Orientation Controllable 2D Colloidal CsPbI₃ Perovskite Nanoplatelets Towards Spectra Stable Pure Red Light-Emitting Diodes with Polarized Light Emission

Perovskite nanocrystal-based light-emitting diodes (NC PeLEDs) have gained tremendous attention due to their superior optoelectronic properties, which are considered as the leading candidates for the next generation of low-cost and high-performance LEDs for display, lighting and optical communication applications. Although green and near-infrared PeLEDs have achieved external quantum efficiencies (EQEs) higher than 20%, there are still challenges with fabricating spectrally stable and highly efficient pure red and blue PeLEDs (colour coordination (0.708, 0.292) and (0.131, 0.046), respectively). Compositional engineering is one of the common methods to produce red and blue NC PeLEDs, low dimensional perovskites provide an alternative route to achieve spectrally stable devices with desirable color emission coordinates. However, the almost unavoidable drawback of mixing halide NCs is the spectral instability (red-shifted emission) associated with halide migration under the application of an electric field. In contrast, 2D low dimensional colloidal perovskites nanoplatelets (NPls) provide an alternative route to achieve spectrally stable devices with desirable color emission coordinates. Another limitation of the current PeLEDs is that their intrinsic light outcoupling efficiency remains considerably lower than the organic counterpart because it is not yet possible to control the transition-dipole-moment (TDM) orientation in QD solids at the device level. Here, we reported orientation controllable CsPbI₃ NPls red LEDs with a stable pure-red emission from 3-7 V, with an EQE of 2.8% (one of the highest EQE among the strongly confined colloidal PeLEDs). The edge-up (the lateral axis of NPls are vertical to the substrate) and face-down configuration (the lateral axis NPls are parallel to the substrate) are studied with TEM and GIWAXS, and potentially indicate that the improvement of LED performance for face-down configuration is due to an increase in horizontal TDMs at a device level. Furthermore, we demonstrate that the NPls exhibit polarized emission compared with their bulk NC counterparts, which can lead to further optoelectronic and photonic applications associated with polarized light-emitting diodes.