An unexpected change in polymer structure opens a new avenue in the search for improved solar cell efficiency

July 25, 2013
Figure 1: A small change in polymer structure changes how the polymer chains stack together in a thin film, resulting in a dramatic improvement in solar cell efficiency. Credit: Itaru Osaka, RIKEN Center for Emergent Matter Science

Solar cells based on organic polymers are of great interest because the materials are both cheaper to make and easier to process than those used in traditional inorganic solar cells. To date, however, the very best power conversion efficiencies for polymer solar cells remain below the threshold for practical application. Itaru Osaka from the Emergent Molecular Function Research Group at the RIKEN Center for Emergent Matter Science and co-workers have now serendipitously discovered that changing the polymer's structure results in a significant enhancement of power conversion efficiency1.

When light energy is absorbed by the in a polymer solar cell, electrons are excited to higher to produce a high-energy electron and a corresponding electron 'hole'. To convert the light energy into electrical current, these electrons and holes have to move through the polymer to the electrodes before they recombine and the energy is lost. Much research has been devoted to understanding how to improve this conversion process.

Osaka and his colleagues had been working with a particular type of containing a repeating naphthodithiophene–naphthobisthiadiazole structure called PNNT-DT. "PNNT-DT has very low solubility," explains Osaka, "so we were interested in attaching additional alkyl side chains to the polymer to improve its processability." As expected, this modification significantly improved the solubility of the polymer, but also significantly and unexpectedly improved the power conversion efficiencies of made with the polymer.

In the solar cells, the polymer is deposited as a thin film, and analysis revealed that these new 'alkylated' polymers were arranged so that the lay flat in stacks on the surface rather than aligned perpendicular to it. This causes the charge carriers—electrons and holes—to move perpendicular to the surface rather than parallel, improving the power conversion efficiency (Fig. 1). "This unexpected change in orientation produced solar cells with an efficiency of up to 8.2% compared with just 5.5% for the unalkylated material," says Osaka.

Ultimately, Osaka and his co-workers hope to exploit this dramatic efficiency enhancement in other polymers in order to move closer to producing polymer solar cells that can truly compete with the 15% or greater efficiency of inorganic solar cells. "We need a greater understanding of why this switch in polymer orientation occurs, and then we need to apply it to other polymers that can absorb a wider range of visible light wavelengths," he says.

Explore further: New polymeric material brings companies one step closer to cheaper plastic solar cells and electronics

More information: Journal of the American Chemical Society 135, 8834–8837 (2013). DOI: 10.1021/ja404064m

Related Stories

The fluorescent future of solar cells

May 9, 2013

( —For some solar cells, the future may be fluorescent. Scientists at Yale have improved the ability of a promising type of solar cell to absorb light and convert it into electrical power by adding a fluorescent ...

Organic polymers show sunny potential

May 29, 2013

( —A new version of solar cells created by laboratories at Rice and Pennsylvania State universities could open the door to research on a new class of solar energy devices.

Recommended for you

Physicists develop new technique to fathom 'smart' materials

November 26, 2015

Physicists from the FOM Foundation and Leiden University have found a way to better understand the properties of manmade 'smart' materials. Their method reveals how stacked layers in such a material work together to bring ...

Mathematicians identify limits to heat flow at the nanoscale

November 24, 2015

How much heat can two bodies exchange without touching? For over a century, scientists have been able to answer this question for virtually any pair of objects in the macroscopic world, from the rate at which a campfire can ...

New sensor sends electronic signal when estrogen is detected

November 24, 2015

Estrogen is a tiny molecule, but it can have big effects on humans and other animals. Estrogen is one of the main hormones that regulates the female reproductive system - it can be monitored to track human fertility and is ...

1 comment

Adjust slider to filter visible comments by rank

Display comments: newest first

not rated yet Aug 26, 2013
Okay now throw an external electric field across the heterojunction. To pull the carriers out.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.