Abstract
We here report polymer light-emitting diodes with substituted poly( p-phenylene ethynylenes) as the emissive layer. Yellow—green electroluminescence was observed for different poly( p-phenylene ethynylene) derivatives. Surprisingly, and importantly in view of stability issues, devices with an aluminium cathode were found to have a higher external quantum efficiency (0.035%) and a lower onset voltage for electroluminescence (10.8 V) than those with a calcium cathode. These results are explained in terms of a lower energy barrier for electron injection than for hole injection, consistent with the ionization potential of poly( p-phenylene ethynylenes) which was determined to be 6.3 eV below vacuum level with ultraviolet photoelectron spectroscopy and 5.8 eV with cyclovoltammetry.
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