  Recent research on twisted bilayer graphene (TBG) uncovered that its
twist-angle-dependent electronic structure leads to a host of unique
properties, such as superconductivity, correlated insulating states, and
magnetism. The flat bands that emerge at low twist angles in TBG result in
sharp features in the electronic density of states (DOS), affecting transport.
Here we show that they lead to superior and tuneable thermoelectric (TE)
performance. Combining an iterative Boltzmann transport equation solver and
electronic structure from an exact continuum model, we calculate thermoelectric
transport properties of TBG at different twist angles, carrier densities, and
temperatures. Our simulations show the room-temperature TE power factor (PF) in
TBG reaches 40 mWm$^{-1}$K$^{-2}$, significantly higher than single-layer
graphene (SLG) and among the highest reported to date. The peak PF is observed
near the magic angle, at a twist angle of $\approx$1.3$^\circ$, and near
complete band filling. We elucidate that its dependence is driven by the band
gap between lowest and second subband, which improves Seebeck but decreases
with twist angle, and Fermi velocity, which dictates conductivity and rises
with twist angle. We observed a further increase in the PF of TBG with
decreasing temperature. The strong TE performance, along with the ability to
fine-tune transport using twist angle, makes TBG an interesting candidate for
future research and applications in energy conversion and thermal sensing and
management.
