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© Imperial College LondonPage 1 Ben Browne
B.C. Browne, A. Ioannides, J.P.Connolly, K.W.J.Barnham
Imperial College London
John Roberts, Geoff Hill, Rob Airey, Cath Calder EPSRC National Centre for III-V Technology
G.Smekens, J. Van BeginEnergies Nouvelles et Environnement, B-1150 Brussels, Belgium
TANDEMTANDEMQUANTUM WELLQUANTUM WELL SOLAR CELLS SOLAR CELLS
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Introduction and motivation
Description of our cells
Modelling
Characterisation of two generations of cells
Predictions under a concentrator spectrum
TANDEMTANDEMQUANTUM WELLQUANTUM WELL SOLAR CELLS SOLAR CELLS
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Single Junction Cells:
The GaAs band gap is below the theoretical optimum
There are no ternary alloys lattice matched to Ge or GaAs with a lower bandgap than GaAs
Efficiency peaks predicted for In0.1Ga0.9As and In0.3Ga0.7As
Tandems: The bandgap of both cells in a GaInP/GaAs
tandem are too high QWs can move the limiting efficiency at 500
suns from 40% to 50%
BANDGAP ENGINEERINGBANDGAP ENGINEERING
InGaP/GaAs
SB-QWSC
Dual SB-QWSC
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GaAsP (barrier)
InGaAs (well)
GaAs (bulk)
STRAIN BALANCINGSTRAIN BALANCINGE
nerg
y
Ef
We are able to grow up to 65 quantum wells with this technique
Strain balanced quantum well solar cells are dislocation free
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p i
Ea
Eg
Ide
al S
ho
ckle
y R
eco
mb
ina
tion
Qu
an
tum
We
ll R
eco
mb
ina
tion
Ba
rrie
r R
eco
mb
ina
tion
Δμ
n
Thermal escape
Thermal escape
Ge
ne
ratio
n
GENERATION AND RECOMBINATIONGENERATION AND RECOMBINATION
•Under concentration, recombination is radiatively dominated
•At short circuit current all generated carriers escape from the wells
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OUR TANDEM SOLAR CELLSOUR TANDEM SOLAR CELLS
Grown by MOVPE:
Bottom Cells: EPSRC National Centre for III-V Technologies, UK
Top Cells: ENE, Belgium
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0
10
20
30
40
50
60
70
80
90
100
400 500 600 700 800 900 1000
Wavelength (nm)
Quantu
m E
ffici
ency
(%
)
QUANTUM EFFICIENCYQUANTUM EFFICIENCYSample
1
AM1.5D
Top Cell Bottom Cell
Tandem QW solar cell grown with 50 InGaAs well in the bottom cellAbsorbing out to 932nm
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1.E-03
1.E-01
1.E+01
1.E+03
1.E+05
0 0.5 1 1.5 2 2.5 3
Bias (V)
Curr
ent
Densi
ty (A
/m²) SRH
Shockley InjectionCurrent
Radiative
1.E- 03
1.E- 01
1.E+01
1.E+03
1.E+05
0 0.5 1 1.5 2 2.5 3
Bias (V)
Curr
ent
Densi
ty (
A/m
²) SRH
Shockley InjectionCurrent
Radiative
DARK CURRENT MODELLINGDARK CURRENT MODELLING
1.E-03
1.E-01
1.E+01
1.E+03
1.E+05
0 0.5 1 1.5 2 2.5 3
Bias (V)
Curr
ent
Densi
ty (A
/m²)
Modelled Bottom Cell
Modelled Top Cell
1.E-03
1.E-01
1.E+01
1.E+03
1.E+05
0 0.5 1 1.5 2 2.5 3
Bias (V)
Curr
ent
Densi
ty (A
/m²)
Modelled Bottom Cell
Modelled Top Cell
Measured DarkCurrent
1.E-03
1.E-01
1.E+01
1.E+03
1.E+05
0 0.5 1 1.5 2 2.5 3
Bias (V)
Curr
ent
Densi
ty (A
/m²)
Modelled Bottom Cell
Modelled Top Cell
Measured DarkCurrent
Modelled DarkCurrent
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Using the measured Jsc, modelling dark current and assuming additivity:
LIGHT CURRENT (1 SUN A0D)LIGHT CURRENT (1 SUN A0D)Sample
1
1ST cell modelling prediction: 29.8±0.3% at 200 suns low AODThis cell had a low top cell emitter doping → poor performance at concentration
0
20
40
60
80
100
120
140
160
180
200
0 0.5 1 1.5 2 2.5Bias (V)
Curr
ent
(µA
)
Top Cell - Model Bottom Cell - Model
Tandem - Model Fraunhofer Measurement
22.1±0.7%
22.0%
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Sample
2IMPROVED QW TANDEM CELLIMPROVED QW TANDEM CELL
A 2nd cell was grown with higher: top cell emitter doping—decreased Rs QW band gap—better current matching
Fill FactorEfficiency
(%)
QW Cell 81.4 30.6
Control 81.9 31.7
Tested at ENE under a red-rich Xenon Lamp, 54 concentration:
0
1
2
3
4
5
400 600 800 1000Wavelength (nm)
Inte
nsi
ty (
W/m
²)
LOW AODXenon Lamp
0
1
2
3
4
5
400 600 800 1000Wavelength (nm)
Inte
nsi
ty (
W/m
²)
LOW AODXenon Lamp
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22NDND CELL PERFORMANCE CELL PERFORMANCESample
2
0
10
20
30
40
50
60
70
80
90
100
400 500 600 700 800 900 1000Wavelength (nm)
Exte
rnal
Quan
tum
Effi
cien
cy (
%)
2nd QW Tandem Top Cell 2nd QW Tandem Bottom CellControl Top Cell Control Bottom Cell
The Control Top Cell absorbs out to longer wavelengthsThis explains the superior performance of the control in a red-rich spectrum
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0
2
4
6
8
10
12
QW cell QW TopCell
QW BottomCell
ControlCell
ControlTop Cell
ControlBottom
Cell
Short
Cir
cuit
Curr
ent
(kA
/m²)
low AOD Xenon Lamp Measured
Sample
222NDND CELL SHORT CIRCUIT CURRENT CELL SHORT CIRCUIT CURRENT
A good quality Top Cell on a quantum well bottom cell would be current matched in a concentrator spectrum
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Sample
222NDND CELL EFFICIENCY CELL EFFICIENCY
Under a low AOD spectrum (1000W/m²) & assuming additivity we expect:
20%
22%
24%
26%
28%
30%
32%
34%
36%
38%
40%
0 200 400 600 800 1000
Concentration
Effici
ency
(%
)
QW Cell + Improved Top Cell QW Cell Control Cell
We are working to improve our top cell in order to realise 34% efficiencies
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CONCLUSIONSCONCLUSIONS
Tandem quantum well solar cells offer a path to increased multi-junction cell efficiency by band gap engineering
We have achieved 30.6% under a Xe lamp at 54 suns
Two junction quantum well solar cells have the potential to reach efficiencies above 34%
Tandem cells with quantum wells in both junctions could perform better still