Unraveling the Transition Dipole Orientation in Lead Halide Perovskite Nanoplatelets and Its Role in QD-LED Light Outcoupling Efficiency

In recent years, quantum dot light-emitting diodes (QD-LEDs) have arisen as ideal candidates to compete with organic light-emitting diodes (OLEDs) in display applications. The outstanding photophysical characteristics of colloidal semiconductor nanocrystals (NC), especially CdSe, InP and lead-halide perovskites (LHP), have led to a rapid surge in their use as active layers in high-efficiency devices, with multiple reports of external quantum efficiencies (EQE) surpassing 20%. To date, the major steps in enhancing device efficiency have come from the development of passivation strategies to reduce NC defects and boost internal quantum efficiency (IQE). As IQE approaches unity, the major bottleneck to device efficiency remains light outcoupling. A well-known intrinsic strategy for improving light extraction in OLEDs is transition dipole moment (TDM) engineering. Accordingly, EQEs approaching 40% have been demonstrated by designing efficient phosphorescent or thermally activated fluorescence (TADF) emitters with preferential orientation of their TDMs parallel to the device substrate.<br/>At the same time, recent studies on few-monolayer-thick colloidal CdSe nanoplatelets (NPLs) have shown highly anisotropic TDM orientation, suggesting that similar strategies might be extended to QD-LEDs. Inspired by these results, we wanted to tackle the following fundamental question: can we develop a model to understand and predict the TDM orientation in semiconductor NC films?<br/>In general, in contrast with organic semiconductor molecules, inorganic materials tend to have highly symmetric crystal lattices, leading to isotropic electronic structure and strong degeneracy at the band-edge. However, nanostructuring offers the possibility of introducing favorable symmetry breaking and anisotropy by controlling the NC shape. In this talk, I will first present a multiscale model of the preferential horizontal TDM orientation in LHP NPLs that highlights three fundamental contributions: (i) anisotropic conduction band Bloch states and large fine exciton splitting, (ii) anisotropic dielectric screening, (iii) ordered assembly in thin film. This model identifies the NPL thickness and the film order parameter as the two fundamental degrees of freedom allowing for continuous tuning of the TDM orientation.<br/>We showed that these criteria can already be satisfied in devices by inducing the formation of 2D ordered assemblies of LHP anisotropic NCs on a hole transport layer with low surface energy. This resulted in a ratio of horizontal dipoles up to 0.75, which allowed our optimized green QD-LEDs to achieve peak EQE up to 24.96%.