Efficient exciton dissociation via two-step photoexcitation in polymeric semiconductors

We report that exciton dissociation occurs within 150 fs during ultrafast photoexcitation at moderately high fluence, with $\sim 10\%$ quantum efficiency, in a model electroluminescent $\pi$-conjugated polymer. This is apparently 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 ${(1B}_u)$ exciton as an intermediate. Efficient exciton dissociation occurs either from the resulting high-energy, even-parity ${(A}_g)$ states, or during ultrafast thermalization. The yield of photoinduced polarons, on the other hand, is $<0.1\%$ under continuous-wave excitation, where access to high-energy states by sequential excitation is not significant.