Operando measurements of top-emission organic light-emitting diode using electric-field-induced doubly resonant sum-frequency generation vibrational spectroscopy

We performed sum-frequency generation (SFG) measurements on top-emission (TE) organic light-emitting diodes (OLEDs) during operation. These measurements require the detection of SFG light using probe laser beams transmitted through a 9 nm thick semitransparent Mg:Ag metal electrode. Only the SFG signals from the hole transport material (4,4′-bis[N-(1-naphthyl)-N-phenylamino]-biphenyl, NPD) are observed for the device configuration of the OLEDs in this study. The SFG peak (at approximately 1603 cm−1) shows a linear dependence on the applied voltage. This indicates that the NPD-derived SFG is enhanced by an electric-field-induced (EFI) effect owing to injected charges. In addition, the SFG efficiency of the TE OLED is considerably higher than that of a bottom-emission (BE) OLED manufactured in the same batch using the same material and film thickness. The difference in the SFG responses of the BE and TE OLEDs is caused by structural differences rather than the layer composition. The results of this study indicate that EFI-doubly resonant-SFG operando measurements can be applied to analyze the carrier behavior and charge transport of not only BE OLEDs but also TE OLEDs, the latter of which are used more commonly as commercial products.