Researchers from the University of Toronto, Peking University and Soochow University have studied the origins of unwanted emission in monolayer perovskite LEDs when the active layer thickness approaches ~5 nm and found that using available fabrication techniques results in a rough perovskite/HTL interface which leads to punch-through and direct electrical interaction between HTL and ETL (electron-transporting layer), and consequently, to undesired exciplex emission in LEDs.
The team sought to control monolayer interfaces in Rec.2100 primary blue perovskite LEDs and recognized that a well-defined, ordered, and compact monolayer film could suppress HTL/ETL interaction. They reasoned that this could be achieved if they could alter the polarity of the CsPbBr3 c-NC surface and thereby induce perovskite self-assembly down to the monolayer limit [i.e., self-assembled monolayer (SAM)] through the use of an HTL-compatible ligand. Self-assembled films with ordered nanocrystal arrangement maximize the interactions between nanocrystals and provide homogeneity needed for monolayer films with ~5-nm thickness.
The scientists reported a self-assembled monolayer (SAM) active layer that improves charge injection. They identified a bifunctional capping ligand that simultaneously enables the self-assembly of CsPbBr3 c-NCs while passivating surface traps. As a result, they reported SAM-based LEDs that exhibited a champion EQE of ~12% [CIE of (0.132, 0.069) at 4.0 V with a luminance of 11 cd/m2], and 10-fold–enhanced operating stability relative to the best previously reported Rec. 2100-blue perovskite LEDs.