Photoelectric Properties of DSSCs Sensitized by Phloxine B and Bromophenol Blue

Phloxine B and bromophenol blue as the sensitizers of dye-sensitized solar cells were investigated via UV-Vis spectra, FT-IR spectra, fluorescence spectra, and current-voltage characteristics. The frontier molecular orbital, vibration analysis, and the first hyperpolarizability were calculated with DFT/6-31G(d). The dipole moment, light harvesting efficiency (LHE), and larger absolute value of driving force of electron injection (ΔGinject)were also discussed.The calculated resultswere comparedwith the experimental results of phloxine B and bromophenol blue. It was found that, comparedwith bromophenol blue, bigger dipolemoment of phloxine B results in larger open circuit voltage (Voc) according to the correlation between dipole moment and Voc. At the same time, for configuration of phloxine B, it has higher LHE and ΔGinject, which are helpful to enhance the abilities of absorbing sunlight and electron injection. Therefore, higher LHE and ΔGinject for phloxine B produced a larger value of Jsc.


Introduction
With gradually aggravated environmental problem, it is hoped that more energies can be found to replace traditional fossil energy.Solar energy as the clean and renewable source has aroused extensive attention.Since O'Regan and Graetzel [1] developed dye-sensitized solar cells (DSSCs), the phototo-electric conversion efficiency () of this type of solar cells has achieved 7.1-7.9%.Due to their friendliness and low cost of production [2][3][4][5], more attention has been paid to investigate the relationship between structure and performance and to design new DSSCs.
The band engineering of the solar cells devices is shown in Figure 1, which indicates the complete energy levels of the different materials used on the DSSCs devices.DSSCs have the characteristics of the following five aspects [6][7][8][9]: (1) Dye molecules absorb sunlight energy, and they are excited from ground state to excited state.(2) After the dye molecules are excited, the electrons are injected into the conduction band of semiconductor and moved into the conducting glass.(3) The electrons diffuse into the external circuit.(4) The dye molecules in oxidation state are deoxidized by the electrolyte in reduction.(5) The electrolyte in oxidation state is deoxidized after receiving the electron at the conducting glass, thereby completing a cycle.
The DSSCs are mainly composed of a nanocrystalline porous semiconductor electrode-absorbed dye, a counter electrode, and an electrolyte containing iodide and triiodide ions [10].The dye as a sensitizer plays a key factor in absorbing sunlight and conversion solar energy into electric energy.DSSCs mainly are divided into organic dyes and inorganic dyes [11][12][13].Inorganic dyes such as N3 and N719 polypyridine complexes [14] have been used as sensitizer in DSSCs.Although DSSCs of inorganic dyes have provided a relatively high efficiency, there are several shortcomings of using inorganic dyes in DSSCs.For example, some inorganic dyes are considered as resources that are limited in amount, which result in more expensive cost in the field of DSSC.By contraries, organic dyes not only are cheaper, but also have been reported to reach efficiency as high as 9.8% [15].Nevertheless, the conversion efficiency  of organic dyes was still lower than that of inorganic dyes, and it was not suitable for commercial production in comparison with silicon cell [3].Torchani et al. [16] studied the henna and mallow (Mloukhya) as the sensitizers of DSSCs.The results showed that the filling factor of mallow solar cells is 55%, and  is 0.215%.Zhou et al. [17] studied twenty natural dyes, extracted from natural materials (such as flowers, leaves, fruits, traditional Chinese medicines, and beverages), and the results showed that the open circuit voltage and  of mangosteen pericarp are the highest, which are 0.686 V and 1.17%, respectively.Now several organic dyes have been utilized as sensitizers in DSSCs, such as coumarin [18], porphyrins [19][20][21], triphenylmethane [22,23], indoline [24,25], and cyanine [26,27].
In recent years, quantum chemistry method has provided a reliable theoretical basis for the rapid screening of high efficiency dye molecules [28][29][30][31][32]. Kumara et al. [33] reported the black tea waste extract (BTE) as a sensitizer for DSSCs in experimental and theoretical studies.The BTE has four theaflavin analogues, and they are calculated via density functional theory (DFT) and time-dependent density functional theory (TD-DFT).The results showed that theaflavin and theaflavin digallate as sensitizers have well performance among four analogues.Beni and Zarandi [34] analyzed the 3-amino-4-nitrofurazan molecule using functional theory (DFT) and MP2 methods.The geometry of the molecule in the gas phase was optimized and compared with that of the crystal.According to DFT and MP2 methods, they obtain the stable gaseous form.El-Shishtawy et al. [35] studied electronic absorption spectra, ground state geometries, and electronic structures of symmetric and asymmetric squaraine dyes with DFT method, and they found that absorption of squaraine dyes can extend into NIR region by straightforward structural modification, and there are well energy match between dye and TiO 2 .Sun et al. [36] investigated the optical and electrical properties of two dyes, purpurin and alizarin complexone, as sensitizers by using UV-Vis spectrum, FT-IR spectrum, cyclic voltammetry, - characteristics, and DFT calculation, and the results indicated that the side chain has an interesting effect on the optical and electrical properties of sensitizers.Li and coworkers [37] investigated the ground state and excited state properties of polymers BSeTT, QTT, BDT-DTBTBPz (Pz), and BDT-DTBTBQx (Qx) and their derivatives D1, D2, and D3 via DFT and TD-DFT methods, which indicated that the molecule BDT-DTBTBPz and designed molecule (D2) had the best optical and electronic properties among the investigated system.Song et al. [38] reported that electron transfer is a key process of light driven charge separation reaction in organic solar cell.
In this work, phloxine B and bromophenol blue were selected as sensitizers to investigate the optical and electrical properties of DSSCs experimentally, and the UV-Vis spectra, fluorescence spectra, and FT-IR spectra are calculated with density functional theory (DFT) and time-dependent density functional theory (TD-DFT).At the same time, the absorption spectra, infrared spectra, and fluorescence spectra of phloxine B and bromophenol blue are analyzed and compared.In addition, the radiative lifetime () and total static first hyperpolarizability of the two dyes are calculated.Combined study of experiment and theory provides deep insight into the relationship between structure and performance for two DSSCS.

Methods
UV-Vis spectra were measured with TU-1900 spectrometer (Beijing, China), and the FT-IR spectra were measured with FT-IR 360 spectrometer (Nicolet, Madison, WI, USA).Solar energy conversion efficiency measurements were done with a solar simulation instrument (Pecell-15, Japan), and light intensity was adjusted via a reference standard Si-solar cell at sunlight intensity of 100 mW cm −2 .The ground state structures of phloxine B and bromophenol blue were optimized with DFT [39] using B3LYP [40] functional at the 6-31G(d) basis set.Based on the optimized ground state structures, the highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO), and energy gaps of the two dye molecules were computed, and vibration frequencies were done at the same level.There was no imaginary vibration frequency in the minimum energy structure in the potential energy landscape.Simulation of absorption spectra was done with TD-DFT at the same level.The total static first hyperpolarizability can be written as follows [41]: Individual static components in the above equation are calculated from where   (, ,  = , , ) are tenser components of hyperpolarizability.Due to the Kleinman symmetry, we finally obtain the equation that has been employed: All of the calculations were performed by using Gaussian 09 package [42].
International Journal of Photoenergy  The experimental absorption spectra of two dyes in ethanol are presented in Figure 3(a).Absorption spectra of phloxine B showed that the absorption range was 450-600 nm, with maximum absorption peak ( max ) at around 550 nm.Absorption spectra of bromophenol blue were displayed within the range of 300-500 nm, and  max was 410 nm, and bromophenol blue had no other obvious absorption peaks (see Figure 3(a)).

Results and Discussion
In addition, absorption spectra of bromophenol blue and phloxine B were computed via TD-DFT/6-31G(d) in solvent (see Figure 3(b)), and the data are listed in Table 1.Phloxine B showed the absorption region in 300-700 nm, with a  max of 472 nm.Bromophenol blue absorption range was 230-330 nm, while  max was 289 nm.The absorptions in visible and near-UV area were important areas for photovoltaic performance, so there are two important factors (the oscillator strength larger than 0.1 and absorption bands with the wavelength longer than 300 nm).The oscillator strength of the first excited state S1 of phloxine B was 0.7582 (at 472 nm), in which the oscillator strength was the strongest among the six excited states.The maximum absorption in absorption spectra was dominated by HOMO → LUMO transition (contribution is about 0.70069), and the corresponding transition energy was 2.63 eV.Meanwhile, for bromophenol blue, at the first excited state S1, the oscillator strength was 0.0322.The oscillator strength of S1 was the strongest in the states, and this state is composed of HOMO → LUMO electron transition.
For more in-depth understanding of the electronic structure of the two dyes, the isodensity plots of the frontier molecular orbitals (MOs) of two dyes were shown in Fig- ure 4. Phloxine B consists of two parts, namely, sodium-2,3,4,5-tetrachlorobenzene and sodium-2,4,5,7-tetrabromo-6-oxo-xanthen-3-olate, respectively.The electron density of HOMO was located on the sodium-2,4,5,7-tetrabromo-6oxo-xanthen-3-olate group, and that of LUMO was located  Bromophenol blue is composed of three parts, namely, two identical 2,6-dibromophenol groups and pyridine, respectively.The electron density of HOMO was located on two identical 2,6-dibromophenol groups and conjugated bridge, and that of LUMO was located on pyridine and conjugated bridge.As the electron transition is from HOMO to LUMO, it was found that a tiny fraction of MOs for HOMO and LUMO was loaded in conjugated bridge, and more electrons were moved to pyridine.Therefore, excitation should result in electron transfer from two identical 2,6-dibromophenol groups to pyridine.The HOMOs and LUMOs in the dyes indicated that the transitions at maximum absorptions had intramolecular charge transfer (ICT) character, and they played an important role in DSSCs.C-H vibrations were studied as follows: C-H swing and stretching vibration in unit 2,6-dibromophenol groups were calculated in the range of 681.42 For absorption peak vibration of phloxine B, the main absorption peaks of FT-IR spectra focus on 1000-2000 cm −1 .They were 1242.93,1355.36,1463.41,1558.18cm −1 , and 1634.31cm −1 (see Figure 5 For C-H, COONa, and O-Na vibrations, it was found that C-H swing and stretching vibration in sodium-2,4,5,7tetrabromo-6-oxo-xanthen-3-olate group were assigned in the region of 936.95, 987.30

Photovoltaic Characterization.
The following formula is used to calculate the maximum power photo-to-electric conversion efficiency () [43]: Here,  was the intensity of the incident light;  sc ,  oc , and FF represent short circuit density, open circuit voltage, and fill factor, respectively.The FF was defined as the ratio of the maximum power  max obtained from the DSSCs and the theoretical maximum power of it [44].Hence, Here,   and   were current and voltage related to the maximum power.In the experiment, we used the phloxine B   and bromophenol blue as sensitizers, to measure the currentvoltage (-) under sunlight intensity of 100 mW cm −2 .And the results are shown in Figure 6, and measured - characteristics of the DSSCs sensitized for the two dyes are listed in Table 2.
The DSSCs sensitized with phloxine B showed  of 0.52%, with  oc of 0.61 V,  sc of 1.31 mA cm −2 , and FF of 0.65, while the DSSCs sensitized with bromophenol blue showed  of 0.05%, with  oc of 0.44 V,  sc of 0.17 mA cm −2 , and FF of 0.68.The results showed that photovoltaic performance of phloxine B is greater than that of bromophenol blue.Then  oc of the DSSCs using the phloxine B as sensitizer was more suitable.The photovoltaic performance of the phloxine B was higher than that of bromophenol blue.This improvement in the photovoltaic performance of the DSSCs with phloxine B could be attributed to the absorption spectra of solar radiations (450-600 nm) (see Figure 3(a)) [43].This result indicated that the range of absorption spectra affected  of DSSCs.
To analyze the difference of two dyes, we performed theoretical calculation to explain the well photoelectric properties (larger  oc and  sc ) of phloxine B. The value of  oc was viewed as the difference between the quasi-Fermi level of semiconductor conduction band edge value and the electrolyte oxygen reductions.Because the electrolyte was typically I − /I 3 − , the redox electrolyte can be considered constant.The value of  oc has a direct dependence on the shift of the reduction potential of the semiconductor conduction band (Δ CB ), and Δ CB is expressed [45]: where  is adsorbed on the surface concentration and  normal is the dipole moment component perpendicular to the direction of TiO 2 surface (where  normal was the -axis direction),  0 is the gas permittivity, and  is the dielectric constant of the organic monolayer.Obviously, according to (6), the larger the value of  normal , the bigger the value of Δ CB .The change of Δ CB has direct influence on  oc .In order to analyze and compare  oc , we calculated  normal , as shown in Figure 7.
normal of phloxine B was 19.80 D, and  normal of bromophenol blue was −1.29 D.  normal of phloxine B was much larger than that of bromophenol blue.Therefore, the increasing  normal for phloxine B results in the larger  oc [46,47].
For the dye sensitizers, the light harvesting efficiency (LHE) and oscillator strength had correlated relationship [48], which is expressed as follows: Here,  is the oscillator strength.According to Table 1, we found that the first excited state of phloxine B and bromophenol blue corresponds to a dominant position in absorption, and the oscillator strength of phloxine B was larger than that of bromophenol blue.The LHE of phloxine B was 0.8255, and that of bromophenol blue was 0.0714.So the correlated relationship between the oscillator strength and light harvesting efficiency (LHE) implied that the dye with larger oscillator strength made more LHE, and thus phloxine B has well utilities of sunlight.
The sunlight absorption and electron injection were important process in DSSCs, which obviously affect the efficiency of  sc [49].Driving force of electron injection (Δ inject ) means that the excited dye provided electron into semiconductor conduction band.Electron injection occurred from the excited state; Δ inject could be calculated by the following equation [50]: where  dye * OX is the oxidation potential of the excited state of the dye, and  SC CB is the reduction potential of the semiconductor conduction band.The reported  SC CB = 4.0 eV [51] for TiO 2 was adopted in this work. dye * OX could be calculated as follows [52]: Here  dye OX is the redox potential of the ground state, and  max was the absorption maximum with ICT character.
The data of phloxine B and bromophenol blue were listed in Table 3.According to the calculation results, Δ inject of phloxine B (−1.82 eV) was higher than bromophenol blue (−1.49eV), meaning that Δ inject of phloxine B was easier than bromophenol blue.Δ inject was an important influence   factor for the electron injection efficiency.At the same time, the LHE showed that phloxine B was stronger than bromophenol blue, as discussed above.Therefore, phloxine B has improved absorption and injection abilities in comparison with bromophenol blue, which cause the larger value of  sc .

Ground and Excited State Properties.
To ensure that electrons could be effectively injected into the conduction band of TiO 2 (about −4.0 eV) [51], LUMO energy level must be higher than the edge of the conduction band of the TiO 2 , and HOMO energy level must be below I − /I 3 − electrolyte (about −4.85 eV) [46].Energy levels of MOs and energy gaps in solvent were studied by using DFT/B3LYP/6-31G(d) method.Energy levels and HOMO-LUMO gap are presented in Figure 8, and the calculated data are listed in Table 4.For phloxine B, the highest occupied MO (HOMO) was −5.44 eV and the lowest unoccupied MO (LUMO) was −2.64 eV.For bromophenol blue, the HOMO was −6.63 eV and the LUMO was −1.77 eV.The band gap of phloxine B was smaller than that of bromophenol blue, meaning there is a red-shifted absorption for phloxine B. From Figure 8, it seems that the LUMOs of phloxine B and bromophenol blue were all higher than the conduction band of TiO 2 , meaning that the electron injection can occur from excited dyes into TiO 2 , and the HOMOs of two dyes were lower than I − /I 3 − (see Figure 8); therefore, two dyes can obtain electron to recovery.
Molecular nonlinear optical properties have a close relationship with external electric field.It reflects the characteristics of the intramolecular charge transfer (ICT), which can affect the electron injection efficiency and the light current.The first hyperpolarizability () is directly proportional to the difference in the dipole moment (Δ eg ) between the ground state and the excited state and the transition dipole moment ( eg ), and it is inversely proportional to the transition energy.The first hyperpolarizabilities could be written as follows [53]: where Δ eg and  eg are difference in the dipole moment for ground state and excited state and the transition dipole,  eg , is transition energy.The first hyperpolarizabilities were calculated, as listed in Table 5.The  tot of phloxine B was higher than that of bromophenol blue.Table 1 and Figure 7 support the results of hyperpolarizabilities.Figure 7 showed that the value of  eg for phloxine B is larger than that of bromophenol blue; at the same time, phloxine B has smaller excitation energy, so phloxine B has larger hyperpolarizabilities with obvious ICT.

Fluorescence Spectroscopy of Dyes. Radiative lifetime (𝜏)
played an important role in DSSCs, and it could affect charge recombination [54]. could be calculated as follows [55]: Here,  is the speed of light,  is the oscillator strength, and  is the fluorescent energy.The experimental spectra of the phloxine B and bromophenol blue are shown in Figure 9, and the calculated fluorescence maxima , oscillator strength ( in a.u.), and relative radiative lifetime are listed in Table 6.
But phloxine B was propitious to charge recombination [54].

Conclusion
The absorption, molecular orbital energies, radiative lifetimes, LHE, and Δ inject were addressed.The oscillator strength (0.7582), vertical dipole moment (19.80 D), and Δ inject (−1.82 eV) for phloxine B were higher than those of bromophenol blue.The  sc ,  oc , and  of phloxine B were 0.61 V, 1.31 mA/cm 2 , and 0.52%, respectively, which were higher than those of bromophenol blue.The results showed that higher dipole moment of phloxine B corresponds to larger  oc .This means that enlarging dipole moment was a possible way to increase  oc of DSSCs.The larger oscillator strength, higher LHE, and larger absolute value of Δ inject correspond to larger  sc .This means increasing oscillator strength and LHE and larger absolute value of Δ inject were a possible way to increase  sc .A similar trend between theory and experiment was observed.Photovoltaic performance of phloxine B was significantly higher than that of bromophenol blue.

2 International 3 −Figure 1 :
Figure 1: Schematic of band edge engineering to electron transfer.

3. 1 .
The Optical Properties of the Dye.The chemical structures of bromophenol blue and phloxine B are shown in Figure2.

Figure 4 :
Figure 4: Electron densities of the selected HOMO and LUMO of the two dyes.
Spectra.Fourier transforms infrared (FT-IR) spectra of phloxine B and bromophenol blue were recorded in the 500-4000 cm −1 range using KBr pellets and calculated at DFT/B3LYP methods with the 6-31G(d) basis set.The experimental FT-IR spectra were shown in Figures 5(a) and 5(b), and calculated FT-IR spectra were given in Figures 5(c) and 5(d).For absorption peak vibration of bromophenol blue, the absorption peaks were distributed in ranges of 1000-2000 cm −1 and 3000-4000 cm −1 .They were 1398.75, 1472.31,1634.19,3440.26cm −1 , and 3851.47 cm −1 (see Figure 5(a)), respectively.Swing vibration of two C-H bonds of 2,6-dibromophenol group for bromophenol blue was observed at 3243.83 cm −1 (see Figure 5(c)).Swing vibration of two O-H bonds of two 2,6-dibromophenol groups was observed at 1389.36 cm −1 .Stretching vibration of two C-H bonds of pyridine group was observed at 1452.16 cm −1 and 3652.02cm −1 , respectively.Pyridine group swing vibration was observed at 1632.13 cm −1 .

Figure 5 :
Figure 5: (a) Measured FT-IR spectra of bromophenol blue in experiment.(b) Measured FT-IR spectra of phloxine B in experiment.(c) Simulated IR spectra of bromophenol blue in theory.(d) Simulated IR spectra of phloxine B in theory.

Figure 6 :
Figure 6: The photocurrent-potential (-) characteristics of the DSSCs with TiO 2 film electrode sensitized by phloxine B and bromophenol blue.

Figure 7 :
Figure 7: Dipole moment of phloxine B and bromophenol blue (gray for C, deep red for Br, red for O, green for Cl, blue for N, purple for Na, yellow for S, and white for H).

7 I − /I 3 −Figure 8 :
Figure 8: Energy levels of phloxine B and bromophenol blue in solvent.

Table 1 :
Transition energy and oscillator strength of phloxine B and bromophenol blue in solvent were calculated by using TD-DFT/6-31(d) in the solvent.

Table 2 :
Current-voltage characteristics of phloxine B and bromophenol blue.

Table 3 :
The driving force of electron injection Δ inject (in eV) for the phloxine B and bromophenol blue in the solvent.

Table 4 :
Energy levels of HOMO, LUMO, and energy gaps in solvent (eV).

Table 6 :
The calculated electron radiative lifetime () for the phloxine B and bromophenol blue in the solvent.