Exciton and Charge Dynamics in Polymer Solar Cells Studied by Transient Absorption Spectroscopy - Organic Solar Cells: Materials and Device Physics - page 107

Ti:sapphire laser. The amplified Ti:sapphire laser provides 800-nm fundamental
pulses at a repetition rate of 1 kHz with an energy of 0.8 mJ and a pulse width of
100 fs (FWHM), which are split into two optical beams with a beam splitter to
generate pump and probe pulses. One fundamental beam is converted into pump
pulses at 400 nm with a second harmonic generator or pump pulses at other
wavelengths with an ultrafast optical parametric amplifier. The other fundamental
beam is converted into white light continuum pulses employed as probe pulses
over the wide wavelength from 400 to 1700 nm. The pump pulses are modulated
mechanically with a repetition rate of 100 Hz for visible and 500 Hz for near-IR
measurements. The temporal evolution of the probe intensity is recorded with a Si
CCD-array photodetector for the visible measurement and with an InGaAs digital
line scan camera for the near-IR measurement. Transient absorption spectra and
decays are collected over the time range from -5 ps to 3 ns. Typically, 200-1000
laser shots are averaged on each delay time to obtain a detectable absorbance
change as small as 10 -4 -10 -3 depending on the monitor wavelength. The polar-
ization direction of the linearly polarized probe pulse is set at a magic angle of
54.7 with respect to that of the pump pulse to cancel out orientation effects on the
dynamics. In order to measure further the transient absorption in a longer time
range, a longer optical delay line would be required. Alternately, additional laser is
employed to provide delayed probe pulses with an electric delay generator syn-
chronized with the pump laser pulse [ 26 ]. Note that it is necessary to correct signal
jitters due to the electric circuit.
5.4 Assignment of Photoexcitations
Let us first describe how to assign transient species such as singlet and triplet
excitons and polarons generated in polymer solar cells. Here, we explain the
assignments on the basis of our recent studies [ 16 - 20 ].
5.4.1 Singlet and Triplet Excitons
Singlet and triplet excitons can be often observed for pristine films upon photo-
excitation. In particular, singlet excitons can be easily observed as the initial
product by photon excitation, because most conjugated polymers have S 0 ? S n
spin-allowed absorption bands from the singlet ground state. In contrast, it is
difficult to detect triplet excitons for highly emissive conjugated polymer films
such as polyfluorene, because the intersystem crossing yield is low due to small
spin-orbit coupling. On the other hand, triplet excitons can be observed for con-
jugated polymer films with high intersystem crossing yield such as regiorandom
poly(3-hexylthiophene) (RRa-P3HT).