New organic materiel based on benzothiadiazole for Photovoltaic application Solar Cells

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ScienceDirect Materials Today: Proceedings 13 (2019) 1188–1196

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ICMES2018

New organic materiel based on benzothiadiazole for Photovoltaic application Solar Cells R. Kacimia, T. Abrama, L. Bejjita, M. Bouachrinea* a

MEM, High School of Technology (ESTM), University Moulay Ismail, Meknes, Morocco.

Abstract In this work, we have studied conjugated polymers based on benzothiadiazole. the quantum chemical calculations on the structure and electronic and optics properties using the density functional theory (DFT), for the ground- and excited-state properties, respectively, using CAM-B3LYP and the 6-31G(d, p) basis set. These results will be devoted to the influence of the substitution benzothiadiazole on the electronic and optoelectronic properties of the polymer. 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 [6,6]-phenyl- C61-butyric-acid methyl ester to estimate the effectiveness of these molecules as electron donors in bulk heterojunction (BHJ) small molecules-fullerene solar cells. The absorption energies have been obtained from TD-DFT calculations performed on the excited-state optimized S geometries. Finally, the theoretical results suggest that both the introduction of benzothiadiazole groups contribute significantly to the electronic and optoelectronic properties of the alternating donor–acceptor–donor conjugated systems studied. © 2019 Elsevier Ltd. All rights reserved. Peer-review under responsibility of the scientific committee of the International Conference on Materials and Environmental Science, ICMES 2018.

Keywords: Benzothiadiazole , Photovoltaic cells, DFT, BHJ, Optoelectronic properties.

1. Introduction Currently, Organic solar cells have been a subject of an increasing interest in recent years due to their advantages of light weight, low cost and potential to make flexible photovoltaic devices in comparison with the traditional

* Corresponding author. Tel.: +212660736921 E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Peer-review under responsibility of the scientific committee of the International Conference on Materials and Environmental Science, ICMES 2018.

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silicon-based solar cells [1]. One category of these devices that has received great attention was the bulkheterojunction (BHJ) solar cells introduced by Tang [2], and that has proved to be a great step forward for organic photovoltaic [3]. 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. In the BHJ polymer solar cells, a conjugated polymer is employed as a donor in combination with fullerene derivatives[4]. However, aspects of BHJ solar cells are that they can be deposited from solution like inks, enabling large-scale production by techniques like printing or roll-to-roll coating. On the other hand, BHJ solar cells still necessitate constant performing to assess their real advantages, other existing technologies cited in literature [5, 6] taking this into consideration, the researchers have devoted a great interest to the design of novel donor and acceptor materials, with particular focus in the design of donor structures with low band gap [7, 8], to be employed in BHJ solar cells. However, aspects of BHJ solar cells are that they can be deposited from solution like inks, enabling large-scale production by techniques like printing or roll-to-roll coating. On the other hand, BHJ solar cells still necessitate constant performing to assess their real advantages, other existing technologies cited in literature [5, 6] taking this into consideration, the researchers have devoted a great interest to the design of novel donor and acceptor materials, with particular focus in the design of donor structures with low band gap [7, 8], to be employed in BHJ solar cells. In this context, two conjugated compounds containing phenyl ester, as shown in (Figure 1), are studied theoretically using DFT at the CAM-B3LYP/6-31G(d, p) level [9]. Various donor electron units were introduced to investigate the effects of various substituents on the geometrical, electronic and optical properties. In addition, the highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) and energy gap (Egap) of the neutral compounds have been calculated and are reported in this paper. The electronic transition energies of the molecules examined in this study were obtained using time-dependent density functional theory (TD-DFT) at the B3LYP/6-31G(d, p) level.23[10]. The calculated results show that the compound M2 can be used as potential electron donors in organic solar cells Heterojunction (BHJ), thank to their better electronic and optical properties and good photovoltaic PV. R

O

O

S S

S

S

S

S

S

S

O

O

O

O

O

R

O

R

R

M1 R

R

O

O

O

O

S N

S N

N

S S

S S

S

M2 Figure 1:Chemical structure of studied compounds Mi (1-2),R=CH(C6H5)2[11].

N

S

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2. Material and Methods In this work, the geometric optimizations for the two molecules studied (Figure.1) were performed without symmetry constraints using the functional theory of density (DFT) and temporal (TD)-DFT calculations were performed using the Gaussian09 package[12].Thanks to its reliability, TD-DFT at this time is the most used in the study of the modeling of electronic excitations and charge transfer effects. In the calculations for the non-metallic atoms, electronic transitions (vertical excitation spectra including wavelengths, oscillator forces (OS) and the main assignment of the configuration) were studied using Functional theory of density, DFT with Beckes threeparameter, functional parameter and functional Lee-Yang-Parr function (B3LYP) [13] and 6-31G (d, p) [14]. These calculation methods of geometry of the ground state and the electronic properties, and the optimization structures, the HOMO, LUMO and gap (HOMO-LUMO) energies of all the molecules or the interest group have been used for the theoretical study of organic solar cells Heterojunction (BHJ)[15]. 3. Results and discussion 3.1. Geometry optimization The optimized geometric structures of the two molecules studied indicated in Figure.2 and their parameters are collected in Tables (1, 2) and Figure 3. The results of the optimized structures, the chosen dihedral angle θi (i=1-7) and the link distance parameters di (i=1-7) show that the corresponding binding distances in these compounds are similar.

M1

M2 Figure 2: Optimized geometries obtained by B3LYP/6-31G (d,p) of the studied molecules.

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Table 1: Obtained distances (Å) of the studied compoundsobtained by B3LYP/6-31G(d,p) calculations.

Inter-ring distances (A˚)

d1

d2

d3

d4

d5

d6

d7

M1

1.454

1.445

1.452

1.446

1.452

1.446

1.457

M2

1.455

1.456

1.452

1.442

1.450

1.456

1.458

Table 2: Dihedral angles θi(◦) of the studied compounds obtained by B3LYP/6-31G(d,p) calculations.

Dihedral angles (θ˚)

θ1

θ2

θ3

θ4

θ5

θ6

θ7

M1

-136.55

175.50

138.03

162.56

-136.53

160.47

-131.30

175.22

-175.66

169.64

-147.38

177.51

-170.47

M2

137.29-

Theoretical calculations show that the obtained torsion angles are assessed at approximately 131.30º and 175.50º for M1, while the other compound pose antigauche angle with a torsional angle means between 137.29° and 177.51° for M2. The effect of alkyl groups is very clear, indeed by comparing the geometrical parameters and especially at the level of the torsion angles, it is noted that the insertion of the alkyl groups causes a distortion between the thiophene rings, and subsequently the structures. This distortion must therefore expect a variation of the electronic properties of parent polymers. These results can be explained by a repulsion of alkyl groups which promote the fragility of these systems. R

O

O

 S

d1

S

 S

d2





d3

d4

O

S

S





d5

d6

O

 d7

S

O

O

R

S

S

O

R

O

R

Figure 3: Scheme of the bond di (i=1-7) lengths and dihedral angles θi (i=1-7),R=CH (C6H5)2.

3.2. Electronic properties and frontier molecular orbital In this part, we have determinate the electronic properties of the two studied molecules and in order to obtain an idea of the distribution patterns of the FMOs. We have also examined the HOMO and LUMO levels. The relative order of these orbitals gives a reasonable qualitative indication of the excitation properties and also gives information on the transport capacity of electron holes [16, 17]. As a consequence, and as indicated in (Figure 4) (LUMO, HOMO); the HOMO has an antisocial character between consecutive subunits. Also, the LUMO of all the compounds studied generally indicates a binding character between the subunits.

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HOMO

LUMO

M1

M2 Figure 4: The contour plots of HOMO and LUMO orbitals of the studied compounds.

The study of the influence of the benzothiadiazole effect on the electronic properties of the compounds studied maintains the results of the energy values HOMO, LUMO and Egap energy gap were theoretically estimated as the difference between the HUMO and LOMO obtained using Functional theory of density, DFT with Beckes threeparameter, functional parameter and functional Lee-Yang-Parr function (B3LYP) and 6-31G (d, p), are shown in (Table 3) and (Figure.5). Tableau 3: Electronic properties parameters (HOMO, LUMO and Egap) obtained by B3LYP/6-31G(d, p) of the studied molecules.

Compounds M1 M2 PCBMC60

EHOMO (eV) -4.948 -4.852 -6.100

ELUMO (eV) -2.031 -2.767 -3.470

Egap (eV) 2.917 2.085

We have observed that M1 has the highest gap energy: 2.917 eV. We first notice that the insertion of benzothiadiazole decreases the gap energy for M2: 2.085 eV, this is explained by the distortion of the aromatic cycles which causes a bad conjugation due to the non flatness. The LUMO for the compound M2 was higher by at least than the energy level of the edge of the CB PCBMC60. This observation indicated that the compound potential in the ground state and the excited state was favorable to an efficient injection of electrons at the CB PCBMC60. This Egap energy results showed that compound M2 can be suggested as a better candidate for the application of organic solar cells. 3.3. Photovoltaic properties Efficiency of the benzothiadiazole compounds considered as photovoltaic devices can be estimated by calculation of the power conversion efficiency (PCE) who measures the amount of power produced by a solar cell relative to the power available in the incident solar radiation (Pin). Photovoltaic efficiency of the photovoltaic cell (power conversion efficiency (ߟ)), can be calculated using Eq. 1[18]:

  FF

Voc J sc Pinc

(1)

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Where Pin is the incident power density, Voc is the open-circuit voltage, Jsc is the short-circuit current, and FF is the fill factor. For solar cells in the bomb (BHJ), the maximum open circuit voltage (Voc) is related to the difference between the electron donor HOMO and the electron acceptor LUMO, energy lost during photo-etching. The theoretical values of the open-circuit voltage Voc were calculated using Eq. 2[19]:

Voc  EHOMO (Donor)  ELUMO ( Acceptor)  0.3

(2)

α i = E LUMO ( D onor) - E LUMO ( A cceptor)

(3)

Table 4:The values of the energy of the molecules studied, ELUMO (eV), EHOMO (eV), Egap (eV) and the open circuit voltage Voc (eV) calculates bay B3LYP/6-31G (d, p).

Compounds M1 M2 PCBMC60

EHOMO (eV) -4.948 -4.852 -6.100

ELUMO (eV) -2.031 -2.767 -3.47

Egap(eV) 2.917 2.085

Voc (eV) 1.178 1.082

α (eV) 1.439 0.703

The optical property and electronic transition, the excitation energy and UV/Vis absorption spectra of all studied molecules in vacuum was performed using CAM-B3LYP/6-31G (d,p) calculations. The results of excited singlet states, transitions energies, oscillator strength and absorption spectra of all studied compounds are shown in Table 5 and Figure 6.The spectra show similar profile for all compounds which present a main intense band at higher energies in the visible region ranging from 407.09 to 521.13 nm, and were assigned to the intramolecular charge transfer (ICT) transitions, these strongest absorption peaks arise from S0 to S1 , which correspond to the dominant promotion of an electron transition from HOMO to LUMO.

Figure 5: Data of the absolute energy of the frontier orbitals HOMO and LUMO for the studied molecules and PCBMC60

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R. Kacimi et al / Materials Today: Proceedings 13 (2019) 1188–1196 Table 5: Absorption maximum λmax obtained by CAM-B3LYB calculations

Compounds M1

λmax(nm) CAM-B3LYB 407.09

Eactivation (eV) 3.04

2.68

HOMO→LUMO (77%)

M2

521.13

2.38

2.22

HOMO→LUMO (62%)

O.S

MO/character

From the results obtained in the Tables (3,4) of the studied molecules indicates that obtained conformations in the two cases of the compounds M1 and M2 are not planes, this confirms the existence of the steric effects exerted by the alkyl groups. It should be noted that the value of the absorption maximum λmax increases from: λmax M1 = 407.09 nm and λmax M2 = 521.13 nm. These results show that the steric interactions are greater and consequently the conjugation is weaker. These results are in agreement with those detailed in the table 4.

Figure 6: Simulated UV–visible optical absorption spectra of title compounds with calculated data at the CAM-B3LYP method.

In the other hand, semiconductor organic materials have a low dielectric constant and a very high absorption coefficient. When a photon was absorbed by an electron donor, the electron was excited to an excited state of energy that binds with the holes to form exciton with a binding energy of about 0.3-0.4 eV. The exciton will be scattered at the interfaces between the electron donor and the acceptor electron under the integrated electric field and be dissociated into free charge carriers. Charge carriers induced by exaction dissociation will be collected by their corresponding electrodes. To be used these molecules in the solar cell and based on the Scharber model [20], the maximum compound conversion rate with M2 / PCBMC60 gives a better conversion rate (see Figure7).

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Figure 7: Contour plots showing the energy-conversion efficient of the studied compounds.

The compound M2 has the best conjugation properties and it can be a candidate for the photovoltaic application. The solubility can be provided by the alkyl group; the compound M1 (soluble) also has interesting electronic and optical properties. In general and according to the results obtained (weak gap, high visible absorption, VOC value and solubility) the molecule M2 it an excellent candidate for application photovoltaic. 4. Conclusions In this work, we have presented a theoretical study of the two M1and M2 molecules using DFT and TD-DFT method. The optimized geometric configurations and the electronic structures of the compounds studied with PCBMC60 as acceptor are discussed. This work allows us to narrowly show that benzothiadiazole affects the performance of Compounds, and as a perspective for the design and development of novel organic materials for the photovoltaic application. We found that the conversion rate could be improved by replacing thiophene with a benzothiadiazole which has an effect on photo-induced charge distributions and improves the efficiency of the light harvest, as well as an effect of absorption, based on the results of this study. I hope that M2-based solar cells will perform better than cell M1. This work may have sound implications for the design of other organic materials with target properties to improve the efficiency yield of organic solar cells. References [1] M.A. Green, A. Ho-Baillie, H.J. Snaith, Nature Photonics, nphoton. Vol 11 Issue 2 (2014) 2134. [2] F.E. Ala'a, J.-P. Sun, I.G. Hill, G.C. Welch, Journal of Materials Chemistry A, Vol 2 Issue 5 (2014) 1201-1213. [3] J. You, L. Meng, T.-B. Song, T.-F. Guo, Y.M. Yang, W.-H. Chang, Z. Hong, H. Chen, H. Zhou, Q. Chen, Nature nanotechnology, Vol 11 Issue 1 (2016) 75. [4] W. Zhao, D. Qian, S. Zhang, S. Li, O. Inganäs, F. Gao, J. Hou, Advanced Materials, Vol 28 Issue 23 (2016) 4734-4739. [5] N. Espinosa, R. García-Valverde, F.C. Krebs, Energy & Environmental Science, Vol 4 Issue 5 (2011) 1547-1557. [6] N. Espinosa, R. Garcia-Valverde, A. Urbina, F.C. Krebs, Solar Energy Materials and Solar Cells, Vol 93 Issue 4 (2011) 1293-1302. [7] K. Hasnaoui, A. MAKAYSSI, M. Hamidi, M. Bouachrine, J. Iran. Chem, Vol 93 Issue 4 (2008) 67-77. [8] A. Operamolla, S. Colella, R. Musio, A. Loiudice, O.H. Omar, G. Melcarne, M. Mazzeo, G. Gigli, G.M. Farinola, F. Babudri , Solar Energy Materials and Solar Cells, Vol 95 Issue 12 (2011) 3490-3503. [9] M.N. Arshad, A.M. Asiri, K.A. Alamry, T. Mahmood, M.A. Gilani, K. Ayub, A.S. Birinji,Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy” Vol 142 (2015) 364-374. [10] Q. Zhang, H. Kuwabara, W.J. Potscavage Jr, S. Huang, Y. Hatae, T. Shibata, C. Adachi, Journal of the American Chemical Society, Vol 136 Issue 52 (2014) 18070-18081. [11] D.M. Shircliff, V.J. Pastore, M.L. Poltash, B.M. Boardman, Materials Today Communications, Vol 8 (2016) 15-22. [12] M. Barbatti, M. Ruckenbauer, F. Plasser, J. Pittner, G. Granucci, M. Persico, H. Lischka,Wiley Interdisciplinary Reviews: Computational Molecular Science, Vol 4 Issue 1 (2014) 26-33. [13] L. Kronik, A. Tkatchenko, Accounts of chemical research, Vol 47 Issue 11 (2014) 3208-3216.

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