New Insights into Organic Dye Regeneration Mechanism in Dye

Research Article pubs.acs.org/journal/ascecg

New Insights into Organic Dye Regeneration Mechanism in DyeSensitized Solar Cells: A Theoretical Study Yohannes Mulugeta Hailu,† Wan-Ru Shie,† Santhanamoorthi Nachimuthu,* and Jyh-Chiang Jiang* Department of Chemical Engineering, National Taiwan University of Science and Technology, No. 43, Keelung Road, Sec. 4, Da’an District, Taipei City 10607, Taiwan, ROC S Supporting Information *

ABSTRACT: Understanding the regeneration mechanism of an oxidized dye with redox couples is significant in improving the efficiency of dye-sensitized solar cells (DSSCs). However, all the previously considered regeneration mechanisms have failed to match the incident photon-to-current efficiency (IPCE) and the UV−vis absorption spectra. Herein, we propose a new regeneration mechanism for organic dyes by assuming the probability of second electron injection from the stable dye−iodide intermediate complex. Thus, the stability and optoelectronic properties of the dye−iodide intermediate complexes of our elementary steps for the regeneration mechanism have been systematically investigated using density functional theory (DFT) and time-dependent TD-DFT calculations. Our results highlight the possibility of two electron injection into the semiconductor surface during dye regeneration, rather than the conventional one electron injection. In addition, the calculated UV−vis absorption spectra for the considered intermediate complexes clearly indicate blue/red-shift compared to that of isolated dye, which reflects the IPCE spectra. KEYWORDS: Dye-sensitized solar cells, Dye regeneration mechanism, Two-electron injection, DFT, TD-DFT, UV−vis spectra, Incident photon-to-current efficiency

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INTRODUCTION

the environment, metal-free organic dyes have attracted great attention in recent years.8−12 The overall photovoltaic performance of DSSCs depends on many factors, such as the molecular design of the dyes, dye adsorption characteristics on the semiconductor surface, and dye aggregation.3,13,14 In the past, a wide variety of efforts has been made to improve the efficiency of DSSCs based on organic sensitizers which include substituting suitable donor/ acceptor moieties for enhancing the optical properties of dyes and modifying the dye structure to suppress the dye aggregation.9,10 Recently, a carboxy-anchor organic dye

Dye-sensitized solar cells (DSSCs) are promising nextgeneration solar cells because of their low production cost, easy fabrication procedures, and high light-to-electricity conversion.1 The operation of a DSSC begins with an excitation of adsorbed dye when the cell is illuminated and the excited electrons are injected into the semiconductor surface. Then, the excited cationic dye is restored to its ground state by means of accepting an electron from the redox electrolyte, which is further returned by receiving electrons from the counterelectrode.2−4 Previous studies on DSSCs with metal sensitizers (Ru and Zn porphyrins) show the highest photoconversion efficiencies of 11−13%.5−7 However, taking the limited noble metal into account and the corresponding adverse impact on © 2017 American Chemical Society

Received: April 17, 2017 Revised: August 20, 2017 Published: August 30, 2017 8619

DOI: 10.1021/acssuschemeng.7b01174 ACS Sustainable Chem. Eng. 2017, 5, 8619−8629

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ACS Sustainable Chemistry & Engineering Scheme 1. Proposed Mechanisms for Photo-oxidized Dye Regeneration

complex in step 2 has high stability and long lifetime. Thus, it can absorb light and inject another electron rather than dissociate into dye and I2•−. The stability, reactivity, and the absorption spectra of the intermediate complexes of the proposed regeneration mechanism have been investigated using the density functional theory (DFT) calculations. Computational Details. The geometries of considered organic dye and dye−iodide complexes were optimized using long-range corrected hybrid density functional method ωB97XD35 in the gas phase. The polarized split-valence triple-ζ 6-311G(d, p)36 basis set is employed for C, H, O, and N atoms, whereas a double-ζ basis set LANL2DZ37 is employed for I atom in all of our calculations. Further, frequency calculations have been performed at the same level of theory to verify the nature of the saddle point. To find the appropriate interaction sites for iodide on the dye, we have calculated Fukui function using the Natural bond orbital (NBO)38 method at the same level of theory. In order to choose more reliable DFT functional for calculating the absorption properties, we have performed benchmark calculations for three organic dyes (L1, L2)39 and (TT2A),40 which are analogous to the present system using different DFT functionals such as B3LYP, M062X, CAM-B3LYP, and ωB97XD in acetonitrile (for L1 and L2) and dichloromethane (for TT2A) solvents. The conductor-like polarizable continuum model (CPCM)41,42 was used to include the solvent effects. All the values discussed in the text are those from the gas-phase calculations unless otherwise stated. The calculated absorption energies along with previous experimental values are given in Table S1 of Supporting Information. The benchmark results show that ωB97XD method provides better performance in predicting the absorption maximum of the reference system than the other functionals. Hence, we have used the timedependent TD-DFT calculations by employing ωB97XD functional to simulate the 20 lowest singlet−singlet excitations of dye−iodide complexes. The simulated UV−vis absorption spectra were described using the Gaussian broadening with half-widths of 4000 cm−1, as shown in the following equation:10

cophotosensitized with an alkoxy silyl anchor dye in conjunction with a cobalt electrolyte showed a record efficiency over 14%.15 Even though the dye design is significant, electron injection from the dye to the surface and dye regeneration by a redox couple are considered to be at the heart of DSSC function.16−19 The process of electron injection from the dye to the surface has been investigated extensively, and it has been found that it occurs in a femtosecond time period.20−22 However, the regeneration mechanism of the dye sensitizer by the redox couple draws lots of attention of researchers in recent years.7,18,19,23−26 Although there are some previous experimental and computational studies related to dye regeneration,16,18,19,26−29 the mechanism of the complete reduction of photo-oxidized dye by a redox couple is still not explained clearly. To achieve a higher efficiency, the cationic dyes need to be reduced by electrolytes faster than the injected electrons. In DSSCs, I3−/I− is one of the widely used redox couples, and most of the previous studies16,19,29−31 considered that it could regenerate the oxidized dyes by the following steps; dye•+ + I− → (dye+ ··· I−)•

(1)

(dye+ ··· I−)• + I− → dye + I 2•−

(2)

2I 2•− → I− + I3−

(3)

and they found that step 1 is the rate-limiting step. Also, recently, Martiniani et al.29 reported that step 2 is the ratelimiting step for some organic dyes. All the previously proposed dye regeneration mechanisms considered one electron injection from the oxidized dye to the semiconductor surface; however, all of these studies have a discrepancy between the incident photon to current conversion efficiency (IPCE) spectrum with that of the absorption spectra of dyes. In general, IPCE plateaus are broader and covers the entire visible region, whereas the UV−vis spectra of the dye sensitizers have absorption maximum only in between these regions.32−34 Therefore, it is necessary to investigate the dye containing iodide ion complexes in detail to understand the real regeneration mechanism involved in the DSSCs. Thus, in this study, we propose a new regeneration mechanism which consists of second electron injection from dye to the semiconductor surface. Herein, we presumed that the [dyeI2]•− intermediate

ε(ω) = C1 ∑ 1

8620

⎛ (ω − ω1)2 ⎞ ⎟⎟ exp⎜⎜ −2.773 2 Δ1/2, I Δ1/2, ⎝ ⎠ I f1


Research Article

ACS Sustainable Chemistry & Engineering where ε is the molar extinction coefficient, ω is the energy of all the allowed transitions, f1 is the oscillator strength, and Δ1/2 is the half-bandwidths. All the calculations in this study were carried out using Gaussian 09 package.43

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RESULTS AND DISCUSSION Proposed Regeneration Mechanism. The oxidized dye can be regenerated either by the single iodide process (SIP)18,25 or via the two-iodide process (TIP).16,19,25 However, the recent studies19,30,44 confirm that two iodide ions are required to reduce the oxidized organic dye completely. Besides, all of these studies on both metal, and metal-free organic dyes, described that the TIP dye regeneration mechanism proceeds via one electron injection (we refer to this as first electron injection) to the semiconductor surface. In their mechanism, they considered the cationic dye (dye•+) initially reacts with I− and formed the complex [dyeI2]•; then with the reaction of the second I− ion, the complex [dyeI2]•− is formed, which further dissociate into dye and I2•−. Conversely, here we assume that the complex [dyeI2]•− should be stable based on thermodynamic and kinetic perspectives. Hence, it can absorb light and inject another electron (we referred as second electron injection) to the TiO2 surface rather than dissociate into dye and I2•−. The proposed detailed regeneration mechanism is illustrated in Scheme 1. As shown in Scheme 1, we considered two reaction pathways, I and II, for both first and second electron injections, respectively. In the reaction pathway I, the complex [dyeI]• is formed through two ways: as described in elementary steps I1 and I2, either the neutral dye absorbs photon, then injects the first electron and thereby the reaction of first I−, [dyeI]• is formed or the neutral dye initially interacts with first I−, then by injecting the first electron after absorbing photon, the [dyeI]• complex is formed. Similarly, in the reaction pathway II, we assumed that the complete regeneration of dye follows in two paths (II1 and II2) from the stable intermediate [dyeI]•. In pathway II1, the intermediate [dyeI]• reacts with the second I− and forms complex [dyeI2]•− (elementary step II1−1) and then it absorbs photon for the second electron injection. After the second electron injection, the [dyeI2] complex is formed (elementary step II1−2), which will further dissociate into dye and I2. On the other hand, in reaction pathway II2, the complex [dyeI]• absorbs light and injects the second electron, which yields [dyeI]+ intermediate (elementary step II2−1); then by reacting with the second I− to yield [dyeI2] (elementary step II2−2) and further dissociates into dye and I2. All the intermediates in both the reaction pathways have been taken into consideration at different possible interaction sites for iodide/iodine on dye and dye•+. Interaction of Dye with Iodide/Iodine Complexes. To validate our proposed regeneration mechanism, we considered a model D−π−A type of organic dye with 4-methoxy-N-(4methoxyphenyl)-N-phenyl benzene amine (MPBA) as an electron donor, thiophene (T) as the π-bridge, thienopyrazine (TP) as an electron acceptor, and COOH as an anchoring group. The chemical structure of the considered dye for this study is shown in Figure 1. The ground-state geometries of the isolated dye and dye-iodide complexes were optimized using the ωB97XD method. The optimized geometry of the isolated dye is given in Figure S1 of the Supporting Information. The selected geometrical parameters of the optimized isolated neutral, cationic and the stable dye-iodide complexes are summarized in Table S2 in the Supporting Information. As can be seen from Table S2, the bond lengths r1, r2, and r3 of the

Figure 1. Chemical structure of triphenylamine, thiophene, thienopyrazine based D−π−A organic dye.

oxidized dye (dye•+) are shorter than neutral dye, indicating the better conjugation of oxidized dye. The shortening of r1 leads to increase the sp2 character of the nitrogen atom in the donor moiety (TPA). It is further confirmed by the calculated atomic charges of a nitrogen atom, which is decreased after the dye oxidized (Table S3). [dyeI]− Complex. As described in the elementary step I2−1 of Scheme 1, the neural dye interacts with I− and forms a stable [dyeI]− complex. The suitable interaction sites for the iodide ion (I−) with dye are determined based on the Fukui functions,45 which are calculated by employing the atomic charges obtained from natural bond orbital analysis.38 The calculation method of Fukui function and the obtained values are given in Supporting Information Table S4. The site which has strong electrophilic tendency can attract nucleophiles,24 as a result, we considered the three strong interaction sites for the investigation of regeneration mechanism. Previously, most of the experimental studies46,47 have considered the substitution of long alkyl side-chain in the π-linker moiety to avoid the iodide interaction with the surface and inhibit the charge recombination. Hence, in this study, we considered the iodide ion interaction sites only on the donor part of dye. The optimized structures of [dyeI]− complexes with different I− binding sites (referred as [dyeI]−A1, [dyeI]−A2, and [dyeI]−A3) are shown in Figure 2. It is found that for the [dyeI]−A1 and [dyeI]−A2 complexes, I− strongly interacts with the neutral dye at the side position of the electrophilic carbon of the phenyl ring which is attached to the methoxy groups in the TPA moiety. Whereas, for the [dyeI]−A3 complex, the I− interacts above the nitrogen

Figure 2. Optimized geometries of [dyeI]−complexes for different iodide ion interaction sites. Carbon atoms are in gray, hydrogen in white, nitrogen in blue, oxygen in red, iodine in purple, and sulfur in yellow (The dashed lines represent the distance between dye and iodide atom, and all distances are in angstroms). 8621


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ACS Sustainable Chemistry & Engineering atom in the TPA moiety. The intermolecular distances between the dye and I− corresponding to three different binding sites are shown in Figure 2, which are shorter than the conventional van der Waals interaction (3.85 Å)48,49 indicating the strong charge−dipole interaction between them. The relative stability based on the free energies is [dyeI]−A2 > [dyeI]−A1 > [dyeI]−A3. This result indicated that the interaction of iodide ion on the above the nitrogen atom of neutral dye ([dyeI]−A3 complex) is thermodynamically less stable compared to another sites, which may be due to the repulsion between the lone-pair electrons of nitrogen atom and the I− ion. Further, the calculated NBO charges are given in Table 1, which shows that the small amount of charge has been transferred from I− to the neutral dye.

Figure 3. Optimized geometries of [dyeI]• complexes for different iodide ion interaction sites. Carbon atoms are in gray, hydrogen in white, nitrogen in blue, oxygen in red, iodine in purple, and sulfur in yellow (The dashed lines represent the distance between dye and iodide atom, and all distances are in angstroms).

Table 1. Bond Distances of I−I (Å), Relative Free Energies (ΔGrel in kcal/mol), and the Atomic Charge of Iodide Ions (e−) for [dyeI]−, [dyeI]•, [dyeI2]•−, and [dyeI2] Intermediate Complexes bond distances complexes [dyeI]−A1 [dyeI]−A2 [dyeI]−A3 [dyeI]•B1 [dyeI]•B2 [dyeI]•B3 [dyeI2]•− C1 [dyeI2]•− C2 [dyeI2]•− C3 [dyeI2]D1 [dyeI2]D2 [dyeI2]D3

atomic charge

I−I

ΔGrel

first I−

second I−

3.37 3.36 3.37 2.81 2.81 2.81

0.47 0.00 3.10 2.04 2.02 0.00 0.29 0.00 1.99 1.09 0.00 0.72

−0.958 −0.962 −0.949 −0.239 −0.237 −0.236 −0.494 −0.494 −0.493 0.004 0.016 0.018

−0.494 −0.495 −0.495 0.003 −0.008 −0.011

demonstrating the intermolecular electron transfer from the first I− ion to the oxidized dye. The calculated relative free energies for the [dyeI]• complexes are given in Table 1, and the order of relative stability is [dyeI]•B3 > [dyeI]•B2 > [dyeI]•B1. As can be seen from Table 1, the NBO charges of I• in the [dyeI]• complexes are about −0.24 e, indicating some amount of charge has transferred from the iodide ion to dye•+. Even though charge has been transferred from iodide ion to the radicaloid cationic dye (dye•+), the oxidized dye has not completely regenerated due to the insufficient charge transfer. Thus, the calculated results show that the [dyeI]• intermediate still needs to interact with another iodide ion in order to be reduced completely. This result confirms an earlier study30 that organic dyes need to be regenerated using two-iodide process (TIP). Therefore, we further considered the second iodide ion interaction with [dyeI]• complexes to complete the regeneration mechanism. [dyeI2]•− Complex. To investigate the reaction of second I− ion with [dyeI]• complex (elementary step II2−2), we have considered the different reaction sites for the second I− ion and found that it prefers to react with the first I− ion in the [dyeI]• complex. Thus, we have considered three [dyeI2]•− complexes (C1, C2, and C3) and the optimized structures are shown in Figure 4. It has been observed that the order of stability based on the relative free energies for these complexes are [dyeI2]•− C3 < •− [dyeI2]•− C1 < [dyeI2]C2 . The I−I interatomic distances are found to be in the range of 3.36−3.37 Å, which are comparable to that of isolated I•− 2 molecule optimized at the same level of theory. The calculated NBO charges show that the atomic charge of the first iodine atom varies from −0.25 to −0.494 e in the [dyeI2]•− complexes, indicating that there is a significant amount of charge has been transferred simultaneously from the second iodide ion to the first I• and also to the oxidized dye in the [dyeI]• complexes. Further, the sum of two iodide atomic charges (−1 e) confirms the formation of diiodide radical ion (I•− 2 ) molecule in the complex. The calculated structural parameters of the [dyeI2]•− complexes are summarized in Table S2 of the Supporting Information. From Table S2, the bond lengths (r1, r2, and r3) in the [dyeI2]•− complexes are found to be elongated compared to those of the oxidized dye (dye•+) and those values are similar to the neutral dye, revealing the reduction of the oxidized dye by second iodide ion interaction.

[dyeI]• Complex. As illustrated in Scheme 1, the intermediate [dyeI]• is formed via two ways; as mentioned in elementary step I1−2, after the first electron injection from the dye to the semiconductor surface, the cationic dye reacts with I− and forms [dyeI]• complex, or the intermediate [dyeI]− absorbs photon and subsequently injects the first electron to the surface, which yields the [dyeI]• complex as illustrated in elementary step I2−2. For the elementary step I1−2, we have considered the different sites at donor part for the interaction of I− with cationic dye and selected preferred electrophilic sites. Additionally, for the elementary step I2−2, we considered the similar reaction sites of [dyeI]− complexes to obtain [dyeI]• intermediate complexes. Our results showed that the formation of the [dyeI]• complex in both the reaction pathways is almost in similar geometry. The considered optimized structures of [dyeI]• complexes ([dyeI]•B1, [dyeI]•B2, and [dyeI]•B3) are shown in Figure 3. As can be seen from Figures 2 and 3, the geometry of dye in the [dyeI]• complexes altered after the electron injection, for instance, the interaction distance between the iodide ion and the dye has significantly changed compared to the [dyeI]− complexes. The observed interaction distances between the dye/dye•+ and I− in [dyeI]•B1, [dyeI]•B2, and [dyeI]•B3 complexes (Figure 3) are shorter than that of the van der Waals interaction, indicating the existence of strong Coulomb attraction between the positively charged TPA moiety and I−. Further, from the Table S2, the bond lengths (r1, r2, and r3) of the [dyeI]• complexes are found to be elongated compared to that of the oxidized dye (dye•+), 8622


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stable [dyeI2]•− complex. The calculated NBO charges for [dyeI2] complex (in elementary step II1−2 and II2−2, Table 1) indicate that the charges of I2 are close to zero revealing that the oxidized dye completely reduced after the second electron injection. In addition, the calculated I−I distance is shortened to 2.81 from 3.38 Å, which is close to the isolated I2 molecule in the gas phase, confirming the formation of I2 from I•− 2 . Also, the calculated bond lengths of dye moiety in the [dyeI2] complexes are found to be similar to those of the neutral dye (Table S2), which further confirms the reduction of the photo-oxidized dye. Hence, the dye regeneration is completed after the second electron injection. Further, I2 in the electrolyte solution reacts with I− and forms I−3 ; then, I−3 ion diffuses to the counter electrode, where it is reduced.16,19,26 To confirm the [dyeI2] complex dissociation, we have calculated the reaction barrier for the dissociation reaction of [dyeI2] into dye and I2. The calculated potential energy profile for this dissociation reaction is shown in Supporting Information Figure S3. The calculated dissociation barrier is 0.29 eV, and the reaction is exothermic by 0.55 eV. The optimized initial, transition, and final state geometries of dissociation reaction are shown in the Figure S4. The I−I bond distance is found to be 4.62 Å in the initial [dyeI2] complex, and it is shortened to 4.46 Å in the transition state, and further, it is shortened to 2.81 Å in the final dissociated complex, which is similar to the isolated iodine molecule.51 Dye Regeneration Energetics. To understand the proposed dye regeneration mechanism, the reaction free energy (ΔGrxn) and interaction free energy (ΔGint) for the formation of intermediate complexes via considered elementary steps corresponding to different interaction sites are calculated, and the values are listed in Tables 2 and 3, respectively. As can be seen in Table 2, the reaction free energies for the formation of [dyeI]• and [dyeI2]•− intermediate complexes (elementary steps I1 and II1−1, respectively) are highly exergonic, revealing that these intermediate complexes can be formed easily. In addition, we found that the reaction free energies for the formation of [dyeI2] complexes (via reaction pathway II2−2) are more negative (−2.83 to −3.52 eV), specifying the tendency of this reaction which favors the formation of [dyeI2] complexes from [dyeI]+ and I−. Further, the calculated interaction free energies (ΔGint) for the [dyeI]− and [dyeI2]•− complexes are negative for all the three different interaction sites, indicating that the neutral dye tends to associate with I− or I•− 2 to form an attractive [dyeI]− or [dyeI2]•− intermediates, and the formation of these complexes is favorable. However, the calculated ΔGint of [dyeI 2 ] is endergonic, which confirms the further dissociation of this complex into dye and I2. Previously, most

Figure 4. Optimized geometries of [dyeI2]•− complexes for different iodide ion interaction sites. Carbon atoms are in gray, hydrogen in white, nitrogen in blue, oxygen in red, iodine in purple, and sulfur in yellow (The dashed lines represent the distance between two iodide atom, and all distances are in angstroms).

Moreover, to confirm this [dyeI2]•− complex formation, we also considered the interaction of second iodide ion with other possible sites in the [dyeI]• complex and investigated the recombination of two iodide ions. The potential energy profile for the recombination of iodide ions is illustrated in Figure S2 of the Supporting Information. As shown in Figure S2, the recombination of iodide ions in all the sites occurs spontaneously because of the large exothermicity and low energy barrier (∼0.1−0.2 eV). The calculated recombination barriers are in agreement with the previous reports.48,50 In addition, the calculated reaction free energy is highly exothermic confirming that the second iodide ion prefers to interact with the first iodide ion in the [dyeI]• complexes. Further, the NBO charges of the iodide ions in the reactants, transition states, and final products are calculated during the recombination reactions, and the values are summarized in Table S5 in the Supporting Information. It has been observed that, even though the second iodide ion interacts with the other sites of [dyeI]• complexes in the reactant, there is almost no charge transfer occurred. However, when the second iodide ion approaches near to the first iodide ion in the final product ([dyeI2]•− complex), we noticed that significant amount of charge has been transferred. Further, the calculated I−I interatomic distances in the [dyeI2]•− complexes are close to the bond length of isolated I•− 2 (3.37 Å). [dyeI2] Complex. According to our regeneration mechanism in elementary step II1−2 and step II2−2, the complex [dyeI2] is formed after the second electron injection by the

Table 2. Calculated Reaction Free Energies (ΔGrxn in eV)a for the Proposed Elementary Steps of the Dye Regeneration Mechanism ΔGGrxn

reaction complexes b

dye•+ + I− → [dyeI]• •

−

+

−

•−

[dyeI] + I → [dyeI 2]

[dyeI] + I → [dyeI 2]

ΔGACN rxn

ΔGEtOH rxn

(B1)

(B2)

(B3)

(B1)

(B2)

(B3)

(B1)

(B2)

(B3)

−3.17

−3.18

−3.26

0.09

0.09

0.05

0.02

0.03

0.01

(C1)b

(C2)

(C3)

(C1)

(C2)

(C3)

(C1)

(C2)

(C3)

−1.31

−1.32

−1.15

0.06

0.19

0.11

0.10

0.09

0.12

(E1)b

(E2)

(E3)

(E1)

(E2)

(E3)

(E1)

(E2)

(E3)

−2.83

−3.33

−3.52

−0.14

−0.25

−0.18

−0.07

−0.19

−0.25

a

The superscript indices to the reaction free energy (ΔGrxn) have the following meaning: G = gas phase, ACN = in acetonitrile solution, and EtOH = in ethanol solution. bLabeling in the parentheses denotes the three different I− interaction sites. 8623


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Table 3. Calculated Interaction Free Energies (ΔGint in eV)a for the Proposed Elementary Steps of Dye Regeneration Mechanism ΔGGint

interaction complexes (A1)

dye + I− → [dyeI]− dye +

I•− 2

−0.37 •−

→ [dyeI 2]

dye + I 2 → [dyeI 2]

b

ΔGACN int

ΔGEtOH int

(A2)

(A3)

(A1)

(A2)

(A3)

(A1)

(A2)

(A3)

−0.39

−0.30

0.09

0.11

0.10

0.14

0.14

0.15

(C1)b

(C2)

(C3)

(C1)

(C2)

(C3)

(C1)

(C2)

(C3)

−0.22

−0.23

−0.15

0.09

0.06

0.05

0.13

0.11

0.06

(D1)b

(D2)

(D3)

(D1)

(D2)

(D3)

(D1)

(D2)

(D3)

0.08

0.04

0.09

0.65

0.69

0.15

0.10

0.11

0.13

a

The superscript indices to the interaction free energy (ΔGint) have the following meaning: G = gas phase, ACN = in acetonitrile solution, and EtOH = in ethanol solution. bLabeling in the parentheses denotes the three different I− interaction sites.

the dye−iodide complexes are shown in Figure 5, and the corresponding absorption energies along with molecular orbital (MO) compositions of vertical transitions for these complexes are tabulated in Table 4. [dyeI]− Complex. As can be seen in Figure 5a, the simulated absorption spectra corresponding to different interaction sites of [dyeI]− complex (A1, A2, and A3) show strong absorption band in the visible region. The maximum absorption wavelength (λmax) of [dyeI]− complexes are 575, 550, and 579 nm, respectively, which is red-shifted by 70, 45, and 74 nm, respectively, compared to isolated dye. In determining the charge-separated states of dye sensitizers, the frontier molecular orbital (FMO) contribution is the most significant. The frontier molecular orbitals correspond to their transitions are plotted in Figure 6, revealing that [dyeI]−complexes have intramolecular charge transfer (ICT) characteristics.58 It is found that the maximum absorption of the [dyeI]− (A1, A2, and A3) complexes are due to the transition from HOMO − 3 to LUMO. Figure 6 shows that the HOMO − 3 of the [dyeI]− complexes mainly delocalized on the donor part (TPA moiety) with some contribution from adjacent π-bridge (thiophene). Meanwhile, the LUMOs are delocalized over the acceptor and adjacent anchoring groups. This ICT characteristic leads to increase the electronic coupling between the dye and surface and thereby increase in electron injection efficiency. However, the absorption maximum for these complexes is in the longer wavelength (∼550−600 nm) region only, whereas typical IPCE spectra have broader absorption bands in the visible region. [dyeI]• Complexes. The simulated absorption spectrum of the oxidized dye with iodide complexes ([dyeI]•) display distinct absorption peaks in the wavelength region of ∼480− 525 nm. The absorption peak in this wavelength region corresponds to ICT transition from the oxidized TPA moiety and iodide complex to acceptor part, as illustrated in the FMO plot (Figure 7). Figure 7 shows HOMOs of three different [dyeI]• complexes are mainly delocalized on the TPA moiety and adjacent π-bridge with some contribution of iodide ion. Meanwhile, the LUMOs are delocalized over the acceptor and adjacent anchoring groups (carboxylic acid) for the (α spin) in all three [dyeI]• complexes. Also, it has been observed that the absorption spectra of [dyeI]•(B1 and B2) complexes are slightly • blue-shifted, whereas the absorption spectra of [dyeI]B3 complex is red-shifted compared to that of neutral dye (Figure 5b). [dyeI2]•− Complex. The calculated UV−vis absorption spectra of [dyeI2]•− complexes display two distinct absorption bands as illustrated in Figure 5c. As can be seen from Figure 5c, the absorption spectra of [dyeI2]•− complexes have dual band

of the theoretical studies used the standard large core effective core potential (ECP) basis set LANL2DZ28,37,52 for the interaction studies of iodine atom with dyes. However, earlier studies point out that the small-core relativistic effective core potentials show better agreement than large core ECPs.53−56 Hence, for the comparison, we also considered the small-core Stuttgart-family ECPs basis set SDDAll and calculated the reaction free energies (ΔGrxn) and interaction free energies (ΔGint), for our proposed elementary steps. The calculated results are summarized in Tables S6 and S7 of the Supporting Information. The data showed that the LC-ECP basis set overestimates the energetics by ∼0.25 eV compared with that of SC-ECP. However, the trend of the values calculated with LANL2DZ basis set is in agreement with that of the small-core SDDAll basis set. Recently, Schreckenbach57 investigated the influence of solvation effects on the reaction equilibria with many examples and found that solvation strongly stabilizes the charged species than neutral in the reaction. Thus, to know the solvent effects on the dye regeneration mechanism, we have calculated the reaction free energies (ΔGrxn) and interaction free energies (ΔGint) of our proposed regeneration elementary steps in acetonitrile (ACN) and ethanol (EtOH) solvent phases using conductor-like polarizable continuum model (CPCM) in selfconsistent reaction field (SCRF) approach at the same level of theory. The calculated values are listed in the Tables 2 and 3, respectively. As can be seen from ΔGrxn values in Table 2, the solvation incredibly stabilize the left-hand side of the reactions that contain the charged species, which is in agreement with the previous studies.50,57 However, the calculated ΔGrxn values in both the solvents are close to zero, indicating that the contribution from both reactant and product are significant in these reactions. Further, as can be seen from Table 3, the calculated interaction free energies of this [dyeI2]•− complexes are exergonic in gas phase, whereas they are endergonic in solvents. Eventhough, this interaction is endergonic in solvents, the obtained interaction free energies (ΔGint) are very minimum (almost close to zero), demonstrating that this [dyeI2]•− complex is stable and we expect that it can absorb another photon and then inject the second electron to the semiconductor surface, rather than dissociate into dye and I•− 2 . Also, the calculated ΔGint for the [dyeI2] complexes is found to be highly endogenic (∼0.69 eV) in ACN (see Table 3), indicating that the dissociation of this complex into dye and I2 are favorable. UV−vis Absorption Spectra. The UV−vis absorption spectra for all the intermediate complexes were simulated using TD-DFT with the ωB97XD method in the gas phase. The simulated absorption spectra for three different binding sites of 8624


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the longer wavelength regions is 570, 538, and 573 nm for the [dyeI2]•− (C1, C2, and C3) complexes respectively, which are significantly red-shifted by ∼30−70 nm compared to that of neutral dye. We found that the complexes which have ICT characteristics via iodide ion have a larger red-shift in absorption spectra, which are in agreement with the previous results.28 Interestingly, we noticed that the absorption bands of [dyeI2]•− complexes in shorter wavelength region are blueshifted to that of neutral dye, which has never been discussed in the previously proposed regeneration mechanism studies.28,32−34 Recently, Zhu et al.28 also investigated the dye regeneration mechanism in gallium based organic dyes and reported only the red-shift in the absorption spectra of [dyeI2]•− complex than that of neutral dye. The simulated absorption spectra revealed that the characteristics of a dual band of intermediate [dyeI2]•− complexes cover a broad absorption spectrum in the visible region, which is consistent with the IPCE spectra. Furthermore, we also calculated the absorption energies of intermediate complexes of elementary steps using small-core SDDAll basis set and the results are summarized in the Table S8 of the Supporting Information. We found that there is no significant difference in the absorption energies calculated with LANL2DZ and SDDAll basis sets, which confirms the reliability of LANL2DZ. The calculated molecular orbital energy levels of the neutral dye and the intermediate complexes of our proposed regeneration mechanism for first and second electron injection ([dyeI]• and [dyeI2]•−) are illustrated in Figure 9 along with the conduction band of TiO2. As can be seen from Figure 9, after the first electron injection, the HOMOs of the [dyeI]• complexes are slightly shifted downward compared to the neutral dye; whereas, LUMOs remains same. We found that the calculated HOMO−LUMO energy gaps of [dyeI]• complexes are higher than the neutral dye, which is consistent with the observed shift in the absorption spectrum (Figure 5b). Further, it has been observed that both HOMO and LUMO levels of [dyeI2]•− complexes shifted upward significantly, indicating the potential electron transfer after second I− ion interaction. The calculated HOMO−LUMO energy gaps for [dyeI2]•− complexes are lower than the [dyeI]• complexes, which leads to a red shift in the absorption spectrum (Figure 5c). Our calculated results indicate that the second electron injection plays a key role in influencing the energy levels and the overall conversion efficiency. The IPCE of a photovoltaic device can be related to light harvesting efficiency (LHE), the quantum yield (Φinj) of electron injection, and the electron collection efficiency (ηcoll) of the injected charge;58−60 and this can be expressed as

Figure 5. Simulated UV−vis absorption spectra of (a) [dyeI]−, (b) [dyeI]•, (c) [dyeI2]•− intermediate complexes along with the neutral dye.

characteristics: one in the shorter wavelength region (∼440− 460 nm) and the other in the longer wavelength region (∼530−585 nm). The FMO plots (Table 4 and Figure 8) of these complexes indicate that absorption band in the shorter wavelength region corresponds to π−π* intramolecular charge transfer transition, whereas, the strong absorption peak in the longer wavelength region is due to ICT transition. Figure 8 shows the HOMO of [dyeI2]•− C1 and C2 complexes mainly delocalized on the iodide ion and TPA donor part connected with the π-linker. Whereas, the HOMO (β spin) of [dyeI2]•− C3 complex is predominantly localized on the diiodide radical ion •− (I•− 2 ), indicates that the [dyeI2]C3 complex could facilitate ICT from the I•− ion to the acceptor part compared to [dyeI2]•− C1 2 and C2 complexes. The LUMO (α and β spin) mainly localized on the anchoring and acceptor part for all interaction site of [dyeI2]•−complexes. The calculated absorption maximum in

IPCE(λ) = LHE(λ)Φinjηcoll

Commonly, the light harvesting efficiency (LHE) of the dye largely affects the IPCE,61,62 and it can be calculated by10,63 LHE(λ) = 1 − 10−f

where f is the oscillator strength of the maximum absorption band of dye. The calculated LHE values for the [dyeI]− and [dyeI2]•− complexes are presented in Table 4. As can be seen in Table 4, the LHE values for [dyeI]−A1, [dyeI]−A2, and [dyeI]−A3, are 0.81, 0.87, and 0.78 respectively. Based on our calculations, it was found that the [dyeI]−A2 complex is the most stable compared to other interaction sites (Table 3), which has larger •− LHE value. Also, the LHE of complexes [dyeI2]•− C1 , [dyeI2]C2 , 8625


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Table 4. Calculated Absorption Energies (λ in nm), Oscillator Strengths (f in au), Light Harvesting Efficiencies (LHE), and the Corresponding MO Transition of Dye−Iodide Complexes state

λ

f

LHE

[dyeI]•B1

S0 S0 S0 S0 S0 S0 S0 S0 S0

→ → → → → → → → →

S1 S3 S3 S7 S4 S10 S4 S8 S6

505.2 346.8 574.7 351.9 550.7 329.2 579.3 339.9 495.3

0.80 0.40 0.72 0.62 0.89 0.65 0.66 0.39 0.65

0.84 0.60 0.81 0.76 0.87 0.78 0.78 0.59 0.78

[dyeI]•B2

S0 → S17 S0 → S6

339.7 496.3

0.57 0.66

0.73 0.78

S0 → S17

341.2

0.45

0.65

S0 → S5 S0 → S18

521.4 334.6

0.50 0.42

0.68 0.62

[dyeI2]•− C1

S0 → S6

570.0

0.85

0.86

[dyeI2]•− C2

S0 → S8 S0 → S6

456.1 537.5

0.36 0.86

0.56 0.86

[dyeI2]•− C3

S0 → S9 S0 → S6

450.3 573.0

0.38 0.80

0.58 0.84

S0 → S8

455.1

0.35

0.55

complexes dye [dyeI]−A1 [dyeI]−A2 [dyeI]−A3

[dyeI]•B3

a

transition assignmenta H H H H H H H H H H H H H H H H H H H H H H H H H H H

→L −1→L −3→L −4→L −3→L −4→L −3→L −3→L →L −1→L →L+1 →L →L −1→L →L+1 −1→L −1→L →L −1→L →L −5→L −4→L →L −5→L −1→L −6→L −5→L

(51%) (23%) (44%) (27%) (50%) (35%) (48%) (20%) α (27%) α (19%) β (10%) β (17%) α (27%) α (11%) β (14%) β (48%) β (14%) β (9%) α (22%) β (21%) β (75%) β (23%) β (21%) β (46%) α (34%) β (16%) β (85%)

+1 +1 +1

+3

+1

+1

+1

+1

H−1→L H→L H−4→L H H H H H H

− − − − − −

→ → → → → →

L L+1 L L+1 L+1 L

(41%) (26%) (27%) (19%) β (25%) β (19%)

H−1→L+1 H−1→L H−1→L+1

β (25%) α (18%) β (11%)

H−4→L+1 H→L+1

β (13%) α (10%)

−6→L −6→L −4→L+1 − 1→ L −6→L −6→L+1 →L −1→L

β (21%) α (13%) β (19%) α (24%) β (11%) β (44%) β (19%) α (10%)

H H H H H H H H

4 3 4 4 1 1

(44%) (22%) (30%)

H and L represent HOMO and LUMO, respectively.

and [dyeI2]•−complexes have a major contribution for the first and the second electron injection, respectively. Most of the previous studies on dye regeneration mechanism considered only one electron injection, which showed only the red shift in absorption spectra as demonstrated for the [dyeI]− complexes. However, the experimental results show that the IPCE is broader at both blue and red sides in the spectrum.32−34 However, in our proposed second electron injection mechanism, we noticed that the [dyeI2]•− complex has both blue/redshifted compared to the isolated dye (Figure 5c). Further, the UV−vis absorption spectra according to our proposed dye regeneration mechanism are in good agreement with the experimental IPCE results. Therefore, our results demonstrated that the dye regeneration is completed by the two electron injection method.

■

CONCLUSION In this study, we have proposed a new mechanism for understanding the dye regeneration process in the metal-free organic sensitizers. The stability and optoelectronic properties of the dye−iodide complexes of different elementary steps have systematically investigated using DFT and TD-DFT calculations. Our results indicate that the formation of the [dyeI2]•− complex is kinetically and thermodynamically stable and possess a long lifetime, thus it absorbs another photon and injects the second electron into the surface instead of dissociating into the dye and I•− 2 as proposed by most of the previous dye regeneration mechanism studies. The calculated reaction energetics indicated that the formation of all the proposed intermediate complexes is thermodynamically favor-

Figure 6. Isosurfaces of selected frontier molecular orbitals of [dyeI]−complexes different iodide ion interaction sites (A1, A2, and A3). The isovalue is 0.02 au.

[dyeI2]•− C3 are 0.86, 0.86, 0.84, respectively, which are slightly higher than that of the isolated dye. Our results indicate that not only the isolated dye is important for photon absorption, but the dye−iodide intermediate complexes [dyeI]− and [dyeI2]•− also have a significant contribution to the photon absorption. In summary, our results confirm that the [dyeI]− 8626


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Figure 7. Isosurfaces of selected frontier molecular orbitals of [dyeI]• complexes for different iodide ion interaction sites (B1, B2, and B3). The isovalue is 0.02 au.

Figure 8. Isosurfaces of selected frontier molecular orbitals of [dyeI2]•− complexes for different iodide ion interaction sites (C1, C2, and C3). The isovalue is 0.02 au.

means of either one or two electron reduction depending on the stability of intermediate complex. We hope that this theoretical study gives a clear understanding of the complex dye regeneration mechanism for the application of dye-sensitized solar cells and it will further motivate future studies to improve the stability of intermediate complexes for promoting the second electron injection by substituting strong donor moieties on the dyes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01174. Figures of optimized isolated organic dye and the potential energy profile for the recombination reaction of iodide ions with [dyeI2]•− complexes, details of Fukui function, results of benchmark calculations, calculated atomic charges, and condensed Fukui functions (PDF)

Figure 9. Molecular orbital energy levels of the neutral and intermediate complexes ([dyeI]• and [dyeI2]•−) of proposed dye regeneration mechanism.

able and we confirmed that the significant amount of charge has transferred from the iodide ion to the oxidized dye and dyeiodide intermediates. Further, the simulated UV−vis absorption spectra of the [dyeI]− and the [dyeI2]•− complexes clearly indicate the blue/red shift compared to that of the isolated neutral dye, which is consistent with the incident photon-tocurrent conversion efficiency (IPCE) spectra. Our findings highlight that the regeneration of oxidized dye followed by

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.C.J.) 8627


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ACS Sustainable Chemistry & Engineering *E-mail: [email protected] (S.N.). Telephone: +886-2-27376653. Fax: +886-2-27376644.

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ORCID

Jyh-Chiang Jiang: 0000-0002-4225-5250 Author Contributions †

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

We thank the Ministry of Science and Technology, Taiwan (MOST 104-2119-M-011-002 and 104-2113-M-011-002) for financial support. We are also grateful to the National Center for High-Performance Computing for donating computer time and facilities.

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