Synthesis and characterization of two fluorenone-based conjugated polymers and their application in solar cells and thin film transistors

Article abstract of DOI:10.1016/j.tetlet.2016.02.048

Two new fluorenone-based conjugated polymers, F1 and F2, were synthesized via the Stille cross-coupling polymerization. Spectral measurements showed that both polymers had optical band gaps of 1.99 eV and 1.89 eV, respectively, calculated based on the film onset absorption edge. Cyclic voltammetric studies gave HOMO/LUMO energy levels of -5.18 eV/-3.19 eV and -5.40 eV/-3.51 eV, respectively, and the polymers were thermally stable at temperatures over 300 °C. Organic field-effect transistor and bulk-heterojunction solar cell performance characteristics of both polymers as p-type semiconductors were evaluated.

Full text of DOI:10.1016/j.tetlet.2016.02.048

Accepted Manuscript
Synthesis and characterization of two fluorenone-based conjugated polymers
and their application in solar cells and thin film transistors
Chia Juan Lim, Yanlian Lei, Bo Wu, Lu Li, Xuyao Liu, Yong Lu, Furong Zhu,
Beng S. Ong, Xiao Hu, Siu Choon Ng
PII:
S0040-4039(16)30157-5
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.02.048
TETL 47323
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
5 July 2015
7 February 2016
12 February 2016
Please cite this article as: Lim, C.J., Lei, Y., Wu, B., Li, L., Liu, X., Lu, Y., Zhu, F., Ong, B.S., Hu, X., Ng, S.C.,
Synthesis and characterization of two fluorenone-based conjugated polymers and their application in solar cells and
thin film transistors, Tetrahedron Letters (2016), doi: http://dx.doi.org/10.1016/j.tetlet.2016.02.048
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Graphical Abstract
Synthesis of two fluorenone-based polymers
and their applications in solar cells and thin
film transistors
Leave this area blank for abstract info.
a
b
b
a
a
a
b
c
d
Chia Juan Lim , Yanlian Lei , Bo Wu , Lu Li , Xuyao Liu , Yong Lu , Furong Zhu , Beng S. Ong *, Xiao Hu ,
a
Siu Choon Ng *

1
Tetrahedron Letters
Synthesis and characterization of two fluorenone-based conjugated polymers and their
application in solar cells and thin film transistors
a
b
b
a
a
a
b
c
Chia Juan Lim , Yanlian Lei , Bo Wu , Lu Li , Xuyao Liu , Yong Lu , Furong Zhu , Beng S. Ong *, Xiao
d
a
Hu , Siu Choon Ng *
a
Nanyang Technological University, School of Chemical & Biomedical Engineering, Division of Chemical and Biomolecular Engineering, 62 Nanyang Drive,
Singapore 637459.
b
Hong Kong Baptist University, Department of Physics, Kowloon Tong, Kowloon, Hong Kong SAR.
Hong Kong Baptist University, Department of Chemistry, Research Centre of Excellence for Organic Electronics, and Institute of Creativity, Kowloon Tong,
c
Kowloon, Hong Kong SAR.
d
Nanyang Technological University, School of Material Science Engineering, Nanyang Avenue, Singapore 639798.
*Corresponding Author Tel: +65 6790 4062; Fax: +65 6794 7553
Email: ngsc@ntu.edu.sg; (S.C. Ng)
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
Two new fluorenone-based conjugated polymers, F1 and F2, were synthesized
via Stille cross-coupling polymerization. Spectral measurements showed that both
polymers had optical band gaps of 1.99 eV and 1.89 eV respectively, calculated
based on the film onset absorption edge. Cyclic voltammetric studies gave
HOMO/LUMO energy levels of -5.18 eV/-3.19 eV and -5.40 eV/-3.51 eV
Available online
Keywords:
Fluorenone
Conjugated Polymers
OPV
o
respectively, and the polymers were thermally stable at temperatures over 300 C.
Organic field-effect transistor and bulk-heterojunction solar cell performance
characteristics of both polymers as p-type semiconductors were evaluated.
OFET
Stille Coupling
2
009 Elsevier Ltd. All rights reserved.
1. Introduction
The growing interest in organic electronics in both academia and
industry over the last decade has spurred intense research in new
organic semiconductor development. These are needed for use as
active materials in organic electronic applications such as organic
solar cells (OSC) and organic field-effect transistors (OFET).
Organic semiconductors, particularly organic polymer
semiconductors, are advantageous for these applications in view
of their highly desirable properties such as being mechanically
flexible, thus enabling manufacturing of thin and lightweight,
mechanically robust and flexible electronic devices using plastic
substrates rather than traditional bulkier, brittle glass substrates.
In addition, manufacturing of organic electronic devices via high-
throughput roll-to-roll printing instead of traditional batch-wise
photolithographic processes has dramatically lowered the
performance of small molecular semiconductors, such as
mobility, varies greatly with evaporation conditions. On the other
hand, the performance of a polymer semiconductor is more
forgiving with respect to the rate of evaporation, and its
crystallinity is often achieved through a post-deposition thermal
annealing step. With respect to polymer semiconductors for use
in organic electronics, recent interest has been directed to
alternating -conjugated donor (D) and acceptor (A) polymers
(D-A polymers) which have shown great promise as ambient
stable, high-performance active materials in both OFET and
2
OSC. We herein report our studies on new fluorenone-based D-
A polymers for OFET and OSC applications.
Fluorenone is a simple electron deficient structure which is an
inexpensive and commercially readily available starting material.
Its relatively planar structure, high thermal stability and good
1
production cost of organic electronics. Organic polymer
semiconductors are in theory superior to organic small molecular
semiconductors for solution processing or printing and offer
better performance reproducibility as the crystallinity of small
molecular semiconductors is far more susceptible to evaporation
rates during small molecule crystallization. Thus, the
electron-transport properties have made it
a potentially3
interesting electron-deficient candidate for D-A polymer design.
For our studies, we have utilized fluorenone as an electron
acceptor moiety in conjunction with structurally equally planar

2
Tetrahedron Letters
donor structures, benzodithiophene and thienothiophene, to
construct D-A polymers F1 and F2 respectively.
2. Results and Discussion
o
2
Scheme 1. (i) NaOH, Zn, H O then RBr, NBu
4
Br, 6 h, 48%; (ii) n-BuLi, THF, Me
3
SnCl, -78 C to rt, 83%; (iii) Pd(PPh
)
3 4
(2.5 mol%),
o
toluene, 100 C, 45 h, 71%.
Benzodithiophene 2 was synthesised from quinone 1 via a single
electron transfer and alkylation reaction followed by stannylation
4
according to literature procedures. Benzothiophene 2 underwent
Stille cross-coupling polymerization with dibromofluorenone 3 to
afford polymer F1 in a good yield of 71% (Scheme 1).
Scheme 2. (i) i-PrMgCl·LiCl in THF then 5, overnight, 76%; (ii) AlCl
3
, LiAlH
4 2 3
, Et O, overnight, 51%; (iii) n-BuLi then Me SnCl, -78
o
o
C, overnight, 96%; (iv) Pd(PPh
)
3 4
(10 mol%), THF, reflux, overnight, 68%; (v) NBS, THF, 40 C, overnight, 53%; (vi) Pd(PPh
3 4
)
(2.5
o
mol%), toluene, 100 C, 45 h, 76%; (vii) PCC, MgSO
4
, CH Cl , overnight, 92%.
2
2
6
(2-Octyldodecyl)thiophene 7, which was synthesized from 3-
3 4
a mixture of AlCl and LiAlH , afforded thiophene 7. Long
bromothiophene, was subjected to stannylation to afford
trimethyltin compound 8. Thereafter, compound 8 was coupled
with dibromofluorenone via Stille coupling to afford dithiophene
fluorenone 9. The latter compound was brominated using N-
bromosuccinimide (NBS) to afford dibromo compound 10. Stille
cross-coupling polymerization of dibromodithiophenefluorenone
branched alkyl chains were employed in both polymers to
promote solubility (Scheme 2). As expected polymers F1 and F2
were soluble in common organic solvents such as hexane, diethyl
ether, THF, chloroform, dichloromethane, chlorobenzene and
dichlorobenzene despite having high molecular weights.
o
1
0 with organotin 11 afforded polymer F2 in 76% yield. For the
synthesis of thiophene 7, 3-bromothiophene was subjected to the
Turbo Grignard” reagent, i-PrMgCl·LiCl, to generate the
Copolymer
Mn
Mw
PDI
7.47
2.86
T ( C)
d
F1
F2
11099
12873
82909
36945
320
390
“
corresponding Grignard reagent, thiophen-3-ylmagnesium
5
bromide in-situ, followed by reaction with 2-octyldodecanal 5,
Table 1. Molecular weights and thermal properties of F1 and F2
which was obtained from oxidation of the corresponding alcohol,
to afford the corresponding alcohol 6. Subsequent
dehydroxylation of 6 with AlH3, which was generated in situ from

3
o
o
C and 390 C, suggesting that both polymers had high thermal
100
stability (Figure 1a). Polymer F1 had a higher PDI (7.47)
compared to polymer F2 (2.86). No glass transitions were
observed in DSC analysis (Figure 1b).
F2
F1
80
60
40
1
.0
.9
0
F1 in CHCl3
F1 Thin film
100
200
300
400
500
600
700
o
0.8
Temperature ( C)
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
F2
F1
Exo
400
500
600
700
800
A
F2 in solution
F2 on thin film
1
.0
.9
Endo
o
F2 on thin film annealed at 200 C
0
50
100
Temperature
150
200
o
C
0.8
.7
0.6
.5
0.4
0
o
-1
Figure 1. (a) TGA plots at a heating rate of 10 C min under
0
o
-1
nitrogen (top) and (b) DSC plots at a heating rate of 10 C min
0.3
0.2
0.1
0.0
under nitrogen (below).
Gel permeation chromatography (GPC) with polystyrene as a
calibration standard was used to determine the molecular weight
and polydispersity index (PDI) showing that both polymers had
high molecular weights (Table 1). Thermogravimetric (TGA)
analysis and differential scanning calorimeter (DSC) were
employed to determine the thermal properties of polymers F1
and F2. Both experiments were conducted with heating under a
continuous flow of nitrogen. From TGA, the onset temperatures
400
500
600
wavelength/ nm
700
800
Figure 2. UV absorption of polymer F1 (top) and polymer F2
(below).
for weight loss (T
d
) of polymers F1 and F2 were respectively 320
UV-Vis absorption spectra
Cyclic Voltammetry
Solution
Film
Annealed at 200 C
o
opt
onset

max

onset

max

onset
E
g
E
ox
HOMO
(eV)
LUMO
(eV)
a
b
a
b
[nm]
[nm]
609
612
[nm]
[nm]
624
656
max
onset
(ev)
(V)
F1 415
F2 427
425
452
-
-
1.99
0.78
-5.18
-5.40
-3.19
-3.51
462
666
1.89
1.00
Table 2. Summary of optical data, including maximum absorption peak, absorption edge wavelength (λonset) and optical band gap
opt
g
(E ) as well as the electrochemical data of polymer F1 and F2.
The UV absorption spectra of polymers F1 and F2 in solution
and as thin films featured broad absorption bands from ca. 350 to
near-infrared region were a direct result of the extensive -
delocalization in the F1 and F2 D-A polymer systems. In the
thin-film spectra, significant red shifts in the absorptions
800 nm (Figure 2). The broad absorptions extending into the

4
Tetrahedron Letters
compared to those of the solution spectra were observed, with
4 6
acetonitrile solution containing 0.1M Bu NPF at a scan rate of
-
1
concomitant appearance of longer-wavelength shoulders,
indicating higher molecular ordering in the solid state. The
maximum absorption, max, of F1 and F2 increased by 10 nm and
50 mV s (Figure 3). The onset of the corresponding oxidative
peaks of the polymers were used to estimate the HOMO and
onset
LUMO levels using the equation HOMO = - ( Eox
+ 4.4) eV,
opt
g
25nm respectively from their absorption spectra in solution to
and LUMO = E
+ HOMO; Polymer F2 showed a lower
that in the thin film state (Table 2).
HOMO energy level (−5.40 eV) in comparison to polymer F1 (-
5.18 eV). The LUMO levels for F1 and F2 were calculated to be
−3.19 and −3.51 eV, respectively (Table 2). Both polymers
possessed deep HOMO levels suggesting that the polymers were
not only chemically stable under ambient conditions but also
have a high open circuit voltage.
o
Thermal annulation at 200 C led to an intensification of the
shoulder peaks in polymer F2, and further red shifts in
absorption, suggesting that F2 had higher crystallinity than F1
arising from stronger - stacking. This is likely due to the more
rigid backbone structure of F2, which promoted closer -
stacking. The alkoxy side-chains on the donor moiety of F1 may
The devices were then characterized using a Keithley 2636B
system with two-channel SourceMeter Units under an ambient
environment. FET mobility was extracted using the following
equation in the saturation regime from the gate sweep:
have led to steric hindrance to - stacking, thus lowering the
opt
g
crystallinity compared to F2. The optical band gaps (E
) for F1
and F2 were determined to be 1.99 eV and 1.89 eV respectively
from their film absorption edges (onset) (Table 2).
I
D
= μCi(V
is the drain current, μ is the field-effect mobility, Ci is
the capacitance per unit area of the gate dielectric layer (SiO
G T
– V )2 (W/2L)
where I
D
2
,
-
2
3
00 nm, Ci = 11 nF cm ), and V
voltage and threshold voltage. V
relationship between the square root of I
and V by extrapolating the measured data to I
G
and V
was derived from the
at the saturated regime
= 0. W and L are
T
are respectively gate
T
D
G
D
F1
F2
respectively channel width and length. The OFET
characterization of polymers F1 and F2 are shown in Table 3 and
Figure 4.
0
1
+
Potential (V vs Ag/Ag )
Figure 3. Cyclic Voltammetry plots of polymer F1 and F2
The electrochemical properties were determined using cyclic
voltammetry from the thin film layer of F1 and F2 in an
2
-1 -1
Mobility (ave.)/cm V s
o
o
As fabricated
Annealed at Annealed at 120 C Annealed at 150 C
o
o
Polymer (dried at 60 C) 90 C for 1 h for 1 h
for 1 h
V
th/V
Ion/Ioff
-
5
-5
-4
-5
-4
-5
-4
3
F1
F2
3.3x10
4.3x10
1.5x10
6.1x10
2.6x10
7.3x10
3.4x10
-10V
-15V
10
-
4
4
0.96x10
~10
Table 3. Mobility and on/off ratio of polymers from OFET devices with L = 80 µm, W = 1500 µm.

5
Figure 4. Transfer characteristics and output curves of F1 (left) and F2 (right).
Both polymers F1 and F2 behaved as p-type semiconductors in
OFET devices. Mobility of the polymers improved upon thermal
annulation as a result of achieving higher molecular ordering
after heat treatment. As expected, F2 exhibited higher mobility
2
.0
.5
1
1.0
.5
700rpm ~ 80nm
0
-
4
2
-1 -1
2
-1 -1
0.0
0.5
1.0
(
3.4x10 cm V s ) than F1 (7.3 x 10 cm V s ) by virtue of its
-
-
higher crystallinity, as reflected from its absorption spectra,
arising from the higher extent of - stacking (Table 3). The
observed low mobility of the two polymers may be understood
from the relatively weak electron acceptor properties of the
fluorenone moiety in the polymers, which was not efficient in
promoting extensive ground-state charge transfer in the D-A
-1.5
-1.0
-0.5
0.0
0.5
1.0
V (V)
3
2
1
0
1
2000rpm ~ 50nm
7
polymer system. The characterization data of OSC devices
comprising of open-circuit voltage (VOC), short circuit current
-
(J
sc), fill factor (FF) and power conversion (PCE) are summarized
-2
in Table 4 and the current density-voltage plots for OSC devices
with active layers of F1 and F2 (right) with PC71BM (1:1) are
shown in Figure 5.
-3
-1.0
-0.5
0.0
0.5
1.0
V (V)
Figure 5. Current density - voltage plots for OSC devices with
active layers of F1 (top) or F2 (bottom) with PC71BM (1:1).
-2
Polymer
F1
V
oc/V
J
sc/mA c
FF
PCE%
0.26
0.84
0.85
0.75
1.71
0.42
0.44
F2
0.64
Table 4. OPV characterization of polymer F1 and F2.
Polymer F2 showed higher PCE than polymer F1, which was
consistent with the result of OFET, as the mobility of F2 was
higher than that of F1. The PCEs for both polymers were
relatively low due to their low hole mobility. The open circuit
voltage for F1 and F2 was determined as 0.84V and 0.85V
respectively. The high open circuit voltage for both polymers was
due to the deep HOMO levels of both polymers. Polymer F1 had
a lower short circuit current (0.75 mA) in comparison to F2 (1.71
mA), which was due to steric hindrance of the alkoxy side chain
which interfered with - stacking. Polymer F1 had a low fill
factor (0.42), due to its wide polydiversity that hindered effective
long range conjugation. On the other hand, polymer F2 has a low
fill factor (0.44), which possibly due to its molecular weight was
insufficiently high for extensive conjugation (Table 4).

6
Tetrahedron
3. Conclusion
References and notes
In conclusions, two fluorenone based D-A polymers, F1 and F2,
were synthesized via Stille cross-coupling polymerization. Both
F1 and F2 have very high decomposition temperatures and low
HOMO levels, and are thus resistant to oxidative doping in air.
These polymers showed polycrystallinity and F2 exhibited higher
molecular ordering than F1 by virtue of its comparatively more
planar backbone structure. However, both showed low OFET and
OSC performance, likely due to the fact that the fluorenone
structure was not a good electron acceptor for building D-A
polymer semiconductors for organic electronic applications.
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5
We would like to thank A*STAR (Grant No. 102 170 0135),
Singapore and Hong Kong Baptist University (Strategic
Development Fund: SDF13-0531-A02) for financial support of
this work, and NTU for the facilities. We would like to thank Dr
Yufeng Cai for his great assistant on GPC spectroscopy.
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