Abstract
Conjugated polymers can be use to provide charge transport and emissive layers in a range of thin-film electroluminescent devices. Electroluminescence results from radiative decay of excitons which are formed by electron-hole capture. Device efficiency is particularly sensitive to the balancing of electron and hole currents injected from opposite electrodes, and this is best achieved at the heterojunction between two polymer layers with different electronegativities. We report here the properties of a conjugated polymer with high electron affinity, a cyano derivative of poly (p-phenylene vinylene), PPV. This polymer shows an electron affinity considerably higher than than that of PPV, and is therefore suitable for use as the electron transporting layer in heterostructure LEDs. It shows efficient photoluminescence due to radiative decay of singlet excitons, and photoinduced absorption in the IR, similar to that in PPV, which we attribute to excitations of photogenerated triplet excitons. EL devices formed with a heterojunction between PPV and this cyano derivative of PPV, with indium/tin oxide as positive electrode, PPV, cyano-PPV layers and an aluminium negative electrode, show internal quantum efficiencies as high as 4%.