R&D R&D sul sul fotovoltaico fotovoltaico in STM in STM Marina Foti IMS R&D STMicroelctronics STMicroelctronics Convegno su Tecnologie, tecniche impiantistiche e mercato del fotovoltaico 15 Ottobre 2012 Mondello (PA)
Outline • Thin film module technology • Amorphous silicon (a-Si:H) and microcrystalline ( µ c-Si) • Tandem and multiple junction solar cells • Enhancement of light absorption in thin film Si • Development of TCO front and back electrodes • Next steps on light trapping • Thin film silicon outlook • TF PV flexible application for smart systems
A New PV Joint Venture: 3SUN A New PV Joint Venture: 3SUN STMICROELECTRONICS, is one of the ENEL GREEN POWER, ENEL Group Company, SHARP CORPORATION , a Japanese largest manufacturers of semiconductors dedicated to the development and Company, which operates at global level in in the world with customers in all management of activities related to energy the manufacturing and distribution of electronics segments. The Corporate production from renewable sources at an consumer products (LCD TV, LED TV, ecc). A headquarter is in Geneva, advanced international level, which operates in Europe leading company at global level in the research and development centers in 10 and the American Continent. It is a leading photovoltaic sector (Solar Cells, and countries, 14 main manufacturing sites Company in this sector at global level. Electronic Devices). and sales offices all around the world.
3SUN 3SUN – Thin Film Multi Thin Film Multi-Junction Modules Junction Modules Fab Fab Numbers: The biggest PV Italian fab destined to compete with the most important players • 240 000 m 2 surface area of the sector • 60 000 m 2 Fab area Thin film multi-junctions modules are manufactured in the innovative plant M6 • 300 employees built in Catania • 160 MW/y (2011) Large area modules: 1m × 1.4 m • 240MW/y, …possible extension
Why TF Solar Cells? Solar cell Si raw material Efficiency Peak power Peak power 1200-1300 g/m 2 160W/m 2 c-Si 16% 0.13W/g 5 g/m 2 100W/m 2 TF-Si 10% 20W/g Amorphous / glass Large area Amorphous or tandem / flex multi-junction / glass
Technology options: thin film Technology options: thin film vs vs Si wafers Si wafers Processing of wafers BULK Si SOLAR CELLS THIN FILMS Series connection of individual solar Monolithic integration cells (series connection by lasering) Mature technology but needs CVD on very large areas a lot of Si Potential for ultra low costs
Large area modules on glass Large area modules on glass Altomonte (CS - Italy): 8,2MW. 11 Millions of kWh. It can satisfy the needs of 4.000 families
Other than Ground PV Plants… Other than Ground PV Plants… Various Applications Innovative Future Solutions Parking area Roof Easy installation and no particular maintenance or cleaning required. No specific accurate angle to the sun. Perfect integration with the environment. Residential, Commercial and Industrial Roof Roof Installation following the roof profile and Integration on Building Design good performance at any slope of the Building front designed with Glass/Glass roof. Nice appearance integrated to the Frameless Thin-Film PV Modules building design. Car, Truck and Trailer PV Roof Deserts and hot climate Countries Supplying high performance even at 50~60°C thanks to the low temperature coefficient (-0,24%/°C). Good performance even when the panels would be partially Stand-alone Applications covered by dust and sand thanks to the powered by PV panels feature to produce energy with diffuse E.g. Water sweetening kit light.
Thin film � PV on flexible substrate
Substrate and superstrate configurations Thin film deposition at low temperatures on large area substrates Transparent sealing Opaque sealing Front TCO Back electrode (Ag, Al, white pvb) BC TCO BC TCO a-Si:H, uc-Si:H or multiJ a-Si:H, uc-Si:H or multiJ a-Si:H, uc-Si:H or multiJ BC TCO Back electrode Front TCO glass Metal, plastic.. Superstrate Substrate
Thin Film Module process flow Thin Film Module process flow 11 Glass with Laser scribe TCO Isolation P4 • The modules are fabricated Laser edge Edge seaming deletion monolithically on a glass substrate during front end Laser scribe P1 In line solar process TCO simulator (IV) Bus bars and Cleaning wires connection • The back end is dedicated to Lamination Lamination add electrical connection, add electrical connection, PECVD with PVB and deposition protection layers, frame and back glass junction box Laser Scribe cell J-box P2 connection • Typical process flow tandem 2 nd in line solar PVD deposition modules Back contact simulator (IV) • 1 x 1.4 m 2 Laser scribe Packaging back contact P3
Thin Thin film on film on glass glass: FEOL : FEOL process process Glass with TCO Layer Cleaning Laser Scribe P1 PECVD Deposition a-Si:H -pin SOIR �c�Si:H � pin Laser Scribe P2 TCO Deposition Laser Scribe Back Contact P3 Cleaning
Scheme of thin film module Scheme of thin film module load back contact - + cell TCO glass
Glass Size Matters for Thin Film High automation PECVD PECVD deposition TCO deposition large area and high throughput is needed to achieve low cost/Wp
TF Silicon Costs breakdown 6% 15% 14% 6% 5% 3% 3% TCO 5% Gas/Chem Target 46% Back glass Encapsulant Terminal Box Silver Paste/ Bus Bar/ Packing/Other Lead Wire / MultiFrame J
Amorphous silicon Amorphous Si: a-Si:H layers were first deposited by R. Chittick (1969) experimenting with SiH4 in a plasma reactor. First systematic study by Spear et al Phil Magaz, 33, 935 (1976) Tetrahedrally Amorphous Si: bonded absence of c-Si structure Long range order
Distribution of density of allowed energy states for electrons due to the disorder direct optical transitions are not forbidden in amorphous Si Eg = ~1.8 eV better light absorption than c-Si 17
• Deposited by plasma-enhanced CVD of SiH4 at 150- Amorphous Si for thin film PV 300 � C. Low gas utilization (10-30%). Heavily hydrogenated 1-10 at.% H. • PN (PIN) junctions formed through boron or phosphorous containing gases. • Total thicknesses in some cases below 1 µm (100 times thinner than c-Si). • Multiple junction devices with two or three junctions grown one upon the other and current matched. The BIG three challenges • Improve efficiency from 6-8% up to 12-15%; • Minimize or eliminate the self-limited degradation • Increase deposition rate
Staebler Staebler-Wronski Wronski Effect Effect • Exposure to light induced degradation, which stabilizes with time • Typically after 1000h of continuous light soaking at 1 Sun AM1.5G L • New dangling bonds (from 1e15 to 1E17 cm-3) are created under light exposure • Degradation is recovered after annealing at T<150C annealing at T<150C Typically 10-13 % of degradation For a-Si:H of 150-300nm Limitation on the thickness
Amorphous a-Si:H: p-i-n Drift charge transport – p-i-n junction p i n • Carriers are photogenerated in the intrinsic region and collected by drift 20
Amorphous and Microcrystalline silicon Two materials with the same process the same process PECVD a-Si:H Eg=1.8eV µ µ c-Si:H Eg=1.1 eV µ µ Columnar microstructure 21
Enhanced absorption: double junction/tandem Enhanced absorption: double junction/tandem Amorphous “spectrum splitting.” Eg=1.8eV nsity (kW/m 2 µ m) «High» absorption in the green-blue Microcrystalline Eg=1.1eV «High» absorption in the red-near I.R. Light intens Wavelength (nm) Micromorph cell efficiency 11-14% Micromorph module efficiency 8.5-10.8%
, Bottom µ µ µ c-Si:H µ µ µ µ µ Tandem configuration: Top Tandem configuration: Top a-Si:H Si:H, Bottom Si:H 1.00 0.90 AL QUANTUM EFFICIENCY 0.80 TCO 0.70 0.60 0.50 a-Si:H a-Si:H EXTERNAL Q 0.40 0.30 µ µ c-Si:H µ µ 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 23
From Single From Single to to Multiple Multiple junctions junctions • Single Junction –aSi:H cell with enhanced light trapping – TCO and Texturing • Double Junction / Tandem cell –highest efficiency: combination of absorber materials having band gap 1.8 eV and 1.1 eV for the top and bottom cell.. • Triple junction –aSiGe:H middle absorber more than 12% on large areas • Best stabilized efficiencies above 12% glass glass • Higher efficiencies (from 12 to 20%) are possible with additional junctions textured TCO a-Si:H top absorber • But so far : a-SiGe:H •Reduced throughput:~ 30% lower for triple middle absorber Junction µ µ c-Si:H µ µ •Costs ~ 20% higher than tandem bottom absorber •Despite the lower efficiency of tandem ZnO technology � higher throughput in MW/years Ag
Issues limiting a-Si:H and µ c-Si:H 25 efficiencies • a-Si:H : Voc too low 0.9V instead of 1.4V (bandtails contacts) • µ c-Si:H: Low Jsc. Improve absorption, light trapping
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