Optoelectronic and transport properties of vacancy-ordered double-perovskite halides: A first-principles study

In the search for stable lead-free perovskites, vacancy-ordered double perovskites (VODPs), ${\mathrm{A}}_{2}{\mathrm{BX}}_{6}$, have emerged as a promising class of materials for solar harvesting owing to their nontoxicity, better stability, and unique optoelectronic properties. Recently, this class has been explored for a wide range of applications, such as photovoltaics, photodetectors, photocatalysis, and light-emitting diodes. Here, we present the stability and the key physical attributes of a few selected compounds in a systematic manner using state-of-the-art first-principles calculations. A careful structural and stability analysis via simulation of convex hulls and compositional phase diagrams for different structural prototypes discloses 14 stable compounds and one metastable compound in this class. Electronic structure calculations using hybrid functionals reveals that six compounds possess band gaps in the ideal visible region. These six compounds, namely ${\mathrm{Cs}}_{2}{\mathrm{Sn}\mathrm{I}}_{6}$, ${\mathrm{Cs}}_{2}{\mathrm{Pd}\mathrm{I}}_{6}$, ${\mathrm{Cs}}_{2}{\mathrm{Te}\mathrm{I}}_{6}$, ${\mathrm{Cs}}_{2}{\mathrm{Ti}\mathrm{I}}_{6}$, ${\mathrm{Cs}}_{2}{\mathrm{Pt}\mathrm{I}}_{6}$, and ${\mathrm{Cs}}_{2}{\mathrm{Pd}\mathrm{Br}}_{6}$, show high optical absorption ($\ensuremath{\approx}{10}^{5}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$), giving rise to high spectroscopic limited maximum efficiency (15--23%) in the thin-film thickness range. Close inspection of the transport properties reveals polar optical phonon scattering to be the dominant mechanism limiting overall mobility. Further analysis of the polaron excitations discloses the possibility of large polaron formation at low to moderate defect concentrations. At high defect concentrations, ionized impurity scattering takes over. Such analysis can be extremely useful for choosing the optimal growth conditions for a given material intended to be used for a desired application. Additionally, a few selected compounds show moderate to high electron mobility values ($\ensuremath{\sim}13--63\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{0.2em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.2em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$) at room temperature. Overall, the present study paves an important path to help design VODPs as lead-free potential candidates for future optoelectronic applications.