Improved efficiencies in light emitting diodes made with CdSe(CdS) core/shell type nanocrystals and a semiconducting polymer

Abstract

We report experiments on bilayer light emitting diodes made with organically capped CdSe(CdS) core/shell type semiconductor nanocrystals and an electroluminescent (EL) semiconducting polymer [poly($p$ -phenylenevinylene) or PPV]. The devices emit from red to green with external quantum efficiencies of up to 0.22% at brightnesses of $\text{600\hspace{0.17em}}{\mathrm{cd/m}}^{\mathrm{2}}$ and current densities of $\text{1\hspace{0.17em}}{\mathrm{A/cm}}^{\mathrm{2}}.$ 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 made 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 output is consistent with a field dependent electron range in the nanocrystal layer limited by carrier trapping.