Plastic-Electronics

For the semiconductor to conduct we need to provide the electrons with energy greater than the bandgap.
There are two possible sources of energy to excite electron across bandgap: • thermal (heat energy) • light
There are two possible sources of energy to excite electron across bandgap: • thermal (heat energy) • light
bound to atom
heat
light
E hf with f c wavelength
bound to atom
heat
light
E hf with f c wavelength
E kBT
semiconductor
Conduction in semiconductors
electron energy
free to move bandgap
For the semiconductor to conduct we need to provide the electrons with energy greater than the bandgap.
Flexible displays
Benefits for Organic Electronics
• • • • • Weight Flexibility Relatively simple processing Large areas (displays) Cost
Disadvantage: Slow compared to silicon
Cu Fe
Doped
Si Si
{
polyethylene
Organic Light-Emitting Diodes Organic light-emitting diode (OLED)
Anode (Al)
Conjugated Material
Cathode (ITO)
V
Glass
Energy
R.H. Friend et al., Nature 397, 121 (1990)
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
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
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
E
Problem: The exciton can only travel < 20 nm before the electron and hole recombine
conductor
Cu Fe
Doped
Si Si
polyethylene
Conductivity = 1 / Resistivity
insulator
semiconductor
{
Nobel Prize for Chemistry 2000
“For the Discovery and Development of Conductive Polymers”
Energy levels in materials
electron energy single atom many atoms
bandgap
metal
insulator
semiconductor
Conduction in semiconductors
electron energy
free to move bandgap
• 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?
Conductivity scale
s (W-1 cm-1) 106 104 102 1 (100) 10-2 10-4 10-6 10-8 10-10 10-12 10-14 10-16
conductor
Cu Fe
Si
polyethylene
Conductivity = 1 / Resistivity
E kBT
semiconductor
Demo: effect of wavelength of light
electron energy
red 650 nm
violet 470 nm
E hf with
semiconductor
c f wavelength
Semiconductors
Energy
Si Si Si Si Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
light
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
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) 106 104 102 1 (100) 10-2 10-4 10-6 10-8 10-10 10-12 10-14 10-16
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)
World in Transition – Towards Sustainable Energy Systems German Advisory Council on Global Change Berlin, 2003
http://www.wbgu.de/wbgu_jg2003_kurz_engl.pdf
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.