シリコンベース新材料を用いた薄膜結晶太陽電池を目指して

(1)

シリコンベース新材料を用いた薄膜結晶太陽電池を目指して

筑波大数理物質科学研究科電子・物理工学専攻

JST-PRESTO

末益崇末益崇

BaSi₂

c=1.158nm

a=0.892nm b=0.680nm

Jan. 25, 2010 日本板硝子工学助成会

(2)

シリコン系

結晶シリコン

単結晶シリコン多結晶シリコン

太陽電池の分類

シリコン系

アモルファス微結晶シリコン

多接合へテロ接合型(HIT) III-V族(GaAs)

材料による分類

化合物系

族( ) CIGS系(CuInSe₂) CdTe

太陽電池

シリサイド系結晶

有機系有機半導体色素増感

動作原理による分類 ^{pn接合型太陽電池}

色素増感太陽電池

殆どの太陽電池は、この型です

厚みによる分類 ^{結晶シリコン太陽電池}

薄膜太陽電池

50-300μm

< 10μm

接合数による分類 ^{単接合型太陽電池}

多接合型太陽電池

市販品の殆どは、この型です

(3)

太陽電池用の半導体材料

(Si

が

95%)

太陽電池の現状

4%

25%

単結晶Si Si リボン

4% 7%

多結晶Si 非晶質Si

60%

_Others

4%

Siの利点 Si

の欠点

・資源が豊富

・成熟した技術

Siの利点

・光吸収係数が小さい⇒厚い太陽電池

Siの大量消費(50μmは必要) 安定確保が困難

Si

の欠点

安定確保が困難

・禁制帯幅

Eg

が小さい

(

理想値

Eg=1.4eV vs Eg

^Si

=1.1eV

将来進むべき方向

資源の豊富な元素で構成される、高効率

&

薄膜太陽電池

(4)

光のエネルギーを吸収するとは？

E₂

エネルギー (光のエネルギー)= (E₂ – E₁)

E₁

(光のエネルギー) < (E₂ – E₁) (光のエネルギー) > (E₂ – E₁)

E₂ (光のエネルギ ) < (E₂ E₁)

光は吸収されない

余分なエネルギーが無駄になる

E

E₂

E₁

(5)

4 5

eV-1 cm-2 s

6500K

太陽光のスペクトル

2 3

ensity 1017 (e

Black-body radiation

AM 0

1

oton flux de

5

m-2 s-1 )

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Pho

Phon energy (eV)

AM1.5 (100mW/cm²)

3 4

017 (eV-1 cm

AM 0 Eg

_Si

=1.1eV

∫

=

5 . 3

)

SC

q F ( E dE J

1 2

oton flux 10

1

θ

cos θ

1

^g

E

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Pho 0

Photon energy(eV)

AM 1

^地表

(6)

太陽電池の出力

4 2

cm )

s

-1

)

₆₄

3 48

sity (mA/ c

10

17

(cm

-2

s

エネルギー変換効率: Egで決まるが太陽電池適す

2 32

rrent den s

x desity 1

1.3−1.6eVが太陽電池に適す

1 16

Photocu r

hoton flu x

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

P h

Enegy gap (eV)

Si

% ) 28

mW/cm (

100 8 . 0 )

mA/cm (

2

photo 2

× × =

≈ J qE

^g

η

(7)

○ 禁制帯幅が太陽電池に相応し

目標：

Si

系薄膜結晶太陽電池

on SiO

₂

Si-based crystalline thin-film solar cells on SiO₂

BaSi

₂の特徴

○ 禁制帯幅が太陽電池に相応しい

Eg: 1.3 eV (BaSi₂)～1.4 eV (Ba_0.5Sr_0.5Si₂) suitable for solar cells

⇒High efficiency solar cell

M it S JJAP45(2006) L390 ^{c=1 158nm}

Nakamura,..,Suemasu, APL81(2002) 1032.

○ 光吸収係数が大きい

Very large optical absorption coefficient α

Morita,…, Suemasu, JJAP45(2006) L390.

(Theory) Imai and Watanabe, TSF515(2007) 8219.

a=0.892nm c=1.158nm

b=0.680nm

Conversion efficiency

y g p p

α～10⁵ cm^-1 at 1.5 eV

More than 100 times larger than crystalline Si

⇒Thin-film solar cell

Morita Suemasu TSF508(2006) 363

○ 資源が豊富

Si & Ba abundant chemical elements

Morita, ..,Suemasu, TSF508 (2006) 363.

(Theory) D. B. Migaset al, PSS(b) 244(2007) 2611.

Si & Ba abundant chemical elements Clarke number: Si(2), Ba(14), Sr(15)

cf. CIGS: Cu(26), In(66), Ga(35), S(16), Se(69)

Ba_0.5Sr_0.5Si₂ Si

○ Si(111)面にエピタキシャル成長可能

○ Si(111)面にピタキシャル成長可能

Epitaxial growth possible on a Si(111) surface

Inomata,..,Suemasu,JJAP43(2004) 4155, L478, L771.

⇒High-quality crystal growth

(8)

他の太陽電池材料との比較：特徴は何か？

Characteristic points of BaSi₂ compared with other materials

BaSi₂系

p ₂ p

BaSi₂系

(9)

研究項目

pn-junction Tandem

Schottky-junction

pn junction Tandem

E_F

1. 不純物ドーピングによる伝導型、キャリア密度制御

Control of electron and hole concentrations by impurity doping

2. 分光感度特性

y p y p g

Photoresponsivity

3. BaSi₂/Siトンネル接合の形成

p y

Formation of heavily doped BaSi₂/Si tunnel junction for an electrical contact

4. 太陽電池動作の実証

y p ₂ j

Demonstration of solar cell

(10)

Growth of BaSi

₂

epitaxial films using a template layer

Inomata,..,Suemasu,Jpn. J. Appl. Phys.43(2004) 4155, L478, L771.

B

BaSi

Reactive deposition epitaxy Tsub: 550℃

MBE growth Tsub: 600℃

Undoped n-BaSi₂

Ba

BaSi₂template (10nm)

Si(111) Si(111)

Si(111)

θ-2θXRD pattern

10³

Si(111) (*) BaSi₂(600)

BaSi₂(400) BaSi₂(200)

](Log Scale)

Pt TEM

10²

Intensity [counts]

Pt

20 30 40 50 60 70 80

10¹

XRD

2θ [deg]

0.2 μm

(11)

Electrical properties of undoped BaSi

₂

film

Zintl phase (A X )

300 250 200 150

Temperature [K]

Ba

Si Zintl phase (A_aX_x)

Si-Si: covalent Ba-Si: ionic

7x10¹⁵ 8x10¹⁵

m

-3

]

300 250 200 150

1200

が置換されす

6x10¹⁵

nsity [c m

₉₀₀

cm

2

/V s]

13 14 15 Baサイトより、Siサイトが置換されやすい Y. Imai et al., Intermetallics 15(2007) 1291.

4x10¹⁵ 5x10¹⁵

ectr on de n

300 600

M obility [

B Al

N P Si

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 3x10¹⁵

El e

0

300

M

_Ga

In

As Rb Y Sb

1 2 3

1000/T [1/K]

Morita,…, Suemasu, Thin Solid Films 508(2006) 363.

Cs Fr

La Ac Ba

(12)

Electrical properties of impurity-doped BaSi

₂

films

Kobayashi,…. Suemasu, Thin Solid Films 515(2007) 8242.

B

Si Ba Sb (V)

In, Al(III)

Kobayashi,…..Suemasu, Appl. Phys. Express1 (2008) 051403.

Si(111) (ρ>1000Ωcm)

BaSi₂(10nm)

Ba

Si(111) (ρ>1000Ωcm)

Impurity –doped BaSi₂

/Vs] 1000

Sb-doped n-BaSi

2

In-doped p-BaSi

₂

(ρ>1000 Ωcm) (ρ>1000 Ωcm)

s] 200

600 800 1000

ility [cm2 /

120 160

ty [cm2 /Vs

200 400

ctron mobi

40 80

ole mobilit

10¹⁵ 10¹⁶ 10¹⁷ 10¹⁸ 10¹⁹ 10²⁰

Elec

Electron concentration [cm^-3]

10¹⁶ 10¹⁷ 10¹⁸

Ho

Hole concentration [cm^-3]

(13)

y p y p g

Photoresponsivity

(14)

Photoresponse properties of BaSi

₂

epitaxial films

Matsumoto,…., Suemasu, Appl. Phys. Express 2(2009) 021101.

hv

Sb-doped n⁺-BaSi₂

1.5 mm Current（applied voltage 1~10 V） hv

E～10 V/cm L～1 mm hv electron

Stripe-shaped electrode

CZ-Si(111) undoped n-BaSi₂

(850nm)

CZ-Si(111) _{h l}

BaSi₂

C S ( ) ( )

Photoresponse spectra of BaSi₂ Photoresponse spectra of Cz-Si hole

band structure

n=3×10¹⁸cm^-3 n=3×10¹⁸cm^-3

0.010 0.015

2%

1%

RT

y (A/W)

1.0V 1.5V 2.0V 2.5V

p p ₂ p p

0.010 0.015

y (A/W)

1 V 3 V CZ-Si, n=3×10¹⁸cm^-3

0.005

otoresponsivity

0.005

toresponsivity

1.0 1.5 2.0 2.5

Pho

Photon energy (eV)

1.0 1.5 2.0 2.5

0.000

Phot

Photon energy (eV)

(15)

Analysis of grain size by Electron Backscatter Diffraction

Matsumoto,..,Suemasu,Jpn. J. Appl. Phys.(2010) in press.

Sample Grain

Electron

Diffraction Pattern

Atomic

Plane Diffraction Electron

ND mapping

Grain size 3~10 μm

TD mapping

100 μm 100 μm

(16)

Formation of (111)-oriented Si layers on SiO

₂

by Al-induced crystallization

O. Nast et al., Appl. Phys. Lett. 73(1998) 3214.

BaSi₂ BaSi₂

sample L N holder

SiO₂

(111)-oriented Si Si(111)

Al(100nm) a-Si(100nm) Al(100nm)

breaking the vacuum to

form a native Al oxide Al

poly-Si L-N₂ p

( )

SiO₂sub.

( )

SiO₂ sub.

Vacuum evaporation RF magnetron sputtering

Annealing at 500^oC

for 10 h in dry N₂ SiO₂sub.

p y

5 mm 5 mm

(17)

Growth of polycrystalline BaSi

₂

films on AIC-Si/SiO

₂

T k d S J C t G th311(2009) 3581

(111) oriented Si Anneal

Tsukada,…..Suemasu, J. Cryst. Growth 311(2009) 3581.

EBSD（ND）

(111)‐oriented Si 500℃, 10 h

SiO₂ SiO₂ SiO₂

a-Si(100 nm)

Al(100 nm) Al

poly-Si

Au/Cr striped electrodes

50μm

Undoped n-BaSi₂ 300 nm( 10¹⁶cm^-3)

1.5 mm

Undoped n-BaSi₂ 300 nm( 10¹⁶cm^-3)

MBE substrates

SiO Sub SiO Sub SiO Sub

(111)-oriented Si (111)-oriented Si (111)-oriented Si

SiO Sub (111)-oriented Si

~300 nm(~10¹⁶cm³)

BaSi₂template ~300 nm(~10¹⁶cm³)

SiO₂ Sub SiO₂ Sub SiO₂ Sub SiO₂ Sub

RDE growth Tsub: 550℃

MBE growth Tsub: 500℃

Evaporation Au/Cr electrodes

(18)

Tsukada,…..Suemasu, Appl. Phys. Express2(2009) 051601.

Photoresponse properties of polycrystalline BaSi

₂

films on SiO

₂

Al striped electrodes hv

1.5 mm

Al striped electrodes

Current (applied voltage 1~5 V)

0.06 8% RT

n-BaSi (300 nm)

0.04 0.05

4% 5 V 6%

vity (A/W)

n-BaSi₂(300 nm)

(111)-oriented Si SiO₂

0.02 0.03

4 V 2 V

3 V 2%

otoresponsiv

1.0 1.5 2.0 2.5

0.00 0.01

2V (AIC-Si ×10)

Pho 1 V

Ph t ( V)

Photon energy (eV)

0.4 μm

(19)

y p y p g

Photoresponsivity

(20)

Band diagrams of BaSi

₂

/Si structure

Suemasu et al., Jpn. J. Appl. Phys. 45(2006) L519 .

χ_BaSi2=3.3eV χ_Si=4.0eV

Vacuum level

ΔE_C=0.7eV Eg_BaSi2=1.3eV Eg_Si=1.1eV

ΔE_V=0.5eV n⁺/p⁺ tunnel junction

n-Si/n-BaSi

₂ ^p ^j

p-Si/p-BaSi

₂

n Si/n BaSi

₂

n-Si B Si

ΔE_C

p Si/p BaSi

₂

p-BaSi

₂

n-BaSi

₂

p Si

n-Si

ΔE

p BaSi

₂

p-Si

ΔE_V

(21)

Formation of n

⁺

-BaSi

₂

/p

⁺

-Si tunnel junction by MBE

B

Ba Si Sb

Sb-doped n⁺-BaSi₂

template d

~1×10²⁰cm^-3

Ba

BaSi₂template BaSi₂template

p-Si(111)

B-doped p⁺-Si(70nm)

~5×10¹⁹cm^-3 4×10¹⁸cm^-3

p-Si(111)

4×10¹⁸cm^-3

p-Si(111)

4×10¹⁸cm^-3

10² 10³

(301)

Si(111) d=0 nm Si(222)

unts)

AFM images of BaSi₂template d=1 nm 2 nm 10 nm

10¹

10³ d=1 nm ₍₆₀₀₎

ntensity (cou

10²

10 d=1 nm

(200) (400)

(600)

XRD In

3 μm

+ Si template

20 30 40 50 60 70

10¹

2θ (deg)

p⁺-Si p-Si(111)

(22)

Dependence of I-V characteristics on template layer thickness

n BaSi /p Si

+ B Si /t l t / ⁺ Si

Undoped n--BaSi₂ Sb-doped n⁺-BaSi₂

240 nm 60 nm

n-BaSi

₂

/p-Si

Sb-doped n⁺-BaSi₂ ~1×10²⁰cm^-3

n⁺-BaSi₂/template/p⁺-Si

1nm 2 nm 10 nm

n⁺-BaSi₂ template

p-Si(111)

template

4×10¹⁸cm^-3

p-Si(111)

template B-doped p⁺-Si(70nm)

d

~5×10¹⁹cm^-3 4×10¹⁸cm^-3

p⁺-Si p-Si(111)

current

10 20

/cm

2

)

10 20

/cm2 ) _d=1nm

d=2nm

d=10nm

d=1nm 4×10 cm

J =20A/cm²at 0 5 V

0 10

ens ity ( A /

n⁺‐BaSi₂/p⁺‐Si

n BaSi /p Si

0 10

density (A/ _{J =20A/cm}²_{at 0.5 V}

-10

C urrent d e

_n‐BaSi₂_/p‐Si

-10

Current d

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

C

-20

Voltage (V)

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

-20

Voltage (V)

(23)

Photoresponsivity of BaSi

₂

layers on tunnel junction

0 10

0 06 0.08 0.10

Reverse 10%

15%

ty (A/W) ^{1 V}_{2 V}

3 V Saito,…..Suemasu, Appl. Phys. Express (2010) in press.

Current flow normal to the sample plane

0 02 0.04 0.06 5%

oresponsivit

4 V n⁺-BaSi₂

hv

1.0 1.5 2.0 2.5 3.0 3.5 4.0

0.02

Photon energy (eV)

Photo

1 0 1 5 2 0 2 5 3 0 3 5 4 0

Undoped BaSi₂(360nm)

-0.02

1.0 1.5 2.0 2.5 3.0 3.5 4.0

ty (A/W)

p-Si(111), 4×10¹⁸cm^-3

n⁺-BaSi₂ p⁺-Si

Tunnel junction

-0.06 -0.04

5%

1 V oresponsivit 3 V

-0.10

-0.08 10%

15%

3 V 4 V

Photo 5 V

Forward

(24)

Wet chemical etching of BaSi

₂

layers on Si

Saito Suemasu Jpn J Appl Phys 48(2009) 106507 Saito,…..Suemasu, Jpn. J. Appl. Phys. 48(2009) 106507.

Au/Cr SiO₂

1mm φ

HCL+H O HF+H O

Si(111) Si(111)

BaSi₂

Si(111) Si(111)

BaSi₂

HCL+H₂O HF+H₂O

HCL

5% 15s 2% 10s 1% 180s 0.5% 120s

HCL

HF

2% 15s

5% 15s 2% 15s 2% 15s

(5，5% 30s) (0.5，0.5% 60s) (0.5，1.5% 60s) (1.5，0.5% 60s)

(25)

Photoresponsivity

特願2007-208729 , US2009/0044862 特願2008-218688 , PCT/WO2009/028560 特願2009-115337,

Schottky-junction

pn-junction Tandem

E_F

(26)

Schottky-barrier diode n-BaSi

₂

/CoSi

₂

sunlight

n

⁺

-BaSi

^ドリフト

~0.5μm

n -BaSi

₂

n-BaSi

₂

(undoped)

CoSi

E_F ^Eg=1.3eV

φ_B～1.6eV

p

⁺

-Si (111) CoSi

₂

CoSi₂/n-BaSi₂/n⁺-BaSi₂

Suemasuet al., J. Cryst. Growth 310(2008) 1250.

Si

Ichikawa,…, Suemasu, Appl. Surf. Sci. 254(2008) 7963.

Epitaxial growth of BaSi

₂

/CoSi

₂

/Si(111) structure

Si(10nm) BaSi

₂

( 22nm) BaSi

₂

( 240nm)

Co Si Ba Ba

Si(111) CoSi

₂

(30nm)

Si(111) CoSi

₂

Si(10nm) BaSi

₂

( 22nm) Si(111)

CoSi

₂

Si(111)

CoSi

₂

(27)

Characterization of n-BaSi

₂

/CoSi

₂

Schottky diode

10000

Si(111)

*

ounts) ^BaSi² ^BaSi²

BaSi₂

(600) BaSi₂

θ-2θ XRD pattern TEM

100 1000

CoSi₂ (222)

ntensity (co ²₍₂₀₀₎ ₍₄₀₀₎

CoSi₂

20 30 40 50 60 70

XRD In 10

5 nm

_Si

20 30 40 50 60 70

2θ (deg)

5 nm

_Si

Si[11-2]

AM1.5, 1Sun

20

A/cm2 )

J_sc=11.3mA/cm² V 0 36 V 10

ent density (A ^V^oc^{=0.36 V}

FF=0.72 V η=2.9%

0.1 0.2 0.3 0.4 0.5

0

Curre

Voltage (V)

(28)

まとめ

○ 不純物ドーピングによる伝導型、キャリア密度制御

○ 分光感度特性

Control of electron and hole concentrations by impurity doping n-type: Sb(10¹⁶Æ1×10²⁰cm^-3), As

p-type: In(10¹⁶ Æ5×10¹⁷cm^-3), Al, Cu

○ BaSi₂/Siトンネル接合の形成

Photoresponsivity

Photocurrent increases sharply for photons greater than 1.25 eV (～Eg).

R～75mA/W at 2.3 eV.

○ BaSi₂/Siトンネル接合の形成

○ W t h i l t hi

n⁺-BaSi₂/p⁺-Si, J =21A/cm²at 0.5 V

○ 太陽電池動作の実証

D t ti f l ll

○ Wet chemical etching

Demonstration of solar cell Schottky-barrier diode

CoSi₂/n-BaSi₂, η～2-3%

(29)

1 h j ti l ll

今後の展開

Future plan

undoped

1. pn homojunction solar cell

表面電極 p⁺-Si p-Si

p-Ba_1-xSr_xSi₂

undoped n-Ba_1-xSr_xSi₂

E_F

反射防止膜 pn接合 n⁺-Ba_1-xSr_xSi₂

E_F

構造が単純 SiO₂基板を利用

SiO 基板

ZnO:Al 反射防止膜

Ba_1-xSr_xSi₂薄膜多結晶(～1μm) Si薄膜多結晶 (～0.1μm)

p 接合トンネル接合

<111>配向・大粒径・

超平坦高品質薄膜結晶

SiO₂基板を利用

効率20%超を目指せる＆多接合への展開

SiO₂基板 ^{超平坦高品質薄膜結晶}

シリコンベース新材料を用いた薄膜結晶太陽電池を目指して