Photonic crystals and related nanostructures for solar light management Christian Seassal, Loïc Lalouat, He Ding, Romain Champory, Ngoc Vu Hoang, Abdelmounaim Harouri, Hai-Son Nguyen, Régis Orobtchouk, Alain Fave, Fabien Mandorlo, Emmanuel Drouard INL, Institut des Nanotechnologies de Lyon, UMR 5270 CNRS-Université de Lyon, Ecole Centrale de Lyon, INSA-Lyon, France In collaboration with: Imec (V. Depauw et al), LPICM (P. Roca i Cabarrocas et al), Obducat (K. Lee et al), LPN (S. Collin et al), ILM (A. Pereira et al), U.Namur (O. Deparis et al)

Efficiency limitations Context: Bulk solar cells, thin film solar cells L’optique pour le photovoltaïque Efficiency limitations Si solar cell Thin film solar cell Which limitations? In-coupling losses R0? AR layer

Efficiency limitations Context: Bulk solar cells, thin film solar cells L’optique pour le photovoltaïque Efficiency limitations Si solar cell Thin film solar cell Which limitations? In-coupling losses EQE/absorption limited in red/IR Rough surface A100%?

Efficiency limitations Context: Bulk solar cells, thin film solar cells L’optique pour le photovoltaïque Efficiency limitations Si solar cell Thin film solar cell Which limitations? In-coupling losses EQE/absorption limited in red/IR Thermalization in the UV/blue No absorption below Eg Third Generation Photovoltaics, Vasilis Fthenakis Ed.

In-coupling and absorption control Efficiency enhancement in PV devices L’optique pour le photovoltaïque In-coupling and absorption control Using the photonic toolbox: Samuelson U. Lund, 2015 wires/holes/ cones/pyramids Bermel MIT, 2007 Metal/dielectric F. J. Haug EPFL, JPV 2015 guided modes / localized modes Boriskina MIT, 2015

Periodic photonic structures for energy harvesting L’optique pour le photovoltaïque High index contrast periodic structures: a platform for Photovoltaics U. Delaware INL INL GIST Wavelength-selective intermediate mirror High efficiency absorber Resonant wavelength converter Antireflecting structure AR “layer” Absorber (Si in this talk) Solar cells Back electrode/reflector

Antireflecting structure Periodic photonic structures for energy harvesting L’optique pour le photovoltaïque High index contrast periodic structures: a platform for Photovoltaics U. Delaware INL INL GIST Wavelength-selective intermediate mirror High efficiency absorber Resonant wavelength converter Antireflecting structure AR “layer” Absorber (Si in this talk) Solar cells Back electrode/reflector

Periodic photonic structures for energy harvesting L’optique pour le photovoltaïque High index contrast periodic structures: a platform for Photovoltaics U. Delaware INL INL GIST Wavelength-selective intermediate mirror High efficiency absorber Resonant wavelength converter Antireflecting structure RE-doped “layer” (AR+converter) Absorber (Si in this talk) Solar cells Back electrode/reflector

Periodic photonic structures for energy harvesting L’optique pour le photovoltaïque High index contrast periodic structures: a platform for Photovoltaics U. Delaware INL INL GIST Wavelength-selective intermediate mirror High efficiency absorber Resonant wavelength converter Antireflecting structure AR “layer” Absorber (Si in this talk) Solar cells Back electrode/reflector

Periodic photonic structures for energy harvesting L’optique pour le photovoltaïque High index contrast periodic structures: a platform for Photovoltaics U. Delaware INL INL GIST Wavelength-selective intermediate mirror High efficiency absorber Resonant wavelength converter Antireflecting structure AR “layer” Top junction Indermediate reflector Solar cells Bottom junction Back electrode/reflector

Solar cells using UV, IR photons How can nanophotonics outperform solar cells? L’optique pour le photovoltaïque Light traping into resonant modes Increased absorption Light localisation Higher absorption, lower pin thickness Radiative losses trapping (direct bandgap cells) Increased Voc Control of absorption/emission of light, wavelength conversion Solar cells using UV, IR photons Increased Jsc Increased angular acceptance

Efficiency enhancement in PV devices L’optique pour le photovoltaïque In-coupling and absorption control Up to which limit? « Yablonovitch » limit, 4n² IEEE Trans. Electron. Dev. 1984 weakly absorbing medium Lambertian limit, below 4n² M.A. Green weakly and strongly absorbing media

In-coupling and absorption control Efficiency enhancement in PV devices L’optique pour le photovoltaïque In-coupling and absorption control Up to which limit? Using the photonic toolbox

Efficiency enhancement in PV devices L’optique pour le photovoltaïque Key questions: Which are the best (nano)structures for PV? Optical, electrical properties How to realize these at low cost? What is the most appropriate absorber thickness Real performances of photonized solar cells

L’optique pour le photovoltaïque Outline L’optique pour le photovoltaïque Introduction: nanophotonics and solar energy conversion Photonic crystals and solar cells Physics and modal engineering, case of a-Si:H Thin c-Si solar cells assisted by PhCs Multi-periodic/complex absorbers Absorption enhancement with pseudo-disordered nanopatterns Design rules for solar cells including complex patterns PhCs for wavelength conversion Rare earth doped photonic metastructures for down shifting Conclusion and outlook

L’optique pour le photovoltaïque Outline L’optique pour le photovoltaïque Introduction: nanophotonics and solar energy conversion Photonic crystals and solar cells Physics and modal engineering, case of a-Si:H Thin c-Si solar cells assisted by PhCs Multi-periodic/complex absorbers Absorption enhancement with pseudo-disordered nanopatterns Design rules for solar cells including complex patterns PhCs for wavelength conversion Rare earth doped photonic metastructures for down shifting Conclusion and outlook

2-Photonic crystal absorbers Case of an ultra-thin a-Si:H layer, 100nm RCWA simulation: absorption spectrum PhC -a=380nm -D/a=0.62 Flat reference Y. Park, Opt. Express 17, 14321 (2009) G. Gomard et al., J. Appl. Phys. 108, 123102 (2010)

2-Photonic crystal absorbers Case of an ultra-thin a-Si:H layer, 100nm which mechanisms control the absorption? For>550nm >80% PhC Flat reference PBG PBG R. Peretti et al., J. Appl. Phys. 111, 123114 (2012)

2-Photonic crystal absorbers Case of an ultra-thin a-Si:H layer, 100nm which mechanisms control the absorption? For>550nm SEM SNOM (weakly) self-localized slow light mode

2-Photonic crystal absorbers Case of an ultra-thin a-Si:H layer, 100nm which mechanisms control the absorption? The resonance = mediator Q0=wt0 The absorbing medium Abs. coef.= For>550nm Absorption peaks due to Bloch mode resonances Critical coupling of a single mode: 50% absorption Additional absorption peaks = Multimode structure  Abs. up to 100% Critical coupling conditions or For a-Si:H : a=1000cm-1  Q0=10-100 Y. Park, Opt. Express 17, 14321 (2009) R. Peretti et al., J. Appl. Phys. 111, 123114 (2012)

2-Photonic crystal absorbers Case of a 100nm thick a-Si:H layer which mechanisms control the absorption? For 450nm No absorption peak but... strong absorption increase PhC Flat reference

2-Photonic crystal absorbers Investigating the physics of absorbing PCs in the blue l-range SNOM experiments Back-side illumination Front-side collection using a SNOM tip FDTD simulations Incident light coupled to vertically guided “channeling modes” Ack.: R. Artinyan, S. Callard G. Gomard et al., APL 104, 051119 (2014)

2-Photonic crystal absorbers Investigating the physics of absorbing PCs in the blue l-range FDTD simulations 70-85% of incident light coupled “channeling modes”, and absorbed in a-Si:H, but… etched sidewalls G. Gomard et al., APL 104, 051119 (2014)

2-Photonic crystals and solar cells Case of a 1µm thick c-Si solar cell stack With periodic nano-pyramids (Λ) Ful parameters scan: (os) 1. Period (Λ) 2. ff=a / Λ 3. Thickness of optical spacer (os)

2-Photonic crystals and solar cells Case of a 1µm thick c-Si solar cell stack 23.59 mA/cm² Period (Λ): 800 nm, ff: 0.85 Optical spacer (os): 110nm Anti-Reflection Effect More modes: e.g. SBM 11.52 mA/cm² 105% increase

Collab.: imec, U. Namur, LPICM, Obducat 2-Photonic crystals and solar cells Nanopattern shape optimization: optical assessment Collab.: imec, U. Namur, LPICM, Obducat

Collab.: imec, U. Namur, LPICM, Obducat 2-Photonic crystals and solar cells Optimized nanopattern: optical assessment, experiments 1µm thick c-Si “Epifree silicon” Over the Lambertian limit for l>500nm Over the 4n² limit for specific resonances Collab.: imec, U. Namur, LPICM, Obducat

Collab.: imec, U. Namur, LPICM, Obducat 2-Photonic crystals and solar cells Nanopattern shape optimization: electrical properties 280-µm-thick FZ p-type wafers, patterned by Nanoimprint, passivated with a-Si:H Carrier lifetime assessment by Quasi Steady-State PhotoConductance Flat wafer TMAH RIE shallow RIE deep ICP 2200 µs 930 µs 780 µs 170 µs 44 µs Lifetimes affected by: The etching process: wet better than dry Surface area enhancement + conformality of passivating layer Collab.: imec, U. Namur, LPICM, Obducat

Collab.: imec, U. Namur, LPICM, Obducat 2-Photonic crystals and solar cells Solar cells, 1µm thick c-Si with periodic nanopatterns 4.5% Collab.: imec, U. Namur, LPICM, Obducat

2-Photonic crystals and solar cells Solar cells, 1µm thick c-Si with periodic nanopatterns 6.5% efficiency, 18mA/cm² 4.5% Strong Jsc increase due to photonic crystals Still room for improvement to reach simulated Jsc : parasitic absorption

Collab.: LPICM, LPN, Total 2-Photonic crystals and solar cells Using c-Si prepared by PECVD (LPICM) low cost low T° scalable Collab.: LPICM, LPN, Total Nanopatterning of PECVD c-Si is feasible Nano Lett 2016, to appear

Collab.: LPICM, LPN, Total 2-Photonic crystals and solar cells Using c-Si prepared by PECVD (LPICM) low cost low T° scalable Collab.: LPICM, LPN, Total Still limited efficiency due to absorption in front and back contacts, and limited absorption in IR Nano Lett 2016, to appear

2-Photonic crystals and solar cells How to further increase the efficiency/get closer to the limits? K.X. Wang Stanford, Nano Lett. 2012 X.Q. Meng U. Lyon, Opt. Express 2012 U.W. Paetzold, Jülich, APL 2014 S. Noda U. Kyoto, ACS Phot 2014 Dual gratings Periodic  controlled disorder  disorder

L’optique pour le photovoltaïque Outline L’optique pour le photovoltaïque Introduction: nanophotonics and solar energy conversion Photonic crystals and solar cells Physics and modal engineering, case of a-Si:H Thin c-Si solar cells assisted by PhCs Multi-periodic/complex absorbers Absorption enhancement with pseudo-disordered nanopatterns Design rules for solar cells including complex patterns PhCs for wavelength conversion Rare earth doped photonic metastructures for down shifting Conclusion and outlook

3-Multi-periodic/complex absorbers What can we expect from disorder? SEM SNOM (weakly) self-localized slow light mode Strongly localized light High confinement in real space. Impact in a solar cell?

3-Multi-periodic/complex absorbers Increasing the absorption bandwidth in the low absorption domain Pseudo-disordered pattern = supercell of randomly located holes, periodically repeated in a square lattice. L. Lalouat et al., SOLMAT (2016) H. Ding et al., Opt. Express 24, A650 (2016)

3-Multi-periodic/complex absorbers Increasing the absorption bandwidth in the low absorption domain Supercell lattice: 3x3 Depth (h) cSi Metal Shift Pseudo-disordered pattern = supercell of randomly located holes, periodically repeated in a square lattice.

3-Multi-periodic/complex PhC absorbers Increasing the absorption bandwidth in the low absorption domain Experiments: 3x3 supercell pseudo-disordered structure (EBL+RIE) New peaks Broaden peaks Smaller amplitude peaks

3-Multi-periodic/complex PhC absorbers Increasing the absorption bandwidth in the low absorption domain Square lattice Pseudo-disorder Relative increase 2.1% Experimental 2.7% Theoretical Shift !! Metrics !!  over 2% relative increase, in the best cases

3-Multi-periodic/complex PhC absorbers Design rules: real space analysis 2µm thick c-Si H. Ding et al. Opt. Express 2016 2 x 2 3 x 3 4 x 4 holes are evenly distributed high !! Metrics !! Jsc (mA/cm²) low Clusters of holes appeared

3-Multi-periodic/complex PhC absorbers Design rules: real space analysis 2µm thick c-Si H. Ding et al. Opt. Express 2016 2 x 2 3 x 3 4 x 4 Efficient design of optimized light-trapping structures !! Metrics !!

L’optique pour le photovoltaïque Outline L’optique pour le photovoltaïque Introduction: nanophotonics and solar energy conversion Photonic crystals and solar cells Physics and modal engineering, case of a-Si:H Thin c-Si solar cells assisted by PhCs Multi-periodic/complex absorbers Absorption enhancement with pseudo-disordered nanopatterns Design rules for solar cells including complex patterns PhCs for wavelength conversion Rare earth doped photonic metastructures for down shifting Conclusion and outlook

Periodic photonic structures for energy harvesting L’optique pour le photovoltaïque High index contrast periodic structures: a platform for Photovoltaics U. Delaware INL INL GIST Wavelength-selective intermediate mirror High efficiency absorber Resonant wavelength converter Antireflecting structure AR “layer” Absorber (Si in this talk) Solar cells Back electrode/reflector

Periodic photonic structures for energy harvesting L’optique pour le photovoltaïque High index contrast periodic structures: a platform for Photovoltaics U. Delaware INL INL GIST Wavelength-selective intermediate mirror High efficiency absorber Resonant wavelength converter Antireflecting structure RE-doped “layer” (AR+converter) Absorber (Si in this talk) Solar cells Back electrode/reflector

4-PhC for wavelength conversion Rare-earth doped photonic meta-structure Rare earth doped layer e.g. Eu-doped Y2O3 SiNx Photonic crystal LDS Layer UV photons (~ 400nm) Visible photons (611 nm) Collab.: B. Moine, A. Pillonnet, A. Pereira

4-PhC for wavelength conversion Rare-earth doped photonic meta-structure, luminescence measurements with an integrating sphere L=250nm and r=80nm Collab.: N.-V. Hoang, B. Moine, A. Pillonnet, A. Pereira Strong Enhancement in Eu3+ emission → Expected improvement of down shifting efficiency N.-V. Hoang et al. (in preparation)

4-PhC for wavelength conversion Photonic resonances at absorption wavelengths: Measured Transmittance RCWA Simulation Histogram from SEM images (1mm2) Bloch modes resonances Clear evidence of the resonant modes Transmittance remains high at longer wavelength N.-V. Hoang et al. (in preparation)

4-PhC for wavelength conversion Photonic modes at emission wavelengths: Excitation (394 nm) q Emission (611 nm) Clear evidence of the resonant modes Oblique emission 611 nm N.-V. Hoang et al. (in preparation)

5-Conclusions, outlook In-plane Bloch modes and vertical “channeling” modes PhC absorbers Mediators to control incident light capture and absorption  Enables to increase Abs/JSC by 100% Photonic crystal solar cells  Dedicated processes developped: nanopatterning, passivation Positive impact of controlled perturbations  Experimental assessment of the integrated absorption increase in a- Si:H and c-Si based stacks. Strong enhancement of down shifting thanks to PhCs  x50, expected PV conversion efficiency Still many work to do in photonics:  Towards Jsc>30mA/cm² with a Si layer of less than 10µm  Develop advanced light trapping for other families of solar cells.  Other light-matter interaction effects, useful for PV (down-conversion…)

Periodic photonic structures for energy harvesting L’optique pour le photovoltaïque High index contrast periodic structures: a platform for Photovoltaics U. Delaware INL INL GIST Wavelength-selective intermediate mirror High efficiency absorber Resonant wavelength converter Antireflecting structure AR “layer” Absorber (Si in this talk) Solar cells Back electrode/reflector

Periodic photonic structures for energy harvesting L’optique pour le photovoltaïque High index contrast periodic structures: a platform for Photovoltaics U. Delaware INL INL GIST Wavelength-selective intermediate mirror High efficiency absorber Resonant wavelength converter Antireflecting structure RE-doped “layer” (AR+converter) Absorber (Si in this talk) Solar cells Back electrode/reflector

Periodic photonic structures for energy harvesting L’optique pour le photovoltaïque High index contrast periodic structures: a platform for Photovoltaics U. Delaware INL INL GIST Wavelength-selective intermediate mirror High efficiency absorber Resonant wavelength converter Antireflecting structure AR “layer” Top junction Indermediate reflector Solar cells Bottom junction Back electrode/reflector

Periodic photonic structures for energy harvesting L’optique pour le photovoltaïque High index contrast periodic structures: a platform for Photovoltaics Perovskite or III-V, or… AR “layer” Top junction Indermediate reflector Bottom junction Back electrode/reflector Silicon or……

Cellules photovoltaïques - introduction Acknowledgements Cellules photovoltaïques - introduction The NANOLYON team EU’s 7th program Grant N°309127 PROGELEC http://nathisol.ec-lyon.fr

3-Multi-periodic/complex absorbers Better angle acceptance? Simulation: Square lattice  3x3 pesudodisorder Higher absorption, mostly close to normal incidence Better stability Current density (mA/cm²) Lambda: 700-1100nm

3-Multi-periodic/complex PhC absorbers Increasing the absorption bandwidth in c-Si cSi TCO Metal P r x40 realizations of the disorder Optimized RCWA simulation: Max achievable Jsc (mA/cm²) c-Si thickness Optimized Square lattice Best pseudo-disordered Size of the best supercell / square lattice Rel. Jsc increase 1µm 19.96 20.02 2x2 +0.3% 2µm 23.71 24.20 +2.1% 4µm 27.46 27.95 3x3 +1.8% 8µm 30.69 31.28 +1.9% 2% relative increase, in the best cases

Record efficiency for ultrathin nanopatterned epifree solar cell 2-Photonic crystals and solar cells Record efficiency for ultrathin nanopatterned epifree solar cell 9.6% Jsc (mA)

Record efficiency for ultrathin nanopatterned epifree solar cell 2-Photonic crystals and solar cells Record efficiency for ultrathin nanopatterned epifree solar cell 9.6%

3-Multi-periodic/complex PhC absorbers Design rules: reciprocal space analysis 2µm thick c-Si State of the art: Magn. of Fourier component  in-coupling:  magnitude of the Fourier components between 2p/L and 4p/L (LC. Andreani et al)  out-coupling:  magnitude of the Fourier components lower than 2p/l0 (T. Krauss et al) Present study: Both criteria are satisfied Small gain in Jsc but without extra cost H. Ding et al. Opt. Express 2016