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
polyethylene
semiconductor
Si
106
104
102
1 (100)
10-2
10-4
10-6
10-8
10-10
10-12
10-14
10-16
insulator
Cu
Fe
conductor
s (W-1 cm-1)
Conductivity = 1 / Resistivity
Energy levels in materials
electron
energy
single
atom
many
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
energy
single
atom
many
atoms
bandgap
metal
insulator
semiconductor
Conduction in semiconductors
For the semiconductor to conduct we
need to provide the electrons with
energy greater than the bandgap.
electron
energy
free to
move
There are two possible sources of energy
to excite electron across bandgap:
• thermal (heat energy)
• light
bandgap
bound to
atom
heat
E  kBT
semiconductor
light
E  hf with
c
f 
wavelength
Conduction in semiconductors
For the semiconductor to conduct we
need to provide the electrons with
energy greater than the bandgap.
electron
energy
free to
move
There are two possible sources of energy
to excite electron across bandgap:
• thermal (heat energy)
• light
bandgap
bound to
atom
heat
E  kBT
semiconductor
light
E  hf with
c
f 
wavelength
Conduction in semiconductors
For the semiconductor to conduct we
need to provide the electrons with
energy greater than the bandgap.
electron
energy
free to
move
There are two possible sources of energy
to excite electron across bandgap:
• thermal (heat energy)
• light
bandgap
bound to
atom
heat
E  kBT
semiconductor
light
E  hf with
c
f 
wavelength
Demo: effect of wavelength of light
electron
energy
semiconductor
red
violet
650 nm
470 nm
E  hf with
c
f 
wavelength
Semiconductors
Energy
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
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
Si
B
Si
Si
Si
Si
Si
Si
As
Si
B
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
Si
{
Si
polyethylene
semiconductor
Doped
106
104
102
1 (100)
10-2
10-4
10-6
10-8
10-10
10-12
10-14
10-16
insulator
Cu
Fe
conductor
s (W-1 cm-1)
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
University of California
at Santa Barbara
Alan MacDiarmid
University of
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
Si
{
Si
polyethylene
polymer
semiconductors
semiconductor
Doped
106
104
102
1 (100)
10-2
10-4
10-6
10-8
10-10
10-12
10-14
10-16
insulator
Cu
Fe
conductor
s (W-1 cm-1)
Organic
Light-Emitting
Diodes(OLED)
Organic
light-emitting
diode
V
Anode (Al)
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
voltage
The plotted voltage is proprtional to light intensity –
this is shown vs. time
time
Organic solar cell
PPV
C60
n
E
Organic solar cell
Glass ITO
Donor
Acceptor
()
PPV
C60
n
E
Al
Organic solar cell
Glass ITO
Donor
Acceptor
Al
() ()
PPV
C60
n
E
Problem: The exciton
can only travel
< 20 nm before the
electron and hole
recombine
Organic solar cell
Glass ITO
Donor
Acceptor
PPV
Al
C60
n
Need to create
exciton <20nm from
an interface
Organic solar cell
Glass ITO
Donor
Acceptor
PPV
C60
n
E
Al
Organic solar cell
Glass ITO
Donor
PPV
Acceptor
Al
C60
+
Organic solar cell
Glass ITO
Donor
PPV
Acceptor
Al
C60
+
Organic solar cell
Glass ITO
Donor
PPV
Acceptor
Al
C60
+
Organicsolar
Solar Cells
Organic
cells
University of Linz
10 x 15 cm ; Active area : 80 cm2
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
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