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量子ドット超格子型中間バンド太陽電池Quantum Dot Superlattice
Intermediate‐Band Solar Cells
岡田至崇Yoshitaka Okada
RCASTThe University of Tokyo
nature photonics Technology Conference, 19‐21/10/2010
2
Scope
What kind of high‐efficiency solar cell are we aiming to build?
Basic principle of intermediate‐band solar cell (IBSC)
Efficiency analysis and material choice
Fabrication technique of quantum dot (QD) superlattice IBSC
How far have we reached?
Photoabsorption by QD superlattice
(1) VB→QD (IB) absorption
(2) QD (IB) →CB absorption : Proof of Concept
3
AM1.5
Wavelength
Efficient absorption by using multiple bandgaps
AM1.5
Wavelength
Efficient absorption by using multiple bandgaps
Efficient use of high-energysolar radiation
AM1.5
Wavelength
Efficient use of high-energysolar radiation
AM1.5
Wavelength
Hot carriers
3 V
Multiple junctions(Quantum size effect)
Eg
Intermediate band(QD superlattice)
Multiple excitongeneration (MEG)
3 I
nanoscaleformats
Innovative PV : > 50% Efficient Solar Cells
4
Intermediate Band Solar Cell:Principle
A. Luque and A. Martí, Phys. Rev. Lett. 78, 5014 (1997)
Conduction Band
Intermediate Band (IB)
p –type Host Semiconductor
n‐type HostSemiconductor
IB material
Valence BandCurrent‐matching
5
Intermediate Band Solar Cell:Theoretical Efficiencies
η = 47% (1sun)η = 63% (Maximum concentration)
Maximum concentration1sun
5010Efficiency (%)
47%Eg=2.4 eVECI=0.9 eV
45
40
20
35
3025
15
1.5 2 2.5 30
0.5
1
E IV
(eV)
Eg (eV)
CB
-IB e
nerg
y ga
p(e
V)
CB-VB energy gap (eV)
Efficiency (%)
63%Eg=1.9 eVECI=0.7 eV
30
60
7010
5040
20
1.5 2 2.5 30
0.5
1
EIV
(eV)
Eg (eV)
CB
-IB e
nerg
y ga
p(e
V)
CB-VB energy gap (eV)
S. Yagi and Y. Okada, 2nd Innovative PV (Tsukuba, 2009)
6
Intermediate Band Realized with Quantum dot Superlattice
VB
CB
miniband
InAs QD
GaAs
VB
CB
InAs QD
GaAs GaAs
GaAs
InAs QD
Single QDSingle QD QD SuperlatticeQD Superlattice
7
Frank‐van der Merwe Volmer‐Weber Stranski‐Krastanov (S‐K)
2D Growth 3D Growth 2D→3D
Self‐Assembled Growth of Quantum Dots
8
InGaAs on (311)BInAs on (100)
K. Akahane et al, APL 73 (1998) 3411
Self‐Organized InGaAs Quantum Dots on (311)B Substrate
QDs on (311)B substrate show;(1) Better size homogeneity(2) Higher in‐plane density(3) Ordered periodic structure(4) Better heterointerface quality
9
Z.R. Wasilewski et al. JCG 201 (1999) 1131
Fabrication of 3D Quantum Dot Superlattice: Strain‐Balancing
Accumulation of lattice strainin conventional approach
Strain‐compensation growth: Strain/layer is balanced out
QD
SpacerStrainfield
10
InGaAlAs
III‐V multijunction solar cells
GaAsP
Strain‐Compensation Materials
11
Intermediate Band Solar Cell:Common Materials
Efficiency (%)
63%Eg=1.9 eVECI=0.7 eV
30
60
7010
5040
20
1.5 2 2.5 30
0.5
1
EIV
(eV)
Eg (eV)
CB
-IB e
nerg
y ga
p(e
V)
CB-VB energy gap (eV)
In(Ga)As/(Al)GaAs
InAs/GaAsP
12
Multi‐Stacked InGaAs QDs on InP (311)B Substrate
Spacer thickness d = 20 nmNumber of stacked QDs = 30 layers
Average diameter = 63.2nmIn‐plane dot density = 2.7×1010cm‐2
Size uniformity ~ 12.3% Y. Okada et al, EU‐PVSEC, Barcelona (2005)
13K. Akahane et al, APL 89 (2006) 151117
Demonstration of 3D Quantum Dot Superlattice
Number of stacked QDs = 150 layers!
14
on GaAs (311)B
InGaAs/GaNAs Quantum Dot Solar Cell : 1sun
ISC(mA/cm2)
VOC(V)
FF Efficiency(%)
QD solar cell on GaAs(311)B 24.26 0.791 0.840 16.12
15R. Oshima et al, Physica E, in press.
Multi‐Stacked QDSCs: Dependence on number of QD layers
Cell size 3mm × 5mm
10 layersi-GaAs i-GaAs
InAs/GaNAsQDs
400nm200nm400nm
200nm600nm
200nm
1000nm
1μm thick-intrinsic layer
p-GaAs
n-GaAs
30 layers
50 layers
10 layersi-GaAs i-GaAs
InAs/GaNAsQDs
400nm200nm400nm
200nm600nm
200nm
1000nm1000nm
1μm thick-intrinsic layer
p-GaAs
n-GaAs
30 layers
50 layers
20 nm GaNAs layer
10, 20, 30, 50 stacked 2.0 MLs InAs QDs/ 20 nm GaNAs spacer layer
n+ - GaAs (001) substrate
250 nm n+ - GaAs buffer layer
1000 nm n - GaAs base layer
20 nm GaNAs layer
50 nm p+ - GaAs contact layer
150 nm p - GaAs emitter layer
Ti/Pt/Au
AuGeNi/Au
SiO2
20 nm GaNAs layer
10, 20, 30, 50 stacked 2.0 MLs InAs QDs/ 20 nm GaNAs spacer layer
n+ - GaAs (001) substrate
250 nm n+ - GaAs buffer layer
1000 nm n - GaAs base layer
20 nm GaNAs layer
50 nm p+ - GaAs contact layer
150 nm p - GaAs emitter layer
Ti/Pt/Au
AuGeNi/Au
SiO2
16
Multi‐Stacked QDSCs: Dependence on number of QD layers
1.5 mA/cm2
for 50 stacks
Current increase solely due to InAs QD absorption is linear up to ~ 50 layers.
17
Multi‐Stacked QDSCs: Dependence on number of QD layers
VOC(V)
ISC(mA/cm2)
FF (%)
η(%)
Diode factor
20 stacks
0.72 21.04 70.0 10.63 1.65
30 stacks
0.67 22.33 70.76 10.59 1.67
50 stacks
0.68 26.36 70.24 12.44 1.59
GaAs SC
0.94 20.26 77.74 14.80 2.00
VOC(V)
ISC(mA/cm2)
FF (%)
η(%)
Diode factor
20 stacks
0.72 21.04 70.0 10.63 1.65
30 stacks
0.67 22.33 70.76 10.59 1.67
50 stacks
0.68 26.36 70.24 12.44 1.59
GaAs SC
0.94 20.26 77.74 14.80 2.00
Isc increases linearly up to ~ 50 layers. Drop in Voc tends to saturate.
18
Stacking up to 100 InAs/GaNAs Strain‐Compensated QDs Layers
SF0
F-1
F+1
F+2
F-2
[001]
[110]
SF0
F-1
F+1
F+2
F-2
[001]
[110]
[001]
[110]Upper region Middle region Lower regionUpper region Middle region Lower region
i - GaAs (001) substrate
250 nm GaAs buffer layer
20 nm GaNAs layer
i - GaAs (001) substrate
250 nm GaAs buffer layer
20 nm GaNAs layer
Up to 100 layers
100 layers of multi‐stacked 2.0 monolayers of InAs QDs and 20 nm of GaN0.01As0.99 SCL
A. Takata et al, 35th IEEE PVSC, Honolulu (2010)
19
Measurement at room temperature
Optical density = -log (T/(1‐R))
T : TransmittanceR : Reflectance
Stacking of up to 100 InAs/GaNAs Strain‐Compensated QDs Layers
Halogenlamp
Monochro‐meter
Sample
Polarizer
InGaAsphotodiode
Chopper
Lock‐inamplifier
Long passfilters
Off‐axis parabolicmirror
Halogenlamp
Monochro‐meter
Sample
Polarizer
InGaAsphotodiode
Chopper
Lock‐inamplifier
Long passfilters
Off‐axis parabolicmirror
20
Stacking of up to 100 InAs/GaNAs Strain‐Compensated QDs Layers
VB→QD(IB) absorption increases linearly with number of QD stacks.
VB→QD(IB) absorption reaches ~ 20% in 100 layer stacked sample.
Further increase of QD density and photon management are helpful.
21
Size Requirement for InAs QDs
10K Large QD Small QDMiddle QD
S. Tomic et al, APL 93 (2008) 263105
InAs/GaNAs QDSC on GaAs (311)B
22
Intermediate Band Solar Cell:Technical Issues
IB
CB
VB
Monochromatic
White bias
IB
CB
VB
Monochromatic
White biasProblem_1: Transition rate is small
Problem_2: Recombination is large
Increase in Isc may not outweigh drop in Voc
23
Conditions for Achieving High Efficiencies
Maximize
Increase of efficiency in IB solar cells
Constant
Constant
Small absorption via IB states
Larger absorption via IB states
Under concentration
① ②
24
Self‐consistent Analysis
• Material parameters: GaAs (300K) except for absorption coefficients.
• Carrier Mobility ModelingElectrons in CB: Negative differential resistance
Dependence on impurity densityHoles in VB : Velocity saturation
Dependence of impurity density
• Boundary ConditionsIdeal Ohmic contacts and Infinite surface recombination velocities
• Effective Density of State for IB (QDs are uniformly distributed)
• Absorption coefficients
Non-Overlap absorption spectrum are assumed.
• Temperatures of the Sun and the cell
• Radiative recombination rates are included.
• Carrier flux in IB state is neglected.
VB
CB
IB
p‐type emitter (0.5μm)
IB region (2μm)
ND = NI /2
n‐type Base (1μm)
Calculated Structure
Case 1. Undoped (intrinsic) Case 2. n‐type doped
[3] A. S. Lin and J. D. Phillips, IEEE Trans. Elec. Dev. 56, 3168 (2009).
K. Yoshida et al, 35th IEEE PVSC, Honolulu (2010)
25
Effect of Doping, Photofilling
Doping in IB region is better to realize a high efficiency.
K. Yoshida et al, APL 97 (2010) 133503
26
Direct Doping of QDs
• Modulation doping
Band diagram Band diagram
Dopant (Si)Quantum dot
Barrierlayer
Dopant (Si)
• Direct doping
Modulation dopingNon‐doped layer 10nmSi‐doped layer 5nm
Barrier 15nmWell In0.4Ga0.6As 5nmSi‐doped to 1018 cm‐3
27
Direct Doping of QDs
• S‐K mode growth (by MBE)
Doping at the self assembling stage enables direct doping
(QD growth step is observed by RHEED pattern)
T. Inoue et al, J. Appl. Phys. 108 (2010) 063524
28
QD solar cell with Direct Si‐doped QDs
T. Morioka et al, 35th IEEE PVSC, Honolulu (2010)
• Strain‐compensated growth by H‐MBE
29
(1) (2) (3)(1) (2) (3)
QD solar cell with Direct Si‐doped QDs
(1)
(2) (3)
GaNAs barrier InAs QDs(1)
(2) (3)
GaNAs barrier InAs QDs
Isc is significantly increased.24.11 mA/cm2 → 30.57 mA/cm2
Non‐doped Direct‐doped
Isc (mA/cm3) 24.11 30.57
Voc (V) 0.480 0.540
FF 0.463 0.656
Efficiency (%) 5.36 10.81
Non‐doped Direct‐doped
Isc (mA/cm3) 24.11 30.57
Voc (V) 0.480 0.540
FF 0.463 0.656
Efficiency (%) 5.36 10.81
30
Photocurrent Production due to IB → CB Optical Transition
Y. Okada, 2010 ssdm, Tokyo (2010)
31
Room temperature
IB→CB optical transition : Thermal escape rate ≈ 1 : 20
Photocurrent Production due to IB → CB Optical Transition
Y. Okada, 2010 ssdm, Tokyo (2010)
33
Summary
Strain‐compensation (Strain‐balanced) growth technique is very effective in stacking multiple layers of QDs to form QD superlattice and solar cells.
For 100‐layer stacked InAs/GaNAs QD superlattice sample, optical absorption is ~ 20%.
In present QD solar cells, electrons generated by VB→QD (IB) absorption escape out of QDs by thermal or field‐assisted tunneling. Electrons are not likely in electrochemical equilibrium within QD states.
QD (IB) →CB absorption is observed at room temperature under IR light illumination of AM1.5 solar spectrum for the first time.
Higher QD density (small QDs) Increased photon absorption by control of quasi‐Fermi level of IBOperation under concentrated sunlightcan significantly improve the efficiency of QD‐IBSC.