Exploration of promising optical and electronic properties of (non-polymer) small donor molecules for organic solar cells

Non-fullerene based organic compounds are considered promising materials for the fabrication of modern photovoltaic materials. Non-fullerene-based organic solar cells comprise of good photochemical and thermal stability along with longer device lifetimes as compared to fullerene-based compounds. Five new non-fullerene donor molecules were designed keeping in view the excellent donor properties of 3-bis(4-(2-ethylhexyl)-thiophen-2-yl)-5,7-bis(2ethylhexyl) benzo[1,2-:4,5-c′]-dithiophene-4,8-dione thiophene-alkoxy benzene-thiophene indenedione (BDD-IN) by end-capped modifications. Photovoltaic and electronic characteristics of studied molecules were determined by employing density functional theory (DFT) and time dependent density functional theory (TD-DFT). Subsequently, obtained results were compared with the reference molecule BDD-IN. The designed molecules presented lower energy difference (ΔΕ) in the range of 2.17–2.39 eV in comparison to BDD-IN (= 2.72 eV). Moreover, insight from the frontier molecular orbital (FMO) analysis disclosed that central acceptors are responsible for the charge transformation. The designed molecules were found with higher λmax values and lower transition energies than BDD-IN molecule due to stronger end-capped acceptors. Open circuit voltage (Voc) was observed in the higher range (1.54–1.78 V) in accordance with HOMOdonor–LUMOPC61BM by designed compounds when compared with BDD-IN (1.28 V). Similarly, lower reorganization energy values were exhibited by the designed compounds in the range of λe(0.00285–0.00370 Eh) and λh(0.00847–0.00802 Eh) than BDD-IN [λe(0.00700 Eh) and λh(0.00889 Eh)]. These measurements show that the designed compounds are promising candidates for incorporation into solar cell devices, which would benefit from better hole and electron mobility.

The optimized geometries found at true minima in potential energy surfaces are shown in Fig. 3.

Frontier molecular orbitals (FMOs) analysis.
FMOs analysis is a very useful tool for the characterization of electronic and optical properties of molecules 37 . According to band theory, HOMO and LUMO orbitals are denoted as valence and conduction band respectively. In photovoltaic materials, FMOs energy difference (ΔE = E LUMO -E HOMO ) is considered as a hallmark of ability to carry charge [38][39][40][41][42][43] . The charge carrier mobility of designed donor molecules can be improved through conjugation due to the electronic delocalization within the molecular systems. Energy of HOMO, LUMO and their difference are fully coupled with PCE of solar cells. It is also illustrated that there is dynamic stability, electron transfer characteristics, chemical softness/hardness and reactivity of the designed compounds 44 . FMOs study for the distribution of charges and principally the ΔE  www.nature.com/scientificreports/ between HOMO/LUMO orbitals is significant to recognize the electronic behavior and optical properties of the investigated compounds throughout the excitation process. FMOs study was performed at TD-DFT/M06/6-31G(d,p) level and HOMO, LUMO energies and their difference in energy ΔE) that are presented in Table 1. Additionally, the pictographic representation for FMOs for BDD-IN, and designed molecules are displayed in Fig. 4. The E HOMO and E LUMO of DMDC are less than all other molecules, signifying the better electron withdrawing effect of the four terminal cyano units of DMDC. Conversely, HOMO-LUMO values in BDD-IN were found greater as compared to the corresponding value of designed structures that predicts the lesser efficiency than the end-caped acceptors of all designed compounds. Moreover, in DMDC, the HOMO-LUMO energy levels are stable than DDHF and DMDH, which designate the significant proficiency of end-caped acceptor moieties.  www.nature.com/scientificreports/   Fig. 4, the HOMO charge is propagated on the central accepter moiety and a little amount of charge is observed on the π-spacer, whereas LUMO is dispersed on the end-capped acceptor units of the studied compounds. This charge dispersion patterns show that occurrence of electrons delocalization is caused by high donor to acceptor charge transfer with the aid of π-bridging unit.
Partial density of states (P-DOS) were computed at M06/6-31G (d, p) level of DFT (Fig. 4). Figure 5 is also in accordance with the factors presented in FMOs study and along with Fig. 4, it reveals that charge is concentrated around LUMO and HOMO because of strong dragging and the electron accepting capability of terminal units. In BDD-IN, the HOMO charge density is occupied primarily on central acceptor part (A1) and π-spacer, while the LUMO is occupied completely on whole molecule except upper part of the central acceptor unit (A1) and half portion of the end-caped acceptor (A2). The HOMO charge density is completely distributed on the central acceptor part (A1) and π-spacer unit and the LUMO is distributed completely on the end-capped acceptor units A2. Overall, these charge density circulation patterns reveal that electron delocalization is happened and huge charge transferred from the central acceptor part (A1) to end-capped acceptor units A2 with the assistance of the bridge part occurred.

Optical properties.
To evaluate the photo-physical responses for BDD-IN, DDHF-DDTC, TD-DFT at M06/6-31G(dp) level of theory was employed to execute UV-Visible absorption spectra in CHCl 3 solvent. The computed results attained from spectral analysis comprising max , transition energy (E x ), oscillator strengths ( f os ), and transition natures of the investigated molecules are arranged in Table 2. Absorption spectra are presented in Fig. 6 showing two absorption peaks for each studied compound representing the major and minor absorption peaks.
Maximum absorption peak ( max = 585 nm) of BDD-IN is in good agreement with experimental ( max = 532 nm) value as can be seen in Table 2. The electron withdrawing groups can potentially be attributed for the red shift in max values in the observed spectra. The TD-DFT based calculations also reveal that absorbance of all studied compounds is found in the visible region within the range of 666-732 nm. The max values of designed compounds are reported considerably larger and red shifted than that of BDD-IN molecule (Fig. 6). Table 2 reveals that the lowest value of max is found in BDD-IN molecule among all the investigated compounds. The strong electron withdrawing capability of four F atoms and four CN groups present in end-capped unit of the compound DDHF successfully caused the red shift to max value to 666 nm which confirms the utility of DDHF end-capped acceptor as compared to other end-capped acceptors of BDD-IN molecule which lack F and CN groups. Replacement of four fluorine with chlorine atoms in the end-capped acceptor of DDHF converts it to the compound DMDH. This change increases the max value to 683 nm showing potential of DMDH over DDHF and BDD-IN molecules. Similarly, the replacement of four chloro groups with cyano groups in DMDH yields compound DMDC, where max value shifts to 732 nm, the largest max value among all the designed compounds. These results confirmed the superiority of DMDC end-capped acceptor containing CN units over all other compounds. The compound DDTF where end-capped unit contains six F atoms successfully shifted the max value to 679 nm, showing that the number of fluorine atoms play their role in causing the red shift. Likewise, the replacement of six fluoro atoms with chloro groups in DDTF produced compound DDTC which also led to the successful red shift of the maximum absorption peak ( max ) value to 692 nm and proved the usefulness of DDTC over DDTF molecule as well as over BDD-IN molecule. Overall, the designed compounds showed red shift of 81, 98, 147, 94 and 107 nm in comparison to that of BDD-IN molecule correspondingly. The maximum absorption peak ( max ) for BDD-IN molecule and designed compounds is in the following escalating order: Excitation energy or charge transfer character exhibits valuable insights and proposes that molecules having smaller transition energy accommodate higher charge transfer capability, easy excitation between the HOMO to LUMO and possess higher PCEs. In case of BDD-IN, maximum value of excitation energy is noticed as 2.11 eV. Strong electron accepting capability of end-caped groups reduces the excitation energy in designed compounds. Hence, the calculated transition energy values show that the reference molecule BDD-IN has greater value of transition energy than the designed compounds. The excitation energy values for DDHF-DDTC are found to be 1.86, 1.81, 1.69, 1.82 and 1.79 eV, respectively. The lowest excitation energy is 1.69 eV in the case of DMDC due to presence of cyano group and extended conjugation. The increasing order for excitation energy of the designed compounds agrees with the decreasing max order: BDD-IN > DDHF > DDTF > DMDH > DDTC > DMDC. The smallest transition energy of DMDC and the highest max value make it a suitable candidate to be used in solar cells due to the better optoelectronic properties. The previous examination concludes that all the designed molecules containing higher max and lower transition energy values possess good potential of optoelectronic properties than that of BDD-IN molecule. Hence, all designed compounds especially DMDC is predicted to be capable of being utilized as an electron donating molecule in OSCs applications. www.nature.com/scientificreports/   (3) and (4). Reorganization energy of all studied compounds was computed at the M06/6-31G (d, p) level of theory (Table 3). The anionic and cationic geometries indicate the transformation of electron and hole towards acceptor from the donor molecule. Reorganization energy (RE) can be utilized to compute the charger transfer (CT) between the electron donating and accepting moieties. This energy is categorized in two segments: internal reorganization energy (REint. ) and external RE ( ext. ). Both ext. and int. specifies the polarization effect on the external environmental and the rapid alterations in the internal geometry, respectively. For this manuscript, the environmental variations have not been considered as they have little effect and only int. is considered. The value of e for DDHF (0.0037 E h ) was found to be less than BDD-IN (0.0070) signifying the dominant electron transfer rate for DDHF as compared to BDD-IN. Likewise, the value of e for DMDH (0.00333 E h ) was noticeably smaller than BDD-IN (0.0070 E h ) and DDHF implying that two terminal chlorine groups work efficiently to tune the intra molecular charge transfer as compared to two fluorines in DDHF. Due to cyano groups modification in DMDC, least value of electron reorganization energy was found to be 0.00285 E h among all the studied compounds indicating the best efficiency of cyano groups as compared to other functional groups present in terminal acceptors. The e for DDTF and DDTC were also found smaller than the reference molecule due to end-capped modifications.
The highest h value of all investigated compounds was noted and compared with BDD-IN molecule. As discussed, the reasons for reduction in electron reorganization energy, hole reorganization energy is also abridged in designed compounds due to end-capped modifications of different functional units. The h values of designed compounds DDHF-DDTC are 0.00826, 0.00802, 0.00847, 0.00824 and 0.00820 E h respectively that are much smaller as compared to that of BDD-IN (0.00889 E h ) and this reveals that designed compounds have a greater rate of transformation of holes in comparison to BDD-IN molecule. Overall, the values of e are smaller as compared to h which proposes that these compounds are inspiring candidates for transfer of electrons.

Open circuit voltage (Voc).
Voc is significant to illustrate the execution of OSCs 45 and reveals the highest value of current that can be taken away from an optical device in this context 46 . It is at the point of maximum  The HOMO energy levels of the studied molecules in comparison with LUMO energy level of well-known acceptor material PC 61 BM are shown in Fig. 7.
The Voc value for compound BDD-IN with regards to HOMO donor -LUMO PC61BM energy difference is calculated to be 1. among all the investigated molecules is computed in case of DMDC that was found to be 0.50 V higher than that of BDD-IN molecule. Likewise, Voc for DDTF is calculated to be 0.28 V larger as compared to BDD-IN molecule and larger than DDHF but less than DMDH and DMDC molecules. Moreover, the Voc value of DDTC is calculated as 0.3 V higher as compared to BDD-IN molecule and greater than DDHF and DMDH molecules but less than DMDC molecule. The least value of Voc 1.54 V amongst all designed compounds was found in the case of DDHF which was still 0.26 V higher than BDD-IN (1.28 V) value. This investigation proves that all designed molecules DDHF-DDTC have the potential to be suitable materials for OSCs applications when blended with well-known acceptor polymer PC 61 BM.

Charge transfer analysis. In the charge transfer (CT) investigations, a complex is established between
DMDC and well-known acceptor polymeric materials, in this study we are using PC 61 BM. Optimized geometry of the DMDC: PC 61 BM complex is shown in Fig. 8. The interactions between donor molecule DMDC and acceptor polymeric material PC 61 BM interact at various points, C3 and polymer sides are parallel. Whereas functional group side of PC 61 BM is positioned to the end-capped acceptor of DMDC, whereas ball side of PC 61 BM points toward the core side of the DMDC molecule (Fig. 8).
The electronic cloud of DMDC: PC 61 BM arrangement is majorly influenced by the relative positioning of the DMDC and PC 61 BM which eases the charge transformation between the electron donating and accepting parts. Dipole moment of the complex largely comes from DMDC to the acceptor and acts as the cause for effective exciton dissociation at the DMDC: PC 61 BM boundary [48][49][50] . The dipole moment is complex because of the electrostatic interactions of permanent dipole moment of PC 61 BM with respect to DMDC. Existing literature supports that the polymer part is largely responsible for the dipole moment within the complex, where, the dipole moment vector originates from the polymer side and point towards the core of the DMDC molecule. The HOMO-LUMO electronic structure and charge circulation pattern were computed at the M06/6-31G(d,p) level of DFT. The HOMO charge concentration in DMDC: PC 61 BM complex is concentrated on the central part and in part on the π-spacer of the donor DMDC molecule (Fig. 8b), while the LUMO charge is dispersed on end-capped group polymer PC 61 BM (Fig. 8b). The orbital diagram illustrates that the HOMO-to-LUMO excitation is a charge transferred from the electron donating DMDC to the electron accepting PC 61 BM molecule. The transformation of charge concentration from the electron donating molecule to the electron accepting is an indication of a good photovoltaic material. www.nature.com/scientificreports/

Exciton binding energy (E b ) and transition density matrix (TDM). The transitions nature is assessed
by calculating the transition density matrixes (TDMs). The M06/6-31G (d, p) level of theory was utilized to calculate the emission and absorption of the S1 state in vacuum, the results are exhibited in Fig. 9. Due to the minute contribution in transitions, the influence of hydrogen atoms is overlooked. TDMs technique allows us to calculate, (1) the interaction within electron donating and accepting moieties in the excited state; (2) the electronic excitation (3) electron hole localization. For the determination of these properties, we distributed our studied molecules into three parts namely, acceptor-1 (A1), π-bridge (B) and acceptor-2 (A2). The TDMs graphs designate that all compounds show analogous behavior in which electron coherence is majorly accessible on the diagonal of π-bridge segment B and a minute portion is present on A1. Similarly, in case of all the designed molecules a major portion of charge is present on the end-capped acceptor A2, while in case of reference molecule BDD-IN, a minute portion is seen on A2. The electron coherence in designed molecules display similar trend that is, majorly present on the π-bridge segment B and end-capped acceptor A2, while a minute portion is present on A1. The TDMs graph for BDD-IN shows that major portion of electron coherence is present on the π-bridge segment B and a minute portion is present on A1, while a minute portion of electron coherence is seen on a diagonal of A2. From TDM diagrams, as shown in Fig. 9, the electron coherence of BDD-IN, DDHF-DDTC confirm that the electrons are successfully transferred from the central acceptor A1 to the π-bridge and lastly the electron charge concentration moves to the electron acceptors. Moreover, the coefficient of interaction between donor and acceptor groups are in order of BDD-IN > DMDH > DDHF > DDTF > DDTC > DMDC. This order suggests that the connection of the hole and the electrons of DMDC may be weaker as compared to the remaining studied compounds, however this exhibited greater and easier exciton dissociation in the excited state.  In Eq. (2), E HOMO−LUMO signifies energy difference of HOMO/LUMO and E opt shows that the smallest quantity of energy required for the first excitation (gained from S 0 to S 1 ), by producing pair of the electron and hole 51,52 . Calculated results for binding energy (E b ) are arranged in Table 4. The

Materials and methods
Gaussian 09 package 53 was utilized to perform the calculations. Initially, GaussView 5.0 program 54 was used to yield three dimensional structures of the molecules and input files for Gaussian 09 package. The geometry optimization of BDD-IN molecule was executed by six DFT based functionals: B3LYP 55 , CAM-B3LYP 56 , MPW1PW91 57 , ωB97XD 58 , LC-BLYP 59 and M06 60 along with 6-31G(d,p) basis set. Later, structural optimization using frequency analysis at true minima of potential energy surface, TD-DFT calculations were employed for calculating the absorption spectra (λ max ) of BDD-IN molecule at same levels of theory and basis set combinations. Among all tested functionals, λ max result of M06/6-31G(d,p) functional was found in agreement to the experimental λ max results for BDD-IN molecule. Therefore, M06/6-31G(d,p) level of theory was considered ideal to be used in this study for computing density of state (DOS), TDM surfaces, FMO analysis, reorganization energies, charge transfer analysis, open circuit voltage (V oc ) , and band gap of BDD-IN as well as designed DDHF, DMDH, DMDC, DDTF and DDTC compounds. The chloroform solvent with conductor-like polarizable continuum (CPCM) model 61 was utilized for estimating λ max values of the investigated compounds.