Pinpointing the origin of the increased driving voltage during prolonged operation in a phosphorescent OLED based on an exciplex host

Abstract

We report on the origin of the reduced power efficiency in a red phosphorescent OLED with an exciplex host after prolonged operation. The power efficiency is reduced solely by an increased driving voltage while the radiant flux remains constant. An electrical model describing the driving voltage increase is, thus, sufficient to explain the reduced power efficiency. The electrical model of the fresh OLED and at different stages of degradation was devised from four different measurement methods. Using multiple measurement methods to determine the model parameters results in a rather unique set of model parameters, despite the large number of model parameters (38) as revealed by a correlation analysis. The increase in driving voltage could be reproduced by modifying only 7 out of the 38 model parameters. A sensitivity analysis identified the parameters with the largest effect (66%) on the driving voltage increase to be the trap density and the mobility of the employed hole transporting layer. This work highlights the benefit of using multiple measurement methods to derive reliable model parameters and the use of a sensitivity analysis to pinpoint the origin of the investigated property.