Researchers at the University of Colorado Boulder have boosted the efficiency of thermal energy harvesting by a factor of 100. The optical rectenna design absorbs light and heat and converts it into electrical power.
"We demonstrate for the first time electrons undergoing resonant tunneling in an energy-harvesting optical rectenna," said Amina Belkadi, a researcher at the Department of Electrical, Computer and Energy Engineering (ECEE). "Until now, it was only a theoretical possibility."
"This innovation makes a significant step toward making rectennas more practical," said Garret Moddel, professor of ECEE at Colorado. "Right now, the efficiency is really low, but it's going to increase."
"You need this device to have very low resistance, but it also needs to be really responsive to light," said Belkadi. "Anything you do to make the device better in one way would make the other worse."
In a traditional rectenna, electrons must pass through an insulator in order to generate power. These insulators add a lot of resistance to the devices, reducing the amount of electricity that engineers can get out. Instead, the design adds two insulators to their devices to create a quantum well. With the right voltages, this enables quantum tunnelling so that the electrons pass through with no resistance, allowing the energy to be captured.
"If you choose your materials right and get them at the right thickness, then it creates this sort of energy level where electrons see no resistance," said Belkadi. The team created an array of 250,000 rectennas using Ni/NiO/Al2O3/Cr/Au metal-double-insulator-metal (MI2M) structures that were able to capture 1 percent of the heat. These are compatible with standard CMOS process technology to allow scaling of the array and with other materials the efficiency will scale.
"If we use different materials or change our insulators, then we may be able to make that well deeper," she said. "The deeper the well is, the more electrons can pass all the way through."
"If you can capture heat radiating into deep space, then you can get power anytime, anywhere," said Moddel.
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