Nanosecond long excited state lifetimes observed in hafnium nitride

Increasing photovoltaic conversion efficiency by overcoming the fundamental loss mechanisms due to hot carrier thermalisation remains one of the challenges in solar energy research. A large proportion of dynamic energy is lost due to thermalisation where hot carriers lose their energy to lattice vibrations within picoseconds. Extending hot carrier lifetimes will potentially improve the efficiency of photovoltaic devices. In this study, the bulk hafnium nitride film is proposed as a potential hot carrier absorber from its phononic properties. Hafnium nitride thin films were deposited by sputtering on quartz substrates. The films are polycrystalline as determined from X-ray diffraction and from X-ray photoelectron spectroscopy the chemical composition of the films are estimated to be HfN_0.9. The absorption coefficient of the film exhibits strong amplitude below 500 nm and free electron absorption at longer wavelengths. Transient absorption spectroscopy was used to investigate carrier dynamics in the visible and near-infrared spectral regions. For the first time we present evidence of up to nanosecond long lifetimes of photo-excited carriers observed in hafnium nitride. We propose that the long lived excited states are due to restricted phonon decay pathways in hafnium nitride. The long lived excited states in the bulk hafnium nitride film provide a simple route to devices designed to utilise hot carriers efficiently.