Solution-processed optoelectronic and electronic devices are attractive owing to the potential for low-cost fabrication of large-area devices and the compatibility with lightweight, flexible plastic substrates. Solution-processed light-emitting diodes (LEDs) using conjugated polymers or quantum dots as emitters have attracted great interest over the past two decades. However, the overall performance of solution-processed LEDs--including their efficiency, efficiency roll-off at high current densities, turn-on voltage and lifetime under operational conditions-remains inferior to that of the best vacuum-deposited organic LEDs. Here we report a solution-processed, multilayer quantum-dot-based LED with excellent performance and reproducibility. It exhibits colour-saturated deep-red emission, sub-bandgap turn-on at 1.7 volts, high external quantum efficiencies of up to 20.5 per cent, low efficiency roll-off (up to 15.1 per cent of the external quantum efficiency at 100 mA cm(-2)), and a long operational lifetime of more than 100,000 hours at 100 cd m(-2), making this device the best-performing solution-processed red LED so far, comparable to state-of-the-art vacuum-deposited organic LEDs. This optoelectronic performance is achieved by inserting an insulating layer between the quantum dot layer and the oxide electron-transport layer to optimize charge balance in the device and preserve the superior emissive properties of the quantum dots. We anticipate that our results will be a starting point for further research, leading to high-performance, all-solution-processed quantum-dot-based LEDs ideal for next-generation display and solid-state lighting technologies.

Metal halide perovskites have shown promising optoelectronic properties suitable for lightemitting applications. The development of perovskite light-emitting diodes (PeLEDs) has progressed rapidly over the past several years, reaching high external quantum efficiencies of over 20%. In this Review, we focus on the key requirements for high-performance PeLEDs, highlight recent advances on materials and devices, and emphasize the importance of reliable characterizations of PeLEDs. We discuss possible approaches to improve the performance of blue and red PeLEDs, increase the long-term operational stability, and reduce toxicity hazards.We also provide an overview of the application space made possible by recent developments of high-efficiency PeLEDs. 3Metal halide perovskites, which have led to great advances in photovoltaic devices, have also proved to be promising candidates for light-emitting diodes (LEDs) 1 . They have shown excellent optoelectronic properties suitable for LEDs, such as high photoluminescence quantum yields (PLQYs), widely tunable bandgap, narrow emission width, and high charge-carrier mobility 2 . Although early reports on perovskite LEDs (PeLEDs) date back to the 1990s 3,4 , room-temperature PeLEDs were not demonstrated until 2014 5 . Since then, benefiting from established experience in both perovskite materials and solution-processed optoelectronic devices, the community has quickly boosted the external quantum efficiencies (EQEs) of PeLEDs to each more than 20% (Box 1) (refs [6][7][8][9][10] ).The rapid development of PeLEDs could lead to a new generation of low-cost and highperformance LEDs for applications including displays, lighting and optical communications 2,11,12 . Compared with other emitters used in commercial devices, such as III-V inorganic semiconductors, organic emitters and conventional colloidal quantum dots (QDs), perovskites have several promising characteristics. Specifically, perovskite emitters with high PLQYs can be straightforwardly fabricated from low-cost precursor solutions, potentially reducing manufacturing costs. Synthesis of colloidal perovskite nanocrystals (PNCs) is also simplified, as PNCs can reach near-unity PLQYs without delicate shell passivation, owing to their unique defect-tolerance nature 13,14 . Furthermore, the optoelectronic properties of perovskite emitters can be readily tailored by engineering composition and dimensionality, enabling continuously tunable light emission from violet to near-infrared (NIR) regions 2,13 . In addition, light emission from perovskites shows narrow linewidths (<100 meV), resulting in high color purity: for example, the photoluminescence full width at half maximum (FWHM) is around 12, 20 and 40 nm for CsPbCl3, CsPbBr3 and CsPbI3 PNCs, respectively 15 . The color gamut of displays made by PNCs can cover up to 140% of the National Television System

A "visible-blind" solution-processed UV photodetector is realized on the basis of colloidal ZnO nanoparticles. The devices exhibit low dark currents with a resistance >1 TOmega and high UV photocurrent efficiencies with a responsivity of 61 A/W at an average intensity of 1.06 mW/cm(2) illumination at 370 nm. The characteristic times for the rise and fall of the photocurrent are <0.1 s and about 1 s, respectively. The photocurrent of the device is associated with a light-induced desorption of oxygen from the nanoparticle surfaces, thus removing electron traps and increasing the free carrier density which in turn reduces the Schottky barrier between contacts and ZnO nanoparticles for electron injection. The devices are promising for use in large-area UV photodetector applications.

High-performance perovskite light-emitting diodes are achieved by an interfacial engineering approach, leading to the most efficient near-infrared devices produced using solution-processed emitters and efficient green devices at high brightness conditions.

Quantum dots are a unique class of emitters with size-tunable emission wavelengths, saturated emission colors, near-unity luminance efficiency, inherent photo- and thermal- stability and excellent solution processability. Quantum dots have been used as down-converters for back-lighting in liquid-crystal displays to improve color gamut, leading to the booming of quantum-dot televisions in consumer market. In the past few years, efficiency and lifetime of electroluminescence devices based on quantum dots achieved tremendous progress. These encouraging facts foreshadow the commercialization of quantum-dot light-emitting diodes (QLEDs), which promises an unprecedented generation of cost-effective, large-area, energy-saving, wide-color-gamut, ultra-thin and flexible displays. Here we provide a Progress Report, covering interdisciplinary aspects including material chemistry of quantum dots and charge-transporting layers, optimization and mechanism studies of prototype devices and processing techniques to produce large-area and high-resolution red-green-blue pixel arrays. We also identify a few key challenges facing the development of active-matrix QLED displays.