In order to meet the clean energy requirement, materials consisting of abundant, eco-friendly, and low-cost elements are of great interest. Therefore in this study, we discussed the importance of BaSi2 and other similar semiconducting compounds which contain inexpensive and earth abundant elements, for solar cell applications. Employing first-principles modeling within the density function theory, we analyze the structural, electronic and optical properties and find that these compounds have fundamental indirect band gaps and the gap energies are in the region of 0.9–1.3 eV, which is suitable for solar cell applications. Furthermore, a lower energy dispersion of the conduction band (CB), which results in a flat shape of the CB minimum, implies a large optical absorption. In fact, our calculations reveal that the photoabsorption of these compounds is stronger than other common PV materials like Si and Cu(Ga,In)Se2

1. Mukesh KUMAR
National Institute for Materials Science
Tsukuba, Japan
Oct, 2015
Barium disilicide (BaSi2):
a low-cost, earth-abundant material for thin-film solar cells

2. BaSi2
CdTe
CuInSe2
Cu2ZnSnS4
One of the most earth-abundant compound than others PV materials
Why BaSi2

3. 1. First time reported by Nakamura et. al (in NIMS) in 2002 for solar cell app
2. Experiment band gap in the range of 1.1 to 1.32 eV
3. Large absorption coefficient (> 3x104 cm-1 at 1.5 eV)
4. Large quantum efficiency (> 70%)
BUT……….
Even after a decade of 1st report, BaSi2 not yet reach to device level
Many questions are still unanswered
• Why there is a large variability (1.1 to 1.32 eV) in exp. band gap?
• Questions related to optical properties such as
1. why these compounds have large absorption coefficient?
2. how large is optical spectra compared to other PV materials?
• Questions related to defect physics of BaSi2 are still unknown
• No well documented computational results as all previous calculations
were performed within the LDA or GGA frameworks
Facts about BaSi2
App Phys Lett 81, 1032, 200
Jpn J appl Phys 41, 4965, 2002
Thin Solid Films 508, 363, 2006
App Phys Lett 102, 112107, 2013
PSSc 10, 1728, 2013
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4.  Revisiting band structures of BaSi2 with an appropriate band gap correction.
 Effect of band gap correction on photo-absorption coefficients
 Reason for the large absorption coefficient of BaSi2 than other PV materials
 Comparative studies for four compounds BaSi2, SrSi2, BaGe2, or SrGe2
 Defect physics of BaSi2
Objectives
 Vienna Ab Initio Simulation Package (VASP)
 PAW method
 GGA-PBE, HSE06 and quasi-particle scGW0
 Spin-orbit-coupling (SOC) effect was included
Band gap correction
Computational details

5. orthorhombic structure with a space group PnmaD h −16
2 (
 The unit cell have 24 atoms
with 8 Ba atoms, and 16 Si
atoms.
 8 Ba with two inequivalent
positions occupy 4c Wyckoff
sites, while Si with three
inequivalent positions occupy
4c and 8c Wyckoff sites
 Wyckoff sites 4c and 8c have
Cs and C1 point-group
symmetry respectively
BaSi2 : structure analysis
b
c a
(a)
(b)
A2
(c)
Ba)
(d)
CP1
CP2 ST
B1
A1
A1
A1
A1
A2
A2
A2
A2
B1
B1
B1
B2
B2
B2
B2
B3
B3
B3
B3
B3 B3
B3 B3
2
b
c a
B1
B2
B3
B3
(a)
(b)
B1
B2
B3
A2
(c)
 Each Si atom in tetradedra is bonded
covalently.
 Ba as isolated separate unit.
 Ba2Si4 : [(Ba2)]4+ and [Si4]4-
 Zintl phase with both covalent and ionic
bonding.
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6. The VB is mainly composed of
Si -p states whereas, the CB is
composed of localized -d states
of Ba atoms.
VBM
CBM
Flat CB
CBM
VBM
M. Kumar et. al., Appl. Phys. Express,7,071203 (2014)
Electronic band structures and density of states
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7. Eg (bulk sample): 1.15 eV [Intermetallics 18, 548, 2010, APL 81, 1032, 2002, PSSc 10, 1728, 2013]
Eg (thin-film) : 1.30 eV [Thin Solid Films 508, 363, 2006, Jpn J appl Phys 41, 4965, 2002]
Why large variability in exp. Eg (1.1 to 1.3 eV) ?
 Thin-film thickness, important
parameter for indirect gap
compound.
 A strain effect on band gap can
cause Eg variation.
 Presence of secondary phases,
impurities or defects
0 1 2 3 4 5 6 7 8 9 10
0.74
0.76
0.78
0.80
0.82
0.84
0.86
0.88
Eg(eV)
Pressure (GPa)
BaSi2
7

8. -1
0
1
2
-1
0
1
2
-1
0
1
2
-1
0
1
2 Eg
=
0.89
eV
Eg
=
1.02
eV
Eg
=
1.17
eV
(c) SrGe2(c) BaGe2
(b) SrSi2
Energy[eV]
(a) BaSi2
Eg
=
1.25
eV
X Γ Y T Z ΓX Γ Y T Z Γ
Energy[eV]
X Γ Y T Z ΓX Γ Y T Z Γ
M. Kumar et. al., J Appl. Phys. 115 (20) 203718 (2014)
Band-engineering with isostructural compounds
Band alignment(Ba,Sr)(Si,Ge)2 solid solutions
0.89 – 1.25 eV
Indirect band gap semiconductors
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9. The absorption coefficients α(ω) at ħω = 1.5 eV is
Calculated (HSE06) = 2.6 x104 cm−1 Expt α(ω) = 3x104 cm−1
Jpn J Appl. Phys. 50, 068001 (2011).Appl. Phys. Express, 7, 071203 (2014)
ExperimentCalculation
Absorption coefficient α(ω) of BaSi2
HSE06 successfully estimated the optical properties

10. 1. Why BaSi2 has large absorption coefficient
1. How large is the absorption spectra of BaSi2 compared to
others PV materials (c-Si, CdTe, CIGS, CZTS)

11. 80 times larger than c-Si
(Agree with Exp observation
reported by Morita et. al. in
Thin solid Film 508, 363, 2006)
2-3 times larger than CISe, CZTS
Does BaSi2 has better optical properties than other PV materials?
at ħω − Eg = 0.5 eV
α(ω) = 8.2x104 cm−1 (BaSi2)
α(ω) = 3.3x104 cm−1 (Cu2ZnSnS4)
α(ω) = 2.5x104 cm−1 (CuInSe2)
α(ω) = 1.2x104 cm−1 (GaAs)
α(ω) = 0.1x104 cm−1 (Si)
M. Kumar et. al., Appl. Phys. Express,7,071203 (2014)

12. M. Kumar et. al., Appl. Phys. Express,7,071203 (2014)
Relation between electronic and optical properties

13. Our first-principles DFT calculation demonstrate that:
 Binary BaSi2 is composed of fairly inexpensive and earth-abundant
elements.
 Fundamental band gap of 1.25 eV is very suitable for PV applications.
 This compound has very high optical activity than other PV materials.
 The intensity of computed photoabsorption spectrum of BaSi2 exhibits
roughly two to eighty times larger than those of the conventional
absorbers like CIGS, CZTS, CdTe and c-Si.
 This can be explained by the presence of -d like states of cation (Sr,Ba) in
CB region, which are very localized and contribute to forming a flat energy
bands.
 With other isostrucral compounds, one can tune the band gap energies in
the region of Eg ≈ 0.89–1.25 eV.
 Overall, BaSi2 and other related compounds (SrSi2, BaGe2 and SrGe2)
can be regarded as a potential absorber materials for thin-film solar cells.
Summary
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