TECHNICAL FIELD

The present application relates to light-emitting devices, and in particular to light-emitting devices including a crosslinked emissive layer containing quantum dots. The light-emitting devices may be implemented in display applications, for example in high resolution, multicolor displays. The present application further relates to methods of fabricating such light-emitting devices.

BACKGROUND ART

A common architecture for a light-emitting device includes an anode, which acts as hole injector; a hole transport layer disposed on the anode; an emissive material layer disposed on the hole transport layer; an electron transport layer disposed on the emissive material layer; and a cathode, which also acts as an electron injector, disposed on the electron transport layer. When a forward bias is applied between the anode and cathode, holes and electrons are transported in the device through the hole transport layer and electron transport layer, respectively. The holes and electrons recombine in the emissive material layer, which generates light that is emitted from the device. When the emissive material layer includes an organic material, the light-emitting device is referred to as an organic light-emitting diode (OLED). When the emissive material layer includes nanoparticles, sometimes known as quantum dots (QDs), the device is commonly referred to as either a quantum dot light-emitting diode (QLED, QD-LED) or an electroluminescent quantum dot light-emitting diode (ELQLED, QDEL).

Each of the layers of the light-emitting device can be deposited by different methods with the common methods including thermal evaporation methods and solution process methods. Thermal evaporation methods are widely used for OLEDs, but they are more complex and have higher costs of fabrication as compared to solution process methods. Solution process methods are thus preferred as cheaper and simpler fabrication methods. In the fabrication of devices with these methods, it is important to find the appropriate solvents such that during the deposition of a layer, the process will not dissolve or otherwise damage the previously deposited layer. Such a non-damaging solvent is typically referred to in the art as “orthogonal” to the previous one (See, Organic Electronics 30 (2016) 18e29; http://dx.doi.org/10.1016/j.orgel.2015.12.008).

To include QLEDs in multicolor high-resolution displays, different manufacturing methods have been designed. These methods typically include depositing three different types of QDs on three different regions of a substrate such that each region emits light (through charge injection that causes electroluminescence) at three different colors, particularly red (R), green (G) and blue (B). Sub-pixels that respectively emit red, green, or blue light may collectively form a pixel, which in turn may be a part of an array of pixels of the display.

WO 2017/117994 (Li et al., published Jul. 13, 2017) describes that through external energy stimuli (e.g. pressure, temperature or UV irradiation), QDs which emit different colors can be selectively attached to bonding surfaces. Surfaces and ligands of QDs must contain particular ending functional groups (e.g. alkenes, alkynes, thiols) to be selectively strongly bonded to each other through chemical reactions. This concept is further expanded in WO 2017/121163 (Li et al., published Jul. 20, 2017), in which QDs with R, G and B emission colors can be patterned separately using cross-linkable ligands and organic connectors through chemical reactions that are activated selectively with UV radiation applied at different monochromatic wavelengths. Park et al., Alternative Patterning Process for Realization of Large-Area, Full-Color, Active Quantum Dot Display (Nano Letters, 2016, pages 6946-6953) describes that QDs with R, G and B emission colors are patterned by combining conventional photolithography and layer by layer assembly. Park describes the deposition of QD layers selectively on activated (i.e., charged) surfaces.

CN106374056A (X. Chao, published Jan. 2, 2017) describes a method where R, G and B subpixels can be patterned on a substrate with an array of electrodes, applying selectively a current to certain electrodes, after immersion in solutions of QDs of specific colors, QD layers are deposited selectively only on charged electrodes.

SUMMARY OF INVENTION

The present application relates to an enhanced structure of a layer including Quantum Dots (QDs) dispersed in a cross-linked material. A layer including QDs dispersed in a cross-linked material is resistant to solvent rinsing and may improve patterned layers in the QLED field. This can be generally called a cross-linked layer in which quantum dots are dispersed (CLQDL). In some embodiments, the quantum dots are dispersed in, but do not form a part of, the cross-linked network of the cross-linked layer. In other embodiments, the quantum dots are dispersed in, and form a part of, the cross-linked network of the cross-linked layer. In embodiments where the cross-linked material is a charge transport material (e.g., hole transport material or electron transport material), this layer may also be referred to as a combined charge transport and emissive layer (CCTEL) of a QLED. The CCTEL may improve performance of existing QLEDs and may simplify fabrication by combining properties of a charge transport layer and an emissive layer.

A CCTEL (CLQDL) may be formed from a solution including QDs, cross-linkable material (e.g., charge transport material), and a solvent by cross-linking the cross-linkable material with the QDs dispersed therein. In some embodiments, the cross-linkable material is crosslinked to form a cross-linked network in which the quantum dots are dispersed (but where the quantum dots do not form a part of the cross-linked network). In other embodiments, the cross-linkable material and the QDs are crosslinked to form a cross-linked network in which the quantum dots are dispersed (where the quantum dots form a part of the cross-linked network). The disposition of the QDs within the cross-linked material can be controlled by engineering the interactions between the different components of the QDs and the surrounding layers. Optionally this solution may include one or more initiators of the polymerization. The initiator can be defined based on an external stimulus that activates the initiator such as light, temperature, pressure, change in pH and the like.

In accordance with one aspect of the present disclosure, a light-emitting device includes: an anode; a cathode; and a combined charge transport and emissive layer disposed on a deposition surface between the anode and the cathode, the combined charge transport and emissive layer including crosslinked charge transport material and quantum dots, the quantum dots distributed unevenly within the crosslinked charge transport material and arranged relative to the deposition layer, wherein the quantum dots include nucleophilic or electrophilic centers and ligands respectively bonded to the quantum dots, and wherein the deposition surface has nucleophilic or electrophilic properties.

In some embodiments, the deposition surface includes chelating ligands.

In some embodiments, the deposition surface is a dithiol functionalized surface.

In some embodiments, the light-emitting device further includes an interlayer disposed in contact with the combined charge transport and emissive layer, wherein the deposition surface is a surface of the interlayer.

In some embodiments, at least 75% of the quantum dots are located in a half portion of the combined charge transport and emissive layer closest to the deposition surface, and at least a portion of the quantum dots are bonded to the deposition layer.

In some embodiments, at least 75% of the quantum dots are located in a half portion of the combined charge transport and emissive layer distal to the deposition surface.

In some embodiments, the deposition surface is a surface of the anode.

In some embodiments, the deposition surface is a surface of the cathode.

In some embodiments, the light-emitting device further includes an electron transport layer disposed between the anode and the cathode, wherein the deposition surface is a surface of the electron transport layer.

In some embodiments, the light-emitting device further includes a hole transport layer disposed between the anode and the cathode, wherein the deposition surface is a surface of the hole transport layer.

In some embodiments, the quantum dots include ligands having an electrophilic center and a nucleophilic center, and at least a portion of the ligands are bonded to the deposition surface.

In accordance with another aspect of the present disclosure, a bank structure of multiple light-emitting devices includes: a first light emitting device and a second light emitting device, each of the first and second light emitting devices being a light emitting device according to any of the above-described embodiments of the light-emitting device; an insulating material that separates at least a portion of the first light-emitting device from the second light emitting device; wherein the first light-emitting device and the second light-emitting device are configured to emit different wavelengths.

In accordance with another aspect of the present disclosure, a method of forming a combined charge transport and emissive layer of a light-emitting device includes the steps of: depositing a mixture on a deposition surface having nucleophilic or electrophilic properties, the mixture including: a solvent; cross-linkable charge transport material; and quantum dots including nucleophilic or electrophilic centers and ligands respectively bonded to the quantum dots; and subjecting at least a portion of the mixture to an activation stimulus to crosslink the cross-linkable material to form a crosslinked charge transporting material; wherein during at least one of the depositing and subjecting steps, the quantum dots are distributed unevenly within the crosslinked charge transport material and arranged relative to the deposition layer.

In some embodiments, the activation stimulus includes at least one of ultraviolet (UV) light exposure, heating, application of pressure, and pH change.

In some embodiments, the mixture further includes a photo initiator, and the activation stimulus includes exposure to UV light.

In some embodiments, the deposition surface includes chelating ligands.

In some embodiments, the deposition surface is a dithiol functionalized surface.

In some embodiments, at least 75% of the quantum dots are located in a half portion of the combined charge transport and emissive layer closest to the deposition surface, and at least a portion of the quantum dots are bonded to the deposition layer.

In some embodiments, wherein at least 75% of the quantum dots are located in a half portion of the combined charge transport and emissive layer distal to the deposition surface.

In some embodiments, the quantum dots include ligands having an electrophilic center and a nucleophilic center, and at least a portion of the ligands are bonded to the deposition surface.

To the accomplishment of the foregoing and related ends, the invention, then, includes the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.

