Exploration of efficient electron acceptors for organic solar cells: rational design of indacenodithiophene based non-fullerene compounds

The global need for renewable sources of energy has compelled researchers to explore new sources and improve the efficiency of the existing technologies. Solar energy is considered to be one of the best options to resolve climate and energy crises because of its long-term stability and pollution free energy production. Herein, we have synthesized a small acceptor compound (TPDR) and have utilized for rational designing of non-fullerene chromophores (TPD1–TPD6) using end-capped manipulation in A2–A1–D–A1–A2 configuration. The quantum chemical study (DFT/TD-DFT) was used to characterize the effect of end group redistribution through frontier molecular orbital (FMO), optical absorption, reorganization energy, open circuit voltage (Voc), photovoltaic properties and intermolecular charge transfer for the designed compounds. FMO data exhibited that TPD5 had the least ΔE (1.71 eV) with highest maximum absorption (λmax) among all compounds due to the four cyano groups as the end-capped acceptor moieties. The reorganization energies of TPD1–TPD6 hinted at credible electron transportation due to the lower values of λe than λh. Furthermore, open circuit voltage (Voc) values showed similar amplitude for all compounds including parent chromophore, except TPD4 and TPD5 compounds. These designed compounds with unique end group acceptors have the potential to be used as novel fabrication materials for energy devices.


Results and discussion
The absorption maxima of the reference chromophore, synthesized TPDR NFA by Li et al. 30 was investigated in chloroform at four functionals: MPW1PW91, ωB97XD , B3LYP and CAM-B3LYP in conjunction with 6-31G (d,p) basis set and λ max of TPDR was calculated to be 714.15, 503.63, 483.12 and 658.89 nm, respectively. The functional, MPW1PW91/6-31G(d,p) showed the best agreement between the computed and experimental values at 632 nm as shown in Fig. 1. Therefore, this λ max value was most appropriate for further investigation on our designed compounds. The terminal acceptor groups of TPDR were substituted sequentially with different acceptor unit as shown in Figure S1 to design efficient non-fullerene OSCs. By replacing end-capped acceptors, six distinct derivatives namely TPD1, TPD2, TPD3, TPD4, TPD5 and TPD6 were obtained, their IUPAC names and two-dimensional (2D) structures are presented in Fig. 2 while their optimized molecular geometries of investigated compounds are presented in Fig. 3.
Frontier molecular orbitals (FMOs) analysis. FMOs analysis helps to understand the intramolecular charge transfer (ICT) characteristics, optoelectronic properties and electron density distribution of chromophores [31][32][33][34][35][36][37] by estimating the charge transition between LUMO and HOMO orbitals 38 . Band theory describes the LUMO and HOMO as valence and conduction bands, respectively. The energy difference among orbitals is explained as bandgap (ΔE) 39,40 . The performance of OSCs can be explained with the help of ΔE or Eg, as there will be greater power conversion efficiency (PEC) of a photovoltaic material with lower bandgap and vice versa [41][42][43] . Herein, we analyze the conducting behavior of electronic density accompanying photon characteristics of the designed compounds. Data in Table 1 shows energies of orbitals and bandgaps of TPDR and TPD1-TPD6.
In terms of charge transfer, chromophores with lesser E g possessed greater charge transfer rate and henceforth, demonstrated larger photovoltaic response 45 . The electronic charge density for HOMO is located significantly over the central donor unit except the methyl groups and A1 unit in most of the designed chromophores. Similarly, for LUMO charge density is concentrated dominantly over the acceptors (A1 and A2) and for a minor part on the central donor moiety (Fig. 4). Overall, all the designed molecules showed significant charge transfer between orbitals indicating their potential to be good photovoltaic materials.

Density of states.
The DOS analysis of TPD1-TPD6 and TPDR are executed using MPW1PW91/6-31G(d,p) functional and DOS spectra are portrayed in Fig. 5.
For DOS analysis, we have divided our compounds into three portions, which are Donor (core unit), Acceptor 1 (bridge) and Acceptor 2 (end-capped acceptor group) represented by blue, green and red lines, respectively in   19.1% and 24.4% to LUMO in TPD1-TPD6, respectively. Similarly, Acceptor-2 contributes 11.7%, 11.5%, 10.5%, 12.5%, 12.7% and 9.9% to HOMO, while 59.4%, 60.1%, 58.1%, 64.0%, 77.3% and 65.2% to LUMO for TPD1-TPD6, respectively. In the same way, donor contributes 68.0%, 68.3%, 68.5%, 68.1%, 77.9% and 69.2% to HOMO, and 12.0%, 11.8%, 12.1%, 10.4%, 3.6%, and 10.4% to LUMO for TPD1-TPD6, separately. By these findings, it is examined from DOS graphs that the HOMOs are largely concentrated on donor as higher peak of blue color which is located nearly -5.6 eV. Similarly, the LUMOs are significantly on A 1 in TPDR while on A 2 in all derivatives as higher peak is located near 6.5 eV hence, these graphs significantly support the FMO diagrams (see Figs. 5,6). Overall, the charge density circulation reveals that significant amount of charge is relocated due to delocalization of electrons in case of TPDR and all its derivatives from the central D to end-capped A units with the assistance of the Acceptor 1.
UV-visible analysis. TD-DFT investigations are utilized to find UV-Vis spectra at MPW1PW91/6-31G(d, p) functional to elucidate optoelectronic properties for entitled chromophores.
The studied compounds are of A 2 -A 1 -D-A 1 -A 2 type with different end-capped acceptors leading to differing optoelectronic responses. In all of the investigated compounds higher λ max and low transition energy values are observed in both gas and chloroform solvent. The significant oscillator strength (f os ), excitation energy and maximum absorption λ max in gas and chloroform are presented in Tables 2 and 3 while other transitions are  shown in Tables S8-S21 and their absorption spectra is shown in Fig. 6. The outcomes illustrate the greater red shift in λ max of novel compounds due to strong electron-withdrawing units at end-capped terminal moiety with extended conjugation. TPD1-TPD6 compounds give higher red shift along lesser excitation energy contrasted with TPDR.
The absorption of all the studied compounds is located in the range of 658.89-810.78 nm in chloroform and 639.29-761.15 nm in gas. Interestingly, it was observed that all the chromophores show red shift in chloroform than in the gaseous phase except for TPD4, which expressed higher absorption wavelength (810.78 nm) in chloroform than in the gas phase (761.15 nm). This might be owing to the interaction of cyano unit on the terminal acceptor with chloroform which stabilized the molecule. Table 3 reveals that the λ max calculated for TPDR is 658.89 nm in chloroform, which correlates well with the experimental value (632 nm). Owing to the solvent effect, observed λ max results in chloroform as red shifted in comparison to while dissolved in gaseous phase. The λ max order is found to be TPDR < TPD5 < TPD3 < TPD1 < TPD6 < TPD2 < TPD4. The lower excitation energies of all new designed compounds depicted the easy excitation between HOMO and LUMO in contrast to TPDR. The excitation energy (E) increasing order is obtained to be TPD4 < TPD2 < TPD6 < TPD3 = TPD1 < TPD5 < TPDR. This confirms that the designed non-fullerene acceptor compounds (TPD1-TPD6) have enhanced optical properties than TPDR.
Reorganization energy. The hole and electron reorganization energy (RE) are considered as a fundamental tool to estimate the performance and working capability of OSCs. Reorganization energy is inversely related to the charge mobility. Chromophores with least RE exhibit greater mobilities of hole and electron or vice versa 46  www.nature.com/scientificreports/ Reorganization energy depends upon myriad factors amongst which geometric shape of cations and anions has the major influence. The cationic geometry displays the hole while anionic geometry shows the electron transportation towards acceptor from donor molecule. The reorganization energy has two major categories: λ ext . responds to exterior environmental changes and λ int . denotes internal reorganization energy and provides information for the internal structural rapid changes. Herein, we ignored the external environmental influence as it does have minimal impact. RE is calculated by utilizing Eqs. (3) and (4) to understand charge mobility of TPDR and TPD1-TPD6 chromophores, and results are displayed in Table 4.
Similarly, Table 4 shows the calculated reorganization energy of hole (λ h ) for TPDR and TPD1-TPD6 is found to be 0.00342, 0.00681, 0.00697, 0.00701, 0.00679, 0.00789 and 0.00785 eV, respectively. All the derivatives exhibited higher value of λ h than TPDR, indicating the lower hole transport capability between D and A. The order of λ h is TPD5 > TPD6 > TPD3 > TPD2 > TPD1 > TPD4 > TPDR. Overall, investigations reveal that the λ e

Open-circuit voltage (Voc).
To analyze the maximum working capacity of OSCs, open-circuit voltage (Voc) plays a vital role. It is the determination of entire quantity of current that is generated by an optical material 47 . A higher value of Voc can be attained whereas the LUMO level of the acceptor has a higher energy value and the HOMO of the donor has a lower value 48 . By utilizing following Equation, Voc can be calculated 46 .
In this study, the chief purpose of Voc is to arrange the HOMO of well-known donor compound J52Cl with the LUMO of the acceptor 30 . The outcomes achieved from Eq. (1) are presented in Table 5 and Fig. 7. Table 5 reveals that TPD1, TPD2, TPD3, and TPD6 have comparable values of V OC in the range of 1.22-1.42 V with the reference chromophore TDPR (1.55 V), while TPD4 and TPD5 have value less than reference chromophore. Highest results of voltage is calculated in TPD1 among all our compounds, may be due effective withdrawing groups with perfect planner geometry that facilitated the supreme shifting of chargers from D to A. V OC order of entitled chromophores is found to be TPDR > TPD1 > TPD3 > TPD6 > TPD2 > TPD 4 > TPD5. A significant value of voltage is obtained for these chromophores which illustrated them as beneficial candidates for NF-OSCs.

Charge transfer analysis.
To predict the potential usage of designed compounds with regards to charge transfer characteristics for OSCs, the studied molecules TPDR and TPD1-TPD6 are blended with J52Cl polymer and complex is optimized using above mentioned level of theory. In complex [J52Cl: TPD1-TPD6 ], designed molecules are used as acceptor materials while J52Cl is used as donor material which is recognized as a well-known polymeric natured compound and frequently utilized in the CT analysis 30 . The effective charge density for HOMO is located at donor polymer J52Cl, whereas LUMO is concentrated over the terminal acceptor unit of TPDR and TPD1-TPD6 as displayed in Fig. 8. The transfer of electronic charge rom D to A provides strong indication of charge mobility from D to A moiety. This charge transfers from D to A provides a piece of information that all our designed derivatives may be used as an efficient acceptor compounds for OSC.
Transition density matrix and exciton binding energy. The interpretation of transition processes in entitled chromophores may also be evaluated by calculating transition density matrix (TDM). The MPW1PW91/6-31G(d,p) level of theory was employed to estimate the behavior of transitions, essentially from the ground state (S 0 ) to an excited state (S 1 ) and interaction between donor and acceptor unit along with electron-hole localization. The hydrogen atoms effect is ignored because of its minute influence in these transitions.  www.nature.com/scientificreports/ For TDM analysis, we divided our molecules (TPDR and TPD1-TPD6) into three fragments namely; central core donor (D), acceptor-1 (A 1 ) and terminal acceptor-2 (A 2 ) and their pictographs are shown in Fig. 9. These TDM heat maps illustrated an efficient diagonal charge transfer coherence in all the designed chromophores. Electron coherence successfully transferred from D to A 1 which facilitated the shifting of electron density towards A 2 without trapping. Interestingly, in TDM map of TPD5, charge is observed with unique pattern only at the D part and this behavior is completely different than TPDR and TPD1, TPD2, TPD3, TPD4 and TPD6. This unique pattern may generate interesting debate as to the origin of this pattern amongst readers of this journal. Nevertheless, findings of TDM heat maps excluding TPD5 implies an facile, easier and higher exciton dissociation in the excited state which would help future solar cell development.
Binding energy (E b ) is also considered a vital factor for evaluating the photovoltaic properties of OSCs particularly exciton dissociation capacity 49,50 . Binding energy is a noticeable parameter for determining the columbic force interaction among hole and electrons. In the excited state, lower columbic interaction between electron and hole and greater the exciton dissociation 51,52 . E opt is a term that refers to the energy of the S 0 -S 1 51,52 . Equation (2) is used to measure E b .
All designed molecules in comparison to TPDR have smaller E b except TPD5 (Table 6). This lowering E b illustrates higher exciton dissociation in the excited state. The descending order of binding energy of all molecular is reported as TPD5 > TPDR > TPD6 > TPD3 = TPD2 > TPD1 > TPD4. Among all the designed chromophores, TPD3 and TPD2 manifest lower binding energy that describes their highest efficiency in exciton dissociation with better optoelectronic properties. The chromophores with 1.9 eV E b could be perfect as an OSCs material with significant Voc.

Conclusion
The solar light active organic chromophores (TPD1-TPD6) are modelled on the compound (TPDR) using efficient end-capped electron accepting groups. Fortunately, photovoltaic properties of the designed compounds show significant improvement on the parent compound. The designed molecules exhibit lower band gap in the range of 2.12-1.71 eV, while TPDR is found with 2.33 eV band gap. Moreover, TPD1-TPD6 are found with broader absorption spectrum as compared to the reference molecule. Interestingly, TPD5 yielded least ΔE value as 1.71 eV, where it is found 0.62 eV less than TPDR among designed compounds owing to the high electron withdrawing influence of end-capped acceptor cyano and ester groups with extended conjugation. Interestingly, e value of all the entitled chromophores is examined to be lower than h except for the reference compound which indicates the higher electron mobility rate in these compounds. Further, lower binding energy (0.33-0.82 eV) of studied molecules are obtained which define higher excitation dissociation. Subsequently, the higher excitation dissociation factor predicting the higher power conversion efficiency of entitled compounds. It is concluded that entitled chromophores obtained by modeling route could be appealing as efficient economically viable organic solar cell materials.

Methods
The Gaussian 09 package was used for the analysis 53 and the GaussView 5.0 54 was employed for DFT calculations. The diverse DFT based levels of theory were applied such as B3LYP 55 , CAM-B3LYP 56 , MPW1PW91 57 and ω B97XD 58 with 6-31G(d,p) basis set combinations for the optimisation of TPDR chromophores. The optimized geometries of TPDR were further used for UV-Vis analysis via TDDFT calculations at aforesaid levels and basis set in chloroform. Subsequently, MPW1PW91 level of theory exhibited absorption maximum values in good agreement with obtained experimental values (see Fig. 1). Thus, MPW1PW91 level was selected for further computations of TPD1-TPD6 chromophores.
To study structure activity relationship and to explore the optoelectronic properties of entitled chromophores, the density of states (DOS), absorption spectra, FMOs analysis, reorganization energy (RE), open-circuit voltage (Voc) and transition density matrices (TDM) were investigated. However, charge transfer analysis was executed using aforesaid level of theory with a 3-21G basis set due to the larger size of complexes (J52CL: TPD1-TPD6). The reorganization energy has two major categories: external reorganization energy (λ ext .) which is used to explain the exterior environmental change and internal reorganization energy (λ int .) agreements with the internal structure rapid variations. In our study, external environmental influence is ignored as it has not much effect. Therefore, following Eq. (3) is used for the calculating the the reorganization energy of electron (λ e ).
where E 0 − is neutral molecule energy obtained via anionic optimized structure, E − is the energy of anions, E − 0 is the single point energy (SPE) of anions and E 0 is the SPE of neutral molecule. In the same way, reorganization energy of the hole (λ h ) can be computed using Eq. (4) [59][60][61] .
Here E 0 + is the neutral molecule energy obtained via optimized cationic structure, E + is the energy of cations and E + 0 is the SPE of cations 47 . Various software's including GaussView 54 , PyMOlyze 62 , Multiwfn 3.7 63 , Avogadro 64 , and Chemcraft 65 were used for data analyses.