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单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,*,Analysis of Optical Absorption in Nanowire Arrays for Photovoltaic Applications,2013-11-7,Wang Weiping,1.F,inite-difference time-domain,(FDTD),simulations,T,ransfer matrix method (TMM),Analysis of Optical Absorption in,Silicon Nanowire Arrays for Photovoltaic,Applications,Apsorption = 1 - Refectance - Transmittance,Geometric parameters: Diameter, Length, Filling ratio,Frequency range：1.14 eV,Fixed diameter: 50 nm,In high,-energy photon regime, the absorption reaches a plateau region shared by,all lengths,In,low-energy photon regime, the absorption in,the film is more efficient.,the reflectance of nanowires is,significantly lower than that of the thin film in the entire spectral range,For low frequencies, the extinction coefficient of,silicon is small and interference effects exist, resulting in,the oscillation of reflectance and transmittance,Fixed period : 100 nm, wire length : 2.33 um,T,he electromagnetic,interaction between nanowires cannot be neglected!,U,ltimate,efficiency,OR,The absorption efficiencies are 5.80%,9.47%, 12.50%, and,15.50%,for the 50, 65, and 80 nm,nanowires and the thin film, respectively,.,Putting a p,erfect,reflecting,mirror (100% reflectivity) on the backside,the overall absorptance efficiency is enhanced from 12.50% to,16.09%,a=100 nm, diameter=80 nm,length=2.33 um,the nanowire structure is,almost isotropic in the x-y plane,TE polarization has lower,absorption than TM polarization because the latter has an electric field component along the wire axis which facilitates the absorption,CONCLUSION: small reflection in a wide spectrum range,in the low-frequency regime, apply light-,trapping techniques or use longer wires,Analysis of optical absorption in GaAs nanowire,arrays,A,perfect refractive index matching at the top interface,between the air and NWAs, hence leads to good coupling of the incident light into the NWAs,Fixed D/P: 0.5, diameters:60240 nm,1.T,he light,absorption is significantly enhanced by the increase of NW diameters from 60 to 180 nm in the long wave length region (photonic resonance mode).,2.Diameter up to 240 nm leads to a decrease in absorption efficiency (enhanced light reflection at the top surface of the NWAs).,3.nanowire acts as a nanoscale cylindrical resonator which can trap light by multiple total internal reflections.,The optical generation rate:,D,/P = 0.5,Length = 2 um, diameter,= 180 nm, wavelength = 800nm.,1.,At,angles of incidence up to 60, the total absorption is maintained above 80% for both TE and TM polarizations.,2.A,higher absorption is,observed in TM polarization than in TE polarization,Conclusion:,The optimal,results for the normal incidence are evaluated as,diameter = 180 nm, length = 2 m, and D/P = 0.5.,2.Mie resonance theory,length=2.5um, diameter=425nm,short-circuit current: 256 pA,open circuit voltage:0.43, filling factor: 0.52,photogenerated current density of 180 mA/cm,2,maximum power: 57pw; efficiency: 40%,Methods-nanowire growth,Nanowires were grown on oxidized Si(111) with 100 nm apertures using a self-catalysed method,The p-doping of the corewas achieved by adding a flux of beryllium during axial growth,The n-type doping was obtained by adding silicon to the growth step,Device fabrication,SU-8 was spun onto the substrate at,4,000 r.p.m. for 45 s and cured with 1 min ultraviolet light and 3 min on a hotplate at 185,o,C,An etch-back with a 13 min oxygen plasma etch wasthen performed to free the nanowire tip,The top contact was defined by electron-beam lithography followed by evaporation of indium tin oxide (ITO),The bottom contact was obtained by silver gluing to the back-side of the wafer,Optical simulations of a single nanowire solar cell,The absorption cross-section is defined as,The absorption cross-section in nanoscale materials is larger than their physical size,The largest absorption is for a nanowire diameter of 380nm,E,xternal quantum efficiency (EQE),and Scanning photocurrent measurements,T,he absorption boost in,the,device is,due to an unexpectedly large absorption cross-section of the vertical nanowiregeometry,Conclusion,A,remarkable boost in absorption,is,observed in single-nanowire solar,cells,that is related to the vertical configuration of the nanowires and to a resonant increase in the,absorption cross-section. These results open a new route to third-generation solar cells, local energy harvesters on nanoscale devices,and photon detectors,.,S,cattering efficiencies,of,Mie,calculations,transversal magnetic (TM) polarization,transversal electric (TE) polarization,non-polarized,AbsorptionandScattering,of Light by Small Particles BohrenCF andHuffmanDR 1998,Optical scattering,properties,of,GaAs,NWs,C,alculated,scattering,efficiencies Q,sca,(red,lines) and,scaled,measured,spectra (black lines),L,eaky-mode resonances (LMRs),This work shows that light,absorption in nanowire devices is not just a function of the,intrinsic optical materials properties, but also can be engineered,through control over the size, geometry and orientation of the,nanostructure.,3.Rigorous coupled wave analysis (RCWA),Numerical simulations of absorption properties of InP,nanowires for solar cell applications,Thanks,24,
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