The Stanford Photonics Research Center

April 7, 2009

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.
 
Plasmonics is an exciting new field of science and technology that aims to exploit the unique optical properties of metallic nanostructures to enable routing and manipulation of light at the nanoscale. Nanometallic objects derive these astounding properties from their ability to support collective electron excitations, known as surface plasmons (SPs). Here, we investigate the possibility to realize broadband absorption enhancements in thin-film solar cells using nanometallic structures. They are applied to a relevant and physically intuitive model system consisting of a 2-dimensional, periodic array of Ag strips on a silicacoated Si film supported by a silica substrate. We illustrate how one can simultaneously take advantage of 1) the high near-fields surrounding the nanostructures close to their surface plasmon resonance frequency and 2) the effective coupling to waveguide modes supported by the thin Si film through an optimization of the array properties. Following this approach, we can attain over 50% enhancements in the short circuit current as compared to a cell without metallic structures.