Perovskite LEDs Performance Enhancement with Nanostructures for Better Light out-Coupling and Stability

Abstract

In the past a few years, perovskite light-emitting diodes (LEDs) have experienced extremely amazing development. Thanks to the decent optoelectronic properties, perovskite material has made itself a competitive candidate for next-generation displays and lighting. With the great efforts of researchers in this field, the external quantum efficiencies (EQEs) have already surpassed the 20% milestone for both red and green colored perovskite LEDs. Lately, the blue perovskite LEDs have been reported to achieve an EQE higher than 12%.With the fantastic material quality, perovskite material can achieve a super high photoluminescent quantum efficiency (PLQY); therefore, the internal quantum efficiency (QE) for perovskite LEDs can also reach the level of higher than 90%. As a result, the key solution to the further enhancement of the perovskite LEDs performance relies on the light outcoupling for perovskite LEDs.During the past five years, we have devoted ourselves to the development of nanophotonic strategies for the performance enhancement of the perovskite LEDs. The strategies are comprised of two main categories. The first category is using a nanophotonic substrate for perovskite LEDs devices. The structure we designed was a combination of a nanodome coupler with the photonic crystal optical antennas. When the geometry of the structure was delicately optimized, the light supposed to be trapped inside the active layer can be first coupled into the photonic crystals and form guided modes. After that, the photonic crystals, working as optical antennas, can convert the guided modes to the leaky modes and finally the leaky modes can emit to the air. With this strategy, we successfully improved the EQE of the perovskite LEDs from about 8% to 17.5%, which was the record for the MAPbBr3 based perovskite LEDs at that time.The second category of the nanophotonic strategies is making nanostructured active materials instead of using the nanostructured out-coupling substrates. With the exploration of varieties of nanostructures, we finally determined to use the nanowire arrays for the perovskite LEDs study. Intriguingly, we came up with a solution method with the nanoporous template for perovskite nanowires growth. By studying the growth with different template geometries, we found out that the nanowire growth had a strong relationship with the capillary effect. Then we carefully designed the template thickness and the nanowire lengths, and we made the three-dimensional nanowire arrays-based perovskite LEDs. Compared to the planar counterparts, the nanowire-based perovskite LEDs showed an enhanced EQE (from 11% to 16%), which is due to the improved light coupling inside the nanowires (from about 20% to about 45%). More importantly, the operational stability of the nanowire-based perovskite LEDs was also enhanced (T50 increased from 9 min to 35 min). And the long-term stability of the nanowire-based perovskite LEDs was also increased with an enhancement factor of 3 times. It’s also worth noting that, the mechanical robustness of the nanowire-based perovskite LEDs was also enhanced a lot due to the protection by the porous template, and much fewer cracks were created inside nanowires during bendings when compared to the planar counterparts.With the above mentioned two nanophotonic strategies for perovskite LEDs performance (both light-coupling efficiency and stability) enhancement, we demonstrated the advantages of applying nanostructures in perovskite LEDs. By further developing the perovskite material qualities and meanwhile improving the deposition techniques that are more compatible with nanostructures, we believe the EQEs of perovskite LEDs can be further improved to a new milestone soon. Last but not least, the nanophotonic strategies we developed here are not just limited to LEDs; they can be also applied for other device applications such as solar cells, photodetectors, and lasers.