2548 Chem. Commun., 2013, 49, 2548--2550 This journal is c The Royal Society of Chemistry 2013

Cite this: Chem. Commun.,2013,49, 2548

Dye-sensitized solar cells based on D–p–A fluorescentdyes with two pyridyl groups as an electron-withdrawing–injecting anchoring group†

Yousuke Ooyama,* Naoya Yamaguchi, Ichiro Imae, Kenji Komaguchi, Joji Ohshitaand Yutaka Harima*

D–p–A fluorescent dye YNI-2 with two pyridyl groups as an

electron-withdrawing–injecting anchoring group possessing a

high coordinate bonding ability to Lewis acid sites on the TiO2

surface, which can lead to high dye loading on the TiO2 film and

efficient electron injection, has been developed as a new type of

D–p–A dye sensitizer for dye-sensitized solar cells.

Dye-sensitized solar cells (DSSCs) based on dye sensitizers, which areone of the third generation photovoltaic cells, have received con-siderable attention from the viewpoint of their interesting construc-tion and operational principles, high power conversion efficiency,and low cost of production.1–9 In particular, donor–acceptorp-conjugated (D–p–A) dyes with both electron-donating (D) andelectron-accepting (A) groups linked by p-conjugated bridges, whichpossess broad and intense absorption spectral features, would beexpected to be one of the most promising organic dye sensitizers.2–9

Most of the D–p–A dyes for DSSCs developed so far possess acarboxylic acid, cyanoacrylic acid or rhodanine-3-acetic acid moietywhich acts as an electron acceptor as well as an anchoring group forattachment on a TiO2 surface. The carboxyl group enables a goodelectron communication between the dye and TiO2 by forming astrong bidentate bridging linkage with Brønsted acid sites (surface-bound hydroxyl groups, Ti–OH) on the TiO2 surface. In our previouswork, on the other hand, we found that a new type of D–p–A dyesensitizers with a pyridyl group as an electron-withdrawing–injectinganchoring group were predominantly adsorbed on the TiO2 filmthrough coordinate bonding between the pyridyl group of the dyeand the Lewis acid site (exposed Tin+ cations) on the TiO2 surface.9 Itwas demonstrated that a new type of D–p–A dye sensitizers caninject electrons efficiently from the pyridyl group to the conductionband (CB) of the TiO2 electrode through coordinate bonding, ratherthan the bidentate bridging linkage of conventional D–p–A dyesensitizers with carboxyl groups. Very recently, photovoltaic perfor-mances of DSSCs based on D–D–p–A organic dyes and a series of

porphyrin-based dyes with pyridyl groups were reported by Zhenget al. and Coutsolelos et al., respectively.10,11 On the other hand,He et al. reported high photovoltaic performance and the long-termstability of DSSC based on a porphyrin dye with 8-hydroxylquinolineas the anchoring group.12 However, the amounts (ca. 5.0–8.0 �1016 molecules per cm2) of dye sensitizers with a pyridyl groupadsorbed on the TiO2 film are lower than those of dye sensitizerswith a carboxyl group,9–11 and thus the low dye loading leads to lowlight-harvesting efficiency (LHE).

In this work, to provide a direction in molecular design towardcreating efficient D–p–A dye sensitizers with pyridyl groups, wehave designed and synthesized D–p–A fluorescent dye YNI-2 withtwo pyridyl groups as electron-withdrawing–injecting anchoringgroup and a thiophene unit as a p-bridge (Scheme 1; see ESI† forthe detailed synthetic procedures). We have demonstrated that thedye YNI-2 exhibits a high coordinate bonding ability, leading to notonly a high dye loading on the TiO2 film, but also efficient electroninjection from the dye to the CB of TiO2, compared with those ofYNI-1 with two pyridyl groups, but without a thiophene unit.

The absorption and fluorescence spectra of YNI-1 and YNI-2 inTHF are shown in Fig. 1 and their spectral data are summarized inTable 1. The dye YNI-2 in THF shows a strong absorption band ataround 380 nm, which is assigned to the intramolecular charge-transfer (ICT) excitation from the electron donor moiety (carbazole)to the electron acceptor moiety (pyridyl group). The absorptionmaximum (labs

max) for the ICT band of YNI-2 occurs at a longerwavelength by ca. 50 nm than that of YNI-1. The molar extinction

Scheme 1 Structures of D–p–A fluorescent dyes YNI1-1 and YNI-2.

Department of Applied Chemistry, Graduate School of Engineering, Hiroshima

University, Higashi-Hiroshima 739-8527, Japan. E-mail: yooyama@hiroshima-u.ac.jp,

harima@mls.ias.hiroshima-u.ac.jp; Fax: +81 82-424-5494

† Electronic supplementary information (ESI) available: Details of experimental pro-cedures, synthesis and characterization of compounds. See DOI: 10.1039/c3cc40498f

Received 21st January 2013,Accepted 7th February 2013

DOI: 10.1039/c3cc40498f

www.rsc.org/chemcomm

ChemComm

COMMUNICATION

Dow

nloa

ded

by G

eorg

e M

ason

Uni

vers

ity o

n 28

Feb

ruar

y 20

13Pu

blis

hed

on 0

8 Fe

brua

ry 2

013

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C3C

C40

498F

View Article OnlineView Journal | View Issue

This journal is c The Royal Society of Chemistry 2013 Chem. Commun., 2013, 49, 2548--2550 2549

coefficient (e) for the ICT band of YNI-2 is ca. 46 000 M�1 cm�1,which is higher than that (ca. 22 000 M�1 cm�1) of YNI-1. Theseresults show that the introduction of two thiophene units on thecarbazole skeleton expands the p conjugation in the dye and thusresults in red-shift and broadening of ICT band, and the enhance-ment of e. The fluorescence maxima (lfl

max) for YNI-1 and YNI-2occur at 387 and 448 nm, respectively. The dye YNI-2 (Ff = 0.67)exhibits a higher fluorescence quantum yield (Ff) than YNI-1 (Ff =0.33). The MO calculations indicate that the longest excitationbands were mainly assigned to the transition from HOMO toLUMO, for which HOMOs were mostly localized on the carbazolemoiety for YNI-1 and the thienylcarbazole moiety for YNI-2, respec-tively, and LUMOs were mostly localized on the pyridylcarbazolemoiety for YNI-1 and the thienylpyridine moiety for YNI-2, respec-tively (see Fig. S2 in the ESI†). The changes in the calculatedelectron density accompanied by the first electron excitation forthe two dyes reveal a strong ICT from the carbazole moiety to thepyridyl group upon photoexcitation (see Fig. S2 in the ESI†).

The absorption spectra of the dyes adsorbed on the TiO2 filmare shown in Fig. 2. The absorption peak wavelengths of YNI-1 andYNI-2 are red-shifted by 23 nm and 27 nm, respectively, comparedwith those in THF. When chenodeoxycholic acid (CDCA) wasemployed as a coadsorbent to prevent dye aggregation on theTiO2 surface, the absorption peak wavelength is blue-shifted by17 nm for both YNI-1 and YNI-2, although the peak wavelength isstill red-shifted compared with that in THF.

Thus, to elucidate the adsorption states of the dyes YNI-1 andYNI-2 on TiO2 nanoparticles, we measured the FTIR spectra of thedye powders and the dyes adsorbed on TiO2 nanoparticles (Fig. 3).For the powders of the two dyes, the characteristic stretching bandsfor CQN or CQC were clearly observed at around 1589 and1473 cm�1 for YNI-1 and 1590, 1481, and 1446 cm�1 for YNI-2.When the dye YNI-1 was adsorbed on the TiO2 surface, the

stretching band at around 1589 cm�1 is shifted by 6 nm to a higherwavenumber, that is, the band at around 1595 cm�1 can be assignedto pyridyl groups hydrogen-bonded to Brønsted acid sites on theTiO2 surface. In the FTIR spectra of YNI-2 adsorbed on TiO2

nanoparticles, on the other hand, the band at 1590 cm�1 dis-appeared completely and a new and strong band appeared ataround 1614 cm�1, which can be assigned to pyridyl groupscoordinated to the Lewis acid sites on the TiO2 surface. Theseobservations indicate that the dye YNI-1 is predominantly adsorbedon the TiO2 surface through hydrogen bonding at Brønsted acidsites, whereas the dye YNI-2 is predominantly adsorbed on the TiO2

surface through coordinate bonding at Lewis acid sites, althoughthere are also a few dye molecules adsorbed at both acid sites. Theseresults reveal that hydrogen bonding and coordinate bonding areresponsible for the large red-shift of the absorption peak wavelengthfor YNI-1 and YNI-2 adsorbed on TiO2 nanoparticles (Fig. 2).

The electrochemical properties of the two dyes were determinedusing cyclic voltammetry (CV; see Fig. S1 and Table S1 in the ESI†).

Fig. 1 (a) Absorption and (b) fluorescence spectra of YNI-1 and YNI-2 in THF.

Table 1 Optical and electrochemical data, HOMO and LUMO energy levels, and DSSC performance parameters of YNI-1 and YNI-2

Dye labsmax/nm (e a/M�1 cm�1) lfl

max/nm (Ff)b Eox

1/2c/V HOMOd/V LUMOd/V Moleculese cm�2 Jsc

f/mA cm�2 Vocf/mV fff Zf (%)

YNI-1 327 (21 900) 387 (0.33) — 1.54 �1.95 4.8 � 1016 g 1.84g 492g 0.63g 0.57g

YNI-2 378 (46 100) 448 (0.67) 0.53 1.22 �1.75 3.1 � 1016 h 4.48h 520h 0.63h 1.47h

9.9 � 1016 g 4.72g 556g 0.61g 1.61g

a In THF: fluorescence quantum yields (Ff) were determined by using a calibrated integrating sphere system (lex = 327 nm and 378 nm for YNI-1and YNI-2, respectively). b In THF: fluorescence quantum yields (Ff) were determined by using a calibrated integrating sphere system (lex = 327 nmand 378 nm for YNI-1 and YNI-2, respectively). c Half-wave potentials for oxidation (Eox

1/2) vs. Fc/Fc+ were recorded in THF–Bu4NClO4 (0.1 M)solution. d Vs. normal hydrogen electrode (NHE). e Adsorption amount per unit area of TiO2 electrode. f Under a simulated solar light (AM 1.5,100 mW cm�2). g Under the adsorption condition of 0.1 mM dye solution in THF (dye coverage (y) of the TiO2 surface is 0.22 for YNI-1 and 0.44 forYNI-2, respectively). h Under the adsorption condition of 0.02 mM dye solution in THF (dye coverage (y) of the TiO2 surface is 0.13).

Fig. 2 Absorption spectra of (a) YNI-1 and (b) YNI-2 adsorbed on the TiO2 film(9 mm) with (–�–) and without (—) CDCA as a coadsorbent.

Fig. 3 FTIR spectra of the dye powders and dyes adsorbed on TiO2 nano-particles for (a) YNI-1 and (b) YNI-2.

Communication ChemComm

Dow

nloa

ded

by G

eorg

e M

ason

Uni

vers

ity o

n 28

Feb

ruar

y 20

13Pu

blis

hed

on 0

8 Fe

brua

ry 2

013

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C3C

C40

498F

View Article Online

2550 Chem. Commun., 2013, 49, 2548--2550 This journal is c The Royal Society of Chemistry 2013

The oxidation peaks for YNI-1 and YNI-2 were observed at 0.99 and0.58 V vs. ferrocene/ferrocenium (Fc/Fc+), respectively. The corre-sponding reduction peak for YNI-2 appeared at 0.48 V, whereas noreduction peak was observed for YNI-1. These results show that theoxidized state of YNI-2 is stable, but that of YNI-1 is unstable. TheHOMO and LUMO energy levels of two dyes were evaluated fromthe spectral analyses and the CV data (Table 1). The HOMO energylevels of YNI-1 and YNI-2 were 1.54 and 1.22 V vs. the normalhydrogen electrode (NHE), respectively, thus indicating that theHOMO energy levels are more positive than the I3

�/I� redoxpotential (0.4 V). This ensures an efficient regeneration of theoxidized dyes by electron transfer from the I3

�/I� redox couple inthe electrolyte. The LUMO energy levels of YNI-1 and YNI-2 were�1.95 and �1.75 V, respectively. Evidently, these LUMO levels arehigher than the energy level of the CB of TiO2 (�0.5 V), so that thetwo dyes can efficiently inject electrons to the TiO2 electrode.Therefore, it was revealed that the red-shift of the ICT absorptionband for YNI-2 relative to YNI-1 is attributed to destabilization ofthe HOMO level and stabilization of the LUMO level by theintroduction of thiophene units to the p-conjugation system ofthe dye, resulting in a decrease in the HOMO–LUMO band gap.

The incident photon-to-current conversion efficiency (IPCE)spectra and the photocurrent–voltage (I–V) curves for the DSSCsbased on YNI-1 and YNI-2 are shown in Fig. 4. The photovoltaicperformance parameters are collected in Table 1. It is worthmentioning that the maximum adsorption amount of dyesadsorbed on TiO2 for YNI-2 is twice as much as that of YNI-1(4.8 � 1016 and 9.9 � 1016 molecules per cm2 for YNI-1 and YNI-2,respectively). An increase in the adsorption amount of YNI-2 relativeto YNI-1 is attributed to the reasonable angle between two thienyl-pyridine moieties as well as the accumulated electron density on thenitrogen atom of the pyridyl groups due to a large planarp-conjugated system extending from the electron donor moiety tothe electron acceptor moiety by the introduction of a thiophene unit,leading to the construction of molecular structures capable offorming strong coordinate bonding between the two pyridyl groupsof dyes and the Lewis acid sites on the TiO2 surface (for a schematicrepresentation of YNI-1 and YNI-2 on the TiO2 surface, see Fig. S4 inthe ESI†). The short-circuit photocurrent density (Jsc) and solarenergy-to-electricity conversion yield (Z) for YNI-2 (4.72 mA cm�2

and 1.61%) are much higher than those for YNI-1 (1.84 mA cm�2

and 0.57%). The IPCE value of YNI-2 exceeds 65% in the range of390 to 435 nm, with a maximum value (67%) at 430 nm, whichis higher than that of YNI-1 (47% at 415 nm). The IPCE spectra are

red-shifted compared with the absorption spectra of the dyesadsorbed on the TiO2 film (Fig. 2). This red-shift may be associatedwith the light absorption of the TiO2 film and/or the interactionbetween the adsorbed dyes and the electrolyte. The open-circuitphotovoltage (Voc) of YNI-1 (492 mV) is lower than that of YNI-2(556 mV). Thus, the lower Voc value for YNI-1 may be attributed tofaster charge recombination between the injected electrons inthe CB of TiO2 and I3

� ions in the electrolyte, arising from theapproach of I3

– ions to the TiO2 surface due to both the relativelycompact molecular size and the low dye loading on the TiO2

electrode (in fact, the dye coverage (y) of the TiO2 surface for YNI-1(y = 0.22) is lower than that of YNI-2 (y = 0.44)). Interestingly, whenthe adsorption amount of YNI-2 is 3.1 � 1016 molecules per cm2,the maximum IPCE and Z values for YNI-2 were 62% and 1.47%(Fig. 4 and Table 1), respectively, which are still higher than thosefor YNI-1 in the maximum adsorption amount of dyes (4.8 �1016 molecules per cm2). Consequently, the higher photovoltaicperformance of YNI-2 is attributed to not only the red-shift andbroadening of the absorption band and the stable oxidized state ofthe dye, but also to efficient electron injection by the formation ofstrong coordinate bonding to Lewis acid sites on the TiO2 surface.

In conclusion, as a new type of D–p–A dye sensitizer for DSSCs,we have designed and synthesized D–p–A fluorescent dye YNI-2with two pyridyl groups as an electron-withdrawing–injectinganchoring group. It was found that the introduction of twothienylpyridines to the p-conjugation system of the dye can leadto the red-shift and broadening of the ICT band, and also high dyeloading on the TiO2 film and efficient electron injection, due to theformation of strong coordinate bonding to Lewis acid sites on theTiO2 surface. We demonstrate that a key point for creating efficientdye sensitizers with pyridyl groups is to design molecular structurescapable of forming strong coordinate bonding between the pyridylgroups of dyes and the Lewis acid sites on the TiO2 surface.

This work was supported by A-STEP (AS242Z00243J) fromJapan Science and Technology Agency (JST), by Grants-in-Aidfor Scientific Research from the Japan Society for the Promo-tion of Science (JSPS) (23350097 and 24550225) and by The KaoFoundation for Arts and Sciences.

Notes and references1 B. O’Regan and M. Gratzel, Nature, 1991, 353, 737.2 Z. Ning and H. Tian, Chem. Commun., 2009, 5483.3 A. Mishra, M. K. R. Fischer and P. Bauerle, Angew. Chem., Int. Ed.,

2009, 48, 2474.4 (a) Y. Ooyama and Y. Harima, Eur. J. Org. Chem., 2009, 2903;

(b) Y. Ooyama and Y. Harima, ChemPhysChem, 2012, 13, 4032.5 A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo and H. Pettersson, Chem.

Rev., 2010, 110, 6595.6 T. Bessho, S. M. Zakeeruddin, C.-Y. Yeh, E. W.-G. Diau and

M. Gratzel, Angew. Chem., Int. Ed., 2010, 49, 6646.7 Z. Ning, Y. Fu and H. Tian, Energy Environ. Sci., 2010, 3, 1170.8 J. Mao, N. He, Z. Ning, Q. Zhang, F. Guo, L. Chen, W. Wu, J. Hua and

H. Tian, Angew. Chem., Int. Ed., 2012, 51, 9873.9 (a) Y. Ooyama, S. Inoue, T. Nagano, K. Kushimoto, J. Ohshita, I. Imae,

K. Komaguchi and Y. Harima, Angew. Chem., Int. Ed., 2011, 50, 7429;(b) Y. Ooyama, T. Nagano, S. Inoue, I. Imae, K. Komaguchi, J. Ohshitaand Y. Harima, Chem.–Eur. J., 2011, 17, 14837.

10 D. Daphnomili, G. Landrou, P. Singh, A. Thomas, K. Yesudas,B. K. G. D. Sharma and A. G. Coutsolelos, RSC Adv., 2012, 2, 12899.

11 M.-D. Zhang, H.-X. Xie, X.-H. Ju, L. Qin, Q.-X. Yang, H.-G. Zheng andX.-F. Zhou, Phys. Chem. Chem. Phys., 2013, 15, 634.

12 H. He, A. Gurung and L. Si, Chem. Commun., 2012, 48, 5910.

Fig. 4 (a) IPCE spectra and (b) I–V curves of DSSCs based on YNI-1 and YNI-2under the adsorption condition of 0.1 mM or 0.02 mM dye solution in THF. CDCAwas not employed.

ChemComm Communication

Dow

nloa

ded

by G

eorg

e M

ason

Uni

vers

ity o

n 28

Feb

ruar

y 20

13Pu

blis

hed

on 0

8 Fe

brua

ry 2

013

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C3C

C40

498F

View Article Online