Strategies for Controlling Emission Anisotropy in Lead Halide Perovskite Emitters for LED Outcoupling Enhancement.

In the last decade, momentous progress in lead halide perovskite (LHP) light-emitting diodes (LEDs) is witnessed as their external quantum efficiency (η) has increased from 0.1 to more than 30%. Indeed, perovskite LEDs (PeLEDs), which can in principle reach 100% internal quantum efficiency as they are not limited by the spin-statistics, are reaching their full potential and approaching the theoretical limit in terms of device efficiency. However, ≈70% to 85% of total generated photons are trapped within the devices through the dissipation pathways of the substrate, waveguide, and evanescent modes. To this end, numerous extrinsic and intrinsic light-outcoupling strategies are studied to enhance light-outcoupling efficiency (η). At the outset, various external and internal light outcoupling techniques are reviewed with specific emphasis on emission anisotropy and its role on η. In particular, the device η can be enhanced by up to 50%, taking advantage of the increased probability for photons outcoupled to air by effectively inducing horizontally oriented emission transition dipole moments (TDM) in the perovskite emitters. The role of the TDM orientation in PeLED performance and the factors allowing its rational manipulation are reviewed extensively. Furthermore, this account presents an in-depth discussion about the effects of the self-assembly of LHP colloidal nanocrystals (NCs) into superlattices on the NC emission anisotropy and optical properties.