This document outlines the course content for a class on organic molecular solids. The course will cover: (1) Introduction to materials and prototypical organic molecules; (2) Insulators and charge transport theory; (3) Transient and steady-state photoconductivity in insulators; (4) Effects of finite charge injection; (5) Organic conductors; (6) Carbon-based nanostructures and superconductors; and (7) Applications of organic conductors. Measurement techniques to characterize electronic properties like conductivity, mobility, and Hall effect will also be discussed. Preparation methods for organic materials are included.

1. Organic Molecular Solids
Prof. Allen M. Hermann
Professor of Physics Emeritus
University of Colorado
Boulder, Colorado USA
and
Vice-President
NanoTech Solutions, Inc.
Lexington, KY USA
allen.hermann@colorado.edu

2. Course Outline

3. Section I.
. Introduction
Materials, crystal structures
Prototypical Molecules, anthracene,
naphthalene, etc.
Molecular Solids
Materials Preparation
Electronic Properties Measurements

4. II. Insulators
Charge Transport Theory, narrow bands
Delocalized (Bloch) Wave Functions
Localized Wave Functions
Excitons
Peirels Distortion (1D systems)

5. III. Transient and Steady-state Photoconductivity in
Insulators, Theory and Experiment
Small-signal limit
Drift Mobility
Trapping (shallow and deep)
IV. Effects of Finite Charge Injection
Boundary Conditions, Space Charge Limited Currents
Pulsed, Steady-state Electric Fields and Light
Excitations
Dispersive transport

6. V. Organic Conductors
Charge-transfer Complexes
Quasi-one-dimensional and two-dimensional
materials, radical-ion salts
Polymers
VI. Carbon-based nanostructures and
Superconductors
Buckyballs, Nanotubes, Graphene
Organic Superconductors

7. VII. Applications
Electrostatic Imaging and Xerographic materials
Organic Light-emitting diodes ) OLEDS and Active
Matrix OLEDS (AMOLEDS) for Display and Lighting
Solar Cells
Field-effect transistors
Batteries
Photo-detectors
Luminescence for Land-mine Sniffing
Lasers
Switches
E-Ink

8. Section I.
. Introduction
Materials, crystal structures
Prototypical Molecules,
anthracene, naphthalene, etc.
Molecular Solids
Materials Preparation
Electronic Properties
Measurements

10. Conductivity
Of Organic
Materials

17. Bonds
Chapter 5 of Solymar

18. Introduction
• When two hydrogen atoms come close to each
other
– They form a chemical bond, resulting in a hydrogen molecule (H2)
• When many silicon atoms come close
– They form many chemical bonds, resulting in a crystal
• What brings them together?
– The driving force is
To reduce the energy

19. Interactions between Atoms
• For atoms to come close and form bonds, there must be
an attractive force
– Na gives up its 3s electron and becomes Na+
– Cl receives the electron to close its n = 3 shell and becomes Cl-
– The Coulomb attractive force is proportional to r-2
• In the NaCl crystal, Na+ and Cl- ions are 0.28 nm apart
– There must be a repulsive force when the ions are too close to
each other
– When ions are very close to overlap their electron orbitals and
become distorted, a repulsive force arises to push ions apart and
restore the original orbitals
– This is a short-range force

20. Equilibrium Separation
• There is a balance point, where the two forces cancel out (Fig.
5.1)
– The energy goes to zero at infinite separation
– As separation decreases, the energy decreases, so the force is
attractive
– At very small separation, the energy rises sharply, so the force is
strongly repulsive
– The minimum energy point (Ec, or the zero force point) corresponds to
the equilibrium separation ro
– The argument is true for both molecules in crystals

21. Mathematical
• Mathematically
A B
E(r )  n  m
– A and B are constants r r
– The first term represents the repulsion and the second attraction
• Minimum energy
B m
EC  m
(  1)
ro n
– It must be negative, so m < n

22. Bond Types
• Four types in total
– Ionic
– Covalent
– Metallic
– van der Waals

23. Metallic Bonds
• Each atom in a metal donates one or more
electrons and becomes a lattice ion
– The electrons move around and bounce back and forth
– They form an “electron sea”, whose electrostatic
attraction holds together positive lattice ions
– The electrostatic attraction comes from all directions,
so the bond is non-directional
– Metals are ductile and malleable

24. Covalent Bonds
• When two identical atoms come together, a covalent bond
forms
• The hydrogen molecule
– A hydrogen atom needs two electrons to fill its 1s shell
– When two hydrogen atoms meet, one tries to snatch the electron
from the other and vice versa
– The compromise is they share the two electrons
• Both electrons orbit around both atoms and a hydrogen molecule
forms
• The chlorine molecule
– A chlorine atom has five 3p electrons and is eager to grab one more
– Two chlorine atoms share an electron pair and form a chlorine atom

25. Group IV
• Carbon 1s22s22p2; Si 1s22s22p63s23p2; Ge
1s22s22p63s23p63d104s24p2
• Each atom needs four extra electrons to fill the p-shell
– They are tetravalent
• sp3 hybridization
– s shell and p shell hybridize to form four equal-energy dangling
electrons
– Each of them pairs up with a dangling electron from a neighbor atom
– There are four neighbor atoms equally spaced
– Each atom is at the center of a tetrahedron
– Interbond angle 109.4
– Covalent bond is directional

26. Group IV
• At 0 K
– All electrons are in bonds orbiting atoms
– None can wander around to conduct electricity
– They are insulators
• At elevated temperatures
– Statistically, some electrons can have more enough energy to
escape through thermal vibrations and become free electrons
– They are semiconductors
• The C–C bond is very strong, making diamond the hardest
material known (Table 5.1)
– Diamond has excellent thermal conductivity
– It burns to CO2 at 700C

27. van der Waals Bonds
• Argon has outer shell completely filled
• When argon is cooled down to liquid helium temperature, it
forms a solid
– The electrons are sometimes here and sometimes there, so the
centers of the positive charge (nucleus) and negative charge
(electrons) are not always coincident
– The argon atom is a fluctuating dipole (instantaneous dipole)
– It induces an opposite dipole moment on another argon atom, so they
attract each other
– Such attraction is weak, so the materials have low melting and boiling
temperatures
– They are often seen in organic crystals

28. Aromatic Hydrocarbon Bonds

35. Conducting Organic Materials

38. Extreme Case – Nearly Ionic Bonds
in Highly Conducting Complexes
“Charge Transfer salts”

39. Discovery of Conducting Organic Crystals

44. Materials Preparation
Techniques

48. S S S S
S S S S

50. Electronic Measurements

51. Conductivity (Resistivity)

52. Conductivity s = enm
n: number of carriers; m: mobility of the carriers

53. 4-probe resistivity measurement

55. Van Der Pauw resistivity measurement

57. Hall effect

60. Drift Mobility from Time of
Flight Measurements and TFT
Structures

69. Some references to this material