Degradation in blue-emitting conjugated polymer diodes due to loss of ohmic hole injection

We report studies of the evolution of the hole injection and transport characteristics of fluorene-based polymer diodes subjected to electrical stressing. Dark injection (DI) transient measurements show that the polyethylenedioxythiophene/polystyrenesulphonate (PEDOT:PSS)-topolymer contact is initially ohmic, but as stressing proceeds, the transients shift to longer times and lose their characteristic temporal profile. A comparison with time-of-flight transient photocurrent measurements led us to conclude that the DI transient is modified by a loss of ohmic injection. Electroabsorption measurements show a drastic reduction in the built-in potential from 1.4 V to 0.6 V. Device simulation shows this to be consistent with a change in the PEDOT:PSS work function, and the introduction of an interfacial resistance at the PEDOT:PSS-to-polymer contact.