Electrical conductivity of tetracyanoquinodimethane crystals

Abstract

The semiconductivity and photoconductivity of a series of single crystals of tetracyanoquinodimethane (TCNQ) of increasing purity are discussed. Evidence for both surface and bulk extrinsic semiconduction is interpreted in terms of ionic impurities, particularly Na⁺TCNQ^–, providing donor levels 0.8 and 1.2 eV below the conduction level. The spectral response of photoconduction is very sensitive to the concentration of these impurities, particularly on the low energy side of the singlet absorption of TCNQ. Photoconduction is discussed in terms of the effect of ionic impurities on the recombination rate of photogenerated charge carriers; mechanisms for generation of such charge carriers are also discussed.