Enhancing the Photovoltaic Properties via Incorporation of Selenophene Units in Organic Chromophores with A2-π2-A1-π1-A2 Configuration: A DFT-Based Exploration

Currently, polymer organic solar cells (POSCs) are widely utilized due to their significant application, such as low-cost power conversion efficiencies (PCEs). Therefore, we designed a series of photovoltaic materials (D1, D2, D3, D5 and D7) by the incorporation of selenophene units (n = 1–7) as π1-spacers by considering the importance of POSCs. Density functional theory (DFT) calculations were accomplished at MPW1PW91/6-311G (d, p) functional to explore the impact of additional selenophene units on the photovoltaic behavior of the above-mentioned compounds. A comparative analysis was conducted for designed compounds and reference compounds (D1). Reduction in energy gaps (∆E = 2.399 − 2.064 eV) with broader absorption wavelength (λmax = 655.480 − 728.376 nm) in chloroform along with larger charge transference rate was studied with the addition of selenophene units as compared to D1. A significantly higher exciton dissociation rate was studied as lower values of binding energy (Eb = 0.508 − 0.362 eV) were noted in derivatives than in the reference (Eb = 0.526 eV). Moreover, transition density matrix (TDM) and density of state (DOS) data also supported the efficient charge transition origination from HOMOs to LUMOs. Open circuit voltage (Voc) was also calculated for all the aforesaid compounds to check the efficiency, and significant results were seen (1.633–1.549 V). All the analyses supported our compounds as efficient POSCs materials with significant efficacy. These compounds might encourage the experimental researchers to synthesize them due to proficient photovoltaic materials.


Introduction
Usually, inorganic solar cell structures were prepared by gallium arsenide (GaAs) and silicon (Si). They have attained great consideration because of their stability and high energy conversion efficiencies moving towards their theoretical limits for their precise bandgaps. Silicon-based solar cells (Si-SCs) used since today have a high proficiency of about 46% [1,2]. However, with the passage of time, the utilization of silicon has been reduced to a remarkable extent due to its fixed composition, non-tunable energy levels, brittleness, high cost and a limited number of atoms and compact structure. Consequently, researchers are now trying to substitute the silicon-based materials. A number of advantages, such as an easy with a selenophene ring and benzothiophene acceptors to improve the V oc. D2, D3, D5 and D7 compounds. These compounds were designed by the incorporation of selenophene units (n = 2-7) in this reference (D1). The influence of structural modifications on electronic and optical behavior is explored in this research paper through DFT. It is predicted that the designed derivatives might be beneficial for the engineering of highly efficient OSCs.

Computational Procedure
The Gaussian 09 program [46] was employed to perform the calculations of current research work. First of all, geometries of aforesaid chromophores were optimized at MPW1PW91 functional with 6-311 G (d, p) basis set to obtain geometries at true minima. With the aid of Gauss View 5.0 [47], the input files were drawn. To find the photovoltaic properties of selenophene derivatives (D1, D2, D3, D5 and D7), various kinds of analyses such as DOS, V oc , E b , µ tot and GRPs were accomplished at the aforesaid level of DFT by utilizing the optimized structures. Nevertheless, the following key electronic properties: TDM, FMO analysis and optical properties, were investigated through TD-TDF at the above-mentioned functional. To interpret the results from output files, multiple software such as; Avogadro [48], Gauss Sum [49], Chemcraft [50], Multiwfn 3.7 [51] and PyMOlyze 2.0 [52], Origin 8.5 program [53] were utilized.

Results and Discussion
Nowadays, polymer organic solar cells (POSCs) are widely utilized as photovoltaic devices due to their low-cost sunlight conversion efficiencies [54,55]. The literature is flooded with many reports in which small units, such as thiophene, selenophene and imidazole, etc. were utilized to improve the charge transference properties of organic systems [8,42,56]. From the literature, we found that by replacing the sulfur with a selenium atom, a significant reduction in band gap can be achieved [57] The current approach aims to explore the effect of selenophene unit on charge transference rate between orbitals and also described the influence on the photovoltaic properties of organic systems. For this purpose, a synthesized fullerene-free organic system (DF-PCIC) is chosen as a parent molecule to design reference compounds (D1) with an A 2 -π 2 -A 1 -π 1 -A 2 framework. First of all, the π-bridge (4H-cyclopenta [1,2-b:5,4-b']dithiophene) on the one side of DF-PCIC is replaced with selenophene and kept the other side preserved. The terminal acceptors (2-(2-methylene-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile) in DF-PCIC are replaced with a benzothiophene-based acceptor (2-(2-methylene-3-oxo-2,3dihydro-1H-benzo[b]cyclopenta[d]thiophen-1-ylidene)malononitrile) in order to improve the electron-withdrawing effect in D1 (Scheme 1 and Figure 1). Then D1 is considered a reference molecule, and further, it is utilized to design other derivatives (D2, D3, D5 and D7). The D2 and D3 are designed by introducing two and three units of selenophene, respectively, keeping the same acceptors as shown in Figure 1. Furthermore, to explore the effect of the high number of selenophene units on the photovoltaic responses of POSCs, we introduce five and seven units of selenophene, respectively, in D5 and D7 (Figure 1). The IUPAC names of reference (D1) and their derivatives (D2, D3, D5 and D7) are explained in the Supplementary File. The true minima structures of the above-mentioned POSCs are displayed in Figure 2, and their cartesian coordinates are tabulated in Tables S1-S5.

Frontier Molecular Orbital (FMO) Analysis
FMO analysis is deliberated as a leading method to estimate the electronic properties of organic systems [58,59]. FMOs are indispensable to accelerating the transmission of electric current and provide the properties of photovoltaic cells with the aptitude for conducting electronic charges [60,61]. In accordance with the valance band theory, HOMO is considered a valence band, while LUMO is regarded as a conduction band. The band gap of HOMO and LUMO is a substantial factor in elucidating several quantum chemical parameters such as chemical reactivity, charge transfer, UV-Visible spectrum, chemical stability and electronic properties [62][63][64]. The potent photovoltaic response is indicated by less energy difference that reasonably determines the efficiency of a compound [65]. The calculated HOMO-LUMO energies as well as their energy gaps for designed molecules, are composed in Table 1.  noticed from D1 to D7 might be due to the continual addition of selenophene monomer in π 1 in each designed compound extending the conjugation and boosting the charge transfer. The charge transfer of a chromophore is indirectly related to the E gap , i.e., the lower the E gap , the greater the charge transfer and vice versa [66,67]. The overall declining trend of E gap is viewed as D1 (2.465 eV) > D2 (2.399 eV) > D3 (2.307 eV) > D5 (2.167 eV) > D7 (2.064 eV). The effective charge mobility from acceptor−2 to acceptor−1 through π-spacer along with the lowest E gap among the molecular orbitals are noticed in D7 chromophore than other designed molecules, which emerged to be an efficient material for use in photovoltaic devices.
The FMOs contour surface diagrams are illustrated in Figure 3, which expresses the distribution of electronic clouds over the molecules. In D1 and D2, charge density is significantly concentrated on the central part, while a little bit of electronic density is noticed on terminal acceptor entities in HOMO and LUMO. In D3, D5 and D7, HOMO is majorly concentrated on the acceptor−2 and π-bridge, whereas LUMO is mainly located in electron-deficient end-capped groups. Hence, the analyzed molecular systems showed charge transmission from acceptor−2 to acceptor−1 through the π-bridge. The energies of HOMO-1/LUMO+1 and HOMO-2/LUMO+2 are illustrated in Table S6, while their FMO diagrams are displayed in Figures S1-S5. Almost the same phenomena for energies and charger transference are seen between higher orbitals (HOMO-1/LUMO+1 and HOMO-2/LUMO+2).

Optical Properties
A UV-Visible analysis is a significant tool to elucidate the probability of charge transference, the nature of electronic transitions and contributing configuration in transitions within the chromophores [68,69]. TD-DFT calculations were performed at MPW1PW91/

Optical Properties
A UV-Visible analysis is a significant tool to elucidate the probability of charge transference, the nature of electronic transitions and contributing configuration in transitions within the chromophores [68,69]. TD-DFT calculations were performed at MPW1PW91/ 6-311 G (d,p) level in chloroform and gaseous phase to assess the photophysical properties of the designed chromophores. The main outcomes of oscillator strength (f os ), transition energy (E) and maximum absorption wavelengths (λ max ) are collected in Tables 2 and 3 in gas and chloroform, respectively, and their graphs are presented in Figure 4. Moreover, other results are exhibited in Tables S8-S17.

.Global Reactivity Parameters (GRPs) Investigations
To investigate the stability and reactivity of a molecule, GRPs are calculated through the energies of HOMOs and LUMOs. The global softness (σ), electron affinity (EA), global hardness (η), global electrophilicity index (ω), chemical potential (µ), ionization potential (IP) and electronegativity (X) were computed by using the band gap of HOMO and LUMO [70][71][72][73][74]. The following Equations (1) and (2) Koopmans's theorem is utilized to determine σ, ω, η, µ and X [75]  . This enhancement might be regarded as the continuous addition of selenophene units in the first π-spacer (π 1 ) in each derivative, which results in extending the conjugation and boosting the charge transfer. Meanwhile, the absorption wavelength of these molecules is compared in the solvent phase to that in the gaseous phase, and it is noticed that λ max in chloroform is also examined to be greater than the gas phase due to the solvent effect. Overall, the maximum bathochromic shift is observed for D7 in both phases, so these designed chromophores can be regarded as excellent solar cell material for future use.
The above parameters were obtained utilizing Equations (1)- (7), and these results are displayed in Table 4. The chemical potential of a molecule expresses the stability and reactivity of a specie. IP signifies the electron donating and accepting ability, which is the energy requires to eradicate the electron from HOMO. The energy gap, chemical potential, stability and hardness are inversely associated with reactivity while directly to one another [69]. Moreover, the stability of a molecule depends upon the electronegativity and the position of its substituents with respect to the electronegative atom [76]. Thus, the molecule with greater energy difference is considered harder, which shows low reactivity and high kinetic stability. The IP values are noted to be greater in magnitude than EA values. The hardness values are noticed as 1. 233, 1.199, 1.154, 1.084 and 1.167 eV for D1, D2, D3, D5 and D7, respectively, and its descending order is found in studied molecules as D1 > D2 > D7 > D3 > D5. The hardness of a molecule is directly linked with the E gap and inversely related to the reactivity. Therefore, a chromophore with a greater energy gap is considered harder and more stable [77]. Another factor that discloses the reactivity of molecules is softness, which is directly associated with polarizability [78]. The softness value calculated for D5 is observed to be 0.461 eV, which reduces to 0.434 eV in D3 and further declines to 0.428 eV for D7, while the least value (0.417 eV) is noted in D2. Interestingly, the highest value of softness (0.461 eV) is viewed in D5, which might be due to an increase in conjugation due to an extended π-spacer. Thus, it is regarded as the most polarizable and exhibits good photovoltaic properties for all the said chromophores.

The Density of State (DOS) Analysis
The DOS analysis is accomplished to estimate the contribution of each fragment of the molecule in the total electronic distribution and absorption band [42,79]. To perform this analysis, the designed molecules are partitioned into four fragments, i.e., acceptor-2, π-spacer-2, acceptor-1 and π-spacer-1. DOS was carried out for D1, D2, D3, D5 and D7 to support the insights obtained from FMO exploration [80]. The DOS pictographs are displayed in Figure 5, where each fragment is presented in different colors (acceptor-2 with red, acceptor-1 with green, π-spacer-2 with blue and π-spacer-1 with pink lines). The pattern of electronic charge dissemination is altered by changing acceptor moieties and extending the π-spacer, which is justified by the DOS percentage of HOMO-LUMO. Herein, acceptor-1 depicted 14. .0% to LUMO, accordingly. It is clear from these outcomes that HOMO is predominantly located on acceptor-1, while LUMO mainly resides on acceptor-2 in the aforementioned compounds. Overall, the pattern of electronic charge distribution unveils that a significant amount of charge is shifted from HOMO to LUMO, exhibiting them as promising candidates for fullerene-free OSCs.

Transition Density Matrix (TDM) Study
The interpretation of the transition process in a conjugated system can be effectively determined by utilizing TDM analysis [81,82]. The TDM investigation presents a threedimensional heat map for transition among two eigenstates. It depicts the scattering of electrons as well as hole pairs and permits to analyze their coherence lengths and delocalization [83,84]. The pictorial representation of interaction among acceptor and donor entities in the S1 (excited) state is represented by the blue region in the spatial map [42,85]. The emission and absorption of studied molecules, i.e., D1, D2, D3, D5 and D7 were examined at TD-DFT/MPW1PW91 functional and 6-311 G (d,p) basis set. The effect of a hydrogen atom is ignored owing to its minor involvement in an electronic transition. The pictographs of TDM are displayed in Figure 6 with different fragments on the left side and bottom, whereas electron density is reported on the y-axis.
The uniform dissemination of electrons over the molecule diagonal transfer can be viewed from the bright portion of TDM graphs for all the computed molecules (D1, D2,  D3, D5 and D7). Moreover, electron-hole pair generation and charge coherence also appeared to proliferate non-diagonally. FMO findings revealed that the charge density is considered observed over the molecule, which causes notable variation in TDM plots. Figure 6 displayed that the electron density effectually transfers from the core to terminal acceptors through π-spacers in D1, D2, D3, D5 and D7 allowing efficient charge transfer.
Binding energy (E b ) is another significant factor to estimate the photovoltaic response of the examined molecules. A lesser E b value results in a greater exciton dissociation in the S1 state due to less coulomb's force between the electron and hole. The E b of D1, D2, D3, D5 and D7 are calculated from the energy of optimization (E opt ) and the HOMO-LUMO energy gap (E gap ) [86] as shown in Equation (8) and the computed outcomes are listed in Table 5. According to the outcomes collected in Table 5, an almost similar trend to the FMOs energy gap is noticed in the first singlet exciton energy, i.e., it decreases gradually from D1, D2, D3, D5 and D7. Moreover, the values of E b for the titled compounds are obtained to be 0.526, 0.508, 0.475, 0.418 and 0.362 eV, respectively. The least value of E b (0.3620 eV) is investigated in D7, among all the designed chromophores, which illustrates that it has the highest capacity of exciton dissociation and enhanced current charge density (J sc ). The decreasing order of E b is obtained as follows: D1 > D2 > D3 > D5 > D7. Interestingly, all the studied chromophores showed lower E b values than that of D1 and might be used for photovoltaic applications.

Dipole Moment (µ tot ) Analysis
The dipole moment (µ tot ) of a molecule is directly influenced by electronegativity (E.N) difference, the greater the E.N difference, the greater would be the dipole moment (µ tot ) [72]. The dipole moment values of D1, D2, D3, D5 and D7 in x, y and z directions are calculated and collected in Table 6. The data from the above table illustrate that D3 depicted the largest value of µ tot (9.9682 D) of all the entitled chromophores. Overall, the decreasing order of µ tot is as follows: D3 > D2 > D5 > D7 > D1. The superior µ tot values of the entitled compounds exploited the greater polarizability in them, which indicates the higher charge transference, resulting in effective photovoltaic responses.

The Open-Circuit Voltage (Voc) Investigations
The open-circuit voltage (Voc) is an interesting approach that plays an important role in determining the performance of the OSCs [87,88]. In fact, it explains the maximum current that may be achieved from an optical substance [89]. The following influential factors affecting the Voc are found: light intensity, light source, external fluorescence proficiency, OSCs device's temperature, charge carrier recombination and various other environmental elements. The Voc is closely related to the energy difference between HOMO and LUMO of the donor (D) and acceptor (A) compounds [90]. In order to attain a higher Voc, in the acceptor molecule, the LUMO level should be lower and for the donor molecule, the HOMO level should be with a high energy level [91]. Equation (9) is used to calculate the Voc for the designed materials, as provided by Scharber and his coworkers [92].
Hence, E is an elementary charge of acceptors, signifies the charge on each molecule, and 0.3 denotes the empirical constant. The chlorinated polymer J52-Cl is a well-known donor polymer widely used in large published reports to blend with acceptor molecules in charge transfer analysis. [93][94][95][96]. Therefore, following the literature, the studied molecules are blended with J52-Cl polymer to predict the potential usage of designed compounds regarding charge transfer characteristics for organic solar cells. The structural representation of J52-Cl is shown in Figure 7. To determine the Voc of the current investigation a donor polymer (J52-Cl) is utilized. In Table 7    The Voc value for D1, D2, D3, D5 and D7 with respect to LUMO acceptor HOMO donor energy difference is determined to be 1.549, 1.584, 1.600, 1.624 and 1.632V, respectively. The V oc of entitled compounds decreases in the following order: D7 > D5 > D3 > D2 > D1. Among all tailored molecules, D7 displayed the highest Voc (1.632 V). Since the transference of electrons from donor (D) to acceptor (A) segments, the HOMO/LUMO energy gap is a crucial tool for improving the PCEs of solar cells. A low-lying LUMO lead to improved Voc values having better optoelectronic properties. Open-circuit voltage (Voc) diagram is illustrated in Figure 8. This form of molecular orbital alignment makes it easier for the electron density to move from the donor polymer to the acceptor, as all our derivatives possess a lower value of LUMO than the J52Cl, which improves optoelectronic behavior.
The Voc value for D1, D2, D3, D5 and D7 with respect to LUMOacceptor HOMOdonor energy difference is determined to be 1.549, 1.584, 1.600, 1.624 and 1.632V, respectively. The Voc of entitled compounds decreases in the following order: D7 > D5 > D3 > D2 > D1. Among all tailored molecules, D7 displayed the highest Voc (1.632 V). Since the transference of electrons from donor (D) to acceptor (A) segments, the HOMO/LUMO energy gap is a crucial tool for improving the PCEs of solar cells. A low-lying LUMO lead to improved Voc values having better optoelectronic properties. Open-circuit voltage (Voc) diagram is illustrated in Figure 8. This form of molecular orbital alignment makes it easier for the electron density to move from the donor polymer to the acceptor, as all our derivatives possess a lower value of LUMO than the J52Cl, which improves optoelectronic behavior.

Conclusions
The organic-based materials (D1, D2, D3, D5 and D7) have been designed through the incorporation of selenophene units in the reference compound (DF-PCIC) up to n = 7. In order to improve the electron-withdrawing effect of terminal acceptors, benzothiophene-based acceptors are also introduced in D1, D2, D3, D5 and D7 compounds. After the addition of selenophene units, diminishing in band gaps (∆E = 2.399−2.064 eV) accompanied by larger bathochromic shift (max= 655.480−728.376 nm) and lower binding ener-

Conclusions
The organic-based materials (D1, D2, D3, D5 and D7) have been designed through the incorporation of selenophene units in the reference compound (DF-PCIC) up to n = 7. In order to improve the electron-withdrawing effect of terminal acceptors, benzothiophenebased acceptors are also introduced in D1, D2, D3, D5 and D7 compounds. After the addition of selenophene units, diminishing in band gaps (∆E = 2.399 − 2.064 eV) accompanied by larger bathochromic shift (λ max = 655.480 − 728.376 nm) and lower binding energies (E b = 0.508 − 0.362 eV) are obtained, and the conjugation is also enhanced. These findings enclosed higher exciton dissociation rate and significant charge transference from HOMO to LUMO, which is further supported by TDM and DOS analyses. The GRP studies and diminishing band gaps revealed that increasing conjugation grants significant stability to the computed chromophores. An efficient value of V oc is noticed for all POSCs materials when determined with respect to JCl52 polymer. Among all the compounds, D7 exhibited a lower bandgap (2.064 eV) and highest λ max (691.953 nm in gas and 728.376 nm in chloroform) and greater open-circuit voltage value (1.632 V), which proves that it is a most suitable chromophore with outstanding photovoltaic characteristics. Consequently, significant photovoltaic materials can be developed by structural tailoring with selenophene units and efficient electron-withdrawing moieties. Moreover, this study also encourages the experimentalist to synthesize these efficient materials.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/polym15061508/s1. It contain cartesian co-ordinates, molecular orbital energies, UV-Vis absorption values, FMOs diagrams showed movement of charges transference between orbitals and ICUPAC names of studied selenophene based compounds.