Charge injection into OLED's during operation studied by Electroabsorption screening

Organic light emitting devices (OLED) were operated using an AC voltage with DC offset (operating voltage). Electroabsorption (EA) signals as a function of a DC-offset were measured at 2f. Higher absolute DC offset voltages lead to a reduction of the EA signal. The effect observed can well be explained in terms of an extended electroabsorption model including Thomas-Fermi screening: The AC component of the electric field is shielded due to charge injection caused by the DC offset This new device characterization technique allows monitoring the charge carrier statistics in the device during operation. It was found that the EA signal indicates the threshold of monopolar and bipolar charge injection. The presence of negative charges enhances drastically the injection and presence of positive charges in the device