Long-lived charge traps in functionalized pentacene and anthradithiophene studied by time-resolved electric force microscopy

Charge trapping in functionalized pentacene and anthradithiophene transistors was studied by time-resolved electric force microscopy (EFM). The spatial distribution of long-lived trapped charge was examined, in TIPS pentacene, for transistors prepared by four different solution deposition techniques. The distribution is markedly different in the four samples, establishing that charge trapping in this material is at least as sensitive to morphology as mobility is. The rate of trap formation in TIPS pentacene depends strongly on the initial free hole concentration, consistent with the view that charge traps in this material should not be viewed as static defect states but as states that are slowly created by reactions of free holes at a localized defect. In one TIPS pentacene sample, the rate of formation and steady-state concentration of trapped charge was found to be independent of the initial free hole concentration, allowing us to estimate the concentration of the impurity giving rise to the trapped charge. Transistors of functionalized anthradithiophene, prepared by spin casting and solvent annealing, showed evidence of grain boundary trapping. In both materials, regions can be found that exhibit essentially no long-lived traps, indicating that the cations of the two materials are not inherently prone to trapping and degradation in an unilluminated film. We find that electrons can be injected from untreated gold electrodes into both materials, and in both materials we observe a finite electron mobility.