SYSTEM AND METHOD FOR MATCHING ELECTRODE RESISTANCES IN OLED LIGHT PANELS
Provided are an OLED device and a method of manufacturing the OLED device that may provide improved luminance uniformity. The disclosed OLED may have a first electrode that has a first sheet resistance Rs, and a second electrode that has a second sheet resistance, wherein the second sheet resistance may be in the range of 0.3 Rs-1.3 Rs. In addition, the disclosed OLED may have a plurality of equal potential difference between points on a first electrode and a second electrode. The equal potential difference may be provided by a gradient resistance formed on at least one of the electrodes.
- 1-43. -43. (canceled)
- 44. A method of fabricating an electrode for an organic light emitting device, comprising:
obtaining a first electrode material source; disposing the first electrode material source over a substrate; arranging the substrate at an angle θ
relative to a line normal to the substrate from the first electrode material source, where 0<
depositing the first electrode material from the first electrode material source onto the substrate to form a first electrode having a non-uniform thickness.
- View Dependent Claims (45, 49, 50, 51, 52, 53, 54, 55)
- 46. A method of fabricating an electrode for an organic light emitting device, comprising:
depositing a plurality of layers of an electrode material through each of a series of shadow masks, each shadow mask in the series of shadow masks having a smaller area through which the electrode material is deposited than the previous shadow mask in the series.
- 47. A method of fabricating an electrode for an organic light emitting device, comprising:
passing a substrate through a plurality of positions in a linear deposition system; and at each position, depositing electrode material over the substrate at a different thickness and over a different portion of an area of the substrate than at each of the other positions.
- View Dependent Claims (48)
The claimed invention was made by, on behalf of, and/or in connection with one or more of the following parties to a joint university corporation research agreement: Regents of the University of Michigan, Princeton University, The University of Southern California, and the Universal Display Corporation. The agreement was in effect on and before the date the claimed invention was made, and the claimed invention was made as a result of activities undertaken within the scope of the agreement.
The present invention relates to organic light emitting devices and, more specifically, to devices that may have matched electrode resistances that provide substantially uniform luminance.
Opto-electronic devices that make use of organic materials are becoming increasingly desirable for a number of reasons. Many of the materials used to make such devices are relatively inexpensive, so organic opto-electronic devices have the potential for cost advantages over inorganic devices. In addition, the inherent properties of organic materials, such as their flexibility, may make them well suited for particular applications such as fabrication on a flexible substrate. Examples of organic opto-electronic devices include organic light emitting devices (OLEDs), organic phototransistors, organic photovoltaic cells, and organic photodetectors. For OLEDs, the organic materials may have performance advantages over conventional materials. For example, the wavelength at which an organic emissive layer emits light may generally be readily tuned with appropriate dopants.
OLEDs make use of thin organic films that emit light when voltage is applied across the device. OLEDs are becoming an increasingly interesting technology for use in applications such as flat panel displays, illumination, and backlighting. Several OLED materials and configurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and 5,707,745, which are incorporated herein by reference in their entirety.
One application for phosphorescent emissive molecules is a full color display. Industry standards for such a display call for pixels adapted to emit particular colors, referred to as “saturated” colors. In particular, these standards call for saturated red, green, and blue pixels.
As used herein, the term “organic” includes polymeric materials as well as small molecule organic materials that may be used to fabricate organic opto-electronic devices. “Small molecule” refers to any organic material that is not a polymer, and “small molecules” may actually be quite large. Small molecules may include repeat units in some circumstances. For example, using a long chain alkyl group as a substituent does not remove a molecule from the “small molecule” class. Small molecules may also be incorporated into polymers, for example as a pendent group on a polymer backbone or as a part of the backbone. Small molecules may also serve as the core moiety of a dendrimer, which consists of a series of chemical shells built on the core moiety. The core moiety of a dendrimer may be a fluorescent or phosphorescent small molecule emitter. A dendrimer may be a “small molecule,” and it is believed that all dendrimers currently used in the field of OLEDs are small molecules.
As used herein, “top” means furthest away from the substrate, while “bottom” means closest to the substrate. Where a first layer is described as “disposed over” a second layer, the first layer is disposed further away from substrate. There may be other layers between the first and second layer, unless it is specified that the first layer is “in contact with” the second layer. For example, a cathode may be described as “disposed over” an anode, even though there are various organic layers in between.
A ligand may be referred to as “photoactive” when it is believed that the ligand directly contributes to the photoactive properties of an emissive material. A ligand may be referred to as “ancillary” when it is believed that the ligand does not contribute to the photoactive properties of an emissive material, although an ancillary ligand may alter the properties of a photoactive ligand.
As used herein, and as would be generally understood by one skilled in the art, a first “Highest Occupied Molecular Orbital” (HOMO) or “Lowest Unoccupied Molecular Orbital” (LUMO) energy level is “greater than” or “higher than” a second HOMO or LUMO energy level if the first energy level is closer to the vacuum energy level. Since ionization potentials (IP) are measured as a negative energy relative to a vacuum level, a higher HOMO energy level corresponds to an IP having a smaller absolute value (an IP that is less negative). Similarly, a higher LUMO energy level corresponds to an electron affinity (EA) having a smaller absolute value (an EA that is less negative). On a conventional energy level diagram, with the vacuum level at the top, the LUMO energy level of a material is higher than the HOMO energy level of the same material. A “higher” HOMO or LUMO energy level appears closer to the top of such a diagram than a “lower” HOMO or LUMO energy level.
As used herein, and as would be generally understood by one skilled in the art, a first work function is “greater than” or “higher than” a second work function if the first work function has a higher absolute value. Because work functions are generally measured as negative numbers relative to vacuum level, this means that a “higher” work function is more negative. On a conventional energy level diagram, with the vacuum level at the top, a “higher” work function is illustrated as further away from the vacuum level in the downward direction. Thus, the definitions of HOMO and LUMO energy levels follow a different convention than work functions.
More details on OLEDs, and the definitions described above, can be found in U.S. Pat. No. 7,279,704, which is incorporated herein by reference in its entirety.
In an embodiment, a light-emitting device having improved luminance uniformity is provided. The device may include a first electrode having a first electrical sheet resistance Rs and a second electrode having a second electrical sheet resistance in the range of about 0.3 Rs-1.3 Rs. Preferably, the second electrical sheet resistance may be in the range of 0.7 Rs-1.0 Rs. An organic emissive layer is disposed between the first electrode and the second electrode. A plurality of organic light emitting devices may be arranged in series.
In an embodiment, an organic light emitting device may include a first electrode, a second electrode, and an organic emissive layer disposed between the first electrode and the second electrode. A gradient resistance of at least one of the first and second electrodes is in a current flow direction.
In an embodiment, an organic light emitting device may include a first electrode, a second electrode, and an organic emissive layer disposed between the first electrode and the second electrode. A potential difference between each of a plurality of points on the first electrode and a corresponding point on the second electrode measured in a direction substantially perpendicular to the first electrode and to the second electrode, is within 5% of each other.
Methods of fabricating organic light emitting devices are also provided. For example, in an embodiment, a device may be fabricated by obtaining a first electrode source material and disposing the material over a substrate. The substrate may be arranged at an angle θ relative to a line normal to the substrate from the first electrode material source, where 0<θ<90°. The first electrode material may be deposited onto the substrate to form a first electrode having a non-uniform thickness.
In embodiment, fabrication of an organic light emitting device may include a plurality of layers of an electrode material that is deposited through a series of shadow masks. Each shadow mask in the series of shadow masks may have a smaller area through which the electrode material is deposited than the previous shadow mask in the series.
According to an embodiment, an organic light emitting device may be fabricated by passing a substrate through a plurality of positions in a linear deposition system. At each position, electrode material may be deposited over a substrate at a different thickness and over a different portion of an area of the substrate than at each of the other positions.
Generally, an OLED comprises at least one organic layer disposed between and electrically connected to an anode and a cathode. When a current is applied, the anode injects holes and the cathode injects electrons into the organic layer(s). The injected holes and electrons each migrate toward the oppositely charged electrode. When an electron and hole localize on the same molecule, an “exciton.” which is a localized electron-hole pair having an excited energy state, is formed. Light is emitted when the exciton relaxes via a photoemissive mechanism. In some cases, the exciton may be localized on an excimer or an exciplex. Non-radiative mechanisms, such as thermal relaxation, may also occur, but are generally considered undesirable.
The initial OLEDs used emissive molecules that emitted light from their singlet states (“fluorescence”) as disclosed, for example, in U.S. Pat. No. 4,769,292, which is incorporated by reference in its entirety. Fluorescent emission generally occurs in a time frame of less than 10 nanoseconds.
More recently, OLEDs having emissive materials that emit light from triplet states (“phosphorescence”) have been demonstrated. Baldo et al., “Highly Efficient Phosphorescent Emission from Organic Electroluminescent Devices,” Nature, vol. 395, 151-154, 1998; (“Baldo-I”) and Baldo et al., “Very high-efficiency green organic light-emitting devices based on electrophosphorescence,” Appl. Phys. Lett., vol. 75, No. 3, 4-6 (1999) (“Baldo-II”), which are incorporated by reference in their entireties. Phosphorescence is described in more detail in U.S. Pat. No. 7,279,704 at cols. 5-6, which are incorporated by reference.
More examples for each of these layers are available. For example, a flexible and transparent substrate-anode combination is disclosed in U.S. Pat. No. 5,844,363, which is incorporated by reference in its entirety. An example of a p-doped hole transport layer is m-MTDATA doped with F.sub.4-TCNQ at a molar ratio of 50:1, as disclosed in U.S. Patent Application Publication No. 2003/0230980, which is incorporated by reference in its entirety. Examples of emissive and host materials are disclosed in U.S. Pat. No. 6,303,238 to Thompson et al., which is incorporated by reference in its entirety. An example of an n-doped electron transport layer is BPhen doped with Li at a molar ratio of 1:1, as disclosed in U.S. Patent Application Publication No. 2003/0230980, which is incorporated by reference in its entirety. U.S. Pat. Nos. 5,703,436 and 5,707,745, which are incorporated by reference in their entireties, disclose examples of cathodes including compound cathodes having a thin layer of metal such as Mg:Ag with an overlying transparent, electrically-conductive, sputter-deposited ITO layer. The theory and use of blocking layers is described in more detail in U.S. Pat. No. 6,097,147 and U.S. Patent Application Publication No. 2003/0230980, which are incorporated by reference in their entireties. Examples of injection layers are provided in U.S. Patent Application Publication No. 2004/0174116, which is incorporated by reference in its entirety. A description of protective layers may be found in U.S. Patent Application Publication No. 2004/0174116, which is incorporated by reference in its entirety.
The simple layered structure illustrated in
Structures and materials not specifically described may also be used, such as OLEDs comprised of polymeric materials (PLEDs) such as disclosed in U.S. Pat. No. 5,247,190 to Friend et al., which is incorporated by reference in its entirety. By way of further example, OLEDs having a single organic layer may be used. OLEDs may be stacked, for example as described in U.S. Pat. No. 5,707,745 to Forrest et al, which is incorporated by reference in its entirety. The OLED structure may deviate from the simple layered structure illustrated in
Unless otherwise specified, any of the layers of the various embodiments may be deposited by any suitable method. For the organic layers, preferred methods include thermal evaporation, ink-jet, such as described in U.S. Pat. Nos. 6,013,982 and 6,087,196, which are incorporated by reference in their entireties, organic vapor phase deposition (OVPD), such as described in U.S. Pat. No. 6,337,102 to Forrest et al., which is incorporated by reference in its entirety, and deposition by organic vapor jet printing (OVJP), such as described in U.S. patent application Ser. No. 10/233,470, which is incorporated by reference in its entirety. Other suitable deposition methods include spin coating and other solution based processes. Solution based processes are preferably carried out in nitrogen or an inert atmosphere. For the other layers, preferred methods include thermal evaporation. Preferred patterning methods include deposition through a mask, cold welding such as described in U.S. Pat. Nos. 6,294,398 and 6,468,819, which are incorporated by reference in their entireties, and patterning associated with some of the deposition methods such as ink-jet and OVJD. Other methods may also be used. The materials to be deposited may be modified to make them compatible with a particular deposition method. For example, substituents such as alkyl and aryl groups, branched or unbranched, and preferably containing at least 3 carbons, may be used in small molecules to enhance their ability to undergo solution processing. Substituents having 20 carbons or more may be used, and 3-20 carbons is a preferred range. Materials with asymmetric structures may have better solution processibility than those having symmetric structures, because asymmetric materials may have a lower tendency to recrystallize. Dendrimer substituents may be used to enhance the ability of small molecules to undergo solution processing.
Devices fabricated in accordance with embodiments of the present invention may further optionally comprise a barrier layer. One purpose of the barrier layer is to protect the electrodes and organic layers from damaging exposure to harmful species in the environment including moisture, vapor and/or gases, etc. The barrier layer may be deposited over, under or next to a substrate, an electrode, or over any other parts of a device including an edge. The barrier layer may comprise a single layer, or multiple layers. The barrier layer may be formed by various known chemical vapor deposition techniques and may include compositions having a single phase as well as compositions having multiple phases. Any suitable material or combination of materials may be used for the barrier layer. The barrier layer may incorporate an inorganic or an organic compound or both. The preferred barrier layer comprises a mixture of a polymeric material and a non-polymeric material as described in U.S. Pat. No. 7,968,146, PCT Pat. Application Nos. PCT/US2007/023098 and PCT/US2009/042829, which are herein incorporated by reference in their entireties. To be considered a “mixture”, the aforesaid polymeric and non-polymeric materials comprising the barrier layer should be deposited under the same reaction conditions and/or at the same time. The weight ratio of polymeric to non-polymeric material may be in the range of 95:5 to 5:95. The polymeric material and the non-polymeric material may be created from the same precursor material. In one example, the mixture of a polymeric material and a non-polymeric material consists essentially of polymeric silicon and inorganic silicon.
Devices fabricated in accordance with embodiments of the invention may be incorporated into a wide variety of consumer products, including flat panel displays, computer monitors, medical monitors, televisions, billboards, lights for interior or exterior illumination and/or signaling, heads up displays, fully transparent displays, flexible displays, laser printers, telephones, cell phones, personal digital assistants (PDAs), laptop computers, digital cameras, camcorders, viewfinders, micro-displays, vehicles, a large area wall, theater or stadium screen, or a sign. Various control mechanisms may be used to control devices fabricated in accordance with the present invention, including passive matrix and active matrix. Many of the devices are intended for use in a temperature range comfortable to humans, such as 18 degrees C. to 30 degrees C., and more preferably at room temperature (20-25 degrees C.).
The materials and structures described herein may have applications in devices other than OLEDs. For example, other optoelectronic devices such as organic solar cells and organic photodetectors may employ the materials and structures. More generally, organic devices, such as organic transistors, may employ the materials and structures.
The terms halo, halogen, alkyl, cycloalkyl, alkenyl, alkynyl, arylkyl, heterocyclic group, aryl, aromatic group, and heteroaryl are known to the art, and are defined in U.S. Pat. No. 7,279,704 at cols. 31-32, which are incorporated herein by reference.
As used herein, “luminance uniformity” refers to the ratio between the minimum luminance and the maximum luminance. Thus, a panel that has the same luminance across the panel will have a luminance uniformity of 100%. It is known that when scaling up to large size, the luminance uniformity of an OLED panel degrades across the panel. The luminance non-uniformity occurs because conventional transparent conducting oxides (TCOs), such as ITO, and IZO, have relatively high sheet resistance, varying from 10 to 100Ω/□ (ohms/sq), which causes resistive loss, i.e. potential drop, as current flows across the panel. Luminance non-uniformity may further cause non-uniform aging, which may lead to a reduced device lifetime. One technique to address this challenge is to embed highly conductive metal bus lines on to the electrode to promote a uniform current distribution. However, the use of bus lines may reduce the fill factor of the OLED panel due to the non-emissive property of metal. In addition, the process steps are more complicated when integrating bus lines, which adds to the manufacturing cost and the total average cycle time (TACT) of manufacturing the device.
Embodiments of the present invention may provide a device that outputs light having improved luminance uniformity, e.g. >80% at 1,000 cd/m2. As used herein, luminance uniformity may be used to compare the brightness of various positions of the light source, i.e. OLED panel in this invention, and does not take chromaticity into account. That is to say, optical distortion, or color variation, may exist even when the luminance uniformity is high.
In some embodiments, the OLED device may have a more uniform potential drop across the two electrodes, and thus an improved luminance uniformity. This may be achieved by matching the sheet resistances of the two electrodes.
In a conventional bottom-emission OLED, it is common practice to design the conductance of the cathode layer 330 to be as great as possible, which means the resistance is as low as possible. Typically, the cathode of a conventional bottom-emission OLED may be considered grounded. The highly-conductive cathode layer 330 is commonly formed from a metal. Conversely, the anode layer 310, which commonly includes ITO or a similar material, may have a greater resistance or lower conductance. By considering segments of the device (represented by dash lines) individually, each single segment may be considered as an individual OLED. The current I (shown by dark arrows) may flow through one electrode contact 312 of anode 310 (e.g. the first electrode) and each of the segments of the organic stack 320 to the opposite electrode contact 332 of cathode 330 (e.g. the second electrode). In a conventional configuration, the luminance typically is greatest in the segment closest to the anode contact 312 and lowest at the segment farthest from the anode contact 312, which happens to be the segment closest to the cathode contact 332. The difference in luminance is due to a greater device voltage at the segment closest to the anode contact 312 than at the segment closest to the cathode contact 332, which is the segment farthest from the anode contact 312. As a result, conventional OLED devices have non-uniform luminance across the OLED device because of the uneven voltage across the panel.
In contrast to conventional techniques and the accepted idea of improving luminance uniformity by attempting to further reduce the resistance of the cathode layer, the inventors have increased the sheet resistance of the cathode layer 330 or portions of the cathode layer 330 to generate a more uniform potential across the OLED segments in the organic layer 320. It has been found that this unexpectedly may provide increased luminance uniformity as well as a substantial increase in fill factor. It has also been found that matching the resistances of the two electrodes in an OLED unexpectedly may provide approximately a 10% increase in luminance uniformity or more, compared to a configuration in which the resistance of one electrode is relatively high and the resistance of the other electrode is very low.
As an example,
For example, as shown in
The potential difference (voltage between the two electrodes) between point P1 on the first electrode 410 and point P1′ on the second electrode 430 may be equal to V. Another point P2 located at (X2, Y2) on the first electrode 410 also may have a corresponding point P2′ located at (X2′, Y2) on the second electrode 430. The coordinates of each point P2 and P2′ are equal, so X2 equals X2′ and Y2 equals Y2′. The potential difference between point P2 and point P2′ may be equal to V′.
In a conventional bottom-emission OLED, where the cathode is highly conductive, the cathode potential 490 shown in
Conversely, as described herein, the second electrode 430 (as shown in
The electrode contact may be positioned at single edge of the electrode. In addition, the electrode contact edge may be less than 25% of the total circumference of the electrode. In some embodiments, the contacts of opposite electrode may be positioned at approximately the farthest locations across the device. The OLED device 400 may be a transparent or top-emission OLED device in which the sheet resistance of a cathode is typically higher than a cathode in conventional bottom-emission OLED devices. Transparent or top-emission OLEDs may more readily benefit for the disclosed techniques because the engineering, e.g. adding sheet resistance, of the cathode is easier to achieve. An implementation of the OLED device 400 may have an active area greater than 50 cm2. More preferably, the OLED device 400 may have an active area of at least 72 cm2. In addition, a side of either the first or second electrode may be at least 2 cm. The resistance of the second electrode may be a function of the thickness of the material, usually a metal, used to fabricate the electrode. Surprisingly, in contrast to a conventional arrangement in which at least one electrode has as small a sheet resistance as possible, an arrangement as described may provide improved uniformity in luminance and potential of the device.
In order to achieve a uniform potential difference between the electrodes, the sheet resistance of the first electrode, second electrode or both may be manipulated by varying the electrode thickness so that an equal potential difference is maintained across the organic layer.
The described structures may be particularly preferred for top-emission and transparent OLED devices. In an embodiment, the electrodes of the OLED device may have sheet resistances that substantially match. In other embodiments, one of the two electrodes of the OLED device may have a sheet resistance that is a gradient resistance. Although the above was described with reference to the second electrode 530 having a non-uniform resistance, the disclosed implementations should not be limited to only the second electrode. It should be understood that the first electrode 510 may be formed with a non-uniform resistance instead of the second electrode.
Examples of the techniques for fabricating the disclosed structures are illustrated in
Step D is shown with an OLED device that includes substrate 640 and electrode 630D. The electrode 630D may, for example, be a second electrode as shown in
It will be understood that the specific example shown in
Generally, the deposition device 715 may apply the electrode source material 750 to a substrate 730 to form an electrode 720 having a non-uniform thickness. Of course, multiple passes beneath the deposition device 715 may also occur with the electrode source material 750 being applied in increasing layers after the substrate jig 740 has passed the spray area 717. In addition, different masks may be used during each pass of the substrate 730 under the deposition device 715. Modifications or alterations to the disclosed fabrication processes within the scope of one of ordinary skill in fabrication processes are envisioned. For example, a linear vacuum deposition system may be used, for example, to deposit source material with various thicknesses at different positions on the substrate when the substrate passes through the deposition chamber.
Benefits of the disclosed devices and structures may include a simplified contact layout that may be nearly one dimensional and the achievement of substantially uniform luminance. In addition, the disclosed structures may provide OLED devices including an OLED having a fill factor of up to 100%. The disclosed structures may also be favored in a series connection configuration.
By designing a cathode resistance to match that of the anode, it has been found that the OLED voltage may be more precisely controlled to achieve better luminance uniformity of the device, as explained herein.
An equivalent circuit of a typical OLED is shown in
A current passing through the mth cathode resistor Ik,m, may be described as
where Ai is the area of the ith OLED segment and Ji is the current density. The current flowing through the mth anode layer Ia,m, may be described with Eq. B:
The potential of any pair of nodes can be written as:
In the case of a typical bottom emission OLED where Rk,m=0,
This means that the OLED segments closer to anode contact are always brighter than the segments that are farther away (because VD is higher), as long as the anode is resistive. This result is well known in a typical bottom emission OLED that includes an ITO anode and a highly conductive cathode.
Comparing Equations 3 and 4, it can be seen that with the introduction of a cathode resistance Rk, it is possible to reduce the voltage difference between neighboring OLED segments (e.g. VDm and VDm-1). In a standard bottom emission OLED where Rk is very small, the region near the anode contact has the highest luminance level. This can be explained by the difference of voltages near the anode (VDn) and cathode (VD1) contacts. When the cathode resistance is very low, Vn≈V1. At the same time, Vn′>V1′ because of the anode sheet resistance, therefore, VDn=Vn′−Vn>V1′−V1=VD1, resulting in a relatively very large difference in luminance of the ith and the nth OLED segments. However, when a cathode resistance is introduced, Vn>V1, it may enable Vn′−Vn=V1′−V1. As a result, the luminance near the anode contact decreases, nevertheless, the total luminance uniformity may be improved.
A simple configuration described above may improve luminance uniformity for a large-area OLED device by tuning the sheet resistance of the cathode to match that of the anode. It is also possible to find an exact solution to Eq. 3 so that equal potential may be maintained across the panel and, therefore, 100% uniformity may be achieved. To do so, the voltage of each OLED segment should be the same: VD,m=VD,m-1. Equation 3 can be rewritten, for a device length L, as:
where, x is the distance from the cathode contact. If the area of each segment is allowed to be the same, the current density of each OLED will be same too, i.e., A(x)=A, J(x)=J. Thus
When the resistances of the anode and cathode meet this requirement, there will be an equal voltage and current among all the OLED segments in the device, which may result in 100% uniformity. This implies that the closer to the anode contact, the less resistive the anode material should be and, similarly, the closer to the cathode contact, the less resistive the cathode material should be.
If the anode resistance is fixed at Ra(x)=Ra, the requirement for the cathode resistance to achieve an equal potential (and thus 100% uniformity) may be described as
For Ra=15 ohm/sq and L=100, the cathode resistance as a function of the distance can be plotted as shown in
Numerically, Eq. 6 can be written as follows:
The case of a 4-segment (m=1, 2, 3, 4) OLED device with n=100 was simulated. In the simulation the anode resistance was maintained at 15Ω/□, and the cathode sheet resistance was given 4 different values depending on the position. The resistance distributions of both electrodes are shown in
The simulation results of luminance uniformity of OLED device with a uniform cathode and a 4-segment varied cathode are shown in
Another technique to obtain matching resistance between the electrodes is to tune the OLED turn-on resistance to achieve luminance uniformity. For example, it is known that the thickness of the hole transporting layer (HTL) has an impact on the device voltage. Therefore, by changing HTL thickness, the OLED turn-on resistance may be changed. An equivalent circuit diagram is illustrated in
In this case,
For a uniform panel,
J i =J
V D,m =D V
AJ·R HTL,m =AJ·R HTL,m-1 +AR a ·mJ
This provides the relationship of the adjacent HTL resistance. Therefore, by changing the thickness of the HTL for each segment, the voltage of each OLED segment may be tuned to be equal across the panel. Note that, any means that controls or modifies the OLED turn-on resistance may be applied here, such as varying thickness of HIL, varying the ETL thickness, or any other suitable modification to the structure of the device.
Fabrication techniques similar to that described with respect to
To determine the impact of this cathode resistance, a computer simulation program was developed to numerically calculate the voltage, current and luminance for each OLED segment, based on the equivalent circuit in
Firstly, uniform resistance was introduced to cathode. The simulation is performed on bottom-emission OLED. The sheet resistance of ITO is 15Ω/□. The cathode sheet resistance is varied from 0.01Ω/□ to 20Ω/□. The OLED device is 2 cm long in one dimension where two electrode contacts were placed at opposite ends. The device is divided into 100 segments (i.e., n=100 in
IR loss, power efficacy, and average luminance of the device with a cathode resistance present were also calculated. The results are summarized in Table 1. For a 2 cm, one-dimensional OLED device, with highly conductive cathode (Rk=0.01Ω/□), the luminance uniformity of the panel, defined as Lmin/Lmax, is 88.2%. With proper tuning of the cathode resistance, the luminance uniformity is improved as Rk increases to Rk=20Ω/□. In particular, when the cathode and anode resistance are the same, i.e., Rk=Ra=15Ω/□, uniformity has an improved value of 94.2%. Notably, even when additional resistance is introduced in the device, the total IR loss is still very small, only 1.34% when Rk=15Ω/□. Since the resistance of the cathode is designed to be the same as that of the anode, the additional power loss due to introducing a matched cathode resistance is the same as that of the anode. Thus the total power loss may be no more than twice the power loss expected for a zero cathode resistance, which typically is a relatively very small number (0.71% in this example). However, if the cathode resistance increases beyond a threshold, for example to Rk=30Ω/□ in the present configuration, the luminance uniformity may be expected to decrease. Thus it is expected that there is an optimized value or value range for the cathode resistance so that a lowest non-uniformity may be achieved. It has been found that this occurs when both resistances are within a small fraction of one another, and preferably Rk=Ra.
From the calculation above, it has been shown that by matching the resistance of two electrodes so as to achieve equal potential drop across the panel, the luminance uniformity of the OLED device may be improved.
In a further test, three OLED panels with the layout shown in
Photo-like images of the three Panels are shown in
It is understood that the various embodiments described herein are by way of example only, and are not intended to limit the scope of the invention. For example, many of the materials and structures described herein may be substituted with other materials and structures without deviating from the spirit of the invention. The present invention as claimed may therefore include variations from the particular examples and preferred embodiments described herein, as will be apparent to one of skill in the art. It is understood that various theories as to why the invention works are not intended to be limiting.