|
Electroluminescence (EL) from heterostructure devices made of organic poly (phenylene vinylene), PPV, and inorganic semiconductor CdSe nanocrystals have been investigated, along with cathodoluminescence (CL) from thin films of ZnS doped with CdSe-ZnS core-shell nanocrystals. In the EL devices, the organic PPV structure, built next to the anode using the technique of molecular layer-by-layer sequential adsorption, serves as the hole transport layer. The inorganic layer, adjacent to the electrode and made of spin cast CdSe nanocrystals passivated with either organic groups or with a thin layer of ZnS, is the emitting layer. The ZnS host film in the CL devices, built using chemical vapor deposition, serves as the support medium for the dispersed nanocrystals, but also provides additional passivation to the surface of those nanocrystals. We find that the EL and CL signals almost exclusively originate from the inorganic nanocrystal in both cases, i.e., EL comes from the nanocrystal layer in the heterostructure device while CL is generated from the dispersed particles in the composite film. The external EL quantum efficiency, (eta) EL, is not enhanced by the presence of ZnS overcoating, opposed to the observed increase in the photoluminescence (PL) quantum yield. However, we find that the CL emission and its stability are substantially improved by the presence of ZnS around the emitting nanocrystal cores. These observations reflect a difference in the effects of overcoating ont he various luminescence processes. On the one hand, a ZnS overlayer is associated with an additional energetic barrier that reduces the efficiency of charge injection into the nanocrystals for EL. On the other hand, PL and CL processes only benefit from the surface passivation with ZnS.
|
