Scientists at the National Institute of Standards and Technology

(NIST) in the US have recently conducted work to deepen their

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understanding of the complex organic films at the heart of the new

solar technology.

Organic photovoltaics, which rely on organic molecules to capture

sunlight and convert it into electricity, in principle have significant

advantages over traditional rigid silicon cells.

Organic photovoltaics start out as a kind of ink that can be applied

to flexible surfaces to create solar cell modules that can be spread

over large areas as easily as unrolling a carpet.This makes them easier

to adapt to a wide variety of power applications, and considerably

cheaper to make than traditional cells.

But there are still improvements needing to be made with the

technology. Currently even the best organic photovoltaics convert less

than six per cent of light into electricity, and last only a few

thousand hours.

“The industry believes that if these cells can exceed 10 per cent

efficiency and 10,000 hours of life, technology adoption will really

accelerate,” said NIST’s David Germack.

“But to improve them, there is critical need to identify what’s

happening in the material, and at this point, we’re only at the

beginning.”

The NIST team has advanced that understanding with their latest

research, which provides a powerful new measurement strategy for

organic photovoltaics that reveals ways to control how they form.

In the most common class of organic photovoltaics, the ‘ink’ is a

blend of a polymer that absorbs sunlight, enabling it to give up its

electrons, and ball-shaped carbon molecules called fullerenes that

collect electrons.

When the ink is applied to a surface, the blend hardens into a film

that contains a haphazard network of polymers intermixed with fullerene

channels. In conventional devices, the polymer network should ideally

all reach the bottom of the film while the fullerene channels should

ideally all reach the top, so that electricity can flow in the correct

direction out of the device.

However, if barriers of fullerenes form between the polymers and the

bottom edge of the film, the cell’s efficiency will be reduced.

In their work, the team were able to change the structure at the

edges of the film by repulsing fullerenes while attracting the polymer.

This was able to reduce the accumulation of fullerenes at the bottom of

the film, and allowed the electrical current produced by the sun’s rays

more opportunities to travel to the right end of it.

Both of these changes could potentially improve the photovoltaic’s efficiency or lifetime.

“We’ve identified some key parameters needed to optimise what

happens at both edges of the film, which means the industry will have a

strategy to optimise the cell’s overall performance,” Germack said.

“Right now, we’re building on what we’ve learned about the edges to

identify what happens throughout the film. This knowledge is really

important to help industry figure out how organic cells perform and age

so that their life spans will be extended.”

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