シリコンベース新材料を用いた 薄膜結晶太陽電池を...
TRANSCRIPT
2010/1/26
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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=0.892nmb=0.680nm
Jan. 25, 2010 日本板硝子工学助成会
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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
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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
4%60% Others
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 EgSi=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
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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
E1
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4
5
eV-1cm
-2s-1
)
6500K 太陽光のスペクトル
2
3
ensi
ty 1
017 (e
Black-body radiationAM 0
1
oton
flux
de
5
m-2s-1
)
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5Pho
Phon energy (eV)
AM1.5 (100mW/cm2)
3
4
017 (e
V-1cmAM 0 EgSi=1.1eV
∫=5.3
)(SC dEEFqJ
1
2
oton
flux
10
1θ θcos
1gE
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.50Ph
o
Photon energy(eV)AM 1 地表
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太陽電池の出力4
cm2 )
s-1) 64
3 48
sity
(mA
/c
1017
(cm
-2s
エネルギー変換効率: Egで決まる
が太陽電池 適す
2 32
rren
t den
s
x de
sity
1
1.3−1.6eVが太陽電池に適す1 16
Phot
ocur
hoto
n flu
x
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5Ph
Enegy gap (eV)
Si
%28)mW/cm(100
8.0)mA/cm(2
2photo =
××≈ gqEJ
η
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○ 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 SiO2Si-based crystalline thin-film solar cells on SiO2
BaSi2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 (Ba0.5Sr0.5Si2) suitable for solar cells ⇒High efficiency solar cell
M it S JJAP 45 (2006) L390 c=1 158nmNakamura,..,Suemasu, APL 81 (2002) 1032.
○ 光吸収係数が大きいVery large optical absorption coefficient α
Morita,…, Suemasu, JJAP 45 (2006) L390.(Theory) Imai and Watanabe, TSF 515 (2007) 8219.
a=0.892nm
c=1.158nm
b=0.680nm
Conversion efficiencyy g p p
α~ 105 cm-1 at 1.5 eVMore than 100 times larger than crystalline Si⇒Thin-film solar cell
Morita Suemasu TSF 508 (2006) 363
○ 資源が豊富Si & Ba abundant chemical elements
Morita, ..,Suemasu, TSF 508 (2006) 363.(Theory) D. B. Migas et al, PSS (b) 244 (2007) 2611.
Si & Ba abundant chemical elementsClarke number: Si(2), Ba(14), Sr(15)cf. CIGS: Cu(26), In(66), Ga(35), S(16), Se(69)
Ba0.5Sr0.5Si2
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, JJAP 43 (2004) 4155, L478, L771.⇒High-quality crystal growth
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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
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研究項目
pn-junction TandemSchottky-junction
pn junction Tandem
EF
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
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Growth of BaSi2 epitaxial films using a template layerInomata,..,Suemasu, Jpn. J. Appl. Phys. 43 (2004) 4155, L478, L771.
B
BaSi
Reactive deposition epitaxyTsub: 550℃
MBE growthTsub: 600℃
Undoped n-BaSi2
Ba
BaSi2 template (10nm)
Si(111) Si(111)
Si(111)
θ-2θ XRD pattern
103
(*)Si(111) BaSi2(600)
BaSi2(400)BaSi
2(200)
](Lo
g Sc
ale)
Pt
TEM
102
Inte
nsity
[cou
nts]Pt
20 30 40 50 60 70 80101
XR
D
2θ [deg]
0.2 μm
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Electrical properties of undoped BaSi2 film
Zintl phase (A X )
300 250 200 150
Temperature [K]Ba
Si
Zintl phase (AaXx)Si-Si: covalentBa-Si: ionic
7x1015
8x1015
m-3
]
300 250 200 150
1200
が置換され す
6x1015
nsity
[cm 900
cm2 /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
ectro
n de
n
300
600
Mob
ility
[ B
Al
N
PSi
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0
3x1015
Ele
0
300 M Ga
In
As
SbRb Y1 2 3
1000/T [1/K]
Morita,…, Suemasu, Thin Solid Films 508 (2006) 363.
Cs
Fr
La
Ac
Ba
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Electrical properties of impurity-doped BaSi2 filmsKobayashi,…. Suemasu, Thin Solid Films 515 (2007) 8242.
B
BaSi Sb (V)In, Al(III)
Kobayashi,…..Suemasu, Appl. Phys. Express 1 (2008) 051403.
Si(111)(ρ>1000 Ωcm)
BaSi2(10nm)
Ba
Si(111)(ρ>1000 Ωcm)
Impurity –doped BaSi2
1000/Vs]
Sb-doped n-BaSi2 In-doped p-BaSi2
(ρ>1000 Ωcm) (ρ>1000 Ωcm)
200
s]
600
800
1000
ility
[cm
2 /
120
160
ty [
cm2 /V
s200
400
ctro
n m
obi
40
80
ole
mob
ilit
1015 1016 1017 1018 1019 1020Elec
Electron concentration [cm-3]
1016 1017 1018Ho
Hole concentration [cm-3]
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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Formation of heavily doped BaSi2/Si tunnel junction for an electrical contact
4. 太陽電池動作の実証Demonstration of solar cell
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Photoresponse properties of BaSi2 epitaxial filmsMatsumoto,…., Suemasu, Appl. Phys. Express 2 (2009) 021101.
hv
Sb-doped n+-BaSi2
1.5 mm Current(applied voltage 1~10 V)
hvE~10 V/cmL~1 mmhv electronStripe-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 structuren=3×1018 cm-3 n=3×1018 cm-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 VCZ-Si, n=3×1018 cm-3
0.005
otor
espo
nsiv
ity
0.005to
resp
onsi
vity
1.0 1.5 2.0 2.5
Pho
Photon energy (eV)1.0 1.5 2.0 2.50.000
Phot
Photon energy (eV)
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Analysis of grain size by Electron Backscatter DiffractionMatsumoto,..,Suemasu, Jpn. J. Appl. Phys. (2010) in press.
Sample
Grain
Electron
Diffraction Pattern
AtomicPlane Diffraction
Electron
ND mapping TD mappingGrain size 3~10 μm
100 μm 100 μm
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Formation of (111)-oriented Si layers on SiO2by Al-induced crystallization
O. Nast et al., Appl. Phys. Lett. 73 (1998) 3214.
BaSi2BaSi2
L NsampleholderSiO2
(111)-oriented SiSi(111)
Al(100nm)a-Si(100nm)
Al(100nm)
breaking the vacuum toform a native Al oxide Al
poly-Si
L-N2p
( )
SiO2 sub.
( )
SiO2 sub.
Vacuum evaporation RF magnetron sputtering
Annealing at 500oC for 10 h in dry N2
SiO2 sub.p y
5 mm 5 mm
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Growth of polycrystalline BaSi2 films on AIC-Si/SiO2
T k d S J C t G th 311 (2009) 3581
(111) oriented SiAnneal
Tsukada,…..Suemasu, J. Cryst. Growth 311 (2009) 3581.
EBSD(ND)
(111)‐oriented Si500℃, 10 h
SiO2 SiO2 SiO2
a-Si(100 nm)Al(100 nm)
Alpoly-Si
Au/Cr striped electrodes
50μm
Undoped n-BaSi2300 nm( 1016 cm-3)
1.5 mm
Undoped n-BaSi2300 nm( 1016 cm-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(~1016 cm 3)BaSi2 template ~300 nm(~1016 cm 3)
SiO2 Sub SiO2 Sub SiO2 Sub SiO2 Sub
RDE growthTsub: 550℃
MBE growthTsub: 500℃
EvaporationAu/Cr electrodes
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Tsukada,…..Suemasu, Appl. Phys. Express 2 (2009) 051601.
Photoresponse properties of polycrystalline BaSi2 films on SiO2
Al striped electrodes
hv
1.5 mmAl striped electrodes
Current (applied voltage 1~5 V)
0.068%
RTn-BaSi (300 nm)
0.04
0.05
5 V4%
6%
vity
(A/W
)n-BaSi2 (300 nm)
(111)-oriented SiSiO2
0.02
0.034 V
2 V3 V
2%
otor
espo
nsiv
1.0 1.5 2.0 2.50.00
0.01
2V (AIC-Si ×10)1 VPh
o
Ph t ( V)Photon energy (eV)0.4 μm
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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Formation of heavily doped BaSi2/Si tunnel junction for an electrical contact
4. 太陽電池動作の実証Demonstration of solar cell
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Band diagrams of BaSi2/Si structureSuemasu et al., Jpn. J. Appl. Phys. 45 (2006) L519 .
χBaSi2=3.3eVχSi=4.0eV
Vacuum level
ΔEC=0.7eV EgBaSi2=1.3eV
EgSi=1.1eVΔEV=0.5eV
n+/p+ tunnel junctionn-Si/n-BaSi2 p-Si/p-BaSi2p jn Si/n BaSi2
B Sin-SiΔEC
p Si/p BaSi2
p-BaSi2p Sin-BaSi2n-Si
ΔE
p BaSi2p-Si
ΔEV
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Formation of n+-BaSi2/p+-Si tunnel junction by MBE
BBa Si Sb
Sb-doped n+-BaSi2
template d
~1×1020cm-3
Ba
BaSi2 template BaSi2 template
p-Si(111)
B-doped p+-Si(70nm) ~5×1019cm-3
4×1018cm-3
p-Si(111)
B-doped p+-Si(70nm)
4×1018cm-3p-Si(111)
B-doped p+-Si(70nm)
4×1018cm-3
102
103
(301)Si(111)
Si(222)d=0 nm
unts
)
AFM images of BaSi2 template
d=1 nm 2 nm 10 nm
101
103 d=1 nm (600)
nten
sity
(cou
102
10 d=1 nm(200) (400)
(600)
XR
D In
3 μm
+ Sitemplate
20 30 40 50 60 70101
2θ (deg)
p+-Sip-Si(111)
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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-BaSi2/p-Si
Sb-doped n+-BaSi2~1×1020cm-3
n+-BaSi2/template/p+-Si
1nm 2 nm 10 nmn+-BaSi2template
p-Si(111)
template
4×1018cm-3p-Si(111)
templateB-doped p+-Si(70nm)
d
~5×1019cm-3
4×1018cm-3
p+-Sip-Si(111)
current
10
20
/cm
2 )
10
20
/cm
2 ) d=1nm
d=2nm
d=10nm
d=1nm
4×10 cm
J =20A/cm2 at 0 5 V
0
10
ensi
ty (A
/
n+‐BaSi2/p+‐Si
n BaSi /p Si0
10
dens
ity (A
/ J =20A/cm2 at 0.5 V
-10C
urre
nt d
e n‐BaSi2/p‐Si
-10
Cur
rent
d
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5-20
C
Voltage (V)-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
-20
Voltage (V)
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Photoresponsivity of BaSi2 layers on tunnel junction0 10
0 06
0.08
0.10Reverse
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
0.065%
ores
pons
ivit 4 V
n+-BaSi2
hv
1.0 1.5 2.0 2.5 3.0 3.5 4.0
0.02
Photon energy (eV)
Phot
o
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+-BaSi2p+-Si
Tunnel junction
-0.06
-0.045%
1 V3 Vor
espo
nsiv
it
-0.10
-0.08 10%15%
3 V 4 V 5 VPh
oto
Forward
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Wet chemical etching of BaSi2 layers on SiSaito Suemasu Jpn J Appl Phys 48 (2009) 106507Saito,…..Suemasu, Jpn. J. Appl. Phys. 48 (2009) 106507.
Au/CrSiO2
1mm φ
HCL+H O HF+H O
Si(111)Si(111)
BaSi2
Si(111) Si(111)
BaSi2HCL+H2O HF+H2O
HCL5% 15s 1% 180s 0.5% 120s2% 10s
HCL
HF2% 15s5% 15s 2% 15s 2% 15s
(5,5% 30s) (0.5,1.5% 60s) (1.5,0.5% 60s)(0.5,0.5% 60s)
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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Photoresponsivity
特願2007-208729 , US2009/0044862特願2008-218688 , PCT/WO2009/028560特願2009-115337,
3. BaSi2/Siトンネル接合の形成Formation of heavily doped BaSi2/Si tunnel junction for an electrical contact
4. 太陽電池動作の実証Demonstration of solar cell
Schottky-junctionpn-junction Tandem
EF
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Schottky-barrier diode n-BaSi2/CoSi2
sunlight
n+-BaSiドリフト
~0.5μm
n -BaSi2n-BaSi2(undoped)CoSi
EF Eg=1.3eV
φB~1.6eV
p+-Si (111)CoSi2
CoSi2 /n-BaSi2/n+-BaSi2
SiSuemasu et al., J. Cryst. Growth 310 (2008) 1250.Ichikawa,…, Suemasu, Appl. Surf. Sci. 254 (2008) 7963.
Epitaxial growth of BaSi2/CoSi2/Si(111) structure
Si(10nm) BaSi2( 22nm)BaSi2( 240nm)
Co BaSi Ba
Si(111)CoSi2(30nm)
Si(111)CoSi2
Si(10nm) BaSi2( 22nm)
Si(111)CoSi2
Si(111)CoSi2
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Characterization of n-BaSi2/CoSi2 Schottky diode
10000Si(111)
*
ount
s)
BaSi2 BaSi2
BaSi2(600) BaSi2
θ-2θ XRD pattern TEM
100
1000
CoSi2(222)
nten
sity
(co 2
(200) (400)
CoSi2
20 30 40 50 60 7010
XR
D In
5 nm Si20 30 40 50 60 702θ (deg) 5 nm Si
Si[11-2]
AM1.5, 1Sun20
A/c
m2 )
Jsc=11.3mA/cm2
V 0 36 V
10
ent d
ensi
ty (A
Voc=0.36 VFF=0.72 Vη=2.9%
0.1 0.2 0.3 0.4 0.50
Cur
re
Voltage (V)
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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Control of electron and hole concentrations by impurity doping
○ 分光感度特性
Control of electron and hole concentrations by impurity dopingn-type: Sb(1016 1×1020cm-3), Asp-type: In(1016 5×1017cm-3), Al, Cu
○ BaSi2/Siトンネル接合の形成
PhotoresponsivityPhotocurrent 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Formation of heavily doped BaSi2/Si tunnel junction for an electrical contact
○ W t h i l t hi
n+-BaSi2/p+-Si, J =21A/cm2 at 0.5 V
○ 太陽電池動作の実証D t ti f l ll
○ Wet chemical etching
Demonstration of solar cellSchottky-barrier diodeCoSi2/n-BaSi2, η~2-3%
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1 h j ti l ll
今後の展開 Future plan
undoped1. pn homojunction solar cell
表面電極p-Sip+-Si
p-Ba1-xSrxSi2undopedn-Ba1-xSrxSi2
EF
反射防止膜 pn接合 n+-Ba1-xSrxSi2
EF
構造が単純
SiO2基板を利用
SiO 基板
ZnO:Al
反射防止膜
Ba1-xSrxSi2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
2. pn heterojunction solar cell