18.4 Doping
477
Fig. 18.6 Position of
HOMO and LUMO for a
variety of organic
semiconductors (relative to
a common vacuum level at
E = 0 eV). Based on
values from [1567]. On the
right hand side the work
functions of several metals
are shown for comparison
(a)
(b)
Fig. 18.7 a Conductivity (at 0.9 V/µm) versus dopant concentration N D for PPEEB films. Experimental data (circles)
and fit (solid line) according to (18.1) with activation energy E a,0 = 0.23 eV and β = 6.5×10 −8 eV cm (μ = 0.2 cm 2 /Vs).
Dashed line denotes linear relation between conductivity and N D . Adapted from [1571]. b Thermal activation energy
E a of carriers (holes) in ZnPc:F 4 -TCNQ as a function of the molar dopant concentration. Adapted from [1568]
The systematic shift of the Fermi level with dopant concentration has been reported in [1568]. Typically,
the conductivity increases superlinearly with the doping concentration (Fig. 18.7a), an effect discussed
in detail in [1569]. While the mobility remains constant, the thermal activation energy E a for carriers
decreases with increasing doping (Fig. 18.7b) due to electrostatic interaction [594, 1570], an effect
already discussed in Sect. 7.5.7. The activation energy in the dilute limit E a,0 is modified to (cmp.
(7.52))
E a = E a,0 − β N
1/3
D .
(18.1)
477
Fig. 18.6 Position of
HOMO and LUMO for a
variety of organic
semiconductors (relative to
a common vacuum level at
E = 0 eV). Based on
values from [1567]. On the
right hand side the work
functions of several metals
are shown for comparison
(a)
(b)
Fig. 18.7 a Conductivity (at 0.9 V/µm) versus dopant concentration N D for PPEEB films. Experimental data (circles)
and fit (solid line) according to (18.1) with activation energy E a,0 = 0.23 eV and β = 6.5×10 −8 eV cm (μ = 0.2 cm 2 /Vs).
Dashed line denotes linear relation between conductivity and N D . Adapted from [1571]. b Thermal activation energy
E a of carriers (holes) in ZnPc:F 4 -TCNQ as a function of the molar dopant concentration. Adapted from [1568]
The systematic shift of the Fermi level with dopant concentration has been reported in [1568]. Typically,
the conductivity increases superlinearly with the doping concentration (Fig. 18.7a), an effect discussed
in detail in [1569]. While the mobility remains constant, the thermal activation energy E a for carriers
decreases with increasing doping (Fig. 18.7b) due to electrostatic interaction [594, 1570], an effect
already discussed in Sect. 7.5.7. The activation energy in the dilute limit E a,0 is modified to (cmp.
(7.52))
E a = E a,0 − β N
1/3
D .
(18.1)