Hafnium nitride for hot carrier solar cells

Abstract

Hot carrier solar cells is an attractive technology with the potential of reaching high energy conversion efficiencies approaching the thermodynamic limit of infinitely stacked multi-junction solar cells: 65% under one sun and 86% under maximally concentrated. The hot carrier solar cell is conceptually simple consisting of two key components: absorber and energy selective contacts. High efficiencies are achieved by minimising the energy lost to thermalisaton of hot photo-generated carriers while absorbing majority of the solar spectrum. For this to be achieved, energy selective contacts are required to allow the extraction of carriers fast enough at an energy level above the electronic band edge. It is critical for the absorber to be able to maintain a hot carrier population for a sufficiently long time period for the extraction of carriers while they are ‘hot’. Bulk materials with a large gap between acoustic and optical branches in the phonon dispersion are predicted to exhibit slow hot carrier thermalisation rates. Hafnium nitride is such a material with a large gap in its phonon dispersion and is identified as a potential material to be used as a hot carrier absorber. Hafnium nitride has been deposited using reactive sputtering and characterised to investigate material properties and carrier cooling rates.