Nanocrystal process promises more efficient white LEDs

Nanocrystal process promises more efficient white LEDs

Technology News |
By Rich Pell

Key to the improvement in color conversion efficiency (CCE) is the reliance on an efficient non-radiative resonant energy transfer instead of the radiative pumping that would typically prevail when combining a blue InGaN/GaN LED with down-conversion materials, such as phosphors or even semiconductor nanocrystals (NCs), they write.

Non-radiative resonant energy transfer (NRET) relies on a strong exciton-exciton coupling. Through a carrier dynamics model, they revealed that NRET can avoid energy loss associated with the intermediate light emission and conversion steps and transfer energy non-radiatively and resonantly to NCs with a higher quantum yield.

They fabricated blue NH-LEDs from InGaN/GaN MQW epitaxial wafers grown by metal organic chemical vapour deposition on a c-plane patterned sapphire substrate, the active area for each LED measuring 300×300μm2. An hexagonal lattice of nanoholes, each 300nm in diameter and disposed at a 600nm pitch, was patterned through the active layers using soft UV-curing nanoimprint lithography. The researchers then dropcast a solution of CdSe/ZnS core/shell nanocrystals onto the devices.

The SEM image of the bare (a) and hybrid InGaN/GaN NH-LEDs (b), revealing the morphologies of the device structures with or without nanocrystals. Further magnification in (c) and (d). Figure 1 (e) shows the uncovered hexagonal lattice of nanoholes, while figure 1 (f) exhibits the TEM image of CdSe/ZnS core/shell 5nm nanocrystals closely packed together within a 10nm diameter.

Another effect they observed and analysed when fabricating blue InGaN/GaN nanohole LEDs (NH-LEDs) with CdSe/ZnS core/shell NCs filled into the nanoholes, is the suppression of efficiency droop, when a large injection current density yields an overflow of carriers in the active region, reducing the overall efficiency of the device.

Analyzing carrier concentrations of InGaN/GaN MQWs in bare LEDs as control devices and in their hybrid NH-LEDs, they found that the carrier concentration in the hybrid NH-LEDs could be reduced via NRET, hence suppressing the efficiency droop. Among other findings, they observed that the quantum yield of the nanocrystal emission within the hybrid NH-LEDs, at 44%, was twice that of the hybrid LEDs without the nanohole patterns.

In such hybrid structures, they note that excitons in the NH-MQWs layer have a greater chance to go through a NRET channel, with NRET decay rates consistently 3 to 4 times faster than on bare (not covered with NCs) nanohole MQW LEDs.

For more, see “Improvement of color conversion and efficiency droop in hybrid light-emitting diodes utilizing an efficient non-radiative resonant energy transfer.”

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