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Electrical properties of polymerizing single-crystalline pTS diacetylene
Synthetic Metals, 37 (1990) 305 - 311
305
ELECTRICAL PROPERTIES OF POLYMERIZING S I N G L E - C R Y S T A L L I N E pTS D I A C E T Y L E N E JULIUSZ SWORAKOWSK! Institute of Organic and Physical Chemistry, Technical University of Wroc~aw, 50-370 Wroc~Iaw (Poland)
Abstract This paper describes the results of measurements of the photoconductivity in thermally polymerizing pTS diacetylene. The evolution of photoresponse in samples during the solid-state polymerization can be explained by assuming the process to be controlled by interchain jumps. The anisotropy of the photoconductivity, as large as three orders of magnitude, results from the anisotropy of the mean free paths of carriers.
Introduction Since late sixties [1], diacetylenes have been known to polymerize in the solid state forming extended n-electron systems. Details of preparation and properties of polymers have been extensiviely studied since then (see, e.g., refs. 2- 4 for recent reviews and collections of papers). Analysis of the geometries of reactive diacetylenes [5, 6] demonstrates that the criteria set by Schmidt [7] for topochemical reactions are entirely fulfilled in this case. The polymerization, which is in fact a 1,4-addition reaction (cf. Fig. 1), depends mainly (but not exclusively) on the arrangement of monomer molecules in the crystal, being thus indirectly dependent on the size and shape of the side groups (see, e.g., a collection of data in ref. 5). The topochemical polymerization yields single crystals of reasonable quality, consisting of linear systems of conjugated bonds parallel to and well separated from one another. This unique feature of polydiacetylenes gives rise to a remarkable anisotropy of several physical properties of these materials and makes them good candidates for studying properties of quasi-one-dimensional systems. In particular, electronic excitations, among them the generation and transport of charge carriers, have been extensively studied (e.g. refs. 8-10 and references therein). Most of the experimental results published to date and describing the electrical properties of these materials have been obtained on two polydiacetylenes: poly[bis(toluenesulfonate) of 2,4-hexadiyne1,6-diol] (R1 =R2=--CH2--O--SO2--CeH4--CH3; hereafter referred to as poly(pTS)), and poly[1,6-bis-(N-carbazolyl)-2,4-hexadiyne] (R1 = R2 = - - C H 2 - 0379-6779/90/$3.50
© Elsevier Sequoia/Printed in The Netherlands
306
-'cx_ R ///C
/C/~CtflR RV~.C~,C
R~.~ /C C/C~--%R
c-,,cWR RV c,C /
R~,C,~/C/C///C~R
Cm c
"
R~(~
(~1 C
Fig. 1. A schematic depiction of the solid-state polymerization of diacetylenes.
C12HsN; poly(DCH)), for which large single-crystalline samples can be readily obtained. The polydiacetylenes are wide-bandgap semiconductors with gaps exceeding 2 e V [11, 12], thus at experimentally available temperatures the concentrations of thermally generated carriers are negligible. To measure observable currents, the carriers must therefore be either injected from electrodes or generated by illumination with photons of suitable energy. An effective injection from alkali-metal electrodes into poly(DCH) has been reported in ref. 13; no similar effect has been found in the case of poly(pTS), although the positions of the energy levels of both polydiacetylenes seem to be quite similar. Photoconductivity has been detected in all polydiacetylenes investigated in this respect. Where measured, the action spectra consist of a broad band starting at c. 2 eV and attributed to excitations of the diacetylene chains [14-20]. In poly(pTS) [14-18] and poly(pFBS) ( R I = I ~ = - - C H 2 - O--SO2--CeH4F) [20], there is also a peak around 1.6 eV, usually attributed to various non-intrinsic processes. Several independent experiments confirm that fully polymerized diacetylenes exhibit a pronounced anisotropy of electrical properties: both dark and photoconductivities measured in the direction parallel to that of polymer chains are a few orders of magnitude higher than those measured perpendicular to the chains (e.g. refs. 18, 20 - 22) estimations of the mobility in the chain direction range from 10° cm2/V s [23 - 25] to 105 cm2/V s [16, 26], although no direct measurements of macroscopic carrier mobilities have been successfully performed to date.
Photoconductivity in the pTS monomer-polymer mixed system While fully polymerized diacetylenes are good photoconductors, no photoconductivity has been reported so far for the respective monomers. In pTS, where growing polymer chains are molecularly dispersed in the monomer matrix thus forming monomer-polymer solid solutions over the
307
entire conversion range, such a situation has interesting implications. Following the photoconductivity build-up during the monomer-to-polymer conversion offers a unique opportunity to study the charge-carrier transport in highly anisotropic, highly ordered mixed molecular systems. The evolution of the photoconductivity during the thermal solid-state polymerization of pTS has been carried out in this laboratory. The measurements (see ref. 18 for experimental details) were carried out in the planeparallel geometry of electrodes in three orthogonal directions: b (the direction of growing polymer chains), a* and c. As shown in Fig. 2, measurable photoconductivity appeared in samples containing more than c. 10% of the polymer, and then was found to increase rapidly between 30% and 60% of conversion. It is important to note that the shapes (but not the magnitudes) of the J ( x ) dependences were found to be identical for measurements carried out in the three directions: in fully polymerized samples the ratio J a : J b : J c ~ 1:1000:2.5. The observed effect can be understood within a simple model [18] that eventually leads to the following dependence describing the photocurrent (J) as a function of the polymer contents (x) J ( x ) oc
Nay(X)
(1)
1 - pdp(X)
where Nay is an average length of unperturbed polymer chains at a given conversion, p is a probability that a given polymer chain has another chain (or chains) in the nearest-neighbour position, and Pd stands for a probability of a carrier executing an interchain jump between the nearest-neighbour chains.
lj
0 Q
~0.8
o~
0"6t 0.4
o
o
2
0"21 0
o
2 I3 ~o .~
r~ ~ ]
0.2
0.4
0.6
0.8 X
Fig. 2. Photocurrent evolution during the thermal polymerization of single crystals of pTS at 333 K. The polymer contents were calculated from the annealing time using the conversion curve measured under identical conditions [27]. Different symbols refer to measurements t a k e n on different samples.
308 1000 ¢u z
100-
10.
0
0'.2
0'.4
O'.S
0'.8
X
Fig. 3. Conversion dependence of the average chain lengths as estimated by different authors: (1) Baughman [28]; (2) Baessler [29]; (3) Kollmar and Sixl [30]. The dashed lines indicate the corrections in the Nay(x) curves introduced in order to fit the postulated dependence to our J(x) results.
The experimental data from Fig. 2 can be reasonably well fitted by eqn. (1) if Nay is assumed to increase with x at low conversions, and then to level off (cf. Fig. 3). At low polymer contents, such behaviour is in reasonable agreement with other estimates [28- 30]. Above x ~ 0.6, however, our results differ substantially from those given in the literature. According to Baessler [29], the average chain length increases over the entire range of conversions, whereas the models put forward by Baughman [28] and Kollmar and Sixl [30] predict a rapid decrease at high conversions (x > 0.8). This discrepancy can be rationalized in the following way: (i) the model of Baessler does not yield correct results for high conversions, as the author himself pointed out in his paper [29]; (ii) Nay estimated from our measurements is essentially the average length of chains forming an 'easy path' for charge carriers. Once the path is formed (which is likely to occur above x ~ 0.5), the production of shorter chains does not influence the transport of carriers. Consequently, shorter chains formed towards the end of the polymerization and giving rise to a decrease of the average chain length, can remain 'invisible' in our experiment. We shall return to the problem of carrier transport along the polymer chains in the following section of this paper.
Fully polymerized crystals The fit of our experimental results to eqn. (1) yields Pa I> 0.9. In other words, a large majority of carriers generated in the sample reach the collecting electrode within the time scale of the experiment (c. 10 -5 s). The average chain length in fully polymerized pTS is likely to amount to c. 102 to 103nm (a few hundred polymer units), therefore, a carrier should experience
309 c. 109 to 104 interchain jumps to travel across a 1-mm thick sample. It seems reasonable to assume t hat these jumps (and, probably, a localization in deeper traps) are the processes determining the macroscopic mobilities of charge carriers. Since independent results seem to indicate t h a t the carrier generation is an isotropic (or, at most, a weakly anisotropic) process, the macroscopic anisotropy of the conductivity and photoconductivity should reflect the anisotropy of the mean free paths of carriers between the rate-determining events, i.e. between interchain jumps. In other words, the anisotropy should come close to the chain l e n g t h - i n t e r c h a i n distance ratio; i.e. it should be of the order 102 to 103, as was indeed found in several experiments [18, 20- 22]. The situation is additionally complicated by the presence of traps. The influence of local states in one-dimensional insulators has already been discussed in our earlier papers [31, 32]. It was demonstrated t hat in strictly one-dimensional solids the kinetics of trapping and detrapping is qualitatively different from that in three-dimensional solids: if the densities of carriers photogenerated or injected into a sample are sufficiently high, then so-called 'degenerate trapping' may take place; i.e. one trap may localize more t han one carrier. However, even at lower densities of carriers, the motion of carriers is strongly influenced by the presence of traps.
~,~E O.025eV 50nm ~,,
5
Fig. 4. A schematic diagram indicating the shape of a potential barrier for carriers moving along chains, built up around a charge carrier localized in a deep trap. The position of the trap is indicated by an arrow.
310 P a r a d o x i c a l l y , a n e m p t y t r a p should influence free c a r r i e r s m o v i n g a l o n g n e i g h b o u r i n g c h a i n s to a f a r lesser d e g r e e t h a n a filled one. Q u a l i t a tively, the effect of local s t a t e s is i l l u s t r a t e d in Fig. 4. A n y l o c a l i z a t i o n of a c a r r i e r in a deep t r a p gives rise to a p o t e n t i a l b a r r i e r e x t e n d i n g o v e r s e v e r a l n e i g h b o u r i n g chains. S u c h a b a r r i e r p r e v e n t s c a r r i e r t r a n s p o r t a l o n g t h o s e chains; t h u s e n h a n c i n g the p r o b a b i l i t y of i n t e r c h a i n jumps. S u c h a process a d d i t i o n a l l y limits the m e a n free p a t h of c a r r i e r s a l o n g the chains, t h u s i n f l u e n c i n g the d e t e r m i n a t i o n of t h e effective c h a i n l e n g t h f r o m the photoconductivity experiments.
Conclusions
M e a s u r e m e n t s of p h o t o c u r r e n t s in p o l y m e r i z i n g pTS single c r y s t a l s show t h a t t h e i n t e r c h a i n j u m p s of p h o t o g e n e r a t e d c a r r i e r s a r e t h e f a c t o r d e t e r m i n i n g the c o n d u c t i v i t i e s of t h e samples. T h e a n i s o t r o p y of the effect is likely to depend p r i m a r i l y on t h e g e o m e t r i c a l a n i s o t r o p y of the system, i.e. on t h e l e n g t h s of the u n p e r t u r b e d p o l y m e r chains.
Acknowledgements
T h e a u t h o r is i n d e b t e d to M r M. O r c z y k for p e r m i s s i o n to use s e v e r a l r e s u l t s q u o t e d in this c o n t r i b u t i o n . T h e r e s e a r c h described in this p a p e r w a s s p o n s o r e d by the Polish A c a d e m y of Sciences w i t h i n the P r o g r a m m e C P B P 01.14.
References
1 G. Wegner, Z. Naturforsch., Teil B, 24 (1969) 824. 2 H. J. Cantow (ed.), Polydiacetylenes, Springer, Berlin, 1984. 3 D. Bloor and R. R. Chance (eds.), Polydiacetylenes: Synthesis, Structure and Electronic Properties, M. Nijhoff, The Hague, 1985. 4 D. J. Sandman (ed.), CrystallographicaUy Ordered Polymers, ACS Symposium Series No. 337,
5 6 7 8 9 10 11 12 13 14 15 16
American Chemical Society, Washington, DC, 1987. V. Enkelmann, in H. J. Cantow (ed.), Polydiacetylenes, Springer, Berlin, 1984, p. 91. D. Bloor, Mol. Cryst. Liq. Cryst., 93 (1983) 183. G. M. J. Schmidt, Solid State Photochemistry, Verlag Chemie, Weinheim, 1979. G. Wegner, in W. E. Hatfield (ed.), Molecular Metals, Plenum, New York, 1977, p. 297. D. Bloor, in J. Ladik, J. M. Andr~ and M. Seel (eds.), Quantum Chemistry of Polymers - - Solid State Aspects, Reidel, Dordrecht, 1985, p. 191. D. Bloor, Philos. Trans. R. SOc. London, Set. A, 314 (1985) 51. L. Sebastian and G. Weiser, Chem. Phys. Lett., 62 (1981) 447. L. Sebastian and G. Weiser, Phys. Rev. Lett., 46 (1981) 1156. W. Spannring and H. Baessler, Chem. Phys. Lett., 84 (1981) 54. B. Reimer and H. Baessler, Phys. Status Solidi (a), 32 (1975) 435. R. R. Chance and R. H. Baughman, J. Chem. Phys., 64 (1976) 3889. K. J. Donovan and E. G. Wilson, J. Phys. C: Solid State Phys., 12 (1979) 4857.
311 17 A. S. Siddiqui, J. Phys. C: Solid State Phys., 13 (1980) 2147. 18 M. Orczyk, J. Sworakowski, M. Schott and M. Bertault, Chem. Phys., 121 (1988) 245. 19 K. Lochner, H. Baessler, L. Sebastian, G. Weiser, G. Wegner and V. Enkelmann, Chem. Phys. Lett., 78 (1981) 366. 20 M. Orczyk, unpublished results. 21 K. Lochner, B. Reimer and H. Baessler, Chem. Phys. Left., 41 (1976) 388. 22 A. S. Siddiqui and E. G. Wilson, J. Phys. C: Solid State Phys., 12 (1979) 4237. 23 B. Reimer and H. Baessler, Phys. Status Solidi (b), 85 (1978) 145. 24 A. S. Siddiqui, J. Phys. C: Solid State Phys., 13 (1983) L1079. 25 D. Moses, M. Sinclair and A. J. Heeger, Phys. Rev. Lett., 58 (1987) 2710. 26 K. J. Donovan and E. G. Wilson, Philos. Mag. B, 44 (1981) 9. 27 M. Bertault, M. Schott, M. J. Brienne and A. Collet, Chem. Phys., 85 (1984) 481. 28 R. H. Baughman, J. Chem. Phys., 68 (1978) 3110. 29 H. Baessler, in H. J. Cantow (ed.), Polydiacetylenes, Springer, Berlin, 1984, p. 1. 30 C. Kollmar and H. Sixl, Proc. VW-Symp. Photoreaktive Festk6rper, M. Wahl Verlag, Karlsruhe, 1984, p. 327. 31 J. Sworakowski and G. F. Leal Ferreira, J. Phys. D: Appl. Phys., 17(1984) 135. 32 J. Sworakowski and M. Samod, IEEE Trans. Electr. Insul., EI-22 (1987) 13.
305
ELECTRICAL PROPERTIES OF POLYMERIZING S I N G L E - C R Y S T A L L I N E pTS D I A C E T Y L E N E JULIUSZ SWORAKOWSK! Institute of Organic and Physical Chemistry, Technical University of Wroc~aw, 50-370 Wroc~Iaw (Poland)
Abstract This paper describes the results of measurements of the photoconductivity in thermally polymerizing pTS diacetylene. The evolution of photoresponse in samples during the solid-state polymerization can be explained by assuming the process to be controlled by interchain jumps. The anisotropy of the photoconductivity, as large as three orders of magnitude, results from the anisotropy of the mean free paths of carriers.
Introduction Since late sixties [1], diacetylenes have been known to polymerize in the solid state forming extended n-electron systems. Details of preparation and properties of polymers have been extensiviely studied since then (see, e.g., refs. 2- 4 for recent reviews and collections of papers). Analysis of the geometries of reactive diacetylenes [5, 6] demonstrates that the criteria set by Schmidt [7] for topochemical reactions are entirely fulfilled in this case. The polymerization, which is in fact a 1,4-addition reaction (cf. Fig. 1), depends mainly (but not exclusively) on the arrangement of monomer molecules in the crystal, being thus indirectly dependent on the size and shape of the side groups (see, e.g., a collection of data in ref. 5). The topochemical polymerization yields single crystals of reasonable quality, consisting of linear systems of conjugated bonds parallel to and well separated from one another. This unique feature of polydiacetylenes gives rise to a remarkable anisotropy of several physical properties of these materials and makes them good candidates for studying properties of quasi-one-dimensional systems. In particular, electronic excitations, among them the generation and transport of charge carriers, have been extensively studied (e.g. refs. 8-10 and references therein). Most of the experimental results published to date and describing the electrical properties of these materials have been obtained on two polydiacetylenes: poly[bis(toluenesulfonate) of 2,4-hexadiyne1,6-diol] (R1 =R2=--CH2--O--SO2--CeH4--CH3; hereafter referred to as poly(pTS)), and poly[1,6-bis-(N-carbazolyl)-2,4-hexadiyne] (R1 = R2 = - - C H 2 - 0379-6779/90/$3.50
© Elsevier Sequoia/Printed in The Netherlands
306
-'cx_ R ///C
/C/~CtflR RV~.C~,C
R~.~ /C C/C~--%R
c-,,cWR RV c,C /
R~,C,~/C/C///C~R
Cm c
"
R~(~
(~1 C
Fig. 1. A schematic depiction of the solid-state polymerization of diacetylenes.
C12HsN; poly(DCH)), for which large single-crystalline samples can be readily obtained. The polydiacetylenes are wide-bandgap semiconductors with gaps exceeding 2 e V [11, 12], thus at experimentally available temperatures the concentrations of thermally generated carriers are negligible. To measure observable currents, the carriers must therefore be either injected from electrodes or generated by illumination with photons of suitable energy. An effective injection from alkali-metal electrodes into poly(DCH) has been reported in ref. 13; no similar effect has been found in the case of poly(pTS), although the positions of the energy levels of both polydiacetylenes seem to be quite similar. Photoconductivity has been detected in all polydiacetylenes investigated in this respect. Where measured, the action spectra consist of a broad band starting at c. 2 eV and attributed to excitations of the diacetylene chains [14-20]. In poly(pTS) [14-18] and poly(pFBS) ( R I = I ~ = - - C H 2 - O--SO2--CeH4F) [20], there is also a peak around 1.6 eV, usually attributed to various non-intrinsic processes. Several independent experiments confirm that fully polymerized diacetylenes exhibit a pronounced anisotropy of electrical properties: both dark and photoconductivities measured in the direction parallel to that of polymer chains are a few orders of magnitude higher than those measured perpendicular to the chains (e.g. refs. 18, 20 - 22) estimations of the mobility in the chain direction range from 10° cm2/V s [23 - 25] to 105 cm2/V s [16, 26], although no direct measurements of macroscopic carrier mobilities have been successfully performed to date.
Photoconductivity in the pTS monomer-polymer mixed system While fully polymerized diacetylenes are good photoconductors, no photoconductivity has been reported so far for the respective monomers. In pTS, where growing polymer chains are molecularly dispersed in the monomer matrix thus forming monomer-polymer solid solutions over the
307
entire conversion range, such a situation has interesting implications. Following the photoconductivity build-up during the monomer-to-polymer conversion offers a unique opportunity to study the charge-carrier transport in highly anisotropic, highly ordered mixed molecular systems. The evolution of the photoconductivity during the thermal solid-state polymerization of pTS has been carried out in this laboratory. The measurements (see ref. 18 for experimental details) were carried out in the planeparallel geometry of electrodes in three orthogonal directions: b (the direction of growing polymer chains), a* and c. As shown in Fig. 2, measurable photoconductivity appeared in samples containing more than c. 10% of the polymer, and then was found to increase rapidly between 30% and 60% of conversion. It is important to note that the shapes (but not the magnitudes) of the J ( x ) dependences were found to be identical for measurements carried out in the three directions: in fully polymerized samples the ratio J a : J b : J c ~ 1:1000:2.5. The observed effect can be understood within a simple model [18] that eventually leads to the following dependence describing the photocurrent (J) as a function of the polymer contents (x) J ( x ) oc
Nay(X)
(1)
1 - pdp(X)
where Nay is an average length of unperturbed polymer chains at a given conversion, p is a probability that a given polymer chain has another chain (or chains) in the nearest-neighbour position, and Pd stands for a probability of a carrier executing an interchain jump between the nearest-neighbour chains.
lj
0 Q
~0.8
o~
0"6t 0.4
o
o
2
0"21 0
o
2 I3 ~o .~
r~ ~ ]
0.2
0.4
0.6
0.8 X
Fig. 2. Photocurrent evolution during the thermal polymerization of single crystals of pTS at 333 K. The polymer contents were calculated from the annealing time using the conversion curve measured under identical conditions [27]. Different symbols refer to measurements t a k e n on different samples.
308 1000 ¢u z
100-
10.
0
0'.2
0'.4
O'.S
0'.8
X
Fig. 3. Conversion dependence of the average chain lengths as estimated by different authors: (1) Baughman [28]; (2) Baessler [29]; (3) Kollmar and Sixl [30]. The dashed lines indicate the corrections in the Nay(x) curves introduced in order to fit the postulated dependence to our J(x) results.
The experimental data from Fig. 2 can be reasonably well fitted by eqn. (1) if Nay is assumed to increase with x at low conversions, and then to level off (cf. Fig. 3). At low polymer contents, such behaviour is in reasonable agreement with other estimates [28- 30]. Above x ~ 0.6, however, our results differ substantially from those given in the literature. According to Baessler [29], the average chain length increases over the entire range of conversions, whereas the models put forward by Baughman [28] and Kollmar and Sixl [30] predict a rapid decrease at high conversions (x > 0.8). This discrepancy can be rationalized in the following way: (i) the model of Baessler does not yield correct results for high conversions, as the author himself pointed out in his paper [29]; (ii) Nay estimated from our measurements is essentially the average length of chains forming an 'easy path' for charge carriers. Once the path is formed (which is likely to occur above x ~ 0.5), the production of shorter chains does not influence the transport of carriers. Consequently, shorter chains formed towards the end of the polymerization and giving rise to a decrease of the average chain length, can remain 'invisible' in our experiment. We shall return to the problem of carrier transport along the polymer chains in the following section of this paper.
Fully polymerized crystals The fit of our experimental results to eqn. (1) yields Pa I> 0.9. In other words, a large majority of carriers generated in the sample reach the collecting electrode within the time scale of the experiment (c. 10 -5 s). The average chain length in fully polymerized pTS is likely to amount to c. 102 to 103nm (a few hundred polymer units), therefore, a carrier should experience
309 c. 109 to 104 interchain jumps to travel across a 1-mm thick sample. It seems reasonable to assume t hat these jumps (and, probably, a localization in deeper traps) are the processes determining the macroscopic mobilities of charge carriers. Since independent results seem to indicate t h a t the carrier generation is an isotropic (or, at most, a weakly anisotropic) process, the macroscopic anisotropy of the conductivity and photoconductivity should reflect the anisotropy of the mean free paths of carriers between the rate-determining events, i.e. between interchain jumps. In other words, the anisotropy should come close to the chain l e n g t h - i n t e r c h a i n distance ratio; i.e. it should be of the order 102 to 103, as was indeed found in several experiments [18, 20- 22]. The situation is additionally complicated by the presence of traps. The influence of local states in one-dimensional insulators has already been discussed in our earlier papers [31, 32]. It was demonstrated t hat in strictly one-dimensional solids the kinetics of trapping and detrapping is qualitatively different from that in three-dimensional solids: if the densities of carriers photogenerated or injected into a sample are sufficiently high, then so-called 'degenerate trapping' may take place; i.e. one trap may localize more t han one carrier. However, even at lower densities of carriers, the motion of carriers is strongly influenced by the presence of traps.
~,~E O.025eV 50nm ~,,
5
Fig. 4. A schematic diagram indicating the shape of a potential barrier for carriers moving along chains, built up around a charge carrier localized in a deep trap. The position of the trap is indicated by an arrow.
310 P a r a d o x i c a l l y , a n e m p t y t r a p should influence free c a r r i e r s m o v i n g a l o n g n e i g h b o u r i n g c h a i n s to a f a r lesser d e g r e e t h a n a filled one. Q u a l i t a tively, the effect of local s t a t e s is i l l u s t r a t e d in Fig. 4. A n y l o c a l i z a t i o n of a c a r r i e r in a deep t r a p gives rise to a p o t e n t i a l b a r r i e r e x t e n d i n g o v e r s e v e r a l n e i g h b o u r i n g chains. S u c h a b a r r i e r p r e v e n t s c a r r i e r t r a n s p o r t a l o n g t h o s e chains; t h u s e n h a n c i n g the p r o b a b i l i t y of i n t e r c h a i n jumps. S u c h a process a d d i t i o n a l l y limits the m e a n free p a t h of c a r r i e r s a l o n g the chains, t h u s i n f l u e n c i n g the d e t e r m i n a t i o n of t h e effective c h a i n l e n g t h f r o m the photoconductivity experiments.
Conclusions
M e a s u r e m e n t s of p h o t o c u r r e n t s in p o l y m e r i z i n g pTS single c r y s t a l s show t h a t t h e i n t e r c h a i n j u m p s of p h o t o g e n e r a t e d c a r r i e r s a r e t h e f a c t o r d e t e r m i n i n g the c o n d u c t i v i t i e s of t h e samples. T h e a n i s o t r o p y of the effect is likely to depend p r i m a r i l y on t h e g e o m e t r i c a l a n i s o t r o p y of the system, i.e. on t h e l e n g t h s of the u n p e r t u r b e d p o l y m e r chains.
Acknowledgements
T h e a u t h o r is i n d e b t e d to M r M. O r c z y k for p e r m i s s i o n to use s e v e r a l r e s u l t s q u o t e d in this c o n t r i b u t i o n . T h e r e s e a r c h described in this p a p e r w a s s p o n s o r e d by the Polish A c a d e m y of Sciences w i t h i n the P r o g r a m m e C P B P 01.14.
References
1 G. Wegner, Z. Naturforsch., Teil B, 24 (1969) 824. 2 H. J. Cantow (ed.), Polydiacetylenes, Springer, Berlin, 1984. 3 D. Bloor and R. R. Chance (eds.), Polydiacetylenes: Synthesis, Structure and Electronic Properties, M. Nijhoff, The Hague, 1985. 4 D. J. Sandman (ed.), CrystallographicaUy Ordered Polymers, ACS Symposium Series No. 337,
5 6 7 8 9 10 11 12 13 14 15 16
American Chemical Society, Washington, DC, 1987. V. Enkelmann, in H. J. Cantow (ed.), Polydiacetylenes, Springer, Berlin, 1984, p. 91. D. Bloor, Mol. Cryst. Liq. Cryst., 93 (1983) 183. G. M. J. Schmidt, Solid State Photochemistry, Verlag Chemie, Weinheim, 1979. G. Wegner, in W. E. Hatfield (ed.), Molecular Metals, Plenum, New York, 1977, p. 297. D. Bloor, in J. Ladik, J. M. Andr~ and M. Seel (eds.), Quantum Chemistry of Polymers - - Solid State Aspects, Reidel, Dordrecht, 1985, p. 191. D. Bloor, Philos. Trans. R. SOc. London, Set. A, 314 (1985) 51. L. Sebastian and G. Weiser, Chem. Phys. Lett., 62 (1981) 447. L. Sebastian and G. Weiser, Phys. Rev. Lett., 46 (1981) 1156. W. Spannring and H. Baessler, Chem. Phys. Lett., 84 (1981) 54. B. Reimer and H. Baessler, Phys. Status Solidi (a), 32 (1975) 435. R. R. Chance and R. H. Baughman, J. Chem. Phys., 64 (1976) 3889. K. J. Donovan and E. G. Wilson, J. Phys. C: Solid State Phys., 12 (1979) 4857.
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