Effect of electric field, solvent, and concentration on the electroluminescence spectra and performance of poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] based light emitting diodes

The electroluminescence (EL) spectra and performance of polymer light emitting diodes based on poly[2-methoxy-5-(2(')-ethyl-hexyloxy)-1,4-phenylene vinylene] are found to depend significantly on the applied electric field as well as the solvent and concentration used in the fabrication of the polymer layer. Drastic changes in the EL spectra with the electric field are observed in devices in which the polymer layer is spin cast from chlorobenzene (CB) at high concentration. At low electric field, the broad reddish-orange emission from these devices originates mostly from aggregate species formed by interchain interactions. However, at high electric field, a narrow yellow emission is observed, which originates from both aggregate species (minor part) as well as single-chain species (major part) arising out of intrachain interactions. The individual emission intensities of the two species are estimated at different electric fields and are found to behave in a different manner with the change in electric field. The fractional contribution of the aggregate species in the overall EL spectra changes from 98% to 17% as the electric field is increased from low to a high value. Either changing the solvent from CB to tetrahydrofuran (THF) or decreasing the concentration (in any particular solvent) reduces the possibility of the formation of aggregate species. At any particular concentration, the turn-on field is found to be always higher in CB-cast films than THF-cast ones. This fact is explained by the presence of deeply trapped holes at low electric field. Devices made from CB are found to give better performance regarding external quantum efficiency, power efficiency, etc., than those made from THF, irrespective of the concentration. Again, devices made from any particular solvent are found to perform better at low concentration than at high concentration. Enhanced carrier mobility accounts for the improved performance of the devices made from CB than those made from THF. On the other hand, device performance becomes poor at high concentration (in any particular solvent) due to the formation of aggregate species. The optimum performance is obtained from the devices in which the polymer layer is spin cast from CB at low concentration. (C) 2003 American Institute of Physics.