Exchange of holes between organic crystals and metal films evaporated in high vacuum

Injection currents in organic crystals have been measured to determine the rate of hole exchange with metal contacts evaporated in high vacuum. At a gold contact the rate of hole injection increased with decreasing ionisation energy of the organic crystal but was depending on the evaporation procedure due to an influence of residual gas molecules. The excitation spectrum of the photocurrent agreed with hole generation through triplet excitons. Below the S0−S1 transition hole generation was due to surface states in the band gap.