Critical Role of Triplet Exciton Interface Trap States in Bilayer Films of NPB and Ir(piq)3

Investigations are carried out into triplet transfer in bilayer films of NPB (N,N´‐diphenyl‐N,N´‐bis(1‐naphthyl)‐1,1´‐biphenyl‐4,4?‐diamine) and Ir(piq)₃ (Iridium (III) Tris(1‐phenylisoquinoline) using laser light pulses to excite the upper surface of the NPB, and thereafter observing the decay dynamics in the Ir(piq)₃ layer situated beneath the NPB. The NPB layer thickness is varied from 13 nm to 80 nm. The results show that up to 200 ns after excitation, the multiexponential decay of directly excited Ir(piq)₃ is observed, thereafter the decay is monoexponential. It is concluded that this monoexponential decay after 200 ns is due to triplets that are transferred to the Ir(piq)₃ via migration from the NPB. The thicker the NPB layer the longer it takes for the reservoir of NPB triplets to deplete via the Ir(piq)₃, with the result that the apparent monoexponential lifetime of the Ir(piq)₃ increases as the thickness of the NPB films increases. Based on time resolved spectra and decays, it is concluded that triplets arriving from NPB are trapped at interface sites of Ir(piq)₃ and do not migrate directly to the bulk states of Ir(piq)₃. A model based on exciton diffusion kinetics, including the presence of interface trap sites, is described, which accurately predicts this behavior.