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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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