Cyanidin-Based Novel Organic Sensitizer for Efficient Dye-Sensitized Solar Cells : DFT / TDDFT Study

Applied Physics Program, Faculty of Sciences, Universiti Brunei Darussalam, Jalan Tungku Link, Gadong BE1410, Brunei Darussalam Physical and Geological Sciences Programme, Centre for Advanced Material and Energy Sciences, Universiti Brunei Darussalam, Jalan Tungku Link, Gadong BE1410, Brunei Darussalam Faculty of Integrated Technology, Universiti Brunei Darussalam, Jalan Tungku Link, Gadong BE1410, Brunei Darussalam


Introduction
Dye-sensitized solar cells (DSSC), first introduced in 1991, are emerging as the most promising alternative to traditional silicon solar cells due to the low cost of fabrication and easy manufacturing process [1][2][3][4].The sensitizer, which plays a major role in DSSC function, absorbs light and injects electrons to the conduction band of TiO 2 from its excited state.Sensitizers based on ruthenium [5,6], zinc porphyrin [7,8], and organic dyes [9] have been discovered and applied as efficient sensitizers for DSSC.Despite of the high efficiency of ruthenium and porphyrin sensitizers, environmental complication and sophisticated synthesizing procedures make it necessary to find for alternative efficient sensitizers.
Natural dye molecules such as flavonoids, betalains, carotenoids, and chlorophylls are currently being tested as alternatives to the expensive sensitizers in the fabrication of DSSC, considering their electron-donating and electronaccepting abilities [12][13][14][15][16].However, the efficiencies of DSSCs sensitized with natural dyes are still very poor, mainly due to the poor light-harvesting properties associated with them [17][18][19][20].Therefore, the need to invent natural dyebased new dye with improved optical properties favorable to enhance efficiency of DSSC is a worthy solution.
In this study, a cyanidin, which belongs to flavonoids, based new organic dye is modeled and its suitability as a novel sensitizer in DSSC is computationally tested using density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations.The new sensitizer, P01-1, is designed by attaching one of the hydroxyl groups on the benzene ring of cyanidin to the benzothiadiazolylbenzoic acid (BTBA) unit as shown in Figure 1.Following IUPAC rules, we have named the molecule (P01-1) as p-{5-[2hydroxy-4-(3,5,7-trihydroxy-2-chromenyl)phenyl]-8-thia-7, 9-diazabicyclo[4.3.0]nona-1(9),2,4,6-tetraen-2-yl}benzoicacid.In the design, we used benzothiadiazolylbenzoic acid as an electron acceptor by considering its strong electron withdrawing ability in some excellent dyes reported by Yao and coworkers [21].The position of the BTBA substitution to cyanidin is based on our previous work [12]; TDDFT calculations of cyanidin revealed that the electron density cloud of the lowest unoccupied molecular orbital (LUMO) is highly localized and denser in the selected hydroxyl group.

Computational Details
The molecular structures of P01-1, cyanidin, and benzothiadiazolylbenzoic acid (BTBA) are computed using Spartan'10 software [22] to retrieve the molecular geometry coordinates.Both DFT and TDDFT calculations are performed using Gaussian'09W software [23].The molecular structures of P01-1, cyanidin, and BTBA are fully optimized using Becke's three-parameter hybrid functional using Lee-Yang-Parr hybrid functional (B3LYP) [24] in ethanol.The electronic structures and total energies are calculated using 6-31g (d) basis set which adopted to describe metal-free atoms.All of the calculations include the solvation effect in ethanol using the polarizable continuum model (PCM) [25].The lowest 5 singlet-singlet excitations are included in the TDDFT calculations.The models of electron density of various energy levels of the P01-1 are visualized, and absorption spectra of P01-1, cyanidin, and BTBA are simulated using GaussView version 5.0 [26].
The synthetic strategy to reach P01-1 is by coupling cyanidin and bromo-benzothiadiazolylbenzoic acid together via Ullmann cross coupling reaction [27,28].Cyanidin can be extracted from natural flowers and fruits; benzothiadiazolylbenzoic acid (BTBA) can be synthesized by following a procedure reported by Yao and coworkers [21].

Results and Discussion
3.1.DFT Calculation.DFT calculations are performed on the P01-1 molecule with a solvent effect on ethanol.For comparative purposes, we also performed calculations on cyanidin and BTBA.The results are summarized in Table 1.It is observed that the band gap value of P01-1, obtained after geometry optimization in ethanol, is smaller than HOMO-LUMO gap values of cyanidin and BTBA in ethanol.   1) show that the P01-1 satisfies the conditions for efficient photo-energy conversion.

HOMO-LUMO Energy
3.3.TDDFT Calculations and Absorption Spectra.TDDFT simulations are performed on the newly designed sensitizer P01-1, cyanidin, and BTBA, to gain perceptions on the excitation energy, electronic transition, optical properties, and UV-Vis absorption spectra for the singlet-singlet transition with hybrid functional B3LYP in ethanol solution.
The lowest 5 singlet-singlet excitations are included in the TDDFT calculations.The excitation energies, oscillator strengths, and molar extinction coefficient for the five states of P01-1 and cyanidin calculated using B3LYP on 6-31g (d) basis set are shown in Table 2, and the computer simulated UV-Vis absorption spectra obtained for the P01-1, BTBA, and cyanidin via TDDFT calculations are depicted in Figure 2.
Oscillator strength expresses the strength of transitions to the excited states.The higher the oscillator strength, the higher the possibility for the molecule to be sensitized [32][33][34][35][36]. Except in the third excitation state, all other excitation states up to fifth excitation state have shown that oscillator strengths of P01-1 are higher than those of cyanidin.
P01-1 has produced four significant oscillator strengths up to the fifth excitation states except third excitation state, while cyanidin only produced three significant oscillator strengths at the first, third, and fifth excitation states.This suggested that P01-1 has more potential to be sensitized as compared to cyanidin.In addition, the λ max values of the P01-1 at each five excitation states shifted to longer wavelengths compared to λ max of cyanidin by 75.59 nm, 36.95 nm, 52.78 nm, 94.45 nm, and 109.11nm, respectively.
The absorption spectra of cyanidin and the BTBA extend only up to 800 nm and 550 nm.Interestingly, the absorption spectra of the P01-1 have shown a redshift up to near IR of 1000 nm (Figure 2).The absorption bands of P01-1 are more intense and broad in the visible region.The absorption maxima of P01-1 has bathochromic shifts of 75.59 nm as compared to that of cyanidin.Furthermore, they have hyperchromic shifts of 8.577 (10 3 M −1 •cm −1 ).These shifts in the modified dyes are favorable to effectively lead to an efficient sensitization.Almost all levels of the ground state energy such as HOMO-4, HOMO-2, HOMO-1, and HOMO levels of P01-1 are π-type.At the HOMO, HOMO-1, and HOMO-3 levels, the electron density clouds are mostly delocalized over the entire molecule of P01-1 but, at the HOMO-2 level, the electron density cloud is more localized in the benzene ring of the cyanidin unit of P01-1.
In the P01-1 molecule, the electron cloud of LUMO is π * type and the molecular orbital is localized at cyanidin unit.Here, the electron density cloud is observed to be denser in a chromenylium unit than the benzene dial on cyanidin unit of P01-1.The photogenerated electrons are excited to  3 International Journal of Photoenergy chromenylium unit on the electrode of TiO 2 .Subsequently, electrons are easily injected into the conduction band of TiO 2 [32].This electron injection is most possible from the chromenylium unit as it has denser electron density cloud in the LUMO π * state.Therefore, the anchoring group of P01-1 molecule that may efficiently inject electrons into the TiO 2 is deduced to be the hydroxyl group.

Conclusions
The molecular geometries, electronic structures, and absorption spectra of the newly designed molecules of the P01-1 sensitizer are investigated by DFT and TDDFT computational analyses.By evaluating HOMO and LUMO energy levels obtained from DFT calculations of geometryoptimized structure of P01-1 molecules with solvent effect in ethanol, P01-1 satisfied the main requirement for efficient electron injection, where the LUMO level of P01-1 is higher than the conduction band of the TiO 2 and the HOMO level of P01-1 is sufficiently lower than the redox couple.Furthermore, the band gap of P01-1 is smaller than that of cyanidin and BTBA in ethanol.
The UV-Vis absorption spectra reveal that the P01-1 complex extends absorption spectra from 800 nm to 1000 nm range with bathochromic and hyperchromic shifts.All these results suggest a more promising sensitization ability of P01-1 which is designed by modifying the sensitizer, cyanidin.International Journal of Photoenergy International Journal of Photoenergy

3. 4 .
Molecular Orbitals.The electron density clouds modelled at the transition of various energy levels that correspond to the first five excited states of P01-1 are shown in Figure3.At different energy levels, it could be observed that electron density clouds are localized in different regions of the P01-1 molecule.

Figure 2 :
Figure 2: UV-Vis absorption spectra obtained for the P01-1, BTBA, and cyanidin via the output of TDDFT calculations with the solvation effect of ethanol solution.

Figure 3 :
Figure 3: The behavior of electron density clouds at the various energy levels corresponding to the first five excited states of P01-1 in ethanol using the TDDFT simulation with isovalue of contour = 0.03.

Table 1 :
Summary of DFT computational calculation of cyanidin, BTBA, and P01-1 constituents in ethanol with geometry optimization.

Table 2 :
Computed excitation energies in (eV), (nm), and the oscillator strength (f ) of the cyanidin and P01-1 obtained by TDDFT calculations at B3LYP/6-31g (d) level with the inclusion of geometry optimization under the solvation effect of ethanol.
* Molecular orbitals with configuration coefficient < 0.1 are not shown.