Accepted Manuscript The electronic structure engineering of organic dye sensitizers for solar cells: the case of JK derivatives Cai-Rong Zhang, Jin-Gang Ma, Jian-Wu Zhe, Neng-Zhi Jin, Yu-Lin Shen, YouZhi Wu, Yu-Hong Chen, Zi-Jiang Liu, Hong-Shan Chen PII: DOI: Reference:

S1386-1425(15)00775-1 http://dx.doi.org/10.1016/j.saa.2015.06.060 SAA 13830

To appear in:

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Received Date: Revised Date: Accepted Date:

19 December 2014 17 June 2015 18 June 2015

Please cite this article as: C-R. Zhang, J-G. Ma, J-W. Zhe, N-Z. Jin, Y-L. Shen, Y-Z. Wu, Y-H. Chen, Z-J. Liu, HS. Chen, The electronic structure engineering of organic dye sensitizers for solar cells: the case of JK derivatives, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), doi: http://dx.doi.org/10.1016/j.saa. 2015.06.060

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The electronic structure engineering of organic dye sensitizers for solar cells: the case of JK derivatives Cai-Rong Zhanga,b*, Jin-Gang Mab, Jian-Wu Zhec, Neng-Zhi Jinc, Yu-Lin Shenc, You-Zhi Wu a, Yu-Hong Chena,b, Zi-Jiang Liu d, Hong-Shan Chene

a

State Key Laboratory of Advanced Processing and Recycling of Non-ferrous

Metals, Lanzhou University of Technology，Lanzhou, Gansu 730050, China; b

School of Sciences, Lanzhou University of Technology, Lanzhou, Gansu 730050, China; c

d

Gansu Computing Center, Lanzhou, Gansu 730030, China;

Department of Physics, Lanzhou City University, Lanzhou, Gansu 730070, China; e

College of Physics and Electronic Engineering, Northwest Normal University, Lanzhou, Gansu 730070, China

*

Corresponding author. Tel.: +86 0931 2973780, Fax: +86 0931 2976040

E-mail address: [email protected] (C.R. Zhang).

1

Abstract: The design and development of novel dye sensitizers are effective method to improve the performance of dye-sensitized solar cells (DSSC) because dye sensitizers have significant influence on photo-to-current conversion efficiency. In the procedure of dye sensitizer design, it is very important to understand how to tune their electronic structures and related properties through the substitution of electronic donors, acceptors, and conjugated bridges in dye sensitizers. Here, the electronic structures and excited-state properties of organic JK dye sensitizers are calculated by using density functional theory (DFT) and time dependent DFT methods. Based upon the calculated results, we investigated the role of different electronic donors, acceptors, and π-conjugated bridges in the modification of electronic structures, absorption properties, as well as the free energy variations for electron injection and dye regeneration. In terms of the analysis of transition configurations and molecular orbitals, the effective chromophores which are favorable for electron injection in DSSCs are addressed. Meanwhile, considering the absorption spectra and free energy variation, the promising electronic donors, π-conjugated bridges, and acceptors are presented to design dye sensitizers.

Keywords: electronic structures, organic dye sensitizers, dye sensitized solar cells, absorption spectra, density functional theory.

2

1. Introduction Dye sensitized solar cells (DSSCs) have attracted much researching attention due to their merits, such as easy fabrication, lower cost, etc[1-5]. The central researching issue of DSSC focuses on how to improve photon-to-current conversion efficiency (PCE). Up to now, it was recognized that all of the main components of DSSC can affect the PCE. Particularly, dye sensitizers have significant influence on the PCE [6-11]. In the past decades, the PCE of DSSC is improved continually through the design and synthesis of novel dye sensitizers. For instance, the 12.3% of PCE had been reported for the DSSC which was co-sensitized by zinc porphyrin dye YD2-o-C8 with another triphenylamine dye Y123 [12]. Recently, the DSSCs sensitized by porphyrin dye SM315 had been achieved PCE about 13% [13]. So, the development of novel dye sensitizer is an effective method to improve the PCE.

The incident PCE (IPCE) can be expressed as IPCE(λ)=LHE(λ)Φinjηcoll, here, the LHE(λ) is the light-harvesting efficiency for photons at wavelength λ, Φinj is the quantum yield for electron injection, and ηcoll is the electron collection efficiency.6 Apparently, for an effective dye sensitizer, the wide absorption band and larger absorption coefficient are expected in order to harvest more photons from solar radiation. Also, the dye should have the suitable energy level of ground state (related to dye regeneration) and excited states with effective charge transfer (CT) character [14]. Certainly, the resistance of degradation and suppressing aggregation should be considered in dye sensitizer design [15, 16]. So, the further developments of novel dye sensitizer depend on the quantitative information of dye sensitizers [9, 11, 17], including electronic structures which dominate energy level alignment, and absorption properties that determine the LHE(λ). 3

Organic dye sensitizers usually contain electronic donor and acceptor which are connected by π-conjugated bridge (CB), commonly known as D-π-A structure. The donor, CB, and acceptor can tune the electronic structures and related properties [18-21]. Ko’s group had been developed many organic dye sensitizers with D-π-A structure, coded as JK series. For JK-1 and JK-2, the electron donor bis-dimethylfluoreneaniline moiety and the electron acceptor cyanoacrylic acid are connected by different thiophene units, and the PCE of JK-2 sensitized DSSC approached 8.01% [15]. In order to expand the absorption bands to longer wavelength region, introducing a vinylene unit into the CB of JK-1 and JK-2 resulted into JK-5 and JK-6 [22]. The JK-13, JK-14, and JK-21, comprising julolidine moieties as electronic donor and bithiophene derivatives as CB, were investigated in order to find the modification effects of CB on PCE [23]. The organic dyes JK-24—JK-29, having N-(9,9-dimethylfluoren-2-yl)carbazole or N-(4-(2,2-diphenylvinyl)phenyl)carbazole as electron donors and cyanoacrylic acid or rhodanine acetic acid as electron acceptors, were designed and synthesized, and the DSSC sensitized by JK-25 generated the PCE about 5.15% [24]. JK-41, JK-42, and JK-43 are composed by bis-dimethylfluoreneaniline and alkyl substituted thiophene unit, and the 7.69% of PCE was reported by JK-41 sensitized DSSC [25]. JK-45 and JK-46 contain dimethylfluorenylamino moiety as donor, thiophene units with aliphatic chains as CB, and cyanoacrylic acid moiety as acceptor. The DSSC based on JK-46 yielded the PCE about 8.60% [26]. JK-51 and JK-52 consist of benzo[cd]indole, and the 8.42% of PCE was obtained by the JK-51 sensitized DSSC [27]. The PCE of DSSCs sensitized JK-51 and JK-52 underlined the dependence of the substituted position on benzo[cd]indole. JK-53 and JK-54 involve trialkylsilyl moiety as anchoring group, and the JK-54 sensitized DSSC gave overall PCE about 4.01% [28]. The 4

bis-dimethylfluorenyl amino donor and cyanoacrylic acid acceptor of JK-57—JK-59 were bridged by p-phenylene vinylene unit, and the PCE of JK-59 sensitized DSSC reached 7.02% [29]. JK-60 and JK-61 contain indolo[1,2-f]phenanthridine unit, and the JK-61 sensitized DSSC gave PCE about 8.34% [30]. JK-62 co-sensitized DSSC achieved considerably improved PCE up to 10.2% [31]. JK-77, JK-78, and JK-79 (containing indole unit) were designed, and the JK-79 sensitized DSSC generated PCE about 7.18% [32]. Five organic sensitizers JK-160—JK-164 contain a fused-planar triphenylamine as electronic donor, and the JK-164 exhibited an excellent stability, yielding an overall PCE about 7.86% [33]. JK-206, JK-207, and JK-208 incorporate thia-bridged heterohelicene unit, thiophene and cyanoacrylic acid, and it was found that the substituent of heterohelicene donor can significantly affect the PCE [34]. These works provided the family of organic dye sensitizers with different electronic donor, acceptor, and CB.

Due to the importance of electronic structures and related properties in the design of dye sensitizers, the above mentioned JK dyes were classified into different groups in terms of their electronic donors, acceptors, and CBs (see Schemes 1—9). The groups 1—6 (G1—G6) can highlight the role of CBs because of the different CBs in each group of G1-G6, while the groups 7—9 (G7—G9) can underline the role of electronic donor. The role of electronic acceptors can be presented by the comparison from the data of JK-24 and JK-26, JK-25 and JK-27. The electronic structures and related properties of the selected JK dyes were calculated by using density functional theory (DFT) and time dependent DFT (TDDFT) method. Based upon the calculations, the role of different electronic donors, acceptors, and CBs in the modification of their electronic structures, absorption properties, as well as the free energy variations for 5

electron injection and dye regeneration were investigated.

2. Computational method The DFT and TDDFT calculations were performed by using Gaussian09 package [35]. Considering computational accuracy and cost, the polarized split-valence 6-31G(d,p) basis sets which are valid for calculating the excitations of organic dyes [36, 37] were applied in the calculations. The B3LYP [38, 39] functional was adopted during the geometry optimization because B3LYP is known to provide molecular geometries in good comparison to experiment [40]. The solvent effects were considered using polarizable continuum model (PCM) method [41, 42]. The electronic excitations were investigated by using TDDFT method based upon the optimized geometries in solvent phase.

It is important to select functional for calculating excitations of dye sensitizers because the TDDFT results calculated with conventional hybrid functionals (B3LYP etc.) usually underestimate CT excitation energies. Also, different functional may generate different molecular orbital (MO) character, and thus the different transition properties (local excitation or CT excitation) [43]. In order to select suitable functional, JK-58 was chosen as representative dye sensitizer, and the TDDFT calculations were performed using different functionals, including the PBE0 [44-46], MPW1K [47], BMK [48], M062X [49], BHandH and BHandHLYP [50] as well as the long range corrected hybrid functional CAM-B3LYP [51], LC-ωPBE [52-55] and 6

ωB97X [56] with default range separation parameter. The table 1 lists the calculated absorption λmax (in nm/eV), the absolute deviation (AD, in eV) which were defined as the difference of calculated excitation energy from experiment, the arithmetic mean absolute deviation (AMAD, in eV), and mean square deviation σ (in eV). The experimental λmax are also listed for comparison. Apparently, for the lowest energy absorption band, the ωB97X functional generated the smallest AD, while the PBE0 functional gave the smallest AD for the second absorption band with higher excitation energy. But the AD of ωB97X (PBE0) are remarkable for the absorption band with the higher (lowest) excitation energy. Also, for the second absorption band, the AD calculated with BHandH, BHandHLYP, CAM-B3LYP, and LC-ωPBE functionals are quite large. In terms of AMAD and σ, the accuracy of M062X is higher than that of other functionals. Furthermore, for long range corrected hybrid functionals, the most suitable range-separation parameter is a function that depend on the electron density of the system under study [57]. The optimized range-separation parameter might be different for different system. The comparability of calculated results will be reduced if we adopt long range corrected functional with optimized range-separation parameter. While for M062X, it doesn’t contain any parameter that depends on system. Therefore, the M062X functional was selected for calculating the electronic structures and excitation properties of JK sensitizers.

The absorption spectra of JK derivatives in UV-vis region were simulated in order to investigate the effects of electronic donors, acceptors, and CBs. The absorption 7

profiles can be calculated using the Pseudo-Voigt model, which is a convolution of the Gaussian and Lorentzian functions [58]

ε (ω ) = Wc1 ∑ i

fi ∆ 12 ,i

 0.25∆212 ,i (ω − ωi )2  + (1 − W )c fi (1) exp − 2.773 ⋅ 2∑ 2 2 2   ∆ ∆ ( ω − ω ) + 0 . 25 ∆ 1 ,i 1 ,i i 1 ,i i 2 2   2

where c1 and c2 are parameters used to determine the height of Gaussian and Lorentzian functions, respectively, ε is the molar extinction coefficient, ω is the energy of all allowed transition included in the equation, W is proportional to the ratio of Gaussian and Lorentzian widths. In this work, we adopted the Gaussian function (W=1), and the half-band width ∆1/2 is assumed to be 0.15 eV. fi denotes the oscillator strength, which is computed as fi =

2 me (ε i − ε j ) µij 3h 2

2

(2)

where me is the mass of electron, h is the reduced Planck constant, εi (εj) corresponds to the ith (jth) electronic levels, and µi, j is the transition dipole moment between the two Kohn-Sham orbitals i and j which are involved in transition, defined as [59]

µij = ∫ Ψi*µˆΨ j d 3r = Ψi µˆ Ψ j 

(3)



µˆ = e ∑ xi , ∑ yi , ∑ zi  

i

i

i

(4)



where µˆ is the dipole moment operator summed over all of the positions of electrons in the system under study.

The free energy variation for electron injection ( ∆ G

inject

) affects on the electron

injection rate and therefore the Jsc in DSSCs. In terms of Preat’s method [60], assuming the electron injection occurs from the unrelaxed excited state of dye, the 8

∆ G inject

can be calculated by the following equations,

dye* ∆G inject = EOX − ECB

(5)

dye * Where E OX is the oxidation potential of the dye in excited state and E CB is the

reduction potential of the conduction band of the semiconductor. The reported E CB dye * (4.0 eV for TiO2) [61] was adopted in this work. The E OX can be calculated as

following [62] dye * dye E OX = E OX − λ max

(6)

dye In which E OX is the redox potential of the ground state and λ max is the absorption

maximum with CT character. Whereas the free energy variation for dye regeneration ( ∆ G regen ) can affect on the rate constant of redox process between the oxidized dyes and electrolyte. The ∆ G regen can be calculated as dye electrolyt e ∆ G regen = E OX − E redox

(7)

electrolyt e electrolyt e where E redox is the redox potential of electrolyte. The E redox of commonly

used redox couple iodide/triiodide is about 4.85 eV (0.35 V vs. NHE) [63].

3. Results and discussion 3.1 Electronic structures 3.1.1 The conjugated effects The MO eigenvalues of JK dyes in G1 are presented in figure 1. The highest occupied MO (HOMO) and the lowest unoccupied MO (LUMO) of JK-1 are about -6.40 and -2.23 eV, respectively, generating HOMO-LUMO gap (HLG) about 4.17 eV. The data of JK-2 gives that the increasing another thiophene in CB of JK-1 elevates HOMO about 0.11 eV, lowers LUMO about 0.15 eV, and reduces HLG about 0.26 eV. The 9

HOMO, LUMO and HLG of JK-5 are about -6.33, -2.37, and 3.96 eV, respectively. The corresponding values for JK-6 are -6.25, -2.48, and 3.77 eV, respectively. Similar to JK-1 and JK-2, the data of JK-5 and JK-6 also indicate that the elongation of CB with another thiophene unit elevate HOMO about 0.08 eV, lower LUMO about 0.11 eV, and reduce HLG about 0.19 eV. Comparison of the data for JK-1 and JK-5, JK-2 and JK-6 suggests that the introduction of vinylene group to CB can elevate HOMO, lower LUMO, and reduce the HLGs more than 0.14 eV. This is resulted from that the elongation of CB enhances the electronic delocalization. The geometrical difference between JK-41 and JK-5 is only the vinylene position. The HLG of JK-41 is slightly smaller than that of JK-5 (0.05 eV). Correspondingly, the dipole moment of JK-41 is also smaller than that of JK-5. The data of JK-41, JK-42, and JK-43 indicates that introducing alkyl chain to thiophene unit in CB generate negligible effects on electronic structures, and therefore alkyl chain in CB relates to the MO with lower energy. The results of JK-43 and JK-45 demonstrate that introducing hexyl chains into thiophene units lower HOMO about 0.03 eV, elevate LUMO about 0.08 eV, and broaden HLG about 0.11 eV. While the substitution of phenyl in JK-45 CB with benzo[b]thiophene, forming JK-46, elevate HOMO about 0.08 eV, lower LUMO about 0.02 eV, and reduce HLG about 0.10 eV. Compared to JK-43, the data of JK-57 show that the substitution of thiophene units with bis(isopentoxy)phenyl increase HOMO (LUMO) about 0.02 (0.13) eV and broaden HLG about 0.11 eV, respectively. The data JK-57, JK-58, and JK-59 suggest that the increasing phenylene vinylene units in CB can elevate HOMO, lower LUMO, and reduce HLG. The substitution of thiophene in JK-41 with phenyl generates JK-62. The HOMO of JK-62 is close to that of JK-41, and the HLG of JK-62 increase about 0.13 eV due to the elevation of LUMO. Generally, the figure 1 indicates that CB can significantly tune the electronic 10

structures. The elongation of CB enhances the electronic delocalization (see MOs in Table S2 in supplementary information (SI)), resulting into HLG reduction. The phenyl

bisthiophene

vinylene

(in

JK-6)

and

the

repetitive

vinylene

bis(isopentoxy)phenyl (in JK-58 and JK-59) are more effective than that of other groups in CB due to the reduction of HLG, so these groups in CB might be helpful to extend the absorption bands to longer wavelength region. The higher HOMO of these dyes corresponds to smaller ground state oxidized potential (GSOP). More interestingly, it can be found from figure 1 that the MO eigenvalue distribution of JK-59, JK-58, and JK-46 are denser than that of other JK dyes in G1, while the experimental short-circuit current density (Jsc) of these dyes are larger than that of others. This means that the dense distribution of MO eigenvalues may be favorable for the improvement of Jsc. This tendency is similar to the Ru-complexes dye sensitizers in our previous work [64].

The electronic structures of the dye sensitizers in G2 are shown in figure 2. The HOMO, LUMO, and HLG of JK-13 are about -6.21, -2.35, 3.86 eV, respectively. Introducing -CH3 moieties to thiophene units in the CB of JK-13, giving JK-14, increase HOMO, LUMO and HLG about 0.02, 0.13 and 0.10 eV, respectively. While introducing bis-EDOT group to the CB of JK-13, generating JK-21, result into a further increasing of HOMO and LUMO about 0.04 and 0.01 eV, respectively, corresponding HLG about 3.93 eV. The slightly increased HOMO which caused by the introducing -CH3 and bis-EDOT moieties to thiophene units in CB can result into the tiny reduction of GSOP.

The electronic structures of dye sensitizers in G3, G4, and G5 are presented in figure 11

3. The HOMO, LUMO, and HLG of JK-24 are about -6.87, -2.18, and 4.69 eV, respectively. Based on JK-24, increasing thiophene unit, giving JK-25, elevate HOMO about 0.22 eV, lower LUMO about 0.18 eV, and reduce HLG about 0.40 eV. Further inserting vinylene unit between thiophene, generating JK-29, elevate HOMO 0.14 eV, lower LUMO about 0.10 eV, and result into HLG 4.05 eV. The data of JK-26 and JK-27 supports that the increasing thiophene units in CB elevate HOMO about 0.18 eV, lower LUMO about 0.12 eV, and reduce HLG about 0.30 eV. Similar to JK-45 and JK-46, the electronic structures data of JK-53 and JK-54 indicate that the substitution of phenyl in CB with benzo[b]thiophene elevate HOMO about 0.05 eV, lower LUMO about 0.27 eV, and reduce HLG about 0.32 eV. The data of G3 and G4 again highlight the effectiveness of bisthiophene, vinylene, and benzo[b]thiophene in the modification of electronic structures and GSOP.

The electronic structures of dye sensitizers in G6 are given in figure 4. The HOMO, LUMO, and HLG of JK-160 are about -6.14, -2.35, 3.79 eV, respectively. Introducing bis-EDOT group to bisthiophene CB, generating JK-161, increase HOMO and LUMO about 0.11 and 0.16 eV, respectively, and thus broaden HLG about 0.05 eV. Substituting the bisthiophene in JK-160 with thieno[3,2-b]thiophen, giving JK-162, lower HOMO about 0.05 eV, elevate LUMO about 0.07 eV, and generate 3.91 eV of HLG.

While

further

extending

CB

with

2-(2-(thieno[3,2-b]thiophen-2-yl)vinyl)thieno[3,2-b]thiophen, corresponding JK-163, elevate HOMO about 0.12 eV, lower LUMO about 0.21 eV, and reduce HLG about 0.33 eV. However, introducing dihexyl in thieno[3,2-b]thiophen in JK-163, giving JK-164, lower HOMO about 0.02 eV, elevate LUMO about 0.04 eV, result into the increase of HLG about 0.06 eV. The bis(3,4-ethylenedioxythiophen) and 12

(2-(thieno[3,2-b]thiophen-2-yl)vinyl)thieno[3,2-b]thiophen are effective CB for tuning electronic structures and GSOP. The denser MO distribution of JK-161 and JK-164 also corresponds to larger Jsc (15.83 and 15.32 mA·cm-2 [33], respectively). The similar electronic structures of JK-163 and JK-164 indicate that the better performance of JK-164 may be resulted from the dihexyl groups reducing dye aggregation and blocking the approach of the electrolyte to the TiO2 surface due to the broken planarity of CB and steric hindrance. This is similar to the role of the tert-butyl and octyloxy groups in porphyrin sensitizers [16].

3.1.2 The donor effects The electronic structures of the JK dyes in G7 are presented in figure 5. The difference among the dyes is their electronic donors. JK-2 was selected as reference dye sensitizer to analyze the role of donor. The substitution of JK-2 donor with julolidine moiety, giving JK-13, elevates HOMO about 0.08 eV, lowers LUMO about 0.03 eV, and reduces HLG about 0.05 eV. Substituting JK-2 donor with N-(9,9-dimethylfluoren-2-yl)carbazole moiety, corresponding JK-25, decreases the HOMO about 0.36 eV, increases LUMO about 0.02 eV, and broadens HLG about 0.38 eV. JK-51 and JK-52 are isomers. The donors of JK-51 and JK-52 are 5-2,6,7,8-tetrahydro-2-(9,9-dimethyl-9Hfluoren-2-yl)-1-phenylbenzo[cd]indol-4-yl and 5-(2,6,7,8-tetrahydro-2-(9,9-dimethyl-9H-fluoren-2-yl)-1-phenylbenzo[cd]indol5-yl), respectively. The HOMO, LUMO, and HLG of JK-51 are about -6.53, -2.37, and 4.16 eV, respectively, while the corresponding values of JK-52 are about -6.78, -2.32, and 4.46 eV, respectively. Apparently, the substituted position has a significant influence on electronic structures for the dyes, and thus on GSOP and λmax. JK-61 contains indole[1,2-f]phenanthridine as donor. The HOMO, LUMO, and HLG are 13

about -6.61, -2.38, and 4.23 eV, respectively. JK-77 and JK-78 are isomers. The donors of JK-77 and JK-78 are 1-(9,9-dimethylfluoren-7-yl)-2-phenylindol and 1-(9,9-dimethylfluoren-2-yl)-2-phenylindol, respectively. The HOMO, LUMO, and HLG of JK-77 are about -6.62, -2.34, and 4.28 eV, respectively. The corresponding values of JK-78 are about -6.77, -2.35, and 4.42 eV. Introducing hexyl in JK-78 generates JK-79 whose donor is (4-hexylphenyl)-1-(9,9-dimethylfluoren-2-yl)-indol, and the HOMO, LUMO, and HLG are about -6.76, -2.36, and 4.40 eV, respectively. The

donor

of

JK-160

is

6,10-di-tert-butyl-4,4,8,8,12,12-hexamethyl-4H,8H,12H-benzo-[1,9]quinolizino[3,4,5, 6,7,-defg]acridine (DHBQA), and the HOMO, LUMO, and HLG are about -6.14, -2.35,

and

3.79

eV,

respectively.

The

donor

of

JK-205

is

2,2′:6′,2′′-dithiatriphenylamine (DTTPA). The HOMO, LUMO, and HLG are about -6.63,

-2.38

and

4.25

eV,

respectively.

The

donor

of

JK-206

is

bis(3-hexyl-6,1-phenylene) -5- (DTTPA-2-yl), and the HOMO, LUMO, and HLG are about -6.53, -2.37, and 4.16 eV, respectively. The donor of JK-207 is bis(4,9-hexylsulfanyl)- 5-(DTTPA-2-yl), and the HOMO, LUMO, and HLG are about -6.72, -2.39 and 4.33 eV, respectively. The electronic structures of these dyes indicate that the donors have significant effect on HOMO, and thus on GSOP, but the influence on LUMO is very slight. The higher HOMOs of JK-2, JK-13, and JK-160 suggest that the donating electron capability of bis-dimethylfluoreneaniline, julolidine, and DHBQA are stronger than that of other donor moieties, resulting into smaller GSOP and HLG.

The Scheme 8 and 9 list the dye sensitizers which have different donors, and their electronic

structures

are

shown

in

figure

6.

The

donor

of

JK-1

is 14

N,N-bis-(9,9-dimethylfluorene-2-yl)phenyl group. The substitution of JK-1 donor with N-(9,9-dimethylfluoren-2-yl)carbazole, giving JK-24, generate HOMO, LUMO, and HLG about -6.87, -2.18, and 4.69 eV, respectively. The substitution of JK-1 donor with N-(4-(2,2-diphenylvinyl)phenyl)carbazole, giving JK-28, generate HOMO, LUMO, and HLG about -6.85, -2.18, and 4.67 eV, respectively. The substitution of JK-1 donor with indolo[1,2-f]phenanthridine unit, giving JK-60, generate HOMO, LUMO, and HLG about -6.69, -2.25, and 4.44 eV, respectively. The higher HOMO and

smaller

HLG

of

JK-1

support

the

effectiveness

of

N,N-bis-(9,9-dimethylfluorene-2-yl)phenyl as electronic donor to tuning electronic structures.

The

donating

electron

capability

of

N,N-bis-(9,9-dimethylfluorene-2-yl)phenyl are stronger than that of other electronic donors in G8. JK-21 and JK-161 can also indicate the role of donor in electronic structures. The HOMO of JK-21 is -6.15, -2.22, and 3.93 eV, respectively. The substitution of JK-21 donor with DHBQA, corresponding JK-161, generate HOMO, LUMO, and HLG about -6.03, -2.19, and 3.84 eV, respectively. The HOMOs (HLG) difference between JK-21 and JK-161 (about 0.10 eV) indicate that the DHBQA group is more effective than the donor of JK-21 to donate electron and reduce HLG.

3.1.3 The acceptor effects The acceptor moiety in dye sensitizers can also tune electronic structure. Comparing the electronic structures of JK-24 and JK-26, JK-25 and JK-27 can help us to understand the role of electronic acceptor. The HOMO, LUMO, and HLG of JK-24 are about -6.87, -2.18, and 4.69 eV, respectively. Substituting cyanoacrylic acid in JK-24 with rhodanine-3-acetic acid, generating JK-26, elevate HOMO about 0.15 eV, lower LUMO about 0.08 eV, and thus reduce HLG about 0.23 eV. The HOMO, 15

LUMO, and HLG of JK-25 are about -6.65, -2.36 and 4.29 eV, respectively. Similarly, substituting cyanoacrylic acid in JK-25 with rhodanine-3-acetic acid, generating JK-27, elevate HOMO about 0.09 eV, lower LUMO about 0.04 eV, and thus reduce HLG about 0.13 eV. The lower LUMO of the JK dyes with rhodanine-3-acetic acid indicates that the withdrawing electron capability of rhodanine-3-acetic acid is stronger than that of cyanoacrylic acid as electronic acceptors.

3.2 Electronic absorption spectra 3.2.1 Transition character and effective chromophores for electron injection The experimental and calculated absorption maximum λmax, as well as the absolute deviations are listed in the table S1 in SI. The data indicates the calculated results agree well with that of experiment. This supports that the calculated results are reliable to analyze the excited state properties. In order to analyze transition properties, it is necessary to distinguish local excitation (LE), effective CT excitation (ECTE) which extends the electron distribution of final state to anchor group, and ineffective CT excitation (ICTE) which is CT excitation where the electron distribution of final state does not extend to the anchor group [16, 43]. The less contribution of anchoring group to the virtual orbitals involved in transitions can result into the lower efficiency because it has been demonstrated that the efficiency of charge injection from the excited dyes having a single carboxylic group is determined by the extent to that the LUMO of the dye fall on its anchoring group [65]. The transition configurations, oscillator strengths, and related molecular orbitals with the excitation wavelength longer than 300 nm are listed in table S2 in SI. Apparently, the relocations of MOs between the initial and final states support that the transitions at λmax are ECTE which is favorable for photon-induced electron injection in DSSC. The occupied MOs in 16

initial states of ECTE transitions indicate bis-dimethylfluorene-aniline, julolidine, N-(9,9-dimethylfluoren-2-yl)carbazole,

indole[1,2-f]phenanthridine,

(9,9-dimethylfluoren-7-yl)-2-phenylindol, DHBQA, and DTTPA moieties are major chromophores which act as electron donors of electron injection. The independence of alkyl chain in the donor parts on the occupied MOs related to ECTE transitions suggests that the alkyl chain moieties are not effective chromophores as electron donor. The role of these alkyl chain moieties may inhibit the dye aggregations.

3.2.1

The conjugation effects

The simulated absorption spectra of JK dyes with different CB are shown in figure 7. The panel a in figure 7 indicates that extending CB with bisthiophene, vinylene, and benzo[b]thiophene can generate red-shift of absorption maximum, while introducing alkyl chain in CB has no effects on tuning of absorption properties because of slight effects on electronic structures. Furthermore, increasing the thiophene units with hexyl chains (from JK-43 to JK-45) result into blue-shift of absorption maximum. However, increasing vinylene bis(isopentoxy)phenyl units (from JK-57 to JK-58, JK-59) can give remarkable red-shift of absorption maximum. Also, JK-59 has the largest absorbance coefficient because the longer CB enhances the overlap between ground state and excited state. The absorbance of JK-41 and JK-62 indicate that thiophene in CB is superior to phenyl for tuning the absorption maximum to longer wavelength region. The data of panel b show that the absorption properties of bisthiophene in CB are very similar to that of bis-EDOT, and introducing -CH3 moieties in thiophene units in CB reduce absorbance with a significant blue-shift of absorption maximum, which is similar to the effects of hexyl chains presented in panel a. Similarly, the data of panels c-e underline the elongation of CB induces the 17

red-shift of absorption maximum and larger absorbance coefficients. In terms of above discussion, introducing bisthiophene, vinylene, and bis-EDOT, as well as increasing thieno[3,2-b]thiophen unit to CB are effective methods to enhance absorbance and shift the absorption bands to longer wavelength region.

3.2.2

The donor effects

The absorbances of JK dyes with different donors are presented in figures 8 and 9. From figure 8, it can be found that the λmax of the dyes with julolidine moiety (JK-13), DHBQA (JK-160), bis-dimethylfluoreneaniline (JK-2) as an electronic donor are longer than that of dyes with other electronic donor parts, while the λmax of the dyes with phenylbenzo[cd]indol (JK-52) and DTTPA (JK-205~JK-207) moieties is shorter than that of dyes with other electronic donor parts. The λmax of other dyes in G7 are comparable.

This

suggests

that

introducing

julolidine

moiety,

DHBQA,

bis-dimethylfluoreneaniline as an electronic donor extend the absorption λmax to longer wavelength. Figure 9 shows that the λmax of JK-24, JK-28, and JK-60 are very close, while the λmax of JK-1 is about 50 nm longer than that of JK-24, JK-28, and JK-60. It is again highlight the effectiveness of bis-dimethylfluoreneaniline as an electronic donor in tuning of absorption bands. The λmax of JK-21 and JK-161 are similar, and the absorbance intensities are also higher. This indicates that julolidine and DHBQA moieties are effective electronic donors which can extend absorption bands to longer wavelength region and increase the light harvesting capability.

3.2.3

The acceptor effects 18

The comparison of absorbance of JK-24 and JK-26, JK-25 and JK-27 exposes the effects of substituting cyanoacrylic acid with rhodanine-3-acetic acid. Apparently, the substitution of cyanoacrylic acid with rhodanine-3-acetic acid increases oscillator strength and extends the λmax to longer wavelength region. So the substitution of cyanoacrylic acid with rhodanine-3-acetic acid is favorable to enhance the light harvesting efficiency.

3.3 The free energy variation of electron injection and dye regeneration The calculated free energy variation of electron injection (∆Ginject, in eV) and dye regeneration (∆Gregen, in eV) are listed in table S3 in SI. The ∆Gregen of JK dyes ranges dye from 1.18 to 2.02 eV. This means that all of EOX are sufficient for dye regeneration.

For the dyes in G1, the data of ∆Ginject indicates that the extension CB of thiophene by bisthiophene or vinylene thiophene results into a reduction of the absolute value of ∆Ginject. The data of JK-41, JK-42, and JK-43 indicate that introducing of alkyl chain of thiophene unit in CB generate negligible effects on ∆Ginject. However, increasing the thiophene units with hexyl chains and the substitution of phenyl in CB of JK-45 with benzo[b]thiophene generate a remarkable increasing of the ∆Ginject absolute value. Furthermore, the data of JK-57—JK-59 indicate that phenylene vinylene and oligo(p-phenylene vinylene) are effective CB for increasing of the ∆Ginject absolute value. JK-62 has larger absolute value of ∆Ginject. The PCE was 9.1% based upon the organic sensitizer TA-St-CA which contains the same CB and acceptor of JK-62 [66]. So, for the dyes in this group, the CB of JK-45, JK-46, JK-57—JK-59, and JK-62 are more effective than that of other dyes due to their larger absolute value of ∆Ginject. The 19

larger absolute value of ∆Ginject is favorable for fast electron injection, and thus for increasing of short-circuit current density (J sc). The experimental Jsc of the dyes support this point. For instance, the Jsc of JK-45, JK-46 are 16.13 and 17.45 mA/cm2, respectively [26].

For the dyes in G2, the introducing of -CH3 moieties and bis-EDOT groups to the thiophene units in CB can give the increasing of the ∆Ginject absolute value about 0.15 and 0.05 eV, respectively. For the dyes in G3—G5, the data indicates that the extension thiophene CB by bisthiophene or vinylene-thiophene and the substitution of phenyl with benzo[b]thiophene results into a reduction of the absolute value of ∆Ginject. This tendency is same to that of dyes in G1.

For the dyes in G6, the absolute value of ∆Ginject for JK-160 is about 0.41 eV. Introducing bis-EDOT to thiophen and substituting of bis-thiophene in JK-160 with thieno[3,2-b]thiophen in JK-162 increase the absolute value of ∆Ginject about 0.02 and 0.06 eV, respectively. However, compared with that of JK-160, the data of JK-163 and JK-164

indicate

that

adopting

2-(2-(thieno[3,2-b]thiophen-2-yl)vinyl)thieno[3,2-b]thiophen as CB or introducing dihexyl in thieno[3,2-b]thiophen CB decrease ∆Ginject absolute value about 0.41 eV. The above analyses suggest that the CB of JK-160—JK162 are superior to that of other dyes in G6 since the increasing of ∆Ginject absolute value.

For the dyes in G7, the ∆Ginject absolute values of JK-160, JK-2, JK-206, and JK-13 are 0.41, 0.29, 0.24, and 0.22 eV, respectively. These are larger than that of other dyes in G7. This indicates that DHBQA, bis-dimethylfluoreneaniline, DTTPA-2-yl, and 20

julolidine moieties as electron donor groups are more effective than other moieties for fast electron injection. For the dyes in G8, the larger ∆Ginject absolute values of JK-1 and JK-60 (0.30 and 0.31 eV, respectively) underline the effectiveness of bis-dimethylfluoreneaniline and indolo[1,2-f]phenanthridine moieties as electronic donors. The larger ∆Ginject absolute values of JK-21 and JK-161 in G9 highlight the effectiveness of cis,cis-1,7-diethoxy-3-isopropyljulolidinyl and DHBQA groups as electron donors. This is similar to that in G7.

The data of JK-24 and JK-26, JK-25 and JK-27 expose the effects of substituting cyanoacrylic acid with rhodanine-3-acetic acid. The reduction of the ∆Ginject and

∆Gregen induced by this substitution suggest that cyanoacrylic acid as acceptor moiety is more favorable for fast electron injection and dye regeneration than that of rhodanine acetic acid.

4. Conclusions In this work, in order to investigate the effects of different donor, CB, and acceptor in the modification of electronic structures and related properties, 39 kinds of JK organic dye sensitizers with D-π-A structures were classified into different groups in terms of their donor, CB, and acceptor. Based upon the DFT and TDDFT calculations, the effects of different electronic donors, acceptors, and CBs on electronic structures, absorption properties, as well as the free energy variations for electron injection and dye regeneration are investigated. The followings are found, (1) The

bis-dimethylfluoreneaniline,

N-(9,9-dimethylfluoren-2-yl)carbazole,

julolidine, indole[1,2-f]phenanthridine, 21

(9,9-dimethylfluoren-7-yl)-2-phenylindol, DHBQA, and DTTPA moieties are major chromophores which act as electron donors of electron injection in DSSCs. The dense distributions of MO eigenvalues for dyes are favorable for improvement of Jsc. (2) The elongation of CB with bisthiophene, vinylene, bis-EDOT, benzo[b]thiophene, and thieno[3,2-b]thiophen, as well as increasing vinylene bis(isopentoxy)phenyl units are effective methods to enhance absorbance and shift the absorption bands to longer wavelength region. The extension of thiophene conjugate bridge by bisthiophene, benzo[b]thiophene, and vinylene thiophene result into a reduction of the

absolute

value

of

∆Ginject.

Furthermore,

2-(2-(thieno[3,2-b]thiophen-2-yl)vinyl)thieno[3,2-b]thiophen

the

adoption or

of

introducing

dihexyl in the CB of thieno[3,2-b]thiophen also decrease the absolute value of ∆Ginject. However, the bis-EDOT, thieno[3,2-b]thiophen, phenylene vinylene, the reptitive thiophene with hexyl chains and phenylene vinylene moieties are effective CB for the increasing of ∆Ginject absolute value. (3) The donors have remarkable effect on HOMO, and thus on GSOP, but the influence on LUMO is very slight. The julolidine, bis-dimethylfluoreneaniline, and DHBQA moieties are effective to extend absorption bands to longer wavelength region and to enhance the light harvesting capability. The DHBQA, DTTPA, bis-dimethylfluoreneaniline, indolo[1,2-f]phenanthridine, and julolidine moieties are more effective than other moieties for fast electron injection because of the larger absolute values of ∆Ginject. (4) The substitution of cyanoacrylic acid with rhodanine-3-acetic acid is favorable to enhance the light harvesting efficiency, while cyanoacrylic acid as acceptor moiety is more favorable for fast electron injection and dye regeneration than that 22

of rhodanine acetic acid. Due to the absorption properties can also be tuned by CB and donor, the organic dye sensitizers with cyanoacrylic acid as acceptor moiety have good performance in DSSCs. (5) For the design of novel dye sensitizers, some electronic donors, CBs, and acceptors generate red-shift of absorption bands, enhancement of absorbance, and reduction of free energy variation for electron injection, or blue-shift of absorption bands and increasing of free energy variation. The overall PCE depends on the competition of these effects. Considering the electronic structures, absorption properties and free energy variation for electron injection, the novel organic dye sensitizers could be designed to contain DHBQA, bis-dimethylfluoreneaniline or julolidine moieties as electronic donors, bis-EDOT, thieno[3,2-b]thiophen, phenylene vinylene and oligo(p-phenylene vinylene) groups as CB, cyanoacrylic acid as acceptor.

Supplementary information The tables about experimental and calculated λmax of JK dyes, the absolute deviations, the transition configurations, excitation energies, oscillator strengths, and related molecular orbitals, as well as the calculated the free energy variation of electron injection and dye regeneration.

Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 11164016 and 11164015). This work was partially supported by the HPC program of LUT. The authors appreciate the helpful computation by Mr. Li-Heng Han 23

and Ms. Li Liu.

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28

Table 1 The absorption λmax (in nm/eV), the absolute deviations (AD, in eV), the arithmetic mean absolute deviations (AMAD, in eV), and mean square deviation σ (in eV) of JK-58 calculated with different functionals. The experimental λmax are also listed for comparison. AD Functionals λmax1 λmax2 AMAD σ λmax1 λmax2 PBE0 639/1.94 379/3.27 0.72 0.05 0.39 0.51 MPW1K 530/2.34 336/3.69 0.32 0.37 0.35 0.35 BMK 544/2.28 344/3.61 0.38 0.29 0.34 0.34 M062X 509/2.44 333/3.72 0.22 0.40 0.31 0.32 BHandH 510/2.43 327/3.79 0.23 0.47 0.35 0.37 BHandHLYP 506/2.45 325/3.82 0.21 0.50 0.36 0.38 CAM-B3LYP 503/2.46 330/3.76 0.20 0.44 0.32 0.34 LC-ωPBE 446/2.78 301/4.12 0.12 0.80 0.46 0.57 ωB97X 459/2.70 308/4.03 0.04 0.71 0.38 0.50 a Experiment 466/2.66 373/3.32 a the experimental results were reported in reference 29.

29

JK-1

JK-2

JK-5

JK-6

JK-41

JK-42

JK-43

JK-45

JK-46

JK-57

JK-58

JK-59

JK-62

Scheme 1. The structures of JK dye sensitizers in G1 have different conjugate bridges. The bis-dimethylfluoreneaniline and cyanoacrylic acid groups act as electronic donor and acceptor, respectively.

30

JK-13

JK-14

JK-21

Scheme 2. The structures of JK dye sensitizers in G2 have different conjugate bridges. The electronic donor and acceptor are julolidine moiety and cyanoacrylic acid, respectively.

31

JK-24

JK-25

JK-29

Scheme 3. The structures of JK dye sensitizers in G3 have different conjugate bridges. The electronic donor and acceptor are N-(9,9-dimethylfluoren-2-yl)carbazole and cyanoacrylic acid, respectively.

32

JK-26

JK-27

Scheme 4. The structures of JK dye sensitizers in G4 have different conjugate bridges. The electronic donor and acceptor are N-(9,9-dimethylfluoren-2-yl)carbazole and rhodanine acetic acid, respectively.

33

JK-53

JK-54

Scheme 5. The structures of JK dye sensitizers in G5 have different conjugate bridges. The electronic donor and acceptor are bis-dimethylfluoreneaniline and trialkylsilyl moieties, respectively.

34

JK-160

JK-161

JK-163

JK-164

JK-162

Scheme 6. The structures of JK dye sensitizers in G6 have different conjugate bridges. The electronic donor and acceptor are fused-planar triphenylamine and cyanoacrylic acid, respectively.

35

JK-2

JK-13

JK-25

JK-51

JK-52

JK-61

JK-77

JK-78

JK-79

JK-160

JK-205

JK-206

JK-207

Scheme 7. The structures of JK dye sensitizers in G7 have different electronic donors. The conjugate bridge and electronic acceptor are bithiophene and cyanoacrylic acid, respectively.

36

JK-1

JK-24

JK-28

JK-60

Scheme 8. The structures of dye sensitizers in G8 have different electronic donors. The conjugate bridge and electronic acceptor are thiophene and cyanoacrylic acid, respectively.

37

JK-21

JK-161

Scheme 9. The structures of dye sensitizers in G9 have different electronic donors. The conjugate bridge and electronic acceptor are bis(3,4-ethylenedioxythiophen) and cyanoacrylic acid, respectively.

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Fig. 1. The electronic structures of dye sensitizer in G1.

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Fig. 2. The electronic structures of dye sensitizer in G2.

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Fig. 3. The electronic structures of dye sensitizer in G3, G4, and G5.

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Fig. 4. The electronic structures of dye sensitizer in G6.

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Fig. 5. The electronic structures of dye sensitizer in G7.

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Fig. 6. The electronic structures of dye sensitizer in G8 and G9.

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a

c

d

b

e

Fig. 7. The simulated absorption spectra of JK dye sensitizers with different conjugate bridges.

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Fig. 8. The simulated absorption spectra of JK sensitizers with different electronic donors.

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Fig. 9. The simulated absorption spectra of JK sensitizers with different electronic donors.

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The electronic structures of selected JK dye sensitizers are calculated The effects of different donors, acceptors, and π-conjugated bridges are analyzed The effective chromophores in dye sensitizers for electron injection are addressed The promising electronic donors, π-conjugated bridges, and acceptors are presented

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