Sum-frequency generation spectroscopy analysis of the charge carrier behavior in blue fluorescent OLEDs fabricated using materials with undisclosed molecular structures

The charge carrier behavior during the operation inside the organic light-emitting diodes (OLEDs) fabricated with materials of undisclosed molecular structures was analyzed using interface-sensitive vibrational sum-frequency generation (VSFG) spectroscopy. Blue fluorescent OLEDs utilizing triplet-triplet annihilation were investigated. Three devices with different electron mobilities in the electron transport layer (ETL) were fabricated by varying only the mixing ratio of their two components. Current density−voltage−luminance (J−V−L) characteristics indicated that the device with a 4:1 ETL ratio exhibited the highest current density and ETL electron mobility. This device also exhibited significantly stronger VSFG peaks originating from the hole transport layer, electron blocking layer (EBL), and emission layer (EML), indicating excess electron accumulation at the EBL/EML interface. Furthermore, J−V−L characteristics revealed that the luminous efficiency increased with higher electron mobility, while transient electroluminescence measurements showed that the prompt-to-delayed fluorescent ratio remained constant. These results suggest that the device with the highest electron mobility exhibits superior luminous efficiency, which can be attributed to increased radiative recombination at the EBL/EML interface. These findings demonstrate that operando VSFG spectroscopy is a powerful tool for analyzing OLEDs, even when the constituent materials have undisclosed molecular structures.