# Surface Scattering Techniques

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```					Nanophysics group

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?
World in Transition –
Towards Sustainable Energy Systems
German Advisory Council on Global Change
Berlin, 2003

http://www.wbgu.de/wbgu_jg2003_kurz_engl.pdf
Conductivity scale
s (W-1 cm-1)
Cu

conductor
106
Fe
104
102
1 (100)

semiconductor
10-2
10-4
Si
10-6
10-8
10-10
10-12

insulator
polyethylene
10-14
10-16

Conductivity = 1 / Resistivity
Energy levels in materials
electron   single   many
energy    atom     atoms

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.
Energy levels in materials
electron   single   many
energy    atom     atoms

bandgap

metal   insulator   semiconductor
Conduction in semiconductors
For the semiconductor to conduct we
electron                   need to provide the electrons with
energy                    energy greater than the bandgap.

There are two possible sources of energy
free to    to excite electron across bandgap:
move        • thermal (heat energy)
• light
bandgap

bound to            heat                light
atom
E  k BT      E  hf with
c
semiconductor                               f 
wavelength
Conduction in semiconductors
For the semiconductor to conduct we
electron                   need to provide the electrons with
energy                    energy greater than the bandgap.

There are two possible sources of energy
free to    to excite electron across bandgap:
move        • thermal (heat energy)
• light
bandgap

bound to            heat                light
atom
E  k BT      E  hf with
c
semiconductor                               f 
wavelength
Conduction in semiconductors
For the semiconductor to conduct we
electron                   need to provide the electrons with
energy                    energy greater than the bandgap.

There are two possible sources of energy
free to    to excite electron across bandgap:
move        • thermal (heat energy)
• light
bandgap

bound to            heat                light
atom
E  k BT      E  hf with
c
semiconductor                               f 
wavelength
Demo: effect of wavelength of light

electron
energy

red           violet
650 nm         470 nm

E  hf with
c
f 
semiconductor           wavelength
Semiconductors

Energy
Si   Si   Si   Si   Si

Si   Si   Si   Si   Si

Si   Si   Si   Si   Si     light

Si   Si   Si   Si   Si

Si   Si   Si   Si   Si
Semiconductors

Si   Si   Si   Si   Si

Si   Si   Si   Si   Si

Si   Si   Si   As
Si   Si

Si   As
Si   Si   Si   Si

Si   Si   Si   Si   Si

Donor
Semiconductors

Si   Si   Si   Si   Si

Si   As
B
Si   Si   Si   Si

Si   Si   Si   As
B
Si   Si

Si   Si   Si   Si   Si

Si   Si   Si   Si   Si

Acceptor
Semiconductors
What happens when we apply a voltage?

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
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 scale
s (W-1 cm-1)
Cu

conductor
106
Fe
104
102
Doped             1 (100)
{

semiconductor
Si               10-2
10-4
Si
10-6
10-8
10-10
10-12

insulator
polyethylene
10-14
10-16

Conductivity = 1 / Resistivity
Nobel Prize in Chemistry 2000
Nobel Prize for Chemistry 2000
“For the Discovery and Development of Conductive Polymers”

Hideki Shirakawa
University of Tsukuba

Alan Heeger                                   Alan MacDiarmid
University of California                             University of
at Santa Barbara                                  Pennsylvania
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 (p-electrons)
Conductivity scale
s (W-1 cm-1)
Cu

conductor
106
Fe
104
102
Doped             1 (100)
{

semiconductor
Si               10-2
polymer
10-4       semiconductors
Si
10-6
10-8
10-10
10-12

insulator
polyethylene
10-14
10-16
Organic light-emitting diode (OLED)
Organic Light-Emitting Diodes

Anode (Al)
V            Conjugated Material
Cathode (ITO)

Glass

Energy

R.H. Friend et al., Nature 397, 121 (1990)
Flexible displays
Benefits for Organic Electronics

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

Disadvantage: Slow compared to silicon
Applications for Molecular Electronics

• Displays

• Electronic paper

• Low-cost chips (e.g. packaging …)

• Solar energy
Solar Cell: demonstration

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

time
Organic solar cell

PPV       C60

n

E
Organic solar cell

Glass ITO   Donor       Acceptor   Al

()
PPV          C60

n

E
Organic solar cell

Glass ITO   Donor       Acceptor   Al

() ()
PPV          C60

n
Problem: The exciton
E                                    can only travel
< 20 nm before the
electron and hole
recombine
Organic solar cell

Glass ITO   Donor       Acceptor   Al

PPV          C60

n
Need to create
exciton <20nm from
an interface
Organic solar cell

Glass ITO   Donor       Acceptor   Al

PPV          C60

n

E
Organic solar cell

Glass ITO   Donor   Acceptor   Al

PPV      C60

-
+
Organic solar cell

Glass ITO   Donor   Acceptor   Al

PPV      C60

-
+
Organic solar cell

Glass ITO   Donor   Acceptor   Al

PPV      C60

-
+
Organic Solar cells
Organic solarCells

University of Linz

10 x 15 cm ; Active area : 80 cm2
Grazing incidence x-ray diffraction
Scanning Probe Microscopy

MDMO-PPV:                      P3HT: PCBM
PCBM blend                     blend
Solarmer
Molecular solar cells
Molecular solar cells
Photosynthesis
Photosynthesis: at the molecular level
Nanophysics group

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

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