Novel light management for ultrathin solar cells

Transcription

1 Novel light management for ultrathin solar cells Ruud E.I. Schropp Energy research Centre of The Netherlands & TU/e Eindhoven INC10 May 15, 2014 Gaithersburg MD, USA

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3 Partners in Solliance ( ECN, the leading energy research institute in the Netherlands; thin film research group in Eindhoven TNO, the leading Dutch institute for applied scientific research Holst Center, a joint research initiative of imec and TNO (and ECN for PV); rollto roll processes, mainly oriented towards OPV on flexible substrates TU/e, Eindhoven University of Technology, fundamental and technological research in PV imec, a world leading research institute in nanoelectronics, based in Flanders, participates with its OPV and CIGS PV research groups Forschungszentrum Jülich, is one of the large interdisciplinary research centres in Europe. The Photovoltaics section of the Institute of Energy and Climate Research (IEK 5) belongs to the world leading research institutions in the range of thin film photovoltaics based on amorphous and nanochrystalline silicon. 3

4 Outline Key Challenges in Thin Film Photovoltaics Light trapping (c.q. concentration ) Utilization of nanostructure in Photovoltaics: Conventional random textures and designed textures Nanorod, nanowire structures Conclusion 4

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6 Status of thin film solar cells (laboratory ~ 1 cm 2 ) 2014/05 Thin film Si: Single junction: µc Si:H 10.7% (EPFL Neuchâtel); a Si:H 10.1% (Oerlikon/TEL Solar) Double junction (tandem): a Si:H/µc Si:H 12.3% (Kaneka); 12.3% (Oerlikon Solar) Triple junction: a Si:H/µc Si:H/µc Si:H 13.44% efficiency (LG Electronics) CIGS: Single junction: 20.8% (rigid; ZSW); 20.4% (flexible; EMPA) CdTe: Single junction: 20.4% (First Solar & GE Global Research partnership) Perovskite Single junction 17.9% (KRICT) OPV: Double junction 12.0% small molecule cell (Heliatek) Single junction 11.1% Mitsubishi Chemical Double junction: 10.6% UCLA Sumitomo Chemical 6

7 Installed PV capacity - world 7

8 Installed PV capacity - Netherlands 2013: 700 MWp (0.5% of total electricity consumption) 8

9 PV system prices (Germany) 9

10 Grid integration Germany explores (and shifts) the frontiers M. Lippert, SAFT Source: Fraunhofer ISE (2013) 10

11 Commercial module efficiencies (selection) wafer Si IBC wafer Si IBC wafer Si IBC wafer Si HIT wafer Si mono CdTe wafer Si multi CIGS tf a/µcsi M.J. de Wild Scholten SmartGreenScans (June 2013) tf asi

12 Thinner is beautiful 1. (local) concentration of light Better ratio of photocurrent over dark current higher V oc s*

13 Concentration can be achieved in two ways Optical concentration outside the cell Concentration of light inside the cell by internal light trapping structures, such as diffractive and plasmonic structures. In both cases: I L I 0 photogenerated current reverse saturation current 13

14 Thinner is beautiful 1. Better ratio of photocurrent over dark current higher V oc s* 2. Collection of carriers also improves higher FF s 3. Better stability and thus higher stabilized efficiency (if J sc is maintained). 4. Better cycle time (lower cost of ownership) 5. Lower materials consumption further cost reduction. R.E.I. Schropp

15 Key Challenges reduce cost (by thinning down) higher efficiency materials resources Light trapping Better spectral matching (multijunction, photon manipulation) use only abundant, environmentally benign materials 15

16 Abundancy table

17 Various random light-scattering morphologies (front TCO) Asahi U Type LPCVD ZnO PVD ZnO+ Wet etch Front TCO Random (bottom up) morphologies

18 Haze Challenge: increase the haze at long wavelengths, and scatter light into large angles. M. Zeman et al., MRS Proc. Ser. (2010)

19 Scalable top down technology: Nanoimprint Lithography

20 Nanoimprint Lithography (NIL) Nano imprinting offers great perspectives to implement sub micron light management structures in solar cells in a cost effective manner. Can periodic textures outperform random textures? Periodic texture Random texture (ZnO) Solution growth (ZnO) Two examples of periodic textures 20

21 1. Light trapping by surface plasmons Design of plasmonic nanoparticles patterns for optimum waveguiding. Fraction scattered into substrate highest for cylinder & hemisphere:strongest near-field coupling Ohmic damping V, scattering V 2, Tradeoff: larger size more scattering, but lower coupling Atwater & Polman, Nature Mater. 9, 205 (2010) metal nanostructures controllably couple light to in-plane waveguide modes

22 Substrate Conformal Imprint Lithography PDMS Stamp Thin glass PDMS stamp (6 ) is bonded on 200 µm AF-45 glass 1 m Full-wafer soft nano-imprint Flexible rubber on thin glass Conform to substrate bow and roughness No stamp damage due to particles Marc Verschuuren, Hans van Sprang Spring MRS 2007, 1002-N03-05

23 Electromagnetic Simulation for Generation Rate Calculations simulation FDTD simulations at discrete wavelengths across the spectrum. AM1.5g Spectrum Assume that one photon absorbed produces one electron: for SR: generation rate at each λ for J sc : solar spectrum weighting around each simulated point V. E. Ferry, et al. Opt. Express, 18, A237-A245 (2010). Vivian Ferry 24

24 Comparing Random and Designed Surfaces = 500 nm = 670 nm Nanopatterned 500 nm = 500 nm = 670 nm Photon Flux (cm -3 sec -1 ) Asahi Controlled spatial curvature of metal layer is critical to avoid losses and increase photocurrent V. E. Ferry, et al. IEEE PVSC Proceedings (2010). Vivian Ferry 25

25 2D FT Periodic arrays Pseudo-random Designed classes of nanoparticles Design constraints: nm, 290 nm, 310 nm diameter - Ag particles should not touch - Same packing fraction as the 400 nm periodic pattern Penrose Replicated Asahi Rounded metal nanostructures: Less parasitic absorption

26 Cell Fabrication 500 nm 90 nm active layer ITO a-si:h ZnO:Al Ag SiO 2 sol-gel 9.5% Cells deposit conformally over nanopatterned back contacts: best pseudorandom pattern and periodic pattern have similar efficiencies Vivian Ferry 27

27 Broadband Light Trapping: Experiment vs. Simulation Mie Experiment: 90 nm i-layers waveguide modes Simulation FDTD - Significant broadband photocurrent enhancement - Good agreement between simulation and experiment Vivian Ferry 28

28 2. Light trapping by scalable periodic texture (with µc-si rather than a-si cells) c-si nip cells on steel foil with various back contacts: 1. Flat back contact 2. BC with 2D periodic texture 3. BC with random texture (replica of SnO 2 :F) (EU FP-7 Energy project Silicon-Light ), Coordinator W. Soppe (ECN). 29

29 30 Light management with R2RR Nano-Imprint Lithography 1. Modeling 2. Mastering 3. Tooling 4. Imprinting

30 Single junction c-si cells: J sc Periodic textures can outperform random textures 31

31 Single junction µc-si cells (750 nm): spectral response 32

32 Single junction uc-si cells: Fill Factor Higher FF for periodic texture! 33

33 Lower FF on random textures due to shunting R.E.I. Schropp et al. Journal of Crystal Growth 311, 760 (2009) Microcracks 34

34 Conformal growth without microcrack formation on periodic texture Dark field TEM (one selected diffraction direction) Bright field TEM ITO c Si ZnO Ag HR-TEM by Martial Duchamp (FZ Jűlich) 35

35 11% efficiency reference cell produced on nanotextured metal foil in a roll-to-roll process 350 nm top cell; 1000 nm bottom cell V oc J sc top cell J sc bottom cell FF [%] Efficiency [mv] [ma/cm2] [ma/cm2] [%] Best cell on CB R ,84 12,31 67,0 11,0 Best cell on CB WR ,90 13,24 65,7 10,7 ECN Eindhoven 36

36 Roughness of which layer is important? 500 nm 90 nm active layer ITO a-si:h ZnO:Al Ag SiO 2 sol-gel

37 Where should the roughness be? AZO 80 or 500 nm AZO Ag Asahi Which roughness is important: that of the ZnO or that of the Ag? Claire van Lare, Albert Polman Frank Lenzmann

38 Periodic structures - simulations No structure in AZO poor light trapping Claire van Lare

39 Periodic structures - simulations No structure in AZO poor light trapping Flat Ag can be better than structured Ag Claire van Lare

40 Absorption in Ag layer Structuring Ag leads to increased parasitic absorption Claire van Lare

41 Photonic roughness rather than geometric roughness?

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43 FLiSS made by polishing down 2D ZnO:Ga grating filled with n-type nc-si:h V oc with FLiSS V oc with flat reference substrate Chemical mechanical polishing H. Sai et al. Applied Physics Letters 98, (2011)

44 Plasmonic Flat Scattering Surfaces: Lourens van Dijk

45 Simulation of Absorption enhancement Flat Scattering Design Lourens van Dijk

46 Absorption Si SiO 2 Ag

47 Nanocylinder or nanowire Yinghuan Kuang, et al., IOP Reports on Progress in Physics, 2013 (accepted for publication)

48 Avoiding the trade-off between optically thick and electronically thin Traditional textured thin film silicon solar cells Glass Light direction Current direction TCO Si p-i-n ZnO Ag/Al Trade-off: Optically thick vs Electrically thin R.E.I. Schropp Nanostructured three dimensional (nano-3d) solar cells Yinghuan Kuang, et al., IOP Reports on Progress in Physics, 2013 (accepted for publication) 50

49 Avoiding the trade-off between optically thick and electronically thin Light direction Current direction + - SUPER scattering! Nanostructured three dimensional (nano-3d) solar cells R.E.I. Schropp Yinghuan Kuang, et al., IOP Reports on Progress in Physics, 2013 (accepted for publication) 51

50 Avoiding the trade-off Elongated nanostructures: Nanorod solar cell Orthogonalize charge carrier path and photon path Super scattering (between nanorods) Anti-reflection texture Light direction Current direction Yinghuan Kuang, et al., Journal of Non-Crystalline Solids, 358, 17, (2012) Challenges Conformal coverage (internal shunting paths low FF). Surface and interface recombination low R.E.I. V oc Schropp Nanostructured three dimensional (nano-3d) solar cells Yinghuan Kuang, et al., IOP Reports on Progress in Physics, 2013 (accepted for publication) 52

51 Conformal n-i-p coverage by Hot Wire CVD; nano-3d cells ITO and grid p layer (PECVD, B(CH 3 ) 3 ) i layer (HWCVD, 25 nm) n layer (PECVD, PH 3 ) ZnO:Al (38 nm) Ag (20 nm) nano 3D cells - Thicknesses determined by HRSEM - For optimal conformal coverage, essential to use HWCVD Yinghuan Kuang et al., Appl. Phys. Lett. 98 (2011) doi: /

52 Synthesis of ZnO nanorods by chemical bath deposition (Zn(CH3COO)2 2H2O) + C6H12N4

53 Hot Wire CVD Set-up at Utrecht University in Eindhoven Two Ø0.5 mm Ta filaments Substrate to filaments distance: 55 mm. Distance between filaments: 40 mm. Temp. at filaments: ~1750 C Temp. at substrate: ~200 C 5

54 Introduction: Hot wire CVD for conformal coverage on high aspect ratio structures Qi Wang et al., NREL and Applied Materials Inc., Appl. Phys. Lett. 84 (2004) 338 Makiko Kitazoe, Shuuji Osono, Hiromi Itoh, Shin Asari, Kazuya Saito and Masahiro Hayama ULVAC, Japan, 3 rd Hot-Wire CVD Conference, Utrecht Y. Kuang et al., Appl. Phys. Lett. 98, (2011). 500 nm 4

55 Cell results Flat, 75 nm Asahi texture, 75 nm Nanorod cell, 25 nm 8.3 ma/cm 2 from a 25 nm layer! Nanorod type solar cell with 25 nm i-layer has higher J sc than a flat or even a textured solar cell with 75 nm i-layer; slight trade off with V oc. Yinghuan Kuang et al., Appl. Phys. Lett. 98 (2011) doi: /

56 ZnO nanorod substrate for thin film solar cells rms:68 Nanorod+Ag+ZnO Shorter rods to prevent V oc loss rms:37 Quite different from Asahi U Asahi U-type +Ag+ZnO rms:13 7/14

57 Latest results Cell type i-layer thickness (nm) J sc (ma/cm 2 ) V oc (V) FF η (%) Flat D Flat D nm 0.6 EQE Flat NR Wavelength (nm) Yinghuan Kuang et al., J. Non Cryst. Solids 358 (17) (2012) /14

58 Conclusions New insight in scattering back reflectors for thin film solar cells Plasmonic and dielectric scatterers have different effects. Index contrast for geometrically flat scattering. These concepts are of interest to all thin film solar cells. Large area manufacturing methods need to be developed. Nanowire/nanorod enhancement schemes can reduce cost of photovoltaically generated kwh s. Approaches for non-lithographic random nanorod-type solar cells are very promising.

59 Acknowledgments Albert Polman Claire van Lare Yinghuan Kuang, Lourens van Dijk, Pim Veldhuizen, Marcel Di Vece, Jatin K. Rath W. Soppe, M. Dörenkämper, N.J. Bakker C.H.M. van der Werf Chinese Scholarship Council Yinghuan Kuang Lourens van Dijk