Zahlou et al. MJC 2015

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Theoreticalinvestigation of new organic materials based on fluorene and
thiophene for photovoltaic applications
Ali ,Zahllou1.Tayeb, Abram4 .Soumia, Boussaidi2,3. Hsaine, Zgou2*.Lahcen,Bejjit1,4 and
Mohammed ,Bouachrine4,*.
1LASMAR, Faculty of Sciences, Moulay Ismail University, B.P. 11201 ZitouneMeknès, Morocco.
2Ibn Zohr University, Polydisciplinary Faculty, B.P. 638, 45000 Ouarzazate, Morocco.
3Ibn Zohr University, Faculty of Sciences, B.P. 8106,80090 Agadir, Morocco.
4MEM, High School of Technology (ESTM), University Moulay Ismail, Meknes, Morocco
*Corresponding author. E-mail: [email protected]
Received 09 Oct 2015, Revised 30 Oct 2015, Accepted 01 Nov 2015
Abstract
In this study,five compounds of novel acceptor-donor organic materials containing 9,9-dimethyl-9H-fluoren, (2,5)
dimethyloxy-4-(thiophen-2yl) thiophenelinked to cyanoacrylic acid via several dyes based alternating donors are
investigated. The geometries, electronic absorption and emission spectra of these six compounds are studied by
Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD/DFT) calculations.The
HOMO, LUMO, Gap energy, λmax, Voc of these compounds have been calculated and reported in this paper. The
objective of this study; is to evidence the relationship between chemical structure of these organic materials and their
properties optoelectronic of ways has conceive thereafter the compounds with effective character for solar cells.
Keyword:9,9-dimethyl -9H-fluoren, [(2,5) dimethyloxy-4-(thiophen-2yl)thiophene, acceptor donor, DFT, solar cells.
1. Introduction:
Recent years have seen a rapid increase of interest in solar cells from polymer materials [1].Conjugated polymer
systems with donor-acceptor architecture, including alternating copolymers; have been widely studied for applications
as transparent conductors [2-4].
Over the past decade, the development of organic semiconductors has focused on organic π-conjugated molecules for
application in electronic and photonic devices by virtue of their ability to afford high operating speeds, large device
densities, low cost, and large area flexible circuits [5]. The key step in developing a suitable candidate for an
electronic system, such as an organic field-effect transistor (OFET), involves the design of a molecule with an optimal
charge carrier ability [6].The reason for the intense research interest in the field of „„plastic electronics‟‟ is the great
opportunity to produce low cost devices on plastic substrates with large areas, which will open up an entirely new
market segment [7].
Increasing energy demands and concerns over global warming have led to a greater focus on renewable energy
sources in recent years [8].Interfacial electron transfer between semiconductor nanoparticles and molecular

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absorption, have been the subject of intense research activities in the past few decades[9]. In terms of solar energy
conversion, the semiconductor–liquid junction solar cell is established as a very promising approach for solar energy
conversion and different strategies have been proposed to generate electrical energy from sunlight. For instance, direct
collection of light by the semiconductor is feasible when the photonic energy exceeds the energy gap between the
valence band and the conduction band of the semiconducting material. In such a case, an electron is promoted from
the valence band to the conduction band leaving a positively charged hole behind. Then, usually the hole migrates to
the semiconductor/solution interface where it oxidizes a redox-active species in solution [10].The conversion of solar
energy directly into electricity is one of the most attractive renewable energy sources that could help replace fossil
fuels and control global warming [11].In the last few decades, polymer solar cells (PSCs) based on conjugated
polymers have attracted considerable attention because of their potential use for future cheap and renewable energy
production [12-15]. In particular, the polymer solar cell has the advantage over all photovoltaic technologies that the
possible manufacturing speed is very high and the thermal budget is low because no high temperatures are needed[16].
In the past10 years, great progresses have been achieved on organic dye based DSSC.Some organic dyes exhibit good
conversion efficiency of about 10% and even higher.Triarylamine and cyanoacrylic acid are most commonly
employed as donor and acceptor, respectively, in the design of donor-π-acceptor structure while thiophene and its
derivatives are served as π bridge to provide conjugation and enhance light absorbance[17].Among many classes of π-
conjugated polymers (polyphenylenes,poly-p-phenylenvinylenes, poly-p-phenyleneethenylenes, polythiophenes and
polycarbazoles,polyfluorenes) have been recognized as unique high band gap polymers for organic electronics and
optoelectronics applications, particularly for organic LEDs[18-19].Recent research has demonstrated
that,polyfluorenehomopolymers and copolymers have emerged as leading electroluminescent materials, which possess
bright blue emission, high hole, and electron mobility, relatively high (for high bandgap polymers)electro- and photo-
and environmental stability[20-21]. In addition to,structural variations in fluorene-based copolymers allow tuning of
the emission from the deep blue to the near-infrared region andmodulation of many other features of these materials
(valence and conducting bands levels, charge mobility, and morphology). Fluorene building blocks have also been
implemented in various copolymers for applications in OFETs, OPVs, and sensors. The concept of alternation of
conjugated building blocks with different highest occupied molecular orbital/lowest unoccupied molecular orbital
(HOMO/LUMO) energies is widely used for tuning the band gap of conjugated donor- π-acceptor copolymers and co-
oligomers thereby modulating the spectral absorption/emission, electrochemical redox potentials, and charge
mobilities in these materials[22].
A fundamental understanding of the ultimate relations between structure and properties of these materials is necessary
to benefit from their adaptive properties to photovoltaic cells. Moreover, poly(hexylthiophene) units have relative
higher charge mobility in comparison with other conjugated polymers and have been widely used as π-conjugating
spacers [23,24] In parallel with the recent experimental work on these new materials, theoretical efforts have indeed
started to be a major source of valuable information that complements experimental studies, thereby contributing to
understand of the molecular electronic structure as well as the optoelectronic properties [25].
In this paper, the ground states of all the molecules (Mi,i=1-5, shown in Fig. 1) are optimized using the DFTB3LYP
method and the low-lying excited state is examined using TD-DFT with 6-31G(d). Furthermore, a theoretical
investigation on the HOMO, LUMO and energy gaps (the orbital energy difference between the highest occupied
molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO)) of these oligomers is very
instrumental in guiding the experimental synthesis to purpose the materials with desired properties in specific
applications such as photovoltaic. We were particularly interested in exploring the potential of thiophene (T), 3,4-
Ethylenedioxythiophene (EDOT),Bithiophene (BT),Dithieno[3,2-b:2′,3′-d]thiophene (DTT), 4H-cyclopenta[2,1-

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b;3',4'-b']-ditiophene-4-thione (CPDS),4-dicyano methyllene-4H-cyclopenta[2,1-:3,4b]dithiophene‟ (CDM), as
electron-donnating moieties (Y) influence on electronic and photovoltaic properties of the materials through exploring
and comparing the HOMO.
S
S
OCH
3
H3CO
Y
CO2H
NC
Y:
S
Mol 1:T
S
O O
MOL 2: EDOT
S
S
Mol 3: BT
Mol4:DTT
SS
S
Mol 5: CPDS
SS
S
Fig. 1: The chemical structures of the studied molecules
2. METHODS:
2.2. Computational methods
In order to further understand the PV performance and electronic properties of all this materials, the molecular
geometries and the electron density of states distribution of five compounds were simulated by using density
functional theory (DFT). The DFT calculations were performed using Gaussian 09 with a hybrid B3LYP correlation
functional[29] and a split valence 6-31G(d) basis set[26].Full geometry optimizations at ground states were
performed under no constraints in the structure with the density functional theory (DFT) by means of the B3LYP
functional (the three parameters of Becke and the Lee-Yang-Parr hybrid functional), using the Gaussian 09 program.
The 6-31G(d) basis set was chosen as a compromise between the quality of the theoretical approach and the high
computational cost associated with the high number of dimensions to the problem for all atoms [27,28].
The HOMO, LUMO and the gap energies were deduced from the stable structures, where the energy gap is the
difference between LUMO and HOMO levels. The absorption properties were calculated starting at the optimized
structures using TD-DFT/B3LYP calculations, with the same basis set.
3. Results and discussion
3.1. Geometry optimization
The chemical structure of all molecules studied is depicted in fig.1 and the optimized geometries of the studied
molecules are plotted in fig.2. In order to determine the geometrical parameters, the molecules are fully optimized in
their ground state using the 6-31G (d) basis set.

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The results of the optimized structures (fig.2) for all studied compounds so that they have similar conformations (quasi
planar conformation). We found that the modification of several groups attached to the basic molecule does not
change the geometric parameters.
Mol1 Mol 2
Mol3Mol4
Mol 5
Figure 2: Optimized geometries obtained by B3LYP/6-31G (d) of the studied molecules.
3.2. Optoelectronic properties
The optoelectronic properties depend essentially on the appropriate HOMO and LUMO energy levels and the electron
and hole mobilities. Everyone knows that (Egap) between the highest occupied molecular orbital (HOMO) and the
lowest unoccupied molecular orbital (LUMO) is an essential parameter that determines the molecular admittance since
it is a measure of the electron density hardness. The band gap is estimated as the difference between the HOMO and
the LUMO level energies (Egap= ∆EHOMO-LUMO)on the ground singlet state. The results of the experiment showed that
the HOMO and LUMO energies were obtained from an empirical formula based on the onset of the oxidation and
reduction peaks measured by cyclic voltammetry [29,30]. However, theoretically speaking the HOMO and LUMO
energies can be calculated by DFT method of calculation. It is remarkable that the solid‒state packing effects are not
calculated in DFT calculations, this fact affects the HOMO and LUMO energy levels in a thin film compared to an
isolated molecule as considered in the calculations. Though these calculated energy levels lack some accuracy still we
can use them to get information by comparing similar oligomers or polymers [31]. In Table 1, we listed the calculated
energies for the EHOMO, ELUMO and Egap for the Moli at ground state.
The theoretical electronic properties parameters (EHOMO, ELUMO) and Gap) are listed in Table 1. The Eg is much
affected by the change of acceptor unit. These results can be explained by the electron-withdrawing power of the
acceptor units Y introduced in each oligomer chains. This implies that different side substituent structures play key
role in electronic properties and the effect of slight structural variations. It can also be found that, the HOMO and
LUMO energies of the studied compounds are slightly different. This implies that different structures play key roles
on electronic properties and the effect of slight structural variations, especially the effect of the motifs branched to the
molecule on the HOMO and LUMO energies is clearly seen. In addition, energy (Egap) of the studied molecules

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differs slightly from 1,421 eV to 2.348 eV depending on the different structures. They are studied in the following
order:
Mol1>Mol2>Mol4>Mol3>Mol5
Table 1: The HOMO energy (EHOMO), LUMO energy (ELUMO), and HOMO–LUMO energy gap (Egap) in eV for
ground state computed at the B3LYP/6-31G level of theories.
Compound
EHOMO (eV)
ELUMO (eV)
Egap (eV)
Mol 1
-5.028
-2.68
2.348
Mol 2
-4.975
-2.63
2.345
Mol 3
-4.93
-2.752
2.178
Mol 4
-4.973
-2.757
2.216
Mol 5
-5.014
-3.593
1.421
Compound
HOMO
LUMO
Mol 1
Mol 2
Mol 3
Mol 4
Mol 5
Fig3: Obtained isodensity plots of the frontier orbital HOMO and LUMO of the studied compounds obtained at
B3LYP/6-31(d) level.
The frontier molecular orbital (MO) contribution is very important in determining the charge-separated states of the
studied molecules because the relative ordering of occupied and virtual orbital provides a reasonable qualitative
indication of excitations properties [32].In general, as shown in fig 3 (LUMO, HOMO), the HOMOs of these
oligomers in the neutral form possess a π-bonding character within subunit and a π-antibonding character between the