Bias-dependent current efficiency in polymer light-emitting diodes

The current efficiency (CE) of single-layer poly-p-phenylene vinylene-based light-emitting diodes (PLEDs) at low voltages is poor and strongly bias dependent, which has been attributed to the quenching of excitons at the metallic cathode. In the absence of exciton quenching the CE is expected to be voltage independent. We have verified this hypothesis by investigating the electro-optical properties of double-layer PLEDs, where all additional polymeric electron transport layer (ETL) is inserted between the light-emitting layer and the cathode. It is confirmed that for ETLs with thicknesses of 40 nm or larger, where exciton quenching is strongly weakened, the conversion efficiency is indeed voltage independent. Reducing the ETL to 20 nm leads to a Pronounced improvement of both quantum and power efficiencies of the PLED at low voltages.