InP-based quantum-dot-on-chip LEDs: excitation with violet or blue light?

Abstract

The external quantum efficiency of light-emitting diodes (LEDs) emitting in the spectral range from 510 to 600 nm is much smaller than blue- or red-emitting LEDs. The current strategy to overcome this so-called green gap is the color conversion of the emitted light by an efficient violet or blue III-nitride-based LED. Colloidal quantum dots (QDs) are promising candidates thanks to their narrow, size-tunable emission spectrum. Single-color QD-on-chip LEDs consisting of a blue pump LED with either green-, amber- or red-emitting QDs have already been reported. In this paper, the question is investigated whether a shorter excitation wavelength, i.e. violet light, offers an interesting alternative to a blue pump LED. After describing the main determining factors for the overall LED efficiency, the paper focuses on the color conversion efficiency, and whether excitation with violet instead of blue decreases re-absorption losses significantly. The performance of green-emitting InP/Zn(Se,S)/ZnS QD-LEDs with either a violet (408 nm) or blue (445 nm) LED is experimentally compared; resulting in a slightly higher color conversion efficiency for the violet-pumped QD-LEDs. The impact of the photoluminescence quantum yield of the QDs, the pump absorbance, and resulting color saturation, on the color conversion efficiency, was further investigated with ray-tracing simulations of green- and amber-emitting QD-LEDs with a violet or blue pump LED. Our findings emphasize the role of the excitation wavelength in optimizing the performance of QD-on-chip LEDs and serve as an important guideline for deciding which pump LED to use.