First-Principles Screening and Design of Novel Triphenylamine-Based

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First Principles Screening and Design of Novel Triphenylamine-Based D-#-A Organic Dyes for Highly Efficient Dye-Sensitized Solar Cells Zhenqing Yang, Chunmeng Liu, Changjin Shao, Chundan Lin, and Yun Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b05745 • Publication Date (Web): 02 Sep 2015 Downloaded from http://pubs.acs.org on September 13, 2015

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The Journal of Physical Chemistry

First Principles Screening and Design of Novel Triphenylamine-Based D-π-A Organic Dyes for Highly Efficient Dye-Sensitized Solar Cells Zhenqing Yang§*, Chunmeng Liu§, Changjin Shao*, Chundan Lin and Yun Liu State Key Laboratory of Heavy Oil Processing, College of Science, China University of Petroleum, Beijing 102249, P R China

Corresponding Author: Zhenqing Yang’s email: [email protected] Changjin Shao ’email: [email protected]

§ These authors contributed equally

ACS Paragon Plus Environment 1


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ABSTRACT

We screen a series of π-conjugated bridge groups and design a range of metal-free organic donor-π-acceptor (D-π-A) SPL101-SPL108 dyes based on the experimentally synthesized C217 dye for highly efficient dye-sensitized solar cells (DSSC) using density functional theory (DFT) and time-dependent DFT (TDDFT), and further calculate their physical and electronic properties, including geometrical structures, electronic cloud distribution, molecular orbital energy levels, absorption spectra, light harvesting efficiency (LHE), driving force of injection ( ∆ Ginj) and regeneration ( ∆ Greg), and electron dipole moment (µnormal). Results reveal that the π-conjugated bridge groups in SPL103 and SPL104 are promising functional groups for D-π-A organic dyes. Especially SPL106 and SPL108 have not only smaller energy gaps, higher molar extinction coefficient and 128 nm and 143 nm redshifts, but also broader absorption spectrum covering entire visible range up to the near-IR region of 1200nm compared to C217 dye. Keywords: metal-free organic dyes, donor-π-acceptor (D-π-A), dye-sensitized solar cells, DFT/TDDFT, absorption spectrum

1. INTRODUCTION The development and utilization of solar energy is the effective means to solve the problem of energy shortage. Silicon solar cells are the most mature solar cell at present, but its high purity requirements of materials, complex production process, and high priced greatly restricted its further wide application. Since Grätzel and his


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team reported dye-sensitized solar cells (DSSC) with photoelectric conversion efficiency (PCE) of 7.1% in 1991,

1

it has quickly become the focus for scientific

works. Dye sensitizer was recognized as the most important part for DSSC to get high performance, and various types of dyes have been explored to improve overall device performance. Conventional Ru-based chromophores such as the N3/N719 dye2-3 and zinc porphyrin sensitizers are the most promising sensitizers for DSSC which have shown the highest record power conversion efficiency of 13%.4 However, taking account of the limited noble metal resource, metal-free organic dyes with wide availability, benign environmental effects and high price-to-performance ratio have attracted more attention in recent years.5 At present, various organic dyes, which generally structure made of electron donors (D), π-conjugated bridge (π) and electron acceptor (A) can be applied in DSSC, mainly including coumarin dyes, polyene dyes,8 indoline dyes,9-10 prophyrin dyes,4,

11-12

6-7

hemicyanine dyes,13-14

triphenylamine dyes15-18 and so on. The donor-π-acceptor (D-π-A) structure with the acceptor directly bounded to the semiconductor surface of metal-free organic dyes, which will favor efficient charge transfer of excited electrons to the conduction band of semiconductor and the regeneration of excited dyes to the ground state by the redox shuttle.19 Due to these advantages as photosensitizer, the organic dyes with D-π-A structure have been intensively developed in DSSC.20-23 In 2009, P.Wang24 synthesized a D-π-A-based triphenylamine dye C217 which is a simple structure of organic chromophore with dihexyloxy-substituted triphenylamine group as donor, 3,4-ethyl-dioxythiophene (EDOT) and thienothiophene as



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π-conjugated bridge，2-cyanoacrylic acid as acceptor. C217-sensitized DSSC device exhibited a high power conversion efficiency (PCE) of 9.8% measured under illumination of AM1.5G full sunlight.24 These molecules have high molar extinction coefficient and electronic transmission capability, and can also inhibit the aggregation of the dye on the semiconductor electrode surface because of its special space sprial structure.15, 18, 25-26 So triphenylamine derivatives have been widely used as sensitizers in DSSC.27-32 Nonetheless, the major issue is how to widen the adsorption band from relatively narrow visible region to larger near infrared region for further improving the PCE of DSSC. The PCE of DSSC devices is mainly determined by the short-circuit current density (Jsc), the open-circuit photovoltage (Voc) and the fill factor (ff). The improvement of short-circuit current density and the open-circuit photovoltage will significantly enhance the power conversion efficiency. The main factors of Jsc are the light harvesting efficiency (LHE) and the electron injection efficiency (Φinj) which are influenced by the oscillator strength (f), driving force of injection ( ∆Ginject ) and regeneration ( ∆Greg ). Besides, the enhancement of electron dipole moment (µnormal) will also improve Voc, which benefit higher efficiency. The efficiencies of DSSC vary with the chemical modification of organic dyes due to their tuned structure and electronic/optical absorption properties.33 Thus, by considering C217 dye as a prototype, we screened a series of new π-conjugated bridge groups, further designed four dyes using dicyanovinyl carboxylic acid and dicyanovinyl sulfonic acid as the acceptor.34


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2. COMPUTATIONAL DETAILS 2.1 D-π-A Molecular Models Considering computational cost, we simplify the molecular structure model of experimentally synthesized C217 by replacing dihexyloxy units with methoxy group; analogous simplification has shown that it could capture the essential character of realistic organic dyes while not changing the relative trend of the obtained results.35 The simplified model of C217 is shown in Figure1. It is named C217 for simplification in this article. In order to explore much higher efficient triphenylamine derivative dyes than C217, we designed a series of dyes by using different π-conjugated bridge groups, SPL101-SPL104. These dyes were obtained by replacing partly of π-conjugated spacers segment which named 3,4-ethylenedioxy thiophene unit (EDOT)，the new π-conjugated bridges we choose are electron-rich and

have

preferable

planarity

than

EDOT.

Then

we

further

designed

SPL105-SPL108 using the π-conjugated bridge of SPL103 and SPL104, dicyanovinyl carboxylic acid and dicyanovinyl sulfonic acid as the acceptor, which screened in our previous work.34, 36



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Figure 1. Molecular structures of C217 simplified model and SPL101-SPL108. We investigated the influence of different π-conjugated bridge groups and accepter groups on the performance of DSSC according to their geometries, electronic structures of ground and excited states, energy levels and absorption properties. All the molecular structures mentioned above are shown in Figure 1.

2.2 Computational Methods The ground state geometries of these dyes were optimized by density functional theory (DFT)37-38 method using B3LYP with 6-31G(d) basis sets in acetonitrile solution. Frequency calculations were performed at the same level of theory to ensure that the geometries correspond to a minimum point (no imaginary frequencies) on the potential energy surface.


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Different exchange-correlation (XC) functionals for charge-transfer excitations often show significant effects. To select suitable functionals, we used different time-dependent DFT (TDDFT) methods with B3LYP,39 CAM-B3LYP,40 and WB97XD41 hybrid functionals to calculate the vertical excitation energy of C217. The previous theoretical studies confirm that the solvent effect is related to describe the absorption spectra of the sensitizers. The calculated C217’s vertical excitation energies of the different functionals were 1.774, 2.422 and 2.520 eV, with errors of 0.472, 0.176 and 0.274 eV, respectively, compared to 2.246 eV for the experimental data. The result of calculation by using CAM-B3LYP functional is in good agreement with the experimental value. Therefore, we adopted the TD-CAM-B3LYP functional with 6-311G(d,p) basis set, combining the conductor-like polarizable continuum model (CPCM) in acetonitrile solution for predicting optical properties of new designed dyes. All calculations are carried out in Gaussian09 program package.42

3. RESULTS AND DISCUSSION 3.1 Screening of π-Conjugated Bridge The π-conjugated bridge is a key factor with D-π-A structure for high performance DSSC device in dye. In order to screen excellent π-conjugated bridges of triphenylamine derivative dyes, we designed SPL101-SPL104 dyes based on C217 dye. The optimized triphenylamine dyes geometries are shown in Figure S1. It could be seen that all dyes have the sprial structure, which can help inhibiting the close



intermolecular aggregation effectively. However, the π-conjugated bridge (EDOT unit) and the acceptor plane in C217 is twist angle of 29.3°, which influences the efficiency of electron transmission. The π-conjugated bridge and acceptor moieties have better planarity in SPL101-SPL104 dyes, which benefit the efficient electron transfer from donor to acceptor moiety, compared to C217.

The calculated energy levels of dye SPL101-SPL104 and dye C217 are shown in Figure 2. The results indicate that the LUMO level of excited dye SPL101-SPL104

-1

-2 LUMO+1 -3 Energy (eV)

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LUMO -4

1.97

1.88

1.91

1.85

1.86

1.41

1.31

1.41

1.31

CB of TiO2

- I /I3 HOMO

-5

HOMO-1 -6

SPL104

SPL102

C217 SPL101

SPL103

SPL108

SPL106 SPL105

SPL107

Figure 2. Calculated energy levels of dye SPL101-SPL108 and C217. Energy gaps between HOMO and LUMO levels calculated from ground-state DFT calculations using the B3LYP exchange-correlation (XC) functional. The red horizontal dashed line indicates the level of conduction band edge (CBE) of the anatase TiO2 surface (-4.0eV), the blue horizontal dashed line indicates the level of I-/I3-redox electrolyte(-4.6eV).43

are higher than the TiO2 conduction band edge (CBE) which located at -4.0eV, shown as red horizontal dashed line, which ensure efficient electron injection from the excited dye into the conduction band of TiO2. Moreover, the HOMO level of excited dye SPL101-SPL104 are lower than the redox potential of I-/I3- redox couple


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(-4.6eV),43 shown as blue horizontal dashed line in Figure 2, which simultaneously ensure these dyes molecules that lose electrons could get electrons quickly from the electrolyte. The energy gap values of SPL101-SPL104 are smaller than the reference dye C217 (1.97eV). It is expected from previous studies that the sensitizers that have smaller energy gap values show several favorable characteristics beneficial to higher light-harvesting and efficiency in the DSSC, therefore our designed dyes SPL102-SPL104 may have better light harvesting ability than C217 (Table 1) excepting SPL101. Interestingly, we noticed that the decrease in energy gap values is mainly due to the decrease of LUMO energy levels. This result confirms that the energy levels of the dye sensitizers can be tuned by the means of substituting the π-conjugated bridge. Table 1. Calculated maximum absorption wavelengths λmax/nm, corresponding vertical excitation energies Eex (eV) (in parentheses), oscillator strengths (f), light harvesting efficiency (LHE) and the corresponding electronic transitions composition by TDCAM-B3LYP/6-311G(d,p). Dyes λmax (Eex) f LHE Main Composition (%) 513(2.42) 2.1327 0.993 H-1→L(23%) H→L(68%) H→L+1(5%) C217 SPL101 502(2.47) 1.7205 0.981 H-1→L(24%) H→L(64%) H→L+1(6%) SPL102 493(2.52) 2.5316 0.997 H-1→L(35%) H→L(49%) H→L+1(7%) SPL103 532(2.33) 2.3654 0.996 H-1→L(29%) H→L(54%) H→L+1(11%) SPL104 532(2.33) 2.3173 0.995 H-1→L(27%) H→L(56%) H→L+1(9%) SPL105 602(2.06) 2.2969 0.995 H-1→L(29%) H→L(54%) H→L+1(9%) SPL106 641(1.93) 2.3405 0.995 H-1→L(29%) H→L(57%) H→L+1(7%) SPL107 613(2.02) 2.2092 0.994 H-1→L(28%) H→L(58%) H→L+1(7%) SPL108 656(1.89) 2.2706 0.995 H-1→L(27%) H→L(61%) H→L+1(6%) H represents HOMO,L represents LUMO The frontier molecular orbital (FMO) contribution is very important in determining the charge separated states of dye sensitizers44. The FMO isodensity plots of C217, SPL101-SPL104 are shown in Figure S2, which are built using the CAM-B3LYP



level of theory. In order to create an efficient charge-separated states, the distribution of molecular orbitals of dyes show a strong D-π-A character; namely, the HOMOs are π orbitals localized on the donor subunit and LUMOs are π* orbitals mainly localized on the acceptor and anchoring groups. The π-characters of these orbitals will contribute to high extinction coefficients of dyes. Chemical modifications in the linker group would tune photoabsorption of dye, we can clearly see from Figure 3 that the designed dyes SPL101-SPL104 have

C217 SPL101 SPL102 SPL103 SPL104

10.0

8.0 4 -1 ε (10 Μ−1. cm )

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6.0

4.0 2.0

0.0 200

300

400

500

600

700

800

900

Wavelength(nm)

Figure 3. Absorption spectra of dyes SPL101-SPL104 and C217

broad absorption spectra to C217. The maximum molar extinction coefficients of SPL102-SPL104 dyes are 10.255×104 M-1·cm-1, 9.582×104 M-1·cm-1 and 9.391×104 M-1·cm-1, respectively, exhibiting an increment of 8.7% - 18.7% compared to C217 except SPL101. As presented in the Supporting Information, the origins of these absorptions are detailed by calculating the singlet electronic transition in the Gaussian


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09W program suite. The maximum absorption peaks of designed dyes SPL103 and SPL104 have 19nm redshifts compared to C217, however, the dye SPL102 has 20 nm blueshifts. So we choose the π-conjugated bridge of SPL103 and SPL104 for further designing of novel dyes. By considering the smaller energy gap, better absorption spectra (including higher molar extinction coefficient and oscillator strengths, larger absorption peak redshift and broader extent of light absorption) and superior calculated electronic properties of SPL103 and SPL104 (see Table 1 and Table 2), We believe that the π-conjugated bridge groups in SPL103 and SPL104 will be better π-conjugated bridge candidates for high-extinction and long-wavelength light absorption, leading to higher Jsc and Voc for better PCE of DSSC. Table 2. Calculated electronic properties of C217 and SPL101- SPL108. Redox potential of the ground state of the dye (Edye), vertical transition energy (E0-0), driving force of injection ( ∆ Ginj), driving force of regeneration ( ∆ Greg), and dipole moment (µnormal). Dyes

Edye/eV

E0-0/eV

C217 SPL101 SPL102 SPL103 SPL104 SPL105 SPL106 SPL107 SPL108

-4.76 -4.95 -4.79 -4.79 -4.73 -4.82 -4.85 -4.77 -4.80

2.42 2.47 2.52 2.33 2.33 2.06 1.93 2.02 1.89

∆

Ginj/eV 1.66 1.52 1.73 1.54 1.60 1.24 1.08 1.25 1.09

∆ Greg/eV

µnormal/Debye

0.16 0.35 0.19 0.19 0.13 0.22 0.25 0.17 0.20

16.92 16.34 16.32 18.42 17.23 20.75 28.44 20.88 28.93

3.2 Design of Novel D-π-A Structure We obtain SPL105-SPL108 by employing the above screened π-conjugated bridges as π bridges, dicyanovinyl carboxylic acid and dicyanovinyl sulfonic acid as



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acceptors. In Figure S1, the optimized geometries of SPL105-SPL108 are depicted, which exhibit a large planar structure in π-conjugated bridges. The calculated energy levels of dyes SPL105-SPL108 and the reference dye C217 are shown in Figure 2. SPL105-SPL108 show smaller HOMO-LUMO energy gaps compared to the reference dye C217, especially SPL105 and SPL107 with the minimal energy gaps 1.41eV, SPL106 and SPL108 with the minimal energy gaps 1.31eV, respectively. This result confirms that the energy levels of the dye sensitizers can be tuned by the means of substituting the π-conjugated bridges and acceptor groups. The simulated absorption spectra of dyes SPL105-SPL108 and the reference dye C217 are shown in Figure 4. SPL105-SPL108 are obviously improved to cover the entire visible region with the maximum absorption peaks of 602, 641, 613, 656nm, respectively, which get extremely large red-shifts from 89 to 143nm, compared to C217. The maximum molar extinction coefficients of SPL105-SPL108 dyes are 9.303×104 M-1·cm-1，9.481×104 M-1·cm-1，8.948×104 M-1·cm-1 and 9.198×104 M-1·cm-1, respectively, maintaining the corresponding oscillator strength to SPL103 and SPL104. Obviously, the introduction of dicyanovinyl carboxylic acid and dicyanovinyl sulfonic acid unit as acceptor red-shift the intramolecular charge transition absorption wavelength and maintain the corresponding oscillator strength. Especially SPL106 and SPL108 with a largely red-shifted at 641nm (1.93eV) by 128nm, 656nm (1.89eV) by 143nm, respectively, which agrees well with having the smallest HOMO-LUMO gaps and covers a broad range in the near-infrared (NIR)


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10.0

C217 SPL105 SPL106 SPL107 SPL108

8.0 -1

ε (10 M .cm )

-1

6.0

4

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4.0

2.0

0.0 200

400

600

800

1000

1200

wavelength(nm) Figure 4. Absorption spectra of dyes SPL105-SPL108 and C217

region. This would make SPL106 and SPL108 become attractive candidates for future development of DSSC devices, especially the π-conjugated bridge groups in SPL103 and SPL104 are promising functional groups for D-π-A organic dyes. The wavelength for maximum absorption λmax show a largely red-shifted as the change of π-conjugated bridges and acceptors (Table 1). The oscillator strength of SPL105-SPL108 is increased, and so does the light harvesting efficiency (LHE). LHE is one of the key factors in DSSC, we can presume that the efficiency of designed dyes SPL105-SPL108 is higher than C217. Moreover, the first absorption peak is predominantly associated with excitations from HOMO to LUMO, which reaches agreement with the rules of most D-π-A organic dye molecules. It shows that the mainly electron transitions come from HOMO to LUMO, and the HOMO-1 and HOMO orbitals to the LUMO and LUMO+1 orbitals, with contributions involving other orbitals amounting to less than 10%. Frontier molecular orbitals of dyes SPL106,



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SPL108 with four orbitals, HOMO-1, HOMO, LUMO, and LUMO+1 are shown in Figure 5. The electron densities of HOMOs of these dyes are mainly populated over the substituted triphenylamine (D) and its adjacent π-spacer unit, while the LUMOs are delocalized through the cyanoacrylic acid fragments (A) and its adjacent thienothiophene group, which indicates a good charge-separated state. The HOMO-1 orbitals of all dyes are more or less delocalized over the entire molecule. This spatial orientation not only facilitates the ultrafast interfacial electron injection from the excited dye molecules to the TiO2 conduction band, but also slows down the recombination of injected electrons in TiO2 with the oxidized dye molecules due to their remoteness, both advantageous for DSSC applications. In addition, the hole localized on the triphenylamine unit is spatially convenient for the electron donor to approach, facilitating the fast dye regeneration. So SPL106 and SPL108 which have the smallest HOMO-LUMO energy gap values, higher molar extinction coefficient and obvious broader spectra absorption scope will be kind of promising novel dyes in the future.


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Figure 5. Frontier molecular orbitals of dye SPL106 and SPL108.Four orbitals (HOMO-1, HOMO, LUMO, and LUMO+1) calculated from TDDFT using CAM-B3LYP functional.

3.3 Overall Efficiencies It is known to us, the PCE of DSSC devices is determined by the short-circuit current density (Jsc), the open-circuit photovoltage (Voc), the fill factor (ff), and the intensity of the incident light (Pinc), is given by Eq.(1)45:

η=

JscVocff Pinc

(1)

In DSSC, Voc can calculate by46:

Voc =

E CB KT nc Eredox + ln( )− q q NCB q

(2)

the open-circuit photovoltage Voc is determined by the energy difference between the



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semiconductor CBE and the electrolyte redox potential. Usually, the solution I-/I3- is used as the electrolyte, so we take it as a constant. ∆CB is the main influent factor of Voc, it can be expressed as47:

∆ECB= -

qµ normal γ ε0ε

(3)

Where q is the elementary charge, γ is the molecular surface concentration, µnormal is the dipole moment of individual molecular perpendicular to the interface of the semiconductor, and ε0, ε as constant. It is obvious that a large µnormal will lead to more shift of CBE which will result in larger Voc. As shown in Table 2, SPL106 and SPL108 have the largest dipole moment, leading to larger Voc which is excellent agreement with better efficiency. The short-circuit current density Jsc is determined by the following equation48:

J sc = ∫ L H E (λ )φ in jη co lld λ

(4)

where LHE(λ) is the light harvesting efficiency at a given wavelength, Φinj evinces the electron injection efficiency and ηcollect denotes the charge collection efficiency. For DSSC the electrode is the same, the only difference is the various sensitizers, so ηcollect can be seen as constant.48 LHE(λ) is mainly determined by the oscillator strength(f) in Eq.(5)49

LHE = 1 − 10−f

(5)

where f represents the oscillator strength of adsorbed dye associated to the λmax. From Table 1, we can know f=2.1327 for C217, the oscillator strength of all dyes become larger than C217 excepting SPL101, so does LHE. Φinj is related to the driving force ∆Ginject of electrons injecting from the excited states of dye molecules


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to the semiconductor substrate. According to the survey of Islam,50 when

∆Ginject >0.2eV the injection efficiency of electrons (Φinj) is approximately equal to one. ∆Ginject can be estimated as51:

∆Ginject = Edye * −ECB = Edye + E0 − 0 − ECB

(6)

Where Edye* is the oxidation potential of the excited dye, ECB is the conduction band edge of the semiconductor (the experimental value -4.0eV), Edye is the redox potential of the ground state of the dye, and E0-0 is the vertical transition energy. Table 2 has listed the calculated ground state of oxidation potential of dyes and corresponding ∆Ginject . From Table 2 the absolute values of ∆ Ginj for C217,

SPL101-SPL108 are much greater than 0.2eV, we can predict these dyes have sufficient driving force for the fast injection of excited electrons. Although ∆ Ginj of

SPL106 and SPL108 is slightly smaller than C217, an energy difference of 1.08eV and 1.09eV between dye HOMO and TiO2 CBE is large enough to guarantee the efficiency of electron injection. On the other hand, too large ∆ Ginj may introduce energy redundancy, further result a smaller Voc and large thermalization losses.52-53 JSC is also influenced by the regeneration efficiency of dye (ηreg) which determined by the driving force of regeneration ∆Greg . It can expressed by Eq.(7)54:

∆Greg = Eredox − Edye

(7)

where Eredox is the redox potential -4.6eV.43 From the survey of Robson,55 the regeneration process of dye can significantly influence the efficiency of DSSC.

SPL105, SPL106 and SPL108 have bigger driving force of regeneration can cause the improvement of ηreg, which benefits the increase of short-circuit current density



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(Jsc). Finally, we find SPL106 and SPL108 are the better ones among all the dyes because they perform nicely on the four key parameters (λmax, LHE, Jsc, Voc) and achieve a good balance among these factors. Therefore, SPL106 and SPL108 will be promising dyes for their good conversion efficiency.

4. CONCLUSION We have systematically investigated the electronic levels, corresponding optical absorption properties, and conversion efficiency of D-π-A type dyes designed by adopting different π-conjugated bridges and electron acceptors. The results indicate that the π-conjugated bridge groups in SPL103 and SPL104 are promising functional group for D-π-A structure. Because the π-conjugated bridge groups are coplanar structure which will benefit efficiently injection from the donor to the acceptor of dye molecules, increasing the dye’s light harvesting efficient and facilitating charge separation upon photoexcitation. Furthermore, by using the above screened promising groups as π-conjugated bridge, dicyanovinyl carboxylic acid and dicyanovinyl sulfonic acid group as electron acceptor, we reconstructed four novel dyes, SPL105-SPL108. The calculated results indicate that dicyanovinyl sulfonic acid group as acceptor are strongly absorbed to provide stable charge transfer channels for fast electron injection and regeneration, increasing the dye’s dipole moment. Therefore, the SPL105-SPL108 dyes show smaller HOMO-LUMO energy gaps, higher molar extinction coefficients and obvious redshifts compared to the


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experimentally synthesized C217 dye. In particular, the newly designed dye SPL106 and SPL108 are more promising candidates for highly photoelectric conversion efficiency DSSC with red light absorption, high molar extinction coefficients, improved vertical dipole moment, larger light-harvesting efficiency (LHE) and better Jsc and Voc values. All of this achievement will considerably encourage further molecular and energy-level engineering of low-cost metal-free organic dyes, boosting the practical application of dye-sensitized solar cells.

ASSOCIATED CONTENT Supporting information Optimized geometry structures of dye C217, SPL101-SPL108 and Frontier molecular orbitals of dye SPL101-SPL105, SPL107 and the reference dye C217. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]

Tel: 8613401096596

*E-mail: [email protected]

Tel: 8615810085561

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is supported by Higher Education Young Elite Teacher Project (2462015YQ0603) from CUP.

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First-Principles Screening and Design of Novel Triphenylamine-Based

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