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Layered Hexagonal Oxycarbides, Mn+1AO2Xn (M = Sc, Y, La, Cr, and Mo; A = Ca; X = C): Unexpected Photovoltaic Ceramics
journal contribution
posted on 2018-06-11, 00:00 authored by Zhenyu Wang, Xin ChenXin Chen, Chunming NiuA family
of layered hexagonal oxycarbides and oxynitrides with
the general formula, Mn+1AO2Xn (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.