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

The open-circuit voltage of organic solar cells is usually lower than the values achieved in inorganic or perovskite photovoltaic devices with comparable bandgaps. Energy losses during charge separation at the donor-acceptor interface and non-radiative recombination are among the main causes of such voltage losses. Here we combine spectroscopic and quantum-chemistry approaches to identify key rules for minimizing voltage losses: (1) a low energy offset between donor and acceptor molecular states and (2) high photoluminescence yield of the low-gap material in the blend. Following these rules, we present a range of existing and new donor-acceptor systems that combine efficient photocurrent generation with electroluminescence yield up to 0.03%, leading to non-radiative voltage losses as small as 0.21 V. This study provides a rationale to explain and further improve the performance of recently demonstrated high-open-circuit-voltage organic solar cells.

promise for low-cost solution processability. [1][2][3][4] Perovskite solar cells have experienced tremendous development in the past few years with power conversion efficiency (PCE) rapidly increased to over 20%. [5,6] Besides efficiency, another critical factor for practical application of perovskite solar cells is stability.Recently, Ruddlesden-Popper perovskites (RPPs) have been reported to have promising stability. [7][8][9] The RPPs can be structurally derived from their 3D counterparts with alternating organic ammonium layers and perovskite layers, giving the general formula of (RNH 3 ) 2 A n−1 M n X 3n+1 , where n represents the number of perovskite layers, RNH 3 is the organic spacer, and the A (small cation), M (divalent metal cation), and X (halide anion) form the perovskite framework. [10,11] The superior stability of RPPs can be attributed to the hydrophobic property of the organic spacer, which inhibits water molecules from penetrating and attacking the inorganic layers.Previous studies reported that RPPs exhibit enhanced stability but at the cost of reduced efficiency due to lower Recently, Ruddlesden-Popper perovskites (RPPs) have attracted increasing interests due to their promising stability. However, the efficiency of solar cells based on RPPs is much lower than that based on 3D perovskites, mainly attributed to their poor charge transport. Herein, a simple yet universal method for controlling the quality of RPP films by a synergistic effect of two additives in the precursor solution is presented. RPP films achieved by this method show (a) high quality with uniform morphology, enhanced crystallinity, and reduced density of sub-bandgap states, (b) vertically oriented perovskite frameworks that facilitate efficient charge transport, and (c) type-II band alignment that favors self-driven charge separation. Consequently, a hysteresis-free RPP solar cell with a power conversion efficiency exceeding 12%, which is much higher than that of the control device (1.5%), is achieved. The findings will spur new developments in the fabrication of high-quality, aligned, and graded RPP films essential for realizing efficient and stable perovskite solar cells. Solar CellsOrganic-inorganic halide perovskites, such as MAPbI 3 (MA = CH 3 NH 3 ), FAPbI 3 (FA = CH(NH 2 ) 2 ), and their mixed cation analogues, have emerged as promising light absorbers for solar cells due to their superior photophysical properties and

High-efficiency, thermally activated delayed-fluorescence organic light-emitting diodes based on exciplex emitters are demonstrated. The best device, based on a TAPC:DPTPCz emitter, shows a high external quantum efficiency of 15.4%. Strategies for predicting and designing efficient exciplex emitters are also provided. This approach allow prediction and design of efficient exciplex emitters for achieving high-efficiency organic light-emitting diodes, for future use in displays and lighting applications.