Madrid, 7 th February 2008 Confidential 1 IBPOWER kick-off Meeting “Growth and Fabrication of Intermediate Band Solar Cells designed for concentration”

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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

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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”

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Overview of device fabricationOverview of device fabrication

Cross-sectional schematic

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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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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

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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