presence of aliphatic ligands. [ 7 ] These perovskite nanocrystals are highly luminescent and emit over the full visible range, making them ideal candidates for luminescent display applications. [ 6 ] The synthetic steps are generally straightforward, and the easy control of halide content allows the perovskite bandgaps to be tailored, both by chemical compositions as well as by quantum size effects. So far, perovskite nanocrystals are shown to have color-pure emission, close to unity photoluminescence yield and low lasing thresholds. [ 8 ] These nanocrystals were also attempted in light-emitting devices, but effi ciencies remain modest at 0.12%. [ 9 ] Here, we show the preparation of highly effi cient perovskite light-emitting diodes (PeLED) using solution-processed nanocrystals. We apply a new trimethylaluminum (TMA) vapor-based crosslinking method to render the nanocrystal fi lms insoluble, thereby allowing the deposition of subsequent charge-injection layers without the need for orthogonal solvents. The resulting near-complete nanocrystal fi lm coverage, coupled with the natural confi nement of injected charges within the perovskite crystals, facilitate electron-hole capture and give rise to a remarkable electroluminescence yield of 5.7%. Here, our electron-injection layer comprises a fi lm of zinc oxide (ZnO) nanocrystals, directly deposited on an indium tin oxide (ITO)-coated glass substrate. [ 4 ] The cesium lead halide nanocrystals were solution-coated onto the ZnO fi lm as the emissive layer. Due to the presence of aliphatic ligands on the nanocrystals, the perovskite fi lm remains soluble to organic solvents, which limits the deposition of subsequent chargeinjection layers using solution methods. We employed a new TMA vapor-phase crosslinking technique to fi x the nanocrystal fi lm in place, thereby enabling us to solution-cast a layer of TFB polymer (poly[(9,9-dioctylfl uorenyl-2,7-diyl)-co -(4,4′-( N -(4-sec-butylphenyl)diphenylamine)]) above without washing the nanocrystals off. TFB serves primarily as a hole-injection and electron-blocking layer. A thin, high work-function molybdenum trioxide (MoO 3 ) interlayer and silver electrode were vacuum-thermal evaporated to complete the device.As shown in Figure 1 c,d, our perovskite nanocrystal devices show saturated and color-pure emission. We control the perovskite bandgap, primarily by tailoring the halide composition, and achieve electroluminescence across a wide range of the visi ble spectrum. Our red, orange, green, and blue devices emit at wavelengths of 698, 619, 523, and 480 nm, respectively.

A significant fraction of global electricity demand is for lighting. Enabled by the realization and development of efficient GaN blue light-emitting diodes (LEDs), phosphor-based solid-state white LEDs provide a much higher efficiency alternative to incandescent and fluorescent lighting, which are being broadly implemented. However, a key challenge for this industry is to achieve the right photometric ranges and application-specific emission spectra via cost-effective means. Here, we synthesize organic–inorganic lead halide-based perovskite crystals with broad spectral tuneability. By tailoring the composition of methyl and octlyammonium cations in the colloidal synthesis, meso- to nanoscale 3D crystals (5–50 nm) can be formed with enhanced photoluminescence efficiency. By increasing the octlyammonium cations content, we observe platelet formation of 2D layered perovskite sheets; however, these platelets appear to be less emissive than the 3D crystals. We further manipulate the halide composition of the perovskite crystals to achieve emission covering the entire visible spectrum. By blending perovskite crystals with different emission wavelengths in a polymer host, we demonstrate the potential to replace conventional phosphors and provide the means to replicate natural white light when excited by a blue GaN LED.

Cesium lead halide nanocrystals, CsPbX3 (X = Cl, Br, I), exhibit photoluminescence quantum efficiencies approaching 100% without the core–shell structures usually used in conventional semiconductor nanocrystals. These high photoluminescence efficiencies make these crystals ideal candidates for light-emitting diodes (LEDs). However, because of the large surface area to volume ratio, halogen exchange between perovskite nanocrystals of different compositions occurs rapidly, which is one of the limiting factors for white-light applications requiring a mixture of different crystal compositions to achieve a broad emission spectrum. Here, we use mixtures of chloride and iodide CsPbX3 (X = Cl, I) perovskite nanocrystals where anion exchange is significantly reduced. We investigate samples containing mixtures of perovskite nanocrystals with different compositions and study the resulting optical and electrical interactions. We report excitation transfer from CsPbCl3 to CsPbI3 in solution and within a poly(methyl methacrylate) matrix via photon reabsorption, which also occurs in electrically excited crystals in bulk heterojunction LEDs.

Fluence-dependent photoluminescence and ultrafast transient absorption spectroscopy are used to study the dynamic behavior of carriers in CsPbCl3 perovskite nanocrystals. At low excitation fluences, the radiative recombination rate is outcompeted by significant trapping of the charge carriers which then recombine nonradiatively, resulting in weak photoluminescence. As fluence is increased, the saturation of trap states deactivates these nonradiative relaxation paths giving rise to an increase in photoluminescence at first. However, with further increases in fluence, Auger recombination of multiexcitons results in a decline in photoluminescence efficiency. Analysis of this behavior yields an absorption cross section at 400 nm (3.1 eV) of (0.24 ± 0.05) × 10–14 cm2. Transient photoluminescence and absorption measurements yielded values for single exciton trapping lifetime (1.6 ± 0.7 ns), biexciton and trion lifetimes (20 ± 3 and 157 ± 20 ps, respectively), single exciton radiative lifetime (12.7 ± 0.2 ns), intraband cooling lifetime (290 ± 37 fs), and exciton–exciton interaction energy (10 ± 2 meV).

Perovskite colloidal nanocrystals have emerged as important new optical materials, with tuneable light emission across the visible spectrum, narrow linewidths for high colour purity, and quantum efficiencies approaching unity. These materials can be solution processed in large volumes at low cost making them promising for optoelectronic devices. The structure of nanocrystals influences the radiative and non-radiative recombination of carriers within them through trap states and Auger recombination. To optimise the emission properties it is vital to understand the relationship between the optical emission of individual nanocrystals and their structure, size, and composition of individual nanocrystals.

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