Chemistry Reference
In-Depth Information
Materials with large
E
ex
values would not be indicated for e.g., photovoltaic de-
vices, since the photogeneration of charge carriers would be inefficient, but could
in principle be used as light emitting diodes. Section 4.2 will show examples of
the experimental determination of the band diagrams of selected organic semicon-
ductors.
E
t
is usually obtained from
e
−
E
t
/
k
B
T
.
However, the derived value for
E
t
is usually termed the activation energy
E
a
and
may contain other contributions in addition to the intrinsic
E
t
value, such as those
arising from the orientation and morphology of the sample. For this reason
E
a
is
most commonly used. In the case of polycrystalline thin films, grain boundaries
may have a dominant effect, as will be discussed in Section 6.4, where examples
of intrinsically metallic materials behaving as semiconductors will be given.
Organic semiconductors can be arbitrarily divided into single component and
multicomponent materials. Table 1.6 provides a list of electronic energies for sin-
gle component materials in alphabetical order. A detailed analysis of the electronic
processes in organic crystals and polymers can be found in Pope and Swenberg,
(1999). Examples of binary or ternary component materials that become semi-
conducting in a given region of their phase-space will be given throughout this
section. From the table we observe that the reported values of
E
ex
lie below 1.4 eV.
However, some discrepancies are found when describing the same material, as in
the case of Alq
3
, where values as low as 0.2 eV and as large as 1.4 eV have been
obtained. Such discrepancies are due to the different experimental methods used for
the determination of
E
ex
and to different sample quality depending on the effective
order range (degree of crystallinity, morphology, etc.). The presence of defects,
especially in films grown from the vapour phase, plays a determinant role, partic-
ularly when localized charges are induced. Therefore, comparison of the obtained
magnitudes is sometimes difficult. We will insist throughout the topic that it is
imperative to evaluate the quality of the samples (single crystals or films) before
discussing intrinsic physical properties. In addition, the phenomenology of charge
generation and transport is complex for organic materials, including polymers, and
fundamental research has to be pursued.
Pentacene is the leading representative of the acenes family (C
4
n
+
2
H
2
n
+
4
), which
are planar organic molecules consisting of
n
aromatic rings arranged linearly, dis-
played in the second column of Table 1.4. Pentacene exhibits high carrier mobility
values,
σ
vs.
T
measurements, since
σ
∝
1cm
2
s
−
1
V
−
1
, close to the values found for amorphous hydrogenated
silicon (a-Si:H), making pentacene a good candidate to compete with silicon micro-
electronics for particular applications (see Table 6.1). Most of the MOMs exhibit
low mobility values,
∼
10
−
3
cm
2
s
−
1
V
−
1
, comparable e.g., to those of
∼
α
- and
10
−
3
cm
2
s
−
1
V
−
1
β
63 K (Loveland
et al.
,
1972). The mobility values are reduced by the presence of grain boundaries and
-N
2
,
∼
2
×
in the range 52
<
T
<
