The electric and optical properties of doped small molecular organic light-emitting devices

Cheon, Kwang-Ohk
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Various aspects of the device physics of doped small molecular organic light-emitting devices (OLEDs) were described and discussed. The doping layer thickness and concentration were varied systematically to study their effects on device performances, energy transfer, and turn-off dynamics. Low-energy-gap [2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[i,j]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene]propane-dinitrile (DCM2) guest molecules, in either N,N'-diphenyl-N,N '-bis(1-naphthylphenyl)-1,1'-biphenyl-4,4 '-diamine (alpha-NPD) or 4,4'-bis(2,2 '-diphenyl-vinyl)-1,1'-biphenyl (DPVBi) host layers, are optically efficient fluorophores but also generate deep carrier trap-sites.;The blue emission increased relative to the red emission band with increasing bias, but the color coordinates of the total emission were well within the white region at V > 10 V. The most lightly doped device, which contained a 5 nm thick layer of 0.5 wt.% of the dye dopant, exhibited the highest power efficiency, 4.1 lm/W, external quantum efficiency of 3.0%, and white brightness Lmax > 50,000 Cd/m 2 (at 1,100 mA/cm2).;The color of the devices shifts from blue to red as the thickness of the doped layer increases from 0 to 35 A. The (nominal) 2 A-thick doped layer device exhibited the highest brightness L ~ 120 Cd/m2 and external quantum efficiency etaext ~ 4.4% at a current density of 1 mA/cm2. By comparing the emission from the DCM2 with that from DPVBi, the energy transfer probability and its dependence on the applied field was determined. The Forster energy transfer radius in this device was determined to be ~30--40A.;The strong electroluminescence (EL) spike and long decay tail occurring at the turn-off of a bias pulse in some small molecular OLEDs. The observed behavior of the spike is found to be in excellent agreement with a model based on correlated charge pairs, with an average charge pair separation of 20--50 A. Immediately following the spike, a long 1/t decay tail is observed. It is formed from the recombination of initially independent charges. It is found that the decay dynamics of the EL spike and tail are strongly dependent on the dynamics of trapped charges in the recombination zone.

Physics and astronomy, Condensed matter physics