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

The selection of π–conjugated core is an essential phenomenon that significantly influences the photovoltaic characteristics of the A–π–A configured organic compounds32. Moreover, the literature study highlights the significance of structural modifications for enhancing optoelectronic properties33. The objective of current study is to design advanced organic materials based on the TTB unit accompanied by the small molecular acceptors (SMAs) to anticipate their optoelectronic and photovoltaic properties for potential use in the organic solar cells (OSCs). Figure S1 shows the structural modeling of TTB parent into TTBR. This reference compound is further modified via the alteration with the end-capped acceptor moieties such as 5-methylene-4-H-cyclopenta[c]thiophene-4,6(5-H)-dione (A1) in TTBR, 1-fluoro-5-methylene-4-H-cyclopenta[c]thiophene-4,6(5-H)-dione (A2) in TTB1, 1,3-difluoro-5-methylene-4-H-cyclopenta[c]thiophene-4,6(5-H)-dione (A3) in TTB2, 1,3-dichloro-5-methylene-4-H-cyclopenta[c]thiophene-4,6(5-H)-dione (A4) in TTB3, 5-methylene-1,3-bis(trifluoromethyl)-4-H-cyclopenta[c]thiophene-4,6(5-H)-dione (A5) in TTB4, 5-methylene-1,3-dinitro-4-H-cyclopenta[c]thiophene-4,6(5-H)-dione (A6) in TTB5 and 5-methylene-4,6-dioxo-5,6-dihydro-4-H-cyclopenta[c]thiophene-1,3-dicarbonitrile (A7) in TTB6. Their structures are represented in the Fig. S2. The optimized views of all the entitled chromophores are presented in the Fig. 1 and their IUPAC names are recorded in Table S1. The spatial arrangement of atoms in the designed chromophores (TTBR and TTBR-TTB6) are displayed in Fig. S3, while the Cartesian coordinates are shown in the Tables S2–S8 (Supporting Information).Fig. 1Optimized structures of the titled compounds (TTBR and TTB1-TTB6).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).Electronic analysisAnalysis of FMOs requires how well a molecule promotes charge mobility and electron density distribution34. 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 FMOs36. The difference between these orbitals is referred to as the band gap (Egap = ELUMO–EHOMO)37 enlisted in the Table S9.It is known from the literature that photovoltaic materials with lower bandgaps showed higher power conversion efficiencies (PCEs) and vice versa38. The EHOMO, ELUMO and ΔE values for TTBR reference are noted as − 6.328, − 3.251 and 3.077 eV, respectively. For the designed chromophores TTB1-TTB6, the EHOMO are − 6.419, − 6.497, − 6.503, − 6.554, − 6.658 and − 6.640 eV, while, the ELUMO are − 3.255, − 3.313, − 3.357, − 3.596, − 4.118 and − 3.825 eV, respectively. Similarly, their corresponding ΔE are found as 3.164, 3.184, 3.146, 2.958, 2.54 and 2.821 eV, respectively. All the designed chromophores exhibit a decrease in ΔE owing to the elevated HOMO and reduced LUMO levels, promoting a significant intramolecular charge transfer (ICT)39. Figure 2 depicts a pictorial representation of essential orbitals (HOMO/LUMO). The higher orbitals such as HOMO-1/LUMO + 1 and HOMO-2/LUMO + 2 are also interpreted and their results are recorded in the Table S10, while their orbital surfaces are shown in the Fig. S4.Fig. 2HOMOs and LUMOs of the designed chromophores (TTBR and TTB1-TTB6).On comparing the HOMO–LUMO band gaps of the designed derivatives with TTBR (3.077 eV), they exhibit lesser energy gaps (2.540–3.184 eV). Among all, the TTB5 demonstrates the smallest value (2.540 eV). It is a promising candidate in terms of energy band gap owing to the presence of potent electron-withdrawing groups such as nitro (–NO2) group on its terminal acceptor moiety (5-methylene-1,3-dinitro-4-H-cyclopenta[c]thiophene-4,6(5-H)-dione) attached to thiophene rings of the π − spacer. The band gap of TTB6 (2.821 eV) is lower than that of TTBR-TTB4 but higher than TTB5 due to the presence of electron-withdrawing cyano (− CN) groups at terminal sides of 5-methylene-4,6-dioxo-5,6-dihydro-4-H-cyclopenta[c]thiophene-1,3-dicarbonitrile acceptor. Further, the TTB4 (2.958 eV) has a higher band gap than TTB6 due to the presence of 5-methylene-1,3-bis(trifluoromethyl)-4-H-cyclopenta[c]thiophene-4,6(5-H)-dione attached to the thiophene rings. Higher band gaps in TTB1-TTB3 (3.164, 3.184 and 3.146 eV) are due to the presence of less electronegative groups (− F >  − Cl) than (− NO2 >  − CN). The largest energy gap observed in TTB2 might be attributed to the decreased conjugation within the compound. The overall band gaps are found to increase in the following order: TTB5 < TTB6 < TTB4 < TTBR < TTB3 < TTB1 < TTB2.Azeem et al. reported the reference molecule SBDT-BDD has a band gap of 4.49 eV40. This band gap is much higher than the current band gap which is in the range of 3.184–2.54 and 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 theorem47 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 ΔNmax48 and it is computed via Eq. (3).$${\Delta N}_{\text{max}} = -\frac{\mu }{\eta }.$$
(3)
A compound with a greater ΔE is considered to be hard, exhibiting higher kinetic stability and less reactive. Conversely, a compound with a smaller ΔE is found to be softer, highly reactive, and less stable49. The ionization energy values of TTBR and TTB1-TTB6 are 6.328, 6.419, 6.497, 6.503, 6.554, 6.658 and 6.646 eV. Similarly, the values for electronegativity and electron affinity are found to be 4.789, 4.837, 4.905, 4.930, 5.075, 5.388, 5.235 eV and 3.251, 3.255, 3.313, 3.357, 3.596, 4.118 and 3.825 eV, respectively are listed in the Table 1. The TTB5 compound exhibits the highest ionization potential (6.658 eV) among all the examined compounds. Moreover, it also showed higher values for X = 5.388 and EA = 4.118 eV. The chemical softness (σ) and hardness (η) are significant parameters in determining the chemical stability and reactivity. A decreasing trend in σ is noticed as follows: TTB5 (0.393) > TTB6 (0.354) > TTB4 (0.338) > TTBR (0.324) > TTB3 (0.317) > TTB1 (0.316) > TTB2 (0.314) in eV–1. Because of the electron-withdrawing characteristics of acceptor moieties, the σ values are observed to be smaller than η values. The hardness descending trend is as follows: TTB2 (1.592) > TTB1 (1.582) > TTB3 (1.573) > TTBR (1.538) > TTB4 (1.479) > TTB6 (1.410) > TTB5 (1.27) in eV. Thus, TTB5 shows the highest σ and least η values as mentioned in the above trends. Furthermore, TTB5 exhibits the highest ΔNmax (4.242 eV). Moreover, the calculated ΔNmax values decrease in the following sequence in eV: TTB5 (4.242) > TTB6 (3.712) > TTB4 (3.431) > TTB3 (3.134) > TTBR (3.113) > TTB2 (3.081) > TTB1 (3.057). The unique characteristics observed in TTB5 due to the presence of –NO2 groups on the terminal acceptors, indicate its suitability for the OSCs.Table 1 Ionization potential (IP), electron affinity (EA), electronegativity (X), chemical potential (μ), global hardness (ƞ), global electrophilicity (ω) and global softness (σ) and maximum charge transfer (ΔNmax) of the investigated compounds.Optical analysisThe absorption profile of the organic chromophores employed for organic photovoltaic cells and is an important parameter for determining the efficiency of solar cells50. 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 rate51. The investigations are conducted to determine the absorption wavelength (λmax), oscillation strength (fos), 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 PCE32. A comparative analysis of data in Table S12 indicates that all the compounds exhibit a bathochromic shift in dichloromethane solvent (486.365–605.895 nm) as compared to TTBR reference (506.451 nm). The λmax values of TTB1-TTB6 are 490.774, 486.365, 493.509, 526.584, 605.895 and 553.205 nm, respectively. The increasing order of λmax in the studied chromophores is: TTB2 < TTB1 < TTB3 < TTBR < TTB4 < TTB6 < TTB5. This shift towards longer wavelengths is due to solvent effects. The outcomes demonstrate the greater λmax values of the designed derivatives are due to strong electron-withdrawing units at their terminal moiety which extended the conjugation. Interestingly, a broader absorption value is obtained for TTB5 (605.895 nm) 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 LUMO52. 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 conjugation53.The UV–Vis absorption spectra of the studied compounds are depicted in dichloromethane phases shown in the Fig. 3. The higher values of λmax (605.895 nm), the lower excitation energy (E) 2.046 eV and oscillator strength (fos) 0.916, with 90% MO contribution from HOMO to LUMO of TTB5 in dichloromethane is due to the participation of –NO2 group at the terminal regions of the compound. The electron-withdrawing group (–NO2), 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 λmax value, leads to a reduction in HOMO–LUMO band gap. This phenomenon facilitates the charge transfer pathway54. From the literature, Azeem et al. reported the optical properties of small molecules with benzodithiophene as a core and the results showed λmax 543 nm. While the current study shows λmax of 605.895 nm which demonstrated the best optical properties of the designed chromophore.Fig. 3Absorption spectrum of the studied compounds in dichloromethane phases.The transition energy (E) demonstrates an inverse correlation with the rate of charge transfer and λmax55. 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) analysisDensity of states (DOS) is an efficient technique for determining the distribution pattern of electronic density on the FMOs50. 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 − spacer54. 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. 4. The acceptor contributions to LUMO and HOMO in TTBR and TTB1-TTB6 are 56.4, 58.1, 57.6, 60.8, 62.4, 86.0 and 71.0% and 29.4, 29.2, 28.7, 29.4, 29.4, 30.6 and 29.9%, respectively. Similarly, the π-spacer shows 43.6, 41.9, 42.4, 39.2, 37.6, 14.0 and 29.0% for LUMO, while, for HOMO 70.6, 70.8, 71.3, 70.6, 70.6, 69.4 and 70.1% are observed for TTBR and TTB1-TTB6, respectively as shown in Table S13.Fig. 4DOS pictographs of the investigated compounds.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 capabilities1.Transition density matrix (TDM) analysisTransition 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 (S0) and the excited state (S1) transitions. The impact of hydrogen atoms is neglected in all compounds since they have only a little contribution toward the excited state transitions56. 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 state57. 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. 558. 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 (–NO2) 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.Fig. 5TDM maps of all the investigated chromophores.Exciton binding energy (Eb)Binding energy (Eb) 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 charges56. The binding energy can be determined by subtracting the minimum energy required for the first excitation (Eopt) from the energy gap between the HOMO–LUMO59. Therefore, a lower value of exciton binding energy is correlated with a weaker electron–hole interaction and an increase in charge transfer57. Lower values of Eb provide high current charge density (Jsc), high charge dissociation and high power conversion efficiency60. The Eq. (4) is used to calculate the binding energy61.$$E_{{\text{b}}} = \, E_{{{\text{gap}}}} – E_{{{\text{opt}}}} ,$$
(4)
Here, the Eb represents binding energy, Egap is the energy gap and Eopt corresponds to the single-point energy.The results obtained are listed in Table 2. The highest value of Eb 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 charge transfer. The decreasing trend of exciton binding energy is TTB1 > TTB2 > TTB3 > TTBR > TTB4 > TTB6 > TTB5. Eb is essential for determining both the efficiency and functionality of photovoltaic cells. Decreased Eb enhanced photovoltaic characteristics, such as increased charge separation efficiency, greater VOC and improved overall energy conversion efficiency.Table 2 Computed binding energies of TTBR and TTB1-TTB6.Hole-electron analysisWhen 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 charges62. This analysis is highly effective and widely utilized for identifying the localization of electron density within a compound63. It is a feasible approach for revealing the nature of electron excitations and charge transfer64. 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.Fig. 6Pictorial illustration of the hole-electron analysis for the titled chromophores.