Nonfullerene Near-Infrared Sensitive Acceptors "Octacyclic Naphtho[1,2-b:5,6-b] Dithiophene Core" for Organic Solar Cell Applications: In Silico Molecular Engineering.

End-capped modification is an efficient approach for enhancing the power conversion efficiency of organic solar cells (OSCs). Herein, five novel acceptor molecules have been designed by end-capper modification of the recently synthesized molecule NTIC (R). Different geometric and photovoltaic properties like frontier molecular orbital analysis, absorption maximum, transition density matrix analysis, reorganizational energy, binding energy, oscillator strength, energy of excitation, and charge transfer analysis of designed and reference molecules have been computed by employing density functional theory and time-dependent density functional theory. Designed molecules expressed a narrow energy band gap (E g) with red-shifting in the absorption spectrum. Additionally, low excitation and binding energies are also noted in designed molecules. Excellent values of hole and electron reorganizational energies suggested that designed molecules are effective contributors to the development of the active layer of the organic solar cells. Further, a complex study is also performed for evaluation of charge transfer between the acceptor molecule and the donor polymer. Results of all analyses recommended that designed molecules are effective candidates for high-performance organic solar cell applications.

Introduction

Nowadays, fulfilling the energy shortage is one of the most debated issues. Fuel consumption is increasing day by day to meet the energy need of humans, and it is assumed that at one stage it will be finished. To overcome this problem, now the world is going to utilize renewable sources and explore new methods which are less costly and more energy-producible.[1−3] Solar energy is one of the most renewable sources which is pollution-free. Due to this property of the solar cell, it gained much attention from the world. According to one analysis, the last 2-decade period is one of the most progressive periods for the development of the solar cell. In the beginning, silicon-based solar cells were made to compensate for energy demand. No doubt silicone-based solar cell has gained much popularity in the market due to their high power conversion efficiency (PCE), but it has high cost and fixed energy levels. To overcome this problem, the scientist went to develop an organic solar containing fullerene acceptor to produce solar energy. However, badly, this idea did not take a longer period due to its some disadvantages like low energy tenability, low PCE, and lesser absorption capability in the visible region. Due to this reason, scientists are now working on the nonfullerene acceptors (NFAs) based bulk-heterojunctions organic solar cells (BHK-OSCs) due to their low cost, flexibility, good tunable energy level, large area of fabrication, transparency, and better power conversion efficiency.[4−8] When a donor is blended with the acceptor, a blend is formed. Various factors are involved in the determination of the PCE of the bulk heterojunction solar cells, like how the transfer of charge occurs from the donor region toward the acceptor region through the external field and reorganizational energy. The literature survey is clear that the transfer of charge in the external field is majorly due to the changes in electronic coupling and it is confirmed by the hole and electron coherence and by the total charge density. To achieve a higher PCE bulk heterojunction, organic solar cells should have a lesser recombination rate of electron and hole as compared to the separation rate of electron and hole.[9] Moreover, the photovoltaic and optoelectronic parameters of active layer acceptor and donor molecules have been improved by using efficient end-capped, core, and donor units.[10−14] The end-capped alteration along with core modification enhances the light harvesting capability of the resulting molecules.[15−18] Motivated from similar reports in the valuable literature, we were inspired and also computed the photovoltaic properties of acceptor molecules by employing DFT and TD-DFT approaches.

In the nonfullerene acceptor-based organic solar cells, end-capped unit modification is one of the promising strategies for enhancing the PCE. In the present work, end-capped group modification is done to design five novel molecules named G1–G5 (Figure ) to checkout their optoelectronic properties like energy band gap, absorption capability, and charge transfer analysis of designed molecules with donor PTB7Th polymer. In this project, we took the NTIC molecule as a reference molecule[19] which contained naphtho[1,2-b:5,6-b′]dithiophene as a central core fused with diarylcyclopentadienylthiophene having 4-hexyl phenyl as a side chain and (3-(1,1-dicyanomethylene)-5,6-difluoro-1-indanone (2FIC) as an end-capped group.[19] This theoretical strategy opens a new path for developing highly efficient organic solar cells containing nonfullerene acceptors.

Figure 1

Schematic diagram of near-infrared sensitive new nonfullerene acceptor structures.

Results and Discussion

In this manuscript, the modification of terminal units of acceptors of NTIC has been done with several proven best efficient end-capped groups G1–G5 as displayed in the scheme of molecular designing. In the present work, we replaced the acceptor group of R with the different efficient groups G1–G5 to determine the enhancement in electronic and photovoltaic properties of the designed molecules. Based on λmax values, functional B3LYP with the 6-31G (d,p) level has been selected for further systematic analysis of all the G1–G5 molecules. The λmax value of the reference molecule was calculated to be 688, 525, 646, and 507 nm at B3LYP, CAM-B3LYP, MPW1PW91, and ωB97XD, respectively, as shown in Figure . The reported experimental λmax value (687 nm)[19] was best in comparison with the theoretical λmax of 689 nm at B3LYP, and it acts as a promoter for the next calculations of the design as well as the reference molecule. Optimized 3D geometries of molecules R and all the tailored G1–G5 molecules at B3LYP with the 6-31G (d,p) level of theory are given in Figure . The Cartesian coordinates (XYZ) optimized geometry of compound R and G1–G5 are presented in the Supporting Information Tables S1–S3.

Figure 2

Graphical representation of the absorption maxima reference R at four different functionals with the 6-31G(d,p) level of TD-DFT.

Figure 3

Optimized geometries of molecule designed G1–G5 and R at the B3LYP/6-31G(d,p) level of DFT.

Frontier Molecular Orbitals Analysis (FMOs)

Analysis of frontier molecular orbitals plays a major role to explore the molecules’ optical as well as electronic properties. From the literature survey, it was observed that any change in an optoelectronic property of a molecule is estimated based on the change in the distribution of FMOs, i.e., HUMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital). Following band theory, the HOMO of a molecule is discussed as a conduction band and the molecule’s LUMO is referred to as a valence band.[20−25]

The ultimate decision of conductivity and photovoltaic properties of a molecule is due to the energy difference between the energy levels, i.e., LUMO and HOMO. The smaller the gap larger will be the conduction. The HOMO and LUMO energy values and their gap are given in Table and Figure .

Table 1

Energies of HOMO and LUMO with a Band Gap of All Studied Molecules

molecules	EHOMO (eV)	ELUMO (eV)	Eg (eV)
R	–5.56	–3.42	2.14
G1	–5.91	–3.89	2.02
G2	–5.82	–3.78	2.04
G3	–5.44	–3.36	2.08
G4	–5.62	–3.50	2.12
G5	–5.61	–3.60	2.00

Figure 4

Frontier molecular orbital diagram of R and G1–G5 molecules at the B3LYP/6-31G(d,p) level of DFT.

The computed HOMO values of reference molecules were found to be −5.562 eV, and designed molecules G1–G5 were found to be −5.910, −5.822, −5.442, −5.616, and −5.613 eV, respectively. The computed LUMO values of the reference molecules were found to be −3.420 eV, and designed molecules G1–G5 were found to be −3.885, −3.781, −3.356, −3.499, and −3.604 eV, respectively. The HOMO–LUMO energy difference of the reference molecule, which is referred to as the energy gap, was found to be 2.141 eV and all for designed molecules G1–G5 was found to be 2.022, 2.041, 2.085, 2.116, and 2.008 eV, respectively as shown in Table . According to the energy gap evaluated data, all titled molecules G1–G5 have a narrow bandgap as compared to the reference molecule, and it was proven that our designing strategy will be fruitful for the synthetic work. Out of five designed molecules, G5 was found with the least energy band gap value when compared to the other designed molecules, which is because of the efficient acceptor groups. The ascending order of energy difference of reference and newly designed molecules are given as G5 < G1 < G2 < G3 < G4 < R. Thus, it is noted that all these designed molecules, due to HOMO orbitals destabilization and LUMO orbitals stabilization, narrow HOMO–LUMO band gaps were found, which suggested that the designed molecules are efficient nominees for high-performance organic solar cells.

Estimation of Density of States

Using B3LYP/6-31G (d,p), the density of state analysis (DOS)[26,27] was also performed for the confirmation of the FMOs notion. The density of state spectra elaborate on the proportion of states at each energy level that are occupied by the system. Simulated DOS graphs are given in Figure . The DOS graph consists of two parts. The left part represents the HOMO region, the right part represents the LUMO region, and the gap between them represents the band gap where no electronic states can exist.

Figure 5

Graphical representation of PDOS round the HOMOs and LUMOs of R and tailored molecules.

DOS plots represent the overlapping of orbitals and their distribution over the entire molecule. The graph provided in Figure shows that in the reference and all the designed molecules, the HOMO density lies mainly on the donor region and in the acceptor region and their distribution behavior is almost comparable with each other. The LUMO density also mainly lies in the donor region and acceptor region in the reference molecules, but in the designed molecules, more electronic density is present in the acceptor region as compared to the reference acceptor region. Out of five designed molecules, G5 is the one molecule in which more LUMO electronic population lies on the acceptor moiety relative to the LUMO acceptor region of other designed as well as reference molecules. Moreover, the overlapping in the LUMO region of all newly fabricated molecules is much better when compared to the reference molecule. This behavior proved that our designing strategy of terminal groups is considerable for enhancing the photovoltaic properties of selected molecules.

Optical Properties

TD-DFT at B3LYP/6-31G (d,p) was utilized for determining the optical properties of R and G1–G5 molecules. Absorption spectrum visualization was done to observe the shift (blue or red). Oscillator strength, excitation energy, concern orbital assignment, computed λmax, dipole moment, and experimental λmax value of R and G1–G5 molecules are tabulated in Table .

Table 2

Tabulated Representation of DFT Based Resultsa

molecules	DFT based λmax (nm)	exptl λmax (nm)	Ex energy in (eV)	oscillator strength	assignment	dipole moment
R	688	687	1.79	2.88	HOMO to LUMO (98%)	0.0186
G1	739		1.67	2.5631	HOMO to LUMO (98%)	0.0034
G2	731		1.69	2.6932	HOMO to LUMO (98%)	5.7486
G3	716		1.73	2.2505	HOMO to LUMO (97%)	9.9269
G4	701		1.76	2.9639	HOMO to LUMO (98%)	0.0019
G5	765		1.62	2.1159	HOMO to LUMO (98%)	8.6182

Excitation energy (Ex), absorption wavelength maximum (λmax), natures of transition, and oscillator strength (f) of R and G1–G5 in eV in the solvent (chloroform) phase.

From Table , it is evident that all the modeled molecules λmax values show a redshift as compared to the experimental λmax value of the reference molecule. The redshift in the λmax value is generally considered the best index of good power conversion efficiency. Out of five designed molecules, the G5 molecule exhibits the highest λmax absorption, which is due to the effective acceptor group. The second highest red-shifted λmax value was found in G1, third value in G2, fourth value in G3, and fifth value in G4. However, all of the tailored molecules have greater λmax values as compared to the theoretical and experimental λmax values of R. This increment in the λmax value of all the designed molecules is due to powerful withdrawing end-capped groups present in the acceptor moieties. So, according to the absorption spectrum, it is proved that G5 is the best candidate for the utilization of getting high-performance OSCs.

Excitation energy is the amount of energy that is used to excite the electron from a lower state to a higher excited state. The relationship between excitation energy and PCE of a solar cell is that smaller excitation energy higher will be the PCE of a photovoltaic device. DFT-based excitation energy values of the designed G1–G5 and R molecule have been tabulated in Table . It was found that all our tailored molecules show lower excitation energy value when compared with the reference molecule. Out of five newly modeled molecules, G5 has the lowest excitation energy about 1.62 eV, and the second-lowest excitation energy was found in the G2 molecule. The up-flowing order of energy of excitation of all molecules is given as G5 < G2 < G1 < G3 < G4 < R. Lower excitation energy is due to their acceptor moieties present in them. From this order, it is concluded that G5 is the best candidate for experimental synthesis to get a high PCE value.

Reorganizational Energy

Reorganizational energy is a major aspect to help in calculating the PCE of OSCs. PCE of an OSC is high if the electron (λe) and hole (λh) reorganization energies are low and vice versa. The energy of reorganization can be categorized into two parts, i.e., external reorganizational energy and internal reorganizational energy.

Any symmetrical change in the geometry of a molecule can be determined through internal reorganization and any molecular external fluctuation can be determined by the external reorganizational energy. In the present investigation, the external factor is ignored because no countable change occurs because of it. So, only internal reorganizational energy is considered here.

The theoretically determined data of reorganizational energy using the B3LYP/6-31G (d,p) level of DFT is represented. The density functional theory based λe value shows by the reference molecule was calculated to be 0.0049 eV, and all the designed molecules G1–G5 have values of 0.0031, 0.0039, 0.0069, 0.0045, and 0.0088 eV, respectively, as shown in Table . Out of five designed molecules, G1, G2, and G4 showed lower reorganizational energy of an electron, and G3 and G5 exhibited higher reorganizational energy of an electron relative to the reference molecule. It is assumed that this higher value shown by G3 and G5 is due to the localization of electrons in these molecules. G1 has the lowest λe value, which exhibits that it has a greater ability for electron transport from the donor region toward the acceptor region. The electron mobility is better in G1, G2, and G4 than the reference molecule because of their acceptor moiety and their end-capped groups presents in the acceptor moiety where no electron localization exists. So, according to λe reorganizational energy values, G1, G2, and G4 were found to be our best-tailored molecules exhibiting high electron mobility and suggested good candidates for high PCE.

Table 3

Tabulated Representation of Energy of Reorganization of R and G1–G5 at the B3LYP/6-31G (d,p) Level of DFT

molecules	λea (eV)	λhb (eV)
R	0.0049	0.0056
G1	0.0031	0.0053
G2	0.0039	0.0058
G3	0.0069	0.0069
G4	0.0045	0.0055
G5	0.0088	0.0068

Reorganizational energy of electron.

Reorganizational energy of the hole.

The computed λh of the reference molecule is 0.056 eV, and the λh values of G1 and G4 were found to be 0.0053 and 0.0055 eV, respectively. This lower λh value shows that there is higher hole mobility in the G1 and G4 as compared to the reference molecule. Thus, these are proven best choices for getting higher PCE in OSCs. The decreasing order in newly modeled molecules and reference molecules is given below in the decreasing order: G5 > G3 > G2 > R > G4 > G1. Overall, the reorganizational data conclude that our designing strategy is fruitful for getting the high PCE of an OSC. Among all five designed molecules, G1 is proven the best choice to get highly efficient OSC.

Open Circuit Voltage

The total open circuit voltage is one of the basic indexes through which one can determine the efficiency of a solar cell. Total current drawn from any photovoltaic device at null voltage is considered the open-circuit voltage. It is evident in the working of organic solar cells, two kinds of current play an efficient role, i.e., photogenerated current and saturation voltage. In this project, some measuring aspects are considered for precise determination of Voc. To complete this assignment, we scaled the HOMO of the polymer donor with the LUMO of the acceptor molecules, i.e., our designed molecules.

Total Voc of the reference and all of the tailored molecules was determined by calculating the difference between the HOMO of the polymer donor and LUMO of the acceptor molecules. For high Voc, the HOMO of the PTB7-Th must be lower as compared to the LUMO of the designed acceptor molecules. In our designed project, we have taken PTB7-Th as the polymer donor due to its frequent use against our tailored acceptor molecules. Open circuit voltage can be estimated through the following eq ,

To get maximal shifting of the charge density, we have lined up the LUMO of R and G1–G5 with the HOMO of PTB7-Th. Interestingly, our all-engineered molecules show a comparable Voc output with R, but the most interesting observation is that G3 exhibits higher Voc when compared to the reference molecule. This is due to the highly efficient acceptor group (2-(5-methylene-6-oxo-5,6-dihydro-4H-cyclopenta[c]thiophen-4-yl) malononitrile) present in G3. The Voc of R is 1.83 V, G1 is 1.37 V, G2 is 1.48 V, G4 is 1.76 V, and G5 is 1.65 V. All of the engineered molecules exhibit countable Voc values. It is concluded that G3 is our best-designed molecule and suggested that this is the suitable candidate for experimental synthesis to get highly efficient OSCs. Figure presents the Voc diagram in which the HOMO of PTB7-Th is displayed with the LUMO of the acceptor molecules, and their difference represents the total Voc values.

Figure 6

Voc of designed and reference acceptor molecules compared to the PTB7-Th polymer donor.

Transition Density Matrix (TDM) and Binding Energy

The transition density matrix is a typical spatial map that displays the distribution of electronic excitation in a photovoltaic cell. The map information is very helpful in understanding the performance of the acceptor groups in the OSCs. To get the map of the TDM and understanding the performance of our titled molecules and the reference molecule, we have done DFT analysis using B3LYP with the 6-31G (d,p) level. The DFT-based TDM map of R and all the tailored molecules at the S1 state is given in Figure .

Figure 7

TDM plots of R and engineered molecules G1–G5.

It is evident that there is no countable effect in the transition, which is why we ignored the hydrogen in taking the TDM map. A plot of the transition density matrix helps us in determining the position of electrons, holes, and excitation of electrons within the molecule. In this study, we portioned our engineered molecules into three parts acceptor (A), donor (D), and core (C) for easy understanding. By visualizing the DFT plot of the transition density matrix, in the graph the electrophilicity is decreasing from the bottom to the top. As we move from the bottom to the top in the plot, then electron density is diagonally shifted from the core donor and finally toward the acceptor region where the nucleophilicity is maximum. It shows that all the engineered (G1–G5) and the reference molecules are highly efficient because the movement of the electron is in the right direction, i.e., toward the acceptor region. However, by comparing the reference molecule with the tailored molecules, the nucleophilicity toward the acceptor region is more than the reference molecule. Out of five tailored molecules, G5 is the excellent candidate who exhibits the highest electronic population in the acceptor part. The second one which shows the greater shifting of the electron population toward the acceptor part is G3 in which mostly the electronic population lies in the acceptor part. All the advancement in our designed molecules is due to the acceptor group modification, which is the substitution of the end-capped groups in the acceptor region as discussed above.

To determine the working efficiency, separation potential, and electronic properties, the binding energy (Eb) is a helpful tool in the field of OSCs. Table represents the results of the binding energy, first excitation energy, and bandgap. A general relationship between the PCE and the binding energy is that the lower the Eb, the higher the PCE[27,28] of the solar cell. In this study, the Eb values of all the designed and reference molecules are determined by eq .

Table 4

Tabulated Representation of Simulated HOMO–LUMO Energy Difference/Gap (EH–L), First Singlet Excitation Energies (Eopt), and Binding Energies of Exciton (Eb) at the B3LYP/6-31G(d,p) Basis Set of DFT

molecules	EH–L (eV)	Eopt (eV)	Eb (eV)
R	2.14	1.79	0.35
G1	2.02	1.67	0.35
G2	2.04	1.69	0.35
G3	2.08	1.73	0.35
G4	2.12	1.76	0.35
G5	2.00	1.62	0.38

Table shows that Eb values of our most tailored molecules and the reference molecule are equal. In G5, it is slightly higher than the reference molecule, and this happens due to the localization of the electron as a result of the force of attraction increasing between the electron and the hole, which needs more energy for separation (Figure ). Overall, the value of Eb of all of the molecules shows that there is a good number of charges present which are easily separable without any need for extra energy.

Figure 8

Optimized geometry of the PTB7-Th–G5 complex.

Charge Transfer Analysis by Making Complex PTB7-Th–G5

The basic need of making a complex of PTB7-Th–G5 in this study is to analyze the transfer of charge between the donor PTB7-Th and G5 as well as to confirm the attribute of our tailored molecules. The reason for making a complex with only a G5 molecule is that it exhibits the smallest Eg value as compared to all other tailored molecules. Theoretical calculation of this complex was done at the above-selected level of DFT, i.e., B3LYP/6-31G (d,p). By visualizing the computed complex, it is clear that the dipole moment is oriented from PTB7-Th to the G5 acceptor molecule. So, it confirms that a successful charge transfer occurs.[29−31] Along with the direction of the dipole moment, the HOMO and LUMO of the complex were examined at B3LYP/6-31G (d,p) to evaluate the distribution of density in the orbitals. From the literature survey, it is evident that the HOMO of a molecule behaves as a donor and the LUMO of a molecule behaves as the acceptor. So, from Figure , the HOMO state electronic density is present on the PTB7-Th, and in the LUMO state, the electron density is present on the G5 molecule. This is solid proof of the transfer of charge from donor PTB7-Th toward the acceptor G5 molecule. Thus, it is solid proof of mobility of electrons from the donor polymer toward the acceptor molecule. Therefore, all of our engineered molecules have been confirmed acceptor materials for future utilization in manufacturing highly proficient OSCs in future.

Figure 9

Distribution model of HOMO and LUMO of PTB7-Th (donor) and G5.

Conclusion

The latest quantum chemical parameters are chosen and applied for tuning optoelectronic properties of the NTIC (R) molecule. By doing end-capped modification in the acceptor moiety of R, great improvement in the optoelectronic properties was seen. The first one is the countable decline within the energy difference/gap in designed molecules (2.00–2.12 eV) as compared to the reference molecule (2.14 eV). The transition density matrix (TDM) and PDOS studies proved the high efficiency of tailored molecules. Voc of all the designed molecules was found to be good (Voc = 1.30–1.90 V). Comparable values of binding energy were observed (Eb = 0.35–0.38 eV). Also, a low value of the energy of reorganization of the electron was observed as compared to the reference molecule in G1, G2, and G4, and lower energies of reorganization of the hole were found in G1 (0.0051) and G4 (0.0054). The complex (PTB7-Th) making strategy was also found to be good in the confirmation of a better transfer of charge from the donor polymer to the acceptor molecule. Overall, designed molecules displayed improved optoelectronic properties as compared to the reference. Out of five designed molecules, G5 was found to be an excellent molecule for future applications to get highly efficient organic solar cells. Based on all of the results of all the analyses, it is concluded that our designed molecules are efficient candidates for organic solar cells. Thus, we recommend a novel kind of system to experimentalists for the future development of organic solar cells.

Computational Methodologies

For the evaluation of computational chemistry simulations, we have utilized Gaussian 09W.[32] For the structural visualization, we used Guassview 6.0.[33] To see the validation of the method, four functionals such as B3LYP,[34] CAM-B3LYP,[35] ωB97XD,[36] and MPW1PW91[37] with the 6-31G (d,p) level of DFT on the reference molecule applied. We compared the experimental absorption maxima of the reference molecule with the λmax values to select the appropriate level of theory. The results showed that B3LYP with level 6-31G (d,p) was a good comparison with the experimental data. Time-dependent DFT at B3LYP/6-31G (d,p) was utilized to observe the absorption spectra of R as well as G1–G5. The maximum absorption was calculated in the solvent phase (chloroform) by applying CPCM. The other parameters like recombination energies of holes and electrons, Voc, TDM, FMOs, PDOS, and dipole moments of R and the G1–G5 molecules were performed at B3LYP functionality with the selected level of theory. The λmax, oscillator strength, and excitation energies (EX) of the R molecule and G1–G5 molecules were calculated by using the Gauss-sum program. Moreover, we calculated the energy of reorganization which is divided into two categories. The first one is the external energy of reorganization (λext) and another one is the internal energy of reorganization (λint). The λext is related to the polarizing effect of the external environment. The λint is related to the change in internal structure. In this analysis, we only deal with λint, not with λext because it plays no significant role. For calculation of electron mobility as well as the hole, we used eqs and 4.[34,38,39]Here, E+ and E– specify cation energy and anion energy correspondingly by using the fully stable geometries of all the molecules. E0+ and E0– indicate the energies of cation and anion for all the molecules at a neutral state. E+0 and E–0 are the cationic single point energies and the anionic single point energies at optimized neutral molecules. Finally, E0 shows the ground state energy molecule.[40]Binding energies can be calculated by using eqs and 6 by determining the energies of the HOMOs and LUMOs.