Si - Enea

STATO E PROSPETTIVE DEL FOTOVOLTAICO IN ITALIA
26 giugno 2014
ENEA – Via Giulio Romano n. 41, Roma
Thin film photovoltaics: industrial strategies
for increasing the efficiency and reducing
costs
Anna Battaglia, Cosimo Gerardi
3SUN R&D
Catania
R&D Group
Strategy on photovoltaic
Innovating on products and solutions
 Increasing conversion efficiency through products
 Dedicated products completing our offer on power and
conversion
Innovating on Si technologies for
photovoltaic modules
 Investing to increase panels efficiency (thin film)
Market evolution
Source: EPIA
Future Trends
Source EPIA
Shift of module demand from EU to sunbelt regions
5
Solar Cell Technology: Why Thin Film Si?
Solar cell
Si raw material
Efficiency
Peak
power
Peak
power
c-Si
1200-1300 g/m2
16%
160W/m2
0.13W/g
TF-Si
5 g/m2
10%
100W/m2
20W/g
Large area
on glass
Transparency
Flexible
plastics
Thin Films for Building integrated PV
Solar panels replace the building materials and provide the electrical power to
the building energy consumptions
Air Mass effect
Low latitude areas are more rich in blue light
Thin film PV has higher sensitivity in blue light
Temperature Effect
TFSi  -0.23% every 1oC
c – Si  -0.45% every 1oC
Light intensity (kW/m2m)
Enhanced absorption: double junction/tandem
“spectrum splitting.”
Amorphous
Eg=1.8eV
«High» absorption
in the green-blue
Microcrystalline
Eg=1.1eV
«High» absorption
in the red-near I.R.
Wavelength (nm)
Micromorph cell efficiency 11-14%
Micromorph module efficiency 8.5-10.8%
Tandem configuration: Top a-Si:H, Bottom c-Si:H
1.00
TCO
EXTERNAL QUANTUM EFFICIENCY
0.90
0.80
0.70
0.60
0.50
a-Si:H
0.40
0.30
c-Si:H
0.20
0.10
0.00
250
350
450
550
650
750
850
950
1050
1150
Wavelength (nm)
Multiple junction devices with two junctions grown one upon
the other and current matched  spectrum splitting enables
higher absorption and higher efficiency
11
The PV Joint Venture in Catania: 3SUN
The biggest PV Italian fab competing
with the most important players of the
sector
Thin film multi-junctions modules are
manufactured in the innovative plant M6
built in Catania
Some number:
– 115.000 mq of usable surface
– 300 employees
– 160 MW/y in 2011
– 200MW/y in 2013
– …possible extension
Large area modules: 1m  1.4 m
More than 1,500,000 PV modules per year !
Large area modules on glass
Altomonte (CS - Italy): 8,2MW. 11 Millions of kWh. It can satisfy the needs of 4.000 families
Continuous focus on Cost/Wp reduction
From Single to Multiple Junctions
•
•
•
Single Junction
–aSi:H cell with enhanced light trapping – TCO and
Texturing  Efficiency: 6 to 8% on module
Double Junction / Tandem cell
–highest theoretical efficiency: combination of
absorber materials having band gap 1.8 eV (a-Si:H)
for the top and 1.1 eV (µc-Si:H) for the bottom cell.
 Efficiency 12.5% on cell  10% on module
Triple junction / Multiple Junction
–a-SiGe:H middle absorber
–a-Si:H/a-SiGe:H/ µc-Si:H
 Efficiency: 14-15% on cell, 12% on module
glass
textured TCO
Eg: 1.752eV
a-Si:H top absorber
Eg: 1.45eV
a-SiGe:H
middle absorber
Eg: 1.1eV
c-Si:H
bottom absorber
ZnO
Ag
Challenges of triple junction:
• Reduced throughput:~ 25% lower with
respect to Tandem
• Power stabilization weakness (light induced
degradation) of a-SiGe:H (15% to 18% LID
degradation factor)
 Multiple Junction no a-SiGe
High Voc and lower LID degradation
Eg gap optimization of a-Si:H
1.78
1.76
Eg (eV)
•
P1
P2<P1
µc-Si
formation
1.74
•
1.72
•
1.70
a-Si:H deposition condition at T=180oC,
RF f=13.56MHz  Eg between 1.7 and
1.76eV
Eg depends on RF power but is mainly
determined by SiH4/H2 ratio
Increasing H2 dilution leads to higher Eg
1.68
1.66
20
40
60
80
100
H2/SiH4 ratio
•
•
•
However the film quality can degrade with an increase of Si-H2 bonds 
transition to µc-Si:H at higher dilution after a critical point
A good trade-off between higher Eg and film quality can be reached with
Eg~1.74eV
The Eg tuning is fundamental for the energy gap matching of a triple or a
quadruple junction solar cell
High Band Gap a:Si with improved current by AR
EU FP7 Project
Light trapping
Asahi VU APCVD
(SnO2:F)
Asahi W
glass
light
TCO ~700nm
p-i-n a-Si:H~250nm
ZnO:B -MOCVD
p-i-n uc-Si:H
~1.6m
W text ZnO
TCO ~50nm
Back reflector
•
•
•
•
•
Texturing causes light scattering, increasing the optical path of photons
in silicon
Natural texturing can be achieved during the CVD deposition process
SnO2: Haze (Diffused T / Total T) is of ~10-15% at 550nm but very low at
longer wavelengths
ZnO Haze can be higher at longer wavelengths ensuring a positive effect
on µc-Si Cell
Double feature texture (possible both for SnO2 and ZnO): higher and
smaller texturing shapes can be reached (but not ready for production!)
High haze: high risk of cracks
uc-Si:H
a-Si:H
TCO
• High haze induce light
scattering and increments
the optical path in Si leading
to Isc increase.
• However the sharp shape of
TCO peaks can easily lead
to the formation of cracks
during the deposition of µcSi
• This causes lower Voc and
shunts
Optimized very high haze front TCO
•
•
•
•
•
•
SnO2
U-Valley AZO
ZnO, doped with B (BZO) or with Al
(AZO), has better transmittance
than SnO2 in the long wavelengths
range
SnO2:F (AGC VU) haze is ~10-15%
at 550nm and is very low at higher
wavelengths
ZnO can be obtained with higher
haze for higher wavelengths
However texturing can induce
cracks in the microcrystalline silicon
 lowering Voc and shunt
resistances
Needed ZnO with smooth U-shape
valley instead of V-shape
Sputtered ZnO:Al (AZO) + surface
treatment
Source: S. Kim et al, Solar Energy
Materials & Solar Cells, 2013
Back contact: n-type µc-SiOx:H: dielectric mirror as back reflector
EU FP7 – FAST TRACK (ECN,
EPFL, TUDELFT, ENEA, Julich,
3SUN, SOLAYER)
• Typically AZO (n=2) is used as
back reflector good mismatch with
n-µc-Si (n=3.8) for reflection.
• AZO/Ag bilayer as back contact
good Rs and providing a textured
surface that increases reflected
light path in the µc-Si
• n-µc-SiOx is studied as back
reflector (dielectric mirror)
because n<2
• Good trade-off n index vs
resistivity and FF at n=1.85 with
thickness of 300 – 500nm (*)
• Anisotropic electrical
performances high resistivity
along the planar direction lower
resistivity along the transversal
direction(**)
(*) S. Kim et al, Solar Energy Materials & Solar Cells,
2013
(**) P.Buehlmann,et al, Applied Physics Letters 91 (2007) 143505-1–143505-3.
C. Das, et al. Applied Physics Letters 92 (2008) 053509-1–053509-3.
Thin Film Si Roadmap
MultiJunc
Full spectrum
TCO
Triple J
Eg1
Eg2
Eg3
Eg4
Double J
Back contact
European and National Research Programs
•
Both ST and 3SUN are strongly involved in collaborative research activities on
PV with Research Centers in Europe and in Italy
Main EU Programs:
• Fast Track EU Project Consortium  Advanced Thin Film Si
–
•
Snapsun and Nascent EU Project Consortium  Quantum Dots Solar Cells
–
–
•
LITEN, Mantis, ECN Juelich, TUDelft, ST, Indian PV Research Consortium
ERG ENIAC EU Project Consortium  Advanced solar cells
–
•
•
•
LITEN, Tyndall, SAFC, Uppsala University, TUDelft, ST
Fraunhofer ISE, Freyburg Univ, Modena Univ, CNR IMM, Barcelona Univ, ST
AGATHA Europa-India Project Consortium  Light Trapping in Thin Film
–
•
EPFL Neuchatel, ECN Juelich, TUDelft, ENEA, 3SUN, Hyet)
Large European program involving main European centers and companies both for technology and electronic
integration in PV (ST is involved both in solar cells technology and electronic system developments)
National PON Programs:
Fotovoltaico: Consortium ST, ENEL, ENEA, CNR, Universities
Energetic, ST and 3SUN, Distretto Tecnologico Sicilia Micro e Nano Sistemi
3SUN is partner of the Bay Area Photovoltaic Consortium (Stanford
and Berkeley, CA)
Creating PV technology that industry will use.
• Collaboration
• Innovation
• Application
[email protected]

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