Loke-Yuen Wong

Polyfluorene-based light-emitting diodes with an azide photocross-linked poly(3,4-ethylene dioxythiophene):(polystyrene sulfonic acid) hole-injecting layer

We used a water-soluble bis(fluorinated phenyl azide) to cross-link a poly(ethylene dioxythiophene):poly(styrene sulphonic acid) (PEDOT:PSS), hole-injection layer, with a view to its future use with water-soluble emitters. To enable direct comparison with conventional PEDOT:PSS, we studied the cross-linked films in diodes incorporating the organic-solvent soluble polymer poly(9,9′-dioctylfluorene-alt-benzothiadiazole). Kelvin probe characterization of the PEDOT:PSS and electroabsorption measurements of the devices consistently show a 0.2eV increase of the PEDOT:PSS work function upon cross-linking. We also observe a 70-fold reduction in resistivity, an increase of the current above threshold and a decrease of the “leakage” current below threshold.

Solvent effects and multiple aggregate states in high-mobility organic field-effect transistors based on poly(bithiophene-alt-thienothiophene)

Franck–Condon absorption analysis reveals the existence of several aggregate states in poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) thin films which impact their recrystallization and the attainable field-effect mobility (μFET). Poor solvents (toluene and mixed-xylenes) lock in both disordered and well-ordered states that cannot be annealed away even in the liquid crystalline phase. This reduces μFET and increases mobility activation energies compared with films from good solvents (chlorobenzene and o-dichlorobenzene). Despite its poor solubility characteristics, PBTTT can be ink-jet printed in dilute chlorobenzene, and devices can be operated unencapsulated in ambient, in the dark (>105cycles over several days) with only a moderate mobility loss.

Electromigration of the conducting polymer in organic semiconductor devices and its stabilization by cross-linking

X-ray photoelectron spectroscopy (XPS) measurement of the ratio of poly(3,4-ethylenedioxythiophene) (PEDT) to polystyrenesulfonate (PSS) reveals accumulation of PEDT+ at the interface between the PEDT:PSSH hole-injection layer and the organic semiconductor during diode operation. This ionic drift of PEDT+ occurs even at low fields of 1Vcm−1, which will have an impact on the operational stability of the characteristics of organic light-emitting diodes. XPS and Raman spectroscopy indicate that dedoping of PEDT+ does not occur significantly in hole-only devices. Cross-linking at the 1mol% level can stabilize the conducting polymer sufficiently against electromigration.