Localized valence states in the forbidden gap of non-crystalline tetracene

Employing the thermally stimulated current technique the existence of three distinct gaussian trap distributions in vapor-deposited tetracene layers has been established. In layers formed at T_f ? 130 K the valence band is split into a distribution of localized states, the width being σ ? 0.06 eV, and the density N_t equal to the molecular density. Upon increasing T_f from 130 to 180 K, both N_t and σ decrease, indicating a decrease of structural disorder. For $\text{T}{}_{\mathrm{<mtext>f</mtext>}}\text{? 180}\text{K}$ the center of the trap distribution is at E₀ = 0.07 eV above the valence band. At T_f = 180 K an optimum quasi amorphous structure is formed which still lacks long-range order, but in which short-range order is present to a degree that in local regions a narrow valence band can be established. Trap distributions centered near 0.4 and 0.7 eV with a width of 0.07 and 0.1 eV, respectively, and containing about 10¹⁵ states/cm^?3, are almost independent of film formation condition, and are probably also of structural origin.