Patent Publication Number: US-2011069732-A1

Title: Enhanced emission of light from organic light emitting diodes

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation of copending U.S. patent application Ser. No. 11/659,991, which is the national phase of PCT/GB2005/003213, filed Aug. 17, 2005, the entire respective disclosures of which are incorporated by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to light emitting devices fabricated using organic light emitting diodes. 
     2. Related Technology 
     Background to OLED Devices 
     Displays fabricated using organic light emitting diodes (OLEDs) provide a number of advantages over other flat panel technologies. They are bright, colourful, fast-switching, provide a wide viewing angle and are easy and cheap to fabricate on a variety of substrates. Organic (which here includes organometallic) LEDs may be fabricated using materials including polymers, small molecules and dendrimers, in a range of colours which depend upon the materials employed. Examples of polymer-based organic LEDs are described in WO 90/13148, WO 95/06400 and WO 99/48160; examples of dendrimer-based materials are described in WO 99/21935 and WO 02/067343; and examples of so called small molecule based devices are described in U.S. Pat. No. 4,539,507. 
     An OLED device comprises a layer of organic light emitting material such as a light emitting polymer (LEP), oligomer or a light emitting low molecular weight material located between an anode for injection of holes and a cathode for injection of electrons. Further layers may be present, for example a layer of a hole transporting material such as a polythiophene derivative or a polyaniline derivative may be located between the anode and the light emitting material. 
     Organic LEDs may be deposited on a substrate in a matrix of pixels to form a single or multi-colour pixellated display. A multicoloured display may be constructed using groups of red, green, and blue emitting pixels. So-called active matrix displays have a memory element, typically a storage capacitor and a transistor, associated with each pixel whilst passive matrix displays have no such memory element and instead are repetitively scanned to give the impression of a steady image. Other passive displays include segmented displays in which a plurality of segments share a common electrode and a segment may be lit up by applying a voltage to its other electrode. A simple segmented display need not be scanned but in a display comprising a plurality of segmented regions the electrodes may be multiplexed (to reduce their number) and then scanned. 
       FIG. 1  shows a vertical cross section through an example of an OLED device  100 . In an active matrix display part of the area of a pixel is occupied by associated drive circuitry (not shown in  FIG. 1 ). The structure of the device is somewhat simplified for the purposes of illustration. 
     The OLED  100  comprises a substrate  102 , typically 0.7 mm or 1.1 mm glass but optionally clear plastic or some other substantially transparent material. An anode layer  104  is deposited on the substrate, typically comprising around 150 nm thickness of ITO (indium tin oxide), over part of which is provided a metal contact layer. Typically the contact layer comprises around 500 nm of aluminium, or a layer of aluminium sandwiched between layers of chrome, and this is sometimes referred to as anode metal. Glass substrates coated with ITO and contact metal are available from Coming, USA. The contact metal over the ITO helps provide reduced resistance pathways where the anode connections do not need to be transparent, in particular for external contacts to the device. The contact metal is removed from the ITO where it is not wanted, in particular where it would otherwise obscure the display, by a standard process of photolithography followed by etching. 
     A substantially transparent hole transport layer  106  is deposited over the anode layer, followed by an electroluminescent layer  108 , and a cathode  110 . The electroluminescent layer  108  may comprise, for example, a PPV (poly(p-phenylenevinylene)) and the hole transport layer  106 , which helps match the hole energy levels of the anode layer  104  and electroluminescent layer  108 , may comprise a conductive transparent polymer, for example PEDOT: PSS (polystyrene-sulphonate-doped polyethylene-dioxythiophene) from H C Starck of Germany. In a typical polymer-based device the hole transport layer  106  may comprise around 200 nm of PEDOT; a light emitting polymer layer  108  is typically around 70 nm in thickness. 
     These organic layers may be deposited by spin coating, dip coating, doctor blade coating (afterwards removing material from unwanted areas by plasma etching or laser ablation). Alternatively selective deposition techniques wherein the organic material is only deposited in desired areas, such as inkjet printing or laser induced thermal imaging (LITI), may be employed. In the case of inkjet printing, banks  112  may be formed on the substrate, for example using photoresist, to define wells into which the organic layers may be deposited. Such wells define light emitting areas or pixels of the display. 
     Cathode layer  110  typically comprises a low work function metal (typically less than 3.5 eV, more preferably less than 3.0 eV) such as calcium or barium (for example deposited by physical vapour deposition or sputtering) covered with a thicker, capping layer of aluminium. Optionally an additional layer may be provided immediately adjacent the electroluminescent layer, such as a layer of lithium fluoride, for improved electron energy level matching. Mutual electrical isolation of cathode lines may achieved or enhanced through the use of cathode separators (not shown in  FIG. 1 ). 
     The same basic structure may also be employed for small molecule devices wherein the light emitting material is typically deposited by vacuum evaporation. 
     Typically a number of displays are fabricated on a single substrate and at the end of the fabrication process the substrate is scribed, and the displays separated before an encapsulating can is attached to each to inhibit oxidation and moisture ingress. 
     To illuminate the OLED power is applied between the anode and cathode, represented in  FIG. 1  by battery  118 . In the example shown in  FIG. 1  light is emitted through transparent anode  104  and substrate  102  and the cathode is generally reflective; such devices are referred to as “bottom emitters”. Devices which emit through the cathode (“top emitters”) may also be constructed, for example by keeping the thickness of cathode layer  110  less than around 50-100 nm so that the cathode is substantially transparent. 
     Organic LEDs may be deposited on a substrate in a matrix of pixels to form a single or multi-colour pixellated display. A multicoloured display may be constructed using groups of red, green, and blue emitting pixels. In such displays the individual elements are generally addressed by way of an active matrix or passive matrix as described above. 
     Referring now to  FIG. 2 , this shows a simplified cross-section through a passive matrix OLED display device  150 , in which like elements to those of  FIG. 1  are indicated by like reference numerals. As shown the hole injection  106  and electroluminescent  108  layers are subdivided into a plurality of pixels  152  at the intersection of mutually perpendicular anode and cathode lines defined in the anode  104  and cathode layer  110  respectively. In the figure conductive lines  154  defined in the cathode layer  110  run into the page and a cross-section through one of a plurality of anode lines  158  running at right angles to the cathode lines is shown. An electroluminescent pixel  152  at the intersection of a cathode and anode line may be addressed by applying a voltage between the relevant lines. The anode metal layer  104  provides external contacts to the display  150  and may be used for both anode and cathode connections to the OLEDs (by running the cathode layer pattern over anode metal lead-outs). 
     The above mentioned OLED materials, and in particular the light emitting polymer material and the cathode, are susceptible to oxidation and to moisture. The device is therefore often encapsulated in a metal can  111 , attached by UV-curable epoxy glue  113  onto anode metal layer  104 , small glass beads within the glue preventing the metal can touching and shorting out the contacts. Preferably the anode metal contacts are thinned where they pass under the lip of the metal can  111  to facilitate exposure of glue  113  to UV light for curing. 
     The Problem of Total Internal Reflection in OLED Devices 
     A problem with OLED devices in general is that often only 20-30% of the light that is generated by the OLED device is actually emitted towards the viewer. The rest of the light that is generated is wasted, due to the effects of total internal reflection. It is a well known optical principle that when light is transmitted through an optical medium adjacent to air, the higher the refractive index of the optical medium, the smaller the critical angle above which total internal reflection will occur within the optical medium. In OLED devices the generated light is transmitted through a substrate having a high refractive index, which (in the case of a typical polymer substrate) gives acritical angle of approximately 40°. Thus, rays which pass through the polymer towards the air at an angle of incidence of greater than 40° are totally internally reflected and are thereby wasted. 
     It is desirable to minimize the amount of generated light that is wasted due to the effects of total internal reflection, and thereby increase the efficiency of the OLED device and enhance the quantity of light that may usefully be extracted. 
     SUMMARY OF THE INVENTION 
     According to a first aspect, the invention provides a device comprising an organic light emitting diode coupled to a cavity, said cavity containing an emitting species, said device being arranged such that light emitted from said organic light emitting diode is at least partially absorbed by the emitting species and re-emitted from the emitting species. Absorbing the light generated by the OLED and re-emitting it provides the advantage that more useful light is output from the device, compared with conventional OLEDs in which a substantial amount of light is wasted. This accordingly improves the efficiency of the device. A further advantage is that it is possible to achieve highly directional emission of light from the emitting species. 
     The term “organic light emitting diode” as used herein should be interpreted broadly, to include organometallic light emitting diodes. 
     Preferably the emitting species is a phosphor. 
     By “phosphor” is meant a material capable of absorbing and re-emitting light. A wide range of suitable emitting species will be apparent to the skilled person including fluorescent or phosphorescent, inorganic or organic materials (it will be appreciated that “phosphors” are not limited to phosphorescent materials). Examples of suitable emitters include fluorescent laser dyes (e.g. rhodamine) and phosphorescent organometallic compounds, for example dendrimers as disclosed in WO 02/066552. 
     The emitter may be dispersed in an inert matrix, e.g. polymethyl methacrylate (PMMA). 
     Preferably the cavity is formed by one or more dielectric Bragg layers. Particularly preferably the cavity is formed between a first dielectric Bragg reflector and a second dielectric Bragg reflector. 
     In a preferred embodiment the first and second dielectric Bragg reflectors are situated between the substrate and the anode of the organic light emitting diode. Preferably the first dielectric Bragg reflector is situated adjacent the substrate and is configured to reflect substantially 100% of the light emitted by the organic light emitting diode, and to reflect a portion and transmit a portion of the light generated by the emitting species. 
     Preferably the second dielectric Bragg reflector is situated adjacent the anode and is configured to transmit substantially all of the light emitted by the organic light emitting diode, and to reflect approximately 100% of the light generated by the emitting species. 
     Alternatively, the cavity may be formed by patterning. 
     The device may be arranged to provide emission of light in the plane of the cavity. Such emission is advantageously highly directional. 
     Light re-emitted from the emitting species may be used to form a pixel in a visual display. 
     Alternatively, the device may be arranged such that the emitting species acts as the gain media of a laser. The organic light emitting diode may be arranged to pump the emitting species. The laser may be arranged to provide forward emission or edge emission of a laser beam. 
     According to a second aspect, the invention provides a method of generating light, said method comprising: coupling an organic light emitting diode to a cavity, said cavity containing an emitting species, said organic light emitting diode and said cavity being arranged such that light emitted from said organic light emitting diode is at least partially absorbed by the emitting species; operating said organic light emitting diode to emit light which is at least partially absorbed by the emitting species; and re-emitting light from the emitting species. 
     The re-emitted light from the emitting species may form a pixel in a visual display. 
     Alternatively, the method may further comprise arranging the emitting species to act as the gain media of a laser. The method may still further comprise arranging the organic light emitting diode to pump the emitting species. 
     Such a laser may be used as a distributed feedback laser for use in telecommunications, or in local area networks. 
     Further aspects of the invention provide a display device and a laser incorporating a device in accordance with the first aspect of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described, by way of example only, and with reference to the drawings in which: 
         FIG. 1  illustrates a vertical cross section through atypical OLED device; 
         FIG. 2  illustrates a cross-section through a passive matrix OLED device; 
         FIG. 3  illustrates a vertical cross section through an OLED device having two dielectric Bragg reflectors and a microcavity phosphor, giving emission of light from the phosphor normal to the plane of the phosphor; 
         FIG. 4  illustrates a vertical cross section through a device providing edge emission of light from a series of microcavity phosphor pixels; 
         FIG. 5  illustrates an OLED device being used as a pump source for a polymer film having a dimpled surface to cause lasing; 
         FIG. 6  illustrates an OLED device being used as a pump source to form a forward emission laser; 
         FIG. 7  illustrates in cross section an OLED device being used as a pump source for an edge emission laser; 
         FIG. 8  illustrates in plan view a series of OLED devices being used as pump sources for a series of edge emission lasers; and 
         FIG. 9  illustrates schematically the desired correspondence between the emission mode profile of light emitted from an OLED device and the absorption profile of a microcavity phosphor, to achieve efficient transfer of energy from an OLED device to a microcavity phosphor. 
     
    
    
     In the figures, like elements are indicated by like reference numerals throughout. The thicknesses of the layers shown in the figures are not to scale. 
     DETAILED DESCRIPTION 
     The present embodiments represent the best ways known to the applicant of putting the invention into practice. However they are not the only ways in which this can be achieved. 
     Rather than using an OLED device directly as an emitter, the present embodiments use an OLED as a pump for a further component. Close-coupled phosphors are provided in microcavities arranged to transfer light from the OLED to the phosphor. The light absorbed by the microcavity phosphor is then re-emitted. In this manner, a greater quantity of light may be extracted, compared with instances in which an OLED is used directly as an emitter. Additionally it is possible to achieve highly directional emission from the phosphor. 
     Background information on the use of optical microcavities as light emitters may be found in U.S. Pat. No. 5,478,658. 
     As shown in  FIG. 3 , a first embodiment  300  comprises a glass substrate  302 , a first dielectric Bragg reflector (DBR)  312  (also known as a “Bragg stack”), a microcavity phosphor  314 , a second DBR  316 , an ITO anode  304 , a PEDOT hole transport layer  306 , a light emitting polymer (LEP) electroluminescent layer  308 , and a cathode  310 . The anode  304 , PEDOT layer  306 , LEP layer  308  and cathode  310  form an OLED device that is close-coupled to the microcavity phosphor  314 . The OLED device  304 ,  306 ,  308 ,  310  is arranged to pump the phosphor  314 , and light is emitted in the direction indicated by arrow  318 . 
     The microcavity is defined by the gap between the Bragg reflectors  312  and  316 , and contains the microcavity phosphor  314 . The thickness of the microcavity (i.e. the distance between the reflectors  312  and  316 ) will be readily apparent to the skilled person. Computer modelling may be employed to optimise the thickness of the microcavity to allow for distributed reflections. 
     In principle, any phosphor (e.g. as typically used for plasma screen displays or fluorescent lighting) may be used to form the microcavity phosphor  314 . Candidate materials include laser dyes (e.g. rhodamine) doped in a transparent matrix (e.g. poly(methyl methacrylate) (PMMA)) to prevent aggregation. Desirable phosphor requirements are a good susceptibility to optical pumping (rather than electrical pumping), high photoluminescence (PL) efficiency, good transparency to emitted wavelengths, and relatively high absorbance above the bandgap. 
     The first Bragg reflector  312  is preferably configured to reflect  100 % of the light generated by the OLED (i.e. light of wavelength λ OLED ), and to reflect a portion and transmit a portion of the light generated by the phosphor  314  (i.e. light of wavelength λ PHOSPHOR ). The high reflectivity (ideally 100%) of the first Bragg reflector  312  to light of wavelength λ OLED  improves the efficiency of the coupling between the OLED and the microcavity phosphor, as any light generated by the OLED that is not absorbed in its first pass through the phosphor is reflected back into the phosphor, providing a further opportunity for the OLED light to be absorbed by the phosphor. 
     The second Bragg reflector  316  is preferably configured to reflect 100% of light of wavelength λ PHOSPHOR , and 0% (i.e. complete transmission) of light of wavelength λ OLED . This configuration enables all the light generated by the OLED to enter the microcavity, and none of the light generated by the phosphor to escape through the reflector  316 , thereby further enhancing the efficiency of the coupling between the OLED and the phosphor. 
     The use of DBRs is well known in the field of laser design to achieve a directional output. Here, the optical structure of the microcavity and surrounding Bragg reflectors  312 ,  316  is preferably such that the intensity of the optical mode is maximised at the light emitting species. As will be apparent to the skilled person, a high-q cavity may be preferable for certain applications. 
     An array of OLED pixels may be used with a corresponding array of microcavity phosphors to produce an array of pixels in a visual display having high light output. A phosphor surrounded by a micro-cavity is placed under an array of OLED pixels. As will be apparent to the skilled person, a high-q cavity may be preferable for certain applications. For an RGB array of pixels, the light emitting polymer may be blue and the phosphors may be patterned in a red, green and blue configuration. The array is arranged such that the phosphor absorbs most of the light generated by the OLED pixels (not just the fraction which would escape into the air) and the absorbed light is emitted with well defined directional properties. 
       FIG. 4  illustrates schematically a portion of a series or array of edge emitting microcavity phosphor pixels, in which three edge-emitting microcavity phosphors  410 ,  412 ,  414  are independently operable to produce directional edge-emitted rays of light  411 ,  413 ,  415 . 
     Microcavity phosphors may also be used to generate lasers. Both forward emission and edge emission lasers may be formed.  FIGS. 5 and 6  illustrate possible arrangements to achieve forward emission lasing, and  FIGS. 7 and 8  illustrate possible arrangements for edge emission lasing. 
     Firstly with reference to  FIG. 5 , a polymer film  514  may be deposited on a glass substrate  512  in which an OLED and microcavity phosphor  510  are situated. The upper surface of the polymer film  514  is provided with a pattern of dimples, shaped in a sinusoidal manner such that the polymer sheet effectively acts as a Bragg grating. The microcavity phosphor  510  effectively acts as a pump source for a forward emission laser. As shown in  FIG. 6 , the OLED and microcavity phosphor emit a first order light output  610  through the polymer film, in the “ 1 ” direction. A small amount of light  612  may be emitted in the opposite direction. Second order light rays  614 ,  616  emitted perpendicularly, in the “ 2 ” directions, are reflected back (rays  615 ,  617 ) towards the microcavity phosphor. These rays combine coherently to provide lasing in the primary “ 1 ” direction  610 . 
       FIGS. 7 and 8  are schematic cross-sectional and plan views respectively, illustrating a series of OLED/microcavity phosphor pump sources  712 ,  722 ,  724 ,  726 ,  728 ,  730  used to produce a series of independently-operable edge emission lasers. In the example shown in  FIG. 8 , sources  712 ,  722 ,  726  and  730  are activated, to generate output laser beams  720 ,  723 ,  727  and  731  respectively. 
     As shown in  FIG. 7 , phosphor  712  is pumped by an OLED (not show)). The material  710  in which the phosphor  712  is located is patterned laterally to the substrate to form a sinusoidal profile in regions  714  and  716 . The material  710  may be a polymer that is patterned by methods well known to the skilled person. For example, the polymer may be patterned by embossing. Region  714  is configured so that 100% of the light that is emitted from the phosphor  712  in the direction of region  714  is reflected back towards the phosphor  712  (the outward and reflected light being indicated by arrow  718  in  FIG. 7 ). Region  716  is arranged to give partial reflection back towards the pump source  712 . The light rays that are reflected back towards the phosphor  712  combine coherently to provide edge emission  720 . The necessary optical thickness of the laterally patterned regions will be apparent to the skilled person. The phosphor  712  could be laterally patterned as an alternative to, or in addition to, lateral pattering of material  710 . 
     To improve the efficiency of the microcavity phosphor, a metal mirror may be provided on the side of the phosphor remote from the OLED to reflect light not absorbed in the first pass through the phosphor. 
     Both forward emission and edge emission lasers obtained using OLEDs and microcavity phosphors may be designed as narrow band distributed feedback lasers (DFBs), for example for telecommunications applications. 
     As those skilled in the art will appreciate, to ensure efficient operation of these devices it is desirable to have strong coupling between the emission profile of the OLED and the optical absorption (pumping) profile of the light emitting species. This is illustrated schematically in  FIG. 9 . A cross section through the OLED and microcavity phosphor of  FIG. 3  is shown on the left of  FIG. 9 , using the same reference numerals as in  FIG. 3 . Alongside the representation of the OLED ( 304 ,  306 ,  308 ,  310 ), DBRs  312 ,  316  and microcavity phosphor  314  is a plot illustrating variations in field strength E with position through the OLED, DBRs and microcavity phosphor, for both light emitted from the OLED and light emitted by the microcavity phosphor. The variation in field strength E with position for light emitted from the OLED is represented by the solid line, and the variation in field strength E with position for light emitted by the microcavity phosphor  314  is represented by the dashed line. As illustrated, it is desirable for the emission mode profile of the light emitted by the OLED device  910  to match the absorption profile  912  of the microcavity phosphor  314  to achieve efficient coupling between the OLED and the microcavity phosphor.