Counterintuitive concept may be the key to harnessing the Sun's energy

Apr 27, 2012 08:59 GMT  ·  By

Experts at the University of California in Berkeley (UCB) say that a counterintuitive concept may be the key to boosting the efficiency of solar cells by a very wide margin. They have recently tested an innovative solar cell design, which also produces light, not only absorbs it.

Usually, engineers create solar cells in such a way that they absorb as much of the light hitting them as possible. The idea here is that more captured light equals more energy, so efforts have been focused on improving this ability.

UCB investigators propose and demonstrate a different approach. They say that solar cells are more effective if they are designed to resemble light-emitting diodes (LED). In other words, they must be able to both absorb and emit light.

In-depth details of how the system works will be presented at the Conference on Lasers and Electro Optics (CLEO: 2012), which will be held in San Jose, California, between May 6-11. With it, it may be possible to exceed the maximum theoretical limit of how many photons can be absorbed by a solar cell.

Back in 1961, a series of studies demonstrated that the absolute maximum efficiency level of a solar cell is around 33.5 percent, meaning that just over a third of photons hitting such a device can be absorbed and transformed into electricity.

But even this efficiency level could not be achieved over the ensuing five decades. “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,” UCB electrical engineering professor Eli Yablonovitch explains.

He was also the principal researcher on the new study. The expert says that the top efficiency level ever reached was achieved in 2010, by a team that created single-junction solar cells capable of absorbing 26 percent of incoming photons.

The 7.5 percent difference between the actual device and the theoretical limit “fundamentally [occurs] because there’s a thermodynamic link between absorption and emission,” UCB graduate student Owen Miller explains.

By developing light-emitting solar cells, photons are not lost in the actual cell. The process naturally leads to an increased voltage inside the device. “If you have a solar cell that is a good emitter of light, it also makes it produce a higher voltage,” Miller explains.

This enables the harvesting of higher amounts of electricity from each solar cell, per sunlight unit.