Plastic solar cells with 80% fill factor show promises

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.

Recently, a research 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 percent -- a first. This number is close to that of silicon solar cells. The study was published Aug. 11 by the journal Nature Photonics.

"Our results indicate that the power-conversion efficiency achievable with polymer solar cells may be far beyond the current levels, heralding a bright future for this technology," Marks said. "With our high fill factors, polymers with very good but not champion light absorption still are able to achieve very good efficiency." Marks is 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.

The team showed that the exceptional fill factors arise 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. The fill factor achieved is more than 10 percent 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 percent is still obtained.

Polymer solar cells work differently from that of traditional silicon solar cells. The active layers of polymer solar cells typically contain a mixture of polymer chains that can pass on electrons and "buckyball" molecules that accept electrons. (Buckminsterfullerene, or buckyball, is a spherical fullerene molecule with the formula C60.) 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) that 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, or lower power generated by the cell as against the power of the sunlight received.