Exploration of the synergistic effect of chrysene-based core and benzothiophene acceptors on photovoltaic properties of organic solar cells

To improve the efficacy of organic solar cells (OSCs), novel small acceptor molecules (CTD1–CTD7) were designed by modification at the terminal acceptors of reference compound CTR. The optoelectronic properties of the investigated compounds (CTD1–CTD7) were accomplished by employing density functional theory (DFT) in combination with time-dependent density functional theory (TD-DFT). The M06 functional along with a 6-311G(d,p) basis set was utilized for calculating various parameters such as: frontier molecular orbitals (FMO), absorption maxima (λmax), binding energy (Eb), transition density matrix (TDM), density of states (DOS), and open circuit voltage (Voc) of entitled chromophores. A red shift in the absorption spectra of all designed chromophores (CTD1–CTD7) was observed as compared to CTR, accompanied by low excitation energy. Particularly, CTD4 was characterized by the highest λmax value of 685.791 nm and the lowest transition energy value of 1.801 eV which might be ascribed to the robust electron-withdrawing end-capped acceptor group. The observed reduced binding energy (Eb) was linked to an elevated rate of exciton dissociation and substantial charge transfer from central core in HOMO towards terminal acceptors in LUMO. These results were further supported by the outcomes from TDM and DOS analyses. Among all entitled chromophores, CTD4 exhibited bathochromic shift (685.791 nm), minimum HOMO/LUMO band gap of 2.347 eV with greater CT. Thus, it can be concluded that by employing molecular engineering with efficient acceptor moieties, the efficiency of photovoltaic materials could be improved.

non-fullerene compounds.This modification involves altering the side chains, end-capped acceptor units, and the core molecule 7,8 .Functionalizing the end groups (EG) of central donor core with non-fullerene acceptors (NFA) demonstrated to be an effective approach for boosting the performance of organic solar cells (OSCs) [7][8][9] .This, in turn, plays a substantial role in shaping the power conversion efficiency (PCE) and various other performance metrics of NFA-based OSCs (NF-OSCs) 9 .The A-π-A architecture, consisting of central electron-donating π-spacer unit combined with two electron-deficient end-capped acceptor groups on both sides is considered as an effective approach in NF-OSCs to improve intramolecular charge transfer (ICT) 10 .A novel electron-donating core, formed by condensing chrysene with two thiophenes via two dihydrobenzene rings, was synthesized by Lu and his coworkers.Utilizing this core along with two electron-accepting end groups of 1,1-dicyanomethylene-3-indanone, a new Z-shaped fused-ring electron acceptor was developed and synthesized.This chrysene-based compound exhibited strong absorption within the 500-850 nm range, a bandgap measuring 1.50 eV, and a charge mobility of 2.5 × 10 −4 cm 2 V −1 s −1 11 .In another report, chrysene based compound with PCE value upto 24.7% on blending with poly (3-hexylthiophene) (P 3 HT) and phenyl-C61-butyric acid methyl ester (PCBM) polymers was stated 12 .Moreover, in a previous study, a chrysene-core based derivative with V oc value of 1.20 V has been reported 13 .
Encouraged by findings, we developed A-π-A type seven novel chrysene-based, non-fullerene, fused-ring electron acceptor materials (CTD1-CTD7).The designing of reference compound CTR has been carried out by the insertion of highly efficient benzothiophene end-capped acceptor at the peripherals of a recently synthesized parent molecule CTIC-4F 24 .The DFT study of compounds containing chrysene core fused with benzodithiophene terminal acceptors has not been reported yet.Introducing the electron-proficient benzothiophene unit into the end-group raises the energy levels of the lowest unoccupied molecular orbitals (LUMO) in non-fullerene acceptors (NFAs), resulting in increased V oc outputs.Moreover, it encourages antiparallel π-π stacking of the end-groups, thereby facilitating efficient electron transport 14 .The reference compound was further fabricated to design CTD1-CTD7 derivatives by introducing various electrophilic groups in to the benzothiophene acceptors.The newly designed molecules CTD1-CTD7 possess unique characteristics attributable to the presence of diverse groups and atoms in their terminal acceptor regions.Hence, employing various electrophilic groups leads to the formation of more efficient OSC molecules from chrysene core.
To assess the photovoltaic and optoelectronic properties of these materials, DFT and TD-DFT calculations have been employed.Analyses including frontier molecular orbital (FMO), density of states (DOS), transition density matrix (TDM), binding energy (E b ), and open circuit voltage (V oc ) have been conducted to elucidate the electronic, photophysical, photovoltaic, and charge transfer characteristics of the reference molecule CTR and its designed compounds (CTD1-CTD7).Noticeably, all the designed derivatives showed good V oc values, revealing their potential for paramount current generation capacity.Thus, it is highly anticipated that these newly designed compounds (CTD1-CTD7) will play a remarkable role in advancing the development of highly efficient organic solar cell (OSC) materials.

Computational procedure
The computational investigation involving density functional theory (DFT) and time-dependent density functional theory (TD-DFT) were carried out using the Gaussian 09 15 software package, with visualization of results facilitated by GaussView 6.0 16 .The M06 17 coupled with the 6-311G(d,p) basis set was employed to perform all analyses.At first geometrical optimization was accomplished to get true minima structures.By utilizing these optimized structures, various photovoltaic and optoelectronic parameters such as frontier molecular orbitals (FMOs), transition density matrix (TDM), global reactivity descriptors (GRDs), density of state graphs (DOS), binding energy (E b ), and open circuit voltage (V oc ) were computed at the entitled functional.Multiple software like Avogadro 18 , Multiwfn 19 , PyMOlyze 20 , GaussSum 21 , Origin 22 and Chemcraft 23 were used to interpret data in tabular and and different graphs (Fig. 1).
The peripheral electron accepting units of CTR were systematically substituted with diverse electron-deficient groups with the objective of formulating efficient non-fullerene OSCs.Through the substitution of terminal groups like -F, -Cl, -CN, -NO2, -CF3, -SO3H, and -CH3COO, seven distinct derivatives CTD1, CTD2, CTD3, CTD4, CTD5, CTD6 and CTD7 were designed, respectively (Fig. 3).The IUPAC names and 2D-structures of these derivatives are arranged in Table S21, while the optimized structures of designed derivatives are presented in Fig. 1.The Cartesian coordinates of investigated compounds are portrayed in Tables S22-S29.
The primary objective of this work is to design NFAs with enhanced photovoltaic properties.In this study, our focus is on designing new NFAs that exhibit significantly improved photovoltaic characteristics, including a narrow optical bandgap, low excitation energy and increased absorption.The theoretically simulated parameters aim to facilitate the development of molecules with superior properties for solar cell applications.

Electronic studies
Investigating the FMOs provides valuable insights into a molecule's reactivity, helping establish the active site through orbital distribution 25 .The determination of effective charge transfer across donor and acceptor components in photovoltaic devices heavily relies on assessing the energy levels of the highest occupied molecular www.nature.com/scientificreports/orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) [26][27][28][29] .Analyzing the HOMO-LUMO relationship is essential for exploring quantum chemical properties.This study aids in predicting reactive sites within π-electron systems and offers explanations for various reactions within conjugated frameworks 30 .The energy band gap in frontier molecular orbitals (FMOs) plays a crucial role in molecular modeling, influencing dynamic structural stability, charge generation, electrical conductivity, chemical reactivity, short-circuit current (J sc ), power conversion efficiency (PCE) and open-circuit voltage (V oc ) 31,32 .A smaller band gap (E g ) in FMOs facilitates the flow of charge from donor's HOMO and acceptor's LUMO, facilitating current generation 33 .FMO     4 portrays surface diagrams of the FMOs, showcasing the spreading of electronic density clouds across molecules.For all the designed compounds (except CTD4), there is a significant concentration of charge density in the central part in HOMO, with a minor presence of electronic cloud observed over the end-capped acceptor groups in LUMO.For CTD4, the HOMO density is predominantly intense on the π-bridge, but the LUMO density is highly concentrated in the nitro group among electron-deficient end-capped groups.Consequently, compounds under analysis exhibited ICT from one terminal acceptor to the other across the π-core.HOMO − 1/ LUMO + 1 and HOMO − 2/LUMO + 2 energies are detailed in Tables S1 and S2 respectively, and their corresponding FMO diagrams are presented in Fig. S1.Similar phenomena in terms of energies and charge transfer are observed among above level orbitals (HOMO − 1/LUMO + 1 and HOMO − 2/LUMO + 2).The GRDs of entitled chromophores are tabulated Table S3.

Optical properties
Optical properties are essential for evaluating overall operational capabilities of organic solar cells (OSCs) 38,39 .The UV-Vis spectra of all the investigated molecules have been thoroughly examined using TD-DFT/M06/6-311G(d,p).Although, UV-Vis data were computed for six excited states (as presented in Tables S5-S20), only data pertaining to the first excited state are shown in Table 2.This selection is justified as the first excited state contains maximum values of wavelength of absorption (λ max ), oscillator strength (f os ), first excitation energy (E), and the highest percentage of intramolecular charge transfer (ICT).
UV-Vis spectra for both the solution and gas phases are illustrated in Fig. 6.In the gaseous phase, the reference CTR and designed molecules CTD1-CTD7 exhibited a red shift in absorbance.But the values of maximum absorption in the chloroform solvent are shifted more bathochromically as a consequence of solvent effect.The optical properties of a compound are also substantially influenced by its internal morphology.The interaction of π-π conjugation between the rings of end groups is crucial in determining intermolecular electronic couplings and facilitating charge transport 40,41 .Moreover, as the electron-pulling capacity of the end groups increases, the intramolecular charge transfer (ICT) between the core and end groups strengthens, resulting in a red-shift in the absorption 42 .The CTR exhibited the λ max of 651.109 nm by showing oscillator strength and excitation energy and at 2.005 and 1.904 eV, respectively.Increase in absorption wavelength is observed as CTD1 (657.497nm), CTD2 (662.061nm), CTD7 (663.301nm) and CTD5 (666.295nm).This increase in wavelength can be attributed to the robust electrophilic nature of their end-capped acceptors.Moreover, a bathochromic shift is also observed in compounds CTD3 (678.510nm) and CTD6 (680.223nm).Interestingly, CTD4 showed the maximum results   www.nature.com/scientificreports/Greater the first molecular orbital (MO) contributions, higher the absorbance wavelength of compounds.The results clearly indicate that excitations originating from the first excited states, specifically HOMO → LUMO transitions, predominantly contribute (between 93 and 97%) to the absorption maxima observed in the designed compounds, regardless of whether they are in solvent or gas phases.It is noteworthy that molecules in which H → L transitions highly contribute tend to have higher λ max values.Therefore, CTD3, CTD4, and CTD6 holding the highest H → L contributions of 95% in solvent and 97% in gas, which account for the red-shift observed in the absorption spectra of these compounds.

Transition density matrix (TDM) and exciton binding energy (E b )
The TDMs of designed compounds has also been examined using Multiwfn, as shown in Fig. 7.The transition density matrix provides insights into the collaboration of donor and acceptor moieties in the excited state, as well as information about location of electron-hole and electronic excitations 43 .It's noteworthy that hydrogen atoms were excluded from the analysis due to their minimal contribution in transitions.
TDM heatmaps elucidated the electronic excitation and nature of the transition in the first singlet excited state.Each molecule was divided into π-spacer (chrysene) and benzothiophene acceptor units with diverse electron-withdrawing groups (refer to Fig. 1).Almost, all the derivatives show a similar pattern of charge transference.The designed molecules (CTD1-CTD7) exhibited robust diagonal electron coherence in the transition density matrix (TDM) heatmaps.It is evident from the graphs that electron coherence effectively transferred from π-bridge towards the terminal acceptors (A), enabling the movement of electron density across the endcapped acceptors without entrapment.The results from TDM heat maps suggest a straightforward, smoother, and enhanced exciton dissociation in the excited state, offering potential advancements in solar cell technology.
The generation of the Frenkel exciton through the photoexcitation of small, conjugated molecules is a widely acknowledged phenomenon.In this phase, charge carriers are tightly bound by electrostatic forces, as opposed to existing as free charges.The successful dissociation of excitons leads to the creation of electrons and holes, a process made feasible by the presence of designed structures with low exciton binding energies 44 .The binding energy (E b ) is determined by subtracting energy required for optical transition (represented by ΔE) and the energy gap (E g ) between the HOMO and LUMO energy levels 45 .The equation used to calculate E b is expressed as follows:

Molecular electrostatic potential (MEP)
MEPs are assessed to elucidate the quantitative representation of intermolecular charge distribution between the donor and acceptor regions of a molecular system 47 .MEP surfaces resemble a cloud with three main colors, each indicating a specific strength of the molecule's electrostatic potential.Red hues denote negative electrostatic potential, indicating electron-rich regions that could potentially serve as attack sites for electrophiles due to the intense electron density.Conversely, blue shades denote positive electrostatic potentials, signifying electrondeficient areas prone to nucleophilic attack and repulsive regions for protons due to the molecular nuclei's presence.Green-yellow shaded sites depict regions with a neutral electrostatic potential 48,49 .MEP graphs for all the studied molecules are illustrated in Fig. 8. Interestingly, a noticeable occurrence of red color is observed at the acceptor regions of all designed compounds CTD1-CTD7 compared to CTR molecule, indicating them as nucleophilic region.A distinct bluish cloud is particularly prominent in the central π-region, characterizing the π-spacer as an electrophilic site.Specifically, this observation is more prominent in the CTD3 and CTD4 compounds.This analysis indicates increased charge separation in investigated compounds, suggesting that these newly devised molecules could be effectively synthesized for OSCs.

Open circuit voltage (V oc )
Measuring open-circuit voltage (Voc) of the device is essential for evaluating the efficiency of an organic solar cell (OSC).The V oc represents the maximum voltage generated by a solar device when applied current is 0. Various factors such as: charge transfer, temperature of photovoltaic device, and light intensity significantly influence V oc

50
. When the highest voltage is achieved, HOMO of donor material couples with LUMO of acceptor substance.To enhance V oc values, the donor's HOMO energy state should be lower, and the acceptor's LUMO energy state should be higher 51 .In this study, the V oc of designed acceptor molecules CTR and CTD1-CTD7 was computed by combining these molecules with a J52Cl donor.According to available data, J52Cl is an efficient donor with HOMO/LUMO energies of − 5.299 and − 2.141 eV, respectively 52 .So, Eq. ( 2) has been utilized to provide a numerical estimate of V oc results of all studied chromophores.Figure 9 elucidates the V oc calculations achieved through blending J52Cl (donor polymer) with variously designed acceptors CTD1-CTD7.Amongst all the studied chromophores CTR and CTD1-CTD7, the CTR depicted the highest value of open circuit voltage (1.72 V).On the other hand, compound CTD4 showed the lowest value (1.421 V) of V oc .Moreover, other entitled molecules represented intermediate results.Thus, the following trend in the open circuit voltage results is observed: CTR > CTD1 > CTD2 > CTD7 > CTD5 > CTD3 > CTD6 > CTD4.It is obvious from the results that all the derivatives exhibited maximum value of V oc , making them proficient OSCs material.

Conclusion
The present research employs theoretical calculations to explore the influence of structural modifications by substituting end-capped acceptor groups in OSCs featuring the chrysene unit.
By structural modeling of chrysene unit with benzothiophene acceptors, the photovoltaic and optoelectronic properties of organic chromophores have been tuned.The quantum chemical findings demonstrated proficient charge transfer from central π-conjugated bridge to end-capped acceptor moieties in all the molecules.Open circuit voltage was calculated by developing a complex between designed chromophore and donor polymer (HOMOdonor-LUMOacceptor) and interestingly, higher photovoltaic response was seen in derivatives.The designed molecule CTD4 demonstrated good properties, i.e. lowest E b value (0.519 eV) and minimum band gap (2.327 eV) with maximum red-shifted absorption in both the solvent (685.791nm) and gaseous phase (647.606nm) amongst all CTD1-CTD7.Moreover, by relating the results of FMOs with GRPs, it is found that, CTD4 derived molecule is observed as softest (0.421 eV −1 ) molecule amongst all the studied derivatives portraying its maximum charge transferring properties.Thus, CTD4 can be considered as a remarkable material for future OSCs applications. https://doi.org/10.1038/s41598-024-65459-6
www.nature.com/scientificreports/ of λ max (685.791nm) accompanying with minimum E (1.8808 eV) owing to the incorporation of powerful electron-pulling nitro groups in end-capped acceptor moieties.Decreasing trend of λ max in the gaseous phase for all compounds can be found as follows: CTD4 > CTD6 > CTD3 > CTD5 > CTD7 > CTD2 > CTD1 > CTR .The increase in absorption spectra is investigated as the electron withdrawing efficacy of moieties increased over the terminal acceptors.Remarkably, exactly similar trend of λ max in the solvent phase has been observed by proving CTD4 as excellent organic material for obtaining maximum efficiency of solar cell materials.

Figure 6 .Figure 7 .
Figure 6.UV-Vis absoption spectra of designed compounds (a) in solvent phase and (b) in gaseous phase.

Table 1 .
Energies of frontier molecular orbitals of the CTR and CTD1-CTD7.Band gap = E LUMO − E HOMO , Units in eV.

Table 2 .
The outcomes of UV-visible absorption spectroscopy for CTR and CTD1-CTD7 in both solvent and gaseous phases.f os = oscillator strength, H = HOMO, L = LUMO, MO = molecular orbital