Light outcoupling strategies in oriented perovskite light-emitting-diodes: recent trends, opportunities, and challenges toward innovation

Metal halide perovskites (MHPs) exhibit outstanding photophysical properties, such as high photoluminescence quantum yield (PLQY), high exciton binding energy, narrow emission spectrum, tailorable bandgap, and facile solution-processibility. MHPs have opened a scientific revitalization in the light-emitting diode (LED) technology. Tremendous research efforts have been carried out, such as balancing electron-hole injection, modulation of stoichiometric compositions, ligand engineering, and minimal non-radiative recombination that boosted the internal quantum efficiency near unity. However, the lower external quantum efficiency and operational stability of perovskite LEDs (PeLEDs) than organic LEDs make them limited to display and lighting applications. One of the main reasons is the trapped photons inside the devices, which reduces light outcoupling. This review includes a detailed analysis of existing challenges in managing light outcoupling of PeLEDs and discusses various strategies adopted for these challenges. Control of dipole orientations, refractive index matching, oriented quasi-two-dimensional structures, and thickness optimization are existing methodologies to address the improved light outcoupling in PeLEDs. We discuss a detailed prospect for high-performance PeLEDs for practical applications.