EFFICIENT GREEN LIGHT-EMITTING-DIODES FROM A PHENYLATED DERIVATIVE OFPOLY(P-PHENYLENE-VINYLENE)

We have used polyvinylcarbazole, PVK, in combination with poly(p-pheny lenevinylene). PPV and a green-emitting, soluble derivative, poly(4,4' -diphenylene diphenylvinylene), PDPV, to fabricate triple-layer light- emitting diodes ITO/PPV/PVK/PDPV/Metal. We report luminances in excess of 1300 cd/m(2) with Al cathodes, turn-on voltages of similar to 8V i n structures 300 nm thick, and internal efficiencies in the range 0.4- 0.8% for Al and Ca cathodes; at least 10 times higher than for single- layer PDPV devices. In addition, the efficiencies for Ca or Al electro des are only a factor 1.5 different. The improvements over single-laye r PDPV devices are interpreted as 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.