LED-like Solar Cell Absorbs, Emits Light

Solar cells theoretically can harvest about 33.5 percent of energy from sunlight, but scientists have yet to reach this efficiency. Now, a solar cell that is designed to be more like an LED, able to emit light as well as absorb it, could achieve efficiencies close to 30 percent.

Since 1961, scientists have known that, under ideal conditions, solar cells at most will absorb and convert 33.5 percent of electrical energy from the incoming sunlight. Yet for five decades, researchers have been unsuccessful in achieving this efficiency: As of 2010, the highest anyone had come was just more than 26 percent.

Eli Yablonovitch and Owen Miller of UC Berkeley worked out the theory for the new solar cell efficiency. The monitor in the picture illustrates the new physics concept where increased light emission yields higher efficiency. (Image: Eli Yablonovitch)

Professor Eli Yablonovitch and colleagues at the University of California, Berkeley, conducted research to understand why such a significant gap remains between the theoretical limit and the limit that researchers have achieved. What they discovered was a relatively simple, if perhaps counterintuitive, answer based on a mathematical connection between absorption and emission of light.

“Fundamentally, it’s because there’s a thermodynamic link between absorption and emission,” said Owen Miller, a graduate student at UC Berkeley and a member of Yablonovitch’s group.

Designing solar cells to emit light, so that photons do not become “lost” within a cell, has the natural effect of increasing the voltage produced.

A high-efficiency solar cell by Alta Devices. (Image: Joe Foster, Alta Devices)

“If you have a solar cell that is a good emitter of light, it also makes it produce a higher voltage,” which would increase the amount of electrical energy that can be harvested from the cell for each unit of sunlight, Miller said.

Photovoltaic manufacturer Alta Devices, co-founded by Yablonovitch, used this concept to create a prototype solar cell made of gallium arsenide. The prototype broke efficiency records, jumping from 26 to 28.3 percent. The company achieved this milestone, in part, by designing the cell to allow light to escape as easily as possible from the cell.

"What we demonstrated is that the better a solar cell is at emitting photons, the higher its voltage and the greater the efficiency it can produce," Yablonovitch said.

The team will present its findings May 11 at the Conference on Lasers and Electro-Optics (CLEO 2012) in San Jose, Calif.

For more information, visit: www.berkeley.edu