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 and origin, compared with the counterpart of other kinds of organic semiconductors, which could provide new perspective to research into amorphous organic materials.

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Carrier transport in organic semiconductors (OSCs) plays an essential role in device performance. OSCs are generally divided into hole-transporting (p-type) and electron-transporting (n-type) materials. The holes should transport in the HOMO energy level, and electrons should transport in the LUMO. Such simple model analysis would easily lead to the primary theoretical result that intrinsic bipolar transport should be a basic characteristic of OSCs. Five widely used hole-transporting materials were utilized for fabricating the corresponding hole-only and electron-only diodes. The results show that the five OSCs are all bipolar transporting materials with hole and electron mobility data in 1 order of magnitude. Considering the simple two electrode structure and that there are not any additional electrode modifications, the data in these experiments should be the real intrinsic bipolar transport characteristic in OSCs. In addition, the experimental mobility versus electric field (μ ∼ E 1/2) curves also showed that there were positive or negative slopes for mobilities of holes or electrons, which leads to further development of the bipolar transport model; it is believed that there is a third kind of traps in the HOMO and LUMO energy spaces, which should possess different work mechanism during the external electric field, and thus result in negative or positive slopes for μ ∼ E 1/2.