Megan Fellman, reporting for Northwestern University in Evanston, Illinois, explains a possible breakthrough in obtaining power conversion efficiency for polymer (plastic) solar cells close to those for more expensive silicon cells.
Fellman lists the benefits of the plastic cells: “Among the various photovoltaic technologies, polymer (plastic) solar cells offer unique attractions and opportunities. These solar cells contain Earth-abundant and environmentally benign materials, can be made flexible and lightweight, and can be fabricated using roll-to-roll technologies similar to how newspapers are printed. But the challenge has been improving the cells’ power-conversion efficiency.”
Faculty members and students led by Professor Tobin J. Marks designed and synthesized new polymer semiconductors, “and reports the realization of polymer solar cells with fill factors of 80 percent – a first. This number is close to that of silicon solar cells.”
Graph of cell output current (red line) and power (blue line) as function of voltage. Also shown are the cell short-circuit current (Isc) and open-circuit voltage (Voc) points, as well as the maximum power point (Vmp, Imp). Click on the graph to see how the curve changes for a cell with low FF. (From PV Education.com
“Fill factor” is a measure of the ratio of the maximum power from the solar cell to the product of Voc (open-circuit voltage) and Isc (short-circuit current). The link to PV Education.org helps explain the importance of this factor.
Marks, the Vladimir N. Ipatieff Research Professor of Chemistry in the Weinberg College of Arts and Sciences and Professor of Materials Science and Engineering in the McCormick School of Engineering and Applied Science, was part of a research team that published their findings in the August 11 edition of the journal Nature Photonics.
The team achieved a fill factor more than 10 percent greater than those for previous polymer solar cells, mainly “from high levels of order in the mixture of polymer donor chains and buckyball acceptor components, the way these two components are distributed within the cell active layer, and a ‘face-on’ orientation of the polymer chains on the electrode surface.”
They managed to garner a record power conversion efficiency for such cells of 8.7 percent, and feel that can be easily increased to 10 percent through bandgap modification. Part of this was from ordering the components in the chemical mix of the cells. Much of the efficiency loss in polymer cells comes from the randomness of the “mix,” which causes “excitons” and matching “holes” generated by the energy of the sun to cancel each other. What would have been free-charge carriers are which would have been collected as electric current at the cell electrodes are negated by an internal self-destruction of the current carriers.
Improvements reached by Northwestern’s team come partly from the neatly-structured materials. “The enhanced performance is attributed to highly ordered, closely packed and properly oriented active-layer microstructures with optimal horizontal phase separation and vertical phase gradation.”
Despite the ability to make polymer solar cells on a large scale, their low efficiency has kept them from wider adaptation. Certainly thin-film cells made at low cost could blanket a vehicle in a power-generating sheath, making them attractive as future range extenders for electric aircraft.
The Argonne-Northwestern Solar Energy Research Center (ANSER Center), an Energy Frontier Research Center funded by the U.S. Department of Energy, the U.S. Air Force Office of Scientific Research, the Institute for Sustainability and Energy (ISEN) at Northwestern University, and Polyera Corp. supported the research.
Marks’ team included Xugang Guo, Nanjia Zhou, Sylvia J. Lou, Jeremy Smith, Daniel B. Tice, Jonathan W. Hennek, Rocío Ponce Ortiz, Shuyou Li, Lin X. Chen, Robert P. H. Chang and Antonio Facchetti, of Northwestern; Juan T. López Navarrete, of the University of Malaga, Spain; and Joseph Strzalka, of Argonne National Laboratory.