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Topics Covered
Background
The Photovoltaic Cell – How Does
It Work ?
Materials and Efficiency
Photovoltaic Device Characterization
Graded
Microcrystalline Silicon Layer
Inhomogeneous ZnO
Layer
Thickness Mapping of SiNx Layer
Conclusion
Background
A photovoltaic cell, or solar cell is a semiconductor device consisting of a
large-area p-n junction diode that in the presence of sunlight is capable of
generating usable electrical energy. This conversion is called the photovoltaic
effect. Solar cells have many applications, and are particularly well suited to
situations where electrical power from the grid is unavailable, such as in
remote area power systems, Earth orbiting satellites, handheld calculators,
remote radiotelephones, water pumping applications, etc.
Much of the research is focussed on making solar cells cheaper and more
efficient, so that they can more effectively compete with other energy sources.
Much of this optimization requires accurate characterization of film thickness
and absorption efficiency for the thin films used to manufacture the cells.
Spectroscopic ellipsometry is an optical measurement technique
used to determine thin film thickness and optical constants simply and
accurately. This article illustrates the ability of the technique to
characterize photovoltaic devices. The materials commonly studied include:
amorphous silicon, poly silicon, ZnO, ITO, SnO2, TiO2, SiNx, MgO, etc.…
The Photovoltaic Cell – How Does It Work?
The photovoltaic effect starts with the absorption of photons in a
semiconductor above its energy band gap, leading to the generation of charge
carriers (electrons and holes). These charge carriers are then separated by an
internal electric field created either by a p-n or p-i-n junction within the
semiconductor, or by a hetero-junction between the semiconductor and another
material.
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Figure 1.
Diagram of a photovoltaic cell.
Finally the charge carriers are collected by electrodes and can be used to
generate a current in the outer circuit. The front electrode of the cell should
be designed to allow high transmission of photons. This can be accomplished
either by a fine grid of metal, or by using a transparent conductive oxide (TCO)
such as Indium-Tin-Oxide (ITO), Tin-Oxide (SnO2) or Zinc-Oxide (ZnO).
Antireflection coatings, used to increase the amount of light coupled into the
solar cell, are typically deposited on the front side of the cell. It is
typically applied in a layer several hundred nanometers thick using plasma
enhanced chemical vapor deposition (PECVD).
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Figure 2.
Schematic cross-section of thin film a-Si:H photovoltaic cell.
Materials and Efficiency
Various materials have been investigated for solar cells. Most large-scale
commercial solar cell factories manufacture screen printed poly-crystalline
silicon solar cells. Single crystalline wafers can be made into excellent high
efficiency solar cells, but they are generally considered to be too expensive
for large-scale mass production. Amorphous silicon cells have low conversion
efficiencies of around 8%. Polymer or organic solar cells are built from ultra
thin layers (typically 100 nm) of organic semiconductors. They are potentially
cheaper to manufacture than silicon, but efficiencies achieved to date are low
and cells are highly sensitive to air and moisture, making commercial
applications difficult.
Photovoltaic Device Characterization
Three examples of samples commonly characterized by spectroscopic
ellipsometers are presented below. The analysis were performed using a HORIBA Jobin
Yvon UVISEL spectroscopic phase modulated ellipsometer controlled
by DeltaPsi2 software. Ellipsometric data were acquired at an angle of incidence
of 70°, across the extended spectral range from 0.6 to 6.5 eV (190 – 2100 nm).
Graded Microcrystalline Silicon Layer
The microcrystalline silicon layer is inhomogeneous in depth. The model
includes a graded layer (linear function) used to specify one value at the
bottom of the layer and another one for the layer top. The optical constants of
the microcrystalline silicon were represented using a dispersion formula.
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Figure 3.
Optical constants of microcrystalline silicon.
The result obtained shows the excellent agreement between the model (line)
and the experimental data (dots) on the whole spectral range, with ?2 =0.81
(result quality parameter).
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Figure 4.
Experimental and generated data.
Inhomogeneous ZnO Layer
The sample consists of a ZnO layer deposited on c-Si. To represent the
inhomogeneity in depth of the ZnO layer, a three layer model was used. The
sample exhibits a small roughness on top, and the density of the ZnO increases
from the c-Si interface (1st layer) to the top (2nd layer). It provides an
increase of the refractive index from the 1st to the 2nd layer.
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Figure 5.
Optical constants of ZnO
Thickness Mapping of SiNx Layer
By using a motorized XY sample stage and a mapping recipe it is simple to
automate the analysis at different positions on the sample. Both thicknesses and
optical constants were determined at each points. The mapping shows a variation
of SiNx thickness between 600 and 750 Å over the surface of the sample.
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Figure 6.
Thickness map of SINx Layer
Conclusion
Spectroscopic ellipsometry is an ideal technique to
characterize film thicknesses and optical constants for photovoltaic
applications. Spectroscopic ellipsometers are also sensitive to the presence
of rough overlayer and graded optical constants. The technique provides the
advantage to be fast, simple to operate and non-destructive for the
characterization of the samples.
Source: Horiba Scientific– Thin Films Division
For more information on this source please visit Horiba
Scientific– Thin Films Division