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Original Article
2025
:18;
3532025
doi:
10.25259/AJC_353_2025

Unveiling the role of peripheral acceptors in ortho-benzodipyrrole-based chromophores for efficient performance in organic solar cells: A comprehensive theoretical study

, , , , ,
aInstitute of Chemistry, Khwaja Fareed University of Engineering & Information Technology, Rahim Yar Khan, 64200, Pakistan
bDepartment of Infectious Diseases, The Affiliated Hospital of Southwest Medical University, Luzhou 646000, China
cDepartment of Chemistry, College of Science, King Saud University, Riyadh 11451, Saudi Arabia

*Corresponding author: E-mail address: suvash_ojha@swmu.edu.cn (S. 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

The exploration of novel non-fullerene acceptor (NFAs) materials for organic solar cells (OSCs) is a frontline area of research. Herein, novel A–π–A chromophores (CBR and CBD1-CBD6) were proposed with reduced energy gaps and improved intramolecular charge transfer (ICT) rates. The ortho-benzodipyrrole was incorporated as the central core, end-capped with strong malononitrile acceptors. The density functional theoretical (DFT) approach was adopted at the M06/6-311G(d,p) level to explore the structure-property relationship, opto-electronic, and photovoltaic (PV) properties of these NFA-based compounds. The suitability of these chromophores for OSCs applications was confirmed by performing their frontier molecular orbitals (FMOs), transition density matrix (TDM), density of states (DOS), UV-Visible, hole-electron, open circuit voltage (Voc), fill factor (FF), and power conversion efficiency (PCE) at the afore-mentioned level. The outcomes of these analyses showed that modifying the terminal acceptors with electron-withdrawing groups significantly lowered the energy gaps (2.20-2.34 eV) of the designed chromophores (CBD1-CBD6) as compared to the CBR reference (2.371 eV). They also showed broader absorption wavelengths (654.75-724.03 nm) with correspondingly lower excitation energies (1.71-1.89 eV). Interestingly, CBD3 showed the most promising results, i.e., minimal highest-occupied molecular orbital (HOMO)/lowest-unoccupied molecular orbital (LUMO) energy gap (2.20 eV), along with high λmax (724.03 nm), and least excitation energy (1.71 eV). All the proposed compounds showed significant open-circuit voltage (Voc) (2.334-2.694 V) and fill factor (FF) (0.937-0.946) values. Consequently, these compounds also possessed greater PCEs, which provides a deep insight into their high charge transfer ability and PV efficiency. Overall, this study is useful in exploring new NFA-based OSCs for high-tech applications.

Keywords

Benzodipyrrole-based chromophores
DFT
Non-fullerene acceptors
Open circuit voltage
Photovoltaic properties

1. Introduction

The increasing demand for energy resources is significantly impacted by the exponential rise in the global population. Although fossil fuels have historically been the main source of energy, their use is limited due to their negative impact on the environment and global warming. In this context, renewable energy has shown an interesting response to a range of energy-related challenges [1]. Solar energy is widely acknowledged as one of the important, modern, and sustainable renewable energy sources [2]. Due to its huge power capacity and minimal CO2 impact, which amounts to 1.1% of the entire world’s emissions, solar technology’s future is bright as one of the renewable energy source types. Sunlight activates the photovoltaic (PV) effect, which generates electrical power. Owing to this, it is considered a renewable energy resource, which is available, cheap, and non-polluting [3]. Different types of solar cells are reported, such as the hybrid solar cells [4], amorphous solar cells [5], crystalline solar cells [6], perovskite solar cells [7], organic solar cells [8], dye-sensitized solar cells (DSSCs) [9], and cadmium telluride (CdTe) solar cells [10]. Silicon-based and inorganic PV cells have dominated the PV industry for many decades due to their high power conversion efficiencies (PCEs) ranging from 15 to 20% [11]. Silicon is extensively utilized in the PV silicon-based energy devices due to its exceptional efficiency, durability, affordability, availability, and environmental responsibility [12]. However, it has lately been observed that silicon is costly, brittle, and has uncontrollable energy levels. On the other hand, OSCs have many desirable properties, including vast production space, easy synthesis, mechanical flexibility, variable energy levels, and low weight [10]. Moreover, they can be utilized in the production of proficient portable electronics, clothing, and transparent electronics. Therefore, the potential of OSCs to replace inorganic PV cells is incredible [13].

Conventionally, the OSCs are composed of an active layer made of acceptor and donor molecules, together with electrodes [14]. When electromagnetic radiation encounters the active layer, electronic transitions occur from their HOMO to LUMO [15]. The performance of OSCs can be enhanced with techniques such as improvement in the architecture of the device, management and control of the heterojunction morphology, and development of new active material layers. The most critical strategy that must be incorporated in the framework for further improving OSC efficiency is the synthesis and utilization of new donor and acceptor materials in the active layer. Conjugated polymers, along with fullerenes, are incorporated as donors and acceptors in the active layer [16].

There are two kinds of OSCs, fullerene-based and non-fullerene-based (NF-OSCs) [10]. Fullerene derivatives are widely used in OSCs, commonly as acceptors, owing to their high carrier mobility and appropriate electron affinity (EA) in the bulk heterojunction (BHJ) active layers. Moreover, fullerene-based OSCs have exhibited PCEs in the range of 10-11% [17]. However, their limitations on a wide scale include low visible light absorption, low Voc, high energy gap, high cost, and limited energy level positions [18]. Scientists are now focusing their attention on the NF-OSCs as they are lighter, possess a larger manufacturing area, tunable energy levels, flexible, and less expensive [13]. The power conversion efficiencies of OSCs based on the non-fullerene acceptors as electron acceptor materials have increased to 19% [15].

Currently, there are two categories of NF-OSCs: the non-fullerene polymer solar cells (NF-PSCs) and non-fullerene small molecular acceptor (NF-SMA) cells [19]. Among them, the latter are superior due to factors such as the better-defined molecular arrangement, and purity [20]. The NF-SMAs usually consist of conjugated units that can contain electron-accepting (A) and electron-donating (D) units in their structures, i.e., A−D−A or A−D−A′−D−A [21]. Similarly, the A−π−A configuration is also widely employed in designing high-performance OSCs. Two novel “quasi-macromolecule” electron acceptors (QM1 and QM2) with a A−π−A molecular structure have been reported in the literature. They achieved promising power conversion efficiencies (17.06 and 16.35%, respectively) along with high open-circuit voltages [22]. In another report, trifluoromethylated π-extended NFA was developed in an environmentally friendly tetrahydro furan (THF) solvent. In conjugation with donor polymer (PTQ10), it shows a high PCE of 14.05% [23].

Keeping in mind the above-stated factors, a NFA-based compound named as 2,2’-((2Z,2’Z)-((11,12-bis(2-hexyldecyl)-3,8-diundecyl-11,12-dihydrothieno[2’’,3’’:4’,5’]thieno[2’,3’:4,5]pyrrolo[3,2-g]thieno[2’,3’:4,5]thieno[3,2-b]indole-2,9-diyl)bis(methaneylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1-H-indene-2,1-diylidene))dimalononitrile [24] is selected as a parent molecule for molecular designing of novel compounds (CBR and CBD1-CBD6). The primary reason for selecting this parent molecule is its high yield (91%) in synthesis process as well as the availability of numerous active sites for designing other compounds. Herein, end-capped acceptor modifications developed six new fullerene-free chromophores (CBD1-CBD6) with an A−π−A structure, using CBR as a reference. Further, theoretical investigation is performed for the newly designed chromophores utilizing the density functional theory approach. Moreover, in the present study, the HOMO/LUMO energy gap, maximum absorption wavelength and open-circuit voltage are considered as standard parameters for evaluating the impact of structural modification in the designed compounds (CBD1-CBD6). To improve the photochemical characteristics of NFAs, several types of electron-withdrawing moieties are incorporated, as shown in the Scheme 1 [25]. However, it is expected that these designed compounds will be indeed better materials which can be employed to fabricate next-generation OSCs in the context of organic PV systems.

Modification of the parent compound into the reference molecule (CBR)
Figure 1.
Modification of the parent compound into the reference molecule (CBR)
Schematic representation of the investigated compounds (CBR and CBD1-CBD6).
Scheme 1.
Schematic representation of the investigated compounds (CBR and CBD1-CBD6).

2. Materials and Methods

Utilizing the Gaussian 16 software package [26], the quantum chemical calculations based on the DFT and time-dependent DFT (TD-DFT) were performed in this research work. Firstly, the designed compounds were optimized at the M06/6-311G(d,p) level [27] to obtain their ground-state geometries visualized via the GaussView software [28]. Based on these optimized structures, the opto-electronic, photophysical, and PV properties were investigated for the reference (CBR) and designed compounds (CBD1-CBD6). They include the dihedral angles, frontier molecular orbitals (FMOs), global reactivity parameters (GRPs), density of states (DOS), transition density matrix (TDM), UV-Visible, open circuit voltage (Voc), and hole-electron analyses. Their absorption spectra were simulated at the afore-mentioned level of conductor-like polarizable continuum model (CPCM) [29]. By employing the Multiwfn 3.7 program [30], their TDM study was conducted to reveal the pattern of delocalization and exciton interaction. To determine the molecular contributions towards the distribution of electron density in each state, the DOS analysis was performed using the PyMOlyze 1.1 software [31]. The data from the output files were evaluated with the help of software, such as the Avogadro [32], GaussSum [33], Chemcraft [34], and the Origin 8.5 program [35].

3. Results and Discussion

In the recent period, fullerene free-organic systems (FF-OSs) containing some specific architectures, such as the D−π−A−π−D, A−π−A−π−A [36], A−D−A [37], and A−π−A become more significant for increasing the efficiency of solar cell materials [38]. In the present study, the parent molecule (CB16) [24] is modified into the reference molecule (CBR) by replacing bulky groups with methyl groups to work with lower computational expense by reducing steric effects, as shown in the Figure 1. Further, six chromophores (CBD1-CBD6) are designed from the CBR reference molecule to enhance the optoelectronic characteristics by altering the terminal acceptor moieties. The Scheme 1 represents the structures of utilized end-capped acceptors. The IUPAC names and abbreviations of reference and designed chromophores are shown in Table S1 (supporting information), while their chemical structures are represented in Figure S1. However, the Figures 2 and 3 show their optimized structures (front-view and side-view). Moreover, Tables S2-S8 displayed the Cartesian coordinates of CBR and CBD1-CBD6 compounds. For all compounds under consideration, several quantum chemical calculations, including the FMOs, DOS, UV-Visible, binding energy (Eb), TDM, hole-electron, and open circuit voltage (Voc) analyses are performed at the afore-mentioned level. This study demonstrates that end-capped tailoring significantly influences the optoelectronic, photophysical and PV properties of the titled NFAs chromophores that may lead to their introduction as potent solar cells, especially with regards to derivatives that contain the electron withdrawing moieties.

Supplementary Information
Optimized ground state structures for all the studied compounds.
Figure 2.
Optimized ground state structures for all the studied compounds.
Sideview geometries of the investigated compounds.
Figure 3.
Sideview geometries of the investigated compounds.

3.1. Ground-state properties

To understand the impact of structural tailoring on the ground-state properties and planarity of the investigated compounds, their dihedral angles and bond lengths are calculated. Figures 2 and 3 show the front-view and side-view of the optimized structures for CBR and CBD1-CBD6 compounds. However, their 2-D chemical structures are shown in Figure S1. It is known from literature that planarity of a molecule is highly influenced by overlapping of π−orbitals and energy band gaps [39]. In this case, all the proposed molecules show A−π−A configuration in which there are two fragments, i.e., acceptor and π-spacer. Bond length is the separation between the nuclei of two atoms that are joined together, whereas bond angle is the angle created by two neighboring bonds in a molecule. A molecule with smaller bond distances and bond angles between its substituent atoms is expected to be stronger than one with larger bond lengths and bond angles [40]. The dihedral angles between the peripheral acceptors and π-spacer moieties are mentioned as θ1 and θ2, respectively, as shown in Figure 2. Similarly, the bond lengths between terminal acceptors and π-spacers are represented as L1 and L2. Tables S9 and S10 show the calculated results for dihedral angles and bond lengths of the investigated chromophores, respectively. In all studied compounds, the bond lengths between fragments range from 1.394-1.402 Å. Further, the dihedral angles (θ1 and θ2) lie close to the zero value, which confirms the planar nature of the designed compounds owing to their little deviation from the x-axis. Moreover, these findings also depict that these compounds have a notable molecular configuration. Nearly all the proposed chromophores are found to be planar, with smaller bond angles between the two fragments.

The geometrical parameters of designed compounds illustrated in Table S11-S17 are calculated with respect to the structures mentioned in Figure S2. It is noted that the bond lengths of C17-S23 and C12-S21 atoms in the central core lie within a range of 1.761 to 1.762 Å in all the investigated compounds, which closely relate to the reported value of C-S bond length (1.77 Å) [41]. Likewise, the bond length of C30-O47 and C39-O48 atoms in the acceptor region are observed to lie in a range of 1.214-1.215 Å, which closely aligns with the reported data of ketonic bond length of C=O (1.21 Å) [42]. In case of bond angles, C-N-C functional groups (C5-N7-C25 and C4-N8-C26) are found in the examined chromophores within a range of 127.9-128°. These numbers are also quite consistent with the bond angle values (124.8°) that have been published [43]. While the bond angle of C-S-C fall within a range of 88.6-88.9° lies in a close harmony with literature data (93.4°) [44].

3.2. Frontier molecular orbitals study

FMOs analysis is regarded significant for assessing the electronic characteristics of the studied organic systems [45]. It is essential for investigating the passage of electric current and endow solar cells with the capability to carry electronic charges [46]. By evaluating the HOMO as valance band and the LUMO as conduction band, the energy difference, which is a classic parameter, (Egap=ELUMO-EHOMO) is determined [47]. Literature study shows that charge transport in the PV OSCs is believed to undergo significant changes in their HOMO and LUMO distribution pattern, which gives information about the PV materials’ efficiency [48]. When the energy gap is minimal, a massive charge transfer between two molecular orbitals becomes feasible. A decrease in the energy gap values in the studied compounds is a key factor which lead towards high charge transfer ability, high efficiency, coupled with high reactivity [49]. The charge carrier mobility is improved in the designed molecules (CBD1-CBD6) by the introduction of electron withdrawing acceptor units which result in greater delocalization of electrons in the designed molecular systems. As a consequence, FMOs investigation is conducted for the proposed chromophores at the M06/6-311G(d,p) level and the outcomes are illustrated in Table 1 of the manuscript. Similarly, results for other orbitals (HOMO-1/LUMO+1 and HOMO-2/LUMO+2) are mentioned in the Table S18.

Table 1. Energies of frontier molecular orbitals (EHOMO and ELUMO) and their corresponding energy gaps (Egap) for CBR and CBD1-CBD6.
Compounds EHOMO ELUMO Egap
CBR -5.84 -3.47 2.37
CBD1 -5.87 -3.53 2.34
CBD2 -5.87 -3.52 2.34
CBD3 -6.04 -3.83 2.20
CBD4 -6.04 -3.79 2.25
CBD5 -6.03 -3.79 2.24
CBD6 -5.95 -3.65 2.31

Egap = ELUMO - EHOMO. All units in eV.

All the designed chromophores show lower Egap values (2.20-2.34 eV) than the CBR reference (2.37 eV). However, the HOMO and LUMO energy values for the derivatives are found to be lower than CBR, which shows EHOMO = -5.84 eV and ELUMO = -3.47 eV. For derivatives (CBD1-CBD6), the EHOMO are found as -5.87, -5.87, -6.04, -6.04, -6.025 and -5.95 eV, while their ELUMO are computed as -3.53, -3.52, -3.83, -3.79, -3.79 and -3.65 eV, respectively. The energy gap is the most reliable parameter for illustrating both intramolecular charge transfer (ICT) and optoelectronic characteristics in the studied molecules. So, the Egap values for all studied chromophores (CBR and CBD1-CBD6) are found as 2.37, 2.34, 2.34, 2.20, 2.25, 2.24, and 2.31 eV, respectively. Derivatives (CBD1 and CBD2) show energy gap values closer to the reference (CBR) due to the fact that they possess halogens substituted at their end-capped acceptor groups. Due to its extended conjugation, the calculated energy gap value of CBD3 is the lowest, i.e., 2.20 eV, among all the compounds under study. Another reason is the presence of nitro group (−NO2) at its terminal acceptor unit, i.e., 2-(2-methylene-5,6-dinitro-3-oxo-2,3-dihydro-1-H-inden-1-ylidene) malononitrile, which shows greater electron withdrawing nature, significantly reduces the energy gap [50]. This may be due to variations in the molecular orbital distribution that may help to enhance charge transfer and separation. The compound (CBD4) shows a slightly higher energy gap (2.25 eV) as compared to CBD3, which is attributed to the end-capped substitution at acceptor groups, i.e., the sulphonic acid (−SO3H) group is substituted in the case of CBD4. In contrast, CBD5 compounds exhibit a higher electron-withdrawing effect when the cyano group is substituted for the sulphonic acid group at its terminal acceptor, i.e., 1-(dicyanomethylene)-2-methylene-3-oxo-2,3-dihydro-1-H-indene-5,6-dicarbonitrile unit, leading to a slight reduction in the energy gap (2.24 eV). Lastly, the derivative (CBD6) shows the largest value of energy gap (2.31 eV) as compared to other derivatives owing to substitution of trifluoromethyl groups (−CF3). A decreasing order of the energy gap is observed as follows: CBR > CBD2 > CBD1 > CBD6 > CBD4 > CBD5 > CBD3.

In addition, as illustrated in Figure 4, the contour sides of the HOMO and LUMO are represented to clarify the electrical cloud transference phenomena using red and green colors to represent electron density regions. The electronic cloud is mainly distributed on the terminal acceptors and slightly on the π-spacers in the LUMO, while it is mainly concentrated on the π-spacer unit in the HOMO. This elucidates efficient charge transfer towards the acceptors from the π-spacer region. Furthermore, the contour surfaces for other orbitals (HOMO-1/LUMO+1 and HOMO-2/LUMO+2) are shown in Figure S3, lie in close correspondence with results obtained in the main orbitals. According to this study, the entitled compounds exhibit proficient ability to be utilized as optoelectronic materials.

HOMO and LUMO pictographs of the reference and designed derivatives.
Figure 4.
HOMO and LUMO pictographs of the reference and designed derivatives.
HOMO and LUMO pictographs of the reference and designed derivatives.
Figure 4.
HOMO and LUMO pictographs of the reference and designed derivatives.

3.3. Density of states

DOS analysis aims at confirming the results obtained from the FMOs study by quantitatively analyzing the electronic charge transfer among individual fragments in the studied compounds [51]. This study is frequently used to calculate the proportion of each unit in a compound contributing to the overall distribution of electronic charge [52]. To clarify the results of CBR and CBD1-CBD6, their DOS analysis is performed at the selected level of theory and basis set. Results show that there is a prominent shift in the electronic distribution pattern by substituting electron-withdrawing groups in the peripheral acceptors. All investigated compounds are fragmented into two parts for the purpose of illustrating their DOS: (i) π-spacer (core), (ii) end-capped terminal acceptor moieties. As seen in the Figure 5, fragments of studied chromophores are represented with distinct colors, such as the π-spacer being red and the peripheral acceptors being green. The total DOS curves for each individual compound are shown by the black curve. For further illustration, the DOS plot for each chromophore shows HOMO and LUMO peaks along with their corresponding energy gaps. The HOMO peaks for all chromophores are present around -5.5 to -6 eV, while the peaks for LUMO are observed at -3.5 to -3.8 eV on DOS plots, which closely relate to the calculated HOMO and LUMO levels from the FMOs investigation (see Table 1). Moreover, the graphs illustrate that high intensity peaks for HOMO are obtained for the π-spacer units in all the studied chromophores, while, for the acceptor moiety, LUMO is the representative energy level. This observation is further verified by the DOS percentage contributions obtained for HOMO and LUMO recorded in Table S19.

Graphical illustration of DOS for the entitled chromophores.
Figure 5.
Graphical illustration of DOS for the entitled chromophores.
Graphical illustration of DOS for the entitled chromophores.
Figure 5.
Graphical illustration of DOS for the entitled chromophores.

Herein, all compounds, including reference, show major contribution towards the acceptor in the LUMO as 55.3, 55.7, 46.6, 69.3, 60.1, 61.9, and 66.0% for CBR and CBD1-CBD6, respectively, while minor contribution towards the π-spacer as 44.7, 44.3, 53.4, 30.7, 39.9, 38.1, and 34.0%. Contrarily, for the HOMO, the maximum charge is contributed by the π-spacer unit as 75.5, 74.9, 75.6, 73.8, 73.9, 73.8, and 73.5% in CBR and CBD1-CBD6, respectively. Whereas, smaller charge contributions are shown by the terminal acceptors as 24.5, 25.1, 24.4, 26.2, 26.1, 26.2, and 26.5%, respectively (see Table S18). In DOS pictographs, HOMO values are shown on the left side of the x-axis, while LUMOs are mentioned towards the right side, and the distance between them is denoted as the energy gap [30]. The highest peaks are seen in both areas, indicating that the acceptor (red peak) exhibits a large density on the LUMO in each compound, while the π-spacer (green peak) donates most of the charge density on the HOMO. As a result, DOS graphs provide significant assistance for FMOs diagrams. This study demonstrates that by utilizing different strong acceptor units, the electrical charge distribution pattern on molecular orbitals is altered significantly.

3.4. UV-visible analysis

To illustrate the optical characteristics, the UV-Visible absorption properties of CBR and CBD1-CBD6 are calculated at the aforementioned level. The studied compounds are examined in both the gaseous and chloroform phases. A summary of their optical properties, including transition energies (E), oscillation strength (fos), excited state transitions (H→L), and maximum absorption wavelength (λmax), is provided in Table 2. Tables S20-S33, however, show detail results at other wavelengths for the investigated compounds. When a compound absorbs photons with a certain energy that matches its corresponding energy gaps, it is excited. It is noted that compounds with extended conjugation, together with potent electron-withdrawing terminal acceptor units, show more bathochromic shifts in their UV-Visible absorption spectra [53]. Another key component that might alter the electron mobility of selected chromophores is the excitation energy [54].

Table 2. Maximum absorption wavelength (λmax), excitation energy (E), oscillator strength (fos) and molecular orbital (MO) contributions of compounds (CBR and CBD1-CBD8) in gaseous and solvent phases.
System TD-DFT λmax (nm) E (eV) fos MO Contributions
aPhase CBR 624.95 1.98 2.152 H→L (97%), H-2→L+1 (3%)
CBD1 633.96 1.95 2.198 H→L (96%), H-2→L+1 (3%)
CBD2 634.65 1.95 2.216 H→L (96%), H-2→L+1 (3%)
CBD3 656.24 1.90 2.061 H→L (96%), H-2→L+1 (2%)
CBD4 653.03 1.90 2.152 H→L (96%), H-2→L+1 (2%)
CBD5 716.38 1.73 2.311 H→L (95%), H-2→L+1 (3%)
CBD6 639.39 1.94 2.176 H→L (97%), H-2→L+1 (2%)
bPhase CBR 675.03 1.84 2.381 H→L (96%), H-2→L+1 (3%)
CBD1 686.44 1.80 2.416 H→L (95%), H-2→L+1 (3%)
CBD2 686.89 1.81 2.430 H→L (95%), H-2→L+1 (3%)
CBD3 724.03 1.71 2.107 H→L (94%), H-2→L+1 (2%)
CBD4 713.70 1.74 2.332 H→L (95%), H-2→L+1 (3%)
CBD5 654.75 1.89 2.150 H→L (96%), H-2→L+1 (2%)
CBD6 695.21 1.78 2.391 H→L (95%), H-2→L+1 (3%)
aPhase: Gaseous phase; bPhase: Solvent phase; TD-DFT: Time-dependent density functional theory

Herein, the studied compounds show improved optical characteristics as compared to reference compound. In the gaseous phase, calculated λmax values for all compounds under investigation fall within a range of 633.96 to 716.38 nm, which is higher than the λmax value of CBR (624.952 nm). All the designed chromophores show prominent red-shifts are depicted in the following increasing order in nm: CBR (624.95) < CBD1 (633.96) < CBD2 (634.65) < CBD6 (639.39) < CBD4 (653.03) < CBD3 (656.24) < CBD5 (716.38). The CBD3 and CBD5 compounds show the highest λmax values with correspondingly least excitation energies (1.90 and 1.73 eV, respectively). These results are correlated to their minimal energy gaps (see Table 1). The reason behind this red shift is their structure-property relationships, which are enhanced due to the attachment of stronger electron-withdrawing nitro (−NO2) and cyano (−CN) groups in CBD3 and CBD5, respectively.

Due to the impact of the solvent, values of the absorption maxima (λmax) in chloroform displayed more improved results than in the gaseous phase. It is anticipated that the polar medium employs appropriate electronic states to stabilize the π–π* state [55]. Similar to the gaseous phase, CBD3 exhibits the highest absorption wavelength at 724.03 nm, least excitation energy (E) at 1.71 eV, and fos of 2.107 with a molecular orbital transition from H→L as 94%. The increasing trend of λmax the studied compounds is as follows in nm: CBD5 (654.75) < CBR (675.03) < CBD1 (686.44) < CBD2 (686.89) < CBD6 (695.21) < CBD4 (713.70) < CBD3 (724.03). A review of the literature indicates that molecules with less energy gap and superior absorption characteristics possess high HOMO to LUMO charge transferability and power conversion efficiency [56]. Figure 6 displays the absorption spectra in both phases for the studied compounds. Recently, the benzodithiophene-based non-fullerene chromophores (T1-T7) are developed for optoelectronic attributes by changing end-capped acceptors. On comparison with our ortho-benzodipyrrole-based designed chromophores (CBD1-CBD6), the previously reported molecules are less efficient as they showed higher energy gaps (3.75-4.24 eV) than CBD1-CBD6 (2.20-2.34 eV). Similarly, in case of UV-Visible absorption, our designed chromophores show higher absorption wavelengths in the visible region ranging from 654.75-724.03 nm, as compared to T1-T7 compounds (466-543 nm) [57].

Absorption spectra of the titled compounds in solvent and gas phases.
Figure 6.
Absorption spectra of the titled compounds in solvent and gas phases.

Looking at the results of TD-DFT analysis, it is inferred that the chromophores under investigation with the highest λmax and least excitation energy values are significant candidates for fullerene-free OSCs.

3.5. Global reactivity parameters

The HOMO and LUMO energy values of the selected compounds are utilized to calculate their global reactivity descriptors. They include the electronegativity (X) [58], ionization potential (IP) [59], electron affinity (EA) [60], global softness (σ) [61], hardness (η) [62], chemical potential (μ) [63], and electrophilicity index (ω) [64]. These factors allow for investigating both the chemical reactivity and kinetic stability of the studied molecules. Eqs (S1-S8) are used to compute GRPs with the aid of Koopman’s theorem [65]. High EA and low IP values accelerate the charge transfer mechanism in the organic chromophores [66]. While a smaller IP decreases the hole-injection barrier, a higher EA value facilitates the injection of electrons into the LUMO [67]. These two characteristics have a considerable relationship with the boundary or orbitals. Calculated results of GRPs for the studied compounds are shown in Table 3.

Table 3. Global reactivity parameters of the studied compounds (CBR and CBD1-CBD6).
Compounds IP EA X η μ ω σ ΔNmax
CBR 5.84 3.47 4.66 1.18 -4.66 9.15 0.42 3.92
CBD1 5.87 3.53 4.70 1.17 -4.70 9.43 0.43 4.02
CBD2 5.87 3.52 4.69 1.17 -4.69 9.40 0.43 4.00
CBD3 6.04 3.83 4.93 1.10 -4.93 11.05 0.45 4.48
CBD4 6.04 3.79 4.92 1.13 -4.92 10.74 0.45 4.37
CBD5 6.03 3.79 4.91 1.12 -4.91 10.74 0.44 4.38
CBD6 5.95 3.65 4.80 1.15 -4.80 10.00 0.43 4.17

IP: Ionization potential; EA: Electron affinity; X: Electro negativity; η: Global hardness; μ: Chemical potential; ω: Electrophilicity index; σ: global softness; ∆Nmax: Charge transfer. All units in eV. σ in eV-1

The Egap is directly correlated with stability, hardness, and chemical potential, while the chemical reactivity is negatively correlated with these parameters [68]. Consequently, it is believed that molecules with larger energy gap are more stable and harder, and vice versa [69]. In this study, we observed that the compound (CBD3) with smallest energy gap has the highest softness (σ) value as 0.45 eV-1 and lowest hardness (η) value as 1.10 eV. Moreover, all the designed compounds (CBD1-CBD6) possess higher softness and lower hardness values than the reference compound due to insertion of different electron withdrawing groups at their peripheral acceptors. It means that energy gap is in inverse relation with softness and directly related to hardness. Further, all the studied compounds show comparable electronegativity values. The highest electronegativity (4.93 eV) is observed for CBD3, as shown in the Table 3.

The charge transfer parameter (∆Nmax = -μ/η) indicates molecule’s ability to drain greater charge density from the environment [70]. A molecule shows a greater tendency to spontaneously accept electrons with high and positive electrophilicity index (ω) and ∆Nmax. But when the level of these indices are less for a compound, it will act as an electron donating group [71]. The values of ∆Nmax lie in a range of 3.92-4.48 eV. The highest value of CBD3 (4.48 eV) revealed that it has a greater tendency to accept the electron. Consequently, all the investigated compounds show efficient results for GRPs due to which they are considered as efficient PV materials with improved chemical reactivity.

3.6. Hole-electron analysis

This analysis is often used to identify the location of an electron density in a chemical compound and is incredibly effective [72]. Electron hole analysis may be used to understand the behavior of excitations and charge carriers in the PV materials [73]. Consequently, we used Multiwfn 3.7 [30] to perform the hole–electron analysis to understand the charge movement in the investigated chromophores. The ICT is shown by the substantial change of electronic charge across various segments in all the chosen chromophores. In the reference molecule (CBR), the hole density is localized at C-1, C-21 and C-23 atoms, respectively. While, in case of electrons, C-27, C-28 and C-40 atoms display stronger intensity in CBR. In CBD1 derivative, the hole intensity is detected at the C-1, C-2, and S-23 atoms of the π-linker, whereas electron intensity is centered over the C-27 and C-28 atoms (π-linker) and C-40 atom of the acceptor units. Figure 7 shows that in CBD2, a hole is formed at the C-1, C-6, C-21, and S-23 atoms of the π-spacer. A significant electronic cloud is visible at C-27, C-28, and C-40 atoms of the π-bridge. For CBD3, a thick hole density is located at C-21 and S-23 atoms and the highest electron density on C-51 and C-52 atoms. The highest intensity of hole is found at C-1, C-21, and S-23 atoms, while electrons are concentrated on the C-27, C-28, C-37, and C-40 atoms in CBD4. The CBD5 and CBD6 compounds exhibit strong hole intensity over C-1, C-2, and S-23 atoms of the π-linker, and high electron density in the terminal acceptors at C-27, C-28, and C-40 atoms. Moreover, the Figure 7 depicts high-intensity holes at different atoms of the π-linker, with charge transfer taking place in the acceptor area. These carbon atoms are linked to the terminal electron withdrawing groups, allowing for efficient ICT in all derivatives (CBD1-CBD6). Thus, all the proposed chromophores have significant electron and hole clouds at different parts within each individual molecule which shows efficient charge mobility.

Pictorial representation of hole-electron transport analysis for the investigated compounds.
Figure 7.
Pictorial representation of hole-electron transport analysis for the investigated compounds.

3.7. TDM

To examine the transfer of charge density in the molecular systems under study, their TDM analysis is employed at the selected level. In this case, the TDM analysis is used to examine the interaction between the acceptor and π-spacer fragments in the excited state. It also offers significant information about the performance aspects of acceptor materials in the OSCs [74]. This technique is essential owing to its tendency to estimate the number of variables, such as the location of electron holes, electronic excitation, and the interaction between excited states in the acceptor and π-spacer units [75].

To study the influence of these effects, the studied molecules (CBR and CBD1-CBD6) are divided into two components, referred to as acceptor and π-spacer. The influence of hydrogen atoms is ignored due to their low participation in transitions in the present study. The TDM maps (Figure 8) demonstrate the existence of charge in a scattered state. Non-diagonal charge transfer is noted in CBR, CBD2, CBD4, and CBD6 compounds. However, there is an unusual display of charge transference for the remaining compounds. Moreover, regions with bright fringes showing more exciton dissociation are indicated with red rings in these maps. Hence, TDM maps enable a simple and effective dissociation of excitons in the excited state for the above-mentioned compounds, indicating their strong potential for use in OSC applications.

TDM pictographs and heatmaps of the entitled molecules.
Figure 8.
TDM pictographs and heatmaps of the entitled molecules.
TDM pictographs and heatmaps of the entitled molecules.
Figure 8.
TDM pictographs and heatmaps of the entitled molecules.
TDM pictographs and heatmaps of the entitled molecules.
Figure 8.
TDM pictographs and heatmaps of the entitled molecules.

3.8. Open circuit voltage (Voc)

Open circuit voltage is another important analysis that is used to assess the efficiency of OSCs [53]. The Voc findings are in direct relation with PCE of the OSCs [76]. External fluorescence, charge carrier recombination, temperature, light source, electrode performance, and other ambient parameters are some of the elements that affect Voc [77]. It shows the entire amount of current that may be derived from any optically active device while the voltage is at zero [78]. Since the acceptor’s LUMO value reduces the energy gap, it should be higher than the donor moiety’s HOMO in order to get a superior Voc [79]. Further, it is found that the difference in HOMO/LUMO between D and A molecules is proportional to Voc. Eq. (1) is published by Scharber and his colleagues, which is used to determine the hypothetical results of the Voc for the studied solar cell materials [80].

(1)
V o c = I / e ( | E HOMO D | − | E LUMO A | ) − 0.3

As shown in the Figure 9, EHOMO of the PTB7 donor polymer is compared to the ELUMO of studied NFA chromophores (CBR and CBD1-CBD6). To examine charge transfer characteristics and determine their Voc, these compounds are therefore mixed with the PTB7 to create a complex. This polymer is selected for the present analysis owing to its favorable EHOMO (6.467 eV). Regarding the energy difference between HOMOdonor and LUMOacceptor, the Voc values of CBR and CBD1-CBD6 are determined as 2.694, 2.637, 2.645, 2.334, 2.375, 2.382, and 2.518 V, as shown in the Table S34. The decreasing trend of open circuit voltage for the investigated compounds is as follows: CBR > CBD2 > CBD1 > CBD6 > CBD5 > CBD4 > CBD3. All the afore-mentioned compounds show exceptional potential for NF-OSCs due to their significant capacity to generate voltage.

Graphical representation of Voc for the entitled chromophores with respect to PTB7 donor polymer.
Figure 9.
Graphical representation of Voc for the entitled chromophores with respect to PTB7 donor polymer.

3.9. Fill factor (FF) and PCE

The performance of a solar cell is determined by important parameters such as the FF, open circuit voltage, and short circuit current density (Jsc). FF is further used to determine the PCE of the studied compounds. Theoretically, the FF is determined using the Eq. (2) [81].

(2)
FF = e V oc K B T − ln ⁡ [ e V oc K B T + 0.72 ] e V oc K B T + 1

In this Equation, the Boltzmann constant (8.61733034 eV) is represented by KB, while the temperature is denoted by T (300 K), in Kelvin. While the term “eVoc/KBT” represents the voltage. The designed compounds have the FF values in a range of 0.937-0.946, as shown in Table S34 among which reference compounds shows the highest value. The order of FF is almost same as that for Voc.

Eq. (3) [82] is used to estimate the PCE of the chromophores under examination.

(3)
PCE = J s c   x F F x Voc P i n

The findings of the above-mentioned Equation yield an over-estimated approximation of PCE, ranging from 32.19-37.40% for the examined chromophores. As a result, the entitled chromophores can be considered as potential candidates for OSCs devices in the future.

These calculated parameters represent upper‐bound indicators, while the actual device performance will also depend on morphology and solid‐state effects.

4. Conclusions

In conclusion, the current study proposes the quantum chemical investigation of novel ortho-benzodipyrrole-based chromophores (CBR and CBD1-CBD6). Their structural tailoring with strong end-capped acceptors, accommodated with different electron-withdrawing groups, resulted in reduced energy gaps (2.20-2.34 eV) and bathochromic shifts (654.75-724.03 nm) in the visible region. Therefore, effective ICT within their structures is facilitated by these acceptor changes. Among all derivatives, the CBD3 compound displayed the least energy gap (2.20 eV) and the highest λmax value (724.03 nm). Additionally, the DOS and TDM graphical representations further demonstrated and supported the above-mentioned results. Furthermore, the studied compounds are blended with the donor polymer (PTB7) in order to investigate their PV potential. Interestingly, favorable results are obtained for the Voc, FF, and PCEs for all compounds under study. Hence, the proposed NFAs chromophores serve as suitable candidates for the PV materials.

Acknowledgment

The authors extend their sincere appreciation to Ongoing Research Funding Program (ORF-2025-253), 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

Mashal Khan: Supervision; investigation; resources; software; project administration; methodology. Sadia Jamal: Formal analysis; investigation; writing - review & editing; visualization. Memoona Arshad: Formal analysis; validation; visualization; writing - review & editing; Wen Qin: Data curation; formal analysis; methodology; reviewing. Norah Alhokbany: Formal analysis; methodology; writing - review & editing; funding. Suvash Chandra Ojha: Data curation; validation; visualization; funding.

Declaration of competing interest

There are no conflicts of interest to declare.

Data availability

All data generated or analyzed during this study are included in this published article and its supplementary information files.

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_353_2025

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