Degradation Mechanism of Solution-Processed Organic Light-Emitting Diodes: Sputter Depth-Profile Study

Abstract

We have studied the origin of degradation in solution-processed organic light-emitting diodes (s-OLEDs) by using X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectroscopy (TOF-SIMS) depth profiling. Successful mechanical exfoliation of top cathode layers from degraded s-OLEDs and Ar gas cluster ion beam sputtering of remaining organic layer stack enabled us to probe chemical and positional changes at the relevant buried interfaces in the degraded device. Our XPS depth profile data showed that device operation induced migration of PEDOT in PEDOT:PSS layer toward the interface between hole transport layer (HTL) and emissive layer (EML), which lowers the hole injection efficiency and shifts the carrier recombination zone toward the interface. TOF-SIMS depth profile data showed that molecular decomposition occurred only in the EML after the device operation. These results depict a scenario in which migration of PEDOT lowers the hole injection efficiency which in turn shifts the recombination zone toward the mixed interface between HTL and EML, where an accumulation of excitons and hole polarons induces strong exciton-polaron quenching that results in molecular break down and device degradation.