ELECTROLUMINESCENCE FROM HETEROSTRUCTURES OF POLY(PHENYLENE VINYLENE)AND INORGANIC CDSE NANOCRYSTALS

Electroluminescence (EL) and photoluminescence (PL) from heterostructu re thin films made of organic poly (phenylene vinylene), PPV, and inor ganic semiconductor CdSe nanocrystals are investigated. In these devic es, the organic PPV structure is built next to an indium tin oxide ano de, using the technique of molecular layer-by-layer sequential adsorpt ion, and serves primarily as the hole transport layer. The inorganic l ayer, adjacent to an Al electrode, is made of spin cast CdSe nanocryst als, passivated with either organic groups or with a wider band gap se miconductor, e.g., ZnS in the present case. We find that the electrolu minescence signal is almost exclusively generated within the inorganic layer, with a very weak contribution from the PPV layer at higher app lied voltage. The performance of these heterostructure devices is infl uenced by the thickness of the dot layer. Lifetime tests reveal promis ing stability, with devices operating continuously over 50-100 h. Valu es of the external quantum efficiency, eta(ext), as high as 0.1% are r eached. The quantum efficiency is not enhanced by the presence of ZnS overcoating, as opposed to the observed increase in the PL quantum yie ld. This reflects a difference in the efficiency of charge injection i nto the nanocrystals due to the ZnS overlayer. (C) 1998 American Insti tute of Physics.