Proceedings Article | 5 December 2006
Proc. SPIE. 6333, Organic Light Emitting Materials and Devices X
KEYWORDS: Thin films, Organic light emitting diodes, Molecules, Composites, Ionization, Aluminum, Electron transport, Heterojunctions, Organic semiconductors, Absorption
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 (9.16x10-10 cm2/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.
