Breakthrough found for 3rd-generation solar cells

In what could be considered a breakthrough, researchers from the US Energy Department’s National Renewable Energy Laboratory (NREL) have found materials that can greatly boost the efficiency of solar cells.

 

NREL researchers reported the first solar cell that produces a photocurrent with an external quantum efficiency greater than 100 percent when photoexcited with photons from the high energy region of the solar spectrum.

 

“The newly reported work marks a promising step toward 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,” they said.

 

Such potentially highly efficient cells, coupled with their low cost per unit area, are considered Third (or Next) Generation Solar Cells, NREL said.

 

It said present-day commercial photovoltaic solar cells are based on bulk semiconductors, such as silicon, cadmium telluride, or copper indium gallium (di)selenide; or on multi-junction tandem cells drawn from the third and fifth (and also in some cases fourth) columns of the Periodic Table of Elements.

 

These cells are referred to as First- or Second-Generation Solar Cells.

 

So far, NREL noted there has been no solar cell that has an external photocurrent quantum efficiency above 100 percent at any wavelength in the solar spectrum.

 

NREL researchers achieved the 114-percent 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.

 

A paper on the breakthrough appeared in the Dec. 16 issue of Science Magazine, titled “Peak External Photocurrent Quantum Efficiency Exceeding 100 percent via MEG in a Quantum Dot Solar Cell.”

 

Its co-authors include NREL scientists Octavi E. Semonin, Joseph M. Luther, Sukgeun Choi, Hsiang-Yu Chen, Jianbo Gao, Arthur Nozik and Matthew Beard.

 

Semonin and Nozik are also affiliated with the University of Colorado at Boulder.

 

Manufacturing process

 

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

 

Nozik had predicted as early as 2001 that MEG would be more efficient in semiconductor quantum dots than in bulk semiconductors.

 

Quantum dots are tiny crystals of semiconductor, with sizes in the nanometer (nm) range of 1-20 nm, where 1 nm equals one-billionth of a meter. At this small size, semiconductors exhibit dramatic effects because of quantum physics, such as:

 

“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,” NREL said.

 

In a 2006 publication, NREL scientists Mark Hanna and Nozik showed that ideal MEG in solar cells based on quantum dots could increase the thermodynamic power conversion efficiency of solar cells by about 35 percent compared to present cells.

 

Also, the fabrication of Quantum Dot Solar Cells is also amenable to inexpensive, high-throughput roll-to-roll manufacturing.

 

Hot electrons

 

A separate article on tech site CNET said a separate team from the University of Texas in Austin developed a plastic semiconductor material that can double the number of electrons produced from a photon on light.

 

It quoted chemist Xiaoyang Zhu at The University of Texas at Austin as saying capturing “hot electrons” normally lost as waste heat promises solar cells that work at 44 percent efficiency, far beyond today’s theoretical limit of 31 percent.

 

But like their NREL counterparts, this group of researchers is working to take advantage of MEG, where a photon striking a solar cell can generate more than one electron.

 

The University of Texas team studied the conditions for the MEG multiple electron process in pentacene, a plastic semiconductor which could lead to cheap solar cells, said Zhu.

 

“Combined with the vast capabilities for molecular design and synthesis, our discovery opens the door to an exciting new approach for solar energy conversion, leading to much higher efficiencies,” he said in a statement. — TJD, GMA News