Thiophene Bridge Solacell

Chemical Physics xxx (2010) xxx–xxx

Contents lists available at ScienceDirect

Chemical Physics

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

Improvement of the efficiency of thiophene-bridged compounds for dye-sensitizedsolar cells

Julien Preat ⇑, Denis Jacquemin, Catherine Michaux, Eric A. PerpèteUnité de Chimie Physique Théorique et Structurale, Facultés Universitaires Notre-Dame de la Paix, rue de Bruxelles, 61, 5000 Namur, Belgium

a r t i c l e i n f o

Article history:Received 28 April 2010In final form 2 August 2010Available online xxxx

Keywords:Triphenylamine dyesTetrahydroquinoline dyesSolar cell sensitizerElectron injectionLight harvesting abilitiesTDDFT

0301-0104/$ - see front matter � 2010 Elsevier B.V. Adoi:10.1016/j.chemphys.2010.08.001

⇑ Corresponding author.E-mail address: [email protected] (J. Preat)

Please cite this article in press as: J. Preat et al.

a b s t r a c t

A quantum-chemical study is conducted in order to provide UV/Vis absorption spectra (with a ±0.20 eVaccuracy) and oxidation potentials (±0.50 eV accuracy) of a series of conjugated metal-free organic dyescontaining triphenylamine (TPA) and thiophene (TH) moieties. These compounds have recently beendeveloped for dye sensitized solar cells (DSSCs), and are here compared to the tetrahydroquinoline(THQ) class of dyes. Our theoretical results reveal that TPAs provide a larger DGinject. variability thanTHQ dyes, and we have therefore chosen to optimize the former structures. Our procedure made it pos-sible to get insights into the geometrical and electronic structures of the dyes, and to unravel the struc-tural modifications needed to optimize the properties of TPA-based DSSCs. In particular, we propose waysto improve the electron injection process, as well as the light harvesting efficiency (LHE) of the dyes. Onthis purpose, we considered a large set of original compounds, and starting from the TPA structure, wereshown to increase the efficiency of the dye: (i) the 18-OH,-COOH, 13,15-diOMe, 1a,1b-diCN functionali-zation of TPA-2; (ii) the 1a,1b-diCN, 14,15-diOMe,17-CN,18-H,-COOH functionalization of TPA-1, thesespecific groups inducing a strongly exergonic free enthalpy of injection; (iii) the 18-diCOOH substitutionof TPA-2 improves the LHE without suffering a deterioration of the exergonic character of the freeenthalpy of injection. Moreover, the molecular topology analysis demonstrates that, due to the lost ofcoplanarity between the anchoring and the bridging unit, the positive charge is not directly brought incontact with the TiO2 surface, consequently limiting the recombination reaction.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction

The present energetic and environmental crisis has stimulatedthe interest in the design of renewable energy sources. Indeed,with the foreseen 1013 W of new power needed for the coming50 years, the carbon oxide (mainly CO2) concentration resultingfrom the fossil fuel-based energy generation will exceed the cur-rent level (�400 ppm) that has to be maintained. In this frame-work, solar photovoltaic devices are likely to be leadingtechnologies in a promising ‘‘low-carbon level” future [1–8].

In order to greatly increase the penetration of photovoltaic de-vices into the global ‘‘green” energy markets one should designcompounds that can offer low-cost per kilowatt whilst presentinga large light-to-electricity conversion yield. In this context, thedye-sensitized solar cells (DSSCs) certainly appear as one of themost promising materials for converting the solar energy. Conse-quently DSSCS received significant attention as low-cost alterna-tives to conventional semiconductor (SC) photovoltaic devices.These DSSCs are composed of a wide band gap semiconductor (typ-ically TiO2) deposited on a translucent conducting substrate, an an-

ll rights reserved.

.

, Chem. Phys. (2010), doi:10.10

chored molecular sensitizer, and a redox electrolyte (the I�=I�3couple) [9–13].

The Ru complexes photosensitizers show a solar energy-to-electricity conversion efficiency of 10% in average. Nevertheless,an increasing interest for purely organic DSSCs as substitutes forRu complexes raised in recent years due to their key advantages,e.g. a high molar extinction coefficient, a simple and relativelyinexpensive preparation processes, a more straightforward compli-ance with environmental rules [14]. Moreover, several solid-stateDSSCs based on organic dyes appear to have equivalent perfor-mances than inorganic complexes, suggesting promising commer-cial applications [15]. Therefore, metal-free dyes like coumarin[16,17], merocyanine [18], indoline [19], xanthene [20], hemi-cya-nine [21], hydroquinones [22,23], perylene and fluorene [24,25]have been tested in this framework.

The molecular architecture of most organic sensitizers includesa donor (D), a bridge (B, typically a p spacer), and an acceptor (A),which are usually combined following a D–p–A (or D–B–A) rod-like configuration (Scheme 1) to maximize the efficiency of thephotoinduced intramolecular charge transfer (ICT). Generally, thecritical factors governing the sensitization are: (i) the excitedstate’s redox potential has to match the energy of the conductionband (CB) of the semiconductor; (ii) the highest occupied

16/j.chemphys.2010.08.001

http://dx.doi.org/10.1016/j.chemphys.2010.08.001

mailto:[email protected]


http://www.sciencedirect.com/science/journal/03010104

http://www.elsevier.com/locate/chemphys


N

N

S

CNHOOC

S

CNHOOC

Donors Bridges Acceptors

Scheme 1. Sketch of typical D-B-A compounds used as dye sensitizer. In thisscheme, the donors are THQ (top) and TPA (bottom), the bridge is constituted ofphenyl or thiophene ring(s), and the acceptor is a phenyl- or a thiophene-cyanoacetic acid.

VB

CB

E (in eV)

I3- / I-Pt4.0

6.0TiO2

Photoexcitation

InjectionDiffusion

Dye RegenerationRecombination

Leak reaction

Fluorescence

E dye*OX

E dyeOX

CE

Fig. 1. Principle of operation of a DSSC. In this scheme, CB and VB are theconduction and valence bands of the TiO2, respectively.

2 J. Preat et al. / Chemical Physics xxx (2010) xxx–xxx

molecular orbital (HOMO) that must fit the iodine/iodide redox po-tential, and the lowest unoccupied molecular orbital (LUMO) thathas to be higher in energy than the conduction band edge of thesemiconductor; (iii) the light harvesting ability of the dye shouldbe large enough so to reach a substantial photocurrent response;(iv) the conjugation across the donor and anchoring group has tobe maximized, as it determines the charge transfer (CT) amplitude;and (v) the electronic coupling strength between dye’s LUMO andthe semiconductor CB. This coupling guides the efficiency of theelectron injection from the dye onto the semiconductor surface.In practice, the major factors leading to the low conversion effi-ciency are the formation of dye aggregates, and the charge recom-bination between the CB electrons of the surface and the positivelycharged dye (or the electrolyte) [26–28].

To improve the metal-free dyes used in DSSCs, appropriate DBAsystems with adequate properties have to be designed. Recently, ithas been reported that triphenylamine-like (TPA) [29] derivativesand the cyanoactetic acid moiety are patterns of choice as D andA (also anchoring) moieties, respectively [26,30]. Indeed, TPA is ex-pected to strongly confine the cationic charge from the semicon-ductor surface, therefore hampering the recombination.Additionally TPA features a steric hindrance that could preventunfavorable aggregation of the dye at the semiconductor surface[30]. The same hold for similar molecules such as tetrahydroquin-oline (THQ) dyes: they show valuable performances for DSSC [31].

The bridging groups steer the light absorption regions of theDSSCs [26,32,22] and subsequently the scale of the electron injec-tion from the excited state of the dye to the semiconductor surface.We herein report the design of new molecules derived from severalorganic dyes recently synthesized by Li et al. [14], Chang and Chow[33], and Chen et al. [22]. These dyes use TPA or THQ as donor, phe-nyl- or thiophene-cyanoacetic acid moieties as acceptor and bythiophenes and/or phenylenes as bridging units.

The goal of this investigation is to gain insights into the geomet-rical and electronic structure of these systems, and to bring out theadequate structural modifications to optimize the properties of theTPA-based DSSCs. Consequently, we focus on the free energy of theelectron injection onto the TiO2 substrate and on the light harvest-ing abilities of the dyes, for which a TDDFT-based procedure is ableto deliver a qualitatively correct description, as demonstrated inour recent methodological investigations [34].

Nowadays, ab initio approaches have become efficient andattractive tools for the interpretation of the experimental data.The progresses in CPU resources now allow to compute the absorp-

Please cite this article in press as: J. Preat et al., Chem. Phys. (2010), doi:10.10

tion spectra of large molecular species, such as the DSSCs dyes [35–38]. For such calculations, one of the most popular schemes re-mains the time-dependent density functional theory (TDDFT) asit provides results that qualitatively agree with experimental datafor a reasonable computational effort, especially when hybrid func-tionals are used [39–54]. Nevertheless, it is known that ‘‘cyanine-like” structures such as triphenylamine derivatives may be prob-lematic to study with DFT [55–58].

The present contribution is organized as follows. In Section 2 wedetail the computational level and the procedure that have beenused to compute all important parameters. In Section 3, we presenta specific way to improve the dye sensitizer properties. More pre-cisely, in this section, we compare the theoretical and experimen-tal key parameters that have been used to estimate the free energychange related to the injection. Our procedure has also been ex-tended to the study of the dye linked to the semiconductor (an-ode). The insights harvested in this section have been used tobring out the optimal structural modifications. We conclude thiswork by some future prospects and challenges.

2. Methodology

2.1. Operation cell and the key parameters

We present in Fig. 1 a schematic representation of the operatingprinciples of the DSSC. The ‘‘heart” of the system is constituted by amesoporous oxide layer generally composed of TiO2 (anatase). Alayer of the sensitizer dye is grafted on this material and the pho-toexcitation of the dye results in the injection of an electron fromthe donor orbital (LUMO) of the excited dye to the CB of the oxidesemiconductor film. Unfortunately, the injected electron canrecombine with the oxidized sensitizer dyes (recombination). Thispunitive reaction is in competition with the regeneration of theoxidized dyes by the redox mediator which is usually an organicsolvent containing redox system, such as the iodide/triiodide cou-ple (regeneration). The remaining electron can be transported (dif-fusion) in the semiconductor film as the conducting electrons(CEs). These CEs can also react with the redox mediator moleculesor with molecules in solution, before reaching the counter elec-trode (leak reaction) [59]. Moreover, the metal-free organic com-pounds used in DSSCs are certainly not shape-persistent: inprinciple they might undergo fluorescence that can obviously inhi-bit the injection step. Nevertheless, it has been recently shown thatthe time scale for a surface-anchored TPA injected electron to enterin the conduction band of TiO2 from the excited state is on the

16/j.chemphys.2010.08.001


Scheme 2. Sketch of the TPA-R with the numbering of the substitution positions.

J. Preat et al. / Chemical Physics xxx (2010) xxx–xxx 3

order of femtoseconds to picoseconds, which is much faster thanthe fluorescence time scale for this kind of compounds (nanosec-ond order) [60]. In other words, the emission is certainly not a fac-tor that would kill the TPAs potential in the present case. Finally,the remaining CEs flow though the external circuit and the iodideis regenerated by the reduction of triiodide at the counter electrode(of platinum, Fig. 1).

2.2. Computational level

All calculations have been performed with Gaussian 03 [61], fol-lowing a two-step procedure: (i) the optimization of the ground-state geometry with DFT with a tight threshold that correspondsto a rms (residual mean square) force smaller than 10�5 a.u. forthe optimal geometry, and (ii) the determination of the verticalelectronic excitation energies by means of TDDFT. For the first step,we have used the popular three-parameter B3LYP functional [62],in which the exchange is a combination of 20% Hartree–Fock (ex-act) exchange, Slater functional, and Becke’s generalized gradientapproximation (GGA) correction [63], whereas the correlation partcombines local and Lee–Yang–Parr (LYP) functionals [64].

We compare in Table 1 the structural parameters (bonds, anglesand dihedral) related to the electron diffraction analysis ofNPh3[65] with the data calculated for TPA-R (see Scheme 2). Theresults listed in this table show a good agreement between bothapproaches with an average error limited to 0.07 Å. This justifiesthe choice of B3LYP for the geometry optimization. Moreover, wehave performed a TDDFT [66] on PBE0 [67] and B3LYP ground-stategeometries. The resulting electronic excitations present similarenergies and oscillator strength f: 3.01 eV vs. 3.01 eV and 1.35 vs.1.36, respectively. This confirms that the form of the hybrid func-tional used to optimize the geometry has a very limited impacton the excitation energies.

For the TDDFT calculations, we used the BHandHLYP hybrid[68,63,64]. In a recent work dealing with the relationship betweenspurious (i.e. charge transfer) excited states and the amount of ex-act exchange, Tretiak and Magyar [69] have examined the perfor-mance of various TDDFT functionals, for a series of polar donor–acceptor systems, including the (E)-4-(4-(methylsufonyl) styryl)-N,N-diphenylbenzenamine, which is similar to TPA-1. For the con-sidered systems, the authors have demonstrated that a good

Table 1Comparison between the experimental [65] gas-phase structure ofthe TPA moiety (see Scheme 2) and their theoretical counterpart.

TPA (NPh3)

Parameters Experiment Theory

Bonds in ÅN-4b 1.418 1.4294b-3b 1.404 1.4033b-2b 1.396 1.3942b-1b 1.397 1.3961b-6b 1.397 1.3966b-5b 1.396 1.3945b-4b 1.404 1.403C-Hmean 1.123 1.085

Angles in degreesN-4b-3b 123.4 120.0N-4b-5b 117.5 120.03b-4b-5b 119.1 119.44b-3b-2b 120.3 120.13b-2b-1b 120.6 120.52b-1b-6b 119.2 119.51b-6b-5b 120.7 120.56b-5b-4b 120.2 120.1

Dihedrals in degrees3b-4b-N-4a �45.2 �48.0


description of the CT states can be achieved when a large fractionof HF exchange is used. While the optimal fraction of exchange re-mains system-dependent, it appears that using a fraction P0.50would result in a qualitatively correct physical description, at theprice of an overestimation of both the excitation energies andthe oscillator strengths [70]. These conclusions therefore confirmthe selection of BHandHLYP, a 50% hybrid, for the UV/Vis spectraevaluation. As an additional test, we found that the range-sepa-rated CAM-B3LYP [47] functional provides kmax that are almostidentical (typical deviation of 0.05 eV) to the BHandHLYP [34] forTPA test structures. This confirms the validity and reliability ofthe latter functional in our case.

We have selected the 6-31G(d,p) [71] basis set (BS) for theground-state optimizations and 6-311G+(2d,2p) [71] for the TD-DFT calculations. These BS have been shown to return convergedkmax for a series of TPA [34].

All the theoretical kmax reported in the following correspond tothe first dipole-allowed singlet excited state from the ground state.The iodine/iodide couple is used as regenerator in DCCS, implyingthat the solar cells work in solvent phase. This is why the UV/Visexperimental data for triphenylamine-based dyes are reported insolution. Therefore the polarizable continuum model (PCM)[72,73,42,74] is used for evaluating bulk solvent effects at allstages. In PCM, one divides the problem into a solute part (thedye) lying inside a cavity, and a solvent part. We have selectedthe so-called non-equilibrium PCM solutions, and we refer thereader to Ref. [42] for extensive details about this procedure.According to the experimental set up, we retained tetrahydrofuran(THF), dichloromethane, ethanol (EtOH), and dimethylformamide(DMF, as this compound is not defined in the Gaussian 03 package,we used DMSO and fixed the relative dielectric constant to36.70 [75]).

2.3. Excited state properties

We propose to establish a reliable theoretical scheme to evalu-ate the dye’s excited state oxidation potential, and quantify theelectron injection onto a titanium dioxide (TiO2) surface. The freeenergy change (in electron volts, eV) for the electron injectioncan be expressed as [76],

DGinject: ¼ Edye�

OX � ESCCB ð1Þ

where Edye�

OX is the oxidation potential of the dye in the excited state,and ESC

CB is the reduction potential of the semiconductor conductionband. It is often difficult to accurately determine ESC

CB experimentally,because it is sensitive to the conditions, e.g. the pH of the solution.There we use ESC

CB ¼ 4:0 eV for TiO2 [77], an experimental value cor-responding to conditions where the semiconductor is in contactwith aqueous redox electrolytes of fixed pH 7.0 [76,78].

16/j.chemphys.2010.08.001


G inject. = E dye* -OX E SCCB

E dye* =OEE X E dye -OX(1)max

VB

CBE dye*

TiO2

OX

E dyeOX

G inject.

(1)mmaaxx

Q

E SCCB

E (inn eV)

QS0

QS1

Fig. 2. Schematic representation of the key parameters evaluated in this work.


Two models can be used for the evaluation of Edye�

OX [79,80]. Thefirst implies that the electron injection occurs from the unrelaxedexcited state. For this reaction path, the excited state oxidation po-tential can be extracted from the redox potential of the groundstate Edye

OX and the vertical transition energy, that is the photoin-duced intramolecular CT (ICT) that is related to kð1Þmax, according to[76]:

Edye�

OX ¼ EdyeOX � kð1Þmax ð2Þ

In the second model, one considers that electron injection occursafter relaxation. Given this condition, Edye�

OX is expressed as [76,81]:

Edye�

OX ¼ EdyeOX � Edye

0�0 ð3Þ

where Edye0�0 is the adiabatic transition energy between the ground

state and the excited state corresponding to the ICT. Though elec-tron injection from unrelaxed excited states has been observed forTiO2 [82] and SnO2 surfaces [83,84], the relative contribution ofan ultrafast injection path remains unclear, and most experimentalgroups commonly assume that electron injection dominantly oc-curs after relaxation [76,81]. In our previous investigation, we foundthat the absolute difference between the relaxed and unrelaxedDGinject. is constant and is of the same order of magnitude as theEdye

OX and Edye�

OX mean average error (MAE). Consequently, the unre-laxed pathway has been chosen for the DGinject. evaluation [34].

To calculate the 0–0 ‘‘absorption” line, we need both the S0 (sin-glet ground state) and the S1 (first singlet excited state) equilib-rium geometries. More precisely, the 0–0 transition energy iscalculated as:

E0�0 ¼ kð1Þmax � Ereorg:S1

ð4Þ

where

Ereorg:S1

¼ ES1 ðQ S0Þ � ES1 ðQ S1

Þ ð5Þ

and

ES1 ðQ S0Þ ¼ ES0 ðQ S0

Þ þ DES1 ð6Þ

Q S0and Q S1

are the equilibrium geometries of the S0 and S1 states,respectively. ES1 ðQ S0

Þ and ES1 ðQ S1Þ denote the internal energies for

the S1 state calculated at Q S0and Q S1

, respectively, whereas DES1

is the S0 ? S1 excitation energy. As geometry optimization in sol-vent phase of the excited singlet states at the TDDFT level is notavailable in QM programs available to us, we cannot determineQ S1

directly. However, Cave et al. calculated the fluorescence spec-tra of coumarin derivatives at the DFT level and propose to use thegeometry of the T1 state as an estimate of the S1 equilibrium geom-etry [85,86]. Indeed, since both the S1 and T1 states are stronglycharacterized by a HOMO ? LUMO contribution, it is reasonableto postulate that their equilibrium geometries are indeed similar.Therefore, the geometry optimization for the S0(T1) is performedat the (U) B3LYP/6-31G(d,p) level, and for the sake of consistency,DES1 has been calculated using the same functional with the 6-311G+(2d,2p) BS. For TPA-R of Scheme 2 this leads to a DES1 thatvalues 2.45 eV and a Ereorg:

S1that has been calculated at 0.72 eV using

Eq. (7):

Ereorg:S1

¼ ES0 ðQ S0Þ � ES1 ðQ S1

Þ þ DES1 ð7Þ

In other words, it turns out that this reorganization energy remainsquite small comparatively, i.e. Ereorg:

S1only amounts to 20% of the ICT

excitation energy ðkð1Þmax values 3.01 eV), in good agreement with theresults presented in our previous work [34].

Considering that the relaxed path would imply unaffordableCPU resources at this stage, we strive for the unrelaxed path ofinjection for computing the DGinject. of novel dyes [34].


2.4. Summary of the model

Fig. 2 clearly depicts the different parameters that are involvedin the injection process and that are calculated as follow:

1. EdyeOX as well as kð1Þmax are directly computed, that is Edye

OX has beenevaluated at the PCM-B3LYP/UB3LYP/6-31G(d,p) level whereasthe kð1Þmax related to the ICT HOMO ? LUMO transition is com-puted at the PCM-TDBHandHLYP/6-311G+(2d,2p) level.

2. DGinject. and Edye�

OX have been indirectly evaluated using Eqs. (1)and (2), respectively.

We want to underline that this procedure has already been pro-ven efficient for Ru based DSSCs [81], allows a fast and reliableevaluation of the DGinject., as the excited state geometry optimiza-tions are useless. Eventually, the possible origins of the major dis-crepancies between theory and experiment can be considered: (i)PCM does not explicitly consider solute–solvent specific interac-tions; (ii) the vibronic effects are not taken into account [87], moreparticularly, for compounds containing a bithiophene unit, a dis-tortion of the bridging chain (low-frequency bending modes) canoccur in solution though in our calculations, the bithiophene andtrithiophene are only considered in their planar conformation;(iii) the CT transition (for which one notices an important changeof the dipolar moment between the ground and excited state) insolution could be more accurately described by using the state-specific PCM [88]; (iv) Eq. (2) is only valid if the entropy changeassociated with the light absorption process is negligible, (v) wehave only considered the unrelaxed path of injection for comput-ing the DGinject. [34].

3. Results

A DSSC comprises four major components: (i) the dye, (ii) the re-dox shuttle, (iii) the SC photoanode and (iv) the cathode (Scheme 1).In order to obtain significant improvements it is necessary to alterat least one of these four key contributors and we focus here onways to improve the dye sensitizer properties. In this section, wepresent a confrontation between the theoretical and experimentalparameters ðkð1Þmax; Edye

OX , and Edye�

OX Þ that have been used to estimatethe free energy change related to the injection. We have also con-sidered the impact of the bonding of the dye to the semiconductor.

As suggested by Fig. 2, we must raise the excitation energy ofthe dye without altering significantly its Edye

OX Þ in order to reach amore exergonic injection reaction.

16/j.chemphys.2010.08.001



3.1. UV/Vis spectra

For the TPA derivatives, our previous investigation has shownthat two allowed excited states characterized by large transitionprobabilities appear in the UV/Vis region [34]. This is in completeagreement with experimental findings, since measurements showan intense first absorption band ½kð1Þmax� in the 400–600 nm region,and a second strong absorption band ½kð2Þmax� of close to 300 nm.These transitions are associated to excitations predominantlyimplying three molecular orbitals: HOMO-1, HOMO and LUMO:HOMO ? LUMO for kð1Þmax and HOMO-1 ? LUMO for kð2Þmax.

kð1Þmax corresponds to an ICT between the TPA donor and thecyanoacetic acid acceptor end group, whereas kð2Þmax (HOMO-1 ? LUMO) is assigned to a standard p ? p* transition. Therefore,kð1Þmax witnesses the electronic excitation that activates the energy-

Fig. 3. Representation of the THQ-2 HOMO (top) and LUMO (bottom). They havebeen obtained at the TDB3LYP/6-31G(d,p)//B3LYP/6-31G(d,p) level with a constantthreshold of 0.05 jej.

Table 2kð1Þmax (in nm) provided by PCM-TDBHandHLYP//6-311+G(2d,2p), Edye

OX (in eV) that aeV). We also provide the corresponding MAE (in eV) for the two series of dyes.

Compounds EdyeOX kð1Þmax

Theory Exp. Theory Exp

TPA-1 4.95 5.25 2.91 2.97TPA-2 4.88 5.19 2.78 2.90TPA-3 4.89 5.19 2.54 2.69TPA-4 4.71 5.50 2.44 2.84TPA-5 4.58 5.45 2.30 2.71TPA-6 4.52 5.44 2.21 2.62TPA-7 4.62 5.49 2.42 2.80DPA 4.73 4.98 2.36 2.65NDPA-1 4.91 5.22 2.90 2.94NDPA-2 4.91 5.22 2.77 2.94NDPA-3 4.87 5.21 2.53 2.69NPA 4.72 5.13 2.35 2.58

MAE 0.50 0.24

THQ-1 4.83 5.35 2.77 2.81THQ-2 4.66 5.21 2.47 2.65THQ-3 4.63 5.26 2.48 2.68THQ-4 4.50 5.13 2.28 2.65THQ-5 4.52 5.21 2.33 2.72THQ-6 4.41 5.11 2.19 2.61THQ-7 3.84 5.12 1.84 2.52THQ-8 4.70 5.37 2.48 2.64THQ-9 4.61 5.28 3.46 2.79

MAE 0.70 0.35

a For the THQ series, the EdyeOX is experimentally evaluated in DMF solution w


to-electricity conversion process of these DSSCs. For the THQ-based dyes, TDDFT predicts only one allowed excitation with animportant oscillator strength (P1.0) in the visible domain. Thistransition corresponds to a HOMO to LUMO electron promotion,and explains the strong band centered between 400 and 500 nm.The molecular orbital analysis (MOA) [89] of THQ-2 (Fig. 3) con-firms the highly-delocalized character of the frontier orbitals.

Table 2 presents a comparison between simulated and experi-mental kmax. We consider a panel of 12 representative compoundsof the DPA/TPA families (see Schemes 3 and 4) [90,33,14], includ-ing four NDPA derivatives (in which a phenyl of the donor groupis replaced by a naphtene group). For the THQ family, 9 dyes havebeen treated. For the majority of them, the bridging group is con-stituted by ethylene and thiopene groups, except for THQ-9, rely-ing on a thieno-thiophene moiety (Scheme 5).

TDDFT systematically underestimates the kð1Þmax and the TPAcompounds listed in Table 2 can be classified into two phenomeno-logical groups: (i) the TPA-4 ? -7 derivatives, with TDDFT devia-tions close to 0.40 eV, (ii) other dyes undergoing small errors(0.10–0.30 eV). For the THQ series, the MAE are even larger. ForTHQ-9, BHandHLYP overestimates the excitation energy by�0.70 eV and we have tested the popular PBE0 [67] hybrid shownto be optimal in average [47]. The comparison between the TDB-HandHLYP and TDPBE0 results shows that: (i) BHandHLYP pro-vides two excitations with a f close to 1.0 (at 3.46 and 3.71 eVwith similar f of 0.8573 and 0.9015, respectively) which are evalu-ated with PBE0 at 2.96 eV (f = 0.9759) and at 3.23 eV (f = 0.7153),and (ii) PBE0 indeed provides a kTHQ-9

max in better agreement withthe experimental value (absolute deviation of 0.17 eV).

Note that, aside from the approximate nature of the availablefunctionals, a second source of error originates in the imperfect(and single) ground-state geometries selected. For example, theunderestimated excitation energies of TPA-4 ? -7 may be relatedto the floppy vinylene link presents in their backbone. This bridg-ing unit may present a variety of torsional conformations experi-mentally, whereas it is perfectly planar in theory. This couldexplain the observed red shifts. In contrast, the backbones of

re obtained at the PCM-B3LYP/6-31G(d,p) level, and the resulting DGinject. (in

DGinject. Solvent(s)a Ref.

. Theory Exp.

�1.96 �1.72 THF [33]�1.90 �1.71 THF [33]�1.65 �1.50 THF [33]�1.73 �1.34 DMF [14]�1.72 �1.17 DMF [14]�1.69 �1.18 DMF [14]�1.80 �1.31 DMF [14]�1.63 �1.67 THF [33]�1.99 �1.72 THF [33]�1.86 �1.72 THF [33]�1.66 �1.48 THF [33]�1.63 �1.45 THF [33]

0.28

�1.94 �1.46 DMF/EtOH [22]�1.81 �1.44 DMF/EtOH [22]�1.85 �1.42 DMF/EtOH [22]�1.78 �1.52 DMF/EtOH [22]�1.81 �1.51 DMF/EtOH [22]�1.78 �1.50 DMF/EtOH [22]�2.00 �1.40 DMF/EtOH [22]�1.78 �1.27 DMF/EtOH [22]�2.85 �1.51 DMF/EtOH [22]

0.51

hereas the UV/Vis spectra have been taken in EtOH.

16/j.chemphys.2010.08.001


Scheme 3. Sketch of the NDPA, NPA, TPA and DPA derivatives listed in Table 1.

Scheme 4. Sketch of TPA-4 to -7 derivatives listed in Table 1.


TPA-1 ? -3 are more rigid and the calculated excitation energiesare in better agreement with experiment. This trend is also notice-able for THQ-7: 2 vinylene links, 0.7 eV red shift.

Despite these limitations, the BHandHLYP deviations remain inthe line of recently published PCM-TDDFT studies for triphenyl-methane dyes similar to TPA [91,92], as well as for other structures


[93–95]. The analysis of the Figs. 4 and 5 leads to the followingconclusions: (i) the p conjugated bridge in TPA-7 is coplanar tothe D/A moieties whereas for NPA-1 the two bridging phenyls un-dergo a 30� out-of-plane distortion, therefore altering the conjuga-tion of the acceptor–donor system, (ii) the MOs clearly indicatethat the kNDPA-1

max corresponds to a CT process.

16/j.chemphys.2010.08.001


N

S

NC

COOH

a b THQ-1 0 1 THQ-2 1 1 THQ-3 0 2 THQ-4 1 2 THQ-5 0 3 THQ-6 1 3

N

S

NC

COOH

THQ-7

N

S

NC

COOH

PhTHQ-8

NH

S

S

S

COOH

NC

THQ-9

Scheme 5. Sketch of the THQ derivatives with the numbering of the substitutionpositions.

Fig. 4. 3-D structures of TPA-7 (top) and NPA-1 (bottom) derivatives.


3.2. Oxidation potential

In Table 2, we provide the DGinject., as well as EdyeOX and Edye�

OX (ineV) for the TPA and THQ series. For the record, the impact of thethermal contribution on the free energy change ðDGdye

OX Þ has beencalculated for TPA-2 by performing a vibrational analysis at thePCM-B3LYP/6-31G(d,p), and it turns out to be negligible (DGdye

OX

amounts 4.83 eV vs. 4.88 eV for the internal energy). In addition,the zero-point correction could also be neglected, well in the lineof our previous investigation [34].

The results listed in Table 2 demonstrate that the cathodic dis-placements of Edye

OX are in good agreement with the experimentaltrends. For instance, in the series TPA-1 ? 2 ? 3, lab measure-ments show a slight cathodic shift (�0.06 eV between TPA-1 and-2) that is perfectly reproduced by the theory: from 4.95 eV


(TAP-1) to 4.88 eV (TPA-2). Also, in the series TPA-4 ? 5 ? 6 ? 7,our theoretical results (4.71 eV ? 4.58 eV ? 4.52 eV ? 4.62 eV)are in qualitative agreement to the measured trends (5.50 eV ?5.45 eV ? 5.44 eV ? 5.49 eV). For the THQ-1 ? 2 ? 3 ? 4 ?5 ? 6 series, the cathodic shift observed when the bridge lengthincreases is also correctly reproduced by the theory, as the exper-imental evolution, 4.83 eV ? 4.66 eV ? 4.63 eV ? 4.50 eV ?4.52 eV ? 4.41 eV, is reasonably reproduced: 5.35 eV ? 5.21 eV ?5.26 eV ? 5.13 eV ? 5.21 eV ? 5.11 eV. On the contrary, ourmethodology is inadequate for THQ-7: the oxidation potential ofthis compound is dramatically underestimated by DFT. This spe-cific error could be related to the free rotation around the bondlinking the ethylene units and the hydroquinoline moiety thatmight be activated. Of course, while the Edye

OX displacementsDEdye

OX

� �are quantitatively well-reproduced, the absolute Edye

OX val-ues are only accurate to ±0.50 eV, and ±0.70 eV for TPAs and THQs,respectively. Nevertheless, since DEdye

OX remains the truly crucialparameter, our procedure can safely be used to optimize newstructures for DSSC applications.

3.3. Electron injection

This section concerns one of the most important part of the pro-cedure, that is the analysis of the free energy change related to theelectron injection. Our procedure provides values of DGinject. thatqualitatively agree with experiment, with average errors of0.28 eV for TPAs and 0.51 eV for THQs, and the dependence uponmolecular systems is quantitatively well-reproduced by the theory.In most cases, the underestimation of both Edye

OX and the excitationenergy leads to an overestimation of the ‘‘exergonic character” ofthe injection free energy change. Note that for DPA, the accurateevaluation of the DGinject. (deviation of �0.03 eV) can be directly re-lated the quality of Edye

OX (�0.15 eV error). On the contrary, for THQ-7, the overestimation of the free enthalpy variation parallels theimportant underestimation of both the oxidation potential andkmax [96].

With respect to DGinject., we pick out four compounds in the TPAset, exhibiting the most exergonic character, mainly TPA-1, NDPA-1, TPA-2 and NDPA-2, for which the experimental DGinject. values

16/j.chemphys.2010.08.001


Fig. 5. Representation of the NDPA-1 (left) and TPA-7 (right) HOMO (top) and LUMO (bottom). They have been obtained at the TDB3LYP/6-31G(d,p)//B3LYP/6-31G(d,p) levelwith a constant threshold of 0.05 jej.


are about �1.70 eV. Concerning the THQ series, all compoundspresent similar properties and no significant outlier emerges.

3.4. Effects of chemical modifications

In the present section, we propose structural modificationsimproving the electron injection efficiency of the TPA-based DSSCs,and more precisely TPA-1 and -2, since they provide a larger DGin-

ject. variability.Of course, all modifications are theoretically possible, and a

large panel of new structures can be tested. Nevertheless, we im-pose that all dyes include at least a terminal -COOH moiety onthe acceptor side, as this group is necessary to link the dye to thesemiconductor surface [30]. Next we applied three criteria: (i)the free energy of injection DGinject. in TiO2 has to be smaller than�1.84 eV, the referential value calculated for TPC-1, as the larger�DGinject., the faster the electron injection from the valence excitedstate [80,97]; (ii) the oxidation potential of the dyes must be morepositive than the I�=I�3 redox couple (4.8 eV ± 0.1 eV) [98], ensuringthat there is enough driving force for a fast and efficient regenera-tion of the dye cation radical; and (iii) the light harvesting effi-ciency (LHE) of the dye has to be as large as possible in order tomaximize the photocurrent response. Here, LHE is expressed as[99]:

LHE ¼ 1� 10�A ¼ 1� 10�f ð8Þwhere A(f) is the absorption (oscillator strength) of the dye associ-ated to kð1Þmax. It is known that TDDFT is less accurate for the evalua-tion of transition probabilities than for transition energies. The LHEcriterion has therefore been underweighted in our classification, asour assessments of DGinject. and Edye

OX are probably more reliable, evenif the dependence of the experimental extinction coefficient withrespect to auxochromic effects can be qualitatively reproduced fortriphenylmethane derivatives [91].

These three criteria have been used to set up an efficiency rank-ing of the compounds listed in Tables 3 and 4 in which we providethe relevant parameters for a set of more than 10 extra species.


Before ranking the compounds, it has been checked that all thecompounds listed in Table 3 show a higher oxidation potentialthan the I�=I�3 redox potential (4.8 eV). Note that we did not ex-clude TPC-13, TPC-14, nor TPA-15, even if their Edye

OX are quite closeto the I�=I�3 redox potential (4.79, 4.77 and 4.68, respectively). In-deed the difference between Edye

OX and the I�=I�3 redox potential issmaller than the theoretical results accuracy.

With respect to the relative free energies of injection(DGinject:

r ¼ DGinject:dye =DGinject:

TPA-2) and the relative LHE (RLHE, is obtainedusing Eq. (8) with the ratio fdye/fTPA-2 replacing f), the compoundslisted in Table 3 show the following trends: (i) TPA-10, -13, -14,and -15 have a LHE significantly superior to TPA-2 (i.e. >0.9200),whereas TPA-16 (18-diCOOH-TPA-2) shows an improved LHE(RLHE of 0.9046) without significant deterioration of the exergoniccharacter of the free enthalpy of injection; (ii) for the set of thecompounds listed in the two tables, it is impossible to isolateone structure showing a huge improvement of both LHE and injec-tion driving force; (iii) for a large majority of dyes, the chemicalmodifications lead to a slight improvement of RLHE and a deterio-ration of the DGinject. with respect to TPA-2; (iv) TPA-11, TPA-18,-21 and -22 are characterized by the worst LHE and DGinject:

r param-eters; (v) a cyano group grafted on the TPA-2 or -10 acceptor site inposition 17 (TPA-18) significantly deteriorates the free energy ofinjection; (vi) the TPA-17 and -20 have DGinject. < �2.00 eV; (vii)for the series TPA-13 to -15 (Scheme 6), the RLHE factor evolveswith the bridge length in the order TPA-14 (0.9291) ’ TPA-13(0.9478) < TPA-15 (0.9609); (viii) on the other hand, by addingone (TPA-13 and -14) or two (TPA-15) ethylene moieties on eachside of the central thiophene bridging group of TPA-2, one signifi-cantly deteriorates the DGinject:

r factor. This is explained by theimportant decrease (�0.2 eV for TPA-13 and -14 and 0.40 eV forTPA-15) of the excitation energies when ethylene subunits areadded; (ix) for the same series, the oscillator strength is alsostrongly affected. For TPC-14, this modification is probably relatedto the gain of coplanarity of the bridging group. Indeed, TPA-2shows a �30� twist, altering the conjugation of the acceptor–donorsystem, whereas TPA-14 is perfectly coplanar.

16/j.chemphys.2010.08.001


Table 3Estimated (relative) DGinject:ðDGinject:

r Þ; EdyeOX and Edye�

OX (in eV) for a series of new structures. For each compound, we also provide the kð1Þmax (in eV), the corresponding oscillator strength(f), and the relative light harvesting efficiency (RLHE). The theoretical parameters presented are obtained at the restricted and unrestricted levels using the B3LYP functionalcombined with the 6-31G(d,p) basis set. The kð1Þmaxs (in eV) are obtained at the TDBHandHLYP/6-311+G(2d,2p) level. For both the ground and excited states, the solvation (THF) hasbeen taken into account by using the PCM model. Note that all the electrochemical parameters obtained for the new structures are compared to the TPA-2 counterparts.

Compounds DGinject.Edye

OX Edye�

OX kð1Þmaxf RLHE DGinject:

r

TPA-2 �1.90 4.88 2.10 2.78 1.7088 0.9000 1.001a,1b-diBr-TPA-2 TPA-8 �1.14 5.68 2.86 2.82 1.8139 0.9132 0.601a,1b-diCl-TPA-2 TPA-9 �1.79 5.03 2.21 2.82 1.8013 0.9117 0.941a,1b-diCN-TPA-2 TPA-10 �1.69 5.26 2.31 2.95 1.8924 0.9219 0.891a,1b-diCN,17-CN-TPA-2 TPA-11 �0.96 5.60 3.04 2.56 1.2079 0.8036 0.511a,1b-diF-TPA-2 TPA-12 �1.82 4.94 2.18 2.76 1.7680 0.9077 0.96

18-diCOOH-TPA-2 TPA-13 �1.76 4.79 2.24 2.55 2.1908 0.9478 0.93TPA-14 �1.74 4.77 2.26 2.51 1.9764 0.9291 0.92TPA-15 �1.70 4.68 2.30 2.38 2.4050 0.9609 0.89TPA-16 �1.87 4.87 2.13 2.74 1.7435 0.9046 0.98TPA-17a �2.87 4.52 1.13 3.39 1.6731 0.9791 1.51TPA-18b �0.85 5.98 3.15 2.83 1.1303 0.7820 0.45TPA-19c �1.91 5.38 2.09 3.29 1.2783 0.7481 1.01

a 1a,1b-diCN,13,15-diOMe,18-OH,-COOH-TPA-2 (see Scheme 7).b 1a,1b-diCN,13,15-diOMe,17-CN-TPA-2 (see Scheme 7).c 1a,1b-diCN,13,15-diOMe,17-CN,18-H,-COOH-TPA-2.

Table 4Estimated (relative) DGinject:ðDGinject:

r Þ; EdyeOX and Edye�

OX (in eV) for a altered structures ofTPA-1 and �3. For each compound, we also provide the kð1Þmax (in eV), the correspondingoscillator strength (f), and the relative light harvesting efficiency (RLHE). Thetheoretical parameters presented are obtained at the restricted and unrestrictedlevels using the B3LYP functional combined with the 6-31G(d,p) basis set. The kð1Þmaxs(in eV) are obtained at the TDBHandHLYP/6-311+G(2d,2p) level. For both the groundand excited states, the solvation (THF) has been taken into account by using the PCMmodel. Note that all the electrochemical parameters obtained for the new structuresare compared to the TPA-2 counterparts.

Compounds DGinject.Edye

OX Edye�

OX kð1Þmaxf RLHE DGinject:

r

TPA-2 �1.90 4.88 2.10 2.78 1.7088 0.9000 1.00TPA-20a �2.36 4.81 1.64 3.17 1.0023 0.7409 1.12TPA-21b �1.47 5.43 2.53 2.90 1.4447 0.8573 0.77TPA-22c �1.76 5.08 2.24 2.84 1.2018 0.7033 0.93

a 1a,1b-diCN,14,15-diOMe,17-CN,18-H,-COOH-TPA-1.b 1a,1b-diCN,9,11-diOMe,17-CN,18-H,-COOH-TPA-1.c 1a,1b-diCN,8,9,12,13-tetraOMe,17-CN,18-H,-COOH-TPA-3.


3.5. Optimal structures

Firstly, we would like to underline that whilst the absolute EdyeOX

values present an accuracy that is not optimal, the experimentalEdye

OX shifts are quantitatively well-reproduced by the procedure.Consequently, in this section we have decided to select the optimalcompounds following relative criteria: DEdye

OX remains the truly cru-cial parameter.

N

S

a b TPA-13 0 1 TPA-14 1 0 TPA-15 1 1

Scheme 6. Sketch of the TPA-13,


From the whole original set of new potential structures listed inTables 3 and 4, only two dyes passed out our test grid of ‘‘best pho-tovoltaic properties”: TPA-20 and -17. Indeed, their DGinject:

r > 1:00(1.12 and 1.51, respectively), a fact related to the important varia-tion of the kð1Þmax (for instance, when going from TPA-2 to -17,Dkð1Þmax ¼ 0:41 eVÞ, and the lowering of the Edye

OX variation (0.36 and0.07 eV for TPA-17 and -20, respectively).

For TPA-19, the increase of the excitation energy (0.41 eV) isquasi identical to the Edye

OX variation (+0.40 eV), therefore inducinga slight improvement of the DGinject:

r (0.01) but unfortunately, animportant deterioration of the RLHE factor (�0.1519). fTPA-19 (aswell as fTPA-20) are very low (1.0023 and 1.2783) and this can beeasily explained by the followings: (i) for TPA-20, his oscillatorstrength decreases originates in a loss of coplanarity of the bridg-ing group; (ii) for TPA-19, the B group is coplanar to D and thesmaller f is probably related to the expected smaller electronmobility in thiophene than in phenyl rings. Moreover, Fig. 6 showsthat the LUMO of TPA-19 and -20 are centered on the anchoringmoiety and would therefore favor the electron injection from dyeto the semiconductor. Furthermore, the evaluation of the atomiccharge carried by the nitrogen qN in jej) at the TPA level confirmsthe DSSC abilities of TPA-2-like structures.

As underlined in the introduction, the cationic TPA moiety con-centrates the positive charge far away from the semiconductor sur-face after injection, and efficiently restricts the recombinationprocess. To evaluate this effect we used the PCM(THF)-B3LYP/6-31G(d,p) charges derived from the electrostatic potential (so-called

HOOC

CN

-14 and TPA-15 derivatives.

16/j.chemphys.2010.08.001


Scheme 7. Sketch of the TPA-1 and -2 derivatives with the numbering of thesubstitution positions.

O O

D

Ti

OHHO

OHOH2

O O

D

Ti Ti

OOH

OH

HO

HO

H2O

OH2H2O

HO

N

CN

D =

Fig. 7. Bidentate chelating (left, 1 Ti) and bidendate binding (right, 2 Ti) modes forTPA-23.


MK or Merz–Singh–Kollman charges [100]). For TPA-2, the varia-tion of the charge (DqN) between the neutral qN

dye ¼ �0:54jejÞ andcationic species ðqN

dyeþ¼ �0:41jejÞ amounts to 0.13jej, whereas for

TPA-17 (for which a larger DGinject:r has been calculated), the varia-

tion is only limited to 0.06jej, the positive charge being completelydiluted in the conjugated bridging group. Of course, several exper-imental factors might affect the recombination. However for TPA-17, since the coplanarity between the -CHCN-CHCOOH anchoringgroup and its bridging unit is broken (�60� out-of-plane distor-tion), it seems likely that the positive charge may not be directly

Fig. 6. Representation of the TPA-19 (left) and TPA-20 (right) HOMO (top) and LUMO (bowith a constant threshold of 0.05 jej.


in contact with the TiO2 surface, subsequently inhibiting therecombination reaction.

3.6. Impact of the semiconductor

In this section we perform a theoretical investigation of the dyelinked to the semiconductor. As a first approximation, we considerthat the dye is bounded to one or two Ti atoms and a set of waterand hydroxyl ligands are added to make sure that the whole com-plex is neutral (Fig. 7). This model is similar to the one of Peng andcoworkers [101].

Experiment have shown that for dyes like TPA-23, a bidentatebridging (2 Ti) or a bidentate chelating (1 Ti) structure is likely tohappen for most anchored dyes [90]. The results stemming from

ttom). They have been obtained at the TDB3LYP/6-31G(d,p)//B3LYP/6-31G(d,p) level

16/j.chemphys.2010.08.001


Table 5Estimated DGinject: ; Edye

OX and its experimental value ðEdye�Exp:OX Þ; ðEdye�

OX and kð1Þmax (in eV) as well as the oscillator strength (f) and the related LHE parameter for TPA-23 upon threeconditions: (i) free in solution, (ii) bounded to Ti (OH)3H2O (1 Ti model) and (iii) to Ti2O (OH)5(H2O)3 (2 Ti model). We also provide the variation (D) between the Free and the 1 Tiand 2Ti models. a

DGinject.Edye

OX Edye-Exp:90OX [90] Edye�

OX kð1Þmaxf LHE

Free �1.14 4.89 5.47 2.86 2.03 1.14 0.93Ti(OH)3H2O �0.99 5.06 – 3.01 2.05 1.21 0.94Ti2O(OH)5(H2O)3 �1.01 5.06 – 2.99 2.03 1.23 0.94D1Ti �0.15 �0.17 – �0.15 �0.02 0.07 0.01D2Ti �0.13 �0.17 – �0.13 0.0 0.08 0.01

a The EdyeOX s presented here are obtained at the restricted and unrestricted levels using the B3LYP functional combined with the 6–31G(d,p) basis set. The kð1Þmaxs are obtained

at the TDB3LYP/6-311+G(2d,2p). For both the ground and excited states, the solvation has been taken into account by using the PCM model. In order to get more details aboutthe procedure, we refer the reader to our previous methodological investigations [34]. The selected solvent is dichloromethane [90].


the extended procedure is organized as the following: firstly, weevaluate the SC effects on the key parameters ðkmax; Edye

OX andEdye�

OX Þ required to estimate the free energy change related to theinjection; secondly, we discuss the impact of the SC on the DGinject.

parameter.All parameters listed in Table 5 have been evaluated using both

the 1 Ti and 2 Ti models and we provide the estimated variations ofkð1Þmax; E

dyeOX ; Edye�

OX and DGinject. (in eV) when going from the free mole-cules in solution to the 1 Ti and 2 Ti models. From results listed inthis table we can conclude that: (i) the differences between 1 Tiand 2 Ti parameters are negligible and the first approach is ade-quate to describe the SC effects; (ii) both kð1Þmax and Edye

OX are affectedby the titanium complex; (iii) when the dye is anchored on theTiO2 surface, the experimental absorption spectra are blue-shiftedby a �0.1 eV compared to the solvated case [90] and this blue-shiftis partially reproduced by the theory, tough within the selectedmodels, Dkð1Þmax is strongly underestimated; (iv) the effect of the SCis more important for Edye

OX for which we notice an anodic displace-ment (�0.2 eV) when going from the free dye to the complexedstructure; (v) the 1 Ti model provides Edye

OX in better agreement withthe experiment value than the free dye approximation (the error is0.41 eV instead of 0.50 eV) [34]; (vi) with the bidendate chelatingscheme (1 Ti model), one observes that Dkð1Þmax < DEdye

OX . This resultsin a decrease of DGinject. to �0.15 eV, which suggests that the QMapproach dealing with free dyes in solution tends to slightly over-estimate the free enthalpy of the electron injection.

4. Concluding remarks

In the present work, we managed to (i) gain insights into thegeometrical and electronic structures of thiophene-bridged organicdyes; and (ii) to bring out the adequate structural modificationsoptimizing the properties of the TPA and THQ-based DSSCs, usingphenyl and thiophene spacers. It is clear that, in agreement withexperimental trends, the BHandHLYP hybrid is a reasonable com-promise functional for the TDDFT calculations. Using this scheme,the mean average error lies in the 0.20�0.30 eV domain. This sat-isfying achievement results in part from the selection of quite ex-tended basis sets and from the explicit consideration of bulksolvent effects.

Admittedly, the absolute accuracy for TPAs EdyeOX is not optimal

and the EdyeOX absolute deviation should be reduced, but the theoret-

ical procedure quantitatively reproduces the experimental dis-placements (e.g. DEdye

OX Þ. Consequently, the ‘‘best” compoundsselection as been performed following relative criteria. Indeed,since DGinject. remains the truly crucial parameter, our procedurecan safely be used to optimize new structures for DSSC applica-tions. This investigation shows that TPAs provide a larger DGinject.

variability (than THQs), and TPA-1 and -2 cores were therefore se-lected for further optimizations. In that context, the electron injec-tion efficiency into the TiO2 surface and the light harvesting


abilities of a panel of dyes have been investigated. Starting withthe TPA-1 and -2 structures, the following modifications help toimprove the properties of the DSSC: (i) the 18-OH,-COOH, 13,15-diOMe, 1a,1b-diCN functionalization of TPA-2 (that is TPC-17);(ii) the displacement of the terminal -CN group from the position18 to 17, the substitution of the hydrogen atoms by two -OMefunctions in positions 14 and 15, as well as the grafting of two -CN groups in positions 1a and 1b on the TPA-1 moiety (leadingto TPC-20). These changes provide a highly exergonic free enthalpyof injection (�2.87 eV ± 0.50 eV and �2.36 eV ± 0.50 eV, for TPA-17and -20, respectively, compared to �1.96 and �1.90 eV ± 0.50 eVfor TPA-1 and -2).

In addition, an improved LHE parameter was predicted for TPA-16 (18-diCOOH-TPA-2, RLHE of 0.9046) without a significant dete-rioration of the exergonic character of the electron injection.

Examination of the atomic charges and molecular orbital topol-ogy demonstrates that the positive charge is completely diluted inthe conjugated bridging group. However, since the coplanarity be-tween the -CHCN-CHCOOH anchoring part and the bridging unit isbroken, the positive charge is not directly in contact with the TiO2

surface and the recombination reaction should therefore beinhibited.

An investigation of the dye linked to the semiconductor as alsobeen performed using a dye bounded to one or two titaniumatoms. This study allowed to conclude that, on one hand, whenthe dye is attached on the TiO2 surface, its experimental kmax isblue-shifted, and, on the other hand, the effect of the SC is moreimportant for Edye

OX for which we notice an anodic displacement(�0.2 eV). Additionally this investigation suggests that the QM ap-proach dealing with free dyes in solution tends to slightly overes-timate the free enthalpy of the electron injection.

To finish, let us state that the absolute computational accuracycan be improved by solving the two principal computational road-blocks which are: (i) the TDDFT framework delivers a too restrictivedescription of the electronic structure of TPAs. This could be fixed byusing more refined methods like CAS-SCF (or CAS-PT2) [102,56] orMR-CI [103], allowing a multideterminental approach; (ii) thelong-range corrected functionals could reduce the TDDFT errorsthough apparently not for cyanine systems [48–50,58,51,52,47].Nevertheless, the use of conventional DFT approaches to computeauxochromic shifts of CT dyes are not uncommon in literature[53,54,104,57].

Similarly, we want to underline the recent work by Hagfeldtet al., who report the synthesis of a novel dye containing twoTPA units connected by a vinyl group, linked to a rhodanine-3-ace-tic acid moiety as electron acceptor. This dye has been synthesizedas a reference to study the intramolecular energy transfer (IET)and charge transfer (ICT) processes and the results suggest thatboth processes show a positive effect on the performance of DSSCs[60].

Undoubtedly, the close future of organic solar cells relies ontheir economic potential that depends upon several critical factors

16/j.chemphys.2010.08.001



like the efficiency, the manufacturing costs, as well as sustainabil-ity, weight, scalability or lifetime. At this point, a couple of condi-tioning techniques can be foreseen: solvent and solvent-freeprocessings. The first process offers a remarkable efficiency andare low-cost. However, these cells use volatile solvents in theirelectrolytes, which is problematic for outdoor solar panels applica-tions, in views of the need of a robust encapsulation. Therefore, sol-vent-free ionic liquids have been looked for to obtain attractivesolutions. Device efficiencies of over 8% were recently achievedby using some low-viscosity formulations containing a ternarymelt of imidazolium iodure [105]. This result is of great importancein the perspective of a large-scale real-life applications of DSSCs.Nevertheless, whilst such a strategy has been widely applied forRu organic cells, this process has not yet been transfered to me-tal-free organic solar cells like triphenylamine. In such a context,a fundamental-level understanding of the interactions betweenthe dye and the ionic solvent would be warmly welcomed to allowa clear optimization strategy [105].

Acknowledgments

The authors thank the Belgian National Fund for Scientific Re-search (FRS-FNRS) for their respective positions. All calculationshave been performed on the Interuniversity Scientific ComputingFacility (ISCF), installed at the Facultés Universitaires Notre-Damede la Paix (Namur, Belgium), for which the authors gratefullyacknowledge the financial support of the FNRS-FRFC and the ‘‘Lote-rie Nationale” for the convention number 2.4578.02 and of theFUNDP.

References

[1] J.P. Holdren, Science 319 (2008) 424.[2] M. Parry, O. Canziani, J. Palutikof, P. van der Linden, C. Hanson (Eds.),

Intergovernmental Panel on Climate Change (IPCC): Climate Change 2007 –Impacts, Adaptation, and Vulnerability, Cambridge University Press,Cambridge, UK, 2007, p. 1.

[3] M.I. Hoffert et al., Science 298 (2002) 981.[4] M.A. Green, Power to the People: Sunlight to Electricity using Solar Cells,

University of New South Wales Press, Sydney, Australia, 2000. pp. 6–91.[5] K. Zweibel, J. Mason, V. Fthenakis, Sci. Am. 298 (2008) 64.[6] R.F. Service, Science 309 (2005) 548.[7] D.M. Kammen, S. Pacca, Annu. Rev. Environ. Sci. 29 (2004) 301.[8] E.A. Alsema, Prog. Photovoltaic 8 (2000) 17.[9] T.A. Heimer, E.J. Heilweil, C.A. Bignozzi, G. Meyer, J. Phys. Chem. A 104 (2000)

4256.[10] M.K. Nazeeruddin, Coord. Chem. Rev. 248 (2004) 1161.[11] P.V. Kamat, M. Haria, S. Hotchandani, J. Phys. Chem. B 108 (2004) 5166.[12] J. Bisquert, D. Cahen, G. Hodes, S. Ruehle, A. Zaban, J. Phys. Chem. B 108 (2004)

8106.[13] A. Furube, R. Katoh, T. Yoshihara, K. Hara, S. Murata, H. Arakawa, M. Tachiya, J.

Phys. Chem. B 108 (2004) 12588.[14] G. Li, K.J. Jiang, Y.F. Li, S.L. Li, L.M. Yang, J. Phys. Chem. C 112 (2008) 11591.[15] M.K. Nazeeruddin, F. De Angelis, S. Fantacci, A. Selloni, G. Viscardi, P. Liska, S.

Ito, T. Bessho, M. Grätzel, J. Am. Chem. Soc. 127 (2005) 16835.[16] S.Z. Wang, Y. Cui, K. Hara, Y. Dan-Oh, C. Kasada, A. Shinpo, Adv. Mater. 19

(2007) 1138.[17] B.M. Wong, J.G. Codaro, J. Chem. Phys. 129 (2008) 214703.[18] K. Sayama, K. Hara, N. Mori, M. Satsuki, S. Suga, S. Tsukagochi, Y. Abe, H.

Sugihara, H. Arakawa, Chem. Commun. (2000) 1173.[19] T. Horiuchi, H. Miura, K. Sumioka, S. Uchida, J. Am. Chem. Soc. 126 (2004)

12218.[20] K. Hara, T. Horiguchi, T. Kinoshita, K. Sayama, H. Sugihara, H. Arakawa, Sol.

Energy Mater. Sol. Cells 64 (2000) 115.[21] E. Stathatos, P. Lianos, A. Laschewsky, O. Ouari, P. Van Cleuvenbergen, Chem.

Mater. 13 (2001) 3888.[22] R. Chen, X. Yang, H. Tian, X. Wang, A. Hagfeldt, L. Sun, Chem. Mater. 19 (2007)

4007.[23] C. Baik, D. Kim, M.S. Kang, K. Song, O.K. Sang, J. Ko, Tetrahedron 65 (2009)

5302.[24] S. Ferrere, A. Zaban, B. Gregg, J. Phys. Chem. B 101 (1997) 4490.[25] S. Ferrere, B. Gregg, New J. Chem. 26 (1997) 1155.[26] D. Liu, R.W. Fessenden, G.L. Hug, P.V. Kamat, J. Phys. Chem. B 101 (1997) 2583.[27] B. Burfeindt, T. Hannappel, W. Storck, F. Willig, J. Phys. Chem. 100 (1996)

16463.


[28] K. Sayama, S. Tsukagochi, K. Hara, Y. Ohga, A. Shinpou, Y. Abe, S. Suga, H.Arakawa, J. Phys. Chem. B 106 (2002) 1363.

[29] A. Hagfeldt, M. Grätzel, Acc. Chem. Res. 33 (2000) 269.[30] Z. Ning, Q. Zhang, W.H.P. Wu, H. Tian, J. Org. Chem. 73 (2008) 3791.[31] R. Chen, X. Yang, H. Tian, L. Sun, Photochem. Photobiol. A 189 (2007) 295.[32] D.P. Hagberg, T. Marinado, K.M. Karlsson, K. Nonomura, P. Qin, G. Boschloo, T.

Brinck, A. Hagfeldt, L. Sun, J. Org. Chem. 72 (2007) 9550.[33] Y.J. Chang, T.J. Chow, Tetrahedron 13 (2009) 4726.[34] J. Preat, C. Michaux, D. Jacquemin, E.A. Perpète, J. Phys. Chem. C 113 (2009)

16821.[35] F. Labat, I. Ciofini, H.P. Hratchian, M. Frisch, K. Raghavachari, C. Adamo, J. Am.

Chem. Soc. 131 (2009) 14290.[36] J.L. Brédas, J.E. Norton, J. Cornil, V. Coropceanu, Acc. Chem. Res. 42 (2009)

1691.[37] D. Jacquemin, E.A. Perpète, I. Ciofini, C. Adamo, Acc. Chem. Res. 42 (2009) 326.[38] V. Barone, A. Polimeno, Chem. Soc. Rev. 36 (2007) 1724.[39] C. Jamorski-Jödicke, H.P. Lüthi, J. Am. Chem. Soc. 125 (2002) 252.[40] C. Jamorski-Jödicke, H.P. Lüthi, J. Chem. Phys. 117 (2002) 4146.[41] J. Preat, D. Jacquemin, V. Wathelet, J.M. André, E.A. Perpète, J. Phys. Chem. A

110 (2006) 8144.[42] M. Cossi, V. Barone, J. Chem. Phys. 115 (2001) 4708.[43] C. Adamo, V. Barone, Chem. Phys. Lett. 330 (2000) 152.[44] W. Adam, O. Krebs, Chem. Rev. 103 (2003) 4131.[45] E.J. Baerends, G. Ricciardi, A. Rosa, S.J.A. van Gisbergen, Coord. Chem. Rev. 230

(2002) 5.[46] D. Jacquemin, J. Preat, V. Wathelet, M. Fontaine, E.A. Perpète, J. Am. Chem. Soc.

128 (2006) 2072.[47] D. Jacquemin, V. Wathelet, E.A. Perpète, C. Adamo, J. Chem. Theory. Comput. 5

(2009) 2420.[48] T. Tawada, T. Tsuneda, S. Yanagisawa, T. Yanai, K. Hirao, J. Chem. Phys. 120

(2004) 8425.[49] M. Kamiya, H. Sekino, T. Tsuneda, K. Hirao, J. Chem. Phys. 122 (2005) 234111.[50] M. Chiba, T. Tsuneda, K. Hirao, J. Chem. Phys. 124 (2006) 144106.[51] M.J.G. Peach, P. Benfield, T. Helgaker, D.J. Tozer, J. Chem. Phys. 128 (2008)

044118.[52] M.A. Rohrdanz, J.M. Herbert, J. Chem. Phys. 129 (2008) 034107.[53] C.A. Bertolino, A.M. Ferrari, C. Barolo, G. Viscardi, S. Caputo, G. Coluccia, Chem.

Phys. 52 (2006) 330.[54] J.B. Prieto, F.L. Arbeloa, V.M. Martinez, I.L. Arbeloa, Chem. Phys. 13 (2003) 296.[55] S. Grimme, F. Neese, J. Chem. Phys. 127 (2007) 154116.[56] M. Schreiber, V. Bub, M.P. Fülscher, Phys. Chem. Chem. Phys. 3 (2001) 3906.[57] D. Jacquemin, E.A. Perpète, G. Scalmani, M.J. Frisch, R. Kobayashi, C. Adamo, J.

Chem. Phys. 126 (2007) 144105.[58] D. Jacquemin, E.A. Perpète, G. Scuseria, I. Ciofini, C. Adamo, J. Chem. Theory.

Comput. 4 (2008) 123.[59] T.W. Hamann, R.A. Jensen, A.B.F. Martinson, H. Van Ryswyk, J.T. Hupp, Energy

Environ. Sci. 1 (2008) 66.[60] H. Tian, X. Yang, J. Pan, R. Chen, M. Liu, Q. Zhang, A. Hagfeldt, L. Sun, Adv.

Funct. Mater. 18 (2008) 3461.[61] M.J. Frisch et al., Gaussian, Revision D.02, in: Gaussian, Inc., (Ed.), Gaussian,

Inc., Wallingford, CT, USA, 2005.[62] A.D. Becke, J. Chem. Phys. 98 (1993) 1372.[63] A.D. Becke, Phys. Rev. A 38 (1988) 3098.[64] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785.[65] V.A. Naumov, S. Samdal, A.V. Naumov, S. Gundersen, H.V. Volden, Russ. J. Gen.

Chem. 75 (2005) 1956.[66] TDBHandHLYP/6-311G+(2d,2p) with PCM (CH2Cl2).[67] C. Adamo, V. Barone, J. Chem. Phys. 110 (1999) 6158.[68] A.D. Becke, J. Chem. Phys. 98 (1993) 5648.[69] R.J. Magyar, S. Tretiak, J. Chem. Theory Comput. 3 (2007) 976.[70] J.L. Brédas, D. Beljonne, V. Coropceanu, J. Cornil, J. Chem. Rev. 104 (2004)

4971.[71] R. Krishnan, J.S. Binkley, R. Seeger, J.A. Pople, J. Chem. Phys. 72 (1980) 650.[72] C. Amovilli, V. Barone, R. Cammi, E. Cancès, M. Cossi, B. Mennucci, C.S.

Pomelli, J. Tomasi, Adv. Quant. Chem. 32 (1998) 227.[73] J. Tomasi, B. Mennucci, R. Cammi, Chem. Rev. 105 (2005) 2999.[74] For the geometry optimisation process, we turned on the following options (i)

Radii = UAKS which selects radii optimized at DFT level of theory for buildingthe solute cavity and (ii) NoAddSph [61] which avoids the generation ofsmoothing spheres on the cavity surface.

[75] A. Sen, P.E. Nielsen, Nucleic Acids Res. 35 (2007) 3367.[76] R. Katoh, A. Furube, T. Yoshihara, K. Hara, G. Fujihashi, S. Takano, S. Murata, H.

Arakawa, M. Tachiya, J. Phys. Chem. B 108 (2004) 4818.[77] J.B. Asbury, Y.Q. Wang, E. Hao, H. Ghosh, T. Lian, Res. Chem. Intermed. 27

(2001) 393.[78] A. Hagfeldt, M. Grätzel, Chem. Rev. 95 (1995) 49.[79] P.F. Barbara, T.J. Meyer, M.A. Ratner, J. Phys. Chem. 100 (1996) 13148.[80] D. Matthews, P. Infelta, M. Grätzel, Sol. Energy Mater. Sol. Cells 44 (1996) 119.[81] F. De Angelis, S. Fantacci, A. Selloni, Nanotechnology 19 (2008) 424002.[82] G. Benkö, J. Kallioien, J.E.I. Korppi-Tommola, A.P. Yartsev, V. Sundström, J. Am.

Chem. Soc. 124 (2002) 489.[83] S. Iwa, K. Hara, S. Murata, R. Katoh, H. Sugihara, H. Arakawa, J. Chem. Phys.

113 (2000) 3366.[84] C. Bauer, G. Boschloo, E. Mukhtar, A. Hagfeldt, Int. J. Photochem. 4 (2002) 17.[85] R.J. Cave, E.W. Castner Jr., J. Phys. Chem. A 106 (2002) 12117.[86] R.J. Cave, K. Burke, E.W. Castner Jr., J. Phys. Chem. A 106 (2002) 9294.

16/j.chemphys.2010.08.001



[87] P. Carbonniere, T. Lucca, C. Pouchan, N. Rega, V. Barone, J. Comput. Chem. 26(2005) 384.

[88] R. Improta, V. Barone, G. Scalmani, M.J. Frisch, J. Chem. Phys. 125 (2006)054103.

[89] The MOA has been performed with Molekel (version 5.4.0.8) of the SwissNational Supercomputing Centre (CSCS).

[90] H. Tian, X. Yang, R. Chen, R. Zhang, A. Hagfeldt, L. Sun, J. Phys. Chem. C 112(2008) 11023.

[91] J. Preat, D. Jacquemin, V. Wathelet, J.M. André, E.A. Perpète, Chem. Phys. 335(2007) 177.

[92] C. Loison, R. Antoine, M. Broyer, J. Dugroud, J. Guthmuller, D. Simon, Chem.Eur. J. 14 (2008) 7351.

[93] D. Guillaumont, S. Nakamura, Dyes Pigm. 46 (2000) 85.[94] Q. Li, X.X. Zhang, Y. Meng, M.B. Chen, Chin. Chem. Lett. 16 (2005) 693.[95] W. Xu, B. Peng, J. Chen, M. Liang, F. Cai, J. Phys. Chem. C 112 (2008) 874.[96] For THQ-9, the excitation energy is overestimated by BHandHLYP, whereas

EdyeOX is underestimated. As previously shown, PBE0 provides a kTHQ-9

max in better


agreement with the experimental value, subsequently giving a theoreticalDGinject: (�2.35 eV) closer to the experimental value (�1.51 eV).

[97] This assumption is valid as long as the injection is restricted to energy levelsclose to the conduction band edge, and is combined to a density of acceptorstates constant in the same energy range.

[98] D. Cahen, G. Hodes, M. Grätzel, J.F. Guillermoles, I. Riess, J. Phys. Chem. B 104(2000) 2053.

[99] H.S. Nalwa, Handbook of Advanced Electronic and Photonic Materials andDevices, Academic, San Diego, CA, 2001. pp. 1–3366.

[100] B.H. Besler, K.M. Merz, P.A. Kollman, J. Comput. Chem. 11 (1990) 431.[101] B. Peng, S. Yang, L. Li, F. Cheng, J. Chen, J. Chem. Phys. 132 (2009) 034305.[102] K. Andersson, P.-A. Malmqvist, B.O. Roos, J. Chem. Phys. 96 (1992) 1218.[103] P.J. Bruna, S.D. Peyerimhoff, R. Buenker, J. Chem. Phys. Lett. 72 (1980) 278.[104] D. Jacquemin, M. Bouhy, E.A. Perpète, J. Chem. Phys. 126 (2006) 204321.[105] Y. Bai, Y. Cao, J. Zhang, M. Wang, R. Li, P. Whang, S.M. Zakeeruddin, M. Grätzel,

Nature Mater. 7 (2008) 626.

16/j.chemphys.2010.08.001


Thiophene Bridge Solacell

Documents

Transcript of Thiophene Bridge Solacell