Nanophotonic light trapping for high efficiency solar cells Kylie Catchpole Centre for Sustainable Energy Systems, Research School of Engineering Australian National University, Canberra.

Why nanophotonics? Thin film eg.  c-Si, a-Si mc-Si Quantum dotOrganic c-Si Applications of nanophotonics ? New types of solar cells are hard to texture or very thin

Nanophotonics for solar cells Localized surface plasmons ≈ (wavelength scale) <<  (sub-wavelength): effective medium >>  : geometrical optics 10  m Scattering back reflectors Diffraction gratings Catchpole et al. (2011) MRS Bulletin 2011; 36(6) :

Nanophotonics for light trapping Mokkapati and Catchpole, Journal of Applied Physics - Focused Review 112, (2012)

Progress for crystalline silicon Mokkapati and Catchpole, Journal of Applied Physics - Focused Review 112, (2012) Open symbols – theory Closed symbols - experiment Nano-cones (Wang et al. Nano Lett. 2012) Skewed pyramids (Chong et al. J. Opt. 2012)

Snow Globe Coating A. Basch, F.J. Beck, T. Söderström, S. Varlamov, K.R. Catchpole, Progress in Photovoltaics, 2012

Silver particles and Snow Globe Coating

Snow Globe Coating combined with Plasmonic Nanoparticles Plain4.0mA/cm 2 Snow Globe 8.0mA/cm 2 coated with plamonic particles 100% increase in Jsc A. Basch, F.J. Beck, T. Söderström, S. Varlamov, K.R. Catchpole, Appl. Phys. Lett. 2012

TiO 2 diffraction gratings Light induced passivation gives lifetimes of 700µs Barbé et al. Progress in Photovoltaics, 20(2), 143 (2011) Wang et al. Progress in Photovoltaics (2012), DOI: /pip.2294 Passivation Light trapping

Plasmonic enhancement Far-field (Scattering): Near-field: Strong local field enhancement - very thin absorbers Increased optical local density of states Parasitic absorption Scattering/absorption cross-sections Diffraction efficiency Mode coupling/light trapping Catchpole & Polman, Opt. Express 2008, Atwater & Polman, Nature Mat

Metallic perfect absorbers I C. M. Watts et al., Advanced Mater. 24, OP98-OP120 (2012).

Metallic perfect absorbers II a) Metamaterial (t, d << ) Effective medium ( ,  ) Impedance matched to free space b) Resonant cavity (t~  c) Plasmonic grating (d~ ) Coupling to SPPs t d dielectric metal C. M. Watts et al., Advanced Mater. 24, OP98-OP120 (2012).

Extremely thin absorber cells Conventional ETA SC Large absorption volume Short carrier path length Transparent transport layers Solution processed Uniformity/infiltration issues

Extremely thin absorber cells Conventional ETA SC Large absorption volume Short carrier path length Transparent transport layers Solution processed Uniformity/infiltration issues Planar ETA SC Local field-enhancement Reduced surface area (recombination?) Physical layer deposition (sputtering/evaporation)

Ultrathin absorber geometry AIR SUPERSTRATE (n = 3.6) Silver stripe, 100nm wide, 25nm high F.J. Beck et al., Opt. Express 19, A146-A156 (2011).

Results Absorbing layer: n = 3.6  = 3.4 x 10 4 cm -1 ~1.8% 5nm E TM Numerical simulations: COMSOL (FEM) TM polarization

Results ~4% 5nm Superstrate n = 3.5 ~1.8% Single pass On superstrate

Results ~16% 5nm Superstrate n = nm 25nm Ag ~4% ~1.8% Single pass On superstrate Grating

Results Single pass On superstrate Grating + mirror 5nm Superstrate n = nm 25nm Ag ~4% ~1.8% ~16% 90% Wang, White & Catchpole, IEEE Photonics Journal 2013.

Results 98% 5nm Superstrate n = nm 25nm 90% ~4% ~1.8% ~16% Single pass On superstrate Grating + mirror Total absorption Wang, White & Catchpole, IEEE Photonics Journal 2013.

Results 5nm Superstrate n = nm 25nm Single pass On superstrate Grating + mirror Total absorption Wang, White & Catchpole, IEEE Photonics Journal x increase in absorption

Results: angular dependence Angle and polarization averaged path length enhancement = 28 Compared to 2D Lambertian limit  n  11 Wang, White & Catchpole, IEEE Photonics Journal 2013.

Crystalline-Si tandems High efficiency c-Si: UNSW PERL Cell (1998):  = 25% (4cm 2 ) Sunpower (2010): 24.2% (155cm 2 ) Panasonic (2014): 25.6% (143cm 2 ) Low-cost thin film Bandgap ~1.7eV Cheap Earth-abundant Efficiencies 25-30%?

Crystalline-Si Tandems Nanotechnology 19, (2008). Janz et al., EU PVSEC (2013). Sunshot projects (Next Generation PV II): III-V Nanowires on c-Si CdSe on c-Si Organic/c-Si tandem: Energy Environ. Sci., 5, 9173 (2012). Nature, 501, 395 (2013): “perovskite cells have now achieved a performance that is sufficient to increase the absolute efficiency of high- efficiency crystalline silicon cells”

Tandem solar cells How good does the top cell need to be? Material requirements Bandgap Diffusion length Luminescence efficiency Optical requirements Low parasitic α Minimal transparent conductor loss Wavelength selective light trapping

How good does a top cell need to be? Top cell bandgap (eV) Bottom cell power (W/m 2 ) T AM1.5G (1000W/m 2 )  = 25% c-Si PERL Cell J sc = 42.7mA/cm 2 V oc = 706mV FF=0.828

How good does a top cell need to be? Top cell bandgap (eV) Required top cell efficiency (%)  = 27.5%  = 30% Top cell bandgap (eV) Bottom cell power (W/m 2 )  = 25% (breakeven efficiency) T AM1.5G (1000W/m 2 )  = 25% c-Si PERL Cell J sc = 42.7mA/cm 2 V oc = 706mV FF=0.828

How good does a top cell need to be? Top cell bandgap (eV) Bottom cell power (W/m 2 ) T AM1.5G (1000W/m 2 )  = 25% c-Si PERL Cell J sc = 42.7mA/cm 2 V oc = 706mV FF=0.828

Four-terminal tandem model Bottom cell: c-Si PERL (25%) J 0 ~ 49 fA/cm 2 FF = 82.8% Top cell (p-i-n): Bandgap E g (direct) Absorption Diffusion length L d => carrier collection [1] Luminescence efficiency  V oc [2] FF = 0.8 Strong absorbers (  0 ~10 4 cm -1 ) Short diffusion lengths ~200nm [1] Taretto, Appl. Phys. A 77, 865 (2003) [2] Smestad, Solar Energy Mat. Solar Cells (1992) White, Lal & Catchpole, IEEE J. Photovolt. (2013), DOI: /JPHOTOV

Results: E g and  dependence L d = 100 nm  0 = 10 4 cm -1 External luminescence efficiency: [Green, Prog. Photovolt: Res. Appl. 20, 472 (2012)] GaAs: > 0.2 c-Si: ~6 x CIGS: a-Si: ~10 -5  = White. Lal & Catchpole, IEEE J. Photovolt. (2013), DOI: /JPHOTOV Light trapping can increase efficiency by 3% absolute. Optimum bandgap increases as  decreases to offset V oc loss.

Results: L d and    dependence E g = 1.95eV  0 = 10 4 cm -1  = (qV oc = 0.6E g ) L d = 35 nm  = L d = 100 nm  = 10 -5

Optical losses TCO Zeng et al. Adv. Mater. 22, (2010). Required top cell efficiency (%) Top cell bandgap (eV)  tandem  = 25%  tandem  = 30% 10% parasitic loss in top cell 20% parasitic loss in top cell Parasitic absorption >20% makes reaching 30% practically impossible

Sub-bandgap absorption Wavelength (nm)  (cm ) a-Si:H CIS Sb 2 S 3 CZTS Top cell thickness (nm) Current lost from bottom cell (mA/cm 2 ) Sb 2 S 3 (E g =1.73eV) CZTS (E g =1.5eV) CIS (E g =1.5eV) a-Si:H (E g =1.7eV) Cell thickness is limited by sub-bandgap loss CZTS Sb 2 S 3 a-Si:H CIS Parasitic absorption of perovskite is very low (similar to a-Si:H) – much more promising than CZTS.

Light trapping Any light trapping must be wavelength selective Broadband trapping for the top cell is detrimental to total efficiency After Green (2002) Prog Photovolt. Res. Appl. (10) 252, to include transparency in the rear reflector Lal, White & Catchpole, submitted

Tandem cells on Si Gaining 5% efficiency on c-Si requires very good top cells How to get there: –III-V or perovskites on Si –Identify new materials: E g, L d,  –High bandgap (E g  =10 -5 ) –Minimize sub-bandgap absorption –Minimize TCO absorption –Wavelength-selective light trapping Top cell bandgap  tandem = 25%  tandem = 30% 1.5eV  top > 17%  top > 22% 1.7eV  top > 12%  top >17% 2eV  top > 9%  top > 14% White, Lal & Catchpole, IEEE J. Photovolt. (2013), DOI: /JPHOTOV

Conventional absorption measurement : 36 Electron-hole pairs Quantifying light trapping A FC (ћω) Free carrier absorption & Parasitic absorption A BB (ћω) for band to band transition Absorptance = 100% - Reflectance - Transmission Conventional absorption measurement makes it hard to identify best light trapping structures.

“A good solar cell makes a good LED and a great LED makes a great solar cell” 37 Emission Absorption higher energy state lower energy state ΔEΔE hν Quantifying light trapping Luminescence

Characterization Photoluminescence spectra of cell structure with and without light trapping hν

39 Constant for certain materials at fixed TWavelength dependent T. Trupke et al., Sol. Energ. Mat. Sol. Cells vol 53, (1998) Characterization

Ag nanoparticles & diffused white coating Structure 1 Structure design Structure 2 c-Si diffused white reflector Ag nanoparticles with metal reflector Ag nanoparticles c-Si metal reflector

41 Plasmonics & DWC on c-Si cell Structure 1 c-Si diffused white coating Dielectric Environment: Passivation layers PECVD α-Si:H PECVD Si 3 N 4 ALD Al 2 O 3 Nanoparticle size Ag film thickness: 15nm 21nm 27nm 33nm

Plasmonics in back contact cell Structure 2 27nm Ag (D=~200nm) Capping layer thickness: 60~150nm PECVD Si 3 N 4 c-Si metal reflector 27 nm 1 μm

Quantifying light-trapping Absolute Absorption Spectrum %A Maximum Possible Photon Current J sc Fraction of Lambertian Enhancement (FLE)

Experimental results C. Barugkin et al., IEEE Journal of Photovoltaics % for Inverted Pyramid Texture with PLE=16 ( T. Trupke et al., Sol. Energ. Mat. Sol. Cells, 1998 ) c-Si diffused white coating c-Si metal reflector 12 Light trapping similar to inverted pyramids but applicable to any cell

Summary Metal particles and scattering back reflectors - can give 100% J sc enhancement. Near-field absorption for planar ETA –90% absorption in 5nm layer Defined optical and electrical requirements for high efficiency tandems on Si Photoluminescence for quantifying light trapping - 62% of Lambertian increase demonstrated PTO for conference slide

Metallic perfect absorbers III K. Aydin et al., Nature Comms. 2, 517 (2011). Can be broadband, angle- and polarization-independent TPV applications (tunable emissivity) X. Liu et al., Phys. Rev. Lett. 107, (2011).

Absorber layer thickness Maximum absorptance Absorber thickness (nm)

Predicted external quantum efficiency Modelled J sc : 1000~1200nm collection η 95% 2.2mA/cm 2 2.6mA/cm 2 18% enhancement c-Si Metal reflector c-Si Metal reflector

Need for light trapping AM 1.5 solar spectrum and solar radiation absorbed in 2 μm c-Si thin film, assuming single pass t optical thickness >> physical thickness ‘t’ glass Thin semiconductor (few µm) Thin solar cells an alternative for low cost PV