Nov
12
New Efficiency Record for Organic Polymer Solar Cells
November 12, 2014 | Leave a Comment
Their findings also open the door to experimentation with different chemical mixtures that comprise the active layers of the cells. The team’ paper has been published in Nature Communications.
Organic Polymer Semi Conducting Solar Cell Improved. Image Credit: North Carolina State University. Click image for the largest view.
Organic polymer solar cells are a delicately controlled mixture of a polymer donor and a fullerene acceptor. The cell is created by adding a solvent to the polymer and fullerene until the mixture becomes a liquid, then spreading the liquid thinly onto a surface. As the solvent evaporates, the thin layer solidifies, with the donor material hardening into tiny, highly ordered “clumps” that are connected by other, disordered donor molecules, and the acceptor weaving around them. Today the most efficient organic solar cells are manufactured using one of only two different fullerenes.
NC State physicist Harald Ade and postdoctoral researcher Wei Ma had previously studied the morphology of solar cells and found that the size scale of the clumps within the donor layer and the aggregation, or interaction between neighboring molecules within the layers, were the main drivers of solar cell efficiency.
Ade, Ma and a team of chemists from the Hong Kong University of Science and Technology led by He Yan show that size scale and aggregation within these devices are strongly temperature dependent. They also show that record efficiencies of up to 10.8 percent, as opposed to the currently published 9.8 percent, are achievable with the substitution of numerous fullerenes.
As a bonus the performance can be achieved in thick film devices, too.
Ade said, “Once we saw how temperature affected the aggregation and morphology of these solar cells, it allowed the chemists more freedom to play with different chemical compositions in the active layer. Yan’s team demonstrated 10% efficiency with 10 different mixtures, and in thicker films. So these solar cells could be compatible with existing methods of mass production, like slot die casting and roll-to-roll processing similar to newspaper printing, rather than the more expensive production methods currently in use that are required for thickness control.”
Ade added, “We hope that these findings will allow others to experiment with different polymer/fullerene blends, further increasing the efficiency of solar cells, decreasing their production costs and leading to a commercially viable alternative source of energy.”
The polymer solar cell research has seen a lot of performance progress over the past few years. Several limitations are holding back its further development. First up is current high-efficiency (>9.0%) cells are based on material combinations with limited donor polymers and just one specific fullerene acceptor. The team’s achievement of higher performance with efficiencies up to 10.8% and fill factors up to 77% thick-film polymer solar cells is a worthwhile achievement.
The new construction is controlled by temperature dependent aggregation behavior of the donor polymers into the “clumps” and its insensitive to the choice of fullerenes. The new “clumping” and design rules yielded three high-efficiency (>10%) donor polymers.
These insights will allow further synthetic material advances and matching of both the polymer and fullerene materials with good prospects for significantly improving performance and increasing design flexibility.
Silicon based cells have a decades long head start, but now polymer based cells are gaining fundamental understanding that may well start closing the performance gap.
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