Chemistry Reference
In-Depth Information
where
m
p
are the mobilities of the negative and positive
carriers, respectively. Photo-conductivity and mobility of the charge carrying species can be deter-
mined from a relationship [
94
]:
e
is the charge on the electron, and
m
n
and
2
m ¼ d
=Vt
where
d
is the thickness of the film,
V
is the applied voltage, and
t
is the carrier drift time. The photo-effect
is evaluated in terms of the effective gain,
. It represents the number of generated carriers reaching the
external circuit per unit time, compared with the number of photons absorbed at the same time [
94
]:
G
G ¼ J
p
=eI
0
ð
1
TÞA
I
0
is the number of incident photons per cm
2
/s,
where
J
p
is the photocurrent,
e
is the electric charge,
T
is the optical transmittance of the film, and
A
is the area of the sample that is being illuminated.
10.7.1 Photoconductive Polymers Based on Carbazole
As stated above, the primary mechanism for charge-carrier generation in poly(vinyl carbazole)
appears to be due to the excitation of the carbazole rings to their excited singlet states [
112
]. While
the singlet excited state is not a conductive species, the conductivity is believed to be the result of an
associated electron-positive hole pair migrating through the solid polymeric material. Dissociation of
the electron pair produces a separate electron and a positive hole in such a way that the electron ends
up in the conducting state [
94
]. This requires acquisition of more energy. One way that can be
accomplished is by exciton-surface interaction [
243
]. Regensburger published an absorption spec-
trum, fluorescence spectrum, and photocurrent spectrum for a 7.6
-vinyl carbazole)
[
244
]. The shape of the response of the photoconductor to the wavelength of the light flash is very
close to the shape of the absorption spectra. Bauser and Klopffer explain this as a result of interaction
of singlet excitons with trapped holes [
245
].
Lyoo used a low-temperature initiator, 2,2
0
-azobis(2,4-dimethyl-valeronitrile), to polymerize
m
m films of poly(
N
N
-
vinyl carbazole in a heterogeneous solution in a mixture of methyl and
t
-butyl alcohols [
247
]. The
10
6
. The author emphasized that this
method provides ultrahigh-molecular-weight polymer and conversions greater than 80%.
polymer that formed has the
M
n
molecular weights
>
3
n
N
The optical transparency of poly(vinyl carbazole) films produced by this room temperature process
appears to be quite high, although transparency decreases at high conversions. In film form, this
material is useful for photoconductors, charge-transfer complexes, and electroluminescent devices.
The higher polymer molecular weight typically enhances film mechanical properties [
247
].
