CASTELLI, PANDEY, THYGESEN, AND JACOBSEN PHYSICAL REVIEW B 91, 165309 (2015)
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22 The lattice parameters of α and β calculated with the PBEsol
functional 30 are 4.12 and 4.08A° , respectively, in very good
agreement with the experimental values (4.11 °A
for both α 31
and β 32).
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26 TDDFT-ALDA does not include excitonic effects. However, the
exciton binding energy Eexc can be estimated using theWannier-
Mott model,
Eexc = R
μ
m
1
2M
,
where R = 13.606 eV, 1
μ
= 1
μ
∗c
+ 1
μ
∗v
∗
c and μ
, with μ
∗
v the
hole and electron effective masses at the CBM and VBM,
respectively, and M is the macroscopic dielectric constant.
Based on this, we estimate the exciton binding energies for
BaSnO3 and BaTaO2N to be of the order of 0.3 and 0.1 eV,
respectively, and about 0.2 eV for the αβ sequence.
27 The efficiency η is given by
η = 1
ntot
∞
gap
phabs(E)nph(E)dE,
where ntot is the total number of photons from the sun measured
at AM1.5, phabs(E) is the photon absorptivity of the material,
and nph(E) is the number of sun photons as a function of
energy E, in eV. The photon absorptivity depends on the the
absorption coefficient α(E) and on the depth of the material
along the absorption direction L: phabs(E) = 1 − −e
α(E)L. The
absorption coefficient is α(E) = 2Ek(E)
c , where and c are the
Planck constant and the speed of light, respectively, and k
is obtained from the absorption spectrum as k2 = 1
2 (−Re +
Re2 + Im2).
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BaSnO3-BaTaO2N, are available in the ComputationalMaterials
Repository at https://cmr.fysik.dtu.dk/.
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