Influence of thickness and position of an ultrathin yellow layer on color tuning (including white) of Organic Light Emitting Diodes

Non-doped white organic light-emitting diodes using (i) a yellow-emitting ultrathin layer of rubrene (5,6,11,12-tetraphenylnaphtacene) inserted on the either side of the interface of a hole-transporting alpha-NPB (4,4´-bis[N-(1-naphtyl)-N-phenylamino]biphenyl) layer and a blue-emitting DPVBi (4,4´-bis(2,2´-diphenylvinyl)-1,1´-biphenyl) layer, (ii) a hole-injecting layer of CuPc (copper pthalocyanine), (iii) an electron-transporting layer of Al(Q)₃ (aluminium tris[8-hydroxyquinolinate]), are described. Both the thickness and the position of the rubrene layer within the hole transporting layer of alpha-NPB and the blue-emitting film of DPVBi allow to achieve the desired color from deep blue with CIE coordinates (x= 0.17, y= 0.15) and an external quantum efficiency eta_ext= 3.4%, (at 30 mA/cm ²), to pure yellow with CIE (0.51, 0.48) and eta_ext =1.3%, via a bright white (WOLED) with excellent CIE coordinates (x= 0.33, y= 0.32), a eta_ext of 1.9%, a color rendering index (CRI) of 70 and a luminance of 2230 cd/m² at 60 mA/cm ². Such a single-well-like structure provides a fine tuning of the emission color by means of the adjustment of the exciton diffusion zone via the exciton-confinement layer of rubrene