Charge carrier dynamics in para-Toluene Sulphonate Polydiacetylene Crystals

ELSEVIER

Synthetic

Metals

102 (1999)

1417-1418

Charge Carrier Dynamics in pat-u-Toluene Sulphonate Polydiacetylene Crystals IRadiation

R.J.O.M. Hoofmanl, L.D.A. Siebbelesl, M.P. de Haas’ M. Szablewski2, D. Bloor2 Chemistry Department, IRI Delft University of Technology, Mekelweg 1.5, 2629 JB Delji, The Netherlands 2Department of Physics, University of Durham, Durham, DHl 3LE,United Kingdom

Abstract The mobility and the decay of charge carriers in pTS crystals with different structure and morphology have been investigated by time-resolved microwave conductivity measurements. The crystal structure of pTS is determined by the method of monomer crystal growth and the polymerisation route. The intrachain mobility was found to depend strongly on the backbone structure, while the interchain transport did not differ much for samples with different morphology. Keywords: 1.

Polydiacetylenes,

Conductivity,

Single Crystal Growth

Introduction

Although polydiacetylenes provide an ideal conjugated system for charge carrier transport, the presence of defects in these polymers affects the charge carrier mobility [l]. Typical defects in polydiacetylene crystals are stacking faults, dislocations and chain ends [2]. Moreover, solid-state polymerisation using high-energy radiation causes chemical changes in the polymer sidechains [3, 41 and even changes in the polymer backbone [5] can not be excluded. In this research, we have studied the effects of the method of monomer crystal growth and polymerisation on the charge carrier mobility and its anisotropy in polydiacetylene single crystals. 2.

Table 1: The pTS-polydiacetylene crystals investigated: appearance, method of monomer crystal growth and way of polymerisation. No .

crystal

appearance

1. 1 small oerfect formed. small gold facets 2. larger less perfect, diamond shaped facets, 1

monomer polymeriI crystal I sation ! growth [ ] slow evaoor. . ] thermal. degassed’ 60°C ’ slow evapor., thermal I air I 60°C

Experimental

2.1. Materials The synthesis of the bis-@-toluene sulphonate) of 2,4hexadiyne-1,6diol (monomer units of TS) followed the route used by Wegner[6], but was modified to ensure high chemical purity as described in the literature [7]. This involves using high purity solvents and a single recrystallisation of the monomer. All monocrystals were grown from acetone solutions (chromatography grade solvent). The most perfect crystals, No’s 1 and 4 in Table 1, were grown by slow evaporation, in which the acetone solution was degassed to remove oxygen. The latter is realized by freezing and melting the acetone solutions several times, while the vessel used was continuously evacuated. When the acetone solution was less pure and not degassed, slow evaporation resulted in samples typical of many of those reported in the literature [S] with a truncated diamond morphology, No.2 in Table 1. These crystals contain both dislocations and stacking faults. The flakes, No.3 in Table 1, were grown by rapid evaporation under a nitrogen atmosphere. Monocrystals 1, 2 and 3 were polymerized by heat, while monocrystal 4 was polymerized using 20 Mrad y-irradiation.

0379-6779/99/$ - see front matter PII: SO379-6779(98)00844-3

0 1999

Elsevier

Science

S.A.

All rights

2.2. Time resolved measurements

microwave

conductivity

(TRMC)

The microwave cell consists of a piece of rectangular waveguide with cross section 7.1x3.55 mm2 closed at one end with a metal plate. The sample was ionised using single pulses of 3 MeV electrons from a Van de Graaff accelerator. Changes in the conductivity of the sample upon pulsed irradiation were monitored in time as changes in the absorbed microwave power. The time-resolved absorbed microwave power per unit beam charge is related to the dose normalised radiationinduced conductivity, Ao(t)/D. The isotropic charge carrier mobility was obtained from the end-of-pulse conductivity for pulverized pTS samples, using the known number density of the charge carriers formed [9, IO]. The isotropic charge carrier mobility, &, is related to the mobility parallel to the polymer backbone, &t, and the average mobility perpendicular to the backbone, . according to &i=(~qi + 2)/3. The anisotropy, (I$ I< pi > ) was determined by rotating a single pTS crystal in the microwave cell according to a method described recently [lo]. reserved.

R.J.O.M. Hoojinan el al. I Synthetic Metals 102 (1999) 1417-1418

1418

The values of pll and can be obtained measured isotropic mobility and the anisotropy. 3.

Results

and

from

the

This can be attributed to the production of chemical defects by the y-irradiation polymerisation procedure used [3-51. Table 2: Charge carrier anisotropy, charge carrier mobility parallel and perpendicular to the polymer backbone, and the decay time r in different pTS-polydiacetylenes

Discussion

Fig.1. shows the radiation-induced conductivity transients obtained for the pulverized pTS samples at room temperature.

No. 10"

anisotropy

1 2 3 4

10" 4.

20

40

80

80

Time (ns) Fig. 1. Dose-normalised radiation-induced conductivity transients obtained for the different pTS crystals. A 2 ns electron pulse (depositing 12.8 kJ/m3) was used. The dashed line through transient 4 is a kinetic fit corresponding to a stretched exponential of the form exp(-(tir)1’3]. The anisotropy in the charge carrier mobility was found to vary by about two orders of magnitude for the different crystals (see Table 2). Table 2 also shows that the charge carrier mobility along the polymer backbone (p.11) varies by more than one order of magnitude for the different crystals. The most perfect pTS crystal, No. 1, shows the highest mobility, i.e. 3.2 cm2V’s-‘. This value is in agreement with the low field mobilities reported earlier for pTS polydiacetylene[ 1 l-143. In the other pTS crystals the intrachain mobility is lower due to the presence of defects and/or less perfect chain conformation. Both crystals 2 and 3 have a high dislocation content. Most of the dislocations are edge dislocations that emerge on the main facet and act to terminate chains, lowering the intrachain mobility. The mobility perpendicular to the polymer backbone in crystal 2 is larger than in the other crystals. In crystal 2, dislocations grow out from the centre into the irregular truncated ends. Hence, the dislocation axes will be normal to the polymer axis, producing misalignment which facilitates interchain transport. The conductivity transients can be well fitted by a stretched exponential of the form exp[-(t/r)1’3], shown by the dashed line in Fig. 1. This decay is characteristic for one-dimensional trapping [15, 161. The values for r are listed in Table 2. The decay in the y-ray polymerised crystal, No. 4, is much faster 1-3. the heat polymerised crystals, than in

115 10 38 2.5

Y (cm2y-1s-1)

3.2 1.0 0.9 0.11


0.03 0.10 0.03 0.04

-r

bs)

126 19 50 2.4

Conclusions

The charge carrier mobility and decay kinetics in pTS crystals were found to depend strongly on the crystal structure. The intrachain mobility decreases strongly and the charge carrier decay becomes faster as the defect concentration increases. In the crystal which was grown from less pure and non-degassed solutions, the interchain mobility was a factor 3 higher than in the other crystals investigated. The larger interchain mobility is attributed to misalignment of the polymer chains, decreasing the interchain distance. The smallest mobility and fastest decay was found for the ‘y-ray polymerized samples, which is attributed to a larger concentration of structural and/or chemical defects on the polymer backbone. References

[l] R. J. Young and J. Petermann, J. Polym. Sci. Polym. Phys. 20 (1982) 961-974 . [2] R. J. Young, R. T. Read and J. Petermann, J Mater. Sci. 16 (1981) 1835-1842 . 131 K. Se, H. Ohnuma and T. Kotaka, Pofym. J. 14 (1982) 895905 [4] R. J. Colton, C. R. K. Marrian, A. Snow and D. J. Dilella, VW. Sci. Technol. B5 (1987) 1353 . [5] G. N. Patel, R&at. Phys. Chem. 15 (1980) 637-641 . [6] G. Wegner, Die Makromol. Chemie 145 (1971) 85-94 [7] G. C. Stevens, D. J. Ando, D. Bloor and J. S. Ghotra, Polymer I7 (1976) 623 . [8] D. Bloor, D. N. Batchelder and F. H. Preston, phys. star. sol. (a) 40 (1977) 279-284 . [9] G. P. Van der Laan, M. P. de Haas, A. Hummel, H. Frey and M. Mdller, J. Phys. Chem. 100 (1996) 5470-5480 . [lo] R. J. 0. M. Hoofman, L. D. A. Siebbeles, M. P. de Haas, A. Hummel and D. Bloor, J. Chem. Phys.109 (1998). [ll] T. Blum and H. Blssler, Phys. Status Solidi B 153 (1989) K57-K61 . [12] D. Moses and A. J. Heeger, J. Phys.: Condens. Mater 1 (1989) 7395-7405 . [13] N. E. Fisher and D. J. Willock, J. Phys.: Condens. Matrer 4 (1992) 2517-2532 . [14] N. E. Fisher, J. Phys.: Condens. Matter4 (1992) 25432555 [15] G. Hunt, D. Bloor and B. Movaghar, J. Phys. C: Solid State Phys. 16 (1983) L623-L628 . [16] U. Seiferheld, H. Bassler and B. Movaghar, Phys. Rev. Len. 51 (1983) 813-616 .