Synthesis of bistriphenylamine- and benzodithiophene-based random conjugated polymers for organic photovoltaic applications

Article abstract of DOI:10.1002/pola.28752

In this study, donor–acceptor random polymers containing benzotriazole acceptor and bistriphenylamine and benzodithiophene donors, P1 and P2, were successfully synthesized by Stille coupling polymerization. The effect of bistriphenylamine moiety and thiophene π-conjugated linker on electrochemical, spectroelectrochemical, and optical behaviors of the polymers were investigated. Optoelectronic properties and photovoltaic performance of the polymers were examined under the illumination of AM 1.5G, 100 mW?cm^?2. The polymers were characterized by cyclic voltammetry, UV-Vis-NIR absorption spectroscopy, gel permeation chromatography. HOMO/LUMO energy levels of P1 and P2 were calculated as ?5.47 eV/–3.41 eV and ?5.43 eV/–3.27 eV, respectively. Bulk heterojunction type solar cells were constructed using blends of the polymers (donor) and [6,6]-phenyl C₇₁ butyric acid methyl ester (PC₇₁BM) (acceptor). Photovoltaic studies showed that the highest power conversion efficiency of these photovoltaic devices were recorded as 3.50% with open circuit voltage; 0.79 V, short circuit current; 9.45 mA?cm^?2, fill factor; 0.53 for P1:PC₇₁BM (1:2, w/w) in 3% o-dichlorobenzene (o-DCB) solution and 3.15% with open circuit voltage; 0.75 V, short circuit current; 8.59 mA?cm^?2, fill factor; 0.49 for P2:PC₇₁BM (1:2, w/w) in 2% chlorobenzene (CB) solution.

Full text of DOI:10.1002/pola.28752

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Synthesis of Bistriphenylamine- and Benzodithiophene-Based Random
Conjugated Polymers for Organic Photovoltaic Applications
1
1
1
1
Asli Cetin, Cagla Istanbulluoglu, Serife Ozdemir Hacioglu, Sevki Can Cevher,
1
,2,3,4
1,2,4,5
Levent Toppare,
Ali Cirpan
1
Department of Chemistry, Middle East Technical University, Ankara 06800, Turkey
2
3
4
5
Department of Polymer Science and Technology, Middle East Technical University, Ankara 06800, Turkey
Department of Biotechnology, Middle East Technical University, Ankara 06800, Turkey
The Center for Solar Energy Research and Application (GUNAM), Middle East Technical University, Ankara 06800, Turkey
Department of Micro and Nanotechnology, Middle East Technical University, Ankara 06800, Turkey
Correspondence to: A. Cirpan (E-mail: acirpan@metu.edu.tr)
Received 23 June 2017; accepted 28 July 2017; published online 00 Month 2017
DOI: 10.1002/pola.28752
ABSTRACT: In this study, donor–acceptor random polymers
containing benzotriazole acceptor and bistriphenylamine and
benzodithiophene donors, P1 and P2, were successfully synthe-
sized by Stille coupling polymerization. The effect of bistriphe-
nylamine moiety and thiophene p-conjugated linker on
electrochemical, spectroelectrochemical, and optical behaviors
of the polymers were investigated. Optoelectronic properties
and photovoltaic performance of the polymers were examined
of the polymers (donor) and [6,6]-phenyl C71 butyric acid
methyl ester (PC71BM) (acceptor). Photovoltaic studies showed
that the highest power conversion efficiency of these photovol-
taic devices were recorded as 3.50% with open circuit voltage;
2
2
0.79 V, short circuit current; 9.45 mA cm , fill factor; 0.53 for
P1:PC71BM (1:2, w/w) in 3% o-dichlorobenzene (o-DCB) solution
and 3.15% with open circuit voltage; 0.75 V, short circuit cur-
22
rent; 8.59 mA cm , fill factor; 0.49 for P2:PC71BM (1:2, w/w) in
22
% chlorobenzene (CB) solution. C 2017 Wiley Periodicals, Inc.
V
2
under the illumination of AM 1.5G, 100 mW cm . The poly-
mers were characterized by cyclic voltammetry, UV-Vis-NIR
absorption spectroscopy, gel permeation chromatography.
HOMO/LUMO energy levels of P1 and P2 were calculated as
J. Polym. Sci., Part A: Polym. Chem. 2017, 00, 000–000
KEYWORDS: benzodithiophene; benzotriazole; bistriphenyl-
amine; bulk heterojunction solar cell; conjugated polymers;
electrochemistry; random conjugated polymers; synthesis
25.47 eV/–3.41 eV and 25.43 eV/–3.27 eV, respectively. Bulk
heterojunction type solar cells were constructed using blends
INTRODUCTION Organic photovoltaics have been engaging
research topics for many years since they may serve as can-
didates to fulfill the energy need of the today’s world. These
promising devices, especially polymer containing ones have
great attention owing to their low cost, capability of flexible
design, solution processability, enabling large area fabrica-
conjugated polymers having broad and intense absorption,
high hole mobility and appropriate HOMO/LUMO energy lev-
9
,10
els have been designed and synthesized.
To obtain poly-
mers having these ideal characteristic properties, various
methods can be applied. Donor–Acceptor (D-A) approach as
one of the most efficient method was frequently applied to
augment the PCE by lowering the band gap of the synthe-
sized polymers and improving light absorption. In this
approach, promising electron rich donor moieties such as
benzodithiophene (BDT), carbazole, fullerene, and heteroaro-
matic electron deficient acceptor moieties like benzotriazole
(BTz), diketopyrrolopyrrole, quinoxaline, benzothiadiazole
derivatives are present on the main backbone of the polymer
1
–3
tion.
OSC), conjugated polymers with low band gaps as donor
units are quite handy as components to create an active
In a bulk heterojunction type organic solar cell
(
4
–6
layer.
More than 11% power conversion efficiency (PCE)
can be achieved by forming great bicontinuous interfacial
6
,7
area which enhances exciton dissociation. Fill factor (FF),
open circuit voltage (V ), and short circuit current (I )
oc
sc
1
1,12
which are very much related with optical and electronic
characters of the donor moiety and morphology of the active
layer are the critical factors to improve PCE of the solar
chain.
Furthermore, the random copolymerization with
D-A terpolymer approach consisting of 2D-1A or 1D-2A units
is generally a preferable method as this method enlarges
absorption ability of the polymer and levels the molecular
2
,5,8
cell.
For this purpose, recent studies show that new
Additional Supporting Information may be found in the online version of this article.
2017 Wiley Periodicals, Inc.
V
C
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orbital energy by adjusting the composition of conjugated
polymers with BTz unit due to higher electron accepting
strength of the benzothiadiazole moiety; however, owing to
the capability of functionalizing NAH bond on the triazole ring
of BTz units, electronic nature, and solubility of the polymers
can be easily improved. In literature, there are many BTz con-
1
3,14
units.
In addition, that develops p-p* transition between
the polymer chains by designing copolymers with planar
1
5
molecular structure. Lee et al. designed and synthesized
copolymers using random D-A terpolymer approach. The
polymer has 600–900 nm wide absorption band with 6.04%
3
1,32
taining polymers with 7% PCE have been reported.
Recent
1
6
PCE. Additionally, Wang and coworkers synthesized fluo-
rine, benzothiadiazole and thiophene containing random
studies showed that photovoltaic performance of D-A conju-
gated polymers have been improved by incorporating conju-
gated linkers such as thiophene, thieno[3,2-b]thiophene, furan,
1
7
copolymer having 3.88% PCE. Moreover, according to Guo
and coworkers’ studies randomly synthesized terpolymer
associating 2D and 1A units showed great PCE (3.63%) in
comparison to PCE (2.67%) of an alternating D-A type
3
3
selenophene because of p-bridge effect. These conjugated
linkers can advance the interaction between D and A units,
thereby improving charge transport, electrochemical and pho-
1
8
34
terpolymer.
tovoltaic character of the polymers.
Due to the outstanding specialties of this random D-A ter-
polymer approach we designed and synthesized the novel
random copolymers comprising BTz, triphenylamine deriva-
tive, and BDT moieties. In literature, triphenylamine and its
derivatives were extensively utilized for organic light emit-
In this study, triphenylamine and BDT derivatives as electron
rich donor moieties and BTz with branch alkyl chain as the
electron deficient acceptor moiety were used to synthesize
two copolymers as shown in Scheme 1. One of the conju-
gated polymers involves thiophene as the conjugated linker.
Effects of the bistriphenyamine and thiophene p-bridge on
photovoltaic studies were examined.
1
9
20
21
ting diodes, OSCs, organic field effect transistor, and
2
2
electrochromic applications as triphenylamine containing
materials demonstrate unlabored oxidation capability
through the nitrogen center and extremely immense hole
EXPERIMENTAL
2
3,24
mobility.
Addition to this, triphenylamine derivatives
General
indicate considerable high LUMO energy level, which brings
about well electron blocking character. Moreover, conjugated
polymers with triphenylamine moiety are possessed of
All chemicals and solvents for the syntheses of monomers
and polymers (P1, P2) were purchased from Sigma-Aldrich
and used as received. Tetrahydrofuran (THF) was freshly dis-
tilled after drying over Na and benzophenone. Reactions
were carried out under argon atmosphere. Structures of syn-
thesized monomers and polymers were confirmed by nuclear
2
5
splendid electrochemical and thermal stability.
According to the examples in literature, p-conjugated BDT
containing polymers demonstrate advanced photovoltaic per-
formance as BDT derivatives have high hole mobility due to
their extraordinary planar nature. Additionally, conjugated
side-group incorporation with BDT unit can enhance planar-
ity of the polymer and adjust energy levels of frontier
1
13
magnetic resonance (NMR) spectra. H and C spectra of the
synthesized molecules in deuterated chloroform (CDCl
)
3
were recorded on a Bruker Spectrospin Avance DPX-400
Spectrometer using trimethylsilane as the internal reference.
Number and weight average molecular weight of polymers
were determined with gel permeation chromatography (GPC)
on Polymer Laboratories GPC 220 using polystyrene as the
2
6,27
orbital
Thanks to having wide planar structural mor-
phology of BDT, p-p stacking gets easier. Attachment of alk-
oxy unit at 4 and 9 positions of the benzene in BDT can
downscale the band gap by minimizing effect of steric hin-
drance and enhance the solution processability of the poly-
reference and chloroform (CHCl ) as the solvent. Perkin
3
Elmer Differential Scanning Calorimetry Diamond and Perkin
Elmer Pyris 1 Thermal Gravimetric Analyzer were preferred
to analyze thermal behaviors of the polymers. Thermal anal-
ysis of the polymers was performed at 10 8C/min under
inert nitrogen atmosphere. Electrochemical, spectroelectro-
chemical, and colorimetry studies were performed by Gamry
Reference 600 Potentiostat/Galvanostat and Varian Cary
5000 UV-Vis spectrophotometer, respectively. For electro-
chemical studies, a three-electrode system which consists of
a polymer coated indium tin oxide (ITO) as the working elec-
trode, a Pt wire as the counter electrode, and a Ag wire as
the pseudo reference electrode. The electrolytic medium was
2
8,29
mers.
Yu and coworkers were synthesized series of alky
and alkoxy substituted BDT as donor moiety and thieno[3,4-
b]thiophene carboxylate as acceptor unit-based D-A conju-
gated polymers. According to their studies, optical band of
polymer incorporated with alkoxy substituted BDT had
lower optical band gap (1.62 eV) compared to optical band
3
0
gap of alky substituted polymer (1.75 eV). Due to the
strong donor characteristic of triphenylamine and BDT deriv-
atives, Xie et al. have preferred to use these two units as
electron blocking moieties and synthesized random 2D-1A
copolymer with a small band gap and broad absorption
2
3
region.
0.1 M tetrabutylammonium hexafluorophosphate (TBAPF )/
6
acetonitrile (ACN) solution.
According to recent studies, electron deficient heteroaromatic
BTz and its derivatives are frequently utilized in the organic
optoelectronic systems by reason of powerful electron accept-
ing and transporting nature of BTz unit. Benzothiazole con-
taining polymers have demonstrated better PCE than
Solar Cell Device Fabrication
Device architecture of P1- and P2-based OSCs are composed
of ITO/PEDOT:PSS (40 nm)/Polymer:PC71BM/LiF(0.6 nm)/
Al(100 nm) in a given order. Cleaning of ITO coated glass
2
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SCHEME 1 Synthetic pathway for the monomer and copolymer.
substrate was performed via toluene, detergent, distilled
water, and isopropyl alcohol during 15 min for each solvent
in ultrasonic bath. After that, substrates were dried by
nitrogen gun and cleaning procedure of them was continued
with oxygen plasma for 5 min. Then, PEDOT:PSS coating was
performed with spin-coating technique on precleaned ITO
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0
substrates thereafter baked at 135 8C for 15 min. To prepare
P1:PC M and P2:PC BM blends in different weight to
weight ratios, o-DCB and CB were used, respectively. After
4,4 -bis[(4-bromophenyl)]phenylamino]biphenyl (74.3 mg,
0.115 mmol), and 2,6-bis(trimethyltin)-4,8-bis(2-ethylhexy-
7
1
71
0
loxy)benzo[1,2-b:4,5-b ]dithiophene (177.6 mg, 0.230 mmol)
these two blends were separately spin coated on PEDOT:PSS
were dissolved in 15 mL dry THF under Ar atmosphere.
Bubbling with Ar for 30 min removes the entire air in the
reaction medium. After bubbling step catalyst and co-catalyst
(tris(dibenzylideneacetone)dipalladium (0) (5 mol %), and
tris(o-tolyl)phosphine (10 mol %) were added into the reac-
tion mixture. Then, the medium was refluxed for 36 h at 70
8C under Ar atmosphere. Then, thiophene-based end cappers
were added and reaction mixture was refluxed overnight.
After solvent was removed under reduced pressure the prod-
uct was precipitated in cold methanol. Purification step was
fulfilled with acetone and hexane. P2 was extracted with
coated ITO substrates in N filled glove box system, thermal
2
evaporation of LiF/Al were carried out in the same system.
To examine device performances, current density–voltage (J–
V) behavior of the devices were studied with Keithley 2400
source meter under 1.5 AM solar simulator while their inci-
dent photon to current efficiency (IPCE) performances were
investigated with Oriel Quantum Efficiency Measurement Kit.
Morphology of the active layer were investigated via atomic
force microscopy (AFM) and transmission electron micros-
copy (TEM) analysis.
CHCl . After evaporation of CHCl3 under reduced pressure,
P2 (140 mg, Yield: 80%) as an orange-red colored end prod-
3
4
Synthesis of Poly(N -(4-(4,8-bis((2-ethylhexyl)oxy)-6-(7-
methyl-2-(2-octyldodecyl)-2H-benzo[d][1,2,3]triazol-4-
yl)benzo[1,2-b:4,5-b ]dithiophen-2-yl)phenyl)-N -(4-(4,8-
uct was obtained.
0
0
4
1
ELEMENTAL ANALYSIS: H NMR (400 MHz, CDCl ), d (ppm): Aro-
bis((2-ethylhexyl)oxy)-6-methylbenzo[1,2-b:4,5-
b ]dithiophen-2-yl)-N ,N -Diphenyl-[1,1 -biphenyl]-4,4 -
diamine) (P1)
3
0
0
4
4
0
0
matic: 8.02, 7.79–6.69, Aliphatic: 4.70 (N-CH ), 4.13 (O-CH ),
.53, 2.02–0.83, 0.77. (GPC: M 5 16,676 Da, M 539,155
Da, D- 5 2.3) Decomposition temperature at 5% mass
loss 5 340 8C (See Supporting Informations).
2
2
2
n w
4
,7-Dibromo-2-(2-octyldodecyl)-2H-benzo[d][1,2,3]triazole (4)
0
(64.6 mg, 0.116 mmol), 4,4 -bis[(4-bromophenyl)]phenylamino]-
biphenyl (74.9 mg, 0.116 mmol), and 2,6-bis(trimethyltin)-4,8-
bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b ]dithiophene (179.0 mg,
0
RESULTS AND DISCUSSION
0.232 mmol) were dissolved in 15 mL anhydrous THF to yield P1
Synthesis
via Stille coupling reaction under Ar atmosphere. Then, bub-
bling with Ar was carried out for another 30 min. After Ar gas
bubbling, tris(dibenzylideneacetone)dipalladium (0) (5 mol %)
as catalyst and tris(o-tolyl)phosphine (10 mol %) as co-catalyst
were added into the reaction medium and the mixture was
refluxed for 36 h at 70 8C. Before the polymerization was ended
via end cappers, catalyst and co-catalyst were added to the
reaction mixture. The reaction mixture was stirred at this tem-
perature for 1 h then bromothiophene and tributyl(thiophen-2-
yl)stanne were added, respectively. After that, THF was
removed under reduced pressure. The product was precipi-
tated in cold methanol. Purification step was done with acetone
Synthetic pathway of monomers, P1 and P2 were shown in
Scheme 1. 9-(Bromomethyl)nonadecane (1), tributyl(thiophen-
2-yl)stannane (2), 4,7-dibromo-1H-benzo[d][1,2,3]triazole (3),
4,7-dibromo-2-(2-octyldodecyl-2H-benzo[d][1,2,3]triazole (4),
4,7-bis(5-bromothiophen-2-yl)-2-(2-octyldodecyl)-2H-benzo[d]
[
1,2,3]triazole (TBT) (6) were synthesized with the same syn-
33,35
thetic procedures reported in literature.
P1 was synthe-
sized via Stille polycondesation coupling reaction between
alkylated dibromo BTz (4), bistriphenlyamine and BDT. Similar
procedure was applied to produce P2; however, reaction was
performed between 4,7-bis(5-bromothiophen-2-yl)-2-(2-octyl-
dodecyl)-2H-benzo[d][1,2,3]triazole (TBT) (6), bistripheyl-
amine and BDT. As given in the experimental part, number and
weight average molecular weights of the polymers were 19,900
Da, 49,800 Da for P1 and 16,676 Da, 39,155 Da for P2, respec-
tively. Polydispersity index (PDI) value of P1 was recorded as
and hexane by Soxhlet extractor. P1 was extracted using CHCl3.
After evaporating CHCl under reduced pressure, P1 (171 mg,
3
Yield: 83%) as an orange-red colored end product was
obtained.
2.5 while PDI value for P2 was 2.3.
1
ELEMENTAL ANALYSIS: H NMR (400 MHz, CDCl ), d (ppm): Aro-
3
matic: 8.66, 7.78–6.82, Aliphatic: 4.71 (N-CH ), 4.50–3.70 (O-
Electrochemical Studies
2
CH
M
2
), 2.64–2.45, 2.05–0.83, 0.75. (GPC: M
5 49,800 Da, D- 5 2.5) Decomposition temperature at 5%
w
n
5 19,900 Da,
Redox features of random copolymers were examined with
cyclic voltammetry (CV) in 0.1 M tetrabutylammonium hexa-
mass loss 5 335 8C (See Supporting Informations).
fluorophosphate/acetonitrile (TBAPF /ACN) electrolyte solu-
6
tion to reveal doping behaviors, HOMO–LUMO energy levels
and electronic band gaps of the polymers. For this purpose,
copolymers were dissolved in chloroform solution (5
4
Synthesis of Poly(N -(4-(4,8-bis((2-ethylhexyl)oxy)-6-(5-
(
7-(5-methylthiophen-2-yl)-2-(2-octyldodecyl)-2H-
benzo[d][1,2,3]triazol-4-yl)benzo[1,2-b:4,5-b ]dithiophen-
-yl)phenyl)-N -(4-(4,8-bis((2-ethylhexyl)oxy)-6-
methylbenzo[1,2-b:4,5-b ]dithiophen-2-yl)phenyl)-N ,N -
0
21
mg mL ) and spray coated on the ITO working electrodes
0
4
2
21
and studies were performed with100 mV s scan rate.
0
4
0
4
0
0
Diphenyl-[1,1 -biphenyl]-4,4 -diamine) (P2)
Electronic features of two copolymers were demonstrated in
Table 1. Both polymers possess p and n doping characters as
given in Figure 1. Doping and dedoping potentials of P1 dur-
ing p-type doping are 1.09 V/0.76 V. Furthermore, P1 has
Same synthetic route with P1 was used to get P2. To achieve
synthesis of P2, 4,7-bis(5-bromothiophen-2-yl)-2-(2-octyldo-
decyl)-2H-benzo[d][1,2,3]triazole (6) (83.0 mg, 0.115 mmol),
4
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TABLE 1 Summary of electrochemical and spectroelectrochemical studies of P1 & P2
el
op
E
E
p-doping
E
p-dedoping
E
n-doping
E
n-dedoping
HOMO
(eV)
LUMO
(eV)
E
k
max
g
g
(V)
(V)
(V)
(V)
(eV)
(nm)
(eV)
P1
P2
1.09
0.96
0.76
0.60
21.72/–1.99
21.75/–2.10
21.63/–1.94
21.62
25.5
25.4
23.4
23.3
2.1
2.1
527/488
481/416
2.16
2.11
two reversible redox couples during n-type doping centered
at 21.72 V/–1.63 V and 21.99 V/–1.94 V. Doping/dedoping
peak potentials of ambipolar P2 at both p- and n-doped sites
were determined as 0.96 V/0.60 V and 21.75 V/–1.62 V.
The different oxidation and reduction potentials of the poly-
mers can be attributed to the varied electron distribution on
the polymer backbone. When redox potentials of P1 are
compared with those of P2, P2 has lower redox potentials as
main backbone of P2 chains incorporate electron rich thio-
phene as p-conjugated linker.
to find out optical features such as maximum absorption
behaviors of polymers and optical band gaps. Polymers with
5 mg mL in CHCl were spray coated on ITO coated glass
3
substrate. P1 was subjected to cycling between 0 and 1.3 V
where P2 was recorded between 0 and 1.2 V.
2
1
Maximum absorption range of the polymers are illustrated
in visible range as reported in Figure 2. Similar to the most
of the D-A type conjugated polymers, both polymers have
two maximum absorption maxima (kmax) due to p-p* transi-
tions on the polymer main backbone and intermolecular
3
4
Owing to ambipolar features of P1/P2, both HOMO and
LUMO energy levels were determined using corresponding
onset potentials according to eqs 1a and 1b.
charge transfer between donor and acceptor blocks. While
these two peaks for P1 were recorded at 527 and 488 nm,
absorption peaks of P2 were observed at 481 and 416 nm.
À
Á
onset
ox
HOMO 5 2 4:75 1 E
(1a)
(1b)
Absorption band observed at neutral state was waned gradu-
ally with the formation of absorption bands of polarons and
bipolarons (Fig. 2). In polaron and bipolaron regions, maxi-
mum absorptions were recorded at 730/1490 nm for P1
À
Á
onset
red
LUMO 5 2 4:75 1 E
op
While HOMO and LUMO energy levels for P1 were calculated
and 715/1480 nm for P2. Optical band gaps (E ) were cal-
g
as 25.47 eV and 23.41 eV, those of P2 were estimated as
culated as 2.16 eV for P1, 2.11 eV for P2 using 574 and
588 nm as the onset wavelength values respectively. All
spectroelectrochemical results are shown in Table 1.
25.43 eV and 23.27 eV, respectively. HOMO energy level of
P2 is relatively higher than that of P1 which can be
explained with higher electron density of P2 due to presence
of thiophene unit on the main backbone.
Colors of the polymers shown in Figure 3 were identified via
International Commission on Illumination (CIE) system
depending on L (luminance), a (hue), and b (saturation) val-
ues of the polymers. While L value is corresponding to col-
or’s brightness, a and b are related with the color between
red-magenta-blue and blue yellow, respectively. P1 and P2
almost possess similar colors at their neutral, intermediate,
and fully oxidized states. Former has orange red color at
Electronic band gaps for P1 and P2 are similar due to differ-
ence between their HOMO and LUMO energy levels.
Spectroelectrochemical Studies
Spectroelectrochemical studies of polymers were carried out
via gradually increasing the potential in 0.1 M TBAPF /ACN
6
21
6
FIGURE 1 Single scan cyclic voltammograms of P1 (a) and P2 (b) in 0.1 M TBAPF /ACN electrolyte solution at 100 mV s scan
rate.
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6
FIGURE 2 Electronic absorption spectra of P1 (a) and P2 (b) in 0.1 M TBAPF /ACN electrolyte solution. [Color figure can be viewed
at wileyonlinelibrary.com]
neutral state, red and gray tones as intermediate colors, and
greenish gray color at fully oxidized state. Both polymers
show light blue colors at 22.0 V.
delocalization ability. Additionally, conjugation length will also
3
9
affect the optical properties of the polymers. According to
optical absorption results of the polymers, P1 has a more red
shifted behavior. The reason of this unexpected result could be
ascribed to the shorter conjugation length of P2 due to lower
molecular weight. That is, lower molecular weight of P2 con-
tributed to short conjugation length, thereby causing this
unforeseeable blue shift. Owing to the blue shifted property of
P2 this polymer has a deeper orange color compared to that of
P1. Similarly, this unexpected situation has been also observed
in the triphenyamine- and BTz-based random copolymers (P1
and P3) synthesized by Hacioglu et al. Triphenylamine-based
P1 showed maximum absorption at 497 nm and this polymer
has red color its neutral state. According to Hacioglu et al. lon-
ger thiophene p-spacer containing P3 revealed blue shifted
behavior compared to P1, unexpectedly. P3 revealed its maxi-
mum absorption at 472 nm and its neutral color was orange
due to this blue shifted behavior. Shorter conjugation length
Optical Studies
As illustrated in Figure 4, red shift features in the absorp-
tion were investigated for the thin films of the polymers
compared to polymer solutions in chloroform. Absorption
peaks of P1 in thin film were examined at 488 and 527 nm
but P1 in CHCl solution exhibited a peak at 483 nm. In the
3
same manner, absorption wavelengths of P2 in thin film
form were recorded at 416 and 481 nm. Whereas, 466 nm
was the value for P2 in CHCl . The reason of this phenome-
3
non is related with absence of pre-aggregation effect,
increasing delocalization of p electrons with planar solid
state, interaction between solvent-polymer and facilitating p-
p* transitions with diminished rotational angles in the poly-
3
6–38
mer backbone.
Optical absorption of P2 is red shifted compared with that of
P1 due to the presence of higher electron density thiophene
spacer on main polymer backbone. Entity of p-bridges such as
thiophene, thiono[3,2-b]thiophene, and so forth. strongly influ-
ence the electronic and optical behaviors of polymers with
increasing planarity, intermolecular p-p stacking and electron
n
due to lower M value of P3 was shown as the reason for this
situation.
4
0
Kinetic Studies
In kinetic studies, percent transmittance change (DT%) and
switching time behaviors of the polymers between neutral
FIGURE 3 Colors of P1 (a) and P2 (b) on oxidation and reduction with their L, a, b value. [Color figure can be viewed at wileyonli-
nelibrary.com]
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FIGURE 4 Normalized absorption spectra of P1 (a) and P2 (b)
in both thin film and CHCl solution. [Color figure can be
3
viewed at wileyonlinelibrary.com]
FIGURE 5 Percent transmittance change (DT%) of P1 (a) and P2
(
6
b) in 0.1 M TBAPF /ACN electrolyte solution at maximum neu-
tral, polaron, bipolaron absorption wavelengths. [Color figure
can be viewed at wileyonlinelibrary.com]
and completely oxidized states were examined. Switching
time can be defined as the time needed for observing the
color switching of the electrochome via oxidation and reduc-
tion processes. During kinetic studies, UV-Vis-NIR spectrome-
try was used by introducing square wave potentials within
proper time intervals to the polymers at maximum absorp-
tion wavelengths which were chosen from the neutral,
polaron, and bipolaron absorption regions in the spectroelec-
trochemical results. With this aim, P1 and P2 coated on ITO
glass substrate were subjected to square-wave potentials
during 5 s for each cycle.
41,42
length.
However, aforementioned lower molecular weight
of P2 might be the reason of rather bad kinetic values com-
pared to P1 as P2 has a lower conjugation length. Therefore,
40
ion diffusion and dopant insertion ability of P2 were weak.
Photovoltaic Studies
Bulk heterojunction type polymer-based solar cells were fab-
ricated with the device architecture of ITO/PEDOT:PSS/
TABLE 2 Summary of transmittance and switching time results
of P1 and P2
Percent transmittance changes were recorded as 20% at
5
2
27 nm, 46% at 730 nm, 76% at 1490 nm for P1 and
1% at 418 nm, 51% at 715 nm, 64% at 1480 nm for P2
Wavelength
(nm)
Transmittance
(%)
Switching
time (s)
(
Figure 5). Switching time values of P1 and P2 were calcu-
lated as 0.9 s, 1.2 s, 1.9 s, and 1.5 s, 0.8 s, 1.8 s at their cor-
responding wavelengths, respectively. All kinetic results were
summarized in Table 2.
5
27
20
46
76
29
28
72
1.9
1.2
0.8
1.8
0.8
1.5
P1
P2
730
1
490
P1 illustrated higher performance than P2 in accordance with
these results. Expectedly, P2 with thiophene should give bet-
ter kinetic results as this spacer may enhance ion diffusion
ability and dopant insertion by increasing the conjugation
481
715
1
480
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TABLE 3 Summary of photovoltaic performances of P1 and P2
J
sc
Thickness
(nm)
22
Polymer:PC71BM
%
V
oc (V)
(mA cm
)
FF (%)
î (%)
a
P1 (1:1)
P1 (1:2)
P1 (1:3)
P1 (1:2)
P1 (1:2)
P2 (1:1)
P2 (1:2)
P2 (1:3)
P2 (1:4)
P2 (1:2)
P2 (1:2)
2
2
2
3
3
2
2
2
2
3
2
0.78
0.79
0.72
0.81
0.79
0.77
0.75
0.75
0.75
0.75
0.76
5.43
7.49
5.65
6.85
9.45
4.82
8.59
8.63
7.66
4.51
6.66
42
50
44
55
47
39
49
42
44
37
38
1.77 (1.71 )
109
106
99
a
2.95 (2.33 )
a
1.80(1.67 )
a
3.05 (2.48 )
114
124
118
112
105
100
118
125
a
3.50 (3.37 )
c
1.45 (1.31 )
b
3.15 (2.82 )
a
2.71 (2.59 )
a
2.52 (2.15 )
a
1.25 (1.21 )
b
1.89 (1.56 )
a
b
c
Average pover conversion effciency for 16 devices, for 15 devices, for 14 devices.
Bold values show the results of best performance photovoltaic devices.
polymer:PC71BM/LiF/Al. To evaluate the photovoltaic behav-
iors of the polymers several optimization studies such as sol-
vent, donor–acceptor ratio, active layer thickness, addition of
additive, and methanol treatments were carried out. Results of
current density–voltage (J–V) behaviors of optimized polymer-
based solar cell devices were shown in Table 3 (Fig. 6). The
best morphology of P1:PC BM and P2:PC BM blends were
1.13 nm and 0.653 for P1 with/without DIO, 0.695 and
0.642 nm for P2 with/without DIO, respectively. Further-
more, to improve morphology of polymer-based devices 3%
vol. DIO (1,8 diiodooctane) used as the additive and metha-
nol treatment was applied. The former mainly increases sol-
ubility of PC BM and enhances formation of donor–acceptor
7
1
7
1
71
domain network while the latter results in homogenous and
stabilize active layer by removing the residual DIO addi-
recorded in o-DCB and CB, respectively. Less symmetric
PC71BM as the acceptor was selected instead of PC61BM due to
higher light absorption in visible region and enhanced current
4
8
tive. After DIO addition and methanol processing were car-
ried out for P1:PC BM active layer, best device performance
7
1
4
3
density ability of the less symmetric fullerene derivative.
Optimization studies of polymer:PC BM weight ratio were
was recorded as 3.50% PCE with 0.79 V V and 9.45
oc
2
2
7
1
mA cm Jsc at 500 rpm spin coating speed (Table 3).
performed as device performance mainly depended on the bal-
ance between absorption and charge transfer of active layer.
4
4
However, when these treatments were applied for P2-based
active layer, photovoltaic performance of the device
decreased from 3.15% to 1.89% owing to reduction in cur-
rent density and FF. The reason of this reduction can be
explained with the formation of less interconnected donor
acceptor network on DIO addition. Additionally, lower FF
values for P2-based device compared to P1-based one may
As a result of this study, polymer:PC BM weight ratio was
7
1
optimized as 1:2 for both polymers. Efficiency of the devices
was improved owing to increased current density and FF
when PC BM ratio was increased from 1:1 to 1:2. However,
7
1
a decrease in device performance was observed for 1:3 ratio
as light harvesting ability of the polymers became less pre-
4
5
dominant with increased PC BM ratio. Additional to these
7
1
optimizations, thickness of the active layer was optimized as
light absorption and charge transport are enhanced with
increased active layer thickness. However, active layer thick-
ness was optimized as light absorption and charge transport
are enhanced with increased active layer thickness. However,
active layer thickness was optimized between 100 and
4
6,47
200 nm due to low diffusion length of organic materials.
Morphology of the active layers was investigated via AFM
and TEM analysis. Optimized active layer thickness was
determined as 124.0 nm for P1-based best performance
device and 112.0 nm for P2-based best performance device
by AFM shown in Figure 7.
FIGURE 6 Current density–voltage (J–V) curves of P1:PC71BM
(
1:2 w/w) and P2:PC71BM (1:2 w/w)-based bulk heterojuntion
According to AFM results, root mean square roughness
organic solar cells with and without 3% DIO 1 MeOH. [Color
(
rms) of the polymer:PC BM blends were specified as
figure can be viewed at wileyonlinelibrary.com]
7
1
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FIGURE 7 TEM images of (a) P1:PC71BM (1:2 w/w) blend without treatment (b) P1:PC71BM (1:2 w/w) blend with treatment (c)
P2:PC71BM (1:2 w/w) blend without treatment (d) P2:PC71BM (1:2 w/w) blend with treatment. AFM images of e) P1:PC71BM (1:2 w/
w) blend without treatment (f) P1:PC71BM (1:2 w/w) blend with treatment (g) P2:PC71BM (1:2 w/w) blend without treatment (h)
P2:PC71BM (1:2 w/w) blend with treatment. (Scale bars are 250 nm.) [Color figure can be viewed at wileyonlinelibrary.com]
4
9
be ascribed to this less interconnected network between
the donor and the acceptor (Fig. 7). In addition, thermal
and solvent annealings were performed to enhance the
device performance; however, any improvement in device
performance was not observed with these annealing meth-
ods. The highest photovoltaic performance of P2-based
below eq 2) of the photovoltaic devices is higher than
experimental Voc.
Voc5ð1=eÞðjE DH oO nMo Or 2E
LUMO
PCBM
jÞ20:3 V
(2)
The deviation from the expected value form the experimental
Voc can be attributed optimizing parameters of the con-
structed bulk heterojunction photovoltaic devices; mainly,
the thickness and the morphology of the active layer, charge
carrier mobility of the materials.
potential of PEDOT:PSS layer can influence the Voc.
device was determined as 3.15% with 0.75 V of V and
oc
22
8.59 mA cm
of Jsc (Table 3). High open circuit voltage
(Voc) values of P1- and P2-based devices were recorded
4
9,50
Also electrochemical
between 0.75–0.81 V owing to low HOMO energy levels of
5
1
9
the polymers. Although the expected Voc (can be calculated
These polymers showed similar optical and electronic behav-
iors, P1 possessed higher photovoltaic performance. The rea-
son of this phenomenon may be attributed to having higher
M of P1 leading to more charge carrier mobility. IPCE meas-
n
urements were performed and 39% and 28% IPCE values
were recorded for best performance P1- and P2-based devi-
n
ces, respectively (Fig. 8). Lower M value of P2 can contrib-
ute to lower photocurrent result with the increasing of
5
2
recombination center.
CONCLUSIONS
Syntheses of the D-A type conjugated random copolymers
comprising BTz, bistriphenylamine and BDT were achieved
successfully via Stille coupling reaction to examine optical,
electronic, and optoelectronic behavior of the polymers. They
demonstrated proper electrochemical, spectroelectrochemical,
and optical behaviors for photovoltaic applications. They
showed not only good film forming performance but also high
thermal stability up to 300 8C. Neutral state color of P1 was
orange-red whereas P2 exhibited more orange color its
FIGURE 8 IPCE curves of best performance P1- and P2-based
organic solar cells. [Color figure can be viewed at wileyonlineli-
brary.com]
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neutral state due to blue shifted absorption for low molecular
weight P2. The polymers showed ambipolar character and
their frontier energy levels were calculated as 25.47 eV/
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