While the sun provides enough total energy to power our society, the efficient capture and conversion of solar photons into a more useful form of energy still eludes us. There are two fundamental challenges here: charge separation and transfer. When a photon induces an electrical excitation in a material, great care is required to keep this excitation from degrading into thermal energy. Inorganic (e.g. Si) solar cells have the advantage of being crystalline so that delocalized charges can be separated by electric fields at junctions created by doping. Their application, however, is limited by production costs and an efficiency on the order of 10%. Part of the problem is that silicon has an indirect band gap so that cells must be made thick enough to capture photons. Understanding how band structure and carrier mobility are controlled by material composition will guide the search for new inorganic photo-active materials.
Organic solar cells have great promise for the future, but they also have greater limitations in their current form. While flexible cells or solar paint could significantly reduce costs, the conversion efficiency in organic cells is still very low, reaching only a few percent. The polymers in organic cells are excellent for manufacturing cells, but the molecular disorder in the material leads to charge trapping. Excitations are relatively localized and are difficult to separate on the time scale of their recombination. Research into new materials may be helpful, but an even more basic model for the motion of energy and charge in such a device is essential.
This workshop will include a poster session; a request for posters will be sent to registered participants in advance of the workshop.
Claudia Draxl, Chair
Jeff Neaton (Lawrence Berkeley Laboratory)
Keith Promislow (Michigan State University, Mathematics)