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Dyes and Pigments 99 (2013) 201e208

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Dyes and Pigments

journal homepage: www.elsevier .com/locate/dyepig

Connection style and spectroscopic properties: Theoreticalunderstanding of the interface between N749 and TiO2 in DSSCs

Jie Chen a, Jian Wang a, Fu-Quan Bai a, Li Hao a, Qing-Jiang Pan b, Hong-Xing Zhang a,*

a State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun 130023, PR ChinabKey Laboratory of Functional Inorganic Material Chemistry of Education Ministry & Laboratory of Physical Chemistry, School of Chemistry & MaterialsScience, Heilongjiang University, Harbin 150080, PR China

a r t i c l e i n f o

Article history:Received 6 July 2012Received in revised form27 March 2013Accepted 5 April 2013Available online 9 May 2013

Keywords:DyeeTiO2 photoanode modelAbsorption spectrumDeprotonationDSSCsTDDFTFMOs

* Corresponding author.E-mail addresses: panqj@yahoo.com.cn (Q.-J. Pa

(H.-X. Zhang).

0143-7208/$ e see front matter � 2013 Published byhttp://dx.doi.org/10.1016/j.dyepig.2013.04.008

a b s t r a c t

N749 has been widely investigated in the dye-sensitized solar cells (DSSCs) for its high incidentphoton-to-current conversion efficiency (IPCE) in the long wavelength range. Absorption spectra of theN749eTiO2 photoanode have been theoretically examined in this article. Depending on rational modelsbetween dye and TiO2 film, the experimental absorption peaks have been well reproduced throughdensity functional theory (DFT) and time-dependent DFT (TDDFT) approaches. The calculation resultsshow that types of N749 connecting to TiO2 electrode affect the utilization of solar energy in DSSCs. Theway of electron injection can be distinguished based on the character of the interfacial charge transfer.In addition, the deprotonation of dye molecule also has a significant effect on the absorption spectrum.It is demonstrated that the dyes with closely-lying unoccupied orbitals, allowing for enhanced orbital-coupling, would improve the total photo-to-current conversion efficiency in DSSCs.

� 2013 Published by Elsevier Ltd.

1. Introduction

Dye-sensitized solar cells (DSSCs) are attracting widespreadacademic and commercial interests due to their low cost, easyfabrication and high light-electric conversion efficiency [1e4].Unfortunately, their overall light-to-electric conversion efficiency(h) is currently still lower than that of silicon cells, evokingextensive research to improve h. In the search to increase the ef-ficiency of DSSCs to at least as high as that of silicon-based solarcells, the sensitizers, the heart of DSSCs, have been heavily inves-tigated. Compared with metal-free organic dyes [5e8] and othermetal dyes [9e12], ruthenium-type dye powered cells are inhigher efficiency under standard (Global Air Mass 1.5) illumina-tion. Among them, the successful examples are N3, N719, N749[13e16] and their derivatives (Fig. 1). It has been noted that thedeprotonation behavior of N749 [Ru (4, 40, 400-tricarboxy-2, 20:60,200-terpyridine) (NCS)3]4� is quite different from that of N3 de-rivatives [17]. In addition, N749-sensitized solar cells possess highIPCE (incident photon-to-current conversion efficiency) even in

n), zhanghx@mail.jlu.edu.cn

Elsevier Ltd.

the range of 600e700 nm, but their overall conversion efficiency isnot as high as that of N3-type cells.

In the view of functionalization, organic ligands of N749 can bedivided into two categories: anchoring (COOH) and electron-donating (NCS) ligands. The NCS ligands afford hole-transport toregenerate the oxidized dyes [18], while anchoring ligands arenecessary to reasonably attach the dye to TiO2 surface, facilitatingthe charge injection into the conduction band (CB) of TiO2.

Nanocrystalline anatase TiO2 is thermodynamically stable, andits (101) surface covers more than 94% application in DSSCs. Theabsorption spectrumof TiO2 is in the ultraviolet region, but is able tosensitized by adsorbing chromophores, allowing that, upon excited,electrons transfer into the CB of TiO2 from dyes. Therefore, charac-teristics of TiO2 surfaces are quite crucial. So far, numerous attemptshave been made to investigate the charge-transfer processesincluding the interaction and charge-injectionmechanism betweendyes and TiO2 surface [19e35]. A time-dependent density functionaltheory (TDDFT) study by De Angelis et al. [36] suggested that thecharge-transfer mechanism might change from nonadiabatic toadiabatic as a result of proton transfer from the dye to the surface;however, the intelligible data of photoelectric properties are notobtained because the systems are too complicated and enormous.

In the present study, we will focus on two aspects. First is whythe saturated tactics ofN749 are adopted in the real device, and TBP

Fig. 1. Structures of N3, N719 and N749.

J. Chen et al. / Dyes and Pigments 99 (2013) 201e208202

(tetrabutylammonium) cations are used to saturate anioniccarboxyl groups. Since the TBP is supposed to obstruct theadsorption of dye molecules onto semiconductor surface, the tac-tics would go against the efficiency of DSSCs. As the saturationwould induce the deprotonation without changing the electronicstructure of N749, the deprotonation of N749 is involved all along.Second, reproduce the ample absorption character and give outdetailed chemical interpretation beyond numerical data andcurves, and then according to these precise spectral data, confirmthe morphology of the dyeeTiO2 interface. Therefore, fouradsorption models (IeIV) have been designed and calculated toexamine which kind of the connection styles between N749 andTiO2 surface is the most possible, see Fig. 2.

In this work, a reasonable fragment of TiO2 crystal structure hasbeen chosen to simulate the contact anatase (101) surface. On thebasis of our models, the corresponding geometry, coupling situa-tion and absorption character will be discussed. Furthermore,charge distribution, spectra and the possible way of electronic in-jection (ultrafast- or reduced-sensitizer injection) could be ob-tained [37]. We hope that the deep theoretical understanding ofsolar cells would help improvement of fabrication and performanceof DSSCs in experiment.

2. Computational details and theory

C1 symmetry was adopted to set the conformation of N749withdifferent degree of deprotonation. The geometry of N749 in theground state was fully optimized using the Becke’s three parameterLeeeYangeParr functional (B3LYP) [38]. Fully relaxed structures ofthe dye adsorbing on the TiO2 surface are not obtained, and thussome indirect approximations are adopted, refer the next sectionfor detail. Then, electronic spectroscopy of the N749 with TiO2surface was acquired by the TD-B3LYP method [39e41]. The abovetheoretical methodology has been successfully used to calculate thesingletesinglet and singletetriplet transitions in literatures [42,43].The polarizable continuum model (PCM) [44] was employed tosimulate the acetonitrile (ACN) solvent effects for all spectral cal-culations. All UV/Vis spectra reported in the work were fitted via aGaussian curve with full-width at half-maximum (FWHM) of800 cm�1.

In the calculations, LanL2DZ and 6e31G* basis sets wereadopted to describe metal atoms and other atoms, respectively. Thequasi-relativistic pseudopotential of Ru and Ti atoms was proposedby Hay and Wadt [45,46], and respective 16 (Ru) and 4 (Ti) valenceelectrons were employed in the calculations. Herein, the basis setswere taken as Ru (8s7p6d/6s5p3d), Ti (8s7p6d1f/6s5p3d1f), S(16s10p1d/4s3p1d), O (10s5p/3s2p), and H (4s/2s). Thus, 1368e1364 basis functions and 742 electrons for IeIV systems wereincluded. All calculations were accomplished by using theGaussian09 (Revision A.02) [47].

3. Results and discussion

3.1. The structures for our nanoparticle models

3.1.1. Adopted models for TiO2 filmThere are diverse connecting styles between dye molecules and

TiO2 film [48,49]. In fact, connecting types could be controlled bythe deprotonation of the dye molecule, structure of TiO2 surfaceand so on. Generally, the monodentate ester-type, namely there isonly one single-bond between one carboxyl and TiO2 surface, is oneof the most fundamental connecting types. In N3 powered solarcells, there are two steady single-bonds between two carboxyl andTiO2 surface [50,51]. Nonetheless, the saturated strategy is adoptedin N749 system, so there is only one carboxyl contribution to theadsorption [17]. Therefore, the connecting type in N3 is notreasonable in our systems. In addition, the single-bond may rotate,leading to the uncertainty in the configuration of the dyeeTiO2interface. Furthermore, within some possible conformations, theother contiguous O atom may also connect with TiO2 due to theweak and/or strong interaction between the O and Ti atoms. Inother words, the monodentate ester-type might be some kind oftransition state configuration. The bidentate chelating type (two Oatoms connect with one Ti atom) is not suitable in pure (101) sur-face because of the strong tension. In fact, this connection style issteady and dominant in TiO2 cluster models [52], but strictlyspeaking, a cluster is not equivalent to pure (101) surface. At last,the bidentate bridging type is applicable in this work. Two single-bonds between one carboxyl and two Ti atoms are unchangeable,which is the most important connecting type in our systems.

The choice of the TiO2 surface model determines the rationalityof theoretical calculation [20,22e26]. Large models of TiO2 arelikely to represent the real TiO2 surface. However, the advisableapproximation of adopted model is necessary, otherwise theoret-ical simulation would die due to the current capacity of computa-tion, if extremely large model of TiO2 surface was involved.

Essentially, we have attempted many models in various scales(from Ti5 toTi20), and found that the contribution of each Ti atom toabsorption is not equal. In consideration of orbital-coupling, themost significant influence on charge transfer is from the Ti atomsconnected to N749 directly, which afford the primary transition-orbital in absorption. For other Ti atoms, if they could afford con-jugated orbitals with the two Ti atoms which connected to dye,certain degree of contribution would appear. It can be concludedthat the distance from Ti to the dye is the key point. In other words,small scale models are reasonable and economical in the presentcalculation, and this strategy has been successfully applied in pre-vious study [53].

Therefore, Ti10O37H36 model was cut from crystal structures ofanatase to simulate the surface of TiO2 in current work. In thismodel, H atomswere used to saturate the covalent bond of O atoms,

Fig. 2. Optimized adsorption structure between N749 (0H) and TiO2 101 surface, TD-B3LYP/Lanl2DZ; 6e31G* level of theory.

J. Chen et al. / Dyes and Pigments 99 (2013) 201e208 203

and this method not onlymaintains bond-orientation in crystal, butalso avoids the chaos of charge and multiplicity in the wholesystem.

3.1.2. The geometry of the whole systemThe fully optimized ground state of N749 shows a pseudo-

octahedral coordination for the Ru6N core, because the Ru (II)atom adopted the low-spin 4d65s0 electronic configuration.Anchoring ligand and one of the NCS ligands are nearly coplanar,and the (101) surface of TiO2 locates in xy plane, see Fig. 2.

Considering the distance for binding between carboxyl and Tiatoms in (101) surface, four connecting models (IeIV) are designedin Fig. 2. Though the TBPs prevent the interaction between othercarboxyl groups and TiO2, they could not change the spectrum ofthe dye molecule or the whole system. Thus, our computationmodel did not contain TBP. By now, we cannot get the completeoptimized-configuration of N749 and TiO2 due to the calculationcapacity. We finally had to adopt some approximate ways. First of

all, one carboxyl with a pyridine was arranged closely to TiO2model, and this structure was optimized with constrained condi-tion of frozen coordinates of TiO2. According to the calculation, thebond length of O and Ti atoms between dye and TiO2 surface is2.14 �A (I and III) and 2.03 �A (II and IV), respectively. Interestingly,the bond length of OeTi in I and III is slightly longer than that in IIand IV, in other words, the interaction between dye and TiO2 sur-face is a little stronger in II and IV. In addition, both 2.14 �A and2.03�A are longer than that in crystal structure (1.93 or 1.97�A). Afterthat, the optimized geometric parameters were kept, and the pyr-idine was replaced by the entire dye molecule (N749). Here, wedeem that TiO2 film has little influence on the structure of dyemolecules, because on the one hand, the configuration of dye isalmost maintained by coordination interaction between Ru andligands, on the other hand, the interaction of dye and TiO2 is weak.Therefore it is reasonable to keep the structure of N749 fixed. Inorder to verify the reliability of data, we have simulated sometypical dye molecules with TiO2 model, and the absorption spectra

Fig. 3. Computational absorption spectra (left) about N749 with TiO2 film and theexperimental one (right) from Ref. [55].

J. Chen et al. / Dyes and Pigments 99 (2013) 201e208204

were obtained in systems of C101 etc [54]. According to the analysisof data, it not only could distinguish different dyes, but also becomparable with the experimental data. The absorption curves ofN749with TiO2 (101) surface in the calculation and experiment are

(a)

-7

-6

-5

-4

-3

-2

Ener

gy (e

V)

ab

1.23 eV 1.08 eV 1.00 eV

I (2H) I (1H) I (0H)

(b)

-7

-6

-5

-4

-3

-2

Ener

gy(e

V)

ab

1.36eV 1.22eV 1.14eV

II (2H) II (1H) II (0H)

Fig. 4. Frontier molecular orbital character for I (a), II (b), III (c) and IV (d), under different dwith Ti and anchoring ligands.)

shown in Fig. 3 (FWHM ¼ 800 cm�1). The two curves, with thestrongest oscillator strengths in calculation, are from the sameconnecting model with different deprotonation, and the twoexperimental peaks could be found in our simulation. In general,the current theoretical model is successful.

We have attempted to employ some kinds of functional such asPBE, B3LYP, M06-2X and so on. Finally, the B3LYP is adopted in thiswork based on the consideration of the following aspects. First,comparing with the experimental data, the B3LYP functional hasprovided the more accurate result than others; second, B3LYP hasbeen broadly used in many similar works. Furthermore, thereasonable result has been obtained in our previous work forsimilar (Ru) systems [54].

3.2. Absorption spectra

3.2.1. Electronic structuresSince the frontier MOs (molecular orbitals) play a relevant role

in electronic transition process, it is necessary to investigate thecharacters of the frontier MOs under the framework of the excitedstates. The frontier MOs are illustrated in Fig. 4. We wish to obtainthe data of energy levels in different degree of deprotonation.Furthermore, the effect of orbital-coupling could be shown

(c)

-7

-6

-5

-4

-3

-2

Ener

gy(e

V)

ab

1.30eV 1.20eV 1.16eV

III (2H) III (1H) III (0H)

(d)

-7

-6

-5

-4

-3

-2

Ener

gy(e

V)

ab

1.42eV 1.34eV 1.24eV

IV (2H) IV (1H) IV (0H)

egree of deprotonation. (Especially, a type means d orbital of Ti atoms, b: orbital mixed

J. Chen et al. / Dyes and Pigments 99 (2013) 201e208 205

unequivocally. Electron density diagrams of frontier MOs, involvedin almost all of absorption in III (0H), are shown in Fig. 5, illus-trating the direct charge injection from dyemolecules into the CB ofthe TiO2 film.

There are more unoccupied MOs than the occupied MOsinvolved in absorption transition. These unoccupied MOs could bedivided into two categories: type a (contribution from d orbital ofTi atom) and type b (contribution from both d orbital of Ti atomand dye molecule). Obviously, there are no more than five type borbitals in each system. The type a orbitals facilitate the directelectron injection from the dye molecule in a short time, however,type b orbitals are quite active in absorption, which could benoticed in the next section. If the major contribution to absorptionis from type b orbitals, the electronwould inject into TiO2 filmwitha long time scale [37]. In addition, all of the correlative occupiedMOs are contributed from N749 molecule.

Because the connected Ti atoms could affect the distributionof unoccupied MOs remarkably, the relative position of Ti atoms

Fig. 5. Electron density diagrams of the frontier molecular orbitals relevant to theabsorptions of III (0H) in ACN solution.

should not be ignored. Thus, we discuss the character of MOs in Iand III systems together, and it is similar for II and IV systems.

There are some similar characteristics in I and III systems.Firstly, the ingredient of LUMO þ 0 to LUMO þ 4 (five of the lowestunoccupied MOs) is all from the two Ti atoms, which connect toN749 directly. Secondly, the energy of occupied MOs would risenoticeably with further deprotonation. Thirdly, the energy of mixedunoccupiedMOs from both orbitals of TiO2 and dye (type b orbitals)is influenced by deprotonation, and the tendency is analogous withthat in occupied MOs. Fourthly; there is an energy gap in unoccu-pied MOs from �3.2 to �3.5 eV in our systems, while the concreteeffect is not clear to absorption.

Oppositely, the differences exist between I and III. The gap be-tween LUMO and HOMO is larger in III, which indicates that theabsorption would afford red-shift in I.

In II and IV, many characteristics of energy levels are identicalwith that of I and III except for the following issues. First, the en-ergy of type b orbitals in II and IV (0H) is much higher than that in Iand III, resulting in the blue-shift in absorption. Second, the packedunoccupied MOs result in no obvious gaps among them.

Generally, the connecting positions of the Ti atoms on theTiO2 (101) surface could affect the distribution of energy levelsdeeply, while the influence on the spectrum is not visible in thissection.

3.2.2. Absorption spectra of I and II systems in acetonitrileFundamental parameters are considered in theoretical calcula-

tions, which control the open-circuit photovoltage (Voc) and theshort-circuit current density (Jsc). These parameters including: (i)light harvesting efficiency (LHE); (ii) injection driving force (DGin-

ject); (iii) reorganization energy (lreorg); (iv) the number of photo-injected electrons transferred from the dye to the semiconductor(nc); (v) the extent of charge recombination between the photo-injected electrons in the conduction band of semiconductor and theelectron acceptors in the electrolyte, and (vi) vertical dipolemoment (mnormal) [55,56]. For the short-circuit current density Jsc inDSSCs, it is determined as

Jsc ¼Z

l

LHEðlÞFinjecthcollectdl;

where hcollect is photo-to-current conversion efficiency and LHE(l)is the light harvesting efficiency related to the oscillator strength (f)at a given wavelength. While the larger f, the stronger LHE, due tothe relationship [57] of

LHE ¼ 1� 10�f :

According to the parameters which can affect Jsc, we know thatbesides the LHE, the electron-injection efficiency Finject could alsoinfluence Jsc, which is closely related to the driving force (DGinject) ofthe electron injection from the photoinduced excited states oforganic dyes to semiconductor surface. In general, the largerDGinject, the larger Finject, and it (in eV) can be expressed as

DGinject ¼ Edye* � ECB;

where ECB is the reduction potential of the conduction band of thesemiconductor, which is sensitive to the conditions, and theexperimental value �4.00 eV (vs. vacuum) is widely used. Edye* isthe excited state oxidation potential of the dye. Edye* is determinedby the redox potential of the ground state of the dye (Edye), and thevertical transition energy (lmax) [58,59]. As only N749 is adopted asthe dye molecule in this work, there are no meaningful differencebetween Voc and Edye*. Therefore, the oscillator strength of ab-sorption is the decisive parameter.

J. Chen et al. / Dyes and Pigments 99 (2013) 201e208206

The absorption spectra of all systems in acetonitrile solution aresimulated by TDDFT (B3LYP) with PCM calculations. The calculatedabsorptions of I and III systems associated with their oscillatorstrengths (>0.02), main configurations (CI > 0.2) and the characterassignments are summarized in Table S1. The fitted Gaussian-typeabsorption curves with the calculated absorption data are shownin Fig. 6. Generally, influence of the connecting positions to ab-sorption is more obvious than that of deprotonation. In otherwords, the connecting model decides the intensity of absorption,while the deprotonation decides the absorption range.

In view of the saturated tactic, the asymmetric connecting modefor N749 is the majority without considering energy factors. Ac-cording to the absorption curves, the absorption peaks from 2H to0H in I system are different. In low degree of deprotonation (2H),there are four visible absorption peaks in the range of wavelength(l) > 450 nm. The lowest-energy absorption in calculation is at1040nm.Owing to the largest coefficient (about 0.70), the excitationH � 1 / L þ 1 is responsible for the absorption. According to thecalculation, HOMO � 1 is formed by 41% dxz(Ru) and 53% NCS P*

orbital, whereas the LUMOþ 1 is delocalized over the d orbital of Tiatoms. Therefore, the absorption at 1040 nm could be assigned ascharge-transition from d orbital of Ru andP* orbital of NCS ligandsto d orbital of Ti, where P is shown that all of the NCS ligands takepart in the conjugation with d orbital of Ru. The largest oscillatorstrength is 0.0754when l is 509 nm in I (2H), and the type b orbitalsdo the overwhelming contribution for this sharp absorptions.

(a)

600 800 1000 1200 1400 1600 1800

0.00

0.02

0.04

0.06

0.08

0.10

Osc

illato

r stre

ngth

s (f)

Wavelength (nm)

I (2H) I (1H) I (0H)

(b)

600 800 1000 1200 1400 1600

0.00

0.01

0.02

0.03

0.04

0.05

Osc

illato

r stre

ngth

s (f)

Wavelength (nm)

II (2H) II (1H) II (0H)

Fig. 6. Simulated absorption spectra of I (a), II

In general, the possible cause is that the orbital-coupling of type b isbetter than that of type a between the dye molecule and TiO2, andthis kind of orbitals is involved easily when absorptions happen.

The strong absorption (713 nm) in I (1H) is red-shifted ascompared to I (2H), and the oscillator strength is the same. How-ever, there are the strongest absorptions at 680 nm at 0H degree ofdeprotonation in this system, of which the contribution is virtuallyfrom type b orbitals. Thus, we suppose that the exhaustive depro-tonation is beneficial to strong absorption in long wavelengthrange, which is the feature of N749 DSSC. The possible reasonshould be that the gap between HOMO and LUMO is small, and thetype b orbitals are the most packed in I (0H).

The absorption of II system is quite different from that of Isystem, especially the oscillator strengths. In II system, the ab-sorption becomes weak and red-shifted as the degree of deproto-nation increases. The reason for theweak absorptions should be thecontorted connecting mode of N749 and TiO2 surface, which couldlead to unfavorable orbital-coupling between dye and semi-conductor in DSSC.

3.2.3. Absorption spectra of III and IV systems in acetonitrileIn fact, the middle carboxyl is the most difficult to be saturated

from the energy point of view, The connecting modes of III and IVsystems are close to the actual condition. Similarly, the effect ofdeprotonation is discussed. The absorption curves are shown inFig. 6.

(c)

600 800 1000 1200 1400

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

Osc

illato

r stre

ngth

s (f)

Wavelength (nm)

III (2H) III (1H) III (0H)

(d)

600 800 1000 1200 1400 1600

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

Osc

illato

r stre

ngth

s (f)

Wavelength (nm)

IV (2H) IV (1H) IV (0H)

(b), III (c) and IV (d) in acetonitrile media.

J. Chen et al. / Dyes and Pigments 99 (2013) 201e208 207

The absorptions are very strong in the III system. Even thoughthe absorptions in III (1H) are quite weaker than 2H and 0H, theoscillator strengths of absorptions are significantly stronger thanthose in other three systems.

In complete deprotonation form of III, the calculated absorp-tions are comparable to the experimental values, and there is onlyone notable peak near 640 nm in the same solvent environment(acetonitrile) [60]. In III (0H) system, the absorption in 639 nm isthe strongest in all of systems in our designedmodels. If the variousconnecting types exist in real condition, the other weak absorptionin long wavelength range would be merged in this strong peak,resulting in the observed unsharp absorption shape. Though theabsorption peak at 510 nm is in agreement with experimental data,the oscillator strength is a little weaker in III (0H) system. Thus, wedeem that the absorption peak at 510 nm in III (2H) system is morecomparable with experimental data. In experiment, the concen-tration of TBP is 1 M, therefore the anchoring groups were not fullysaturated, resulting in the spectrum observed in experiment.

Similarly, the weak absorptions are obtained in IV. Since theutilization of solar energy is not satisfactory, the connecting modelof II and IV should be deserted in real DSSC device.

Interestingly, there is no linear dependence between the wholeof absorption strengths and the degree of deprotonation. Of allpossible connection styles, III (0H) exhibits extremely strong ab-sorptions. We speculate two reasons are significant, that one is thegap factor as the same as I system, and the other is that three type borbitals own low and proximate energy (LUMO þ 8: �2.83 eV,LUMO þ 9: �2.81 eV and LUMO þ 14: �2.63 eV) in III (0H) system.In brief, distribution of type b orbitals in frontier molecular orbitalswould make far-reaching influence on absorptions.

4. Conclusions

Electronic structure and spectroscopy of the N749-sensitizedTiO2 surface as well as connecting styles of dye and surface weretheoretically investigated in this work.

The experimental spectrum of N749 in DSSC has been wellreproduced by our theoretical approaches. Moreover, chemicalconnection between dye and TiO2 surface leads to numerousorbital-coupling states, and thus the different connecting stylesinevitably affect efficiency of DSSC greatly. It is shown that theconnecting mode with a high degree of symmetry allows forintense absorptions and Jsc. In DSSCs, the saturated tactic for N749could control the connecting positions effectively. However, N749intrinsically possesses more connecting positions in its anchoringligand than those of N3 derivatives (such as N719 and C101). Thus,the invalid connecting modes would play a more important role inactual device, which may be the reason that the IPCE in N749 islower than N3 derivatives in the range from 400 to 600 nm. If theconnecting modes were controlled effectually, the efficiency ofDSSC would be promoted. Therefore, attempt to decrease thenumber of anchor such as carboxyl acid in N749 is one of the goodways to improve performance of its sensitized solar cells.

In fact, the sign of well orbital-coupling is the appearance of theunoccupied MOs, which are consisted of both d orbital of Ti and p*

of anchoring ligand (type b orbital). And these orbitals significantlycontribute to the absorptions, especially in the range 450e750 nmin our systems. In this research, it should be an edification that fulldeprotonation would affect the distribution of type b orbitals, andbring out the strong absorption in N749-sensitized cells. If the typeb orbitals could be energetically lowered and be more abundant,the power-enhanced DSSC would be expected.

In summary, we have theoretically simulated the absorptionspectra of the N749-type DSSCs. Other relevant dye-sensitizedDSSC systems would be described using the similar theoretical

protocol. And thus, accurate calculation and theoretical simulationare credible, based on which, the potential DSSC with high effi-ciency would be found with a nonpolluting method.

Acknowledgments

This work was supported by the Natural Science Foundation ofChina (20973076, 21003057 and 21273063) and Program for NewCentury Excellent Talents in Heilongjiang Provincial University ofChina (1154-NCET-010) and Specialized Research Fund for theDoctoral Program of Higher Education (20110061110018).

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.dyepig.2013.04.008.

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