Hafnium Oxynitride as a possible luminescent material for an Optical Hot Carrier Solar Cell

The optical hot carrier solar cell (OHCSC) has potential to achieve an efficiency above 60%. Compared to its electrical counterpart, the OHCSC has several advantages, such as the ability to separately optimise the absorber for long hot carrier lifetime, the optical contact for a narrow window with high transmission and the collecting solar cell for high conversion efficiency. Hafnium oxynitride has shown hot carrier lifetimes of several nanoseconds, whilst these are enhanced when the HfON is sandwiched between high acoustic impedance materials in an AIM structure. Long hot carrier lifetimes are essential in a HCSC and also in an OHCSC in order to maintain the energy of carriers above the band gap. Also critical in an OHCSC is that the absorber material has a high radiative efficiency such that this energy is recycled into emitted photons without any energy loss. HfON is a luminescent material with reasonable radiative effiociency due to its bandgap of about 1.8eV. This work presents the design of an OHCSC with a luminescent HfON absorber, coupled to a distributed Bragg reflector as a narrow band pass filter at 1.8-2eV and a high efficiency GaN or perovskite collector cell of appropriate band gap. The collector cell will operate at high power output because the light filtered by the band pass filter is close to monochromatic, the filter will have high transmission in the window and reflect energies either side of the window with high efficiency, the absorber will have a long hot carrier lifetime. Results will be presented that indicate the principle of high efficiencies with such an OHCSC.