Enhanced light out-coupling efficiency and reduced efficiency roll-off in phosphorescent OLEDs with a spontaneously distributed embossed structure formed by a spin-coating method

Abstract

A simple and low-cost spin-coating method was proposed to form a spontaneously distributed embossed structure. With such an embossed structure produced by Al₂O₃ nanospheres (NPs) with different diameters, the out-coupling efficiency is increased in all viewing directions without introducing significant spectral changes and the efficiency roll-off is suppressed at the same time. When the concentration of Al₂O₃ ethanol solution is chosen as 0.08 mg ml⁻¹, a peak current efficiency (CE) of 52.1 cd A⁻¹, peak power efficiency (PE) of 32.7 l m W⁻¹, and peak external quantum efficiency (EQE) of 20.9% are obtained. From the maximum efficiency value to the case where the brightness is 3000 cd m⁻², the roll-off of CE, PE and EQE for device B are only 6.53%, 26.30% and 6.22%, respectively. The improved properties are attributed to greatly enhanced light extraction and an enlarged charge carrier recombination region.