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Microstructure analysis of n-doped μ c-SiO x :H reflector layers and their use in stable a-Si:H p-i-n cells. Pavel Babal * , Johan Blanker, Ravi Vasudevan, Arno Smets, and Miro Zeman Photovoltaic Materials and Devices, Delft University of Technology. *. Motivation.

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Microstructure analysis of n-doped μc-SiOx:H reflector layers and their use in stable a-Si:H p-i-n cells

Pavel Babal*, Johan Blanker,

Ravi Vasudevan, Arno Smets,

and Miro ZemanPhotovoltaic Materials and Devices, Delft University of Technology



Microcrystalline hydrogenated silicon oxide (c-SiOx:H) has been successfully applied in solar cells, improving their performance, yet the microstructure of this material is not fully understood. Parameters of c-SiOx:H layers have been optimized and the heterogeneous microstructure has been studied with Raman and FTIR spectroscopy revealing correlations between deposition parameters, the material properties of c-SiOx:H, and solar cell performance. c-SiOx:H/Ag back reflectors have been integrated in a-Si:H single junction cells, achieving through improved light trapping an initial efficiency of 11.1%. The best stable efficiencies are achieved for cells with an intrinsic a-Si:H film of around 200 nm.

n-doped μc-SiOx:H development and characterization

Raman analysis to study crystalline grains

Material properties

  • Properties and trends
  • 5 deposition parameters varied
  • Trade-off between electrical and optical properties
  • Lower refractive index (n) ~2.3 - higher reflection
  • Higher bandgap ~2.5 eV – less absorption losses
  • More PH3 - smaller lateral conductivity
  • H2 dilution dependence
  • Determines crystallinity
  • Highest influence on conductivity
  • Optimal value at H2:SiH4=100:1; lowest activation energy of 38 meV
  • Crystalline peak shift
  • Silicon crystals embedded in a-SiOx:H matrix
  • Peak shifted from 521 cm-1 to 518-516 cm-1
  • Towards greater peak shift –> density of crystals increases, size of crystals decreases
  • Estimated crystal grain size: 4-6 nm
  • Amorphous samples showed poor conductivity
  • Lower CO2 flow - peak shift to lower wavenumbers
  • Higher pressure - peak shift to lower wavenumbers
  • Optimized recipe
  • Peak shift to 518 cm-1
  • XRD - (111) reflection
  • High lateral conductivity implies quality a-SiOx:H tissue
  • Conductivity not solely accounted for by crystalline grains

Table of trends associated with increase in each parameter; s=strong trend, w=weak trend, σ=conductivity, n600=refractive index at 600 nm, k600=absorption coefficient at 600 nm, EA=activation energy, Ebg=bandgap.

Raman spectroscopy of n-doped c-SiOx:H. The peak is left of the 521 crystalline silicon peak, evidence of 4-6 nm silicon crystal grains.

Raman peak overview of the CO2 (a) and pressure (b) series.

FTIR analysis to study a-SiOx:H tissue

  • Analysis
  • FTIR data collected in range 400-6000 cm-1
  • Oxygen-related peaks at 1000-1200 cm-1 and 2000-2300 cm-1 analyzed
  • 1135 cm-1 dominates over 1050 cm-1 when 2250 cm-1 dominates over 2180, 2140, and 2100 cm-1
  • Higher bandgap with higher 1135 and 2250 cm-1
  • Lower 2100 cm-1 – less voids/defects – less recombination
  • PH3 variation – insignificant effect on modes
  • Small carbon modes at 740-820 cm-1; variation insignificantly small
  • Pressure influence
  • High enough for crystals to form and for oxygen incorporation
  • Reasonably low to minimize defects/voids (2100 cm-1)
  • CO2 influence
  • More CO2, more oxygen incorporation
  • Less CO2, less defects/voids (2100 cm-1)
  • H2 influence
  • Higher H2 flow, more H incorporation
  • H2:SiH4>50 - no significant variation of oxygen content
  • Optimal recipe when more equality between contributions from 2100 to 2250 cm-1

Gaussian fits of FTIR scans for 2 c-SiOx:H recipes. A microcrystalline sample deposited with pressure of 2 mbar (top) and an amorphous sample deposited with a H2:SiH4 ratio of 0 (bottom).

Assignments of wagging and stretching modes in c-SiOx:H.

Changes in normalized relative contributions of different stretching modes in c-SiOx:H as a function of pressure, CO2:SiH4, and H2:SiH4. Yellow regions show amorphous material.

Solar cell integration of n-μc-SiOx:H

Higher stable efficiency with varying i-layer thickness

JSC improvement with varying μc-SiOx:H thickness

  • Degradation
  • H2 diluted i-layer thickness varied from 100-300 nm and light soaked
  • Lowest relative degradation for sample with 200 nm –> optimal thickness
  • JSC of 200 nm higher after degradation –> confirmed in multiple samples
  • 250 and 300 nm show a strong reduction in blue part

Deposition parameters

for optimized recipe

  • Gains
  • Best at red response –> reflection of high wavelength photons back into i-layer
  • Reduction in size of the a-Si n-layer as the reference cell's n-layer is twice as thick
  • Blue and green response:
  • 1. Reduction in recombination in n-layer/Ag contact interface
  • 2. Better electron collection -> facilitated by lower activation energy of μc-SiOx:H than a-Si n-layer
  • Total current enhancement – 6.3%

EQE spectra of degraded cells with c-SiOx:H and different H2 diluted i-layer thicknesses.

EQE of a-Si cells with different thicknesses of c-SiOx:H. *Cell has double standard a-Si n-layer thickness.

Initial and degraded (in brackets) parameters of cells with different R=5 i-layer thickness.


  • FTIR analysis: relations of stretching modes with oxygen and hydrogen content in material
  • FTIR analysis: best performing material - equal contribution of 2100 cm-1-2250 cm-1 modes
  • Optimized c-SiOx:H films enhance performance of single junction a-Si solar cells over whole spectrum
  • Best single junction a-Si:H cells with c-SiOx:H: 11.1% initial, 7.71% stable efficiency
  • N-doped c-SiOx:H layers with wide array of optical and electrical properties deposited
  • Raman spectroscopy: good c-SiOx:H material consists of crystalline silicon grains embedded in amorphous SiOx:H matrix
  • Raman spectroscopy: peak shift to 518-516 cm-1 - signature of grains 4-6 nm in size

*) Contact:Pavel Babal

Feldmannweg 17

2628 CT, Delft Netherlands

Photovoltaic Materials and Devices Laboratory