Electronic traps and percolation paths in electroluminescent polymers

Abstract

Trap related properties of the electroluminescent polymers poly(2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene (MDMO-PPV, OC1C10–PPV) and poly[(2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene]-[(4′-(3″,7″-dimethyloctyloxy)phenyl-5)-1,4-phenylenevinylene] are investigated in organic light emitting diode (OLED) devices. By means of the thermally stimulated currents (TSC) technique the densities of occupied states, trap polarities and charge transport mechanisms are studied. The fractional TSC technique in combination with optical trap filling reveals broad density-of-occupied-states distributions for both polymers, ranging from 200 to 600 meV located between the highest occupied molecular orbital and lowest unoccupied molecular orbital levels. Conventional TSC experiments in combination with electrical trap filling allow to distinguish between electron and hole traps. In OC1C10–PPV both trap polarities are detected whereas in the copolymer only hole traps and no electron traps were found. In addition, a TSC study on optical and electrical trap filling in OLED devices strongly suggests the presence of percolation paths. Despite larger charging currents in the electrical trap filling case no or much smaller TSC signals were detected in comparison with the optical trap filling case.