Design and photovoltaic properties of new molecules based bithiophene for bulk heterojunction solar cells
Y. A. Sadiki
a, SM. Bouzzine
b, L. Bejjit
a, M. Hamidi
band M. Bouachrine
*ca LASMAR, Faculté des Sciences, Université Moulay Ismail, Meknès, Maroc
bUCTA/URMM, Faculté des Sciences et Techniques, Errachidia, Maroc.
cMEM, EST Meknes, Université Moulay Ismaïl, Meknes, Maroc
*Corresponding author. E-mail : [email protected]
Received 7 Apr 2014, Revised 10 May 2014, Accepted 10 May 2014
ABSTRACT
In the present work, new π-conjugated compounds based on bithiophene have been studied by quantum chemistry using DFT (Density Functional Theory) at B3LYP/6-31G(d,p) level to examine the structural and electronic properties. The absorption spectra were simulated by TD-DFT (Time Dependent Density Functional Theory) at the same level.The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels of these compounds were calculated and compared to LUMO of fullerenes C60. Electronic, optical and photovoltaic properties have been reported in order to predict the BHJ solar cell device efficiency for studied compounds.
Keyword: DFT, bithiophene, low band gap, PCBM, bulk heterojunction, solar cells.
INTRODUCTION
New solar cells absed on organic compounds are a subject of an increasing interest in recent years due to their advantages of low cost, light weight, processability of organic materials and potential to make flexible photovoltaic devices in comparison with the traditional silicon-based solar cells [1]. One class of these devices that received great attention was the bulk-heterojunction (BHJ) solar cells. The bulk-heterojunction (BHJ) architectures are based on charge generation at the interface between two different blended organic semiconductors, which proceed as donor and acceptor. The organic conjugated molecule is employed as a donor in combination with PCBM as acceptor [2]. Recently, new organic molecules based on pi-conjugated system with low band gap began to have great interest as donors in this kind of devices. In the same way, materials based on heterocyclic molecules have been the subject of several studies where these compounds show a potential for applications in photovoltaic cells [3]. Thus, numerous syntheses have been mainly developed. The conception of these materials was especially made for polymer or dye-sensitized solar cells but rarely for small molecules for bulk-heterojunction organic solar cells.
In this context, we present herein theoretical study of the structural and optoelectronic properties of new systems based on bithiophene (Fig. 1) by quantum chemistry using DFT (Density Functional Theory) at B3LYP/6-31G(d,p) level. The absorption spectra were simulated by TD-DFT (Time Dependent Density Functional Theory) at the same level.The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels of these compounds were calculated and compared to LUMO of fullerenes C60. Electronic, optical and photovoltaic
properties have been reported in order to predict the BHJ solar cell device efficiency for studied compounds. Few molecules of this series were prepared by Raposo et al [4]
S N
S
S
N N
1
S N
S S
S
N N 2
S N S
S N N
S
7
S N S
S S
N N
N N OH
OH 3
S N
S S
N N
N N
S
OH OH 8
S N S
S S
N N
N N
4
S N
S
S N N
N N
S
9
S N S
S S
N N N
O OH 5
S N
S S
N N N
O OH S
10
S N
S S
S
N N N
O
6
S N
S S
N N N
O S
11
Fig. 1: The sketch map of studied structures
I. THEORETICAL METHODOLOGY
DFT method of three-parameter compound of Becke (B3LYP) [5] was used in all the study of the neutral and polaronic compounds. The 6-31G (d,p) basis set was used for all calculations [6]. To obtain the charged structures, we start from the optimized structures of the neutral form. The calculations were carried out using the GAUSSIAN 09 program [7]. The geometry structures of neutral and doped molecules were optimized under no constraint. We have also examined HOMO and LUMO levels; the energy gap is evaluated as the difference between the HOMO and LUMO energies. The ground state energies and oscillator strengths were investigated using the TD DFT, calculations on the fully optimized geometries. In fact, these calculation methods have been successfully applied to other conjugated molecules and polymers [8]. All calculations were done on cluster machines in the IPREM of Pau in France.
II. RESULTS AND DISCUSSION
3.1. Molecular design and geometry structures
The chemical structures of all studied molecules in this work are displayed in Figure 1. All the molecular geometries have been calculated with the hybrid B3LYP function combined with 6-31G (d,p) basis sets using Gaussian 09 program suite. The calculated bond lengths of molecules are listed in table 1. For each model six inter- ring bond lengths di (with i = 1, 2, 3, 4, 5 or 6) which greatly contribute to the internal energy, were compared in table 1. It was found in other works [17] that the DFT-optimized geometries were in excellent agreement with the data obtained from X-ray analyses.
TABLE 1: GEOMETRICAL PARAMETERS OF STUDIED COMPOUNDS F1 TO F5 OBTAINED BY B3LYP/6-31G (D,P) IN THEIR NEUTRAL STATES.
S N
S S
S
N N N
O OH d1
d2
d3 d4
d5
d6
d1 d2 d3 d4 d5 d6
COMPOUNDS
1 - 1.443 1.360 1.276 1.378 -
2 - 1.416 1.370 1.278 1.386 -
3 1.374 1.410 1.363 1.284 1.379 1.414 4 1.367 1.405 1.352 1.290 1.376 1.412 5 1.369 1.406 1.355 1.288 1.379 1.461 6 1.370 1.406 1.356 1.287 1.379 1.469
7 - 1.446 1.367 1.279 1.385 -
8 1.372 1.437 1.359 1.285 1.379 1.414 9 1.365 1.429 1.349 1.291 1.376 1.412 10 1.367 1.431 1.352 1.289 1.378 1.461 11 1.367 1.432 1.353 1.288 1.379 1.469
We note firstly a decrease of the inter-ring double bonds d2 when we make the bridge either by the sulfur (S) or by the carbone-sulfure (C=S), so the difference by the component without the bridge and the other one with the bridge
is about 0.01 Å and also for the d6 bond especially for compounds 3 to 6 and 8 to 11, but on the other hand a slight increase in d4 bond, as far as concerned the other bonds (d5 & d3) we see that there is no change, we have the same value for all studied compound.
This is probably due to greater extension of conjugation. These modifications can explain the gap variations found in table 3. On the other hand, adding the bridge of sulfur or carbon-sulfur leads to decrease the gap energy
Fig 2 Optimized structures of studied oligomers (F1, F2, F3 and F4)
3.2. Opto-electronic properties
It is important to examine the HOMO and the LUMO for these oligomers because the relative ordering of occupied and virtual orbital provides a reasonable qualitative indication of excitation properties [20]. In general, and as plotted in Figure 3; the HOMO possesses an antibonding character between the consecutive subunits. On the other hand, the LUMO of all oligomers generally shows a bonding character between the subunits.
HOMO LUMO
1
2
3
4
5
6
7
8
9
10
11
Fig 3 The contour plots of HOMO and LUMO orbitals of the studied compounds F1, F2, F3 and F4
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 voltametry. But in the theory, the HOMO and LUMO energies can be calculated by DFT calculation [21]. However, it is noticeable that solid-state packing effects are not included in the DFT calculations, which tend to affect the HOMO and LUMO energy levels in a thin film compared to an isolated molecule as considered in the calculations. Even if these calculated energy levels are not accurate, it is possible to use them to get information by comparing similar oligomers or polymers.
TABLE 3:VALUES OF HOMO(EV),LUMO(EV) AND EG (EV) ENERGIES
Studied
compounds EHOMO (eV) ELUMO (eV) Egap (eV)
1 -5.591 -2.903 2.688
2 -5.616 -2.849 2.767
3 -4.808 -2.432 2.375
4 -5.262 -3.001 2.261
5 -5.137 -2.835 2.302
6 -5.123 -2.830 2.293
7 -5.673 -3.502 2.171
8 -4.849 -3.202 1.646
9 -5.300 -3.591 1.708
10 -5.178 -3.461 1.717
11 -5.162 -3.450 1.711
The highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) are not only used to understand the charge transfer within the molecule but also used to know a lot of informations namely to estimate the chemical reactivity and kinetic stability of the molecules [7,49] .The calculated frontier orbital energies HOMO and LUMO and energy gaps between highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) are listed in Table 3. As shown in Table 3, the calculated electronic parameters (Gap, LUMO, HOMO) of the studied compounds are : (2,688 eV; -2,903 eV; -5,591 eV) ; (2,767eV, - 2,849eV, -5,616eV); (2,375eV, -2,432 eV, -4,808 -eV), (2,261eV, -3,001 eV, -5,262eV) ; (2,302eV, -2,835eV, - 5,137eV) ; (2,293eV, -2,830eV, -5,123 eV) ; (2,171eV, -3,502 eV, -5,673-eV) ;(1,646eV, -3,202eV, -4,849eV) ; (1,708eV, -3,591eV, -5,300eV); (1,717eV, -3,461 eV, -5,178eV) ;(1,711eV, -3,450eV, -5,162eV) respectively.
Comparing these molecules the increased ICT characters make the energies of HOMO and LUMO stabilized and the energy gaps between HOMO and LUMO decrease, which would make the Optical absorption spectra red shifted. The order of energy gaps between HOMO and LUMO is : 8(1,646 eV)< 9(1,708 eV) < 11(1,711 eV)<
10(1,717 eV)< 7(2,171 eV)< 6(2,293 eV)< 4(2,261 eV) <5(2,302 eV) < 3(2,375 eV) < 1(2,688 eV) < 2(2,767 eV).
The comparison between the energies of all compounds shows that the low gap energy is found in molecules with carbone-sulfur bridge. On the other hand, we note that the Energy Egap decreases when we add bridges to the studied molecules.Molecule 8 with this lowest energy gap is expected to have the most outstanding photophysical properties.
3.3. Photovoltaic properties
Generally, the most efficient material solar cells are based on the bulk hetero-junction structure of the blend of π- conjugated molecule or polymer donors and fullerene derivative acceptors [22]. Here, we studied the photovoltaic properties of the compounds as donor blended with [6.6]-phenyl-C61-butyric acid methyl ester (PCBM), which is the most broadly used as an acceptor in solar cell devices.
The HOMO and the LUMO energy levels of the donor and acceptor components are very important factors to determine whether effective charge transfer will happen between donor and acceptor. Figure 4 shows detailed data
of absolute energy of the frontier orbitals for studying compounds and PCBM (C60) is included for comparison purposes. It is deduced that the nature of donor or acceptor pushes up/down the HOMO/LUMO energies in agreement with their electron character. To evaluate the possibilities of electron transfer from the excited studied molecules to the conductive band of PCBM, the HOMO and LUMO levels were compared.
Fig 4 B3LYP/6-31G (d, p) calculated energies of the HOMO, LUMO level of studied molecules.
As shown in Figure 4, both HOMO and LUMO levels of the studied molecules agreed well with the requirement for an efficient photosensitizer. On the one hand, the difference in the LUMO energy levels of the studied compounds (1 to 11) and PCBM was in the range of 0.1 to 1.27 eV, suggesting that the photoexcited electron transfer from the studied molecule to the acceptor PCBM may be sufficiently efficient to be useful in photovoltaic devices [23]. On the other hand, the power conversion efficiency (PCE) was calculated according to the following equation:
Where Pin is the incident power density, Jsc is the short-circuit current, Voc is the open-circuit voltage, and FF denotes the fill factor. The maximum open circuit voltage (Voc) of the BHJ solar cell is related to the difference between the highest occupied molecular orbital (HOMO) of the electron donor and the LUMO of the electron acceptor, taking into account the energy lost during the photo-charge generation [24]. The theoretical values of open-circuit voltage Voc have been calculated from the following expression:
The calculated Voc of the studied molecules ranges from 0.8 eV to 1.69 eV. These values are sufficient for a possible efficient electron injection. Therefore, all the studied molecules can be used as sensitizers because the electron injection process from the excited molecule to the conduction band of PCBM and the subsequent regeneration is possible in an organic solar cell.
0 2 4 6 8 10 12
-6,0 -5,5 -5,0 -4,5 -4,0 -3,5 -3,0 -2,5 -2,0
1 2 3 4 5 6 7 8 9 10 11 LUMO
HOMO
Egap 2.68 2.76
2.37
2.26
2.3 2.29
2.17 1.64
1.70 1.71 1.71
TABLE 4:ENERGY VALUES OF ELUMO(EV),EHOMO(EV) AND THE OPEN CIRCUIT VOLTAGE VOC(EV).
Studied compounds
EHOMO
(eV) ELUMO (eV) Voc (eV) ELUMO (i) – ELUMO
(PCBM)
1 -5.591 -2.903 1.69 0.8
2 -5.616 -2.849 1.61 0.86
3 -4.808 -2.432 0.8 1.27
4 -5.262 -3.001 1.26 0.7
5 -5.137 -2.835 1.13 0.87
6 -5.123 -2.830 1.12 0.87
7 -5.673 -3.502 1.67 0.2
8 -4.849 -3.202 0.84 0.5
9 -5.300 -3.591 1.3 0.1
10 -5.178 -3.461 1.17 0.24
11 -5.162 -3.450 1.16 0.25
PCBM C60 - 6.1 - 3.7 [25-26] - -
3.4. Absorption properties
Based on the optimized molecular structures with B3LYP/6-31G (d,p) method. We have calculated the UV-vis spectra of each studied compound using TD-DFT method. As illustrated in fig 5, we can find the values of calculated wavelength max and oscillator strengths O.S. We note firstly that the principle electronic transitions occur in the visible region (400-750nm). Excitation to the S1 state corresponds almost exclusively to the promotion of an electron from the HOMO to the LUMO orbital.
Fig 5 Simulated UV-visible optical absorption and emission spectra of each compound with the calculated data at the TDDFT B3LYP/6-31G(d,p) and OPT-TDDFT B3LYP/6-31G(d,p) level
500 1000 1500
0 10000 20000 30000 40000 50000 60000
Energy (ua)
wavelength()
1 2 3 4 5 6 7 8 9 10 11
The absorption wavelengths arising from S0→S1 electronic transition increase progressively with the increasing of conjugation lengths. It is reasonable, since HOMO→LUMO transition is predominant in S0→S1 electronic transition; the results are a decrease of the LUMO and an increase of the HOMO energy. Data in fig 5 shows that there is a bathochromic shift when passing from 1 to 11. This effect is obviously due to insertion of different donor or acceptor to the π-spacers unit. Those interesting points are seen both in the theoretical and experimental results.
III. CONCLUSIONS
New π-conjugated compounds, based on bithiophene have been theoretically investigated using DFT(Density Functional Theory). Geometric, electronic and photovoltaic parameters were studied using DFT (Density Functional Theory) at B3LYP/6-31G(d,p) level. Absorption spectra were simulated by TD-DFT (Time Dependent Density Functional Theory) at the same level.
- We note firstly a decrease of the inter-ring double bonds d2 when we make the bridge either by the sulfur (S) or by the carbone-sulfure (C=S), this is probably due to greater extension of conjugation
- The order of energy gaps between HOMO and LUMO is : 8(1,646 eV)< 9(1,708 eV) < 11(1,711 eV)<
10(1,717 eV)< 7(2,171 eV)< 6(2,293 eV)< 4(2,261 eV) <5(2,302 eV) < 3(2,375 eV) < 1(2,688 eV) <
2(2,767 eV).
- we note that the Energy Egap decreases when we add bridges to the molecule, Molecule 8 with this lowest energy gap is expected to have the most outstanding photophysical properties
- The absorption spectra of the studied molecules exhibited red-shifted maximum wavelength The bathochrom effect observed from 1 to 11. This effect is obviously due to insertion of different donor or acceptor to the π-spacers unit.
- The difference between the energy of LUMO level of PCBM and the energy of LUMO level of the studied molecules ranges from 0.8 eV to 1.69 eV. These values are sufficient for a possible efficient electron injection
- All the studied molecules can be used as sensitizers because the electron injection process from the studied molecule (donor) to the conduction band of PCBM (acceptor) and the subsequent regeneration are feasible in the organic sensitized solar cell.
This calculation procedure can be used as a model system for understanding the relationships between electronic properties and molecular structure and also can be employed to explore their suitability in electroluminescent devices and in related application. Presumably, the procedures of theoretical calculations can be employed to predict and assume the electronic properties on yet prepared and efficiency proved the other materials, and further to design novel materials for organic solar cells.
ACKNOWLEDGMENT -THIS WORK WAS SUPPORTED BY VOLUBILIS PROGRAM (N°MA/11/248), AND THE CONVENTION CNRST/CNRS(PROJECT CHIMIE1009).WE ARE GRATEFUL TO THE “ASSOCIATION MAROCAINE DES CHIMISTES THÉORICIENS”(AMCT) FOR ITS PERTINENT HELP CONCERNING THE PROGRAMS
REFERENCES
[1] T. Yuhua, S. Ping, D. Tianpeng, H. Huang, B. Zhao, S. Tan, European Polymer Journal 46 (2010) 2033–2041.
[2] T. Kitagawa, Y. Murata, K. Komatsu, in: M.M. Haley, R.R. Tykwinski (Eds.), Carbon-Rich Compounds, Wiley-VCH Verlag GmbH & Co. KGaA,Weinheim, (2006) 383–420.
[3] Eva Bundgaard, Sean E. Shaheen, Frederik C. Krebs, David S. Ginley, Solar Energy Materials & Solar Cells, 91 (2007) 1631–1637.
[4] M. Manuela M. Raposo, M. Cidália R. Castro, M. Belsley, A. Maurício C. Fonseca, Dyes and Pigments. 91(3) (2011) 454–465
[5] J.A Mikroyannidis, D.V. Tsagkournos, P. Balraju, GD. Sharma, Journal of Power Sources. 196 (2011) 4152.
[6] R. Ditchfield,. W.J. Hehre,. J.A. Pople. J. Chem. Phys. 54 (1971) 54
[7] Gaussian 09, Revision C.01, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A.
Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N.
Staroverov, T. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J.
Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J.
Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R.
L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D.
Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, and D. J. Fox, Gaussian, Inc., Wallingford CT, (2010).
[8] S. V. Meille, A. Farina, F. Bezziccheri., MC. Gallazi, Adv. Mater. 6 (1994) 848.
[9] M. Bouachrine,.A. Echchary, SM. Bouzzine,M. Amine, M. Hamidi,A. Amine, A.; T. Zair, Phys. Chem.News.
58 (2011) 61.
[10] J. Casado, V. Hernàndez, FJ. Ramirez, JT. López Navarrete,. J. Mol. Struct. (Theochem). 463 (1999) 211.
[11] S.M. Bouzzine, S. Bouzakraoui, M. Bouachrine, M. Hamidi,. J. Mol. Struct. (Theochem). 271 (2005) 726.
[12] L. Yang, J. Feng, A. Ren, Polymer. 46 (2005) 10970.
[13] , J.-L. Brèdas, M.D. McGehee, Z. J Bao. Mater. Chem. 20 (2010) 5823.
[14] H. Derouiche, V. Djara, Sol. Energy Mater. Sol. Cells. 91 (2007) 1163
[15] F.B. Kooistra, J. Knol, F. Kastenberg, LM. Popescu, WJH.Verhees,JM. Kroon, JC. Hummelen, Org. Lett. 9 (2007) 551.
[16] A. Gadisa, M. Svensson, MR. Andersson, O. Inganas, Appl. Phys. Lett. 84 ( 2004) 1609.
[17] B. C. Thompson, Y. Kim, R. Reynolds,. Macromolecules. 38 (2005) 5359.
[18] E. J. Meijer, D. M. De Leeuw,S. Setayesh, E.Van Veenendaal, BH. Huisman, PWM. Blom, JC. Hummelen, U.
Scherf, TM. Klapwijk, Nat. Mater. 2 (2003) 678.
[19] S. Kato, T.Matsumoto, M. Shigeiwa, H.Gorohmaru, S. Maeda, T. Ishi-i, S. Mataka, Chem Eur J. 12 (2006) 2303.
[20] J.-S.K. Yu, W.-C. Chen, C.-H. Yu,. J Phys Chem A107 (2003) 4268.
