1. Field of the Invention
The aspects of the present disclosure relate generally to the field of light emitting electrical packages, and in particular to supplying power, reduce operating voltage and electrical/optical instability to organic electroluminescent devices.
2. Description of Related Art
An Organic Light Emitting Diode (OLED) is a type of electroluminescent device in which light is generated within an organic compound formulated to emit light when electric current is applied. OLEDs are well known in the art and are typically built as a laminate on top of a suitable substrate material such as glass or a polymer. An OLED consists of one or more layers of organic material sandwiched between two electrodes. One electrode is a negatively charged cathode usually made from a highly reflective metal and the other is a positively charged anode usually made from a transparent conductive metal oxide. Photons generated in the organic material will be reflected off the metallic cathode or pass through the transparent anode to exit the device as light. When a voltage is applied across the two electrodes, a current of electrons flows from the cathode, through the organic material to the anode. Electrons enter the lowest unoccupied molecular orbit (LUMO) of the organic material from the cathode and exit from the highest occupied molecular orbit (HOMO) of the organic material to the anode. Electrons exiting the organic material leave behind positively charged regions called holes. When these electrons and holes meet at a luminescent center, usually in an organic molecule or polymer, they combine to form excitons which will decay releasing photons. The released photons have a frequency proportional to the energy gap between the HOMO and LUMO of each emitting molecule. The generated photons can then pass through the transparent substrate and exit from the bottom of the device as light.
An OLED is commonly fabricated from two types of organic materials, small molecules and polymers. Commonly used small molecules include organometallic chelates, fluorescent and phosphorescent dyes and conjugated dendrimers. A second type of OLED is constructed from conductive electroluminescent or electro-phosphorescent polymers. These devices are sometimes referred to as Polymer Light Emitting Diodes (PLED) or Polymer Organic Light Emitting Diodes (P-OLED). Typical polymers used in P-OLED construction include electroluminescent derivatives of poly(p-phenylene vinylene) and polyfluorene or electro-phosphorescent materials such as poly(vinylcarbazole). Traditionally, the term OLED referred only to devices constructed from small molecules. However in recent years OLED has been used to refer to both small molecule and polymer type of devices. For the purposes of this disclosure the terra Organic Light Emitting Diode and the acronym “OLED′” is defined to refer generally to electroluminescent devices constructed using both types of organic material. When referring to a specific type of organic material, SM-OLED is used to describe a Small Molecule Organic Light Emitting Diode, and P-OLED is used to refer to a Polymer Organic Light Emitting Diode.
The organic material serves three main functions: hole transport, electron transport, and emission. A basic three layer device uses a layer of n-type material for the electron transport layer (ETL), a layer of p-type material for the hole transport layer (HTL), with emissive layer (EML) of electroluminescent material, usually fluorescent or phosphorescent dyes, in between. The emissive layer can be either a separate layer in between the ETL and HTL or a dopant, close to the recombination zone, in one of these layers. Layers may be combined or additional layers may or may not be included in the light emitting structure without straying from the fundamental concept of an organic light emitting diode (OLED) presented here.
OLEDs have the potential for being very efficient light sources. In order to optimize OLED efficiency, the distribution of charges must be balanced within the device. The calculation of carrier distribution in the organic layers can be quite complex due to the presence of both types of charge carriers, electrons as well as holes, in working devices. Consequently recombination and neutralization of carriers must be regarded. OLED device operation is determined by three processes: charge injection, charge transport and recombination. The dominant effect in hole and electron currents has been long discussed. The basic equations describing electron transport and distribution are well known (see J. Appl. Phys. 100, 084502-2006) and are the current-flow equation for carrier drift and the Poisson equation,
where μis the drift mobility, which is assumed constant, n is the electron carrier density, e is the elementary charge, J0 is the steady-state current density, which is constant across the sample, ε is the dielectric constant, and E is the electric field intensity. The solution of the above equations with boundary condition E=0 at the cathode is given by the Mott-Gurney square-law equation,
where V0 is the voltage across the organic layer. Note that current flow and efficiency of the device depends on defect formation and electric field. Space charge formation due to electric field can be great. Space charge can build up at the electrodes and at abrupt interlayer boundaries between the organic materials used in the ETL, EML, and HTL.
Space charge causes a number of problems in OLEDs. Space charge can cause chromatic variations across the surface of an OLED, and instability of the space charge regions can lead to undesirable flickering effects. In large area OLEDs, such as those used in lighting applications, these effects become even more noticeable and therefore more undesirable. When space charge is sustained over extended periods of time, it can damage the organic materials thereby limiting the useful lifetime of the device. Because space charge tends to oppose voltage applied to the device it increases the required operating voltage resulting in lower efficiency.
For many purposes, one may desire light emitting devices or OLEDs to be generally flexible, i.e. are capable of being bent into a shape having a radius of curvature of less than about 10 cm. These light emitting devices are also preferably large-area, which means they have a dimension of an area greater than or equal to about 10 cm2, and in some instances are coupled together to form a generally flexible, generally planar OLED panel comprised of one or more OLED devices, which has a large surface area of light emission. Flexible OLED devices usually comprise a flexible polymeric substrate, which while flexible, does not prevent moisture and oxygen penetration.
Accordingly, it would be desirable to provide an organic light emitting diode device that addresses at least some of the problems identified above.
As described herein, the exemplary embodiments overcome one or more of the above or other disadvantages known in the art.
One aspect of the exemplary embodiments relates to an organic electroluminescent device. In one embodiment the device includes an organic light emitting diode having a first electrode and a second electrode. A power supply is electrically coupled to the first electrode and the second electrode and is configured to generate a forward bias voltage and a reverse bias voltage pulse. The power supply is configured to alternately connect a forward bias voltage to the first electrode and the second electrode, and a reverse bias voltage pulse to the second electrode and the first electrode.
Another aspect of the exemplary embodiments relates to a method for reducing space charge in an organic light emitting diode that includes a first and second electrode. In one embodiment, the method includes applying a forward bias voltage across the first and second electrode of the organic light emitting diode such that light is generated in an electroluminescent layer of the organic light emitting diode; applying a reverse bias pulse across the first and second electrode of the organic light emitting diode such that space charge is removed; and re-applying the forward bias voltage across a first and second electrode of the organic light emitting diode.
Another aspect of the exemplary embodiments relates to an organic electroluminescent device. In one embodiment, the device includes an organic light emitting diode having a first electrode and a second electrode; an H-Bridge drive circuit having a center leg, a first upper leg, a second upper leg, and two lower legs; a signal generator; and a power supply having a first voltage and a second voltage, wherein each of the first upper leg, second upper leg and two lower legs of the H-Bridge comprises a switch; wherein the first and second electrode are electrically connected across the center leg of the H-Bridge, the first upper leg is configured to receive the first voltage from the power supply, the second upper leg is configured to receive the second voltage from the power supply, the two lower legs are electrically connected to an electrical ground, and the signal generator is configured to alternately energize the switches as a first pair and a second pair; and wherein when the first pair of switches is energized the first electrode is electrically connected to the first voltage and the second electrode is electrically connected to the electrical ground, and when the second pair of switches is energized the second electrode is electrically connected to the second voltage and the first electrode is electrically connected to the electrical ground.
These and other aspects and advantages of the exemplary embodiments will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. Additional aspects and advantages of the invention will be set forth in the description that follows, and in part will be obvious from the description, or may be learned by practice of the invention. Moreover, the aspects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
In the drawings:
Referring to
In the exemplary embodiment illustrated in
In the OLED device 100 shown in
For context, hereinbelow are described additional features of an organic light emitting electrical package of the present disclosure. The organic light emitting electrical package as a whole is configured to be flexible and/or conformal; that is, the light emitting electrical package comprises flexibility sufficient to “conform” to at least one predetermined shape, at least once. For example, as will generally be understood, a “conformal” light emitting electrical package may be initially flexible enough to wrap around a cylinder body to form a fixture, and then not be flexed again during its useful lifetime. Alternatively, the light emitting electrical package may remain generally flexible over its useful lifetime such as for a flexible display that may be rolled up for storage. The light emitting electrical packages according to the present disclosure are generally flexible (or conformable).
Generally, the anode layer 102 may be comprised of a substantially transparent nonmetallic conductive material. The requirements for a good transparent conductive nonmetallic coating (e.g., ITO) for OLED applications can be summarized by high light transmission (>than about 90%), low sheet resistance of 1 to 50 Ω/sq, high work function (sometimes as high as 5.0 eV) and low roughness below 1 nm (RMS). However, as a practical matter such desired parameters are not always easily achieved. Furthermore, transparent conductive nonmetallic coatings are typically brittle and may have defects due to processing conditions. Suitable materials for embodiments of the present disclosure include, but are not limited to, transparent conductive oxides such as indium tin oxide, indium gallium oxide (IGO), aluminum doped zinc oxide (AZO), fluorine doped tin oxide (FTO), zinc oxide, zinc-oxide-fluoride (fluorine doped zinc oxide), indium doped zinc oxide, magnesium indium oxide, and nickel tungsten oxide; conductive polymers such as poly(3,4-ethylenediosythiophene)poly(styrenesulfonate) (PEDOT:PSS); and mixtures and combinations or alloys of any two or more thereof. Other substantially transparent nonmetallic conductive materials would be apparent to those of ordinary skill in the field.
Cathodes, such as cathode 106 shown in
In certain embodiments, the organic light-emitting layer 104 is built up over the first electrode layer 102 by solution-phase deposition, followed by solvent-assisted wiping or other patterning, and then a cathode layer 106 is deposited over the organic light emitting layer by a vapor deposition, e.g., 100-1000 nm thick aluminum film. In one embodiment, the OLED device 100 comprises a continuous un-patterned anode layer 102 and a discontinuous cathode layer 106 configured in a plurality of ribbon-like structures. The term “ribbon-like” refers to the dimensions of the lighted areas of the device 100, which may be long and narrow and thin in cross-section.
Organic light emitting diodes tend to be very efficient making them attractive for lighting applications. Large area devices being designed for these lighting applications often require a driving voltage excess of 10 volts and it is not uncommon for these devices to operate with driving voltages between 18 to 25 volts. To turn the OLED on, a forward biased voltage is applied to the device 100 with the positive voltage being applied to the anode 102 and the negative voltage applied to the cathode 106 causing current to flow from the anode 102 to the cathode 106 (i.e. electrons flow from the cathode to the anode).
When holes are injected into the HTL 103 from the anode 102 excess electrons are left behind in the anode 102. Likewise, when electrons are injected into the ETL 105 excess positive charge is left in the cathode 106. In perfect conductors these excess charges would be continuously drained off, however materials used for electrodes 102, 106, as well as the organic semiconductors are not perfect conductors and limit mobility of the charge carries. This results in the build-up of an electric charge, known as a space charge, in regions of the device 100. Space charge builds up around the electrodes 102, 106, and can also buildup at abrupt interlayer boundaries between the organic materials used for the ETL 105, EML 104, and HTL 103. This space charge tends to oppose the forward bias voltage thereby reducing the amount of current flowing in the device 100. In effect the space charge is limiting the amount of current flowing through the device 100. Space charge build up causes a number of undesirable effects in OLED lighting devices. It can cause instabilities resulting in flickering or it can manifest as uneven light output across the surface of the device. Space charge can cause chromatic variations that result in undesirable color variations. When the organic compounds are exposed to space charge for extended periods of time, as would be the case in lighting applications, the organic compounds can be damaged reducing the usable lifetime of these electroluminescent devices.
Aspects of the present disclosure address methods of removing space charge from an OLED. Applying a reverse bias voltage, which means applying a negative voltage to the anode 102 and a positive voltage to the cathode 106, can quickly remove built up space charge from an OLED device 100. Applying the reverse bias voltage, Vrb, for an extended period of time will stop generation of light and turn the device 100 off which can cause undesirable effects. If however, the reverse bias voltage is applied as a pulse 301 of relatively short duration, the light output of the device 100 is not noticeably interrupted.
Perception speed of the eye is a complex question but it is generally accepted that events with duration of less than a few milliseconds will not be negatively perceived. For example, the power grid in North America operates at 60 Hz and in Europe at 50 Hz, resulting in flicker of fluorescent lighting with a period of about 5 to about 8 milliseconds. This is generally viewed as acceptable. The electrical time constant of OLEDs is on the order of about 10 μs (microseconds). A parameter can be defined that represents the time elapsed before the intensity of delayed electroluminescence (EL) decreases to half of its value at the time the forward bias voltage is turned off, referred to as delayed EL half-life and denoted by t1/2. For a typical OLED device 100 t1/2 is about 900 μs. Thus, a reverse bias pulse of one to a few microseconds applied to OLED device 100 will cause little variation in light output of the device 100. Charge neutralization can be accomplished in less than 1 microsecond (μs). One exemplary embodiment of the drive signal illustrated in
An exemplary embodiment of an OLED driving device that can be used to apply drive signal 300 to an OLED 100 is shown in
Referring now to
Embodiments of the invention may comprise a method for reducing space charge in an organic light emitting diode, which includes a first electrode and a second electrode. Unless otherwise indicated, the method steps are performed by an apparatus, such as a circuit, a processor, a power supply, etc., and may be performed in any suitable order.
For example, one such exemplary method may comprise applying a forward bias voltage across the first electrode and the second electrode of the organic light emitting diode such that light is generated in an electroluminescent layer of the organic light emitting diode. The method may further comprise applying, using an embodiment of the power supply described herein, a reverse bias pulse across the first electrode and the second electrode of the organic light emitting diode such that space charge is removed. The method may also further comprise re-applying, via the power supply, the forward bias voltage across the first electrode and the second electrode of the organic light emitting diode.
The method may further comprise varying the forward bias voltage, via the power supply, over a luminescent voltage range of the organic light emitting diode thereby dimming a light generated by the organic light emitting diode. In one embodiment, the steps of applying a forward bias voltage, applying a reverse bias pulse, and re-applying the forward bias voltage can be performed such that a series of two or more reverse bias pulses are applied to the organic light emitting diode, wherein the reverse bias pulses are separated by regular intervals. Alternatively, the steps of applying a forward bias voltage, applying a reverse bias pulse, and re-applying the forward bias voltage can be performed such that a series of two or more reverse bias pulses are intermittently applied to the organic light emitting diode.
Thus, while there have been shown, described and pointed out, fundamental novel features of the invention as applied to the exemplary embodiments thereof, it will be understood that various omissions and substitutions and changes in the form and details of devices and method(s) illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit and scope of the invention. Moreover, it is expressly intended that all combinations of those elements and/or method steps, which perform substantially the same function in substantially the same way to achieve the same results, are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.