Direct observations of the charge behavior of a high-efficiency blue organic light-emitting diode under operating conditions using electric-field-induced doubly resonant sum-frequency-generation vibrational spectroscopy

We investigated the carrier behavior in a high-efficiency multilayer blue organic light-emitting diode (OLED) that employs the triplet–triplet annihilation (TTA) phenomenon using electric-field-induced doubly resonant sum-frequency generation (EFI-DR-SFG) vibrational spectroscopy. We observed enhancements of the EFI-DR-SFG signals—particularly the peaks at 1404, 1464, and 1601 cm−1—when we applied forward- or reverse-bias voltages to the blue OLEDs. The results of our EFI-DR-SFG analyses of the carrier behavior in an operating blue OLED showed changes in the peak intensities at 1404 and 1464 cm−1, indicating that the electrons accumulate at the electron-blocking layer/emission layer interface. Conversely, the changes in the intensities of the peaks obtained from the hole-transport materials are caused by the cancelation of the interfacial polarization charges at the hole-injection layer/hole-transport layer interface. These results demonstrate that the EFI-DR-SFG technique is a powerful and effective tool for directly investigating the carrier behavior in practical multilayer organic devices with complicated operating mechanisms.