LEDs based on transition metal dichalcogenides display reduced efficiency losses

Light-emitting diodes (LEDs), semiconductor-based devices that emit light when an electric current flows through them, are key building blocks of numerous electronic devices. LEDs are used to light up smartphone, computer, and TV displays, as well as light sources for indoor and outdoor environments.

Past studies consistently observed a decline in the performance and efficiency of LED devices based on two-dimensional (2D) materials at high current densities. This loss of efficiency at high current densities has been linked to high levels of interaction between excitons, which cause a process known as exciton-exciton annihilation (EEA).

Essentially, the properties of some 2D materials prompt excitons to strongly interact with each other, causing excitons to “deactivate” one another. This results in a significant waste of energy that could otherwise contribute to the lighting of LEDs.

Researchers at Southeast University, Beijing Institute of Technology and other institutes recently developed new LEDs that exhibit weaker exciton-exciton interactions and could thus attain improved efficiencies. These devices, introduced in a paper published in Nature Electronics,

“Dielectric or strain engineering can be used to reduce exciton–exciton annihilation rates in monolayer transition metal dichalcogenides, but achieving electroluminescence in two-dimensional LEDs without efficiency roll-off is challenging,” wrote Shixuan Wang, Qiang Fu and their colleagues in their paper.

“We describe pulsed LEDs that are based on intercalated transition metal dichalcogenides and offer suppressed exciton–exciton annihilation at high exciton generation rates.”

To fabricate their LEDs, the researchers first fabricated quantum-well-like superlattices using a technique known as one-step oxygen-plasma intercalation. This allowed them to stack a few quasi-monolayers of materials on top of each other, altering their electronic structure and enhancing the photoluminescence of the LEDs.

“We intercalate oxygen plasma into few-layer molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂) to create LEDs with a suppressed efficiency roll-off in both photo-excitation and electro-injection luminescence at all exciton densities up to around 10²⁰ cm⁻² s⁻¹,” wrote Wang, Fu, and their colleagues. “We attribute this suppression to a reduced exciton Bohr radius and exciton diffusion coefficient, as extracted from optical spectroscopy measurements.”

The researchers evaluated their LEDs in a series of tests and found that they outperformed previously designed LEDs based on pristine monolayer 2D materials. Their device displayed weakened exciton-exciton interactions, which reduced efficiency roll-off and boosted their luminescence.

“LEDs based on intercalated MoS₂ and WS₂ operate at maximum external quantum efficiencies of 0.02% and 0.78%, respectively, at a generation rate of around 10²⁰ cm⁻² s⁻¹,” wrote the researchers.

The new design and fabrication strategies employed by Wang, Fu and their colleagues could soon inspire the development of similar LED devices that do not display significant efficiency losses at high current densities. These highly performing LEDs could have various real-world applications, ranging from more efficient lighting sources to optoelectronic devices.

For instance, the LEDs could be used to realize high-speed on-chip data communication in optical interconnects (i.e., components that use light to transmit data between different parts of systems or nodes in a network). In this context, they could help boost the speed at which optical interconnects can transmit data while reducing their power consumption.

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