Conducting polymers as infrared detectors

Conducting polymers as infrared detectors

Srnthctic Metals, 28 (1989)('775 ('780 C775 CONDUCTING POLYMERS AS INFRARED DETECTORS C. DAVIES, P.M. PETTY and M.R. WILLIS Department of Chemistry...

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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.

REFERENCES

1

A.B.P. Lever, Adv Inorg Chem Radiochem, 7, 27, 1965

2

J. Simon, J-J. Andre, "Molecular Semiconductors: Photoelectrical

3

S.M. Ryvkin, Soviet Phys JETP, 20, 139 1950

4

V. Nadjiev, J Phys D: Appl Phys, 16, 1529-1531,

5

D.D. Eley, S. Kinnear and M.R. Willis, Phys Stat Sol (b), 108, 373, 1981

6

D.D. Eley, S. Kinnear and M.R. Willis, Phys Stat Sol (b), 105, 677, 1981

Properties and Solar Cells", Springer-Verlag

1983

7

C. Davies and M.R. Willis, in preparation

8

J. Castonguay and A. Theoret, Thin Solid Films, 69(I}, 85-97,

9

Y. Osada, A. Mizumuto and H. Tsurata, J. Macromol Sci-Chem, A 24 {3 & 4),

10

D. Wohrle, V. Schmidt, B. Schumann, A. Yamada and K. Shigehara, Ber, 91,

11

D. Wohrle, R. Bannehr, B. Schumann, G. Meyer and N. Jaeger, J Mol Catal,

403-418,

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1987

1987

21, 255-263, 12

1980

1983

T. Narba, Y. Mizushima, H. Naake, A. Imamura, Y. Igarishi, Y. Torihashi and A. Nishioka, Jap J Appl Phys 4, 977, 1965