Transient and Steady-State Photocurrent in an Emissive Layer of Green Electrophosphorescent Devices(Characterization and Abilities of Organic Electronic Devices)(<Special Section>Recent Progress in Organic Molecular Electronics)

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Charge carrier generation, transport, and exciton diffusion in fac tris (2-phenylpyridine) iridium (III) Ir(ppy)_3 doped in 4, 4'-N, N'-dicarbazole-biphenel (CBP) thin films, an emissive layer of green electrophosphorescent devices, have been studied in terms of time-of-flight (TOF) transient photocurrent, steady-state photocurrent and time-resolved photoluminescence (PL) spectroscopies. It is found that the excitation energy rapidly transfer from CBP to Ir(ppy)_3, and that the charge carriers are generated on Ir(ppy)_3 sites. With increasing Ir(ppy)_3 concentration, the electron drift mobility is slightly decreased, while the hole transit signals become unobservable. The electron and hole transport properties of Ir(ppy)_3 doped CBP thin films result from the energy levels of the lowest unoccupied molecular orbital and the highest occupied molecular orbital of Ir(ppy)_3 with respect to those of CBP. From steady-state photocurrent measurement, the diffusion lengths of 3.5% and 7.0% Ir(ppy)_3 doped CBP thin films are determined to be 21 nm and 50 nm, respectively.
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2004-12-01