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Microscopic rake boosts PV cell efficiency

Posted: 18 Aug 2015     Print Version  Bookmark and Share

Keywords:Stanford University  solar cell  PV  silicon  wafer 

Scientists from the Department of Energy's SLAC National Accelerator Laboratory and Stanford University have developed a manufacturing process that has the potential to double the electricity output of inexpensive solar cells by using a microscopic rake when applying light-harvesting polymers.

When commercialised, this improvement could help make polymer solar cells an economically attractive alternative to those made with much more expensive silicon-crystal wafers, the researchers stated.

In experiments, solar cells made with the tiny rake double the efficiency of cells made without it and are 18 per cent better than cells made using a microscopic straightedge blade.

The research was led by Zhenan Bao, a chemical engineering professor at Stanford and a member of the Stanford Institute for Materials and Energy Sciences (SIMES), which is run jointly by SLAC and Stanford. The team reported its results August 12 in Nature Communications.

"The fundamental scientific insights that come out of this work will give manufacturers a rational approach to improving their processes, rather than relying simply on trial and error," Bao said.

FLUENCE rake

A scanning electron microscope image shows the rigid pillar-like bristles of the FLUENCE rake, which is used to apply light-harvesting polymers to a solar cell. The distance between the pillars is 1µm. (Z. Bao et al, Nature Communications)

"We also expect this simple, effective and versatile concept will be broadly applicable to making other polymer devices where properly aligning the molecules is important."

The Problem with Polymers

Although prices for silicon-based solar cells are dropping, it still takes five to 15 years before they produce enough electricity to offset their purchase and installation. Silicon solar cells also require a large amount of energy to manufacture, which partly offsets their value as renewable energy sources.

Polymer-based photovoltaic cells are much cheaper because they're made of inexpensive materials that can be simply painted or printed in place. They are also flexible and require little energy to manufacture. While small, lab-scale samples can convert more than 10 per cent of sunlight into electricity, the large-area coated cells have very low efficiency, typically converting less than five per cent, compared with 20-25 per cent for commercial silicon-based cells.

Polymer cells typically combine two types of polymers: A donor, which converts sunlight into electrons, and an acceptor, which stores the electrons until they can be removed from the cell as usable electricity. But when this mixture is deposited on a cell's conducting surface during manufacturing, the two types tend to separate as they dry into an irregular assortment of large clumps, making it more difficult for the cell to produce and harvest electrons.

The SLAC/Stanford researchers' solution is a manufacturing technique called "fluid-enhanced crystal engineering," or FLUENCE, which was originally developed to improve the electrical conduction of organic semiconductors.

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