New acceptor–donor–acceptor (A–D–A) type copolymers for efficientorganic photovoltaic devices

S. Ghomrasni, S. Ayachi, K. Alimi n

Unité de Recherche: Matériaux Nouveaux et Dispositifs Electroniques Organiques (UR 11ES55), Faculté des Sciences de Monastir,Université de Monastir, Monastir 5000, Tunisia

a r t i c l e i n f o

Article history:Received 22 May 2014Received in revised form18 August 2014Accepted 29 August 2014Available online 6 September 2014

Keywords:A. Organic compoundsC. Ab-initio calculationsD. Electronic structureD. Optical properties

a b s t r a c t

Three new conjugated systems alternating acceptor–donor–acceptor (A–D–A) type copolymers havebeen investigated by means of Density Functional Theory (DFT) and Time-Dependent DFT (TD-DFT) atthe 6-31g (d) level of theory. 4,40-Dimethoxy-chalcone, also called the 1,3-bis(4-methoxyphenyl)prop-2-en-1-one (BMP), has been used as a common acceptor moiety. It forced intra-molecular S⋯O interactionsthrough alternating oligo-thiophene derivatives: 4-AlkylThiophenes (4-ATP), 4-AlkylBithiophenes(4-ABTP) and 4-Thienylene Vinylene (4-TEV) as donor moieties. The band gap, HOMO and LUMOelectron distributions as well as optical properties were analyzed for each molecule. The fully optimizedresulting copolymers showed low band gaps (2.2–2.8 eV) and deep HOMO energy levels ranging from�4.66 to �4.86 eV. A broad absorption [300–900 nm] covering the solar spectrum and absorptionmaxima ranges from 486 to 604 nm. In addition, organic photovoltaic cells (OPCs) based on alternatingcopolymers in bulk heterojunction (BHJ) composites with the 1-(3-methoxycarbonyl) propyl-1-phenyl-[6,6]–C61 (PCBM), as an acceptor, have been optimized. Thus, the band gap decreased to 1.62 eV, thepower conversion efficiencies (PCEs) were about 3–5% and the open circuit voltage Voc of the resultingmolecules decreased from 1.50 to 1.27 eV.

& 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The challenge of using polymers is to obtain flexible electronicdevices with the highest efficiency. Organic polymers, in particularπ-conjugated oligomers, continue to attract great interest for theiruse as an active layer in organic light emitting diodes (OLEDs) [1]and organic photovoltaic cells (OPCs) [2]. In fact, a wide variety ofstructure architectures has been deeply investigated theoreticallyand experimentally [3]. Thus, we have recently reported thedesign of two new copolymers involving poly(N-vinylcarbazole)PVK blended with poly(p-phenylenevinylene) (PPV) [4] and poly(3-methylthiophene) (P3MeT) [5,6]. Such copolymers presentinteresting photo-physical properties.

In parallel, conjugated small molecules still attract much attentiondue, in particular, to their electronic and optical properties. Further-more, OPCs consisting of donor (D) and acceptor (A) moleculestogether as an active layer seem to improve the photoconversion [7].Likewise, symmetrical A–D–A, D–A–D or simply D–A represents themost successful molecular architecture [8,9]. These systems allowintra-molecular charge transfer (ICT) process that leads to a broadabsorption spectrum and a small energy band gap. In other words,

the matching of the energy level of the donor material to that of theacceptor compound is one of the key steps to improve the totalefficiency of the cell [10,11]. However, the charged defects could actas dopants and affect the exciton dissociation and charge transport inthe active layer of OPCs [12].

In order to design new organic materials with lower band gaps,quantum-chemical methods have been applied to investigate avariety of polymers as well as to optimize their optoelectronicproperties. This paper aimed at theoretically designing and mod-eling of three new conjugated π-copolymers with A–D–A archi-tecture. For that, 4,40-dimethoxy-chalcone (BMP) as a commonacceptor unit linked to three different appropriate donors such as4-ATP, 4-ABTP and 4-TEV were chosen (henceforth they areabbreviated as P1, P2 and P3, respectively). Their correspondingchemical structures are depicted in Fig. 1. The band gap, HighestOccupied Molecular Orbital (HOMO), Lowest Unoccupied Molecu-lar Orbital (LUMO) electron distributions, electronic structure aswell as the optical properties were theoretically analyzed for eachmolecule by using Density Functional Theory (DFT) and Time-Dependent DFT (TD-DFT) at the 6-31g (d) level of theory. Likewise,the donor-pended effects on the optoelectronic parameters werediscussed.

Besides, for a better understanding of the optoelectronic para-meters that affect OPCs efficiencies, we have elucidated the struc-ture–property relationship of the studied compounds. Accordingly,

Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/jpcs

Journal of Physics and Chemistry of Solids

http://dx.doi.org/10.1016/j.jpcs.2014.08.0130022-3697/& 2014 Elsevier Ltd. All rights reserved.

n Corresponding author. Tel.: þ216 73 500 274; fax: þ216 73 500 278.E-mail address: kamel.alimi@fsm.rnu.tn (K. Alimi).

Journal of Physics and Chemistry of Solids 76 (2015) 105–111

we have analyzed the studied copolymers for their hosting PCBMinto a single layer BHJ solar cell. Then, we selected the compoundhaving the best optoelectronic properties to be blended with PCBMin an organic bulk heterojunction (BHJ) solar cell [13].

2. Computational details

All calculations were carried out using the Density FunctionalTheory (DFT) implanted in the GAUSSIAN (09) program [14,15]for isolated gas-phase molecules. The geometric structure ofeach molecule was optimized using the most popular Becke'sthree parameters hybrid functional, B3 [16], with non-local corre-lation of Lee–Yang–Parr, LYP, abbreviated as B3LYP hybrid func-tional [17]. DFT calculations with B3LYP were performed with the6-31g (d) level of theory. The electronic properties such as HOMO,LUMO energies as well as their associated gaps were also eluci-dated for studied copolymers. Therefore, the optimized geometrieswere calculated for cationic and anionic charged states. Hence, theenergies for the different charged states in the relevant geometrieswere obtained for calculating Ionization Potential (IP) and electro-nic affinity (EA).

As for theoretical optical properties, the well-known Config-uration Interaction Singles (CIS) method [18] with the same basisset was used to optimize the lowest singlet excited-state geo-metries. Besides, Time Dependent Density Functional Theory(TD-DFT) level was employed to predict the optical absorptionspectra on S0 optimized structures [19,20]. Electronic transitionassignments and oscillator strengths were also calculated usingthe same method of calculation. The theoretical absorption spectra

were simulated using the SWizard program [21]. Absorptionprofiles, however, were calculated using Gaussian model (1) withthe half-bandwidths (Δ1/2) of 3000 cm�1.

3. Results and discussion

3.1. Ground and excited states geometry

DFT and TD-DFT with 6-31 (d) level of theory were employed toperform the geometry on S0 and S1 optimized structures, respec-tively. For the sake of comparison, the obtained results are displayedin Table 1. In their ground states (Fig. 2) and unlike the compoundP3 which had a planar structure related to the presence of theethylene groups in its main chain, P1 and P2 structures were twistedto about 221 (φ2Eφ4�221). In fact, the order of coplanarityincreased from P1, P2 to P3 when the dihedral angle φ3 decreasedfrom 54.051, 6.921 to 4.681 for the three copolymers, respectively.This effect was strictly related to the number of methyl substitutedthiophene rings. It was also noted that the 4-ABTP and 4-TEV asdonors improved the molecular coplanarity and were responsiblefor electronic delocalization along the chain backbone of P2 and P3compounds. In addition, in all structures, the particularity of usingBMP units resulted in intra-molecular attractive interaction forcestaking place between the oxygen atoms (negatively charged) ofcarbonyl groups and the sulfur atoms (positively charged) ofthiophene units. Consequently, non-covalent interactions lead tothe molecules' conformational rigidification. In fact, the distanceS—O of �2.64 Å, which corresponded to �79% of the sum of theirVan der Waals radii, fall inside the Van der Waals contact distance

Fig. 1. Chemical structures of copolymers P1 (a), P2 (b), P3 (c) and that of PCBM (d).

S. Ghomrasni et al. / Journal of Physics and Chemistry of Solids 76 (2015) 105–111106

of the S—O (3.32 Å) and outside their covalent contacts of 1.70 Å forS―O in the same case of other copolymers based thiophene [22,23].In that case, for the all studied compounds, the planar conformations

were stabilized by the non-bonded S—O interactions, in which thedihedral angles φ1 and φ5 were twisted by about 181 between theBMP acceptor unit and oligothiophene derivatives as donor units.

The lowest singlet excited-state geometries for copolymers wereperformed at CIS/6–31g (d) level of theory. First, it was found thatthe excited copolymers almost reached planarity. The dihedralangles, φ2 and φ4 were deviated to 0.561, 2.921 for P1 and 0.291,1.351 for P2. We also noted that φ3 decreased to reach 0.01 in thecase of P2 and P3. Whatever the state is, the non bonded S—Ocontacts were found to be considerably shorter than the sum oftheir Van der Waals radii. These distances (S—O) varied from 2.64 Ǻin the ground state to 3.11 Ǻ in the excited state, which could berelated to the more twisted conformations (φ1�781, φ5�851).

3.2. Electronic structures analysis

We calculated the band gap, HOMO, LUMO distributions as wellas their related electronic structure for each molecule. Based onground state calculations, the band gap was estimated as thedifference between the HOMO and the LUMO energy levels(Eg¼εHOMO�εLUMO) [24]. Furthermore, the energy band structureswere calculated and are diagramed in Fig. 3. The calculated opticalband gaps were about 2.8, 2.5 and 2.2 eV for P1, P2 and P3,respectively. The reduced band gap in the case of the lastcopolymer (P3) was directly related to the improved conjugationdue to the presence of 4-TEV as a donor group. On the other hand,recent developments on low-band-gap polymers based on theincorporation of electron-donating properties of thiophene ringsystem are reported [25–27].

For charge separation, key quantities are the ionization poten-tial (IP) and electron affinity (EA) of donor and acceptor materials.In Table 2, the calculated IP and EA values are listed. It could be

Fig. 2. Optimized geometries obtained by B3LYP/6-31g (d) of the studied moleculesin their ground states.

Fig. 3. Energy levels diagram of P1, P2 and P3 copolymers as well as those of PCBM.

Table 1Selected dihedral angles for copolymers P1, P2 and P3 in their ground and excitedstates, calculated at DFT/B3LYP/6-31g (d) method.

Dihedralangle (deg)

Studied copolymers

P1 P2 P3

Groundstate

Excitedstate

Groundstate

Excitedstate

Groundstate

Excitedstate

φ1 �18.97 �78.05 �18.08 �77.94 �16.83 �77.41φ2 22.23 0.56 21.26 0.29 – –

φ3 54.05 51.94 6.92 0.02 4.68 0.04φ4 22.22 2.92 21.17 1.35 – –

φ5 �18.97 �84.89 �18.12 �84.87 �17.83 �84.72

Table 2Optoelectronic parameters and open circuit voltage Voc (eV) of P1, P2 and P3copolymers calculated using DFT/B3LYP/6-31g (d) level of theory.

Studiedcompounds

εHOMO

(eV)εLUMO

(eV)Eg (eV) IP (eV) EA (eV) Voc

(eV)

Copolymer P1 �4.89 �2.02 2.8 5.43 1.41 1.50Copolymer P2 �4.73 �2.18 2.5 5.49 1.50 1.34Copolymer P3 �4.66 �2.36 2.2 5.42 1.69 1.27PCBM �5.65 �3.09 2.5 – – –

S. Ghomrasni et al. / Journal of Physics and Chemistry of Solids 76 (2015) 105–111 107

noted that IP values were quite similar. So, the energy required tocreate a hole was �5.4 eV. However, the EAs energy needed toaccept an electron were of 1.41, 1.50 and 1.69 eV for P1, P2 and P3,respectively. The obtained results were strictly related to themolecular orbitals (MOs) values.

3.3. Optical properties and electronic transitions

The vertical singlet–singlet electronic transition energies andoptical absorption spectra of copolymers were calculated usingTD-DFT/B3LYP//6-31g (d). Fig. 4 showes the simulated optical absorp-tion spectra of P1, P2 and P3. The detailed computed absorptionwavelengths, oscillator strengths (f), and the major MOs contributionsof electronic transitions are listed in Table 3. Qualitatively, we noted

from optical analyses that all compounds exhibited at least twoseparated absorption peaks attributed to the oligothiophene derivativemoieties and intra-molecular charge transfer (ICT) process originatingfrom BMPmoiety, respectively. In fact, the absorption maxima rangingfrom 486 to 604 nmwere associated to the ICT type π-πn transitionfrom the HOMO located at the electron-donating 4-ATP, 4-ABTP and 4-TEV moieties to the LUMO located at the electron-withdrawingcarbonyl moiety of BMP [28]. These electronic transitions have largeoscillator strength (f) values. In fact, when modifying the core of thetwo studied copolymers (P2 and P3) with un-substituted bithiopheneor by introducing π-spacer between un-substituted thiophene rings,this factor increases from 1.44 for P1 to 2.50 for P3.

Compared to P1 and P2, the π-πn absorption band of P3became broader within the visible region, which would facilitatemore efficient sunlight absorption [7]. The bathochromic shifts of118 nm and 54 nm in comparison to P1 and P2, respectively, wereattributed to the more extended π-conjugation throughout themolecular backbone [29–33]. These results were in close agree-ment with the oscillator strength's values.

The small band gap observed in the case of P3 was not onlyassociated to the improvement of conjugation but also to thereduction of the steric hindrance effects in the absence of methylgroups substituted thiophene rings. The presence of such sub-stituted groups could yield regioregular head to head linkage [34]and could justify the twisted molecule.

3.4. Frontier molecular orbitals (MOs) analysis

Since the relative ordering of the occupied and virtual orbitalprovides a reasonable indication of the exciting properties and theability of an electron or a hole transport [32], we have plotted thecontour plots of MOs including HOMO and LUMO of all com-pounds in their ground state, as shown in Fig. 5. In all these cases,we deduced that the MO spread over the central conjugated

Fig. 4. Simulated UV–visible optical absorption spectra of studied copolymers withTD-DFT/B3LYP/6-31g (d) level of theory.

Table 3Main transition states, their assignments, wavelengths and oscillator strengths for copolymers P1, P2 and P3 obtained at TD-DFT/B3LYP//6-31g (d).

Compounds Transition Wavelength (λ, nm) f MO/character

Copolymer P1 S0-S1 493.6 1.441 H-L (97%)S0-S2 364.3 0.161 H-Lþ2(þ61%) H-2-L (þ25%)

Copolymer P2 S0-S1 551.6 1.893 H-L (99%)S0-S2 373.9 0.307 H-1-Lþ1 (87%)

Copolymer P3 S0-S1 604.2 2.508 H-L (99%)S0-S2 384.3 0.340 H-2-L (54%)S0-S3 325.5 0.017 H-5-Lþ1(þ53%)

Fig. 5. The contour plots of HOMO and LUMO orbitals of the studied copolymers in their ground states.

S. Ghomrasni et al. / Journal of Physics and Chemistry of Solids 76 (2015) 105–111108

backbone resulting in novel electron distribution associated to thedonor groups. The HOMO possessed an anti-bonding π characterbetween the two adjacent subunits [35]. However, the LUMO has abonding character between the two subunits. When passing fromground to excited states (Fig. 6), the HOMO state density wasmuch localized on the donor moiety, while the electron density ofLUMO was mainly localized on the acceptor moiety. Thus, thefrontier molecular orbital (MO) contribution is very important inthe determination of the charge-separated states of the modelcopolymers. To create an efficient charge separate state, the HOMOenergy level must be localized on the extended donor moiety andthe lowest LUMO energy level on the acceptor moiety.

4. Bulk-heterojunction solar cell (BHJ) optimization

Generally, the most efficient polymer solar cells are based onthe bulk heterojunction (BHJ) structure containing a blend ofπ-conjugated copolymer as electron donor and fullerene deriva-tives as electron acceptor [36,37]. The original three-dimensionalstructure of fullerenes and their derivatives, denoted as [6,6]-phenyl-C61 butyric acid methyl ester (PCBM), have demonstratedtheir potentiality for energy storage and photovoltaic con-version [38]. In order to reinforce the optoelectronic propertiesof the studied copolymers, we firstly compared the MOs' energylevels between the donors units with those derived from DFT/B3LYP/6-31g (d) optimized structure of PCBM (Fig. 2 and Table 2).Secondly, particular attention was paid to the higher LUMO energyvalue of the studied compounds compared to the PCBM one.Finally, we analyzed the blended copolymers with PCBM for theiradopted band structure in developing efficient OPCs.

The power conversion efficiency (PCE) can be extracted fromthe following equation [13]:

PCE¼ FF Voc JscPin

ð1Þ

where Pin is the incident power density, Jsc is the short-circuitcurrent, Voc is the open-circuit voltage, and FF denotes the fillfactor. However, its determination required the knowledge of theopen circuit voltage parameter (Voc) [39]. In another related work[40], it was possible to achieve the value of Voc just theoretically.The properties of frontier molecular orbital of polymer donor arerelated to the open-circuit voltage (Voc) of organic solar cells andoptical properties [41]. In fact, the maximum Voc of the BHJ solarcell was calculated from the difference between the HOMO of the

electron donor and the LUMO of the electron acceptor, taking intoaccount the energy lost during the photo-charges generation [42].The theoretical values of the Voc were determined from thefollowing expression [13]:

Voc ¼ EðHOMOÞdonor���

���� EðLUMOÞacceptor

��

���0:3 ð2Þ

We deduce Voc of the resulting studied molecules whichdecreased from 1.50 to 1.27 eV (see Table 2). Additionally, copoly-mers exhibit power conversion efficiencies (PCEs) ranging from 3%to 5% (see Fig. 7).

Although the two studied copolymers (P2 and P3) have insuredtheir hosting PCBM into a single layer BHJ solar cell, it seemsinteresting to develop BHJ structure preferably using copolymer P2as donor (Fig. 8). This choice cannot unfortunately be adopted dueto the hole injection barrier [42], as in the case of the P3copolymer. For this reason, bulk heterojunction (BHJ) organicphotovoltaic cell was generated on the basis of P2:PCBM in weightratio of 1:1. The resulting blend geometry optimized structure atDFT/B3LYP/6-31g (d) is displayed in Fig. 9.

Fig. 6. The contour plots of HOMO and LUMO orbitals of the studied copolymers in their excited states.

Fig. 7. Power efficient conversion (PEC) diagram of bulk heterojunction of solar cell(P2:PCBM).

S. Ghomrasni et al. / Journal of Physics and Chemistry of Solids 76 (2015) 105–111 109

Further insights were obtained from the density of states (DOS)of the isolated P2 and PCBM as well as P2/PCBM blend forcomparison (Fig. 10). It was interesting to note that the ground-state interaction between the P2 copolymer and the PCBM inducedICT states lying inside the gap of the P2: PCBM. This resultcorroborated recent optoelectronic findings measured in differentpolymer/acceptor blends [43].

Based on the comparison between the donor and the acceptorcompounds, the resulting blend showed some interesting electronic

properties, laying in a low band gap of 1.62 eV and a deeper HOMOenergy level. The difference in the LUMO energy levels of P2 andPCBM was close to 0.9 eV40.3 eV, suggesting that the photo-excited electron transfer between the compounds might be suffi-ciently efficient to be useful in photovoltaic devices.

5. Conclusion

We have designed new molecules based on alternating sym-metric A–D–A conjugated copolymers for efficient OPCs. Thealternating 4,40-dimethoxy chalcone as acceptor with oligo-thiophene derivatives as donor could yield an intra-molecularinteraction responsible for the conformational rigidification aswell as an ICTs. The studied compounds showed relatively smallband gaps, lower MOs energy levels and optical spectra over-lapping solar spectrum. Based on the obtained properties, thisvariety of polymers could be used as acceptor molecules blendedwith PCBM in developing optimized BHJ within OPCs. The studiedcompounds have also allowed power conversion efficiencies(PCEs) of 3–5%, and open circuit voltage Voc ranging from 1.27 to1.50 eV. Likewise, we have proposed a bulk heterojunction (BHJ)organic photovoltaic cells band structure.

References

[1] A. Mabrouk, A. Azazi, K. Alimi, J. Phys. Chem. Sol 71 (2010) 1225.[2] A. Iwan, M. Palewicz, A. Chuchmala, L. Gorecki, A. Sikora, B. Mazurek, Synth.

Met. 162 (2012) 143.[3] S. Ayachi, A. Mabrouk, M. Bouachrine, K. Alimi, Photophysical properties of

two new donor–acceptor conjugated copolymers and their model com-pounds: applications in polymer light emitting diodes (PLEDs) and polymerphotovoltaic cells (PPCs), in: Jai Singh (Ed.), Organic Light Emitting Diode,InTech, Croatia, 2012.

[4] M. Mbarek, F. Massuyeau, J.-L. Duvail, J. Wery, E. Faulques, K. Alimi, J. Appl.Polym. Sci. 130 (2013) 2839.

[5] M. Chemek, J. Wéry, M. Bouachrine, M. Paris, S. Lefrant, K. Alimi, Synth. Met.160 (2010) 2306.

[6] M. Chemek, S. Ayachi, A. Hlel, J. Wéry, S. Lefrant, K. Alimi, J. Appl. Polym. Sci.122 (2011) 2391.

[7] J.Y. Lee, S.H. Kim, I.S. Song, D.K. Moon, J. Mater. Chem. 21 (2011) 16480.[8] P.L.T. Boudreault, A. Najari, M. Leclerc, Chem. Mater. 23 (2011) 456.[9] Z. Linghai, P. Ke, Z. Hongbo, W. Sujuan, W. Yang, G. Jinwei, Chem. Phys. Lett.

543 (2012) 199.[10] P. Cheng, X. Zhao, W. Zhou, J. Hou, Y. Li, X. Zhan, Org. Electron. 15 (2014) 2270.[11] M.L. Keshtov, G.D. Sharma, V.S. Kochurov, A.R. Khokhlov, Synth. Met. 166

(2013) 7.[12] H. Bässler, A. Köhler, Top. Curr. Chem. 312 (2012) 1.[13] A. Mabrouk, A. Azazi, K. Alimi, Polym. Eng. Sci. 53 (2013) 1040.[14] W. Kohn, L.J. Sham, Phys. Rev. A 140 (1965) 1133.[15] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman,

et al., Gaussian 09, Revision B.01, Gaussian, Inc., Wallingford, CT, 2009.[16] A.D. Becke, Phys. Rev. A 38 (1988) 3098.[17] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785.[18] L. Yang, A.M. Ren, J.K. Feng, X.D. Liu, Y.G. Ma, H.X. Zhang, Inorg. Chem. 43

(2004) 5961.[19] K. Burke, J. Werschnik, E. Gross, J. Chem. Phys. 123 (2005) 062206.[20] M. Mbarek, M. Chemek, J. Wery, J.L. Duvail, K. Alimi., J. Phys. Chem. Sol 75

(2014) 752.[21] S.I. Gorelsky, Swizard Program, University of Ottawa, Ottawa, Canada, 2009.[22] S. Ayachi, S. Ghomrasni, K. Alimi, J. Appl. Polym. Sci. 123 (2012) 2684.[23] S. Ayachi, S. Ghomrasni, M. Bouachrine, M. Hamidi, K. Alimi, J. Mol. Struct.

1036 (2013) 7.[24] N.A. Sánchez-Bojorge, L.M. Rodríguez-Valdez, N. Flores-Holguín, J. Mol. Model.

19 (2013) 3537.[25] R. Kroon, M. Lenes, Jan C. Hummelen, Paul W.M. Blom, B.D. Boer, Polym. Rev.

48 (2008) 531.[26] T.T. Do, Y.E. Ha, J.H. Kim, Org. Elecron. 14 (2013) 2673.[27] K. Ranjith, S.K. Swathi, A. Malavika, Praveen C. Ramamurthy, Sol. Energy

Mater. Sol. Cells 105 (2012) 263.[28] Y.K. Lee, Y.M. Nam, W.H. Jo, J. Mater. Chem. 21 (2011) 8583.[29] N. Bouzayen, B. Zaidi, A. Mabrouk, M. Chemek, K. Alimi, Comput. Theor. Chem.

984 (2012) 1.[30] K. Fujimoto, H. Shimizu, M. Furusyo, S. Akiyama, M. Ishida, U. Furukawa,

T. Yokoo, M. Inouye, Tetrahedron 65 (2009) 9357.[31] H. Wang, G. Chen, X. Xu, H. Chen, S. Ji, Dyes Pigm. 86 (2010) 238.[32] H.-Y. Fu, X.-D. Gao, G.-Y. Zhong, Z.-Y. Zhong, F. Xiao, B.-X. Shao, J. Lumin. 129

(2009) 1207.

Fig. 8. Schematic energy diagram of the proposed bulk heterojunction solar cell(ITO/P2:PCBM/Mg).

Fig. 9. Geometry optimized structure of P2:PCBM composite at the DFT/B3LYP/6-31g (d) method.

Fig. 10. Density of states of copolymer P2 and its blended composite with PCBM aswell as those of PCBM, for the sake of comparison.

S. Ghomrasni et al. / Journal of Physics and Chemistry of Solids 76 (2015) 105–111110

[33] S. Jungsuttiwong, R. Tarsang, T. Sudyoadsuk, V. Promarak, P. Khongpracha,S. Namuangruk, Org. Electron. 14 (2013) 711.

[34] E. Bundgaard, F.C. Krebs, Sol. Energy Mater. Sol. Cells 91 (2007) 954.[35] V. May, O. Kühn, Charge and Energy Transfer in Molecular Systems, Wiley,

VCH, Berlin, 2000.[36] Z.C. Rong, L.Z. Jiang, C.Y. Hong, C.H. Shand, W.Y. Zhi, Y.L. Hua, J. Mol. Struct.

(THEOCHEM) 899 (2009) 86.[37] H.Y. Chen, H. Jianhui, Z. Shaoqing Li. Yongye, Y. Guanwen, Y. Yang, Yu Luping,

Wu Yue, Li Gang, Nat. Photonics 35 (2009) 649.

[38] J. Bandara, K. Shankar, C.A. Grimes, M. Thelakkat, Thin Solid Films 520 (2011)582.

[39] M.C. Scharber, D. Mühlbacher, M. Koppe, P. Denk, C. Waldauf, A.J. HeegerC.J. Barbec, Adv. Mater. 18 (2006) 789.

[40] A. Gadisa, M. Svensson, M.R. Andersson, O. Inganas, Appl. Phys. Lett. 84 (2004)1609.

[41] X. Peng, W. Shen, X. Liu, Y. Zhang, M. Li, J. Phys. Org. Chem. 27 (2014) 99.[42] A. Azazi, A. Mabrouk, K. Alimi, Comput. Theor. Chem. 978 (2011) 7.[43] M. Hallermann, S. Haneder, E. Da Como, Appl. Phys. Lett. 93 (2008) 053307.

S. Ghomrasni et al. / Journal of Physics and Chemistry of Solids 76 (2015) 105–111 111