Hybrid organic-inorganic metal halide perovskites are particularly promising for light-emitting diodes (LEDs) due to their attractive optoelectronic properties such as wavelength tunability, narrow emission linewidth, defect tolerance, and high charge carrier mobility. However, the undercoordinated Pb and halide at the perovskite nanocrystal (NC) surface causes traps and nonradiative recombination. In this work, the external quantum efficiency of iodide-based perovskite LEDs is boosted to greater than 15%, with an emission wavelength at 750 nm, by engineering the perovskite NC surface stoichiometry and chemical structure of bulky organoammonium ligands. To the stoichiometric precursor solution for the 3D bulk perovskite, 20% molar ratio of methylammonium iodide is added in addition to 20% excess bulky organoammonium iodide to ensure that the NC surface is organoammonium terminated as the crystal size is decreased to 5-10 nm. This combination ensures minimal undercoordinated Pb and halide on the surface, avoids 2D phases, and acts to provide nanosized perovskite grains which allow for smooth and pinhole-free films. As a result of time-resolved photoluminescence (PL) and PL quantum yield measurements, it is possible to demonstrate that this surface modification increases the radiative recombination rate while reducing the nonradiative rate.photodetectors, [7,8] memories, [9] etc. Perovskite light-emitting diodes (PeLEDs) are particularly interesting due to certain advantages over both organic LEDs (OLEDs) and quantum dot LEDs (QLEDs). [6] For example, full width at half-maximum of PeLEDs reaches 20 nm at an emission wavelength around 520 nm, smaller than that of either OLEDs (>40 nm) or QLEDs (≈30 nm). [5] Therefore, the emission color is more pure. The emission wavelength of PeLEDs can be tuned by component engineering such as by alloying the halide component. [10,11] In addition, the carrier mobility of hybrid perovskites can exceed 100 cm 2 V −1 s −1 , an order of magnitude higher than organic semiconductors and quantum dots (QDs). [3] Therefore, the current density through PeLEDs and thus the highest brightness has the potential to be higher than that of OLEDs and QLEDs. [6] The performance of PeLEDs has experienced dramatic improvement over the last 4 years. [12][13][14][15][16][17][18][19] Since the first report of PeLEDs using 3D perovskite phases, CH 3 NH 3 PbI 3 (MAPbI 3 ) and MAPbBr 3 , with an external quantum efficiency (EQE) of 0.76% and 0.1%, respectively, in 2014, [20] various approaches have been developed to improve the performance of PeLEDs. For example, Cho et al. incorporated a 5% molar excess of MABr to passivate surfaces and

Perovskite light-emitting diodes (LEDs) require small grain sizes to spatially confine charge carriers for efficient radiative recombination. As grain size decreases, passivation of surface defects becomes increasingly important. Additionally, polycrystalline perovskite films are highly brittle and mechanically fragile, limiting their practical applications in flexible electronics. In this work, the introduction of properly chosen bulky organo-ammonium halide additives is shown to be able to improve both optoelectronic and mechanical properties of perovskites, yielding highly efficient, robust, and flexible perovskite LEDs with external quantum efficiency of up to 13% and no degradation after bending for 10 000 cycles at a radius of 2 mm. Furthermore, insight of the improvements regarding molecular structure, size, and polarity at the atomic level is obtained with first-principles calculations, and design principles are provided to overcome trade-offs between optoelectronic and mechanical properties, thus increasing the scope for future highly efficient, robust, and flexible perovskite electronic device development.

Metal halide perovskites are the first solution processed semiconductors that can compete in their functionality with conventional semiconductors, such as silicon. Over the past several years, perovskite semiconductors have reported breakthroughs in various optoelectronic devices, such as solar cells, photodetectors, light emitting and memory devices, and so on. Until now, perovskite semiconductors face challenges regarding their stability, reproducibility, and toxicity. In this Roadmap, we combine the expertise of chemistry, physics, and device engineering from leading experts in the perovskite research community to focus on the fundamental material properties, the fabrication methods, characterization and photophysical properties, perovskite devices, and current challenges in this field. We develop a comprehensive overview of the current state-of-the-art and offer readers an informed perspective of where this field is heading and what challenges we have to overcome to get to successful commercialization.

Metal halide perovskite light-emitting diodes (PeLEDs) have gained significant interest for next-generation optoelectronic devices, since PeLEDs exhibit narrow emission bandwidth that allows for vivid and clear images based on their high color purity. [1][2][3][4][5][6] The emission color of PeLEDs is tunable in the visible and near-infrared (NIR) spectral regions and they offer low operating and turn-on voltages, along with promising efficiency values. [3,4,[7][8][9] In addition, thin films have shown nearunity photoluminescence quantum yield (PLQY) and population inversion at room temperature, [10][11][12][13][14] potentially allowing for electrically pumped lasers with various emission colors.There has recently been rapid growth in the external quantum efficiency (EQE) of PeLEDs, to values of over 20%, [9,[15][16][17][18][19][20][21][22][23][24][25][26][27] since early reports of PeLEDs in 2014 with efficiency below 0.25%. [28] Numerous strategies to improve the EQE of PeLEDs are being actively pursued in order to bring their performance in line with other, more established, LED technologies. [8] However, a disparity of refractive index (n) between organic transport layers (typically in the range of 1.6-1.8) and the perovskite emissive layer (≈2.3 near the emission wavelength) holds back performance. [29][30][31][32] Due to the high n of the perovskite layer, the maximum EQE of PeLEDs is limited by outcoupling efficiency and restricted to ≈20%, with the remainder of light being trapped within the thin film and substrate materials, as well as parasitic absorption. [31,32] Therefore, it is necessary to investigate alternative device architectures that are able to enhance outcoupling efficiency and realize direct benefits to EQE.In this study, we demonstrate EQE of 14.6% in methylammonium lead iodide (MAPbI 3 ) based red/NIR LEDs using a randomly distributed nanohole array (NHA) embedded in a SiN layer between the indium tin oxide (ITO) anode and glass substrate. The SiN layer with a high n of 2.02 at the peak emission wavelength possesses a high-index contrast with the voids of the NHA with n of 1.0. This layer effectively compensates for the high n of the perovskite emissive layer and aids outcoupling of waveguided and substrate modes. As a result, PeLEDs with NHAs show 1.64 times higher light extraction than PeLEDs without NHAs. Figure 1a displays the device structure of PeLEDs with and without NHAs, as well as the molecular structures of transport Organic-inorganic hybrid perovskite light-emitting diodes (PeLEDs) are promising for next-generation optoelectronic devices due to their potential to achieve high color purity, efficiency, and brightness. Although the external quantum efficiency (EQE) of PeLEDs has recently surpassed 20%, various strategies are being pursued to increase EQE further and reduce the EQE gap compared to other LED technologies. A key point to further boost EQE of PeLEDs is linked to the high refractive index of the perovskite emissive layer, leading to optical losses of more than 70% of ...

Using an ultrasensitive, exfoliated 2D perovskite single-crystal sheet/graphene heterostructure device, spontaneous iodide loss is revealed as an important degradation pathway of perovskites, which n-dopes perovskites by generating positively charged iodide vacancies. Furthermore, covering perovskites with graphene can suppress iodide loss, significantly improving perovskite stability. Our work not only provides important insights for future stable perovskite optoelectronic device development, but also demonstrates the potential of graphene as an encapsulant as well as a sensitive diagnostic tool for device and material degradation studies.