WO2008034210A2

WO2008034210A2 - Transparent external electrodes fluorescent lamp (teefl)

Info

Publication number: WO2008034210A2
Application number: PCT/BR2007/000255
Authority: WO
Inventors: Alaide Pellegrini Mammana; Daniel Den Engelsen
Original assignee: Alaide Pellegrini Mammana; Daniel Den Engelsen
Priority date: 2006-09-21
Filing date: 2007-09-20
Publication date: 2008-03-27
Also published as: WO2008034210A3; BRPI0604090A

Abstract

The invention refers to a fluorescent lamp with external transparent electrodes (2), characterized in that said electrodes (2) are made of conductive transparent material in the form of a thin film, which is deposited directly on the glass envelope (1 ) of the lamp or deposited on a transparent flexible substrate which is wrapped around the envelope (1 ) of the lamp, said material consisting preferably of tin oxide (SnO2), which may be deposited by a vapor decomposition process, using tin tetrachloride and methanol as reactants for the chemical deposition reaction and an inert carrier gas for transporting the vapors of these reagents.

Description

TRANSPARENT EXTERNAL ELECTRODES FLUORESCENT LAMP (TEEFL)

Field of the invention

The present invention refers to a new type of fluorescent lamp with external electrodes, usually called External Electrode Fluorescent Lamp (EEFL), which according to this invention has transparent external electrodes over the envelope of the lamp.

Description of the state of the art

Although fluorescent lamps are being applied in all kinds of illumination, an important application is in backlight units (BLUs) for Liquid Crystal Displays (LCDs), which are used in television sets, monitors, notebooks and in displays in general.

Fluorescent lamps are manufactured according to various technologies; however for backlight units the majority of lamps are Cold Cathode Fluorescent Lamps (CCFLs).

For notebooks and monitors, 1 up to 4 CCFLs are mounted at the border of the display and the light is coupled into a flat light guide positioned just behind the LCD panel realizing in this way a uniform luminance distribution. For TV-applications with large screens this arrangement is not appropriate, being necessary to use many lamps in parallel to each other to cover the whole area of the panel in order to get sufficient brightness and uniformity.

Before illuminating, CCFLs require a high voltage to start the discharge. Since the strike voltage varies from one lamp to another it is difficult to drive the lamps in a parallel arrangement with only one inverter, because when the first lamp has started, the voltage drops as the current in the lamp increases with the ionization of the gas (plasma) inside the lamp. Therefore, the voltage is not, anymore, enough to start the other lamps. For this reason CCFLs in backlight units need separate inverters for driving. This is not only a cost issue but it has also consequences for mounting and space requirements in backlight units for LC-TV as well as regarding the power consumption. A way to solve the problem just described is to apply fluorescent lamps with external electrodes. Those lamps, known as External Fluorescent Lamps (EEFLs)₁ also present some problems that need to be addressed. The applied voltage in EEFLs polarizes the internal glass surface, yields a surface charge and induces a flux of electrons that can initiate a gas discharge. The discharge is maintained by the current that is capacitively coupled into the lamp through the external electrodes and the glass as the dielectric material. This capacitive coupling permits that a plurality of parallel arranged EEFLs can be driven as one unit and needs therefore only one inverter, whereas CCFLs need individual inverters, as we have seen above.

Another advantage of EEFL is its good life performance as compared with CCFLs, because the external electrodes are not in contact with the plasma inside the lamp and thus these are not bombarded by the positive ions, which are largely responsible for limiting the lifetime of CCFLs. Because of the capacitive coupling, the current in an EEFL is limited by the reactance of this capacitor as well as by the series resistance of the electrodes. Said reactance can be lowered by increasing the frequency of the driving alternating voltage or by increasing the capacitance. As the frequency cannot be increased ad libitum, to avoid electro-magnetic interference, there is a need to increase the capacitance, mainly for long lamps to be applied in large format LC-TVs. This can be realized by using larger areas of the electrodes, since very thin glass envelopes are undesirable and significantly changing the dielectric constant of the glass is expensive. However, the external electrodes are usually made of opaque metals and increasing their area leads to more blocking of the light and hence it will decrease the light flux instead of increasing it.

Objectives of the invention

As explained before the first objective of the invention is the design a new type of external electrode enabling many parallel arranged EEFLs, to be driven simultaneously with one inverter without any loss of light and efficiency as presently restricted by the state of the art.

A second objective of the invention is the development of various types, formats and sizes of EEFLs without the restrictions of the present state of the art.

A third objective of the invention is enabling a wider field of applications including systems for general lighting. The final objective of the invention is enabling the development and construction of highly efficient backlight units for all types of LCDs. Abstract of the invention

The objectives described above are met through the present invention where said external electrodes are made of a conductive and transparent material and applied as a thin film, which is deposited directly on the glass envelope of the lamp.or deposited on a transparent flexible substrate which is wrapped around the envelope of the lamp

Another feature of the invention is that said transparent and conductive material consists of transparent and conductive oxides of metals and semi-metals or other transparent conductors such as thin films of metals, organic or polymeric films and may contain nanostructured materials to increase their electrical conductive without limiting their optical transmittance.

Another feature of the invention is that said transparent and conductive material can be deposited onto a foil or sheet of plastic or polymer coated with transparent and conductive oxides of metals and semi-metals, or other transparent conductors such as thin films of metals or polymers. This plastic foil is wrapped around the envelope of the lamp with said transparent and conductive coating in contact with the glass envelope, and tightly fixed to it.

Another feature of the invention refers to a preferred embodiment viz. the transparent and conductive material should be tin oxide.

According to another feature of the invention, the deposition of said electrode materials in the form of thin films can be done with techniques such as evaporation in vacuum (thermal or e-beam), sol-gel deposition, sputtering, spraying, chemical vapor deposition (CVD) or dipping.

According to another feature of the invention the preferred process of depositing transparent and conductive thin films on glass is the technology of Vapor Decomposition Process (VDP) in which the vapors of the reagents are produced by bubbling a carrier gas through their liquids.

According to another feature of the invention the preferred process of depositing transparent and conducting thin films on a plastic substrate is the technology of sputtering or vacuum evaporation or any other low-temperature process which the plastic can withstand.

According to another feature of the invention said transparent electrodes can be made in various formats and sizes.

According to another feature of the invention said transparent electrodes can be deposited in patterns through the use of masks to define the areas to be coated.

According to another feature of the invention said transparent thin film can be deposited on the whole outer surface of the lamp or in parts of it and this coating is subsequently patterned by means of mechanical abrasion or by selective dry or wet etching using photolithographic techniques in order to define the electrodes with shapes, dimensions and distribution that optimizes the operation of the lamp.

According to another feature of the invention said transparent thin film can be selectively deposited in areas of the outer surface of the lamp or of the polymeric foil of sheet to be wrapped around the envelope of the lamp or in parts of it to create electrodes with formats, dimensions and distributed in such a way that optimize the operation of the lamp.

According to the invention the transparent and conductive film deposited on the glass envelope or on the polymeric foil of sheet can be patterned with various configurations of electrodes by selectively depositing the film on certain regions by using shadow masks or by selectively removing certain regions and keeping others. This can be done by mechanical abrasion processes or by photolithographic processes, which can be wet chemical or dry processing as reactive plasma etching. In this case the image must be transferred to photosensitive thin films previously applied on the top of the Snθ2 films, by using shadow masks. The dimensions and architectures of the electrodes are essentially not limited and allow a plurality of electrode configurations, including those to selectively activate regions of the lamp through the use of special electrical circuitry that enables local and temporal driving. According to another feature of the invention said envelope of the lamp may have any shape such as a cylindrical, rectangular, rounded, oval, flat, twisted tube or another shape appropriate for the final application.

Thus, the invention refers to a new lamp type, which will be called TEEFL which stands for Transparent External Electrode Fluorescent Lamp. It can be produced in various shapes and sizes and may be used in many applications, substituting advantageously present-day fluorescent lamps. Description of the figures

Other objectives, features and advantages of the present invention will be evaluated in more depth through the description of the preferred embodiments of the invention, given as examples and not in a limitating sense, and in the following figures that refer to these embodiments, in which:

Figure 1 depicts a cylindrical lamp made according to the invention. The figures 1a, 1b and 1c show the transparent electrodes made in three different sizes. In order to facilitate visualization said electrodes are represented in the figures as opaque instead of transparent. Figure 2 shows an arrangement of parallel mounted lamps to be used in a backlight unit of a LCD television screen. The electrodes are not indicated in the figure.

Figure 3 shows a backlight that employs a single flat lamp mounted in the place of the parallel lamps in figure 2.

Figure 4 shows a cylindrical lamp, whose transparent electrodes represented in the figure as being opaque, are deposited in the form of long lines or tracks along the envelope of the lamp.

Figure 5 shows a cylindrical lamp with two intermediate electrodes in the form of rings.

Detailed description of the invention According to the objectives of the invention the external electrodes are thin films of transparent and conductive materials, which are deposited directly onto the external surface of the lamp. Therefore these electrodes can be made in any size and format without blocking the transmission of light. Since the electrodes are not in contact with the plasma, they are not suffering any sputtering degradation caused by the bombardment of the positive ions, which will increase the lifetime of the lamps. Furthermore, the lamps do not require metal parts and, consequently, neither any glass- metal joint, which implies that the manufacturing process of the lamps can be much simpler and cheaper. Also, the lifetime of the lamps is increased, since there is no metal-glass joints where leakage normally can occur. It is advantageous to use tin oxide (Snθ2) as the electrode material, which is a quasi-degenerate semiconductor in its polycrystalline and non-stoichiometric form, having a high electrical conductivity and high transmittance for visible light and being much cheaper than other transparent and conductive oxides, notably indium tin oxide (ITO), which is normally applied in the manufacturing of transparent electrodes in Flat Panel Displays such as LCDs, OLEDs and PDPs. Various technologies can be applied to get thin films of transparent and conductive materials, for instance: evaporation in vacuum (thermal or e-beam), sol-gel deposition, sputtering, spraying, chemical vapor deposition (CVD) or dipping.

The vapor decomposition process (VDP), which is used in this invention, is based on chemical reactions in a gas flow over a hot surface. The vapors are produced either by heating the corresponding liquid precursors or by forcing inert carrier gases through them. VDP is a variant of CVD, especially suitable for making thin films at low temperatures (<450°C) and therefore it is a simple and low cost process. For example, the good performance of the tin oxide films, obtained with this technology, is represented by the high conductivity (>103 Ω^"1 cm^"1), high transmittance for visible light, high uniformity, high chemical, mechanical and electrical stability, good adherence to different types of substrates, especially glass and quartz, which are conventionally used in lamps. The high chemical and mechanical resistance of these films enables their use as an external coating, capable to support the friction of mechanical holders and electrical contacts. These properties make tin oxide (Snθ2) very suitable to be applied as external electrodes in lamps.

Another advantage of tin oxide (Snθ2) thin films is that they can be doped with suitable elements, e.g. antimony (Sb) or fluorine (F), to increase their electrical conductivity without impairing the light transmission. An additional advantage of Snθ2 films is their ability of making good ohmic contacts with conventional metals, which are applied in lamp sockets.

The conductive and transparent films presents another advantage that is, after being deposited onto the whole surface of the lamp envelope, they can be patterned with various configurations of electrodes by removing certain regions and keeping others. This selective removal can be done by mechanical abrasion processes or by photolithographic processes, which can be wet chemical or dry processing such as reactive plasma etching. In this case the image must be transferred to photosensitive thin films previously applied on the top of the Snθ2 films, by using shadow masks. The dimensions and architectures of the electrodes are essentially not limited and allow a plurality of electrode configurations, including those to selectively activate regions of the lamp through the use of special electrical circuitry that enables local and temporal driving. The Vapor Decomposition Process (VDP) is realized on glass substrates or glass envelopes, which are heated by thermal conduction or thermal radiation. Various precursors for the chemical deposition process may be used and the required vapors may be produced in various ways such as heating the reagents, dragging by a carrier gas, spraying ultrasound or other means. In the case of Snθ2 films one of the options is to use tin tetrachloride (SnCI-J) and methanol (CH₃OH) as reagents and nitrogen as carrier gas. Ethanol or any other aliphatic alcohol may substitute methanol. The process comprises bubbling of the carrier gas in two separate flows through the liquid phases of said reagents, which are then mixed upon entering the deposition chamber.

In the case of using methanol the chemical reaction (not stoichiometric) can be simplified according to:

SnCI₄ (V) + CH₃OH (v) → SnO₂ (s) + HCI (g) + CO₂ (g) + H₂O (v)

The deposition conditions such as the temperature of the substrates, flux and concentration of the reagents and the time of deposition are defined by the required properties of the thin films. For example the following ranges of conditions could have been chosen:

Deposition time t: 2 < t < 20 minutes. Substrate temperature T: 300⁰C < T < 45O⁰C. SnCI₄ flux/CHsOH flux with ratios of 1:2, 1:1, 2:1

Total flux Φ: 250 seem < Φ < 750 seem (cubic centimeters per minute at standard temperature and pressure).

Doped SnO₂ films can easily be obtained by adding the desired dopants (in whatever composition) to the precursors. For example, films doped with antimony or fluorine can be obtained by respectively adding antimony tetrachloride or ammonium fluoride to the reaction precursors. Lamps having external electrodes may offer risk of electrical shocks to those that manipulate them, especially if the electrodes are transparent (invisible). In the invention described here, this problem can be easily solved covering said electrodes with a transparent insulating cover. This cover can be a case, a tube or a foil or sheet that can be made of various transparent and insulating materials. The sheet or foil may be added to the lamp or integrated into it to create a safe package, e.g. as a varnish or enamel, a polymer film or sheet, or a layer deposited onto the electrodes. These embodiments eliminate the risk of accidents without impairing the lumen flux of the lamps.

In a preferred embodiment of the invention said insulating foil or sheet is coated in its surface facing the envelope of the lamp with a transparent conducting film that acts as the external electrode. In this embodiment, it is not necessary to coat the glass envelope with said conducting film. Said coated foil or sheet is tightly wound around the glass envelope in order to avoid air spacing that would decrease the capacitance. This foil or sheet is applied over selected portions of said envelope i.e., it can completely cover the glass envelope or only parts of it, said parts being the regions in which the electric field is to be applied.

In this embodiment the transparent and conductive film may also be patterned with electrodes with a plurality of shapes, dimensions and distributions that can optimize the functioning of lamps of several geometries and dimensions.

The coating of the polymeric foil or sheet with the transparent and conductive film is done with any low temperature process that can be sustained by the polymeric material, such as, for example, vacuum evaporation, sputtering and screenprinting.

The electrical contact to the transparent electrodes is provided by depositing pads and stripes (or tracks) of conductive materials, preferably metals, that make ohmic junction with the transparent electrode. The thickness, format, dimensions and distribution of these pads and tracks must be defined according to the geometry of the lamp and such as to permit the application of conductors of sockets or any other kind of electrical connections that can provide the contact with external circuits.

In figure 1, which shows three examples of diagrams of a cylindrical lamp, it can be seen that the transparent electrodes have been deposited directly onto the glass envelope of the lamp. Said electrodes are connected with an inverter (not shown in the figure), which supplies the voltage for switching on the lamp. In figure 1a the electrodes

(2) have been deposited at the ends of the lamp envelope (1). Figure 1b represents a similar lamp where each transparent electrode (2) occupies approximately a quarter of the surface of the envelope (1). In figure 1c the electrodes (2) cover almost the complete external surface of the envelope (1). Since the electrodes (2) are transparent they do not affect the light flux of the lamp and can be made in any number, size or shape in order to optimize the lamp behavior. This architecture makes it easy to develop lamps of any shape for a wider field of application.

A group of lamps, placed in a parallel arrangement as shown in figure 2, may be driven by only one inverter (not indicated in the figure). Said mounting refers to a backlight unit (3) that is used to illuminate a LC-panel (4). A backlight unit with said lamp arrangement is especially advantageous in illuminating large LC-panels in order to get a uniform brightness distribution.

Another possible embodiment of the invention is indicated in figure 3 where only one flat lamp (6) is applied in a backlight unit. In this example, the transparent electrodes (2), which are indicated shaded in that figure, have been deposited onto the two opposite sides of said flat lamp (6). This configuration of the electrodes (2) is only an example since there are no restrictions for different designs and dimensions. By using this deposition technique to make the external electrodes (2), any configuration can be made to optimize the functioning of the lamp (6).

Another example of a possible configuration of the electrodes is represented in figure 4, which shows a lamp with a cylindrical envelope (1) having two opposite electrodes (2) as longitudinal stripes or tracks along the surface of said envelope (1).

Conventional fluorescent lamps may show, among other shortcomings, a tendency to develop regions with different luminance, so-called striations, which can move in the longitudinal direction of the tube. This effect is especially troublesome when these lamps are used in scanners and copiers. By placing transparent electrodes (2) along the length of the envelope (1), it is possible to make lamps with intermediate electrodes (7) as shown in figure 5. When said electrodes (7) are uniformly distributed along the tube (1) the striations can effectively be suppressed which means that with this technology it is possible to produce better lamps for scanners and copiers. The application of lamps made according to the technology described here has also advantages in respect to temperature stabilization and better resistance against mechanical shocks and vibrations in comparison with lamps made according to the conventional technology.

Although the invention has been described based on examples of practical embodiments, experts in the art of lamp making can introduce modifications within the basic concept of this invention. Thus, for example, it may be possible to make lamps in diverse sizes and shapes with different patterns of electrodes aiming various or specific applications, from general lighting to backlights for LCDs.

Claims

1. Fluorescent lamps with external electrodes, characterized in that said electrodes are made of transparent and conductive film, which is placed over the envelope of the lamp.

2. Fluorescent lamps with external electrodes, as claimed in claim 1, characterized in that said transparent and conductive film is deposited directly onto the glass envelope.

3. Fluorescent lamps as claimed in claim 1 , characterized in that said film comprises a transparent and conductive oxide.

4. Fluorescent lamps as claimed in claim 3, characterized in that said transparent and conductive oxide is a metal oxide.

5. Fluorescent lamps as claimed in claim 3, characterized in that said transparent and conductive oxide is a semi-metal oxide.

6. Fluorescent lamps as claimed in claims 3, 4 or 5, characterized in that said transparent and conductive oxide is a mixture of two or more transparent and conductive oxides.

7. Fluorescent lamps as claimed in claim 1 , characterized in that said transparent and conductive film comprises a transparent thin film of metal.

8. Fluorescent lamps as claimed in claim 1 , characterized in that said transparent and conductive material is a conductive organic material or polymer.

9. Fluorescent lamps as claimed in claim 4, characterized in that said material is a transparent and conductive tin oxide.

10. Fluorescent lamps as claimed in claim 2, characterized in that said deposition is done by evaporation in vacuum.

11. Fluorescent lamps as claimed in claim 2, characterized in that said deposition is done by a sol-gel technique

12. Fluorescent lamps as claimed in claim 2, characterized in that said deposition is done by a sputtering technique.

13. Fluorescent lamps as claimed in claim 2, characterized in that said deposition is done by spraying.

14. Fluorescent lamps as claimed in claim 2, characterized in that said deposition is done by chemical vapor deposition (CVD).

15. Fluorescent lamps as claimed in claim 2, characterized in that said deposition is done by chemical deposition in a solution (dip coating).

16. Fluorescent lamps as claimed in claim 2, characterized in that said deposition is performed by the Vapor Decomposition Process (VDP).

17. Fluorescent lamps as claimed in claim 16 characterized in that said process uses tin tetrachloride and methanol as precursors for the chemical deposition reaction.

18. Fluorescent lamps as claimed in claim 16, characterized in that said process uses tin tetrachloride and ethanol as precursors for the chemical deposition reaction.

19. Fluorescent lamps as claimed in claim 16, characterized in that said process uses tin tetrachloride and any aliphatic alcohol as precursors for the chemical deposition reaction.

20. Fluorescent lamps as claimed in claim 16, characterized in that said process uses any method to produce the vapors of the reactants which are used in the deposition of the tin oxide film.

21. Fluorescent lamps as claimed in claim 20, characterized in that said process uses a spray of the reagents used in the deposition of the transparent and conductive film.

22. Fluorescent lamps as claimed in claim 2, characterized in that said process uses any method of making a spray or vapors of the reagents used in depositing the tin oxide film

23. Fluorescent lamps as claimed in claim 2, characterized in that said electrodes are made in any number, size or shape.

24. Fluorescent lamps as claimed in claim 2, characterized in that said electrodes are coated with an insulating layer of any transparent material.

25. Fluorescent lamps as claimed in claim 1, characterized in that said transparent and conductive electrodes comprise a transparent and conductive thin film which is deposited onto a transparent, flexible and insulating foil that is tightly wrapped around selected portions of said envelope, said film facing said envelope.

26. Fluorescent lamps as claimed in claim 25, characterized in that said foil is a polymeric foil.

27. Fluorescent lamps as claimed in claim 26, characterized in that said thin film is deposited by any low-temperature process that can be withstood by the polymeric foil.

28. Fluorescent lamps as claimed in claim 27, characterized in that said deposition process comprises the technology of sputtering.

29. Fluorescent lamps as claimed in claim 27, characterized in that said deposition process comprises the technology of vacuum evaporation.

30. Fluorescent lamps as claimed in claim 25, characterized in that said transparent and conductive film is patterned to create electrodes with a plurality of shapes, sizes and distributions to optimize the operation of lamps of a plurality of shapes and dimensions.

31. Fluorescent lamps as claimed in claim 30, characterized in that said patterning is obtained by depositing the transparent and conductive film in selected regions through the use of shadow masks.

32. Fluorescent lamps as claimed in claim 30, characterized in that said patterning is obtained by depositing the transparent and conductive film by screenprinting in selected regions.

33. Fluorescent lamps as claimed in claim 30, characterized in that said patterning of the transparent and conductive film is done by photolithography with selective dry etching.

34. Fluorescent lamps as claimed in claim 30, characterized in that said patterning of the transparent and conductive film is done by photolithography with selective wet etching.