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Conducting polymers as electrochromic materials
SvntheticMctals. 2S (1989) (.'501 C50(~
CONDUCTING
POLYMERS AS E L E C T R O C H R O M I C
CSO1
MATERIALS.
M . M A S T R A G O S T I N O , A . M . M A R I N A N G E L I , A.CORRADINI, S . G I A C O B B E D i p a r t i m e n t o di c h i m i c a " G . C i a m i c i a n " , U n i v e r s i t a ' di B o l o g n a
Italy
ABSTRACT The data of a c o m p a r a t i v e study on e l e c t r o c h r o m i c p h e n o m e n a of p o l y - 3 m e t h y l t h i o p h e n e , p o l y - 2 , 2 ' b i t h i o p h e n e and p o l y - d i t h i e n o ( 3 , 2 b;2' , 3 ' - d ) t h i o p h e n e in propylene carbonate with IM LiCIO 4 are r e p o r t e d . T h e best r e s u l t s were found with p o l y m e t h y l t h i o p h e n e . T h i s electrosynthesized conducting polymer is a very promising electrochromic material with high e l e c t r o c h r o m i c efficiency, fast s w i t c h i n g response, long l i f e - t i m e and good o p t i c a l memory.
INTRODUCTION The h i g h - c o n t r a s t electrochromic phenomenon associated with electrochemical doping of conducting polymers, and the r e v e r s i b i l i t y of the process, make these materials attractive candidates for electrochromic display devices. Although s p e c t r o e l e c t r o c h e m i c a l studies have been of great i m p o r t a n c e in u n d e r s t a n d i n g the nature of c h a r g e - s t o r a g e states in the chargetransfer doping reactions, few reports to date [1,2] have been d e v o t e d to a d e t a i l e d study of the e l e c t r o c h r o m i c phenomenon. This paper d i s c u s s e s the use of t h i o p h e n e d e r i v a t i v e s p o l y m e r s as e l e c t r o c h r o m i c m a t e r i a l s by examining spectra at different d o p i n g levels, e l e c t r o c h r o m i c efficiency, s w i t c h i n g time, l i f e t i m e and o p t i c a l memory. The p e r f o r m a n c e data of p o l y - 3 m e t h y l t h i o p h e n e ( p M e T ) , p o ! y - 2 , 2 ' b i t h i o p h e n e (pBT) and poly-dithieno(3,2-b:2,3-d) t h i o p h e n e (pDTT) are reported.
EXPERIMENTAL
All the electrochromic experiments were p e r f o r m e d in d e g a s s e d electrolyte, propylene c a r b o n a t e (PC) with IM LiCIO 4 ,using a rectangular pyrex cell. The p o l y m e r samples were grown on indiumtin oxide (ITO) c o n d u c t i n g glass, 2 cm 2 s u r f a c e . A Pt wire as c a t h o d e and Ag wire as q u a s i - r e f e r e n c e (+50 mV vs. SCE under our conditions) were u s e d . A l l the p o t e n t i a l data are r e f e r r e d to Ag. The e l e c t r i c a l and the o p t i c a l r e s p o n s e s were m e a s u r e d by a p p l y i n g 0379-6779/89/$3.50
© Elsevier Sequoia/Printed in The Netherlands
C502
square wave bias voltage. The electrochemical apparatus was a galvanostat-potentiostat driven by a programmable function g e n e r a t o r . The optical apparatus was a q u a r t z h a l o g e n l a m p and a monochromator as s o u r c e and a photomultiplier as d e t e c t o r . The curves were r e c o r d e d by a double-channel oscilloscope interfaced with a microcomputer. The optical response was measured by d e t e c t i n g the trasmitted light i n t e n s i t y at 530, 480 and 490 n m for pMeT, p B T and p D T T , r e s p e c t i v e l y . The electrical response data were analyzed in t e r m s of the exchanged charge a n d the o p t i c a l data in t e r m s of the p e r c e n t a g e of trasmitted light intensity (L%) vs. time. T h e 0 and I00 v a l u e s of L~ are the t r a n s m i t t e d l i g h t intensities when the p o l y m e r is f u l l y u n d o p e d a n d w h e n d o p e d w i t h complete oxidation for the a p p l i e d p o t e n t i a l , r e s p e c t i v e l y . Both values were r e a d j u s t e d for e a c h a n a l y z e d step. T h e r e a d i n g s w e r e t a k e n 20 s a f t e r s t e p p i n g p o t e n t i a l in o r d e r to a l l o w e q u i l i b r i u m to be r e a c h e d . T h e same time e l a p s e d b e f o r e r e c o r d i n g the in s i t u absorption spectra. A l l the s a m p l e s w e r e q a l v a n o s t a t i c a l l y electrosynthesized: pMeT f i l m s in n i t r o b e n z e n e w i t h 0.02 M t e t r a b u t y l a m m o n i u m hexafluoropho s p h a t e and 0.2 M methylthiophene at I = 5 m A / c m 2 and T = 5°C, as in ref. [3] , with the a b s o r p t i o n m a x i m u m of the f u l l y u n d o p e d polymer b e i n q 530 nm and the c o r r e s p o n d i n g a b s o r b a n c e 1.2 ± 0.1 when 50 m C / c m 2 is used as deposition charge; pBT films in acetonitri!e with 0 . 5 M LiCl04 and 0.01 M b i t h i o p h e n e at I = 0.5 m A / c m 2 and r o o m t e m p e r a t u r e ; p D T T f i l m s in m e t h y l e n e c h l o r i d e w i t h 0.2 M tetrabutylammonium perchlorate and 3mM dithienothiophene at I = 0.25 mA/cm 2 and T = 15°C. The a b s o r b a n c e (A) of t h e s e polymers increases linearly with the a m o u n t of charqe used for e l e c t r o p o l y m e r i z a t i o n ( 20 - 60 m C / c m 2 tested range). Ditienothiophene was s y n t h e s i z e d as in ref. [4]. A l l the other chemical compounds were reagent grade products, further purified b e f o r e use. RESULTS
AND
DISCUSSION
Figure 1 s h o w s a s e r i e s of s p e c t r a of pMeT, e l e c t r o s y n t h e s i z e d w i t h 56 mC/cm 2 t a k e n in s i t u at various potentials. The charge amounts involved in the d o p i n g p r o c e s s at d i f f e r e n t v o l t a g e s are s h o w n in the i n s e t of the f i g u r e . The e l e c t r o c h r o m i c efficiency ( d e f i n e d as the r a t i o of the absorbance change to the c o n s u m e d c h a r g e ) of p M e T at 530 n m w h e n the p o t e n t i a l is s w i t c h e d f r o m 0.55 to 1.00 V is ca. 0.24 (mC/cm 2 )-I. (The e l e c t r o c h r o m i c efficiency data are s i g n i f i c a n t o n l y w h e n the s w i t c h i n g p o t e n t i a l v a l u e s and the e v o l u t i o n of the s p e c t r a w i t h the d o p i n g l e v e l are both known ) . The switching time performances of pMeT were tested by potential step f r o m - 0 . 5 5 to + 1 . 0 0 V. A s a m p l e e l e c t r o s y n t h e s i z e d w i t h 52 m C / c m 2 ( F = 2.5 10 -7 m o l / c m 2 ) r e a c h e d c o m p l e t e o x i d a t i o n for the applied potential (4.6 m C / c m 2 ) a n d the i00 v a l u e of L~ in 6 s time. F i g u r e 2a s h o w s the o p t i c a l r e s p o n s e (L%) vs. time f o l l o w i n g the a p p l i c a t i o n of r e p e a t e d s q u a r e w a v e s of 2 s p e r i o d . The values of the c h a r g e in the o x i d a t i o n (Qa ) and in the r e d u c t i o n ( Q c ) s t e p s are in the f i g u r e ' s inset. To evidence signs of material degradation to repeated s w i t c h i n g , the in s i t u s p e c t r a of the f u l l y u n d o p e d (at - 0 . 5 5 V ) and doped (at 1.00 V) p o l y m e r and the c y c l i c v o l t a m m e t r i e s (CV) a f t e r 50th, 5000th, 1 0 0 0 0 t h , 1 5 0 0 0 t h and 2 0 0 0 0 t h double potential s t e p s w e r e r e c o r d e d . T h e r e s u l t s are s h o w n in f i g u r e 2b,c. -
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4 eV Fig.6 pDTT/ITO ( / ' = Z . 4 10 -7 m o l / c m Z ) , a) L% v s . t i m e , b) situ spectre, c) CV ( Z 0 0 m V / s ) , a t ( 1 , Z } 50th, (2,2') 5000th, (3,3') 10000th, (4,4') 15000th.y
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C506
F r o m the L% d a t a in f i g u r e 2a a n d the d a t a in f i g u r e 2b the absorbance difference , ~ A , b e t w e e n the u n d o p e d a n d d o p e d s a m p l e at 530 nm, w h i c h is d u e to p o t e n t i a l s t e p p i n g , c a n be c a l c u l a t e d o v e r time. At 1 s the I A v a l u e s are 0.90 a n d 0 . 7 5 for the 5 0 t h and 20000th step, respectively. ~A v a l u e of a b o u t 0.7 c a n be calculated at 500 ms for e v e r y o x i d a t i o n step. The switching time and the l i f e t i m e of p M e T are s a t i s f a c t o r y . After 20000 s t e p s the a b s o r b a n c e of the f u l l y undoped polymer still represents 79% a n d the c h a r g e exchanged during the cyclic voltammetry 7 3 % of t h e i r i n i t i a l v a l u e , as s h o w n in f i g u r e 2b,c, respectively. Comparable experiments were carried out with pBT and pDTT. The pBT results are shown in figures 3, 4. The electrochromic e f f i c i e n c y of p B T is 0 . I I ( m C / c m 2 )-I at 480nm when the p o t e n t i a l is switched from - 0 . 5 5 to 1.20 V: the s w i t c h i n g t i m e was tested by stepping between these potentials using a sample electrosynthesized with 35 m C / c m 2 ( F = 1.5 i0 ~ m o l b i t h i o p h e n e /cm 2 ) pBT reached complete oxidation for applied potential and the I00 v a l u e of L% in 20 s time. T h e p e r i o d of the s q u a r e w a v e was adjusted at 8 s. T h e s w i t c h i n g t i m e a n d the l i f e t i m e of p B T are not satisfactory, as s h o w n in f i g u r e 4. The pDTT r e s u l t s a r e s h o w n in f i g u r e s 5,6 . T h e e l e c t r o c h r o m i c efficiency is 0.!0 (mC/cm 2 )-I at 490 n m w h e n the p o t e n t i a l is switched from - 0 . 5 5 to 1 . 2 0 V. The switching time was tested between these potential values using a sample electrosynthesized w i t h 35 mC/cm 2 ( F = 1.4 10 *7 m o l / c m 2 ) . p D T T r e a c h e d c o m p l e t e o x i d a t i o n for applied potential a n d t h e i00 v a l u e of L% in 10s The period of the square wave was adjusted at 2s . T h e d a t a of p D T T are satisfactory, e v e n if its p e r f o r m a n c e is w o r s e t h a n t h a t of pMeT. As far as the o p t i c a l m e m o r y is c o n c e r n e d , the s t a b l e f o r m in open circuit c o n d i t i o n s of all t h e s e p o l y m e r s is the n e u t r a l . T h e poor charge-retention of the o x i d i z e d f o r m of t h e s e m a t e r i a l s is known [5]. H o w e v e r , for u s e as electrochromic m a t e r i a l s , it d o e s not s e e m to be a problem. Over 1 h the absorbance i n c r e a s e of samples was 0 . 0 1 at 530nm, 0 . 0 6 at 4 8 0 n m a n d 0.07 at 4 9 0 n m for pMeT, p B T a n d p D T T , r e s p e c t i v e ! y . The best results,to conclude, were obtained with pMeT. This electrosynthesized c o n d u c t i n g p o l y m e r is a v e r y p r o m i s i n g m a t e r i a l with high electrochromic efficiency, fast s w i t c h i n g r e s p o n s e , long life-time and good optical memory.
ACKNOWLEDGEMENT The research was funded by a grant from Finalizzato Energetica 2 (n.86.00876.59).
CNR,
Progetto
REFERENCES ! H. Y a s h i m a et al., J . E l e c t r o c h e m . Soc., 134 (1987) 46 2 A. C o r r a d i n i et al. Solid State Ionics, in,press. 3 J. R o n c a l i et al., J . P h y s . C h e m . , 91 (1987) 6 7 0 6 4 F. de J o n g et al., J . O r ~ . C h e m . , 36 (1971) 1 6 4 5 5 M. M a s t r a g o s t i n o et al., E l e c t r o c h i m i c a A c t a , 32 (1987)
1589
CONDUCTING
POLYMERS AS E L E C T R O C H R O M I C
CSO1
MATERIALS.
M . M A S T R A G O S T I N O , A . M . M A R I N A N G E L I , A.CORRADINI, S . G I A C O B B E D i p a r t i m e n t o di c h i m i c a " G . C i a m i c i a n " , U n i v e r s i t a ' di B o l o g n a
Italy
ABSTRACT The data of a c o m p a r a t i v e study on e l e c t r o c h r o m i c p h e n o m e n a of p o l y - 3 m e t h y l t h i o p h e n e , p o l y - 2 , 2 ' b i t h i o p h e n e and p o l y - d i t h i e n o ( 3 , 2 b;2' , 3 ' - d ) t h i o p h e n e in propylene carbonate with IM LiCIO 4 are r e p o r t e d . T h e best r e s u l t s were found with p o l y m e t h y l t h i o p h e n e . T h i s electrosynthesized conducting polymer is a very promising electrochromic material with high e l e c t r o c h r o m i c efficiency, fast s w i t c h i n g response, long l i f e - t i m e and good o p t i c a l memory.
INTRODUCTION The h i g h - c o n t r a s t electrochromic phenomenon associated with electrochemical doping of conducting polymers, and the r e v e r s i b i l i t y of the process, make these materials attractive candidates for electrochromic display devices. Although s p e c t r o e l e c t r o c h e m i c a l studies have been of great i m p o r t a n c e in u n d e r s t a n d i n g the nature of c h a r g e - s t o r a g e states in the chargetransfer doping reactions, few reports to date [1,2] have been d e v o t e d to a d e t a i l e d study of the e l e c t r o c h r o m i c phenomenon. This paper d i s c u s s e s the use of t h i o p h e n e d e r i v a t i v e s p o l y m e r s as e l e c t r o c h r o m i c m a t e r i a l s by examining spectra at different d o p i n g levels, e l e c t r o c h r o m i c efficiency, s w i t c h i n g time, l i f e t i m e and o p t i c a l memory. The p e r f o r m a n c e data of p o l y - 3 m e t h y l t h i o p h e n e ( p M e T ) , p o ! y - 2 , 2 ' b i t h i o p h e n e (pBT) and poly-dithieno(3,2-b:2,3-d) t h i o p h e n e (pDTT) are reported.
EXPERIMENTAL
All the electrochromic experiments were p e r f o r m e d in d e g a s s e d electrolyte, propylene c a r b o n a t e (PC) with IM LiCIO 4 ,using a rectangular pyrex cell. The p o l y m e r samples were grown on indiumtin oxide (ITO) c o n d u c t i n g glass, 2 cm 2 s u r f a c e . A Pt wire as c a t h o d e and Ag wire as q u a s i - r e f e r e n c e (+50 mV vs. SCE under our conditions) were u s e d . A l l the p o t e n t i a l data are r e f e r r e d to Ag. The e l e c t r i c a l and the o p t i c a l r e s p o n s e s were m e a s u r e d by a p p l y i n g 0379-6779/89/$3.50
© Elsevier Sequoia/Printed in The Netherlands
C502
square wave bias voltage. The electrochemical apparatus was a galvanostat-potentiostat driven by a programmable function g e n e r a t o r . The optical apparatus was a q u a r t z h a l o g e n l a m p and a monochromator as s o u r c e and a photomultiplier as d e t e c t o r . The curves were r e c o r d e d by a double-channel oscilloscope interfaced with a microcomputer. The optical response was measured by d e t e c t i n g the trasmitted light i n t e n s i t y at 530, 480 and 490 n m for pMeT, p B T and p D T T , r e s p e c t i v e l y . The electrical response data were analyzed in t e r m s of the exchanged charge a n d the o p t i c a l data in t e r m s of the p e r c e n t a g e of trasmitted light intensity (L%) vs. time. T h e 0 and I00 v a l u e s of L~ are the t r a n s m i t t e d l i g h t intensities when the p o l y m e r is f u l l y u n d o p e d a n d w h e n d o p e d w i t h complete oxidation for the a p p l i e d p o t e n t i a l , r e s p e c t i v e l y . Both values were r e a d j u s t e d for e a c h a n a l y z e d step. T h e r e a d i n g s w e r e t a k e n 20 s a f t e r s t e p p i n g p o t e n t i a l in o r d e r to a l l o w e q u i l i b r i u m to be r e a c h e d . T h e same time e l a p s e d b e f o r e r e c o r d i n g the in s i t u absorption spectra. A l l the s a m p l e s w e r e q a l v a n o s t a t i c a l l y electrosynthesized: pMeT f i l m s in n i t r o b e n z e n e w i t h 0.02 M t e t r a b u t y l a m m o n i u m hexafluoropho s p h a t e and 0.2 M methylthiophene at I = 5 m A / c m 2 and T = 5°C, as in ref. [3] , with the a b s o r p t i o n m a x i m u m of the f u l l y u n d o p e d polymer b e i n q 530 nm and the c o r r e s p o n d i n g a b s o r b a n c e 1.2 ± 0.1 when 50 m C / c m 2 is used as deposition charge; pBT films in acetonitri!e with 0 . 5 M LiCl04 and 0.01 M b i t h i o p h e n e at I = 0.5 m A / c m 2 and r o o m t e m p e r a t u r e ; p D T T f i l m s in m e t h y l e n e c h l o r i d e w i t h 0.2 M tetrabutylammonium perchlorate and 3mM dithienothiophene at I = 0.25 mA/cm 2 and T = 15°C. The a b s o r b a n c e (A) of t h e s e polymers increases linearly with the a m o u n t of charqe used for e l e c t r o p o l y m e r i z a t i o n ( 20 - 60 m C / c m 2 tested range). Ditienothiophene was s y n t h e s i z e d as in ref. [4]. A l l the other chemical compounds were reagent grade products, further purified b e f o r e use. RESULTS
AND
DISCUSSION
Figure 1 s h o w s a s e r i e s of s p e c t r a of pMeT, e l e c t r o s y n t h e s i z e d w i t h 56 mC/cm 2 t a k e n in s i t u at various potentials. The charge amounts involved in the d o p i n g p r o c e s s at d i f f e r e n t v o l t a g e s are s h o w n in the i n s e t of the f i g u r e . The e l e c t r o c h r o m i c efficiency ( d e f i n e d as the r a t i o of the absorbance change to the c o n s u m e d c h a r g e ) of p M e T at 530 n m w h e n the p o t e n t i a l is s w i t c h e d f r o m 0.55 to 1.00 V is ca. 0.24 (mC/cm 2 )-I. (The e l e c t r o c h r o m i c efficiency data are s i g n i f i c a n t o n l y w h e n the s w i t c h i n g p o t e n t i a l v a l u e s and the e v o l u t i o n of the s p e c t r a w i t h the d o p i n g l e v e l are both known ) . The switching time performances of pMeT were tested by potential step f r o m - 0 . 5 5 to + 1 . 0 0 V. A s a m p l e e l e c t r o s y n t h e s i z e d w i t h 52 m C / c m 2 ( F = 2.5 10 -7 m o l / c m 2 ) r e a c h e d c o m p l e t e o x i d a t i o n for the applied potential (4.6 m C / c m 2 ) a n d the i00 v a l u e of L~ in 6 s time. F i g u r e 2a s h o w s the o p t i c a l r e s p o n s e (L%) vs. time f o l l o w i n g the a p p l i c a t i o n of r e p e a t e d s q u a r e w a v e s of 2 s p e r i o d . The values of the c h a r g e in the o x i d a t i o n (Qa ) and in the r e d u c t i o n ( Q c ) s t e p s are in the f i g u r e ' s inset. To evidence signs of material degradation to repeated s w i t c h i n g , the in s i t u s p e c t r a of the f u l l y u n d o p e d (at - 0 . 5 5 V ) and doped (at 1.00 V) p o l y m e r and the c y c l i c v o l t a m m e t r i e s (CV) a f t e r 50th, 5000th, 1 0 0 0 0 t h , 1 5 0 0 0 t h and 2 0 0 0 0 t h double potential s t e p s w e r e r e c o r d e d . T h e r e s u l t s are s h o w n in f i g u r e 2b,c. -
C503 1
1.5
A
N
~ \
o~,
/~\
i ~
1.0
2 3
/
\ \
/
\\
I/
4 5 6
\\
7
/- ~\\
Q
530nm -0.55 1.45 o.3o 1.07 0.40 0.83 o.5o 0.52 0.60 0.39 0.70 0.31 o.8o 0.28 0:90 0.27 0.27
i
/ \ / \
A
V
=
mClcm 2 0.34 0.83 1.69 2.50 3.00 3.49 4.09 4.90
0.5
1-
1
eV F i d . l In different
situ spectra of doping levels.
pMeT/ITO
( i'= 2.7
10 -7 m o l / c m z)
at
100 o ~ 5 4
L%
• ///
N
Qa
Qc
i 2
2.oo 1.88
2.oo 1.s9
3
2.00
2.00
\\
/'//
/~,\\
"
I
Qa
50
i
mc/cm 2 4.5
o
I
tls
b
A
/'~2 //~\3
N
. 0.5
, \
f/
s3o,,,, 1' o.25 2' 0.25 3' 0.24
\
4' o. 2~
s, o.22 ~ .
i
.
~
/111 ~
/ .
.
.
.
2
;3 4
f
Q¢
1.31
C/c,,2
21.21 3 1.17 4 1.o8
I° \\\
\\5 1.o4
I I "-..4 ///t~
/I
I
\\
\\~A'
\",-A
3.6 3.4
\~ \\\~ \"~
~
c
A
1
It)-~\\ ~ II1 \\\ #, %
\
3.3/f
s 3o~,,
II ~ \\4
~
1.0
1
2
4.0 3.6
~!
//~
,, ~ eV
Fig.2 pMeT/ITO (P: 2 . 5 10 -7 m o l / c m 2 ) . a) L% v s . t i m e , b) in situ spectra, c) C V ( 1 0 0 m V / s ) , at ( l , l ' ) 5 0 t h , (2,2')5000th, (3,3') 1 0 0 0 0 t h , (4,4')!5000th, (5,5')20000th. Only one spectrum of d o p e d p o l y m e r has been reported.
C504 N
A
7---~ 6 ~
~ /
1.o
0.5
3
A
g
480n~ me/ca2 1 -0.55 1.14 2 0.70 0.94 0.55
I
3 4 5 6
\
/ \\\
V
//
\\
0.80 0.90
0.78 0.55
1.40 2.90
1.00
0.,2
°.30
1.1o
o.38
6.35
~ "
2~ I ~ i
1
2
,
,
3
4
eV Fig.3 In situ spectra of pBT/ITO ( I ~= 1.5 10 -7 mol 2,2'bithiophene/cm z) at different doping levels. 100 L%
Q N
ga
gc
1
5.44
5.37
=C/c=:'
2 2.37 2.32
50 i
u
.Qa mC/cm2
~o
3.84
~
2. O0
~
I
/ t2
///
//l (N
051
•
y_/
~.7s
I
1.88
1' 2'
I
1
2
3
4
eV Fig.4 pBT/ITO ( F = 1.5 10 -7 mol/cmz), a) L% vs. time, b) in situ spectra, e) CV ( 100mV/s ), at (I,I') 50th, (2,2') 3000th.
C
C505 1 N
2
1 2 3 4 5 6 7 8
10
05
F i g . 5 In different
situ spectra of doping levels.
eV ( I'= 1.4
DDTT/ITO
0
v
A
-0.55 0.60 0.70 0.80 0.90 1.00 i. I0 1.20
490nm 1.26 1.07 0.95 0.83 0.74 0.68 0.63 0.59
A
mC/cm 2 1.4 2.2 2.6 3.6 4.5 5.6 6.6
10 -7 m o l / c m z)
at
100 (3
N
Qc
Qa
L%
mClcm2 /%1 ,/ \
1 2 3
3.04 2.85 2.27
3.00 2.90 2,41
o
Qa mC/cm2
=
s.87 5.39
~
I]2
/// ////
3~
C
/ o<
///
\
\ \ / ,c/~,~ %, \ ~_./_4 5 73 \\ <-4/3 ~'18
t/s b i,~.
A
_
~
i ~/~2 I~\\3
N
~I~-.\\\4
~
A 490nm 1.36 1.29
4
1.12
1
3'
Qc
,:,,
°
"-"l 0,5
3.45
i' 0 55
3' 0.52
l/
,
{--~-
3
4 eV Fig.6 pDTT/ITO ( / ' = Z . 4 10 -7 m o l / c m Z ) , a) L% v s . t i m e , b) situ spectre, c) CV ( Z 0 0 m V / s ) , a t ( 1 , Z } 50th, (2,2') 5000th, (3,3') 10000th, (4,4') 15000th.y
zn
C506
F r o m the L% d a t a in f i g u r e 2a a n d the d a t a in f i g u r e 2b the absorbance difference , ~ A , b e t w e e n the u n d o p e d a n d d o p e d s a m p l e at 530 nm, w h i c h is d u e to p o t e n t i a l s t e p p i n g , c a n be c a l c u l a t e d o v e r time. At 1 s the I A v a l u e s are 0.90 a n d 0 . 7 5 for the 5 0 t h and 20000th step, respectively. ~A v a l u e of a b o u t 0.7 c a n be calculated at 500 ms for e v e r y o x i d a t i o n step. The switching time and the l i f e t i m e of p M e T are s a t i s f a c t o r y . After 20000 s t e p s the a b s o r b a n c e of the f u l l y undoped polymer still represents 79% a n d the c h a r g e exchanged during the cyclic voltammetry 7 3 % of t h e i r i n i t i a l v a l u e , as s h o w n in f i g u r e 2b,c, respectively. Comparable experiments were carried out with pBT and pDTT. The pBT results are shown in figures 3, 4. The electrochromic e f f i c i e n c y of p B T is 0 . I I ( m C / c m 2 )-I at 480nm when the p o t e n t i a l is switched from - 0 . 5 5 to 1.20 V: the s w i t c h i n g t i m e was tested by stepping between these potentials using a sample electrosynthesized with 35 m C / c m 2 ( F = 1.5 i0 ~ m o l b i t h i o p h e n e /cm 2 ) pBT reached complete oxidation for applied potential and the I00 v a l u e of L% in 20 s time. T h e p e r i o d of the s q u a r e w a v e was adjusted at 8 s. T h e s w i t c h i n g t i m e a n d the l i f e t i m e of p B T are not satisfactory, as s h o w n in f i g u r e 4. The pDTT r e s u l t s a r e s h o w n in f i g u r e s 5,6 . T h e e l e c t r o c h r o m i c efficiency is 0.!0 (mC/cm 2 )-I at 490 n m w h e n the p o t e n t i a l is switched from - 0 . 5 5 to 1 . 2 0 V. The switching time was tested between these potential values using a sample electrosynthesized w i t h 35 mC/cm 2 ( F = 1.4 10 *7 m o l / c m 2 ) . p D T T r e a c h e d c o m p l e t e o x i d a t i o n for applied potential a n d t h e i00 v a l u e of L% in 10s The period of the square wave was adjusted at 2s . T h e d a t a of p D T T are satisfactory, e v e n if its p e r f o r m a n c e is w o r s e t h a n t h a t of pMeT. As far as the o p t i c a l m e m o r y is c o n c e r n e d , the s t a b l e f o r m in open circuit c o n d i t i o n s of all t h e s e p o l y m e r s is the n e u t r a l . T h e poor charge-retention of the o x i d i z e d f o r m of t h e s e m a t e r i a l s is known [5]. H o w e v e r , for u s e as electrochromic m a t e r i a l s , it d o e s not s e e m to be a problem. Over 1 h the absorbance i n c r e a s e of samples was 0 . 0 1 at 530nm, 0 . 0 6 at 4 8 0 n m a n d 0.07 at 4 9 0 n m for pMeT, p B T a n d p D T T , r e s p e c t i v e ! y . The best results,to conclude, were obtained with pMeT. This electrosynthesized c o n d u c t i n g p o l y m e r is a v e r y p r o m i s i n g m a t e r i a l with high electrochromic efficiency, fast s w i t c h i n g r e s p o n s e , long life-time and good optical memory.
ACKNOWLEDGEMENT The research was funded by a grant from Finalizzato Energetica 2 (n.86.00876.59).
CNR,
Progetto
REFERENCES ! H. Y a s h i m a et al., J . E l e c t r o c h e m . Soc., 134 (1987) 46 2 A. C o r r a d i n i et al. Solid State Ionics, in,press. 3 J. R o n c a l i et al., J . P h y s . C h e m . , 91 (1987) 6 7 0 6 4 F. de J o n g et al., J . O r ~ . C h e m . , 36 (1971) 1 6 4 5 5 M. M a s t r a g o s t i n o et al., E l e c t r o c h i m i c a A c t a , 32 (1987)
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