Findings at NIST and NRL reveal how to test a candidate organic photovoltaic material's viability at converting light to electricity both quickly and directly

Findings at NIST and NRL reveal how to test a candidate organic photovoltaic material's viability at converting light to electricity both quickly and directly. (Credit: Cappello/Georgia Tech, Courtesy National Renewable Energy Lab)

Scientists at the National Institute of Standards and Technology (NIST; Gaithersburg, MD) and the Naval Research Laboratory (NRL; Washington, DC) have developed a quick way to test potential new organic photovoltaic materials without having to build a whole solar cell first.1 The team shows that it is possible to test a candidate material quickly and directly using off-the-shelf ultrafast laser technology, bypassing the costly, time-consuming step of constructing a prototype solar cell for each different material to be evaluated.

"We'd like to give companies and manufacturers an alternative to trial and error," says NIST research chemist Ted Heilweil. "It takes a long time to develop photovoltaic materials for market. Screening them using our method would be much faster."

At this point, organics are far less efficient at converting sunlight to electricity than traditional silicon-based technology, but ideas for better materials come at a fast clip. Unfortunately, sifting through these candidates and zeroing in on the most promising ones is expensive and arduous. Because it entails building a prototype cell for each prospective material, relatively few candidates get tested.

Terahertz photoconductivity spike

The team's new method sidesteps this problem by using ultrafast lasers to probe a candidate material's abilities directly and without electrical contacts. They found that when shining pulses of visible light onto a sample to mimic the sun, they could probe the sample's electronic behavior with a second laser pulse in the terahertz region. When the sample absorbs the terahertz radiation, a picoseond-scale spike in terahertz photoconductivity results. The details of the spike are dependent on the material's viability at converting light to electricity.

To test their method, the team looked at a number of mixed organic molecules and polymers whose abilities were well-understood from conventional prototyping.

"We looked at small organics and polymers that people in the solar industry have been using as benchmarks, and we saw the same relative behavior with our terahertz measurements," Heilweil says. "We're pretty confident that our method can tell you what is useful to know."

The team is using the method as part of its own ongoing materials search, Heilweil says.

Source: http://www.nist.gov/pml/div682/201507_organic_solar.cfm

REFERENCE:

1. P. A. Lane et al., Nature Communications (2015); doi: 10.1038/ncomms8558