Degradation in organic light-emitting devices (OLEDs) is generally driven by reactions involving excitons and polarons. Accordingly, a common design strategy to improve OLED lifetime is to reduce the density of these species by engineering an emissive layer architecture to achieve a broad exciton recombination zone. Here, the effect of exciton density on device degradation is analyzed in a mixed host emissive layer (M-EML) architecture which exhibits a broad recombination zone. To gain further insight into the dominant degradation mechanism, losses in the exciton formation efficiency and photoluminescence (PL) efficiency are decoupled by tracking the emissive layer PL during device degradation. By varying the starting luminance and M-EML thickness, the rate of PL degradation is found to depend strongly on recombination zone width and hence exciton density. In contrast, losses in the exciton formation depend only weakly on the recombination zone, and thus may originate outside of the emissive layer. These results suggest that the lifetime enhancement observed in the M-EML architectures reflects a reduction in the rate of PL degradation. Moreover, the varying roles of excitons and polarons in degrading the PL and exciton formation efficiencies suggest that kinetically distinct pathways drive OLED degradation and that a single degradation mechanism cannot be assumed when attempting to model the device lifetime. This work highlights the potential to extract fundamental insight into OLED degradation by tracking the emissive layer PL during lifetime testing, while also enabling diagnostic tests on the root causes of device instability.
Bibliographical noteFunding Information:
This work was supported by The Dow Chemical Company. J.S.B. acknowledges support from the National Science Foundation Graduate Research Fellowship under grant no. 00039202. The authors acknowledge useful discussions with Dr. D. Wayne Blaylock, Dr. Peter Trefonas, Dr. Hong-Yeop Na, and Dr. Dominea Rathwell.
© 2018 American Chemical Society.
- exciton formation
- exciton quenchers
- recombination zone