TRAP DISTRIBUTION NEAR THE ENERGY BAND GAP EDGE IN MDMO-PPV

Abstract

Carrier transport and trapping was investigated in poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) layers by thermally stimulated currents (TSC) depending on the exciting light spectral range. The upper edge of the light spectra was varied from 1.77 eV up to 3.1 eV to assure selective excitation of the defect states. We had shown that material conductivity is affected by several thermally activated processes, i.e., carrier generation from trapping states and thermally stimulated mobility growth. If the below band gap excitation was used, the effective photoconductivity activation energy values of 0.13–0.15 eV were obtained. After the above band gap excitation, the effective photoconductivity activation energy values decreased to 0.05 eV. The energy distribution of the trapping state density was shown to follow the Gaussian distribution function. The deeper states with activation energies of 0.28–0.3 eV and 0.8–0.85 eV were identified too. The results are direct indication by photo-thermo-electrical methods of distributed in energy trapping and transport states with the standard deviation of the density of states of about 0.015 eV.