Tiny and efficient, light-emitting diodes (LEDs) are supposed to be the bright future of illumination.
But they perform best at only low power, enough for a flashlight or the screen of your cellphone. If you increase the current enough for them to light a room like an old-fashioned incandescent bulb, their vaunted efficiency nosedives. It’s called LED droop, and it’s a real drag on the industry. Now, researchers have found a way to grow more efficient LEDs that get more kick from the same amount of current—especially in the hard-to-manufacture green and blue parts of the spectrum.
LEDs look like a sandwich, with layers of semiconductors slapped between metal electrodes. When a voltage is applied between the electrodes, negatively charged electrons and positively charged “holes” are pushed into the middle of the sandwich and combine, giving off light.
It sounds straightforward. But there’s often a problem. In blue and green LEDs, the semiconductor layer at the center is typically made from gallium nitride. As the current passing through the LED increases, the diode’s gallium nitride layer generates its own electric field. This second field pushes apart positive and negative charges rushing in from the electrodes and prevents them from combining and giving off light. The larger the current grows, the bigger the second field gets, and the harder it is for electrons and the holes to combine. The efficiency goes down. This droop quickly becomes so bad that LEDs lose their cost advantage over the far less efficient but cheaper-to-manufacture fluorescent and regular incandescent bulbs.