Ribbon Silicon for Cost Reduction in Photovoltaics. Advantages and Challenges

Transcription

1 Ribbon Silicon for Cost Reduction in Photovoltaics Advantages and Challenges Giso Hahn University of Konstanz Photos: Light (greek) Volt: Alessandro Volta Photovoltaics Electricity from (sun)light

2 UKN Zakopane

3 Outline - Why photovoltaics? - A short history of PV - How does it work? - Why ribbon silicon? What is it? - Material properties and solar cell results - Summary

4 Motivation Present CO 2 and Temperature - Link between T and [CO 2 ] - Since a stable T (Holocene) - (when) does T follow increase of [CO 2 ]? [ [

5 Motivation Limitation of fossile fuels ( Hubbert-Peak ) [after: M. King Hubbert, Science, February 4, 1949] Our ignorance is not so vast as our failure to use what we know. M. King Hubbert

6 Motivation Examples of PV Applications

7 A Short History Of PV 1839: Alexandre Edmond Becquerel discovers interaction between light and electrons 1886: Heinrich Hertz and Wilhelm Hallwachs: systematic investigations 1905: Albert Einstein explains photoelectric effect and quantum nature of light (Nobel prize 1921) 1954: Daryl Chapin, Calvin Fuller, Gerald Pearson fabricate first (Si) solar cell (η = 6%) 1958: First application in Vanguard satellite (space) 1973: Oil crisis, ideas for terrestrial use 1990: 1000 roofs program (Germany), sunshine program (Japan) 2000: Law for renewable energies (EEG) in Germany, market stimulation programs 2006: PV electricity in Germany for the first time cheaper than conventional electricity (temporarily!)

8 PV: Operation Principle Crystalline Si Solar Cell photon Absorption in c-si Ag n + emitter (P) p base (B) Al ε charge carriers (electron hole pairs) - - L diff µm L diff 1/α(600nm) 2 µm 1/α(1000nm) 100 µm = D τ diffusion constant D lifetime τ of minority carriers Absorption in c-si: Absorption α dependent on λ, 1/α(600nm) 2 µm, 1/α(1000nm) 100 µm usable spectrum nm high diffusion length L diff high lifetime τ of electrons low losses (recombination) high material quality necessary [A. Götzberger: Sonnenenergie, Teubner, Stuttgart 1995]

9 PV: Operation Principle Generation of Charge Carriers in Semiconductors by Absorption of Light (Photo Effect, Einstein) C E g band gap energy E g [unknown artist: Konstanz] Excess energy: Thermalisation (heat) E ph < E g : Transmission

10 Use Of Spectrum Si Solar Cell [S. Glunz, DPG-Meeting 2004] - Losses due to thermalisation and transmission - Maximum efficiency η max, thermodyn 30% (realised: 24.7%, 4 cm 2 ) [J. Zhao et al., Appl. Phys. Lett. 73, 1998, 1991]

11 PV Production Technologies absolute relative sold mo odule power [MWp] CIS CdTe a-si ribbon-si multi-si mono-si chnology [% %] te CIS CdTe a-si asi ribbon-si multi-si mono-si [ : PV News, 2003, : Photon] - Trend from mono- to multicrystalline t lli Si - (still) low market share increase for thin film technologies (CdTe, a-si, CIS) - Crystalline Si dominates short/medium term

12 From Sand To Solar Cells [Bayer Solar AG] Wire Table Column

13 Cz Si Production Czochralski mono-si (~2 m length) Necking (to prevent forming of dislocations) [ - high quality (single crystal) - more losses (square shped wafers) - higher h costs

14 Si Losses Si Losses (mc Ingot) Ingot Column 30% Column Wafer 34% Process 4% Data for 2008 wire: 120 µm (kerf loss 160 µm) Thickness wafer: ~200 µm Total losses: 68%! [D. Sarti et al, Sol. En. Mat. & Sol. Cells 72 (2002) 27]

15 Si Module Costs Ingot based crystalline Si Module Cost Distribution - Higher module efficiencies (e.g. back contacted cells) - New fabrication methods - Lower Si consumption per W p (thinner wafers and wires, larger ingots) - Alternative Si feedstock ( Solar Grade Si) - Avoiding kerf losses (ribbon Si) - Higher efficiencies (better understanding of material, novel processing steps) [after: C. del Canizo et al., Prog. Photovolt: Res. Appl., 2009]

16 Ribbon Si Techniques Methode year Jahr Dendritic web DW M Stepanov S M Edge-defined film-fed growth EFG M Horizontal ribbon growth HRG M Ribbon-against-drop RAD M Ribbon-to-ribbon RTR M Silicon on ceramic SOC M Capillary action shaping technique CAST M Contiguous capillary coating CCC M Inverted Stepanov IS M Roller quenching RQ M Edge supported pulling ESP M Low angle silicon sheet growth LASS M Interface-controlled crystallization ICC M Supported web SW M Ramp assisted foil casting technique RAFT M Silicon-Film TM SF M Ribbon growth on substrate RGS M Horizontal supported web HSW M String ribbon SR M Two shaping elements TSE M Silicon sheets from powder SSP M Hoxan cast ribbon HCR M Hoxan spin cast HSC M3 1991

17 Ribbon Si Techniques Edge-defined Film-fed Growth (EFG) Commercialised 1994 Schott Solar

18 Ribbon Si Techniques String Ribbon (SR) Commercialised 2001 Evergreen Solar Inc. Similar crystal quality as EFG, elongated crystals, SR: less complicated technique, lower throughput

19 Ribbon Si Techniques Pulling Velocity: Vertical Ribbon Techniques (EFG, SR) v p,max ( W + ) 5 1 σε d KmT m = L ρ m Wd 1/ 2 L : latent heat of fusion ρ m : Si density at melting point T m σ : Stefan-Boltzmann constant ε : emissivity of crystal K m : thermal conductivity of crystal at T m W : ribbon width d : ribbon thickness Max. 8 cm/min In production: 1-2 cm/min (internal stresses)

20 Ribbon Si Techniques Ribbon Growth on Substrate (RGS) casting frame (open) gassing R&D phase ECN + SolarWorld (Bayer AG) Substrate plates Very high throughput, small crystals, more defects

21 Ribbon Si Techniques Pulling Velocity: Horizontal Ribbon Techniques (RGS) v p 4α Kms = ΔT α t dlρ ( 2 K ) m m α : effective coefficient i of heat transfer s : length of liquid/solid interface (in pulling direction) ΔT : temperature gradient between melt and substrate [, A. Schönecker, J. Phys.: Condensed Matter 16 (2004) R1615] In production: 650 cm/min

22 Ribbon Si Techniques Properties material V p [cm/min] width [cm] throughput [cm 2 /min] thickness [µm] EFG x12,5 ~ SR 1-2 4x8 ~ RGS (15.6) 6500(10140) ~300 material crystals resistivity [Ωcm] [O i ] [10 cm ] [C s ] [10 cm ] L diff as grown [µm] EFG cm 2-4 (p) < SR cm 2-5 (p) < RGS <mm 3 (p)

23 Defects in Ribbon Si TEM Studies in RGS 1 µm grain boundaries (GB), dislocations (D), stress fields around O-precipitates (P)

24 Defects in Ribbon Si TEM Studies in RGS Decorated Dislocation (O) Clean Dislocation 100 nm Decorated dislocations more detrimental (more recombination active)

25 Defects in Ribbon Si Current Distribution in RGS Material quality - Inhomogeneous - Spatially resolved characterisation! image: 1x1 cm 2

26 Gettering & Hydrogenation (Theory) Gettering Hydrogenation E 1 2 E CB 3 VB DL SL p-si Al x 1) Freeing of metal impurity (thermally, ~ C) 2) Diffusion in wafer 3) Capture at gettering site (higher solubility) Energy levels in middle of band gap most detrimental (deep levels) H-atoms: energy level of defect shifted Deep level (highly recombination active) shallow level (less recombination active) Less recombination: removal of impurities (gettering) & shift of defect levels in band gap (hydrogenation)

27 Gettering & Hydrogenation (Exp.) EFG as grown Typical behavior: - elongated areas of high τ (> 50 µs) - wide areas of lower τ (~ 1 µs) P-gettering + hydrogenation - SiN:H (800 C, 1 s): poor areas > 10 µs - MIRHP (H-plasma): poor areas 10 µs T (350 C, 60 min) too low for MIRHP? SiN:H based cell process necessary! [M. Kaes et al., Proc. 31st IEEE PVSC, 923] [M. Kaes et al., Proc. 20th EC PVSEC, 1063]

28 Synchrotron Investigations Precipitation Kinetics of Transition Metals in mc-si Formation kinetics of metal clusters dependant on crystallization and chemical species Slow crystallization (ingot) larger clusters, fast crystallization (ribbon) smaller clusters [T. Buonassisi i et al., Progr. Photovolt: Res. Appl ]

29 Synchrotron Investigations XAFS (X-ray absorption fine structure) Gettering efficiency highly dependent on elemental composition of precipitate (dissolution kinetics) FeSi 2 : easily dissolvable Fe 2 O 3 : hardly dissolvable [T. Buonassisi i et al., Progr. Photovolt: Res. Appl ]

30 Efficiencies & Si Consumption Industrial cell process: EFG, SR 15-16%, RGS 13% Lab cell process: EFG, SR 18%, RGS 14-15% EFG, SR of higher but more inhomogeneous quality Industrial-type process material Thickness [µm] η [%] g Si /W p mc-si ingot ~250 ~ EFG SR RGS 300 (150) 13 (11 * ) 5.4 (3.2) Less g Si /W p = lower cost/w p? p yield, plant utilization, etc. Energy Payback Time p * : first try Assumptions: ribbon Si: 100% Si usage, record values, mc-si ingot: average value [ et al., Proc. 4 th WC PEC, Waikoloa 2006, 972]

31 Summary & Outlook Summary -PV one form of electricity generation in future renewable energy mix - history of PV, current market situation, future trends (Si) -module costs wafer dominated d - Si ribbon production techniques (EFG, SR, RGS) - highest throughput for RGS, still R&D, pilot production - higher defect densities in ribbon Si optimised/new gettering and hydrogenation steps - cell efficiencies (lab + industrial process) - better Si usage (g/w p ) Outlook - use full potential for cost reduction in photovoltaics by e.g. ribbon Si - reaching grid parity (short term: southern Europe, mid term: central Europe) - more research and understanding necessary to exploit full potential!

32 Co-workers: S. Seren, M. Käs, P. Geiger, J. Junge, U. Hess, (UKN) T. Buonassisi (MIT) A. Schönecker, (RGS Development) A. Metz, H. Nagel (Schott Solar) Thank you for your Attention! Greenpeace