Formation of polaron pairs and time-resolved photogeneration of free charge carriers in pi-conjugated polymers

We have performed in the present work time-resolved experiments on poly(3-d odecyl-thiophene) (P3DDT) and poly(2,5-dioctyloxy-p-phenylene vinylene) (OO -PPV) films by directly probing the formation of charge carriers responsibl e for the cw photoconductivity within the time domain of -10 ps to 1 ns. La ser light pulses of 400 nm wavelength, 150 fs width, induced photoconductiv ity in a sample with a frequency 1 kHz. Red 800 nm light pulses delayed in respect to blue ones were revealed to affect the photoconductivity. The eff ect of the second pulses increased with the delay time. Red light induced c hanges of the photoconductivity were positive in OO-PPV, and negative in P3 DDT. These results are rationalized as an evidence of delayed not immediate formation of free charge carriers. The carriers seem to be formed within 1 0 ps after the pumping pulse. A mechanism of formation of free polarons fro m polaron pair is suggested, which has permitted to explain main feature of the results including different signs of the effect of the red light in di fferent polymers.