Hybrid nanohole LED design suppresses efficiency droop
Key to the improvement in colour 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.
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.