Operando sum-frequency generation spectroscopy of high-efficiency OLEDs for probing the charge carrier behavior

Operando analysis of organic devices is crucial for understanding the device operation mechanism and intrinsic degradation mechanism. Buried organic layers in OLEDs during operation have been investigated using electric-field-induced doubly resonant sum-frequency generation (EFI-DR-SFG) and time-resolved EFI-DR-SFG experiments. The time-resolved EFI-SFG technique allows us to observe the carrier transportation directly and non-destructively. DC electric field leads to a significant increase in the SFG intensity produced by the charged carriers. Electric-field-induced SFG results clearly show that charge accumulation occurs at the interface between the light emitting layer and the electron-blocking layer in the blue OLED and that this charge accumulation strongly contributes for the light emission. Particularly in the TTA process, high charge accumulation is effective in generating triplet excitons and improving the luminescence efficiency, but at the same time, it reduces the lifetime of the device.