Photonics for High-efficiency Crystalline Silicon Solar Cells

Transcription

1 Photonics for High-efficiency Crystalline Silicon Solar Cells Stefan Glunz Fraunhofer Institute for Solar Energy Systems ISE Workshop Nanophotonics - essential ingredient for efficient and cost-effective solar cells? EU-PVSEC, Paris, Sept Fraunhofer ISE

2 AGENDA Crystalline Silicon: How to handle an indirect semiconductor? Theoretical efficiency limit Light absorption Increasing light absorption: Classical approaches Pyramids Internal reflection layers Increasing light absorption: New concepts Plasmonic structure Diffractive optics Using the full spectrum Up-conversion 2

3 AGENDA Crystalline Silicon: How to handle an indirect semiconductor? Theoretical efficiency limit Light absorption Increasing light absorption: Classical approaches Pyramids Internal reflection layers Increasing light absorption: New concepts Plasmonic structure Diffractive optics Using the full spectrum Up-conversion 3

4 Shockley-Queisser-Limit Maximum efficiency as function of bandgap Detailed balance between sun (temp. T 1 ) and solar cells (temp. T 2 ) Maximum efficiency ~ 33% High thermalization for low band gap energy High non-absorption for high band gap energy Silicon and GaAs are close to the optimum band gap Actual record efficiency of GaAs closer to limit than Silicon Why? 4 Efficiency Thermalization Ge GaSb c-si mc-si InP GaAs CdTe Shockley-Queisser Record Efficiencies a-si AlGaAs E G [ev] Non-absorption

5 Shockley-Queisser-Limit Other recombination channels Ideal solar cell has only radiative recombination (Shockley and Queisser, J. Appl. Phys. 1961) But silicon is an indirect semiconductor therefore direct recombination has no high probability (three particle process) Radiative recombination in an indirect semiconductor 5

6 Shockley-Queisser-Limit Other recombination channels Ideal solar cell has only radiative recombination (Shockley and Queisser, J. Appl. Phys. 1961) But silicon is an indirect semiconductor therefore direct recombination has no high probability (three particle process) In silicon solar cells Auger recombination (non-radiative) is limiting loss mechanism. Auger recombination process in an indirect semiconductor 6

7 What is the maximum efficiency? Including Auger recombination Shockley, Queisser (1961): Efficiency limit of solar cell made of one material = 33% (AM1.5) Theoretical efficiency for silicon = 29.4% 1 Best silicon cell = 25% 85% of theoretical efficiency! 1 Richter, Hermle, Glunz et al., IEEE J. Photovolt. (2013) 7

8 What is the maximum efficiency? Including Auger recombination Shockley, Queisser (1961): Efficiency limit of solar cell made of one material = 33% (AM1.5) Theoretical efficiency for silicon = 29.4% 1 Best silicon cell = 25% Efficiency c-si mc-si Shockley-Queisser Record Efficiencies GaAs InP CdTe 85% of theoretical efficiency! Ge GaSb a-si AlGaAs E G [ev] 8

9 AGENDA Crystalline Silicon: How to handle an indirect semiconductor? Theoretical efficiency limit Light absorption Increasing light absorption: Classical approaches Pyramids Internal reflection layers Increasing light absorption: New concepts Plasmonic structure Diffractive optics Using the full spectrum Up-conversion 9

10 Light absorption Indirect semiconductors as silicon exhibit a low absorption for photons with energies around the band gap energy IR UV P. Würfel, Physik der Solarzellen 10

11 AGENDA Crystalline Silicon: How to handle an indirect semiconductor? Theoretical efficiency limit Light absorption Increasing light absorption: Classical approaches Pyramids Internal reflection layers Increasing light absorption: New concepts Plasmonic structure Diffractive optics Using the full spectrum Up-conversion 11

12 Light trapping in thin silicon solar cells Reduction of direct reflection due to multiple chances to enter the cell Oblique path of light increases optical thickness D. Kray, M. Hermle, and S. W. Glunz, Progress in Photovoltaics 16 (2008) 12

13 Light trapping in thin silicon solar cells Perfect scattering Goetzberger, IEEE-PVSC 1981: Perfect rear scatterer (Lambertian) Yablonovitch, J. Opt. Soc. 1982: Absorption enhancement limit 4n 2 J. Eisenlohr, Diploma Thesis, Uni Freiburg (2012) 13

14 High-efficiency cells with dielectrically passivated rear Rear reflector Optimized Cell with improved optical and electrical performance Rear passivation layer serves as internal reflector (low n) Passivation Layer (SiO 2, Al 2 O 3 /SiN X ) Reduced Contact Area 14

15 Cell parameters as a function of thickness Diffusion length (Material quality) High 15 (L>> 250 μm) Medium (L 250 μm) Low (L<< 250 μm) Thin cells can improve electrical characteristics (carrier confinement) But: Light trapping has to be excellent! [%] Thickness [µm]

16 Cell parameters as a function of thickness Short-circuit current as a function of cell thickness of a PERC cell on high-quality material Measurement can be described by parameters extracted from test structures Short-circuit current J sc [ma/cm 2 ] Measurement Simulation Cell Thickness W [µm] Thin cells can improve electrical characteristics (carrier confinement) But: Light trapping has to be excellent! 16

17 Ultra-thin high-efficiency cells 20.2% on 37 μm thin wafer using LFC technology 750 mv with HIT cell structure by Sanyo/Panasonic on 98 μm thickness S.W. Glunz, Sol. Energy and Solar Cells 90 (2006) 17

18 AGENDA Crystalline Silicon: How to handle an indirect semiconductor? Theoretical efficiency limit Light absorption Increasing light absorption: Classical approaches Pyramids Internal reflection layers Increasing light absorption: New concepts Plasmonic structure Diffractive optics Using the full spectrum Up-conversion 18

19 Improving light absorption Plasmonics Scattering at metal nanoparticles at front longer effective path length Metal nanoparticles embedded in semiconductor Near field excitation of carriers Corrugated semiconductor/metal back surface longer effective path length Atwater and Polman, Nature Materials 9 (2010) 19

20 Improving light absorption Plasmonics Plasmonic on rear side: Only small wavelength range has to be considered Less parasitic absorption in Ag particles possible Separation of Ag particles from semiconductor by thin passivation layer (e.g. Al 2 O 3 ) Better electrical performance Uniform size distribution crucial Jüchter et al., EUPVSEC (2013) 20

21 Fabrication of Nanoparticles Interference Lithography, Imprint, Metallization, Lift-off Interference lithography masters up to m 2 possible Uniform distribution of particle size achieved Proof of concept at cell level under investigation ~31 nm ~150 nm 21 Jüchter et al., EUPVSEC (2013)

22 AGENDA Crystalline Silicon: How to handle an indirect semiconductor? Theoretical efficiency limit Light absorption Increasing light absorption: Classical approaches Pyramids Internal reflection layers Increasing light absorption: New concepts Plasmonic structures Diffractive optics Using the full spectrum Up-conversion 22

23 Increasing light absorption Diffractive rear surfaces Heine and Morf, Applied Optics 34 (1995) Diffractive optics But: Structuring of silicon can increase surface recombination Heine and Morf, Applied Optics 34 (1995) 23

24 Increasing light absorption Diffractive rear surfaces Diffractive allows longwavelength light to be redirect in very shallow angles Optical properties decoupled from rear surface passivation Optically rough, Electrically flat Important especially for very thin cells incoming light passivation diffractive rear side structure antireflection layer emitter diffracted light base rear side metal Janz et al., EU-PVSEC 2009 Voisin et al., EU-PVSEC

25 Increasing light absorption Diffractive rear surfaces Optimization using rigorous coupled wave analysis (RCWA 1 ) simulation photo current density [ma/cm 2 ] Simulated photo current densities AM1.5g ( nm) front: planar (no ARC) - rear: mirror front: planar (DARC) - rear: mirror front: planar (no ARC) - rear: perfect scatterer front: planar (DARC) - rear: perfect scatterer front: inverted pyramids with SARC - rear: mirror front: planar (DARC) - rear: sphere grating cell thickness [ m] 1 RCWA P. Lalanne, Reticolo 2D 25

26 Increasing light absorption Diffractive rear surfaces Realization by self-organized opaline structures Hexagonal photonic structure Deposition of ALD Al 2 O 3 layer (surface passivation) Spin Coating of spherical nanoparticles (SiO 2 ) Eisenlohr et al., EU-PVSEC

27 Increasing light absorption Diffractive rear surfaces Realization by self-organized opaline structures Hexagonal photonic structure Deposition of ALD Al 2 O 3 layer (surface passivation) Spin Coating of spherical nanoparticles (SiO 2 ) Structure filled up using ALD of TiO 2 (alternative SolGel) Eisenlohr et al., EUPVSEC (2013) 27

28 Increasing light absorption Diffractive rear surfaces Realization by self-organized opaline structures Hexagonal photonic structure Deposition of ALD Al 2 O 3 layer (surface passivation) Spin Coating of spherical nanoparticles (SiO 2 ) Structure filled up using ALD of TiO 2 (alternative SolGel) Current gain 1.5 ma/cm 2 Surface passivation < 10 cm/s absorption enhancement A-A ref [%] wafer thickness 100 m, measured wafer thickness 250 m, measured wavelength [nm] Eisenlohr et al., EUPVSEC (2013) 28

29 AGENDA Crystalline Silicon: How to handle an indirect semiconductor? Theoretical efficiency limit Light absorption Increasing light absorption: Classical approaches Pyramids Internal reflection layers Increasing light absorption: New concepts Plasmonic structures Diffractive optics Using the full spectrum Up-conversion 29

30 Specifications for solar cell upconverter devices Bifacial solar cell 30 Large transmittance of subband-gap photons Large EQE of UC photons from the rear Upconverter Large absorptance Large upconversion quantum yield (UCQY) Broad absorption range Er 3+ based upconverter Shockley-Queisser limit enhanced from 30% to 40% 1 1 T. Trupke, et al., Sol. Energy Mater. Sol. Cells 90, 3327 (2006)

31 Upconversion (UC) Er 3+ based upconverter Symbolic example Laser 1523 nm Energy transfer most efficient UC process (ETU) Subsequent absorption of photons (GSA, ESA) UC luminescence due to spontaneous emission (SPE) Multi-phonon relaxation (MPR) F. Auzel, Chemical Review 104, (2004) K. W. Krämer, et al., Chemistry of Materials 16(7), (2004) 31

32 Solar cell upconverter device Short-circuit current density Monochromatic laser excitation 1.79% at 1000 W/m 2 (0.179 cm 2 /W) Broad-band excitation 0.77% at 1063 W/m 2 (0.072 cm 2 /W) 2.2 ma/cm 2 under 78 suns Solar concentrator 13.3 ma/cm 2 under 207 suns Efficiency increase of 0.19% 1 S. Fischer et al., submitted to SOLMAT (2013) 32

33 Conclusion Q: Nanophotonics - essential ingredient for efficient and cost-effective solar cells? A: Yes, but Don t deteriorate the surfaces electrically! Structures should be applicable and not only scientifically sexy! 33

34 Acknowledgements My sincere thanks to my all my coworkers at Fraunhofer ISE especially to Johannes Eisenlohr, Stefan Fischer, Benedikt Bläsi, Jan-Christoph Goldschmidt for the fruitful and pleasant national and international cooperation for the budget from industry, German Government and European Commission 34

35 AGENDA Crystalline Silicon: How to handle an indirect semiconductor? Theoretical efficiency limit Light absorption Increasing light absorption: Classical approaches Pyramids Internal reflection layers Increasing light absorption: New concepts Diffractive optics Plasmonic structure Using the full spectrum Up-conversion Spectral splitting 35

36 Using the full spectrum Spectral splitting Spectral splitting of the spectrum using beam splitters Different cells adapted to each part of the spectrum Realized system: Efficiency 32.1% B. Mitchell et al., Progress in Photovoltaics 19 (2011) 36