Exciton and Charge Dynamics in Polymer Solar Cells Studied by Transient Absorption Spectroscopy - Organic Solar Cells: Materials and Device Physics - page 115

5.5.1 Exciton Delocalization

As mentioned above, the absorption band of singlet excitons in P3HT films is red-
shifted with time, suggesting delocalization of singlet excitons. Figure 5.10
summarizes the transient absorption spectra of RRa-P3HT and RR-P3HT pristine
films. The singlet exciton band is red-shifted from 900 to 1060 nm for RRa-P3HT
and from 1200 to 1250 nm for RR-P3HT from 0 to 100 ps after the excitation at
400 nm. Such a peak shift suggests that singlet excitons efficiently migrate on a
time scale of picoseconds, resulting in the exciton delocalization into longer
conjugated segments and the singlet-singlet exciton annihilation under an intense
excitation. The stabilization energy corresponds to DE = 0.21 eV for RRa-P3HT
and DE = 0.04 eV for RR-P3HT. The large DE for RRa-P3HT is probably
because RRa-P3HT amorphous films have a relatively wide distribution of ener-
getic disorders compared to RR-P3HT crystalline films. Interestingly, as shown in
Fig. 5.10 c, no peak shift is observed for RR-P3HT films excited at 620 nm that is

Fig. 5.10 Transient
absorption spectra of
P3HTpristine films measured
at 0, 1, 10, 100, and 3000 ps
from top to bottom in each
panel: a RRa-P3HT excited at
400 nm, b RR-P3HT excited
at 400 nm, and c RR-P3HT
excited at 620 nm. Adapted
with permission from [ 18 ].
Copyright 2009 American
Chemical Society

(a)

(b)

(c)

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