bk-1985-0278.ch016

16 Recent Advances in Organic Materials

Downloaded by UNIV OF SYDNEY on May 3, 2015 | http://pubs.acs.org Publication Date: May 14, 1985 | doi: 10.1021/bk-1985-0278.ch016

FRED WUDL Department of Physics, Institute for Polymers and Organic Solids, University of California, Santa Barbara, C A 93106

Organic conductors can naturally be subdivided into two types: single crystals and polymers. The former have experienced a rapid development from semiconductors (1950's - I960's) (1) to metals (1970's) (2) to superconductors (1980's) (3). Highly conducting organic polymers (except "pyropolymers" and composites) have had a shorter history. Whereas the mechanisms responsible for metallic conductivity as well as the loss of conductivity at lower temperatures in single crystals are now understood (4), the same cannot be said for poly meric conductors because there is s t i l l some controversy centered around the nature of the transport process and change carriers (5) For that reason and due to spatial requirements, this article will deal only with polymeric conductors and will emphasize poly(heterocycles), particularly poly(thiophene). The only certainty, insofar as requirements to observe high conductivities in polymers is concerned, is that the polymers must be partially oxidized or reduced ("p-doped" or "n-doped", respect ively). It is therefore not an unreasonable assumption that any polymer with an extended π system will be a good candidate for the achievement of activated (semiconductor) or non-activated (metal) conductivity upon doping (6). Clearly, the simplest π-conjugated polymer is poly(acetylene) (Figure 1) and has therefore been the most extensively studied. The reasons for popularity of a polymer in this highly specialized area of research are ease of preparation and "form" (7) (film, fiber, or powder). The l a t t e r can be o v e r whelming; f o r example, p o l y a c e t y l e n e has e x i s t e d f o r decades as b l a c k , amorphous powder and was, i n f a c t , doped i n t h e I 9 6 0 ' s ( 8 ) but a t t r a c t e d l i t t l e a t t e n t i o n because n o t much c o u l d be done w i t h an i n t r a c t a b l e ( n o n - f i b r i c a b l e ) " b r i c k d u s t " . When Shirakawa d i s c o v e r e d t h e p r o c e s s t o produce t h e same polymer i n f i l m f o r m , t h e a t t e n t i o n i t a t t r a c t e d was phenomenal. 0097-6156/85/0278-0257$06.00/0

© 1985 American Chemical Society

In Organic Phototransformations in Nonhomogeneous Media; Fox, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


ORGANIC PHOTOTRANSFORMATIONS IN NONHOMOGENEOUS MEDIA

*=(·),(-),(+)

X = NH , NMe

,S,0

F i g u r e 1. Examples o f c o n d u c t i n g polymers and s p e c i e s r e s p o n s i b l e f o r charge s t o r a g e . T o p , p o l y ( a c e t y l e n e ) and s o l H o n : 2 n d , p o l y - p - p h e n y l e n e and b i p o l a r o n : 3 r d , poly-rj»-phenylene s u l f i d e ; 4 t h , p o l y ( h e t e r o c y c l e s ) and b i p o l a r o n . Bipolarons in poly(furan) have n o t y e t been e s t a b l i s h e d .


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S i n c e p o l y ( p y r r o l e ) and p o l y ( t h i o p h e n e ) (PT) can be p r e p a r e d i n f i l m form by a s i m p l e p r o c e d u r e , t h e y a l s o f u l f i l l the above r e quirements f o r p o p u l a r i t y . However, i t i s c l e a r , p a r t i c u l a r l y from s p e c t r o s c o p i c i n v e s t i g a t i o n s , t h a t t h e n a t u r e o f the charge s t o r a g e (and c a r r i e r s ) i n the p o l y ( h e t e r o c y c l e s ) i s d i f f e r e n t from p o l y (acetylene) (9).


The t o p i c s t o be c o v e r e d i n t h i s a r t i c l e a r e : (a) b r i e f review of p o l y ( h e t e r o c y c l e s ) , ( b ) d e s c r i p t i o n o f the l a t e s t r e s u l t s on poly(thiophene), and (c) description of poly(isothianaphthene), (PITN). Brief

Review o f

Poly(heterocycles)

These polymers, particularly poly(pyrrole), are most con v e n i e n t l y p r e p a r e d from the p a r e n t m o l e c u l e v i a e l e c t r o l y s i s . So f a r , f u r a n , p y r r o l e , t h i o p h e n e , and v a r i o u s m e t h y l a t e d d e r i v a t i v e s have been p o l y m e r i z e d by t h i s p r o c e d u r e ( 1 0 ) . The a n o d i c p o l y merization apparently also works for relatively electron rich a r o m a t i c compounds such as a n i l i n e and a z u l e n e ( 1 1 ) . In the c a s e o f the five-membered h e t e r o c y c l e s , p o l y m e r i z a t i o n o c c u r s p r e d o m i n a n t l y t h r o u g h t h e 2,5 p o s i t i o n s and when t h e 3 , 4 p o s i t i o n s o f p y r r o l e are b l o c k e d by methyl g r o u p s , number average m o l e c u l a r w e i g h t s on the o r d e r o f 1000 have been o b t a i n e d ( 1 2 ) . Of a l l the heterocycles, unsubstituted pyrcole y i e l d s the most highly c o n d u c t i n g polymer [= 200 S/cm (S = Ω ) ] . S i n c e the p o l y m e r i z a t i o n i s performed under o x i d a t i v e c o n d i t i o n s , the a s - f o r m e d polymer i s p-doped w i t h the c o u n t e r i o n c o r r e s p o n d i n g t o the a n i o n o f the supporting e l e c t r o l y t e . These doped f i l m s a r e c o n s i d e r a b l y more s t a b l e t o the atmosphere than t h e i r p o l y ( a c e t y l e n e ) counterparts. However, when t h e y are c o n n e c t e d t o the cathode o f a b a t t e r y , t h e y can be "dedoped" ( b r o u g h t t o a n e u t r a l s t a t e ) and c o n v e r t e d t o an u n s t a b l e form. This conversion u s u a l l y occurs with a concomitant change i n c o l o r ( f r o m b l u e - b l a c k t o brown o r o r a n g e - r e d ) , i n d i c a t i n g t h a t t h e s e polymers are a l s o e l e c t r o c h r o m i c ( 1 3 ) . The bandgap f o r p o l y ( p y r r o l e ) i s = 3eV (= 410nm), f o r p o l y ( t h i o p h e n e ) i t i s = 2eV (= 620nm) and f o r p o l y ( a c e t y l e n e ) i t i s = 1.6eV (= 780nm). From t h e s e numbers one c o u l d imagine t h a t p o l y ( a c e t y l e n e ) and PT would be w e l l s u i t e d f o r s o l a r energy c o n v e r s i o n devices. However, w h i l e the bandgap o f p o l y ( a c e t y l e n e ) i s the b e s t of the three polymers to match the solar spectrum, from a t h e o r e t i c a l p o i n t o f v i e w , i t appears as though t h i s m a t e r i a l may be doomed t o have v e r y low s o l a r energy c o n v e r s i o n e f f i c i e n c y because the p h o t o g e n e r a t e d c a r r i e r s can form i n t r i n s i c l o c a l i z e d gap s t a t e s ( s o l i t o n - a n t i s o l i t o n p a i r ) a t a c a l c u l a t e d r a t e o f = 1013 sec . In fact, the observed efficiency of a liquid junction poly( a c e t y l e n e ) s o l a r c e l l i s v e r y low ( 1 4 ) . At the m o l e c u l a r l e v e l , s t u d i e s o f t h e doping mechanism i n the poly(heterocycles) reveal that very short lived radical cations ("polarons") i m m e d i a t e l y decay (combine) t o form d i c a t i o n s ("bi-


260


p o l a r o n s " ) ; i . e . , charge s t o r a g e appears t o i n v o l v e d i c a t i o n s r a t h e r than r a d i c a l c a t i o n s ( 9 , 1 5 ) . Whether b i p o l a r o n s w i l l be as d e t r i mental t o t h e e f f i c i e n c y o f s o l a r c e l l s based on p o l y ( h e t e r o c y c l e s ) as a r e s o l i t o n s t o s o l a r energy c o n v e r s i o n d e v i c e s based on p o l y ( a c e t y l e n e ) remains t o be e s t a b l i s h e d .


Recent R e s u l t s

on P o l y ( t h i o p h e n e )

U n l i k e p y r r o l e , t h i o p h e n e can be c o u p l e d through the 2,5 p o s i t i o n s by a n o n - e l e c t r o c h e m i c a l approach i n v o l v i n g G r i g n a r d i n t e r mediates ( 1 6 ) . Careful p u r i f i c a t i o n of s t a r t i n g materials ( i n t h i s case 2 , 5 - d i i o d o t h i o p h e n e ) a l l o w e d t h e i s o l a t i o n o f an e l e c t r i c a l l y i n s u l a t i n g sample o f PT w i t h a m o l e c u l a r weight o f c a 4000 and c l e a n i r and u v - v i s s p e c t r a as w e l l as e l e m e n t a l a n a l y s i s c o r r e s p o n d i n g t o the e m p i r i c a l f o r m u l a CiogHg-jIS/.g. This composition indicates t h a t , on t h e a v e r a g e , f o r e v e r y 4b t h i o p h e n e u n i t s t h e r e i s one b u t a d i e n e (17). Electronic and i n f r a r e d spectroscopy revealed that the " c h e m i c a l l y c o u p l e d " PT was c l e a n e r ( p r a t i c a l l y no a b s o r p t i o n below t h e bandgap) than a sample o f dedoped e l e c t r o c h e m i c a l l y p o l y m e r i z e d thiophene. Since the i o n i z a t i o n p o t e n t i a l of thiophene i s r e l a t i v e l y h i g h , the e l e c t r i c fields required f o r i t s anodic polymerization are r a t h e r s t e e p (= 20V vs S C E ) . In a d d i t i o n , t h e s i m p l e s t s u p p o r t i n g e l e c t r o l y t e f o r t h i s o p e r a t i o n i s L i B F . and d e p o s i t i o n o f L i a t the cathode ( u s u a l l y P t ) i s a l s o e n e r g e t i c a l l y u n f a v o r a b l e . Recently, Druy (13) r e p o r t e d t h a t s u b s t i t u t i o n o f 2 , 2 ' - b i t h i o p h e n e f o r t h i o phene gave b e t t e r q u a l i t y f i l m s , p r o b a b l y due t o t h e lower i o n i z a t i o n p o t e n t i a l o f t h e dimer r e l a t i v e t o t h i o p h e n e . An a d d i t i o n a l improvement c o n s i s t e d i n r e p l a c i n g t h e Pt c o u n t e r e l e c t r o d e by A l (9). S p e c t r o s c o p y r e v e a l e d t h a t dedoped PT f i l m s produced w i t h t h e above improvements were indistinguishable in quality from the c h e m i c a l l y c o u p l e d PT. With t h e s e e x c e l l e n t f i l m s on hand we were a b l e t o do h i g h l y sophisticated experiments of i n s i t u d o p i n g and dedoping w h i l e p e r f o r m i n g a n o t h e r measurement such as e l e c t r o n i c s p e c t r o s c o p y ( 9 ) . The r e s u l t s o f such experiments showed t h a t c h a r g e s , i n PT, a r e s t o r e d as d i c a t i o n s ; a f i n d i n g t h a t p a r a l l e l s o b s e r v a t i o n s on p o l y (pyrrole (15). E l e c t r o n s p i n resonance experiments r e v a l e d t h a t samples o f t h e c h e m i c a l l y c o u p l e d polymer had v e r y few s p i n d e f e c t s (= 65 ppm p e r carbon). P r e l i m i n a r y r e s u l t s o f e p r experiments d u r i n g i n s i t u d o p i n g seem t o c o r r o b o r a t e t h e p r o p o s a l o f charge storage~~in t h e form o f weakly c o n f i n e d b i p o l a r o n s because d o p i n g , p a r t i c u l a r l y a t h i g h dopant c o n c e n t r a t i o n s , shows almost no paramagnetism ( s p i n l e s s c a r r i e r s ) (18). S o l i d State

Photoeffects

The t e c h n i q u e o f p h o t o i n d u c e d s p e c t r o s c o p y has been used v e r y e f f e c t i v e l y t o probe t h e l i f e t i m e and n a t u r e o f s t a t e s i n t h e gap o f



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polyacetylene (19"25_); i . e . , both the l a t t i c e d i s t o r t i o n and the a s s o c i a t e d e l e c t r o n i c s t r u c t u r e o f t h e s e p h o t o g e n e r a t e d s p e c i e s were i n v e s t i g a t e d by r e c o r d i n g the small changes i n s p e c t r o s c o p y (ir, epr) that occur during p h o t o e x c i t a t i o n ( 1 9 - 2 5 J . Frequency depen dence o f the p h o t o g e n e r a t e d s i g n a l s r e v e a l e d t h a t s o l i t o n s c o u l d be g e n e r a t e d by photons w i t h ηω < Ε (Ε i s the s e m i c o n d u c t o r energy gap). S i m i l a r l y , e l e c t r o n s p i n reSonaSce measurements d u r i n g p h o t o excitation revealed the spinless nature of the photogenerated species (25). Because the c h a r a c t e r i s t i c f e a t u r e s o f the i n f r a r e d spectrum o f the c h a r g e d p h o t o g e n e r a t e d s p e c i e s were found t o be i n a o n e - t o - o n e c o r r e s p o n d e n c e w i t h the s p e c t r a l f e a t u r e s o f an i n d e p e n d e n t l y doped sample o f t r a n s - ( C H ) , a p r i n c i p a l c o n c l u s i o n was t h a t i n both c a s e s the charge was s x o r e d i n the form o f s p i n l e s s charged s o l i t o n s . Coupling of e l e c t r o n i c e x c i t a t i o n s to nonlinear conformational changes can be an intrinsic feature of conducting polymers. G e n e r a l i z a t i o n o f t h i s i d e a and a p p l i c a t i o n t o the l a r g e r f a m i l y o f non d e g e n e r a t e g r o u n d - s t a g e , c o n j u g a t e d polymers was done f o r t h e f i r s t time w i t h p o l y ( t h i o p h e n e ) ( 2 6 ) . The p h o t o i n d u c e d a b s o r p t i o n experiments on a 0 . 3 wt.% p o l y ( t h i o p h e n e ) on KBr were c a r r i e d out by H. S c h a f f e r u s i n g an IBM Instruments IR/98 vacuum F o u r i e r - t r a n s f o r m i n t e r f e r o m e t e r m o d i f i e d t o a l l o w an e x t e r n a l beam o f an Ar laser (ηω = 2.41 eV) t o i r r a d i a t e the sample s i m u l t a n e o u s l y w i t h t h e p r o b i n g i n f r a r e d beam. The o b s e r v e d s p e c t r u m , which i s a d i f f e r e n c e spectrum ( s p e c t r u m due to laser irradiation minus dark spectrum), consists Of four r e l a t i v e l y narrow l i n e s a t 1020, 1120, 1200, and 1320 cm plus a v e r y broad band p e a k i n g a t ^3600 cm . The c l o s e c o r r e s p o n d e n c e o f the f o u r narrow peaks w i t h t h o s e - , r e p o r t e d by H o t t a (27) (1330-1310, 1200, 1120-1080, 1030-1020 cm" ), is strong evidence that the e l e c t r o n i c s t r u c t u r e o f t h e p h o t o i n d u c e d s p e c i e s i s the same as t h a t o f the doped m a t e r i a l . R e s u l t s o f s i m i l a r e x p e r i m e n t s c a r r i e d out by M o r a e s on the same KBr s u s p e n s i o n o f n e u t r a l p o l y ( t h i o p h e n e ) u s i n g e l e c t r o n s p i n resonance as a p r o b e , r e v e a l e d t h a t c o n t r a r y t o the c a s e o f (CH) , a s p i n b e a r i n g s p e c i e s was g e n e r a t e d ; s t r o n g l y i m p l y i n g the f o r m a t i o n o f radical cations (polarons). In t h i s c o n t e x t , the broad band w i t h a maximum a t 0.45 eV (3600 cm ) can be a s s i g n e d t o the l o w e s t - e n e r g y e l e c t r o n i c t r a n s i t i o n o f t h e s e p h o t o generated, d e l o c a l i z e d radical cations (polarons) (26). 2 7

1

2 6

S i n c e we had shown e a r l i e r t h a t the s p e c i e s g e n e r a t e d upon ( d a r k ) d o p i n g were d e l o c a l i z e d c a t i o n s ( p o l a r o n s ) , one can c o n c l u d e from the above experiments that in poly(thiophene) irradiation g e n e r a t e s both c a t i o n s and c a t i o n r a d i c a l s ; i n sharp c o n t r a s t t o what i s o b s e r v e d i n ( Ε Η ) . 1 8

χ

Finally, a n o t h e r s e t o f p r e l i m i n a r y experimetns sowed t h a t S c h o t t k y b a r r i e r s and d i o d e s c o u l d be p r e p a r e d from PT f i l m s o r pressed p e l l e t s . Efficiency o f photoenergy c o n v e r s i o n by t h e s e d e v i c e s i s c u r r e n t l y under i n t e n s e i n v e s t i g a t i o n by M. I s o g a i (28) i n our group.


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on P o l y ( i s o t h i a n a p h t h e n e )

PITN has r e c e n t l y been prepared i n our group by c h e m i c a l and e l e c t r o c h e m i c a l p r o c e d u r e s ( 2 9 ) . The reason c r e a t i o n i s b e s t e x p l a i n e d w i t h Scheme I , below.

several for its


Scheme I I f both resonance forms l a and l b were i s o e n e r g e t i c , then t h e r e would be no bond a l t e r n a t i o n " ™ the hydrocarbon backbone o f PT and the polymer would p r o b a b l y e x h i b i t m e t a l l i c c o n d u c t i v i t y w i t h o u t t h e need t o dope. In r e a l i t y , as shown i n Scheme I , t h e two forms a r e not i s o e n e r g e t i c and t h e r e s u l t i n g bond a l t e r n a t i o n i n resonance form l a o f PT, g i v e s r i s e t o t h e o b s e r v e d bandgap o f c a 2eV. On t h e o t h e r hand, b e n z a n n u l a t i o n i n t h e 3 , 4 p o s i t i o n s o f PT, would produce t h e n e a r l y d e g e n e r a t e s t r u c t u r e s 2a and 2b (Scheme I ) because t h e g a i n i n a r o m a t i c i t y o f t h e benzene r i n g T n 2b i s exp e c t e d t o outweigh the l o s s o f t h i o p h e n e a r o m a t i c i t y i n 2a. The r e s u l t would be a m e l i o r a t i o n o f bond a l t e r n a t i o n ( " P e i e r l s distortion" o f the backbone) w i t h a c o n c o m i t a n t d e c r e a s e i n bandgap. Recent r e s u l t s o f i n s i t u d o p i n g s p e c t r o s c o p y ( F i g u r e 2) as w e l l as c y c l i c voltammetry by N. C o l a n e r i and M. Kobayashi i n o u r l a b o r a t o r y showed t h a t indeed t h e bandgap o f PITN i s c a one eV lower than t h a t o f PT. An i n t e r e s t i n g " f a l l o u t " o f t h i s r e s u l t i s t h a t upon d o p i n g , t h e a b s o r p t i o n moves i n t o t h e i n f r a r e d r e g i o n and t h e material becomes a t r a n s p a r e n t ( d e p e n d i n g on sample t h i c k n e s s ) , yellow conductor; i.e., PITN i s a h i g h c o n t r a s t electrochromic material. The energy gap i s now low enough so t h a t S c h o t t k y b a r r i e r experiments showed Ohmic, r a t h e r than d i o d e b e h a v i o r , as was obs e r v e d w i t h PT. E l e c t r o n m i c r o s c o p y r e v e a l e d t h a t t h e morphology o f 6% CI doped PITN depends on t h e s u b s t r a t e on which i t i s d e p o s i t e d and t h a t i t i s a r e l a t i v e l y "open" s t r u c t u r e , a l t h o u g h not as open as poly(acetylene). S e l e c t e d a r e a e l e c t r o n d i f f r a c t i o n on t h e same sample showed t h e m a t e r i a l t o be p a r t i a l l y c r y s t a l l i n e ( t h r e e d i f f r a c t i o n r i n g s c o u l d be s e e n ) . C o n t r a r y t o PT, the f u l l y dedoped s e m i c o n d u c t o r ; an o b s e r v a t i o n which i s energy gap o f t h i s new polymer. Conclusions

PITN i s b l u e - b l a c k and i s a i n agreement w i t h t h e small

and O u t l o o k

We have p r e s e n t e d e v i d e n c e t o prove t h e s t r u c t u r e o f e l e c t r o c h e m i c a l l y g e n e r a l l y p o l y ( t h i o p h e n e ) from d i t h i o p h e n e both by i n d e pendent s y n t h e s i s and s p e c t r o s c o p y . Diodes and p h o t o d i o d e s were fabricated from lightly doped chemically and electrochemically s y n t h e s i z e d PT.



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la

lb

ΔΕ l b > ΔΕ

la

ΔΕ 2 a s 2 b ;

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Materials

ΔΕ 2 a > 2b ?

Scheme I

τ

ι

1

1

1

1

1

1

1

1

1

1

1

1

1

1

r

ENERGY (eV)

F i g u r e 2. In s i t u e l e c t r o n i c s p e c t r o s c o p y o f PITN. Solid line a t 2.5V i s a 6% C T doped sample and the 3.5V l i n e c o r r e s p o n d s to the same sample, f u l l y doped. V o l t a g e s a r e \is_ L i .



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U s i n g p h y s i c a l o r g a n i c c h e m i c a l r a t i o n a l i z a t i o n s , we were a b l e t o modify t h e e l e c t r o n i c s t r u c t u r e o f a p o l y ( h e t e r o c y c l e ) . The r e s u l t was p o l y ( i s o t h i a n a p h t h e n e ) ; a polymer which i s a l r e a d y a s e m i c o n d u c t o r i n t h e n e u t r a l (undoped) s t a t e . In t h e f u l l y doped s t a t e , PITN i s a t r a n s p a r e n t c o n d u c t i n g polymer.


Our r e s u l t s w i t h PITN a r e v e r y e n c o u r a g i n g t o t h e development o f an o r g a n i c semimetal ( z e r o gap s e m i c o n d u c t o r ) s i n c e a r e l a t i v e l y minor m o d i f i c a t i o n a l l o w e d t o reduce t h e energy gap w i t h i n a f a m i l y o f p o l y ( h e t e r o c y c l e s ) by 1 eV (23 K c a l ) . F u r t h e r m o d i f i c a t i o n s by a n n u l a t i o n and s u b s t i t u t i o n ( e l e c t r o n donor o r a c c e p t o r ) a r e e x p e c t e d t o f u r t h e r reduce t h e energy gap o f a PT-based p o l y m e r i c conductor. We a r e c u r r e n t l y a c t i v e l y engaged i n t h e p r e p a r a t i o n o f such n e u t r a l o r g a n i c c o n d u c t o r s .

Acknowledgments We a r e i n d e b t e d t o Showa Denko Κ. K. and t h e O f f i c e o f Naval Research through g r a n t N00014-83-K-0450 f o r support o f t h i s work.

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