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Light trapping with particle plasmons Kylie Catchpole1,2, Fiona Beck2 and Albert Polman1 1Center for Nanophotonics, FOM Institute AMOLF Amsterdam, The Netherlands 2Australian National University Canberra, Australia Poor absorption below the bandgap Indirect bandgap Semiconductor (Si): poor absorption just below the bandgap thick cell required Eg solar spectrum Si solar cell Solution: light trapping fair fsubs f subs Goal: • Increased efficiency (IR response) and/or • Reduced thickness (=cost) Plasmon-enhanced photocurrent: 5 examples Stuart and Hall, APL 69, 2327 (1996) SOI Derkacs et al., APL 89, 93103 (2006) Schaadt et al., APL 86, 63106 (2005) a-Si Si Pillai et al., JAP 101, 93105 (2007) Nakayama et al., APL 93, 121904 (2008) Si SOI GaAs Plasmon-enhanced photocurrent: 5 examples Stuart and Hall, APL 69, 2327 (1996) SOI Derkacs et al., APL 89, 93103 (2006) Schaadt et al., APL 86, 63106 (2005) a-Si Si What are the physical principles Nakayama et al., APL 93, 121904 (2008) Pillai et al., JAP 101, 93105 and (2007) limitations Si SOI GaAs Light scattering Rayleigh scattering from point dipole Scattering from point dipole above a substrate p p E p p 2 4 sin 2 I s 32 2c3 0 r 2 4 % Preferential scattering into high-index substrate 96 % See, e.g.: J. Mertz, JOSA-B 17, 1906 (2000) (a) Resonant scattering TOT, DIP / R REF Metal nanoparticle scattering 150 Material: Ag (Palik) 100 Scattering vs Ohmic losses 50 Albedo 1 for D > 100 nm 0 F Albedo 1,0 m 4 0 R 2 m 3 Plasmon resonance: = -2m() Ag (b) 0,8 0,6 Absorption ~ r3 Scattering ~ r6 0,4 0,2 0,0 0 50 100 Sphere diameter (nm) 150 Metal nanoparticle scattering Cross section > 1 All light captured and scattered into substrate (=AR coating) Resonance tunable by dielectric environment Ag, D=100 nm Si3N4 (n=2.00) Q D O Si (n=3.5) Q D H Optics Express (2008), in press From point dipole to particle plasmon 96 % FDTD calculations Fraction scattered into substrate 1 0.8 0.6 0.4 dipole cylinder hemisphere sphere 100nm sphere 150nm 0.2 0 500 550 600 650 700 Wavelength (nm) 750 0 800 Fraction scattered into substrate highest for cylinder & hemisphere: Strongest near-field coupling Tradeoff: larger size larger albedo but lower coupling Appl. Phys. Lett. 93, 191113 (2008) Maximum path length enhancement fair fsubs f subs Highest path length enhancement for cylinder and hemisphere enhancement length Path maximum path length enhancement Geometric series horizontal dipole 100 Lambertian hemisphere 30 x cylinder 10 sphere 100nm sphere 150nm (A=0.90) (A=0.95) 1 0.6 0.7 0.8 0.9 fraction into substrate Fraction scattered into substrate Appl. Phys. Lett. 93, 191113 (2008) 1 Scattering cross-section with dielectric spacer σscat normalized to particle area Larger spacing: 14 12 Q Qscat, Qsubs 10 30 nm 30nm Interference in driving field tot 8 sub 6 But: lower coupling fraction D 4 2 0 500 (+ local density of states variation modifies albedo) 10 nm10nm 600 700 800 wavelength (nm) 900 1000 Appl. Phys. Lett. 93, 191113 (2008) Ag nanoparticle formation on SiO2/Si3N4/TiO2 on Si Thermal evaporation of 14 nm Ag + 300 °C anneal Thermal SiO2 dave= 135 nm f = 26% n=1.46 LPCVD Si3N4 dave= 220 nm f = 28% n=2.00 APCVD TiO2 dave= 215 nm f = 30% n=2.50 Optical absorption (1-R-T) in Si wafers Integrating sphere 30 nm 100 μm SiO2 Si3N4 TiO2 c-Si Si3N4 c-Si SiO2 Ref. Ref. TiO2 AR effect, interference for shorter wavelength + redshift Strongly enhanced near-IR absorption egineered by dielectric spacer TiO2 Si3N4 SiO2 Photocurrent, external quantum efficiency front SiO2 back Si3N4 front TiO2 front back back Red-shifted EQE enhancement with refractive index of underlying dielectric Decrease at short wavelength due to phase shift Small increase at long wavelength for TiO2 Relative photocurrent, EQE enhancement TiO2 back front Si3N4 SiO2 SiO2 TiO2 coated Si: EQE enhancement 2.7 fold at λ = 1050 nm Si3N4 TiO2 Note: particle size and distribution are not optimized Design principles for plasmon-enhanced solar cells 1) Metal nanoparticles scat > 1 2) Coverage ~ 10-20 % required 3) D>100 nm albedo > 0.95 i.e. Ohmic losses < 5% 4) Angular distribution (=path length) increased 5) Coupling fraction f = 0.96 for point dipole 6) f reduces for larger particle size 7) scat increases with spacer thickness 8) f decreases with spacer thickness Design parameter optimization Include: inter-particle coupling Appl. Phys. Lett. 93, 191113 (2008) For details/references visit: www.erbium.nl VACANCIES in nano-photovoltaics see: www.amolf.nl Substrate Conformal Imprint Lithography PDMS Stamp Thin glass PDMS stamp (6”) on 200 µm AF45 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 Angular dependence of scattered light fair W dav Lambertian dav=2 Dipole dav~1.5 Increased power around critical angle for dipole compared to isotropic Lambertian less oblique path K.R Catchpole and A. Polman, APL (2008) Tadeoff between cross section and incoupling Point dipole Optics Express (2008), in press