Several developments in polymers and polymer blends are underway across the globe to make solar cells lighter, more flexible, more efficient. Current commercially produced solar panels use silicon cells to efficiently convert sunlight to energy. But silicon panels are too heavy to be used in energy-producing coatings for buildings and cars, or for flexible and portable power supplies for use in remote areas. Polymer cells are better suited to these potential uses.

Paving the way for lighter and more flexible solar devices, UCLA researchers have identified the key principles for developing high-efficiency polymer solar cells. Researchers led by Yang Yang, the Carol and Lawrence E. Tannas Jr. Professor of Engineering at the UCLA Henry Samueli School of Engineering and Applied Science, demonstrated improvement in the architecture and performance of polymer cells. The group successfully blended different pairs of polymers to enable devices to absorb light from a larger part of the solar spectrum. They also identified criteria that could lead to even greater solar cell efficiency and absorption of light as researchers develop new polymers. The research was published in Nature Photonics. Solar cells based on different materials or molecular structures show different potential in terms of efficiency, flexibility and cost-effectiveness, according to Yang, the principal investigator on the research. Silicon cells are the current choice for efficient energy conversion. Perovskite cells, developed over the last few years, show great promise in terms of efficiency, but are still being studied. Polymer cells, which have been studied for many years, have the advantage of being light and inexpensive to manufacture. “As polymer solar cells become more efficient,” said Yang, “they could have a profound impact on our ability to tap the power of the sun.” Over the years, researchers have invented polymers with different molecular structures in an effort to create materials that can absorb light from different parts of the solar spectrum. They also have blended two or more polymers together on one device to further improve absorption. However, blending has not yielded great improvement. In the new study, UCLA researchers demonstrated the problem could be solved by carefully selecting polymers with molecular structures that are compatible with each other.  Using different combinations of polymers and device architectures, they determined which blends improved the solar cells’ efficiency and which were incompatible with each other. Yang “Michael” Yang, the first author of the paper says, “By experimenting with the molecular organization and crystallite size of the polymers, and the architecture of the cell, we found a rule for selecting the proper pair of polymers to enhance efficiency,” he said. “These findings can help chemists design better pairs of polymers and reach even higher efficiency in the future.”

A University of Cincinnati research partnership is reporting advances on how to make solar cells stronger, lighter, more flexible and less expensive when compared with the current silicon or germanium technology on the market. Yan Jin, a UC doctoral student in the materials science and engineering program, Department of Biomedical, Chemical, and Environmental Engineering reported results. A blend of conjugated polymers resulted in structural and electronic changes that increased efficiency three-fold, by incorporating pristine graphene into the active layer of the carbon-based materials. The technique resulted in better charge transport, short-circuit current and a more than 200% improvement in the efficiency of the devices. “We investigated the morphological changes underlying this effect by using small-angle neutron scattering (SANS) studies of the deuterated-P3HT/F8BT with and without graphene,” says Jin. The partnership with the Oak Ridge National Laboratory, U.S. Department of Energy, is exploring how to improve the performance of carbon-based synthetic polymers, with the ultimate goal of making them commercially competitive. 
Unlike the silicon-or germanium-powered solar cells on the market, polymer substances are less expensive and more malleable. “It would be the sort of cell that you could roll up like a sheet, put it in your backpack and take it with you,” explains Vikram Kuppa, Jin’s advisor and a UC assistant professor of chemical engineering and materials science. One of the main challenges involving polymer-semiconductors is that they have significantly lower charge transport coefficients than traditional, inorganic semiconductors, which are used in the current solar technology. Although polymer cells are thinner and lighter than inorganic devices, these films also capture a smaller portion of the incoming light wavelengths and are much less efficient in converting light energy to electricity. “We’re finding that these enhancements resulted from improvements in both charge mobility and morphology,” says Jin. “The morphology is related to the physical structure of the blend in the polymer films and has a strong impact on the performance and the efficiency of the organic photovoltaic (OPV) cells.

A new antireflective coating inspired by the compound lenses in moth eyes could help boost the efficiency of solar cells and sharpen the view of image sensors. Using a simple method to stamp patterned lenses over large areas, researchers in Singapore have come up with a process that could make manufacturing such coatings easier, as per cens.acs.org. Antireflective coatings help solar cells collect as much of the sun’s light as possible, boosting the power output. But typically, these thin-film coatings work best at preventing the reflection of a specific wavelength of light, hitting perpendicular to the surface. They don’t catch light of different wavelengths, coming in at other angles. Layering films of different materials of varying thicknesses yields more absorptive coatings, but that approach is expensive and difficult to do over large areas. Nature provides an alternative design strategy for an affordable, broadband antireflective coating.  Nocturnal moths navigate under the dim light of the moon and stars thanks to eyes made of arrays of microsized lenses called ommatidia, which are further patterned with dome-shaped nanostructures. This hierarchical design reduces reflection and also prevents water from beading up on the creatures’ eyes. But re-creating such a design in the lab, by using moth eyes as tiny stamps or by plasma etching, has proved laborious. To solve this problem, Raut and Mohammad S. M. Saifullah of the Agency for Science, Technology & Research, Singapore, turned to nano imprint lithography, a method for stamping high-resolution, nanoscale patterns over large areas. To create a reusable stamp, the two first made two sets of nickel molds, one patterned with 200-nm-diameter domes and one with micro lenses 2 to 25 μm in diameter. The researchers then used these molds to pattern films of polycarbonate. First they stamped the nano domes and protected that pattern by spinning a thin coating of a sacrificial polymer on top. Afterwards, they stamped the larger micro lenses. Finally, they washed away the sacrificial polymer, leaving a polycarbonate micro lens array. The researchers then tested the moth-inspired arrays, comparing them with micro lens arrays without the nano domes, to see how much light they reflected. From 400 to 1,000 nm in wavelength, the moth-inspired arrays reflected just 4.8% of light, compared with 8.7% reflected by the simple micro lenses. When they varied the incident angle of the light, the nano dome-decorated arrays continued to perform about twice as well. The nano domes also repelled water, which could help keep solar cells clean. The antireflective coatings perform impressively, says Jiang, who is also making bioinspired antireflective coatings. The group now needs to demonstrate that these methods can scale up to make coatings square meters in size.