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