Axis-Dependent Conduction Polarity: Design Principles and High-Throughput Discovery of Transverse Thermoelectrics

Some semiconductor materials exhibit axis-dependent conduction
polarity (ADCP), where carrier transport is dominated by electrons
along one crystallographic axis and holes along another. This unconventional
transport behavior enables transverse thermoelectricity and other
functionalities that are inaccessible in conventional isotropic/unipolar
semiconductors. However, only a few ADCP materials have been identified,
largely because the underlying electronic design principles have remained
unclear. Here, we establish a quantitative framework that defines
the electronic conditions required for ADCP to emerge and remain robust.
Using a minimal two-band tight-binding model, we clarify that ADCP
requires two key electronic conditions: a sufficiently small band
gap that enables simultaneous electron–hole transport and strong
anisotropy in carrier effective masses that causes their transport
contributions to differ between axes. These parameters define a chemical-potential
window in which axis-resolved Seebeck coefficients take opposite signs,
identifying narrow-gap semiconductors and semimetals with anisotropic
band edges as prime ADCP candidates. Guided by these criteria, we
conduct a first-principles screening of 4282 anisotropic narrow-gap
semiconductors and metals and identify 361 ADCP materials, which are
predominantly found among chalcogenides, pnictides, and tetrel-based
compounds, including 57 potential transverse thermoelectrics. Analysis
of two representative materials, AlReGe and ZrSe3, reveals
that ADCP originates from anisotropic band-edge states derived from
low-dimensional bonding networks, resulting in spatially separated
electron and hole transport on different crystal sublattices. These
results provide chemically intuitive design rules for ADCP materials
and establish a comprehensive data set for accelerating the development
of transverse thermoelectrics and other next-generation electronic
devices.