LED-like solar cell absorbs, emits light

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

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

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 lesser 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.

A high-efficiency solar cell by Alta Devices. Courtesy of Alta Devices.

“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.

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

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. Designing the cell to allow light to escape from it as easily as possible was one reason for the increase.

“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 presented its findings at CLEO:2012, the Conference on Lasers and Electro-Optics, in San Jose, Calif.

Erratum

The article “Full spectrum boosts solar cell power” (GreenLight, May 2012, p. 39) contained a factual error. Harry Atwater of California Institute of Technology and his colleague Albert Polman of the FOM Institute for Atomic and Molecular Physics in Amsterdam did not claim to have achieved solar-cell efficiency of 70 percent. The researchers say that such goals are realistic and potentially achievable, but that the milestone has not yet been achieved.