Elucidating the performance limits of perovskite nanocrystal light emitting diodes

The unique optical properties of lead halide perovskites have drawn significant attention towards their application in light emitting devices (LEDs) in recent years. While quantum yield, emission wavelength and stability are already in the focus of many research groups, the orientation of the emissive transition dipole moments (TDM) has rarely been investigated. As known from other thin film applications such as organic LEDs, this quantity can severely affect the light outcoupling of the device and thereby limit the external quantum efficiency. In this work, we investigate CsPbBr3 nanoplatelets of variable thickness and determine the orientation of their TDMs from thin film radiation pattern analysis. We then apply optical simulations to elucidate the performance limits of perovskite based blue LEDs in prototypical device architectures. We find that with increasingly beneficial horizontal orientation, the maximum efficiency achievable increases to values close to 30%. However, since the photoluminescence quantum efficiency degrades considerably for decreasing thickness, the overall device efficiency does not significantly improve. Thus, for the currently available material sets we can conclude that while for nanocubes the non-ideal orientation limits device performance, devices with nanoplatelets are limited by non-optimal photoluminescence quantum yields. •Reducing the thickness of CsPbBr3 nanocrystal platelets from 6 to 2 monolayers allows tuning their emission colour from green to blue.•Concomitantly the transition dipole moment is confined towards more in-plane orientation.•This has strong potential for improved light outcoupling from perovskite light-emitting diodes.