Patent Application
20170150573
This invention relates to organic electronic lighting systems such as OLED (organic light emitting diode) lighting systems and, in particular, to techniques for reducing the cost and increasing the lifetime of such systems.
Reducing the cost and increasing the lifetime of organic light emitting devices is important to the success of this technology. Typically such devices incorporate one transparent electrode, generally ITO (indium tin oxide) to enable the light to escape from the structure. Use of such material provides relatively good lateral conductivity and performance but is expensive and, in the long term, is not desirable for large scale production. The degradation of ITO-based devices has been investigated in “White-Light Generation and OLED Lifetime Issues”, by Aaron R. Johnson, Thesis, 2008; and in “Effect of driving method on the degradation of organic light emitting diodes”, P. Cusumano, F. Buttitta, A. Di Cristofalo, C. Cali, Synthetic Metals 139 (2003) 657-661. In this context it appears that pulsed operation of an OLED may mitigate degradation, although the observations are somewhat contradictory. An OLED pre-charge circuit is described in U.S. Pat. No. 7,079,092.
One alternative to the use of ITO is to employ an anode metal grid rather than a continuous anode electrode; in this case the metal grid provides for current distribution within the panel and the conductive polymer layer needs to exhibit sufficiently good lateral conductivity to provide even current distribution within a cell. In such a system it is observed that improved lifetimes are obtained when using gold. It would, however, be desirable to be able to use a wider range of materials, in particular copper. However in practice devices with copper anode electrode tracks are observed to have only short life times, typically a few hours.
According to the present invention there is therefore provided an ITO-free organic lighting system, the system comprising: a substrate bearing a set of anode electrode metal tracks; a conductive organic layer over said metal tracks; an organic light emitting layer over said conductive organic layer; and a cathode electrode layer over said organic light emitting layer; and a driver system having an output electronically coupled to said anode electrode metal tracks and to said cathode electrode layer, to drive said organic light emitting layer with a drive current to emit light; wherein said drive current is pulsed such that an operational lifetime of said lighting system is increased.
Experimentally this approach has been demonstrated to mitigate device failure, in particular where a copper anode electrode grid is employed—which facilitates good performance at low cost. Without wishing to be bound by theory it is speculated that this may be related to a reduction in migration of copper ions into the conductive organic layer.
In embodiments a percentage on-time of the light emitting layer is reduced from 100%, and a peak value of the drive current is increased, such that for a given, for example maximum, light output, the light output is maintained whilst driving the organic light emitting device with a reduced on-time.
In some embodiments the conductive organic layer is deposited directly onto the metal (copper) tracks. In other embodiments a protective, barrier layer may be provided between the metal tracks and the conductive organic layer; this layer may be a doped conductive polymer layer.
Although the copper may be in the form of a copper alloy to mitigate electromigration, surprisingly embodiments of the invention work well with low cost “pure” copper, that is copper which is not a deliberate alloy but which nonetheless may contain impurities up to, for example, 0.1% 0.5%, 1%, 2%, 3% or even 5%.
Nonetheless, in embodiments of this aspect of the invention, and also in embodiments of the other aspects of the invention described below, the anode electrode metal comprises a NiP alloy (a mixture of nickel and phosphorus), in particular a copper NiP alloy. Then, preferably, the NiP alloy forms a protective or a capping layer on the anode electrode metal, for example copper. This may be deposited by electro- or electrolessly plating the NiP alloy on the metal, for example copper. It has been found that this helps to increase device lifetime.
Also surprisingly, it has been found that with drive schemes of the type described the anode electrode metal may be deposited by electroplating or electroless deposition (for example on a thin printed conductive template). These types of deposition inherently require mobile ions and thus are techniques which one might expect should particularly be avoided, despite the advantage of being able to deposit a relatively thick layer of metal in a short time. However, the techniques we describe provide sufficient benefit to enable this type of deposition process to be employed. As previously mentioned, a preferred metal is copper but silver may also be used.
In embodiments the light emitting structure is an organic light emitting diode (OLED) structure. This may comprise a light emitting polymer (LEP), or small-molecule, stack between a hole injection layer and a cathode layer. As the skilled person will appreciate, the LEP stack may comprise multiple layers depending on the construction of the OLED. Similarly, as described later, the cathode layer may comprise multiple layers of different materials. The skilled person will further appreciate that where in this specification one layer is described as being over another layer this does not necessarily mean that the layers are directly on top of one another.
In preferred embodiments the conductive organic layer is a hole injection layer, preferably comprising a doped conducting polymer, in embodiments (polyethylene dioxythiophene) (PEDT), optionally substituted. The dopant may be a charge-balancing polyacid such as polystyrene sulphonate (PSS).
In embodiments the light emitting structure is a bottom-emitting structure; this may be in the form of a lighting tile. The driver system may be integrated with such a structure/tile or separate from the structure/tile.
In a related aspect the invention provides a method of increasing the lifetime of an organic lighting system, in particular an ITO-free organic lighting system, the method comprising: providing an organic lighting element comprising: a substrate bearing a set of anode electrode metal tracks; a conductive organic layer over said metal tracks; an organic light emitting layer over said conductive organic layer; and a cathode electrode layer over said organic light emitting layer; wherein said metal comprises copper, and wherein said conductive organic layer comprises a doped conducting polymer; and driving said organic lighting element with a pulsed current drive.
As used herein the lifetime of an organic lighting system may be defined either by the “T50 luminance”, the time in hours it takes the luminance of a device to decrease to half its value at turn on, or by the “T90 luminance”, the time in hours it takes the luminance of a device to decrease to 90% of its value at turn on.
In some preferred embodiments the current drive employs a biphasic pulse having a first, on-phase in which the lighting element is driven with a defined current and a second, off-phase in which the lighting element is driven with a defined potential difference across the element (OLED), in embodiments zero volts.
Thus in embodiments the current drive is a pulsed current drive with forward drive and substantially zero level drive portions. Where the anode electrode metal has a higher work function than the cathode electrode layer during the zero level drive portion of the current drive these different work functions provide a reverse bias electric field within the lighting element which, it is speculated, helps to clear unwanted charged impurities from the conductive organic layer. More generally, to achieve this a portion of an anode structure of the organic lighting element should have a higher work function than a portion of the cathode electrode structure of the element, in particular a cathode metal part of the cathode electrode structure. Alternatively the relevant internal electric field may be generated by one or both of the hole injection layer and an electron injection layer (if present) in addition to or instead of the anode electrode metal/cathode electrode layer, in which case the cathode electrode layer may be considered as including an electron injection layer (where present). Where the current drive is a pulsed current drive with forward drive and substantially zero level drive portions, preferably the zero level drive is an active drive to zero volts rather than just leaving the one electrode of the lighting element floating.
The frequency of the pulsed current drive apparently effects the device lifetime; preferably a frequency of less than 1000 Hz is employed. Preferably the frequency is greater than 30 Hz to avoid visual flicker. Similarly the duty cycle of the pulsed current (the percentage of time for which the forward drive is on) also appears to effect lifetime although a low percentage on-time also reduces the brightness of the lighting element. In some preferred embodiments the duty cycle (percentage on time) is at least 40%; an upper limit of the range may be 85%, 90%, 95% or 98%.
In a further related aspect the invention provides an OLED lighting system including an OLED lighting element with at least one copper electrode and a driver, wherein said driver is configured to drive said OLED lighting element with a pulsed current drive at a maximum light output of the system.
In some preferred embodiments the driver is configured to drive the OLED lighting element with a biphasic pulse. The biphasic pulse has a first, on-phase in which the OLED lighting element is driven with a defined current and a second, off-phase in which the OLED lighting element is actively driven with a defined potential difference across the OLED, in embodiments zero volts.
These and other aspects of the invention will now be further described, by way of example only, with reference to the accompanying Figures in which:
In this specification references to organic LEDs include organometallic LEDs, and OLEDs fabricated using either polymers or small molecules. 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.
OLED devices (which here includes organometallic devices and devices including one or more phosphors) may be fabricated using either polymers or small molecules in a range of colours and in multi-coloured displays depending upon the materials used. For general background information reference may be made, for example, to WO90/13148, WO95/06400, WO99/48160 and U.S. Pat. No. 4,539,570, as well as to “Organic Light Emitting Materials and Devices” edited by Zhigang Li and Hong Meng, CRC Press (2007), ISBN 10: 1-57444-574X, which describes a number of materials and devices, both small molecule and polymer.
To aid in understanding embodiments of the invention it is helpful to describe an example structure of an OLED lighting tile. Thus referring to
A hole injection layer 16 is deposited over the anode electrode tracking, for example a conductive transparent polymer such as PEDOT:PSS (polystyrene-sulphonate-doped polyethylene-dioxythiophene). This is followed by a light emitting polymer (LEP) stack 18, for example comprising a PPV (poly(p-phenylenevinylene)—based material: The hole injection layer helps to match the hole energy levels of this layer to the anode metal. This is followed by a cathode layer 20, for example comprising a low work function metal such as calcium or barium with an optional electron injection layer (EIL; not shown) such as lithium fluoride or, more preferably, sodium fluoride, or a charge transporting polymer for energy matching, over which is deposited a reflective back (cathode) electrode 22, for example of aluminium or silver. Where an electron injection layer is employed the low work function metal may be omitted. Preferably the light emitting structure is encapsulated to reduce oxygen/moisture ingress and increase device lifetime.
The example of
Referring now to
Referring again to
The anode may comprise any material with a work function suitable for injection of holes into the light emitting layer/stack. The hole injection layer preferably comprises a conducting material; it assists hole injection from the anode into the light emitting layer. Representative examples of materials that may be used to form the hole injection layer include PEDOT:PSS, PANI (polyaniline), polypyrole, optionally substituted, doped poly(ethylene dioxythiophene) (PEDT), in particular PEDT doped with a charge-balancing polyacid such as polystyrene sulfonate (PSS) as disclosed in EP0901176 and EP0947123, polyacrylic acid or a fluorinated sulfonic acid, for example Nafion®; polyaniline as disclosed in U.S. Pat. No. 5,723,873 and U.S. Pat. No. 5,798,170; and optionally substituted polythiophene or poly(thienothiophene). Other suitable materials are summarized in the book by Zigang Li and Hong Meng, Chapter 3.3 page 303-12. Examples of conductive inorganic materials include transition metal oxides such as VOx, MOx and RuOx as disclosed in Journal of Physics D: Applied Physics (1996), 29(11), 2750-2753. Suitable materials for use as the hole injection layer are commercially available, e.g. from Plextronics Inc. Where a hole injection layer is employed an auxiliary layer of organic conductive material may optionally be included between the anode electrode tracks and the hole injection layer.
Preferably the hole injection layer is deposited by a solution-based processing method over the anode tracks. Any conventional solution-based processing method may be used. Representative examples of solution-based processing methods include spin coating, gravure printing, flexographic printing, dip coating, slot die coating, doctor blade coating and ink-jet printing. In preferred methods, however, depositing is by spin coating. The parameters used for spin coating the hole injection layer such as spin coating speed, acceleration and time are selected on the basis of the target thickness for the layer. After deposition, the hole injection layer is preferably annealed by heating, e.g. at 150 to 200° C. for 2 to 30 minutes in air. The thickness of the hole injection layer may be in the range 15 to 200 nm, for example around 130 nm. The rest of the LEP stack (including the interlayer where present) may similarly have a thickness of order 100-200 nm.
The electrode metal may comprise aluminium, titanium, tantalum, molybdenum or steel but copper is preferred as it is highly conductive and is cheap. Where the structure is such that light shines through an electrode the electrode may be in the form of a regular or irregular grid and/or thin enough to allow light through. Use of a NiP alloy as part of an electrode, in particular the anode, can help to increase device lifetime, it is speculated by mitigating electromigration as well as in other ways.
The NiP may be present as a layer of an electrode; in embodiments the layer comprising NiP alloy has a thickness of 1 nm to 1000 nm, more preferably 1 nm to 200 nm and still more preferably 5 nm to 100 nm. Preferably the layer comprising NiP alloy has a substantially uniform thickness. In embodiments at least one polymeric layer is deposited onto the electrode, for example a hole injection layer. Preferably the polymeric layer comprises acidic groups. Representative examples of hole injection layers include poly(3,4-ethylenedioxythiophene) (PEDOT), PEDOT:PSS, polythiophene conductive polymer, polyaniline (PANI), polypyrole, polyacrylic acid or a fluorinated sulfonic acid, for example Nafion. Preferably the polymeric layer is solution processed. Advantageously the NiP alloy constitutes a hydrophilic layer on the metal and facilitates deposition of polymeric layers by solution processing from water.
In embodiments an electrode comprising an NiP alloy, for example an NiP alloy layer, comprises 1 to 15% wt phosphorus, more preferably 5 to 12% wt and still more preferably 8 to 12% wt phosphorus. The phosphorus content of the NiP alloy is preferably determined by the test set out in ISO4527 Annex D. The amount of phosphorus present in the NiP alloy is important as it can affect the microstructure and performance characteristics of the alloy. Usually NiP alloys comprising greater than about 10% wt phosphorus are amorphous and those comprising less than about 8% wt phosphorus are microcrystalline. Alloys containing an intermediate amount of phosphorus, e.g. 8 to 10% wt are generally semi-crystalline. An NiP alloy may be present in one or both the electrodes of the present invention and may be semi-crystalline or amorphous, more preferably amorphous. Semi-crystalline and particularly amorphous NiP alloys comprise fewer grain boundaries that may act as sites for intergranular corrosion and therefore tend to provide improved corrosion resistance compared to crystalline NiP alloys.
Some preferred NiP alloys present in an electrode comprise 85-99% wt nickel, 1 to 15% wt of phosphorus and 0-2% wt of impurities, e.g. other metal such as Pd. Still further preferred NiP alloys comprise 90-92% wt nickel, 8-10% wt phosphorus, and 0-2% wt of impurities, e.g. other metal such as Pd. Further preferred NiP alloys present in an electrode may consist essentially of, e.g. consist of, nickel and phosphorus. Low amounts (e.g. 0 to 2% wt) of impurities such as a second metal used as a catalyst may be present. Particularly preferred NiP alloys consist of 85-99% wt nickel and 1 to 15% wt of phosphorus, wherein the total weight of nickel and phosphorus is 100%. Still further preferred NiP alloys consist of 90-92% wt and 8-10% wt of phosphorus, wherein the total weight of nickel and phosphorus is 100%.
Copper is a low cost metal with a suitable work function for OLED devices, and is potentially useful for anode metal tracking, but devices fabricated using copper suffer from failure due to shorting. Copper is also highly reactive and oxidises when in contact with aqueous materials. These problems are compounded by the preferred method of deposition, electroplating. The inventors have investigated the problems associated with the use of copper and it appears that the presence of the copper: PEDT interface may lead to electromigration of ionic impurities, which is believed to be responsible for the observed phenomena, in particular the very short device lifetime. The device lifetime can be improved by including a protective barrier layer between the copper and the overlying layers, typically an overlying doped interlayer. In the case of copper, protection may be provided by one or more self-assembled monolayers of aromatic thiols in alkaline solutions (Phys Chem Chem Phys. 2010 Aug. 28; 12(32):9230-8, “Copper protection by self-assembled monolayers of aromatic thiols in alkaline solutions”, Caprioli F et al.). Nonetheless the performance of such structures falls well short what is desirable.
The inventors have established that using a pulsed drive scheme with copper electrodes, with or without an intermediate doped barrier layer, can significantly increase the device lifetime as well as improving the initial luminance decay.
Experiments have further established that a biphasic, rectangular pulse drive is preferred with the pulses having a first, forward drive portion and a second, device protection portion. In the forward drive portion of the waveform the OLED device is preferably driven by a defined or programmed forward current (that is, in embodiments, the forward drive portion of the waveform is a current-programmed rather than voltage-programmed drive portion). In the second, device-protect phase of the rectangular pulse waveform preferably the OLED is driven to a defined voltage, in embodiments zero volts. More particularly the OLED is driven so that there is a defined potential difference across the OLED—that is across the anode and cathode electrodes (rather than simply leaving one or other electrode floating). Although in principal a negative, reverse biased may be employed in this second portion of the pulse waveform in practice, surprisingly, this has been found to be less effective than driving to zero volts, which is equivalent to shorting the OLED and cathode during the off phase.
Investigation into the effect of changing the frequency of the waveform suggests that frequencies of around 1 KHz are less effective than lower frequencies. An effective frequency range appears to be around 300-500 Hz, for example around 400 Hz. It may be preferable to employ a higher than 50% on-time duty cycle (percentage of time during which the device is forward driven) to reduce to the overdrive, where this is employed to offset the time-averaged reduction in overall luminosity.
Without wishing to be bound by theory we believe that driving the device to zero voltage during a negative cycle is equivalent to reverse biasing the device, since charged impurities will tend to equilibrate due to the built-in field from the differing work functions of the anode and cathode metals. Surprisingly, it is apparently better to actively drive the voltage across the device to zero volts rather than simply disconnecting one or other of the anode and cathode electrodes to leave the device floating, an effect which appears to be related to parasitic capacitance.
Providing an off-portion of the duty cycle may also help to offset any ionic drift induced by the forward bias because, during the off-cycle, more heat can be dissipated than would be the case for a device driven by a DC drive (always on), which may provide a second order benefit to device lifetime from improved heat dissipation during the off-portion. This is counterintuitive since the time-averaged power applied to the device is the same, or for typical OLEDs higher than in the DC case (depending on the drive conditions and the shape of the OLED characteristic curve of lumens per watt efficiency versus luminance).
It has been demonstrated that OLED devices with copper metal tracking can have an improved lifetime if a pulsed drive scheme is employed. This facilitates the use of cheaper technology and potentially also removes a requirement for intermediate protection/barrier layers, thus potentially bringing a further cost reduction.
No doubt many other effective 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 and lying within the scope of the claims appended hereto.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1411493.8 | Jun 2014 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2015/063835 | 6/19/2015 | WO | 00 |
