Exciton and Charge Dynamics in Polymer Solar Cells Studied by Transient Absorption Spectroscopy - Organic Solar Cells: Materials and Device Physics - page 123

words, the time constant of the delayed formation of polarons is limited by the
exciton migration in relatively large crystalline domains of RR-P3HT. This is
consistent with the recent transient studies [ 57 , 58 ]. On the other hand, the prompt
polaron formation ([10 13 s -1 ) is ascribed to the charge generation at the interface
of RR-P3HT/PCBM. The rate constant of the prompt polaron formation is 10 4
times faster than the deactivation rate constant (3.0 9 10 9 s -1 ) of singlet excitons
in RR-P3HT pristine films. Thus, the charge transfer efficiency (g CT ) is estimated
to be *100 % at the interface of RR-P3HT/PCBM. Because g CT & 100 %, the
exciton diffusion efficiency is estimated to be g ED = g q g CT & g q = k q /
(k F ? k q ) = 93 % before the thermal annealing and 89 % after the thermal
annealing in RR-P3HT:PCBM blend films. In other words, there is almost 10 %
loss in g ED in either case, although it is still high enough to collect singlet excitons
into the interface of RR-P3HT/PCBM.
5.6.2 Monomolecular and Bimolecular Recombination
The charge recombination between electrons and holes can be typically classified
into monomolecular (geminate) recombination and bimolecular recombination. It
is important to distinguish between the two, because the photocurrent generation is
critically dependent on whether the electron and hole are bound as a geminate pair
at the interface or dissociated into free carriers. However, it is impossible to
distinguish between geminate charge pairs and free carriers from transient
absorption spectra alone, because the charge species are identical in either case.
Monomolecular recombination is the first-order reaction, and hence the decay
constant is, as shown in Fig. 5.15 a, independent of the concentration of the
transient species. On the other hand, bimolecular recombination is the second-
order reaction, and hence the half-life is, as shown in Fig. 5.15 b, dependent on the
concentration of the transient species: it should be theoretically half at twice
concentration. Therefore, we can distinguish whether electron and hole are bound
as a geminate pair at the interface or dissociated into free carriers by analyzing the
intensity dependence of the decay dynamics.
In RRa-P3HT:PCBM blend films, as described in Sect. 5.6.1 , polarons are
promptly generated in a picosecond with g ED = 100 %. However, as shown in
Fig. 5.16 , the PCBM anion band at 1,030 nm decays monoexponentially to 30 %
with a time constant of *0.8 ns. The photobleaching at 480 nm also recovers with
the same constant of *0.8 ns. Furthermore, as shown in Fig. 5.17 , all the decay
dynamics of the P3HT polarons at 850 nm, the PCBM anions at 1030 nm, and the
photobleaching at 480 nm are independent of the excitation intensity, and there-
fore can be ascribed to the monomolecular (geminate) recombination of P3HT
polarons and PCBM anions. We therefore conclude that 70 % of P3HT polarons
geminately recombine with PCBM anions to the ground state and the remaining
30 % of polarons can be dissociated into free carriers. The dissociated polarons
survive until longer time domains, and hence can be observed on a time scale of