Electronic Structure of Molecular Organic Semiconductor - Metal Interfaces

1
Key concepts for electrical circuits

• Electrical circuits
• Electron
• Conductor
• Insulator
• Current
• Battery and Voltage
• Switch
• Resistance
• Energy

Chemical
Electrical
Heat
Light
Mechanical
2
Electric current and conductor

• Motion of electrons in a conductor

I
Some electrons form bonds, and are not free to
move
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-
-

-
-
-
electrons
Atom
_
_
_

nucleus


protons neutrons


_
-
_
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Some electrons are free to move
-
-
-

• Typical conductors metals like copper, gold,
  silver, aluminum, .
• In a metal, there are about 1023 free electrons /
  cm3
• Can apply a potential difference across the metal
  to move electrons
• 
  electric current

3
Insulator
I0
All the electrons are used in forming chemical
bonds. No electron is free to move

• Typical insulators plastics, ceramics, wood, ..

4
Mechanical analogy of an electric circuit with
battery and motor
water flow electron flow
elevator battery
potential energy
wheel motor
Potential energy given to electrons by battery is
used to do work with the motor, but could also
produce light in light-bulb, produce heat in
radiator, etc..
5
Electric circuit
light heat
heat
light bulb
radiator
electromagnet
connecting wires
battery
current
switch
motor
Key point circuit must be closed all parts
must conduct electricity
6
Electrochemical Battery
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-
-
electric wire
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• Zn atoms at surface of the anode lose two
  electrons to become Zn
• Zn ions combine with SO4 to form ZnSO4

• The two electrons recombine with two H ions at
  the graphite cathode
• Two H atoms form a molecule, and hydrogen bubbles
  off

Carbon
Zinc
H2
H
SO4--
Acid electrolyte molecules break up in three
ions two H and one SO4--
H
H2SO4
Electrodes are zinc and manganese-oxide, with an
alkaline electrolyte
The process is very similar in a regular Duracell
or Energizer battery
-

This chemical reaction wants to happen. It is
exothermic. The strength of the reaction
determines how strongly electrons can be pushed
through the circuit, and what the voltage, or
potential, across the battery is.
7
Resistance

• Element of a circuit made to oppose resistance to
  the flow of electrons
• made of a poor conductor
• V I x R, where V is the voltage across the
  resistance (in volts, V), I is the current (in
  amperes, A) and R is the value of the resistance
  (in ohms, ?)

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Analogy
Symbol
Liquid flow
V
0
Diode

• Element of a circuit that lets current flow only
  in one direction

Analogy
I
Symbol
V
0