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

Bright and efficient blue emission is key to further development of metal halide perovskite light-emitting diodes. Although modifying bromide/chloride composition is straightforward to achieve blue emission, practical implementation of this strategy has been challenging due to poor colour stability and severe photoluminescence quenching. Both detrimental effects become increasingly prominent in perovskites with the high chloride content needed to produce blue emission. Here, we solve these critical challenges in mixed halide perovskites and demonstrate spectrally stable blue perovskite light-emitting diodes over a wide range of emission wavelengths from 490 to 451 nanometres. The emission colour is directly tuned by modifying the halide composition. Particularly, our blue and deep-blue light-emitting diodes based on three-dimensional perovskites show high EQE values of 11.0% and 5.5% with emission peaks at 477 and 467 nm, respectively. These achievements are enabled by a vapour-assisted crystallization technique, which largely mitigates local compositional heterogeneity and ion migration.

Photodetectors are critical parts of an optical communication system for achieving efficient photoelectronic conversion of signals, and the response speed directly determines the bandwidth of the whole system. Metal halide perovskites, an emerging class of low-cost solution-processed semiconductors, exhibiting strong optical absorption, low trap states, and high carrier mobility, are widely investigated in photodetection applications. Herein, through optimizing the device engineering and film quality, high-performance photodetectors based on all-inorganic cesium lead halide perovskite (CsPbI Br ), which simultaneously possess high sensitivity and fast response, are demonstrated. The optimized devices processed from CsPbIBr perovskite show a practically measured detectable limit of about 21.5 pW cm and a fast response time of 20 ns, which are both among the highest reported device performance of perovskite-based photodetectors. Moreover, the photodetectors exhibit outstanding long-term environmental stability, with negligible degradation of the photoresponse property after 2000 h under ambient conditions. In addition, the resulting perovskite photodetector is successfully integrated into an optical communication system and its applications as an optical signal receiver on transmitting text and audio signals is demonstrated. The results suggest that all-inorganic metal halide perovskite-based photodetectors have great application potential for optical communication.

Metal halide perovskites are emerging as promising semiconductors for cost-effective and high-performance light-emitting diodes (LEDs). Previous investigations have focused on the optimisation of the emissive perovskite layer, for example, through quantum confinement to enhance the radiative recombination or through defect passivation to decrease non-radiative recombination. However, an in-depth understanding of how the buried charge transport layers affect the perovskite crystallisation, though of critical importance, is currently missing for perovskite LEDs. Here, we reveal synergistic effect of precursor stoichiometry and interfacial reactions for perovskite LEDs, and establish useful guidelines for rational device optimization. We reveal that efficient deprotonation of the undesirable organic cations by a metal oxide interlayer with a high isoelectric point is critical to promote the transition of intermediate phases to highly emissive perovskite films. Combining our findings with effective defect passivation of the active layer, we achieve high-efficiency perovskite LEDs with a maximum external quantum efficiency of 19.6%.

The integration of optical signal generation and reception into one device – and thus allowing bidirectional optical signal transmission between two identical devices – is of value in the development of miniaturized and integrated optoelectronic devices. However, conventional solution-processable semiconductors have intrinsic material and design limitations that prevent them from being used to create such devices with high performance. Here, we report an efficient solution-processed perovskite diode that is capable of working in both emission and detection modes. The device can be switched between modes by changing the bias direction, and it exhibits light emission with an external quantum efficiency of over 21% and a light detection limit on a sub-picowatt scale. The operation speed for both functions can reach tens of megahertz. Benefiting from the small Stokes shift of perovskites, our diodes exhibit high specific detectivity (more than 2×10 12 Jones) at its peak emission (~804 nm), allowing optical signal exchange between two identical diodes. To illustrate the potential of the dual-functional diode, we show that it can be used to create a monolithic pulse sensor and a bidirectional optical communication system.

Interfacial reactions between the perovskite emitters and the interlayers are detrimental to the operational stability of the perovskite light-emitting diodes. Incorporating dicarboxylic acids into the precursor efficiently eliminates reactive organic ingredients in the perovskite emitters through an in situ amidation process, which is catalyzed by the alkaline zinc oxide substrate underneath. The formed amides improve the stability of the perovskite emitters and the charge injection contacts, ensuing notably improved operational stability of the resulting perovskite light-emitting diodes.

This review summarizes the advantages of non-fullerene acceptors and their applications in ternary blend organic solar cells.