Upconversion and downconversion processes for photovoltaics

The main efficiency losses of all single threshold solar cells resulting in energy-conversion efficiencies fundamentally constrained by Shockley–Queisser (S-Q) limits to practical values below 30% arises mainly from the photons that are not absorbed due to energy less than the threshold (so-called sub-bandgap or transmission losses) and due to the energy absorbed in excess of the threshold that is converted to heat (so-called lattice thermalization losses). There are many approaches to address these fundamental losses and to exceed the S-Q limits: concentrating the sunlight, restricting cell acceptance angles so that cells convert light only from a limited directional range, by steering different wavelength bands of sunlight to cells of appropriate bandgap for efficient conversion (multi-juction solar cells), multiple-exciton generation in quantum-confined cells, and last but not least via spectral conversion. Of these, the spectral conversion is the most promising cost-effective approach, which does not involve any major modifications in the solar cell architecture or usage of complicated costly optics. The present chapter gives an overview of the spectral conversion processes (upconverison and downconversion) mainly discussing the experimental results on integration with solar cell and resulting enhancements in device characteristics reported in literature.