High contrast electroluminescent display panels

A d.c. or a.c. electroluminescent panel comprises a transparent substrate, a transparent first electrode film, a thin film phosphor layer, a control layer and a second electrode film. A black or dark colored material, less than 1 micron thick, is interposed between the thin film phosphor layer and the control layer to enhance the contrast of the panel whenever a voltage is applied across the thin film phosphor layer causing it to emit light.

BACKGROUND OF THE INVENTION 
This invention relates to electroluminescent (EL) phosphor panels and 
displays designed for unidirectional voltage operation, known as DCEL 
panels. 
DESCRIPTION OF THE PRIOR ART 
Thick film powder DCEL panels (which are also capable of ACEL operation) 
are conventionally manufactured by a process comprising the steps of: 
(a) depositing a transparent front electrode film e.g. of tin oxide, onto a 
transparent insulating substrate, e.g. glass; 
(b) spreading an active layer, comprising phosphor particles, such as zinc 
sulphide (ZnS) doped with an activator such as manganese (Mn) and coated 
with copper suspended in a binder medium, on the front electrode; this 
layer is typically 10-50 .mu.m thick (hence `thick film` device); 
(c) depositing a back electrode film, e.g. of aluminium on the active 
layer; 
(d) applying a unidirectional voltage to the electrode films for a 
predetermined time, so that in the region of the positively biased front 
electrode the copper coating is stripped from phosphor particles to form a 
high resistivity, high light output layer, typically 1-2 .mu.m thick. The 
relatively thick layer of unstripped phosphor particles then remaining 
behind this thin light-emitting layer constitutes a highly conductive 
control layer. 
The last step, (d) in the manufacturing process, is known as `forming` and 
is more particularly described in U.K. Pat. No. 1,300,548. The electrodes 
can of course be laid down in any desired shape to produce a particular 
display, e.g. if the electrodes comprise mutually perpendicular strips a 
matrix of active phosphor elements, or `dots` will be defined each of 
which may be addressed and driven using conventional electronic techniques 
to form alphanumeric characters. Having such a process we have designed 
and built a 2000 character DCEL panel suitable for use with a computer as 
a monitor display and replacing the conventional bulky cathode ray tube 
monitor display. 
A disadvantage of the all powder panels is that the display elements can 
presently only produce a light output which, whilst acceptable in all but 
the highest ambient light conditions, is difficult to maintain throughout 
the life of the display. Moreover, since the quiesent colour of the 
phosphor material is a very light shade of grey such high light output 
levels are required to provide an adequate display contrast. 
The powder panels described above are known as `self-healing`, i.e. the 
copper-coated powder backlayer, the control layer, protects the thin, high 
resistance, light-emitting `formed` layer from catastrophic breakdown due 
to excessive current density at defects or points of weakness by further 
copper stripping or `forming` at such `hot spots`. 
To ensure a more reproducible manufacturing technique, not requiring the 
expensive and time-consuming forming operations, a composite thin film 
power electroluminecent panel has been proposed (see `A Composite ZnS Thin 
Film Powder Electroluminescent Panel` C. J. Alder et al, Displays, January 
1980, at page 191). Such panels are in effect a hybrid structure in which 
a thin film, equivalent to the light-emitting formed layer in conventional 
DCEL panels, is coated with the copper coated phosphor backlayer, i.e. 
control layer. The thin film is of semi-insulating activator-doped 
phosphor, such as ZnS doped with Mn, and is typically 200 A to 1 .mu.m 
thick. This light-emitting film is deposited onto the transparent front 
electrode of the panel by sputtering, evaporation, electrophoretic plating 
or any of the known ways of depositing thin films on substrates. The 
conventional control layer and the back electrode are spread and 
vacuum-deposited onto the light-emitting film in the known manner. The 
control layer need not contain Mn since the light emitted by the device 
originates from the thin film. U.S. Pat. No. 4,137,481 describes such a 
hybrid panel which may or may not require the application of a forming 
current before it is ready for use. If a forming current is required, 
forming is found to occur at much lower current densities than those 
required for conventional thick film DCEL panels. 
The hybrid DCEL panel is protected by the control layer from catastrophic 
breakdown due to excessive current density at defects and points of 
weakness by retaining its forming properties in the same way as the thick 
film powder only DCEL panels. However the known hybrid panels using 
conventional control layers still suffer from the effects of further 
forming during extensive use leading to brightness degradation with time. 
Again, the contrast provided by such known hybrid devices is poor. 
It is an object of the present invention to provide a thin film powder 
composite DCEL (hybrid) panel with improved brightness maintenance during 
its operational lifetime and providing siginficant contrast enhancement. 
SUMMARY OF THE INVENTION 
According to the present invention, an electroluminescent d.c. panel 
includes in serial order, a transparent electrically insulating substrate, 
a transparent first electrode film, a first thin film layer of 
semi-insulating self-activated or activator-doped phosphor, a second layer 
of black or dark coloured, electrically resistive material and a third 
layer comprising an electrically conducting powder control layer, the said 
second layer having an average thickness of less than 1 micron and 
effective to provide contrast enhancement and to allow injection of 
electrons thereacross from said powder control layer into said first thin 
film layer. 
The third or control layer is preferably elected from the group consisting 
of transition metal oxides, transition metal sulfides, rare earth metal 
oxides and rare earth metal sulfides. 
The second layer, hereinafter referred to as the thin film interlayer, may 
be for example ZnTe (dark brown), CdTe (black), CdSE (black/brown), a 
Chalcogenide glass (black), or Sb.sub.2 S.sub.3 (black/brown), or any 
other suitable dark material, e.g. a compound of a transition metal or of 
a rare earth metal, e.g. an oxide sulfide or other Chalcogenide, for 
example PbS, PbO, CuO, MnO.sub.2, Tb.sub.4 O.sub.7, Eu.sub.2 O.sub.3, 
PrO.sub.2 or Ce.sub.2 S.sub.3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, the panel, indicated by reference numeral 1 includes a 
transparent tin oxide or indium tin oxide electrode 2 laid for example, by 
sputtering, on part of the upper surface of a glass substrate 3. The 
electrode 2 can be etched to any desired shape or pattern depending on the 
type of display required; for example the display required may be a dot 
matrix display in which case the electrode 2 will take the form of a 
plurality of parallel strips of width and spacing determined by the 
desired `dot` (pixel) size. 
A semi-insulating thin film 4 of self-activated or activator-doped 
phosphor, not more than 5 microns thick, is deposited on the electrode 2. 
The film for example may be ZnS activated with Mn in which case the 
display will exhibit a yellow colour in operation. Alternative colours may 
be effected by using activators other than Mn in ZnS, and other lattices 
with Mn and activators such as rare earth metals. For example, other 
phosphor lattices which may also be used are the alkaline earth sulphides 
e.g. BaS, CaS, SrS, fluorides such as LaF.sub.3 and YF.sub.3, oxides such 
as Y.sub.2 O.sub.3 or any other suitable phosphor. 
A black thin film interlayer 5, not more than 1 micron thick, is deposited 
on the thin film light emitting layer 4. The interlayer 5 may be for 
example ZnTe (dark brown), CdTe (black), CdSe (black/brown), a 
Chalcogenide glass (black), or Sb.sub.2 S.sub.3 (black/brown), or any 
other suitable dark material. The interlayer 5 enables the combination of 
contrast enhancement and the current-controlling properties associated 
with a control layer 6. 
The control layer 6 is a conventional layer of copper coated activated 
phosphor powder suspended in a binder medium, e.g. ZnS:Mn. It could also 
however, be a non-activated, copper coated phosphor powder so suspended; 
but preferably it is a high contrast layer of the type described in the 
aforesaid co-pending application. The control layer 6 is deposited on the 
interlayer 5. 
An aluminum electrode is deposited, for example, by evaporation, onto the 
control layer 6. This electrode can be mechanically scribed to provide a 
shape corresponding or related to the electrode 2 to form the desired 
display pattern, for example, if a dot matrix display is required the 
electrode 7 will take the form of a plurality of parallel strips mutually 
perpendicular to the strips of electrode 2 so that the `intersection` of 
the two sets of strips define the display pixels. If the control layer 6 
is conductive, the electrode 7 can be omitted and means can then be 
provided for supplying electrical power direct to the control layer. 
In operation, a DC or AC voltage typically between 20 and 200 V is applied 
across the electrodes 7 and 2. 
Electrode 2 can be either positively or negatively biased. Light is emitted 
from the thin film 4 in a pattern determined by the electrode shape. The 
contrast between the light-emitting regions of the thin film 4 and the 
non-light-emitting regions is enhanced by the black interlayer 5 so that 
the display may be read by an observer even in relatively high ambient 
light conditions and with `display brightness` of only a few foot 
lamberts, typically 4-8 fL. The presence of the black interlayer 5 may 
reduce brightness and efficiency, but this is more than compensate for by 
the improved contrast ratio. 
For Chalcogenide glass, however, as the interlayer, practical levels of 
brightness of over 80 fL with efficiencies of 0.01-0.02% W/W have been 
achieved. Contrast ratios of 14:1 have been reported for 50 fL brightness, 
in ambient light conditions of 100 fL. 
The panel shown in FIG. 2 is identical to that shown in FIG. 1 (and like 
reference numbers have been used to indicated like parts) with the 
exception that the rear electrodes 7 have been omitted and the powder 
layer 6 has been formed into discrete ridges 8 separated by furrows or 
grooves 9. An electrical connection (not shown) is made to each of the 
ridges 8 of the powder layer 5. 
The embodiment shown in FIG. 2 is intended for multiplex addressing on an 
X-Y matrix and so transparent electrode film 2 is formed in strips running 
perpendicular to (or intersecting) the furrows 9. 
The black interlayer 5 may be conductive, semi-conductive or insulating and 
if conductive, the grooves 9 should of course extend through the 
interlayer. The same may be true if the interlayer is semi-conductive but 
this depends on the conductivity of the layer concerned. 
The interlayer 5 may be made of a material that has self-healing 
properties, i.e. a material that changes conductivity in response to 
applied voltage, but this is not necessary since the control powder layer 
6 can act through the interlayer 5 to provide these properties. In this 
case, the interlayer must be sufficiently thin to allow the control layer 
6 to act through the interlayer but not so thin as to mean that the 
interlayer loses its dark colour.