C. Silva et al., Efficient exciton dissociation via two-step photoexcitation in polymeric semiconductors - art. no. 125211, PHYS REV B, 6412(12), 2001, pp. 5211
We report that exciton dissociation occurs within 150 fs during ultrafast p
hotoexcitation at moderately high fluence, with similar to 10% quantum effi
ciency, in a model electroluminescent pi -conjugated polymer. This is appar
ently inconsistent with the otherwise well-supported view that spin-singlet
electron-hole pairs (excitons) are the primary photoexcitations. However,
we demonstrate that resonant sequential transitions account quantitatively
for the photoinduced polaron-pair yield, with the lowest (1 B-a) exciton as
an intermediate. Efficient exciton dissociation occurs either from the res
ulting high-energy, even-parity (A(g)) states, or during ultrafast thermali
zation. The yield of photoinduced polarons, on the other hand, is <0.1% und
er continuous-wave excitation, where access to high-energy states by sequen
tial excitation is not significant.