Cyclopentadithiophene bridged organic sensitizers with different auxiliary acceptor for high performance dye-sensitized solar cells

Dyes and Pigments 137 (2017) 165e173

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Cyclopentadithiophene bridged organic sensitizers with different auxiliary acceptor for high performance dye-sensitized solar cells Weixia Hu, Zemin Zhang, Wei Shen, Ming Li, Rongxing He* Key Laboratory of Luminescence and Real-Time Analytical Chemistry (Southwest University), Ministry of Education, College of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 August 2016 Received in revised form 7 October 2016 Accepted 12 October 2016 Available online 13 October 2016

A series of triphenylamine-based dye sensitizers were designed by modifying the auxiliary acceptors and their positions for the potential use in dye-sensitized solar cells (DSSCs). The geometrical structures, photoinduced charge transfer character together with optical properties for these dyes adsorbed on (TiO2)6 have been investigated using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods. The effects of auxiliary units and their positions on the optoelectronic properties of the dyes are demonstrated. The results illustrate that the dyes with auxiliary acceptors close to the cyanoacrylic acid show very narrow band gap (HOMO-LUMO), leading to an obvious red-shifted absorption band in contrast to the dyes with additional acceptors next to the donor part. Further analyses of the dye-(TiO2)6 systems manifest that there has strong electronic coupling between the dyes and the (TiO2)6 surface. The results are expected to provide a useful reference to the future design and optimization for new highly efficient metal-free organic dyes. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Metal-free sensitizers Cyclopentadithiophene Auxiliary acceptor High performance Dye-sensitized solar cells

1. Introduction Tremendous efforts have been devoted to DSSCs due to their environmental compatibility and low cost, since O'Regan and €tzel reported the groundbreaking work in the field of dye Gra sensitized solar cells (DSSCs) based on polypyridyl Ruthenium (Ru) dye in 1991 [1]. In a typical DSSC, the photovoltaic performance is mainly determined by sensitizers, photoanode, counter electrode, electrolyte and their combination [2]. As the key components, the sensitizer and the redox electrolyte have been confirmed to be particularly essential for the photoelectric conversion efficiency and battery properties [3,4]. Over the past decades, numerous dyes have been used in DSSCs. Comparatively, the metal-free organic dyes, especially those based on the “push
pull” structure of donorp conjugated bridge-acceptor (D-p-A), have displayed rather high performances considering the advantages of their high molar extinction coefficient, flexible molecular structures, as well as good compatibility to the cobalt bipyridyl complex-based electrolytes [5e10]. It is noteworthy that the power conversion efficiency of 10.65% has been reached by using metal-free organic dyes YA422 in

* Corresponding author. E-mail address: [email protected] (R. He). http://dx.doi.org/10.1016/j.dyepig.2016.10.011 0143-7208/© 2016 Elsevier Ltd. All rights reserved.

conjunction with Co(II/III) tris-bipyridyl redox shuttle [9]. 2þ/3þ Commonly, iodide/triiodide ðI
=I
are most 3 Þ and [Co(bpy)3] frequently used as redox mediators to attain highly efficient photovoltaic performances [11]. Recently, Kakiage and co-workers have successfully achieved high efficiency over 14% by means of collaborative sensitization [12]. In general, the performance of DSSCs depends on some key parameters: open-circuit photovoltage (Voc), short-circuit photocurrent density (Jsc), fill factor (FF), and power conversion efficiency (h) [2]. Structural modification is the common method to improve the parameters. As is known, most organic sensitizers consist of the D-p-A configuration, which the electron transfers from the donor section to the acceptor/anchoring group through a conjugated linker [13,14]. A novel donor-acceptor-p conjugated bridgeacceptor (D-A-p-A) configuration which incorporates an electronwithdrawing auxiliary acceptor such as diketopyrrolopyrrole [15e18], bithiazole [19,20], isoindigo [9], benzothiadiazole [7,14,21], benzotriazole [22], and quinoxaline [23e25] was proved to be an efficient tactic to extend the absorption spectra and improve the photovoltaic performances. More recently, Chaurasia et al. [26] focused on incorporating electron-deficient entity in the conjugated spacer to develop PyTbased sensitizers. They found that the introduction of an electron-deficient entity, benzothiadiazole (BT), pyrido[2,1,3]

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W. Hu et al. / Dyes and Pigments 137 (2017) 165e173

thiadiazole (PyT), as the auxiliary acceptor in the conjugated bridge of the D-p-A framework [27], was an effectual way to broad the light absorption band. Wang et al. reported the D-A-p-A sensitizers D2 (named BT-CPDT as the model molecule in our work) with a broad visible light absorption range up to 800 nm and a power conversion efficiency over 9% using cobalt electrolytes [28]. The bulky triphenylamine donor offers the probability to inhibit charge recombination and hinder molecular aggregation, especially for the cobalt redox system. Meanwhile, the cyclopentadithiophene (CPDT) unit has been documented as an effective building block for organic photovoltaics [29e31]. The entirely rigid coplanar structure together with good electron-donating ability is anticipated to decrease the energy gap and boost the intramolecular charge transfer process. More importantly, evidence from recent research shows that the insertion of long alkyl chain into dye structure can effectively offer double protection [32]. However, the absorption bands of most organic dyes are restricted to a comparatively narrow absorption range compared with Ru dyes and other dyes. Until now, the effects of auxiliary acceptors and their positions near the anchor group or close to the donor group on the photovoltaic properties have not been systematically studied. Based on the previous study, we begin with a cyclopentadithiophene unit as the spacer to design efficient DSSCs dyes with cobalt electrolytes (Fig. 1). PyNT (the N atom facing toward the acceptor) and NPyT (the N atom facing away from the acceptor), which are just one atom of difference from BT, have a higher electron affinity than BT. In order to extend the absorption range, we incorporate PyNT and NPyT entities as additional acceptors into D-A-p-A molecule owing to the following reasons: (1) the more electron affinity than BT is helpful for red shifting of the absorption spectra; (2) the asymmetric feature of pyridine ring can form two isomers. With the purpose of finding the potential sensitizers, the absorption spectra, electronic properties and energy gap for these dyes are considered using DFT and TD-DFT methods. What's more, to simulate the real

working conditions, the optical properties of investigated dyes adsorbed on (TiO2)6 surface are further calculated. It is hoped that the perceptions obtained in this study will offer guidance for the future experimental research on design and optimization of new efficient dyes for DSSC applications.

2. Computational methods 2.1. Theoretical background As described above, the power conversion efficiency (h) of the DSSCs is closely associated with three factors: short-circuit photocurrent density (Jsc), open-circuit photovoltage (Voc) and the fill factor (FF). And it can be expressed as follows [33]:

h ¼ FF

Voc Jsc Pinc

(1)

where Pinc represents the incident solar power on the cell. Correspondingly, Jsc can be described as the following equation [11,34,35]:

Z Jsc ¼ e

LHEFinject hreg hcollect Is ðlÞdl

(2)

l

where LHE is the light-harvesting efficiency. Finject is the electron injection efficiency, and hreg and hcollect denote the regeneration efficiency of the oxidized dye and the charge collection efficiency, respectively. The LHE can be expressed as

LHE ¼ 1
10
f

(3)

where f is the oscillator strength of the dye molecule at the maximum wavelength. Finject can be estimated through the driving force of electron injection (DGinject) from the excited dye to the semiconductor. Additionally, hreg can also be evaluated by the driving force of regeneration (DGreg) between the oxidized dye and the electrolyte. They are defined as [36,37]

DGinject ¼ Edye
ECB 

.

(4) 

DGreg ¼ E Co2þ Co3þ
ECB

(5)

where Edye* is the excited-state oxidation potential energy of the dye and ECB is the redox potential of the excited dye. Edye* can be expressed as [38]

Edye* ¼ Edye
Elmax

(6)

where Edye is the ground-state oxidation potential energy of the dye and Elmax is the vertical transition energy [39]. As for Voc in DSSCs, it can be described by Ref. [38]

Voc ¼

Fig. 1. Molecular structures of the metal-free organic sensitized dyes investigated in this work.

  ECB þ DECB kb T nc E ln þ
redox q q NCB q

(7)

where q is the unit charge, ECB is the conduction band edge of the semiconductor substrate, kb is the Boltzmann constant, T is the absolute temperature, nc is the number of electrons in conduction band, NCB is the density of accessible states in the conduction band. DECB is the shift of ECB when the dyes are absorbed on substrate and can be expressed as [40]

W. Hu et al. / Dyes and Pigments 137 (2017) 165e173

qmnormal g ε0 ε

DECB ¼


(8)

Here, mnormal denotes the dipole moment of individual dye molecular perpendicular to the surface of the semiconductor substrate. g is the surface concentration of dyes. ε0 and ε represent the vacuum permittivity and the dielectric permittivity, respectively. It can be drawn from Eqs. (7) and (8) that nc and mnormal show vital influence on Voc. 2.2. Computational details All the calculations were implemented with DFT and TD-DFT using the Gaussian 09 package [41]. With the intention of selecting an appropriate HOMO energy which will agree well with the experiment value, a series of hybrid functional including B3LYP, BHandHLYP, PBE0, and M062X together with different basis sets were applied to optimize the ground-state (S0) geometry of investigated model compound BT-CPDT (shown in Table S1, Supplementary material). We find that PBE0/6-311G(d,p) method is proper for the present work. The frequency analyses were also carried out at the same theoretical level as geometry optimization to guarantee the local minimum of potential energy surface. In the process of TD-DFT calculation, optical absorption spectra at the optimized ground-state geometries with the 30 lowest singletsinglet excitations were calculated. To find a proper functional, we used different exchangeecorrelation (XC) functional including BMK, PBE0, and CAM-B3LYP. By comparing the theoretical maximum absorption wavelength with the experimental data, it is found that the Boese and Martin's s-dependent hybrid functional (BMK) together with 6-311G(d,p) is the most suitable one used for their vertical electronic excitation energies (listed in Table S2). In addition, to study the coupling interaction between dye and TiO2 surface further, the geometry optimizations and TD-DFT calculations for dyes binding to (TiO2)6 were explored by using the same functional as isolated dyes for the C, H, O, N, S, and LANL2DZ basis set [42] for Ti atoms. The conductor-like polarized continuum model (C-PCM) (in dichloromethane environment) was also taken into consideration [43]. Besides, the electron density differences were calculated with the code Multwfn 2.5 to assess the efficiency of charge separation [44]. The distance of charge transfer and the fraction of charge exchange were calculated with DctViaCube [45]. 3. Results and discussion

167

as well as between cyclopentadithiophene unit and cyanoacrylic acid moiety (g) are listed in Table 1. The results indicate that the donor and acceptor moieties are fully conjugated through the p bridge. For all the dyes, the dihedral angles (a) are computed to be 33.0 ,
22.8 , 14.8 , 19.7,
31.2 , and
19.3 . It can be easily found that the changes of auxiliary groups not only can be benefit to the increase of conjugation but also can inhibit the p-aggregation due to the decline of the a. For the dyes of auxiliary group near the anchor group, CPDTPyNT has the smaller a than that of CPDT-BT and CPDT-NPyT, which perhaps due to the strong electron withdrawing ability of N atom. This may lead to the bathochromic shift of the absorption band. The dihedral angles b are 14.7, 1.2 ,
7.9 , 0.3 ,
0.3 , and 0.7, respectively. The results indicate that the good coplanarity may be beneficial to the electron injection process. As for the dihedral angles g, all are close to 0 . Generally, intramolecular charge transfer characteristics have close relation to the orbital spatial distribution and the compositions of the frontier molecular orbital [46], which have a substantial impact on the electronic excitation of dyes. For a suitable sensitizer, the HOMO is mostly localized on the donor segment whereas the LUMO is localized on the acceptor part. The orbital spatial distribution and components of the HOMO and LUMO for these molecules are depicted in Fig. 2. For the investigated dyes BT-CPDT, PyNT-CPDT, the HOMO orbitals are almost populated over the donor and additional moiety, while the LUMOs are centered at the “A-p-A” segment, with primely overlap on the auxiliary acceptors. As for the dyes CPDT-BT, CPDT-NPyT and CPDT-PyNT, the HOMOs mainly show localization at electron-donor and linker moiety, while LUMOs primarily are centered at the p-conjugated bridge and acceptor part. The electron transfer from HOMO to LUMO induces efficient charge separations, and can be visually displayed by the electronic density difference (EDD) plots in Table 2. On the contrary, the dye NPyT-CPDT exhibits an insufficient spatial charge distribution, the HOMO shows location at the “A-p-A” segment, whereas the LUMO extends over the “A-p-A” section. Such a delocalized HOMO is considered to produce electron-hole recombination, which is disadvantageous to cell performance. Besides, the energy level is another pivotal feature. To gain a deep insight into the driving force for the electron injection and the regeneration of the sensitizers, Fig. 3 and Table S3 illustrate the energy levels of HOMOs and LUMOs of designed dyes BT-CPDT,

Table 1 Brief structure and calculated dihedral angles between the neighbouring units of dyes.

3.1. Geometrical structures and electronic properties The geometrical structures of investigated sensitizers are shown in Fig. 1, in which triphenylamine is used as electron-donating unit (D), BT, PyNT, NPyT acting as auxiliary acceptors (A), a cyclopentadithiophene unit as spacer (p), and cyanoacrylic acid as electron acceptor (A), respectively. A potential dye molecule must have a high LUMO energy level for efficient electron injection into TiO2, a sufficiently low HOMO energy level for efficient regeneration of the oxidized state dye and a low energy gap to ensure the redshift of the absorption band. The incorporation of an auxiliary electron-withdrawing acceptor between the donor and conjugated bridge can tune energy level, extend light absorption range, and promote electron transfer from the donor part to anchor group. In this paper, the influence of the electron withdrawing ability and their positions of the auxiliary acceptor was taken into account in detail. The dihedral angles between donor group and auxiliary acceptor (a), between auxiliary acceptor and cyclopentadithiophene unit (b)

Dyes

a

b

g

BT-CPDT CPDT-BT NPyT-CPDT CPDT-NPyT PyNT-CPDT CPDT-PyNT

33.0
22.8 14.8 19.7
31.2
19.3

14.7 1.2
7.9 0.3
0.3 0.7


0.2
0.1 1.8 0.2 1.1
0.1

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W. Hu et al. / Dyes and Pigments 137 (2017) 165e173

Fig. 2. Frontier molecular orbitals of all the dyes at level of PBE0/6-311G(d,p).

Table 2 Electron density difference plots of the electronic transition S0/S1 for all the dyes performed in dichloromethane solvent using the BMK functional together with the 6311G(d,p) basis set. Scheme

BT-CPDT

CPDT-BT

NPyT-CPDT

CPDT-NPyT

PyNT-CPDT

CPDT-PyNT

10.046 1.063 0.2536

11.816 1.101 0.0539

3.744 0.950 0.3665

11.353 1.076 0.0891

8.529 1.102 0.3380

12.281 1.140 0.0445

Picture

L (Ang) De

U

L is the electron transfer distance (Å), De is the fraction of electron exchange (je
j), U is overlap between the regions of density depletion and increment (isovalue: 4  10
4 e au
3).

Fig. 3. Energy level diagram of the sensitizers relative to TiO2 and [Co(bpy)3]2þ/3þ in CH2Cl2.

CPDT-BT, NPyT-CPDT, CPDT-NPyT, PyNT-CPDT, CPDT-PyNT at PBE0/6-311G(d,p) level in CH2Cl2. The LUMO levels of all of the sensitizers are established above the conduction band (CB) of TiO2 semiconductor, thus ensuring an effective electron injection from the excited state dye to TiO2 surface, whereas the corresponding HOMO levels are adequately lower than these of the Cobalt (II/III) electrolyte redox potential so that all dyes can be restored by electrons from electrolyte [47]. As Fig. 3 demonstrated, for all the designed dyes, the HOMO

energies are all up-shifted and the LUMO energies are downshifted considerably except for the dye NPyT-CPDT in comparison with the model molecule BT-CPDT. That is to say, due to their much lower LUMO energies, the HOMO-LUMO energy gaps of all the dyes become narrower than that of BT-CPDT except for the dye NPyTCPDT. As testified in other literature, sensitizers with smaller energy gaps may lead to the electrons being more easily excited and thus favorable for providing broader wavelength light range [39]. The band gaps between HOMO and LUMO in CH2Cl2 for the dyes are in the sequence of NPyT-CPDT > BT-CPDT > PyNT-CPDT > CPDTBT > CPDT-NPyT > CPDT-PyNT. The outcome reveals that the calculated band gaps of different complexes are determined by the position of the additional acceptors. The additional acceptors adjacent to the anchor side can lead to narrow band gaps. We speculate that the reason why the band gap of the dyes with auxiliary group next to the anchor group is narrower than that of near the donor part is the good planarity of the p-bridge. It is evident that the dyes with the additional moiety close to the anchor group have the potential for getting bathochromic shift of the absorption band, which may make contributions to higher power conversion efficiency [48]. In this work, in order to study the charge transfer properties deeply, electron density difference plots of all isolated dyes were calculated. Besides, three parameters including the electron transfer distance (L in Å), the fraction of electron exchange (De in je
j), together with the overlap between the zones of density depletion and increment (U, isovalue: 4  10
4 e au
3 ) are summed up in Table 2 [45]. Apparently, the region of the electron density depletion (green color) mostly localizes on the donor part, while the region of the

W. Hu et al. / Dyes and Pigments 137 (2017) 165e173

electron density increment (purple color) is largely aligned with the p-conjugation spacer and acceptor regions. Therefore, the intramolecular charge transfer (ICT) excitation is important for reaching fast electron injection from the excited dye to the TiO2 conduction band. Normally, with the longer distance and less electron density overlap, a better charge separation can be achieved. The electron transfer distance is in the following order: CPDT-PyNT (12.281) > CPDT-BT (11.816) > CPDT-NPyT (11.353) > BT-CPDT (10.046) > PyNT-CPDT (8.529) > NPyT-CPDT (3.744). When the electron-withdrawing entities of sensitizers are in the vicinity of the acceptor, the fraction of electron exchange is larger than that of near the donor. Moreover, we can observe that the electron density depletion of CPDT-PyNT whose PyNT is directly connected to the anchoring group is the largest, which may be caused by an enhanced charge accumulation around the cyanoacrylic acid and its surrounding area. It also can be demonstrated by the largest fraction of charge exchange De for CPDT-PyNT (1.140). In addition, the longer electron transfer distance (L) of CPDT-PyNT (12.281) is also the evidence of the better charge separation when PyNT (the N atom facing toward the acceptor) is directly linked to acceptor part. In addition to De and L, the overlaps between the regions of density depletion and increment also can well evaluate the charge separation. Notably, weak overlap is advantageous to good charge separation. As presented in Table 2, with the changes of the additional acceptors, the overlaps decrease in the order of NPyT-CPDT (0.3665) > PyNT-CPDT (0.3380) > BT-CPDT (0.2536) > CPDT-NPyT (0.0891) > CPDT-BT (0.0539) > CPDT-PyNT (0.0445). All in all, when a series of strong electron-withdrawing units of the dyes are linked to the acceptor entities, the charge separation efficiency can be substantially improved.

3.2. Photophysical properties The UVevis absorption spectra are more trustworthy to evaluate their light-harvesting performances in the solar cells. The simulated electronic spectra of the sensitizers in CH2Cl2 which were obtained at the TDDFT/BMK/6-311G(d,p) level are shown in Fig. 4, and it has been demonstrated previously that the absorption peak was in good agreement with the experimental result. The relevant electronic transition data of the dyes are summarized in Table 3, more detailed analyses of the molecular orbital compositions with the transition information including the excitation energies, the maximum wavelength (lmax), oscillator strengths (f) and major electron transitions, together with absorption bands with f > 0.1

Fig. 4. Simulated absorption spectra and oscillator strength of the free dyes at the BMK/6-311G(d,p) level of theory in CH2Cl2.

169

and l > 350 nm in the UVevis regions for all the dyes in CH2Cl2 are listed in Table S4. Obviously, the absorption spectrum displays two major intense bands in the ranges of 320e450 nm for B band and 520e750 nm for Q band, respectively. The prominent Q band can be attributed to the intramolecular charge transfer from the triphenylamine donor to the cyanoacrylic acid acceptor, while the B band can be ascribed to the localized p-p* electron transition within the whole dyes. From Fig. 4, it is easy to observe that the different sites of the auxiliary group have a great influence on the absorption spectra. The maximum absorption wavelength of BT-CPDT appears at 560 nm. Apparently, the absorption spectra of the designed dyes have significantly broadened and red-shifted, whereas dye NPyTCPDT is blue-shifted owing to insufficient charge separation. Particularly, for each isometric pair molecules, the large red-shift can be achieved for the dyes CPDT-BT, CPDT-NPyT, CPDT-PyNT with additional acceptors next to the anchor moiety, which are in well agreement with their narrow HOMO-LUMO energy gaps. We conjecture that the reason for the broaden absorption range is the higher coplanarity and lower energy gaps. Furthermore, albeit weak intensity of the absorption peak, the dye CPDT-PyNT with the wavelength at 736 nm has a great bathochromic shift compared with the reference dye BT-CPDT, which is beneficial for light harvesting. By comparison, the intensities of the absorption peak for CPDT-BT, CPDT-NPyT are increasing gradually by substituting H atom for N atom on the auxiliary receptor, which is possibly caused by the growing contribution of electron transition from HOMO to LUMO. Moreover, we can see from Table 3 that the orbital contributions of electronic transition for designed dyes are larger than the experimental molecule. As we all known, f reveals the lightharvesting efficiency (LHE) to some degree. As described in Eq. (3), a larger f will lead to a larger LHE. Obviously, the largest LHE of 0.9858 achieved in CPDT-NPyT is not conspicuous in contrast to the reference dye. As a whole, we can draw the conclusion that the LHE has no marked changes for dyes in the maximum absorption bands. 3.3. Charge injection (DGinject) and dye regeneration (DGreg) According to the above discussion, Finject and hreg are also important means to estimate the photoelectric conversion efficiency, and it can be roughly reckoned by means of DGinject and DGreg. Based on the related reference, we know that the ground state oxidation potential energy (Edye) can be evaluated as negative EHOMO and Edye* can be calculated by Eq. (4). As shown in Table 4, the free energy changes DGinject of all the designed dyes are slightly lower than that of BT-CPDT, suggesting an efficient electron injection from the dyes into the conduction band. Beyond that, low DGreg is required for the fast electron transfer. The spin density distributions for the oxidized systems were analysed to study dye regeneration [49]. Based on the (U) DFT/PBE0/6311G(d,p) calculations, the contour plots of the spin densities as well as the contributions from the donor moiety and bridge are presented in Fig. 5. The spin density for all oxidized dyes is mainly populated over the donor and p-conjugated spacer. Meanwhile, the contribution from the electron donor and p-linker is in the sequence of NPyT-CPDT (88.1%) < CPDT-PyNT (95.5%) < CPDT-BT (96.8%) < CPDT-NPyT (97.9%) < BT-CPDT (98.8%) < PyNT-CPDT (98.9%), which shows a very faintly difference between the reference dye and the new designed ones. At the same time, from Table 4, the DGreg values show a small changes between the reference dye BT-CPDT and other dyes except for NPyT-CPDT. More importantly, the favorable spin density distribution of the cationic radical, sufficient DGreg, and the larger absorption spectra ranges can induce a higher Jsc which may contribute to the improvement of the cell performance.

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W. Hu et al. / Dyes and Pigments 137 (2017) 165e173

Table 3 Main electron transitions, oscillator strengths (f), and absorption bands with f > 0.1 and l > 350 nm in the UVevis regions at the BMK/6-311G(d,p) level for all the dyes in CH2Cl2. (H¼HOMO, L ¼ LUMO, Lþ1 ¼ LUMOþ1, etc.) Sensitizer

Excited energy (eV)

lmax (nm)

f

Composition

BT-CPDT

2.21(S0/S1)

560

1.7899

CPDT-BT NPyT-CPDT CPDT-NPyT PyNT-CPDT

1.84(S0/S1) 2.33(S0/S1) 1.79(S0/S1) 2.06(S0/S1)

675 531 692 603

1.7447 1.8409 1.8478 1.5695

CPDT-PyNT

1.68(S0/S1)

736

1.5396

H-1/L (15%) H/Lþ1 (7%) H-1/L (12%) H/L (96%) H-1/L (10%) H/L (84%) H/Lþ1 (4%) H-1/L (10%)

H/L (75%) H/L (85%) H-3/Lþ1 (2%) H/L (87%) H-1/L (10%) H/L (88%)

Table 4 Calculated electronic properties of the isolated dyes. Sensitizer

LHEa

Edye (eV)b

Edye* (eV)c

DGinject (eV)d

DGreg (eV)e

mnormal (Debye)f

BT-CPDT CPDT-BT NPyT-CPDT CPDT-NPyT PyNT-CPDT CPDT-PyNT

0.9838 0.9820 0.9856 0.9858 0.9731 0.9711

5.25 5.14 5.82 5.21 5.28 5.16

3.04 3.30 3.49 3.42 3.22 3.48


0.96
0.70
0.51
0.58
0.78
0.52

0.25 0.14 0.82 0.21 0.28 0.16

19.559 25.451 21.404 23.412 15.981 22.466

a b c d e f

The The The The The The

light-harvesting efficiency at maximum wavelength (LHE). oxidized potential of ground state (Edye, in eV). oxidized potential of the first excited state (Edye*, in eV). injection driving force (DGinject, in eV) related to the electronic transition in Q band. regeneration energy (DGreg, in eV). dipole moment for the free dyes absorbed on (TiO2)6 cluster (mnormal, in debye).

Fig. 5. Contour plot of the spin density for the optimized geometry of the investigated dyes.

3.4. Dye absorption on (TiO2)6 In addition to Jsc, possessing a high Voc value would lead to high

h. The Voc is directly related to the CB energy (ECB). It has been reported that the shift of ECB is closely correlated with mnormal of the absorbed dyes [50,51]. From Eqs. (7) and (8), we can find that the larger mnormal can exert a significant impact on Voc. In the previous nchez-de-Armas performed real-time calculations on work, Sa different model (TiO2)n (n ¼ 1, 2, 3, 6, 9, 15, and 38) nanoclusters. They regarded that a unit of (TiO2)6 as the smallest nanocluster model was able to simulate semiquantitatively all the features in the electronic structure of the system [52,53]. The (TiO2)6 cluster has also been employed to study the influence of position of auxiliary acceptor in D-A-p-A photosentitizers on photovoltaic

performances of dye-sensitized solar cells [54]. Therefore, we used a (TiO2)6 cluster which was proved to be reasonable to simply simulate the dye-TiO2 systems. Typically, there are three possible modes through which dyes are adsorbed onto a TiO2 surface: monodentate, chelated, and bridged bidentate [55,56]. Bidentate bridging, in which one hydrogen atom (H) is bound to the oxygen surface, is proved to be the most stable [57,58]. Therefore, we employed the bidentate chelation for all the sensitizers absorbed on the TiO2 film, and the corresponding optimized structures for the dye-(TiO2)6 complexes are presented in Fig. 6. As shown in Table 4, the decreased mnormal for the dyes with additional acceptor closing to the triphenylamine moiety may result in a lower Voc. On the contrary, the great increased mnormal for the dyes with additional acceptor next to the cyanoacrylic acid may induce more

W. Hu et al. / Dyes and Pigments 137 (2017) 165e173

171

Fig. 6. The optimized structures together with the HOMO and LUMO energy levels (in eV) of all the dyes when adsorbed on the (TiO2)6 surface, as obtained by the PBE0 functional with the 6-311G(d,p) basis set (a); contour plots of the HOMO and LUMO of dyes on (TiO2)6 surface (b).

negative charges populated closer to the TiO2 surface than that of the reference dye BT-CPDT, resulting in a greater ECB upshift and a larger Voc values. Besides, the total and partial densities of states [59] for dye-TiO2 complexes were conducted to elucidate electronic coupling between the dye and cluster (shown in Fig. S2). For all dye-TiO2 complexes, the electronic densities of LUMO orbitals are primarily localized on the dyes with subtle contribution on adsorbates, suggesting weak electronic coupling between dye and TiO2. The small contribution plays a key role in electron transfer. The contributions from the LUMOs of the designed dyes are higher than these of the model molecule as we can see from Table S5 which shows the collected contributions of selected unoccupied molecular orbitals. We can conclude that dyes with additional acceptors adjacent to cyanoacrylic acid have stronger electronic coupling with the CB film than others, which will boost electron injection. To simulate the performance of the DSSCs more vividly, we explore the electronic and optical properties of the dyes adsorbed on the (TiO2)6 based on wholly optimized structures of dye-(TiO2)6. Bond distances between the carboxylic oxygen atoms and the Ti atoms (TieO bond lengths) of the dyes are gathered in Table S6. The calculated bond distances between the Ti and O atoms of dyes are in the range of 2.007e2.043 Å. The results suggest that all the dyes are adsorbed on the TiO2 substrate tightly which will certainly increase the electron-transfer rate and improve the photovoltaic performance. The optical properties of dye-(TiO2)6 are presented in Fig. 7. The spectral character is well simulated and their spectra comply with the isolated dyes. We can obviously observe that the maximum absorption wavelength of the investigated dyes is redshifted in the scope of 10e75 nm contrasted with the isolated dyes. The largest absorption peaks for the isomers of dye CPDT-BT and CPDT-NPyT are redshifted by 50 nm and 34 nm, respectively. In addition, the relevant molecular orbitals for dye-TiO2 systems during photoexcitation are depicted in Fig. S3. The HOMOs of the

Fig. 7. Simulated absorption spectra and oscillator strength of the dyes absorbed systems at the BMK/6-311G(d,p) level of theory in CH2Cl2.

dyes are located on the triphenylamine section, whereas the HOMO of dye NPyT-CPDT spreads across the whole dye molecule which hampers the efficient electron transfer. LUMO þ 2 and LUMO þ 3 (see Table S7) are centered on the TiO2 substrate, which manifest the dyes have the capacity of electron injection from dyes to the semiconductor. Furthermore, there are puny changes about 0.13 eV in the HOMO when dyes binding to the TiO2 surface in comparison to the HOMO of the isolated sensitizer. The LUMO energy of each dye-(TiO2)6 system is much lower (by about 0.25 eV) than the corresponding LUMO of the isolated dye, indicating that the LUMO has the major contribution to the relatively strong electron coupling of each dye with the TiO2 surface, which facilitates electron injection.

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4. Conclusions In summary, we have designed a series of sensitizers by changing the auxiliary acceptors and their positions using DFT and TD-DFT approaches. The geometrical structures, photoinduced charge transfer character, optical properties are analysed in detail. The additional acceptors positions have a significant influence on the absorption wavelength because of differences in the geometrical structures and the energy gaps together with the MO contributions. It is found that the dyes with the auxiliary acceptors close to the cyanoacrylic acid exhibit an improved DSSC photovoltaic performance with a larger short-circuit current density JSC and a higher open-circuit photovoltage VOC. By comparison, the dyes with additional acceptors next to the donor part may lead to a lower VOC due to possessing a smaller dipole moment, which could be detrimental to the performance of DSSCs. Analyses of the geometrical structures and frontier molecular orbitals of the dye(TiO2)6 system demonstrate that there has strong electronic coupling between the dyes and the TiO2 semiconductor. It is hoped that this theoretical approach will provide guideline for the future design of pushepull organic dyes towards more efficient dyesensitized solar cells. Acknowledgement We acknowledge generous financial support from Natural Science Foundation of China (21173169, 20803059), Chongqing Municipal Natural Science Foundation (cstc2013jcyjA90015). And program for Innovation Team Building at Institutions of Higher Education in Chongqing (CXTDX201601011). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.dyepig.2016.10.011. References [1] O’regan B, Grfitzeli M. A low-cost, high-efficiency solar cell based on dyesensitized. Nature 1991;353(6346):737e40. [2] Hagfeldt A, Boschloo G, Sun L, Kloo L, Pettersson H. Dye-sensitized solar cells. Chem Rev 2010;110(11):6595e663. [3] Yu Z, Vlachopoulos N, Gorlov M, Kloo L. Liquid electrolytes for dye-sensitized solar cells. Dalton Trans 2011;40(40):10289e303. €tzel C, Zakeeruddin SM, Gra €tzel M. Recent developments in redox [4] Wang M, Gra electrolytes for dye-sensitized solar cells. Energy Environ Sci 2012;5(11): 9394e405. [5] Qu S, Hua J, Tian H. New D-p-A dyes for efficient dye-sensitized solar cells. Sci China Chem 2012;55(5):677e97. [6] Yang J, Guo F, Hua J, Li X, Wu W, Qu Y, et al. Efficient and stable organic DSSC sensitizers bearing quinacridone and furan moieties as a planar p-spacer. J Mater Chem 2012;22(46):24356e65. [7] Wu H-P, Ou Z-W, Pan T-Y, Lan C-M, Huang W-K, Lee H-W, et al. Molecular engineering of cocktail co-sensitization for efficient panchromatic porphyrinsensitized solar cells. Energy Environ Sci 2012;5(12):9843e8. [8] Bessho T, Zakeeruddin SM, Yeh CY, Diau EWG, Gr€ atzel M. Highly efficient mesoscopic dye-sensitized solar cells based on donoreacceptor-substituted porphyrins. Angew Chem Int Ed 2010;49(37):6646e9. [9] Yang J, Ganesan P, Teuscher J, Moehl T, Kim YJ, Yi C, et al. Influence of the donor size in D-p-A organic dyes for dye-sensitized solar cells. J Am Chem Soc 2014;136(15):5722e30. [10] Marszalek M, Nagane S, Ichake A, Humphry-Baker R, Paul V, Zakeeruddin SM, et al. Structural variations of DepeA dyes influence on the photovoltaic performance of dye-sensitized solar cells. RSC Adv 2013;3(21):7921e7. [11] Yella A, Lee H-W, Tsao HN, Yi C, Chandiran AK, Nazeeruddin MK, et al. Porphyrin-sensitized solar cells with cobalt (II/III)ebased redox electrolyte exceed 12 percent efficiency. Science 2011;334(6056):629e34. [12] Kakiage K, Aoyama Y, Yano T, Oya K, Fujisawa JI, Hanaya M. Highly-efficient dye-sensitized solar cells with collaborative sensitization by silyl-anchor and carboxy-anchor dyes. Chem Commun 2015;51(88):15894e7. [13] Hong Y, Liao J-Y, Cao D, Zang X, Kuang D-B, Wang L, et al. Organic dye bearing asymmetric double donor-p-acceptor chains for dye-sensitized solar cells. J Org Chem 2011;76(19):8015e21.

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