476
18 Organic Semiconductors
Table 18.1 Properties of oligoacene crystals. Melting point and unit cell parameters. Data from [1559]
Property
Naphtalene
Anthracene
Tetracene
Pentacene
Melting point ( ◦ C)
80
217
357
> 300 ◦ C
Crystal system
Monoclinic
Monoclinic
Triclinic
Triclinic
a (nm)
0.824
0.856
0.798
0.793
b (nm)
0.600
0.604
0.614
0.614
c (nm)
0.866
1.116
1.357
1.603
α ( ◦ )
90
90
101.3
101.9
β ( ◦ )
122.9
124.7
113.2
112.6
γ ( ◦ )
90
90
87.5
85.8
Fig. 18.5 Schematic band
structure of a polymer
originating from the states
of the benzene molecule
(see Fig. 2.11)
p z
sp 2
6
18
18.3 Electronic Structure
The p z orbitals in benzene are partially filled and there is an energy gap between HOMO and LUMO
(Fig. 2.11). A similar consideration is valid for polymers. The coupling of orbitals along the polymer
chain leads to broadening of the π and π
∗ states into a (filled) valence and an (empty) conduction band,
respectively (Fig. 18.5).
The HOMO and LUMO positions of various organic semiconductors are shown in Fig. 18.6 relativ to the vacuum level (cmp. Fig. 12.21 for inorganic semiconductors). The HOMO is also known
as ionization energy (IE), the LUMO as electron affinity (EA). With layered organic semiconductors
heterostructures can be built, e.g. to design recombination pathways (recombination layer, electron
blocking layers (EBL) and hole blocking layers (HBL)). For electron injection and electron extraction (hole injection) contacts, metals with appropriate work function (in connection with a possible
interface dipole layer) have to be used. More details on injection and extraction contacts to organic
semiconductors are discussed in Sect. 21.2.7.
18.4 Doping
The doping of organic semiconductors can be achieved by
• partial oxidation or reduction of the organic molecule,
• substitution of atoms in the organic molecule,
• mixing of the matrix with ‘dopant’ molecules.
18 Organic Semiconductors
Table 18.1 Properties of oligoacene crystals. Melting point and unit cell parameters. Data from [1559]
Property
Naphtalene
Anthracene
Tetracene
Pentacene
Melting point ( ◦ C)
80
217
357
> 300 ◦ C
Crystal system
Monoclinic
Monoclinic
Triclinic
Triclinic
a (nm)
0.824
0.856
0.798
0.793
b (nm)
0.600
0.604
0.614
0.614
c (nm)
0.866
1.116
1.357
1.603
α ( ◦ )
90
90
101.3
101.9
β ( ◦ )
122.9
124.7
113.2
112.6
γ ( ◦ )
90
90
87.5
85.8
Fig. 18.5 Schematic band
structure of a polymer
originating from the states
of the benzene molecule
(see Fig. 2.11)
p z
sp 2
6
18
18.3 Electronic Structure
The p z orbitals in benzene are partially filled and there is an energy gap between HOMO and LUMO
(Fig. 2.11). A similar consideration is valid for polymers. The coupling of orbitals along the polymer
chain leads to broadening of the π and π
∗ states into a (filled) valence and an (empty) conduction band,
respectively (Fig. 18.5).
The HOMO and LUMO positions of various organic semiconductors are shown in Fig. 18.6 relativ to the vacuum level (cmp. Fig. 12.21 for inorganic semiconductors). The HOMO is also known
as ionization energy (IE), the LUMO as electron affinity (EA). With layered organic semiconductors
heterostructures can be built, e.g. to design recombination pathways (recombination layer, electron
blocking layers (EBL) and hole blocking layers (HBL)). For electron injection and electron extraction (hole injection) contacts, metals with appropriate work function (in connection with a possible
interface dipole layer) have to be used. More details on injection and extraction contacts to organic
semiconductors are discussed in Sect. 21.2.7.
18.4 Doping
The doping of organic semiconductors can be achieved by
• partial oxidation or reduction of the organic molecule,
• substitution of atoms in the organic molecule,
• mixing of the matrix with ‘dopant’ molecules.