Exciton and Charge Dynamics in Polymer Solar Cells Studied by Transient Absorption Spectroscopy - Organic Solar Cells: Materials and Device Physics - page 105

often called bound radical pairs to be distinguished from tightly bound electron-
hole pairs of excitons in the bulk. Some bound radical pairs can be dissociated into
free carriers (charge dissociation) in competition with the geminate recombination
to the ground state or triplet state (charge recombination) on a time scale of 10 -12 to
10 -9 s[ 19 ]. The dissociated free carriers are transported to each electrode through
repetitive charge hoppings in an energetically disordered matrix (charge transport).
Some of them escaping from the bimolecular recombination are collected to the
electrode (charge collection). The charge collection time is dependent on the charge
mobility, the thickness of the active layer, and the electric field applied to the layer.
Recent studies have shown that it takes 10 -6 to 10 -5 s for charge carriers to be
collected to the electrode [ 25 ]. Finally, the photocurrent is generated as a result of
the series of photovoltaic conversion events. In other words, the device perfor-
mance of J-V characteristics is just the final result of the series of photovoltaic
conversion events ranging from 10 -14 to 10 -5 s (nine orders of magnitude on a
temporal scale). In this chapter, we focus on such rapid photovoltaic conversion
events studied by transient absorption spectroscopy and discuss the findings
obtained from the kinetics analysis.
5.3 Transient Absorption Spectroscopy
Transient absorption spectroscopy is the most useful method for directly observing
photovoltaic conversion events ranging from 10 -14 to 10 -5 s. However, it is
difficult to measure the absorption of thin films such as polymer solar cells where
the active layer is typically as thin as 100 nm (=10 -5 cm). For example, the
absorbance would be as small as 10 -5 in the case of a molar absorption coefficient
of 10 4 M -1 cm -1 , which is a typical value for organic dye molecules, a molar
concentration of 0.1 mM, and an optical path length of 10 -5 cm. To detect an
absorbance change of 10 -5 , it is necessary to measure only 1/50000 of the change
in the optical probe signal separately from various noises.
Figure 5.2 shows a block diagram of the highly sensitive microsecond transient
absorption spectroscopy system [ 17 ]. In this system, a probe light is provided from
a tungsten lamp with a power source stabilized to reduce fluctuation of the probe
intensity. To reduce unnecessary scattering light, stray light, and emission from the
sample, two monochromators and appropriate optical cut-off filters are placed
before and after the sample. An excitation light is supplied from a dye laser
pumped by a nitrogen laser, which can excite the absorption peak of thin-film
samples to give a high yield of photoexcitations. The probe light passing through
the sample is detected with a PIN photodiode such as Si or InGaAs depending on
the measuring wavelength. The signal from the photodiode is pre-amplified and
sent to the main amplification system with electronic band-pass filters to improve
the signal to noise ratio. The amplified signal is collected with a digital oscillo-
scope, which is synchronized with a trigger signal of the laser pulse from a
photodiode. In our system, the detectable absorbance change is as small as 10 -5 to