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

BaSi2

CdTeCuInSe2

Cu2ZnSnS4

One of the most earth-abundant compound than others PV materials

Why BaSi2

1. First time reported by Nakamura et. al (in NIMS) in 2002 for solar cell app2. Experiment band gap in the range of 1.1 to 1.32 eV3. 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, 200Jpn J appl Phys 41, 4965, 2002Thin Solid Films 508, 363, 2006 App Phys Lett 102, 112107, 2013PSSc 10, 1728, 2013

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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

orthorhombic structure with a space group PnmaD h −162 (

The unit cell have 24 atomswith 8 Ba atoms, and 16 Siatoms.

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

bc 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

2bc 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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The VB is mainly composed of Si -p states whereas, the CB is composed of localized -d states of Ba atoms.

VBMCBM

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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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 defects0 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 (e

V)

Pressure (GPa)

BaSi2

7

-1

0

1

2

-1

0

1

2

-1

0

1

2

-1

0

1

2

Eg = 0.8

9 eV

Eg = 1.0

2 eV

Eg = 1.1

7 eV

(c) SrGe2(c) BaGe2

(b) SrSi2

Energ

y [eV

]

(a) BaSi2

Eg = 1.2

5 eV

X Γ Y T Z Γ

X Γ Y T Z Γ

Energ

y [eV

]

X Γ Y T Z Γ X 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 solutions0.89 – 1.25 eV

Indirect band gap semiconductors

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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

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)

80 times larger than c-Si(Agree with Exp observationreported 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)

M. Kumar et. al., Appl. Phys. Express,7,071203 (2014)

Relation between electronic and optical properties

Our first-principles DFT calculation demonstrate that:

Binary BaSi2 is composed of fairly inexpensive and earth-abundantelements.

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 conventionalabsorbers like CIGS, CZTS, CdTe and c-Si.

This can be explained by the presence of -d like states of cation (Sr,Ba) inCB region, which are very localized and contribute to forming a flat energybands.

With other isostrucral compounds, one can tune the band gap energies inthe 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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