MATERIAL SCIENCE
PROJECT REPORT
ON
THIN FILM SOLAR CELLS
(Amorphous and Organic)
Submitted by:
Isheeka Dasgupta
Shivani Deshpande
Umang Murawat
Introduction
This work is concerned with the theory, design and operation of some photovoltaic solar cell.
However, we need to first ask the fundamental question- why consider solar cells at all? The
answer to this question involves energy. As a form of life Homo sapiens requires, as do all
other living things, energy in form of food and energy in the form of heat ( oftentimes
supplied the food, but sometimes by sunlight or hot water or ..). We also use energy for
number of other purposes, such as clothing, shelter, transportation, entertainment, cooling and
construction of tools.
Currently the world consumes an average of 13 terawatts (TW) of power. By the year
2050, as the population increases and the standard of living in developing countries
improves, this amount is likely to increase to 30 TW. If this power is provided by burning
fossil fuels, the concentration of carbon dioxide in the atmosphere will more than double,
causing substantial global warming, along with many other undesirable consequences.
Therefore, one of the most important challenges facing engineers is finding a way to provide
the world with 30 TW of power without releasing carbon into the atmosphere. Although it is
possible that this could be done by using carbon sequestration along with fossil fuels or by
greatly expanding nuclear power plants, it is clearly desirable that we develop renewable
sources of energy. The sun deposits 120,000 TW of radiation on the surface of the earth, so
there is clearly enough power available if an efficient means of harvesting solar energy can
be developed.
Only a very small fraction of power today is generated by solar cells, which convert solar
energy into electricity, because they are too expensive (Lewis and Crabtree, 2005). More
than 95 percent of the solar cells in use today are made of crystalline silicon (c-Si). The
efficiency of the most common panels is approximately 10 percent, and the cost is $350/m2.
In other words, the cost of the panels is $3.50/W of electricity produced in peak sunlight.
When you add in the cost of installation, panel support, wiring, and DC to AC converters, the
price rises to approximately $6/W. Over the lifetime of a panel (approximately 30 years), the
average cost of the electricity generated is $0.3/kW-hr. By comparison, in most parts of the
United States, electricity costs about $0.06/kW-hr. Thus, it costs approximately five times as
much for electricity from solar cells. If the cost of producing solar cells could be reduced by a
factor of 10, solar energy would be not only environmentally favorable, but also
economically favorable.
Amorphous Silicon
Amorphous – from the Greek for “without form” – refers not to materials that
have no shape, but rather to materials with no particular structure. The atoms or
molecules of amorphous materials are arranged in essentially the same manner
as they are in a liquid. An amorphous material is still solid – the molecules are
closely packed and chemically bonded, and the material exhibits an elastic
response to shear stresses – but the spatial arrangement of the atoms is nearly
random. In contras t, the building blocks of crystalline solids are arranged in
orderly, 3-dimensional, periodic arrays
Amorphous solid crystalline solid
Amorphous materials are characteristized by lack of long range order (long
range, may be considered to be any distance in excess of one lattice constant)
when the locations of the constituent atoms are considered. The resulting
devices, in general, exhibit wider energy gap than the crystalline varieties of the
same material, higher (often dramatically higher) absorption coefficients and,
owing to a large number of non-symmetrical interatomic bonds, a significant
number of energy states within the forbidden gap.
A variety of amorphous materials have undergone investigation as solar cell
candidates. The preponderance of such materials has involved silicon(because
of its high availibilty and optimum band gap properties). Among materials
showing some promise are a-Si, a-SiGe and a-SiC (amorphous Si is alloyed
with carbon and germanium which changes its band gap).
CHEMICAL CHARACTERISTICS OF PURE AMORPHOUS
SILICON
The large no. of energy states within the forbidden gap result in poor overall
material charge carrier transport properties, excessive recombination and a
much reduced optical-to-electrical energy conversion efficiency. Also, on the
debit side of a the situation, most amorphous material based solar cells exhibit a
degradation in efficiency with prolonged exposure to light. This decrease in
performance can exceed 30% under conditions commonly encountered and
combat this problem.
Why hydrogenated amorphous silicon and not pure amorphous silicon?
Elemental amorphous silicon does not appear to posses any commercially
valuable photovoltaic properties. Solar cells are constructed of less than a
percent and have electrical characteristics which will vary with time. The reason
for this is the existence of large numbers of vacancies, and other imperfections
within the semiconductor. These defects prevent doping or proper
photoconductivity. When coupled with the non-peroidic arrangements of the
silicon atoms, this creates vast numbers of allowed energy states. These states,
more-or-less, span the energy gap and impede the manufacture’s ability to
adequately “dope” the amorphous semiconductor. These states also drastically
reduce the carrier lifetime.
The density of states observed within the energy gap of intrinsic amorphous
silicon is outlined in fig. Note that the density of the induced states is extremely
high.
The volume density of states in the energy band gap for intrinsic amorphous
silicon-
this density of states is roughly constant across the forbidden gap. As noted
earlier, this high density of states makes the use of added impurity atoms
(doping) to change the type of semiconductor effectively impossible.
In 1975, it was reported that amorphous silicon films produced by the glow
discharge decomposition of silane (SiH4) could be doped to produce pn
junctions. Such films clearly contain hydrogen, a byproduct of decomposition of
silane, at a five to ten percent atomic proportion level. It is theorized that these
hydrogen atoms saturate the dangling bonds that are a feature of the interval
vacancies and atomic structure. The role of dangling bond has a decisive effect
on the properties of amorphous silicon (a-Si). This saturation reduces the state
density within the energy gap. By 1976 amorphous silicon ( or more precisely
amorphous silicon containing hydrogen or a-Si:H) based solar cells with an
efficiency of 5.5% were produced.
Note that a-Si:H films typically exhibit an energy gap (Ec-Ev) on the order of
1.5 to 1.7 eV when optical phenomena are considered. This effective energy gap
is considerably wider than the 1.1 eV exhibited by single crystal silicon and
represents a much better match for the solar spectrum. As a result, the observed
absorption coefficients for a-Si are significantly higher than for c-Si when
photon energies in excess of the a-Si are significantly higher than for c-Si when
photon energies in excess of the a-Si:H optical energy gap are considered.
The increased absorption coffiecient for a-Si means that solar cells can be
constructed of films of this material with less than a micron in thickness of
absorption material. Such films can be deposited on many different substrates
using a variety processes. Glow discharge of silane, sometimes known as
PECVD or plasma enhanced chemical vapour deposition, appears to be
favoured by many experiments. Other techniques for production of a-Si include
sputtering, pyrolysis, chemical vapour deposition and photodecomposition
combined with chemical vapour deposition.
Defects and Gap States
Between the bandtails lie defect levels; in undoped a-Si:H, these levels appear
to be due entirely to the dangling bonds (“D-centers”) measured by electron
spin resonance.The presence of these dangling bonds decrease the efficiency of
the solar cell as they act as recombination centres and hence capture the
electron which reduces the photocurrent.
SWE
The observation of metastable changes in a-Si:H goes back to the work of
Staebler and Wronski, who found in 1977 that the dark conductivity and
photoconductivity of glow-discharge deposited amorphous silicon can be
reduced significantly by prolonged illumination with intense light. The observed
changes were found to be reversible by annealing of the a-Si:H samples at
elevated temperatures (_150_C), and were attributed to a reversible increase in
density of gap states acting as recombination centres for photoexcited carriers
and leading to a shift of the dark Fermi level EF toward midgap . Since this first
report, light-induced metastable changes in the properties of hydrogenated
amorphous silicon are referred to as the Staebler-Wronski effect (SWE).
Many theories have evolved in explanation to the SW effect .Historically, the
most favoured model has been the hydrogen bond switching model which
proposes that photoexcited electrons and holes recombine at weak Si-Si bond
locations, that the accompanying non-radiative energy release is sufficient to
break the bond, and that a back-bonded H atom prevents restoration of the
broken bond by a bond switching event.
This basically means that light provides enough energy to break and shift
hydrogen and silicon bonds which ultimately increases the no of dangling bonds
and hence increase recombination.
Methods to reduce SWE effect
Incorporate fluorine in the gas mixture during production as fluorine bond
together to Si than H.
- Fluorine bonds tighter to silicon than hydrogen, and is less mobile in the a-Si
network
- Fluorinated a-Si cells show much better stability under light soaking
DOPING
Doped layers are integral to pin solar cells. Doping itself, which is the
intentional incorporation of atoms like phosphorus and boron in order to shift
the Fermi energy of a material, works very differently in amorphous silicon than
in crystals. For example, in crystalline silicon (c-Si), phosphorus (P) atoms
substitute for silicon atoms in the crystal lattice. P has five valence electrons, so
in the “fourfold coordinated” sites of the Si lattice, four electrons participate in
bonding to neighboring silicon atoms. The fifth “free” electron occupies a state
just below the bottom of the conduction band, and the dopants raise the Fermi
energy to roughly this level.
In a-Si, most phosphorus atoms bond to only three silicon neighbors; they are
in “threefold coordinated” sites. This configuration is actually advantageous
chemically; phosphorus atoms normally form only three bonds (involving the
three valence electrons in “p” atomic orbitals). The final two electrons are
paired in “s” atomic orbitals, do not participate in bonding, and remain tightly
attached to the P atom. The reason that this more favorable bonding occurs in a-
Si, but not in c-Si, is the absence of a rigid lattice. As a thin film of a-Si grows,
the network of bonds adjusts to incorporate impurity atoms in a nearly ideal
chemical arrangement. In c-Si, it would be necessary to grossly rearrange
several Si atoms in the lattice and to leave a number of dangling Si bonds, in
order to accommodate the P atom in this configuration. The extra energy for this
rearrangement is larger than what would be gained from more ideal bonding of
P, and substitutional doping is favored.
Thus, phosphorus doping is a paradox in amorphous silicon. It is, at first,
unclear why it occurs at all, since doping involves fourfold coordinated P, and P
atoms are generally threefold coordinated in a-Si. This puzzle was first solved
in 1982 by Street,who realized that independent formation of both a positively
charged, fourfold coordinated P4 + and a negatively charged dangling bond D−
can occur occasionally instead of the more ideal threefold coordination . This
understanding leads to two important consequences. First, doping is inefficient
in a-Si; most dopant atoms do not contribute a “free” electron and do not raise
the Fermi energy. Second, for each dopant atom that does contribute an
electron, there is a balancing, Si dangling bond to receive it. These defect levels
lie well below the conduction band, so the fourfold coordinated phosphorus
atoms are less effective in raising the Fermi energy than that in c-Si.
Additionally, the negatively charged dangling bonds induced by doping are very
effective traps for holes. Since bipolar transport of both electrons and holes is
essential to photovoltaic (PV) energy conversion, photons absorbed in doped
layers do not contribute to the power generated by solar cells.
Alloying and Optical Properties The structural and optical properties we have described can be varied
substantially by changes in deposition conditions. For example, changing the
substrate temperature or the dilution of silane by hydrogen (in plasma
deposition) causes a change in the optical band gap for a-Si:H films over at least
the range 1.6 to 1.8 eV ; these changes can be ascribed to changes in the
hydrogen microstructure of the films. Even larger changes canbe effected by
alloying with additional elements such as Ge, C, O, and N; alloying is readily
accomplished by mixing the silane (SiH4) source gas with gases such as GeH4,
CH4, O2 or NO2, and NH3 , respectively. The resulting alloys have very wide
ranges of band gaps, as we illustrate for a-Si1−xGex:H. For simplicity, we shall
usually refer to these alloys using the abbreviated notation: a-SiGe for a-
Si1−xGex:H, and so on.
Only some of these materials have proven useful in devices. In particular, a-
SiGe alloys with optical gaps down to about 1.45 eV are employed as absorber
layer in multijunction pin cells; the narrower band gap of a-SiGe compared to a-
Si allows for increased absorption of photons with lower energies illustrates
how the spectrum of the absorption coefficient α(hν) changes for a-SiGe alloys
with different atomic percentages x; the different optical band gaps are indicated
as labels. Two features of these data should be noted. First, the Urbach slopes
remain constant (at about 50 meV) over the entire range of band gaps. Second,
the plateau in the absorption coefficient at the lowest photon energies increases
steadily as the band gap diminishes, which is indicative of a corresponding
increase in defect density. Fig. is a contour plot showing how the optical band
gap of a-Si1−xGex:H varies with the Ge-ratio x and with atomic fraction h of
hydrogen. The figure reflects experimental results for a-Si:H alloys of varying
H-fraction and for a-SiGe:H alloys for which both x and h were reported. Note
that, for constant fraction h, the band gap decreases about 0.7 eV as the Ge ratio
x increases from 0 to 1. The band gap increases with atomic fraction of
hydrogen h. should be viewed as a useful approximation; in particular, the
atomic fraction h is only one aspect of the hydrogen microstructures in a-SiGe
alloys, and quantitative deviations from the contour plot are likely.
Additionally, only some of the materials represented in the figure are useful as
absorber layers. In particular, as the Ge ratio x rises to about 0.5, the
optoelectronic properties become so poor that these alloys are no longer useful
in solar cells. Similarly, only limited ranges of the atomic fraction of hydrogen
h yield useful absorber layers. It might be thought that a-SiC would be equally
useful as a wider band gap absorber; despite some promising research, this
material is not being used as an absorber layer by manufacturers. B-doped a-SiC
is used extensively as a p-type, window layer. a-SiO and a-SiN are used as
insulators in thin-film transistors, but are not major components in solar cells.
To enhance the fill factor of cells made using a-SiGe, band gap grading is used
to enhance the collection of holes [152, 153]. In such a design, an asymmetric
“V”-shaped band gap profile is created by adjusting the Ge content across the i-
layer. Wider band gap material lies closest to the n- and p-layers. The plane of
narrowest band gap lies closer to the p-layer (through which the photons enter
into the device). Such a grading scheme allows more light to be absorbed near
the p-layer so that “slower” holes do not have to travel far to get collected. Also,
the tilting of the valence band assists holes generated in the middle or near the
n-side of the i-layer to move toward the p-layer. With appropriate hydrogen
dilution during growth and band gap grading, a-SiGe cells can be made to
generate up to 24.4 mA/cm2 (27 mA/cm2 as the bottom cell in a triple cell)
when a light enhancing back reflector is used.
The band gap of a-SiC can be adjusted between 1.7 and 2.2 eV, depending
mainly on the C concentration. After extensive research, most workers decided
that a-SiC is not suitable for use as the i-layer of the uppermost cell in a
multijunction structure. After light soaking, a-SiC material that has an
appreciable band gap increase over a-Si is fairly defective and must be used in
very thin layers; these layers do not absorb enough sunlight to be optimal. The
wide band gap material presently used in triple-junction cells is a-Si with a
relatively higher concentration of H (achieved by using lower substrate
temperature and H dilution).
Deposition Methods
The usual methods of depositing a-Si:H is by plasma decomposing of silane Gas
,SiH4 ,with other gases added for doping and alloying. Silane decomposes in
the absence of the plasma above 450 C and high temperature pyrolitic
decomposition is used. Amorphous films can be grown in this way if the
temperature is less than 550C but these films are mostly of low quality as the
temp is too high to retain hydrogen which is very crucial. The decomposition of
hydrogenated films at lower temperature requires a source of energy to
dissociate the silane and this is the role of plasma.
Few methods adopted to grow a-Si:H are:
-PECVD
-Sputtering
-Pyrolysis
-CVD
PECVD (plasma enhanced chemical vapour deposition)
Deposition usually takes place at a gas pressure of .01 to 1 torr which is the
optimum pressure to sustain the plasma. The reactor consists of a gas inlet
arrangement, the decomposition chamber which holds the substrate, a pumping
sytem and the source of power for the discharge.
PECVD variables that determine the quality of the gfilm
Substrate temperature –controls the reaction of the growing surface
Pressure in the chamber-mean free path for collisions and influences the
reaction
Flow rate of the gases-determines the residence tme
Plasma power-controls the rate of dissociation
Frequency of the plasma
Source gas
Alloy gases
n, p dopants
RF-PECVD
1. Silicon containing gas, SiH2 and H2 flows into a vacuum chamber.
2. RF power applied across two electrode plates.
3. A plasma will occur at a given RF voltage for a specific range of gas
pressures.
4. Plasma excites and decomposes the gas and generates radicals and ions.
5. Thin hydrogenated silicon films grow on heated substrates mounted on the
electrode.
Deposition Conditions
• Gas pressure
– Higher for preparing microcrystalline films.
– Lower for uniform deposition.
• RF Power
– Higher power for higher deposition rate.
– Above 100 mW/cm2, rapid reactions create silicon polyhydride powder that
contaminates the growing Si film.
• Substrate temperature
– Lower Temperature, more H incorporated in the film, increases the bandgap
of a-Si:H.
• Below 150 deg. C., makes the powder formation worse.
– Higher Temperature, less hydrogen is incorporated and the bandgap is slightly
reduced.
• Above 350 deg. C., the quality of the material degrades due to loss of
hydrogen and increasing defect density (dangling bonds).
• Electrode spacing
– Smaller spacing for uniform deposition.
– Larger spacing makes maintaining plasma easier.
Advantages of RF-PECVD:
-Highly uniform
-controlling changes can be carried out during deposition.
p-i-n diodes
The fundamental photodevice inside a solarcell has 3layers deposited in either
p-i-n/n-i-p a built in electric field (more than 4 V/cm) is formed when diffusion
current and drift current equalizers.
Sunlight enters the photodiode as a stream of photons that pass through the p-
type layer, which is a nearly transparent “window” layer. The solar photons are
mostly absorbed in the much thicker intrinsic layer; each photon that is
absorbed will generate one electron and one hole photocarrier. The
photocarriers are swept away by the built-in electric field to the n-type and p-
type layers, respectively – thus generating solar electricity!
For amorphous Si bases cells, photons invariably enter through the p type
window layer.
Substrate and superstrate designs
One of the advantages of amorphous silicon–based solar cells is that they absorb
sunlight very efficiently: the total thickness of the absorbing layers in
amorphous silicon solar cells is less than 1 μm. Consequently, these layers need
to be supported on a much thicker substrate. Two totally different designs for
amorphous silicon solar cells have evolved corresponding to transparent and
opaque substrates. We have illustrated the two designs in Fig. In the
“superstrate” design, sunlight enters through the transparent substrate, which is
usually glass or a transparent plastic. The insulating substrate needs a
conducting layer, which is typically a “transparent conductive oxide” (TCO)
such as SnO2. The amorphous silicon photodiode layers are then deposited onto
the TCO, starting with a p-type window layer. Finally, a “back” reflector is
deposited onto the photodiode; the back reflector acts as an electrode to the n-
type photodiode layer.
In the “substrate” design, sunlight enters the photodiode before it reaches the
substrate. Starting with the substrate, the cell is fabricated in the reverse order
compared to the superstrate design: first a back reflector, then the photodiode
layers (starting with an n-type layer), and finally a TCO layer to act as an
electrode to the topmost, window layer of the photodiode.
These two designs permit a very wide range of applications for amorphous
silicon solar cells. The superstrate design (light enters through the substrate) is
particularly suited to building-integrated solar cells in which a glass substrate
can be used as an architectural element. The substrate design has generally been
applied to solar cells using flexible, stainless steel (SS) substrates. The detailed
construction of a deposition facility of course depends upon whether the
substrate is rigid or flexible. Finally, it turns out that there is a profound effect
of the substrate upon the properties of the first photodiode layers deposited upon
it; this effect has led to fairly different photodiode structures for the superstrate
and substrate designs.
Multijunction solar cells
The conversion efficiency of the relatively simple, amorphous silicon pin
photodiode structure just described can be significantly improved by depositing
two or three such photodiodes, one on top of another, to create a
“multijunction” device. A tandem device, which shows a combination of two
pin diodes. Note that the “bottom” cell is not based on a-Si:H, but rather upon
an amorphous silicon–germanium alloy made by including germane (GeH4) gas
in the plasma-deposition recipe.
The main advantage of the tandem design over the simpler single-junction one
is due to “spectrum splitting” of the solar illumination. Since the absorption
coefficient of light rises rapidly with the photon energy, the topmost layer of a
tandem cell acts as a “low-pass” optical filter. This effect is illustrated in Figure
12.2, which shows that a 0.5-μm layer of a-Si:H absorbs photons with energies
larger than 1.9 eV and passes photons with smaller energies. The “wasted”
lower energy photons can be efficiently harvested by amorphous silicon-
germanium, which has a much larger optical absorption coefficient below 1.9
eV than does a-Si:H, hence a lower threshold energy. Overall, the advantages of
the multijunction design are sufficiently compelling that they usually overcome
the additional complexity and cost of the deposition facility. Both tandem and
triple-junction devices are being manufactured today.
The main advantage of the tandem design over the simpler single-junction one
is due to “spectrum splitting” of the solar illumination. Since the absorption
coefficient of light rises rapidly with the photon energy, the topmost layer of a
tandem cell acts multijunction solar cell consisting of two pin solar cells
deposited in series.
Double-junction (or “tandem,” as shown) and triple-junction designs can be
significantly more efficient than single-junction designs. Substrate texturing,
which is important in real devices, is not indicated;
Efficiency
While crystalline silicon achieves a yield of about 18 percent, amorphous solar
cells’ yield remains at around 7 percent. The low efficiency rate is partly due to
the Staebler-Wronski effect, which manifests itself in the first hours when the
panels are exposed to sunlight, and results in a decrease in the energy yield of
an amorphous silicon panel from 10 percent to around 7 percent.
A German researcher from Delft University of Technology has demonstrated
how to raise the energy output of amorphous silicon solar panels from around 7
percent to 9 percent. In his doctoral research, Gijs van Elzakker investigated
adaptations in the production processes of amorphous silicon modules to
increase the output without any additional costs using Silane Gas to reduce the
Staebler-Wronski effect.
This is just one approach being tried today. UniSolar's, laminate efficiency is
currently at 8.2%; however, by late spring 2011, the company expects to be at
10% using their triple coating / triple junction technology.
Advantages of amorphous Si beams cells
Amorphous Si-based PV technology is unique compared with other PV
technologies. Amorphous Si absorbs sunlight more strongly than c-Si and poly-
Si because it is amorphous; the selection rules that weaken absorption in c-Si
(an “indirect band gap” semiconductor) do not apply to a-Si. A rather thin layer
of a-Si is sufficient to absorb sunlight. Amorphous Si can be made at a low
temperature on inexpensive substrates.
The product is made through a low-cost process. The energy payback time (the
time required for an a-Si module to generate the energy used in its production)
was estimated as one to two years in 1989, and has probably shrunk
substantially since then [1994]. One expects that the cost will continue to
decline as the production volume is increased.
When deposited on selected substrates, the product can be made lightweight and
flexible, which is important for many applications. The output power of a-Si PV
products also has a positive temperature coefficient: at higher ambient
temperature, for example, in areas with more sunshine, the efficiency is higher.
The principal advantage of amorphous silicon solar cells is their lower
manufacturing costs, which makes these cells very cost competitive.
One of the main advantages of a-Si over crystalline silicon is that it is much
more uniform over large areas. Since amorphous silicon is full of defects
naturally, any other defects, such as impurities, do not affect the overall
characteristics of the material too drastically.
Amporphous silicon can be produced in a variety of shapes and sizes (e.g.,
round, square, hexagonal, or any other complex shape. This makes it an ideal
technology to use in a variety of applications such as powering electronic
calculators, solar wristwatches, garden lights, and to power car accessories.
Small solar cells used in pocket calculators have been made with a-Si for many
years.
Unlike crystalline solar cells in which cells are cut apart and the recombined,
amorphous silicon cells can be connected in series at the same time the cells are
formed, making it is easy to create panels in a variety of voltages (e.g, for use in
solar battery rechargers).
The human eye is sensitive to light with wavelengths of 400 nm to 700 nm.
Since amorphous silicon solar cells are sensitive to light with essentially the
same wavelengths, this means that in addition to be used as solar cells they can
also be used as light sensors (e.g., outdoor sensor lights, etc).
Disadvantages
As mentioned previously, these panels have a lower efficiency than mono-
crystalline solar cells, or even poly-crystalline solar cells. Attempts to increase
the efficiency, such as building multi-layer cells or alloying with germanium to
reduce its band gap and further improve light absorption all have an added
complexity. Namely, the processes are more complex and process yields are
likely to be lower and costs are likely to be higher as a result – thus reducing the
cost advantage of this type of solar cell.
The expected lifetime of amorphous cells is shorter than the lifetime of
crystalline cells, although how much shorter is difficult to determine, especially
as the technology continues to evolve. From reading through the literature, it
appears that the expected life is still in the order of 25 years or so. For example,
Uni-Solar offers the following performance guarantee on their 144 Wp panels:
92% at 10 years, 84% at 20 years , 80% at 25 year (of minimum power).
Current state
a-Si cells have been made with 15.2% initial efficiency and 13%
stable efficiency
• Rapid deposition processes are being refined so that high rate, high
quality can be achieved
• Research into light degradation remedies will provide for cells with
efficiencies comparable with c-Si cells
• New applications for a-Si cells are being sought such as building-
integrated PV, space power, consumer electronics, grid integration,
and large scale power generation
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