Picosecond transient photoconductivity in a soluble derivative of poly(p-phenylene vinylene)

We report the results of picosecond transient photoconductivity (PC) measurements in films of poly[2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV), a soluble derivative of poly(p-phenylene vinylene) (PPV). The onset of picosecond transient PC and the onset of steady-state PC are both found to coincide with that of optical absorption. The peak transient PC is proportional to the electric field, independent of temperature, and proportional to the light intensity. Since the transient PC is an order of magnitude smaller than that of PPV, early recombination is more probable due to the large side groups of MEH-PPV. This has been checked through photoinduced charge-transfer studies; both the magnitude and lifetime of the transient PC increase substantially on addition of dilute concentrations of C60 into MEH-PPV. Thus, ultrafast photoinduced electron transfer reduces early recombination and enhances the quantum yield for photogeneration of free charge carriers. These results indicate direct photogeneration of free carriers via an interband transition.