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)
Source: EPIA Market evolution
Source EPIA Future Trends
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/m 2 16% 160W/m 2 0.13W/g TF-Si 5 g/m 2 10% 100W/m 2 20W/g Large area on glass Flexible plastics Transparency
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 1 o C c Si -0.45% every 1 o C
Light intensity (kw/m 2 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%
EXTERNAL QUANTUM EFFICIENCY Tandem configuration: Top a-si:h, Bottom c-si:h 1.00 0.90 TCO 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 a-si:h c-si:h 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 Large area modules: 1m 1.4 m Some number: 115.000 mq of usable surface 300 employees 160 MW/y in 2011 200MW/y in 2013 possible extension 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 Eg: 1.75 2eV Eg: 1.45eV glass textured TCO a-si:h top absorber a-sige:h middle absorber 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 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) Eg: 1.1eV c-si:h bottom absorber Multiple Junction no a-sige High Voc and lower LID degradation ZnO Ag
Eg (ev) Eg gap optimization of a-si:h 1.78 1.76 1.74 1.72 P1 P2<P1 µc-si formation a-si:h deposition condition at T=180 o C, RF f=13.56mhz Eg between 1.7 and 1.76eV Eg depends on RF power but is mainly determined by SiH4/H2 ratio 1.70 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 (SnO 2 :F) Asahi W light glass 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 µc- Si This causes lower Voc and shunts
Optimized very high haze front TCO 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 SnO2 U-Valley AZO 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 Triple J MultiJunc Full spectrum TCO 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 EPFL Neuchatel, ECN Juelich, TUDelft, ENEA, 3SUN, Hyet) Snapsun and Nascent EU Project Consortium Quantum Dots 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 LITEN, Mantis, ECN Juelich, TUDelft, ST, Indian PV Research Consortium ERG ENIAC EU Project Consortium Advanced solar cells 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
Anna.Battaglia@3sun.com