Synthesis of random copolymers of 2,6-di(thiophen-2-yl)aniline and 2,2'-(thiophen-2,5-diyl)dianiline with aniline and its evaluation in organic solar cells

Article abstract of DOI:10.4067/S0717-97072014000100031

The present work deals with copolymers synthesized by reacting aniline units or blocks with 2,6-di(thiophen-2-yl)aniline and 2,2'-(thiophen-2,5-diyl) dianiline monomers. Characterization by spectroscopic techniques such as FT-IR, UV-vis, 1H-NMR corroborated the formation of true copolymers. Synthesized products showed improved solubility and better photovoltaic efficiency than the respective homopolymers. The latter is due to the presence of a greater number of quinoid units, capable of promoting charge carriers generation. However, the low mobility of the charge carriers, that increases copolymers resistivity, would be responsible for an efficiency improvement of just one order of magnitude with respect to the homopolymers.

Full text of DOI:10.4067/S0717-97072014000100031

J. Chil. Chem. Soc., 59, Nº 1 (2014)
SYNTHESIS OF RANDOM COPOLYMERS OF 2,6-DI(THIOPHEN-2-YL)ANILINE AND 2,2’-(THIOPHEN-2,5-
DIYL)DIANILINE WITH ANILINE AND ITS EVALUATION IN ORGANIC SOLAR CELLS
I.A. JESSOP1, P.P. ZAMORA2, F.R. DÍAZ2*, M.A. DEL VALLE3, J.C. BERNEDE4
1 Universidad de Tarapacá, Facultad de Ciencias, Departamento de Química, Av. General Velásquez 1775, Arica, Chile.
2 Pontificia Universidad Católica de Chile, Facultad de Química, Departamento de Química Orgánica, Laboratorio de Polímeros,
Av. Vicuña Mackenna 4860, Santiago de Chile
3 Pontificia Universidad Católica de Chile, Facultad de Química, Departamento de Química Inorgánica, Laboratorio de Electroquímica de Polímeros,
Av. Vicuña Mackenna 4860, Santiago de Chile
4 Université de Nantes, MOLTECH-Anjou,CNRS, UMR 6200, 2 rue de la Houssinière, BP 92208, Nantes, F-44000 France
(Received: December 18, 2013 - Accepted: January 30, 2014)
ABSTRACT
The present work deals with copolymers synthesized by reacting aniline units or blocks with 2,6-di(thiophen-2-yl)aniline and 2,2’-(thiophen-2,5-diyl)dianiline
monomers. Characterization by spectroscopic techniques such as FT-IR, UV-vis, 1H-NMR corroborated the formation of true copolymers. Synthesized products
showed improved solubility and better photovoltaic efficiency than the respective homopolymers. The latter is due to the presence of a greater number of
quinoid units, capable of promoting charge carriers generation. However, the low mobility of the charge carriers, that increases copolymers resistivity, would be
responsible for an efficiency improvement of just one order of magnitude with respect to the homopolymers.
Keywords: Aniline derivative copolymers, optical and thermal studies, bandgap, photovoltaic efficiency.
cycles. All measurements were accomplished under nitrogen at 20 °C min-1
heating rate. Photovoltaic devices were assembled according to the following
INTRODUCTION
structure: indium tin oxide (ITO) anode/anode buffer layer (MoO3, CuI or CuI/
MoO )/undoped electron donor polymer (coPANIA or coPANIB)/fullerene
(C60)/3exciton blocking layer (BCP)/Al cathode. The device was prepared by
sublimation at 1×10-7 atm. The thin film deposition rate and thickness were
estimated in situ using a quartz monitor. The deposition rate of the organic
layers was 0.05 nm s-1. The film thickness was 8-20 nm, 40 nm and 9 nm
for the electron donor polymer, C60 and BCP respectively. The ITO and
Al films were 100 nm thick. The effective area of each cell was 0.25 cm2.
Electrical characterization was conducted on an automated I-V tester, in the
dark and under global AM 1.5G simulated solar illumination. Photovoltaic cell
efficiency was measured using a calibrated solar simulator (Oriel 300W) at 100
mW cm-2 light intensity.
In recent decades intrinsically conductive polymers have attracted
considerable attention, giving rise to the developed of variety of
a
technological applications, e.g. light emitting diodes (LEDs) [1], anti-corrosion
coatings [2], integrated circuits and transistors [3], rechargeable batteries [4]
and photovoltaic devices [5], among others. The wide versatility of these
compounds stems mainly from their electronic and optical properties, high
thermal and environmental stability and ease of preparation [6]. Polyaniline
(PANI), polythiophene (PTH) and their corresponding derivatives occupy a
privileged place within this family of materials and have been extensively
studied [7-12]. In previous works the possibility of enhancing the properties
of these polymers by designing monomers containing both aniline and
thiophene units, i.e. hybrid precursors, was set forth. This fact was successfully
corroborated in such investigations [13-14].
Mononers and polymers synthesis
The monomeric units showed in Fig. 1 were previously prepared [13] using
the Suzuki-Miyaura cross-coupling reaction [15] between 2,6-dibromoaniline
and 2-thienylboronic acid for 2,6-di(thiophen-2-yl)aniline (MonA) and
between 2,5-dibromothiophene and 2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-
2-yl)aniline for 2,2’-(thiophen-2,5-diyl)dianiline (MonB), in the presence of
Pd[(PPh3)4]2Cl2 as catalyst and K2CO3 as base in 1,4-dioxane. The compounds
were purified by column chromatography to afford a 95 % yield.
To complement the work, in the present study the polymerization of
2,6-di(thiophen-2-yl)aniline and 2,2’-(thiophen-2,5-diyl)dianiline monomers
has been carried out by adding aniline units during the course of the reaction.
It was demonstrated [13] that monomers growth occurs only through aniline
or anilines unit(s), whereby the incorporated aniline units would increase
the PANI-like character of the macromolecules and the spacing between
monomers, which would provide greater flexibility to the polymer chains,
favoring its processability in solution, and would modify its optical and
electronic properties and thermal stability. The novel obtained products were
characterized spectroscopic, thermal and electrochemically and evaluated as
the active layer in organic solar cells.
EXPERIMENTAL
Chemicals used in this research were purchased from Sigma-Aldrich while
solvents were Merck and used as received, except aniline that was bi-distilled
at reduced pressure.
Instrumentation
1H NMR spectra were recorded on a 400 MHz Bruker spectrometer. FT-
IR spectra of the polymers dispersed in KBr pellets were recorded on a Bruker
Vector 22 spectrometer. UV-Vis spectra were obtained on a SPECORD 40
Analitik Jena spectrophotometer using 1 cm optical path quartz cells. The
solubility was determined in various commonly used organic solvents, e.g.
DMSO, NMP, THF, CHCl , etc. at 25 °C. Thermogravimetric analysis (TGA)
was performed on a Mett3ler TA-3000 Thermal Analysis System equipped
with a TC-10A processor and a TG-50 thermo-balance with a Mettler MT5
microbalance, heating the samples from 25 to 900 °C at a rate 10 °C min-
Figure 1. Synthesis of MonA and MonB monomer units.
Chemical polymerization was performed using Chan et al. method [16].
1 mmol of MonA or MonB and 4 mmol of anhydrous FeCl was dissolved
in CHCl3 in the dark at 0-5 °C and the mixture was stirred for31 h. 1 mmol of
aniline in 10 mL CHCl3 was then slowly added and the mixture was stirred for
24 h at room temperature. The obtained polymers, coPANIA and coPANIB,
1
under nitrogen. Glass transition temperature (Tg) was measured using
differential scanning calorimetry (DSC) technique on a Mettler-Toledo DSC
821 calorimeter. The first step was to remove sample adsorbed moisture by
heating from -100 to 110 °C, and then from 25 to 300 °C during the subsequent
2394

J. Chil. Chem. Soc., 59, Nº 1 (2014)
were washed with a methanol/water mixture for 24 h, dedoped by stirring with
50% aqueous hydrazine for 24 h, washed with CHCl3 using a Soxhlet apparaus
and vacuum dried for 24 h. The copolymers yield was 55% of a black solid
(Fig. 2).
coPANIA and coPANIB 1H NMR spectra were run in DMSO-d solvent in
which they are partially soluble. Because very broad bands were ob6served that
precluded their correct interpretation, oligomers, obtained two hours after the
polymerization onset, were prepared.
Figure 2 Synthesis of coPANIA and coPANIB random copolymers.
Figure 4. coPANIA (up) and coPANIB (down) 1H NMR oligomers
spectra in DMSO-d6.
RESULTS AND DISCUSSION
Despite the short time for the reaction to proced, it is possible to distinguish
two relevant changes as compared to the spectra of the monomeric units. For
coPANIA, the band at 6.78 ppm, assigned to the para proton of the aniline
central unit, decreases its intensity relative to the rest of the aromatic signals;
for coPANIB, one of the signals between 6.85 and 6.75 ppm, ascribed to ortho
and para protons of aniline side units, changes its resonance pattern. As for the
signals associated to thiophene units, they exhibited no modifications.. This
means the polymer growth occurs just through aniline unit(s) of the monomers,
corroborating thus the results of a previous work [13]. On the other hand,
in both cases the appearance of weak signals at about 7.50 ppm, ascribed to
PANI chain benzenoid and quinoid aromatic protons, were observed [14, 19-
20]. At 4.00 ppm, where the signal associated to amino groups shows up, a
second band, downfield shifted, starts emerging. This would account for the
formation of C-NH-C bonding and, hence, for the conjugation extent of the
polymer chains. These results are consistent with those found in the FT-IR
studies that would indicate the random incorporation of aniline units to the
monomers through their aniline moieties, obtaining real copolymers displaying
substituted PANI-like character.
FT-IR and NMR spectroscopy
Figure 3 shows FT-IR spectra of both coPANIA and coPANIB random
copolymers. At about 3450-3350 cm-1 a strong absorption band is observed
corresponding to N-H stretching of aniline units. C-H stretching vibrations of
aromatic rings were observed between 3100-3000 cm-1. Two absorption peaks
characteristic of PANI and its derivatives appeared close to 1600 and 1450 cm-1.
These signals correspond to quinone and benzene ring-stretching deformation,
respectively. The amount of oxidizing agent used was optimized in such a way
that the ratio of the relative intensities of the two signals is close to unity, as
long as the copolymers are obtained as emeraldine, their most conductive state
[17]. As for thiophene unit or units, a highly intensive absorption band at 698
cm-1 for coPANIA and 750 cm-1 for coPANIB was obtained. As previously
described [18], these signals are generated by C-H out of plane (oop) bending
from 2-substituted and 2,5-disubstituted thiophene units, respectively.
In contrast to infrared spectra of the corresponding PolyA and PolyB [13]
homopolymers, some differences were found. A new signal was exhibited at
1510 cm-1 for coPANIA and 1570 cm-1 for coPANIB, that would reveal the
presence of aniline units or blocks inserted during the polymerization. This
would arise from C-N stretching or C-C stretching vibrations and from C-Nsec
deformation, respectively [17]. On the other hand, the absorption bands
assigned to the out of plane vibration of aniline ring or rings, 802 and 813 cm-1
for MonA and MonB homopolymers [13], respectively, appeared shifted to
793 and 824 cm-1. This would reflect the addition of aniline units would occur
via such rings, conferring greater substituted polyaniline-like character to the
synthesized polymers.
Solubility
Table 1 summarizes solubility results for copolymers derived from MonA
and MonB. These assays were conducted at 25 °C and a concentration of 1 mg
of polymer/1 mL of solvent.
Table 1. Solubility of the studied polymers in commonly used organic
solvents. aSlightly soluble. bInsoluble.
Solvent
Polymer
CH3CN
CH3Cl
DMSO
NMP
±
DMF
±
THF
Tol.
a
b
coPANIA
coPANIB
±
±
±
±
±
-
-
-
±
±
±
-
Because of the low solubility of the products, it was not possible to
carry out GPC studies nor viscosimetric measurements. Compared to the
corresponding homopolymers [13], a slight improvement in the solubility
of the products was observed. The random incorporation of aniline units or
blocks would promote the formation of less regular polymers, which affects
the strength of intermolecular interactions and provides more flexibility to the
chains. Notwithstanding the above, none of them was completely soluble in
the used solvents.
Thermal Studies
Figure 3. FT-IR spectra of coPANIA and coPANIB in undoped state.
Figure 5 shows the thermal degradation profiles and the respective
2395

J. Chil. Chem. Soc., 59, Nº 1 (2014)
differential curves for coPANIA and coPANIB copolymers, obtained by
thermogravimetric analysis (TGA).
Figure 6. coPANIA (left line) and coPANIB (right line) UV-vis spectra
Soluble fraction absorption bands were almost the same as those found for
the homopolymers, hence their assignments can be found in the corresponding
work [13]. However, in the case coPANIA the signal appearing near 450
nm, which could be related to polaron formation [25-27], is much more
defined. This finding was ascribed to an easier oxidation of the aniline units
that are incorporated into the polymer. This signal is also observed in the
coPANIB spectrum, but now appears superimposed with the signal assigned
to π-π* transitions of the thiophene ring. coPANIB also displayed a broad
band reaching over the 800 nm, that would reflect the electronic population
associated to quinoid structures [14].
The optical bandgap, the energy barrier between HOMO and LUMO
molecular orbitals, of each copolymer was determined from the intersection
of the tangent line passing through the less energetic absorption band and the
baseline. The obtained wavelength divides the constant 1242 (Eg = hc/l), giving
an estimate bandgap value in eV [13-14, 28]. According to the data shown in
Fig. 6, the magnitude of this quantity is 2.30 and 2.92 eV for coPANIA and
coPANIB, respectively. These values are slightly higher than those determined
for PolyA and PolyB [13], which would reflect a lower capacity to capture solar
photons and hence lower efficiency of photovoltaic devices fabricated from
them. Consequently, the addition of aniline units has generated an impairment
in the optical properties of the copolymers.
Figure 5. Thermogram (left) and differential thermogram (right) for
coPANIA and coPANIB.
It is observed that coPANIA presents higher thermal stability than
coPANIB. The 10% mass loss thermal decomposition temperature TDT10% is
365 °C and 304 °C, respectively. In addition, it was observed that above 250 °C
both compounds exhibited a significant mass loss that would be associated with
polymer degradation. During this process crosslinking phenomenon already
reported for PANI and its derivatives [21] would take place. The greater
degradation rate for coPANIA and coPANIB occurs at 563 °C and 339 °C,
respectively.
Comparison of the thermal behavior of homo- [13] and copolymers,
demonstrated the following:
· MonA derived polymers are more thermally stable than those derived
from MonB: as MonA shows a single growing point (aniline central unit), the
formed linear polymers would present more symmetry than those branched
polymers derived from MonB (side aniline units), whereupon ordering and
packing between chains would be facilitated.
coPANIA is less thermally stable than PolyA, despite its mass loss
maximum rate takes place at similar temperatures. It was assumed that the
introduction of aniline units between MonA promotes a loss of polymer
symmetry, thereby decreasing the magnitude of the intermolecular forces
between the chains.
coPANIB is thermally more stable than PolyB: due to the insertion of
aniline units or blocks between MonB, the probability that the copolymer
crosslinks through the ortho positions of benzenamines would be enhanced.
As for the results obtained from the study of differential scanning
calorimetry (DSC), in both cases it can be seen an endothermic process above
200 °C, which would be related to morphological changes of the PANI-
derived polymers [22-23]. A unique and distinct glass transition temperature
for coPANIB and coPANIA was detected at 136 and 189 °C, respectively,
corroborating thus the formation of true copolymers. In comparison with
homopolymers [9], coPANIA and coPANIB Tg is higher in both cases. The
conjugation increase of the central chain provokes a shift of this property
towards higher temperatures [24].
Photovoltaic devices
In order to obtain reproducible and comparable results to those reported
for the homopolymers [13], bilayered solar cells prototypes were fabricated.
Due to coPANIA and coPANIB low solubility they were deposited on the
anode (FTO) using high-vacuum sublimation. Different buffer layers were
tested being a double-layered made of MoO3/CuI the one that provided the best
efficiency [29]. Active layers (synthesized copolymers) of various thicknesses
were tried. All of them exhibited high series resistance. Figure 7 and Table 3
show the electrical features of the prototypes.
Table 2 summarizes the results obtained from thermal studies of MonA
and MonB homo- and copolymers.
Table 2. Thermal properties of MonA and MonB homo- and copolymers
derivatives.
Polymer
PolyA
TDT10% (°C)
Tg (°C)
133
512
365
222
304
coPANIA
PolyB
136
Figure 7. J-V characteristics of photocells fabricated from MonA and
MonB copolymers with aniline. Assembly: ITO (100 nm)/MoO3 (3 nm)/CuI
(2.5 nm)/coPANIA (10 nm) or coPANIB (15 nm)/C60 (40 nm)/BCP (8 nm)/
Al (100 nm)/Se (20 nm).
117
coPANIB
189
Table 3. Photocells electrical parameters.
Optical properties
Jsc (mA
Figure 6 depicts UV-vis spectroscopy studies for coPANIA and coPANIB.
To this purpose, 10 mg of the sample was suspended in 2 mL of DMSO and
stirred overnight. The sample was then filtered and the recovered extract was
employed to perform the measurements.
Polymer
coPANIA
coPANIB
V (V)
FF (%)
31.54
26.84
h (%)
cm-2)
0.57
oc
0.38
0.06
6.80∙10-2
1.51∙10-2
1.02
2396

J. Chil. Chem. Soc., 59, Nº 1 (2014)
Comparison with the data presented in a previous work demonstrated that
the prototypes made from MonB derivatives exhibited higher current density,
therefore it might be said that they are more conductive. However, analysis
of the short circuit current Jsc of all photocells (including MonA products),
revealed it was very low. This can be ascribed to the high resistivity of the
polymers, as a result of branching in their structures (polyB and coPANIB)
or main chain torsion (PolyA and coPANIA), which would hinder the charge
carrier mobility in the polymeric chains [13] and would account for the
high value of their bandgaps. These hypotheses will be assessed through the
respective study of hole-mobility.
Contrary to what it was assumed, prototypes prepared with coPANIA and
coPANIB exhibited better efficiency than the respective homopolymers (one
order of magnitude higher). It was assumed that this difference was due to
a greater amount of photo-generated charge carriers from quinoid units that
provoked a current density increase with respect to the monomers, or a greater
difference between the copolymer LUMOs with C60, that would explain a better
excitons separation. The presence of quinoid units is consistent with FT-IR,
UV-vis and 1H-NMR spectroscopic results. However, these results are still low.
Consequently, this series of homo- and copolymers proved to be unsuitable for
the proposed application. Nevertheless, further work with these products in
other technological applications has not been ruled out.
3. J. D. Yuen, F. Wudl, Energy Environ. Sci. 6, 392, (2013).
4. L. Nyholm, G. Nyström, A. Mihranyan, M. Strømme, Adv. Mater. 23,
3751, (2011).
5. D. Gendron, M. Leclerc, Energy Environ. Sci. 4, 1225, (2011).
6. M.S. AlSalhi, J. Alam, L.A. Dass, M. Raja, Int. J. Mol. Sci. 12, 2036,
(2011).
7. D. Li, J. Huang, R.B. Kaner, Acc. Chem. Res. 42, 135, (2009).
8. M.R.A. Alves, H.D.R. Calado, T. Matencio, C.L. Donnici, Quim. Nov. 33,
2165, (2010).
9. E. Ortega, F. Armijo, I. Jessop, M.A. del Valle, F.R. Díaz, J. Chil. Chem.
Soc. 58 (4), 1959 (2013).
10. C.O. Sánchez, A. Isla, C. Bustos, F. Díaz, N. Gatic, J. Chil. Chem. Soc.
55 (2), 233 (2010).
11. M.A. del Valle, M.A. Gacitúa, L.I. Canales, F.R. Díaz, J. Chil. Chem. Soc.
54 (3), 260 (2009).
12. A.S. Ribeiro, W.A. Gazotti, V.C. Nogueiera, D.A. Machado, P.F. dos
Santos Filho, M.A. De Paoli, J. Chil. Chem. Soc. 49 (3), 197 (2004).
13. I.A. Jessop, P.P. Zamora, F.R. Díaz, M.A. del Valle, A. Leiva, L. Cattin,
M. Makha, J.C. Bernède, Int. J. Electrochem. Sci. 7, 9502, (2012).
14. P.P. Zamora, M.B. Camarada, I.A. Jessop, F.R. Díaz, M.A. del Valle, L.
Cattin, G. Louarn, J.C. Bernède, Int. J. Electrochem. Sci. 7, 8276, (2012).
15. A.E. Thompson, G. Hughes, A.S. Batsanov, M.R. Bryce, P.R. Parry, B.
Tarbit, J. Org. Chem. 70, 388, (2005).
CONCLUSIONS
16. S.C. Ng, L.G. Xu, H.S.O. Chan, J. Mat. Sci. Lett. 16, 1738, (1997).
17. N.V. Bhat, D.T. Seshadri, R.S. Phadke, Synth. Met. 130, 185, (2002).
18. D.A. Marani, A.A. Entezami, Iran. J. Pol. Sci. Tech. 3, 2, (1994).
19. X. Wang, T. Sun, C. Wang, C. Wang, W. Zhang, Y. Wei, Macromol.
Chem. Phys. 211, 1814, (2010).
20. P. Anilkumar, M. Jayakannan, J. App. Pol. Sci. 114, 3531, (2009).
21. W.F. Alves, E.C. Venancio, F.L. Leite, D.H.F. Kanda, L.F. Malmonge,
J.A. Malmonge, L.H.C. Mattoso, Thermochim. Acta 502, 43, (2010).
22. S. Wang, F. Wang, X. Ge, Synth. Met. 16, 99, (1986).
23. H.S.O. Chan, S.C. Ng, W.S. Sim, S.H. Seow, K.L. Tan, B.T.G. Tan,
Macromolecules 26, 144, (1993).
The controlled introduction of aniline units during MonA and MonB
chemical polymerization allows true copolymers to be obtained. These
products exhibit, thanks to overlapping increase of their molecular orbitals,
improved solubility and higher glass transition temperature with respect to
those of the corresponding homopolymer.
Photovoltaic modules assembled from coPANIA and coPANIB displayed
better efficiency compared to PolyB and PolyA [13]. However, the high
resistivity of their deposits and large bandgap were responsible for the
efficiency to increase by just one order of magnitude.
ACKNOWLEDGEMENTS
24. J.C. Chen, K. Rajendran, S.W. Huang, H.W. Chang, J. Polym. Res., 18,
1693, (2011).
Financial support through project FONDECYT 1095165 is kindly
acknowledged. IAJ and PPZ thank CONICYT for a Doctoral Scholarship and
Project ECOS/CONICYT C09E02.
25. M. Wan, J. Yang, J. Appl. Polym. Sci. 55, 399, (1995).
26. S.A. Chen, L.C. Lin, Macromolecules 28, 1239, (1995).
27. P. Ghosh, S.K. Siddhanta, S.R. Haque, A. Chakrabarti, Synth. Met. 123,
83, (2001).
REFERENCES
28. D.A.M. Egbe, B. Cornelia, J. Nowotny, W. Günther, E. Klemm,
Macromolecules 36, 5459, (2003).
1. H. Sasabe, J. Kido, J. Mater. Chem. C 123, 1699, (2013).
2. T. Ohtsuka, Int. J. Corros. 2012, 915090, (2012).
29. J.C. Bernède, L. Cattin, M. Makha, V. Jeux, P. Leriche, J. Roncali, V.
Froger, M. Morsli, M. Addou, Sol. Energ. Mat. Sol. Cells 110, 107, (2013).
2397