Field of invention:
The present invention relates to an alternating current plasma display panel (hereinafter referred to as AC PDP) and Its manufacturing method and more particularly related to a PDP cell structure which provides lower firing voltage, lower sustain voltage, lower power consumption and high brightness to improve luminous efficacy and hence applicable for manufacturing high picture quality PDP.
Background of the invention:
An AC PDP 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 also the surface between the barrier ribs and then the cell volume is filled with Neon and Xenon gas mixture. During PDP operation, the gas discharge takes place between electrodes in the cells that result in the generation of Vacuum Ultra Violet (VUV) photons with wavelengths of 147 nm and 173 nm. These phosphors absorb VUV photons (147 nm and 173 nm) and emit visible light to display a picture including characters and graphics. Such PDPs are self-emitting type fiat panel displays and have excellent characteristics such as large size, wide horizontal and vertical view-angle, slim look, lightweight etc. The drawback of the PDP is lower luminous efficacy in comparison to other display devices such as cathode ray tubes. At present several scientists are putting their efforts to improve the luminous efficacy of PDP especially by cell structure modifications. The conventional AC 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 Indium-Tin-Oxide (ITO) material. The bus electrodes of highly electrically conducting material are introduced over the ITO electrodes to decrease the sheet resistance of 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 working gas ions (Neon + Xenon). The back plate has formed therein a plurality of address electrodes that are orthogonal to the scan and sustain electrodes. A pair of sustain and scan electrodes along with an address electrode form a sub-pixel. A sub-pixel consists of either Red, Green or Blue color phosphor that make either Red, Green or Blue sub-pixel respectively. The 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 (sustain and scan electrodes) by square pulse voltage. The VUV radiation produced by the
discharge phenomenon excites phosphors and hence visible light is emitted. This type of conventional AC PDP is described in US patent no. 0183441A1- 2004.
The luminous efficacy 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 which 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 PDP cell structure for visible light transmission loss etc. It is seen from the electric field profile between sustain and scan electrodes of conventional PDP structure that a strongest applied electric field path is passed through the transparent dielectric (TD) in discharge gap where the working gases (Neon and Xenon) are not available to create co-planar discharge at lower voltage. The cell structure modification is required so that the strongest electric field applied to the gas is useful to create co-planar discharge. In this case, the requirement of firing voltage or sustain voltage is reduced even for getting higher amount of visible light output from the cell as most of the visible light transmission loss occurs through TD in case of conventional PDP cell structure. This modification reduces the discharge power and enhances luminance and hence improves the luminous efficacy. In this invention the inventor proposes an AC PDP cell structure where there is a very small amount of TD material which is applied to protect the edges of the ITO display electrodes keeping most of the portion of discharge gap material free and hence utilization of the strongest applied electric field path, by the initial charge particles, produced by the sharp edges of the ITO electrodes in the discharge gap. Since this cell structure provides more area for coating of protective and secondary electron emissive layer than the conventional cell structure hence more emission of secondary electrons and therefore, less sustain voltage. This helps in improving discharge characteristics of plasma display panel.
Object of the invention:
It has already been proposed that luminous efficacy of the conventional AC driven PDP is low because of low brightness and high panel power consumption. The requirements of high sustain voltage approximately 200 volts is a considerable contributor for high power consumption resulting in low luminous efficacy. The sustain voltage is reduced by the utilization of strongest electric field (shortest path between sharp edges of ITO display electrodes) by initially available charge particles in the discharge gap with removal of most of the TD material.
The front plate structure in the conventional PDP is provided with sustain and scan electrodes made of ITO (Indium-tin-oxide) material. The removal of significant amount of TD material from discharge gap decreases the panel capacitance and increases the surface area for coating of secondary electron
emissive layer by creating extra vertical surface area of TD, which helps in lowering the panel firing voltage or sustain voltage and also improve panel discharge characteristics.
The principal object of the present invention is to provide a novel cell structure for PDP which operates at low sustain voltage with high luminance.
Yet another object of the present invention is to increase the brightness in terms of almost material free discharge gap and hence prevents visible light transmission loss.
Yet another object of the present invention is to improve the luminous efficacy of the AC PDP.
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) are formed over said reflective dielectric layer (8) and layers of Red (10a), Green (10b) and 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 ITO electrodes (11) arranged in the first direction, a space between ITO display electrodes is almost material free for utilization of the strongest applied electric field, by the initial charge particles, produced by the sharp edges of the ITO electrodes in the discharge gap and is provided for preventing visible light transmission loss, said ITO and electrically conducting electrodes (12) are covered with transparent electrically insulating layers (13a and 13b) and an electron emissive layer (14) is formed over the transparent electrically insulating layers (13a and 13b).
Brief description of Drawings:
Figure 1(a) illustrates the cross-sectional view of conventional AC PDP with straight barrier ribs
Figure 1(b) shows the top view of the conventional AC PDP structure.
Figure 2(a) illustrates the schematic view of the AC PDP structure according to
the present invention.
Figure 2(b) shows the top view of the AC PDP structure according to the present invention.
Figure 3(a) shows the cross sectional view of the front glass substrate with patterned ITO electrode according to the present invention.
Figure 3(b) shows the cross sectional view of the patterned ITO and electrically conducting electrodes over front glass substrate according to the present invention.
Figure 3(c) shows the cross sectional view of patterned ITO and electrically conducting electrodes and also transparent dielectric layer 1 (13a) over front glass substrate according to the present invention.
Figure 3(d) shows the cross sectional view of patterned ITO, electrically conducting electrodes, transparent dielectric layer 1 (13a) and layer 2 (13b) over front glass substrate according to the present invention.
Figure 3(e) shows the cross sectional view of the deposited electron emissive layer (14) and also almost material free channel (5) according to the present invention.
Detailed description of the invention :
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 on 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 on the transparent dielectric layer (5). On the back glass substrate (2), a plurality of address electrodes (7) are formed. 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) and 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 display electrodes which are made of transparent ITO display electrode (11). A plurality of sustain (X) and scan (Y) opaque electrically conducting bus electrodes arranged in the first direction on the surface of transparent ITO electrode (11) to increase the conductivity of the ITO electrodes. The width of the electrically conducting electrodes (12) is in the range of 60 - 120 \xm and height is in the range of 5 - 15 |.im. The width of the ITO display electrodes (11) is in the range of 250 - 350 \xrr\ and height is in the range of 150 - 250 nm. The space between inside edges (or discharge gap) of ITO electrodes (11) is 60 - 150 |jm. The front glass plate (1) having display electrodes (11 and 12) are covered with a transparent dielectric layers (13a and 13b) having height in the range of 20 - 60 jam. The first transparent dielectric layer (13a) having height in the range of 10 - 30 |am and is patterned by removing transparent dielectric materials to make almost material free channels of width in the range of 50-100 ^tm in the first direction to provide almost material free discharge gap. The second transparent dielectric layer (13b) having height in the range of 10 - 30 |am is printed on the first layer in such a way which further increases the material free channel width within the range of 450 - 530 ^m .
The patterning of dielectric layers(13a and13b) are done for utilization of the strongest electric field path produced by the sharp edges of ITO display electrode in the discharge gap by the initially available charge particles. These almost material free channels provide more volume for working gas (Neon and Xenon) and hence more VUV photons providing more visible light conversion. These material free channels provide more visible light transmission as discharge gap is almost free of transparent dielectric material and hence more brightness which helps in improving luminous efficacy. As compare to conventional AC PDP, in present invention material free channels provide more surface area by creating steps, removing transparent dielectric providing large area for emissive layer coating resulting in more secondary electron emission thereby reducing firing voltage, sustaining voltage or power consumption. The discharge gap or material free channels (5) can be increased by decreasing ITO electrode width which leads to more transmittance of visible light. As firing voltage is already reduced considerably low as compare to conventional structure and hence the total working gas pressure and partial pressure of Xenon can be increased for higher brightness.
The display electrodes are covered with electrically insulating layers (13a and 13b) to limit the discharge current and then the electron emissive layer (14) is
deposited on the electrically insulating layers (13a and 13b) to prevent erosion of display electrode by Xenon and Neon ion bonnbardment. 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. 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) and B (10c) phosphor layers are formed in the barrier rib (9) channel spaces. To create the VUV light through gas discharge, Neon + Xenon gas mixture is enclosed between the front (1) and back glass substrate (2).
Figure 2(b) shows the top view of the PDP structure according to the present invention. This clarifies the orientation of plurality of display electrodes (11) and plurality of address electrodes (7) placed orthogonal to the display electrodes in the pixels (10a, 10b and 10c). 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. During 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. 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 PDP can be increased by increasing the number of sustain pulses.
According to the present invention the X-Y capacitance of the present electrode structure is also low because of almost material free discharge gap in comparison to the conventional AC PDP having full of TD in the discharge gap (see figure 1(a)). Low X-Y capacitance leads to the low displacement current that results to low capacitive loss. By keeping almost material free discharge gap between the display electrodes on the front glass substrate (1), the electric field intensity in the discharge gap gets significantly enhanced 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 edges of the ITO electrodes (11) along with co-planar discharge.
In figure as shown 3(a) - 3(e), the process details for new cell structure development according to the present invention are given. Initially a thin layer of ITO having thickness 150 - 200 nm is coated on the entire front glass substrate
(1) by electron beam evaporation or sputtehng process. This thin layer is patterned through photolithography process for the formation of ITO display electrode (11) (see Figure 3(a)).
Figure 3(b) shows the cross sectional view of the patterned ITO (11) and electrically conducting electrodes (12) over front glass substrate according to the present invention. The electrically conducting electrodes(12) are formed by screen-printing process.
Figure 3(c) shows the cross sectional view of patterned ITO (11) and electrically conducting electrodes (12) and also dielectric layer 1 (13a) over front glass substrate according to the present invention. The dielectric layer 1 (13a) (having height 10-30 pm) is formed initially by screen printing process and then dielectric layer 1 is removed from the discharge gap followed by sand blasting process. The height or depth of the material free channel in discharge gap is 10 -30 pm.
Figure 3(d) shows the cross sectional view of patterned ITO, electrically conducting electrodes, transparent electrically insulating layer 1 (13a) and layer 2 (13b) on front glass substrate. The patterned transparent electrically insulating layer 2 (13b) (having height 10-30 pm) is formed over transparent electrically insulating layer 1 (13a) by screen printing process.
Figure 3(e) shows the cross-sectional view of electron emissive layer (14) having height 500 - 1000 nm and is deposited on transparent electrically insulating layer (13a and 13b). 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 fodel 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 ribs. Further a thick layer (100 - 160 pm) of barrier rib material (9) is coated over the dielectric layer (8) using screen-printing process. The straight channel barrier ribs (9) are made by patterning the barrier rib layers (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 and B) phosphors (10a, 10b and 10c) are deposited by screen-printing process on the inner walls of the barrier ribs 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 - 20 pm. 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 back glass substrate (2). The Xenon + Neon 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 PDP where sustain and scan electrodes are kept separately with almost transparent dielectric material free in between and these electrodes are provided on front glass substrate. The present invention helps in creating plasma at low voltage as there are charge particles initially available in the highest electric field path of the material free discharge gap whereas this is not the case of conventional AC PDP. This invention provides faster avalanche process to create plasma than the conventional structure as the charge particles utilizes the highest electric field produced by the sharp edges of display ITO electrodes and hence strong force(acceleration) is experienced by the charge particles for the initial discharge to start up the cascading avalanche process. The present structure is able to provide improved luminous efficacy by reducing the power consumption with low sustain voltage and high brightness with preventing visible light transmission loss through almost material free discharge gap. This structure can be designed for different PDP resolutions such as SD, XGA and HD, etc with the optimization of cell dimensions.
To make this PDP 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.
Claims:

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) are formed over said reflective dielectric layer (8) and layers of Red (10a), Green (10b) and Blue (10c) phosphors are formed in said barrier rib (9) channel spaces
Wherein,
the front glass substrate (1) having a plurality of highly electrically conducting electrodes printed on sustain (X) and scan (Y) display ITO electrodes (11) produced in the first direction, a space between ITO display electrodes (11) is almost free of TD matehal and is provided for high visible light transmission (brightness) and utilization of the strongest electric field produced by the sharp edges of ITO display electrodes in the discharge gap by the initially available charge particles to reduce panel firing or sustaining voltage, to reduce panel capacitive loss and said display electrodes are covered with electrically insulating layers (13a and 13b) and an electron emissive layer (14) is formed on the electrically insulating layers (13a and 13b), on additional vertical area created by almost material free discharge gap which provides improved discharge characteristics of panel as it provides additional secondary electrons and also on material free channel (5) which includes channel (5a) and channel (5b).
2) The PDP as claimed in claim 1, wherein, a main sustain discharge is initiated face-to-face between two sharp edges of said ITO display electrodes (11) along with co-planar discharge.
3) The PDP as claimed in claim 1, wherein, the width of the display electrodes (11) is in the range of 250 - 350 µm.
4) The PDP as claimed in claim 1, wherein, the space between two edges of sustain (X) and scan (Y) ITO electrodes (11) is 60 - 150 pm.
5) The PDP as claimed in claim 1, wherein, the thickness of electrically insulating layer (13a + 13b) covering said display electrodes (11 and 12) can be varied within the range of 20 - 60 µm .
6) A Process for manufacturing a PDP 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 (9) over the reflective dielectric layer (8) in second direction.
d) depositing three color (R, G and B) phosphors (10a,10b and 10c) on the inner walls of the barrier ribs (9) in second direction.
e) patterned ITO display electrodes (11) are made on the front glass substrate (1),
f) printing an electrically conductive electrode (12) over the patterned ITO and forming a plurality of pair of sustain and scan display electrode (11) in the first direction orthogonal to the address electrodes (7),
g) printing electrically insulating layers (13a and 13b) over both the electrically conductive electrodes (12) and ITO display electrodes (11) that are formed in the first direction excluding the discharge gap.
h) forming an electron emissive layer (14) having the height in the range of 500 -1000 nm on the said electrically insulating layers (13a and 13b) of the front plate (1),
i) joining the front (1) and back glass (2) substrates using frit seal material and a Neon + Xenon gas mixture is filled in the discharge spaces formed by the barrier ribs between the front and back glass substrates to develop an AC PDP.
7) The process as claimed in claim 6, wherein, the plurality of ITO electrodes having width in the range of 250 - 350 pm and the height in the range of 150 -250 nm.
8) The process as claimed in claim 6, wherein, the thickness of electrically insulating layer (13a) covering said display electrodes (11 and 12) can be varied within the range of 10 - 30 pm.
9) The process as claimed in claim 6, wherein, the thickness of transparent electrically insulating layer (13b) covering said display electrodes (11 and 12) can be varied within the range of 10 - 30 µm.
10) The process as claimed in claim 6, wherein, the width of the material free channel 1(5a) in discharge gap is in the range of 50 - 100 µm.
11) The process as claimed in claim 6, wherein, the width of the material free channel 2 (5b) in discharge gap is in the range of 450 - 530 pm.
12) The process as claimed in claim 6, wherein, the width of the discharge gap between two ITO electrodes is in the range of 60-150 µm.