No abstract

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.

In this paper, four pyrene–fluoroene derivatives with conjugated and nonconjugated pyrene substitution were designed and synthesized. In PFP1 and PFP2, there are nonconjugated pyrene substitution on C9 and conjugated pyrene on C2 and/or C7 of the fluorene moiety, and in the control molecules BP1 and BP2, there is only the conjugated pyrene in the C2 and/or C7 of the fluorene core. There is a special π–π hyperconjugation effect between nonconjugated pyrene and the pyrene–fluorene conjugation in the system (PFP1 and PFP2), which means the electron cloud of such two isolated conjugation systems (nonconjugated pyrene group and pyrene–fluorene group) could be delocalized and transferred to each other. Because of delocalization of electron cloud, the molecule size of PFP1 and PFP2 might have been decreased and led to decreased phase transition temperature compared with that of BP1 and BP2. Also due to the electron transfer between the molecules, the intermolecular force between PFP1 and PFP2 has been improved, which is the reason that they are more amorphous than BP1 and BP2. The easy electron transfer also makes the PFP1 and PFP2 show improved hole injection and device performance compared with that of BP1 and BP2.

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.