Photoconductivity and small-polaron effects in tetracyanoquinodimethan

Photoconductivity has been observed in high-purity tetracyanoquinodimethan (TCNQ) single crystals and polycrystalline evaporated thin films. The photocurrent was measured as a function of wavelength, light intensity, and temperature. A photoconducting edge is observed at about 1.7 eV. The spectral response of the crystals and films are essentially identical. Comparison with optical-absorption, reflection, and resistivity data imply that the 1.7-eV edge is a measurement of the band gap. The relatively small value is attributed to the high electron affinity of TCNQ and its polaron binding energy. Pulsed photocurrent measurements give a drift mobility for both electrons and holes of about 0.4 ${\mathrm{cm}}^2$/V sec in single crystals and somewhat lower in thin films. The electron mobility is about 10% higher than the hole mobility, and both are independent of temperature from 204 to 306 °K. The results suggest that conduction occurs by diffusion of small polarons in a conduction band whose width is comparable to the phonon energies. The relevance of this study to other systems is discussed.