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Principles and Outlook of Photovoltaics

International Symposium on Energy and Sustainability , Ciemat, Madrid, Spain, June 16, 2008. Principles and Outlook of Photovoltaics. Rolf Brendel Institute for Solar Energy Research (ISFH) in Hamelin and Institute for Solid State Physics Leibniz University of Hanover.

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Principles and Outlook of Photovoltaics

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  1. International Symposium on Energy and Sustainability , Ciemat, Madrid, Spain, June 16, 2008 • Principles and Outlook • of • Photovoltaics Rolf Brendel Institute for Solar Energy Research (ISFH) in Hamelin and Institute for Solid State Physics Leibniz University of Hanover Institut für Solarenergieforschung Hameln

  2. Institute for Solar Energy Research Hamelin • Applied research on photovoltaic and solar thermal energy conversion • Owned by Sate of Lower Saxony and associated to Leibniz University Hanover • 132 employees • Budget: 8.7 M€ • Industry projects: 46% • Public projects: 24% • Institutional funding: 30% • Director: R. Brendel Institut für Solarenergieforschung Hameln

  3. Physical principles of photovoltaics Institut für Solarenergieforschung Hameln

  4. Solar energy: Remote nuclear power • Solar energy is available anywhere (less transport) and plentiful (104) • Emits approximately a black body spectrum at Ts=6000 K • Potentially high efficiency for a Carnot process: • Solar cells exhibit additional entropy generating losses Institut für Solarenergieforschung Hameln

  5. Physical processes in solar cells hn Energy Position hn Eg< hn Process: carrier generation balances recombination and extraction SQ-limit: non-absorption hn<Eg radiative plus thermalization Real world: non-absorptionfor hn>Eg non-radiative Joule heating, at surface & bulk diffusion, membrane Institut für Solarenergieforschung Hameln

  6. What efficiency is theoretically possible with a single semiconductor? • 33% efficiency at 1 sun • Optimum energy gap reflects maximum of solar spectrum • Many materials such as Si, GaAs, CdTe, CIGS, a-Si have suitable band gaps • Higher limiting efficiencies for spectral splitting (e.g. 49% for triple junction cells) and concentration  Prof. Sala (Shockley and Queisser, 1961) 0.3 0.2 Efficiency 0.1 2 1 kW/m , AM1.5G 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Energy gap E [eV] g Institut für Solarenergieforschung Hameln

  7. Working principle of solar cells: Membranes make the cell work! - - - - - + - + - - + + + + + + + + n+-type semiconductor semiconductor p+-type semiconductor • p-type layer has high conductivity for holes and blocks electrons • n-type layer has low conductivity for electrons and blocks holes Institut für Solarenergieforschung Hameln

  8. Drastic increase in electric power from PV on a yet very small scale • Solar power that hits the earth is 104 x power consumption of the world • 1% contribution in 2013 if constant rate of market growth • Bavaria > 1 % contribution in 2007 • Germany 0.6 % in 2007 (3.5TWh/a) Institut für Solarenergieforschung Hameln

  9. Costs of photovoltaic energy • PV energy costs [€/kWh] System priceSI [€/kWp] Interest rate iAnnual energy E [kWh/kWp] • Fast decreasing costs are required for keeping the market growth rate. • Politicians have high responsibility not to pay too much per kWh. Otherwise price will not come down as fast as is technically feasible. Hannover Madrid Institut für Solarenergieforschung Hameln

  10. Production volume in GWp/a • Growth by 25 to 60% per year • Crystalline Si is the dominatingtechnology (90%)(multi, mono, ribbons) • Thin-film is catching up (CdTe, a-Si, CIS) Institut für Solarenergieforschung Hameln

  11. Learning curve: 20% price reduction of PV-modules per doubled production (Source: EPIA) Installed cumulated power [GWp] (Source: E. Weber, G. Willeke, 2007) • About -8 % per a in 1980…2004 • Research and production to keep or accelerate this! Institut für Solarenergieforschung Hameln

  12. Crystalline Si technology Institut für Solarenergieforschung Hameln

  13. Value chain of crystalline Si solar cells 20062010 100% 61 % 42 % 26 % • Cost reduction of 8% p. a. expected (M. Rogol, photon consulting) • Increased cell efficiency accounts for largest part (36%) of expected cost reduction (Data from Photon, April 2007) Poly Si Crystal Wafer Cell Module System Institut für Solarenergieforschung Hameln

  14. Average Si-wafer-module efficiencies • 13.1 % average efficiency in 2008 • Much smaller than efficiency limit! • +2% abs. per decade • Current efficiency is far below the physical limits! (G. Willeke, 33rd IEEE PVSC, 2008, Data from Photon market survey 2003-2008) Institut für Solarenergieforschung Hameln

  15. Impact of wafer costs and module efficiency on system cost Improved screen printing, Cell 18% Status New high h processCZ Cell 22% -17 % -31 % -37 % Ultra thin wafer Cell 20% 1.4 • Efficiency enhancement 13  20% has a cost-impact similar to reducing wafer costs to zero! • Ultra-thin monocrystalline Si attractive at 18% efficieny 1.2 1.0 rel. absorber Relative system costs [€/kWp] costs 1.3 0.8 1 0.6 0.33 0 0.4 10 12 14 16 18 20 22 24 Module efficiency [%] Institut für Solarenergieforschung Hameln

  16. Three approaches for lowering costs of c-Si-PV-energy • Improved screen printing Example here: Alu+ technology Back-side junction with screen-printed emitters • New high h process Example here: RISE technology: Laser-processed back contact cells • Thin-film wafer Example here: PSI technology: Layer transfer of epitaxial layers using porous Si Potential for low cost reduction Technological challenge Institut für Solarenergieforschung Hameln

  17. Improved screen printingExample: Alu+-cellScreen printingSiN-Passivation Institut für Solarenergieforschung Hameln

  18. Metal contaminations are less harmful in n-type than in p-type Si QSSPC measurements teff > 3 ms in low-level injection for as grown n-type Cz Si starting material Institut für Solarenergieforschung Hameln

  19. n-type wafer with screen printed Al-p+-type emitter Screen-printed Ag contact SiN ARC n+ (P) n-type Si p+ (Al) Al-Si eutectic P diffusion SiN deposition Ag, Al screen- printing Co-firing Institut für Solarenergieforschung Hameln

  20. SEM micrograph of a screen-printed Al-alloyed emitter n-Si bulk p+ 50 µm • Closed p+-type region along rear surface • Thickness about 8 µm Institut für Solarenergieforschung Hameln

  21. Alu+ cells with full-area screen-printed Al-alloyed rear emitter AM 1.5 G, 100 mW/cm2, 25°C *Independently confirmed C. Schmiga et al., EU-PVSEC, Dresden 2006 Institut für Solarenergieforschung Hameln

  22. New high h processExample: RISE-EWT cellLaser structuringSiN- or SiO2/SiN-PassivationSiO2-Ablation by laserEvaporated Al-contactsB-Diffusion (optional) Institut für Solarenergieforschung Hameln

  23. High efficiency RISE-EWT cell • using multiple laser processing Rear Interdigitated Single Evaporation-Emitter Wrap Through • Both contacts on the rear • No shadowing on the front • Carrier collection on two sides • Rear-side SiO2 passivation • Laser processing for  grooves,  holes and  contact openings • Single Al evaporation • ISFH lab result on 10x10 cm2: (Hermann et al., Proc. 22nd EU-PVSEC (2007), in press) Institut für Solarenergieforschung Hameln

  24. EWT-version ofRISE-technology Institut für Solarenergieforschung Hameln

  25. Laser ablation of SiO2: Nanosecond-laser vs. picosecond pulses Nd:YVO4 ( = 355nm); pulse 30ns Nd:YVO4 ( = 532nm); pulse 10ps (P. Engelhart et al., Prog. Photovolt. 15, 521, 2007) Institut für Solarenergieforschung Hameln

  26. SunPower: 22.4% efficiency pilot production. 20% module demonstrated (T.L. Jester et al, 33rd IEEE PVSC, 2008) Institut für Solarenergieforschung Hameln

  27. Thin-film wafer Example: Layer transfer of epitaxial Si filmsPorous Si formationAtmospheric pressure CVDEfficient light trapping Institut für Solarenergieforschung Hameln

  28. Why thin? Energy Position Thickness W • Carriers carry entropy • Only free energy Fn = En- Tcs may be converted to entropy free electric energy • More free energyat higher densities n • Reducing thickness W Light concentration ( Prof Sala) • Excellent surface passivation requiredfor thin cells (a-Si, Al2O3) Institut für Solarenergieforschung Hameln

  29. Layer transfer with porous Silicon(PSI-process) Institut für Solarenergieforschung Hameln

  30. Layer transfer with porous Silicon(PSI-Prozess) glass carrier Si cell Si cell porous Si Si substrate Si substrate poröses Si glass carrier Si substrate Si cell porous Si Si substrate textured Si substrate detach cell re-use substrate R. Brendel, 14th EU-PVSEC, (1997), p.1354. Institut für Solarenergieforschung Hameln

  31. Stable process since structureevolves into minimum free energy N. Ott, M. Nerding, G. Müller,R. Brendel, and H. P. Strunk, J. Appl. Phys. 95, 497 (2004). First report on surface closure: V. Labunov et al., Thin Solid Films 137, 123 (1986) First report on separation layer formation: H. Tayanaka et al., in Proc. 2nd World Conf., (Vienna 1998), p.1272 Institut für Solarenergieforschung Hameln

  32. Epitaxy saves a crystallization step,wafering and Si material recycling 33% 3% … 33% 33% 33% wafer costs per area 3% … 33% 100% • Thin film epitaxy replaced poly-CVD • No melting of poly-Si • No CZ-growth • No wafering Costs are 1/3 • 10 to 100 times less chlorosilane per area further cost reduction • High throughput epitaxy reactors required! Institut für Solarenergieforschung Hameln

  33. Generating a textured thin c-Si film for efficient light trapping 20 µm 20 µm 15 µm Solarzelle Epitaxial film (15 µm thick) Substrate (300 µm) Substrat Institut für Solarenergieforschung Hameln

  34. Cell result on 10x10 cm2 a-Si Al SiN n+-type emitter p-type base p+-type bsf Structured by shadow mask • Cell area 95 cm2Thickness 26 µm VOC = 616 mV JSC = 29.0 mA/cm2 FF = 78.8 % h = 14.1% Al B. Terheiden, R. Horbelt, and R. Brendel, in Technical Digest , 15th Intern. Photov. Sci. and Eng. Conf. (Shanghai, 2005) p. 196. Institut für Solarenergieforschung Hameln

  35. Efficiency potential of thin-film c-Si wafers µs t µs µs µs h = 18 % µs µs W = 2.5 µm • Assumptions: • Good optics • t = 1 µs • S = 100 cm/s • Simulated efficiency: • h= 18 % • W = 2.5 µm 90% of Lambetian √ 16 µs measured √ 120 cm/s measured √ R. Brendel, Solar Energy 77, 969, (2004). Institut für Solarenergieforschung Hameln

  36. Thin-film technologies (non-c-Si):Large area processing and little material consumptiona-Si a-Si/µc-Si CdTe CIGS Institut für Solarenergieforschung Hameln

  37. Value chain of crystalline Si solar cells Poly Si Si -Crystal c-Si-Wafer c-Si Cell c-Si Module System integrated into a single line! Raw material Thin-film module System Institut für Solarenergieforschung Hameln

  38. Thin-film CIGS-solar cell (Abbildungen: M. Powalla, ZSW) Institut für Solarenergieforschung Hameln

  39. Integrated series connection i- ZnO Zn:Al CdS CIGS Mo Glass Active Area (Source: M. Powalla, ZSW) • 13 % module efficiency = best performing thin-film module • Module efficiency similar to average c-Si module Institut für Solarenergieforschung Hameln

  40. Glass in … module out: Large area processing  savings on module fabrication (Source: Antec Solar, CdTe-module factory) Institut für Solarenergieforschung Hameln

  41. Average thin-film module efficiencies • 5.5 % average efficiency in 2003 • 7.0 % average efficiency in 2008 • +4% abs. per decade • Data from Photon market surveys 2003-2008 • Thin-film –Si-wafer competition helps to bring prices down (CdTe) (G. Willeke, 33rd IEEE PVSC, 2008) Institut für Solarenergieforschung Hameln

  42. Summary Institut für Solarenergieforschung Hameln

  43. Summary Photovoltaic market is rapidly growing • 30 to 60 % per a in recent years.Module price reduction of around 8% p.a., • This costs reduction will probably hold on in the future • Market has reacted on Si shortage. Upgraded metallurgical grade Si is now available Cell physics permits significant improvements of crystalline Si module efficiency • Average module efficiency is currently only 13% • 20%-efficient crystalline Si module already demonstrated (SunPower) • 20% cell efficiency is even physically possible with only a few µm of c-Si consumption New processing technology for very thin wafer cells need to be developed • High efficiency c-Si cell designs require local processing • Laser processing is an attractive candidate for local structuring, drilling, ablating of dielectrics ( and doping and crystallization and soldering…) Institut für Solarenergieforschung Hameln

  44. Acknowledgements • Industry partners for cooperation and financing • Federal Ministry for the Environment, Nature Conservation and Nuclear Safety for project funding and 6” processing equipment • State of Lower Saxony for project funding Hamelin Madrid (Source: JRC, http://re.jrc.ec.europa.eu) Thank you for your attention!www.isfh.de Institut für Solarenergieforschung Hameln

  45. Summary: Sorting the Si-world 20 15 Module efficiency h [%] 10 5 0 1 3 10 30 100 300 Cell thickness W [µm] Saving cost and material Thin-film wafer Thin Wafer Many wafer projects Future work Wafer LTPas today Recombination hurdle µ-morph thin-film Mech. stability hurdle CSG thin-film thin-film a-Si Institut für Solarenergieforschung Hameln

  46. (Source: J. Schmid ISET/ WBGU) Institut für Solarenergieforschung Hameln

  47. (Source: WGBU, Politikpaier, renewales 2004) Institut für Solarenergieforschung Hameln

  48. Results for J0e on 100 Ω/sq. J0e,pass Si n+ SiO2 J0e (P. Engelhart, N.-P. Harder, R. Grischke, A. Merkle, R. Meyer, and R. Brendel, Progr. Photov. 15, 237, 2007). Institut für Solarenergieforschung Hameln

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