Thermally transferred emitting layer at low pressure for residual solvent-free organic light-emitting diodes

Abstract

We utilize thermal transfer technology to suggest a novel baking process for the emitting layer (EML) of residual-solvent-free organic light-emitting diodes (OLEDs). We deposited the EML onto an intermediate substrate using a conventional spin-coating process, after which the EML was transferred to the device by evaporating the EML through heating of the intermediate substrate. The EML films were fully transferred from an intermediate substrate to the target substrate at a temperature exceeding 350 °C under both a low vacuum of ∼10−3 Torr and a high vacuum of ∼10−6 Torr. The quantity of residual solvent was carefully investigated using gas chromatography-mass spectrometry (GC-MS) and nuclear magnetic resonance (NMR) analysis methods. While residual solvent peaks were found to exist 1.69 min after the beginning of the measurement process in the conventional spin-coating EML films despite the baking condition of 150 °C, solvent peaks were not observed in the EML films created with the novel baking process under conditions identical to those used in the GC-MS measurement. Moreover, according to the NMR analysis, the solvent peak of toluene at 2.3 ppm was not found in the thermally transferred EML films. Furthermore, we investigated the device performance capabilities between samples fabricated using the conventional baking process and those created with the novel thermal transfer process. In spite of low pressure of ∼10−3 Torr, the current efficiency and the lifetime of the OLED device created via the thermal transfer process were enhanced owing to the elimination of the residual solvent.