Monolithic, Ultra-Thin GaInP/G

Tandem Solar Cells

Mark Wanlass, Phil Ahrenkiel, David Albin, Jeff Ca

Keith Emery, Daniel Friedman, John Geisz, Kim J

James Kiehl, Sarah Kurtz, William McMahon

Jerry Olson, Aaron Ptak, Manuel Romero, a

National Renewable Energy Laboratory (NREL), 1617 Cole 

Email: mark_wanlass@nrel.gov; Phone: (303) 384-6532; 

NREL/PR-520-39852Presented at the 2006 IEEE 4th World Conference on Photovoltaic Energy Conversion (WMay 7-12, 2006 in Waikoloa, Hawaii.
Disclaimer and Government LicenseThis work has been authored by Midwest Research Institute (MRI) under Contract No. DE-AC36Department of Energy (the “DOE”). The United States Government (the “Government”) retains awork for publication, acknowledges that the Government retains a non-exclusive, paid-up, irrevocor reproduce the published form of this work, or allow others to do so, for Government purposes.Neither MRI, the DOE, the Government, nor any other agency thereof, nor any of their employeeimplied, or assumes any liability or responsibility for the accuracy, completeness, or usefulness o

product, or process disclosed, or represents that its use would not infringe any privately owned rispecific commercial product, process, or service by trade name, trademark, manufacturer, or othits endorsement, recommendation, or favoring by the Government or any agency thereof. The viand/or presenters expressed herein do not necessarily state or reflect those of MRI, the DOE, the thereof. 
Monolithic, Ultra-Thin GaInP/GaAs/GaInAs Ta

Top

Middle

Bottom

• NREL IR # 05-05,patent pending.• Near-optimum subcellbandgaps.• ~300 mV voltage output boost compared to conventional Ge-based triple-junction tandems.• Bottom subcell E

g

is variable.• Basic approach is expandable to 4-6 

subcells.

~1-eV GaInAsN

GaInabsoMCC 
Inverted GaInP/GaAs/GaInAs Tan

GaAs substrate

n+-GaInP etch stop

n+-GaAs contact

p+/n+-GaAs tunnel junction

n/p GaAs/GaInP DH

p+/n+-GaAs tunnel junction

Compositionally step-graded n-GaInP

p+-GaInAs contact

1-eVBottomSubcell1.4-eVMiddleSubcell1.9-eVTopSubcellTransparentGradedLayer

n/p GaInAs/GaInP DH

n/p GaInP/AlInP DH

Solar radiation

Back surface: Handle mount, back co
Ultra-thin Tandem Cell Processin

Back contact/BSR

Contact layer

Multi-bandgap device layers

Contact layer

Etch-stop layer

Parent substrate

Contact layer

Multi-bandgap device layers

Contact layer

Back contact/BSR

Handle (support substrate)

Multi-b

Ha

Multi-b

Han

Contactlayer

1) Inverted tandem structure is grown on the parent substrate, and the back contact/BSR is formed. 2) Epistructure 

handle materia

3) Parent substrate is removed.

4) Front-surfac
Advantages of Ultra-Thin, 

Mounted Tandem Solar 

• Handle material can be engineered to have advantageous characteristics.• Thermal management can be optimized. • Highest specific power (W/kg) for space app• Reuse and/or reclamation of the parent subpossible, reducing cost.• Parent substrate can be impure to reduce co• Benefits of BSR: thin GaInAs subcell, lower 

radiation hardness, reduced operating tempe
Semi-Realistic Performance 

GaInP/GaAs/GaInA

SubcellAbsorber

Eg (eV)

Voc (V)

Jsc(A/cm2)

FF (%)

GaInP 1.87 1.53 3.40 91.57

GaAs 1.42 1.12 3.40 89.23

GaInAs 1.01 0.74 3.63 85.21

Voc (V) Jsc

(A/cm2)

FF (%)

3.38 3.40 90.32 

Low-AOD Direct Spectrum, 250 suns, 25QE = 0.95, realistic J

0(E

g), no parasitic lo

Subcell parameters Series-connected 

Tandem effic
Semi-Realistic Performance 

SubcellAbsorber

Eg (eV)

Voc (V

GaInP 1.87 1.393 

GaAs 1.42 0.981 

GaInAs 1.02 0.608 

Voc (V) Jsc 

(mA/cm2)

FF (%)

2.98 17.00 89.27 

ConditionsAM0, one sun, 25°CQE = 0.95, Realistic J

0(E

g

)

Series-connected ta

Subcell pa

Tandem efficiency~33% (one sun)~36% (10 suns)
Defects in LMM, ~1-eV 

TD de

XTEM Pla
1.00.80.60.40.20.0

1.0 1.2 1.4 1.6

Photon energy (eV)

Single-junction

GaInAs 

with grade

Internal Quantum Efficiency

~1-eV, LMM (2.2%) GaInAs

DH Cell Performance

Curre

-20-15-10

-5

0

Current (mA/cm2)

0.0 0.1 0

Single-j

GaIn

Low AOD 

Voc = 

Jsc = 18.6 

FF = 
Ultra-Thin, Handle-Moun

GaInP/GaAs/GaInAsTand

AEQE & R Data

• QE is excellent for all subcells, but some improvement is still possible(reduce parasitic absorption and reflection).• ZnS/MgF

2 ARC is not 

optimal.

• Interference evident in thin bottom subcell.
Reported Performa

PVSC 1/3-7/05 Global, 25°C SPW 4/18-21/05 AM0, 1 sun, 25°C ISCC 5/1-5/05 Direct, 10.1 sunsSPRAT 9/20-22/05 AM0, 1 sun, 25°C Kapton handle SPRAT 9/20-22/05 AM0, 8.9 suns, 2
Voc, FF (Concentration R

<n> = 1.36

<n> = 
I-V Data at

Peak Efficiency

•

η = 37.9% (10.1 suns).

• On 4/25/05, a new record for solar PV conversion.• With continued development, η > 40% possible at higher concentration ratios.
Recent progress

~200 suns
Research Issues

• High-yield processing of handle-mounted, u• Develop process enabling reuse of parent s• Tandem cell efficiency testing more difficult even more difficult as we consider more than • Inverted tunnel junctions.• Radiation effects.• Push efficiency limits by including more subachieve 40-50% (terr. conc.), 35-40% (AM0)? 
Advanced Design Opt

Contact layer

LMM DH subcell m+1

Contact layer

Etch-stop layerParent substrate

Transparent tunnel junctionTransparent tunnel junction

LM DH subcell 1

LM DH subcell 2

Transparent tunnel junction

Transparent graded layer

LM DH subcell m

LMMepi.

LMepi.

Legend:LM = lattice matched.LMM = lattice mismatched.DH = double heterostructure.

• Concept applies to subcells.• A wide range of sumaterials, tunnel junand transparent comare possible.• Substrates: GaAs, • Subcells, etc.: AlGa