Layered Hexagonal Oxycarbides, M_n+1AO₂X_n (M = Sc, Y, La, Cr, and Mo; A = Ca; X = C): Unexpected Photovoltaic Ceramics

A family of layered hexagonal oxycarbides and oxynitrides with the general formula, M_n+1AO₂X_n (MAOX) is discovered using first-principles DFT calculations, where n = 1–3, M is an early transition metal, A is an alkaline earth metal in Group IIA or a late transition metal in Groups IB and IIB, X is C and/or N. Thermodynamically, the MAOX phases are very stable. Tuning the composition, MAOX can be metals, semimetals, or semiconductors. To date, five 2121 oxycarbide MAOX phases, M₂CaO₂C (M = Sc, Y, La, Cr, and Mo), are found to be semiconductors with band gaps from 0.39 to 1.14 eV. To our strong surprise, they have superior photovoltaic (PV) properties and their theoretical solar cell efficiencies are on par with GaAs. In particular, the efficiency of Cr₂CaO₂C reaches 27.7%, which is above 90% of the Schottky–Queisser (SQ) limit. Furthermore, amazingly, the five MAOX semiconductors possess outstanding strength and machinability, e.g., their Young’s moduli are comparable to ceramics and MAX phases, and Poisson’s ratios higher than MAX and even comparable to metals. MAOX semiconductors are promising multifunctional ceramics. The unique combination of the photovoltaic and mechanical properties will certainly enable the MAOX semiconductors to find broad applications in the photovoltaic industry.