Yb 2 O 3 / Mo O 3 connecting electrodes for tandem organic semiconductor devices

This paper reports that ${\mathrm{Yb}}_{2}{\mathrm{O}}_{3}/{\mathrm{Mo}\mathrm{O}}_{3}$ stacks can be used effectively as a connecting electrode, enabling highly efficient tandem organic light-emitting diodes (OLEDs). The current efficiency of these tandem OLEDs more than doubles, from approximately 90 to 200 cd/A, compared to the single-layer devices. Additionally, this connecting electrode introduces little increase in the tandem OLED driving voltage, maintaining a value close to the sum of two single-layer devices. However, the energy-level alignments measured by ultraviolet photoemission spectroscopy show significant energy barriers in the ${\mathrm{Yb}}_{2}{\mathrm{O}}_{3}/{\mathrm{Mo}\mathrm{O}}_{3}$ stack. Analysis using photoemission measurements revealed the presence of gap states in the oxide stack and a high differential work function, 2.28 vs 6.38 eV on each side of the ${\mathrm{Yb}}_{2}{\mathrm{O}}_{3}/{\mathrm{Mo}\mathrm{O}}_{3}$ stack. Variable-temperature current-voltage measurements on ${\mathrm{Yb}}_{2}{\mathrm{O}}_{3}/{\mathrm{Mo}\mathrm{O}}_{3}$ stacks revealed that the gap states behave like Anderson-Mott localized quantum states, which form efficient hopping conduction paths for electrons and holes. These findings demonstrate that the gap states in electrodes can be effectively used to transport charges in tandem semiconductor devices.