Lead halide perovskite nanocrystals (NCs) have shown remarkable properties for emission applications, but their toxicity and instability are a hindrance to many commercial uses. Herein, we report the synthesis of lead-free all-inorganic Cs 3 Cu 2 X 5 (X = I, Br/I, Br, Br/Cl, Cl) colloidal nanocrystals as members of the metal−metal halide family of materials. These nanocrystals have uniform sizes less than 10 nm in diameter and show excellent optical properties, including composition-tunable emission spectra over the spectral region of 440−530 nm; high photoluminescence quantum yields of ∼100, 20, and 30% for X = Cl, Br, and I, respectively; and large effective Stokes shifts of over 100 nm for all species. Nanocrystals are synthesized by a room temperature, antisolvent method, but the precursors and ligands are also shown to be effective in hot -injection synthesis. Pure-and mixed-halide materials show tunable emission with the halide concentration, with a large fwhm of 80−110 nm due to a widely reported exciton self-trapping emission mechanism. Notably, the Cu 3 Cu 2 Cl 5 NCs exhibit a near-unity quantum yield with an emission at 520 nm, high crystallinity, and good stability. These materials can be processed and maintained in adequately stable dispersions to enable inkjet printing of these materials into arbitrary patterns. These results indicate that cesium copper chloride NCs may have great potential for the future display or lighting applications.

In this Perspective, we review the recent work on metal halide perovskite light-emitting devices (peLEDs) with a focus on current challenges and the prospect of commercialization of perovskite displays. Metal halide perovskites have shown remarkable hole and electron mobilities, high photoluminescence quantum yield and color tunability, and thin line width emission peaks making them ideal materials for the emissive layer in LEDs. Red- and green-emitting perovskites have already achieved very high external quantum efficiencies of greater than 20% in the last year and may be ready to begin the commercialization process. However, blue peLEDs still lag behind in efficiency and stability. After reviewing recent attempts to improve the blue peLED efficiency, we then list the current challenges in the field and suggest possible avenues of future research, including improvements in materials chemistry and device architecture. In particular, we find that advancements are needed in color stability and lifetime, by using doping and nanostructured materials to avoid ion migration effects, and in the synthesis of novel compounds. We then compare the current state of the art in peLEDs with the competitive landscape and conclude that fixing the blue peLEDs is the key to building micropixel LEDs from perovskites. Without blue-emitting perovskites, high PLQY perovskite films can still be useful for down-conversion color filters but may have difficulty displacing existing technologies. Thus, for perovskites, the future is indeed blue (LEDs).

Ternary metal halides, including perovskites, have become a popular field of study over the last decade. Recently many groups have attempted to replace the lead in lead-based perovskites with other...