Combined auxiliary electrode and reflective bank for three-dimensional QLED pixel

A top emitting quantum dot light emitting diode (QLED) apparatus for an emissive display device sub-pixel, with at least one bank defining an emissive region of the emissive display device sub-pixel, includes an emissive layer deposited in the emissive region between a first electrode and a second electrode. The first electrode comprising a reflective metal, and the second electrode has a transparent conductive electrode and an auxiliary electrode. The bank has a sloped portion adjacent the emissive region. The auxiliary electrode includes a reflective conductive metal and is configured to cover the sloped portion, and the sloped portion is configured at an angle, such that the auxiliary electrode reflects internally reflected light out of the sub-pixel in a viewing direction and a first area of the transparent conductive electrode covering the sloped portion is thinner than a second area of the transparent conductive electrode in the emissive region.

FIELD

The present disclosure is generally related to quantum dot light-emitting diodes (QLEDs), and more particularly related to a QLED display device formed of three-dimensional pixels, where light extraction enhancement is achieved with a transparent electrode and a reflective bank auxiliary electrode.

BACKGROUND

Quantum dot Light Emitting Diodes (QLEDs) are a promising emissive display technology predicted to outperform liquid crystal displays (LCDs) and organic light emitting diode (OLED) displays. OLEDs and QLEDs are both examples of emissive display technologies, where light is generated in response to current passing through a diode. In the diode, electrons and holes are injected into an emissive layer where they combine and emit light.

Control of the light emitted from an OLED/QLED emissive device is enabled by a driving circuit such as a thin film transistor (TFT). The circuitry for driving the device may be printed on a substrate such as plastic or glass. If light is emitted through the substrate, then the device is said to be bottom emitting. If light is emitted in a direction away from the substrate, then the device is said to be top emitting. One issue of bottom emitting OLED/QLED devices is that the emission area is small due to the presence of the driving circuitry. Top emitting OLED/QLED devices can have much larger emission areas as the driving circuitry is positioned beneath the emissive areas of the device.

To create a top emitting OLED/QLED device, a transparent conductive electrode above the emissive region is required. The transparent conductive electrode allows current to spread across the sub-pixels of the display, while enabling high transmission of the emitted light. OLED displays commonly use a 15-20 nm thin film of Ag/Mg alloy which is semi-transparent and has good conductivity. This semi-transparent electrode has the benefit of creating an interior space, or ‘resonance cavity’ which narrows the emission band of the device and leads to a high efficiency device having high color saturation.

For a QLED pixel, the emission band for the emissive layer is considerably narrower than that present in an OLED. The advantage of band narrowing in QLEDs is therefore not as effective, as the colors are already saturated. Also, a sub-pixel with an interior space as described above has a strong angular dependence of color shift. Specifically, at wide angles the color may vary considerably.

Additionally, 15-20 nm Ag/Mg alloy is suitable for small (e.g., mobile device, etc.) top emitting OLED displays. However, for larger displays, an issue arises with voltage drop across the display. Such a thin metal transparent conductive electrode is generally not conductive enough to transport current evenly across the whole display. This leads to a voltage drop between the edges and the center of the display. For small displays (such as the aforementioned mobile device), this voltage drop is small and can be compensated for with the driving circuitry. For a larger display, such as a tablet or a television, however, the voltage drop is large and leads to considerable brightness variation across the display.

One solution proposes a nanoparticle transparent conductive electrode in combination with an auxiliary wire grid electrode to solve the problem of insufficient conductivity. The nanoparticle transparent conductive electrode is highly transparent and so removes the unwanted resonance cavity. The auxiliary electrode is positioned in the non-emissive regions of the pixel and therefore may be relatively thick, leading to a very high conductivity which enables current to be effectively and evenly transported across larger displays.

For QLED displays to outperform OLEDs in terms of efficiency, the light extraction from each sub-pixel needs to be optimized. This requires a special design of the QLED sub-pixel structure so that the maximum amount of light can be directed outwards from the display. One manner of accomplishing maximum output proposes a 3-dimensional pixel configuration with reflective banks and filler material to help increase extraction. This design is improved by creating a detuned interior space along with an optimized wave-guiding structure which allows for light emitted at wide angles to reflect off the reflective banks and be redirected to a narrower viewing angle.

The nanoparticle transparent conductive electrode with auxiliary wire grid structure requires the patterning of the auxiliary electrode on top of the nanoparticle layer. The patterning process must be compatible with the other QLED materials (i.e., with no harsh conditions). One of the preferred methods for mass production is to use a wet etch method for patterning the auxiliary electrode. On a banked substrate, the materials deposited on the bank slope are generally thinner and not necessarily as uniform as in the center of the emissive area. During a wet etch process, these thinner areas may be more porous, and therefore less resilient, which means they are more susceptible to attack from the etchant used. Attack in these areas can lead to considerable damage to the QLED layers and very inconsistent quality across a display area.

One solution uses pixel structures with high banks, and the banks have a reflective layer deposited on the slope of the bank. To avoid crosstalk and current leakage, the reflective layer must either be extremely carefully patterned, or an additional insulating layer must be carefully patterned on top of the reflective bank. These extra high precision processing steps are undesirable for mass production. This design also requires the light, which is designed to be reflected off the bank, to pass through the transparent electrode and possibly other QLED materials multiple times which can lead to unwanted absorption.

The present disclosure seeks to improve upon the methods described above.

The basic QLED structure comprises an emissive layer (EML) disposed between a top electrode and a bottom electrode. The emission in the emissive layer comes from quantum dots (QDs) which may be composed of a range of materials such as CdSe, ZnN, InP, and ZnSe. The QDs may have a core-shell structure and may have ligands surrounding each quantum dot.

To aid charge transport from the electrodes into the EML, there is generally a charge transport layer (CTL) between the EML and the top electrode, and another CTL between the EML and the bottom electrode. There are two types of charge transport layers: electron transport layers (ETLs) and hole transport layers (HTLs). When an ETL is deposited between the EML and the top electrode, the QLED is said to have a standard structure. When an HTL is deposited between the EML and the top electrode, the QLED is said to have an inverted structure. Common ETL materials include ZnO, MgZnO, and AlZnO. Common HTL materials include TFB, PVK, and OTPD.

To further aid charge balancing in the QLED device, the QLED structure may also include hole injection layers (HILs) (such as HIL-8 and PEDOT:PSS), electron injection layers (EILs) (such as LiF and MoO3), hole blocking layers (HBLs), and electron blocking layers (EBLs) (such as PMMA, PETE, and PEI).

The present disclosure does not relate to a specific QLED device structure and therefore is not limited to any combination of the layers and materials stated above.

SUMMARY

Disclosed is a top emitting quantum dot light emitting diode (QLED) apparatus for an emissive display device sub-pixel. The QLED includes at least one bank defining an emissive region of the emissive display device sub-pixel. The top emitting QLED apparatus has an emissive layer deposited in the emissive region between a first electrode and a second electrode. The first electrode comprises a reflective metal. The second electrode comprises a transparent conductive electrode and an auxiliary electrode. The bank (or banks) comprises a sloped portion adjacent the emissive region.

The auxiliary electrode comprises a reflective conductive metal and is configured to cover the sloped portion. The sloped portion is configured at an angle, such that the auxiliary electrode reflects internally reflected light, including totally internally reflected light, out of the sub-pixel in a viewing direction.

The QLED may also comprise a transparent filler material configured to fill an interior space above the sub-pixel and surrounded by the sloped portion of the bank (or banks). The transparent filler material may have a refractive index of between 1.5 and 2.0. The QLED may also include a transparent low refractive index layer covering the transparent filler material, the transparent low refractive index layer having a refractive index of between 1.0 and 1.5.

The transparent low refractive index layer and the transparent filler material form an internal reflection interface for reflecting light onto the auxiliary electrode over the sloped portion.

The bank (or banks) may include a top portion adjacent the sloped portion. Additionally, the auxiliary electrode may cover the sloped portion and the top portion. The QLED may include a first charge transport layer between the first electrode and the emissive layer, and a second charge transport layer between the second electrode and the emissive layer.

In another implementation, a top emitting QLED apparatus for an emissive display device having a plurality of sub-pixels separated by a bank structure, with each sub-pixel having an emissive region, includes a plurality of emissive layers, each disposed in the emissive region between a plurality of first electrodes and a second electrode. The plurality of first electrodes preferably comprises a reflective metal, individually separated by the bank structure. The second electrode comprises a transparent conductive electrode and an auxiliary electrode. The bank structure comprises a plurality of sloped portions surrounding each emissive region.

The auxiliary electrode comprises a reflective conductive metal configured to cover the plurality of sloped portions, and wherein the plurality of sloped portions are each configured at an angle, such that the auxiliary electrode reflects internally reflected light out of each sub-pixel in a viewing direction.

The QLED may include a transparent filler material configured to fill an interior space above each of the plurality of sub-pixels and surrounded by at least one of the plurality of sloped portions of the bank structure. The transparent filler material has a refractive index of between 1.5 and 2.0. The QLED may also include a transparent low refractive index layer covering the transparent filler material, the transparent low refractive index layer having a refractive index of between 1.0 and 1.5.

The transparent low refractive index layer and the transparent filler material form an internal reflection interface for reflecting light onto the auxiliary electrode over the at least one of the plurality of sloped portions. The bank structure may comprise at least one top portion adjacent to the plurality of sloped portions. The auxiliary electrode covers the plurality of sloped portions and the at least one top portion.

A first area of the transparent conductive electrode preferably covers the plurality of sloped portions is thinner than a second area of the transparent conductive electrode covering the corresponding emissive region. Further, the QLED further comprises a first charge transport layer between the first electrodes and each of the emissive layers, and a second charge transport layer between the second electrode and each of the emissive layers.

In yet another implementation, a top emitting QLED apparatus for an emissive display device sub-pixel, with at least one bank structure defining an emissive region of the sub-pixel, includes a bottom substrate layer, with the bank structure (or bank structures) deposited on the bottom substrate layer. A plurality of reflective metal bottom electrodes is deposited on the bottom substrate layer and separated by the bank structure (or bank structures). An emissive layer is deposited in the emissive region between each bottom electrode and a top electrode layer. The top electrode layer comprising a transparent conductive electrode covers the at least one bank structure and the emissive region, and an auxiliary electrode covers only the at least one bank structure. The bank structure (or bank structures) comprises a sloped portion surrounding each emissive region.

The auxiliary electrode comprises a highly reflective metal configured to cover the sloped portion. The transparent conductive electrode may be is thinner where it covers the sloped portion than where it covers each emissive region. And the sloped portion is configured at an angle, such that the auxiliary electrode reflects internally reflected light out of the sub-pixel in a viewing direction. The bank structure (or bank structures) may comprise a top portion adjacent the sloped portion, the top portion also covered by the auxiliary electrode.

DETAILED DESCRIPTION

The following description contains specific information pertaining to exemplary implementations of the present disclosure. The drawings in the present disclosure and their accompanying detailed description are directed to merely exemplary implementations. However, the present disclosure is not limited to merely these exemplary implementations. Other variations and implementations of the present disclosure will occur to those skilled in the art. Unless noted otherwise, like or corresponding elements among the figures may be indicated by like or corresponding reference numerals. Moreover, the drawings and illustrations in the present disclosure are generally not to scale and are not intended to correspond to actual relative dimensions.

For consistency and ease of understanding, like features are identified (although, in some examples, not shown) by numerals in the exemplary figures. However, the features in different implementations may differ in other respects, and thus shall not be narrowly confined to what is shown in the figures.

The description uses the phrases “in one implementation,” or “in some implementations,” which may each refer to one or more of the same or different implementations. The term “coupled” is defined as connected, whether directly or indirectly through intervening components, and is not necessarily limited to physical connections. The term “comprising,” when utilized, means “including, but not necessarily limited to”; it specifically indicates an open-ended inclusion or membership in the so-described combination, group, series, and the equivalent.

Additionally, for purposes of explanation and non-limitation, specific details, such as functional entities, techniques, protocols, standards, and the like are set forth for providing an understanding of the described technology. In other examples, detailed description of well-known methods, technologies, systems, architectures, and the like are omitted so as not to obscure the description with unnecessary details.

Disclosed herein is a top emitting (TE) quantum dot light emitting diode (referred to in the art as QLED, QDLED, or QDEL; hereinafter “QLED”). QLEDs may be part of an array of independently controllable red, green, and blue (RGB) QLED sub-pixels, which together, typically form the individual pixels of a full color display.

Referring toFIGS.1A and1B, top emitting QLEDs may exhibit either a standard layer structure or an inverted layer structure.FIG.1Ashows a related art standard (e.g., non-inverted) top emitting QLED10, whileFIG.1Billustrates a related art inverted top emitting QLED12. The standard QLED10and inverted QLED12typically include a substrate layer14on which is formed a reflector layer15. The standard QLED10and inverted QLED12also include an anode layer18, cathode layer20, hole injection layer (HIL)22, hole transport layer (HTL)26, electron injection layer (EIL)24, and electron transport layer (ETL)28, in addition to an emissive layer (EML)30. In the standard QLED10and inverted QLED12, the substrate layer14and the reflector layer15form the bottom layers.

Referring toFIG.1A, in the standard structure QLED10, an anode layer18layer is placed over the reflector layer15to serve as a bottom electrode. The reflector layer15and the anode layer18may be the same material forming a single reflective bottom electrode38as illustrated inFIGS.2-3,5-7, and9-11. An HIL22is formed over the anode layer18. An HTL26is formed over the HIL22. The EML30is formed over the HTL26. An ETL28is formed over the EML30. An EIL24is formed over the ETL28. The cathode layer20is formed over the EIL24to serve as a top electrode and complete the standard structure QLED10diode.

Referring toFIG.1B, in the inverted QLED12, a cathode layer20is placed over the reflector layer15to serve as the bottom electrode. The reflector layer15and the cathode layer20may also be the same material forming a single reflective bottom electrode38as illustrated inFIGS.2-3,5-7, and9-11. An EIL24is placed over the cathode layer20. An ETL28is placed over the EIL24. The EML30is formed over the ETL28. An HTL26is placed over the EML30. An HIL22is placed over the HTL26. The anode layer18is formed over the HIL22to serve as a top electrode and complete the inverted QLED12.

FIG.2illustrates an elevation view of a first related art three-dimensional top emitting QLED sub-pixel structure (hereinafter, “first related art QLED sub-pixel”)32, which may have either a standard (non-inverted) or an inverted QLED diode structure as discussed above. The first related art QLED sub-pixel32includes a substrate layer14atop which a thin film transistor (TFT) circuitry16is formed. Each first related art QLED sub-pixel32includes an emissive region34, including an EML30(FIGS.1A/1B) for providing luminance. The emissive region34is defined by one or more bank structures36. The first related art QLED sub-pixel32is controlled by a reflective bottom electrode38. Reflective bottom electrodes38of adjacent first related art QLED sub-pixels32are separated by bank structures36(or a surrounding bank structure36). Each reflective bottom electrode38is separated between each first related art QLED sub-pixel32to prevent shorting between individual reflective bottom electrodes38since the reflective materials comprising the reflective bottom electrode38are generally conductive. The reflective bottom electrodes38are driven by a driving circuit (not shown) to control the first related art QLED sub-pixels32individually. In contrast to having separate reflective bottom electrodes38, all first related art QLED sub-pixels32in a pixel array (not shown) share a common top transparent conductive electrode (TCE)40, which is formed over the emissive region34and bank structures36.

The bank structures36include sloped surfaces44extending downward from a top portion58of the bank structure36toward the emissive region34. A patterned reflective layer42is formed over each sloped surface44. A patterned insulating layer46is formed over each patterned reflective layer42. The patterned insulating layer46separates the patterned reflective layer42from the TCE40, extending over the bank structure(s)36, including the sloped surface44. The primary role of the insulating layer46is that it prevents shorting between the TCE40and the reflective bottom electrode38. If the patterned reflective layer42lacks an insulator such as the insulating layer46, there is a potential for the reflective layer42to contact the reflective bottom electrode38due to manufacturing errors. This could result in the TCE40and the reflective bottom electrode38coming into electrical contact. A filler material48is formed over the TCE40, filling the space over the emissive region34and sloped surfaces44of the bank structures36. A continuous low refractive index layer50is formed over the filler material48and the bank structures36. In a first related art QLED sub-pixel array, all first related art QLED sub-pixels32, including emissive regions34and bank structure(s)36, are covered by the low refractive index layer50.

FIG.3illustrates in elevation view a second related art three-dimensional top emitting QLED sub-pixel structure (hereinafter, “second related art QLED sub-pixel”)52, which may have either a standard (non-inverted) or an inverted QLED diode structure as discussed above. Similar to the first related art QLED sub-pixel32, the second related art QLED sub-pixel52includes a substrate layer14and a TFT circuitry16. Each second related art QLED sub-pixel52includes an emissive region34, including an EML30(FIGS.1A/1B) for providing luminance. The emissive region34is defined by one or more bank structures36. The second related art QLED sub-pixel52is also controlled by an independent reflective bottom electrode38. Reflective bottom electrodes38of adjacent second related art QLED sub-pixels52are separated by bank structures36(or a surrounding bank structure36). The reflective bottom electrodes38are driven by a driving circuit (not shown) to control second related art QLED sub-pixels52individually. In contrast to having separate bottom electrodes, all second related art QLED sub-pixels52in a pixel array (not shown) share a common top nanoparticle TCE54, which is formed over the emissive region34and bank structures36.

Using a nanoparticle TCE54in the second related art QLED sub-pixel52results in a thinner portion56of the nanoparticle TCE54being deposited on the sloped surfaces44of the bank structures36. These thinner portions56are generally not as uniform as the portions of nanoparticle TCE54formed over the emissive region34and on the top portions58of the bank structures36. A patterned auxiliary electrode60is formed over the top portions58of the bank structures36. Generally, TCEs54in such an arrangement are not sufficiently conductive to transport current across an entire display but are sufficiently conductive to transport current from the auxiliary electrode60to the sub-pixel52.

FIG.4illustrates how a wet etch process may work for patterning metal, in the process of creating three dimensional QLED sub-pixels. For example, the first related art QLED sub-pixel32and the second related art QLED sub-pixel52might be manufactured in part using such a method. InFIG.4, a continuous metal layer62is deposited on a substrate64(for example, by vapor deposition, thermal evaporation, or another method). A photoresist66is then deposited on top of the metal layer62. The photoresist66is then selectively exposed to UV light68. Exposure to the UV light68alters the solubility of the photoresist66in different solvents.

The photoresist66is then patterned by washing away the soluble regions70of the photoresist66in a developer solution (not shown). The exposed regions72of the metal layer62beneath the photoresist soluble regions70are then etched away using a chemical etchant76(FIGS.5and6). The remaining photoresist66can then be washed away using a remover solution (not shown). This wet etch process allows patterned metal layers62to be created on surfaces, and the process is a good candidate for patterning an auxiliary electrode60such as the one shown inFIG.3. However, the various layers of a top emitting QLED, such as the second related art QLED sub-pixel52(both standard and inverted) may be damaged by a such a wet etch process.

FIG.5illustrates how the metal layer62(e.g., as shown inFIG.4) removal step in a wet etch process may work when manufacturing the second related art QLED sub-pixel52shown inFIG.3, after soluble regions of photoresist66(e.g., those areas over the emissive region34and the sloped surfaces44of the bank structures36) have been removed. The chemical etchant76is shown in this view attacking and etching away the exposed regions of the metal layer62of the second related art QLED sub-pixel52(FIG.3).

The auxiliary electrode60structure of the second related art QLED sub-pixel52(FIG.3) is preferable to the TCE40of the first related art QLED sub-pixel32(FIG.2). As discussed, deposition of the nanoparticle TCE54(FIG.3) results in the thinner portion56of the TCE54forming on the sloped surface44of the bank structures36. Additionally, during a wet etch process, deposition of the metal layer62over the nanoparticle TCE54results in a reduced thickness portion74of the metal layer62on the sloped surface44of the bank structures36.

Having the reduced thickness portion74of the metal layer62on the sloped surfaces44of the bank structures36may lead to a chemical attack on the thinner portions56(i.e., weaker regions) of the nanoparticle TCE54. Specifically, the metal etchant76can damage the thinner portion56of the nanoparticle TCE54on the sloped surfaces44of the bank structures36. Because the metal in the reduced thickness portion74is thinner, it is etched away in a shorter time, compared to the metal of the emissive region34. Therefore, the thinner (i.e., weaker) portion56region of the TCE54is exposed to chemical attack for a period of time while the thicker metal regions are still being etched. As discussed, the resulting damage in these areas can lead to damage to the QLED layers and inconsistent quality across a display area.

Referring toFIG.6, an action in a wet etch process is shown for manufacturing a combined auxiliary electrode and reflective bank for a three-dimensional QLED sub-pixel78(e.g., as shown inFIG.7), in accordance with an example implementation of the present disclosure. InFIG.6, the design of photoresist66patterning protects the aforementioned weaker regions of the nanoparticle TCE54and metal layer62on the sloped surfaces44of the bank structures36. The photoresist66is patterned such that only the portion of the metal layer62covering the emissive region34is exposed for etching away by the metal etchant76. The portion of the metal layer62covering the bank structures36, including the sloped surfaces44and top portions58, is protected by the photoresist66and thereby left intact after the wet etch process. This design prevents the metal etchant76from attacking the thinner nanoparticle TCE54.

FIG.7shows a completed combined auxiliary electrode and reflective bank for a three-dimensional QLED sub-pixel78, in accordance with an example implementation of the present disclosure. Driving thin film transistor (TFT) circuitry16is printed on the substrate layer14. The QLED sub-pixel78includes a combined reflective bottom electrode38(serving as a reflective layer and a bottom electrode) deposited on, and in electrical contact with, the TFT circuitry16. The emissive region34has a width (“w”) between, and defined by, the bank structures36. The bank structures36have height (“h”) and the bank structures36isolate the bottom electrode38from other bottom electrodes in adjacent QLED sub-pixels. The nanoparticle TCE54is deposited continuously across the entire QLED sub-pixel78, including the emissive region34and entire bank structures36(sloped surfaces44and top portions58), and over adjacent QLED sub-pixels so that for any particular QLED sub-pixel array, all QLED sub-pixels are covered by the nanoparticle TCE54.

For each QLED sub-pixel78, the various QLED layers (e.g., as shown inFIGS.1A and1Bfor standard and inverted structures), including HILs22, EILs24, HTLs26, ETLs28, and their related EMS30, are deposited between the reflective bottom electrode38and the nanoparticle TCE54. Different EMLs30are patterned in adjacent QLED sub-pixels78, to produce adjacent red, green and blue (RGB) sub-pixels (thereby enabling a full color pixel display). Other layers such as the HIL22, EIL,24, HTL26, and ETL28may be common to all sub-pixels or may be patterned. The metal layer62(as shown inFIG.6) has been selectively etched, thereby forming a patterned auxiliary electrode60covering non-emissive regions of the QLED sub-pixel78, i.e. covering the bank structures36, including the top portions58and the sloped surfaces44. The patterned auxiliary electrode60extends down the sloped surfaces44towards the emissive region34, at an angle θ relative to the horizontal plane of the QLED sub-pixel78. In various implementations, the angle θ may be modified by controlling the angle of the bank structure35slope surfaces.

Referring toFIG.8a QLED sub-pixel array80is shown, including a series of adjacent red (R), green (G) and blue (B) QLED sub-pixels78arranged in an RGB stripe pattern customary for a color display. Although an RGB stripe pattern is shown in the illustrated implementation, the present disclosure is not limited to such a structure. A section of the QLED sub-pixel array80, including a QLED sub-pixel78is also shown in elevation view as a call out. The emissive regions34are defined by the bank structures36which isolate the red (R), green (G) and blue (B) QLED sub-pixels78from one another. Light is emitted from the emissive region34of each QLED sub-pixel78.

FIG.9illustrates an improved QLED sub-pixel including a filler material, in accordance with an example implementation of the present disclosure. InFIG.9, the QLED sub-pixel78includes essentially the features disclosed inFIG.7, and in addition includes a filler material82which is patterned to fill a volume of the QLED sub-pixel78, as defined by the emissive region34and the sloped surfaces44of the bank structures36. The filler material82is highly transparent and has a high refractive index.

FIG.10illustrates an improved QLED sub-pixel including a low refractive index layer and cover glass, in accordance with an example implementation of the present disclosure. InFIG.10, the QLED sub-pixel78includes a low refractive index layer84added and is formed above the entire QLED sub-pixel78structure (i.e., above the emissive region34and filler material82, and above the bank structures36and patterned auxiliary electrode60). Thus, the low refractive index layer84extends across an entire array of QLED sub-pixels78. A cover glass86, or similar cover layer material, is formed over the low refractive index layer84, thereby also extending across an entire array of QLED sub-pixels78.

FIG.11illustrates a luminance pattern of the improved QLED sub-pixel, in accordance with an example implementation of the present disclosure. InFIG.11, the luminance emission profile from the emissive region34into the filler material82occurs along a first direction88(or ‘lobe’), and a second direction90(or ‘lobe’). The first direction88has strong first intensity92on axis. The second direction90has strong second intensity94at a predetermined wider angle. The QLED sub-pixel78, with the filler material82, low refractive index layer84, is designed so that the angle of emission in the second direction90(and therefore the angle of the second intensity94) is within an angle range required for total internal reflection (TIR) at the boundary of the filler material82and the low refractive index layer84. This causes light emitted along the second direction90to be internally reflected in the QLED sub-pixel78. As such, the luminance of the QLED sub-pixel78may be of optimum functionality.

InFIG.11, one TIR is shown at the boundary of the filler material82and the low refractive index layer84. In other implementations, there may be several TIR reflections between the filler material82/low refractive index layer84boundary and the reflective bottom electrode38, depending on the angles in various implementations. Preferably in all implementations, including those shown in the illustrations, TIR light96emitted along the second direction90(i.e., second intensity94) eventually strikes and reflects off the portion of the patterned auxiliary electrode60extending over the sloped surface44of the bank structure36. Due to the predetermined angle θ (FIG.7) of the sloped surface44relative to the horizontal plane of the QLED sub-pixel78, the TIR light96is then directed out of the QLED sub-pixel78at a much narrower angle.

In a standard OLED/QLED structure, secondary lobes can usually occur depending on the stack design, but these modes of emission are usually lost within the device and not emitted. An emission mode as shown inFIG.11would not usually be desirable in a standard structure as a user would detect a strong luminance variation (with the naked eye) when moving with respect to a display. The design disclosed herein takes these lobes, which would usually be undesirable or lost within the device and redirects them to a narrow angle. This increases the amount of light emitted and on-axis brightness (also known as the collimation ratio).

In various implementations, with reference to the aforementioned figures, the top emitting QLED sub-pixels78may be part of an array of independently controllable pixels which form a display. Each QLED sub-pixel78has an emissive region34which is defined by a bank structure36. Each QLED sub-pixel78is controlled by an independent bottom electrode38which is driven by a driving circuit. All QLED sub-pixels have a common top electrode (e.g., TCE40or nanoparticle TCE54).

The reflective bottom electrode38comprises a reflective metal such as aluminum or silver. The reflective bottom electrode38may further comprise a transparent conductive layer such as indium tin oxide (ITO) or indium zinc oxide (IZO).

The top electrode comprises a TCE54that is continuous across all of the QLED sub-pixels78and a patterned auxiliary electrode60forms a wire grid around the emissive areas of the QLED sub-pixels78. The TCE54, in conjunction with the patterned auxiliary electrode60, has a conductivity that is high enough to transport current evenly from the grid of the auxiliary electrode60across the emissive region34of each QLED sub-pixel78. The TCE54is highly transparent in order to minimize the “resonance cavity” effect.

The TCE54may be selected from a variety of materials. It may be a transparent conductive nanoparticle layer such as ITO nanoparticles. The nanoparticles may be uniform in size or the nanoparticle layer may be a blend of nanoparticles ranging in diameter from 3 nm to 100 nm. Other TCEs may be formed from an ultrathin metal such as 5 nm silver or aluminum, a conductive polymer such as PEDOT:PSS, or a conductive mesh, such as carbon nanotubes or silver nanowires. In the case of a conductive mesh such as carbon nanotubes or metal nanowires, the conductive mesh may be in a transparent matrix material which aids with adhesion. In the case of a thin metal, it may be formed of a material such as aluminum, silver, or platinum. The thin metal may be thinner than state of the art thin metal electrodes to achieve higher transmission. For example, the thin metal may have a thickness of between 5 nm and 20 nm.

The bank structures36between QLED sub-pixels78provide regions where different colored EMLs30of different QLED sub-pixels78can be deposited but also provide insulation between the different sub-pixel areas so that crosstalk between QLED sub-pixels78is minimized. The bank structures36have a large height relative to the thickness of the conductive, emissive, and other layers (HIL22, EIL24, HTL26, ETL28, and EMLs30, including the reflective bottom electrode38and nanoparticle TCE54) of the QLED sub-pixel78. The bank structures36have a gradual slope at the edges of the QLED sub-pixels78, making an angle θ to the horizontal plane of a QLED display. The bank structures36also provide insulation from the nanoparticle TCE54, ensuring that current flows across and into the emissive region34of the QLED sub-pixel78rather than directly down from the patterned auxiliary electrode60.

Since patterning of the patterned auxiliary electrode60on top of a QLED apparatus should not damage the layers of the QLED, the temperature should be low so as not to damage the quantum dots (QDs) or the substrate (if plastic). QLED fabrication is aiming to be much cheaper than OLED fabrication and so ideally the process is solution process compatible. One feasible method is the aforementioned chemical wet etch to pattern the auxiliary electrode.

As discussed above and shown in the aforementioned figures (in particular inFIG.4), a continuous metal layer62is deposited on a substrate (by vapor deposition, thermal evaporation or other method). Photoresist is then deposited on top of the metal layer62. The photoresist is selectively exposed to UV light68. Exposure to UV light68alters the solubility of the photoresist in different solvents. The photoresist is then patterned by washing away the soluble regions of the photoresist in a developer solution. The exposed regions of the metal layer62are then etched away using a chemical etchant. The remaining photoresist can then be washed away using a remover solution. This process allows for a metal layer to be patterned and is a good candidate for patterning a patterned auxiliary electrode60.

Chemical etchants can damage or wash away the layers in a QLED device. It is therefore important that the TCE is resilient to the wet etch process. In the case of a QLED sub-pixel78with bank structures36in between emissive regions34, the nanoparticle TCE54layer is uniform and resilient. However, on sloped surfaces44of the bank structures36, the nanoparticle TCE54layer can become thinner and less uniform. This can lead to the nanoparticle TCE54layer being more porous and less resilient to the chemical etchant. Increased porosity means the etchant can permeate through in the sloped surfaces44of the bank structures36and attack the QLED layers. Even though the etching process is time dependent, close control over timing cannot be relied upon as the thickness of the metal layer62may also be thinner at the sloped surfaces44of the bank structures36, meaning it is etched away more quickly.

In a first implementation, the patterned auxiliary electrode60is redesigned so that the thinner more porous regions on the sloped surfaces44of the bank structures36are protected during the wet etch process. The patterned auxiliary electrode60caps the bank structure36and extends down the sloped surface44to the edge of the emissive region34. The patterning of the photoresist during the wet etch process prevents the etchant from attacking these regions, as illustrated inFIGS.6and7.

This patterned auxiliary electrode60that extends over the bank structure36, follows the shape of the bank structure36. If the patterned auxiliary electrode60is formed from a highly reflective metal such as aluminum or silver, then the patterned auxiliary electrode60provides a reflective surface at a similar angle to the sloped surface44of the bank structure36.

The first implementation therefore further provides a reflective bank structure36that can be used in the 3-D pixel design. This replaces the need to precisely pattern an isolated reflective layer on the banks before the QLED layers are deposited. This reflective bank structure36does not need isolation as it is all doubling up as the auxiliary electrode60which is in electrical contact with the TCE54.