We report cyclic voltammetry measurements for the blue electroluminescent conjugated polymer poly(9,9-dioctylfluorene). Both oxidation and reduction potentials are determined and thus estimates of both the ionization potential Ip and electron affinity Ea of the polymer are obtained for the same sample under the same experimental conditions. We estimate Ip=5.80 eV and Ea=2.12 eV. These results disagree with the common assumption that Ea is, to good approximation, given by the difference between Ip and the optical gap. Measurements on indium tin oxide/polyfluorene/calcium light emitting diode structures are consistent with the deductions from the electrochemical data.

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The origin of electrophosphorescence from a doped polymer light emitting diode ͑LED͒ has been investigated. A luminescent polymer host, poly͑9,9-dioctylfluorene͒ ͑PFO͒, was doped with a red phosphorescent dye, 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum͑II͒ ͑PtOEP͒. The maximum external quantum efficiency of 3.5% was obtained at a concentration of 4% PtOEP by weight. Energy transfer mechanisms between PFO and PtOEP were studied by absorption, photoluminescence, and photoinduced absorption spectroscopy. Even though electroluminescence spectra were dominated by PtOEP at a concentration of only 0.2 wt % PtOEP, Förster transfer of singlet excitons was weak and there was no evidence for Dexter transfer of triplet excitons. We conclude that the dominant emission mechanism in doped LED's is charge trapping followed by recombination on PtOEP molecules.

We investigate the influence of the hole-transporting layer ͑HTL͒ on the performance of bilayer vapor-deposited organic light-emitting diodes. Three different HTL materials were used: m-MTDATA, triphenyl-diamine, and naphthyl-phenyl-diamine. In all cases, Alq 3 was the electron-transporting layer ͑ETL͒. We measure and compare the current density-voltage (J -V) and luminance-voltage ͑L-V͒ characteristics of these devices and we conclude that the operating voltage is controlled by the type of HTL used and the nature of the hole-injecting indium tin oxide/HTL interface. We found that the device quantum efficiency depends not only on the electron transport characteristics of the ETL but also on the energetics of the HTL/ETL interface. Analysis of the J -V characteristics suggests that current flow in bilayer devices cannot be described sufficiently by a single carrier theory; both hole and electron currents should be considered.

We have investigated the hole-injection characteristics from indium tin–oxide (ITO) into 4,4′,4″-tris{N,-(3-methylphenyl)-N-phenylamino}triphenylamine (m-MTDATA) and have measured the hole-carrier drift mobility of this compound in single-layer ITO/m-MTDATA/Au structures. We have found that ITO is able to provide trap-free space-charge-limited currents over a wide range of film thicknesses and have established unambiguously that the ITO/m-MTDATA is an ideal Ohmic contact at high electric fields. Our observations clarify the role of m-MTDATA as a voltage-lowering hole-injecting buffer layer in organic light-emitting diodes.