Field of invention:
The present invention relates to a plasma display panel (hereinafter referred to as POP) and its manufacturing method and more particularly related to a POP cell structure capable of operating at lower sustain voltage, lower power consumption to give improved luminous efficacy and applicable for making high picture quality flat panel displays.
Background of the invention:
POP is composed of a matrix of discharge cells that are formed by partitions made by barrier ribs between a pair of glass substrates. Three colors (Red, Blue and Green) phosphors are provided on the surface of the respective barrier ribs, and the cell volume is filled with a gas mixture (Ne+Xe). During POP operation, the gas discharge takes place between electrodes in the cells that result in the generation of Vacuum Ultra Violet (VUV) photons with a wavelength of 147 nm and 173 nm. These phosphors absorb these VUV photons and emit visible light to display a picture including characters and graphics. Such PDPs are self-emitting type flat panel displays and have excellent characteristics such as large size, wide horizontal and vertical view-angle, slim look, lightweight etc. The drawback of the POP is lower luminous efficacy in comparison to other display devices such as cathode ray tubes. At present various efforts are on to improve the luminous efficacy of PDP especially by cell structure modifications.
The conventional AC (Alternating Current) driven PDP possesses three-electrode structure on two glass plates forming front plate and back plate. The front plate has formed therein a plurality of pairs of display electrodes known as sustain and scan electrodes. These electrodes are formed of ITO (Indium-Tin-Oxide) material. The ITO electrode sheet resistance is decreased with the introduction of metal bus lines of electrically conducting material over the ITO electrodes. The display electrodes are covered with a transparent dielectric layer to limit the discharge current. A thin electron emissive layer is formed over the transparent dielectric layer to emit secondary electrons and to protect the transparent dielectric layer from sputtering by ion bombardment. The back plate has formed therein a plurality of address electrodes that are orthogonal to the display electrodes. A pair of sustain and scan electrodes along with an address electrode form a sub-pixel. A sub-pixel comprises of Red, Green or Blue color phosphor that make Red, Green or Blue sub-pixel respectively. A combination of Red, Green and Blue sub-pixels forms a pixel. The straight channel barrier ribs are formed on the back plate to create the discharge volume and also to separate different sub-pixels. The main co-planar discharge is created with the display electrodes by square pulse voltage. The VUV radiation produced in the discharge phenomenon excites the phosphor to emit visible light. This type of conventional AC PDP is described in US Patent no. 5661500.
The luminous efficacy i.e. visible light output per applied electrical power of PDP is significantly low in comparison to other display device such as Cathode Ray
Tube (CRT). The low luminous efficacy is caused by various power losses at different stages of POP operation. These losses include ion heating loss, radiation loss for generation of VUV photons, the VUV photons not directed to phosphor layer, conversion loss VUV to visible light and transparency of POP cell structure for visible light transmission etc. It is seen that from the electric field profile between sustain and scan electrodes of conventional POP structure that a significant amount of applied electric field is passed through the front glass plate. This leads to reduction of electric field applied through the Ne-Xe gas mixture to create co-planar discharge during sustain period. The cell structure modification is required so that the amount of electric field applied to the gas is increased. In this case, the requirement of sustain voltage is reduced for getting similar light output from the cell. This modification reduces the discharge power and subsequently improves the luminous efficacy. In this invention the inventors propose a POP cell structure where sustain and scan electrodes are kept far from front glass plate and these electrodes are provided over the barrier ribs made on front glass plate.
Object of the invention:
It has already been proposed that luminous efficacy of the conventional AC driven POP is low because of low brightness and high panel power consumption. The requirements of high sustain voltage ~ 200 volts is a considerable contributor for high power consumption and hence the luminous efficacy. The sustain voltage can be reduced if the voltage drop at the front glass plate is prevented. Due to small cell volume the discharge efficiency is low. The short discharge gap giving short length of positive column in the produced glow discharge is also responsible for low discharge efficiency or VUV output.
The front plate structure in the conventional POP is provided with sustain and scan electrodes made of ITO (Indium-tin-oxide) material. There is always possibility for the occurrence of defects such as ITO leakage and electrode shorting during development process. The removal of ITO material is required to reduce the panel capacitance for improved luminous efficacy and also to solve the pixel defects caused by ITO.
The principal object of the present invention is to provide a novel cell structure for POP which can operate at low sustain voltage.
Yet another object of the present invention is to increase the brightness by increasing the cell volume and creating face-to-face discharge along with co-planer discharge.
Yet another object of the present invention is to improve the luminous efficacy of the AC POP.
Statement of the invention:
Accordingly the invention provides a Plasma Display Panel comprising a pair of front and back glass substrates, and gas enclosed between the substrates; said back glass substrate having a plurality of address electrode (7) in second direction orthogonal to the first direction covered with a reflective dielectric layer (8), a plurality of straight channel barrier ribs (9) of second type are formed over said reflective dielectric layer (8) and layers of Red (10a), Green (10b), Blue (10c) phosphors are formed in said barrier rib (9) channel spaces Wherein, front glass substrate (1) having a plurality of sustain (X) and scan (Y) display electrodes arranged in the first direction over a plurality of straight channel barrier ribs of first type (11) formed in the first direction, a space between display electrodes or the barrier ribs of first type is provided for visible light transmission, said display electrodes are covered with an electrically insulating layer (13) and an electron emissive layer (14) is formed over the electrically insulating layer (13).
Brief description of Drawings:
Figure 1(a) illustrates the cross-sectional view of conventional AC POP with straight barrier ribs
Figure 1 (b) shows the top view of the conventional AC POP structure.
Figure 2(a) illustrates the schematic view of the AC POP structure according to the present invention.
Figure 2(b) shows the top view of the AC POP structure according to the present invention.
Figure 3(a) shows the cross sectional view of the front glass substrate with formed layer of dielectric according to the present invention.
Figure 3(b) shows the cross sectional view of the patterned dielectric layer forming barrier ribs over front glass substrate according to the present invention.
Figure 3(c) shows the cross sectional view of the electrically conducting layer formed over the front plate barrier ribs according to the present invention.
Figure 3(d) shows the cross sectional view of the dielectric covering over the electrically conducting layer formed over the front plate barrier ribs according to the present invention.
Figure 3(e) shows the cross sectional view of the deposited electron emissive layer according to the present invention.
Detailed description of the invention with reference to drawings:
Before starting the detailed description of the present invention, it is necessary to discuss the conventional AC PDP for clear understanding of the present invention. Figure 1(a) illustrates the cross-sectional view of conventional AC PDP with straight barrier ribs and figure 1(b) shows the Red (R), Green (G) and Blue (B) pixel arrangement.
As in figure 1(a), the front glass substrate (1) and back glass substrate (2) are shown. In the front glass substrate (1), display electrodes are made of transparent ITO sheet (3). To reduce the resistance of the display electrode, opaque electrically conducting bus electrodes (4) are made over the ITO electrodes. The display electrode is covered with a transparent dielectric layer (5) to limit the discharge current. Then the electron emissive layer (6) is deposited over the transparent dielectric layer (5). On the back glass substrate (2), a plurality of address electrodes (7) are formed with one address electrode (7) is formed in each sub-pixel. The address electrodes (7) are covered with a dielectric layer (8) to limit the discharge current and for light reflection. The straight channel barrier ribs (9) are formed over the dielectric layer (8). The R (10a), G (10b), B (10c) phosphor layers are formed in the barrier rib (9) channel spaces. Figure 1(b) shows the top view of the conventional AC PDP structure
Figure 2(a) illustrates the schematic view of new AC PDP structure according to the present invention. This AC PDP comprising of front (1) and back (2) glass substrates. The front glass substrate (1) is used to visualize the picture elements. The front glass substrate (1) is provided with a plurality of sustain (X) and scan (Y) display electrodes arranged in the first direction. The display electrodes X and Y (12) exist over the straight channel barrier ribs of first type (11) formed using dielectric material at the front glass substrate (1) in the first direction. The barrier ribs (11) may also be of box shape. The thin layer of Red, Green and Blue phosphor can also be provided in this box shape ribs of first type (11) to increase the visible light. The width of the barrier ribs of first type (11) is in the range of 40-300 micron and the height is in the range of 20-300 micron. The display electrodes (12) are made of electrically conducting material at the top of barrier ribs of first type. The width of the display electrodes is in the range of 30-290 micron and height is in the range of 5-100 micron. The width of the display electrode can be equal to width of the barrier ribs of first type (11). The space between the barrier ribs of the first type having sustain (X) and scan (Y) electrode is 45-300um. The aim of forming display electrodes over the barrier ribs is to keep them at far distance from the front glass substrate (1) to minimize the voltage drop across front glass substrate (1) during PDP operation. The display electrodes are covered with an electrically insulating layer (13) to limit the discharge current and to prevent erosion of display electrode by ion bombardment. The thickness of the electrically insulating layer (13) can be varied with in range of 10um to 50um and can be made of dark color to enhance the bright room contrast ratio. Then the electron emissive layer (14) is deposited over
the electrically insulating layer (13). The back plate comprising a plurality of address electrode (7) made of electrically conducting material in second direction that is orthogonal to the first direction. On the back glass substrate (2), a plurality of address electrodes (7) are formed with one address electrode (7) is formed in each sub-pixel. The address electrodes (7) are covered with a dielectric layer (8) to limit the discharge current and for light reflection. The straight channel barrier ribs (9) of second type are formed over the dielectric layer (8). The R (10a), G (10b), B (10c) phosphor layers are formed in the barrier rib (9) channel spaces. To create the VUV light through gas discharge, Ne+Xe gas mixture is enclosed between the front (1) and back glass substrate (2).
Figure 2(b) shows the top view of the PDF structure according to the present invention. This clarifies the orientation of plurality of display electrodes (12) and plurality of address electrodes (7) placed orthogonal to the display electrodes in the pixels (11,12,13). The top of barrier rib walls that differentiates the R, G, and B sub-pixels is also visualized.
The plasma display panel is driven by applying a scan pulse for line selection to the scan electrode in a specific order while applying an address pulse respectively to the address electrode according to display data in synchronization with the application of scan pulse. By the addressing, a proper wall charge is prepared in each cell on a line basis. Further strong discharge is created between X and Y electrodes of the addressed cell by applying sustain pulses of proper amplitude and frequency during the sustain period utilizing already present wall charges of address period, no discharge or illumination is created at un-addressed cells. This VUV radiation generated by sustain discharge is used to excite R, G, and B phosphors to emit visible light. The output luminance of the POP can be increased by increasing the number of sustain pulses.
According to the present invention the display electrodes are made of electrically conducting material having much higher thickness in comparison to conventional structure (see figure 1(a)). Hence the electrode resistance R is reduced by 50-80% in comparison to conventional structure. The low resistance is favorable for reduced resistive heating (H=i2R), where H is the resistive heating and i is the current passing through the display electrode. Also, the voltage drop (i.R) across the display electrode will be low to improve its current carrying capability. The X-Y capacitance of the present electrode structure is also low because of reduced electrode area and large sustain gap in comparison to the conventional AC POP having ITO electrodes (see figure 1(a)). Low X-Y capacitance leads to the low displacement current that results to low capacitive loss. By keeping the display electrodes far from front glass substrate (1), the electric field intensity at front glass substrate region is significantly reduced and the requirement of sustain voltage is reduced to get similar sustain discharge as in conventional structure. The main sustain discharge is initiated face-to-face between two display electrodes (12) along with co-planar discharge that forms long path positive discharge column. The discharge remains floating in the discharge space and does not diffuse towards the front glass substrate to prevent VUV loss. The long positive column in the discharge is possible because of long discharge space.
Due to large positive column or short negative glow region, the major part of the applied power is utilized to produce VUV production by electron excitation. The discharge space is large equal to the separation between two barrier rib walls having sustain (X) and scan (Y) electrodes respectively. Using the proposed cell structure the long positive column discharge is possible even with the use of low sustain voltage. With positive column discharge, the applied voltage or power is efficiently used to produce VUV photons that after phosphor excitation generate visible light. Therefore present invented structure leads to improved luminous efficacy by 10% to 60% as compared to the conventional structure.
In figure as shown 3(a)-3(e), the process details for new cell structure development according to the present invention are given. At first a thick solid layer (11) (20 - 300 micron) of electrically insulating material is printed over the entire front glass substrate (1) by screen-printing process (see figure 3(a)). This layer (11) may be made of transparent dielectric material.
Figure 3(b) shows the cross-sectional view of barrier ribs of first type (11) made at the front glass substrate (1) according to the present invention. The barrier ribs (11) are formed by patterning the dielectric layer (11) using sand blasting process. The screen-printing process may be used for making the barrier ribs of first type.
Figure 3(c) shows the cross-sectional view of the display electrodes (12) formed over the barrier ribs of first type. The display electrodes are formed by thick film photo process (Fodal process). The height of the display electrode (12) is in the range of 5-100 micron. The width of the display electrodes is less than or equal to the width of the barrier ribs of first type.
Figure 3(d) shows the cross-sectional view of the transparent electrically insulating layer (13) formed over the display electrodes. This transparent electrically insulating layer is formed by screen-printing process. Other processes like sand blasting or lamination can also be used to form insulating layer (13). The thickness of layer (13) is in the range of 10-50 micron.
Figure 3(e) shows the cross-sectional view of electron emissive layer (14) deposited over transparent electrically insulating layer (13). The processes like electron beam evaporation method or sputtering method is used to deposit the electron emissive layer (14). All the layers formed during the above processes can be provided by lamination method also.
The processes for making back substrate (not shown) structure is discussed. The address electrodes (7) made of electrically conducting material is printed on the back glass substrate (2) using screen-printing or fodal process. A layer of dielectric material (8) is also printed over the address electrodes (7) for insulation and light reflection. The dielectric layer is also used to prevent address electrode deformation by absorbing the solvent from pastes for the address electrodes and the barrier rib. Further a thick layer (20 - 300 micron) of Barrier rib material (9) is coated over the dielectric layer (8) using screen-printing process. The straight
channel barrier ribs of second type (9) are made by patterning the barrier rib layer (9) by sand blasting or screen printing process printed over dielectric layer (8). The barrier rib bottom width may be more than 10% to 50% of the width of the barrier rib top. The three color (R, G, B) phosphors (10a, 10b, 10c) are deposited by screen-printing process on the inner walls of the second type barrier rib in the paste form so as to fill nearly 10% to 100% volume of the cavity and is deposited on the inner walls of the barrier rib cavity forming a layer of thickness 5-50 urn. The phosphor paste is fired to form a layer. In this process the volume of the phosphor paste is constant as it is determined by the volume of the cavity formed by barrier ribs. The composition of the phosphors is chosen such that on simultaneous excitation of the phosphors by the VUV gives white color. All the layers formed during the above processes can be provided by lamination method also.
The front and back glass substrates are joined using frit seal material (not shown) applied on front glass substrate (1). The Xe+Ne gas mixture is filled in the discharge spaces formed by barrier ribs between the front and back glass substrates.
The present invention provides an AC POP where sustain and scan electrodes are kept far from front glass substrate and these electrodes are provided over the barrier ribs made on front glass substrate. The present structure is able to provide improved luminous efficacy by reducing the power consumption with low sustain voltage. The present structure is cost effective because the ITO material and its processing are removed. This structure can be designed for different POP resolutions such as SD, XGA, and HD etc with the optimization of cell dimensions.
To make this POP functional, it is provided with the drive control system (not shown) consists of an analog-to-digital converter, a frame memory, a scan control portion, an X-electrode driving circuit, a Y-electrode driving circuit and an A-electrode driving circuit. The drive control system is electrically connected to the X, Y and A-electrodes via a flexible printed circuit board (not shown).
Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in limiting sense. Various modifications of the disclosed embodiments, as well as alternate embodiments of the invention, will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore contemplated that such modifications can be made without departing from the spirit or scope of the present invention as defined.

We Claim:
1) A Plasma Display Panel comprising
a pair of front(1) and back glass substrates(2), enclosing gas between the substrates; said back glass substrate(2) having a plurality of address electrode (7) in second direction orthogonal to the first direction covered with a reflective dielectric layer (8), a plurality of straight channel barrier ribs (9) of second type are formed over said reflective dielectric layer (8) and layers of Red (10a), Green (10b), Blue (10c) phosphors are formed in said barrier rib (9) channel spaces
Wherein,
the front glass substrate (1) having a plurality of sustain (X) and scan (Y) display electrodes(12) arranged in the first direction over a plurality of straight channel barrier ribs of first type (11) formed in the first direction, a space between display electrodes or the barrier ribs of first type(11) is provided for visible light transmission, said display electrodes are covered with an electrically insulating layer (13) and an electron emissive layer (14) is formed over the electrically insulating layer (13).
2) The POP as claimed in claim 1, wherein, the plurality of barrier ribs of first
type (11) having width in the range of 40-300 urn and height in the range of
20-300 urn as plurality of sustain (X) and scan (Y) display electrodes (12)
arranged over the ribs in the first direction .
3) The POP as claimed in claim 1, wherein, a main sustain discharge is initiated
face-to-face between two said display electrodes (12) along with co-planar
discharge.
4) The POP as claimed in claim 1, wherein, the width of the display electrodes
(12) is in the range of 30 -290 urn.
5) The POP as claimed in claim 1, wherein, the space between the barrier ribs
of the first type (11) having sustain (X) and scan (Y) electrode (12) is 45-
300um.
6) The POP as claimed in claim 1, wherein, the thickness of electrically
insulating layer (13) covering said display electrodes(12) can be varied with
in range of 10um to 50um .
7) A Process for manufacturing a POP comprising the steps of:
a) making a plurality of address electrodes(7) by patterning an electrically conductive material on a back glass substrate(2) in second direction,
b) forming a layer of reflective dielectric material(8) over the plurality address
electrodes,
c) making a plurality of barrier ribs of second type (9) over the reflective
dielectric layer(8),
d) depositing three color (R, G, B)(10a,10b,10c) phosphors on the inner walls
of the barrier ribs of second (9) type in second direction,
e) making a plurality of barrier ribs of first (11) over a front glass substrate(1),
f) printing an electrically conductive paste over the barrier ribs of first type(11)
and forming a plurality of pair of sustain and scan display electrode(12) in
the first direction orthogonal to the address electrodes(7),
g) forming an electrically insulating layer(13) over the electrically conductive
electrodes that are formed over the said barrier ribs of first type,
h) forming an electron emissive layer(14) on the front plate(1) over the said electrically insulating layer,
i) Joining the front (1) and back glass (2) substrates using frit seal material and a Ne+Xe gas mixture is filled in the discharge spaces formed by the barrier ribs between the front and back glass substrates to develop an AC POP.
8) The process as claimed in claim 7, wherein , the plurality of barrier ribs of
first type is having width in the range of 40-300 urn and the height in the
range of 20-300 Mm.
9) The process as claimed in claim 7, wherein , the thickness of electrically
insulating layer (13) covering said display electrodes can be varied with in
range of 10um to 50um .
10) A Plasma Display Panel and a process for manufacturing POP substantially
herein described with reference to the accompanying drawings.