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The problem is what’s called efficiency droop. LEDs function most efficiently at low currents. Turn the current up to levels needed for room lighting, and the efficiency falls off markedly. The lights don’t dim, but as you turn up the amount of electricity, you don’t get more light, so the efficiency goes down, a problem that has made it impossible for LED bulbs to be as cost-effective as incandescent or fluorescent home lighting. Other theories suggest a process called carrier leakage, in which at high currents the carriers start to spill away from areas of high performance, or another process, called carrier delocalization, where electrons simply fail to find a paired hole at high electric currents because they are pulled away from the active region where electrons and holes are supposed to recombine and emit light.
2/20 NY Times