OPTOELECTRONIC PROPERTIES AND PHOTOVOLTAIC
APPLICATIONS OF SUBSTITUTED POLYTHIOPHENE AND
OLIGOTHIOPHENE. EXPERIMENTAL AND THEORETICAL INVESTIGATIONS
Rchid KACIMI1, Khawla KHOUZAMi2, Tayeb ABRAM1, Lahcen BEJJIT1, Mohammed Nassiri BENNANI4, Mohammed BOUACHRINE1,3*
1MEM,
(ESTM), University Moulay Ismail, Meknes, Morocco.
2
MIFT, University of Messina, Italy
3MCNS
Laboratory, Faculty of Science, University Moulay Ismail, Meknes, Morocco
4CBAE
laboratory, Faculty of Science, University Moulay Ismail, Meknes, Morocco
Abstract
In This work, we have studied conjugated polymers based on thiophene:
polythiophene, poly(3-alkythiophene) and poly(alkylbithiophene). The
discussion and interpretation of the obtained results will be devoted to
the influence of substitution on the spectroscopic and electronic
properties of the polymer, based firstly on an experimental analysis (NMR
and UV), then on a DFT theoretical study by the oligomeric approach.
Finally, the optoelectronic properties of the proposed oligomers will be
determined and discussed. The possibility of their photovoltaic application
will also be mentioned and studied.
Results
S
S
S
S
S
(8T)
C6H13
C6H13
S
C6H13
C6H13
S
S
S
S
S
S
S
C6H13
C6H13
C6H13
C6H13
Key words: Polythiophene , DFT, electronic properties, Photovoltaic.
(8AT, HT)
C6H13
Introduction
Conjugated polymers have been widely studied by several researchers over
many years [1]. These new materials were born in 1977, following the work
of A. J. Heeger and A. G. Shirakawa at the University of Pennsylvania. The
main idea of the work was to investigate the dramatic increase of the
conductivity of polyacetylene doped with iodine. The obtained results have
opened up perspectives for many industrial applications such as for the
conception of new organic photovoltaic cells and blue-emitting material for
novel organic light emitting diodes (OLED) and organic field-effect
transistors[2]. Among conjugated polymers, Poly(3-alkyl)thiopheneis gaining
an increasing interest thanks to its stability and solubility in many organic
solvents. These advanced materials that display a wealth of appealing
properties were studied by several physicochemical techniques [3].
Specifically, the use of solution characterization techniques allows a good
identification of the sequences in the polymer chain. Furthermore, since
alkyl chains were found to play an important role in the molecular
properties and spectral characteristics of polymer, it is indispensible to
obtain a thorough knowledge of the structure and backbone of the polymer
chains. The present work deals with a theoretical evaluation of the
geometric, optoelectronic, absorption and photovoltaic properties of
Polythiophene (8T), Poly(3-alkylthiophene) (8AT) and Poly (alkylbithiophene)
(8ABT). The effects of the alkyl substitution and the regularity of the
sequence on these properties were investigated
S
S
S
These results reported by the literature [7] revealed that the position of the
maximum absorption (λmax) strongly depends on the proportion of
sequences. The superabundance of the alkyl chains and therefore the
existence of the (HH) chain defects lead to steric constraints. The polymer
adopts in this case conformations which lead to a rupture of the conjugation
and a degradation of the properties of the polymer. In order to confirm these
results, the energy values ​EHOMO, ELUMO and Egap were theoretically
evaluated, from the optimized geometries obtained by B3LYP / 6-31G (d,p).
The results revealed that the excellent electronic properties were detected for
poly(thiophene) with regular sequences (HT): the polymer (8AT) with 100%
HT, possesses an energy gap of 2.84eV while a value of 3.57 eV was recorded
for the same polymer with 0% HT. This reveals the existence of repulsive
forces between the groups which appear clearly in the molecule (8AT, HH)
having the highest gap energy. For the study the photovoltaic properties, all αi
values of the thiophene-based oligomers (8T), (8AT, HT), (8ABT) and (8AT, HH)
studied, are positive and the calculated values of Voc indicates that such
molecules can be used as organic solar cells especially that they absorb in the
visible in the surroundings of 478 nm
C6H13
S
C6H13
C6H13
S
S
S
S
S
S
S
(8ABT)
C6H13
C6H13
S
S
S
C6H13
C6H13
S
S
S
S
S
C6H13
C6H13
C6H13
C6H13
(8AT, HH)
*Geometrical parameters of study compounds (8T), (8AT, HT),
(8ABT) and (8AT, HH). obtained byB3LYP/6-31G(d, p).
*Obtained distances (Å) of the studied compounds.
Inter-ring distances (A˚)
d1
D2
d3
d4
d5
d6
d7
1.446
1.441
1.441
1.441
1.441
1.441
1.446
1.457
1.449
1.449
1.449
1.451
1.452
1.454
1.447
1.449
1.442
1.445
1.442
1.448
1.447
1.466
1.449
1.464
1.449
1.464
1.449
1.466
(8T)
* Signals corresponding to the protons of αCH2 of the alkyl chain.
(8AT,HT)
(8ABT)
(8AT,HH)
* Dihedral angles θi(◦) of the studied compounds obtained by
B3LYP/6-31G(d,p) calculations.
Theoretical Methodology
Dihedral angles (θ˚)
θ1
θ2
θ3
θ4
θ5
θ6
θ7
179.97
-179.98
179.98
-179.99
179.99
-179.99
179.97
139.51
156.73
154.90
-151.83
-149.59
-149.95
-150.61
(8T)
Density function theory (DFT) method of three-parameter compound of
Becke (B3LYP) was used in all the study of the neutral and polaronic
compounds. The 6-31G(d,P) basis set was used for all calculations [4]. To
obtain the charged structures, we start from the optimized structures of
the neutral form. The calculations were carried out using the GAUSSIAN
09program [5]. The geometry structures of neutral molecules were
optimized under no constraint. We have also examined HOMO and
LUMO levels; the energy gap is evaluated as calculated by the difference
between the HOMO and LUMO energies. The ground state energies and
oscillator strengths were investigated using the CAM-B3LYB, calculations
on the fully optimized geometries. In fact, these calculation methods
have been successfully applied to other conjugated polymers [6].
(8AT,HT)
(8ABT)
161.86
-151.08
166.75
-162.88
177.09
153.75
-164.02
-106.01
-168.42
168.59
-167.31
108.01
161.44
102.29
* Energy gap and frontier molecular orbital energy levels of PCBM and
thiophene-based oligomers (8T), (8AT, HT), (8ABT) and (8AT, HH).
(8AT,HH)
*Energy values of ELUMO (ev), EHOMO (ev) and the Voc (ev).
Compounds
EHOMO
ELUMO
Egap
Voc (V)
α (eV)
1.53
0.59
1.48
1.05
2.0-2.3
(8T)
-4.73
-2.31
2.42
Polymers
PHT1
PHT2
PHT1
PHT1
PHT1
λmax(CHCl3)
515
460
438
390
384
%HT
98%
98%
70%
0%
0%
-
(8AT,HT)
-4.68
-1.85
2.84
-
(8ABT)
-4.65
-2.09
2.56
1.45
0.81
1.95
1.32
-
(8AT,HH)
2.4.6-OMe-PCBM
Egap exp
* Influence of the percentage of HT on the value of λmax.
-5.15
-1.57
-5.45
-2.90
Conclusion
3.57
* Absorption λabs(nm) obtained by the CAM-B3LYB method.
Compounds
Three thiophene-based oligomers were designed. Quantum chemistry calculations were
performed employing (DFT) B3LYP with 6-31G (d, p). The results of the theoretical study
obtained agree with those obtained by NMR and UV. the band gap energy decreases with the
lowering of the number of alkyl groups and the rise of the number of HT into the backbone. From
the calculated photovoltaic parameters, the best values of Voc are indicated for the studied
oligomers blended with 2.4.6-OMe-PCBM (1.45 V to 1.95 V). These values are sufficient for a
possible efficient electron injection. Finally, based on Scharber model, the compound (8T) had
the best conversion rate and it serves as a candidate for the photovoltaic application. The
substitution of (8T) oligomer by alkyl groups to overcome its low solubility issue resulted to obtain
the (8AT, HT) oligomer with interesting photovoltaic properties.
λmax(nm)
Eactivation (eV)
O.S
MO/character
CAM-B3LYB
(8T)
478.05
2.59
2.96
HOMO→LUMO (81%)
(8AT.HT)
421.39
2.94
2.62
HOMO→LUMO (80%)
(8ABT)
457.05
2.71
2.83
HOMO→LUMO (82%)
(8AT.HH)
345.21
3.59
2.45
HOMO→LUMO (69%)
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* Contour plots showing the energy-conversion efficiency
of the investigated polymers
* Absorption spectrum of study compounds (8T),
(8AT, HT), (8ABT) and (8AT, HH) using the
CAM/B3LYP method.