Time-resolved optical and electrical study of second-order processes responsible for the formation of free polarons in conjugated polymers

A technique based on the action of pairs of correlated laser light pulses (400 nm wavelength) with a controlled delay time between them was applied for studying second-order processes responsible for the formation of free charge carriers in a conjugated polymer poly(2,5-dioctyloxy-$p$-phenylenevinylene). Time-resolved experiments within the picosecond time domain enabled the attainment of direct evidence of the existence of intermediate states preceding free charge carriers at the photoexcitation of conjugated polymers. These states are shown to take part in second-order processes responsible for the generation of free charge carriers in conjugated polymers at a sufficiently high excitation density. The short lifetime of species involved in the processes led to the speculation that they are primarily formed polaron pairs, the majority of them being generated without any delay at the absorption of the light.