Abstract:
In accordance with the invention, a night-light or safety light comprises a layer of organic semiconducting material including a light-emitting organic semiconductor disposed between a substrate-supported bottom electrode and a top electrode. Electricity (AC or DC) applied between the electrodes stimulates low-level illumination. The electrodes or the organic material can be patterned to display text or aesthetic design, and a plurality of different organic light emitting materials can patterned to produce a multicolored pattern.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to night-lights and safety lights for providing low-level safety illumination in homes, businesses and vehicles, and, in particular, to lights using organic light-emitting material. Advantageous embodiments can provide patternable illumination regions and multicolored patterns.  
         BACKGROUND OF THE INVENTION  
         [0002]    Night-lights and safety lights are useful in providing orientation in a darkened building or vehicle and guidance in the event of an unexpected power failure. These lights have traditionally used incandescent bulbs although some recently marketed devices use crystalline semiconductor light-emitting diodes.  
           [0003]    Both bulb-based and crystalline semiconductor diode lights are expensive to fabricate and limited in design flexibility. Bulb-based lights require numerous mechanical processing steps. Diode lights are typically limited to a single color. Accordingly there is a need for night-lights and safety lights that are inexpensive to fabricate, and it would be advantageous if such lights could provide patterned illumination regions in one or more colors.  
         SUMMARY OF THE INVENTION  
         [0004]    In accordance with the invention, a night-light or safety light comprises a layer of organic semiconducting material including a light-emitting organic semiconductor disposed between a substrate-supported bottom electrode and a top electrode. Electricity (AC or DC) applied between the electrodes stimulates low-level illumination. The electrodes or the organic material can be patterned to display text or aesthetic design, and a plurality of different organic light emitting materials can patterned to produce a multicolored pattern.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]    The advantages, nature and various additional features of the invention will appear more fully upon consideration of the illustrative embodiments now to be described in detail in connection with the accompanying drawings. In the drawings:  
         [0006]    [0006]FIG. 1 is a schematic cross section of a typical illumination source for a light in accordance with the invention;  
         [0007]    [0007]FIG. 2 is a schematic flow diagram of the steps involved in an exemplary process for making the source of FIG. 1;  
         [0008]    [0008]FIG. 3A and 3B are front and side views of a typical night-light or safety light employing the source of FIG. 1;  
         [0009]    [0009]FIG. 4 schematically illustrates an exemplary electrical connection arrangement for the light of FIGS. 3A and 3B; and  
         [0010]    [0010]FIG. 5 is a schematic cross-section of an alternative illumination source. 
     
    
       [0011]    It is to be understood that these drawings are for purposes of illustrating the concepts of the invention and are not to scale.  
       DETAILED DESCRIPTION  
       [0012]    Referring to the drawings, FIG. 1 is a schematic cross section of an illumination source  100  comprising a substrate  101  supporting a bottom electrode  102 , a layer of one or more organic semiconducting material(s)  103  and a top electrode  104  in electrical contact with the organic material(s)  103 . The term organic semiconducting material as used herein refers to an organic material exhibiting weak electron or hole conductivity. At least one of the electrodes  102 ,  104  is transparent for light emission. An insulating spacer layer  105  is conveniently provided to keep bottom electrode  102  from shorting with top electrode  104 , and the layer of organic material(s)  103  is encapsulated by an air impermeable encapsulant  106  and an impermeable lid  107  to retain an inert gas ambient  108 . Either the substrate  101  or the lid  107  is transparent. Contact metal  109  is advantageously deposited for wire bonding. The layer  103  can be a composite of organic semiconducting layers, at least one of which is light-emitting material.  
         [0013]    The substrate  101  can be any impermeable insulating material such as glass or plastic and is preferably polyester. The bottom electrode  102  can be any conductive material compatible with the substrate, and is preferably indium tin oxide (ITO). Glass or plastic (e.g. polyester) sheets precoated with ITO are commercially available. The organic layers preferably comprise a hole transport layer (HTL) such as 4,4′-bis[N-(1-napthyl)-N-phenyl-amino]biphenyl (α-NPD), and an electron transport and light-emitting layer (ETL/LL) such as tris-(8-hydroxyquinoline) aluminum (Alq 3 ). The HTL and ETL/LL thicknesses are preferably in the range of 200 to 500 Å. Alternatively, a single carrier transport and light-emitting layer can be used. This single organic layer preferably comprises a hole transporting layer such as poly(N-vinylcarbazole)(PVK) and contains dispersed electron transporting molecules such as 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) and a fluorescent dye such as coumarin 6. The top electrode  104  can be any conductive material but is preferably a layer of Mg:Ag alloy having a thickness in the range of 1000 to 2000 Å. The encapsulant  106  can be epoxy, and the lid  107  can be glass or a plastic such as polyester. The inert gas ambient can be a relatively inert gas such as N 2  or a true inert gas such as argon.  
         [0014]    [0014]FIG. 2 is a schematic flow diagram of the steps involved in an exemplary process for making the light source  100  of FIG. 1. The process starts with a commercially available polyester sheet  101  pre-coated with an ITO film  102 . The first step, shown in Block A of FIG. 1, is to pattern the bottom electrode  102 . This can be done by photolithography and wet or dry etching in accordance with techniques well known in the art.  
         [0015]    The second step, shown in Block B, is to deposit and pattern insulating spacer layer  105 . The layer  105  can be SiN x  deposited and patterned by well-known techniques.  
         [0016]    The next step (Block C) is to deposit and pattern contact metal  109  to form bonding pads.  
         [0017]    The fourth step (Block D) is to form the layer(s) of organic material(s)  103 . The materials can be deposited by spin coating, vacuum deposition, organic vapor phase deposition or ink jet printing. Organic materials emitting in different colors can be selectively deposited and patterned to form a multi-colored light pattern. Usually only the ETL/LL needs to be patterned in the case of multiple organic layers.  
         [0018]    The next step (Block E) is to form the top electrode layer  104 . It can be a layer of low work function metal or alloy probably capped by a relatively inactive metal film such as Ag. The top electrode can be deposited through a shadow mask to form a graphic or text pattern.  
         [0019]    The sixth step shown in Block F of FIG. 2 is to encapsulate the light emitting region in inert gas and finish the device. This involves applying encapsulating material  106 , such as epoxy, around the periphery of the light-emitting region. The lid  107  can then be joined to the epoxy in a dry nitrogen or inert gas atmosphere to form a hermetic encapsulation. In the case of a plastic substrate and lid, a watertight film such as SiO 2  may be deposited over the outer surfaces to ensure hermetic encapsulation.  
         [0020]    Once the light source  100  is complete, a night-light or safety light is finished by attaching the source  100  to an appropriate package including plug blades for an electrical power outlet.  
         [0021]    [0021]FIG. 3A and 3B are front and side views of an exemplary finished light comprising a package  30  with an open window  31  for the light source  100 . Plug blades  32  are dimensioned to fit into a standard electrical outlet. Electrical wires (not shown) connect the light source  100  to a voltage down converter/rectifier (not shown), which, in turn, is connected to the blades  32 .  
         [0022]    [0022]FIG. 4 illustrates. the electrical connection of the light source  100  (represented as an organic light emitting diode—an OLED), a voltage down converter/rectifier  40 , and the plug blades  32 . The light source  100  is represented by one diode symbol, although the light source may consist of multiple OLEDs in parallel in the case of multi-color light source. In order to supply electric power to the light source  100 , a voltage down converter/rectifier  40  down converts electricity from the AC voltage of ˜110 V (or 220 V, depending on the country where the night light is to be used) to an AC voltage of a few volts to about 20 volts, and possibly rectifies the down converted AC voltage to a single polarity pulse or DC voltage. A transformerless down converter/rectifier, composed of a capacitor and a rectifying bridge or diode, is preferred for compactness and light weight. Rectification is not necessary since the light sources can operate at AC voltages. Pulsed or AC operation may be preferred for simplicity and elongated lifetime. The input terminals of the voltage downconverter/rectifier are connected to the plug blades  32 .  
       ALTERNATIVE EMBODIMENT  
       [0023]    [0023]FIG. 5 shows the cross-section of an alternative light source  200  comprising a substrate  201  supporting a capacitor bottom electrode  211 , a dielectric layer  212 , an OLED bottom electrode  202  (electrode  202  is also the top electrode for the capacitor), a single or composite layer of organic semiconducting material(s)  203  in electrical contact with the bottom electrode  202 , (in the case of multiple organic layers, only one layer is in contact with the bottom electrode  202 ). A top electrode  204  is disposed in electrical contact with the organic material  203  or the topmost layer of  203  in the case of composite organic layers. The substrate  201 , electrodes  211  and  202 , and the dielectric layer  212  are transparent for light emission. An insulating spacer layer  205  is conveniently provided to keep bottom electrode  202  from shorting with top electrode  204 , and the organic materials  203  should be encapsulated within an air impermeable encapsulant  206  and an impermeable lid  207  to retain an inert gas ambient  108  around the light emitting material. Contact metal  209  is deposited for wire bonding.  
         [0024]    In this embodiment, the electrodes  211  and  202  and the dielectric layer  212  form a capacitor, which will assume most of the applied AC voltage and thereby reduce the voltage across the organic layer. Hence, no voltage down converter is needed, and contacts  209  are directly connected to the plug blades  32 . The capacitance value and hence thickness of layer  212  are determined by the OLED resistance and operating voltage, and by the total applied AC voltage. In light source  200 , parts  203  through  209  are identical to  103  through  109  in light source  100 .  
         [0025]    Fabrication of light source  200  starts with a commercially available polyester sheet  201  pre-coated with an ITO film  211 . The first step is to deposit and pattern the dielectric layer  212 . This can be done by selectively depositing a dielectric such as SiN x  through a shadow mask, leaving uncovered area for later deposition of contact  209  to the capacitor bottom electrode  211 .  
         [0026]    The second step is to deposit and pattern the OLED bottom electrode  202 . This can be done by sputtering or thermal evaporation through a shadow mask.  
         [0027]    The next steps are identical to the process steps illustrated by Blocks B through F in FIG. 2.  
         [0028]    Once the light source  200  is complete, a night-light or safety light is finished by attaching the source  200  to an appropriate package including plug blades for an electrical power receptacle.  
         [0029]    The light source  200  is packaged as shown in FIGS. 3A and 3B. Plug blades  32  are dimensioned to fit into a standard electrical outlet. Electrical wires (not shown) connect the light source  200  to the blades  32 .  
         [0030]    It is to be understood that the above-described embodiments are illustrative of only a few of the many possible specific embodiments, which can represent applications of the principles of the invention. Numerous and varied other arrangements can be readily devised by those skilled in the art without departing from the spirit and scope of the invention.