Introduction

The world now faces a serious energy crisis due to ever increasing energy demands. Primarily fossil fuels are used to meet current energy demands; however, they have certain limitations. For example, fossil fuels are non-renewable energy sources and cause environmental pollution [1]. These limitations have motivated scientists to explore alternate renewable energy sources that are environmentally friendly, such as, wind energy, hydro power energy, geothermal energy, solar energy, etc. [2]. Solar energy is of particular interest among the potential alternative to fossil fuels as a renewable and environmentally friendly energy source [3].

Different types of solar cells, including silicon-based solar cells, mono crystalline silicon solar cells, solution-processed solar cells and gallium-selenide-based solar cells, have been used previously [4]. Although these solar cells showed remarkable efficiency, inorganic solar cells have many disadvantages, such as rigid structure [5], high cost, non-tunable energy levels, etc. Organic solar cells have many advantages over silicon-based inorganic solar cells, such as low cost [6], light weight [7], flexibility, tunable structure and batch-to-batch reproducibility [8,9,10,11,12]. Over the past decade, organic photovoltaic cells have attained remarkable power conversion efficiency (PCE). For example, PCE values >10% can be achieved with polymer solar cells [13,14,15,16,17,18]. More recently, efforts have been exerted to design small molecules that can be used in solar cells. In this regard, a variety of donor molecules with diverse structures, e.g., star shaped, X-shaped, linear and others, have been reported in the literature in conjunction with PCBM acceptor ([6,6]-phenyl-C61-butyric acid methyl ester). Many fascinating characteristics of these small molecule organic solar cells reflect their potential as alternatives to solution-processed polymer solar cells (PSCs).

Although small molecule organic solar cells (SMOSCs) have emerged as an alternative to solution-processed PSCs, they still suffer from several limitations. For example, the frontier molecular orbitals (FMOs) of designed donor molecules do not align properly with the FMOs of PCBM, which significantly affects the open circuit voltage due to poor film morphology and low charge transport ability. To overcome the above cited problems, scientists have designed small molecule donor materials with special emphasis on solubility, absorption bands, suitable energy levels, and charge mobility.

The literature reveals several studies where computational tools have been used to design new molecules with improved efficiencies for solar cell applications. For example, computational studies were carried out for the design of dibenzosilol donor unit based A-D-A type molecules with different end-capped acceptor units for application in organic solar cells [19]. In the latter study, density functional theory (DFT) methods were used to evaluate different optoelectronic properties of organic solar cells [19]. In another DFT study, A-D-A type donor molecules based on the benzodithiophene donor core unit flanked with different acceptor moiety were evaluated for their optoelectronic properties. The study revealed that CAM-B3LYP/6-31G(d,p) level of theory is best for this class of compounds. Furthermore, the designed donor is blended with well-known acceptor PCBM [20].

To improve the performance of small molecules, we applied a dithieno (DTT) donor moiety to the organic solar cell due to its better optical properties [21]. Herein, we designed dithieno-based organic solar cells with donor and acceptor groups, linked together via a suitable bridge. Three molecules, M1, M2 and M3, were designed, where the donor dithieno(2,3-b:3,2-d)thiophene and acceptor 2-(3-methyl-5-methylene-4-oxothiazolidin-2-ylidene)malononitrile are linked through 3-methylthiophene (M1), 3-methlyfuran (M2) and 3-methylselenophen (M3) groups, respectively. The calculated properties are compared with well-known model molecule R, which is structurally comparable to the designed molecules.

Computational details

All calculations were performed using the Gaussian 09 program package [22]. Molecular structures were drawn with the GaussView 5.0 program [23]. Initially, for selection of basis set, the geometries and absorption spectra of the reference compound R [24] were calculated with different functionals, including B3LYP [25] CAM-B3LYP [26], and ωB97XD [27] at 6-31G (d,p) basic set [28], and the results compared with available experimental data. CAM-B3LYP gave closest agreement between theoretical and experimental results. The basis set 6-31G (d,p) gave the most reliable result for geometry optimization as well as for electronic properties [29, 30]. We evaluated different basis sets, such as 6–31 + G(d,p), 6–311 + G(d,p), 6–311++G(2d,2p), 6–311 + G(d,p) and 6-31G(d,p); however, the 6-31G(d,p) basis set proved best in term of results and computational cost (Table S1). Further, all calculations for the cationic, anionic and neutral species were performed at CAM-B3LYP/6-31G (d,p) level of theory.

For calculation of absorption spectra, time-dependent density functional theory (TD-DFT) with CAM-B3LYP at 6-31G (d,p) level of theory was used in gas as well as in solvent (chloroform) phase. The solvent effect was incorporated through the integral equation formalism polarizable continuum model (IEPCM) with chloroform solvent. UV-visible absorption spectra were drawn using the origin 6.0 program. Density of states (DOS) around HOMO–LUMO (highest occupied molecular orbital to lowest unoccupied molecular orbital gap) were calculated using PyMolyze software [31]. Reorganization energies of hole (λh) and electron (λe) were estimated with the selected functional. Reorganization energy was further divided into two parts: internal reorganization energy (λint) [32] and external reorganization energy (λext). The external environmental relaxation and effect of polarization on the external surrounding medium were explained by λext, while, on the other hand, λint revealed the fast changes in internal geometry. Herein, we are working on small donor molecules that have low dielectric constant value. Hence, we neglected the external surrounding interference on our study, i.e., only internal reorganization energy λint is discussed. Finally, the reorganization energy of hole (λh) and electron (λe) were calculated by the following Eqs. (1 and 2).

$$ \uplambda \mathrm{e}=\left[{E}_0^{-}-{E}_{-}\right]+\left[{E}_{-}^0-{E}_0\right] $$
(1)
$$ \uplambda \mathrm{h}=\left[{E}_0^{+}-{E}_{+}\right]+\left[{E}_{+}^0-{E}_0\right] $$
(2)

Where, \( {E}_0^{+},{E}_0^{-} \)shows the energy of cations and anions calculated at optimized structure of neutral molecules. The \( {E}_{-}^0,{E}_{+}^0 \) are the energies of neutral molecules calculated via optimized geometries of anion and cation, respectively. E0 is the single point energy of optimized structure of neutral molecule. Finally E+, E− are the energies of cation and anion at optimized geometry of cations and anions.

Results and discussion

Different DFT functionals including B3LYP, CAM-B3LYP, and ωB97XD with 6-31G (d,p) basis set were used for selecting suitable functionals. The maximum absorption values with B3LYP, CAM-B3LYP, and ɷB97XD are 626 nm, 450 nm, and 424 nm, respectively, while the experimental value is 532 nm (see Table S2). The UV-vis spectrum of reference R simulated with CAM-B3LYP provided the best match with the experimental spectrum. Hence, all further calculations were performed with this selected functional. We evaluated different basis sets: 6–31 + G(d,p), 6–311 + G(d,p), 6–311++G(2d,2p), 6–311 + G(d,p) and 6-31G(d,p). The λmax values at 6–311 + G(d,p), 6–311++G(2d,2p), 6–311 + G(d,p), and 6–31 + G(d,p) were 451.6 nm, 450.1 nm, 438.09 nm, and 448.7 nm, respectively. The 6-31G(d,p) basis set was chosen for this study by considering the efficiency and cost of the different basis sets tested here. The structure of reference molecule R, as well as the designed molecules M1, M2 and M3 are shown in Fig. 1. Optimized geometries at CAM-B3LYP/6-31G (d,p) level of theory are shown in Fig. 2. Donor and acceptor groups are arranged in one plane in optimized geometry.

Fig. 1
figure 1

Molecular structures of reference R and designed molecules M1–M3

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Fig. 2
figure 2

Optimized structures of reference R and designed molecules M1–M3 at the CAM-B3LYP/6-31G (d,p) level of theory

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Frontier molecular orbitals

Charge transition character is strongly influenced by the distribution of FMOs [33]. Optical and electronic properties can be studied with the help of electronic distribution pattern of FMOs, i.e., HOMO and LUMO [34, 35]. The FMO diagram shows the relationship between conjugation and electron mobility. There is a direct relationship between conjugation and charge carrier mobilities. If conjugation/delocalization in the structure increases, the mobility of electrons also increases (Fig. 3). The calculated HOMO energy levels of designed molecules M1, M2 and M3 are −6.71 eV, −6.47 eV, and − 6.72 eV, respectively, while the LUMO energies levels are −2.32 eV, −2.19 eV, −2.36 eV, respectively (Table 1).

Fig. 3
figure 3

Frontier molecular orbitals (FMOs) and highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) distribution pattern at the ground state of reference R molecule and molecules M1 to M3

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Table 1 The highest occupied molecular orbital (HOMO) energy (EHOMO), lowest unoccupied molecular orbital (LUMO) energy (ELUMO) and HOMO–LUMO energy gap (Eg) in eV at CAM-B3LYP/6-31G (d,p) level of theory
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The HOMO value of M2 (−6.47 eV) is higher than that of M1 (−6.71 eV), which can be attributed to the 3-methylfuran bridge group (linked with donor group) in M2 as compared to 3-methylthiophene in M1. The bridge 3-methylthiophene shows better conjugation with the donor group, due to the presence of the thio group in the ring. The LUMO values of M2 and M1 are −2.19 eV and − 2.32 eV, respectively. As a result, M2 has a low energy gap (4.28 eV) as compared to M1 (4.40 eV). The HOMO value of M3 (−6.72 eV) is even lower than that of M1 (−6.71 eV), which is due to the more conjugated 3-methylselenophene group (with donor) in M3 compared to 3-methylthiophene in M1. As a result, M3 has a low energy gap than M1. The HOMO and LUMO values of the reference molecule R are −6.61 eV and − 2.35 eV, respectively, which are higher than those of designed molecules M1 and M3.

The low HOMO value of M3 is useful in obtaining high open circuit voltage in organic solar cells. The designed molecule M2 has an orbital energy (eV) comparable to the corresponding value for the reference molecule R, as shown in Fig. 4. The density of states (DOS) calculations for all molecules were performed at CAM-B3LYP/6-31G (d,p) level of theory. DOS helps to find the density of electrons throughout the molecule. The presence of different bridge groups with donor, dithieno [2,3-b:3,2-d] thiophene, affects the distribution pattern of electron density over all the molecules. Density of states support the inferences obtained from FMO analysis, as shown in Fig. 5.

Fig. 4
figure 4

Evaluation of the calculated HOMO and LUMO energies for reference molecule R and designed donor molecules M1–M3 at CAM-B3LYP/6-31G (d,p) level of theory

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Fig. 5
figure 5

Density of states around HOMO and LUMO for reference molecule R and designed donor molecules M1–M3

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The FMO diagram shows the distribution of electron density around HOMO and LUMO. The HOMO of M1 is distributed all over the molecule including donor, bridge as well as acceptor sites, while the LUMOs are spread over the donor and 3-methylthipehene bridge groups only. The HOMO of M2 is distributed over the entire molecule except the spacer group, while the LUMO is distributed uniformly over the entire molecule. The HOMO of M3 shows a pattern similar to that of M2, i.e., it is distributed on donor and spacer groups while the LUMOs are distributed all over the molecule. The HOMO of the reference molecule R is distributed all over the molecule, except the acceptor groups, while the LUMOs are spread all over the molecules. DOS (Fig. 5) calculations were also performed with selected functionals to support the findings illustrated in the FMO diagram. The DOS further supports the idea of distribution of the electron pattern according to the donor moiety. DOS also shows the energies of HOMO–LUMO gap of reference R and designed molecules M1, M2 and M3.

Furthermore, the molecular electrostatic potential (MEP) shows the distribution of electron-rich and electron-deficient sites in a molecule [36]. The color MEP maps for R, M1, M2 and M3 are illustrated in Fig. 6. The same scale is used for all molecules. Three colors are present in MEP diagrams. The red region show the accumulation of positive charge (electron deficient) while blue illustrates negative charge (electron rich) and green the electrically neutral part. All designed molecules (M1–M3), including reference R, show similar MEP pattern. The end capped acceptor units contain positive charge (red color) whereas bridge units bear negative charge (blue color). The central donor core unit is neutral (green color). It can be seen that M1, M2 and R have greater positive and negative charges on acceptor and bridge units, respectively (with respect to M3). The lesser charge in the case of M3 may be due to the silicon atom on the bridge unit.

Fig. 6
figure 6

Molecular electrostatic potential (MEP) analysis of R, M1, M2 and M3

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Optical properties

TD-DFT calculations at the CAM-B3LYP/6-31G (d,p) level were applied in gas phase as well as with solvent (chloroform) to obtain the absorption spectra (Tables 2 and 3). The calculated absorption value (λmax), excitation energy (Ex), dipole moment, and oscillator strength (f) in the gas phase are presented in Table 2.

Table 2 Absorption values (λmax), excitation energies (Ex), dipole moment, and oscillator strength (f), are calculated at time-dependent (TD)-CAM-B3LYP/6-31G (d,p) level of theory in the gas phase
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Table 3 λmax, Ex, dipole moment and f calculated at TD-CAM-B3LYP/6-31G (d,p) level of theory in chloroform solvent
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The λmax of the designed molecules in gas phase ranged from 437 nm to 474 nm. The theoretical calculated λmax values for M1, M2 and M3 are 437 nm, 474 nm and 441 nm, respectively. The designed molecule M2 shows the highest λmax value as compared to the reference R, which is attributed to extended conjugation in the 3-methylfuran bridge group with donor (DTT) group. We can clearly see from the results that λmax values are strongly affected by extended conjugation with the donor unit, which causes a red shift in the absorption spectrum.

The simulated absorption spectra (Fig. 7) show two high intensity peaks in the case of M1. The peak around 310 nm is due to a 3-methylfuran moiety. M3 has a higher absorption wavelength than M1 due to bridge group 3-methylselonophene. The designed molecules M1 and M3 have similar absorption profiles with respect to the reference molecule R. The absorption value of M3 is 9 nm blue shifted as compared to R. The absorption value (λmax), excitation energy (Ex), dipole moment, and oscillator strength (f) with TD-CAM-B3LYP/6-31G (d,p) in chloroform solvent are shown in Table 3.

Fig. 7
figure 7

Simulated absorption spectra of reference R and designed donor molecules (M1–M3) in gas phase and chloroform solvent

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The designed molecules M1, M2 and M3 show maximum absorption in chloroform solvent at 459 nm, 500 nm and 462 nm, respectively. Hence, the designed molecule M2 shows the highest λmax value (500 nm) among all designed and reference molecules. The reason for the high absorption of the M2 molecule is the extended conjugation between 3-methylfuran and the donor group (DTT). The simulated absorption spectrum in chloroform solvent shows two high intensity peaks due to the 3-methylfuran bridge group. M3 shows the largest red shift as compared to M1, due to the presence of the 3-methylselenophene group in conjugation with the donor group (DTT). The λmax value of the reference compound is 467 nm. The designed molecules M1 and M3 shows comparable λmax values with reference molecule R.

Reorganization energy

The performance of solar cells is evaluated by charge mobility values, which can be calculated with the assistance of reorganization energy values of electron mobility (λe) and hole mobility (λh). An inverse relation is found between reorganization energy and charge mobility. To obtain high charge mobility, there must be a low reorganization energy value of donor material [37, 38]. The reorganization energy value depends on different factors but is affected primarily by cation geometry and anion geometry (as defined by Eqs. 1 and 2). Reorganization energies of all molecules (M1–M3) including reference R are shown in Table 4.

Table 4 Reorganization energies of the designed molecules M1–M3 and the reference R at CAM-B3LYP/6-31G (d,p) level of theory
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In our study, we neglected external environmental relaxation and external reorganization and dealt only with internal reorganization energy. The results revealed that the designed molecules have better electron (λe) and hole (λh) motilities as compared to the reference molecule R. The lower value of hole reorganization energy, as compared to electron reorganization energy revealed that the designed molecules are better suited for hole mobility. The λe values of the reference R, and the designed molecules M1, M2 and M3 are 0.02407 eV, 0.02448 eV, 0.01916 eV, and 0.02564 eV, respectively. The designed molecule M2 has the lowest λe value; therefore, it shows the highest electron mobility among all designed and reference molecules. The λe value of M1 and M2 are almost comparable to the λe value of reference molecule R; hence, both M1 and R have same electron mobility. Therefore, the designed molecule M2 is recommended for use in organic solar cells due to its highest hole mobilities. The λe values of all studied molecules are in the order of M2 > M1 > M3.

On the other hand, hole mobility λh for the reference molecule is 0.02099 eV, while the λh of the designed molecules M1, M2 and M3 are 0.02090 eV, 0.01495 eV, and 0.02100 eV, respectively. The designed molecule M2 is the best for hole transport mobility due to having the lowest λh value (0.01495 eV) of all the designed and reference molecules. The designed molecules M1 and M3 have comparable hole transfer abilities to the reference molecule R. The λh values are in the order M2 > M1 > M3.

The above discussion shows that the designed molecules are good candidates for electron and hole mobilities. Furthermore, the designed molecule M2 shows the lowest λe as well as λh values among all the designed and reference molecules. All the designed molecules show good hole and electron mobility, but M2 is the lead molecule, with lowest λe and λh values.

Dipole moment

Another promising factor to evaluate the performance of OSCs is dipole moment. Dipole moments of all designed and reference molecules (M1, M2, M3 and R) were also calculated with CAM-B3LYP/6-31G(d,p) level of theory. The values of dipole moment have a great influence on the fabrication of organic solar cells, and a direct relationship with solubility in organic solvent. The higher the dipole moment, the greater the solubility in organic solvents. From Table 2, it is obvious that the designed molecule M2 has the highest dipole moment among of all designed molecules, as evidenced by its good solubility in organic solvent. The dipole moments are even higher in chloroform solvent (Table 3) than in gas phase (Table 2). The value of dipole moments of designed molecules M1, M2, and M3 in chloroform solvent are in order of M2 > M3 > M1. The dipole moments of M1, M2, and M3 are 3.14 D, 3.79 D, and 3.13 D in gas phase, and 3.66 D, 4.15 D, and 3.81 D, respectively, in chloroform solvent. The higher values of dipole moments facilitate the self-assembly of molecules and formation of long chains that provide a strong pathway for charge mobility if the designed molecules have the ability to pack. The dipole moment at ground as well as in excited state and their difference are shown in Table 5.

Table 5 Calculated dipole moment values of the designed molecules and reference molecule at CAM-B3LYP/6-31G (d,p) level of theory
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Open circuit voltage

Open circuit voltage (Voc), which shows the maximum amount of voltage that can be taken from any device, is an important parameter to check the performance of organic solar cells [39]. Voc shows the bias of junction of current taken from the solar cell. Both light-generated current and saturation current rely on recombination in the devices, and Voc relies on these both currents. Open circuit values depend on the difference between donor and acceptor molecules (corresponding to HOMO and LUMO energies, respectively) and are approximately proportional to Voc. Herein, our designed molecules are donors. Therefore, we compared our designed molecules with the well-known acceptor, PCBM [28, 21].

Figure 8 presents a molecular orbital diagram, where the energy differences between the HOMOs of designed molecules and LUMOs of the acceptor PCBM can be observed. The open circuit voltages of reference R and designed molecules M1, M2 and M3 with respect to PCBM are 2.91 eV, 3.01 eV, 2.77 eV, and 3.02 eV, respectively. The calculated open circuit voltages are in the order of M3 > M1 > R > M2. The comparison between of HOMOs–LUMOs of the reference R and designed molecules M1, M2 and M3 with respect to PCBM are shown in Fig. 9. The above result shows that M1 and M3 have higher Voc values than M2.

Fig. 8
figure 8

Molecular orbital energy level diagrams of reference R and designed donors M1–M3 and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) acceptors

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Fig. 9
figure 9

Expected open circuit voltage (Voc) of references R and designed donor molecules (M1–M3) with respect to PCBM

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Transition density matrix and exciton binding energy

The transition density matrix (TDM) of dithieno [2,3-b:3,2-d] thiophene (DTT)-based small molecules donor were computed to estimate the nature of the transition. CAM-B3LYP/6-31G (d,p) level of theory was used to compute absorption and emission of the S1 state in a vacuum, and the results are illustrated in Fig. 10. TDM explains the interaction between donor and acceptor groups in the excited state, in addition to explaining hole-electron localization and electronic excitation [40, 41]. Due to the small contribution of hydrogen atoms in transition, the effect of hydrogen atoms was ignored by default in the present study.

Fig. 10
figure 10

Transition density matrix (TDM) of reference R and designed molecules (M1–M3)

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We divided our designed molecules into three types to evaluate the TDM results: A, B and D (A for end capped acceptor, B for bridge unit, and D for central donor unit). From the TDM diagram, it is obvious that the electron coherence of reference R and designed molecules (M1–M3) is partially available on the diagonal of donor and the bridge segment, while there is very little on the end-capped acceptor moieties. Additionally, the coefficient of interaction between donor and acceptor group of designed molecules are in the order of M1  > M3 > M2. The coupling of hole and electron of M2 may be lower with respect to other three molecules, but perhaps it shows higher and easier exciton dissociation in the excited state. Exciton binding energy (Eb) reveals that M2 has the highest number of charges, which may cause easy dissociation into separated charges. Therefore, as a result, it has higher charge dissociation energy with respect to M1 and M3.

Binding energy holds a promising key to evaluating the performance of organic solar cells. It helps to determine exciton dissociation potential, interaction of columbic force between hole and electron, and the optoelectronic properties of OSCs. Binding energy and columbic interactions have a direct relationship. The lower the resultant binding energy, the lower the columbic interaction between electron and hole. Lower binding energy leads to higher exciton dissociation in the excited state. Binding energy can be calculated by taking the difference between energy gap (Eg) and single point energy Eopt forms S0 to S1 by producing pair of electron and hole [42,43,44]. The binding energy (Eb) values of reference R and designed molecules M1–M3 are calculated using following Eq. 3.

$$ {E}_b={E}_{H-L}-{E}_{opt} $$
(3)

The theoretical calculated binding energies of designed molecules (M1–M3) including reference R are shown in Table 6.

Table 6 Calculated HOMO–LUMO energy gap EH-L, and Eopt first singlet excitation energies, exciton binding energies (Eb)
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All newly designed molecules have binding energies comparable to that of the reference molecule R. Moreover, the order of binding energy of all molecules is M1 (1.774) > M3 (1.592) > M2 (1.488) > R (1.433), which is in excellent agreement with the TDM result.

Charge transfer analysis of our designed donor and acceptor PCBM

In order to estimate the charge transfer (CT) between designed donor molecules (M1) and PCBM, the complex was subjected to CT analysis, as shown in Fig. 11.

Fig. 11
figure 11

Charge transfer (CT) between the M1 and PCBM, the complex analyzed geometry for CT analysis at CAM-B3LYP/6-31G (d,p) level of theory

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The PCBM acceptor interacts with the designed molecule M1 in such a way that the designed molecule is aligned parallel to the acceptor PCBM. The relative orientation of M1 and PCBM has significant effect on the electronic structure of the complex. The dipole from PCBM to M1 suggests efficient excitation at the M1/PCBM interface [45,46,47,48]. The dipole μ in the M1:PCBM complex is due to electrostatic interactions of permanent electric moment of PCBM with that of M1. The orientation of the dipole in our case is in accordance with this statement. The dipole moment vector originates from the polymer side, and points towards the end capped acceptor group of designed molecule M1.

The HOMO–LUMO distribution pattern and electronic structure of the complex are calculated at CAM-B3LYP/6-31G (d,p) level of theory (Fig. 12). The HOMO is spread mainly on the donor molecule, while the LUMO is present mainly on the acceptor PCBM. The fact shown in the orbital diagram shows the HOMO to LUMO excitation as CT from donor M1 to the acceptor PCBM molecule. Shifting of density from donor to acceptor is concrete evidence for CT between different groups.

Fig. 12
figure 12

Distribution patterns of FMOs (HOMO and LUMO) of M1 and PCBM at CAM-B3LYP/6-311G (d,p) level of theory

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Conclusion

Three new molecules M1, M2 and M3 are designed to enhance the optical, photophysical and electronic properties of OSCs. These molecules comprising dithieno [2,3-b:3,2-d] thiophene (DTT) donor group linked with acceptor 2-(3-methyl-4-oxothiazolidin-2-ylidine)malononitrile through three different bridge groups, 3-methylthiophene (M1), 3-methylfuran (M2) and 3-methylselenophene (M3), respectively. Optical properties, electronic properties, photophysical and excited state energy were calculated and compared with the well-known reference molecule R, which was recently published. All designed molecules (M1–M3) have suitable FMO diagrams for CT. From their binding energy, M2 has the highest number of charges, which may cause easier dissociation into separated charges. Therefore, as a result, it has higher charge dissociation energy with respect to M1 and M3. All designed molecules show potential photovoltaic parameters with respect to the reference molecule R. The designed molecule M2 exhibits the lowest band gap (4.28 eV) and shows the highest λmax value at 474 nm in gas phase and 500 nm in chloroform solvent. The lower λe and λh of M2 (among all molecules) revealed its higher charge mobility (electron and hole) with respect to all designed and reference molecule R. The lower λh value compared to λe values revealed that all designed molecules have higher hole mobility than that of the electron mobility. Conclusively, all designed donor molecules (M1–M3), especially M2, hold promising optoelectronic properties, and, therefore, are suitable donor materials for their use in organic solar cells.