Luminescent spectral changes in polymer light-emitting diodes after heat treatments

Abstract

Luminescent spectra of polymer light-emitting diodes (LEDs) would depend upon the heat treatment condition and procedure greatly. We have tried thermal annealing at the temperature above T-g of poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV), before and after Al deposition respectively, in ITO/MEH-PPV/Al structure. The pre-deposition annealing makes the PL and EL spectrum red-shifted and more broadened. After annealing, the optical hand-gap of MEH-PPV thin film decreases by about 0.1 eV relative to that of the dilute solution. The annealing also makes an excimer emission dominant in PL and EL spectra. In contrast, after post-deposition annealing, the EL spectrum was blue-shifted and narrowed. In addition, the excimer emission is withdrawn remarkably after the port-annealing.