Abstract:
The invention provides a light-emitting arrangement, comprising: a light source adapted to emit light of a first wavelength; a wavelength converting member comprising an organic wavelength converting compound adapted to receive light of said first wavelength and to convert at least part of the received light to light of a second wavelength, said wavelength converting member and said light source being mutually spaced apart; and a sealing structure at least partially surrounding said wavelength converting member to form a sealed cavity containing at least said wavelength converting member, said sealed cavity containing an inert gas and oxygen gas, the concentration of oxygen gas being in the range of from 0.05 to 3% based on the total volume within said sealed cavity. An oxygen concentration in this range has been found to have very limited influence on the life time of the organic wavelength converting compound.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a light-emitting arrangement comprising an organic phosphor maintained under a controlled environment and to a lamp comprising such light-emitting arrangements. 
     BACKGROUND OF THE INVENTION 
     Light-emitting diode (LED) based illumination devices are increasingly used for a wide variety of lighting applications. LEDs offer advantages over traditional light sources, such as incandescent and fluorescent lamps, including long lifetime, high lumen efficacy, low operating voltage and fast modulation of lumen output. 
     Efficient high-power LEDs are often based on blue light emitting materials. To produce an LED based illumination device having a desired color (e.g., white) output, a suitable wavelength converting material, commonly known as a phosphor, may be used which converts part of the light emitted by the LED into light of longer wavelengths so as to produce a combination of light having desired spectral characteristics. The wavelength converting material may be applied directly on the LED die, or it may be arranged at a certain distance from the phosphor (so-called remote configuration). For example, the phosphor may be applied on the inside of a sealing structure encapsulating the device. 
     Many inorganic materials have been used as phosphor materials for converting blue light emitted by the LED into light of longer wavelengths. However, inorganic phosphors suffer from the disadvantages that they are relatively expensive. Furthermore, inorganic phosphors are provided in the form of particles, thus always reflecting a part of the incoming light, which leads to loss of efficiency in a device. Furthermore, inorganic phosphors have limited quantum efficiency and a relatively broad emission spectrum, in particular for the red emitting inorganic phosphors, which leads to additional efficiency losses. Currently, organic phosphor materials are being considered for replacing inorganic phosphor in LEDs where conversion of blue light to green to red light is desirable, for example for achieving white light output. Organic phosphors have the advantage that their luminescence spectrum can be easily adjusted with respect to position and band width. Organic phosphor materials also often have a high degree of transparency, which is advantageous since the efficiency of the lighting system is improved compared to systems using more light-absorbing and/or reflecting phosphor materials. Furthermore, organic phosphors are much less costly than inorganic phosphors. However, since organic phosphors are sensitive to the heat generated during electroluminescence activity of the LED, organic phosphors are primarily used in remote configuration devices. 
     The main drawback hampering the application of organic phosphor materials in remote phosphor LED based lighting systems is their photo-chemical stability, which is poor. Organic phosphors have been observed to degrade quickly when illuminated with blue light in the presence of oxygen. 
     U.S.2007/0273274 (Horiuchi et al.) discloses a translucent laminate sheet comprising a light-emitting device and comprising an organic phosphor arranged in an airproofed space. The space is filled with the organic phosphor in a state where the concentration of oxygen is kept at 100 ppm and preferably at 20 ppm or less in a vacuum or ambient atmosphere of inert gas, to avoid deterioration of the phosphor. However, performing this operation under such low concentrations of oxygen is difficult and costly. 
     Hence, there remains a need in the art for improved light-emitting devices employing organic phosphor materials. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to overcome the above-mentioned problems, and to provide a light-emitting arrangement using an organic phosphor in which degradation of the phosphor material is kept at an acceptable level. 
     According to a first aspect of the invention, this and other objects are achieved by a light-emitting arrangement comprising: a light source adapted to emit light of a first wavelength; a wavelength converting member comprising an organic wavelength converting material adapted to receive light of said first wavelength and to convert at least part of the received light to light of a second wavelength; and a sealing structure at least partially surrounding said wavelength converting member to form a sealed cavity containing at least said wavelength converting member, said sealed cavity containing an inert gas and oxygen gas, the concentration of oxygen gas being in the range of from 0.05 to 3% based on the total volume within the cavity. 
     The inventors have surprisingly found that oxygen concentrations in said range only slightly influence the decay rate of an organic phosphor. These low oxygen contents at ambient or near ambient pressure is easier to achieve and less costly than hermetic sealing of the phosphor under vacuum or a completely inert atmosphere, as have been suggested previously. In addition, it may be difficult or costly to prevent any outgassing of the wavelength converting member or any other components present within the sealing structure during the lifetime of the device. However, the present invention provides the possibility of maintaining the oxygen concentration at an acceptable level. As a result, the lifetime of the organic phosphor increased. The desired oxygen concentration is typically maintained during the entire lifetime of the light-emitting arrangement. 
     In embodiments of the invention, the concentration of oxygen gas within the cavity containing the wavelength converting member, i.e. the phosphor, may be in the range of from 0.05 to 1%, and preferably from 0.05 to 0.6%, based on the total gas volume in said sealed cavity. Below a concentration of 0.6%, the influence of the oxygen gas on the phosphor degradation rate is predicted to have a very limited effect on the phosphor degradation rate. For example, a concentration of oxygen gas of 0.1% has been demonstrated to have very little effect on the lifetime of organic phosphors. 
     According to embodiments of the invention, an oxygen getter may be arranged in the sealed cavity. The oxygen getter enables sealing the cavity containing the phosphor under an atmosphere containing a higher oxygen concentration than desired, and/or using a permeable seal which allows oxygen to enter the cavity at a rate that would without the getter result in an undesirably high oxygen concentration. 
     In embodiments of the invention the sealing structure comprises a hermetic seal, which is thus impermeable to oxygen gas and other degrading components. Hence, the oxygen concentration within the cavity can easily be maintained at the desired level. 
     Alternatively, according to other embodiments of the invention the sealing structure may comprise a seal which is non-hermetic and permeable to oxygen. Thus, oxygen permeation into the cavity may be allowed. The concentration of oxygen gas may still be maintained in the desired range, in particular if the oxygen permeation rate is slow and/or if an oxygen getter is used. 
     According to embodiments of the invention, the wavelength converting material comprises a perylene derivative. Perylene derived phosphors have been found to have particularly good stability in a low oxygen atmosphere. Preferably, the wavelength converting material comprises a compound selected from the group consisting of perylene derivatives of the following general formula: 
     
       
                 
         
             
             
         
      
         
         
           
             in which 
             G 1  is a linear or branched alkyl group or oxygen-containing alkyl group C n H 2n+1 O m , n being an integer from 1 to 44 and m&lt;n/2, or Y; 
             each of A, B, C, J and Q independently is hydrogen, isopropyl, t-butyl, fluorine, methoxy, or unsubstituted saturated alkyl C n H 2n+1 , n being an integer from 1 to 16; 
             each of G 2 , G 3 , G 4  and G 5  independently is hydrogen, fluorine, methoxy, or unsubstituted saturated alkyl group C n H 2n+1 , n being an integer from 1 to 16, or X; and each of D, E, I, L and M independently is hydrogen, fluorine, methoxy, or unsubstituted saturated alkyl group C n H 2n+1 , n being an integer from 1 to 16. 
           
         
       
    
     In one example, G 1  is Y. Advantageously, when G1 is Y, each of G 2 , G 3 , G 4  and G 5  is X, each of A and C is isopropyl, and each of B, J, Q, D, E, I, L and M is hydrogen. This wavelength converting compound has been found to have particularly good stability in a low oxygen atmosphere compared to other perylene derived compounds. 
     According to embodiments of the invention, the wavelength converting member comprises a matrix material. The matrix material may be selected from the group consisting of poly(ethylene terephthalate) (PET), PET copolymers, polyethylene naphthalate (PEN) and PEN copolymers. In particular, incorporating the organic phosphor in a PET matrix has been found to greatly enhance the lifetime of the phosphor. However, other suitable matrix materials may also be used. 
     According to embodiments of the invention the light source comprises at least one LED. Typically the wavelength converting member and said light source are mutually spaced apart, i.e., the wavelength converting member is arranged in a remote position with respect to the light source. 
     In another aspect, the invention provides a lamp, e.g. a retrofit lamp, comprising a light-emitting arrangement as described herein. 
     It is noted that the invention relates to all possible combinations of features recited in the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       This and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing embodiment(s) of the invention. As illustrated in the figures, the sizes of layers and regions may be exaggerated for illustrative purposes and, thus, are provided to illustrate the general structures of embodiments of the present invention. 
         FIG. 1  is a graph showing the decay of an organic phosphor as a function of time. 
         FIG. 2  is a graph showing the decay rate of an organic phosphor as a function of temperature for various oxygen concentration. 
         FIG. 3  is a graph showing the decay rate of an organic phosphor measured as a function of oxygen concentration in nitrogen gas. 
         FIGS. 4   a  and  4   b  illustrate light-emitting arrangements according to embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present inventors have found that oxygen concentrations many times higher than those disclosed in U.S. 2007/0273274 may be acceptable with respect to the rate of degradation of an organic phosphor compound. Quite surprisingly, oxygen concentrations in the range of 0.05-3% were found to be highly acceptable, only slightly increasing the phosphor degradation rate. In particular, the present inventors have found that the degradation rate of a red-emitting organic phosphor in a polymer matrix illuminated with blue light is very little affected by increasing oxygen concentrations as long as the oxygen concentration is approximately 0.6% or below. According to the present invention, the oxygen concentration within a sealed compartment containing the organic phosphor can be maintained at such acceptable level during the entire lifetime of the light-emitting arrangement. 
       FIG. 1  is a graph showing, as a function of time, the emission from a layer of organic phosphor represented by 0.1% Lumogen® Red F-305 dye (commercially available from BASF) in PMMA matrix illuminated by a laser emitting light of 450 nm with a flux density of 4.2 W/cm 2 . Due to degradation of the Red F-305 phosphor under blue light irradiation, the emission intensity of the Red F-305 phosphor decreases with time. The initial absorption in the layer was 10% and thus the intensity decrease can be directly related to the concentration of phosphor molecules that have degraded and thus no longer emit light. It can be seen that the change in light intensity is an exponential function of time, c(t)=c(0)*e −kt , with a decay constant k representing the degradation rate of the organic phosphor compound. 
     Next, the decay rate k was measured under illumination with blue light at a flux density of 4.2 W/cm 2  as a function of temperature for various oxygen concentrations (percentage) in nitrogen. The result is presented in  FIG. 2  (logarithmic scale). 
       FIG. 3  is a graph showing the decay rate k of the organic compound measured at 120° C. as a function of oxygen concentration in nitrogen gas on a logarithmic scale. Interestingly, two different regimes can be identified. Up to the measured point of 0.1% oxygen, the decay constant increases very little with increasing oxygen concentration. For the measured concentrations of 1% and above, the decay rate increases rapidly with increasing oxygen concentration. Drawing straight lines through the measured point as illustrated in  FIG. 3 , these regimes can be clearly seen. Furthermore, these lines intersect at an oxygen concentration of approximately 0.6%. Hence, it can be predicted that for an oxygen concentration of about 0.6% or lower, the oxygen content will have a very limited influence on the degradation rate of the organic phosphor, whereas for an oxygen concentration of more than approximately 0.6%, the oxygen content will highly influence the phosphor degradation rate. 
     The magnitude of the decay constant k depends on the light flux density, the temperature, the oxygen concentration and type of phosphor compound. The flux density and temperature are highly related to the device configuration. It is noted that the light flux density of 4.2 W/cm 2  is higher than what is commonly used in LED based lighting devices comprising organic phosphor compounds. Also the temperature of 120° C. is higher than in most remote phosphor applications. Hence, the tests from which the graphs of  FIGS. 1 to 3  are obtained represent accelerated conditions. 
     In view of these insights, it is suggested that an organic phosphor should be kept in a controlled atmosphere containing a limited amount, preferably not more than about 0.6%, of oxygen gas or other degrading gas in otherwise inert gas. However, oxygen concentrations of up to 3%, or even up to 5%, are considered acceptable, since the lifetime of the phosphor then will be sufficient for application at least in certain LED based lighting systems. 
       FIG. 4   a  schematically illustrates a light-emitting arrangement  400  according to an embodiment of the invention. The light-emitting arrangement  400  of this embodiment is provided as a retrofit lamp. The phrase retrofit lamp is well known to the person skilled in the art and refers to a LED based lamp having an outer appearance of an older type of lamp which did not have a LED. A light source  401  comprising a plurality of LEDs  401   a  each comprising an electroluminescent layer connected to an anode and a cathode (not shown) is arranged on a base part  402 , which is provided with a traditional cap, such as an Edison screw cap or a bayonet cap. A sealing structure  403  comprising a bulb shaped light outlet member  404  is arranged over the LEDs  401   a  and enclosing a cavity  405 . A wavelength converting member  406  comprising an organic wavelength converting compound is arranged within the cavity  405  on the inside of the light outlet member  404  (i.e. on the side of the light outlet member facing the cavity  405 ) Typically, the wavelength converting compound is dispersed in a polymeric matrix or carrier. The wavelength converting member and the light source are arranged mutually spaced apart, meaning that the wavelength converting member is arranged at a remote position in relation to the LEDs (so-called remote phosphor configuration). 
     The sealing structure  403  also comprises a seal  407 , which extends along the rim of the bulb-shaped light outlet member  404 . The seal  407  may be a hermetic seal or a gas-permeable seal. The light outlet member  404  of the sealing structure is formed of a gas impermeable material. 
     When the seal  407  is a hermetic seal, the sealing structure provides a gas impermeable barrier between the cavity  405  and the outside atmosphere surrounding the sealing structure. According to the present invention, the atmosphere inside the sealing structure, i.e., in the cavity  405 , is mainly composed of an inert gas, such as nitrogen or argon but may contain minor amounts of other, non-inert gases, such as oxygen. Alternatively, in embodiments of the invention, the cavity  405  need not be hermetically sealed. In such embodiments, the seal  407  may be permeable such as to allow a low rate of gas (e.g. oxygen) permeation into the cavity  405 . A permeable seal is typically an organic adhesive, such as an epoxy adhesive. 
     It should be noted that throughout this application the sealing structure  403  comprises one or more walls, which may be formed of glass, ceramic, metal or a polymeric material optionally provided with a barrier coating or film. The sealing structure may be at least partly light transmissive. For example, in the embodiment of  FIG. 4   a  the light outlet member is formed of a light transmissive material. In embodiments of the invention, the sealing structure may comprise a plurality of walls or wall portions which may be joined together by an adhesive which may be gas permeable or gas impermeable. 
     Oxygen may be present in the cavity  405  as a result of sealing under an oxygen-containing atmosphere, and/or it may enter the cavity  405  via a permeable seal, and/or it may be released or produced from a material or component within the cavity  405 , e.g. the matrix material of the wavelength converting member, during operation of the light-emitting arrangement. 
       FIG. 4   b  illustrates a light-emitting arrangement according to another embodiment of the invention. The light emitting arrangement  400  comprises a plurality of LEDs  401   a  arranged on a base part  402  and a dome shaped sealing structure  403  comprising a light outlet member  404  covering the LEDs  401   a . However, in this embodiment the light outlet member comprises a sandwich structure of an inner wall  404   a  forming an inner barrier, and an outer wall  404   b  forming an outer barrier, and the remote wavelength converting member  406  arranged between the outer wall  404   b  and the inner wall  404   a . The outer and inner walls  404   b ,  404   a  are connected with the base part and with each other by means of the seal  407  extending along circumferential edge portions of the inner and outer walls. The light outlet member  404  thereby forms the sealed cavity  405  between the mutually spaced inner and outer walls  404   a ,  404   b . The sealed cavity  405  containing the wavelength converting member is separate from the compartment  409  formed by the sealing structure  404  and the base part  402  and containing the LEDs  401   a . Since there are no particularly oxygen sensitive components within the further compartment  409  it does not require a special environment or atmosphere, but may contain air. However it is also possible to initially provide the compartment  409  with the same atmosphere as the cavity  405 , since the compartment  409  is in fact sealed from the environment by the seal  407 , because it is additionally used to attach the light outlet member  404  to the base part  402 . 
     As shown in  FIG. 4   b , the wavelength converting member is arranged in a remote position in relation to the light source  401 . 
     It is contemplated that two or more wall portions may be used to form the sealing structure  404 . Also, the walls  404   a ,  404   b  need not be hemispherical, dome-shaped or even curved, but may have any suitable shape, and may for example comprise a plurality of portions. 
     In accordance with an embodiment of the light emitting arrangement, the sealing structure may comprise a light outlet member formed as a cylindrical tube, e.g. a glass tube, wherein the sealing structure further comprises end caps each attached to the cylindrical tube by means of a seal as described above in relation to  FIG. 4   a . This embodiment can be arranged as for example a retrofit fluorescent tube, the interior of the tube forming a sealed cavity corresponding to the cavity  405  described above e.g. in relation to  FIG. 4   a.    
     The sealing of the cavity  405  may be performed using methods and conditions which reduce the content of degrading gas, such as oxygen, within the cavity. Such methods and conditions are known to person skilled in the art and include vacuum pumping and filling the cavity with an inert gas before sealing thereof; flushing the cavity with an inert gas during sealing thereof; or sealing the cavity in an oxygen-free environment such as in a glovebox. 
     The wavelength converting member may be a polymeric matrix or carrier for the organic wavelength converting compound. Examples of suitable polymeric material for the matrix comprise poly(ethylene terephthalate) (PET) and copolymers thereof, polyethylene naphthalate (PEN) and copolymers thereof, poly(methyl methacrylate) (PMMA), polystyrene, polycarbonate, silicone, polysiloxane, and acrylate polymers. 
     The wavelength converting compound used in the device according to the present invention may be any conventional organic phosphor. For example, the wavelength converting compound may be a perylene derivative. In particular, perylene derivatives having the following general formula may be used in the light-emitting arrangement according to the invention: 
     
       
                 
         
             
             
         
      
         
         
           
             in which
 
G 1  is a linear or branched alkyl group or oxygen-containing alkyl group C n H 2n+1 O m , n being an integer from 1 to 44 and m&lt;n/2, or Y;
 
             each of A, B, C, J and Q independently is hydrogen, isopropyl, t-butyl, fluorine, methoxy, or unsubstituted saturated alkyl C n H 2n+1 , n being an integer from 1 to 16; 
             each of G 2 , G 3 , G 4  and G 5  independently is hydrogen, fluorine, methoxy, or unsubstituted saturated alkyl group C n H 2n+1 , n being an integer from 1 to 16, or X; and each of D, E, I, L and M independently is hydrogen, fluorine, methoxy, or unsubstituted saturated alkyl group C n H 2n+1 , n being an integer from 1 to 16. 
           
         
       
    
     Typically G 2 , G 3 , G 4  and G 5  independently may be hydrogen or X, and at least one of D, E, I, L and M may be hydrogen. Also, at least one of J and Q may be hydrogen. For example, at least two of D, E, I, L and M may be hydrogen. In one example, G 1  is Y. Advantageously, when G 1  is Y, each of G 2 , G 3 , G 4  and G 5  is X, each of A and C is isopropyl, and each of B, J, Q, D, E, I, L and M is hydrogen. These wavelength converting compounds have been found to have particularly good stability in a PET matrix. 
     Typically, at least one of D, E, I, L and M may be hydrogen. For example, at least two of D, E, I, L and M may be hydrogen. Alternatively or additionally, at least one of J and Q may be hydrogen. The function of the moieties A, B, C, J, Q, D, E, I, L and M is to improve the stability of the structure. 
     Phosphor compounds corresponding to the above general formula were tested and found to have good stability compared to other organic phosphors, including other perylene derived organic phosphors. 
     In embodiments of the invention, an oxygen getter  408  is provided in the cavity  405 , together with the wavelength converting member. By “oxygen getter” is meant a material which absorbs or reacts with oxygen, thus removing oxygen from the atmosphere within the cavity  405 . 
     The getter may be any getter conventionally used in LED phosphor applications. The getter  408  is capable of absorbing a gas which enters the cavity  405 . The getter is arranged to absorb a gas that would be detrimental to the organic wavelength converting member  406 , in particular the wavelength converting compound. With this structure of the LED device  400  it is possible to provide a non-hermetic seal, i.e. a permeable seal. The getter is typically made of a solid material and arranged adjacent to the seal  407   a . The position is chosen inter alia in order to avoid that the getter  408  interferes with an output light path, i.e. the light that is output from the LED device  400 . The getter can be placed behind a reflector. The getter itself can also be made reflective. In embodiments of the invention, the getter may be a particulate material, applied in or on a permeable carrier material, e.g. contained in a permeable patch, or applied on an inner surface of the sealing structure for example as a coating. 
     In embodiments of the invention, in addition to an inert gas, the cavity  405  may contain a further gas which reacts with oxygen in the cavity  405 . For example hydrogen gas may be used as an oxygen getter. For instance, LED components or other parts arranged in the cavity  405  may produce a degrading gas which compromises the operation or the lifetime of the light-emitting arrangement  400 . It is then possible to choose a reactant gas which reacts chemically with the degrading gas and produces a stable component or a component that can easily be absorbed by an additional getter. 
     Example 
     The lifetime of different organic phosphor compounds was tested under different conditions. The compounds used were as follows: 
     Compound I: 
                                
Compound II:
 
                                
Compound III:
 
     
       
                 
         
             
             
         
      
     
     Compound III is available from BASF as Lumogen® Red F-305 and corresponds to the above general formula in which each of A and C is isopropyl, B is hydrogen and each of D, E, I, L and M is hydrogen. 
     Each compound was incorporated in two different polymeric matrices, formed into layers, and placed in air or in controlled atmosphere containing 0.1% oxygen. The layers containing the phosphor materials were illuminated with blue light at 4.1 W/cm 2  at a temperature of 60° C. The phosphor concentration and the layer thickness were chosen such that the transmission of blue light was 90%. The lifetime of the phosphor was estimated as a 10% reduction in the luminescence intensity. The resulting lifetimes are presented in Table 1. 
     
       
         
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Matrix material;  
                   
                   
                   
               
               
                 atmosphere 
                 Compound I 
                 Compound II 
                 Compound III 
               
               
                   
               
             
             
               
                 PMMA; air 
                 10 minutes 
                 3 minutes 
                  40 hours 
               
               
                 PMMA; 
                 2 hours 
                 7 hours 
                  500 hours 
               
               
                 0.1% oxygen 
                   
                   
                   
               
               
                 PET; air 
                 minutes 
                 minutes 
                  300 hours 
               
               
                 PET; 
                 6 hours 
                 5 hours 
                 3200 hours 
               
               
                 0.1% oxygen 
               
               
                   
               
             
          
         
       
     
     As can be seen in Table 1, the tested compounds showed considerably longer lifetime when kept under an atmosphere containing a reduced amount of oxygen. In particular, Compound III shows extraordinary stability when comprised in a PET matrix and maintained under a low oxygen atmosphere. 
     The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims.