Crystalline Silicon
Thin-Film Solar Cells

Market Area

2

Fraunhofer ISE –

Crystalline Silicon Thin-Film Solar Cells

The Solar Cell of
the Next Generation

Solar cells are fascinating. They generate electric-
ity without damaging the environment, operate
silently and are maintenance-free for decades.
They are powered by the energy of the numer-
ous photons which are incident as sunlight on
the solar cell surface.

The goal of world-wide research and develop-
ment activities is to reduce the costs of photo-
voltaics. As well as optimising the silicon wafer
solar cells which presently dominate the market,
thin-film solar cells, the solar cells of the second
generation, are being intensively developed.
Thin-film solar cells can be realised with a variety
of materials. At the Fraunhofer Institute for Solar
Energy Systems ISE, research is being carried out
on the crystalline silicon (c-Si) thin-film solar cell.

The principle is simple: On an economical sub-
strate at temperatures of over 1 000 °C we pro-
duce an extremely thin layer of highly pure
crystalline silicon to create a thin-film solar cell.
We are convinced that with this method thin-
film solar cells with efficiencies comparable to
those of conventional wafer solar cells can be
produced at much lower costs.

The advantages of the c-Si thin-film solar cell in
comparison to other thin-film solar cells are con-
vincing:

- They do not contain toxic and environmentally

harmful materials.

- They profit similarly from all advances made in

the area of conventional silicon wafer solar
cells.

- Like all crystalline solar cells, they show no

degradation over a long period of time.

- Silicon is widely available. Silicon ore is simply

quartz sand, which can be found all over the
world.

Additionally, as valid for all thin-film solar cells, it
is possible to automatically manufacture large
area c-Si thin-film solar cells, e.g. in the size of a
table top. In short, the c-Si thin-film solar cell
unites the advantages of c-Si wafer solar cells
with those of thin-film solar cells.

Our competence covers all aspects of research
and development in the area of c-Si thin-film
solar cells. Our concentration of know-how on
this subject is unique world-wide, and our exper-
tise includes constructing the systems, manufac-
turing the layers, processing and characterising
the solar cells and layers. This all-encompassing
know-how guarantees excellent results both for
us and for our customers. In spite of this, there
is still much to do in this area. As the partner on
your side, we are in the process of bringing the
next generation of solar cells to the market.

Structure of a silicon
wafer solar cell in com-
parison to a c-Si thin-
film solar cell. For the
silicon wafer solar cell,
more than ten times the
amount of highly pure
silicon is required.

Interdigitated grid c-Si
thin-film solar cell from
Fraunhofer ISE on high-
quality SIMOX-
Substrate. World
record efficiency of up
to 19.2% shows the
enormous potential of
this technology.

Breakdown of costs of a
conventional solar cell
module. Almost two
thirds of the costs are
attributed to the solar
cell, and alone two
thirds of these are attri-
buted to the silicon
wafer.

Module
Glass, frame, interconnection
40%

Solar cell technology
Diffusion, metalisation
20%

Silicon wafer
Silicon, crystallisation, sawing
40%

Silicon wafer

250–350 µm

Silicon wafer 20 µm

Substrate (ceramic, low
cost silicon)

300–800 µm

Intercon-
nection

Insulation
trench

Base
contact

Emitter
contact

Active silicon
layer

Substrate

Insulating
intermediate
layer

“Integrated module” concept

“Innovation through integration”, the
secret of success for the chip industry,
can be applied also to thin-film solar
cells. Thin-film solar cells are pro-
cessed simultaneously on a large sub-
strate, and the electrical connections
to form a module are integrated into
the metalisation step.

In addition to the decreased amount
of work required compared to that
for the conventional electrical
connection of solar cells, potential
sources of failure in the module
connections, i.e. soldered points and
cell connectors, are eliminated.

For our integrated module concept,
we just substitute the conductive
substrate of the wafer equivalent
with an electrically insulated substrate
and modify the solar cell production
process.

The idea is irresistibly easy. The reali-
sation remains our challenge for the
future.

Our Cell Concepts

Crystalline Silicon Thin-Film Solar Cells

– Fraunhofer ISE

3

Schematic of an integrated series
connection of single thin film solar
cells on a substrate to form an inte-
grated module.

“Wafer equivalent” concept

Not only must a successful concept
be good, but also it must be able to
be easily adopted into existing struc-
tures. Our wafer equivalent concept
fulfils both of these requirements.

From the outside, a wafer equivalent
looks like a silicon wafer. The active
part, however, is only a thin silicon
layer. It can be directly processed to
solar cells without changes to existing
production lines.

To produce a wafer equivalent, we
choose two methods: The first and
the fastest method is to deposit a
high grade silicon layer on a substrate
of low cost silicon. The second
method offers somewhat more free-
dom, whereby a silicon layer is first
deposited on an arbitrary conductive
substrate and then melted. By the
following recrystallisation, an impro-
ved quality is then achieved.

Both methods have advantages, and
costs will ultimately be the deciding
factor as to which method will domi-
nate.

First integrated c-Si thin-film solar cell module
on high-quality SIMOX substrate. It holds the
world record with over 17% efficiency.

Structure of a wafer equivalent on a
given conductive foreign substrate
with a conductive intermediate layer.

The simplest wafer equivalent struc-
ture: The thin silicon solar cell layer
is deposited on a cost effective sili-
con substrate.

c-Si thin-film solar cell on an inactive multi-
crystalline silicon wafer with an epitaxially
deposited silicon layer. Efficiencies to date of
up to 13% are an excellent basis for further
optimisation.

There are many ways possible to produce the c-Si thin-film
solar cell. In our opinion, processes using atmospheric pres-
sure and high temperature are the most economical and
energetical ways to success. This forms the basis of both
our concepts.

Antireflection
coating

Antire-
flection
coating

Substrate

Emitter

Emitter

Emitter
contact

Emitter
contact

Base

Silicon-
substrate

Base
contact

Base
contact

Base

Conductive
intemedi-
ate layer

Crystalline Silicon Thin-Film Solar Cells

– Fraunhofer ISE

4

Silicon ribbon of 20 cm width
made by the SSP procedure.

After the zone melting recrystallisation, the Si
layer is made up of long, high-quality crystals,
shown here on a 10 cm wide SSP substrate.

Competence and Service
Available for You

From the substrate...

We manufacture SSP silicon substra-
tes and can assess the quality of your
ceramic substrate. A high quality c-Si
thin-film solar cell can be made only
with a substrate that can withstand
all the demands of the manufacturing
process. Our substrate manufacturing
process, the SSP (Silicon Sheets from
Powder) ribbon process, is optimally
suited in this case. With our reactor
developed specifically for this pur-
pose, we can manufacture ribbons
up to 20 cm wide and 1 m long by
melting silicon powder.

In dealing with ceramics, we also
have a strong expertise, which is con-
stantly being expanded. From alumi-
nium-nitride to zirconium silicate, we
have tested to date many different
types of ceramics for c-Si thin-film
solar cells. We would furthermore be
glad to test your ceramic.

...over the layers....

Layer deposition and zone melting
recrystallisation (ZMR) belong to our
main areas of competence. In these
areas also, we gladly offer you our
services.

We use layers made out of:
- Silicon dioxide and silicon nitride as

intermediate layer and passivation
layer

- Silicon carbide as intermediate layer
- Silicon as the active solar cell layer

doped with boron or phosphorus

Depending on the cell concept, the
crystal quality of a deposited silicon
layer must be improved prior to pro-
cessing. For years we have used the
ZMR procedure to change micro-
crystalline silicon layers into coarse
grained ones. In our lamp heated
zone melting furnaces we can melt
layers with a width of up to 20 cm.

...to the cell...

For our customers’ samples, we first
find the optimal solar cell process and
then carry it out. Only a well-fitting
solar cell process brings out the best
capabilities of the silicon layer.

At our disposal we have all the neces-
sary equipment that one needs to
produce solar cells with peak perfor-
mance.
- Wet chemical processing, tube fur-

nace diffusion, photolithography,
vacuum metalisation and electro-
plating for highly efficient solar cell
processes

- RTP and in-line diffusion, screen

and pad printing for fast industrial
processes

- Plasma technology and remote

hydrogen passivation for processing
completely without wet chemicals

...and to the characterisation

Layer and cell form a whole. We char-
acterise both and compile the single
measurement results to achieve an
overall result.

For this we use both standard
methods and special methods:
- We characterise layers with optical

microscopy and scanning electron
microscopy on polished sections,
with mass spectroscopy as well
as with spreading resistance
profilometry.

- We analyse solar cells with illumina-

ted and dark IV characteristics, with
internal and external quantum effi-
ciency as well as with spectrally
resolved light beam induced current
topography to name a few.

By the electrical characterisation of the solar
cell, like the lifetime topography shown here,
the weak areas can be immediately identified.

The quality of the system as a whole is a direct function of
the quality of every part. At Fraunhofer ISE successful cell
concepts are realised using our collective experience and
know-how for each production step along the way. This is
to your advantage.

Clean room for wet chemical
processing of silicon solar cells.

x [mm]

30

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y [mm]

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eff

[µm]

Equipment for melting silicon...

For many years now melting silicon
has belonged to one of our main
areas of expertise. The know-how
gained over the years in this area is
transferred into the development of
our melting systems for c-Si thin-film
solar cells. We can offer you two
types of systems and willingly can
adapt them to fit your specific
demands.

- SSP melting system for manufac-

turing SSP silicon ribbons. The
manually operative SSP120 is de-
signed for a maximum ribbon width
of 10 cm, and the SSP250 is design-
ed for a maximum ribbon width
of 20 cm, respectively.

- Zone melting recrystallisation system

for the automatic melting and
recrystallisation of thin silicon layers.
Three apparatuses are available for
processing a maximum substrate
width of 10 cm, 20 cm or 40 cm,
respectively.

Crystalline Silicon Thin-Film Solar Cells

– Fraunhofer ISE

5

Development and
Construction of
the Equipment

Installation in China of a SSP120 system for
SSP ribbons with a width of 10 cm.

Zone melting recrystallisation apparatus
ZMR100 for 10 cm wide layers in operation.
Like all of our melting equipment, it is heated
with halogen lamps.

...and for coating with silicon

In the microchip industry, the chemi-
cal vapor deposition process (CVD)
has been successfully used for dec-
ades for the deposition of silicon. For
c-Si thin-film solar cells we develop
specially customised silicon-CVD reac-
tors. Our simple reactor system real-
ises a concept that enables high
throughputs by an efficient gas use
during in-line processing.

- With our newly developed compact

laboratory system RTCVD160, sub-
strates having an area up to
10 x 30 cm

2

are coated with silicon

at temperatures of 900 – 1300 °C.
This apparatus is ideally suited for
the wide variety of tasks carried out
in the field of research.

- Together with Centrotherm GmbH,

we are developing the ConCVD,
which will go into operation for the
first time at our Institute. The
ConCVD is a continuous in-line pro-
cessing deposition system for sili-
con. It is customised for substrates
with a maximum width of 20 cm.
With an annual projected coating
area of 10 000 m

2

, this system is

large enough to be used for a pilot
production.

Our newest development: optically heated Si-
deposition system, RTCVD160, for substrates
with a width of 10 cm.

We develop and build special laboratory equipment for
manufacturing substrates and layers for c-Si thin-film solar
cells, which are destined not only for the market but espe-
cially for our own purposes. Our self-interests in process
development, error elimination and technical advancement
are also of major benefit to our customers.

Are you interested in our research
and technical services? We would be
glad to further inform you either by
personal consultation or through our
internet page.

Several customers

We have numerous customers, both
national and international
- German Federal Ministry of

Economics and Technology

- European Union
- Bayer AG
- Guangzhou Institute of Energy

Conversion, China

- RWE Solar GmbH

... and our projects

- Development and building

of deposition and melting
equipment

- Process development for

the silicon deposition

- Aptitude test for ceramic

substrates

- Production of c-Si thin-film

solar cells

Several Examples

- First c-Si thin-film solar cell on

industrial ceramic

- Construction and installation

of an SSP and silicon CVD
apparatus in China

- Development of fast screen

printing processes for c-Si
thin-film solar cells

- Design and manufacture of

zone melting equipment for
recrystallisation of silicon layers
up to 40 cm in width

Addresses

Fraunhofer Institute for Solar
Energy Systems, ISE
Heidenhofstraße 2
D-79110 Freiburg
Tel: [phone]
Fax: [phone]
http://www.ise.fhg.de

Area of Business
Solar Cells

Market Area
Crystalline Silicon Thin-Film Solar Cells

Co-ordination
Dr. Stefan Reber
Tel: [phone]
Fax: [phone]
E-Mail: [e-mail]

Contact Persons

Silicon-CVD
Dr. Albert Hurrle
Tel: [phone]
Fax: [phone]
E-Mail: [e-mail]

SSP ribbons and Recrystallisation
Dr. Achim Eyer
Tel: [phone]
Fax: [phone]
E-Mail: [e-mail]

October 2001

Visit us on the Internet!
http://www.ise.fhg.de

Our Customers
and Our Team

Dielectric Layers
Dr. Friedrich Lutz
Tel: [phone]
Fax: [phone]
E-Mail: [e-mail]

We work within a network of 56
Fraunhofer Institutes with a total of
ca. 11 000 employees. For our cus-
tomers this means direct contacts and
additional competence.

Title page
Left: Melting furnace for SSP
ribbons with a width of 20 cm.

Middle: Back side of a SSP ribbon,
taken with a scanning electron
microscope.

Right: Interdigitated grid (one-
sided contact) with industrial
screen printing procedure for
c-Si thin-film solar cells.