The decline in efficiency is the main reason for hindering the GaN-based LEDs in the important emerging applications of high current density. But RPI researchers say this shortcoming can be overcome by using a polarity-matched epitaxial structure.
LED manufacturers are currently focusing on new markets such as automotive headlights, large screen displays and general lighting. To some extent, lowering prices can help LEDs break into these areas, but that's not enough. LED chips also need to achieve high efficiency even at large drive currents. This also means that there is a need to address the widespread concern of power reduction.
By comparing the output of LEDs with electrical and optical excitations, researchers at RPI can help researchers investigate the causes of power reduction in GaN-based devices.
In particular, the power drop refers to the phenomenon that the efficiency of blue, green, and white LEDs decreases as the current increases. GaN-based LEDs typically peak at efficiencies only at current densities of 10 A cm-2, with efficiency dropping to half of the peak at current densities of 100 A cm-2. This has caused everyone's attention, and today's high-brightness chips need to work efficiently at current densities much larger than 10A cm-2.
It is obviously very important to overcome this problem, and it has attracted the interest of a large number of researchers in the global industry and academia, including our research team at Rensselaer Polytechnic Institute in Troy, New York. We have been in the past few years. The roots of the problem of declining efficiency have been explored. We believe we can find the crux and overcome this problem with a completely different LED design.
We worked with the Mary Crawford research team at the Sandia National Laboratory in New Mexico to study the causes of the power decline problem. We focus on the effect of dislocation density on LED efficiency and find that dislocations reduce efficiency at low current densities, but do not affect efficiency degradation at high current densities.
At low current densities, carriers are typically lost in a trap-assisted process (ie, SRH recombination) and become more severe as dislocation density increases. By increasing the spontaneous emission, increasing the current density will initially increase the efficiency, but as the current increases further, another carrier-loss mechanism that counteracts it will cause a decrease in efficiency.
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