Influence of azacycle donor moieties on the photovoltaic properties of benzo[c][1,2,5]thiadiazole based organic systems: a DFT study

Fullerene free organic chromophores are widely utilized to improve the efficacy of photovoltaic materials. Herein, we designed D-π-A-π-D form chromophores (TAZD1-TAZD5) via end-capped redistribution of donor moieties by keeping the same π-bridge and central acceptor unit for organic solar cells (OSCs). To analyze the photovoltaic characteristics of these derivatives, DFT estimations were accomplished at B3LYP/6–311 G (d,p) functional. Different investigations like frontier molecular orbital (FMO), absorption spectra (UV–Vis), density of states (DOS), binding energy (Eb), open circuit voltage (Voc), and transition density matrix (TDMs) were performed to examine the optical, photophysical and electronic characteristics of afore-mentioned chromophores. A suitable band gap (∆E = 2.723–2.659 eV) with larger bathochromic shift (λmax = 554.218–543.261 nm in acetonitrile) was seen in TAZD1-TAZD5. An effective charge transference from donor to acceptor via spacer was observed by FMO analysis which further supported by DOS and TDM. Further, lower binding energy values also supported the higher exciton dissociation and greater CT in TAZD1-TAZD5. Among all the designed chromophores, TAZD5 exhibited the narrowest Egap (2.659 eV) and maximum red-shifted absorption in solvent as well as gas phase i.e. 554.218 nm and 533.219 nm, respectively which perhaps as a result of the phenothiazine-based donor group (MPT). In a nutshell, all the tailored chromophores can be considered as efficient compounds for promising OSCs with a good Voc response, interestingly, TAZD5 is found to be excellent chromophores as compared to all these designed compounds.

of benzodithiophene based organic (TAZD1-TAZD5).To choose the suitable functional for current investigation, a relative investigation of X94FIC λ max outcomes among several TD-DFT functionals and experimental results was performed.For this purpose, the reference chromophore X94FIC was subjected to geometry optimization using four different functionals, including B3LYP 42 , M06 43 , MPW1PW91 44 and ɷB97XD 45 in acetonitrile solvent as range-separated functionals estimates HOMO-LUMO gaps and excited-state energies better [46][47][48][49] .Then these optimized geometries were applied to execute UV-Vis analysis in acetonitrile solvent and 818.864, 736.686, 676.142, and 495.461 nm values of λ max were obtained at aforesaid functionals, respectively.The λ max values of X94FIC obtained using these functionals were compared to the experimentally determined maximum absorption value of 783 nm 39 of X94FIC chromophore.At B3LYP/6-311G (d,p) functional, closed harmony was seen with experimental results.Moreover, we also compared the band gap values of X94FIC calculated at aforesaid functional of TD-DFT (1.774, 2.142, 2.032 and 4.971 eV, respectively) with experimental ΔE value (1.41 eV) 39 and interesting, good harmony with experimental results was seen at B3LYP, hence, this functional was selected for this study.First of all, structures of designed systems were optimized at B3LYP/6-311G(d,p) to get true minima geometries in acetonitrile solvent.The absence of any imaginary frequency specified that structures were at true minima potential energy surface.After the successful optimization of geometries, different analyses; FMOs, DOS, UV-Vis, V oc , E b and TDMs were attained to inspect the optical, electronic and photophysical characteristics of afore-mentioned chromophores at B3LYP/6-311G (d,p) level of DFT/TDDFT in acetonitrile solvent.Nevertheless, in order to understand the effect of different media on UV-Vis properties, we performed absorption analysis in gas and acetonitrile at foresaid functional.For the extraction of data from output files, Gauss View 5.0 program 50 , Avogadro 51 , Chemcraft 52 , PyMOlyze 2.0 53 , and Origin 54 software were utilized and the data was recorded in the form of graphs and tables.

Results and discussion
In current era, fullerene free-organic systems (FF-OSs) with some special architectures like D-π-A-π-D, A-π-A-π-A 55 , A-D-A 56 and A-π-A gain significant importance in improving the efficiency of solar cell materials [57][58][59] .Therefore, in current study we formulated a range of donor nature chromophores (TAZD1-TAZD5) with D-π-A-π-D framework from a synthesized system X94FIC (A-π-A-π-A) 40 by molecular replacement at the terminals with efficient azacycle donor moieties (see Fig. 2).First of all, we designed TAZD1 from X94FIC by replacing its terminal acceptors with four rings azacycle donor unit (9-phenyl-9H-carbazole) keeping the central 'π-linker' and ' A' same as shown in Fig. 1.After that TAZD2-TAZD5 are designed by replacing the four member azacycle donor rings unit with three, five and six member ring azacycle donor unit as exhibited in Fig. 2. The optimized structures of aforesaid systems are displayed in Fig. 3 while their Chemdraw structures are shown in Fig. S2, however, their IUPAC names are tabulated in Table S1.The utilized azacycle donor moieties: 9-phenyl-9H-carbazole (THC), 5-phenyl-10,11-dihydro-5H-dibenzo[b,f]azepine (THA), 5-phenyl-5H-dibenzo[b,f]azepine (TBA), 9,9,10-triphenyl-9,10-dihydroacridine (PTH), 3-methyl-10H-phenothiazine (MPT) and their structures can be seen in Fig. S1.We have calculated different parameters like FMOs, DOS, UV-Vis, V oc , E b and TDMs of all the studied compounds.The modifications in derivatives with their respective donor moieties might prove as a significant step towards introducing efficient solar cells.
Frontier molecular orbitals (FMOs) analysis.The optoelectronic properties i.e. charge transfer, electronic features, reactivity, chemical stability and molecular interactions 60 are investigated via utilizing FMOs [61][62][63] .The band gap of HOMO/LUMO orbitals is closely linked to these parameters 64 .As HOMO is the electronically filled highest orbital, so it is considered as an electron contributor, whereas, LUMO is considered to be an electron acceptor as it is an empty or unfilled orbital.Molecules having high energy gap (E gap ) values are hard, because they resist changes in electronic configurations, resulting in lower reactivity and increased kinetic stability.Conversely, the compounds with low energy gap are attributed as soft molecules owing to their less stability and higher reactivity.These compounds reveal strong intramolecular charge transfer (ICT) possibilities due to their highly polarized nature and are extremely efficient molecules in the production of solar cell materials 65 .In addition, the HOMO-LUMO band difference is important in calculating a molecule's total V oc and E b 64 .So, FMO analysis is used to compute E HOMO , E LUMO and E gap of TAZD1-TAZD5 and the outcomes are exhibited in Table 1.The pictographs showing charge transference among orbitals are depicted in Fig. 4.
The above table reveals HOMO energy values for TAZD1, TAZD2, TAZD3, TAZD4 and TAZD5 as − 5.177, − 5.185, − 5.185, − 5.199 and − 5.104 eV while energies of LUMO are − 2.469, − 2.467, − 2.463, − 2.476 and − 2.445 eV, correspondingly.E gap is used to calculate molecules conductivity and net charge transfer 66,67 .The E gap  values of designed chromophores (TAZD1-TAZD5) are revealed as 2.708, 2.718, 2.722, 2.723 and 2.659 eV, correspondingly.Highest energy difference between HOMO and LUMO (2.723 eV) is observed in TAZD4 among all the other derivatives which may be due to the 9,9-diphenyl-10-(p-tolyl)-9,10-dihydroacridine (PTH) donor moiety.The E gap value is abridged to 2.722 eV in TAZD3 due to the substitution of PTH with 5-(p-tolyl)-5Hdibenzo[b,f]azepine (TBA) donor moiety which may be due to the decreased in hindrance in charge transfer of TBA as compared to that of PTH.Furthermore, the replacement of donor of TAZD3 i.e.TBA with 5-(p-tolyl)-10,11-dihydro-5H-dibenzo[b,f]azepine (THA) in TAZD2 resulted in further reduction of bandgap to 2.718 eV owing to the enhancement in conjugation in the newly introduced donor moiety.TAZD1 is designed via replacing THA with carbazole containing donor moiety i.e. 9-(p-tolyl)-9H-carbazole (THC) in which nitrogen atom of carbazole exhibit the electron donating capability.As a result of this, the energy difference is lessen to 2.708 eV in TAZD1 because of the enhanced push pull mechanism.Moreover, TAZD5 has exhibited minimum energy gap as compared to all of the studied chromophores owing to the use of phenothiazine-based donor moiety such as 3-methyl-10H-phenothiazine (MPT) instead of THC in TAZD1.The extra electron-rich sulphur atom in phenothiazine might give an improved electron-donating capacity compared to donors that just include nitrogen atoms, like carbazole.Overall, the band gap descending order in the studied compounds is; TAZD4 > TAZD3 > TAZD2 > TAZD1 > TAZD5.
The electron density in HOMO of TAZD1-TAZD4 is predominantly located over the center ' A' and 'π-spacer' parts of the organic systems and minor over some atoms of donor, while in TAZD5 the electron density is dispersed on entire system.For LUMO, the electron density is majorly located over π-bridge and core acceptor in TAZD1-TAZD5.Among all the investigated compounds, TAZD5 is found to be the appropriate candidate for future OSCs with enhanced PV behavior due to less energy band gap and adequate charges transition from terminal donors to center acceptor (see Fig. 4).    2 as well as Table 3 and other transitions are represented in Tables S2-S11, whereas the absorption spectra of studied compounds TAZD1-TAZD5 is displayed in Fig. 5.
In solvent (acetonitrile), all the investigated compounds have revealed maximum absorbance in visible spectrum (Fig. 5).The designed molecules (TAZD1-TAZD5) exhibit absorption range from 543.361 to 554.218 nm in acetonitrile.In solvent phase, λ max values are found to be more red-shifted contrary to gas phase because of solvent effect.Furthermore, the absorption spectra of studied compounds (TAZD1-TAZD5) are dominated by   π-π interactions 68 .The polar medium results in the stabilization of π-π* state associated with n-π* characteristics by the use of an efficient electronic state 69 .This indicates that, in the stabilization of first singlet state, hydrogen bonding and dipole interactions are imperative 70 and the molecules exhibit red-shifted absorption as a result of enhancement of solvent polarity.
It is seen that, max values are controlled efficiently by end-capped donor moieties which successively drive the red shifted absorption spectra 71,72 .The absorption band of TAZD4 is noticed at 543.361 nm having 2.282 eV energy of transition, 1.031 f os by exhibiting 98% molecular orbital contribution from HOMO to LUMO.The computed λ max value is shifted towards bathochromic shift in TAZD3 due to the replacement of PTH donor of TAZD4 by TBA so, TAZD3 has exhibited λ max at 544.101 nm, 2.279 eV transition energy, and 0.964 oscillator strength via showing HOMO → LUMO MO contribution of 98%.Furthermore, the substitution of TBA with THA donor moiety resulted in red-shifted absorption of 544.483 nm in TAZD2 along with lower transition energy (2.277 eV) and 1.006 oscillator strength via same MO contributions.Additionally, TAZD1 absorption spectra further shifted towards bathochromic shift (546.427nm), owing to the deposition of another donor moiety i.e., THC in TAZD1 in the replacement of THA in TAZD2.Finally, the substitution of TTC with MPT donor moiety resulted in maximum red shift of 554.218 nm in TAZD5 due to phenothiazine group in MPT by showing 96% HOMO → LUMO and 2% HOMO-2 → LUMO molecular orbital contribution at 1.005 oscillator strength and minimum transition energy (2.237 eV) owing to its lowest band gap.max of all the compounds in acetonitrile solvent is found to be in increasing order as TAZD4 < TAZD3 < TAZD2 < TAZD1 < TAZD5.
In gaseous phase (Table 3), all the entitled compounds have almost exhibited equivalent order as well as characteristics as in solvent phase.The absorption spectrum shifts towards the red shift as the dielectric constant of media enhanced.Therefore, greater bathochromic shift is seen in acetonitrile due to its higher dielectric constant than that of gas phase.Nevertheless, it can be concluded from above discussion that, TAZD5 compound has exhibited maximum absorption wavelength, the lowest transition energy and minimum band gap which implies that, it can be used as an efficient material for photophysical characteristics in non-fullerene OSC materials.

Open circuit voltage. Open circuit voltage (V oc
) is another significant study that provides insights into the performance of OSCs i.e. their maximum working capability.The total current that can be produced via any optical system can be estimated by V oc 31,55   .So, V oc shows direct relation with E HOMO and E LUMO of donor and acceptor molecules, correspondingly.Thus, V oc outcomes of TAZD1-TAZD5 are determined via Eq.(1) proposed by Scharber and his coworkers.
The major purpose of the calculation of V oc is to associate HOMO of the investigated donors with LUMO of PC 61 BM acceptor which is a well-known acceptor having energy of HOMO = − 6.10 eV and energy of LUMO = − 3.70 eV 73 and the results obtained are represented in Table 4.
As Table 4 reveals that the values of V oc for TAZD1-TAZD5 by considering the energy gap of HOMO donor -LUMO PC61BM are found to be 1.446, 1.454, 1.454, 1.468, and 1.373 V, respectively.Among all, TAZD4 shows maximum results of V oc .The descending order of open circuit voltage of all the studied molecules is: TAZD4 > TAZD2 = TAZD3 > TAZD1 > TAZD5.From literature, we have found that for a significant transference from D HOMO towards A LUMO, the LUMO of acceptor should be at lesser energy level than that of the LUMO of donor molecules 46,71 .Interestingly, the LUMO of our compounds is higher than the PC 61 BM.These higher values of V oc elucidate the higher ICT from donor HOMO TAZD1-TAZD5 towards PC 61 BM which shows highly efficient donating capability of all the studied donors as shown in Fig. 6.
(1)  74 .The FMO diagrams in the Fig. 7 signify the electronic transitions that demonstrate the intramolecular charge transfer (ICT).In DOS pictographs, the negative values characterize HOMOs while, the positively charged outcomes depict the LUMOs and the difference among their values represents the energy gap on x-axis 75 .
The maximum density on LUMO is noticed at − 2.5 to 4 eV in all the investigated chromophores (TAZD1-TAZD5), while on HOMO highest density is observed from − 7 to − 12 eV as shown in Fig. 7.In TAZD2, TAZD4 and TAZD5 both HOMO and LUMO have comparable charge which depict their equal contribution toward FMOs.In TAZD1-TAZD5, the donor contributes 7.7, 5.3, 4.8, 4.6 and 22.0% to HOMO, whereas to LUMO its participation is 5.5, 5.5, 5.2, 5.9 and 4.0%, respectively.In the same fashion, π-spacer participates 72.5, 74.4,74.8, 74.8 and 61.7% to HOMO, while 35.0, 35.2, 34.2, 35.9 and 32.6% to LUMO in TAZD1-TAZD5, respectively.Likewise, acceptor shows its percentage participation 19.8, 20.3, 20.4,20.5 and 16.2% to HOMO in TAZD1-TAZD5, respectively, while 59.5, 59.3, 60.6, 58.2 and 63.4% to LUMO, respectively.Overall, the pattern of electronic charge distribution elucidates the delocalization of charges and large amount of charge transfer has taken place in all the modulated chromophores.Interestingly, all the designed derivatives portray almost alike contributions and the electron density is more prominent on the central unit (π-spacer and A units).
Transition density matrix (TDM) analysis.TDM is considerably utilized for the evaluation of electronic transitions along with their nature for TAZD1-TAZD5 in solvent phase.The study of the charge carriers localization along with delocalization and the interaction between donor and acceptor groups followed by electronhole delocalization as calculated by TDM analysis 76 .The role of hydrogen atoms has been neglected because of their small contribution.The TDM heat maps of all the formulated molecules (TAZD1-TAZD5) are presented in Fig. S3.To study the transition of electrons within molecules in detail, we split our compound into fragments i.e. donor (D), π-spacer and acceptor (A).
It has been seen that adequate charge is transmitted out of donor towards acceptor moieties as the electron-hole pair is constituted diagonally on the entire TDM plot and represented by clear red and green spots near the acceptor portion.The electron delocalization is seen in the diagonals of A and π-spacers and very little in the D region.Moreover, the charge coherence and electron-hole pair generation are similarly observed in the off-diagonal portions of TDM heat maps (see Fig. S3).
Hole-electron analysis.Hole-electron analysis is popularly accomplished by utilizing the Multiwfn 3.8 software.It is a very useful method for revealing the nature of electron excitations.Moreover, it offers a deep understanding of all different electron transfer properties 77,78 .In this study, hole-electron analysis is performed at B3LYP/6-311G (d,p) to understand the charge transmission in our studied molecules.Figure 8 shows that hole intensity is found maximum at sulphur atom (S15 and S16) of the thiophene ring of π-linker in parent compound (X94FIC) while, the hole intensity is observed at C36 and C38 of the acceptor region.Furthermore, it is also clear from Fig. 8 that electron intensity is found at its peak at sulphur atom (S9) of the acceptor region in compounds TAZD2, TAZD3 and TAZD4 whereas, hole intensity is observed to maximum at sulphur atoms  (S15 and S16) of the π-spacer.However, in TAZD5 hole intensity is higher at methyl group of the π-spacer and electron density is intense at nitrogen atoms (N7 and N8) of the acceptor portion.Moreover, electronic cloud is observed to be thick at nitrogen (N7 and N8) and sulphur (S9) atoms of the acceptor in TAZD6 however, hole intensity is maximum at carbon atoms (C12 and C14) of the π-linker.The labeled structures of entitled chromophores without hydrogen atoms are illustrated in Fig. S5.In conclusion, all the designed molecules except TAZD5, are electron type materials as electronic cloud is observed thick at electronic band in contrast to hole intensity at hole band.In TAZD5, hole intensity is found higher at hole band therefore, it is a hole type material.

Exciton binding energy (E b ).
One more consideration to estimate the working proficiency, optoelectronic characteristics, and separation potential is binding energy (E b ) 79 .The Eq. ( 2) is used to compute the binding energy of the studied systems.
( Here, E opt represents the least energy that is obtained when an electron moves from S 0 (ground state) to S 1 (excited state) during the first electronic transition.E H-L signifies the energy difference among HOMO and LUMO whereas, E b is the binding energy that is obtained by the difference in band gap between molecular orbitals and first singlet exciton energy.Usually, the lower the value of E b , greater would be the charge separation and current charge density J sc which results in higher PCE 80 .The outcomes for the studied compounds calculated in acetonitrile are formulated in Table 5.  www.nature.com/scientificreports/According to our obtained results, the values of E opt decreases progressively in all the designed compounds and is found as minimum in TAZD5 and the same behavior is observed in the HOMO and LUMO energy gap.The binding energy values in TAZD1-TAZD5 are found to be 0.439, 0.441, 0.443, 0.441 and 0.422 eV, correspondingly.The lowermost E b value of TAZD5 depicts that it has excessive charges that can be separated into isolated charges.It is noted that TAZD5 exhibits high segregation of charges along with high J sc which indicates that it is the leading candidate to improve the efficiency of organic photovoltaics.Furthermore, E b data unveil a good agreement with TDM outcomes.

Charge transfer analysis.
In order to understand the intermolecular charge transfer between donor and acceptor, a complex is developed between a donor molecule (TAZD5) and acceptor (PC 16 BM) polymer and FMO is investigated as shown in Fig. S4.For charge transfer analysis, we selected TAZD5 due to its unique properties such as reduced band gap and greater UV-Vis absorption spectra etc. among all fabricated chromophores.According to Fig. S4, in HOMO the charge is located over the TAZD5 donor chromophore and significantly transferred towards the acceptor polymer in LUMO which elucidates the significantly charge transfer from donor towards acceptor.

Conclusion
In a nutshell, through the molecular engineering with azacycle donor moieties in an organic system (X94FIC) fullerene free donor based chromophores (TAZD1-TAZD5) were designed.To comprehend their photophysical properties, the behavior of charge transfer, and structure-activity relationship, various analyses were performed at quantum chemical approach.A reasonable energy gap between LUMO/HOMO (∆E = 2.723-2.659eV) and significant charge transfer with wider absorption spectra (λ max = 554.218-543.261nm in acetonitrile) was examined in all non-fullerene donor chromophores.Additionally, the less binding energy outcomes (E b = 0.422-0.411eV) in formulated compounds specified higher rate of exciton dissociation that also reinforce the tremendous charge transition out of HOMO towards LUMO as shown by FMOs, DOS and TDMs analyses.Moreover, the V oc values are also determined with regarding to HOMO donor − LUMO PC61BM and interesting data was found with this order; TAZD4 (1.199 V) > TAZD2 (1.185 V) = TAZD3 (1.185 V) > TAZD1(1.177V) > TAZD5(1.104V).Consequently, significant photovoltaic materials can be developed by structural tailoring with efficient azacycle donor moieties.Moreover.this study also encourages the experimentalist to synthesize these efficient materials for practical use.
www.nature.com/scientificreports/UV-Vis analysis.UV-Vis analysis is significant to investigate the possibility of ICT, kind of configurations of transitions and electronic transitions in a compound.To calculate the absorption spectra of the excited states, the TD-DFT calculations are accomplished in gas and acetonitrile solvent.The observed oscillator strength (f os ), transition energy (E), transition type and maximum absorption wavelength (λ max ) are shown in Table

Figure 6 .
Figure 6.Pictographic representation of V oc of TAZD1-TAZD5 with respect to PC 61 BM.

Figure 8 .
Figure 8. Graphical representation of hole-electron analysis of investigated compounds.

Table 1 .
Computed orbital energies of TAZD1-TAZD5 and their energy gap.Band gap = E LUMO − E HOMO , units in eV.

Table 3 .
Wavelength ( ) , excitation energy (E), oscillator strength (f os ) and nature of molecular orbital contributions of TAZD1-TAZD5 in gas phase.

Table 4 .
Open circuit voltage and energy driving force of TAZD1-TAZD5.E = E A LUMO − E D HOMO .

Table 5 .
Calculated HOMO-LUMO band gap (E H-L ), first singlet excitation energy (E opt ) and exciton binding energies (E b ) of TAZD1-TAZD5.Units in eV.