Hybrid nanohole LED design suppresses efficiency droop

October 12, 2016 // By Julien Happich
Publishing their results in the Applied Physics Letters, a team of Chinese researchers have drastically improved the efficiency of white light LEDs through a hybrid approach, filling nanoholes in a blue InGaN/GaN LED structure with nanocrystals.

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.

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.