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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A major efficiency limit for solution-processed perovskite optoelectronic devices (e.g. light-emitting diodes, LEDs) is trap-mediated non-radiative losses. Defect passivation using organic molecules has been identified as an attractive approach to tackle this issue. However, implementation of this approach has been hindered by a lack of deep understanding of how the molecular structures affect the passivation effectiveness. We show that the so far largely ignored hydrogen bonds play a critical role. By weakening the hydrogen bonding between the passivating functional moieties and the organic cation featuring the perovskite, we significantly enhance the interaction with defects sites and minimize non-radiative recombination losses. Consequently, we achieve exceptionally high-performance near infrared perovskite LEDs (PeLEDs) with a record external quantum efficiency (EQE) of 21.6%. In addition, our passivated PeLEDs maintain a high EQE of 20.1% and a wall-plug efficiency of 11.0% at a high current density of 200 mA cm-2 , making them more attractive than the most efficient organic and quantum-dot LEDs at high excitations.

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

Perovskite-based optoelectronic devices have gained significant attention due to their remarkable performance and low processing cost, particularly for solar cells. However, for perovskite light-emitting diodes (LEDs), non-radiative charge carrier recombination has limited electroluminescence (EL) efficiency. Here we demonstrate perovskite-polymer bulk heterostructure LEDs exhibiting record-high external quantum efficiencies (EQEs) exceeding 20%, and an EL half-life of 46 hours under continuous operation. This performance is achieved with an emissive layer comprising quasi-2D and 3D perovskites and an insulating polymer. Transient optical spectroscopy reveals that photogenerated excitations at the quasi-2D perovskite component migrate to lower-energy sites within 1 ps. The dominant component of the photoluminescence (PL) is primarily bimolecular and is characteristic of the 3D regions. From PL quantum efficiency and transient kinetics of the emissive layer with/without charge-transport contacts, we find non-radiative recombination pathways to be effectively eliminated. Light outcoupling from planar LEDs, as used in OLED displays, generally limits EQE to 20-30%, and we model our reported EL efficiency of over 20% in the forward direction to indicate the internal quantum efficiency (IQE) to be close to 100%. Together with the low drive voltages needed to achieve useful photon fluxes (2-3 V for 0.1-1 mA cm -2 ), these results establish that perovskite-based LEDs have significant potential for light-emission applications.

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.