Movement of carrier recombination zone in organic light emitting devices by applied voltage

We fabricated organic light emitting devices that consist of three different emitting layers in series between hole transport layer (HTL) and electron transport layer (ETL), in order to investigate the carrier recombination zone in the devices. Since the three different emitting layers are constructed to emit different colors, the carrier recombination zone can be observed from the luminescence. The predominant recombination zone was found to be relatively far from the HTL-EML hetero-interface at low applied voltage (around 7V). When the applied voltage increases to 11V, the recombination zone tends to shift towards the hetero-interface. As the applied voltage increases further, interestingly the recombination zone tends to go back away from the HTL-EML interface due to the electron crossover to the HTL or electron diffusion back to the far side from the interface.