Probing charge behaviour in multilayer organic light-emitting diodes via electronic sum-frequency generation spectroscopy†

Abstract

Understanding the charge behavior inside organic layer interfaces in multilayer organic light-emitting diodes (OLEDs) is essential for improving device efficiency and lifetime. However, examining charge transport during voltage application passing through these organic interfaces in ultrathin and in encapsulated OLEDs is extremely challenging. To address this, electronic sum-frequency generation (ESFG) spectroscopy, a non-invasive technique, offers interface-selective information on the electronic structure of organic interfaces under light-emitting conditions. This study demonstrates the capabilities of ESFG spectroscopy by comparing the spectra of three different OLED devices with buried interfacial electronic structures under operation. The ESFG spectra revealed ESFG signal increases in intensity at the absorption band of the hole transport material upon voltage application and decreases in ESFG intensity at the absorption band of the light emitting layer. This observation is attributed to the electrical potential balance of the specific organic layers inside the devices caused by charge injection into the devices. Time-resolved ESFG measurements using square-wave pulse voltages have also enabled a detailed investigation of the electric field formation process caused by charge injection into the devices. This technique is an innovative, highly effective, and nondestructive spectroscopic approach for investigating electric-field formation owing to injected charges in solid-state thin-film devices.