Patent Application
20100102706
The present invention relates to electroluminescent devices having spaced-apart electrodes with one of the electrodes being patterned to form two individual electrode segments. Specifically, the invention relates to an electroluminescent device capable of producing light within the area between the individual electrode segments to obtain improved fill factor.
Many display and lighting devices exist within the market today. Among the technologies that are employed within these markets are thin-film electroluminescent devices, including organic light-emitting diode (OLED) devices. Electroluminescent (EL) devices are generally constructed by forming an EL layer between a pair of electrodes. Generally, at least one of these electrodes is patterned, producing gaps between adjacent patterned electrode segments. In these devices, electrons and holes are introduced into the EL layers by the electrodes and are localized onto the EL molecules that are located between the two electrodes. As a result, these light-emitting devices emit light in the regions defined by the overlap of the two electrodes but do not emit light in the regions where no electrode is present or regions where only one electrode is present.
Despite these advantages, the localization of light-emission within EL devices to the areas of the electrodes has some significant disadvantages. First, as these electrode segments are formed, the size of the gaps between the electrode segments directly influence the fill factor of the light-emitting element, wherein the fill factor represents the ratio of the size of the light-emitting portion of each light-emitting element to the size of the area allocated for each light-emitting element on the EL device. As the size of the gap between electrode segments is increased relative to the overall light-emitting element size, this fill factor is decreased. Unfortunately, decreasing the size of the gap between electrode segments typically requires more expensive manufacturing processes and decreases overall manufacturing yield. Therefore, it is desirable to permit devices to be formed with as large a gap between electrode segments as possible. However, in most EL devices, and especially in OLED devices, the fill factor is related to lifetime of the device. This relationship is typically modeled through the use of a second order or higher polynomial fit, therefore, increases in fill factor are highly desirable to improve the overall lifetime of the device.
Having light-emitting elements with limited fill factors also produces significant artifacts within displays. For example, it is well understood that limited fill factor produces images with artifacts such as increased visibility of so called “jaggies”, in which diagonal lines appear as stair steps rather than smooth lines, and the screen door effect, in which the inactive areas between light-emitting regions are perceptible and appear to overlay regions of uniform luminance. As a result, the literature provides multiple methods for improving the fill factor of displays. For example, it is known that optical elements can be included to improve the fill factor of pixels in a display. Chiu et al. in U.S. Pat. No. 5,929,962 discusses the use of optical elements to increase the perceived fill factor of a liquid crystal display. Thielemans in European Patent Application EP 1 780 798 describes the formation of curved reflectors behind LEDs with relatively small aperture ratios to increase the fill factor to nearly 100%. Unfortunately, while these methods increase the perceived fill factor of the device, they do not increase the actual fill factor and therefore do not have positive effects upon the lifetime of the device.
Significant work has also been done to simply reduce the area between light-emitting regions. In active matrix displays, the size of the areas between light-emitting elements is not only influenced by the ability to pattern gaps but also by active electronics that are often formed between these electrode segments. In such displays, it is known to decrease the area required for forming electronics. For instance, commonly used electrical components can be stacked vertically and insulated from one another as described by Gu in U.S. Pat. No. 5,920,084. While such processes add additional cost to the development of such displays, this patent demonstrates the importance of increasing the fill factor of light-emitting elements within display applications.
In lighting devices, it is typically important to provide diffuse, uniform illumination. The visibility of these segments can reduce the uniformity of the light, however, external diffusing layers, typically including arrays of lenses or diffusing particles can be used to improve uniformity. For instance Foust et al. in U.S. Pat. No. 7,348,738 describes the use of a diffusing layer together with a pixelated OLED. This diffusing layer has the ability to diffuse the light generated within one OLED into the inter-area between OLEDs, increasing the perceived uniformity and fill factors of the pixilated OLEDs. However, depending upon the location and design of these diffusing layers, they can reduce the light output from the device and again do not affect the area of the electroluminescent layer that is stimulated to produce light output.
There is a need for an improved EL device having an increased fill factor to provide an improved lifetime and their luminance uniformity. This increase in fill factor should be achieved without increasing the resolution of patterning technology to reduce the area between electrodes. This object is achieved by providing an electroluminescent device including:
(a) at least two spaced-apart electrodes wherein at least a portion of each of the two spaced-apart electrodes overlap within a first area and a second portion of the two spaced-apart electrodes do not overlap within a second area;
(b) a light-emitting layer having a first resistivity formed between the two electrodes, the light-emitting layer disposed to overlap at least a portion of both the first and second areas;
(c) a carrier-diffusing layer formed between the light-emitting layer and one of the spaced-apart electrodes; the carrier-diffusing layer disposed to overlap the light-emitting layer in at least a portion of both the first and second areas; and
(d) wherein the carrier-diffusing layer has a second resistivity selected to be lower than the first resistivity to cause light to be produced by the light-emitting layer within the first and second areas.
The present invention enables EL devices to emit light in areas, which do not contain electrode segments. As such, these devices can emit light between pairs of neighboring electrode segments, thereby improving the fill factor and consequently the lifetime and uniformity of the resulting device. This technology can also increase the gap size between electrode segments without loss of fill factor, thereby reducing manufacturing tolerances for patterning of electrode segments.
The need is met by providing an electroluminescent (EL) device as shown in
As shown in
Although the device shown in
The EL device 62 depicted in
The remainder of the device of
Within these embodiments, the spread of the carriers and therefore the distribution of light that is produced by the light-emitting layer can be controlled by controlling the relative resistivity of the layers, thickness of the layers, the size of the individual segments 64a, 64b and the space between adjacent individual segments 64a, 64b. Specifically, assuming that the light-emitting layer 32 or 52 has a thickness d1 and a resistivity r1, the carrier-diffusing layer 34 or 54 has a thickness d2 and a resistivity r2, the smallest dimension of one of the adjacent, individual segments is s and the space between two of the adjacent, individual segments is g, light will be emitted over a significant portion of the second area 56, 58, 60 as long as the relationship specified by the following inequality is satisfied:
(r2/r1)×s×g<9×d1×d2.
Note that the distances s and g are depicted in
In each of the previous embodiments, at least one carrier transport layer (e.g., a hole or electron transport layer was located between one of the electrodes and the light-emitting layer. The presence of such a layer is significant as it provides both a carrier transport and provides a high resistivity spacer between the electrode and the carrier-diffusing layer to prevent shorts. That is this carrier transport layer provides the function of a short reduction layer. It is therefore, important that this shorting reduction layer will typically have a resistivity that is significantly (often more than an order of magnitude) higher than the resistivity of the carrier-diffusing layer.
To demonstrate the concept of a device according to the present invention, a pair of devices was constructed. Each of these devices used an arrangement of cathode and anode segments as depicted in
In this example, a device that was nearly identical to the device provided in the comparative example was formed. The layers of this device are provided in Table 2. It should be noted that these layers are identical except that the 100 Angstrom thick carrier-diffusing layer was formed between the connecting layer and the hole injecting layer of the second EL structure. In this particular device, this layer was formed from silver to insure that it would have a low resistivity as compared to the resistivity of the light-emitting layer.
In this device, the carrier-diffusing layer served to diffuse the electrons within the device, permitting light to be emitted within the area of the cathode even though the anode was only located be coincident with a small region of the cathode. The embodiment shown in
In addition to these areas, light is also emitted along other portions of the cathode, such as in region 280. It is important to note, however, that the actual luminance provided by the light-emitting element does decrease somewhat as the distance from the areas 272, 274, 276, and 278 is increased. This can be illustrated through photometric measurements recorded at each of four measurement locations within a light-emitting element, including locations 282, 284, 286, and 288. Within this device, the relative intensity at location 282 was 4.49 nits. This value decreases to 0.709 nits at location 284, 0.644 nits at location 286 and 0.600 nits at location 288. This demonstrates the fact that this carrier-diffusing layer permits a larger portion of the light-emitting layer to be activated including portions of the light-emitting layer that is outside the spatial region defined by the intersection of the anode and cathode segments (i.e., the two spaced-apart electrodes). Further, it demonstrates that as the distance from this intersection is increased, the luminance output by the light-emitting layer decreases.
The cross section of the devices shown in
In this example, the electroluminescent device included at least two spaced-apart electrodes wherein at least a portion of each of the two spaced-apart electrodes overlap within a first area and a second portion of the two spaced-apart electrodes do not overlap within a second area; two separate EL structures disposed between the two spaced-apart electrodes and a connecting layer connecting the two EL structures, each EL structure having a light-emitting layer having a particular resistivity, each light-emitting layer disposed to overlap at least a portion of both the first and second areas; a carrier-diffusing layer formed between one of the light-emitting layers and one of the spaced-apart electrodes; the carrier-diffusing layer disposed to overlap the light-emitting layer in at least a portion of both the first and second areas; and wherein the carrier-diffusing layer has a second resistivity selected to be lower than the resistivity of one of the light-emitting layers to cause light to be produced by the light-emitting layer within the first and second areas. As shown, the carrier-diffusing layer affected only the light output of one of the two separate EL structures due to its placement. However, it should be noted that placing the carrier-diffusing layer in this example in the device such that it is separated from the anode by at least one organic layer, such as a hole injection layer, the carrier-diffusing layer can be made to affect the light output of both of the EL structures. It should further be noted that the carrier-diffusing layer can be placed either within or outside of one or more than one of the EL structures in order to be effective.
Although the previous example provided a discussion of an OLED device the present invention can be applied in any thin film coated electroluminescent diode device according to the present invention. This device can be any thin film, coated electroluminescent device that can be used to form light-emitting diodes between a pair of electrodes. These devices can include organic electroluminescent materials, including a light-emitting layer 32, 52, employing purely organic small molecule or polymeric materials, typically including organic hole transport, organic light-emitting and organic electron transport layers as described in the prior art, including U.S. Pat. No. 4,769,292 to Tang et al., and U.S. Pat. No. 5,061,569 to VanSlyke et al. The electroluminescent materials, including the light-emitting layer 32, 52, can alternately be formed from a combination of organic and inorganic materials, typically including organic hole transport and electron transport layers in combination with inorganic light-emitting layers, such as the light-emitting layers described in U.S. Pat. No. 6,861,155 to Bawendi et al. Other layers, such as the electron or hole transport layers can alternatively be formed from inorganic semiconductors and applied with either organic or inorganic light-emitting layers. These inorganic hole or electron transport layers can be annealed to alter their resistivity and permit them to serve as the carrier-diffusing layer.
In yet another alternative embodiment, the electroluminescent materials, including the light-emitting layer 32, 52, can be formed from fully inorganic materials such as the devices described in U.S. Patent Application Publication No. 2007/0057263. Note such devices can include a carrier-diffusing layer that is formed by annealing an inorganic semiconductor material. In such devices, the resistivity of the layer can be controlled by the annealing conditions and the ratio of the resistivity of the light-emitting layer to the resistivity of the carrier-diffusing layer can be controlled to control the length of light emission beyond the boundary of the electrodes. These devices can also include quantum dots within their light-emitting layer, as described within this patent application.
Although, the basic concept of the present invention can be applied within devices of each of the classes that are mentioned in the previous paragraph, the exact mechanism by which this concept will be implemented will likely be different. For example, the resistances of most organic semiconductors, which are useful in the formation of the layers of an OLED device, are often very similar to each other. Therefore, in these devices, the carrier-diffusing layer 34, 54 will likely be formed from a class IB transition metal such as silver but can be formed from a composite layer including these transition metals or can include an inorganic semiconductor or organic semiconductor having a lower resistance than the light-emitting layer. In a device formed predominantly from inorganic semiconductors, the resistance of potential materials can vary over well more than an order of magnitude and therefore, the carrier-diffusing layer 34, 54 will likely be formed from an inorganic semiconductor material. This inorganic semiconductor can serve other purposes within the device, such as serving as the hole or electron transport layer while also serving as the carrier-diffusing layer 34, 54. Further the ratio of the resistance of vertical resistance through the light-emitting layer to the vertical resistance through the carrier-diffusing layer 34, 54 can be adjusted by adjusting the thickness of one or more of these layers. That is, the thickness of the inorganic light-emitting layer 32, 54 can be adjusted to increase its resistance, such that its vertical resistance is higher than the lateral resistance through the carrier-diffusing layer 34, 54. This fact is due to the fact the ratio of the thickness of the light-emitting layer to the thickness of the carrier-diffusing layer is proportional to the ratio of the second resistivity to the first resistivity.
Based on this discussion, the carrier-diffusing layer can be formed from Group IB transition metals or from type II-VI and III-V semiconductors. The carrier-diffusing layer can also be formed from organic semiconductor materials having a high mobility, for example PEDOT. Group IB transition metals include silver as demonstrated in the previous example. Type II-VI and III-V semiconductors include ZnSe or ZnS. Additional dopants such as Al, In, or Ga can be used to dope the Type II-VI and III-V semiconductors, which are commonly n-type semiconductors, to form p-type semiconductors using these same materials. Carrier-diffusing layers formed from n-type semiconductors will typically be useful to transport electrons while p-type semiconductors will typically be more useful to transport holes. As noted earlier, changes in layer thickness, annealing conditions and others can be used to form a carrier-diffusing layer that has a lower resistivity than the light-emitting layer in order to form a carrier-diffusing layer within a device that employs a carrier-diffusing layer formed from type II-VI or III-V semiconductors. While the type II-VI and III-V semiconductors can be applied in devices such as the ones described by Kahen, hybrid devices can also be created.
As electrical shorts between the carrier-diffusing layer and the electrodes can provide devices with undesirable attributes, any of these devices can include thin layers of high resistivity directly over the electrode to serve as a shorting reduction layer. For instance, in devices employing primarily organic electroluminescent materials, wherein the carrier-diffusing layer is a Group IB transition metal, this carrier-diffusing layer can be separated from each of the electrodes by at least one layer of organic material. This organic material will typically provide the function of carrier injection or transport and will prevent shorts between individual segments within a single electrode. Other materials having a high resistivity, such as indium oxide, gallium oxide, zinc oxide, tin oxide, molybdenum oxide, vanadium oxide, antimony oxide, bismuth oxide, rhenium oxide, tantalum oxide, tungsten oxide, niobium oxide, or nickel oxide, can be employed to form an appropriate shorting reduction layer between one of the electrodes and the carrier-diffusing layer. These oxides can further be combined with one another, and or an electrically insulating oxide, fluoride, nitride, or sulfide material to form appropriate shorting reduction layers. Further, in inorganic devices that employ annealing to decrease resistivity of the carrier-diffusing layer, flash heating or other methods can be used to anneal only portions of the carrier-diffusing layer that is farthest from the electrode such that resistance of the material is higher near the electrode than further from the electrode.
Particularly in devices in which the resistivity of the different layers is selectable or controllable, the distance over which light-emission will occur beyond the edge of a first area, corresponding to the location of overlapping portions of spaced-apart electrode segments, can be controlled by changing the resistivity or the thickness of the layers. For example, the thickness of the light-emitting layer can be varied to satisfy the following relationship:
d1>=((r2/r1)×sL/9)/d2
to control the length (L) over which light-emission will occur beyond the edge of the first area. In this equation the thickness (d1) of the light-emitting layer is determined as the ratio of the resistivity of the carrier-diffusing layer to the resistivity of the light-emitting layer multiplied by the quantity achieved by multiplying the smallest dimension (s) of the electrode segment by the length L, divided by 9. The resulting value is then divided by the thickness (d2) of the carrier-diffusing layer to obtain the thickness d1 of the light-emitting layer. Generally, the light obtained will not end simultaneously but will decrease gradually within the second areas of the device. In some preferred embodiments, the distribution of light that is produced by the light-emitting layer between the two individual segments decreases as the distance between the adjacent two individual segments increases such that the point of half amplitude occurs at or before the midpoint between the two individual, spaced, adjacent segments. The midpoint is the point half way between the nearest edges of two adjacent, spaced individual segments. As such, while the light will be diffused due to the presence of the carrier-diffusing layer, light emission will generally be confined to the area of a single addressable element within the final device, resulting in a minimal loss of sharpness as compared to a device without the carrier-diffusing layer while having the positive benefit of increased aperture ratio.
Although not shown in the previous embodiments, it is also possible for both of the two spaced-apart electrodes to be patterned and for light to be produced outside the area defined by the intersection of the segments of the two spaced-apart electrodes. One embodiment of such a device can include a passive matrix of anode and cathode elements wherein the EL layers between these two spaced-apart electrodes includes a carrier-diffusing layer.
Devices of the present invention can be useful in various applications. As noted in the prior art section, diffusers have been used in conjunction with displays, particularly large displays to provide a uniform appearance. Therefore, it is reasonable that such a device can be employed to form a display in which each of the individual segments are individually addressable to permit images to be created.
Other applications of the current technology include illumination sources. It is known to construct lamps as illumination sources from coatable EL materials such as discussed within this disclosure. These illumination sources typically are required to create a diffuse illumination profile. To achieve this diffuse illumination profile, devices known in the art can include external diffusers, which require the construction of external structures which increase the cost of the device or that absorb a portion of the light that is emitted by the device, reducing the efficiency of the device. However, by applying devices of the present invention in the construction of an illumination source the efficiency of the device is not decreased by the external diffuser, the addition of this carrier-diffusing layer is very cost effective as compared to external diffusers and devices of the present invention will have a higher effective fill factor than devices formed with patterned electrodes. These higher effective fill factors will decrease the average current density in the device, increasing the average lifetime of the device.
The use of a carrier-diffusing layer of the present invention is particularly useful when diffuse light is required from an EL source that produces polarized light. Coatable EL devices, such as described by Culligan et al. in U.S. Pat. No. 7,037,599 are known in the art for producing polarized light. Such devices are particularly useful when creating backlights for displays, for creating other illumination sources where polarized light is required or for creating polarized light from a display for directing light as can be required in a stereographic display. In these devices, the light-emitting layer forms polarized light. However, the use of external diffusive elements for diffusing the light generally scatters the light and will therefore depolarize at least a portion of the emitted light. Devices of the present invention, however, diffuse the carriers before light-emission occurs. Therefore, when the light-emitting layer in a device of the present invention creates polarized light, it is capable of emitting this light across the entire surface of the device. Therefore, the device is capable of emitting diffuse, polarized light.
In a particularly desirable embodiment, a device of the present invention can be used to create a patterned, addressable polarized backlight for a light modulator, such as a liquid crystal display. The top view of one such backlight is shown in
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.
