Ultrafast photogeneration of charged polarons in conjugated polymers

Ultrafast photoinduced absorption by infrared-active vibrational modes (IRAV) is used to detect charged photoexcitations (polarons) in conjugated luminescent polymers. The experiments, carried out in zero applied electric field, show that polarons are generated instantaneously (within ∼100 fs) with appreciable quantum efficiencies, ${\phi }_{\mathrm{ch}}\sim 10\%.$ The ultrafast photoinduced IRAV absorption, the weak pump-wavelength dependence of ${\phi }_{\mathrm{ch}}$ and linear dependence of charge density on pump intensity indicate that both charged polarons and neutral excitons are independently generated even at the earliest times. Several mechanisms for charge generation proposed in the literature are eliminated by these results.