Hot carrier solar cell absorber prerequisites and candidate material systems

The hot carrier cell aims to extract the electrical energy from photo-generated carriers before they thermalise to the band edges. Hence it can potentially achieve a high current and a high voltage and hence very high efficiencies up to 66% for unconcentrated sunlight and 85% under maximum concentration. To slow the rate of carrier thermalisation is very challenging, but modification of the phonon energies and the use of nanostructures are both promising ways to achieve some of the required slowing of carrier cooling. Required absorber material properties have been identified, relating to phononic and electronic properties as well as to practical considerations. Candidate materials evaluated in terms of these prerequisites include large mass anion, transition metal nitrides, Group IV compounds and nanostructures. Promising candidate materials include InN, HfN and SnSi. Initial measurements indicate slowed carrier cooling in III–Vs with large phonon band gaps in multiple quantum wells. It is expected that soon proof of concept of hot carrier devices will pave the way for their development to fully functioning high efficiency solar cells.