Characterisation of bifacial solar cells

1. Characterisation of bifacial solar cells
Jochen Hohl-Ebinger, Wilhelm Warta
Fraunhofer Institute for
© Fraunhofer ISE
Fraunhofer Institute for
Solar Energy Systems ISE
Bifi-Workshop Konstanz
23.04.2012
www.ise.fraunhofer.de

2. Calibration of bifacial solar cells
Jochen Hohl-Ebinger, Wilhelm Warta
Fraunhofer Institute for
© Fraunhofer ISE
Fraunhofer Institute for
Solar Energy Systems ISE
Bifi-Workshop Konstanz
23.04.2012
www.ise.fraunhofer.de

3. 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

4. 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
spectralirradiance[Wm
-2
nm
-1
]
wavelength [nm]
But:
Bifaciality not addressed in STC AM1.5G Edition 2

5. 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%
Encapsulated
2x2 cm² Solar Cell < 0.7%
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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. €
Industrial
Solar Cell < 2.0%

6. 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
Chuck
T(λ)
E(λ)
R(λ)
© Fraunhofer ISE
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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

7. Estimation of Bifaciality Impact
Set 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 side
Irradiation from front Irradiation from rear
© Fraunhofer ISE
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400 600 800 1000 1200
0.00
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

8. 80
100
Reflection[%]
black foil
black anodized
brown anodized
Cu
brass1
brass2 with25
30
35
Transmission[%]
bifacial cell
A
B
C
D
Estimation of Bifaciality Impact
Additional Irradiation from Back Side
Transmission through test cells Chuck reflection
© Fraunhofer ISE
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200 400 600 800 1000 1200
0
20
40
60
Reflection[%]
Wavelength [nm]
brass2 with
ad. foil
gold plated
400 500 600 700 800 900 1000 1100 1200
0
5
10
15
20
Transmission[%]
Wavelength [nm]
D

9. Estimation of Bifaciality Impact
Calculated Impact
Back current contribution
20
25
30
35
100
120
140
160
1.4
1.6
1.8
2.0
2.2
2.4
8
10
12
Cell B Cell C
Backcurr.contrib.[%]
black foil brown anod. brass1 gold plated
black anod. Cu brass w. ad. foil
Cell A Cell D
front
contribbackrel
contribback
SR
SR
SR ,
, =
© Fraunhofer ISE
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1000 1200
0
5
10
15
20
1000 1200
0
20
40
60
80
1000 1200
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1000 1200
0
2
4
6
Backcurr.contrib.[%]
Wavelength [nm]
Usual Cu or gold plated chucks may add significant uncertainty

10. Estimation of Bifaciality Impact
Calculated Impact
Increase of Current
back current contribution
Cell 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%

11. Experimental Test
Special Chuck for Bifacial Cells
Exchangeable surface plates Measured reflectivity
30
40
50
60 black plastic foil
brown anodized
black anodized
grey anodized
Reflection[%]
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200 400 600 800 1000 1200
0
10
20
Reflection[%]
Wavelength [nm]

12. Experimental Test
Comparison to Simple Model
Measurement with black foil
as reference
Model fits for brown surface
Measurement 25% higher for
black and grey anodized
surfaces
12
14
16
18
20
22
24
26
28
backcurr.contrib.[%]
calculated
brown anodized
black anodized
grey anodized
measured
brown anodized
black anodized
© Fraunhofer ISE
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Likely reason:
SR for calculation measured
with small irradiation angle
Anodized surfaces reflect
highly diffusive
Light trapping increases current
200 400 600 800 1000 1200
0
2
4
6
8
10
12
backcurr.contrib.[%]
Wavelength [nm]
black anodized
grey anodized

13. Reference Chuck Reflection
black 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
backcurr.contrib.[%]
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 1200
0
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

14. White as Reference
Reflections (1000-1200nm):
65 to 95%
60
80
100
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200 400 600 800 1000 1200
0
20
40
60
R[%]
Wavelength [nm]
Paint
Paper
PTFE
deco-foil
gold plated
w-module foil

15. 80
100
Reference chuck reflection
How 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 1200
0
20
40
60
80
R[%]
Wavelength [nm]
Paint
Paper
PTFE
deco-foil
gold plated
w-module foil
200 400 600 800 1000 1200
0
20
40
60
80
Reflection[%]
Wavelength [nm]
brown anodized
Cu
brass1
brass2 with
ad. foil
gold plated

16. Reference Chuck Reflection
Black, White or Grey?
R = 0%
R = 100%
© Fraunhofer ISE
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R = 100%
R ≈ 90%

17. Reference Chuck Reflection
Black, 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

18. 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

19. Thank You!
© Fraunhofer ISE
19
Fraunhofer ISE CalLab PV Cells