Electroluminescence from heterostructures of poly(phenylene vinylene) and inorganic CdSe nanocrystals

Electroluminescence (EL) and photoluminescence (PL) from heterostructure thin films made of organic poly (phenylene vinylene), PPV, and inorganic semiconductor CdSe nanocrystals are investigated. In these devices, the organic PPV structure is built next to an indium tin oxide anode, using the technique of molecular layer-by-layer sequential adsorption, and serves primarily as the hole transport layer. The inorganic layer, adjacent to an Al electrode, is made of spin cast CdSe nanocrystals, passivated with either organic groups or with a wider band gap semiconductor, e.g., ZnS in the present case. We find that the electroluminescence signal is almost exclusively generated within the inorganic layer, with a very weak contribution from the PPV layer at higher applied voltage. The performance of these heterostructure devices is influenced by the thickness of the dot layer. Lifetime tests reveal promising stability, with devices operating continuously over 50–100 h. Values of the external quantum efficiency, ${\eta }_{\mathrm{ext}},$ as high as 0.1% are reached. The quantum efficiency is not enhanced by the presence of ZnS overcoating, as opposed to the observed increase in the PL quantum yield. This reflects a difference in the efficiency of charge injection into the nanocrystals due to the ZnS overlayer.