Page - 71 - in Photovoltaic Materials and Electronic Devices

Thestructuralmodelconstructedforeachsampleconsistsofastratifiedlayer stackofopticallydistinctmaterials,whichmaybecontinuousfilms, interfaceswith unique ε and thickness, or Bruggeman effective medium approximation layers. Ellipsometric spectra are modeled using a scattering matrix formalism [44] in which 2ˆ2 matrices based on Fresnel coefficients and wave propagation of light throughmedia isgeneratedfor thesemi-infinitesubstrateandambient, each layer offinite thickness,andthe interfacesbetweeneachopticallydistinct layer. Matrices are calculated for the components of the incident electric field both parallel and perpendicular totheplaneof incidence. Ingeneral,ellipsometricspectraaresensitive to spectra in ε for each material, including the ambient and substrate, and the thicknessesofopticallyfinite layers. 3. ResultsandDiscussion 3.1. OpticalCharacterizationofBackReflectorComponentsandStructure The first layers deposited for n-i-p configuration a-Si:H solar cells comprise the ZnO/Ag BR structure. Therefore, ellipsometric spectra from 0.04 to 5.0 eV are collected and analyzed for a ZnO/Ag BR structure. The models and thicknesses describedherefirstcorrespondtoaZnOcoatedAgBRsample,whilevariations in propertiesduetogrowthofover-depositedn-and i-typea-Si:Hlayers isdescribedin Section3.3.1. All layersweredepositedwithoutvacuumbreakwithconditionsgiven inTable1. InsituSEdata from0.734eVto5.88eVwascollectedforeachdeposited layer and the model generated was used for extended spectral range IR-SE analysis. 3.1.1. AgandZnO+AgInterfaceProperties Data collected for semi-infinite Ag substrate were taken before ZnO layer depositionat roomtemperatureandwereanalyzedintheenergyrangefrom0.734 to5.88eV.Figure1showsspectra in ε forAgparameterizedbyacombinationofa Drude oscillator [45], a higher energy transition assuming critical point parabolic bands (CPPB) [46], and a constant additive term to ε1 denoted as ε8. The surface roughness is represented by two Lorentz oscillators [47] with ε8= 1. The Drude oscillator is representedby: εpEq“ ´} 2 ε0ρpτE2` i}Eq (3) where h¯ is the reduced Planck’s constant, ε0 is the permittivity of free space,τ is the scatteringtime,andρ is theresistivity. EachCPPBoscillator is representedby: εpEq“Aeiφ " Γ r2En´2E´ iΓs *µ (4) 71