High Electrical Conductivity in Organic Single-Component Systems Based on 2,5-Dimethylthio-Tcnq
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J. S. ZAMBOUNIS*, J. MIZUGUCHI*, H. HEDIGER*, J. PFEIFFER*, B. SCHMIDHALTER*, G. RIHS*, 0. CHAUVET** AND L. ZUPPIROLI** * Corporate Materials Research, Ciba-Geigy AG, 1723 Marly 1, Switzerland ** Laboratoire de Physique des Solides Semi-cristallins, Ddpartement de Physique, Ecole Polytechnique Frddrale, CH- 1015 Lausanne, Switzerland
ABSTRACT 2,5-dimethylthio-TCNQ has been newly synthesized, and its optical and electrical properties have been investigated in evaporated films. A high electrical conductivity of (7=2x 10-5 Scm- 1 has been measured at room temperature. The present single-component system is found to contain 2x10 17 spins/cm 3 . The charge carriers are presumably due to incorporated impurities which give the ESR signals. Carrier hopping is considerably facilitated by close intermolecular S-N contacts between the S atom of the -SCH 3 group of one molecule and the N atom of -C=sN group of the neighboring molecule.
1. INTRODUCTION The discovery of TTF-TCNQ 111, the first organic metal, has triggered an intensive research effort over the last two decades. The list of the organic conductors known today contains a number of compounds whose electrical conductivity varies from the semiconducting to the superconducting region 121. Most of them are based on charged radicals of ir-donors and/or 7r-acceptors. Lately, some tetrathiafulvalene derivatives beating long alkyl chains 131 or methyltelluro groups 141 have been found to exhibit considerable conductivity. They have been added to the fairly large list of single-component organic semiconductors which also includes phthalocyanines [51. However, we should bear in mind that the above compounds are ;relectrons donors and are therefore easily oxidized to give charge carriers. Cu-phthalocyanine 161 and Pb-phthalocyanine 171 are the typical examples to show enhanced hole conductivity due to the formation of 02- species. This means that the electrical conductivity depends largely on the oxygen concentration in the bulk and on the surface, leading often to irreproducible results. On the other hand, single-component strong acceptors are basically inert under oxidative conditions 181 and are therefore expected to show stable, intrinsic conductivity. In the case of tetramethyltetratelluro-tetrathiafulvalene 141, the high electrical conductivity is assumed to arise from strong Te-Te contacts which allow considerable electron delocalization. Similarly, S-S and S-N contacts [91 are also expected to provide high electron delocalization. These considerations prompted us to synthesize the new strong acceptor 2,5-dimethylthiotetracyanoquinodimethane (2,5-DMeS-TCNQ). In fact, the present system has been found to exhibit a high conductivity of (TRT=2xl0- 5 Scm- 1 . In this paper, we report on the optical and electrical properties of evaporated 2,5-DMeS-TCNQ. 289 Mat. Res. Soc. Symp. Proc. Vol. 328. ©1994 Materials Research Society
2. EXPERIMENTAL 2,5-DMeS-TCNQ was prepared in analogy to the DuPont procedure [101 in four steps starting from 1,4-bis(methylthio)-benzene. The dark
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