  Polarised light is critical for a wide range of applications, but is usually
generated by filtering unpolarised light, which leads to significant energy
losses and requires additional optics. Herein, the direct emission of
linearly-polarised light is achieved from light-emitting diodes (LEDs) made of
CsPbI3 perovskite nanoplatelet superlattices. Through use of solvents with
different vapour pressures, the self-assembly of perovskite nanoplatelets is
achieved to enable fine control over the orientation (either face-up or
edge-up) and therefore the transition dipole moment. As a result of the
highly-uniform alignment of the nanoplatelets, as well as their strong quantum
and dielectric confinement, large exciton fine-structure splitting is achieved
at the film level, leading to pure-red LEDs exhibiting a high degree of linear
polarisation of 74.4% without any photonic structures. This work unveils the
possibilities of perovskite nanoplatelets as a highly promising source of
linearly-polarised electroluminescence, opening up the development of
next-generation 3D displays and optical communications from this highly
versatile, solution-processable system.
