A common architecture for a light-emitting device includes an anode and cathode; an emissive layer (EML) containing a material which emits light upon electron and hole recombination, such as an inorganic or organic semiconductor layer or layer of quantum dots (QDs); at least one hole transporting layer (HTL) between the anode and the emissive layer providing transport of holes from the anode and injection of holes into the emissive layer; and at least one electron transporting layer (ETL) between the cathode and the emissive layer providing transport of electrons from the cathode and injection of electrons into the emissive layer. U.S. Pat. No. 9,525,148 (Kazlas et al., issued Dec. 20, 2016), discloses a light-emitting device including such an architecture.
When the emissive layer includes an organic material, the light-emitting device is referred to as an organic light emitting diode (OLED). When the emissive material includes nanoparticles, sometimes known as quantum dots (QDs), the device is commonly called either a quantum dot light emitting diode (QLED, QD-LED) or an electroluminescent quantum dot light emitting diode (ELQLED).
Current methods of patterning QLEDS into regions or subpixels (e.g., on a substrate) include various printing and stamping methods (e.g., inkjet, offset lithography, gravure, screen-printing, nano-imprinting, and dip coating). The QLEDS patterned into these regions or subpixels may be different respective QDs such that they emit (through electrical injection, i.e., by electroluminescence) different respective colors (e.g., 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.
However, there remains an issue of the ability to provide a cost effective, high-resolution display. For example, generally there is a significant trade-off between speed and resolution, with currently industrially available printing techniques generally being fast (>˜1 m2/s) only if the resolution is poor (>100 um pitch).
U.S. Pat. No. 7,569,248 (Jang et al., issued Aug. 4, 2009) discloses a process to build a multilayer stack of nanocrystals via a series of UV curing steps on individual monolayers, but without reference to different colored regions.
U.S. Pat. No. 8,344,615 (Chae et al., issued on Jan. 1, 2013) discloses a device structure to achieve patterning of differently colored emitting regions by means of hydrophilic interlayers.
Choi et al., Wearable red-green-blue quantum dot light-emitting diode array using high-resolution intaglio transfer printing, Nature Communications, pages 1-8, May 2015 discloses that stacking and patterning different colors or types of QDs can be achieved in a layer-by-layer method such as transfer printing. Bae et al., Multicolored Light-Emitting Diodes Based on All-Quantum-Dot Multilayer Films Using Layer-by-Layer Assembly Method, Nano Letters, pages 2368-2373 and Liu et al., All-quantum-dot emission tuning and multicolored optical films using layer-by-layer assembly method, Chemical Engineering Journal 324, pages 19-25 (2017) disclose that stacking and patterning different colors or types of QDs can be achieved in a layer-by-layer method by using QD layers with alternately charged capping groups. However, such methods are slow, as they can only build the emissive layer one QD monolayer at a time.
Bae et al., Multicolored Light-Emitting Diodes Based on All-Quantum-Dot Multilayer Films Using Layer-by-Layer Assembly Method, Nano Letters, pages 2368-2373 also discloses that light emission preferentially occurs in the first monolayer from the limiting injection layer (either electron transfer layer or hole transfer layer).