U.S. patent application number 09/754181 was filed with the patent office on 2001-07-12 for active matrix electroluminescent display device.
This patent application is currently assigned to U.S. Philips Corporation. Invention is credited to Battersby, Stephen J..
Application Number | 20010007413 09/754181 |
Document ID | / |
Family ID | 9883305 |
Filed Date | 2001-07-12 |
United States Patent
Application |
20010007413 |
Kind Code |
A1 |
Battersby, Stephen J. |
July 12, 2001 |
Active matrix electroluminescent display device
Abstract
An active matrix electroluminescent display device (2) comprises
an array of display pixels (4) arranged in rows (6) and columns
(8). Each pixel (4) comprises a current-driven electroluminescent
display element (10). Each row (6) of pixels (4) shares a common
line (20), currents through the display elements (10) of a row (6)
of pixels passing along the common line (20). The width of each
common line (20) tapers from one end (20a) to the other (20b), the
common line (20) being coupled to row driver circuitry at the wider
end (20a). The resistance of the common lines is thus reduced at
the end of the common line near to the row driver circuitry, where
the current flowing is derived from all pixels within the row. The
invention thereby reduces voltage drops along the common line,
which contribute to cross-talk between pixels within a row.
Inventors: |
Battersby, Stephen J.;
(Haywards Heath, GB) |
Correspondence
Address: |
Jack E. Haken
Corporate Patent Counsel
U.S. Philips Corporation
580 White Plains Road
Tarrytown
NY
10591
US
|
Assignee: |
U.S. Philips Corporation
|
Family ID: |
9883305 |
Appl. No.: |
09/754181 |
Filed: |
January 3, 2001 |
Current U.S.
Class: |
315/169.3 |
Current CPC
Class: |
G09G 2320/0223 20130101;
G09G 3/3233 20130101; G09G 2320/0209 20130101; H01L 27/3279
20130101; G09G 2300/0842 20130101 |
Class at
Publication: |
315/169.3 |
International
Class: |
G09G 003/10 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 7, 2000 |
GB |
0000290.7 |
Claims
1. An active matrix electroluminescent display device comprising an
array of display pixels arranged in rows and columns, each pixel
comprising an electroluminescent display element and a switching
means for controlling the current through the display element based
on a signal voltage applied to the pixel, each row of pixels
sharing a common line, currents through the display elements of a
row of pixels passing along the common line, wherein the width of
each common line tapers from one end to the other, the common line
being coupled to row driver circuitry at the wider end.
2. A display device as claimed in claim 1, wherein the common lines
of alternate rows taper in opposite directions.
3. A display device as claimed in claim 2, wherein first and second
row driver circuits are provided at opposite ends of the rows of
pixels.
4. A display device as claimed in any preceding claim, wherein the
common lines are arranged over a substrate, and the display pixels
are arranged over the common lines, and wherein the display pixels
emit light away from the substrate.
Description
[0001] This invention relates to active matrix electroluminescent
display devices, comprising an array of electroluminescent display
pixels arranged in rows and columns. The invention is particularly
concerned with display devices in which rows of pixels share a
common line, with currents through the display elements of the row
passing along the common line.
[0002] Matrix display devices employing electroluminescent,
light-emitting, display elements are well known. The display
elements may comprise organic thin film electroluminescent
elements, for example using polymer materials, or else light
emitting diodes (LEDs) using traditional III-V semiconductor
compounds. Recent developments in organic electroluminescent
materials, particularly polymer materials, have demonstrated their
ability to be used practically for video display devices. These
materials typically comprise one or more layers of a semiconducting
conjugated polymer sandwiched between a pair of electrodes, one of
which is transparent and the other of which is of a material
suitable for injecting holes or electrons into the polymer layer.
An example of such is described in an article by D. Braun and A. J.
Heeger in Applied Physics Letters 58(18) p.p. 1982-1984 (May 6,
1991).
[0003] The polymer material can be fabricated using a CVD process,
or simply by a spin coating technique using a solution of a soluble
conjugated polymer.
[0004] Organic electroluminescent materials exhibit diode-like I-V
properties, so that they are capable of providing both a display
function and a switching function, and can therefore be used in
passive type displays.
[0005] However, the invention is concerned with active matrix
display devices, with each pixel comprising a display element and a
switching device for controlling the current through the display
elements. Examples of an active matrix electroluminescent display
are described in EP-A-0653741 and U.S. Pat. No. 5,670,792, the
contents of which are incorporated herein by way of reference
material.
[0006] A problem with display devices of this type arises from the
fact that they have current driven display elements. Display
devices of the type with which this invention is concerned include
a common line on which the currents from all pixels in a row pass.
Compounding currents from the pixels in a row give rise to
different voltages along the common line. These voltages depend
upon the currents through all pixels in the row, since these
currents all pass to the common line. These different voltages give
rise to undesired changes to the outputs from the display pixels,
which vary as a function of the full set of signals applied to the
row. Consequently, there is cross-talk between the pixels within
the row.
[0007] According to the invention, there is provided an active
matrix electroluminescent display device comprising an array of
display pixels arranged in rows and columns, each pixel comprising
an electroluminescent display element and a switching means for
controlling the current through the display element based on a
signal voltage applied to the pixel, each row of pixels sharing a
common line, currents through the display elements of a row of
pixels passing along the common line, wherein the width of each
common line tapers from one end to the other, the common line being
coupled to row driver circuitry at the wider end.
[0008] By making the common lines wedge-shaped, the resistance of
the common line can be reduced dramatically at the end of the
common line near to the row driver circuitry. It is at this end of
the common line that the current flowing is derived from all pixels
within the row, and that voltage drops are therefore highest.
[0009] Preferably, the common lines of alternate rows taper in
opposite directions, thereby defining an interleaved pattern of
wedge shaped common lines. In this case, first and second row
driver circuits may be provided at opposite ends of the rows of
pixels.
[0010] Preferably, the common lines are arranged over a substrate,
and the display pixels are arranged over the common lines, and
wherein the display pixels emit light away from the substrate. The
tapered shape of the common lines does not then result in any
visual artifacts in the display image, and the pixels may be
arranged as a regular array of equal size pixels.
[0011] The invention will now be described by way of example, with
reference to and as shown in the accompanying drawings, in
which:
[0012] FIG. 1 shows part of an electroluminescent active matrix
display device to which the invention may be applied;
[0013] FIG. 2 schematically illustrates the current flowing in a
row of electroluminescent display pixels, to illustrate the
cross-talk resulting from the common signal line; and
[0014] FIG. 3 shows a display device according to the
invention.
[0015] FIG. 1 shows a known pixel configuration for an
electroluminescent active matrix display device. Various types of
electroluminescent display devices are known, which utilise
current-controlled electroluminescent or light emitting diode
display elements. One example of the construction of such a display
is described in detail in U.S. Pat. No. 5,670,792.
[0016] As shown schematically in FIG. 1, a display device 2
comprises an array of pixels 4 arranged in rows 6 and columns 8.
Each pixel 4 comprises a display element 10 and a switching element
12 in a form of a thin film transistor, which controls the
operation of the display element 10 based on a signal voltage
applied to the pixel 4. As one example, the display element 10
comprises an organic light emitting diode comprising a pair of
electrodes between which one or more active layers of organic
electroluminescent material is sandwiched. At least one of the
electrodes is formed of a transparent material such as ITO. Various
electroluminescent materials are available, for example as
described in EP-A-0717446.
[0017] The signal voltage for a pixel is supplied via a control
signal line 14 which is shared between a respective column 8 of
pixels. The control signal line 14 is coupled to the gate of the
switching transistor 12 through an address transistor 16. The gates
for the address transistors 16 of a row 6 of pixels are coupled
together to a common address line 18.
[0018] Each row 6 of pixels 4 also shares a common voltage supply
line 20 usually provided as a continuous common electrode covering
all pixels, and a common line 22. The display element 10 and the
switching element 12 are arranged in series between the voltage
supply line 20 and the common signal line 22, which acts as a
current drain for the current flowing through the display element
10, as represented by arrows 24. The current flowing through the
display element 10 is controlled by the switching element 12 and is
a function of the gate voltage on the transistor 12, which is
dependent upon the control signals supplied to the control signal
line 14.
[0019] A row of pixels is selected by applying a selection pulse to
the address line 18 which switches on the address transistors 16
for the respective row of pixels. A voltage level derived from the
video information is applied to the control signal line 14 and is
transferred by the address transistor 16 to the gate of the
switching transistor 12. During the periods when a row of pixels is
not being addressed by the address line 18, the address transistor
16 is turned off, but the voltage on the gate of the switching
transistor 12 is maintained by a pixel storage capacitor 26 which
is connected between the gate of the switching transistor 12 and
the common line 22. The voltage between the gate of the switching
transistor 12 and the common line 22 determines the current passing
through the display element 10 of the pixel 4. Thus, the current
flowing through the display element is a function of the
gate-source voltage of the switching transistor 12 (the source of
the transistor 12 being connected to the common line 22, and the
drain of the transistor 12 being connected to the display element
10). This current in turn controls the light output of the
pixel.
[0020] The switching transistor 12 is arranged to operate in
saturation, so that the gate-source voltage governs the current
flowing through the transistor, irrespectively of the drain-source
voltage. Consequently, slight variations of the drain voltage do
not affect the current flowing through the display element 10. The
voltage on the voltage supply line 20 is therefore not critical to
the correct operation of the pixels. However, voltage fluctuations
on the common line 22, which couples together the sources of the
switching transistors 12, will influence the current flowing
through the display element 10 for a given control voltage on the
control signal line 14.
[0021] A problem therefore arises that the resistance of the common
line 22 gives rise to voltage drops along that line, which voltage
drops are a function of the currents supplied to the line from the
individual pixels 10. The voltages on the common signal line 22 at
the location of different pixels will depend in a complex manner on
the currents passed by all of the pixels in the row. The
gate-source voltage of the switching transistor 12 will depend upon
the voltage on the common signal line 22 at the location of that
pixel, so that these voltage variations will affect the brightness
of the pixels. The result is non-uniformity and horizontal
cross-talk of the picture information shown on the display. This is
explained further with reference to FIG. 2.
[0022] FIG. 2 shows the common line 22 with the currents i.sub.1,
i.sub.2, . . . i.sub.n associated with the pixels shown. These
currents are the currents flowing through the pixels. A current
summation occurs at each pixel location, as shown, and the voltage
drop along each section of the common line 22 between adjacent
pixels is a function of the current flowing in that section.
[0023] FIG. 3 shows a display device according to the invention.
Each row 6 of pixels 4 has a corresponding common line 20 along
which the currents through the display elements of the row 6 of
pixels pass. The width of each common line 20 tapers from one end
20a to the other 20b. The common line 20 of each row is coupled to
row driver circuitry at the wider end 20a.
[0024] The common lines 20 are referred to hereinafter as row
conductors. The tapered row conductor arrangement of the invention
reduces the resistance at the wider end of the row conductor. As
the row conductor connects to the row driver circuitry at the wider
end, this is where the highest current flows, as the row driver
circuitry supplies and drains the drive current using the two
common lines 20 and 22 (of FIG. 1).
[0025] In FIG. 3, the row conductors of alternate rows taper in
opposite directions. This requires first and second row driver
circuits to be provided at opposite ends of the rows of pixels,
because the row driver circuitry is connected to the wider end 20a
of the row conductors 20.
[0026] The alternating taper also allows the row conductors to
occupy a large proportion of the area of the substrate. It is
necessary to ensure that only the desired connection is made to the
row conductors, and for this purpose a contact point 30 is
indicated schematically in FIG. 3 between each pixel 4 and the row
conductor 20. As will be appreciated by those skilled in the art,
the contact points may be implemented as an array of wells etched
into an insulator layer which overlies the conductor layer defining
the row conductors 20.
[0027] The common lines may thus be arranged over a common
substrate, beneath the components of the display pixels. The
display pixels may then be arranged to emit light away from the
substrate. This ensures that the tapered row conductor arrangement
does not casue any distortion of the displayed image. It is,
however, equally possible to provide transparent row conductors and
to arrange the display pixels to emit light through the
substrate.
[0028] The use of large row conductors requires additional layers
to those conventionally used, if the pixel components are to
overlap the row conductors. However, the invention will also
provide a benefit for a relatively small taper angle, for example
giving the row conductor a width at the wider end of double the
width at the narrow end. This may be implemented with no
significant modification to the layers and processing steps used
for conventional pixel arrangements, and without requiring any or
any significant reduction in the area of the electroluminesent
display element.
[0029] Each pixel 4 is represented as a schematic box in FIG. 3. It
will be appreciated that each pixel 4 includes the same components
as described with reference to FIG. 1. For clarity, the other
control lines 18, 22 have not been represented in FIG. 3, but they
will equally be required for the pixel arrangement of FIG. 1. Of
course, numerous other pixel configurations will be apparent to
those skilled in the art. The row conductor arrangement of the
invention will benefit any pixel arrangement in which each row of
pixels shares a common signal line which carries the currents from
all pixels in the row.
[0030] Details of the precise layout and processing requirements
for producing a display device using electroluminescent pixels have
not been described. The invention may be implemented with minor
modification to the known processes, and any modifications required
to an existing pixel configuration in order to put the invention
into practice will be apparent to those skilled in the art.
* * * * *