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. 2021 Oct 20;6(43):28923–28935. doi: 10.1021/acsomega.1c03975

Tuning of a A–A–D–A–A-Type Small Molecule with Benzodithiophene as a Central Core with Efficient Photovoltaic Properties for Organic Solar Cells

Urwah Azeem †, Rasheed Ahmad Khera †,*, Ayesha Naveed †, Muhammad Imran ‡, Mohammed A Assiri ‡, Muhammad Khalid §, Javed Iqbal †,∥,*
PMCID: PMC8567361  PMID: 34746584

Abstract

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With the aim of upgrading the power conversion efficiency of organic solar cells (OSCs), four novel non-fullerene, A1–A2–D–A2–A1-type small molecules were designed that are derivatives of a recently synthesized molecule SBDT-BDD reported for its efficient properties in all-small-molecule OSCs (ASM-OSCs). Optoelectronic properties of the designed molecules were theoretically computed with a selected CAM-B3LYP functional accompanied by the 6-31G(d,p) basis set of density functional theory (DFT), and excited-state calculations were performed through the time-dependent self-consistent field. The parameters of all analyzed molecules describing the charge distribution (frontier molecular orbitals, density of states, molecular electrostatic potential), absorption properties (UV–vis absorption spectra), exciton dynamics (transition density matrix), electron–hole mobilities (reorganization energies), and exciton binding energies were computed and compared. All the designed molecules were found to be superior regarding the aforesaid properties to the reference molecule. Among all molecules, SBDT1 has the smallest band gap (3.88 eV) and the highest absorption maxima with broad absorption in the visible region. SBDT3 has the lowest binding energy (1.51 eV in chloroform solvent) ensuring easier and faster dissociation of excitons to produce free charge-carriers and has the highest open-circuit voltage (2.46 eV) with PC61BM as the acceptor. SBDT1 possesses the highest hole mobility because it has the lowest value of λ+ (0.0148 eV), and SBDT4 exhibits the highest electron mobility because it has the lowest value of λ– (0.0146 eV). All the designed molecules are good candidates for ASM-OSCs owing to their superior and optimized properties.

Introduction

Continuously escalating energy crisis has mobilized researchers’ attention toward exploring new means of energy, especially renewable energy sources. Renewable energy resources are endless sources of energy that are optimistic means to fulfill the energy demands of human beings without causing adverse effects to the environment. Among all the renewable resources, Sun is the most important one since its energy is totally free, ample, and pollution-free. It provides 3.8 million EJ (EJ = 1018 J) energy per year.1−3 Solar energy is exploited by the solar cells (SCs) to produce energy based on the principle of photovoltaic effect, which states that on the absorption of a photon of specific energy, holes and electrons are formed either free (as in inorganic SCs) or combined as excitons (as in organic SCs) which moves toward opposite electrodes, thus generating electricity.4−6 In the mid-twentieth century, the first silicon-based SC devised by Russel Ohl reached the power conversion efficiency (PCE) of 24.7% through continuous revolutionary changes.7 The high cost on account of many complex fabrication steps, rigidity, and being eco-unfriendly are the shortcomings of silicon-based SCs that led to the development of second-generation SCs. A thin layer of amorphous silicon, CIGS, CdTe, GaAs, and so forth was used as the active layer in the second generation of SCs. Researchers have been focusing on exploring other materials to find more efficacious and reliable photovoltaic materials.

Chlorophyll, an organic compound created by Nature, absorbs solar energy helping plants in entrapping and converting solar energy into chemical energy. Inspired by Nature, organic compounds were explored for efficient optoelectronic properties. Organic SCs (OSCs) are preferred to others because of their lower cost, simple device structure, unique electronic properties, easier roll-to-roll (R2R) processing, being light weight, and ability to be fabricated on flexible substrates such as plastic and fabric. In OSCs, the electron donor–acceptor bilayer of organic molecules was employed as an active layer. Due to the less interfacial area and the high diffusion length of charge carriers, the PCE attained by a bilayer heterojunction was not very high.

PCE is the ratio of maximum power of energy conversion of a SC Pmax to the incident radiation power Pir. However, Pmax is determined by three important parameters, viz, the open circuit voltage VOC, fill factor (FF), and short-circuit current density JSC. In 1995, a bulk heterojunction (BHJ) of polymer–fullerene and polymer–polymer was used as an active layer in third-generation SCs. Huang and co-workers employed a BHJ of polymer (donor)/fullerene (acceptor) blends as an active layer in OSCs. Zhang and co-workers used an active layer containing a BHJ of a polymer (donor)/polymer (acceptor) blend and achieved an improved PCE.8,9 Traditionally, a polymer-conjugated donor was paired with fullerene acceptor molecules such as PC61BM and PC71BM and attained a PCE of up to 11%, which is ascribed to good device fabrication and innovative donor materials.10 The PCE of fullerene-based OSCs failed to expand because of the inherent constraints in fullerene acceptors such as weak light-harvesting ability in the UV–visible region, fewer chances of remodeling of the molecule for tuning the energy levels, and high sensitivity to light and oxygen.11−17 Therefore, other molecules were explored as acceptor molecules in place of fullerenes that include perylene diimide derivatives,18 rylene diimide,19 and others. Fused ring electron acceptors are being extensively studied because of advantages such as the ability to tune energy levels, stronger absorption in the UV–visible region, and greater thermal stability.20,21

Small-molecule donors are preferable to polymer donors possessing a well-defined structure, easier purification, and perfect batch-to-batch replicability, and these are important parameters in the commercialization of all-small-molecule organic SCs (ASM-OSCs). For achieving a higher PCE of ASM-OSCs, several criteria should be met such as the complementary absorption of donors and acceptors, a well-optimized structure of the blended film, and matched molecular energy levels of donors with that of acceptors. There is an immediate need to optimize the optoelectronic properties of donors and acceptors to enhance the PCE of ASM-OSCs. End-group functionalization, side-chain engineering, central core modification, and π-bridge insertion are various techniques for the remodeling of small molecules.22,23 A computational study of spectral, structural, and electronic properties of molecules provides an alternative approach to the experimental method to explore the potential of different molecules for commercial use as a photovoltaic material.24 In 2018, Huo and co-workers have developed an excellent small-molecule donor named SBDT-BDD exhibiting a unique dual-accepting unit A1–A2–D–A2–A1 structure. It has a planar conjugated structure comprising a BDT core that is reported for its potential to serve as a good electron-donating moiety due to the presence of sulfur atoms in thiophene rings and the advantage of facile synthesis. This dual-accepting unit donor design has been proved a promising design to increase the PCE of ASM-OSCs by enhancing the JSC and FF. This medium band gap small-molecule donor has reported a PCE of 9.2% for the SBDT-BDD:IDIC blend binary device and 10.9% for the SBDT-BDD:IDIC:PC71BM ternary device.25−27

In the present study, we have reported four novel molecules of A1–A2–D–A2–A1 structure for ASM-OSCs. SBDT-BDD is taken as the reference molecule, it contains alkylthio-substituted benzo[1,2-b:4,5-b]dithiophene (BDT) as a central donor moiety(D), two BDD (1,3-bis(thiophen-2-yl)-5,7-bis(2-ethylhexyl)benzo-[1,2-c:4,5-c]dithiophene-4,8-dione) units as acceptor moieties (A2), and two 3-ethylrhodanine dye units as terminal acceptor moieties (A1), all the constituent units being connected through thiophene bridges.28 Four new molecules are designed by substituting the acceptors of SBDT-BDD with four electron-withdrawing acceptor groups29−31 that are already reported to enhance the optical properties by enhancing the conjugation resulting in a better charge distribution. The thiophene bridges also take part in extending the conjugation to facilitate charge transfer (CT) from the core toward terminals. These new donor molecules are expected to improve the PCE of ASM-OSCs.

Results and Discussion

Designed Molecules and Optimized Geometries

Four new molecules (SBDT1–SBDT4) were designed and derived from SBDT-BDD that was experimentally synthesized and characterized for its potential to increase the PCE of ASM-OSCs. We substituted A1 with 2-(5,6-difluoro-2-methylene-3-oxo-2,3-dihydroinden-1-ylidene)malononitrile and 2-(2-methylene-3-oxo-2,3-dihydroinden-1-ylidene)malononitrile in SBDT1 and SBDT2, respectively, while A2 was replaced with a benzo[c][1,2,5]thiadiazole substituent in both. The terminal acceptor A1 in SBDT-BDD was substituted with 2-(5,6-difluoro-2-methylene-3-oxo-2,3-dihydroinden-1-ylidene)malanonitrile in SBDT3. SBDT4 was designed by replacing both acceptors (A1 and A2) in reference molecule with benzo[c][1,2,5]thiazole and thiophene moieties at both ends. The easy availability, low cost, and validity were the benchmarks that were taken into consideration while deciding an appropriate method with a suitable basis set. The geometry of the reference molecule was optimized to the lowest potential energy using four different functionals, viz, CAM-B3LYP, B3LYP, MPW1PW91, and ωB97XD with the basis set 6-31G(d,p); their absorption maxima (λmax) in chloroform solvent were estimated, and the values obtained were 466, 669, 615, and 444 nm, respectively, whereas the experimentally determined λmax was at 521 nm, as depicted by Figure 1. CAM-B3LYP was found to be in sensible agreement with the absorption maximum value found in literature. Then, the geometries of the designed molecules (SBDT1–SBDT4) were optimized with this selected functional together with the basis set 6-31G(d,p). The chemical structures of designed molecules and the reference molecule are displayed in Figure 2 in which the central BDT core, thiophene bridges, and acceptor moieties are highlighted in blue, red, and black colors, respectively. The optimized geometries of all the designed molecules in the ball and stick model including the reference are shown in Figure 3.

Figure 1.

Figure 1

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Absorption profile of SBDT-BDD calculated with four different hybrid functionals with a magenta line indicating its literature value.

Figure 2.

Figure 2

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Molecular structures of designed chromophores (SBDT1–SBDT4) including the reference SBDT-BDD drawn using ChemDraw Ultra software.

Figure 3.

Figure 3

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Geometries of SBDT-BDD, SBDT1, SBDT2, SBDT3, and SBDT4 in the ball and stick model optimized at the CAM-B3LYP/6-31G(d,p) level of theory.

2. Quantum Mechanical Descriptors (Highest Occupied Molecular Orbital and Lowest Unoccupied Molecular Orbitals)

The lowest unoccupied molecular orbitals (LUMO) and the highest occupied molecular orbital (HOMO) are collectively referred to as frontier molecular orbitals (FMOs). Frontier molecular orbitals quantum mechanically describe the optoelectronic properties of molecules by displaying an electronic distribution pattern in the HOMO and LUMO of the molecules. By analyzing the electronic distribution in FMOs, the intramolecular charge transfer (ICT) can be estimated. A pictographic display of FMOs of all investigated molecules is given in Figure 5. The electron density transfers from the valence band at the ground state, that is, HOMO to the conduction band at the excited state, that is, LUMO during ICT. In SBDT-BDD and SBDT1-SBDT3 at the ground state, the electron density is spread mainly on the BDT core, alkylthio substitution along with adjacent thiophene moieties and slightly on acceptor A2 (BDD or benzothiazole), while in the excited state the electron density is shifted to terminal acceptors (A1) along with thiophene bridges between the acceptors and more on A2 acceptors. In SBDT4, HOMO has the electron density uniformly spread all over the molecule, while LUMO has the electron density mainly on acceptor sections and thiophenes. HOMO is more stable and lower in energy than LUMO. The HOMO and LUMO values along with band gaps of reference and designed molecules are listed in Table 1. HOMO is stabilized in reported molecules in the order of SBDT3 > SBDT1 > SBDT-BDD > SBDT2 > SBDT4, and their LUMO are stabilized in the order of SBDT1 > SBDT3 > SBDT2 > SBDT-BDD > SBDT4. LUMO in SBDT1 is most stable due to the joint effect of extended conjugation and strong electron-withdrawing abilities of benzothiazole acceptors as well as terminal electron acceptors containing a benzene ring, two cyano groups, and two high electronegative fluorine atoms.

Figure 5.

Figure 5

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Quantum mechanical descriptors (HOMO–LUMO) of SBDT-BDD and SBDT1-SBDT4 at their optimized geometries.

Table 1. HOMO and LUMO Energy Levels and Band Gap of Designed Molecules (SBDT1–SBDT4) and the Reference SBDT-BDD Molecule.

molecules –EHOMO (eV) –ELUMO (eV) Eg (eV)
SBDT-BDD –6.39 –1.90 4.49
SBDT1 –6.42 –2.54 3.88
SBDT2 –6.35 –2.41 3.94
SBDT3 –6.46 –2.42 4.03
SBDT4 –6.15 –1.66 4.49
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Molecules having a planar structure, push–pull assembly, and extended conjugation are expected to have a narrow band gap and subsequently exhibit high CT and efficient absorption in the longer wavelength region. The energy difference between the LUMO and HOMO energy levels is called the band gap denoted as Eg [Eg = ELUMO – EHOMO]. The shorter the frontier orbital gap, the more the chemical reactivity, the lower the stability, and the easier the CT will be, thus the rate of CT increases that results in more photon utilization and an increase in the PCE.32 The band gap of the reference molecule SBDT-BDD is 4.49 eV. The band gap of all molecules is in the descending order of SBDT-BDD = SBDT4 > SBDT3 > SBDT2 > SBDT1. All the designed molecules have a shorter band gap than the reference molecule except that SBDT4 that has a frontier gap of 4.49 eV equal to SBDT-BDD because the benzothiazole moiety has an electron-accepting strength equal to a combination of BDD and 3-ethylrhodanine dye moieties. SBDT1 has the lowest value of HOMO–LUMO gap attributed to the stabilized LUMO of the molecule because of its stronger end-capped electron acceptors at terminals containing diflouro benzene and efficient benzothiazole acceptors that mutually extend conjugation ultimately resulting in a narrowing band gap. SBDT2 and SBDT3 have band gaps still lower than that of the reference molecule that correspond to the benzoid rings of end-capped acceptors. The band gap plot of all molecules is displayed in Figure 4.

Figure 4.

Figure 4

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Band gap plot of designed and reference molecules plotted using Origin2021b software.

3. Absorption Properties

A quantum absorption spectrum is important in predicting the photo-electronic properties of the molecules. Chromophores absorb photons of specific energy corresponding to their band gaps and get excited. The absorption profile (λmax), oscillating strength (f), excitation energy (Ex), and dipole moment (μ) estimate the ability of molecules to utilize solar energy efficiently to result in better CT. All the spectral data of investigated chromophores in the solvent phase and the gaseous phase are presented in Tables 2 and 3 respectively. Oscillating strength is the probability of transition and excitation energy is the energy required for transition, thus a high oscillating strength, a low excitation energy, and broader absorption at a higher molar absorption coefficient (λmax) are expected to produce efficient ICT.22 The energy was calculated theoretically at the excited state time-dependent self-consistent field (TD-SCF) of all optimized chromophores (SBDT-BDD and SBDT1–SBDT4) using density functional theory (DFT) theory with CAM-B3LYP/6-31G(d,p) in the gaseous phase and in the solvent phase using an integral equation formalism for the polarizable continuum model (IEFPCM) and chloroform as the solvent up to 20 excited states. The first excitation S0→S1 contributes largely to HOMO–LUMO CT, having a high oscillating strength, hence it is interpreted here. Origin 6.0 software was used to plot the absorption spectra between the normalized absorption coefficient in M–1cm–1 along the y-axis and the wavelength in nanometer along the x-axis. Absorption maxima, like the band gap, is greatly influenced by extended conjugation and the electron-withdrawing effect of acceptor moieties. All the designed molecules exhibited good absorption in the visible region due to the combined effect of chromophoric and auxochromic moieties. The absorption maximum in the gaseous state is in the increasing order of SBDT-BDD < SBDT4 < SBDT3 < SBDT2 < SBDT1, while in chloroform solvent, absorption maxima were largely red-shifted in a similar order of SBDT-BDD = SBDT4 < SBDT3 < SBDT2 < SBDT1. All the molecules showed a bathochromic shift in the chloroform solvent because the polar solvent stabilized the polar excited state LUMO causing a decrease in the band gap and an increase in the wavelength since the energy and the wavelength have an inverse relation. All chromophores showed a red-shifted λmax value except SBDT4 that exhibited an overlapping absorption profile (apart from the shoulder peak in the range of 250–400 nm) with the reference SBDT-BDD molecule on account of acceptor moieties of equivalent strengths; besides, both molecules have equivalent band gaps. SBDT1 showed the largest bathochromic shift among all the designed molecules due to its strong electron-accepting constituents, the benzothiazole acceptor and the terminal electron acceptor containing a benzene ring in conjugation with cyanide, ester, and fluorine atoms. SBDT1 showed large red shifts of 75 and 78 nm in the gaseous and solvent phases, respectively, as compared with SBDT-BDD. All designed molecules have a broad absorption in the visible region and a shoulder peak in the 250–400 nm region. SBDT2 has absorption maxima that is 7 nm blue-shifted than SBDT1 and 72 nm red-shifted than the reference molecule in the solvent phase. SBDT1–SBDT3 have large oscillating strengths and low excitation energies and are promising chromophores to exhibit excellent photon utilization and thus a good CT in organic photovoltaics. The UV–vis spectra as a multiple line plot for all chromophores in gaseous and solvent phases are displayed in Figure 6.

Table 2. Absorption Profile, Excitation Energy, Oscillating Strength, Assignments, and Dipole Moments of all Designed and Reference Molecules in the Chloroform Solvent Phase.

molecules λmax (nm) calc. λmax (nm) exp. Ex (eV) f assignments dipole moment
SBDT-BDD 465 521 2.66 4.1645 H–L (+31%) 8.74
SBDT1 543   2.28 4.3375 H–L (+39%) 3.36
SBDT2 537   2.31 4.3038 H–L (+40%) 3.48
SBDT3 490   2.52 3.7607 H–L (+23%) 7.41
SBDT4 465   2.66 2.4787 H–L (+57%) 4.00
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Table 3. Absorption Profile, Excitation Energy, Oscillating Strength, Assignments, and Dipole Moments of all Designed and Reference Molecules in the Gaseous Phase.

molecules λmax (nm) calc. Ex (eV) f assignments dipole moment excited (Debye)
SBDT-BDD 458 2.71 3.9339 H–L (+32%) 7.12
SBDT1 532 2.32 4.0319 H–L (+38%) 2.78
SBDT2 527 2.35 3.9961 H–L (+39%) 2.89
SBDT3 479 2.58 3.6544 H–L (+32%) 6.44
SBDT4 460 2.69 2.2090 H–L (+54%) 3.28
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Figure 6.

Figure 6

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UV-vis spectra of targeted molecules (SBDT1–SBDT4) including SBDT-BDD in the gaseous state and in the solvent (chloroform) phase simulated using Origin 6.0 software at CAM-B3LYP/6-31G(d,p).

4. Density of State Analysis

To further validate the inference of FMOs, the density of states (DOS) was estimated with CAM-B3LYP/6-31G(d,p), and a curve plot was obtained using the PyMOlyze 1.1 package. The DOS plot has relative intensity along the vertical axis, and the energy in the electron volt unit along the horizontal axis. The region with negative values of energy denotes the ground state (HOMO) and the region with positive values of energy indicates the excited state (LUMO). The DOS plot illustrates the number of electronic states present at a specific energy and also displays the electron density on different segments of molecule in the valence band (HOMO) as well as in the conduction band (LUMO), and the difference in the energies of these bands is called the band gap. The molecules are divided into three fragments to understand their individual contribution toward the total electron density, viz, the donor, the bridge and the acceptor. In DOS plots, the black line indicates the total absorption (total DOS) of the whole molecule, while the red, green, and blue lines indicate the individual contribution (partial DOS) of the acceptor, bridge, and donor core fragments, respectively, toward total absorption.24 The peaks of the plot varies from reference to designed molecules due to the fact that all chromophoric molecules have different acceptor groups of varying strength and conjugation. In SBDT-BDD, an acceptor has a major contribution in the HOMO and LUMO, while the bridge and the donor have comparatively less contribution. In SBDT1–SBDT3, the contribution of acceptor moieties is greater than that of the donor and the bridge in the ground state, but acceptors have more prominent peaks at the excited state, as indicated by the well-parted peaks. In SBDT4, all the three fragments contribute equally in HOMO and LUMO. DOS analysis of molecules verifies the inference obtained through FMO analysis, and DOS plots are displayed in Figure 7.

Figure 7.

Figure 7

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DOS curve plots of designed and reference molecules calculated with CAM-B3LYP/6-31G(d,p) and plotted using the PyMOlyze-1.1 package (H represents HOMO and L represents LUMO).

Furthermore, the molecular electrostatic potentials (MEPs) of all the molecules (SBDT-BDD, SBDT1–SBDT4) have been calculated at the CAM-B3LYP/6-31G(d,p) level of DFT theory and are displayed in Figure 8. MEPs are shown as colored maps of molecules, in which different colors on the molecules code areas of different electrostatic potentials. They designate electron-rich or donor and electron-deficient or acceptor sites in the molecules. The order of decreasing potential is blue > green > yellow > red. The blue color in the molecule indicates a high potential (nucleophilic region or donor) region that arises from the accumulation of negative charge. The red colored region is the region of the lowest potential (electrophilic or acceptor) that indicates the accumulation of positive charges in the molecule. Neutral sites in the molecules have zero potential and are green colored.22 All the designed molecules (SBDT1–SBDT4) including SBDT-BDD have a similar pattern of colors, red and yellow colors are present mainly on regions containing electronegative atoms such as sulfur of the thiophene group, nitrogen of cyanide and benzothiazole groups, as well as oxygen of the carbonyl group, thus highlighting the donor sites. The blue color is distributed on BDD acceptors, end-capped acceptors (except cyanides), and other regions containing methyl groups that indicate electron-deficient sites of the molecules.

Figure 8.

Figure 8

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MEP colored maps of designed (SBDT1–SBDT4) and SBDT-BDD molecules.

Fluorine atoms on terminal acceptors are high potential (electron-rich) sites indicted by blue color. MEP maps confirm that designed molecules have more efficient end-capped acceptors having a high positive potential, as indicated by blue sites, ensuring good ICT on excitation.

5. Transition Density Matrix Analysis

Transition density matrix (TDM) is another tool to evaluate the exciton dynamics, that is, generation, motion, and separation of the electron–hole pair and to estimate the type of interaction between the donor and acceptor components of the BDT-based molecule. TDM actually is the analysis of electron-filled simulated orbitals; it indicates electron density transfer within a molecule and the transient position of holes and electrons in the molecule during excitation.33 The TDM energy was calculated using a DFT model and CAM-B3LYP/6-31G(d,p) at the first excited state, then it was plotted using Multiwfn_3.7 (Multiwavefunction) software in the form of a two-dimensional colored plot. The left y-axis and the bottom x-axis of the plot contain the numbering of non-hydrogen atoms from one to the total number of atoms since the software neglected hydrogens by default due to their very little contribution in excitation. The right vertical axis contains electron density which increases as the color changes from blue to red.29 All the simulated TDM plots at the excited state of reference and designed molecules are shown in Figure 9. Two basic trends are observed in the TDM plot; first is the locally excited states (LE states along the diagonal) and second is the CT behavior (CT along off-diagonal). All the molecules are divided into three components, C, A, and B indicating the donor core, acceptors, and thiophene bridge, respectively, thus these components are labeled on a 2D-TDM plot to analyze the contribution of various components toward CT. All the molecules including the reference SBDT-BDD and designed chromophores SBDT1–SBDT4 showed diagonal bright fringes and off-diagonal bright portions for all components, indicating a uniform distribution and good delocalization of electrons on the whole molecule. SBDT4 showed the greatest π-delocalization behavior, as indicated by the off-diagonal bright portions on the overall molecule. TDM analysis described that electron density is efficiently transferred from the donor core through thiophene bridges toward both acceptors in all the reported chromophores that can increase the PCE of OSCs.34

Figure 9.

Figure 9

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2D-TDM map of all designed molecules (SBDT1–SBDT4) and the reference molecule SBDT-BDD plotted through the Multiwfn_3.7 package at the CAM-B3LYP/6-31G(d,p) level of theory (C = core, B = bridge, and A = acceptors).

6. Exciton Binding Energy

Organic chromophores absorb photons and produce a pair of charge carriers (exciton). For charges to move toward the respective electrodes, their separation from the exciton is crucial. Owing to the low dielectric constant value, organic conjugated molecules have strong Coulombic attractive forces between charge carriers and thus a difficult dissociation as the binding energy has an inverse relation with exciton dissociation. Introduction of electron acceptor groups at the terminals helps in the reduction of Coulombic forces and consequently easier dissociation.35 The reported molecules (reference and designed molecules) have terminal acceptors of varying strengths, thus they have different exciton binging energies. The exciton binding energy refers to the energy needed to dissociate excitons into free charge carriers. The binding energy (Eb) is calculated by subtracting the first excitation energy (Ex) from the energy difference of valence and conduction bands (EL–H = Eg), as given in eq 1

6. 1

The reported molecules showed a similar trend as the band gap for Eb and have the exciton binding energy in the descending order of SBDT4 > SBDT-BDD > SBDT2 > SBDT1 > SBDT3. Binding energies of all investigated molecules in the gaseous state and the solvent phase are tabulated in Table 4. Eb has a greater value in chloroform than in the gaseous state because polar solvents interact and tightly bind with the excitons. Like the band gap, both SBDT-BDD and SBDT4 have comparable values of binding energy, ascribed to the constituent electron acceptors of equivalent strength. SBDT3 has the least value of Eb due to the lowest excitation energy that ensures easier exciton production, easier dissociation of excitons, and subsequently high charge carrier generation for achieving a high JSC.

Table 4. Exciton Binding Energy of all Molecules Calculated in the Gaseous State and the Solvent (Chloroform) Phase.

molecules Eg (eV) Ex (eV) gaseous Eb (eV) gaseous Ex (eV) chloroform Eb (eV) chloroform
SBDT-BDD 4.49 2.71 1.78 2.66 1.83
SBDT1 3.88 2.32 1.56 2.28 1.60
SBDT2 3.94 2.35 1.59 2.31 1.63
SBDT3 4.03 2.58 1.45 2.52 1.51
SBDT4 4.49 2.69 1.80 2.66 1.83
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7. Reorganization Energy

Reorganization energy is a parameter to measure the charge carrier mobilities to estimate the CT properties in chromophores. After the exciton splits into holes and electrons, these charge carriers must move toward the respective electrodes before they recombine. There are two types of reorganization energies, one is internal λint. that depends upon the internal geometry of the molecule and the other is external λext. that varies with external factors such as the polarization of the surrounding environment. λext. is ignored here, and the internal reorganization energy is calculated using eqs 3 and 4 of Marcus theory.32,36 Hole and electron mobilities are calculated by estimating the energies of neutral, anionic, and cationic optimized geometries at DFT using CAM-B3LYP/6-31G(d,p). Electron and hole reorganization values of all investigated molecules are tabulated in Table 5. The λ– of chromophores is in the descending order of SBDT-BDD > SBDT2 > SBDT1 > SBDT3 > SBDT4. All designed molecules have a lower reorganization energy for electrons as compared to the reference molecule, indicating higher electron mobilities and therefore are efficient electron transporters. The lower the reorganization energy, the higher the charge mobility will be, so SBDT4 has the lowest λ– value and the highest electron mobility among all the reported chromophores due to more efficient end-capped acceptor groups. λ+ of chromophores is in the decreasing order of SBDT4 > SBDT3 > SBDT-BDD > SBDT2 > SBDT1. The order of hole mobility in designed molecules is a reverse of their order of electron mobility. SBDT1 and SBDT2 have equivalent reorganization energies for electrons and holes owing to equally efficient end-capped acceptor groups (the difference in acceptors is of two fluorine atoms), thus both have equivalent hole and electron mobilities. There is a small difference in the hole mobilities of all the investigated molecules; all have good hole mobilities due to extended conjugation and good electron-accepting groups, while SBDT1 has the highest hole mobility of all molecules. Donor materials in the active layer should have a high hole mobility that is advantageous to its efficient transport; SBDT1 and SBDT2 have the highest hole mobilities and hence are superior candidates as donor molecules for OSCs.

Table 5. Reorganization Energies for Electrons (λ–) and Holes (λ+) of the Designed and Reference Molecules.

molecules λ– (eV) λ+ (eV)
SBDT-BDD 0.0217 0.0156
SBDT1 0.0179 0.0148
SBDT2 0.0182 0.0149
SBDT3 0.0171 0.0161
SBDT4 0.0146 0.0165
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8. Dipole Moment

Fill factor (FF) is an important parameter deciding the PCE of OSCs which in turn is a function of morphology of the active layer. Dipole moment (μ) determines the morphology by describing the polarity and solubility of the molecule. Dipole moment is a measure of polarity in chromophores; the higher the μ value, the greater will be its solubility in organic polar solvents, and vice versa.37 Polar molecules align themselves in an antiparallel fashion that opposite poles of molecules attract each other resulting in an increase in the self-assembly and a decrease in the intermolecular disorder.38 Enhanced self-assembly and reduced disorder cause better CT and less recombination of charges. Dipole moments at the ground state and the excited state of the gaseous phase as well as in chloroform solvent, calculated with CAM-B3LYP combined with the basis set 6-31G(d,p) of DFT theory, are tabulated in Tables 2 and 3. The dipole moment of investigated molecules observed at the ground state, gaseous excited state, and solvent excited state was found to be in the same increasing order of SBDT1 < SBDT2 < SBDT4 < SBDT3 < SBDT-BDD. All molecules (SBDT-BDD, SBDT1–SBDT4) had the same value of the dipole moment at the ground and gaseous excited states because of the same polarity at both states. The polarity of molecules increased when they were solvated with the polar chloroform solvent due to the fact that the polar solvent decreases the symmetry of molecules and thus increases the polar character. All designed molecules have a lower dipole moment than the reference molecule due to the presence of the benzothiazole acceptor that increased the symmetry in the designed molecules. SBDT3 has the highest dipole moment among all designed molecules due to the greater charge separation endowed by the good electron-withdrawing effect of BDD acceptors and terminal acceptor groups, thus it is promising to increase CT in the active layer by improving its morphology.

9. Device Performance

Among other parameters determining the value of PCE of OSCs, an important parameter is the open-circuit voltage (VOC). It is dependent on the HOMO (donor)–LUMO (acceptor) difference and the rate of carrier generation and recombination and is is the highest value of voltage that can be obtained from a SC as a result of the photoelectric effect when the electric circuit contains no load. It depends upon various factors such as the difference between the energy value of HOMO of the donor and LUMO of the acceptor, the intensity of light, charge mobilities, and the temperature of the device. In our computational study of designed molecules, VOC is estimated by comparing the theoretically calculated HOMO energy levels of designed molecules to the LUMO energy level of PC61BM as the acceptor molecule that is reported in literature. Expression 2 is used to calculate the value of VOC of combinations of our designed molecules as a donor with PC61BM as the common acceptor

9. 2

For a high value of VOC, there should be more stabilized (low energy) HOMO level of the donor and more de-stabilized (high energy) LUMO level of the acceptor. In equation 2, the term e in the denominator is the charge on the molecule that is equal to 1, and 0.3 is a constant value.39,40 PC61BM has HOMO and LUMO energy levels at −6.1 and −3.7 eV, respectively. VOC calculated for the reference molecule SBDT-BDD is 2.39 eV, while for the designed molecules SBDT1–SBDT4, it is 2.42, 2.35, 2.46, and 2.15 eV, as sequentially displayed in Figure 10. All combinations of designed molecules with the PC61BM acceptor have a higher VOC than the reference molecule except SBDT4 and SBDT2 due to their less stabilized (high energy) HOMO energy levels. SBDT3 has the highest value of VOC among all molecules having the most stabilized HOMO energy level due to its end-capped difluorobenzoid acceptor. SBDT1 has a VOC value lower than that of SBDT3 but higher than that of the reference molecule due to its benzothiazole acceptors that extended the conjugation in the molecule, lowered the band gap, and increased the hole mobility of the molecule, resulting in the lowering of its HOMO energy level.

Figure 10.

Figure 10

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VOC of designed molecules including the reference molecule estimated with PC61BM as the acceptor.

Conclusions

In the present work, four A1–A2–D–A2–A1-type new molecules have been designed by substituting the acceptor moieties of the reference molecule with different electron-withdrawing groups that are reported in literature for their better properties. The designed molecules (SBDT1–SBDT4) are derivatives of the BDT-based reference molecule SBDT-BDD. To plot the simulated UV–vis spectra of the reference molecule, four methods, namely, CAM-B3LYP, B3LYP, MPW1PW91, and ωB97XD with the basis set 6-31G(d,p) of DFT theory were employed, and CAM-B3LYP was found in good agreement with the experimental data (λmax). Quantum chemical calculations were performed and compared for all the molecules. All designed chromophores had a broad absorption in the visible region and exhibited a large bathochromic shift from the reference molecule except SBDT4 that showed absorption properties similar to those of reference SBDT-BDD. SBDT1 exhibited broad absorption with the highest peak value at 532 nm in the gaseous state while at 543 nm in chloroform solvent, and it has the lowest HOMO–LUMO gap of all chromophores (3.88 eV). Exciton binding energies of all molecules were calculated in the gaseous phase as well as in the solvent phase; SBDT3 was found to have the least value of binding energy, promising to give easier dissociation of the exciton. The HOMO–LUMO display, DOS graph, MEP maps, and TDM plot of all molecules verified the good delocalization of electrons, ensuring better CT in all designed molecules. Reorganization energies of all molecules were computed; SBDT1 showed the least value of the hole reorganization energy (0.0148 eV), thus possessing the highest hole mobility and SBDT4 exhibited the least value of electron reorganization energy (0.0146 eV) and so has the highest electron mobility. The energy difference between HOMO of the designed molecules and LUMO of the PC61BM acceptor molecule was in the range of 2.15–2.46 eV; SBDT3 exhibited the largest VOC (2.46 eV), promising to improve the efficiency of organic photovoltaics. These designed molecules SBDT1–SBDT4 showed superior properties than those of the reference molecule, and they should be targeted to synthesize ASM-OSCs with high performance.

Computational Details

All the computational calculations were performed using Gaussian 09, Revision D.0141 software that was developed by Ogliaro and co-workers, and the files were visualized using GaussView 6.0.16 software.42 Long alkyl chains in the structure were replaced with methyl groups because long alkyl chains have little contribution toward CT.43 Electronic, spectroscopic, and thermodynamic properties were computed using the most reliable DFT. To select the appropriate hybrid functional for calculations, the geometry of SBDT-BDD was optimized with four different functionals, namely, CAM-B3LYP,44 B3LYP,45 ωB97XD,46 and MPW1PW9147 of DFT48 with the 6-31G split basis set at the (d,p) level. Ground-state and excited-state energy calculations were performed up to 20 excited states with the TD-SCF49 method without solvent and implementing IEFPCM37 with chloroform solvent to estimate the λmax. The results of absorption profile obtained were processed through SWizard software and plotted using the Origin 6.0 program.29 The λmax obtained by CAM-B3LYP/6-31G(d,p) was in close agreement with the experimental value of the reference molecule. All the theoretical calculations of molecules were performed on restricted spins. All the designed molecules were optimized with the selected functional CAM-B3LYP of the DFT level of theory in conjunction with the basis set 6-31G(d,p), and these optimized geometries were estimated through TD-SCF with IEFPCM employing chloroform solvent to calculate the theoretical value of λmax. To evaluate the charge mobilities in the molecule, reorganization energy (λ) for electrons and holes were computed using the following expressions given by Marcus theory. Equation 3 is for calculating the electron mobility

graphic file with name ao1c03975_m003.jpg 3

Here, E–0 is the energy of the neutral molecule that is calculated from the optimized geometry of anions. E0– is the energy of the anion that is calculated from the optimized geometry of the neutral molecule. E0 is the energy of the molecule estimated from the neutral optimized geometry. Equation 4 is used for calculating the hole mobility

graphic file with name ao1c03975_m004.jpg 4

Here, E+0 is the energy of the neutral molecule calculated from the cationic optimized file of molecule. E0+ is the energy of the cation calculated from the optimized neutral molecule. E+ is the energy of the cation calculated from the optimized geometry of the cation. Reorganization energies of electrons and holes of all the reference and designed molecules were calculated to estimate the electron and hole mobilities, respectively.50

To further validate the contribution of each component in electron transfer, DOS was estimated with PyMOlyze 1.1 software29 using a selected method of DFT theory. TDM was computed and plotted with the Multiwfn_3.7 program51 package to estimate the electronic excitation processes. All the files were generated using the same selected functional of DFT theory.

Acknowledgments

All the computational work was accomplished in the computational laboratory of Punjab Bioenergy Institute, University of Agriculture (UAF), 38000, Faisalabad Pakistan funded by the Govt. of Punjab, Pakistan. M.I. expresses appreciation to the Deanship of Scientific Research at King Khalid University, Saudi Arabia, through the research groups program under the grant number R.G.P. 1/37/42. The authors also thank Dr. Khurshid Ayub, COMSATS University Islamabad, Abbottabad campus, Pakistan for additional resources.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.1c03975.

  • Cartesian coordinates of internally optimized geometries of all molecules (reference molecule SBDT-BDD and designed molecules SBDT1, SBDT2, SBDT3, and SBDT4) along the X, Y, and Z axes at the CAM-B3LYP/6-31G(d,p) level of DFT (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao1c03975_si_001.pdf (122.1KB, pdf)

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Supplementary Materials

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