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 degrees C and 290 degrees 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 degrees 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 degrees 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 degrees C, whereas the OLEDs annealed at 290 degrees 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 degrees C, whereas the spectrum of the similar devices annealed at 190 degrees C had an additional blue component corresponding to PFO emission. (C) 2014 Elsevier Ltd. All rights reserved.