Studies of Hole-Trap Distribution and Valence-Band Structure in Anthracene

Abstract

Measurements are reported of the spectral yield and other characteristics of the photocurrent arising from photoexcitation of holes out of bulk traps into the valence band of anthracene. Vapour-grown crystals were found to contain two main sets of hole traps: an essentially discrete shallow set, about 1 eV above the valence-band edge and present with a density of ∼ 10¹¹ cm⁻³, and a deeper set lying 1.8–2.1 eV above the valence-band edge with a density of ∼ 5×10¹² cm⁻³. The trap density in meltgrown samples is one to two orders of magnitude lower. The spectral yield of the photocurrent arising from photoexcitation of trapped holes injected by an ohmic contact revealed the following information. A well-defined peak near the photocurrent threshold (∼1eV) indicates clearly the presence of a narrow conducting sub-band at the top of the valence band of anthrancene. The sub-band is less than 0.2eV in width and is separated from a broad continuum of lower conducting states by at most 0.2eV. This structure is similar to that observed previously for the conduction band of anthracene except that vibronic splittings of the narrow sub-band are not apparent here. Also, de-trapping of holes by photo-generated triplet excitons is much weaker than in the case of electron traps.