Ultrahigh efficiency green polymer light-emitting diodes by nanoscale interface modification

Abstract

We report highly efficient green polymer light-emitting diodes (PLEDs) achieved by introducing a nanoscale interfacial layer, made of calcium (2) acetylacetonate [Ca(acac)2], between the aluminum cathode and the green polyfluorene polymer. Ca(acac)2 is solution processible, therefore it is ideal for the fabrication of PLEDs. It is believed that the Ca(acac)2 layer plays multiple roles in enhancing the device performance. Firstly, it enhances the injection of electrons, which are the minority carriers in our green polyfluorene PLEDs. Secondly, it provides a buffer layer, preventing the quenching of luminescence from the aluminum electrode. Thirdly, it behaves as a hole-blocking layer, and subsequently enhances exciton formation. Based on Ca(acac)2/aluminum cathode, we obtained device efficiency as high as 28 cd/A at 2650 cd/m2 brightness, which is an improvement by a more than a factor of 3 over devices using calcium/aluminum as the cathode.