Charge injection into OLED's during operation studied by Electroabsorptionscreening

Organic light emitting devices (OLED) were operated using an AC voltage wit h DC offset (operating voltage). Electroabsorption (EA) signals as a functi on of a DC-offset were measured at 2f. Higher absolute DC offset voltages l ead to a reduction of the EA signal. The effect observed can well be explai ned in terms of an extended electroabsorption model including Thomas-Fermi screening The AC component of the electric field is shielded clue to charge injection caused by the DC offset This new device characterization techniq ue allows monitoring the charge carrier statistics in the device during ope ration. It was found that the EA signal indicates the threshold of monopola r and bipolar charge injection, The presence of negative charges enhances d rastically the injection and presence of positive charges in the device.