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

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Abstract
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.

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