All-solution-processed fluorene/dibenzothiophene-S,S-dioxide blue co-oligomer light-emitting diodes with an electron transporting PEI/ultrafine-ZnO-nanoparticle bilayer

Abstract

In this article, an all solution-processed blue organic light-emitting diode (OLED) comprising the emissive fluorene/dibenzothiophene-S,S-dioxide co-oligomer, 3,7-bis[7-(9,9-di-n-hexylfluorenyl-2,7-diyl)-9,9-di-n-hexylfluoren-2-yl]-dibenzothiophene-S,S-dioxide (HFSO), and a poly-ethyleneimine (PEI)/ultrafine-ZnO-nanoparticle (NP) bilayer in the conventional device structure is proposed for the first time. The device configuration can be described as indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/HFSO/PEI/ZnO-NP/Al. By carefully choosing the solvent for PEI, the PEI interlayer can be deposited without dissolving the underlying HFSO. Compared to our normal-sized ZnO NPs with the average size (ψ_avg) of 3.9 nm, the device with the ultrafine ZnO NPs (ψ_avg ∼ 2.9 nm) decreases the turn-on voltage from 3.3 to 2.9 V due to the reduction of the electron-encountered energy barrier. Furthermore, by inserting a PEI interlayer between the HFSO and ZnO layers, the device with an electron transporting bilayer can further improve the turn-on voltage and output luminance for the formation of a molecular dipole while avoiding the exciton quenching of HFSO from the adjacent ZnO. Thereby, the OLED with a PEI/ZnO-NP bilayer exhibits a blue electroluminescence peaked at 452 nm purely from HFSO, a very low turn-on voltage of ∼2.6 V, a high maximum luminance of ∼12 000 cd m⁻² at 7.6 V, a highest current efficiency of ∼1.55 cd A⁻¹, and stable CIE coordinates of (0.144, 0.137).