ZEBRA is an IBC bifacial solar cell technology developed by ISC Konstanz. Indoor measurements show the ZEBRA cell achieves a higher efficiency of 21.0% compared to 20.3% for the BiSoN n-PERT cell under front illumination. However, the BiSoN cell has a higher bifaciality ratio of 87.5% versus 75.0% for the ZEBRA cell. Outdoor module testing found the BiSoN module provided a 19.8% yield gain compared to 15.1% for the ZEBRA module, demonstrating the ZEBRA technology can enable bifacial performance despite having a lower bifaciality ratio.

1. ZEBRA: bifacial IBC technology
Valentin D. Mihailetchi, Andreas Halm, Corrado Comparotto, Severin Aulehla,
Radovan Kopecek
International Solar Energy Research Center e.V., Konstanz, Germany

2. Mihailetchi et.at., bifiPV workshop, Chambery, May 2014
Outline
2
General
• Bifacial cells and modules
• ISC Konstanz bifacial cell concepts
• BiSoN: n-PERT cell technology
• ZEBRA: IBC cell technology
Indoor STC results (front vs. rear illumination)
• IV data
• Spectral Response
• Comparisons, predictions, …
• BiSoN and ZEBRA modules
Outdoor test results
• ZEBRA vs. BiSoN (IBC vs. n-PERT)
Summary

3. Mihailetchi et.at., bifiPV workshop, Chambery, May 2014 3
n-type solar panels producers on the market:
Producer
Cell
technology
Cell
efficiency
Module
efficiency
SunPower IBC (Maxeon) >24% 21.5% Monofacial cells
Panasonic HIT 22.0% 19.4% Bifacial cells
Yingli n-PERT (Panda) 19.5% 17.1% Bifacial cells
PVGS n-PERT (EarthOn) 19.5% 16.2% Bifacial cells and panels
LG n-PERT (NeoN) 21.3% 18.3% Bifacial cells
Bifacial cells and modules
Screen printed metallization

4. Mihailetchi et.at., bifiPV workshop, Chambery, May 2014 4
n-type solar panels producers on the market:
Producer
Cell
technology
Cell
structure
SunPower IBC (Maxeon) Monofacial cells
Panasonic HIT Bifacial cells
Yingli n-PERT (Panda) Bifacial cells
PVGS n-PERT (EarthOn) Bifacial cells and panels
LG n-PERT (NeoN) Bifacial cells
Bifacial cells and modules
All (produced) bifacial cells have both sides contacts (BSC) !!

5. Mihailetchi et.at., bifiPV workshop, Chambery, May 2014 5
Is BSC the ultimate bifacial cell technology ?
Bifacial cells and modules
front side rear side

6. Mihailetchi et.at., bifiPV workshop, Chambery, May 2014 6
ISC’s bifacial cell concepts
BiSoN (n-PERT)
• BSC cell technology
• n-type 6-inch Cz Si
• homogeneous p+ and n+ diffusions
• front side textured, rear side flat
• IBC cell technology
• n-type 6-inch Cz Si
• homogeneous p+ and n+ diffusions
• front side textured, rear side flat or
textured
ZEBRA (IBC)

7. Mihailetchi et.at., bifiPV workshop, Chambery, May 2014 7
ISC’s bifacial cell concepts
BBr3 diffusion, p+ emitter
POCl3 diffusion, n+ BSF
Saw damage etching, alkaline texturization, cleaning steps
SiO2/SiNx passivation & ARC
Screen printing and firing of p and n fingers, busbars
PECVD masking
Laser ablation
BiSoN (n-PERT) ZEBRA (IBC)

8. Mihailetchi et.at., bifiPV workshop, Chambery, May 2014 8
ISC’s bifacial cell concepts
BiSoN (n-PERT) ZEBRA (IBC)
What if both cell concepts cost the same (in €/Wp)?

9. Mihailetchi et.at., bifiPV workshop, Chambery, May 2014 9
IV data
Cell type
area
[ cm2 ]
JSC
[ mA/cm2 ]
VOC
[ mV ]
FF
[ % ]
h
[ % ]
Pmax
[W]
239 39.4 652 78.9 20.3* 4.85
239 41.2 646 79.1 21.0 5.03
IV measurements of best cells (front side illumination):
- on a reflective chuck
*Independently confirmed by CalLab PV cells at Fraunhofer ISE

10. Mihailetchi et.at., bifiPV workshop, Chambery, May 2014 10
IV data
Cell type illumination
JSC
[ mA/cm2 ]
h
[ % ]
Pmax
[W]
Metallization area
[ % ]
front 39.3 20.1 4.8 6.3 ± 0.24
rear 34.7 17.6 4.2 7.7 ± 0.32
front 40.8 20.9 5 0
rear 30.5 n/a(15.6*) n/a(3.7*) 17.5 ± 0.64
IV measurements of best cells (front and rear side illumination):
- on a black coated chuck
Is ZEBRA cell concept interesting for bifacial applications?
?OK
*Estimated value assuming the same Voc and FF from front side illumination

11. Mihailetchi et.at., bifiPV workshop, Chambery, May 2014 11
IQE data
IQE measurements from front and rear side:
1. front side response:
ZEBRA  BiSoN
2. rear side response:
ZEBRA < BiSoN
0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
BiSoN (front)
BiSoN (rear)
ZEBRA (front)
ZEBRA (rear)
IQE(-)
Wavelength (µm)

12. Mihailetchi et.at., bifiPV workshop, Chambery, May 2014 12
Comparisons, predictions
IV measurements of best cells (front and rear side illumination):
ƞ rear / ƞ front = 75 %ƞ rear / ƞ front = 87.5 %
1. ZEBRA is the best for
front side illumination
2. BiSoN has the best
bifacial ratio
BiSoN ZEBRA
15
16
17
18
19
20
21
Efficiency(%)
rear illumination
front illumination
(5 W)
(4.8 W)

13. Mihailetchi et.at., bifiPV workshop, Chambery, May 2014 13
BiSoN and ZEBRA modules
STC indoor measurements (25°C, 1 sun)
illumination
Pmax
[W]
Prear/Pfront
(module)
Prear/Pfront
(best cell)
Front 17.68
0.88 0.87
rear 15.71
front 18.22
0.65* 0.75**
rear 11.84*
4 cells modules with transparent back sheet:
Front side photograph
Front side photograph
* with 3 mm wide ribbons (24% shaded area)
** estimated from JSC(rear)/JSC(front)

14. Mihailetchi et.at., bifiPV workshop, Chambery, May 2014 14
Comparisons, predictions
Calculated modules performance with additional rear side illumination:
1. ZEBRA is as good as a
dedicated bifacial module
for rear side light
intensities <0.3 sun.
2. ZEBRA has the
advantage of generating
more power in monofacial
or at low albedo
contributions
0.0 0.1 0.2 0.3 0.4 0.5
17
18
19
20
21
22
23
24
25
26
rear side light intensity (suns)
4cellsmodulepower(W)
BiSoN, 2mm ribbons (Prear
/Pfront
=88%)
ZEBRA, 3mm ribbons (Prear
/Pfront
=65%)
ZEBRA, 2mm ribbons (Prear
/Pfront
=75%)

15. Mihailetchi et.at., bifiPV workshop, Chambery, May 2014 15
Comparisons, predictions
Cells (or modules) fabrication:
• can have asymmetric front/rear performance
• should be optimized based on front side performance
Reflected
sunlight
Direct
sunlight
Slanted
Reflected
sunlight
Direct
sunlight
Horizontal
Direct
light
Reflected
light>
Typical outdoor installation of a bifacial module:
rear side irradiance ≤ 0.3 × front side irradiance !?

16. Mihailetchi et.at., bifiPV workshop, Chambery, May 2014 16
Outdoor test results
Outdoor test conditions of the 4-cells modules:
• ground reflectivity →  70% (300-1200nm) (white back sheet)
• orientation and inclination → South, 30°
• measurement interval → every 5 min. (10 sec. module measure time)
• all modules assumed to have the same temperature during an IV scan
• front glass taped black outside cell area
• one additional module was used to measured the back side light intensity
Photograph of the outdoor setup
ZEBRA
black BS
ZEBRA
transparent BS
ZEBRA
(rear irradiance)
BiSoN
transparent BS
BiSoN
black BS
ZEBRA (2)
transparent BS

17. Mihailetchi et.at., bifiPV workshop, Chambery, May 2014 17
Outdoor test results
15-May-14
16-May-14
17-May-14
18-May-14
19-May-14
20-May-14
21-May-14
0
200
400
600
800
1000
1200
1400
Directirradiance(W/m
2
)
One sunny day chosen for modules comparison !
Outdoor test conditions of the 4-cells modules:

18. Mihailetchi et.at., bifiPV workshop, Chambery, May 2014 18
Outdoor test results
Outdoor modules performance (bifacial versus monofacial):
Module temp. at
noon: 48°C !!!
1. Monofacial modules:
ZEBRA = BiSoN
2. Bifaical modules:
ZEBRA ≤ BiSoN
08:00 10:00 12:00 14:00 16:00 18:00
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
ZEBRA (transparent BS)
ZEBRA (black BS)
Bison (transparent BS)
Bison (black BS)
Direct irradiance (suns)
Indirect irradiance (suns)
time (HH:mm)
Pmax
/Pmax(STC)
(-)

19. Mihailetchi et.at., bifiPV workshop, Chambery, May 2014 19
Outdoor test results
One day yield comparison (bifacial versus monofacial):
0.26
0.28
0.30
0.32
0.34
0.36
Energyyield(Wh/Wp)
BiSoN
blackBS
BiSoN
transparentBS
ZEBRA
blackBS
ZEBRA
transparentBS
+ 15.1%
+ 19.8%
08:00 10:00 12:00 14:00 16:00 18:00
5
10
15
20
25
30
35
40
45
50 Bifacial gain:
BiSoN
ZEBRA
Irradiance:
Indirect/Direct
time (HH:mm)
(%)
ZEBRA is a bifacial IBC technology !!!

20. Mihailetchi et.at., bifiPV workshop, Chambery, May 2014 20
Summary
Best cell efficiency:
 20.3%, n-PERT BiSoN
 21.0%, IBC ZEBRA
Bifaciality ratio (@ cell level):
 87.5%, n-PERT BiSoN
 75.0%, IBC ZEBRA
Bifacial/Monofacial yield gain:
 19.8%, n-PERT BiSoN
 15.1%, IBC ZEBRA
Acknowledgements:
The project HERCULES has received funding from the European Union’s Seventh Programme for research,
technological development and demonstration under grant agreement No 608498.