1 / 35

Photovoltaics

Photovoltaics. F.-J. Haug Ecole Polytechnique Fédérale de Lausanne (EPFL), Institute of Microengineering (IMT), Photovoltaics and Thin Film Electronics Laboratory (PV-Lab), 2000 Neuchâtel, Switzerland. Outline. Why Solar Cells? How do they work? Why thin film silicon solar cells?

Download Presentation

Photovoltaics

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Photovoltaics F.-J. Haug EcolePolytechniqueFédérale de Lausanne (EPFL), Institute of Microengineering (IMT), Photovoltaics and Thin Film Electronics Laboratory (PV-Lab), 2000 Neuchâtel, Switzerland.

  2. Outline • Why Solar Cells? • How do they work? • Why thin film silicon solar cells? • What do we do at PV-Lab? • thin film solar cells on glass and plastic substrates • thin films for wafer-based cells (hetero-junction cells) • transparent conducting oxides • (sensor applications) • (module design and reliability) F.-J. Haug – Photovoltaics

  3. The power source: Our Sun • A low to medium size star • Heated by fusion(essentially , processes for , etc.) • Power output: (convert to energy) • Delivers to the earth • 10’000 the global energy consumption F.-J. Haug – Photovoltaics

  4. Solar irradiation atlas Central Europe: ca 1 kW/m2 under clear sky at noon Yearly average: 1000 kWh/m2, ca 3 hours every day SolarGIS F.-J. Haug – Photovoltaics

  5. The solar spectrum • The solar spectrum resembles white-hot body glowing at T=5700 K • Usually measured with respect to wavelength (use a grating or a prism) • In PV: often necessary to convert to photon energy: See www.pvlighthouse.com.au F.-J. Haug – Photovoltaics

  6. Filtering by atmosphere: air mass (AM) AM0: space integrated: 1366 W/m2 AM1: equator AM1.5: central Europe 1000 W/m2 AM2: 30° above horizon AM3: 20° above horizon Ozone Water Air mass number: , latitude PV standard: => , central Europe, Canada/US border F.-J. Haug – Photovoltaics

  7. Is 1 kW/m2 a low energy density? • Example Germany: 30% of electricity supply by lignite (brown coal) • exploitation in open strip mines • some data for Hambach(Germany): • lower ground water table by 400 m • dislocate 3 villages (2 more planned) • 85 km2 open mine and waste-deposit • 40 Mio tons of coal per year~7GW electricity • Formation of lignite: 25 Mio y • exploitation: 1984-2050 • Perspective: • solar illumination on 80 km2: 80 GW • cover with solar cells of 10% efficiency => 8 GWp (alas, not continuously) F.-J. Haug – Photovoltaics

  8. Other “conventional” energy sources MTR (mountain top removal) for Appalachian coal (USA) Strip mining of tar sands in Alberta (Canada) Less obvious, but still impact on large area: dropping ground over coal shafts, shale-gas fracking, deep-sea oil drilling, etc. F.-J. Haug – Photovoltaics

  9. Solar electricity (photovoltaics) Teplin Airstrip, Germany: 130 MW on 2 km2 installed in 4 months Roof top installation (Kaneka a-Si modules) c.f. Centrale de GdDixence: 2 GW, 8 km2 lakes + catchment area Suitable roof top area in Switzerland: 138 km2 (residential, commercial, etc.) (IEA) F.-J. Haug – Photovoltaics

  10. e e h h What is inside a solar cell? silicon wafer solar cell p-n junction thin film silicon solar cell p-i-n junction glass glass transparent front contact encapsulation p-layer n-contact semiconductor absorber layer p-type wafer n-layer reflecting back electrode metallic back contact encapsulation encapsulation glass glass F.-J. Haug – Photovoltaics

  11. What is inside a solar cell? active part: semiconducting silicon (crystalline) active part: semiconducting silicon (thin film) (also used: CdTe, CIGS etc.) glass glass encapsulation n-contact 2-3 μm 200-300 μm encapsulation encapsulation glass glass F.-J. Haug – Photovoltaics

  12. What is a semiconductor? • It’s conducting, but not as good as metals • Electric conductivity is normally associated with the flow of electrons in metals • Conductivity in semiconductors is different • The flow of water can serve as analogue F.-J. Haug – Photovoltaics

  13. Conductivity in metals Analogue: a pond of water Flow of electrons similar to flow of water Horizontal surface, no (very small) potential difference F.-J. Haug – Photovoltaics

  14. Electronic conductivity in a semiconductor Winter: the pond is covered with ice F.-J. Haug – Photovoltaics

  15. Electronic conductivity in a semiconductor Winter: the pond is covered with ice Winter: the pond is covered with ice Flow of electrons similar to flow of water (like metals) Difference: Slower movement, needs potential difference F.-J. Haug – Photovoltaics

  16. Hole conductivity in a semiconductor Imagine: blow air bubbles under the ice sheet Anew phenomenon, exclusive to semiconductors ! Bubbles under the barrier move upwards In semiconductors: positive charge carriers called holes F.-J. Haug – Photovoltaics

  17. Why semiconductors for solar cells? e- h+ F.-J. Haug – Photovoltaics Metals normally reflect light; semiconductors can absorb it Absorbed light creates pairs of electrons and holes (water droplets above and bubbles below the ice sheet) An electric field between doped regions separates electron-hole pairs (imagine you inclined the ice sheet)

  18. - - + + Absorption in semiconductors: bandgap Photons with energy less than the band gap are not absorbed Photon energy in excess of band gap is lost to thermalization High bandgap: high el. potential low current Low bandgap: low el. potential high current F.-J. Haug – Photovoltaics

  19. Limiting efficiency Theoretical limit of ~33%, optimum bandgap 1.44 eV certified records: 28.8%: crystalline GaAs (Alta Devices) 25.6%: crystalline silicon (Panasonic) 20.5%: thin film Cu(In.Ga)Se2, (Solibro) F.-J. Haug – Photovoltaics

  20. Better use of incident light: tandem cells high gap glue low gap Theoretical limit of ~43%, optimum bandgaps 1.1 and 1.7 eV certified records (triples): 37.9: crystalline (In,Ga)P/GaAs/(In,Ga)As (Sharp) 13.4%: thin film a-Si/nc-Si/nc-Si (LG) F.-J. Haug – Photovoltaics

  21. Wafers or thin films? Thin film modules: size determined by glass and machinery e.g. thin film silicon tandem modules 5.3 m2(Applied Materials machinery) 1.4 m2(TEL Solar (ex Oerlikon) machinery) Crystalline cells: size determined by wafer e.g. c-Si: single-crystalline: max 30cm diameter multi-crystalline: typically cut to 15x15 cm2 high efficiency – high cost moderate efficiency – low cost F.-J. Haug – Photovoltaics

  22. Wafers or thin films? Polycrystalline or amorphous materials: defects and grain boundaries recombination and short lifetime Crystalline material: perfect properties excellent lifetime and collection e- e- e- e- Electric field: Electric field: h+ h+ h+ h+ F.-J. Haug – Photovoltaics

  23. Module interconnection SaFlex/TEL Solar SunPower Wafer based: solder cells into strings (e.g. SunPower: 22.4%) Thin film: 3 x laser scribing during process (e.g. TEL Solar: 12.2%) Rowell, En. Env. Sci F.-J. Haug – Photovoltaics

  24. Thin film advantage Colorful TF-Si modules (PV-Lab prototypes) Semi-transparent a-Si modules (Schott) F.-J. Haug – Photovoltaics

  25. Thin film advantage Roof tile laminate (Bressler) Flexible a-Si modules (Flexcell) • (UniSolar) F.-J. Haug – Photovoltaics

  26. Thin film dilemma • PV modules are sold by power, currently: • ~0.8 $/W crystalline • ~0.6 $/W thin film 3.5$/W in 3.5 y Balance of System (BOS) “cost baseline” 3.5$/W in 3.5 y • PV system cost (currently ~5 $/W) • includes items that scale with area • (mount, wiring, labour, etc.) • For given power kWp, a larger area is needed, higher cost takes longer for amortisation F.-J. Haug – Photovoltaics

  27. PV landscape in Switzerland PV-Lab systens inverters 3S Laminators MB wafer sawing Pasansun simulators Roth and Rau (CH) flexible CIGS Neuchâtel Equipment (end 2014) PV-Lab c-Si, TF-Si PV-Centre dye sensitized solar cells module testing dye cells F.-J. Haug – Photovoltaics

  28. What we do at PV-Lab TF-Si challenge: weak absorberption especially for long wavelengths (red light) Solution: add back reflector => 2x absorption add texture for scattering => up to 50x Superstrate (glass) TCO p growth i n TCO back reflector p-i-n (usually on rigid glass) F.-J. Haug – Photovoltaics

  29. Textured ZnO:Belectrodes • Requirements: • conductivity: add B-dopant (diborane) • surface texture for light scattering • Examples: ZnO:B (PV-Lab std.) SnO2:F (commercial, Asahi) W. Wenas, Jap. J. Appl. Phys (1991) S. Faÿ, EU PV Conf. (2000) F.-J. Haug – Photovoltaics

  30. Take care with texturing! Collision of growth fronts on facets yields defective material ! Here: μc-Si, but also observed in a-Si H. Sakai, Jap. J. Appl. Phys. (1990) Y. Nasuno, Jap. J. Appl. Phys. (2001) M. Python, J. Non-Cryst. Sol., (2008) F.-J. Haug – Photovoltaics

  31. Cells on flexible plastic substrate Nano-imprinting used for texturing of plastic (here: periodic grating structure) T. Söderström, Appl. Phys. Lett, (2009) K. Söderström, Prog. in PV, (2010) J. Escarré, J. Optics, (2012) R. Biron, Sol. En. Mat. (2013) F.-J. Haug – Photovoltaics

  32. Heterojunction solar cells Surface passivation • c-Si is a perfect crystal; cell-performance is not bulk- but surface-limited • Intrinsic (undoped) a-Si:H provides excellent passivation of c-Si surface • Charge isextractedthroughpassivatinga-Si:H bi-layers • High efficiency cells >22% (certified) • A. Descoedre, EU-PV Conf., (2012) F.-J. Haug – Photovoltaics

  33. Research on hetero-junction cells • Thin a-Si layers: max 5 nm • Pure a-Si layers: avoid epitaxy on underlying c-Si • Chemical annealing: alternate SiH4 and H2 plasma • Other: • Texture etch (avoid sharp valleys) • Highly transparent TCOs (very high transmission at 1100 nm) • Replace screen printing of Ag contacts by galvanic Cu F.-J. Haug – Photovoltaics

  34. Thanks for your attention • Acknowledgementfor funding • SwissFederal Office for EnergyEU FP6 and FP7, CTI, FNS, CCEM-CH, SwissElectric Research, AxpoNaturstromfonds, Velux-Stiftung, IBM… • Bosch, OerlikonSolar, Pasan, Flexcell, Solvay, Dupont, Metalor,Meyer-Burger (3S Moduletec, Roth and Rau), Indeotec SA, … • Thanksalsoto the PV-Labmembers F.-J. Haug – Photovoltaics

  35. F.-J. Haug – Photovoltaics

More Related