Development of photovoltaic materials comprised of non-toxic, abundant elements is an important step toward increasing the economic viability of solar energy to meet growing global energy needs. The quaternary kesterite, Cu2ZnSnS4 (CZTS), is a potential photovoltaic absorber material that has recently gained attention.
We are developing film crystal silicon technology on inexpensive foreign substrates for photovoltaics. Silicon is abundant, non-toxic and highly manufacturable, but the wafer presently accounts for about half the photovoltaic module cost. Our goal is an film silicon alternative with the efficiency of crystal silicon and a cost structure more like amorphous silicon. However, there are many materials science and engineering challenges to be overcome.
Photovoltaic (PV) cells can provide virtually unlimited amounts of energy by effectively converting sunlight into clean electrical power. Thin-film solar cells have attracted significant attention as they provide a viable pathway towards reduced materials and processing costs. Unfortunately, the materials quality and resulting energy conversion efficiencies of such cells is still limiting their rapid large-scale implementation. The low efficiencies are a direct result of the large mismatch between electronic and photonic length scales in these devices; the absorption depth of light in popular PV semiconductors tends to be longer than the electronic (minority carrier) diffusion length in deposited thin-film materials, especially for photon energies close to the bandgap. As a result, charge extraction from optically thick cells is challenging due to carrier recombination in the bulk of the semiconductor. If light absorption could be improved in ultra-thin layers of active material it would lead directly to lower recombination currents, higher open circuit voltages, and higher conversion efficiencies. In this presentation, I will show how the emerging field of Plasmonics may provide new opportunities towards this goal.
The performance of a solar cell is a sensitive function of the microstructure of the component materials. Recombination of photo-excited carriers at defects is one of the main contributors to low efficiency. For example, in polycrystalline thin films, high angle grain boundaries have been shown to greatly reduce the minority carrier diffusion length.
I will present our results on how nanostructures can help in understanding of two types of solar cells: amorphous silicon and CuIn(Ga)Se2, which can lead to the improvement of solar cell processing and device performance.