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ENERGY & SMART BUILDINGS

8

No 20 | October 2017 |

OPE

journal

I

n times of smart homes and wireless

sensor networks, it is of great importance

to generate the required electrical energy

directly on the relevant devices to avoid

maintenance of batteries. The field of

energy harvesting is related to this topic

and covers the conversion of waste energy

to usable electric energy. One possible

way is the conversion of waste heat using

thermoelectric generators (TGs). PROFAC-

TOR GmbH developed fully inkjet-printed

thermocouples on flexible substrates. Two

different silver inks with different post

processing requirements were used and

compared. Different designs have been

inkjet-printed and respective voltages and

Seebeck coefficients were determined.

Thermocouples and

the Seebeck effect

Thermocouples are normally composed

of two different metals or alloys, which

are firmly attached on one side. If the two

ends are exposed to a temperature differ-

ence a small voltage in the region of µV is

produced. When connected to a load an

electrical current is flowing and therefore

electrical power can be produced. This

behaviour is based on the Seebeck effect.

To reach higher voltage and power outputs

semiconductors are

used and single couples

are assembled in series.

The performance

of TGs is related to

high electrical con-

ductivity and low

thermal conductivity.

The disadvantage of

these discrete systems

is their inflexible and

rigid structure, which

prevents the usage

on curved or uneven

surfaces. Further-

more, they are rather

expensive compared

to other electronic

parts. Thermocouples based on organic

semiconductors on flexible substrates

can be a more affordable and flexible

alternative for usage in room temperature

applications. Inkjet printing is an interesting

method for their production as it allows

a contactless deposition on various sub-

strates. Furthermore, it is a digital process,

which allows graphical designs without

the use of physical master templates.

PEDOT:PSS

and silver as

flexible Counterparts to

conventional inorganic

semiconductors

Organic semiconductors exhibit low

thermal conductivities and reasonable

electrical conductivities, which qualifies

them for the use in thermoelectric applica-

tions.

PEDOT:PSS

is a widely used conduc-

tive organic material in printed electronics

applications. As a second material for the

thermocouple silver was used. Silver-based

inkjet inks are widely available nowadays.

Demonstrators were inkjet printed on flex-

ible PET foils. Due to the thermal sensitivity

of PET substrates, thermal sintering is dif-

ficult. Thus, two different silver ink systems

were used: a reactive silver ink as well as

photonic sinterables of silver nanoparticle

(NP) inks. The reactive silver ink contains a

diamine silver complex, which is reduced to

bulk silver upon moderate thermal treat-

ment. In a second approach, photonic

sintering of NP containing silver inks was

used to create thermocouples. The advan-

tage is the higher conductivity of sintered

silver without destroying the PET substrate.

Fully inkjet-printed

thermocouples

Inkjet-printed thermocouples with

reactive silver ink were dried on a hotplate

at 60°C for a few minutes before inkjet

printing of

PEDOT:PSS

on top. The dem-

onstrators with particle based silver ink

were dried for at least an hour for effec-

tive solvent evaporation. After photonic

sintering,

PEDOT:PSS

was printed on top. In

performance tests one part of the ther-

mocouples was mounted on a hotplate.

The generated temperature difference was

measured with an infrared thermometer.

In combination with the generated voltage

the Seebeck coefficient was calculated.

Improving the

Seebeck coefficient

It is known that the Seebeck coefficient

can be increased by treating

PEDOT:PSS

with ethylene glycol (EG) due to removal

of excessive PSS which acts as an insulator.

Therefore, inkjet-printed demonstrators

were immersed in an EG bath. This treat-

ment was performed with an EG bath at

room temperature or at 50°C. A comparison

of untreated and treated demonstrators is

depicted in the table on the left and shows

that especially the treatment at 50 °C

leads to an improved Seebeck coefficient.

The inkjet printing of devices, which

are now discrete and silicon-based, is an

exciting area of development of wearables

and flexible devices. It allows individual

production at low cost in future.

Comparison of Seebeck coefficients

Energy for a smart future

Florian Durst and Julia Kastner from Profactor GmbH (Steyr-Gleink, Austria)

on inkjet-printed thermocouples and thermoelectric generators