Quantum Chemical Investigation on D-π-A Based Phenothiazine Organic Chromophores with Spacer and Electron Acceptor Effect for DSSCs

In this work, the newly designed phenothiazine based organic dye (PT-BTBA, PT-EBTBA and PT-EBTEBA) derivatives were screened and investigated for dye-sensitized solar cells (DSSCs) application. The literature dye of SB covers the electron-donor (D) in phenothiazine and cyanoacrylic acid in electron-acceptor (A) based on D-A structure. In order to improving the π-conjugation and acceptor effects on the SB dye were investigated. The effect of D-π-A designed dyes on the optical absorption spectra and photovoltaic (PV) parameters were implemented by the density functional theory (DFT) and time ‐ dependent DFT (TD-DFT) calculations. Also, the hybrid functionals were initially evaluated to establish an accurate methodology for calculating the first-singlet absorption peak of SB dye. Consequently, TD-CAM-B3LYP functional and 6-311++G(d,p) theory were well match with the literature data. According to this result, phenothiazine-4-((7-ethynylbenzo[c][1,2,5]thiadiazol-4-yl)ethynyl)benzoic acid (PT-EBTEBA) dye has the strong group for more red-shifted and successfully electron inject into TiO 2 surface. It is expected to provide some theoretical guidance on designing photosensitive with new metal-free organic dyes for use in DSSCs yielding highly efficient performance.

Highly efficient organic photosensitizer for DSSCs must have the following properties [30]: First, dye absorbs most of the sunlight or light rays, a broad absorption range that covers together the Ultraviolet-Visible (UV-Vis) and near-infrared (N-IR) regions. Secondly, an intramolecular charge transfer (ICT) from electronic-D and A moieties is one of the main characteristics of organic DSSCs [31][32][33][34][35]. Third, the suitable energy level of the highest occupied molecular orbitals (HOMOs) must be below the redox potential 3 / II  of the electrolyte.
Consequently, the oxidized sensitizer that have lose electrons is then turned back to the ground state of a redox system. Also, lowest unoccupied MOs (LUMOs) of the dyes must be above the conduction band edge (CBE) of the TiO2 semiconductor metal oxide. Thus, excited state molecules electrons can be efficiently injected into the TiO2 surface. The D-π-A structure with acceptor directly bonded to the semiconductor surface, which favor efficient charge transfer (CT) to the CBE of TiO2 and regeneration to the ground state of dyes [36].
In 2011, Derong Cao synthesized a D-A based SB dye is a simple structure of the organic chromophore with a phenothiazine group as a donor and 2-cyanoacrylic acid as acceptor. The electron rich nature of a phenothiazine group were delivered a good spread for the electron movement from D to A. According to SB-based DSSCs were exhibits a power conversion efficiency (PCE) 2.91%, high photocurrent (6.13 mA cm -2 ) and photovoltage (709 mV) measured at under AM 1.5G irradiation [37]. The PCE of DSSCs was generally determined by The calculated result PT-EBTEBA were represent that the smaller energy gap, vertical absorption coefficient obviously red-shift and development of organic DSSCs.

Computational Details
The quantum chemical investigation were recognized as a powerful instrument. It has been used to the electronic configuration and numerous belongings of dye sensitizers. The ground-state geometries of the designed dyes were fully optimized by DFT with the B3LYP [39] and 6-31G(d,p) theory. The optimized geometry results were executed by without any symmetry constraints, it has been broadly used for theoretical DSSCs research. The optimized dye structures were found at its local minima (no imaginary frequency modes) on the potential energy surfaces.
The previous TD-DFT valuation suggested that extremely efficient and reliable calculation of vertical excitation energies, photoabsorption spectra and PV properties [40,41]. In reliability of DSSCs, the great effects of the exchange (XC) and long-range corrected (LC) functionals were used for absorption peak. In this work, XC & LC functionals such as B3LYP, CAM-B3LYP [42], P3PW91 [43] and ɷB97XD [44] methods were analyzed for UV-Vis peak.
From the functionals, SB dye absorption peak calculated values were listed in  [45,46]. During the research, as per literature tetrahydrofuran (THF) solvent were used in SB dye. Also, CAM-B3LYP was successfully employed for calculating the absorption spectra of the designed dyes. The absorption spectra were calculated by GaussSum 3.0 version [47]. The DFT and TD-DFT calculations of the dyes were accomplished by Gaussian 09w suite [48].

Screening of the Dyes and Optimized Structures
The spacer and electron acceptor groups are a key parameter with D-π-A structure for highperformance of organic DSSCs. We selected three functional types, which might be made for the addition of chemical modified in conjugation and acceptor. The substituent groups have been given as the collected from literature as discussed above. From the preceding study, those molecules are good performing for organic DSSCs application. In this manner, those dyes may be stimulated to the phenothiazine based dye derivatives. A newly designed dyes in PT-BTBA, PT-EBTBA and PT-EBTEBA were investigated by the DFT and TD-DFT systematically. The optimized geometric structures of the SB and designed dyes were performed by the B3LYP/6-31G(d,p) level of theory and shown in Fig. 1. As shown in figure, the geometry structures of the dyes were shown that the better coplanar configurations. Also, the coplanar structure designates that the photosensitizers were fully conjugated throughout the D-π-A pattern. It has been favorable for the photo-induced CT from the electron D-A and also accelerating electron injection to the CBE of TiO2 surface. Additionally, the introduced π-conjugation and acceptor part were favored to the broadening absorption wavelengths.

Electronic Structures and Frontier Molecular Orbitals (FMOs) of the Planned Dyes
Frontier molecular orbitals (FMOs) is a very crucial factor in defining the charge separated states of dye sensitizers [49].  [50]. Similarly, all the LUMOs energies have to be capable of injecting electrons into the CBE of TiO2 (-4.0 eV) surface [50]. According to fig. 3, all LUMOs energies dyes lie over the CBE of TiO2 and the HOMOs were situated below the redox couple. As shown in figure 3 and table 2, energy levels of the all designed dye compounds has smaller g E , compared to SB. In particular, PT-EBTEBA dye can be identified that the positive response to wider absorption region and regeneration with photo-oxidation manner.

Optical Absorption Properties
Firstly, the optical absorption peak of SB dye was used four different functionals (B3LYP, CAM-B3LYP, P3PW91 and ɷB97XD) with THF solvent and TD-DFT calculation (Table 1 and Figure 1). According to the best of functional, designed dye derivatives have been calculated at TD-CAM-B3LYP/6-311++G(d,p) theory with THF medium. The simulated absorption spectra and corresponding calculated vertical excitation energy (E), oscillator strengths (f), major and minor orbital contributions (%) of the dyes have been listed table 3 and shown in Fig. 4 and.
From the figure, all the organic dyes have shown extensive visible around 600 nm, which is related to ICT character. Normally, the molecular dyes with broader wavelengths and enhanced molar extinction coefficients are predicted to have higher photo-to-current conversion efficiency of DSSCs performance [51].
As shown in / II  is used as the redox shuttle, it is able to be taken as a constant. The shift of CBE can be determined by using the formulation to earlier studies [53]. The normal  is another crucial characteristic that delivers the vertical electronic charge dispersal in the dye molecules. As proven in figure S0 (supporting information) V , compared to SB (4.55 Debye). Among three dyes, PT-EBTEBA may be the best performance for excessive conversion efficiency of DSSCs. As for SC J , it is a crucial part for DSSCs calculated by the formulations previous study [54] and the corresponding factors are discussed below. In order to attain extreme SC J , LHE of the dye molecules necessity to be as high as possible.
LHE (λ) can be described by the following equation [55]: (2) Where f signifies oscillator strength of associated dye molecules related to the absorption peak.

Electron Injection and Dye Regeneration
Another method of increasing SC J influence factors of the dye regeneration ( ) and oxidation potential energy ( * dye OX E ) was calculated by using principles as defined in a previous study [56]

Open-Circuit Photovoltage
Theoretical analysis of  Table 4 and presented in figure 5b. From the table, PT-BTBA, PT-EBTBA and PT-EBTEBA values are given in a range from 0.99 to 1.08 eV, in these values is a promising response to the efficient electron injection. Also, larger values of the LUMOs will produce the higher OC eV , which is provided to PCE of the DSSCs. According to figure 5b, it has shown that the PT-BTBA and PT-EBTEBA dyes larger OC eV value, compared to SB dye. In specific, PT-EBTEBA dye has a higher OC eV , which identify that the extraordinary performance of organic DSSCs. Therefore, PT-EBTEBA dye has used for the dye-sensitized, owing to their electron injection technique from the excited state sensitizers into the CBE of TiO2 have been successful.

Exciton Binding Energy (
To get high PCE in DSSCs, the electron-hole pairs should be split into separate negative and positive charges to escape from recombination owing to coulombic forces. During this procedure, the binding energy need be overcome; that is, the dyes must take lower exciton binding energy to attain high PCE of DSSCs. The exciton binding energy was followed by using  and table 4, PT-EBTEBA dye is smaller value than the other molecules and SB. It can be noted that PT-EBTEBA dye has lesser than the other sensitizers and SB (0.61 eV), which denotes that the excellent act for DSSCs.

Conclusion
A sequence of three isolated metal-free organic DSSCs totally on the PT-BTBA, PT-EBTBA and PT-EBTEBA became successful with spacer and electron acceptor. The molecular orbital electronic structures, absorption properties and PCE influence parameters of the dyes were investigated systematically. In this study, the DFT and TD-DFT methods were discussed in detail to planning dye products. The calculated outcomes suggest that the screened with a spacer and the electron acceptor in PT-EBTEBA has promising functional organization for D-π-A form.
Furthermore, the calculated results imply that the PT-EBTEBA dye has strong light harvesting efficiency, electron injection, electron transition, dye regeneration, open-circuit photovoltage, exciton binding energy and increasing the dipole moment, which benefits to higher PCE of DSSCs. It has concludes that the choice of the appropriate screened with spacer and electron acceptor. Finally, these are very important for the molecular design of phenothiazine based highly efficient organic solar cells.

Figure Captions Scheme 1
Outlining of molecular structures for D-π-A System.

Fig. 1
Optimized geometric structures of SB and designed dye molecules were calculated at B3LYP/6-31G(d,p) level of set.

Fig. 2
The selected frontier molecular orbitals of the Planned dyes and SB were implemented on B3LYP/6-31G(d,p) level of theory.