Mechanistic studies of Yb2O3/HAT-CN connection electrode in tandem semiconductor devices

Abstract

The optically transparent connecting electrode is much desired in fabrication of tandem optoelectronic devices. Yet, optically transparent materials, such as oxides, are electrically insulating. In this work, we show that low work function oxides Yb2O3 combing with high work function 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) molecule can be used as effective connecting electrodes to make high performance tandem organic light emitting diodes with negligible voltage loss. For instance, in a tandem device with two emission zones, yielding a brightness of 100 cd/m2, the voltage required is 5.3 V, which is approximately twice that of a single emission zone device. To gain insights into the band alignment of this electrode, we conducted the measurements, including ultraviolet photoelectron spectroscopy to analyze the electronic structures of occupied valence and gap states and reflection electron energy loss spectroscopy to study the unoccupied states. To understand the charge transport and injection behavior of this electrode, we conducted variable temperature charge transport measurements. Our findings reveal the presence of localized gap states within the Yb2O3/HAT-CN structure. These gap states effectively form a conduction pathway for facilitating the transport of charge carriers. At higher temperatures (≥200 K), charge transport is primarily limited by the Efros–Shklovskii type of hopping conduction through the localized states in the Yb2O3. Conversely, at lower temperatures (<200 K), the electrical current is limited by the properties of HAT-CN. These discoveries suggest that localized gap states at the oxides/organic heterojunctions can be effectively utilized in the fabrication of tandem semiconductor devices.