Highly efficient green polymer light-emitting diodes through interface engineering

By interface engineering, we have improved the quantum efficiency for green polyfluorene polymer light-emitting diodes by three fold: the efficiency improved from 9 to 28 cd/A. This interface engineering was achieved by inserting a thin layer of calcium (2) acetylacetonate, denoted as Ca(acac)(2), at the polymer/ metal cathode interface. The Ca(acac)(2) layer behaves in a multifunctional way. It assists the electron injection by lowering the electron-injection barrier. In the meanwhile, hole injection is enhanced by the accumulated electrons in the polymer layer. This effect is believed to lower the barrier height for hole-injection through the Schottky effect. Finally, the Ca(acac)(2) layer works as a hole block layer to block the holes. As a result of the charge balance and charge confinement, the device quantum efficiency increases dramatically.