The charge carrier transport properties of neat thin films of red-, green-and blue-phosphorescence Ir complexes have been investigated in terms of time-of-flight transient photocurrent measurement. Bis(2-(2 0 -benzo[4,5-a]thienyl)pyridinato-N,C 3 0 )iridium(acetylacetonate) (btp 2 Ir(acac)) (red-phosphorescence emitter) and iridium(III) bis[(4,6-di-fluorophenyl)pyridinato-N,C 2 0 ]picolinate (FIrpic) (blue-phosphorescence emitter) thin films exhibit bipolar charge transport. In contrast, the electron and hole transit signals of fac-tris(2-phenylpyridine)iridium(III) (Ir(ppy) 3 ) (green-phosphorescence emitter) thin films are unobservable. The charge carrier transport properties of thin films of an Ir-complex-doped 4,4 0 -N,N 0 -dicarbazolebiphenyl (CBP) and the appropriateness of FIrpic as a triplet-exciton and hole blocking layer are shown.
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Charge-carrier transport and triplet exciton diffusion in iridium͑III͒ bis͓͑4,6-di-fluoropheny͒pyridinato-N,C 2Ј ͔picolinate ͑FIrpic͒ doped in 4,4Ј-N, NЈ-dicarbazole-biphenyl ͑CBP͒ thin films have been studied by time-of-flight ͑TOF͒ 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.
Temperature dependences of lifetime and intensity of photoluminescence have been investigated in neat fac-Ir(ppy)3 thin films, which have a nonradiative decay channel due to aggregate quenching. Both temperature dependences can be well understood by a model which consists of three substates of the triplet metal-to-ligand-charge-transfer (3MLCT) state and a nonemissive state. Good agreement between the experimental results and the calculations based on the model suggests that nonradiative decay occurs through a higher lying excited state in fac-Ir(ppy)3 molecules, and that the nonemissive excited state has a decay rate of 3.1×109 s-1 and is located at 0.12 eV above the lowest substate of the 3MLCT state.
We propose a carrier trapping (CT) dopant method to improve the luminous efficiency of organic light-emitting diodes (OLEDs). The CT dopant did not emit light itself, but adjusted the charge balance in the OLEDs. The cell structure of the OLED in the experiment was as follows: Indium tin oxide (ITO)/hole transport layer (50 nm, 2TNATA)/emitting layer (20 nm, NPB+5% rubrene)/electron transport layer (30 nm, Alq+4% CT dopant)/MgIn. (2TNATA: 4,4′,4′′-Tris(N-(2-naphthyl)-N-phenyl-amino)-triphenylamine, NPB; N,N′-Di(naphthalen-1-yl)-N,N′-diphenyl-benzidine). 4,4′-Bis(carbazol-9-yl)-biphenyl (CBP) was used as a CT dopant. This cell had a yellow emission originating from the rubrene with the luminous efficiency of 8.7 cd/A at a current density of 10 mA/cm2. On the other hand, when the CT dopant was not used, the luminous efficiency was 7.5 cd/A. The luminous efficiency was improved by the CT dopant method.
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