Perovskite light-emitting diodes (PeLEDs) are attracting increasing attention owing to their impressive efficiencies and high luminance across the full visible light range. Further improvement of the external quantum efficiency (EQE) of planar PeLEDs is limited by the light out-coupling efficiency. Introducing perovskite emitters with directional emission in PeLEDs is an effective way to improve light extraction. Here, we report that it is possible to achieve directional emission in mixed-dimensional perovskites by controlling the orientation of the emissive center in the film. Multiple characterization methods suggest that our mixed-dimensional perovskite film shows highly orientated transition dipole moments (TDMs) with the horizontal ratio of over 88%, substantially higher than that of the isotropic emitters. The horizontally dominated TDMs lead to PeLEDs with exceptional high light out-coupling efficiency of over 32%, enabling a high EQE of 18.2%.

Quantum-dot light-emitting diodes (QLEDs) are multilayer electroluminescent devices promising for next-generation display and solid-state-lighting technologies. In the state-of-the-art QLEDs, hole-injection layers (HILs) with high work functions are generally used to achieve efficient hole injection. In these devices, Fermi-level pinning, a phenomenon often observed in heterojunctions involving organic semiconductors, can take place in the hole-injection/hole-transporting interfaces. However, an in-depth understanding of the impacts of Fermi-level pinning at the hole-injection/hole-transporting interfaces on the operation and performance of QLEDs is still lacking. Here, we develop a set of NiO_x HILs with controlled work functions of 5.2–5.9 eV to investigate QLEDs with Fermi-level pinning at the hole-injection/hole-transporting interfaces. The results show that despite that Fermi-level pinning induces identical apparent hole-injection barriers, the red QLEDs using HILs with higher work functions show improved efficiency roll-off and better operational stability. Remarkably, the devices using the NiO_x HILs with a work function of 5.9 eV demonstrate a peak external quantum efficiency of ~ 18.0% and a long T₉₅ operational lifetime of 8,800 h at 1,000 cd·m⁻², representing the best-performing QLEDs with inorganic HILs. Our work provides a key design principle for future developments of the hole-injection/hole-transporting interfaces of QLEDs.

We report the formation of high-quality Cs_0.4MA_0.6PbBr₃ thin films with nearly full surface coverage and good emission properties upon the introduction of Cs⁺ into perovskite crystals. The Cs_0.4MA_0.6PbBr₃ thin films were applied as emissive layers in light-emitting diodes. A maximum external quantum efficiency of ~2.0% was achieved for these green-emitting devices.

We demonstrate that charge carrier diffusion lengths of two classes of perovskites, CH₃NH₃PbI_3-xCl_x and CH₃NH₃PbI₃, are both highly sensitive to film processing conditions and optimal processing procedures are critical to preserving the long carrier diffusion lengths of the perovskite films. This understanding, together with the improved cathode interface using bilayer-structured electron transporting interlayers of [6, 6]-phenyl-C61-butyric acid methyl ester (PCBM)/ZnO, leads to the successful fabrication of highly efficient, stable and reproducible planar heterojunction CH₃NH₃PbI_3-xCl_x solar cells with impressive power-conversion efficiencies (PCEs) up to 15.9%. A 1-square-centimeter device yielding a PCE of 12.3% has been realized, demonstrating that this simple planar structure is promising for large-area devices.