Theoretical Insights into DD-Ï-A Sensitizers Employing N-Annulated

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Theoretical Insights into D-D-#-A Sensitizers Employing N-Annulated Perylene for Dye-Sensitized Solar Cells Liezel Labrador Estrella, Mannix Padayhag Balanay, and Dong Hee Kim J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b03331 • Publication Date (Web): 11 Jul 2018 Downloaded from http://pubs.acs.org on July 12, 2018

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The Journal of Physical Chemistry

Theoretical Insights into D-D-π-A Sensitizers Employing N-Annulated Perylene for Dye-Sensitized Solar Cells

Liezel L. Estrella,a Mannix P. Balanay,b Dong Hee Kim*a

a

b

Department of Chemistry, Kunsan National University, Kunsan, 573-701, Republic of Korea

Department of Chemistry, School of Science and Technology, Nazarbayev University, Astana, Kazakhstan

Corresponding author: Tel: +82-63-469-4576 Fax: +82-63-469-4571 E-mail: [email protected]

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The Journal of Physical Chemistry 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

ABSTRACT

This paper reports new D-D-π-A dyes based on N-annulated perylene, emphasizing on the enhanced dye-to-semiconductor charge transfer mechanism. A series of DFT calculations for new tPA-perylene-based dyes was conducted, starting from the systematic selection of DFT methods by reproducing the experimentally obtained properties of known perylene-based sensitizers. Accordingly, using the LC-ωPBE xc functional with 6-31+G(d) basis set for the time-dependent calculations of the excitation energies, a damping parameter of ω=0.150 Bohr-1 was found most appropriate for dyes having spatial orbital overlap value of 0.21 < ΛHL < 0.38, while ω=0.175 Bohr-1 is suitable for analogues with 0.43 < ΛHL < 0.57. Moreover, the mPWHandHPW91/6-31G(d) method gave high accuracy in GSOP calculations. The comparison between the properties of tPA-based donor groups has revealed that the semi-rigid tPA-based D4 unit is an effective donor group for perylene-based dye. Initial screening of the acceptor designs resulted in PLz4 dye with promising charge transfer mechanism and highly favorable dye-TiO2 interaction based on the calculated dipole moment of the dye and dye-TiO2 complex. The attachment of the substituted-hydroacridine donor unit (D4) to PLz4 afforded a bathochromically shifted absorbance and improved molar absorptivity signifying its effective electron donating ability. Among the D-D-π-A dyes, DP46 is expected to render a relatively high Voc and Jsc supported by the calculated optical properties, oxidation potentials, ionization potential, and electron affinity values.

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1. INTRODUCTION The statistical records for the past years documented the appreciated changes in the photovoltaic (PV) industry such as an amazing increase in the manufacturing capacities and drastic reduction of the market price of photovoltaic devices. Accordingly, solar energy is now the world’s fastest-growing form of renewable energy, with net solar generation increasing by an average of 8.3%/year.1 While crystalline silicon-based devices currently dominate the PV market with around 90% share, these devices are more expensive than the new solar cell designs. Dye-sensitized solar cells (DSSCs) have many advantages over their silicon-based counterparts in terms of transparency and low cost. However, to date, the overall efficiency of DSSCs is still lower than the silicon-based solar cells, mostly because of the inherent voltage loss during the dye regeneration process and poor long-term stability. Even so, the DSSC still remains one of the most promising photovoltaic technology in the near future owing to the ease and economically favorable fabrication, outstanding performance in darker condition, and easy printing on glass and flexible substrate.2 As of to date, the highest efficiency recorded for DSSC is 14.7% for the solar cell device by ADEKA1 + LEG4 (1.0 : 0.25 for ADEKA-1 : LEG4) co-sensitization in conjunction with cobalt electrolyte.3 The dye sensitizer, which plays a crucial role for the charge generation and transport, has been one of the focus in the development of the DSSCs yet the design and synthesis of dyes still need substantial efforts to realize a highly efficient sensitizer. Metal-free organic dyes offer a wide range of practical advantages for DSSC applications and for this reason, numerous research efforts have been dedicated to the search of the promising metal-free sensitizer.4 Polyaromatic hydrocarbons (PAHs) have been receiving sizeable research attention attributable to their vast applications in photovoltaics. One of these is the perylene, 3



which gained popularity in solar cell application primarily due to its high degree of conjugation, favorable photoelectrochemical properties, and excellent thermal and photostability.5,

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The N-annulated perylene (1H-phenanthro[1,10,9,8-cdefg]carbazole, PC)

has been widely studied both as donor and π-linker moiety in dye sensitizers due to its satisfactory properties. In fact, for DSSC devices that employ no co-adsorbent nor cosensitizer, the highest recorded efficiency of 13.0% was attained by the C281 dye which utilized PC spacer.7 Thus, recent researchers have become more interested in the molecular engineering of N-annulated perylene-based dyes. While the triphenylamine (tPA) is among the arylamines employed as an electron donating unit in a dye sensitizer because of its excellent electron donating ability making it a subject of tremendous experimental modifications and theoretical investigations,8-11 studies have revealed that rigid or more planar tPA can increase the sp2 character of the amine unit which can augment the charge separation lifetime after the excitation process. As previously pointed out in several studies, the rotation of the free phenyl rings of the conventional tPA could result in energy loss which could utterly compromise the device performance.12-14 In our previous work, we presented a novel idea of semi-rigid tPA wherein the two phenyl rings in the tPA were bridged forming a hydroacridine unit while leaving the third phenyl ring at an almost perpendicular orientation with respect to the hydroacridine plain resulting to 9,9diethyl-10-phenyl-9,10-dihydroacridine (D4) as the electron donating group of Dhk4 dye whose structure is shown in Figure 1. The properties of the novel Dhk4 dye were investigated using computer simulations and were compared to the properties of the known reference dye named DIA3. The results of the calculations suggest the superior properties of Dhk4 compared to the reference dye.15 Therefore, this mode of binding in D4 could render better Voc as a result of reduced energy loss due to free rotation of the phenyl rings. In addition, with 4


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the planar configuration between the bridged phenyl rings on the donor group, the charge separation lifetime can be increased by delocalization of the positive charge over the planar electron donating amine unit.

Figure 1. Chemical structure and properties of Dhk4 dye. In this study, we designed a series of D-D-π-A dye sensitizers by incorporating a semirigid tPA as the electron donating group with a highly conjugated perylene moiety as donor/πbridge. This study is divided into four main sections: (1) the DFT and TD-DFT methods were assessed by reproducing the experimentally obtained properties of several known perylenebased sensitizers; (2) with the most reliable method, the properties of the semi-rigid tPA (D4) as a new electron donating group for perylene-based dyes were evaluated in comparison to conventional tPA; (3) a series of PLz dyes were designed to realize promising acceptor design that is capable of attaining a favorably strong electronic interaction with TiO2 semiconductor upon adsorption; (4) a group of D-D-π-A dyes were designed by incorporating a semi-rigid tPA-based D4 donor group to the outstanding PLz dye, which was subsequently followed by the modification of the perylene unit. The properties of these dyes were investigated using DFT and TD-DFT methods. Further evaluation of various key parameters of the isolated form of the dye and dye-TiO2 complexes was implemented to gain a proper understanding of the structure-property relationships of the dyes.

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2. COMPUTATIONAL METHODOLOGY The general expression for the power conversion efficiency of a photovoltaic device is as follows:16

η=

Voc ⋅ J sc ⋅ FF Io

(1)

where Voc, Jsc, FF, and I0 stands for the open circuit photovoltage, photocurrent density, fill factor, and the incident solar power, respectively. The Voc of DSSCs is calculated using the following equation:17

Voc =

Ec + ∆CB kT  nc  Eredox + ln  − q q  NCB  q

(2)

where the q, Ec, and ∆CB in the first term stand for the unit charge, conduction band (CB) edge, and the shift in the conduction band of the TiO2 when a dye is adsorbed onto it, respectively. In the second term, k is the Boltzmann constant and T is the absolute temperature. The Eredox is the redox potential of the electrolyte. The Jsc is mathematically expressed as follows:18

Jsc = ∫ LHE(λ)Φinjectηcollect dλ λ

(3)

where LHE is the light-harvesting efficiency, Φinject is the electron injection efficiency, and ηcollect stands for the effectiveness of electron collection. The LHE can be obtained by the following mathematical expression: LHE = 1 – 10–f

(4)

where f represents the oscillator strength of the dye corresponding to the highest absorption 6


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wavelength (λabs).19 Φinject is associated with the thermodynamic driving force (∆Ginject) for the transfer of electrons from photoexcited dye molecules to the CB of the semiconductor substrate. The ∆Ginject can be evaluated using the following equations: ∆Ginject = ESOP – ECB

(5)

ESOP = GSOP + E0–0

(6)

where ESOP is the excited-state oxidation potential of the dye which can be calculated using eq. 6, and ECB = -4.05 eV is the energy of the CB of TiO2 at pH 7,20 GSOP is the ground-state oxidation potential, and E0-0 is equal to the energy of the lowest excitation state which can be obtained from the TD-DFT calculations. The mathematical expression of the ESOP is based on the experimental observation that the dye-to-semiconductor injection in a solar cell device, with TiO2 or SnO2 semiconductors, occurs from the unrelaxed excited-state dye species to the CB of the semiconductor.21, 22 The free energy of dye regeneration can be calculated as follows: ∆Greg = Eredox – GSOP

(7)

In this paper, the Eredox = -4.80 eV (0.35 V vs NHE) corresponding to the experimentally known redox potential of the I−/I3− electrolyte.23

The GSOP is the difference between the energy of the neutral specie and the oxidized groundstate form of the dye as expressed in the equation below: GSOP = −

(8)

Intramolecular charge transfer (ICT) of the dye can be further explained by examining the ionization potential (IP) and electron affinity (EA). Herein, an adiabatic approach was used to 7



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calculate the IP and EA, which was recognized to agree well with the experimental values in comparison to the vertical approach.24, 25 Eqs. 9 and 10 are the mathematical expression for the adiabatic IP and EA, respectively. IP = −

(9)

EA = −

(10)

In the equations above, and ± are the neutral and cationic/anionic forms of the molecule at their lowest energies. Density functional theory (DFT) calculations were employed for the optimization of the molecules at mPWHandHPW91 method with 6-31G(d) basis set, followed by frequency calculations using the same method. The reliability of this method is based on our previous publications.26,

27

The absence of imaginary frequencies confirmed that the optimized

structures correspond to the minimum on the potential energy surface. The solvation effect was taken into account by employing the conductor-like polarized continuum model C-PCM model28 which allows the calculations to be done under realistic solvent environment. The TD-DFT calculations were performed using the exchange correlation functional LC-ωPBE29 at 6-31+G(d) basis set in THF solution by employing C-PCM framework. For the range-separated hybrid (RSH) xc functional LC-ωPBE, separation parameter (ω) of 0.150 Bohr-1, 0.175 Bohr-1 and 0.20 Bohr-1 were used. The dye sensitization mechanism can be further understood by detailed analysis of the dyes adsorbed on the TiO2 surface. Herein, investigation of the dye-TiO2 complexes was performed following the self-consistent charge density functional tight-binding (SCC-DFTB) 8


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method30 wherein the interaction between the atoms is based on Slater-Koster mio-1-131 and tiorg-0-132 parameters. The optimization of the clusters was performed using the DFTB+ code with self-consistent charges tolerance and maximum force component set to 10-4. Subsequent excitation energy calculation and single-point energy calculation at TD-CAMB3LYP/3-21G(d) and B3LYP/3-21G(d), respectively, were performed on the optimized dye and dye-(TiO2)38 structures. The adsorption energies (Eads) were calculated using the equation:

= (( ) + ) – ( )

(11)

where , ( ) , and ( ) are the energies of the isolated dyes, semiconductor, and dye-TiO2 complexes, correspondingly. All of the DFT/TD-DFT calculations presented in this study were performed using the Gaussian 09 software package.33 The simulation of the UV-Vis absorption spectra was done by GaussSum 3.0 program.34

3. RESULTS AND DISCUSSION 3.1. Assessment of Theoretical Method 3.1.1 First Singlet Excitation Energy (E0-0) The validation of the theoretical method is among the most vital step in computational modelling. This can be done by reproducing the experimentally known properties to the highest accuracy as much as possible. In designing a dye sensitizer, it is pivotal to validate the method for the simulation of the important properties such as the UV-Vis absorption and the molecular energy levels of the dyes. These are the determining properties to evaluate the 9



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light-harvesting and the charge-transfer properties of the dye sensitizers. In the study which assess the quality of TDDFT excitation, Tozer et al. have examined the influence of overlap between the occupied and virtual orbitals involved on the excitation process.35 When the excitation process corresponds to HOMO LUMO transition, that is the HOMO and LUMO orbitals are the major contributing orbitals to the excitation, the spatial orbital overlap ( ΛHL ) can be determined by:

Λ HL =

N

∑

ciH c jL

(12)

i = j =l

where ciH and cjL in the above equation correspond to the MO coefficients of HOMO and LUMO, respectively. The spatial overlap takes the value 0 ≤ ΛHL ≤ 1 . In long-range excitation where ΛHL → 0 , there is a minimal spatial overlap between the involved in the excitation process of the contributing orbitals. As ΛHL → 1 , the excitation is said to be a short-range excitation and that the occupied and virtual orbitals are likely to occupy the same region of space.27, 35 According to Marcus theory, the electronic coupling between different states during an electron transport process is related to the rate of electron transport.36 The electronic coupling between two adjacent electronic states is dictated largely by orbital overlap. Given that the transitions are mostly HOMO LUMO, with increased electronic coupling between these states as depicted by the decreasing spatial orbital overlap upon increased conjugation, the energy gap between the HOMO – LUMO is expected to be shortened as a result of reinforced charge transfer communications between the electron donating and electron withdrawing groups.37-39 The dependence of the TD-DFT calculations employing long range exchange 10


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correlation (xc) functionals on the ΛHL between the HOMO and LUMO of the dyes have been made known in previous publications.27,

40

In the conducted evaluation of the separation

parameter using representative dye sensitizers, it was shown that the xc functional LC- ωPBE can satisfactorily reproduce the excitation energy of push-pull dye designs by tuning the separation parameter (ω). For instance, for the time-dependent calculation of the excitation energy of the dyes employing fluorenyl and tetrahydroquinoline donor groups, thiophene πbridge, and cyanoacrylic acid anchoring unit, ω= 0.20 Bohr-1 is appropriate to reproduce the experimental results for the dye analogues with 0.42≤ΛHL≤0.52. However, for the tPA-based dye analogues with higher CT character whose spatial overlap measures 0.32≤ΛHL≤0.37, the use a lower separation parameter value of ω =0.150 Bohr-1 was found to have better correlation with the experimental values.27 C2 H5

C2 H5

C4 H9

NC

C4 H9

COOH

N

COOH

S

N

N

NPS-4

NPS-1 C6H13

C8H17

C6H17

C6H13

C6H13 C H 8 17 C6H13

S N

N

NC

N

C8H17

N

N

N

S

N COOH

COOH

HW-1

HW-2

C6H13 C6H13 C8 H17

C6H13

C6 H13O

C8 H17

N

N

S

N

N

C6H17 S N N

N COOH

N

COOH

C6 H13O

C262

HW-3

C6H13

C6 H13O

C6 H13

C6H17 N N

C6 H13O

S N

C8H17

N

C8 H17 COOH

C6 H13

N O

C289

11


S

N COOH

C272

Figure 2. Molecular structure of the reference dyes.

N


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In this paper, we present a highly accurate calculation for the excitation energies of known push-pull dyes based on N-annulated perylene π-spacer whose structures are presented on Figure 2. The ΛHL values are calculated using eq. 12 from the optimized ground-state geometry and the resulting values are presented in Table 1. For the presented known perylene dyes, the alteration of the donor group does not significantly affect the ΛHL value, as can be seen among the HW dye series. However, the ΛHL values of the analogues are significantly affected with changes between the π-linker and the acceptor/anchoring units. For example, comparing C262 and C289, extending the pibridge of by insertion of ethynylene unit in C289 resulted in the decreased ΛHL. The same phenomena are observed for the NPS-1 and NPS-4. The increased degree of charge-transfer upon extension of the conjugation length is evidenced by the lower ΛHL value, suggesting a long-range excitation where there is a small degree of spatial overlap between the occupied and virtual orbitals involved in the electronic excitation.41 The ΛHL values of the reference dyes range from 0.21 to 0.57, which match with the

ΛHL values of the previously assessed dyes designs.27 Thus, for the TD-DFT calculations employing the xc functional LC-ωPBE, three separation parameters, ω=0.150, 0.175, and 0.20 Bohr-1 were used in an effort to reproduce the excitation energies of the reference dyes. Table 1 lists the calculated first singlet excitation energy along with the corresponding experimentally obtained

excitation energy and maximum

absorption

wavelength.

Consequently, for perylene-based dyes having spatial orbital overlap value of range 0.21 < ΛHL < 0.38, the calculation using ω=0.150 Bohr-1 gave a mean absolute error (MAE) of 0.024 while the use of ω=0.175 Bohr-1 was found appropriate for the calculation of the excitation energies of the analogues with lower CT character whose ΛHL values fall in the range 0.43 < 12


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ΛHL < 0.57, resulting to 0.035 MAE value. Since recent theoretical calculations involving perylene-based dyes use CAM-B3LYP method for excitation energy calculations,42, 43 timedependent calculation using this method was also performed. However, the TD calculation using CAM-B3LYP resulted in higher error compared to the LC-ωPBE. Based on these findings, accurate calculations of the excitation energies of the perylene-based analogues can be performed using LC-ωPBE following the proper assessment of the ω based on the chargetransfer properties.

Table 1. The spatial orbital overlap (ΛHL) and first singlet excitation energy (E0-0, ev/|error|) of reference dyes calculated using TD-LC-ωPBE/6-31+G(d) with three different separation parameters and TD-CAM-B3LYP/6-31+G(d). Optimization was performed at mPWHandHPW91/6-31G(d).

dye

ΛHLa

NPS-1b NPS-4b HW-1c HW-2c HW-3c C262d C289d C272e

0.43 0.52 0.37 0.38 0.37 0.57 0.21 0.38

First singlet excitation energy (E0-0) LC-ωPBE (eV / |error|) CAM-B3LYP Experimental (eV / |error|) (eV / nm) ω=0.150 ω=0.175 ω=0.20 2.47 / 0.16 2.60 / 0.03 2.70 / 0.07 2.71 / 0.08 2.63 / 472 2.37 / 0.05 2.46 / 0.04 2.53 / 0.11 2.58 / 0.16 2.42 / 512 2.46 / 0.03 2.58 / 0.09 2.68 / 0.19 2.72 / 0.23 2.49 / 498 2.41 / 0.01 2.53 / 0.11 2.63 / 0.21 2.66 / 0.24 2.42 / 512 2.40 / 0.00 2.53 / 0.13 2.63 / 0.23 2.66 / 0.26 2.40 / 517 2.43 / 0.11 2.58 / 0.04 2.70 / 0.24 2.77 / 0.23 2.54 / 489 2.30 / 0.05 2.42 / 0.17 2.53 / 0.28 2.59 / 0.34 2.25 / 550 2.40 / 0.02 2.53 / 0.11 2.63 / 0.21 2.66 / 0.24 2.42 / 512

a

Calculated from optimized structure Experimental values taken from references b44, c45, d46, and e5.

3.1.2 Ground-State Oxidation Potential

Considering that proper molecular energy levels of the sensitizer determines the efficiency of the charge transfer mechanisms in a DSSC device, it is therefore of high importance to assess the method for the calculation of the oxidation potentials of the dyes. 13



The ground-state oxidation potential (GSOP) must be sufficiently below the redox potential of the electrolyte for an efficient dye regeneration while the excited-state oxidation potential (ESOP) must be suitably higher than the CB of the semiconductor to permit electron injection. For the evaluation of the GSOP, four different DFT methods namely B3LYP,47 CAMB3LYP,48 M06-2X,49 and mPWHandHPW91, were employed to reproduce the experimental GSOP of the HW dye series. These dyes are structurally related to the target dye designs. All calculations were performed using 6-31G(d) basis set, with inclusion of solvent effect by employing C-PCM framework. The GSOP and ESOP were calculated using eqs. 8 and 6, respectively. For the calculation of the respective ESOP of the dyes, the E0-0 value used is the vertical excitation energy derived from the optimized TD-DFT method, LC-ωPBE with ω=0.150 Bohr-1, as discussed in the preceding section. Figure 3 shows the calculated GSOP and ESOP of the reference dyes. The calculated GSOP values using B3LYP and CAMB3LYP are higher than the experimental GSOP values. On the contrary, M06-2X provided a GSOP value lower than the actual GSOP. These deviations are consistent among the three reference dyes. However, for the mPWHandHPW91, the calculated GSOP values for HW-1 and HW-2 are slightly more positive than the actual GSOP while the result for HW-3 is slightly more negative. As Summarized in Table S1, mPWHandHPW91 method gives MAE value of 0.03, indicating the high reliability of this method for the calculation of the GSOP for this group of dyes.

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Figure 3. The GSOP and ESOP of the reference dyes calculated at different DFT/TD-DFT methods in THF solution using the C-PCM framework. The band gap (E0-0) corresponds to the singlet excitation energy calculated at TD-LC-ωPBE/6-31+G(d)//mPWHandHPW91/631G(d) with ω=0.150 Bohr-1. Experimental values are taken from reference 45.

3.2 Semi-rigid tPA as a new donor group for perylene-based dyes.

3.2.1 Geometry and absorption characteristics

N

NC

Donor

COOH

N

N

DL-4

NPS-4

Figure 4. The chemical structure of DL-4 and NPS-4.

Herein, we examine the properties of the semi-rigid tPA as a donor group for perylene-based dye.

The D4 donor was incorporated in a N-annulated perylene forming

DL-4 dye, as illustrated in Figure 4. The known NPS-4 dye having a conventional tPA donor group was used as a reference.44

As discussed in section 3.1.1, the separation parameter (ω)

for xc functional LC-ωPBE is dependent on the spatial orbital overlap (ΛHL) value of the analogues in consideration. The calculated ΛHL of DL-4 and NPS-4 dyes are 0.51 and 0.52, respectively, thus a separation parameter of 0.175 Bohr-1 was used in the TD-DFT calculation. 15



The absorption spectra of the dyes were simulated by TD-DFT calculations for the 15 lowest singlet-singlet excitations. Figure 5 illustrates the optimized geometries and the simulated absorption spectra of the dyes. The calculated absorption maxima of NPS-4 is located at 505 nm which is in good agreement with the experimental value of 512 nm. Compared to NPS-4, DL-4 dye with a semi-rigid tPA donor is slightly bathochromically shifted with a higher molar absorptivity. This indicates that the DL-4 design has better light harvesting property than the NPS-4 dye.

As previously pointed out in several studies, the rotation of the free

phenyl rings of the conventional tPA could result in energy loss which could utterly compromise the device performance.12 Therefore, it is worth pointing out that the mode of binding in the tPA donor of DL-4, where the two phenyl rings in the tPA were bridged to prevent free-rotation, could effectively reduce the energy loss. In addition, compared to the conventional tPA, DL-4 enables delocalization of positive charge of the dye cation over the planar amine group resulting in an induced a better charge separation lifetime.13

Figure 5. The (a) optimized geometry of the dyes along with their dihedral angles and the (b) simulated absorption spectra calculated at TD-LC-ωPBE/6-31+G(d)//mPWHandHPW91/631G(d) in THF solution; ω=0.175 Bohr-1.

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3.2.2 Oxidation potentials and charge transfer character

Figure 6. The calculated oxidation potential and the frontier orbital electron density distribution of the dyes.

The mismatch between the molecular energy levels of the dyes and other components of the solar cell device, specifically the conduction (CB) of the semiconductor and the redox potential of the electrolyte, is among the common reasons for the unsatisfactory device performance. Such misalignment between the molecular energy level with the CB of the semiconductor and redox potential of the electrolyte results in rather ineffective electron injection and dye regeneration. The electron injection and dye regeneration in a DSSC can be assessed by examining the oxidation potential level of the dyes. The calculated oxidation potentials of the analogues are presented in Figure 6. The GSOP corresponds to the difference between the energies of the neutral and oxidized ground-state forms of the dyes in solution, while the ESOP can be derived by getting the sum of the GSOP and the first excited-state 17



energy (E0-0) of the dye, as shown in eqs. 8 and 6, respectively. When a semi-rigid tPA donor group was introduced, there is a destabilization of GSOP brought by the stronger electron donating ability of the donor group of DL-4 with respect to the conventional tPA donor of NPS-4. As shown in Figure 6, both DL-4 and NPS-4 dye portrays pronounced intramolecular charge separation with the HOMO electron density delocalizes from the donor group to the perylene-unit while the electron density of the LUMO extends from the perylene unit down to the acceptor group. To examine the electron injection and dye regeneration processes between the dyes, the ∆Ginject and ∆Greg were calculated using eqs. 5 and 7, respectively. The results are summarized in Table 2. The ∆Ginject of DL-4 is higher than that of NPS-4. This imply that the semi-rigid D4 donor group allows faster electron injection compared to the conventional tPA. Since the Jsc of a solar cell device is linearly related to the electron injection process as shown in eq. 3, DL-4 is expected to render a relatively higher Jsc value. Moreover, it is well known that a ∆Greg value of 0.2 – 0.3 eV is required for an efficient regeneration of the dye.50, 51 When there is no enough ∆Greg, there is a risk of recombination between the oxidized dye molecule and the injected electrons in the CB of the semiconductor. On one hand, an excessive ∆Greg results in potential loss during the regeneration process. These processes bring negative effect to both Jsc and Voc. The calculated ∆Greg of DL-4 and NPS-4 are 0.470 eV and 0.533 eV, respectively. While both dyes have ∆Greg values that are higher than the limit, DL-4 has lower ∆Greg implying that the potential loss is lesser compared to NPS-4.

18


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Table 2. The calculated spatial orbital overlap (ΛHL), first excitation energy (E0-0, eV), driving force of electron injection (∆Ginject, eV), driving force of dye regeneration (∆Greg, eV), ionization potential (IP, eV), electron affinity (EA, eV), total reorganization energy (λtot, eV), and light harvesting efficiency (LHE) of the dyes. DL-4 NPS-4

ΛHL 0.51 0.54

E0-0 2.44 2.46

∆Ginject 1.217 1.172

∆Greg 0.470 0.533

IP 5.270 5.333

EA 3.027 3.043

λtot 0.725 0.764

LHE 0.934 0.930

The ionization potential (IP) and electron affinity (EA) of the dyes were calculated to further assess the charge-transfer properties. Previous studies have stated that lower IP means favored release of electrons and creation of holes while a small EA value can favor efficient electron transfer from the excited dye to the semiconductor.15 The adiabatic IP and EA are calculated using eqs. 9 and 10 and the values are summarized in Table 2. The IP for DL-4 is lower than NPS-4 indicating that the hole formation is more favored in DL-4. In addition, the smaller EA of DL-4 tells of a better electron injection and less favored recombination between the injected electron and the dye. To further evaluate the charge recombination, the total reorganization energies of the dyes were calculated using the following equation:

= ± − ± + ± −

(13)

where the ± are the energies of the cationic/anionic form derived from the neutral specie of the dyes, ± stands for the energies of the neutrally charged compound calculated from the charged molecular species, and and ± are the neutral and cationic/anionic forms of the molecule at their lowest energies. As explained in detail in our previous paper, a small λtot value is favorable for more effective hole-charge separation which is desirable for preventing the parasitic charge recombination process. 15 Interestingly, the λtot of DL-4 is lower than NPS-4. Thus, employing a semi-rigid tPA is beneficial for reduced rate of charge recombination. Following the evaluation on the absorption and charge-transfer properties, the DL-4 dye having a semi-rigid tPA is expected to render a relatively higher Jsc and Voc values. 19



Page 20 of 44

3.3. Screening of acceptor unit N

N

N

NC COOH

S

N

H3C

H 3C

N

N

S

N

H3C

COOH

COOH

NC

NC

Plz3

Plz2

Plz1

N

N

S

N

NC

N

COOH H3C

N

S

NC

COOH

N S

H3C

Plz4

Plz5

Figure 7. Molecular structure of perylene-based D-π-A dyes. Among the effective ways of extending the absorption of a dye sensitizer to the near IR region is the use of rigid π-linker and extending the π-conjugation. Herein, we introduce five simple dyes designed by forming a combination of N-annulated perylene (PC) and either of ethynylene (ET), benzothiadiazole (BTD), benzene (Bz), thiophene (Th), and cyanoacrylic acid (CA). The molecular structures of these dyes were shown in Figure 7. Perylene and its derivatives has been getting wide attention in designing molecules for photovoltaic applications owing to their excellent photophysical and charge transport properties, as well as attractive thermal and photostability.5, 6 BTD on the other hand has been highly studied in dye-sensitized solar cell designs. Its electron deficient nature makes it an attractive auxiliary acceptor that can improve the light-harvesting property by inducing a noteworthy bathochromic shift along with a significant increase in the molar extinction coefficient.52 The ET moiety has been strategically employed in the designs of molecules for photovoltaic devices because it can systematically enhance the interaction between the neighboring units by providing a planar orientation.53 PLz1 dye is simply composed of a PC unit and a cyanoacrylic acid (CA), as in the 20


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acceptor unit of NPS-4 dye.

In PLz2, the conjugation length was extended by introducing

the auxiliary acceptor BTD between the PC and CA. However, it was found that there is a large strain in PLz2 dye, thus in PLz3 dye, ET unit was inserted between the PC and the BTD to improve the coplanarity between the moieties. On the other hand, studies on the dyes incorporating BTD unit in the π-bridge have shown that employing BTD adjacent to the CA acceptor unit would suffer from electron recombination between the TiO2 and the dye, compromising the charge transfer process.54 Thus, to suppress the charge recombination, Ph and Th units were introduced in PLz4 and PLz5, respectively, between the BTD and the CA. The properties of these dyes were compared to the known HW-1 dye (Figure 2) whose acceptor unit design is adapted by most of the highest performing perylene-based dyes.

3.3.1 Geometric structure, absorption spectra and charge transfer properties of PLz dyes. Based on the ΛHL of the analogues listed in Table 3, ω = 0.150 Bohr-1 is appropriate for the calculation of the UV-Vis absorption spectra of the PLz dyes except for PLz1 whose ΛHL=0.57 and matches with ω = 0.175 Bohr-1. The simulated absorption spectra are shown in Figure 8 showing that in comparison to PLz1, adding the BTD unit between the PC and CA groups of PLz2 ensued a bathochromic shift in the absorption spectrum. However, a decrease in the molar extinction coefficient was also observed, which could be attributed to the large twisting between the PC and the BTD resulting in less effective conjugation. Subsequently, the insertion of ET between the PC and BTD in PLz3 rendered a more than doubled molar extinction coefficient with a slight bathochromic shift in the absorption spectrum compared to the PLz2. This is due to the improved coplanarity in the dye backbone. On the contrary to what was theorized, the extension of the conjugation length by the incorporation of Bz and Th units in PLz4 and PLz5, respectively, did not extend the ICT band. While both PLz4 and 21



Page 22 of 44

PLz5 exhibited an augmented molar extinction coefficient in the ICT transition compared to that of the PLz3, a blue-shift in the ICT band of both dyes was observed. The blue shift in the absorption band of the PLz4 is more intense than that of the PLz5. The variation between the absorption property of PLz4 and PLz5 is due to the difference in the coplanarity along the BTD, Bz or Th, and the cyanoacrylic acid acceptor. As presented in Figure 9, there is an almost coplanar orientation between the BTD and Th units in the PLz5 dye having a measure of dihedral angle of ~2° thus the observed blue-shift is minimal.

Conversely, a dihedral of

~34° has been recorded between the BTD and Bz units in the PLz4 dye, which explains the degree of blue-shifting of the maximum absorption wavelength.

Figure 8. Simulated UV-Vis absorption spectra of PLz dyes with their corresponding λmax calculated at TD-LC-ωPBE/6-31+G(d)//mPWHandHPW91/6-31G(d) in THF solution.

Figure 9. Optimized structure of the dyes calculated at MPWHandHPW91/6-31G(d).

22


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Figure 10. The oxidation potential of the dyes calculated at TD-LC-ωPBE/631+G(d)//mPWHandHPW91/6-31G(d) in THF.

The calculated oxidation potentials of the dyes are presented in Figure 10. The PLz dyes exhibit GSOP values that are more negative than the potential of the I-/I3- redox couple (-4.80 eV vs. vacuum),23 denoting a thermodynamically favorable dye regeneration of the oxidized dye species. Also, because the ESOP of the dyes are situated above the CB of the TiO2 , it is expected that all dyes possess enough driving force for the injection of electron and thus the transfer of electron from the excited dye species to the TiO2 substrate can happen efficiently. With respect to PLz1, the insertion of BTD unit in PLz2 lead to the decrease in the band gap brought by the destabilization of the HOMO level and stabilization of the LUMO level, which tells that the electron deficient nature of the BTD improves the electron withdrawing ability of the acceptor moiety. However, even though the incorporation of ET unit between the PC and BTD units in PLz3 has caused a highly improved coplanarity, such alteration has brought a minimal change in the GSOP and ESOP (0.01 and 0.02 eV, respectively). The extension of the π-conjugation backbone by inserting either Bz or Th in the PLz4 and PLz5, respectively, initiated a destabilization of the corresponding GSOP by 0.07 and 0.06 eV. While the addition of the Bz π-spacer resulted in an increase of the band gap due to the destabilization of the ESOP, the incorporation of the Th π-spacer caused a 23



Page 24 of 44

favorable decrease of the band gap with stabilized ESOP. The position of MO energy levels tells of the energetically favored electron injection and dye regeneration. Figure S1 shows the electronic structures of the HOMO and LUMO of the dyes. Except for PLz1, there is a distinct charge separation along each dye design. The electron density of the HOMO is mainly delocalized on the perylene moiety which means that in this group of dyes, the perylene group acts as the electron donor supported by its electron-rich nature. Conversely, the electron density of the LUMO is primarily delocalized on the BTD unit and distributed down to the anchoring group, indicating an efficient intramolecular charge transfer from the perylene to the acceptor group.

Table 3. The calculated spatial orbital overlap (ΛHL), first excitation energy (E0-0, eV), driving force of electron injection (∆Ginject, eV), driving force of dye regeneration (∆Greg, eV) ionization potential (IP, eV), electron affinity (EA, eV), and the light harvesting efficiency (LHE) of the dyes. Dye

ΛHL

E0-0a

∆Ginject

∆Greg

IP

EA

LHE

PLz1

0.57

2.520

1.156

0.614

5.414

3.000

0.871

PLz2

0.26

2.119

0.954

0.415

5.215

3.440

0.768

PLz3

0.31

2.096

0.927

0.419

5.219

3.603

0.957

PLz4

0.22

2.347

1.243

0.354

5.154

3.118

0.970

PLz5

0.34

2.155

1.047

0.358

5.158

3.403

0.975

HW-1

0.39

2.465

1.304

0.411

5.211

2.940

0.944

a Excitation energy for PLz1 was calculated using ω =0.175 Bohr-1 while the calculation for other dyes was performed using ω=0.150 Bohr-1

The IP values increases as PLz4 < PLz5 < HW-1 < PLz2 < PLz3 < PLz1. The lowest IP value was attained by PLz4, telling of the more favored hole formation in this dye relative to the other dyes within the group. The calculated EA increases in the order: HW-1 < PLz1 < PLz4 < PLz5 < PLz2 < PLz3. The HW-1 dye possesses EA the lowest value. Within the PLz group, the release of electron is fastest for PLz1 followed by PLz4 while the slowest 24


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release of electron is recorded for PLz3. The calculated ∆Ginject value of PLz4 is higher than any of the PLz dyes and is close to that of HW-1 dye. These properties indicate that among the PLz dyes, PLz4 dye has the most favorable electron injection process and is expected to attain a higher Jsc value. In addition, PLz4 has a competently high LHE among the dyes which further supports the estimated Jsc trend.

Figure 11.

The hole density distribution and the distance from the centroid of hole (r) to the TiO2 surface.

Another criteria for Jsc is the efficiency of charge collection (ηcollect). The major competing process of charge collection is the recombination between the photoinjected electron and the oxidized dye. This occurs when the charge separation distance is not sufficient to overcome the electrostatic attraction between the generated holes and electrons. Accordingly, the degree of charge recombination could be estimated by measuring the distance (r) separating the photoinjected electrons in the CB of the TiO2 and hole center of the dye cation as demonstrated by Durant and coworkers.55, 56 Herein, the hole distribution and the coordinates of hole center was located using Multiwfn 3.3.4 software.57 The distance r from the hole center to the surface of the TiO2 was measured as illustrated in Figure 11. The r values for PLz1, PLz2, PLz3, PLz4, PLz5, and HW-1 are 6.23, 9.65, 10.60, 14.55, 12.23 25



and 12.58, respectively. The degree of recombination in PLz5 can be estimated to be comparable to that of HW-1 since these dyes have comparable r values. The hole center of the Plz4 is situated farthest from the TiO2 surface indicative of the least degree of charge recombination between the injected electrons and the positive carriers. The order of calculated ∆Greg are as follows PLz4 (0.354 eV) < PLz5 (0.358 eV) < HW-1 (0.411 eV) < PLz2 (0.415 eV) < PLz3 (0.419 eV) < PLz1 (0.614 eV). PLz4 has ∆Greg value closest to the ideal ∆Greg value needed for efficient regeneration process. As mentioned earlier, it is of intention to further modify the PLz dye by introducing a rigid tPA-based donor group in the next section. Since introduction of a donor group is expected to destabilize the GSOP of the dye, it is worth mentioning that the PLz dyes have ∆Greg values that provide room for destabilization when subjected to further structural modification.

3.3.2 Dye adsorption on TiO2 and the influence of the dipole moment to Voc. The electronic and optical properties of the SCC-DFTB optimized dye-(TiO2)38 complexes were evaluated to gain further understanding on the sensitization mechanism. The use of (TiO2)38 cluster for the simulation of dye adsorption of TiO2 semiconductor surface is well known to provide reliable estimation of dye-TiO2 interaction following the investigation conducted by De Angelis et al. where a good agreement between the experimental and theoretical excitation energy of the TiO2 substrate was attained.58 The optimized geometries are presented in Figure S2 wherein the dyes were adsorbed on the TiO2 surface through a bidentate mode of chemisorption. This anchoring mode was proven to be the most energetically ideal in DSSCs.59, 60 Table S2 lists the adsorption energies (Eads), selected bond lengths as well as the total dipole moments of the isolated dye (µfreedye) 26


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and dye-(TiO2)38 complexes (µdye-TiO2). The resulting Eads are all in positive values and fall in the range of 149.9 – 169.3 kJ mol-1 indicative of strong dye-TiO2 coupling.61 The calculated bond distances for the selected Ti – O bonds were 2.113 – 2.148 Å, consistent with the reported theoretical Ti – O bond distances (2.03 – 2.24 Å) for dye – TiO2 complexes ensuring strong chemisorption of these dyes on the surface of the TiO2.59, 60, 62 The transfer mechanism of the charge carriers can be investigated by examining the electron density of the molecular orbitals involved in the electronic transition.63 The characterization of the electronic transition of the dye-(TiO2)38 complexes are listed in Table S4. The main highest energy transition arises from the HOMO LUMO + n transition. Figure S2 shows the electron density distribution upon photoexcitation of the dyes adsorbed on the TiO2 surface. Accordingly, the HOMOs are delocalized on the perylene units. Except for PLz1 and PLz3, the LUMOs of the dyes are delocalized mainly on TiO2 cluster with contribution from acceptor group. Strong dye-to-TiO2 coupling happens when the acceptor group of the dye and the TiO2 cluster both contributes to the LUMO of the composite, where the electron density is mostly delocalized on the TiO2 cluster for retarded recombination between the injected electron and the oxidized dye.41, 64, 65 We therefore examine the percentage contribution of the acceptor unit and the TiO2 cluster to the main molecular orbitals of the dye-TiO2 complex that participates in the respective excitation process. The contributions of the fragments (donor, (π-bridge)-acceptor, and TiO2 cluster) for the HOMO and LUMO + n were evaluated by Hirshfield method using Multiwfn 3.3.4 program.57 The reliability of this method in evaluating the orbital composition has been previously reported.57, 66 From the results summarized in Table S5, the perylene unit is the primary contributor to the HOMO of the composite system while the π-bridge and acceptor units contribute to the LUMO + n and have negligible contribution to the HOMO. For PLz dyes, the contribution of the TiO2 complex on the LUMO + n is higher than the contribution 27



Page 28 of 44

of the respective acceptor unit. This mode of charge density separation indicates that the electrons are efficiently injected to the TiO2 surface.65, 67 This is in contrary with the HW-1 dye which is likely to have a higher probability of charge recombination most of the contribution to the LUMO + n orbital comes from the acceptor group rather than the TiO2 cluster. It is known that the adsorption of a sensitizer on the surface of TiO2 semiconductor causes a positive shift in the conduction band edge (∆CB) of the TiO2.68 The upshift in the CB edge brings an increase in the Voc of the solar cell device.61 As per eq. 2, the Voc of the dye sensitizer is directly proportional to the ∆CB upon adsorption of the dye. As pointed by Ruhle et al., there is a linear relationship between the ∆CB normal dipole moment of the dye adsorbed on the semiconductor’s surface as in eq. 14. Thus, the ∆CB can be reasonably estimated from the dipole moment of the dye.58, 69

∆CB = −

qµnormalγ

(14)

ε0ε

In the above equation, γ is the dye concentration, and ε0 and ε are the vacuum and dielectric permittivity, respectively. Since the molecular plane of the dyes presented in this paper adapt a nearly perpendicular orientation with respect to the TiO2 surface, the µnormal is assumed to be equal to the total dipole moment of the dyes when chemisorbed to the TiO2( ). The calculated values are listed in Table S2. Among the designed dyes, PLz4 attained the highest dipole moment when adsorbed on the surface of the semiconductor. A clear comparison between the dipole moment of the dyes is shown in Figure 12 which also show the calculated dipole moment of the isolated dyes. Notably, PLz4 adsorbed on the TiO2 surface achieved the highest dipole moment. In addition, the change in the dipole moment from the isolated form and the chemisorbed dye entities is highest in PLz4 dye indicating the 28


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stronger interaction between the dye and the TiO2 semiconductor relative to the other designs. This indicates that the mode of anchorage of PLz4 dye rendered a favorable electronic coupling between the dye and the TiO2 semiconductor. The high dipole moment of PLz4 adsorbed on the TiO2 is expected to afford a sufficiently higher shift of the CB edge of the TiO2 with respect to the redox potential. These phenomena is known to be beneficial to the Voc of the solar cell device based on this dye.69 In addition, studies have shown that having a large dipole moment lead to better dye-to-TiO2 charge transfer as a result of enriched directional flow of the electrons to the TiO2 electrode.70 Therefore, a large dipole moment could aid in averting the charge recombination resulting to a high Voc.

Figure 12. The dipole moment of the free-dyes and dye-(TiO2)38 complexes.

3.4 Design of novel perylene-based D-D- π -A dyes N

N

S

N

NC COOH

N

DP44 S N

N

N

N

S N

N

N

S

DP45

DP46

DP47

Figure 13. Molecular structure of D-D-π-A dyes. From the previous group of dyes, the PLz4 dye gave the most promising properties 29



such as favorable intramolecular charge transfer and high dipole moment for improved Voc. Based on PLz4, another group of dyes were designed by incorporating a donor group and modifying the perylene moiety, as shown in Figure 13. Also, it was pointed out in section 3.2 that the properties of semi-rigid tPA-based D4 donor group is better than the conventional tPA unit. Thus, we introduced the D4 to the outstanding D-π-A system to make a series of dyes with D-D-π-A structure in an effort to design a high-performing novel perylene-based dye. DP44 dye is formed by introducing the electron donating D4 moiety to PLz4 dye. In DP45 and DP46, the perylene unit was modified by attaching a thiophene or quinoxaline, respectively, between D4 and perylene unit. The quinoxaline derivative introduced on DP47 is based on the work of Zhu’s group who demonstrated that the introduction of two alkylsubstituted thiophene units on the quinoxaline with twisted orientation could be beneficial in preventing the redox mediator in approaching the TiO2 surface.22 The conjugation was maintained by tethering the thiophene or quinoxaline derivatives to the perylene moiety.

Figure 14. Simulated UV-Vis absorption spectra of dyes with their corresponding λmax calculated at TD-LC-ωPBE/6-31+G(d)// mPWHandHPW91/6-31G(d) in THF, ω = 0.150 Bohr-1.

Time-dependent DFT calculations were performed to evaluate the charge transfer properties of the dyes and the UV-Vis absorption spectra are shown in Figure 14. The ΛHL values of the DP dye series were calculated to be 0.28 – 0.30 thus ω = 0.150 Bohr-1 was used for the excitation energy calculations of the dyes. As expected, the incorporation of the D4 30


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moiety with PLz4 resulted to the expansion of the absorbance of DP44 dye towards longer wavelength accompanied by the increase of the molar extinction coefficient, a notable proof of the strong electron donating ability of the D4 unit. As summarized in Table S6, the maximal absorption of the DP44 dye arises mainly from HOMO LUMO transition with contributions from HOMO – 1 LUMO and HOMO LUMO + 1 excitations. In Figure S3, the electron density distribution of these molecular orbitals showed that the D4 unit exhibit a significant contribution to the ICT of the DP dyes. This transition gave an oscillator strength higher than that of the PLz4 (1.825 and 1.523 for DP44 and PLz4, respectively) as summarized in Table S3 and Table S6. Since LHE of the dye can be calculated in terms of the oscillator strength as in eq. 4, a higher LHE was attained by DP44 compared to PLz4. The LHE is directly related to the molar absorptivity of the dye expressed as LHE = 1 – 10-A and the absorbance (A) is linearly related to the product of the molar absorptivity (ɛ), film thickness (b) and dye concentration (c) such that A= ɛbc.19 Therefore, the observed increase in the LHE of the dyes is an implication that the addition of the D4 donor effectively enhanced the molar absorptivity by enforcing a more pronounced intramolecular charge transition. The HOMOs extend from the D4 unit to the perylene whereas the LUMOs are delocalized from the BTD unit to the cyanoacrylic anchoring group. An overlap of the HOMO and LUMO electron densities can be seen along the perylene and the BTD unit suggesting an effectual push-pull effect from donor to acceptor unit through the π-spacer.

31



Figure 15. Optimized structure of DP dyes calculated at mPWHandHPW91/6-31G(d).

The addition of the thiophene unit in DP45 decreased the degree of twisting between the D4 and the perylene units by ~21˚ as shown in Figure 15. The enhance coplanarity has brought a broader and bathochromically shifted spectrum with higher molar absorptivity. While the attachment of quinoxaline derivatives in DP46 and DP47 reduced the dihedral angle by only ~5˚, the same degree of bathochromic shift was observed for both dyes.

Figure 16. The calculated oxidation potential of the DP dyes.

32


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Table 4. The calculated spatial orbital overlap (ΛHL), first excitation energy (E0-0, eV), driving force of electron injection (∆Ginject, eV), driving force of dye regeneration (∆Greg, eV), ionization potential (IP, eV), electron affinity (EA, eV), total reorganization energy (λtot, eV), and light harvesting efficiency (LHE) of the dyes. E0-0 2.310

∆Ginject

∆Greg

IP

EA

λtot

LHE

DP44

ΛHL 0.28

1.268

0.292

5.092

3.120

0.763

0.985

DP45

0.28

2.178

1.329

0.099

4.899

3.123

0.790

0.993

DP46

0.30

2.203

1.246

0.207

5.007

3.122

0.717

0.994

DP47

0.30

2.186

1.249

0.188

4.988

3.122

0.723

0.993

Dye

The calculated optical and electronic properties of the DP dyes are summarized in Table 4 and Figure 16. The introduction of the D4 unit in DP44 resulted to the decrease of the band gap accompanied with minor destabilization of the GSOP and the ESOP with respect to the PLz4 dye. The calculated ∆Greg values are 0.292 eV, 0.099 eV, 0.207 eV, and 0.188 eV for DP44, DP45, DP46, and DP47, respectively. As stated in the earlier section, the ∆Greg of the dyes should be within 0.20 – 0.30 eV for a feasible dye regeneration. This means that the dye regeneration process of DP45 dye could be hindered by the high-lying GSOP level. The low rate of dye regeneration could lead to the recombination of the injected electrons and the oxidized dye species, subsequently leading to low Voc. While the ∆Greg of DP47 falls slightly below the threshold limit, the ∆Greg values of DP44 and DP46 perfectly fall within the desirable range denoting sufficient driving force for the regeneration processes of these dyes. The ESOP of the dyes are situated above the CB of the TiO2 providing enough driving force for electron injection (∆Ginject). The calculated values for the ∆Ginject of the dyes are listed in Table 4. While anchorage of the dye sensitizer on the TiO2 substrate could allow an adsorption-induced band bending of the CB, experimental investigations on the effect of band bending on the photochemistry of TiO2 semiconductor revealed that the band bending could occur at ~0.2 to ~0.4 eV positive shift.71, 72 To allow the electron injection process, the ∆Ginject 33



should be at least 0.2 eV.73 It is worth noting that even if there is band bending upon adsorption of these dye designs, these systems will have sufficient driving force for electron injection of ∆Ginject > 0.85 eV. The IP values increases such as DP45 < DP47 < DP46 < DP44. The attachment of thiophene unit in the perylene moiety of DP45 improved the hole generation. The same phenomenon was observed when the perylene was modified by attaching the quinoxaline derivatives in DP46 and DP47. The lowest IP of DP45 suggests that among the DP dyes, the formation of holes is more favored in DP45. The EA values are as follows: DP44 < DP46 = DP47 < DP45. The EA values of these dyes fall in a narrow range (~0.003 eV difference) thus the rate of release of electrons are of comparable degree. In addition, the calculated λtot values increases in the order DP46 < DP47 < DP45 < DP44. While the addition of thiophene in DP45 resulted in the increase of the λtot the incorporation of quinoxaline derivatives in DP46 and DP47 resulted in the significant decrease of the λtot with respect to DP44. Among the dyes, DP46 has the lowest λtot suggesting its more efficient hole-charge separation and lower rate of charge recombination. Considering these findings, DP46 could have the highest Voc benefiting from the lowest estimated rate of recombination and properly aligned energy level.

34


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Figure 17.

The hole density distribution and the distance (r) from the centroid of hole to the TiO2 surface.

The calculated ∆Ginject increases such as DP47 (1.236 eV) < DP46 (1.246 eV) < DP44 (1.268 eV) < DP45 (1.329 eV). Since DP45 attained the highest ∆Ginject, it is expected that DP45 would have the fastest electron injection. In Figure 17, the measured r values are 15.4 Å, 17.0 Å, 17.1 Å, and 17.3 Å, for DP44, DP45, DP46, and DP47 respectively. Compared to PLz4, the r of DP44 dye is higher by ~1.0 Å implying the improvement of charge collection upon attachment of D4 unit to PLz4. Modifying the perylene unit in DP45, DP46, and DP47 has increased the r distance by ~2.0 Å thus an enhanced ηcollect is expected for these dyes with respect to DP44. In addition, the calculated LHE values of the DP45 (0.993), DP46 (0.994), and DP47 (0.993) dyes are higher than DP44 (0.985). Considering the ∆Ginject, r, and LHE as criteria for assessing the Jsc, the expected Jsc of the designed DP dyes would increase such as DP44 < DP47 ≈ DP46 < DP45. Nonetheless, it should not be ignored that the high-lying GSOP of DP45 and DP47 provided an insufficient ∆Greg as previously discussed. The incompetent dye regeneration process could lower the Jsc and Voc. Thus, benefiting from the proper energy level, favorable absorption, charge transfer properties, and highest estimated Voc, the designed DP46 dye is expected to render a 35



Page 36 of 44

relatively higher PCE and is found to be a potential candidate for solar cell application. Most importantly, the calculated properties for the DP44 and DP46 D-D-π-A dyes are better than that of the PLz4 D-π-A prototype signifying the dependable electron donating ability of the designed D4 donor unit.

4. CONCLUSION Density functional theory (DFT) and time-dependent DFT methodologies were used to conduct a computational study of the geometry and electronic structure of the new dye designs based on semi-rigid triphenylamine donor and perylene π-linker.

Initially,

benchmark calculations were performed to identify a proper methodology to calculate the excited-state energy and oxidation potentials of the reference dyes. The results showed that the use of LC-ωPBE and fine-tuned ω value could provide a highly accurate calculation of the excitation while calculation using mPWHandHPW91 method with 6-31G(d) basis set gave high accuracy in reproducing the experimental GSOP of the reference dyes. Analysis of the properties of perylene-based dyes with different tPA-based donor group has revealed that the semi-rigid tPA rendered better properties than the conventional tPA. In addition, a new set of simple D-π-A dyes, named as PLz dyes, were designed by modulating the coplanarity and extending the conjugation length. The use of the rigid perylene as donor/π-linker provided a broad absorption wavelength while the extension of the conjugated backbone using proper moieties afforded tuned properties. Analysis on the dye-TiO2 clusters revealed the strong chemisorption of these dyes on the semiconductor surface. The electron density distribution of the dye-TiO2 clusters showed the favorable charge transfer from the dye to the semiconductor. From the assessment of various parameters, it was concluded that PLz4 dye bestowed the most promising properties. The attachment of the electron donating D4 unit to 36


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the PLz4 resulted in bathochromic shift of the ICT band signifying an effective electron donating ability of the semi-rigid tPA-based D4 donor group. The D-D-π-A DP46 dye displayed the most favorable charge-transfer mechanism and light-harvesting properties and is expected to have the highest Voc and Jsc values within the group making it a promising candidate as a photosensitizer for DSSC application.

SUPPORTING INFORMATION AVAILABLE The supporting information includes: the calculated error and the mean absolute error (MAE) of the DFT methods used in the calculation of the ground-state oxidation potential (GSOP) of the reference dyes; the optimized bond length (Ti – O, Å) adsorption energy (Eads, kJ/mol) and total dipole moment (µ, Debye) of the free-dyes and dyes-(TiO2)38 complexes; the maximum absorption (λmax, nm) and percentage MO character of the orbitals involved in the maximum absorbance of dye-(TiO2)38 complexes of PLz dyes and HW-1 dye; The maximum absorption (λmax, nm) and percentage MO character of the orbitals involved in first singlet excitation of the DP dyes; the electron density distribution of frontier molecular orbitals of the PLz dyes and HW-1 dye; the SCC-DFTB optimized geometries of dye(TiO2)38 with their corresponding spatial orientations of the major contributing orbitals of their highest excitation energy; and the electron distribution of the orbitals involved in first singlet excitation of the DP dyes.

ACKNOWLEDGEMENT This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF-2017R1D1A1B03028356) and the KISTI supercomputing center 37



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