Mechanism of carrier generation in poly(phenylene vinylene): Transient photoconductivity and photoluminescence at high electric fields.

The carrier generation mechanism in poly~phenylene vinylene!, 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 is proportional to E at low fields, but increases nonlinearly for E.10 V/cm. The field at which the photoconductivity becomes nonlinear ~the onset field, E0 ) depends on the degree of alignment: the higher the draw ratio, the lower E0 pc . The dependence of the photocurrent on E is similar to the dependence of the dark current on E; both imply a field-dependent mobility ~rather than fielddependent carrier generation!. The onset field for the nonlinear photoconductivity is, however, different from the onset field for quenching the luminescence (E0 ). 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 field-induced carrier generation is not significant. @S0163-1829~96!08231-8#