Hafnium and zirconium nitrides with rock-salt and Th3P4 structures: electronic and phonon band structure calculations to examine hot carrier solar cell and thermoelectric properties

Abstract

Hafnium and zirconium mononitrides (HfN and ZrN) have been of interest for the hot carrier solar cell (HCSC) concept as a bulk absorber material and in thermoelectric power generation as a superlattice structure. Slowed hot carrier (HC) cooling is a fundamental requirement in an HCSC device because the carriers must be collected at high energy levels via energy selective contacts before they cool down to the band edges. However, achieving longer thermalization time is challenging in bulk materials. Here, we have calculated electronic and phonon properties of rock-salt HfN and ZrN and the nitrogen-rich thorium phosphide (Th3P4) structures: c-Hf3N4 and c-Zr3N4 using a first principle density functional theory method. Electronic and phonon band structures of alloyed Hf and Zr in the nitrogen-rich phases are presented for the first time. Our calculations show that the mononitrides of Hf and Zr have wide optical phonon (OP) to acoustic phonon (AP) band gap and simultaneous smaller OP dispersion that have the potential to slow down the HCs cooling rates, making them suitable for an HCSC absorber. Further, the results provide a first step towards exploring both HCSC and thermoelectric properties in metal/semiconductor superlattice structure of nitrides of Hf and Zr only.