Using an improved time-of-flight (TOF) technique, the drift
mobilities of electrons and holes in organic films prepared on silicon
or indium-tin-oxide (ITO)-coated glass substrates have been
determined. For the samples on silicon, the silicon was also used as a
carrier-generating layer. This substantially increased the number of
charge carriers generated and thus resulted in a higher intensity
electrical signal. Consequently, the thickness of the organic layers
can be reduced to less than 1/10 of the typical values (several
microns) required in the conventional TOF measurement. The typical
thickness of the organic layer in the present work is 400 nm. For
organic materials with a high optical absorption coefficient, samples
for the TOF measurement can be prepared by directly depositing these
materials onto ITO glass substrates with a thickness of about
1000 nm. For both types of substrate, the thickness of the organic
layer is much closer to the typical value used in organic
electroluminescent devices. The signal, and thus the accuracy, in the
present measurement were much improved over those of the conventional
TOF measurement. The logarithm of the drift mobility changed linearly
with the square root of the applied electric field.
A doping technique for fabricating organic multiple-quantum-well electroluminescent (EL) devices is demonstrated. This device consists of N,N′-Bis(3-methyphenyl)-N,N′-diphenylbenzidine used as a hole transporter, undoped tris(8-quinolinolato) aluminum (Alq) as a barrier potential or electron transporter, and Alq doped with 5,6,11,12-tetraphenylnaphthacene as a potential well and an emitter. Our experimental results suggest that the double-quantum-well EL devices show the optimum emission characteristics. The efficiency and the luminance of the device achieve 15.7 lm/W and 7500 cd/m2, respectively.
By using a series of tris-(8-hydroxyquinoline) metal chelates with
central metal ions of Al3+, Ga3+, In3+ as the host materials,
red organic light-emitting diodes have been fabricated, which used a
red fluorescent dye,
4-(dicyanomethylene)-2-t-butyl-6-(8-methoxy-1,1,7,7-tetramethyljulolidyl-
9-enyl)4H-pyran
(DCJMTB), as the emitter or guest dopant material. The doped devices with
Gaq3 as the host materials produce high efficiencies and saturated red
colour chromaticity. The device (1% DCJMTB doped in Gaq3) showed a current
efficiency of 2.64 cd A-1, which is about 40% higher than that of the
doped Alq3 device, and nearly two times higher than that of the doped
Inq3 device at 1% DCJMTB dopant concentration. The colour coordinates of
the Gaq3:1% DCJMTB device in the Commission internationale del'Eclairage
chromaticity chart are x = 0.63 and y = 0.36.
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