  Thermoelectric power generation has been recognized as one of the most
important technologies, and high-performance thermoelectric materials have long
been pursued. However, because of the large number of candidate materials, this
quest is extremely challenging, and it has become clear that a firm theoretical
concept from the viewpoint of band-structure engineering is needed. In this
study, we theoretically demonstrate that pnictogen-dichalcogenide layered
compounds, which originally attracted attention as a family of superconductors
and have recently been investigated as thermoelectric materials, can exhibit
very high thermoelectric performance with elemental substitution. In
particular, we clarify a promising guiding principle for materials design and
find that LaOAsSe$_2$, a material that has yet to be synthesized, has a
powerfactor that is six times as large as that of the known compound LaOBiS$_2$
and can exhibit a very large $ZT$ under some plausible assumptions. This large
enhancement of the thermoelectric performance originates from the
quasi-one-dimensional gapped Dirac-like band dispersion, which is realized by
the square-lattice network. Our study offers one ideal limit of the band
structure for thermoelectric materials. Because our target materials have high
controllability of constituent elements and feasibility of carrier doping,
experimental studies along this line are strongly awaited.
