10.1021/jp500414c.s001
Hung-Yu Hsu
Hung-Yu
Hsu
Chi-Wen Cheng
Chi-Wen
Cheng
Wei-Kai Huang
Wei-Kai
Huang
Yuan-Pern Lee
Yuan-Pern
Lee
Eric Wei-Guang Diau
Eric Wei-Guang
Diau
Femtosecond
Infrared Transient Absorption Dynamics
of Benzimidazole-Based Ruthenium Complexes on TiO<sub>2</sub> Films
for Dye-Sensitized Solar Cells
American Chemical Society
2014
TiO 2 Films
N 719 dye
Transient Absorption Dynamics
electron injections
RD dyes
3 MLCT electron injection
triplet 3 MLCT states
nonsubstituted RD 5 dye
4.3 μ m
TiO 2
singlet 1 MLCT
BET
2014-07-31 00:00:00
Journal contribution
https://acs.figshare.com/articles/journal_contribution/Femtosecond_Infrared_Transient_Absorption_Dynamics_of_Benzimidazole_Based_Ruthenium_Complexes_on_TiO_sub_2_sub_Films_for_Dye_Sensitized_Solar_Cells/2269861
By means of femtosecond infrared
transient absorption spectra,
we measured the interfacial electron-transfer dynamics for benzimidazole-based
heteroleptic ruthenium dyes (RD5, RD12, RD15–RD18) sensitized
on TiO<sub>2</sub> thin films. For all measurements, the first singlet
metal-to-ligand charge-transfer states (<sup>1</sup>MLCT) of the ruthenium
complexes were excited at 519 nm and the injected electrons in the
conduction band of TiO<sub>2</sub> were probed at 4.3 μm. All
transient signals featured two rising components on a femtosecond–picosecond
scale due to a two-step electron injection and an offset (N719, RD16–RD18)
or a slow-decay (RD5, RD12, and RD15) component on a nanosecond–microsecond
scale due to a back electron transfer. A complicated two-step kinetic
model was derived analytically to interpret the observed two rising
components for which the rapid (τ<sub>1</sub> < 300 fs) and
slow (τ<sub>2</sub> = 10–20 ps) electron injections arose
from the singlet <sup>1</sup>MLCT and triplet <sup>3</sup>MLCT states,
respectively. The amplitudes of the two electron-injection components
(<i>A</i><sub>1</sub> and <i>A</i><sub>2</sub>) were controlled by the rate coefficient of the <sup>1</sup>MLCT
→ <sup>3</sup>MLCT intersystem crossing; the variations of <i>A</i><sub>1</sub> and <i>A</i><sub>2</sub> are consistent
with the trend of the corresponding Stokes shifts rationalized with
a conventional energy-gap law for nonradiative transitions. Compared
with the kinetics observed for the N719 dye, the involvement of a
benzimidazole ligand in RD dyes had the effect of accelerating the
two electron injections, thus improving the short circuit current
of the device. RD dyes substituted with fluorine atoms and/or thiophene
units in the benzimidazole ligands showed a retardation of <sup>3</sup>MLCT electron injection relative to that of the nonsubstituted RD5
dye. Acceleration of the BET process was observed for the RD5 dye
(9 ns), and both fluoro-substituted dyes (14 ns for RD12 and 21 ns
for RD15) and thiophene-substituted dyes (nonobservable for RD16–RD18)
had significantly retarded BET kinetics. The observed kinetics of
the <sup>3</sup>MLCT electron injection for all RD dyes is satisfactorily
simulated with the Marcus theory.