madrid, 7 th february 2008 confidential 1 ibpower kick-off meeting “growth and fabrication of...
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Madrid, 7Madrid, 7thth February 2008 February 2008 ConfidentialConfidential
11
IBPOWER kick-off MeetingIBPOWER kick-off Meeting
“Growth and Fabrication of
Intermediate Band Solar Cells
designed for concentration”
Corrie Farmer and Colin Stanley
University of Glasgow
Madrid, 7Madrid, 7thth February 2008 February 2008 ConfidentialConfidential
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Standard (Al,Ga)As heterojunction solar cell
GaAs reference/IBSC layer structureGaAs reference/IBSC layer structure
QD density ~1-2x1010 cm-2
Si -doping to give one electron per dot 10-50 layers of QDs
Donor impurities
QD QD WL
n+ -substrate
n+ -Al 0.2Ga 0.8As
n-GaAs
p+p+ -GaAs
n-metal contact
Metal contact on p+-GaAs layer
SiNx ARC, Al0.85Ga0.15As window layer
IBSC - InAs QD layers incorporated into n-GaAs “base”
Madrid, 7Madrid, 7thth February 2008 February 2008 ConfidentialConfidential
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Overview of device fabricationOverview of device fabrication
Cross-sectional schematic
Madrid, 7Madrid, 7thth February 2008 February 2008 ConfidentialConfidential
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Solar cell characteristics (one sun)Solar cell characteristics (one sun)
Good efficiency of GaAs reference cell.
Lower efficiency for prototype IBSC.
Reduced VOC.
Photocurrent from IBSC due to absorption of sub-
bandgap photons (2+2) with EIB-EVB<Ehf<Eg.
Photocurrent due to absorption of two sub-bandgap
photons (2+3), one with Ehf~ECB-EIB
IB
CB
VB
2 1
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Madrid, 7Madrid, 7thth February 2008 February 2008 ConfidentialConfidential
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Plans for next 6 monthsPlans for next 6 months
Design for concentration Minimize series resistance Increase the open circuit voltage, Voc
MBE growth of IBSC structure Maximize IB-CB separation (total In content per QD
layer) Maximize current produced by absorption of sub-
bandgap photons (total number of QD layers) Optimize spacer thickness between QD layers (coupled
or uncoupled QDs??)
Fabrication of solar cells using; ~5 μm thick electro-plated Au-grid for top electrode Optimal grid geometry Two-layer ARC Heat sink
Characterisation by PL/EL/FTPS using FTIR spectrometer 400 500 600 700 800
0
10
20
30
40
50
60
70
Day 0 Day 3 Day 6 Day 20 Day 20+ ZnS/MgF
2 ARC
Ref
lect
aan
ce (
%)
Wavelength (nm)
Stabilised uncapped Al0.9
Ga0.1
As Window Layer
1
o
LBoc J
Jn
q
TnkV
Madrid, 7Madrid, 7thth February 2008 February 2008 ConfidentialConfidential
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Further afield – strain balancing on InPFurther afield – strain balancing on InP
V M Ustinov et al., Semiconductors Vol. 31 (10), p1080, 1997
In(Al,Ga)AsInAs QD layer
In(Al,Ga)As layer with In fraction reduced below lattice-matched value
InAs QDs embedded in In0.53Ga0.47As on InP
InP
At 300 K;Eg(InP) = 1.35 eV, Eg(InAlAs) = 1.46 eV
In theory, this should permit an “unlimited” number of QD layers to be stacked one on top of the other
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