Thick polymer light-emitting diodes with very high power efficiency using Ohmic charge-injection layers

Abstract

We demonstrate the optimization of single-layer polymer LED structures with active layers with thicknesses of the order of 1 micron. By using a combined approach of the addition of MoO3, as a bottom hole-injection layer, and the incorporation of such thick active layers, exceptionally high performance metrics are achieved. In particular, brightnesses of 1000 cd m−2 at driving voltages of only 6.8 V, corresponding to a power efficiency of 7.8 lm W−1, a current efficiency of 17.2 cd A−1, and external quantum efficiency of 5.6%, are reported for devices based on F8BT (Poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)]). A side-by-side comparison, between the standard LED structure and hybrid structures, demonstrates that with MoO3 as bottom hole-injection layer, the electron and hole charge carriers are both giving space-charge limited current for both carriers due to the Ohmic contacts. The devices hence show improved charge carrier balance, and, most importantly, high brightness at low operational voltage. Such thick active-layer devices with high performance metrics, in addition to improved engineering and processing tolerances, are thus especially important for application to high-throughput device fabrication methods.