70
Figure 2. Single-scan cyclic voltammograms of polymer films in 0.1
TBAPF6/ACN electrolyte solution a) P1 b) P2 c) P3
M
On the other hand, the donor group on polymer backbone
affects the absorption spectra, band gap value and all optical
6 | J. Name., 2012, 00, 1-3
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Figure 3. UV-Vis-NIR spectra and colors at different potentials of a) P1 , b) P2 and c) P3 in 0.1 M TBAPF6 /ACN electrolyte solution
properties of polymers. In literature, random copolymer of Kinetic Studies
CoP1, CoP2 and CoP3 published by our group, including
Kinetic studies were performed in order to investigate
switching times and optical contrast for copolymers. Optical
contrast basically is the percent transmittance change at a
given wavelength and switching time is the time required
switching between neutral and oxidized states. For kinetic
studies, polymer coated ITO glass was exposed to sweeping
potential between the two extreme states with 5 s time
intervals in 0.1 M TBAPF6/ACN.
thiophene unit as donor and same quinoxaline and
benzotriazole units as acceptor demonstrate lower optical
band gap values compared to P1, P2 and P3.38
Benzodithophene unit has a more electron donor character
with its benzene fused thiophene structure and also offers
longer conjugation length, thus polymer synthesized in this
study are expected to show lower band gap compared to
those of CoP1, CoP2 and CoP3. The underlying reason behind
this unexpected result can be the stronger π stacking ability of
small thiophene unit, that reduces steric hindrance and
enhance interchain π-π stacking for effective charge carrier
mobility.46 Also, because of the differences during the
polymerization processes, P1, P2 and P3 could have lower
solubility than their counterparts and low solubility of
polymers may cause low conjugation length that results with
lower band gaps.
Table 4. L, a, b values of P1, P2 and P3
P1
-2.1 V
0.0 V
1.1 V
1.2 V
L
47
20
22
26
a
b
-4
0
6
3
5
3
5
-3
P2
-2.0 V
0.0 V
1.0 V
1.2 V
L
a
b
47
-4
-3
21
6
-3
40
5
-1
44
2
0
For the scientific expression of colors, colorimetric
measurements of the thin film state polymers were performed
at all stages of the oxidation process and colors of polymers
were identified using CIE (Commission Internationale de
l’Eclairage) coordinates. CIE coordinate system comprises
three components including luminance (L), hue (a), and
saturation (b). Colors of natural and oxidized-reduced state of
polymers are demonstrated in Figure 3 and colorimetric
studies results including L, a, b values are summarized in
Table 4.
P3
-2.1 V
0.0 V
0.9 V
1.1 V
L
a
b
51
-5
2
12
6
2
17
4
5
23
4
2
Photovoltaic Studies
Photovoltaic properties of synthesized polymers were
investigated using conventional device structure of
ITO/PEDOT:PSS/Polymer: PC71BM/LiF/Al. Polymers P1, P2 and
P3 were used as the donor materials and blended with PC71BM
which was used as the acceptor material. Solvent
optimizations were carried out and 1,2 dichlorobenzene (o-
DCB) processed PCSs showed superior performance compared
to the ones with chlorobenzene. In all optimizations, polymer
concentration was fixed to 15 mg/mL. Current density-voltage
characteristics of the devices were tested under illumination of
As seen in Figure 3, all polymers exhibit different tones of
purple color in the neutral states, and they turn to gray in their
oxidized states with different intermediates. While P1 and P2
have darker purple and gray in the corresponding states, both
tones are lighter for P2 which can be explained with the higher
molecular weight and lower solubility of P2. All polymers have
ambipolar behaviour and show grayish blue color in their
reduced states.
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Table 5. Optical contrast and switching time values of P1,P2 and P3
Optical contrast
(ΔT %)
Switching times (s)
43
1080 nm
1.7
1.4
1.3
P1
P2
P3
52
11
760 nm
540 nm
35
42
31
1470 nm
750 nm
536 nm
0.9
1.2
2.4
51
53
13
1700 nm
760 nm
534 nm
0.7
1.1
2.9
extractions.49,50 Also, reason behind this increment could be
thanks to the optimization of the phase separation and
improvement of the interface between active layer and
PEDOT:PSS.51 Another explanation states that solvent
treatment causes to enhancement of the surface charge
density and the built in voltage by eliminating the traps on
surface of the solar cell devices.52
Figure 4. Percent transmittance changes of a) P1 b) P2 c) P3 in 0.1 M TBAPF6
ACN electrolyte solution at given wavelengths
/
AM 1.5 G with a power of 100 mW/cm2. The J–V curves of the
PSCs were shown in Figure 5. In addition, polymer: PC71BM
weight ratio optimizations were carried out.
As depicted in Table 6, 1:3, 1:4 and 1:2 polymer:PC71BM
weight ratios were found as optimum weight ratios for P1, P2
and P3 based PSCs, respectively. Due to its deepest HOMO
energy level P2 incorporated PSCs showed highest VOC values
which refer to the energy difference between HOMO of the
polymer and the LUMO of the PC71BM.47,48 With MeOH
treatment, PCE of PSCs were improved from 1.80% to 2.03%,
1.49% to 2.22% and 1.37% to 1.52% for P1, P2 and P3
incorporated PSCs, respectively. In addition to these
optimization studies, thermal annealing and addition of DIO
addition were carried out, however no improvement was
observed.
Although exact mechanism of the enhancement by MeOH
treatment could not explained yet, many different approaches
were developed to clarify this phenomenon. One of them
states that by MeOH treatment interface dipoles are
generated which reduce the injection barriers by increasing
the vacuum level of cathode and this situation improves the
contact between active layer and the cathode material. Then,
PCE is improved by enhancing charge transportations and also
Figure 5. Current density-voltage curve of devices with P1, P2 and P3
8 | J. Name., 2012, 00, 1-3
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Polymer:PCBM
VOC
(V)
JSC
VMP
(V)
JMP
(mA/cm2)
FF
PCE
(%)
Treatment
Thickness
(nm)
(w:w)
(mA/cm2)
(%)
P1 (1:2)
0.69
0.69
0.70
0.71
4.22
7.68
7.85
6.74
0.47
0.41
0.43
0.41
2.49
4.39
4.72
4.23
40
34
37
36
1.16
1.80
2.03
1.72
-
130
138
138
142
P1 (1:3)
P1 (1:3)
P1 (1:4)
-
MeOH
-
P2 (1:1)
P2 (1:2)
P2 (1:3)
P2 (1:4)
P2 (1:4)
0.76
0.74
0.75
0.75
0.78
3.08
3.09
3.56
4.00
4.74
0.47
0.52
0.52
0.55
0.62
1.92
2.21
2.63
2.72
3.61
38
50
51
50
60
0.89
1.15
1.36
1.49
2.22
-
102
106
110
113
123
-
-
-
MeOH
P3 (1:1)
P3 (1:2)
P3 (1:3)
P3 (1:2)
0.73
0.73
0.73
0.76
2.97
3.90
3.73
4.08
0.48
0.49
0.49
0.51
1.67
2.80
2.71
2.99
37
48
49
49
0.80
1.37
1.33
1.52
-
115
113
102
124
-
-
MeOH
Table 6. Summary of the photovoltaic studies of P1,P2 and P3
Although P2 has the highest band gap value among them, its
photovoltaic device has the highest PCE value of 2.22 % which
could be due to the molecular weight that enhances
intermolecular charge hoping process by increasing polymer
chain length.53 This phenomenon could also be the reason for
the low PCE of P3 which has a lower molecular weight
compared to P1 and P2. For P1, the best PCE result was
calculated as 2.03 % for P1: PC71BM (1:3, w:w) with VOC value
of 0.70 V and JSC value of 7.85 mA/cm2 via applying methanol
treatment. Fill factor value for the best device is 37 % for P1.
For P2, best PCE result was obtained as 2.22 % from the device
of P2:PC71BM (1:4, w:w) with MeOH treatment. VOC value is
0.78 V, JSC is 4.74 mA /cm2 and FF is 0.60 for this device
configuration. For P3 highest device performance was
recorded with P3:PC71BM (1:2, w:w) ratio with MeOH
treatment as 1.52 %. VOC, JSC and FF values were estimated as
0.76V, 4.08 mA/cm2 and 49 % respectively. A previous study in
2016 shows that benzotriazole, thiophene and benzene
containing quinoxaline bearing random copolymer (CoP2)
based PSC achieved the highest PCE value of 1.72 %. PCE was
improved from 1.72 % to 2.22 % due to enhancement of VOC
value. The reason behind this increment could be the BDT unit
that provides low-lying HOMO level, which enhances the VOC
and also PCE.54,55
Figure 6. IPCE curves of P1, P2 and P3 based solar cells
(TEM) were utilized in order to get information on
morphology of the devices. For this purpose, AFM and TEM
analyses of active layers showing best performance were
conducted and images are shown in Figure 7. Root mean
square roughness (Rms) of the active layers were measured
from active layers of P1: PC71BM (1:3, w:w), P2:PC71BM (1:4,
w:w) and P3:PC71BM (1:2, w:w) as 1.54 nm, 5.42 nm and 2.15
nm, respectively. Also, thicknesses of the best performance
active layers were estimated as 138 nm for P1: PC71BM (1:3,
w:w), 123 nm for P2:PC71BM (1:4, w:w) and 124 nm for
P3:PC71BM (1:2, w:w). The darker areas in TEM images
correspond to the PCBM rich areas while lighter ones are
contributions of the polymer rich areas.
Incident photon to current efficiency (IPCE) measurements
were carried out for the best performance solar cells. 43.72%,
21.60% and 19.51% IPCE values were obtained for P1, P2 and
P3 incorporated solar cells. Corresponding IPCE curves are
depicted in Figure 6.
FF is a degree of rectangularity of the current density vs.
voltage curve. It mainly depends on the morphology of the
active layer. For a high fill factor device, interpenetrated
bicontinous nanoscale network active layer morphology is
It is a well known fact that morphology of the polymer-
fullerene blend affects the PCE of solar cell devices.56 Atomic
force microscopy (AFM) and transmission electron microscopy
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required. As seen in TEM and AFM images absence of were calculated as -5.15, -5.52 and -5.35 eV while their LUMO
interpenetrated donor acceptor morphology may be the levels were -3.45, -3.47 and 3.63 eV respectively. Electronic
reason of low fill factor values obtained in P1, P2 and P3 based band gaps were calculated as 1.70, 2.05, and 1.72 eV for P1, P2
solar cells.
and P3. In the light of these information, bulk heterojunction
type solar cells were successfully constructed and due to the
broad visible light absorption character of the copolymers,
best results were recorded as 2.03 % for P1 with P1: PC71BM
(1:3, w:w) ratio, 2.22 % for P2:PC71BM (1:4, w:w) ratio and
1.52 % for P3 with P3: PC71BM (1:2, w:w) ratio. Comparing
with literature examples, P1, P2 and P3 demonstrate high VOC
values for locating the BDT unit on the polymer backbone.
Among them, highest PCE was achieved for benzene
substituted quinoxaline based P2 with a high VOC and FF values
which makes this quinoxaline unit a good candidate for
incorporating this unit with different donors for further
studies.
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Improvement of power conversion efficiency of polymer solar cells was successfully achieved by
selecting proper quinoxaline substituents on polymer backbone.