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2010 ANNUAL REPORT
Photon Sciences Directorate at Brookhaven National Laboratory
Science Highlights

A Straightforward Solution for Increasing Solar Cell Performance

An exciting development in the field of solar cells has been the discovery of organic semiconductors, which can, in principle, lower the manufacturing costs of large-area solar devices. But solar cells made from semiconducting organic materials, based on carbon, have their own drawbacks. Although this class of solar cells may provide a more cost-effective manufacturing route, they also are less efficient.

Solar cells made from organic plastic materials may deform a little bit when put out in the hot sun, and their electrical properties may change as they move. For solar cells made out of such materials to really work, they will need to withstand significant changes in conditions.

Researchers from Brookhaven Lab report that one way to increase the stability of organic solar cells is "locking" in place the semiconducting base layer, the first layer upon which the solar cell is built. To do this, a chemical crosslinker, which interconnects the polymer chains in the material's base layer, was added to the solution-based starting materials. This high degree of interconnection immobilizes the polymer structure and helps preserve its properties during changes in temperature, for example.

The crosslinker mechanically stabilizes the polymer and increases its conductivity by as much as five times. The efficiency of model solar cells made from the crosslinked polymer also increased, up to three fold. The solution-based process is cost-effective and allows for large-scale uses, such as in spray-on or roll-to-roll manufacturing methods.

Using x-ray diffraction at NSLS beamline X6B, the group investigated whether structural changes in the polymer film induced by crosslinking were related to the observed improvements in conductivity and device efficiency. The measurements showed that the polymer chains orient in such a way that electronic charge follows a more direct pathway through the film.

The new technique provides a way to create a stable polymer foundation upon which additional semiconducting materials can be layered - a necessary component of realizing more complex solar cell designs.

I.R. Gearba, C-Y Nam, R. Pindak, C.T. Black, "Thermal Crosslinking of Organic Semiconducting Polythiophene Improves Transverse Hole Conductivity," Appl. Phys. Lett., 95, 173307 (2009).

Left: Ioana Gearba (right), a former researcher at the CFN, and Ron Pindak, Physical and Chemical Sciences Division Head at NSLS, display the enhanced polythiophene-blended solar cells.

Right: Properties of polythiophene polymer after crosslinking with radical initiator ditert butyl peroxide. The intensity of light absorption (blue circles) is unchanged with up to 70 radicals per alkyl chain of crosslinker. Conductivity (red circles) increases, up to five times, as increasing concentrations of crosslinker are added. The addition of 70 radicals per alkyl chain of crosslinker immobilizes the polymer, preserves its absorption properties, and increases conductivity by three times (shown by solid line).

Figure 1 Figure 2