The impact of high bias voltages on the luminance characteristics of light-emitting electrochemical cells

We report on the opto-electronic characteristics (in particular with respect to the efficiency behaviour) of organic light-emitting diodes (oLEDS) and organic light-emitting electrochemical cells (oLECS), both prepared from a blue-green emitting conjugated polymer, (methyl substituted ladder type poly (p-phenylene), mLPPP), as electronically conductive component. Different from the oLEDs, the oLECs are characterized by a distinct decrease of efficiency upon bias voltage increase. From scan rate dependent studies we conclude that this efficiency drop is not only due to device decomposition upon operation above the window of electrochemical stability but also reflects (and thereby sheds some more light on) the intrinsic mode of oLEC operation. Due to the declining width of the intrinsic region upon ongoing pin-junction formation field induced and electrode induced luminance quenching processes also play a decisive role.