Luminescence and efficiency of an ideal photovoltaic cell with charge carrier multiplication

We discuss the potential of an ideal photovoltaic cell with charge carrier multiplication by impact ionization. Thermodynamics requires that carrier multiplication modifies luminescence as well. Such a cell may be described as a hot-carrier cell where interband equilibrium is inhibited and where thermalization is restricted to the band edges. We show that in the limit of low band-gap energy this cell is equivalent to a thermal absorber coupled to a Carnot engine. In conjunction with a spectral filter, a finite band-gap cell with carrier multiplication can even surpass the work obtainable from a selective thermal absorber. In principle, charge carrier multiplication can be tailored such that a single-gap cell matches a fully selective conversion device. A high-efficiency, thin-film cell with carrier multiplication may be thought of as a single-band-gap cell on the basis of a low-band-gap material, with efficiency close to the value of a fully selective device. Low absorptivity for photons below the threshold for carrier multiplication might be realized with an indirect transition. When the solar irradiance exceeds a certain threshold all photons above gap energy may be used.