Plastic Solar Cells See Bright Future

Plastic Solar Cells See Bright Future

Evanston, Ill---Energy consumption is growing rapidly in the 21th century, with rising energy costs and sustainability issues greatly impacting the quality of human life. Harvesting energy directly from sunlight to generate electricity using photovoltaic technologies is considered to be one of the most promising opportunities to produce electricity in an environmentally benign fashion. Among the various photovoltaic technologies, polymer (plastic) solar cells offer unique attractions and opportunities. These solar cells can be fabricated using roll-to-roll technologies similar to how newspapers are printed. The working principle of polymer solar cells differs greatly from that of traditional silicon solar cells. The active layers of polymer solar cells typically contain a mixture of polymer chains which can donate electrons and “bucky-ball” molecules which accept electrons. Under solar irradiation, electronic excitation generates mobile electron-hole pairs called excitons. The excitons then diffuse through the active layer of the cell, separating at donor-acceptor interfaces into free charge carriers (electrons and holes) which are collected as electrical current when they reach the cell electrodes. 

   In spite of the attractions of polymer solar cells, their large scale application has been limited by the relatively low power conversion efficiency, which is defined as the percentage of the power generated by the cell versus the power of the incident sunlight. The power produced by a solar cell is the product of three cell performance parameters, the open circuit voltage, the short circuit current, and the fill factor. Various strategies are now being developed to increase these parameters to maximize the power conversion efficiency of the cells. While there are reliable approaches to increase the cell open circuit voltage and short circuit current, the realization of high fill factors has proven elusive, with fill factors of most polymer solar cells typically well below 70%, versus 80% for conventional silicon solar cells.

   In polymer solar cells, achieving high fill factors is largely prevented by the recombination (self-annihilation) of the photogenerated electrons and holes before they reach the cell electrodes to be collected as current. In typical polymer solar cells, the randomly distributed donor and acceptor components lead to formation of disordered domains and isolated islands, as well as energy-wasting contacts with the electrodes. Such disordered structures combined with low mobilities of holes and electrons in such materials means that many recombine before they can be swept to the electrodes for collection. Thus, decoding and implementing design principles which produce high fill factors would represent a significant advance in polymer solar cell technology.

    In a paper appearing August 11, 2013 in the journal Nature Photonics1, a team of faculty members and students led by Professor Tobin J. Marks of Northwestern University reports the design and synthesis of new polymer semiconductors and reports the realization of polymer solar cells with fill factors of 80%. The team showed that the exceptional fill factors arise from high levels of order in the mixture of polymer donor chains and bucky ball acceptor components, how these two components are distributed within the cell active layer, and a “face-on” orientation of the polymer chains on the electrode surface. The fill factor achieved is more than 10% greater than previously achieved by the polymer solar cell community, and in the present study, although the polymer semiconductors have non-optimal light absorption characteristics, a near-record power conversion efficiency as high as 8.7% is still obtained. Note that the 80% fill factor is close to that of silicon solar cells. These results indicate that the power conversion efficiency achievable with polymer solar cells may be far beyond the current levels, and herald a bright future for this technology.

Marks Plastic Solar Cells

1 “Polymer solar cells with enhanced fill factors” published online by Nature Photonics on August 11, 2013 (DOI : 10.1038/nphoton.2013.207). In addition to Tobin J. Marks, the authors are 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, Antonio Facchetti, of Northwestern U.; Juan T. López Navarrete, of the University of Malaga, Spain; and Joseph Strzalka, of Argonne National Laboratory.

The work was supported by the Argonne-Northwestern Solar Energy Research Center (ANSER Center), an Energy Frontier Research Center funded by the U.S. Department of Energy, the US Air Force Office of Scientific Research, the Institute for Sustainability and Energy (ISEN) at Northwestern U., and Polyera Corp.

Figure caption: Polymer solar cells with unprecedented fill factors. Northwestern researchers have developed polymer solar cells with fill factors of 80%. The figure shows the close packed donor-acceptor microstructure with horizontal separation and vertical gradation of the electron donor and acceptor regions. This results in low electron-hole recombination, efficient charge sweep-out, and thus, high fill factor and power conversion efficiency.

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