Investigation of carrier transport and trapping by oxygen-related defects in MEH-PPV diodes

Carrier transport and capture were investigated in poly [2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) Schottky diodes by thermally stimulated currents, thermally stimulated depolarization and current-voltage characteristics. After the white light excitation, we observed two well-marked thermally stimulated current peaks. Their maxima were located in the temperature regions 214-244 K and 304-394 K, respectively. The detailed numerical modelling revealed that the full TSC is a superposition of several thermally activated processes. The peaks themselves could be attributed to the carrier generation from the traps with activation energies 0.45-0.55 eV, 0.76-0.8 eV and 0.76-0.9 eV. They were observed in the background of thermally stimulated mobility growth according to the Poole-Frenkel or the Gaussian disorder models. It is worth noting that the trap filling could be increased significantly by exposing the sample to the air, indicating that the traps are oxygen related. Therefore, they could be identified as electron traps. The trap with activation energy 0.45-0.55 eV is likely distributed over the sample depth, meanwhile the two deepest traps are most probably located near to the surface. It is worth noting that the traps could not be recharged by applied voltage. Instead, excitation by electric field resulted in a sample polarization. The non-exponential depolarization lasted for several thousands of seconds and was not thermally activated even above the glass-transition temperature. These facts imply that possibly different physico-chemical mechanisms, e.g., reversible chemical reactions or chain structure reorganization induced by electric field, have to be taken into account.