  Thermoelectric materials (TMs) can uniquely convert waste heat into
electricity, which provides a potential solution for the global energy crisis
that is increasingly severe. Bulk Cu2Se, with ionic conductivity of Cu ions,
exhibits a significant enhancement of its thermoelectric figure of merit zT by
a factor of ~3 near its structural transition around 400 K. Here, we show a
systematic study of the electronic structure of Cu2Se and its temperature
evolution using high-resolution angle-resolved photoemission spectroscopy. Upon
heating across the structural transition, the electronic states near the corner
of the Brillouin zone gradually disappear, while the bands near the centre of
Brillouin zone shift abruptly towards high binding energies and develop an
energy gap. Interestingly, the observed band reconstruction well reproduces the
temperature evolution of the Seebeck coefficient of Cu2Se, providing an
electronic origin for the drastic enhancement of the thermoelectric performance
near 400 K. The current results not only bridge among structural phase
transition, electronic structures, and thermoelectric properties in a condensed
matter system, but also provide valuable insights into the search and design of
new generation of thermoelectric materials.
