Effects on the spectrum of the blue organic light-emitting diodes by the indium tin oxide surface treatment

Abstract

Deionized water, sulfur acid (98%) (H2SO4), sodium hydroxide (NaOH) solution and oxygen plasma are used to treat ITO substrates, then blue organic light emitting devices (OLEDs) with the structure of ITO/NPB(50 nm)/BAlq₃(40 nm)/Alq₃(25 nm)/Mg:Ag are fabricated in the vacuum. The experiment results show that ITO treatments influence the electroluminescence (EL) spectrum of the blue OLEDs, the OLEDs which are treated by four methods the EL spectrums change from 496 to 455 nm. According to atom force microscope (AFM) and X-ray photoelectron spectroscopy (XPS) analysis, the surface morphology and surface composition are greatly changed, so the surface work function is changed, then recombination zone of the carrier is moved, which leading to the emission spectrums alter. The ITO substrate is treated by oxygen plasma, which has the highest oxygen concentration, so the surface work function is the greatest, the hole can jump the barrier and recombine in BAlq₃-Alq₃ interface, the spectral peak is at 496 nm. The ITO treated by acid solution, the oxygen concentration is less than that of plasma treatment, so the surface work function is also less than it, thus the hole can inject into the BAlq₃ layer, the spectral peak is at 480 nm. Roughness of deionized water treated ITO is the largest, which can became local high electric field, so some hole are excitated into the BAlq₃-Alq₃ interface, most of the hole are blocked by BAlq₃ barrier, the light from BAlq₃-Alq₃ interface and NPB layer, the spectral peak is at 474 nm. The ITO treated by NaOH solution, the oxygen concentration is the lowest, so the surface work function is also the lowest, all of the hole are blocked by BAlq₃ layer, they can only recombine in the NPB layer, the spectral peak is at 455 nm.