Reduced Spin-Phonon Coupling and Increased Hole Mobility of CuO by Li doping from First-principles Calculations*

Cupric oxide (CuO) is a promising material with many desirable characteristics as a photocathode in photoelectrochemical cells and owns exotic physical properties due to its similarity to high T_C superconductors. Unfortunately, it suffers from low carrier mobilities that is characterized by an activated polaronic hopping conduction. Therefore, it is believed that CuO, similar to other well-studied transition metal oxides (e.g. Fe₂O₃, BiVO₄) exhibits strong electron-phonon coupling leading to formation of small polarons and low mobility. However, we find that electron-phonon coupling in this system is uncharacteristically low and well-established first principles methods of computing polaron hopping barriers based in Marcus theory, severely underestimate the hopping barrier in the pristine system. We propose instead that limitations in the transport of carriers in this system is dominated by strong spin-phonon coupling due to carriers forming localized spin states which must hop through the magnetic lattice. Furthermore, we investigate Li doped CuO (known from experiment to introduce more carriers and improve carrier mobility) and how Li leads to changes in the magnetic couplings in this system that may enhance the carrier mobility in CuO.