Wafer-based silicon PV technology Status, innovations and outlook

Wafer-based silicon PV technology Status, innovations and outlook Wim Sinke ECN Solar Energy, Utrecht University & European PV Technology Platform www.ecn.nl

Contents Wafer-based silicon photovoltaics - features - market position - history and state-of-the-art Cell and module efficiencies - achievements so far - limiting factors - options for further improvement Towards integration of cell and module designs Cost reduction potential Outlook

Wafer-based crystalline silicon Bell - ½ century of manufacturing experience - huge technology base (materials, processes & device designs) - extensive track record (performance, lifetime & reliability - highest performance of flat-plate technologies - further cost reduction (preserving efficiency) is main overall challenge

Cell & module technologies ( flat plate ) Commercial: wafer-based crystalline silicon - monocrystalline (cut) - multicrystalline (cut) - ribbons ( 80% of global market) ECN Commercial: thin films - silicon - copper-indium/gallium-diselenide (CIGS) - cadmium telluride (CdTe) ( 20% of global market) Helianthos ECN/Holst Centre Nanosolar Pilot production and laboratory: emerging and novel technologies - super-low-cost concepts (printed organic & inorganic, etc.) - super-high-efficiency concepts

Cell & module technologies ( flat plate ) Commercial: wafer-based crystalline silicon module efficiencies 13 ~ 19% ECN Commercial: thin films module efficiencies 6 ~ 12% Helianthos Pilot production and laboratory: emerging and novel technologies ECN/Holst Centre Nanosolar (various efficiencies; most not yet commercially available)

The silicon PV value chain Silicon feedstock Crystal Wafer Solar cell Solar module PVsystem Solar electricity

Typical current industrial silicon solar cell B-doped substrate (base) P-doped front (emitter) Al-doped rear (back surface field, BSF) SiN anti-reflection coating / passivation layer Ag contacts

Cell design options Standard: front emitter Rear emitter / front surface field Sanyo Heterojunction Metallisation Wrap Through (MWT) Emitter Wrap Through (EWT) SunPower Back Junction Back Contact (BJBC) Carrier collection at front Carrier collection at rear Front and rear contacted All rear contacted 9

World record monocrystalline silicon cell (efficiency 25.0%) Passivated Emitter and Rear Locally diffused (PERL) cell Zhao, Wang & Green, UNSW (1999)

World record monocrystalline silicon large-area module (efficiency 21.4%) based on Interdigitated Back Junction, Back Contact cells Courtesy SunPower Corp. ancestor (1986): Point Contact Solar Cell (>28% under concentration) Swanson, Sinton & King

Very high efficiency monocrystalline silicon large-area module (efficiency %) based on HIT (Heterojunction with Intrinsic Thin layer) cells Courtesy Sanyo Electric Co., Ltd.

Historic efficiency development crystalline silicon cells and modules (rounded values) 30 best laboratory cells (monocrystaline Si) best laboratory cell (multicrystalline Si) 20 15 10 typical commercial modules 5 Year 2015 2010 2005 2000 1995 1990 1985 1980 1975 1970 1965 1960 1955 0 1950 Efficiency [%] 25

Photovoltaic conversion: basic process and losses recombination energy gap (Si: 1.12 ev) X X X generation

Solar spectrum and spectral losses 1.6 UV visible infrared solar spectrum (Air Mass 1,5; 1000 W/m2) power [W/(m2.nm)] available for conversion in crystalline Si 1.2 X 1100 nm 1.1 ev = Si bandgap 0.8 0.4 X 0.0 400 800 1200 1600 2000 2400 wavelength [nm] courtesy John Schermer, KUN Courtesy John Schermer, RUN, NL

current Solar cell electrical characteristic: voltage and curve factor losses Voc Jsc FF 1 Pmax Vmax voltage Voc Jsc FF Plight Imax Jmax J J0 e qv kt 1 JL Pmax VOC kt J L ln 1 Egap q J0

Crystalline silicon solar cell conversion efficiencies: limits and losses (indicative)

From lab to fab Trade-off between cost & performance Small area large area Best average Lower Si material quality, highly doped regions, surfaces and contacts additional recombination: Jsc, Voc Additional optical losses (reflection & transmission): Jsc Additional resistive losses: FF Efficiency range multicrystalline Si cells: ~14-17% Efficiency range monocrystalline Si cells : ~16-22%

Selected options for further improvement Minimize recombination: - improve material quality manage defects and impurities, use n-type Si - reduce heavy doping effects local doping, selective emitters - effective surface passivation SiNx, SiO2, Al2O3, asi, etc. - low-recombination contacts heterojunctions Minimize optical losses - reduce reflection, apply light trapping coatings & textures, plasmonic structures? - reduce shadow losses rear contact designs Minimize resistive losses - increase conductance advanced electrode architectures and materials, rear contact designs

Example: understanding and managing impurities

Understanding and managing impurities: effects of Fe, Ni, Cr added to Si feedstock Bottom Middle Top Efficiency [%] 16 14 Ref Fe 50 ppm wt 12 Cr 40 ppm wt 10 Ni 40 ppm wt Fe 200 ppm wt 8 0% 20% 40% 60% Position in the ingot [%] 80% 100%

Understanding and managing impurities: replace p-type Si by n-type Si (n+) Phosphorous emitter (p+) Boron emitter Tio/+ Ti+/o Fe+/o Back contact B-O2i p-type silicon substrate Feo/+ (n+) n-type silicon substrate Phosphorus BSF Major metal impurities positively charged in p-type, neutral in n-type. p-type dopant B forms B-O recombination center

Example: the importance of low-recombination contacts VOC kt J L ln 1 q J 0 e J 0b J 0e fcont J 0e, cont (1 fcont ) J 0, pass Well-passivated emitter: ~30 fa/cm2 or less Ohmic contacts: ~1000-2000 fa/cm2 With only 5% contact coverage, 50-100 fa/cm2 from contacts

Low-recombination contacts Transfer majority carriers without (resistive) loss Reflect minority carriers without recombination loss minority carrier mirrors Practical solution: - silicon heterojunction contacts Graph: Miro Zeman, DUT, NL, 2010 25 12-10-2010

Example: apply light trapping Full absorption (even) in very thin substrates low bulk recombination (high Voc) combined with high Jsc allow the use of low-quality materials plasmon pictures Amolf

Example: The best of both worlds (SunPower & Sanyo) the IBC-HIT cell No shadow losses on front No optical absorption losses on front Very low contact recombination at rear (beyond asi) M. Tucci et al., BEHIND cell concept 27 12-10-2010 Work in collaboration with Univ. Rome, ENEA, ECN, Univ. Utrecht, and others

Anatomy of a standard module Module consists of: - glass superstrate - encapsulant (EVA) - interconnected solar cells - encapsulant (EVA) - rear-side foil Finishing: - framing - junction box - cabling and wiring

Example: Metallisation Wrap-Through (MWT) cell A n+ Base: p n+ A Base: p Cross-section AA n+ Base: p n+ Base: p n+ Base: p

MWT cells & module: single-shot module manufacturing inspired by Surface-Mount Technology (SMT) Photo: GEC, Inc.

Design and manufacture of MWT module Rear of cell Section of conductive foil Equipment by Eurotron (NL)

MWT cells and modules Technology - 120 µm (Deutsche Solar) - and 160 µm (REC) mc-si wafers Conductive adhesive (alternative: low-t solder) Patterned rear-side foil Novel module line; zero cell breakage Module results - Aperture area efficiency: - 16.0 % (120 µm cells) - 17.0 % (160 µm; 17.8% cells)

How far can wafer Si module cost go down? h

Cost structure of wafer Si PV (2009) From Peter Fath, Centrotherm, 2009 36 12-10-2010

Crystalline silicon: first generation PV? picture

Energy pay-back time of turn-key PV systems Mariska de Wild-Scholten, Environmental Sustainability of Thin Film PV 2nd EPIA International Thin Film Conference, 12 November 2009, Munich

Carbon footprint of selected electricity generating technologies Mariska de Wild-Scholten, Environmental Sustainability of Thin Film PV 2nd EPIA International Thin Film Conference, 12 November 2009, Munich 40

Greenpeace 9 March 201041 41