Charge Mobility of Mixed Organic Semiconductors: A NPB-A1Q_3 Study

摘要

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 (AlQ_3). Based on Poole-Frenkel model, the extracted zero-field hole mobility of pure NPB was 2.6x10~(-4) cm~2/Vs which is much larger than that of pure AlQ_3 (9.16x10~(-10) cm~2/Vs). As the AlQ_3 concentration is increased, the hole mobility decreases exponentially. In this case, AlQ_3 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 AlQ_3 is much smaller (5.28x10~(-6) cm~2/Vs vs. 1.51x10~(-7) cm~2/Vs) and the field-free electron mobility of the mixed films exhibits a minimum as the AlQ_3/NPB fraction ratio reaches about 75%. The LUMO energy level of AlQ_3 is 0.6 eV lower than that of NPB, making AlQ_3 become "traps" to the electron transport. When the amount of AlQ_3 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.