Planar perovskite light-emitting diodes (LEDs) are high-performance and cost-effective electroluminescence devices that are ideal for large-area display and lighting applications. By exploring the emission layers with high ratios of horizontal transition dipole moments (TDMs), researchers can boost the photon outcoupling of planar LEDs. The LEDs that are based on anisotropic perovskite are inefficient due to the challenges of regulating the orientations of TDMs as well as the difficulties of achieving high photoluminescence quantum yields, including challenges of realizing charge balance in the films of assembled nanostructures. In this work, Jieyuan Cui and a research team in chemistry, materials science and optics in China, showed efficient electroluminescence emanating from an in-situ perovskite film made of a monolayer of nanoplatelets. The team achieved LEDs with a peak external quantum efficiency (EQE) of 23.6 percent to represent highly efficient planar perovskite LEDs.