The effect of π-spacer on D-A'-π-a system organic dyes for dye-sensitized solar cells (DSSCs) technology: a computational approach”

Abstract

The globe is consuming more energy as a result of population growth and economic development. One of the most important forms of renewable energy for human usage is solar energy. By modifying the π-spacers, four D-A'-π-A of novel organic dye molecules (D1–D4) have been created in this study. To evaluate the optoelectronic capabilities and photovoltaic qualities of four D-A'-π-A new organic dyes created molecules, density functional theory (DFT) and time-dependent DFT (TD-DFT) theory methodologies through the B3LYP and 6-311G basis set have been employed. To ascertain the effect of developed π-spacer on enhancing intramolecular charge transfer (ICT) and enhancing light-absorbing capacities, a number of crucial factors, including molecular geometry, energy bandgap and light-harvesting efficiency (LHE), have been studied. Based on the available data, D4 outperforms the other four developed organic dye molecules, with energy bandgap of 1.4896 and 1.4253 eV for gas and solvent phase, respectively, regeneration driving forces (ΔG_reg) of 0.0469 and 0.0300 eV for the gas phase and solvent phase, respectively, and open-circuit voltages (Voc) of 0.6427 and 0.5953 eV for the gas phase and solvent phase, respectively. Additionally, the maximum absorption wavelengths (λ_max) for the gas phase and solvent phase are 932.03 and 1013.81 nm, respectively. Consequently, it was found that the D4 dye molecule was a more promising option for the use of dye-sensitized solar cells (DSSCs) technology hence advised for more practical research to provide efficient advancements in the D-A'-π-A system organic dye for the production of sustainable energy.

Introduction

Humans have diligently investigated and utilized traditional fossil fuels like coal, oil and natural gas to meet the world’s growing energy needs. These carbon-based nonrenewable resources have a finite amount of storage capacity. The greenhouse effect and environmental contamination are the results of the exploitation of these resources. Consequently, the creation of environmentally friendly as well as renewable energy sources is necessary to address these issues. Renewable energy sources like wind, hydropower and solar power have many advantages over traditional energy sources. Solar energy is especially crucial because the sun gives earth a vast quantity of energy every day [1]. There are three primary methods for producing electrical energy from sun energy including thermocouples, photovoltaics and solar thermal technology [2, 3]. Light from the sun is straightly converted into electricity by a photovoltaic solar cell. When compared to photovoltaic solar cell technology, the power conversion efficiency (PCE) of alternative indirect methods of producing energy from sunlight is poor. Consequently, the topic of developing photovoltaic devices has attracted a lot of attention from researchers in recent years [4].

The silicon-based mono and poly-crystalline solar cells, which have thicknesses ranging from 100 to 300 micrometer and a PCE of roughly 26%, currently dominate the photovoltaic solar cell market [5]. The primarily obstacle to using these solar technologies is their high cost, which results from the high cost of semiconductor materials as well as high temperature processing techniques for photovoltaic modules [6]. In an attempt to lower the cost of semiconductor ingredients and fabrication, a novel organic solar cells with thicknesses of a few thousand nanometer has been proposed [7, 8]. Due to their high efficiency and simple manufacturing, dye-sensitized solar cells are the most popular organic solar cells that replace silicon-based solar cells [9].

The dyes play a crucial role in capturing sunlight to produce excited electrons, which are then injected into the semiconductor of DSCCs, serving as a central component of a conventional DSSCs device [10, 11]. As a result, the primary area of study for DSSCs is now dye inquiry, and numerous dye kinds have been created [12, 13]. The dyes can be broadly categorized as metal-free organic dyes, zinc porphyrin dyes and ruthenium complexes [14, 15]. With regard to molecular engineering of dyes in DSSCs, metal-free organic dyes have gradually emerged as the most popular option. This is primarily because of their advantages in terms of ease of preparation, straightforward structural optimization, affordability and environmental stability [16, 17].

It is well recognized that metal-free organic dyes have traditional electron donor (D)-π-bridge (π)-electron acceptor (A) configuration [18, 19], which has been shown to be advantageous for the dyes’ intramolecular charge transfer (ICT) action [20]. Nevertheless, there may be non-negligible restriction to the D-π-A metal-free organic dyes due to flaws in the low spectrum response into near infrared (NIR) as well as poor photothermal stability [21]. Zhu and colleagues proposed a novel type of D-A-π-A framework in response to this shortcoming; this framework is implemented by adding an extra electron withdrawing unit to act as auxiliary electron acceptor between the electron donor and the π-bridge of the D-π-A architecture [22–24]. Interestingly, it was determined that the extra acceptor was crucial in extending the absorption spectra and enhancing the dyes’ photostability through the manipulation of the electron donor’s electron distribution. A survey of the literature over the last several years indicates that in order to create D-A-π-A dyes, a number of benzoheterocycles, including quinoxaline, benzotriazole, benzothiadiazole and benzodioxazole were used as the additional acceptor [25, 26].

Thus, the goal of this work was to create the novel organic dye molecule by utilizing quantum chemical techniques to investigate the effect of π-spacer of D-A'-π-A for the application of DSSCs. To carry out the study, five π-spacers were substituted to the D-A'-π-A: thiophene, 4-(thiophene-3-yl)pyridine, 2-(4-(thiophene-2-yl)thiophene and (E)-5,6-dihydro-6-(4,5-dihydrocyclopenta[b]thiophene-6-ylidene)-4H-cyclopenta[b]thiophene. This resulted to five different molecules, D1, D2, D3 and D4, respectively.

Methodology

Molecular design and computational details

Four novel organic dye molecules, based on the D-A'-π-A configuration, were designed in this study by adjusting the π-spacers through the use of thiophene, 4-(thiophene-3-yl)pyridine, 2-(4-(thiophene-2-yl)thiophene and (E)-5,6-dihydro-6-(4,5-dihydrocyclopenta[b]thiophene-6-ylidene)-4H-cyclopenta[b]thiophene. Figure 1a illustrates constant donor, internal acceptor and external acceptor that π-spacer, donated as π1, π2, π3 and π4, respectively, attaches to. As shown in Fig. 1b, the design was completed by replacing the π-spacers π1, π2, π3 and π4 with the dye molecules D1, D2, D3 and D4, respectively. The Gaussian 09 program [27] was used to study the proposed dye compounds using DFT and TD-DFT techniques through B3LYP hybrid function and 6-311G basis set in gas and solvent phases. Using DFT approach, the ground state geometries were painstakingly tweaked without symmetry restrictions, while the excited state molecules were investigated using TD-DFT. Using acetonitrile solvent, the conductor-like polarizable continuum model (CPCM) was utilized to incorporate the effect of solvent at both the DFT and TD-DFT theoretical levels.

Theoretical calculations

Several metrics were computed in order to fully assess the compounds under study.

In this case, △G_reg is equivalent to the electrolyte’s redox potential, which is −4.8 eV [29].

Results and discussion

Geometrical optimization

The methods used to minimize a molecule’s model energy and ascertain its three-dimensional atomic arrangement are referred to as geometry optimization. Using the B3LYP/6-311G, the geometry for D1, D2, D3 and D4 was optimized. In order to evaluate the conformation of the dye molecules being investigation, particular geometric factors were taken into account, including dihedral angle. The ideal configuration of every dye molecule studied in solvent phase is depicted in Fig. 2, where comparable patterns are also seen in the gas phase. Acetonitrile, a polar aprotic solvent, interacts through dipole-dipole interactions with the electron-rich regions of D4, potentially changing the molecule's ideal geometry. In comparison to its gas-phase geometry, these interactions may stabilize or destabilize particular conformations of D4, resulting in modifications to the general molecular structure.

Table 1, also shows the chosen dihedral angles that came from the optimized geometries. Every dye molecule was observed to have a consistent planar conformation in its optimized geometry, as demonstrated by the optimum structures displayed in Fig. 2. The dihedral angle values of every dye molecule under study, which were almost 180° and suggested a planar shape, were employed to assess this. Due to its conjugated π-system and fused aromatic rings, which limit rotational freedom and preserve a set geometric structure, D4 is rigidity [35, 36]. In addition to improving the molecule's photophysical characteristics of D4, such as light absorption and emission efficiency, this rigidity reduces non-radiative decay processes. Furthermore, when adsorbed onto surfaces, the planar shape promotes effective π-π stacking, supporting charge transfer mechanisms. D4's stability and performance in optoelectronic applications are largely dependent on its overall planarity and rigidity. Therefore, the molecules underwent twisting as a result of steric interactions between the inserted acceptor and π-spacer, which improved their absorption efficiency by modifying the π-spacers [37].

Frontier molecular orbitals

Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are two examples of frontier molecular orbitals (FMOs) that provide important information about the optoelectronic characteristics of dyes. These descriptors of quantum mechanics aid in the analysis of the dissemination and stabilization of charge density among molecules [38]. It is crucial to comprehend FMOs so as to comprehend dye molecule charge-separated states as well as ICT [39]. For the dye molecules under investigation, the solvent phase contour plots of HOMO and LUMO orbitals are displayed in Fig. 3. Even after geometry optimization, these trends are true in the gas phase. The FMOs analysis revealed that the HOMO electrons were primarily found on the electron-rich side for all four dye molecules (D1-D4), with the donor groups having the highest densities and the internal acceptor groups having lowest electron concentrations. Conversely, the majority of the LUMO electrons were dispersed on the side that lacked electrons for D1, D3 and D4, with external acceptor, π-spacer and internal acceptor having the highest densities, whereas, for the D2 dye molecule, the LUMO electrons were extensively dispersed with maximal density on the internal acceptor only that can act as an anchoring group in this dye and minimal density on the π-spacer. According to the results from the FMOs, the HOMO is usually found at the donor group, whereas the LUMO is found at the external acceptor group, internal acceptor group along with the π-spacer. Consequently, this shows that the D-A'-π-A system as intended is working well since it permits electrons to flow from the donor group to the acceptor group on their own.

For effective charge transfer between the donor and acceptor, the energies of the HOMO and LUMO are essential. The photosensitizer material needs to have right energy levels in order to support electron injection and allow material regeneration, according to the DSSC working principles. The HOMO energy levels must be lower than the electrolyte’s −4.8 eV redox potential in order to inhibit charge recombination between the photoinjected electron and the oxidized organic dye and to promote dye regeneration. Furthermore, the LUMO energy levels must exceed the TiO₂ conduction band (CB) edge at −4.0 eV. This suggests a robust thermodynamic driving force for effective electron injection into the conduction band of the semiconductor from the excited state of the dye [40]. The data in Table 2 makes it abundantly evident that the HOMO energy levels of the dye sensitizer are organized as follows: D4 (−4.8469 eV) > D3 (−4.9163 eV) > D2 (−4.9786 eV) > D1 (−5.0540 eV) and D4 (−4.8300 eV) > D3 (−4.8646 eV) > D2 (−4.8948 eV) > D1 (−5.0009 eV) in both gas and solvent phases, respectively. The results demonstrated a high degree of performance and regeneration, suggesting that D4 has a greater capacity for regeneration compared to the other dye molecules. In contrast, the LUMO energy levels are ranked as follows in both the gas and solvent phases: D1 (−3.4621 eV) > D4 (−3.3573 eV) > D3 (−3.3333 eV) > D2 (−3.2898 eV) and D1 (−3.5293 eV) > D4 (−3.4047 eV) > D3 (−3.3541 eV) > D2 (−3.2398 eV), respectively, and an injection capability is indicated by these values. According to these findings, D1 has higher capacity of injection compared to other dye molecules. Therefore, due to the minor variations in injection values of 0.1048 and 0.1246 eV in the gas and solvent phases, respectively, rather than the significant differences in regeneration values of 0.2035 and 0.1709 eV in both phases, makes D4 outperformed D1. Actually, electronic characteristics of D4 are directly influenced by the adsorption structure on a substrate. Charge transfer pathways are determined by the orientation and closeness of the molecule to the surface. Strong binding to the substrate and assistance with energy level alignment are provided by the anchoring group in D4, such as cyanoacrylic group, which improve electron injection efficiency. This group has an impact on dye’s stability as well, maintaining the proper charge separation needed for solar cell application.

Energy sensitizers with a narrow bandgap are better at absorbing light and are more efficient at triggering electrons. Thus, the energy gap has a major impact on molecular activity. This enhances the PCE and short-circuit current (J_SC) of sensitizers by enabling them to absorb more visible light [41]. The dye molecules’ energy bandgaps are demonstrated in Table 2, with the following rankings for the gas and solvent phases, respectively: D4 (1.4896 eV) < D3 (1.5830 eV) < D1 (1.5919 eV) < D2 (1.6888 eV) and D4 (1.4253 eV) < D1 (1.4716 eV) < D3 (1.5105 eV) < D2 (1.6550 eV). In light of this, the D4 was supposed to have a smaller energy bandgap than other dye molecules in both the gas and solvent phases. This indicates that D4 should perform better than the other dye molecules in terms of J_SC and reactivity. Because D4 has a narrower energy bandgap than the other dye molecules, its UV-Vis spectra are therefore probably going to be red-shifted.

The dipole moment in Table 2 illustrates the symmetry of the molecule’s electrical charge distribution. Notably, a greater dipole moment is associated with asymmetric behavior in the electronic charge distribution, which increases the sensitivity of molecules to external electric fields [42]. Table 2 indicates the following dipole moment order for the gas and solvent phases: D3 (16.4627) > D4 (15.4503) > D1 (14.3879) > D2 (12.4246) and D1 (23.3192) > D4 (23.1813) > D3 (21.2873) > D2 (14.7648), respectively. Based on the results, it can be concluded that the action of the acetonitrile solvent caused the dipole moments of all suggested dye molecules in the solvent phases to be bigger than those of all proposed dye molecules in the gas phases. So, D1 and D4 exhibit greater dipole moment than the other dye molecules by considering dipole moment in the solvent phases. Therefore, D1 and D4 are highly vulnerable to the external electric field.

Absorption properties

The transition energy, oscillator strength (f) and vertical excited singlet state of every dye molecule were investigated in both the gas and solvent phases so as to evaluate the dyes’ absorption behavior. More absorption strengths and wider absorption bands generally translate into more efficient dyes. Table 3 details all of the dye compounds, including their computed vertical excitation energy (E_ex), transition energy (HOMO to LUMO), transition character (TC %) and maximum wavelength (λ_max). The order of the first vertical excited energy in the gas phase is D4 < D3 < D1 < D2 and the solvent phase exhibits the same sequence: D4 < D3 < D1 < D2. The results show that, in comparison to the other dye molecules, D2 have larger vertical excitation energy in both phases.

The oscillator strength and the efficiency of light harvesting are highly correlated. In both the gas and solvent phases, the oscillator strengths of all dye molecules are sorted as follows: D4 > D3 > D1 > D2. Therefore, in both the gas and solvent phases, D4 dye molecule exhibits higher strengths than the other dye molecules, suggesting that D4 is likely to perform better and enhance the efficiency of DSSC application. Based on Table 3 and Fig. 4, the maximum absorption wavelengths of all the dye compounds under investigation are lowered in the following order: Specifically, for the gas phase, D4 (932.03 nm) > D3 (882.13 nm) > D1 (835.28 nm) > D2 (826.13 nm) and for the solvent phase, D4 (1013.81 nm) > D3 (963.86 nm) > D1 (931.39 nm) > D2 (887.91 nm). According to the results reveled that, D4 dye molecule have higher maximum absorption wavelengths when compared to the other dye molecules. Because acetonitrile is polar, it can change the electrical environment surrounding D4, which can cause variations in the molecule's absorption spectrum. Changes in the energy levels of electronic transitions arising from this interaction can cause absorption maxima to shift in a bathochromic (red). Furthermore, the solvent can affect the absorption peaks' strength and broadening because solute-solvent interactions change oscillator strengths and transition probabilities. Therefore, D4 dye molecule is the most suggested option for DSSC applications because of their longer wavelengths and red shift.

Photovoltaic properties

The photovoltaic characteristics of the dye molecules under evaluation are listed in Table 4. The metrics that are mentioned are the open-circuit voltage (V_OC), ground state oxidation potential energy (E^dye), excited state oxidation potential energy (E^dye∗), electronic injection-free energy (ΔG^inject) and light-harvesting efficiency (LHE). These crucial factors account for the dye’s capacity to take in sun photons and inject photoexcited electrons from the LUMO into conduction band (CB). As can be seen in Table 4, every dye molecule has a negative ΔG^inject value, suggesting a spontaneous electron injection mechanism. Furthermore, the excited states of all dye molecules are above the TiO₂ CB, which guarantees the high electron injection efficiency. The values of electron injection efficiency are ranked as follows: D2 (−0.5222 eV) > D3 (−0.4892 eV) > D4 (−0.4834 eV) > D1 (−0.4303 eV) in the gas phase, while D2 (−0.5016 eV) > D3 (−0.4217 eV) > D4 (−0.3930 eV) > D1 (−0.3303 eV) in the solvent phase. D2 demonstrates the largest ΔG^inject value in both phases, indicating significant benefits for DSSC applications for electron injection driving force. This suggests that the thermodynamically advantageous injection of electrons from the dyes into TiO₂ semiconductor is responsible for the high electron injection efficiency. The electronic regeneration-free energy (ΔG_reg) should be kept to a minimum for efficient electron transport. Table 4 displays the ΔG_reg trend in the following order: D4 (0.0469 eV) < D3 (0.1163 eV) < D2 (0.1786 eV) < D1 (0.2540 eV) and D4 (0.0300 eV) < D3 (0.0646 eV) < D2 (0.0948 eV) < D1 (0.2009 eV) for the gas and solvent phases, respectively. These results that D4 dye molecule has a higher PCE because to its lower ΔG_reg value when compared to the other dye molecules. However, the D4 dye molecule was predicted to perform better than the D2 dye molecule because of the minor variation in electron injection efficiency value of 0.0388 eV and significant difference in electron regeneration efficiency value of 0.1317 eV. Therefore, this makes D4 to outperform D2.

For every dye molecules as shown in Table 4, the LHE values in the gas phase are ranked as follows: D4 (0.9494) > D3 (0.9364) > D1 (0.9057) > D2 (0.7780); in the solvent phase, the ranking is as follows: D4 (0.9815) > D1 (0.9641) > D3 (0.9621) > D2 (0.8212). These differences in LHE values are caused by the various π-spacers that are affixed between the internal and external acceptor. Since D4 has a greater LHE value compared to the other dye molecules, it appears to be a promising for the application in DSSC. LHE, ΔG^inject and J_SC all get more effective as oscillator intensity rises.

Another important factor in determining the PCE of DSSC is the open-circuit voltage (V_OC), where a higher V_OC indicates a stronger electron injection power in the molecular dye. Table 4 illustrates the decreasing V_OC trends in the gas phase: D2 (0.7102 eV) > D3 (0.6667 eV) > D4 (0.6427 eV) > D1 (0.5379 eV), and in the solvent phase: D2 (0.7602 eV) > D3 (0.6459 eV) > D4 (0.5953 eV) > D1 (0.4707 eV). The findings reveal that the D2 dye molecule consistently exhibits higher V_OC value compared to the other dyes in both phases. However, D4 was performed better than the D2 due to the easily of D4 to regenerate electrons as well as higher maximum absorption wavelengths compared to D2. Since PCE and V_OC have a high correlation, using D4 dye in DSSC applications more frequently is advised because of its higher practical efficiency.

Conclusion

The study utilized DFT and TD-DFT methodologies to successful create and analyze four innovative series of organic dyes found on D-A'-π-A system and four distinct π-spacers. The results display that the molecular dyes can be adjusted to produce the desired photovoltaic characteristics by varying the π-spacers. To find viable sensitizer for solar cell applications, the study examined ideal geometries, photovoltaic trait, absorption qualities and charge transfer capacities. Due to its smaller energy bandgap value of 1.4896 and 1.4253 eV for gas and solvent phase, respectively, low ΔG_reg of 0.0469 and 0.0300 eV for gas and solvent phase, respectively, V_OC value of 0.6427 and 0.5953 eV for gas and solvent phase, as well as higher maximum absorption wavelength of 932.03 and 1013.81 nm for gas and solvent phase, respectively, D4 dye molecule performed better than the other dye molecules in general. As a result, D4 dye molecule becomes a more appealing option for DSSC application. Furthermore, this work may provide useful guidance in the development of D-A’-π-A organic dye for DSSCs.

Acknowledgements

We are grateful for the cooperation provided by the University of Dodoma's computational laboratory and chemistry department staff in completing this work.

Author contributions

Michael Kennedy Sanama(Investigation [equal], Writing—original draft [equal]), Ismail Abubakari (Formal analysis [equal], Supervision [equal]), and Surendra Babu Numbury (Conceptualization [equal], Supervision [lead])

Conflict of interest: None declared.

Funding

The authors affirm that there was no funding available for this research work and that no known conflicting financial interests or personal relationship might have influenced any of the work presented in this study.

Data availability

The article itself contains the data that supported it.

References