A low driving voltage organic light-emitting diode using a yellow phosphor bis[2-(4-tert-butylphenyl)benzothiazolato-N,C2′] iridium (acetylacetonate) as a dopant and 4,4′-bis[N-1-napthyl-N-phenyl-amino]biphenyl as a host was fabricated. The device without p- or n-doped transporting layer shows a turn-on voltage as low as 2.45V, and a luminance of 1000cd∕m2 at 4.3V. A maximum luminance of 23230cd∕m2 at 10V was achieved. The decrease of the device turn-on voltage may result from direct charge carrier trapping in the dopant and hole only transporting characteristic of the host material. Both charge carrier trapping and energy transfer mechanisms were found in the electroluminescence process.
Multilayer phosphorescent organic light-emitting diodes with bright yellow light emission based on novel bis[2-(4-tert-butylphenyl) benzothiazolato-N,C 2 ] iridium (acetylacetonate) [(t-bt) 2 Ir(acac)] were fabricated. The device structure is ITO/CuPc (15 nm)/NPB (15 nm)/CBP: (t-bt) 2 Ir(acac) (5 wt%, 30 nm)/BCP (20 nm)/Alq (20 nm)/ LiF (1nm)/Al (100 nm). Electroluminescent (EL) spectra of the device have a main peak at 560 nm with a shoulder at 596 nm ascribed to (t-bt) 2 Ir(acac) phosphor and a weak peak from the NPB layer. A maximum luminance of 10 500 cd m −2 at a driving voltage of 12.5 V and current density of 188 mA cm −2 and a maximum external quantum efficiency of 6.1% at 4.2 V were achieved.
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