Hot carrier solar cells: hafnium nitride as an absorber material

The hot carrier solar cell (HCSC) is a device concept for photovoltaics that has the potential to achieve high power conversion efficiencies. The HCSC aims to capture the energy of hot carriers that is otherwise lost to the lattice as heat, as in conventional solar cell devices. This relies on the photon absorbing material to maintain hot carriers for sufficiently long time for extraction through energy selective contacts.While HCSCs have been extensively studied theoretically, a practical device has yet to be fabricated. A reason for this is the lack of absorber materials available that exhibit sufficiently slow thermalisation rates. This research aims to solve this problem by finding a suitable absorber material and subsequently develop a working HCSC. Hafnium nitride (HfN) and its phonon properties have lead it to be identified as a potential photon absorbing material for the HCSC. This research was focused on exploring the prospects of using HfN as the absorber.This work involved developing deposition processes for HfN thin films by reactive sputtering such as determining the relative nitrogen and argon gas flow rates. Characterisation of HfN thin films was performed to judge the quality of the material and its potential to be used as a hot carrier absorber. This included X-ray Diffraction to study material quality. It is possible for single crystalline films to be deposited on MgO substrates whereas it is polycrystalline on Si and quartz substrates. Raman spectroscopy and Inelastic X-ray Scattering was performed to study the phonon properties of HfN. The HfN thin films exhibited the large phononic gap between optical and acoustic modes, consistent with predicted values, which influences the hot carrier lifetimes in materials. The carrier dynamics were investigated in a number of HfN thin films by means of ultrafast transient absorption spectroscopy. Observed in these measurements in HfN are hot carrier lifetimes on the order of several hundred picoseconds to nanoseconds.These properties of HfN suggest it to be a good candidate absorber for the HCSC. However there are other technical challenges to be resolved before a practical HCSC utilising HfN can be realised such as light trapping and issues around hot carrier collection