The ITO glass plate has a sheet resistance of 30 ohm sq−1.
The thin polymer film was spin-coated from a chloroform
solution onto the ITO glass with a thickness of ca. 100 nm.
The surface profile was monitored using a Talysurf machine.
On top of the polymer layer, an aluminium electrode was
vacuum evaporated at a pressure of <8×10−6 mbar and had
a final thickness of 200 nm. Large emitting areas in the range
of 10–20 mm2 were used in the study.
Current, voltage and relative light intensity were measured
using a Keithley 236 source measure unit combined with an
amplified silicon photodiode and a digital multimeter. The
measurements of current, voltage and light were performed
simultaneously and fully controlled through a computer. The
light detection was carried out by placing the photodetector
as close as possible to the top of the electroluminescent
electrode. Forward voltage or current pulses with a 100 ms
device has not been predictable. Apart from the nature of each
of the materials, the interaction between the different compo-
nents can play an important role as well.
In our further investigation of the photoluminescent proper-
ties of the PAQ4 doped PVK matrices, we found that the
interaction between PVK and PAQ4 is strong. PAQ4 was able
to quench PVK photoluminescence effectively and transfer
energy from PVK to PAQ4. From Fig. 3, it can be seen that
PVK photoluminescence was completely quenched at a con-
centration of about 10 wt% of PAQ4 dopant. At the same
time, the photoluminescent emission wavelengths from PAQ4
shifted continuously from 438 to 464 nm as the concentration
increased from 10−5 to 12 wt% (Fig. 3). These results suggest
that at low concentration, PAQ4 and PVK interaction was
dominant. The photoluminescence of PAQ4 was maintained
around 440 nm. At higher concentration, dopant–dopant inter-
action may occur. The emission wavelength shifted significantly
as the dopant concentration increased. In the comparison of
the two curves in Fig. 3, it was realised that the photoluminesc-
ent intensities from PVK started to decrease around the
concentration level at which the dopant–dopant interaction
occurred. Therefore, the energy transfer which occurred in the
photoluminescence from this doped matrix was a process of
the transfer of the excitation energy in PVK to the new species
formed through the dopant–dopant interaction, but not to
that species formed through the dopant–polymer interaction.
In the electroluminescence case, the 442 nm centred emission
for the PAQ4 device implies a difference between the photo-
luminescent and electroluminescent excitation mechanisms.
However, it is reasonable to suggest that this emission centre
may be dominated by the species formed through the PAQ4
and PVK interaction, i.e. the dopant–polymer interaction,
rather than the dopant–dopant interaction. The explanation
of why this happens in PAQ4, but not in PAP1, is not yet
known. This will be the subject of our further investigations
of these materials.
‘
on’ time and a 500 ms ‘off ’ time were used unless otherwise
specified. Fig. 2 shows an example of the current–voltage and
light–voltage characteristics from the PAQ4 device, in which
the light output is almost linear in relation to the current
density. The corresponding electroluminescent spectra were
taken using a constant dc current drive. The device structure
has not been optimised and the accurate efficiency measure-
ment of these devices has not yet been carried out. An
estimation of device efficiency for these initial devices was
made according to the responsivity of the photodetector, which
has an external quantum efficiency at a level of 0.1%.
Interesting results were obtained from a comparison of the
electroluminescent spectra and the photoluminescent spectra
(
Fig. 1). It was found that the emission peak maximum of the
electroluminescence from the PAQ4 device is shifted ca. 20 nm
to the blue in comparison with its photoluminescence, while
the electroluminescence spectrum fell within the same spectral
range as the photoluminescent spectrum for the PAP1 device.
This implies that the EL and PL may originate from a different
recombination process in the PAQ4 device and from the same
recombination process in the PAP1 device.
The mechanism of the operation of an organic electrolumi-
nescent device is not always straightforward. It is even more
complicated in a multiple component system, such as polymer
blends or polymers doped with one or more low molecular
weight dyes. In these systems, the interaction between the
different components cannot be ignored. Exciplex formation
and energy transfer are also involved. Some typical devices
have been reported. For example, exciplex formation is the
principal electroluminescence obtained from 2,5-bis(5-tert-
butyl-2-benzoxazolyl)thiophene with an aromatic amine in a
multilayer type device9 and energy transfer is the principal
Summary
We have demonstrated blue electroluminescence from the
novel materials 1,3-diphenyl-4-methylpyrazolo[3,4-b]quino-
line (PAQ4) and 4-phenyl-1,3,5,7-tetramethyl-1,7-dihydrodipy-
razolo-[3,4-b; 4∞,3∞-e]pyridine (PAP1). These materials have
been incorporated into a simple device: a single layer consisting
of a poly(N-vinylcarbazole) host, which was previously doped
with either PAQ4 or PAP1. ITO and aluminium were used as
hole and electron injection electrodes, respectively. The electro-
luminescent spectra were found to have an emission maximum
in the blue spectral range, with a peak wavelength of 442 nm
(showing a 20 nm shift from the photoluminescent spectrum)
electroluminescence in
a 1,1,4,4-tetraphenylbuta-1,3-diene
doped PVK multilayer device.10 However, up to the present
time, the type of emission produced by an electroluminescent
Fig. 3 A comparison of (2) the concentration dependence of PL
emission wavelength at maximum peak intensity from PAQ4 and
($) PVK PL intensities as a function of the PAQ4 concentration of
the PAQ4 doped PVK matrices
Fig. 2 Light and current density dependence from an EL device
consisting of PAQ4 doped PVK layer
2
324
J. Mater. Chem., 1997, 7(12), 2323–2325
He
