  Bi2Se3 and Bi2Te3 are layered compounds of technological importance, being
excellent thermoelectric materials as well as topological insulators. We report
density functional theory calculations of the atomic, electronic and
thermoelectric properties of strained bulk and thin film Bi2Se3 and Bi2Te3,
focusing on an appropriate description of van der Waals interactions. The
calculations show that the van der Waals Density functional with Cooper's
exchange can reproduce closely the experimental interlayer distances in
unstrained Bi2Se3 and Bi2Te3. Interestingly, we predict atomic structures that
are in much better agreement with the experimentally determined structure from
Nakajima than that obtained from Wyckoff, especially for Bi2Se3 where the
difference in atomic structures qualitatively changes the electronic band
structure. The band structure obtained using the Nakajima structure, and the
theoretically optimized structure, are in much better agreement with previous
reports of photoemission measurements, than that obtained using the Wyckoff
structure. Fully optimizing atomic structures of bulk and thin film Bi2Se3 and
Bi2Te3 under different in-plane and uniaxial strains, we predict that the
electronic band gap of both the bulk materials and thin films decreases with
tensile in-plane strain and increases with compressive in-plane strain. We also
predict, using the semiclassical Boltzmann approach, that the magnitude of the
n-type Seebeck coefficient of Bi2Te3 can be increased by the compressive
in-plane strain, while that of Bi2Se3 can be increased with tensile in-plane
strain. Further, the in-plane power factor of n-doped Bi2Se3 can be increased
with compressive uniaxial strain, while that of n-doped Bi2Te3 can be increased
by compressive in-plane strain. ...
