Tunable emission directionality in transparent quantum-dot LEDs via photonic interface engineering

Transparent quantum-dot LEDs (Tr-QLEDs) have potential applications in see-through displays. In Tr-QLEDs, light is emitted bidirectionally from the top and bottom surfaces due to the high transparency of the electrodes. However, to maximally utilize the photons, the emission directionality must be tailored so that the photons are directed toward the viewers, which remains a critical challenge. Here, we reveal that the emission directionality, efficiency, and transparency of Tr-QLEDs are closely related to surface reflection. By engineering the surface reflection, tunable emission directionality, ranging from symmetrically bidirectional to highly unidirectional, is demonstrated in Tr-QLEDs. Specifically, by reducing the reflection of both the top and bottom electrodes, Tr-QLEDs with symmetric bottom and top emissions, high averaged transparency (90%), and high external quantum efficiency (EQE, 11%) for each side emission are achieved, making them ideal candidates for see-through, dual-sided displays. In addition, by introducing a bottom electrode with low refractive index and a top electrode with metallic/dielectric/dielectric structure, 90% of the generated photons can be directed to the bottom side, resulting in a highly unidirectional emission with a bottom-to-top luminance ratio of 10:1, a unidirectional EQE of 19.5%, and an averaged transparency of 40.7%, which enables Tr-QLEDs’ applications in see-through, unidirectional displays. This work offers theoretical and practical guidance for developing Tr-QLEDs with tunable emission directionality for various see-through display applications.