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New solar cell achieves 114% efficiency

Posted: 22 Dec 2011     Print Version  Bookmark and Share

Keywords:solar cell  quantum dots  photocurrent  solar photon  research 

Researchers from the US Department of Energy's National Renewable Energy Laboratory (NREL) have created the first solar cell that produces more electrons in photocurrent than solar photons entering cell.

NREL claims that it has reached an external quantum efficiency reached 114 per cent. The external quantum efficiency for photocurrent, usually expressed as a percentage, is the number of electrons flowing per second in the external circuit of a solar cell divided by the number of photons per second of a specific energy (or wavelength) that enter the solar cell.

"None of the solar cells to date exhibit external photocurrent quantum efficiencies above 100 per cent at any wavelength in the solar spectrum," claimed NREL.

"The newly reported work marks a promising step towards developing next-generation solar cells for both solar electricity and solar fuels that will be competitive with, or perhaps less costly than, energy from fossil or nuclear fuels."

Researchers achieved the 114 per cent external quantum efficiency with a layered cell consisting of antireflection-coated glass with a thin layer of a transparent conductor, a nanostructured zinc oxide layer, a quantum dot layer of lead selenide treated with ethanedithol and hydrazine, and a thin layer of gold for the top electrode.

The mechanism for producing a quantum efficiency above 100 per cent with solar photons is based on a process called Multiple Exciton Generation (MEG), whereby a single absorbed photon of appropriately high energy can produce more than one electron-hole pair per absorbed photon.

NREL scientist Arthur J. Nozik first predicted in a 2001 publication that MEG would be more efficient in semiconductor quantum dots than in bulk semiconductors. Quantum dots, by confining charge carriers within their tiny volumes, can harvest excess energy that otherwise would be lost as heat – and therefore greatly increase the efficiency of converting photons into usable free energy.

Quantum dots are tiny crystals of semiconductor, with sizes in the nanometre range of 1-20 nm, where 1 nm equals one-billionth of a metre. At this small size, semiconductors exhibit dramatic effects because of quantum physics, such as formation of correlated electron-hole pairs (called excitons) at room temperature; enhanced coupling of electronic particles (electrons and positive holes) through Coulombic forces; and enhancement of the MEG process.

The new results published in Science by the NREL research team is the first report of MEG manifested as an external photocurrent quantum yield greater than 100 per cent measured in operating quantum dot solar cells at low light intensity; these cells showed significant power conversion efficiencies as high as 4.5 per cent with simulated sunlight. While these solar cells are un-optimised and thus exhibit relatively low power conversion efficiency (which is a product of the photocurrent and photovoltage), the demonstration of MEG in the photocurrent of a solar cell has important implications because it opens new and unexplored approaches to improve solar cell efficiencies.

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