Role of the Charge Generation Layer in Tandem Organic Light-Emitting Diodes Investigated by Time-Resolved Electroluminescence Spectroscopy

In the tandem organic light-emitting diodes (tandem OLEDs), the charge generation layer (CGL) plays an essential role in the performance, which is composed of an n-doped layer and a p-doped layer for injection of electrons and holes, respectively. However, the working mechanism of the CGL is still not thoroughly clear so far. By using time-resolved electroluminescence (TREL) measurements, we found that, at the initial stage of device operation, electrons and holes are supplied from the CGL to start the OLED units for electroluminescence. At the subsequent stage, the consumed electrons and holes in the CGL are refilled by the electrons and holes injected from the cathode and anode, respectively; then, the bipolar currents reach a steady state gradually. The time required for charge refilling in the CGL decreases with charge fluxes supplied from the electrodes. Such understanding on the working mechanism of the CGL could inspire design concepts for tandem OLEDs.