Electrical Current Aging of Mixed-Host Organic Light-Emitting Devices with Thin Doped Layer

Abstract

Time-evolved electrical and optical characteristics of an organic light-emitting device (OLED), which exhibited a bipolar emitting layer (EML) with thin doped layer insertion at different positions, were investigated. When the OLED was “fresh,” the dopant near the recombination zone peak of the bipolar EML resulted in a voltage reduction due to the enhancement of the recombination current, and a strong red dopant emission was observed from the electroluminescence (EL) spectra. After constant electrical current aging, spectral changes were observed by analyzing the time-evolved EL spectra, which exhibits two discernible peaks from the green host and the red dopant. By optically exciting OLED at different wavelengths, it was observed that the loss of green emission from the host varied with different dopant positions, which resulted in a nonuniform degradation in the EML. For the dopant near the recombination zone peak, a reduction in green emission was most serious. It suggested that the enhancement of the recombination current induced by the dopant resulted in an increase in local temperature, which helped to deteriorate the organic material nearby, resulting in such nonuniform degradation behaviors.