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23 April 2012

Luminescent ‘LED-type’ solar cell design breaks efficiency record

To produce the maximum amount of energy, solar cells are designed to absorb as much light from the Sun as possible, but researchers from the University of California, Berkeley (UCB) have now demonstrated a counter-intuitive concept: solar cells should be designed to be more like LEDs, able to emit light as well as absorb it.

The findings will be presented in paper CF2J.1 ‘The Opto-Electronics which Broke the Efficiency Record in Solar Cells’ by Eli Yablonovitch and Owen D. Miller at the Conference on Lasers and Electro Optics (CLEO 2012) in San Jose, CA (6-11 May).

“The better a solar cell is at emitting photons, the higher its voltage and the greater the efficiency it can produce,” says principal researcher Yablonovitch, professor of electrical engineering.

The researchers say that since 1961 scientists have known that, under ideal conditions, the absolute limit to the amount of electrical energy that can be harvested from sunlight hitting a typical solar cell is, theoretically, about 33.5% (i.e. 33.5% of the energy from incoming photons can be absorbed and converted into useful electrical energy).

Yet researchers were unable to come close to achieving this efficiency: as of 2010, the highest achieved was just more than 26% (flat-plate, single-junction solar cells, which absorb light above a specific frequency, rather than multi-junction cells with multiple layers and absorption frequencies, which can achieve higher efficiencies).

More recently, Yablonovitch and his colleagues aimed to understand why there has been such a large gap between the theoretical limit and the limit that researchers have been able to achieve. As they worked, a ‘coherent picture emerged’, says graduate student and group member Owen Miller. They came across a relatively simple, yet counterintuitive, solution based on a mathematical connection between absorption and emission of light.

“Fundamentally, it's because there’s a thermodynamic link between absorption and emission,” Miller says. 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 by the solar cell. “If you have a solar cell that is a good emitter of light, it also makes it produce a higher voltage,” which in turn increases the amount of electrical energy that can be harvested from the cell for each unit of sunlight, he adds.

The theory that luminescent emission and voltage go hand in hand is not new, but the idea had never been considered for the design of solar cells before now, Miller continues.

In the past year, Alta Devices of Santa Clara, CA, USA, which was co-founded by Yablonovitch in 2007, used the new concept to create a prototype solar cell made of gallium arsenide (GaAs) that raised the efficiency record from 26% to 28.3%. The firm achieved this milestone partly by designing the cell to allow light to escape as easily as possible - using techniques that include, for example, increasing the reflectivity of the rear mirror, which sends incoming photons back out through the front of the device.

Solar cells produce electricity when photons from the Sun impact the semiconductor material within a cell. The energy from the photons knocks electrons loose from this material, allowing the electrons to flow freely. But the process of knocking electrons free can also generate new photons through luminescence. The idea behind the solar cell design is that these new photons - which do not come directly from the Sun - should be allowed to escape from the cell as easily as possible.

“The first reaction is usually, why does it help [to let these photons escape]?” Miller says. “Don’t you want to keep [the photons] in, where maybe they could create more electrons?” However, mathematically, allowing the new photons to escape increases the voltage that the cell is able to produce.

The work is “a good, useful way” of determining how scientists can improve the performance of solar cells, as well as of finding creative new ways to test and study solar cells, says Leo Schowalter, co-founder & chief technology officer of Crystal IS Inc of Green Island, NY, USA and visiting professor at Rensselaer Polytechnic Institute (RPI), who is chairman of the CLEO committee on LEDs, photovoltaics, and energy-efficient photonics.

Yablonovitch hopes that researchers will be able to use the new technique to achieve efficiencies close to 30% in the coming years. Also, since the work applies to all types of solar cells, the findings have implications throughout the field.

Tags: Luminescent Solar cell

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