(Co-PIs-Mark Brongersma, Yi Cui, Shanhui Fan) Stanford University. GCEP Research Symposium 2013 Stanford, CA October 9, 2013

Transcription

1 High-efficiency thin film nano-structured multi-junction solar James S. cells Harris (PI) (Co-PIs-Mark Brongersma, Yi Cui, Shanhui Fan) Stanford University GCEP Research Symposium 2013 Stanford, CA October 9, 2013

2 Solar Cell Efficiency and Cost Efficiency 40% (<<1 $/W) CPV 30% High efficiency Low cost csi (~1 $/W) 20% glass Single-crystalline Thin film III-V PV (>>1 $/W) 10% Konarka Cost per area Higher efficiency reduces entire system cost (BOS) 2

3 Lowering Cell Cost--Ultra-Thin Films Estimated cost distribution for III-V solar cells Processing 20% Substrate 30% 3% 10% 6% Epitaxial growth 50% 81% Ultra-thin films decrease material cost Increased throughput decreases capital cost (10X) Scaled processing (3X) Substrate recycling (10X) Substrate Epitaxial growth Processing Save Cost cost savings 3

4 Outline Photon management in thin film solar cells Key to cost reduction and efficiency improvement Enhanced optical absorption by nano-structuring Nanostructured III-V solar cells Conventional designs GaAs nano-junction cell performance and challenges New designs for high efficiency nanostructured solar cells Dielectric nanostructure window layer solar cell Ultra-thin film Si cells for tandem junction cells Conclusions 4

5 Current density (ma/cm2) Benefits of Photon Management Black: Thin film GaAs cell Blue: Conventional GaAs cell Red: Ultra-thin film GaAs cell with photon management Photon Management Voltage (V) Carrier confinement Y. Kang et al. 39 th PVSC, 2013 Photon management is crucial for ultra-thin film solar cells 5

6 Metal Nanowire Transparent Conducting Electrodes 2Ω/square at 90% transmittance and 10Ω/square at 95% transmittance H. Wu, D. Kong, S. Fan, Y. Cui: Nature Nanotechnology 8, 421 (2013) 6

7 Meso- and Nano- Wire Transparent Electrodes Meso-scale metal wires reduce sheet resistance by at least one order of magnitude with the same transmittance. P. Hsu, S. Wang, Y. Cui et. al. Nature Communication 4: 2522,

8 Enhancing Open Circuit Voltage with Nanophotonic Design bulk GaAs 44nm GaAs A. Niv et al PRL (2012); S. Sandhu, Z. Yu, S. Fan, Optics Express 21, 1209 (2013); 8

9 Nanostructure Light Trapping Wave optics regime: Optimizing absorption = Optimizing resonance excitation Model cell: 1 m thick csi cell Light absorption vs angle & wavelength 1: Local (Mie) resonance 2:Fabry-Perot Resonance 3:Guided mode resonance 4:Diffracted resonance Total absorption = Aggregate of contributions of all narrow resonances Z. Yu et al., PNAS 107, 17491, (2010). 9

10 Outline Photon management in thin film solar cells Key to cost reduction and efficiency improvement Enhanced optical absorption by nano-structuring Nanostructured III-V solar cells Conventional designs GaAs nano-junction cell performance and challenges New designs for high efficiency nanostructured solar cells Dielectric nanostructure window layer solar cell Ultra-thin film Si cells for tandem junction cells Conclusions 10

11 Prior Nanostructured Solar Cells Antireflection Light trapping absorbers Xi et al. Nat. Photonics, 2007 Oh et al. Nat. Nanotechnol Back reflector Nano-structured, flexible GaAs thin film Hsu et al. Adv. Energy. Mat D. Liang, et.al. Adv. Energy Mat.,

12 Nanostructured p-n Junctions B. M. Kayes, et al. J. Appl. Phys. 97, (2005) Nanostructured p-n junction: 1) Enhance absorption by antireflection and light trapping 2) Decouple the absorption length and carrier transport 12

13 Current Density (ma/cm 2 ) Challenges for III-V Nanostructured Solar Cells 200nm Planar and nano-pyramid cells with 200nm thick p-n junction Planar Nano-pyramid Nano-dome 200nm Voltage (V) 200 nm Cell Jsc (ma/cm2) Voc (V) Fill Factor Eff. (%) Planar Nano-dome Nano-pyramid Nano-wire * Nanostructured GaAs cells enhance J sc, however degrade V oc and F.F. New designs required to improve V oc and F.F. * Czaban et al. Nano. Letter

14 Metal Contact Problems and New Nano-Cell Design Shunts at valleys Add Insulation layer, still some shunts Planar junction/mesa separates contact from nanostructures Metal Mesa grid design eliminates shunts 14

15 Outline Photon management in thin film solar cells Key to cost reduction and efficiency improvement Enhanced optical absorption by nano-structuring Nanostructured III-V solar cells Conventional designs GaAs nano-junction cell performance and challenges New designs for high efficiency nanostructured solar cells Dielectric nanostructure window layer solar cell Ultra-thin film Si cells for tandem junction cells Conclusions 15

16 Nano-Pyramid Cell Design A heterojunction window in III-V solar cell Metal p-algaas 1.0 um p-gaas n-gaas 0.3 um 2.0 um n-algaas 0.1 um Window layer has wide bandgap --- transparent to most light Window layer repels minority carriers back to the junction Transport Electron-hole pairs in nanocones to p-n junction Replace nanocone GaAs with nanocone window (AlGaAs, InGaP) 16

17 Nano-Pyramid Window Solar Cell Metal mesa Nanocone window p Al 0.8 Ga 0.2 As p GaAs n GaAs D. Liang et al. Nano Letters

18 Nano-Pyramid Absorption Nanocone window Planar window 500nm Completely black even under 1-sun illumination D. Liang et al. Nano Letters 2013 Reflection < 3% from 450nm to 850nm 18

19 J (ma/cm 2 ) Performance: Measured J-V (1 sun) Nanocone window Planar window nm Voltage (V) D. Liang et al. Nano Letters 2013 Voc (V) Jsc (ma/cm 2 ) Fill Factor Efficiency(%) Planar window Nanocone window Improvement 0.3% 15% 13% 30% 19

20 Bandgap Offset (ev) Eg-Voc Bandgap Offset (E g - qv oc ) Czaban et al. Nano.Lett Mariani et.al. Nano.Lett Putnam et al. Energy Environ. Sci Fan et al. Nat. Mat Zhu et al. Nano. Lett.2010 Hsu et al. Adv. Energy. Mat Oh et al. Nat. Nanotechnol Zhao et al. Appl. Phys. Lett Our work May 2012 Our work Nov 2012 Kayes et al. 37 th PVSC c-si GaAs CdTe a-si Best V oc in nanostructure-based solar cells 20

21 Benefits of Nano-Pyramid Window Enhanced light absorption and carrier collection from nanocone window % improvement in J sc (24.4 ma/cm 2 ) High-quality and low-area junction minimize J high V oc (1.003 V) Metal mesa contact improves shunt and series resistance good FF (71%) % enhancement in efficiency (17%) 21

22 Reflectance (A.U.) Band gap (ev) Reflection (A.U.) Nanostructured Dielectric Window SiO 2 MgF 2 Al 2 O 3 MgO Si 3 N 4 ZnO TiO 2 ZnS Refractive index Transparent, large band gap (5.5eV) Refractive index ~2 and tunable Widely used as anti-reflective coating Wavelength ( m) SLARC DLARC DLARC Si 3 N 4 Nano Si AlGaAs N Nano 3 4 Nano Planar n=1.9 n=2.0 n=2.1 n=2.2 Reflection < 10% Incident Angle (degree) 22

23 Absorption (A.U.) 1 Ultra-Thin Nano-Pyramid Cells Absorption in 200nm GaAs slab 0.8 J sc = ma/cm J sc = ma/cm Si 3 N 4 window Planar Wavelength ( m) Strong light trapping, 88% absorption enhancement Guided lateral propagating mode 23

24 Outline Photon management in thin film solar cells Key to cost reduction and efficiency improvement Enhanced optical absorption by nano-structuring Nanostructured III-V solar cells Conventional designs GaAs nano-junction cell performance and challenges New designs for high efficiency nanostructured solar cells Dielectric nanostructure window layer solar cell Ultra-thin film Si cells for tandem junction cells Conclusions 24

25 Nano-Pyramid Si Cell Structure Metal contact 0.3um 0.9um n-type [intrinsic Si] 1 x Si 3 N 4 window 1.7um p-type 3 x Si Cell p-substrate 1 x Y. Kang et al. to be published,

26 Current density (ma/cm^2) J-V Characteristics Si3N4 nano window Si3N4 SL ARC w/o Si3N Voltage (V) Si 3 N 4 nano-cone enhances J sc by 32% Eff. by 43% Voc (V) Jsc (ma/cm^2) Eff. (%) F.F. (%) Si 3 N 4 Nano window Si 3 N 4 SL ARC W/O Si 3 N Y. Kang et al. to be published,

27 EQE (%) External Quantum Efficiency Si3N4 nano window Si3N4 SL ARC w/o Si3N Wavelength (nm) In the visible light region, EQE is enhanced by over 30%; In the near infrared region, EQE is enhanced by ~15%, due to the low absorption in thin film. Y. Kang et al. to be published,

28 Summary Demonstrated Highest Efficiency Nano-structured III-V solar cell Nano-scale photon management enables ultra-thin film solar cells to achieve high efficiency The nano-window, planar junction, mesa contact design simultaneously improves junction quality, light absorption, carrier collection and eliminates shunting defects The AlGaAs nano-cone window solar cell showed and enhanced efficiency of 17% Demonstrated highest transparency nano-metal contact grid Demonstrated thin film lift-off and flexible GaAs cell The Si 3 N 4 nanostructure window design reduces the optical losses in window layer and enhances the efficiency by 43% 28

29 Acknowledgements STUDENTS and POSTDOCS Brongersma Group Fan Group Dianmin Lin Cui Group Hui Wu Desheng Kong Po-Chun Hsu Shuang Wang Corporate Collaborators Solar Junction Solexel OEpic Thank You Sunil Sandhu Harris Group Yangsen Kang Yusi Chen Yijie Huo Research Support 29