Srnthctic Metals, 28 (1989)('775 ('780
C775
CONDUCTING POLYMERS AS INFRARED DETECTORS
C. DAVIES, P.M. PETTY and M.R. WILLIS Department of Chemistry,
Univer'sity off Nottingham,
Nottingham,
NG7 2RD, [JK
ABSTRACT The photoconductive response of polymeric phtha]ocyanines infrared region is reported.
in the near'
Thin-film samples were irradiated with chopped
light and phase sensitive detection was employed.
The use of variable frequency
Results are reported on polymers
prepared by the reaction of copper (films) with tetracyanobenzene or tetnacyanoethylene,
and plasma polymer~sed phthalocyanJnes.
The films show an
infrared photoconduction which is linear with light intensity, only weakly temperature dependent, range.
and in some cases independent off wavelength oven a wide
They thus possess some of the properties required for cheap,
large area
infrared detectons.
iNTRODUCTION Organic materials have considerable potential as radiation detectors and solar energy converters.
Their principle advantages are that they ape often
0379-6779/89/$3.50
The third
in part, to film quality which is also being investigated.
© Elsevier Sequoia/Printed in The Netherlands
C776
The phthalocyanine
molecule represents a good starting point because of its
remarkable
stability
absorption
and its photoconduction
photoconductive reported
response,
response
[2].
its intense visible
In this paper the
using light chopped at a range of frequencies,
for phthalocyanine-related
photoresponse
MEASURING
[I], its film forming capability,
polymers modified
to give absorption and
in the near infrared.
TECHNIQUE
With the exception of mobility measurements majority of measurements steady-state
of photoconductivity
illumination.
complications
in organics have been made using
effects,
heating effects,
improvements
in sensitivity
in the presence of a dark current
However the results can be misleading when significant conduction kinetics occur, eg at a phase change [6]. frequency chopping has the added advantage different
heating effect
or
severe for
carriers.
can be achieved using chopped light and a phase sensitive detector
significantly
but
since the low band-gap gives rise to a significant
of thermally generated
Considerable
the vast
is normally perfectly satisfactory,
The problem of dark current is particularly
infrared photoconductors concentration
The technique
using pulsed light,
can arise when there are polarisation
a large dark current.
separated
is
time constants
The use of variable
that it enables processes with
to be separated.
For instance,
(which is a common problem in polymeric systems)
from the true photocurrent.
technique enables the photoconduction
[3,4,5].
changes in the photo-
Furthermore,
the
can be
in favourable
cases the
kinetics to be explored.
MATERIALS The following materials (a) Chloroindium [7].
have been studied:
phthalocynaine
is a good photoconductor
When plasma polymerised
good strength and adhesion. intact with polymerisation hydrogen atoms [8,9].
it yields a good quality polymeric The phthalocyanine
film with a
ring system appears to remain
occurring by edge linking following the loss of
Samples were fabricated
of sandwich cells 1000-]0,000 (b) A two-dimensional
in the near infrared
from this material
A thick, using gold electrodes
phthalocyanine
in the form
as contacts.
polymer formed by the reaction of a fresh
copper film deposited on a ceramic substrate with tetracyanobenzene Samples were approximately
3 microns thick, contacted
[10,]I].
by two gold electrodes
separated by a gap of 2.5mm, giving a surface cell configuration. {c) A two-dimensional tetracyanoethylene
[]2].
polymer,
similar to (b), but formed by reaction of
with copper, giving a more tetraazaporphyrin-like
material
C777
RESULTS AND DISCUSSION CHOPPED LIGHT RESPONSE The sample is irradiated with chopped light
(5-300 Nz) using monochromated
tight from a quartz halogen lamp in the region 400-4000 nm. A typical response is shown in Fig I.
It
¢i
II l I'l
't,.''
~ ,'~ t'- ¢~ t, I I I II I I j I I I II It II I II 1111111111 I I , '1'
!t',llI', I ~1 tl ii i1
r 11
t ii
II II II
I I1 II
rI II
"
II I I t
[
Ill|l II I I II II
I I II
h
i I
,,
~
II ii II II I III
I
'1'
I
I I I
,,,
II IiI II I tl vI v i O ~
\
~k_ 1
5
I
I
I
I0
I
20
15
Time (s)
FIG I: Response of a photoconductor The peak-to-peak
response,
of chopping frequency, Typical
converted
to a variety of chopping frequencies to conductivity,
applied voltage,
is measured as a function
light intensity and wavelength.
plots of the variation of photoresponse
with chopping frequency are
shown in figs 2 and 3.
3 , 0 x l O 1~
/
r
2 . 0 x l O I~
3'g
I . O x l O 18
0.0
/
i
I
0.02
I
I
0.04.
l/f (s)
FIG 2. First order response of PcInCl plasma polymer as a function of I/chopping frequency
C778
/ o
j
8.0
6.C
4.6
-d 2.0
0.01
0.03 1/f (s)
FIG 3. Second order response of TCNE-Cu polymer as a function of q/chopping frequency The curves can be fitted to one of two equations.
(a) The RyvkSn Equation [3]:
Ao = ~stanh(1/(4xf)) W%lere AU is the measured photoconductivity at chopping frequency f, o
is the s
true steady state photoconductivity and tau the carrier lifetime. The equation is based on first order rise and decay, both processes having the same lifetimes.
Computer modelling shows that where the two processes are
first order but with different lifetimes, the apparent lifetime is the mean of the rise and decay lifetimes. The plasma polymer samples show a close fit to this model with a lifetime tau -I of 2 ms, and a steady state photoconductivity, C~s , of 3 x 10-]6 ohm -1 cm at 100 W m -2 at 280K.
(b) The Hadjiev equation
[4]:
tanh(t4x) Ac~ = cs((l+(,Ito)tanh(to/Xi)
C779 The equation is based on a second order rise and decay, both processes having the same rate constant.
The precise physical model
presumably dominated by bimolecular processes. steady state photoconductivity
is not specified but is
Again the equation enables the
to be obtained as well as a kinetic parameter tau.
The p]asma polymer' deriw~d form PcInC] show a close fLt to this mode] with
(~'S
10-18
: ~ x
ohm-lcm -1 at
lO0 Wm -2 and 280K.
The observatfon of kinetic orders,
including fra
related observations of voltage and inbensity dependence provide a means of exploring the kinetics and mechanism of the photoconduction process.
WAVELENGTH DEPENDENCE Using values (0r the photoconductivity ebtaJnpd from chopped light measurements,
the wavelength dependence can be explored.
polymer is shown in FL8~ 4.
The behaviour of the p]asma
The response extends further into the near infrared
than is normally Found in monomeric
materials.
The iwo-dimensional
polymers,
which are expected to have a distribution of molecular weights and optical absorptions,
show a response which
is Largely independent of wavelength o w r
a
wide range.
S × IC) m
i
E
g
4 ",, 10 le
4(X)
'
600 Wavelenglh l, (nm)
FIG 4: Wavelength dependence off photoconductivity
8&)
O ~I ( X ) )
J n Pc lnCl plasma polymer
TEMPERATURE AND INTENSITY DEPENDENCE
The films show a response which is proportional to light intensity of' the range studied. materials,
The photoeonductivity
is weakly temperature dependent
in all
indicating that the effect observed is a true photocurrent rather than
C780 a heating effect.
The photocurrent for the TCNB-Cu polymer obeys the relation
whereas tau is virtually temperature independent, indicating that temperature dependence arises from an activated mobility or quantum yield. The dark conductivity,(~d
, of the TCNB-Cu polymer shows an activation energy of 0.13 e V
The technique of chopped light is a valuable means of investigating photoconduction.
The polymers possess many of the qualities required for an
infrared detector, but higher sensitivity is required.
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1
A.B.P. Lever, Adv Inorg Chem Radiochem, 7, 27, 1965
2
J. Simon, J-J. Andre, "Molecular Semiconductors: Photoelectrical
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D.D. Eley, S. Kinnear and M.R. Willis, Phys Stat Sol (b), 105, 677, 1981
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