Atomically-thin LEDs could pave way for transparent displays
Reporting their findings in a paper titled “Large-area and bright pulsed electroluminescence in monolayer semiconductors” published in Nature Communications, the researchers describe a three atoms thick device using a single metal-semiconductor contact as its source (instead of typically two contacts for hole and electron injection).
By laying a monolayer semiconductor such as MoS2, WS2, MoSe2, or WSe2 on an insulator (50nm thick SiO2) and placing electrodes on the monolayer and underneath the insulator, the researchers simplified device fabrication yet demonstrated efficient bipolar carrier injection and light emission using a simple AC voltage across the two-terminal device. All four monolayer materials yielded devices emitting light at different wavelengths corresponding to their respective electroluminescence.
As the authors explain in their paper, under a bipolar square wave voltage (-6V to +6V), alternating electron and hole populations are injected into the monolayer TMDC from the same source contact, with large tunnelling currents present at the source during the gate-voltage (Vg) transients, allowing for the modulation of both carrier densities.
It is the excess electron and hole populations simultaneously present and their recombination during the AC transient that results in a pulsed light emission. The pulsed electroluminescence was observed at each Vg transition, with a full-width half-maximum of 8ns, and an intensity increasing linearly with frequency.
The 0.7nm thick monolayer transient-EL (t-EL) device as the authors call it, was capable of outputting up to 193μW cm−2 visible in ambient room lighting. The researchers then fabricated the same devices on a transparent quartz substrate, using transparent indium tin oxide (ITO) as the gate and source electrodes and Al2O3 as the gate dielectric. They obtained t-EL devices that were fully transparent in the off-state.
They went further by fabricating a seven-segment display based on monolayer WSe2 with optimized Ni contacts (patterned as a grid), scaling the t-EL device to millimetre dimensions by using large-area monolayer films of WSe2 grown by CVD. Individual elements of the 3×2mm display could be lit sequentially by grounding their corresponding source electrodes.
The researchers anticipate that such atomically-thin devices could pave the way for displays on walls and windows that would remain invisibly transparent when turned off but would suddenly appear bright when turned on.
They also note that because t-EL devices have their light-emitting surface exposed (unlike traditional LEDs), they would allow for the direct integration of plasmonic structures, nano-antennas, and photonic crystals, leading to the fabrication of high-speed devices or the development of electrically pumped 2D lasers.
UC Berkeley – www.berkeley.edu
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