(E)-2,2'Dibromo-7,7'-bis(diphenylamino)-9,9'-bifluorenylidene as a new electron acceptor for organic photovoltaic cells

Article abstract of DOI:10.1166/jnn.2014.9887

(E)-2,2'-Dibromo-7,7'-bis(diphenylamino)-9,9'-bifluorenylidene (BDPABF) was synthesized as a new non-fullerene-type electron acceptor for organic photovoltaic cells. The UV-visible absorption spectra of BDPABF showed two main bands at 319 and 474 nm in a

Full text of DOI:10.1166/jnn.2014.9887

Article
Journal of
Nanoscience and Nanotechnology
Vol. 14, 8225–8230, 2014
Copyright © 2014 American Scientific Publishers
All rights reserved
Printed in the United States of America
www.aspbs.com/jnn
ꢀ
ꢀ
ꢀ
(
E)-2,2 Dibromo-7,7 -Bis(diphenylamino)-9,9 -
Bifluorenylidene as a New Electron Acceptor
for Organic Photovoltaic Cells
∗
On You Park, Hee Un Kim, Jong Baek Park, and Do-Hoon Hwang
Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University,
Busan 609-735, Republic of Korea
ꢀ ꢀ ꢀ
E)-2,2 -Dibromo-7,7 -bis(diphenylamino)-9,9 -bifluorenylidene (BDPABF) was synthesized as a new
(
non-fullerene-type electron acceptor for organic photovoltaic cells. The UV-visible absorption spec-
tra of BDPABF showed two main bands at 319 and 474 nm in a chloroform solution and 320 and
4
81 nm as a solid thin film. The optical band gap of BDPABF was determined to be 2.34 eV by
measuring the onset absorption wavelength of the solid thin film. The ionization potential of BDPABF
was determined to be 5.59 eV using photoelectron spectroscopy. The measured lowest unoccupied
molecular orbital energy level of BDPABF was − 3.25 eV. Its electron-accepting ability was inves-
tigated through a Stern–Volmer quenching experiment. The intensity of the photoluminescence of
P3HT dramatically decreased upon the addition of BDPABF. The measured Stern–Volmer quench-
Delivered by Publishing Technology to: McMaster University
4
−1
ing constant was 5ꢀ3 × 10 M . Photovoltaic devices were fabricated using P3HT as the electron
IP: 221.224.81.206 On: Wed, 10 Feb 2016 06:40:00
donor and BDPABF as theC eo l pe cy trr i og nh t a: cAc me p et or ir c aa tn v Sa rc i oi eu ns ti cf ioc mPp uo bs li ti si o hn e rr as tios. The optimized device
showed a maximum power conversion efficiency of 0.27% with an open-circuit voltage of 0.71 V,
short-circuit current density of 1.29 mA/cm , and fill factor of 0.29 after thermal annealing at 100 C
for 5 min.
2
ꢁ
ꢀ
Keywords: Organic Solar Cell, Electron Acceptor, 9,9 -Bifluorenylidene Derivative.
1
. INTRODUCTION
used as electron acceptors for OPVs because of their tun-
able lowest unoccupied molecular orbital (LUMO) energy
level, which enables a high Voc, and high electron mobil-
ity, which is obtained through three-dimensional charge
transfer and enables a high Jsc. In 2009, Li et al. reported
a P3HT-based BHJ OPVs containing an indene C60 bis-
adduct (ICBA), which achieved a maximum power conver-
Bulk-heterojunction (BHJ) organic photovoltaic cells
(OPVs) are promising alternative energy sources, and thus
have been extensively investigated in recent years. OPVs
have many advantages over silicon-based inorganic photo-
voltaic cells, such as low manufacturing cost, lightweight,
and flexibility. The active layer in OPVs comprises an
electron donor such as a low band-gap polymer, and elec-
tron acceptor such as a fullerene derivative. The active
6
sion efficiency (PCE) of 6.48%. However, there are some
drawbacks to using fullerene derivatives as acceptors,
including difficult synthesis and purification, high manu-
facturing cost, and poor light absorption in the visible-
1
layer is the key component of OPVs. For the donor
material, some design rules have been established: The
donor material should have a low band gap for broad light
absorption, high hole mobility for a high short-circuit cur-
rent density (J ꢀ, and a deep highest occupied molecular
orbital (HOMO) energy level for a high open-circuit volt-
7
near-IR region. Therefore, the design and synthesis of
new non-fullerene-type electron acceptors is important in
OPVs esearch. Several non-fullerene-type electron accep-
sc
8
tors have been studied, such as naphthalene diimide,
9
10
ꢀ
age (V ꢀ, etc.
diketopyrrolopyrrole (DPP), benzothiadiazole, and 9,9 -
oc
1
1
[6,6]-Phenyl-C-butyric acid methyl ester (PC BM and
bifluorenylidene (99’BF).
6
1
2
–6
PC BM) and fullerene derivatives
are predominantly
Among these, 99’BF derivatives have attracted signif-
icant scientific attention because of their unique photo-
physical properties as electron acceptors. One of the
71
∗
Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2014, Vol. 14, No. 11
1533-4880/2014/14/8225/006
doi:10.1166/jnn.2014.9887
8225

BDPABF as a New Electron Acceptor for Organic Photovoltaic Cells
Park et al.
CrO3
1
0.7 mmol), and sodium tert-butoxide (1.1 g, 11.4 mmol)
Br
Br
Br
Br
Acetic anhydride
ꢁ
in toluene (100 mL). The mixture was refluxed at ∼110 C
under a nitrogen atmosphere for 2 d. Cooling, extrac-
tion with ethyl acetate and the usual work-up resulted in
crude material, which was purified by column chromatog-
O
1
(
CH3)3CONa
Pd2 (dba)3
(CH3)3C]3P
raphy (dichloromethane/hexane = 1:10) to give the desired
[
N
Br
1
1
product (0.98 g, 21.42%). H NMR (300 MHz, CD CO
3
Toluene, 110 ºC
N
H
CD ; ꢂ [ppm]): 7.75 (d, 1H), 7.72 (d, 1H) 7.61–7.65
3
2
O
1
3
(m, 4H), 7.35–7.40 (m, 4H), 7.14–7.18 (m, 6H).
C
O
NMR (75 MHz, CDCl ; ꢂ [ppm]): 192.59, 149.79, 147.12,
143.66, 137.46, 136.64, 136.21, 135.40, 129.84, 128.07,
127.76, 125.24, 124.16, 121.82, 121.49, 121.25, 11898.
FT-IR (KBr, cm ꢀ: 3055 (multi, aromatic C–H stretch),
1
3
S
P
S
P
S
S
Br
N
O
−
1
Lawesson’s reagent
Toluene, 110 ºC
2
705 (single, C O), 1589, 1487 (single, aromatic C C).
N
Br
ꢀ
ꢀ
2
.4. Synthesis of (E)-2,2 -Dibromo-7,7 -
ꢀ
BDPABF
Bis(diphenylamino)-9,9 -Bifluorenylidene
(
BDPABF)
ꢀ
ꢀ
Scheme 1. Synthetic route to (E)-2,2 -dibromo-7,7 -bis(diphenyl-
amino)-9,9 -bifluorenylidene (BDPABF).
ꢀ
A mixture of compound 2 (0.98 g, 1.9 mmol) and Lawes-
son’s reagent (0.54 g, 1.3 mmol) in toluene (50 mL)
was refluxed for 12 h. Cooling, extraction with ethyl
acetate and the usual work-up resulted in crude mate-
rial, which was purified by column chromatography
ꢀ
fluorene rings in 99 BF can easily accept one electron from
the donor material to satisfy the 14 ꢁ-electron requirement
for aromaticity.
(
dichloromethane/hexane = 1:3) to give the desired prod-
ꢀ
Herein, we report a new 99’BF derivative, (Eꢀ-2,2 -
1
uct (0.36 g, 19%). H NMR (300 MHz, CD COCD ; ꢂ
ꢀ
ꢀ
3
3
dibromo-7,7 -bis(diphenylamino)-9,9 -bifluorenylidene
BDPABF), for use as an OPVs electron acceptor. By
Delivered by Publishing Technology to: McMaster University
[
1
ppm]): 8.36(s, 1H), 8.25 (s, 1H), 8.11 (s, 1H), 7.85 (s,
(
H), 7.60–7.73 (m, 4H), 7.53 (dd, 1H), 7.44 (dd, 1H),
introducing a diphenylamine moiety, we expected to raise
IP: 221.224.81.206 On: Wed 7, . 12 50 – F7 e. 3 b5 2( m0 1, 68 H0 6) ,: 47 0. 0: 00 –0 7.13 (m, 12H), 6.97 (d, 2H).
the LUMO energy level of the 99’BF derivative. We also
1
3
C
Copyright: American S cNi eMn Rt if i(c7 5P uM b Hl i sz h, Ce rDs Cl ; ꢂ [ppm]): 147.68, 140.94, 140.26,
3
introduced two electron-withdrawing bromine atoms to
promote the abstraction of an electron from the donor
material. The synthetic route to BDPABF and its chemical
structure are shown in Scheme 1.
1
1
40.08, 139.33, 135.55, 132.35, 129.75, 129.55, 126.21,
24.60, 124.21, 123.34, 122.98, 121.04, 120.74, 120.16
FT-IR (KBr, cm ꢀ: 3032 (multi, aromatic C–H stretch),
−
1
1
8
588, 1488 (single, aromatic C C). MALDI-TOF (m/z =
20ꢃ27ꢄM+ꢀꢀ. Elemental analysis for C H Br N : cal-
5
0
32
2
2
2
. EXPERIMENTAL DETAILS
culated, C: 73.18; H: 3.93; N: 3.41; found, C: 71.64, H:
3
.72, N: 4.88.
2
.1. Materials
Fluorene and Lawesson’s reagent were purchased from
Alfa Aesar, and chromium oxide, diphenyl amine,
tris(tert-butyl) phosphine, and sodium tert-butoxide were
purchased from Aldrich. Tris(dibenzylideneacetone) dipal-
ladium(0) was purchased from Strem Chemicals, Inc.
All chemicals were used as received without further
purification.
3
. RESULTS AND DISCUSSION
3.1. Characterization
BDPABF was synthesized by the reductive coupling of 2-
bromo-7-(diphenylamino)-9H-fluoren-9-one using Lawes-
son’s reagent, as shown in Scheme 1. The synthesis of
BDPABF was confirmed by H and C NMR spec-
troscopy and MALDI-TOF mass spectrometry, as shown in
Figure 1. The FT-IR spectra of compound 2 and BDPABF
1
13
2
.2. Material Synthesis
2
,7-Dibromo-9H-fluorene and 2,7-dibromo-9H-fluoren-9-
(Fig. 2) showed aromatic C–H stretching at around 3055
one (1) were prepared according to previously reported
−1
and 3032 cm , respectively, and aromatic C C stretch-
ing at 1589, 1487 and 1588, 1488 cm , respectively. The
literature.12
−1
spectrum of compound 2 shows a specific carbonyl absorp-
tion peak at 1705 cm that is not present in the spectrum
of BDPABF.
−
1
2
.3. Synthesis of 2-Bromo-7-(Diphenylamino)-
H-Fluoren-9-One (2)
9
A prepared solution of tris(dibenzylideneacetone) dipalla-
dium(0) (0.2 g, 0.2 mmol) and tris(tert-butyl) phosphine
The thermal properties of BDPABF were investigated
using thermogravimetric analysis (TGA) under a nitro-
(
0.04 g, 0.2 mmol) in toluene, as the catalyst, was added
gen atmosphere. The decomposition temperature (T ꢀ of
BDPABF was 392 C which is comparable to those of
d
ꢁ
to a solution of 1 (3.0 g, 8.9 mmol), diphenylamine (1.8 g,
8
226
J. Nanosci. Nanotechnol. 14, 8225–8230, 2014

Park et al.
BDPABF as a New Electron Acceptor for Organic Photovoltaic Cells
820.27 (M+)
1.0
Solution
Film
0
0
0
0
0
.8
.6
.4
.2
.0
500
600
700
800
900
1000
300
400
500
600
700
800
Mass (m/z)
Wavelength (nm)
Figure 1. MALDI-TOF mass spectrum of BDPABF.
Figure 3. UV-vis absorption spectra of BDPABF in chloroform solution
and as a solid thin film.
5
fullerene derivatives. The melting point (T ꢀ of BDPABF
m
was determined to be 150 °C using differential scanning
calorimetry (DSC). BDPABF exhibited good thermal sta-
bility, losing less than 5% of their weight upon heating
to temperatures greater than 386 C according to thermo-
gravimetric analysis (TGA).
was 2.34 eV, and the ꢅmax and ꢅonset values were red-
shifted with respect to those of 99’BF (458 and 513 nm,
1
1
respectively) because of the extension of the effective
conjugation length by the incorporation of the dipheny-
lamine moiety.
ꢁ
The ionization potential was measured to be 5.59 eV
using photoelectron spectroscopy, as shown in Figure 4.
The LUMO energy level (ELUMOꢀ of BDPABF was calcu-
3
.2. Optical and Electrochemical Properties
Figure 3 shows the UV-visible absorption spectra of
BDPABF in chloroform solution and as a solid thin film.
The absorption spectrum of the chloroform solution shows
opt
lated by combining the measured Eg and HOMO energy
opt
Delivered by Publishing Techno lloe gv ey lt o( E: McMꢀ a, si t. ee .r , UE nivers=ityE
+ Eg . The HOMO
HOMO
LUMO
HOMO
two main absorption bands at aI Pr o: u2n 2d 1 .321 29 4 .a 8n 1d . 24 07 64 On nm :;Wed, 10 Feb 2016 06:40:00
and LUMO energy levels of BDPABF were −5.59 and
∗
these bands correspond to the ꢁ–ꢁ tr Ca no s pi tyi or ing ho tf : dA i pm h ee nr i yc -a n Scientific Publishers
−
3.25 eV, respectively. As expected, the measured LUMO
∗
lamine and ꢁ–ꢁ transition of the dimerized fluorene
11
energy level was higher than that of 99’BF (−3.37 eV).
structure in BDPABF, respectively. The absorption in the
ultraviolet region is stronger than in the visible region,
and the absorption band of BDPABF is much broader
than that of PC BM. The maximum absorption (ꢅmaxꢀ
and onset absorption wavelength (ꢅonsetꢀ of the BDPABF
solid thin film were 481 and 530 nm, respectively (see
To evaluate the electron-accepting ability of BDPABF,
we conducted a fluorescence quenching experiment on
poly(3-hexylthiophene) (P3HT). The intensity of the pho-
toluminescence (PL) of P3HT in chloroform was measured
with increasing BDPABF concentration. At a low concen-
tration of the quencher, the relationship between the flu-
orescence intensity and concentration of BDPABF obeys
the Stern–Volmer equation, as follows:
6
1
opt
Table I). The optical band-gap energy (Eg ꢀ of BDPABF
0
ꢆ /ꢆ = 1+K ꢇquencherꢈ
sv
N
Br
0
where ꢆ and ꢆ represent the PL intensity in the absence
O
and presence of the quencher, respectively, and Ksv is
the Stern–Volmer quenching constant. The obtained Stern–
Volmer plot for BDPABF is shown in Figure 5(a). The plot
is linear in the quencher concentration range of 0 to 9ꢃ8×
aromatic C-H
C=O stretch
aromatic C=C
−
5
4
−1
10
M. The obtained K value is 5ꢃ3×10 M , which is
sv
Br
N
N
Table I. Optical and electrochemical properties of BDPABF.
ꢅmax, abs (nm)
Br
opt
a
Solution
Film
ꢅonset (nm) Eg (eV) HOMO (eV) LUMO (eV)
3
000
2500
2000
1500
1000
500
Wavenumbers (cm–1
)
319, 474 320, 481
530 2.34 −3.25
−5.59
Note: aCalculated from ꢅonset of the absorption spectrum of the film.
Figure 2. FT-IR spectra of compound 3 and BDPABF.
J. Nanosci. Nanotechnol. 14, 8225–8230, 2014
8227

BDPABF as a New Electron Acceptor for Organic Photovoltaic Cells
Park et al.
Table II. Device performance of OPVs based on P3HT and BDPABF
2
under AM 1.5G illumination at 100 mW/cm .
4
3
2
1
0
0
0
0
0
Voc
(V)
Jsc
(mA/cm2ꢀ
PCE
(%)
Rsh
(ꢉ cm2ꢀ
Rs
(ꢉ cm2ꢀ
Ratio
FF
a
1:0.3
1:0.5
1:0.7
1:1.0
0.63
0.65
0.81
0.71
1.19
1.00
1.29
0.88
1.13
0.28
0.28
0.27
0.29
0.29
0.29
0.27
0.30
0.11
0.16
0.11
0.26
0.18
0.27
0.13
0.20
1136
1024
892
762
760
680
697
653
632
613
353
337
339
300
406
222
b
a
b
a
b
a
b
0
.73
0.57
.75
0.64
.71
0.56
.59
5.59 eV
0
0
0
4.0
4.5
5.0
5.5
6.0
a
b
ꢁ
Note: pristine, annealing at 100 C for 10 min.
Photon energy (eV)
Figure 4. Photoelectron spectra of BDPABF.
are shown in Figure 5(b). The PL intensity of the P3HT
film was also dramatically reduced by the presence of
BDPABF.
1
3
comparable to those of other reported non-fullerene and
1
4
fullerene-based fluorescence quenchers. The PL quench-
ing experiment and high Stern–Volmer quenching constant
suggest that BDPABF can efficiently accept an electron
from P3HT. The PL emission spectra of P3HT and the
active layer (P3HT:BDPABF = 1:0.7, w/w) in the film state
3.3. Device Performance
Photovoltaic devices were fabricated using BDPABF as
the acceptor and P3HT as the donor with a device struc-
ture of ITO/PEDOT:PSS/P3HT:BDPABF/LiF/Al. Four dif-
ferent compositions (P3HT:BDPABF = 1:0.3, 1:0.5, 1:0.7,
and 1:1.0, w/w) and two different conditions (pristine and
(
a) 10
ꢁ
1
1
40
20
1
2
3
4
5
after thermal annealing at 100 C for 5 min) were tested
to determine the optimum fabrication conditions. The Voc
8
6
4
2
0
100
Delivered by Publishing Technology to: McMaster University
8
6
4
2
0
0
0
0
0
values of the pristine devices were ∼ 0.60 V; after ther-
IP: 221.224.81.206 On: Wed, 10 Feb 2016 06:40:00
mal annealing, the V values increased to 0.70 V, except
oc
Copyright: American Scientific Publishers
for the device with a P3HT:BDPABF ratio of 1:1.0 (w/w).
Even though the Voc values of the fabricated devices were
slightly higher than that of a P3HT-based device with a
500
550
600
650
700
750
Wavelength (nm)
3
PC BM acceptor (∼ 0.65 V), they had much lower J
6
1
sc
values and fill factors (FFs). The J values of the pristine
sc
2
devices were less than 1 mA/cm ; after thermal annealing,
the J values increased slightly to 1.29 mA/cm .
2
sc
0
2
4
6
8
10
All the fabricated devices showed relatively low FFs.
The FF of an OPVs is related to the internal resistance of
[
BDPABF] / 10–5
M
(
b)
Pure P3HT
P3HT:BDPABF(1:0.7)
0
0
0
0
0
1
1
1
.0
.2
.4
.6
.8
.0
.2
.4
P3HT:BDPABF
:0.3
1:0.5
:0.7
1
1
1:1.0
600
650
700
750
800
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Wavelength (nm)
Voltage (V)
Figure 5. (a) Stern–Volmer quenching plots for BDPABF: PL quench-
ing spectra of P3HT:BDPABF in CHCl with increasing concentration
of BDPABF (10 M): (1) 0.0, (2) 2.4, (3) 4.9, (4) 7.3, (5) 9.8. (b) PL
quenching spectra of P3HT:BDPABF (1:0.7, w/w).
Figure 6. J–V curves of OPVs fabricated using P3HT:BDPABF active
3
−5
2
layers under AM 1.5G illumination at 100 mW/cm after thermal anneal-
ꢁ
ing at 100 C for 5 min.
8
228
J. Nanosci. Nanotechnol. 14, 8225–8230, 2014

Park et al.
BDPABF as a New Electron Acceptor for Organic Photovoltaic Cells
Figure 7. AFM images of P3HT:BDPABF devices fabricated with ratios (a) 1:0.3, (b) 1:0.5, (c) 1:0.7 and (d) 1:1.0 (w/w, after thermal annealing at
ꢁ
1
00 C for 5 min).
the active layer and resistance between the active layer and
and 3.44 nm, respectively. The surfaces of the active lay-
electrodes. The series resistance (R ꢀ and shunt resistance
ers were smoother than that of the P3HT:PC BM film
s
61
1
6
(
R ꢀ of the fabricated OPVs were measured and are sum-
(∼ 8 nm after annealing). Generally, the surface rough-
sh
marized in Table II. The R values of the fabricated devices
ness of the active layer of P3HT-based OPVs is increased
s
were relatively high, while tDh ee l Ri ver ve ad lu be ys Pw ue rb eli sl oh wi ne gr tTh ea nc hno lbo yg yt h teor :m Ma lc tMr e aa st mt eern Ut bn ei vc ea ur ss ei t yo f the increased crystallinity
sh
4
IP: 221.224.81.206 On: Wed, 10 Feb172016 06:40:00
those of high efficiency OPVs ; this is the primary rea-
of P3HT, which suggests that P3HT is not sufficiently
15
Copyright: American Scientific Publishers
son for the low FFs of the devices. We expect that the
electron mobility of BDPABF would be much lower than
the hole mobility of P3HT, although the charge mobilities
of the active layer were not measured in this preliminary
study. The poor balance between the hole and electron
mobility would also contribute to the low FFs. The use
crystalline in the active layers of our fabricated devices;
this is a potential reason for the low Jsc values of the
devices.
4
. CONCLUSION
We synthesized BDPABF as a non-fullerene-type electron
acceptor for organic photovoltaic cells. The UV-visible
absorption band of BDPABF was much broader than that
of fullerene derivatives. BDPABF effectively quenched the
photoluminescence of P3HT. BDPABF has a smaller opti-
ꢀ
of new 99 BF derivatives that have higher mobilities and
an interfacial layer between the active layer and electrodes
could improve the FFs and PCE values of the OPVs.
The current–density–voltage (J–V ꢀ curves of the fab-
ricated devices are shown in Figure 6. The maximum
PCE of 0.27% was obtained using the active layer with
P3HT:BDPABF = 1:0.7 that underwent thermal annealing
ꢀ
cal band gap and higher LUMO energy level than 99 BF.
In this preliminary study, the photovoltaic devices fab-
ricated using BDPABF and P3HT showed low power-
conversion efficiency because of their low short-circuit
currents and fill factors. The performance of these OPVs
could be improved by further optimization of the devices;
further modification of the molecular structure of BDPABF
to impart higher electron mobility.
ꢁ
at 100 C for 5 min. The V , J , and FF values of the
oc
sc
2
device were 0.71 V, 1.29 mA/cm , and 0.29, respectively.
The performances of the fabricated OPVs are summarized
in Table II.
3
.4. Surface Morphology of Active Layer
Surface morphology of the active layer is an important
factor in OPVs. The surface conditions of the fabricated
devices were investigated using atomic force microscopy
Acknowledgments: This work was supported by New
and Renewable Energy Program of the Korea Institute
of Energy Technology Evaluation and Planning (KETEP)
funded by the Korea government Ministry of Trade, indus-
try and Energy (20113010010030), and National Research
Foundation (NRF) grants funded by the Korean gov-
ernment (MEST) (Nos. 2014007318 and 2011-0030668
through GCRC SOP).
(AFM). Figures 7(a)–(d) show AFM images of the films
with P3HT:BDPABF ratios of 1:0.3, 1:0.5, 1:0.7, and 1:1.0
ꢁ
(
w/w, after thermal annealing at 100 C for 5 min), respec-
tively. In the AFM images, the root mean square (RMS)
values of the four active layer films were 3.39, 3.51, 3.58,
J. Nanosci. Nanotechnol. 14, 8225–8230, 2014
8229

BDPABF as a New Electron Acceptor for Organic Photovoltaic Cells
Park et al.
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Received: 10 April 2013. Accepted: 9 December 2013.
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IP: 221.224.81.206 On: Wed, 10 Feb 2016 06:40:00
Copyright: American Scientific Publishers
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