Chemistry Reference
In-Depth Information
10.7 Photo-Conducting Polymers
Unless polymers contain long sequences of double bonds, they are fairly good insulators, particularly
in the dark. Nevertheless, a number of common polymers show measurable increase in conductivity,
when irradiated with light. When polymeric materials, like poly(vinyl fluoride), poly(vinyl acetate),
poly(vinyl alcohol), or poly(
-vinyl carbazole), are exposed to light, they develop charged species.
The species can migrate under an electric field and thus conduct electricity. When poly(
N
-vinyl
carbazole) is doped with photosensitizers or compounds that form charge-transfer complexes, the
photosensitivity can be increased and even extended into the visible region of the spectrum. Since
discovery in 1957 that poly(
N
-vinyl carbazole) has photoconductive properties, there has been
increasing interest in the synthesis and study of this and other polymeric materials with similar
properties that allow various photonic applications. Related polymers are presently utilized in
photocopiers, laser printers, and electro-photographic printing plates.
Photoconductive polymers can be p-type (hole-transporting), n-type (electron-transporting), or
bipolar (capable of transporting both holes and electrons). To date, most photoconductive charge-
transporting polymers used commercially are p-type.
Poly(vinyl carbazole) and other vinyl derivatives of polynuclear aromatic polymers, such as poly
(2-vinyl carbazole) or poly(vinyl pyrene), have high photoconductive efficiencies. These materials
may take up a helical conformation with successive aromatic side chains arranged parallel to each
other in a stack. In such an arrangement, the transfer of electrons is facilitated. Also, it is believed that
the primary mechanism for poly(vinylcarbazole) charge carrier generation is due to excitation of the
carbazole rings to the first excited singlet state. This polymer absorbs ultraviolet light in the 360-nm
region and forms an exciton that ionizes in the electric field. The excited state by itself is not a
conductive species. The addition of an equivalent amount of an electron acceptor, like 2,4,7-
trinitrofluorenone, shifts the absorption of this polymer into the visible region by virtue of formation
of charge transfer states. The material becomes conductive at 550 nm. This associated electron-
positive hole pair can migrate through the solid polymeric material. Upon dissociation of this pair into
charged species, an electron and a positively charged hole, the electron becomes a conductive state.
To achieve this, additional energy is required and can be a result of singlet-singlet interaction [
239
],
singlet-triplet interaction [
240
], singlet-photon interaction [
239
], triplet-photon interaction [
239
],
and two-photon interaction [
240
]. Kepler carried out fluorescence quenching studies and concluded
that the migration of the exciton is the most probable energy transfer mechanism of poly(vinyl
carbazole) [
241
]. He, furthermore, suggested that the exiton can visit 1,000 monomer units during its
lifetime [
241
]. This is a distance of about 200
˚
.
Kang and coworkers [
242
] also explored steady state and pulsed photo-conductivities in 4-8
N
m
m
thick films of
-polyphenylacetylene films doped with
inorganic and organic electron acceptors, particularly iodine and 2,3-dichloro-5,6-dicyano-
trans
-polyphenylacetylene and also
trans
-
modulated by shallow electron traps in the undoped polymer and by trapping the charge-transfer
complex in the doped polymer [
242
]. Guellet [
94
] states that photo-conductivity
p
s
is equal to the
current density
is aperture/unit electrode area. This is
related to the number of negative-charge carriers (usually electrons) per unit volume, and
J
divided by the applied field strength
e
, where
J
p
is the
number or positive charge carriers (or positive holes) per unit [
94
]
s ¼ J=e ¼ nem
n
þ pem
p
