Luminescence and spectrum variations caused by thermal annealing in undoped and doped polyfluorene OLEDs

• The effect of annealing was investigated on undoped and doped polyfluorene OLEDs.
• Performances of non-annealed and annealed (190 °C or 290 °C) OLEDs were compared.
• Annealing at 190 °C was beneficial for undoped OLEDs.
• Non-annealed OLEDs showed best results for the doped OLEDs.
• Influence on luminance and EL spectra was observed in both types of the OLEDs.

The effect of thermal annealing on electroluminescent (EL) properties of polyfluorene-based organic light-emitting diodes (OLEDs) was studied. Two types of light-emitting layers were investigated: undoped layers made of poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO) and doped layers consisting of 95 wt% of PFO and 5 wt% of poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)] (F8BT). Thermal annealing at 190 °C and 290 °C was performed prior to top contact thermal deposition. The experiments indicated that the annealing treatment results in significant changes of EL properties in both types of the considered OLEDs. However, the annealing affects them differently. For the undoped OLEDs, the devices annealed at 190 °C presented the highest luminance and current density. In the case of doped OLEDs, the highest luminance was observed in the non-annealed devices and the highest current density in the devices annealed at 190 °C. Remarkably, the annealing was noticed to affect the EL spectra of both types of OLEDs. In the undoped OLEDs the purest blue emission was observed from the OLEDs annealed at 190 °C, whereas the OLEDs annealed at 290 °C and the non-annealed OLEDs in particular had pronounced undesired green emission, a common problem for the polyfluorene-based blue light-emitting polymers. Green emission originating from F8BT was observed for the doped OLEDs in the case of non-annealed devices and the devices annealed at 290 °C, whereas the spectrum of the similar devices annealed at 190 °C had an additional blue component corresponding to PFO emission.