Mechanism of carrier generation in conducting polymers

The carrier generation mechanism in poly(phenylene vinylene), PPV, is addressed by studying the transient photoconductivity and the photoluminescence as a function of the external electric field, E. In samples oriented by tensile drawing, the transient photocurrent in PPV is proportional to E at low fields, but increases nonlinearly for E > 10(4) V/cm. The field at which the photoconductivity becomes nonlinear (the onset field, E-0(pc) depends on the degree of alignment: the higher the draw ratio, the lower E-0(pc). The dependence of the photocurrent on E is similar to the dependence of the dark current on E, implying a field dependent mobility rather than field dependent carrier generation. The onset field for the nonlinear photoconductivity is, however, different from the onset field for quenching the luminescence (E-0(pl). Thus, contrary to expectations for strongly bound neutral excitons as the elementary excitations, the high field increase in photocurrent and the corresponding decrease in photoluminescence are not proportional, indicating that the carrier generation induced by the external field is not significant. The transient photocurrent is independent of photon energy in the measured range between 2.33-3.67 eV, indicating that carriers are the prime excitation in PPV.