Exciton and Charge Dynamics in Polymer Solar Cells Studied by Transient Absorption Spectroscopy - Organic Solar Cells: Materials and Device Physics - page 106

Power Stabilized
Tungsten Lamp

Detector

MC S MC

Preamplifier

N 2 Laser Pumped
Dye Laser

Bandpass Filters

PD

Main Amplifier

Function
Generator

PC

Digital Oscilloscope

Fig. 5.2 Block diagram of highly sensitive microsecond transient absorption measurement
system: MC monochromator, S sample, PC computer, and PD PIN photodiode to detect a part of
a pump laser pulse as a trigger signal, which is sent to the digital oscilloscope. The detector is
replaceable: Si PIN photodiode for the visible wavelength range and InGaAs PIN photodiode for
the near-IR wavelength range. The black and gray lines represent electric signals and optical
probe and pump light, respectively

10 -6 depending on the measuring time domain after appropriate accumulation
owing to the amplification and noise reduction system.
For ultrafast phenomena on a time scale of \1 ns, the pump and probe method
is widely employed. In this method, ultrashort laser pulses serve as the pump light
for the sample excitation and the probe light for the transmittance measurement.
Transient absorption can be measured at various delay times by controlling the
arrival time of each laser pulse at the sample. The arrival time of each laser pulse
can be tuned with an optical delay line: the probe light is delayed relative to the
pump light because of the additional path length in the optical delay line. For
example, when the total optical delay length is set at 3 cm the probe light is
delayed by 100 ps relative to the pump light, and therefore, the transmittance at
100 ps after the laser excitation can be measured. Figure 5.3 shows a block dia-
gram of a typical pump and probe spectroscopy system we employ [ 17 - 21 ]. This
system consists of a transient absorption spectrometer and a regenerative amplified

Probe
800 nm

D

ODL

SHG Regeneratively

Amplified
Ti:Sapphire
Laser

WLG

Pump
400 nm

C

S

Fig. 5.3 Block diagram of pump and probe femtosecond transient absorption measurement
system: SHG second harmonic generator, ODL optical delay line, C chopper, WLG white light
generator, S sample, D detector. The detector is replaceable: a linear CCD array for the visible
wavelength range and a digital line scan InGaAs camera for near-IR wavelength range