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Original Article
2025
:18;
612025
doi:
10.25259/AJC_61_2025

Exploration of optoelectronic and photovoltaic properties for azaborinine-based materials via symmetrically coupled acceptors: A DFT/TD-DFT approach

, , , , ,
aInstitute of Chemistry, Khwaja Fareed University of Engineering & Information Technology, Rahim Yar Khan, 64200, Pakistan
bDepartment of Chemistry, College of Science, King Saud University, Riyadh 11451, Saudi Arabia
cDepartamento de Química Fundamental, Instituto de Química, Universidade de São Paulo, Av. Prof. Lineu Prestes, 748, São Paulo, 05508-000, Brazil
dDepartment of Infectious Diseases, The Affiliated Hospital of Southwest Medical University, Luzhou 646000, China

*Corresponding authors: E-mail addresses: khalid@iq.usp.br, muhammad.khalid@kfueit.edu.pk (M. Khalid), suvash_ojha@swmu.edu.cn (S. C. Ojha)

Received: , Accepted: ,
© 2025 Arabian Journal of Chemistry - Published by Scientific Scholar
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This is an open-access article distributed under the terms of the Creative Commons Attribution-Non Commercial-Share Alike 4.0 License, which allows others to remix, transform, and build upon the work non-commercially, as long as the author is credited and the new creations are licensed under the identical terms.

Abstract

A set of azaborinine-based compounds with B-N covalent linkage (BNT1-BNT7)z, featuring an A–D–A configuration, were designed from the BNTR reference by incorporating benzothiophene (BT)-based, symmetrically coupled acceptor moieties. To determine the photovoltaic and electronic properties of the BNTR and BNT1-BNT7 compounds, density functional theory (DFT) and time-dependent DFT (TD-DFT) were utilized. Therefore, various analyses such as the frontier molecular orbitals (FMOs), binding energy (Eb), UV-Visible, density of states (DOS), open circuit voltage (Voc), transition density matrix (TDM), and electron-hole were conducted. The incorporated terminal acceptor moieties in the designed derivatives influenced their electronic properties and resulted in a narrow energy gap (ΔE = 2.611−2.450 eV). The energy gaps of each of the designated compounds were found to decrease in the following order: BNT1 > BNT3 > BNT2 > BNTR > BNT7 > BNT5 > BNT4 > BNT6. They also showed the bathochromic shifts (λmax = 617.297-650.665 nm) as compared to BNTR (623.224 nm). The BNT6 compound showed the least energy gap (2.450 eV), highest absorption (λmax =650.665 nm), and minimal binding energy value (Eb = 0.451 eV) among all derivatives. Moreover, the effective delocalization of electrons in the studied compounds was confirmed by the DOS and transition density matrix (TDM) illustrations. Furthermore, Voc was carried out in conjunction with donor (PBDB-T), and all the tailored chromophores showed reasonable results with respect to the reference compound (BNTR). Therefore, it is anticipated that the compounds under investigation will be considered attractive for the development of good organic solar cells (OSCs).

Keywords

Azaborinine
Benzothiophene acceptors
DFT
FMOs
Photovoltaic properties

1. Introduction

Organic solar cells (OSCs) have drawn a lot of interest in recent years because of their low production costs, low weight, and enhanced flexibility [1]. Over the recent decades, the two main materials, fullerene and non-fullerene-based acceptors, attained significant importance in bulk heterojunction (BHJ) OSCs. The BHJ-OSCs based on fullerene acceptors have shown improved charge mobility and a 12% power conversion efficiency (PCE) [2]. Nevertheless, some disadvantages such as untunable energy levels, a large energy gap, a reduced open-circuit voltage (Voc), poor visible absorption, and high cost are also found [3]. Researchers have been trying to create OSCs without fullerene to get over these restrictions and improve solar cell performance [4]. In this regard, non-fullerene based OSCs (NF-OSCs) offer several benefits such as enhanced stability, notable absorption in the visible region, greater solubility, lower cost, and easier modification of energy levels [5,6].

The NF-OSCs are used in both forms, either in polymers or as small molecules in OSCs. The small molecule OSCs (SM-OSCs) are superior to polymer-based OSCs because they offer various benefits, including increased stability, purification, and a highly ordered molecular arrangement [7]. Photovoltaic materials based on the NF-SMAs have advanced significantly owing to various architectures like A–D–A–D–A and A–D–A configurations, which demonstrate significant PCEs of 17–18% [8]. Heteroatom side-chain engineering has been explored in A–D–A–D–A compounds in NF-SMA, demonstrating a strong structure-property relationship and achieving optimal charge mobility [9]. The NF-SMAs consisted of central electron-donating cores, such as perylene diimide (PDI), benzothiophene (BT), azaborinine, and indacenodithiophene (IDT). Having terminal acceptor processes efficient charge mobility and photovoltaic properties [10]. In azaborinine, the incorporation of boron (B) and nitrogen (N) atoms into the conjugated π-system of organic chromophores enhances charge transport, molecular stability, and tunability of energy levels. These compounds exhibit strong electron affinity due to the presence of boron, facilitating efficient charge separation and transport in bulk BHJ-OSCs [11]. Additionally, azaborinine derivatives offer excellent photostability, high absorption coefficients in the visible to near-infrared region, and reduced recombination losses, making them attractive candidates for high-efficiency photovoltaic applications. Their structural versatility allows fine-tuning of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels, optimizing exciton dissociation and charge collection. Moreover, heteroatom substitution in azaborinine frameworks can further modulate optoelectronic properties, enhancing their compatibility with donor materials and interfacial layers. As a result, these materials hold great potential for improving the PCE and long-term stability of OSCs, paving the way for the development of more efficient and durable photovoltaic technologies [12]. Moreover, molecular engineering at terminal acceptors is an efficient approach in tuning the photovoltaic properties of organic chromophores. Literature studies reveal that integrating BT-based acceptors with electron-withdrawing groups enhances charge transfer toward the acceptor domains. This results in higher short-circuit current (Jsc) and open-circuit voltage (Voc) in NFAs. The improvement is primarily due to the selective stabilization of the lower-lying LUMO while maintaining a nearly unchanged HOMO. Furthermore, the absorption energy is significantly broadened, further contributing to enhanced device performance [13]. Currently, density functional theory (DFT) has been instrumental in uncovering the electronic and optical properties of NFA-OSCs. Through DFT simulations, researchers obtain critical insights into molecular interactions, charge transport dynamics, and the energetics of these advanced materials. This computational methodology not only supports the strategic design of high-efficiency non-fullerene acceptors (NFAs) but also enhances the understanding of the intricate relationship between molecular architecture and device performance. Ultimately, these insights contribute to the development of next-generation high-performance OSCs [14].

By considering the importance of the azaborinine core in tuning the electrical and photovoltaic properties of OSCs, currently, an azaborinine-based synthesized compound BNTT2F, with a B-N covalent linkage, was taken as the parent molecule for the newly designed derivatives for photovoltaic materials. The BNTT2F is an effective non-fullerene fused ring electron acceptor with distinctive properties, including red-shifted absorption spectra, minimal excitation energies, and a lower energy gap (Egap = ELUMO-EHOMO), along with 8.3% PCE [15]. It is modified into the BNTR reference compound by replacing long-chain alkyl groups in the central core with methyl groups to diminish steric hindrance and reduce the computational costs, as shown in Figure 1. Moreover, sulphur atoms (S) in the central core were replaced with selenium atoms (Se), as the Se-atom is more polarizable than the S-atom due to its larger atomic size and larger electron cloud. It shows enhanced charge mobility and Se-Se intermolecular interactions [16]. The electron-rich BT acceptors are utilized for structural modeling at the terminal positions as they elevate the LUMO energy level, improve charge transfer, Voc, and enhance the π-conjugation in the organic systems [17]. To determine the photovoltaic and electronic properties of the BNTR and BNT1-BNT7 compounds, DFT and time-dependent DFT (TD-DFT) approaches were utilized. Therefore, various analyses such as the frontier molecular orbitals (FMOs), binding energy (Eb), UV-Visible, density of states (DOS), open circuit voltage (Voc), transition density matrix (TDM), and electron-hole analyses were conducted. Newly designed derivatives are expected to be synthesized by experimentalists in the future as efficient photovoltaic OSCs.

Conversion of parent compound (BNTT2F) into reference compound (BNTR) by (i) replacement of long chain alkyl group with a methyl group (ii) replacement of sulphur with selenium.
Figure 1.
Conversion of parent compound (BNTT2F) into reference compound (BNTR) by (i) replacement of long chain alkyl group with a methyl group (ii) replacement of sulphur with selenium.

2. Materials and Methods

The Gaussian 09 package [18] was employed for all the quantum chemical calculations. Gauss View 5.0 [19] program was utilized to visualize the true minima structures of the studied compounds (BTNR and BNT1-BNT7). The DFT method was carried out at the M06/6-311G(d, p) level [20] to calculate the opto-electronic and photovoltaic properties of the afore-mentioned compounds. Various analyses, such as the FMOs, UV-Visible spectra, DOS, TDM, open-circuit voltage (Voc), and electron-hole analyses, were performed to uncover the photovoltaic characteristics of the entitled chromophores. The FMOs diagrams for the HOMO, LUMO, HOMO-1 and LUMO+1 levels etc. and their energies were obtained with the aid of Avogadro software from Gaussian outputs, and their energies were obtained from the Avogadro software [21]. The global reactivity parameters (GRPs) were calculated via the HOMO-LUMO energies by using (Eqs. 1–8).

(1)
I P = − E HOMO

(2)
E A = − E LUMO

(3)
X = [ I P + E A ] 2

(4)
η = [ I P − E A ]

(5)
μ = E HOMO + E LUMO 2

(6)
σ = 1 η

(7)
ω = μ 2 2 η

(8)
Δ N m a x = − μ η

The DOS maps were generated by using the PyMOlyze 1.1 program [22] by utilizing Gaussian outputs to measure the percentage of charge distribution on the molecular orbitals. To elucidate the TDM maps from the Gaussian log files, the Multiwfn 3.7 software [23] was utilized. Moreover, different software such as origin 8.5 [24], Gaussum [25], and Chemcraft [26] were also utilized to interpret the findings obtained from the output files.

3. Results and Discussion

An A–D–A-structured BNTT2F parent compound is chosen, featuring a B–N covalent linkage along with a selenophene ring acting as a donor, which is further attached with electron-accepting terminal acceptor units [15]. To prevent steric hindrance and reduce computational expenditure, bulky alkyl groups, i.e., –C6H13, –C11H23, and –C8H17 are replaced with small methyl (–CH3) units, as displayed in Figure 1. Moreover, the sulfur (S) atoms in the central core are replaced with selenium (Se) atoms, as selenium is more polarizable than sulfur due to its larger atomic size and electron cloud. This results in enhanced charge mobility and strong Se-Se intermolecular interactions [27].

By doing the aforesaid structural modifications, the parent chromophore (BNTT2F) is converted to the reference compound (BNTR). The BNTR is further used to design seven different derivatives (BNT1-BNT7) by replacing terminal acceptors with BT-type acceptors. The chemical structures of these chromophores have been shown in the Figure S1, while Tables S1-S8 provide their Cartesian coordinate values. density functional theory/time dependent density functional theory (DFT/TD-DFT) analyses are performed to examine the impact of the azaborinine core and BT end-capped acceptor units on the photovoltaic characteristics of the above-mentioned chromophores.

Figure S1

Tables S1-8

To determine the planarity, dihedral angles, and bond lengths for BNTR and BNT1-BNT7 chromophores were performed, and their optimized geometries have been illustrated in Figures 2 and 3. Two dihedral angles (θ1 and θ2) and bond lengths (L1 and L2) were calculated at the point of attachment of the core with BT acceptors (Tables S9 and S10). The results in Table S9 show that all simulated carbon-carbon bond length values (1.405-1.413 Å) were in agreement with reported experimental values of C-C (1.54 Å) and C=C (1.34 Å) bonds [28]. Further, the dihedral angles are obtained close to 0° in a range of -0.777° to 2.733° for θ1 and -0.760° to 2.849° for θ2. These values show their non-twisted or planar nature as they are least deviated from the X-axis. Thus, the results infer that the investigated compounds showed a planar arrangement, which illustrated good ICT in these molecules [29].

Table S9

Table S10
Optimized structures at the ground state for the titled compounds (BNTR and BNT1-BNT7).
Figure 2.
Optimized structures at the ground state for the titled compounds (BNTR and BNT1-BNT7).
Lateral view of optimized structures for the studied compounds.
Figure 3.
Lateral view of optimized structures for the studied compounds.

3.1. FMOs

The FMOs analysis provides an effective way for probing the optoelectronic and chemical properties of the molecule [30]. The LUMO shows electron-accepting property, while the electron-donating propensity is evaluated by HOMO [31]. The energy difference between HOMO/LUMO is depicted as the energy gap (Egap = ELUMO–EHOMO) [32,33]. It is a significant parameter that demonstrates the chemical reactivity, electrical behavior, chemical hardness, dynamic stability, and chemical softness of the investigated chromophores [34]. Moreover, the FMOs analysis reveals the pattern of electronic distribution among different energy states [35]. The energy values of HOMO (EHOMO), LUMO (ELUMO) and their difference (ΔE = ELUMO–EHOMO) for BNTR and BNT1–BNT7 have been recorded in the Table 1, while the Table S11 shows similar information regarding other orbitals.

Table S11
Table 1. Energies of FMOs of BNTR and BNT1–BNT7.
Compounds EHOMO ELUMO ΔE
BNTR -6.115 -3.533 2.582
BNT1 -5.974 -3.363 2.611
BNT2 -6.060 -3.472 2.588
BNT3 -6.022 -3.427 2.595
BNT4 -6.152 -3.690 2.462
BNT5 -6.098 -3.564 2.534
BNT6 -6.164 -3.714 2.450
BNT7 -6.042 -3.478 2.564

Units in eV

According to Table 1, comparable energy difference values are observed for the derivatives (BNT1–BNT7), such as 2.611, 2.588, 2.595, 2.462, 2.534, 2.450, and 2.564 eV. Compounds (BNT4-BNT7) show a consistent reduction in their Egap than that of BNT1-BNbT3, which demonstrated slightly increased energy gaps (2.611, 2.588, and 2.595 eV) as compared to BNTR (2.582 eV). The following terminal acceptors were present in BNT1-BNT3: 2-(2-methylene-1-oxo-1,2,3a,8b-tetrahydro-3-H-benzo[b]cyclopenta[d]thiophen-3-ylidene)malononitrile; (2-(6,7-dichloro-2-methylene-3-oxo-2,3,3a,8b-tetrahydro-1-H-benzo[b]cyclopenta[d]thiophen-1-ylidene)malononitrile) and (2-(6,7-difluoro-2-methylene-3-oxo-2,3,3a,8b-tetrahydro-1-H-benzo[b]cyclopenta[d]thiophen-1-ylidene)malononitrile), respectively. The decline in Egap of BNT4-BNT7 as compared to the BTNR might be due to the integration of strong electron-withdrawing groups incorporated in their terminal acceptor regions [36]. The introduction of BT acceptors enhances the charge delocalization due to the existence of thiophene rings [13]. A prominent decrease in the energy gap was noticed in BNT4 (2.462 eV), where the sulfonic acid (–SO3H) groups at the acceptor unit, i.e., (1-(dicyanomethylene)-2-methylene-3-oxo-2,3,3a,8b-tetrahydro-1-H-benzo[b]cyclopenta[d]thiophene-6,7-disulfonicacid) play a significant role. A slight increase in the Egap of BNT5 (2.534 eV) was observed as compared to BNT4, where the sulfonic acid groups (–SO3H) are replaced with trifluoromethyl (–CF3) group at terminal acceptor moieties i.e., 2-(2-methylene-3-oxo-6,7-bis(trifluoromethyl)-2,3,3a,8b-tetrahydro-1-H-benzo[b]cyclopenta[d]thiophen-1-ylidene) malononitrile. This might be due to the presence of resonance in the sulfonic acid groups. The BNT6 compound showed the lowest Egap value (2.450 eV) among all chromophores due to the presence of strong electron-withdrawing nitro (–NO2) groups at the acceptor unit, i.e., 2-(2-methylene-6,7-dinitro-3-oxo-2,3,3a,8b-tetrahydro-1-H-benzo[b]cyclopenta[d]thiophen-1-ylidene) malononitrile. The –NO2 group has a larger negative inductive effect (-I) as compared to the –CF3 unit at the end of the acceptor. The last derivative (BNT7) showed a higher Egap value (2.564 eV) ming be owing to the –CH3COOH group at the terminal acceptor, i.e., dimethyl 1-(dicyanomethylene)-2-methylene-3-oxo-2,3,3a,8b-tetrahydro-1-H-benzo[b]cyclopenta[d]thiophene-6,7-dicarboxylate. The Egap exhibits the following decreasing order: BNT1 > BNT3 > BNT2 > BNTR > BNT7 > BNT5 > BNT4 > BNT6.

The energy values of HOMO/LUMO for BNTR reference were calculated as -6.115 and -3.533 eV. Whereas the EHOMO values of BNT1–BNT7 were -5.974, -6.060, -6.022, -6.152, -6.098, -6.164, and -6.042 eV. In contrast, their ELUMO values were -3.363, -3.472, -3.427, -3.690, -3.564, -3.714, and -3.478 eV. Among all the designed compounds, BNT6 demonstrated the lowest value of EHOMO (-6.164 eV) and ELUMO (-3.714 eV). Incorporation of the –NO2 group in the end-capped acceptor of the BNT6 compound effectively lowered the HOMO-LUMO energy values, enhancing the stability and promoting charge transfer as compared to other derivatives [37]. A comparison with reported similar analogues from literature having the same A-D-A configuration showed comparable energy gaps with the current study’s designed derivatives [38].

The electronic charge distribution over the MOs surface areas of BNT1-BNT7 has been demonstrated in Figure 4 and S2. For reference chromophore, the charge density was majorly located over core in HOMO whereas in LUMO it was concentrated all over the molecule. The electronic clouds in HOMO were majorly occupied by the azaborinine core region of the derivatives, while they were mainly concentrated over the terminal acceptor moieties for LUMO and minutely over the donor unit. This shows that charge transfer from azaborinine core towards acceptor region is significantly facilitated in BNT1-BNT7. In conclusion, it is evident from earlier discussions that modulating terminal acceptors with BT acceptors is a useful method for influencing the electronic characteristics of organic chromophores [39]. By reducing the energy gaps for acquiring good performance OSCs, BNT6 might be a reasonable candidate for photovoltaic materials.

Figure S2
HOMO and LUMO of the investigated compounds (BNTR and BNT1-BNT7) illustrating the charge distribution.
Figure 4.
HOMO and LUMO of the investigated compounds (BNTR and BNT1-BNT7) illustrating the charge distribution.

3.2. GRPs

To understand the chemical reactivity and kinetic stability of BNTR and BNT1-BNT7, their GRPs are investigated by utilizing Koopmans’ theorem [40]. The ionization potential (IP) [41] and electron affinity (EA) [42] describe the electron-donating and electron-accepting characteristics of chromophores, respectively [43]. Another crucial factor is the electronegativity (X) [44], which elucidates the capability of a molecule to draw electron density towards itself [45]. The electrophilicity index (ω) [46] characterizes the energy differences concerning the maximum charge transfer magnitude. Another significant parameter related to global electrophilicity is ΔNmax [47], which represents the maximum electric charges absorbed by a compound from its environment [48]. The stability and reactivity of the compounds are significantly influenced by the global softness (σ) [49], hardness (η) [50], and chemical potential (μ) [51].

The values of the ionization potential of the designed compounds are in the range of 5.974 – 6.164 eV. The declining order of IP is: BNT6 > BNT4 > BNT5 > BNT7 > BNT2 > BNT3 > BNT1. The entitled chromophores exhibit greater electron acceptance rates (EA values) in comparison to BNTR, which supports their tendency as electron acceptors. Table 2 illustrates the negative values of the chemical potential (μ) for investigating compounds showing their stable nature. The assessed global hardness (η) values decline from BNT1 to BNT7, the utmost value of η is observed in BNT1 (1.305 eV), which drops to 1.225 eV in BNT6 chromophore. The reducing order of η in the modified chromophores is as follows: BNT1 > BNT3 > BNT2 > BNT7 > BNT5 > BNT4 > BNT6. For softness (σ), the BNT6 shows the highest value (0.408 eV-1) among all derivatives owing to the least energy gap value. Due to these unique characteristics, like a lower energy gap with the least global hardness and higher softness values, indicating the greater reactivity with efficient charge transfer in BNT6, which illustrates its potential as a good photovoltaic material.

Table 2. Global reactivity descriptors of BNTR and BNT1-BNT7.
Compounds IP EA X η µ ω σ ΔNmax
BNTR 6.115 3.533 4.824 1.291 -4.824 9.012 0.387 3.736
BNT1 5.974 3.363 4.668 1.305 -4.668 8.347 0.382 3.576
BNT2 6.060 3.472 4.766 1.294 -4.766 8.776 0.386 3.683
BNT3 6.022 3.427 4.724 1.297 -4.724 8.601 0.385 3.641
BNT4 6.152 3.69 4.921 1.231 -4.921 9.836 0.406 3.997
BNT5 6.098 3.564 4.831 1.267 -4.831 9.210 0.394 3.812
BNT6 6.164 3.714 4.939 1.225 -4.939 9.956 0.408 4.031
BNT7 6.042 3.478 4.76 1.282 -4.76 8.836 0.390 3.712

All units in eV; σ is in eV-1

3.3. UV-visible analysis

UV-Visible absorption study offers a comprehensive insight into the spectral and optoelectronic characteristics, hence elucidating the light-harvesting capacity of an OSC device [52,53]. The evaluation of absorption spectra was done in both the gaseous and solvent phases (chloroform) at the aforementioned level. This examination shows essential optical elucidations, including the maximum absorption wavelength (λmax), transition energies (E), oscillation strength (fos), excited state transitions (H→L), and within BNTR and BNT1-BNT7. The molecular excitations occur when photons with a certain energy are absorbed, which corresponds to energy gaps. It is noticed that compounds having strong terminal acceptor units together with extended conjugation show more red-shifted λmax values and vice [54]. The absorption spectra of the titled chromophores have been shown in Figure 5, while Table S12 shows major results for the above-mentioned factors.

Table S12
UV–Visible spectra of BNTR and BNT1-BNT7 in chloroform and gaseous phase.
Figure 5.
UV–Visible spectra of BNTR and BNT1-BNT7 in chloroform and gaseous phase.

In the solvent phase (chloroform), the range of maximum absorption wavelengths is found as 617.297-650.665 nm, as shown in the Table S12. The highest λmax of 650.665 nm with minimal excitation energy value as 1.906 eV is obtained for BNT6 chromophore due to the strong electron-accepting −NO2 group in the terminal acceptor moiety [55]. While, the λmax for other derivatives (BNT1-BNT5 and BNT7) were investigated as 617.297, 627.355, 620.604, 648.860, 632.960, and 627.863 nm. All derivatives except BNT1 and BNT3 showed higher λmax values as compared to the reference chromophore (BNTR = 623.224 nm). The descending order of λmax was observed in the chloroform solvent phase as: BNT6 > BNT4 > BNT5 > BNT7 > BNT2 > BNTR > BNT3 > BNT1. The absorption spectra for six excited states for studied compounds in chloroform solvent have been mentioned in the Table S13. Comparative study with similar architecture literature data (tricyanofuran-based donor-acceptor chromophores:514-589 nm) illustrated that these chromophore showed bathochromic shift [38].

Table S13

In the gaseous phase, the λmax was observed within a range of 588.412-625.677 nm. for BNT1, all derivatives possessed higher wavelengths than the BNTR (590.738 nm) in this phase. Compounds (BNT4 and BNT6) showed the highest λmax among the derivatives i.e., 625.677 and 620.262 nm. Table S12 shows a modest decrease in λmax values for designed compounds, possibly due to the solvent effect [56]. The other results of absorption parameters for studied compounds in gas phase have been mentioned in the Table S14. The declining order of λmax in gaseous phase was as follows: BNT4 > BNT6 > BNT5 > BNT7 > BNT2 > BNT3 > BNT1. Hence, the compounds (BNT4 and BNT6) showed the most red-shifted λmax values and correspondingly least transition energy values due to lower energy gaps and thus regarded as the appropriate materials for the OSCs applications [57].

Table S14

3.4. DOS

The DOS study was conducted to measure the distinct electronic charge contributions at different portions of individual molecular [58]. These computations made it easier to evaluate electron distribution by quantitative estimation of the HOMO and LUMO contributions in terms of percentage obtained via the FMOs analysis [59,60]. To execute this analysis, the designed molecules underwent fragmentation into two segments, namely donor (D) and terminal acceptor units (A). Further, this present analysis for the reference compound (BNTR) and its derivatives (BNT1-BNT7) was accomplished to support the FMOs analysis. The DOS pictographs have been displayed in the Figure 6, where every fragment has been shown by different colored curves (donor with red and acceptor with green).

DOS of entitled chromophores represented graphically.
Figure 6.
DOS of entitled chromophores represented graphically.

The DOS percentage contributions recorded in the Table S15 clearly depict the charge distribution on HOMO and LUMO for the investigated chromophores and shows the influence of substituting the acceptor moieties in varying electronic charge distribution patterns, as shown previously in the FMOs analysis. Herein, the acceptor showed less charge contribution towards the HOMO as 27.0, 25.0, 25.3, 26.8, 25.8, 26.3, 24.9, and 25.1% for BNTR and BNT1-BNT7. However, for LUMO, the acceptor showed higher contributions as 39.0, 7.1, 58.9, 63.1, 63.4, 75.2, 68.5, 56.3, and 54.9%. An opposite behavior was observed for the donor fragment, which revealed the maximum electronic distributions towards the HOMO, i.e., 73.0, 74.4, 74.7, 74.4, 75.3, 74.7, 75.1, and 76.1% for BNTR and BNT1-BNT7. While 61.0, 41.1, 36.9, 39.1, 30.5, 34.8, 43.7, and 48.1% were the LUMO contributions for BNTR and BNT1-BNT7 (see Table S15).

Table S15

The DOS pictogram showed energy on the X-axis in eV, and relative intensity on the y-axis. It is to be noted that the left side on x-axis shows the energy values for HOMOs and right side denotes for the LUMO. The statistical data of DOS was validated from the graphical peaks obtained for each chromophore, which showed foremost higher green peaks (A) in the right side and highest red peaks (D) on the left side. Moreover, the region of zero intensity between two peaks is the energy gap for each individual compound, which corresponds with the FMOs results. In summary, the analysis of charge distribution indicated the presence of electron delocalization, with a substantial charge transference from donor moiety to the terminal acceptor unit. Thus, the studied compounds were considered as good candidates for the future OSCs devices.

3.5. TDM

TDM was also utilized to illustrate the intramolecular charge transference (ICT) from the ground state to the excited state (S0→S1) [61,62]. This is a significant quantum chemical technique, which allows estimation of the location of electron holes, electronic excitation, and the excited state interactions in the acceptor and donor units [63]. Figure 7 illustrates the absorption and emission of electronic charges from the S0 to the S1 state. The minimal involvement of the hydrogen atoms leads to their neglect during transition. The TDM analysis is essential for assessing the phenomena associated with excited electronic states, including electronic excitation and localization of electron-hole pairs [64]. The BNTR and BNT1-BNT7 are divided into two segments: the donor (D) and end-capped acceptor (A). Figure 7 shows the presence of effective charge over the acceptor groups, validated by prominent and large coherence lengths. This shows greater rates of exciton dissociation in these molecules. It can be shown from the analysis of the TDM map data that all the compounds under investigation exhibited effective charge coherence and diagonal charge transfer from the donor part to the acceptor regions without any charge trapping. Thus, TDM maps indicated that the selected molecules are able to contribute as effective candidates for OSCs applications.

The TDM maps of BNTR and BNTD1-BNTD7 compounds illustrating the charge transfer from S0 to S1.
Figure 7.
The TDM maps of BNTR and BNTD1-BNTD7 compounds illustrating the charge transfer from S0 to S1.

3.6. Exciton binding energy

The Eb holds significance in estimating the capacity for exciton dissociation and the interaction strength of Coulombic forces. Coulomb forces between electrons and holes are reduced as the Eb lowers [65]. These factors collectively contribute to an increased rate of exciton dissociation in the excited state of the studied chromophore [66]. The Eb represents a significant parameter for assessing the photovoltaic properties of the investigated compounds (BNTR and BNT1-BNT7). It is defined as the difference between the energy gap (EL-H) and the minimal energy necessary for the initial excitation (Eopt) [67]. A lower Eb corresponds to higher Jsc [68] and enhanced PCE. Eq. (9) was used to calculate the Eb of compounds that have been examined [69].

(9)
E b =   E L − H − E opt

Hence, Eopt is the energy of the initial singlet-singlet excitation, where Eb is the energy difference. Table 3 displays the computed values of Eb for all the candidates.

Table 3. Calculated Eb of the studied compounds.
Compounds EH–L Eopt Eb
BNTR 2.582 2.099 0.483
BNT1 2.611 2.107 0.504
BNT2 2.588 2.071 0.517
BNT3 2.595 2.092 0.503
BNT4 2.462 1.982 0.480
BNT5 2.534 2.050 0.484
BNT6 2.450 1.999 0.451
BNT7 2.564 2.065 0.499

Units in eV

From the results presented in Table 3, the Eb values for the entitled chromophores (BNTR and BNT1-BNT7) are 0.483, 0.504, 0.517, 0.503, 0.480, 0.484, 0.451, and 0.499 eV. The least value of Eb (0.451 eV) was observed in BNT6, which depicts that it had the highest potential for exciton separation. The decreasing tendency of Eb was as follows: BNT2 > BNT3 > BNT1 > BNT7 > BNT5 > BNT4 > BNT6. Concludingly, the studied compounds exhibited lower Eb values with higher exciton dissociation, which were also supported by the TDM analysis. The least energy gaps, red-shifted absorption spectra, and lower binding energies with efficient ICT indicated that these designed derivatives can be utilized as effective photovoltaic materials for OSCs.

3.7. Open circuit voltage (Voc)

The open circuit voltage (Voc) analysis is a fundamental parameter in photovoltaic devices, denoting the maximum potential difference across the terminals in the absence of an external load [35,70]. The OSC device’s fluorescence efficiency, temperature, electrode work function, light intensity, and recombination of charge carriers rates are some of the variables that affect it [71]. The Voc is directly related to the energy difference of the HOMO of donor and LUMO of the acceptor i.e., HOMO(donor) − LUMO(acceptor) [72]. To attain higher values of Voc, the HOMO level of the charge-donating molecule should be at lower energy, while the LUMO level of the charge-accepting molecule must be present relatively at high energy [73]. For the current analysis, the PBDB-T donor polymer was used to determine the open-circuit voltage values for the selected acceptor molecules. The energy values of HOMO and LUMO of PBDB-T were EHOMO = -5.401 eV and ELUMO = -2.328 eV, which are significantly coherent with the EHOMO and ELUMO values of the designed derivatives [74]. An approximated theoretical approach is established, which is employed to estimate the findings by using the Scharber’s Equation (Eq. 10) [75].

(10)
V o c = I / e ( | E HOMO D | − | E LUMO A | ) − 0.3

The Voc calculations for all the entitled chromophores (BNTR and BNT1-BNT7) have been presented in Table 4. The energy gap values for the HOMO(PBDB-T) and LUMO(acceptor) were calculated as 1.868, 2.038, 1.929, 1.974, 1.711, 1.837, 1.687, and 1.923 eV for BNTR and BNT1-BNT7. Among all the studied compounds, BNT1 showed the utmost Voc value of 1.738 V. The Voc findings of all the designed chromophores were observed in the following descending order: BNT1 > BNT3 > BNT2 > BNT7 > BNTR > BNT5 > BNT4 > BNT6. A reduction in the LUMO energy level of the acceptor by Voc promotes sufficient charge transference from the donor moiety HOMO to the acceptor LUMO. Additionally, the PCE was also influenced by the energy difference among the acceptor and donor units. The Voc pictograms for the investigated compounds in relation to the PBDB-T donor have been shown in Figure 8. Concluding the discussion, the proposed molecules showed potential for voltage generation and are reasonable OSCs candidates.

Table 4. Open circuit voltage of entitled compounds.
Compounds VOC (V) Δ E
BNTR 1.568 1.868
BNT1 1.738 2.038
BNT2 1.629 1.929
BNT3 1.674 1.974
BNT4 1.411 1.711
BNT5 1.537 1.837
BNT6 1.387 1.687
BNT7 1.623 1.923

Δ E= E LUMO A − E HOMO D

Diagrammatic representation of Voc for the selected compounds.
Figure 8.
Diagrammatic representation of Voc for the selected compounds.
Graphical representations of electron-hole analysis for the studied compounds.
Figure 9.
Graphical representations of electron-hole analysis for the studied compounds.

3.8. Fill factor (FF)

Another critical factor impacting the proficiency of OSCs is the fill factor (FF) of the molecular structure. The FF of OSCs quantifies the effectiveness of the cell in transforming incident light into electrical energy [76,77]. Enhancements in the FF of OSCs can be achieved via the utilization of materials exhibiting favorable transport characteristics, including high carrier mobility [78-80]. Following Eq. (11) [81] is used to evaluate the FF of the BNTR and BNT1-BNT7.

(11)
F F = e V O C K B T − l n ( e V O C K B T ) + 0.72 e V O C K B T + 1

In this Equation, e stands for the molecular charge and is always equal to 1. The K B is a Boltzmann constant taking 8.61733034 × 10−5 electron volts per kelvin, and T stands for temperature, which is constant (300 K). The term “eVoc\KBT “ is termed as the normalized voltage, which is also calculated with the FF, and the values have been given in the Table S14. The outcomes reveal that in comparison to the reference molecule, BNT1 shows better normalized Voc and FF (67.104 and 0.958, respectively). It follows that all recently designed compounds could lead to the development of effective OSCs.

3.9. Hole electron analysis

The electron-hole coupling is an effective and extensively utilized technique for analyzing the charge transfer phenomena in the studied molecules. As depicted in Figure 9, the C-33 atom situated within the selenophene ring of the donor in the BNT1 compound undergoes hole localization, whereas the C-38 atom coupled to the opposite selenophene ring in the donor region shows a notable electron density. The same trend is observed for hole and electron density in BNT2 and BNT3 compounds. In case of BNT4, the hole intensity is located over the C-27 atom of the benzene ring of the donor, while the electron intensity is observed at the same position (C-38 atom) of the selenophene ring in the donor. Among all the designed chromophores, BNT6, featuring a nitro group in the terminal acceptor, exhibits the highest electron density. This phenomenon may arise due to resonating effects and a significant inductive effect (-I) exerted by the nitro group [82]. The great electronegativity of the oxygen atoms in the nitro group withdraws electron density from the rest of the molecule via sigma bonds, resulting in a partial positive charge on adjacent carbon atoms [83]. In all studied chromophores, charge transfers via electron-hole coupling take place in the following decreasing order: BNT6 > BNT4 > BNT5 > BNT7 > BNT2 > BNT3 > BNT1. This descending sequence indicates that the investigated molecules demonstrated remarkable electron-hole coupling. Consequently, they represent themselves as reasonable candidates for solar cells due to their good electron-hole coupling.

4. Conclusions

The present quantum chemical study is performed to evaluate the photovoltaic and optoelectronic characteristics of the azaborinine-based chromophores (BNTR and BNT1-BNT7). Molecular engineering via symmetrically coupled BT-based acceptors is accomplished to tune the electrical and photovoltaic characteristics. The derivatives showed reduced energy gaps (Egap = 2.611−2.450 eV) and red-shifted λmax values (617.297−650.665 nm in the chloroform and 588.412–625.677 nm in the gas phase), accompanied by the lower excitation energy values. Increasing order of the λmax was found to be BNT1 < BNT2 < BNT3 < BNT7 < BNT5 < BNT4 < BNT6 in chloroform. The results of the GRPs revealed less hardness and higher global softness in derivatives as compared to the reference chromophore with illustrated their greater reactivity. Moreover, the DOS and TDM plots displayed good charge transfer from the azaborinine core towards BT acceptors. The smaller Eb values supported the good exciton dissociation with significant ICT. Comparable Voc results are obtained for all the examined chromophores with respect to the PBDB-T donor. Among all derivatives, BNT6 is regarded as the most efficient chromophore owing to its lowest energy gap (2.450 eV) and highest absorption values (650.665 nm). Moreover, BNT6 also showed the least value for exciton binding energy, as 0.451 eV. These results concluded that the incorporation of BT-based electron-withdrawing acceptors increased the charge transfer efficiency for achieving better photovoltaic materials in the OSCs.

Acknowledgment

Dr. Muhammad Khalid gratefully acknowledges the financial support of HEC Pakistan (project no. 20-14703/NRPU/R&D/HEC/2021). A.A.C.B. acknowledges the financial support of the São Paulo Research Foundation (FAPESP) (Grants 2014/25770-6 and 2015/01491-3), the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) of Brazil for academic support (Grant 309715/2017-2), and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) that partially supported this work (Finance Code 001). The authors thank the Ongoing Research Funding Program (ORF-2025-6) King Saud University, Riyadh, Saudi Arabia. S.C.O. acknowledges the support from the doctoral research fund of the Affiliated Hospital of Southwest Medical University.

CRediT authorship contribution statement

Muhammad Khalid: Methodology; Investigation; Resources; software; project administration; Methodology. Supervision; Formal analysis, review, Shahzad Murtaza: Conceptualization; Investigation; Writing - original draft; Validation; Visualization, Ayesha Mussarat: Data curation; Visualization; Writing - review & editing, Tansir Ahamad: Conceptualization; Formal analysis; Investigation; Writing - original draft; Visualization, Ataualpa Albert Carmo Braga: Resources; software; Conceptualization; Formal analysis; Supervision; Validation, Suvash Chandra Ojha: DFT data acquisition, Writeup, Analysis, Funding acquisition.

Declaration of competing interest

The authors declare no conflicts of interest.

Declaration of Generative AI and AI-assisted technologies in the writing process

The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript and no images were manipulated using AI.

Supplementary data

Supplementary material to this article can be found online at https://dx.doi.org/10.25259/AJC_61_2025.

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