Hydrostatic-pressure dependence of the photoconductivity of single-crystal pentacene and tetracene

Pentacene and tetracene show readily observable photoconductivity when illuminated with light in the blue part of the visible spectrum. We measured the change of photoconductivity with hydrostatic pressure in single-crystal samples of both materials. Possible mechanisms for the observed increase in photoconductivity with pressure are discussed. We conclude that a carrier-mobility increase under pressure is most likely to cause the increase in photoconductivity in the case of pentacene. For tetracene, changes in the absorption spectrum in the range of the excitation wavelengths may also be significant. We also observe a phase transition near 0.3 GPa in tetracene, in agreement with previous results. (C) 2001 American Institute of Physics.