Hot carrier cells: an example of third generation photovoltaics

Third generation photovoltaic approaches aim to use multiple energy level approaches to circumvent the Schockley- Queisser limit but to still allow use of thin film approaches. Hence they offer significant potential to reduce cost per Watt and move solar cell technologies towards the levels necessary to achieve LCOE values that give grid parity. The Hot Carrier solar cell is a Third Generation device that aims to tackle the carrier thermalisation loss after absorption of above band-gap photons. It is theoretically capable of extremely high efficiencies, 65% under one sun, very close to the maximum thermodynamic limit. However, it relies on slowing the rate of carrier cooling in the absorber from ps to ns. This very tough challenge can perhaps be addressed through nanostructures and modulation of phonon dispersions. The mechanisms of carrier cooling are discussed and methods to interrupt this process investigated to give a list of properties required of an absorber material. Quantum well or nano-well structures and large mass difference compounds with phonon band gaps are discussed in the context of enhancing phonon bottleneck and hence slowing carrier cooling. Materials for these structures are discussed and potential combined structures to maximize phonon bottleneck and slow carrier cooling are suggested.