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Integrated probing the influence of dye acceptor with several electron withdrawing groups for dye-sensitized solar cells
Solar Energy 195 (2020) 491–498
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Integrated probing the influence of dye acceptor with several electron withdrawing groups for dye-sensitized solar cells
T
⁎
Zhu-Zhu Suna, Shuai Fengb, , Wei-Lu Dingc a
Energy-Saving Building Materials Innovative Collaboration Center of Henan Province, Xinyang Normal University, Xinyang 464000, China College of Chemistry and Chemical Engineering, Taishan University, Taian 271021, China c Beijing Key Laboratory of Ionic Liquids Clean Process, CAS Key Laboratory of Green Process and Engineering, State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Dye-sensitized solar cells Organic dyes Electron withdrawing group Charge transfer Density functional theory
In order to explore the influence of diverse acceptors with varying amounts of electron withdrawing groups on the organic dye in dye-sensitized solar cell (DSCs), a series of JF419 organic analogues with several cyano groups (CN) on acceptor were engineered. By performing theoretical studies on these investigated dyes and the corresponding dye/(TiO2)38 complexes, two benzoic acid units (benzothiadiazole benzoic acid and vinyl benzoic acid) with more -CNs were identified as alternative acceptors for DSCs. Particularly, S3 and S5 dyes exhibited impressive red-shift in spectra (ca. 77 nm for S3 and 48 nm for S5 compared with JF419), more accessible charge separation state and large driving force for dye regeneration (ΔGreg = 0.51 eV for S3 and ΔGreg = 0.61 eV for S5). According to quantifying the charge transfer (CT) on semiconductor surface, it found that the growing electron withdrawing groups on acceptor can strengthen the interaction between dyes and TiO2 and provide favorable conditions for electron injection. A brilliant balance between the electron injection time (τ = 6.8 fs for S3 and τ = 7.3 fs for S5) and the electron-hole recombination distance (r = 20.5 Å for S3 and r = 19.2 Å for S5) could be acquired for S3 and S5 dyes. Synthetically, introducing electron withdrawing groups on acceptor for organic dyes is a valid approach for gaining great performance of DSCs.
1. Introduction Dye-sensitized solar cells (DSCs) have been extensively investigated due to their comparable efficiency and cost-effectiveness since the breakthrough by O’Regan and Grätzel in 1991. With continually exploring and efforts, the power conversion efficiency (PCE) of DSCs has improved from the initial 7% to 14.3% in 2015 under the standard air mass 1.5 global (AM1.5G) condition (O'Regan and Grätzel, 1991; Kakiage et al., 2015). The outstanding PCE of DSCs can display over 20% under artificial indoor lights (Freitag et al., 2017; Tingare et al., 2017). Gaining from the tailorability of dye molecules (Purnama et al., 2019; Xu and Mallouk, 2019), DSCs draw more attraction to their practical applicability such as building-integrated photovoltaic (BIPV) windows (Fakharuddin et al., 2014; Yoon et al., 2011; Heiniger et al., 2013; Zhang et al., 2014). Recently, a stable blue photosensitizer was reported by Wang et al. for color palette of DSCs, and an impressive PCE of 12.6% was achieved (Ren et al., 2018). Meanwhile, the effect of acceptor position modulation has been investigated by selecting benzothiadiazole as a representative acceptor (Chai et al., 2019). As the
⁎
heart of DSCs, the dye is responsible for light- harvesting, charge separation at dye/semiconductor interface, and dye regeneration, etc (Feng et al., 2019). By reasonably designing and synthesizing dyes, especially donor-acceptor metal-free organic dyes, the PCE of DSCs can be enhanced availably. Fluorene-based photosensitizers, which can impede undesired electron recombination and extend the electron lifetime of dyes in the semiconductor, have been successfully employed in the DSCs recently (Yella et al., 2013). Representative fluorene-based dye JF419 (Yella et al., 2013); whose donor is constituted by fluorene unit and with 4Hcyclopenta [2,1-b:3,4-b′] dithiophene (labeled CPDT) unit as π bridge, displayed its distinguished photovoltaic performance (PCE = 10.3%) in the solar cell. However, the conventional acceptor cyanoacrylic acid in JF419 may deplete the durability, weaken the light harvesting capacity and decrease the electron injection of dye, which are obstacles for the practical application of DSCs. Searching for a substitutable acceptor for JF419 dye may be available for solving aforementioned problems. Enlightened by the remarkable performance of benzothiadiazole benzoic acid (BATA) unit in C259 dyes (Zhang et al., 2013); in this research we
Corresponding author. E-mail address: hff[email protected] (S. Feng).
https://doi.org/10.1016/j.solener.2019.11.076 Received 3 September 2019; Received in revised form 12 November 2019; Accepted 21 November 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
Solar Energy 195 (2020) 491–498
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Scheme 1. Chemical structure of JF419 and dyes with different accepters employed with BTBA and VBA building blocks for the S-series dyes, respectively.
the most potential organic dyes toward more efficient DSCs.
replaced cyanoacrylic acid with BATA unit on the JF419 backbone. Meanwhile, vinyl benzoic acid (VBA) was also introduced in the JF419 backbone as alternative acceptor. According to our previous studies (Feng et al., 2019; Sun et al., 2018), some subtle modulation on the acceptor of dye can bring apparent variations in the cell performance. The replaced acceptors were embellished by some electron withdrawing groups. In order to deeply detect the effects of different acceptors with varying amounts of electron withdrawing groups on organic dye in DSCs, a series of JF419 organic analogues were designed by introducing cyano groups (CN) on these two different blocks (BATA and VBA). The newly designed organic dyes were labeled as S1, S2, S3, S4 and S5, respectively (see Scheme 1). The organic dyes including JF419 and all designed dyes were performed theoretical simulations. Systematic investigations demonstrate that the increased electron-withdrawing groups will improve these sensitizers’ light-harvesting capability, charge separation and interface electron transfer properties on semiconductor surface. The S3 and S5 with multi-CNs were screened out as
2. Computational details In our works, all the dyes (JF419 and designed dyes) were optimized using density functional theory (DFT) with the hybrid functional B3LYP, coupled with the 6-311G** basis set (Becke, 1993; Chen et al., 2013). By performing frequency analyses on these optimized dyes at the same level of theory, no imaginary frequency confirmed their stationary nature at energy minimum. Solvent effects of dichloromethane (DCM) were considered by means of the Conducting Polarizable Continuum Model (C-PCM) (Tomasi et al., 2005). Moreover, in order to illustrate the effects of different functionals on the geometric optimization, the JF419 dye was optimized with the B3LYP, MPW1K, M062X, and CAMB3LYP functionals, respectively. As listed in Table S1, the B3LYP/6311G** method can produce the stablest molecular geometry, and therefore this method was selected for the molecular optimizations. The UV–vis spectra were simulated by performing vertical transitions of the 492
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can be captured a track from the contributions of different acceptors on the HOMO and LUMO: more proportions were assigned for the LUMOs and ignorable distributions were dispensed on the HOMOs as listed in Table1. The stable HOMOs and the energetic down-shift LUMOs result in narrowed molecular energy gaps (△EHL), which ensured charge separation upon photo-excitation and provided more valid electron transport channel across the dye/TiO2 interface. Noticeably, the shrinkage for △EHL went along with the electron-withdrawing capability on acceptors evidenced by the data in Figure S1 (△EHLJF419 = 2.05 eV > △EHL-S1 = 1.90 eV > △EHL-S2 = 1.67 eV > △EHL-S3 = 1.66 eV, △EHL-JF419 = 2.05 eV > △EHL-S4 = 1.90 eV > △EHL-S5 = 1.81 eV). In the light of previous work (Pastore et al., 2013); the lowest dyedye excited state was analogous to the experimental optical band gaps. In this work, we simulated the lowest dye-dye excited state by adding the first singlet excitation energy to the HOMOs and compared it with corresponding simulated absorption spectra. The data of all newly designed dyes were illustrated in Fig. 1 and Fig. 2a. All the simulated absorption spectra had almost spread from 300 to 700 nm presenting amazing capability in light harvesting and showed red-shifted bands at 540–600 nm compared with the original JF419 dye. Moreover, the contracted dyes’ lowest dye-dye excited states(△EHL-S2 = 2.04 eV < △EHL-S3 = 2.06 eV < △EHL-S1 = 2.21 eV < △EHL-JF419 = 2.36 eV, △EHL-JF419 = 2.36 eV > △EHL-S4 = 2.29 eV > △EHL-S5 = 2.16 eV) were consistent with the redshifts of absorption spectra (λmax,S2 = 608 nm > λmax, S3 = 603 nm > λmax,S1 = 560 nm > λmax,JF419 = 526 nm; λmax,S5 = 574 nm > λmax,S4 = 543 nm > λmax,JF419 = 526 nm). As listed in Table 2, the dominating contribution to the λmax is the transition from HOMO to LUMO for each dye and the redshifts also followed the shrinking energy gaps. Additionally, the negligible distinction for S2 and S3 λmax as well as the more red shifted λmax,S5 for S5 than λmax,S1 for S1 demonstrated that the increasing amount of electron-withdrawing group (-CN) on acceptor could reduce the losses from lower conjugate in dyes for spectrum. Notably, the absorption spectra of investigated dyes were calculated by several methods (MPW1K, CAM-B3LYP, B3LYP and M062X) at the same levels to verify the simulation’s reliabilities. As listed in Table S2, by comparing the calculated maximum absorption wavelengths and the experimental value of JF419 (554 nm (Yella et al., 2013), the MPW1K method could describe the absorption spectra preferably. Moreover, the mean errors between the MPW1K functional and the other functionals (CAM-B3LYP, B3LYP and M062X) were also estimated. It can be found that the absorption spectra were largely overestimated by the B3LYP method and were underestimated by the CAM-B3LYP and M062X methods. The similar phenomena were also observed in the calculation of vertical emission wavelengths (Table S3). All the results above manifested the incremental electron-withdrawing groups on the acceptor could shrink the energy gaps and optimize the spectral properties. Additionally, an excellent acceptor in dye to improve the spectral properties should balance the relationship between the introduced acceptor blocks and the numbers of the electronwithdrawing groups on acceptor.
lowest 50 singlet–singlet excited states in the DCM solution employing single-point time-dependent DFT (TD-DFT) calculations. The vertical emission energies were also simulated by carrying out single-point TDDFT calculations based on the optimized first exited state. In view of the accuracy, we employed the most appropriate method CAM-B3LYP/631G* in DCM solution (Yang et al., 2015) to simulate the first exited state. In order to identify the reliable functional that can reproduce the experimental spectra of JF419 dye best, several methods were tested for absorption spectrum (MPW1K, CAM-B3LYP, B3LYP and M062X) and vertical emission energies (MPW1K, CAM-B3LYP, B3LYP and M062X), and the data see supporting information. Finally, the hybrid functional MPW1K and 6-311G** basis set (Feng et al., 2015; Lynch et al., 2000) were regarded as the suitable methodology to describe spectrum properties. The interfacial characteristics of dye on TiO2 surface were obtained from the optimized dye/(TiO2)38 geometries, which were performed by DFT with B3LYP, coupled with Lanl2dz for Ti atom and the 6-31G* for other atoms with Gaussian 09 program package. Based on previous works (De Angelis et al., 2010); a (TiO2)38 cluster cutting from an anatase slab with bare (1 0 1) surface was adequate to describe the surface effects. Additionally, all the models of dye absorbed on TiO2 were adopted the bidentate-bridging configuration due to favorable energy (Pastore and De Angelis, 2012; Ding et al., 2017). To be consistent with singular dyes, the single-point DFT and TD-DFT calculations in DCM solution were carried out to simulate the electronic and optical properties with the B3LYP and MPW1K functions and 6-311G** basis set, respectively. All the calculations above were performed with the Gaussian 09 program package (Frisch et al., 2009).
3. Results and discussions 3.1. Configurations and electronic properties of single dyes The dyes’ configurations, energy levels and light-harvesting capabilities have been scrutinized based on DFT and TD-DFT approaches. The data of torsional angles for the investigated dyes (labeled on Scheme 1) were listed in Table 1. Clearly, the torsional angles θ1 between benzothiadiazole (vinyl) unit and benzoic acid were affected by the steric hindrance of different building blocks. The values of θ1 are enlarged obviously when the acceptor is BTBA blocks, while the values of θ2 are significantly susceptible to the numbers of electron withdrawing cyano group (see Table 1). Moreover, all the sensitizer energy levels of the HOMOs and LUMOs were controlled within a reasonable range, which were lower than the redox potential of I−/I− 3 (−4.60 eV vs in vacuum) and higher than the conduction band of TiO2 (−4.00 eV vs in vacuum), respectively (see details in Figure S1 and Table 1). The appropriate energy levels could guarantee the sufficient driving force for dye regeneration and unidirectional electron transfer process of DSCs. As shown in Figure S1, the LUMO levels were affected distinctly by the incorporated electron withdrawing groups, while the variation of HOMO levels was slight. It Table 1 The torsional angles, orbital energy levels and molecular orbital coefficients for the investigated dyes (H = HOMO, L = LUMO, A = Acceptor and D = Donor). Dye
JF419 S1 S2 S3 S4 S5
Torsional angles
Energy levels
HOMO
3.2. Characteristic for the intramolecular charge transfer It is well known that the extent of the intramolecular charge transfer (ICT) is critical for the charge separated state of dyes, which guaranteed unidirectional electron transfer process of DSCs. Charge transfer and separation are important processes governing numerous chemical reactions. Obtaining fundamental understanding of these processes and the underlying mechanisms will be helpful for photochemistry (Chi et al., 2019). Here, the charge-transfer parameters containing the transferred charge (qCT ), the charge-transfer distance (dCT ), and the exciton binding energy (EB ) were evaluated to explore the electron-hole separation of designed dyes.
LUMO
θ1
θ2
H
L
A
D
A
D
0.3 −36.1 −37.3 −31.8 1.5 13.0
– −0.1 −0.2 24.9 −17.0 −20.4
−4.92 −4.79 −4.81 −4.82 −4.85 −4.93
−2.87 −2.89 −3.13 −3.16 −2.94 −3.12
3.3% 5.0% 5.2% 4.5% 4.9% 4.6%
84.3% 73.3% 73.6% 77.4% 76.4% 82.0%
39.2% 81.1% 87.1% 82.0% 74.0% 61.1%
7.7% 2.1% 1.3% 2.1% 3.0% 4.8%
The key dihedral angles are shown in Scheme 1. 493
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Fig. 1. Simulated the frontier molecular orbitals (HOMOs and LUMOs) and energy levels alignment (HOMOs and Lowest dye-dye excited state) of investigated dyes related to the experimental conduction band edge of TiO2.
configuration in the subsequent works. (Pastore and De Angelis, 2012) As shown in Fig. 2b, all UV–vis spectra for the adsorbed dyes displayed distinctly red-shifted (ca. 20 nm) contrasted with free dyes under the identical computational procedure. The tendency for spectra is similar to free dyes with more red shifted for BTBA blocks and stronger molar absorption coefficient with VBA blocks. The λmax of S3/ (TiO2)38 complexes (622 nm) are more red-shifted ca. 50 nm than that of the JF419/(TiO2)38 complexes (567 nm). Moreover, the increased numbers of cyano group facilitate the amounts of red-shifted absorption spectrum (λmax,S3 = 622 nm > λmax, S1 = 572 nm, λmax,S5 = 598 nm > λmax,S4 = 564 nm). Further analyzing the molecular orbital distribution (Figure S3), it can be found that all the LUMOs of dye/(TiO2)38 complexes mainly fixed on the TiO2 nanoparticle, while the HOMOs delocalize over whole dye molecules. Such single-particle excitations facilitate the electron injection at the dye/ semiconductor interface and are indispensable for a high-efficiency cell assembly. According to the definition τ = 1.499/(fE 2) , the exited-state lifetimes (τ, Table 3) of the investigated dyes are simulated to evaluate the decay of the exited state from S1 to S0, where f and E correspond to the oscillator strength and the excitation energy of different electronic states, respectively (Zhang et al., 2013; Rohatgi-Mukherjee, 1978). Obviously, the lower oscillator strength and vertical emission energy result in more extended exited-state lifetime. As the absorption spectrum, the dyes with BTBA blocks had smaller vertical emission energy and lower oscillator strength, while these values were larger for the dyes with VBA blocks (Table S3). The sequence for τ was S5 (2.55 ns) < S4 (2.57 ns) < S3 (3.93 ns) < S1 (4.01 ns) < S2 (4.65 ns). It is found that the strengthened conjugative effect contribute more to extend the exited-state lifetimes. But the comparable τ for S4 and S5 as well as S1 and S3 indicated the slight effect for the increased electron-withdrawing groups on τ values. Although the longer exitedstate lifetime (τ) for dye should be concerned as one affecting elements for the electron injection from dye to the TiO2 conduction band (CB), various aspect affections should also be taken into account. The driving forces for the electron injection (ΔGinj) and dye regeneration (ΔGreg) are other elements to take into account because of the direct correlation with the charge generation step in DSCs. The parameters associated with the driving forces (excited-state oxidation potential ESOP and ground-state oxidation potential GSOP) were calculated and listed in Table 3. Using our formerly reported technique (Feng et al., 2019; Sun et al., 2018), the relevant parameters including the energies of dyes excited, oxidized and ground states were calculated
Following the method reported by Ciofini et al. (1955); Le Bahers et al. (2011); the amounts of qCT and dCT were calculated. As shown in Figure S2, all the transferred charge of designed dyes are larger than that for JF419, demonstrating superior charge separated state for these dyes. By analyzing the numerical values of qCT and the geometry parameters (Table 3), the qCT for dyes with BATA blocks was found following the order: S2 (qCT = 0.85e ) ≈ S3 (qCT = 0.84e ) > S1 (qCT = 0.78e ), while the qCT for dyes with VBA blocks were approximate (S4qCT = 0.76e and S5qCT = 0.74e ). It suggested that the increased cyano groups facilitating the charge separated state. Meanwhile, the simulated charge-transfer distance (dCT ) emphasized the analogous suggestion: S3 (dCT = 6.52Å ) > S4 (dCT = 6.29 Å ) > S5 (dCT = 6.17 Å ) > S2 (dCT = 6.11Å ) > S1 (dCT = 5.68 Å ). The longer distance was beneficial to charge separated state of dyes. The charge transfer characteristic was further appraise by exciton binding energy (EB ), which was simulated through the difference between the electronic and optical band gaps (Scholes and Rumbles, 2006). The electronic band gaps identified with the difference between the HOMO and the LUMO energies and the first singlet excitation energies were equal to the optical gaps (Li et al., 1873; Duan et al., 2013). The EBs listed in Table 3 displayed a sequence of S5 (0.29 eV) < S3 (0.33 eV) < S4 (0.35 eV) < S2 (0.36 eV) < S1 (0.45 eV), which presented upward trend with the weakened ability of electron withdrawing on dyes. The less the Coulomb interaction, the easier the dissociation of the exciton. Above all, the more transferred charges and the lesser exciton binding energies can further guarantee unidirectional electron transfer process and facilitate interfacial photoelectron injection of DSCs. Although there were slightly differences on the trends of the three charge-transfer parameters for the investigated dyes, the newly designed dyes can effortlessly achieve the charge separation state compared with that of JF419. Furthermore, the increasing amount of electron-withdrawing group (–CN) on acceptor indeed promoted intramolecular charge transfer. The conclusion was consistent with the electronic and spectral properties. 3.3. Characteristic for dye/(TiO2)38 complexes. To assess the feasibility of these investigated dyes in DSCs, the performances of dye/(TiO2)38 complexes were studied in this section, including the absorption spectrum, the excited-state time, the driving force of electron injection and regeneration and electron injection rate. Considering the energetic favorable architecture, all the model of dye/ (TiO2)38 compounds were employed the bidentate-bridging 494
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Fig. 2. Calculated optical absorption spectra of the investigated dyes.
from an excited state dye to the semiconductor. Learned from Figure S4, all the investigated dyes have higher ESOP values than the CB edge of TiO2 in energy and ΔGinj of these dyes are in the scope of 0.7–1.1 eV. Although the increased amounts of electron-withdrawing groups resulted in a cathodic shift for ESOP compared to JF419, the ΔGinj values could satisfy the injection requirement thermodynamically. Moreover, the GSOP values are lower than the oxidation potential of redox couple ([Co(bpy)3]2+/3+) revealing thermodynamically favorable regeneration of oxidized dyes. Particularly, there are significant declining trend of GSOP values with the growing numbers of withdrawing groups and decreasing conjugated groups. Comparing all regeneration driving force of dyes (ΔGreg, Table 3), the energy difference between GSOP and the oxidation potential of [Co(bpy)3]2+/3+, the ΔGreg with the sequence was S2 (0.27 eV) < S1 (0.29 eV) < S4 (0.48 eV) < S3 (0.51 eV) < S5 (0.61 eV). Obviously, the extended conjugative units (S2 and S1 dyes) were adverse to regeneration, while the increased amounts of electron-withdrawing groups on acceptors could be made up the detriment.
Table 2 The spectral properties for the investigated dyes and: calculated maximum absorption wavelengths for isolated dyes and complexes (λ in nm), the lowest excitation energy (in eV), oscillator strengths (f), Light-harvesting efficiency (LHE) and the main orbital contributions (H = HOMO, L = LUMO). Dye
λdye(/TiO2)
△EEX
f
LHE
Main assignment
JF419 S1 S2 S3 S4 S5
526 560 608 603 543 574
2.36 2.21 2.04 2.06 2.29 2.16
1.78 1.51 1.38 1.59 2.14 2.20
0.98 0.97 0.96 0.97 0.99 0.99
H H H H H H
(567) (572) (624) (622) (564) (598)
→ → → → → →
L L L L L L
(77%), (68%), (69%), (69%), (70%), (73%),
H-1 H-1 H-1 H-1 H-1 H-1
→ → → → → →
L L L L L L
(18%) (26%) (25%) (25%) (21%) (20%)
at the CAM-B3LYP/6-31G+(d,p) level. The difference between ESOP and the CB edge of TiO2 was used to estimate the electron injection driving force. As is known to all, the driving force should be on the order of tenths of an eV to guarantee an effective electron injection 495
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Table 3 Intramolecular charge transfer parameters (qCT in e, dCT in Å, EB in eV and τ in ns), charge transfer amount (Δq, e), Ground-state and excited-state oxidation potentials (GSOP and ESOP), driving forces (△Ginj and △Greg), and recombination distance (r). Dye
qCT
dCT
EB
τ
Δq
GSOP
ESOP
△Ginj
△Greg
r
JF419 S1 S2 S3 S4 S5
0.73 0.78 0.85 0.84 0.76 0.74
5.43 5.68 6.11 6.52 6.29 6.17
0.35 0.45 0.36 0.33 0.35 0.29
2.66 4.01 4.65 3.93 2.57 2.55
0.69 0.77 0.74 0.91 0.71 0.82
−5.52 −5.29 −5.27 −5.51 −5.48 −5.61
−3.16 −3.08 −3.23 −3.45 −3.19 −3.45
1.01 1.09 0.94 0.72 0.98 0.72
0.52 0.29 0.27 0.51 0.48 0.61
15.2 20.0 22.1 20.5 18.4 19.2
ℏΓ =
where the pi and εi are the portion of the i-th molecular orbital and its energy, respectively. According to Fig. 4, the mixing values for the adsorbate LUMO (ELUMO (ads ) ) are downshift from −2.91 eV to −3.32 eV along with the increased numbers of electron-withdrawing groups. However, the broadening width (ℏΓ ) is different from the trend of ELUMO (ads ) , which are relevant to the location of the electron-withdrawing group and the conjugated unit. It is observed that the VBA block facilitates broadening the ℏΓ compared to the BTBA block, while the cyano group on the conjugated unit narrowed the ℏΓ . Based on the formula τ (fs ) = 658/ℏΓ(meV ) , which was used to evaluate the heterogeneous electron-transfer time, the wider of the broadening width between adsorbate and TiO2 surface lead to the fast electron-injection time. Therefore, the S3, S4 and S5 displayed a positive influence on JSC, whose τ are relative less than that of other dyes (τS4 =4.6fs < τS3 =6.8fs < τS5 =7.3fs < τS1 =10.7fs < τS2 =28.8fs). It suggested that the increased electron-withdrawing groups on acceptor could accelerate the photoelectron injection. Noticeably, the electron injection rate for S1 and S2 dyes were sluggish markedly rooting in the prolonged conjugated groups. However, the shortened electron-injection time for S3 testified that the compensation could be offered by adding electron-withdrawing groups on acceptor. The effect of electron recombination from the injected photo-electrons to the oxidized dyes is also taken into account to assess its negative influence on the open-circuit voltage in DSCs. Because of the sensitivity to the spatial separation of the dye cation hole and the semiconductor surface, the recombination rate was roughly estimated by the distance between the dye cation HOMO and the semiconductor surface (Clifford et al., 2004). The distances for the investigated dyes (see Table 3) were elongated in various degree compared with that of the JF419 (15.2 Å). It indicated that these new tailored JF-series dyes are expected to reduce the charge recombination possibility in DSCs. Notably, the spatial distance between the dye cation hole and the semiconductor surface (shown in Figure S5) was stretched with the growing numbers of cyano group on the acceptors, such as the distances for S5 (19.2 Å) and S3 (20.5 Å) are larger than those of S4 (18.4 Å) and S1 (20.0 Å), respectively. It is illustrated that the introduced electron drawing groups are beneficial to lower the recombination rate, which will help to improve the performance of DSCs. It is necessary to mention that although these parameters were roughly estimating the electron injection and recombination progress in this work, the conclusion is referable for the performance of dyes with variable electron withdrawing groups by evaluating the identical model. 4. Conclusions In this work, a series of JF419 organic analogues with alternative acceptors (BTBA and VBA) and varying amounts cyano groups (CN) on acceptor were engineered to comprehensively assess the effects including the substituted acceptors and the amounts of electron withdrawing groups on the organic dye in DSCs. It can be found that the embedded benzothiadiazole unit in dyes (such as S1, S2 and S3) could strengthen π-conjugacy to promote the spectrum red-shift and prolong excited-state lifetimes. However, further exploring the intramolecular charge transfer and interface electronic transfer properties, the increased benzothiadiazole groups in dyes showed adverse effect on these properties including regeneration driving force, the amount of interface charge transfer and the efficiency of electron-injection. Fortunately, these negative impacts could be offset by increasing cyano groups on acceptor to gain better performance for the JF-based dyes. Particularly, the S3 dye (BTBA acceptor) with two CN groups on benzoic acid shows more red-shift absorption spectra and the S5 dye (VBA acceptor) with one CN group on vinyl and two CN groups on benzoic acid shows
( ) ℏΓ
1 2 π (E − E 2 LUMO (ads )) +
ℏΓ 2 2
( )
where ELUMO (ads ) signifies the dye’s LUMO after adsorbed on the TiO2. ELUMO (ads ) and ℏΓ can be characterized by:
ELUMO (ads ) =
|εi − ELUMO (ads )|
i
Apart from the driving force, the dye reorganization energy is another important parameter to determine the charge transfer process in photoelectron injection and dye regeneration, which reflects the energy alteration of the geometry relaxation during the charge transfer process. The reorganization energy was calculated with the formula of λ = [E 0 (M+) − E 0 (M 0 ) + E + (M 0 ) − E + (M+)], where E 0 and E + represent the energies of neutral and cationic states, and M 0 and M+ represent the optimized geometries of neutral and cationic dyes, respectively. In this work, the equilibrium (λ eq ) represented the inner reorganization energy and the outer reorganization energy was also calculated by non-equilibrium model (λneq ) to evaluate the contribution of the surrounding solvent molecules. As listed in Table S4, λ for all investigated dyes changed rarely and displayed slightly lower values compared to JF419, suggesting beneficial properties for promoting the charge transfer in electron injection and dye regeneration processes. The charge transfer occurred at the dye–titanium interface was also studied to manifest the effect of dyes on the electronic injection. It can be qualitatively described by the isodensity sketches of electron density difference (Ronca et al., 2012; Belpassi et al., 2008) and performed in Multiwfn 3.4 (Lu; Lu and Chen, 2011). As shown in Fig. 3; there is a distinct rearrangement of electron density in the dye/(TiO2)38 systems along with an accumulation at the dye–TiO2 interface and a depletion around the anchoring group. It indicated that the charge transfer were indeed taken place at the dye/(TiO2)38 interface for all the designed dyes. Furthermore, the amount of charge transfer (CT) was quantitatively characterized by charge displacement analysis (CD) (Fig. 3). Obviously, the amount of CT was ascending accompany with the growing number of electron-withdraw groups on the anchoring group. Among all dye/(TiO2)38 systems, the S3/(TiO2)38 (0.91 e−) and S5/ (TiO2)38 system (0.82 e−) had bigger CT, while, the CT were smaller for S1/(TiO2)38 (0.77 e−), S2/(TiO2)38 (0.74 e−) and S4/(TiO2)38 (0.71 e−) (as listed in Table 3 and Fig. 3). The result indicated that the growing electron-withdraw groups on dyes could strengthen the interaction between the dyes and TiO2 compared to the dyes with increased conjugative groups, which benefited for the electron injection from dye to the TiO2 conduction band. Deeply discussing the effect of dye’s acceptor on the interfacial electron-injection, the electron-injection time on the surface was adopted to appraise the efficiency of electron injection by NewnsAnderson method (Muscat and Newns, 1978). Because of the splitting energy levels of dye’s LUMO after adsorbing on the substrate TiO2, the shifts in energy for dye’s LUMO and the broadening width (ℏΓ ) were described by the Lorentzian distribution (Persson et al., 2006):
ρLUMO (E ) =
∑ pi
∑ pi εi i
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Fig. 3. Charge displacement curves (CDC) for (a) JF419/(TiO2)38 (olive), (b) S1/(TiO2)38 (magenta), (c) S2/(TiO2)38 (red), (d) S3/(TiO2)38 (purple), (e) S4/(TiO2)38 (green) and (f) S5/(TiO2)38 (blue), respectively. Isodensity sketches of electron density difference for the studied complexes with the isovalue to 0.001. The regions of density diminution and accumulation are marked in blue and purple, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
befitting acceptor for organic dyes.
stronger molar absorption coefficient than JF419, suggesting remarkable light-harvesting capability. Moreover, S3 and S5 displayed profound performance in other aspects, including the accessible charge separation state, large driving force for dye regeneration (ΔGreg = 0.51 eV for S3 and ΔGreg = 0.61 eV for S5), and brilliant balance between the electron injection time (τ = 6.8 fs for S3 and τ = 7.3 fs for S5) and the electron-hole recombination distance (r = 20.5 Å for S3 and r = 19.2 Å for S5). Synthetically, S3 and S5 dye were screened as prospective organic sensitizers to exhibit heightened DSC photovoltaic performance and the corresponding acceptor could substitute for the cyanoacrylic acid group. These results high lighten the profitability of electron withdrawing groups regardless of on which kind of acceptors and illuminate the designing guide for more valid and
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments We appreciatively acknowledge financial supported by the Henan province natural science foundation of China (162300410232), the Shandong province natural science foundation of China, 497
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Fig. 4. Lorentzian broadening ℏΓ for JF419 (olive), S1 (magenta), S2 (red), S3 (purple), S4 (Green) and S5 (blue), respectively. The corresponding fitting parameters ELUMO, and broadening width are listed in the upper left table. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
(ZR2017BB050, ZR2017BEM014), the National Natural Science Foundation of China (51702228), and Talent introduction start-up foundation of Taishan University (Y-01-2016003), the Nanhu Scholars Program for Yong Scholars of XYNU, and Doctoral Scientific Research Foundation of Xinyang Normal University. This work is also supported by the high-performance computing platform of Xinyang Normal University.
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Integrated probing the influence of dye acceptor with several electron withdrawing groups for dye-sensitized solar cells
T
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Zhu-Zhu Suna, Shuai Fengb, , Wei-Lu Dingc a
Energy-Saving Building Materials Innovative Collaboration Center of Henan Province, Xinyang Normal University, Xinyang 464000, China College of Chemistry and Chemical Engineering, Taishan University, Taian 271021, China c Beijing Key Laboratory of Ionic Liquids Clean Process, CAS Key Laboratory of Green Process and Engineering, State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Dye-sensitized solar cells Organic dyes Electron withdrawing group Charge transfer Density functional theory
In order to explore the influence of diverse acceptors with varying amounts of electron withdrawing groups on the organic dye in dye-sensitized solar cell (DSCs), a series of JF419 organic analogues with several cyano groups (CN) on acceptor were engineered. By performing theoretical studies on these investigated dyes and the corresponding dye/(TiO2)38 complexes, two benzoic acid units (benzothiadiazole benzoic acid and vinyl benzoic acid) with more -CNs were identified as alternative acceptors for DSCs. Particularly, S3 and S5 dyes exhibited impressive red-shift in spectra (ca. 77 nm for S3 and 48 nm for S5 compared with JF419), more accessible charge separation state and large driving force for dye regeneration (ΔGreg = 0.51 eV for S3 and ΔGreg = 0.61 eV for S5). According to quantifying the charge transfer (CT) on semiconductor surface, it found that the growing electron withdrawing groups on acceptor can strengthen the interaction between dyes and TiO2 and provide favorable conditions for electron injection. A brilliant balance between the electron injection time (τ = 6.8 fs for S3 and τ = 7.3 fs for S5) and the electron-hole recombination distance (r = 20.5 Å for S3 and r = 19.2 Å for S5) could be acquired for S3 and S5 dyes. Synthetically, introducing electron withdrawing groups on acceptor for organic dyes is a valid approach for gaining great performance of DSCs.
1. Introduction Dye-sensitized solar cells (DSCs) have been extensively investigated due to their comparable efficiency and cost-effectiveness since the breakthrough by O’Regan and Grätzel in 1991. With continually exploring and efforts, the power conversion efficiency (PCE) of DSCs has improved from the initial 7% to 14.3% in 2015 under the standard air mass 1.5 global (AM1.5G) condition (O'Regan and Grätzel, 1991; Kakiage et al., 2015). The outstanding PCE of DSCs can display over 20% under artificial indoor lights (Freitag et al., 2017; Tingare et al., 2017). Gaining from the tailorability of dye molecules (Purnama et al., 2019; Xu and Mallouk, 2019), DSCs draw more attraction to their practical applicability such as building-integrated photovoltaic (BIPV) windows (Fakharuddin et al., 2014; Yoon et al., 2011; Heiniger et al., 2013; Zhang et al., 2014). Recently, a stable blue photosensitizer was reported by Wang et al. for color palette of DSCs, and an impressive PCE of 12.6% was achieved (Ren et al., 2018). Meanwhile, the effect of acceptor position modulation has been investigated by selecting benzothiadiazole as a representative acceptor (Chai et al., 2019). As the
⁎
heart of DSCs, the dye is responsible for light- harvesting, charge separation at dye/semiconductor interface, and dye regeneration, etc (Feng et al., 2019). By reasonably designing and synthesizing dyes, especially donor-acceptor metal-free organic dyes, the PCE of DSCs can be enhanced availably. Fluorene-based photosensitizers, which can impede undesired electron recombination and extend the electron lifetime of dyes in the semiconductor, have been successfully employed in the DSCs recently (Yella et al., 2013). Representative fluorene-based dye JF419 (Yella et al., 2013); whose donor is constituted by fluorene unit and with 4Hcyclopenta [2,1-b:3,4-b′] dithiophene (labeled CPDT) unit as π bridge, displayed its distinguished photovoltaic performance (PCE = 10.3%) in the solar cell. However, the conventional acceptor cyanoacrylic acid in JF419 may deplete the durability, weaken the light harvesting capacity and decrease the electron injection of dye, which are obstacles for the practical application of DSCs. Searching for a substitutable acceptor for JF419 dye may be available for solving aforementioned problems. Enlightened by the remarkable performance of benzothiadiazole benzoic acid (BATA) unit in C259 dyes (Zhang et al., 2013); in this research we
Corresponding author. E-mail address: hff[email protected] (S. Feng).
https://doi.org/10.1016/j.solener.2019.11.076 Received 3 September 2019; Received in revised form 12 November 2019; Accepted 21 November 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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Scheme 1. Chemical structure of JF419 and dyes with different accepters employed with BTBA and VBA building blocks for the S-series dyes, respectively.
the most potential organic dyes toward more efficient DSCs.
replaced cyanoacrylic acid with BATA unit on the JF419 backbone. Meanwhile, vinyl benzoic acid (VBA) was also introduced in the JF419 backbone as alternative acceptor. According to our previous studies (Feng et al., 2019; Sun et al., 2018), some subtle modulation on the acceptor of dye can bring apparent variations in the cell performance. The replaced acceptors were embellished by some electron withdrawing groups. In order to deeply detect the effects of different acceptors with varying amounts of electron withdrawing groups on organic dye in DSCs, a series of JF419 organic analogues were designed by introducing cyano groups (CN) on these two different blocks (BATA and VBA). The newly designed organic dyes were labeled as S1, S2, S3, S4 and S5, respectively (see Scheme 1). The organic dyes including JF419 and all designed dyes were performed theoretical simulations. Systematic investigations demonstrate that the increased electron-withdrawing groups will improve these sensitizers’ light-harvesting capability, charge separation and interface electron transfer properties on semiconductor surface. The S3 and S5 with multi-CNs were screened out as
2. Computational details In our works, all the dyes (JF419 and designed dyes) were optimized using density functional theory (DFT) with the hybrid functional B3LYP, coupled with the 6-311G** basis set (Becke, 1993; Chen et al., 2013). By performing frequency analyses on these optimized dyes at the same level of theory, no imaginary frequency confirmed their stationary nature at energy minimum. Solvent effects of dichloromethane (DCM) were considered by means of the Conducting Polarizable Continuum Model (C-PCM) (Tomasi et al., 2005). Moreover, in order to illustrate the effects of different functionals on the geometric optimization, the JF419 dye was optimized with the B3LYP, MPW1K, M062X, and CAMB3LYP functionals, respectively. As listed in Table S1, the B3LYP/6311G** method can produce the stablest molecular geometry, and therefore this method was selected for the molecular optimizations. The UV–vis spectra were simulated by performing vertical transitions of the 492
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can be captured a track from the contributions of different acceptors on the HOMO and LUMO: more proportions were assigned for the LUMOs and ignorable distributions were dispensed on the HOMOs as listed in Table1. The stable HOMOs and the energetic down-shift LUMOs result in narrowed molecular energy gaps (△EHL), which ensured charge separation upon photo-excitation and provided more valid electron transport channel across the dye/TiO2 interface. Noticeably, the shrinkage for △EHL went along with the electron-withdrawing capability on acceptors evidenced by the data in Figure S1 (△EHLJF419 = 2.05 eV > △EHL-S1 = 1.90 eV > △EHL-S2 = 1.67 eV > △EHL-S3 = 1.66 eV, △EHL-JF419 = 2.05 eV > △EHL-S4 = 1.90 eV > △EHL-S5 = 1.81 eV). In the light of previous work (Pastore et al., 2013); the lowest dyedye excited state was analogous to the experimental optical band gaps. In this work, we simulated the lowest dye-dye excited state by adding the first singlet excitation energy to the HOMOs and compared it with corresponding simulated absorption spectra. The data of all newly designed dyes were illustrated in Fig. 1 and Fig. 2a. All the simulated absorption spectra had almost spread from 300 to 700 nm presenting amazing capability in light harvesting and showed red-shifted bands at 540–600 nm compared with the original JF419 dye. Moreover, the contracted dyes’ lowest dye-dye excited states(△EHL-S2 = 2.04 eV < △EHL-S3 = 2.06 eV < △EHL-S1 = 2.21 eV < △EHL-JF419 = 2.36 eV, △EHL-JF419 = 2.36 eV > △EHL-S4 = 2.29 eV > △EHL-S5 = 2.16 eV) were consistent with the redshifts of absorption spectra (λmax,S2 = 608 nm > λmax, S3 = 603 nm > λmax,S1 = 560 nm > λmax,JF419 = 526 nm; λmax,S5 = 574 nm > λmax,S4 = 543 nm > λmax,JF419 = 526 nm). As listed in Table 2, the dominating contribution to the λmax is the transition from HOMO to LUMO for each dye and the redshifts also followed the shrinking energy gaps. Additionally, the negligible distinction for S2 and S3 λmax as well as the more red shifted λmax,S5 for S5 than λmax,S1 for S1 demonstrated that the increasing amount of electron-withdrawing group (-CN) on acceptor could reduce the losses from lower conjugate in dyes for spectrum. Notably, the absorption spectra of investigated dyes were calculated by several methods (MPW1K, CAM-B3LYP, B3LYP and M062X) at the same levels to verify the simulation’s reliabilities. As listed in Table S2, by comparing the calculated maximum absorption wavelengths and the experimental value of JF419 (554 nm (Yella et al., 2013), the MPW1K method could describe the absorption spectra preferably. Moreover, the mean errors between the MPW1K functional and the other functionals (CAM-B3LYP, B3LYP and M062X) were also estimated. It can be found that the absorption spectra were largely overestimated by the B3LYP method and were underestimated by the CAM-B3LYP and M062X methods. The similar phenomena were also observed in the calculation of vertical emission wavelengths (Table S3). All the results above manifested the incremental electron-withdrawing groups on the acceptor could shrink the energy gaps and optimize the spectral properties. Additionally, an excellent acceptor in dye to improve the spectral properties should balance the relationship between the introduced acceptor blocks and the numbers of the electronwithdrawing groups on acceptor.
lowest 50 singlet–singlet excited states in the DCM solution employing single-point time-dependent DFT (TD-DFT) calculations. The vertical emission energies were also simulated by carrying out single-point TDDFT calculations based on the optimized first exited state. In view of the accuracy, we employed the most appropriate method CAM-B3LYP/631G* in DCM solution (Yang et al., 2015) to simulate the first exited state. In order to identify the reliable functional that can reproduce the experimental spectra of JF419 dye best, several methods were tested for absorption spectrum (MPW1K, CAM-B3LYP, B3LYP and M062X) and vertical emission energies (MPW1K, CAM-B3LYP, B3LYP and M062X), and the data see supporting information. Finally, the hybrid functional MPW1K and 6-311G** basis set (Feng et al., 2015; Lynch et al., 2000) were regarded as the suitable methodology to describe spectrum properties. The interfacial characteristics of dye on TiO2 surface were obtained from the optimized dye/(TiO2)38 geometries, which were performed by DFT with B3LYP, coupled with Lanl2dz for Ti atom and the 6-31G* for other atoms with Gaussian 09 program package. Based on previous works (De Angelis et al., 2010); a (TiO2)38 cluster cutting from an anatase slab with bare (1 0 1) surface was adequate to describe the surface effects. Additionally, all the models of dye absorbed on TiO2 were adopted the bidentate-bridging configuration due to favorable energy (Pastore and De Angelis, 2012; Ding et al., 2017). To be consistent with singular dyes, the single-point DFT and TD-DFT calculations in DCM solution were carried out to simulate the electronic and optical properties with the B3LYP and MPW1K functions and 6-311G** basis set, respectively. All the calculations above were performed with the Gaussian 09 program package (Frisch et al., 2009).
3. Results and discussions 3.1. Configurations and electronic properties of single dyes The dyes’ configurations, energy levels and light-harvesting capabilities have been scrutinized based on DFT and TD-DFT approaches. The data of torsional angles for the investigated dyes (labeled on Scheme 1) were listed in Table 1. Clearly, the torsional angles θ1 between benzothiadiazole (vinyl) unit and benzoic acid were affected by the steric hindrance of different building blocks. The values of θ1 are enlarged obviously when the acceptor is BTBA blocks, while the values of θ2 are significantly susceptible to the numbers of electron withdrawing cyano group (see Table 1). Moreover, all the sensitizer energy levels of the HOMOs and LUMOs were controlled within a reasonable range, which were lower than the redox potential of I−/I− 3 (−4.60 eV vs in vacuum) and higher than the conduction band of TiO2 (−4.00 eV vs in vacuum), respectively (see details in Figure S1 and Table 1). The appropriate energy levels could guarantee the sufficient driving force for dye regeneration and unidirectional electron transfer process of DSCs. As shown in Figure S1, the LUMO levels were affected distinctly by the incorporated electron withdrawing groups, while the variation of HOMO levels was slight. It Table 1 The torsional angles, orbital energy levels and molecular orbital coefficients for the investigated dyes (H = HOMO, L = LUMO, A = Acceptor and D = Donor). Dye
JF419 S1 S2 S3 S4 S5
Torsional angles
Energy levels
HOMO
3.2. Characteristic for the intramolecular charge transfer It is well known that the extent of the intramolecular charge transfer (ICT) is critical for the charge separated state of dyes, which guaranteed unidirectional electron transfer process of DSCs. Charge transfer and separation are important processes governing numerous chemical reactions. Obtaining fundamental understanding of these processes and the underlying mechanisms will be helpful for photochemistry (Chi et al., 2019). Here, the charge-transfer parameters containing the transferred charge (qCT ), the charge-transfer distance (dCT ), and the exciton binding energy (EB ) were evaluated to explore the electron-hole separation of designed dyes.
LUMO
θ1
θ2
H
L
A
D
A
D
0.3 −36.1 −37.3 −31.8 1.5 13.0
– −0.1 −0.2 24.9 −17.0 −20.4
−4.92 −4.79 −4.81 −4.82 −4.85 −4.93
−2.87 −2.89 −3.13 −3.16 −2.94 −3.12
3.3% 5.0% 5.2% 4.5% 4.9% 4.6%
84.3% 73.3% 73.6% 77.4% 76.4% 82.0%
39.2% 81.1% 87.1% 82.0% 74.0% 61.1%
7.7% 2.1% 1.3% 2.1% 3.0% 4.8%
The key dihedral angles are shown in Scheme 1. 493
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Fig. 1. Simulated the frontier molecular orbitals (HOMOs and LUMOs) and energy levels alignment (HOMOs and Lowest dye-dye excited state) of investigated dyes related to the experimental conduction band edge of TiO2.
configuration in the subsequent works. (Pastore and De Angelis, 2012) As shown in Fig. 2b, all UV–vis spectra for the adsorbed dyes displayed distinctly red-shifted (ca. 20 nm) contrasted with free dyes under the identical computational procedure. The tendency for spectra is similar to free dyes with more red shifted for BTBA blocks and stronger molar absorption coefficient with VBA blocks. The λmax of S3/ (TiO2)38 complexes (622 nm) are more red-shifted ca. 50 nm than that of the JF419/(TiO2)38 complexes (567 nm). Moreover, the increased numbers of cyano group facilitate the amounts of red-shifted absorption spectrum (λmax,S3 = 622 nm > λmax, S1 = 572 nm, λmax,S5 = 598 nm > λmax,S4 = 564 nm). Further analyzing the molecular orbital distribution (Figure S3), it can be found that all the LUMOs of dye/(TiO2)38 complexes mainly fixed on the TiO2 nanoparticle, while the HOMOs delocalize over whole dye molecules. Such single-particle excitations facilitate the electron injection at the dye/ semiconductor interface and are indispensable for a high-efficiency cell assembly. According to the definition τ = 1.499/(fE 2) , the exited-state lifetimes (τ, Table 3) of the investigated dyes are simulated to evaluate the decay of the exited state from S1 to S0, where f and E correspond to the oscillator strength and the excitation energy of different electronic states, respectively (Zhang et al., 2013; Rohatgi-Mukherjee, 1978). Obviously, the lower oscillator strength and vertical emission energy result in more extended exited-state lifetime. As the absorption spectrum, the dyes with BTBA blocks had smaller vertical emission energy and lower oscillator strength, while these values were larger for the dyes with VBA blocks (Table S3). The sequence for τ was S5 (2.55 ns) < S4 (2.57 ns) < S3 (3.93 ns) < S1 (4.01 ns) < S2 (4.65 ns). It is found that the strengthened conjugative effect contribute more to extend the exited-state lifetimes. But the comparable τ for S4 and S5 as well as S1 and S3 indicated the slight effect for the increased electron-withdrawing groups on τ values. Although the longer exitedstate lifetime (τ) for dye should be concerned as one affecting elements for the electron injection from dye to the TiO2 conduction band (CB), various aspect affections should also be taken into account. The driving forces for the electron injection (ΔGinj) and dye regeneration (ΔGreg) are other elements to take into account because of the direct correlation with the charge generation step in DSCs. The parameters associated with the driving forces (excited-state oxidation potential ESOP and ground-state oxidation potential GSOP) were calculated and listed in Table 3. Using our formerly reported technique (Feng et al., 2019; Sun et al., 2018), the relevant parameters including the energies of dyes excited, oxidized and ground states were calculated
Following the method reported by Ciofini et al. (1955); Le Bahers et al. (2011); the amounts of qCT and dCT were calculated. As shown in Figure S2, all the transferred charge of designed dyes are larger than that for JF419, demonstrating superior charge separated state for these dyes. By analyzing the numerical values of qCT and the geometry parameters (Table 3), the qCT for dyes with BATA blocks was found following the order: S2 (qCT = 0.85e ) ≈ S3 (qCT = 0.84e ) > S1 (qCT = 0.78e ), while the qCT for dyes with VBA blocks were approximate (S4qCT = 0.76e and S5qCT = 0.74e ). It suggested that the increased cyano groups facilitating the charge separated state. Meanwhile, the simulated charge-transfer distance (dCT ) emphasized the analogous suggestion: S3 (dCT = 6.52Å ) > S4 (dCT = 6.29 Å ) > S5 (dCT = 6.17 Å ) > S2 (dCT = 6.11Å ) > S1 (dCT = 5.68 Å ). The longer distance was beneficial to charge separated state of dyes. The charge transfer characteristic was further appraise by exciton binding energy (EB ), which was simulated through the difference between the electronic and optical band gaps (Scholes and Rumbles, 2006). The electronic band gaps identified with the difference between the HOMO and the LUMO energies and the first singlet excitation energies were equal to the optical gaps (Li et al., 1873; Duan et al., 2013). The EBs listed in Table 3 displayed a sequence of S5 (0.29 eV) < S3 (0.33 eV) < S4 (0.35 eV) < S2 (0.36 eV) < S1 (0.45 eV), which presented upward trend with the weakened ability of electron withdrawing on dyes. The less the Coulomb interaction, the easier the dissociation of the exciton. Above all, the more transferred charges and the lesser exciton binding energies can further guarantee unidirectional electron transfer process and facilitate interfacial photoelectron injection of DSCs. Although there were slightly differences on the trends of the three charge-transfer parameters for the investigated dyes, the newly designed dyes can effortlessly achieve the charge separation state compared with that of JF419. Furthermore, the increasing amount of electron-withdrawing group (–CN) on acceptor indeed promoted intramolecular charge transfer. The conclusion was consistent with the electronic and spectral properties. 3.3. Characteristic for dye/(TiO2)38 complexes. To assess the feasibility of these investigated dyes in DSCs, the performances of dye/(TiO2)38 complexes were studied in this section, including the absorption spectrum, the excited-state time, the driving force of electron injection and regeneration and electron injection rate. Considering the energetic favorable architecture, all the model of dye/ (TiO2)38 compounds were employed the bidentate-bridging 494
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Fig. 2. Calculated optical absorption spectra of the investigated dyes.
from an excited state dye to the semiconductor. Learned from Figure S4, all the investigated dyes have higher ESOP values than the CB edge of TiO2 in energy and ΔGinj of these dyes are in the scope of 0.7–1.1 eV. Although the increased amounts of electron-withdrawing groups resulted in a cathodic shift for ESOP compared to JF419, the ΔGinj values could satisfy the injection requirement thermodynamically. Moreover, the GSOP values are lower than the oxidation potential of redox couple ([Co(bpy)3]2+/3+) revealing thermodynamically favorable regeneration of oxidized dyes. Particularly, there are significant declining trend of GSOP values with the growing numbers of withdrawing groups and decreasing conjugated groups. Comparing all regeneration driving force of dyes (ΔGreg, Table 3), the energy difference between GSOP and the oxidation potential of [Co(bpy)3]2+/3+, the ΔGreg with the sequence was S2 (0.27 eV) < S1 (0.29 eV) < S4 (0.48 eV) < S3 (0.51 eV) < S5 (0.61 eV). Obviously, the extended conjugative units (S2 and S1 dyes) were adverse to regeneration, while the increased amounts of electron-withdrawing groups on acceptors could be made up the detriment.
Table 2 The spectral properties for the investigated dyes and: calculated maximum absorption wavelengths for isolated dyes and complexes (λ in nm), the lowest excitation energy (in eV), oscillator strengths (f), Light-harvesting efficiency (LHE) and the main orbital contributions (H = HOMO, L = LUMO). Dye
λdye(/TiO2)
△EEX
f
LHE
Main assignment
JF419 S1 S2 S3 S4 S5
526 560 608 603 543 574
2.36 2.21 2.04 2.06 2.29 2.16
1.78 1.51 1.38 1.59 2.14 2.20
0.98 0.97 0.96 0.97 0.99 0.99
H H H H H H
(567) (572) (624) (622) (564) (598)
→ → → → → →
L L L L L L
(77%), (68%), (69%), (69%), (70%), (73%),
H-1 H-1 H-1 H-1 H-1 H-1
→ → → → → →
L L L L L L
(18%) (26%) (25%) (25%) (21%) (20%)
at the CAM-B3LYP/6-31G+(d,p) level. The difference between ESOP and the CB edge of TiO2 was used to estimate the electron injection driving force. As is known to all, the driving force should be on the order of tenths of an eV to guarantee an effective electron injection 495
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Table 3 Intramolecular charge transfer parameters (qCT in e, dCT in Å, EB in eV and τ in ns), charge transfer amount (Δq, e), Ground-state and excited-state oxidation potentials (GSOP and ESOP), driving forces (△Ginj and △Greg), and recombination distance (r). Dye
qCT
dCT
EB
τ
Δq
GSOP
ESOP
△Ginj
△Greg
r
JF419 S1 S2 S3 S4 S5
0.73 0.78 0.85 0.84 0.76 0.74
5.43 5.68 6.11 6.52 6.29 6.17
0.35 0.45 0.36 0.33 0.35 0.29
2.66 4.01 4.65 3.93 2.57 2.55
0.69 0.77 0.74 0.91 0.71 0.82
−5.52 −5.29 −5.27 −5.51 −5.48 −5.61
−3.16 −3.08 −3.23 −3.45 −3.19 −3.45
1.01 1.09 0.94 0.72 0.98 0.72
0.52 0.29 0.27 0.51 0.48 0.61
15.2 20.0 22.1 20.5 18.4 19.2
ℏΓ =
where the pi and εi are the portion of the i-th molecular orbital and its energy, respectively. According to Fig. 4, the mixing values for the adsorbate LUMO (ELUMO (ads ) ) are downshift from −2.91 eV to −3.32 eV along with the increased numbers of electron-withdrawing groups. However, the broadening width (ℏΓ ) is different from the trend of ELUMO (ads ) , which are relevant to the location of the electron-withdrawing group and the conjugated unit. It is observed that the VBA block facilitates broadening the ℏΓ compared to the BTBA block, while the cyano group on the conjugated unit narrowed the ℏΓ . Based on the formula τ (fs ) = 658/ℏΓ(meV ) , which was used to evaluate the heterogeneous electron-transfer time, the wider of the broadening width between adsorbate and TiO2 surface lead to the fast electron-injection time. Therefore, the S3, S4 and S5 displayed a positive influence on JSC, whose τ are relative less than that of other dyes (τS4 =4.6fs < τS3 =6.8fs < τS5 =7.3fs < τS1 =10.7fs < τS2 =28.8fs). It suggested that the increased electron-withdrawing groups on acceptor could accelerate the photoelectron injection. Noticeably, the electron injection rate for S1 and S2 dyes were sluggish markedly rooting in the prolonged conjugated groups. However, the shortened electron-injection time for S3 testified that the compensation could be offered by adding electron-withdrawing groups on acceptor. The effect of electron recombination from the injected photo-electrons to the oxidized dyes is also taken into account to assess its negative influence on the open-circuit voltage in DSCs. Because of the sensitivity to the spatial separation of the dye cation hole and the semiconductor surface, the recombination rate was roughly estimated by the distance between the dye cation HOMO and the semiconductor surface (Clifford et al., 2004). The distances for the investigated dyes (see Table 3) were elongated in various degree compared with that of the JF419 (15.2 Å). It indicated that these new tailored JF-series dyes are expected to reduce the charge recombination possibility in DSCs. Notably, the spatial distance between the dye cation hole and the semiconductor surface (shown in Figure S5) was stretched with the growing numbers of cyano group on the acceptors, such as the distances for S5 (19.2 Å) and S3 (20.5 Å) are larger than those of S4 (18.4 Å) and S1 (20.0 Å), respectively. It is illustrated that the introduced electron drawing groups are beneficial to lower the recombination rate, which will help to improve the performance of DSCs. It is necessary to mention that although these parameters were roughly estimating the electron injection and recombination progress in this work, the conclusion is referable for the performance of dyes with variable electron withdrawing groups by evaluating the identical model. 4. Conclusions In this work, a series of JF419 organic analogues with alternative acceptors (BTBA and VBA) and varying amounts cyano groups (CN) on acceptor were engineered to comprehensively assess the effects including the substituted acceptors and the amounts of electron withdrawing groups on the organic dye in DSCs. It can be found that the embedded benzothiadiazole unit in dyes (such as S1, S2 and S3) could strengthen π-conjugacy to promote the spectrum red-shift and prolong excited-state lifetimes. However, further exploring the intramolecular charge transfer and interface electronic transfer properties, the increased benzothiadiazole groups in dyes showed adverse effect on these properties including regeneration driving force, the amount of interface charge transfer and the efficiency of electron-injection. Fortunately, these negative impacts could be offset by increasing cyano groups on acceptor to gain better performance for the JF-based dyes. Particularly, the S3 dye (BTBA acceptor) with two CN groups on benzoic acid shows more red-shift absorption spectra and the S5 dye (VBA acceptor) with one CN group on vinyl and two CN groups on benzoic acid shows
( ) ℏΓ
1 2 π (E − E 2 LUMO (ads )) +
ℏΓ 2 2
( )
where ELUMO (ads ) signifies the dye’s LUMO after adsorbed on the TiO2. ELUMO (ads ) and ℏΓ can be characterized by:
ELUMO (ads ) =
|εi − ELUMO (ads )|
i
Apart from the driving force, the dye reorganization energy is another important parameter to determine the charge transfer process in photoelectron injection and dye regeneration, which reflects the energy alteration of the geometry relaxation during the charge transfer process. The reorganization energy was calculated with the formula of λ = [E 0 (M+) − E 0 (M 0 ) + E + (M 0 ) − E + (M+)], where E 0 and E + represent the energies of neutral and cationic states, and M 0 and M+ represent the optimized geometries of neutral and cationic dyes, respectively. In this work, the equilibrium (λ eq ) represented the inner reorganization energy and the outer reorganization energy was also calculated by non-equilibrium model (λneq ) to evaluate the contribution of the surrounding solvent molecules. As listed in Table S4, λ for all investigated dyes changed rarely and displayed slightly lower values compared to JF419, suggesting beneficial properties for promoting the charge transfer in electron injection and dye regeneration processes. The charge transfer occurred at the dye–titanium interface was also studied to manifest the effect of dyes on the electronic injection. It can be qualitatively described by the isodensity sketches of electron density difference (Ronca et al., 2012; Belpassi et al., 2008) and performed in Multiwfn 3.4 (Lu; Lu and Chen, 2011). As shown in Fig. 3; there is a distinct rearrangement of electron density in the dye/(TiO2)38 systems along with an accumulation at the dye–TiO2 interface and a depletion around the anchoring group. It indicated that the charge transfer were indeed taken place at the dye/(TiO2)38 interface for all the designed dyes. Furthermore, the amount of charge transfer (CT) was quantitatively characterized by charge displacement analysis (CD) (Fig. 3). Obviously, the amount of CT was ascending accompany with the growing number of electron-withdraw groups on the anchoring group. Among all dye/(TiO2)38 systems, the S3/(TiO2)38 (0.91 e−) and S5/ (TiO2)38 system (0.82 e−) had bigger CT, while, the CT were smaller for S1/(TiO2)38 (0.77 e−), S2/(TiO2)38 (0.74 e−) and S4/(TiO2)38 (0.71 e−) (as listed in Table 3 and Fig. 3). The result indicated that the growing electron-withdraw groups on dyes could strengthen the interaction between the dyes and TiO2 compared to the dyes with increased conjugative groups, which benefited for the electron injection from dye to the TiO2 conduction band. Deeply discussing the effect of dye’s acceptor on the interfacial electron-injection, the electron-injection time on the surface was adopted to appraise the efficiency of electron injection by NewnsAnderson method (Muscat and Newns, 1978). Because of the splitting energy levels of dye’s LUMO after adsorbing on the substrate TiO2, the shifts in energy for dye’s LUMO and the broadening width (ℏΓ ) were described by the Lorentzian distribution (Persson et al., 2006):
ρLUMO (E ) =
∑ pi
∑ pi εi i
496
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Fig. 3. Charge displacement curves (CDC) for (a) JF419/(TiO2)38 (olive), (b) S1/(TiO2)38 (magenta), (c) S2/(TiO2)38 (red), (d) S3/(TiO2)38 (purple), (e) S4/(TiO2)38 (green) and (f) S5/(TiO2)38 (blue), respectively. Isodensity sketches of electron density difference for the studied complexes with the isovalue to 0.001. The regions of density diminution and accumulation are marked in blue and purple, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
befitting acceptor for organic dyes.
stronger molar absorption coefficient than JF419, suggesting remarkable light-harvesting capability. Moreover, S3 and S5 displayed profound performance in other aspects, including the accessible charge separation state, large driving force for dye regeneration (ΔGreg = 0.51 eV for S3 and ΔGreg = 0.61 eV for S5), and brilliant balance between the electron injection time (τ = 6.8 fs for S3 and τ = 7.3 fs for S5) and the electron-hole recombination distance (r = 20.5 Å for S3 and r = 19.2 Å for S5). Synthetically, S3 and S5 dye were screened as prospective organic sensitizers to exhibit heightened DSC photovoltaic performance and the corresponding acceptor could substitute for the cyanoacrylic acid group. These results high lighten the profitability of electron withdrawing groups regardless of on which kind of acceptors and illuminate the designing guide for more valid and
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments We appreciatively acknowledge financial supported by the Henan province natural science foundation of China (162300410232), the Shandong province natural science foundation of China, 497
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Fig. 4. Lorentzian broadening ℏΓ for JF419 (olive), S1 (magenta), S2 (red), S3 (purple), S4 (Green) and S5 (blue), respectively. The corresponding fitting parameters ELUMO, and broadening width are listed in the upper left table. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
(ZR2017BB050, ZR2017BEM014), the National Natural Science Foundation of China (51702228), and Talent introduction start-up foundation of Taishan University (Y-01-2016003), the Nanhu Scholars Program for Yong Scholars of XYNU, and Doctoral Scientific Research Foundation of Xinyang Normal University. This work is also supported by the high-performance computing platform of Xinyang Normal University.
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