Abstract:
The results of an investigation into carrier dynamics in several novel functionalized and solution-processable pentacene and anthradithiophene derivatives are reported. Measurements were made of real-time photoresponse of polycrystalline thin films of these materials to ultrafast laser pulses, on picosecond to microsecond time-scales, as well as measurements of dark current and current under steady illumination. This data was taken over varied field-strength, light intensity and temperature. The results support a model for carrier generation and transport with the following features. Carrier photo-generation is assisted weakly, if it is assisted at all, thermally or by applied fields. Carriers are initially (picosecond to nanosecond time-scales) in extended states and transport is "bandlike." Carriers then relax into more localized states, transported via thermally assisted hopping (nanosecond to second time-scales). This model was supported by further experiments with the electric behavior of films prepared from a pure anthradithophene derivative, doped with either the buckminsterfullerene C₆₀ or with other molecular dopants. These results also show that samples with traps of known density and depth can be prepared, as a means of manipulating transport dynamics. The electronic and photo-electronic behaviors of films with self-anodized aluminum and of films with gold electrodes were compared, and a model of the particular energy profile and dynamics which exist at the different interfaces between the films and the different contacts was developed. This model views the metal-organic-metal system as an anode-to-anode Schottky strucure, whose I-V relation is shaped both by the nature of the interface dynamics for different metal contacts, and by the different distributions of space-charge in the thin film between different electrodes.
