Diffusion at Interfaces in OLEDs Containing a Doped Phosphorescent Emissive Layer

A common feature of organic light-emitting diodes is their stacked multilayer structure, which is critical for efficient charge injection and transport, and light emission. In this study, it is found that a blended layer of the hole-transport material tris(4-carbazol-9-ylphenyl)amine with 6 wt% fac-tris(2-phenylpyridyl)iridium(III) [Ir(ppy)₃] readily undergoes interdiffusion with adjacent layers of typical charge transport materials: bathocuproine; 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene; N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine; and N,N′-bis(naphthalen-1-yl)-N,N′-diphenylbenzidine. This process is followed using combined neutron reflectometry and in situ photoluminescence measurements. The temperature at which diffusion occurred is found to correlate with the glass transition temperature of the materials. Importantly, the layer of the material with the lowest T_g is the material that acts as a diffusive host for the adjacent layer, which has a higher T_g. That is, a high T_g material does not necessarily act as a blocking layer for diffusion. Furthermore, the results show that the order of structural change within a film can be predicted on the basis of the thermal properties of the materials. These results confirm the necessity of using materials with high glass transition temperatures throughout the device to minimize performance degradation by layer interdiffusion.