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Investigating Dynamics of Dye Sensitized Solar Cells with Time Resolved
Terahertz Spectroscopy
Jason B. Baxter, Department of Chemistry, Yale University, New Haven, CT
ZnO is a wide band gap semiconductor that has many potential optoelectronic applications. We have studied
the photoconductivity and native conductivity of ZnO wafers as a function of temperature by measuring their
terahertz permittivity using time-resovled terahertz spectroscopy (TRTS). ZnO absorbance increases and
refractive index decreases with increasing temperature (10 K to 120 K shown). These changes arise from
increases in conductivity due to electrons being thermally excited from trap states. The complex-valued
conductivity is fit well by the Drude model, with electron mobility ~1500 cm2/V-s. Increasing conductivity was
also measured when electrons were photoexcited by absorption of above-band-gap photons.
300
T
200
100
0
Refractive Index
3.0
2.8
2.6
T
2.4
2.2
0.5
1.0
1.5
Frequency (THz)
For nanoparticle films, photons can also be absorbed by
sensitizer molecules adsorbed to the semiconductor surface.
Subsequent interfacial electron transfer (IET) increases
semiconductor photoconductivity. Using TRTS to study a Ru
terpy dye commonly used in dye sensitized solar cells, we
found that IET occurs on sub-picosecond time scales for IET
into TiO2 nanoparticles with 400 nm excitation, 10 ps time
scales for TiO2 with 800 nm excitation, and 100 ps time scales
for ZnO.
We
TiO2 400nm
0.0
believe
the
ZnO 800nm
differences are
TiO2 800nm
-0.2
primarily related
-0.4
to the overlap
between
the
-0.6
dye’s
excited
-0.8
states and the
semiconductor
-1.0
density of states.
0
50
100
150
200
Normalized Transmission Change
Absorbance (cm-1)
400
2.0
2.5
Pump-probe delay time (ps)