Electroluminescence of different colors from polycation/CdTe nanocrystal self-assembled films

Water soluble thiol capped CdTe nanocrystals are assembled into ultrathin f ilms in combination with poly(diallyldimethylammonium chloride) (PDDA) by t he self-assembly method of layer-by-layer adsorption of oppositely charged polyelectrolytes. Electroluminescent devices, which produce different color emissions, are fabricated by sandwiching CdTe/PDDA films between indium-ti n-oxide (ITO) and aluminum electrodes using CdTe nanocrystals of different sizes. It is shown that the electroluminescence (EL) spectra of the CdTe/po lymer films are nearly identical to the photoluminescence spectra of the co rresponding CdTe nanocrystals in aqueous solutions. The devices produce roo m-light visible light output with an external quantum efficiency up to 0.1% . Light emission is observed at current densities of 10 mA/cm(2) and at low onset voltages of 2.5-3.5 V, which depends on the thickness of the film in dicating field-dependent current injection. A variation of the EL efficienc y with the size of the CdTe particles is observed and explained by the size dependent shift of the CdTe energy levels with respect to the work functio n of the electron injecting Al electrode. This is confirmed by the behavior of two-layer devices prepared from two differently sized CdTe particles be ing spatially separated, i.e., one size CdTe near ITO and the other size Cd Te near Al by using the self-assembly method. (C) 2000 American Institute o f Physics. [S0021-8979(00)06404-5].