This document summarizes several bifacial solar cell technologies: 1) Heterojunction solar cells show similar efficiency potential for bifacial (20%) and non-bifacial configurations but rear efficiency is lower (17%) due to reflectivity limitations. 2) N-type PERT cells initially showed lower rear efficiency (16%) but modifying the process to use a lighter rear doping achieved 92% of front efficiency (18%). 3) Ion implantation and co-diffusion processes simplify PERT cell fabrication and could achieve the highest efficiency (19.5%) at lowest cost through fewer steps.

1. Overview of several bifacial
solar cells technologies
1
solar cells technologies
Y. Veschetti, R. Cabal, D. Munoz, S. Harrison, S. Gall
INES-CEA

2. Outline
•Two cells technologies compatible for bifacial application
a-Si:H/c-Si Heterojunction (HET) and PERT n-type Si
• HET R&D and Industrial development
Bifacial properties
• n-type PERT solar cells development
bifiPV Workshop Y. Veschetti
• n-type PERT solar cells development
Overview of various processes
Initial bifacial properties
Process adaptation to improve bifacial behaviour
•Conclusion et perspectives

3. Two bifacial solar cells on n-type silicon
SiO2 / SiNx
SiO2/
SiNx
Si(n)
P- BSF
B-Emitter
Rear electrode
Front electrode
a-Si:H/c-Si heterojunction PERT n-type cell
Courtesy of Sanyo HITTM cell
bifiPV Workshop Y. Veschetti
nmax 23.7% (lab)
n = 21% prod [1]
T (°C) coefficient
Simple process < 10 steps
Low T - process
Thin substrate compatible
nmax = 19.7% (lab) [2]
n ~ 18.5 - 19% prod
Simple process < 10 steps (potentially)
Compatible with p-type prod line
Thin substrate compatible
SiNx
Rear electrode
[1] 2AO.2.6, SANYO Electric, 26th EPVSEC (2011)
[2] Boescke, Bosch Solar, n-type workshop, Konstanz (2011)

4. Heterojunction development at INES
(n) c-Si
(i)/(p) a-Si:H
(i)/(n) a-Si:H
ITO
ITO
Ag
(n) c-Si
(i)/(p) a-Si:H
(i)/(n) a-Si:H
ITO
ZnO:B
Ag
Non bifacial configuration Bifacial configuration
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ITO
Ag
ZnO:B
Al
Best cell result certified S (cm²)
Jsc
(mA.cm²)
Voc (mV) FF (%) η (%)
Non bifacial cell (FZ) 105.0 36.3 732 77.9 20.7
Bifacial cell (Cz) 148.6 35.2 729 77.9 20.0
• Same amount of fabrication steps for both configuration
• Similar efficiency potential
Respective interest will be determined at the module level

5. HET: Bifacial characteristic
Increased reflectivity on the rear side due to a larger metal coverage
Substrate & ITO resistivity limitations
Illumination Reff (%)
Jsc
(mA.cm²)
Voc (mV) FF (%) η (%)
Front side 6.7 34.6 719 74.3 18.5
Rear side 9.6 31.8 717 74.4 17.0
90
100
InternalQuantumefficiency
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0
10
20
30
40
50
60
70
80
90
300 500 700 900 1100
Wavelength (nm)
InternalQuantumefficiency
(%)
Front side
Rear side
Efficiency ratio
RS/FS = 92%
Room for improvement by
optimizing the a-Si:H(n+)
layers

6. • 35MW HET Labfab at INES Start up by the end of 2011
Towards Industrial Application in Europe :
Silicon Heterojunction solar cells :LABFAB
bifiPV Workshop Y. Veschetti
• The PECVD and TCO line are working since december 2011
• Wet & screen printing hardware set up ongoing
• First cells after 3 months show high efficiency!!! >19%
• Uniformity tests ongoing on 90x90 cells 125PSQ

7. N-type PERT cell – Reference INES process
Si(n) Phosphorus BSF
Boron Emitter
Thermal SiO2 SiNx
Rear s.p. grid
Front s.p. grid
Thermal SiO2 SiNx ARC
Texturing
Rear diffusion barrier
BCl3 emitter diffusion
BSG and barrier etching
POCl3 BSF diffusion
Front diffusion barrier
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POCl3 BSF diffusion
Dry oxidation
Front & Rear PECVD SiN
PSG and barrier etching
Front & Rear SP grid
Contact firing
148.6
cm² Cz-Si
Jsc
(mA/cm²)
Voc (mV) FF (%) η (%)
Best 38.1 635.0 79.5 19.3
13 steps

8. Heading towards higher efficiencies
N Cz-Si
p+
N+
SiO2/SiN
N Cz-Si
p+
N+
SiO2/SiN
Implied Voc Cell Voc
Measurement of implied Voc
prior to metallization steps
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Gap between implied Voc and final cell Voc
SiO2/SiN SiO2/SiN
Issues on Voc limitation:
Impact of SP metallization

9. Co-diffused n-type cell process
3 fewer steps
Process simplification 1: co-diffusion
Reference n-type cell process
KOH texturing
Diffusion barrier
BCl3 diffusion
Barrier & BRL removal
Diffusion barrier
POCl3 diffusion
BSG/PSG removal
PECVD SiO2(B) deposition
Co-diffusion POCl3 furnace
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Cell result
Jsc
(mA.cm²)
Voc (mV) FF (%) η (%)
Cz (4Ω.cm; 180µm;
138.3cm²)
Ave. (x9) 37.2 631.1 79.2 18.6
best 37.2 632.9 80.2 18.9
BSG/PSG removal
Thermal oxidation
Front & rear SiN
Front & rear screen-printing
Co-firing

10. Process simplification 2: Ion Implantation
Standard n-type process Implantation process
Front B-implant
Back P-implant
Texturing
Rear side diffusion barrier
Front Boron diffusion
Barrier and BRL etching
Front diffusion barrier
Back POCl3 diffusion
Standard n-type process Implantation process
Front B-implant
Back P-implant
Texturing
Rear side diffusion barrier
Front Boron diffusion
Barrier and BRL etching
Front diffusion barrier
Back POCl3 diffusion
Collaborative project with
- Potentially lowest $/Wp solution
Higher efficiency at lower cost
- Higher cell yield
4 Fewer steps and lower breakage
- Best wafer-wafer doping repeatability
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Thermal oxidation
Front and back PECVD SiN
Back P-implant
Thermal oxidation
Front/back metallization
Back POCl3 diffusion
Barrier and PSG etching
Thermal oxidation
Front and back PECVD SiN
Back P-implant
Thermal oxidation
Front/back metallization
Back POCl3 diffusion
Barrier and PSG etching
19.5% efficiency potential
Process under development
FF limitation being addressed
- Best wafer-wafer doping repeatability
better binning, higher value
Cell result
Implied
Voc (mV)
Voc (mV)
Jsc
(mA.cm²)
FF (%) η (%)
Cz (239cm²) 655 630.7 38.4 76.2 18.5
Co-firing

11. Cell results: 156PSQ Cz wafers
Bifacial performance of reference BCl3 process
80
100
InternalQuantum
Illumination Reff (%) Voc (mV) Jsc (mA.cm²) FF (%) η (%)
Front side 5.3 630.7 38.6 78.3 19.1
Rear side 6.7 624.8 33.2 78.6 16.3
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Ratio efficiency
rearside/front side = 85%
Higher reflectivity
Heavy BSF non adapted
0
20
40
60
80
300 500 700 900 1100
Wavelength (nm)
InternalQuantum
Efficiency Front side
Rear side

12. Process modification for bifacial application
Texturing
Rear diffusion barrier
BCl3 emitter diffusion
BSG and barrier etching
POCl3 BSF diffusion 840°C
Front diffusion barrier
Identical grid on both side
Use of wet oxidation at 700°C
No distribution of P- BSF
Lighter doping (60 Ω/ )
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Wet oxidation
Front PECVD SiN
PSG and barrier etching
Front & Rear SP grid
Contact firing
Rear PECVD SiN
Pconcentration(cm-3)

13. Bifacial properties with adapted process
Illumination Voc (mV) Jsc (mA.cm²) FF (%) η (%)
Front side 627.4 38.8 77.0 18.7
Rear side 624.5 35.3 78.0 17.2
60
80
100
InternalQuantum
efficiency(%)
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0
20
40
300 500 700 900 1100
Wavelength (nm)
InternalQuantum
efficiency(%)
Front side
Rear side
Ratio η rear side/ front side = 92%
Positive effect of a lighter BSF

14. CONCLUSION
Overview of two high efficiencies cell technologies HET & PERT cell
HETEROJUNCTION:
Efficiency ratio RS/FS = 92%
Room for improvement (work on a-Si:H(n+))
Limitation of rear side reflectivity (grid density)
PERT cell:
bifiPV Workshop Y. Veschetti
PERT cell:
Description of simple fabrication processes
Ratio of efficiency RS/FS = 92% using adapted process
Room for improvement without degrading the front performance
Perspectives:
Fabrication of bifacial modules
B. Soria, presentation Tuesday at 11.20

15. Acknowledgements :
Workshop organizers
LCP team
bifiPV Workshop Y. Veschetti
Merci de votre attention
1525/04/2012
Many thanks for your attention !
CONTACT : yannick.veschetti@cea.fr