Photopyroelectric spectroscopy of poly(3-butylthiophene) films

Synthetic Metals, 55-57 (1993) 269-274

PHOTOPYROELECTRIC

R.M. FARIA,

SPECTROSCOPY

269

OF POLY(3-BUTYLTHIOPHENE)

W.L.B. MELO*, A.PAWLICKA,

FILMS

R. SANCHES, M. Y O N A S H I R O ~

I n s t i t u t o de F i s i c a e Q u ~ m i c a de Sio Carlos, U n i v e r s i d a d e de S~o P a u l o C a i x a P o s t a l 369,

13560 - S~o Carlos-SP,

BRAZIL *

ABSTRACT

P h o t o t h e r m a l s p e c t r a of u n d o p e d p o l y ( 3 - b u t y l t h i o p h e n e )

films w e r e o b t a i n e d

u s i n g the p h o t o t h e r m a l s p e c t r o s c o p y t e c h n i q u e in w h i c h a p y r o e l e c t r i c B - P V D F film was e m p l o y e d as a detector. The m e a s u r e m e n t s were p e r f o r m e d in the v i s i b l e range fordifferent

c h o p p i n g frequencies,

and the data were a n a l y s e d a c c o r d i n g to the

m o d e l d e v e l o p e d by M a n d e l i s and Zver. The fitting of the e x p e r i m e n t a l d a t a to the m o d e l a l l o w e d us to o b t a i n severals thermal p a r a m e t e r s and also the o p t i c a l gap of t h e material. KEYWORDS:

Photothermal

spectroscopy,

poly(3-butylthiophene).

INTRODUCTION

Since

the

discovery

of the

conducting polymers

most

c o n c e n t r a t e d m a i n l y on t h e i r optical and e l e c t r i c p r o p e r t i e s

attention

has been

[1]. B e c a u s e this

n e w c l a s s of p o l y m e r i c m a t e r i a l s is of great t e c h n o l o g i c a l interest, w e take the v i e w t h a t the k n o w l e d g e of their thermal b e h a v i o u r and t h e r m a l p a r a m e t e r s is also of

of

thermal

p r o p e r t i e s of undopped, c h e m i c a l l y s y n t h e s i z e d [2] p o l y ( 3 - b u t y l t h i o p h e n e )

fundamental

importance.

This

paper

presents

a

detailed

study

(P3BT),

u s i n g the p h o t o p y r o e l e c t r i c spectroscopic m e t h o d (PPES) [3,4]. This t e c h n i q u e is also

an a p p r o p r i a t e

m e t h o d to m e a s u r e the optical gap of the P 3 B T t o a great

accuracy.

tPresent address: * D e p a r t a m e n t o d e Eng. El~trica-UNESP. A v Brasil, 364 - I. S o l t e i r a - S P ( B r a z i l ) D e p a r t a m e n t o de Qu[mica, U F S C a r - Rod. W a s h i n g t o n Luis K m 235, 13560 - S~o Carlos-SP(Brazil)

Elsevier Sequoia

270 EXPERIMENTAL

PROCEDE~KES

The monomer 3-butylthiophene was prepared by the Grignard method coupling l-butylmagnesium

bromide with 3-bromothiophene,

and using Ni(dppp)Cl 2 as the

catalyst [5]. Polymerization was carried out tl,rough chemical oxidation of the monomer with FeCl3, and the resulting P3BT showed a good solubility in THF, CHCI3, and CH2C12. The thickness of the films prepared by casting was about 12.5pm. The mass density p was obtained from the ratio between the volume and the weight of a piece of the film, which gave 1 g/cm 3 with an accuracy of 5%. The PPES spectra were obtained in a home-made double beam photopyroelectric, whose schematic experimental set-up is shown in Fig. i. The

%

~inL~

L~ns C~

Chopper

IRef

~0n0chromat0r

==3 U

~

c::~ ~

Printer

[

Cable

•

Ref.Cel~

Microcomputer

Optical

~ l e

I

Lock-in I

Fig. I - Schematic experimental set-up of the photothermal system. light source was an ORIEL 450 W Xe lamp. The light beam was modulated by a mechanical chopper (PAR 194A) in the range from 500 to 1500 Hz, and pass through a monochromator (Jarrell-Ash) controled by a microcomputer through a step-motor. The monochromic light was driven to the PPES cells (one with the sample and the other as the references cell) through a split optical cable and the electric signals of the cells were detected by lock-in amplifiers (SR 530 and PAR 5210) loocked at the chopper frequency. All measurements were carried out at room temperature. The pyroelectric detector of the PPES cell was a 20 ~m thick 8-PVDF film with vacuum evaporated Ni-AI electrodes of approximately 20 mm 2 on both surfaces. The characteristic values of the pyroelectric coefficient, the dielectric constant, the thermal diffusivity, and the thermal conductivity of the PVDF film, provided by the manufacturer (Pennwalt KYNAR) are, respectively 3.0x10" 9C/cm2.K, 12, 5.4xi0 ~ cmZ/s, and 3.1x10 ~ cal/cm.s.k, at room temperature. The detector was also painted with black ink to maximize the optical absorption and to ensure its optical opacity condition. The signal from this black surface was used as the reference.

271

RESULTS

AND

DISCUSSION

Figures 2 and 3 show both amplitude V N and phase difference # spectra of the

P3BT's

photopyroelectric

response

in the

range

from

550

to

700

nm,

for

chopping frequences between 500 and 1500 Hz. The spectra are similar to the UVvisible transmission spectrum of the sample [6], whose maximum optical absorption

0.60

/ 500 700 900 II00 1300

~ o

~ 040

HZ

/ /

N

~30.20 z c-

0,00 500.0

55 .0

600.0

650.0

700.0

Wovelength(nm) 2 - Normalized amplitude several frequencies.

Fig.

spectra of the

P3BT's

photopyroelectric

is around 490 nm. In the region of weak optical absorption, high

degree

normalized

of

light

signal.

to

heat

conversion

For wavelength

of the

smaller than

detector 630 nm,

for

above 700 nm, the generates

the PPES

a highly

spectra

are

considered to be in the saturated region, since the optical absorption depth, ~.i, of the sample is smaller than its thermal diffusion length data

(Fig.

conditions

2) of

the

transition

the

P3BT

sample

region is

between

comprised

the

H,- For the amplitude

opacity

between

640

and

and

transparency

700

nm.

In the

difference phase spectra 0, this step becomes more abrupt for increasing chopping frequencies altered

(Fig.

3). The transition region,

suggesting

transition.

that

this

is

the

situated at around 635 nm, is not

wavelength

associated

with

the

~-~*

This wavelength corresponds to 1.95 eV. The same value of energy was

found by extrapolating the linear part of the transition region of the amplitude spectra. A quantitative analysis of the experimental results can be carried out if the Mandelis and Zver theoretical model [3] is used. The thermal diffusion length Hj - where j can be g (gas), s (sample), p (pyroelectric detector), material) thermal

- which

is the inverse of aj is given by

diffusivity

of

the material

j and

(aj/~f) ~,

f is the

chopping

or b (back

where aj, is the frequency.

For

chopping frequencies above i00 Hz the detector is considered as thermally thick

272 0.00

-0.44

I ,I,

ID

500 700 900 zloo 1300

Hz

/

~

-0.88 "

cO - 1 . 3 2 n

-1.76 -

-2.20 500.0

i

i

5500

i

600.0

i

6500

7000

Wavelength(nm) Fig. 3 - Phase frequencies.

difference

spectra of P3BT's photopyroelectric

for

several

and since it is optically opaque, the phase difference in the saturated region (below 640 nm) is given by,

~=-tan-Z[(b~pa+l)c°sh(asLs)+(b~s+bps)sinh(asLs) (bgs+bgs)c o s h (asLs)+ ( b g ~ p s + l ) s i n h (asLs)t a n where

bj01, is

equal

to

~aj/kdad,

and

~

is

the

thermal

]-~0 (awl 8)

conductivity

of

the

material j. Figure 4 presents the fitting of the equation to experimental data in the saturated region.

From the equation

it can be seen that @ practically

varies

linearly with a., since bm << 1 and bm was approximately i. From the slope of this curve we obtain directly the thermal diffusivity of the sample, a., which is ca. 19.5x10 ~ m2/s, and from bm the value 2.46x10 "L W/mK. This value of u. agrees well with that obtained from the inflexion point on V w

versus Ln(f~), shown in Fig.

5, which is directly related to the transition between the thermally thick and thin region of the sample. With the help of the expression pC = k/u, where C is the specific heat, we found C = 1.3 W/gK.

C O N C L U S I O N S

The use of the photopyroelectric spectroscopy to obtain thermal parameters of

poly(3-butylthiophene)

films

has

been

reported.

The

fitting

of

the

experimental data to the phase difference of the Mandelis' model provided us with important values of the thermal diffuslvlty, the thermal conductivity, and the specific heat for the P3BT. The technique also provided an accurate value for the ~-~* transition.

273 -080

-0

- 1.00

"O G) - 1 . 2 0 N O

E L.

0 -1.40

Z

q~ b3 c-~ - 1.60 -

(3. -

1.80

i 18.0

14.0

22'.0

26'.0

Fig. 4 - F i t t i n g of the experimental (continuous line) t h e o r e t i c a l curve.

30'.0

data.

(O)

experimental

points.

- 1.00

0 ~-1.50

0 0 -2.00 Z

>C c -2.50

2.6

2.~

i

~o

i

~

Ln(~

~.~

~.;

3;

)

Fig. 5 - L o g - l o g c u r v e of n o r m a l i z e d amplitude v o l t a g e v e r s u s square root of the f r e q u e n c y . Inflexionlx~int represents the transition between the thermally t h i c k and t h i n regions.

274 ACKNOWLEDGEMENTS The authors

want to thank the financial

assistance

of the FAPESP,

CNFq

(Brazil).

REFERENCES

1

Handbook of Conducting Polymers,

Ed. by A.T. Skotheim,

Marcel Dekker New

York (1986). 2

Kulszewicz-Bajer, A. Pawlicka, J. Plenkiewicz, and A. Pr6n, Synth.Met., 30

3

A. Mandelis and M.M. Zver, J.Appl.Phys.,

4

A. Mandelis,

(1989) pp.335. R.E. Wagner, K.Ghandi,

57 (9),

(1985), pp. 4421.

and R. Baltman,

Phys.Rev.B.,

39 (8),

(1989), pp. 5254. 5

R. Sugimoto,

S. Takeda,

H.B. Gu, Nad K. Yoshiro,

Chemistry Express.

11,

(1986), pp. 635. 6

S. Hotta, 20,

7

S.D.D.V.

Rughooputh, A.J. Heeger,

and F. Wudl, Macromolecules,

(1987), pp. 212.

R. Linton, C.W. Frank, and D.D.V. Rughooputh,

Synth.Met.,

28 (1989) C393.