Investigating energy level alignments at organic–organic interfaces in practical devices

Energy level alignments are crucial for designing high-performance semiconductor devices. However, the reported energy levels, especially the lowest unoccupied energy levels (LUMOs), exhibit significant variability for a given molecular compound. This variability often leads to misunderstanding of device working mechanisms. In this study, single-carrier devices with organic/interlayer/organic structures are proposed to probe the energy level alignments at organic–organic heterojunctions. It is observed that carrier transport characteristics deviate significantly depending on charge scattering or trapping. Five organic molecules, including 1, 3, 5-tri(m-pyrid-3-ylphenyl)benzene (TmPyPB), 4, 4′-bis(arbazole-9-y1)biphenyl (CBP), 2, 2′, 2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi), bis[2-(2-hydroxyphenyl)-pyridine] beryllium (Bepp2), and tris(8-hydroxyquinolinato)aluminum (Alq3), are utilized to test the method. The deduced LUMO level order for these materials is found to deviate significantly from reported values. Furthermore, the effect of differences in the energy level arrangement on the performance of electroluminescent devices is investigated. This work suggests that determining LUMO energy alignments via single-carrier analysis is a valuable method for understanding device working mechanisms.