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Scientific Reports volume 16, Article number: 9842 (2026) Cite this article
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Recent advances in the non-fullerene acceptors (NFAs) have achieved remarkable attention owing to their significant photovoltaic and optoelectronic characteristics in organic solar cells. Herein, new A–π–A configuration-based NFAs (TPBD-Cl to TPBD-CF₃) were designed by incorporating the strong electron withdrawing acceptor units at their terminals. Theoretical investigation was performed for these chromophores by using DFT/TD-DFT methods at M06/6-311G(d, p) level. The results demonstrate that the end-capped acceptor substitution has promoted the charge transfer and efficiently reduced the energy gap (2.25–2.13 eV). Further, it was revealed that the structural modifications elevated that absorption wavelengths in the studied chromophores (721.97–754.58 nm). The transition density matrix (TDM) and density of states (DOS) analyses demonstrated an efficient charge transfer from the donor (π–spacer) moieties towards terminal acceptors. Notably, TPBD-NO₂ compound exhibited the least energy gap (2.13 eV), highest absorption wavelength (λ_max = 754.58 nm) and correspondingly least excitation energy (1.64 eV). Moreover, it showed the minimal exciton binding energy (0.50 eV) among all other derivatives which highlight its potential for the organic solar cell materials. Photovoltaic parameters were calculated by blending the donor (PBDB-T) with designed acceptors showed the good open-circuit voltages (V_oc). These findings suggest that the end-capped engineering in NFAs chromophores can significantly enhance the optoelectronic performance, offering promising materials for next-generation OSC devices.
The growing demand of energy worldwide and decreasing reserves of fossil fuels prompted the need to establish effective energy sources. Solar energy is one of the most promising technologies of all renewable energy sources due to its abundance and environmentally friendly nature. Organic photovoltaic (OPV) systems have particularly received great researchers’ interest owing to their affordability, flexibility and adjustable optoelectronic characteristics^1,2,3,4. Nevertheless, to obtain high power conversion efficiency (PCE) in OPV devices, one must design new organic semiconductor materials with an optimal set of electronic and optical characteristics.
Several OSCs employed the fullerene-based acceptors, which have significant benefits with a reported PCE of about 10%⁵. Even though the fullerene-based OSCs achieved great success, they faced a number of challenges, including high purification, low stability, and limited absorption in the visible wavelength region⁶. Considerable study has been devoted to the non-fullerene acceptors, especially the non-fullerene small-molecule acceptors (NF-SMAs), in an attempt to overcome these challenges⁷. Compared to fullerene derivatives, NFAs have several advantages, including cost effectiveness, adjustable energy levels and efficient absorption of visible light⁸. These materials have been analyzed in a variety of conformations, including the acceptor–donor–acceptor (A–D–A), acceptor–π–donor–π–acceptor (A–π–D–π–A), acceptor–donor–acceptor–donor–acceptor (A–D–A–D–A) and donor–acceptor–donor (D–A–D) reported in literature^9,10.
Recent work on the A–D–A type NFAs has resulted in remarkable progress in the organic solar cells. Structural modifications of the donor (D) core, acceptor end groups, and side chains are shown to effectively tune the energy levels, absorption spectra, molecular packing, and charge transport¹⁰. The photovoltaic efficiency in such compounds is improved by the asymmetric central core to achieve directional intramolecular transfer of charges between the donor center and the terminal acceptors. This asymmetry causes unequal distribution of charges which enhances the electric field strength inside and promotes effective dissociation of excitons. Furthermore, a decrease in the molecular symmetry decreases over-aggregation resulting in optimized molecular packing and fewer losses of recombination. The extended π-conjugation facilitates the reduction of energy gaps and high absorption shifts. The combination of these effects leads to better charge separation, transport and performance of the photovoltaic materials¹¹.
Literature shows that side-chain engineering in a series of non-fullerene acceptors (TTCn-4 F) demonstrated a high PCE of 15.34% in the TTC8-4 F molecule blending with PM6 due to favorable morphology and balanced charge mobility¹². Recently, an ortho-benzodipyrrole (o-BDP) based non-fullerene acceptors (CFB and CMB) demonstrated significant performance. The PM6:CFB and PM6:CMB blends achieved 16.55% and 16.46% PCEs, respectively, owing to reduced energy losses and efficient packing¹³. These results highlight that systematic optimization of NFAs via the central core design, end-group tuning, and side-chain modification continues to drive OSC’s performance towards higher efficiencies and practical device applications¹⁴.
In recent years, the A–D–A configuration-based NFA molecules are developed by isomerization to study its influence on the ICT. Comparison showed that TPBTT-4 F is much better than its counterpart in exhibiting the red-shifted absorption and elevated HOMO levels, which lead towards reduced energy gap¹⁵. Moreover, it showed a high power conversion efficiency (15.72%) due to reduced nonradioactive energy loss (E_nonrad) for organic solar cells. These properties impelled the authors to design new NFAs using the above-mentioned compound by utilizing the end-capped tailoring strategy. So, to improve the charge transfer, the designed compounds have strong electron withdrawing groups in their acceptors as shown in the Fig. 1. Moreover, these compounds are not reported previously in the literature and are unique in the field of solar cells. The objective of this research work is to investigate the impact of various acceptor moieties on the photovoltaic and optoelectronic properties of the newly designed derivatives. For this purpose, DFT and TD-DFT methods are employed which determined various parameters including the optical, electronic and photovoltaic properties. The results provide useful insights into the design and optimization of small-molecule NFAs, hence leading to more efficient solar cell materials.
To conduct the quantum chemical calculations, the Gaussian 16 suite program^16,17 was utilized in this work. The designed compounds (TPBR and TPBD-Cl toTPBD-CF₃) were optimized at the ground state using the M06 functional¹⁸ and 6-311G(d, p) basis set¹⁹. The true minima structures with no imaginary frequency were obtained and visualized via the GaussView 6.0 software²⁰. The frontier molecular orbitals (FMOs), global reactivity parameters (GRPs), density of states (DOS), transition density matrix (TDM), open circuit voltage (V_oc), and UV-Visible absorption were accomplished at the above-mentioned functional to investigate the impact of end-capped acceptors on the designed chromophores. The solvent effect was studied using the conductor like polarizable continuum (CPCM) model in the UV-Visible spectra²¹. For the calculation of data from the output files, Avogadro²², Multiwfn 3.8²³, PyMOlyze 2.0²⁴, GaussSum²⁵, Chemcraft²⁶ and Origin 8.5 software²⁷ were utilized.
The electron push-pull effects on NFAs are significant to enhance the intramolecular charge transfer (ICT) and intrinsic photophysical properties to develop efficient OSCs. In this study, TPBTT-4 F compound is selected from the literature to design new NFAs chromophores which possess strong end-capped acceptor groups to achieve efficient solar cell materials. TPBTT-4 F exhibits elevated HOMO level (-4.19 eV) owing to which impressive power conversion efficiency was achieved (15.72%)¹⁵. Herein, TPBTT-4 F is utilized as a reference compound (TPBR) which is further used to develop different derivatives, such as TPBD-Cl, TPBD-Br, TPBD-NO₂, TPBD-SO₃H, TPBD-CN and TPBD-CF₃ having electron withdrawing functional groups (–Cl, –Br, –NO₂, –SO₃H, –CN and –CF₃, respectively) attached to benzene rings of their terminal acceptors. A schematic representation is shown in the Fig. 1 to understand the designing approach in these derivatives. The Fig. S1 shows their chemical structures, while Fig. 2 represents the optimized structures for the designed chromophores. The Table S1 shows IUPAC names of the reference compound and its derivatives, while their Cartesian coordinates are shown in the Tables S2–S8.
Schematic representation of the designed compounds.
True minima geometries of TPBR and TPBD-Cl – TPBD-CF₃.
Frontier molecular orbitals study is an important tool for estimating the stability, light absorption, and electrical characteristics of organic systems^28,29. The highest occupied molecular orbital (HOMO) evaluates the electron donating tendency, while the lowest unoccupied molecular orbital (LUMO) estimates the electron withdrawing capability³⁰. To characterize the distribution of electronic cloud in the HOMOs and LUMOs, frontier molecular orbitals study is the key analysis³¹. The reactivity and stability of a chemical can be assessed by the energy gap = E_LUMO−E_HOMO calculation^32,33. The FMOs analysis for the reference and designed derivatives (TPBR and TPBD-Cl – TPBD-CF₃) is performed using the M06/311G(d, p) level and the results are presented in Table 1. The Fig. 3 shows the visual representation of electronic cloud density in these orbitals.
Results indicate that the reference compound (TPBR) shows the highest energy gap (2.27 eV) along with corresponding HOMO/LUMO energies of -5.79 and − 3.52 eV. The calculated HOMO level energies for TPBD-Cl to TPBD-CF₃ derivatives are − 5.82, -5.81, -6.01, -6.01, -5.99, and − 5.91 eV, whereas their LUMO energy values are − 3.57, -3.56, -3.87, -3.85, -3.83, and − 3.69 eV, respectively. Similarly, they exhibit the following energy gap values: 2.25, 2.25, 2.13, 2.15, 2.15, and 2.21 eV, respectively. Previous similar studies show that greater electron attracting capability of acceptor groups results in narrowing of energy gap in the organic chromophores³⁴.
For the reference compound (TPBR), a wider energy gap (2.27 eV) which might be due to the presence of weak electron withdrawal effect at the terminal acceptors, hence resulting in less LUMO stabilization and charge transfer as compared to its derivatives. The tuning of HOMO and LUMO levels as a result of electron withdrawing substituents is a determinant of E_gap in the organic chromophores³⁵. With the introduction of chlorine substituent (TPBD-Cl), the energy gap (2.25 eV) is reduced significantly. Chlorine has a strong –I (inductive) effect but has a moderate π-donation capability via the dπ–pπ delocalization, which increases the electronic communication of the terminal acceptor to the conjugated backbone. This two-fold act stabilizes the two frontier orbitals although the LUMO is more stabilized giving a smaller HOMO-LUMO gap.
In contrast, TPBD-Br results in almost similar but slightly less pronounced energy gap reduction (2.25 eV). Although, the bromine is heavier than chlorine and also induced more polarization, its orbital overlap efficiency with the π-system is reduced due to which it does not significantly increases the conjugation. The most prominent reduction in energy gap is observed for TPBD-NO₂ (2.13 eV), which contains the –NO₂ substituents. Nitro groups are substituents which possess strong − I effect and − M (mesomeric) effect due to which it effectively lowers the LUMO levels owing to efficient π-electron delocalization³⁶. So, the ICT is elevated from the π-spacer to the terminal acceptor, substantially lowering the energy gap. In TPBD-SO₃H, the presence of –SO₃H groups slightly shift the energy gap (2.15 eV) compared to TPBD-NO₂, despite their strong electron-withdrawing nature. This behavior can be attributed to their reduced planarity and increased steric hindrance, which partially disrupt conjugation along the π-framework.
The TPBD-CN compound exhibits a moderate energy gap (2.15 eV) that is governed by the cumulative − I effect of multiple –CN groups. These groups effectively lower the LUMO level to retain the molecular planarity. However, the absence of strong resonance interaction compared to nitro groups results in a slightly wider energy gap in TPBD-CN than TPBD-NO₂. This indicates the phenomena of controlled tuning of energy gap which is favored by a balanced inductive withdrawal without excessive structural distortion. In last derivative (TPBD-CF₃), the incorporation of –CF₃ groups show 2.21 eV as the E_gap. Although –CF₃ group possess strong − I effect, it does not participate in the π-conjugation. So, the LUMO in this compound is stabilized via the inductive effect, however, its ICT is limited. This results in comparatively larger E_gap than nitro- or cyano-substituted analogues. Overall, the designed chromophores are listed in the following decreasing order of E_gap: TPBR > TPBD-Cl = TPBD-Br > TPBD-CF₃ > TPBD-CN > TPBD-SO₃H > TPBD-NO₂. This order highlights that the energy gap tuning requires not only strong electron withdrawing substituents, rather it is also favored by effective delocalization of orbitals and molecular planarity³⁷.
FMOs analysis also clarifies ICT among the orbitals of the organic chromophores in addition to their corresponding orbital energies³⁸. The pictograms of HOMO and LUMO for the designed compounds are shown in the Fig. 3. All the studied compounds show an electron density on the π-spacer for the HOMO, but for the LUMO, the charge is shown on the acceptor region. In order to further investigate the charge transfer propensity including intramolecular charge transfer, density distribution analysis is done. The isodensity amplitude plots of HOMO and LUMO are plotted for the studied compounds as shown in the Fig. 4. It is noticed that for LUMO, the electronic coud is spread over the acceptor units, while in HOMO it is localized on the π-spacers. Thus, the obtained HOMO-LUMO energy gaps indicate a suitable balance between molecular stability and optical absorption, suggesting that the designed molecule can effectively facilitate charge separation and enhanced photovoltaic performance³⁷.
HOMOs and LUMOs for the investigated compounds.
Percentage contribution of the molecular orbital density corresponding to the partitioned segments obtained from the HOMO and LUMO of the entitled chromophores.
The number of accessible electronic states of a molecule at a given energy level is indicated by the density of states. A higher DOS value at a specific energy indicates that more states exist at that particular energy level³⁹. The goal of this study is to determine how each molecule fragment contributes to the formation of different energy levels, particularly HOMO and LUMO. DOS analysis shows the percentage contributions of each individual fragment in determining the HOMO and LUMO charge densities in the studied chromophores which are displayed in the Table S19, while the Fig. 5 shows the graphical representations of these results. In this research, the designed chromophores (TPBR and TPBD-Cl to TPBD-CF₃) are divided into two fragments (π–spacer and acceptor) to establish the A–π–A configuration. The HOMO (valence band) is shown by the energy values to the left side along the x-axis, while the LUMO (conduction band) is represented by energy values towards the right side. The energy gap is the distance between these valence and conduction bands which is represented as the region in between HOMO and LUMO peaks showing negligible intensity (see Fig. 5). Each individual fragment is shown by separate peaks i.e., red curves show the π–spacer, while, the green peaks represent the acceptor unit. The overall DOS peaks are shown by black curves which represent the combine contribution of all the fragments in determining the charge distribution on the HOMO and LUMO.
The terminal acceptors exhibited the charge distribution percentages as 26.5, 27.5, 27.5, 29.8, 29.6, 29.7 and 28.5% to HOMO for TPBR and TPBD-Cl to TPBD-CF₃, respectively. However, 62.6, 63.0, 63.0, 71.8, 67.4, 66.8 and 63.8% are contributions for the LUMO formation in TPBR and TPBD-Cl to TPBD-CF₃ chromophores, respectively. Similarly, the π-spacer contributes 73.5, 72.5, 72.5, 70.2, 70.4, 70.3 and 71.5% of the charge density towards HOMO and 37.4, 37.0, 37.0, 28.2, 32.6, 33.2, and 36.2% to LUMO in the studied compounds, correspondingly. These results demonstrate that the π-spacer in all the designed derivatives occupy maximum charge density for HOMO, whereas the charge in LUMO is mostly found over the terminal acceptors. Further, the DOS spectra in Fig. 5 corroborates the above-mentioned results. Since the green curves indicate the maximum peak intensity in LUMO for all chromophores, which is close to -3.5 to -3.8 eV, the charge is significantly concentrated over the acceptor in all the studied molecules. While, in HOMO, the maximum charge is situated above the π-spacer unit that is indicated by the red maximum peaks close to -6.0 eV for these chromophores. These results validate the FMOs analysis (see Table 1) which mark the significance of this analysis in determining the electronic characteristics of the designed solar cell materials.
DOS representation for the investigated compounds.
The global reactivity parameters of the studied compounds are evaluated using the energies of the frontier molecular orbitals, since the HOMO-LUMO energy gap obtained from FMOs analysis directly determines the chemical reactivity and stability descriptors. The global softness (σ)⁴⁰, hardness (η)⁴¹, global electrophilicity index (ω)⁴², chemical potential (µ)⁴³, ionization potential (IP)⁴⁴, electron affinity (EA)⁴⁵, electronegativity (X)⁴⁶ and charge transfer (CT) within a molecule (ΔN_max) are some of the GRPs⁴⁷ which are calculated through HOMO/LUMO energies. Furthermore, the Koopman’s theorem⁴⁸ is used to compute the above-mentioned parameters for the TPBR and TPBD-Cl to TPBD-CF₃.
Table 2 displays the computed values of GRPs for the studied compounds which indicate that efficient charge transfer relies on the EA and IP; their high values indicate the stability of a compound. In descending order, the computed ionization potential (IP) values are as follows: TPBD-SO₃H (6.01) = TPBD-NO₂ (6.01) > TPBD-CN (5.99) > TPBD-CF₃ (5.91) > TPBD-Cl (5.82) > TPBD-Br (5.81) > TPBR (5.79) in eV. A molecule is said to be softer if its energy gap is less, which denotes greater reactivity and less stability. The descending order of global softness is as follows: TPBD-NO₂ (0.46) > TPBD-SO₃H (0.46) > TPBD-CN (0.46) > TPBD-CF₃ (0.45) > TPBD-Cl=TPBD-Br (0.44) > TPBR (0.43) in eV^− 1. The utmost electronegativity value (4.940 eV) is displayed by TPBD-NO₂ compound, which indicates its acceptor nature. Furthermore, the most negative chemical potential (µ) obtained is -4.94 eV obtained for TPBD-NO₂ which confirmed its strong acceptor nature, emphasizing its improved CT capabilities. Reactivity is dependent on two fundamental properties that are inversely proportional: softness (σ) and hardness (η). Molecules with higher σ and lower η values exhibit improved reactivity, decreased stability, and the least energy gap⁴⁹. Moreover, the enhanced polarizability of all the proposed compounds is also demonstrated by their lower hardness and higher softness values⁵⁰. Interestingly, in the studied series of chromophores listed above, TPBD-NO₂ displayed the greatest σ value (0.46 eV^− 1) and the least η as 1.06 eV. The obtained values reveal a consistent relationship between the HOMO-LUMO energy gap and the calculated descriptors, confirming the reliability of FMOs-based reactivity predictions.
UV-Visible spectroscopy is an essential tool to analyze the electronic transitions in a molecule as well as their opto-electronic characteristics for the OSCs^51,52. The wavelength and intensity of absorbed light provide useful information about the electronic structure, conjugation, π–π* and n–π* transitions, and the overall optical properties of the compounds⁵³. This analysis is commonly applied to estimate absorption maxima (λ_max), optical band gap, and light-harvesting ability of organic molecules, which is particularly valuable in designing materials for the optoelectronic and photovoltaic applications^54,55. Herein, the study is performed at the afore-mentioned TD-DFT level in the gaseous and chloroform phases. The CPCM is utilized for analyzing the solvent effects in this study. The chloroform solvent acts as a continuous dielectric medium surrounding the solute molecule. This approach accounts for bulk solvation effects on the electronic excitation energies and provides more realistic absorption spectra compared to gas phase calculations⁵³. The main results are presented in the Table 3, while detailed analysis is represented in the Tables S20-S33. The Fig. 6 displays their absorption spectra obtained in both gas and solvent media. The results show that lower excitation energy values are obtained by substitution of the strong electron accepting moiety in derivatives which elevates their λ_max values⁵⁶.
The above data shows that the TPBD-Cl to TPBD-CF₃ have much higher λ_max than the TPBR molecule. The λ_max of these investigated chromophores are situated in the visible region in chloroform (709.86-754.57 nm) as well as in the gaseous phases (653.20-684.57 nm). The solvent environment has a significant impact on absorption maxima, oscillator strengths, and other key optical characteristics. A red-shift happens when the absorption peak shifts to longer wavelengths, which is caused by polar solvents stabilizing chromophores’ excited states more than nonpolar solvents. Aprotic solvents, on the other hand, cause a blue shift, in which the absorption peak shifts to shorter wavelengths owing to the excited states’ inferior stability. A crucial element in regulating the transition probabilities within a molecule is the interplay between the polarity and viscosity of the solvent, which influences the oscillator strengths (f_os)⁵⁷.
In chloroform solvent, the λ_max values for the investigated compounds (TPBR and TPBD-Cl to TPBD-CF₃) are noted as follows: 709.86, 721.97, 723.74, 754.57, 748.33, and 750.28 and 730.82 nm, respectively. Contrarily, they have lower excitation energy values as 1.74, 1.71, 1.71, 1.64, 1.65, 1.65, and 1.69 eV for TPBR and TPBD-Cl to TPBD-CF₃, respectively. Correspondingly, their oscillation strengths are 2.63, 2.75, 2.78, 2.42, 2.60, 2.63, and 2.61. Among all derivatives, TPBR, TPBD-Cl and TPBD-CF₃ possess maximum H→L transition of 91% as compared to other derivatives. TPBD-CF₃ displays the second highest H→L transitions as 90%. Rest of the derivatives, such as TPBDNO₂, TPBD-SO₃H and TPBD-CN exhibit 84, 88 and 89% of the H→L transitions, respectively. In the case of chloroform solvent, the absorption maxima (λ_max) in solvent phase in nm are observed in following decreasing trend: TPBD-NO₂> TPBD-CN> TPBD-SO₃H> TPBD-CF₃> TPBD-Br> TPBD-Cl> TPBR.
In the gaseous phase, TPBR shows the λ_max = 653.20 nm along with excitation energy (E) of 1.89 eV. The oscillation strength (f_os) of TPBR is 2.40, and the H→L transition is noted as 93%. The derivatives (TPBD-Cl-TPBD-CF₃) show the following λ_max values: 663.05, 664.65, 684.35, 683.52, 684.57, and 669.57 nm, respectively. Contrarily, they exhibit reduced E values of 1.87, 1.86, 1.81, 1.81, 1.81, and 1.85 eV. Furthermore, their respective f_os are indicated as 2.55, 2.59, 2.37, 2.46, 2.48, and 2.42. Derivatives: TPBR, TPBD-Cl, TPBD-Br, TPBD-CN and TPBD-CF₃ exhibit a 93% of H→L contributions, while, TPBD-NO₂ and TPBD-SO₃H exhibit about 92% H→L percentages. The absorption maxima (λ_max) in nm for the gaseous phase among the studied chromophores exhibit the following decreasing trend: TPBD-CN> TPBDNO₂> TPBD-SO₃H> TPBD-CF₃> TPBD-Br> TPBD-Cl> TPBR. Hence, data shows that maximum λ_max is obtained for TPBD-NO₂ in both phases along with the least transition energy as compared to other compounds. The red-shifted absorption in TPBD-NO₂ is in correspondence with reduced energy gap as shown in its FMOs study. This might be due to the synergistic electronic effects of –NO₂ group in its acceptor unit. As, the nitro group exhibits both strong − I effect and resonance (− M) electron-withdrawing behavior, it causes an efficient π-conjugation and charge delocalization in the molecule. Its high electron affinity and low-lying π*-orbitals significantly stabilize the LUMO and increase the ICT³⁶. In contrast, limited resonance is shown by –CN groups, while –CF₃ acts mainly through an inductive effect with negligible conjugation. Thus, –NO₂ more effectively lowers the HOMO–LUMO gap and promotes bathochromic absorption shifts.
To further assess the robustness of the TDDFT simulation results of UV-Visible analysis, the studied compounds are additionally evaluated using the CAM-B3LYP, M06-2X and ωB97XD functionals. This is done to check the validity of the selected level of theory i.e., M06/6-311G(d, p). The calculated results are depicted in the Tables S34-S36 which indicate almost similar results with TPBD-NO₂ and TPBD-CN revealing the highest λ_max and least excitation energies with only minor variations (≤ 30 nm). While, the nature of the dominant transitions and charge-transfer characteristics remained unchanged, confirming the reliability of the chosen methodology. Moreover, these results are in close correspondence with its FMOs and GRPs data as they collectively state these compounds are the most suitable candidates for solar cells.
UV-Visible spectra of the investigated compounds in gaseous and solvent phases.
The mobility and transition of an electronic charge density in an organic system is observed using their transition density matrix plots. It is a three-dimensional representation of electron-hole pair distribution and delocalization which help to clarify the location of excited electrons, holes, and electrons within the organic solar cells⁵⁸. Identifying the improved exciton dissociation in the excited states of molecules is most easily accomplished with the TDM heat maps, which is essential for the development of solar cells⁵⁹. Owing to least contribution of hydrogen atoms, they overlooked in this analysis. All the designed derivatives are A–π–A configuration-based structures with two key components: (i) the π-spacer and (iii) the acceptor. As depicted in the Fig. 7, the local excitations (LE) are indicated by bright diagonal regions, where the electron and hole remain on the same molecular fragment. In contrast, off-diagonal intensity is represented by the charge-transfer (CT) excitations, which are particularly present between π-spacer and A regions. This reflects sufficient electron migration from the donor (π-conjugated core) to the acceptor terminal units.
Herein, the TDM heat maps confirm that the designed compounds exhibit strong intramolecular charge transfer characteristics, because the excitation is not localized at one site. Rather, it involves electron transition between donor and acceptor fragments of the molecule. In these molecules, the bright regions are located between the π-spacer and acceptor segments, confirming donor → acceptor charge transfer. All compounds have strong electrical charge coherence, according to the transition density maps. Overall, this behavior is desirable for organic solar cell chromophores and supports their photovoltaic efficiency.
TDM maps of the entitled compounds.
Hole-electron analysis refers to the migration of an excited electron from the hole area to the electron. In case of the studied OSCs, charge transfer (CT) happens at the interface of the π-spacer and acceptor which is necessary for the separation of charges⁶⁰. This study is frequently used to determine the location of electron density within a chemical compound⁶¹. It is a practical method for identifying the characteristics of charge transfer and electron excitations⁶². Figure 8 shows that the reference compound (TPBR) has the highest electron intensity located at C35 atom, and maximum hole intensity at the C24 atom. The highest electron intensity in the TPBD-Cl is found at C43 atom, while the highest hole intensity is found at C24 atom. The highest electron intensity of TPBD-Br is found at C43, while the highest hole intensity is found at C24 atom. C24 atom shows the maximum hole intensity in the TPBDNO₂, whereas the electron intensity is the highest at C37 atom. The highest electron intensities in the TPBD-SO₃H are found at C37 atom, while major hole intensity found at C24 atom. The electron intensity in the TPBD-CN peaks at C35, O49, and C52 atoms and maximum hole intensity at C24 atom. The TPBD-CF₃ exhibits a maximum electron intensity at C35, S49, and C52 atoms and a maximum hole intensity at C33 atom.
Overall, the hole electron analysis clearly demonstrates that electron density is mainly localized on specific carbon atoms depending on the molecular structure and substituent pattern. In all designed derivatives, the electron and hole distributions are well separated, indicating an efficient charge transfer character. In particular, most molecules show maximum hole accumulation at the C24 atom, while the positions of maximum electron density vary among different substituent types. This spatial separation of hole and electron densities supports the effective ICT behavior, which is a desirable feature for enhancing exciton dissociation and improving photovoltaic performance for the organic solar cell applications.
Hole electron analysis representation for the studied compounds.
One important factor affecting a compound’s optoelectronic characteristics and charge separation efficiency is the exciton binding energy (E_b)⁶³. It is inversely proportional to charge mobility and exciton dissociation, and directly related to the energy gap and optimization energy values⁶⁴. Greater exciton dissociation in the compound is indicated by a lower E_b, while decreased dissociation is reflected by a higher E_b⁶⁵. As seen in Eq. (1), the binding energy can be computed by subtracting the energy gap (E_gap) from the optimization energy⁶⁶.
In this case, E_H−L stands for HOMO-LUMO energy gap, E_b for binding energy, and E_opt for the first excitation energy⁶⁷.
According to the data mentioned in the Table 4, TPBD-SO₃H has the lowest exciton binding energy (0.49 eV) as compared to other derivatives. While, TPBD-NO₂ has the second least value for E_b as 0.50 eV which corresponds with its structural properties and charge transfer ability. Moreover, TPBD-NO₂ is the compound obtained with the least energy gap (2.13 eV) and highest λ_max (754.57 nm) due to its strong electron-withdrawing and high charge transfer nature, TPBD-SO₃H shows a lower exciton binding energy. This can be attributed to the highly polar nature of –SO₃H group, which enhances dielectric screening and reduces electron-hole Coulombic attraction. Overall, the examined chromophores in relation to their E_b values are arranged as follows: TPBD-Br> TPBR> TPBD-Cl> TPBD-CF₃> TPBD-CN> TPBD-NO₂> TPBD-SO₃H. As a result, it is clear that TPBD-NO₂ has possessed a higher degree of dissociation into free electrons and more photo-electronic properties, making it an effective material for the OSCs⁶³.
Another parameter that aids in assessing the degree of charge transfer between the donor and acceptor sites is molecular electrostatic potential⁶⁸. It is a three-dimensional depiction of the charge density at different points in the molecules under study. The MEP, which corresponds with a molecule’s reactive potential, highlights its electrophilic and nucleophilic centers⁶⁹. In this case, colors like green, blue, and yellow are utilized to show where different charges (positive, negative, or neutral) are placed over a molecule. The sequence of potential reductions is blue > yellow > dark blue > red. The blue color of the molecule indicates a region of positive charge, primarily over the central core. Figure 9 shows that the blue color of π-spacers in all the studied compounds (TPBR and TPBD-Cl – TPBD-CF₃) showing a nucleophilic propensity, whereas, the dark blue color near hydrogen atoms connected to aromatic carbon groups indicates the strongest positive potential. On the other hand, substituent groups exhibit the yellow hue over heteroatoms and have a somewhat negative potential (electrophilic). These results indicate that every molecule under study has substantial electrostatic potential.
ESP plots for the studied chromophores.
To understand the improved optoelectronic properties in the studied chromophores, their charge transfer analysis is performed⁷⁰. As know from previous studies, the molecular orbital distributions effectively show an electronic interaction among the donor and acceptor components⁷¹. This involves the formation of donor–acceptor complex to reveal the intermolecular charge transfer characteristics. The FMOs investigation shows that the derivative (TPBD-NO₂) exhibits the least energy gap as 2.13 eV. Similarly, the UV-Visible absorption wavelength is also maximum for TPBD-NO₂ as 754.57 nm. So, it is considered as the most suitable candidate for complex formation owning to its good charge transfer nature and promising optoelectronic properties. Among various donor, the PBDB-T is utilized in this study owing to its favorable properties. In order to explore the behavior of intermolecular charge transfer, the PBDB-T donor polymer and the TPBD-NO₂ acceptor are coupled in this case to form a donor–acceptor complex. As shown in Fig. 10, the in HOMO, the charge cloud is mostly centered on the PBDB-T, signifying it as the principal electron donor. While, the TPBD-NO₂ has the highest concentration in LUMO, indicating that it effectively accepts the electrons. Moreover, the electronic excitation and charge transfer from the donor towards the acceptor molecule are shown by the arrow pointing from HOMO to LUMO. Thus, this investigation shows good charge separation between them which essential in their optoelectronic functionality.
Charge transfer analysis of PBDB-T: TPBD-NO₂ donor: acceptor complex.
Open circuit voltage (V_oc) is important measure to demonstrate the performance of an organic solar cell material⁷². It accounts for the maximum amount of current obtained from any optical substance at zero voltage conditions⁷³. Presence of charge carriers, light source, external fluorescence, electrode function, OSC’s temperature, light intensity, and a variety of environmental conditions can influence the value of V_oc. The energy difference between the HOMO and LUMO of donor and acceptor molecules (HOMO_donor−LUMO_acceptor) is directly related to their open circuit voltages⁷⁴. The V_oc values in the current study are calculated using the well-known donor (PBDB-T) with a E_HOMO of -5.401 eV obtained from its optimization at the afore-mentioned level of theory. In theory, the V_oc of the OSCs is calculated using the Eq. (2), as described by Scharber and colleagues⁷⁵.
The Table S35 shows the energy difference for the HOMO_{PBDB T}−LUMO_acceptor for TPBR and TPBD-Cl to TPBD-CF₃ as 1.89, 1.83, 1.83, 1.53, 1.55, 1.57 and 1.70 eV, respectively. Similarly, their V_oc values are obtained as follows: 1.59, 1.53, 1.53, 1.23, 1.25, 1.27 and 1.40 V, respectively. The TPBR compound shows the utmost V_oc value of 1.59 V as compared to the designed derivatives. For these titled compounds, the V_oc findings are decreasing in the following order: TPBR> TPBD-Br> TPBD-Cl> TPBD-CF₃> TPBD-CN> TPBD-SO₃H> TPBD-NO₂. As mentioned before, the difference for the HOMO_{PBDB T}−LUMO_acceptor determines the V_oc value. Better optoelectronic properties and a higher V_oc value are the outcomes of lower acceptor origin LUMO. A low-lying LUMO of the acceptor increases the migration of electrons from the HOMO of D molecules, improving the photovoltaic characteristics. The orbital energy diagram of the aforementioned chromophores with respect to PBDB-T donor polymer is displayed in the Fig. 11 which shows that the donor polymer has a greater LUMO level than the designed acceptor chromophores. It should be emphasized that the molecular-level parameters presented here provide upper-limit estimates, since the actual device performance (such as V_oc) might be influenced by the morphology and solid-state packing. Nevertheless, these findings suggest that the proposed organic molecules possess promising characteristics and can be considered as strong potential candidates for future OSC’s applications.
Graphical representation of V_oc for the investigated chromophores with respect to PBDB-T.
In summary, a series of NFA-based organic chromophores (TPBR and TPBD-Cl to TPBD-CF₃) are designed using the end-capped structural modeling with malononitrile-based acceptors. The impact of these acceptors on the optoelectronic and photovoltaic properties is explored using the quantum chemical approach. A thorough examination of frontier molecular orbitals, optical characteristics, and photovoltaic factors reveals that TPBD-NO₂ is the most promising candidate for use in OSCs. It exhibits the smallest HOMO-LUMO energy gap (2.13 eV), red-shifted λ_max values in both the solvent (754.57 nm) and gas (684.35 nm) phases. Moreover, the DOS and TDM visual analyses further validated the above-discussed findings. Further, the designed compounds are blended with the PBDB-T donor to examine their photovoltaic features. Notably, all the designed systems exhibited favorable values of V_oc, which inferred that these chromophores can be regarded as the promising candidates for high-performance photovoltaic materials.
All data generated or analyzed during this study are included in this published article and its supplementary information files.
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This Project was funded by the Deanship of Scientific Research (DSR) at King Abdulaziz University, Jeddah, Saudi Arabia under grant no. (IPP:455-961-2025). The authors, therefore, acknowledge with thanks DSR for technical and financial support.
Institute of Chemistry, Khwaja Fareed University of Engineering & Information Technology, Rahim Yar Khan, 64200, Pakistan
Mashal Khan, Fatima Sarwar, Khansa Gull, Memoona Arshad, Iqra Shafiq & Rifat Jawaria
Center of Excellence for Advanced Materials Research, Faculty of Science, King Abdulaziz University, PO Box 80203, Jeddah, 21589, Saudi Arabia
Muhammad Nadeem Arshad & Khalid A. Alzahrani
Chemistry Department, Faculty of Science, King Abdulaziz University, PO Box 80203, Jeddah, 21589, Saudi Arabia
Khalid A. Alzahrani
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Mashal Khan: Investigation; Formal analysis; Writing – original draft; Visualization.Fatima Sarwar: Formal analysis; Data curation; Validation.Khansa Gull: Literature review; Methodology; Validation.Memoona Arshad: Visualization; Figure preparation; Data curation.Iqra Shafiq: Investigation; Formal analysis; Writing – original draft; Visualization, supervision Muhammad Nadeem Arshad: Methodology; Software; Resources; Technical support.Khalid A. Alzahrani: Resources; Project administration; Supervision; Funding acquisition.Rifat Jawaria: Conceptualization; Supervision; Validation; Writing – review & editing.
Correspondence to Iqra Shafiq or Rifat Jawaria.
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Khan, M., Sarwar, F., Gull, K. et al. DFT insights into the photovoltaic performance of A–π–A non-fullerene acceptors for organic solar cells. Sci Rep 16, 9842 (2026). https://doi.org/10.1038/s41598-026-40331-x
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DOI: https://doi.org/10.1038/s41598-026-40331-x
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