Solar Cells

Current Practice

All photovoltaic systems require a light-induced source of electrical carriers and an intrinsic electrical asymmetry to force those carriers into an external circuit before being lost to internal de-excitation effects. In semiconductor based photovoltaics the light-induced carriers are negatively charged electrons and positively charged holes and the intrinsic asymmetry is a built-in potential of a diode. The built in potential of the pn-diode both separates the electrons and holes and sweeps them into the external circuit as usable electricity. In such a photovoltaic system, input photons (sunlight) must overcome a temperature dependent band-gap (energy threshold). The current state of affairs for solar cells is that there exists a complete dependence on materials with an intrinsic band-gap and which are also compatible with photovoltaics. These materials are typically hard to find, are often rare and expensive, and become less efficient at energy conversion as their temperature increases---a big disadvantage when using concentrated sunlight, which tends to make the solar cell hot. This has significantly narrowed the potential candidate materials for solar cells and created a need for new directions in solar cell technology.