New low band-gap conjugated organic materials based on fluorene, thiophene and phenylene for photovoltaic applications: Theoretical study

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ScienceDirect www.materialstoday.com/proceedings Materials Today: Proceedings 3 (2016) 2578–2586

SURFOCAP 2015

New low band-gap conjugated organic materials based on fluorene, thiophene and phenylene for photovoltaic applications: Theoretical study H. Zgoua, *, S. Boussaidi a,b, A. Eddiouane a,b, H. Chaiba, R. P. Tripathic, T. Ben Haddad, M. Bouachrinee, and M. Hamidif a.

Ibn Zohr University, Polydisciplinary Faculty, B.P. 638, 45000 Ouarzazate, Morocco. b

Ibn Zohr University, Faculty of Sciences, B.P. 8106, 80090 Agadir, Morocco.

C

Medicinal and Process Chemistry Division, Central Drug Research Institute, Lucknow, 226001, India. d

LCM, Mohammed 1stUniversity, Faculty of Sciences, 60000 Oujda, Morocco.

e

Moulay Ismail University , High School of Technology (ESTM), B.P 3103 Toulal, 50040 Meknes, Morocco. f

Moulay Ismail University, Faculty of Sciences and Technology, B.P. 509, 52000 Errachidia, Morocco.

*Corresponding author.E-mail address: [email protected]

Abstract We use the density functional theory (B3LYP/6-31G(d)) to determine structural parameters band gap, absorption and photovoltaic properties of several conjugated organic materials. The chemical structure of these molecules contains fluorene, thiophene and phenylene rings, together with different unities (e.g. Ethylenedioxythiophene EDOT). The theoretical calculations were performed by using Gaussian 09 program supported by GaussView 5.0.8 Interface. The results reveal that the geometrical parameters (dihedral angles, bond lengths) of all molecules possess nearly planar conformations. Moreover, the optoelectronic properties (HOMO, LUMO, Egap…) were determined from the fully optimized structures by using B3LYP/6-31G(d) level. The absorption data (λmax, Etr, OS) of these systems were obtained by TD-B3LYP/6-31G(d) method. The studied molecules Mol-i (with i = 1-6) have low band-gaps with appropriate energy levels of HOMO and LUMO which are desired in photovoltaic applications, especially the Molecules with acceptor unities in their chain. We conclude that these materials are good candidates for bulk heterojunctions used in organic solar cells.

©2015 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the Conference Committee Members of International Workshop on Functionalized Surfaces for Sensor Applications (Surfocap’15). Keywords: Conjugated; materials; Organic; Thiophene; Phenylene; Photovoltaic properties; DFT; TD.

1. Introduction: In 2000 the Nobel Prize in chemistry was awarded to A.J Heeger, A. G. Mc Diamid and H. Shirakawa for the discovery and development of conjugated polymers. This discovery of organic materials, which previously was thought to be exclusively isolating, opened a new fascinating research field because of their several advantages among them: their low cost solar energy harvesting, flexibility, light-weight, and by the possibility of fabricating large area devices [1]. These materials have recently been used increasingly as active semi-conductors in organic field effect transistors [2], light emitting diodes [3], photovoltaic cells [4] and sensory materials [5].

2214-7853©2015 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the Conference Committee Members of International Workshop on Functionalized Surfaces for Sensor Applications (Surfocap’15).

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The development and research of new conjugated polymers/oligomers as active components in bulk heterojunction1 (BHJ) photovoltaic devices could help to significantly increase power conversion efficiency (PCE). Over this decade PCE risen from 1% [6] to 12% [7]. Lately, several models have been proposed to estimate the polymer performance in BHJ solar cells [8, 9, 10]. In this context some research teams have proposed a oligomer model including an alternating acceptor donor blocs into the oligomeric backbone [11, 12, 13.]. On the other hand, the polymer must be air-stable; therefore, the HOMO energy level needs to be below the air oxidation threshold (-5.27eV) [14]. Furthermore, this relatively low value assures a relatively high open circuit potential (VOC) in the final device [15]. Second, the LUMO energy level of the polymer or oligomer must be positioned above the LUMO energy level of the acceptor by at least 0.2-0.3eV to ensure efficient electron transfer from the polymer to the acceptor [16,17]. Finally, the optimal band gap, considering the solar emission spectrum and the open circuit potential of the resulting solar cell, should range between 1.2 and 1.9eV [18,19]. In this work, we chose as acceptor semiconductor the BisPCBM and as donor semiconductor six oligomers based on thiophène and phenylene TPT ; where it has strengthened the donor blocks TPT by the fluorine and the acceptor block A by a chromophore involving the highly acceptors functions (CN et CO2H). The chemical structure of every oligomer has a private unit A, we replace it by Thiophene (Th) for oligomer1 , EthylenDiOxyThiophène (EDOT) for oligomer 2, DiThiophene (DT) for oligomer 3, Dithieno[3,2-b:2',3'd]thiophene (DTT) for oligomer 4, 4H-CycloPenta[2,1-b;3,4-b']-4-Dithiophene Sulfone (CPDS) for oligomer 5 and 4-dicyanomethylene-4H-cyclopenta[2,1-b;3,4-b']-dithiophene (CDM) for oligomer 6 as schematized in Fig.1. Therefore, this work focus on structural, optical and electrical properties investigation of six new oligomers to arrive finally the most promising ones based photovoltaic devices. OCH3 S

A

S H3CO O

O

S

CO2H CN

S

S S S

S

A=Th

A= EDOT Oligomer 2

Oligomer 1

S

A= 2Th

A=DTT

Oligomer 3

Oligomer 4

NC

CN

S

S

A= CPDS Oligomer 5

S

S

A= CDM

S

Oligomer 6

Fig. 1: Chemical structures of the studied oligomers and the BisPCBM.

BisPCBM

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2. Computational Methodology The theoretical quantum calculations have been the famous tool in the study of π-conjugated systems because they can be used to rationalize the properties of oligomers and also can serve as guidance to experiment. The Density Functional Theory (DFT) method represents a practical and accurate way for the correct description of the structural and optoelectronic properties of these systems. It is found that the B3LYP [20] results are in good agreement with experimental band gaps [21, 22]. The geometries of the studied oligomers were totally optimized by using the B3LYP level. The 6-31G(d) [23] basis set was used for all atoms. The energy gap is evaluated as the difference between the LUMO and HOMO energies. Time-dependent TDDFT [24], which is a developed tool from DFT, has been used to compute optical properties, including UV-vis spectra, excitation energies, oscillator strengths and configuration interaction coefficient [25, 26]. All quantum chemistry calculations were done with Gaussian 09 program [27], supported with the GaussView 5.1.8 interface [28]. 3. Results and discussion 3.1. Optimized geometries By using the B3LYP/6-31G(d) method, we optimized the ground state of the six oligomers. Furthermore we obtain the most stable conformations of studied compounds as shown in Fig. 2:

Oligomer 1

Oligomer 2

Oligomer 3

Oligomer 4

Oligomer 5

Oligomer 6

Fig. 2. The most stable conformations of the studied oligomers.

The inter-ring bond lengths (Å) and dihedral angles (°) values of the optimized oligomers i, (i =1–6) are summarized in Tables 1 and 2.

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H. Zgou et al./ Materials Today: Proceedings 3 (2016) 2578–2586 OCH3 d10

d2 d1

d3

d8 d4

d5

d9

d7

d11 S

d16

d13

d12

d15

d6

S d17

d14

d20

A

d21

CO2H

d19

d18

H3CO

CN

H3 C H

H

O S

A θ1

θ2

S

θ3

θ5

θ4

O

CO2H

θ6 CN

CH3

Fig. 3. The labeled bond lengths and dihedral angles. Table 1. Bond-length (Å) values of the studied oligomers. di

Oligo 1

Oligo 2

Oligo 3

Oligo 4

Oligo 5

Oligo 6

d1

1.399

1.399

1.399

1.399

1.399

1.399

d2

1.389

1.389

1.389

1.389

1.389

1.389

d3

1.411

1.411

1.411

1.411

1.411

1.411

d4

1.466

1.466

1.466

1.466

1.466

1.466

d5

1.397

1.397

1.396

1.397

1.397

1.397

d6

1.392

1.392

1.392

1.392

1.392

1.392

d7

1.410

1.410

1.411

1.410

1.411

1.410

d8

1.465

1.465

1.465

1.465

1.465

1.465

d9

1.380

1.380

1.380

1.379

1.380

1.380

d10

1.413

1.413

1.413

1.413

1.413

1.412

d11

1.385

1.385

1.384

1.384

1.385

1.385

d12

1.463

1.463

1.463

1.463

1.463

1.462

d13

1.417

1.417

1.417

1.417

1.418

1.418

d14

1.389

1.389

1.390

1.390

1.389

1.389

d15

1.409

1.409

1.409

1.409

1.409

1.410

d16

1.461

1.462

1.462

1.462

1.461

1.460

d17

1.389

1.389

1.388

1.387

1.388

1.389

d18

1.406

1.405

1.408

1.408

1.407

1.406

d19

1.384

1.386

1.383

1.383

1.384

1.385

d20

1.438

1.436

1.440

1.441

1.438

1.437

d21

1.422

1.419

1.423

1.423

1.421

1.423

d22

1.372

1.372

1.372

1.372

1.373

1.371

With the change of A unit, the bond lengths values are not have a noteworthy variation. The inter-ring bond lengths (di), as given away in table 1, are in the average of 1.4667Ǻ for d4, 1.4650Ǻ for d8, 1.4630Ǻ for d12, 1.4610Ǻ for d16 and 1.4380Å for d20. Likewise these inter-ring distances for each oligomer are decrease with approaching to the A unit. We notice also, that the single bond lengths, for everyone, are decreasing while those of the double bonds rise.

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H. Zgou et al. / Materials Today: Proceedings 3 (2016) 2578–2586 Table 2. Dihedral angle (°) values of the studied compounds. Oligomers

θ1

θ2

θ3

θ4

θ5

θ6

Oligo 1 Oligo 2 Oligo 3 Oligo 4 Oligo 5 Oligo 6

0.170 0.163 0.171 0.166 0.173 0.191

26.971 26.797 26.764 26.931 26.966 27.091

13.681 14.129 15.563 14.402 13.270 14.252

8.252 6.164 4.630 7.124 5.398 3.916

0.181 0.845 4.187 4.237 1.094 3.778

0.085 0.011 0.031 0.073 0.140 0.072

The optimized geometries for the studied oligomers show that all oligomers possess almost planar torsional angles (θ 1 = θ 5 = θ 6 ~ 0°), while the dihedral angles θ2 is anti-left side with a value in average close to 26.92°; This value is relatively high. We suggest that this is likely due to the steric hindrance of the hydrogen atoms of the adjacent thiophene and phenylene cycles. While the dihedral angles θ3 and θ4 are relatively low 14.216° and 5.914° respectively; and that, is due to the Sulfur-Oxygen interaction (fig.3) [22]. The geometric parameters show that the structure of all oligomers favors the quinoidic form once we approach the A moiety, especially for the oligomers 5 and 6 with the acceptor moieties. 3.2. Electronic properties To discover electronic properties of the studied oligomers, we presented in Table 3 values of the calculated HOMO (highest occupied molecular orbital) and LUMO (the lowest unoccupied molecular orbital) levels and band gap. Table 3. The EHOMO, ELUMO and Egap energies of the studied molecules obtained by B3LYP/6-31G(d). Oligomer

EHOMO

ELUMO

Egap

Oligo 1

-5.03

-2.68

2.35

Oligo 2

-4.97

-2.63

2.34

Oligo 3

-4.93

-2.75

2.18

Oligo 4

-4.97

-2.75

2.21

Oligo 5

-5.01

-3.59

1.42

Oligo 6

-5.08

-3.74

1.34

In general, the all values of band gap are weak for the six oligomers; it is between 2.18 and 2.35eV for oligomers 1, 2, 3 and 4, but for oligomer 5 and oligomer 6, we get a significant band gap drop, with 1.42eV and 1.34eV respectively. The main reason for this of band-gap is due to the electron-withdrawing effect on the propagation of the electronic density. This phenomenon is very clearly seen in Fig.4; the electronic density of LUMO is stronger around the A unit for the oligomers 5 and 6. 3.3. Frontier Molecular Orbitals The excited-state properties are strongly related to the electron density in the frontier molecular orbitals (FMO); The HOMO and LUMO can provide a reasonable qualitative indication of the excitation properties and the ability of electron or hole transport [29]. The contour plot of HOMO and LUMO orbitals of the six oligomers obtained by B3LYP/6-31G(d) are shown in Fig.4

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H. Zgou et al./ Materials Today: Proceedings 3 (2016) 2578–2586 Oligomers

HOMO

LUMO

Oligo 1

Oligo 2

Oligo 3

Oligo 4

Oligo 5

Oligo 6

Fig. 4. The contour plot of HOMO and LUMO orbitals of the studied oligomers obtained by B3LYP/6-31G(d).

From this figure we observe that the electron density of the HOMO orbital is distributed all over the oligomer chain, while it focuses on the side of the A unit for LUMO orbitals, especially for the oligomers 5 and 6. This confirms the results obtained in the geometrical properties. 3.4. Optical Properties The optical properties of the six oligomers were investigated by UV-Vis absorption spectroscopy (fig. 5). The electronic absorption data including the absorption peak wavelength (λab), oscillator strength (O.S) and CI coefficients along with the main excitation configuration of all the oligomers are presented in Table 4.

Fig. 5. Calculated UV-vis spectra (wavelength λ(nm) and absorbance ε(mol-1.L.cm-1)) of the studied oligomers obtained by TD-DFT/B3LYP/6-31G(d).

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Table 4. Calculated wavelength λ, Transition Energies and Oscillator Strengths (f) of all oligomers obtained by TD/B3LYP/6-31G(d) method. Oligomer Oligo 1

Oligo 2

Oligo 3

Oligo 4

Oligo 5

Oligo 6

λab(nm) 552.29 433.14 403.41 560.41 448.95 427.68 597.01 469.72 432.00 588.05 462.20 427.59 1115.09 759.68 696.92 1088.09 752.64 585.06

Etr(eV) 2.24 2.86 3.07 2.21 2.76 2.90 2.07 2.64 2.87 2.11 2.68 2.90 1.112 1.63 1.78 1.14 1.65 2.12

OS 1.36 0.78 0.47 1.18 0.03 1.07 1.38 2.06 0.50 1.41 0.22 0.44 0.178 0.009 0.000 0.231 0.0006 0.011

MO /charaters H→L (0.703) H-1→L (0.624), H→L+1 (0.325) H-2→L (0.446), H-1→L(-0.240),H→L+1 (0.48) H→L ( 0.702) H-2→L (0.428),H-1→L( 0.544) H-2→L ( 0.426), H-1→L (-0.318), H→L+1 ( 0.456) H→L (0.704) H-1→L (0.631), H→L+1 (-0.312) H-2→L (0.245), H-1→L (0.298), H→L+1 ( 0.579) H→L ( 0.703) H-1→L (0.518), H→L+1(-0.337) H-2→L (-0.295), H-1→L (0.308), H→L+1 (0.552) H-1→L (-0.206), H→L (0.668) H-2→L (0.164), H-1→L (0.640),H→L(0.225). H-3→L (0.695), H-3→L+1(-0.126) H-1→L (-0.166),H→L ( 0.681) H-2→L (-0.181), H-1→L (0.646),H→L ( 0.184) H-2→L(0.66), H-1→L (0.20)

The simulated absorption spectra of these oligomers are shown in fig.5. The oligomer 5 and oligomer 6 show their absorption peak at 1115.09nm and 1088.09nm respectively. However, the oligomers 1, 2, 3 and 4 show their absorption peak at 552.29nm, 560.41nm, 597.01nm and 588.05nm respectively. The absorption results found for the oligomers 5 and 6 indicate that these organic materials could yield more light at the longer wavelength, which is beneficial to further increase the photocurrent. The latter is proportional to the conversion efficiency of the corresponding solar cell. 3.5. Photovoltaic Properties The study of the relation between open-circuit voltage Voc and the energy levels of Donor/Acceptor in bulk heterojunction polymer (or oligomer) solar cells has stimulated interest in modifying the Voc by tuning the energy levels of the oligomers. Therefore, the HOMO and LUMO energy level of the donor and acceptor components are very important factors to get photovoltaic properties of these components. In this context, we try to study photovoltaic properties of bulk heterojunction cells of one of the six oligomers as a donor component, blended with the acceptor bis adduct of phenyl-C61-butyric acid methyl ester (BisPCBM). The latter is characterized by -3.8eV for the LUMO energy level and -6.1eV for the HOMO energy level [30]. To realize that, we list in Table 5 the theoretical values of open-circuit voltage Voc and the difference in the LUMO energy levels of the studied oligomers with those of the BisPCBM. They have been calculated respectively from the following expressions (1 and 2) [31]: Donor Acceptor Voc = E HOMO − E LUMO − 0.3

(1)

Acceptor Donor α = ELUMO − ELUMO

(2)

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H. Zgou et al./ Materials Today: Proceedings 3 (2016) 2578–2586 Table 5. Energy values of HOMO (eV), LUMO (eV), Egap (eV), open circuit Voltage Voc (eV) and α (eV) of oligomers. Oligomers

EHOMO (eV)

ELUMO (eV)

EGap (eV)

Voc (eV)

α (eV)

Oligo 1

-5.03

-2.68

2.35

0.93

1.12

Oligo 2

-4.97

-2.63

2.34

0.87

1.17

Oligo 3

-4.93

-2.75

2.18

0.83

1.05

Oligo 4

-4.97

-2.75

2.21

0.87

1.05

Oligo 5

-5.01

-3.59

1.42

0.91

0.21

Oligo 6

-5.08

-3.74

1.34

0.98

0.06

-2,5

HOMO levels LUMO levels

-3,0

Energy (eV)

-3,5 -4,0 -4,5 -5,0 -5,5 -6,0 Oligo 1

Oligo 2 Oligo 3

Oligo 4

Oligo 5

Oligo 6

BisPCBM

-6,5

Fig. 6: Energy diagram scheme illustrating the positions HOMO and LUMO of all oligomers with relative to those of BisPCBM. From Table 5 and Fig. 6, the differences in the LUMO energy levels of the six oligomers and Bis-PCBM, which equal -3.8eV, are 1.12eV, 1.17eV, 1.05eV, 1.05eV, 0.21eV, and 0.06eV respectively (Table 5). It is clear that the oligomers 5 and 6 have the lowest values. This allows a good transfer of LUMO level of the electron donor to the LUMO level of the acceptor. Voc also increases to 0.91eV and 0.98eV for oligomers 5 and 6, respectively. This leads us to suggest that the photoexcited electron transfer from the LUMO level of the molecules to BisPBM may be sufficiently efficient to be useful in photovoltaic devices [32,33]. 4. CONCLUSION

In this theoretical investigation we applied DFT quantum chemical method by means of the DFT-B3LYP/631G(d) method, to predict the structural, electronic and photovoltaic properties of organic molecules bearing sulfur, oxygen and nitrogen atoms. The absorption spectra were carried out at the TD- B3LYP/6-31G(d) level. The obtained results lead us to suggest, two organic materials as donor blended with Bis-PCBM acceptor: The oligomers 5 and 6 owing to this indicators: they have (i) the lowest band gaps (1.42 eV, 1.34 eV respectively), (ii) the highest absorption spectra about 1115.09 nm for oligomer 5 and 1088.09 nm for oligomer 6, (iii) the most important open circuit tension Voc values (0.91eV for oligomer 5, and 0.98eV for oligomer 6). ACKNOWLEDGMENTS: The authors are grateful to the “Association Marocaine des Chimistes Théoriciens” (AMCT) for its pertinent help concerning the programs.

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