Charge-carrier transport and triplet exciton diffusion in a blue electrophosphorescent emitting layer

Charge-carrier transport and triplet exciton diffusion in iridium(III) bis[(4,6-di-fluoropheny)-pyridinato-N,C2′]picolinate (FIrpic) doped in 4,4′-N, N′-dicarbazole-biphenyl (CBP) thin films have been studied by time-of-flight (TOF) transient photocurrent and steady-state photocurrent measurements. With increasing FIrpic concentration, hole TOF transients become highly dispersive and the hole drift mobility is decreased. In contrast, the electron transit signals become unobservable. The TOF results are found to be consistent with those of steady-state photocurrent measurements of FIrpic-doped CBP thin films; the photocurrent spectra are symbatic and antibatic when the illuminated electrode is positively and negatively biased, respectively. The diffusion lengths of triplet excitons in FIrpic-doped CBP thin films are determined from the symbatic photocurrent spectra.