Characterisation of bifacial solar cells

Jochen Hohl-Ebinger, Wilhelm Warta

Fraunhofer Institute for

© Fraunhofer ISE

Fraunhofer Institute for Solar Energy Systems ISE

Bifi-Workshop Konstanz23.04.2012

www.ise.fraunhofer.de

Calibration of bifacial solar cells

Jochen Hohl-Ebinger, Wilhelm Warta

Fraunhofer Institute for

© Fraunhofer ISE

Fraunhofer Institute for Solar Energy Systems ISE

Bifi-Workshop Konstanz23.04.2012

www.ise.fraunhofer.de

Frame and Outline

Not only bifacial cells are bifacial!

• Back contact solar cells

• Back grid on standard cells

(thin cells, cost savings in new cell concepts)

© Fraunhofer ISE

� Context

� Estimation of bifaciality impact on current

� Experimental test

� Proposals to obtain inter-lab comparability

Context I: Standard Testing Conditions (STC)How to Reach International Comparability of Results?

Usual path:

� Realize STC according to IEC 60904

� Estimate uncertainty budget

Spectral distribution

Temperature 25°C

Irradiance 1000 W/m²

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500 1000 1500 2000 2500 3000 3500 4000

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

spec

tral

irra

dian

ce [W

m-2nm

-1]

wavelength [nm]

But:

Bifaciality not addressed in STC AM1.5G Edition 2

Context II: Uncertainty of Reference Calibration Example: Traceability Chain at ISE CalLab PV Cells

Planck spectrum, small diode

Synthetic irradiation, small cell

Cryoradiometer < 0.01%

Photodiode < 0.1%

Encapsulated2x2 cm² Solar Cell < 0.7%

© Fraunhofer ISE

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Simulator irradiation, large area

Contributions > 0.1 % count!

Strong economic impact:0.1 % of 20 GWp/a PV world production 200 Mill. €

IndustrialSolar Cell < 2.0%

Estimation of Bifaciality Impact Effect on Spectral Response and Short Circuit Current

Light transmitted through cell: T(λ)

Reflected by surface of measurement chuck

R(λ) depends on individual chuck:Unpredictable differences between labs

Solar Cell

ChuckT(λ)

E(λ)

R(λ)

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Simple model:

Non-linearities neglected- may be severe e.g. due to injection dependent surface recombination

)()()()()()()( , λλλλλλλ backChuckCellfrontcontribbackfrontmeas SRRTSRSRSRSR +=+=

λλλλλ ∂= ∫ )()()()(, backChuckCellcontribback SRRTEI

Estimation of Bifaciality ImpactSet of Test Cells with Variable Bifaciality

Measured Spectral Response

0.04

0.05

0.06

0.07

SR

[A/W

]

Cell A Cell B Cell C Cell D

Front side

0.04

0.05

0.06

0.07

SR

[A/W

]

Cell A Cell B Cell C Cell D

Back sideIrradiation from front Irradiation from rear

© Fraunhofer ISE

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400 600 800 1000 12000.00

0.01

0.02

0.03

SR

[A/W

]

Wavelength [nm]400 600 800 1000 1200

0.00

0.01

0.02

0.03

SR

[A/W

]

Wavelength [nm]

Cell B: fully bifacial

Cells A, C, D back contact cells with different metal coverage

80

100

Ref

lect

ion

[%]

black foil black anodized brown anodized Cu brass1 brass2 with 25

30

35

Tra

nsm

issi

on [%

]

bifacial cell A B C D

Estimation of Bifaciality ImpactAdditional Irradiation from Back Side

Transmission through test cells Chuck reflection

© Fraunhofer ISE

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200 400 600 800 1000 12000

20

40

60

Ref

lect

ion

[%]

Wavelength [nm]

brass2 with ad. foil

gold plated

400 500 600 700 800 900 1000 1100 12000

5

10

15

20

Tra

nsm

issi

on [%

]

Wavelength [nm]

D

Estimation of Bifaciality ImpactCalculated Impact

Back current contribution

20

25

30

35

100

120

140

160

1.41.61.82.02.22.4

8

10

12

Cell B

Cell C

Bac

k cu

rr. c

ontr

ib. [

%]

black foil brown anod. brass1 gold plated black anod. Cu brass w. ad. foil

Cell A Cell D

front

contr ibbackrelcontribback SR

SRSR ,

, =

© Fraunhofer ISE

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1000 12000

5

10

15

20

1000 12000

20

40

60

80

1000 12000.00.20.40.60.81.01.21.4

1000 12000

2

4

6

Bac

k cu

rr. c

ontr

ib. [

%]

Wavelength [nm]

Usual Cu or gold plated chucks may add significant uncertainty

Estimation of Bifaciality ImpactCalculated Impact

Increase of Current

back current contributionCell A Cell B Cell C Cell D

black plastic foil 0.00% 0.05% 0.01% 0.03%

black anod. 0.01% 0.22% 0.06% 0.11%

brown anod. 0.00% 0.18% 0.05% 0.09%

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Usual Cu or gold plated chucks may add significant uncertainty

brown anod. 0.00% 0.18% 0.05% 0.09%

Cu 0.02% 0.66% 0.17% 0.33%

brass1 0.02% 0.69% 0.19% 0.34%

brass w. ad. foil 0.02% 0.77% 0.20% 0.38%

gold plated 0.03% 1.07% 0.30% 0.52%

Experimental TestSpecial Chuck for Bifacial Cells

Exchangeable surface plates Measured reflectivity

30

40

50

60 black plastic foil brown anodized black anodized grey anodized

Ref

lect

ion

[%]

© Fraunhofer ISE

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200 400 600 800 1000 12000

10

20Ref

lect

ion

[%]

Wavelength [nm]

Experimental TestComparison to Simple Model

� Measurement with black foilas reference

� Model fits for brown surface

� Measurement 25% higher forblack and grey anodized surfaces

121416182022242628

back

cur

r. c

ontr

ib. [

%]

calculated brown anodized black anodized grey anodized

measured brown anodized black anodized

© Fraunhofer ISE

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Likely reason:

� SR for calculation measuredwith small irradiation angle

� Anodized surfaces reflect highly diffusive

Light trapping increases current

200 400 600 800 1000 1200

02468

1012

back

cur

r. c

ontr

ib. [

%]

Wavelength [nm]

black anodized grey anodized

Reference Chuck Reflectionblack or white?

200 400 600 800 1000 1200-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

black

back

cur

r. c

ontr

ib. [

%]

black (R = 0%)

� Simple realization with plastic foil

� No further definition necessary

© Fraunhofer ISE

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200 400 600 800 1000 1200

Wavelength [nm]

200 400 600 800 1000 12000

20

40

60

80

100

R [%

]

Wavelength [nm]

Paint Paper PTFE deco-foil gold plated w-module foil

white (R = 100%)

� Realization with 80 - 90% possible

(paint, gold, foil)

� Tight definition of albedo incl. angular

distribution of reflection necessary

� Higher c2m-loss

White as Reference

Reflections (1000-1200nm):

65 to 95%

60

80

100

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200 400 600 800 1000 12000

20

40

60

R [%

]

Wavelength [nm]

Paint Paper PTFE deco-foil gold plated w-module foil

80

100

Reference chuck reflectionHow to achieve comparable measurements?

Reflections (1000-1200nm):

65 - 95%

80 - 90 %

80

100

black foil black anodized brown anodized

back current contribution:

0.8 - 1.2%

1 - 1.13%

© Fraunhofer ISE

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200 400 600 800 1000 12000

20

40

60

80

R [%

]

Wavelength [nm]

Paint Paper PTFE deco-foil gold plated w-module foil

200 400 600 800 1000 12000

20

40

60

80

Ref

lect

ion

[%]

Wavelength [nm]

brown anodized Cu brass1 brass2 with

ad. foil gold plated

Reference Chuck ReflectionBlack, White or Grey?

R = 0%

R = 100%

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R = 100%

R ≈ 90%

Reference Chuck ReflectionBlack, White or Grey?

R = 0%

Easy definition

Easy to realize (Rfoil ≈ 4%)

High lab inter-lab comparability

Higher internal reflection improves calibration value not energy yield

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R = 100%

R ≈ 85%

Hard to realize (R ≈ 80-90% realistic)

Angular and spectral distribution makes influence

Improving calibration value improves energy yield

Conclusions

� Impact of bifaciality can be significant

� Efficiencies should be stated with relevant information:

area definitionmodule ready or not

© Fraunhofer ISE

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η(STC) = 29 %t, ap, damr, nmr

bl, w, av. 92%

reflection of back sheet

cusi, sf

contact unit shadow included/shadow free

Thank You!

© Fraunhofer ISE

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� Fraunhofer ISE CalLab PV Cells