Exciton and Charge Dynamics in Polymer Solar Cells Studied by Transient Absorption Spectroscopy - Organic Solar Cells: Materials and Device Physics - page 109

under a nitrogen atmosphere, while it decays more rapidly under an oxygen
atmosphere. Molecular oxygen is a general and efficient quencher of singlet and
triplet excitons in organic materials [ 27 ]. Thus, such a long lifetime is ascribable to
triplet excitons, because it is too long to be assigned to singlet excitons. Although,
electron spin resonance (ESR) is a useful method for distinguishing between
singlet and triplet states as reported in the literatures [ 28 - 30 ], the oxygen
quenching is most widely employed to assign triplet excitons. Note that not all
triplet excitons are quenched by molecular oxygen because quenching is a
diffusion-limited reaction. Roughly speaking, triplet excitons with a long lifetime
([ls) can be effectively quenched by molecular oxygen even in solid films
depending on the oxygen permeability of the film.
In most cases, as described above, singlet excitons are observed as the initial
product by photon absorption. However, this is not true for RR-P3HT pristine films
excited by intense laser pulses. Figure 5.5 shows the transient absorption spectra
of RR-P3HT pristine films under different excitation conditions. At an excitation
intensity of 15 lJcm -2 , a large absorption band is observed at around 1200 nm
immediately after the laser excitation. As the excitation intensity is increased from
15 to 120 lJcm -2 , this band disappears and instead a new absorption band is
observed at around 1000 nm. From the intensity dependence of the initial transient
signals, we ascribe the 1200-nm band to a singlet exciton and the 1000-nm band to
polarons. At higher excitation intensities, polarons are generated even at 0 ps from
hot exciton states formed by the singlet-singlet exciton annihilation. Note that no
exciton diffusion is involved in the singlet-singlet exciton annihilation because the
duration is too short. The energy of two photons at 400 nm corresponds to 6.2 eV,
which is much larger than the ionization potential of
RR-P3HT films. Thus, it would be more appropriate to assign the polaron for-
mation to two-photon ionization. This finding suggests that the initial product by
photon absorption is not always a singlet exciton particularly at a high excitation
intensity. In conclusion, for the assignment of singlet excitons, it is essential to
analyze not only the decay kinetics but also the intensity dependence of the
spectrum and kinetics.
5.4.2 Polarons
As mentioned above, polarons are efficiently generated from excitons at a donor-
acceptor interface. Thus, they can be more clearly observed in polymer-fullerene
blend films rather than in pristine conjugated polymer films. Some polarons
escaping from the geminate recombination at the interface can survive up to a time
scale of microseconds or more. On such a longer time scale, therefore, polarons
can often be observed separately from singlet and triplet excitons. Figure 5.6 a
shows the transient absorption spectra of RRa-P3HT:PCBM blend films from
0 to 3 ns after the laser excitation. In this time domain, the absorption spectrum
varies with time, suggesting that the transient species changes with time. As will