Abstract:
An optocoupler including: a substrate comprising a photodetector; a transparent electrically-insulating layer disposed over the photodetector; and an organic electroluminescent device having an organic electroluminescent layer disposed between a first and a second electrode disposed over the transparent electrically-insulating layer; the photodetector arranged to detect light emitted from the organic electroluminescent device; wherein the optocoupler comprises a second current path between the first and second electrodes in addition to a first current path between the first and second electrode which in operation causes the organic electroluminescent layer to emit light.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to optocouplers, in particular to optocouplers incorporating organic electroluminescent devices. 
     2. Related Technology 
     Organic light emitting diodes (OLEDs) comprise a particularly advantageous form of light-emitting device. They are easy and cheap to fabricate on a variety of substrates. Organic (which here includes organometallic) LEDs may be fabricated using either polymers or small molecules in a range of colours, depending upon the materials used. Examples of polymer-based OLEDs are described in WO 90/13148, WO 95/06400 and WO 99/48160; examples of so called small molecule based devices are described in U.S. Pat. No. 4,539,507. 
     At their most basic, organic electroluminescent devices generally comprise an organic light emitting material which is positioned between a hole injecting electrode and an electron injecting electrode. The hole injecting electrode (anode) is typically a transparent tin-doped indium oxide (ITO)-coated glass substrate. The material commonly used for the electron injecting electrode (cathode) is a low work function metal such as calcium or aluminium. 
     The materials that are commonly used for the organic light emitting layer include conjugated polymers such as poly-phenylene-vinylene (PPV) and derivatives thereof (see, for example, WO-A-90/13148), polyfluorene derivatives (see, for example, A. W. Grice, D. D. C. Bradley, M. T. Bernius, M. Inbasekaran, W. W. Wu, and E. P. Woo,  Appl. Phys. Lett.  1998, 73, 629, WO-A-00/55927 and Bernius et al.,  Adv. Materials,  2000, 12, No. 23, 1737), polynaphthylene derivatives and polyphenanthrenyl derivatives; and small organic molecules such as aluminium quinolinol complexes (Alq3 complexes: see, for example U.S. Pat. No. 4,539,507) and quinacridone, rubrene and styryl dyes (see, for example, JP-A-264692/1988). The organic light emitting layer can comprise mixtures or discrete layers of two or more different emissive organic materials. 
     Typical device architecture is disclosed in, for example, WO-A-90/13148; U.S. Pat. No. 5,512,654; WO-A-95/06400; R. F. Service,  Science  1998, 279, 1135; Wudl et al.,  Appl. Phys. Lett.  1998, 73, 2561; J. Bharathan, Y. Yang,  Appl. Phys. Lett.  1998, 72, 2660; T. R. Hebner, C. C. Wu, D. Marcy, M. L. Lu, J. Sturm,  Appl. Phys. Lett.  1998, 72, 519); and WO 99/48160; the contents of which references are incorporated herein by reference thereto. 
     The injection of holes from the hole injecting layer such as ITO into the organic emissive layer is controlled by the energy difference between the hole injecting layer work function and the highest occupied molecular orbital (HOMO) of the emissive material, and the chemical interaction at the interface between the hole injecting layer and the emissive layer. The deposition of high work function organic materials on the hole injecting layer, such as poly(styrene sulfonate)-doped poly(3,4-ethylene dioxythiophene) (PEDOT/PSS), N,N′-diphenyl-N,N′-(2-naphthyl)-(1,1′-phenyl)-4,4′-diamine (NBP) and N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD), provides “hole transport” layers which facilitate the hole injection into the light emitting layer, transport holes stably from the hole injecting electrode and obstruct electrons. These layers are effective in increasing the number of holes introduced into the light emitting layer. 
     We will now describe a basic structure of a typical OLED. A glass or plastic substrate supports a transparent anode layer comprising, for example, indium tin oxide (ITO) on which is deposited a hole transport layer, an electroluminescent layer, and a cathode. The electroluminescent layer may comprise, for example, a PPV (poly(p-phenylenevinylene)) and the hole transport layer, which helps match the hole energy levels of the anode layer and electroluminescent layer, may comprise a conductive transparent polymer, for example PEDOT:PSS (polystyrene-sulphonate-doped polyethylene-dioxythiophene) from Bayer AG of Germany. The cathode layer typically comprises a low work function metal such as calcium or barium and may include an additional layer immediately adjacent the electroluminescent layer, such as a layer of lithium fluoride, for improved electron energy level matching. Contact wires connected to the anode and the cathode respectively provide a connection to a power source. The same basic structure may also be employed for small molecule devices. 
     In general, OLEDs consist of a multi-layer sandwich of indium-tin-oxide (ITO) as anode contact, one or more organic layers including a light emitting polymer (LEP) layer, and a metal layer as cathode, deposited on a planar substrate, usually of glass or a high refractive index plastic such as polycarbonate. In so-called “bottom emitter” devices, the multi-layer sandwich is deposited on the front surface of a planar glass substrate, with the reflecting electrode layer, usually the cathode, furthest away from the substrate, whereby light generated internally in the LEP layer is coupled out of the device through the substrate. We will describe an example of a bottom emitter, where light is emitted through transparent anode and substrate and the cathode is reflective. Conversely, in a so-called “top emitter”, the multi-layer sandwich is disposed on the back surface of the substrate, and the light generated internally in the LEP layer is coupled externally through a transparent electrode layer without passing through the substrate. Usually the transparent electrode layer is the cathode, although devices which emit through the anode may also be constructed. The cathode layer can be made substantially transparent by keeping the thickness of cathode layer less than around 50-100 nm, for example. 
     Organic electroluminescent devices may be advantageously employed in optocouplers, as described, for example, in U.S. Pat. No. 6,509,574. Previously, optocouplers employed inorganic light-emitting diodes. Commercial light emitting diodes (LEDs) typically constitute a p-n junction of inorganic, doped semiconducting materials such as gallium arsenide (GaAs) or aluminium gallium arsenide (AlGaAs). At these junctions between the doped layers, recombination of electrons and holes results in interband emission of light. 
     Heteroepitaxial growth of direct bandgap III-V compound semiconductors such as GaAs, InP, and GaP on silicon substrates, from which LEDs can be fabricated, yields highly defective material due to mismatches in lattice parameters and thermal expansion coefficients. These LEDs do not perform well, and the silicon circuits are affected during the heteroepitaxy due to the required high growth temperatures (typically &gt;600° C.). Further, achieving good electrical isolation is not easy in these approaches. 
     As an alternative, III-V LEDs have been integrated with a silicon drive circuit at the package level. But these approaches are expensive and not suitable for wafer-level integration. 
     Recently, organic light-emitting diodes (OLEDs) have drawn much attention, especially for emissive display applications. Since OLEDs can be fabricated on any smooth surface, such as silicon wafers, and at low (&lt;100° C.) temperatures, they are also very promising for many optoelectronic applications. 
       FIG. 1  shows an OLED optocoupler according to the prior art. An optocoupler, or optoisolator, is a coupling device in which a light-emitting diode, energized by the input signal, is optically coupled to a photodetector, such as a light-sensitive output diode, transistor or silicon-controlled rectifier. In  FIG. 1 , the optocoupler, generally designated  100 , is constructed on a semiconductor substrate  101 . The semiconductor material can be silicon, silicon germanium, gallium arsenide or any other semiconducting material used in photolithographic manufacturing. Integral to substrate  101  is an integrated circuit (not shown in  FIG. 1 ) and a semiconductor device  102  sensitive to electromagnetic radiation in a certain wavelength range. The radiation-sensitive device  102  is frequently referred to as photodetector. 
     Laying over photodetector  102  is a flat portion of an optically transparent, electrically insulating layer  103 , which has first surface  103   a  and second surface  103   b . The radiation-sensitive device  102  is integral with second surface  103   b , and the organic diode  104 , operable to emit electromagnetic radiation, is integral with the first surface  103   a . The radiation-emitting diode is commonly referred to as OLED (organic light-emitting diode). Consequently, photodetector  102  is electrically isolated from OLED  104  and positioned in the path of the emitted radiation. 
     However, the input current to an optocoupler incorporating an OLED emitting device may not necessarily be the correct value to produce the optimum OLED current. 
     For a particular device with a standard specified output current (for example 1 mA), the standard specified input current (for example 1 mA) may be too high for the OLED. Driving OLEDs at above their rated currents leads to reductions in device lifetime, at a rate approximately proportional to the square of the driving current. Increasing the device area increases the cost of the device, but reducing the device area (and increasing the brightness of the OLED) reduces the signal to noise ratio, as well as reducing the lifetime of the device. For a given standard input current and output current for an optocoupler, the input current may be too high for the optimum size of OLED. 
     SUMMARY OF THE INVENTION 
     We will describe the control of current through the OLED device by the use of the OLED device architecture (an alternative method would be to limit current using silicon circuitry). An advantage of this method is that it allows a standard input current to the optocoupler to be used without having additional circuitry on either the input (which would limit the device&#39;s applicability) or on the detector silicon. 
     According to an aspect of the present invention there is provided an optocoupler including: a substrate comprising a photodetector; a transparent electrically-insulating layer disposed over the photodetector; and an organic electroluminescent device having an organic electroluminescent layer disposed between a first and a second electrode disposed over the transparent electrically-insulating layer; the photodetector arranged to detect light emitted from the organic electroluminescent device; wherein the optocoupler comprises a second current path between the first and second electrodes in addition to a first current path between the first and second electrodes which in operation causes the organic electroluminescent layer to emit light. 
     By incorporating leakage paths within the device structure the optimum current can be delivered to the OLED. This may also provide enhanced lifetime for the OLED. 
     Preferably, the second current path is parallel to the first current path. 
     Preferably the substrate is a semiconductor substrate, more preferably silicon. The photodetector is preferably a light-sensitive semiconductor device comprising a transistor such as an opto-transistor, a diode such as an opto-diode, or any other suitable semiconductor device (e.g. thyristor, triac, silicon-controlled rectifier etc). Preferably the transparent electrically insulating layer is an optical waveguide and more preferably comprises an oxide layer such as silicon dioxide. 
     Preferably the organic electroluminescent device comprises an organic light-emitting diode (OLED), more preferably a bottom-emitter OLED, although top-emitting types may also be used. Preferably the organic electroluminescent device comprises a multi-layered structure, and the second current path comprises at least one layer of the multi-layered structure. For example, the organic electroluminescent device may comprise a hole transport layer (e.g. PEDOT:PSS), and the second current path may be fabricated from the same material in the same fabrication step. 
     Alternatively the second current path may comprise interlayer material, or a combination of these and/or other layers. Depending on the layers used, the second current path may have resistive and/or diode-like electrical characteristics. The organic electroluminescent device may comprise bank material, for example photoresist, and the second current path may be formed by opening up the bank and providing a hole or window in the bank for the second current path. 
     The electrical characteristics of the second current path may be configured to meet a particular specification, such as 1 mA output current at 1 mA input current. More generally, the relationship between the output current and the input current may be linear, for example 1:1. 
     According to another aspect of the present invention there is provided an optocoupler comprising a substrate bearing a light sensitive semiconductor device, an electrically-insulating layer disposed over the light-sensitive semiconductor device to conduct light to the light-sensitive semiconductor device, a first electrically-conducting layer disposed over the electrically-insulating layer, an organic electroluminescent layer disposed over a first part of said first electrically-conducting layer and a resistive layer disposed over a second part of said first electrically-conducting layer, and a second electrically-conducting layer disposed over the organic electroluminescent layer and the resistive layer. 
     According to a further aspect of the present invention there is provided an optocoupler comprising a substrate bearing a light sensitive semiconductor device, an electrically-insulating layer disposed over the light-sensitive semiconductor device to conduct light to the light-sensitive semiconductor device, a first electrically-conducting layer disposed over the electrically-insulating layer, an organic electroluminescent layer disposed over a first part of said first electrically-conducting layer, and a second electrically-conducting layer disposed over the organic electroluminescent layer and over a second part of said first electrically-conducting layer. 
     According to a yet further aspect of the present invention there is provided a method of fabricating an optocoupler, the method comprising fabricating a substrate comprising a photodetector; depositing a transparent electrically-insulating layer over the photodetector; and forming an organic electroluminescent device having an organic electroluminescent layer disposed between a first and a second electrode disposed over the transparent electrically-insulating layer; the photodetector arranged to detect light emitted from the organic electroluminescent device; wherein the method includes: forming a second current path between the first and second electrodes in addition to a first current path between the first and second electrodes which in operation causes the organic electroluminescent layer to emit light. 
     According to a further aspect of the present invention there is provided a method of optocoupling an input current to an output current, the method comprising driving an organic electroluminescent device with a portion of said input current to emit light, receiving said light at an inorganic semiconductor light-sensitive device to generate said output current, and shunting excess input current through an alternative current path to a current path through said organic electroluminescent device. 
     A further method of fabricating an optocoupler includes fabricating a substrate comprising a photodetector; depositing a transparent electrically-insulating layer over the photodetector; forming a first electrode layer; patterning the first electrode layer; depositing an organic electroluminescent layer over the first electrode layer; forming a second current path between the first electrode and a second electrode formed over the organic electroluminescent layer in addition to a first current path between the first and second electrodes which in operation causes the organic electroluminescent layer to emit light. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects of the present invention will now be described in detail, with reference to the accompanying Figures, in which: 
         FIG. 1  shows an OLED optocoupler of this prior art; 
         FIG. 2   a  shows a cross-section view of an optocoupler incorporating an organic light-emitting diode according to an aspect of the present invention; 
         FIG. 2   b  shows a cross-section view of an alternative optocoupler incorporating an organic light-emitting diode according to an aspect of the present invention; 
         FIG. 3  shows a cross-section view of an optocoupler incorporating an organic light-emitting diode according to another aspect of the present invention; and, 
         FIG. 4  shows a plan view of an indium tin oxide (ITO) layer of an optocoupler incorporating an organic light-emitting diode according to another aspect of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     We will describe an example of an optocoupler that is specified 1 mA input and 1 mA output. If we use an OLED device with an internal quantum efficiency 8.3% and an outcoupling efficiency of 60% which we may reasonably expect for this kind of device then an output of 1 mA is achieved with only 0.35 mA of input current. Without any form of sinking the additional current the OLED will be overdriven by a factor of three—resulting in a decrease in lifetime of up to a factor of 10. By sinking 0.65 mA the OLED can be driven at its optimum condition to deliver the required output. 
       FIGS. 2   a  and  2   b  illustrate an optocoupler  200  incorporating an alternative current path for the excess current. In this example the OLED provided is a bottom-emitter type, although it will be apparent to those skilled in the art that top-emitter OLEDs could also be provided. The OLED is configured to emit light through the anode, but OLEDs may also be provided which emit light through the cathode. 
     An N-type silicon substrate  201  has a p-type well  202  forming the base of a light-sensitive phototransistor into which light from an organic light-emitting diode may pass. N+ region  203  forms the emitter of the transistor and bond wires may be connected to emitter connection  204 . Silicon substrate  201  forms the collector of the transistor and contact metal  205  deposited on the underside of the substrate forms the collector connection. Collector and emitter connections together form the output side of the optocoupler; in alternative embodiments these may comprise anode and cathode connections of a photodiode; more generally input and output connections of a semiconductor device. 
     Insulating layers  206  and  207  provide electrical isolation between the phototransistor and the OLED. These may be made of silicon nitride, silicon dioxide, silicon oxynitride or any other suitable insulating material known in the art. Electrical isolation may be provided in excess of 1 kV, making the optocoupler useful for interfacing between low voltage control circuits and high voltage power devices in a variety of applications. Insulating layer  206  may be deposited and patterned before emitter connection  204  is made, and insulating layer  207  may be deposited afterwards. 
     Anode material  208  may comprise a transparent conductor such as indium tin oxide (ITO). This is deposited on the top surface of insulating layer  207 . Anode contact  209  may comprise a metal such as aluminium. This is useful for making bond wire connections to the anode. OLED layers  210 ,  211  and  212  are exemplary of a typical OLED layer structure. There must be one light-emitting polymer layer; other layers which may be present may comprise one or more of hole transport layers, electron transport layers and interlayers. Cathode material  213  may comprise a low work function metal such as aluminium, barium or calcium. This may be deposited on top of the OLED layer structure and bank material  214  (for example photoresist). 
     By opening the bank it is possible to create parallel current paths around the OLED device. The resistance of the parallel path may be controlled both by dimensions and by the use of PEDT:PSS and interlayer. 
     Thus, cathode metal  215  and OLED layer material  216  form a current path in parallel with the OLED to provide an alternative path for current to flow from the anode to the cathode. This path may be resistive in terms of its current-voltage characteristic or it may have diode-like characteristics similar to the OLED, depending upon the choice of materials for 216. Optionally  216  may comprise more than one layer, and it may also comprise materials not used in the OLED layer structure—but in this example OLED materials because they may be deposited in the same process steps used to fabricate the OLED (i.e. additional processing steps are not required). 
     For example, by using hole transport layer material such as PEDOT:PSS, a resistive parallel current path may be formed. The resistance of this path may be controlled by adjusting the dimensions of the structure, for example the thickness of the layer or the area. Area may be controlled at mask level, whereas thickness may be controlled during the fabrication process. 
     Alternatively, by using hole transport layer material and interlayer material, or interlayer by itself, a parallel current path with a diode-like characteristic may be formed. The parameters of this path may be controlled by adjusting the dimensions of the structures, and by adjusting the doping of (e.g.) the hole transport layer. Other combinations of OLED layer materials may also be employed. 
     The dimensions may be controlled to achieve a specified output current for a specified input current—for example 1 mA output current at 1 mA input current. The characteristics of the alternative current path may be controlled to achieve one particular current specification or they may be controlled over a range of input currents and output currents. The parallel current path may be ohmic, that is to say it may have a substantially constant resistance over a range of input currents and temperatures, or it may have a different characteristic. One series of characteristics with particular advantages is the series of characteristics proportional to the organic light-emitting diode characteristic. Thus, over a range of input currents to the optocoupler, the current through the alternate current path may be proportional to the current through the OLED. 
     The alternative current path may be provided at any point between the anode and the cathode, and it need not be provided in one place.  FIG. 2   b  shows an alternative optocoupler  300  having a different location for the alternative current path  315  and  316 , and it will be apparent to those skilled in the art that it could equally well be provided in both places or anywhere else between the anode and cathode. Furthermore, the Figures show cross-sections of the optocoupler; it will be appreciated that optocouplers according to the present invention may be three-dimensional structures, and the parallel current path may also be provided in different places in the axis perpendicular to the page. 
     In addition the resistance may also be controlled by etching ITO features on the anode.  FIG. 3  shows an optocoupler  400  according to this embodiment of the invention. Anode material  208  is extended underneath the cathode contact  401 , and a feature  402  is provided between the anode and the material under the cathode contact. This is shown diagrammatically in  FIG. 3  as being thinner than the rest of the ITO material, but it may advantageously comprise the same thickness, as it may be formed in the same process step as the rest of the ITO material. 
       FIG. 4  shows a plan view. By opening up the bank to the contact pad the resistance from cathode to anode contact can be limited by the resistance defined by the ITO geometry. The Figure shows a contact pad with current limiting etched ITO feature. The ITO layer  500  comprises anode material  508 , material  501  under the cathode and alternate current path feature  502 . The dimensions of this feature may be controlled to vary the resistance. An exemplary resistance that may be obtained using this feature is 40 ohms. 
     No doubt alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto.