Charge mobility of mixed organic semiconductors: a NPB-AlQ 3 study

Abstract

We report a time-of-flight study of drift mobilities of hole and electron in mixed thin films of N,N'-diphenyl-N,N'- bis(1-napthyl)-1,1'-biphenyl-4,4'-diamine (NPB) and tris(8-hydroxyquinoline) aluminum (AlQ₃). Based on Poole- Frenkel model, the extracted zero-field hole mobility of pure NPB was 2.6x10^-4 cm²/Vs which is much larger than that of pure AlQ₃ (9.16x10^-10cm²/Vs). As the AlQ₃ concentration is increased, the hole mobility decreases exponentially. In this case, AlQ₃ molecules act as blocking "hills" to the hole transport, since its HOMO energy level is 0.4 eV lower than that of NPB. In contrast, the difference in the electron mobilities of pure NPB and AlQ₃ is much smaller (5.28x10^-6 cm²/Vs vs. 1.51x10^-7 cm²/Vs) and the field-free electron mobility of the mixed films exhibits a minimum as the AlQ₃/NPB fraction ratio reaches about 75%. The LUMO energy level of AlQ₃ is 0.6 eV lower than that of NPB, making AlQ₃ become "traps" to the electron transport. When the amount of AlQ₃ reaches a certain level such that they form connected transport network, the electrons are then driven mostly in this network and the NPB molecules become blocking "hills". In summary, the HOMO and LUMO energy levels, the charge mobilities of pure compounds and the characteristics of their microscopic networks can greatly influence the resultant transport behaviors. These results may create challenges for existing transport models of disordered organic semiconductors and will be useful in designing organic light-emitting devices based on mixed-layer structures.