Defect Imide Double Antiperovskites <i>AE</i> ₅ As <i>Pn</i> (NH) ₂ ( <i>AE</i> =Ca, Sr; <i>Pn</i> =Sb, Bi) as Potential Solar Cell Absorber Materials

An abundance of oxide, halide and chalcogenide perovskites have been explored, demonstrating outstanding properties, while the emerging nitride perovskites are extremely rare due to their challenging synthesis requirements. By inverting the ion type in the perovskite structure, the corresponding antiperovskite structure is obtained. Among them, ternary antiperovskite nitrides X₃AN (X=Ba, Sr, Ca, Mg; A=As, Sb) have recently been identified as exhibiting excellent optoelectronic properties. To explore the unrealized composition space of nitride perovskites, the ammonothermal method was applied, yielding three new layered quaternary imide-based defect-antiperovskites, namely AE₅AsPn(NH)₂ (AE=Ca, Sr; Pn=Sb, Bi). These new compounds feature distorted square-pyramidal coordination around the imide-group (Ca₅NH). Layers with Ca²⁺ vacancies are found with an alternating As^3- and Pn^3- (Pn^3-=Sb^3-, Bi^3-) coordination along the A-site, forming a two-dimensional (2D) structure. All three AE₅AsPn(NH)₂ compounds show suitable direct band gaps within the visible light spectrum. Density functional theory calculations reveal favorable band dispersion, as well as transport and optical properties, especially along the out-of-plane direction, demonstrating their 3D character of electronic transport. The narrow tunable direct band gaps and favorable charge carrier properties make AE₅AsPn(NH)₂ promising candidates for solar cell absorber materials.