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

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

Conductivity = 1 / Resistivity (10 0 ) Cu Fe polyethylene Si conductor insulator semiconductor ( -1 cm -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 insulatorsemiconductor bandgap electron energy

Conduction in semiconductors semiconductor bandgap thermal (heat energy) light heat light 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 light 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 light 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 electron energy red 650 nm violet 470 nm

Semiconductors Si light Energy

Semiconductors Donor Si As

Semiconductors Si As B B Acceptor

Semiconductors Si What happens when we apply a voltage?

Semiconductors Si + -

Conductivity = 1 / Resistivity (10 0 ) Cu Fe polyethylene Si { Doped Si conductor insulator semiconductor ( -1 cm -1 ) Conductivity scale

Nobel Prize in Chemistry 2000 For the Discovery and Development of Conductive Polymers Alan Heeger University 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 (sp 2 ) A cloud of partly free electrons above and below the ring ( -electrons)

(10 0 ) Cu Fe polyethylene Si { Doped Si conductor insulator semiconductor ( -1 cm -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 voltage

Organic solar cell n C 60 PPV E

n C 60 ( ) PPV E Glass ITO DonorAcceptor Al Organic solar cell

n C 60 ( ) PPV ( ) Problem: The exciton can only travel < 20 nm before the electron and hole recombine E Glass ITO DonorAcceptor Al Organic solar cell

n C 60 PPV Need to create exciton <20nm from an interface Glass ITO DonorAcceptor Al Organic solar cell

n C 60 PPV E Glass ITO DonorAcceptor Al Organic solar cell

C 60 PPV + - Glass ITO DonorAcceptor Al Organic solar cell

C 60 PPV + - Glass ITO DonorAcceptor Al Organic solar cell

C 60 PPV + - Glass ITO DonorAcceptor Al Organic solar cell

Organic Solar Cells University of Linz 10 x 15 cm ; Active area : 80 cm 2 Organic solar cells

Grazing incidence x-ray diffraction

Scanning Probe Microscopy MDMO-PPV: PCBM blend P3HT: PCBM blend

Solarmer

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