Chinese researchers use a hybrid nanocrystal to fill nanocrystals in a nanoporous indium gallium nitride (InGaN)/gallium nitride (GaN) blue LED structure, which is said to significantly increase white LEDs. effectiveness.
Researchers from Nanjing University (NJU) pointed out in the study published in Applied Physics Letters that the key to improving color conversion efficiency (CCE) depends on effective non-radiative resonance energy transfer, rather than A radiation pump that often occurs when combining blue InGaN/GaN LEDs with down-converting materials such as phosphorus or even semiconductor nanocrystals (NC).
Non-radiative resonance energy conversion (NRET) relies on strong exciton-exciton coupling. Through the mode of carrier flow, the researchers found that NRET is immune to the losses caused by the intermediate source radiation and conversion steps, and converts and recombines energy into nanocrystals with higher quantum ratios in a non-radiative and resonant manner.
The researchers used metal organic chemical vapor deposition to grow InGaN/GaN MQW epitaxial wafers on a c-plane patterned sapphire substrate to produce NH-LEDs with a blue nanohole (NH) structure. The effective area of ​​each LED is 300 × 300 μm ^ 2 .
The soft-UV-cured nanoimprint lithography technique was used to pattern the active layer to realize a hexagonal nanopore lattice with a diameter of 300 nm and a pitch of about 600 nm. Next, the researchers applied droplets of the CdSe/ZnS core/shell nanocrystal solution onto the element.
An SEM image of the LED bare crystal (a) and the mixed InGaN/GaN NH-LED (b) shows that the element structure has a morphology of a nanocrystal or a nanocrystal. Further enlarged to (c) and (d). Figure 1 (e) shows a bare hexagonal nanopore lattice, while Figure 1 (f) shows an SEM image of a CdSe/ZnS core/shell 5 nm nanocrystal densely packed at 10 nm diameter.
When the researchers made blue InGaN/GaN nanohole LEDs with CdSe/ZnS core/shell NC fill, they also observed and analyzed another effect that inhibited the reduction in efficiency - when a large amount of current density is injected The carrier flow generated in the active region reduces the overall efficiency of the component.
By analyzing the concentration of InGaN/GaN MQW carriers in the LED bare crystal as a control element and its mixed NH-LED, the researchers found that the carrier concentration in the mixed NH-LED can be reduced by NRET, thereby suppressing the decrease in efficiency. In addition, the researchers also observed a quantum efficiency of 44% for nanocrystal radiation in hybrid NH-LEDs, which is two times higher than hybrid LEDs with a nanoporous pattern. In this hybrid structure, they noticed that the excitons of the NH-MQW layer have a greater chance of passing through the NRET channel, while the NRET decay rate is evenly 3-4 times faster than the nanopore MQWLED die.
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