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
- 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
- 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
- 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
for optimized recipe
- 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
2628 CT, Delft Netherlands [email protected]
Photovoltaic Materials and Devices Laboratory