Patent Application
20070018153
An organic light emitting diode (“OLED”) device is typically comprised of: (1) a transparent anode on a substrate; (2) a hole injection layer (“HIL”); (3) an electron injection and light emitting layer (“emissive layer”); and (4) a cathode. When a forward bias is applied, holes are injected from the anode into the HIL, and the electrons are injected from the cathode into the emissive layer. Both carriers are then transported towards the opposite electrode and allowed to recombine with each other, the location of which is called the recombination zone. The recombination of holes and electrons in the emissive layer produce excitons which then emit light.
The emissive layer in an OLED typically is composed of one or more organic compounds (such as monomers or polymers) dissolved in a solvent. The organic solution may contain other elements such as wetting agents, cross-linking agents, side-groups and so on. The emissive layer is fabricated by depositing this organic solution onto the HIL or other underlying layer and allowing or causing (by baking or cross-linking) the solution to dry into a film. The organic solution may be deposited using selective deposition techniques such as inkjet printing or non-selective deposition techniques such as spin-coating.
Displays made from OLED pixels may be either passive-matrix or active-matrix. Active-matrix displays are fabricated by including switching elements within each OLED pixel so that they can be individually activated or inactivated. Passive matrix displays have no pixel-internal switching elements and are driven instead by line by line scanning or multiplexing. As a result, passive-matrix displays require a higher voltage to drive them than active matrix or other displays. The high driving voltage increases typically when even more rows of display need to be addressed. This high driving voltage can tend to degrade the performance of the emissive polymer, and especially so over time, leading to lower lifetimes.
One problem with PPV and polyfluorene-based light emitting polymers, and generally any class of polymeric light emitting material, is that they exhibit lifetimes, particularly under multiplexed operation for passive matrix display applications that are too short for many commercially attractive applications. Until now, OLED devices have been fabricated with LEP thicknesses on the order of 70-80 nm which provide good photopic efficiency and reasonably low voltage requirements (<10 V). However, with very few exceptions, these device structures do not exhibit the required lifetimes.
Therefore, there is a need to improve OLED device efficiency and lifetime especially for particular applications of OLED displays.
In at least one embodiment of the invention, a “thick” light emitting polymer (LEP) layer is disclosed which has a thickness of more than eighty (80) nanometers and in some embodiments, a thickness of between eighty (80) and two hundred (200) nanometers. OLEDs utilizing thick LEP layers have been shown in experiments to provide better photopic efficiency and increase in lifetime than their thinner counterparts. Some applications of thick LEP include, but are not limited to, low multiplex rate passive matrix displays, low brightness displays, and products that can provide 12 to 20 Volts or more (such as lighting products powered with 110V or 220V AC) over the device lifetime. In other embodiments of the invention, the total thickness of the “organic stack” (typically consisting of the HIL layer and LEP layer) is held fixed by reducing the HIL layer thickness while the LEP layer thickness is increased.
An increase in LEP thickness is typically associated with an increase in required drive voltage. This might be expected to decrease efficiency and lifetime because of the additional stress on the device. To avoid this anticipated decrease in performance, and for the reason that many low voltage applications require thin LEP layers rather than thick LEP layers, it is atypical to use a thick LEP layer. However, as discussed above and demonstrated below, the thick LEP layer actually and unexpectedly increases efficiency and lifetime.
Thick LEP devices may also exhibit the following characteristics:
Considerable reduction in leakage current. The potential reduction in leakage current stems from the larger path (thicker LEP layer) through which current has to travel. This tends to reduce the possibility that a path for leakage current (due to materials defects) will be present. By providing inherently less leakage current in the LEP layer, the thickness of the HIL layer can be reduced. This material is typically made rather thick (>100 nm) in order to provide good coverage of surface defects. However, it has been demonstrated that longer device lifetimes can be achieved when the thickness of the HIL layer is reduced. By increasing the LEP thickness while holding the total organic layer thickness constant, additive improvements in device performance can be achieved.
Significantly better wetting properties of the LEP, thus reducing the number of pinholes; and
Significantly different Tg (glass transition temperature) compared with a thin layer, allowing better processing conditions. A higher Tg will enable the LEP layer to be processed (baked) at a higher temperature while still avoiding molecular ordering. Molecular ordering within the LEP layer may lead to increased leakage current as the path for conductivity is better defined in the ordered material.
In
As illustrated in
Again, the same five OLED devices tested with respect to
In other embodiments of the invention, the use of a thick LEP layer would allow the fabrication of EC-OLEDs by adding a dissociable salt into the LEP and using non-reactive metal electrodes whose work functions are not critical for the operation of the device. One fraction of the thickness of the LEP thick film would become p-doped at the anode and another fraction would become n-doped at the cathode, the middle fraction would then be the LEP layer. This would be most appropriate for lighting application where switching speed is not critical.
One or more organic materials is deposited to form one or more organic layers of an organic stack 416. The organic stack 416 is on the first electrode 411. The organic stack 416 includes a hole injection (conducting polymer) layer (“HIL”) 417 and light emitting polymer (LEP) layer 420. If the first electrode 411 is an anode, then the HIL 417 is on the first electrode 411. Alternatively, if the first electrode 411 is a cathode, then the active electronic layer 420 is on the first electrode 411, and the HIL 417 is on the LEP layer 420. The electronic device 405 also includes a second electrode 423 on the organic stack 416. Other layers than that shown in
Substrate 408:
The substrate 408 can be any material that can support the organic and metallic layers on it. The substrate 408 can be transparent or opaque (e.g., the opaque substrate is used in top-emitting devices). By modifying or filtering the wavelength of light which can pass through the substrate 408, the color of light emitted by the device can be changed. The substrate 408 can be comprised of glass, quartz, silicon, plastic, or stainless steel; preferably, the substrate 408 is comprised of thin, flexible glass. The preferred thickness of the substrate 408 depends on the material used and on the application of the device. The substrate 408 can be in the form of a sheet or continuous film. The continuous film can be used, for example, for roll-to-roll manufacturing processes which are particularly suited for plastic, metal, and metallized plastic foils. The substrate can also have transistors or other switching elements built in to control the operation of the device. A single substrate 408 is typically used to construct a larger OLED display containing many pixels (devices) such as device 405 arranged in some pattern.
First Electrode 411:
In one configuration, the first electrode 411 functions as an anode (the anode is a conductive layer which serves as a hole-injecting layer and which comprises a material with work function greater than about 4.5 eV). Typical anode materials include metals (such as platinum, gold, palladium, indium, and the like); metal oxides (such as lead oxide, tin oxide, ITO (Indium Tin Oxide), and the like); graphite; doped inorganic semiconductors (such as silicon, germanium, gallium arsenide, and the like); and doped conducting polymers (such as polyaniline, polypyrrole, polythiophene, and the like).
The first electrode 411 can be transparent, semi-transparent, or opaque to the wavelength of light generated within the device. The thickness of the first electrode 411 is from about 10 nm to about 1000 nm, preferably, from about 50 nm to about 200 nm, and more preferably, is about 100 nm. The first electrode layer 411 can typically be fabricated using any of the techniques known in the art for deposition of thin films, including, for example, vacuum evaporation, sputtering, electron beam deposition, or chemical vapor deposition.
In an alternative configuration, the first electrode layer 411 functions as a cathode (the cathode is a conductive layer which serves as an electron-injecting layer and which comprises a material with a low work function). The cathode, rather than the anode, is deposited on the substrate 408 in the case of, for example, a top-emitting OLED. Typical cathode materials are listed below in the section for the “second electrode 423”. In the configuration used in obtaining the experimental results shown in FIGS. 1(a)-(b), 2 and 3, the first electrode 411 was an anode comprised of ITO.
HIL 417:
The HIL 417 has a much higher hole mobility than electron mobility and is used to effectively transport holes from the first electrode 411 to the substantially uniform organic polymer layer 420. The HIL 417 is made of polymers or small molecule materials. For example, the HIL 417 can be made of tertiary amine or carbazole derivatives both in their small molecule or their polymer form, conducting polyaniline (“PANI”), or PEDOT:PSS (a solution of polyethylenedioxythiophene (“PEDOT”) and polystyrenesulfonic acid (“PSS”) available as Baytron P from HC Starck). The HIL 417 has a thickness from about 5 nm to about 1000 nm, preferably from about 20 nm to about 500 nm, and more preferably from about 50 to about 250 nm.
The HIL 417 can be formed using selective deposition techniques or nonselective deposition techniques. Examples of selective deposition techniques include, for example, ink jet printing, flex printing, and screen printing. Examples of nonselective deposition techniques include, for example, spin coating, dip coating, web coating, and spray coating. The hole injection material is deposited on the first electrode 411 and then allowed to dry into a film. The dried material represents the hole transport layer. In the configuration used in obtaining the experimental results shown in FIGS. 1(a)-(b), 2 and 3, the HIL 417 was PEDOT:PSS solution (such as that available from HC Starck) which was dried into a film of 200 nanometers.
LEP Layer 420:
For organic LEDs (OLEDs), the LEP layer 420 contains at least one organic material that emits light. These organic light emitting materials generally fall into two categories. The first category of OLEDs, referred to as polymeric light emitting diodes, or PLEDs, utilize polymers as part of LEP layer 420. The polymers may be organic or organometallic in nature. As used herein, the term organic also includes organometallic materials. Preferably, these polymers are solvated in an organic solvent, such as toluene or xylene, and spun (spin-coated) onto the device, although other deposition methods are possible. Devices utilizing polymeric active electronic materials in LEP layer 420 are especially preferred. Optionally, LEP layer 420 may include a light responsive material that changes its electrical properties in response to the absorption of light. Light responsive materials are often used in detectors and solar panels that convert light energy to electrical energy.
The light emitting organic polymers in the LEP layer 420 can be, for example, EL polymers having a conjugated repeating unit, in particular EL polymers in which neighboring repeating units are bonded in a conjugated manner, such as polythiophenes, polyphenylenes, polythiophenevinylenes, or poly-p-phenylenevinylenes or their families, copolymers, derivatives, or mixtures thereof. More specifically, the organic polymers can be, for example: polyfluorenes; poly-p-phenylenevinylenes that emit white, red, blue, yellow, or green light and are 2-, or 2,5-substituted poly-p-pheneylenevinylenes; polyspiro polymers; or their families, copolymers, derivatives, or mixtures thereof.
If the organic electronic device 405 is an organic solar cell or an organic light detector, then the organic polymers are light responsive material that changes its electrical properties in response to the absorption of light. The light responsive material converts light energy to electrical energy.
If the organic electronic device 405, is an organic transistor, then the organic polymers can be, for example, polymeric and/or oligomeric semiconductors. The polymeric semiconductor can comprise, for example, polythiophene, poly(3-alkyl)thiophene, polythienylenevinylene, poly(para-phenylenevinylene), or polyfluorenes or their families, copolymers, derivatives, or mixtures thereof.
In addition to polymers, smaller organic molecules that emit by fluorescence or by phosphorescence can serve as a light emitting material residing in LEP layer 420. Unlike polymeric materials that are applied as solutions or suspensions, small-molecule light emitting materials are preferably deposited through evaporative, sublimation, or organic vapor phase deposition methods. Combinations of PLED materials and smaller organic molecules can also serve as active electronic layer. For example, a PLED may be chemically derivatized with a small organic molecule or simply mixed with a small organic molecule to form LEP layer 420.
In addition to active electronic materials that emit light, LEP layer 420 can include a material capable of charge transport. Charge transport materials include polymers or small molecules that can transport charge carriers. For example, organic materials such as polythiophene, derivatized polythiophene, oligomeric polythiophene, derivatized oligomeric polythiophene, pentacene, compositions including C60, and compositions including derivatized C60 may be used. LEP layer 420 may also include semiconductors, such as silicon or gallium arsenide.
In accordance with at least one embodiment of the invention, the LEP layer 420 has a thickness of greater than 80 nm and preferably, between 80 and 200 nm. “Thickness of the LEP layer” as used in describing this and other embodiments of the invention, refers to the distance between bottom of the second electrode 423 and the top of the HIL 417 in a vertical direction. The thicker LEP layer 420 has been show to increase the photopic efficiency and lifetime of device 420. In other embodiments of the invention, the combined thickness of the layers in the organic stack, i.e. LEP layer 420 and HIL 417, is held at a constant. For example, assume the combined thickness of the layers in the organic stack was fixed to be 275 nanometers. If the LEP layer 420 were 150 nanometers thick, then the HIL 417 would be 125 nanometers. Likewise, if the LEP layer 420 thickness were 165 nanometers, then the HIL 417 would be 110 nanometers.
All of the organic layers such as HIL 417 and LEP layer 420 can be ink-jet printed by depositing an organic solution or by spin-coating, or other deposition techniques. This organic solution may be any “fluid” or deformable mass capable of flowing under pressure and may include solutions, inks, pastes, emulsions, dispersions and so on. The liquid may also contain or be supplemented by further substances which affect the viscosity, contact angle, thickening, affinity, drying, dilution and so on of the deposited drops.
The LEP layer 420 is fabricated by depositing this solution, using either a selective or non-selective deposition technique, onto HIL 417. To obtain a thicker LEP layer 420, in accordance with the invention, more drops or a greater concentration of polymer solution or a slower rotational speed while spin coating is required to be deposited.
Second Electrode (423)
In one embodiment, second electrode 423 functions as a cathode when an electric potential is applied across the first electrode 411 and second electrode 423. In this embodiment, when an electric potential is applied across the first electrode 411, which serves as the anode, and second electrode 423, which serves as the cathode, photons are released from active electronic layer 420 that pass through first electrode 411 and substrate 408.
While many materials, which can function as a cathode, are known to those of skill in the art, most preferably a composition that includes aluminum, indium, silver, gold, magnesium, calcium, and barium, or combinations thereof, or alloys thereof, is utilized. Aluminum, aluminum alloys, and combinations of magnesium and silver or their alloys can also be utilized.
Preferably, the thickness of second electrode 423 is from about 10 to about 1000 nanometers (nm), more preferably from about 50 to about 500 nm, and most preferably from about 100 to about 300 nm. While many methods are known to those of ordinary skill in the art by which the first electrode material may be deposited, vacuum deposition methods, such as physical vapor deposition (PVD) are preferred. Other layers (not shown) such as a barrier layer and getter layer may also be used to protect the electronic device. Such layers are well-known in the art and are not specifically discussed herein.
Often other steps such as washing and neutralization of films, the addition of masks and photo-resists may precede the cathode deposition. However, these are not specifically enumerated as they do not relate specifically to the novel aspects of the invention. Other steps (not shown) like adding metal lines to connect the anode lines to power sources may also be included in the workflow. Also, for instance, after the OLED is fabricated it is often encapsulated to protect the layers from environmental damage or exposure. Such other processing steps are well-known in the art and are not a subject of the invention.
In other embodiments of the invention, a thick LEP layer enables the fabrication of electro-chemical OLEDS (EC-OLEDS). An electrochemically stable, dissociable salt, such as lithium triflate, tetrabutyl ammonium tetrafluoroborate, or the salts used in lithium batteries, thin film batteries or electrochromic devices can be added into the LEP layer. Under bias, holes are injected into the LEP layer, causing the LEP to be oxidized, while negatively charged ions from the salt can now diffuse to stabilize the positively charged LEP. Thus, a thin layer of the LEP thick film becomes p-doped and able to efficiently transport holes into the undoped LEP region. Simultaneously at the other electrode, electrons are injected into the LEP layer, causing the LEP to be reduced, while positively charged ions from the salt diffuse to stabilize the negatively charged LEP. Thus, a thin layer of the LEP thick film becomes n-doped and able to efficiently transport electrons into the undoped LEP region. These p-doped and n-doped layers created in-situ act as HIL and ETL, while the middle region is the LEP layer where recombination and light emission takes place. Non-reactive metal electrodes whose work functions are not critical for the operation of the device can be used in this type of devices.
An EC-OLED stack is illustrated in
In yet other embodiments of the invention, a thick LEP layer can also be utilized in fabricating active matrix OLED displays. This is based upon testing done on passive matrix OLED displays under DC conditions, rather than on a multiplexed basis. It is expected that the DC results translates to a reasonable likelihood of similar performance in an active matrix setting where each OLED pixel is individually controlled by its own switching mechanism. Operating at lower voltages, active matrix displays can use thick LEP layers between 80 nm and 150 nm, approximately, but probably not much higher, unless a way can be found to support the higher voltages required to drive the LEP. Since active-matrix displays themselves operate at lower voltages than passive matrix displays, and since the power consumption of switching effects are less, overall power consumption is less a factor than with passive matrix displays.
While the embodiments of a thicker LEP layer are illustrated in which it is incorporated within an OLED device, this concept may be applied to other electronic devices that use an active electronic layer. For example, with a solar cell, the light responsive layer (i.e., the active electronic layer) can be comprised of a thick film polymer. The OLED device described earlier can be used in applications such as, for example, area, general, industrial and medical lighting, back lighting, computer displays, information displays in vehicles, television monitors, telephones, printers, and illuminated signs.
As any person of ordinary skill in the art of electronic device fabrication will recognize from the description, figures, and examples that modifications and changes can be made to the embodiments of the invention without departing from the scope of the invention defined by the following claims.
