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Thin Film Photovoltaics by Spectroscopic Ellipsometry
What do we mean by photovoltaics? First used in about 1890, the word has two parts: photo, derived from
the Greek word for light, and volt, relating to electricity pioneer Alessandro Volta. So, photovoltaics could
literally be translated as light-electricity. And that's what photovoltaic (PV) materials and devices do - they convert light energy into electrical energy (Photoelectric Effect), as French physicist Edmond Becquerel discovered
as early as 1839.
Commonly known as solar cells, individual PV cells are electricity-producing devices made of semiconductor
materials. PV cells come in many sizes and shapes - from smaller than a postage stamp to several inches
across. They are often connected together to form PV modules that may be up to several feet long and a few
feet wide. Modules, in turn, can be combined and connected to form PV arrays.
Did you know that PV systems are already an important part of our lives? Simple PV systems provide power
for many small consumer items, such as calculators and wristwatches. More complicated systems provide
power for communications satellites, water pumps, and the lights, appliances, and machines in some people's
homes and workplaces. Many road and traffic signs along highways are now powered by PV. In many cases,
PV power is the least expensive form of electricity for performing these tasks.
The Photoelectric Effect
Light and the PV Cell
The photoelectric effect is the basic physical process by which a PV
cell converts sunlight into electricity. When light shines on a PV cell,
it may be reflected, absorbed, or pass right through. But only the
absorbed light generates electricity.
The energy of the absorbed light is transferred to electrons in the
atoms of the PV cell. With their newfound energy, these electrons
escape from their normal positions in the atoms of the semiconductor PV material and become part of the electrical flow, or current, in an electrical circuit.
The sun emits almost all of its energy in a range of wavelengths
from about 2x10-7 to 4x10-6 meters. Most of this energy is in the
visible light region. Solar cells respond differently to the different
wavelengths of light. In summary, light that is too high or low in energy is not usable by a cell to produce electricity. Rather, it is transformed into heat.
Bandgap Energies of Semiconductors and
Light
To induce the built-in electric field within a PV cell, two layers of
somewhat differing semiconductor materials are placed in contact
with one another. One layer is an «n-type» semiconductor with an
abundance of electrons, which have a negative electrical charge.
The other layer is a «p-type» semiconductor with an abundance of
«holes», which have a positive electrical charge. Sandwiching these
together creates a p/n junction at their interface, thereby creating
an electric field.
Only photons with a certain level of energy can free electrons in
the semiconductor material from their atomic bonds to produce an
electric current. This level of energy is known as the «bandgap energy,». To free an electron, the energy of a photon must be at least
as great as the bandgap energy. However, photons with more energy than the bandgap energy will expend that extra amount as
heat when freeing electrons. So, it's important for a PV cell to be
«tuned»-through slight modifications to the silicon's molecular
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structure-to optimize the photon energy. A key to obtaining an efficient PV cell is to convert as much sunlight as possible into electricity.
Crystalline silicon has a bandgap energy of 1.1 electron-volts (eV).
(An electron-volt is equal to the energy gained by an electron when
it passes through a potential of 1 volt in a vacuum.) The bandgap
energies of other effective PV semiconductors range from 1.0 to
1.6 eV. In this range, electrons can be freed without creating extra
heat.
The photon energy of light varies according to the different wavelengths of the light. The entire spectrum of sunlight, from infrared
to ultraviolet, covers a range of about 0.5 eV to about 2.9 eV. For
example, red light has an energy of about 1.7 eV, and blue light
has an energy of about 2.7 eV. Most PV cells cannot use about
55% of the energy of sunlight, because this energy is either below
the bandgap of the material or carries excess energy.
Polycrystalline thin-film cells have a heterojunction structure, in which
the top layer is made of a different semiconductor material than the
bottom semiconductor layer. The top layer, usually n-type, is a window
that allows almost all the light through to the absorbing layer, usually ptype. An «ohmic contact» is often used to provide a good electrical
connection to the substrate.
Different PV materials have different energy band gaps. Photons with
energy equal to the band gap energy are absorbed to create free
electrons. Photons with less energy than the band gap energy pass
through the material.
Thin Film Solar Cells
Thin film solar cells can be made from:
• Polycrystalline thin films-including copper indium diselenide
(CIS), cadmium telluride (CdTe), and thin-film silicon
• Single-crystalline thin films-including high-efficiency material
such as gallium arsenide (GaAs)
Polycrystalline Thin Film
Thin film cells use much less material: the cell's active area is usually only 1 to 10 micrometers thick, whereas thick films typically are
100 to 300 micrometers thick. Also, thin-film cells can usually be
manufactured in a large-area process, which can be an automated, continuous production process. Finally, they can be deposited
on flexible substrate materials.
Unlike most single-crystal cells, a typical thin-film device doesn't
have a metal grid for the top electrical contact. Instead, it uses a
thin layer of a transparent conducting oxide, such as tin oxide.
These oxides are highly transparent and conduct electricity very
well. A separate antireflection coating might top off the device, unless the transparent conducting oxide serves that function.
The typical polycrystalline thin film cell has a very thin (less than
0.1 micron) layer on top called the «window» layer. The window
layer's role is to absorb light energy from only the high-energy end
of the spectrum. It must be thin enough and have a wide enough
bandgap (2.8 eV or more) to let all available light through the interface (heterojunction) to the absorbing layer. The absorbing layer
under the window, usually doped p-type, must have a high absorptivity (ability to absorb photons) for high current and a suitable
band gap to provide a good voltage. Still, it is typically just 1 to 2
microns thick.
Thin film PV structures characterized by
spectroscopic ellipsometry
Proper understanding of thin film materials and thin film stack, tailoring numerous properties of thin films required for an efficient solar cell demands a range of characterization techniques.
Spectroscopic ellipsometry is an optical technique used for the
measurement of thin film thickness and optical constants. The
technique provides the advantages to be non-destructive, fast, precise to the Angstrom level, and the capability to measure multi-layer stacks.
1 Characterization of anti-reflective coatings
Silicon is a shiny gray material and can act as a mirror, reflecting
more than 30% of the light that shines on it. To improve the conversion efficiency of a solar cell, antireflection (AR) coating is applied to the top layer of the cell helping to optimize light
absorption. Single AR layer or multiple AR layers basically use SiOx
and TiOx materials. Another way to reduce reflection is to texture
the top surface of the cell, which causes reflected light to strike a
second surface before it can escape, thus increasing the probability of absorption.
To make an efficient solar cell, we try to maximize absorption, minimize
reflection and recombination, and thus maximize conduction.
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Experimental Conditions
SiNx Refractive Index Mapping
(n value at 633 nm)
- Spectroscopic Ellipsometer: UVISEL
- Spectral range: 0.75 - 5.0 eV ⇔ 248 - 1653 nm
SiO2
Graded TiO2
SiO2
Glass
Cr
158 Å
1242 Å
500 Å
Full structure characterization
9 Thicknesses
9 TiO 2 Graded optical constants
TiO 2 G ra de d O p tic a l C o n s ta n ts
4
1.4
3 .8
1.2
3 .6
1
3 .4
0.8 k
n
3 .2
0.6
3
0.4
2 .8
0.2
3 Characterization of TCO electrodes film
The properties of the contacts and windows layers are critical to
device performance.
At least one contact must be electrically conducting and transparent to photons in the spectral range where the absorber creates
carriers. Transparent conducting oxides are the material of choice
for this purpose (ZnO, ITO, TiO2, SnO2).
The highest efficiency devices rely on diffusion to transport carriers
to the junction of collection.
2 .6
1
2
3
P h o to n E n e rg y (eV )
tio 2 _ f ilm to p.ds p ( n)
tio 2 _ f ilm to p.ds p ( k)
4
5
0
tio 2 _ f ilm bo tto m.ds p ( n)
tio 2 _ f ilm bo tto m.ds p ( k)
Experimental Conditions
- Spectroscopic Ellipsometer: UVISEL
- Spectral range: 0.6 - 4.0 eV ⇔ 306 - 2066 nm
One can notice that the bottom of the TiO2 film exhibits a higher
refractive index than the top of the film.
Full structure characterization
9 Thicknesses
Roughness
2 Thickness mapping of SiNx thin film
SnO2 SnO2
Graded
SiCOx
Glass
Experimental Conditions
- Spectroscopic Ellipsometer: UVISEL
- Spectral range: 0.6 - 6.5 eV ⇔ 190 - 2100 nm
532 Å
6582 Å
9 SiCOx optical constants
256 Å
9 Surface roughness
9 SnO2 Graded optical constants
SnO 2 G raded O ptic al C onstants
2.2
9 Film thickness uniformity
SiNx
~ 700 Å
mc-Si
1.1
2
9 Optical constants (n,k)
uniformity
1
0.9
1.8
0.8
1.6
0.7
n
SiNx Thickness Mapping
1.4
0.6 k
1.2
0.5
1
0.4
0.3
0.8
0.2
0.6
0.1
0.4
500
1 000
1 500
W avelen g th (n m )
S nO2_ film top.ds p (n)
S nO2_ film top.ds p (k)
2 000
S nO2 _ film bottom.ds p (n)
S nO2 _ film bottom.ds p (k)
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Experimental Conditions
S iC xO y O p tic a l C o n s ta n ts
- Spectroscopic Ellipsometer: UVISEL
- Spectral range: 0.75 - 5.0 eV ⇔ 248 - 1653 nm
4 .4
4 .2
4
3 .8
9 Thicknesses
3 .6
3 .4
n
83 Å
278 Å
Roughness
Si
µm c-Si
cc-Si
3 .2
9 Micro crystalline Si optical constants
9 Surface roughness
3
2 .8
2 .6
O p tic a l C o ns ta n ts o f M ic ro C ry s tallin e S ilic o n
2 .4
4
5
2 .2
50 0
1 00 0
W a vele n g th (n m )
1 500
2 00 0
3 .5
4 .5
3
4
2 .5
Experimental Conditions
n
- Spectroscopic Ellipsometer: UVISEL
- Spectral range: 0.6 - 6.5 eV ⇔ 190 - 2100 nm
3 .5
3
1 .5
Optical Bandgap
2 .5
Full structure characterization
27 Å
Roughness
Graded ZnO
900 Å
c-Si
k
2
1
2
0 .5
9 Thicknesses
9 Graded ZnO optical constants
1
2
3
P h o to n E n erg y (e V )
9 Surface roughness
4
5
0
Experimental Conditions
- Spectroscopic Ellipsometer: UVISEL
- Spectral range: 0.6 - 6.5 eV ⇔ 190 - 2100 nm
Z n O G ra de d O p tic a l C o n s ta n ts
2 .4
0 .9
2 .2
0 .8
2
0 .7
0 .6
1 .8
0 .5
n 1 .6
9 Thickness
k
0 .4
1 .4
0 .3
1 .2
Graded µm Si
1226 Å
c-Si
9 Graded Micro crystalline Si
optical constants
0 .2
1
0 .1
0 .8
1
2
3
4
Ph o to n E n erg y (e V )
Z nO film to p.c lc (n)
Z nO film to p.c lc (k)
5
6
M ic ro C ry s ta lline S ilic o n G rad e d O p tic a l C o n s ta n ts
0
Z nO film bo tto m.c lc (n)
Z nO film bo tto m.c lc (k)
3
4
3 .5
2 .5
3
2
n 2 .5
4 Characterization of Micro crystalline & Amorphous
Silicon thin films
k
1 .5
2
1
1 .5
0 .5
Microstructure of silicon films can be varied from one extreme of
amorphous/nanocrystalline to highly oriented and/or epitaxial
growth.
The characterization of silicon thin films by spectroscopic ellipsometry provides a wealth of information such as:
- Optical bandgap
- Inhomogeneities of silicon films (gradient)
- The shape of optical constants is directly linked to the microstructure of silicon materials
1
1
2
3
4
Ph o to n E n erg y (e V )
m icro S i film to p.c lc (n)
m icro S i film to p.c lc (k )
5
6
m ic roS i film b o ttom .c lc (n)
m ic roS i film b o ttom .c lc (k )
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Experimental Conditions
Experimental Conditions
- Spectroscopic Ellipsometer: UVISEL
- Spectral range: 1.14 - 3.54 eV ⇔ 350 - 1088 nm
- Spectroscopic Ellipsometer: UVISEL
- Spectral range: 0.6 - 4.2 eV ⇔ 295 - 2066 nm
9 Thicknesses
Roughness
am Si
Glass
Full CIGS PV Structure Characterization
9 Amorphous Si optical constants
26 Å
2518 Å
9 Surface roughness
9 Film Thicknesses
109 nm
110 nm
ITO
ZnO
Graded CIGS
9 CIGS, ITO, ZnO Optical constants
2218 nm
9 CIGS Film gradient
Mo-SS
O p tic a l C o n s ta n ts o f A m o rp h o us S ilic o n
2
4 .8
1 .8
1 .6
4 .6
1 .4
4 .4
1 .2
k
1
n
4 .2
0 .8
0 .6
4
0 .4
3 .8
0 .2
400
500
600
700
800
W a ve len g th (n m )
900
1 00 0
0
5 CIGS PV Cell
Copper Indium Selenide (CuInSe2) have extremely high optical absorption coefficients that allow nearly all of the available light within the terrestrial spectrum to be absorbed in the first micrometer of
the material. Therefore, the total thickness of the active layers is in
the order of 2 micrometers, resulting in efficient use of materials
without negatively impacting the conversion efficiency.
The addition of controlled amounts of gallium and/or sulfur into
the CuInSe2 absorber layer allows to adjust its energy gap to provide device with higher voltage, better carrier collection and higher
conversion efficiency.
Typical GIGS PV Cell
Transparent Layer
CdS
CuInGaSe2
Contact
Substrate
Z n O O p tic a l C o n s ta n ts
0.5
1 .9 5
1 .9
1 .8 5
0.4
1 .8
1 .7 5
0.3
n
1 .7
k
1 .6 5
0.2
1 .6
1 .5 5
1 .5
0.1
1 .4 5
1 .4
2
3
P h o to n E n erg y (e V )
4
0
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I T O O p t ic a l C o n s ta n ts
P3 H T O p tic a l C o n s ta n ts
2 .4
1 .64
0 .3 2
0.8
2 .3
0.7
1 .6
0 .3
2 .2
1 .56
0 .2 8
2 .1
1 .52
0 .2 6
n
0.6
n
0 .2 4
1 .48
0.5
2
k
0 .2 2
1 .44
1 .9
0.4
1 .8
0.3
1 .7
0 .2
k
0.2
1 .6
1 .4
0 .1 8
0.1
1 .5
1 .36
0 .1 6
3
P h o to n E n e rg y (e V)
4
1
2
6 Polymer solar cells
3
P h o to n E n e rg y (eV )
4
5
0
Z nO O p tic al C on s tants
Organic solar cells and polymer solar cells are built from thin films
(typically 100 nm) of organic semiconductors such as polymers
and small-molecule compounds like polyphenylene vinylene, copper phthalocyanine and carbon fullerenes.
Energy conversion efficiencies achieved to date using conductive
polymers are low at 6% efficiency. However, these cells could be
beneficial for some applications where mechanical flexibility and
disposability are important.
0.220
0.200
1.760
1.720
n
0.180
1.680
0.160
1.640
0.140
0.120
1.600
0.100
k
0.080
Experimental Conditions
1.560
0.060
- Spectroscopic Ellipsometer: UVISEL
- Spectral range: 0.6 - 5.0 eV ⇔ 248 - 2066 nm
1.520
0.040
0.020
1.480
1
2
3
4
Ph o ton E nerg y (e V)
5
0.000
Organic PV Structure Characterization
ZnO
P3HT
PEDOT:PSS
Glass
167 Å
143 Å
543 Å
Conclusion
9 Film thicknesses
9 PEDOT: PSS, P3HT, ZnO Optical
constants (n,k)
P E D O T : P S S U n ia xia l A n is o tro p ic O p tic a l C o n s t a n ts
0.3
2 .1
2
0.2 5
1 .9
0.2
1 .8
k
0.1 5
n
1 .7
0.1
1 .6
0.0 5
1 .5
2
3
p e d o t o .ud f (n )
4
P h o to n E n e rg y (e V)
p e d o t e .u d f (n )
p e d o t o .u d f (k)
5
6
p e d o t e .ud f (k)
0
Spectroscopic ellipsometry is an ideal technique to characterize
film thicknesses and optical constants and optical bandgap 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.
Sources and Acknowledgements
- http://www1.eere.energy.gov/solar/tf_polycrystalline.html
- Céline Eypert, Li Yan, Michel Stchakovsky, Marzouk Kloul, Assia
Shagaleeva, Roland Seitz, application engineers - HORIBA Jobin
Yvon ellipsometry.
This document is not conractually binding under any circumstances - Printed in France - ©HORIBA Jobin Yvon - RCS EVRY B 837 150 366 - 03/2008
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