Unveiling peripheral symmetric acceptors coupling with tetrathienylbenzene core to promote electron transfer dynamics in organic photovoltaics

Non-fullerene organic compounds are promising materials for advanced photovoltaic devices. The photovoltaic and electronic properties of the derivatives (TTBR and TTB1-TTB6) were determined by employing density functional theory (DFT) and time-dependent density functional theory (TD-DFT) analyses using the M06/6-311G(d,p) functional. To enhance the effectiveness of fullerene-free organic photovoltaic cells, modifications were applied to end-capped acceptors by using strong electron-withdrawing moieties. The structural tailoring showed a significant electronic impact for HOMO and LUMO for all chromophores, resulting in decreased band gaps (3.184–2.540 eV). Interestingly, all the designed derivatives exhibited broader absorption spectra in the range of 486.365–605.895 nm in dichloromethane solvent. Among all derivatives, TTB5 was observed to be the promising candidate because of its lowest energy gap (2.54 eV) and binding energy (0.494 eV) values, along with the bathochromic shift (605.895 nm). These chromophores having an A–π–A framework might be considered promising materials for efficient organic cells.


Computational procedure
The chromophores (TTBR and TTB1-TTB6) were subjected to theoretical calculations using the Gaussian 09 software 22 .To perform the DFT\TD-DFT analyses, their structures were optimized at the Minnesota 06 exchangecorrelational functional (M06) 23 and 6-311G(d,p) basis set 24 .The optoelectronic and photovoltaic properties which included UV-Visible, FMOs, DOS, V oc , electron-hole and TDM analyses were determined for the studied compounds at the afore-mentioned functional.Moreover, the HOMO-LUMO band gaps were utilized to estimate their GRPs.TD-DFT analysis was used to analyze the excited state property such as UV-Visible spectra and FMOs.The absorption spectra of studied chromophores were predicted in dichloromethane solvent.To interpret the data from the input files, software programs such as Avogadro 25 , Origin Pro 8.5 26 , Chemcraft 27 , GausSum 28 , Multiwfn 3.7 29 , Gauss View 6.0 30 and PyMOlyze 2.0 31 were utilized.
Various quantum chemical investigations of TTB-based organic chromophores are performed which showed the lower band gaps and significant bathochromic shifts.Moreover, their photovoltaic properties are remarkable and they have sufficient ICT.Thus, the present approach is highly favorable in developing unique organic solar cells (OSCs).www.nature.com/scientificreports/

Electronic analysis
Analysis of FMOs requires how well a molecule promotes charge mobility and electron density distribution 34 .FMOs distribution patterns are helpful in describing the optoelectronic characteristics of the studied compounds (TTBR and TTB1-TTB6).It also aids in understanding how charge transmission occurs in the OSCs.According to the molecular orbital theory (MOT), the highest occupied molecular orbital (HOMO) is classified as a valence band, whereas the lowest unoccupied molecular orbital (LUMO) is classified as a conduction band.As a result of excitation, electrons move from the valence band (HOMO) to the conduction band (LUMO) 35 .The HOMO/LUMO orbitals are commonly termed as FMOs 36 .The difference between these orbitals is referred to as the band gap (E gap = E LUMO -E HOMO ) 37 enlisted in the Table S9.
Azeem et al. reported the reference molecule SBDT-BDD has a band gap of 4.49 eV 40 .This band gap is much higher than the current band gap which is in the range of 3.184-2.54and showed the best photovoltaic properties.The above discussion concluded that TTB5 exhibits the narrowest ΔE among all designed chromophores (TTBR and TTB1-TTB6) and is the best candidate for photovoltaic OSCs.The majority of the electronic cloud is centered on the π-spacer in HOMO, whereas in the LUMO, it is mostly focused on the over-terminal acceptors and to a lesser extent on the π-spacer.This shows that there is significant facilitation of charge transfer from the π-spacer to the acceptors in all the entitled compounds.

Global reactivity parameters (GRPs)
The global reactivity descriptors, including ionization potential (IP) 41 , electron affinity (EA), global hardness (η) 42 , chemical potential (μ) 43 , electronegativity (X) 44 , global softness (σ) 45 and electrophilicity index (ω) 46 are calculated using the Koopman's theorem 47 listed in Eqs.S1-S5 for TTBR and TTB1-TTB6.The energy of HOMO determines the IP.Similarly, the EA reflects the capacity to accept an electron from a donor which is determined by the energy of LUMO.IP and EA are calculated through Eqs.(1) and ( 2).
The capability of the compound to absorb additional electrical charge from its surroundings is represented by ΔN max 48 and it is computed via Eq.(3).

Optical analysis
The absorption profile of the organic chromophores employed for organic photovoltaic cells and is an important parameter for determining the efficiency of solar cells 50 .The compounds (TTBR and TTB1-TTB6) are analyzed by using the TD-DFT computations to investigate their UV-Visible characteristics in dichloromethane solvent.The UV-Visible spectral analysis provides valuable information about electronic transitions in all the investigated molecules and also determines the charge transfer rate 51 .The investigations are conducted to determine the absorption wavelength (λ max ), oscillation strength (f os ), energy of excitation (E) and the molecular orbital exhibit a red-shift and lower excitation energies than TTBR.Moreover, the literature study shows that molecules with lower transition energy values have higher charge transport ability and may easily undergo excitation between the HOMO and LUMO, resulting in high PCE 32 .A comparative analysis of data in Table S12 indicates that all the compounds exhibit a bathochromic shift in dichloromethane solvent (486.365-605.895nm) as compared to TTBR reference (506.451nm).The λ max values of TTB1-TTB6 are 490.774,486.365, 493.509, 526.584, 605.895 and 553.205 nm, respectively.The increasing order of λ max in the studied chromophores is: TT B2 < TTB1 < TTB3 < TTBR < TTB4 < TTB6 < TTB5.This shift towards longer wavelengths is due to solvent effects.The outcomes demonstrate the greater λ max values of the designed derivatives are due to strong electronwithdrawing units at their terminal moiety which extended the conjugation.Interestingly, a broader absorption value is obtained for TTB5 (605.895nm) relative to other designed chromophores.
The optical properties of compounds are highly affected by internal morphology.Increased crystallinity and optimal molecular arrangement facilitate an increase in conjugation and enhance the efficient overlap of orbitals, resulting in a reduction of the energy difference between the HOMO and LUMO 52 .This promotes effective movement of charges and minimizes losses due to recombination, while internal molecular structures and strong π-π interactions in highly crystalline regions cause a shift towards longer wavelengths (red-shift) in the absorption spectra.This enhances the efficiency of light absorption over a wider range of wavelengths by increasing the electron's mobility and extending conjugation 53 .
The UV-Vis absorption spectra of the studied compounds are depicted in dichloromethane phases shown in the Fig. 3.The higher values of λ max (605.895nm), the lower excitation energy (E) 2.046 eV and oscillator strength (f os ) 0.916, with 90% MO contribution from HOMO to LUMO of TTB5 in dichloromethane is due to the participation of -NO 2 group at the terminal regions of the compound.The electron-withdrawing group (-NO 2 ), effectively pulls the electrons from the π-spacer toward the terminals.As the electron-withdrawing effects of end-capped acceptor groups increase, there is an increase in the λ max value, leads to a reduction in HOMO-LUMO band gap.This phenomenon facilitates the charge transfer pathway 54 .From the literature, Azeem et al. reported the optical properties of small molecules with benzodithiophene as a core and the results showed λ max 543 nm.While the current study shows λ max of 605.895 nm which demonstrated the best optical properties of the designed chromophore.
The transition energy (E) demonstrates an inverse correlation with the rate of charge transfer and λ max 55 .The E values of TTB1-TTB6 are 2.526, 2.549, 2.512, 2.355, 2.046 and 2.241 eV, respectively, whereas the value of TTBR is 2.448 eV in the solvent phase listed in the Table S11.The decreasing order of E for TTBR-TTB6 is as follows: TTB2 > TTB1 > TTB3 > TTBR > TTB4 > TTB6 > TTB5 in eV.
Concluding the entire discussion, compounds exhibiting a red shift possess lower energy gaps and increased charge transfer rates, suggesting their excellent photovoltaic response.Thus, they can be utilized as proficient OSCs.

Density of states (DOS) analysis
Density of states (DOS) is an efficient technique for determining the distribution pattern of electronic density on the FMOs 50 .It is an essential study which aids in investigating the activities of each fragment in the designed (TTBR and TTB1-TTB6) chromophores i.e. acceptor and π − spacer 54 .It graphically displays the relative intensity on the y-axis and energy over the x-axis.The green colored line shows the π-spacer and the red lines display the acceptor.The x-axis displays the HOMO (valence band) on the right side and the LUMO (conduction band) on the left side in the Fig.   S13.A variety of electron-withdrawing groups are responsible for the different electrical charge distribution patterns in the DOS analysis.The HOMO in these compounds has a reduced electron density on the acceptor and a higher electron density at the π-spacer regions.Contrarily, the LUMO displays a lower electron density on the π-spacer and a larger density on the terminal acceptors.The DOS analysis effectively demonstrates significant charge transfer from the π − bridge towards the peripheral acceptor moieties in all the investigated compounds.Therefore, it is concluded from the DOS plots that end-capped acceptor alteration is a successful method for designing non-fullerene acceptor molecules with excellent optoelectronic and photovoltaic capabilities 1 .

Transition density matrix (TDM) analysis
Transition density matrix (TDM) analysis plays a key role in the charge transfer and various transitions present within a molecule.It also assists in comprehending some of the characteristics of the neutral state (S 0 ) and the excited state (S 1 ) transitions.The impact of hydrogen atoms is neglected in all compounds since they have only a little contribution toward the excited state transitions 56 .TDM analysis facilitates the assessment of (i) electronic excitation (ii) localization of the electron holes (iii) interactions between the π-spacer and acceptor moieties in the excited state 57 .For TDM analysis, the designed compounds (TTBR and TTB1-TTB6) are partitioned into two segments such as the π-spacer (yellow line) and acceptors (red line) as shown in Fig. 5 58 .As per TDM maps, all molecules exhibit coherence in charge distribution.The number of atoms is shown on the x-axis and left y-axis in TDM maps, while, the relative intensity is represented by the right vertical axis.Extended conjugation enhances the charge transfer facilitated via the π-bridge.Subsequently, charge transfer occurs smoothly from one end to another without hindrance.Results show that in all derivatives, the electrical charges are effectively transferred diagonally from the π-spacer to the acceptor components.In compound (TTB5), the charge-shift is most noticeable.This phenomenon could arise due to the potent electron-withdrawing nitro (-NO 2 ) groups at the terminal side acceptors.Thus, incorporating end-capped electron-withdrawing acceptors in the newly designed organic chromophores improves the electron transport from the π-spacer to the acceptor regions, making them efficient for OSC applications.

Exciton binding energy (E b )
Binding energy (E b ) is another factor for studying the optoelectronic properties and excitation dissociation potential in the OSCs.It is known as the Columbic interactions between negative and positive charges 56 .The binding energy can be determined by subtracting the minimum energy required for the first excitation (E opt ) from the energy gap between the HOMO-LUMO 59 .Therefore, a lower value of exciton binding energy is correlated with a weaker electron-hole interaction and an increase in charge transfer 57 .Lower values of E b provide high current charge density (J sc ), high charge dissociation and high power conversion efficiency 60 .The Eq. ( 4) is used to calculate the binding energy 61 .
Here, the E b represents binding energy, E gap is the energy gap and E opt corresponds to the single-point energy.The results obtained are listed in Table 2.The highest value of E b is 0.638 eV in the case of TTB1.However, the lowest value is shown by TTB5 (0.494 eV), indicating the highest levels of charge dissociation and rate of (4) www.nature.com/scientificreports/

Hole-electron analysis
When an excited electron in the hole region migrates to the electron region, this phenomenon is known as hole-electron analysis.In OSCs, the charge transfer (CT) state occurs at the interface between π-spacer and electron acceptor.This CT state is essential for the separation of charges 62 .This analysis is highly effective and widely utilized for identifying the localization of electron density within a compound 63 .It is a feasible approach for revealing the nature of electron excitations and charge transfer 64 .The designed chromophores possess tetra thienyl benzene as the π-spacer and electron-withdrawing acceptors at terminals.Moreover, it is observed that in the reference (TTBR) and designed compounds (TTB2-TTB4), the hole arises from a carbon atom within the thiophene ring (π-spacer) and the electron density is maximum in the electron band at the carbon atom in between the terminal acceptor groups and π-spacer.But in TTB1, the hole band has medium density and in the electron band, maximum intensity is same as mentioned for the above designed chromophores.Notably, all pictographs demonstrate that the hole emerges within the π-linker segment at various atoms, while the electron intensity reaches its peak at the carbon atom of the π-linker and acceptor as demonstrated in the Fig. 6.Moreover, in TTB5 and TTB6, the electron intensity is high in the hole band at C-24.The results indicate that TTBR-TTB4 are observed as electron-type material, whereas TTB5-TTB6 compounds demonstrate themselves as hole-type materials.

Conclusion
In summary, a series of A-π-A non-fullerene acceptor molecules (TTBR and TTB1-TTB7) are designed to explore their photovoltaic properties for OSC applications.Quantum chemical analysis is performed to analyze their photovoltaic, photophysical and electronic characteristics.All designed chromophores have demonstrated favorable outcomes for various computational analyses.Among all the designed compounds, TTB5 displayed the least band gap value of 2.540 eV and the bathochromic shift (λ max = 605.895nm), highest softness (0.393 eV −1 ) and lowest hardness (1.27 eV).The decreasing trend of X is TTB5 > TTB6 > TTB4 > TTB3 > TTB2 > TTB1 > TTBR.The E b of TTBR-TTB6 is comparable among them, leading to higher exciton dissociation and lower binding energy (E b = 0.494-0.638eV).The results from the analysis of all geometrical parameters demonstrate that modifications of end-capped acceptors represent highly efficient photovoltaic materials with superior optoelectronic properties.In short, it is clear that all afore-mentioned compounds are obtained by using structural modifications and they show potential for OSCs.

Fig. 6 .
Fig. 6.Pictorial illustration of the hole-electron analysis for the titled chromophores.