Organic Electronics

J Emyr Macdonald,School of Physics and Astronomy

Nanophysics group

Issues

We have had electronics and solar cells made from semiconductors like silicon for years.

• Could we make electronics from molecules or plastic?

• What would the benefits be?– Cheaper than silicon to produce– Flexible sheets

• Has anyone seen solar cells made from molecules? Today?

Nanophysics group

http://www.wbgu.de/wbgu_jg2003_kurz_engl.pdf

World in Transition –Towards Sustainable Energy SystemsGerman Advisory Council on Global ChangeBerlin, 2003

Conductivity = 1 / Resistivity

106

104

102

1 (100)

102

104

106

108

1010

1012

1014

1016

CuFe

polyethylene

Si

cond

ucto

rinsulator

semicon

ductor

(-1cm-1)

Conductivity scale

Energy levels in materials

Electrons can only occupy one level.

The first electron will occupy the lowest energy level. The next electron will have to go into a higher energy level.

many atoms

single atom

electron energy

Energy levels in materials

single atom

many atoms

metal insulator semiconductor

bandgap

electron energy

Conduction in semiconductors

semiconductor

bandgap

• thermal (heat energy)• light

heat

BE k T

light

E hf

cf

wavelength

with

For the semiconductor to conduct we need to provide the electrons with energy greater than the bandgap.

electron energy

bound to atom

free to move

There are two possible sources of energy to excite electron across bandgap:

Conduction in semiconductors

semiconductor

bandgap

• thermal (heat energy)• light

heat

BE k T

light

E hf

cf

wavelength

with

For the semiconductor to conduct we need to provide the electrons with energy greater than the bandgap.

electron energy

bound to atom

free to move

There are two possible sources of energy to excite electron across bandgap:

Conduction in semiconductors

semiconductor

bandgap

• thermal (heat energy)• light

heat

BE k T

light

E hf

cf

wavelength

with

For the semiconductor to conduct we need to provide the electrons with energy greater than the bandgap.

electron energy

bound to atom

free to move

There are two possible sources of energy to excite electron across bandgap:

Demo: effect of wavelength of light

semiconductor

E hf

cf

wavelength

with

electron energy

red

650 nm

violet

470 nm

Semiconductors

Si Si Si Si Si

Si Si Si Si Si

Si

Si Si Si Si Si

Si Si Si Si

Si Si Si Si Si

light

Energy

Semiconductors

Donor

Si Si Si Si Si

Si Si Si Si Si

Si

Si Si Si Si Si

Si Si Si Si

Si Si Si Si Si

As

AsAs

Semiconductors

Si Si Si Si Si

Si Si Si Si Si

Si

Si Si Si Si Si

Si Si Si Si

Si Si Si Si Si

AsB

AsB

Acceptor

Semiconductors

Si Si Si Si Si

Si Si Si Si Si

Si

Si Si Si Si Si

Si Si Si Si

Si Si Si Si Si

What happens when we apply a voltage?

Semiconductors

Si Si Si Si Si

Si Si Si Si Si

Si

Si Si Si Si Si

Si Si Si Si

Si Si Si Si Si

+-

Conductivity = 1 / Resistivity

106

104

102

1 (100)

102

104

106

108

1010

1012

1014

1016

CuFe

polyethylene

Si

{Doped

Si

cond

ucto

rinsulator

semicon

ductor

(-1cm-1)

Conductivity scale

Nobel Prize in Chemistry 2000

“For the Discovery and Development of Conductive Polymers”

Alan HeegerUniversity of California at Santa Barbara

Alan MacDiarmid University of Pennsylvania

Hideki Shirakawa University of Tsukuba

Nobel Prize for Chemistry 2000

How do molecules act as semiconductors?

We must have alternating single and double bonds

We have:• bound electrons between the atoms in the ring

(sp2) • A cloud of partly free electrons above and below

the ring (-electrons)

106

104

102

1 (100)

102

104

106

108

1010

1012

1014

1016

CuFe

polyethylene

Si

{Doped

Si

cond

ucto

rinsulator

semicon

ductor

(-1cm-1)

polymer semiconductors

Conductivity scale

Organic Light-Emitting Diodes

Glass

Cathode (ITO) Conjugated Material

Anode (Al)V

R.H. Friend et al., Nature 397, 121 (1990)

Energy

Organic light-emitting diode (OLED)

Flexible displays

Benefits for Organic Electronics

• Weight• Flexibility• Relatively simple processing• Large areas (displays)• Cost

Disadvantage: Slow compared to silicon

Applications for Molecular Electronics

• Electronic paper

• Low-cost chips (e.g. packaging …)

• Solar energy

• Displays

Solar Cell: demonstration

The plotted voltage is proprtional to light intensity – this is shown vs. time

time

volt

age

Organic solar cell

n

C60PPV

E

n

C60

( )PPV

E

Glass ITO Donor Acceptor Al

Organic solar cell

n

C60

( )PPV

( )

Problem: The exciton can only travel < 20 nm before the electron and hole recombine

E

Glass ITO Donor Acceptor Al

Organic solar cell

n

C60PPV

Need to create exciton <20nm from an interface

Glass ITO Donor Acceptor Al

Organic solar cell

n

C60PPV

E

Glass ITO Donor Acceptor Al

Organic solar cell

C60PPV

+

-

Glass ITO Donor Acceptor Al

Organic solar cell

C60PPV

+

-

Glass ITO Donor Acceptor Al

Organic solar cell

C60PPV

+

-

Glass ITO Donor Acceptor Al

Organic solar cell

Organic Solar Cells

University of Linz

10 x 15 cm ; Active area : 80 cm2

Organic solar cells

Grazing incidence x-ray diffraction

Scanning Probe Microscopy

MDMO-PPV: PCBM blend

P3HT: PCBM blend

Solarmer

Molecular solar cells

Molecular solar cells

Photosynthesis

Photosynthesis: at the molecular level

Summary

• Metals, insulators and semiconductors• Molecules and energy levels• Some new devices made from plastic

electronics• Solar energy and world energy requirements• Current developments in molecular solar cells• Photosynthesis: the oldest and most advanced

solar cell technology

Nanophysics group