The photoconductivity of metal-free and copper phthalocyanine crystals in high and ultra-high vacua

Abstract

Two-probe d.c. photoconductivity measurements are reported for crystals of metal-free and copper phthalocyanines in the frequency range 5000–20,000 cm.^–1. Crystals were prepared by sublimation both in nitrogen and in nitrogen–hydrogen ambients. Experiments were carried out in air, and at pressures of 10^–6 torr and 10^–9 torr, the latter after baking the crystals at 200°C for 12 hr. Evacuation of nitrogen sublimed crystals at 10^–6 torr at room temperature removed most of the photoconductivity associated with the intense visible absorption bands and caused a smaller decrease in the near-infrared photoconductivity. When air was re-admitted the photoconductivity returned. Baking under vacuum reduced the photoconductivity even further, and it did not return on exposure to air. Crystals grown in nitrogen–hydrogen mixtures already had photoconductivity spectra in air resembling nitrogen sublimed samples which had been evacuated and baked. It is suggested that the photoconductivity in the visible region results from traps formed by the interaction of oxygen with free radical impurities near the surface of the crystals.