Dependence of carrier recombination mechanism on the thickness of the emission layer in green phosphorescent organic light emitting devices

Delayed EL measurements are used to elucidate carrier recombination and light emission mechanism in phosphorescent organic light emitting devices (PhOLEDs) based on 4,4'-bis(9-carbazolyl)-1,1'-biphenyl (CBP) and fac-tris(2-phenylpyridine) iridium (Ir(ppy)(3)) host: guest system. The results show that changing the thickness of the emitting layer (EML) leads to marked changes in charge-trapping and host-host triplet-triplet-annihilation (TTA) patterns, suggesting that carrier transport and recombination processes change depending on EML thickness. The results suggest a change in carrier recombination and exciton formation mechanism, depending on EML thickness, from a scenario (for EMLs < 20 nm) in which recombination occurs mostly on the guest rather than the host thus creating excitons directly on the guest, and hence is not strongly influenced by host-to-guest energy transfer; to another scenario (for EMLs > 20 nm) where carrier recombination and exciton creation on the host is not negligible, and hence also the role of host-to-guest energy transfer. (C) 2011 Elsevier B.V. All rights reserved.