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Influence of π spacer of donor-acceptor-π-acceptor sensitizers on photovoltaic properties in dye-sensitized solar cells
Organic Electronics 76 (2020) 105429
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Influence of π spacer of donor-acceptor-π-acceptor sensitizers on photovoltaic properties in dye-sensitized solar cells
T
Kenan Suna,b, Li Wanga,b,∗, Lemin Maoa,b, Yuanmi Zhanga,b, Fang Liua,b, Jinglai Zhanga,b,∗∗ a
Institute of Upconversion Nanoscale Materials, Henan Provincial Engineering Research Center of Corrosion and Protection for Magnesium Alloys, College of Chemistry and Chemical Engineering, Henan University, Kaifeng, Henan, 475004, PR China b Institute of Upconversion Nanoscale Materials, Henan Provincial Engineering Research Center of Green, Anticorrosion Technology for Magnesium Alloy, College of Chemistry and Chemical Engineering, Henan University, Kaifeng, Henan, 475004, PR China
ARTICLE INFO
ABSTRACT
Keywords: Photon-to-electron conversion efficiency Aggregation Electronic coupling Molecular dynamics simulations Dye sensitized solar cells
On the basis of organic dyes (E)-3-(6-(7-(bis(4-methoxyphenyl)amino)benzo[c] [1,2,5]thiadiazol-4-yl)-4,4-dimethyl-4H-cyclopenta[2,1-b:3,4-b']dithiophen-2-yl)-2-cyanoacrylic acid (1) and (E)-3-(6-(7-(bis(4-methoxyphenyl)amino)benzo[c] [1,2,5]thiadiazol-4-yl)-4-methyl-4H-dithieno[3,2-b:2′,3′-d]pyrrol-2-yl)-2-cyanoacrylic acid (2), two new D-A-π-A dyes, (E)-3-(7-(7-(bis(4-methoxyphenyl)amino)benzo[c] [1,2,5]thiadiazol-4-yl)benzo [2,1-b:3,4-b']dithiophen-2-yl)-2-cyanoacrylic acid (3) and (E)-3-(9-(7-(bis(4-methoxyphenyl)amino)benzo[c] [1,2,5]thiadiazol-4-yl)-2,3-dihydrodithieno[3′,2':3,4; 2″,3'':5,6]benzo[1,2-b] [1,4]dioxin-6-yl)-2-cyanoacrylic acid (4) are designed with diphenylamine as donor, 2-cyanoacrylic acid as acceptor along with different π group. Besides the frontier orbital energy levels and absorption spectra, the interfacial properties of dye-TiO2 adsorbed system is considered. After adsorption, the energy level of both dye and TiO2 surface shifts, which would affect the overall performance. More important, the short-circuit current density (JSC), open-circuit voltage (VOC), and photon-to-electron conversion efficiency (PCE) are calculated, which is helpful to determine the effect of π group on the donor-acceptor-π-acceptor dyes. The negative influence of aggregation for the dye-TiO2 adsorbed system is also considered to evaluate the overall photovoltaic performance. The molecular simulations are carried out to explore the dynamic properties of aggregation.
1. Introduction In past two decades, dye sensitized solar cells (DSSCs) have attracted considerable attentions since the breakthrough work reported by Grätzel et al., in 1991 [1]. It is regarded as a promising alternative for the classical silicon-based solar cell because of low cost fabrication and easy manufacturing process. Although the perovskites solar cells (PSCs) have become the new favorable topics due to the rapid increase of photon-to-electron conversion efficiency (PCE), their applications is still limited because of the toxicity and instability. Therefore, it is still necessary to further develop DSSCs to enrich the solar cell family. Ru complexes is one of the most popular dyes applied in DSSCs with the recorded PCE of 11% [2]. However, its potential application is limited from the cost-effective and environmental friendly viewpoint. Organic dyes have the mature synthetic method, abundant materials, and high molar absorption. The prominent shortcoming of organic dyes is the
relative narrow light harvesting region leading to the lower PCE. In recent years, some organic dyes have been reported with PCE over 12%. However, the number of organic dyes with excellent PCE is deserved to be enriched. The variation of architecture and/or groups is an usual and effective approach to refine the overall performance [3–9]. From the experimental viewpoint, various new configurations are developed in succession. Zhu et al. firstly reported the D-A-π-A configuration, WS2, that contains indole derivatives as donor, benzothiadiazole (BTZ) and cyanoacetic acid as acceptor bridged by thiophene [10]. The insertion of one more acceptor is favorable for electron distribution and tuning the energy level. Benzothiadiazole presents widely applications as auxiliary acceptor. Katono et al. reported a D-A-π-A dye, KM10 (1, See Scheme 1), containing BTZ unit linked with diphenylamine as donor moiety leading to the PCE of 7.1% [11]. Later, Li et al. developed a new dye, FS16 (2, See Scheme 1) [12], with the same donor and auxiliary acceptor. The π group is the only difference
∗ Corresponding author. Institute of Upconversion Nanoscale Materials, Henan Provincial Engineering Research Center of Green Anticorrosion Technology for Magnesium Alloy, College of Chemistry and Chemical Engineering, Henan University, Kaifeng, 475004, PR China. ∗∗ Corresponding author. Institute of Upconversion Nanoscale Materials, Henan Provincial Engineering Research Center of Green Anticorrosion Technology for Magnesium Alloy, College of Chemistry and Chemical Engineering, Henan University, Kaifeng, 475004, PR China. E-mail addresses: [email protected] (L. Wang), [email protected] (J. Zhang).
https://doi.org/10.1016/j.orgel.2019.105429 Received 12 April 2019; Received in revised form 25 June 2019; Accepted 23 August 2019 Available online 25 August 2019 1566-1199/ © 2019 Elsevier B.V. All rights reserved.
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Scheme 1. The sketch structures of all the investigated dyes.
between KM10 and FS16. The slight variation of π group results in the deviation of PCE. Following the same rule, other two new dyes 3 and 4 are designed with benzo[2,1-b:3,4-b']dithiophene and 2,3-dihydrodithieno[3′,2':3,4; 2″,3'':5,6]benzo[1,2-b] [1,4]dioxine as π group with the goal to estimate the effect of different π group (See Scheme 1). In this work, their performance is theoretically studied and compared to uncover the relationship between their structure and property. The properties of organic dyes are qualitatively compared from the energy levels of frontier molecular orbitals (FMOs), electronic density distributions, absorption properties, interaction between organic dye and surface of TiO2, and others in previous reports [13–16]. Although some progresses have been made, the accuracy of theoretical studies is still required to be improved. Due to the lack of accurate quantitative result including short-circuit current density (JSC), open-circuit voltage (VOC), and PCE, the development of organic dyes still follows the trialerror pathway rather than from theoretical screening to experimental synthesis. The overall performance not only is determined by the properties of dyes but also is related with the aggregation process [17,18]. Moreover, the latter item is more complicate including both positive and negative influence on the PCE. However, they are rarely considered to design the organic dyes along with the configuration of dyes. In this work, JSC, VOC, and PCE are quantitatively calculated along with the aggregation to improve the reliability of theoretical evaluation.
2. Computational details The following electronic calculations about isolated dyes are carried out by the Gaussian 09 program [19]. The ground-state geometries were optimized by Becke's three-parameter hybrid exchange functional combined with the Lee-Yang-Parr correlation functional method (B3LYP) [20,21] with a standard 6-31G(d,p) basis set. Based on the optimized ground-state geometries, the absorption spectra were simulated at the time dependent (TD) LC-BLYP/6-31G(d,p) method [22,23] along with the polarized continuum model (PCM) in dichloromethane [24,25]. To get better insight into the interfacial properties of the dye-TiO2 system, the following calculations were completed in VASP (Vienna abinitio simulation package) program [26,27]. The 6 × 7 × 4 TiO2 anatase (101) supercell is taken as the adsorption surface, since the TiO2 (101) facet has been testified to be the most favorable surface in TiO2 anatase crystals [28]. A vacuum buffer space of 30 Å was added in z direction. The isolated dye, bare TiO2 surface, and dye-TiO2 system were optimized by means of projector-augmented wave method with the generalized gradient approximation (GGA) using Perdewe-BurkeeErnzerhof (PBE) exchange-correlation function [29,30]. Despite GGA/ PBE method has achieved the widespread success, it fails to accurately evaluate the non-local correlation interactions. Therefore, Grimme-D2 dispersion correction was added to the function to take into account dispersion effects [31]. Another popular method is local density 2
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approximation (LDA), which would significantly underestimate the band gap resulting in the incorrect result for the metallic system. The band gap is an important item to determine the overall performance of DSSCs. So the GGA/PBE method is finally chosen along with the Grimme-D2 [32,33]. The energy cutoff was set to be 400 eV and the optimization would stop when the force on each atom was smaller than 0.1 eV Å−1. Furthermore, the density of states (DOS) and projected density of states (PDOS) were calculated at the same level to deeply analyze the adsorption properties. For the dyes-TiO2 adsorbed system, molecular dynamics simulations were also carried out for 10 ps with an integration time step of 1 fs at T = 298 K in the constant volume constant internal energy (NVE ensemble). The dynamic simulation was implemented in the DFTB+1.3 program package [34]. The SK-parameters of mio-1-1 [35–37] for C, N, O, H, and S and tiorg-0-1 [38] for Ti were employed. 3. Results and discussion 3.1. Reliability of theoretical calculation
Fig. 2. Energy diagram of HOMO and LUMO for all dyes along with the CB of TiO2, redox potential of iodine/iodide, and experiments values for dyes 1* [11] and 2* [12], respectively. aHOMO values are calculated according to Ref. [42]. b LUMO = HOMO + Eg.
On the basis of the optimized structures, the absorption spectrum of 1 is simulated by five different functions, TD-B3LYP, TD-CAM-B3LYP [39], TD-LC-BLYP, TD-M06-2X [40], and TD-PBE0 [41] along with the same basis set of 6-31G(d,p). As shown in Fig. S1, Both absorption wavelength and strength calculated by TD-LC-BLYP/6-31G(d,p) method are more close to the corresponding experimental value as compared with other methods. Therefore, the absorption spectra for other dyes are also simulated by the same method, which are shown in Fig. 1. The maximum absorption wavelengths (λmax) of 3 (572 nm) and 4 (576 nm) are shorter than those of 1 (606 nm) and 2 (598 nm). The smaller maximum absorption wavelength indicates the narrower light harvesting region resulting in the less overall performance. Since the highest occupied molecular orbital (HOMO, H) and the lowest unoccupied molecular orbital (LUMO, L) energy levels of dyes calculated at the density functional theory (DFT) is not accurate due to its intrinsic defect, they are fitted according to the equation reported by Chi et al. [42].
Y = 1.107X
0.118
falls in a reasonable region. The LUMO energy level is calculated by the relationship between HOMO energy level and the energy gap, i.e., LUMO = HOMO + Eg. Not only the calculated absorption wavelength but also the energy levels of frontier molecular orbitals show good agreement with the corresponding experimental results from the different literature. Therefore, it is reasonable to believe the accuracy of employed methods in this work. On the basis of the calculated energy levels, they are the potential dyes for DSSCs with the HOMO energy level being lower than the redox couple iodine/triiodide electrolytes and the LUMO energy level being higher than the conduction band (CB) of TiO2. 3.2. Overall PCE
(1)
According to the following equation:
Here, Y is the fitting value for the HOMO energy level, X denotes the calculated HOMO value in gas phase. The fitting HOMO energy levels of four organic dyes are plotted in Fig. 2. The calculated values deviate from the experimental values of 0.06 eV for 1 and 0.17 eV for 2, which
PCE =
VOC JSC FF Pin
(2)
PCE is proportional to JSC, VOC, fill factor (FF), and inverse proportional to the input power of incident solar light (Pin). The FF and Pin are regarded as the same for all studied dyes. Then, the PCE is only determined by JSC and VOC. The JSC is defined by the relationship [43]:
JSC = e
LHE ( )
inj reg coll ph.AM1.5G (
)
(3)
in which LHE is the light-harvesting efficiency, Φinj is the electron-injection efficiency, ηreg is the dye regeneration efficiency, ηcoll is the collection efficiency, and ϕph.Am1.5G is the photon flux under AM1.5G solar irradiation spectra. The Фinj, ηreg, and ηcoll are normally regarded as the unit. Then, the LHE is defined by the equation: LHE = 1 10 ( ) (4), where λ is the wavelength of UV/Vis absorption spectra, ε(λ) is the molar absorption coefficient at certain wavelength, and Γ is the surface loading of sensitizers (mol·cm−2) determined by the corresponding experimental results [9]. The LHE curves and their cormax responding maximum JSC are shown in Fig. 3. The simulated JSC are −2 −2 −2 37.92 mA cm for 1, 37.77 mA cm for 2, 35.85 mA cm for 3, and max 36.28 mA cm−2 for 4, respectively. The JSC of dye 1 is slightly larger than that of dye 2, however, they are very close. There is also slight max difference for experimental results of JSC between dye 1 and dye 2. It max is ascribed to their similar absorption region and strength. The JSC of 3 and 4 are less than that of 1 and 2, which is attributed to their narrower
Fig. 1. The UV/Vis absorption spectra of all dyes calculated in dichloromethane at the LC-BLYP/6-31G(d,p) level of theory along with the experimental value for 1 [11] and 2 [12], respectively. 3
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Table 1 The calculated adsorption energy (Eads), the energy difference between the CB of TiO2 and the LUMO of dye (ΔE), and electron injection time (τinj) for the dyeTiO2 adsorbed system. Dye
Eads (eV)
ΔE (eV)a
ΔE (eV)b
τinj (fs)
1 2 3 4
−2.62 −1.47 −1.78 −1.83
0.25 0.26 0.23 0.23
0.71 0.88 0.85 0.81
2.03 1.56 0.94 0.98
a Refers to the energy difference between the CB of clean TiO2 surface and the LUMO of isolated dye. b Refers to the energy difference between the CB of TiO2 and the LUMO of adsorbed dye.
toward the more positive values. In contrast, the CB of TiO2 shifts toward the more negative value (See Fig. 5). As a result, the difference between the LUMO of dye and the CB of TiO2 is enlarged, which is beneficial for the electron injection. The Fermi energy level is artificially varied to keep the fixed value. On the basis of dye-TiO2 adsorbed system, the calculated VOC of dye 1 (See Table 2) is still larger than that of dye 2. As a result, the calculated PCE of dye 1 is the larger than that of dye 2, which is consistent with the corresponding experimental values. Although the VOC(s) of dye 3 and dye 4 are the larger than that of dye 2, especially for dye 4, which is even the larger than that of dye 1, their PCE values are still less than max those of both 1 and 2 due to the much smaller JSC . Although the max calculated values for JSC and VOC deviate large from the corresponding experimental values, the relative sequence is still consistent. It is reasonable to infer that the comparison between 3 and 4 determined by the calculated value is also reliable. Although there is still great deviation between theoretical and experimental result, the quantitative calculation at least affords the possibility to determine the performance of DSSCs under the same or different experimental conditions. The great deviations should be attributed to the following reasons: (1) NCB and surface loading of sensitizers are not the same for different experimental studies. The value of NCB is simply treated as the same in ref. 44. (2) The surface loading of four dyes is cited from Ref. [12]. The inaccurate surface loading of sensitizers would affect the JSC value. (3) The value of VOC is mainly determined by the ECBM and the number of injected electrons. The absolute values calculated by DFT method could not be compared with the experimental values, especially when the lattice is enlarged.
max Fig. 3. Simulated JSC (in mA·cm−2) and light-harvesting efficiency LHE(λ) of all dyes. The gray line is the Air Mass 1.5 Global (AM 1.5G) solar spectrum.
absorption region and less absorption strength. VOC is the other critical factor to determine the overall performance, which is defined by the following relationship:
VOC =
ECBM
Eredox + CB k T nc + B ln q q NCB
(5)
Here, ECBM is conduction band minimum of TiO2, ΔCB is the shift of ECBM when the dyes are adsorbed on TiO2, q is Coulomb charge with the value of 1.6 × 10−19, kB is the Boltzmann constant, nc is the number of injected electrons into TiO2 due to dye adsorption, Eredox is reductionoxidation potential of electrolyte, T is temperature, and NCB is the accessible density of conduction band states in the semiconductor. The temperature of 300 K and typical NCB of 7 × 1020 cm−3 are adopted according to the experiment [44]. The standard iodide/triiodide redox potential, −5.04 eV, is regarded as the reduction-oxidation potential of electrolyte (Eredox) [44]. The ECBM is calculated on the basis of dye-TiO2 system involving the energy shift due to the dye adsorption. If only the isolated dye is considered, the calculated result is not reliable since the energy level would shift after the dye is adsorbed on the TiO2 surface. It is necessary to consider the dye-TiO2 adsorbed system rather than the isolated dye. The isolated dye, clean TiO2 (101) surface, and dye-TiO2 system are all optimized by PBE method. Optimized results are plotted in Fig. 4. The bidentate bridging adsorbed model is employed, which has been popularly reported in previous literature [45]. The adsorbed energies are all negative for four dyes suggesting that the adsorption is spontaneous and stable (See Table 1). When the dye is adsorbed on the TiO2 surface, the LUMO of dye shifts
3.3. Aggregation effect Aggregation is one of inevitable properties for organic dyes. For positive side, the aggregation would enhance the dye loading on the surface and widen the absorption region. For negative side, the charge
Fig. 4. The graph a) is the adsorption configuration of dye-TiO2. The graph b) is the detail of adsorption including the adsorption energy (eV) and Ti–O distance (Ǻ). 4
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Fig. 5. Calculated DOS and PDOS for clean TiO2 surface and interfaces of TiO2 adsorbed with 1 (a), 2 (b), 3 (c), and 4 (d).
5
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Table 2 max Estimations of nc, VOC, JSC , and PCE for all dyes and corresponding experiment electrochemical parameters for 1 [11] and 2 [12]. dye
nc (cm−3)
ECBM(eV)
VOC (mV)
max JSC (mA cm−2)
PCE (%)
VOC (exp; mV)
JSC (exp; mA cm−2)
PCE (exp; %)
1 2 3 4 5
1.08 × 1021 1.07 × 1021 1.03 × 1021 1.05 × 1021 0.96 × 1021
−3.43 −3.48 −3.46 −3.40 −3.47
1619 1575 1588 1647 1582
37.92 37.77 35.85 36.28 12.50
45.54 44.14 42.26 44.33 14.67
653 609
14.5 15.92
7.1 5.24
transfer between aggregated dyes would be further increased leading to the excited electrons quenching. As a result, the less electrons would be injected into the TiO2. The intermolecular charge transfer would compete with the intramolecular charge transfer. The later item determine the electron injection time from dye into semiconductor [46,47]. The larger electron coupling between dye and semiconductor would result in the faster electron injection rate, which is favorable to improve the PCE. However, the large electron coupling between two dyes would reduce the electron injection from dye into semiconductor. The electronic coupling between stacking monomers is studied by the direct method, which is defined via the relationship: 0, site1 LUMO / HOMO
V12 = +
( l (occ )
0 l
)
F0
0, site 2 LUMO / HOMO
0, site1 0 LUMO / HOMO l
=
0, site1 LUMO / HOMO
0, site2 LUMO / HOMO
hcore
that on the initial structure. Dye 1 has the smaller intermolecular electronic coupling with the value of 0.205 than dye 2 with the value of 0.373 (See Table S1). Dye 1 not only has the larger PCE but also the less negative effect suggesting the better performance. The electronic coupling for dye 3 is the smaller than that of dye 2 but larger than that of 1, which could be the potential dye with acceptable property. Dye 4 is not suitable to be regarded as the promising dye for DSSCs with the smaller PCE and the larger intermolecular electronic coupling. 3.4. The influence of auxiliary acceptor
0, site 2 LUMO / HOMO
A new D-π-A dye, dye 5 (See Scheme 1), is designed by depletion of ancillary acceptor from dye 3. By the same method, the energies of frontier molecular orbitals and absorption spectrum are calculated for dye 5. The HOMO energy of dye 5 is stabilized and its LUMO energy is unstabilized results in the larger H-L energy gap. Correspondingly, the blue shift of λmax would be expected, which obeys the following calculated absorption spectrum (See Fig. S3). The narrower absorption max region would lead to the smaller JSC. The calculated JSC of dye 5 (12.50 mA cm−2) is almost three times less than that of dye 3. On the basis of the adsorption system, the VOC of dye 5 is only 1582 mV, which is similar with that of dye 3 (1588 mV). As a result, the overall PCE of dye 5 is worse than that of dye 3. Although lots of D-π-A dyes have been developed with satisfied PCE, it is not suitable for every group. At least, the insertion of auxiliary group is a favorable pathway to greatly refine the PCE.
0, site1 0, site2 0 LUMO / HOMO LUMO / HOMO l
(6)
where V12 is the charge transfer integral for the electron/hole and 0, site1 0, site 2 LUMO / HOMO and LUMO / HOMO represent the LUMOs/HOMOs of two adjacent molecules 1 and 2 when no intermolecular interaction is presented. F0 is the Fock operator and its density matrix is constructed from orbitals of two adjacent molecules [48,49]. To study the electronic coupling, the dimer is optimized to consider the aggregation (See Fig. S2). Taken the optimized dimer-TiO2 adsorbed system as the initial guess, the MD is performed for 10 ps to include the effect of deformation of organic dyes. As shown in Fig. 6, the perpendicular adsorbed status is broken with the obvious bent for the organic dyes over time. The average electronic coupling in the 10 ps is evaluated rather than
Fig. 6. Evolutions of the total electronic energy as a function of the simulation time.
6
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4. Conclusion
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The properties of four organic dyes are explored by combination of DFT and MD simulation to evaluate the possibility of them to be potential organic dyes in DSSCs. The λmax(s) of 3 and 4 presents blue-shift as compared with those of 1 and 2 leading to the narrower absorption region. The less λmax(s) of 3 and 4 are consistent with their larger H-L energy gap. After the dye is adsorbed on the TiO2 surface, the energy difference between LUMO of dye and CB of TiO2 is further enlarged, which is favorable for the electron injection. The calculated PCE of 3 and 4 are slightly less than that of 1 and 2, which is mainly attributed to their narrower absorption region and smaller absorption strength. However, the difference among them is not large. Moreover, the electronic coupling for dye 3 is the smallest, which is favorable for the electron injection. Therefore, dyes 3 would also be taken as the potential organic dyes. Conflicts of interest There are no conflicts of interest to declare. Acknowledgements We thank the National Supercomputing Center in Shenzhen (Shenzhen Cloud Computing Center) for providing computational resources and softwares. This work was supported by the National Natural Science Foundation of China (21503069, 21676071, 21703053). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.orgel.2019.105429. References [1] B. O'Regan, M. Grätzel, Nature 353 (1991) 737–740. [2] Y. Cao, Y. Bai, Q. Yu, Y. Cheng, S. Liu, D. Shi, F. Gao, P. Wang, J. Phys. Chem. C 113 (2009) 6290–6297. [3] Y.K. Eom, I.T. Choi, S.H. Kang, J. Lee, J. Kim, M.J. Ju, H.K. Kim, Adv. Energy Mater. 5 (2015) 1500300. [4] S.H. Kang, M.J. Jeong, Y.K. Eom, I.T. Choi, S.M. Kwon, Y. Yoo, J. Kim, J. Kwon, J.H. Park, H.K. Kim, Adv. Energy Mater. 7 (2017) 1602117. [5] Y.K. Eom, S.H. Kang, I.T. Choi, Y. Yoo, J. Kim, H.K. Kim, J. Mater. Chem. A 5 (2017) 2297–2308. [6] J.M. Ji, H. Zhou, H.K. Kim, J. Mater. Chem. A 6 (2018) 14518–14545. [7] L.L. Estrella, S.H. Lee, D.H. Kim, Dyes Pigments 165 (2019) 1–10. [8] H. Cheema, A. Baumann, E.K. Loya, P. Brogdon, L.E. McNamara, C.A. Carpentar, N.I. Hammer, S. Mathew, C. Risko, J.H. Delcamp, ACS Appl. Mater. Interfaces 11 (2019) 16474–16489. [9] J.K. Roy, S. Kar, J. Leszczynski, J. Phys. Chem. C 123 (2019) 3309–3320. [10] W. Zhu, Y. Wu, S. Wang, W. Li, X. Li, J. Chen, Z. Wang, H. Tian, Adv. Funct. Mater. 21 (2011) 756–763. [11] M. Katono, M. Wielopolski, M. Marszalek, T. Bessho, J.E. Moser, R. Humphry-Baker, S.M. Zakeeruddin, M. Grätzel, J. Phys. Chem. C 118 (2014) 16486–16493.
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Influence of π spacer of donor-acceptor-π-acceptor sensitizers on photovoltaic properties in dye-sensitized solar cells
T
Kenan Suna,b, Li Wanga,b,∗, Lemin Maoa,b, Yuanmi Zhanga,b, Fang Liua,b, Jinglai Zhanga,b,∗∗ a
Institute of Upconversion Nanoscale Materials, Henan Provincial Engineering Research Center of Corrosion and Protection for Magnesium Alloys, College of Chemistry and Chemical Engineering, Henan University, Kaifeng, Henan, 475004, PR China b Institute of Upconversion Nanoscale Materials, Henan Provincial Engineering Research Center of Green, Anticorrosion Technology for Magnesium Alloy, College of Chemistry and Chemical Engineering, Henan University, Kaifeng, Henan, 475004, PR China
ARTICLE INFO
ABSTRACT
Keywords: Photon-to-electron conversion efficiency Aggregation Electronic coupling Molecular dynamics simulations Dye sensitized solar cells
On the basis of organic dyes (E)-3-(6-(7-(bis(4-methoxyphenyl)amino)benzo[c] [1,2,5]thiadiazol-4-yl)-4,4-dimethyl-4H-cyclopenta[2,1-b:3,4-b']dithiophen-2-yl)-2-cyanoacrylic acid (1) and (E)-3-(6-(7-(bis(4-methoxyphenyl)amino)benzo[c] [1,2,5]thiadiazol-4-yl)-4-methyl-4H-dithieno[3,2-b:2′,3′-d]pyrrol-2-yl)-2-cyanoacrylic acid (2), two new D-A-π-A dyes, (E)-3-(7-(7-(bis(4-methoxyphenyl)amino)benzo[c] [1,2,5]thiadiazol-4-yl)benzo [2,1-b:3,4-b']dithiophen-2-yl)-2-cyanoacrylic acid (3) and (E)-3-(9-(7-(bis(4-methoxyphenyl)amino)benzo[c] [1,2,5]thiadiazol-4-yl)-2,3-dihydrodithieno[3′,2':3,4; 2″,3'':5,6]benzo[1,2-b] [1,4]dioxin-6-yl)-2-cyanoacrylic acid (4) are designed with diphenylamine as donor, 2-cyanoacrylic acid as acceptor along with different π group. Besides the frontier orbital energy levels and absorption spectra, the interfacial properties of dye-TiO2 adsorbed system is considered. After adsorption, the energy level of both dye and TiO2 surface shifts, which would affect the overall performance. More important, the short-circuit current density (JSC), open-circuit voltage (VOC), and photon-to-electron conversion efficiency (PCE) are calculated, which is helpful to determine the effect of π group on the donor-acceptor-π-acceptor dyes. The negative influence of aggregation for the dye-TiO2 adsorbed system is also considered to evaluate the overall photovoltaic performance. The molecular simulations are carried out to explore the dynamic properties of aggregation.
1. Introduction In past two decades, dye sensitized solar cells (DSSCs) have attracted considerable attentions since the breakthrough work reported by Grätzel et al., in 1991 [1]. It is regarded as a promising alternative for the classical silicon-based solar cell because of low cost fabrication and easy manufacturing process. Although the perovskites solar cells (PSCs) have become the new favorable topics due to the rapid increase of photon-to-electron conversion efficiency (PCE), their applications is still limited because of the toxicity and instability. Therefore, it is still necessary to further develop DSSCs to enrich the solar cell family. Ru complexes is one of the most popular dyes applied in DSSCs with the recorded PCE of 11% [2]. However, its potential application is limited from the cost-effective and environmental friendly viewpoint. Organic dyes have the mature synthetic method, abundant materials, and high molar absorption. The prominent shortcoming of organic dyes is the
relative narrow light harvesting region leading to the lower PCE. In recent years, some organic dyes have been reported with PCE over 12%. However, the number of organic dyes with excellent PCE is deserved to be enriched. The variation of architecture and/or groups is an usual and effective approach to refine the overall performance [3–9]. From the experimental viewpoint, various new configurations are developed in succession. Zhu et al. firstly reported the D-A-π-A configuration, WS2, that contains indole derivatives as donor, benzothiadiazole (BTZ) and cyanoacetic acid as acceptor bridged by thiophene [10]. The insertion of one more acceptor is favorable for electron distribution and tuning the energy level. Benzothiadiazole presents widely applications as auxiliary acceptor. Katono et al. reported a D-A-π-A dye, KM10 (1, See Scheme 1), containing BTZ unit linked with diphenylamine as donor moiety leading to the PCE of 7.1% [11]. Later, Li et al. developed a new dye, FS16 (2, See Scheme 1) [12], with the same donor and auxiliary acceptor. The π group is the only difference
∗ Corresponding author. Institute of Upconversion Nanoscale Materials, Henan Provincial Engineering Research Center of Green Anticorrosion Technology for Magnesium Alloy, College of Chemistry and Chemical Engineering, Henan University, Kaifeng, 475004, PR China. ∗∗ Corresponding author. Institute of Upconversion Nanoscale Materials, Henan Provincial Engineering Research Center of Green Anticorrosion Technology for Magnesium Alloy, College of Chemistry and Chemical Engineering, Henan University, Kaifeng, 475004, PR China. E-mail addresses: [email protected] (L. Wang), [email protected] (J. Zhang).
https://doi.org/10.1016/j.orgel.2019.105429 Received 12 April 2019; Received in revised form 25 June 2019; Accepted 23 August 2019 Available online 25 August 2019 1566-1199/ © 2019 Elsevier B.V. All rights reserved.
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Scheme 1. The sketch structures of all the investigated dyes.
between KM10 and FS16. The slight variation of π group results in the deviation of PCE. Following the same rule, other two new dyes 3 and 4 are designed with benzo[2,1-b:3,4-b']dithiophene and 2,3-dihydrodithieno[3′,2':3,4; 2″,3'':5,6]benzo[1,2-b] [1,4]dioxine as π group with the goal to estimate the effect of different π group (See Scheme 1). In this work, their performance is theoretically studied and compared to uncover the relationship between their structure and property. The properties of organic dyes are qualitatively compared from the energy levels of frontier molecular orbitals (FMOs), electronic density distributions, absorption properties, interaction between organic dye and surface of TiO2, and others in previous reports [13–16]. Although some progresses have been made, the accuracy of theoretical studies is still required to be improved. Due to the lack of accurate quantitative result including short-circuit current density (JSC), open-circuit voltage (VOC), and PCE, the development of organic dyes still follows the trialerror pathway rather than from theoretical screening to experimental synthesis. The overall performance not only is determined by the properties of dyes but also is related with the aggregation process [17,18]. Moreover, the latter item is more complicate including both positive and negative influence on the PCE. However, they are rarely considered to design the organic dyes along with the configuration of dyes. In this work, JSC, VOC, and PCE are quantitatively calculated along with the aggregation to improve the reliability of theoretical evaluation.
2. Computational details The following electronic calculations about isolated dyes are carried out by the Gaussian 09 program [19]. The ground-state geometries were optimized by Becke's three-parameter hybrid exchange functional combined with the Lee-Yang-Parr correlation functional method (B3LYP) [20,21] with a standard 6-31G(d,p) basis set. Based on the optimized ground-state geometries, the absorption spectra were simulated at the time dependent (TD) LC-BLYP/6-31G(d,p) method [22,23] along with the polarized continuum model (PCM) in dichloromethane [24,25]. To get better insight into the interfacial properties of the dye-TiO2 system, the following calculations were completed in VASP (Vienna abinitio simulation package) program [26,27]. The 6 × 7 × 4 TiO2 anatase (101) supercell is taken as the adsorption surface, since the TiO2 (101) facet has been testified to be the most favorable surface in TiO2 anatase crystals [28]. A vacuum buffer space of 30 Å was added in z direction. The isolated dye, bare TiO2 surface, and dye-TiO2 system were optimized by means of projector-augmented wave method with the generalized gradient approximation (GGA) using Perdewe-BurkeeErnzerhof (PBE) exchange-correlation function [29,30]. Despite GGA/ PBE method has achieved the widespread success, it fails to accurately evaluate the non-local correlation interactions. Therefore, Grimme-D2 dispersion correction was added to the function to take into account dispersion effects [31]. Another popular method is local density 2
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approximation (LDA), which would significantly underestimate the band gap resulting in the incorrect result for the metallic system. The band gap is an important item to determine the overall performance of DSSCs. So the GGA/PBE method is finally chosen along with the Grimme-D2 [32,33]. The energy cutoff was set to be 400 eV and the optimization would stop when the force on each atom was smaller than 0.1 eV Å−1. Furthermore, the density of states (DOS) and projected density of states (PDOS) were calculated at the same level to deeply analyze the adsorption properties. For the dyes-TiO2 adsorbed system, molecular dynamics simulations were also carried out for 10 ps with an integration time step of 1 fs at T = 298 K in the constant volume constant internal energy (NVE ensemble). The dynamic simulation was implemented in the DFTB+1.3 program package [34]. The SK-parameters of mio-1-1 [35–37] for C, N, O, H, and S and tiorg-0-1 [38] for Ti were employed. 3. Results and discussion 3.1. Reliability of theoretical calculation
Fig. 2. Energy diagram of HOMO and LUMO for all dyes along with the CB of TiO2, redox potential of iodine/iodide, and experiments values for dyes 1* [11] and 2* [12], respectively. aHOMO values are calculated according to Ref. [42]. b LUMO = HOMO + Eg.
On the basis of the optimized structures, the absorption spectrum of 1 is simulated by five different functions, TD-B3LYP, TD-CAM-B3LYP [39], TD-LC-BLYP, TD-M06-2X [40], and TD-PBE0 [41] along with the same basis set of 6-31G(d,p). As shown in Fig. S1, Both absorption wavelength and strength calculated by TD-LC-BLYP/6-31G(d,p) method are more close to the corresponding experimental value as compared with other methods. Therefore, the absorption spectra for other dyes are also simulated by the same method, which are shown in Fig. 1. The maximum absorption wavelengths (λmax) of 3 (572 nm) and 4 (576 nm) are shorter than those of 1 (606 nm) and 2 (598 nm). The smaller maximum absorption wavelength indicates the narrower light harvesting region resulting in the less overall performance. Since the highest occupied molecular orbital (HOMO, H) and the lowest unoccupied molecular orbital (LUMO, L) energy levels of dyes calculated at the density functional theory (DFT) is not accurate due to its intrinsic defect, they are fitted according to the equation reported by Chi et al. [42].
Y = 1.107X
0.118
falls in a reasonable region. The LUMO energy level is calculated by the relationship between HOMO energy level and the energy gap, i.e., LUMO = HOMO + Eg. Not only the calculated absorption wavelength but also the energy levels of frontier molecular orbitals show good agreement with the corresponding experimental results from the different literature. Therefore, it is reasonable to believe the accuracy of employed methods in this work. On the basis of the calculated energy levels, they are the potential dyes for DSSCs with the HOMO energy level being lower than the redox couple iodine/triiodide electrolytes and the LUMO energy level being higher than the conduction band (CB) of TiO2. 3.2. Overall PCE
(1)
According to the following equation:
Here, Y is the fitting value for the HOMO energy level, X denotes the calculated HOMO value in gas phase. The fitting HOMO energy levels of four organic dyes are plotted in Fig. 2. The calculated values deviate from the experimental values of 0.06 eV for 1 and 0.17 eV for 2, which
PCE =
VOC JSC FF Pin
(2)
PCE is proportional to JSC, VOC, fill factor (FF), and inverse proportional to the input power of incident solar light (Pin). The FF and Pin are regarded as the same for all studied dyes. Then, the PCE is only determined by JSC and VOC. The JSC is defined by the relationship [43]:
JSC = e
LHE ( )
inj reg coll ph.AM1.5G (
)
(3)
in which LHE is the light-harvesting efficiency, Φinj is the electron-injection efficiency, ηreg is the dye regeneration efficiency, ηcoll is the collection efficiency, and ϕph.Am1.5G is the photon flux under AM1.5G solar irradiation spectra. The Фinj, ηreg, and ηcoll are normally regarded as the unit. Then, the LHE is defined by the equation: LHE = 1 10 ( ) (4), where λ is the wavelength of UV/Vis absorption spectra, ε(λ) is the molar absorption coefficient at certain wavelength, and Γ is the surface loading of sensitizers (mol·cm−2) determined by the corresponding experimental results [9]. The LHE curves and their cormax responding maximum JSC are shown in Fig. 3. The simulated JSC are −2 −2 −2 37.92 mA cm for 1, 37.77 mA cm for 2, 35.85 mA cm for 3, and max 36.28 mA cm−2 for 4, respectively. The JSC of dye 1 is slightly larger than that of dye 2, however, they are very close. There is also slight max difference for experimental results of JSC between dye 1 and dye 2. It max is ascribed to their similar absorption region and strength. The JSC of 3 and 4 are less than that of 1 and 2, which is attributed to their narrower
Fig. 1. The UV/Vis absorption spectra of all dyes calculated in dichloromethane at the LC-BLYP/6-31G(d,p) level of theory along with the experimental value for 1 [11] and 2 [12], respectively. 3
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Table 1 The calculated adsorption energy (Eads), the energy difference between the CB of TiO2 and the LUMO of dye (ΔE), and electron injection time (τinj) for the dyeTiO2 adsorbed system. Dye
Eads (eV)
ΔE (eV)a
ΔE (eV)b
τinj (fs)
1 2 3 4
−2.62 −1.47 −1.78 −1.83
0.25 0.26 0.23 0.23
0.71 0.88 0.85 0.81
2.03 1.56 0.94 0.98
a Refers to the energy difference between the CB of clean TiO2 surface and the LUMO of isolated dye. b Refers to the energy difference between the CB of TiO2 and the LUMO of adsorbed dye.
toward the more positive values. In contrast, the CB of TiO2 shifts toward the more negative value (See Fig. 5). As a result, the difference between the LUMO of dye and the CB of TiO2 is enlarged, which is beneficial for the electron injection. The Fermi energy level is artificially varied to keep the fixed value. On the basis of dye-TiO2 adsorbed system, the calculated VOC of dye 1 (See Table 2) is still larger than that of dye 2. As a result, the calculated PCE of dye 1 is the larger than that of dye 2, which is consistent with the corresponding experimental values. Although the VOC(s) of dye 3 and dye 4 are the larger than that of dye 2, especially for dye 4, which is even the larger than that of dye 1, their PCE values are still less than max those of both 1 and 2 due to the much smaller JSC . Although the max calculated values for JSC and VOC deviate large from the corresponding experimental values, the relative sequence is still consistent. It is reasonable to infer that the comparison between 3 and 4 determined by the calculated value is also reliable. Although there is still great deviation between theoretical and experimental result, the quantitative calculation at least affords the possibility to determine the performance of DSSCs under the same or different experimental conditions. The great deviations should be attributed to the following reasons: (1) NCB and surface loading of sensitizers are not the same for different experimental studies. The value of NCB is simply treated as the same in ref. 44. (2) The surface loading of four dyes is cited from Ref. [12]. The inaccurate surface loading of sensitizers would affect the JSC value. (3) The value of VOC is mainly determined by the ECBM and the number of injected electrons. The absolute values calculated by DFT method could not be compared with the experimental values, especially when the lattice is enlarged.
max Fig. 3. Simulated JSC (in mA·cm−2) and light-harvesting efficiency LHE(λ) of all dyes. The gray line is the Air Mass 1.5 Global (AM 1.5G) solar spectrum.
absorption region and less absorption strength. VOC is the other critical factor to determine the overall performance, which is defined by the following relationship:
VOC =
ECBM
Eredox + CB k T nc + B ln q q NCB
(5)
Here, ECBM is conduction band minimum of TiO2, ΔCB is the shift of ECBM when the dyes are adsorbed on TiO2, q is Coulomb charge with the value of 1.6 × 10−19, kB is the Boltzmann constant, nc is the number of injected electrons into TiO2 due to dye adsorption, Eredox is reductionoxidation potential of electrolyte, T is temperature, and NCB is the accessible density of conduction band states in the semiconductor. The temperature of 300 K and typical NCB of 7 × 1020 cm−3 are adopted according to the experiment [44]. The standard iodide/triiodide redox potential, −5.04 eV, is regarded as the reduction-oxidation potential of electrolyte (Eredox) [44]. The ECBM is calculated on the basis of dye-TiO2 system involving the energy shift due to the dye adsorption. If only the isolated dye is considered, the calculated result is not reliable since the energy level would shift after the dye is adsorbed on the TiO2 surface. It is necessary to consider the dye-TiO2 adsorbed system rather than the isolated dye. The isolated dye, clean TiO2 (101) surface, and dye-TiO2 system are all optimized by PBE method. Optimized results are plotted in Fig. 4. The bidentate bridging adsorbed model is employed, which has been popularly reported in previous literature [45]. The adsorbed energies are all negative for four dyes suggesting that the adsorption is spontaneous and stable (See Table 1). When the dye is adsorbed on the TiO2 surface, the LUMO of dye shifts
3.3. Aggregation effect Aggregation is one of inevitable properties for organic dyes. For positive side, the aggregation would enhance the dye loading on the surface and widen the absorption region. For negative side, the charge
Fig. 4. The graph a) is the adsorption configuration of dye-TiO2. The graph b) is the detail of adsorption including the adsorption energy (eV) and Ti–O distance (Ǻ). 4
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Fig. 5. Calculated DOS and PDOS for clean TiO2 surface and interfaces of TiO2 adsorbed with 1 (a), 2 (b), 3 (c), and 4 (d).
5
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Table 2 max Estimations of nc, VOC, JSC , and PCE for all dyes and corresponding experiment electrochemical parameters for 1 [11] and 2 [12]. dye
nc (cm−3)
ECBM(eV)
VOC (mV)
max JSC (mA cm−2)
PCE (%)
VOC (exp; mV)
JSC (exp; mA cm−2)
PCE (exp; %)
1 2 3 4 5
1.08 × 1021 1.07 × 1021 1.03 × 1021 1.05 × 1021 0.96 × 1021
−3.43 −3.48 −3.46 −3.40 −3.47
1619 1575 1588 1647 1582
37.92 37.77 35.85 36.28 12.50
45.54 44.14 42.26 44.33 14.67
653 609
14.5 15.92
7.1 5.24
transfer between aggregated dyes would be further increased leading to the excited electrons quenching. As a result, the less electrons would be injected into the TiO2. The intermolecular charge transfer would compete with the intramolecular charge transfer. The later item determine the electron injection time from dye into semiconductor [46,47]. The larger electron coupling between dye and semiconductor would result in the faster electron injection rate, which is favorable to improve the PCE. However, the large electron coupling between two dyes would reduce the electron injection from dye into semiconductor. The electronic coupling between stacking monomers is studied by the direct method, which is defined via the relationship: 0, site1 LUMO / HOMO
V12 = +
( l (occ )
0 l
)
F0
0, site 2 LUMO / HOMO
0, site1 0 LUMO / HOMO l
=
0, site1 LUMO / HOMO
0, site2 LUMO / HOMO
hcore
that on the initial structure. Dye 1 has the smaller intermolecular electronic coupling with the value of 0.205 than dye 2 with the value of 0.373 (See Table S1). Dye 1 not only has the larger PCE but also the less negative effect suggesting the better performance. The electronic coupling for dye 3 is the smaller than that of dye 2 but larger than that of 1, which could be the potential dye with acceptable property. Dye 4 is not suitable to be regarded as the promising dye for DSSCs with the smaller PCE and the larger intermolecular electronic coupling. 3.4. The influence of auxiliary acceptor
0, site 2 LUMO / HOMO
A new D-π-A dye, dye 5 (See Scheme 1), is designed by depletion of ancillary acceptor from dye 3. By the same method, the energies of frontier molecular orbitals and absorption spectrum are calculated for dye 5. The HOMO energy of dye 5 is stabilized and its LUMO energy is unstabilized results in the larger H-L energy gap. Correspondingly, the blue shift of λmax would be expected, which obeys the following calculated absorption spectrum (See Fig. S3). The narrower absorption max region would lead to the smaller JSC. The calculated JSC of dye 5 (12.50 mA cm−2) is almost three times less than that of dye 3. On the basis of the adsorption system, the VOC of dye 5 is only 1582 mV, which is similar with that of dye 3 (1588 mV). As a result, the overall PCE of dye 5 is worse than that of dye 3. Although lots of D-π-A dyes have been developed with satisfied PCE, it is not suitable for every group. At least, the insertion of auxiliary group is a favorable pathway to greatly refine the PCE.
0, site1 0, site2 0 LUMO / HOMO LUMO / HOMO l
(6)
where V12 is the charge transfer integral for the electron/hole and 0, site1 0, site 2 LUMO / HOMO and LUMO / HOMO represent the LUMOs/HOMOs of two adjacent molecules 1 and 2 when no intermolecular interaction is presented. F0 is the Fock operator and its density matrix is constructed from orbitals of two adjacent molecules [48,49]. To study the electronic coupling, the dimer is optimized to consider the aggregation (See Fig. S2). Taken the optimized dimer-TiO2 adsorbed system as the initial guess, the MD is performed for 10 ps to include the effect of deformation of organic dyes. As shown in Fig. 6, the perpendicular adsorbed status is broken with the obvious bent for the organic dyes over time. The average electronic coupling in the 10 ps is evaluated rather than
Fig. 6. Evolutions of the total electronic energy as a function of the simulation time.
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4. Conclusion
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The properties of four organic dyes are explored by combination of DFT and MD simulation to evaluate the possibility of them to be potential organic dyes in DSSCs. The λmax(s) of 3 and 4 presents blue-shift as compared with those of 1 and 2 leading to the narrower absorption region. The less λmax(s) of 3 and 4 are consistent with their larger H-L energy gap. After the dye is adsorbed on the TiO2 surface, the energy difference between LUMO of dye and CB of TiO2 is further enlarged, which is favorable for the electron injection. The calculated PCE of 3 and 4 are slightly less than that of 1 and 2, which is mainly attributed to their narrower absorption region and smaller absorption strength. However, the difference among them is not large. Moreover, the electronic coupling for dye 3 is the smallest, which is favorable for the electron injection. Therefore, dyes 3 would also be taken as the potential organic dyes. Conflicts of interest There are no conflicts of interest to declare. Acknowledgements We thank the National Supercomputing Center in Shenzhen (Shenzhen Cloud Computing Center) for providing computational resources and softwares. This work was supported by the National Natural Science Foundation of China (21503069, 21676071, 21703053). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.orgel.2019.105429. References [1] B. O'Regan, M. Grätzel, Nature 353 (1991) 737–740. [2] Y. Cao, Y. Bai, Q. Yu, Y. Cheng, S. Liu, D. Shi, F. Gao, P. Wang, J. Phys. Chem. C 113 (2009) 6290–6297. [3] Y.K. Eom, I.T. Choi, S.H. Kang, J. Lee, J. Kim, M.J. Ju, H.K. Kim, Adv. Energy Mater. 5 (2015) 1500300. [4] S.H. Kang, M.J. Jeong, Y.K. Eom, I.T. Choi, S.M. Kwon, Y. Yoo, J. Kim, J. Kwon, J.H. Park, H.K. Kim, Adv. Energy Mater. 7 (2017) 1602117. [5] Y.K. Eom, S.H. Kang, I.T. Choi, Y. Yoo, J. Kim, H.K. Kim, J. Mater. Chem. A 5 (2017) 2297–2308. [6] J.M. Ji, H. Zhou, H.K. Kim, J. Mater. Chem. A 6 (2018) 14518–14545. [7] L.L. Estrella, S.H. Lee, D.H. Kim, Dyes Pigments 165 (2019) 1–10. [8] H. Cheema, A. Baumann, E.K. Loya, P. Brogdon, L.E. McNamara, C.A. Carpentar, N.I. Hammer, S. Mathew, C. Risko, J.H. Delcamp, ACS Appl. Mater. Interfaces 11 (2019) 16474–16489. [9] J.K. Roy, S. Kar, J. Leszczynski, J. Phys. Chem. C 123 (2019) 3309–3320. [10] W. Zhu, Y. Wu, S. Wang, W. Li, X. Li, J. Chen, Z. Wang, H. Tian, Adv. Funct. Mater. 21 (2011) 756–763. [11] M. Katono, M. Wielopolski, M. Marszalek, T. Bessho, J.E. Moser, R. Humphry-Baker, S.M. Zakeeruddin, M. Grätzel, J. Phys. Chem. C 118 (2014) 16486–16493.
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