Intermolecular Charge Transfer in

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Solvent Effects on Intra/Intermolecular Charge Transfer in Indoloquinoxaline-Based Dyes Fatemeh Barati-Darband, Mohammad Izadyar, and Foroogh Arkan J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b00812 • Publication Date (Web): 07 Mar 2019 Downloaded from http://pubs.acs.org on March 12, 2019

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The Journal of Physical Chemistry

Solvent Effects on Intra/Intermolecular Charge Transfer in Indoloquinoxaline-Based Dyes Fatemeh Barati-darband, Mohammad Izadyar*, Foroogh Arkan Computational Chemistry Research Lab., Department of Chemistry, Faculty of Science, Ferdowsi University of Mashhad, Mashhad, Iran * Email: [email protected]

Abstract In this work, kinetics and dynamics of the functionality of indoloquinoxalinebased dye sensitized solar cells (DSSCs), QX22-QX25, were investigated in the gas and solvent media. Quantum chemistry properties of the dyes at the excited states show that each moiety of (D)2-A-π-A system has a specific effect on the photovoltaic properties. Solvent effect analysis shows that among ethanol, toluene, tetrahydrofuran, methylene dichloride, toluene is the preferred medium for intra/intermolecular charge transfer, dynamically and kinetically. Moreover, the behavior of the light harvesting efficiency (LHE) and incident photon-to-current efficiency (IPCE) are not similar, due to a strong effect of the Gibbs energy of electron injection on the energy conversion efficiency. Finally, the dye composed of -COOH as the anchoring group and thiophene as the π-spacer is the best candidate to be applied in DSSC due to its better efficiency originated from a lower electrophilicity and electronic chemical potential. Keywords: efficiency, solvent effect, photovoltaic process, dynamic, kinetic exciton

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Introduction Since one of the most primary needs of the human is energy, research on the abundant and clean energy sources seems required

1-3

. Many studies have focused

on the solar energy as the world's most abundant energy resource due to its advantages such as a clean, renewable, and applicable energy source 4,5. Due to the unique properties of the sunlight, the study of the systems that can effectively convert the solar energy is of interest for researchers. One of the most efficient systems in this field is the photovoltaic (PV) devices, such as the dye-sensitized solar cells (DSSCs), which have received a widespread attention in recent years 6–13. In DSSCs, dye molecules absorb the solar light, accompanied by the excitation of electrons to the excited states. The excited electrons, after injection into the conduction band (CB) of the semiconductors, such as TiO2, FeO, CuO are transferred to the anode. The oxidized dye molecules are regenerated by the iodide redox couple or hole transfer materials (HTMs). Therefore, the performance of DSSC strongly depends on some factors, such as the solar light absorption ability of the photosensitizer, electron injection probability from the dye to semiconductors and the electron transfer probability from the electrolyte to the oxidized dye Moreover, kinetic aspects of the charge transfer are important, too 15. 2 ACS Paragon Plus Environment

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A theoretical study on the structural/electronic properties of of triphenylamine dyes containing variable thiophene units as the spacers (TPA1-TPA3) were done 16. Molecular analysis shows that the addition of thiophene units leads to an improvement in the absorption properties of dyes. On the other hand, this unit enlarges the distance between electron donor groups and semiconductor surface, which turns reduces the dynamics/kinetics of the electron injection and overall conversion efficiency. Therefore, the choice of appropriate conjugate bridge can play a key role in designing the efficient dyes 16. Another series of triphenylamine dyes were studied as the sensitizers for DSSCs

17

. Different substituted phenylene units, 2,2′;5′,2′′-terthiophene (TT) and

dithieno[3,2-b;2′,3′-d]thiophene (DTT) selected as the π-spacers, and cyanoacrylic acid or rhodanine-3-acetic acid units are applied as the electron acceptors. According to the open - circuit voltage, VOC, and short-circuit current, JSC, it was found that cyanoacrylic acid unit is a preferred unit than rhodanine-3-acetic acid as an electron acceptor. Also, the electron-withdrawing groups on the phenylene units as π-spacers have a reducing effect on the performance of the DSSCs. Park and co-workers, first demonstrated the application of quinoxaline in the photosensitizers 18. They have used quinoxaline as a π-spacer and acceptor in the dyes. The dye containing quinoxaline as a π-linker due to a red-shifted absorption, showed a better efficiency (5.56%) of, which used quinoxaline as an acceptor 3 ACS Paragon Plus Environment


(3.30%). Moreover, Yamashita and co-workers reported quinoxaline and thienopyrazine-based dyes 19. Co-sensitization using quinoxaline and thienopyrazine dyes leads to a high power conversion efficiency (PCE) of 6.2% in DSSC 19. Zhu et al. also reported efficiency as high as 9.24% using a dye containing quinoxaline πspacer and indoline donor 20. Moreover, Grätzel and co-workers reported the dyes, having pride [3,4-b] pyrazine units in the π-conjugation pathway within an efficiency of 3.11 to 7.12% depending on the donor strength and peripheral groups on the quinoxaline core 21. The stability of quinoxaline unit encourages the researchers to design new dyes based on quinoxaline for DSSC. Based on the emission properties of the several dyes containing indoloquinoxaline core, the researchers found that the fusion of electron-rich indole fragment on the quinoxaline unit develops the electrochemical prospects of the dyes by an increase in the band gap 22,23. Since the surrounding of the photosensitizer affects the solar cell efficiency, the application of the proper solvents is of importance to achieve better results. 24,25. On the basis of several reports, the interactions of the solvent-sensitizer affects the final efficiency of the solar cells 26-29.


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Organic dyes containing indolo[2,3-b]quinoxaline as the donor are applied in DSSCs as photosensitizers 30. Their optical properties are tunable by the nature of the conjugation bridge. Since the dyes have a solvent-dependent absorption properties, they were prepared from two different solvents, methylene dichloride (DCM) and acetonitrile/tert-butanol/dimethylsulphoxide mixture, which leads to an efficient DSSC 30. To improve the excited state and optical properties of the D-A-π-A structure of the organic dyes, a new molecular engineering strategy suggests through the extension of the conjugation size and rigidity of the auxiliary acceptor functional group. For example, a series of phenanthrene-fused-quinoxaline (PFQ) based D-Aπ-A organic sensitizers were studied 31. In comparison to 2,3-diphenylquinoxaline (DPQ) -based dye, IQ-4 and PFQ dyes show an extended absorption spectrum and higher VOC. Moreover, an elongation of alkyl chain and π-spacer moiety decrease the solubility and act as a high energy barrier in intramolecular charge transition (ICT) in PFQ dyes 31. Solvent affects the photovoltaic properties of the photosensitizers and the photovoltaic processes and represents more reliable results

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, therefore the

investigation of the solvent effects on DSSC systems seems essential.



In this research, the ground/excited states properties of the metal-free organic dyes (D2-A-π-A) composed of triphenylamine (D unit) and indoloquinoxaline (A unit) (QX22-QX25) were theoretically investigated by density functional theory (DFT)/time dependent DFT (TD-DFT) methods. To analyze the medium effects on the solar cell performance, ethanol, toluene, tetrahydrofuran (THF) and methylene dichloride (DCM) were applied. After describing the ground state properties of the dyes, the study of the excited state properties of the quinoxaline-based dyes in different solvent media was continued. Non-equilibrium version of the conductor-like polarizable continuum (CPCM) model

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has used for evaluating the solvent effects. Also, exciton

behavior, intra/intermolcular charge transfer, spectroscopic properties and quantum chemistry properties of the dyes in different media were analyzed. Through a molecular approach, it is possible to have insight into the intrinsic properties of the dyes.

Computational methods All calculations were performed on Gaussian 09 program package 38. Groundstate geometries were fully optimized at the M062X/6-31G(d,p) level of theory 39. 6 ACS Paragon Plus Environment

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Natural bond orbital (NBO) analysis was applied to evaluate the quantum chemistry reactivity indices

40

. In addition to the simulation of UV–Vis absorption spectra,

excitation energies, oscillator strength of the lowest singlet-singlet transitions were obtained by TD-DFT calculations at M062X/6-311++G(d,p) level of theory. Solvent effects were evaluated by the CPCM model

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for both geometry optimizations

and excited state calculations. One of the important parameters of the DSSCs is the incident photon-tocurrent efficiency, IPCE, which is theoretically obtained by Eq. 1 44: IPCE= LHE(λ).Φinject. ηcollect

(1)

where, LHE is the light harvesting efficiency (Eq. 2), ƞcoll is the electron collection efficiency, and ϕinj is the net electron injection efficiency. LHE = 1- 10-f (2) where f is the oscillating strength. Gibbs energies of electron/hole injection, ΔGinj./∆Greg. are obtained by Eqs. 3 and 4, respectively 45: ∆Ginj. = EOX(dye*) – ECB,TiO2 ∆Greg.(dye) = EOX(dye) – Eredox(electrolyte)


(3) (4)


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where, EOX(dye*) is the excited state oxidation potential of the dye, ECB(TiO2) is the energy of the charge transfer (CB) of the semiconductor and Eredox(electrolyte) is the redox potential of the electrolyte. Electron injection rate constant of the dye/TiO2 interface (kinj.) can be calculated by the Marcus electron transfer theory (Eq. 6) 46. k inj. =

π0.5 ћ(λkB T)0.5

|2

|VRP exp [

−(∆Ginj. +λ)2 4λkB T

]

(5)

where, ħ is the reduced Planck constant, kB is the Boltzmann thermal energy, λ is the reorganization energy of the system and |VRP| is the coupling constant between the dye and TiO2 surface. When light is absorbed by a sensitizer, an exciton as the Frenkel type is created in the organic dye with a low dielectric constant (ε=3-4)47. The rate of photon absorption for a singlet excitation, RSa , is obtained by Eq. 6 48: RSa =

4ke2 (ELUMO − EHOMO )3 ax 2

(6)

3c3 ε1.5 ћ4

where, c is the light speed, ε is the dielectric constant of the donor and ax is the exciton Bohr radius, which is calculated by Eq. 7 for the singlet excitons 49: ax =

α2 με

a (α−1)2 μx 0

(7)


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where, μ is the reduced mass of electron in a hydrogen atom, μx is the reduced mass of the exciton, α is the material dependent constant, which shows the ratio of the coulomb and exchange interactions between the excited electron and hole and ao is the Bohr radius. For singlet excitons, exciton dissociation rate, Rd, is calculated by Eq. 8 50: Rd=

8π2 3ћ3 ε2 EB

D A [ELUMO − ECB − EB ]2 (ћωϑ )μx ax 2

(8)

where, ωv is the frequency of the incident phonon to the dyes.

Results and discussion Dynamics and kinetics of the charge transfer Figure 1 shows the optimized structures of the dyes in the gas phase within electronic energies calculated at M062X/6-31G(d,p) level.



Figure 1: Optimized structures of the studied indoloquinoxaline derivatives in the gas phase at M062X/6-31G(d,p) level.

The redox properties of the dyes were described by NBO analysis. All dyes represent an irreversible reduction corresponding to electron release from the quinoxaline segment and an irreversible oxidation originating from the indole unit. The theoretical trend of the oxidation potential of the dyes is as follows: QX24

Intermolecular Charge Transfer in

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