Efficient green light emitting diodes from a phenylated derivative of poly(p-Phenylene-Vinylene)

We have used polyvinylcarbazole, PVK, in combination with poly(p-phenylenevinylene), PPV and a green-emitting, soluble derivative, poly(4,4ʹ-diphenylene diphenylvinylene), PDPV, to fabricate triple-layer light-emitting diodes ITO/PPV/PVK/PDPV/Metal. We report luminances in excess of 1300 cd/m2 with Al cathodes, turn-on voltages of ~8V in 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 electrodes are only a factor 1.5 different. The improvements over single-layer 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.