Solution-processed planar perovskite light-emitting diodes (LEDs) promise high-performance and cost-effective electroluminescent devices ideal for large-area display and lighting applications. Exploiting emission layers with high ratios of horizontal transition dipole moments (TDMs) is expected to boost the photon outcoupling of planar LEDs. However, LEDs based on anisotropic perovskite nanoemitters remain to be inefficient (external quantum efficiency, EQE <5%) due to the difficulties of simultaneously controlling the orientations of TDMs, achieving high photoluminescence quantum yields (PLQYs) and realizing charge balance in the films of assembled nanostructures. Here, we demonstrate efficient electroluminescence from an in situ grown perovskite film composed of a monolayer of face-on oriented nanoplatelets. The ratio of horizontal TDMs of the perovskite nanoplatelet film is ~84%, which leads to a light-outcoupling efficiency of ~31%, substantially higher than that of isotropic emitters (~23%). In consequence, LEDs with a peak EQE of 23.6% are achieved, representing highly efficient planar perovskite LEDs.

Distribution of the transition dipole moments (TDMs) of light emitters can intrinsically affect the light out-coupling efficiency of planar light-emitting diodes (LEDs). Lacking the control of TDM distribution has limited the efficiency of nanocrystal-based LEDs to 20%. Here, we present a method that deposits uniform nanocrystal films with unity in-plane TDM distribution. Combining an inkjet printing technique and colloidal nanocrystal self-assembly, we achieved direct printing and in situ assembly of colloidal CdSe/CdS nanoplatelets to all orient “face-down” on various substrates. With motorized translation stages, pattern printing is realized, which demonstrates the potential for integration in industrial-scale fabrication. The method is applied to achieve uniform nanoplatelet films with unity in-plane TDM distribution on zinc-oxide films, a commonly used electron-transport layer. Thus, our work paves the way to break the light out-coupling efficiency limitation of 20% in state-of-the-art nanocrystal-based LEDs, which exclusively possess an isotropic TDM distribution.

Cadmium‐based nanoplatelets as optical display and lasing materials are widely explored and exhibit great advantages, owing to their narrow emission linewidths, anisotropic transition‐dipole distributions, and low lasing thresholds. However, in the green range, the photoluminescence quantum yield (PLQY) and emission tunability of nanoplatelets are still inferior to that of quantum dots. In this work, a new synthesis protocol is developed, enabling core/crown nanoplatelets to grow continuously from elementary precursors to their final form. A new heterostructure of CdSe/CdSeS core/alloyed‐crown nanoplatelets is produced that realizes 100% PLQY, the continuous tunability of emission peaks in between 502 and 550 nm, and low full‐width‐at‐half‐maximum (FWHM) of less than 15 nm. Achieving these excellent properties in all three aspects at the same time is unprecedented. In addition, the time‐resolved photoluminescence (TRPL) spectra of these nanoplatelets show a mono‐exponential decay characteristic, and the nanoplatelet film can also show 100% PLQY and a mono‐exponential decay characteristic, indicating the suppression of trap states. The high‐quality nanoplatelets achieved in this work provide a solid foundation for developing nanoplatelet‐based light sources, like light‐emitting diodes and lasers, with much higher efficiency, color purity, and lower working thresholds.

The development of chip-level near-infrared laser sources using two-dimensional semiconductors is imperative to maintain the architecture of van der Waals integrated optical interconnections. However, the established two-dimensional semiconductor lasers may have either the disadvantages of poor controllability of monolayered gain media, large optical losses on silicon, or complicated fabrication of external optical microcavities. This study demonstrates room-temperature near-infrared lasing from mechanically exfoliated γ-phase indium selenide (InSe) microflakes free from external optical microcavities at a center wavelength of ∼1030 nm. The lasing action occurs at the sub-Mott density level and is generated by exciton−exciton scattering with a high net modal optical gain of ∼1029 cm −1 . Moreover, the lasing is sustained for microdisks fabricated by a simple laser printing with a reduced threshold. These results suggest that InSe is a promising material for near-infrared microlasers and can be employed in a wide range of applications, including imaging, sensing, and optical interconnects.

Colloidal semiconductor CdSe nanoplatelets (NPLs) feature ultranarrow and anisotropic emissions. However, the optical performance of blue‐emitting NPLs is deteriorated by trap states, currently exhibiting tainted emissions and inferior photoluminescence quantum yields (PLQYs). Here, near trap‐free blue‐emitting NPLs are achieved by the controlled growth of the core/crown. Deep trap states in NPLs can be partially suppressed with the asymmetrical crown growth and are further suppressed with the growth of the small core and the alloyed symmetrical crown, yielding NPLs with pure blue emissions and near‐unity PLQYs. Exciton dynamic research based on these NPLs indicates that the trap emission stems from surface traps. Besides, light‐emitting diodes exhibiting ultranarrow emission centered around 461 nm with full‐width‐at‐half‐maximums down to 11 nm are fabricated using these NPLs.