In a joint experimental and theoretical work, Lei shows that binary cations of Cs and Rb improves the operation stability of perovskite LEDs. The work is published open access on Advanced Materials. A short summary of the work can be found below:
Perovskite light emitting diodes (PeLEDs) have recently attracted significant attention in the light emitting fields with comparable external quantum efficiency to the organic light emitting diodes (OLEDs). The superior properties of perovskites, such as low cost, high photoluminescence quantum efficiency and narrow emission linewidth, have endowed PeLEDs with great potential for industrial application. However, the operation stability is still challenging due to the severe decay of the luminescence efficiency under bias.
We carefully investigated the operation stability of FAPbI3-based PeLEDs and for the first time provided direct experimental evidence of I– migration in operating PeLEDs. To promote the operation stability, we explored a composition engineering strategy to suppress the defects-assisted I– migration by incorporating binary alkali cations (Cs+ and Rb+) in FAPbI3 perovskites.
Using combined experimental data from X-ray photoelectron spectroscopy profiles and first principle density functional theory (DFT) calculations, we reveal that although both Cs+ and Rb+ play a positive role in improving operation stability of PeLEDs, the underlying mechanisms of Cs+-induced and Rb+-induced enhancement are different. We found that while Cs+ ions are present throughout the bulk, Rb+ ions are spontaneously located on the surface and grain boundaries (GBs) of perovskite films. Further chemical bonding analysis show that both Cs+ and Rb+ raise net atomic charges of surrounding I– anions, leading to stronger Coulomb interactions between cations and the inorganic framework, potentially increasing the formation energy of I– vacancies. As such, the presence of Cs+ and Rb+ ions efficiently blocks the pathways of ion migration in the bulk and along GBs, respectively. As a result, the Cs+-Rb+-incorporated PeLEDs has reached a record external quantum efficiency of 15.84% among alkali-cation-incorporated FAPbI3 LEDs. More importantly, the PeLEDs show significantly enhanced operation stability with half lifetime over 3600 min, comparable to that of the near infrared organic light emitting diodes.