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.

Perovskite light-emitting diodes based on solution-processed self-organized multiple quantum wells, Nature Photonics, 2016. 10(11) Encouraging performance metrics of light-emitting diodes (LEDs) based on 3Dperovskites, such as low turn-on voltages and external quantum efficiencies (EQEs) of up to 3.5% at high current densities, have been demonstrated 9 . However, the EL quantum efficiency is far behind the limit predicated by ~70% PLQE of the 3D perovskites, mainly due to the existence of current losses caused by incomplete surface coverage of the perovskite films and the fact that the high PLQE can only be obtained at high excitations 8,9 . By using thick (>300 nm) perovskite films, Cho et al.obtained LEDs with over 8% EQE 10 . However, for this device, the turn-on voltage is high and the power efficiency is low, which may result from the thick perovskite layer used. In order to further enhance the performance of 3D perovskite-based LEDs, it is 3 essential to obtain perovskite thin films with both complete surface coverage and high PLQE [8][9][10] . Moreover, device stability, which was proven to be a vital issue in organic-inorganic halide perovskite-based photovoltaics 11 , has not been addressed in perovskite LEDs.The 3D perovskites are actually an extreme case of layered organometal halide perovskites with a general formula of L2(SMX3)n-1MX4, where M, X, L, and S are a divalent metal cation, a halide, and organic cations with long and short chains, respectively ( Fig. 1a) [12][13][14] . Here n is the number of semiconducting MX4 monolayer sheets within the two organic insulating layers (cation L), with n=∞ corresponding to the structure of a 3D perovskite SMX3. With smaller numbers of MX4 layers, quantum confinement effects, such as an increase in bandgap and exciton energy, become important 6,15 . In consequence, the layered perovskites naturally form quantum-well structures. At the opposite extreme, when n=1, the layered perovskites form a monolayer structure of a two-dimensional (2D) perovskite L2MX4. The 2D L2MX4 perovskites generally have good film-formation properties 13 . Nevertheless, the PLQEs of the 2D perovskites are rather low at room temperature, owing to fast exciton quenching rates 6,7 . LEDs based on the 2D perovskites have been attempted, while the devices are either very low in efficiency or only operational at cryogenic temperatures [16][17][18] . Here we demonstrate very efficient (up to 11.7% EQE) and high-brightness EL achievable at room temperature by using solution-processed perovskite multiple quantum wells (MQWs) with an energy cascade, which can combine the advantages of 2D and 3D perovskites. We note, a relevant perovskite LED work 19 which shows a peak EQE of 8.8% has been published online during the revision of this paper.A precursor solution of 1-naphthylmethylamine iodide (NMAI), formamidinium iodide (FAI), and PbI2 with a molar ratio of 2:1:2 dissolved in N,N-dimethylformamide (DMF) was used to deposit perovskite films (see Methods for details), which are abbreviated as NFPI...

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