F R A U N H O F E R I N S T I T U T E F O R S O l A R E N E R g Y S Y S T E m S I S E

Co-Diffusion for n-Type solar Cells

N-type solar cells offer the highest

conversion efficiencies due to the absence

of light induced degradation and the

lower sensitivity to common impurities

like iron. These advantages are expected

to strongly increase the market share

of n-type solar cells in the near future.

Large scale industrial production of n-type

solar cells requires cost effective process

solutions.

At the Fraunhofer Institute for Solar Energy

Systems ISE we develop process sequences

for n-type cells that considerably reduce the

number of process steps. A key technology

is the simultaneous diffusion of p and n

dopants by co-diffusion. Our expertise

covers the application and characterization

of a wide range of dopant sources as well

as process development for different cell

structures from prototyping to industrial

implementation.

Our services include the integration of new

dopant sources in existing processes. We also

provide cell half fabricates for evaluating

customer process lines. Our expertise and

excellent infrastructure enable us to trans-

form our customers‘ ideas and visions for

n-type solar cells into functioning processes.

Co-Diffusion Approach

With our co-diffusion approach we offer a

cost-effective method for simultaneously

generating p- and n-doped regions within

one single thermal process. The use of

conventional tube furnaces allows for high

throughput and easy integration in existing

production lines. Prototyping at our clean

room facilities (ETALab) ensures demon-

stration on a high efficiency level while

processing at the Photovoltaic Technology

Evaluation Center (PVTEC) enables a fast

and direct transfer of results into industrial

production.

1 High throughput diffusion furnace.

2 Bifacial solar cell (left: front side,

right: rear side).

3 Back contact back junction (BC-BJ)

solar cell.

Fraunhofer Institute for

Solar Energy Systems

Heidenhofstrasse 2

79110 Freiburg

Germany

Phone +49 761 4588-0

Fax +49 761 4588-9000

www.ise.fraunhofer.de

Silicon Photovoltaics –

Doping and Diffusion

Dr Andreas Wolf

Phone +49 761 4588-5580

sipv.doping @ise.fraunhofer.de

September 2014

1 3 2

Equipment and Processes

We offer co-diffusion processes using

combinations of

n POCl3- or BBr3 tube furnace diffusion

n inline diffusion

n phosphorus and boron ion implantation

n phosphorus and boron-doped layers

deposited by CVD

n printed dopant sources

Our flexible doping profiles are applicable

for a wide range of solar cell concepts. In-

line characterization allows for contactless

in-depth characterization of large batch

sizes. Possible wafer sizes are 125, 156,

and 210 mm edge length.

Research and Testing Facilities

The cleanroom facilities (ETALab), the

Photovoltaic Technology Evaluation Center

(PVTEC) and the PV Module Technology

Center (Module-TEC) at Fraunhofer ISE

are the ideal platforms for advanced

concepts and processing on a pilot line

scale. Our accredited characterization labs

CalLab PV Cells and CalLab PV Modules

enable high precision IV-measurements

while the reliability according to IEC 61215

is tested in our Testlab PV Modules.

Expertise and Services

n industrially relevant process sequences

for numerous n-type solar cell concepts:

- screen printed bifacial

- back contact back junction

- others like PERL and PERT

n optimization of diffusion processes

and dopant concentration profiles in

co-diffusion processes

n combination of diffusion and implant

anneal processes

n control and characterization of cross

doping effects and reproducibility

n detailed characterization of doped

regions regarding dopant concentration

profile, carrier recombination and

contact resistance including device

modeling

n high efficiency processes at the clean

room facilities

n solar cell fabrication under industrially

relevant conditions with batch sizes of

several thousand solar cells

n adjustment of doping processes to

different wafer materials and surface

conditions

n characterization of solar cells, also with

regard to module integration

n module integration at the PV Module

Technology Center (Module-TEC)

n detailed cost of ownership calculation

for individual process and full process

sequences

1 Scanning electron microscope cross section

image of a doped region formed by co-diffusion.

2 Structured doped layers for BC BJ solar cells.

1

4 Boron and phosphorus dopant concen-

tration profiles generated by co-diffusion

processes using different dopant sources.

3 Example for co-diffusion setup for back

contact back junction (left) and bifacial solar

cells (right)

N -TYPE WAFER 1 µm

BORO

N D

OPED

EmITTER

BORON DOPED CVD lAYER

PSG

n++-Si

n+-Si

BSG

p++-Si

POCl3 BSG

p++-Si

n+-Si

POCl3 SiOx

depth (nm)

500 1000

phosphorus

boron

carr

ier

conc

entr

atio

n (c

m-3)

1021

0

1020

1019

1018

1017

1500

2 200 µm

PSg/S iO x/BSg

BSg

S iO x