DECAY MECHANISMS OF PHOTOEXCITATIONS IN CONDUCTING POLYMERS PROBED BY PICOSECOND PHOTOINDUCED ABSORPTION

The dynamics of picosecond photoinduced absorption (PA) in a host of conducting polymers with varying degrees of ground-state degeneracy are compared for low-density excitation (less-than-or-equal-to 10(18) carriers/cm3). The results are consistent with dispersive recombination associated with the 1-D diffusion of the photoexcited species on chain segments distributed in length in the polymers polythiophene, trans-polyacetylene, and in polydiacetylene at high pressure. We find that, within the 5 ps time resolution of the experiment, the transient electronic response is approximately independent of probe wavelength in the range 1.2 to 2.1 eV for all polymers except polydiethynylsilane. In that material recombination occurs within 1.4 ps after photoexcitation at all energies except those between 1.8 and 2.0 eV, coincident with a narrow band in the PA spectrum that is linked to thermal modulation of the absorption edge.