Just about every manner of nanoparticle and nanomaterial has been applied to polymer solar cells. Despite all of this work, conversion efficiencies for single p-n junction polymer solar cells are mired at around 9 percent, while cells with more than one p-n junction have mustered efficiencies only as high as 10.6 percent.
All those frustrated efforts made it reasonable to wonder whether nanoparticles would ever provide much of a boost to polymer solar cells.
Now, an X-ray study performed at the Deutsches Elektronen-Synchrotron (DESY) by a team from the Technical University of Munich (TUM) using DESY’s synchrotron radiation source, PETRA III, has demonstrated that magnetic nanoparticles can improve the performance of polymer solar cells—if the mix is right.
In research published in the journal Advanced Energy Materials, the German-based researchers demonstrated that by making sure the solar cell material contains just about one percent of magnetic nanoparticles by weight, they were able to boost the solar cell’s efficiency.
“The X-ray investigation shows that if you mix a large number of nanoparticles into the material used to make the solar cell, you change its structure”, explains coauthor Stephan Roth, who runs DESY’s microfocus small- and wide-angle x-ray scattering beamline at PETRA III, in a press release. “The solar cells we looked at will tolerate magnetic nanoparticle doping levels of up to one percent by mass without changing their structure.”
How to exploit the nanoparticles is where the Germany-based researchers departed from recent research. Solar cell material doped with gold nanoparticles had already been demonstrated to absorb additional sunlight—which, in turn, produced additional electrical charge carriers when the energy was released again by the gold particles.
“The light creates pairs of charge carriers in the solar cell, consisting of a negatively charged electron and a positively charged hole, which is a site where an electron is missing,” explained the main author of the current study, Daniel Moseguí González, in a press release. “The art of making an organic solar cell is to separate this electron-hole pair before they can recombine. If they did, the charge produced would be lost. We were looking for ways of extending the life of the electron-hole pair, which would allow us to separate more of them and direct them to opposite electrodes.”
To extend the life of the electron-hole pair, the researchers exploited the spin of the electrons. The positively charged hole also has a spin. If the two spins are in the same direction, they can add up to a value of one, or cancel each other out, for a value of zero, if they are oriented in opposite directions. Pairs that have an overall spin value of one last longer than those that have an overall spin of zero.
The key was finding a material capable of converting an electron-hole pair’s overall spin state from zero to one. To accomplish this, the researchers needed nanoparticles made from heavy elements, because they can flip the spin of the electron or the hole so that spins are aligned in the same direction.
The material they hit upon was iron oxide magnetite. By adding just the right amount of the magnetite (doping the substrate with 0.6 percent nanoparticles by weight) they were able to increase the energy conversion efficiency by 11 percent, from 3.05 to 3.37 percent.
“The combination of high-performance polymers with nanoparticles holds the promise of further increases in the efficiency of organic solar cells in the future,” said Peter Müller-Buschbaum of TUM in the release. “However, without a detailed examination, such as that using the X-rays emitted by a synchrotron, it would be impossible to gain a fundamental understanding of the underlying processes involved.”
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.