  Perovskite nanoplatelets (NPls) hold great promise for light-emitting
applications, having achieved high photoluminescence quantum efficiencies
(PLQEs) approaching unity in the blue wavelength range, where other
metal-halide perovskites have typically been ineffective. However, the external
quantum efficiency (EQE) of blue-emitting NPl light-emitting diodes (LEDs) have
only reached 0.12%, with typical values well below 0.1%. In this work, we show
that the performance of NPl LEDs is primarily hindered by a poor electronic
interface between the emitter and hole-injector. Through Kelvin Probe and X-ray
photoemission spectroscopy measurements, we reveal that the NPls have
remarkably deep ionization potentials (>=6.5 eV), leading to large barriers for
hole injection, as well as substantial non-radiative decay at the interface
between the emitter and hole-injector. We find that an effective way to reduce
these non-radiative losses is by using poly(triarylamine) interlayers. This
results in an increase in the EQE of our blue LEDs emitting at 464 nm
wavelength to 0.3%. We find that our results can be generalized to thicker
sky-blue-emitting NPls, where we increase the EQE to 0.55% using the
poly(triarylamine) interlayer. Our work also identifies the key challenges for
further efficiency increases.
