Electroluminescent device with silazane-containing luminescent zone

DETAILED DESCRIPTION Although recent performance improvements in organic EL devices have suggested a potential for widespread use, most practical applications require limited voltage input or light output variance over an extended period of time.
Consequently, the limited thermal stability of materials within many prior art devices has remained a deterrent to widespread use.
Device degradation results in obtaining progressively lower current densities when a constant voltage is applied.
Lower current densities in turn result in lower levels of light output.
With a constant applied voltage, practical EL device use terminates when light emission levels drop below acceptable levels--e.
g.
, readily visually detectable emission levels in ambient lighting.
If the applied voltage is progressively increased to hold light emission levels constant, the field across the EL device is correspondingly increased.
Eventually a voltage level is required that cannot be conveniently supplied by the EL device driving circuitry, or which produces a field gradient (volts/cm) exceeding the dielectric breakdown strength of the layers separating the electrodes, resulting in a catastrophic failure of the EL device.
It has been discovered quite surprisingly that stability and sustained operating performance of the organic EL device of Van Slyke et al, cited above, can be markedly improved by forming the hole transporting zone of the organic luminescent medium to contain a silazane hole transporting agent.
In one aspect this invention is directed to an electroluminescent device comprising in sequence, an anode, an organic hole injecting and transporting zone, an electron injecting and transporting zone, and a cathode, characterized in that the organic hole transporting zone is comprised of a layer containing a hole transporting silazane in contact with the anode and interposed between the hole injecting layer and the electron injecting and transporting zone.
When organic EL devices according to this invention are constructed with cathodes formed of a plurality of metals other than alkali metals, at least one of the metals having a work function of less than 4 eV, as taught by Tang et al (I), cited above, further advantages are realized