Delayed electroluminescence in small-molecule-based organic light-emitting diodes: Evidence for triplet-triplet annihilation and recombination-center-mediated light-generation mechanism

We measured delayed electroluminescence in small-molecule-based organic light-emitting diodes based on N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine hole-transport molecule and tris(8-hydroxyquinoline) aluminum electron-transport and emitter molecule after the excitation currents are switched off and reverse bias applied to the sample. The experiments indicate that delayed light emission is a result of two distinct processes: emissive excited singlet-state generation by either triplet-triplet annihilation or recombination of trapped positive and negative charges in the device. Under reverse device bias these two mechanisms have distinctly different signatures. We also found that upon device aging, delayed light emission decreases faster (by about a factor of 4–5) than prompt electroluminescence, which is attributed to an increase of the triplet decay rate due to the presence of aging induced spin-12 trapped charges in the device.