The optoelectronic properties of new dyes based on thienopyrazine

Mohamed Bourass; Adil Touimi Benjelloun; Mohammed Benzakour; Mohammed Mcharfi; Mohammed Hamidi; Si Mohamed Bouzzine; Françoise Serein-Spirau; Thibaut Jarrosson; Jean-Marc Sotiropoulos; Mohammed Bouachrine

doi:10.1016/j.crci.2016.12.004

The optoelectronic properties of new dyes based on thienopyrazine

Mohamed Bourass ¹ ; Adil Touimi Benjelloun ¹; Mohammed Benzakour ¹; Mohammed Mcharfi ¹; Mohammed Hamidi ²; Si Mohamed Bouzzine ^2,³; Françoise Serein-Spirau ⁴; Thibaut Jarrosson ⁴; Jean-Marc Sotiropoulos ⁵; Mohammed Bouachrine ⁶

¹ ECIM/LIMME, Faculty of Sciences Dhar El Mahraz, University Sidi Mohamed Ben Abdallah, Fez, Morocco
² Centre Régional des Métiers d'Éducation et de Formation, BP 8, Errachidia, Morocco
³ Equipe d'Électrochimie et Environnement, Faculté des Sciences et Techniques, Université Moulay, Morocco
⁴ Institut Charles-Gerhardt de Montpellier, UMR CNRS 5253, Architectures Moléculaires et Matériaux Nanostructurés, France
⁵ Université de Pau et des Pays de l'Adour, UMR5254 – IPREM, Équipe Chimie-Physique, Hélioparc, Pau, France
⁶ ESTM, (LASMAR), University Moulay Ismaïl, Meknes, Morocco

Comptes Rendus. Chimie, Volume 20 (2017) no. 5, pp. 461-466.

Abstract

In this article, we present the quantum result of six dyes based on thienopyrazine (D1–D6) with donor–π–acceptor structure (D–π–A) using DFT/B3LYP/6-31G(d,p) and TD-DFT/CAM-B3LYP/6-31G(d,p) levels. The donor unit varied and the influence was investigated. The study of structural, electronic, and optical properties of these dyes could help design more efficient functional photovoltaic organic materials. Key parameters in close connection with the short-circuit current density (J_sc) including light harvesting efficiency, injection driving force (ΔG_inject), and total reorganization energy (λ_total) are discussed in this work.

Metadata

Received: 2016-07-13
Accepted: 2016-12-12
Published online: 2017-01-11

DOI: 10.1016/j.crci.2016.12.004

Keywords: Thienopyrazine derivatives, DFT, Photovoltaic, Optoelectronic properties

Author's affiliations:

Mohamed Bourass ¹; Adil Touimi Benjelloun ¹; Mohammed Benzakour ¹; Mohammed Mcharfi ¹; Mohammed Hamidi ²; Si Mohamed Bouzzine ^{2, 3}; Françoise Serein-Spirau ⁴; Thibaut Jarrosson ⁴; Jean-Marc Sotiropoulos ⁵; Mohammed Bouachrine ⁶

¹ ECIM/LIMME, Faculty of Sciences Dhar El Mahraz, University Sidi Mohamed Ben Abdallah, Fez, Morocco
² Centre Régional des Métiers d'Éducation et de Formation, BP 8, Errachidia, Morocco
³ Equipe d'Électrochimie et Environnement, Faculté des Sciences et Techniques, Université Moulay, Morocco
⁴ Institut Charles-Gerhardt de Montpellier, UMR CNRS 5253, Architectures Moléculaires et Matériaux Nanostructurés, France
⁵ Université de Pau et des Pays de l'Adour, UMR5254 – IPREM, Équipe Chimie-Physique, Hélioparc, Pau, France
⁶ ESTM, (LASMAR), University Moulay Ismaïl, Meknes, Morocco

@article{CRCHIM_2017__20_5_461_0,
     author = {Mohamed Bourass and Adil Touimi Benjelloun and Mohammed Benzakour and Mohammed Mcharfi and Mohammed Hamidi and Si Mohamed Bouzzine and Fran\c{c}oise Serein-Spirau and Thibaut Jarrosson and Jean-Marc Sotiropoulos and Mohammed Bouachrine},
     title = {The optoelectronic properties of new dyes based on~thienopyrazine},
     journal = {Comptes Rendus. Chimie},
     pages = {461--466},
     publisher = {Elsevier},
     volume = {20},
     number = {5},
     year = {2017},
     doi = {10.1016/j.crci.2016.12.004},
     language = {en},
}

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AU  - Mohammed Mcharfi
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AU  - Si Mohamed Bouzzine
AU  - Françoise Serein-Spirau
AU  - Thibaut Jarrosson
AU  - Jean-Marc Sotiropoulos
AU  - Mohammed Bouachrine
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JO  - Comptes Rendus. Chimie
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%A Mohamed Bourass
%A Adil Touimi Benjelloun
%A Mohammed Benzakour
%A Mohammed Mcharfi
%A Mohammed Hamidi
%A Si Mohamed Bouzzine
%A Françoise Serein-Spirau
%A Thibaut Jarrosson
%A Jean-Marc Sotiropoulos
%A Mohammed Bouachrine
%T The optoelectronic properties of new dyes based on thienopyrazine
%J Comptes Rendus. Chimie
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%P 461-466
%V 20
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%R 10.1016/j.crci.2016.12.004
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%F CRCHIM_2017__20_5_461_0

Mohamed Bourass; Adil Touimi Benjelloun; Mohammed Benzakour; Mohammed Mcharfi; Mohammed Hamidi; Si Mohamed Bouzzine; Françoise Serein-Spirau; Thibaut Jarrosson; Jean-Marc Sotiropoulos; Mohammed Bouachrine. The optoelectronic properties of new dyes based on thienopyrazine. Comptes Rendus. Chimie, Volume 20 (2017) no. 5, pp. 461-466. doi : 10.1016/j.crci.2016.12.004. https://comptes-rendus.academie-sciences.fr/chimie/articles/10.1016/j.crci.2016.12.004/

Version originale du texte intégral

1 Introduction

The π-conjugated molecules have been drawing broad research attention because of their practical applications in device technology such as solar cells [1,2]. In the field of organic solar cells, conjugated molecules are a subject of an increasing interest in recent years because of their advantages of light weight, low cost, and potential to make flexible photovoltaic devices in comparison with the traditional silicon-based solar cells.

Dye-sensitized solar cells (DSSCs) reported by O'Regan and Grätzel in 1991 attracted attention of the scientific community as new devices for solar cell application. These solar cells are composed of wide gap semiconductors (such as TiO₂, ZnO, and so on) having a high surface area on which is adsorbed a dye (such as a ruthenium complex or an organic dye). The cell also contains a redox couple electrolyte (typically I⁻/I₃⁻) [1,3], which is in contact with an electrode (usually platinum). In the excited state (D^∗), after the absorption of light, the dye injects an electron into the conduction band of the semiconductor. The oxidized dye is reduced by a reducing agent (I⁻), the latter is regenerated against the electrode [4]. In the end, no species are consumed or formed during operation of the cell. So, there is appearance of a photocurrent.

The semiconductor used must have a wide gap to have no overlap between the absorption and the solar spectrum. The most widely used semiconductor is titanium oxide (TiO₂) in the anatase form. For a high power conversion efficiency, the DSSCs must have the following characteristics: the HOMO (highest occupied molecular orbital) level of dyes must be down than the HOMO level of electrolyte for making easy the reception of the electron from a redox electrolyte pair (I⁻/I₃⁻), the LUMO (lowest unoccupied molecular orbital) must be located on the conduction band of TiO₂ for facilitating the injection phenomenon into its conduction band_.

The dyes having a donor–π–acceptor (D–π–A) structure are the most studied currently, which exhibit several advantages such as low cost of production and high molar extinction coefficients [5]. In this structure, the process photoexcitation has been done by the intramolecular charge transfer (ICT) between D (donor) and A (acceptor) via π-spacer. The electron injects into the conduction band of the semiconductor (TiO₂) at the anchoring unit, usually a 2-cyanoacrilic acid function (acceptor unit). Modification of the donor group affects the HOMO and LUMO energy levels [6]. However, many researchers have carried out with the aim to built new π-conjugated materials having a D–π–A structure. Recently, special interest was expressed for compounds containing thienopyrazine unit as a donor [7,8].

The compounds containing thienopyrazine were proved to be excellent precursors to produce organic materials having a low band gap and facilitating the ICT. This effect could be explained by the existence of the thiophene unit, which shares a portion of its electron density to the neighboring pyrazine unit [9–11]. Recently, photovoltaic devices consisting of thienopyrazine-based low band gap compounds [12–15] as hole-transporting materials have shown improved efficiencies. In this context, the aim of this work was to study theoretically a new family of organic dyes based on thienopyrazine (D1, D2, D3, D4, D5, and D6), to determine their capacity to generate a photocurrent in the field of DSSC (Fig. 1).

Fig. 1
Chemical structure of studied compounds Di (i = 1–6).

2 Theoretical methodology

In this work, all calculations are made with the Gaussian 09 program [16] using the density functional theory (DFT) with the hybrid functional B3LYP [17–19] in which this functional has shown that the calculated electronic properties of compounds based on thienopyrazine are in good agreement with those obtained experimentally [20,21]. The structure of the ground state of each dye was optimized with the base Pople 6-31G(d,p) and a frequency calculation was performed to ensure that the geometry corresponded to a minimum. The vertical electronic transitions were evaluated using the TD-DFT and coulomb-attenuating method based on B3LYP (CAM-B3LYP)/6-31G(d,p) [22]. As for the absorption, Céron-Carrasco et al. [23] and Dessì et al. [24] have shown that for a set of thienopyrazine and thiazolothiazole organic dyes, the most CAM-B3LYP functional gave accurate transition energies, which are in good agreement with the experimental results with an error of 0.1 eV. Therefore, the vertical electronic transitions and electronic absorption spectra were evaluated using TD-CAM-B3LYP in this work. The solvent effects were included in the calculation by using an approach of the polarizable continuum (PCM) and more particularly the “conductor-like-PCM” such as implemented in the Gaussian program.

3 Results

3.1 Frontier molecular orbitals

To enhance the light harvesting efficiency (LHE) of DSSCs, the choice of the appropriate donor and acceptor unit is essential. Strong electron donating groups give a high E_HOMO energy level, whereas strong accepting electron groups cause a low E_LUMO energy level. The calculated HOMO/LUMO energies using B3LYP/6-31G(d,p) are −5.025/−3.057 for D1, −5.276/−3.293 for D2, −5.091/−3.099 for D3, −5.139/−3.124 for D4, −5.155/−3.140 for D5, and −3.140/−3.159 eV for D6, corresponding to energy gaps of 1.968 for D1, 1.983 for D2, 1.992 for D3, 2.015 for D4, 2.015 for D5, and 2.171 eV for D6 (Table 1). The increase in the conjugation of these π-systems permits to decrease the HOMO–LUMO gap, giving a red-shifted absorption.

Table 1

Dyes	E_HOMO	E_LUMO	E_g	λ_h	λ_e	λ_total	E₀₀	E^dyes	E^dyes∗	ΔG_inject	LHE	V_OC
D1	−5.025	−3.057	1.968	0.22	0.27	0.49	2.0963	5.025	2.9287	−1.0713	0.95	0.943
D2	−5.276	−3.293	1.983	0.22	0.14	0.36	2.1215	5.276	3.1545	−0.8455	0.94	0.707
D3	−5.091	−3.099	1.992	0.22	0.27	0.49	2.1183	5.091	2.9727	−1.0273	0.94	0.901
D4	−5.139	−3.124	2.015	0.23	0.25	0.48	2.1334	5.138	3.0046	−0.9954	0.95	0.876
D5	−5.155	−3.140	2.015	0.22	0.27	0.49	2.1362	5.155	3.0188	−0.9812	0.94	0.860
D6	−5.330	−3.159	2.171	0.30	0.36	0.66	2.2618	5.330	3.0682	−0.9318	0.91	0.841

We notice that the energy gap increases when going from D1 to D6 in the following order: D1 < D2 < D3 < D4 < D5 < D6. Finally, the energy gaps of D1, D2, and D3 are much smaller than the three other compounds. This is because of the strong electron donor property of the donor unit (R) that enhances the ICT character, which is an important factor to the red-shift in the absorption spectra.

The narrow band gap E_g in D1 and D2 is attributed to an increase in the conjugation caused by their donor units, which possess a heteroatom (oxygen in the dyes D1 and D2). On the contrary, the result indicates that a phenylene group is not appropriate to extend the conjugation [25].

The frontier orbitals in the ground state of all compounds are presented in Fig. 2. The graphical representation of orbitals shows that the HOMO of D1, D2, and D3 is almost located on the whole molecule, but that from D4, D5, and D6 is mainly located on the π-bridge (thiophene–thienopyrazine–thiophene) and only a little part of it is located on the acceptor unit (carboxyl groups), whereas the LUMO for the six compounds is located on the π-bridge groups and near the carboxyl groups (acceptor of electrons). These results show that electrons are moving from the donor unit to acceptor unit via thiophene–thienopyrazine–thiophene and then to TiO₂ during the excitation process_.

Fig. 2
The contour plots of HOMOs and LUMOs of the studied compounds Di (i 1–6).

3.2 Photovoltaic properties

The analysis of the frontier orbitals (HOMO and LUMO) of the six dyes allows finding their donor behavior in the face of semiconductor TiO₂. Comparing the LUMO levels from these six dyes with the conduction band of TiO₂ (−4.00 eV) [26] (Table 1), we deduce that all compounds are likely to inject an electron in the conduction band of the semiconductor, in the excited state. This suggests that these compounds can be useful in photovoltaic devices.

The power conversion efficiency (η) was calculated according to Eq. 1:

η = FF \frac{V_{oc} J_{sc}}{P_{inc}}

(1)

where P_inc is the incident power density, J_sc is the short-circuit current density, V_oc is the open-circuit voltage, and FF denotes the fill factor. V_OC can be expressed as [27]

V_{oc} = E_{LUMO} - E_{CB}

(2)

The obtained values of V_oc of all studied dyes range from 0.707 to 0.943 eV (Table 1). These values are sufficient for a possible efficient electron injection.

3.2.1 Theoretical background

Generally, in DSSCs, the short-circuit current density J_sc is determined as

J_{sc} = \int_{λ}^{0} LHE (λ) Φ_{inject} η_{collect} d λ

(3)

where LHE (λ) is the light harvesting efficiency at a given wavelength, Φ_inject is the electron injection efficiency, and η_collect is the charge collection efficiency. As a result, to understand the relationship between the J_sc and η theoretically, we investigated the LHE, Φ_inject, and total reorganization energy (λ_total). As can be seen in Eq. 3, the large LHE leads to high J_sc, therefore enhances the efficiency of DSSCs. The LHE can be expressed as [28]

LHE = 1 - 10^{- f}

(4)

where ƒ is the oscillating strength of the dyes associated with the absorption energy (E₀₀).

On the basis of Eq. 3, the large Φ_inject leads to a high J_sc, in which Φ_inject is related to the driving force (ΔG^inject) of the electron injection from the photoinduced excited states of organic dyes to the semiconductor surface. Generally, the larger the ΔG^inject, the larger will be Φ_inject, and it can be expressed (in eV) as [28]

Δ G^{inject} = E^{dye*} - E_{CB}

(5)

where E^dye∗ represents the oxidation potential energy of the dye in the excited state and can be estimated by the following equation [27]:

E^{dye*} = E^{dye} - E_{00}

(6)

where E^dye is the energy of oxidation potential of dye in the ground state, and E₀₀ is the energy of electronic vertical transition corresponding to λ_max.

As can be seen in Eq. 3, the high value of J_sc needs the weak value of total reorganization energy (λ_total), in which λ_total is defined as the sum of reorganization energy of the hole and those of the electron. Hence, we computed the hole and the electron reorganization energy (λ_h and λ_e) according to the following formula [29]:

λ_{i} = [E_{0}^{\pm} - E_{\pm}^{\pm}] - [E_{\pm}^{0} - E_{0}]

(7)

where

E_{0}^{\pm}

is the energy of the cation or anion calculated with the optimized structure of the neutral molecule,

E_{\pm}^{\pm}

is the energy of the cation or anion calculated with the optimized cation or anion structure,

E_{\pm}^{0}

is the energy of the neutral molecule calculated at the cationic or anionic state, and the E₀ is the energy of the neutral molecule at ground state.

On the basis of Eq. 3, we notice that LHE and the electronic injection free energy (ΔG^inject) are the two main influencing factors on J_sc and consequently on the efficiency of the organic dyes. The LHE is considered as a very important factor for any dyes, in which we could appreciate the role of these dyes in the DSSCs, that is, absorbing photons and injecting photoexcited electrons to the conduction band of TiO₂. To give an impression as how the donor unit (D) influences the LHE in our compounds, we simulated the UV–vis absorption spectra of the six dyes. We found that when we change the donor unit, the oscillator strengths were changed slightly. As shown in Table 1, the LHE of all dyes ranged from 0.91 to 0.95. The LHE values of the dyes are in narrow ranges. However, this means that all of the dyes will give similar photocurrent.

To enhance the value of J_sc, another factor was introduced which is the electronic injection free energy ΔG^inject. The details of the calculation of ΔG^inject have been described in Eq. 5. The ground state oxidation potential energy is related to ionization potential energy according to the Koopmans theorem [29]. Furthermore, E^dye can be estimated as negative E_HOMO and E^dye∗ is calculated based on Eq. 6. Through the results shown in Table 1, we observe that the electron donor significantly influences E^dye∗. We also notice that for all studied dyes, ΔG^inject is negative; this reveals that the electron injection process is spontaneous, and the calculated ΔG^inject for these six dyes is decreased in the following order: D2 > D6 > D5 > D4 > D3 > D1. Among these six dyes, we observe that the dyes D2, D6, D5, and D4 have the largest ΔG^inject, this may be because of the influence of the donor spacer on our dyes. As a result and on the basis of LHE and ΔG^inject related to J_sc, we could conclude that the cell containing the dyes D2, D6, D5, and D6 should have the highest J_sc because of its high value of LHE compared with the dyes D1, D3.

On the other hand, the reorganization energy λ_total is also an important influencing factor in the kinetics of electron injection. Therefore, to analyze the relationship between the electronic structure and the J_sc it is necessary to calculate the reorganization energy (λ_total). As mentioned previously, the small λ_total increase the J_sc. We notice that the calculated λ_total of all compounds proposed for use as dyes in a solar cell is increased in the following order: D2 < D4 < D1 = D3 = D5 < D6 (Table 1). These results show that the compounds D2 and D4 have the smallest λ_total, whereas the compound D6 has the largest λ_total. Furthermore, for high efficient DSSCs, the balanced transport of both holes and electrons is important, that is, the more the balanced reorganization energy between the hole and electron is obtained, the higher will be the luminous efficiency will obtained [30]. From Table 1, we also notice that the differences between λ_h and λ_e of six compounds are 0.05, 0.08, 0.05, 0.02, 0.05, and 0.06 eV successively. As a result, combined with discussion of the electron injection driving force, LHE, and the reorganization energies of the studied organic dyes, we could predict that these six dyes are favorable candidates for DSSCs and can exhibit a favorable J_sc

3.3 Absorption and emission properties

TD-DFT/CAM-B3LYP/6-31G(d,p) level was performed to simulate the optical property of the six compounds used as dyes in the solar cell. The maximum (λ_max) wavelengths for UV–vis absorption spectra of all dyes are tabulated in Table 2. The absorption wavelengths increase progressively with the increase in conjugation lengths. As shown in Table 2 and Fig. 3, the absorption wavelengths of D1, D2, D3, D4, D5, and D6 are 625.38, 618.01, 620.04, 615.49, 613.46, and 574.33 nm respectively, indicating that all molecules have only one band in the visible region (λ_abs > 400 nm) and D1 has the maximum absorbent wavelength.

Table 2

Compounds	D1	D2	D3	D4	D5	D6
λ_{max emi} (nm)	805.02	794.65	801.53	793.82	790.72	727.01
λ_{max abs} (nm)	625.38	618.01	620.04	615.49	613.46	574.33
SS (nm)	179.64	176.64	181.49	178.33	177.26	152.68

Fig. 3
Simulated UV–vis optical absorption spectra of the title compounds with the calculated data at the TD-DFT/CAM-B3LYP/6-31G(d,p) level in chloroform solvent.

The emission spectra data obtained at the TD-DFT/CAM-B3LYP/6-31G(d,p) level of all compounds recorded in chloroform are presented in Table 2 and depicted in Fig. 3. The normalized photoluminescence spectrum of the studied compounds shows a maximum at 805.02 for D1, 794.65 for D2, 801.53 for D3, 793.82 for D4, 790.72 for D5, and 727.01 nm for D6. The red-shifted emission of the photoluminescence spectra observed is in reasonable agreement with the obtained results of absorption. We can also note that relatively high values of Stokes shift (SS) are obtained from all dyes D1 (179.64), D2 (176.64), D3 (181.49), D4 (178.33), D5 (177.26), and D6 (152.68 nm) (Table 2), indicating that the compounds having a weak SS present a minimal conformational reorganization between the ground state and excited state. Indeed, this stops the intermolecular transfer charge and delays the injection phenomenon from LUMO of the dye to the conduction band of TiO₂ [31,32].

The SS is the difference between the vertical absorption energy (E_VA) and the vertical emission (E_VE) of the same electronic transition, which is usually related to the bandwidths of both, the absorption and emission bands [33,34].

4 Conclusions

We have used the DFT/B3LYP and TD-DFT/CAM-B3LYP methods to investigate theoretically the optoelectronic properties of the six dyes based on thienopyrazine. The band gaps obtained using DFT-B3LYP with 6-31G(d,p) as a basis set range from 1.968 to 2.171 eV, indicating that these compounds have a high order of conjugation. The LUMO levels of all dyes are much higher than the conduction band of TiO₂, suggesting that the photoexcited electron escapes easily from Di to TiO₂. The V_oc range from 0.706 to 0.942 eV, indicating that these values are sufficient for an efficient electron injection. The absorption wavelengths of D1, D2, D3, D4, D5, and D6 are 625.38, 618.01, 620.04, 615.49, 613.46, and 574.33 nm, respectively, indicating that spectra of these dyes cover the spectra emitted from sunlight. Finally, this study has been carried out to propose the synthesis of novel materials with specific electronic properties.

Acknowledgments

This work was supported by Volubilis Program (No. MA/11/248) and the convention CNRST/CNRS (Project chime 1009).

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