IMPROVED EFFICIENCIES IN LIGHT-EMITTING-DIODES MADE WITH CDSE(CDS) CORE SHELL TYPE NANOCRYSTALS AND A SEMICONDUCTING POLYMER/

We report experiments on bilayer light emitting diodes made with organ ically capped CdSe(CdS) core/shell type semiconductor nanocrystals and an electroluminescent (EL) semiconducting polymer [poly(p-phenylenevi nylene) or PPV]. The devices emit from red to green with external quan tum efficiencies of up to 0.22% at brightnesses of 600 cd/m(2) and cur rent densities of 1 A/cm(2). They have operating voltages as low as 4 V and lifetimes under constant current flow of hundreds of hours. Most of these numbers are significant improvements over similar devices ma de with CdSe nanocrystals. The devices show either nanocrystal-only EL or a combination of nanocrystal and PPV EL, depending on nanocrystal layer thickness. The nanocrystal EL is dependent on nanocrystal size. Some devices show a voltage dependent spectral output. The spectral ou tput is consistent with a field dependent electron range in the nanocr ystal layer limited by carrier trapping. (C) 1997 American Institute o f Physics.