Solar Cells

Current Limitations

If the photon has a lower energy than the band-gap then no electron-hole pairs are generated; if a photon has a much greater energy than the band-gap then the excess energy above the band gap has a tendency to dissipate as lattice vibrations (phonons) instead of liberating electron-hole pairs. Thus, an energy mismatch causes unwanted heating of the solar cell material---instead of providing electricity. This is one of the primary impediments to energy conversion in a semiconductor photovoltaic cell. Moreover, in silicon based photovoltaics, there is a need for electrons in the material to already possess the proper momentum state prior to the reception of the sunlight for electron-hole pairs to be generated instead of heating the solar cell. This is a constraint that limits the available photo-current and decreases overall efficiency. Materials like silicon are called indirect band-gap materials. Materials like gallium arsenide, which have no momentum restrictions, are called direct band-gap materials.