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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 Khattabi1,2 · Mohamed Hachi1 · Asmae Fitri1 · Adil Touimi Benjelloun1 · Mohammed Benzakour1 · Mohammed Mcharfi1 · Mohammed Bouachrine3
Received: 26 June 2018 / Accepted: 20 November 2018 / Published online: 28 November 2018
© Springer Nature B.V. 2018
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.
Frontier molecular orbitals
Intramolecular charge transfer (ICT) is strongly related to the electron distribution of the frontier molecular orbitals (FMOs). The FMOs for all eight compounds [com- puted at B3LYP/6-31G(d,p) level] are shown in Fig. 4.
The electronic distribution of the HOMO of P1-CN, P2-CN, P5-CN, P7-CN, and P8-CN is almost delocalized over the whole molecule, while that of P3-CN, P4-CN, and P6-CN is mainly distributed over the donor and π-spacer. The LUMO for the eight molecules has a greater distribution on the π-spacer group (thiophene, benzene) and near the MMN acceptor. Examination of the HOMO and LUMO of these compounds indicates that HOMO–LUMO excitation moves the electron distribution from the donor unit to the acceptor. On the one hand, the HOMO of all the compounds presents a binding character on the bonds that connect the donor group and π-spacer, and the acceptor group and π-spacer. This also facilitates mobility of electrons of the donor compound to the LUMO of the acceptor compound (PCBM) in the photovoltaic cell. Meanwhile, the LUMO of all the compounds presents an antibonding character on the bonds that connect the donor group and π-spacer, and the acceptor group and π-spacer.
Absorption is achieved in this system by transition of a π-electron from the HOMO to LUMO level. Therefore, one can deduce that electrons on the donor/π-spacer units are responsible for the absorption in the ultraviolet, while the acceptor/π-spacer units are mainly responsible for the absorption phenomenon in the visible and near-UV.
Optical properties
We calculated the longest absorption wavelength (λabs), vertical excitation energy (ΔEexcit), and oscillator strength (f) with the solvent (chloroform) phase for the eight studied compounds using TD-DFT at CAM-B3LYP/6-31(d,p) level [39, 40]. The results are presented in Table 3 and depicted in Fig. 6.
From the results in Table 3, we remark that the maximum absorption value cal- culated for compounds P1-CN, P2-CN, P3-CN, P4-CN, P5-CN, P5-CN, P6-CN, P7-CN, and P8-CN was 472.14, 506.99, 522.93, 527.93, 427.02, 396.10, 445.33, 459.52, and 602.00 nm, respectively. As shown in Fig. 6, all the compounds exhib- ited a strong absorption band in the visible region at around 396–602 nm, which can be assigned to intramolecular charge transfer (ICT) between the various donat- ing units and the electron acceptor malononitrile unit. Furthermore, we remark that compound P4-CN could harvest more light at longer wavelengths, which is beneficial to increase further the photoelectric conversion efficiency of correspond- ing solar cells. Thus, the lowest-lying transition can be tuned by changing the mor- phology of the π-spacer via elongation based on introduction of thiophene.
Photovoltaic properties
To study the photovoltaic properties of these compounds, it is necessary to evaluate certain parameters such as the short-circuit current (JSC), open-circuit voltage (Voc), fill factor (FF), efficiency (η), and incident photon-to-current efficiency (IPCE) or external quantum efficiency (EQE), using Eq. (1):
The short-circuit current (JSC) depends on the open-circuit voltage (Voc), which is the voltage measured when no current flows in the photovoltaic device. This voltage depends essentially on the type of solar cell (p–n or Schottky junction), the lifetime and mobility of the charge carriers, the active layer materials, and the nature of the active-electrode contacts [41, 42].
In this context, we studied the photovoltaic properties of bulk heterojunction cells formed using one of the eight compounds as a donor component, blended with phe- nyl-C61-butyric acid methyl ester (PCBM) as acceptor material. The latter is charac- terized by a LUMO energy level of − 3.8 eV and HOMO energy level of − 6.1 eV [43]. where q is the elementary charge, the LUMO energy level of PCBM is −3.75 eV, and a value of 0.3 is used as an empirical factor.
Another parameter is determined by the difference between the LUMO energy level of the studied compounds Pi and the LUMO energy level of PCBM as follows: Note that the values obtained for the open-circuit voltage Voc of all the studied molecules vary from 0.912 to 1.410 eV, being sufficient to ensure efficient electron injection into the cell. On the other hand, Table 4 shows that the values of the energy differences between the LUMO level of the electron donors (P1-CN, P2-CN, P3-CN, P4-CN, P5-CN, P6-CN, P7-CN, and P8-CN) and the electron acceptor (PCBM) are greater than 0 eV, indicating that electron transfer will occur from the donor (Pi) to acceptor (PCBM).
Table 4 also shows that the difference [LD(Pi) – LA(PCBM)] between the LUMO energy level of the designed donors (P1-CN, P2-CN, P3-CN, P4-CN, P5-CN, P6-CN, P7-CN, and P8-CN) and the PCBM acceptor is larger than 0 eV and var- ies between 0.715 and 1.018, ensuring efficient electron transfer from the donor to acceptor (PCBM). This suggests that all the studied molecules could be used as donors in BHJ solar cells, because the electron injection process from the excited molecule to the conduction band of PCBM and the subsequent regeneration are pos- sible. The lowest αi value was found for compound P4-CN, again suggesting that this compound would offer the best photovoltaic performance, since the smaller the difference between the LUMO of the Pi compound and PCBM, the more the elec- tron injection process into PCBM is favored and the better the performance of the resulting organic solar cell.
To evaluate the performance of the molecules designed as donors for BHJ cells, Fig. 7 presents a contour plot of the power conversion efficiency of a bulk hetero- junction solar cell using PCBM as acceptor material versus the [ELUMO(donor) − ELUMO(acceptor)] energy and bandgap value. Analysis of this plot shows that the theoretical efficiency of the designed compounds varies between 2 and 6%. Com- pound P4-CN achieves efficiency greater than 6%, reconfirming that this compound would be the best donor for use in BHJ solar cells.
Conclusions
Quantum-chemical calculations of the geometry and electronic properties of various compounds based on indole were performed to reveal the effect of their molecular structure on the optoelectronic properties and assess their potential for application in organic solar cells. We applied the DFT method at B3LYP/6- 31G(d,p) level to predict the structural, electronic, and photovoltaic properties of eight new D–π–A compounds (P1-CN, P2-CN, P3-CN, P4-CN, P5-CN, P6-CN, P7-CN, and P8-CN), and calculated their absorption spectra using TD-DFT at CAM-B3LYP/6-31G(d,p) level. Based on the results, the following conclusions can be drawn:
•The energy gap predicted for the eight molecules using DFT at B3LYP/6- 31G(d,p) level varies between 1.927 and 2.728 eV. These low values indicate easy electron transfer between the donor and acceptor;
•Increasing the number of thiophenes in the π-spacer narrowed the energy gap;
•The calculated values of the open-circuit voltage Voc of the studied molecules vary from 0.912 to 1.410 eV, being sufficient for very efficient electron injec- tion;
•The UV–Vis absorption properties were obtained using TD-DFT calculations at CAM-B3LYP/6-31G(d,p) level. The obtained absorption maxima lie in the range of 396.10–522.93 nm. The results of this computational study suggest that these dyes are good candidates for application in BHJ solar cells.
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