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1. IPCE and APCE measurements

The measurements of photoabsorption cross sections and output

short-circuit currents allowed us to determine the incident photon-to-current efficiency (IPCE) and theabsorbed photon-to-current efficiency (APCE). The IPCE is calculated from the number of electrons
JSCS/e fed into an external circuit divided by that of incident photons on the cell, !P=IPS=PS/h". The
APCE is derived from JSCS/e divided by the number of photons absorbed by the dye #!P. Here, JSC isthe density of the output short-circuit current; S is the area of the DSSC; e is the electron charge; IP andP are the intensity and power density of SR, respectively.
The DSSC was prepared by applying nanocrystalline mesoporous TiO2 paste on an FTO glass andsintering the paste at 450°C. After soaking the glass in dye solution (“N3” or “N719”), iodide
electrolyte (50 mM tri-iodide in acetonitrile, ionic liquid and 4-tert-butyl pyridine) was dropped on theTiO2 electrode which was combined with a counter electrode covered with Pt. The formula of N719,
for instance, is written as RuL2(NCS)2:2TBA (L = 2,2'-bipyridyl-4,4'-dicarboxylic acid; TBA =tetrabutylammonium). Here, dye molecules are firmly grafted onto the surface of TiO2 through theircarboxylate substituents. Our DSSC exhibits a maximum energy conversion efficiency of $7 %.
!

0.6

1.4(b)

1.2

0.2

APCE(a)

!

IPCE

0.3

0.8(b)

0.6
0.4

0.1
0.0400(a)1.00.4!

0.5

0.2
500

600700

Wavelength / nmFig. 1. IPCE curves obtained using (a) the SR
source and (b) an exclusive IPCE spectrometer.

0.0400500600
Wavelength / nm

700Fig. 2. APCE curves calculated in different
ways of estimation of #!P.

The IPCE curve a in Fig. 1 calculated from JSC

the IPCE measurements. An IPCE value wasimproved to be 0.7 when the layer of TiO2 film isthicker than 16 %m. Figure 2 shows the two APCE
curves calculated in different ways of estimationof #!P, according as whether the scattered SR in"(&"(%"(""injection is expected to be more than 0.8 at shorter
wavelengths, whereas it gradually decreases atlonger wavelengths.
2. Transient fluorescence spectroscopy of DSSCElectron injection is known to occur in a timescale of 10 ps or faster, if TiO2 film together with Rudyes is covered in inert solvents. In contrast, the rate

#"

$"

%"

&"'""

&"'""

'("!%&"!1E!<#"!1E!&""!1E"(&B"(%!

)*+,-./0.10.!213.1/435!6!7-8(!+143/from the dye. The quantum yield of the electron!;<'=!>*70?!@5.
!@AAB!,C!;<'="(#or not (curve a). It is clear that such multiple
wavelengths and contributes to electron injection

97:!
98:!
90:!

"($the TiO2 film is partly absorbed by dye (curve b)
absorption may favorably take place in shorterA!

using an instrument fitting a Xe lamp dedicated for'(")*+,-./0.10.!213.1/435!6!7-8(!+143/

and !P agrees with the curve b that was obtained"($"(#"(""#"$"%"

D4E.!!6!!1/Fig. 3. Fluorescence decay curves of DSSC andphotovoltaic electrodes, obtained by FELphotolysis.

of injection is drastically reduced by one or twoorders of magnitude in a typical redox active electrolyte. Several authors argued that the compounds inthe electrolyte added for reducing undesirable electron-hole recombination have an effect of raising
the conduction band edge of TiO2. Eventually, the high-efficiency DSSCs often show lifetimes as longas 1 ns with respect to electron injection, that is, injection dynamics which is just fast enough tocompete with the fluorescence decay (10 $ 100 ns) of the excited state of the Ru dyes.
We observed fluorescence decay by time-resolved single photon counting using FEL of $ 580 nm
at a repetition rate of 11.3 MHz. The decay curves in Fig. 3A were monitored at 720 nm for (a) aphotovoltaic electrode of TiO2 film prepared with N719, (b) that with Black dye, and (c) a complete
DSSC fabricated using N719. The apparent fluorescence lifetime of the complete DSSC is longer thanthose of the two photovoltaic electrodes, due to either slower electron injection induced by one of theadditives in the electrolyte or aggregation of the dye molecules at the surface of TiO2. Figure 3B
shows the dependence of the decay curves of the DSSC on the fluorescence wavelength. The
fluorescence lifetime appears to increase with increasing wavelength. This suggests that theconversion rates to other electronic states (e.g. that to the lowest triplet state) are comparable to thoseof the electron injection and fluorescing emission.
References[1] B. O’Regan, M. Grätzel, Nature, 353 (1991) 737. [2] Y. Tachibana et al., J. Phys. Chem., 100(1996) 20056. [3] A. Reynal et al., J. Am. Chem. Soc., 130 (2008) 13558. [4] S.E. Koops, J.R. Durrant, Inorg.
Chim. Acta, 361 (2008) 663.