Routes to high efficiency photovoltaic power conversion

The thermodynamic efficiency limit for solar power conversion sits at 87% yet the majority of photovoltaic devices operate at efficiencies of 20% or less. Multi-junction solar cells presently represent the most accessible technological route to high efficiency and are now operating close to the radiative limit, where optical coupling between sub-cells becomes significant. Alternatively, sequential absorption of photons within a single material can be used to achieve efficiencies comparable to a triple junction solar cell, however the relatively low flux of solar photons places constraints around the acceptable lifetime and mobility of excited carriers. Finally thermal gradients in solar cells have been shown to yield small efficiency enhancements. The only means to achieve high efficiency using thermal gradients is by isolating the carrier population from the lattice as proposed in the hot carrier solar cell.