Blue organic light emitting devices with improved colour purity and efficiency through blending of poly(9,9-dioctyl-2,7-fluorene) with an electron transporting material

Organic light emitting devices (OLEDs) containing a single blended layer of poly(9,9- dioctyl-2,7-fluorene) (PFO) or poly[9,9-bis(4-octyloxyphenyl)-2,7-fluorene] (PFOPh) and an electron transporting material 2,7-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazol-5-yl]-9,9-dihexylfluorene (DFD) have been fabricated. The long-wavelength emission (> 480 nm) observed for pure PFO and PFOPh structures was reduced significantly and a pure blue emission, with Commission Internationale de l'Eclairage (CIE) colour coordinates (0.157, 0.037), could be achieved. The blended-layer devices also possessed considerably higher efficiencies than the pure PFO, pure PFOPh or the pure DFD structures. It is suggested that the presence of DFD results in an improved balance of charge carriers and provides a micro-encapsulation environment for the polymer chains, hindering the formation of fluorenone defects and their red-shifted emission.