Nature of Defect States within Amorphous NPB Investigated through Drive-Level Capacitance Profiling

In this contribution, the double-peak phenomenon was observed in the capacitance–voltage characteristics of device ITO/NPB/Al; such a phenomenon was attributed to defect states within NPB. To verify it, several capacitance measurements have been applied to this device, including capacitance–voltage, capacitance–frequency, and, in particular, the drive-level capacitance profiling, which is common in researches of inorganic devices and is first applied to study the properties of defects in organic semiconductors. The results showed that capacitance–voltage and capacitance–frequency methods could not help to find any clues about the defect states in this material, but drive-level capacitance profiling could determine the defect states and other properties, which comprised the density of defect states, the spatial and energetic distribution of defect states, and the spectral density of states. The results also indicated that the defect states in this amorphous organic material probably have special properties...