Volume 91, Issue 10, 15 May 2002
Index of content:
- DEVICE PHYSICS (PACS 85)
Effect of carbazole–oxadiazole excited-state complexes on the efficiency of dye-doped light-emitting diodes91(2002); http://dx.doi.org/10.1063/1.1469692View Description Hide Description
Interactions between hole-transporting carbazole groups and electron-transporting 1,3,4-oxadiazole groups were studied by photoluminescence and electroluminescence(EL)spectroscopy, in blends of poly(N-vinylcarbazole) with 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole (PVK:PBD) and in random copolymers with carbazole and oxadiazole groups attached as side chains. Different excited-state complexes form in the blends, which exhibit exciplexes, and in the copolymers, which manifest electroplexes, due to topological constraints on the position of carbazole and oxadiazole units in the polymer. Both types of complex red-shift the EL spectra of the matrices compared with pure PVK homopolymer, although the shift is significantly greater for the electroplex. The presence of these complexes has a profound effect on the external quantum efficiency of dye-doped organic light-emitting diodes employing the blends or copolymers as matrices, as it strongly affects the efficiency of Förster energy transfer from the matrix to the dye. Single-layer devices doped with either coumarin 47 (C47), coumarin 6 (C6), or nile red (NR) were compared. Among the three dye-doped PVK:PBD devices, C6 doping yields the highest efficiency, while NR doping produced the most efficient copolymer devices, consistent with the degree of overlap between the ELspectrum of the matrix material and the absorptionspectrum of the dye.
Observation of current staircase due to large quantum level spacing in a silicon single-electron transistor with low parasitic series resistance91(2002); http://dx.doi.org/10.1063/1.1471928View Description Hide Description
We have fabricated a siliconpoint-contact channel single-electron transistor (SET) with an ultrasmall dot. By narrowing only the point-contact region and suppressing the parasitic series resistance, a peak conductance as large as 8.8 μS and single-electron addition energy as large as 128 meV are simultaneously obtained. A current staircase due to the large quantum level spacing is clearly observed at low temperatures. From numerical calculations, it is found that the staircase feature due to discrete quantum levels stands out even at room temperature in future silicon SETs with an ultrasmall dot.