Computational Study on the Intramolecular Charge Separation of D-A

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Computational Study on the Intramolecular Charge Separation of D-A-#-A Organic Sensitizers with Different Linker Groups Jing Yang, Xingzhu Wang, Wai-Leung Yim, and Qing Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b09117 • Publication Date (Web): 10 Nov 2015 Downloaded from http://pubs.acs.org on November 16, 2015

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The Journal of Physical Chemistry

Computational Study on the Intramolecular Charge Separation of D-A-π π-A Organic Sensitizers with Different Linker Groups Jing Yang1, Xingzhu Wang1, Wai-Leung Yim2, Qing Wang1* 1

Department of Materials Science and Engineering, NUSNNI-NanoCore, National University of Singapore, Singapore, 117576

2

Institute of High Performance Computing, Agency for Science, Technology and Research, Singapore 138632 Email: [email protected]

ABSTRACT A series of D-A-π-A dyes based on the structure of 2-Cyano-3-{6-{4-[N,N-bis(4hexyloxyphenyl)amino]phenyl}-4,4-dihexyl-4H-cyclopenta[2,1-b:3,4-b΄]dithiophene2-yl} acrylic acid (D1) were comprehensively investigated by computational methods, in order to understand the roles of C=C (D2) and benzothiadiazole moiety (D3) as the linker groups in dye-sensitized solar cells (DSCs). Despite that both dyes exhibit similar energetics and bathochromic shifts in the absorption spectra, it was found that the different linker groups result in distinct solar cell performance. While DFT calculations reveal favorable energetics of these dyes for ultrafast electron injection into the conduction band of TiO2, charge density difference analysis suggests that the C=C group adversely hinders, while the benzothiadiazole group promotes the intramolecular electron transfer of the dyes upon photoinduced excitation, which leads to a great disparity of device performance. And the good agreement of theoretical calculations with the experimental findings provides interesting insights into the understanding of the influence of linker groups on cell performance, as well as rational designs of D-π-A dyes for high efficiency DSCs.

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1. INTRODUCTION Dye-sensitized solar cells (DSCs) have received considerable attention since the work by O’Regan and Grätzel in 1991.1 In DSC, a thin film of nanocrystalline TiO2 grafted with sensitizing dye molecules is generally used as photoanode, which harvests light and generates charge carriers. Numerous molecular sensitizers have been developed to achieve high power conversion efficiency. Among these sensitizers, the metal-free organic dyes are especially intriguing owing to their advantages of high extinction coefficient, great diversity in structures and consequently tunable energetics, as well as good compatibility to the cobalt bipyridyl complex-based electrolytes.2 While promising, the power conversion efficiency of metal-free organic dyes except for the porphyrin derivatives, is generally lower than the metal complexes dyes. This is because the most efficient metal complex dyes are able to harvest solar irradiation up to near infrared region (NIR) but the metal-free dyes usually absorb light in shorter wavelength region.3-4 Hence, tremendous efforts have been devoted to modifying the structure of organic dyes to enhance their absorption in NIR while without sacrificing other properties.5-7 Among all the efforts, additional acceptor chromophores that introduced in D–π–A dyes between the donor and the π-spacer, leading to a D–A–π–A architecture, have recently been reported to be able to effectively tailor the band gap energy for harvesting more NIR light. Thus, the D–A– π–A sensitizers have been regarded as a promising approach to enhance the cell efficiency.8-12 2-Cyano-3-{6-{4-[N,N-bis(4-hexyloxyphenyl)amino]phenyl}-4,4-dihexyl-4Hcyclopenta[2,1-b:3,4-b΄]dithiophene-2-yl}acrylic acid (Dye D1, as shown in Figure 1) is an interesting D–π–A sensitizers for DSCs because of its high power conversion


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efficiency of 8.9% and great flexibility in tuning the structure and energetics.11 For instance, the introduction of a carbon-carbon double bond (C=C) between the triphenylamine (TPA) and conjugated thiophene heterocycle moieties (D2, Figure 1) leads to a broadening of the absorption spectrum, while unfortunately reduced cell efficiency of 7.4% in cobalt bipyridyl-based electrolyte .11 Our recent study has also revealed that by inserting a benzothiadiazole group into D1 (to form the new dye D3, Figure 1), light absorption of the dye is considerably shifted to NIR region yielding a power conversion efficiency of >9.0% in [Co(bpy)3]2+/3+–based electrolyte.13 As the photovoltaic parameters of the three dyes listed in Table S1 and S2, it is interesting to note that D2 and D3 present considerably different cell efficiency, despite that both dyes have favorable energy levels for charge injection and identical bathochromic shift in the absorption spectrum. In this work, based on rigorous ab initio computations we aim to understand the reason underlying the disparity of cell performance caused by the subtle structural changes between these two dyes, which we believe is critical to the rational design of D–π–A sensitizers with improved power conversion efficiency. To correlate the cell efficiency to molecular properties, here based on density functional theory (DFT) calculations we present by far the most comprehensive characterizations on the UV-Vis absorption optical spectra, geometrical and electronic structures of the three dyes, density of state (DOS) and Γ-point crystalline orbitals of dye/TiO2, as well as electron density difference upon photo excitation. It was found that although both the introduced acceptor moieties have brought forth a similar bathochromic shift in the absorption spectra, the C=C group hinders the intramolecular electron transfer. In contrast, the benzothiadiazole group promotes spatial charge separation and leads to better cell performance. The good agreement of


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theoretical calculations with the experimental findings discloses at molecular length scale useful insights into the elementary process in DSCs.

Figure 1. Molecular structures of the dyes investigated in this work. Dyes D2 and D3 are derivatives of dye D1 with an additional carbon-carbon double bond and benzothiadiazole group as the π-linker, respectively.

2. METHODOLOGY 2.1 Electronic Energy Level Alignment and Electron Injection Mechanism A two-dimensional slab model consisting of 40 TiO2 chemical units in one supercell was used to model the most stable (1 0 1) surface of anatase TiO2. DFT calculations on the Γ-point crystalline orbitals and the electron injection time of the dye/TiO2 systems were performed by using the plane-wave technique implemented in Vienna ab initio simulation package (VASP).14-17 The generalized gradient approximation with the Perdew-Burke-Ernzerhof functional has been employed to describe the exchange-correction potential in all calculations.18 The projectoraugmented wave method was applied to describe the electron-ion interactions, and a


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cutoff energy was set to 350 eV.19 It is noted that the standard functionals generally underestimate the band gap while the hybrid functional gives more accurate calculations.20-22 In this study, since the exact energy is not the focus but the energy alignment and the time scales, we believe the traditional functional is still a fine choice as a good compromise between the accuracy and computing resource.23-24 Here all atomic positions were optimized by the conjugated gradient method with a convergence threshold of 0.02 eV Å-1 for the force on all atoms. The electron injection time (τ, in femtosecond) from the dye to TiO2 was estimated by a model derived from the Newns-Anderson approach, which states that

= 658/Γ

(1)

where τ is inversely proportional to the broadening (Γ, in meV) of the donor orbital of the dye upon adsorption. More detailed information on Newns-Anderson model can be found in the supporting information. This model has been extensively applied to DSCs systems, such as perylene derivatives/TiO2,25 N3/TiO2,26 Esoin Y/TiO2,27 metal dicorrole/TiO2 systems,21 and was reported to typically provide about one-half of the experimental results.25, 27 2.2 Optical Absorption Calculations by Time Dependent Density Functional Theory The DFT calculations on the isolated dyes were performed with the Gaussian09 software package.28 Full geometry optimizations and electronic structure calculations were firstly carried out in vacuum at B3LYP/6-311G(d, p) level of theory.29 To obtain relatively accurate energies comparable to the experimental results, the ground-state geometries of the studied dyes in solvent dichloromethane (DCM) were re-optimized by using the polarized continuum model (PCM).30 After geometry optimization, the UV-Vis spectra of these dyes were obtained by time-dependent


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density functional theory (TD-DFT). We have chosen to study the excited state properties with the Coulomb attenuated approximation (CAM-B3LYP) as this calibrated DFT/TDDFT functional has been reported to lead to a better description of the charge transfer interaction of the triphenylamine-donor dyes.31-33 In real process, the electron transition is generally dominated by the transition between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Hence, the calculations on isolated dyes, including this paper, are focused on the HOMO-LUMO electron transition.33-36

2.3 Charge Separation Indices To evaluate the charge transfer (CT) ability of the dyes upon photo-induced excitation, the CT parameters (qCT, dCT, t index) were calculated with the method proposed by carlo Adamo and Ilaria Ciofini.37-38 This method is implemented to the code of Multiwfn 3.0 which has been used for our charge transfer calculation.39 The electron density difference is defined as the density variation between the excitation state and the ground state (∆ρ = ρ

− ρ ). The transferred charge (qCT) is the

magnitude of the integral electron density reduction and enhancement zone. In particular, the charge transfer distance (dCT) defines the spatial distance between the two barycenters (R-, R+) of the density depletion (ρ-) and density enhancement (ρ+) zones upon excitation.37-38

R = (r)dr⁄ ()

(2)

R = (r)dr⁄ ()

(3)

In addition, a diagnostic t index evaluating the through space CT character upon the excitation is defined as t = dCT − H, H being half of the sum of the root-mean-square distribution of the ρ- and ρ+ zone.37-38 The t may be regarded as the degree of


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separation of ρ+ and ρ- region. The larger is t, the less overlap between the distribution of ρ+ and ρ- region.37-38.

3. RESULTS AND DISCUSSIONS 3.1 Photoinduced Electron Injection The electronic structure of dye adsorbed system plays a crucial role in the DSC operation, in which at least two requirements must be fulfilled for an efficient cell operation: (1) the HOMO of the sensitizer must lie within the semiconductor’s band gap, and (2) the LUMO of the dye must locate above and overlap with the CB edge of TiO2 for efficient electron injection. Herein, the DOS of the dye/TiO2 systems depicting the interactions between the sensitizers and the semiconductor are shown in Figure 2. The band gaps of the dye/TiO2 systems are reduced due to a significant adsorbate (sensitizers) contribution at the top of the valence band (VB). In comparison, the bottom of the CB derives exclusively from substrate states. The partial DOS results indicate clearly that the HOMOs of all the dyes lie above the VB while well below the CB of TiO2. Moreover, the excited states of the dyes overlap with the semiconductor conduction band energetically, implying a favorable electron transfer from the LUMO of the dyes to the CB of TiO2.


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Figure 2. Electronic DOS of the pure TiO2 (bottom panel) and the dye/TiO2 systems (upper panels). The solid black line represents the total dos and the filled orange curve represents the partial electronic DOS on the adsorbate. Fermi level of the bare TiO2 supercell is set as 0 eV.

The computed Γ-point crystalline orbitals in Figure 3 illustrate how the electrons are transferred from the dye to the semiconductor. The calculations clearly show that the HOMOs of all dye/TiO2 systems are exclusively localized on the dye; upon photoinduced excitation, the electron will be promoted to the excited states from which it will be injected into the TiO2 CB. The predicted electron-hole separation of the three dyes adsorbed on TiO2 agrees with the well-accepted mechanism in DSCs. That is, electrons are injected into the semiconductor indirectly through the excited states of the sensitizers. The favorable electron injection mechanism is confirmed by the electron transfer rate calculations. The Newns-Anderson approach stated in Eq. (1) estimates the injection time for D1, D2 and D3 to be 3 fs, 9 fs and 7 fs, respectively. Such an ultrafast rate is deemed to be sufficient for an efficient electron injection.


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Figure 3. The illustration on the photo-induced electron injection mechanism based on Γpoint crystalline orbitals calculation for adsorbed D1, D2 and D3 on TiO2 surface. The energy level increases from the bottom to top direction and the Fermi energy level (EF, red dash line) of each system is aligned for clarity. The blue arrow indicates the electron transfer mechanism: the electron is firstly excited from the ground state to the excited state and then injected to the semiconductor conduction band. The electron injection times that estimated by the NewnsAnderson model are presented. The isovalue of electron density is set to |0.0008 a.u.|.

3.2 Frontier Molecular Orbital Spatial Distribution Figure 4 shows the isosurface plots of the frontier orbitals of D1, D2 and D3. The LUMOs of these molecules are primarily populated on the acceptor (cyanoacrylic acid) and the linker groups, with only little contribution from the donor group. For dyes D1 and D3, the HOMOs are mainly located at the TPA donor and linker moieties. This is highly desirable in the dye design for DSCs as it leads to sufficient charge separation upon photo-excitation that in turn, reduces geminate recombination and facilitates dye regeneration. In contrast, dye D2 exhibits an ineffective spatial charge distribution, in which a more extended HOMO crosses the C=C linker moiety and


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even spreads to the acceptor group. Such a delocalized HOMO is believed to induce a higher electron-hole recombination rate, resulting in lower photovoltage and cell efficiency. As the band gap calculation results listed in Figure 4, with an additional benzothiadiazole group in the π-linker, dye D3 presents a reduced band gap of 1.93 eV in DCM solution in good agreement with our experimental data of 1.83 eV.13 It is worth mentioning that both dyes D2 and D3 demonstrate smaller band gaps than D1 (2.14 eV and 1.95 eV vs. 2.33 eV in vacuum and 2.03 eV and 1.93 eV vs. 2.18 eV in DCM solution). So the presence of additional linker group favorably enhances light harvesting of the new dyes by red shifting the absorption spectra, which would be advantageous for improved photovoltaic performance.

Figure 4. Optimized structures and electron distribution in HOMO and LUMO levels of dye D1, D2 and D3. The band gap energies (∆ = − , in eV) are calculated under vacuum and in DCM solution (the value outside and inside the parentheses, respectively). Calculations are based on ground state geometry by DFT at the B3LYP/6-311G(d, p) level. The isovalue of electron density is set to 0.02 e/a.u..


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3.3 Optical Absorption Spectra The above calculated band gap energies revealed that the additional linker group in dyes D2 and D3 will result in bathochromic shifts in the optical absorption spectra, which is further confirmed by the TD-DFT calculations and supported by the UV-Vis measurement as shown in Figure 5. Based on the TD-DFT results, the first excitation of D1, corresponding to the direct electron transition from HOMO to LUMO, has a λmax at 497 nm, close to the previous experimental data of 512 nm measured in 1:4 EtOH/DCM solution.13 These results (both experimental and computational) appear to be distinct from the literature value for which the λmax of D1 was reported to be 552 nm in cholorobenzene.11 The divergent optical behavior in different solvents is attributed to the fact that the polarities of the solvent are different. With the benzothiadiazole as the linker, dye D3 shows a calculated λmax at 534 nm, which is in line with the UV-Vis measurement result of 556 nm.13 In addition, it is interesting to note that dye D3 presents a new peak at 424 nm because of the electron excitations of HOMO-1 →

LUMO and HOMO →

LUMO+1, which are

advantageous for the light harvesting in the short wavelength region. In comparison, the C=C bond drives the absorption spectrum of D2 to a more bathochromic region which shows a calculated λmax of 532 nm.


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Figure 5. Simulated spectral absorption of dyes D1 (black), D2 (green) and D3 (red) in DCM solution at TDDFT//CAM-B3LYP/6-311G(d, p) level of theory based on the optimized ground state geometries as showed in Figure 4. The experimental molar absorption coefficients of dyes D1 and D2 as a function of wavelength in chlorobenzene solutions is included as inset (a)11 and the absorption spectra of dyes D1 and D3 measured in DCM as inset (b).13

The red shift of the absorption is beneficial to the light harvesting of DSCs, especially that it will enhance the light absorption in the long wavelength region. While both C=C and benzothiadiazole linker groups shift the optical spectra bathochromically, the previous reports showed that only the benzothiadiazole group enhanced the overall cell efficiency.11, 13 This reveals that, given the complex charge transfer processes in DSCs, light absorption and charge injection rate of the dyes are insufficient to explain the opposite trends of the cell efficiency. In the below section, intramolecular charge separation of the three dye molecules will be scrutinized in details. It is found that the intramolecular spatial charge separation upon photoinduced


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excitation is an important factor accounting for the distinct cell performance between D2 and D3.

Figure 6. Representations of charge density difference associated with the first electronic transition along with the corresponding transferred charge amount (qCT, in e) and charge transfer distance (dCT, in Å) computed for dyes D1, D2 and D3. Red and blue zones correspond to electron density depletion and enhancement zones, respectively. The isosurface of electron density is set to |0.004 a.u.|.

3.4 Photoinduced Intramolecular Charge Separation As the initial step of light harvesting, photoinduced excitation of the dye is of great significance to the operation of DSCs. Along with photo excitation, intramolecular charge transfer of electron from ground state to excited state takes place, in which sufficient spatial charge separation within the sensitizing dye is desired to enhance the subsequent charge transfer processes. For D-π-A dyes, both


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experimental and theoretical studies have shown that the first photoinduced electron excitation of these dyes is dominated by the electron transition between HOMO and LUMO, for which the spatial separation of the molecular orbitals of the molecule is critical to the actual charge separation upon excitation. To examine intramolecular charge separation of these dyes, the CT parameters for all the three dyes were analyzed. The electron density differences associated with the first electronic transition computed for the three dyes are depicted in Figure 6. It is evident that both dye D1 and D3 present a marked charge transfer by excitation, where significant electron density differences are observed between the TPA group (electron donor) and cyanoacrylic acid moiety (electron acceptor). In the HOMO-LUMO transition, dye D3 reveals a charge separation (qCT) of 0.89 e-, which is slightly larger than the 0.84 eby dye D1. Besides qCT, charge transfer distance (dCT) represents another useful parameter quantitatively describing the effectiveness of intramolecular charge transfer. As shown in Figure 6, the presence of benzothiadiazole linker increases the charge transfer length from 6.75 Å for D1 to 7.23 Å for D3, indicating that the benzothiadiazole group effectively facilitates the intramolecular charge transfer from the TPA to cyanoacrylic acid moiety upon photo excitation. In contrast, the C=C linker in D2 presents less effective charge separation upon excitation, where the electron enrichment and depletion zones are not well separated (qCT = 0.54 e-, dCT = 3.28 Å). In addition, as shown in Figure 6, the TPA segment in dye D2 is no longer the main electron donor as that in dye D1. Further calculations suggest that the barycentre of the electron depletion zone partially locates at the carbon-carbon double bond, indicating the C=C moiety plausibly serves as the electron donor to some extent which is however not desirable for the design of D-π-A dyes. Moreover, t-index was calculated to further evaluate the intramolecular charge separation in these dyes. It


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shows that after photo excitation the positive charge center strongly overlaps with that of the negative charge in dye D2, which leads to a negative t-index (t = -1.99 Å). On the contrary, dyes D1 and D3 have positive t-index values of 1.41 Å and 1.21 Å, respectively, suggesting a better spatial charge separation as compared to dye D2. The above analyses have unambiguously demonstrated the opposite effects of C=C and benzothiadiazole linker groups on the photo-induced intramolecular charge redistribution. The C=C bond in dye D2 hinders the charge separation which is believed to directly lead to an inferior charge injection into the semiconductor and accelerated geminate electron-hole recombination (the distance between holes in the oxidized dyes and electrons in TiO2 becomes closer, while that for dye regeneration becomes further). Thus, we propose that the weak intramolecular charge separation of dye D2 is an important factor accounting for the worse cell efficiency. In contrast, the benzothiadiazole moiety in dye D3 facilitates the above process with more effective charge redistribution onto a longer distance, and hence gives rise to superior cell performance.

4. CONCLUSIONS Based on DFT calculations, the above has presented a systematic and comprehensive study on the optical and electronic properties of a series of D-A-π-A dyes and their implications to solar cell operation. The effects of π-linker groups on light absorption, intramolecular charge transfer, as well as charge injection were scrutinized to elucidate the factors dictating the disparity of cell perfromance. While the electronic structures of all three dyes (D1, D2 and D3) are energetically favorable for untrafast electron injection, the distinct intramolecular charge separation properties arisen from the change of linker groups (C=C vs. benzothiadiazole) are


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believed to be an important factor for the divergent cell performance. The above findings have important implications to the rational design of D-A-π-A dyes for high efficiency DSCs. Hence, besides the optical and energetic properties of dyes, which are essential to the light harvesting and charge injection, the photoinduced spatial separation of electron and hole in the dye molecule should also be taken into consideration. We understand that the calculations presented in this study are based on isolated dye molecules and those with TiO2. The operation of DSCs however relies on multiple kinetic processes, some of which are not addressed in the computation. For instance, geminate recombination of the injected electrons with holes resided in the dye molecules, and dye regeneration, the two key processes determining the charge separation of DSCs may also be influenced by the linker groups. While these processes are being investigated, we anticipate that the computational study presented here would provide interesting insights into the understanding of influences of linker groups on cell performance.

ACKNOWLEDGMENTS This research was supported by Ministry of Education Tier 2 research grant (MOE2011-T2-2-130). The authors acknowledge A*STAR Computational Resource Centre (A*CRC) for computing facilities.

ASSOCIATED CONTENT: Detailed information on Newns-Anderson model; Typical photovoltaic parameters of DSCs with D1 and D2 as sensitizer; Typical photovoltaic parameters of DSCs with D1 and D3 as sensitizer.


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Table of Contents (TOC) Graphic:


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Computational Study on the Intramolecular Charge Separation of D-A

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