Synthesis, characterization and optoelectronic properties of a new perylene diimide-benzimidazole type solar light harvesting dye

Article abstract of DOI:10.1016/j.saa.2014.02.131

A perylene diimide type small molecule (BI-PDI) has been synthesized through Suzuki coupling reaction between N,N′-bis(2,6-diisopropylphenyl)- 1,7-dibromoperylene-3,4,9,10-tetracarboxylic diimide and 2-(2-hydroxyphenyl)-7- phenyl-1H-benzimidazole-4-boronic acid. BI-PDI small molecule has showed an absorption band between 350 and 750 nm on thin films. HOMO and LUMO energy levels of BI-PDI dye have been calculated to be about -5.92 eV and -3.82 eV, respectively. Solution-processed bulk heterojunction (BHJ) solar cells have been constructed using BI-PDI as donor and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) as acceptor or poly(3-hexylthiophene) (P3HT) as donor and BI-PDI as acceptor. The external quantum efficiencies (EQE) of the devices cover the most of the visible region between 400 and 700 nm for both configurations. Photovoltaic performances of BI-PDI-based organic solar cells are limited by the aggregation tendency of PDI structure and poor hole/electron mobilities of the active layer.

Full text of DOI:10.1016/j.saa.2014.02.131

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 197–206
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, characterization and optoelectronic properties of a new
perylene diimide–benzimidazole type solar light harvesting dye
a
b
b
c,
Haluk Dinçalp a, , Oguzhan Çimen , Tayebeh Ameri , Christoph J. Brabec , Sıddık Içli
⇑
⇑
_
a Department of Chemistry, Faculty of Arts and Science, Celal Bayar University, Muradiye, 45030 Manisa, Turkey
b Institute of Materials for Electronics and Energy Technology (I-MEET), Friedrich-Alexander-University, Martensstraße 7, 91058 Erlangen, Germany
c Solar Energy Institute, Ege University, Bornova, 35100 Izmir, Turkey
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
Benzimidazole-substituted PDI dye
for solution-processed BHJC was
synthesized.
Aggregation tendency of dye in the
presence of P3HT polymer was
observed.
Electron mobility of PDI dye is
calculated to be higher than the hole
mobility.
a r t i c l e i n f o
a b s t r a c t
Article history:
A perylene diimide type small molecule (BI-PDI) has been synthesized through Suzuki coupling reaction
between N,N0-bis(2,6-diisopropylphenyl)-1,7-dibromoperylene-3,4,9,10-tetracarboxylic diimide and
2-(2-hydroxyphenyl)-7-phenyl-1H-benzimidazole-4-boronic acid. BI-PDI small molecule has showed
an absorption band between 350 and 750 nm on thin films. HOMO and LUMO energy levels of BI-PDI
dye have been calculated to be about ꢁ5.92 eV and ꢁ3.82 eV, respectively. Solution-processed bulk het-
erojunction (BHJ) solar cells have been constructed using BI-PDI as donor and [6,6]-phenyl-C61-butyric
acid methyl ester (PC61BM) as acceptor or poly(3-hexylthiophene) (P3HT) as donor and BI-PDI as accep-
tor. The external quantum efficiencies (EQE) of the devices cover the most of the visible region between
400 and 700 nm for both configurations. Photovoltaic performances of BI-PDI-based organic solar cells
are limited by the aggregation tendency of PDI structure and poor hole/electron mobilities of the active
layer.
Received 22 December 2013
Received in revised form 31 January 2014
Accepted 19 February 2014
Available online 10 March 2014
Keywords:
Perylene diimide
Benzimidazole
Photoluminescence
Charge transfer
Light harvesting
Photovoltaics
Ó 2014 Elsevier B.V. All rights reserved.
Introduction
as well-ordered molecular structures, simple synthetic pathways,
well-defined molecular weights, easily changeable spectral tunings
Small molecule semiconductors have been extensively used in
BHJ solar cells for decades due to their exclusively properties such
by variation of functional groups and advantageous on thin film
morphology [1,2]. Nowadays most of the organic photovoltaic
(OPV) materials have been configured by using well-known conju-
gated small molecules including phthalocyanines [3,4], boron-
dipyrromethenes [5,6], fused acenes [7–9], thiazoles [10,11],
triphenylamines [12,13], diketopyrrolopyrroles [14,15], isoindigos
[16], tetraphenylphthalic-based diimides [17], and fullerenes
⇑
Corresponding authors. Tel.: +90 236 2013158; fax: +90 236 2412158
(H. Dinçalp). Tel./fax: +90 232 3740118 (S. Içli).
_
E-mail addresses: haluk.dincalp@cbu.edu.tr (H. Dinçalp), siddik.icli@ege.edu.tr
_
(S. Içli).
http://dx.doi.org/10.1016/j.saa.2014.02.131
1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

198
H. Dinçalp et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 197–206
[18]. However, they have had still lower power conversion efficien-
cies (PCEs) with respect to their polymeric counterparts. It will be
expected that higher efficiencies can be obtained by the develop-
ment of suitable molecular designs for small molecule organic
semiconductors in near future studies. Among the preferred small
molecule-based BHJ configurations, donor–acceptor networks con-
taining perylene diimides (PDIs) as acceptors and appropriate do-
nor materials lead to efficient photoinduced charge separation in
OPVs [19–21].
Generally, PDIs show two reduction waves with their planar
structure responsible for high degree of stacking [35–37].
p–p
The introduction of bulky substituents in bay positions of the ring
disturbs the planarity of the perylene core. These deviations from
planarity may decrease multi-dimensional electron transfer pro-
cesses. Also, deformations from molecular planarity results the
inhibition of aggregation which may be helpful to improve PCE of
a desirable device. In such efforts, bay-functionalized groups on
the perylene ring are the most important molecular building blocks
for an effective OPV device [38]. The report herein is based to
decrease the molecular planarity of perylene core by attaching with
special benzimidazole derivative in 1- and 7-positions of the pery-
lene ring. There are a limited number of studies about small organic
molecules based on benzimidazole for OPV devices in the literature.
A recent study is related to 2,2-dimethyl-2H-benzimidazole small
molecule-based BHJ solar cell comprising benzimidazole derivative
with PC61BM giving FF of 27 with a low PCE of 0.37 [39].
In present work, we also targeted to synthesize a low band gap
material by incorporating benzimidazole group to the perylene
core. This kind of benzimidazole substituted PDI-type small mole-
cule (BI-PDI) may ascribe the intramolecular charge generation.
The absorption and emission properties of the dye in common sol-
vents of different polarities and also on thin films have been inves-
tigated in detail. Also, hole and electron mobility performances of
some selected devices have been tested. Synthetic routes and dye
structures are shown in Schemes 1 and 2.
PDIs are suitable candidates as an acceptor in device structures
due to their stronger visible absorptions within the range of
400–450 nm (B band) and 500–700 nm (Q band), better charge
transporting properties [22], higher molar absorption coefficients
[23], higher fluorescence quantum yields [24] and better photo-
stabilities [25] under solar irradiation as compared to their
conjugated counterparts. In the literature survey, PCE values given
for OPV devices using PDI small molecules as acceptor are found
within the wide range of 0.002–3.17 depending on both the nature
of the active layer and the used techniques [26–33]. Sharma et al.
have blended 1,7-substituted perylene-anthracene diimide with a
donor molecule containing dithyenyl-benzothiadiazole central unit
and obtained PCE of 2.85% with short-circuit current density of
6.8 mA/cm2 [27]. A diphenoxylated PDI small molecule in bay posi-
tions of perylene ring has been used as acceptor and p-phenylene-
vinylene type small group has been used as donor group in OPV
device withZnO layer giving PCE of 3.17% by Sharma et.al in different
study [33]. In that device, the PCE has been improved by both incor-
porating ZnO layer between the active layer and the Al electrode and
also annealing to increase the crystalline of the active layer. Han et.al
have manufactured the ternary structure for OPV device consisting
of P3HT:PCBM with the addition of a butoxyphenoxy substituted
PDI molecule and obtained a considerable improvement of over
70% in PCE with respect to reference cell without an additive [34].
Experimental
General procedures
1H NMR and 13C NMR spectra for characterization of the
synthesized compounds were recorded on a Bruker 400 MHz
Scheme 1. Reaction pathway for the synthesis of BI-B(OH)2. (i) Br2/HBr, 120 °C, 93% [40]; (ii) phenylboronic acid, Na2C03, benzene, Pd(PPh3)4, 80 °C, 10% (for 2b) [41]; (iii)
EtOH/THF, NaBH4, CoCl2ꢂ6H2O, 80 °C [42]; (iv) Salicyl aldehyde, Me-CN, CoCl2ꢂ6H2O, room temp., 95% [43]; (v) n-BuLi, THF, ꢁ78 °C, B(OMe)3, 75% [44].
Scheme 2. Reaction pathway for the synthesis of target BI-PDI. (i) H2SO4/Oleum, Br2, 85 °C, 84% [45]; (ii) 2,6-diisopropylaniline, propionic acid, 130 °C, 72% [45]; (iii) 2-(2-
hydroxyphenyl)-7-phenyl-1H-benzimidazole-4-boronic acid, BI-B(OH)2, Na2CO3, Ar, Pd(OAc)2, 80 °C, 80% [35].

H. Dinçalp et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 197–206
199
ꢀ
ꢁ
Vnin
L3
0:89 ꢃ b
pffiffi
pffiffiffi
9
8
spectrometer. FT-IR spectra (KBr pellets) were recorded on a Perkin
Elmer-Spectrum BX spectrophotometer. UV–visible spectra were
measured on a Perkin Elmer Lambda 950 UV/VIS/NIR spectropho-
tometer in the solutions and on thin films. Fluorescence measure-
ments were performed on a FLSP 920 Edinburg fluorescence
phosphorescence spectrophotometer.
The fluorescence decays of the dye in solutions were occupied
with laser system. The Edinburgh Instruments F900 reconvolution
software based on the Marquardt Levenberg algorithm [46] was
utilized for calculations. Up to 4 lifetimes could be fitted from up
to 10,000 data channels with iterative shift fit, background fit
and parameter fixing options. The fitted decay curve, together with
lifetime and amplitude values, weighted residuals, v2 < 1.2 good-
ness of fit, autocorrelation function, etc. were all calculated to
allow an assessment of the quality of the fit. The instrument re-
sponse function (IRF) was measured using a ludox scattering solu-
tion. Solutions of the samples were contained in 1 cm path-length
quartz cells at the optical density between 0.10 and 0.18 for
fluorescence studies.
The cyclic voltammetry was performed on a CH instruments
(Electrochemical Workstation) with a standard three-electrode
electrochemical compartment in 100 mM [TBA][PF6] solution in
Me-CN in room temperature under nitrogen purge. Ag/Ag+ refer-
ence electrode, a glassy carbon working electrode and a Pt wire
counter electrode were used. The scanning rate was 50 mV/s. Fer-
rocene-ferrocenium (Fe/Fe+) couple which was exhibited at about
+0.55 V was used as internal reference for the calculation of the on-
set values of Ered. Redox potentials of BI-PDI were calculated from
reversible waves with the formula [Ep(ox) + Ep(red)]/2.
JSCL
¼
e0er
l
exp
V
L
where J was the current density, V was applied voltage, er (2.7 for
blends) and e0 were the relative dielectric constants of the organic
layer and permittivity of free space (8.85 ꢃ 10ꢁ12 As/Vm), respec-
tively,
l was the electron mobility and L (100 nm) was the thickness
of the organic layer.
Materials
1,7-Dibromo-perylene-3,4,9,10-tetracarboxylic
dianhydride
(P1) and N,N0-bis(2,6-diisopropylphenyl)-1,7-dibromoperylene-
3,4,9,10-tetracarboxylicdiimide (P2) were synthesized according
to the well-known procedure as shown in Scheme 2. Phenyl boro-
nic acid was synthesized according to the standard procedure
given in the literature [44]. 2,1,3-benzothiadiazole, NaBH4,
Tetrakis(triphenylphosphine)palladium(0) Pd(PPh3)4, palladium
acetate, salicyl aldehyde, bromobenzene, n-butyllithium (1.6 M in
hexane), hydrochloric acid were purchased from Sigma Adrich. Tri-
methyl borate, perylene-3,4,9,10-tetracarboxylic acid dianhydride,
propionic acid, cobalt(II) chloride hexahydrate and bromine were
purchased from Merck company. 2,6-Diisopropylaniline (Acros
Organics) and hydrogen bromide (Riedel-de Haen) were used as re-
ceived. Other chemical reagents and organic solvents were analyt-
ical grade and used without further purification.
ITO coated glass with a surface resistance lower than 20
X/sq
(Osram) was used as transparent electrode. PEDOT:PSS was
obtained from Baytron PH H.C. Starck and P3HT was supplied from
Rieke Inc. for the device preparation. Technical grade PCBM was
purchased from Solenne.
Device preparation and testing
Synthesis
All the organic solar cells were constructed on indium tin oxide
(ITO)-covered glass substrates (2.5 ꢃ 2.5 cm) as a normal structure
configuration. First of all, the substrates were cleaned in an ultra-
sonic bath of isopropanol and acetone, respectively prior to device
preparation. Poly(3,4-ethylenedioxy thiophene)/poly(styrene sul-
fonic acid) (PEDOT:PSS) was coated by a doctor-blade technique
at a substrate temperature of 50 °C. PC61BM and other photoactive
components were dissolved at concentrations of 2 wt% from o-
dichlorobenzene (oDCB) solution in different mass ratios. Active
layers consist of different configurations such as BI-PDI:PC61BM,
P3HT:BI-PDI or BI:PC61BM structures and deposited with a typical
film thickness of around 80–100 nm, which was measured by pro-
filometer. Films were then annealed at 65 °C for 10 min under air.
The Ca/Ag (15 nm/80 nm) metal electrode was thermally depos-
ited. All the devices were constructed in air and then stored in
glove box.
Solar cells were illuminated under AM1.5G irradiation on an
OrielSol 1A Xenon solar simulator (100 mW cmꢁ2). The current–
voltage characteristics were measured using a Keithley 2400
SMU under nitrogen atmosphere. The surface of the thin films
was investigated by Ambios Qscope 250 Model Atomic Force
Microscope. Thicknesses of the active layers of the thin films were
measured by Ambios Q1 Profilometry instrument. The EQE was de-
tected with a lock-in amplifier (SR830, Stanford Research Systems)
with current preamplifier (HMS-74) under monochromatic illumi-
nation, which was calibrated with a mono-crystalline silicon diode.
We fabricated the hole and electron only devices with the struc-
tures ITO/PEDOT:PSS/Active Layer/PEDOT:PSS/Ag(100 nm) and
ITO/AZO(HT)/Active Layer/Ca(15 nm)/Ag(80 nm), respectively. Alu-
minum-doped zinc oxide (AZO) nanoparticles at high temperature
were used in electron only device fabrication. These devices
were prepared by the same way as organic solar cell production.
We estimated the electron and hole mobilities according to the
equation [33]:
Synthesis of 4,7-dibromo-2,1,3-benzothiadiazole (1)
2,1,3-Benzothiadiazole (3 g, 22.0 mmol) was dissolved in 65 mL
of HBr (47%). On the other hand, Br2 (3.5 mL, 68.3 mmol) was di-
luted with 45 mL of HBr. A solution containing Br2 was added drop-
wise to the 2,1,3-benzothiadiazole solution. Then, extra 30 mL of
HBr was added to the solution and the solution was stirred at
120 °C under an argon atmosphere for 15 h. Then, the mixture
was cooled to room temperature and a sufficient amount of satu-
rated solution of NaHSO3 was added to the solution to remove
the excess Br2 completely. The solution was filtered off and then
washed with Et2O. The yellow solid was purified by column chro-
matography on silica gel using dichloromethane:n-hexane (3:2) as
an eluent. Yield: 93%, FT-IR (KBr pellet, cmꢁ1): 3078, 3045, 1574,
1498, 1476, 1310, 1273, 1184, 937, 875, 826, 587, 488 cmꢁ1 1H
.
NMR (400 MHz, DMSO-d6 d 2.48 ppm): d = 7.92 (s) ppm. 13C NMR
[100 MHz, DMSO-d6 d 40.2 ppm (7 peaks)]: d = 162.9, 133.4,
113.7 ppm.
Synthesis of 4,7-diphenyl-2,1,3-benzothiadiazole (2a) and 4-bromo-7-
phenyl-2,1,3-benzothiadiazole (2b)
To a 250 ml two-necked round-bottomed flask were added
compound 1 (1 g, 3.40 mmol) in 32 mL of benzene and Na2CO3
(2 M, 21 mL).
A solution of phenylboronic acid (415 mg,
3.73 mmol) in 15 mL of ethanol was added to two-necked flask
at 60 °C under an argon atmosphere. Pd(PPh3)4 catalyst (118 mg,
0.10 mmol) was added to the solution and the solution was stirred
at 80 °C for 15 h. Small amount of water was poured into the solu-
tion. The reaction mixture was extracted with chloroform and con-
centrated under vacuum. The resulting green solid was purified by
column chromatography on silica gel using dichloromethane:n-
hexane (3:2) as an eluent. However, the mixture of 2a and b were
not separated from each other. They were used as a mixture in the

200
H. Dinçalp et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 197–206
next step. Only a small amount of compound 2a was isolated for
the analysis. Yield for 2a and b mixture: 80%, FT-IR (KBr pellet,
cmꢁ1): 3448, 3056, 3033, 2957, 2925, 1527, 1478, 1447, 1328,
1266, 1184, 1154, 1075, 1031, 933, 882, 843, 790, 757, 694,
3 mL of ethanol was added to two-necked flask at 80 °C under an
argon atmosphere. Pd(PPh3)4 catalyst (3 mg, 2.6 mol) was added
l
to the mixture and then the solution was stirred at 80 °C for
72 h. Finally, the solvent was evaporated. The crude product was
purified by column chromatography on silica gel using dichloro-
methane:n-hexane (10:1) as an eluent. Yield: 80%, FT-IR (KBr
pellet, cmꢁ1): 3402 (OAH stretching), 2962 and 2926 (aliphatic
517 cmꢁ1 1H NMR (400 MHz, CDCl3 d 7.22 ppm): d = 7.96 (4H, d,
.
J = 7.6 Hz), 7.76 (2H, s), 7.53 (4H, t, J = 7.6 Hz), 7.44 (2H, t,
J = 7.6 Hz) ppm. 13C NMR [100 MHz, CDCl3 d 78.8 (3 peaks)]:
d = 154.3, 137.6, 133.6, 129.4, 128.8, 128.5, 128.3 ppm.
m
CAH), 1704 (imide
m
m
C@O), 1666 (imide
m
C@O), 1592 (aromatic
mC@C),
1464, 1422, 1344 (
CAN), 1264, 1202, 942, 810, 750 cmꢁ1
.
1H NMR
Synthesis of [1,10:40,100]-terphenyl-20,30-diamine (3a) and 4-
bromobiphenyl-2,3-diamine (3b)
(400 MHz, CDCl3 d 7.26 ppm): d = 9.78 (2H, d, J = 8.4 Hz), 8.81 (3H,
d, J = 2.2 Hz), 8.79 (4H, d, J = 2.2 Hz), 8.77 (2H, s), 8.74 (2H, s), 8.72
(1H, s), 8.69 (2H, d, J = 8.4 Hz), 8.59 (2H, s), 7.60 (6H, m), 7.36 (10H,
m), 2.77 (4H, septet, J = 6.8 Hz), 1.19 (24H, d, J = 6.8 Hz) ppm.
13C NMR [100 MHz, CDCl3 d 77.2 (3 peaks)]: d = 164.0, 163.8,
163.6, 158.2, 146.9, 146.8, 136.4, 136.3, 136.1, 134.7, 132.6,
131.6, 131.2, 131.1, 130.9, 130.7, 130.0, 129.9, 129.8, 129.7,129.1,
128.9, 127.7, 124.3, 124.2, 124.0, 123.2, 122.4, 119.6, 29.4,
24.2 ppm.
To the mixture of 2a–b (1 g) in 120 mL of ethanol:THF (3:1)
solution was added CoCl2ꢂ6H2O (11.3 mg, 47.5
lmol) and NaBH4
(144 mg, 3.81 mmol), respectively. The mixture was stirred at
80 °C under an argon atmosphere for 20 h. Black solid Co2B was
formed instantly and, in a few minutes, H2S evolution was noted.
Then, the solution was cooled to room temperature and filtered
off. The black solid was separated and then solvent was evapo-
rated. 50 mL of water was poured into the solution and the reac-
tion mixture was extracted with Et2O. The solid product couldn’t
be purified by column chromatography because the diamine prod-
uct does not stable in air. Diamines were used immediately in the
next step to avoid fast decomposition.
Results and discussion
Optical properties
Synthesis of 2-(4-bromo-7-phenyl-1H-benzimidazole-2-yl)phenol (BI)
BI-PDI dye exhibits three characteristic PDI core absorption
bands between 480 and 560 nm belonging to its electronic S0–S1
transition in solutions. Also, absorption bands in the region of
380–410 nm are attributed to its electronic S0–S2 absorption indi-
To the mixture of 3a–b (1.3 g) and salicylaldehyde (579 lL,
5.43 mmol) were dissolved in 25 mL of acetonitrile. CoCl2ꢂ6H2O
(120 mg, 0.50 mmol) was added to the reaction mixture and the
mixture was stirred at room temperature overnight. After comple-
tion, the solvent was evaporated and the crude product was dis-
solved in 25 mL of ethanol and then treated with 2 mL of NH4OH,
followed by 25 mL of water. The mixture was heated and then
cooled to room temperature and then the precipitate was washed
with solvent mixture of n-hexane/ethanol. The crude solid was
purified by column chromatography on silica gel using dichloro-
methane:n-hexane (3:2) as an eluent. Yield: 95%, FT-IR (KBr pellet,
cmꢁ1): 3420, 3062, 2918, 1628, 1595, 1488, 1417, 1384, 1257,
cating the
p–
p* transition of the aromatic rings. The maximum
and the shoulder of the absorption band of BI-PDI in more polar
ethanol and benzonitrile solutions are shifted to blue and red
region, respectively, as shown in Fig. 1a. No aggregation behavior
was observed in studied solutions according to the absorption
spectra. Fig. 1b illustrates the steady-state emission spectra of
BI-PDI in different solvents of increasing polarities. Higher the
polarities of the solvents, the more increase the red shifted wave-
length of emission maxima of the dye. In ethanol solution, charge
transfer is enhanced by the intra- or inter-molecular hydrogen
bond formation with the unshared electron pairs of the heteroat-
oms on benzimidazole group of the dye. This leads to significant
shift to longer wavelength on its emission spectrum. Larger Stokes
shift (Table 1) in ethanol solution with respect to other solutions
supports the change in electronic spectra, leading to the possible
formation of excimer for perylene core. Fig. 1c illustrates the de-
cays of single photon timing experiments in Bz-Cl solution. Decay
analysis in both less polar Bz-Cl and more polar Bz-CN solutions
gave two-exponential decays at the emission wavelength of
570 nm for BI-PDI as summarized in Table 2. Fast decay compo-
nents at 0.47 and 0.86 may be attributed to the photoinduced en-
ergy hopping process between the different BI-PDI conformers
generated because of deformation of its molecular planarity. The
occurence of faster energy trapping may be explained by the
through-space orientation of benzimidazole subunits toward to
the perylene core. Other decay values at 4.54 and 4.70 in solutions
may be attributed to the stationary fluorescence of PDI. Energy
transfer from the perylene core to the benzimidazole group is
unlikely at the excitation wavelength of 485 nm, because S1 state
of PDI chromophore is lower than that of benzimidazole subunit.
Thin films of BI-PDI, BI-PDI:PC61BM and P3HT:BI-PDI blends (as
shown in Fig. 1a, Fig. 2a and b, respectively) are prepared by doctor
blading 2 wt% Bz-Cl solution to form films. The absorption spec-
trum of BI-PDI gives a maximum signal at 571 nm and a shoulder
at 530 nm originating from the PDI core absorption. The absorption
spectrum of 1:1 (w:w) ratio of blend film for BI-PDI:PC61BM dis-
plays maxima at 564 nm with a shoulder at 525 nm and gives a
marked hypsochromic shift (about 5–7 nm) in maxima compared
1219, 772, 698 cmꢁ1 1H NMR (400 MHz, DMSO-d6 d 2.47 ppm):
.
d = 13.28 (1H, s), 8.07 (1H, d, J = 3.4 Hz), 7.96 (1H, d, J = 7.1 Hz),
7.59 (1H, d, J = 7.7 Hz), 7.51 (2H, t, J = 7.1 Hz), 7.38 (4H, m), 7.02
(2H, d, J = 7.7 Hz) ppm. 13C NMR [100 MHz, DMSO-d6 d 40.2 ppm
(7 peaks)]: d = 168.7, 162.6, 138.6, 132.4, 129.2, 129.1, 128.1,
126.8, 124.4, 122.0, 119.7, 117.8, 113.1, 111.4 ppm.
Synthesis of 2-(2-Hydroxyphenyl)-7-phenyl-1H-benzimidazole-4-
boronic acid (BI-B(OH)2)
Compound BI (110 mg, 0.30 mmol) was dissolved in 7 mL of
THF and the solution was cooled to ꢁ78 °C in cooling bath of dry
ice/acetone. n-butyllithium solution (1.6 M in n-hexane, 1 mL,
1.6 mmol) was added dropwise to solution under an argon atmo-
sphere. The mixture was stirred at ꢁ78 °C for 1 h and then
trimethyl borate (89 lL, 0.80 mmol) was added to solution slowly.
The solution was cooled to room temperature within the time per-
iod of 3 h and then stirred at room temperature overnight. The
solution was cooled to 0 °C and pH of the solution was adjusted
to 2–3 by adding 2 M of HCl solution. The reaction mixture was ex-
tracted with THF. Yield: 75%, FT-IR (KBr pellet, cmꢁ1): 3218 (boro-
nic acid OAH stretching), 2958, 2927, 2866, 1630, 1557, 1385
(m .
BAO), 1265, 1152, 1060, 772, 757, 700 cmꢁ1
Synthesis of N,N0-bis(2,6-diisopropylphenyl)-1,7-bis[2-(2-
hydroxyphenyl)-7-phenyl-1H-benzimidazole-4-yl]perylene-3,4,9,10-
tetracarboxylic diimide (BI-PDI)
To a 50 ml two-necked round-bottomed flask were added P2
dye (50 mg, 0.058 mmol) in 6 mL of benzene and Na2CO3 (2 M,
3 mL). A solution of compound BI-B(OH)2 (40 mg, 0.12 mmol) in

H. Dinçalp et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 197–206
201
Fig. 1. Normalized (a) UV–Vis absorption and, (b) fluorescence spectra of BI-PDI dye in solvents of different polarities at the concentrations of 10ꢁ6 mol/L and on thin films
(kexc = 485 nm). (c) Fluorescence decay of BI-PDI dye in chlorobenzene (kdetection = 570 nm, time increment = 194 ps).
Table 1
blends. The addition of P3HT to the BI-PDI results in the generation
of new emission peaks at 724 nm and bathochromic shifts at the
absorption maximum of BI-PDI from 656 to 666 nm. Both
the shifting of PDI emission maximum and also broad shape of
the emission peak support the aggregation behavior of BI-PDI
The UV–visible absorptions, fluorescence emissions and Stokes shifts (cmꢁ1) data of
BI-PDI in solvents of different polarities and on thin films (k (nm), e (L molꢁ1 cmꢁ1))
(kexc = 485 nm).
Solvent
ea
k1
k2
k3
k4(max)
kem(max)
ðmA ꢁ mFÞ
Chloroform
Chlorobenzene
Ethanol
Benzonitrile
Thin films
4.8
387
388
386
389
–
409
409
410
411
–
517
519
517
522
530
555
557
552
560
571
573
576
588
583
656
566
592
1109
704
dye. Photoluminescence studies indicate that p–p stacking of
P3HT is disturbed by blending with BI-PDI dye.
5.6
24.5
26.0
CV measurements
2269
a
Dielectric constant values were taken from the Ref. [47].
Fig. 3 illustrates the cyclic voltammograms of BI and BI-PDI
dyes and the results are given in Table 3. BI-PDI dye gives reduc-
tion waves at ꢁ0.43 and ꢁ0.65 V, indicating the formation of stable
PDI monoanion and dianion radicals, respectively. Those values are
well-correlated with reference values for bay-substituted PDI dyes
in the literature [35]. Also, BI dye exhibits oxidation wave at 1.25 V
and reduction wave at ꢁ1.21 V, indicating the formation of benz-
imidazole cation and anion radical, respectively. There is a good
correlation between the experimentally (ꢄ2.01 eV) and theoreti-
cally calculated (ꢄ2.10 eV) band gap values indicating the S0–S1
transition of BI-PDI dye in solution. The HOMO energy level of
BI-PDI dye is calculated to be about ꢁ5.92 eV. HOMO level of the
dye is lower than that of the HOMO level of the P3HT (ꢄꢁ5.1 eV)
molecule, indicating the more efficient hole transfer from PDI to
P3HT structure. The corresponding LUMO energy level of BI-PDI
dye is estimated to be ꢁ3.82 eV which is slightly higher than that
of PC61BM (ꢄꢁ3.9 eV) molecule. Also, BI-PDI dye has a suitable en-
ergy level as p-type organic semiconductor material towards to
PC61BM small molecule for OPV devices.
Table 2
Fluorescence decay times (sf(i) (ns)) and associated relative amplitudes (ai (%))a and
fluorescence lifetimes (sf (ns)) of BI-PDI in chlorobenzene and benzonitrile solutions
(kexc = 485 nm).
BI-PDI
sf(1)
a1
sf(2)
a2
sf
Chlorobenzene
Benzonitrile
0.47
0.86
4.1
5.0
4.54
4.70
95.9
95.0
2.51
2.78
a
Relative amplitude values,
ai, were calculated with the formula [48]: Wai =
P
sf ꢃ Pi/ si ꢃ Pi, where si was the decay time of the compound, and Pi was the
number of free parameters in the fit function (kdetection ¼ 575nm, v2 = 1.1). Fluo-
P
rescence life times are calculated with the formula: sf
=
sf(n)/n.
to the same spectrum for pristine BI-PDI. Absorption spectra for
thermally annealed films of P3HT:BI-PDI blends give the same
absorption maxima like pristine BI-PDI dye. However, it can be
seen from the Fig. 2b that both the intensity and the shape of the
signal are changed. These differences may be explained by the
aggregation behavior of PDI dye and the occurrence of the do-
nor–acceptor electronic interactions.
Atomic force microscopy (AFM) images
The emission of BI-PDI on thin film is centered at 656 nm, as
shown in Fig. 2c. The other observed peaks can be attributed to
the emission of PDI and P3HT films which were thermally annealed
To gain a better understanding of surface morphology which
gives us important data to form optimized structure of the device,
we have used atomic force microscopy images. Fig. 4a and b show

202
H. Dinçalp et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 197–206
Fig. 2. Ground state absorption spectra obtained on thin films of (a) BI-PDI:PC61BM and, (b) P3HT:BI-PDI blends under investigation. (c) Normalized steady-state
photoluminescence spectra of pristine BI-PDI dye and P3HT:BI-PDI blend films (kexc = 485 nm).
Table 3
Cyclic voltammeter data and molecular orbital energies of BI and BI-PDI dye with respect to the vacuum level.a
E0red2 ðVÞ
E0red1 ðVÞ
E0ox1 ðVÞ
Dye
LUMO-1 (eV)
LUMO (eV)
HOMOb (eV)
Egap (eV)
BI
BI-PDI
ꢁ1.21
ꢁ0.43
1.25
1.58
–
ꢁ3.6
ꢁ3.04
ꢁ3.82
ꢁ5.69
ꢁ5.92
2.65
2.10
ꢁ0.65
HOMO and LUMO energy levels of the dyes were determined by the formulas: ELUMO ¼ ꢁð4:8 þ E
Þ; E
¼ E0red ꢁ Eo0xðferroceneÞ; EHOMO ¼ ELUMO ꢁ Egap [49].
a
onset
red
onset
red
b
Band gap values were calculated from the onset values of the absorption bands.
the AFM images of thermally annealed BI-PDI:PC61BM (1:4, w/w)
and P3HT:BI-PDI (3:1, w/w) films, and also Fig. 4c shows BI:PC61BM
(1:2, w/w) blend films which are taken after device fabrication. As
seen, the root-mean-square (rms) roughness of BI-PDI:PC61BM
blend is smaller than that of P3HT:BI-PDI blend film. The rms
roughness of the active layers were 0.40 nm for BI-PDI:PC61BM,
2.01 nm for P3HT:BI-PDI and 0.47 nm for BI:PC61BM, respectively.
It is concluded that the addition of BI-PDI dye to P3HT polymer re-
duces the
backbone.
p–p interaction of PDI structure with polymeric
It is known that highly twisted geometric structure of bay-
substituted PDI molecules may adversely impact on molecular
packing and thin film morphology [38]. Also, similar rotation was
observed for benzimidazole substituted BI-PDI dye, which may
inhibits the regular p–p stacking of perylene core with thiophene
rings of P3HT structure when blending with the polymer.
Fig. 3. Cyclic voltammograms of BI-PDI dye in 0.1 M [TBA][PF6]/Me-CN at 50 mV/s
scan rate on glassy carbon working electrode. Inset shows BI voltammogram.
Device performance and external quantum efficiency (EQE)
obtained maximum PCE of 1.60% by using PDI type donor–acceptor
PDIs not only can be used as n-type semiconductor towards do-
nor materials, but also they can be employed as p-type semicon-
ductor in OPV devices. There is no sufficient data about the PDIs
performance as p-type material in OPV devices in literature. Only
a few reports are seen concerning OPV structures using perylenes
as donor material towards acceptors. Choi and co-workers have
structure containing p-extended structure that are incorporated
into the perylene units [50].
Benzimidazole substitution in 1-,7-positions for BI-PDI small
molecule is a good choice to minimize the aggregation effect of
PDI molecule. Also, the charge recombination between PDI mole-
cules may be minimized by substitution of functional groups in

H. Dinçalp et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 197–206
203
Fig. 4. Representative AFM images of (a) BI-PDI:PC61BM (1:4, w/w), (b) P3HT:BI-PDI (3:1, w/w) and (c) BI:PC61BM (1:2, w/w) blend films on the glass substrate coated from
chlorobenzene.

204
H. Dinçalp et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 197–206
Fig. 5. I–V curves of photovoltaic devices using (a) BI-PDI:PC61BM, (b) P3HT:BI-PDI and (c) BI:PC61BM blends as active layers.
Table 4
Table 5
Photovoltaic parameters of ITO/PEDOT:PSS/Active Layer/Ca(15 nm)/Ag(80 nm) bulk
heterojunction solar cells.
SCLC hole and electron mobilities of different combinations of thin film cast from
chlorobenzene solvent.
Active Layer
Voc (V)
Isc (mA/cm2)
FF (%)
g
(ꢃ10ꢁ3 %)
Solar cell structure
Donor:acceptor
Hole mobility
, cm2/Vs)
Electron mobility
(l
, cm2/Vs)
(
l
Donor:acceptor
w:w
w:w
P3HT:PC61BM
BI-PDI:PC61BM
1:1
1:1
1:2
1:4
1:5
1:1
2:1
3:1
1:2
0.55
0.08
0.47
0.60
0.64
0.28
0.28
0.26
0.31
7.91
61.3
9.4
2720
0.002
3.2
4.8
9.7
2.1
2.3
2.5
3.9
BI-PDI:PC61BM
BI:PC61BM
P3HT:BI-PDI
1:5
1:2
3:1
3.70 ꢃ 10ꢁ6 (R2:0.99)
1.01 ꢃ 10ꢁ6 (R2:0.99)
0.0003
0.025
0.032
0.055
0.032
0.036
0.039
0.033
26.0
25.4
27.2
23.8
22.8
24.9
35.8
2.58 ꢃ 10ꢁ5 (R2:0.99)
P3HT:BI-PDI
BI:PC61BM
In the first type of device production, BI-PDI small molecule is
used as donor material in typical sandwich configuration of ITO/
PEDOT:PSS/BI-PDI:PC61BM/Ca/Ag (Fig. 5a) instead of P3HT poly-
mer. Because absorption band of P3HT almost overlaps with the
shape of the absorption spectrum of BI-PDI dye in the visible re-
gion so that BI-PDI dye may collect the visible photon-flux at least
similar to P3HT polymer. The open-circuit voltage (VOC), short-cir-
bay positions of the perylene core [38]. Do and co-workers have
modified a perylene dye to imide–imidazole derivative comprised
of a benzimidazole moiety giving a PCE of 0.20% with the increase
in perylene content [51].
cuit current (ISC), fill factor (FF) and power conversion efficiency (g)
Fig. 6. EQE spectra of (a) BI-PDI:PC61BM (1:2, w/w) and (b) P3HT:BI-PDI (2:1, w/w) under investigation.

H. Dinçalp et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 197–206
205
Fig. 7. Dark current density-effective voltage characteristics of SCLC mobilities in (a) BI-PDI:PC61BM (1:5, w/w) and BI:PC61BM (1:2, w/w) and, (b) P3HT:BI-PDI (3:1, w/w) thin
film cast from chlorobenzene.
of all the device combinations are summarized in Table 4. The best
device using BI-PDI small molecule as donor with donor:acceptor
weight ratio of 1:5 exhibits a VOC of 0.64 V, ISC of 0.055 mA/cm2
and FF value of 27.2 with PCE of 0.01. ISC and VOC values of the de-
vices improve when PC61BM content of active layer blend in-
creases. These low efficiencies may be related to the weak
interaction of fullerene additive with the perylene core which is ro-
tated out of plane with bulky benzimidazole substituent due to
mobilities in BI-PDI:PC61BM and BI:PC61BM. The hole mobilities
for BI-PDI:PC61BM and BI:PC61BM blends are found to be
3.70 ꢃ 10ꢁ6 and 1.01 ꢃ 10ꢁ6 cm2/Vs, respectively. Also, electron
mobility for P3HT:BI-PDI blend is found to be about
2.58 ꢃ 10ꢁ5 cm2/Vs (Fig. 7b).
In generally, electron and hole mobilities of the devices are very
low and in the order of 10ꢁ6 cm2/Vs, which explain the poor effi-
ciencies of the BHJ solar cells. The low mobilities may be related
to the non-planar structure of PDI core which reduces the inter-
steric hindrance. However, PDI structure shows better p–p interac-
tion when the PC61BM content is increased in the blend. Finally,
charge injection increase and efficiencies can reach to slightly
higher values. Fig. 5b illustrates the I–V curves of the second type
of device with ITO/PEDOT:PSS/P3HT:BI-PDI/Ca/Ag configuration,
where BI-PDI was used as acceptor. PCE values of the devices are
very low and are not affected by the differences in blend ratios
of the active layer. PDI molecules aggregate in the presence of
P3HT polymer, as understood from PL experiments. This generates
intermolecular recombination centers which inhibit the charge
injection.
We have also tried device performance of pristine BI structure
in the device configuration of ITO/PEDOT:PSS/BI:PC61BM/Ca/Ag in
order to understand the benzimidazole group effect on photovol-
taic performance (Fig. 5c). In our study, BI structure in device gives
a FF of 35.8, which is lower than that of BI-PDI small molecule.
The external quantum efficiency (EQE) curves of BI-PDI:PC61BM
and P3HT:BI-PDI devices under illumination are shown in Fig. 6a
and 6b, respectively. The EQEs of the device cover the most of
the visible region wavelength in the range from 400 to 700 nm
for both configurations. Contribution of visible light to photocur-
rent is only about 0.44% in blend of BI-PDI with PC61BM device.
In the presence of P3HT films blended with the BI-PDI dye, the
EQE value is enhanced in the PDI region and reaches to the its high-
est value of 0.64%, which results from the overlap of PDI signal
with P3HT absorption in the same visible region. Although the
EQE shows relatively better overlap of the absorption spectrum
with the solar spectrum, its value is extremely low in order to
absorb sufficient light.
chain p–p interactions [55]. These interactions may be inhibited
by the bulky benzimidazole group in bay positions of the perylene
ring. It seems to be electron mobilities of BI-PDI small molecule in
blends are approximately ten times higher than hole mobilities of
BI-PDI dye so that this dye is more suitable to be used as acceptor
material in given components.
Conclusions
In this study, we have tried to disrupt the planarity of the per-
ylene core and also to prevent the aggregate formation by attach-
ing bulk benzimidazole groups in bay positions of the central ring
for the purpose of application in solution-processed BHJ solar cell.
All OPVs based on BI-PDI small molecule as donor and PC61BM as
acceptor or P3HT as donor and BI-PDI as acceptor have been inves-
tigated systematically. PL and absorption measurements support
the aggregation tendency of BI-PDI dye in the presence of P3HT
polymer, which results the deformation of
p–p stacking of P3HT
polymer with perylene core giving poor efficiencies. In the device
of BI-PDI:PC61BM blend, BI-PDI dye exhibits weak interaction with
fullerene due to the rotation out of plane resulting poor efficien-
cies. EQE and electron/hole mobility measurements also indicate
these low performances.
Twisted form of BI-PDI dye caused by bulk benzimidazole
group does not help to increase the short circuit current density
and device efficiency. Potential reason for those observations
may be related to the weak interaction of the dye with the compo-
nents when it is blended. In summary, twisted form, molecular
shape and morphology of BI-PDI dye play a promising role in
BHJ solar cell performance. Hence the morphological properties
of donor–acceptor couple will remain a highly essential issue for
the next researches.
Transport studies
The hole and electron mobilities of the components for the active
layers play an important role in discussing the photovoltaic perfor-
mance of the device [52,53]. We have used the space charge limited
current (SCLC) [54] regime to get information about the BI-PDI small
molecule and other blends given in Table 5. We have fabricated the
devices in a configuration of ITO/AZO(HT)/Active Layer/Ca/Ag for
electron mobilities and ITO/PEDOT:PSS/Active Layer/PEDOT:PSS/
Ag for hole mobilities. Fig. 7a shows I–V characteristics of SCLC
Acknowledgements
Financial support for this work was provided by the Research
Council of Celal Bayar University (BAP/2011-056) and the High
Education Council of Turkey. We thank to the State Planning
Organization of Turkey (DPT).

206
H. Dinçalp et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 197–206
[29] V. Kamm, G. Battagliarin, I.A. Howard, W. Pisula, A. Mavrinskiy, C. Li, K. Müllen,
References
F. Laquai, Adv. Energy Mater. 1 (2011) 297–302.
[30] W. Fu, C. He, Z. Li, J. Polym. Sci. A1 (50) (2012) 1333–1341.
[31] P.E. Keivanidis, V. Kamm, W. Zhang, G. Floudas, F. Laquai, I. McCulloch, D.D.C.
Bradley, J. Nelson, Adv. Funct. Mater. 22 (2012) 2318–2326.
[32] X. Guo, L. Bu, Y. Zhao, Z. Xie, Y. Geng, L. Wang, Thin Solid Films 517 (2009)
4654–4657.
[33] G.D. Sharma, P. Suresh, J.A. Mikroyannidis, M.M. Stylianakis, J. Mater. Chem. 20
(2010) 561–567.
[34] S. Jeong, Y.S. Han, Y. Kwon, M.-S. Choi, G. Cho, K.-S. Kim, Y. Kim, Synth. Met.
160 (2010) 2109–2115.
[35] C.C. Chao, M.K. Leung, Y.O. Su, K.Y. Chiu, T.H. Lin, S.J. Shieh, S.C. Lin, J. Org.
Chem. 70 (2005) 4323–4331.
[36] M. Queste, C. Cadiou, B. Pagoaga, L. Giraudet, N. Hoffmann, New J. Chem. 34
(2010) 2537–2545.
[37] G. Türkmen, S. Erten-Ela, S. Icli, Dyes Pigm. 83 (2009) 297–303.
[38] C. Li, H. Wonneberger, Adv. Mater. 24 (2012) 613–636.
[39] S. Song, H. Han, Y. Kim, B.H. Lee, S.H. Park, Y. Jin, I. Kim, K. Lee, H. Suh, Sol.
Energy Mater. Sol. Cells 95 (2011) 1838–1845.
[40] F.S. Mancilha, B.A. DaSilveira Neto, A.S. Lopes, P.F. Moreira, F.H. Quina, R.S.
Gonçalves, J. Dupont, Eur. J. Org. Chem. 2006 (2006) 4924–4933.
[41] A.S.D. Sandanayaka, K. Matsukawa, T. Ishi-i, S. Mataka, Y. Araki, O. Ito, J. Phys.
Chem. B 108 (2004) 19995–20004.
[1] Y. Lin, Y. Li, X. Zhan, Chem. Soc. Rev. 41 (2012) 4245–4272.
[2] X.H. Zhu, J. Peng, Y. Cao, J. Roncali, Chem. Soc. Rev. 40 (2011) 3509–3524.
[3] H.-S. Shim, H.J. Kim, J.W. Kim, S.-Y. Kim, W.-I. Jeong, T.-M. Kim, J.-J. Kim, J.
Mater. Chem. 22 (2012) 9077–9081.
[4] B.P. Rand, D. Cheyns, K. Vasseur, N.C. Giebink, S. Mothy, Y. Yi, V. Coropceanu, D.
Beljonne, J. Cornil, J.-L. Brédas, J. Genoe, Adv. Funct. Mater. 22 (2012) 2987–2995.
[5] S.Y. Leblebici, L. Catane, D.E. Barclay, T. Olson, T.L. Chen, B. Ma, ACS Appl.
Mater. Interfaces 3 (2011) 4469–4474.
[6] T. Mueller, R. Gresser, K. Leo, M. Riede, Sol. Energy Mater. Sol. Cells 99 (2012)
176–181.
[7] V.S. Barlier, C.W. Schlenker, S.W. Chin, M.E. Thompson, Chem. Commun. 47
(2011) 3754–3756.
[8] L. Valentini, D. Bagnis, A. Marrocchi, M. Seri, A. Taticchi, J.M. Kenny, Chem.
Mater. 20 (2008) 32–34.
[9] K.N. Winzenberg, P. Kemppinen, G. Fanchini, M. Bown, G.E. Collis, C.M. Forsyth,
K. Hegedus, T.B. Singh, S.E. Watkins, Chem. Mater. 21 (2009) 5701–5703.
[10] Q.Q. Shi, P. Cheng, Y.F. Li, X.W. Zhan, Adv. Energy Mater. 2 (2012) 63–67.
[11] P. Dutta, W. Yang, W.H. Lee, I.N. Kang, S.H. Lee, J. Mater. Chem. 22 (2012)
10840–10851.
[12] P. Dutta, W. Yang, S.H. Eom, S.H. Lee, Org. Electron. 13 (2012) 273–282.
[13] J. Zhang, J. Yu, C. He, D. Deng, Z.-G. Zhang, M. Zhang, Z. Li, Y. Li, Org. Electron. 13
(2012) 166–172.
[14] S. Qu, H. Tian, Chem. Commun. 48 (2012) 3039–3051.
[15] G.D. Sharma, J.A. Mikroyannidis, S.S. Sharma, M.S. Roy, K.R. Justin, Thomas,
Org. Electron. 13 (2012) 652–666.
[16] J.G. Mei, K.R. Graham, R. Stalder, J.R. Reynolds, Org. Lett. 12 (2010) 660–663.
[17] A. Iwan, E. Schab-Balcerzak, M. Siwy, A. Sikora, M. Palewicz, H. Janeczek, M.
Sibinski, Opt. Mater. 33 (2011) 958–967.
[18] J. Roncali, Chem. Soc. Rev. 34 (2005) 483–495.
[19] X. Zhan, A. Facchetti, S. Barlow, T.J. Marks, M.A. Ratner, M.R. Wasielewski, S.R.
Marder, Adv. Mater. 23 (2011) 268–284.
[20] P.E. Keivanidis, I.A. Howard, R.H. Friend, Adv. Funct. Mater. 18 (2008) 3189–
3202.
[21] E. Kozma, M. Catellani, Dyes Pigm. 98 (2013) 160–179.
[22] C. Zafer, M. Kus, G. Turkmen, H. Dincalp, S. Demic, B. Kuban, Y. Teoman, S. Icli,
Sol. Energy Mater. Sol. Cells 91 (2007) 427–431.
[23] H. Quante, Y. Geerts, K. Mullen, Chem. Mater. 9 (1997) 495–500.
[24] H. Langhals, J. Karolin, L.B.A. Johansson, J. Chem. Soc. Faraday Trans. 94 (1998)
2919–2922.
[25] H. Dinçalp, S. Içli, Sol. Energy 80 (2006) 332–346.
[26] D.K. Susarova, P.A. Troshin, D. Höglinger, R. Koeppe, S.D. Babenko, R.N.
Lyubovskaya, V.F. Razumov, N. Serdar Sariciftci, Sol. Energy Mater. Sol. Cells 94
(2010) 803–811.
[27] G.D. Sharma, P. Balraju, J.A. Mikroyannidis, M.M. Stylianakis, Sol. Energy Mater.
Sol. Cells 93 (2009) 2025–2028.
[28] W.S. Shin, H.-H. Jeong, M.-K. Kim, S.-H. Jin, M.-R. Kim, J.-K. Lee, J.W. Lee, Y.-S.
Gal, J. Mater. Chem. 16 (2006) 384–390.
[42] B.A.D. Neto, A.S. Lopes, M. Wust, V.E.U. Costa, G. Ebeling, J. Dupont,
Tetrahedron Lett. 46 (2005) 6843–6846.
[43] A.T. Khan, T. Parvin, L.H. Choudhury, Synth. Commun. 39 (2009) 2339–2346.
[44] S.-J. Liu, Q. Zhao, Q.-L. Fan, W. Huang, Eur. J. Inorg. Chem. 2008 (2008) 2177–
2185.
_
[45] H. Dinçalp, Sß. Kızılok, S. Içli, Dyes Pigm. 86 (2010) 32–41.
[46] J.R. Knutson, J.M. Beechem, L. Brand, Chem. Phys. Lett. 102 (1983) 501–507.
[47] J.C. Scaiano, CRC Handbook of Organic Photochemistry, Taylor & Francis Group,
1989.
[48] M. Maus, R. De, M. Lor, T. Weil, S. Mitra, U.M. Wiesler, A. Herrmann, J. Hofkens,
T. Vosch, K. Mullen, F.C. De Schryver, J. Am. Chem. Soc. 123 (2001) 7668–
7676.
[49] J. Pommerehne, H. Vestweber, W. Guss, R.F. Mahrt, H. Bassler, M. Porsch, J.
Daub, Adv. Mater. 7 (1995) 551–554.
[50] M.H. Kim, M.J. Cho, K.H. Kim, M.H. Hoang, T.W. Lee, J.-I. Jin, N.S. Kang, J.-W. Yu,
D.H. Choi, Org. Electron. 10 (2009) 1429–1441.
[51] B.R. Sim, B.G. Kim, J.K. Lee, J.Y. Do, Thin Solid Films 519 (2011) 8091–8094.
[52] N.I. Craciun, J. Wildeman, P.W.M. Blom, Phys. Rev. Lett. 100 (2008) 056601–
056605.
[53] M.M. Mandoc, L.J.A. Koster, P.W.M. Blom, Appl. Phys. Lett. 90 (2007).
133504:1ꢁ133504:3.
[54] P.W.M. Blom, M.J.M. deJong, M.G. vanMunster, Phys. Rev. B 55 (1997) R656–
R659.
[55] J.-H. Kim, H.U. Kim, D. Mi, S.-H. Jin, W.S. Shin, S.C. Yoon, I.-N. Kang, D.-H.
Hwang, Macromol. 45 (2012) 2367–2376.