Exploration Of Intrinsic Bipolar Carrier Transport Characteristic And Defect States Within The Organic Semiconductors

Carrier transport in organic semiconductors?OSCs?plays the essential role in the device performance.In the common ideas,the OSCs are generally divided into hole-transporting?p-type?and electron-transporting?n-type?materials.The holes should transport in HOMO energy level,and electrons should transport in LUMO.Such simple model analysis would easily lead to the primary theoretical results that the intrinsic bipolar should be the basic characteristics of OSCs.Then five widely used hole-transporting materials?Figure 2-4?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 one 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 mobility verse electric field(?^E1/2)curves in bipolar experiments showed that there were positive or negative slopes for mobilities of hole or electron.Typically,this phenomenon was attributed to the influence of defect states.Then the carrier transporting of two different materials,NPB and?-NPB were further investigated.The results indicated that traditional theories of defect states cannot explain this phenomenon,which makes the further development of the bipolar transport model:it is believed that there are special 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 the?^E1/2.To verify this bipolar transport model,the survey to the special traps within OSCs was conducted.Several capacitance measurements have been applied to ITO/NPB/Al 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 defect states in NPB 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 was electron defects and located within LUMO energy space,which affirmed the bipolar transport model.In addition,the origin of this defect states was also determined.This research could provide new perspective to research into the nature of carrier transporting and the relationship with defect states in OSCs.