  Surface-defect passivation is key to achieving high photoluminescence quantum
yield in lead halide perovskite nanocrystals. However, in perovskite
light-emitting diodes these surface ligands also have to enable balanced charge
injection into the nanocrystals to yield high efficiency and operational
lifetime. In this respect, alkaline halides have been reported to passivate
surface trap states and increase the overall stability of perovskite light
emitters. On the one side, the incorporation of alkaline ions into the lead
halide perovskite crystal structure is considered to counterbalance cation
vacancies, while, on the other side, the excess halides are believed to
stabilise the colloids. Here, we report an organic lithium salt, viz. LiTFSI,
as a halide-free surface passivation on perovskite nanocrystals. We show that
the treatment LiTFSI has multiple beneficial effects on lead halide perovskite
nanocrystals and LEDs derived from them. We obtain higher photoluminescence
quantum yield and longer exciton lifetime, and a radiation pattern that is more
favourable for light outcoupling. The ligand-induced dipoles on the nanocrystal
surface shift their energy levels toward lower hole-injection barrier. Overall,
these effects add up to a four- to seven-fold boost of the external quantum
efficiency in proof-of-concept LED structures, depending on the color of the
used lead halide perovskite nanocrystal emitters.
