Abstract:
A net negative charge is used to eliminate silver migration in an EL lamp. An electrode containing silver, either the entire electrode, a bus bar on the electrode, or a contact area on the electrode, receives a net negative charge while the EL lamp is being driven with alternating current. The net negative charge can be provided by the driver itself or by a current source, such as a battery or a charged capacitor; or by asymmetric current means in the current path of the EL lamp, including the lamp itself.

Description:
FIELD OF THE INVENTION 
     This invention relates to a lighting system including a thick film electroluminescent lamp and, in particular, to an EL lamp driven without causing silver migration. 
     GLOSSARY 
     A bridge is a circuit having four arms with two pairs of arms connected in parallel between a first pair of terminals. In each pair of arms, the arms are connected in series. The junctions of the arms in each series pair are a second pair of terminals. With unidirectional current elements in the arms and alternate arms conducting simultaneously, a bridge has a DC diagonal across one pair of terminals and an AC diagonal across the second pair of terminals. 
     “Asymmetric conducting means” is any electrical device or circuit that conducts approximately the same current in both directions but conducts more current in one direction than in the opposite direction. 
     “Thick film” refers to one type of EL lamp and “thin film” refers to another type of EL lamp. The terms only broadly relate to actual thickness and actually identify distinct disciplines. In general, thin film EL lamps are made by vacuum deposition of the various layers, usually on a glass substrate or on a preceding layer. Thick film EL lamps are generally made by depositing layers of inks on a substrate, e.g. by roll coating, spraying, or various printing techniques. The techniques for depositing ink are not exclusive, although the several lamp layers are typically deposited in the same manner, e.g. by screen printing. A thin, thick film EL lamp is not a contradiction in terms and such a lamp is considerably thicker than a thin film EL lamp. 
     An EL “panel” is a single sheet including one or more luminous areas, wherein each luminous area is an EL “lamp.” An EL lamp is essentially a capacitor having a dielectric layer between two conductive electrodes, one of which is transparent. The dielectric layer can include phosphor particles or there can be a separate layer of phosphor particles adjacent the dielectric layer. The phosphor particles radiate light in the presence of a strong electric field, using relatively little current. 
     BACKGROUND OF THE INVENTION 
     Current, flexible, thick film EL lamps are typically formed on a release layer, such described in U.S. Pat. N 0 . 5,856,030 (Burrows). A first insulating layer overlies the release layer and is covered by a first electrode. A phosphor layer overlies the first electrode and a dielectric layer overlies the phosphor layer. A second electrode is deposited over the dielectric layer. A second insulator overlies the second electrode. The electrodes are a conductive polymer, such as PEDOT/PSS (Poly-3,4-ethylenedioxythiophene/ polystyrenesulfonic acid) sold under the tradename “Orgacon™” by Agfa-Gevaert N.V. The conductive polymer can be translucent or opaque, depending upon composition or additives. 
     Bus bars, located at least along one edge of the lit area, are used to improve the conductivity of the electrodes. Typically, the bus bars are screen printed from an ink containing resin and conductive particles, such as carbon or silver. Silver particles are desirable because they provide good conductivity and reflect light. 
     Silver particles contribute to a problem known as “silver migration”, a dendritic growth of silver oxide crystals into the lit area of the lamp, causing black spots and electrical short circuits. The problem has long been known in the art and has been associated with DC bias on a lamp; e.g. see U.S. Pat. No. 4,143,297 (Fischer). The Fischer patent proposes a symmetrical structure to avoid “one-sided space charge conditions”, i.e., DC bias. More typically, barrier layers have been proposed to minimize silver migration. For example, see U.S. Pat. No. 5,697,322 (Hay et al.), U.S. Pat. No. 5,811,930 (Krafcik et al.), U.S. Pat. No. 6,586,873 (Mizutani et al.), and U.S. Pat. No. 6,965,196 (Murasko et al.). 
     In portable electronic devices, automotive displays, and other applications where the power source is a low voltage battery, an EL lamp is powered by a driver that converts low voltage direct current into high voltage alternating current. In order for an EL lamp to glow sufficiently, a peak-to-peak voltage in excess of about one hundred and twenty volts is necessary. The actual voltage depends on the construction of the lamp and, in particular, the field strength within the phosphor powder. 
     U.S. Pat. No. 4,210,848 (Suzuki et al.) discloses a driver having a switching bridge output. The bridge acts as a double pole double throw switch to alternate current through the EL lamp at low frequency. U.S. Pat. No. 5,313,141 (Kimball) discloses a driver that produces AC voltage without a bridge. A plurality of drivers are commercially available in integrated circuit form using either technology. 
     Unless components are carefully matched, and they are not because it is too expensive, there is usually a slight asymmetry in the waveform applied to an EL lamp. The slight asymmetry causes a DC bias to accumulate over time and several techniques have been proposed for preventing the accumulation of charge or for more thoroughly discharging a lamp, thereby reducing DC bias. Unless the DC bias is removed completely, or silver is not used, the problem of silver migration exists in the prior art. There are also test results suggesting that moisture and field intensity contribute to silver migration. 
     In view of the foregoing, it is therefore an object of the invention to provide an apparatus and a method for eliminating silver migration in EL lamps without eliminating silver. 
     A further object of the invention is to eliminate silver migration regardless of moisture or field intensity. 
     Another object of the invention is to simplify the construction of EL lamps, and reduce costs, by eliminating barrier layers for reducing silver migration. 
     SUMMARY OF THE INVENTION 
     The foregoing objects are achieved in this invention in which an electrode containing silver receives a net negative charge while the EL lamp is being driven with alternating current. The net negative charge can be provided by the driver itself or by a current source, such as a battery or a charged capacitor, or by asymmetric current means in the current path of the EL lamp, including the lamp itself. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       A more complete understanding of the invention can be obtained by considering the following detailed description in conjunction with the accompanying drawing, in which: 
         FIG. 1  illustrates the operation of a bridge circuit for converting DC to AC; 
         FIG. 2  is a schematic of a commercially available driver having a bridge output; 
         FIG. 3  is a partial cross-section of an EL lamp biased in accordance with the invention; 
         FIG. 4  is a schematic of an asymmetric current means; and 
         FIG. 5  is a schematic of another asymmetric current means. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As note above, an electroluminescent lamp requires an alternating current for operation. When a direct current source is all that is available, alternately reversing the connections of an EL lamp and a source of direct current will provide an alternating current. As illustrated in  FIG. 1 , the terminals of EL lamp  11  are coupled to respective poles of double pole, double throw (DPDT) switch  12  through resistors  13  and  14 . The throws of switch  12  are connected to capacitor  16 , which stores high voltage DC from a suitable source, not shown. When switch  12  is closed to the left, voltage at a first polarity is applied to EL lamp  11 . When the switch is closed to the right, the polarity of the voltage applied to EL lamp  11  is reversed, producing, over successive cycles, an alternating current through the lamp. If resistors  13  and  14  are not identical, a net charge accumulates on EL lamp  11 . The net charge is a DC bias voltage on EL lamp  11 . 
       FIG. 2  is a schematic of a commercially available driver that includes the electronic analog of a DPDT switch. Driver  20  includes inductor  21  and switching transistor  22  operating in a well known boost configuration to charge capacitor  23  to a high voltage. A bridge circuit, including SCR  25 , SCR  26 , switching transistor  27 , and switching transistor  28 , converts the voltage on capacitor  23  to alternating current. EL lamp  24  is connected to the AC diagonal of the bridge. Capacitor  23  is connected across the DC diagonal of the bridge. SCR  25  and transistor  28  conduct simultaneously to pass current in a first direction through EL lamp  24 . SCR  26  and transistor  27  conduct simultaneously to pass current in a second direction through EL lamp  24 , alternating with SCR  25  and transistor  28 . As in the case of  FIG. 1 , if the electrical components are not identical for each half of the bridge, a net charge accumulates on EL lamp  24 . 
       FIG. 3  illustrates an EL lamp in cross-section, with the layers not drawn to scale, coupled to a driver in accordance with the invention. EL lamp  30  includes transparent front electrode  32  overlying substrate  31 . Front electrode  32  is a transparent conductor, such as a transparent layer of Orgacon™ polymer coupled to bus bars (not shown) containing carbon particles or other conductivity enhancing particles but not silver particles. Phosphor layer  35  overlies the front electrode and dielectric layer  36  overlies the phosphor layer. Layers  35  and  36  are combined in some applications. Overlying dielectric layer  36  is opaque rear electrode  38 . An optional backing layer (not shown) may also be provided, e.g. for insulating the rear electrode or sealing the lamp. Coated phosphor particles are used, eliminating the need for a sealing layer. In accordance with one aspect of the invention, rear electrode  38  is preferably deposited from an ink containing silver particles, thereby providing the functions of reflection and conduction. 
     In accordance with another aspect of the invention, driver  39  is coupled to EL lamp  30  in such a way as to cause negative charge to accumulate on rear electrode  38 . Specifically, rear electrode  38  is biased negatively with respect to front electrode  32 . A net negative charge is all that is necessary. The negative voltage can be a fraction of a volt and still be effective. 
     For a given driver, the DC bias is readily determined by simply running the driver with a capacitive load and measuring the bias. The connections to the driver are noted and, for all drivers of that type, one pin is noted as the positive pin and the other pin is noted as the negative pin. It has been found that drivers of the same type, e.g. D381, behave the same way. Although the amount of DC bias may vary from device to device of a given type, the polarity of the DC bias does not. In a test of over one hundred drivers of the same type, the DC bias varied from 0.10 to 2.90 volts with an average of 1.12 volts and a standard deviation of 0.53 volts. The polarity of the bias was always the same. 
     In accordance with another aspect of the invention, rather than testing drivers, one can provide negative charge from an external voltage or by introducing an asymmetry in the current path of an EL lamp. In  FIG. 3 , optional voltage source  41  provides direct current to EL lamp  30 . The direct current causes an offset in the alternating current to the lamp from driver  39 , making the waveform of the current asymmetrical about zero and resulting in a net negative charge on electrode  38 . 
     Asymmetric current means ( 42 ,  45 ) in the current path of an EL lamp can be a diode in anti-parallel with two or more series connected diodes, as illustrated in  FIG. 4 , or a resistor in parallel with a diode, as illustrated in  FIG. 5 . Using either external bias or asymmetry provides the necessary bias for EL lamps having three or more electrodes. The use of external bias or asymmetric current means overcomes the inherent polarity, if any, in a driver because the bias is forced in a preferred direction. 
     The invention thus provides an apparatus and a method for eliminating silver migration in EL lamps without eliminating silver. A net negative charge eliminates silver migration in EL lamp regardless of moisture or field intensity. The invention reduces the cost of making an EL lamp by eliminating the need for barrier layers. 
     Having thus described the invention, it will be apparent to those of skill in the art that various modifications can be made within the scope of the invention. For example, the invention is independent of the kind of EL lamp or the type of driver. If rear electrode  38  does not contain silver, the electrode can include bus bars, such as bus bar  41 , containing silver particles, or contact areas, such as contact area  42 , containing silver particles. Whatever the construction, the silver particles are associated with only one electrode in a two electrode lamp. In a lamp having plural electrodes, one electrode must have a net positive charge and the other electrodes can include silver.