Thin Film SC

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Crystalline Silicon

Thin-Film Solar Cells

Market Area


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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 SolarEnergy 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

250350 m

Silicon wafer 20 m

Substrate (ceramic, low

cost silicon)

300800 m


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Intercon-

nectionInsulation

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 electricalconnection 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 seriesconnection 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 ofup 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

BaseConductive

intemedi-

ate layer


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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 ribbonsup 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 layerdoped 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 toproduce 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 opticalmicroscopy 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

15

0

00

2

4

6

8

10

2 4 6 8 10

y[mm]

Leff [m]


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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.


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 heatedwith 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 compactlaboratory system RTCVD160, sub-

strates having an area up to

10 x 30 cm2 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 m2, 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.


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Are you interested in our research

and technical services? We would be

glad to further inform you either bypersonal 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 onindustrial 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

Heidenhofstrae 2

D-79110 Freiburg

Tel: +49 (0) 7 61/45 88-0

Fax: +49 (0) 7 61/45 88-90 00

http://www.ise.fhg.de

Area of Business

Solar Cells

Market AreaCrystalline Silicon Thin-Film Solar Cells

Co-ordination

Dr. Stefan Reber

Tel: +49 (0) 7 61/45 88-52 48

Fax: +49 (0) 7 61/45 88-92 48

E-Mail: [email protected]

Contact Persons

Silicon-CVD

Dr. Albert HurrleTel: +49 (0) 7 61/45 88-52 65

Fax: +49 (0) 7 61/45 88-92 65


SSP ribbons and Recrystallisation

Dr. Achim Eyer

Tel: +49 (0) 7 61/45 88-52 61

Fax: +49 (0) 7 61/45 88-92 61


Visit us on the Internet!

http://www.ise.fhg.de

Our Customers

and Our Team

Dielectric Layers

Dr. Friedrich Lutz

Tel: +49 (0) 7 61/45 88-52 67

Fax: +49 (0) 7 61/45 88-92 67


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.

Documents

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