Page - 85 - in Photovoltaic Materials and Electronic Devices

Table 5. Parameters describing ε and structure for a ZnO/Ag BR coated with n-type and intrinsic a-Si:H. Experimental ellipsometric spectra were collected ex situusingnear infraredtoultraviolet (0.734to5.0eV)andinfrared(0.04 to0.734eV) spectral range instruments and fit jointly using least squares regression analysis withanunweightedestimatorerror function,σ=11ˆ10´3. Parametersdescribing ε for Ag and the ZnO + Ag interface were fixed from Tables 2 and 3 respectively. Parameters describing ε for the n-layer were determined from RTSE analysis of data collected at T = 200˝C, parameterized by a Cody-Lorentz oscillator, and then parameter values extrapolated to room temperature. The ZnO and intrinsic a-Si:H bulk layer thicknesseswereallowedtovaryseparately foreachsetofspectra;all other parameters are common to both analyses. For ZnO, the parameterization of εconsistedof twoCPPB oscillatorswith allparameters except theamplitudes fixedtothevalues inTable4, fourLorentzoscillators,and ε8. Fora-Si:Hlayers, the parameterization of εwas based on a Cody-Lorentz oscillator and ε8. A Sellmeier oscillatorandthreeGaussianoscillatorswereaddedtotheparameterizationof ε for intrinsica-Si:H. Layer Oscillators i-typea-Si:H db (Near IRtoUV)=3623˘1Å db (IR)=3619˘2Å ds =29.0˘0.3Å i-typea-Si:H Cody-LorentzEg (T&R)=1.780˘0.001; ε8=1.50˘0.01 Gaussian A(Unitless) Γ (eV) E0 (eV) 1.732˘0.06 0.013˘0.001 0.079˘0.001 0.28˘0.01 0.010˘0.001 0.249˘0.001 0.41˘0.04 0.016˘0.002 0.106˘0.001 Sellmeier 0.0050˘0.0002eV2 - 0 i- a-Si:H/n-typea-Si:H Interface=30˘1Å n-layer db =278˘1Å n-typea-Si:H/ZnO Interface=84Å n-typea-Si:H Cody-Lorentz;ε8=1 A(eV) Γ(eV) E0 (eV) Eg (eV) Ep (eV) 62 2.01 3.99 1.65 1.05 ZnO db (Near IRtoUV)=2763˘3Å db (IR)=2738˘5Å ZnO CPPB(µ=0.5)ε8=1.91˘0.02 A Γ (eV) En (eV) Materials  2016,  9,  128  9  of  23  Table  3.  Parameters  describing  ε  and  structure  for  a  ZnO  film  deposited  on  Ag  and  the  ZnO  +  Ag  interface  formed.  Experimental  ellipsometric  spectra  were  collected  in  situ  after  deposition  at  room  temperature  in  the  spectral  range  from  0.734  to  5.0  eV  and  fit  using  least  squares  regression  analysis  with  an  unweighted  estimator  error  function,     =  7  ×  10−3.  Parameters  describing  ε  for  Ag  were  fixed  from  Table  2.  For  ZnO,  the  parameterization  of  ε  consisted  of  two  CPPB  oscillators,  a  Sellmeier  oscillator,  and  ε .  For  the  ZnO  +  Ag  interface,  the  parameterization  of  ε  consisted  of  a  Drude  oscillator,  a  Lorentz  oscillator,  and  ε .    Layer  Oscillators  ZnO  db  =  3060  ±  3  Å  ds  =  80  ±  1  Å  CPPB  (μ  =  0.5)  ε =  2.27  ±  0.01  A  (Unitless)     (eV)  En  (eV)  Ө  (degrees)  2.63  ±  0.02  0.199  ±  0.002  3.363  ±  0.001   20.1  ±  0.5  1.41  ±  0.02  3.83  ±  0.08  4.36  ±  0.03  0  (fixed)  Sellmeier  A  (eV2)     (eV)  En  (eV)  0.080  ±  0.002  ‐  0  ZnO/Ag  Interface  =  108  ±  11  Å  Lorentz  ε =  1  A  (Unitless)     (eV)  E0  (eV)  2.8  ±  0.2  0.57  ±  0.05  2.83  ±  0.01  Drude     (   cm)     (fs)  3.7  ±  0.5  x10−5  2.7  ±  0.3  3.1.2.  Phonon  Modes  in  ZnO  The  analysis  was  extended  to  the  IR  by  fitting  parameters  defining  ε  for  ZnO  only  and  fixing  those  defining  ε  for  Ag  and  the  ZnO  +  Ag  interface  as  well  as  the  interface  layer  thickness.  This  analysis  approach  was  chosen  because  free  carrier  absorption  represented  by  the  Drude  feature  dominates  the  IR  response  of  Ag  and  the  ZnO  +  Ag  interface  layers  and  is  already  established  from  near  IR  to  UV  spectral  range  analysis.  A  common  parameterization  of  ε  for  the  ZnO  was  applied  for  4.04˘0.05 0.2 9 . 4 ´20.8 1.31˘0.02 3.95 3.94 0 Lor ntz A(Unitless) Γ (eV) E0 (eV) 3.89˘0.1 0.233˘0.001 0.162˘0.002 82.0˘4.0 0.0030˘0.0003 0.0506˘0.0001 16.4˘0.4 0.039˘0.002 0.085˘0.001 13.0˘3.0 0.004˘0.002 0.047˘0.001 85