Electrical and magnetic properties of organic semiconductors: Electrical conductivity and electron spin resonance studies of semiconducting, organic, charge transfer salts.

摘要

Charge transfer salts of Tetracyanoquinodimethane (TCNQ) wereudsynthesised and their electrical and magnetic properties wereudinvestigated. These salts show unusual electrical and magneticudbehaviour in contrast to conventional organic compounds. Theseudsalts have crystal structures which in general consist of TCNQudradical ions stacked in chains, isolated from each other by theuddiamagnetic cations. They are thus regarded as "one-dimensional"udelectrical and magnetic systems. The ESR spectra of these salts areudattributed to triplet excitons showing that the spin-spin and electronelectronudcorrelation effects are important. In the ESR spectraud(Chapter III) of some TCNQ salts dipolar splitting is observedudconfirming the spin-spin interaction. These triplet excitons areudregarded as bound electron-hole pairs. The experimentally determineduddipolar splitting tensors are presented in Chapter III and the intensityuddata in Chapter IV. A large number of fine structure lines areudobserved in the ESR spectra of Pyridinium-TCNQ and 4-Aminopyridinium-TCNQudapart from regular triplet exciton lines (Chapter III). These linesudare attributed to the trapping of excitons on an extended formulaudfinit (TCNQ2 )n. In Chapter IV the temperature dependent magneticudsusceptibilities are discussed in terms of Heisenberg antiferromagnetismudand Pauli paramagnetism. In Chapter V temperatureuddependent behaviour of electrical conductivity is discussed in termsudof an exciton band model, the lattice structure of the salts andudone-dimensional lattice consisting of defects giving rise to high andudlow conducting segments. Low temperature electrical and magneticudphases are discussed (Chapters IV and VII) in terms of a band andudhopping mechanisms.In Chapter VI self consistent field calculations are made withudreference to the tight binding one electron band theory usingudsimplified Roothaan equations considering CNDO approximations.udTheoretical results are related to experimental band gaps, spinspinudinteractions and charge alteration.