The effect of spacer in hybrid white organic light emitting diodes

Two kinds of highly efficient, bright hybrid white organic light emitting diodes have been fabricated by utilizing ambipolar material 4,4-N,N-dicarbazolebiphenyl (CBP) and unipolar material N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB) as the spacer, respectively. By adjusting the thickness of spacers together with effectively utilizing combinations of exciton-harnessed orange-phosphorescence/blue-fluorescence, the current efficiency of optimal device with CBP spacer and device with NPB spacer are 31.0 and 38.9 cd/A, the maximum power efficiency are 23.9 and 29.1 lm/W, the maximum brightness are 87,249 and 75,501 cd/m², and the Commission Internationale de l’Eclairage (CIE) coordinates are (0.42, 0.43) and (0.47, 0.44) at a voltage of 11 V, respectively. Furthermore, systematic investigations focused on the effects of spacers on device performance have been performed. By virtue of the multifunctional spacer, energy transfer between the phosphorescent and fluorescent emitters can be avoided while the carriers still can pass through this spacer bridge, which is important to realize such high device performance. In addition, a novel concept that both ambipolar materials and unipolar materials can be the promising candidates for spacers has been proposed if the thickness of spacers with high triplet energy is optimized.