Opto-electronic properties of Ta$_3$N$_5$: a joint experimental and theoretical study

Tantalum nitride (Ta$_3$N$_5$) is considered a promising material for use in photoelectrochemical cells, due to its suitable band gap for visible light absorption and favorable band-edge positions for water splitting. However, Ta$_3$N$_5$ films have been recently shown to exhibit low photocurrent (i.e. less than 50\% of the theoretical limit). We report a joint experimental and ab initio theoretical study of the opto-electronic properties of Ta$_3$N$_5$, aimed at understanding possible reasons for the limited photocurrent. Our experimental optical spectra of films with different thicknesses show two absorption edges at 2.1 and 2.5 eV. To provide an interpretation of these features, we performed ab initio calculations, at several levels of theory, of the electronic band structure and optical absorption spectra of Ta$_3$N$_5$. We employed density functional theory with semi-local (PBE/LDA) and hybrid (PBE0/HSE06) functionals and many body perturbation theory at the G$_0$W$_0$ level, and we obtained optical spectra by solving the Bethe-Salpeter equation within density matrix perturbation theory.