Effects of residual solvent in printed phosphorescent emissive thin films as, the origin of limited efficiency in organic light emitting diodes

Abstract

Using printed organic materials to fabricate organic light emitting diodes (OLEDs) is a low-cost process and one of the most innovative deposition technologies. Nozzle jet printing is a facile method with the advantages of high speed and large area fabrication. However, the printing process leaves residual solvent in the deposited organic films. How that remaining solvent affects the performance of the OLEDs, and the fundamental mechanism of that effect, is still unclear. In this study, we investigated the effect of residual solvent in a jet printed phosphorescent emissive layer. Gas chromatography-mass spectrography (GC–MS) was used to measure the amount of residual solvent, which was dependent on the film drying environment. The printed phosphorescent films exhibited weak intersystem crossing and decreased coupling between the vibrational states of the tris[2-(p-tolyl)pyridine]iridium(III) (Ir(mppy)3) in measurements of ultraviolet–visible absorption (UV–vis Abs), photoluminescence (PL), and electroluminescence (EL). The deterioration of the photophysical characteristics of the phosphorescence was attributed to the polaronic effect of the residual solvent molecules. The dipoles of the solvent molecules influenced the orbitals of surrounding organic molecules, and those perturbations induced nonradiative relaxation of the excitons. There was no shift in the peaks, but the intensities of the photophysical measurements decreased, indicating that the deterioration in phosphorescence was not chemical degradation. Finally, when the amount of residual solvent increased, the current density and the efficiency of the fabricated OLED decreased. The residual solvent also resulted in efficiency roll-off at high luminance in the devices.