Synthesis and polymerisation of novel methacrylates with carbazolyl and benzofuranyl pendant groups for photovoltaic applications

Article abstract of DOI:10.1080/15421400801904732

This article reports properties of photovoltaic devices containing new donor materials - copolymers referred as PCEMB, composed of 2-(carbazol-9-yl) ethyl methacrylate (CEM) and benzofuran-2-carboxylic acid 2-(2- methylacryloyloxy)ethyl ester (MB). Manufactured photovoltaic cells consisted of donor and acceptor (PBT or P3OT) layers sandwiched between ITO covered glass and evaporated aluminum electrode. Performance of the cell was optimized for a copolymer composed of 8% CEM and 92% MB. The efficiency, found equal c.a. =0,03%, was low comparing to other photovoltaic cells reported in the literature. However, following the observed tendencies, it can be supposed that there exists a large reserve for the enhancement of the open circuit voltage through further chemical modifications of the chromophore. In the case of the PDT containing cells, measured Uoc was substantially lower. This fact may be attributed to difference in carrier mobility between this polymer and PCEMB. Simultaneously, the larger sizes of the MB molecule may favor separation of the charges and leads to enhancement of the corresponding voltage.

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Synthesis and Polymerisation
of Novel Methacrylates
with Carbazolyl and
Benzofuranyl Pendant Groups
for Photovoltaic Applications
M. Pokladko a , J. Sanetra a , E. Gondek a , D. Bogdal
b , J. Niziol c & I. V. Kityk d
a Institute of Physics, Cracow University of
Technology , Krakow, Poland
b Cracow University of Technology, Faculty of
Chemical Engineering and Technology , Krakow,
Poland
c Faculty of Physics and Applied Computer Science,
AGH University of Science and Technology , Kraków,
Poland
d Institute of Physics, J. Dlugosz University
Czêstochowa , Czêstochowa, Poland
Published online: 22 Sep 2010.
To cite this article: M. Pokladko , J. Sanetra , E. Gondek , D. Bogdal , J. Niziol & I. V.
Kityk (2008) Synthesis and Polymerisation of Novel Methacrylates with Carbazolyl and
Benzofuranyl Pendant Groups for Photovoltaic Applications, Molecular Crystals and
Liquid Crystals, 484:1, 335/[701]-344/[710], DOI: 10.1080/15421400801904732
To link to this article: http://dx.doi.org/10.1080/15421400801904732
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Mol. Cryst. Liq. Cryst., Vol. 484, pp. 335=[701]–344=[710], 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400801904732
Synthesis and Polymerisation of Novel Methacrylates
with Carbazolyl and Benzofuranyl Pendant Groups
for Photovoltaic Applications
M. Pokladko1, J. Sanetra1, E. Gondek1, D. Bogdal2,
J. Niziol3, and I. V. Kityk4
1Institute of Physics, Cracow University of Technology, Krakow, Poland
2Cracow University of Technology, Faculty of Chemical Engineering and
Technology, Krakow, Poland
3Faculty of Physics and Applied Computer Science, AGH University of
ꢀ
Science and Technology, Krakow, Poland
4
ˆ
ˆ
Institute of Physics, J. Dlugosz University Czestochowa, Czestochowa,
Poland
This article reports properties of photovoltaic devices containing new donor
materials – copolymers referred as PCEMB, composed of 2-(carbazol-9-yl)ethyl
methacrylate (CEM) and benzofuran-2-carboxylic acid 2-(2-methylacryloylox-
y)ethyl ester (MB). Manufactured photovoltaic cells consisted of donor and
acceptor (PBT or P3OT) layers sandwiched between ITO covered glass and evapo-
rated aluminum electrode. Performance of the cell was optimized for a copolymer
composed of 8% CEM and 92% MB. The efficiency, found equal c.a. g ¼ 0,03%,
was low comparing to other photovoltaic cells reported in the literature. However,
following the observed tendencies, it can be supposed that there exists a large
reserve for the enhancement of the open circuit voltage through further chemical
modifications of the chromophore.
In the case of the PDT containing cells, measured Uoc was substantially lower.
This fact may be attributed to difference in carrier mobility between this polymer
and PCEMB. Simultaneously, the larger sizes of the MB molecule may favor
separation of the charges and leads to enhancement of the corresponding voltage.
Keywords: fill factor; open circuit voltage; photovoltaic cell; short circuit current
density
The research was partially supported by the grant number PB-0687=T09=2002=23.
Address correspondence to M. Pokladko, Institute of Physics, Cracow University of
Technology, Podchora˛z˙ych 2, Krakow, Poland. E-mail: mpokladko@if.pk.edu.pl
335=[701]

336=[702]
M. Pokladko et al.
INTRODUCTION
The conversion of the sun light into the electric power requires gener-
ation of both negative and positive charge, their space separation and
finally, a driving force pushing these charges trough an external elec-
tric circuit. In organic semiconductors, absorption of photons leads to
formation of bound electron-hole pairs (excitons) rather than free
charges. These excitons can be separated at some number of sites.
Next, the separated charges need to travel to respective electrodes,
holes to the anode and electrons to the cathode. This separation pro-
vides voltage drop between electrodes and can be injected into an
external circuit. All these processes are crucial for efficiency of the
solar cells.
During the last decade, a whole range of conjugated polymers, copo-
lymers, polymers blends, and fluorescent dye-doped polymers have
been reported to issue photovoltaic properties while applied in appro-
priates structures. Already developed semiconducting and conducting
conjugated polymers represents typical advantages like small weight,
flexibility, ultra-fast optoelectronic response, nearly continuous ten-
ability of materials energy levels and band-gaps possible to be altered
via molecular design and synthesis. They are also versatile materials
for processing and device fabrication and low cost on large scale indus-
trial manufacturing [1,2]. Recently, poly(N-vinylcarbazole) (PVK) has
attracted attention in applications related to photovoltaic devices and
it serves as one of layers either pristine or in or in blends with other
materials [3].
In view of design of new materials for photovoltaic devices, we have
investigated a group of novel carbazole copolymers, applied as active
layer in photovoltaic cells together with poly(3-decylthiophene)
(PDT) or poly(3-octylthiophene) (P3OT). The copolymers are composed
of 2-(9-carbazolyl) ethyl methacrylate (CEM) and benzofuran-2-car-
boxylic acid 2-(2-methylacryloyloxy)ethyl ester (MB) (Fig. 1).
MATERIALS
Composition and structure of all synthesised compounds were verified
by mean of NMR, IR, GC=MS techniques. 1H-NMR spectra were
recorded in CDCl3 on Tesla 80 MHz NMR spectrometer. IR spectra
were recorded using BioRad FTS 165 FT-IR spectrophotometer. Mass
spectra and purity of all the intermediates were measured using
Hewlett-Packard 5971 mass detector. The molecular weight of the
reaction was estimated in THF by gel permeation chromatography
(GPC) using a Knauer Instrument chromatograph equipped with PL

Synthesis and Polymerisation of Novel Methacrylates
337=[703]
FIGURE 1 2-(9-Carbazolyl) ethyl methacrylate (CEM) and benzofuran-2-
carboxylic acid 2-(2-methylacryloyloxy)ethyl ester (MB).
gel 10 ml mixed columns. The calibration curve was obtained using
polystyrene standards.
Monomer Synthesis
Both CEM (Fig. 2) and MB (Fig. 3) were prepared through novel syn-
thetic methods that were recently developed in our laboratory [4,5].
N–(2-hydroxyethyl)carbazole (I): The mixture of carbazole (1.0 g,
0.006 mol) and ethylene carbonate (2.1 g, 0.024 mol) was stirred and
irradiated in the microwave reactor NOVA 2004 (Ertec, Poland) for
30 minutes at 150ꢀC. A trace amount of NaOH (0.1 g, 2.5 mmol) was
added to the reaction mixture prior to the reaction. After the reaction,
the product was precipitated from solvent by water ꢁC2H5OH solution
and recrystallized from cyclohexane. Yield: 75%, m.p. 69–71ꢀC.
MS ¼ 212 ((M þ 1), 5%), 211 (Mþ , 30%), 180 (100%), 181 (15%),
152 (18%).
IR (KBr): 3419 (w), 3213 (w), 3048 (w), 2976 (w), 2955 (w), 2937 (w),
2918 (w), 2868 (w), 1687 (m), 1656 (m), 1595 (m), 1485 (s), 1458 (s),
1388 (m), 1364 (s), 1350 (s), 1326 (s), 1245 (m), 751 (s), 720 (s), 559
(w), 491 (m), 423 (m) cmꢁ1.
2–(9–Carbazolyl)ethyl methacrylate (CEM): was prepared in the
reaction conducted at ambient temperature: N-(2-hydroxyethyl)
carbazole (2.11 g, 10 mmol) was mixed with methacrylic acid (0.9 g,

338=[704]
M. Pokladko et al.
FIGURE 2 Synthesis of 2-(9-carbazolyl) ethyl methacrylate (CEM).
10.5 mmol) and 4-pyrrolidinopyridine (0.2 g, 1.3 mmol) as a catalyst in
ethylene dichloride (10 ml). During first 30 minutes of the reaction, an
equimolar amount of N,N’-dicyclohexylcarbodiimide (DCC) (1.4 g,
10 mmol) in ethylene dichloride (5 ml) was added. The reaction was
FIGURE 3 The synthesis of and benzofuran-2-carboxylic acid 2-(2-methyla-
cryloyloxy)ethyl ester (MB).

Synthesis and Polymerisation of Novel Methacrylates
339=[705]
completed after 24 hours to afford CEM conversion higher than 80%.
A white solid of the side-product, N,N0-dicyclohexylurea (DCU), was
filtered from the solution, and the filtrate was stored and slowly con-
centrated until white crystals of the desired product precipitated.
Then the crude product was recrystallized from ethanol. Yield: 85%.
MS (70 eV): 280 ((M þ 1) 7%), 279 (Mþ 34%), 193 (39%), 181 (14%),
180 (100%), 152 (17%).
IR (KBr): 3050 (m), 2978 (m) 2054 (m), 1719(s), 1627 (m), 1596 (m),
1485 (m), 1454 (s), 1358 (m), 1318 (s), 1295 (s), 1171 (s), 1153 (s), 946
(m), 816 (m), 745 (s), 720 (s), 648 (w), 559 (w), 472 (w), 420 (m) cmꢁ1
1H NMR (80 MHz, CDCl3): 1.79–1.82 (3H, s, CH2 CꢁCH3), 4.53–
=
=
4.60 (4H, m, CH2ꢁCH2), 5.45–5.49 (1H, m, trans C CH2), 5.92 (1H,
=
s, cis C CH2), 7.13–7.50 (6H, m, aromatic ring), 8.03–8.1 (2H, m, aro-
matic ring) ppm.
Ethyl 6-diethylamino-2-benzo[b]furancarboxylate (II): Potassium
carbonate (2.70 g, 20 mmol), TBAB (0.16 g, 0.50 mmol), and 4-diethyla-
minosalicylaldehyde (5.0 mmol) were thoroughly mixed and placed in
an round-bottom reaction flask. Then chloroacetate ester (10 mmol)
was added dropwise The mixture was thoroughly stirred with a spat-
ula for a few seconds and placed in the microwave reactor NOVA 2004
(Ertec, Poland) then irradiated for 10 minutes at 140ꢀC. Upon com-
pletion of the reaction, monitored by GC=MS, the mixture was
extracted with methylene chloride, and the solvent was then removed.
The crude product was purified by column chromatography (cyclo-
hexane=methyl chloride 8 : 2) yielding ethyl 6-diethylamino-2-
benzo[b]furancarboxylate as a red oil, yield 80%.
MS (70 eV): 261 (M1, 42), 247 (16), 246 (100), 0218 (23), 190 (10), 116
(6.7%).
IR (KBr): 2973(s), 2931(m), 2899(m), 2872(w), 1716(s), 1626(s),
1578(m), 1556(m), 1509(s), 1398(m), 1372(s), 1357(m), 1303(m),
1270(s),1241(s), 1180(s), 1120(s), 1095(m), 1017(m), 844(w), 795(m),
759(m) cmꢁ1;
1H NMR (80 MHz, CDCl3): 7.49–7.26 (2H, m, aromatic protons),
6.77–6.40 (2H, m aromatic protons), 4.40 (2H, q, CH2ꢁMe), 3.40 (4H,
q, NCH2Me), 1.40 (3H, t, CH2ꢁMe), 1.19 (6H, t, Hz, NCH2Me) ppm.
6-diethylaminobenzo[b]furan-2-carboxylic
6-diethylamino-2-benzo[b]furancarboxylate was converted into
6-diethylaminobenzo[b]furan-2-carboxylic acid by hydrolysis
acid
(III): Ethyl
a
reaction in a 0.5 mol NaOH (40 ml) and methanol (40 ml) solution,
which was conducted still in the same vessel under microwave
irradiation (reactor NOVA 2004, Ertec, Poland) for 25 minutes.
Finally, 6-diethylaminobenzo[b]furan-2-carboxylic acid was precipitated

340=[706]
M. Pokladko et al.
from the solution by the addition of water and hydrochloric acid until
pH of the solution reached 2. The crude product was dried and used in
the next stage without purification.
Benzofuran-2-carboxylic acid 2-(2-methylacryloyloxy)ethyl ester
(MB): A mixture of 6-diethylaminobenzo[b]furan-2-carboxylic acid
(0.49 g, 2.1 mmol), 2-hydroxyethyl methacrylate (0.40 g, 3 mmol), 4-
pyrrolidinopyridine (0.08 g, 0.5 mmol) as a catalyst in a THF solution
was stirred at ambient temperature for 24 hours. During the first
30 min. of the reaction, DCC (0.55 g, 2.7 mmol) in a THF solution
was added dropwise. Next, DCU was filtered off from the solution,
and the solvent was evaporated under reduced pressure to give crude
MB as a heavy oil; yield 70% (Fig. 3).
1H NMR (80 MHz, CDCl3): 7.46–7.22 (2H, m, aromatic protons), 6.70–
=
6.40 (2H, m aromatic protons), 5.92 (1H, s, cis C CH2), 5.44–5.49 (1H, m,
=
trans C CH2), 4.57–4.62 (4H, m, CH2ꢁCH2), 3.40 (4H, q, NꢁCH2Me),
=
1.76–1.82 (3H, s, CH2 CꢁCH3), 1.19 (6H, t, NCH2Me) ppm.
Polymerisation
The monomers CEM and MB (total amount 0.5 g) taken in desired
molar ratio (see first column of Table 1) were dissolved in toluene
(10 ml) and AIBN (1%) was added as initiator. The reaction mixture
was flushed with argon for 20 min. and then heated in a water bath
at 60ꢀC to initiate polymerisation. After 48 hr the reaction was termi-
nated by adding THF (15 ml) and the polymer precipitated into a large
amount of methanol. The product was several times redissolved in
THF and reprecipitated into methanol.
The molecular masses of the copolymers were estimated in THF by
gel permeation chromatography (GPC) using polystyrene as a
standard to give mass and number average masses shown in Table 1.
PDT and P3OT polymers were obtained by courtesy from Prof.
Malgorzata Zagorska, Warsaw University of Technology.
TABLE 1 Number and Weight Average Molecular Masses of the Copolymers
of MB and CEM
Copolymer
Mn
Mw
Pd
MB (1%) CEM (99%)
MB (2%) CEM (98%)
MB (5%) CEM (95%)
MB (8%) CEM (92%)
1,34 ꢂ 104
1,47 ꢂ 104
1,25 ꢂ 104
1,25 ꢂ 104
3,43 ꢂ 104
4,15 ꢂ 104
3,80 ꢂ 104
4,01 ꢂ 104
2,56
2,82
3,04
3,20

Synthesis and Polymerisation of Novel Methacrylates
341=[707]
PHOTOCELL PREPARATION
Figure 4 shows the photovoltaic cell architecture. Electron donor (D)
and electron acceptor (A) materials were deposited subsequently by
spin-coating method from solvents unique for the given polymer to
avoid deterioration of deposited layers. Glass substrates were partially
coated by indium–tin oxide (ITO). The average thickness of the actives
layers was estimated to be about 100 nm. At the end cells were covered
with 100 nm thick layer of vacuum evaporated aluminum to ensure
good electrical contact.
Both exciton and charge transport in organic materials usually
require hopping from chromophore to polymer. In order to meet these
specific requirement for efficient photon to charge conversion different
device architectures have been developed in the past [1].
Charge separation normally occurs at the interface between D and
A polymers. Double layer cell structure benefits from the single charge
transport in each layer what provides electrical connectivity to the
right electrode and gives tiny probability of carriers mutual
recombination.
The difference of work functions of electrodes DWf can cause the
bands tilting, which creates a constant electrostatic across the bulk.
According to this picture the driving force i.e. Voc is mainly determined
by the offset at the D=A junction although the increased built in elec-
tric field can help to transport charges through the bulk. The behav-
iour of photovoltaic cell can be seen in traditional the fourth
quadrant of the current-voltage characteristic in [6].
The overall efficiency g of a photovoltaic cell can be calculated by
standard formula:
ꢃ
g ¼ Voc JscꢃFF=Plight
ð1Þ
where Voc is the open circuit voltage, Jsc the short circuit current
density in A=m2, FF is the fill factor and, Plight is the incident solar
FIGURE 4 Device designs used for cells comprising two components, an elec-
tron donor (D) and an electron acceptor material (A).

342=[708]
M. Pokladko et al.
radiation in mWatt=cm2. Because of the spectral and intensity depen-
dences the power conversion efficiencies are only meaningful for a
given spectral distribution and intensity [6].
RESULTS AND DISCUSSION
Absorption and photoluminescence spectra of four prepared PCEMB
copolymers, were measured, because these characteristics play princi-
pal role in the photovoltaic response. Samples were prepared in form
of free standing few micrometer thick films, obtained from THF solu-
tions at room temperature followed by drying at 40ꢀC in vacuum for
24 h. Alternatively, copolymers were transformed in sub-micrometer
thin layers by spin-casting [3].
Both absorption and photoluminescence spectra of all the copoly-
mers are presented in Figure 5.
The photoluminescence maximum was observed at about 430 nm.
Current–voltage characteristics of a studied photovoltaic cells in the
dark and under illumination (halogen lamp Plight ¼ 1255 mWatt=cm2)
are shown in Figure 6. In the dark the devices exhibit a clear current
rectification, whereas illuminated, they present a short–circuit cur-
rent density and an open–circuit voltage. Such behaviors reflect sub-
stantial influence of MB content on the output open circuit voltage.
Table 2 presents photovoltaic parameters for double layer photovol-
taic cells consisting of ITO=PCEMMB=PDT=Al. The best values of the
JSC and VOC parameters for double layer architecture were obtained
for the cell number 8, with P3OT and the photovoltaic response for this
case is equal to about g ¼ 0,03%. This value is low compared to the
known photovoltaic cells, however following the principal tendencies
FIGURE 5 Examples of absorption (left) and photoluminescence (right) spec-
tra of copolymer series PCEM ꢂ MB.

Synthesis and Polymerisation of Novel Methacrylates
343=[709]
FIGURE 6 J-V Characteristics dark and under illumination of ITO=
PCEMMB=Al devices (4th run).
an opportunity appears for the enhancement of the open circuit volt-
age by further chemical modification of the chromophores.
In the case of the PDT polymers the Uoc is substantially lower and
this fact may be related to different carrier mobility for these two types
of polymers. Paralelly, the larger sizes of the MB molecule may favor a
separation of the charges and leads to enhancement of the correspond-
ing voltage. The similar effects were shown during investigations of

344=[710]
M. Pokladko et al.
TABLE 2 Photovoltaic Parameters for a Sequence of Layers of ITO=Active
Layers=Al Devices
Cell no.
Active layers
Jsc [mA=cm2]
Uoc [mV]
FF
1
2
3
4
5
6
7
8
PCEM1MB=PDT
PCEM2MB=PDT
PCEM5MB=PDT
PCEM8MB=PDT
PCEM1MB=P3OT
PCEM2MB=P3OT
PCEM5MB=P3OT
PCEM8MB=P3OT
0,18
0,09
0,43
0,17
1,36
1,496
0,699
3,120
540
410
720
520
810
840
630
860
0,19
0,23
0,17
0,24
0,16
0,10
0,18
0,15
stilbene chrmophore incorporated into the photopolymer matrices and
were manifested in nonlinear optical effects [7].
CONCLUSIONS
Best manufactured in frame of current work photovoltaic cells, cannot
compete with leading architectures, already proposed in the extensive
literature.
However, the synthesized series of copolymers can serve a starting
point for future developing of carbazole-based photovoltaic polymer
materials. This direction seems very promising due to well known
thermal stability and good charge transport by PVK and its
derivatives.
REFERENCES
[1] Skotheim, T. A., Elsenbaumer, R. L., & Reynold, J. R. (1998). Handbook of Conduct-
ing Polymers, 2nd ed. Marcel Dekker: New York.
[2] Sanetra, J. (2006). Efekt fotowoltaiczny w organicznych ogniwach słonecznych –
ꢀ
wybrane zagadnienia, Oficyna Wydawnicza Politechniki Krakowskiej, Krakow.
[3] Pokladko, M., Sanetra, J., & Gondek, E. (2006). e-Polymers, Polymeric Photovoltaic
Devices (http://www.e-polymers.org).
[4] Sanetra, J., Bogdal, D., & Warzala, M. (2002). Chem. Mat., 14, 89.
[5] Bogdal, D. & Warzala, M. (2000). Tetrahedron, 56, 8769.
[6] Pokladko, M., Sanetra, J., Gondek, E., Nizioł, J., & Jaglarz, J. (2006). Polimeric
Photovoltaic devices. e-Polymers.
[7] Kityk, I. V., Makowska-Janusik, M., Gondek, E., Krzeminska, L., Danel, A.,
Plucinski, K. J., Benet, S., & Sahraoui, B. (2004). J. Phys.: Condens. Matter, 16, 231.