Direct probing of charge carrier behavior in multilayered organic light-emitting diode devices by time-resolved electric-field-induced sum-frequency generation spectroscopy

Time-resolved electric-field-induced sum-frequency generation (EFI-SFG) spectroscopy was employed to study the charge behavior of multilayer organic light-emitting diodes (OLEDs). Through application of the square wave pulse bias to the OLEDs, compensation for the polarization charges in the electron transport layer and the generation of 4,4'-bis[N-(1-naphthyl-N-phenylamino)-biphenyl](alpha-NPD) cations were observed. When the pulse voltage was turned off, the a-NPD cations immediately disappeared, confirming that charge recombination occurs at the interfaces. We therefore concluded that time-resolved EFI-SFG is useful for directly probing the carrier behavior in OLEDs in addition to identifying the origin of the charge carriers present in OLEDs. (C) 2017 The Japan Society of Applied Physics