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Home > DFT and TD‑DFT calculations on thieno[2,3‑b]indole‑based compounds for application in organic bulk heterojunction (BHJ) solar cells
DFT and TD‑DFT calculations on thieno[2,3‑b]indole‑based compounds for application in organic bulk heterojunction (BHJ) solar cells
Rahma El Mouhi1 · Souad El Khattabi1, · Mohamed Hachi1 · Asmae Fitri1 · Adil Touimi Benjelloun1 · Mohammed Benzakour1 · Mohammed Mcharfi1 · Mohammed Bouachrine
Introduction
During the last 10 years, organic photovoltaics (OPVs) have become a highly popular research topic. Bulk heterojunction photovoltaic cells were first reported in 1995 [1] and have attracted worldwide attention due to their potential low cost, high absorption coefficient, solution processability, and easy fabrication [2–6]. Generally, organic BHJ solar cells are based on a mixture of an electron acceptor material such as (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) or its deriva- tives [7–10] and an electron donor (organic material) with the aim of harvesting sunlight. The choice of the electron donor is among the main factors influencing photovoltaic performance. In principle, the strategies that can be used to improve the efficiency of BHJ solar cells include lowering the highest occupied molec- ular orbital (HOMO) of the organic material and reducing its bandgap, which increases the open-circuit voltage (Voc) and power conversion efficiency (PCE) [11]. To facilitate intramolecular charge transfer (ICT) upon excitation, electron donors with D–π–A structure are most widely used, where D and A are the donor and acceptor, and π is a conjugated linker between D and A. Extensive research has been carried out to design and synthesize more efficient D–π–A systems.
Compared with popular donors such as thieno[3,2-b][1]benzothiophene [12], carbazole [13], triphenylamine [14–19], and indoline [20–26], thieno[2,3-b] indole is rarely adopted as a donor. Recently, Irgashev et al. [27–29] synthesized six novel organic dyes (IK1–IK6) based on indole for use in dye-sensitized solar cells (DSSCs) with different π-spacers and N-alkyl groups. The power conversion efficiency (PCE) of the synthesized compounds reached 6.3%. Motivated by these values, we investigated in this work the performance of eight designed indole- based compounds for use as donor moieties for BHJ systems. The designed mol- ecules have thieno[2,3-b]indole as donor and malononitrile (MMN) as acceptor, whereas the π-spacer is composed of thiophene and phenyl or their derivatives. In this work, theoretical analysis of the geometry and electronic properties of the designed compounds based on the indole 3-(5-(8-ethyl-8H-thieno[2,3-b]indole- 2-yl)thiophen-2-yl)-2-cyanoacrylic acid synthesized by Irgashev et al. [27] was carried out.
The aim of this work is to evaluate whether these molecules could be used as donors in BHJ cells with a methylene malononitrile (MMN) instead of cyanoacr- ylic acid (CA) acceptor. In this study, we used density functional theory at B3LYP/6-31G(d,p) level to determine the optimized geometry, electronic prop- erties, photovoltaic properties, and quantum-chemical parameters, and TD-DFT to determine the optical properties. The newly designed compounds (P1-CN, P2-CN, P3-CN, P4-CN, P5-CN, P6-CN, P7-CN, and P8-CN) are shown in Fig. 1.
Computational details
Theoretical quantum calculations are the most well-known tool to study π-conjugated systems, as they can be used to rationalize the properties of such compounds and provide guidance for experimental work. In this study, the ground-state geometry in gas phase was optimized using density functional theory (DFT) with Becke’s three-parameter functional and the Lee–Yang–Parr functional B3LYP/6-31G(d,p) basis set [28–31]. None of the calculations gener- ated imaginary frequencies, indicating that the optimized geometries were real energy minima. The optical properties, including ultraviolet–visible (UV–Vis) spectrum, excitation energy, and oscillator strength, were obtained by TD-DFT calculations using the CAM-B3LYP [32] functional with solvation (chloroform) effects included. Consideration of the solvent effect in theoretical calculations is important to reproduce or predict experimental spectra with reasonable accuracy. In this work, to calculate the excitation energy, we used the integral equation for- malism of the polarizable continuum model (IEF-PCM) [33, 34]. All quantum- chemistry calculations were done using Gaussian 09 software [35], supported by the GaussView 5.1.8 interface [36].
Results and discussion
Molecular design and geometry
All calculations were carried out at DFT B3LYP/6-31G(d,p) level. The studied com- pounds Pi-CN are listed in Table 1. In these D–π–A molecules, the π-conjugated group is employed as an intramolecular charge transfer (ICT) bridge from the elec- tron-donor to electron-acceptor group. Considering the bond lengths, we note that their values for the eight compounds lie between 1.434 and 1.459 Å for d1 (donor group/π-spacer) and between 1.418 and 1.447 Å for d2 (π-spacer/acceptor group).
For the dihedral angles, note that the value of Ф1 formed between the donor group and π-spacer varies between 159.27° and 177.83° We deduce that, in these compounds, the thieno[2,3-b]indole donor is slightly twisted, which can prevent intermolecular aggregation. Meanwhile, it can be seen that, for all molecules, the value of the dihedral angle Ф2 is 179°, thus the MMN acceptor and π-spacer are per- fectly coplanar, indicating a strong conjugation effect. This facilitates charge transfer during the transition of excited electrons from the donor unit to acceptor group.
The values of the bond lengths and dihedral angles of the Pi-CN compounds are very similar to each other and comparable to those obtained by Hachi et al. [37] for the Mi systems comprising the same donor and π-spacer but a different acceptor. Indeed, the acceptor in the molecules studied by Hachi et al. is cyanoacrylic acid (CA), since they were interested in using these molecules as dyes in DSSCs [37]. This indicates that the acceptor group and π-spacer have little effect on these geo- metric parameters (Fig. 2).
Electronic properties
The energies of the highest occupied molecular orbital (HOMO), lowest unoccu- pied molecular orbital (LUMO), and bandgap calculated at B3LYP/6-31G(d,p) level for the eight compounds (P1-CN, P2-CN, P3-CN, P4-CN, P5-CN, P6-CN, P7-CN, and P8-CN) are presented in Table 2 and shown in Fig. 3. Comparison of these values shows that the modifications in the π-spacer had a great effect on the HOMO and LUMO energy levels. Indeed, with increasing number of thiophenes, the energy gap decreased from 2.634 to 1.927 eV (0.707 eV). This trend was also observed by Hachi et al. for the Mi compounds, whose energy gap ranged from 2.694 to 2.068 eV (0.626 eV) [37]. Moreover, note that, with increasing number of benzene units in the π-spacer, the energy gap decreased, albeit not as strongly as in the case of thiophene. Also, the sequence of the thio- phene and benzene in the π-spacer had some effect on the EHOMO, ELUMO, and Egap values. The analysis in Fig. 3 shows that elongation of the π-spacer by suc- cessive introduction of thiophene and benzene units destabilized the HOMO energy level but stabilized the LUMO, facilitating electron transfer from the LUMO of compounds Pi-CN to the LUMO of the acceptor PCBM.
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