Aging induced traps in organic semiconductors

Trap states in organic single layer devices have been detected by the method of thermally stimulated currents (TSC). These devices have been exposed to potentially harmful atmospheres of oxygen and humid nitrogen in order to investigate the influence of ambient atmosphere on the trap spectrum of materials used for organic light-emitting diodes. Discrete traps with activation energies of 105 and 140 meV have been found in α-NPD (N,N′-di(1-naphthyl)-N,N′-diphenylbenzidin). In 1-NaphDATA (4,4′,4″-Tris(N-2-naphthyl)-N-phenylamino-triphenylamin) discrete trap levels with activation energies of 100 and 235 meV are present. The exposure to humidity forms a deep hole trap of 0.5 eV in 1-NaphDATA, whereas in α-NPD no new trap was detected. The influence of the so generated traps on the charge transport is demonstrated by I-V characteristics.