Hot carrier dynamics in HfN and ZrN measured by transient absorption spectroscopy

• We did transient absorption (TA) measurements on two hot carrier absorber candidates HfN and ZrN.
• We extracted temperature information from the TA data using a dynamics theory developed in this work.
• The proposed theory uses the whole range of the TA data and outputs the instantaneous carrier occupation.
• A carrier temperature of 330 K was maintained for more than 2 ns in HfN.
• A cooler temperature and a shorter cooling time was observed in ZrN.

The hot carrier solar cell is a promising concept for high efficiency photovoltaics. Obtaining the dynamic temperature information for the hot carriers in an absorber is of crucial importance to this concept. In this paper, we present a dynamical hot carrier theory that extracts the carrier temperature and the instantaneous carrier occupation from transient absorption spectra. We have applied this model to two transition metal nitride materials, HfN and ZrN. The analysis showed that for HfN the initial carrier temperature rose to 360 K immediately after a femtosecond pulse excitation and then dropped rapidly to 330 K within 0.13 ps. After this initial process, the carrier temperature still remained relatively elevated at above 320 K for as long as 2 ns. The initial fast process was attributed to the strong carrier–carrier scattering in the material while the slow decay of temperature might be due to unusually weak electron–phonon interactions. A shorter cooling time and overall lower hot carrier temperatures were observed in ZrN.