Efficient Molecular Modelling of Butterfly-Shaped Hole Transport Materials With Remarkable Photovoltaic Properties For High-Performance Organic Solar Cells

doi:10.21203/rs.3.rs-182560/v1

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Efficient Molecular Modelling of Butterfly-Shaped Hole Transport Materials With Remarkable Photovoltaic Properties For High-Performance Organic Solar Cells

https://doi.org/10.21203/rs.3.rs-182560/v1

This work is licensed under a CC BY 4.0 License

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posted 12 Feb, 2021

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Currently, organic solar cells (OSCs) with non-fullerene electron acceptors offer the highest efficiencies among all reported OSCs. To further improve the efficiencies and stabilities of fullerene-free organic solar cells, end-capped acceptor variations is built with strong electron withdrawing groups. In this report, we have theoretically calculated five new butterfly-shaped fullerene-free acceptors (FD1-FD6) by making end-capped modifications on reference molecule (R) with the purpose to study the improvement in photophysical, opto-electronic, and photo-voltaic properties of newly designed molecules by employing density functional theory (DFT) and time dependent (TD-DFT). Besides, some properties like position of frontier molecular orbitals (FMOs), excitation and binding energy, hole-electron overlap, density of states, overlap density of states, molecular electrostatic potential, open circuit voltage, transition density matrix, and reorganizational energy of electron and hole are also considered and associated with experimentally synthesized reference compound. All calculated molecules displayed a good red-shifting with high charge mobility of electrons among low binding and excitation energies as opposed to reference molecule. Furthermore, all designed molecules (FD1-FD6) and the reference R shows narrow band-gap along-with great charge shifting capability. This theoretical framework proves that end-capped acceptors variation is a modest and effective strategy to accomplish the desirable opto-electronic properties. Therefore, FD1-FD6 are suggested to experimentalist for out-looking future developments to fabricate highly efficient solar cells devices.

General Biochemistry

Hole Transport Materials

Organic solar cells

Power conversion efficiency

End-capped modifications

Butterfly shaped designing

Butterfly-shaped efficient hole transport molecules (FD1-FD6) are studied for high performance solar cells applications.
Significant lowering of energy gap with concomitant red shifting of the absorption spectra is achieved with end-group engineering.
These HTMs show lower binding energy and excellent electron and hole reorganizational energies.
All HTMs have remarkable optoelectronic properties compared to reference R.

Nowadays hybrid perovskite semiconductors are newly emerged semiconductor and are potentially focus on research in solar cells due to their high efficacious nature¹. A semiconductor Spiro-OMETAD have high molecular weight, high cost and difficult to synthesize replace by low molecular weight hole transporting material (HTMs) by using recently computational research techniques to ensure the high yield ^2,3. For the synthesis of HTMs electron-donating moieties are being used like triphenylamines (TPA), diphenylamine’s (DPA), as these had been already studied in Dye Sensitize Solar Cells (DSSC) ^1,3−6.

The benzothiadiazole (BT) group is a backbone of frequently used organic semiconductor ^7,8. Basically, it is a heterocyclic system with electron withdrawing nature can be used in transportation of charge carriers like negative charge and create hole. But in this case, we were not use it for this purpose. In this it acts as fluorophore. The structure bearing the BT-group has crystalline and planner structure and show conjugation in polymers, perovskite solar cells. Because it creates π-π interaction ⁹ with other moieties in polymers and solar cells. So through conjugation it must be used as a charge carrier ^10,11. Recently, C.R Seco and his co-workers had successfully elaborated and synthesized the novel hole transport materials for solar cell applications ¹². All synthesized molecules behaved well under given parameters but among all CS01 ¹² disclosed high power conversion efficiency with narrow band gap. So, motivated from that report, our group decided to design and elaborate the photovoltaic properties of newly designed molecules by considering CS01 as reference molecule (R) ¹². In this work, CS01 is selected as reference molecule which have butterfly-shape, hence some chemical modifications were done by adding phenyl ring to make a binary connection between molecular structure of HTMs and device performance. On the behalf of quantum techniques used in CS01 we had done the quantum calculation of new designed molecule.

Motivated from these findings we presented six new novel butterfly-shaped HTMs (FD1-FD6). The BT-group is used as core unit and covalently bonded with other ring system for making these molecules Donor-Accepter-Donor semiconductor. This D-A-D structure leading the transfer of charge to colored chromophore. And this BT-group tuned the highest occupied orbitals (HOMO) to achieve high value of Voc in solar cells. This lowest occupied orbital (LUMO) affected due to electron-withdrawing group and transportation of holes formed by the blocking of electrons. The examination of designed molecules is done by different quantum and chemical techniques. Also compare with reference molecule, the properties like photo-physical, opto-electronic properties and geometrical parameters, density of states, absorption maxima, excitation, binding and reorganizational energies of FD1-FD6 were exclusively studied. So finally, findings about newly designed molecules suggest that these can be used for the development of highly efficient solar cells.

The calculations of newly designed solar cells have been done by using Gaussian 09 program and Gauss-View program visualized them. Firstly the reference molecule R optimized by using commands as B3LYP ¹³, CAM-B3LYP ¹⁴, MPW1PW91 ¹⁵ and ωB97XD ¹⁶ levels in conjunction with 6-31G(d,p) ^17–20 basis set. By using above mentioned levels calculated the Time-dependent density functional theory (TDDFT) for reference compound to simulate the absorption spectrum. The absorbed maximum wavelength of reference compound is compared with reported experimental data. The absorption maximum (λ_max) value of Cam-B3LYP/6-31G(d,p) level is nearest to experimental data ¹² as compare to other levels. At this level (Cam-B3LYP/6-31G(d,p) ) of theory the λ_max for reference R molecule and designed molecules ( FD1, FD2, FD3, FD4, FD5 and FD6) solvent phases. The analysis of TDM, dipole moment, reorganization energies, DOS calculations, FMOs and charge transfer of reference R and designed molecules have performed at above level (B3LYP/6-31G(d,p) ) of DFT.

Reorganization energies has two sub categories:

(i) Internal reorganization energy (λ_int.) (ii) External reorganization energy (λ_ext)

The internal structure changes related to internal energy and external environment changes related to external energy. So we could have neglected the internal energy as can have neglected the external energy because it did not affect the current studies so much. For the calculation of reorganization energy below mentioned equations are helpful for electron (λe) and hole (λ_h) ^21–23.

λe = [E₀⁻ - E₋] + [E₋ ⁰-E₀] (1)

λ_h = [E₀⁺ - E₊] + [E₊ ⁰- E₀] (2)

The energies of neutral molecule are obtained by optimized anionic and cationic structures respectively.

3.1 Selection of basis set

In this contribution, the reference molecule CS01 (R) was initially optimized at four functionals like Cam-B3LYP, MPW1PW91, ωB97XD, and B3LYP along with 6-31G(d,p) basis set. The DFT based observed absorption maxima in dichloromethane solvent is 486 nm (Cam-B3LYP/6-31G(d,p)), 625 nm (MPW1PW91/6-31G(d,p)), 475 nm (ωB97XD/6-31G(d,p)), 685 nm (B3LYP/6-31G(d,p)), while experimental calculated absorption maxima 529 nm ¹² as shown in Figure 1. From above data and Figure 1, it is evidently concluded that CAM-B3LYP/6-31G(d,p) basis set disclosed best agreement with experimental data.

3.2 Designing of molecules

In this contribution, the reference molecule CS01 is taken as reference molecule (R) ¹². end-capped donor modifications of R (reference molecule) are made with N,N-dialkylaniline (FD1), triphenylamine (FD2), N, N-diphenylnapthalein-1-amine (FD3), indoline (FD4), carbozole (FD5), and phenothiazine (FD6) as shown in Figure 2. After modifications, six new molecules are designed which were further analyzed by employing DFT calculations at Cam-B3LYP/6-31G(d,p) basis set.

3.3 Alignment of Frontier molecular orbital

Lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) are known frontier molecular orbitals. In molecular orbital theory, the LUMO is considered as valence band and HOMO is considered as conduction and the gap between these two bands are known as band gap or in simple words it is known as HOMO-LUMO energy gap ^20,24–28. Energy gap is very important tool for analyzing the overall photovoltaic properties of a solar cell as it directly affected the rate of charge transfer between two molecular orbitals. So, the photovoltaic devices with narrow band gap offers high charge transfer between two molecular orbitals and vice versa. The optimized geometries of all studied molecules at Cam-B3LYP/6-31G(d,p) basis set is given in Figure 3.

Energies of both orbitals (HOMO and LUMO) also play vital role for demonstrating the overall efficiency of a photovoltaic device. The frontier molecular orbitals analysis is considered at B3LYP/6-31G(d,p) basis set of DFT. Table 1 describes the values of frontier molecular orbitals energies and HOMO-LUMO energy gap. Reference molecule disclosed HOMO and LUMO energy values as -5.60, and -0.80 eV. The designed molecules FD1-FD6 displayed HOMO values of -5.69, -5.57, -5.71, -5.45, -5.45, -5.92, and -5.93 eV, and LUMO values of -0.90, -0.83, -0.95, -0.74, -1.15, and -1.24 eV, respectively. The DFT based calculated band gap of reference molecule is 4.80 eV. Design molecules (FD1-FD6) displayed band gap of 4.79, 4.74, 4.76, 4.71, 4.77, and 4.69 eV, respectively. All designed butterfly-shaped molecules disclosed narrow band gap as compared to reference molecule. Narrow band-gap in designed molecules is might be due to addition of extra and efficient end-capped donor units in end-capped donor moieties. Narrowest energy band gap is observed in FD6 molecule which is due to phenothiazine unit. Second and third lowest energy band gap values are seen in FD4 and FD2 molecules which is probably due to indoline and triphenylamine units as shown in Figure 4. FD3 and FD5 molecules have very close values of band gap which is might be due to addition of extra functional group in donor moieties. The decreasing order of energy band gap of all studied molecules is: R> FD1> FD5> FD3> FD2> FD4> FD6.

From order and proceeding discussion, it is concluded that our designed molecules are superior than reference molecules, thus these molecules are fine aspirant for high performance solar cells.

Table 1: Energy of HOMO (E_HOMO), energy of LUMO (E_LUMO) and energy gap (E_g) of all studied molecules

Molecules	E_HOMO (eV)	E_LUMO (eV)	E_g (eV)
R (CS01)	-5.60	-0.80	4.80
FD1	-5.69	-0.90	4.79
FD2	-5.57	-0.83	4.74
FD3	-5.71	-0.95	4.76
FD4	-5.45	-0.74	4.71
FD5	-5.92	-1.15	4.77
FD6	-5.93	-1.24	4.69

3.4 Density of states analysis

Density of states (DOS) analysis has been done to unveil the position of frontier molecule orbitals ^{17,18,26,29–34}. This analysis is also useful for locating the HOMO and LUMO densities on a molecule. DOS analysis provides the evidences of efficient designing of hole transport materials by end-capped donor modifications.

Density of states analysis of all designed molecules is performed at Cam-B3LYP/6-31G(d,p) basis set and DOS plots are given in Figure 5. From figure it is came to know that HOMO density in molecules are present on donor part. In reference and designed molecules same distribution is observed. In designed molecules end-capped efficient donor units donate electron density and core unit being acceptor in nature accept the electron density. The presence of LUMO density on central acceptor unit supportive of our notion. So, from above analysis it is concluded that all designed molecules are efficient donor molecules and donate the electron density efficiently and presence of same distribution pattern of electron density in reference and designed molecules suggested that our end-capped donor modification is a fine approach for development of high performance solar cells.

3.5 Photovoltaic Properties

Excitation energy, molecular oscillating strength, concern HOMO to LUMO molecular assignment and absorption maxima in solvent phase are known as photovoltaic properties and behavior of a solar cell ³⁵. All these properties are comprehensively studied and analyzed by employing time dependent-density functional theory at Cam-B3LYP method with conjunction of 6-31G(d,p) basis set.

The absorption maxima of all studied molecules is estimated in dichloromethane solvent as shown in Table 2. The experimental recorded absorption maxima were 529 nm, while DFT based observed absorption maxima is 486.01 nm. Usually, red-shifting in absorption spectrum offer high and intense absorption in particular region of light which in return enhances the overall power conversion efficiency of a solar cell. The designed butterfly-shaped donor molecules expressed large red-shifting in absorption spectrum which might be due to addition of different end-capped donor units in end-capped donor moieties of reference molecule. The absorption maxima values of designed molecule FD1 is 506.33 nm, similarly other designed molecules have absorption maxima values of 528.21 nm (FD2), 530.93 nm (FD3), 551.44 nm (FD4), 549.01 nm (FD5), and 568.65 nm (FD6), respectively. All butterfly-shaped designed molecules have red-shifted absorption spectrum as compared to reference molecule. Large red-shifting in absorption spectrum is seen in FD6 followed by FD4, which is might be due to phenothiazine and indoline end-capped units. Similarly, FD5, FD3, FD2 and FD1 also exhibited red-shifting in absorption spectrum which is probably due to carbazole, N, N-diphenylnapthalein-1-amine, triphenylamine, and N,N-dialkylaniline end-capped units. In short, all designed molecules displayed red-shifting in absorption spectrum which mean that these designed molecules are better hole transport materials as compared to reference molecule for solar cells.

Table 2. Absorption maxima (experimental and DFT based), excitation energy, oscillating strength and major concerned molecular orbitals assignments

Molecule	DFT.λ_max	Exp.λ_max	E_x (eV)	f	Assignment
R (CS01)	486.01	529	2.01	2.17	HOMO→LUMO (99%)
FD1	506.33	---	1.99	2.37	HOMO→LUMO (99%)
FD2	528.21	---	1.81	2.92	HOMO→LUMO (99%)
FD3	530.93	---	1.80	3.19	HOMO→LUMO (99%)
FD4	551.44	---	1.74	3.26	HOMO→LUMO (98%)
FD5	549.01	---	1.76	3.15	HOMO→LUMO (99%)
FD6	568.65	---	1.69	3.35	HOMO→LUMO (99%)

Excitation energy is yet another tool which measure the efficiency of a solar cell. The lower value of excitation energy offers high power conversion efficiency and rapid transportation of charge density from excited HOMO to excited LUMO in a solar cell. Therefore, excitation energy analysis of all designed molecules is estimated at Cam-B3LYP/6-31G(d,p) basis set. According to Table 2, all designed molecules disclosed lower values of excitation energy as compared to reference molecule which is might be due to addition of efficient donor moieties in the end-capped of all donor molecules. The decreasing order of excitation energy is: [R (E_x=2.01 eV)]> [FD1 (E_x=1.99 eV)]> [FD2 (E_x=1.81 eV)]> [FD3 (E_x=1.80 eV)]> [FD5 (E_x=1.76 eV)]> [FD4 (E_x=1.74 eV)]> [FD6 (E_x=1.69 eV)]. From discussion, it is suggested that designed molecules are better candidates for solar cell applications as compared to reference molecule.

Simulated UV graph of all studied molecules is also considered in solvent phase and displayed in Figure 6. From Figure 6, it is easily located that all designed molecules have great red-shifting in absorption spectrum as compared to reference molecule. So, results of photovoltaic parameters suggested that designed butterfly-shaped donor molecules are efficient aspirants for solar cell applications.

3.6 Reorganizational Energy

Reorganizational energy is one of the important factor which decided the overall efficiency of a solar cell. Charge mobility and reorganizational energy have inverse relationship with each other, i.e. lower value of reorganizational energy allows high charge mobilities and vice versa. Low reorganizational energy of a solar cell offers high hole and electron mobility in a solar cell. High hole and electron transportability of a solar cell also provides high power conversion efficiency in a solar cell.

The reorganizational energy of hole and electro is considered at Cam-B3LYP/6-31G(d,p) basis set. The reorganizational energy of hole of reference molecule is 0.0171, while designed molecules have reorganizational energy of hole of 0.0168 (FD1), 0.0168 (FD2), 0.0165 (FD3), 0.0166 (FD4), 0.0158 (FD5), and 0.0149 (FD6). All designed molecules displayed low values of hole reorganizational energy as shown in Figure 7. Lowest value is noted for FD6 molecule which is due to phenothiazine unit that is present in end-capped moiety of FD6. Second and third lowest value of hole reorganizational energy is seen FD5 and FD3 molecules which is might be due to carbazole and N, N-diphenylnapthalein-1-amine end-capped units. Overall, it is observed that designed molecules are better hole transport materials for solar cell applications.

Reorganizational energy of electron is also estimated at Cam-B3LYP/6-31G(d,p) basis set. The calculated reorganizational energy of electron of reference molecules is 0.0198. The reorganizational energy of electron of designed molecules are 0.0206, 0.0221, 0.0208, 0.0201, 0.0211, and 0.199 for FD1-FD6, respectively. All designed molecules have comparable values of reorganizational energy of electron as compared to reference molecule. Among designed molecules, FD6 expressed highest electron mobility and lowest reorganizational energy of electron which is might be due to addition of extra functional group in end-capped acceptor moiety. Overall, from discussion, it is evidently proved that all designed molecules are fine candidates for solar cell applications, thus designed are recommended for development of high performance solar cells.

Table 3. Reorganizational energy of hole and electron of all studied molecules at CAM-B3LYP/6-31G(d,p) basis set of DFT.

Molecules	λ_e	λ_h
R (CS01)	0.0198	0.0171
FD1	0.0206	0.0168
FD2	0.0221	0.0168
FD3	0.0208	0.0165
FD4	0.0201	0.0166
FD5	0.0211	0.0158
FD6	0.0199	0.0149

3.7 Open Circuit Voltage

The total current drawn for any optical device is known as open circuit voltage. Open circuit voltage depends on saturated and degenerated current of a photovoltaic device. Open circuit voltage measures the power conversion efficiency of a solar cell. Open circuit voltage (V_oc) also affects the current charge density of a solar cell. Open circuit voltage in the case of charge density donor molecule, is calculated by lying the HOMO of donor molecules and LUMO of a acceptor polymer.

Our reference and designed molecules (FD1-FD6) are donor in nature with good charge donating aptitude. So, we use the HOMO orbital of all these molecules that are lying with LUMO of acceptor polymer which is PC₆₁BM. PC₆₁BM is well known and very famous acceptor polymer and mostly used for solar cell applications. Figure 8, expressed the open circuit voltage of all molecules with respect to HOMO_Donor-LUMO_PC61BM. The Voc of reference molecule is 1.60 V, and open circuit voltage for designed molecules FD1-FD6 is 1.69, 1,57, 1.71, 1.43, 1.92, and 1.93 V as shown in Figure 8. All designed molecules (except FD2, and FD4) have higher value of open circuit voltage. The higher values of open circuit voltage suggested that designed molecules are efficient candidates for solar cell applications. Among designed molecules, FD6 molecule disclosed highest open circuit voltage which is due to phenothiazine end-capped unit. Second and third highest values of open circuit voltage is seen in FD5 and FD3 molecules which is probably due to carbazole and N, N-diphenylnapthalein-1-amine end-capped units. Similarly, FD1 due to N,N-dialkylaniline end-capped unit disclosed high open circuit voltage as compared to reference molecule as shown in Figure 8. So, from above discussion we concluded that designed molecules especially FD6 and FD5 are more efficient candidates for solar cell applications.

Orbitals position donor molecules and acceptor polymer is very important for charge shifting ³⁶. It is suggested that low lying LUMO of acceptor polymer and high level of HOMO od donor molecules offer high charge shifting in donor: acceptor blend. In current report, same condition is seen which suggested that designed donor molecules are efficient molecules that contributes much in overall charge transfer from donor to acceptor molecule as shown in Figure 9.

3.8 Transition density matrix and binding energy analysis

Transition density matrix (TDM) were carried out to investigate the electronic transition processes occurred in photovoltaic materials. TDM study of the designed materials provides a useful insight about electronic transition and has a characteristic to produce three dimensional (3D) maps which corresponds to the linked electron-hole pairs. These instigations also provide information about the delocalization and coherence lengths of the materials. TDM analysis also useful to estimate the nature of transition appeared during S₁ emission state. Mainly, we can employ these calculations to understand the charge transfer excitation behavior of the molecules. The calculated TDM plots of all the designed and the reference molecule R are shown in Figure 10.

Table 4: Binding energy, first principle excitation energy and HOMO-LUMO energy gap of all studied HTMs

Molecules	E_H-L	E_opt	E_b
R (CS01)	4.80	2.01	2.79
FD1	4.79	2.02	2.77
FD2	4.74	1.99	2.75
FD3	4.76	1.97	2.79
FD4	4.71	2.00	2.71
FD5	4.77	2.09	2.68
FD6	4.69	2.22	2.47

In order to understand this phenomenon, we have divided the molecule into three different parts; donor as core unit (D), donor part (D) while terminal acceptors as (A), respectively. We realized that the end-capped donors as well as bridged part are covered with electronic charge mobility of the corresponding material. The presence of various electronic charge associations in reference molecule R and in FD1-FD6 designed systems revealed that charge transformation occurred from donor to bridged part of the molecule with any hindrance. The other important parameter that is related to the evaluation of proficiency, excited splitting potential and the related electronic properties is binding energy. The columbic forces present in the molecule was responsible for the interaction of hole and electrons which is directly influenced by E_b and inversely proportional to excitation segregation potential at excited state. Hereafter, any increment in E_b can directly influence the columbic force present in between the electron and holes. The calculation of the related E_b can be done by the following equation.

E_b = E_H-L_E_opt (3)

In this equation, E_H-Lcorresponds to the difference of HOMO and LUMO, while, E_opt relates to the amount of energy required for first excitation (S₀-S₁) known as singlet excitation. As we see Table 4, FD6 has the ability to produce extra charges that can dissociate easily and that’s why we can assume that it would be more helpful to improve the photocurrent density of the devices.

Figure 11, showed the calculated Eb, Eopt and Eh-L spectrum. It is interesting that all the designed materials exhibited low Eb values which provides an evidence of our efficient designing of materials. The corresponding Eb, EH-L and Eopt values have been shown in Table 4. All of the calculated values of the designed materials lie closer to the reference molecule R. Among these, FD6 showed the lowest possible Eb value which is due to attachment of an efficient end-capped unit. The other designed materials FD1 to FD5 have also quite lower Eb values as compared with reference R. Hence, due to this study we can estimate the efficient charge transformation behavior of the molecules which will ultimately help to boost the J_sc values of these materials. Based on these investigations, we can claim that these designed materials are best fitted for solar cells applications.

3.9 Molecular Electrostatic Potential (MEP) analysis

Figure 12 shows the computational characterization of MEP analysis of all the designed (FD1-FD6) and the reference R. These MEP investigations was done by employing well known Cam-B3LYP/6-31G(d,p) level of theory. The purpose behind these investigations is to understand the behavior of various charge sites present in the molecule. These calculations are done using iso-surface value of 0.02e/Å³for all the newly designed (FD1-FD6) and also for the reference molecule R. In these outcomes, we realized various cloud densities and every cloud color density is related to specific property of the molecule. For instance, blue color showed presence of positive charge accumulation, red for negative and green color represent the most possible potential area of the molecule i.e. neutral region among all designed (FD1-FD6) and reference molecule R.

Interestingly, we realized almost similar charge sites over all the designed molecules especially on FD1-FD4 as compared with reference R. The designed materials FD5 and FD6 shows the specific uniform distribution of charges which spreads on the whole molecule equally. Therefore, with these results we can predict our efficient end-capped designing of the molecules for photovoltaic applications. As shown in figure 12, more charge density spreads over FD6, indicating its superiority over others designed materials (FD1-FD5) for the most efficient charge transfer reactions.

3.10 Study of Complex FD6/PC₆₁BM

We have also performed some specific calculations related to charge transfer analysis (CTA) for all the newly designed as well as for reference molecule R. For this purpose, we have selected one best working designed material (FD6) for these investigations and makes its complex with the well-known polymer acceptor PC₆₁BM. The polymer acceptor PC₆₁BM and the D-A units FD6:PC₆₁BM were initially optimized at Cam-B3LYP/6-31G(d,p) level of theory. The selection of donor FD6 was done due its outstanding hole mobility and charge transfer abilities as compared with other designed materials (FD1-FD5), respectively. The optimized geometries of newly designed FD6 and polymer acceptor PC₆₁BM revealed that both of the materials are oriented parallel to each other, as shown in figure 13. The FD6: PC₆₁BM formed a unique orientation where PC₆₁BM came parallel on that side of FD6 where there is a chance for maximum charge transformation can happen in between these D-A interfaces.

In addition, we have also optimized FD6 and PC₆₁BM units individually to investigate their HOMO and LUMO levels, as shown in figure 13. For this, we have also used the same methodology (Cam-B3LYP/6-31G(d,p)) as we did for other designed materials and the reference R. The HOMO-LUMO distribution patterns are shown in figure 14. With these studied, we realized that the HOMO density resides over donor part i.e. FD6, while, LUMO density spreads over PC₆₁BM. At HOMO-LUMO interface, we suspect that charge transformation occurred from D to A interface, ensuring clear understanding for charge density transformation between two oppositely charge moieties.

In this work, we designed butterfly shaped non-fullerene acceptors for solar cells applications. Remarkably, after successful theoretical optimizations, all the newly designed (FD1-FD6) have showed quite interesting photo-physical, optoelectronic, and photovoltaic properties. We efficiently designed these new FD1-FD6 molecules by end-capped mutation of reference material R. The HOMO-LUMO energy-gap contractions are quite narrower in FD1-FD6 designed molecules (E_g=1.69-1.99 eV) as compare to reference compound R (E_g=2.01 eV), which are quite desirable and attractive. This end-capped modification strategy revealed our efficient designing and explore the ways how we can tune the band-gaps of the materials efficiently and investigate their photo-physical and opto-electronic properties theoretically first instead after synthesizing them. The distribution pattern of HOMO and LUMO depends upon the charge density and overlap density of states. High absorption efficiency was achieved by observing highly red- shifting behavior in absorption spectrum, sideways with maximum charge shifting from excited HOMO to LUMO. All the theoretically optimized molecules (FD1-FD6) revealed suitable red-shifting absorption behavior as opposed to reference material R. Besides, the resultant molecule (FD6) has minimal excitation and binding energy which might have ability to produce high current density. Usually, low energies of electron and hole are responsible for high charge mobilities, therefore, our designed molecules (FD1-FD6) displayed high electron mobilities as opposed to reference material R. Open circuit voltages and complicated study of PC₆₁BM:FD6 complex is also achieved by realizing great charge shifting from HOMO to LUMO molecular orbitals. Transition density matrix and hole-electron overlap study of all calculated molecules (FD1-FD6) divulge the position of charges in a molecule. Briefly, the fallouts of all analysis show that our designed compounds FD1-FD6 have high photo-voltaic and opto-electronic properties than reference material R. That’s why, the conceptualized fullerene-free molecules should be aimed for synthesis of high-performance organic solar cells.

Acknowledgement: We are highly thankful to Prof, Jae Kwan Lee from Chosun University, Republic of Korea for his useful guidance about the project and to provide his state-of-the-art lab facilities.

Conflict of interest

All authors declared no conflicts of interest.

Associated Content

Optimized Cartesian coordinates of all studied molecules (R and FD1-FD6) are available in supporting information file.

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Efficient Molecular Modelling of Butterfly-Shaped Hole Transport Materials With Remarkable Photovoltaic Properties For High-Performance Organic Solar Cells

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1. Introduction

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