© 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