QM/MM investigation of the degradation mechanism of the electron-transporting layer

The mechanism of charge transfer among tris(8-hydroxyquinolinate)aluminum (Alq₃) molecules in the electron-transporting layer (ETL) under amorphous conditions was theoretically investigated using both quantum mechanical/molecular mechanical (QM/MM) calculations and molecular dynamics (MD) simulations. The rate constant of the electron transfer was estimated for the equilibrated structure taken from the QM/MM MD simulations, based on the hopping model and Marcus theory. It was found that the coordination of a (LiF)₄ cluster in ETL drastically lowers the energy of the lowest unoccupied molecular orbital in the Alq₃ molecule. The small rate constant, namely the slow charge mobility, in ETL is believed to be causally related to the low-lying delocalized unoccupied molecular orbital of Alq₃ coordinated by the (LiF)₄ cluster. The results suggest that their interaction has a considerable influence on efficiency and is attributed in part to ETL degradation in organic light-emitting diodes.