# Experimental and Theoretical Investigation of the Photoelectrical Properties of Tetrabromophenol Blue- and Bromoxylenol Blue-Based Solar Cells

Authors: Qian Liu; Penghui Ren; Xiaofei Wang; Yuanzuo Li; Yanhui Yang
Journal: Journal of Nanomaterials
 (2018)

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## Abstract

Tetrabromophenol blue and Bromoxylenol blue as the sensitizers of dye-sensitized solar cells (DSSCs) are measured in experiments. In order to better understand the photoelectrical properties of the two dyes, we obtain the UV-Vis spectra, fluorescence spectra, and current-voltage characteristics. The frontier molecular orbital, energy levels, the first hyperpolarizability, the first hyperpolarizability density, and molecular electrostatic potential are calculated with density functional theory (DFT) and time-dependent DFT (TDDFT). The critical factors including the light harvesting efficiency (LHE (Tetrabromophenol blue for 0.0284 and Bromoxylenol blue for 0.0290), the driving force of electron injection (ΔGinject), x-axis direction dipole moment (μnormal), the conduction band of edge of the semiconductor (ΔECB), and the excited-state lifetime (τ)) are computed, which have a close connection to the short-circuit current density (Jsc) and open-circuit voltage (Voc). The results show that the Jsc (0.09 mA/cm2) and Voc (0.39 V) of Tetrabromophenol blue have larger values, which can be explained by a larger absolute value of ΔGinject, absolute value of μnormal, τ, and ΔECB. Therefore, Tetrabromophenol blue displays well photoelectric conversion efficiency compared with Bromoxylenol blue.


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## Body

## 1. Introduction

Currently, the utilities of fossil fuels as a major energy are applied to 80% [1, 2]. Especially, coal is one kind of fossil fuel, which holds a 42% margin in comparison with other energy sources [3, 4]. Fossil fuels as nonrenewable will eventually dry up; therefore, scientific researches has been paid attention to exploit inexhaustible or renewable sources, such as hydroelectric, geothermal, wind, and solar power [5–8], and these sources do not release the greenhouse gases, not contributing to global warming [9].Solar energy is one type of renewable energy, which is propitious to protect the environment [10, 11]. And DSSCs are viewed as a kind of promising solar energy-harvesting device for generation of electricity, which have received a great deal of interest due to their low-cost and light-weight advantages [12]. Since DSSCs were reported by O’Regan and Grätzel et al. [13], most efforts have been focused on DSSCs.Working on DSSCs has five steps [14–16]: (1) dyes can gain electrons from the ground state to an excited state by incident photons; (2) the dyes are excited by incident photons, and then electrons inject into the conduction band of the semiconductor; (3) the electrons are gathered in the TiO2 film, which can transfer to the outer circuit, and the electrons return to the counter electrode; (4) the dye molecules in oxidation state are deoxidized by the electrolyte in reduction; and (5) the electrolyte in oxidation state is reduced after receiving the electron at the conducting glass, accordingly, completing a cycle.The DSSCs are made up of a nanocrystalline porous semiconductor electrode-absorbed dye, a counter electrode, and an electrolyte containing iodide and triiodide ions [17, 18]. From the structure and principle of solar cells, it can be found that dyes are a core part of DSSCs, which are divided into organic dyes and inorganic dyes [19, 20]. Inorganic dyes (such as pyridline and N719) are used as sensitizer in DSSCs, which can show high efficiency; however, the inorganic dyes have more expensive cost in the field of DSSCs [21]. Therefore, organic dyes as the main object have the advantages of inexpensive medium, environmental friendliness, ease in controlling their properties, and so on. Several kinds of dyes have been reported, such as coumarin [22–24], porphyrins [25, 26], phenylene [27, 28], carbazole dyes [29], and indoline dyes [30]. About the organic dyes, researchers have performed the improved experimental strategies for the rapid screening of highly efficient DSSCs. Ma et al. [31] used three novel quinoxaline-based donor-π-acceptor-type organic dyes as sensitizers for DSSCs, showing the influence of various π-conjugation structures that contain different quinoxaline derivatives on the absorption properties, electrochemical properties, and photovoltaic performances. The results show that 2,3-dioctylquinoxaline as the spacer exhibited the highest efficiency of 8.20% with a short-circuit photocurrent density of 17.83 mA cm−2, an open-circuit photovoltage of 0.71 V, and a fill factor of 0.65, under standard global AM 1.5G solar light condition. Freitag et al. [32] designed sensitizers (D35 and XY1) with the copper complex Cu(II/I)(tmby) as a redox shuttle (tmby, 4,4′,6,6′-tetramethyl-2,2′-bipyridine), showing that the highest photoelectronic conversion efficiency was 28.9%.During the past decades, quantum chemistry methods had been used to reveal the relationship between structures and properties of dyes [33–37]. He et al., [34] theoretically demonstrated that the relative position of π-spacer center on DSSCs (such as a rigidified-aromatic as π-spacer or an appropriate trimming of the position) has an influence on DSSC performance. The molecular controlling mechanism of the optical and electronic properties for organic dyes and Ru complexes has been revealed on the basis of DFT and TD-DFT [36, 37], and results also found that there was obviously different performance between the cis and trans isomers [36]. El-Shishtawy et al. [38] used DFT/TD-DFT to study the optical electronic properties of symmetric and asymmetric squaraine dyes of SQD1–SQD4, finding that there are well energy math between dye and semiconductor, indicating possible electron injection from the excited dyes to TiO2. Theoretical calculation provides an in-depth understanding on the micromechanism behind the experiment.Here, Tetrabromophenol blue and Bromoxylenol blue with multibranched structures are chosen as sensitizers to study the optical and electrical properties for DSSCs, which structures and mutihydroxyl are hoped to enhance the charge transfer and adsorptivity. In order to comprehend the experimental results in depth, the absorption spectra, electronic properties, and energy gaps of the two dyes are measured and computed. The UV-Vis spectra and fluorescence spectra are computed by DFT and TD-DFT. In addition, the excited-state lifetime, total static first hyperpolarizability, first hyperpolarizability density, molecular electrostatic potential, and natural bond orbital of two dyes are computed via DFT, and the value of experiment and computational work provides deeply insight into the relationship between structure and performance for two DSSCs.


## 2. Experimental and Computational Methods

### 2.1. Experiment

Tetrabromophenol blue and Bromoxylenol blue with a purity of greater than 98.0% was obtained from Aladdin (Shanghai). Tetrabromophenol blue and Bromoxylenol blue were dissolved in ethanol solution, which were measured with TU-1900 spectrometer (Beijing, China). Solar photon-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 solar cell at sunlight intensity of 100 mW cm−2. Cyclic voltammetry experiments were performed using CH Instruments CHI615E Electrochemical Workstation (shanghai Chenhua Instrument Co. Ltd., Shanghai, China). The redox potentials of the dyes were measured in a tetrahydrofuran (THF) solution, using 0.1 M KNO3 as the supporting electrolyte. The scan range was between −1000 mV and +1000 mV, and the initial scan potential was −1000 mV at a scan rate of 50 mV/s, with a three-electrode system consisting of a glassy carbon working electrode, a platinum counter electrode, and an Ag/AgCl reference electrode.


### 2.2. Computation

Ground-state structures and electronic structures of Tetrabromophenol blue and Bromoxylenol blue are optimized with DFT [39] using PBEPBE functionals [40–43] with 6-31G (d) basis set. Optimization in ethanol solution has been done with the conductor-like polarizable continuum model (CPCM) [44]. Absorption spectrums (UV-Vis) are calculated using the TD-DFT method [45–47] with PBEPBE/6-311G(d, p), which the result shows a closed trend with the experiment for small dendrimer system in comparison with calculations of B3LYP and Cam-B3LYP (see Table S1). Fluorescence spectroscopy of Tetrabromophenol blue and Bromoxylenol blue is simulated by TD-PBEPBE/6-311G(d, p) and TD-CAM-B3LYP/6-311G(d, p). The polarizability and hyperpolarizability tensors (αxx, αxy, αyy, αxz, αyz, αzz and βxxx, βxxy, βxyy, βyyy, βxxz, βxyz, βyyz, βxzz, βyzz, βzzz) can be obtained by a frequency job output file of Gaussian. However, α and β values of Gaussian output are in atomic units (au). So, they have been converted into electronic units (esu) (α: 1 au = 0.1482 × 10−24 esu, β: 1 au = 8.6393 × 10−33 esu). The mean values of the polarizability (△α) and the first hyperpolarizability βtot can be calculated using the following equations [48].
(1)αtot=13αxx+αyy+αzz,Δα=12αxx−αyy2+αyy−αzz2+αzz−αxx2+6αxz2+6αxy2+6αyz21/2,βTol=βx2+βy2+βz2.Individual static components in the above equation are calculated from(2)βi=βiii+13∑i≠jβijj+βjij+βjji,where βijk (i, j, k = x, y, z) are tensor components of hyperpolarizability. Due to the Kleinman symmetry, the equation was obtained [49]:
(3)βtot=βxxx+βxyy+βxzz2+βyyy+βyzz+βyxx2+βzzz+βzxx+βzyy21/2.All the quantum chemical computations are performed by using Gaussian 09 package [50]. Multiwfn 3.39 was applied for wave function analyses [51].


## 2.1. Experiment

Tetrabromophenol blue and Bromoxylenol blue with a purity of greater than 98.0% was obtained from Aladdin (Shanghai). Tetrabromophenol blue and Bromoxylenol blue were dissolved in ethanol solution, which were measured with TU-1900 spectrometer (Beijing, China). Solar photon-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 solar cell at sunlight intensity of 100 mW cm−2. Cyclic voltammetry experiments were performed using CH Instruments CHI615E Electrochemical Workstation (shanghai Chenhua Instrument Co. Ltd., Shanghai, China). The redox potentials of the dyes were measured in a tetrahydrofuran (THF) solution, using 0.1 M KNO3 as the supporting electrolyte. The scan range was between −1000 mV and +1000 mV, and the initial scan potential was −1000 mV at a scan rate of 50 mV/s, with a three-electrode system consisting of a glassy carbon working electrode, a platinum counter electrode, and an Ag/AgCl reference electrode.


## 2.2. Computation

Ground-state structures and electronic structures of Tetrabromophenol blue and Bromoxylenol blue are optimized with DFT [39] using PBEPBE functionals [40–43] with 6-31G (d) basis set. Optimization in ethanol solution has been done with the conductor-like polarizable continuum model (CPCM) [44]. Absorption spectrums (UV-Vis) are calculated using the TD-DFT method [45–47] with PBEPBE/6-311G(d, p), which the result shows a closed trend with the experiment for small dendrimer system in comparison with calculations of B3LYP and Cam-B3LYP (see Table S1). Fluorescence spectroscopy of Tetrabromophenol blue and Bromoxylenol blue is simulated by TD-PBEPBE/6-311G(d, p) and TD-CAM-B3LYP/6-311G(d, p). The polarizability and hyperpolarizability tensors (αxx, αxy, αyy, αxz, αyz, αzz and βxxx, βxxy, βxyy, βyyy, βxxz, βxyz, βyyz, βxzz, βyzz, βzzz) can be obtained by a frequency job output file of Gaussian. However, α and β values of Gaussian output are in atomic units (au). So, they have been converted into electronic units (esu) (α: 1 au = 0.1482 × 10−24 esu, β: 1 au = 8.6393 × 10−33 esu). The mean values of the polarizability (△α) and the first hyperpolarizability βtot can be calculated using the following equations [48].
(1)αtot=13αxx+αyy+αzz,Δα=12αxx−αyy2+αyy−αzz2+αzz−αxx2+6αxz2+6αxy2+6αyz21/2,βTol=βx2+βy2+βz2.Individual static components in the above equation are calculated from(2)βi=βiii+13∑i≠jβijj+βjij+βjji,where βijk (i, j, k = x, y, z) are tensor components of hyperpolarizability. Due to the Kleinman symmetry, the equation was obtained [49]:
(3)βtot=βxxx+βxyy+βxzz2+βyyy+βyzz+βyxx2+βzzz+βzxx+βzyy21/2.All the quantum chemical computations are performed by using Gaussian 09 package [50]. Multiwfn 3.39 was applied for wave function analyses [51].


## 3. Results and Discussion

### 3.1. The Optical Properties of the Dye

The chemical structures of the two dyes (Tetrabromophenol blue and Bromoxylenol blue) are shown in Figure1. Figure 2(a) showed the experimental absorption spectra of the two dyes in ethanol, and the data of the absorption peaks are summarized in Table S1. As shown in Figure 2(a), Tetrabromophenol blue and Bromoxylenol blue only have one absorption peak at 400 nm~450 nm, at which the maximum absorption peaks (λmax) of Tetrabromophenol blue and Bromoxylenol blue are 433 nm and 426 nm, respectively.Figure 1
Chemical structures of two molecules Tetrabromophenol blue and Bromoxylenol blue.
(a)
(b)Figure 2
The experimental UV-Vis spectrum of Tetrabromophenol blue and Bromoxylenol blue in ethanol (a); the theoretical UV-Vis spectrum of Tetrabromophenol blue and Bromoxylenol blue by TDDFT/PBEPBE/6-31G(d) basis set in gas (b) and ethanol (c).
(a)
(b)
(c)In addition, the optical properties of Tetrabromophenol blue and Bromoxylenol blue are computed via TD-DFT/PBEPBE/6-311G(d, p) in gas and solvent and the absorption spectra of two dyes are displayed in Figures2(b) and 2(c). The absorption peaks and oscillator strengths were investigated, which were listed in Table S1. The absorption region of Tetrabromophenol blue is 300 nm–550 nm, and the absorption region of Bromoxylenol blue is 275 nm–500 nm. About the photovoltaic performance, oscillator strength is an important factor and they can be seen from Table S1. For gas (see Figure 2(b)), the strongest oscillator strength of Tetrabromophenol blue is the excited state S2, and the oscillator strength is 0.0072. The electron transition of Tetrabromophenol blue is from H-1 → L, and the excited states S1 and S3 are from H → L and H → L+1, respectively. The largest oscillator strength of Bromoxylenol blue is 0.0092 (for S1), and electrons transferring are from H → L, and the excited states S2 and S3 are from H → L+1 and H-1 → L+1, respectively. For solvent (see Figure 2(c)), the absorption region of Tetrabromophenol blue is 200 nm–550 nm. The absorption region of Bromoxylenol blue is 270 nm–475 nm. The absorption peak for Bromoxylenol blue is smaller than that of Tetrabromophenol blue, reflecting the trend of the experiment. There is also difference between calculation and experiment probably because the single molecule was considered in calculation, which can not completely respond to experimental conditions. The optical performance and oscillator strength of the dye are showed in Table S1. The third excited state S3 of Tetrabromophenol blue has the strongest oscillator strength, and the electron transition of Tetrabromophenol blue is from H → L+1. The strongest oscillator strength of Bromoxylenol blue is the first excited state S1 (0.0128), and electrons transferring are from H → L.Moreover, the selected frontier molecular orbitals of the two dyes are shown in Figure3. Tetrabromophenol blue has three parts, which are sulfonic acid, 1,2,3,4-tetrabromobenzene, and 2,6-dibromophenol. For S2 (HOMO-1-LUMO), electron density of HOMO-1 level centers on two 2,6-dibromophenol groups, which transmits to the sulfonic acid and 1,2,3,4-tetrabromobenzen (LUMO level). Electron density of HOMO level is focused on 2,6-dibromophenol groups that it transits to sulfonic acid and 1,2,3,4-tetrabromobenzen at S1. For S3 (HOMO-LUMO+1), electron density of HOMO level transmits to sulfonic acid and 1,2,3,4-tetrabromobenzen (LUMO+1 level) in gas. For solvent, electron densities of HOMO level and LUMO+1 level are transmitted between two 2,-dibromophenol groups at S1. For S3, electron density in 2,6-dibromophenol groups was moved to sulfonic acid and 1,2,3,4-tetrabromobenzen (LUMO+1 level). For S3 (HOMO-1-LUMO), electron density of HOMO-1 level is focused on two 2,6-dibromophenol groups, and it was transferred to sulfonic acid and 1,2,3,4-tetrabromobenzen. Bromoxylenol blue has three parts, which are 2-bromo-3,6-dim ethylphenol, sulfonic acid, and benzene ring. For S1 (HOMO-LUMO), electron density of HOMO level is focused on two 2-bromo-3,6-dim ethylphenol groups, which transits to sulfonic acid and benzene ring (LUMO level) in gas. For S2 (HOMO-LUMO+1), electron density of HOMO level is focused on a 2-bromo-3,6-dim ethylphenol group, which was moved to sulfonic acid and benzene ring (LUMO+1 level). For S3, electron density is from HOMO-1 to LUMO+1 level. For solvent, electron density of HOMO level is focused on a 2-bromo-3,6-dim ethylphenol group that it transmits to the other 2-bromo-3,6-dim ethylphenol group (LUMO level) for S1. For S3 (HOMO-1), electron density of HOMO-1 level is focused on two 2-bromo-3,6-dim ethylphenol groups, which transmits to a 2-bromo-3,6-dim ethylphenol group (LUMO level). For S3 (HOMO-LUMO+1), electron density of HOMO level transmits to sulfonic acid and benzene ring (LUMO+1).Figure 3
Electron densities of the selected HOMO and LUMO of the Tetrabromophenol blue and Bromoxylenol blue in gas and solvent.


### 3.2. Electrochemical Characteristic

The characteristics of electron transition are from an excited state to the conduction band of the semiconductor, and it can be achieved from the investigation of electrochemical properties. The electrochemical characteristics of Tetrabromophenol blue and Bromoxylenol blue are measured by cyclic voltammetry measurements in ethanol solvent, and the supporting electrolyte is KNO3 [52], and the results are listed in Figures 4(a) and 4(b), showing that the onset oxidation potentials are 0.41 eV and −0.32 eV for Tetrabromophenol blue and 0.55 eV and −0.38 eV for Bromoxylenol blue, and they are obtained by the intersection of two tangent lines for the rising current curve and the starting current curve. The HOMO and LUMO energy corresponds to the onset oxidation potential of Tetrabromophenol blue and Bromoxylenol blue. While the working electrode is glassy carbon electrode, the reference electrode is saturated calomel electrode. So, the HOMO energy and LUMO energy can be calculated according to the equation reported by de Leeuw et al. [53]: EHOMO = −(EOX + 4.40 eV) and ELUMO = −(ERed + 4.40 eV), where EOX and ERed represent the onset oxidation potential of Tetrabromophenol blue and Bromoxylenol blue. Therefore, the HOMO energy and LUMO energy of Tetrabromophenol blue are −4.81 eV and −4.08 eV, respectively, and those of Bromoxylenol blue are −4.90 eV and −4.02 eV, respectively. In general, the energy gap (Eg) of Tetrabromophenol blue is smaller than that of Bromoxylenol blue. In view of those, Tetrabromophenol blue has the greater donating electron ability.Figure 4
Cyclic voltammograms of (a) Tetrabromophenol blue and (b) Bromoxylenol blue.
(a)
(b)In order to gain a better understanding of experimental values, the HOMOs, LUMOs, and energy gaps of Tetrabromophenol blue and Bromoxylenol blue are compared in the gas and solvent phases (see Figure5), and the data are listed in Table S2. Dyes have smaller energy gaps, which means that dyes are easier excited. Energy levels of TiO2 and electrolyte (I−/I−3) are shown in Figure 5, and the LUMO energy is higher than those of TiO2 (−4 eV), and the HOMO energy is lower than those of I−/I−3 (−4.8 eV) [54]. For Tetrabromophenol blue, the HOMO levels of gas and solvent are −6.69 eV and −6.64 eV, respectively, and the LUMO levels are −2.17 eV and −2.10 eV, respectively. The HOMO levels and LUMO levels of Bromoxylenol blue are computed, and the results are −6.20 eV, −6.27 eV, −1.32 eV, and− 1.29 eV. The data show that the energy gap of Tetrabromophenol blue is smaller than that of Bromoxylenol blue, which means that Tetrabromophenol blue is easier excited.Figure 5
Calculated HOMOs, LUMOs, and energy gaps of Tetrabromophenol blue and Bromoxylenol blue.


### 3.3. Polarizability and First Hyperpolarizability Calculations

The dipole moment (μ), linear polarizability (α), anisotropy of the polarizability (Δα), and first hyperpolarizability (β) are important factors of nonlinear optical (NLO) properties, and β is defined as the third derivative of the total energy. The data of μ, α, Δα, and β are calculated, and the results are listed in Table 1. The parameters of dipole moment [μi (i = x, y, z) and total dipole moment μtot] are showed in Table 1, and the total dipole can be expressed [49]:
(4)μtot=μx2+μy2+μz21/2.Table 1
The dipole momentsμ (D), the polarizability α (au), the average polarizability αtot (esu), the anisotropy of the polarizability Δα (esu), and the first hyperpolarizability βtot (esu) of Tetrabromophenol blue and Bromoxylenol blue.
Tetrabromophenol blue
Bromoxylenol blue
μx = −1.3838
βxxx = 166.279
μx = −0.6012
βxxx = 135.324
μy = 0.6285
βxxy = −74.417
μy = −0.0071
βxxy = 325.867
μz = 3.6474
βxyy = 325.971
μz = 9.0028
βxyy = −91.592
μtot = 3.9514
βyyy = −291.467
μtot = 9.0229
βyyy = 405.071
αxx = 440.475
βxxz = 38.380
αxx = 336.744
βxxz = 38.166
αxy = 4.858
βxyz = −57.081
αxy = 8.223
βxyz = 63.245
αyy = 354.282
βyyz = 58.637
αyy = 6.803
βyyz = 98.242
αxz = 10.068
βxzz = −69.811
αxz = −14.491
βxzz = −11.785
αyz = 4.041
βyzz = 30.701
αyz = 6.545
βyzz = 44.779
αzz = 350.652
βzzz = 38.947
αzz = 263.435
βzzz = 207.694
αtot = 381.803
βtot = 556.137
αtot = 202.327
βtot = 549.022
Δα = 90.45
Δα = 300.124Due to the important application of dipole moment in the research of polarizability and the first hyperpolarizability, theμ of Tetrabromophenol blue and Bromoxylenol blue is calculated to be 3.9514 D and 9.0229 D, respectively, and μx is the dipole moment component perpendicular to the direction TiO2 surface, and they are 1.3838 D and 0.6012 D, respectively. The β values are 556.137 × 10−30 esu and 549.022 × 10−30 esu, respectively. The μ, α, Δα, and β of Tetrabromophenol blue and Bromoxylenol blue can affect the effective electron injection from the dye molecule to the conduction band of the semiconductor.


### 3.4. Analysis of the First Hyperpolarizability Density

In order to more deeply understand the result of hyperpolarizability, which is studied by hyperpolarizability density analysis and is a powerful technique to characterize contribution to hyperpolarizability from various molecular regions, the electron density of a systemρr,F may be expressed as Taylor expansion with respect to the externally applied electric field F [55, 56]:
(5)ρr,F=ρ0r+∑jρj1rFj+12!∑j∑kρjk2rFjFk+13!∑j∑k∑iρjki3rFjFkFi+….From the above equation and expansion formula of dipole in the power ofF, a component of the first hyperpolarizability can be expressed:
(6)βijk=−12!∫riρjk2rdr,where the first hyperpolarizability density is defined as
(7)ρjk2r=∂2ρr∂Fj∂FkF=0.The first hyperpolarizability is componential byZZZ, which is the most remarkable component of βtot. According to the finite difference technique, the first hyperpolarizability density can be achieved by means of electron densities yielded under different manually applied electric fields:
(8)ρZZ2r=ρr,FZ+ρr,−FZ−ρr,0FZ2.Here,ρZZ2r is the electron density at a spatial point r in the presence of a very weak electric field FZ, which is a step size of finite difference (chosen to be 0.003 au). The first hyperpolarizability can be achieved from the integrand of ZZZ, namely, ρZZ2r of various systems presented in Figure 6. Solid and dash lines correspond to positive and negative parts, respectively, and the positive and negative regions have an important influence on nonlinear optical (NLO).Figure 6
Contour maps of integrand ofβZZZ, namely, ρZZ2r on the Z plane. (a) Tetrabromophenol blue; (b) Bromoxylenol blue. Solid and dash lines correspond to positive and negative parts, respectively.
(a)
(b)


### 3.5. Molecular Electrostatic Potential

For molecular electrostatic potentials (MEP) about the nuclear and charge distribution of Tetrabromophenol blue and Bromoxylenol blue, they are viewed as a tool for interpretation and prediction of chemical reactivity and a plot of the MEP of a chemical species is helpful in understanding its reactive behavior [57]. The MEP maps of Tetrabromophenol blue and Bromoxylenol blue are shown in Figure 7. The electron-rich (negative) region, electron-poor (positive) region, and zero electrostatic potential are denoted in red color, blue color, and green color, respectively, which means that electrophilic attack appears in the negative region and nucleophilic reactions happen on the positive region. So, the electron concentrations are shown by different colors, and the values of electron density (V(r)) increase in the following order [58]: red > yellow > green > blue.Figure 7
Molecular electrostatic potential map calculated at the DFT/6-31G(d) level (Tetrabromophenol blue and Bromoxylenol blue).
(a)
(b)The (V(r)) range of Tetrabromophenol blue and Bromoxylenol blue is from −0.06 au (deepest red) to 0.06 au (deepest blue) in MEP. Electrophilic attack and nucleophilic reactions of Tetrabromophenol blue and Bromoxylenol blue happen on oxygen atoms and hydrogen atoms, which are –SO3 and –OH, respectively. Moreover, MEP is also shown that two dyes display the redistribution of electron density with the positive region and negative region, which are identified with the transfer of electron density.


### 3.6. Photovoltaic Characterization of Dyes

Efficiency as an important factor is an important criterion for solar cells. Moreover, it has a close relationship with the spectra and intensity of the incident sunlight. The photo-to-electric conversion efficiency (η) is obtained by the short-circuit density (Jsc), open-circuit voltage (Voc), and fill factor (FF). The η is equal to these three factors than the incident solar power (Pinc) demonstrated as the following equation [59]:
(9)η=VocJscFFPinc.FF is defined as the ratio of the maximum power obtained from the DSSC and the theoretical maximum power, which is formulated as [60]
(10)FF=ImVmJscVoc.Here,Im and Vm were the current and voltage, respectively, related to the maximum power. For experiment, Tetrabromophenol blue and Bromoxylenol blue are measured with the current-voltage (I-V) instrument under sunlight intensity of 100 mW cm−2, and they are shown in Figure 8, and the photoelectrochemical parameters of DSSCs are given in Table S3. Comparing the photovoltaic characteristics of the DSSCs between Tetrabromophenol blue and Bromoxylenol blue, photoelectric conversion efficiency of Tetrabromophenol blue is higher (0.02%), with Voc of 0.39 V, Jsc of 0.09 mA cm−2, and FF of 0.59.Figure 8
The photocurrent-potential (I-V) characteristics of the DSSCs with TiO2 film electrode sensitized by Tetrabromophenol blue and Bromoxylenol blue.


### 3.7. Theoretical Analysis forJsc and Voc

According to (9), Jsc, Voc, and FF play a decisive role for photoelectric conversion efficiency. Theoretical analyses for Jsc and Voc are computed as follows [61]:
(11)Voc=ECB+ΔECBq+KTqlnncNCB−Eredoxq.Here,ECB is the conduction band edge of the semiconductor substrate, ΔECB is the shift of ECB, q is the unit charge, K is the Boltzmann constant, T is the absolute temperature, nc is the number of electrons in the conduction band, NCB is the density of accessible states in the conduction band and Eredox is the reduction-oxidation potential of the electrolyte. While in DSSCs, Voc is closely related with ΔECB and ΔECB [62]:
(12)ΔECB=−qμnormalγε0ε.Here,γ is adsorbed on the surface concentration and μnormal is the dipole moment component perpendicular to the direction TiO2 surface (where the μnormal was the x-axis direction), ε0 is the gas permittivity, and ε is the dielectric constant of the organic monolayer. Based on (12), the values of μnormal and ΔECB have proportional relationship. μnormal values of Tetrabromophenol blue and Bromoxylenol blue are −1.3838 D and −0.6012 D (see Table 1), respectively. The absolute value of μnormal of Tetrabromophenol blue is larger than that of Bromoxylenol blue. According to (11) and (12), μnormal is directly proportional to Voc.Jsc in DSSCs is determined by the following equation [63]:
(13)Jsc=∫LHEλ⋅ΔGinjectdλ.This equation includes two important factors that have direct influence on DSSCsη, LHE, and ΔGinject. LHE is light harvesting efficiency and ΔGinject is injection efficiency of the electrons in the excited state. For (13), to reach high Jsc, the DSSCs should have a large ΔGinject that is computed by [64]
(14)ΔGinject=Eoxdye∗−ECBTiO2.Here,Eoxdye∗ is the oxidation potential of excited dye and ECBTiO2 is the reduction potential of conduction band TiO2 (ECBTiO2 = 4.0 eV) and Eoxdye∗ can be calculated as [65]:
(15)Eoxdye∗=Eoxdye−ΔE,where Eoxdye is the oxidation potential of the dye in the ground state and ΔE is an experiment electronic vertical energy related to λmax.The data of Tetrabromophenol blue and Bromoxylenol blue are listed in Table2. The absolute value of ΔGinject of Tetrabromophenol blue is 2.25 eV, which is larger than that of Bromoxylenol blue (2.21). LHE of Tetrabromophenol blue and Bromoxylenol blue are 0.0284 and 0.0290, respectively. According to (13), ΔGinject of Tetrabromophenol blue is bigger than that of Bromoxylenol blue, which is helpful to improve the Jsc ability for Tetrabromophenol blue.Table 2
The driving force of electron injectionΔGinject (in eV) for the Tetrabromophenol blue and Bromoxylenol blue in the solvent.
Dye
EOXdye
EOXdye∗
ΔGinject
Tetrabromophenol blue
4.81
1.95
−2.25
Bromoxylenol blue
4.90
1.99
−2.21


### 3.8. Natural Bond Orbital Analysis

Natural bond orbital (NBO) exploits the second-order Fock matrix to get the donor-acceptor interactions [66]. For the sake of illuminating intramolecular rehybridization and delocalization of electron density within the molecule, NBO analyses of Tetrabromophenol blue and Bromoxylenol blue are calculated and the information is listed in Table S4 and Table S5. As shown, E(2) means that the energy of hyper conjugative interaction (stabilization energy)) has higher value and electron donors and electron acceptors have stronger interaction and larger extent of conjugation of the entire system.Stabilization is caused by intramolecular hyperconjugative interaction of molecules. Intramolecular hyperconjugative interaction ofπ∗ (C17-C18) distributed to π∗ (C16–C21) leads to the strongest stabilization of 285.88 KJ/mol (for Tetrabromophenol blue). The strong intramolecular hyperconjugative interaction of π∗ (C14-C15), π∗ (C5-C6), π∗ (C19-C20), and π∗ (C1-C2) distributes to π∗ (C10-C11), π∗ (C3-C4), π∗ (C21-C22), and π∗ (C3-C4), and E(2) values are 269.58 KJ/mol, 152.56 KJ/mol, 181.99 KJ/mol, and 160.11 KJ/mol (see Table S4). For Bromoxylenol blue, the electron delocalization is distributed form donor to acceptor that are π∗→π∗ and they are C18-C19 → C20-C21 (278.95 KJ/mol), C18-C19 → C10-C11 (271.65 KJ/mol), C12–C14 → C15-C16 (246.97 KJ/mol), C12–C14 → C13–C17 (271.69 KJ/mol), and C1-C2 → C3-C4 (176.83 KJ/mol) (see Table S5). The above results show that intramolecular hyperconjugative interaction of Tetrabromophenol blue is higher than that of Bromoxylenol blue.


### 3.9. Mobility Calculation

The mobility as an important factor that can affect the performance of DSSC, and it is computed to provide the calculation of value for applications of organics in electronic devices. So, the drift mobility of hopping (μ) is expressed [67]:
(16)μ=e2nkBT∑ir2WiPi,where n=3 is the dimensionality, e is the electronic charge, kB is the Boltzmann constant, T is the temperature, r is the distance of the center of mass, and P is the relative probability for charge carrier to a particular ith neighbor
(17)Pi=Wi∑iWi.According to the standard Marcus theory,Wi can be compared by the rate of charge motion between neighboring molecules and it is obtained by the reorganization energy λ and the coupling matrix element V and it can be written as [68]
(18)Wi=V2ℏπλkBT1/2exp−λ4kBT.where V and λ play important roles for Wi and they are achieved by framework of the Marcus-Hush theory. V is closely correlated with EHOMO and EHOMO-1, and λ is divided into hole reorganization energy (λe) and electron reorganization energy (λh). So, V, λe, and λh are expressed [69]:
(19)V=12EHOMO−EHOMO‐1,λh=E0∗−E0+E+∗−E+,λe=E0∗−E0+E−∗−E−,where E+∗E−∗ represents the cationic (anionic) state with the geometry of the neutral species; E+ and E− represent the energies of the cationic and anionic with the structure of cationic and anionic species; and E0 and E0∗ represent the energies of neutral species and state, respectively. The data of V, λe, and λh are listed in Table 3. V, λe, and λh of Tetrabromophenol blue are 0.03 eV, 0.82 eV, and 0.27 eV, respectively, and those of Bromoxylenol blue are 0.004 eV, 0.54 eV, and 0.27 eV, respectively. According to the Marcus theory, V and λ of dyes are higher and lower, which get larger μ.Table 3
Calculated molecular reorganization energies (eV) and coupling matrix element (eV) of Tetrabromophenol blue and Bromoxylenol blue.
Dye
λh
λe
V
Tetrabromophenol blue
0.27
0.82
0.03
Bromoxylenol blue
0.27
0.54
0.004


### 3.10. Molecular Descriptors

The electron affinity (A) and ionization potential (I) can affect the energy barrier of electrons. The data of the energy barrier between HOMO level and LUMO level are −4.52 eV and− 4.54 eV in gas and solvent, respectively, (for Tetrabromophenol blue) and those of Bromoxylenol blue are −4.88 eV and− 4.98 eV, respectively (see Table 4). So, the absolute value of the energy barrier of Tetrabromophenol blue is lower than that of Bromoxylenol blue. Moreover, the total and FMO energies (ETOTAL), electronegativity (χ), chemical hardness (η), and chemical softness (S) are computed by 6-31G(d) in gas and solvent. Parr et al. found that the lower η results in the lower resistance to intramolecular charge transfer [70]. η values of Tetrabromophenol blue (2.260 eV and 2.270 eV) are lower than those of Bromoxylenol blue (2.440 eV and 2.490 eV). The chemical hardness is an important factor, which is a measure of resistance to charger transfer.Table 4
The calculated ionization energy (eV), electron affinity (eV), electronegativity (eV), hardness (eV), softness (eV−1), electrophilicity index, and total molecular energies (Hartree) for the Tetrabromophenol blue and Bromoxylenol blue with the 6-31G(d) basis set.
Dye
I
A
χ
η
S
ΔE
ETOTAL
φ
Tetrabromophenol blue
Gas
6.69
2.17
4.430
2.260
1.480
4.52
−22075.48
4.323
Solvent
6.64
2.10
4.370
2.270
1.463
4.54
−22075.50
4.206
Bromoxylenol Blue
Gas
6.20
1.32
3.760
2.440
1.207
4.88
−6806.13
2.897
Solvent
6.27
1.29
3.780
2.490
1.259
4.98
−6806.15
2.869
ΔE = EHOMO − ELUMO; I = −EHOMO; A = −ELUMO; χ = (I + A)/2; η = (I − A)/2; S = I/2η; φ = χ2/2η.


### 3.11. Fluorescence Spectroscopy of Dyes

Excited-state lifetime (τ) plays an important role in DSSCs, and it can affect the efficiency of charge transfer. The longer τ can keep the dye in the cationic form for a long time, which can be expressed as [71]:
(20)τ=ac3u22fe2,where c is the speed of light, f is the oscillator strength, e is the fluorescent energy, and u is a constant. Figure 9 shows the experimental spectra and academic spectra; the fluorescence maxima λ, oscillator strength (f in au), and τ are listed in Table 5. For experiment and computational work, the maximum absorption peaks of Tetrabromophenol blue are 548 nm and 537 nm, respectively, and those of Bromoxylenol blue are 540 nm and 533 nm, respectively, and the fluorescence energy, f, and τ are 1.28 eV, 0.0066, and 2.16 × 10−5 s, respectively. While, for Bromoxylenol blue, the fluorescence energy, f, and τ are 2.33 eV, 0.0087, and 4.90 × 10−7, respectively. Those results showed that τ of Tetrabromophenol blue is higher, which means that Tetrabromophenol blue can remain stable in the cationic stat for a long time.Figure 9
The experimental (a) and theoretical (b) fluorescence spectra of Tetrabromophenol blue and Bromoxylenol blue.
(a)
(b)Table 5
Experimental absorption peaks, calculated transition properties, and excited-state lifetime of Tetrabromophenol blue and Bromoxylenol blue.
Dye
E (eV)
λ (nm)
Strengthf
Τ (s)
Exp (nm)
Tetrabromophenol blue
1.28
537
0.0066
2.16 × 10−5
548
Bromoxylenol blue
2.33
533
0.0087
4.90 × 10−7
540


## 3.1. The Optical Properties of the Dye

The chemical structures of the two dyes (Tetrabromophenol blue and Bromoxylenol blue) are shown in Figure1. Figure 2(a) showed the experimental absorption spectra of the two dyes in ethanol, and the data of the absorption peaks are summarized in Table S1. As shown in Figure 2(a), Tetrabromophenol blue and Bromoxylenol blue only have one absorption peak at 400 nm~450 nm, at which the maximum absorption peaks (λmax) of Tetrabromophenol blue and Bromoxylenol blue are 433 nm and 426 nm, respectively.Figure 1
Chemical structures of two molecules Tetrabromophenol blue and Bromoxylenol blue.
(a)
(b)Figure 2
The experimental UV-Vis spectrum of Tetrabromophenol blue and Bromoxylenol blue in ethanol (a); the theoretical UV-Vis spectrum of Tetrabromophenol blue and Bromoxylenol blue by TDDFT/PBEPBE/6-31G(d) basis set in gas (b) and ethanol (c).
(a)
(b)
(c)In addition, the optical properties of Tetrabromophenol blue and Bromoxylenol blue are computed via TD-DFT/PBEPBE/6-311G(d, p) in gas and solvent and the absorption spectra of two dyes are displayed in Figures2(b) and 2(c). The absorption peaks and oscillator strengths were investigated, which were listed in Table S1. The absorption region of Tetrabromophenol blue is 300 nm–550 nm, and the absorption region of Bromoxylenol blue is 275 nm–500 nm. About the photovoltaic performance, oscillator strength is an important factor and they can be seen from Table S1. For gas (see Figure 2(b)), the strongest oscillator strength of Tetrabromophenol blue is the excited state S2, and the oscillator strength is 0.0072. The electron transition of Tetrabromophenol blue is from H-1 → L, and the excited states S1 and S3 are from H → L and H → L+1, respectively. The largest oscillator strength of Bromoxylenol blue is 0.0092 (for S1), and electrons transferring are from H → L, and the excited states S2 and S3 are from H → L+1 and H-1 → L+1, respectively. For solvent (see Figure 2(c)), the absorption region of Tetrabromophenol blue is 200 nm–550 nm. The absorption region of Bromoxylenol blue is 270 nm–475 nm. The absorption peak for Bromoxylenol blue is smaller than that of Tetrabromophenol blue, reflecting the trend of the experiment. There is also difference between calculation and experiment probably because the single molecule was considered in calculation, which can not completely respond to experimental conditions. The optical performance and oscillator strength of the dye are showed in Table S1. The third excited state S3 of Tetrabromophenol blue has the strongest oscillator strength, and the electron transition of Tetrabromophenol blue is from H → L+1. The strongest oscillator strength of Bromoxylenol blue is the first excited state S1 (0.0128), and electrons transferring are from H → L.Moreover, the selected frontier molecular orbitals of the two dyes are shown in Figure3. Tetrabromophenol blue has three parts, which are sulfonic acid, 1,2,3,4-tetrabromobenzene, and 2,6-dibromophenol. For S2 (HOMO-1-LUMO), electron density of HOMO-1 level centers on two 2,6-dibromophenol groups, which transmits to the sulfonic acid and 1,2,3,4-tetrabromobenzen (LUMO level). Electron density of HOMO level is focused on 2,6-dibromophenol groups that it transits to sulfonic acid and 1,2,3,4-tetrabromobenzen at S1. For S3 (HOMO-LUMO+1), electron density of HOMO level transmits to sulfonic acid and 1,2,3,4-tetrabromobenzen (LUMO+1 level) in gas. For solvent, electron densities of HOMO level and LUMO+1 level are transmitted between two 2,-dibromophenol groups at S1. For S3, electron density in 2,6-dibromophenol groups was moved to sulfonic acid and 1,2,3,4-tetrabromobenzen (LUMO+1 level). For S3 (HOMO-1-LUMO), electron density of HOMO-1 level is focused on two 2,6-dibromophenol groups, and it was transferred to sulfonic acid and 1,2,3,4-tetrabromobenzen. Bromoxylenol blue has three parts, which are 2-bromo-3,6-dim ethylphenol, sulfonic acid, and benzene ring. For S1 (HOMO-LUMO), electron density of HOMO level is focused on two 2-bromo-3,6-dim ethylphenol groups, which transits to sulfonic acid and benzene ring (LUMO level) in gas. For S2 (HOMO-LUMO+1), electron density of HOMO level is focused on a 2-bromo-3,6-dim ethylphenol group, which was moved to sulfonic acid and benzene ring (LUMO+1 level). For S3, electron density is from HOMO-1 to LUMO+1 level. For solvent, electron density of HOMO level is focused on a 2-bromo-3,6-dim ethylphenol group that it transmits to the other 2-bromo-3,6-dim ethylphenol group (LUMO level) for S1. For S3 (HOMO-1), electron density of HOMO-1 level is focused on two 2-bromo-3,6-dim ethylphenol groups, which transmits to a 2-bromo-3,6-dim ethylphenol group (LUMO level). For S3 (HOMO-LUMO+1), electron density of HOMO level transmits to sulfonic acid and benzene ring (LUMO+1).Figure 3
Electron densities of the selected HOMO and LUMO of the Tetrabromophenol blue and Bromoxylenol blue in gas and solvent.


## 3.2. Electrochemical Characteristic

The characteristics of electron transition are from an excited state to the conduction band of the semiconductor, and it can be achieved from the investigation of electrochemical properties. The electrochemical characteristics of Tetrabromophenol blue and Bromoxylenol blue are measured by cyclic voltammetry measurements in ethanol solvent, and the supporting electrolyte is KNO3 [52], and the results are listed in Figures 4(a) and 4(b), showing that the onset oxidation potentials are 0.41 eV and −0.32 eV for Tetrabromophenol blue and 0.55 eV and −0.38 eV for Bromoxylenol blue, and they are obtained by the intersection of two tangent lines for the rising current curve and the starting current curve. The HOMO and LUMO energy corresponds to the onset oxidation potential of Tetrabromophenol blue and Bromoxylenol blue. While the working electrode is glassy carbon electrode, the reference electrode is saturated calomel electrode. So, the HOMO energy and LUMO energy can be calculated according to the equation reported by de Leeuw et al. [53]: EHOMO = −(EOX + 4.40 eV) and ELUMO = −(ERed + 4.40 eV), where EOX and ERed represent the onset oxidation potential of Tetrabromophenol blue and Bromoxylenol blue. Therefore, the HOMO energy and LUMO energy of Tetrabromophenol blue are −4.81 eV and −4.08 eV, respectively, and those of Bromoxylenol blue are −4.90 eV and −4.02 eV, respectively. In general, the energy gap (Eg) of Tetrabromophenol blue is smaller than that of Bromoxylenol blue. In view of those, Tetrabromophenol blue has the greater donating electron ability.Figure 4
Cyclic voltammograms of (a) Tetrabromophenol blue and (b) Bromoxylenol blue.
(a)
(b)In order to gain a better understanding of experimental values, the HOMOs, LUMOs, and energy gaps of Tetrabromophenol blue and Bromoxylenol blue are compared in the gas and solvent phases (see Figure5), and the data are listed in Table S2. Dyes have smaller energy gaps, which means that dyes are easier excited. Energy levels of TiO2 and electrolyte (I−/I−3) are shown in Figure 5, and the LUMO energy is higher than those of TiO2 (−4 eV), and the HOMO energy is lower than those of I−/I−3 (−4.8 eV) [54]. For Tetrabromophenol blue, the HOMO levels of gas and solvent are −6.69 eV and −6.64 eV, respectively, and the LUMO levels are −2.17 eV and −2.10 eV, respectively. The HOMO levels and LUMO levels of Bromoxylenol blue are computed, and the results are −6.20 eV, −6.27 eV, −1.32 eV, and− 1.29 eV. The data show that the energy gap of Tetrabromophenol blue is smaller than that of Bromoxylenol blue, which means that Tetrabromophenol blue is easier excited.Figure 5
Calculated HOMOs, LUMOs, and energy gaps of Tetrabromophenol blue and Bromoxylenol blue.


## 3.3. Polarizability and First Hyperpolarizability Calculations

The dipole moment (μ), linear polarizability (α), anisotropy of the polarizability (Δα), and first hyperpolarizability (β) are important factors of nonlinear optical (NLO) properties, and β is defined as the third derivative of the total energy. The data of μ, α, Δα, and β are calculated, and the results are listed in Table 1. The parameters of dipole moment [μi (i = x, y, z) and total dipole moment μtot] are showed in Table 1, and the total dipole can be expressed [49]:
(4)μtot=μx2+μy2+μz21/2.Table 1
The dipole momentsμ (D), the polarizability α (au), the average polarizability αtot (esu), the anisotropy of the polarizability Δα (esu), and the first hyperpolarizability βtot (esu) of Tetrabromophenol blue and Bromoxylenol blue.
Tetrabromophenol blue
Bromoxylenol blue
μx = −1.3838
βxxx = 166.279
μx = −0.6012
βxxx = 135.324
μy = 0.6285
βxxy = −74.417
μy = −0.0071
βxxy = 325.867
μz = 3.6474
βxyy = 325.971
μz = 9.0028
βxyy = −91.592
μtot = 3.9514
βyyy = −291.467
μtot = 9.0229
βyyy = 405.071
αxx = 440.475
βxxz = 38.380
αxx = 336.744
βxxz = 38.166
αxy = 4.858
βxyz = −57.081
αxy = 8.223
βxyz = 63.245
αyy = 354.282
βyyz = 58.637
αyy = 6.803
βyyz = 98.242
αxz = 10.068
βxzz = −69.811
αxz = −14.491
βxzz = −11.785
αyz = 4.041
βyzz = 30.701
αyz = 6.545
βyzz = 44.779
αzz = 350.652
βzzz = 38.947
αzz = 263.435
βzzz = 207.694
αtot = 381.803
βtot = 556.137
αtot = 202.327
βtot = 549.022
Δα = 90.45
Δα = 300.124Due to the important application of dipole moment in the research of polarizability and the first hyperpolarizability, theμ of Tetrabromophenol blue and Bromoxylenol blue is calculated to be 3.9514 D and 9.0229 D, respectively, and μx is the dipole moment component perpendicular to the direction TiO2 surface, and they are 1.3838 D and 0.6012 D, respectively. The β values are 556.137 × 10−30 esu and 549.022 × 10−30 esu, respectively. The μ, α, Δα, and β of Tetrabromophenol blue and Bromoxylenol blue can affect the effective electron injection from the dye molecule to the conduction band of the semiconductor.


## 3.4. Analysis of the First Hyperpolarizability Density

In order to more deeply understand the result of hyperpolarizability, which is studied by hyperpolarizability density analysis and is a powerful technique to characterize contribution to hyperpolarizability from various molecular regions, the electron density of a systemρr,F may be expressed as Taylor expansion with respect to the externally applied electric field F [55, 56]:
(5)ρr,F=ρ0r+∑jρj1rFj+12!∑j∑kρjk2rFjFk+13!∑j∑k∑iρjki3rFjFkFi+….From the above equation and expansion formula of dipole in the power ofF, a component of the first hyperpolarizability can be expressed:
(6)βijk=−12!∫riρjk2rdr,where the first hyperpolarizability density is defined as
(7)ρjk2r=∂2ρr∂Fj∂FkF=0.The first hyperpolarizability is componential byZZZ, which is the most remarkable component of βtot. According to the finite difference technique, the first hyperpolarizability density can be achieved by means of electron densities yielded under different manually applied electric fields:
(8)ρZZ2r=ρr,FZ+ρr,−FZ−ρr,0FZ2.Here,ρZZ2r is the electron density at a spatial point r in the presence of a very weak electric field FZ, which is a step size of finite difference (chosen to be 0.003 au). The first hyperpolarizability can be achieved from the integrand of ZZZ, namely, ρZZ2r of various systems presented in Figure 6. Solid and dash lines correspond to positive and negative parts, respectively, and the positive and negative regions have an important influence on nonlinear optical (NLO).Figure 6
Contour maps of integrand ofβZZZ, namely, ρZZ2r on the Z plane. (a) Tetrabromophenol blue; (b) Bromoxylenol blue. Solid and dash lines correspond to positive and negative parts, respectively.
(a)
(b)


## 3.5. Molecular Electrostatic Potential

For molecular electrostatic potentials (MEP) about the nuclear and charge distribution of Tetrabromophenol blue and Bromoxylenol blue, they are viewed as a tool for interpretation and prediction of chemical reactivity and a plot of the MEP of a chemical species is helpful in understanding its reactive behavior [57]. The MEP maps of Tetrabromophenol blue and Bromoxylenol blue are shown in Figure 7. The electron-rich (negative) region, electron-poor (positive) region, and zero electrostatic potential are denoted in red color, blue color, and green color, respectively, which means that electrophilic attack appears in the negative region and nucleophilic reactions happen on the positive region. So, the electron concentrations are shown by different colors, and the values of electron density (V(r)) increase in the following order [58]: red > yellow > green > blue.Figure 7
Molecular electrostatic potential map calculated at the DFT/6-31G(d) level (Tetrabromophenol blue and Bromoxylenol blue).
(a)
(b)The (V(r)) range of Tetrabromophenol blue and Bromoxylenol blue is from −0.06 au (deepest red) to 0.06 au (deepest blue) in MEP. Electrophilic attack and nucleophilic reactions of Tetrabromophenol blue and Bromoxylenol blue happen on oxygen atoms and hydrogen atoms, which are –SO3 and –OH, respectively. Moreover, MEP is also shown that two dyes display the redistribution of electron density with the positive region and negative region, which are identified with the transfer of electron density.


## 3.6. Photovoltaic Characterization of Dyes

Efficiency as an important factor is an important criterion for solar cells. Moreover, it has a close relationship with the spectra and intensity of the incident sunlight. The photo-to-electric conversion efficiency (η) is obtained by the short-circuit density (Jsc), open-circuit voltage (Voc), and fill factor (FF). The η is equal to these three factors than the incident solar power (Pinc) demonstrated as the following equation [59]:
(9)η=VocJscFFPinc.FF is defined as the ratio of the maximum power obtained from the DSSC and the theoretical maximum power, which is formulated as [60]
(10)FF=ImVmJscVoc.Here,Im and Vm were the current and voltage, respectively, related to the maximum power. For experiment, Tetrabromophenol blue and Bromoxylenol blue are measured with the current-voltage (I-V) instrument under sunlight intensity of 100 mW cm−2, and they are shown in Figure 8, and the photoelectrochemical parameters of DSSCs are given in Table S3. Comparing the photovoltaic characteristics of the DSSCs between Tetrabromophenol blue and Bromoxylenol blue, photoelectric conversion efficiency of Tetrabromophenol blue is higher (0.02%), with Voc of 0.39 V, Jsc of 0.09 mA cm−2, and FF of 0.59.Figure 8
The photocurrent-potential (I-V) characteristics of the DSSCs with TiO2 film electrode sensitized by Tetrabromophenol blue and Bromoxylenol blue.


## 3.7. Theoretical Analysis forJsc and Voc

According to (9), Jsc, Voc, and FF play a decisive role for photoelectric conversion efficiency. Theoretical analyses for Jsc and Voc are computed as follows [61]:
(11)Voc=ECB+ΔECBq+KTqlnncNCB−Eredoxq.Here,ECB is the conduction band edge of the semiconductor substrate, ΔECB is the shift of ECB, q is the unit charge, K is the Boltzmann constant, T is the absolute temperature, nc is the number of electrons in the conduction band, NCB is the density of accessible states in the conduction band and Eredox is the reduction-oxidation potential of the electrolyte. While in DSSCs, Voc is closely related with ΔECB and ΔECB [62]:
(12)ΔECB=−qμnormalγε0ε.Here,γ is adsorbed on the surface concentration and μnormal is the dipole moment component perpendicular to the direction TiO2 surface (where the μnormal was the x-axis direction), ε0 is the gas permittivity, and ε is the dielectric constant of the organic monolayer. Based on (12), the values of μnormal and ΔECB have proportional relationship. μnormal values of Tetrabromophenol blue and Bromoxylenol blue are −1.3838 D and −0.6012 D (see Table 1), respectively. The absolute value of μnormal of Tetrabromophenol blue is larger than that of Bromoxylenol blue. According to (11) and (12), μnormal is directly proportional to Voc.Jsc in DSSCs is determined by the following equation [63]:
(13)Jsc=∫LHEλ⋅ΔGinjectdλ.This equation includes two important factors that have direct influence on DSSCsη, LHE, and ΔGinject. LHE is light harvesting efficiency and ΔGinject is injection efficiency of the electrons in the excited state. For (13), to reach high Jsc, the DSSCs should have a large ΔGinject that is computed by [64]
(14)ΔGinject=Eoxdye∗−ECBTiO2.Here,Eoxdye∗ is the oxidation potential of excited dye and ECBTiO2 is the reduction potential of conduction band TiO2 (ECBTiO2 = 4.0 eV) and Eoxdye∗ can be calculated as [65]:
(15)Eoxdye∗=Eoxdye−ΔE,where Eoxdye is the oxidation potential of the dye in the ground state and ΔE is an experiment electronic vertical energy related to λmax.The data of Tetrabromophenol blue and Bromoxylenol blue are listed in Table2. The absolute value of ΔGinject of Tetrabromophenol blue is 2.25 eV, which is larger than that of Bromoxylenol blue (2.21). LHE of Tetrabromophenol blue and Bromoxylenol blue are 0.0284 and 0.0290, respectively. According to (13), ΔGinject of Tetrabromophenol blue is bigger than that of Bromoxylenol blue, which is helpful to improve the Jsc ability for Tetrabromophenol blue.Table 2
The driving force of electron injectionΔGinject (in eV) for the Tetrabromophenol blue and Bromoxylenol blue in the solvent.
Dye
EOXdye
EOXdye∗
ΔGinject
Tetrabromophenol blue
4.81
1.95
−2.25
Bromoxylenol blue
4.90
1.99
−2.21


## 3.8. Natural Bond Orbital Analysis

Natural bond orbital (NBO) exploits the second-order Fock matrix to get the donor-acceptor interactions [66]. For the sake of illuminating intramolecular rehybridization and delocalization of electron density within the molecule, NBO analyses of Tetrabromophenol blue and Bromoxylenol blue are calculated and the information is listed in Table S4 and Table S5. As shown, E(2) means that the energy of hyper conjugative interaction (stabilization energy)) has higher value and electron donors and electron acceptors have stronger interaction and larger extent of conjugation of the entire system.Stabilization is caused by intramolecular hyperconjugative interaction of molecules. Intramolecular hyperconjugative interaction ofπ∗ (C17-C18) distributed to π∗ (C16–C21) leads to the strongest stabilization of 285.88 KJ/mol (for Tetrabromophenol blue). The strong intramolecular hyperconjugative interaction of π∗ (C14-C15), π∗ (C5-C6), π∗ (C19-C20), and π∗ (C1-C2) distributes to π∗ (C10-C11), π∗ (C3-C4), π∗ (C21-C22), and π∗ (C3-C4), and E(2) values are 269.58 KJ/mol, 152.56 KJ/mol, 181.99 KJ/mol, and 160.11 KJ/mol (see Table S4). For Bromoxylenol blue, the electron delocalization is distributed form donor to acceptor that are π∗→π∗ and they are C18-C19 → C20-C21 (278.95 KJ/mol), C18-C19 → C10-C11 (271.65 KJ/mol), C12–C14 → C15-C16 (246.97 KJ/mol), C12–C14 → C13–C17 (271.69 KJ/mol), and C1-C2 → C3-C4 (176.83 KJ/mol) (see Table S5). The above results show that intramolecular hyperconjugative interaction of Tetrabromophenol blue is higher than that of Bromoxylenol blue.


## 3.9. Mobility Calculation

The mobility as an important factor that can affect the performance of DSSC, and it is computed to provide the calculation of value for applications of organics in electronic devices. So, the drift mobility of hopping (μ) is expressed [67]:
(16)μ=e2nkBT∑ir2WiPi,where n=3 is the dimensionality, e is the electronic charge, kB is the Boltzmann constant, T is the temperature, r is the distance of the center of mass, and P is the relative probability for charge carrier to a particular ith neighbor
(17)Pi=Wi∑iWi.According to the standard Marcus theory,Wi can be compared by the rate of charge motion between neighboring molecules and it is obtained by the reorganization energy λ and the coupling matrix element V and it can be written as [68]
(18)Wi=V2ℏπλkBT1/2exp−λ4kBT.where V and λ play important roles for Wi and they are achieved by framework of the Marcus-Hush theory. V is closely correlated with EHOMO and EHOMO-1, and λ is divided into hole reorganization energy (λe) and electron reorganization energy (λh). So, V, λe, and λh are expressed [69]:
(19)V=12EHOMO−EHOMO‐1,λh=E0∗−E0+E+∗−E+,λe=E0∗−E0+E−∗−E−,where E+∗E−∗ represents the cationic (anionic) state with the geometry of the neutral species; E+ and E− represent the energies of the cationic and anionic with the structure of cationic and anionic species; and E0 and E0∗ represent the energies of neutral species and state, respectively. The data of V, λe, and λh are listed in Table 3. V, λe, and λh of Tetrabromophenol blue are 0.03 eV, 0.82 eV, and 0.27 eV, respectively, and those of Bromoxylenol blue are 0.004 eV, 0.54 eV, and 0.27 eV, respectively. According to the Marcus theory, V and λ of dyes are higher and lower, which get larger μ.Table 3
Calculated molecular reorganization energies (eV) and coupling matrix element (eV) of Tetrabromophenol blue and Bromoxylenol blue.
Dye
λh
λe
V
Tetrabromophenol blue
0.27
0.82
0.03
Bromoxylenol blue
0.27
0.54
0.004


## 3.10. Molecular Descriptors

The electron affinity (A) and ionization potential (I) can affect the energy barrier of electrons. The data of the energy barrier between HOMO level and LUMO level are −4.52 eV and− 4.54 eV in gas and solvent, respectively, (for Tetrabromophenol blue) and those of Bromoxylenol blue are −4.88 eV and− 4.98 eV, respectively (see Table 4). So, the absolute value of the energy barrier of Tetrabromophenol blue is lower than that of Bromoxylenol blue. Moreover, the total and FMO energies (ETOTAL), electronegativity (χ), chemical hardness (η), and chemical softness (S) are computed by 6-31G(d) in gas and solvent. Parr et al. found that the lower η results in the lower resistance to intramolecular charge transfer [70]. η values of Tetrabromophenol blue (2.260 eV and 2.270 eV) are lower than those of Bromoxylenol blue (2.440 eV and 2.490 eV). The chemical hardness is an important factor, which is a measure of resistance to charger transfer.Table 4
The calculated ionization energy (eV), electron affinity (eV), electronegativity (eV), hardness (eV), softness (eV−1), electrophilicity index, and total molecular energies (Hartree) for the Tetrabromophenol blue and Bromoxylenol blue with the 6-31G(d) basis set.
Dye
I
A
χ
η
S
ΔE
ETOTAL
φ
Tetrabromophenol blue
Gas
6.69
2.17
4.430
2.260
1.480
4.52
−22075.48
4.323
Solvent
6.64
2.10
4.370
2.270
1.463
4.54
−22075.50
4.206
Bromoxylenol Blue
Gas
6.20
1.32
3.760
2.440
1.207
4.88
−6806.13
2.897
Solvent
6.27
1.29
3.780
2.490
1.259
4.98
−6806.15
2.869
ΔE = EHOMO − ELUMO; I = −EHOMO; A = −ELUMO; χ = (I + A)/2; η = (I − A)/2; S = I/2η; φ = χ2/2η.


## 3.11. Fluorescence Spectroscopy of Dyes

Excited-state lifetime (τ) plays an important role in DSSCs, and it can affect the efficiency of charge transfer. The longer τ can keep the dye in the cationic form for a long time, which can be expressed as [71]:
(20)τ=ac3u22fe2,where c is the speed of light, f is the oscillator strength, e is the fluorescent energy, and u is a constant. Figure 9 shows the experimental spectra and academic spectra; the fluorescence maxima λ, oscillator strength (f in au), and τ are listed in Table 5. For experiment and computational work, the maximum absorption peaks of Tetrabromophenol blue are 548 nm and 537 nm, respectively, and those of Bromoxylenol blue are 540 nm and 533 nm, respectively, and the fluorescence energy, f, and τ are 1.28 eV, 0.0066, and 2.16 × 10−5 s, respectively. While, for Bromoxylenol blue, the fluorescence energy, f, and τ are 2.33 eV, 0.0087, and 4.90 × 10−7, respectively. Those results showed that τ of Tetrabromophenol blue is higher, which means that Tetrabromophenol blue can remain stable in the cationic stat for a long time.Figure 9
The experimental (a) and theoretical (b) fluorescence spectra of Tetrabromophenol blue and Bromoxylenol blue.
(a)
(b)Table 5
Experimental absorption peaks, calculated transition properties, and excited-state lifetime of Tetrabromophenol blue and Bromoxylenol blue.
Dye
E (eV)
λ (nm)
Strengthf
Τ (s)
Exp (nm)
Tetrabromophenol blue
1.28
537
0.0066
2.16 × 10−5
548
Bromoxylenol blue
2.33
533
0.0087
4.90 × 10−7
540


## 4. Conclusion

The photoelectrical properties of Tetrabromophenol blue and Bromoxylenol blue as the sensitizers of DSSCs are measured in experiments. The absorption and fluorescence spectra, frontier molecular orbital, energy gaps, electronic structures, and MEP of Tetrabromophenol blue and Bromoxylenol blue are computed via DFT and TDDFT methods. As the key parameters (excited-state lifetime (τ), the driving force of electron injection (ΔGinject), and the vertical dipole moment (μnormal)), they have a close connection with Jsc and Voc. τ (2.16 × 10−5 s), absolute value of ΔGinject (2.25 eV), and absolute value of μnormal (1.3838 D) of Tetrabromophenol blue are larger than those of Bromoxylenol blue. Jsc and Voc of Tetrabromophenol blue are 0.39 V and 0.09 mA/cm2, respectively, which are higher than those of Bromoxylenol blue. The results show that the larger τ, ΔGinject, and μnormal can be in favor of improving the values of Jsc and Voc. Therefore, the photovoltaic performance of Tetrabromophenol blue is significantly better than that of Bromoxylenol blue.



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*Source: 9720595-2018-07-22.xml*