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Properties of electronically excited states of four squaraine dyes and their complexes with fullerene C70: A theoretical investigation
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 184 (2017) 82–88
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Properties of electronically excited states of four squaraine dyes and their complexes with fullerene C70: A theoretical investigation Jian Zhang ⁎, Tingyu Li School of Chemistry and Material Science, Shanxi Normal University, Linfen 041004, China
a r t i c l e
i n f o
Article history: Received 30 November 2016 Received in revised form 28 March 2017 Accepted 30 April 2017 Available online 01 May 2017 Keywords: Electronically excited states Organic photovoltaic devices Squaraine dyes Fullerene C70
a b s t r a c t Solar cells sensitized by polypyridyl Ru(II) complexes exhibit relatively high efficiency, however those photosensitizers did not absorb the photons in the far-red and near-infrared region. At present, squaraine dyes have received considerable attention as their attractively intrinsic red light absorption and unusual high molar extinction coefficient. Here we applied density functional theory and time dependent density functional theory to investigate the properties of electronically excited states of four squaraine dyes and their complexes with fullerene C70. The influences of different functionals, basis sets and solvent effects are evaluated. To understand the photophysical properties, the investigations are basing on a classification method which splits the squaraine dyes and their complexes with fullerene C70 into two units to characterize the intramolecular density distribution. We present the signatures of their electronically excited states which are characterized as local excitation or charge-transfer excitation. The relationship between open-circuit voltage and the number of intramolecular hydrogen bonds in squaraine dyes are discussed. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Many different photovoltaic materials based on donor-acceptor organic heterojunction are being developed for large-scale solar energy conversion [1–5]. Due to low cost, low temperature, nontoxicity and high plasticity, these materials have attracted great attention like inorganic devices [6–10]. Organic photovoltaics (OPV) has come into the research focus of materials and device architecture during the past few years [11–13]. Monomeric donors have attracted growing attention for use in photovoltaic devices, as they can be easily organized in self-assembled structures [14]. Among the monomeric donors explored for photovoltaics, squaraine dyes have been intensely studied [15–18]. In addition to photovoltaics, squaraines dyes are used for a wide range of applications such as ion sensing, nonlinear optics, photodynamic therapy and imaging [19–22]. They absorb in the visible to near-IR region with high absorption coefficient. It was observed that the power-conversion efficiency (PCE) could be enhanced up to 5.7% with SQ/C60 heterojunction [23]. Chen and coworkers used fullerene C70 as an acceptor instead of C60 due to insufficient green light capturing in SQ/C60 cell. The optical absorbance of fullerene C70 is much greater than that of C60, so the SQ/C70 heterojunction provides a higher photocurrent than C60-based device [24]. Four SQ dyes were designed and synthesized with various chemical structures using different numbers of OH groups and side chains in 2014 ⁎ Corresponding author. E-mail address: [email protected] (J. Zhang).
http://dx.doi.org/10.1016/j.saa.2017.04.086 1386-1425/© 2017 Elsevier B.V. All rights reserved.
[25]. SQ dyes 1 has branched side chains at the 4-position of the two phenyl rings and four OH groups at the 2, 6-position of the two phenyl rings, 2 has linear side chains and four OH groups, 3 has linear side chains and two OH groups, while 4 has linear side chains without OH group. Experimental data such as single-crystal X-ray crystallography and XRD show that the molecular structures of these SQ dyes are very similar. Meanwhile, their molecular packing, material properties, and OPV performance are vastly different. For instance, SQ dyes 1 exhibits J-aggregation, 2 exhibits both H-aggregation and J-aggregation, 3 exhibits preferential H-aggregation, 4 exhibits J-aggregation. It is important to note that optical absorption is dictated by the physical structures and electronic structures of these SQ-C70 heterojunction, which are not fully understood. The theoretical calculation has its necessity and feasibility, such as providing assistance in regards to understanding the mechanism of SQ-C70 heterojunction and selection of SQ dyes. density functional theory (DFT) and time dependent density functional theory (TDDFT) have become the most popular approaches for calculating structures and electronic spectra of molecules with no less than 200 atoms [26], it is used to investigate electronic properties of excited states of SQ-fullerene C70 heterojunction in this study. 2. Theoretical and computational aspects All the quantum chemistry calculations were carried out with the Gaussian 09 program suite [27]. The ground-state structures of the monomers and complexes were performed using modern hybrid
J. Zhang, T. Li / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 184 (2017) 82–88
functional ωB97XD including long range and dispersion correction [28]. The 6-31G(d) basis set was used and the optimizations were unconstrained [29]. Since every one of these molecules has a closed-shell
83
electronic configuration in the ground state and does not contain transition metal atoms, which means monoconfigurational system, excited state calculations were performed by TDDFT method, assuming
Fig. 1. The optimized molecular structures of four SQ dyes.
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Table 1 The vertical excitation energies corresponding to maximum oscillator strengths, absorption maxima wavelengths and involved orbitals of four SQ dyes in gas phase and chloroform solution.
1 In gas phase 2 In gas phase 3 In gas phase 4 In gas phase 1 In chloroform 2 In chloroform 3 In chloroform 4 In chloroform
Excited states
ΔE (ev)
λmax (nm)
Oscillator strengths
Orbitals involved in the transition
1 1 1 2 1 1 1 1
2.52 2.54 2.46 2.42 2.32 2.33 2.26 2.24
491.9 488.8 503.2 511.4 534.3 531.8 547.5 553.7
1.747 1.782 1.688 1.568 1.990 2.011 1.961 1.934
HOMO → LUMO HOMO → LUMO HOMO → LUMO HOMO → LUMO HOMO → LUMO HOMO → LUMO HOMO → LUMO HOMO → LUMO
closed-shell monoconfigurational wave functions. To evaluate the dependence of the electronic structures on the functionals, the excitation properties were simulated using different parameter ω values ωB97XD, comparing with popular conventional hybrid functional B3LYP [30], empirical exchange-correlation functional M06-2 × [31], coulomb-attenuated functional CAM-B3LYP [32], and exchange-correlation functional PBE1PBE [33,34]. To evaluate the influence of basis set extension, basis sets 6–31+G(d), 6–311+G(d), 6–311++G(d,p) and TZVP (triple-zeta-valence plus polarization) [35] were used in calculations to compare the 6-31G(d) basis set. The basis set superposition errors (BSSE) were corrected by counterpoise method to accurately estimate interaction energies [36]. Computational procedure for the solvation effect in chloroform and toluene (dielectric constants (ε) are 4.70 and 2.38 at 25 °C) was based on the SMD [37] continuum solvation model implemented in the Gaussian 09. For SMD model, an intrinsic atomic SMD-Coulomb radii for the cavity and van der Waals surface with α = 1.0 were used. This method is based on the self-consistent reaction field (SCRF) of bulk electrostatic contribution that involves the solution of nonhomogeneous Poisson equation (NPE) and the cavity-dispersion-solvent-structure protocol for the solvent-accessible surface areas of the individual atoms. The investigations were basing on a classification method which splits the SQ dyes and SQ-C70 heterojunction into two units to characterize the intramolecular density distribution. The quantitative Table 2 Under different parameter ω, ε, basis sets and functionals, vertical excitation energies corresponding to maximum oscillator strengths, absorption maxima wavelengths and involved orbitals of SQ dye 3. Excited ΔE states (ev) ωB97XD/6-31G(d) ω = 5 ωB97XD/6-31G(d) ω = 10 ωB97XD/6-31G(d) ω = 15 ωB97XD/6-31G(d) ω = 20 ωB97XD/6-31G(d) ω = 25 ωB97XD/6-31G(d) ω = 30 ωB97XD/6-31G(d) ω = 35 ωB97XD/6-31G(d) ω = 40 ωB97XD/6–31+g(d) ωB97XD/6–311+g(d) ωB97XD/6–311++g(d,p) ωB97XD/tzvp pbe1pbe/6-31g(d) b3lyp/6-31g(d) cam-b3lyp/6-31g(d) m06-2×/6-31g(d) Chloroform (ε = 4.70) Toluene (ε = 2.38)
λmax (nm)
Oscillator strengths
Orbitals involved in the transition
1 1
2.47 502.4 1.696 2.47 502.6 1.692
HOMO → LUMO HOMO → LUMO
1
2.47 502.8 1.689
HOMO → LUMO
1
2.46 503.2 1.688
HOMO → LUMO
1
2.46 503.6 1.683
HOMO → LUMO
1
2.46 504.1 1.679
HOMO → LUMO
1
2.46 504.7 1.676
HOMO → LUMO
1
2.45 505.3 1.672
HOMO → LUMO
1 1 1 1 1 1 1 1 1 1
2.44 2.45 2.45 2.44 2.47 2.43 2.47 2.46 2.26 2.25
HOMO → LUMO HOMO → LUMO HOMO → LUMO HOMO → LUMO HOMO → LUMO HOMO → LUMO HOMO → LUMO HOMO → LUMO HOMO → LUMO HOMO → LUMO
507.4 505.5 506.7 508.4 502.6 509.8 501.7 503.4 547.5 550.4
1.705 1.711 1.705 1.708 1.751 1.696 1.707 1.776 1.961 1.919
indicator of electron excitation mode (Δr) is given to understand the photophysics of the SQ dyes and SQ-C70 heterojunction [38]. The smaller the Δr index is, the more likely the excitation is local excitation (LE). The bigger the Δr index is, the more likely the excitation is charge transfer excitation (CT). The t index is the main criterion of the separation of electron and hole. The t index is positive number, the separation of electron and hole is easy to see. The t index is negative number in every xyz direction, the electron and hole overlap with each other definitely. These indexes were calculated with the Multiwfn program [39]. Meanwhile, Multiwfn represented the hole and electron distributions as isosurface. The hole distribution was shown as blue and electron distribution was shown as green. 3. Results and discussion 3.1. Four SQ dyes The molecular structures of four investigated SQ dyes are shown in Fig. 1. There are four intramolecular hydrogen bonds in SQ dyes 1 and 2, two intramolecular hydrogen bonds in 3, while it is zero in 4. As a result, the twisting angle around the central 4-membered ring in 3 is significantly greater than those in 1 and 2. The vertical excitation energies obtained by TDDFT were computed at the optimized ground state minimum. Table 1 shows our calculated vertical excitation energies corresponding to maximum of oscillator strengths, absorption maxima wavelengths and involved orbitals of these SQ dyes in gas phase and chloroform solution, respectively. Maximum of oscillator strengths is associated with HOMO to LUMO transition in every case. The computed vertical excitations of different SQ dyes look the same, as an example, the gap between vertical excitation energies in 1 and 2 is only 0.02 eV in gas phase and meanwhile 0.01 eV in the chloroform solution. Taking 3 as example, the influences of different functionals, basis sets and solvent effects are evaluated and shown in Table 2. Especially in the long-range corrected functional ωB97XD, parameter ω is adjusted in order. It has a different value, describing the separation of the Coulomb operator, with the long-range part given by the exact exchange integral. From these results, it can be found that the lowest excitation energy of 3 shows a steady decline when adjusting ω from 0.05 to 0.40 a−1 0 . On the whole, variation of vertical excitation energies in 3 from choosing basis sets is 0.01 eV. Considering over the basis sets with more polarization, more diffuse, and triple-ζ functions, variation of vertical excitation energies in 3 is 0.03 eV. The above-
Table 3 The first excited state of SQ dye 3. Excited states
ΔE (ev)
λ (nm)
Oscillator strengths
1
2.46
503.2
1.688
Δr (Å)
Types of t (x,y,z) excitations
0.01496 −3.748 LE (A) −1.070 −0.740
J. Zhang, T. Li / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 184 (2017) 82–88
Fig. 2. The isosurface of hole and electron distributions in the first excited state of SQ dye 3.
mentioned data calculated at all kinds of functionals associated with 631G(d) basis set distribute in the interval from 2.43 eV to 2.47 eV. From the high dielectric constant (ε = 4.70) to the low dielectric constant (ε = 2.38) in the solvation phases, it can be found that the vertical excitation energy reduces 0.01 eV. In order to understand the electronically excited states and to characterize the intramolecular density distribution between unit A (corresponds to cyclobutene ring, benzyl and hydroxyl group) and unit B (corresponds to end amine group) in these SQ dyes, taking the first excited state of 3 as example, the Δr and t indexes are given in Table 3. As can be seen in this table, it is highly possible that the first excited state of 3 possesses strong local excitation character since it has very small Δr index. Moreover, the t indexes are all negative in any xyz direction, which means hole and electron overlap with each other strictly. Fig. 2 illustrates the isosurface of hole and electron distributions of the same excited state mentioned above. It can be observed that there is no evident spatial separation between hole and electron, clearly exhibiting both hole and electron are mainly localized on unit A region (LE(A)). We can finally conclude that the first excited state of 3 is LE state after we look into the hole and electron distributions. 3.2. SQ-fullerene C70 heterojunction In order to analyze the properties of SQ-fullerene C70 complexes, the optimized minimum-energy geometries were determined in their gas phase using DFT calculations and illustrated in Fig. 3. We can find an interesting phenomenon: the SQ starts bending and come in packages which enclose the fullerene C70. The twisting angle around the central 4-membered ring is significantly increased compared to original SQ. The binding energy is one of the key parameters reflecting the physics of photovoltaic devices. After geometry optimizations, the binding energies were corrected for BSSE at the ωB97XD/6-31G(d) level. The complex 1-C70 has a BSSE-corrected binding energy of − 24.2 kcal/mol, 2C70 is − 23.2 kcal/mol, 3-C70 is − 26.4 kcal/mol, while 4-C70 is − 26.1 kcal/mol. It can be found that the BSSE-corrected binding energy is related to the amount of intramolecular hydrogen bonds in these four SQ-C70 complexes. SQ dyes with two and zero intramolecular hydrogen bonds better combine the fullerene C70 than four intramolecular hydrogen bonds SQ dyes. The most nearest C\\C distances between the central 4-membered ring of these four SQ dyes and the fullerene C70 are as follows: 3.33 Å, 3.22 Å, 3.23 Å, and 3.44 Å. The electron-deficient cyclobutene ring is linked to different end amine groups in four SQ dyes. In the contributions of end amine groups, charge density distributions may be affected differently and cause local excitation or charge-transfer excitation. In order to understand the electronically excited states and to characterize the charge-transfer between unit A (corresponds to SQ) and unit B (corresponds to C70), the Δr and t indexes of the lowest three excited states in these four SQ-C70 complexes are given in Table 4. One can observe from this table that
Fig. 3. The optimized molecular structures of four SQ-C70 complexes.
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Table 4 The lowest three excited states of four SQ-C70 complexes. Excited states
ΔE (ev)
λ (nm)
Δr (Å)
t (x,y,z)
Types of excitations
3 of 1-C70
2.60
476.9
5.9229
CT (A → B)
2 of 1-C70
2.45
505.2
1.5242
1 of 1-C70
2.44
509.2
0.2200
3 of 2-C70
2.59
479.3
5.7030
2 of 2-C70
2.47
503.0
1.1530
1 of 2-C70
2.44
509.1
0.2971
3 of 3-C70
2.47
502.4
1.2024
2 of 3-C70
2.37
523.0
3.0255
1 of 3-C70
2.36
525.1
5.7717
3 of 4-C70
2.45
505.3
1.0969
2 of 4-C70
2.41
514.6
1.9008
1 of 4-C70
2.22
558.1
5.7856
−3.372 2.970 −1.683 −2.570 −2.275 −2.242 −3.453 −1.054 −1.115 −3.328 2.785 −1.689 −2.596 −2.170 −2.263 −3.644 −0.786 −1.070 −2.532 −2.099 −2.227 −3.478 −1.543 −1.065 −2.615 2.859 −1.173 −2.366 −2.233 −2.169 −3.251 −1.114 −1.191 −2.528 2.695 −1.407
LE (B)
LE (A)
CT (A → B)
LE (B)
LE (A)
LE (B)
LE (A)
CT (A → B)
LE (B)
in 3-C70 and 4-C70, the second and third excited states possess local excitation character since they have small Δr indexes, and meanwhile the corresponding t indexes are all negative in any xyz direction, and in consequence the overlap between hole and electron is significant. At the same time, the first excited states possess charge transfer excitation character since they have very large Δr indexes greater than 5.0, and meanwhile there exist positive numbers in corresponding t indexes, and in consequence the separation of hole-electron is clear. Fig. 4 illustrates the isosurfaces of hole and electron distributions of the lowest three excited states of these four SQ-C70 complexes. We can finally conclude these excited states belong to LE or CT states by visualizing the hole and electron distributions. From this figure, it appears that excited states 1 of 1-C70, 1 of 2-C70, 2 of 3-C70 and 2 of 4-C70 have no evident spatial separation between hole and electron, distinctly exhibiting both hole and electron are localized on the SQ region without perturbation from C70 region (LE (A)). On the contrary, in excited states 2 of 1C70, 2 of 2-C70, 3 of 3-C70 and 3 of 4-C70, both hole and electron are localized on the C70 region (LE (B)). Undoubtedly 3 of 1-C70, 3 of 2-C70, 1 of 3C70 and 1 of 4-C70 are typical CT transition states, corresponding to charge transfer from SQ region to C70 region (CT (A → B)). It is well-known that one of the most vital parameters of OPV devices is open-circuit voltage (VOC). The linearly dependent relationship between VOC and CT states has been widely discussed using experimental and theoretical methods. Besides that it has been proposed that the energy of CT states (ECT) determines the VOC data directly [40]. Vandewal et al. deduce a consistent description of the energetic losses between ECT and qVOC for a range of polymer-fullerene-blend solar 3 2
LE (A)
CT (A → B)
in 1-C70 and 2-C70 the first and second excited states possess local excitation character since they have very small Δr indexes, and meanwhile the corresponding t indexes are all negative in any xyz direction, and in consequence the overlap between hole and electron is significant. At the same time, the third excited states possess charge transfer excitation character since they have very large Δr indexes greater than 5.0, and meanwhile there exist positive numbers in corresponding t indexes, and in consequence the separation of hole-electron is clear. By contrast,
J SC h c cells [41]: V OC ¼ EqCT þ ðkBqT Þ ln ð2πfqðE Þ þ ðkBqT Þ ln ðEQEEL Þ; where kB CT −λ0 Þ
and T are Boltzmann's constant and absolute temperature, JSC and h are short-circuit current density and Planck's constant, c and f are the speed of light and proportional parameter, respectively. λ0 is reorganization energy and EQEEL is the electroluminescence quantum efficiency. By applying a best fit to the data of different studied photovoltaic devices, Hörmann et al. simplify this equation so that a linear dependence relation between VOC and ECT which can be described as [42]: VOC = ECT/ q − 0.55 V. This formula shows that there is an energy loss of 0.55 V between VOC and ECT at room temperature. Therefore the VOC of these four SQ-C70 complexes are calculated and illustrated in Fig. 5 by using this formula. We can see that the VOC values are ordered as 1-C70 ≈ 2-C70 N 3-C70 N 4-C70 which has a good agreement with the number of intramolecular hydrogen bonds in SQ dyes. This indicates that VOC in SQ-C70 OPV devices is controlled by intramolecular hydrogen bonding
Fig. 4. The isosurfaces of hole and electron distributions in the lowest three excited states of four SQ-C70 complexes.
J. Zhang, T. Li / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 184 (2017) 82–88
Fig. 5. Open-circuit voltage of four SQ-C70 complexes.
interactions in SQ dyes. There are four intramolecular hydrogen bonds in SQ dyes 1 and 2, so 1-C70 and 2-C70 achieve high VOC values (N2 V). It is zero in SQ dye 4, and consequently VOC in 4-C70 drop to 1.67 V. These results bring out the relation between molecular structures of photovoltaic devices and their photophysical properties.
4. Conclusions From the results reported in this paper, we investigated the electronic properties of four SQ dyes and their complexes with fullerene C70. The computed vertical excitations of different SQ dyes are similar in both the gas phase and chloroform solution. Their maximum of oscillator strengths are all associated with HOMO to LUMO transition. The lowest excitation energy of SQ 3 shows a steady decline when adjusting ω from 0.05 to 0.40 a−1 0 . The Δr and t indexes show that the first excited state of SQ 3 is LE state. By visualizing the hole and electron distributions in four SQ-C70 complexes, it appears that excited states 3 of 1-C70, 3 of 2C70, 1 of 3-C70 and 1 of 4-C70 are typical CT transition states, corresponding to charge transfer from SQ region to C70 region. For the SQ-C70 heterojunction, SQ dyes with two and zero intramolecular hydrogen bonds have a stronger association with fullerene C70 than four intramolecular hydrogen bonds SQ dyes. By contrast, four intramolecular hydrogen bonds SQ dyes achieve high VOC values.
Acknowledgements This research was supported by the Educational Reform Project for Postgraduates of Shanxi Province, China (Grant No. 20142043) and the Educational Science Foundation of Shanxi Normal University, China (Grant No. YJ1504). References [1] S. Jin, X. Ding, X. Feng, et al., Charge dynamics in a donor-acceptor covalent organic framework with periodically ordered bicontinuous heterojunctions [J], Angew. Chem. Int. Ed. 52 (7) (2013) 2017–2021. [2] S. Prasanthkumar, S. Ghosh, V.C. Nair, et al., Organic donor-acceptor assemblies form coaxial p-n heterojunctions with high photoconductivity [J], Angew. Chem. Int. Ed. 54 (3) (2015) 946–950. [3] K. Paudel, B. Johnson, M. Thieme, et al., Enhanced charge photogeneration promoted by crystallinity in small-molecule donor-acceptor bulk heterojunctions [J], Appl. Phys. Lett. 105 (4) (2014), 043301. [4] H. Huang, C.E. Chou, Y. Che, et al., Morphology control of Nanofibril donor-acceptor heterojunction to achieve high photoconductivity: exploration of new molecular design rule [J], J. Am. Chem. Soc. 135 (44) (2013) 16490–16496.
87
[5] H. Li, C. Fan, W. Fu, et al., Solution-grown organic single-crystalline donor-acceptor heterojunctions for photovoltaics [J], Angew. Chem. 127 (3) (2015) 970–974. [6] Q. Xu, Q. Cheng, Z. Zhang, et al., Synthesis of ZnO nanostructures by spontaneous oxidation of Zn films on p-type silicon substrates [J], J. Alloys Compd. 590 (2014) 260–265. [7] M.A. Baklar, Processing Organic Semiconductors [M], Queen Mary, University of London, 2010. [8] D. Vak, S.S. Kim, J. Jo, et al., Fabrication of organic bulk heterojunction solar cells by a spray deposition method for low-cost power generation [J], Appl. Phys. Lett. 91 (8) (2007), 081102. [9] H.J. Snaith, Perovskites: the emergence of a new era for low-cost, high-efficiency solar cells [J], J. Phys. Chem. Lett. 4 (21) (2013) 3623–3630. [10] Y. Liang, Z. Xu, J. Xia, et al., For the bright future-bulk heterojunction polymer solar cells with power conversion efficiency of 7.4% [J], Adv. Mater. 22 (20) (2010) 135–138. [11] L. Schmidt-Mende, A. Fechtenkötter, K. Müllen, et al., Self-organized discotic liquid crystals for high-efficiency organic photovoltaics [J], Science 293 (5532) (2001) 1119–1122. [12] S.E. Shaheen, D.S. Ginley, G.E. Jabbour, Organic-based photovoltaics: toward lowcost power generation [J], MRS Bull. 30 (01) (2005) 10–19. [13] C.J. Brabec, Organic photovoltaics: technology and market [J], Sol. Energy Mater. Sol. Cells 83 (2) (2004) 273–292. [14] T. Sahin, M.A. Harris, P. Vairaprakash, et al., Self-assembled light-harvesting system from chromophores in lipid vesicles [J], J. Phys. Chem. B 119 (32) (2015) 10231–10243. [15] S.F. Völker, S. Uemura, M. Limpinsel, et al., Polymeric squaraine dyes as electron donors in bulk heterojunction solar cells [J], Macromol. Chem. Phys. 211 (10) (2010) 1098–1108. [16] S.J. Chung, S. Zheng, T. Odani, et al., Extended squaraine dyes with large twophoton absorption cross-sections [J], J. Am. Chem. Soc. 128 (45) (2006) 14444–14445. [17] J.M. Hales, J. Matichak, S. Barlow, et al., Design of polymethine dyes with large thirdorder optical nonlinearities and loss figures of merit [J], Science 327 (5972) (2010) 1485–1488. [18] C. Toro, L. De Boni, S. Yao, et al., Linear and nonlinear optical characterizations of a monomeric symmetric squaraine-based dye in solution [J], J. Chem. Phys. 130 (21) (2009) 214504. [19] A. Ajayaghosh, Chemistry of squaraine-derived materials: near-IR dyes, low band gap systems, and cation sensors [J], Acc. Chem. Res. 38 (6) (2005) 449–459. [20] M. Furuki, L.S. Pu, F. Sasaki, et al., Monomolecular layer of squarylium dye J aggregates exhibiting a femtosecond optical response of delocalized excitons [J], Appl. Phys. Lett. 72 (21) (1998) 2648. [21] D. Ramaiah, I. Eckert, K.T. Arun, et al., Squaraine dyes for photodynamic therapy: mechanism of cytotoxicity and DNA damage induced by halogenated squaraine dyes plus light (N600 nm) [J], Photochem. Photobiol. 79 (1) (2004) 99–104. [22] K.Y. Law, Organic photoconductive materials: recent trends and developments [J], Chem. Rev. 93 (1) (1993) 449–486. [23] G. Wei, X. Xiao, S. Wang, et al., Arylamine-based squaraine donors for use in organic solar cells [J], Nano Lett. 11 (10) (2011) 4261–4264. [24] G. Chen, D. Yokoyama, H. Sasabe, et al., Optical and electrical properties of a squaraine dye in photovoltaic cells [J], Appl. Phys. Lett. 101 (8) (2012), 083904. [25] G. Chen, H. Sasabe, Y. Sasaki, et al., A series of squaraine dyes: effects of side chain and the number of hydroxyl groups on material properties and photovoltaic performance [J], Chem. Mater. 26 (3) (2014) 1356–1364. [26] A. Dreuw, G.R. Fleming, M. Head-Gordon, Charge-transfer state as a possible signature of a zeaxanthin-chlorophyll dimer in the non-photochemical quenching process in green plants [J], J. Phys. Chem. B 107 (27) (2003) 6500–6503. [27] M.J. Frisch, et al., GAUSSIAN 09, Revision B.01, Gaussian, Inc., Wallingford, CT, 2009. [28] Q. Liu, K. Wu, F. Tang, et al., Amino acid ionic liquids as chiral ligands in ligand-exchange chiral separations [J], Chem. Eur. J. 15 (38) (2009) 9889–9896. [29] W.J. Hehre, R. Ditchfield, J.A. Pople, Self-consistent molecular orbital methods. XII. Further extensions of Gaussian-type basis sets for use in molecular orbital studies of organic molecules [J], J. Chem. Phys. 56 (5) (1972) 2257–2261. [30] A.D. Becke, Density-functional thermochemistry. III. The role of exact exchange [J], J. Chem. Phys. 98 (7) (1993) 5648–5652. [31] Y. Zhao, D.G. Truhlar, The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: two new functionals and systematic testing of four M06-class functionals and 12 other functionals [J], Theor. Chem. Accounts 120 (1–3) (2008) 215–241. [32] T. Yanai, D.P. Tew, N.C. Handy, A new hybrid exchange-correlation functional using the Coulomb-attenuating method (CAM-B3LYP) [J], Chem. Phys. Lett. 393 (1) (2004) 51–57. [33] J.P. Perdew, Density-functional approximation for the correlation energy of the inhomogeneous electron gas [J], Phys. Rev. B 33 (12) (1986) 8822. [34] J.P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made simple [J], Phys. Rev. Lett. 77 (18) (1996) 3865. [35] A. Schäfer, C. Huber, R. Ahlrichs, Fully optimized contracted Gaussian basis sets of triple zeta valence quality for atoms Li to Kr [J], J. Chem. Phys. 100 (8) (1994) 5829–5835. [36] S.F. Boys, F. Bernardi, The calculation of small molecular interactions by the differences of separate total energies. Some procedures with reduced errors [J], Mol. Phys. 19 (4) (1970) 553–566.
88
J. Zhang, T. Li / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 184 (2017) 82–88
[37] A.V. Marenich, C.J. Cramer, D.G. Truhlar, Universal solvation model based on solute electron density and on a continuum model of the solvent defined by the bulk dielectric constant and atomic surface tensions [J], J. Phys. Chem. B 113 (18) (2009) 6378–6396. [38] C.A. Guido, P. Cortona, B. Mennucci, et al., On the metric of charge transfer molecular excitations: a simple chemical descriptor [J], J. Chem. Theory Comput. 9 (7) (2013) 3118–3126. [39] T. Lu, F. Chen, Multiwfn: a multifunctional wavefunction analyzer [J], J. Comput. Chem. 33 (5) (2012) 580–592.
[40] C.J. Brabec, A. Cravino, D. Meissner, et al., Origin of the open circuit voltage of plastic solar cells [J], Adv. Funct. Mater. 11 (5) (2001) 374–380. [41] K. Vandewal, K. Tvingstedt, A. Gadisa, et al., Relating the open-circuit voltage to interface molecular properties of donor: acceptor bulk heterojunction solar cells [J], Phys. Rev. B 81 (12) (2010) 125204. [42] U. Hörmann, J. Kraus, M. Gruber, et al., Quantification of energy losses in organic solar cells from temperature-dependent device characteristics [J], Phys. Rev. B 88 (23) (2013) 235307.
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Properties of electronically excited states of four squaraine dyes and their complexes with fullerene C70: A theoretical investigation Jian Zhang ⁎, Tingyu Li School of Chemistry and Material Science, Shanxi Normal University, Linfen 041004, China
a r t i c l e
i n f o
Article history: Received 30 November 2016 Received in revised form 28 March 2017 Accepted 30 April 2017 Available online 01 May 2017 Keywords: Electronically excited states Organic photovoltaic devices Squaraine dyes Fullerene C70
a b s t r a c t Solar cells sensitized by polypyridyl Ru(II) complexes exhibit relatively high efficiency, however those photosensitizers did not absorb the photons in the far-red and near-infrared region. At present, squaraine dyes have received considerable attention as their attractively intrinsic red light absorption and unusual high molar extinction coefficient. Here we applied density functional theory and time dependent density functional theory to investigate the properties of electronically excited states of four squaraine dyes and their complexes with fullerene C70. The influences of different functionals, basis sets and solvent effects are evaluated. To understand the photophysical properties, the investigations are basing on a classification method which splits the squaraine dyes and their complexes with fullerene C70 into two units to characterize the intramolecular density distribution. We present the signatures of their electronically excited states which are characterized as local excitation or charge-transfer excitation. The relationship between open-circuit voltage and the number of intramolecular hydrogen bonds in squaraine dyes are discussed. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Many different photovoltaic materials based on donor-acceptor organic heterojunction are being developed for large-scale solar energy conversion [1–5]. Due to low cost, low temperature, nontoxicity and high plasticity, these materials have attracted great attention like inorganic devices [6–10]. Organic photovoltaics (OPV) has come into the research focus of materials and device architecture during the past few years [11–13]. Monomeric donors have attracted growing attention for use in photovoltaic devices, as they can be easily organized in self-assembled structures [14]. Among the monomeric donors explored for photovoltaics, squaraine dyes have been intensely studied [15–18]. In addition to photovoltaics, squaraines dyes are used for a wide range of applications such as ion sensing, nonlinear optics, photodynamic therapy and imaging [19–22]. They absorb in the visible to near-IR region with high absorption coefficient. It was observed that the power-conversion efficiency (PCE) could be enhanced up to 5.7% with SQ/C60 heterojunction [23]. Chen and coworkers used fullerene C70 as an acceptor instead of C60 due to insufficient green light capturing in SQ/C60 cell. The optical absorbance of fullerene C70 is much greater than that of C60, so the SQ/C70 heterojunction provides a higher photocurrent than C60-based device [24]. Four SQ dyes were designed and synthesized with various chemical structures using different numbers of OH groups and side chains in 2014 ⁎ Corresponding author. E-mail address: [email protected] (J. Zhang).
http://dx.doi.org/10.1016/j.saa.2017.04.086 1386-1425/© 2017 Elsevier B.V. All rights reserved.
[25]. SQ dyes 1 has branched side chains at the 4-position of the two phenyl rings and four OH groups at the 2, 6-position of the two phenyl rings, 2 has linear side chains and four OH groups, 3 has linear side chains and two OH groups, while 4 has linear side chains without OH group. Experimental data such as single-crystal X-ray crystallography and XRD show that the molecular structures of these SQ dyes are very similar. Meanwhile, their molecular packing, material properties, and OPV performance are vastly different. For instance, SQ dyes 1 exhibits J-aggregation, 2 exhibits both H-aggregation and J-aggregation, 3 exhibits preferential H-aggregation, 4 exhibits J-aggregation. It is important to note that optical absorption is dictated by the physical structures and electronic structures of these SQ-C70 heterojunction, which are not fully understood. The theoretical calculation has its necessity and feasibility, such as providing assistance in regards to understanding the mechanism of SQ-C70 heterojunction and selection of SQ dyes. density functional theory (DFT) and time dependent density functional theory (TDDFT) have become the most popular approaches for calculating structures and electronic spectra of molecules with no less than 200 atoms [26], it is used to investigate electronic properties of excited states of SQ-fullerene C70 heterojunction in this study. 2. Theoretical and computational aspects All the quantum chemistry calculations were carried out with the Gaussian 09 program suite [27]. The ground-state structures of the monomers and complexes were performed using modern hybrid
J. Zhang, T. Li / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 184 (2017) 82–88
functional ωB97XD including long range and dispersion correction [28]. The 6-31G(d) basis set was used and the optimizations were unconstrained [29]. Since every one of these molecules has a closed-shell
83
electronic configuration in the ground state and does not contain transition metal atoms, which means monoconfigurational system, excited state calculations were performed by TDDFT method, assuming
Fig. 1. The optimized molecular structures of four SQ dyes.
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Table 1 The vertical excitation energies corresponding to maximum oscillator strengths, absorption maxima wavelengths and involved orbitals of four SQ dyes in gas phase and chloroform solution.
1 In gas phase 2 In gas phase 3 In gas phase 4 In gas phase 1 In chloroform 2 In chloroform 3 In chloroform 4 In chloroform
Excited states
ΔE (ev)
λmax (nm)
Oscillator strengths
Orbitals involved in the transition
1 1 1 2 1 1 1 1
2.52 2.54 2.46 2.42 2.32 2.33 2.26 2.24
491.9 488.8 503.2 511.4 534.3 531.8 547.5 553.7
1.747 1.782 1.688 1.568 1.990 2.011 1.961 1.934
HOMO → LUMO HOMO → LUMO HOMO → LUMO HOMO → LUMO HOMO → LUMO HOMO → LUMO HOMO → LUMO HOMO → LUMO
closed-shell monoconfigurational wave functions. To evaluate the dependence of the electronic structures on the functionals, the excitation properties were simulated using different parameter ω values ωB97XD, comparing with popular conventional hybrid functional B3LYP [30], empirical exchange-correlation functional M06-2 × [31], coulomb-attenuated functional CAM-B3LYP [32], and exchange-correlation functional PBE1PBE [33,34]. To evaluate the influence of basis set extension, basis sets 6–31+G(d), 6–311+G(d), 6–311++G(d,p) and TZVP (triple-zeta-valence plus polarization) [35] were used in calculations to compare the 6-31G(d) basis set. The basis set superposition errors (BSSE) were corrected by counterpoise method to accurately estimate interaction energies [36]. Computational procedure for the solvation effect in chloroform and toluene (dielectric constants (ε) are 4.70 and 2.38 at 25 °C) was based on the SMD [37] continuum solvation model implemented in the Gaussian 09. For SMD model, an intrinsic atomic SMD-Coulomb radii for the cavity and van der Waals surface with α = 1.0 were used. This method is based on the self-consistent reaction field (SCRF) of bulk electrostatic contribution that involves the solution of nonhomogeneous Poisson equation (NPE) and the cavity-dispersion-solvent-structure protocol for the solvent-accessible surface areas of the individual atoms. The investigations were basing on a classification method which splits the SQ dyes and SQ-C70 heterojunction into two units to characterize the intramolecular density distribution. The quantitative Table 2 Under different parameter ω, ε, basis sets and functionals, vertical excitation energies corresponding to maximum oscillator strengths, absorption maxima wavelengths and involved orbitals of SQ dye 3. Excited ΔE states (ev) ωB97XD/6-31G(d) ω = 5 ωB97XD/6-31G(d) ω = 10 ωB97XD/6-31G(d) ω = 15 ωB97XD/6-31G(d) ω = 20 ωB97XD/6-31G(d) ω = 25 ωB97XD/6-31G(d) ω = 30 ωB97XD/6-31G(d) ω = 35 ωB97XD/6-31G(d) ω = 40 ωB97XD/6–31+g(d) ωB97XD/6–311+g(d) ωB97XD/6–311++g(d,p) ωB97XD/tzvp pbe1pbe/6-31g(d) b3lyp/6-31g(d) cam-b3lyp/6-31g(d) m06-2×/6-31g(d) Chloroform (ε = 4.70) Toluene (ε = 2.38)
λmax (nm)
Oscillator strengths
Orbitals involved in the transition
1 1
2.47 502.4 1.696 2.47 502.6 1.692
HOMO → LUMO HOMO → LUMO
1
2.47 502.8 1.689
HOMO → LUMO
1
2.46 503.2 1.688
HOMO → LUMO
1
2.46 503.6 1.683
HOMO → LUMO
1
2.46 504.1 1.679
HOMO → LUMO
1
2.46 504.7 1.676
HOMO → LUMO
1
2.45 505.3 1.672
HOMO → LUMO
1 1 1 1 1 1 1 1 1 1
2.44 2.45 2.45 2.44 2.47 2.43 2.47 2.46 2.26 2.25
HOMO → LUMO HOMO → LUMO HOMO → LUMO HOMO → LUMO HOMO → LUMO HOMO → LUMO HOMO → LUMO HOMO → LUMO HOMO → LUMO HOMO → LUMO
507.4 505.5 506.7 508.4 502.6 509.8 501.7 503.4 547.5 550.4
1.705 1.711 1.705 1.708 1.751 1.696 1.707 1.776 1.961 1.919
indicator of electron excitation mode (Δr) is given to understand the photophysics of the SQ dyes and SQ-C70 heterojunction [38]. The smaller the Δr index is, the more likely the excitation is local excitation (LE). The bigger the Δr index is, the more likely the excitation is charge transfer excitation (CT). The t index is the main criterion of the separation of electron and hole. The t index is positive number, the separation of electron and hole is easy to see. The t index is negative number in every xyz direction, the electron and hole overlap with each other definitely. These indexes were calculated with the Multiwfn program [39]. Meanwhile, Multiwfn represented the hole and electron distributions as isosurface. The hole distribution was shown as blue and electron distribution was shown as green. 3. Results and discussion 3.1. Four SQ dyes The molecular structures of four investigated SQ dyes are shown in Fig. 1. There are four intramolecular hydrogen bonds in SQ dyes 1 and 2, two intramolecular hydrogen bonds in 3, while it is zero in 4. As a result, the twisting angle around the central 4-membered ring in 3 is significantly greater than those in 1 and 2. The vertical excitation energies obtained by TDDFT were computed at the optimized ground state minimum. Table 1 shows our calculated vertical excitation energies corresponding to maximum of oscillator strengths, absorption maxima wavelengths and involved orbitals of these SQ dyes in gas phase and chloroform solution, respectively. Maximum of oscillator strengths is associated with HOMO to LUMO transition in every case. The computed vertical excitations of different SQ dyes look the same, as an example, the gap between vertical excitation energies in 1 and 2 is only 0.02 eV in gas phase and meanwhile 0.01 eV in the chloroform solution. Taking 3 as example, the influences of different functionals, basis sets and solvent effects are evaluated and shown in Table 2. Especially in the long-range corrected functional ωB97XD, parameter ω is adjusted in order. It has a different value, describing the separation of the Coulomb operator, with the long-range part given by the exact exchange integral. From these results, it can be found that the lowest excitation energy of 3 shows a steady decline when adjusting ω from 0.05 to 0.40 a−1 0 . On the whole, variation of vertical excitation energies in 3 from choosing basis sets is 0.01 eV. Considering over the basis sets with more polarization, more diffuse, and triple-ζ functions, variation of vertical excitation energies in 3 is 0.03 eV. The above-
Table 3 The first excited state of SQ dye 3. Excited states
ΔE (ev)
λ (nm)
Oscillator strengths
1
2.46
503.2
1.688
Δr (Å)
Types of t (x,y,z) excitations
0.01496 −3.748 LE (A) −1.070 −0.740
J. Zhang, T. Li / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 184 (2017) 82–88
Fig. 2. The isosurface of hole and electron distributions in the first excited state of SQ dye 3.
mentioned data calculated at all kinds of functionals associated with 631G(d) basis set distribute in the interval from 2.43 eV to 2.47 eV. From the high dielectric constant (ε = 4.70) to the low dielectric constant (ε = 2.38) in the solvation phases, it can be found that the vertical excitation energy reduces 0.01 eV. In order to understand the electronically excited states and to characterize the intramolecular density distribution between unit A (corresponds to cyclobutene ring, benzyl and hydroxyl group) and unit B (corresponds to end amine group) in these SQ dyes, taking the first excited state of 3 as example, the Δr and t indexes are given in Table 3. As can be seen in this table, it is highly possible that the first excited state of 3 possesses strong local excitation character since it has very small Δr index. Moreover, the t indexes are all negative in any xyz direction, which means hole and electron overlap with each other strictly. Fig. 2 illustrates the isosurface of hole and electron distributions of the same excited state mentioned above. It can be observed that there is no evident spatial separation between hole and electron, clearly exhibiting both hole and electron are mainly localized on unit A region (LE(A)). We can finally conclude that the first excited state of 3 is LE state after we look into the hole and electron distributions. 3.2. SQ-fullerene C70 heterojunction In order to analyze the properties of SQ-fullerene C70 complexes, the optimized minimum-energy geometries were determined in their gas phase using DFT calculations and illustrated in Fig. 3. We can find an interesting phenomenon: the SQ starts bending and come in packages which enclose the fullerene C70. The twisting angle around the central 4-membered ring is significantly increased compared to original SQ. The binding energy is one of the key parameters reflecting the physics of photovoltaic devices. After geometry optimizations, the binding energies were corrected for BSSE at the ωB97XD/6-31G(d) level. The complex 1-C70 has a BSSE-corrected binding energy of − 24.2 kcal/mol, 2C70 is − 23.2 kcal/mol, 3-C70 is − 26.4 kcal/mol, while 4-C70 is − 26.1 kcal/mol. It can be found that the BSSE-corrected binding energy is related to the amount of intramolecular hydrogen bonds in these four SQ-C70 complexes. SQ dyes with two and zero intramolecular hydrogen bonds better combine the fullerene C70 than four intramolecular hydrogen bonds SQ dyes. The most nearest C\\C distances between the central 4-membered ring of these four SQ dyes and the fullerene C70 are as follows: 3.33 Å, 3.22 Å, 3.23 Å, and 3.44 Å. The electron-deficient cyclobutene ring is linked to different end amine groups in four SQ dyes. In the contributions of end amine groups, charge density distributions may be affected differently and cause local excitation or charge-transfer excitation. In order to understand the electronically excited states and to characterize the charge-transfer between unit A (corresponds to SQ) and unit B (corresponds to C70), the Δr and t indexes of the lowest three excited states in these four SQ-C70 complexes are given in Table 4. One can observe from this table that
Fig. 3. The optimized molecular structures of four SQ-C70 complexes.
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Table 4 The lowest three excited states of four SQ-C70 complexes. Excited states
ΔE (ev)
λ (nm)
Δr (Å)
t (x,y,z)
Types of excitations
3 of 1-C70
2.60
476.9
5.9229
CT (A → B)
2 of 1-C70
2.45
505.2
1.5242
1 of 1-C70
2.44
509.2
0.2200
3 of 2-C70
2.59
479.3
5.7030
2 of 2-C70
2.47
503.0
1.1530
1 of 2-C70
2.44
509.1
0.2971
3 of 3-C70
2.47
502.4
1.2024
2 of 3-C70
2.37
523.0
3.0255
1 of 3-C70
2.36
525.1
5.7717
3 of 4-C70
2.45
505.3
1.0969
2 of 4-C70
2.41
514.6
1.9008
1 of 4-C70
2.22
558.1
5.7856
−3.372 2.970 −1.683 −2.570 −2.275 −2.242 −3.453 −1.054 −1.115 −3.328 2.785 −1.689 −2.596 −2.170 −2.263 −3.644 −0.786 −1.070 −2.532 −2.099 −2.227 −3.478 −1.543 −1.065 −2.615 2.859 −1.173 −2.366 −2.233 −2.169 −3.251 −1.114 −1.191 −2.528 2.695 −1.407
LE (B)
LE (A)
CT (A → B)
LE (B)
LE (A)
LE (B)
LE (A)
CT (A → B)
LE (B)
in 3-C70 and 4-C70, the second and third excited states possess local excitation character since they have small Δr indexes, and meanwhile the corresponding t indexes are all negative in any xyz direction, and in consequence the overlap between hole and electron is significant. At the same time, the first excited states possess charge transfer excitation character since they have very large Δr indexes greater than 5.0, and meanwhile there exist positive numbers in corresponding t indexes, and in consequence the separation of hole-electron is clear. Fig. 4 illustrates the isosurfaces of hole and electron distributions of the lowest three excited states of these four SQ-C70 complexes. We can finally conclude these excited states belong to LE or CT states by visualizing the hole and electron distributions. From this figure, it appears that excited states 1 of 1-C70, 1 of 2-C70, 2 of 3-C70 and 2 of 4-C70 have no evident spatial separation between hole and electron, distinctly exhibiting both hole and electron are localized on the SQ region without perturbation from C70 region (LE (A)). On the contrary, in excited states 2 of 1C70, 2 of 2-C70, 3 of 3-C70 and 3 of 4-C70, both hole and electron are localized on the C70 region (LE (B)). Undoubtedly 3 of 1-C70, 3 of 2-C70, 1 of 3C70 and 1 of 4-C70 are typical CT transition states, corresponding to charge transfer from SQ region to C70 region (CT (A → B)). It is well-known that one of the most vital parameters of OPV devices is open-circuit voltage (VOC). The linearly dependent relationship between VOC and CT states has been widely discussed using experimental and theoretical methods. Besides that it has been proposed that the energy of CT states (ECT) determines the VOC data directly [40]. Vandewal et al. deduce a consistent description of the energetic losses between ECT and qVOC for a range of polymer-fullerene-blend solar 3 2
LE (A)
CT (A → B)
in 1-C70 and 2-C70 the first and second excited states possess local excitation character since they have very small Δr indexes, and meanwhile the corresponding t indexes are all negative in any xyz direction, and in consequence the overlap between hole and electron is significant. At the same time, the third excited states possess charge transfer excitation character since they have very large Δr indexes greater than 5.0, and meanwhile there exist positive numbers in corresponding t indexes, and in consequence the separation of hole-electron is clear. By contrast,
J SC h c cells [41]: V OC ¼ EqCT þ ðkBqT Þ ln ð2πfqðE Þ þ ðkBqT Þ ln ðEQEEL Þ; where kB CT −λ0 Þ
and T are Boltzmann's constant and absolute temperature, JSC and h are short-circuit current density and Planck's constant, c and f are the speed of light and proportional parameter, respectively. λ0 is reorganization energy and EQEEL is the electroluminescence quantum efficiency. By applying a best fit to the data of different studied photovoltaic devices, Hörmann et al. simplify this equation so that a linear dependence relation between VOC and ECT which can be described as [42]: VOC = ECT/ q − 0.55 V. This formula shows that there is an energy loss of 0.55 V between VOC and ECT at room temperature. Therefore the VOC of these four SQ-C70 complexes are calculated and illustrated in Fig. 5 by using this formula. We can see that the VOC values are ordered as 1-C70 ≈ 2-C70 N 3-C70 N 4-C70 which has a good agreement with the number of intramolecular hydrogen bonds in SQ dyes. This indicates that VOC in SQ-C70 OPV devices is controlled by intramolecular hydrogen bonding
Fig. 4. The isosurfaces of hole and electron distributions in the lowest three excited states of four SQ-C70 complexes.
J. Zhang, T. Li / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 184 (2017) 82–88
Fig. 5. Open-circuit voltage of four SQ-C70 complexes.
interactions in SQ dyes. There are four intramolecular hydrogen bonds in SQ dyes 1 and 2, so 1-C70 and 2-C70 achieve high VOC values (N2 V). It is zero in SQ dye 4, and consequently VOC in 4-C70 drop to 1.67 V. These results bring out the relation between molecular structures of photovoltaic devices and their photophysical properties.
4. Conclusions From the results reported in this paper, we investigated the electronic properties of four SQ dyes and their complexes with fullerene C70. The computed vertical excitations of different SQ dyes are similar in both the gas phase and chloroform solution. Their maximum of oscillator strengths are all associated with HOMO to LUMO transition. The lowest excitation energy of SQ 3 shows a steady decline when adjusting ω from 0.05 to 0.40 a−1 0 . The Δr and t indexes show that the first excited state of SQ 3 is LE state. By visualizing the hole and electron distributions in four SQ-C70 complexes, it appears that excited states 3 of 1-C70, 3 of 2C70, 1 of 3-C70 and 1 of 4-C70 are typical CT transition states, corresponding to charge transfer from SQ region to C70 region. For the SQ-C70 heterojunction, SQ dyes with two and zero intramolecular hydrogen bonds have a stronger association with fullerene C70 than four intramolecular hydrogen bonds SQ dyes. By contrast, four intramolecular hydrogen bonds SQ dyes achieve high VOC values.
Acknowledgements This research was supported by the Educational Reform Project for Postgraduates of Shanxi Province, China (Grant No. 20142043) and the Educational Science Foundation of Shanxi Normal University, China (Grant No. YJ1504). References [1] S. Jin, X. Ding, X. Feng, et al., Charge dynamics in a donor-acceptor covalent organic framework with periodically ordered bicontinuous heterojunctions [J], Angew. Chem. Int. Ed. 52 (7) (2013) 2017–2021. [2] S. Prasanthkumar, S. Ghosh, V.C. Nair, et al., Organic donor-acceptor assemblies form coaxial p-n heterojunctions with high photoconductivity [J], Angew. Chem. Int. Ed. 54 (3) (2015) 946–950. [3] K. Paudel, B. Johnson, M. Thieme, et al., Enhanced charge photogeneration promoted by crystallinity in small-molecule donor-acceptor bulk heterojunctions [J], Appl. Phys. Lett. 105 (4) (2014), 043301. [4] H. Huang, C.E. Chou, Y. Che, et al., Morphology control of Nanofibril donor-acceptor heterojunction to achieve high photoconductivity: exploration of new molecular design rule [J], J. Am. Chem. Soc. 135 (44) (2013) 16490–16496.
87
[5] H. Li, C. Fan, W. Fu, et al., Solution-grown organic single-crystalline donor-acceptor heterojunctions for photovoltaics [J], Angew. Chem. 127 (3) (2015) 970–974. [6] Q. Xu, Q. Cheng, Z. Zhang, et al., Synthesis of ZnO nanostructures by spontaneous oxidation of Zn films on p-type silicon substrates [J], J. Alloys Compd. 590 (2014) 260–265. [7] M.A. Baklar, Processing Organic Semiconductors [M], Queen Mary, University of London, 2010. [8] D. Vak, S.S. Kim, J. Jo, et al., Fabrication of organic bulk heterojunction solar cells by a spray deposition method for low-cost power generation [J], Appl. Phys. Lett. 91 (8) (2007), 081102. [9] H.J. Snaith, Perovskites: the emergence of a new era for low-cost, high-efficiency solar cells [J], J. Phys. Chem. Lett. 4 (21) (2013) 3623–3630. [10] Y. Liang, Z. Xu, J. Xia, et al., For the bright future-bulk heterojunction polymer solar cells with power conversion efficiency of 7.4% [J], Adv. Mater. 22 (20) (2010) 135–138. [11] L. Schmidt-Mende, A. Fechtenkötter, K. Müllen, et al., Self-organized discotic liquid crystals for high-efficiency organic photovoltaics [J], Science 293 (5532) (2001) 1119–1122. [12] S.E. Shaheen, D.S. Ginley, G.E. Jabbour, Organic-based photovoltaics: toward lowcost power generation [J], MRS Bull. 30 (01) (2005) 10–19. [13] C.J. Brabec, Organic photovoltaics: technology and market [J], Sol. Energy Mater. Sol. Cells 83 (2) (2004) 273–292. [14] T. Sahin, M.A. Harris, P. Vairaprakash, et al., Self-assembled light-harvesting system from chromophores in lipid vesicles [J], J. Phys. Chem. B 119 (32) (2015) 10231–10243. [15] S.F. Völker, S. Uemura, M. Limpinsel, et al., Polymeric squaraine dyes as electron donors in bulk heterojunction solar cells [J], Macromol. Chem. Phys. 211 (10) (2010) 1098–1108. [16] S.J. Chung, S. Zheng, T. Odani, et al., Extended squaraine dyes with large twophoton absorption cross-sections [J], J. Am. Chem. Soc. 128 (45) (2006) 14444–14445. [17] J.M. Hales, J. Matichak, S. Barlow, et al., Design of polymethine dyes with large thirdorder optical nonlinearities and loss figures of merit [J], Science 327 (5972) (2010) 1485–1488. [18] C. Toro, L. De Boni, S. Yao, et al., Linear and nonlinear optical characterizations of a monomeric symmetric squaraine-based dye in solution [J], J. Chem. Phys. 130 (21) (2009) 214504. [19] A. Ajayaghosh, Chemistry of squaraine-derived materials: near-IR dyes, low band gap systems, and cation sensors [J], Acc. Chem. Res. 38 (6) (2005) 449–459. [20] M. Furuki, L.S. Pu, F. Sasaki, et al., Monomolecular layer of squarylium dye J aggregates exhibiting a femtosecond optical response of delocalized excitons [J], Appl. Phys. Lett. 72 (21) (1998) 2648. [21] D. Ramaiah, I. Eckert, K.T. Arun, et al., Squaraine dyes for photodynamic therapy: mechanism of cytotoxicity and DNA damage induced by halogenated squaraine dyes plus light (N600 nm) [J], Photochem. Photobiol. 79 (1) (2004) 99–104. [22] K.Y. Law, Organic photoconductive materials: recent trends and developments [J], Chem. Rev. 93 (1) (1993) 449–486. [23] G. Wei, X. Xiao, S. Wang, et al., Arylamine-based squaraine donors for use in organic solar cells [J], Nano Lett. 11 (10) (2011) 4261–4264. [24] G. Chen, D. Yokoyama, H. Sasabe, et al., Optical and electrical properties of a squaraine dye in photovoltaic cells [J], Appl. Phys. Lett. 101 (8) (2012), 083904. [25] G. Chen, H. Sasabe, Y. Sasaki, et al., A series of squaraine dyes: effects of side chain and the number of hydroxyl groups on material properties and photovoltaic performance [J], Chem. Mater. 26 (3) (2014) 1356–1364. [26] A. Dreuw, G.R. Fleming, M. Head-Gordon, Charge-transfer state as a possible signature of a zeaxanthin-chlorophyll dimer in the non-photochemical quenching process in green plants [J], J. Phys. Chem. B 107 (27) (2003) 6500–6503. [27] M.J. Frisch, et al., GAUSSIAN 09, Revision B.01, Gaussian, Inc., Wallingford, CT, 2009. [28] Q. Liu, K. Wu, F. Tang, et al., Amino acid ionic liquids as chiral ligands in ligand-exchange chiral separations [J], Chem. Eur. J. 15 (38) (2009) 9889–9896. [29] W.J. Hehre, R. Ditchfield, J.A. Pople, Self-consistent molecular orbital methods. XII. Further extensions of Gaussian-type basis sets for use in molecular orbital studies of organic molecules [J], J. Chem. Phys. 56 (5) (1972) 2257–2261. [30] A.D. Becke, Density-functional thermochemistry. III. The role of exact exchange [J], J. Chem. Phys. 98 (7) (1993) 5648–5652. [31] Y. Zhao, D.G. Truhlar, The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: two new functionals and systematic testing of four M06-class functionals and 12 other functionals [J], Theor. Chem. Accounts 120 (1–3) (2008) 215–241. [32] T. Yanai, D.P. Tew, N.C. Handy, A new hybrid exchange-correlation functional using the Coulomb-attenuating method (CAM-B3LYP) [J], Chem. Phys. Lett. 393 (1) (2004) 51–57. [33] J.P. Perdew, Density-functional approximation for the correlation energy of the inhomogeneous electron gas [J], Phys. Rev. B 33 (12) (1986) 8822. [34] J.P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made simple [J], Phys. Rev. Lett. 77 (18) (1996) 3865. [35] A. Schäfer, C. Huber, R. Ahlrichs, Fully optimized contracted Gaussian basis sets of triple zeta valence quality for atoms Li to Kr [J], J. Chem. Phys. 100 (8) (1994) 5829–5835. [36] S.F. Boys, F. Bernardi, The calculation of small molecular interactions by the differences of separate total energies. Some procedures with reduced errors [J], Mol. Phys. 19 (4) (1970) 553–566.
88
J. Zhang, T. Li / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 184 (2017) 82–88
[37] A.V. Marenich, C.J. Cramer, D.G. Truhlar, Universal solvation model based on solute electron density and on a continuum model of the solvent defined by the bulk dielectric constant and atomic surface tensions [J], J. Phys. Chem. B 113 (18) (2009) 6378–6396. [38] C.A. Guido, P. Cortona, B. Mennucci, et al., On the metric of charge transfer molecular excitations: a simple chemical descriptor [J], J. Chem. Theory Comput. 9 (7) (2013) 3118–3126. [39] T. Lu, F. Chen, Multiwfn: a multifunctional wavefunction analyzer [J], J. Comput. Chem. 33 (5) (2012) 580–592.
[40] C.J. Brabec, A. Cravino, D. Meissner, et al., Origin of the open circuit voltage of plastic solar cells [J], Adv. Funct. Mater. 11 (5) (2001) 374–380. [41] K. Vandewal, K. Tvingstedt, A. Gadisa, et al., Relating the open-circuit voltage to interface molecular properties of donor: acceptor bulk heterojunction solar cells [J], Phys. Rev. B 81 (12) (2010) 125204. [42] U. Hörmann, J. Kraus, M. Gruber, et al., Quantification of energy losses in organic solar cells from temperature-dependent device characteristics [J], Phys. Rev. B 88 (23) (2013) 235307.