Exciton and Charge Dynamics in Polymer Solar Cells Studied by Transient Absorption Spectroscopy - Organic Solar Cells: Materials and Device Physics - page 118

5.5.3 Triplet Formation

Triplet excitons are generally converted through the intersystem crossing from the
lowest singlet exciton. As mentioned in Sect. 5.4.1 (see Fig. 5.4 ), triplet excitons
are generated from singlet excitons in RRa-P3HT films. Indeed, as shown in
Fig. 5.12 , the triplet rise (*4 ps) is in good agreement with the singlet decay
(*4 ps), suggesting that triplet excitons are rapidly converted from singlet exci-
tons. The rise and decay time is dependent on the excitation intensity, suggesting
that the singlet exciton-exciton annihilation contributes to the rapid interconver-
sion. Interestingly, the formation yield of triplet excitons is estimated to be as high
as *25 % at 10 ps from the decay analysis [ 18 , 38 ], even though singlet excitons
are significantly quenched by the singlet exciton-exciton annihilation. If triplet
excitons were formed via the intersystem crossing from relaxed singlet excitons,
the triplet yield at 10 ps should be as small as *1 % because the intersystem
crossing rate has been reported to be *1ns -1 for poly(3-octylthiophene) in a
xylene solution [ 38 ]. Therefore, such a short interconversion time of *4 ps cannot
be simply explained in terms of the intersystem crossing from the lowest singlet
exciton state to triplet exciton state. Rather, the rapid triplet formation results from
a higher singlet exciton state generated by the singlet exciton-exciton annihilation
in the picosecond time domain. The triplet formation from a higher exciton state
should be completed before the relaxation to the lowest singlet exciton state,
because the interconversion in the relaxed exciton states is limited by the slow
intersystem crossing rate. We therefore conclude that the rapid triplet formation is
in competition with the vibrational relaxation to the lowest exciton or polaron pair
states within \100 fs. Such a rapid triplet formation is indicative of efficient spin-

10 -12 10 -11 10 -10 10 -9

Time / s

Fig. 5.12 Normalized transient absorption decays of RRa-P3HT pristine films excited at 400 nm
(*30 lJcm -2 ). The closed circles represent time evolution of singlet excitons measured at
1000 nm. The open circles represent time evolution of triplet excitons, which is evaluated by
subtracting the transient signals of singlet excitons at 1000 nm from that at 825 nm. Adapted with
permission from [ 18 ]. Copyright 2009 American Chemical Society