ELECTROOPTICAL PROPERTIES OF THIN FILMS OF POLYHETEROCYCLES

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ELECTROOPTICAL PROPERTIES OF THIN FILMS OF POLYHETEROCYCLES

M. Gazard, Jessica Dubois, M. Champagne, F. Garnier, G. Tourillon

To cite this version:

M. Gazard, Jessica Dubois, M. Champagne, F. Garnier, G. Tourillon. ELECTROOPTICAL PROP- ERTIES OF THIN FILMS OF POLYHETEROCYCLES. Journal de Physique Colloques, 1983, 44 (C3), pp.C3-537-C3-542. �10.1051/jphyscol:19833108�. �jpa-00222617�

J O U R N A L DE P H Y S I Q U E

Colloque C3, supplement au n06, T o m e 44, juin 1983 page C3-537

E L E C T R O O P T I C A L P R O P E R T I E S OF T H I N F I L M S OF POLYHETEROCYCLES

M. ~ a z a r d * , J . C . ~ u b o i s * , M. Champagne*, F. ~ a r n i e r ~ a n d G. ouri ill on*

*Thornson-CSF L a b o r a t o i r e C e n t r a l d e Recherches, Domaine de CorbeviZZe, 91 4 0 1 Orsay Cedes, France

**C. N. R. S. L a b o r a t o i r e de Photochimie S o l a i r e , 94320 T h i a i s , France

R6sm6 - Des f i l m s minces de polymeres conducteurs prkpar6s par polymkri- s a t i o n klectrochimique d ' h k t k r o c y c l e s changent de c o u l e u r reversiblement l o r s q u l o n l e s soumet B des c y c l e s d'oxydation-rkduction. Par exemple l e poly- p y r r o l e s y n t h k t i s k sous une forme oxydke passe du brun au jaune l o r s q u ' i l e s t r 6 d u i t , l e s p o t e n t i e l s 21 a p p l i q u e r B l l C l e c t r o d e Ctant respectivement 0 e t - 0 , 7 v o l t par r a p p o r t B une k l e c t r o d e de r k f k r e n c e au calomel. D ' a u t r e s cou- l e u r s peuvent i t r e obtenues avec d ' a u t r e s h k t 6 r o c y c l e s t e l s l e s t h i o p h h e s . C e t t e p r o p r i k t k peut t r o u v e r d e s a p p l i c a t i o n s dans l e domaine ae l a v i s u a l i - s a t i o n . Les p r o p r i k t k s klectrochimiques de quelques polyhktkrocycles o n t kt6 dkterminkes e t l e s c a r a c t k r i s t i q u e s d e n s i t k o p t i q u e , temps de rkponse, durke de v i e s o n t comparCes B c e l l e s des a u t r e s matkriaux 6lectrochromes.

Abstract - Some conducting p o l y h e t e r o c y c l e s prepared by e l e c t r o i n i t i a t e a polymerization a s t h i n f i l m s on e l e c t r o d e s change c o l o r r e v e r s i b l y when they a r e e l e c t r o c h e m i c a l l y cycled. A s an example, p o l y p y r r o l e i s s y n t n e t i z e d i n an o x i d i z e d form and may be reduced r e v e r s i b l y between 0 and - 0,7 v o l t v s SGE.

The modified e l e c t r o d e change from brown t o yellow a u r i n g t h e c y c l i n g of t h e p o t e n t i a l . Other c o l o r s may be obtained w i t h o t h e r n e t e r o c y c l e s . The e l e c t r o - chemical p r o p e r t i e s of some p o l y h e t e r o c y c l e s have been determined and t h e i r c h a r a c t e r i s t i c s - o p t i c a l d e n s i t y , s w i t c h i n g speed, a g e i n g e f f e c t s - a r e com- pared t o t h o s e of o t h e r electrochromic o r g a n i c m a t e r i a l s .

I N T O W C T I O N

Chemically o r e l e c t r o c h e m i c a l l y doping o r undoping of conducting polymers inauces a m o d i f i c a t i o n of t h e i r p h y s i c a l p r o p e r t i e s such a s e l e c t r i c a l c o n d u c t i v i t y and ab- s o r p t i o n spectrum. Conductivity may be c o n t r o l l e d through e l e c t r o c h e m i c a l doping and t h i s p r o p e r t y al.lows t h e use of conducting polymers i n rechargeable b a t t e r i e s ( 1 ) . O p t i c a l absorpti.on spectrum depends on t h e e x t e n t of doping and changes i n t h e v i s i b l e range have been observed f o r a l l t h e conducti.ng polymers : polyacetylene ( 2 ) , polyparaphenylene ( 3 ) , polypyrrole ( 4 ) ; t h e doging process may be chemical o r electrochemical.. M a t e r i a l s i n which a colour change i s induced by t h e passage of a charge a r e c a l l e d electrochromic and some of them have been proposed f o r a p p l i c a - t i o n s i n d i s p l a y s . One of t h e f i r s t p r o p e r t i e s whicn a r e r e q u i r e d f o r an e l e c t r o - cnromic m a t e r i a l i s t h e a b i l i t y t o form uniform t h i n fil.ms on e l e c t r o d e . Electro- chemically i n i t i a t e d polymerization i s an a t t r a c t i v e method t o d e p o s i t f i l m and has been used t o make f i l m s of conducting p o l y p y r r o l e s ( 5 ) . The f i l m s a r e obtained i n t h e oxidized s t a t e and may be r e v e r s i b l y reduced with a c o l o u r change from brown t o yellow. Polypyrrole and N s u b s t i t u t e d polypyrrol.es nave been proposed a s elec- trocnromic m a t e r i a l s ( 6 ) . I n a previous paper t h e e l e c t r o c h e m i c a l polymerization of o t h e r h e t e r o c y c l e s was d e s c r i b e d : tniophene, f u r a n , i n a o l e , azulene and t n e physi- cochemical and conducting p r o p e r t i e s of t h e polymers were given ( 7 ) . We i n v e s t i g a - t e d o t h e r el.ectrochemically polymerized thiophenes ana i n t h i s paper we r e p o r t t n e e l e c t r o c n e m i c a l and e l e c t r o o p t i c a l p r o p e r t i e s of t n i n f i l m s of t h e s e polymers and we compare them t o t h o s e of polypyrrole f i l m s .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19833108

JOURNAL DE PHYSIQUE

EXPERIMENTAL

films preparation

Electrochemical polymerization was carried out in a single compartment cell from deaecrated solutions containing 0.05 d monomer in acetonitrile. The monomers were purified by distillation, chromatography and/or recrystallization. Acetonitrile was used as received or after distillation. Tetraethylammonium tetrafluoroborate (Eta N BF4) was used as supporting electrolyte in a concentration of 0.1 M. The working electrode was a unpolished platinum foil or Sn 02 coated glass ana the counter- electrode was a platinum foil or a platinum wire. All potentials were measured with respect to a calomel reference electrode (SCE).

The films were prepared at constant current density (1 50 v~/cm2) or at constant potential. After polymerization the working electrodes were removed from the cell, rinsed with acetonitrile and dried witn argon. The film thicknesses were determined from measurements using a Sloan Dektak profiler.

Measurements on the films

Electrochemical measurements on the films were made in the cell describea before and in 0.1 M (Et4 N BF4) - CH3 CN. Cyclic voltammetry and cnronoamperometry were carried out using Tacussel apparatus. Spectra were recorded witfi a Perkin Elmer spectro photometer.

Table 1

data of in CH3 CN electrochemically polymerized heterocycles

( I ) current density 150 uii/cm2. Electrolyte salt Et4 N BF4 0.1 N

(2) non uniform films (3) not measured a tnin films b pressed pellets

Conductivity measurements were made on films or pressed pellets at room temperature by a four probe technique using a Keithley current source and a Keithley elec- trometer.

RESULTS AND DISCUSSION Polymer deposition

Six monomers were polymerized ; they are listed in table 1 where electrochemical data of the polymers are given. The films were deposited at potential slightly higher than the oxidation potential of the corresponding monomer. Uniform films were obtained except for thiophene and 3 methylthiophene, the oxidation potential of w9ich being the highest. The growth rate for the uniform films was a few 100 ~ / m . The films had good ad'nering properties when the thicknesses were lower than a few microns.

The polymers are conducting but their conductivities depend on the nature of the monomer.

C y c l i c voltammetry

All the films exhibit cyclic voltammetric responses and this behavior is illustra- ted in figure 1 where typical voltammograms are represented. It is interesting to compare the electrochemical responses of films grown from similar monomers : poly (2 2' bithiophene) voltammogram is different from this of polythiophene : the wave in the anodic region is sharper and there are two not clearly defined waves in the cathodic region which are not well understood. Poly (3,4 dimethylthiopnene) and

~oly(3 methylthiophene) possess two waves in the cathodic region. The peaks are very broad in the case of poly (3 meth~lthio~hene). It can be seen tnat except for polypyrrole the reduction and the oxidation of the polymers occur at potentials which are in the positive range versus SCE.

V v s SCE

F i g . 1 - Typical cyclic voltammograms of films of electrochemically ,polymerized he-

terocycles on Pt in 0.1 M Et4 N B F ~ / C H ~ C N solutions : a) pyrrole b) N methyl pyrro- le c) thiophene d) 2 2' bithiophene e) 3 methyl thiophene f ) 3,4 dimethyipyrrole.

The anodic and cathodic peak currents have noi exactly the same value except for poly N methyl pyrrole. They are proportionnal to the tnickness and to the scan rate up to 0.2 V/S. But wnen the scan rate increases the anodic wave is shifted towara higher potentials.

Film thickness has an influence : the redox waves become broader and the peak to peak separation increases when the thickness increases.

The quantities of electricity consumed in the oxidation and in the reduction are almost equal. They are given by the areas under the 2 peaks and they vary linearly with the thickness of the film showing that the electroactive sites are uniformly distributed in the film. Tne value of the charges represent only a fraction of the charge consumed for the synthesis : 20 % for poly (2 2' bithiophene) and 6 % for polypyrrole. The charge was independant of the scan rate.

C3-540 JOURNAL DE PHYSIQUE

S p e c t r o e l e c t r o c h e m i c a l s t u d i e s

The a b s o r p t i o n s p e c t r a f o r a l l t h e f i l m s have been measured i n t h e oxidized and i n t h e reduced s t a t e ( f i g u r e 2 ) . The two p y r r o l e f i l m s have s i m i l a r s p e c t r a with a s t r o n g a b s o r p t i o n band i n t h e iJ.V.-blue range f o r t h e reduced s t a t e . The oxidized s t a t e s a r e s l i g h t l y d i f f e r e n t . Poly ( 3 methylthiophene) and poly ( 2 2 ' b i t n i o p h e n e ) a r e c h a r a c t e r i z e d by a band peak a t 470 nm i n t h e reduced s t a t e and a very broad band i n t h e r e d range i n t h e oxidized s t a t e . Poly thiophene i s very s i m i l a r .

Fig. 2 - Absorption s p e c t r a of p o l y h e t e r o c y c l e s

a ) p o l 5 - ( w methyl ~ y r a o l e ) (2. 0.3 ~ m ) b ) p o l y p y r r o l e ^(2. 0 . 1 vm) c ) poly ( 2 2 ' b i t h i o ~ h e n e ) ( s 0.1 ~ m ) d ) poly ( 3 methylthiophene)

-

Spectrum of poly ( 3 , 4 dimethylthiophene) does n o t change very much under o x i d a t i o n o r r e d u c t i o n . I n t a b l e 2 a r e l i s t e d t h e observed c o l o r changes f o r t h e monomers which polymerize i n uniform f i l m s .

Figure 3 shows t h e t y p i c a l c u r r e n t response when a s t e p of p o t e n t i a l i s applied t o t h e e l e c t r o d e . The v o l t a g e p u l s e depends on t h e polymer and was deduced from t h e e l e c t r o c h e m i c a l s t u d y : - 0.7 -0 V f o r p o l y ( p y r r o l e ) , 0-0.8 V f o r poly(N me- t h y l p y r r o l e ) and 0-1.3 V f o r p o l y ( 2 , 2 ' b i t h i o p h e n e ) . The o x i d a t i o n was found t o be slower t h a n t n e r e d u c t i o n process. The response times d e f i n e d a s t n e times r e - q u i r e d f o r c u r r e n t t o f a l l t o 1 U % of i t s maximum, depend on t h e n a t u r e of t n e f i l m and on i t s t h i c k n e s s ( s e e f i g u r e 4 ) .

Table 2 E l e c t r o o p t i c a l d a t a

F i g . 3 - Chronoamperometry t y p i c a l curves f o r p o l y h e t e r o c y c l e f i l m s s u b j e c t e d t o square wave p o t e n t i a l p u l s e .

Color change

Polymer Oxiciized Reduced

s t a t e s t a t e

f8)

H n

Ky,

Q ) n blue grey r e d

rn

71) measured a t 500 nm i n t h e oxidized

-

-

0.05 0.1 0.15

t h i c k n e s s (pm) (2) measured a t 470 nm i n t h e reduced s t a t e

Absorption c o e f f i c i e n t

(cm-l)

4. lo4 ('1

1.104

8.104 ( 2 )

-

s t a t e

r e d u c t i o n

L

oxidation

1

Fig. 4 - Zesponse time-thickness depen- dance f o r poly?yrrole ( a ) and poly (2. 2 ' b i t h i o p h e n e ) ( b ) f i l m s .

Life- time

2.104

103

103

-

Electroc'nromic e f f i c i e n c y A O U / ( ~ C / C ~ ~ )

0.1

0.05

0.05

-

time ( a .u)

Response time

(ms )

100

100

100

-

JOURNAL DE PHYSIQUE

Electrochromic properties

The t h i c k n e s s e s of t h e f i l m s which g i v e a s u f f i c i e n t c o n t r a s t on platinum f o i l s were found t o be about 0 . 1 vm. I n t h e s e c o n d i t i o n s t h e o p t i c a l d e n s i t y change i s between 0.1 and 0 . 3 and a few mC/cm2 have t o be i n j e c t e d in orcier t o o b t a i n t h i s v a l u e . The r a t i o of t h e o p t i c a l d e n s i t y change t o t h e consumed charge i s an i n d i - c a t i o n of t h e s e n s i t i v i t y of t h e m a t e r i a l and i s c a l l e d t h e e l e c t r o c h r o m i c e f f i - c i e n c y . The v a l u e s of t h i s parameter a r e given i n t a b l e 2 f o r t h e most i n t e r e s t i n g polymers. There i s a f a c t o r 2 between t h e e f f i c i e n c i e s of p o l y p y r r o l e s and p o l y ( 2 2 ' b i t h i o p h e n e ) b u t t h e a b s o r p t i o n c o e f f i c i e n t of t h i s l a s t polymer i s higner.

Two o t h e r parameters c h a r a c t e r i z e e l e c t r o c h r o m i c d i s p l a y s : response time and l i f e time. Response time was lower t h a n 150 ms on platinum e l e c t r o d e s ' w h i c h were used i n t h e s e experiments. L i f e time was d e f i n e d a s t h e number of o x i d a t i o n - r e d u c t i o n cy- c l e s w i t h 0.5 seconde p u l s e d u r a t i o n which caused a 50 % d e c r e a s e i n t h e c u r r e n t peak h e i g h t . The v a l u e s a r e l i s t e d i n t a b l e 2.

Electrochromic p a r a m e t e r s of p o l y h e t e r o c y c l e s may be compared t o t h o s e of o t h e r e l e c t r o c h r o m i c m a t e r i a l s such a s t u n g s t e n e o x i d e , i r r i d i u m o x i d e , l u t e t i u m d i p h t a - l o c y a n i n e ( 8 ) , e l e c t r o a c t i v e polymers o b t a i n e d by a t t a c h i n g an e l e c t r o c ' n r o m ~ c mole- c u l e ( t e t r a t h i a f u l v a l e n e ) t o a l l n e a r polymer c h a i n ( 9 ) o r p o l y e l e c t r o l y t e comple- xes ( 1 0 ) ( 1 1 ) . For t h e s e polymeric m a t e r i a l s t h e e l e c t r o c n r o m i c e f f i c i e n c y i s i n t h e range 0.05 t o 0.160 (mC/cm2)-1, t h e r e s p o n s e time i s a few 100 ms and t h e l i f e time i s 104 c y c l e s .

CONCLUSION

As p o l y p y r r o l e s , e l e c t r o c h e m i c a l l y polymerized t n i o p n e n e s a r e e l e c t r o a c t i v e poly- mers and undergo c o l o r changes when t h e y a r e swi.tchea between t h e o x i d i z e d s t a t e and t h e reduced s t a t e . The b l u e t o r e a c o l o r change which may be observed i s more a t t r a c t i v e t h a n t h e brown t o yellow c o l o r change o b t a i n e d w i t h p o l y p y r r o l e . Zlec- t r o o p t i . c a 1 measurements on t h e s e two kinds of polymers d e p o s i t e d on u n p o l i s h e d platinum f o i l s showed t h a t , even n o t optimized t h i n f i l m s e x h i b i t good switcrling speeds ( < 200 ms) compared t o o t h e r e l e c t r o c h r o m i c m a t e r ' a l s . L i f e time i s lower ( 1 0 3 c y c l e s ) f o r polythiopnene t h a n f o r p o l y p y r r o l e ( 2 . 1 0 c y c l e s ) . Recent s t u d i e s d-

show t h a t t h e s e two p a r a m e t e r s may be improved a l l o w i n g t h e s e polymers t o be good c a n d i d a t e s f o r el.ectrochromic d i s p l a y s .

ACKNOWLEDGMENT

This work was s u p p o r t e d i n p a r t by t h e D e l e g a t i o n Generale B l a Recherche S c i e n t i - f i q u e e t Technique.

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