Abstract
We have used polyvinylcarbazole, PVK, in combination with poly(p-phenylene vinylene), PPV and a green-emitting, soluble derivative, poly(4,4'-diphenylene diphenylvinylene) or briefly PDPV, to fabricate triple-layer light-emitting diodes PPV/PVK/PDPV/metal, with indium tin oxide as the anode. We report luminances in excess of 1300 cd/m(2) with Al cathodes, turn-on voltages of similar to 8 V in structures 300 nm thick, and internal efficiencies in the range 0.4%-0.8% for Al and Ca cathodes: at least ten times higher than for single-layer PDPV devices. Quantum efficiencies for Ca or Al electrodes are only a factor 1.5 different. The improvements over single-layer PDPV devices are considered to be due to better hole injection by PPV and to the formation of a hole barrier at the PPV/PVK interface, which forces a better balance of the electron and hole currents. (C) 1996 American Institute of Physics.