High-efficiency red electroluminescence from a narrow recombination zone confined by an organic double heterostructure

Abstract

Red light-emitting diodes (LEDs) with both a conventional bilayer structure and a double heterostructure (DH) have been investigated. In these LEDs, ${\mathrm{N,N}}^{\prime }\mathrm{-bis-(1-naphthl)-diphenyl-1},$ $1^{\prime }{\mathrm{-biphenyl-4,4}}^{\prime }\mathrm{-diamine}$ (NPB), 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), and tris(8-quinolinolato) aluminum $({\mathrm{Alq}}_3)$ were used as hole-transporting, hole-blocking, and electron-transporting layers, respectively. The bilayer and DH LEDs had a configuration of ${\mathrm{ITO/NPB/Alq}}_3:\mathrm{red}$ ${\mathrm{dopant/Alq}}_3\mathrm{/MgAg}$ and ${\mathrm{ITO/NPB/Alq}}_3:\mathrm{red}$ ${\mathrm{dopant/BCP/Alq}}_3\mathrm{/MgAg,}$ respectively. Three kinds of red fluorescent dyes—nile red, DCJTB, and DCM—were used as dopants. Compared with the bilayer structures, the luminance efficiencies of the DH LEDs were found to increase as much as 100%. We attribute the efficiency enhancement to the formation of a narrow recombination zone, in which both charge carriers and excitons were confined. High charge concentrations in the emissive layer resulted in efficient collision capture in the electron–hole recombination process. Exciton confinement led to improved energy transfer. The two factors were simultaneously operating and consequently benefitted from efficiency enhancement.